Studies in Surface Science and Catalysis 137 H. van Bekkum, E.M. Flanigen, P.A. Jacobs and J.C. Jansen (Editors) 9 2001 Elsevier Science B.V. All rights reserved. 175 Chapter 5A THE PREPARATION OF OXIDE MOLECULAR SIEVES A. Synthesis of zeolites B. Synthesis of A1PO4 -based molecular sieves J.C. Jansen S.T. Wilson A. Synthesis of zeolites J.C. Jansen Delft University of Technology, Laboratory of Applied Organic Chemistry and Catalysis, Julianalaan 136, 2628 BL Delft, The Netherlands I. INTRODUCTION a. General Nature provided mankind with zeolites (ref. 1). Massive zeolite deposits have been discovered at many places in the world (ref. 2). The occurrence of natural zeolites can be assigned to certain geological environments or hydrological systems (refs. 3,4). Natural zeolites generally form by reaction of mineralizing aqueous solutions with solid aluminosilicates. The main synthesis parameters are: (i) the composition of the host rock and interstitial solutions; pH -0 10 (ii) the time; thousands of years (iii) the temperature; often < 100~ The first systematic studies on zeolite syntheses could thus be guided by the geological and mineralogical findings of the natural species (ref. 5). From 1946 on many additional zeolite types without a natural counterpart have been synthesized (ref. 6). The evolution in the preparation of one of the most studied zeolites is illustrated in Figure 1 by the number of papers and patents on the material denoted as ZSM-5 (ref. 7 and Section XII of this chapter). 176 Number of reports / year 200 100 9 ,, il 1970 1980 i i 1990 2000 Fig. 1. The annual number of papers ( D ) and patents ( 1 ) on the synthesis of zeolite ZSM-5 since the first publication in 1972. Throughout the last six decades molecular sieves were mainly prepared by reactive crystallization or precipitation from an aqueous mixture of reagents at 6 < pH < 14 and temperatures between 100-200 ~ As shown in Scheme 1 a relatively large effort is needed in the optimization of the synthesis mixture, effort .- time Scheme 1. The effort in the optimization of the synthesis mixture (1), the reaction (2) and the isolation (3) versus time. whereafter the hydrothermal reaction process (2) runs autoclaved for a few minutes, days or weeks without intervention. Isolation (3) of the crystalline material is a simple final step in the synthesis procedure. 177 The zeolite synthesis field is not only extended and refined by useful data from modem zeolite characterization and application techniques but also by the interfacing activities with physical, chemical and mathematical science, see Scheme 2. NMR, SAXS, WAXS Modem chemicals and physical methods Zeolite synthesis Nucleation/ crystallization Sol-gel chemistry Computational modelling Combinatorial chemistry Scheme 2. Areas of chemical, physical and mathematical science interfacing the zeolite preparation field. Studies in the sol-gel chemistry and NMR analysis area have contributed substantially to the knowledge of the hydrothermal reaction process. A new approach, combinatorial chemistry is a recently developed technique to screen and optimize with a high throughput a zeolite synthesis procedure (ref. 12). Papers and reviews regarding subjects within the different areas which are mentioned in Scheme 2 and which are of interest for zeolite synthesis are given in Table 1. Besides the annually new zeolite preparations the extensive exploratory efforts of \"zeolite scientists\" in the last four decades has resulted in the synthesis of porous materials like the A1PO4 -group (part B of this chapter), the metal-sulfides (ref. 30) and the clathrasils (ref. 31). Accordingly, zeolite synthesis appears to remain a promising area for future research. The crystallinity of different synthesis products is well illustrated in Plate 1. The morphology and forms of the crystals give a first indication of the type of zeolite present and the purity of the product. 178 Table 1. Examples of subjects from areas of physical, chemical and mathematical science which delivered contributions to the knowledge of the zeolite synthesis process together with references. Area Subject Reference Sol-gel chemistry Hydrolysis and condensation of silicates The sol-gel process Structure of (alumino)silicate-clusters in solution Determination of nano-sized particles Efficient routes to synthesise zeolite materials Lattice energy calculation Local interactions, template-framework 8, 9 10 1 la 1 l b 12 13 14 15 16 17 18 19 20 21 NMR SAXS, WAXS Combinatorial chemistry Computational modelling Modem chemical and Alkoxides as reagents physical methods Fluorides as reagents and mineralizing agents Gravity - reduced - elevated CVD (chemical vapour deposition) Microwave Nucleation/ crystallization theory- practice Mathematical analyses of zeolite crystallization. A review. Are the general laws of crystal growth applicable to zeolite synthesis 22 23 Zeolite Characterization Application ZSM-5/- 11 intergrowth Catalysis - The catalytic site activity - The catalytic properties and the crystal size 24-26 27 179 b. This chapter In this part of the chapter the preparation of two subgroups of the micro-porous tectosilicates (see Chapter 3) i.e. the aluminosilicates and silicates, both including the clathrasils, will be presented. The division between aluminosilicates and silicates is often discussed on Al-poor rather than A1- free level (ref. 30). The aluminosilicates, starting from Si/A1 ratio 1 up to e.g. Si/A1 ratio of 10000, do reveal the presence of A1 in synthesis, in characterization as well as in application, see Fig. 2 (ref. 31). The Al-poor zeolites show no, at least no detectible, Al-dependent behaviour and are therefore, together with the Al-free materials, denoted as silicates. The presence of aluminum, the guest-host interaction and the nucleation and crystallization all contribute to the synthesis events which are chronologically described in Sections II to VII of this chapter. Section VIII is focussed on the reaction parameters. In Sections IX and X the silicates and clathrasils are presented. Examples of research syntheses performed with certain procedures and/or mixture compositions are listed in Section XI. Sections XII and XIII contain literature sources on zeolite preparations and the references, respectively. Rel. catal, activity 100 10 1 0.1 1 10 100 1000 10000 ppm Al Fig. 2. The relative catalytic activity of H-ZSM-5 versus AI content on ppm scale (ref. 31). 180 Plate 1. The crystalline nature of zeolites, a) Single crystals of zeolite A and b) and c) of analcime and of natrolite, respectively, d) A batch of zeolite L, e) typical needle aggregates of zeolite 181 mordenite and f) of Nu-10. II. PREPARATION a. Reactants The chemical sources which are in principle needed for zeolite syntheses are given in Table 2. Table 2. Chemical sources and their function in zeolite synthesis. Sources Func ti on (s) SiO2 A102- OH- Alkali cation, template Water Primary building unit(s) of the framework Origin of framework charge Mineralizer, guest molecule Counterion of A102-, guest molecule Solvent, guest molecule Within each type of source a variety of chemicals (ref. 32), has been used as the differences in physical nature and chemical impurities strongly influence the zeolite synthesis kinetics (ref. 34) and sometimes the properties as catalysts (refs. 24-27). Data on the specifications of regularly used chemical sources are given in the following survey. - Si02 -sources Recent synthesis papers of the Proceedings of the International Zeolite Conferences (refs. 34-37) and of other zeolite conferences (refs. 38-40) reveal that for laboratory scale particular Si-sources are often used, see Table 3a. Depending upon the particular synthesis a certain Si-source might favour a specific crystallization. For instance, the Aerosil 200 product can be readily dissolved compared to the Optipur and Gold label material because of the difference in particle size, see Figure 3. As the rate of dissolution can influence the rate of nucleation and crystallization (ref. 41) the product formation can be affected. At the same time the A1 and other potentially Si replacing impurities are more than 10000 times higher in the Aerosil 200 product compared to the Optipur and Gold label materials. The influence of impurities can change the crystal form (ref. 42) and the chemical properties (refs. 24-26). 