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 adsorption for a series of simple amines in H-ZSM-5 as a function of coverage. For each molecule studied, the heats of adsorption were approximately constant to a coverage of close to 1 per Al. The average heats of adsorption in the low coverage regime were as follows: ammonia (145 kJ/mol), methylamine(185 kJ/mol), ethylamine (195 kJ/mol), isopropylamine (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 adsorption increase in increments that are identical to the gas-phase, proton-affinity differences, except for relatively small deviations with trimethylamine and n-butylamine. The deviations for trimethylamine and n-butylamine imply that there are some interactions between these bound molecules and the zeolite that are specific to those amines. For example, the deviation for trimethylamine could be explained by a decrease in the ability of this molecule to form hydrogen bonds, while the deviation for n-butylamine could be due to interactions between the alkyl group and the zeolite walls. In contrast to the excellent correlation between heats of adsorption and gas-phase acidity scales, the correlation with solution-phase acidities is very poor. These results demonstrate the utility of adopting a gas-phase reference condition for understanding the acid-base properties of zeolites.