St

JID

a

ARR1A

KHpWTc

1

ttaosms

Aaoasepi

1d

Chemical Engineering Journal 166 (2011) 1083–1089

Contents lists available at ScienceDirect

Chemical Engineering Journal

journa l homepage: www.e lsev ier .com/ locate /ce j

ynthesis of ZSM-5 hierarchical microsphere-like particle by two stage varyingemperature crystallization without secondary template

ianhua Yang, Suxia Yu, Huiye Hu, Yan Zhang, Jinming Lu, Jinqu Wang ∗, Dehong Yinnstitute of Adsorption and Inorganic Membrane, State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, 158, Zhongshan Road,alian 116012, PR China

r t i c l e i n f o

rticle history:eceived 31 May 2010eceived in revised form1 November 2010ccepted 19 November 2010

eywords:ierarchical ZSM-5 microsphere-likearticles

a b s t r a c t

In the present paper, Hierarchical ZSM-5 microsphere-like particles (ZSM-5-HM) were successfully syn-thesized by separate hydrolysis combined two stage varying temperature crystallization (SHTSVTC)without any secondary template. XRD, SEM and TEM analysis revealed that the ZSM-5-HM parti-cles were 300–600 nm in size and constructed by primary ZSM-5 nanocrystals of 30–50 nm. The BJHpore size based on the N2 adsorption isotherm and desorption isotherm was 9.7 nm and 9.4 nm,respectively. For comparison the synthesis was carried out by the constant temperature crystalliza-tion (CTC) and two stage varying temperature crystallization methods (TSVTC) under cohydrolysis,respectively. A time trial growth study was conducted for insight into the growth of the hierarchical

ithout secondary templatewo stage varying temperaturerystallization

ZSM-5-HM. It is suggested that the growth of ZSM-5-HM products involves aggregation and assemblycombined crystallization process. The synthesis by the TSVTC method gave rise to similar hierarchicalZSM-5 particles when the second stage crystallization time was extended for more 24 h. It indicatesthat the TSVTC is mainly responsible for the formation of the ZSM-5-HM products but the separatehydrolysis can reduce the crystallization time. The demonstrated facile and highly effective approachto mesoporous zeolites can be extended to the synthesis of other zeolites with unique hierarchicalstructure.

. Introduction

Zeolites are a class of microporous crystalline aluminosilicateshat possess numerous and unique structures with characteris-ic cavities and/or pores of molecular sizes. Zeolites have manypplications related with molecular recognition in a broad rangef areas, such as catalysis, adsorption, separation, ion exchange,ensing and optoelectronics based on their strong acidity, uniformicroporosity, unique structure and high thermal/hydrothermal

tability [1].Since ZSM-5 zeolite was first synthesized in 1965 by Landolt and

rgauer, for example, it has widespread application as a fluid cat-lytic cracking additive for both propylene production and gasolinectane improvement [2]. Besides, ZSM-5 exhibits excellent cat-lytic properties in the aromatics compounds synthesis due to its

hape-selective medium-pore [3] such as p-xylene synthesis [4],thylbenzene and cumene synthesis [5], etc. However, the micro-orous character of the zeolite limits the diffusion of molecules to

nternal catalytic sites, particularly when the diffusion in the micro-

∗ Corresponding author. Tel.: +86 411 3989 3632; fax: +86 411 8365 3220.E-mail address: wjinqu@dlut.edu.cn (J. Wang).

385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2010.11.071

© 2011 Elsevier B.V. All rights reserved.

pores is significantly slower than the reaction, for example whenthe reactions are performed in the liquid phase, which adverselyaffect catalytic performance [6].

To overcome this problem, several novel materials such as ultralarge micropore zeolite [7], ordered mesoporous materials [8], zeo-lite nanocrystals [9] and mesoporous zeolites were synthesized[10]. Among these materials, mesoporous zeolite was considered tobe one of most promising materials because it combines the advan-tage of zeolites and mesoporous materials as proved by catalyticand diffusion tests [11].

