green routes for synthesis of zeolites

Green Routes for Synthesis of ZeolitesXiangju Meng and Feng-Shou Xiao*

Department of Chemistry, Zhejiang University (XiXi Campus), Hangzhou 310028, China

CONTENTS

1. Introduction: Zeolites and Green Chemistry 15212. Green Routes for Use of Organic Templates in

Zeolite Synthesis 15222.1. Organic Templates in Zeolite Synthesis 15222.2. Synthesis of Zeolites with Low-Toxicity and

Cheap Organic Templates 15222.3. Synthesis of Zeolites with Recyclable Organ-

ic Templates 15242.4. Organotemplate-Free Routes for Synthesiz-

ing Zeolites 15252.4.1. Adjusting Molar Ratios of Starting Gels 15252.4.2. Zeolite Seed Solution-Assisted Ap-

proach 15262.4.3. Zeolite Crystal Seed-Directed Approach 1527

3. Synthesis of Zeolites under Relatively LowPressure: Use of Ionic Liquids as Solvents 15323.1. Ionic Liquids 15323.2. Ionothermal Synthesis of Aluminophos-

phate Zeolites 15323.3. Ionothermal Synthesis of Silica-Based Zeo-

lites 15334. Solvent-free Synthesis of Zeolites 1534

4.1. Solvent-free Synthesis 15344.2. Solvent-free Synthesis of Aluminosilicate

Zeolites 15344.3. Solvent-free Synthesis of Aluminophos-

phate-based Zeolites 15355. Synthesis of Zeolites with Relatively High

Efficiency: Use of Microwave Radiation 15355.1. Microwave-Assisted Synthesis 15355.2. Microwave-Assisted Hydrothermal Synthesis

of Zeolites 15365.2.1. Hydrothermal Synthesis of Zeolites

Assisted by Microwave Radiation 15365.2.2. Microwave-Assisted Crystallization of

Zeolite Crystals with Preferred Orienta-tion 1536

5.2.3. Microwave-Assisted Hydrothermal Syn-thesis of Zeolite Membranes 1537

5.3. Microwave-Enhanced Ionothermal Synthesisof Zeolites 1537

5.3.1. Ionothermal Synthesis of Zeolites As-sisted by Microwave Radiation 1538

5.3.2. Microwave-Enhanced Ionothermal Syn-thesis of Zeolite Membranes 1538

6. Summary and Perspectives 1538Author Information 1539

Corresponding Author 1539Notes 1539Biographies 1539

Acknowledgments 1539References 1540

1. INTRODUCTION: ZEOLITES AND GREENCHEMISTRY

Zeolite crystals with intricate micropores, strong acidity, andredox sites have been widely used as heterogeneous catalysts inthe petrochemical and fine chemical industries.1−4 For example,Y zeolites are widely used as solid acid catalysts in refiningprocesses and petrochemistry.1 The applications of TS-1zeolites in phenol hydroxylation and olefin epoxidation areregarded as milestones in green oxidation.5−7

Notably, most zeolites are usually synthesized underhydrothermal conditions from silicate or aluminosilicate gelsin alkaline media at temperatures between about 60 and 200°C.4,8 The main discoveries and advances in thinking in thefield of zeolite synthesis, especially in hydrothermal synthesisfrom the 1940s up to the present, have been carefully discussedby classical books and several recent extensive reviews.4,8−13

However, the hydrothermal synthesis of zeolites is not agreen process (that is, based on the concept of greenchemistry) due to the following:Organic templates. Modern synthetic methodologies for

synthesizing zeolites typically involve the use of organicmolecules that direct the assembly pathway and ultimately fillthe pore space.8 Removal of these templates normally requireshigh-temperature combustion that destroys these high-costcomponents, producing hazardous (NOx) as well as greenhousegases (CO2). The associated energy released, in combinationwith the formed water, can be extremely detrimental to theinorganic structure of zeolites.14

High pressure. Conventional hydrothermal synthesis ofzeolites involves heating the reaction mixture (80−200 °C) in apoly(tetrafluoroethylene)- (PTFE-) lined steel autoclave athigh autogenous pressure of solvent (mainly water) for a periodof time.8 The safety of the equipment is always of concern dueto this high autogenous pressure.Low efficiency. The hydrothermal synthesis of zeolites

sometimes takes long times (1−20 days) even at relatively high

Received: March 7, 2013Published: November 4, 2013

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temperature (80−200 °C),8 which is considered a high energycost process.The “green chemistry” concept was introduced in the

scientific community in the early 1990s and entails thatchemicals and chemical processes be designed to reduce oreliminate negative environmental impacts, involving reductionof waste and improvement of efficiency.15−18 Green chemistryencompasses several major research areas: (1) use of alternativesynthetic pathways (e.g., natural processes such as photo-chemistry and renewable biomass); (2) alternative reactionconditions (e.g., use of solvents with a reduced impact onhuman health) or increased selectivity and reduced wastes andemissions; and (3) design of ecocompatible chemicals withlower toxicity than current alternatives.To overcome the above disadvantages of conventional zeolite

preparation processes, alternative routes for synthesizingzeolites in a green or sustainable manner have been sought.Recently, important advances have been made in the synthesisof zeolites,19 and some typical examples are described below:(a) Zeolite synthesis by use of recyclable, low-cost, or

degradable templates: Davis and co-workers14,21−25 haveattempted to recycle organic templates in the syntheses ofzeolites, while Xiao and co-workers20 have synthesized AlPOzeolites using cheap and degradable guanidine as templates.(b) Organotemplate-free zeolite synthesis: Several groups

have sought to synthesize a series of zeolites in the absence oforganic templates.26−38

(c) Ionothermal zeolite synthesis: Morris and co-work-ers39−50 have successfully prepared zeolites using ionic liquidsas solvent, eliminating safety concerns associated with highpressure.(d) Solvent-free zeolite synthesis: Ren et al.51 reported a

solvent-free route for synthesizing zeolites starting from solidraw materials.(e) Microwave zeolite synthesis: The use of microwave

heating results in energy and time savings during synthesis.Moreover, by combining the advantages of ionic liquids andmicrowave heating, a novel synthesis of zeolites was successfullydemonstrated by Tian and Yan and co-workers.52−55

These examples show the potential for zeolite synthesis viagreen routes. When the enormous amount of zeolite productsused globally is considered, green routes for synthesizingzeolites are of great importance. In this review, a brief survey isgiven of recent developments in the green synthesis of zeolites.

2. GREEN ROUTES FOR USE OF ORGANIC TEMPLATESIN ZEOLITE SYNTHESIS

2.1. Organic Templates in Zeolite Synthesis

In 1961, two groups of researchers disclosed their observationof the effect of introducing quaternary ammonium cations intozeolite synthesis, opening a door to the synthesis of novelzeolites in the presence of organic templates.4,56,57 The key step

followed shortly after in 196758 with the disclosure of the firsthigh-silica zeolite, zeolite Beta (10 < Si/Al < 100), made withthe tetraethylammonium cation as template. Since that time,more than 200 types of zeolites have been prepared in thelaboratory in the presence of organic templates. There is nodoubt that modern synthetic methodologies for preparingzeolites are based on the wide applications of organictemplates.4

However, the use of organic templates bring about somedisadvantages, mainly those listed below:Toxicity. The most common organic templates are

quaternary ammonium or amines, which are usually toxic andnot environmentally benign.Removal of templates. Generally, organic templates are

removed by calcination at high temperature to obtain the openpores characteristic of zeolites. The combustion step is alwaysaccompanied by the release of hazardous gases (mainly NOxand CO2), high energy consumption, and some amount ofstructural destruction of the zeolites.Cost. Most organic templates are costly, and calcinations for

removing organic templates would result in increased cost forproduction of zeolites.Thus, new green routes such as using low-toxicity organic

templates and recycling the organic templates, as well asorganotemplate-free synthesis, is significantly important for theproduction of zeolites on a large scale in industry.19

2.2. Synthesis of Zeolites with Low-Toxicity and CheapOrganic Templates

Generally, the synthesis of high-silica zeolites requires thepresence of organic templates, which are toxic and expensive.Zones and Hwang59 have developed a new approach for thepreparation of zeolites using multiorganic amines instead ofexpensive organic templates. As a typical example, the zeolitestructure of MWW is usually templated from hexamethyleni-mine in aluminosilicate gels. However, Zones and Hwang useda cheap isobutylamine together with a small amount ofaminoadamantane to template SSZ-25 (MWW). This routeoffers economic benefits by reducing the cost associated withstructure-directing agents (SDAs) and waste stream cleanupcosts, as well as time in the reactor. Furthermore, a series ofzeolites such as SSZ-13 (CHA), SSZ-33 (CON), SSZ-35(STF), and SSZ-42 (IFR) have also been prepared in the samemanner by the same group.60

It has been reported that EMT zeolites exhibit much bettercatalytic properties than Y zeolites, the most importantcomponents in fluid catalytic cracking (FCC) catalysts.1,2

However, the industrial applications are significantly limited byuse of the costly and toxic template 18-crown-6 in the synthesisof EMT zeolites.61,62 Recently, polyquaternium-6, a componentof shampoo, was successfully used as a template to synthesizeEMT-rich faujasite.63 As shown by its extensive use in dailyhuman life, polyquaternium-6 is nontoxic and inexpensive. The

Figure 1. Mechanism of Cu-TEPA-templated Cu-SSZ-13 zeolite synthesis. Reprinted with permission from ref 64. Copyright 2011 Royal Society ofChemistry.

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successful synthesis of EMT-rich faujasites offers the possibilityof industrial applications of EMT zeolites.Due to the urgent need for selective catalytic reduction of

NOx by ammonia, industrial applications of Cu-SSZ-13catalysts are strongly influenced by the need for expensivetemplates in the synthesis of SSZ-13 zeolites. To obtain lessexpensive templates, Ren et al.64 have theoretically comparedthe configuration of the CHA cage, a building unit of SSZ-13zeolites, with a series of inexpensive inorganic or organiccompounds. They found that a low-cost copper complex (Cu2+

coordinated with tetraethylenepentamine, Cu-TEPA) matchesthe CHA cage well. The copper complex was then successfullyused to synthesize the zeolite Cu-SSZ-13 (designated as Cu-

ZJM-1; Figure 1), which showed superior catalytic activity inNH3 selective catalytic reduction (SCR) reactions.64

Scientists at UOP have developed the charge densitymismatch (CDM) approach to prepare zeolites via additionof alkali and alkaline earth cations at low levels, whichcooperate with organic templates.65−67 Such cooperation allowsthe use of commercially available organic templates for thediscovery of new materials. For example, hexagonal 12-ringzeolites UZM-4 (BPH) and UZM-22 (MEI) were prepared byuse of a choline−Li−Sr template system based on the CDMapproach.67 Notably, the CDM approach to zeolite synthesiswas initially proposed as a cheaper alternative to the trend ofusing ever more complicated quaternary ammonium species.Recently, the CDM approach has proven to be an efficient tool

Figure 2. (A, top) Dependence of AlPO-5 crystallinity on crystallization time (arrow indicates a large change in crystallinity). (Inset) Structures oftetramethylguanidine and triethylamine. (B, lower left) Nitrogen isotherms and (C, lower right) pore size distribution from adsorption branch forcalcined aluminophosphate spheres synthesized at 5 and 6 h in the presence of the organic template tetramethylguanidine. Reprinted withpermission from ref 20. Copyright 2009 Elsevier.



