Influence of Gel Composition and Crystallization Conditions on the

1 Feb 1997 - structure zeolites were prepared and were crystallized under different conditions in a design of a factorial experiment. The synthesized ...
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Ind. Eng. Chem. Res. 1997, 36, 1622-1631

Influence of Gel Composition and Crystallization Conditions on the Conventional Synthesis of Zeolites Sanyuan Yang, Athanasios G. Vlessidis, and Nicholaos P. Evmiridis* Laboratory of Analytical Chemistry, Department of Chemistry, University of Ioannina, 45110 Ioannina, Greece

A detailed study of the conventional hydrothermal synthesis of zeolites in aqueous sodium alkaline solutions without the use of templates or organic bases is presented. Gel compositions representing most areas of the silica-rich half-triangle of the crystallization field that yield openstructure zeolites were prepared and were crystallized under different conditions in a design of a factorial experiment. The synthesized products were examined for their crystal structure and their Si/Al ratio. The data prove that the gel-alkalinity factor (a) increases the rate of crystallization to a point but further increase reveals a decrease of the phase transformation rate, thus stabilizing metastable structures, and (b) decreases the Si/Al ratio of the zeolitic framework. The increase of the gel-aluminicity (a) generates new crystalline structures that grow simultaneously with those that are formed in low-aluminicity gels and (b) decreases the slope of the framework Si/Al ratio versus alkalinity. An increase of crystallization temperature produces structures that are more dense and stable. Structure stability sequences were established in experiments performed at relatively high gel alkalinity and aluminicity. Introduction Hydrothermal synthesis is a naturally-occurring process that produces several classes of inorganic natural minerals such as smectites, micas, feldspars, crystalline silicas, and zeolites (found in sedimentary tuffs and in cavities of basaltsspecifically termed vugs of basalt). The process starts by the influence of aqueous solutions in an alkaline matrix. The hydrothermal synthesis of zeolites can be described by the following chemical changes SiO2 + Al2O3 + Na2O + H2O

gelation

aluminosilicate gel

aging, crystallization temperature

crystalline zeolites

There are numerous studies concerning the hydrothermal synthesis of zeolites; a small collection of early and later studies classified in categories is as follows: (1) Investigations related to the influence of the synthesis factors (especially the type of cations, template organic ions, and basic organic molecules) on the production of specific crystalline zeolitic structures of high purity are found in J. Chem. Soc. by the title The Hydrothermal Chemistry of Silicates (1951-1978)1 and by the title The Chemistry of Soil Minerals (19651978);2 in the monographs by Barrer of Hydrothermal Chemistry of Zeolites (1982)6 and Zeolite and Clay Minerals as Sorbents and Molecular Sieves (1978),5 by Breck of Zeolite and Molecular Sieves (1974),11 and by Szostak of Molecular Sieves: Principles of Synthesis and Identification (1989);44 in articles of ACS Adv. Chem. Ser. such as those by Zhdanov and by Kokotailo and Ciric in Molecular Sieve Zeolite I (1971),32,49 or those by Robson et al. and by Meise and Schwochow in Molecular Sieves (1973),37,40 or that by Rollman in Inorganic Compounds with Unusual Properties (1979);39 in ACS Symp. Ser. such as that of Kacirec and Lechert in Molecular Sieves II (1977)29 or Hasegawa and Sakka in Zeolite Synthesis (1989);25 in articles of Proceedings * Author to whom correspondence is addressed. S0888-5885(96)00228-X CCC: $14.00