182 Table 3. Specifications and the suppliers of recent, regular used sources, and high-pure, Si-, and Al-sources. Si-source (a) Al-source (b) Specifications Phys. Chem. impurities (ppm) Reference manufacturer (a) Silicon compounds Si(OCH3 )4 Tetramethyl orthosilicate (TMeOS) Si(OC2 H5 )4 Tetraethyl orthosilicate (TEOS) Na2SiO3.9H20 liquid Na,Ca < .5 Merck liquid A1,Pt < .2 A1 < 200 Fe < 120 Ti <60 heavy metals < 50 \"N\" Philidelphia Quartz Co. Water glass Colloidal silica Ludox-AS-40 sol A1 < 500 DuPont de Nemours SiO2 40 wt % NH4 + (counterion) Ludox-HS-40 SiO2 40 wt % Na + (counterion) Fumed silica Aerosil-200 CAB-O-S IL M- 5 * Silica Optipur Gold label Zr Fe < 50 Ti B<10 Dp-- 12 nm AI<10 Fe < .6 Ti<10 Degussa BDH Dp- 200 mm A1 < .001 Dp -- 800 mm Fe < 0.01 Merck Aldrich 183 Table 3, continued (b) - NaA102 Na20 54% Sodium aluminate Fe<4 Riedel de Hahn Carlo Erba BDH Ltd. - A1OOH Pseudo-boehmite A1203 70% H20 30% Catapal-B - AI(OH)3 Gibbsite - AI(NO3 )3.9H20 Dp ~ mm Fe < 4 Ti <40 Vista Fe < 3 Merck - A1203 Dp - nm * Aluminiumoxide (Ultrex) Fe < 0.01 Baker Therefore, a careful choice of the reactants is needed. The high grade Si- alkoxides of which even double alkoxides like -Si-O-A1- are available (ref. 43) do not have the above discussed disadvantages, except for the rate of hydrolysis of the alkoxide groups. Fig. 3. SEM photographs of (a) Optipur, (b) Gold label, and (c) Aerosi1200. 184 9 A102- sources Often used Al-sources, collected from the same references as given for the Si-sources, are listed in Table 3b together with the main chemical impurity. Though the very pure A1203 product consists of small particles it is not easily dissolved. 9 Alkali cation/template The inorganic cations in the zeolite synthesis are mainly alkaline or ammonium ions. The organic cations/templates used may be divided in charged and neutral molecules containing functional atoms or groups. The large number of organic molecules used in zeolite synthesis is extensively listed in several publications (refs. 44, 45) together with the specific zeolite product formed. To illustrate in general the variation in organic template molecules some of the more common templates are listed in Table 4. 9 OH- Most zeolite syntheses are performed under basic conditions using OH- as a mineralizing agent. A second agent is F- (refs. 16 and 46) of which the different nature compared to OH-will be discussed in the section on reaction parameters. Both anions are the counterion of the inorganic or organic cations used for the syntheses. Depending upon the quality of the mineralizing agent impurities such as A13+ and Fe 3+ are present at ppm scale. 9 The overall reactant mixture In general the chemical behaviour of impurities like Fe 3+ and Ti 4+ are of minor importance compared to A13+ in high Si/AI zeolites in the heterogeneous catalysis when based on Bronsted activity. However, in the case of an all silica zeolite, or modified zeolites like B-ZSM-5, Fe- ZSM-5 and Ga-ZSM-5 traces of A13+ from reactants as given in Table 3 may play an unexpected dominant role in the Bronsted activity (refs. 24-26). Extensive information on this point is given in Chapter 6 on the modification of zeolites. Another example of the influence of impurities from reactants is K +. The crystallization time can be retarded by factors when K + is present in the synthesis of e.g. zeolite Na-A or Na-ZSM-5 (refs. 47, 48). Impurities like trivalent metal ions sometimes change physical conditions in the reaction mixture indicated by the crystal form or morphology (ref. 42). 185 Table 4. Type of organic templates, functional atoms/groups and references. Organic template amine Functional atom/group Ref. Organic templ ate Functional atom/group Ref -N,,\" 49 -N\"'~ Cn n=4,5 N~ di-amine - N-Cn- N~~. 3~n~9 ammon i um N~N 50 penta- erythri tol amine + alcohol C-OH l HO-C-C-C-OH I C-OH ~N- (Cn-OH)x n=2,3 x=l-3 65 51 52 53 54 55 66 -~+- I ~.+.v-~ *N~-'Cn ammoni um+ alcohol i+ -N -Cn-OH n=2 67 n=4,5 acetal .,.0-. ! 0..,.~0 I 68 sG 1.-~+z.,. ~ Cn.,,,N,,.~ Cn 57 n=4,5 di - ammon i um amine + ether -NLjO 69 -N -C_-N - I il I I + I + 58 N-oxide + ammonium 70 3
150 - Physical and chemical properties - 3D, cages connected pore arrangements via windows 1.28 density (g/cm 3 ) .37 pore volume (cm 3/g) Na + , H20 1 low hydrophylic lattice stabilization Si/A1 Bronsted activity affinity 2D, intersecting channels 1.77 .18 TPA § >12 high hydrophobic 189 b. Zeolite product versus synthesis mixture The most simple zeolite product composition can be given by the overall Si/A1 ratio and the cation type/content. More often the unit cell composition of the zeolite crystal is expressed, e.g. * At higher loadings than 4 A1/uc, TPA + is replaced by the smaller TPA-ZSM-5:4 TPA[AlnSi96-nO192 ]H20 cation Na + (ref. 81). *n<4 The zeolite reaction mixture is often formulated in the molar oxide ratio of the reactants, e.g. SiO2 :A1203 :Na20:(TPAzO):HaO. The ratios of H20/SiO2, OH- /SiO2 , SiO2 /A1203 and (Na20/TPA20) then give an impression of the concentration, solubility and the expected zeolite types, respectively (ref. 82). Correlation between the synthesis mixture and the product can be obtained from ternary composition diagrams (see Fig. 6a,b) (refs. 83-86), or from graphs of crystallization fields of zeolite types as a function of reactant ratios, see Fig. 6c and Section XI.b.3. S ::)2 ~, Composi l ion.$ ~,,~.. .... ~ lVllXrure 600 Na-A: Nal2 [A112Si12048 ].27 H20 / + Product 40(1-- Na20 $i0 Z A1203 200~ /~ Co o i \" ..,, I Mixture \"~,I/ Product IN o -0.4 ! ! 0 ]n m i . , .... 0A. OH/'SIO 2 so'/./ ('rpA. ~o)xO ..... ~ 5o% At201 Fig. 6. Zeolite product versus the synthesis mixture, a) and b), ternary composition diagrams with an inorganic and organic cation/template, respectively, c) Crystallization fields, indicating (O) ZSM-5, ( I1 ) ZSM-35, and (A) ZSM-39 (ref. 87). 190 The product fields at certain P,T, depicted in Fig. 6, are obtained from experimental data which are not always expected from a thermodynamic point of view. As the inevitably heterogeneous synthesis mixture contains micro-domains with different reactant ratios, kinetic parameters might induce other product phases than those derived from the ternary synthesis composition diagram. Because particularly the nucleation is kinetically determined it is thus of interest to understand the different factors, e.g. type of Si-source, cation, Al-source, additives and physical parameters, influencing the kinetic stage of nucleation. The influence of these factors can be recognized in the subsequent events of the zeolite preparation which are given in Table 7 and discussed hereafter in detail. Table 7. The subsequent events present in the course of the zeolite preparation. Temperature Subsequent events Low (< 60 ~ Reactant solutions Reactant mixture - gel formation Gel rearrangement Dissolution of gel Dissociation of silicate Pre-nucleation phase Nucleation Crystallization Isolation Low T high (< 60~ T < 200 ~ High (< 200 ~ Low (< 60 ~ IV. THE LOW TEMPERATURE REACTION MIXTURE a. Introduction The reaction mixture events occurring at low temperature (< 60 ~ reasons. will be discussed for two i) Reaction mixtures are prepared at low temperature. Chemical and physical changes should take place through adequate aging. For example, zeolite A mixtures might still comprise silicate and aluminate phases which are not mixed on a molecular level, after poor aging,. This is observed, in contrast to heating in a hot air oven, when the mixture is fast heated with microwaves to the synthesis temperature. The product distribution then shows pure zeolite, silicate, aluminate and an amorphous aluminosilicate (ref.88). 191 ii) Substantial knowledge about the zeolite reaction mixture at low temperature has been obtained using characterization methods such as the molybdate method (ref. 89a), the paper chromatography method (ref. 89b), the trimethylsilylation method (ref. 90), IR- and laser- Raman spectroscopy (ref. 91), single crystal structure analysis (refs. 92, 93) and the NMR technique (ref. 94). Mostly, starting reaction mixtures typically consist of a gel phase and a liquid phase which means that nucleation is initiated at high temperature on/in the residual gel phase, though there are a few exceptions (refs. 91, 95). The (alumino)silicate gel phase consists of either a homogeneous dispersed phase of branched chains of sol particles, see Fig. 7a, or a more separated solid phase of an ordered aggregate of sol particles (like opals), see Fig.7b. The mechanisms and kinetics of formation have been extensively investigated and reviewed (refs. 96a and b, 97). Fig. 7. Alkaline gel forms. Schematic representation and micrograph picture of I) a dispersed low density gel (ref. 96b) of branched chains of sol particles and II) a separated high density gel form resulting in spheres consisting of an ordered aggregate of sol particles (like opals). 192 si(w) % 100 . si(lv) % 100 80 - - ), /\\ \\ sJ4 o8 (OH)~ - /.