Olefin Metathesis by Supported MoOx/Al2O3 Catalysts

Meeting Program — October 2017

Anisha Chakrabar­ti — Stu­dent Speak­er

Advi­sor: Israel E. Wachs
Operan­do Mol­e­c­u­lar Spec­troscopy & Catal­y­sis Lab­o­ra­to­ry
Depart­ment of Chem­i­cal and Bio­mol­e­c­u­lar Engi­neer­ing
Lehigh Uni­ver­si­ty, Beth­le­hem, PA 18015 USA
 

Abstract — The olefin metathe­sis reac­tion was com­mer­cial­ized in the late 1960s to pro­duce eth­yl­ene and 2-butene from propy­lene in the Phillips Tri­olefin Process. The reverse reac­tion, how­ev­er, is cur­rent­ly desired due to a glob­al propy­lene short­age caused by the shift to lighter feed­stocks derived from shale gas frack­ing. Het­ero­ge­neous sup­port­ed MoOx/Al2O3 cat­a­lysts are employed for olefin metathe­sis in the Shell High­er Olefin Process (SHOP) that oper­ates between room tem­per­a­ture and ~200°C.

To probe the mol­e­c­u­lar details of the sup­port­ed MoOx/Al2O3 cat­a­lysts, a mod­ern in situ spec­troscopy approach was under­tak­en. In situ UV-vis mea­sure­ments (Eg val­ues) con­firmed the pres­ence of iso­lat­ed and oligomer­ic MoOx sur­face sites, with the lat­ter increas­ing with molyb­de­na load­ing. In situ Raman spec­troscopy revealed that at low load­ings of molyb­de­na (1 Mo atoms/nm2), only iso­lat­ed dioxo (O=)2MoO2 sur­face sites are present. As the molyb­de­na load­ing is increased (1–4.6 Mo atoms/nm2), oligomer­ic mono-oxo O=MoO4 sur­face sites co-exist with the iso­lat­ed dioxo (O=)2MoO2 sur­face sites. Above mono­lay­er load­ings (>4.6 Mo atoms/nm2), crys­talline MoO3 nanopar­ti­cles are also present. In situ IR indi­cates that the iso­lat­ed dioxo MoO4 sites are anchored at more basic HO-μ1-AlIV sur­face hydrox­yls, while the sur­face oligomer­ic mono-oxo sites are anchored to more acidic HO-μ1/3-AlV/VI sur­face hydrox­yls. Propy­lene metathe­sis at reac­tion con­di­tions sug­gest that the iso­lat­ed dioxo (O=)2MoO2 sur­face site may still be present after acti­va­tion of the mono-oxo sur­face sites with propy­lene. In situ UV-vis dur­ing propy­lene metathe­sis indi­cates that Mo+6 sites are dom­i­nant dur­ing propy­lene metathe­sis due to the pres­ence of unre­duced sur­face dioxo Mo+6O4 sites and re-oxi­da­tion of reduced Mo+4 sites by propy­lene back to Mo+6=CH2 and Mo+6=CHCH3 reac­tion inter­me­di­ates. The sur­face chem­istry was chem­i­cal­ly probed by C3H6-TPSR that ini­tial­ly formed oxy­genat­ed prod­ucts (CH3CHO, H2CO, CH3COCH3, H2O and CO/CO2) dur­ing cat­a­lyst acti­va­tion. The reac­tiv­i­ty of the acti­vat­ed cat­a­lysts to butene pro­gres­sive­ly increased with molyb­de­na load­ing, indi­cat­ing that the oligomer­ic mono-oxo MoOx sites are much more active than iso­lat­ed dioxo MoO4 sites for olefin metathe­sis. The crys­talline MoO3 nanopar­ti­cles, how­ev­er, were found to be inac­tive for metathe­sis. This pre­sen­ta­tion will address the fun­da­men­tal mol­e­c­u­lar and struc­tur­al details of the sup­port­ed MoOx/Al2O3 cat­a­lysts dur­ing propy­lene metathe­sis and estab­lish their struc­ture-activ­i­ty rela­tion­ships.