A Tian−Calvet type calorimeter is applied to the simultaneous determination of adsorption isotherms and heats of adsorption. This is the first of a series of studies of the effect of adsorbate size and polarity on the energetics of adsorption in zeolites. The adsorbate gases used in this study are quadrupolar (N2 and CO2) and nonpolar (Ar, O2, CH4, C2H6, and SF6). The heats of adsorption of both polar and nonpolar gases are either constant or increase with coverage, so silicalite may be classified as a relatively homogeneous adsorbent compared to X type zeolite. Reversibility was established by comparing adsorption and desorption isotherms. Reproducibility was studied by comparing runs for different samples of the same adsorbent. The average experimental error in loading is ±0.6%. The error in the isosteric heat of adsorption is ±2% for heats larger than 20 kJ/mol and ±5% for heats smaller than 20 kJ/mol.

We have used microcalorimetry to measure the differential heats of adsorption of both a series of alkylamines and a series of substituted pyridines in H-ZSM-5 and H-Mordenite. With few exceptions, the differential heats are approximately constant to coverages close to the expected Brønsted site concentration. Both zeolites show good correlations between the average differential heats of adsorption and the gas-phase proton affinities of the basic adsorbates. Slopes of the correlation lines for the two zeolites are similar; the intercepts differ by about 15 kJ/mol. We use this data set to demonstrate that a self-consistent, quantitative Brønsted acidity scale for solid acids cannot be obtained from heats of adsorption of ammonia or pyridine or any other single reference base. However, the correlation between heats of adsorption and gas-phase proton affinities does provide a useful starting point for a more complete description of the thermochemistry of proton transfer reactions in zeolites. Deviations from the correlation curves for specific zeolite/adsorbate pairs can be used to infer how the strengths of Coulombic, hydrogen-bonding, or van der Waals interactions change with structure of either the zeolite acid or the adsorbate base.

The principal components of the carbonyl carbon chemical shift tensor of the hydrogen-bonded 1:1 stoichiometric acetone-H-ZSM-5 adsorption complex have been determined from an analysis of 13 C NMR spectra of static and magic angle sample spinning powder samples at 78 and 130 K, respectively. In a similar manner the principal elements of this tensor have been determined for physisorbed acetone in silicalite and the pure solid in order to separate changes due to hydrogen bonding of the acetone molecule in the zeolite complex from confinement effects defined as interactions of the adsorbed molecule with the siliceous cavity. The energetics associated with such changes have also been measured using microcalorimetry. The differential heats of adsorption of acetone adsorbed in H-ZSM-5 and silicalite over a wide range of surface coverage are reported. The results are compared with ab-initio calculations of the reaction of acetone with model zeolite structures to form a stoichiometric hydrogen-bonded cluster-molecule complex. At the Hartree-Fock level the agreement with respect to both energetics and isotropic shifts is good but only fair for the shielding anisotropy. The magnitudes of the chemical shifts due simply to confinement of the acetone molecule are of the same order of magnitude as those associated with hydrogen bonding.