The crystal morphology of zeolite A.The effects of the source of the reagents
Catherine I. Round, Susan J. Hill, Kay Latham, Craig D. Williams *School of Applied Sciences, University of Wolverhampton, Wulfruna Street, Wolverhampton, WV1 1SB, U.K.
1. Introduction Zeolite A, discovered by Milton in 1956 [3,4],is of great industrial importance as an adsorbentand detergent builder. Large crystals have beenZeolite crystal morphology results from the gelgrown in sodium systems to study their catalytic,composition, the nucleation process, the chemistrysorptive and ion-exchange properties [5 ]. Workat the crystal surface and the kinetics of the crystalwith many known zeolite structure types has beengrowth. The rates of the reaction correlate directlycarried out to study the influential factors affectingwith the basic units involved in the growth of thegeneral synthesis. This includes molar compositioncrystal [1]. Industrial synthesis is guided by usingof the starting gels, synthesis time and temperature,economical hydrothermal conditions. It has devel-the cation source, ageing procedures, stirring, seed-oped to allow the construction of defined frame-ing and the order of the mixing of the gels [6–13].works with specific geometry, adjustable acidIt is generally accepted that the nucleation processstrengths and selectable electric fields [2].is kinetically controlled and the chemical andphysical nature of the reactants before crystallisa-
* Corresponding author. tion is a kinetic variable [14]. The primary influ-
ence of the molar ratios of reagents is also well with 0.88% Fe; deionised water; fumed silica asCab-O-Sil M5 ‘‘Scintran’’ (B.D.H.), with 0.3% Al;documented [15], as are the effects of organic
substances used in synthesis procedures [16]. The sodium aluminate laboratory reagent (B.D.H.);sodium hydroxide as pearls, (General purposesource of the reagents including mixed alkali–
organic base systems (i.e. NaOH, KOH and reagent), with 1.0% K; sodium silicate solution assodium oxide, 7.5–8.5%, silicate acidimetricTMAOH) have been studied [17]. Single and
binary cation systems using cations from both 25.5–28.5% (B.D.H.); tetraethyl-orthosilicate astetraethoxysilane 98% (Fluka).inorganic and organic sources [18] have also
been used.The effects of using different sources of alumin-
ium and/or silicon have been studied more recently 3. Mixing procedurein zeolites NaX [19], Omega [20], Beta [21],ZSM-5 [22] and Mordenite [23]. However, recent A preliminary investigation was undertaken. All
sources of reagents were used to establish an orderwork appears to have neglected the study of effectsof the starting materials used in the synthesis of of addition and a mixing procedure which pro-
duced the most consistently crystalline product.zeolite A, in particular the source of the alumin-ium. This research investigates the use of inorganic The mixing of each system differed slightly due to
the nature of the source reagent. The proceduresand organic silicon and aluminium compounds,and their effects on morphology and crystallinity, are described on the basis of the aluminium source,
the silica source and the method of combination.in the synthesis of zeolite A.
3.1. Procedure A2. Experimental
The cation source was dissolved in ~40%volume of the deionised water. The aluminiumThe molar composition of the starting reaction
gel is given in Table 1. Chemical analysis of the reagent was added and the silica source was thenadded to the same vessel at the same time as thestarting materials was determined by X-ray fluo-
rescence ( XRF) using an ARL 8410 sequential remaining water. The resulting mixture was stirredbriskly by hand for 5 min, then aged for 30 min atX-ray spectrometer. Samples of ~1 g were sand-
wiched between 4 mm of Prolene (X-ray transpar- room temperature.ent film) prior to analysis. The reagents used werealuminium isopropoxide as aluminium isopropyl- 3.2. Procedure Bate (98+% Aldrich Chimie) analysed as containing0.9% Ca; aluminium metal as fine powder, B.D.H., The aluminium source was added to the solution
of the cation source (40% volume H2O) and brisklystirred until dissolved. The silica source was dis-
Table 1 solved in the remaining water and stirred by handMolar compositions of the starting reaction gels until a homogeneous slurry was obtained. The two
solutions were combined and the resulting mixtureSystem No. Molar composition expressed as oxide ratioswas stirred and aged as described in procedure A.
