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Y zeolite from kaolin taken in Yen Bai-Vietnam: synthesis, characterization and catalytic activity for the cracking of n-heptane
159 Studies in Surface Science and Catalysis, volume 159 Hyun-Ku Rhee, In-Sik Nam and Jong Moon Park (Editors) © 2006 Elsevier B.V. All rights reserved
197 197
Y zeolite from kaolin taken in Yen Bai-Vietnam: synthesis, characterization and catalytic activity for the cracking of n-heptane Ta Ngoc Don(ll), Vu Dao Thang(ac), Pham Thanh Huyen*(bc), Pham Minh Hao(a), Nguyen Khanh Dieu Hong*' (a) Department of organic chemistry, [email protected]. vn; (b> Department of organic and petrochemical technology, [email protected]; (c) Laboratory of petrochemical and catalysis materials, Faculty of chemical technology, Hanoi university of technology, 1 Dai Co Viet str., Hanoi, Vietnam ABSTRACT This paper is concerned with the synthesis of Y zeolite with SiC^/A^Oa ratio of 4.5 from kaolin taken in Yen Bai-Vietnam and their catalytic activity for the cracking of n-heptane. The synthesized sample (NaYl) showed the Y zeolite crystallinity of 53% and PI zeolite crystallinity of 32%, and exhibited good thermal stability up to 880°C. The activity and the stability of HY1 turned out to be lower than those of standard sample (HYs), but the toluene selectivity was higher. The conversion of n-heptane to toluene might be due to the metal oxide impurities, which was present in the raw materials and this indicates the potential application of this zeolite for the conversion of n-paraffm to aromatics. 1. INTRODUCTION Y zeolites synthesized from pure chemicals have now been used as the main composition of FCC catalysts [1-4]. However, the application of Y zeolites synthesized from kaolin in the catalytic processes is still limited. The refinery and petrochemical industry is being built in Vietnam, so the synthesis of Y zeolites from domestic materials and minerals is necessary [4]. In this paper, the initial results in the synthesis of Y zeolites with SiCVAkCh ratio of 4.5 from kaolin taken in Yen Bai-Vietnam and their catalytic activity for the cracking of n-heptane are reported. 2. EXPERIMENTAL NaY zeolite was synthesized with the gel composition of 3.5Na2O.Al2O3.7SiO2.150H2O from Vietnamese kaolin, liquid glass and organic template, NaOH, NaCl and distilled water. The crystallization was carried out at 95°C for 24 hours. Synthesized sample (noted as NaYl) was converted to acid form (noted as HY1) and subsequently calcined at 650°C in nitrogen. Standard NaY zeolite (noted as NaYs) synthesized from pure chemicals was treated simultaneously and the obtained acid form was noted as HYs. NaYl, NaYs, HY1 and HYs were characterized by XRD (Siemens D5005), IR (Shimadzu FTIR 8100), SEM (JSM 5410 LV), BET (Coulter SA3100), cation exchanging capacity (CEC) and benzene adsorption (ACeHe). The catalytic tests for the cracking of n-heptane on HY1 and HYs were carried out by flow method at 450-600°C and reaction time of 5-30 minutes.
