(727h) Spectroscopic Elucidation of Propylene Metathesis Reaction Intermediates Using Modulation Excitation-Phase Sensitive Detection-Diffuse Reflectance Techniques

This work focuses on the in situ and operando spectroscopic characterization of surface reaction intermediate species during propylene metathesis over Mo and W based catalysts. A Mo(4.4%)- TUD-1 catalyst was studied at temperatures as high as 723 K by means of modulation excitation-phase sensitive detection diffuse reflectance infrared Fourier transform (ME-PSD-DRIFTS) and UV-vis spectroscopies (ME-PSD-DR-UV-vis). The ME-PSD technique was based on 1) collection of rapid scan data during periodic changes (at a fixed modulation frequency) in gas feed concentration; 2) spectra conversion from time domain to frequency domain via Fourier transform; 3) data filtering at the feed modulation frequency, and 4) spectra reconstruction from frequency domain to phase domain by inverse Fourier transform to yield only species that responded to the feed modulation frequency. Such observed species are more likely to be surface intermediate species. ME-PSD-DRIFTS detected peaks that responded to propylene (90-101 kPa) and water (0-1 kPa) feed concentration modulation frequency during propylene metathesis at 723 K. These species were assigned to adsorbed ethylene and propylene, Mo carbenes and silanol species. Additionally, ME-PSD-DR-UV-vis allowed the sensitive detection of Mo(VI) reduction during propylene modulation experiments. These results combined provided evidence for Mo(VI)-methylidene, Mo(VI)-ethylidene, and Mo(VI)-cyclobutanes as likely reaction intermediates in propylene metathesis and their possible formation via a pseudo-Wittig generation mechanism assisted by neighboring silanols which adsorbed the olefin and reduced Mo(VI) precursor species. Moreover, the promoting role of TiO₂ on WO_x/TiO₂/SiO₂ catalysts for propylene metathesis will also be discussed in regards to the nature and reduction of WO_x species and the stabilization of metal carbene species at reaction conditions.