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126 Epoxidation of ethene over silver catalysts supported on crystalline α-alumina carriers
Science and Technology in Catalysis 2002 Copyright 9 2003 by Kodansha Ltd.
509
126 Epoxidation of Ethene over Silver Catalysts Supported on Crystalline -Alumina Carriers
Akimi AYAME', Yoshio UCHIDA ~, Hiroyuki ONO', Tomoyuki SATO', and Hirofumi HAYASAKA 1 'Department of Applied Chemistry, Muroran Institute of Technology; 27-1 Mizumoto-cho, Muroran, Hokkaido 050-8585, Japan ~Tsukuba Research Laboratory, Sumitomo Chemical Co. Ltd.; 6 Kitahara, Tsukuba, Ibaraki 300-3292, Japan
Abstract The a-alumina carders with homogeneous pore structure and high purity were prepared from primary a-alumina crystal particles. The mean pore diameter was controlled by changing the primary particle size. In ethene epoxidation, the carriers having the mean pore diameter of 0.7-1.0 ~tm resulted in very stable and valuable performances.
1. INTRODUCTION Due to the industrial importance of the silver-catalyzed epoxidation of ethene, the researches on the preparation of the silver catalysts with high selectivity and long life time have been continued. In recent years, the attention is focused on the selection of additives and preparation method of carriers with desirable and effective pore structure. In the present study, attempts were made to prepare a new cartier consisting of a-alumina crystal particles and to reveal the performance of silver catalysts supported on the crystalline a-alumina carriers. 2. EXPERIMENTAL
Primary a-alumina crystal particles with polyhedral structure and high purity above 99.95 % were prepared by treating transient aluminas in hydrogen chloride gas at 1073-1473 K [1]. The primary particles were molded into lamellas using polyvinyl alcohol as binder and calcined at 1473-1873 K. The calcined lamellas were crushed and sieved to 0.25-0.5 and 1-2 mmr parts, which were used as crystalline a-alumina catalyst cartier (CAC). CsReAg catalyst precursor supported on the CAC were prepared by the amine method using ethylenediamine-water solution including given amounts of silver oxalate, cesium nitrate and rhenium oxide. The precursor was dried and activated in a stream of hydrogen at 573 K. The catalyst composition was Cs/Re/ Ag/carrier = 0.012/0.012/12/88 by weight. Ethene epoxidation was carried out using a pulse technique (catalyst 0.2 g, pulse size 0.192 ml, C 2 H 4 / O z / H e = 3/20/77 and 12/3/85 by volume, He-carrier 12.4 mlmin 1) [2] and a conventional fixed bed flow reactor (catalyst 0.2 or 9 g, feed gas C2H4/O2/He = 30/8/62 by volume, flow rate 26 ml min 1, no addition of organic halides) .
510 A. Ayame et
al.
3. R E S U L T S AND DISCUSSION Physicochemical properties of the CACs prepared from the primary a-alumina particles of 2 -10 Ixm are summarized in Table 1. The CACs have large surface areas and large mean pore diameters (MPD) of 0.33 -2.2 gm. The pore structure was very homogeneous: the pores in the range of 0.5-2.0 times the MPD occupied 60-95 % of the whole of pore. Since the pores of the CAC are cavities surrounded by crystal surfaces of the primary crystal particles, the MPD becomes large with increasing the primary crystal size. For the promoted catalysts, Fig.1 shows the relationships between the yield factor (fv), which was given by the product of ethene conversion and selectivity to ethene oxide, and the MPD of the CAC used. In the range of 0.7-1.0 grn, a maximum yield factor appeared. Also, the relationships between the yield factor and the MPD of the promoted catalysts showed a maximum at about 0.1 lain. From these results, it was revealed that the most desirable pore size of a-alumina carder for silver-catalyzed ethene epoxidation was 0.7-1.0 jim. The CsReAg/AA5 and CsReAg/ZM catalysts indicated more stable ethene conversion and selectivity for 200 h and comparatively higher selectivity even if ethene conversion was elevated, compared to those for the CsReAg/SA5561 catalyst which showed slight deactivation phenomena with reaction time. From the results shown in Table 2, it is clear that the promoted Agsupported CAC catalysts possess very excellent performance. 4. References [1] Sumitomo Chem. Co. Ltd., Japan Patent H6-191835. [2] L. Feng, A. Uchiyama, and A. Ayame, Sekiyu Gakkaishi, 39 (1996) 236.