1, 2 and 3 Na2O: Al2O3: 2SiO2: 100 H2O4 2NaAlO2: Na2SiO3: 100 H2O 3.3. Procedure C5 C9H21AlO3: Na2SiO2: 100 H2O6 0.5Na2O: 2NaAlO2: 2SiO2: 100 H2O The cation source was dissolved as previously7 0.5Na2O: 2NaAlO2: 2Si (OC2H5)4: 100 H2O4a Na2O: 2NaAlO2: Na2SiO3: 100 H2O described and added to a dispersion of the silica5a Na2O: 2C9H21AlO3: Na2SiO3: 100 H2O source in water. The aluminium source was added6a Na2O: 2NaAlO2: 2SiO2: 100 H2O last. The mixture was stirred and aged as described7a Na2O: 2NaAlO2: 2Si (OC2H5)4: 100 H2O in procedure A.
215C.I. Round et al. / Microporous Materials 11 (1997) 213–225
These three procedures produced gels whichwere sampled over a period of 14 days at temper-atures of 100 (±1)°C and 70(±1)°C.
The crystallinity of the sampled products wasdetermined from the X-ray diffraction data. Thesum of the peak counts, for three standard refer-ence pattern peak angles, were compared as a ratioof the sum of the same three peak counts for themost crystalline sample produced. This allowedinter-system and intra-system comparison. A com-parison of the crystallinity of the resulting products(Figs. 1 and 2) showed that procedure B producedgels which yielded the most crystalline samples.
4. Experimental methods—the systems investigated Fig. 2. Mixing procedures A, B and C, comparing the percen-tage crystallinity obtained at 70°C.
The stoichiometry as calculated produced 200 gof zeolite gel using mixing procedure B. The pHwas measured after the 30 min ageing period, and washed with deionised water. The samples were
then dried overnight in an oven at ~40°C, andthe characteristic behaviour of the solutions ineach system was observed. The gel was divided ground to homogenous small particles before
analysis.between six PTFE bottles which were then sealed.No intentional seeding of the gels was employed. Characterisation was carried out between 5 and
40° 2h on an automated Philips 1710 X-rayThe gels were placed in an oven, controlled atthe synthesis temperature, and sampled over a 14 diffractometer, using CuKa radiation, interfaced
to a DEC Microvax 3100 minicomputer, withday period. After removal from the oven thesamples were quenched and the pH was recorded. Philips APD software. This included a search and
match facility using the Joint Committee onThe solid sample was separated from the motherliquor by Buchner filtration and the products were Powder Diffraction Standards (JCPDS ) database.
For comparable diffraction results standard sampleholders of 0.10 cm3 volume were used. The pro-ducts were compared with standard patternnumber: 38–241 (Na2 Al2 Si1.85 O7.7 · 5.1 H2O), azeolite A type, and the degree of crystallinity wasdetermined from the X-ray diffraction ( XRD)data. XRD crystallographic analysis was used toestablish the unit cell parameters. XRF analysisdetermined the chemical composition of the finalproduct. The crystal morphology was determinedusing a Philips 515 scanning electron microscopeon samples that had been mounted on aluminiumpegs and then gold sputtered. Photomicrographswere obtained at varying magnifications to showparticle size and uniformity of morphology, crystalsize and shape and crystal surface appearance. Theframework infrared spectra of the samples wereFig. 1. Mixing procedures A, B and C, comparing the percen-
tage crystallinity obtained at 100°C. obtained using a Nicolet Impact 404 spectropho-
tometer, interfaced with a 486DX microprocessor took several minutes to dissolve. The addition ofwith maths coprocessor and Fourier transform the silica source was exothermic with a distinctinfrared (FTIR) software OMNIC version 2. The odour of alcohol. Stirring resulted in an opaquesamples were uniformly mixed with dry potassium smooth gelatinous liquid. Over the 14 day samplingbromide using a 1:200 ratio of sample: KBr, the period the pH value reduced by 1.8. This systemmixture was then pressed using 9 tons of pressure reached a maximum of 62% crystallinity (Table 4),to form a transparent fused halide window (13 mm and matched XRD reflections with reference todiameter). The spectra were recorded at room JCPDS pattern 38–241. SEM (Table 5) showedtemperature. Thermogravimetric analysis (TGA) the sample to consist of regular well defined cubeswas performed on a Mettler TG50 thermobalance with chamfered edges with zeolite NaP apparentand Mettler TA3000 processor, under nitrogen, at at day 13 in the 100°C system but not in thea heating rate of 20 K min−1 between temperatures system at 70°C. Zeolite NaP is commonly foundof 40 and 800°C and using ~15 mg of product as an over-run product in zeolite A synthesis. TGAsample. revealed weight losses corresponding to the
desorption/dehydration of physically absorbedwater molecules occluded within the pore system.