Nano-sized zeolite crystals (nanozeolites) have received muchinterest due to their properties of high external surface area, moreexposed active sites as well as reduction of diffusion path length forsubstrate in catalysis. Crystal size can be decreased by modifyingsynthesis parameters like the gel composition or the crystalliza-tion temperature. Jacobs et al. [12] reported the existence of X-rayamorphous ZSM-5, which contained crystals of <8 nm size in anamorphous matrix of silica. Camblor et al. [13] were able to synthe-

size zeolite beta with a crystal size as small as 10 nm determinedby TEM. Recently, Schmidt et al. [14] have synthesized nano-sizedzeolites of ZSM-5, zeolite beta, zeolite X, and zeolite A with highinterparticle mesoporosity by confined space synthesis. Gia-Thanhet al. [15] synthesized nanozeolites with the hydrophobic external

1 ering J

sots

ficp1doailcvpoahatzbcfchst[frlczephlmosnz

achcfiazsahostplaemc

084 J. Yang et al. / Chemical Engine

urface in an organic solvent. Unfortunately, zeolite crystallizationften requires very specific conditions in terms of composition andemperature which considerably limits the range of feasible crystalizes.

When the MCM-41 type ordered mesoporous materials wererst introduced in the 1990s it was anticipated that these materialsould function as catalysts for bulky molecules. Ordered meso-orous materials (OMMs) lead to improved transport due to the0 times larger pore size, but their limited acidity, stability cooledown the OMM revolution in catalysis. On this basis, the devel-pment of multi-structured zeolitic materials with controlled porerchitecture on different length scales has attracted great attentionn recent years. Introduction of mesopore into the microporous zeo-ites, namely hierarchical zeolite, is a new approach to improve theatalyst effectiveness in chemical reactions. The mesopores pro-ide improved access to the micropores and shorten the diffusionath length, thereby enhancing the rate of diffusion because thevercome mass transport to/from the microporous zeolite, the hier-rchical zeolite can exhibit significantly improved conversion origh selectivity or high resistance to catalysis deactivation. Theyre attractive for a wide range of applications in catalysis reac-ions. There are several approaches to prepare the hierarchicaleolite. Post-treatment, dealumination or desilication [16–18], haseen widely adopted to create mesopores in zeolites. Groen ando-worker have synthesized hierarchical ZSM-5 with excellent dif-usion properties by desilication of uniform Al-distributed ZSM-5rystals [17–18]. Templating method as a new and promising routeas been received increasing attentions in this area [19–24]. Jacob-en et al. prepared 12–30 nm mesoporous ZSM-5 by impregnatinghe synthesis gel components with multiwall carbon nanotubes19–20]. Using carbon aerogel template Tao et al. synthesized uni-orm mesopore-modified Y zeolites [21]. Christensen et al. [22]eported a new family of zeolite catalysts, the mesoporous zeo-ite single crystals which exhibited a significantly improvement ofatalytic activities and selectivities, as compared to conventionaleolite catalysts, in the alkylation of benzene with ethene. Xiaot al. reported the use of a cationic polymer as mesopore tem-late for the synthesis of mesoporous Beta zeolite which exbitedigher conversion and better selectivity than a conventional zeo-

ite in catalytic alkylation of benzene with 2-propanol [23]. Theesoporous MFI zeolite could be obtained using amphiphilic

rganosilanes, [(CH3O)SiC3H6N(CH3)2CnH2n+1]Cl and exhibiteduperior catalytic activity and selectivity in the jasminaldehyde (�--amylcinnamaldehyde) compared to a purely microporous MFIeolite [24].

