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Study on the mechanism of zeolite Y formation in the process of liquor recycling
Microporous and Mesoporous Materials 25 (1998) 1–6
Study on the mechanism of zeolite Y formation in the process of liquor recycling Conghua Liu *, Xionghou Gao, Yanqing Ma, Zhongliang Pan, Rongrong Tang Petrochemical Research Institute of Lanzhou Oil Refinery, Lanzhou, 730060, People’s Republic of China Received 14 April 1996; received in revised form 23 October 1997; accepted 23 February 1998
Abstract Crystallization of zeolite Y formation in the process of mother liquor recycling (MLR) has been studied. The zeolite crystallization mother liquor comprises a little sodium silicate as well as minor zeolite, Upon addition of aluminum sulfate, the silicate existed in the mother liquor reacts to form finely divided silica–alumina hydrogel (MLHG) which can be utilized later in zeolite Y formation. The particle of MLHG is relatively small and Si/Al of the gel’s framework is a little great. Since amorphous aluminosilicate gel passes through two stages of gelation, the pseudoequilibrium between solid phase and liquid phase of gel is attained rapidly and the time of zeolite Y formation is much reduced. In addition, experiments have also showed that the quantities and the properties of zeolite Y added affect the process of crystallization heavily. Minor quantities of zeolite in the mother liquor have detrimental effect on the zeolite Y formation. Based on studies of crystallization from recycling mother liquor, a model of two-phase transformation for zeolite Y formation has been proposed. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Mechanism; Zeolite Y; Liquor; Silica–alumina hydrogel
1. Introduction Research on mechanism of zeolite formation through hydrothermal chemistry of aluminosilicates has been a challenging subject. There has been appeared two alternative mechanistic points all the time. One is ‘solid-phase transformation’ mechanism in which the amorphous gel is thought to recognize to form crystals [1], another is the ‘solution-mediated transport’ mechanism, in which amorphous gel is thought to dissolve, the depletion of the liquid components of the gel lead to formation of zeolite [2]. Increasingly, it has been recog* Corresponding author.
nized that both these processes may be present in the zeolite system [3–5]. Zeolite Y is one of the most extensively used commercial product. Research on the mechanism of zeolite Y formation has received extensive attention from many investigators [6,7]. There are various reports on the use of n.m.r spectroscopy for the study of soluble aluminosilicate species in the zeolite Y synthesis solution. Thangaraj et al. [8] have identified many kinds of the aluminosilicate polymers and the secondary building blocks present in the aqueous seed solution, and constructed a model of ‘liquid mediate process’ for zeolite Y formation. Ginter et al. [9] have shown that the rate of crystallization and the yield of zeolite Y from colloidal silica depend strongly on
1387-1811/98/$ – see front matter © 1998 Elsevier Science B.V. All rights reserved. PII: S1 3 8 7 -1 8 1 1 ( 9 8 ) 0 0 14 9 - 8
2
C. Liu et al. / Microporous and Mesoporous Materials 25 (1998) 1–6
Fig. 1. Flow-chart of zeolite Y preparation with MLR.
the means of aging. Another report [10] points out that amorphous gel prepared from colloidal silica passes through rearrangement constructively during aging, and produces zeolite Y nuclei. Dutta et al. [11] have found that the mechanism of zeolite formation is related to gel–seed interaction, and the liqud phase of gel provides nutrients for crystal formation. In 1979, Elliott et al. provided an improved method for preparing zeolite Y [12] (Fig. 1). The zeolite crystallization mother liquor comprises a relatively dilute solution of sodium silicate, and contains minor quantities of alumina as well as finely divided particles of zeolite. Upon addition of aluminum sulfate, the silicate existed in the mother liquor reacts to form finely divided silica–alumina hydrogel (MLHG). The MLHG is recovered by a filtration, which will be utilized later in the zeolite synthesis reaction. On average, the utility factor of silicate rises to more than 90%, vs about 50% with no mother liquor recycling (MLR). The process of reaction and the components of this kind of amorphous gel are characterized intrinsically, which lead to different crystallization process and zeolite product. The aim of the present study is to discuss the mechanism of zeolite Y formation in the process of MLR.