198 198
3. RESULTS AND DISCUSSION 3.1. Synthesis and characterization of Y zeolite. Only single crystalline phase was observed in the XRD pattern of NaYs (Fig.l), and this indicates that the crystallinity of NaYs is 100%, NaYl was found to contain mainly the crystalline phase of NaY. The presence of the NaPi crystalline phase and trace of quartz crystalline phase are also observed. Since there is no kaolinite crystalline phase, it is confirmed that the kaolin in the raw materials was completely converted. NaY zeolite in NaYl has the PDF 43-0168 with the formula of Na2Al2Si4.5O13.xH2O similar to that of NaYs. It is crystallized in cubic form with ao=24.676 A and SiO2/Al2O3 ratio of 4.5 [3]. NaPi zeolite in NaYl has the PDF 39-0219 with the formula of Na6Al6Siio032.12H20, and it is crystallized in tetragonal form with ao=10.043 A and SiO2/Al2O3 ratio of 3.33 [4]. 300
Lin (Cps)
Y
(a)
(a)
200
P1 100
742 724
1152
600
0 300
55
10 10
20 20
30 30
Lin (Cps)
Y Y
(b)
40 40
575 998
(b)
200
1136
100 100
1
0 5
n 1\i -.1,
10
20
456
789 724 575 464 1017
! '. 1
30
40 2-Theta-Scale
Fig. 1. XRD patterns of NaYl (a) and NaYs (b)
1300
1200
1000
800
600
cm–11 400
Fig. 2. IR spectra of NaYl (a) and NaYs (b)
The IR spectra of NaYl and NaYs (Fig.2) are very similar to each other. In the IR spectrum of NaYl, beside the bands at 575 and 724 cm"' assigned for the Y zeolite, there are additional bands at 600 and 742 cm"1 representing Pi zeolite [5]. The other bands are shifted to lower wave lengths compared to that in IR spectrum of NaYs and thus the SiO2/Al2O3 ratio in NaYl is lower than that in NaYs. This is in accordance with the result obtained from XRD analysis. The band at 575 cm"1 in NaYl spectrum has a much stronger intensity than the band at 600 cm"1. This again indicates that NaYl sample contains mainly Y zeolite.
Fig. 3. SEM micrographs of NaYl (left-hand side) and NaYs (right-hand side). Fig. 3 shows that NaYl contains two types of crystals: Y zeolite in cubic form and Pj zeolite in spherical form with diameter of about 5 um and 3 um, respectively. On the other hand, NaYs contains only crystals of Y zeolite in spherical form with diameter of about 0.5 um. It is clearly demonstrated that the diameter of zeolite synthesized from kaolin is much larger than that of zeolite synthesized from pure chemicals. The data of CEC and ACeHs, thermal stability and SiO2/Al2O3 ratio... are presented in Table 1 and these results show that NaY] is a porous material which contains a large number
199 199
of negative charges in the network. NaYl seems to meet the requirement as a good catalyst for the conversion of hydrocarbon, and thus we carried out ion exchange in NH4CI solution and then calcination at 650°C to obtain the acidic form HY1 and HYs. The Na+ exchange levels of HY1 and HYs are 87.15 and 95.84%, respectively. Calcination at 650°C not only increases the Na+ exchange levels but also brings about the collapse of the Pi structure and keeps the stable structure of Y zeolite in HY1 [6]. The BET surface area of HY1 is 284 m2/g with a crystallinity of 40%. The conversion of Pi zeolite to the amorphous phase which supports Y zeolite makes HY1 suitable for the cracking of hydrocarbon [1,6]. Table 1. Characteristics of NaYl and NaYs zeolites. Sample CEC, meq AC6H<, Y zeolite PI zeolite Thermal SiO2/Al2O3 Fe2O3 FeO TiO2 Ba2+/100g % crystallinity crystallinity stability, "C ratio wt.% wt. % wt.% NaYl NaYs
216 235
18,0 20,2
53 100
32 0
880 900
4.38 4.52
0.40
0.05
0.08
3.2. Catalytic activity in the cracking of n-heptane Conversion (C) of n-heptane, composition of products and selectivities of toluene and gas products at different temperatures are presented in Table 2 and Fig. 4. Clearly, the conversion of n-heptane and the selectivity of toluene increase with temperature, whereas the selectivity of gas products decreases. At the same temperature the conversion and selectivity of gas products on HY1 are slightly lower than that on HYs, but the selectivity of toluene is higher. Table 2. Conversion (C) of n-heptane and composition of products at different temperatures with the reaction time of 20 minutes Sample
HY1
HYs
Reaction n-heptane Product composition, % wt. Selectivity (S), % temperature conversion n-heptane Toluene Gas Other liquid Toluene Gas °C (Q, % products product products 450 18.1 81.9 5.5 10.8 1.8 30.39 59.67 32.6 500 67.4 10.6 19.3 2.7 32.52 59.2 54.1 550 45.9 22.7 20.3 2.9 49.46 44.23 600 51.7 48.3 26.3 21.2 4.2 50.87 41.01 450 21.4 78.6 3.7 14.9 2.8 17.29 69.63 500 35.8 64.2 8.6 23.9 3.3 24.02 66.76 550 51.8 48.2 18.1 29.6 4.1 34.94 57.14 600 58.7 23.4 50.77 41.3 29.8 5.5 39.86
As shown in Table 2, during the cracking process, two main types of product are obtained: gas products, including light paraffins and olefins, and toluene. These products are formed by the breaking of C-C bonds to form light paraffins and olefins, the oligomerization of light olefins and then the cyclization of obtained oligomers, the hydrogen transfer between cyclized oligomers and olefins to form aromatics. Moreover, because of the large amount of toluene in the products, dehydro-cyclization of n-heptane directly to toluene might occur. The reforming process is carried out on bifunctional catalyst [2] and the metal sites promote the dehydrogenation and cyclization. Hence, the impurities in HY1, such as Fe2O3, FeO, TiC«2 with the content of 0.4; 0.05 and 0.08 wt%, respectively (table 1), can promote the formation of toluene and thus the selectivity of toluene on HY1 catalyst is higher than that on HYs. 2-methylhexane and 3-methylhexane are also found in the liquid product. So another reaction, isomerization of n-heptane, might have taken place. However, the activation energy of isomerization is higher than that of dehydro-cyclization [2], the content of isomers in the product is much lower than that of other products.