Compared with the above methods, direct synthesis withoutny post treatment or secondary template is regarded as a new,ost-effective, highly efficacious and simple route to preparation ofierarchical zeolites. Mesoporous aggregate of zeolite nanocrystalsan be obtained without any secondary template through care-ully controlling the synthesis conditions because they have a largenfluence on the size and crystallinity of zeolite nanocrystals. Fangnd coworkers reported the synthesis of mesoporous aggregate ofeolite nanocrystals without secondary template by enhancing theupersaturation degree via concentration of the synthesis gel or bydding the nucleation promoter [25]. In our own research work, weave obtained hierarchical MCM-22 and ZSM-5 aggregates with-ut any secondary template by means of carefully controlling theynthesis parameters (the degree of supersaturation and ageingimes) and by separate hydrolysis combined two stage varying tem-erature crystallization respectively. In our previous work, their

argely improved catalytic performance in methane dehydroarom-tization reaction was presented [26]. In the present work, themphasis was placed on the synthesis of the hierarchical ZSM-5icrosphere-like particles (ZSM-5-HM) of 300–600 nm which are

onstructed by primary nanocrystals 30–50 nm in size by a facile

ournal 166 (2011) 1083–1089

method namely the separate hydrolysis combined two stage vary-ing temperature crystallization (SHTSVTC) without any secondarytemplate. The crystallization synthesis by two stage varying tem-perature crystallization method (TSVTC) was first carried out atlower temperature and rapidly changed into higher temperature.Li et al. proposed the TSVTC method to investigate the period ofnucleation and crystal growth for the system of discrete colloidalparticles of TPA-silicalite-1 and pointed out that lower tempera-ture is favorable for nucleation and higher temperature for crystalgrowth [27]. Inspired by this findings, the TSVTC combined separatehydrolysis was applied to the synthesis of the unique hierarchicalzeolite structure of ZSM-5. For comparison, the ZSM-5 zeolite syn-thesized by the conventional constant temperature crystallization(CTC) and TSVTC respectively was also investigated. The time trialstudy of ZSM-5-HM was carried out to gain the insight into thegrowth of the products.

2. Experimental

2.1. Synthesis of ZSM-5-HM

All chemical reagents used in this work were of analyticalgrade. The ZSM-5-HM was obtained by the separate hydrolysiscombined varying temperature crystallization without any sec-ondary template or post-treatment. At first tetraethylorthosilicate(TEOS) as silica source and aluminium isopropoxide (AIP,) as alu-mina source was hydrolyzed in TPAOH solution separately foran specified period. Then the two separate solutions were mixedtogether to form a precursor solution. Then through varying tem-perature crystallization the final ZSM-5-HM could be obtained. Thetypical synthesis procedures were described as follows: 16.3 g oftetrapropylammonium hydroxide (TPAOH, 25 wt%, purchased fromAldrich) as structure directing agent was dissolved in 15 g of deion-ized water under stirring at room temperature for 1 h, then 20.8 gof TEOS (>98%, purchased from Aldrich,) was added into the solu-tion and reacted at room temperature for 24 h under stirring. Theresultant synthesis is designed as the solution A. Another 16.3 gof TPAOH and 15 g of deionized water were mixed and stirred atroom temperature for 1 h, then 0.68 g of AIP (>99%, purchased fromAldrich) was added and stirred at room temperature for 24 h. Theresultant solution is designed as the solution B. Finally, this twosolutions were mixed together and stirred for 4 h to obtain thesynthesis solution. The composition of the resultant synthesis solu-tion is 60SiO2:Al2O3:24TPAOH:1800H2O. This separate hydrolysisof TEOS and AIP is referred to separate hydrolysis. In contrast thenormal hydrolysis is referred to the cohydrolysis which is done byadding TEOS into AIP solution containing all the specified amountof water and TPAOH. The synthesis solution then aged at room tem-perature for 3 h without stirring. Then the aged synthesis mixturewas transferred into a Teflon-lined stainless steel autoclave. Thecrystallization reaction was first carried out at 373 K for 24 h andthen rapidly changed into second stage temperature of 443 K for24 h. The solid product was collected, centrifuged, filtered thenwashed with deionized water and dried in air at 393 K overnightand calcined at 773 K for 6 h to remove TPAOH. Finally, the prod-uct was obtained. For comparison, the synthesis was carried out bythe CTC method namely, the hydrolysis of the TEOS and AIP at roomtemperature for 24 h followed by constant temperature crystalliza-tion at 373 K and 443 K for 24 h, respectively. Instead of separatehydrolysis, the synthesis was also carried out by cohydrolysis of

the TEOS and AIP at room temperature for 24 h then followed byTSVTC under the same temperature conditions as that of ZSM-5-HM. For both of the CTC and TSVTC methods the composition ofthe synthesis solution is 60SiO2:Al2O3:24TPAOH:1800H2O, sameas that of SHTSVTC.

