Anisole disproportionation on HZSM-5: The key role of Si/Al ratio on auto-inhibition effect

The conversion of anisole has been studied at 773 K on HZSM-5 zeolites with nine Si/Al ratios ranging from 9 to 201. The influence of the distribution of Brønsted Acid Sites on catalytic activity and reaction products selectivity has been studied experimentally. The adsorption energies for reactant and products on isolated and paired aluminium have been modelled by DFT. The Si/Al ratio does not affect any selectivity. Higher concentration of Brønsted acid sites leads to higher activity from Si/Al = 201 to 29. The shift from mostly isolated to increasingly paired Al strongly inhibits the conversion and initial activity. This is attributed to product desorption becoming the limiting step in the Sabatier volcano plot. This finding is consistent with the significant increase in adsorption energies between isolated and paired sites. When the adsorption energy of gaseous anisole becomes similar to that of its products, an auto-inhibition effect takes place as already known for liquid-phase processes.