2. Experimental The overall stoichiometry of the amorphous gel was ~2.7 Na O, 1 Al O , 8.5 SiO , ~185 H O, 2 2 3 2 2
in the process of zeolite Y formation with MLR or not. The molar ratio of seeds solution was 1 Al O , 16 Na O, 15 SiO , 320 H O. Crystallization 2 3 2 2 2 mother liquor: SiO 40 g/l, moduli 1.95±0.05. 2 MLHG were prepared by adding aluminum sulfate solution to mother liquor, giving overall molar ratio Si/Al=4.0, which were homogenized at 800 rpm, and left to age for 0.5 h at room temperature. MLHG were then filtrated using a Buchner funnel in the condition of vacuum 0.05 Mpa, giving a filter cake containing approximatly 12% solid. Typically, the synthesis reactants were mixed together in orderly way,such as seeds, sodium silicate solution and/or MLHG cake, aluminum sulfate solution and sodium aluminate solution. The SiO recovered from mother liquor is about 2 35% of the overall. The freshly prepared gels were homogenized at 800 rpm, poured into 3 l stainless steel buckets, and left to age at room temperature. After aging for a period of 0.5 h, each batch was heated at 95°C in a water bath to promote crystallization. Samples of the gel were taken as scheduled. The compositions of the solid and liquid phases of gel were provided by elemental chemical analysis. Identification of the solid phase was done using a Rigaku (D/Max-3C ) X-ray diffractometer with Cu radiation and a Ni filter. The Si/Al ratio of the framework was abtained from the diffraction data using the strongest peaks in the area 2 theta= 31°~32°, using sodium chloride as internal standard sample. The relative crystallinity (RC ) of the products were determined using the ratio of
3
C. Liu et al. / Microporous and Mesoporous Materials 25 (1998) 1–6
diffraction peak intensity in the area of Miller indices 533 (hkl ). The particle sizes of the gel were examined by the Malvern (3600Ec) Laser diffraction particle sizers.
3. Results and discussions 3.1. Solid-phase composition and particle size Upon heating, the gel changed gradually, the Si/Al ratios of the gel framework determined by elemental analysis decreased and the Si/Al ratios determined by X-ray diffraction increased during crystallization, in which the former was a little more than the later in the final zeolite product, and the results are show in Table 1. It has been proposed that the silica dissolve slowly, releasing silicate anions into the liquid phase, and Al species deplete from the solution crystallization. Besides, decrease of the Na/Al ratio of the gel framework is probably owing to the following changes during the formation of crystalline framework [11]
Table 2 Effect of the gel particle sizes on crystallization time Sample
Initial gel average particle sizes (mm)
Time (h)
Max. RC (%)
A5-39* A13-117* A11-116 A3-37 A12-106 A5-303 A16-325 A10-326
30.0 21.7 12.1 13.3 14.6 20.0 23.0 24.5
28 30 8 14 16 18 22 22
90 91 82 83 82 86 85 85
*Without MLHG.
¬Si−O−Na++HO−T¬ ¬Si−O−T¬+Na++OH− ( T=Si or Al ).
Table 1 Composition of gel framework during the crystallization Crystallization
Elemental analysisa
XRD
time (h)
Si/Al
Na O/Al O 2 2 3
Si/Al
0b 0 2 4 6 8 10 12 14 16 18 20
3.5 3.2 3.1 3.1 3.0 2.7 2.6 2.6 2.6 2.6 2.6 2.5
1.32 1.28 1.25 1.24 1.21 1.10 1.11 1.10 1.10 1.10 0.95 0.99
/ / 1.25 1.50 1.90 2.00 2.00 2.25 2.32 2.39 2.40 2.40
aBefore determining, washed by water (50°C ), which is five times the volume of the gel in three parts. bSample is MLHG.
Fig. 2. The change of the gel particle sizes during crystallization.