~ .L. /--...~,o,(o,),'- ', I ', I ,! Y / \\ 80 6o 40 6o - SltOH)~, / , / ;, /\"\\ ' / \\ \\ 4o - 20 - 9 SIO 2(0~ ~ 9 6 \\ /t .K\" \\ I 10 ,/r , U.-----~.----J /~,o=(o.~;- ~ 20 o ,,!, ~ __ I,~,,,, 12 8 12 pH 6 8 10 pH a) O.lm Si(IV) b) lo'Srn si(Iv) Fig. 8. Silicate distribution versus pH at a) high and b) low silicate concentration (ref. 98). The pH of the liquid phase, in the case of OH- as the mineralizing agent, lies generally between 8-12. As depicted in Fig. 8 the most abundant form(s) of Si-species at relatively high pH are the monomeric ions, whereas at lower pH value monomeric neutral Si-species can be formed, when the Si-concentration is low. At high concentration, however, cyclic tetramers are the most abundant species (ref. 98). b. Hydrolysis and condensation of silicate Monomers and oligomers in solution are in equilibrium with the gel phase. At this ambient stage of the reaction mixture monomeric silica species can be released from the gel via hydrolysis reactions and are present in solution as e.g. Si(OH)30- and Si(OH)202 e- . The dissolution of the gel is promoted by the OH -coordination of silicon above four, thus weakening the other siloxane bonds to the gel network. This nucleophilic mechanism may occur via an Sne-type transition state as shown in Scheme 3a (ref. 9). ~,OH o OH- + Si(OR)4--,T- O ~Si '''OR OH I -- ] ~OR OR \"--;'. RO---~=.. OR ~\" OR I ~.~OR 04 OR =\" J Si(OR):3OH + RO- 193 OR OR OR y il S]O ~,:,~ l6\\ OR o. OR ~ =slo - i / [ \\ ~ OR ~ - .sJo---. bY ~ - OR \" 2.; -oR ~ .si / o \\ Sl \" + RO'. -- b Scheme 3. a) Hydrolysis and b) condensation mechanism of silicate species at room temperature. 0 J O- liquid phase gel phase I I H H I I 0 0 H \ H \\ I 0 0 0JSi i ~o \"\\/ / / 0 0 H0--Si --0--Si --0--Si \" Si~ \" /\\ 0 0 .. Si \\ Si --====~ Relative < acidity branching > .of SiOH Scheme 3c. Growth site in the gel phase for monomers from solution. The mechanism of the condensation reactions in aqueous systems at high pH involves the attack of a nucleophilic deprotonated silanol group on a monomeric neutral species as represented in Scheme 3b (ref. 9). The acidity of the silanol group depends on the number and type of substituents on the silicon-atom. The more silicon substituents are present, the more acidic the OH-groups of the central silicon atom. As shown in Scheme 3c, at high pH the most favourable polymerization is the reaction between large most highly branched species and the monomer silica species. At more neutral pH, hydrolysis and condensation of clusters, containing specific bonding configurations, see Fig. 9, indicate that inversion in the pentacoordinate state of Si, illustrated in Scheme 3, is not essential (ref. 99). 194 -~*--u'7\"~i- .o-I ~1 _,./ _ ? _,./. 9 -~o-~_ l -&~-o--~i_ _~ /o I -,=-I-~ '-o-. o/ H-O--Si~O-T-S i_ ,/ ? ,/ o + .o' /'o' r -si-l-o--st_ J 1 ,/ ?_,./ o o/i ./,- -\" r-\"- ~-7%- ~, o ,./- ?/-6, I n -'\" -- -- --~-,--uq-si- -o/I - Si-- 0--Si - I | o/l - ~o--~i + H20 Fig. 9. Condensation of octamers, A) with retention of the configuration. The pentacoordinate silicon intermediate state is, however, confirmed crystallographically (ref. 100). Condensation of the monomers leads, as the pH of the zeolite synthesis mixture is above the isoelectric point of silica (ref. 101) to ramified clusters. Such clusters can be reorganized into fewer larger particles with a corresponding reduction in surface energy, according to the Ostwald ripening principle. The structural evolution of a growing cluster is schematically given in Fig. 10. -Si-~ I -Si--O--Si- ~ i I -Si--0--Si .I I .---- .. ,/ Si .... 0 .... Si- o,/ I I ' ' ' ! , o I o,,o '- , I -~'-1-~ I -Si---O---IS i - ,/ o/I -~o--~i- o, I 0,/ , ]// -Si-I-O-- ~i--O-- Si--O--Si- ,/ o/i o o I j~-o-~_ I / ! o/I0 l ! t Fig. 10. Structural evolution of silicate clusters. 195 c. Evidence for silicate clusters In the course of the gel dissolution the monomers form dimers, according to 29 Si-NMR studies (ref. 94), via condensation reactions whereafter trimers and tetramers, cyclic trimers and tetramers and higher order tings are observed as condensation products, see Fig. 11. Fig. 11. Numerous oligomers characterized in solution at low temperature by ~'Si-NMR (ref. 101). Evidence for the existence of e.g. double four rings resulted from the single crystal structure analysis of so-called pseudo-A, a material, not a zeolite, crystallized at ambient temperature from a mixture of SiO2, TBAOH and H20, see Fig. 12 (ref. 93). Fig. 12. Model of a part of the framework of pseudo-A; the double four ring units are indicated by asterisks (*). The silicate species identified in the liquid phase by a.o. NMR, SAXS (ref. 102) and IR, (ref. 103) are products in a simple reaction mixture of SiO2, NaOH and H20 at room temperature. 196 The interaction of alkali-ions in such systems is not clear. It is often suggested (ref. 104) that the ordered hydration sphere of a.o. Na + stabilizes silicate species. Recent NMR results indicate that interactions between cations and silicate species (ref. 105) do occur. An organic cation/template added as ingredient(s) to the simple reaction mixture shows in typical experiments according to NMR measurements interaction with the gel and silicate species, respectively (refs. 106-108). However, the highly complicated set of interactions and fast changing equilibria, due to the increased number of type of species after addition of template and/or A1 has not been unravelled yet. V. THE TEMPERATURE RAISE OF THE REACTION MIXTURE Temperature raise, from < 60 ~ up to < 200 ~ can be performed in several ways as shown in Fig. 13 for one type of autoclave and reaction mixture. The different heating rates are achieved in static systems. 200 -*C 150 - cl 100\" 50 \"Art ' claw Ol \" .... i .... + +| .............I. ........... +. .............. i ~ ---i 2 4 6 8 10 12 0 t (rain) Figure 13, different heating rates for one type of autoclave achieved by (a) microwave, (b) hot sand bath and (c) hot air oven. The size of the autoclave, the viscosity of the reaction mixture and the way of agitating e.g. static, tumbling or turbo-stirring are factors modulating the temperature raise of the reaction mixture. During the temperature raise of the reaction mixture from ambient to reaction conditions primary events are: - Accelerated dissolution of the gel into monomeric silicate species. - Dissociation of silicate oligomers in solution and increase of monomers as measured by NMR up to -- 100 ~ (refs. 109-112). As shown in Fig. 14 a model study with NMR on trimethylsilylated silicate confirms (ref. 109) a shift of the silicate anion equilibrium from relatively high-molecular, mainly double four rings, to low-molecular weight, monomers and dimers. 197 (%) mol 80- \"\" -.~.cubic octamer. 60 \\ \\ 40- monomer..,,, i \\ 20- \"~ dim e r... _- -r\" O 9 0 IJa20 40 60 g I0 100 ~ Fig. 14. Main changes in composition (% mol) of trimethylsilylated silicate solution versus temperature. - Higher concentration and mobility of monomeric silicate- and eventually aluminate species. - Association of primary building units. Possible nucleation and crystallization of unwanted (metastable) phases. Some secondary events are: The start of the degradation of quaternary ammonium ions, which can be substantial in a ZSM-5 synthesis (ref. 42) as depicted in Fig. 15. - Start of the drop in pH caused by the Hoffman degradation. 100 % t 50 \\ 10 24 50 100 150 ...... 200 -'~t(hr) Fig. 15. Degradation of tetrapropylammonium versus time. 198 VI. THE HIGH TEMPERATURE REACTION PROCESS a. Introduction The main event occurring in the synthesis mixture at the reaction temperature is the formation of zeolites from amorphous material. Chemical reaction processes accelerated by the high temperature lead to: i) further reorganization of the low temperature synthesis mixture; ii) whereafter primary (homogeneous or heterogeneous) and secondary (seed crystals (ref. 113)) nucleation; iii) and finally, precipitation (based on reactions) as a form of crystallization. b. Nucleation At the high temperature of the reaction mixture the zeolite crystallization is expected after an induction period in which the nucleation occurs. During the induction period the gel and species in solution (aforementioned in the low temperature section) rearrange from a continuous changing phase of monomers and clusters, e.g. polysilicates and aluminosilicates. These clusters form and disappear through inhomogeneities in the synthesis mixture via condensation and hydrolysis processes. The continuous dissolution of the gel phase increases, however, the amount of clusters and the possibility of further association of the clusters and cations. In the course of this process particles become stable. Nuclei of certain dimensions, e.g. 1.0 nm for zeolite Na-A (ref. 114) and 2.0 nm for zeolite ZSM-5 (ref. 115), are formed and crystallization starts. c. Crystallization The lines along which ideas on zeolite crystal formation are developed, either based on bulk and macroscopic observations or on molecular mechanistic scale are described in this paragraph. Four cases of nucleation and crystallization are schematically presented in Table 8. (a) Zeolite crystallizations, which might occur in clear synthesis solutions, or, more often, in heterogeneous reaction mixtures where (b) highly dispersed or (c) dense gel forms are present, see also Fig. 7. In some occasions (d) metastable solid phases undergo transformation during synthesis. Homogeneous nucleation whereafter crystallization has been observed in (a) clear solution experiments (refs. 91, 92). 