IMINE CHEMISTRY IN ZEOLITES - OBSERVATION OF GEM-AMINO-HYDROXY INTERMEDIATES BY IN-SITU C-13 AND N-15 NMR

Variable-temperature C-13 NMR and N-15 NMR with magic angle spinning w ere used to characterize the formation of imines by the reactions of b enzaldehyde or acetone with ammonia on zeolites. When ammonia was adso rbed at cryogenic temperatures on zeolite HY (FAU) which was preloaded with benzaldehyde, it formed an appreciable amount of the tetrahedral gem-amino-hydroxy intermediate at temperatures as low as 163 K. As th e sample was heated to slightly above room temperature; the intermedia te dehydrated to form imine. The intermediate is persistent at 298 K a nd yields distinct isotropic chemical shifts at 78 ppm for C-13 and -3 20 ppm for N-15. Benzaldehyde was cleanly converted to imine by reacti on with ammonia on the medium-pore zeolite HZSM-5 (MFI), but no gem-am ino-hydroxy intermediate was seen. We attribute this effect to the ste ric and reactivity differences that favor dehydration to form the less bulky imine product on HZSM-5. The experience of solution chemistry s uggests that gem-amino-hydroxy intermediates from ketones are less sta ble than those from aldehydes, and this was also confirmed for reactio ns on zeolites. However, a small C-13 Signal at 81 ppm was observed du ring the low-temperature reaction of acetone and ammonia on HY, and th is shift is identical to that seen for 2-aminopropan-2-ol in liquid am monia. Low loadings of acetone and ammonia on HZSM-5 formed the imine cleanly, and this was stable with little secondary chemistry to 523 K. Secondary reactions were more pronounced for acetone and ammonia on H Y, and these were used to explore and compare several reactant adsorpt ion protocols. Sample heating generated a sequence of nitrogen-substit uted products analogous to those seen in aldol reactions of acetone on the same zeolite. These species were partially converted to aromatic hydrocarbons at 523 K, but no pyridines were formed with these reactio n conditions. The imines showed evidence of protonation-and hydrogen b onding to the conjugate base site of the zeolite, We also characterize d the temperature-dependent dynamics of imine products that exhibited broad C-13 signals at room temperature, but sharp lines at low and hig h temperatures. The results of this study were used in combination wit h published theoretical studies of the interaction of ammonia with zeo lites to suggest models of the association of reactants, intermediates , and products with the framework site.