5. The hydroxide concentration The weight loss of 15.8% ranged from 43 to 213°C.A second weight loss of 6.3% occurred between
Sufficient sodium cations were combined with 213 and 440°C (Table 6). This two-stage weightthe reagent sources in systems 4 and 5, using
loss of occluded water molecules was found to besodium silicate and sodium aluminate, to omit the
characteristic of the three systems with cubic,NaOH (Table 1). In systems 6 and 7, using sodiumchamfered edged morphology (systems 1, 3 and 5,aluminate, the stoichiometric volume of NaOHFig. 3). The infrared spectra of the product of thiswas reduced by 50% as the cations were availablesystem closely agreed with literature values [24]in the aluminium source. This adjustment also(Fig. 4). The three main characteristic trans-reduced the [OH]− by the equivalent mole frac-mission peaks were recorded at ~565 cm−1,tion. These four syntheses were then repeated to ~660 cm−1 and ~975 cm−1. Literature values areinclude the stoichiometric quantity of [OH]−.550, 660 and 996 cm−1, respectively [24].Table 2 lists the combinations of reagents sources
System two, using tetraethyl-orthosilicate andinvestigated and Table 3 lists the systems repeatedaluminium powder produced a grey, opaque liquidto compare the effects of the [OH]−.on mixing. After ageing, a clear organic layer,immiscible with aqueous solutions, was seen onthe surface of the gel. Crystallinity of the products6. Resultsreached 87% at day 13 in the 70°C system, butonly 13% at 100°C. SEM (Fig. 5(a)) shows theSystem one, using tetraethyl-orthosilicate and
aluminium isopropoxide, the aluminium source product to be regular elongated crystals (~5 mm)
Table 2The systems investigated. Reagent source and weight in grams used
System No. Si source (g) Al source (g) Cation source (g) H2O (g)
The syntheses were repeated at 70°C and include the stoichiometric quantities of the OH− ion concentrations. The sodium ions werein excess.
with a hexagonal cross-section, and someTable 4unreacted/amorphous material. XRD analysisMaximum crystallinity obtained for each synthesis and the day
maximum crystallinity achieved (%) identified zeolite A, (Fig. 6(d )). The unit cell wasidentified as face centred cubic, with the unit cell
System No. Maximum crystallinityconstant calculated at a=24.59 A ( literature valuea=24.64 A [24]). Plots of the XRD patterns indi-70° 100°cated an impurity with reflections at 18.3, 22.9 and
(%) day (%) day 28.3° 2h. This impure phase was consistentthroughout all samples. The phase was not iden-1 56 13 62 6
2 87 13* 13 6 tified but was calculated to represent less than 5%3 100 13* 68 2 of the material analysed. XRF analysis indicated4 64 7* 34 7 a small Fe content in the chemical composition,5 † † † †
this was consistent with the small iron impurity in6 52 6 5 97 36 14 44 7* the aluminium powder. TGA showed a 4.5%4a 72 2 weight loss between 123 and 188°C and a sharp5a 97 7* weight loss of 26.8% between 188 and 320°C. This6a 49 13
system was anomalous, in that the main weight7a 76 14loss occurs at the higher temperature and that the
*Refer to Fig. 5 showing SEM images of crystal morphologies. overall weight loss is increased by 50% (Table 6).†Denotes reaction system that remained amorphous.
