Olefin feedstocks such as ethylene, propylene, and styrene contain ~2-5% terminal alkynes. The alkynes poison olefin polymerization catalysts, which drives the need to convert the alkynes into olefins. Industrial reactors typically contain heterogeneous Pd catalysts, though the hydrogenation is not 100% selective and the surface must be modified to prevent hydrogenation to the alkane. Au catalysts are a promising alternative due to the remarkable selectivity to olefins originating in strong alkyne-Au bonds and weak olefin-Au bonds. A majority of the Au-catalyzed alkyne hydrogenation literature is focused on the optimization of process variables like temperature, pressure, and H2/alkyne feed ratios. A fundamental understanding of the reaction would be invaluable in directing optimization efforts. We studied the selective hydrogenation of 1-octyne with Au catalysts supported on a variety of metal oxide supports. A colloidal synthesis method was developed to generate Au catalysts with consistent particle sizes regardless of support properties. Catalyst supports included Al2O3, TiO2, SiO2, Fe2O3, MoO2, CuO, and ZrO2. Kinetic parameters were also determined to obtain a mechanistic understanding of alkyne hydrogenation. The 1-octyne conversion was &gt; 80% for most catalysts by 200 °C with selectivity to 1-octene between 90-95% at all temperatures. The supports effects, which ranged from reducible TiO2 to inert Al2O3, were relatively small. In contrast, the commercial PdAg/Al2O3 catalyst was so active that it reached 100% conversion with 0% 1-octene selectivity at room temperature. This result is surprising; H2 is known to activate at the Au-MOx interface, so variations in support properties were expected to shift the location of the lightoff curve to either higher or lower temperatures. The high selectivity is characteristic of Au catalysts, which suggests that the reaction takes place on the Au NP itself. The apparent activation barriers were ~25-45 kJ/mol, which corresponds to H2 activation. The order of 1-octyne was 0.0 to -0.4, so 1-octyne acted as an inhibitor. The order of H2 was found to be 0.6 to 1.2, so the only relevant kinetic step was the activation of H2. Previous studies also found that alkynes inhibited Au catalysts at 3000-10,000 ppm, but this was also true at the low concentration of 200 ppm in this study. The 1-octyne likely binds so strongly to the Au NP that the reaction is limited to the availability of H2 activation sites.

Support effects for the selective hydrogenation of 1-octyne

255th National Meeting (2018)

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