Activation and Self-Initiation in the Phillips Ethylene Polymerization Catalyst

Meeting Program — February 2016

Susannah Scott
Susan­nah Scott
Dun­can and Suzanne Mel­lichamp Chair in Sus­tain­able Catal­y­sis
Chem­i­cal Engi­neer­ing and Chem­istry & Bio­chem­istry
Uni­ver­si­ty of Cal­i­for­nia, San­ta Bar­bara

Abstract — The mech­a­nism of spon­ta­neous acti­va­tion of the Phillips (Cr/SiO2) eth­yl­ene poly­mer­iza­tion cat­a­lyst in the absence of an alky­lat­ing co-cat­a­lyst is one of the longest-stand­ing prob­lems in het­ero­ge­neous catal­y­sis. Exper­i­men­tal and com­pu­ta­tion­al evi­dence has long point­ed to organochromium(III) active sites, and the prepa­ra­tion of graft­ed (SiO)2CrCH(SiMe3)2 sites by the reac­tion of Cr[CH(SiMe3)2]3 with par­tial­ly dehy­drox­y­lat­ed sil­i­ca sup­ports this con­clu­sion. How­ev­er, a plau­si­ble mech­a­nism for their for­ma­tion from the inter­ac­tion of chro­mate and eth­yl­ene alone remains to be found. A key issue is the incom­men­su­rate nature of the required redox reac­tions, since Cr(VI) must be reduced by an odd num­ber of elec­trons (three), while only closed-shell organ­ic oxi­da­tion prod­ucts are detect­ed. For the CO-reduced cat­a­lyst, Cr K-edge XANES, EPR and UV-vis spec­tro­scopies are con­sis­tent with ini­tial step-wise reduc­tion of Cr(VI) in two-elec­tron steps, first to Cr(IV), and ulti­mate­ly to Cr(II). Accord­ing to Cr K-edge EXAFS and UV-vis spec­troscopy, the Cr(II) sites have a coor­di­na­tion num­ber high­er than two, most like­ly through inter­ac­tion with neigh­bor­ing silox­ane oxy­gens. After removal of adsorbed CO, the Cr(II) sites react with eth­yl­ene in an over­all one-elec­tron redox reac­tion to gen­er­ate organochromium(III) sites and organ­ic rad­i­cals.
Biog­ra­phy — Scott received her B.Sc. in Chem­istry from the Uni­ver­si­ty of Alber­ta (Cana­da) in 1987, and her Ph.D. in Inor­gan­ic Chem­istry from Iowa State Uni­ver­si­ty in 1991, where she worked with J. Espen­son and A. Bakac on the acti­va­tion of O2 and organ­ic oxi­da­tion mech­a­nisms. She was a NATO Post­doc­tor­al Fel­low with Jean-Marie Bas­set at the Insti­tut de recherch­es sur la catal­yse (CNRS) in Lyon, France, before join­ing the fac­ul­ty of the Uni­ver­si­ty of Ottawa (Cana­da) in 1994 as an Assis­tant Pro­fes­sor of Chem­istry. She held an NSERC Women’s Fac­ul­ty Award, a Cot­trell Schol­ar Award, a Union Car­bide Inno­va­tion Award and was named a Cana­da Research Chair in 2001. She moved to the Uni­ver­si­ty of Cal­i­for­nia, San­ta Bar­bara in 2003, where she is cur­rent­ly holds the Dun­can and Suzanne Mel­lichamp Chair in Sus­tain­able Catal­y­sis, with joint fac­ul­ty appoint­ments in both Chem­i­cal Engi­neer­ing and Chem­istry & Bio­chem­istry. She directs the NSF-spon­sored Part­ner­ship for Inter­na­tion­al Research and Edu­ca­tion in Elec­tron Chem­istry and Catal­y­sis at Inter­faces, a col­lab­o­ra­tive research pro­gram involv­ing UCSB and sev­er­al promi­nent catal­y­sis research groups in Chi­na. Her research inter­ests include sur­face organometal­lic chem­istry, olefin poly­mer­iza­tion, nano­ma­te­ri­als, bio­mass con­ver­sion, envi­ron­men­tal catal­y­sis and the devel­op­ment of new kinet­ic and spec­tro­scop­ic meth­ods to probe reac­tion mech­a­nisms at sur­faces. In 2013, Scott became an Asso­ciate Edi­tor for the jour­nal ACS Catal­y­sis.