of the 5th International Conference of Zeolites such as that by Barrer et al. (1980)7 or that by Erdem and Sand (1980).19 (2) Investigations related to the influence of synthesis factors on phase transformations and metastability sequence of zeolite structures are found in the monographs by Barrer (1982)6 and by Breck (1974);11 in ACS Symposium Series such as that by Milton (1989)38 and in articles of periodicals such as that of Chao et al. (1981).13 (3) Investigations related to the influence of the synthesis factors (especially alkalinity, Si/Al ratio, and temperature) on the mechanism and kinetics of the crystallization process are found in the monographs by Barrer (1982),6 by Breck (1974),11 and by Szostak (1989);44 in articles in ACS Advances in Chemistry Series such as that by Zhdanov (1971);49 in articles in ACS Symposium Series such as that by Hayhurst and Sand (1977)26 and that by Angell and Flank (1977)4 in Molecular Sieves II or that of Milton (1989)38 and that of Katovic et al. (1989)30 in Zeolite Synthesis; in articles of Proceedings of the 5th International Conference on Zeolites such as that by Aiello et al. (1980)3 and that by Vaughan and Lussier (1980);47 in articles of periodicals such as that of Chao et al. (1981).13 (4) Investigations related to oligomerization of reactants in solutions and studies of the influence of the type of species (oligomers) on the resulting zeolitic structure under various combinations of synthesis factor levels are found in the monographs by Barrer (1982)6 and Breck (1974);11 in ACS Symposium Series such as that by Bell (1989)10 and that by Harvey and Glasser (1989);24 in articles of Proceedings of the 5th International Conference on Zeolites such as that by Guth et al. (1980).23 (5) Investigations related to the influence of synthesis factors on specific features such as zoning, crystal defects, morphology, sorption behavior, order of elements in framework T-atoms, location, and orientation of cations and molecules hosted in the pores (especially template ions and molecules) such as those found in the monograph by Barrer (1982);6 in ACS Symposium Series such as that by Lyman et al. (1983)33 and that by Milton (1989);38 in Proceedings of the 5th International Confer© 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1623 Table 1. Compositions of Working Solutions A and B and Quantities Used for Preparation of the Various Aluminosilicate Gels solution A solution B synthesis mixture designation of aluminosilicate gels NaOH (g) sodium aluminate (g) water (g) H2SO4 (g) water (g) waterglass (g) solution A (g) solution B (g) T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14 T15 T16

3.40 2.06

5.44 9.41 12.3 21.9 25.0 5.96

15.3

4.63 8.98 8.15 4.19 2.56 3.37 9.59 5.41 11.6 6.07 1.78 1.78 1.98 2.16 3.03 3.39

148 147 64.3 70.0 75.3 200 155 171 186 189 201 202 100 100 80.4 198

ence on Zeolites such as that by Erdem and Sand (1980);19 in articles of periodicals such as that by Ueda and Koizumi (1979).46 These areas of research are still expanding as evidenced by recent reports of numerous research groups such as that by Feijen et al. (1994),20 by Dai et al. (1986),15 by Guth et al. (1990),22 by Jacobs (1992),27 by Gilson (1992),21 by McCormick and Bell (1989),35 by Vaughan (1991),48 by Dwyer et al. (1991),18 by Jansen and Wilson (1991),28 by Dougnier et al. (1992),17 by Martens et al. (1993),34 by Terasaki et al. (1993),45 by Burkett and Davies (1993),12 by Skeels et al. (1993),43 and by many others in Studies in Surface Science and Catalysis (Vol. 84) book series, in ACS (Vol. 398) Symposium Series, in NATO ASI (Vol. 352) series, and in Proceedings of the 7th International Conference (Tokyo). This volume of research work has led to preparations of synthetic zeolites with new topologies, sorption, and separation properties. Furthermore, Feijen et al. (1994)20 review the expansion of zeolite applications to areas such as photochemistry, electrochemistry, and supramolecular catalysis which impose extra demands on their physical properties. This is why so much effort is made toward unravelling the pathways that lead to the formation of zeolites from its precursors; such knowledge can lead to “tailor-made synthesis”. The monographs by Barrer (1982)6 and Breck (1974)11 and the one by Szostak (1989)44 cover most of the early and later literature on the zeolite hydrothermal synthesis. In Chapter 4 of the monograph by Breck, one can find crystallization field diagrams for sodium zeolites crystallized at low temperatures that were obtained by few research groups. However, there is no information about the number of crystalline phases and their purity that are present in the solid product of the test preparation. Such information is provided in studies targeted at the synthesis of specific zeolites of a certain structure and composition. In addition no information is provided about the influence of the crystallization and aging conditions on the final product. The above information is of utmost importance in tailoring the zeolitic structure since zeolite formation in most cases is a nonequilibrium process involving nucleation and crystal growth steps. Factorial experiments employing a unique hydrothermal synthesis method that covers the entire crystallization field with identical source materials is the key for obtaining true relationships between the yield of each crystalline phase and the

