title: Structural and mechanistic investigation of a phosphate-modified HZSM-5 catalyst for methanol conversion.
abstract: Phosphorus modification of a HZSM-5 (MFI) zeolite by wet impregnation has long been known to decrease aromatic formation in methanol conversion chemistry. We prepared and studied a catalyst modified by introducing trimethylphosphine under reaction conditions followed by oxidation. Magic-angle spinning (MAS) NMR shows that extensive dealumination occurs, resulting in a catalyst with a much higher framework SiO2/Al2O3 ratio, as well as extraframework aluminum and approximately 1.4 equiv of entrained phosphoric acid (under working conditions) per aluminum. Upon dehydration or regeneration, the phosphoric acid is converted, reversibly, to entrained P4O10. The aromatic selectivity of the modified catalyst is significantly lower than that of an unmodified zeolite with a similar, increased framework SiO2/Al2O3 ratio. By comparing the rates of H/D exchange in propene under conditions similar to those in methanol conversion chemistry, we determined that the acid site strength is indistinguishable on modified and unmodified zeolites, and this is consistent with theoretical modeling. On the phosphorus-modified zeolite, the rate of propene oligomerization is greatly suppressed, suggesting that entrained phosphate is an impediment to sterically demanding reactions.
authors.affiliation: Department of Chemistry, University of Southern California, University Park, Los Angeles, California 90089-1661, USA.
authors.forename: Saifudin M
authors.initials: SM
authors.lastname: Abubakar
authors.forename: David M
authors.initials: DM
authors.lastname: Marcus
authors.forename: Jeffrey C
authors.initials: JC
authors.lastname: Lee
authors.forename: Justin O
authors.initials: JO
authors.lastname: Ehresmann
authors.forename: Ching-Yeh
authors.initials: CY
authors.lastname: Chen
authors.forename: Philip W
authors.initials: PW
authors.lastname: Kletnieks
authors.forename: Darryl R
authors.initials: DR
authors.lastname: Guenther
authors.forename: Miranda J
authors.initials: MJ
authors.lastname: Hayman
authors.forename: Mari
authors.initials: M
authors.lastname: Pavlova
authors.forename: John B
authors.initials: JB
authors.lastname: Nicholas
authors.forename: James F
authors.initials: JF
authors.lastname: Haw