to investigate the fine effects in templating interactions and tomore thoroughly evaluate the potential of an organic templateto crystallize zeolitic structures. Park et al.68 investigated thepathway for formation of UZM-9 zeolites (LTA) from theirdiscrete building units by employing a combination of ex situ13C magic-angle spinning (MAS) NMR and IR techniquesbased on the CDM system. They found that the nucleationbegan with the formation of lta cages rather than the smallersod or d4r cages in the tetraethylammonium (TEA+)/tetramethylammonium (TMA+)/Na+ mixed-SDA system. Inthis CDM synthesis, both Na+ and TMA+ play an importantrole in the initial condensation and nucleation, while the crystalgrowth takes place in an aluminosilicate solution with theincorporation of Na+, TMA+, and TEA+ into the solid phase.Compared with aluminosilicate zeolites mainly templated

from organic ammonium cations, the organic templates usedfor synthesizing crystalline microporous aluminophosphatezeolites, usually poisonous amines, have relatively high toxicity.8

Therefore, the search for low-toxicity organic templates is verydesirable. Recently, Wang et al.20 reported a method foridentifying low-toxicity organic templates for the synthesis ofaluminophosphate zeolites. They suggested that nontoxic andinexpensive organic compounds containing nitrogen atoms(e.g., tetramethylguanidine, TMG) found in the products ofanimal metabolism could be suitable for templating micro-porous aluminophosphate zeolites.20 This group has used theTMG template to successfully synthesize AlPO-5 zeolites.Interestingly, the use of TMG for the synthesis of AlPO-5resulted in relatively high crystallization rates compared withconventional templates such as triethylamine, which might bedue to the unique structure of guanidines (Figure 2A).Guanidines, which contain three nitrogen atoms, might offerstronger coordination ability to aluminum species thantriethylamine, with only one nitrogen atom.Notably, the TMG-templated AlPO-5 sample exhibited

typical type IV plus type I adsorption curves. A steep increaseoccurred in the curve at relative pressure 10−6 < P/P0 < 0.01,which is due to the filling of the micropores. Another step canbe identified in the adsorption curve at a relative pressure 0.4 <P/P0 < 0.9, which is due to the presence of mesoporosity in thesamples. Correspondingly, the mesopore size distribution of thesample indicated narrow uniform pores. The formation of

mesoporosity in the samples can possibly be attributed tointercrystalline voids in the spherical particles (Figure 2B,C).Furthermore, the TMG-templated synthesis was extended to

prepare heteroatom-substituted AlPO-5 crystals such as SAPO-5, MnAPO-5, and CoAPO-5. Interestingly, the TMG templateconcept is not limited to the preparation of aluminophosphatewith AFI structures (AlPO-5); crystalline microporousaluminophosphate with AWO structures can also be preparedwith TMG as a template.

2.3. Synthesis of Zeolites with Recyclable OrganicTemplates

From an economic standpoint, organic templates are often themost expensive components in the synthesis of zeolites.14

Therefore, a way of potentially reducing the cost of zeolites thatrequire organic templates in their synthesis is to developtechniques that will remove the organic templates bynondestructive means, thereby potentially allowing for recoveryand reuse of the expensive organic templates. One method thatappears to be promising for some porous materials is solventextraction. For example, it is well-known that organic templatesfor the preparation of ordered mesoporous materials (e.g.,surfactants) can be extracted from the mesoporous channels viasolvent extraction, which could eliminate any high-temperatureprocessing steps and the destruction of the organictemplates.69−71 However, removal of organic templates frommicroporous zeolites is more difficult, as the size of the organictemplates is generally closely related to the size of themicropores, and easy diffusion of the organic templates outof the pores is therefore impeded.14 In addition, in many cases,the interaction between the organic templates and the silicateframework via charge-balancing ionic interactions is ratherstrong.23 Davis and co-workers14,21−25 have performed pioneer-ing work in the field of extracting organic templates from themicropores of zeolites.In 1999, Takewaki et al.21 prepared zincosilicate CIT-6 with

a BEA-type structure under hydrothermal conditions in thepresence of tetraethylammonium hydroxide (TEAOH), Li+,and Zn2+, finding that TEA+ cations could be easily extractedfrom CIT-6 with acetic acid-containing solutions, because ofthe weak interaction between the TEA+ cations and CIT-6framework. The extraction at 135 °C simultaneously removedTEA+ cations and zinc to form a highly hydrophobic Si-CIT-6with very few defects. They then systemically investigated the

Table 1. Synthetic and Physical Parameters of Samplesa

sample SDAb Si/Xc % extractedd porositye (cm3 of liquid N2/g of SiO2 TGAf (°C) % loss <400 °Cg

CIT-6 TEAOH 33.3, 22.1, >500 >99 0.233 (0.237) 575 61Si-Beta-F TEAF >99 0.239 (0.237) 375 100Al-Beta-F TEAF 20.0, 18.2, 18 49 0.085 (0.233) 680 48B-Beta-F TEAF 40.0, 29.2, 74 85 0.222 (0.241) 455 79Si-Beta-OH bis-PIP h h 680 68Al-Beta-OH TEAOH NDi 45 ND 680 45B-Beta-OH TEAOH/DABCO ND, 15.6, ND 75 0.099 (0.250) 640 61Si-Beta-OH-meso TEAOH >99 0.248 (0.242) 385 100Si-Beta-F-meso TEAF >99 0.193 (0.204) 385 100Si-MFI-OH TPABr h h 450 13Si-MFI-F HMDA/HF 98 0.136 (0.146) 325 100

aAdapted with permission from ref 25. Copyright 2001 Elsevier. bSDA, structure-directing agents; TEAOH, tetraethylammonium hydroxide; TEAF,tetraethylammonium fluoride; bis-PIP, bis-piperizine; DABCO, 1,4-diazabicyclo[2.2.2]octane; HMDA, hexamethylenediamine. cGel composition, as-synthesized composition (X = B, Al, or Zn), and composition after extraction at 80 °C. dAs determined by TGA after extraction at 80 °C. ePorosityafter extraction; porosity of the calcined sample is given in parentheses in cubic centimeters of liquid N2 per gram of SiO2; P/P0 = 0.2. fTemperatureat which weight loss in TGA ceases. gPercent mass loss x, where 200 °C < x < 400 °C. hNone. iND, not determined.



removal of organic templates via solvent extraction inzincosilicates, aluminosilicates, borosilicates, and pure silicateswith BEA topology (Table 1).25 The ease of liberation ofcharge-balancing tetraethylammonium (TEA) cations from thevarious metallosilicates was shown to reduce along the series Zn> B >Al. Additionally, it was shown that these tightly boundorganic templates were removed by extraction under conditionsthat simultaneously hydrolyze part of the framework. Forexample, TEA+ cations charge-balancing boron atoms in thesilicate framework were removed with concomitant hydrolysisof the B−O−Si bonds, releasing the tightly bound TEA+

cations with subsequent desorption of the boron and TEA+

cations from the molecular sieve pores. This method had alsobeen utilized in pure-silica molecular sieves with MFI topology(Table 1). The authors concluded that the amount of organictemplates that could be removed by extraction was found to bedependent on the size of organic templates and the strength ofinteraction of organic templates with the molecular sieveframework.25

Furthermore, they showed a complete recycling of theorganic template in the synthesis of ZSM-5.14 They chose acyclic ketal as an organic template that would remain intactunder zeolite synthesis conditions (high pH) and be cleavableat conditions that would not destroy the assembled zeolite(Figure 3). The 13C cross-polarization (CP) MAS NMR

spectrum showed that the as-synthesized material containsintact 8,8-dimethyl-1,4-dioxa-8-azaspiro[4,5]decane (1, Figure4a). When the ZSM-5 was treated with 1 M HCl solution at 80°C for 20 h, the 13C CP MAS NMR spectrum obtained wasconsistent with the presence of the ketone fragment (Figure4b), suggesting that 1 could be cleaved into the desired piecesinside the zeolite pore space. After ion-exchange treatment witha mixture of 0.01 M NaOH and 1 M NaCl at 100 °C for 72 h,1,1-dimethyl-4-oxopiperidinium (2) could be completelyremoved as shown in the 13C CP MAS NMR spectrum (Figure4c). Conceptually, this strategy involves assembly of an organictemplate from at least two components, using covalent bondsand/or noncovalent interactions that are able to survive the

conditions for assembly of the zeolite and yet can be reversedinside the microporous void space. The fragments formed fromthe organic template in the zeolite can then be removed fromthe inorganic framework and be recombined for reuse. Otherzeolites such as ZSM-11 and ZSM-12 can also be synthesized inthe same manner, suggesting that the route can be used as ageneral methodology in the field of zeolite preparation.72

2.4. Organotemplate-Free Routes for Synthesizing Zeolites

An alternative route for solving the problems caused by organictemplates is to avoid their use entirely. Several methodsincluding adjusting molar ratios of the starting gels26,73−81 orthe addition of zeolite seed solutions27,29,31 or zeolite crystalseeds28,30,32,33,35−38,82−85 into the starting gels, have beenreported for synthesis of aluminosilicate zeolites in the absenceof organic templates.86 The zeolite seed solutions referred tohere are zeolite precursor solutions or solutions containing theprimary and secondary building units of zeolites, while the term“zeolite seeds” refers to solid zeolite crystals remaining in thesynthesis system.

2.4.1. Adjusting Molar Ratios of Starting Gels. Thesynthesis of ZSM-5 zeolites in the presence of tetrapropy-lammonium (TPA) can be regarded as a milestone in thehistory of hydrothermal zeolite synthesis.87 Following thediscovery of ZSM-5, there developed a belief that this zeolitecould only be made by use of a suitable organic template(usually TPA+) or through the addition of existing ZSM-5seeds.4,87 Grose and Flanigen73−75 prepared well-crystallizedZSM-5 from the Na2O−SiO2−Al2O3−H2O system, which isthe first example of an organotemplate-free synthesis of ZSM-5.Around the same time, another two groups reported that ZSM-5 with good crystallinity can be successfully synthesized in theabsence of any organic templates.76,77 Later, Shiralkar andClearfield78 reported that the Si/Al and Na/Al ratios are keyfactors for the organotemplate-free synthesis of ZSM-5 zeolites.T h e c om p o s i t i o n o f t h e s t a r t i n g g e l w a saSiO2:Al2O3:bNa2O:1500H2O. When a is less than 30, theproducts were composed of mordenite as the major phase

Figure 3. Schematic representation of the synthetic methodology forpreparation of ZSM-5 using 1 as template. Step 1, assemble the SDAwith silica precursor, H2O, alkali metal ions, etc., for zeolite synthesis.Step 2, cleave the organic molecules inside the zeolite pores. Step 3,remove the fragments. Step 4, recombine the fragments into theoriginal SDA molecule. Reprinted with permission from ref 14.Copyright 2003 Nature Publishing Group.

