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Crystallization kinetics of zeolite omega, the synthetic analog of mazzite
Microporous and Mesoporous Materials 90 (2006) 221–228 www.elsevier.com/locate/micromeso
Crystallization kinetics of zeolite omega, the synthetic analog of mazzite M.L. Guzman Castillo a
a,1
, F. Di Renzo
a,*
, F. Fajula a, J. Bousquet
b
Laboratoire de Mate´riaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS-ENSCM-UM1, Institut C. Gerhardt, FR 1878, ENSCM, 8 rue Ecole Normale, 34296 Montpellier, France b Total SA, Direction Scientifique, BP 22, 69360 Solaize, France Received 29 August 2005; received in revised form 11 October 2005; accepted 13 October 2005 Available online 15 December 2005
Dedicated to the late Denise Barthomeuf, George Kokotailo and Sergey P. Zhdanov in appreciation of their outstanding contributions to zeolite science
Abstract The synthesis of zeolite omega, the synthetic analog of mazzite, was carried out by using commercial reagents and a low amount of tetramethylammonium template. The use of a slow-release source of aluminium allowed to avoid the formation of a precursor gel and to concentrate the nucleation at the beginning of the synthesis. The rates of crystal growth in radial and axial direction depends on the alkalinity and the aluminium concentration. The changes of the aspect ratio and morphology of the crystals can be accounted for by the relative growth rates of different crystal faces. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Zeolite crystallization; Growth kinetics; Growth rates; Mazzite; Zeolite omega; Crystal size; Aspect ratio
1. Introduction The zeolite omega or ZSM-4, the synthetic analog of mazzite, is known for its strong Brønsted acidity and its activity as a catalyst of alkylation and isomerization [1,2]. It is an example of zeolite with intermediate aluminium content (usual Si/Al between 3 and 5) and was originally synthesized in the presence of sodium and tetramethylammonium (TMA) cations. When, in 1970, Aiello and Barrer revealed the synthesis of zeolite omega [3], patents for the very similar syntheses of ZSM-4 [4] and zeolite omega [5] had already been filed by, respectively, Mobil and Union Carbide. The row on industrial property which followed was settled when mazzite, a rare natural zeolite discovered by Ermanno Galli, was shown to present the same structure of zeolite omega [6,7]. The original synthesis of zeolite omega was plagued by phase selectivity problems [8]. Several zeolites with smaller *
1
Corresponding author. Tel.: +33 467163479; fax: +33 467163470. E-mail address: [email protected] (F. Di Renzo). Present address: Istituto Mexicano del Petroleo, Mexico, DF, Mexico.
1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.10.022
pores and higher aluminium content, like phillipsite, gismondine, and analcime may easily contaminate the mazzite phase. Zeolite Y is formed in the earliest steps of the syntheses with a low TMA content and is metastable towards zeolite omega [9–11]. A high TMA content was shown to speed up the formation of zeolite omega, but non-porous sodalite was also formed in these conditions [12]. The use of slow-release sources of aluminium, viz. zeolite Y [13] or silica–alumina [14], was found to improve the selectivity of the synthesis. The solubility of the source of aluminium was shown to affect the morphology of zeolite omega, spherical grains being formed from highly soluble sources of aluminium and elongated hexagonal prisms from slow-release sources [15]. The importance of the concentration of aluminiumbearing species in the synthesis solution was confirmed by an experiment of continuous crystallization, in which an aluminate solution was fed to the crystallization system and larger and flatter crystals were obtained [16]. The significance of the aspect ratio in the case of zeolite omega is not limited to the influence of the length of the crystals on the diffusion properties in a phase with
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2. Experimental The reagents used in the syntheses were precipitated silica Zeosil 175MP by Rhoˆne-Poulenc (Na 0.7%, Al 0.17%, H2O 13.8% (w/w)), Siliporite by CECA (from an industrial batch of zeolite X with 20% zeolite A, Si/Al 1.26, corresponding to a molar fraction 0.443 Al/(Si + Al)), tetramethylammonium (TMA) bromide by Aldrich, sodium hydroxide by Merck, deionized water. The compositions of the synthesis batches were in the range 0.41–0.48 Na2O/0.024 TMA2O/0.059–0.075 Al2O3/SiO2/26.6 H2O, corresponding to three levels of Al/(Si + Al) ratio (0.105, 0.116, and 0.135) and two levels of alkalinity ratio (0.7 and 0.8). The alkalinity ratio was evaluated from the molar composition as OH/Si = (Na + TMA–Al–Br)/Si. The syntheses have been carried out at 378 K in a 900 cm3 stirred stainless-steel autoclave equipped for the on-line sampling of the crystallization medium. The solid and liquid fractions of samples withdrawn at different times of the synthesis were separated by filtration. The solid phase was washed with water and characterized by X-ray diffraction (CGR Theˆta 60 diffractometer, Cu Ka radiation) and scanning electron microscopy (Cambridge Stereoscan 260 apparatus). The relative fraction of zeolite omega XMAZ = MAZ/(MAZ + FAU + LTA) was evaluated from the intensity ratios of selected lines of the X-ray diffractograms. The size of the crystals was averaged on 30–60 measurements on micrographs. The pH of the solutions was measured at 298 K. The solutions were diluted for storage after filtration and analyzed for elemental composition at the Service Central dÕAnalyse of the CNRS in Solaize.
1 0.8 X (MAZ)
monodimensional porosity [17]. It has been shown that the thermal stability of zeolite omega depends on the crystal morphology, being higher for hexagonal prisms than for spherical or cylindrical crystals [18–20]. This phenomenon suggests the thermal degradation of zeolite omega to be initiated on the surface defects of the non-flat faces, with a mechanism quite different from the bulk nucleation of the decomposed phase observed in MFI [21]. The crystallization kinetics of zeolite omega has deserved some interest in the literature some years ago [12,15,16]. The purpose of this paper is to show that it is possible to improve both the understanding of the crystallization process and the synthesis procedure of a specific zeolite by operating in conditions near to industrial standards, as is the case when commercial reagents and bench-scale stirred autoclaves are used.
0.6 0.4 0.2 0 0
20
40 t/h
60
80
Fig. 1. Crystalline fraction of zeolite omega as a function of synthesis time at 378 K. Batch compositions: OH/Si 0.7 (filled symbols) or 0.8 (void symbols); Al/(Si + Al) 0.105 (circles), 0.116 (squares) or 0.135 (triangles). The lines are fittings of the experimental data by the Avrami equation.
zeolites and crystallization of zeolite omega was completed after nearly 30 h for the syntheses at the highest alkalinity (OH/Si 0.8), which presented virtually no induction time. The syntheses at the lowest alkalinity (OH/Si 0.7) presented a significant induction time. The Al fraction affected in a significant way the kinetics of the syntheses, the Al-richest syntheses being completed in 42 h and the Al-poorest one in 82 h. The ratio FAU/LTA did not change during the synthesis. The initial crystals of zeolite A were 5 lm cubes and the crystals of zeolite X were 1 lm octahedra. It seems likely that a faster dissolution kinetics of the zeolite A compensated for the larger crystal size and allowed both phases to be present up to the end of the synthesis. The morphology of the crystals of zeolite omega evolved during the synthesis. As already observed for syntheses of zeolite omega with slow-release aluminium sources [15,21], the crystals presented a cylindrical habit in the earliest stages of the crystallization and became hexagonal prisms in later stages. The patterns of growth along the axis and the radius of the crystals were significantly different. The evolution of the average length of the crystals during the synthesis is presented in Fig. 2. For all syntheses,
2.5 2.0 L/µm
222
1.5 1.0 0.5
3. Results The final product of each synthesis presented the crystal structure of mazzite. No other crystalline phase was observed in the final X-ray diffractograms. The evolution of the relative fraction of zeolite omega is represented in Fig. 1. The process of dissolution of the parent X and A
0.0 0
20
40
60
80
t/h Fig. 2. Length of the crystals of zeolite omega as a function of synthesis time at 378 K. Batch compositions: OH/Si 0.7 (filled symbols) or 0.8 (void symbols); Al/(Si + Al) 0.105 (circles), 0.116 (squares) or 0.135 (triangles). The lines are a guide for the eye.
