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Hydrogenolysis of α-methylbenzyl alcohol over bifunctional catalysts
Applied Catalysis A: General 238 (2003) 141–148
Hydrogenolysis of ␣-methylbenzyl alcohol over bifunctional catalysts Byong-Sung Kwak∗ , Tae-Jin Kim, Sang-Il Lee Laboratory of Fine Chemicals, R&D Center, SK Corporation, 140-1 Wonchon-dong, Yusung-gu, Taejon 305-370, South Korea Received 30 January 2002; received in revised form 23 March 2002; accepted 5 June 2002
Abstract Hydrogenolysis of ␣-methylbenzyl alcohol (MBA), a co-product of propylene oxide (PO) in hydrocarbon peroxide oxidation processes, was studied to develop a by-product-free PO process. The conversion of MBA was mainly affected by the acidity of the support. Among the metals tested, Pd was most active and selective for the formation of ethylbenzene (EB). The hydrogenation of MBA exhibited type II selectivity, meaning that the selectivity to main products is controlled by reaction conditions and metals chosen. On Pd or Ni, the formation of EB is predominant while on Ru or Pt, that of 1-cyclohexylethanol (CHE) prevails. By carefully designing the catalyst and selecting the reaction condition, one could obtain a very high yield of EB with only a small amount of side products. Based on the results, a bifunctional reaction mechanism was proposed. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydrogenolysis; ␣-Methylbenzyl alcohol; Propylene oxide; Reaction mechanism; Bifunctional
1. Introduction Propylene oxide (PO) is a very important raw material in the petrochemical industry, with many applications. There are a few processes for the commercial production of PO: for example, PO/styrene monomer (SM) co-production process [1,2], PO/t-butanol co-production process [3,4] and chlorohydrin process [5]. The major drawback of these commercial processes is the co-production of almost equimolar amounts of by-products. Therefore, there have been ceaseless efforts to develop new processes that either produce PO without by-products or have higher productivity. These research and development activities can be classified into four groups, viz. the reaction ∗ Corresponding author. Tel.: +82-42-866-7402; fax: +82-42-866-7402. E-mail address: [email protected] (B.-S. Kwak).
of propylene with hydrogen peroxide [6,7], direct oxidation of propylene with oxygen [8,9], oxidation with hydrogen and oxygen [10–14], and oxidation with organic peroxides [15,16]. Of course, among these, the second and third groups are most desirable as relatively cheap starting materials are used. However, the yields achieved so far are still too low to commercialize. Recently, two processes that belong to the first and fourth categories, which exhibit remarkable technological advances, were announced by Degussa [17] and Degussa and Sumitomo [18], respectively. The Degussa process is known to produce PO through the catalytic epoxidation of propylene with hydrogen peroxide by utilizing the company’s strong position in the world hydrogen peroxide market. The Sumitomo process, on the other hand, yields PO by reacting propylene with cumene hydroperoxide (Fig. 1). After reaction, cumene hydroperoxide is converted to cumene alcohol, which then is dehydrated
0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 2 ) 0 0 3 4 7 - 2
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B.-S. Kwak et al. / Applied Catalysis A: General 238 (2003) 141–148
Fig. 1. Schematic diagram of a new Sumitomo process.
to ␣-methyl styrene, followed by hydrogenation to cumene. Cumene again is oxidized to make cumene hydroperoxide. By adopting so-called cumene hydroperoxide extinction process, it becomes possible to produce PO without any co-products. Before this extinction process, there were some trials to apply the same concept to the PO/SM process (Fig. 2) [19,20]. Here, PO is synthesized by the epoxidation of propylene with ethylbenzene hydroperoxide (EBHP). During the reaction, EBHP turns into ␣-methylbenzyl alcohol (MBA). By hydrogenolysis, MBA can be converted in a step to ethylbenzene (EB), which then is oxidized to become EBHP. This MBA extinction process, if installed at
the existing PO/SM process which has the largest world market share, can either easily control the amount of the co-product, SM or produce PO even without SM. However, former research efforts were focused on using batch processes that restrict the large scale use of the MBA extinction process. Also, the details of the hydrogenolysis are not well known as there is no study on this reaction other than a few patents. In this paper, therefore, we report for the first time the hydrogenolysis of MBA in an attempt to design bifunctional catalysts that can be effectively exploited in the control of the amount of SM co-produced in the PO/SM process.
Fig. 2. Schematic diagram of MBA extinction process.