14.8 6.78 3.05

53.7 59.4 80.0

44.5 80.2 30.8

106 70.3 65.8

57.6 47.3 37.5 56.8 68.5 77.9 47.0 68.4 60.4 74.4 83.8 92.3 89.8 104 76.1 89.2

140 148 59.0 72.3 73.6 170 152 172 202 195 174 191 82.3 86.9 73.7 202

58.2 64.1 73.8

103 132 92.9

tested factors. Such studies contribute to the full understanding of the nature of hydrothermal synthesis as a whole and reveal the function of the various individual factors and the mechanistic paths. This kind of study is not available in the literature to the extent needed for establishing the basic rules for the purpose of tailor-made synthesis. In this work we try to meet the target of tailoring the zeolitic hydrothermal synthesis to a specific structure and skeletal composition by studying the influence of the aluminosilicate gel composition, the aging conditions, and the crystallization conditions on the resulting crystal structure of the zeolite product as well as on the Si/Al ratio of its framework. In order to carry out the investigation over a wide range of aluminosilicate gel compositions and keep the number of syntheses to a relatively small number, we have chosen the compositions and conditions in accordance to values found in the literature. Experimental Section Materials and Stock Solutions. The following materials and stock solutions were used: (1) Waterglass (8.44% w/w Na2O, 28.3% w/w SiO2, 63.3% w/w H2O; Merck). (2) Sodium aluminate (38.5% w/w Na2O, 54.2% w/w Al2O3, 7.3% w/w H2O; BDH). (3) Sodium hydroxide (>99% w/w NaOH; Merck). (4) Sulfuric acid (98% w/v H2SO4; Merck). (5) Sulfuric acid stock solution (14.23% w/w H2SO4, 84.4% w/w H2O). (6) Distilled water was used for all synthesis samples. (7) Sodium aluminate working solution A was prepared by dissolving variable amounts of sodium aluminate in variable amounts of water of variable alkaline content. The quantities of sodium aluminate, sodium hydroxide, and water are shown in Table 1 for each sample. (8) Sulfuric acid working solution B was prepared by diluting variable weights of sulfuric acid stock solution in variable weights of water as shown in Table 1. Characterization. IR spectra were recorded with a Perkin Elmer 783 spectrophotometer. The characteristic stretching vibrations were used for the detection of the zeolitic structure. XRD powder patterns were recorded by Philips PW 1710 X-ray diffractometer with Cu KR radiation. The d-spacing and relative peak heights of unknown samples were matched to reference crystal structures. The intense peaks of relatively high 2Θ values within each specific crystalline phase were used to obtain the phase composition of multicrystalline phase zeolitic products. Overlapping peaks between

1624 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 Table 2. Aging and Crystallization Conditions sample Tn-1 Tn-2 Tn-3 Tn-4 Tn-5 Tn-6 Tn-7 Tn-8

aging temp (°C)

aging time (h)

cryst. temp (°C)

cryst. time (h)

36 36

24 24

100 100

24 24

100 100 100 100 170 170 170 170

30 192 30 192 30 192 30 192

different crystalline phases were not used in this respect because when more than two phases are present, the quantitative analysis of the detected phases is liable to large errors. Conventional methods of chemical analysis were used: the quantitative gravimetric method with oxine and the titrimetric method with EDTA were used for the determination of Al; the quantitative gravimetric method with concentrated HCl and the quantitative spectrophotometric method with molybdate reagent were used for the determination of Si; the quantitative flame photometric method was used for the determination of Na. A Hitachi Model 100-80 double-beam UVvis spectrophotometer was used for the quantitative spectrophotometric determination of Si; a Jenway PFP7 flame photometer was used for the flame photometric determination of Na; a WTW pH-meter was used for controlling the pH values of the analysis samples. All materials used for chemical analysis were of analytical reagent grade, and demineralized-distilled water was used throughout. Preparation of Aluminosilicate Gel Samples. Variable weights of sodium aluminate working solution A were slowly added to variable weights of waterglass into a 500 mL beaker with stirring at room temperature. The sodium aluminate working solution A contained an excess of sodium hydroxide for the preparation of relatively high alkalinity aluminosilicate gels. For the preparation of relatively low alkalinity aluminosilicate gels, the working solution B was added in varying weights to the above mixture. The exact weights of the reactants and the working solutions are given in Table 1. The prepared homogeneous aluminosilicate gel is then ready for the crystallization process. Crystallization of Aluminosilicate Gel Samples. Part of the aluminosilicate gel was distributed into four Teflon reactors (each with a volume capacity of about 30 mL) for crystallization at 100 °C, and another part was distributed to four stainless steel reactors (each with a volume capacity of about 50 mL) for crystallization at 170 °C; the stainless steel reactors with the aluminosilicate gel followed procedures identical with the Teflon ones. After the crystallization process, the reactors were left to cool at ambient temperature, and the content of the reactor was filtered, washed repeatedly with distilled water, and dried overnight in the oven at 100 °C. The dried product was weighed and the percent yield was calculated by eq 1

yield (%) ) Wsolid/Wgel

(1)

where Wsolid and Wgel are the weights (g) of the solid and gel, respectively. Table 2 shows the designations for the different gelcomposition samples, Tn-m; the number n indicates the