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3.0 2.5
L/D
2.0 1.5 1.0 0.5 0.0 0
20
40
60
80
t/h Fig. 4. Aspect ratio (length/width) of the crystals of zeolite omega as a function of synthesis time at 378 K. Batch compositions: OH/Si 0.7 (filled symbols) or 0.8 (void symbols); Al/(Si + Al) 0.105 (circles), 0.116 (squares) or 0.135 (triangles). The lines are a guide for the eye.
13.5 13 pH
the growth curve accelerated in the earliest stages of the synthesis and slowed down in the latest ones, roughly following a sigmoidal trend. The axial growth rate was the lowest for the syntheses at the lowest alkalinity. The final length of the crystals was 1.15–1.30 lm for zeolite omega formed at OH/Si ratio 0.7 and 1.70–2.00 lm for crystals formed at OH/Si 0.8. The evolution of the average width of the crystals with time, reported in Fig. 3, significantly differed from the pattern observed for the length of the crystals. The highest radial growth rate was observed at the beginning of the crystallization. After this initial burst, the radial growth followed a sigmoidal trend similar to the one observed for the axial growth. The width of the crystals remained constant after the morphological transition from cylinders to prisms, albeit the crystals were still growing along their axis. The final width of the crystals strictly depended on the alkalinity ratio and was 0.53–0.59 lm at OH/Si 0.7 and 0.71–0.73 lm at OH/Si 0.8. The evolution of the aspect ratio of the crystals is reported in Fig. 4. The earliest crystals observed were flat cylinders, with the diameter larger than the length along the crystal axis. The initial aspect ratio L/D was about 0.7 for all syntheses. The aspect ratio steadily increased during the synthesis. After the morphological transition from cylinders to prisms, the width of the crystals remained constant and the aspect ratio followed the increase of the crystal length. The evolution of the pH of the synthesis batches is reported in Fig. 5. The inital pH was about 12 for the syntheses at OH/Si 0.7 and between 12.2 and 12.8 for the syntheses at OH/Si 0.8. The pH steadily increased during the crystallization, as normally observed in the formation of zeolites as an amorphous aluminosilicate material is consumed [22]. The final pH values were in the range 12.9–13.3. Final pH values above 13 have already been reported for the synthesis of zeolite omega. The formation of zeolite omega from zeolite X and A is not a congruent process. Zeolite omega is a much more
223
12.5 12 11.5 0
20
40
60
80
t/h Fig. 5. pH of the synthesis solution as a function of synthesis time at 378 K. pH measurement at 298 K. Batch compositions: OH/Si 0.7 (filled symbols) or 0.8 (void symbols); Al/(Si + Al) 0.105 (circles), 0.116 (squares) or 0.135 (triangles). The lines are fittings of the experimental data by the Avrami equation.
silicic zeolite than X or A and SiO4 tetrahedra issued from amorphous silica take part in the crystallization. The evolution of the concentration of silica in the synthesis solution is reported in Fig. 6. The initial concentration of silica was between 1.8 and 2 mol L1. If all the silica of the system had been dissolved,
2.5
[Si] mol L-1
2.0 1.5 1.0 0.5 0.0 Fig. 3. Width of the crystals of zeolite omega as a function of synthesis time at 378 K. Batch compositions: OH/Si 0.7 (filled symbols) or 0.8 (void symbols); Al/(Si + Al) 0.105 (circles), 0.116 (squares) or 0.135 (triangles). The lines are a guide for the eye. A star indicates, for each synthesis, the completion of the transition from cylindrical crystals to hexagonal prism.
0
20
40
60
80
t/h Fig. 6. Concentration of silicon in solution as a function of synthesis time at 378 K. Batch compositions: OH/Si 0.7 (filled symbols) or 0.8 (void symbols); Al/(Si + Al) 0.105 (circles), 0.116 (squares) or 0.135 (triangles). The lines are a guide for the eye.
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the concentration in solution would have been 2.09 mol L1. The fraction of total silicon issued from the reagent zeolites X and A was 14.4%, 16.1%, and 18.5% for Si/(Al + Si) ratios, respectively, 0.105, 0.116, and 0.135. The silica concentration levels are the beginning of the synthesis indicated that virtually all amorphous silica, as well as a fraction of the reagent zeolites, had been dissolved. The concentration of silica decreased as the crystallization proceeded and the final concentrations were in the range 0.8–1.2 mol L1. The evolution of the concentration of aluminate in solution is reported in Fig. 7. The initial concentrations were in the range 13–17 mmol L1. In the case of a complete dissolution of the parent zeolites X and A, the concentration of aluminium would have been 239, 267, 307 mmol L1 for Si/(Al + Si) ratios, respectively, 0.105, 0.116, and 0.135. The initial concentrations of Fig. 7 indicated that about 5% aluminium of the parent zeolites had been dissolved before the beginning of the synthesis. At the beginning of the crystallization, the concentration of aluminium rapidly decreased to values of 2–3 mmol L1. After this sudden fall, the concentration slowly decreased to final values of 0.4–0.75 mmol L1 for the syntheses at OH/Si 0.7 and 0.85–1 mmol L1 for the syntheses at OH/Si 0.8. The final composition of zeolite omega is reported in Fig. 8. The Si/Al ratios cover the range 3.7–4.5 (0.182 < Al/(Si + Al) < 0.213). The aluminium content of the zeolite was much higher than the aluminium content of the synthesis batch, as normally observed for syntheses in alkaline conditions. The aluminium content of the zeolite was roughly proportional to the aluminium content of the synthesis batch, as often observed in zeolites with a disordered aluminium distribution [23]. More aluminium is incorporated in the syntheses at lower alkalinity, in agreement with the lower solubility of silica at less alkaline pH [24–27]. In the samples withdrawn at the beginning of the syntheses, the mass of solid was about 50 mg cm3 of crystallization bath, marginally lower than the mass of initial zeolite X and A. At the end of the syntheses, the mass of solid was
[Al] mmol L-1
20 15 10 5 0 0
20
40
60
80
t/h Fig. 7. Concentration of aluminium in solution as a function of synthesis time at 378 K. Batch compositions: OH/Si 0.7 (filled symbols) or 0.8 (void symbols); Al/(Si + Al) 0.105 (circles), 0.116 (squares) or 0.135 (triangles). The lines are a guide for the eye.
0.25
0.20 Al/(Si+Al) MAZ
224
0.15
0.10
0.05
0.00 0.00
0.05
0.10
0.15
0.20
0.25
Al/(Si+Al) initial Fig. 8. Aluminium content of zeolite omega as a function of the aluminium fraction of the synthesis batch. OH/Si 0.7 (filled triangles) or 0.8 (void triangles).
between 80 and 110 mg cm3. This range of mass variation is in good agreement with the ratio between the molar masses of faujasite and zeolite omega if the aluminium balance is respected. The molar mass of a hydrated faujasite with Si/Al ratio 1.26 (Al/(Si + Al) = 0.442) is 212 Daltons per Al atom, while the molar masses of zeolites omega with Si/Al ratios 3.7 and 4.5 (Al/(Si + Al), respectively, 0.213 and 0.182) are, respectively, 430 and 370 Daltons per Al atom.
4. Discussion TMA remains the most specific template for the synthesis of zeolites of the mazzite structural type, albeit other templates have been successfully used [28–30]. It has been suggested that at least 1 TMA ion per 20 Si + Al is needed for the formation of zeolite omega [12]. The crystallization experiments related in this article show that the use of a low-cost commercial zeolite as slow-release source of aluminium allows to form zeolite omega in a reproducible way from synthesis batches with 1 TMA per 24 Si + Al. The unusual feature of this kind of synthesis is that the main source of silica is not present as a solid phase. Albeit some silica can be lost in the filtration as silica colloids and analyzed in the liquid phase, at the pH of the synthesis silica is expected to be mainly present as oligomeric and monomeric silicates [25]. At the beginning of most syntheses of zeolites, the high concentration of aluminate—a very effective condensating agent for silica [31]—induces the formation of an aluminosilicate gel. The use of a slow-release source of aluminium decreases the aluminate concentration and prevents the formation of a precursor gel. The absence of an intermediate reservoir of silica is expected to affect the kinetics of crystallization.
