Crystallisation of zeolitic molecular sieves: direct measurements of the growth behaviour of single crystals as a function of synthesis conditions

摘要

Faraday Discuss., 1993,95,235-252 Crystallisation of Zeolitic Molecular Sieves: Direct Measurements of the Growth Behaviour of Single Crystals as a Function of Synthesis Conditions Colin S. Cundy* Research and Technology Department, ICI Chemicals Polymers Ltd., The Heath, Runcorn, Cheshire, UK WA7 4QD the late Barrie M, Lowe? and Douglas M. Sinclair Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh, UK EH9 3JJ A technique is described for the milligram-scale synthesis of zeolitic molecular sieves in sealed capillary tubes. The method is ideally suited to the measurement of the linear growth rates of individual crystals. Spectroscopic characterisation of individual crystals within the sealed capillaries has been demonstrated.Detailed results for growth rates as a function of synthesis temperature and composition are presented for the isostructural zeolite ZSM-5 and silica molecular sieve silicalite (MFI framework). Length growth rates follow the Arrhenius law over a wide temperature range and have an apparent activation energy of 80 f3 kJ mol-I. Width growth rates show more complex behaviour and yield apparent activation energies in the range 62-81 kJ mol-I, depending upon the temperature range chosen and the synthesis conditions. The crystal aspect ratio is found to increase with increasing temperature and also when the crystals are growing under chemical constraint. The maximum value observed for the length growth rate (0.5 dl/dt = (1.9-2.0) x lo-*pm h-If at 368 K agrees well with that found in earlier studies, and it is suggested that this represents a rate limit dependent upon the integration of growth species into the crystal surface.Addition of aluminium compounds, tetramethylam- monium salts or ethanol reduces the growth rate, and a number of possible mechanisms are suggested. Some observations have also been made on zeolites EU-1 (EUO), ZSM-39 (MTN) and ZSM-48. 1. Introduction In an earlier paper,' the method of Zhdanov and Samulevich* was used to analyse patterns of crystallisation behaviour for ~ilicalite,~ a silica molecular sieve. Size analysis of crystal samples from batch reactions was used to deduce crystal growth rates and nucleation behaviour. In the present work, a method has been developed to enable the response of individual crystals to changes in synthesis conditions to be monitored by direct observation.One of the advantages of the reaction compositions used previously' was that they allowed the zeolite product to be crystallised directly from a clear sol without interference from solid amorphous material. Further experimentation demonstrated that conditions could be found which enabled crystals to be grown large enough (50-250 pm) to be observed easily by optical microscopy. Reaction mixtures were sealed into glass capillaries, and the shape and size of the resulting crystals were observed in situ as they grew, without any invasive procedure. A preliminary account of this technique has already a~peared.~ t We report with regret that Dr.Lowe, a man of great scientific insight, enthusiasm and kindness, passed away shortly after the completion of this paper. 235 236 Direct Measurement of Zeolite Crystal Growth 2. Experimental 2.1. Materials The following materials were used: tetraethyl silicate, aluminium hydroxide, tetramethyl- ammonium chloride, tetramethylammonium iodide (BDH, GPR grade); sodium hydrox- ide, bromide, chloride and iodide, and lithium hydroxide (Fisons analytical grade); tetramethylammonium hydroxide, tetrapropylammonium hydroxide, caesium hydroxide and bromide (Fluka pract. grade), tetramethylammonium bromide, tetrapropylammo- nium bromide and hexamethylenebis (trimethylammonium bromide) (Fluka, purum grade), fumed silica (BDH Cab-o-sil M5), ethanol (BDH AnalaR grade).Distilled water was employed throughout. 2.2. Preparation of Reaction Mixtures Mixtures were prepared on an approximately 50 g scale in 60 ml polyethylene bottles. The alkali hydroxide was dissolved in the required amount of water, followed by addition of tetraethyl silicate to the solution. The resulting mixtures were then stirred magnetically in sealed bottles until all the tetraethyl silicate had hydrolysed to give a clear sol. Addition of tetrapropylammonium bromide (TPABr) or any other salt or component was carried out after the hydrolysis had been completed. For aluminous compositions, the tetraethyl silicate was hydrolysed in two-thirds of the required amount of water containing half the necessary alkali hydroxide.The remainder of the hydroxide and water were used to dissolve the aluminium hydroxide, and any salt was added to the resulting solution. The aluminate solution was then added, with very rapid stirring, to the silicate solution. 