Modification of H-mordenite with silane and diborane. A comparative study of the reaction parameters

Abstract

Silane and diborane have been found to be very suitable for the structural modification of H-mordenite. The reaction with silane or diborane reduces, in a controlled way, the effective pore size of zeolites by implantation of additional atoms or atom groups. As a result, molecular sieving effects can be controlled by pore size engineering, and gases can be encapsulated under moderate conditions of temperatures and pressures by closing the zeolitic pores by silanation or boranation techniques.

The main parameter determining the efficiency of a pore closure, is the degree of modification: the amount of silane or diborane chemisorbed in the zeolite. The effect of reaction temperature, reaction pressure and reaction time on the degree of modification is studied for silanation and boranation of H-mordenite.

Generally, for silanation and boranation it was observed that the modification degree increases with increasing pressure of the modifying agent. For silanation we observed that a higher reaction temperature enhances primary and secondary reactions. Increasing the pressure of silane decreases the ratio secondary/primary reactions, while the chemisorption of silane by primary reaction is favoured at higher pressure. Compared to the silanation, the boranation reaction is very fast because of the higher reactivity of diborane towards zeolitic hydroxy groups. Even at room temperature and low pressure the ultimate boranation degree could be reached. For the boranation we observed an indirect pressure effect which has an important influence on the ultimate modification and hence on the resulting porosity of the zeolite. The amount of added diborane, and hence the number of boranation steps, used to reach the ultimate boranation degree is a critical parameter in determining the pore size reduction.