Encapsulated Products inside Pores of H-Mordenite in the

Chapter 19

Encapsulated Products inside Pores of H-Mordenite in the Isopropylation of Biphenyl Downloaded by UCSF LIB CKM RSCS MGMT on November 25, 2014 | http://pubs.acs.org Publication Date: November 2, 1999 | doi: 10.1021/bk-2000-0738.ch019

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K. Nakajima , T. Hanaoka , Y. Sugi , T. Matsuzaki , Y. Kubota, A. Igarashi , and K. Kunimori 4

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Institute of Materials Science, University of Tsukuba, Tsukuba 305-8577, Japan National Institute of Materials and Chemical Research, Tsukuba 305-8565, Japan Department of Chemistry, Gifu University, Gifu 501-1193, Japan Department of Chemical Engineering, Kogakuin University, Tokyo 192-0015, Japan 3

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Selective formation of 4,4'-diisopropylbiphenyl (4,4'-DIPB) was observed in the isopropylation of biphenyl (BP) over H-mordenite (HM). The selectivity of 4,4'-DIPB was dependent on the SiO /Al O ratio, and it was low for HMs with the lower ratio. Encapsulated products showed that the selectivities of 4,4'-DIPB inside the pores were higher than 85% for all HMs. The selectivity of 4,4'-DIPB was influenced by propene pressure in the isopropylation of BP over highly dealuminated H M . The selectivity was up to 90% under 0.8 MPa of propene pressure, whereas the decrease of the selectivity occurred under 0.1 MPa. The selectivties of 4,4'-DIPB in encapsulated products for the isopropylation of BP was more than 90 % under all pressures. These results suggest that the formation of 4,4' -DIPB occurred in the pores. The low selectivity of 4,4'-DIPB over H M with low SiO /Al O ratio is due to non-regioselective isopropylation at the external acid sites. The high selectivity of 4,4'-DIPB under high propene pressure is due to the prevention of the adsorption of 4,4'-DIPB on acid sites because of preferential adsorption of propene. The isomerization of 4,4'-DIPB under low pressures occurred on external acid sites where no propene was adsorbed. 2

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Polynuclear aromatics such as biphenyl and naphthalene have attracted the attention of many researchers as a key component of advanced materials such as heat-resistant and liquid crystalline polymers. Shape selective alkylation using zeolite is a promising way for manufacturing symmetrically dialkylated polynuclear aromatics (1,2). H M has been © 2000 American Chemical Society

In Shape-Selective Catalysis; Song, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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272 found to be the potential catalyst for shape selective isopropylation of BP with propene (1-7). Especially, highly dealuminated HM gave high catalytic activity and high selectivity of 4,4-DIPB, while catalyst performances were low for HMs with the low Si02/Al 0 ratio (¥-6). It is still unclear where the reaction occurs because the selectivity of 4,4'-DIPB is changed much by many factors such as reaction conditions and the nature of the catalyst. In this paper, we describe relationships between encapsulated products inside the pores and bulk products in the isopropylation of BP and in the isomerization of 4,4'DIPB, and discuss the mechanism of shape selective catalysis. Downloaded by UCSF LIB CKM RSCS MGMT on November 25, 2014 | http://pubs.acs.org Publication Date: November 2, 1999 | doi: 10.1021/bk-2000-0738.ch019

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Experimental Catalysts and Reagents. H-Mordenites (HM, SKyAl 0 = 10» 15, 20, 25, 30, 73, 110, 128, 206, and 220) were obtainedfromTosoh Corporation, Tokyo, Japan, and calcined at 550 °C in an air stream just before use. BP and 4,4'-DIBP were purchased from Tokyo Kasei Kogyo Co., Ltd, Tokyo, Japan, and 4-isopropyIbiphenyl (4-IPBP) from Aldrich Chem. Co., Inc., Milwaukee, WI, USA. These reagents were used without further purification. Isopropylation. The isopropylation of BP was carried out in a 100-ml SUS-316 autoclave under propene pressure. The autoclave containing BP and HM was purged with nitrogen before heating. After reaching reaction temperature, propene was introduced to the autoclave, and the pressure was kept constant throughout the reaction. After cooling the autoclave, the bulk products were diluted with toluene as a solvent. The products were analyzed by a Hewlett-Packard 5890 II Gas Chromatograph equipped with a Ultra-1 capillary column (25 m χ 0.2 mm) and identified by a Hewlett-Packard 5978 series II Gas Chromatograph with a 5971A Mass Selective Detector. The yield of each product is calculated on the basis of consumed BP, and the selectivities of each IPBP and DIPB isomers are expressed based on total amounts of IPBP and DIPB isomers, respectively. The analysis of encapsulated products in the catalyst used for the reaction was carried out as follows. The catalyst wasfilteredoft washed well with 200 ml of acetone, and dried at 110 °C for 12 h. The resulting catalyst was dissolved using a 3 ml of aqueous hydrofluoric acid (47%) at room temperature. This solution was basified with solid potassium carbonate, and the organic layer was extracted three times with 20 ml of dichloromethane. After removal of the solvent in vacuo, the residue was dissolved in 5 ml of toluene, and 10 mg of naphthalene was added to the resulting mixture as an internal standard. The GC analysis was done according to the procedure for bulk products. Thermogravimetric analysis (TG) of HM was carried out after the reaction with the temperature programmed rate of 10 °C/min in an air stream by TG-DTA 2000 (MAC Science Co. Ltd., Tokyo, Japan). The weight loss due to adsorbed water was corrected by blank measurement of corresponding HM before being used for the reaction. 2