199 Table 8. Four cases of crystal growth environment and schematic representation of nucleation and crystallization. Crystal growth environment (a) Clear solution (b) Dispersed low density gel (c) Separated high density gel (d) Solid phase m -fix m m ~\"~ Nucleation (e) Crystallization ( ~.-) Fig. 16. a) Powder and b) a twinned elongated prismatic crystal of ZSM-5 from a dispersed gel phase and c) a cubic form of ZSM-5 from a dense gel phase. 200 Nucleation (heterogeneous) occurs at the liquid-gel interface in the dispersed gel-solution mixtures (b) (ref. 108). The forms of the crystallization products in the case of a dispersed gel phase are shown for ZSM-5 in Fig. 16a,b. Similarly to the clear synthesis solutions, the driving force for crystallization is equal in all directions as shown in Table 8a,b. In the case of a dense gel phase present in the synthesis mixture, see Table 8c, crystallization proceeds into the gel (ref. 42) as shown schematically in Fig. 17. Deviating crystal forms compared to crystal forms from dispersed gel systems are then observed, as shown in Fig. 16c. Generally, the typical form and morphology of a zeolite crystal reveals not only information on the type of the zeolite formed but also on the crystal growth history, as shown above. a/c ratios Pyramldal crystals basal plane 2nd a/c plane views on gelsphere surface perpendicular a along c gel g'pher e 1 .4 O Fig. 17. Average a/c ratios of developing crystals and schematic drawing of growth process in the gel spheres. As a liquid phase is continuously present between the dissolving dense gel phase and the growing crystal, the crystallization is, however, still solvent mediated. When a metastable solid phase, e.g. a zeolite, is present in the synthesis mixture, a transformation into a more stable phase is possible, according to the Ostwald rule of successive transformations (ref. 116). 201 The nucleation and crystallization of the new phase, illustrated in Table 8d, occurs in the supersaturated solution generated by the dissolution of the former phase (ref. 117). In the last three cases of Table 8 dynamic equilibria between successive steps of dissolution, ion transportation and precipitation, can be recognized (ref. 118). Especially, the precipitation/crystallization step, i.e. the type of crystal building units and the way of crystal growth on molecular level, has been subject to many studies. d. Crystal building units At least three types of crystal building units have been suggested which are described below. d.1. The primary building unit Arguments that primary building units, i.e. tetrahedral monomeric species, can be involved in the crystallization are: i) The general view from crystal growth theories that crystals are formed via primary building units (ref. 119); ii) The general view in sol/gel chemistry (refs. 8, 10) that the most favoured condensation reaction occurs between a monomeric and polymeric species. In terms of the zeolite crystallization: between a primary building unit and a crystal surface; see Section IVb; iii) At raising temperatures (up till 100 ~ the concentration of monomers increases (ref. 109) at the expense of clusters. Though in situ measurements (till 200 ~ are not actually performed, the above experimental results might indicate that at reaction temperatures mainly monomers are present; iv) Studies on the crystallization of zeolite have shown that the growth of a zeolite occurs by a surface reaction of monomeric anions (ref. 120). d.2. A typical cluster as building unit As shown in Chapter 4 of this book secondary building units (SBU's) are relatively low (F 16-Si- tetrahedra) polymer units. SBU's were introduced several decades ago (ref. 121) and used since to present structural (ref. 6) and further physical features of the zeolites. At the same time SBU's acting as non-chiral independent units can generate a certain zeolite structure. It is, however, though the SBU's show sometimes a superficial resemblance to silicate anions, not likely that SBU's are the building blocks of the growing crystal (ref. 122). On the other hand, the building of the porous and different zeolite frameworks with monomers condensating in the right topology seems less favourable compared to a typical cluster building unit (ref. 123). From this point of view suggestions are raised about a typical or common cluster building unit for all zeolite structures. d.3. The cation templating theory Organic as well as inorganic cations show structure directing, i.e. water-ordering, properties. Typical examples are given in a review of single crystal structure analysis of organic water clathrated cations (ref. 124 a). The water molecules comprising a tetrahedral network in the first layer around the cation might be partly replaced by silicate and aluminate anionic tetrahedra. 202 The clathrated cations might serve this way as crystal building units. An example of such a templating/clathrating role is the formation of sodalite with tetramethylammonium (TMA) cations (ref. 124 b). d.4. Particular building blocks for one type of zeolite Recently such blocks have been identified in the formation of MFI frameworks indicating that building units can be present for a specific type of zeolite. (ref. 125) The high temperature events, discussed above, are summarized in Scheme 4. I I I I metastable phase I ion transportation stable phase gel or crystal monomer i I[---~ small clusters large clusters ,~[ clathrates[ hydrolysis condonsatlon I9, !t crystallizatiohn nucleation I / I 99P'I association ,, preopltatlon I!!!!Scheme 4. Representation of successive steps in the evolution of the reactive crystallization of zeolite Nucleation and crystallization events are generally illustrated by characteristic S-shaped crystallization curves (ref. 126). The yield (wt % of crystalline material), often determined by indirect methods, plotted against time gives an impression of the nucleation and crystallization time and certain reaction temperatures. More accurate information on the crystallization kinetics can be provided when, based on crystal size and size distribution, the linear crystal growth rate and the rate of nucleation can be determined. Of the studies (ref. 127) on zeolite crystallization, one contribution (ref. 128) reporting on a method to collect kinetic data is briefly described here. 203 d(um I 'lO L(um| ,[ -- .......~-o .\"-----'--o ZO 3E0~- ~o. I :50 40 : ,cl1~ 10- 0 5 a / / 40 ~0 p o2/i j ~ ,I , , ~ 80 120 Time ( h ) ~0 , , , t 200 240 b ..o..---~1oo % conversion of - n /o \"t the-mass of crystals 0 0 6 40 .... 80- ~ i ..l. I 1ZO\" 160 200 Time ( h ) Z40 Fig. 19. a) Histogram of the crystal size distribution in the final product, b) diameter of the largest crystals of different unfinished crystallization runs versus time, resulting in the crystal growth rate graph and c) (i) the nucleation rate (number of crystals of each unfinished crystallization run versus time) together with (ii) the yield curve. A number of identical synthesis experiments, but differing in total synthesis time, were performed. The average diameter of the largest crystals which could be collected from the various products was measured. In the case of zeolite Na-X it was found that in a plot of crystal size versus time the linear crystal growth rate (.5 L/ t) was constant, irrespective of the crystal size, even until near exhaustion of the crystal building units, see Fig. 19b. The nucleation time can be determined now for any crystal in the final product of this Na- X crystallization, knowing the growth rate. For instance, a crystal of 16.5 mm nucleated at t -- 90 h. Together with the particle size distribution curve, Fig. 19a, the rate of nucleation was found, see Fig. 19c. The nucleation rate curve and the yield curve calculated from both the growth rate and particle size distribution curve, indicate that as soon as the crystallization starts the chemical nutrients are consumed for crystal growth. 