Calorimetric Study of Alcohol and Nitrile Adsorption Complexes in H-ZSM-5

Differential heats of adsorption have been measured for a series of alcohols, a series of nitriles, water, and diethyl ether in H-ZSM-5 and silicalite. Higher differential heats in H-ZSM-5 demonstrated that most of the adsorbates had a significant affinity for the Brønsted sites, and the affinity increased with the gas-phase proton affinity of the adsorbate. For the nitriles and diethyl ether, the differential heats were approximately constant up to a coverage of 1/site, after which values fell to the level on silicalite. The heat of formation of the 1:1, stoichiometric adsorption complexes increased with the proton affinity with a slope of 0.37. Heats of formation of stoichiometric adsorption complexes were difficult to identify for water and the alcohols since the differential heats did not fall at a coverage of 1/site. This is evidence for adsorbate clusters, the result of alcohols being both hydrogen-bond acceptors and donors. The implications of these results for understanding zeolite acidity and acid catalysis are discussed.