200
80
S, %
C, %
70
70 60
HY1
50
60
Toluene 50
HYs
HY1 HYs
40 HY1
30
HYs
20
Gas product
40 30
10 0 450
500
550
600
T,
20 5
10
15
20
25
30
t, min
Fig.4. Selectivities to toluene and gas products Fig.5. Influence of reaction time (t) on the at different temperatures (with reaction conversion of n-heptane (C) at 550°C. time of 20 minutes) The stability of catalyst is one of the most important criteria to evaluate its quality. The influence of time on stream on the conversion of n-heptane at 550°C is shown in Fig. 5. The conversion of n-heptane decreases faster on HY1 than on HYs with time, so the question is "Could the formation of coke on the catalyst inhibit diffusion of reactant into the caves and pores of zeolite and decrease the conversion?" According to Hollander [8], coke was mainly formed at the beginning of the reaction, and the reaction time did not affect the yield of coke. Hence, this decrease might be caused by some impurities introduced during the catalyst synthesis. These impurities could be sintered and cover active sites to make the conversion of n-heptane on HY1 decrease faster. 4. CONCLUSION NaY zeolite with SiCVAkCh ratio of 4.5 has been synthesized successfully from Vietnamese kaolin under hydrothermal condition, at 95°C and atmospheric pressure, synthesis time of 24 hours, under the presence of organic template. Y zeolite synthesized from kaolin can be used for the cracking of hydrocarbons to obtain lighter ones and for the conversion of n-paraffin to aromatics. The metal oxide impurities introduced during the synthesis of catalyst cannot be avoided. However, these impurities can contribute to the increase of catalytic activity to some extent for the formation of toluene in the cracking of n-heptane. These results indicate that Y zeolite can be synthesized from plentiful raw materials and minerals in Vietnam to apply as catalysts for the petrochemical and refining industry. ACKNOWLEDGEMENTS Financial support of this work by Project VLIR/HUT IUC/PJ1 from the VLIR/HUT research fund in the, co-operation between Hanoi University of Technology, Vietnam and Flemish Universities, Belgium is gratefully acknowledged. REFERENCES [1]. Scherzer J., Catal. Rev. - Sci. Eng., 31(3), 1989, pp. 215-354. [2]. Gates B.C., Kazer J.R. and G.C.A Schuit, Chemistry of catalytic processes, McGraw-Hill, New York, 1979. [3]. Bergeret G. et al, Unit cell data, J. Phys. Chem., 87,1983, p. 1160 ( ICDD Grant-in-Aid, 1991). [4]. Barlocher Ch., Meier W. M., Unit cell data, J. Kristallchem., 135,1972, p. 339. [5]. K. Kurita, K. Tomita, T. Tada, S. Ishii, F. S. Nishimura, K. Shimoda. J. Polym. Sci., Part A Polym. Chem. Vol. 31,1993, pp. 485-491. [6]. Breck D.W., Zeolite Molecular Sieves, A Wiley, publication, New York, 1974. [7]. Ta Ngoc Don, Ph. D Thesis, 2002, Hanoi, Vietnam. [8]. Hollander Ir. M.A., PhD-projects of the OSPT, 1996.
