Preparation of highly active zeolite-based hydrodesulfurizationcatalysts: zeolite-supported Rh catalysts

Catalysts in Petroleum Refining and Petrochemical Industries 1995 M. Absi-Halabi et al. (Editors) 9 1996 Elsevier Science B.V. All fights reserved.

551

PREPARATION OF HIGHLY ACWIVE ZEO[ATE-BASED HYDRODESULFURIZATION CATALYSTS: ZEOIATF.,-SUPPORTED Rh CATALYSTS M. Sugioka, C. Tochiyama, F. Sado and N. Maesaki

Department of Applied Chemistry, Muroran Institute of Technology, 27-1 Mizumoto-cho, Muroran 050, Japan ABSTRACT It was revealed that Rh/USY showed the highest activity among Rh supported on various zeolites and its catalytic activity was higher than commercial CoMo/A1203 catalyst for the hydrodesulfurization of thiophene at 400~ The catalytic activity of Rh/USY decreased gradually with the reaction time. However, the catalyst deactivation of Rh/USY with reaction time was remarkably improved by the addition of small amounts of alkali metal salts. It is concluded that Rh/USY modified with alkali metal salts are potential highly active second generation hydrodesulfurization catalysts for petroleum feedstocks. 1. INTRODUCTION Hydrodesulfurization of petroleum feedstocks is one of the important processes in the petroleum industry to produce clean fuels. CoMo/AI203 catalyst has been widely used in hydrodesulfurization process of petroleum. However, recently, the development of highly active hydrodesulfurization catalysts with higher activity than commercial CoMo/AI203 hydrodesulfurization catalyst has been claimed in the petroleum industry to produce lower sulfur content fuels because of serious problems of air pollution on global scale by burning petroleum feedstocks. It has been reported that metal-zeolite catalysts have high possibility as new hydrodesulfurization catalysts for petroleum [1-9]. The authors have been investigating the catalytic desulfurization of organic sulfur compounds over zeolites [ 10-12] and also developing highly active zeolite- based hydrodesulfurization catalysts [ 13-19]. In the present work, the authors prepared various zeolite-supported Rh catalysts and examined their catalytic activities for the hydrodesulfurization of thiophene in order to develop highly active second generation hydrodesulfurization catalysts for petroleum feedstocks. 2. EXPEREVIENTAL Hydrodesulfurization of thiophene was carried out at 400~ under 1 atm by use of a conventional fixed bed flow reactor. Thiophene was introduced into the reactor by passing hydrogen through thiophene trap cooled at 0~ The reaction products were analysed by gas chromatograph(FID). Zeolite-supported Rh catalysts were prepared by impregnation method using RahC13 aqueous solution and the Rh loading was 0.5-5 wt%. Alkali metal-modified Rh/USY catalysts were prepared by addition of alkali metal salt aqueous solutions to Rh/USY catalyst. All

552 Rh/zeolite catalysts were calcined at 500~ for 4 hr in air and were reduced at 450~ for 1 hr prior to use. Presulfiding of Rh/USY catalyst was carried out at 400~ for 1 hr by using 5%HzS-H2 mixture. 3. RESULTS AND DISCUSSION 3.1 Activities of Rh/zeolite catalysts

The catalytic activities of Rh supported on various zeolites such as NaY, NaX, NaA, NaZSM-5, NaMordenite, HY, USY, HZSM-5, HMordenite, etc. for the hydrodesulfurization of thiophene were examined at 400~ Table 1 shows the catalytic activities of various Rh/zeolite catalysts in the hydrodesulfurization of thiophene. It was revealed that the activities of Rh/zeolite catalysts were markedly changed by the kind of zeolites. Rh supported on proton type zeolite(HZ) showed high catalytic activity but Rh supported on sodium zeolites(NaZ) except NaY showed low activity. Especially, Rh supported on HZ with large pore diameter such as USY and HY zeolites showed considerably high catalytic activity for the hydrodesulfurization of thiophene in comparison with that on HZ with small pore diameter like HZSM-5 and HMordenite. It was found that Rh/USY showed the highest initial activity and this activity was higher than commercial CoMo/A1203 catalyst as shown in Figure 1. The reaction products in the hydrodesulfurization of thiophene over Rh/USY catalyst were mainly hydrogen sulfide and C4 hydrocarbons and small amount of C1 -C3 hydrocarbons were also formed. Furthermore, the effect of presulfiding with 5%H2S-H2 mixture on the catalytic activity of Rh/USY was examined. It was revealed that the catalytic activity of Rh/USY was enhanced by the presulfiding treatment as shown in Figure 2. This indicates that Rh/USY catalyst is not poisoned by sulfur compounds and this catalyst has high sulfur-tolerant ability in the hydrodesulfurization of thiophene. On the other hand, the catalytic activities of various H-zeolites used as carriers in Rh/Hzeolite catalysts for the cracking of thiophene and cumene were also examined at 400~ by use of a pulse reactor under helium stream. It was ascertained that USY zeolite showed the Table 1. Catalytic activities of Rh/zeolite catalysts for the hydrodesulfurization at 400~ (W/F = 37.9g.hr/mol.; H2/Thiophene=30). Catalyst 5wt%Rh/NaY 5wt%Rh/NaZSM-5 5wt%Rh/NaMord. 5wt%Rh/NaX 5wt%Rh/NaA a) After 10 min.

Conversion(%) a) 79.5 34.3 23.5 22.3 17.5

Catalyst 5wt%Rh/USY 5wt%Rh/HY 5wt%Rh/HZ SM- 5 5wt%Rh/HMord.

