Oxidation and Reduction of Molybdenum Disulfide Catalyst and their Effects on the Decomposition of 2-Propanol

Transcription

1 Oxidation and Reduction of Molybdenum Disulfide Catalyst and their Effects on the Decomposition of 2-Propanol Masatoshi SUGIOKA* and Fujimi KIMURA Faculty of Engineering, Hokkaido University, North 13, West 8, Kita-ku, Sapporo 060 (Received August 8, 1984) The catalytic activity and selectivity of variously pretreated molybdenum disulfide for the pretreatment of MoS2 with hydrogen and oxygen influenced remarkably the catalytic activity and selectivity of MoS2 in the decomposition of 2-propanol. The dehydrogenation of 2-propanol proceeded preferentially on the reduced MoS2 catalyst, and the dehydration proceeded on the oxidized MoS2 catalyst in the decomposition of 2-propanol. The active sites of reduced and oxidized MoS2 catalysts for the decomposition of 2-propanol were assumed to be the coordinatively unsaturated sites and the acidic sites of molybdenum oxysulfide on MoS2 surface formed by the reduction and oxidation of MoS2, respectively. 1. Introduction Recently, a number of articles have reported on the hydrodesulfurization reactions over molybdenum based catalysts. It has been reported that molybdenum disulfide acts as the principal constituent in the CoO-MoO3-Al2O3 and NiO-MoO3- Al2O3 hydrodesulfurization catalysts after presulfurization of these catalysts with H2/H2S mixture.1),2) However, little attention has been given to the acid property of MoS2.3) In the previous paper,4) one of authors reported on the catalytic decomposition of 2-propanol (2- PA), which has been widely employed as a test reaction for the acid-base catalysis,5) over MoS2 to clarify the acid property of MoS2 by use of a pulse reactor with helium as a carrier gas. It was found that the dehydration of 2-PA to propylene rather than the dehydrogenation to acetone proceeded predominantly over variously pretreated MoS2 catalysts. It was concluded that there were a great many acidic sites on MoS2 surface, which accelerate the dehydration of 2-PA. However, there existed some possibilities that the MoS2 surface was oxidized by a trace amount of oxygen contained in the carrier gas and, as a result, high dehydration activity of MoS2 was observed even after MoS2 catalyst was pretreated with hydrogen. Therefore, there still exist many points unclarified regarding * To whom correspondence should be addressed. the active site of MoS2 for the decomposition of 2-PA. In the present work, we studied again the decomposition of 2-PA over variously pretreated MoS2 catalysts by use of a closed circulation reactor system in order to clarify the effects of pretreatment conditions on the activity and selectivity of MoS2 catalyst and the catalyst active site for the decomposition of 2-PA. By use of a closed circulation system, the possibility of oxidation of MoS2 by a trace amount of oxygen contained in a carrier, which was encountered in the previous work, can be completely eliminated. Furthermore, the decomposition of 2-PA over MoS2 catalyst may be related to the fundamental research for the hydrodeoxygenation of organic oxygen compounds contained in heavy oil and coalderived liquids over sulfided CoO-MoO3-Al2O3 and NiO-MoO3-Al2O3 2. Experimental catalysts. 2.1 Experimental Apparatus and Procedure The catalytic decomposition of 2-PA over MoS2 closed circulation reactor system made from pyrex glass and 20torr of initial pressure of 2-PA. The dead volume of the apparatus including the reaction vessel is about 450ml. The reactor was U type glass tube. Circulation of 2-PA through the reactor was carried out by means of magnetic

2 307 pumping after 2-PA was introduced into the system. The analysis of reaction products in the decomposition of 2-PA was made on Hitachi 023 gaschromatograph using PEG-1000 column with flame ionization detector. 2.2 Catalyst Molybdenum disulfide, employed as a catalyst, was provided from Kanto Chemical Co., Tokyo, in powder form. The purity of MoS2 powder was above 99.2% and the impurities were silica, below 0.05%; ferric oxide, below 0.2%; cupric oxide, below 0.05%; water soluble substance, below 0.5%. The sample of MoS2 used in this work was found to have a hexagonal structure and preferred orientation parallel to the (001) plane by X-ray diffraction (XRD) analysis. MoS2 was evacuated at 10-5torr of hydrogen or 30torr of oxygen for 2hrs at various temperatures before use. 2.3 Reagents Super special grade 2-PA was provided from Wako Pure Chemical Co., Tokyo, and purified by trap-to-trap distillation after being dried with molecular sieve 5A. Hydrogen ( %) and oxygen (99.99%) were provided from Nippon Oxygen Co., Tokyo, and Hoxan Co., Sapporo. Hydrogen and oxygen were dried by passage through the molecular sieve 4A before storage. 3. Results and Discussion 3.1 Catalytic Activity and Selectivity of MoS2 Pretreated under Various Conditions The catalytic activity and selectivity of MoS2, which were pretreated under various conditions, for the decomposition of 2-PA were examined at of reaction products with reaction time in the decomposition of 2-PA over MoS2 catalyst evacuated and acetone, and no other products were formed. The ratio of propylene to acetone with reaction time was almost constant. As can be seen in Fig. 1, both dehydration and dehydrogenation of 2-PA catalytic reaction proceeded independently on the different active sites of the catalyst. The dehydration activity of MoS2 was higher than the dehydrogenation, indicating that the number of effective active sites for the dehydration is greater than that for dehydrogenation over MoS2 evacuated at 450 Fig. 3 Effect of Reduction Temperature on Catalytic Activity and Selectivity of MoS2 for Decomposition of 2-Propanol

