Structure and site evolution of molybdenum carbide catalysts upon exposure to oxygen

Sullivan, Mark M.; Held, Jacob T.; Bhan, Aditya

doi:10.1016/j.jcat.2015.03.011

Cited by 77 publications

(85 citation statements)

References 52 publications

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“…The data of Table 8 also illustrate that the Mo-oxides are active for 4-MP HDO and as noted elsewhere, electrophillic coordinatively unsaturated sites present on MoO 3 and MoO 2 under reaction conditions, result in a high selectivity for C-O hydrogenolysis or DDO [21]. Previous studies have also concluded that oxygen-containing species change the surface properties of the Mo 2 C during HDO [47]. Lee et al [20] reported that the presence of D 2 O suppresses the hydrogenation ability of Mo 2 C significantly and this change is irreversible suggesting that the loss in activity is not due to competitive adsorption.…”

Section: The Analysis Of the 4-mp Hdo Reaction Kinetics Shows Very Sisupporting

confidence: 73%

Synthesis and Hydrodeoxygenation Activity of Carbon Supported Molybdenum Carbide and Oxycarbide Catalysts

2016

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Carbothermal hydrogen reduction (CHR) of ammonium heptamolybdate impregnated activated charcoal (AC) yields a mixed Mo 2 C/MoO x C y catalyst. As the CHR temperature increases (from 600 to 800 °C) the Mo 2 C content increases. At 675 °C graphite networks are generated that attach to the β-Mo 2 C particles, and at ≥700 °C agglomeration and sintering occur, all of which decrease catalyst activity. An optimal CHR temperature of ∼650 °C is identified based on the catalyst activity for the hydrodeoxygenation (HDO) of 4-methylphenol (4-MP) at 350 °C and 4.3 MPa H 2 and the high selectivity for direct deoxygenation (DDO: to yield toluene) versus hydrogenation (HYD: to yield cyclohexane). The Mo 2 C/MoO x C y catalysts have higher DDO selectivity than MoP, MoO 2 , or MoS 2 when operated at similar conditions. The apparent activation energies for DDO (125 kJ/mol) and HYD (89 kJ/mol) are invariant among the catalysts with varying Mo 2 C content, but the rate per g Mo correlates with the CO uptake. The fact that the kinetics are not strong functions of the CHR reduction temperature and hence relative content of Mo 2 C versus MoO x C y suggests that the active site of the catalyst is a result of O adsorption and/or exchange with the catalyst during reaction, and these active sites occur on both Mo 2 C and MoO x C y during the HDO reaction.

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Section: The Analysis Of the 4-mp Hdo Reaction Kinetics Shows Very Sisupporting

confidence: 73%

Synthesis and Hydrodeoxygenation Activity of Carbon Supported Molybdenum Carbide and Oxycarbide Catalysts

2016

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“…In fact, in al ater study XPS was used to examine the effect of passivationo nt he Mo 2 Cs urface. [73] No Mo 2 + was detected. Instead,a bout half of the molybdenum was fully oxidized to Mo 6 + (51 %), with the remaining being partially oxidized to Mo 4 + (23 %) and Mo 3 + (26 %).…”

Section: Hdo Over Transition Metal Catalystsmentioning

confidence: 97%

“…Even though a bulk oxide or oxycarbide phase was not detected, it was postulated that surface and sub‐surface oxidation of the Mo 2 C was one of the mechanisms of catalyst deactivation. In fact, in a later study XPS was used to examine the effect of passivation on the Mo 2 C surface . No Mo 2+ was detected.…”

Section: Vapor‐phase Deoxygenation Over Novel Catalystsmentioning

confidence: 99%

“…The calculated benzene synthesis rate was ∼10 times greater over Mo 2 C than for bulk MoO x , indicating that the active sites under the experimental reaction conditions were linked with molybdenum carbide and not the bulk oxide. In subsequent work examining isopropanol dehydration, the product distribution changed with and without an oxygen co‐feed . In the absence of oxygen, the catalyst showed metallic and alkaline activity by promoting dehydrogenation and carbonyl condensation reactions to form acetone and C 6+ products.…”

Section: Vapor‐phase Deoxygenation Over Novel Catalystsmentioning

confidence: 99%

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A Perspective on Catalytic Strategies for Deoxygenation in Biomass Pyrolysis

