Mechanisms for High Selectivity in the Hydrodeoxygenation of 5-Hydroxymethylfurfural over PtCo Nanocrystals

Abstract

Carbon-supported, Pt and PtCo nanocrystals (NCs) with controlled size and composition were synthesized and examined for hydrodeoxygenation (HDO) of 5-hydroxymethylfurfural (HMF). Experiments in a continuous flow reactor with 1-propanol solvent, at 120 to 160 °C and 33 bar H2, demonstrated that reaction is sequential on both Pt and PtCo alloys, with 2,5-dimethylfuran (DMF) formed as an intermediate product. However, the reaction of DMF is greatly suppressed on the alloys, such that a Pt3Co2 catalyst achieved DMF yields as high as 98%. XRD and XAS data indicate that the Pt3Co2 catalyst consists of a Pt-rich core and a Co oxide surface monolayer whose structure differs substantially from that of bulk Co oxide. Density functional theory (DFT) calculations reveal that the oxide monolayer interacts weakly with the furan ring to prevent side reactions, including overhydrogenation and ring opening, while providing sites for effective HDO to the desired product, DMF. We demonstrate that control over metal nanoparticle size and composition, along with operating conditions, is crucial to achieving good performance and stability. Implications of this mechanism for other reactions and catalysts are discussed

Base metal-Pt alloys: A general route to high selectivity and stability in the production of biofuels from HMF

“…In another example involving the reaction of HMF, Wang et al [10] reported that catalysts based on Pt-Co nanoparticles, encapsulated in hollow carbon spheres, could give DMF yields as high as 98%. We have recently confirmed this result on carbon-supported Pt-Co nanocrystals (NCs), without the carbon hollow-spheres, and further showed that the bimetallic catalysts exhibit much better stability than the monometallic catalysts [11]. The high selectivity of the Pt-Co catalysts resulted from the fact that DMF is unreactive over the bimetallic catalyst.…”

“…It is not likely that the higher yields are due to the increase in temperature. In addition to the fact that previous work indicated that selectivities are not very different between 100 and 200 °C [11,22], one would expect the reaction of DMF to over-hydrogenated products to increase with temperature. The maximum in the yield as a function of space time in Figure 6 is clearly less steep than that found for Pt 6 Ni/C and PtNi/C catalysts.…”

“…All these intermediates can react further to form DMF (C), which in turn reacts to overhydrogenated products (D), such as DMTHF 2-hexanone, 2-hexanol, 2,5-hexanedione, and their etherification derivatives, 1-propoxy-1-methylpentane (2-propoxyhexane) and 1,4-dipropoxy-1,4-dimethylbutane (2,5-dipropoxyhexane). All of the studied monometallic catalysts, including carbon-supported Pt, Pd, Ir, Ru, Co, and Ni, were found to exhibit relatively poor selectivity for DMF formation [8,11,22]. DMF yields varied with the particular metal catalyst and the reaction conditions but were typically less than about 50%.…”

“…The bimetallic catalyst, Pt-Co, can be highly selective for production of DMF, with 98% yield for optimized metal compositions and reaction conditions [11]. The high selectivity in that case was the result of DMF being unreactive, so that the sequential reaction stopped at DMF.…”

“…Such an orientation of the furan ring prevents side reactions on the ring itself [9,15,16]. Finally, recent work by some of us on the Pt-Co system showed that Co tends to form a CoO x shell on a Pt-rich core and that this oxide layer prevents interactions between the Pt and the furan ring, enhancing DMF yields from HDO of HMF [11].…”