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Skeletal isometrization of 1-hexene to isohexenes over zeolite catalysts
Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
747
Skeletal Isomerization of 1-Hexene to Isohexenes over Zeolite Catalysts Zhihua Wu, Qingxia Wang, Longya Xu and Sujuan Xie Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian 116023, E R. China Several zeolite catalysts such as SAPO-11, ZSM-11, ZSM-12, etc. were selected to convert 1-hexene to branched hexenes in this work. Pore size of the zeolite catalyst plays an important role on the yield and the distribution of branched isohexenes. And the zeolite catalysts with the pore size of 0.6nm are optimum to produce dimethylbutenes (DMB). SAPO-11 zeolite is a suitable skeletal isomerization catalyst, especially in the production of methyl pentenes. Under the following reaction conditions: WHSV=I.0 h~, HJhexene=8, T=250 ~ P=0.2 MPa, the yield of skeletal isohexenes remains above 80% at the prolonged time-on stream of 80 h, accompanying low C5., C7+ products and low carbon deposition on the catalyst. 1. INTRODUCTION The catalytic reactions for converting unbranched olefins into branched olefins, such as the skeletal isomerization of n-butenes to isobutene, are important processes for the large-scale production of raw materials for chemical industry. To guide the screening of catalysts for the desired processes, tremendous of work has also been devoted to the mechanistic studies of these processes. To date, there are at least two proposed models for the skeletal isomerization of olefins, monomer model and dimerization model. Guisnete [1] reported that there were three steps from n-butenes to isobutene: (i) dimerization of n-butenes, (ii) skeletal isomerization of dimers, and (iii) cracking of the octene isomers. In contrast, Houzvickn [2] proposed that the dominating process for the skeletal n-butene isomerization was monomolecular and the bimolecular mechanism was mainly responsible for the formation of byproducts, such as propene and pentenes. Also, the results of Mooiweer [3] favored the mechanism of skeletal n-pentenes isomerization to isopentenes to be monomolecular. Isomerization reactions of olefins are affected by various factors. Asensi reported that the selectivity of n-butene to isobutene was greatly improved with the increased Si/A1 ratio in MCM-22. Further characterization of these catalysts revealed that the increased Si/A1 ratio led to a lower acid site density. Since these acid sites were proposed to the sites for the bimolecular side-reaction, a decreased acid site density in those catalysts was attributed to the increased isobutene selectivity [4]. Besides the acid site density, the pore size of the zeolites also affects the selectivity of the isomerization reaction. The results of Feng [5] indicated that the outcome of 2-methyl-2-pentene isomerization reaction was also greatly influenced by the pore sizes of zeolites. This was supported by the feeding experiment with several octenes over open-surface and microporous materials and it was found that the 10-membered ring (10-MR) channels were hardly accessible to double-branched hydrocarbons and the diffusion through the 10-MR by triple-branched were denied [2].
748 In the present paper, the catalytic performance of zeolites for the isomerization of 1-hexene to branched hexenes was investigated in a continuous-flow fixed bed reactor. Reported herein are the preliminary skeletal isomerization results. 2. EXPERIMENTAL
2.1. Catalyst preparation ZSM-11 (Si/AI=700), ZSM-35 (Si/AI=15) and ZSM-12 (Si/AI=50) zeolites were synthesized in our laboratory. SAPO-11 and Y-type zeolites were produced by another laboratory in our Institute of Chemical Physics. Si-ZSM11 and Si-SAPO11 were prepared by binding the zeolite and silica sol according to a definite weight ratio together, while the catalyst, A1-SAPOll, was prepared by binding A1203 and SAPO-11. The solids were calcined in air at 550 ~ for 3 h before reaction. Si-ZSM35 was prepared by binding silica sol and ZSM-35 zeolite, then was calcined in air at 550 ~ for 3 h. The catalyst was exchanged with 0.8 M ammonium nitrate solution two times (for 2 h each time), then impregnated with magnesium nitrate aqueous solution, calcined at 500 ~ for 2 h. The catalyst was about 8 wt % Mg loading. Si-ZSM12 was prepared as the catalyst Si- ZSM35, and the catalyst was about 1 wt % Mg loading. The catalyst, Si-Y, was prepared from NaY by exchanging with 0.8 M ammonium nitrate solution only one time.
2.2. Reaction performance 1-Hexene of 96.92% purity obtained from Acros Organics was used. The major impurities were 3-methyl-1-pentene (0.66%), 2- and 3-hexenes (2.41%). Olefin isomerization reaction was carried out in a microreactor (9 mm I. D., 39mm O.D.), with 3.5g catalyst (20-40 mesh). The reactor was heated from room temperature to 400 ~ at a rate of 200 ~ in a flow of hydrogen then maintained at 400 ~ for an hour. After that, it was cooled to the reaction temperature. As the desired reaction temperature was reached, the mixture feed of 1-hexene and hydrogen (1:8 molar ratio) was passed through the reactor instead of hydrogen. The tail gas was analyzed by an on-line gas chromatography equipped with a 9-m squalane column and TCD, while the liquid product was analyzed by a Varian 3800 gas chromatography with a 100-m Pond capillary column and FID. Yields to the different reaction products are calculated according to the following equation: % Yield (i) =100 •
weight of product i formed Weight of 1-hexene fed
2.3. Catalyst characterization 2.3.1. NH3 temperature programmed desorption (NH3-TPD) A catalyst sample of 140mg was first heated from room temperature to 600 ~ at a ramping rate of 25 ~ and then held at 600 ~ for 30 min under a flow of 30ml/min pure helium. The system was then cooled to 150 ~ in a He stream. At 150 ~ the adsorption of the catalyst was carried out in a He stream containing ammonia until it was saturated. Then, the sample was swept with helium. When the baseline of gas chromatography was stable, the NH3 desorption profile of the catalyst was performed from 150 ~ to 600 ~ at a heating rate of 20 ~ The amount of desorption NH3 was monitored by a thermal conductivity detector and quantified by the pulse method.
