A CALORIMETRIC STUDY OF SIMPLE BASES IN H-ZSM-5 - A COMPARISON WITH GAS-PHASE AND SOLUTION-PHASE ACIDITIES

We have used microcalorimetry to measure the differential heats of ads orption for a series of simple amines in H-ZSM-5 as a function of cove rage. For each molecule studied, the heats of adsorption were approxim ately constant to a coverage of close to 1 per Al. The average heats o f adsorption in the low coverage regime were as follows: ammonia (145 kJ/mol), methylamine(185 kJ/mol), ethylamine (195 kJ/mol), isopropylam ine (205 kJ/mol), n-butylamine (220 kJ/mol), dimethylamine (205 kJ/mol ), and trimethylamine (205 kJ/mol). An excellent correlation is found between these heats and gas-phase, proton affinities. The heats of ads orption increase in increments that are identical to the gas-phase, pr oton-affinity differences, except for relatively small deviations with trimethylamine and n-butylamine. The deviations for trimethylamine an d n-butylamine imply that there are some interactions between these bo und molecules and the zeolite that are specific to those amines. For e xample, the deviation for trimethylamine could be explained by a decre ase in the ability of this molecule to form hydrogen bonds, while the deviation for n-butylamine could be due to interactions between the al kyl group and the zeolite walls. In contrast to the excellent correlat ion between heats of adsorption and gas-phase acidity scales, the corr elation with solution-phase acidities is very poor. These results demo nstrate the utility of adopting a gas-phase reference condition for un derstanding the acid-base properties of zeolites.