B.-S. Kwak et al. / Applied Catalysis A: General 238 (2003) 141–148
2. Experimental For support materials, commercial samples were kindly supplied by Sud Chemie (HMOR90, SiO2 / Al2 O3 = 90; IS-28N, amorphous silica–alumina), Crosfield (HY5, SiO2 /Al2 O3 = 5.3), Grace-Davison (GD8625, silica), Crosfield (HTC-500, 20.7% Ni/alumina), and Norton (SA3177 and SA6173, alumina). All zeolites were ion exchanged twice with 1.0 mol/dm3 aqueous solution of ammonium hydroxide at room temperature for 12 h. After drying at 110 ◦ C overnight, they were calcined at 500 ◦ C for 2 h under a flowing oxygen atmosphere to obtain H-forms of zeolites. Metal loaded catalysts were prepared by incipient wetness impregnation of aqueous solutions of metal chlorides at room temperature. The concentration and amount of the solution were controlled in such a way as to fix the loading at 2 wt.%, as analyzed by XRF (Philips, Type 9430). After impregnation, the samples were dried and calcined as described previously. Reaction tests were carried out using either a 100 ml autoclave batch reactor or a fully automatic continuous reactor system (Altamira, AMI-100) with 2–3 g of powder catalyst. For batch tests, pre-reduced catalysts immersed in 69 ml of ethanol (Aldrich) were loaded with 3 g of ␣-methylbenzyl alcohol (Aldrich). After purging the system with nitrogen, the pressure was set at 2.0 bar with H2 , followed by reaction at 40 ◦ C for 5 h. For continuous tests, the sample was loaded into a 1/2 in. stainless steel reactor, followed by reduction with H2 at 400 ◦ C for 6 h. H2 flow rate was 80 standard cubic centimeter per minute (sccm) and the ramp rate was 5 ◦ C/min. After reduction, the system was cooled to room temperature. The reaction temperature was
143
varied between 30 and 150 ◦ C; the pressure range was between 2.0 and 6.8 bar. The feed (5 g MBA + 115 ml ethanol) was pumped at the rate of 10 ml/min while the hydrogen flow rate was set at 20 sccm. The liquid collected at the separator of the reaction system was periodically taken and analyzed by gas chromatography (Hewlett-Packard 5890 with a flame ionization detector) using a Hewlett-Packard PONA column. For comparison purposes, the hydrogenation of MBA to 1-cyclohexylethanol (CHE) was carried out at a pressure of 27.2–88.4 bar, a temperature of 80–180 ◦ C and weight hourly space velocity (WHSV) value of 0.1–0.64 h−1 with various catalysts. In this case, 10 g catalysts (16–25 mesh) were tested with the feed containing 10% MBA and 90% methylcyclohexane (Junsei). Other details are the same as mentioned previously. The catalysts were characterized with NH3 -TPD (Altamira, AMI-1), nitrogen adsorption/desorption (Micromeritics, ASAP 2010), CO chemisorption (Micromeritics, ASAP 2010C) and dehydration of isopropyl alcohol, following the procedures described elsewhere [21].
3. Results and discussion 3.1. Characterization studies Table 1 shows the results of characterization studies. Based on the Tmax value from NH3 -TPD spectra of the samples, the Brönsted acid strength increases in the order [21]: IS-28N ≤ HY5 < HMOR90
Table 1 Results of characterization studies Sample
BET surface area (m2 /g)
External surface area (m2 /g)
IPA conversion at 150 ◦ C
Tmax (◦ C)a
Dispersion (%)b
GD8625 SA3177 SA6173 IS-28N HY5 HMOR90
234 102 215 340 995 482
– – – – 52 94
1.0c 6.5 10.5 100 100 100
– – – 400 425 550
– 12.3 – 24.6 8.1 28.9
a
Tmax was decided from NH3 -TPD spectra [21]. CO chemisorption was measured at 35 ◦ C over samples with the Pd loading of 2 wt.%. c IPA conversion at 250 ◦ C. b
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In the case of IS-28N, which contains binder materials, the Tmax value was shifted to lower temperatures due to the binders. A binderless sample from the same company was found to have the value of 450 ◦ C, confirming that the intrinsic acidity of IS-28N is stronger than that of HY5. From t-plot analysis, the fractions of the external surface areas of HMOR90 are larger than that of HY5. The acid strength of the samples was also measured by IPA conversion at 150 ◦ C [22]. Unfortunately, at this temperature it was not possible to quantitatively differentiate the acid strength among zeolite samples. However, in general, zeolites were found to be much stronger in acidity than aluminas. It should be noted that the IPA conversion on GD8625 was very low even at 250 ◦ C. Combining the two results, one find that the acidity increases in the order: GD8625 < SA3177 < SA6173 IS-28N ≤ HY5 < HMOR90 CO chemisorption results showed that 2% Pd/IS-28N and 2% Pd/HMOR90 have relatively smaller Pd crystallites while 2% Pd/SA3177 and 2% Pd/HY5 have bigger crystallites. 3.2. Hydrogenolysis 3.2.1. Effect of metal The hydrogenolysis of MBA resulted in the production of EB and styrene as major products. Acetophenone was also formed. Besides these major products, ethyl ether of MBA, ethylcyclohexane, CHE, and benzene were formed as by-products. In Fig. 3 is shown the effect of metal using HY5 as a support in a batch reactor. Three metals were arbitrarily chosen according to a patent by Lansink-Rotgerink et al. [20] who claimed that all the metals in group VIII of the periodic table were active in the hydrogenolysis of MBA if the acid activity index of the support exceeded 90%. They defined the acid activity index as the conversion of IPA at 250 ◦ C under a specified reaction conditions. For all samples, the conversion reached almost 100%. This high conversion was obtained due to the strong acidity of the samples, for it is well known that MBA is dehydrated to form styrene on acidic sites [21,23]. On Pd/HY5, the selectivity to EB was ca. 58%. However, in the cases of Fe/HY5 and Ag/HY5, the major product was
Fig. 3. Effect of metal on the hydrogenolysis of MBA over HY5 supported catalysts at 30 ◦ C and 2.0 bar.