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Experimental S-shaped crystallization curves can be described by the Avrami–Kholmogorov equation a ¼ 1 expðKtn Þ
ð1Þ
in which a is the fraction of crystallized zeolite. The equation, originally developed for the solidification of melts [32–34] and also applied to solid state reactions [35], has been early applied to the kinetics of zeolite crystallization [36–39]. The value of the exponent n provides some information on the pattern of nucleation. Values lower than 4 correspond to a decreasing nucleation rate after an initial burst, while values higher than 4 corresponds to a regime of autocatalytic nucleation, commonly observed for zeolite crystallization from gels, in which the nucleation rate increases after an induction lag [39,40]. Best fitting Avrami curves for the evolution of the fraction of zeolite omega are reported in Fig. 1. The agreement with experiment was reasonable in the first part of the syntheses, while the Avrami model underevaluated the crystal fraction during the termination of the crystallization. The exponents of the Avrami equations are reported in Table 1. The exponent n is about 3 for the syntheses at OH/Si 0.7 and about 1.5 for the syntheses at OH/Si 0.8. The higher exponent n for the syntheses at lower alkalinity corresponds to the longer induction time observed in the crystallization curves of Fig. 1 [41]. Anyway, the exponent of the Avrami equation is lower than 4 for all syntheses. This indicates that the nucleation is concentrated in the very first stages of the crystallization. The aluminate measured in solution is in the presence of a strong excess of silicate and is probably always bonded with silica in aluminosilicate species. The very high excess of silicate—the Si/Al ratio in solution is never lower than 60—suggests that most silicate species are not bound to aluminate tetrahedra. The nucleation of zeolite omega takes place at the relatively high aluminate concentration of about 15 mmol L1. It is likely that this value includes some aluminosilicate colloids retained in the liquid phase during the filtration. Indeed, no zeolite omega crystals smaller than 0.2 lm are observed. The earliest and smallest crystals are probably lost in the filtration. The nucleation of zeolite omega brings the aluminate concentration down to less than 3 mmol L1. The insuring growth of the crystals of zeolite omega is fed by the dissolution of the parent zeolite X and A. The concentration of aluminate in solution depends on the relative rates of the Table 1 Exponent n of the Avrami equation as a function of the synthesis conditions OH/Si
Al/(Si + Al)
Avrami exponent n
0.7 0.7 0.7 0.8 0.8 0.8
0.105 0.116 0.131 0.105 0.116 0.131
3.58 3.66 2.44 1.58 1.36 1.88
225
dissolution and growth processes and decreases with the decrease of the surface of dissolution of the parent zeolites and the increase of the surface of growth of zeolite omega. The excess silicate in solution allows zeolite omega to be formed from the aluminium-richer parent zeolites. The earliest crystals of zeolite omega present the characteristic shape of flat cylinders, so-called aspirin tablets. The radial surface of the cylinders is quite peculiar, as it cannot be described as a crystallographic face and corresponds to the envelope of the (h k 0) faces. As the aluminate concentration decreases, the radial growth rate decreases until the (h k 0) surface is superseded by (1 0 0) faces. Further growth of the (1 0 0) faces is so slow that no further increase of the width of the crystals is observed after their formation. No data on the distribution of aluminium inside the crystals are available and a change of the Si/Al ratio with the variation of the concentration of aluminate in solution cannot be ruled out. Slender crystals at lower aluminium content were also observed in the synthesis of zeolite omega in the presence of TMA and diaminohexane [30]. The role of the concentration of aluminium is also underlined by the comparison of the compositions of the synthesis batches and of the final zeolites. The composition of the synthesis batches normalized on the amount of aluminium is in the range 5.65–7.8 Na/0.33–0.41 TMA/Al/ 6.65–8.5 Si, while the composition of the final products is 0.69–0.75 Na/0.22–0.28 TMA/Al/3.69–4.58 Si. The ratios between each component and aluminium are always lower in zeolite omega than in the synthesis batch. This indicates that excess silicate, Na+ and TMA+ cations are left in solution at the end of the crystallization. Aluminate seems an excellent candidate as growth limiting species. In principle, the rate of growth of the face of a crystal (dL/dt) usually depends on the concentration of the limiting species by an equation n
dL=dt ¼ Kð½Al ½Aleq Þ ;
ð2Þ
in which [Al] [Al]eq indicates the supersaturation of the solution and the exponent n varies between 1 and 2 according to the mechanism of growth [42,43]. The final concentration of aluminium in each synthesis has been assumed as equilibrium concentration [Al]eq. The attempts to correlate the crystallization kinetics of zeolites to the concentrations in solution have especially dealt with the problems related to the equilibria between silicate species in the system [44–46] or to the evaluation of the crystal size distribution by population balance models [47,48]. The extremely low Avrami exponents for the crystallization curves of this paper indicate that no further nucleation takes place after an initial burst. In this case, it seems reasonable to consider the crystal population as monodispersed and apply Eq. (2) to the average crystal size. In Fig. 9, the rate of growth of the crystals of zeolite omega in axial and radial direction are reported as a function of the aluminate supersaturation in solution. The rates of growth (dL/dt) are measured as half the size variation of
226
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0.03 dLr /dt (µm h-1)
dLa/dt (µm h-1)
B
A
0.03
0.02
0.01
0.02
0.01
0
0 0
1 0.5 [Al]-[Al]eq (mmol L-1)
0
1.5
1 0.5 [Al]-[Al]eq (mmol L-1)
1.5
Fig. 9. Rates of growth of the crystals of zeolite omega at 378 K in axial (A) and radial (B) direction as a function of the aluminate supersaturation. Batch compositions: OH/Si 0.7 (filled symbols) or 0.8 (void symbols); Al/(Si + Al) 0.105 (circles), 0.116 (squares) or 0.135 (triangles).
1.2
NaAlSi1:25 O4:5 + 0.25TMAþ + 2.75Si(OH)3 O = Na0:75 TMA0:25 AlSi4 O10 + 0.25Naþ + 2.75H2 O + 2.75OH ð3Þ
The real variation of pH would depend on the degree of ionization of the silicate species in solution and the buffer effect of the silicate species not involved in the crystallization. The rise of the concentration of hydroxyl species should correspond to a rise of the rate of silicate condensation. In this way, the rise of alkalinity during the synthesis should increase the rate of growth of the zeolite and partially counteract the decrease of the rate of growth due to the decrease of aluminate concentration. The effect of alkalinity can be taken into account by defining a new rate constant K 0 = K/[OH] and explicitly introducing the concentration of the hydroxyl catalyst into the growth rate equation n
dL=dt ¼ K 0 ½OH ð½Al ½Aleq Þ .
A 1 0.8 0.6 0.4 0.2 0
ð4Þ
In Fig. 10, the rates of growth corrected by the hydroxyl concentrations (dL/dt)/[OH] are reported as a function of the aluminate supersaturation. The introduction of alkalinity in the rate equation significantly reduces the scattering
(dLr /dt)/[OH] (µm h-1 mol-1 L)
(dLa/dt)/[OH] (µ mh-1 mol-1 L)
the crystals between two experimental points and the concentration of aluminium is the average value between the concentrations at the corresponding experimental points. The first experimental points of each crystallization curve, corresponding to the appearance of zeolite omega, are not included in this representation. No significant measurement of the rate of growth for these points is possible, due to the incertitude on the lower limit of crystal size. Moreover, the observed sudden fall of Al concentration prevents any evaluation of a significant average value. The rates of growth presented in Fig. 9 can only be defined as clouds of random-scattered points. It seems clear that the aluminate concentration alone cannot account for the rates of growth of the crystals of zeolite omega. Another parameter to be taken into account is the pH of the crystallization system. The alkalinity is a well-known accelerating factor in zeolite crystallization [49–51]. The OH groups catalyze the nucleophilic attack of silicate species to silica tetrahedra which represents the slow step of silicate condensation [42,52]. In most zeolite syntheses, the pH increases during the crystallization, as amorphous or dissolved silicate species are depleted [22]. The charge balance of the crystallization of zeolite omega from zeolite X can ideally be schematized as
1.2 B 1 0.8 0.6 0.4 0.2 0
0
0.5 1 [Al]-[Al]eq (mmol L-1)
1.5
0
0.5 1 [Al]-[Al]eq (mmol L-1)
1.5
Fig. 10. Rates of growth of the crystals of zeolite omega at 378 K corrected by the alkalinity as a function of the aluminate supersaturation. Axial (A) and radial (B) growth. Batch compositions: OH/Si 0.7 (filled symbols) or 0.8 (void symbols); Al/(Si + Al) 0.105 (circles), 0.116 (squares) or 0.135 (triangles).
