(506c) Mo@Pt Overlayer As Effective Catalysts for Hydrodeoxygenation of Guaiacol and Anisole

Recently, the transformation of lignocelluloseic biomass into chemicals and fuels has attracted much attention. Pyrolysis of biomass is one of the most promising processes for converting biomass to liquid fuels. However, the crude bio-oils from pyrolysis process are mixtures of different oxygenated compounds, which cause corrosiveness and reduce stability, heating value and combustion performance. In addition, crude bio-oils are immiscible with conventional liquid fossil fuel from petroleum refinery, ruining any opportunity to blend with fossil fuels. Therefore, deoxygenation of crude bio-oils is required for utilizing bio-oils as fuels. The catalytic hydrodeoxygenation (HDO) reaction is one of most common process to remove the undesired oxygen from crude bio-oils in the form of water in the presence of hydrogen. Lignin-derived bio-oils contain large number of oxygenated aromatic compounds and it is desirable to upgrade these phenolics to saturated hydrocarbons or aromatic hydrocarbons via catalytic HDO. To simplify the catalyst development, representative model compounds such as phenol, anisole and guaiacol are used for the HDO studies of lignin-derived bio-oils. These model compounds contain functional groups found in lignin and are found in the lignin-derived bio-oils.

Preparing Mo based bimetallic catalysts has been used as a strategy to enhance catalytic activity and selectivity of HDO reaction. The pseudomorphic overlayer (with a monolayer of metal atop a different bulk metal) bimetallic catalysts might also enhance HDO activity. The computational and experimental studies have shown that the pseudomorphic overlayer bimetallic catalyst would have different surface properties than the single metal and alloy counterparts.[1,2] In particular, the overlayer bimetallic catalyst would show an altered adsorbate binding strength because of the shift of the d-band center energy. The Mo@Pt bimetallic overlayer catalyst with decreased adsorbate binding strength might exhibit improved performance for the HDO reaction, as strong binding of model phenolic compounds (guaiacol or catechol) by the catalyst has been reported to block surface sites.[3] In addition, the Mo@Pt bimetallic catalyst might favor the C-O bond cleavage compared to a Pt only catalyst and facilitate the dissociation of H₂ in comparison to monometallic Mo only catalyst.[4] Thus, the Mo@Pt overlayer bimetallic catalysts were synthesized to improve the catalytic activity of guaiacol and anisole HDO while reducing the necessary loading of costly platinum.

A silica-alumina catalyst support was employed as support for all samples. The monometallic Mo and Pt catalyst as well as non-structured bimetallic Mo-Pt catalyst were synthesized using a co-impregnation method. Silica-alumina supported bimetallic Mo@Pt overlayer catalysts were synthesized using the directed deposition method.[2] Hydrogen chemisorption, ethylene hydrogenation descriptor reaction, powder XRD and H₂-TPR studies were employed to characterize the catalysts. The guaiacol and anisole hydrodeoxygenation tests were performed in a fixed-bed tubular quartz reactor operated at atmospheric pressure. The HDO reaction conditions were 200 mg catalyst, 0.72 ml/h pure guaiacol or anisole liquid and 60 ml/min H₂. Reaction temperatures were varied from 350°C to 450°C. Liquid samples collected by cold trap at 1 h intervals were analyzed off-line by gas chromatograph.

Mo@Pt overlayer showed reduced heats of hydrogen and carbon monoxide adsorption. The decreased ethylene hydrogenation reactivity for Mo@Pt overlayer catalysts also suggested decreased hydrogen adsorption strength compared to Pt only catalyst. These results agree with computationally predicted d-band shifts from the literature that Mo@Pt overlayers would cause weaker hydrogen adsorption on the metal surface, decreased surface coverage, and ultimately reduced activity for ethylene hydrogenation when compared to Pt metal alone. The H₂-TPR results demonstrated the close interaction between Pt and Mo species for Mo@Pt overlayer catalysts and enhanced reducibility of the molybdenum oxides via deposition of Pt to Mo. Active sites of overlayer catalysts showed significantly improved deoxygenation turnover frequency (TOF) for both guaiacol and anisole HDO reactions compared with that of monometallic Pt and Mo catalysts. For the guaiacol HDO reaction at 350°C, the Mo@Pt DD showed highest HDO TOF, twice as high as that of Pt and Mo catalysts. For the Mo@Pd DD catalyst, the selectivity of benzene, toluene and xylene (BTX) could achieve above 80% at a W/F of 2.65 h for guaiacol HDO and a W/F of 1.33 h for anisole HDO. This result is similar with that reported by Sun et al., an 83% selectivity of benzene/toluene/trimethylbenzene on the PdFe/C catalyst at 450°C and a W/F of 1.9 h.[5] The data fitting results showed that the overall guaiacol conversion was second order reaction with respect to guaiacol, while the anisole conversion was first order reaction with respect to anisole. Catalysts showed better stability in HDO of anisole than in HDO of guaiacol. In summary, Mo@Pd DD has been demonstrated as an effective catalyst for HDO of guaiacol into BTX.

Reference

1. Christoffersen E., Liu P., Ruban A., Skriver H.L., Norskov J.K., J. Catal. 2011, 199, 123-131

2. Lai Q., Skoglund M.D., Zhang C., Morris A.R., Holles J.H. Energy & Fuels 2016, 30, 8587-8596.

3. Gonzalez-Borja, M. A.; Resasco, D. E., Energy & Fuels 2011, 25, 4155-4162.

4. Robinson A.M., Montemore M.M., Tenney S.A., Sutter P., Medlin J.W., J. Phys.Chem. C. 2013, 117, 26716-26724.

5. Sun J. M., Karim A. M., Zhang H., Kovarik L., Li X. H. S., Hensley A. J., McEwen J. S., Wang Y., J. Catal. 2013, 306, 47-57.