2.3. Crystallisation Samples for direct observation were drawn into glass capillary tubes, usually ‘Microslides’ (obtainable from Camlab Ltd., Cambridge, UK). These are flat, rectangular, open-ended microcapillary tubes with precision optical pathlengths. They are made from heat-resistant glass and can easily be filled by capillary action or by using an adapted microsyringe. The optimum optical pathlength for the present work was found to be 0.1 mm. The filling of the capillary tubes was carried out immediately after solution preparation to avoid problems arising from the viscosity increase which occurs in some cases on standing.The tubes were then sealed by melting the ends. Heating was accomplished by placing the tubes in a drilled metal block situated in a thermostatted oven (temperature f 1 “C). Crystal growth was followed using a Vickers M41 Photoplan optical microscope fitted with a Pentax ME Super camera. The glass tubes were briefly removed from the block, photographed and returned to the block at regular intervals. By means of identification marks drawn on the outside of the tubes, growth rates could usually be obtained by the measurement of the same crystal at different stages in its growth. In other cases, results were obtained by averaging the dimensions of the largest crystals.Although not a subject of detailed study, comparisons made between different crystals in a given reaction mixture always showed them to grow at comparable rates. Significant dispersion of growth rates5 was thus not observed in these systems for single crystals, although there may be anomalies in the apparent growth rates of oriented aggregates (see Section 3.8). To obtain enough product for normal characterisa- tion purposes (verification of phase purity), larger quantities of reaction mixture were heated in unstirred 30 ml PTFE-lined stainless-steel autoclaves. 3. Results and Discussion The majority of the work was concerned with crystals having the MF16 structure, i.e.that of the ZSM-57 family of zeolites. Silicalite3 is the notionally aluminium-free end-member of this series. Some results for other zeolites will also be described (Section 3.8). C. S. Cundy, B. M. Lowe and D.M. Sinclair 3.1. Temperaure Dependence of Growth Rate The temperature dependence of the rate of crystal growth for silicalite was determined using the reaction composition 1Na,O :60Si02:3TPABr :1500H20:240EtOH Reactions were carried out at 368 K, 393 K, 413 K, 433 K and 448 K. Linear growth rates for both the crystal length and width were measured at each temperature. These are given as 0.5 dl/dt for the length parameter (I), and similarly for the width (w).An idealised crystal of the type under study is shown in Fig. 1. The observed length-growth curves are shown in Fig.2. The width-growth curves corresponded very closely, with the crystal aspect ratio (I/w) being constant for each temperature. It was not possible to obtain depth (d)growth rates in view of the small size of this dimension, and also the fact that most crystals grew with this face (100) parallel to the observation direction. (However, an attempt, which must be regarded as approximate, is made to quantify I/din Section 3.4.) Growth rates were estimated from the linear portion of each curve and are given in Table 1. The corresponding Arrhenius plots are shown in Fig. 3. The apparent activation energies for length and width growth E,(I) and E,(w) are 79 f1 and 62 f2 kJ mol-*, respectively. The size of these energies shows clearly that the 7-W 1-II Fig.1 Idealised silicalite crystal P 60 J'_*.______1_1 160 260 300 400 500 600 tlh Fig. 2 Crystal length growth for silicalite crystals from a reaction mixture of composition 1Na,O: 60Si0,: 3TPABr: 1500H,O :240EtOH. The crosses indicate a second reaction at 393 K carried out in order to check the reproducibility of the technique. T/K: .,368; 0,393; A, 413; 0,433; 0,448. Direct Measurement of Zeolite Crystal Growth Table 1 Growth rates for silicalite as a function of temperature TK length-growth rate, R,/pm h-' width-growth rate R,/pm h-I 368 0.01 1 0.005 393 0.061 0.027 413 0.228 0.074 433 0.55 0.126 448 1.17 0.195 Synthesis composition: 1Na20: 60Si02: 3TPABr: 1500H20: 240EtOH.crystallisation process is controlled by a chemical step, probably surface integration of growth species into the framework. Diffusional control would have given a value related to the solution viscosity, and of ca. 16 kJ mol-1.8 The measured growth rates are comparable with those obtained from similar direct measurements by Feoktistova et al.,9although the present apparent activation energies are ca. 20 higher in value. The discrepancy could be due to differences in reaction composition and conditions, and certainly the visible response of crystal aspect ratio to temperature is more pronounced under the regime described in this current report (Fig. 4). However, the two studies were carried out over significantly different temperature ranges: 95-175 "C in this work and 150-200 "C in ref. 9.The activation energies so derived are I nI I I I I 1-I2.2 2.3 2.4 2.5 2.6 2.7 2.8 lo3K/ T Fig. 3 Arrhenius plot for silicalite crystallisation from a reaction mixture of composition 1Na20: 60Si02: 3TPABr: 1500H,O: 240EtOH. The equations of the lines are: 0,R,(length-growth rate) = 2.0 x lo9 exp (-7.9 x 104/RT);0,R, (width-growth rate) = 3.4 x lo6exp (-6.15 x 104/RT). C. 7. Cundy, B. M. Lowe and D. M. Sinclair 6.0-5.0-4.0-3 1-3.0-2.0. 