Infrared analysis of this system (Fig. 4) shows
Table 5Summary of crystal morphologies —consistent for both temperatures
System No. Source of reagents Crystal morphology
1 Organic Si, Organic Al Poor crystallinity—chamfered edges to cubes visible, much unreacted material andtransformation to NaP
2 Organic Si, Al Powder Slow to crystallise—irregular hexagonal shapes over-run Nap and amorphousmaterial at 100°C
3 Fumed Si, Organic Al Uniform deep chamfered edges. Regular clean cubes ~2 mm. 97% crystallinity sustaineddays 2–14.
4 Sodium Si, Sodium Al Poor crystallinity with low [OH−]. Sharp edged cubes with increased [OH].5 Sodium Si, Organic Al Amorphous with low [OH−]. Regular sized cubes with deep chamfers and 96%
crystallinity with increased [OH−].6 Fumed Si, Sodium Al Poor crystallinity—some sharp edged cubes. Much amorphous material.7 Organic Si, Sodium Al Poor crystallinity, sharp edged cubes and zeolite NaP at 100°C. Sharp edged
*Weight loss confined to lower the temperature range, consistent for systems producing sharp edged cubic morphology.
Fig. 4. Infrared traces of the most crystalline samples to show(a) system 2 with distortion of the main characteristic T–O–T
Fig. 3. TGA plots showing thermal behaviour of the most crys- transmission peaks (~1000–550 cm—1) and in the hydroxyltalline samples: (a) systems 4 and 7; (b) systems 3 and 5; (c)
region (~3300–3750 cm—1), (b) typical I.R. transmission forsystem 2.
zeolite A and representative of systems 1 and 3 to 7.
‘‘bridging’’ hydroxyl groups. The vibrations, atpronounced distortion of the main characteristiczeolite A transmission peaks. In the regions of ~3740 cm−1 show distortion. These vibrations are
known as the ‘‘catalysis absorbency band’’ and are500–650 cm−1, characteristic of double ring exter-nal linkages, indicating distortion of the frame- diagnostic of terminal hydroxyl groups arising at
the surface, or from defect sites within the frame-work’s double rings. Considerable distortion canbe seen of the symmetrical T–O–T stretching work. These distortions or defects are supported
by the thermogravimetric analysis which showsvibrations (750–820 cm−1) and the asymmetricalT–O–T stretching vibrations region (1050– irregular weight loss behaviour.
System 3, using fumed silica and aluminium1150 cm−1). Also distortion in the internal tetrahe-dral symmetrical O–T–O stretching vibration isopropoxide, initially formed a thick white gelati-
nous paste which liquified on stirring. The organicregion (650–720 cm−1) [24] is seen. The behaviourof the hydroxyl groups is also anomalous [25], the layer was visible after allowing the sample to stand.
This system produced the most crystalline product,vibrations at ~3600 cm−1 are assigned to OHgroups associated with Brønsted sites, these are stable between 85–100% for 11 days at 70°C.
219C.I. Round et al. / Microporous Materials 11 (1997) 213–225
221C.I. Round et al. / Microporous Materials 11 (1997) 213–225
Fig. 5. SEM images showing the crystal morphology of the products obtained from the different systems: (a) system 2, 70°C, day 13;(b) system 3, 70°C, day 7; (c) system 4, 70°C, day 7; (d ) system 5a, 70°C day 7; (e) system 7, 100°C day 7.
Fig. 6(a) and (b) show XRD patterns for this combination. This paste softened on stirring andthe reaction was slightly exothermic. This synthesissystem at 70°C after 1 day and 14 days. Reflections
matched the reference pattern, but as in the other produced larger (~6 mm3) sharp edged, cubic mor-phology (Fig. 5(c)), with maximum crystallinitysystems, three impurity peaks were observed. Unit
cell parameters were calculated as face centred of 72% in system 4a. XRD data indicated a mixtureof zeolites A and X. Unit cell analysis, aftercubic, with a=24.65A. SEM (Fig. 5(b)) showed
an even distribution of good quality, clean, regular removal of zeolite X reflections, indicated face-centred cubic system with a=24.55A. TGAcubes (~3 mm3) with deep chamfered edges, the
type preferred by detergent manufacturers [25]. revealed a single, broad temperature range ofweight loss, viz. 46–508°C. This single stage lossThe synthesis at 100°C exhibited the same mor-
phology, but had transformed to NaP by day 13. was found to be characteristic of the three systemsproducing zeolite A with sharp edged cubic mor-XRF analysis indicated a calcium content in the
chemical composition consistent with the calcium phology, systems 4, 6 and 7 (Fig. 3(a)). The totalweight loss, 21.3%, of occluded water moleculesimpurity identified in the alumnium isopropoxide.
