Microporous and Mesoporous Materials 144 (2011) 57–66

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Microporous and Mesoporous Materials

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Efficient, low-cost, minimal reagent syntheses of high silica zeolites usingextremely dense gels below 100 �C

C.A. Fyfe ⇑, R.J. Darton ⇑, H. Mowatt, Z.S. LinChemistry Department, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1

a r t i c l e i n f o

Article history:Received 21 December 2010Received in revised form 13 January 2011Accepted 14 February 2011Available online 19 February 2011

Keywords:High-silica zeolitesLow-temperature synthesesDense gelsMFI

1387-1811/$ - see front matter � 2011 Elsevier Inc. Adoi:10.1016/j.micromeso.2011.02.020

⇑ Corresponding authors. Present address: SchoolSciences, Lennard–Jones Laboratories, Keele Universi(R.J. Darton).

E-mail addresses: fyfe@chem.ubc.ca (C.A. Fyfe),(R.J. Darton).

a b s t r a c t

By using very dense, minimal water content synthesis gels, it is possible to prepare highly siliceous ZSM-5(MFI) zeolite at temperatures below 100 �C using very simple apparatus and experimental procedures inshort reaction times. The materials are highly crystalline and can be made using both fumed silica andLudox reagents under both high alkali and fluoride synthesis conditions. Aluminium is easily incorpo-rated into the framework in the high-alkali syntheses.

Preliminary experiments on ZSM-11 suggest that the method may be applicable to other high silicazeolite frameworks.

� 2011 Elsevier Inc. All rights reserved.

1. Introduction

High-silica zeolites are usually synthesized by reaction of a sil-ica source such as fumed silica or Ludox with a templating organicquaternary ammonium salt with, in addition, an aluminium sourceif desired, under very basic hydrothermal conditions at elevatedtemperatures (140–210 �C) for periods of 2–5 days [1,2]. Becauseof these conditions, the reactions must be carried out in high pres-sure, stainless steel vessels, which limit the size of the reactionsand require appropriate safety precautions to be made.

Potentially there would be considerable simplification possibleat low temperatures (specifically less than 90 �C) and a number oflow temperature syntheses of MFI zeolite have previously been re-ported [3–12]. Indeed, the development of low-temperature semi-continuous [13,14] and pilot plant scale batch processes have evenbeen described [15–17]. However, the conditions used, which haveinvolved the use of seed crystals, high template to silica ratios, highdilutions or long crystallization times differ from those used in thepresent work in which we demonstrate how these types of hydro-thermal reactions may be carried out efficiently at temperaturesbelow 100 �C, under static conditions, in similar reaction times tothose at higher temperatures, with a stoichiometric or loweramount of template and without the use of seed crystals, therefore

ll rights reserved.

of Physical and Geographicalty, Keele, Staffs ST5 5BG, UK

r.j.darton@chem.keele.ac.uk

avoiding the use of complex and expensive equipment and withsignificant savings in energy consumption.

Typically, hydrothermal zeolite syntheses follow a multi-stepprocess that can be characterized in its simplest form as involvingan initiation period with gel formation and then the relatively fastcrystallization of product with the elimination of water from thehydrophobic structure [18]. Many studies of the proposed ‘‘nucle-ation’’ or ‘‘initiation’’ process have been carried out but the detailsof these complex reactions are still poorly understood. Althoughmany solution silicate species have been identified in the very ini-tial stages after mixing, it is not clear how or whether these are in-volved in the crystallization process [19–22].

In light of this, a proper kinetic analysis of the species involvedwould be very difficult indeed, since only the reactants and prod-ucts have been properly identified and some of these can havemore than one role; for example, water is a product of the crystal-lization from the gel but is also the reaction medium and is neededfor hydration and mobilization of the ionic species present.

However, two general assumptions could be made: the rate ofthe overall reaction should decrease as the temperature is loweredand there is likely to be some kind of power dependence of therates on the concentrations of the reactant species. Of course, thedesired framework structure may not be formed at all tempera-tures, further complicating the situation. Nevertheless, based onthis reasoning, we have investigated whether increasing the reac-tant concentrations could be used to offset the reduction in ratesfrom lowering the temperature enough that the syntheses couldbe carried out efficiently below 100 �C. This is achieved by reduc-ing the water content as much as possible; the H2O to SiO2 ratiosin the case of MFI syntheses being between three and eight in

58 C.A. Fyfe et al. / Microporous and Mesoporous Materials 144 (2011) 57–66

the present study. We have found that this approach is successfulfor different frameworks and mineralization media and presentdata illustrating this for completely siliceous and aluminium con-taining ZSM-5 under both high pH and fluoride synthesis condi-tions, from different silica sources and for both organic templatesalone and mixtures of these with sodium cations and the replace-ment of the template hydroxide with the iodide and bromideforms. In order to demonstrate that the approach is not limitedto ZSM-5 alone, some very preliminary data on zeolite ZSM-11(MEL) are also reported.

