Use of unsupported and silica supported molybdenum carbide to treat chloroarene gas streams

Abstract

The gas phase catalytic hydrodechlorination (HDC) of mono- and di-chlorobenzenes (423K≤T≤593K) over unsupported and silica supported Mo carbide (Mo2C) is presented as a viable means of detoxifying Cl-containing gas streams for the recovery/reuse of valuable chemical feedstock. The action of Mo2C/SiO2 is compared with MoO3/SiO2 and Ni/SiO2 (an established HDC catalyst). The pre- and post-HDC catalyst samples have been characterized in terms of BET area, TG-MS, TPR, TEM, SEM, H2 chemisorption/TPD and XRD analysis. Molybdenum carbide was prepared via a two step temperature programmed synthesis where MoO3 was first subjected to a nitridation in NH3 followed by carbidization in a CH4/H2 mixture to yield a face-centred cubic (α-Mo2C) structure characterized by a platelet morphology. Pseudo-first order kinetic analysis was used to obtain chlorobenzene HDC rate constants and the associated temperature dependences yielded apparent activation energies that decreased in the order MoO3/SiO2 (80±5kJmol−1)≈MoO3 (78±8kJmol−1)>Ni/SiO2 (62±3kJmol−1)≈α-Mo2C (56±6kJmol−1)≈α-Mo2C/SiO2 (53±3kJmol−1). HDC activity was lower for the dechlorination of the dichlorobenzene reactants where steric hindrance influenced chloro-isomer reactivity. Supporting α-Mo2C on silica served to elevate HDC performance, but under identical reaction conditions, Ni/SiO2 consistently delivered a higher initial HDC activity. Nevertheless, the decline in HDC performance with time-on-stream for Ni/SiO2 was such that activity converged with that of α-Mo2C/SiO2 after three reaction cycles. A temporal loss of HDC activity (less extreme for the carbides) was observed for each catalyst that was studied and is linked to a disruption to supply of surface active hydrogen as a result of prolonged Cl/catalyst interaction.