TGA showed a weight loss pattern consistent with corresponds to all systems investigated exceptsystem 2. The infrared spectra again agreed closelysystems 1 and 5 (Fig. 3(b)). The total weight loss,
in two stages, was 21.6%. The infrared spectra of with literature values and with all other systems(Fig. 4(b)) except system 2 (Fig. 4(a)).the product agreed closely with the literature
values. System 5, using sodium silicate and aluminiumisopropoxide in combination, the solutionsSystem 4, using sodium silicate and sodium
aluminate, dissolved quite slowly in the deionised remained aqueous with no visible reaction takingplace and no heat or odour being produced. Thewater and produced a gelatinous white paste on
was consistent with the calcium identified in thealuminium isopropoxide and the potassium impu-rity in the increased sodium hydroxide content.TGA revealed a two stage weight loss of 24.8%over the temperature ranges shown in Fig. 3. Thiswas consistent with other systems producing zeoliteA cubes with chamfered edged morphology. Thissystem again displayed classic zeolite A infraredtransmission bands.
System 6, using fumed silica and sodium alumi-nate, formed a gelatinous paste which softened onstirring and produced an opaque liquid. Thissystem was less efficient at a maximum 52% crys-tallinity at day 6, 70°C. There was no improvement(49% crystallinity) in the repeated 6a system. SEMshowed irregular sharp edged cubes with a lot ofamorphous material. XRD data confirmed thereference pattern matching. Weight loss duringthermogravimetric analysis was confined to a singlebroad temperature range (Fig. 3a) as in systems 4and 7. Infrared analysis showed transmissionbands characteristic zeolite A.
System 7, using tetraethyl-orthosilicate andsodium aluminate, formed a clear aqueous liquidwhich was easy to mix. There was no visiblereaction or odour on mixing. Ageing allowed aclear organic surface layer to form. This systemwas less efficient with 36% crystallinity at 70°Cincreasing to 76% in the 7a synthesis at 70°C.Crystal morphology showed sharp edged cubes(~4 mm3) with over-run product zeolite NaP atday 7 at 70°C (Fig. 5e). XRD data confirmedreference pattern matching. Weight loss duringthermogravimetric analysis was confined to a singleFig. 6. XRD plots compared with reference pattern: (a) systembroad temperature range as in systems 4 and 6.3, 70°C, day 1; (b) system 3, 70°C, day 14; (c) system 5, 70°C,Infrared analysis indicated characteristic zeolite Aday 4; (d ) system 2, 70°C, day 13; (e) JCPDS reference
pattern 38–241. transmission bands with assigned peak values closeto literature values.