Figure 1. Crystallization field of sodium alluminosilicate compositions. Table 3. Aluminosilicate Gel Compositions and Associated Composition Factors factors sample

prepared SiO2-Al2O3 gel composition

Aa

Rb

T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14 T15 T16

4.50 Na2O, 0.700 Al2O3, 8.60 SiO2, 300 H2O 4.50 Na2O, 1.43 Al2O3, 7.14 SiO2, 300 H2O 3.00 Na2O, 1.43 Al2O3, 7.14 SiO2, 300 H2O 3.00 Na2O, 0.700 Al2O3, 8.60 SiO2, 300 H2O 3.00 Na2O, 0.370 Al2O3, 9.26 SiO2, 300 H2O 4.50 Na2O, 0.370 Al2O3, 9.26 SiO2, 300 H2O 7.00 Na2O, 1.43 Al2O3, 7.14 SiO2, 300 H2O 7.00 Na2O, 0.700 Al2O3, 8.60 SiO2, 300 H2O 10.0 Na2O, 1.43 Al2O3, 7.14 SiO2, 300 H2O 10.0 Na2O, 0.700 Al2O3, 8.60 SiO2, 300 H2O 4.50 Na2O, 0.192 Al2O3, 9.62 SiO2, 300 H2O 3.00 Na2O, 0.192 Al2O3, 9.62 SiO2, 300 H2O 2.00 Na2O, 0.192 Al2O3, 9.62 SiO2, 300 H2O 1.00 Na2O, 0.192 Al2O3, 9.62 SiO2, 300 H2O 2.00 Na2O, 0.370 Al2O3, 9.26 SiO2, 300 H2O 7.00 Na2O, 0.370 Al2O3, 9.26 SiO2, 300 H2O

0.900 0.900 0.600 0.600 0.600 0.900 1.40 1.40 2.00 2.00 0.900 0.600 0.400 0.200 0.400 1.40

0.140 0.286 0.286 0.140 0.074 0.074 0.286 0.140 0.286 0.140 0.038 0.038 0.038 0.038 0.074 0.074

a

Alkalinity. b Aluminicity.

gel composition (n ) 1, ..., 16), and the number m gives the aging and crystallization conditions of the synthesis. Results and Discussion Synthesis. The aluminosilicate gels had the general molecular composition of aNa2O‚bAl2O3‚(c - 2b)SiO2‚ 300H2O with c ) 10 and a and b varying with the aluminosilicate gel sample. An alkalinity factor, A, is defined as A ) 2a/c, and an aluminicity factor ()zeolite framework aluminium content), R, is defined as R ) 2b/c. The aluminosilicate gels cover a large area of the silica-rich-half of the composition-triangle-area that is shown in Figure 1. The prepared aluminosilicate gels are designated T1, T2, ..., T16; their molecular compositions and their A and R factors are shown in Table 3. It is important to point out that the 2a value used for estimating the alkalinity value is the number of hydroxide anions in the prepared gel and that, in calculating this gel hydroxide content of a crystallization mixture, the [OH] is conventionally calculated by summing up the moles of OH added as NaOH, sodium silicate (defined as NaOH + SiO2), and sodium aluminate (NaOH + Al2O3) and subtracting any moles of acid that have been added as free acid of salt as mentioned by Szostak (1989).44 Identified Phases. The solid phase of the crystallized gels was examined by IR spectroscopy, and the

Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1625 Table 4. Unit Cell Composition of Synthesized Samples Compared to Reference Zeolites of ASTM X-ray Cards sample

unit cell composition

faujasite (ref.) faujasite (T7-1) Na-P1 (ref.) Na-P1 (T11-2) sodalite (ref.) sodalite (T9-4) analcime (ref.) analcime (T4-6) cancrinite (ref.) cancrinite (T9-8) mordenite (ref.) mordenite (T15-6)

Na88[Al88Si104O384](H2O)220 (Si/Al ) 1.18) Na74.7[Al72.2Si119.8O384](H2O)337 (Si/Al ) 1.66) Na6[Al6Si10O32](H2O)12 (Si/Al ) 1.67) Na4.3[Al4.2Si11.8O32](H2O)17 (Si/Al ) 2.84) Na6[Al6Si6O24](NaOH)2(H2O)2 (Si/Al ) 1.00) Na5.5[Al5.2Si10.8O24](H2O)17 (Si/Al ) 1.30) Na16[Al16Si32O96](H2O)16 (Si/Al ) 2.00) Na10.5[Al10.3Si37.7O96](H2O)21.3 (Si/Al ) 3.64) Na7.0Ca0.9[Al6Si6O24](CO3)1.4(H2O)2.1 (Si/Al ) 1.00) Na5.9[Al5Si7O24](H2O)8.7 (Si/Al ) 1.39) Na8[Al8Si40O96](H2O)24 (Si/Al ) 5.00) Na5.7[Al5.6Si42.4O96](H2O)23 (Si/Al ) 7.51)

