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

Abstract BACKGROUND Trace 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/Al 2 O 3 and Pd/Al 2 O 3 in gas phase continuous catalytic hydrogenation of phenylacetylene. RESULTS Temperature 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 (Al 2 O 3 ) carrier. Pd/Al 2 O 3 exhibited a greater specific hydrogen (H 2 ) uptake capacity than Au/Al 2 O 3 under reaction conditions to deliver appreciably higher turnover frequencies (TOF) for reaction in excess H 2 . Stepwise hydrogenation predominated over Au/Al 2 O 3 with 100% selectivity to styrene at 353 K where an increase in temperature favoured subsequent hydrogenation to ethylbenzene. Under the same conditions, Pd/Al 2 O 3 was non‐selective, activating styrene to generate ethylbenzene with a greater contribution of direct phenylacetylene hydrogenation to ethylbenzene at higher temperature. CONCLUSION Kinetic analysis has revealed stepwise phenylacetylene hydrogenation in excess H 2 over Au/Al 2 O 3 with 100% selectivity to styrene. Stepwise hydrogenation also prevailed over Pd/Al 2 O 3 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/Al 2 O 3 lowered rate where the activity/selectivity profile overlapped that exhibited by Au/Al 2 O 3 in excess H 2 . © 2019 Society of Chemical Industry