3 308 catalyst. Fig. 3 shows the effects of reduction temperature on the catalytic activity and selectivity of MoS2 for the decomposition of 2-PA. Although the catalytic activity of MoS2 was little affected by the reduction temperature, the selectivity of the formation of acetone increased remarkably and that of propylene decreased with increase in the reduction temperature. These results indicate that the number of effective active sites of MoS2 for the dehydrogenation of 2-PA increased and that for dehydration decreased with the reduction treatment. Fig. 4 shows the changes in the composition of reaction products in the decomposition of 2-PA over MoS2 catalyst, which was oxidized at Fig. 5 Effect of Oxidation Temperature on Catalytic Activity and Selectivity of MoS2 Catalyst for crease in the oxidation temperature. These results suggest that the effective active sites of reduced MoS2 catalyst for the dehydrogenation of 2-PA were easily changed to those for the dehydration of 2-PA by oxidation with oxygen. Effects of various pretreatment conditions on the catalytic activity and selectivity of MoS2 for the decomposition of 2-PA are summarized in Table 1. Table 1 shows that the surface of MoS2 catalyst was very sensitive to the pretreatment conditions and that the catalytic activity and selectivity of the MoS2 catalyst for the decomposition of 2-PA were remarkably influenced by the pretreatment with hydrogen and oxygen. As the catalytic activity and selectivity of oxidized MoS2, for the decomposition of 2-PA, were restored again to those of initially reduced MoS2, after the oxidized MoS2 was reduced Decomposition of 2-Propanol

4 309 That is to say, the dehydrogenation of 2-PA to form acetone proceeds predominantly on the reduced MoS2 surface and the dehydration of 2-PA to propylene proceeds on the oxidized MoS2 surface. The selectivities of MoS2 catalyst for the dehydrogenation and dehydration of 2-PA are determined by the states of MoS2 surface, i.e, reduced or oxidized surface states. 3.2 Active Sites of MoS2 Catalysts We will, hereinafter, discuss the active sites of MoS2 catalysts, which were pretreated under various conditions, for the dehydration and dehydrogenation of 2-PA. As we showed in Figs. 1 and 4, tivity for 2-PA, we shall first discuss the active sites of evacuated and oxidized MoS2 catalysts for the dehydration of 2-PA. It is generally accepted that the dehydration of aliphatic alcohol to olefin proceeds preferentially over acidic site of solid acid catalysts.5) Furthermore, it has been thought that the acidic metal oxysulfide species is easily formed on metal sulfide surface when metal sulfides come into contact with oxygen or air.6) One of the authors examined previously the i.r. spectra of MoS2 after it was treated with air, and observed the absorption band at 1,050-1,150cm-1 which was assigned to SO42- or S=O bond.4) From these results, it can be concluded that the molybdenum oxysulfide species like MoS2O or MoO2S are present on evacuated and oxidized MoS2 surface, and the acidic sites of molybdenum oxysulfide species on the catalyst surface accelerate predominantly the dehydration of 2-PA. On the other hand, MoS2 catalyst reduced at decomposition of 2-PA as shown in Fig. 2, which is contrary to the case of evacuated and oxidized MoS2 catalysts. Next, we shall discuss the active site of reduced MoS2 catalyst for the dehydrogenation of 2-PA. It has been reported that the dehydrogenation of aliphatic alcohols, to form ketone or aldehyde, proceeds on the metallic catalysts and solid base catalysts.7) The possibility that metallic molybdenum on MoS2 surface, formed by the reduction of MoS2, acts as the active site for the dehydrogenation of 2-PA was denied by the facts that MoS2 is hardly reduced by hydrogen even at elevated temperature and XRD analysis of reduced MoS2 Fig. 6 Effect of Introduction of Hydrogen Sulfide on Decomposition of 2-Propanol over Reduced denum on MoS2 surface. We speculated previously that the sulfur anion, S2-, on MoS2 surface acts as the basic site for the dehydrogenation of 2-PA without any experimental evidence.4) However, since it has not been reported that MoS2 acts as the solid base catalyst, it would be unreasonable to assume that the sulfur anion acts as the basic site for the dehydrogenation of 2-PA. Thus, it would be necessary to consider active sites other than metallic site and basic site on MoS2 surface for the dehydrogenation of 2-PA. The MoS2 crystal possesses a hexagonal structure characterized by sandwich-like layer form. The molybdenum atom is surrounded by six sulfur atoms. The sulfur atom bonded with molybdenum atom at the crystal corner has a single bond. Since such sulfur atom at the crystal corner is unstable and can be easily removed as hydrogen sulfide by reduction with hydrogen, the coordinatively unsaturated sites on MoS2 surface are formed.8) One of the authors reported previously that the catalytic activity of MoS2 for the decomposition of hydrogen sulfide and methanol was remarkably enhanced by reduction with hydrogen but was not appreciably increased with increase in the evacuation temperature.9),10) In the decomposition of methanol over MoS2 catalyst,9) the selectivity for the methanol dehydrogenation to carbon monoxide and hydrogen was remarkably enhanced by the reduction of MoS2. Thus, we concluded that the catalytic decomposition of hydrogen sulfide and methanol proceeded on the coordinatively unsaturated site on MoS2 surface formed by hydrogen reduction. MoS2 Catalyst