products that showed structure sensitive vibration absorption bands were examined by XRD and identified, making use of the Collection of Simulated XRD Powder Patterns for Zeolites by von Ballmoos and Higgins (1990).9 The crystalline structures of faujasite (FAU), gismodine (GIS), chabazite (CHA), sodalite (SOD), analcime (ANA), mordenite (MOR), and cancrinite (CAN) were identified among the 128 test preparations. Their identification was made possible because in some test preparations a crystalline phase of a single structure, SCSP ()single crystalline structure phase), appeared. The identification procedure was based on peaks ranging from 5 to 35 deg Θ. Test preparations with crystalline phases of more than one structure, MCSP ()multicrystalline structure phase), were found to be mixtures of some of the above seven structures. The XRD powder pattern d-spacings of the SCSP test preparations match nicely to the pure reference zeolites in the sense that, apart from some low-intensity lines that may be missing or in some cases the appearance of one or two extra lines, the majority of the existing line d-spacings are approximated to the second decimal digit and in a few cases to the first. The XRD data of the test preparations and reference zeolites are given in Supporting Information. The small differences in d-spacing between test preparations and reference zeolite powder patterns may be due to poor zeroing of the diffractometer, misalignment of the sample or diffractometer, errors in measurements, or changes in chemical composition, especially the Si/Al in the framework. The latter is reasonable since the Si-O bond (1.61 Å) is little shorter than the Al-O bond (1.75 Å), and therefore the dimensions of the unit cell decrease with an increase in Si/Al. Evidence for the difference in the Si/Al ratio between test preparations and reference zeolites is given in Table 4. Such differences in the Si/Al ratio were not desired but were revealed from the unit cell formula of the synthesis product calculated by using the data of chemical analysis normalized to the number of framework (Si + Al) atoms of the structure it belongs to (assuming that the product is completely crystalline and of a single structure). The Na/Al ratios of prepared samples in Table 4 are found to be in most cases larger than 1, but their deviation is about 5%, giving evidence that they are within the limits of analytical error; however, we cannot exclude the possibility of the existence of small quantities of the amorphous phase or occlusion of reagents within the zeolitic structure. Structure Density and Stability. The physical characteristics of the crystalline zeolites are are found in the Atlas of Zeolite Structure Types by Meier and Olson (1987).36 The crystalline zeolitic structures prepared in this work belong to different point and space symmetries and therefore possess different pore struc-

Figure 2. Effect of gel composition and crystallization time on the type of crystalline structure that is formed at a crystallization temperature of 100 °C. Flag register: upper/aged gels; lower/ unaged gels.

ture systems, pore openings, and densities. Structures of increased density are less soluble, and from a physicochemical point of view the denser structure is more stable; thus, according to Atlas of Zeolite Structure Types, the stability of the crystal structures obtained in this work should follow the sequence ANA > MOR, SOD > CAN > GIS > CHA > FAU. Influence of Experimental Factors on Selectivity of Structure. The structures of solid phases of test preparations crystallized under different factor levels at 100 and 170 °C are shown in Figures 2 and 3, respectively. The structures of the solid phases of test preparations that have undergone the aging process and have crystallized under fixed factor levels for 30 and 192 h are shown in Figure 4. Conclusions from these data will be discussed in a later section. Influence of Temperature. The solid phases produced by crystallizing gels at 170 °C are amorphous, MOR, ANA, and CAN, and at 100 °C are amorphous, GIS (Na-P), CHA, SOD, and FAU (Figure 4). The experimental findings, generally, support the rule that the more stable phases are favored by crystallization at relatively high temperatures. The amorphous phase is always found when the crystallization time is not sufficient for the formation of crystalline phases. Influence of the Gel Composition. At a crystallization temperature of 170 °C the most stable (analcime) structure is dominant at alkalinity values of 0.92.0 for low aluminicity gels and of 0.6-2.0 for relatively high aluminicity gels. The less stable structure of mordenite is observed for low alkalinity and aluminicity gels. The even less stable structure of cancrinite is

1626 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997

Figure 3. Effect of gel composition and crystallization time on the type of crystalline structure that is formed at a crystallization temperature of 170 °C. Flag register: upper/aged gels; lower/ unaged gels.