The occurrence of such a process is supported by the observation of an increase in the Na+ concentration in solution during the crystallization (see Table 3). Zhdanov investigated, in detail, the chemical compositions of the liquid and solid components of the amorphous gel in the process of zeolite formation [13]. The particle sizes of the gel become smaller and smaller during crystallization (see Fig. 2), which decline a great deal, then keep the same point at the last stage of crystallization. The end point of crystallization can be judged by the tendency of the gel particle size change. The particle sizes of the initial gel containing MLHG is much smaller, and
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C. Liu et al. / Microporous and Mesoporous Materials 25 (1998) 1–6
Table 3 Relationship between zeolite products and compositions of the mother liquor Sample
Zeolite products
A12-72 A13-106 A5-99 A4-100 A1-90 A13-115 A6-80
Na O* % 2
Mother liquor
Time (h)
Max. RC %
Moduli
Na O % 2
18 20 20 16 16 16 16
44 60 85 89 80 77 61
1.80 1.79 1.99 2.03 1.97 1.81 1.79
1.39 1.71 1.89 2.03 2.69 3.38 3.61
1.70 2.01 2.86 3.00
*Which is Na O(%) in the liquid phase of the initial gel. 2
the time of crystallization is significantly reduced (see Table 2). 3.2. The compositions of the crystallization mother liquor The RC of the products depend heavily on the compositions of the crystallization mother liquor. The Na+ concentration of the mother liquor which is too high or too low has detrimental effect on the process of crystallization. The Na+ concentration of the liquid phase of the initial gel is less than the mother liquor’s, which shows that the sodium oxides in the framework of gel dissolve partially during crystallization (see Table 3).
zeolite. The effect of the zeolite on the crystallization shows in the Tables 4 and 5. The zeolite which is dried (100°C×2 h) and added to the synthesis mixture in the form of dry powder improved the synthesis product to some extent. When zeolite crystals are dried or during handling of the dry powder, a lot of microcrystallite dusts are typically formed, which can become viable growing entities in a fresh synthesis solution. This process is called ‘Initial Breeding’, which is enhancing the crystallization rate [14]. The zeolite crystals in the mother liquor are in the form of wet state, and will do harm to the crystallization in the process of the MLR. When the quantities of zeolite crystals are
3.3. The effect of minor quantities zeolite The crystallization mother liquor generally contains minor quantities of finely divided particles of Table 4 Effect of the quantities of zeolite crystals added on the crystallization Samples
Zeolite crystal added (%)b
Time (h)
Max. RC (%)
94-NaY-1a 94-NaY-2 94-NaY-3 94-NaY-4 94-Nay-5 94-NaY-6
0 0 1.0 3.0 6.0 16.9
25 21 18 15 15 15
93 92 90 82 83 82
aWithout MLHG. bRC: 84%, Si/Al: 2.45, in slurry.
Fig. 3. Comparison of crystallization curves.
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C. Liu et al. / Microporous and Mesoporous Materials 25 (1998) 1–6 Table 5 Effect of the properties of zeolite crystals added on the crystallization Zeolite crystals added Samples 1 2 3 4 5 6 7
# # # # # # #
Time (h)
Quantities %
RC %
Si/Al
State
0 7.2 10.8 1 2.0 2.0 3.0
/ 85 85 85 85 85 87
/ 2.40 2.40 2.45 2.45 2.45 2.35
/ dried dried slurry slurry slurry slurry
25 25 25 15 15 15 15
Product Max.RC %
Si/Al
94 96 97 92 85 83 83
2.46 2.42 2.45 2.35 2.29 2.30 2.32
Fig. 4. A model of two-phase transformation of zeolite Y formation in the process of MLR.
more than 2% ( Wt) in the mother liquor, the crystallization time is much more reduced, and the max. RC, Si/Al ratio of the synthesis product drop somewhat in the process of MLR. That is to say, ‘the second generation is not better than the first one’. 3.4. The curves of crystallization Fig. 3 shows the curves of crystallization in the process with either MLR or not. In the former, the time of crystallization is typically 16-18 h, and the max. RC is smaller, while it is 30–36 h without MLR. It is explained that the silicates coming from mother liquor is considerably active and there are minor quantities of finely divided particles of zeolite crystals which act as seed crystals to enhance the crystallization rate.