1.o 3 I 380 I 400 I 420 I 440 TIK Fig. 4 Effect of temperature upon silicalite crystal aspect ratio (l/w).Reaction mixture composition lNa,O: 6OSi0, :3TPABr: 1500H,O: 240EtOH. compound quantities having many factors rolled up within them, since the crystal-growth conditions at different temperatures are not truly comparable: supersaturation, distribu- tion of solution species etc.are not independent of temperature. Nevertheless, such measurements probably provide the best values currently obtainable for zeolite-type syntheses. They are certainly more informative than experiments relying on the temperature-dependence of the slopes of crystallisation curves obtained by measurement of bulk quantities, for example XRD crystallinity.lOJ1 Even so, the response of the growth rates observed here to temperature changes raises further questions, and these are discussed in Section 4. 3.2. Effect of Added Aluminate For this study the base level in the synthesis composition was increased to ensure that the reaction mixtures remained clear and free from macroscopic gel: (3 + x)Na20:60Si02:yA120,: 1.STPABr :4500H20:240EtOH Reactions were carried out both at constant base (x = 0) and at constant ‘free’ base (x = y). The molar proportion of aluminium (y)was varied from 0 to 1, corresponding to a change in silica to alumina ratio from (nominally) infinity to 60. A series of reactions were carried out at temperatures between 363 and 433 K in a similar manner to that described in Section 3.1. The results are set out in Table 2. No significant differences were found between growth rates for the pairs of compositions (x = 0 or x = y).Addition of aluminium sources reduces the length-growth rate, but the activation energy remains approximately unchanged at an average value of 80 f 3 kJ mol-l.However, for width growth, there is no clear trend in growth rate as y is varied, although the activation energy tends to increase with increasing h,P 0 b ?Table 2 Effect of aluminate level on ZSM-5crystal length-growth rate (R,) and width-growth rate (R,) at a series of temperatures ch2 R,/pm h-I R,/p,m h-' chEd 1) Ea(w) 8 x y 363 K 368 K 391 K 393 K 413K 433 K /kJmol-' 363K 368K 391 K 393 K 413 K 433K /kJmol-l s 0.00 1.0 0.011 0.063 0.25 79 0.008 0.047 0.2 1 81 s 1.00 1.00 0.012 0.073 0.26 83 0.010 0.059 0.22 78 0.00 0.75 0.018 0.079 0.29 77 0.014 0.061 0.18 70 0.75 0.75 0.017 0.081 0.29 0.84 79 0.012 0.053 0.16 0.48 74 20.00 0.50 0.014 0.087 0.32 79 0.011 0.056 0.17 70 0.50 0.50 0.014 0.093 0.34 81 0.010 0.061 0.19 73 3 0.00 0.25 0.019 0.100 0.35 80 0.015 0.063 0.18 70 0 0.25 0.25 0.020 0.093 0.33 0.99 79 0.014 0.059 0.15 0.43 5680.00 0.00 0.015 0.096 0.37 81 0.010 0.058 0.15 68 P Synthesis composition: (3 + x)Na,O:60Si02:yA120,:1.5TPABr:4500H20: 240EtOH.C. S. Cundy, B. M. Lowe and D. M. Sinclair 241 aluminium. The overall effect on crystal shape is to reduce the aspect ratio (Z/w)as the crystals become more aluminous. Apart from its effect on crystal growth, the addition of aluminate also has an effect upon nucleation. Data for numbers of crystals observed in a fixed Microslide volume are given in Table 3 for aluminous reactions at 41 3 K.Crystal numbers remain approximately constant for values of y between 0.0 and 0.50. However, for yBO.50, reactions enter the compositional range in which ZSM-5 can be synthesized in the absence of an organic template. l2 The increased nucleation rate in these experiments probably reflects the greater facility for assembling the structure within this optimum region. A recent paper13 discusses nucleation and growth of zeolite Na, TPA-ZSM-5 at 443 K for a composition 3Na20:yAl,0,: 60Si02: 5.3(TPA),O: 750H,O but with y = 0.075-0.592 (in the nomenclature of the present paper). There is in general good agreement with the results given above in that (i) overall crystal size is found to decrease (i.e. total nucleation increases) with increasing aluminium content, (ii) linear growth rate also decreases as y is increased, and (iii) the numerical values of growth rates appear to be in broad agreement, although a figure of 2.25 pm h-l for y = 0.106 in ref.13 run (B) with x = 0.177 in the terminology of that paper does seem anomalously high. 