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Results and Discussion Effect of SiOa/AljOa ratio of H M . The isopropylation of BP with propene over HMs yielded mixtures of IPBP, DIPB, and triisopropylbiphenyls (TrIPB) (1-5). Figure In Shape-Selective Catalysis; Song, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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1 shows the effect of S i 0 / A l 0 ratio of H M on the compositions of bulk products and of encapsulated products in the pores under 0.8 MPa of propene. In the bulk products, the percentage of IPBP and DIPB isomers increased with increasing the ratio. TrIPB was also observed in a small amount for all catalysts. On the other hand, the percentages of BP in encapsulated products were much lower than that in bulk products except for the case of HM(IO) . DIPB isomers were predominant in the encapsulated products, particularly with highly dealuminated HMs, whereas TrIPB isomers were not observed in the encapsulated products with any catalysts. These results show that the isopropylation of BP proceeds rapidly to yield IPBP and DIPB, but not TrIPB. The absence of TrIPB in encapsulated products was due to steric restriction of the pores against the isopropylation of DIPB. The conformation of DIPB isomers, especially 4,4'-DIPB, to form TrIPB should not be allowed in the pores. The low percentages of IPBP and DIPB isomers over HMs with the low S i 0 / A l 0 ratio are due to deactivation by severe coke-deposition inside the pores. Figure 2 shows the effects of S i 0 / A l 0 ratio of H M on the selectivities of 3,4'and 4,4'-DIPB under 0.8 MPa of propene. In bulk products, the selectivity of 4,4'DIPB increased with S i 0 / A l 0 ratio, whereas that of 3,4'-DIPB was low and almost constant for all catalysts. Highly selective formation of 4,4'-DIPB was observed for dealuminated HMs, especially for HM(206). However, the selectivity of 4,4'-DIPB with HM(10) was particularly low, and other DIPB isomers were obtained with high proportions. The features of encapsulated products are quite different from that of bulk products. The selectivities of 4,4'-DIPB were higher than 85 % with all HMs, even with HM(10). The results show that shape selective isopropylation occurs inside all H M pores. Therefore, the low selectivity of 4,4'-DIPB in bulk products for HM(10) is not due to the lack of shape selectivity of the pores. It should be explained by non-regioselective isopropylation on the external acid sites which are still active in spite of coke-deposition. The high selectivities of 4,4'-DIPB in both bulk and encapsulated products for dealuminated H M show that the isopropylation occurs preferentially inside the pores, and that the external acid sites do not work as principal catalytic sites. Highly shape selective catalysis for the formation of 4-IPBP and 4,4'-DIPB is ascribed to the steric restriction at the transition states at microporous environments of the H M pores to produce the least bulky products. It is important to elucidate the participation of external acid sites. Table 1 shows the isomerization of 4,4'-DIPB over HM(25) and HM(206) under 0.8 MPa of propene pressure. The isomerization of 4,4'-DIPB was very slow with both catalysts, and 4,4'DIPB was found as an exclusive encapsulated product inside the pores. These results suggest that DIPB isomers were scarcely changed to other products during the reaction at the external acid sites after they were formed. Therefore, at least, under high propene pressure, the low selectivity of 4,4'-DIPB for HMs with the low S i 0 / A l 0 ratio is not due to the isomerization of 4,4'-DIPB, but to non-regioselective isopropylation at external acid sites. Details of the isomerization will be discussed in the next section. 2

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Downloaded by UCSF LIB CKM RSCS MGMT on November 25, 2014 | http://pubs.acs.org Publication Date: November 2, 1999 | doi: 10.1021/bk-2000-0738.ch019

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The number in parenthesis expresses the S1O2/AI2O3 ratio of HMs.