J. Yang et al. / Chemical Engineering Journal 166 (2011) 1083–1089 1085

Fs

2

dwrna(sTo

3

3

smtotFFr1ai

ig. 1. (a) The XRD pattern and (b) FT-IR pattern of the as-synthesized product byeparate hydrolysis combined varying temperature crystallization.

.2. Characterization

As-synthesized product was characterized by X-ray powderiffraction (XRD) on a Rigaku-Dmax 2400 diffractometer equippedith graphite monochromatized CuK� radiation in the 2� angle

anging from 4◦ to 40◦, IR spectrum (Bruker EQUINOX55), scan-ing electron micrograph (SEM, JEOL-6360LV). Nitrogen adsorptionnalyses was carried out with an Autosorb-1 adsorption analyzerQuantachrome Instruments) at 77 K. Prior to the adsorption mea-urements, the sample was degassed at 473 K for at least 6 h.ransmission electron microscopy (TEM) measurement was carriedut on a Philips Tecnai G2 20 instrument, operating at 200 KV.

. Results and discussion

.1. Synthesis and characterization of ZSM-5-HM product

Fig. 1(a) shows the XRD pattern of synthesized products byeparate hydrolysis combined varying temperature crystallizationethod. The very sharp reflections of the synthesized product in

he wide-angle XRD pattern represent for the characteristic peaksf ZSM-5, with no evidence of other crystalline phases, confirminghe formation of ZSM-5 zeolite with high pure crystalline phase.T-IR test was carried out to further identify the crystal phase.

or the products in the IR spectrum of the framework absorptionegion as shown in Fig. 1(b), absorption bands are observed at 1224,150–1050, 795, 550, and 455 cm−1. The spectra of all the samplesre identical with respect not only to band positions but also to peakntensity. Absorption bands at 1224 cm−1 (external asymmetric

Fig. 2. SEM image of synthesized hierarchical ZSM-5 in this work.

stretch), 1150–1050 cm−1 (internal asymmetric stretch), 795 cm−1

(external symmetric stretch), and 455 cm−1 (T–O bend) are typicalfor highly siliceous materials, while the framework vibration bandat 550 cm−1 had been assigned to the double five rings of the char-acteristic structure of MFI-type zeolites [28]. Both the XRD and theIR pattern can indicate that MFI-type zeolite has been successfullysynthesized.

SEM analysis was further carried out for the obtained productsas shown in Fig. 2. Relatively uniformed sphere-like particles ofabout 300–600 nm were observed. Interestingly, TEM images ofobtained ZSM-5 sample in Fig. 3 revealed that these particles wereconstructed by many primary nanocrystals of 30–50 nm. In highresolution TEM image of the part marked by the circle (Fig. 3(a)),clear lattice fringes could be observed, revealing the high crys-tallinity of this sample. It can be seen that some mesopores appearon the surface and edge of ZSM-5 crystals. Fig. 4 shows TEM imagesof the ZSM-5 sample by the CTC method synthesized at 373 and443 K, respectively. The typical coffin-shape morphology of theZSM-5 zeolite at both 373 and 443 K was observed. But the crys-tallinity of ZSM-5 at 373 K was lower than that of ZSM-5 at 443 K(judged from XRD patterns, not shown here), though the size ofZSM-5 at 373 and 443 K was similar to each other. It is clear that thehierarchical spherical architecture of ZSM-5-HM by the SHTSVTCis entirely distinct from the typical coffin-shape morphology of theZSM-5 by the CTC method.