3.3. Salt Addition: Effect of Sodium and Tetramethylammonium Ions There are several ways in which the addition of soluble salts may influence the course of the synthesis reaction, e.g. (i) by changing the water activity, (ii) by altering the distribution of silicate and other species, or (iii) by offering foreign ions to compete in growth integration processes at the crystal surface. As with the study on aluminate addition, some modification of reaction composition was necessary to maintain clear homogeneous reaction sols.In the present case, this mainly involved dilution. Compositions were as follows: 3Na,0 :60Si02:1TPABr :6000H20:240EtOH :zsalt where z = 1,4 or 16 and salt = NaCl, NaBr, NaI or the corresponding tetramethylammo- nium (TMA) halides. pH values are listed in Table 4. Addition of salts always lowers the initial pH, the effect being greater for the sodium system. The effects upon growth rate were examined by running reactions at 41 3 K. The influence of the sodium salts was found to be minimal. The initial (linear) part of the growth curves was identical in all cases, giving growth rates of R, = 0.27 pm h-l and R, = 0.10 pm h-*. At longer times (40-100 h), growth died away at slightly different rates, leading to some scatter in final crystal sizes (ca.35-50 pm in length by ca. 9-14 pm in width). There was no clear pattern to the effects observed. Table 3 Number of crystals present in a fixed volume of capillary as a function of added aluminate (reaction temperature 41 3 K) Y no. of crystals mean length (std. deviation), IlW 1.oo 945 0.75 628 0.50 394 0.25 449 0.00 48 1 Capillary volume = 0.9 mm x 0.9 mm x 0.1 mm (0.081 mm3). Synthesis composition: 3Na20: 60Si02:yA1203: 1.5TPABr:4500H20: 240EtOH. Direct Measurement of Zeolite Crystal Growth Table 4 Initial pH for different amounts of added salts pH as function of halide anion cation z Cl Br I Na 1 11.08 11.08 11.09 4 10.92 10.92 10.90 16 10.57 10.56 10.47 TMA 1 11.02 11.06 11.06 4 10.92 10.92 10.92 16 10.75 10.75 10.77 ~ ~~ ~ Synthesis composition: 3Na20: 6OSiO2: 1TPABr: 6000H,O: 240EtOH: zsalt.For the tetramethylammonium (TMA) system at a given concentration, the growth rates were again essentially independent of the anion. However, in this case, there was a major dependence on salt concentration, as shown in Fig. 5.(In this diagram, the points for TMA = 0 were taken from the study on sodium salt addition described above.) It is known that the TMA cation can change the relative concentrations of silicate anions in solution: the concentration of the cubic octamer Si,0208- is particularly enhanced, l4 although the effect is most marked for homocationic solutions.However, it is considered most probable that the predominant mode of action of the added TMA is one of competition, with the TMA cation interfering at the crystal surface with the incorporation of either Na+ or TPA+ cations, or both. (However, see also Section 3.6.) Once again, the shape of the crystals was affected, the aspect ratio altering from 2.7at z = 0 to 4.2at z = 16. Rossin and Davis have studied a mixed TP-TMA system under related condition^,^^ and show from TG and I3C-MASNMR data that both types of quaternary cation become included in the product crystals. 3.4. Addition of Tetrapropylammonium Bromide The tetrapropylammonium cation plays a very important part in the formation of MFI structures.It is trapped inside the growing crystals, filling the available void space almost completely. It is one of the best examples of a ‘template’ molecule in zeolite synthesis. However, it is not indispensable: other templates can be used, and it is also possible to form the structure in the absence of organic materials, although in this case the compositional (Si:Al) range is greatly restricted.I2 As TPA is a participant in the crystallisation, the supersaturation of the mother liquor will be affected by its concentration. Also, since the TPA becomes part of the crystal, it is effectively removed from solution as the product crystallises. The amount of TPA present would therefore be expected to influence the number and size of the crystals obtained.The reaction composition chosen for this part of the study was 1.5Na,0:60Si0,:qTPABr:2250H20:240EtOH The ideal unit-cell composition of silicalite is (TPAOH),(SiO,),, with the TPA cations situated at the channel intersections. This means that only when q 22.5is there enough TPA present to fill the structure and utilise all the available silica. The length-growth rates and associated data obtained for each value of q are given in Table 5. The results from Section 3.3 confirm that the differences observed must be attributed to the cation and not the bromide anion. In Fig. 6 are plotted the growth rates and relative mass yields (calculated from the data in Table 5 assuming that the reactions with q = 3 and q = 15 have gone to completion) as a function of TPA molar proportion.For values of q below ca. 0.9, there is a strong dependence of growth rate and aspect ratio upon TPA level. There are two possible I 1 I 1 I 4 8 12 16 moles of TMA (z) Fig. 5 Effect of tetramethylammonium (TMA) ion concentration on silicalite crystal growth rate at 413 K for a reaction mixture of composition 3Na20:60Si02:1TPABr :6000H,O :240EtOH :zTMAX (X = C1, Br, I). 