styrene; the selectivity to EB was rather low. Therefore, one may conclude that Fe and Ag are not very efficient in the hydrogenation of styrene. 3.2.2. Effect of support As Pd showed the best selectivity to EB, a few catalysts prepared with Pd chloride were tested, as shown in Fig. 4. Among the four samples, HMOR90 showed the highest conversion (Fig. 4a). This is presumably because HMOR90 is strongest in acidity and has the largest external surface area, which facilitates the transport of the molecules. At ca. 20 h on stream where the activity of catalysts became stable, the activity was in the order: SA3177 < HY5 < IS-28N < HMOR90 which is in accordance with the acid strength. It is remarkable that the activity of HY5 is similar to that of SA3177. All the samples other than IS-28N exhibited quite stable performance for over 20 h. HMOR90 was the highest in the case of the selectivity to EB as shown in Fig. 4(b). IS-28N was also very selective in the formation of EB. The high selectivity to EB of these two samples is quite in line with their high dispersion values. This suggests that the hydrogenolysis is favored on small metal crystallites. On the other hand, the HY5 sample produced styrene as a major product (Fig. 4c). It should be noted that the dispersion value of 2% Pd/HY5 is the lowest among the samples, leading to a low selectivity
B.-S. Kwak et al. / Applied Catalysis A: General 238 (2003) 141–148
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Fig. 4. Hydrogenolysis of MBA over bifunctional catalysts at 30 ◦ C and 2.0 bar: (a) conversion; (b) selectivity to ethylbenzene; (c) selectivity to styrene.
to EB as well as a low activity, owing to weak acid strength. 3.2.3. Effect of temperature and pressure In Fig. 5 is shown the effect of temperature for HMOR90 sample at 2.0 bar. As temperature is increased from 30 to 75 ◦ C, the conversion increased
from 72 to 100%. The selectivity to EB increased with temperature at the expense of styrene. It should be noted that the selectivity to EB reached 99.4% at 75 ◦ C. The high selectivity, namely almost negligible formation of ethylcyclohexane, is highly important, as ethylcyclohexane is not easy to separate by distillation and, therefore, accumulates upon recycling
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B.-S. Kwak et al. / Applied Catalysis A: General 238 (2003) 141–148 Table 2 Hydrogenation of MBA at high pressure and WHSV of 0.64 h−1 Sample
HTC 500 1% Pt/SA6173 1% Pd/SA6173 5% Ru/SA3177 5% Ru/GD8625a a
T (◦ C)
80 127 98 166 130
P (bar)
32.7 88.8 13.6 86.9 88.4
Conversion (%)
85.4 82.7 74.7 48.2 100
Selectivity (%) to CHE
EB
11.9 74.3 0.9 71.2 96.3
87.5 9.3 98.2 16.3 0
WHSV, 0.32 h−1 .