4. Conclusions The crystallization process and products depend heavily on the nature of the initial gel. In the process of MLR, Si species and Al species are mixed through two steps. First, the MLHG is formed in the mother liquor by adding aluminum sulfate in which the reactant mixture is in a state of weak alkali and low concentration, and results in enhancement of the Si/Al ratio of gel framework advantageously. In the process of filtration, storage and transport, the MLHG is aged deeply, and become a quite active reactant. When the MLHG is put into crystallization reactor, in which additional reactants are added such as sodium silicate solution, sodium aluminate solution et al. the second gelation is carried out promptly, and the reactants react quietly. And so the polyaluminosili-
6
C. Liu et al. / Microporous and Mesoporous Materials 25 (1998) 1–6
cate ions formed come mostly into the liquid phase and the formed gel’s structure is not very stable. The pseudoequilibrium between the solid phases and liquid phases of the gel is attained easily. The crystallization mother liquor generally contains minor quantities of finely divided particles zeolite crystals acting as seeds, and the particles sizes of gel in the process of MLR is comparably small, which lead to reduction of the crystallization time greatly and decrease of RC and Si/Al ratio of the synthesis products. In summary, it is shown that there may occur the dissolution, polycondensation, transport, nucleation in the liquid phase as well as the ordering process in the solid phase in the process of MLR. Based on these results, we can propose a model of two-phase transformation for zeolite Y formation in the process of MLR (see Fig. 4).
Shen Lan for providing the elemental chemical analysis.
Acknowledgement
[11] [12] [13]
The author would like to thank Wu Jianqiang for his helpful discussions and encouragement, and
References [1] [2] [3] [4] [5] [6 ] [7] [8] [9] [10]
[14]
D.W. Breck, E.M. Flanigen, Molecular Sieves, 1968, p.49. S.P. Zhdanov, Adv. Chem. Ser. 101 (1971) 20. Q.H. Shi, Journal of Scientific Research 3, 1984. Z. Gabelica et al., Zeolites Synthesis, Structure, Technology and Application Abstracts, 1984, p. 20. L.E. Iton et al., Langmuir 8 (1992) 1045. S. Kasahara, et al., Studies in Surface Science and Catalysis, vol. 28, Elsevier, Amsterdam, 1986, p. 185. P. Bodart et al., J. Chem. Phys. 83 (1986) 777. A. Thangaraj et al., Zeolites 10 (1990) 117. D.M. Ginter et al., Zeolites 12 (1992) 733–742. R.M. Barrer, The Hydrothermal Chemistry of Zeolites. Academic Press, London, 1982. P.K. Dutta et al., Zeolites 14 (1994) 250. US Patent, 4164551 (1979). S.P. Zhdanov, Molecular Sieves, Society of the Chemical Industry, London, 1968. L.Y. Hou et al., Zeolites 9 (1989) 526.
Study on the mechanism of zeolite Y formation in the process of liquor recycling Conghua Liu *, Xionghou Gao, Yanqing Ma, Zhongliang Pan, Rongrong Tang Petrochemical Research Institute of Lanzhou Oil Refinery, Lanzhou, 730060, People’s Republic of China Received 14 April 1996; received in revised form 23 October 1997; accepted 23 February 1998
Abstract Crystallization of zeolite Y formation in the process of mother liquor recycling (MLR) has been studied. The zeolite crystallization mother liquor comprises a little sodium silicate as well as minor zeolite, Upon addition of aluminum sulfate, the silicate existed in the mother liquor reacts to form finely divided silica–alumina hydrogel (MLHG) which can be utilized later in zeolite Y formation. The particle of MLHG is relatively small and Si/Al of the gel’s framework is a little great. Since amorphous aluminosilicate gel passes through two stages of gelation, the pseudoequilibrium between solid phase and liquid phase of gel is attained rapidly and the time of zeolite Y formation is much reduced. In addition, experiments have also showed that the quantities and the properties of zeolite Y added affect the process of crystallization heavily. Minor quantities of zeolite in the mother liquor have detrimental effect on the zeolite Y formation. Based on studies of crystallization from recycling mother liquor, a model of two-phase transformation for zeolite Y formation has been proposed. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Mechanism; Zeolite Y; Liquor; Silica–alumina hydrogel
1. Introduction Research on mechanism of zeolite formation through hydrothermal chemistry of aluminosilicates has been a challenging subject. There has been appeared two alternative mechanistic points all the time. One is ‘solid-phase transformation’ mechanism in which the amorphous gel is thought to recognize to form crystals [1], another is the ‘solution-mediated transport’ mechanism, in which amorphous gel is thought to dissolve, the depletion of the liquid components of the gel lead to formation of zeolite [2]. Increasingly, it has been recog* Corresponding author.