0, R,;0,R, reasons for this. First, the near-linear dependence of growth rate upon TPA concentration at low conversions may be due to pseudo-first-order kinetics limited by TPA availability. At ca. q = 0.9, the surface integration step of crystal growth becomes dominant, and the limiting value of the growth rate no longer contains a TPA-dependent term.However, a second possibility stems from the observation (see below, Section 3.5) that the crystals grow more slowly in the presence of large quantities of ethanol. If ethanol at high concentration is able to compete with TPA for adsorption sites at the crystal surface, then it may not be coincidental that the decrease in rate observed above begins at a:qvalues (molar ratios of ethanol to TPA) between 160 and 800 and becomes significant for ethanol addition at a:q between 400 and 560, i.e. that the threshold value (a:q>ca. 500) is the same in both cases. There is a trend towards increasing numbers of crystals as q increases, confirming the influence of TPA level upon nucleation. Within the limits of the estimation, the mass of crystals formed is linearly proportional to the TPA concentration, suggesting that the product crystallises at near-constant composition (with close to 4 TPA per unit cell) but limited in yield by the overall quantity of TPA available. This is the pattern of behaviour observed previously.l6 In some of the studies described in earlier sections, the synthesis compositions are TPA- deficient (q R,(Na). For a caesium-based synthesis, the composition 10Cs20:60Si02: 6TPABr: 8250H20 gave a value for linear growth rate of 0.12 pm h-' at 423 K. By comparison with the results in previous sections and with other a value of at least 0.5 pm h-l would have been expected for a sodium system under comparable conditions. It is reasonable to assume that integration of the larger caesium cation into the crystal lattice is responsible for the lower growth rate in this case. However, if this is correct, then it may be valid to advance a similar argument for the reduction in growth rate brought about by the effectively spherical and even larger TMA cation (Section 3.3).Although the molecular size of the base could also play a role, it is almost certainly the lower pH17J9 which is responsible for the low values of growth rates found in the alkali-free amine-based synthesesI9 summarised in Table 7. For compositions (where PIPZ = piperazine) bPIPZ :60Si02:1.5TPABr:nH20:240EtOH crystal growth was measured at 433 K and 473 K and found to be proceeding at only a fraction (0.1-0.5) of the rate found in comparable sodium-based reactions. However, the absence of small, mobile cations may also be significant. Acceleration in the rate of PIPZ- based syntheses on addition of sodium salts has been observed previously in a study of ZSM-39 crystallisation.20 Table 6 Growth rates for silicalite as a function of the ethanol content of the reaction mixture U 393 K 413 K 433 K 240 0.067 0.23 0.62 480 0.065 0.22 - 720 1200 0.063 - 0.21 - 0.56 0.53 1680 0.047 0.14 0.36 Synthesis composition: 1 .SNa,O:60Si0,: 3TPABr: 3000H,O:uEtOH.Direct Measurement of Zeolite Crystal Growth Table 7 Growth rates at 433 and 473 K for PIPZ, TPA-silicalite (PIPZ = piperazine)' 3.O 6750 0.07 0.23 3.O 9000 0.10 0.19 1.5 9000 0.08 0.21 Synthesis composition: bPIPZ: 6OSiO2:I STPABr: nH,O: 240EtOH. Results obtained by S. J. Bainbridge, Edinburgh University. 3.7. Effect of Concentration Parameters Growth rates at 413 K for Na, TPA-silicalite from the studies described above are summarised in Table 8, together with some additional values.Results for a related dilution experiment are set out in Table 9, and in this case a lithium-based preparation is also reported. Using a reaction mixture of composition 1.5Li20:60Si02:3TPABr :nH,O :240EtOH syntheses were run at 423 K for n = 9000, 12000 and 15 000. With n = 9000, a further reaction was run with the base level raised to 3.0 Li,O. Taken overall, the data set indicates that growth rate is not a sensitive function of the concentration parameters varied within the present range, although there is a tendency for growth rate to increase with concentration and with increasing base.This is as expected for a system which is highly buffered in terms of true solution species and not dependent on nominal silica concentrations. 