M.L. Guzman Castillo et al. / Microporous and Mesoporous Materials 90 (2006) 221–228
of the experimental points. The alkalinity-corrected rates are proportional to the aluminate supersaturation, in the case of both axial and radial growth. If the data of Fig. 10 are best-fitted by power laws, the following rate equations are obtained: dLa =dt ¼ 0:74½OH ð½Al ½Aleq Þ1:07 ;
ð5Þ
dLr =dt ¼ 0:22½OH ð½Al ½Aleq Þ1:08 ;
ð6Þ
respectively, for the axial and radial growth. The mean square errors on the exponents of the rate laws are 0.10 for the axial growth and 0.13 for the radial growth. As a consequence, both rate laws scantily differ from a linear correlation. The rate constant of the axial growth is more than three times the rate constant of the radial growth. These rate laws do not represent the initial stages of the crystallization, for which the quality of the available data is too poor to establish any quantitative correlation. The fast radial growth of the primitive aspirin-tablet crystals has still to be explained. The formation of flat crystals at the beginning of zeolite syntheses has been observed in extremely different systems. The phenomenon can be justified by a mechanism of colloid aggregation in the case of the formation of slab crystals of silicalite from organic-rich sols [53] but primitive flat crystals have also been observed in the crystallization of mordenite from thick gels [54]. For aluminate concentrations lower than 2 mmol L1, Eqs. (5) and (6) account for the growth rates. The interplay between rise of pH and decrease of aluminate concentration can justify the curious sigmoidal trend observed for both axial and radial growth. The disappearance of the cylindrical (h k 0) surfaces clearly correspond to their superseding by slower-growing (1 0 0) faces. It can be assumed that the (1 0 0) growth rate depends on aluminate concentration by an order significantly higher than 1, while the (h k 0) growth rate depends nearly linearly on aluminate concentration. In this case, the (h k 0) growth can be slower than the (1 0 0) growth above a given threshold of aluminate concentration. The (1 0 0) faces will appear below this concentration threshold, when their growth will be slower than the (h k 0) growth. 5. Conclusions The use of a sparingly soluble source of aluminate allows to perform nucleation and growth of zeolite omega in an aluminium-starved system. In these conditions, the usual formation of an intermediate aluminosilicate gel is prevented. This allows to avoid the induction period usually connected to the ripening of the gel and described by autocatalytic kinetics. In the high-pH and low-[Al] environment of these syntheses, the synthesis sol is depolymerized enough to be treated as a true solution. This allows to establish significant correlations between the experimental rates of growth of zeolite omega and the concentrations in solution. The concentration of aluminium, probably present as aluminosili-
227
cate species, is a rate-limiting factor. However, its influence on the growth rate cannot be evidenced if the catalytic effect of hydroxyl anions on silicate condensation is not explicitly taken into account. When both aluminate supersaturation and hydroxyl concentration are taken into account, the growth rates of the crystals of zeolite oemga in both axial and radial direction can be successfully modelled. The control of the length of the crystals, the most significant parameter for diffusion in a catalytic system, depends on the interplay between nucleation and growth. The assessment of the nucleation process is outside the field of experimental conditions of this study. However, nucleation corresponds to a burst of radial growth, possibly taking place by aggregation of nanosized precursors. The initial useful aspirin-tablet morphology has to be preserved by reducing the further lengthening of the crystals. Slow syntheses at low alkalinity are quite successful in producing stable crystals slightly longer than 1 lm. References [1] R. Shigeishi, B. Chiche, F. Fajula, Micropor. Mesopor. Mater. 43 (2001) 211. [2] D. McQueen, B. Chiche, F. Fajula, A. Auroux, F. Guimon, P. Schulz, J. Catal. 161 (1996) 587. [3] R. Aiello, R.M. Barrer, J. Chem. Soc. A (1970) 1470. [4] E. Bowes, J.J. Wise, US Pat. 3,578,723, 1971. [5] E. Flanigen, Neth. Pat. 6,710,729, 1968. [6] E. Galli, Cryst. Struct. Commun. 3 (1974) 339. [7] A. Martucci, A. Alberti, M.L. Guzman-Castillo, F. Di Renzo, F. Fajula, Micropor. Mesopor. Mater. 63 (2003) 33. [8] F. Fajula, M. Vera-Pacheco, F. Figueras, Zeolites 7 (1987) 203. [9] F.A. Dwyer, US Pat. 3,642,434, 1972. [10] F.G. Dwyer, P. Chu, J. Catal. 59 (1979) 263. [11] A.J. Perrotta, C. Kirby, B.R. Mitchell, E.R. Tucci, J. Catal. 55 (1978) 240. [12] H. Lechert, H. Weyda, in: L. Occelli, H. Robson (Eds.), Synthesis of Microporous Materials, vol. I, Van Nostrand Reinhold, New York, 1992, p. 77. [13] C.J. Plank, E.J. Rosinski, M.K. Rubin, British Pat. 1,297,256, 1972. [14] J.F. Cole, H.W. Kouwenhoven, Adv. Chem. Ser. 121 (1973) 583. [15] F. Di Renzo, F. Fajula, F. Figueras, S. Nicolas, T. Des Courie`res, Stud. Surf. Sci. Catal. 49 (1989) 119. [16] F. Fajula, S. Nicolas, F. Di Renzo, C. Gue´guen, F. Figueras, in: M.L. Occelli, H.E. Robson (Eds.), Zeolite Synthesis, ACS Symp. Ser. 398 (1989) 493. [17] F. Di Renzo, F. Fajula, F. Figueras, T. Des Courie`res, US Pat. 5,165,906, 1992. [18] A. Araya, T.J. Barber, B.M. Lowe, D.M. Sinclair, A. Varma, Zeolites 4 (1984) 263. [19] P. Massiani, F. Fajula, D. Di Renzo, F. Remoue´, F. Figueras, Stud. Surf. Sci. Catal. 52 (1989) 215. [20] A. Maubert, R. Dutartre, L.C. de Menorval, F. Figueras, Zeolites 13 (1993) 587. [21] J.L. Tallon, R.G. Buckley, J. Phys. Chem. 91 (1987) 1469. [22] J.L. Casci, B.M. Lowe, Zeolites 3 (1983) 186. [23] F. Hamidi, A. Bengueddach, F. Di Renzo, F. Fajula, Catal. Lett. 87 (2003) 149. [24] S.P. Zhdanov, in: Molecular Sieves, Society of Chemical Industry, London, 1968, p. 62. [25] P. Caullet, J.L. Guth, in: M.L. Occelli, H.E. Robson (Eds.), Zeolite Synthesis, ACS Symp. Ser. 398 (1989) 83. [26] H. Lechert, H. Kacirek, Zeolites 11 (1991) 720.
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[27] M. Pamba, G. Maurin, S. Devautour, J. Vanderschueren, J.C. Giuntini, F. Di Renzo, F. Hamidi, Phys. Chem. Chem. Phys. 2 (2000) 2027. [28] S. Yang, N.P. Evmiridis, Stud. Surf. Sci. Catal. 84 (1994) 155. [29] B. De Witte, J. Patarin, J.L. Guth, T. Cholley, Micropor. Mater. 10 (1997) 247. [30] A.M. Goossens, E.J.P. Feijen, G. Verhoeven, B.H. Wouters, P.J. Grobet, P.A. Jacobs, J.A. Martens, Micropor. Mesopor. Mater. 35 (2000) 555. [31] R.K. Iler, The Chemistry of Silica, Wiley-Interscience, New York, 1979. [32] A.N. Kholmogorov, Izv. Akad. Nauk SSSR, Ser. Math. 3 (1937) 353. [33] M. Avrami, J. Chem. Phys. 7 (1939) 1103. [34] M. Avrami, J. Chem. Phys. 8 (1940) 212. [35] J. Sestak, G. Berggren, Thermochim. Acta 2 (1971) 42. [36] J. Ciric, Coll. Interf. Sci. 28 (1968) 315. [37] S.P. Zhdanov, in: Molecular Sieve Zeolites, Adv. Chem. Ser. 101 (1971) 20. [38] S.P. Zhdanov, N.N. Samulevich, in: L.V.C. Rees (Ed.), Proceedings of the 5th International Conference on Zeolites, Heyden, London, 1980, p. 75. [39] B. Subotic, in: M.L. Occelli, H.E. Robson (Eds.), Zeolite Synthesis, ACS Symp. Ser. 398 (1989) 110. [40] B. Subotic, A. Graovac, Stud. Surf. Sci. Catal. 24 (1985) 199.