2. Experimental

2.1. Materials

All materials used were from commercial sources: Fused silica,Sigma S-2128; Ludox (30%) HS-30 Du Pont; Ludox (40%) HS-40;Ludox (50%) Sigma TM-50; tetrapropylammonium hydroxide,Aldrich; tetrapropylammonium bromide and iodide, Acros Organ-ics; sodium hydroxide, Fisher; N,N-diethyl-3,5-dimethylpiperidini-um hydroxide synthesized from 3,5-dimethylpiperidine (Aldrich)and iodoethane (Aldrich) by the previously described method[23,24].

2.2. Syntheses

Appropriate amounts of SiO2 (fused silica or Ludox) and tetra-propylammonium hydroxide (1 M aqueous solution, Aldrich) orbromide (Across) were added together in a new unused 30 ml or60 ml (Nalgene)™ polypropylene screwcap bottle and sodiumhydroxide, ammonium fluoride, hydrofluoric acid and distilledwater added as required for the different syntheses. The compo-nents were thoroughly mixed by hand until homogeneous, theresulting reaction mixtures ranged from dry powders to homoge-neous liquids in appearance. The bottle was then sealed and heatedat 90 �C in an oven. It was found that the syntheses were morereproducible if as large a fraction of the internal volume was filledwith the reactant mixture and the caps well-sealed with Teflon™tape. After the required time, the reaction was quenched and thesamples subsequently treated by two cycles of dilution with waterand isolation by centrifugation at 3500–4500 rpm for up to onehour before being dried at 90 �C.

2.4. Analyses

Electron microscopy experiments were performed on a HitachiS-2300 system using graphite as conductive coating. Images shownin the text are at a magnification of �10 K. Powder X-ray diffrac-tion patterns were obtained from Bruker D8 Advance and Discoverdiffractometers using a flat disc sample holder and 1.0 mm capil-lary samples respectively. Solid state NMR measurements wereperformed using a Bruker Avance 400 spectrometer operating atfrequencies of 400.13, 376.55, 104.267, 100.622, 79.494, and29.91 MHz for 1H, 19F, 27Al, 13C, 29Si and 14N, respectively, usingBruker probes with either 7 mm or 4 mm MAS rotors. 29Si chemicalshifts were referenced to tetramethylsilane (TMS) with Q8M8 (thecubic octamer Si8O12[OSi(CH3)3]8) as external secondary reference,19F to CFCl3 with the 19F resonance of octadecasil as external sec-ondary reference, 27Al shifts to 1 M aluminium nitrate aqueoussolution as external reference, 13C chemical shifts to TMS with ada-mantane as external secondary reference and 14N chemical shiftsdirectly to solid ammonium chloride. In some cases, further exper-imental details of individual NMR experiments are given in the fig-ure captions.

3. Results and discussion

The main focus of this study was ZSM-5 (MFI framework) [25]which have previously been extensively investigated and fullycharacterized and were considered as representative of the classof highly siliceous zeolites. Preliminary experiments were also car-ried out on the closely related, but much less well-studied ZSM-11(MEL framework) [26].

The MFI syntheses were carried out at 90 �C using a variety ofoxide sources and reagents, all chosen because they are readilyavailable and inexpensive. In order to facilitate comparisons be-tween different reactions, most syntheses used equal stoichiome-tric amounts of silica and template. The gel composition and theindividual reactant concentrations were varied by changing thewater content.

All products were characterized by a combination of 29Si MASand 13C CP MAS NMR and measurements on other nuclei as appro-priate, powder X-ray diffraction and scanning electron microscopy.In order to conserve space and for clarity of presentation, the reac-tions are discussed under separate headings and only representa-tive characterization data are presented as figures. A summary ofthese syntheses is given in Table 1. Additional data are presentedin the Supplementary Data associated with the article.