3.8 Other Zeolite Systems The experiments described above illustrate the effect of reaction conditions upon the nucleation and growth of zeolite crystals having the MFI structure (silicalitelZSM-5). Similar but less detailed work was carried out on zeolite EU-1 (EUO) prepared using the Table 8 Summary of growth rates at 413 K for Na, TPA-silicalite (including data from Sections 3.1-3.5) results section X 4 n RJpm h- 11 W 3.1 1.o 3.0 1500 0.23 3.1 3.2 3.0 1.5 4500 0.37 2.5 3.3 3.0 1.o 6000 0.27 2.7 3.4 1.5 2 1.5 2250 0.28 2.6 3.5 1.5 3.O 3000 0.23 2.6 U 1.5 3.O 2250 0.23 - 0 a 1.5 1.5 2.3 1.5 2250 2250 0.27 0.24 -- ll 2.1 3.0 2250 0.42 - Synthesis composition: xNa20: 6OSiO2 :qTPABr :nH20:240EtOH.Results obtained by S. J. Bainbridge, Edinburgh University. C. S. Cundy, B. M. Lowe and D.M. Sinclair Table 9 Growth rates for M,TPA-silicalite as a function of dilution (a) M = Naa, temperature = 433 K Rll length average n pm h-1 crystal, l/pm ~~ 2250 0.42 110 4500 0.28 100 6750 0.24 50 9000 0.22 40 Synthesis composition: 1.5Na20: 6OSi0,: 1 STPABr:nH,O: 240EtOH. Results obtained by S. J. Bainbridge, Edinburgh University (b)M = Li, temperature = 423 K ~ R,/ length largest X ~~ 4 n pm h-I crystal, l/pm 1.5 3.0 9 000 0.49 65 1.5 3.0 12 000 0.42 65 1.5 3.0 15 000 0.42 50 3.0 3.O 9 000 0.53 95 1.5 1.5 4 500 0.43 105 Synthesis composition: xLi,O: 60Si02 :qTPABr:nHzO:240EtOH. hexamethylenebis(trimethy1ammonium)template21 (as bromide, HexBr,), and on zeolites ZSM-39 (MTN)22 and ZSM-4823 synthesized in the tetramethylammonium system.Measurement of growth rates for EU-1 and ZSM-48 is complicated by their tendency to grow as oriented aggregates, as shown for EU-1 in Fig. 7. Using the composition (for EU-1) 3Na,0 :6OSi0, :0.9A1,03:3HexBr, :2250H20 with fumed silica as silica source at 473 K, values for apparent linear growth rates of ‘dumb- bell’-shaped crystal aggregates of 0.32 and 0.69 pm h- * were obtained from two separate experiments. Although not true single-crystal values, they are probably of the right order, and confirm the practical observation that EU-1 appears to crystallise more slowly than ZSM-5.Crystals of ZSM-48 are shown in Fig. 8(a) and (b),whilst (c) illustrates the co- crystallisation of ZSM-39 and ZSM-48. 3.9. Zn situ Measurements The primary advantage of the capillary-tube method as illustrated by the present work is the ability to study the crystallisation of very small quantities of reactants by direct observation under the optical microscope. At the end of the reaction, it is possible to break open the tubes and remove the contents for further characterisation. The scanning electron micrographs shown in Fig. 7 and 8(b)were acquired in this way. However, the progress of the reaction and the characterisation of the product can be obtained by other means without recourse to invasive procedures.For example, the Raman spectrum shown in Fig. 9 was determined in situ from a single crystal of mass ca. 7 x 10-lo g located inside an unopened Microslide. An Anaspec-33 FT Raman spectrometer (argon laser) fitted with a Nikon Raman microscope of beam resolution 1.0 pm was aligned on the crystal and the total spectrum recorded. The spectrometer was then aligned with a blank section of the tube and the spectrum of the tube and supernatant solution subtracted to give the resultant shown. As observed el~ewhere,,~ the MFI framework absorbs only weakly in the Raman, so Direct Measurement of Zeolite Crystal Growth Fig.7 Scanning electron micrograph of zeolite EU-1 that the spectrum observed derives mainly from the tetrapropylammonium template located in the void space of the crystals. Attempts to record in situ XRD diffractograms were unsuccessful owing to excessive attenuation by the glass of the microslide. The use of a Lindemann capillary would have overcome this difficulty, but could not be contemplated with the strongly alkaline reaction mixtures employed in this work. Even with the microslides, considerable attack upon the borosilicate glass was occasionally observed, as shown in Fig. 10. 4. Conclusions In an earlier study,' it was demonstrated that a single reaction composition crystallised at a fixed temperature could give rise to a variety of reaction profiles and product crystal size distributions.The differences were brought about by changes in preliminary treatment (order of addition of reagents, ageing) and reaction conditions (reactor type, stirring regime). However, although these variations served to illustrate some of the factors that can influence the kinetics of nucleation, the crystal growth rate was found to be the same (within experimental error) in all cases. Thus, relatively small changes occurring early in the synthesis can alter the competitive kinetics of processes leading to n~cleation,~~ whereas the crystallisation process itself, once fully underway, is largely dependent on synthesis composition and temperature only. It