[41] F. Di Renzo, F. Remoue´, P. Massiani, F. Fajula, F. Figueras, T. Des Courie`res, Zeolites 11 (1991) 539. [42] J.L. Guth, P. Caullet, J. Chim. Phys. 83 (1986) 155. [43] J.W. Mullin, Crystallization, third ed., Butterworth-Heinemann, London, 1993. [44] B.M. Lowe, Zeolites 3 (1983) 300. [45] C.S. Cundy, M.S. Henty, R.J. Plaisted, Zeolites 15 (1995) 342. [46] C.S. Cundy, P.A. Cox, Chem. Rev. 103 (2003) 663. [47] R.W. Thompson, A. Dyer, Zeolites 5 (1985) 202, 292, 302. [48] A.G. Dixon, R.W. Thompson, Zeolites 6 (1986) 154. [49] R. Mostowicz, J.M. Berak, Stud. Surf. Sci. Catal. 24 (1985) 65. [50] J. Kornatowski, G. Finger, W. Schmitz, Pol. J. Chem. 61 (1987) 155. [51] F. Di Renzo, Catal. Today 41 (1998) 37. [52] C.J. Brinker, W.G. Scherer, Sol–Gel Science, Academic Press, New York, 1990. [53] R. Ravishankar, C.E.A. Kirschhock, P.P. Knops-Gerrits, E.J.P. Feijen, P.J. Grobet, P. Vanoppen, F.C. De Schryver, G. Miehe, H. Fuess, B.J. Schoeman, P.A. Jacobs, J.A. Martens, J. Phys. Chem. B 103 (1999) 4960. [54] F. Hamidi, R. Dutartre, F. Di Renzo, A. Bengueddach, F. Fajula, in: M.M.J. Treacy, B.K. Marcus, M.E. Bisher, J.B. Higgins (Eds.), Proceedings of the 12th International Zeolite Conference, vol. 3, Materials Research Society, Pittsburgh, 1999, p. 1803.
Crystallization kinetics of zeolite omega, the synthetic analog of mazzite M.L. Guzman Castillo a
a,1
, F. Di Renzo
a,*
, F. Fajula a, J. Bousquet
b
Laboratoire de Mate´riaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS-ENSCM-UM1, Institut C. Gerhardt, FR 1878, ENSCM, 8 rue Ecole Normale, 34296 Montpellier, France b Total SA, Direction Scientifique, BP 22, 69360 Solaize, France Received 29 August 2005; received in revised form 11 October 2005; accepted 13 October 2005 Available online 15 December 2005
Dedicated to the late Denise Barthomeuf, George Kokotailo and Sergey P. Zhdanov in appreciation of their outstanding contributions to zeolite science
Abstract The synthesis of zeolite omega, the synthetic analog of mazzite, was carried out by using commercial reagents and a low amount of tetramethylammonium template. The use of a slow-release source of aluminium allowed to avoid the formation of a precursor gel and to concentrate the nucleation at the beginning of the synthesis. The rates of crystal growth in radial and axial direction depends on the alkalinity and the aluminium concentration. The changes of the aspect ratio and morphology of the crystals can be accounted for by the relative growth rates of different crystal faces. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Zeolite crystallization; Growth kinetics; Growth rates; Mazzite; Zeolite omega; Crystal size; Aspect ratio
1. Introduction The zeolite omega or ZSM-4, the synthetic analog of mazzite, is known for its strong Brønsted acidity and its activity as a catalyst of alkylation and isomerization [1,2]. It is an example of zeolite with intermediate aluminium content (usual Si/Al between 3 and 5) and was originally synthesized in the presence of sodium and tetramethylammonium (TMA) cations. When, in 1970, Aiello and Barrer revealed the synthesis of zeolite omega [3], patents for the very similar syntheses of ZSM-4 [4] and zeolite omega [5] had already been filed by, respectively, Mobil and Union Carbide. The row on industrial property which followed was settled when mazzite, a rare natural zeolite discovered by Ermanno Galli, was shown to present the same structure of zeolite omega [6,7]. The original synthesis of zeolite omega was plagued by phase selectivity problems [8]. Several zeolites with smaller *
1
Corresponding author. Tel.: +33 467163479; fax: +33 467163470. E-mail address: [email protected] (F. Di Renzo). Present address: Istituto Mexicano del Petroleo, Mexico, DF, Mexico.
1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.10.022
pores and higher aluminium content, like phillipsite, gismondine, and analcime may easily contaminate the mazzite phase. Zeolite Y is formed in the earliest steps of the syntheses with a low TMA content and is metastable towards zeolite omega [9–11]. A high TMA content was shown to speed up the formation of zeolite omega, but non-porous sodalite was also formed in these conditions [12]. The use of slow-release sources of aluminium, viz. zeolite Y [13] or silica–alumina [14], was found to improve the selectivity of the synthesis. The solubility of the source of aluminium was shown to affect the morphology of zeolite omega, spherical grains being formed from highly soluble sources of aluminium and elongated hexagonal prisms from slow-release sources [15]. The importance of the concentration of aluminiumbearing species in the synthesis solution was confirmed by an experiment of continuous crystallization, in which an aluminate solution was fed to the crystallization system and larger and flatter crystals were obtained [16]. The significance of the aspect ratio in the case of zeolite omega is not limited to the influence of the length of the crystals on the diffusion properties in a phase with
M.L. Guzman Castillo et al. / Microporous and Mesoporous Materials 90 (2006) 221–228
2. Experimental The reagents used in the syntheses were precipitated silica Zeosil 175MP by Rhoˆne-Poulenc (Na 0.7%, Al 0.17%, H2O 13.8% (w/w)), Siliporite by CECA (from an industrial batch of zeolite X with 20% zeolite A, Si/Al 1.26, corresponding to a molar fraction 0.443 Al/(Si + Al)), tetramethylammonium (TMA) bromide by Aldrich, sodium hydroxide by Merck, deionized water. The compositions of the synthesis batches were in the range 0.41–0.48 Na2O/0.024 TMA2O/0.059–0.075 Al2O3/SiO2/26.6 H2O, corresponding to three levels of Al/(Si + Al) ratio (0.105, 0.116, and 0.135) and two levels of alkalinity ratio (0.7 and 0.8). The alkalinity ratio was evaluated from the molar composition as OH/Si = (Na + TMA–Al–Br)/Si. The syntheses have been carried out at 378 K in a 900 cm3 stirred stainless-steel autoclave equipped for the on-line sampling of the crystallization medium. The solid and liquid fractions of samples withdrawn at different times of the synthesis were separated by filtration. The solid phase was washed with water and characterized by X-ray diffraction (CGR Theˆta 60 diffractometer, Cu Ka radiation) and scanning electron microscopy (Cambridge Stereoscan 260 apparatus). The relative fraction of zeolite omega XMAZ = MAZ/(MAZ + FAU + LTA) was evaluated from the intensity ratios of selected lines of the X-ray diffractograms. The size of the crystals was averaged on 30–60 measurements on micrographs. The pH of the solutions was measured at 298 K. The solutions were diluted for storage after filtration and analyzed for elemental composition at the Service Central dÕAnalyse of the CNRS in Solaize.
1 0.8 X (MAZ)
monodimensional porosity [17]. It has been shown that the thermal stability of zeolite omega depends on the crystal morphology, being higher for hexagonal prisms than for spherical or cylindrical crystals [18–20]. This phenomenon suggests the thermal degradation of zeolite omega to be initiated on the surface defects of the non-flat faces, with a mechanism quite different from the bulk nucleation of the decomposed phase observed in MFI [21]. The crystallization kinetics of zeolite omega has deserved some interest in the literature some years ago [12,15,16]. The purpose of this paper is to show that it is possible to improve both the understanding of the crystallization process and the synthesis procedure of a specific zeolite by operating in conditions near to industrial standards, as is the case when commercial reagents and bench-scale stirred autoclaves are used.
