Role of surface methoxy species in the conversion of methanol to dimethyl ether on acidic zeolites investigated by in situ stopped-flow MAS NMR spectroscopy

In situ continuous-flow (CF) MAS NMR experiments indicated that the investigation of the role of surface methoxy species in the conversion of methanol to dimethyl ether requires the preparation of pure methoxy groups on the catalyst surface. Applying the novel in situ stopped-flow (SF) MAS NMR technique, methoxy species were selectively retained on zeolites H-Y (n(Si)/n(Al) = 2.7) and H-ZSM-5 (n(Si)/n(Al) = 22.0) and on the silicoaluminophosphate H-SAPO-34 [n(Si)/(n(Al) + n(Si) + n(P)) = 0.088]. These pure methoxy species were obtained after conversion of I (CH3OH)-C-13 under CF conditions at 423 K and purging the catalyst materials with dry nitrogen at 473 K. For H-Y, H-ZSM-5, and H-SAPO-34, the C-13 MAS NMR signals of selectively prepared methoxy species occurred at 56.2, 59.4, and 56.6 ppm, respectively. Application of cross polarization led to a strong enhancement of the above-mentioned signals accompanied by sideband patterns which indicates that these alkoxy species are rigidly bound on the catalyst surfaces. By conversion of methanol with C-13-isotopes in natural abundance (denoted (CH3OH)-C-12) on zeolite H-Y covered with C-13-enriched methoxy species, the formation of partially C-13-enriched dimethyl ether ((CH3OCH3)-C-13-C-12) was observed at 60.5 and 63.5 ppm. This finding gives experimental evidence for the formation of dimethyl ether on acidic zeolites by the reaction of methanol with surface methoxy species formed in a former reaction step.