To see if MBA can be hydrogenated to CHE, which is another unwanted major side product as described in the previous section, we tested a few catalysts under
severer conditions than in the case of hydrogenolysis. As MBA gets dehydrated readily on strong acidic sites, we carried out tests with the supports having relatively similar lower acid strength, i.e. aluminas and silica (Table 1) in order to exclude the possibility of predominantly forming EB over all catalysts. Another purpose of these experiments was to see if EB could be easily produced over supported Pt catalysts prepared using aluminas without chloride ions; there had been such an effort over pre-chlorided catalysts [19]. In Table 2 are shown initial activity data at 3 h on stream at the WHSV of 0.64 h−1 . For these experiments, the temperature and pressure were varied to keep the conversion values nearly equal to or higher than 50%. Among the catalysts tested, 1% Pt/SA6173 deactivated most rapidly, whereas, other catalysts were rather stable for more than 10 h. It is surprising that Pt and Pd catalysts showed quite different selectivity patterns on the same support. The Ru catalyst exhibited a similar pattern to that of the Pt catalyst,
Fig. 6. Effect of pressure on 2% Pd/HMOR90 at 30 ◦ C.
Fig. 7. Type II selectivity depending on the catalyst used.
Fig. 5. Effect of temperature on 2% Pd/HMOR90 at 2.0 bar.
the hydrogenolysis product in the extinction process newly proposed by us, is almost negligible. The effect of pressure is as shown in Fig. 6 over HMOR90 at 50 ◦ C. Both MBA conversion and selectivity to EB increased slightly with pressure up to 6.8 bar. Over 6.8 bar, there was no increase in the catalytic performance. This low dependence on pressure of the reaction results from the fact that the phases of reactants and products are liquid under these reaction conditions. 3.3. Hydrogenation to 1-cyclohexylethanol
B.-S. Kwak et al. / Applied Catalysis A: General 238 (2003) 141–148
147
Fig. 8. Reaction mechanism of the hydrogenolysis of MBA.
mainly producing CHE, while the Ni and Pd catalysts predominantly produced EB. When the reaction condition and catalyst (5% Ru/silica) were optimized, the selectivity to CHE approached to nearly 100%, suggesting that hydrogenation of the benzene ring is favored in the presence of Ru and weakly acidic sites. Therefore, the hydrogenation of MBA is a typical example exhibiting ‘type II selectivity’ [24,25] depending mainly on the metal used (Fig. 7). It can also be concluded that without chloride ions, supported Pt catalysts are not useful for the preparation of EB.
co-product of PO, to EB in a step without forming a significant amount of side products under a suitable reaction condition. Ethylbenzene thus formed is recycled to prepare EBHP, which is then reacted with propylene to make PO. By this method, consequently, one can easily produce PO either without a co-product or with a controlled amount of the co-product, e.g. SM. We are now studying this reaction extensively and will soon report the result in subsequent papers.
3.4. Reaction mechanism
Financial support from Ministry of Science and Technology of Korea under the Grant number of 2000-N-NL-01-C-145 is gratefully acknowledged.
Considering the results described so far, we propose the following sequential dehydration–hydrogenation mechanism on bifunctional Pd and/or Ni catalysts for the hydrogenolysis of MBA (Fig. 8). First, MBA is protonated to form a carbenium ion, which is dehydrated to make styrene and a proton. On metal crystallites, styrene thus formed is rapidly hydrogenated to EB. Consequently, the strong acidity and small crystallites enhance the hydrogenolysis of MBA, indicating a concerted mechanism between metal and acidic functions. Further hydrogenation of EB gives a small amount of ethycyclohexane as a by-product. Formation of ethers of MBA is assumed to occur following the mechanism described elsewhere [19,22]. The formation of CHE is more affected by the metal adopted for hydrogenation. In conclusion, it looks possible to develop a new process for the production of PO by designing a proper bifunctional catalyst that can hydrogenolyze MBA, a
Acknowledgements
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A.C. Butler, C.P. Nicolaides, Catal. Today 18 (1993) 443. J.J. Coyle, US Patent 4,059,598 (1977). J. Kollar, US Patent 3,351,635 (1967). M. Gelb, D.W. Leyshon, J.A. Sofranko, C.A. Jones, US Patent 4,988,830 (1991). G.T. White, US Patent 3,247,227 (1966). T. Chang, US Patent 6,066,750 (2001). W. Laufer, R. Meiers, W. Hoelderich, J. Mol. Catal. A 141 (1999) 215. R.G. Bowman, A. Kuperman, H.W. Clark, G.E. Hartwell, G.R. Meima, US Patent 6,323,351 (2001). I. Yamanaka, K. Nakagaki, K. Otsuka, Appl. Catal. A 171 (1998) 309. G.H. Grosch, U. Muller, M. Schultz, N. Rieber, H. Wurz, US Patent 6,008,389 (1999). R.N. Cochran, J.C. Jubin Jr., M.A. Liepa, R. Nedwick, R. Pitchai, W.W. Wentzheimer, US Patent 5,973,171 (1999).
