Chemical Titration and Transient Kinetic Studies of Site Requirements in Mo₂C-Catalyzed Vapor Phase Anisole Hydrodeoxygenation

Abstract

The turnover frequency (TOF) of benzene synthesis from vapor phase anisole hydrodeoxygenation (HDO), estimated via in situ CO titration, was found to be invariant (1.1 ± 0.3 × 10 −3 s −1 ) over molybdenum carbide (Mo 2 C) catalysts with varying CO chemisorption uptakes (∼70 to ∼260 μmol g −1 , measured ex situ at 323 K). Accumulation of oxygen (∼0.29 monolayer) over Mo 2 C catalysts was determined by an oxygen mass balance during the transient of anisole HDO at 423 K under ambient pressure (H 2 /anisole molar ratio ∼ 110). Similar product selectivity, apparent activation energy, and TOF of benzene synthesis for an oxygen treated (with oxygen incorporation: O/Mo bulk (molar ratio) = 0.075) and freshly prepared Mo 2 C catalysts (no exposure to air prior to kinetic measurements) demonstrate that the effect of oxygen at these low concentrations is solely to reduce the number of active sites for anisole HDO, resulting in a lower (∼3 times) benzene synthesis rate per gram of catalyst for the oxygen-modified material. The observed benzene synthesis rates per CO chemisorption site for bulk molybdenum oxide (MoO x ) catalysts were found to be ∼10 times lower than those for Mo 2 C catalysts, suggesting that bulk molybdenum oxide phases are not associated with the dominant active sites for anisole HDO at 423 K under ambient pressure.

A Perspective on Catalytic Strategies for Deoxygenation in Biomass Pyrolysis

“…By using an oxygent reatment duringa nisoled eoxygenation, ad ecrease in the benzene synthesis rate by one-third was observed. [74] However, there was no changei np roduct selectivity,a pparent activation energy of the benzene formation, or turnover frequency,w hich suggestedt he presence of oxygenr educed the number and not the chemistry of the active sites.T he calculatedb enzene synthesis rate was 10 times greater over Mo 2 Ct han for bulk MoO x ,i ndicating that the active sites under the experimental reaction conditions were linked with molybdenum carbide and not the bulk oxide.I ns ubsequent work examining isopropanol dehydration, the product distribution changed with and withouta n oxygenc o-feed. [73] In the absence of oxygen, the catalyst showedm etallica nd alkaline activity by promoting dehydrogenationa nd carbonyl condensation reactionst of orm acetone and C 6 + products.T he presence of oxygenr epressed the metallic and alkalinea ctivity and, in turn, promoted the acidic dehydration of isopropanol to propylene and di-isopropylether.…”

“…By using an oxygen treatment during anisole deoxygenation, a decrease in the benzene synthesis rate by one‐third was observed . However, there was no change in product selectivity, apparent activation energy of the benzene formation, or turnover frequency, which suggested the presence of oxygen reduced the number and not the chemistry of the active sites.…”

Bifunctional Molybdenum Polyoxometalates for the Combined Hydrodeoxygenation and Alkylation of Lignin‐Derived Model Phenolics

“…[34] Unfortunately,t he catalyst lost 50 %o fi ts activity by 4h on stream.Higher Pt loadings resulted in less deactivation. [36,37] Molybdenumc arbide-am aterialf eaturing redox sites and tunable Brønsted acidity [38][39][40] -has been shown to be ah ighly active HDO catalyst, but with low selectivity for alkylation (< 1%), [41,42] whereas nickel phosphides have shown improved selectivity for alkylation approaching 15 %. [36,37] Molybdenumc arbide-am aterialf eaturing redox sites and tunable Brønsted acidity [38][39][40] -has been shown to be ah ighly active HDO catalyst, but with low selectivity for alkylation (< 1%), [41,42] whereas nickel phosphides have shown improved selectivity for alkylation approaching 15 %.…”

Advances in Upgrading Lignin Pyrolysis Vapors by Ex Situ Catalytic Fast Pyrolysis

“…The results show that in situ oxidation of molybdenum carbide by the reactant (anisole) during HDO cannot be avoided, and the dominant active sites for the HDO of anisole at 150 °C under ambient pressure are associated with molybdenum carbide but not bulk molybdenum oxide. Besides pure Mo 2 C, Mo 2 C embedded on different supports has also been investigated . Mortensen et al.…”

“…Furthermore, the site requirementso fm olybdenum carbide catalysts for the vapor-phase HDO of anisole have been investigated by in situ CO titration, transientk inetic studies,a nd temperature-programmed surface reaction with H 2 .T he results show that in situ oxidation of molybdenum carbide by the reactant (anisole) during HDO cannot be avoided, and the dominant active sites for the HDO of anisolea t1 50 8Cu nder ambientp ressurea re associated with molybdenum carbide but not bulk molybdenum oxide.B esides pure Mo 2 C, Mo 2 Ce mbedded on different supports has also been investigated. [60] Mortensen et al have used Mo 2 C/ZrO 2 to catalyze the HDO of phenoli n1octanol. [61] Mo 2 C/ZrO 2 not only selectively converts phenol into benzene but also effectively converts 1-octanol into octenea nd octane.H owever, the carbide catalyst shows fast deactivation by oxidation with water, which is ac hallenge of the catalyst system.…”