HETEROGENEITY OF OH GROUPS IN H-MORDENITES TPD AND IR STUDIES OF AMMONIA DESORPTION

We have investigated the temperature-programmed desorption (TPD) of am monia 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 het erogeneity of the bridging Si-OH-Al groups in HNa-mordenites. This was concluded from the width of the distribution functions and from the p resence of submaxima. For HNa-mordenites of exchange degrees below 50% , containing only hydroxy Is in the broad channels, two distinct subma xima are present, thus suggesting the presence of at least two kinds o f bridging hydroxyls of various acid strengths. In HNa-mordenites of e xchange 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 amm onium ions inside narrow channels. The maximum concentrations of hydro xyls 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 pe r unit cell (u.c.). These values agree well with our previous IR resul ts 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 concern ing ammonia desorption. The IR band of OH groups restoring upon satura tion of all the hydroxyls with ammonia and subsequent step-by-step des orption at increasing temperatures shifts to lower frequencies indicat ing that there are hydroxyls of various acid strengths and the less ac idic hydroxyls restore first at lower desorption temperatures.