204 The formation of fresh nuclei is from then on largely suppressed. In conclusion, it can be said that zeolite synthesis, resulting in a good crystalline product can deliver accurate information on nucleation and crystallization kinetics. f. Energy of activation Though zeolitic material can be prepared at low temperature (20-60 ~ most nucleation and crystallization processes are performed at temperatures between 60 and 250 ~ The choice of the reaction temperature is governed by the energy of activation required for the zeolite crystallization. Table 9 shows the energy of activation (E a) as a function of the Si/A1 ratio. Table 9. Ea's of different zeolite framework types and Si/AI ratios. Guest molecule Framework Si/A1 E a (kcal/mol) Ref. 129 Na + ; H20 Y 1.5 1.8 2.2 2.5 30 oo 80 11.8 12.3 14.1 15.6 7 11 18 a TPA + TPA + Na +; H20 MFI MFI MFI b c It appears that the Ea's are not related to diffusion of crystal building units in solution (E, (diff.) < 5 kcal mol ) but to condensation reactions between the crystal surface and crystal building unit. As shown in Table 9 the Ea of Na-Y changes as a function of the Si/A1 ratio which indicates that the more silicious the zeolite, the larger the Ea. Generally, this trend is also observed between different zeolites, although the contribution to Ea of cations and templates, as shown for ZSM-5, can be substantial. VII. ISOLATION OF THE ZEOLITE PRODUCT Products of zeolite preparations can be composed of either one pure zeolitic phase, a mixture of zeolitic phases or a mixture of a zeolitic phase and e.g. quartz, cristobalite or gel phase. Mostly the product is isolated by decantation/centrifugation or filtration. 205 If the product consists of crystals with a uniform crystal form which is recognized as characteristic for the expected product, the zeolite can be separated by decanting the mother liquor followed by washing with water. If there is, however, e.g. some gel phase present, this may be either co- precipitated as a separate phase or adsorbed on the crystals. Careful dissolution of the gel phase with e.g. a dilute basic OH- solution at slightly elevated temperature is strongly advisable prior to the isolation of the zeolite. Especially in the case of adsorbed gel on the crystal surface elemental analysis (AAS, ICP or EMPA) is required to control the Si/A1 ratio of the crystals before and after the washing procedure (ref. 130). The final step in the zeolite preparation is the drying or calcination procedure after which the zeolite void volume is free for different modification and/or application. VIII. REACTION PARAMETERS a. Introduction The type of reactants, the way the reactant mixture is made, the pH and the temperature typically affect the crystallization kinetics and product formation. Furthermore the pre-treatment of the reaction mixture, the addition of crystal growth inhibitors, the reaction mixture temperature trajectory and the use of seeds have an influence on the zeolite preparation. Some aspects of the type of the above mentioned factors are discussed in the following paragraphs. Illustrations are mainly given on the zeolite A and ZSM-5 formation. b. The Si-source As mentioned in Section II of this chapter the different types of the Si-sources contain impurities which may affect zeolite crystallization. Another parameter, the specific surface area of these sources, can result in different nucleation and crystallization times as shown for zeolite A in Fig. 20a (ref. 47). The shorter induction and crystallization times lead to more and smaller crystals, see Fig. 20b. Zeoi~e A % t00- -1, -,,/-,,, / 9 ,,,,,,,~ ....,#'. / / / ~ , ,i .i source II III / ICrystals tel. number size 48 .7 30 2.6 15 4.8 time (h) Fig. 20. a) The yield of zeolite A versus time of different silica sources, b) The specific surface areas of the silica sources (I > II > III) result in different amounts and sizes of crystals in pm. 206 b.The type of template Many types of template are regularly used (see e.g. Section II of this chapter). The surprising performance of certain templates on stabilizing the type of zeolite framework formed is illustrated in Table 10. One type of template can be used to crystallize various zeolites whereas the same type of zeolite may be crystallized while using different templates. Table 10. Single and mixture of templates/cations in the preparation of different zeolite types. Single template Zeolite Ref. Mixture of Zeolite template/cation Ref. Sodalite TMA + Gismondine 131 TMA+, Na + 132 A, X, Y L(+ K) Sodalite, P, S and R ZSM-6 and ZSM-47 Omega 135 136 136 137 138 TPA § ZSM-5 Na + 133 134 TEA EDA Ethanolamine Propanolamint Na + ZSM-5 139 Ethanol Glycerol Morpholine Hexanediol TPA § The role of the single template/cation in stabilizing subunits of different zeolite types is not unravelled yet. A common factor, however, appears to be the size of a certain free void diameter in the structures of sodalite and gismondine, 6.8 A and 7.0 A, respectively, and the diameter of. 6.7 A of the template TMA, see Fig. 21. 207 Fig. 21. Models of a) the sodalite and b) the gismondine void and the void filler/template/cation TMA § . Although TPA + and Na + are rather different templates/cations a common factor might be the stabilization of voids (either intersection of channels or channel windows), see Fig. 22. Fig. 22. View along straight channels of wire model of ZSM-5 with either TPA § (*) or hydrated Na § (O) on intersections of channels and channel windows, respectively. (courtesy St. Bromley) Charged templates compensate negative framework charges, due to isomorphous substitution of Si 4+ by A1 3+ . A range of Si/A1 ratios is possible, see Scheme 5. If, however, the number of charged templates required for charge compensation cannot be accommodated for dimensional reasons the zeolite combines charged templates with e.g. Na § . This way, still various Si/A1 ratios for one zeolite type are possible as shown in Scheme 5 for zeolite ZSM-5. 208 Sodalite can be prepared with two different Si/A1 ratios. ZSM-5 TPA + TPA +/Na + Na + Si/A1 23 - <10000 23- 11 11 Na + 1 Sodalite TMA + Si/A1 5 Scheme 5. Different Si/AI ratios for ZSM-5 and sodalite. d. 1 The reactant mixture The way reactant mixtures are made, e.g. the addition sequence of the reactants, the stirring and gel aging can result in method-dependent factors influencing nucleation. As shown in Fig. 23 crystals of zeolite A started growing in a zeolite X synthesis mixture whereafter zeolite X crystals started growing on and over the zeolite A crystal (refs. 140 a and b). Though the thermodynamic variables were correctly chosen to prepare zeolite X, synthesis mixtures of zeolite A and X, given below, do have comparable elements and apparently local kinetic factors initiated the synthesis of zeolite A. Na2SiO3.9H20 NaA102 Triethanolamine H20 Ref.(141) zeolite A: zeolite X: .4 .4 .1 .05 .7 .7 28 (molar) 28 209 d.2 nano particles from clear solutions The synthesis of so-called nano particles of zeolite ( short diffusion path) has gained interest in the last decennium. Though high concentrations and temperature induce high nucleation rates, thus many nuclei, it is also possible to use diluted solutions and relatively low temperatures, typically below 100 ~ in order to synthesize nano-particles ( ref. 142 a,b,c ). d.3 in-situ crystallization of zeolite on supports Another recent development is the nucleation of zeolite from/on supports of metal or ceramics. Crystals, ranging in size from a few tens of nanometers parallel to the support surface up to a few tens of gm perpendicular to the support surface can be grown in a controlled way ( 142 d ). For the important field of zeolites grown to a closed layer onto a porous support so as to give zeolitic membranes the reader is referred to the recent reviews (142 e-i). e. The pH e. 1. Introduction The pH and the solubility of reactants in the synthesis mixture are governed by the presence of OH-or F-. An advantage of F- compared to OH- is the higher solubility of e.g. Fe 3+ and Ti 4+ and the condensation capability for e.g. Ge 4+. A too high concentration of F-, however, prevents the polycondensation mechanisms. A compromise between solubility of certain elements and inhibition of zeolite framework formation leads to F- synthesis mixtures which are less supersaturated than OH- media. Hence, zeolite types are obtained this way as well, (ref. 16 a and b). e.2. OH- Raising pH of synthesis mixtures using OH-, mainly influences the crystallization of a certain zeolite in a positive way within the synthesis field. As depicted in Fig. 24a and b for zeolite A and zeolite ZSM-5, respectively, increasing the pH shows an increase in the crystallization rate. The OH is a strong mineralizing agent for bringing reactants into solution. The higher the pH and thus the concentration of dissolved reactants the more the rate of crystal growth of zeolites is enhanced (refs. 47, 143). 