Heterogeneity of OH Groups in H-Mordenites TPD and IR studies of ammonia desorption

Abstract

We have investigated the temperature-programmed desorption (TPD) of ammonia during the activation of NH4Na-mordenites of different exchange degrees. Using a regularization method, desorption energy distribution functions have been calculated. The obtained results indicate the heterogeneity of the bridging Si-OH-Al groups in HNa-mordenites. This was concluded from the width of the distribution functions and from the presence of submaxima. For HNa-mordenites of exchange degrees below 50%, containing only hydroxyls in the broad channels, two distinct submaxima are present, thus suggesting the presence of at least two kinds of bridging hydroxyls of various acid strengths. In HNa-mordenites of exchange degrees above 50%, the hydroxyls appear in narrow channels and the distribution of ammonia desorption energy broadens on the side of higher energies. This may be related to a strong stabilization of ammonium ions inside narrow channels. The maximum concentrations of hydroxyls of desorption energies between 95 and 135 kJ mol-1 and between 135 and 165 kJ mol-1 calculated from TPD data were 3.9 and 3.3 OH per unit cell (u.c.). These values agree well with our previous IR results of concentrations of hydroxyls in broad and in narrow channels (3.7 and 2.8 OH per u.c.). The TPD data obtained for the heterogeneity of OH groups in HNa-mordenites are in accordance with the IR data concerning ammonia desorption. The IR band of OH groups restoring upon saturation of all the hydroxyls with ammonia and subsequent step-by-step desorption at increasing temperatures shifts to lower frequencies indicating that there are hydroxyls of various acid strengths and the less acidic hydroxyls restore first at lower desorption temperatures.