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[12] C. Qi, T. Akita, M. Okumura, M. Haruta, Appl. Catal. A 218 (2001) 81. [13] R. Meiers, U. Dingerdissen, W.F. Holderich, J. Catal. 176 (1998) 376. [14] E.E. Strangland, K.B. Stavens, R.P. Andres, W.N. Delgass, J. Catal. 191 (2000) 332. [15] J. Yamamoto, J. Tsuji, US Patent 6,323,147 (2001). [16] J. Tsuji, J. Yamamoto, A.C. Canos, F.R. Garcia, US Patent 6,211,388 (2001). [17] Chemical Market Reporter, 259 (2) (2001) 3. [18] Chemical Week 163 (13) (2001) 21.
[19] C.W.A. Schram, F. Wattimena, US Patent 3,944,627 (1976). [20] H. Lansink-Rotgerink, M. Scholtz, A. Freund, G. Kunz, US Patent (1998). [21] B.S. Kwak, T.J. Kim, Appl. Catal. A 188 (1999) 99. [22] B.S. Kwak, S.I. Lee, T.J. Kim, Catal. Comm., in preparation. [23] B.S. Kwak, T.J. Kim, Catal. Lett. 59 (1999) 55. [24] G.W. Roberts, in: P.N. Rylander, H. Greenfield (Eds.), Catalysis in Organic Synthesis, Academic Press, New York, 1976, p. 1. [25] A. Wheeler, Adv. Catal. 3 (1951) 249.
Hydrogenolysis of ␣-methylbenzyl alcohol over bifunctional catalysts Byong-Sung Kwak∗ , Tae-Jin Kim, Sang-Il Lee Laboratory of Fine Chemicals, R&D Center, SK Corporation, 140-1 Wonchon-dong, Yusung-gu, Taejon 305-370, South Korea Received 30 January 2002; received in revised form 23 March 2002; accepted 5 June 2002
Abstract Hydrogenolysis of ␣-methylbenzyl alcohol (MBA), a co-product of propylene oxide (PO) in hydrocarbon peroxide oxidation processes, was studied to develop a by-product-free PO process. The conversion of MBA was mainly affected by the acidity of the support. Among the metals tested, Pd was most active and selective for the formation of ethylbenzene (EB). The hydrogenation of MBA exhibited type II selectivity, meaning that the selectivity to main products is controlled by reaction conditions and metals chosen. On Pd or Ni, the formation of EB is predominant while on Ru or Pt, that of 1-cyclohexylethanol (CHE) prevails. By carefully designing the catalyst and selecting the reaction condition, one could obtain a very high yield of EB with only a small amount of side products. Based on the results, a bifunctional reaction mechanism was proposed. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydrogenolysis; ␣-Methylbenzyl alcohol; Propylene oxide; Reaction mechanism; Bifunctional
1. Introduction Propylene oxide (PO) is a very important raw material in the petrochemical industry, with many applications. There are a few processes for the commercial production of PO: for example, PO/styrene monomer (SM) co-production process [1,2], PO/t-butanol co-production process [3,4] and chlorohydrin process [5]. The major drawback of these commercial processes is the co-production of almost equimolar amounts of by-products. Therefore, there have been ceaseless efforts to develop new processes that either produce PO without by-products or have higher productivity. These research and development activities can be classified into four groups, viz. the reaction ∗ Corresponding author. Tel.: +82-42-866-7402; fax: +82-42-866-7402. E-mail address: [email protected] (B.-S. Kwak).
of propylene with hydrogen peroxide [6,7], direct oxidation of propylene with oxygen [8,9], oxidation with hydrogen and oxygen [10–14], and oxidation with organic peroxides [15,16]. Of course, among these, the second and third groups are most desirable as relatively cheap starting materials are used. However, the yields achieved so far are still too low to commercialize. Recently, two processes that belong to the first and fourth categories, which exhibit remarkable technological advances, were announced by Degussa [17] and Degussa and Sumitomo [18], respectively. The Degussa process is known to produce PO through the catalytic epoxidation of propylene with hydrogen peroxide by utilizing the company’s strong position in the world hydrogen peroxide market. The Sumitomo process, on the other hand, yields PO by reacting propylene with cumene hydroperoxide (Fig. 1). After reaction, cumene hydroperoxide is converted to cumene alcohol, which then is dehydrated
0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 2 ) 0 0 3 4 7 - 2
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Fig. 1. Schematic diagram of a new Sumitomo process.
