Evolution of C-C Bond Formation in the Methanol-to-Olefins Process: From Direct Coupling to Autocatalysis

Methanol conversion during the induction period of methanol-to-olefin (MTO) process has been investigated by solid-state nuclear magnetic resonance (ssNMR), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), gas chromatography mass spectroscopy (GC-MS), and time-of-flight mass spectroscopy (TOF-MS), over HSAPO-34 molecular sieve. The two-dimensional (2D) C-13-C-13 MAS NMR spectra revealed the correlation of surface methoxy species (SMS) and dimethyl ether (DME)/methanol was enhanced with the temperature increase, supporting that the carbon carbon (C-C) bond can be possibly formed through the direct coupling of SMS and the surface-adsorbed Cl reactant. The evolution of surface species was monitored continuously by the aid of in situ ssNMR and in situ DRIFTS. With the consumption of SMS, alkenyl or/and phenyl carbocations were formed and accumulated as the successive intermediates for methanol conversion. Based on these direct observations, we propose that the first C-C bond is derived from SMS-mediated DME/methanol activation while alkenyl/phenyl carbocations can take over SMS and convert methanol efficiently in the autocatalysis stage of the MTO process.