0.6 0.4 0.2 0 0
20
40 t/h
60
80
Fig. 1. Crystalline fraction of zeolite omega as a function of synthesis time at 378 K. Batch compositions: OH/Si 0.7 (filled symbols) or 0.8 (void symbols); Al/(Si + Al) 0.105 (circles), 0.116 (squares) or 0.135 (triangles). The lines are fittings of the experimental data by the Avrami equation.
zeolites and crystallization of zeolite omega was completed after nearly 30 h for the syntheses at the highest alkalinity (OH/Si 0.8), which presented virtually no induction time. The syntheses at the lowest alkalinity (OH/Si 0.7) presented a significant induction time. The Al fraction affected in a significant way the kinetics of the syntheses, the Al-richest syntheses being completed in 42 h and the Al-poorest one in 82 h. The ratio FAU/LTA did not change during the synthesis. The initial crystals of zeolite A were 5 lm cubes and the crystals of zeolite X were 1 lm octahedra. It seems likely that a faster dissolution kinetics of the zeolite A compensated for the larger crystal size and allowed both phases to be present up to the end of the synthesis. The morphology of the crystals of zeolite omega evolved during the synthesis. As already observed for syntheses of zeolite omega with slow-release aluminium sources [15,21], the crystals presented a cylindrical habit in the earliest stages of the crystallization and became hexagonal prisms in later stages. The patterns of growth along the axis and the radius of the crystals were significantly different. The evolution of the average length of the crystals during the synthesis is presented in Fig. 2. For all syntheses,
2.5 2.0 L/µm
222
1.5 1.0 0.5
3. Results The final product of each synthesis presented the crystal structure of mazzite. No other crystalline phase was observed in the final X-ray diffractograms. The evolution of the relative fraction of zeolite omega is represented in Fig. 1. The process of dissolution of the parent X and A
0.0 0
20
40
60
80
t/h Fig. 2. Length of the crystals of zeolite omega as a function of synthesis time at 378 K. Batch compositions: OH/Si 0.7 (filled symbols) or 0.8 (void symbols); Al/(Si + Al) 0.105 (circles), 0.116 (squares) or 0.135 (triangles). The lines are a guide for the eye.
M.L. Guzman Castillo et al. / Microporous and Mesoporous Materials 90 (2006) 221–228
3.0 2.5
L/D
2.0 1.5 1.0 0.5 0.0 0
20
40
60
80
t/h Fig. 4. Aspect ratio (length/width) of the crystals of zeolite omega as a function of synthesis time at 378 K. Batch compositions: OH/Si 0.7 (filled symbols) or 0.8 (void symbols); Al/(Si + Al) 0.105 (circles), 0.116 (squares) or 0.135 (triangles). The lines are a guide for the eye.
13.5 13 pH
the growth curve accelerated in the earliest stages of the synthesis and slowed down in the latest ones, roughly following a sigmoidal trend. The axial growth rate was the lowest for the syntheses at the lowest alkalinity. The final length of the crystals was 1.15–1.30 lm for zeolite omega formed at OH/Si ratio 0.7 and 1.70–2.00 lm for crystals formed at OH/Si 0.8. The evolution of the average width of the crystals with time, reported in Fig. 3, significantly differed from the pattern observed for the length of the crystals. The highest radial growth rate was observed at the beginning of the crystallization. After this initial burst, the radial growth followed a sigmoidal trend similar to the one observed for the axial growth. The width of the crystals remained constant after the morphological transition from cylinders to prisms, albeit the crystals were still growing along their axis. The final width of the crystals strictly depended on the alkalinity ratio and was 0.53–0.59 lm at OH/Si 0.7 and 0.71–0.73 lm at OH/Si 0.8. The evolution of the aspect ratio of the crystals is reported in Fig. 4. The earliest crystals observed were flat cylinders, with the diameter larger than the length along the crystal axis. The initial aspect ratio L/D was about 0.7 for all syntheses. The aspect ratio steadily increased during the synthesis. After the morphological transition from cylinders to prisms, the width of the crystals remained constant and the aspect ratio followed the increase of the crystal length. The evolution of the pH of the synthesis batches is reported in Fig. 5. The inital pH was about 12 for the syntheses at OH/Si 0.7 and between 12.2 and 12.8 for the syntheses at OH/Si 0.8. The pH steadily increased during the crystallization, as normally observed in the formation of zeolites as an amorphous aluminosilicate material is consumed [22]. The final pH values were in the range 12.9–13.3. Final pH values above 13 have already been reported for the synthesis of zeolite omega. The formation of zeolite omega from zeolite X and A is not a congruent process. Zeolite omega is a much more
223
12.5 12 11.5 0
20
40
60
80
t/h Fig. 5. pH of the synthesis solution as a function of synthesis time at 378 K. pH measurement at 298 K. Batch compositions: OH/Si 0.7 (filled symbols) or 0.8 (void symbols); Al/(Si + Al) 0.105 (circles), 0.116 (squares) or 0.135 (triangles). The lines are fittings of the experimental data by the Avrami equation.
silicic zeolite than X or A and SiO4 tetrahedra issued from amorphous silica take part in the crystallization. The evolution of the concentration of silica in the synthesis solution is reported in Fig. 6. The initial concentration of silica was between 1.8 and 2 mol L1. If all the silica of the system had been dissolved,
2.5
[Si] mol L-1
2.0 1.5 1.0 0.5 0.0 Fig. 3. Width of the crystals of zeolite omega as a function of synthesis time at 378 K. Batch compositions: OH/Si 0.7 (filled symbols) or 0.8 (void symbols); Al/(Si + Al) 0.105 (circles), 0.116 (squares) or 0.135 (triangles). The lines are a guide for the eye. A star indicates, for each synthesis, the completion of the transition from cylindrical crystals to hexagonal prism.
0
20
40
60
80
t/h Fig. 6. Concentration of silicon in solution as a function of synthesis time at 378 K. Batch compositions: OH/Si 0.7 (filled symbols) or 0.8 (void symbols); Al/(Si + Al) 0.105 (circles), 0.116 (squares) or 0.135 (triangles). The lines are a guide for the eye.
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the concentration in solution would have been 2.09 mol L1. The fraction of total silicon issued from the reagent zeolites X and A was 14.4%, 16.1%, and 18.5% for Si/(Al + Si) ratios, respectively, 0.105, 0.116, and 0.135. The silica concentration levels are the beginning of the synthesis indicated that virtually all amorphous silica, as well as a fraction of the reagent zeolites, had been dissolved. The concentration of silica decreased as the crystallization proceeded and the final concentrations were in the range 0.8–1.2 mol L1. The evolution of the concentration of aluminate in solution is reported in Fig. 7. The initial concentrations were in the range 13–17 mmol L1. In the case of a complete dissolution of the parent zeolites X and A, the concentration of aluminium would have been 239, 267, 307 mmol L1 for Si/(Al + Si) ratios, respectively, 0.105, 0.116, and 0.135. The initial concentrations of Fig. 7 indicated that about 5% aluminium of the parent zeolites had been dissolved before the beginning of the synthesis. At the beginning of the crystallization, the concentration of aluminium rapidly decreased to values of 2–3 mmol L1. After this sudden fall, the concentration slowly decreased to final values of 0.4–0.75 mmol L1 for the syntheses at OH/Si 0.7 and 0.85–1 mmol L1 for the syntheses at OH/Si 0.8. The final composition of zeolite omega is reported in Fig. 8. The Si/Al ratios cover the range 3.7–4.5 (0.182 < Al/(Si + Al) < 0.213). The aluminium content of the zeolite was much higher than the aluminium content of the synthesis batch, as normally observed for syntheses in alkaline conditions. The aluminium content of the zeolite was roughly proportional to the aluminium content of the synthesis batch, as often observed in zeolites with a disordered aluminium distribution [23]. More aluminium is incorporated in the syntheses at lower alkalinity, in agreement with the lower solubility of silica at less alkaline pH [24–27]. In the samples withdrawn at the beginning of the syntheses, the mass of solid was about 50 mg cm3 of crystallization bath, marginally lower than the mass of initial zeolite X and A. At the end of the syntheses, the mass of solid was
[Al] mmol L-1
20 15 10 5 0 0
20
40
60
80
t/h Fig. 7. Concentration of aluminium in solution as a function of synthesis time at 378 K. Batch compositions: OH/Si 0.7 (filled symbols) or 0.8 (void symbols); Al/(Si + Al) 0.105 (circles), 0.116 (squares) or 0.135 (triangles). The lines are a guide for the eye.
0.25
0.20 Al/(Si+Al) MAZ
224
0.15
0.10
0.05
0.00 0.00
0.05
0.10
0.15
0.20
0.25
Al/(Si+Al) initial Fig. 8. Aluminium content of zeolite omega as a function of the aluminium fraction of the synthesis batch. OH/Si 0.7 (filled triangles) or 0.8 (void triangles).
between 80 and 110 mg cm3. This range of mass variation is in good agreement with the ratio between the molar masses of faujasite and zeolite omega if the aluminium balance is respected. The molar mass of a hydrated faujasite with Si/Al ratio 1.26 (Al/(Si + Al) = 0.442) is 212 Daltons per Al atom, while the molar masses of zeolites omega with Si/Al ratios 3.7 and 4.5 (Al/(Si + Al), respectively, 0.213 and 0.182) are, respectively, 430 and 370 Daltons per Al atom.