2016

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Biomass fast pyrolysis is a technology that is able to convert biomass, a low‐density solid, into primarily a liquid bio‐oil product that is denser and more easily storable and transportable. However, utilizing raw bio‐oil is difficult, mainly due to its high oxygen content. Bio‐oil must be upgraded and deoxygenated first before it can be co‐fed into a petroleum refinery. One method of upgrading involves condensing the pyrolysis vapors into bio‐oil and then hydrotreating the bio‐oil. Significant deoxygenation can be achieved; however, the hydrotreatment is typically performed under high pressure (>6900 kPa) and with multiple temperature stages. Also, lignin‐derived phenolic compounds can be prone to polymerize and plug reactors. Generally, the catalysts that have been tested are expensive noble metals or transition metal sulfides. Another upgrading approach is to react the pyrolysis vapors over a catalyst to produce a deoxygenated liquid product upon condensation. Common petroleum refining catalysts, such as zeolites, have been found to produce hydrocarbons although their yields are generally low to moderate whereas the yields of solids (char/coke) and gases (CO and CO2) can be high. In addition, zeolites can suffer rapid deactivation through coking and loss of acidity. Other, new catalysts that have been tested for vapor‐phase deoxygenation include noble‐metal and transition‐metal catalysts. Noble metals are active but are also prone to saturating aromatic rings. More cost‐effective catalysts, that is, transition metal‐based catalysts, have also been tested for hydrodeoxygenation efficacy. Particularly, two molybdenum‐based catalysts, MoO3 and Mo2C, were found to deoxygenate biomass model compounds under low pressure and moderate temperatures. In addition, MoO3 has been shown to deoxygenate pyrolysis vapors obtained from complete biomass. From our perspective, the most feasible upgrading strategy would likely involve catalytically upgrading pyrolysis vapors under low pressure, moderate temperature, and with minimal hydrogen consumption to produce unsaturated alkenes and aromatics, which can then be further processed and refined in a traditional petroleum refinery.

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“…[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 %.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2017

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Reductive catalytic fractionation of biomass has recently emerged as a powerful lignin extraction and depolymerization method to produce monomeric aromatic oxygenates in high yields. Here, bifunctional molybdenum-based polyoxometalates supported on titania (POM/TiO ) are shown to promote tandem hydrodeoxygenation (HDO) and alkylation reactions, converting lignin-derived oxygenated aromatics into alkylated benzenes and alkylated phenols in high yields. In particular, anisole and 4-propylguaiacol were used as model compounds for this gas-phase study using a packed-bed flow reactor. For anisole, 30 % selectivity for alkylated aromatic compounds (54 % C-alkylation of the methoxy groups by methyl balance) with an overall 72 % selectivity for HDO at 82 % anisole conversion was observed over H PMo O /TiO at 7 h on stream. Under similar conditions, 4-propylguaiacol was mainly converted into 4-propylphenol and alkylated 4-propylphenols with a selectivity to alkylated 4-propylphenols of 42 % (77 % C-alkylation) with a total HDO selectivity to 4-propylbenzene and alkylated 4-propylbenzenes of 4 % at 92 % conversion (7 h on stream). Higher catalyst loadings pushed the 4-propylguaiacol conversion to 100 % and resulted in a higher selectivity to propylbenzene of 41 %, alkylated aromatics of 21 % and alkylated phenols of 17 % (51 % C-alkylation). The reactivity studies coupled with catalyst characterization revealed that Lewis acid sites act synergistically with neighboring Brønsted acid sites to simultaneously promote alkylation and hydrodeoxygenation activity. A reaction mechanism is proposed involving activation of the ether bond on a Lewis acid site, followed by methyl transfer and C-alkylation. Mo-based POMs represent a versatile catalytic platform to simultaneously upgrade lignin-derived oxygenated aromatics into alkylated arenes.

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Structure and site evolution of molybdenum carbide catalysts upon exposure to oxygen

Cited by 77 publications

References 52 publications

Synthesis and Hydrodeoxygenation Activity of Carbon Supported Molybdenum Carbide and Oxycarbide Catalysts

Synthesis and Hydrodeoxygenation Activity of Carbon Supported Molybdenum Carbide and Oxycarbide Catalysts

A Perspective on Catalytic Strategies for Deoxygenation in Biomass Pyrolysis

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

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