3.1. High-alkali ZSM-5 syntheses using fumed silica

Thoroughly mixing 1.0 g of fumed silica with 1.4 g of 1.0 MTPAOH by hand for several minutes yields a dry powder. No addi-tional water is added to the reaction mixture whose oxide ratiocomposition is 1.0SiO2:0.085 TPAOH:3.72H2O, almost exactly theideal stoichiometry of the silica and template and the maximumconcentrations of the two that can be obtained using this commer-cially available template solution. The product crystallizes in 2–4 days at 90 �C giving a solid precipitate below a mobile liquid con-sisting mainly of bulk water. Two treatments of shaking with alarge excess of water and then centrifugation, followed by dryingat 90 �C yields a fine white powder as final product (Sample A). Ki-netic analysis of the reaction indicates that the crystallization be-gins between one and two days. At reaction times of three andfour days, the yield of product is ca. 1.15 g (92% based on theMFI structure and assuming complete occupancy of templatesites).

The powder XRD pattern (Fig. 1a) indicates the product is highlysiliceous template-containing ZSM-5, based on literature data [2].This is confirmed by the 13C CPMAS spectrum (Fig. 1c); whosechemical shifts and particularly the splitting of the methyl reso-nance are diagnostic of the TPA template being located at the chan-nel intersection of the MFI framework [22]. SEM (Fig. 1b) showsthe particle morphology is spherical, particle diameters of approx-imately 0.2–0.3 lm.

Almost identical reactions performed at 140 �C in Teflon-linedstainless steel autoclaves yield �1.2 g of highly crystalline MFI inless than 48 h. These reactions were followed as a function of timeby removing small samples from the rapidly quenched autoclavesand analyzing the samples using capillary X-ray diffraction and so-lid state NMR. The results show that crystallization begins afterapproximately 8 h of heating and is followed by a rapid increasein crystallization with over 90% of the product being formed within24 h. Similar methods can be applied to these lower temperaturesyntheses with the results showing that a decrease in reactiontemperature increases the initial induction period from 8 h toapproximately 24 h. This is then followed by the characteristic ra-pid crystallization period with about 90% of the product beingformed within 48 h. This type of crystallization profile is expectedto be same for all the syntheses performed based on the general

Table 1Summary of synthesis conditions for samples referred to in figures.

Sample SiO2(FS) SiO2(L) TPAOH TPABr NaOH NH4F H2O* Time Yield

A 1.0 g 1.4 g(1 M) [1.1 g] 4d 1.17 gB 3.3 g(30%) 1.4 g(1 M) [3.4 g] 4d 1.24 gC 2.5 g (40%) 1.4 g(1 M) [2.1 g] 4d 1.14 gD 2.0 g(50%) 1.4 g(1 M) [2.1 g] 4d 1.15 gE 2.0 g(50%)� 1.4 g(1 M) [2.1 g] 4d 0.75 gF 10.0 g(40%) 5.6 g(1 M) [10.4 g] 5d 4.31 gG 1.0 g 1.4 g(1 M) [1.1 g] 5d 1.12 gH 3.3 g(30%) 0.159 g 0.046 g [2.0 g] 3d 1.23 gI 2.0 g(50%)� 0.159 g 0.069 g [1.0 g] 4d 0.75 gJ 2.4 g 0.86 g 3.0 g 10 g 45–69 h Kinetic runK 1.2 g 0.43 g 0.75 g 2 g 4d 1.14 gL 1.2 g 0.30 g 0.75 g 3 g 3d 1.15 gM 2.4 g 0.86 g 3.29 g 2.9 ml (49% HF) 10 g 7d 2.12 gN 3.0 g (40%) 0.43 g 1.50 g 1.2 g 3d 1.23 gO 2.4 g (50%)� 0.43 g 1.50 g 1.8 g 3d 0.87 gP 1.0 g 3.0 g(1 M) DECDMPOH 1.66 g (1 M) 5d 1.09 g

� Hand-made 50% silica water mixture.* Water solely derived from other reactants shown in square brackets.

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Fig. 1. (a) Powder X-ray diffraction pattern, (b) SEM image and (c) 13C{1H} MAS NMR spectrum (29314 scans, 6 kHz spinning rate, 1 ms contact time and 2 s recycle time) ofSample A.

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appearances of the reaction mixtures, and these could all be stud-ied in a similar manner.

3.2. High-alkali ZSM-5 syntheses using ludox

Ludox™, a commercial colloidal aqueous silica suspension, isanother cheap and easily accessible silica source and is often pre-ferred over fused silica because of ease of handling, its availabilityin various concentrations with differing pH’s, and a full range ofthese reagents were studied in the present work.