is this second aspect of zeolite crystallisation which has formed the subject of the present paper.However, it should be borne in mind that all the MFI results refer to a synthesis system which is unlikely to produce more than a single zeolite species. As demonstrated in Section 3.8 for ZSM-39 and ZSM-48, many zeolite syntheses have the potential for producing several microporous crystalline phases (often metastable), and all the factors noted above will contribute to deciding the actual product isolated under specific conditions. By far the greatest influence upon zeolite crystal growth rate is temperature. Very large changes occur, and the activation energies show that the crystallisation process is controlled by a chemical step, the surface integration reaction being the most likely.Introduction of C. S.Cundy, B. M. Lowe and D. M. Sinclair Fig. 8 (a)Optical micrograph and (b)scanning electron micrograph of zeolite ZSM-48; (c) optical micrograph of co-crystallisation of ZSM-48 needle aggregates and a large single ZSM-39 crystal aluminate to the MFI system does not cause any fundamental change, but does have a definite effect. Length-growth rates are reduced, and nucleation is enhanced. Crystals therefore become smaller and squarer. There is also an increased tendency towards multiple twinning, so that the crystal surfaces become increasingly irregular. Crystal-growth kinetics are surprisingly tolerant of salt additions and changes in concentration, although the underlying trend is for growth rate to increase with both increasing overall concentration and with increasing hydroxide ion concentration.The main effect of dilution is to cause earlier termination of reaction, since more silica is left in solution at the end. However, quarternary salts have a much more positive effect. Reduction in TPA level leads ultimately to reaction limitation by this reagent, both in terms of linear growth rate and also mass of material crystallised. Foreign organic cations (e.g. TMA+) compete with the TPA and reduce the growth rate. Large quantities of neutral organic molecules such as ethanol also cause a reduction in growth rate, but whether the mechanism is one of competition with Direct Measurement of Zeolite Crystal Growth I I I I I I I 1 200 400 600 800 1000 1200 1400 1600 wavenumber/cm-' Fig.9 Raman spectrum of Na, TPA-silicalite recorded from a single 12 pm crystal (mass x 730 pg)residing in its mother liquor inside a sealed capillary tube (background spectrum subtracted) TPA or more simply a result of the changing relative permittivity of the reaction medium is not clear. Growth rates for comparable Na+ and Li+ cation systems appear to lie in the order R,(Li) >R,(Na), with the much larger Cs+cation bringing about a further reduction. The rates therefore follow the trend of ionic size. This would seem to be due to the ease of incorporation into the crystal since considerations of desolvation energy would lead to the reverse of the observed order. The maximum value observed for length growth rate at 368 K (1.9-2.0) x pm h-I, Table 21 is the same as the value found in an earlier study,' and seems to represent the approximate maximum normally seen for Na, TPA-silicalite at this temperature.It is therefore proposed that such limiting values observed under optimum synthesis conditions are probably the ultimate rate, limited by the surface integration step of crystal growth itself. Once the chemistry and hydrodynamics are optimised, the only major factor to affect this process will be temperature. However, at any given temperature, it will be possible to experience constraints beyond the surface-integration limit by moving away from the optimum in other parts of the system. This could be a physical process (e.g. diffusional limitation in an unstirred reaction mixture of high viscosity) or chemical (e.g.low dissolution rate of silica source, base level insufficient to maintain an adequate flux of growth species, or exhaustion of a necessary template molecule). Examples of such chemical limitation are responsible for most of the lower growth-rate values recorded earlier. A further interesting feature is the observed variation in crystal aspect ratio. It is clear that there is an energetic difference between growth on the major faces of the crystal such that (in the Na, TPA-ZSM-5 system) length growth through advance of (101) almost always exceeds growth in other directions, the more so as temperature is increased (Fig. 4 and Table 5). In the Arrhenius plots (Fig. 3), it can be seen that the length-growth points form a good straight line.However, the width-growth plot is definitely curved. Further work (to be reported elsewhere) shows that R1 approximates quite well to Arrhenius behaviour down to ambient temperature, whereas the crystals grown at 295 K have the same l/w ratio as