4. Discussion TMA remains the most specific template for the synthesis of zeolites of the mazzite structural type, albeit other templates have been successfully used [28–30]. It has been suggested that at least 1 TMA ion per 20 Si + Al is needed for the formation of zeolite omega [12]. The crystallization experiments related in this article show that the use of a low-cost commercial zeolite as slow-release source of aluminium allows to form zeolite omega in a reproducible way from synthesis batches with 1 TMA per 24 Si + Al. The unusual feature of this kind of synthesis is that the main source of silica is not present as a solid phase. Albeit some silica can be lost in the filtration as silica colloids and analyzed in the liquid phase, at the pH of the synthesis silica is expected to be mainly present as oligomeric and monomeric silicates [25]. At the beginning of most syntheses of zeolites, the high concentration of aluminate—a very effective condensating agent for silica [31]—induces the formation of an aluminosilicate gel. The use of a slow-release source of aluminium decreases the aluminate concentration and prevents the formation of a precursor gel. The absence of an intermediate reservoir of silica is expected to affect the kinetics of crystallization.
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Experimental S-shaped crystallization curves can be described by the Avrami–Kholmogorov equation a ¼ 1 expðKtn Þ
ð1Þ
in which a is the fraction of crystallized zeolite. The equation, originally developed for the solidification of melts [32–34] and also applied to solid state reactions [35], has been early applied to the kinetics of zeolite crystallization [36–39]. The value of the exponent n provides some information on the pattern of nucleation. Values lower than 4 correspond to a decreasing nucleation rate after an initial burst, while values higher than 4 corresponds to a regime of autocatalytic nucleation, commonly observed for zeolite crystallization from gels, in which the nucleation rate increases after an induction lag [39,40]. Best fitting Avrami curves for the evolution of the fraction of zeolite omega are reported in Fig. 1. The agreement with experiment was reasonable in the first part of the syntheses, while the Avrami model underevaluated the crystal fraction during the termination of the crystallization. The exponents of the Avrami equations are reported in Table 1. The exponent n is about 3 for the syntheses at OH/Si 0.7 and about 1.5 for the syntheses at OH/Si 0.8. The higher exponent n for the syntheses at lower alkalinity corresponds to the longer induction time observed in the crystallization curves of Fig. 1 [41]. Anyway, the exponent of the Avrami equation is lower than 4 for all syntheses. This indicates that the nucleation is concentrated in the very first stages of the crystallization. The aluminate measured in solution is in the presence of a strong excess of silicate and is probably always bonded with silica in aluminosilicate species. The very high excess of silicate—the Si/Al ratio in solution is never lower than 60—suggests that most silicate species are not bound to aluminate tetrahedra. The nucleation of zeolite omega takes place at the relatively high aluminate concentration of about 15 mmol L1. It is likely that this value includes some aluminosilicate colloids retained in the liquid phase during the filtration. Indeed, no zeolite omega crystals smaller than 0.2 lm are observed. The earliest and smallest crystals are probably lost in the filtration. The nucleation of zeolite omega brings the aluminate concentration down to less than 3 mmol L1. The insuring growth of the crystals of zeolite omega is fed by the dissolution of the parent zeolite X and A. The concentration of aluminate in solution depends on the relative rates of the Table 1 Exponent n of the Avrami equation as a function of the synthesis conditions OH/Si
Al/(Si + Al)
Avrami exponent n
0.7 0.7 0.7 0.8 0.8 0.8
0.105 0.116 0.131 0.105 0.116 0.131
3.58 3.66 2.44 1.58 1.36 1.88
225
dissolution and growth processes and decreases with the decrease of the surface of dissolution of the parent zeolites and the increase of the surface of growth of zeolite omega. The excess silicate in solution allows zeolite omega to be formed from the aluminium-richer parent zeolites. The earliest crystals of zeolite omega present the characteristic shape of flat cylinders, so-called aspirin tablets. The radial surface of the cylinders is quite peculiar, as it cannot be described as a crystallographic face and corresponds to the envelope of the (h k 0) faces. As the aluminate concentration decreases, the radial growth rate decreases until the (h k 0) surface is superseded by (1 0 0) faces. Further growth of the (1 0 0) faces is so slow that no further increase of the width of the crystals is observed after their formation. No data on the distribution of aluminium inside the crystals are available and a change of the Si/Al ratio with the variation of the concentration of aluminate in solution cannot be ruled out. Slender crystals at lower aluminium content were also observed in the synthesis of zeolite omega in the presence of TMA and diaminohexane [30]. The role of the concentration of aluminium is also underlined by the comparison of the compositions of the synthesis batches and of the final zeolites. The composition of the synthesis batches normalized on the amount of aluminium is in the range 5.65–7.8 Na/0.33–0.41 TMA/Al/ 6.65–8.5 Si, while the composition of the final products is 0.69–0.75 Na/0.22–0.28 TMA/Al/3.69–4.58 Si. The ratios between each component and aluminium are always lower in zeolite omega than in the synthesis batch. This indicates that excess silicate, Na+ and TMA+ cations are left in solution at the end of the crystallization. Aluminate seems an excellent candidate as growth limiting species. In principle, the rate of growth of the face of a crystal (dL/dt) usually depends on the concentration of the limiting species by an equation n
dL=dt ¼ Kð½Al ½Aleq Þ ;
ð2Þ
in which [Al] [Al]eq indicates the supersaturation of the solution and the exponent n varies between 1 and 2 according to the mechanism of growth [42,43]. The final concentration of aluminium in each synthesis has been assumed as equilibrium concentration [Al]eq. The attempts to correlate the crystallization kinetics of zeolites to the concentrations in solution have especially dealt with the problems related to the equilibria between silicate species in the system [44–46] or to the evaluation of the crystal size distribution by population balance models [47,48]. The extremely low Avrami exponents for the crystallization curves of this paper indicate that no further nucleation takes place after an initial burst. In this case, it seems reasonable to consider the crystal population as monodispersed and apply Eq. (2) to the average crystal size. In Fig. 9, the rate of growth of the crystals of zeolite omega in axial and radial direction are reported as a function of the aluminate supersaturation in solution. The rates of growth (dL/dt) are measured as half the size variation of
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0.03 dLr /dt (µm h-1)
dLa/dt (µm h-1)
B
A
0.03
0.02
0.01
0.02
0.01
0
0 0
1 0.5 [Al]-[Al]eq (mmol L-1)
0
1.5
1 0.5 [Al]-[Al]eq (mmol L-1)
1.5
Fig. 9. Rates of growth of the crystals of zeolite omega at 378 K in axial (A) and radial (B) direction as a function of the aluminate supersaturation. Batch compositions: OH/Si 0.7 (filled symbols) or 0.8 (void symbols); Al/(Si + Al) 0.105 (circles), 0.116 (squares) or 0.135 (triangles).
1.2
NaAlSi1:25 O4:5 + 0.25TMAþ + 2.75Si(OH)3 O = Na0:75 TMA0:25 AlSi4 O10 + 0.25Naþ + 2.75H2 O + 2.75OH ð3Þ
The real variation of pH would depend on the degree of ionization of the silicate species in solution and the buffer effect of the silicate species not involved in the crystallization. The rise of the concentration of hydroxyl species should correspond to a rise of the rate of silicate condensation. In this way, the rise of alkalinity during the synthesis should increase the rate of growth of the zeolite and partially counteract the decrease of the rate of growth due to the decrease of aluminate concentration. The effect of alkalinity can be taken into account by defining a new rate constant K 0 = K/[OH] and explicitly introducing the concentration of the hydroxyl catalyst into the growth rate equation n
dL=dt ¼ K 0 ½OH ð½Al ½Aleq Þ .