CoMo/AI203

Conversion(%) a) 98.2 93.4 38.2 34.3 77.2

553

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highest activity among H-zeolites for the cracking of both thiophene and cumene. This indicates that the strong BrOnsted acid sites of USY in Rh/USY catalyst play an important role for the hydrodesulfurization of thiophene. That is to say, it is assumed that the strong BrOnsted acid sites of USY in Rh/USY catalyst act as active site for the activation of thiophene, whereas Rh acts as active site for the activation of hydrogen in the hydrodesulfurization ofthiophene. In other words, Rh/USY catalyst behaves as bifunctional catalyst for the hydrodesulfurization of thiophene as well as the reduced MeY zeolite catalysts as described in our previous papers[ 13-15]. 3.2 Improvement of catalyst deactivation of Rh/USY

It was revealed that Rh/USY showed higher catalytic activity than commercial

CoMo/A1203 catalyst in the hydrodesulfurization ofthiophene. However, the catalytic activity of Rh/USY decreased gradually with the reaction time as shown in Figure 1. This may be due to the accumulation of carbonaceous deposit on Rh/USY catalyst surface. Thus, we tried to improve the catalyst deactivation of Rh/USY by various procedures. We attempted to change the dispersion of Rh on USY by changing Rh content in Rh/USY catalyst in order to enhance the hydrogenating ability for carbonaceous deposit on Rh/USY catalyst. Figure 3 shows the effect of Rh content on the catalytic activity and catalyst life of Rh/USY in the hydrodesulfurization of thiophene. It was found that the catalyst deactivation of Rh/USY was not improved by changing the Rh content in Rh/USY catalyst as shown in Figure 3. It is assumed that the strong BrOnsted acid sites are prerequisite for the activation of thiophene in the hydrodesulfurization ofthiophene on Rh/USY. However, strong Br6nsted

555

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Figure 4. Effect of amount ofNa loading(NaOH) on the activity and catalyst life of Rh/USY. sites also act as active sites for the formation of carbonaceous deposit which brings about catalyst deactivation. Therefore, it is necessary to control the strength and number of strong Br6nsted acid sites in Rh/USY in order to prepare highly active Rh/USY catalyst with long catalyst life. The modification of Rh/USY with alkali metal salts such as NaOH, NaNO3, Na2CO3, NaCI, etc. was, therefore, performed in order to control the strength and number of strong Br6nsted acid sites of Rh/USY catalyst. It was revealed that the catalyst deactivation of Rh/USY was remarkably improved by the addition of small amount of alkali metal salts. Modification with NaOH was the most effective and 0.5wt% addition of Na using NaOH was optimal amount for the improvement of the catalyst deactivation with reaction time as shown in Figure 4. It is evident that 0.5wt%Na-5wt%Rh/USY catalyst shows higher and more stable catalytic activity for the hydrodesulfurization of thiophene than 5wt% Rh/USY and CoMo/AI203 catalysts. Therefore, it can be concluded that there is a possibility of usage ofNa- Rh/USY as highly active second generation hydrodesulfurization catalyst for petroleum feedstocks. 3.3 Mechanism of hydrodesulfurization of thiophene on Rh/USY catalyst It was revealed that Rh/USY showed higher catalytic activity than commercial CoMo/Al203 catalyst in the hydrodesulfurization of thiophene. We also studied the mechanism of hydrodesulfurization of thiophene over RH/USY catalyst. As mentioned above, Rh/USY catalyst acts as bifunctional catalyst for the hydrodesulfurization of thiophene, in which both Br6nsted acid sites of USY and Rh in RH/USY catalyst act as active site.

556

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Figure 5. Hydrodesulfurization of thiophene over Rh/quartz(A), USY(B)and mechanically mixed (Rh/quartz(A) + USY(B)) catalysts at 400~ Furthermore, it was assumed the existence of spillover hydrogen in the hydrodesulfurization of thiophene over RhAJSY catalyst. Thus, we tried to confirm the existence of spillover hydrogen in the hydrodesulfurization of thiophene over RH/USY catalyst. The catalytic activity of Rh/SiO2(quartz) mixed mechanically with USY in the hydrodesulfurization ofthiophene was examined. It was found that the activity of mixed catalyst obtained experimentally was higher than that calculated theoretically as shown in Figure 5. This implies that there exists the spillover hydrogen on Rh/USY catalyst in the hydrodesulfurization of thiophene. Therefore, we Propose a possible mechanism for the hydrodesulfurization of thiophene over Rh/USY catalyst as shown below; In this mechanism, thiophene is adsorbed on the Br6nsted acid sites and hydrogen is activated on Rh to form spillover hydrogen. The spillover hydrogen formed on Rh attacks the reaction intermediate like S=C=CH-CH=CH2, which is formed by the decomposition of thiophene adsorbed on the strong Br6nsted acid sites of H- zeolite [ 16]. On the basis of the proposed mechanism, it can be possible to develop much more highly active zeolite-based hydrodesulfurization catalysts for petroleum feedstocks. 4. CONCLUSION It was revealed that Rh/USY showed higher catalytic activity than commercial CoMo/ A1203 in the hydrodesulfurization of thiophene. The catalyst deactivation of R h ~ S Y with

557

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Scheme 1. A Possible mechanism for the hydrodesulfurization of thiophene over Rh/USY catalyst. reaction time was remarkably improved by the addition of small amount of NaOH. Therefore, there is a possibility of use of Rh/USY modified with NaOH as highly active second generation hydrodesulfurization catalyst for petroleum feedstocks.

Acknowledgment A part of this work has been carried out as a research project of The Japan Petroleum Institute commissioned by the Petroleum Energy Center with the subsidy of the Ministry of International Trade and Industry. REFERENCES 1. 2. 3. 4. 5. 6. 7.

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