749 Table 1 Influence of temperature on the performance of Si-ZSM11 (H2/1-hexene=8, P=0.2 MPa, SV=I.0h "l) Temp. Yield of product (wt%) ~ Cs. 1-hexene hexene(-2,-3) branched hexenes 350 0.00 96.27 3.13 0.61 400 0.00 52.98 40.06 6.96 500 0.00 17.12 67.20 15.57
C7+ 0.00 0.00 0.11
2.3.2. Thermogravimetric Thermogravimetric (TG) data was acquired on a Perkin Elmer Pyrisl TGA apparatus. The used catalyst of about 10 mg was heated to 150 ~ and held at 150 ~ for 30 min under a flow of 20ml/min N2. Then N2 was switched to air and the catalyst was heated from 150 ~ to 800 ~ at a rate of 10 ~ and the weight of catalyst was monitored by the thermo-balance and recorded. 3. RESULTS AND DISCUSSION
3.1. Reaction performance of Si-ZSMll for skeletal isomerization of 1-hexene The effect of temperature on the performance of skeletal isomerization of 1-hexene to branched hexenes (BH) over Si-ZSM11 catalyst was studied. The results are shown in Table 1. The skeletal isomerizaion reaction does not occur until the reaction temperature rises up to 400 ~ And the amount of branched hexenes increases from 6.96% to 15.57% when the temperature increases from 400 ~ to 500 ~ The C7+ products appear at 500 ~ due to the polymerization of hexenes. Si/A1 ratio in ZSM-11 zeolite is 700, and the average distance of an A13§ ion in zeolite to the closest one is 4.23 nm, while the length of a 1-hexene molecule is 1.03nm. This means that the closest distance between A13+is 2 times greater than the size of a 1-hexene molecule. This excludes the possibilities of the interaction of 1-hexene absorbed on different A1> sites. Thus, the branched isohexenes in the product without C5. and Cv+ at 400 ~ might come from monomolecular hexenes adsorbed on the catalyst. In a word, the skeletal isomerization of 1-hexene to branched hexenes is monomolecular. However the farmation of C7+ at 500 ~ might come from the direct reaction between the hexene absorbed on the acid site of catalyst surface and the 1-hexene existed in the gas phase. The formation of C7+ is agreement with that of Eley-Riedeal mechanism. Thus, it can be inferred from the above results that the skeletal isomerization of 1-hexene to isohexenes over Si-ZSM-11 zeolite catalyst is monomolecular mechanism. 3.2. Reaction performance over difference zeolites Here, we investigated the relation between skeletal isomerization of 1-hexene to BH and the acid density of catalysts with similar acid strength, and table 2 shows reaction performance. The results from Figure 1 show that the acid densities of catalysts decrease as the following: Si-Y >> Si-ZSM35>Si-ZSM12>>Si-SAPOll, while the values of isohexenes over the catalysts from Table 2 are: Si-ZSM35>Si-SAPOll> Si-ZSM12 >Si-Y. The results show that the yield of branched BH over Si-ZSM35 is highest and that of Si-Y is the lowest. Since the acid site density of Si-Y is the highest among the catalysts used. The above results indicated that the acid density of a catalyst is not the sole factor directly related with the
750 Table 2 Reaction results of 1-hexene isomerization to isohexenes over catalysts (H2/1-hexene=8, P=0.2 MPa, SV=I.0h l, T=270 ~ Yield of product (wt %) Catalyst Pore diameter Acidity* (nm) (mmol/g) C5. Hexene (-1, BH (DMB)
-2,-3)
Si-Y 0.80-0.90 0.439 Si-ZSM12 0.57x0.61 0.244 Si-ZSM35 0.42x0.54 0.290 Si-SAPO11 0.39x0.63 0.035 * Values calculated from NH3-TPD
0.31 2.20 0.58 0.00
93.99 35.22 25.96 46.68
5.09 (0.13) 48.20 (3.34) 69.11 (3.95) 53.32 (1.07)
C7+ 0.61 14.38 4.35 0.00
skeletal isomerization of 1-hexene to BH if it plays an important role. Microporous materials produce less dimerization reaction than open-surface materials [2]. This implies that the pore size of the catalyst might also be crucial for the catalytic production of the desired products. And this led to our further studies on the effect of catalyst pore diameter on the selectivity of the catalyst. The results from Table 2 indicate that the catalyst pore diameter between about 0.4 nm to 0.6 nm was optical for the 1-hexene skeletal isomerization. When the catalyst pore diameter is above 0.8nm, the value of branched hexenes is very low. Our data clearly indicate that, besides the acid site density, the micropore size of the zeolite is responsible for the highest selectivity shown in the case of Si-SAPO 11. The percentage of each branched isohexene in the product mixture may be affected by the catalyst pore diameter. Figure 2 shows the ratio of the dimethylbutenes (DMB) percentage in the branched isohexene mixture over catalysts tested in our experiment to that of calculated equilibrium value (Equilibrium value refers to ref. [6]). Although the yield of branched isohexenes over Si-SAPOll is more than that over Si-ZSM12, the yield of DMB over SiSAPO 11 is less than that over Si-ZSM12. The differences in the product distribution over the catalysts used might be attributed to the pore size differences. Microporous materials such as SAPO-11 do not allow free diffusion of tribranched or even dibranched hydrocarbon [2]. It is highly possible that the wall of 0.39 nm wide pore 0.5 0.4
0.3 0.2
0. i 150
250
350
450
550
Desorption Temperature(~
Fig. 1 NH3-TPD profiles of different zeolites
Si-Y
Si-ZSMI2 Si-ZSM35 Si-SAP011
Fig.2 Ratio of DMB percentage in BH in this test to that of equilibrium.
