Sulfur tolerance of methanol synthesis catalysts: Modelling of catalyst deactivation

A series of supported Pd and commercial Cu-based catalysts was tested for methanol synthesis activity and activity maintenance in the presence of 2 ppm H₂S in a differential fixed-bed reactor operated at 523 K and 1.5 MPa. Sulfur breakthrough curves were obtained by monitoring the H₂S concentration in both the feed and exit streams using a gas chromatograph with a flame photometric detector. A comparison of the resistance to sulfur poisoning of the various catalysts is given using a qualitative examination of activity maintenance plots as well as a mathematical model for catalyst deactivation. The rates of deactivation of the Cu-based catalysts in the absence or presence of H₂S were very similar, with 70-90% of the initial activity being lost during 300 h on stream. No H₂S breakthrough was observed with these catalysts whereas the Pd catalysts exhibited breakthroughs after short times (10-30 h) on stream. The Pd catalysts retained a much higher residual activity in the presence of H₂S than those reported for Fe, Co, Ni and Ru at much lower H₂S pressures, and Pd/SiO₂ possessed a stable, lined-out activity in 2 ppm H₂S that was near half its original activity. Problems that arise in catalyst evaluation when a single deactivation model does not apply in all cases are discussed. The Szepe-Levenspiel approach, with a modification recently proposed by Fuentes for poison-tolerant catalysts, was found to give meaningful quantification of sulfur resistance of Pd-based catalysts.