Table 1 Physicochemical properties Carder PSPa PVb SA~ Porosity MPDd AA2 2 0.02 0.24 7.4 0.33 AA3 3 0.05 0.39 15.8 0.48 AA4 4 0.11 0.47 30.5 1.01 1.22 AA5 5 0.09 0.30 26.5 AASb 5 0.09 0.40 25.4 0.84 AA10 10 0.15 0.29 38.1 2.15 1.04 ZM 5 0.09 0.36 SA5561e 0.18 2.00 0.36 a Primary eoarticle size/lam~ -1 b pore volume/ ml g-l, surface area m g , mean pore diameter/~n, 9 carder supplied by Norton Co.
.......
Reaction temp. 473 K n 453 K
J
Table 2 Steady conversion and selectivity obtained using the fixed bed flow reactor. _Carder AA5 AASb SA5561 ZM f
PS' 1-2 1-2 1-2 1-2
SVb/h"1 TRC/h Cvd/% se/% 250 200 8.60 70.1 230 100 7.45 63.3 160 200 9.27 57.3 1160 200 7.50 82.4 290 52 13.0 81.5 ZM 0.25-0.5 15300 10 6.10 68.9 SA5561 0.25-0.5 17700 9 4.06 68.2 Reaction temperature 473 K. ' particle size of carder /mm, b space velocity, Creaction time, a ethene conversion, e selectivity, f calcined again in air at -1873 K for 6 h before catalyst preparation.
.....
0
i
l
I
l
0.5
1
1.5
2
M e a n p o r e d i a m e t e r / /~ m
Fig.1
Relationships between the yield factor, f v, and the mean pore diameter (MPDI of the crystalline a-alumina carrier. Theft, was determined by the pulse reaction data. Particle size of the CACs used was 0.25-0.5 mm.
509
126 Epoxidation of Ethene over Silver Catalysts Supported on Crystalline -Alumina Carriers
Akimi AYAME', Yoshio UCHIDA ~, Hiroyuki ONO', Tomoyuki SATO', and Hirofumi HAYASAKA 1 'Department of Applied Chemistry, Muroran Institute of Technology; 27-1 Mizumoto-cho, Muroran, Hokkaido 050-8585, Japan ~Tsukuba Research Laboratory, Sumitomo Chemical Co. Ltd.; 6 Kitahara, Tsukuba, Ibaraki 300-3292, Japan
Abstract The a-alumina carders with homogeneous pore structure and high purity were prepared from primary a-alumina crystal particles. The mean pore diameter was controlled by changing the primary particle size. In ethene epoxidation, the carriers having the mean pore diameter of 0.7-1.0 ~tm resulted in very stable and valuable performances.
1. INTRODUCTION Due to the industrial importance of the silver-catalyzed epoxidation of ethene, the researches on the preparation of the silver catalysts with high selectivity and long life time have been continued. In recent years, the attention is focused on the selection of additives and preparation method of carriers with desirable and effective pore structure. In the present study, attempts were made to prepare a new cartier consisting of a-alumina crystal particles and to reveal the performance of silver catalysts supported on the crystalline a-alumina carriers. 2. EXPERIMENTAL
Primary a-alumina crystal particles with polyhedral structure and high purity above 99.95 % were prepared by treating transient aluminas in hydrogen chloride gas at 1073-1473 K [1]. The primary particles were molded into lamellas using polyvinyl alcohol as binder and calcined at 1473-1873 K. The calcined lamellas were crushed and sieved to 0.25-0.5 and 1-2 mmr parts, which were used as crystalline a-alumina catalyst cartier (CAC). CsReAg catalyst precursor supported on the CAC were prepared by the amine method using ethylenediamine-water solution including given amounts of silver oxalate, cesium nitrate and rhenium oxide. The precursor was dried and activated in a stream of hydrogen at 573 K. The catalyst composition was Cs/Re/ Ag/carrier = 0.012/0.012/12/88 by weight. Ethene epoxidation was carried out using a pulse technique (catalyst 0.2 g, pulse size 0.192 ml, C 2 H 4 / O z / H e = 3/20/77 and 12/3/85 by volume, He-carrier 12.4 mlmin 1) [2] and a conventional fixed bed flow reactor (catalyst 0.2 or 9 g, feed gas C2H4/O2/He = 30/8/62 by volume, flow rate 26 ml min 1, no addition of organic halides) .