751
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---o-- Br C7+anehed ProduetIsohexenes --a~
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Reaction T e l n p e r a t u r e (~ Space Velocity (h -~)
Fig.3 Effect of space velocity
Fig.4 Effect of reaction temperature
suppresses DMB production. Or even if the DMB were produced in the pore of SAPO-11/Si with high percentage, the very slow diffusion rate of DMB would make DMB stay in the pore. All of these might explain the low percentage of DMB in the product mixture. It is conceivable that catalyst with pore size about 0.6 nm might generate a product mixture with higher DMB percentage. 3.3 Reaction performance of Si-SAPOll under different reaction conditions Based on the discussion above, it can been seen that Si-SAPOll is a good catalyst for skeletal isomerization of 1-hexene. The effects of space velocity and reaction temperature on the performance of skeletal isomerization of 1-hexene to isohexenes over Si-SAPO 11 catalyst were investigated. The results are shown in Figure 3 and 4. The higher the space velocity is, the lower the yield of skeletal isohexenes is. The DMB percentage in the branched isohexene mixture decreases with increasing space velocity. Table 3 exhibits the production rate of methyl pentenes (MP) and DMB at various space velocities. The result shows that the production rate of MP is larger than that of DMB. If every acid site can convert 1-hexene to skeletal isohexenes, at the utmost about 180 MP and 2.3 DMB can be produced one hour at one site over Si-SAPO 11 catalyst at 250 ~ Since DMB come from the skeletal isomerazation of monomethyl pentenes, which in turn come from the skeletal isomerizatiion of 1-hexene [7]. Thus, two consecutive isomerization steps are required to produce DMB from 1-hexene. As an intermediate for the production of DMB, it is reasonable that monomethyl pentene production is higher than that of DMB.
Table 3 Influence of space velocity on the produce rate (H2/1,hexene=8, P=0.2 MPa, T=250 ~ WHSV( h 4) ............. 0,5 ....... 1.0 MP Produce rate(mmol, h 1. g-l) 3.10 5.27 DMB produce rate(mmol, h "1. g4) 0.051 0.080
1.5 6.35 0.074
2.0 6.15 0.063
752 Table 4 Influence of binder on the reaction performance (H2/1-hexene=8, WHSV=I h "l, P=0.2 MPa, T=250 ~ Catalyst Yield of produce (%) DMB/BH C 5. BH C7+ (%) Si'SAPO 11 0.00 44.97 0.00 1.5 A1-SAPO11 0.37 79.19 7.84 7.1
Acidity (~tmol/g) 350 ~ 450 ~ 33.0 1.8 62.5 16.5
600 ~ 2.2
When the reaction temperature rises from 250 ~ to 310 ~ the yield of skeletal isohexenes increases and achieves the highest value at 310 ~ The results also reveal that the C1 and C2 product are not observed in the temperature range between 250 ~ and 340 ~ The yield of Cs is larger than that of C4 in the temperature range between 280 ~ and 340 ~ The yields of C5. and C7+ products, especially that of propane, rise quickly with increased temperature. The yield of propane increases from zero at 280 ~ to 4.36% at 340 ~ while that of Cs only increases from 0.06% to 0.72%. At 340 ~ the ratio of C9/C3 is only 0.036, while the ratios of C7/Cs and C8/C4 are 2.014 and 6.653 respectively. The wide production ratio distribution indicates that there might be multiple mechanisms employed in this catalytic process. The relative low C9/C3 ratio compared to these of C7/C5 and C8/C4 implies that the production mechanism of C3 might be different from that of C4 and C5. The dimerization- cracking process produces C4 and Cs product, however maybe 13-scission of polymers produces C3 product [7]. The high reaction temperature is more benefit for 13-scission than dimerizationcracking. 3.4. Influence of binder on the reaction performance on SAPO-11
Table 4 shows the effect of the binder on the reaction performance of SAPO-11. And Figure 5 shows the NH3-TPD of A1-SAPO 11 and Si-SAPO 11. Compared to Si-SAPO 11, A1SAPOll is more acidity. And the yields of both DMB and by-product are higher. The alumina, which is impregnated in the form of an acidic aluminium nitrate, has enough acidity to efficiently convert 1-pentene to skeletal isomers [8]. In the process of binding SAPO-11 and A1203, nitric acid was added and aluminium nitrate is produced and then impregnated onto A1203,. This process is equivalent to the direct impregnation of aluminium nitrate on A1203. This is why the acidity of A1-SAPO 11 is higher than that of A1-SAPO 11. Since these acidic sites are responsible for the isomerization of 1-hexenes to BH, it is expected that the yield of BH over A1-SAPO11 is higher than that over Si-SAPO 11. The surface of alumina is opensurface, and open-surface favors the dimerization of olefins and cracking ' ' AI-'SAP() 1 1' l, Si-SAPO 1 1 ] dimerization [2]. Thus, the higher yields of both C5. and C7+ products over A1r SAPO 11 catalyst, as indicated in Table 4, Eare expected and consistent with the ...::::::::::l- .... properties of the corresponding catalyst 2;o3;o 4;o s;o 600 used. Desorl)tion Temperature(~ Neither Cl, C2, nor C3 product is observed in the product mixture in the case of A1-SAPO 11 at 250 ~ This is Fig.5 NH3-TPD profiles of different catalysts