210 Zeolite A ~] 11:)13- HzO/Na20-20 H2OINa20 -\" 30 I # !I l f # H20/Na20-40 Cr tstallization rote I0.0 8.O ~'*\" 50- I' l I I t .,**. 6.0 4.0 2.0 ---: ........... O /j\" 2 4 6 8 IOH] a b. Fig. 24. The influence of alkalinity on a) zeolite A and b) ZSM-5 crystallization. e.3. F After the first publications on synthesis with F (ref. 46) extensive studies have been undertaken to investigate the effect of F- and possibilities in zeolite synthesis (refs. 16, 46). Replacing OH- by F- the pH values of the synthesis mixture lies generally between 3 and 10. A typical synthesis formulation is given in Section XII of this chapter. Zeolites obtained so far by this route are silica-rich materials and include: ZSM-5 Ferrierite Theta-1 ZSM-23 and Beta f. The temperature It has been shown for many zeolites that raising synthesis temperatures within a certain zeolite synthesis field increases the crystal growth rate (refs. 47, 144, 145). As shown in Fig. 25a and b for zeolite A and zeolite ZSM-5, increasing temperature influences the crystal growth rate whereas in the case of zeolite A the crystal size does not change significantly compared to substantial variations in the ZSM-5 product. 211 Crystal size (~rn) ] 9 9 9 200 \"C 9 185 \"C Zr 100 ^ 1%] 30 60oC 50. i!( \" O.,O~ 70~ If 170 \"C 20 9 9 9 r ) i / / ,~ ....... ~ 9 1590 \"C 10 i' 0 Time [hours] b o ~'o....2. '0 ..... 3'0 t(h) Fig. 25. Influence of temperature on the crystallization of a) zeolite A and b) zeolite ZSM-5. IX. ALL SILICA MOLECULAR SIEVES a. Introduction Two preparation routes can be followed to obtain all silica molecular sieves: i) A direct synthesis to crystallize molecular sieves with an SiO2 composition and well known zeolite topologies. ii) A secondary synthesis. After the direct synthesis of a zeolite a de-alumination procedure, e.g. acid oxalic treatment, steaming (ref. 146), ammonium silicon hexafluoride (ref. 147) or silicon tetrachloride (ref. 148) can lead to an all silica molecular sieve. b. Synthesis Though the neutral all silica molecular sieves do formally not need to be stabilized with cations the silica structures usually contains the cations used in the synthesis. For example, tetrapropylammonium for silicalite-1, tetrabutylammonium for silicalite-2, and tetraethylammonium for silica-ZSM-12. Recently, amines, di-amines (ref. 149) and poly-amines (ref. 150) have been used as templates. Table 11 contains a list of all silica molecular sieves with two examples of synthesis recipes and references. 212 Table 11. All silica zeolites, recipes and references. Product Ref. Silica-ZSM-48 Recipe: 14.6 g of triethylenetetramine is dissolved in 18 ml H20 whereafter the solution is stirred into dry 1.2 g SiO2. The smooth dispersion is then autoclaved between 120-180 ~ for 28-105 days, respectively. Silica-ferrierite Recipe: 0.75 g Si(OCH3 )4 is added to a solution of 1.2 g ethylenediamine (EDA) in 10 ml H20. After adding 2 ml 1 M aqueous boric acid the solution is sealed in a silica tube and heated at. 170 ~ for 56 days. Silicalite-1 (Silica-ZSM-5) Silicalite-2 (Silica-ZSM-11) Silica-ZSM-22 c. Remark 150 151 152 153 149 The main property of the silica molecular sieves is the strong hydrophobic character of the pores. The preferential uptake of e.g. traces of organic compounds (ref. 152) from water, which is not accommodated (ref. 154), in silicalite-1 is a good example. X. CLATHRASILS or actually silicates and zeolite molecular sieves? a. Introduction The name \"clathrasil\" has been introduced for a subclass of porous tectosilicates different from zeolites. The windows of the framework, connecting the cages, are too small to let guest species, stabilized during the synthesis, pass. This characteristic of a clathrate together with the all silica composition is considered as specific for the members of the clathrasils (ref. 29). There are, however, exceptions. The recently synthesized decadodecasil-3R (DD-3R) (ref. 155) contains windows of eight-rings of oxygen, indicating that diffusion of small molecules through the porous structure is possible after calcination. This structure can therefore be considered to form an interface between the clathrasils and the silica molecular sieves. A modified type of DD-3R denoted as Sigma-1 (ref. 156) can, however, be seen as a link between clathrasils and zeolites, because some Si-framework sites are isomorphously substituted by A1. Finally, there is a tectosilicate, 213 Sigma-2 (ref. 157), of which the structure contains two different polyhedra, see Fig. 26, with a free diameter of .75 nm. Sigma-2 has been prepared in the silicalite as well as in the zeolite form and can thus be considered as an intermediate between clathrasils, zeolites and silicates. Fig. 26. The nonahedral and eikosahedral cages of Sigma-2. b. Experimental The clathrasils can be synthesized generally from .5 M silica, prepared by hydrolyzing an alkoxysilane, e.g. Si(OCH3 )4, in solutions containing an amine as guest template molecule9 The syntheses are mainly carried out between 160-240 ~ The clathrasils, together with detailed synthesis data and products expected, are given in Table 12. Table 12. Clathrasils with references to synthesis prescriptions and two examples of synthesis recipes. Product Ref. Melanophlogite Dodecasil 3C Dodecasil 1H Silica-sodalite Sigma-1 DD-3R (silica) Recipe: 0.75 g Si(OCH3)4 is added to a solution of 1.2 g ethylenediamine (EDA) in 10 ml H20. After adding 350 mg 1- adamantaneamine the solution is sealed in a silica tube and heated at 170 ~ for 9 70 days. 158 159 160 161 156 155 214 Table 12, continued, Sigma-2 (silica and aluminosilicate) Synthesis example: The molar oxide ratio of the synthesis system is: Na20 3 AN 20 (1-adamantanamine) A1203 (0.6) (Al-wire) SiO2 60 (colloidal silica) H20 2400 The system was crystallized at 180 ~ and continuously stirred for a few days. 157 c. Remark The templating role of some of the guest molecules is illustrated in Fig. 26. Polyhedra of different clathrasils are filled with a guest molecule. As only 4-, 5- or 6-ring faces are present in most of the polyhedra it looks like the crystal building units have formed around the guest molecule as this molecule is too large to pass through one of the tings. The clathrate formation might therefore be obtained and based on single building units in solution and/or at the growing crystal surface (ref. 162). Fig. 27. Orientation of various guest molecules in clathrasils (ref. 163). a) H3CNH2 in [51262 ] and b) adamantylamine in [51268] of melanophlogite and dodecasil 1H, respectively. 215 XI. EXAMPLES OF SYSTEMATIC RESEARCH in the field of molecular sieves preparation to reach various objectives a. Introduction A main thrust of research is: - to synthesize new molecular sieves - further optimization of recipes to gain knowledge on the essential functions of reactants, e.g. structure directing role of cation/template to prepare relative large single crystals for fundamental studies The list can be longer, however, the examples given below illustrate generally the purpose and variety in the research of molecular sieves preparation. b. Research examples Objective 1) Preparation of zeolites 2) Preparation of zeolites 3) Investigation of crystallization fields with pyrrolidine as template 4a) Investigation of template-zeolite interaction 4b) Directing role of template in the crystallization 5) Large single crystals Parameter(s) Non-aqueous solvents F- as mineralizing agent Na20-A1203 -S iO2 -H20 system was varied Systematic variation of template Use of bis-quaternary ammonium compounds Knowledge on nucleation/ crystallization Change of [SiO2 ], template, cation or additives 6) Morphology and form of zeolite products b.1. The use of non-aqueous solvents In contrast to the rich crop of zeolite types synthesized in aqueous systems the results in non-aqueous solvents are poor (refs. 164, 165). Solvents used, of which the choice was a.o. based on boiling point (100-200 ~ and relative permittivity (10-45) (water: 78), are given in Table 13. 