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Bulk p r o d u c t s

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Encapsulated products

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S1O2/AI2O3 ratio

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S1O2/AI2O3 ratio

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Figure 1. Effects of the S i 0 / A l 0 ratio of H M on bulk and encapsulated products in the isopropylation of BP. Reaction conditions: BP, 200 mmol; H M , 1 g; propene, 0.8 MPa; temperature, 250 °C; period, 4 h. 2

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S1O2/AI2O3 ratio

Figure 2. Effects of the S i 0 / A l 0 ratio of H M on the selectivity of 3,4'- and 4,4'DIPB in bulk and encapsulated products in the isopropylation of BP. The reaction conditions are the same as Figure 1. 2

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275 Figure 3 shows T G profiles of HMs used for the isopropylation of BP. The amounts of cokes observed as a peak around 600 °C decreased with increasing the S i 0 / A l 0 ratio. Volatile organic compounds, which are ascribed to isopropylated biphenyls encapsulated inside the pores discussed above, were also found at 300-350 °C for dealuminated HMs ( S i 0 / A l 0 > 70). These results are explained by preferential removal of strong acid sites by the dealumination (5,9). The cokedeposition easily occurs on strong and dense acid sites by the dehydrogenation of alkylated aromatics (10-12). It is considered that the most of the volatile compounds inside H M pores with the low S i 0 / A l 0 ratio was converted to coke-deposits, and that major parts of acid sites in the pores could not act as catalytic active sites by choking pore entrance. Highly dealuminated HMs, such as HM(206), effectively catalyze shape selective isopropylation of BP without the inhibition by coke-deposition. 2

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Effects of propene pressure on the isopropylation. The formation of 4,4-DIPB over HM(220) was much influenced by propene pressure as shown in Figure 4. The selectivities were as high as 80 % under all pressure conditions at early stages, and kept almost constant during the reaction under higher pressures than 0.3 MPa. However, the decrease of the selectivity was observed under lower pressures than 0.2 MPa. The decrease of the selectivity of 4,4-DIPB corresponded to the increase of that of 3,4DIPB. The yields of 3,4'- and 4,4'-DIPB were in linear relations to the yield of combined DIPB isomers under higher pressure than 0.3 MPa as shown in Figure 5. These results show that the isopropylation occurs in steady state under high propene pressures. However, the yield of 4,4-DIPB under such a low pressure as 0.1 MPa was deviated downwards from the linear plot under higher pressures, and the amount of 4,4-DIPB decreased after reaching the maximum at higher conversion. On the contrary, the upward deviation occurred for the yield of 3,4'-DIPB. The increase of the yield of 3,4-DIPB corresponded to the decrease of that of 4,4-DIPB. The change of the selectivities of 4,4'-DIPB is not due to the change of shape selectivity of the pore, but to its isomerization to 3,4'-DIPB, because 3,4'-DIPB is thermodynamically more stable isomer than 4,4-DIPB (13). Isomerization of 4,4'-DIPB under propene pressure. The behavior of 4,4-DIPB during the reaction is one of the essential factors for product distributions. Figure 6 shows the stability of 4,4'-DIPB under propene pressures over HM(220). Without propene or under low propene pressure, 4,4-DIPB was isomerized significantly to 3,4DIPB accompanying IPBP isomers formed by the dealkylation. However, the formtaion of 3,4-DIPB decreased with the increase of propene pressure. These tendencies correspond well with the influences of propene pressure in the isopropylation of BP. The inhibition of the isomerization under high propene pressure shows that propene is adsorbed more preferentially than 4,4-DIPB. Adsorbed propene prevents the adsorption of 4,4-DIPB, and retards its isomerization. However, the adsorption of 4,4-DIPB predominates over that of propene under low propene pressure to result in the enhancement of its isomerization. Further isopropylation of 4,4-DIPB under the conditions was observed only in small amounts even under high propene pressure; i.e., the isopropylation of 4,4'-DIPB is prevented inside the pores. This is one of the reasons why shape selective


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Table 1. The isomerization of 4,4'-DIPB under propylene pressure f


Composition of 4,4 -DIPB (%) Catalyst

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Encapsulated products

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HM(206)

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Reaction conditions: 4,4'-DIPB, 100 mmol; H M , l g ; propene, 0.8 MPa; temperature, 250 °C; period, 4 h.