those grown at 363 K. The temperature behaviour of the width-growth rate cannot therefore be represented by a simple Arrhenius plot, but must be a compound process such that R1/R,attains a constant value below a temperature of ca. 420 K, i.e. the activation energy for width growth must become the same as that for length growth at lower temperatures. Simple Donnay-Harker26 calculations using the space group Pnma indicate that the equilibrium morphology for the structure should in any case be more complex than that shown in Fig. 1.C. S. Cundy, B. M. Lowe and D. M. Sinclair 25 1 Fig. 10 Etch marks on a Microslide capillary tube observed after running a reaction mixture of composition 3Na,0: 6OSiO2: 0.9A1203: 1 .5TPABr:4500H20:240EtOH at 448 K for 4 days Chemical constraints to the system (e.g. reduction of TPA, competition from TMA, addition of ethanol), cause an increase in Z/w ratio, and restriction of OH- is known to have the same For these changes, growth along the major axis of the crystal is less affected than growth in directions at right angles to it. It appears that, as nutrients become scarce, they are preferentially adsorbed by the (101) faces. The apparent exception is aluminium addition, which appears to decrease the Z/w ratio.However, if A1 is regarded as a growth poison (which is not unreasonable in view of the charge distortion it will cause), the picture is not inconsistent. The species is once again preferentially attracted to the potentially faster-growing face, but in this case causes charge anomalies and site blocking, the result of which is a net reduction in growth rate for that face over those receiving a lower flux of aluminate. Financial support from the SERC and ICI plc. is gratefully acknowledged. The authors would also like to thank R. J. Davey, S. J. Maginn and M. D. Shannon for helpful discussions, and B. W. Cook for obtaining the Raman spectrum. References 1 C. S. Cundy, B.M. Lowe and D. M. Sinclair, J. Crystal Growth, 1990, 100, 189. 2 S. P. Zhdanov and N. N. Samulevich, in Proc. 5th Znt. Con5 Zeolites, ed. L. V. C. Rees, Heyden, London, 1980, p. 75. 3 E. M. Flanigen,J. M. Bennett, R. W. Grose, J. P. Cohen, R. L. Patton, R. M. Kirchner and J. V. Smith, Nature (London), 1918,271, 512; R. W. Grose and E. M. Flanigen, US Patent 4 061 724, 1977. 4 B. M. Lowe, in Innovation in Zeolite Materials Science, Studies in Surface Science and Catalysis Vol. 37, ed. P. J. Grobet, W. J. Mortier, E. F. Vansant and G. Schulz-Ekloff, Elsevier, Amsterdam, 1988, p. 1. 5 J. Garside, in Industrial Crystallisation '78, ed. E. J. de Jong and S. J. JanEiC, North Holland, Amsterdam, 1979, p. 143, and references therein. 252 Direct Measurement of Zeolite Crystal Growth 6 W.M. Meier and D. H. Olson, Atlas of Zeolite Structure Types, Butterworths, London, 2nd edn., 1987, and references therein. 7 G. T. Kokotailo, S. L. Lawton, D. H. Olson and W. M. Meier, Nature (London), 1978, 272,437; R. J. Argauer and G. R. Landolt, US Patent 3 702 886, 1972. 8 R. M. Barrer, Hydrothermal Chemistry of Zeolites, Academic Press, London, 1982, ch. 4, p. 152. 9 N. N. Feoktistova, S. P. Zhdanov, W. Lutz and M. Bulow, Zeolites, 1989, 9, 136. 10 A. Culfaz and L. B. Sand, in Molecular Sieves, ACS Adv. Chem. Ser. No. 121, ed. W. M. Meier and J. B. Uytterhoeven, American Chemical Society, Washington D.C., 1973, p. 140. 11 C. J. J. den Ouden and R. W. Thompson, Ind. Eng. Chem. Res. 1992,31,369.12 A. Araya and B. M. Lowe, Zeolites, 1986,6, 11 1; R. Aiello, F. Crea, A. Nastro and C. Pellegrino, Zeolites, 1987, 7, 549. 13 G. Golemme, A. Nastro, J. B. Nagy, B. SubotiC, F. Crea and R. R. Aiello, Zeolites, 1991, 11, 776. 14 W. M. Hendricks, A. T. Bell and C. J. Radke, J. Phys. Chem., 1991,95,9513. 15 J. A. Rossin and M. E. Davis, Indian J. Technol., 1987, 25, 621. 16 F. Crea, A. Nastro, J. B. Nagy and R. Aiello, Zeolites, 1988, 8, 262. 17 S. G. Fegan and B. M. Lowe, J. Chem. Soc., Chem. Commun., 1984,427; S. G. Fegan and B. M. Lowe, J. Chem. Soc., Faraday Trans. I, 1986,82, 785. 18 A. Nastro and L. B. Sand, Zeolites, 1983,3, 57. 19 S. G. Fegan and B. M. Lowe, J. Chem. Soc., Faraday Trans. I, 1986,82,801. 20 K. R. Franklin and B. M. Lowe, Zeolites, 1987,7,433.21 N. A. Briscoe, D. W. Johnson, M. D. Shannon, G. T. Kokotailo and L. B. McCusker, Zeolites, 1988,8,74, and references therein. 22 J. L. Schlenker, F. G. Dwyer, E. E. Jenkins, W. J. Rohrbaugh, G. T. Kolotailo and W. M. Meier, Nature (London), 1981,294,340. 23 J. L. Schlenker, W. J. Rohrbaugh, P. Chu, E. W. Valyocsic and G. T. Kokotailo, Zeolites, 1985,5,355. 24 P. K. Dutta and M. Puri, J. Phys. Chem., 1987,91,4329. 25 C. J. J. den Ouden and R. W. Thompson, J. Colloid Interface Sci.,1991, 143, 77. 26 J. D. H. Donnay and D. Harker, Am. Mineral., 1937,22,446. 27 G. H. Kuhl, Eur. Pat. Appl. 93519, 1983. Paper 21063595; Received 26th November, 1992