Figure 4. 13C CP MAS NMR spectra: (a) intact 1 inside as-synthesized ZSM-5; (b) after cleavage of 1 inside ZSM-5 pores by useof 1 M HCl solution; and (c) after ion exchange with 0.01 M NaOHand 1 M NaCl solution. Reprinted with permission from ref 14.Copyright 2003 Nature Publishing Group.



along with traces of ZSM-5. However, when a is above 60, thecontribution from α-quartz increases, coexisting with ZSM-5and mordenite. Almost 100% pure α-quartz was obtained whenthe gel used was free of alumina. Increasing the value of b alsoresulted in the formation mordenite or α-quartz as impurephase. At a starting composition of a = 40 and b = 4.5−6.0,pure ZSM-5 with the occlusion of Na+ in excess of chargecompensation on the zeolite framework can be well crystallized.Notably, the thermal stability of such ZSM-5 synthesized in theabsence of organic templates was less than that of conventionalZSM-5, due to occluded excess sodium species.78

The organotemplate-free synthesis of ECR-1 zeolite is also asuccessful example of adjusting the molar ratios of startinggels.26ECR-1, a large-pore aluminosilicate zeolite, is an intimatetwin of the mordenite-like sheets between layers of mazzite-likecages, which was first discovered by use of the organic templatebis(2-hydroxyethyl)dimethylammonium chloride.88−90 Later,other organic templates such as adamantine-containingdiquaternary alkylammonium iodides and tetramethylammo-nium (TMA+) were also used in synthesis of ECR-1.91,92 Inthese cases, organic templates are necessary in the synthesis ofECR-1. However, gallosilicate zeolite (TNU-7), an analogue ofaluminosilicate ECR-1, is hydrothermally synthesized in theabsence of organic templates, a process attributed to thestructure-direction effect of inorganic Ga3+ species.79 Thesuccess of TNU-7 suggested that ECR-1 might be prepared inthe absence of organic templates.Song et al.26 synthesized aluminosilicate zeolite of ECR-1

under hydrothermal conditions at 100−160 °C for 1−14 daysby carefully adjusting the molar ratios of Na2O/SiO2 in theabsence of organic templates for the first time (Figure 5). The

molar ratio of Na2O/SiO2 in the synthesis significantlyinfluences the final products of zeolites. When the ratio was0.33, a pure phase of zeolite Y was formed; when the ratio was0.28, a mixture of zeolite Y and ECR-1 was crystallized; whenthe ratio was 0.25, a pure phase of ECR-1 was successfullysynthesized; and when the ratio was 0.20, the product wasamorphous silica. Furthermore, it was found that thecrystallization rate of ECR-1 notably increases with temperature

in the synthesis. For example, when the temperature in thesynthesis was 160 °C, ECR-1 with an impurity crystallized over24 h. In contrast, crystallization of ECR-1 at 100 °C took 14days. Notably, although the synthesis at 100 °C took longer, itwas a pure phase of ECR-1. Like the gallium in TNU-7,hydrated alkali-metal cations in the synthesis may organizeECR-1 structural subunits and effect solution-mediatedcrystallization of the amorphous gel.26

Recently, Zhang et al.80 reported the organic template-freesynthesis of ZSM-5/ZSM-11 zeolite intergrowth with differentSiO2/Al2O3 ratios, ZSM-5 percentages, and various morphol-ogies by adjusting compositions of the starting gels. It wasfound that this organic template-free system is well-suited tothe synthesis of aluminum-rich zeolites. As the initial Si/Alratios increase, the crystal sizes and ZSM-5 percentage increase,and the product morphology changes from nanorod aggregateto microspindle and then to single and twinned hexagonalcrystals. Moreover, increasing the concentration of Na+ andOH− in the initial reaction gel enhanced the crystallization rateremarkably by shortening the induction period, and the length/width ratios of the product decreased. Notably, the addition ofK+ disfavors the organotemplate-free synthesis of the ZSM-5/ZSM-11 zeolite intergrowth.More recently, Ng et al.81 reported organotemplate-free

synthesis of ultrasmall hexagonal EMT zeolite nanocrystals (6−15 nm in size) at very low temperature from sodium-richprecursor suspensions. Normally, compared with FAU zeolitesas FCC catalysts, EMT zeolites show interesting catalyticproperties, but their very high cost currently precludes theirpractical application. The synthesis of pure EMT-type zeolitesusually involves the use of expensive and toxic 18-crown-6 astemplate.61,62 The novel organotemplate-free synthesis of EMTzeolites offers a good opportunity to develop new FCCcatalysts in the future. Ng et al.81 point out that the gelcomposition, nucleation temperature and times, and type ofheating strongly influence the synthesis of EMT zeolites. Whenthe crystallization time is longer or the temperature is higher,the nanoscale EMT materials could be converted into the well-known FAU and SOD structures. This phenomenon isinterpreted by noting that the EMT is the first kinetic,metastable product in this synthesis, followed by conversioninto the more stable cubic FAU and more dense SODstructures, which is strongly supported by several studies onEMT/FAU intergrowths.93−95 It is worth emphasizing that theEMT synthesized from the organotemplate-free route still hasrelatively low Si/Al ratios compared with conventional EMTsynthesized in the presence of 18-crown-6 as template. Theorganotemplate-free synthesis of EMT with high Si/Al ratiosstill remains a challenge.

2.4.2. Zeolite Seed Solution-Assisted Approach. Thealuminosilicate zeolite ZSM-34 is an intergrowth of offretite(OFF) and erionite (ERI) zeolites containing zeolitic buildingunits of cancrinite (CAN) cages.96−99 ZSM-34 zeolite was firstdiscovered by Rubin et al.96 using the organic template choline[(CH3)3NCH2CH2OH], and later ZSM-34 samples weresuccessfully synthesized in the presence of different diamines(NH2CnH2nNH2, n = 4, 6, 8, 10).100 It is well-known that thosezeolites (e.g., CAN, OFF and ERI) composed of CAN cages arenatural zeolites, formed in the absence of organic templates.Thus, the synthesis of ZSM-34 without using organic templatesis possible.Xiao and co-workers27,29 proposed a new strategy for the

organotemplate-free synthesis of ZSM-34. The authors have

Figure 5. Scanning electron microscopy (SEM) image and (inset) X-ray diffraction (XRD) patterns of ECR-1 synthesized in the absence oforganic template. Reprinted with permission from ref 26. Copyright2006 American Chemical Society.



prepared the seed solution containing zeolitic building unitsmade up of CAN cages, which can induce the crystallization ofZSM-34 at suitable conditions. On the other hand, L zeolitescontain CAN cages, and thus their seed solution, of course, alsocontains CAN cages.8,101−104 Thereby, Xiao and co-work-ers27,29 have used zeolite L seed solutions to induceorganotemplate-free synthesis of ZSM-34 zeolites (Figure 6).

The amount of zeolite L seed solution and the molar ratios ofSiO2/Na2O in the starting aluminosilicate gels were regarded asthe key factors for preparation of pure ZSM-34 zeolites (Table2). Amorphous solid product was obtained in the absence ofzeolite L seed solution. When a small amount of zeolite L seedsolution (0.88−1.31 mL) was added to the starting gel, ZSM-34with low crystallinity was the main product. A further increaseof zeolite L seed solution to 1.75 mL resulted in completecrystallization of ZSM-34 zeolites. When the amount of zeoliteL seed solution was above 2.19 mL, the product showed animpurity of zeolite L. Moreover, the alkalinity of the starting gelshould be carefully controlled, and only when the SiO2/Na2Oratio was kept at about 2.76 was pure ZSM-34 zeolite obtained.Otherwise, high alkalinity led to the formation of PHI zeolite asan impure phase, and MOR appeared at low alkalinity.Additionally, the crystallization temperature and time alsoinfluenced the crystallinity of ZSM-34 zeolite significantly. Themain products at low crystallization temperature (80−100 °C)remained amorphous for a long time (7−14 days). However,high temperatures (130 °C or higher) always resulted in theformation of impure phases (e.g., PHI zeolite or orthoclase).Furthermore, heteroatom-substituted ZSM-34 (B, Ga, and

Fe) can also be prepared via the same route.29 UV−vis andNMR spectroscopy confirmed that these heteroatoms havebeen located in the framework of ZSM-34 zeolites. All as-synthesized samples exhibit typical Langmuir-type curves withlarge surface areas (388−433 m2/g) and high microporevolume (0.16−0.19 cm3/g) as well as narrow pore sizedistribution (5.2 Å), confirming the existence of openmicropores in these uncalcined ZSM-34 samples synthesizedin the absence of organic templates. Catalytic tests in theconversion of methanol to olefins over the aluminosilicate

ZSM-34 zeolite, and its analogues containing framework B andGa heteroatoms, show that these ZSM-34 catalysts exhibithigher selectivities for ethylene and propylene (>80%) thanZSM-5 zeolite (<55%), which can presumably be attributed tothe shape selectivity of ZSM-34 with smaller pore size (5.2 Å)than ZSM-5 (5.6 Å).Encouraged by the success in organotemplate-free synthesis

of ZSM-34 by use of zeolite seed solutions, otheraluminosilicate zeolites synthesized with zeolite seed solutionshave also been investigated. Ferrierite (FER) zeolite, with ananisotropic framework composed of two-dimensional straightchannels including a 10-membered ring channel (0.42 × 0.54nm) along the [001] direction and a eight-membered ringchannel (0.35 × 0.48 nm) along the [010] direction,105,106 hasbeen carefully studied due to its excellent catalytic perform-ance.107−109 Notably, FER zeolites with low ratios of Si/Alcould be synthesized in the absence of organic templates,110 buthigh-silica FER zeolites (ZSM-35) are always prepared in thepresence of organic templates.111−114 Recently, Zhang et al.31

have demonstrated the successful synthesis of high-silica FERzeolites (Si/Al at 14.5) by the introduction of RUB-37 zeolite(CDO structure) in the absence of organic templates (Figure7). This sample was hydrothermally synthesized from startinggels with molar ratios of SiO2/(0.154−0.244)Na2O/(0.024−0.035)Al2O3/35H2O in the presence of RUB-37 (5% mass inSiO2) at 150 °C for 72−168 h.It is well-known that the building units of FER and CDO are

the same, and their difference is only a shift of layers in thehorizontal direction.115 Therefore, it is reasonable to use thebuilding units of RUB-37 zeolite to induce the crystallization ofFER-type zeolites. X-ray diffraction (XRD) patterns of thealuminosilicate-gel-added RUB-37 zeolite exhibit a weak peak at9.6° associated with the CDO structure,116 suggesting that theRUB-37 seeds are still crystalline in nature; 12 h later, the peakat 9.6° had completely disappeared, giving an amorphousproduct. This phenomenon indicates that the seeds of RUB-37crystals could be dissolved into zeolite secondary building unitsunder alkaline conditions. After 24 h, a weak peak at 9.4° wasobserved, corresponding to FER-type zeolite, which isdesignated as ZJM-2. A further increase of crystallization timeto 72 h results in the complete crystallization of FER-typezeolites. In contrast, a mixture of amorphous products isobtained with a small amount of MOR zeolite in the absence ofRUB-37 seeds.31 These results suggest that RUB-37 seedsadded to the aluminosilicate gel play an important role in thecrystallization of FER-type zeolites. Furthermore, UV−Ramanspectroscopy suggests that the starting aluminosilicate gelexhibits a stronger peak at 430 cm−1 associated withcontribution of five-membered rings (5MRs) in the sample,117

resulting from the building units of RUB-37 zeolite added tothe starting gel. This result also confirms the importance ofRUB-37 seeds for the preparation of high-silica FER zeolite inthe absence of organic templates, in good agreement withresults obtained from XRD patterns.