Fig. 5 provides the nitrogen adsorption and desorption isothermof the ZSM-5-HM synthesized in this work. The nitrogen adsorptionisotherm contains a steep uptake below P/P0 = 0.02 and a hys-teresis loop from P/P0 = 0.45 to about P/P0 = 1, which suggests theco-existence of micropores and intercrystal mesopores. The paral-lel disposition of the adsorption and desorption branches impliesthat there were open mesopores connected to the external surface[29]. Pore size distribution (Fig. 5(b)) revealed that the micropores,mesopores and macropores co-existed in the ZSM-5-HM productwith two pore peaks at the micropore of 0.56 nm and macropore of70 nm, respectively. The BET total surface area is about 402.8 m2/gwith micropore and external area of 246.3 and 156.5 m2/g, respec-tively. Compared with the ZSM-5 by the CTC method, ZSM-5-HMshows a higher external area as shown in Table 1 due to the pres-ence of the mesopore and macropores. The BJH pore size basedon the adsorption isotherm and desorption isotherm was 9.7 nm

and 9.4 nm, respectively. The N2 adsorption results further confirmthe formation of the ZSM-5-HM. These hierarchical mesopores areimportant to catalytic performance [11,24,26].

1086 J. Yang et al. / Chemical Engineering Journal 166 (2011) 1083–1089

Fig. 3. TEM image of synthesized hierarchical ZSM-5-HM in this work by the SHTSVTC (crystallization condition: 373 K 24 h, 443 K 24 h).

Table 1Pore structural parameters of the ZSM-5-HM and ZSM-5 zeolite.

Sample Si/Ala BET specific surface area (m2/g) t-plot micropore area (m2/g) Micropore volume (cm3/g)

ZSM-5-HM 30 402.8 246.3 0.1130

ZSM-5b 30 395.5

a Si/Al ratio in the starting synthesis solution.b ZSM-5 synthesized by the CTC method at temperature 443 K 48 h.

Fig. 4. TEM images of products by constant temperature cryst

325.9 0.1551

allization at temperatures, (a) 373 K 48 h; (b) 443 K 48 h.

J. Yang et al. / Chemical Engineering Journal 166 (2011) 1083–1089 1087

ore s

3

gtp

rat

Fig. 5. N2 adsorption and desorption isotherm (a) and p

.2. Formation process of ZSM-5-HM products

A time trial growth study was conducted for insight into therowth of the hierarchical ZSM-5-HM. The morphology and crys-alline characteristic of the products with various reaction durationeriods were analyzed using XRD and TEM technique.

The synthesis solution was clear after 24 h of crystallizationeaction (first stage) at 373 K. It turned creamy-white and opaquefter further crystallization of 8 h at 443 K. Fig. 6(a) and (b) showshe TEM images of the products at this stage. Primary gel par-

Fig. 6. TEM images of products by constant temperature crystallization

ize distribution (b) of the obtained ZSM-5-HM product.

ticles with a diameter of about 20–30 nm were observed in theHRTEM image and theses primary gel particles aggregated intolarger gel particles. Dried samples from this solution gave amor-phous XRD reflections as shown in Fig. 7. With the increase in thereaction time, the ZSM-5 crystalline phase emerged from the cen-tre of the gel particles. The crystalline particles with outer edge

being amorphous gel is observed in TEM images Fig. 6(c) and (d)for the sample after 24 h of crystallization at 373 K and 16 h at443 K. Dried sample from this solution presents characteristic peakof ZSM-5 and part of amorphous phase in the XRD experiments

, (a and b) 373 K 24 h, 443 K 8 h; (c and d) 373 K 24 h, 443 K 16 h.

1088 J. Yang et al. / Chemical Engineering J

6040200

intensity

2θ

a

b

F8

(twacppp

ohf36eaa

ZTotgtnan5seiaTiup

gmo2pHr

[ration, characterization, and applications, Chem. Rev. 106 (2006) 896–910.

ig. 7. XRD pattern of products at different reaction periods, (a) 373 K 24 h, 443 Kh; (b) 373 K 24 h, 443 K 16 h.