nized that both these processes may be present in the zeolite system [3–5]. Zeolite Y is one of the most extensively used commercial product. Research on the mechanism of zeolite Y formation has received extensive attention from many investigators [6,7]. There are various reports on the use of n.m.r spectroscopy for the study of soluble aluminosilicate species in the zeolite Y synthesis solution. Thangaraj et al. [8] have identified many kinds of the aluminosilicate polymers and the secondary building blocks present in the aqueous seed solution, and constructed a model of ‘liquid mediate process’ for zeolite Y formation. Ginter et al. [9] have shown that the rate of crystallization and the yield of zeolite Y from colloidal silica depend strongly on
1387-1811/98/$ – see front matter © 1998 Elsevier Science B.V. All rights reserved. PII: S1 3 8 7 -1 8 1 1 ( 9 8 ) 0 0 14 9 - 8
2
C. Liu et al. / Microporous and Mesoporous Materials 25 (1998) 1–6
Fig. 1. Flow-chart of zeolite Y preparation with MLR.
the means of aging. Another report [10] points out that amorphous gel prepared from colloidal silica passes through rearrangement constructively during aging, and produces zeolite Y nuclei. Dutta et al. [11] have found that the mechanism of zeolite formation is related to gel–seed interaction, and the liqud phase of gel provides nutrients for crystal formation. In 1979, Elliott et al. provided an improved method for preparing zeolite Y [12] (Fig. 1). The zeolite crystallization mother liquor comprises a relatively dilute solution of sodium silicate, and contains minor quantities of alumina as well as finely divided particles of zeolite. Upon addition of aluminum sulfate, the silicate existed in the mother liquor reacts to form finely divided silica–alumina hydrogel (MLHG). The MLHG is recovered by a filtration, which will be utilized later in the zeolite synthesis reaction. On average, the utility factor of silicate rises to more than 90%, vs about 50% with no mother liquor recycling (MLR). The process of reaction and the components of this kind of amorphous gel are characterized intrinsically, which lead to different crystallization process and zeolite product. The aim of the present study is to discuss the mechanism of zeolite Y formation in the process of MLR.
2. Experimental The overall stoichiometry of the amorphous gel was ~2.7 Na O, 1 Al O , 8.5 SiO , ~185 H O, 2 2 3 2 2
in the process of zeolite Y formation with MLR or not. The molar ratio of seeds solution was 1 Al O , 16 Na O, 15 SiO , 320 H O. Crystallization 2 3 2 2 2 mother liquor: SiO 40 g/l, moduli 1.95±0.05. 2 MLHG were prepared by adding aluminum sulfate solution to mother liquor, giving overall molar ratio Si/Al=4.0, which were homogenized at 800 rpm, and left to age for 0.5 h at room temperature. MLHG were then filtrated using a Buchner funnel in the condition of vacuum 0.05 Mpa, giving a filter cake containing approximatly 12% solid. Typically, the synthesis reactants were mixed together in orderly way,such as seeds, sodium silicate solution and/or MLHG cake, aluminum sulfate solution and sodium aluminate solution. The SiO recovered from mother liquor is about 2 35% of the overall. The freshly prepared gels were homogenized at 800 rpm, poured into 3 l stainless steel buckets, and left to age at room temperature. After aging for a period of 0.5 h, each batch was heated at 95°C in a water bath to promote crystallization. Samples of the gel were taken as scheduled. The compositions of the solid and liquid phases of gel were provided by elemental chemical analysis. Identification of the solid phase was done using a Rigaku (D/Max-3C ) X-ray diffractometer with Cu radiation and a Ni filter. The Si/Al ratio of the framework was abtained from the diffraction data using the strongest peaks in the area 2 theta= 31°~32°, using sodium chloride as internal standard sample. The relative crystallinity (RC ) of the products were determined using the ratio of
3
C. Liu et al. / Microporous and Mesoporous Materials 25 (1998) 1–6
diffraction peak intensity in the area of Miller indices 533 (hkl ). The particle sizes of the gel were examined by the Malvern (3600Ec) Laser diffraction particle sizers.
