Ohkatsu, Y; Uchida, Y; Misono, A
Effects of the metals supported on active carbon and those of the chemical environment under which the metal was placed were examined with respect to the dehydrogenation reaction of hexane. Dehydrogenation was carried out using a fixed bed flow reactor packed with a catalyst. Products were qualitatively and quantitatively analyzed by gas liquid chromatography. Active carbon itself catalyzed dehydrogenation, but its catalytic activity decreased nearly to zero within a period from thirty minutes to one hour. On the other hand, the supported catalyst catalyzed dehydrogenation at a constant rate over thirty minutes from the start of reaction. This fact could be ascribed to so-called reverse spill-over. Effects of the metals supported were in the order of Cr>Pt>WâNi>V>Pd>Mo for conversion of hexane, and Pt>Mo>W>Pd>Cr>V>Ni for selectivity of hexenes (Table 1). From the above results, a combination of Pt and Cr, the most effective metals for selectivity and conversion, respectively, was used to examine the catalytic activity (Table 3 and Fig. 1). As shown in Fig. 1, the conversion of hexane was minimum when the catalyst contained 60% to 80% of platinum based on total amount of the metals, while the selectivity of hexenes increased with increase of content of platinum, This result suggest that the catalytic activity of the combination could only be explained by the dependent interaction of both metals. Table 4 shows the results obtained by using catalysts containing platinum from different salts. The anion of the platinum salts seemed to remain near the platinum and affect the catalytic activity. To clarify the effect of such chemical environment surrounding the platinum, the salt, H2PtCl6, of which ligand was considered to be easily lost during reaction, was used as catalyst in the presence of an additive. (In this reaction, some measurements were carried out at stationary state after 30 minutes from the start). When the additive was a carboxylic acid, the selectivity of hexenes was in the order of adipic acid⥠o-phthalic acidâ«succinic acid> sebacic acid (Fig. 2). This order could be arranged according to the acidity of the acids (Fig. 3), i.e., the higher the acidity, the more effective was the acid. This result coincides with the discussion in terms of counteranions shown in Table 4 (H2SO4: pKa=â3, HCl: pKa=â7, H3PO4: pKa=2.1, H2O: pKa=15.7). When a base such as imidazole or quinoline was added, conversion decreased markedly, though selectivity was comparable (Table 6). However, in the presence of a base, hexene-1 was also produced, but it was not produced by a catalyst which contained no additive or which had been treated with a carboxylic acid. © 1982, The Japan Petroleum Institute. All rights reserved.