510 A. Ayame et
al.
3. R E S U L T S AND DISCUSSION Physicochemical properties of the CACs prepared from the primary a-alumina particles of 2 -10 Ixm are summarized in Table 1. The CACs have large surface areas and large mean pore diameters (MPD) of 0.33 -2.2 gm. The pore structure was very homogeneous: the pores in the range of 0.5-2.0 times the MPD occupied 60-95 % of the whole of pore. Since the pores of the CAC are cavities surrounded by crystal surfaces of the primary crystal particles, the MPD becomes large with increasing the primary crystal size. For the promoted catalysts, Fig.1 shows the relationships between the yield factor (fv), which was given by the product of ethene conversion and selectivity to ethene oxide, and the MPD of the CAC used. In the range of 0.7-1.0 grn, a maximum yield factor appeared. Also, the relationships between the yield factor and the MPD of the promoted catalysts showed a maximum at about 0.1 lain. From these results, it was revealed that the most desirable pore size of a-alumina carder for silver-catalyzed ethene epoxidation was 0.7-1.0 jim. The CsReAg/AA5 and CsReAg/ZM catalysts indicated more stable ethene conversion and selectivity for 200 h and comparatively higher selectivity even if ethene conversion was elevated, compared to those for the CsReAg/SA5561 catalyst which showed slight deactivation phenomena with reaction time. From the results shown in Table 2, it is clear that the promoted Agsupported CAC catalysts possess very excellent performance. 4. References [1] Sumitomo Chem. Co. Ltd., Japan Patent H6-191835. [2] L. Feng, A. Uchiyama, and A. Ayame, Sekiyu Gakkaishi, 39 (1996) 236.
Table 1 Physicochemical properties Carder PSPa PVb SA~ Porosity MPDd AA2 2 0.02 0.24 7.4 0.33 AA3 3 0.05 0.39 15.8 0.48 AA4 4 0.11 0.47 30.5 1.01 1.22 AA5 5 0.09 0.30 26.5 AASb 5 0.09 0.40 25.4 0.84 AA10 10 0.15 0.29 38.1 2.15 1.04 ZM 5 0.09 0.36 SA5561e 0.18 2.00 0.36 a Primary eoarticle size/lam~ -1 b pore volume/ ml g-l, surface area m g , mean pore diameter/~n, 9 carder supplied by Norton Co.
.......
Reaction temp. 473 K n 453 K
J
Table 2 Steady conversion and selectivity obtained using the fixed bed flow reactor. _Carder AA5 AASb SA5561 ZM f
PS' 1-2 1-2 1-2 1-2
SVb/h"1 TRC/h Cvd/% se/% 250 200 8.60 70.1 230 100 7.45 63.3 160 200 9.27 57.3 1160 200 7.50 82.4 290 52 13.0 81.5 ZM 0.25-0.5 15300 10 6.10 68.9 SA5561 0.25-0.5 17700 9 4.06 68.2 Reaction temperature 473 K. ' particle size of carder /mm, b space velocity, Creaction time, a ethene conversion, e selectivity, f calcined again in air at -1873 K for 6 h before catalyst preparation.
.....
0
i
l
I
l
0.5
1
1.5
2
M e a n p o r e d i a m e t e r / /~ m
Fig.1
Relationships between the yield factor, f v, and the mean pore diameter (MPDI of the crystalline a-alumina carrier. Theft, was determined by the pulse reaction data. Particle size of the CACs used was 0.25-0.5 mm.