to ␣-methyl styrene, followed by hydrogenation to cumene. Cumene again is oxidized to make cumene hydroperoxide. By adopting so-called cumene hydroperoxide extinction process, it becomes possible to produce PO without any co-products. Before this extinction process, there were some trials to apply the same concept to the PO/SM process (Fig. 2) [19,20]. Here, PO is synthesized by the epoxidation of propylene with ethylbenzene hydroperoxide (EBHP). During the reaction, EBHP turns into ␣-methylbenzyl alcohol (MBA). By hydrogenolysis, MBA can be converted in a step to ethylbenzene (EB), which then is oxidized to become EBHP. This MBA extinction process, if installed at
the existing PO/SM process which has the largest world market share, can either easily control the amount of the co-product, SM or produce PO even without SM. However, former research efforts were focused on using batch processes that restrict the large scale use of the MBA extinction process. Also, the details of the hydrogenolysis are not well known as there is no study on this reaction other than a few patents. In this paper, therefore, we report for the first time the hydrogenolysis of MBA in an attempt to design bifunctional catalysts that can be effectively exploited in the control of the amount of SM co-produced in the PO/SM process.
Fig. 2. Schematic diagram of MBA extinction process.
B.-S. Kwak et al. / Applied Catalysis A: General 238 (2003) 141–148
2. Experimental For support materials, commercial samples were kindly supplied by Sud Chemie (HMOR90, SiO2 / Al2 O3 = 90; IS-28N, amorphous silica–alumina), Crosfield (HY5, SiO2 /Al2 O3 = 5.3), Grace-Davison (GD8625, silica), Crosfield (HTC-500, 20.7% Ni/alumina), and Norton (SA3177 and SA6173, alumina). All zeolites were ion exchanged twice with 1.0 mol/dm3 aqueous solution of ammonium hydroxide at room temperature for 12 h. After drying at 110 ◦ C overnight, they were calcined at 500 ◦ C for 2 h under a flowing oxygen atmosphere to obtain H-forms of zeolites. Metal loaded catalysts were prepared by incipient wetness impregnation of aqueous solutions of metal chlorides at room temperature. The concentration and amount of the solution were controlled in such a way as to fix the loading at 2 wt.%, as analyzed by XRF (Philips, Type 9430). After impregnation, the samples were dried and calcined as described previously. Reaction tests were carried out using either a 100 ml autoclave batch reactor or a fully automatic continuous reactor system (Altamira, AMI-100) with 2–3 g of powder catalyst. For batch tests, pre-reduced catalysts immersed in 69 ml of ethanol (Aldrich) were loaded with 3 g of ␣-methylbenzyl alcohol (Aldrich). After purging the system with nitrogen, the pressure was set at 2.0 bar with H2 , followed by reaction at 40 ◦ C for 5 h. For continuous tests, the sample was loaded into a 1/2 in. stainless steel reactor, followed by reduction with H2 at 400 ◦ C for 6 h. H2 flow rate was 80 standard cubic centimeter per minute (sccm) and the ramp rate was 5 ◦ C/min. After reduction, the system was cooled to room temperature. The reaction temperature was
143
varied between 30 and 150 ◦ C; the pressure range was between 2.0 and 6.8 bar. The feed (5 g MBA + 115 ml ethanol) was pumped at the rate of 10 ml/min while the hydrogen flow rate was set at 20 sccm. The liquid collected at the separator of the reaction system was periodically taken and analyzed by gas chromatography (Hewlett-Packard 5890 with a flame ionization detector) using a Hewlett-Packard PONA column. For comparison purposes, the hydrogenation of MBA to 1-cyclohexylethanol (CHE) was carried out at a pressure of 27.2–88.4 bar, a temperature of 80–180 ◦ C and weight hourly space velocity (WHSV) value of 0.1–0.64 h−1 with various catalysts. In this case, 10 g catalysts (16–25 mesh) were tested with the feed containing 10% MBA and 90% methylcyclohexane (Junsei). Other details are the same as mentioned previously. The catalysts were characterized with NH3 -TPD (Altamira, AMI-1), nitrogen adsorption/desorption (Micromeritics, ASAP 2010), CO chemisorption (Micromeritics, ASAP 2010C) and dehydration of isopropyl alcohol, following the procedures described elsewhere [21].
