Examining Metal Identity and Proximity Effects on Acetylene Hydrogenation with Azolate-Based MOFs

Liquid organic hydrogen carriers (LOHCs) are an attractive fuel source due to their compatibility with existing transportation methods and ease of use. However, they suffer from sluggish (de)hydrogenation kinetics. One promising platform for developing next-generation catalysts is metal-organic frameworks (MOFs), which can enable systematic interrogation into the influence of metal identity and spatial arrangement. In this study, the effect of the coordination environment was investigated using Ni- and Co-based azolate MOFs: MFU-4<i>l</i>-OH (M_<i>x</i>Zn_5-<i>x</i>(OH)₄(BTDD)₃; <i>x</i> = 4 for M = Co and <i>x</i> = 3 for M = Ni, H₂BTDD = bis(1H-1,2,3-triazolo[4,5-<i>b</i>][4',5'-i])dibenzo[1,4]dioxin), composed of single-site nodes, and M(OH)₂BBTA (M = Ni, Co; H₂BBTA = 1H,5H-benzo(1,2-d:4,5-d')bistriazole), composed of extended chain-type nodes. The catalysts were characterized by isotherms, powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), inductively coupled plasma-optical emission spectroscopy (ICP-OES), and X-ray photoelectron spectroscopy (XPS) analysis. Acetylene hydrogenation activity under steady state conditions (150 °C, 1:1 C₂H₂:H₂) revealed higher turnover frequencies (TOFs) up 1.17 × 10^-2 mol C₂H₂ mol^-1 Ni min^-1 and 1.98 × 10^-3 mol C₂H₂ mol^-1 Co min^-1 for Ni-MFU-4<i>l</i>-OH and Co-MFU-4<i>l</i>-OH, respectively, compared to their BBTA analogues. However, the Co-based MOFs, particularly Co₂(OH)₂-BBTA, exhibited greater selectivity (up to 19%) for the fully hydrogenated ethane product. Isosteric heat of adsorption (<i>Q</i>_st) measurements for ethylene and ethane revealed that the BBTA framework had stronger binding to the products than MFU-4<i>l</i>. These findings demonstrate that metal identity and coordination environment may modulate acetylene hydrogenation performance, leading to design principles for tuning LOHC hydrogenation catalysts.