Fig. 7). As can been seen from the TEM images, crystalline par-icles composed of many small nanocrystals in size of 30–50 nmith high crystallinity were formed after further 8 h crystallization

t 443 K (Fig. 3), clearly revealing the formation of the hierarchi-al particles. XRD patterns (Fig. 1) of the dried sample at this stageresents characteristics peak of ZSM-5 without no other impurityhase, revealing that the formed particles are highly pure ZSM-5hase.

It is accepted that precursor nanoparticles (NPs) consistingf a disordered silica-core/TPA-shell structure spontaneously onydrolysis of tetraethylorthosilicate in aqueous solutions of TPAOH

or a clear dilute synthesis systems [30]. The NPs are in size of aboutnm. The nuclei and crystal are evolved from secondary particles of–50 nm which are evolved by the aggregation of these NPs. Lobot al. reported that the tertiary particles larger than 200 nm weredditionally observed in the Beta synthesis solutions containingluminium source when the solution was heated to 393 K [31].

According to these observations, the formation process of theSM-5-HM can be depicted as follows: In the separate hydrolysis ofEOS and lower temperature steps the NPs particle and small sec-ndary aggregates are formed. With the increasing temperature,he NPs particle and small secondary gel particles further aggre-ated together. In parallel with aggregation process, nucleationake place among the primary or secondary gel particle and a largeumber of nuclei which are favorable for the hierarchical structurere formed because of the lower temperature. The evolved ZSM-5anocrystals tend to assembly together to form hierarchical ZSM-zeolite aggregates because the nanocrystals are unstable in the

tudied condition. The nanoparticles in the centre intergrow withach other, whereas the crystalline parts in the outer edge developnto nanocrystals through solution-mediated transport mechanismnd oriented attachment with each other during crystallization.he crystallization reaction will stop after the amorphous materials totally consumed. In summary the growth of ZSM-5-HM prod-cts involves aggregation and assembly combined crystallizationrocess.

The effect of separate hydrolysis on the ZSM-5-HM was investi-ated by comparing the product of SHTSVTC with that of the TSVTCethod. Instead of separate hydrolysis, the synthesis was carried

ut by cohydrolysis of the TEOS and AIP at room temperature for

4 h then followed by crystallization reaction under the same tem-erature conditions as that of SHTSVTC as mentioned in Section 2.1.owever amorphous products were obtained, suggesting the sepa-

ate hydrolysis facilitates the formation of ZSM-5-HM probably via

[

[

ournal 166 (2011) 1083–1089

facilitating the formation of nano precursor particles for the solu-tion A due to the higher super-saturation degree resulting from thehalf reduced water content in solution A. But the similar hierarchi-cal products (not shown here) to ZSM-5-HM were obtained whenthe crystallization was extended for 24 h at 443 K, suggesting thetwo stage varying temperature crystallization mainly contribut-ing to the formation of hierarchical ZSM-5-HM but the separatehydrolysis can short the crystallization time of ZSM-5-HM.

4. Conclusions

In conclusion, the hierarchical ZSM-5 zeolite microsphere-likeparticles were successfully synthesized by separate hydrolysiscombined two stage varying temperature crystallization withoutany secondary template. The hierarchical microsphere particlesare 300–600 nm in size each of which is constructed by primaryZSM-5 nanocrystals of 30–50 nm. A time trial growth study wasconducted for insight into the growth process of the hierarchicalZSM-5-HM. It was suggested that the growth of ZSM-5-HM prod-ucts involves aggregation and assembly combined crystallizationprocess. And the crystallization of nanocrystals constituting ZSM-5-HM is governed by solution-mediated transport mechanism. Thesynthesis by cohydrolysis combined varying temperature crystal-lization give rise to similar hierarchical ZSM-5 particles when thesecond stage crystallization time is extended for more 24 h. It indi-cates that the two stage varying temperature crystallization ismainly responsible for the formation of the ZSM-5-HM productsbut the separate hydrolysis can reduce the crystallization time. Thiswork also demonstrates a facile and highly effective, and industrialapplicable approach without any secondary template to meso-porous zeolites. It can be extended to the synthesis of other zeoliteswith unique hierarchical structure thus unique properties.