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753 90 L~
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/_~_mm __| ~ n__m ~__m.lm__| __m~__n 9 --~"~'-m~mm~m
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Dim ethylbutenes C~+ P r o d u e t
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Fig.6 Infuence of time-on-stream over A1-SAPO 11 (H2/1-hexene=8, WHSV=I.0h 1, P=0.2 MPa, T=250 ~
different to that over Si-SAPO 11 at 340 ~ Since the production of C 3 products is proposed to be the result of the [3-scission mechanism, the higher yield of C3 product at higher temperature indicates that the [3-scission mechanism is favored at higher temperature. 3.5. Stability test of AI-SAPOll catalyst Figure 6 shows the stability of A1-SAPOll catalyst at WHSV = 1.0 h 1, HJhexene = 8, T-250 ~ P=0.2MPa. The yield of branched hexenes is usually 80% or above when the reaction time is less than 78 hours. After 126 hours the yield of skeletal isohexenes is still higher than 60% at the same reaction condition. The test shows that the catalyst is robust and has a relatively stable performance over long time. The C~, C2 and C3 products have not been observed from the start to the end of the stability test, while a few of C4 and Cs products are observed. The yield of Cs. is always less than 0.4%. The sum of C5 is larger than that of C4. It is also shown that the yield of C7+ is always larger than that of C5.. When the carbon number of product is larger than 6, the yield of the corresponding product decreases with the increasing chain length. Also, both C5. and C7+ products decrease with increasing time on stream. The DMB yield always decreases with increasing time on stream, even when the yield of BH keeps above 80%. The yield of DMB decreases from 5.60% at the start to 95 2.21% at 78 h, and to 1.37% at 126 h. Accordifigly, the ratio of DMB/BH ,.c:: decreases from 7.07% to 2.78%, and to 93 2.27%. These results imply that the active sites responsible for the production of the DMB lost activity at a rate faster than that 2;0 360 460 560 660 760 of MR Figure 7 shows TG of used A1-SAPO11 Temper ature (~ catalyst. The coke is about 4.3 w. %. The Fig.7 TG of used A1-SAPO11 abruptly temperature point of weight lost is "~
~JO
94'
754 about 470 ~ active sites.
This shows they might be responsible for the lost of activity of the catalytic
4. CONCLUSIONS Our data presented in this paper favors the monomolecular skeletal isomerization of 1hexene to branched isohexenes over Si-ZSMll zeolite catalyst. Also, the skeletal isomerization of 1-hexene to isohexenes is not only influenced by the acid strength and acid site density, but also by the zeolite catalyst pore size. It has also been found that the A1-SAPO 11 catalyst is an excellent catalyst for the skeletal isomerazation of 1-hexene. High yield of skeletal isohexenes with monomethyl pentenes as the major product, and high catalyst stability are obtained. Furthermore, the low yields of the side products, such as C5 and C7+ products, and low carbon deposition on the catalyst over long time make this catalyst attractive for future target for further optimization. ACKNOWLEDGEMENT We thank Dr. L. Xu for providing SAPO-11 zeolite used in this paper. REFERRENCES 1. M. Guisnet, E Andy, N. S. Gnep, E. Benazzi and C. Travers, J. Catal., 158 (1996) 551. 2. J. Houzvicka and V. Ponec, Ind. Eng. Chem. Res., 36 (1997) 1424. 3. H. H. Mooiweer, K.P. de Jong, B.Kraushaar-Czametzki, W.H.J. Stork and B.C.H. Krutzen, Stud. Surf. Sci. Catal., 84 (1994) 2327. 4. M. A. Asensi, A. Corma, and A. Martinez, J. Catal., 158 (1996) 561. 5. X. Feng, J. S. Lee, J. W. Lee, J. Y. Lee, D. Wei and G. L. Haller, Chem. Eng. J., 64 (1996) 255 6. J. E. Kilpatrick, E. J. Prosen, K. S. Pitzer and E D. Rossini, J. Res. Nati. Bur. Standarts., 36 (1946) 559. 7. W. A. Groten and B. W. Wojciechowski, J. Catal., 122 (1990) 362. 8. C. Lin, H. Yang, C. Lai, C. Chang, L. L. K. Kuo and K. Yung, Skeletal Isomerization of Olefins with an Alumina Based Catalyst, US Patent No. 5 321 193 (1991)
747
Skeletal Isomerization of 1-Hexene to Isohexenes over Zeolite Catalysts Zhihua Wu, Qingxia Wang, Longya Xu and Sujuan Xie Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian 116023, E R. China Several zeolite catalysts such as SAPO-11, ZSM-11, ZSM-12, etc. were selected to convert 1-hexene to branched hexenes in this work. Pore size of the zeolite catalyst plays an important role on the yield and the distribution of branched isohexenes. And the zeolite catalysts with the pore size of 0.6nm are optimum to produce dimethylbutenes (DMB). SAPO-11 zeolite is a suitable skeletal isomerization catalyst, especially in the production of methyl pentenes. Under the following reaction conditions: WHSV=I.0 h~, HJhexene=8, T=250 ~ P=0.2 MPa, the yield of skeletal isohexenes remains above 80% at the prolonged time-on stream of 80 h, accompanying low C5., C7+ products and low carbon deposition on the catalyst. 