2.4.3. Zeolite Crystal Seed-Directed Approach. Seedcrystallization is a widely used methodology in the large-scaleproduction of zeolites,118−123 where the roles of zeolite seedsare to (i) enhance the crystallization rate, (ii) suppress theformation of undesired phases, and (iii) control the crystalsizes. However, zeolite crystal seed-directed synthesis in theabsence of organic templates had never been reported before itssuccessful application in the preparation of Beta zeolites,reported by Xiao and co-workers.28 After this example,

Figure 6. SEM image and (inset) XRD patterns of ZSM-34synthesized in the absence of organic template. Reprinted withpermission from ref 27. Copyright 2008 American Chemical Society.



syntheses of other important zeolites such as RUB-13 (RTH),78

ZSM-12 (MTW),35,38 RUB-50 (LEV),30,32 Heulandite(HEU),30 and SUZ-481 were also reported via the sameroute, indicating that the seed-directed synthesis is a generalapproach for synthesizing zeolites in the absence of organictemplates.2.4.3.1. Beta Zeolites. Beta zeolites with three-dimensional

12-membered micropores are widely applied in industrialapplications, and their organotemplate-free synthesis is of greatimportance. Xie et al.28 reported an organotemplate-free andfast route for synthesizing Beta zeolites by addition of calcinedBeta crystals as seeds in the starting aluminosilicate gel in theabsence of any organic templates for the first time (Figure 8).Because of the discovery of Beta zeolites in natural minerals,124

it is understood that the organotemplate-free synthesis of Betazeolites is possible due to the fact that no organic templates areused in the synthesis of Beta zeolites in nature. Interestingly,after addition of calcined Beta crystals as seeds, Beta zeoliteswith high crystallinity can be obtained from a startingaluminosilicate gel with a molar ratio of 40SiO2/Al2O3/10Na2O/570H2O by crystallization at 140 °C for 17−19 h.Nitrogen sorption isotherms of as-synthesized samplesexhibited a steep increase in the curve at a relative pressure10−6<P/P0 < 0.01, characteristic of Langmuir adsorption due tothe filling of micropores, which confirmed that the as-synthesized sample had open micropores already and therefore

the combustion of the sample could be avoided. Notably, thecrystallization time of seed-directed Beta is very short (17−19 hat 140 °C) compared with that of conventional Betasynthesized in the presence of TEA+ (3−4 days at 140 °C).This phenomenon is explained in that the growth of Betazeolite crystals was greatly accelerated after the addition of Betaseeds to the starting aluminosilicate gel. Generally, hydro-thermal synthesis of zeolites includes an induction andcrystallization period. The addition of Beta seeds in thesynthesis remarkably reduces the induction period.Kamimura et al.36 systematically studied various parameters

of the seed-directed synthesis of Beta zeolites in the absence oforganic templates, such as the molar ratios of SiO2/Al2O3,H2O/SiO2, and Na2O/SiO2 in the starting gels, the amount andSi/Al ratios of seeds, and crystallization time. Beta zeolites canbe successfully synthesized with a wide range of chemicalcompositions of the initial Na+-aluminosilicate gel (SiO2/Al2O3

= 40−100, Na2O/SiO2 = 0.24−0.325, and H2O/SiO2 = 20−25)by addition of calcined Beta seeds with Si/Al ratios in the range7.0−12.0 (Table 3). Importantly, such seed-directed Beta seedcrystals can be used as renewable seed crystals to establish acompletely organotemplate-free process for the production ofBeta zeolite, which is a vital development from the viewpoint ofgreen chemistry. Thus, Beta zeolite prepared by this seed-directed route was termed “green Beta zeolite” by the authors.

Table 2. Syntheses of Aluminosilicate ZSM-34 Products under Various Conditionsa

runb SiO2/Na2O SiO2/Al2O3 L seed solution (mL) temp (°C) time (days) products

1 2.76 47 0 120 7 amorc

2 2.76 47 0.88 120 7 amor + ZSM-343 2.76 47 1.31 120 7 ZSM-34 + amor4 2.76 47 1.75 120 7 ZSM-345 2.76 47 2.19 120 7 ZSM-34 + L6 2.76 47 2.63 120 7 ZSM-34 + L7 2.76 47 3.0 120 7 ZSM-34 + L

8 5.52 47 1.75 120 14 amor9 3.68 47 1.75 120 14 amor10 2.94 47 1.75 120 9 ZSM-34 + MOR11 2.47 47 1.75 120 7 ZSM-34 + PHI12 2.21 47 1.75 120 7 ZSM-34 + PHI13 1.84 47 1.75 120 7 PHI

14 2.76 47 1.75 80 14 amor15 2.76 47 1.75 100 14 ZSM-34 + amor16 2.76 47 1.75 110 7 ZSM-3417 2.76 47 1.75 130 5 ZSM-34 + PHI18 2.76 47 1.75 140 5 ZSM-34 + PHI19 2.76 47 1.75 160 5 orthoclase

20 2.76 94 1.75 120 5 ZSM-3421 2.76 31 1.75 120 9 ZSM-34

22 2.76 47 1.75 120 7 amor + ZSM-34

23 2.76 47 120 7 ZSM-34 + CHA + PHIaAdapted with permission from ref 29. Copyright 2010 American Chemical Society. bSynthetic conditions: Runs 1−7, 47 SiO2:1 Al2O3:17Na2O:1457 H2O, 120 °C, 0−3.0 mL of zeolite L seed solution. Runs 8−13, 47 SiO2:1 Al2O3:8.5−25.5 Na2O:1457 H2O, 120 °C, 1.75 mL of zeoliteL seed solution. Runs 14−19, 47 SiO2:1 Al2O3:17 Na2O:1457 H2O, 80−160 °C, 1.75 mL of zeolite L seed solution. Runs 20 and 21, 47 SiO2:0.5−1.5 Al2O3:17 Na2O:1457 H2O, 120 °C, 1.75 mL of zeolite L seed solution. Run 22, 47 SiO2:1 Al2O3:17 Na2O:1457 H2O, 120 °C, 1.75 mL of unaged“zeoliteL seed solution”. Run 23, 47 SiO2:1 Al2O3:17 Na2O:1457 H2O, 120 °C in the presence of KOH. cAmorphous solid product.



It is worth noting that when the seed-directed synthesis ofBeta zeolite (Beta-SDS) is well controlled, the as-synthesizedBeta-SDS exhibits much higher surface area than calcined Betasynthesized in the presence of TEAOH (Beta-TEA), as shownin Figure 9.30 After calcination at 550 °C for 4 h, Beta-SDS stillhas high crystallinity and pure tetracoordinate Al species, whileBeta-TEA is significantly lower in crystallinity and producespartial hexacoordinate Al sites. After 100% steaming treatmentat 750 °C for 8 h, Beta-SDS retains its crystallinity, while Beta-TEA is remarkably reduced in crystallinity. These features arepossibly related to the fact that Beta-SDS has much fewerdefects than conventional Beta usually produced by calcination

of organotemplates at high temperature. These results are ingood agreement with the data characterized from 29Si NMRspectroscopy that Beta-SDS has many fewer terminal Si-OHgroups associated with the defects of the Beta zeolite structurethan Beta-TEA.It should be emphasized that Beta-SDS is Al-rich. Mintova

and co-workers33 have shown that Beta zeolite with Si/Al ratioas low as 3.9 can be synthesized. Obviously, the Al-rich Beta isfavorable for catalytic cracking reactions. Fluid catalyticcracking (FCC) tests showed that the Beta-SDS exhibitedmuch higher activities than those of conventional Betasynthesized in the presence of TEA+, giving higher yields ofLPG in oil refining.30

The mechanism of seed-directed synthesis of Beta zeolite hasbeen independently discussed by Xiao and co-workers30 andOkubo and co-workers37 at around the same time. By using aseries of modern techniques [e.g., XRD, transmission electronmicroscopy (TEM), SEM, X-ray photoelectron spectroscopy(XPS), Raman, MAS NMR], Xie et al.30 have extensivelyinvestigated the seed-directed synthesis of Beta-SDS undervarious conditions, suggesting that seed-directed Beta zeolitesare grown from solid Beta seeds, and final Beta-SDS crystals aremainly core−shell structure. The core parts of Beta seeds haverelatively high Si/Al ratios, and the shell parts grown fromaluminosilicate gels have relatively low Si/Al ratios. Figure 10shows TEM images of Beta-SDS zeolite crystallized over arange of time periods. When the crystallization time was 1 h, itwas directly observed that crystalline Beta seeds are embeddedin an amorphous aluminosilicate phase. When the crystal-lization time was increased to 4 h, a crystal is visible in theamorphous aluminosilicate gel, and this crystal has a dark“core”. The EDS analysis shows that the Si/Al ratio of the dark“core” is about 13.3, which is close to that (Si/Al at 10.2 byICP) of Beta seeds. In contrast, the shell part of the crystal

Figure 7. (a) N2 sorption isotherm of H-form of ZJM-2 sample (inset, HK pore size distribution), (b) SEM image of as-synthesized ZJM-2 sample,(c) 27Al NMR MAS spectrum of as-synthesized ZJM-2 sample, and (d) NH3−temperature-programmed desorption (TPD) curve of H-form of ZJM-2 sample. Reprinted with permission from ref 31. Copyright 2011 Royal Society of Chemistry.

Figure 8. SEM image and (inset) XRD patterns of Beta zeolitesynthesized in the absence of organic template. Reprinted withpermission from ref 28. Copyright 2008 American Chemical Society.



shows a Si/Al ratio of about 5.3, which is the same as that (5.3)of amorphous aluminosilicates. After crystallization for 18.5 h,TEM images showed pure crystalline phase and amorphousaluminosilicates completely disappeared, demonstrating the fullconversion from amorphous phase to Beta crystals. In addition,XPS measurements of Beta samples show that the Si/Al ratio(5.90) of the central part of the crystals is notably higher thanthat (4.17) on crystal surface, further confirming the presenceof nonuniform Si/Al ratios in Beta crystals. On the other hand,by using XRD, TEM, Raman spectroscopy, and solid-state 27Aland 23Na MAS NMR as well as high-energy XRD analyses, etc.,Kamimura et al.37 suggest that seed growth without nucleationwas predominant in the organotemplate-free synthesis of Betazeolite. When the seed-embedded Na+-aluminosilicate gel wasin the early stages of hydrothermal treatment, the Beta seedswere partly dissolved and disaggregated into small pieces, and

most of them were embedded in the amorphous aluminosili-cate. Upon dissolution of the amorphous aluminosilicate, theembedded Beta seeds should be exposed on the surface of theamorphous material and/or released into the liquid phase, andthese Beta seeds in contact with the liquid phase would providea surface for crystal growth by consuming aluminosilicateprecursors in the liquid phase. Eventually, all of the amorphousaluminosilicates were dissolved, and crystal growth wascomplete.

