Active Sites in Ni₂P/USY Catalysts for the Hydrodeoxygenation of 2-Methyltetrahydrofuran

A series of nickel phosphide catalysts supported on ultrastable Y zeolites USY (Si/Al = 40) with microporous/mesoporous structure were prepared by impregnation and temperature-programmed reduction and were studied for the hydrodeoxygenation (HDO) of 2-methyltetrahydrofuran (2-MTHF). The loading of the active phase was varied from 0.58, 1.16, and 1.74 to 2.3 mmol (g support)⁻¹ and the corresponding samples were denoted as 0.5, 1.0, 1.5, and 2.0 Ni₂P/USY. The two lowest loading supports did not show X-ray diffraction (XRD) lines, but X-ray absorption fine-structure spectroscopy (XAFS) indicated the formation of a Ni₂P phase, with low Ni–Ni coordination, consistent with high dispersion. The two highest loading supports showed XRD patterns typical of Ni₂P, and XAFS indicated similar bond distances to bulk Ni₂P and high Ni–Ni coordination. Furthermore, the XAFS data indicated that the low-loading samples had shorter bond distances and more Ni in square-pyramidal coordination compared to the high loading samples and the reference Ni₂P material, suggesting that there were differences in structural properties in the samples. This likely was due to preferred termination of the small crystallites with pyramidal Ni sites. The HDO of 2-MTHF was studied at 0.5 MPa and 513–593 K and the main products were n-pentane and n-butane for all catalysts. The low-loading samples showed higher turnover frequency (based on sites titrated by CO chemisorption), and this was attributed to the higher intrinsic activity of the pyramidal Ni sites. In addition, the low-loading samples showed higher selectivity to n-pentane, and this was attributed to lower C–C hydrogenolysis type reactions, which are favored by metallic ensembles as found in the high-loading samples.