(138d) Overlayer Catalyst for Hydrodeoxygenation of Lignin-Derived Model Compounds

Recently, in an effort to build a more sustainable society, the transformation of lignocelluloseic biomass into chemicals and fuels has attracted extensive attention. The lignin fraction of biomass is a three-dimensional amorphous polymer composed of methoxylated phenylpropane structures, which contains approximately 40% of the possible energy of the biomass. Lignocelluloseic biomass can be converted into crude bio-oil via thermochemical treatment such as pyrolysis or liquefaction. The crude bio-oils are multicomponent mixtures of a large number of oxygenated compounds. However, the high oxygen content of crude bio-oils, usually 20 to 50 wt%, leads to low heating value, poor stability, poor volatility, high viscosity and corrosiveness. Therefore, oxygen removal from bio-oils is required for upgrading bio-oils to fuels suitable for replacement of conventional liquid transportation fuels. Hydrotreating, in which crude bio-oils are reacted with hydrogen in the presence of a catalyst, is the most common method to upgrading bio-oil to hydrocarbons via hydrodeoxygenation (HDO) reaction to remove the undesired oxygen.

For elucidation of HDO mechanism and kinetics, HDO studies of lignin-derived oils employ model compounds such as phenols, anisole and guaiacol instead of bio-oil. Research has identified that surface properties of the catalyst could be modified by bimetallic alloys to enhance the efficiency of catalysts for the HDO[1]. Bimetallic catalysts have proven capable of improving catalytic activity, selectivity, and catalyst stability via the bimetallic effect causing on interaction between metals. Based on previous work, the pseudomorphic overlayer bimetallic catalysts with a monatomic layer atop a different bulk metal would also enhance HDO activity. These catalysts have received widespread attention from computational and single crystal studies because of their notably different adsorption properties when compared to their constituent metals. These changes are due to electronic interactions between the two metals causing a shift in the d-band center of the overlayer metal. First principle computational work has demonstrated a linear relationship between the center of the d-band and heats of adsorption. For certain metal combinations, pseudomorphic overlayer catalysts have shown shifted surface d-band center and reduced metal-adsorbate binding energy when compared to single metal and alloy counterparts.[2] This slightly reduced binding strength for reactants and intermediates in HDO reaction makes overlayer catalysts ideal for HDO because strong surface-reactant binding have been shown to inhibit HDO of lignin derived phenols.[3] Thus, the pseudomorphic overlayer type bimetallic catalysts were synthesized to increase the activity of the catalyst for hydrodeoxygenation of guaiacol while reducing the necessary loading of costly palladium and platinum.

All catalysts were supported with a silica-alumina catalyst support. The monometallic 5 wt% Ni and Mo parent catalysts and non-structured bimetallic Ni-Pd, Ni-Pt and Mo-Pt catalysts were prepared by using incipient wetness co-impregnation. Silica-alumina supported bimetallic overlayer catalysts, Ni@Pd, Ni@Pt and Mo@Pt have been prepared by the directed deposition synthesis technique.[4, 5] Hydrogen chemisorption, ethylene hydrogenation descriptor reaction, powder XRD, H₂-TPR, XAS and TEM studies were employed to characterize the catalysts.

The guaiacol and anisole hydrodeoxygenation tests were performed a fixed-bed tubular quartz reactor operated at atmosphere 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.

Hydrogen chemisorption studies showed that the heat of adsorption for hydrogen decreased for the overlayer catalysts (Ni@Pd, Ni@Pt and Mo@Pt) compared to Pd or Pt catalyst, as expected. For ethylene hydrogenation, Pd or Pt was highly active and the Ni and Mo baseline catalysts showed lower activities. As expected, the deposition of the Pd or Pt overlayer resulted in catalysts (Ni@Pd, Ni@Pt and Mo@Pt) that were more active for ethylene hydrogenation than the pure Ni or Mo parent catalyst. However, when compared to pure Pd or Mo, the overlayer catalysts showed decreased activity. These results agree with computationally predicted d-band shifts from the literature that would cause weaker hydrogen adsorption on the metal surface, decreased surface coverage, and ultimately reduced activity for ethylene hydrogenation when compared to Pd or Pt metal alone. Thus, the chemisorption and ethylene hydrogenation reactivity descriptors indicated that Ni@Pd, Ni@Pt and Mo@Pt catalysts are promising candidate for subsequent guaiacol HDO studies.

Guaiacol and anisole HDO studies suggested that the catalysts are capable of producing deoxygenation products under desirable reaction condition. The HDO results showed that Pd and Pt active sites of overlayer catalysts showed significantly enhanced deoxygenation activity compared with that of Pd or Pt only catalyst. This experimental result supports the speculation that Ni@Pd, Ni@Pt and Mo@Pt overlayer may result in slightly reduced binding strength for reactants and intermediates, and then enhancing HDO activity. Further studies showed that guaiacol and anisole could be completely deoxygenated over the silica alumina supported metal catalysts at high W/F, yielding benzene, toluene, and xylenes as major products.

Reference

1. Wang, H.; Male, J.; Wang, Y., ACS Catal. 2013, 3 (5), 1047-1070.

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

3. Gonzalez-Borja, M. A.; Resasco, D. E., Energy & Fuels 2011, 25 (9), 4155-4162.

4. Skoglund M.D., Jackson C.L., McKim K.J., Olson H. J., Sabirzyanov S., Holles J. H. Appl. Catal. A: General 2013, 467, 355-362.

5. Latusek, M. P.; Spigarelli, B. P.; Heimerl, R. M.; Holles, J. H., J. Catal. 2009, 263, 306-314.