216 Table 13. Zeolite products formed. Solvents Na + K + Li + Ca ++ Glycol HS Glycerol HS DMSO HS Sulfolane HS C6 C7 alcohol HS Ethanol HS - - - - - - - - - - - - - - - - - - HS: Hydroxysodalite. Generally mixtures within the following molar oxyde ratio were used: MeO A1203 SiO2 Solvent and MeO/SiO2 1-20 1 1-100 5-350 0.1-10 As shown in the Table zeolite products could only be obtained in the case of Na. The use of other inorganic and organic cations was not successful. As the boiling point is a less critical factor than high relative permittivity (reduces the Coulomb force between ions and polar compounds thus enhancing dissolution) other non- aqueous solvents for zeolite crystallization which might be subject to zeolite synthesis tests are given below. Non-aqueous solvent Er formic acid formamide hydrogen peroxide hydrocyanic acid 57 84 93 95 b.2. The use of U (ref 166) The compositional ratios of the reaction mixtures used were: A1 or B/Si F/Si Templ./Si H20/Si in molar ratio 0-0.5 0.05-6 0.05-6 4-500 217 with the pH of the mixtures between 1.5-10. The reaction mixtures were heated at 60-250 ~ C and autoclaved for a few hours to a few months. After isolation the products were washed with water and dried. A typical example of a \"F \" synthesis of ZSM-5 is given below: Reaction mixture composition: 36 g Ammonium aluminiumsilicate (Si/A1 -- 7; NH4/A1 -- 1) 18.5 g NH4F pH =7 33.2 g TPABr temp. = 172 ~ 180 g H20 time =lldays Product Unit cell composition: 1.8 NH4 + + 4.1 TPA + [A12.9 8i93.1 O192 ] Crystal size 30x 12 ~tm Advantages and differences using F- instead of OH- as concluded so far: - Low pH compared to OH- - Incorporation in the framework of elements sparingly soluble in alkaline medium, e.g. Fe +++ Synthesis without alkaline cations New possibility to directly incorporate cations as NH4 + and divalent cations such as Co ++ as well - Good stability of usual templates such as TAA in this medium - Highly crystalline materials b.3. Pyrrolidine as template (ref 87) The crystallization of zeolites in the system Na20-A1203 -SiO2 -H20 + pyrrolidine as a template was studied. The reaction mixture compositions used are given in Table 14 in molar oxyde ratio. Table 14. Reaction mixture ranges in molar oxide ratios. Na20 A1203 SiO2 H2SO4 H20 0.05-0.5 0.002-0.05 1 0-0.4 20-80 --. 0.7 pyrrolidine 218 Two procedures were used: I. To a stirred aluminium sulfate solution, calculated amounts of sodium silicate, sulfuric acid and pyrrolidine were added dropwise. II. Calculated amounts of aluminium nitrate, colloidal silica and pyrrolidine were added to a stirred sodium hydroxide solution. 3OO St02/A1203 A 9 4, 200 lOO 0 '_ I ! I I 50 l I ! I I H20/$102 100 Fig. 28. Crystallization fields of product SiO2/A1203 versus H20/SiO2 of ZSM-39 ( & ), ZSM-48 ( 9 ) and KZ-1 (A). The follow-up of both procedures was to autoclave the reaction mixture for 7-40 h at 423-435 K with stirring. After isolation the product was washed with water and dried. The results of the experiments are given in Fig. 6 for procedure A whereas the results of the experiments with procedure B are given in Fig. 28. The main conclusion of the study is that pure ZSM-5, ZSM-35, ZSM-39, ZSM-48 and KZ-1 can be crystallized with pyrrolidine in the aforementioned synthesis system. No common factor, based on the use of pyrrolidine, could be recognized in the various zeolite products. b.4.a. The use of bis-quaternary ammonium compounds in molecular sieves synthesis (ref 58) The objective in this study was the systematic variation of template in the synthesis. An example of the synthesis is given below together with the products formed, see Table 15. The general formula of this bis-quaternary template (T) is: T = [(CH3 )3 N(CH2 )x N(CH3 )3 ]2+ 219 Table 15. Synthesis mixtures and product formation with bis-quat as template. Synthesis conditions molar oxide ratio SiO2 60 A1203 1 Na20 10 TBr2 10 H20 3000 Product formation 3 4 5,6 7,8 Zeolite phases ZSM-39 ZSM-12 EU- 1 ZSM-23 Silica phases EU-4 EU-2 3 (without A1203 ) 4,9 The reaction conditions were 180 ~ three days and crash-cooling after the synthesis was terminated. b.4.b. Another example (ref 167) Systematic variation of the chain length of the template (T) given below in the general formula T = H2 N-(CH2)x-N-H2 resulted in the products, given in Table 16. Table 16. Zeolite formations, obtained with c~ ,6o-diamines x Zeolite phase 2-5 5-6 7-10 Ferrierite ZSM-5 ZSM-5 ZSM-11 The full synthesis description is given in ref. 167. 220 b.5. The synthesis of relatively large single crystals of molecular sieves b.5.1. Introduction Pertaining to e.g. the viscosity of the synthesis mixture several systems, clear solution, diluted gel and dense gel phase have been investigated. b.5.2. Crystallization of ZSM-22 from a clear solution (ref. 149) In a typical experiment tetramethoxysilane was hydrolyzed in 3 M aqueous diethylamine (DEA) according to the following reactions: DEA Si(OCH3 )4 + 2 H20 ....... > SiO2 + 4 CH3OH ...... > 2 SiO2 (C2H5)2 NH 180 ~ 100 days Single crystals of silica-ZSM-22 of 45 x 100 x 225 lam were isolated and used for structure determination. b.5.3. Synthesis of elongated prismatic ZSM-5 crystals The objective of this study was to obtain large single crystals of ZSM-5. Systems using Na + -TPA + , Li + -TPA + and NH4 + -TPA + were investigated applying a reaction mixture given in molar oxide ratio for e.g. NH4 + -TPA + : TPA20 4 (NH4)20 123 A1203 1 SiO2 59 H20 2280 T = 453 K t= 7 days Products Alkaline-free, homogeneous elongated prismatic single crystals of ZSM-5 of 350 lam in length at maximum (ref. 168). b.5.4. Synthesis of cubic shaped single crystals of ZSM-5 (ref 169) The synthesis of this type of crystals developed recently (ref. 169) was subject of a study on the crystal growth history (ref. 42) of this type of crystals. The objective was: to pinpoint the driving forces which change the ZSM-5 crystal form from elongated prismatic into cubic. The crystal growth history study revealed that the cubic crystal growth occurred in a dense gel phase. Perfect single crystals up to 500 lam of zeolite (ZSM-5) and all silica (silicalite-1) molecular sieve type, see Fig. 16c, could be obtained using the following molar oxyde ratio: 221 ZSM-5 SiO2 12 A1203 1 Na20 44 TPA20 44 H20 2000 Silicalite- 1 12 44 44 2000 After 5 days at 180 ~ crystals could be isolated and selected from the product. b.5.5. Synthesis of single crystals of zeolite A and X (ref 142) Single crystals of zeolite A and X up to 100-500 mm in size could be obtained using the following procedures. Procedure for zeolite A: Solution I: 100 g Na2SiO3.9H20 in 350 ml H20 + 50 ml TEA Solution II: 80 g NaA102 in 350 ml H20 + 50 ml TEA Both solutions are filtered with milipore filters, whereafter solution II is added to solution I with stirring. The crystallization is performed at 75-85 ~ for 2-3 weeks, without stirring. Procedure for zeolite X: Identical to the procedure for zeolite A, only 40 g of NaA102 is used in solution II now. The crystallization time is 3-5 weeks. Remark Careful filtering of the starting solutions substantially reduces the amount of heterogeneous nuclei such as dust and foreign particles in the starting chemicals. The lower the number of nuclei, the larger the crystals. b.6. Morphology and form of mordenite and ZSM-5 The morphology and/or form of zeolite crystals appear generally to be influenced by: - [SiO2 ] - Guest molecule type - Cation (ref. 171) - Crystal growth inhibitors A frequently observed crystal form of mordenite is the needle form (with pore channel system parallel to needle direction), see Fig. 29a. 222 Fig. 29. Different forms of mordenite. The needle form a), the intermediate forms b) and c) and the disk form d). The pore direction is indicated by a bar (ref. 85). As shown in Fig. 29b, c and d, completely different crystal forms of mordenite can be prepared. According to the synthesis system used (ref. 85) the main influence in the shape of the crystals seems to be the [SIO2 ]. The higher the [SIO2 ], i.e. the more the crystallization occurs in a dense gel, the more the elongated form is reduced and changed into a disk form. The increase in pore entries and decrease in pore length going from needle to disk form is evident and may be of interest in catalysis (ref. 170). The elongated prismatic form is the most frequently found crystal form of ZSM-5. Changing the [SIO2 ] can change the crystal form as shown in Fig. 30a and b for relatively low and high [SIO2 ] concentrations, respectively. Fig. 30. The elongated prismatic crystal form (a) and the cubic crystal form (b) of zeolite ZSM-5. Changing the template type, i.e. replacing TPA for the divalent bi-quaternary ammonium ion, hexapropyl-1,6-hexanediammonium, resulted in different crystal forms for low as high [SiO2 ] as well, see Fig. 31a and b. 223 Fig. 32. Additional crystal face {001} compared to regular elongated prismatic form of zeolite ZSM-5. 