In situ MAS NMR investigation of the hydrogenation of acrylonitrile on Pt- and Rh-containing zeolites Y

Abstract

This work presents first ex situ and in situ MAS NMR spectroscopic studies of the effect of noble metal reduction on the hydroxyl coverage of zeolites 0.9[Pt]Na-Y and 0.4[Rh]Na-Y and the hydrogenation of a double-bond-containing organic reactant, acrylonitrile. By 1H MAS NMR spectroscopy, it was found that the number of Brønsted acidic bridging OH groups (SiOHAl) formed upon noble metal reduction under flowing hydrogen nearly agrees with the number of the initially introduced complexes of bivalent metal cations. Hence, about 50% of the bivalent metal cations formed two SiOHAl groups per metal site upon reduction. After adsorption of reactants on the reduced metal-containing catalysts, no effect of the noble metals on the chemical shieldings of 13C nuclei of these molecules occurred. On the other hand, the 1H MAS NMR signals of reactant molecules adsorbed on zeolite 0.9[Pt]Na-Y have shown a significantly larger residual line width compared with those on zeolite 0.4[Rh]Na-Y, which indicates a higher mobility of reactants adsorbed on the latter zeolite. By in situ1H MAS NMR spectroscopy of the acrylonitrile hydrogenation carried out under semi-batch conditions at 295K, a reaction velocity on zeolite 0.4[Rh]Na-Y (r=7.0×10−3mmol/s) was determined, which is more than a factor six higher in comparison with that on zeolite 0.9[Pt]Na-Y (r=1.1×10−3mmol/s). During the initial period (t=12–24s) of the acrylonitrile hydrogenation on zeolite 0.4[Rh]Na-Y, weak 1H MAS NMR signals of temporary intermediates were observed. These temporary signals may be due to half-hydrogenated intermediates with short lifetime.