syntheses remained amorphous at both 70°C and100°C. When repeated with the formula stoichio-metric volume of sodium hydroxide (system 5a), 7. Discussion97% crystallinity was produced at day seven. SEMshowed a clean regular cubic morphology From the summary of crystal morphologies
(Table 5) it is clear that the source of the alumin-(~2–4 mm3), each cube having deep chamferededges (Fig. 3(d )). X-ray diffraction data show ium reagent had a predictable effect. Using organic
alumina the cubes always had deep chamferedsustained crystal stability over 6 days (Fig. 6(c)).XRF analysis indicated potassium and calcium in edges (systems 1, 3 and 5). In contrast, the systems
using sodium aluminate always produced cubesthe chemical composition of the final product. This
223C.I. Round et al. / Microporous Materials 11 (1997) 213–225
with sharp edges (systems 4, 6 and 7). This was effects, need to be identified and investigated. Anequilibrium model of crystal growth, as a functionconsistent in systems with a low hydroxide concen-
tration (systems 4, 5, 6, and 7), and in systems of synthesis conditions [27], confirms that theconcentration of solution species is controlled bywhich produced poorly crystalline material (sys-
tems 1, 4a, 6 and 7). System 2, using an organic the solubilities of the solid phases, which initiallyare the amorphous reactants. Tetraethyl-orthosili-silica source and aluminium powder, produced an
irregular morphology, defined as cubic but with cate is a four co-ordinated monomeric silica sourcewhich would influence the solution phase moreconsiderable distortion. SEM showed hexagonal
cross-sections. The irregular morphology is sup- quickly, as it is an uncharged species in truesolution as opposed to the fumed silica whichported by TGA analysis which shows a reduced
dehydration weight loss, but an increased total initially forms a colloidal dispersion in water.Aluminium isopropoxide is an uncharged, threeweight loss. Also, the infrared analysis of system
2 shows distortion in both symmetrical and asym- co-ordinated aluminium source requiring basicconditions to form the tetrahedral chargedmetrical T–O–T and O–T–O stretching, and in the
region of the hydroxyl signals. Al(OH)4− species necessary to catalyse the forma-tion of the aluminosilicate gel. These organicFajula [1] states that in the investigation of the
crystal faces of zeolite omega, there was a clear sources on hydrolysis produce ethanol and isopro-pyl alcohol, respectively. These heterogeneous sys-correlation between the aluminium concentration
in the gel solution and the crystal habit. Low tems exhibit different behaviour and the rheologyof the solutions is changed.concentrations of aluminium producing high
aspect, elongated crystals. The behaviour of the sodium systems is wellknown [2,13]. The ease of Na+ hydration changesThe initial zeolite nucleation process is not fully
understood yet. The composition of the gel affects the supersaturation of the solution, and also thetemplating or structure directing effects. Thesethe crystal surface chemistry, resulting in different
morphologies depending on the source of the three different solution chemistries affect the kinet-ics of the systems and the nucleation processes.aluminium and silica used. This was evident as a
consistent crystal growth along the three crystal The results can be observed in the morphologiesof zeolite A produced by these systems.axes. Crystal growth is controlled by a chemical
step [12] in which the lattice forming secondary The increased hydroxide concentrations in sys-tems 4a to 7a affected each system differentlybuilding units (SBUS) are believed to undergo
condensation and polymerisation with the crystal (Table 4). This agrees with previous theories thatthe hydroxide ion can increase and decrease crys-surface to produce a particular morphology. The
reference to ‘‘SBUS’’ is used as the hypothetical tallisation time according to its concentration, i.e.the Na oxide to silica ratio in the reaction mixillustration of progressive crystal growth, often
quoted as ‘‘visually tempting but mechanistically [7,10]. It speeds up crystal growth and shortensnucleation time [25]. According to Hayhurst andchallenging’’ [26 ]. Material adding to the growing
crystal must ‘‘fit’’ the underlying surface. It could co-workers [28], changes in base concentration,studied for its effect on silicalite-1, found that thebe argued that any alteration in growth direction,
to produce a chamfered edge as opposed to a growth rate and the crystal aspect ratio were astrong function of the hydroxide concentration.sharp edged crystal, must be apparent from nucle-
ation onwards. According to Fajula [1] it is the The Na+ content of these four systems was inexcess according to the stoichiometric formulasynthesis conditions that decide the crystal habit,
and the initial high supersaturation of the gel (Table 1). It has been reported that the alkalication is strongly and stoichiometrically associatedproduces only spherical crystals. Towards the end
of crystallisation when the nutrients have been with the aluminate species [29]. The ratio of thesespecies in the reaction gel has previously beenconsumed, low-index crystal faces develop. To
investigate the process of zeolite crystal growth, measured, and reported to be near to a value ofone, earlier on in the crystallisation reaction. Thethe precursor molecules, and any templating
report concluded that an ion pairing, or an associ- chamfered edges. Systems using sodium aluminatealways produced sharp edged cubic morphology.ated species of [Na+] [AlOH4−] or NaAlO2, were
the reacting or diffusing species and the precipitat- The system using teraethyl-orthosilicate with ele-mental aluminium produced an irregular hexago-ing agent [12]. As sodium aluminate was used as
a source reagent in the systems which produced nal morphology.These morphologies were not affected bythe sharp edged cubic morphology, the theory
could be used to explain the fact that three different changes in the hydroxide concentration.The system which produced the most crystallinemorphologies were produced. These associated
species, together with the increased hydroxide ion sample used fumed silica and aluminium isoprop-oxide. The system using sodium silicate andconcentration, may also explain the change in
crystallinity of system 5a, when the stoichiometric organic aluminium sources showed a strikingimprovement, i.e. from amorphous to 97% crystall-sodium hydroxide concentration was included. The
increased hydroxide concentration alone did not inity when the stoichiometric volume of NaOHreagent was included. The system which was theaffect the crystal morphology peculiar to each
system. most thermodynamically stable at both reactiontemperatures used both silicon and aluminiumThe phase separation in the heterogeneous
organic systems may have influenced the overall from sodium sources, with the stoichiometricvolume of NaOH included.fluidity and the mechanical behaviour of the gels.
