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

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/Al₂O₃ and Pd/Al₂O₃ 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 (Al₂O₃) carrier. Pd/Al₂O₃ exhibited a greater specific hydrogen (H₂) uptake capacity than Au/Al₂O₃ under reaction conditions to deliver appreciably higher turnover frequencies (TOF) for reaction in excess H₂. 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. CONCLUSION: Kinetic analysis has revealed stepwise phenylacetylene hydrogenation in excess H₂ 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 H₂ dissociation generated significant ethylbenzene. Decreasing inlet H₂/phenylacetylene (to 1 mol/mol) over Pd/Al₂O₃ lowered rate where the activity/selectivity profile overlapped that exhibited by Au/Al₂O₃ in excess H₂. © 2019 Society of Chemical Industry