1. INTRODUCTION The catalytic reactions for converting unbranched olefins into branched olefins, such as the skeletal isomerization of n-butenes to isobutene, are important processes for the large-scale production of raw materials for chemical industry. To guide the screening of catalysts for the desired processes, tremendous of work has also been devoted to the mechanistic studies of these processes. To date, there are at least two proposed models for the skeletal isomerization of olefins, monomer model and dimerization model. Guisnete [1] reported that there were three steps from n-butenes to isobutene: (i) dimerization of n-butenes, (ii) skeletal isomerization of dimers, and (iii) cracking of the octene isomers. In contrast, Houzvickn [2] proposed that the dominating process for the skeletal n-butene isomerization was monomolecular and the bimolecular mechanism was mainly responsible for the formation of byproducts, such as propene and pentenes. Also, the results of Mooiweer [3] favored the mechanism of skeletal n-pentenes isomerization to isopentenes to be monomolecular. Isomerization reactions of olefins are affected by various factors. Asensi reported that the selectivity of n-butene to isobutene was greatly improved with the increased Si/A1 ratio in MCM-22. Further characterization of these catalysts revealed that the increased Si/A1 ratio led to a lower acid site density. Since these acid sites were proposed to the sites for the bimolecular side-reaction, a decreased acid site density in those catalysts was attributed to the increased isobutene selectivity [4]. Besides the acid site density, the pore size of the zeolites also affects the selectivity of the isomerization reaction. The results of Feng [5] indicated that the outcome of 2-methyl-2-pentene isomerization reaction was also greatly influenced by the pore sizes of zeolites. This was supported by the feeding experiment with several octenes over open-surface and microporous materials and it was found that the 10-membered ring (10-MR) channels were hardly accessible to double-branched hydrocarbons and the diffusion through the 10-MR by triple-branched were denied [2].
748 In the present paper, the catalytic performance of zeolites for the isomerization of 1-hexene to branched hexenes was investigated in a continuous-flow fixed bed reactor. Reported herein are the preliminary skeletal isomerization results. 2. EXPERIMENTAL
2.1. Catalyst preparation ZSM-11 (Si/AI=700), ZSM-35 (Si/AI=15) and ZSM-12 (Si/AI=50) zeolites were synthesized in our laboratory. SAPO-11 and Y-type zeolites were produced by another laboratory in our Institute of Chemical Physics. Si-ZSM11 and Si-SAPO11 were prepared by binding the zeolite and silica sol according to a definite weight ratio together, while the catalyst, A1-SAPOll, was prepared by binding A1203 and SAPO-11. The solids were calcined in air at 550 ~ for 3 h before reaction. Si-ZSM35 was prepared by binding silica sol and ZSM-35 zeolite, then was calcined in air at 550 ~ for 3 h. The catalyst was exchanged with 0.8 M ammonium nitrate solution two times (for 2 h each time), then impregnated with magnesium nitrate aqueous solution, calcined at 500 ~ for 2 h. The catalyst was about 8 wt % Mg loading. Si-ZSM12 was prepared as the catalyst Si- ZSM35, and the catalyst was about 1 wt % Mg loading. The catalyst, Si-Y, was prepared from NaY by exchanging with 0.8 M ammonium nitrate solution only one time.
2.2. Reaction performance 1-Hexene of 96.92% purity obtained from Acros Organics was used. The major impurities were 3-methyl-1-pentene (0.66%), 2- and 3-hexenes (2.41%). Olefin isomerization reaction was carried out in a microreactor (9 mm I. D., 39mm O.D.), with 3.5g catalyst (20-40 mesh). The reactor was heated from room temperature to 400 ~ at a rate of 200 ~ in a flow of hydrogen then maintained at 400 ~ for an hour. After that, it was cooled to the reaction temperature. As the desired reaction temperature was reached, the mixture feed of 1-hexene and hydrogen (1:8 molar ratio) was passed through the reactor instead of hydrogen. The tail gas was analyzed by an on-line gas chromatography equipped with a 9-m squalane column and TCD, while the liquid product was analyzed by a Varian 3800 gas chromatography with a 100-m Pond capillary column and FID. Yields to the different reaction products are calculated according to the following equation: % Yield (i) =100 •
weight of product i formed Weight of 1-hexene fed
2.3. Catalyst characterization 2.3.1. NH3 temperature programmed desorption (NH3-TPD) A catalyst sample of 140mg was first heated from room temperature to 600 ~ at a ramping rate of 25 ~ and then held at 600 ~ for 30 min under a flow of 30ml/min pure helium. The system was then cooled to 150 ~ in a He stream. At 150 ~ the adsorption of the catalyst was carried out in a He stream containing ammonia until it was saturated. Then, the sample was swept with helium. When the baseline of gas chromatography was stable, the NH3 desorption profile of the catalyst was performed from 150 ~ to 600 ~ at a heating rate of 20 ~ The amount of desorption NH3 was monitored by a thermal conductivity detector and quantified by the pulse method.