3. Results and discussion 3.1. Characterization studies Table 1 shows the results of characterization studies. Based on the Tmax value from NH3 -TPD spectra of the samples, the Brönsted acid strength increases in the order [21]: IS-28N ≤ HY5 < HMOR90
Table 1 Results of characterization studies Sample
BET surface area (m2 /g)
External surface area (m2 /g)
IPA conversion at 150 ◦ C
Tmax (◦ C)a
Dispersion (%)b
GD8625 SA3177 SA6173 IS-28N HY5 HMOR90
234 102 215 340 995 482
– – – – 52 94
1.0c 6.5 10.5 100 100 100
– – – 400 425 550
– 12.3 – 24.6 8.1 28.9
a
Tmax was decided from NH3 -TPD spectra [21]. CO chemisorption was measured at 35 ◦ C over samples with the Pd loading of 2 wt.%. c IPA conversion at 250 ◦ C. b
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In the case of IS-28N, which contains binder materials, the Tmax value was shifted to lower temperatures due to the binders. A binderless sample from the same company was found to have the value of 450 ◦ C, confirming that the intrinsic acidity of IS-28N is stronger than that of HY5. From t-plot analysis, the fractions of the external surface areas of HMOR90 are larger than that of HY5. The acid strength of the samples was also measured by IPA conversion at 150 ◦ C [22]. Unfortunately, at this temperature it was not possible to quantitatively differentiate the acid strength among zeolite samples. However, in general, zeolites were found to be much stronger in acidity than aluminas. It should be noted that the IPA conversion on GD8625 was very low even at 250 ◦ C. Combining the two results, one find that the acidity increases in the order: GD8625 < SA3177 < SA6173 IS-28N ≤ HY5 < HMOR90 CO chemisorption results showed that 2% Pd/IS-28N and 2% Pd/HMOR90 have relatively smaller Pd crystallites while 2% Pd/SA3177 and 2% Pd/HY5 have bigger crystallites. 3.2. Hydrogenolysis 3.2.1. Effect of metal The hydrogenolysis of MBA resulted in the production of EB and styrene as major products. Acetophenone was also formed. Besides these major products, ethyl ether of MBA, ethylcyclohexane, CHE, and benzene were formed as by-products. In Fig. 3 is shown the effect of metal using HY5 as a support in a batch reactor. Three metals were arbitrarily chosen according to a patent by Lansink-Rotgerink et al. [20] who claimed that all the metals in group VIII of the periodic table were active in the hydrogenolysis of MBA if the acid activity index of the support exceeded 90%. They defined the acid activity index as the conversion of IPA at 250 ◦ C under a specified reaction conditions. For all samples, the conversion reached almost 100%. This high conversion was obtained due to the strong acidity of the samples, for it is well known that MBA is dehydrated to form styrene on acidic sites [21,23]. On Pd/HY5, the selectivity to EB was ca. 58%. However, in the cases of Fe/HY5 and Ag/HY5, the major product was
Fig. 3. Effect of metal on the hydrogenolysis of MBA over HY5 supported catalysts at 30 ◦ C and 2.0 bar.
styrene; the selectivity to EB was rather low. Therefore, one may conclude that Fe and Ag are not very efficient in the hydrogenation of styrene. 3.2.2. Effect of support As Pd showed the best selectivity to EB, a few catalysts prepared with Pd chloride were tested, as shown in Fig. 4. Among the four samples, HMOR90 showed the highest conversion (Fig. 4a). This is presumably because HMOR90 is strongest in acidity and has the largest external surface area, which facilitates the transport of the molecules. At ca. 20 h on stream where the activity of catalysts became stable, the activity was in the order: SA3177 < HY5 < IS-28N < HMOR90 which is in accordance with the acid strength. It is remarkable that the activity of HY5 is similar to that of SA3177. All the samples other than IS-28N exhibited quite stable performance for over 20 h. HMOR90 was the highest in the case of the selectivity to EB as shown in Fig. 4(b). IS-28N was also very selective in the formation of EB. The high selectivity to EB of these two samples is quite in line with their high dispersion values. This suggests that the hydrogenolysis is favored on small metal crystallites. On the other hand, the HY5 sample produced styrene as a major product (Fig. 4c). It should be noted that the dispersion value of 2% Pd/HY5 is the lowest among the samples, leading to a low selectivity
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Fig. 4. Hydrogenolysis of MBA over bifunctional catalysts at 30 ◦ C and 2.0 bar: (a) conversion; (b) selectivity to ethylbenzene; (c) selectivity to styrene.