3. Results and discussions 3.1. Solid-phase composition and particle size Upon heating, the gel changed gradually, the Si/Al ratios of the gel framework determined by elemental analysis decreased and the Si/Al ratios determined by X-ray diffraction increased during crystallization, in which the former was a little more than the later in the final zeolite product, and the results are show in Table 1. It has been proposed that the silica dissolve slowly, releasing silicate anions into the liquid phase, and Al species deplete from the solution crystallization. Besides, decrease of the Na/Al ratio of the gel framework is probably owing to the following changes during the formation of crystalline framework [11]
Table 2 Effect of the gel particle sizes on crystallization time Sample
Initial gel average particle sizes (mm)
Time (h)
Max. RC (%)
A5-39* A13-117* A11-116 A3-37 A12-106 A5-303 A16-325 A10-326
30.0 21.7 12.1 13.3 14.6 20.0 23.0 24.5
28 30 8 14 16 18 22 22
90 91 82 83 82 86 85 85
*Without MLHG.
¬Si−O−Na++HO−T¬ ¬Si−O−T¬+Na++OH− ( T=Si or Al ).
Table 1 Composition of gel framework during the crystallization Crystallization
Elemental analysisa
XRD
time (h)
Si/Al
Na O/Al O 2 2 3
Si/Al
0b 0 2 4 6 8 10 12 14 16 18 20
3.5 3.2 3.1 3.1 3.0 2.7 2.6 2.6 2.6 2.6 2.6 2.5
1.32 1.28 1.25 1.24 1.21 1.10 1.11 1.10 1.10 1.10 0.95 0.99
/ / 1.25 1.50 1.90 2.00 2.00 2.25 2.32 2.39 2.40 2.40
aBefore determining, washed by water (50°C ), which is five times the volume of the gel in three parts. bSample is MLHG.
Fig. 2. The change of the gel particle sizes during crystallization.
The occurrence of such a process is supported by the observation of an increase in the Na+ concentration in solution during the crystallization (see Table 3). Zhdanov investigated, in detail, the chemical compositions of the liquid and solid components of the amorphous gel in the process of zeolite formation [13]. The particle sizes of the gel become smaller and smaller during crystallization (see Fig. 2), which decline a great deal, then keep the same point at the last stage of crystallization. The end point of crystallization can be judged by the tendency of the gel particle size change. The particle sizes of the initial gel containing MLHG is much smaller, and
4
C. Liu et al. / Microporous and Mesoporous Materials 25 (1998) 1–6
Table 3 Relationship between zeolite products and compositions of the mother liquor Sample
Zeolite products
A12-72 A13-106 A5-99 A4-100 A1-90 A13-115 A6-80
Na O* % 2
Mother liquor
Time (h)
Max. RC %
Moduli
Na O % 2
18 20 20 16 16 16 16
44 60 85 89 80 77 61
1.80 1.79 1.99 2.03 1.97 1.81 1.79
1.39 1.71 1.89 2.03 2.69 3.38 3.61
1.70 2.01 2.86 3.00
*Which is Na O(%) in the liquid phase of the initial gel. 2
the time of crystallization is significantly reduced (see Table 2). 3.2. The compositions of the crystallization mother liquor The RC of the products depend heavily on the compositions of the crystallization mother liquor. The Na+ concentration of the mother liquor which is too high or too low has detrimental effect on the process of crystallization. The Na+ concentration of the liquid phase of the initial gel is less than the mother liquor’s, which shows that the sodium oxides in the framework of gel dissolve partially during crystallization (see Table 3).
zeolite. The effect of the zeolite on the crystallization shows in the Tables 4 and 5. The zeolite which is dried (100°C×2 h) and added to the synthesis mixture in the form of dry powder improved the synthesis product to some extent. When zeolite crystals are dried or during handling of the dry powder, a lot of microcrystallite dusts are typically formed, which can become viable growing entities in a fresh synthesis solution. This process is called ‘Initial Breeding’, which is enhancing the crystallization rate [14]. The zeolite crystals in the mother liquor are in the form of wet state, and will do harm to the crystallization in the process of the MLR. When the quantities of zeolite crystals are
3.3. The effect of minor quantities zeolite The crystallization mother liquor generally contains minor quantities of finely divided particles of Table 4 Effect of the quantities of zeolite crystals added on the crystallization Samples
Zeolite crystal added (%)b
Time (h)
Max. RC (%)
94-NaY-1a 94-NaY-2 94-NaY-3 94-NaY-4 94-Nay-5 94-NaY-6
0 0 1.0 3.0 6.0 16.9
25 21 18 15 15 15
93 92 90 82 83 82
aWithout MLHG. bRC: 84%, Si/Al: 2.45, in slurry.