5 310 On the other hand, Tanaka, et al.11) showed that 2-PA is dissociatively adsorbed on the coordinatively unsaturated site on MoS2 surface in the study of the hydrogen transfer reaction between 2- PA and 1,3-butadiene over MoS2 catalyst. Furthermore, the catalytic activities of reduced MoS2 for the dehydrogenation and dehydration in the decomposition of 2-PA were strongly suppressed by the introduction of hydrogen sulfide during the course of the decomposition of 2-PA as shown in Fig, 6. On the basis of these results, it can be concluded that the coordinatively unsaturated site on MoS2 surface also acts as the active site for the dehydrogenation of 2-PA. The dehydration of 2-PA was also suppressed by hydrogen sulfide as shown in Fig, 6, indicating that hydrogen sulfide also poisoned the acidic site of reduced MoS2 catalyst. The dehydration of 2-PA proceeded slightly even This result suggests that thiol groups (SH) on MoS2 surface formed by the reduction of MoS212) or the trace amount impurities contained in MoS2 act as the acidic site for the dehydration of 2-PA. 4. Conclusion The catalytic activity and selectivity of MoS2 for the decomposition of 2-PA were remarkably changed by the pretreatment with hydrogen or oxygen. The dehydration and dehydrogenation of 2-PA proceeded preferentially on the oxidized and reduced MoS2 catalyst, respectively. It is concluded that the active site of oxidized MoS2 catalyst for the dehydration of 2-PA is the acidic site of molybdenum oxysulfide species formed by the oxidation of MoS2, and the active site of reduced MoS2 catalyst for the dehydrogenation of 2-PA is the coordinatively unsaturated site formed by the reduction of MoS2. The detailed structure of molybdenum oxysulfide species formed by the oxidation of MoS2, the nature of thiol groups formed by the reduction of MoS2 and the effect of trace amount of impurities in MoS2 will be the subjects of future study. References 1) Massoth, F. E., MuraliDhar, G., Proceedings of Fourth International Conference of the Chemistry and Uses of Molybdenum, p. 343 (1982). 2) Ternan, M., Can. J. Chem. Eng., 61, 133 (1982). 3) Hou, P., Wise, H., J. Catal., 78, 469 (1982). 4) Sugioka, M., Sasaki, M., Hosotsubo, T., Aomura, K., Sekiyu Gakkaishi, 23, 218 (1980). 7) Hattori, H., Shokubai, 26, 250 (1984). 8) Tanaka, K., Okuhara, T., Catal. Rev., 15, 249 (1977). 9) Sugioka, M., Aomura, K., Proceedings of Fifth World Hydrogen Energy Conference, 2, 477 (1984). 10) Sugioka, M., Aomura, K., International J. Hydrogen Energy, 9, 891 (1984). 11) Tanaka, K., Yaegashi, I., Aomura, K., Bull. Fac. of Eng. Hokkaido Univ., No. 102, 113 (1981). 12) Ratanasamy, P., Fripiat, J., J. Chem. Soc. Faraday Trans., I, 66, 2897 (1970).

6 Keywords Acid site, Dehydration, Dehydrogenation, Molybdenum disulfide, 2-Propanol