Abstract
The gas-phase hydrodeoxygenation (HDO) of furfural, a model compound for bio-based conversion, was investigated over transition metal phosphide catalysts. The HDO activity decreases in the order Ni2P ≈ MoP > Co2P ≈ WP ≫ Cu3P > Fe2P. Nickel phosphide phases (e.g., Ni2P, Ni12P5, Ni3P) are the most promising catalysts in the furfural HDO. Their selectivity to the gasoline additives 2-methylfuran and tetrahydro-2-methylfuran can be adjusted by varying the P/Ni ratio. The effect of P on catalyst properties as well as on the reaction mechanism of furfural HDO were investigated in depth for the first time. An increase of the P stoichiometry weakens the furan-ring/catalyst interaction, which contributes to a lower ring-opening and ring-hydrogenation activity. On the other hand, an increasing P content does lead to a stronger carbonyl/catalyst interaction, i.e., to a stronger η2(C, O) adsorption configuration, which weakens the C1O1 bond (Scheme 1) in the carbonyl group and enhances the carbonyl conversion. Phosphorus species can also act as Brønsted acid sites promoting C1O1 (Scheme 1) hydrogenolysis of furfuryl alcohol, hence contributing to higher production of 2-methylfuran.
Highlights
Second generation biomass is a renewable feedstock with a lower carbon footprint than fossil crudes and is a promising alternative raw material to produce fuels and chemicals [1,2,3]
We focus on the production of MF from furfural HDO, because the involved hydrogen consumption is lower, while the carbon yield is higher compared with products like THMF, tetrahydrofurfuryl alcohol (THFOL), and furan [5,6]
Since Co2P is the third active catalyst in furfural HDO (Fig. 6) and the active phases of cobalt phosphide are adjustable by varying the molar P/Co ratio, we investigated the effect of the P stoichiometry in CoP(x) phases on the furfural HDO as well (Figure S6)
Summary
Second generation biomass is a renewable feedstock with a lower carbon footprint than fossil crudes and is a promising alternative raw material to produce fuels and chemicals [1,2,3]. Metal phosphides have extensively been investigated in hydrodesulfurization (HDS) [42,43,44] and hydrodenitrogenation (HDN) [44,45,46,47,48] reactions in recent years Due to their promising catalytic performance, they are increasingly applied in hydrodeoxygenation (HDO) reactions [49,50,51,52]. We decided to explore the catalytic performance of different metal phosphides in the furfural HDO reaction with a special interest in obtaining MF as the most desired product. In addition to changing the metal component, we varied the phosphorus/metal ratios in our samples and explored catalytic properties in response to the changed P/M stoichiometries for both Ni and Co. Previous literature reports the influence of the P/Ni ratio on the performance of nickel phosphides in HDS, HDN and HDO [70,71,72,73]. We provide deeper insight into how the P/Ni ratio influences in each of the active phases the catalytic performance during furfural HDO, by means of furfural-IR and CO-IR spectroscopy
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