Silica gels are generally classified as thixotrophicwith a characteristic period of hysteresis afterstirring and pouring. In this respect the compo- Acknowledgementnents of the gels, in particular the organics, wouldhave the effect of reducing and/or confining the The authors thank Mr Jon Allen for XRFwater activity and the deformation and flow of the analysis, Mr Brian Bucknall for XRD analysis,systems. The viscosity of the heterogeneous sys- Mr David Crane for electron microscopy and Drtems differed in their relativity to particle size C.V.A. Duke for FTIR analysis.distribution. All of the systems, however, con-tained a high molar ratio of water which wouldhave reduced the impact of particle size distribu- Referencestion. The changes in crystal habit are indicative ofall of the elements of the gel which affect the [1] F. Fajula, NATO ASI Series B (1989) 53.crystal surface chemistry from nucleation onwards. [2] R.M. Barrer, Hydrothermal Chemistry of Zeolites,
Academic Press, 1982.In particular, the morphology produced by system[3] D.W. Breck, W.G. Eversole, R.M. Milton, T.B. Reed, T.L.2, and the change in the rheology of the gel were
Thomas, J. Am. Chem. Soc. 78 (1956) 5963.manifested by the distortion of the growing crystal[4] T.B. Reed, D.W. Breck, J. Am. Chem. Soc. 78 (1956)
surface forming the cavities and channels of zeolite 5972.A. This distortion is supported by XRD, SEM, [5] J.F. Charnell, J. Cryst. Growth 8 (1971) 291.
[6 ] G.T. Kerr, J. Phys. Chem. 70 (1966) 1047.TGA and infrared analysis.[7] J. Ciric, J. Colloidal Interface Sci. 28 (1968) 315.[8] W. Meise, F.E. Schwochow, Adv. Chem. Ser. 121
[10] J.A. Kostinko, Am. Chem. Soc. Symp. Ser. 218 (1983) 3.[11] F. Roozeboom, H.E. Robson, S.S.Chan, in: L.P. RollmanThe source of the reagents used in the synthesis
(Ed.), Zeolite Science and Technology, Martinus Nijhoffof zeolite A affects the gel rheology, the kineticsPublishers, 1984, p. 127.of the gel chemistry and the activity at the crystal
[16] B.M. Lok, T.R. Cannan, C.A. Messina, Zeolites (1983) Reinhold, New York, 1992, p. 266.[25] A. Dyer. An Introduction to Zeolite Molecular Sieves,282.
[17] R. Aiello, R.M. Barrer, J. Chem. Soc. (1970) 1470. John Wiley and Sons, Chichester, 1988, pp. 55 and 121.[26 ] C.G.T. Knight, R.T. Syvitski, S.D. Kinrade, in:[18] B.M. Lowe, N.A. MacGilp, T.V Whittam, in: L.V.C. Rees
(Ed.), Proceedings of the 5th International Conference on J. Klinowski, P.J. Barrie (Eds.), Studies in Surface Scienceand Catalysis, Part 1, vol. 97, Elsevier, Amsterdam,Zeolites, Naples, Heydon Press, London, 1980, p. 85.