224 XII. LITERATURE SOURCES PERTAINING ZEOLITE PREPARATION ASPECTS Though most of the literature sources are given in Section XIII, a more extended list of sources is given below for reasons of clarity and ease. - Atlas of Verified Zeolite Synthesis Microporous and Mesoporous Materials, vol.22 no 4-6 (1998), updated in 2001. - Chemical Abstracts A literature search in the Chemical Abstracts (CA) can be successful when Controlled Vocabulary Index Terms (CVIT's) are used. As CVIT's after 1976 are not only assigned to words in the title and the abstract, but also throughout the text of the paper (open literature or patent) the search will be thoroughly. The choice of CVIT's must be correct. In the case the word \"synthesis\" is used instead of \"preparation\" the main part of the search \"hits\" will pertain to reactions with the help of zeolites whereas the preparation of zeolites is then difficult to extract. - Proceedings of lnternational Zeolite Conferences (IZC) 1. \"Molecular Sieves\", Soc. Chem. Ind., London, 1968; Proceedings of the 1st IZC, London, U.K., 1967. 2. \"Molecular Sieves I and II\", Adv. Chem. Ser., 101 and 102, ACS, Washington, D.C., 1971; Proceedings of the 2nd IZC, Worcester, Mass., U.S.A., 1970. 3. \"Molecular Sieves\", Adv. Chem. Ser., 121, ACS, Washington, D.C., 1973; W.M. Meier and J.B. Uytterhoeven, Eds., Proceedings of the 3rd IZC, Ztirich, Switzerland, 1973. 4. \"Molecular Sieves-II\", ACS Symp. Ser., 40, ACS, Washington, D.C., 1977; J.R. Katzer, Ed., Proceedings of thre 4th IZC, Chicago, Ill., U.S.A., 1977. 5. \"Proceedings of the 5th International Conference on Zeolites\", Heyden, London, Philadelphia, Rheine, 1980; L.V.C. Rees, Ed., Proceedings of the 5th IZC, Naples, Italy, 1980. 6. \"Proceedings of the 6th International Conference on Zeolites\", Butterworths, Guildford, 1984; D. Olson and A. Bisio, Eds., Proceedings of the 6th IZC, Reno, Nev., U.S.A., 1983. 7. New Developments in Zeolites Science and Technology\", Kodansha, Tokyo, Elsevier, Amsterdam, Oxford, New York, Tokyo, 1986, Stud. Surf. Sci. Catal., 28; Y. Murakami, A. Iijima and J.W. Ward, Eds., Proceedings of the 7th IZC, Tokyo, Japan, 1986. 8. \"Zeolites: Facts, Figures, Future\", Elsevier, Amsterdam, Oxford, New York, Tokyo, 1989, Stud. Surf. Sci. Catal., 49; P.A. Jacobs and R.A. van Santen, Eds., Proceedings of the 8th IZC, Amsterdam, Netherlands, 1989. 9. Proceedings from the 9 th IZC, Butterworth-Heinemann, Boston, London, 1992, R. von Ballmoos, J.B.Higgins and M.M.J.Treacy, Montreal, Canada,1992 10 \"Zeolites and Related Microporous Materials\", State of the Art 1994, Elsevier, Amsterdam, Oxford, New York, Tokyo, 1994, Stud.Surf.Sci.Catal. 84; J.Weitkamp, H.G.Karge, H.Pfeifer and W.Holderich, Eds. Proceedings of the 10 th IZC, Garm isch Partenkirchen, Germany, 1994. 225 11 \"Progress in Zeolite and Microporous Materials\", Elsevier, Amsterdam, Oxford, New York, Tokyo, 1996, Hakze Chon, Son-ki Ihm and Young Sun Uh, Eds. Proceedings of the 11 th IZC, Seoul, Korea, 1996. 12 \"Proceedings of the 12t h IZC\", MRS, Warrendale, Pennsylvania, M.M.J.Treacy, B.K.Marcus, M.E.Bisher and J.B.Higgins, Eds. Baltimore, Maryland, U.S.A., 1998 - Synthesis part on International Conferences \"Zeolites, Synthesis, Structure, Technology and Application\", Elsevier, Amsterdam, Oxford, New York, Tokyo, 1985, Stud. Surf. Sci. Catal., 24; B. Drzaj, S. Hocevar and S. Pejovnik, Eds. \"Innovation in Zeolite Materials Science\", Elsevier, Amsterdam, Oxford, New York, Tokyo, 1988, Stud. Surf. Sci. Catal., 37; P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff, Eds. \"Zeolite Synthesis\", ACS Symp. Ser., 398, ACS, Washington, D.C., 1989; M.L. Occelli and H.E. Robson, Eds. - Journals Zeolites, L.V.C. Rees and R. yon Ballmoos, Eds., Publishers, Butterworth, Heinemann, Stoneham, MA, U.S.A., until 1993. Microporous Materials, J.Weitkamp, Ed., P.A.Jacobs Consulting Ed., Elsevier, Amsterdam- London-New York-Tokyo., until 1997. Microporous and Mesoporous Materials, J.Weitkamp Ed., S.L.Suib, R.W.Thompson, and K.Kuroda, Regional Eds., Elsevier, Amsterdam-London-New York-Tokyo., as from 1998. - Books \"Zeolite Molecular Sieves\", Structure, Chemistry and Use, John Wiley & Sons, New York, London, Sydney, Toronto, 1974; D.W. Breck. \"Hydrothermal Chemistry of Zeolites\", Academic Press, London, New York, 1982; R.M. BaiTer FRS. \"Synthesis of High-Silica Aluminosilicate Zeolites\", Elsevier, Amsterdam, Oxford, New York, Tokyo, 1987, Stud. Surf. Sci. Catal., 33; P.A. Jacobs and J.A. Martens, Eds. \"Molecular Sieves, Principles of Synthesis and Identification\", Van Norstrand Reinhold, New York, 1989; R. Szostak. \"An Introduction to Zeolite Molecular Sieves\", John Wiley and Sons, Chichester, 1988, A. Dyer. \"Molecular Sieves\", Vol. I, 'Synthesis',, Springer, Berlin, Heidelberg, New york, Tokyo, 1998, J.Weitkamp and H.G.Karge, Eds. Chapter I of J.L.Guth and H. Kessler in \"Catalysis and Zeolites\", Fundamentals and Applications, J.Weitkamp and L.Puppe(Eds.), Springer, Berlin (1999) ACKNOWLEDGMENT. I like to thank Dr. H. W. Kouwenhoven for reading the manuscript. 226 XIII. REFERENCES 1 G. Gottardi and E. Galli, Minerals and Rocks, Natural Zeolites, Springer-Verlag, Berlin, 1985. 2 L.B. Sand and F.A. Mumpton, Natural Zeolites, Occurrence, Properties and Use, Pergamon Press, Oxford, 1978. 3 R.L. Hay, Geologic Occurrence of Zeolites, in: L.B. Sand and F.A. Mumpton (Eds.), Natural Zeolites, Pergamon, Oxford, 1978, pp. 135-143. 4 a) A. Iijima, Geology of Natural Zeolites and Zeolitic Rocks, in: L. Rees (Ed.), Proc. 5th Int. Conf. on Zeolites, Naples, Italy, June 2-6, 1980, Heyden, London, 1980, pp. 103-118., b) R.W. Tschernich, Zeolites of the World, Geoscience Press, Inc. Phoenix, Arizona,USA, 1992. 5 R.M. Barrer, Synthesis of Molecular Sieve Zeolites, in: Molecular Sieves, London, England, Soc. Chem. Ind., London, 1968, pp. 39-46. 6 W.M. Meier and D.H. Olson, Atlas of Zeolite Structure Types, 2nd edn., Butterworths, London, 1987. 7 C.A.G. Konings, Delft University of Technology, Central Library. The Library search was performed using Controlled Vocabulary Index Terms of the Chemical Abstracts. 8 C.J. Brinker, D.E. Clark and D.R. Ulrich (Eds.), Symp. Proc. Mat. Res. Soc., Vol. 32, Better Ceramics through Chemistry, Albuquerque, U.S.A., February, 1984, Elsevier, New York, 1984. 9 C.J. Brinker, J. Non-Crystalline Solids, 100, 1988, pp. 31-50. 10 C.J. Brinker, D.E. Clark and D.R. Ulrich (Eds.), Symp. Proc. Mat. Res. 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Patarin, A. Seive, J.M. Chezeau and R. Wey, Zeolite Synthesis in the Presence of Fluoride Ions, in: M.L. Occelli and H.E. Robson (Eds.), Zeolite Synthesis, ACS Symp. Ser., 398, ACS, Washington, DC, 1989, pp. 176-195. b) P.A.Barrett, E.T.Boix, M.A.Camblor, A.Corma, M.J. Diaz-Cabanas, S.Valencia and L.A. Villaescusa, Proc. 12 th IZC, M.M.J.Treacy,B.K.Marcus,M.E.Bisher and J.B.higgins, (Eds.) (1998), Baltimore U.S.A., MRS, Warrendale, Pennsylvania. 227 17 L.B. Sand, A. Sacco, Jr., R.W. Thomson and A.G. Dixon, Zeolites, 7, 1989, pp. 387-392. 18 D.T. Hayhurst, P.J. Melling, Wha Jung Kim and W. Bibbey, Effect of Gravity on Silicalite Crystallization, in: M.L. Occelli and H.E. Robson (Eds.), Zeolite Synthesis, ACS Symp. Ser., 398, ACS, Washington, DC, 1989, pp. 233-243. 19 M. Niwa and Y. Murakami, J. Phys. Chem. Solids, 50, 1989, 487-496. 20 Chu Pochen, F.G. Dwyer and V.J. Clarke, Crystallization Method Employing Microwave Radiation, E.P. 0358827. 21 R.W. Thomson and A. Dyer, Zeolites, 5, 1985, pp. 202-210. 22 F. Di Renzo, F. Fajula, F. Figueras, S. Nicolas and T. des Courieres, Are the General Laws of Crystal Growth Applicable to Zeolite Synthesis?, in: P.A. Jacobs and R.A. van Santen (Eds.), Stud. Surf. Sci. Catal., 49A, Elsevier, Amsterdam, 1989, pp. 119-132. 23 L.Y. Hou and L.B. Sand, Determinations of Boundary Conditions of Crystallization of ZSM-5/ZSM-11 in one System, in: D. Olson and A. Bisio (Eds.), Proc. 6th Int. Conf. on Zeolites, Reno, U.S.A., July 10-15, 1983, Butterworths, London, 1984, pp. 887-893. 24E.G. Derouane, L. Baltusis, R.M. Dessau and K.D. Schmitt, Quantitation and Modification of Catalytic Sites in ZSM-5, in: B. Imelik (Ed.), Catalysis by Acids and Bases, Elsevier, Amsterdam, 1985, pp. 135-146. 25C.T-W. Chu, G.H. Kuehl, R.M. Lago and C.D. Chang, J. of Catal., 93, 1985, pp. 451-458. 26M.F.M. Post, T. Huizinga, C.A. Emeis, J.M. Nanne and W.H.J. Stork, An Infrared and Catalytic Study of Isomorphous Substitution in Pentasil Zeolites, in: H.G. Karge and J. Weitkamp (Eds.), Stud. Surf. Sci. Catal., 46, Proc. of an Int. S ymp., Sept. 4-8, 1988, W