Mixing 3.3 g of 30 wt.% Ludox (1.0 g SiO2) with 1.4 g of 1.0 MTPAOH, oxide formulation: 1.0SiO2:0.086 TPAOH:11.69H2O (tem-plate to silica again stoichiometric for the MFI structure, assumingcomplete occupancy of template sites), gives a clear, homogeneoussolution, quite different from the first example using fumed silicaas the Si source. Crystallization takes place in 4 days and isolatingthe final product, as previously, yields a fine white powder (SampleB), identified as highly crystalline MFI from the XRD pattern [2](Fig. 2a), yield 1.15 g. The particles are approximately spherical,diameters 0.7–0.8 lm (Fig. 2b).

An important reason for the differences in behaviour of the tworeaction mixtures is the water concentration. There are 3.4 g ofwater per 1 g of SiO2 in the present case, compared to 1.12 g whenfumed silica is used. The water content can be reduced by using aLudox reagent with a higher silica concentration; 2.5 g of 40 wt.%Ludox provides the same amount of silica but contains only1.48 g of water and when combined with 1.4 g of 1 M TPAOH pro-duces a reaction mixture with the same stoichiometric silica totemplate ratio as before but with the water content now reducedto 2.6 g. The corresponding oxide ratios of the reaction mixtureare 1.0SiO2:0.085 TPAOH:8.8H2O. Mixing the two by hand againgives a clear solution which becomes cloudy after one day but isstill mobile. Further reaction leads to precipitation of solid belowa clear liquid layer. Workup after 3–4 days yields ca. 1.15 g ofhighly crystalline MFI (Sample C), as shown by the powder XRDpattern (Fig. 2c). The particles are still spherical, diameters 0.5–0.8 lm (Fig. 2d).

The water content can now be lowered further by using 2.0 g of50% commercial Ludox with 1.4 g of 1.0 M TPAOH giving clear li-quid reaction mixture containing 2.1 g of water, oxide formula-

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Fig. 2. (a) Powder X-ray diffraction pattern and (b) SEM image of Sample B. (c) Powder X-ray diffraction pattern and (d) SEM image of Sample C. (e) Powder X-ray diffractionpattern and (f) SEM image of Sample D. (g) Powder X-ray diffraction pattern and (h) SEM image of Sample E.

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tion:1.0SiO2:0.085 TPAOH:7.1H2O. Three to four days reactionleads to separation of solid below a clear liquid yielding ca.1.15 g MFI (XRD pattern Fig. 2e), Sample D. The particle morphol-ogy is again spherical, diameters 0.7–0.8 lm, Fig. 2d. Similar re-sults are obtained when a handmade silica source made bythoroughly mixing 50 wt.% fumed silica plus 50 wt.% water to agranular powder is used. Hand-mixing gives a mobile phase whichappears somewhat heterogeneous. The reaction proceeds similarlyto the others, beginning after one day and being complete in threedays and yielding high-quality MFI as sub-micron spherical parti-cles, Sample E, Fig. 2g and h. However, yields appear to be consis-tently lower in this reaction (ca.0.75 g) and all other synthesesusing this silica source. It is probable that this situation could beimproved by better and more thorough mixing of the components.

It is clear from these two series of reactions that the water con-tent is critical in determining the rate of reaction and that evenlower temperatures could be used at lower water contents. In fact,ZSM-5 can be prepared at a temperature of 75 �C, still within5 days, using either fumed silica or 40% Ludox as the silica source

and 1 M TPAOH as the template. The initial reaction mixture hadratios of 1SiO2:0.084 TPAOH and water to silicon ratio of 3.72 or8.73 depending on whether fumed silica or 40% Ludox were used,respectively. Fig. 3a–d show the appropriate powder XRD and SEMdata for samples F, G.

3.3. Substitution of sodium ions for tetrapropylammonium iontemplate

Minimization of the amount of organic template used in thesereactions is a very important consideration in these syntheses be-cause of the cost of the reagent and the disposal of the productsfrom calcination. Although ‘‘template-free’’ syntheses have beenreported using sodium hydroxide, these usually require the addi-tion of ca. 10% of a precursor species of pre prepared MFI nanocrys-tals or the use of relatively high temperatures >180 �C.

