Gas phase selective hydrogenation of phenylacetylene to styrene over Au/Al2O3

BACKGROUNDTrace quantities of phenylacetylene can poison styrene polymerisation catalysts. The phenylacetylene content must be less than 10 ppm and selective hydrogenation (to styrene) is viewed as a viable process solution. High styrene selectivities have been achieved in batch liquid phase operations while a switch from conventional batch liquid to continuous gas phase reaction presents process advantages in terms of higher throughput and enhanced productivity. We aim to provide the first direct comparison of Au/Al2O3 and Pd/Al2O3 in gas phase continuous catalytic hydrogenation of phenylacetylene. RESULTSTemperature programme reduction (TPR) generated metal particles at the nano-scale (mean size = 3.0-4.3 nm), with evidence of electron donation from the aluminium oxide (Al2O3) carrier. Pd/Al2O3 exhibited a greater specific hydrogen (H-2) uptake capacity than Au/Al2O3 under reaction conditions to deliver appreciably higher turnover frequencies (TOF) for reaction in excess H-2. Stepwise hydrogenation predominated over Au/Al2O3 with 100% selectivity to styrene at 353 K where an increase in temperature favoured subsequent hydrogenation to ethylbenzene. Under the same conditions, Pd/Al2O3 was non-selective, activating styrene to generate ethylbenzene with a greater contribution of direct phenylacetylene hydrogenation to ethylbenzene at higher temperature. CONCLUSIONKinetic analysis has revealed stepwise phenylacetylene hydrogenation in excess H-2 over Au/Al2O3 with 100% selectivity to styrene. Stepwise hydrogenation also prevailed over Pd/Al2O3 at the lower temperature but surface activation of styrene coupled with enhanced H-2 dissociation generated significant ethylbenzene. Decreasing inlet H-2/phenylacetylene (to 1 mol/mol) over Pd/Al2O3 lowered rate where the activity/selectivity profile overlapped that exhibited by Au/Al2O3 in excess H-2. (c) 2019 Society of Chemical Industry