2.4.3.2. RTH-type Zeolite. Since the successful synthesis ofBeta zeolite, seed-directed synthesis in the absence of organictemplates has been applied to a series of zeolites.30,32,35,38,82−85

RTH-type zeolite (e.g., borosilicate RUB-13, aluminosilicateSSZ-50), consists of RTH cages with eight-membered ring(8MR) openings and 2D channels with aperture sizes of 0.41 ×0.38 and 0.56 × 0.25 nm, parallel to the a and c axis,respectively.125,126 This kind of zeolite has shown excellentcatalytic properties in methanol-to-olefin reactions. However,the synthesis of RUB-13 and SSZ-50 always requires the use oforganic templates such as 1,2,2,6,6-pentamethylpiperidine(PMP), ethylenediamine (EDA), or N-ethyl-N-methyl-5,7,7-trimethylazoniumbicyclo[4.1.1]octane cation.125,126 Interest-ingly, Yokoi et al.82 have reported a successful synthesis ofRTH-type zeolites (denoted as TTZ-1) without using organictemplates by addition of calcined B-RUB-13 as seeds in astarting gel composed of SiO2:0.25H3BO3:0.2NaOH:200H2O.The key factors influencing the seed-directed synthesis of

TTZ-1 are the addition of sodium hydroxide and the amount ofwater.82 Without addition of sodium hydroxide, the productswere a mixture of amorphous silica and RTH-type zeolite. Themajor role of sodium hydroxide is to compensate for the lack ofalkalinity resulting from the absence of organic amines. Highalkalinity (e.g., Na/Si = 0.5) would result in the appearance ofα-quartz as an impure phase, and a further increase in the ratioof Na/Si to 1.0 led to the formation of pure α-quartz. Inaddition, the introduction of water should be carefully

Table 3. Composition of Initial Reactant Gel, Reaction Conditions, and Products in Synthesis of OSDA-free Betaa

run SiO2/Al2O3 Na2O/SiO2 H2O/SiO2 seed (%) time (h) phase cryst (%) Si/Al

1 30 0.25 20 10 70 Beta + amorb 202 40 0.25 25 10 70 Beta 1003 40 0.275 25 10 22 Beta + amor 304 40 0.275 25 10 46 Beta 100 5.55 40 0.275 25 10 70 Beta + MOR 806 40 0.300 25 10 48 Beta 1007 40 0.350 25 10 60 Beta + unknown8 40 0.325 25 10 38 Beta 100 5.29 50 0.240 24 10 94 Beta 100 6.810 50 0.325 24 10 35 Beta 10011 60 0.240 22 10 82 Beta 10012 60 0.275 24 10 70 Beta 10013 60 0.300 22 10 46 Beta 100 6.014 70 0.325 20 10 24 Beta 100 5.915 80 0.300 20 10 34 Beta 100 6.316 100 0.225 20 10 142 Beta + amor 2017 100 0.300 20 10 27 Beta 100 6.618 100 0.300 20 5 30 Beta 10019 100 0.300 20 2.5 30 Beta + amor 8020 100 0.300 20 1 30 Beta + amor 2021 100 0.300 20 1 70 Beta + amor 8022 100 0.300 20 1 100 Beta 100

aAdapted with permission from ref 36. Copyright 2010 Wiley−VCH Verlag GmbH & Co. KGaA, Weinheim, Germany. bAmorphous solid product.

Figure 9. N2 isotherms of (a) as-synthesized Beta-SDS and (b) Beta-TEA calcined at 550 °C for 4 h, Beta-SDS was obtained bycrystallization at 120 °C for 120 h in the presence of 4.6% Beta seeds(Si/Al = 11.6). Reprinted with permission from ref 30. Copyright 2011Royal Society of Chemistry.



controlled, because a mixture of amorphous aluminosilicate andRTH-type zeolite would be obtained with more or less wateraddition.Interestingly, direct introduction of Al and Ga heteroatoms

into the RTH framework during crystallization of B-TTZ-1 inthe absence of organic templates was also successfullyperformed. NMR spectra confirmed the tetrahedrally coordi-nated heteroatoms in the framework. Furthermore, the authorsprepared pure aluminosilicate with an RTH topology (i.e., SSZ-50) employing the same route, except that deboronated RUB-13 crystals were used as seeds instead of B-RUB-13.82 Notably,the Si/Al ratios of Al-TTZ-1 varied from 37 to 57, while that of[Al,B]-TTZ-1 can be very high (e.g., >100). Such a feature isdifferent from other organotemplate-free syntheses of zeoliteswith relatively low Si/Al ratios (e.g., 4−15).The catalytic properties of these seed-directed RTH-type

zeolites in methanol-to-olefin reactions have also been tested.82

The selectivity for propene is higher than that of SAPO-34 andZSM-5 zeolite, and the catalytic life of RTH-type zeolites wasmuch longer. The excellent catalytic performance is presumablyassociated with their unique structure.125,126

2.4.3.3. ZSM-12 Zeolite. ZSM-12, a high-silica aluminosili-cate zeolite with framework code MTW, which has a 1Dchannel with 12-membered ring (12MR) openings along the b-axis, is a useful catalyst in petrochemical processes.127,128

Generally, ZSM-12 zeolites are synthesized in the presence of

organic templates such as TEA+ cations.129,130 Recently, Okuboand co-workers35,38 reported the organotemplate-free synthesisof MTW-type aluminosilicate zeolites by addition of ZSM-12crystal seeds in the starting gels. The Si/Al ratios of seed-directed ZSM-12 samples could be varied from 23.2 to 34.3,which was lower than that of conventional ZSM-12 zeolites.Notably, the alkali metal ion of Li+ was essential for seed-

directed synthesis of ZSM-12. MFI zeolites (ZSM-5) mixedwith a small amount of MTW are obtained from the seed-directed route if lithium cations are absent. These resultssuggest that the seeds and the lithium cations cooperativelydirect the synthesis of MTW zeolites. Presumably, alkali metalcations (e.g., Li+, Na+, K+, Cs+) strongly influence thenucleation and crystallization process. For example, thesynthesis of L zeolites requires K+,8 and RHO zeolites aresynthesized in the presence of Cs+.131 It is expected thateffective use of these alkali metal cations would expand thepossibilities of organotemplate-free synthesis with the additionof seed crystals.

2.4.3.4. LEV Zeolites. RUB-50 zeolites with LEV structuresare also very important catalysts for shape-selective catalysisand are initially templated by diethyldimethylammonium(DEDMA) at 150 °C.30 Recently, Zhang et al.32 reported theorganotemplate-free and seed-directed synthesis of LEVzeolites (LEV-SDS) in the presence of RUB-50 seeds and asmall amount of alcohol. Characterizations showed that the

Figure 10. TEM images of Beta-SDS samples crystallized for (a) 1, (b−d) 4, (e−g) 8, and (h, i) 18.5 h at a temperature of 140 °C by addition of10.3% Beta seeds (Si/Al = 10.2) in the starting aluminosilicate gels. Areas α, β, δ, and γ in panels b, c, e, and f are enlarged as panels c, d, f, and g,respectively. Reprinted with permission from ref 30. Copyright 2011 Royal Society of Chemistry.



zeolite product has good crystallinity, high surface area, uniformcrystals, tetrahedral Al3+ species, and abundant acidic sites. Inthis synthesis, the alcohol plays an important role in thesynthesis of LEV-SDS zeolites by inhibiting the formation ofMOR zeolites. Catalytic tests in methanol-to-olefin processesshow that the H form of LEV-SDS zeolites exhibits goodconversion of methanol and high selectivities for ethylene andpropylene.2.4.3.5. SUZ-4 Zeolite. SUZ-4 zeolite is an aluminosilicate

zeolite with a three-dimensional topology consisting of 5-, 6-,8-, and 10-membered rings and has been used as a catalyst forthe conversion of methanol to dimethylether.132 SUZ-4 zeolitewas first discovered by use of organic templation of TEAOHand quinuclidine under stirring or rotation conditions.133

Recently, Zhang et al.85 reported an organotemplate-freeroute for hydrothermally synthesizing zeolite SUZ-4 understatic conditions by adding the calcined SUZ-4 seeds into thestarting aluminosilicate gels. It is suggested that addition of theseed crystals causes the deposition of amorphous gel particleson its surface and induces the crystallization therein, throughwhich the use of organic templates is avoided. Also, the staticconditions could make the synthesis more convenient forpractical use.In sum, zeolite crystal seed-directed synthesis in the absence

of organic templates opens a new door to the synthesis of low-cost zeolites, which would be potentially important forindustrial applications as catalysts, catalyst supports, adsorbents,and detergent builders in the future.More recently, Sano and co-workers83,84 have developed a

new strategy for synthesizing zeolites from the transformationof raw zeolites in the presence of zeolite seeds. For example,faujasite-type (FAU) zeolites were hydrothermally convertedinto BEA and levynite (LEV) zeolites in the presence of BEAand LEV seeds, respectively. Notably, the zeolite seeds in thesecases are noncalcined. Calcined seeds have also been used butonly amorphous products were formed, which was attributed tothe fact that seed crystals containing organotemplates are morestable and consequently the crystal surfaces contribute to thecrystal growth of zeolites BEA and LEV.82 This strategy offersan alternative organotemplate-free synthesis of zeolites.

3. SYNTHESIS OF ZEOLITES UNDER RELATIVELY LOWPRESSURE: USE OF IONIC LIQUIDS AS SOLVENTS

3.1. Ionic Liquids

Ionic liquids (IL) have received great attention in many fields,with particular emphasis on the drive to replace organicsolvents for the purpose of green chemistry.45,47,134−139 A novelsynthesis methodology known as ionothermal synthesis hasthus been developed, which has been widely used in thepreparation of functional materials such as zeolites, metal−organic frameworks (MOF), and other inorganic−organichybrids.45,47,139 The term “ionothermal” was used to describedpreparation processes that are performed in ionic liquids at hightemperature.Generally, ionic liquids are defined as salts composed solely

of ions with melting points below 100 °C, or even below roomtemperature.45,140 For the purpose of ionothermal synthesis, abroader definition of an ionic liquid is used: a salt that meltsbelow the temperatures used in the synthesis of zeolites,typically 150−220 °C.45,47,138 The cationic parts of the mostcommon ILs are organic moieties such as imidazolium,pyridinium, quaternary ammonium, quaternary phosphonium,

or nitrogen-rich alkyl-substituted heterocyclic cations.139 Theanionic part can be organic or inorganic such as halides, nitrate,acetate, hexafluorophsphate (PF6), tetrafluoroborate (BF4),trifluoromethylsulfonate, or bis(trifluoromethanesulfonyl)imide.139 Compared with other solvents, ILs show specialfeatures including low toxicity, negligible vapor pressure, lowmelting temperatures, low viscosity, nonvolatility, nonflamm-ability, high thermal and chemical stabilities, ionic conductivity,and heating behavior under microwave radiation.139,141

Therefore, compared with traditional hydrothermal orsolvothermal synthesis, the ionothermal synthesis has obviousadvantages, such as the following: (i) ILs can act as bothsolvent and template as well as charge-compensation groups;(ii) ionothermal synthesis can be performed at ambientpressure, avoiding the high pressure used in traditionalhydrothermal and solvothermal synthesis and thus eliminatingthe safety concerns associated with high pressure; (iii) ILs offera novel chemical environment for organic and inorganicreactants; and (iv) excellent absorption of microwaves ensuresthat microwave techniques can be safely applied in ionothermalsynthesis.139,141

3.2. Ionothermal Synthesis of Aluminophosphate Zeolites

The first example of ionothermal synthesis of zeolite andzeotype materials was reported by Cooper et al.3939 in 2004,using the ionic liquid 1-ethyl-3-methylimidazolium bromide([EMIm]Br) and urea/choline chloride deep eutectic solventsto synthesize aluminophosphate zeolites (named as SIZ-n,Figure 11). After this discovery, a number of zeolites andzeotype materials with both known and unknown structureshave been synthesized.45,47 For example, when [EMIm]Br isinvolved in the ionothermal synthesis, new forms ofaluminosilicate SIZ-1 and SIZ-6 zeolites can be prepared inthe absence of fluoride, while previously known structures suchas SIZ-3 (AEL), SIZ-4 (CHA), and SIZ-6 (AFO) can beobtained in the presence of fluoride.39 These results indicatethat fluoride, a conventionally used mineralizer for zeo-lites,142−144 plays an important role in determining phaseselectivity in the ionothermal synthesis of zeolites.39,145 Inaddition, water is also a critical factor for ionothermalsynthesis.39,45,146 Ma et al.146 reported quantitative studies ofthe effect of water on the ionothermal synthesis ofaluminophosphate, showing that water could accelerate thehydrolysis and condensation reactions. Water, as a nucleophilicreagent, would improve hydrolysis reactions and therebyfacilitate the formation of solution-active species for zeolites.On the other hand, water could also promote synthesis kineticsby promoting both production and transport of H+ and OH−

hydrates.146

To achieve more new structures, organic templates such asamines have been introduced into the ionothermal synthesissystem.39,45 The structure-directing role of amines inionothermal synthesis in the presence of IL 1-butyl-3-methylimidazolium bromide ([BMIm]Br) has been discussedby Wang et al.147 They found that the addition of amines to theIL strongly influenced the dynamics of the crystallizationprocess and improved the phase selectivity of the crystallization,leading to the formation of pure AFI and ATV structures. Xinget al.148 prepared a novel aluminophosphate (denoted as JIS-1)consisting of an anionic open framework [Al6P7O28H]