A 1 0.8 0.6 0.4 0.2 0
ð4Þ
In Fig. 10, the rates of growth corrected by the hydroxyl concentrations (dL/dt)/[OH] are reported as a function of the aluminate supersaturation. The introduction of alkalinity in the rate equation significantly reduces the scattering
(dLr /dt)/[OH] (µm h-1 mol-1 L)
(dLa/dt)/[OH] (µ mh-1 mol-1 L)
the crystals between two experimental points and the concentration of aluminium is the average value between the concentrations at the corresponding experimental points. The first experimental points of each crystallization curve, corresponding to the appearance of zeolite omega, are not included in this representation. No significant measurement of the rate of growth for these points is possible, due to the incertitude on the lower limit of crystal size. Moreover, the observed sudden fall of Al concentration prevents any evaluation of a significant average value. The rates of growth presented in Fig. 9 can only be defined as clouds of random-scattered points. It seems clear that the aluminate concentration alone cannot account for the rates of growth of the crystals of zeolite omega. Another parameter to be taken into account is the pH of the crystallization system. The alkalinity is a well-known accelerating factor in zeolite crystallization [49–51]. The OH groups catalyze the nucleophilic attack of silicate species to silica tetrahedra which represents the slow step of silicate condensation [42,52]. In most zeolite syntheses, the pH increases during the crystallization, as amorphous or dissolved silicate species are depleted [22]. The charge balance of the crystallization of zeolite omega from zeolite X can ideally be schematized as
1.2 B 1 0.8 0.6 0.4 0.2 0
0
0.5 1 [Al]-[Al]eq (mmol L-1)
1.5
0
0.5 1 [Al]-[Al]eq (mmol L-1)
1.5
Fig. 10. Rates of growth of the crystals of zeolite omega at 378 K corrected by the alkalinity as a function of the aluminate supersaturation. Axial (A) and radial (B) growth. Batch compositions: OH/Si 0.7 (filled symbols) or 0.8 (void symbols); Al/(Si + Al) 0.105 (circles), 0.116 (squares) or 0.135 (triangles).
M.L. Guzman Castillo et al. / Microporous and Mesoporous Materials 90 (2006) 221–228
of the experimental points. The alkalinity-corrected rates are proportional to the aluminate supersaturation, in the case of both axial and radial growth. If the data of Fig. 10 are best-fitted by power laws, the following rate equations are obtained: dLa =dt ¼ 0:74½OH ð½Al ½Aleq Þ1:07 ;
ð5Þ
dLr =dt ¼ 0:22½OH ð½Al ½Aleq Þ1:08 ;
ð6Þ
respectively, for the axial and radial growth. The mean square errors on the exponents of the rate laws are 0.10 for the axial growth and 0.13 for the radial growth. As a consequence, both rate laws scantily differ from a linear correlation. The rate constant of the axial growth is more than three times the rate constant of the radial growth. These rate laws do not represent the initial stages of the crystallization, for which the quality of the available data is too poor to establish any quantitative correlation. The fast radial growth of the primitive aspirin-tablet crystals has still to be explained. The formation of flat crystals at the beginning of zeolite syntheses has been observed in extremely different systems. The phenomenon can be justified by a mechanism of colloid aggregation in the case of the formation of slab crystals of silicalite from organic-rich sols [53] but primitive flat crystals have also been observed in the crystallization of mordenite from thick gels [54]. For aluminate concentrations lower than 2 mmol L1, Eqs. (5) and (6) account for the growth rates. The interplay between rise of pH and decrease of aluminate concentration can justify the curious sigmoidal trend observed for both axial and radial growth. The disappearance of the cylindrical (h k 0) surfaces clearly correspond to their superseding by slower-growing (1 0 0) faces. It can be assumed that the (1 0 0) growth rate depends on aluminate concentration by an order significantly higher than 1, while the (h k 0) growth rate depends nearly linearly on aluminate concentration. In this case, the (h k 0) growth can be slower than the (1 0 0) growth above a given threshold of aluminate concentration. The (1 0 0) faces will appear below this concentration threshold, when their growth will be slower than the (h k 0) growth. 5. Conclusions The use of a sparingly soluble source of aluminate allows to perform nucleation and growth of zeolite omega in an aluminium-starved system. In these conditions, the usual formation of an intermediate aluminosilicate gel is prevented. This allows to avoid the induction period usually connected to the ripening of the gel and described by autocatalytic kinetics. In the high-pH and low-[Al] environment of these syntheses, the synthesis sol is depolymerized enough to be treated as a true solution. This allows to establish significant correlations between the experimental rates of growth of zeolite omega and the concentrations in solution. The concentration of aluminium, probably present as aluminosili-
227
cate species, is a rate-limiting factor. However, its influence on the growth rate cannot be evidenced if the catalytic effect of hydroxyl anions on silicate condensation is not explicitly taken into account. When both aluminate supersaturation and hydroxyl concentration are taken into account, the growth rates of the crystals of zeolite oemga in both axial and radial direction can be successfully modelled. The control of the length of the crystals, the most significant parameter for diffusion in a catalytic system, depends on the interplay between nucleation and growth. The assessment of the nucleation process is outside the field of experimental conditions of this study. However, nucleation corresponds to a burst of radial growth, possibly taking place by aggregation of nanosized precursors. The initial useful aspirin-tablet morphology has to be preserved by reducing the further lengthening of the crystals. Slow syntheses at low alkalinity are quite successful in producing stable crystals slightly longer than 1 lm. References [1] R. Shigeishi, B. Chiche, F. Fajula, Micropor. Mesopor. Mater. 43 (2001) 211. [2] D. McQueen, B. Chiche, F. Fajula, A. Auroux, F. Guimon, P. Schulz, J. Catal. 161 (1996) 587. [3] R. Aiello, R.M. Barrer, J. Chem. Soc. A (1970) 1470. [4] E. Bowes, J.J. Wise, US Pat. 3,578,723, 1971. [5] E. Flanigen, Neth. Pat. 6,710,729, 1968. [6] E. Galli, Cryst. Struct. Commun. 3 (1974) 339. [7] A. Martucci, A. Alberti, M.L. Guzman-Castillo, F. Di Renzo, F. Fajula, Micropor. Mesopor. Mater. 63 (2003) 33. [8] F. Fajula, M. Vera-Pacheco, F. Figueras, Zeolites 7 (1987) 203. [9] F.A. Dwyer, US Pat. 3,642,434, 1972. [10] F.G. Dwyer, P. Chu, J. Catal. 59 (1979) 263. [11] A.J. Perrotta, C. Kirby, B.R. Mitchell, E.R. Tucci, J. Catal. 55 (1978) 240. [12] H. Lechert, H. Weyda, in: L. Occelli, H. Robson (Eds.), Synthesis of Microporous Materials, vol. I, Van Nostrand Reinhold, New York, 1992, p. 77. [13] C.J. Plank, E.J. Rosinski, M.K. Rubin, British Pat. 1,297,256, 1972. [14] J.F. Cole, H.W. Kouwenhoven, Adv. Chem. Ser. 121 (1973) 583. [15] F. Di Renzo, F. Fajula, F. Figueras, S. Nicolas, T. Des Courie`res, Stud. Surf. Sci. Catal. 49 (1989) 119. [16] F. Fajula, S. Nicolas, F. Di Renzo, C. Gue´guen, F. Figueras, in: M.L. Occelli, H.E. Robson (Eds.), Zeolite Synthesis, ACS Symp. Ser. 398 (1989) 493. [17] F. Di Renzo, F. Fajula, F. Figueras, T. Des Courie`res, US Pat. 5,165,906, 1992. [18] A. Araya, T.J. Barber, B.M. Lowe, D.M. Sinclair, A. Varma, Zeolites 4 (1984) 263. [19] P. Massiani, F. Fajula, D. Di Renzo, F. Remoue´, F. Figueras, Stud. Surf. Sci. Catal. 52 (1989) 215. [20] A. Maubert, R. Dutartre, L.C. de Menorval, F. Figueras, Zeolites 13 (1993) 587. [21] J.L. Tallon, R.G. Buckley, J. Phys. Chem. 91 (1987) 1469. [22] J.L. Casci, B.M. Lowe, Zeolites 3 (1983) 186. [23] F. Hamidi, A. Bengueddach, F. Di Renzo, F. Fajula, Catal. Lett. 87 (2003) 149. [24] S.P. Zhdanov, in: Molecular Sieves, Society of Chemical Industry, London, 1968, p. 62. [25] P. Caullet, J.L. Guth, in: M.L. Occelli, H.E. Robson (Eds.), Zeolite Synthesis, ACS Symp. Ser. 398 (1989) 83. [26] H. Lechert, H. Kacirek, Zeolites 11 (1991) 720.
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