The syntheses discussed in the previous sections involved noexcess template, but this can be reduced further by the use of an-other base. Substitution of sodium hydroxide for TPAOH has sev-

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Fig. 3. (a) Powder X-ray diffraction pattern and (b) SEM image of Sample F. (c) Powder X-ray diffraction pattern and (d) SEM image of Sample G.

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eral important consequences in the present context: firstly, anhy-drous NaOH can be used, reducing the water content of the reac-tion mixture and potentially increasing the reaction rate;secondly, less costly TPABr can be substituted for TPAOH andthirdly, the minimum amount of TPA templating cation can bedetermined. If the material is to be used as a catalyst, some quater-nary salt must be present but the minimum possible amount ofTPA cation can be used. A range of TPABr to NaOH ratios was stud-ied to find the minimum quantity of organic template necessary toform the fully crystalline MFI framework. The minimum oxide ra-tios at 90 �C were determined to be 1.0SiO2:0.035 TPABr:0.103NaOH:3.33H2O or 1.0SiO2:0.035 TPABr:0.069 NaOH:7.67H2O usingeither hand-made 50:50 silica to water mixture described above orLudox-30, respectively. The reaction mixture using either silicasource starts out as a granular powder and the reaction proceedsin a similar manner to the previously described syntheses and is

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Fig. 4. (a) Powder X-ray diffraction pattern and (b) SEM image of Sample H

complete within three days. The products of the reaction are whitepowders with yields of approximately 1.23 g and 0.75 g dependingon whether Ludox-30 or the hand-made 50:50 mixture was used.The products were confirmed as ZSM-5 using powder X-ray dif-fraction and the particle morphologies by SEM (Fig. 4), SamplesH, I.

These results show that over 60% of the organic template re-quired for a stoichiometric synthesis can be replaced with sodiumions added in the form of sodium hydroxide under these condi-tions. The yields for the syntheses performed with both Ludoxand the handmade 50:50 silica water mixture are in close agree-ment with those syntheses carried out with the stoichiometricquantities of TPA cations. A comparison of the X-ray powder dif-fraction patterns and the SEM images show that replacing the or-ganic template with sodium ions has had little if any effect onthe overall crystallinity of the sample.

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. (c) Powder X-ray diffraction pattern and (d) SEM image of Sample I.

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3.4. MFI syntheses using fluoride as mineralizing agent

The addition of fluoride in the synthesis of high-silica zeoliteswas first reported by Flannigan and Patton [27]. Guth and cowork-ers further explored its effects [28–31] and, more recently, Camb-lor and Morris and coworkers [32–35] have independentlyextended its use to a wide variety of framework structures. Inthe earliest work, hydroxide was also present, making the reactionmixtures very basic, but fluoride has also been used alone as min-eralizing agent and the syntheses are at much lower pH. In general,the presence of fluoride tends to give products of higher crystallin-ity and favors the formation of larger crystals. In the present work,we have studied systems where fluoride is the sole mineralizingspecies present and the TPA cation is present as a halide salt (bro-mide in most cases).

3.5. NH4F syntheses

Hand-mixing 2.4 g fumed silica, 0.86 g of TPABr, 3.0 g of NH4Fand 10 g of water gives a smooth homogeneous gel-like phase thatbecomes lumpy and heterogeneous with free water produced asthe reaction proceeds. Crystallization takes place over a period of45–69 h (Sample J), while a similar mixture where the NH4F con-tent has been lowered to 1.5 g crystallizes over 69–74 h, indicatingthat the reaction rate is sensitive to the NH4F content, as would beexpected from its role as a mineralizing agent. The product is highquality and has exactly the same characteristics as previously re-ported for materials synthesized by a similar method [2]. Powder

