Influence of alkali metal (Li and Cs) addition to Mo₂N catalyst for CO hydrogenation to hydrocarbons and oxygenates

Abstract

Abstract The aim of this work is to understand the catalytic behaviour of Li and Cs promoted Mo 2 N for CO hydrogenation to hydrocarbons and oxygenates at the reaction conditions 275–325 °C, 7 MPa, and 30 000 h −1 GHSV. Molybdenum nitrides were synthesized via temperature programmed treatment of ammonium heptamolybdate (AHM) and alkali metal (AM) precursors under continuous gaseous ammonia flow. Unpromoted Mo 2 N and AM‐Mo 2 N catalysts were characterized using BET‐pore size, X‐ray diffraction, TPD‐mass of CO, HR‐TEM, and XPS techniques. Nominal loadings of 1, 5, and 10 wt% of Li and Cs were selected for these studies. At a 10 % CO conversion level, the total oxygenate selectivity of 28, 11, and 6.5 % was observed on 5Cs‐Mo 2 N, 5Li‐Mo 2 N, and unpromoted Mo 2 N, respectively. The decreased oxygenate selectivity for unpromoted Mo 2 N was mainly associated with CO dissociative hydrogenation on Mo δ+ sites. On the other hand, improved molecular CO insertion into −C x H y intermediate accelerates the total oxygenate formation on the Cs‐Mo‐N catalyst. However, during nitridation, crystal structure changes were observed in Li‐Mo‐N and the obtained oxygenates selectivity was attributed to the Li 2 MoO 4 phases. At lower AM loadings, the active sites corresponding to oxygenates formation were inadequate, and at higher AM loadings, surface metallic molybdenum decreased the total oxygenate selectivity.