Acknowledgments

We are grateful to the financial support of the National KeyTechnology R&D Program (2006BAE02B05) and National NaturalScience Foundation of China (20606004).

References

[1] J. Cheng, S. Pei, B. Yue, Synthesis and characterization of hollow zeolitemicrospheres with a mesoporous shell by O/W/O emulsion and vapor-phasetransport method, Micropor. Mesopor. Mater. 115 (2008) 383–388.

[2] T.F. Degnan, G.K. Chitnis, P.H. Schipper, History of ZSM-5 fluid catalytic crack-ing additive development at Mobil, Micropor. Mesopor. Mater. 35–36 (2000)245–252.

[3] N. Viswanadham, J.K. Gupta, G. Murali Dhar, M.O. Garg, Effect of synthesismethods and modification treatments of ZSM-5 on light alkane aromatization,Energy Fuels 20 (2006) 1806–1814.

[4] D. Van Vu, M. Miyamoto, N. Nishiyama, Y. Egashira, K. Ueyama, Selective forma-tion of para-xylene over H-ZSM-5 coated with polycrystalline silicalite crystals,J. Catal. 243 (2006) 389–394.

[5] O.E. Kartal, I. Onal, synthesis of ZSM-5 from modified clinoptilolite and its cat-alytic activity in alkyl action of benzene to ethylbenzene, Chem. Eng. Commun.195 (2008) 1043–1057.

[6] J.C. Groen, L.A.A. Peffer, J.A. Moulijn, On the introduction of intracrystallinemesoporosity in zeolites upon desilication in alkaline medium, Micropor.Mesopor. Mater. 69 (2004) 29–34.

[7] M.E. Davis, C. Saldarriaga, C. Montes, C. Garces, C. Crowder, A molecular sievewith eighteen-membered rings, Nature 331 (1988) 698–699.

[8] C.T. Kresge, M.E. Leonowicz, W.J. Roth, Ordered mesoporous molecular sievessynthesized by a liquid-crystal template mechanism, Nature 359 (1992)710–712.

[9] L. Tosheva, V.P. Valtchev, Nanozeolites: synthesis, crystallization mechanism,and applications, Chem. Mater. 17 (2005) 2494–2513.

10] Y.S. Tao, H. Kanoh, L. Abrams, K. Kaneko, Mesopore-modified zeolites: prepa-

11] M. Hartmann, Hierarchical zeolites: a proven strategy to combine shapeselectivity with efficient mass transport, Angew. Chem. Int. Ed. 43 (2004)5880–5882.

12] P.A. Jacobs, E.G. Derouane, J. Weitkamp, Evidence for X-ray-amorphous zeolites,J. Chem. Soc., Chem. Commun. (1981) 591–593.

ering J

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

as a pore-directing agent, Chem. Eur. J. 11 (2005) 4983–4994.

J. Yang et al. / Chemical Engine

13] M.A. Camblor, A. Corma, S. Valencia, Characterization of nanocrystalline zeolitebeta, Micropor. Mesopor. Mater. 25 (1998) 59–74.

14] I. Schmidt, C. Madsen, C.J.H. Jacobsen, Confined space synthesis. A novel routeto nanosized zeolites, Inorg. Chem. 39 (2000) 2279–2283.

15] V. Gia-Thanh, D. Trong-On, A new route for the synthesis of uniform nanoze-olites with hydrophobic external surface in organic solvent medium, J. Am.Chem. Soc. 129 (2007) 3810–3811.

16] D. Ma, Y. Lu, L. Su, et al., Remarkable improvement on the methane aromatiza-tion reaction: a highly selective and coking-resistant catalyst, J. Phys. Chem. B106 (2002) 8524–8530.