197 197
Y zeolite from kaolin taken in Yen Bai-Vietnam: synthesis, characterization and catalytic activity for the cracking of n-heptane Ta Ngoc Don(ll), Vu Dao Thang(ac), Pham Thanh Huyen*(bc), Pham Minh Hao(a), Nguyen Khanh Dieu Hong*' (a) Department of organic chemistry, [email protected]. vn; (b> Department of organic and petrochemical technology, [email protected]; (c) Laboratory of petrochemical and catalysis materials, Faculty of chemical technology, Hanoi university of technology, 1 Dai Co Viet str., Hanoi, Vietnam ABSTRACT This paper is concerned with the synthesis of Y zeolite with SiC^/A^Oa ratio of 4.5 from kaolin taken in Yen Bai-Vietnam and their catalytic activity for the cracking of n-heptane. The synthesized sample (NaYl) showed the Y zeolite crystallinity of 53% and PI zeolite crystallinity of 32%, and exhibited good thermal stability up to 880°C. The activity and the stability of HY1 turned out to be lower than those of standard sample (HYs), but the toluene selectivity was higher. The conversion of n-heptane to toluene might be due to the metal oxide impurities, which was present in the raw materials and this indicates the potential application of this zeolite for the conversion of n-paraffm to aromatics. 1. INTRODUCTION Y zeolites synthesized from pure chemicals have now been used as the main composition of FCC catalysts [1-4]. However, the application of Y zeolites synthesized from kaolin in the catalytic processes is still limited. The refinery and petrochemical industry is being built in Vietnam, so the synthesis of Y zeolites from domestic materials and minerals is necessary [4]. In this paper, the initial results in the synthesis of Y zeolites with SiCVAkCh ratio of 4.5 from kaolin taken in Yen Bai-Vietnam and their catalytic activity for the cracking of n-heptane are reported. 2. EXPERIMENTAL NaY zeolite was synthesized with the gel composition of 3.5Na2O.Al2O3.7SiO2.150H2O from Vietnamese kaolin, liquid glass and organic template, NaOH, NaCl and distilled water. The crystallization was carried out at 95°C for 24 hours. Synthesized sample (noted as NaYl) was converted to acid form (noted as HY1) and subsequently calcined at 650°C in nitrogen. Standard NaY zeolite (noted as NaYs) synthesized from pure chemicals was treated simultaneously and the obtained acid form was noted as HYs. NaYl, NaYs, HY1 and HYs were characterized by XRD (Siemens D5005), IR (Shimadzu FTIR 8100), SEM (JSM 5410 LV), BET (Coulter SA3100), cation exchanging capacity (CEC) and benzene adsorption (ACeHe). The catalytic tests for the cracking of n-heptane on HY1 and HYs were carried out by flow method at 450-600°C and reaction time of 5-30 minutes.
198 198
3. RESULTS AND DISCUSSION 3.1. Synthesis and characterization of Y zeolite. Only single crystalline phase was observed in the XRD pattern of NaYs (Fig.l), and this indicates that the crystallinity of NaYs is 100%, NaYl was found to contain mainly the crystalline phase of NaY. The presence of the NaPi crystalline phase and trace of quartz crystalline phase are also observed. Since there is no kaolinite crystalline phase, it is confirmed that the kaolin in the raw materials was completely converted. NaY zeolite in NaYl has the PDF 43-0168 with the formula of Na2Al2Si4.5O13.xH2O similar to that of NaYs. It is crystallized in cubic form with ao=24.676 A and SiO2/Al2O3 ratio of 4.5 [3]. NaPi zeolite in NaYl has the PDF 39-0219 with the formula of Na6Al6Siio032.12H20, and it is crystallized in tetragonal form with ao=10.043 A and SiO2/Al2O3 ratio of 3.33 [4]. 300
Lin (Cps)
Y
(a)
(a)
200
P1 100
742 724
1152
600
0 300
55
10 10
20 20
30 30
Lin (Cps)
Y Y
(b)
40 40
575 998
(b)
200
1136
100 100
1
0 5
n 1\i -.1,
10
20
456
789 724 575 464 1017
! '. 1
30
40 2-Theta-Scale
Fig. 1. XRD patterns of NaYl (a) and NaYs (b)
1300
1200
1000
800
600
cm–11 400
Fig. 2. IR spectra of NaYl (a) and NaYs (b)
The IR spectra of NaYl and NaYs (Fig.2) are very similar to each other. In the IR spectrum of NaYl, beside the bands at 575 and 724 cm"' assigned for the Y zeolite, there are additional bands at 600 and 742 cm"1 representing Pi zeolite [5]. The other bands are shifted to lower wave lengths compared to that in IR spectrum of NaYs and thus the SiO2/Al2O3 ratio in NaYl is lower than that in NaYs. This is in accordance with the result obtained from XRD analysis. The band at 575 cm"1 in NaYl spectrum has a much stronger intensity than the band at 600 cm"1. This again indicates that NaYl sample contains mainly Y zeolite.
