Ligand and Ensemble Effects in Bimetallic NiFe Phosphide Catalysts for the Hydrodeoxygenation of 2-Methyltetrahydrofuran

A series of supported nickel–iron phosphide catalysts were used to study the hydrodeoxygenation (HDO) of 2-methyltetrahydrofuran (2-MTHF), a model compound for biomass-derived pyrolysis oil. The catalysts were prepared by incipient wetness impregnation of the active components onto potassium ion-exchanged USY zeolites followed by temperature-programmed reduction. The samples were denoted as Ni₂P/KUSY, NiFeP(3:1)/KUSY, NiFeP(1:1)/KUSY, NiFeP(1:3)/KUSY, and FeP/KUSY, where the numbers in parenthesis are the Ni:Fe molar ratios. The results of the studies can be understood from ensemble and ligand effects. X-ray diffraction analysis indicated the presence of alloys in the mixed composition samples, and this was supported by CO chemisorption and infrared measurements. Uptakes of CO decreased as the iron content increased, suggesting that iron present on the surface of the catalysts blocked chemisorption sites. Fourier transform infrared (FTIR) measurements showed a single peak for linearly adsorbed CO whose intensity decreased with Fe content, in agreement with the uptake results. Moreover, the FTIR peak shifted monotonically consistent with a ligand effect in an alloy formed in the Ni₂P and FeP particles. The reactivity in 2-MTHF HDO was studied at 250–325 °C and 0.5 MPa and it was found that the conversion was highest for the Ni₂P/KUSY sample and decreased steadily with Fe content. However, the turnover frequency did not change significantly, indicating that the rate-determing step was activation of 2-MTHF on single Ni sites. The selectivity at low conversion (3 %) changed from mostly n-pentane and n-butane for Ni₂P/KUSY to mostly 1-pentanol for the iron-containing samples, suggesting that in subsequent steps an ensemble effect was operational with the iron influencing the reaction chemistry. At high conversion (70 %) all the samples produced mostly n-pentane and n-butane.