2− with 1-methylimidazole (MIA) and [EMIm]Br as cotemplates.Protonated [MIAH]+ cations along with [EMIm]+ cations actas cotemplates and were found to coexist in the intersection of



the three-directional channels in the structure. More recently,Wei et al.149 demonstrated the successful ionothermal synthesisof thermally stable aluminophosphate zeolites (denoted asDNL-1) with 20-membered ring pore openings (CLO) by useof 1,6-hexanediamine (HDA) and [EMIm]Br as cotemplatesfor the first time (Figure 12). Both [EMIm]+ and protonatedHDA remained intact upon occlusion inside the CLO structure,suggesting that the protonated HDA is essential and acts as acotemplate together with the ionic liquid cations in thecrystallization process of DNL-1. Notably, not all amines addedwere included in the porous structure during the ionothermalsynthesis.147

Ionothermal synthesis is also useful to synthesize metal-substituted aluminophosphate zeolites.44 Three cobalt alumi-nophosphate zeolites, SIZ-7 (SIV), SIZ-8 (AEI), and SIZ-9(SOD), were synthesized ionothermally by use of [EMIm]Br.44

SIZ-7 exhibits a novel zeolite framework structure featuringdouble-crankshaft chains, which run parallel to the crystallo-graphic a-axis, characteristic of a family of zeolites such as thePHI, GIS, and MER structure types.44 Furthermore,magnesium, gallium, and silicon can also be incorporated intothe ionothermally prepared aluminophosphate zeolites.150,151

The successful introduction of heteroatoms as catalyticallyactive sites in the framework of aluminophosphate zeolites isvery important for designing and preparing novel alumino-phosphate-based zeolite catalysts in the future.3.3. Ionothermal Synthesis of Silica-Based Zeolites

Although ionothermal synthesis of aluminophosphate zeoliteshas been widely investigated,39,47,139 there are very fewsuccessful examples of ionothermal syntheses of silica-basedzeolites.47,152 This problem is due to the dissolution of silicon-containing reagents in the presence of ILs.39,47 As a result, onlya silica polymorph and a sodalite prepared from an ionic liquidhave been reported in the initial stage.47 In many of thesuccessful attempts at synthesis of silica-based zeolites, theorganic additives have been utilized in their hydroxide form inhydrothermal synthesis.13 Therefore, exchanging the anions ofthe ILs to those of the hydroxide type is strongly desirable forthe ionothermal synthesis of silica-based zeolites.Very interestingly, Wheatley et al.153 reported the successful

ionothermal synthesis of the siliceous zeolites silicalite-1 (MFI)and theta-1 (TON) using 1-butyl-3-methylimidazolium hydrox-ide ([bmIm]OH) as the ionic liquid. Notably, the [bmIm]OHwas obtained from [bmIm]Br via ion-exchange with an anion-exchange resin in water. The amount of exchange isapproximately 65%. The ionic liquid used in the reaction istherefore a ternary liquid of approximate formula [bmIm]-OH0.65Br0.35. The approximate initial molar composition was20IL:tetraethyl orthosilicate (TEOS):4H2O:0.38HF, confirm-ing that the IL is indeed the major solvent and that this is a trueionothermal preparation.Notably, the siliceous zeolite crystals obtained from the

ionothermal synthesis were too small for data collection on alaboratory X-ray source; thus, the data were collected on asynchrotron source and the structure was determined to be[bmIm]-silicalite-1. XRD and MAS NMR results show that (1)the aromatic five-membered rings of the imidazolium cationsreside at the intersections; (2) fluoride species exist in theframework close to imidazolium cations, balancing the charges;and (3) there are ethanol molecules (from hydrolysis of TEOS)

Figure 11. Synthesis of zeotypes by using ionic liquids and eutecticmixtures. (a) 1-Ethyl-3-methylimidazolium bromide can be used asboth solvent and template to prepare SIZ-1, SIZ-3, SIZ-4, and SIZ-5.SIZ-3 and SIZ-4 are prepared in the presence of fluoride and SIZ-5 inthe presence of excess water. (b) A choline chloride/urea eutecticmixture can be used to prepare SIZ-2 in the absence of fluoride orexcess water and to prepare AlPO-CJ2 in the presence of fluoride orexcess water. Orange, cyan, and red spheres correspond tophosphorus, aluminum, and oxygen atoms, respectively. Reprintedwith permission from ref 39. Copyright 2004 Nature PublishingGroup.

Figure 12. A [001] perspective view of the framework structure ofDNL-1 obtained from the structure refinement. The bridging oxygenatoms in the framework and all hydrogen atoms have been omitted forclarity. Al, yellow; P, green; and O, red. Reprinted with permissionfrom ref 149. Copyright 2010 Wiley−VCH Verlag GmbH & Co.KGaA, Weinheim, Germany.



in the pores of the structure. Therefore, the chemical formula ofthe c ry s t a l s was de te rmined to be [S i 4 8O9 6 ] -F4(C8N2H15)2(C2H7O)2.

153

The above successful example indicates that it is possible toalter the chemistry of ILs so that they are suitable for thesynthesis of crystalline silica-based zeolites, which might openup a new route for synthesizing silica-based zeolites underionothermal conditions.

4. SOLVENT-FREE SYNTHESIS OF ZEOLITES

4.1. Solvent-free Synthesis

Traditional chemists like to carry out their reactions in solution,even when a special reason for the use of solvent cannot befound.154 Indeed, in many cases, solid-state reactions occurmore efficiently and more selectively than their solutioncounterparts, providing various advantages such as reducedpollution, low cost, and simplicity in process and han-dling.154−157 Organic chemists have known since the 1980sthat organic reactions can occur by mixing powdered reactantsand reagents in the absence of solvent and that reactionproducts can be obtained efficiently. Solvent-free thermalorganic synthesis seems to be a highly useful technique,especially for industry.154−157

Solvent-free thermal synthesis has also been applied in thepreparation of ceramics, hydrides, and nitrides, which oftenrequire very high temperatures and repeat firings to ensure thatthe bond making/breaking and organization processes haveenough energy for the formation of crystalline phases.However, solvent-free syntheses of nanoporous materials arestill scarce.158−161

In 1990, Xu et al.162 reported the first example of thesynthesis of zeolites by dry gel conversion (DGC) and vapor-phase transport (VPT) techniques, in which a prepared dampor dried sodium aluminosilicate gel was suspended above a

liquid in an autoclave and subjected to a mixed vapor of amineand water at elevated temperature and pressure. Later, Rao andMatsutaka163 designed a similar method (steam-assistedconversion, SAC) to prepare zeolites. Encouraged by theseadvances,162−164 Schuth and co-workers165,166 have reportedhydrothermal zeolite syntheses from “dry” starting materials.These compositions had the appearance of free-flowingpowders although they contained 20−44% water by weight.Their thermal conversion to zeolites must involve conditionsclosely related to those applied in the SAC methods notedabove. Notably, solvents (e.g., water and alcohols) are stillnecessary for preparation of the homogeneous gels, indicatingthat these methods are not “true” solvent-free syntheses.162−166

4.2. Solvent-free Synthesis of Aluminosilicate Zeolites

In recent years, the ionothermal synthesis of zeolites has seengreat success, due to the elimination of high-pressureconditions and the creation of new structures. However,compared with conventional solvents (e.g., water and alcohols),the cost of ILs is still high, which is not suitable for large-scaleproduction in industry. Recently, Ren et al.51 reported ageneralized solvent-free route for synthesizing zeolites bymixing, grinding, and heating solid raw materials. Such asolvent-free route is quite distinct from the DGC method forsynthesizing zeolites,162−164 where the solvents are necessaryfor preparation of homogeneous gels, followed by vaporizationof the solvents to obtain dry gels. Compared with hydrothermalsynthesis, the solvent-free route has obvious advantages, forinstance, (1) high yields of zeolites, (2) better utilization ofautoclaves, (3) significant reduction of pollutants, (4) reducedenergy use and simplified synthetic procedures, and (5)significant reduction of reaction pressure.As a typical example, the solvent-free synthesis of pure silica

MFI zeolites (S-Si-ZSM-5) was performed by mechanicalmixing of the solid raw materials NaSiO3·9H2O, fumed silica,template (TPABr), and NH4Cl. After being ground for 10−20

Figure 13. (A) Photographs, (B) XRD patterns, (C) UV−Raman spectra, and (D) 29Si NMR spectra of samples crystallized at (a) 0, (b) 2, (c) 10,(d) 12, (e) 18, and (f) 24 h for synthesizing silicalite-1 zeolite via a solvent-free route. Reprinted with permission from ref 51. Copyright 2012American Chemical Society.



min, the powder mixture was transferred to an autoclave forfurther crystallization. The crystallization of S-Si-ZSM-5 hasbeen investigated by XRD, UV−Raman, and 29Si NMRtechniques (Figure 13). Before crystallization, the XRDpatterns showed each raw material, all of which disappearedafter treatment at 180 °C for 2 h. At the same time, the bandsassigned to TPA+ species in the UV−Raman spectra are greatlyreduced; this phenomenon can be explained by thespontaneous dispersion of solid salts on the amorphoussupport. Moreover, the Q4 signal is dominant in 29Si NMRspectra, suggesting a significant condensation of silica species.Furthermore, a peak related to the cubic NaCl phase (resultingfrom the reaction of NaSiO3 and NH4Cl) is observed. Aftercrystallization for 10 h, the XRD patterns exhibited a series ofcharacteristic peaks assigned to an MFI structure, and the UV−Raman spectra showed an obvious band at 374 cm−1 assignedto the five-membered Si−O−Si ring of MFI zeolites, indicatingthat a small amount of S-Si-ZSM-5 crystals are formed. Uponincreasing the crystallization time to 18 h, the intensities of theXRD patterns and Raman bands gradually increased, suggestingthat more S-Si-ZSM-5 crystals are formed. After treatment for18 h, S-Si-ZSM-5 was completely crystallized. The photographsof the samples crystallized at various times confirm that thesamples are always in a solid phase, and the sample volume wasreduced remarkably after the treatment, due to the con-densation of silica species during the crystallization.Notably, a small amount of water from the raw materials

(e.g., hydrated forms of sodium silicate or silica) is a criticalparameter for the formation of zeolites via the solvent-freeroute, which might be favorable for facilitating hydrolysis andcondensation of Si−O−Si bonds during the synthesis.Importantly, this solvent-free route was not limited to the

synthesis of MFI zeolites; other zeolites such as ZSM-39, SOD,MOR, Beta, and FAU have also been successfully prepared inthe absence of solvents. Moreover, this route can also beextended to the synthesis of mesoporous zeolites. For example,mesoporous MFI zeolites have been successfully prepared byuse of nanosized CaCO3 as solid templates in the absence ofsolvents.Notably, the yield of zeolites in conventional hydrothermal

synthesis is less than 80% (based on the yield of silica),suggesting that there is a large amount of raw material still inthe waste mother liquor due to the dissolution of silica in thealkaline medium. From an economic viewpoint, the wastemother liquor has to be recycled to increase the utilization ofthe raw materials (up to 95%) and consequently reduce thecost of industrial processes. Great efforts have been devoted toreusing the waste mother liquor.167 In the solvent-free synthesisof zeolites, alkaline media have been minimized and the yield ofsilica can be greatly improved up to 93−95%, at which pointrecycling of the waste mother liquor is not necessary.