Fig. 3. SEM micrographs of NaYl (left-hand side) and NaYs (right-hand side). Fig. 3 shows that NaYl contains two types of crystals: Y zeolite in cubic form and Pj zeolite in spherical form with diameter of about 5 um and 3 um, respectively. On the other hand, NaYs contains only crystals of Y zeolite in spherical form with diameter of about 0.5 um. It is clearly demonstrated that the diameter of zeolite synthesized from kaolin is much larger than that of zeolite synthesized from pure chemicals. The data of CEC and ACeHs, thermal stability and SiO2/Al2O3 ratio... are presented in Table 1 and these results show that NaY] is a porous material which contains a large number
199 199
of negative charges in the network. NaYl seems to meet the requirement as a good catalyst for the conversion of hydrocarbon, and thus we carried out ion exchange in NH4CI solution and then calcination at 650°C to obtain the acidic form HY1 and HYs. The Na+ exchange levels of HY1 and HYs are 87.15 and 95.84%, respectively. Calcination at 650°C not only increases the Na+ exchange levels but also brings about the collapse of the Pi structure and keeps the stable structure of Y zeolite in HY1 [6]. The BET surface area of HY1 is 284 m2/g with a crystallinity of 40%. The conversion of Pi zeolite to the amorphous phase which supports Y zeolite makes HY1 suitable for the cracking of hydrocarbon [1,6]. Table 1. Characteristics of NaYl and NaYs zeolites. Sample CEC, meq AC6H<, Y zeolite PI zeolite Thermal SiO2/Al2O3 Fe2O3 FeO TiO2 Ba2+/100g % crystallinity crystallinity stability, "C ratio wt.% wt. % wt.% NaYl NaYs
216 235
18,0 20,2
53 100
32 0
880 900
4.38 4.52
0.40
0.05
0.08
3.2. Catalytic activity in the cracking of n-heptane Conversion (C) of n-heptane, composition of products and selectivities of toluene and gas products at different temperatures are presented in Table 2 and Fig. 4. Clearly, the conversion of n-heptane and the selectivity of toluene increase with temperature, whereas the selectivity of gas products decreases. At the same temperature the conversion and selectivity of gas products on HY1 are slightly lower than that on HYs, but the selectivity of toluene is higher. Table 2. Conversion (C) of n-heptane and composition of products at different temperatures with the reaction time of 20 minutes Sample
HY1
HYs
Reaction n-heptane Product composition, % wt. Selectivity (S), % temperature conversion n-heptane Toluene Gas Other liquid Toluene Gas °C (Q, % products product products 450 18.1 81.9 5.5 10.8 1.8 30.39 59.67 32.6 500 67.4 10.6 19.3 2.7 32.52 59.2 54.1 550 45.9 22.7 20.3 2.9 49.46 44.23 600 51.7 48.3 26.3 21.2 4.2 50.87 41.01 450 21.4 78.6 3.7 14.9 2.8 17.29 69.63 500 35.8 64.2 8.6 23.9 3.3 24.02 66.76 550 51.8 48.2 18.1 29.6 4.1 34.94 57.14 600 58.7 23.4 50.77 41.3 29.8 5.5 39.86
As shown in Table 2, during the cracking process, two main types of product are obtained: gas products, including light paraffins and olefins, and toluene. These products are formed by the breaking of C-C bonds to form light paraffins and olefins, the oligomerization of light olefins and then the cyclization of obtained oligomers, the hydrogen transfer between cyclized oligomers and olefins to form aromatics. Moreover, because of the large amount of toluene in the products, dehydro-cyclization of n-heptane directly to toluene might occur. The reforming process is carried out on bifunctional catalyst [2] and the metal sites promote the dehydrogenation and cyclization. Hence, the impurities in HY1, such as Fe2O3, FeO, TiC«2 with the content of 0.4; 0.05 and 0.08 wt%, respectively (table 1), can promote the formation of toluene and thus the selectivity of toluene on HY1 catalyst is higher than that on HYs. 2-methylhexane and 3-methylhexane are also found in the liquid product. So another reaction, isomerization of n-heptane, might have taken place. However, the activation energy of isomerization is higher than that of dehydro-cyclization [2], the content of isomers in the product is much lower than that of other products.