Fig. 3. Comparison of crystallization curves.
5
C. Liu et al. / Microporous and Mesoporous Materials 25 (1998) 1–6 Table 5 Effect of the properties of zeolite crystals added on the crystallization Zeolite crystals added Samples 1 2 3 4 5 6 7
# # # # # # #
Time (h)
Quantities %
RC %
Si/Al
State
0 7.2 10.8 1 2.0 2.0 3.0
/ 85 85 85 85 85 87
/ 2.40 2.40 2.45 2.45 2.45 2.35
/ dried dried slurry slurry slurry slurry
25 25 25 15 15 15 15
Product Max.RC %
Si/Al
94 96 97 92 85 83 83
2.46 2.42 2.45 2.35 2.29 2.30 2.32
Fig. 4. A model of two-phase transformation of zeolite Y formation in the process of MLR.
more than 2% ( Wt) in the mother liquor, the crystallization time is much more reduced, and the max. RC, Si/Al ratio of the synthesis product drop somewhat in the process of MLR. That is to say, ‘the second generation is not better than the first one’. 3.4. The curves of crystallization Fig. 3 shows the curves of crystallization in the process with either MLR or not. In the former, the time of crystallization is typically 16-18 h, and the max. RC is smaller, while it is 30–36 h without MLR. It is explained that the silicates coming from mother liquor is considerably active and there are minor quantities of finely divided particles of zeolite crystals which act as seed crystals to enhance the crystallization rate.
4. Conclusions The crystallization process and products depend heavily on the nature of the initial gel. In the process of MLR, Si species and Al species are mixed through two steps. First, the MLHG is formed in the mother liquor by adding aluminum sulfate in which the reactant mixture is in a state of weak alkali and low concentration, and results in enhancement of the Si/Al ratio of gel framework advantageously. In the process of filtration, storage and transport, the MLHG is aged deeply, and become a quite active reactant. When the MLHG is put into crystallization reactor, in which additional reactants are added such as sodium silicate solution, sodium aluminate solution et al. the second gelation is carried out promptly, and the reactants react quietly. And so the polyaluminosili-
6
C. Liu et al. / Microporous and Mesoporous Materials 25 (1998) 1–6
cate ions formed come mostly into the liquid phase and the formed gel’s structure is not very stable. The pseudoequilibrium between the solid phases and liquid phases of the gel is attained easily. The crystallization mother liquor generally contains minor quantities of finely divided particles zeolite crystals acting as seeds, and the particles sizes of gel in the process of MLR is comparably small, which lead to reduction of the crystallization time greatly and decrease of RC and Si/Al ratio of the synthesis products. In summary, it is shown that there may occur the dissolution, polycondensation, transport, nucleation in the liquid phase as well as the ordering process in the solid phase in the process of MLR. Based on these results, we can propose a model of two-phase transformation for zeolite Y formation in the process of MLR (see Fig. 4).
Shen Lan for providing the elemental chemical analysis.
Acknowledgement
[11] [12] [13]
The author would like to thank Wu Jianqiang for his helpful discussions and encouragement, and
References [1] [2] [3] [4] [5] [6 ] [7] [8] [9] [10]
[14]
D.W. Breck, E.M. Flanigen, Molecular Sieves, 1968, p.49. S.P. Zhdanov, Adv. Chem. Ser. 101 (1971) 20. Q.H. Shi, Journal of Scientific Research 3, 1984. Z. Gabelica et al., Zeolites Synthesis, Structure, Technology and Application Abstracts, 1984, p. 20. L.E. Iton et al., Langmuir 8 (1992) 1045. S. Kasahara, et al., Studies in Surface Science and Catalysis, vol. 28, Elsevier, Amsterdam, 1986, p. 185. P. Bodart et al., J. Chem. Phys. 83 (1986) 777. A. Thangaraj et al., Zeolites 10 (1990) 117. D.M. Ginter et al., Zeolites 12 (1992) 733–742. R.M. Barrer, The Hydrothermal Chemistry of Zeolites. Academic Press, London, 1982. P.K. Dutta et al., Zeolites 14 (1994) 250. US Patent, 4164551 (1979). S.P. Zhdanov, Molecular Sieves, Society of the Chemical Industry, London, 1968. L.Y. Hou et al., Zeolites 9 (1989) 526.