Effect of phosphorus on acidity and performance of HZSM-5 for the isomerization of styrene oxide to phenylacetaldehyde

•Catalyst lifetimes are prolonged with phosphorus addition at P/Al ratio≤1.•Trimerization of phenylacetaldehyde occurs at external acid sites.•External acid sites can be deactivated by phosphorus modification.•Phosphorus modified HZSM-5 zeolites exhibit an improved anti-coking ability. A series of phosphorus modified HZSM-5 (SiO2/Al2O3=25, P/Al=0.5, 1.0, 1.5, 2.0) samples were prepared and tested in the isomerization of styrene oxide to phenylacetaldehyde under a gas-phase atmosphere free of solvents. The strength and concentration of Brønsted acid sites decreased with increased phosphorus content, while all samples exhibited initial conversions of styrene oxide greater than 99%. External acid sites were gradually deactivated with phosphorus addition, improving the phenylacetaldehyde selectivity by suppressing its trimerization which occurred at external acid sites. When the P/Al ratio≤1, catalyst lifetimes extended because of the decrease in strong acid strength. Hard coke, which preferentially formed on strong acid sites, was considerably inhibited in the presence of phosphorus.