Hydrogenation of Acetylenes Over Supported Metal Catalysts

[摘要] The hydrogenation reactions of propyne, 2-butyne and phenylacetylene have been studied using a series of silica- and alumina-supported palladium and platinum catalysts, each containing a nominal metal loading of 1%. All catalysts have been the subject of numerous characterisation techniques including temperature programmed reduction, selective chemisorption, UV-Vis diffuse reflectance spectroscopy, thermal gravimetric analysis, BET measurements, atomic absorption spectroscopy and transmission electron microscopy. The reaction of hydrogen and propyne was performed in a pulse-flow microcatalytic reactor using reaction mixtures of varying hydrogen concentration; (1:1 :: C3H4 : H2) and (1:3 :: C3H4 : H2). All propyne hydrogenation reactions were performed at ambient temperature. Using low concentrations of hydrogen, both Pd/SiO2 and Pt/gamma-Al2O3 were completely selective for the formation of propene; Pt/gamma-Al2O3 and Pt/SiO2 exhibiting lower selectivities. This behaviour is believed to be the result of a particle size effect, with the former catalysts containing large metal particles which act less electrophilically towards the acetylenic bond of the reactant, and thus permit desorption of the olefin before complete saturation occurs. With higher coverages of hydrogen, the formation of the alkane was favoured over all catalysts, with an "induction period" being observed before the production of propene was evident. This behaviour has been attributed to the initial dissociative adsorption of propyne to produce surface hydrocarbonaceous overlayers and alkane precursors simultaneously. The associative adsorption of propyne on this hydrocarbonaceous overlayer, with the latter acting as a hydrogen transfer medium, is proposed as the route to propene. The formation of either the alkane or the surface hydrocarbonaceous residues is therefore postulated as being a function of the rate of hydrogen supply to the relevant surface sites. All catalysts were prone to deactivation which is believed to occur through a site-blocking mechanism. The gas phase hydrogenation of 2-butyne favoured the formation of the cis-olefin isomer when low concentrations of hydrogen were used. Stereospecific reduction of an associatively adsorbed acetylenic species (2,3-di-sigma/pi-butyne) is proposed as the main route to this olefin. At high reaction temperatures, trace amounts of the trans-olefin isomer were observed. This reaction is believed to occur by either (i) adsorption on surface defect sites or (ii) by isomerisation of the cis isomer via a radical intermediate. With higher coverages of hydrogen, all catalysts displayed complete conversion of 2-butyne over a series of thirty pulses. Formation of the alkane was the predominant reaction using all catalysts except highly dispersed Pt/SiO2, which was selective for cis-2-butene formation. A direct mechanism for the hydrogenation of 2-butyne via an associatively adsorbed surface species is proposed as the route to 2-butane. The propensity of 2-butyne to undergo dissociative adsorption is evident from the production of methane. Similar to the reactions of propyne, the formation of surface hydrocarbonaceous residues and the hydrogenation products is believed to be governed by the availability of surface hydrogen. Reaction of phenylacetylene and hydrogen in the liquid phase proceeds to yield both styrene and ethylbenzene, with the adsorption and hydrogenation of styrene predominating after the removal of all of the acetylene from the system. Co-hydrogenation experiments performed using an equimolar mixture of styrene/phenylacetylene indicates that the adsorption of both the olefin and acetylene occurs at different surface sites, with the hydrogenation of each adsorbate occurring independently of the other. It is believed that olefin chemisorption in the presence of the acetylene has the effect of reducing the amount of available hydrogen for phenylacetylene hydrogenation. The hydrogenation of phenylacetylene in the gas phase produces only ethylbenzene, the formation of which is proposed to occur via (i) a di-pi-adsorbed species analogous to that involved in propyne hydrogenation to propene or (ii) from dissociative adsorption to yield a surface alkylidyne. In conclusion, we can state that the surface hydrogen concentration plays a crucial role in determining the selectivity and activity of the catalysts during the hydrogenation reactions. In the presence of high coverages of hydrogen, all catalysts favoured the production of the alkane, except highly dispersed Pt/SiO2 in 2-butyne hydrogenation. The observed deactivation phenomenon during propyne hydrogenation has been attributed to the possible formation of surface oligomers which reduce the number of exposed active sites. Since the reactions between 2-butyne and excess hydrogen exhibited no signs of catalyst deactivation, it is proposed that the formation of surface oligomers from this acetylene would be less probable due to the steric effects experienced by the substituent methyl groups. Therefore, the likelihood of extensive oligomerisation and hence, deactivation occurring is reduced.

[发布机构] University:University of Glasgow

[关键词] Organic chemistry