Figure 4. Effect of gel composition and crystallization temperature on the type of crystalline structure that is formed from aged gel samples. Flag register: upper/crystallization time, 30 h; lower/ crystallization time, 192 h.

observed at the combined levels of relatively high gelalkalinity and gel-aluminicity (Figure 4). At crystallization temperatures of 100 °C (except the sodalite structure) the most stable structure is gismodine (NaP) and is dominant at alkalinity values in the range of 0.9-2.0 when the aluminicity of the gel is low; the range is increased to 0.6-2.0 when the aluminicity of the gel is increased. The amorphous structure is dominant at gels of low alkalinity combined with low aluminicity. The faujasite structure is found at alkalinities of about 1.4 in coexistence with gismodine but on its own at lower crystallization times. Finally, the chabazite and sodalite phases appear at relatively high aluminicityalkalinity gels. Influence of Experimental Factors on Crystallization Rates. The structures obtained by crystallizing aluminosilicate gels of various compositions at 100 °C and at fixed crystallization times are shown in Figure 2. Apart from the amorphous phase, all structures are crystalline and are found in SCSP or MCSP form. The rate of formation of a crystalline structure phase, rCSF, can be deduced from the change of the quantity of a particular crystalline phase with crystallization time in preparations of identical synthesis factor levels that lead to SCSP zeolites. For example in Figure 2, the gel sample T14 shows a very low rCSF (amorphous phase up to 192 h); the gel sample T6 shows a relatively rapid rCSF (G is formed in less than 30 h); the gel sample T8 shows a relatively rapid rCSF (F is formed in less than 30 h); gel samples T3 and T4 show intermediate values of rCSF (G is formed between 30 and 192 h). Within a reasonable error the rate of crystalline structure transformation, rCST, can be deduced from the change in quantity with crystallization time of the less

stable crystalline structure, determined by the XRD powder pattern peak intensities in test preparations of identical factor levels. For example in Figure 2 the gel sample T1 shows zero rCST of G (no change between 30 and 192 h of the crystallization process); the gel sample T8 shows a relatively rapid rCST (complete transformation from F to G in 160 h); the gel sample T10 shows intermediate rCST (partial transformation from F to G). From data in Figure 2 it is deduced that (1) the aging process of the gel (at the level tested in this work) plays a small or insignificant role in the rates of crystalline structure formation and transformation, (2) the rate of crystalline structure formation depends on the crystallization field triangle position of the aluminosilicate gel composition [the maximum is observed at alkalinityaluminicity of intermediate values (A ) 0.9; R ) 0.14) and the lowest at low alkalinity values (A = 0.20-0.40)], (3) the rate of crystalline structure transformation is high at alkalinity values of 0.60-0.90 (the stable G structure is formed before 30 h) but decreases with the further increase of gel alkalinity (the least stable F structure is formed before 30 h and is slowly transformed to G structure thereafter). Similarly, Figure 3 shows the structures obtained by crystallizing aluminosilicate gels of various composition at 170 °C, and from the data the following conclusions are deduced: (1) The rates of crystalline structure formation are far more rapid compared to crystallization at 100 °C since in most cases the most stable crystalline structure appears in less than 30 h; relatively slower rates are observed only at the lowest alkalinity-aluminicity values applied in this work.

Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1627

Figure 5. Crystal growth curve of unaged T7 gel sample at a crystallization temperature of 100 °C.

Figure 6. Crystal growth curve of unaged T9 gel sample at a crystallization temperature of 100 °C.

(2) The only significant phase transformation that is observed in the data is that of sample T4 from the crystalline phase of Na-P to analcime; in almost all other cases the most stable structure is reached in less than 30 h crystallization time. (3) At relatively high alkalinity-aluminicity values the structure of cancrinite is observed but from the data is not clear if there is a transformation from cancrinite to analcime structure. The decrease of the induction period and the increase of the crystallization rate with the increase of alkalinity was observed, also, by Domine and Quobex (1968)16 in mordeneite synthesis, by Hayhurst and Sand (1977)26 in (Na,K)-phillipsites, by Meise and Schwochow (1973)37 in synthesis of NaA, by Campbell and Fyfe (1960),14 by Kerr (1966)31 in zeolite synthesis, etc. Crystallization Process and Structure Sequence. The observed structures at different crystallization times are the result of the consecutive or parallel growth of crystalline structures. The transformation process between the crystalline phases is made slower by crystallizing gels of relatively high alkalinity at 100 °C; therefore, to find out the sequence of appearance of crystalline structures, we monitored by XRD powder patterns the crystalline structures that appeared by crystallizing gel compositions of T7 and T9 at 100 °C; the crystallization profile of each crystalline structure is shown in Figures 5 and 6 for T7 and T9 gel composition samples, respectively. The simultaneous increase of the chabazite and faujasite crystalline phases (Figure 5) reveals that the two phases are grown in parallel. The consumption of the chabazite phase with time contributes to the analcime phase and Na-P1 (GIS) phase, while the consumption of the faujasite phase with time contributes to the Na-P1 (GIS) phase; finally the consumption of the NaP1 (GIS) phase contributes to the analcime phase. Initially, in profiles of Figure 6, there is a parallel increase of the faujasite and sodalite crystalline phases, but the rate of faujasite crystalline structure formation is very rapid; the consumption of the faujasite phase contributes to the Na-P1 (GIS) phase, while the sodalite phase continues to increase until the analcime phase