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Figure 3. Thermogravimetric profiles of H-mordenite used for the reaction under air atmosphere. T G conditions: H-mordenite, 10 mg; programmed rate, 10 °C/min. The reaction conditions are the same as Figure 1.


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Q Downloaded by UCSF LIB CKM RSCS MGMT on November 25, 2014 | http://pubs.acs.org Publication Date: November 2, 1999 | doi: 10.1021/bk-2000-0738.ch019

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Figure4. Effects of propene pressure on the selectivity of 4,4-DIPB in the isopropylation of BP. Reaction conditions: BP, 400 mmol; HM(220), 2 g; propene, 0.10.8 MPa; temperature, 250 °C.

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Figure5. Effects of propene pressure on the yield of 3,4- and 4,4-DIPB in the isopropylation of BP. Reaction conditions were the same as Figure 4.


278 isopropylation occurs in the catalysis over H M . H M pore allows the formation of 3and 4-IPBP, and 3,4 - and 4,4'-DIPB, while the formation of TrIPB is forbidden in the pores. The higher steric restriction of 3,4-DIPB compared with that of 4,4-DIPB results in the lower selectivity for 3,4-DIPB because molecular diameter of the former isomer is larger than that of the latter. Relatively high amount of TrIPB was observed in the isomerization of 4,4-DIPB under 0.1 MPa. TrIPB is formed by the isomerized DIPB isomers at the external surface because of low propene pressure.


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Effect of propene pressure on encapsulated products inside the pores. Figures 7 and 8 show the effect of propene pressure on the selectivities of 4,4'-DIPB in bulk and encapsulated products in the isopropylation of BP and the isomerization of 4,4'-DIPB over HM(206). The selectivity of 4,4'-DIPB encapsulated inside the pores in the isopropylation was almost constant under all pressures, whereas that of bulk products decreased under low propene pressure as discussed above. The effects of the pressure on the isomerization of 4,4'-DIPB showed similar tendency to the isopropylation of BP. The selectivity of 4,4'-DIPB in encapsulated DIPB isomers was almost constant under any pressure condition similar to the case of the isopropylation. On the other hand, 4,4'-DIPB itself isomerized significantly to 3,3'- and 3,4'-DIPB over HM(206) in the absence of propene, whereas no significant isomerization of 4,4'-DIPB occurred in the presence of sufficient propene pressure. These results indicate that the isomerization does not occur inside the pores, but at the external acid sites. Preferential adsorption of propene on acid sites is considered to inhibit the isomerization of 4,4'-DIPB under high pressures. However, the adsorption of 4,4'DIPB should predominate over that of propene under the low pressure, and thus, the isomerization of 4,4'-DIPB occurs at external acid sites. CONCLUSION Shape selective formation of 4,4'-DIPB was observed in the isopropylation of BP over dealuminated HMs. The selectivity of 4,4'-DIPB was dependent on the S K V A l ^ ratio of H M , and it was low for HMs with the lower ratio. However, encapsulated products showed that the selectivities of 4,4'-DIPB inside the pores were higher than 85% for all HMs regardless of the ratio. These results show that shape selective catalysis occurs inside the pores for all HMs in spite of the fact that non-regioselective isopropylation is predominant in bulk products for HMs with the lower ratio. This means that acid sites inside the pores do not always act as principal active sites for the isopropylation. Especially on HMs with the lower S i 0 / A l 0 ratio, coke-deposition occurs on these acid sites via the dehydrogenation of isopropylated biphenyls, and blocks the pores. The effects of propene pressure on the isopropylation of BP were investigated over highly dealuminated HMs. The selectivity of 4,4-DIPB was up to 90% under 0.8 MPa of propene pressure, whereas the decrease of the selectivity occurred under 0.1 MPa. The isomerization of 4,4'-DIPB occurred extensively in the absence of propene or under low pressures, but decreased with increasing the pressure. The selectivities of 4,4-DIPB in encapsulated products for the isopropylation of BP and for the isomerization of 4,4-DIPB was more than 90 % under all pressures. These results show that the decrease of the selectivity of 4,4-DIPB during the isopropylation of BP is not ascribed to non2

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Figure6. The isomerization of 4,4-DIPB under propene pressure. Reaction conditions: 4,4'-DIPB, 100 mmol; HM(220), 1 g; propene, 0-0.8 MPa; temperature, 250 °C; period, 4 h.

100

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Encapsulated Products inside Pores of H-Mordenite in the

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