749 Table 1 Influence of temperature on the performance of Si-ZSM11 (H2/1-hexene=8, P=0.2 MPa, SV=I.0h "l) Temp. Yield of product (wt%) ~ Cs. 1-hexene hexene(-2,-3) branched hexenes 350 0.00 96.27 3.13 0.61 400 0.00 52.98 40.06 6.96 500 0.00 17.12 67.20 15.57
C7+ 0.00 0.00 0.11
2.3.2. Thermogravimetric Thermogravimetric (TG) data was acquired on a Perkin Elmer Pyrisl TGA apparatus. The used catalyst of about 10 mg was heated to 150 ~ and held at 150 ~ for 30 min under a flow of 20ml/min N2. Then N2 was switched to air and the catalyst was heated from 150 ~ to 800 ~ at a rate of 10 ~ and the weight of catalyst was monitored by the thermo-balance and recorded. 3. RESULTS AND DISCUSSION
3.1. Reaction performance of Si-ZSMll for skeletal isomerization of 1-hexene The effect of temperature on the performance of skeletal isomerization of 1-hexene to branched hexenes (BH) over Si-ZSM11 catalyst was studied. The results are shown in Table 1. The skeletal isomerizaion reaction does not occur until the reaction temperature rises up to 400 ~ And the amount of branched hexenes increases from 6.96% to 15.57% when the temperature increases from 400 ~ to 500 ~ The C7+ products appear at 500 ~ due to the polymerization of hexenes. Si/A1 ratio in ZSM-11 zeolite is 700, and the average distance of an A13§ ion in zeolite to the closest one is 4.23 nm, while the length of a 1-hexene molecule is 1.03nm. This means that the closest distance between A13+is 2 times greater than the size of a 1-hexene molecule. This excludes the possibilities of the interaction of 1-hexene absorbed on different A1> sites. Thus, the branched isohexenes in the product without C5. and Cv+ at 400 ~ might come from monomolecular hexenes adsorbed on the catalyst. In a word, the skeletal isomerization of 1-hexene to branched hexenes is monomolecular. However the farmation of C7+ at 500 ~ might come from the direct reaction between the hexene absorbed on the acid site of catalyst surface and the 1-hexene existed in the gas phase. The formation of C7+ is agreement with that of Eley-Riedeal mechanism. Thus, it can be inferred from the above results that the skeletal isomerization of 1-hexene to isohexenes over Si-ZSM-11 zeolite catalyst is monomolecular mechanism. 3.2. Reaction performance over difference zeolites Here, we investigated the relation between skeletal isomerization of 1-hexene to BH and the acid density of catalysts with similar acid strength, and table 2 shows reaction performance. The results from Figure 1 show that the acid densities of catalysts decrease as the following: Si-Y >> Si-ZSM35>Si-ZSM12>>Si-SAPOll, while the values of isohexenes over the catalysts from Table 2 are: Si-ZSM35>Si-SAPOll> Si-ZSM12 >Si-Y. The results show that the yield of branched BH over Si-ZSM35 is highest and that of Si-Y is the lowest. Since the acid site density of Si-Y is the highest among the catalysts used. The above results indicated that the acid density of a catalyst is not the sole factor directly related with the
750 Table 2 Reaction results of 1-hexene isomerization to isohexenes over catalysts (H2/1-hexene=8, P=0.2 MPa, SV=I.0h l, T=270 ~ Yield of product (wt %) Catalyst Pore diameter Acidity* (nm) (mmol/g) C5. Hexene (-1, BH (DMB)
-2,-3)
Si-Y 0.80-0.90 0.439 Si-ZSM12 0.57x0.61 0.244 Si-ZSM35 0.42x0.54 0.290 Si-SAPO11 0.39x0.63 0.035 * Values calculated from NH3-TPD
0.31 2.20 0.58 0.00
93.99 35.22 25.96 46.68
5.09 (0.13) 48.20 (3.34) 69.11 (3.95) 53.32 (1.07)
C7+ 0.61 14.38 4.35 0.00
skeletal isomerization of 1-hexene to BH if it plays an important role. Microporous materials produce less dimerization reaction than open-surface materials [2]. This implies that the pore size of the catalyst might also be crucial for the catalytic production of the desired products. And this led to our further studies on the effect of catalyst pore diameter on the selectivity of the catalyst. The results from Table 2 indicate that the catalyst pore diameter between about 0.4 nm to 0.6 nm was optical for the 1-hexene skeletal isomerization. When the catalyst pore diameter is above 0.8nm, the value of branched hexenes is very low. Our data clearly indicate that, besides the acid site density, the micropore size of the zeolite is responsible for the highest selectivity shown in the case of Si-SAPO 11. The percentage of each branched isohexene in the product mixture may be affected by the catalyst pore diameter. Figure 2 shows the ratio of the dimethylbutenes (DMB) percentage in the branched isohexene mixture over catalysts tested in our experiment to that of calculated equilibrium value (Equilibrium value refers to ref. [6]). Although the yield of branched isohexenes over Si-SAPOll is more than that over Si-ZSM12, the yield of DMB over SiSAPO 11 is less than that over Si-ZSM12. The differences in the product distribution over the catalysts used might be attributed to the pore size differences. Microporous materials such as SAPO-11 do not allow free diffusion of tribranched or even dibranched hydrocarbon [2]. It is highly possible that the wall of 0.39 nm wide pore 0.5 0.4
0.3 0.2
0. i 150
250
350
450
550
Desorption Temperature(~
Fig. 1 NH3-TPD profiles of different zeolites
Si-Y
Si-ZSMI2 Si-ZSM35 Si-SAP011
Fig.2 Ratio of DMB percentage in BH in this test to that of equilibrium.