appears and grows; finally, the sodalite phase is consumed in favor of the analcime phase. The hydrothermal stability sequences from these experiments are, therefore, found to be ANA > GIS > FAU ) CHA (Figure 5) and ANA > GIS ) SOD > FAU (Figure 6). However, the structures of CHA and SOD are selectively formed by the high-aluminicity gel compositions as shown in Figure 2. Such a selectivity would suggest that these structures are preferred at higher aluminium content. Based on this observation, it is reasonable to suggest that in light of data in Figures 5 and 6 there is a parallel crystallization of two groups of consecutive structures. The first group of Figure 5 is CHA, GIS, and ANA, and the second is FAU, GIS, and ANA related with their transformation processes; correspondingly, the first group in Figure 6 is SOD and ANA, while the second is FAU and GIS. Since SOD is formed in the most alkaline gel while CHA in less alkaline, we obtain the evidence that the more stable structure of SOD is favored in gels of higher alkalinity. Influence of the Experimental Factors on Recovery and Si/Al Ratio. The recovery of a substance B in the precipitate, RB, is given by definition

(% B)sp Wsp (% B)sp ) × yield (% B)gel Wgel (% B)gel (% B)sp % RB ) × % yield (% B)gel RB )

(2)

When B is SiO2, the % RB is the percent recovery of SiO2 in the precipitated solid phase; when B is Al2O3, the % RB is the percent recovery of Al2O3. The Si/Al ratio is calculated by eq 3

Si/Al )

(% SiO2)sp/MWSiO2 2(% Al2O3)sp/MWAl2O3

(3)

where MWSiO2 and MWAl2O3 are the molecular weights of SiO2 and Al2O3, respectively. The test preparations were analyzed for % SiO2, % Al2O3, and % Na2O, and, with the above equations, %

1628 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997

Figure 7. Effect of gel alkalinity on percent recovery of SiO2 for samples crystallized at 100 (a) and 170 °C (b) that have R values of 0.038 (1), 0.074 (2), 0.140 (3), and 0.286 (4).

RSiO2, % RAl2O3, and the Si/Al ratio are calculated and tabulated in Table S1 of the supporting information. Influence of Experimental Factors on Percentage of Silica Recovery. Parts a and b of Figure 7 show the percent recovery of SiO2 as a function of the alkalinity factor at different gel-aluminicity levels crystallized at 100 and 170 °C, respectively. From Figure 7 it is realized that the percent recovery of SiO2 in the solid phase decreases with an increase of alkalinity. This behavior was originally observed and reported by Senderov (1965, 1966).41,42 The decrease of percent recovery versus alkalinity is large by crystallizing gels at ranges of low gel-alkalinity values and levels out as the gelalkalinity increases to relatively high values. In addition, the decrease of the percent recovery versus alkalinity is significantly dependent on the gel-alumina content; i.e., there is a decrease of 80% (from 90% to 10%) recovery of silica at R ) 0.038 as the gel alkalinity increases from 0.2 to 0.9, while there is a decrease of only 30% (from 80% to 50%) recovery at R ) 0.286 as the gel alkalinity increases from 0.6 to 2.0. This trend is consistent for both crystallization temperatures and demonstrates that the increase of alumina content in the gel decreases the solubility of silica in the alkaline gel-liquid phase during the hydrothermal process, suggesting that the frameworks richer in aluminum aluminosilicate are more resistant to alkaline solutions. The conditions of the crystallization process as well as the aging of the gel do not have a significant effect on the recovery of SiO2, and the small differences that are observed may be justified by the different solubility products of the different crystalline structures that are observed after the crystallization process at each particular test preparation. Influence of Experimental Factors on Percentage of Alumina Recovery. Parts a and b of Figure 8 show the percent recovery of Al2O3 as a function of alkalinity factor for different gel-aluminicity levels crystallized at 100 and 170 °C, respectively. From

Figure 8. Effect of gel alkalinity on percent recovery of Al2O3 for samples crystallized at 100 (a) and 170 °C (b) that have R values of 0.038 (1), 0.074 (2), 0.140 (3), and 0.286 (4).