to EB as well as a low activity, owing to weak acid strength. 3.2.3. Effect of temperature and pressure In Fig. 5 is shown the effect of temperature for HMOR90 sample at 2.0 bar. As temperature is increased from 30 to 75 ◦ C, the conversion increased
from 72 to 100%. The selectivity to EB increased with temperature at the expense of styrene. It should be noted that the selectivity to EB reached 99.4% at 75 ◦ C. The high selectivity, namely almost negligible formation of ethylcyclohexane, is highly important, as ethylcyclohexane is not easy to separate by distillation and, therefore, accumulates upon recycling
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B.-S. Kwak et al. / Applied Catalysis A: General 238 (2003) 141–148 Table 2 Hydrogenation of MBA at high pressure and WHSV of 0.64 h−1 Sample
HTC 500 1% Pt/SA6173 1% Pd/SA6173 5% Ru/SA3177 5% Ru/GD8625a a
T (◦ C)
80 127 98 166 130
P (bar)
32.7 88.8 13.6 86.9 88.4
Conversion (%)
85.4 82.7 74.7 48.2 100
Selectivity (%) to CHE
EB
11.9 74.3 0.9 71.2 96.3
87.5 9.3 98.2 16.3 0
WHSV, 0.32 h−1 .
To see if MBA can be hydrogenated to CHE, which is another unwanted major side product as described in the previous section, we tested a few catalysts under
severer conditions than in the case of hydrogenolysis. As MBA gets dehydrated readily on strong acidic sites, we carried out tests with the supports having relatively similar lower acid strength, i.e. aluminas and silica (Table 1) in order to exclude the possibility of predominantly forming EB over all catalysts. Another purpose of these experiments was to see if EB could be easily produced over supported Pt catalysts prepared using aluminas without chloride ions; there had been such an effort over pre-chlorided catalysts [19]. In Table 2 are shown initial activity data at 3 h on stream at the WHSV of 0.64 h−1 . For these experiments, the temperature and pressure were varied to keep the conversion values nearly equal to or higher than 50%. Among the catalysts tested, 1% Pt/SA6173 deactivated most rapidly, whereas, other catalysts were rather stable for more than 10 h. It is surprising that Pt and Pd catalysts showed quite different selectivity patterns on the same support. The Ru catalyst exhibited a similar pattern to that of the Pt catalyst,
Fig. 6. Effect of pressure on 2% Pd/HMOR90 at 30 ◦ C.
Fig. 7. Type II selectivity depending on the catalyst used.
Fig. 5. Effect of temperature on 2% Pd/HMOR90 at 2.0 bar.
the hydrogenolysis product in the extinction process newly proposed by us, is almost negligible. The effect of pressure is as shown in Fig. 6 over HMOR90 at 50 ◦ C. Both MBA conversion and selectivity to EB increased slightly with pressure up to 6.8 bar. Over 6.8 bar, there was no increase in the catalytic performance. This low dependence on pressure of the reaction results from the fact that the phases of reactants and products are liquid under these reaction conditions. 3.3. Hydrogenation to 1-cyclohexylethanol
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Fig. 8. Reaction mechanism of the hydrogenolysis of MBA.
mainly producing CHE, while the Ni and Pd catalysts predominantly produced EB. When the reaction condition and catalyst (5% Ru/silica) were optimized, the selectivity to CHE approached to nearly 100%, suggesting that hydrogenation of the benzene ring is favored in the presence of Ru and weakly acidic sites. Therefore, the hydrogenation of MBA is a typical example exhibiting ‘type II selectivity’ [24,25] depending mainly on the metal used (Fig. 7). It can also be concluded that without chloride ions, supported Pt catalysts are not useful for the preparation of EB.
co-product of PO, to EB in a step without forming a significant amount of side products under a suitable reaction condition. Ethylbenzene thus formed is recycled to prepare EBHP, which is then reacted with propylene to make PO. By this method, consequently, one can easily produce PO either without a co-product or with a controlled amount of the co-product, e.g. SM. We are now studying this reaction extensively and will soon report the result in subsequent papers.
3.4. Reaction mechanism
Financial support from Ministry of Science and Technology of Korea under the Grant number of 2000-N-NL-01-C-145 is gratefully acknowledged.
Considering the results described so far, we propose the following sequential dehydration–hydrogenation mechanism on bifunctional Pd and/or Ni catalysts for the hydrogenolysis of MBA (Fig. 8). First, MBA is protonated to form a carbenium ion, which is dehydrated to make styrene and a proton. On metal crystallites, styrene thus formed is rapidly hydrogenated to EB. Consequently, the strong acidity and small crystallites enhance the hydrogenolysis of MBA, indicating a concerted mechanism between metal and acidic functions. Further hydrogenation of EB gives a small amount of ethycyclohexane as a by-product. Formation of ethers of MBA is assumed to occur following the mechanism described elsewhere [19,22]. The formation of CHE is more affected by the metal adopted for hydrogenation. In conclusion, it looks possible to develop a new process for the production of PO by designing a proper bifunctional catalyst that can hydrogenolyze MBA, a
Acknowledgements
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