751
,
'
i
,
i
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~
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Reaction T e l n p e r a t u r e (~ Space Velocity (h -~)
Fig.3 Effect of space velocity
Fig.4 Effect of reaction temperature
suppresses DMB production. Or even if the DMB were produced in the pore of SAPO-11/Si with high percentage, the very slow diffusion rate of DMB would make DMB stay in the pore. All of these might explain the low percentage of DMB in the product mixture. It is conceivable that catalyst with pore size about 0.6 nm might generate a product mixture with higher DMB percentage. 3.3 Reaction performance of Si-SAPOll under different reaction conditions Based on the discussion above, it can been seen that Si-SAPOll is a good catalyst for skeletal isomerization of 1-hexene. The effects of space velocity and reaction temperature on the performance of skeletal isomerization of 1-hexene to isohexenes over Si-SAPO 11 catalyst were investigated. The results are shown in Figure 3 and 4. The higher the space velocity is, the lower the yield of skeletal isohexenes is. The DMB percentage in the branched isohexene mixture decreases with increasing space velocity. Table 3 exhibits the production rate of methyl pentenes (MP) and DMB at various space velocities. The result shows that the production rate of MP is larger than that of DMB. If every acid site can convert 1-hexene to skeletal isohexenes, at the utmost about 180 MP and 2.3 DMB can be produced one hour at one site over Si-SAPO 11 catalyst at 250 ~ Since DMB come from the skeletal isomerazation of monomethyl pentenes, which in turn come from the skeletal isomerizatiion of 1-hexene [7]. Thus, two consecutive isomerization steps are required to produce DMB from 1-hexene. As an intermediate for the production of DMB, it is reasonable that monomethyl pentene production is higher than that of DMB.
Table 3 Influence of space velocity on the produce rate (H2/1,hexene=8, P=0.2 MPa, T=250 ~ WHSV( h 4) ............. 0,5 ....... 1.0 MP Produce rate(mmol, h 1. g-l) 3.10 5.27 DMB produce rate(mmol, h "1. g4) 0.051 0.080
1.5 6.35 0.074
2.0 6.15 0.063
752 Table 4 Influence of binder on the reaction performance (H2/1-hexene=8, WHSV=I h "l, P=0.2 MPa, T=250 ~ Catalyst Yield of produce (%) DMB/BH C 5. BH C7+ (%) Si'SAPO 11 0.00 44.97 0.00 1.5 A1-SAPO11 0.37 79.19 7.84 7.1
Acidity (~tmol/g) 350 ~ 450 ~ 33.0 1.8 62.5 16.5
600 ~ 2.2
When the reaction temperature rises from 250 ~ to 310 ~ the yield of skeletal isohexenes increases and achieves the highest value at 310 ~ The results also reveal that the C1 and C2 product are not observed in the temperature range between 250 ~ and 340 ~ The yield of Cs is larger than that of C4 in the temperature range between 280 ~ and 340 ~ The yields of C5. and C7+ products, especially that of propane, rise quickly with increased temperature. The yield of propane increases from zero at 280 ~ to 4.36% at 340 ~ while that of Cs only increases from 0.06% to 0.72%. At 340 ~ the ratio of C9/C3 is only 0.036, while the ratios of C7/Cs and C8/C4 are 2.014 and 6.653 respectively. The wide production ratio distribution indicates that there might be multiple mechanisms employed in this catalytic process. The relative low C9/C3 ratio compared to these of C7/C5 and C8/C4 implies that the production mechanism of C3 might be different from that of C4 and C5. The dimerization- cracking process produces C4 and Cs product, however maybe 13-scission of polymers produces C3 product [7]. The high reaction temperature is more benefit for 13-scission than dimerizationcracking. 3.4. Influence of binder on the reaction performance on SAPO-11
Table 4 shows the effect of the binder on the reaction performance of SAPO-11. And Figure 5 shows the NH3-TPD of A1-SAPO 11 and Si-SAPO 11. Compared to Si-SAPO 11, A1SAPOll is more acidity. And the yields of both DMB and by-product are higher. The alumina, which is impregnated in the form of an acidic aluminium nitrate, has enough acidity to efficiently convert 1-pentene to skeletal isomers [8]. In the process of binding SAPO-11 and A1203, nitric acid was added and aluminium nitrate is produced and then impregnated onto A1203,. This process is equivalent to the direct impregnation of aluminium nitrate on A1203. This is why the acidity of A1-SAPO 11 is higher than that of A1-SAPO 11. Since these acidic sites are responsible for the isomerization of 1-hexenes to BH, it is expected that the yield of BH over A1-SAPO11 is higher than that over Si-SAPO 11. The surface of alumina is opensurface, and open-surface favors the dimerization of olefins and cracking ' ' AI-'SAP() 1 1' l, Si-SAPO 1 1 ] dimerization [2]. Thus, the higher yields of both C5. and C7+ products over A1r SAPO 11 catalyst, as indicated in Table 4, Eare expected and consistent with the ...::::::::::l- .... properties of the corresponding catalyst 2;o3;o 4;o s;o 600 used. Desorl)tion Temperature(~ Neither Cl, C2, nor C3 product is observed in the product mixture in the case of A1-SAPO 11 at 250 ~ This is Fig.5 NH3-TPD profiles of different catalysts
ill
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~i111,11.