4.3. Solvent-free Synthesis of Aluminophosphate-basedZeolites

Encouraged by the success in the field of solvent-free synthesisof aluminosilicate zeolites, Jin et al.168 have reported in a recentpaper solvent-free synthesis of silicoaluminophosphate (SAPO-34, SAPO-11, SAPO-20, and SAPO-43), aluminophosphate(APO-11), and heteroatom- (M = Co or Mg) containingaluminophosphate (M-APO-11 and M-SAPO-46) zeolites viamixing, grinding, and heating of the raw materials.Chosen as a model, the solvent-free synthesis of SAPO-34

(S-SAPO-34) is performed by mechanically mixing the solid

raw materials NH4H2PO4, boehmite, fumed silica, and template(morpholine). After being ground for 10−20 min, the powdermixture was transferred to an autoclave for further crystal-lization. The obtained sample was thoroughly characterized byvarious techniques including XRD, adsorption, SEM, TEM, andsolid MAS NMR. XRD patterns confirmed the CHA zeolitestructure; 27Al, 31P, and 29Si MAS NMR spectra of as-synthesized and calcined S-SAPO-34 suggested the coordina-tion of tetrahedral Al, P, and Si species. Very interestingly, ahysteresis loop occurred at a relative pressure of 0.50−0.98,indicating the presence of mesoporosity and macroporosity inthe samples. SEM and TEM images confirmed the presence ofhierarchical macroporosity. S-SAPO-34 samples prepared in asolvent-free manner were found to have a unique micromeso-macroporous structure, which has benefits for designing andpreparing efficient catalysts. In the methanol-to-olefin reaction,such S-SAPO-34 zeolites showed comparable activity toconventional SAPO-34 synthesized by the hydrothermalroute, but enhanced selectivity for propylene and butylenedue to the hierarchical porosity of S-SAPO-34.169

5. SYNTHESIS OF ZEOLITES WITH RELATIVELY HIGHEFFICIENCY: USE OF MICROWAVE RADIATION

5.1. Microwave-Assisted Synthesis

The utilization of microwave radiation as an energy source forchemical reactions and processes has been widely studied sincethe last century.141,170,171 Microwave energy has been found tobe more efficient for selective heating in many processes,making them require less energy than conventional processes,and is in general regarded as a green chemistry process.170,172

Previous studies over the past decade have demonstrated thatmicrowave-assisted syntheses not only reduce the time andenergy consumption of a process but also provide uniformstructures and versatile composition of the products.171,173

Such features are attributed to more rapid nucleation of theinitial crystallites as well as more uniform growth processesduring microwave-assisted synthesis. Several hypotheses havebeen advanced to explain the enhancement of syntheses ofnanoporous crystalline solids (e.g., zeolites):141,171 (i) micro-wave heating can rapidly transfer microwave energy into thereaction system, leading to an increase of heating rate andconsequent crystallization rate;174 (ii) microwave energyaccelerates dissolution of the reaction gels;175,176 (iii) micro-wave heating results in more uniform temperature distributionin the reaction mixture;177 (iv) microwave heating can generatesuperheating in the reaction system; (v) microwave heating cangenerate hot spots in the reaction system, and such hot spotstogether with superheating may affect the orientation of thereactant as well as products;178,179 and (vi) microwave heatingchanges the association between reaction species in the reactionmixtures.176 As described above, microwave-assisted synthesiscan be a promising alternative to conventional hydrothermalsynthesis of zeolites.Notably, great care should be taken when microwave heating

is used. The buildup of high pressure is possible for reactionsperformed in closed vessels in the presence of low-boilingsolvents. Thus, considerable change in internal pressure mustbe anticipated and controlled to prevent vessel rupture,microwave oven damage, and possible personal injury. As asafety precaution, vessels with pressure release mechanisms arerecommended. On the other hand, oven modification should



be carried out with caution, since leaked microwave radiationposes a health hazard.

5.2. Microwave-Assisted Hydrothermal Synthesis ofZeolites

5.2.1. Hydrothermal Synthesis of Zeolites Assisted byMicrowave Radiation. The initial examples of zeolitesynthesis assisted by microwave heating were NaA and ZSM-5 zeolites, reported by Mobil in 1988.180 After this discovery,many research groups became involved in microwave-assistedsynthesis of aluminosilicate zeolites under hydrothermalconditions.141,171,181 Compared with traditional hydrothermalsynthesis, microwave-assisted synthesis has unique “microwaveeffects”.141,171,176−183 Particularly, zeolite synthesis via micro-wave heating is much faster than that under conventionalhydrothermal conditions. For example, zeolite synthesis iscurrently a time-consuming process, often requiring days underconventional hydrothermal conditions. However, if microwaveheating is employed during the synthesis, the time may bereduced to several hours or even several minutes.,138,171,174,181

Following the early studies on the microwave-assistedsynthesis of zeolites, many attempts have been carried out toprepare various aluminosilicate zeolites including NaA(LTA),178,184−188 sodalite (SOD),189 analcime (ANA),190

NaY (FAU),10,191−195 NaY (EMT),196 Na−P (GIS),180 L(LTL),171 ZSM-5 (MFI),179,180,196,197 and Beta (BEA).179

Among these aluminosilicate zeolites, NaY was one of themost important zeolites due to its wide application as a FCCcatalyst.1,2 Conventionally, the synthesis of NaY suffers fromlong reaction times (10−30 h). However, NaY zeolites withhigh crystallinity can be obtained by microwave heating for 10−15 min without any impure phase.171,179 Even if the microwaveheating time was extended to 2 h, no other impure structurewas detected by XRD, suggesting that the microwave-assistedsynthesis is an effective route for synthesis of pure NaY zeolites.In addition, ZSM-5 zeolites with uniform crystal sizes of about3−4 μm can be prepared via a microwave-assisted route with areagent compos i t ion of 5 .0Na2O:0 .2Al 2O3 :60S i -O2:4.0TPA

+:900H2O at 170 °C for about 3 h.141 The sampleXRD patterns show that the nucleation requires about 1.65 h

and the crystal growth takes ca. 1 h. Notably, very well-crystallized ZSM-5 crystals with uniform size about 0.3−0.5 μmcan be obtained upon microwave heating at 175 °C for only 5min in the presence of 5% seeds.141,171 Furthermore,investigation of microwave-assisted synthesis of Ti-ZSM-5(TS-1) revealed that the nucleation required about 7.5 h andcrystallization needs only 1.5 h even in the presence offluoride.198 All these results suggest that, in the microwave-assisted synthesis of MFI zeolites, nucleation is the key stepthat determines the crystallization rate. Thus, crystallization willbe greatly increased in the presence of seeds.Microwave-assisted synthesis of zeolites is not limited to

aluminosilicate zeolites; aluminophosphate zeolites can also beprepared under microwave heating.178,199−204 For example,AlPO-5 can be synthesized at 180 °C for 1 min undermicrowave heating.181 Heteroatom-substituted aluminophos-phate zeolites such as Si, V, Co, and Mn have also beensynthesized in microwave-assisted processes.205−210

5.2.2. Microwave-Assisted Crystallization of ZeoliteCrystals with Preferred Orientation. Due to the uniqueheating method, the microwave-assisted synthesis of zeolitesnot only increases the crystallization rate but also results innarrow particle size distribution, as well as controllable crystalorientation.Park and co-workers211−213 reported the fabrication of MFI-

type zeolite crystals with a fibrous morphology incorporating Tiunder microwave irradiation. These crystals are stacked on topof one another along their b-direction to form fibers, with thedegree of self-assembly depending on the nature of thetetravalent metal ion used. Importantly, self-assembly of thezeolite crystals and the resultant fibrous morphology areobserved only when the substituting metal ions are present.The fibrous morphology was ascribed to condensation of theterminal hydroxyl groups between crystal surfaces to inducemultiple stacking of flat crystals.Later, Chen et al.214−216 systematically studied the depend-

ence of solvent on the crystallization behavior of silicalite-1under microwave irradiation. The properties of the solventwere finely turned by introduction of a second and a third

Figure 14. SEM images of Si-MFI crystals crystallized from microwave-assisted solvothermal synthesis system in the presence of diols: (a) ethyleneglycol, (b) diethylene glycol, (c) triethylene glycol, and (d) tetraethylene glycol. A schematic identifying the crystal faces is shown at the right side ofthe figure. Reprinted with permission from ref 215. Copyright 2009 Elsevier.



alcohol as cosolvents. These alcohols include ethylene glycol,methanol, ethanol, 1-propanol, 2-propanol, n-butanol, andhexanol. They found that the polarity of the alcohols used ascosolvents significantly influenced the morphology of thezeolite crystals. Alcohols with relatively high polarity (dielectricconstant) led to isolated single crystals, while alcohols withrelatively low polarity resulted in self-stacked crystals.215 Thefibers formed by these self-stacked crystals are stable andcannot be destroyed even under strong and prolongedultrasonication, which suggests the existence of strong chemicalbonds between the individual crystals due to condensation ofsurface Si-OH groups among individual crystals at the earlystages of the synthesis. Thus, alcohols with low polarity(dielectric constant) might favor the formation of an abundanceof Si-OH groups on the surface of nanocrystals formed in theearly stages of microwave-assisted synthesis. Furthermore, diolswere also used as cosolvents in microwave-assisted synthesis. Inthis case, silicalite-1 crystals become longer, narrower, andthinner, with an increasing ratio of the number of the carbonatoms and hydroxyl groups of the diols (Figure 14).216

All these results indicate that the unique microwave heatingmethod strongly influences the growth rates of MFI crystalsalong different directions. The combination of microwaveheating and application of alcohols remarkably changed thegrowth kinetics of MFI zeolite crystals.141

5.2.3. Microwave-Assisted Hydrothermal Synthesis ofZeolite Membranes. Preparation of zeolite membranes hasbecome a very attractive field due to their wide applications inseparation, catalysis, and electronic devices since the mid-1990s.217,218 Generally, zeolite membranes can be prepared viathree routes: in situ hydrothermal synthesis,219−223 secondary(seeded) growth synthesis,224−227 and vapor-phase transportsynthesis.228−230 Notably, the preparation of zeolite membranesunder conventional hydrothermal routes generally takes a longtime and the quality is not as high as desired. Upon microwaveheating, the formation of zeolite particles with small anduniform sizes makes it possible to prepare thin, dense,orientated, and aligned membranes (Figure 15).231−254

Currently, zeolite membranes such as LTA, MFI, AFI, FAU,SOD, and ETS-4 have been successfully synthesized bymicrowave heating.

