(277d) Ni@Pd Overlayer Catalyst for Hydrodeoxygenation of Guaiacol

The transformation of lignocellulosic biomass into chemicals and fuels has attracted much attention as an alternative to conventional petroleum. 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. It 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 similar to 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 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 by bimetallic effect causing by 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. 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. Thus, the pseudomorphic overlayer type bimetallic catalysts were synthesized to increase the activity of the catalyst in hydrodeoxygenation of guaiacol while reducing the necessary loading of costly palladium.

All catalysts were supported with a silica-alumina catalyst support. The monometallic 5 wt% Ni/SiO₂-Al₂O₃, parent catalysts and non-structured bimetallic Ni-Pd/ SiO₂-Al₂O₃ catalysts were prepared by using incipient wetness co-impregnation. The pseudomorphic overlayer catalysts used were prepared by using the directed deposition technique. 

Hydrogen chemisorption studies for isotherm analysis and to determine hydrogen heats of adsorption were done using a Micromeritics ASAP 2020. Ethylene hydrogenation studies were performed at standard reaction conditions: 1atm, 20% H₂, 5% C₂H₄, 75% N₂, flow rate = 750 ml/min. The reactor effluent was monitored using a GCMS.  The guaiacol hydrodeoxygenation tests were performed a fixed-bed tubular quartz reactor operated at atmosphere pressure. The APR reaction conditions were 200 mg catalyst, 0.72 ml/h pure guaiacol 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 compared to Pd catalyst, as expected. For ethylene hydrogenation, Pd/SiO₂-Al₂O₃ is highly active and the Ni/ SiO₂-Al₂O₃ baseline catalysts show lower activities. As expected, the deposition of the Pd overlayer resulted in catalysts that were more active for ethylene hydrogenation than the pure Ni parent catalyst. However, when compared to pure Pd, the Ni@Pd 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 palladium metal alone. Thus, the chemisorption and ethylene hydrogenation reactivity descriptors indicated that Ni@Pd catalyst is promising candidate for subsequent guaiacol HDO studies.

Guaiacol HDO studies suggested that the catalysts are capable of producing deoxygenation product under desirable reaction condition. Single deposition Ni@Pd SD overlayer catalyst exhibited a significantly enhanced activity for guaiacol HDO reaction. Among the tested catalysts at 350 °C, TOF of deoxygenation product formation of Ni@Pd SD catalyst was highest, three times as high as that of monometallic Pd catalyst and twice as high as that of Ni catalyst. The double deposition Ni@Pd DD overlayer catalyst showed lower deoxygenation activity than single deposition Ni@Pd SD overlayer catalysts, due to that the  SD  catalysts may  have  the  largest  d-band  shift  as  compared to the DD catalysts, which have been demonstrated in our previous Ni@Pt and Co@Pt SD overlayer catalyst system [2]. Though the monometallic Pd catalyst had the highest TOF of non-deoxygenation product formation, it’s TOF of deoxygenation product formation was lowest. HDO results are consistent with hydrogen chemisorption and ethylene hydrogenation results. 

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

2. 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.