17] J.C. Groen, W.D. Zhu, S. Brouwer, S.J. Huynink, F. Kapteijn, J.A. Moulijn, J. Pérez-Ramírez, Direct demonstration of enhanced diffusion in mesoporous ZSM-5zeolite obtained via controlled desilication, J. Am. Chem. Soc. 129 (2007) 355.

18] J.C. Groen, T. Bach, U. Ziese, A.M. Paulaime-van Donk, K.P. deJong, J.A. Moulijn,J. Pérez-Ramírez, Creation of hollow zeolite architectures by controlled desilli-cation of Al-Zoned ZSM-5 crystals, J. Am. Chem. Soc. 127 (2005) 10792–10793.

19] I. Schmidt, A. Boisen, E. Gustavsson, K. Stahl, S. Pehrson, S. Dahl, A. Carlsson,C.J.H. Jacobsen, Carbon nanotube templated growth of mesoporous zeolite sin-gle crystals, Chem. Mater. 13 (2001) 4416–4418.

20] A. Boisen, I. Schmidt, A. Carlsson, S. Dahl, M. Brorson, C.J.H. Jacobsen, TEMstereo-imaging of mesoporous zeolite single crystals, Chem. Commun. (2003)

958–959.

21] Y. Tao, H. Kanoh, K. Kaneko, Uniform mesopore-donated zeolite Y using carbonaerogel templating, J. Phys. Chem. B 107 (2003) 10974–10976.

22] C.H. Christensen, K. Johannsen, I. Schmidt, Catalytic benzene alkylation overmesoporous zeolite single crystals: improving activity and selectivity with anew family of porous materials, J. Am. Chem. Soc. 125 (2003) 13370–13371.

[

[

ournal 166 (2011) 1083–1089 1089

23] F.S. Xiao, L. Wang, C. Yin, K. Lin, Y. Di, J. Li, R. Xu, D. Su, S.R. Schlgl, T. Yokoi, T. Tat-sumi, Catalytic properties of hierarchical mesoporous zeolites templated witha mixture of small organic ammonium salts and mesoscale cationic polymers,Angew. Chem. Int. Ed. 45 (2006) 3090–3093.

24] M. Choi, H.S. Cho, R. Srivastava, C. Venkatesan, D. Choi, R. Ryoo, Amphiphilicorganosilane-directed synthesis of crystalline zeolite with tunable mesoporos-ity, Nat. Mater. 5 (2006) 718–723.

25] Y. Fang, H.Q. Hu, G.H. Chen, In situ assembly of zeolite nanocrystals intomesoporous aggregate with single-crystal-like morphology without secondarytemplate, Chem. Mater. 20 (2008) 1670–1672.

26] N.B. Chu, J.H. Yang, J.Q. Wang, S.X. Yu, J.M. Lu, Y. Zhang, D.H. Yin, A feasible wayto enhance effectively the catalytic performance of methane dehydroaromati-zation, Catal. Commun. 11 (2010) 513–517.

27] Q. Li, D. Creaser, J. Sterte, The nucleation period for TPA-silicalite-1 crystal-lization determined by a two-stage varying-temperature synthesis, Micropor.Mesopor. Mater. 31 (1999) 141–150.

28] Y.S. Tao, H. Kanoh, K. Kaneko, ZSM-5 monolith of uniform mesoporous channels,J. Am. Chem. Soc. 125 (2003) 6044–6045.

29] J.C. Groen, L.A.A. Peffer, J.A. Moulijn, J. Pérez-Ramírez, Mechanism of hierarchi-cal porosity development in MFI zeolites by desilication: the role of aluminium

30] T.M. Davis, T.O. Drews, H. Ramanan, C. He, J.S. Dong, Mechanistic principles ofnanoparticle evolution to zeolite crystals, Nat. Mater. 5 (2006) 400–408.

31] Nathan D. Hould, Raul F. Lobo, Nanoparticle precursors and phase selec-tivity in hydrothermal synthesis of zeolite, Chem. Mater. 20 (2008) 5807–5815.