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Fig.6 Infuence of time-on-stream over A1-SAPO 11 (H2/1-hexene=8, WHSV=I.0h 1, P=0.2 MPa, T=250 ~
different to that over Si-SAPO 11 at 340 ~ Since the production of C 3 products is proposed to be the result of the [3-scission mechanism, the higher yield of C3 product at higher temperature indicates that the [3-scission mechanism is favored at higher temperature. 3.5. Stability test of AI-SAPOll catalyst Figure 6 shows the stability of A1-SAPOll catalyst at WHSV = 1.0 h 1, HJhexene = 8, T-250 ~ P=0.2MPa. The yield of branched hexenes is usually 80% or above when the reaction time is less than 78 hours. After 126 hours the yield of skeletal isohexenes is still higher than 60% at the same reaction condition. The test shows that the catalyst is robust and has a relatively stable performance over long time. The C~, C2 and C3 products have not been observed from the start to the end of the stability test, while a few of C4 and Cs products are observed. The yield of Cs. is always less than 0.4%. The sum of C5 is larger than that of C4. It is also shown that the yield of C7+ is always larger than that of C5.. When the carbon number of product is larger than 6, the yield of the corresponding product decreases with the increasing chain length. Also, both C5. and C7+ products decrease with increasing time on stream. The DMB yield always decreases with increasing time on stream, even when the yield of BH keeps above 80%. The yield of DMB decreases from 5.60% at the start to 95 2.21% at 78 h, and to 1.37% at 126 h. Accordifigly, the ratio of DMB/BH ,.c:: decreases from 7.07% to 2.78%, and to 93 2.27%. These results imply that the active sites responsible for the production of the DMB lost activity at a rate faster than that 2;0 360 460 560 660 760 of MR Figure 7 shows TG of used A1-SAPO11 Temper ature (~ catalyst. The coke is about 4.3 w. %. The Fig.7 TG of used A1-SAPO11 abruptly temperature point of weight lost is "~
~JO
94'
754 about 470 ~ active sites.
This shows they might be responsible for the lost of activity of the catalytic
4. CONCLUSIONS Our data presented in this paper favors the monomolecular skeletal isomerization of 1hexene to branched isohexenes over Si-ZSMll zeolite catalyst. Also, the skeletal isomerization of 1-hexene to isohexenes is not only influenced by the acid strength and acid site density, but also by the zeolite catalyst pore size. It has also been found that the A1-SAPO 11 catalyst is an excellent catalyst for the skeletal isomerazation of 1-hexene. High yield of skeletal isohexenes with monomethyl pentenes as the major product, and high catalyst stability are obtained. Furthermore, the low yields of the side products, such as C5 and C7+ products, and low carbon deposition on the catalyst over long time make this catalyst attractive for future target for further optimization. ACKNOWLEDGEMENT We thank Dr. L. Xu for providing SAPO-11 zeolite used in this paper. REFERRENCES 1. M. Guisnet, E Andy, N. S. Gnep, E. Benazzi and C. Travers, J. Catal., 158 (1996) 551. 2. J. Houzvicka and V. Ponec, Ind. Eng. Chem. Res., 36 (1997) 1424. 3. H. H. Mooiweer, K.P. de Jong, B.Kraushaar-Czametzki, W.H.J. Stork and B.C.H. Krutzen, Stud. Surf. Sci. Catal., 84 (1994) 2327. 4. M. A. Asensi, A. Corma, and A. Martinez, J. Catal., 158 (1996) 561. 5. X. Feng, J. S. Lee, J. W. Lee, J. Y. Lee, D. Wei and G. L. Haller, Chem. Eng. J., 64 (1996) 255 6. J. E. Kilpatrick, E. J. Prosen, K. S. Pitzer and E D. Rossini, J. Res. Nati. Bur. Standarts., 36 (1946) 559. 7. W. A. Groten and B. W. Wojciechowski, J. Catal., 122 (1990) 362. 8. C. Lin, H. Yang, C. Lai, C. Chang, L. L. K. Kuo and K. Yung, Skeletal Isomerization of Olefins with an Alumina Based Catalyst, US Patent No. 5 321 193 (1991)