Mesoporous solid acid catalysts: relationship between amine TPD data and catalytic activities

The concentrations of surface acid sites on Al3+- and H+-exchanged AlMCM-41 catalysts have been measured by temperature-programmed desorption (TPD) of a series of primary, secondary and tertiary amines, and by adsorption of ammonia. Surface acid site concentrations have been compared with cation exchange capacities (CEC's) and catalytic activities in the alkylation of toluene with benzyl alcohol. The results show that the catalytic activities of the two ion-exchanged forms of the AlMCM-41 are similar. This is reflected in the acid site concentrations measured with tri-n-butylamine, and to a lesser extent tri-i-butylamine, which are similar for both ion-exchanged forms of the catalyst, and close to the CEC. Acid site concentrations measured with smaller amines and ammonia are similar to those based on tributylamine TPD for H+-exchanged AlMCM-41, but the Al3+ form of the catalyst appears to chemisorb significantly more of the smaller amines and ammonia, resulting in higher than expected acid site concentrations when these adsorbates are used. Larger amines than the tributylamines show very low chemisorption capacities on both forms of AlMCM-41, and appear to access only a small proportion of the surface acid sites. The primary amines tested show poor separation of chemisorbed and physisorbed amine in TPD experiments, making the determination of acid site concentration difficult. The results illustrate the importance of choosing an appropriate amine in using TPD to measure catalyst surface acidities.