LTA membranes have been extensively investigated due totheir convenient and facile synthesis.233−239,244,246−250 Han etal.233 reported microwave-assisted hydrothermal synthesis ofNaA zeolite membranes on alumina supports in 1999. It wasfound that microwave heating greatly accelerated thecrystallization rate, giving crystallization in only 10 min. Thethickness of the membrane could be controlled by varying thecomposition of the reaction mixture, and surface seeding isnecessary to promote the formation of NaA zeolite membranesand suppress the formation of impurity phases.234−236 Yang andco-workers237 developed a new strategy called “in-situ aging−microwave heating” for the preparation of zeolite membraneswithout seeding. High-quality NaA zeolite membranes with H2/N2 permselectivity of 5.6 were successfully synthesized by thismethod. The authors initially rearranged the synthesis mixtureto form the germ nuclei on the support surface, which wereobtained by in situ aging. Then nucleation and crystal growthon the support were achieved by consequential crystallizationunder fast and homogeneous microwave heating.237

Due to the relatively high temperatures (>150 °C) requiredfor MFI zeolite crystallization and the fast degradation of theTPA+ template under microwave conditions,176 the microwave-assisted synthesis of MFI zeolite membranes is more difficultthan that of LTA zeolite membranes. Koegler et al.249

demonstrated microwave-assisted hydrothermal synthesis ofsilicalite-I zeolite membranes, which were in situ synthesized ona silicon wafer via rapid heating and cooling. Combiningmicrowave heating with secondary growth strategy, Motuzas etal.250 successfully synthesized silicalite-I zeolite membranes onalumina supports with (101) and (001) preferred orientation.Later, they developed an ultrarapid and reproducible synthesismethod for thin and good-quality MFI membranes by couplingmicrowave-assisted synthesis with a rapid template removalmethod (ozone treatment).251

Microwave-assisted hydrothermal synthesis of zeolite mem-branes was also extended to the preparation aluminophosphatezeolite membrane.252−254 Mintova et al.252 used microwaveheating to synthesize thin films of AlPO4−5 on gold-coatedquartz crystal microbalances. The temperature, microwaveheating time, power, and aging time are important factors forthe control of membrane thickness and crystal orientation.Later, Tsai et al.253 prepared well-aligned SAPO-5 membranesusing microwave heating on an anodized aluminum substrate.The effects of various synthetic parameters on the degree ofpreferred orientation along the c-crystal axis of the AFIstructure, and the zeolite coverage on the alumina support,were discussed.Compared with synthesis of zeolite membranes under

conventional heating, microwave-assisted synthesis of zeolitemembranes led to obvious differences in morphology andcomposition and a significant improvement in permanence,permselectivity, and compactness, besides the remarkabledecreasing of synthetic time. These features are potentiallyimportant for industrial applications of zeolite membranes.

5.3. Microwave-Enhanced Ionothermal Synthesis ofZeolites

Although microwave-assisted hydrothermal synthesis of zeoliteshas obvious advantages compared with conventional hydro-thermal synthesis, drawbacks are apparent in that theexperiment process is sometimes not safe, and organictemplates or volatile solvents still cause problems with excessivepressure production, especially from hot spots. In the previous

Figure 15. Comparative synthesis model of zeolite membraneprepared by microwave heating and conventional heating. Reprintedwith permission from ref 248. Copyright 2008 Elsevier.



section we have discussed the features of ILs, which have beenproven a good medium for absorbing microwaves. Thus, goodmicrowave absorption combined with the low-pressureevolution at high temperature of ILs will open up manypossibilities for the use of microwaves in zeolite synthesis.52

5.3.1. Ionothermal Synthesis of Zeolites Assisted byMicrowave Radiation. For the first time, Xu et al.52 reporteda microwave-enhanced ionothermal synthesis of aluminophos-phate zeolites in 2006. They prepared AEL-type aluminophos-phate zeolites (AlPO-11 and SAPO-11) in [emim]Br underionothermal conditions. Products with cubic-like crystals can beexclusively obtained in 20−60 min (Figure 16). On the

contrary, complete crystallization under conventional heatingrequired 20−40 h. The rapid crystallization of zeolites duringmicrowave-enhanced ionothermal synthesis is attributed tolocal superheating. Additionally, it seems likely that microwavescan enhance the reaction of fluoride ions with Al species,namely, enhancing the digestion of the reactant into the liquidphase, as well as facilitating the formation of the zeolitestructure during ionothermal synthesis. Therefore, the micro-wave-enhanced ionothermal synthesis of zeolites can beexpected to be a promising approach to preparation of zeolites.Recently, Yan and co-workers55 have successfully developed

a new strategy for the preparation of silica-based zeolites (MFI)at ambient pressure that combines the advantages ofionothermal synthesis, dry-gel conversion, and microwaveradiation, making it a promising, safe, fast, and continuousprocess for industrial applications. They used microwaveheating instead of conventional heating to convert the driedgel precursor (DGP) containing [Bmim]Br as structure-directing agents to MFI zeolites at ambient pressure. TheDGP was successfully converted to MFI crystals after 2 h ofmicrowave radiation at 175 °C at ambient pressure in thepresence of excess water. The crystallinity increased with

microwave radiation time for the first 3 h but remained thesame after that. The role of ionic liquids is to retain a sufficientamount of water at high temperature and ambient pressure forthe gel crystallization, as well as being hygroscopic, nonvolatile,and efficient at absorbing microwave energy that can lead to asuperheated fluid due to inverse heating, together withstabilization of the SDA by an ion-exchange process.

5.3.2. Microwave-Enhanced Ionothermal Synthesis ofZeolite Membranes. Yan and co-workers54 have alsoreported the microwave-enhanced ionothermal synthesis ofextremely well-oriented zeolite coatings on copper-containingaluminum alloys, which are used extensively in the aerospaceindustry without corrosion problems. They prepared alumi-nophosphate (AlPO) and Si-substituted aluminophosphate(SAPO) zeolites with the same structure topology (AEL)(Figure 17). They found that the SAPO coating crystallizes

more slowly but in such a way that it is highly aligned to thesurface of the metal, while AlPO coatings crystallize quicklywith almost randomly oriented coating. The coatings adherewell to the metal surface, and direct current (dc) polarizationresults indicate that the coatings make excellent anticorrosionbarriers.The success of Yan’s work proves that microwave-enhanced

ionothermal synthesis methods can be used as novel, simple,fast, environmentally benign, and safe ways to prepare orientedzeolite membranes.

6. SUMMARY AND PERSPECTIVESThe modern synthesis of zeolites mainly involves use of organictemplates, addition of solvent, preparation of starting gels, andheating of the gels. Each step could be made greener in thefuture.

Figure 16. SEM micrographs of aluminophosphate molecular sieves(AEL type) prepared by ionothermal synthesis: (a, b) samples after 68h of crystallization with conventional heating; (c, d) samples after 20min of crystallization with microwave heating. Synthesis conditions:150 °C, ambient pressure, ionic liquid [emim]Br. Reprinted withpermission from ref 52. Copyright 2006 Wiley−VCH Verlag GmbH &Co. KGaA, Weinheim, Germany. Figure 17. SEM images of different as-synthesized AEL coatings on

AA 2024-T3: (a) AlPO-11 (surface); (b) AlPO-11 (cross section); (c)SAPO-11 (surface, inset is higher magnification); (d) SAPO-11 (crosssection, mildly polished surface); (e) SAPO-11 with spin-on BTSM-MEL (surface); (f) SAPO-11 with spin-on BTSM-MEL (crosssection). Reprinted with permission from ref 54. Copyright 2008Wiley−VCH Verlag GmbH & Co. KGaA, Weinheim, Germany.



This survey presents a brief overview of recently reportedgreen routes for synthesizing zeolites, including reduction orelimination of organic templates, use of green solvents such asILs or complete elimination of solvent, and efficient heating ofthe starting gels. To overcome the disadvantages of usingorganic templates, nontoxic templates and template recyclingsteps have been employed in zeolite syntheses. In addition,organotemplate-free syntheses have become a popular anduniversal methodology for synthesizing zeolites. Particularly,seed-directed synthesis in the absence of organic templates is ageneral route for synthesizing a series of zeolites. To reduce theaqueous wastes and high pressure required in the synthesis ofzeolites, ionic liquids (ILs) as green solvents have been widelyused in recent years. Of course, the best way to reduce theaqueous wastes is a solvent-free (solventless) synthetic route.To heat the starting gels efficiently, microwave radiation wasused to crystallize zeolites very rapidly.Notably, most green approaches referred to above are

separated, and as a consequence, there is always a balance ofcompeting aspects. For example, many kinds of zeolites can beprepared in the absence of solvent but still require the presenceof organic templates; zeolite seed solutions assist the synthesisof zeolites without use of organic templates but water isnecessary for the solution; microwave-assisted synthesis savesenergy and increases efficiency but also can cause high pressurein the presence of low-boiling solvents. Thus, the combinationof various green routes may have a promising future forsynthesizing zeolites from an industrial perspective.It has been demonstrated that the combination of

ionothermal synthesis and microwave heating is a simple, fast,environmentally benign, and safe route for synthesizingzeolites.52,55 However, the combination of solvent-free syn-thesis and organotemplate-free strategies is still challenging,although this would represent a truly green route,51 as itcompletely avoids the use of organic templates and solvents.Furthermore, microwave-assisted synthesis of zeolites in theabsence of solvents and organic templates would be a simple,fast, low-cost, environmentally benign, and safe route, whichcompletely fulfills the requirements of green chemistry. Webelieve this is needed to move forward in the field.The solvent-free (solventless) synthesis of zeolites is

particularly emphasized. This approach raises many questionsin terms of its potential for large-scale applications, includingthe role of the initial grinding, the mechanism of solvent-freesynthesis, the interaction between the organic SDAs (in thecase involved) and the silica, and the properties of theproducts.158 Synthetic chemistry under green conditions is stilla fertile ground for fundamental studies of reactivity.

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected].

Notes

The authors declare no competing financial interest.

Biographies

Dr. Xiangju Meng received his B.S. (1999) and Ph.D. (2004) from theCollege of Chemistry, Jilin University, China. Subsequently, he joinedthe Catalytic Chemistry Division, Chemical Resources Laboratory,Tokyo Institute of Technology, Japan, for postdoctoral work on thesynthesis and applications of nanoporous materials under the guidanceof Prof. T. Tatsumi. After postdoctoral work at the National Instituteof Advanced Industrial Science and Technology (AIST), Japan, hereturned to China and joined Prof. Xiao’s group. He was promoted toassociate professor at Zhejiang University in 2009. His researchinterests include zeolites and heterogeneous catalysis.

Prof. Feng-Shou Xiao received his B.S. and M.S. degrees from theDepartment of Chemistry, Jilin University, China. From there hemoved to the Catalysis Research Center, Hokkaido University, Japan,where he was involved in collaborative research between China (JilinUniversity and Dalian Institute of Chemical Physics) and Japan. Hewas a Ph.D. student there for two years, and was awarded his Ph.D.degree at Jilin University in 1990. After postdoctoral work at theUniversity of California at Davis, USA, he joined the faculty at JilinUniversity in 1994, where he is a full and distinguished professor ofChemistry. Since 2009, He moved to Zhejiang University from JilinUniversity, and now he is a full and distinguished professor ofChemistry in Zhejiang Univeristy. For his research in porous catalyticmaterials, Prof. Xiao has been recognized with the NationalOutstanding Award of Young Scientists of the National ScienceFoundation of China in 1998 and Thomson Reuters ScientificResearch Fronts Award in 2008.

ACKNOWLEDGMENTSThis work was supported by the National High-Tech Researchand Development program of China (2013AA065301) and theNational Natural Science Foundation of China (21333009,21273197, and U1162201).



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