Figure 8 we observe once more that the crystallization conditions are not affecting significantly the recovery of Al2O3. The greater scatter of the data points may be due to the low precision of analysis of the Al content at low Al content levels. The observed slope of the percent recovery versus gel-alkalinity is very small; it is lower at relatively high gel-aluminicity levels. However, the percent recovery of Al2O3 from the gels is quite high, i.e., 70-95%. The crystallization temperature shows only a small effect on the percent recovery of Al2O3 between gel samples of the same alkalinity-aluminicity level. Influence of Experimental Factors on Percentage of Si/Al Ratio. The effect of gel alkalinity on the percent recovery of SiO2 and Al2O3 controls the relative content between Si and Al of the zeolitic framework expressed by the Si/Al ratio. Plots of Si/Al ratio vs gel alkalinity at different gel-aluminicity levels are shown in parts a and b of Figure 9 for test preparations crystallized at 100 and 170 °C, respectively. From Figure 9 it is made clear that test preparation products of high value Si/Al are obtained with low gel-alkalinity levels and that the increase of gel alkalinity decreases the Si/Al ratio value; the decrease is lower at relatively high gel-aluminicity levels. Factor Influence on Structure Selectivity and Mechanistic Considerations. The mechanistic pathway of zeolite synthesis reported by Barrer et al. (1959),8 by Breck (1974),11 by Zhdanov (1971),49 and by Angell and Flank (1977)4 involves gel formation, nucleation, crystal structure formation, and a sequence of structure transformations as shown in the following manner:

Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1629

stable when the gel aluminicity increases. The evidence that structures like faujasite and/or chabazite with D6R SBUs are formed by crystallizing at 100 °C suggests that larger oligomers are more stable at lower temperatures. The chemical composition of the D6R SBUs found in this work have at least three Al elements in their structure, and, therefore, at least three counterions balance their charge, forming a charge-crowded entity for a dense structure. Gel alkalinity, on the other hand, increases the dissolution rate; when the rate of precipitation is much higher than the rate of dissolution, a phase transformation takes place by rearranging the D6R SBUs to S6R SBUs that give more stable structures. The phase transformations become slower when we increase the gel alkalinity; this may be due to a decrease of the rate of precipitation in comparison to the rate of dissolution with an increase of the gelalkalinity factor. Finally, when gel alkalinity becomes relatively high, the rate of dissolution becomes even more rapid and the rate of precipitation is comparably much slower; thus, we observe a porous structure of low yield. Since the silicates are more soluble than aluminosilicates in alkaline solutions, the crystalline phases formed in alkaline gels are poorer in silica when the gel has a relatively high aluminicity (i.e., sodalite formation). Acknowledgment

Figure 9. Effect of gel alkalinity on the Si/Al ratio for samples crystallized at 100 (a) and 170 °C (b) that have R values of 0.038 (1), 0.074 (2), 0.140 (3), and 0.286 (4). rgf

rnf

solutes w gel formation w rcf

rct1

nucleii formation w crystalijformation w

We thank the General Secretary of Ministry of Research and Technology of Greece for funding this work and the Scientific and Tecnological Institute of Chemical Processes for providing facilities to obtain the XRD powder patterns. Supporting Information Available: Tables of crystal structure recoveries and Si/Al ratios of synthesized sample zeolites (Table S1) and XRD data of crystalline zeolitic structures (Table S2) (9 pages). Ordering information is given on any current masthead page.

rct2

crystalij transformation w etc. The crystallization process comprises the steps of structure formation and transformations; the relative rates for the individual steps of the crystallization process decide the quantity and the lifetime of each intermediate structure. A low rate in a specific structure transformation step increases the quantity and lifetime of previous crystal structures; the equilibrium conditions are established, then, very slowly. The selectivity of the particular structure under fixed factor levels found in this work is based on the quantity and lifetime of the particular structure as a result of the effect of the factors on the step rates. The evidence that crystallization at 170 °C results in dense structures (i.e., mordenite (Si/ Al ) 5) at low aluminicity gels, analcime (Si/Al ) 2) at intermediate aluminicity gels, and cancrinite (Si/Al ) 1) at relatively high aluminicity gels) characterized by secondary building units (SBU) of 5-1, S6R, and S6R, respectively, suggests that single-ring SBUs with six members are stable at high temperatures and the stability of each stereochemical isomer depends on the silicate content of the gel; therefore, the substituted fivemembered-ring isomer is more stable when the gel is very rich in silica; the six-membered-ring isomer is more

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Received for review April 23, 1996 Revised manuscript received November 19, 1996 Accepted November 19, 1996X IE960228O X Abstract published in Advance ACS Abstracts, February 1, 1997.