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XRD confirms that highly crystalline MFI has been formed(Fig. 5a) and SEM shows the particles have a well-defined morphol-ogy of inter-grown obloids, Fig. 5b. The crystals are considerablylarger (5–10 lm) than those from the high alkali procedures ashas been previously observed for the corresponding dilute synthe-sis mixtures. As expected, the 29Si CPMAS spectrum (Fig. 5c) has farfewer Q3 SiOH defect signals than the previous samples from thehigh pH syntheses, indicating a very high crystallinity and orderin the as-synthesized material. In fact, the Si Q4 signal shows someslight indication of unresolved fine structure. The 19F MAS NMRspectrum obtained at a high spinning rate of 12 kHz (Fig. 5d)shows a major resonance with associated sidebands, indicatingfluoride covalently incorporated into the zeolite framework inthe product (isotropic chemical shift = �64.9 ppm with respect toCFCl3), forming a 5-coordinate silicon at Si9, as reported for previ-ous FMFI products synthesized at high temperatures. In the ini-tially isolated material, there is also a signal at ca.�127 ppm dueto adsorbed fluoride that is removed by thorough washing andthere is a broad, low-intensity background signal due to the probe(�120 to �140 ppm) that can be completely removed by spectralsubtraction, but not done for the spectra presented here. However,there is a second smaller signal with associated sidebands, isotro-pic resonance at �78 ppm. At a spinning rate of 15 kHz, the 14NMAS spectrum (not shown) of the crude solid shows a signal at27.3 ppm with multiple sidebands due to the TPA cation and asharp resonance at �16.2 ppm with no sidebands. On thoroughwashing, the latter signal is removed, indicating that it is due toweakly adsorbed ammonium ions. Thus, the smaller resonance at

CP MAS NMR spectrum (13412 scans, 6 kHz spinning rate, 7 ms contact time, 5 snning rate, 10 s recycle time, isotropic peak indicated by⁄) of Sample J, (e) 29Si MAScination.

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�78 ppm in the 19F spectrum described above does not appear tobe due to intercalated ammonium fluoride. From the present data,it is unclear whether it is due to covalently bound fluoride or someother species. Further information on this topic is presented below.

After calcination, at 350 and then 500 �C for 16 h in air, the 29SiMAS NMR spectrum (Fig. 5e) shows a much reduced Q3 signal andclear resolution of a significant number of the Q4 resonances thatcharacterize the material as the empty monoclinic MFI framework.Further calcination of the sample at 600 �C produced no furtherchanges. Reactions involving the substitution of TPAI for TPABrhad no effect on the quality or yield of the FMFI product and thereaction time was unchanged.

The effect of the water content is less clear in the NH4F basedsynthesis procedure. Lowering it indirectly increases the concen-tration of the other species in the mixture, but water is also neces-sary to facilitate the dissociation and migration of the highconcentrations of ionic species in the reaction mixture. Thus, a sys-tematic study of the effects of the concentrations of these two spe-cies was carried out, starting with relatively high concentrations ofboth. The reactions progressed in a similar fashion to that de-scribed above except that the initial hand-mixed reaction mixturesbecame more solid-like as the water content was decreased. Se-lected XRD and SEM results are summarized in fig. 6.

A mixture of 1.2 g of fumed silica, 0.43 g of TPABr, 1.5 g NH4Fand 5 g water (oxide ratios 1.02SiO2:0.08 TPABr:2.06 NH4F:14.06H2O) yields a near quantitative yield of very high crystallinityZSM-5 after 2–3 days at 90 �C, in agreement with the crystalliza-tion studies above. Lowering the NH4F content to 0.75 g whilekeeping the TPABr and water contents the same yields 1.33 g ofcrystalline MFI product in 3 days. Lowering the water content to3 g while keeping the silica, TPABr and NH4F contents constant at1.2 g, 0.43 g and 0.75 g, respectively, (oxide ratios 1.0SiO2:0.08TPABr:1.03 NH4F:8.43H2O) yields 1.19 g MFI in 3 days and lower-ing the water content further to 2 g (oxide ratios 1.0SiO2:0.08TPABr:1.03 NH4F:5.64H2O) yields 1.14 g MFI in 4 days, Sample K(Fig. 6a and b). Decreasing the NH4F content to 0.5 g from 0.75 gwhile keeping the water at 2.0 g increases the reaction time to5 days but the yield of MFI is still near-quantitative at 1.15 g. Whenthe water content is now lowered to 1.0 g with the NH4F kept at0.50 g, 0.98 g of MFI is formed in 9 days.

A useful variation in the synthesis from the point of view of re-agent cost would be to minimize the TPABr content on the assump-

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Fig. 6. (a) Powder X-ray diffraction pattern and (b) SEM image of Sample K

tion that some of the template sites could be occupied byammonium ions as they were by sodium ions in the high-alkalisyntheses. Using the reference reaction of 1.2 g fused silica,0.43 g TPABr, 0.75 g NH4F and 3 g of water which crystallizes in3 days as described above, the TPABr content was now systemati-cally lowered to 0.30 g (1.0SiO2:0.056 TPABr:1.03 NH4F:8.43H2O),Sample L and 0.20 g (1.0SiO2:0.037 TPABr:1.03 NH4F:8.43H2O). Inboth of these cases highly crystalline FMFI was obtained in nearquantitative yields (Fig. 6c and d show the XRD and SEM data ofthe former). When the TPABr content was lowered to 0.10 g, crys-talline MFI was again obtained after 7 days but the XRD patternindicated the presence of some amorphous material. Thus, thereis considerable opportunity to minimize the quantities of the reac-tants in these syntheses and an optimum reaction compositioncould be found for any given set of priorities.

