A series of catalysts with Keggin-structure polyoxometalates (POMs) based on H3PMo12O40 dispersed on silica were used to investigate the role of counter-cation replacement during methanol oxidation and dehydration. The rates and turnover frequencies for both methanol selective oxidation to formaldehyde and methanol dehydration to dimethyl ether were decreased by the addition of sodium as the POM counter-cation. The decrease in turnover frequencies for the oxidation and dehydration pathways were highly correlated, suggesting a role of POM acidity in creating an intermediate common to both pathways. NH3 TPD was demonstrated to be a reasonable probe of the proton-methanol interaction and was used to correlate acid site strength changes during cation exchange with dehydration and oxidation activity. The usefulness of cation addition as a tool to tune POM properties is limited by the adverse impact on POM acidity and consequent destabilization of relevant reaction intermediates. (C) 2019 Elsevier Inc. All rights reserved.