200
80
S, %
C, %
70
70 60
HY1
50
60
Toluene 50
HYs
HY1 HYs
40 HY1
30
HYs
20
Gas product
40 30
10 0 450
500
550
600
T,
20 5
10
15
20
25
30
t, min
Fig.4. Selectivities to toluene and gas products Fig.5. Influence of reaction time (t) on the at different temperatures (with reaction conversion of n-heptane (C) at 550°C. time of 20 minutes) The stability of catalyst is one of the most important criteria to evaluate its quality. The influence of time on stream on the conversion of n-heptane at 550°C is shown in Fig. 5. The conversion of n-heptane decreases faster on HY1 than on HYs with time, so the question is "Could the formation of coke on the catalyst inhibit diffusion of reactant into the caves and pores of zeolite and decrease the conversion?" According to Hollander [8], coke was mainly formed at the beginning of the reaction, and the reaction time did not affect the yield of coke. Hence, this decrease might be caused by some impurities introduced during the catalyst synthesis. These impurities could be sintered and cover active sites to make the conversion of n-heptane on HY1 decrease faster. 4. CONCLUSION NaY zeolite with SiCVAkCh ratio of 4.5 has been synthesized successfully from Vietnamese kaolin under hydrothermal condition, at 95°C and atmospheric pressure, synthesis time of 24 hours, under the presence of organic template. Y zeolite synthesized from kaolin can be used for the cracking of hydrocarbons to obtain lighter ones and for the conversion of n-paraffin to aromatics. The metal oxide impurities introduced during the synthesis of catalyst cannot be avoided. However, these impurities can contribute to the increase of catalytic activity to some extent for the formation of toluene in the cracking of n-heptane. These results indicate that Y zeolite can be synthesized from plentiful raw materials and minerals in Vietnam to apply as catalysts for the petrochemical and refining industry. ACKNOWLEDGEMENTS Financial support of this work by Project VLIR/HUT IUC/PJ1 from the VLIR/HUT research fund in the, co-operation between Hanoi University of Technology, Vietnam and Flemish Universities, Belgium is gratefully acknowledged. REFERENCES [1]. Scherzer J., Catal. Rev. - Sci. Eng., 31(3), 1989, pp. 215-354. [2]. Gates B.C., Kazer J.R. and G.C.A Schuit, Chemistry of catalytic processes, McGraw-Hill, New York, 1979. [3]. Bergeret G. et al, Unit cell data, J. Phys. Chem., 87,1983, p. 1160 ( ICDD Grant-in-Aid, 1991). [4]. Barlocher Ch., Meier W. M., Unit cell data, J. Kristallchem., 135,1972, p. 339. [5]. K. Kurita, K. Tomita, T. Tada, S. Ishii, F. S. Nishimura, K. Shimoda. J. Polym. Sci., Part A Polym. Chem. Vol. 31,1993, pp. 485-491. [6]. Breck D.W., Zeolite Molecular Sieves, A Wiley, publication, New York, 1974. [7]. Ta Ngoc Don, Ph. D Thesis, 2002, Hanoi, Vietnam. [8]. Hollander Ir. M.A., PhD-projects of the OSPT, 1996.