3.6. HF syntheses

High quality FMFI can also be obtained at 90 �C with the morecommon synthesis method using HF. To make comparisons be-tween the two methods as direct as possible, a reaction mixturewith similar ratios of Si, F, TPABr and water to those of Sample J de-scribed above was used. Mixing 2.4 g of fumed silica with 0.86 g ofTPABr, 2.9 ml of 49% HF, 3.29 g solid sodium hydroxide and 10 g ofwater (oxide ratios 1.0SiO2:0.08 TPABr:2.06 NaOH:2.04 HF:16.28H2O) gave a smooth white paste that yielded 2.12 g of crystal-line product after 7 days at 90 �C (Sample M). The XRD and SEMdata (not shown) indicate the material to be highly crystalline FMFI,the SEM shows the crystals are of similar size and inter-grown, butwith sharper faces and ‘‘coffin-like’’ shapes. Particularly interestingare the 19F and 29Si spectra shown in Fig. 7. The 19F spectrum(Fig. 7a) now shows mainly the isotropic signal at ca.�65 ppm withassociated spinning sidebands, due to covalently bound fluorineand only a very small signal at ca. �78 ppm due to a second site.This result is confirmed by the 14N MAS spectrum (not shown) thatshows only a single resonance at 24.3 ppm due to the TPA cation.Syntheses at high temperatures using HF or NH4F show no indica-tion of the presence of a second fluorine site, so it would appear thatthis only occurs to a substantial extent with NH4F at low tempera-ture. The situation could, however, be different for other zeolites.The crystallinity of this sample is even higher than those of samplesmade using NH4F; the 29Si CPMAS spectrum of the as-synthesized

b

d

0

. (c) Powder X-ray diffraction pattern and (d) SEM image of Sample L.

Fig. 7. (a) 19F spin-echo MAS NMR spectrum (1084 scans, 12 kHz spinning rate, 10 s recycle time, isotropic peak indicated by⁄), (b) 29Si{1H} CP MAS NMR spectrum (6400scans, 6 kHz spinning rate, 10 s recycle time, 7 ms contact time) of sample M, (c) 29Si MAS NMR spectrum (9872 scans, 6 kHz spinning rate, 10 s recycle time) of Sample Mafter calcination.

64 C.A. Fyfe et al. / Microporous and Mesoporous Materials 144 (2011) 57–66

material (Fig. 7b) shows a very small Q3 resonance and clear chem-ical shift resolution of a number of signals. Simple calcination at 350and then 500 �C gives a silicon MAS spectrum with excellent reso-lution for such a simple procedure (Fig. 7c).

3.7. Use of Ludox™ in fluoride MFI syntheses

In order to test whether Ludox solutions could substitute forfumed silica as silica source in syntheses by the fluoride methodunder these conditions, reaction mixtures were made up using40%, and 50% Ludox as well as the hand-mixed 50% solid formula-tion that had the same oxide ratios as the mixture from fumed sil-ica with 1.2 g SiO2, 0.43 g TPABr, 1.5 g NH4F and 3 g H2O describedabove. Each mixture contained 0.43 g TPABr and 1.5 g NH4F andbulk water was added to each Ludox used so that each reaction

a

Inte

nsity

0

100

200

300

400

500

600

700

2-Theta

5 10 20 30 40 50

c

Inte

nsity

0

100

200

300

2-Theta 5 10 20 30 40 50

Fig. 8. (a) Powder X-ray diffraction pattern and (b) SEM image of Sample N

mixture contained 1.2 g SiO2 and the total water content was3.0 g (3.0 g 40% Ludox plus 1.2 g water; 2.4 g 50% Ludox plus1.8 g water; 2.4 g hand-mixed solid 50% formulation plus 1.8 gwater). While the 40% and 50% formulations gave near quantitativeyields (1.23 g) of highly crystalline fluoride-containing ZSM-5(FMFI), the hand-mixed solid formulation again gave a lower yieldin the same reaction time (0.87 g). Fig. 8a shows the XRD and SEMdata, Samples, N, and O.

3.8. Introduction of aluminium

For the high pH syntheses, Al was easily introduced using alu-minium salts such as aluminium sulphate, sodium aluminate andfrom zeolite A as an aluminium source, as evidenced by the cleantetrahedral resonance at ca. 50 ppm (spectra not shown). For the

b

d

. (c) Powder X-ray diffraction pattern and (d) SEM image of Sample O.

ba

c d

Inte

nsit

y

0

100

200

300

400

2-Theta 5 10 20 30 40 50

Fig. 9. (a) Powder X-ray diffraction pattern and (b) SEM image of Sample P. (c) 13C{1H} CP MAS NMR (40918 scans, 6 kHz spinning rate, 0.5 ms contact time, 2 s recycle time ;(d) 29Si{1H} CP MAS NMR (15804 scans, 6 kHz spinning rate, 7 ms contact time, 5 s recycle time) of Sample P.

C.A. Fyfe et al. / Microporous and Mesoporous Materials 144 (2011) 57–66 65

fluoride syntheses, there was no incorporation of aluminium at90 �C and this topic merits further study.

3.9. Phase-pure, high silica ZSM-11 (MEL)

In order to also confirm that the general approach was not lim-ited to only the case of ZSM-5, some exploratory experiments werecarried out on the closely related but less well studied ZSM-11framework. In these, the template described by Nakagawa was usedto obtain phase-pure ZSM-11 [23,24]. Pure silica phase-pure ZSM-11, MEL structure type, was synthesized using the template N,N-diethyl-3,5-dimethylpiperidinium hydroxide in the form of a1.0 M aqueous solution. In a typical synthesis, 1.0 g of fumed silicawas thoroughly mixed by hand in a 30 ml polypropylene bottlewith 3.0 g of the 1.0 M template solution and 1.66 g of 1.0 M NaOHto give a thick gel (final reactant ratios 1.0SiO2:0.18 DECDMPOH:0.1NaOH:13.46H2O) and the mixture heated at 90 �C for 5 days until ithad separated into a white solid with a clear solution above it. Thereaction yielded 1.09 g of dry product (Sample P) that was con-firmed to be phase-pure ZSM-11 [36–38], by powder XRD as shownin Fig. 9a. The corresponding SEM data are shown in Fig. 9b. How-ever, this reaction is much more sensitive to unknown synthesisvariables and is not nearly as easily reproduced at the MFI synthe-ses described above. Duplicate experiments gave similar yields ofphase-pure MEL of similar quality as this initial synthesis but onlyafter much longer reaction times of between 14 and 28 days. Thisreaction is currently being investigated in more detail.

4. Conclusions

In the present work we have demonstrated that highly crystal-line MFI materials can be synthesized in a highly reproduciblemanner in both highly alkaline and fluoride synthesis media attemperatures below 100 �C in short periods of time and using sim-ple synthesis apparatus if very dense gels with minimal water con-tent (molar water to silica ratios in the range of 3–8 in most cases)are used.

Both fumed silica and Ludox reagents can be used as silicasources in both media and ammonium fluoride is the only fluoridesource in the fluoride synthesis method.

In the high-alkali syntheses, over 60% of the TPA template canbe replaced by sodium ions. Aluminium is smoothly introducedinto the frameworks of these materials in these syntheses by theaddition of a variety of aluminium sources.

In the fluoride syntheses, the structure directing TPA can beused as the iodide as well as the bromide salt with no effect onthe efficiency of the reaction. The fluoride MFI material is identicalto that reported from high-temperature syntheses under differentreaction conditions in which the fluorine is covalently bonded tosilicon in the framework.

Preliminary studies indicate that the MEL framework can beproduced under similar conditions but the reaction is slower andmore sensitive to the reaction conditions. This suggests that thelow-temperature, dense-gel approach to the synthesis of high silicaporous materials may be more generally applicable.

The use of temperatures below 100 �C makes it possible to usevery simple equipment which, coupled with minimal quantities ofreagents, should produce significant reductions in both the finan-cial and environmental costs of the syntheses described here.

Acknowledgements

The authors acknowledge the financial assistance of the NaturalSciences and Engineering Research Council (NSERC) of Canada inthe form of Equipment and Discovery grants to CAF. RJD would liketo thank the Keele University research institute for the Environ-ment, Physical Sciences and Applied Mathematics (EPSAM) forfunding.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.micromeso.2011.02.020.

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