Non-oxidative methane dehydroaromatization reaction over highly active $${\upalpha }$$ α - $$\hbox {MoC}_{1{-}\mathrm{x} }$$ MoC 1 - x ZSM-5 derived from pretreatment

Abstract

The catalytic active-phase of reduced Mo species plays a vital role in non-oxidative methane dehydroaromatization (MDA) reaction. Pretreatment effect of one of the gases containing $$\hbox {N}_{2}$$ , $$\hbox {H}_{2}$$ and (90 vol%) $$\hbox {CH}_{4}+\hbox {H}_{2}$$ over 15% Mo-loaded HZSM-5 catalyst has been investigated in the present work. Various spectroscopic investigations viz., XRD, TPR, TPO, XPS, etc., show that the pretreatment of 15% Mo-HZSM-5 catalyst with (90 vol%) $$\hbox {CH}_{4}+\hbox {H}_{2}$$ gas stream exclusively leads to the formation of $$\hbox {MoO}_{\mathrm{x}}\hbox {C}_{\mathrm{y}}\hbox {H}_{\mathrm{z}} $$ which acts as precursor moieties for the formation of highly active metastable fcc ( $${\upalpha }$$ - $$\hbox {MoC}_{1{-}\mathrm{X}})$$ and $$\hbox {MoO}_{\mathrm{x}}\hbox {C}_{\mathrm{y}}$$ phases during the induction period. Comparatively, $$\hbox {H}_{2}$$ and $$\hbox {N}_{2}$$ -pretreated catalysts showed major formation of hcp ( $$\upbeta $$ - $$\hbox {Mo}_{2}\hbox {C}$$ ) species that are found to be a less-active phase in MDA reaction. The active fcc ( $${\upalpha }$$ - $$\hbox {MoC}_{1{-}\mathrm{X}})$$ phases are immune to inert coking and assist primary ethylene products formed on carbonized Mo associated Brønsted acid sites to travel in the zeolite channels which is further aromatized over Brønsted acid sites deep inside the channels. XPS analysis of the catalyst shows that $${\upalpha }$$ - $$\hbox {MoC}_{1{-}\mathrm{x}}$$ and $$\upbeta $$ - $$\hbox {Mo}_{2}\hbox {C}$$ are major catalytic phases that are covered with graphitic carbon and amorphous carbon present on the surface. The active phases $${\upalpha }$$ - $$\hbox {MoC}_{1{-}\mathrm{x}}$$ and $$\hbox {MoO}_{\mathrm{x}}\hbox {C}_{\mathrm{y}}$$ , associated with Brønsted acid sites along with the vacant Brønsted acid sites in catalyst pretreated with (90 vol%) $$\hbox {CH}_{4}+\hbox {H}_{2}$$ mixture are responsible for high activity in methane conversion ( $$\sim $$ 13%), excellent aromatic selectivity (38%), and high stability of the catalyst. The catalytically active $${\upalpha }$$ -MoC $$_{1{-}}$$ x phase over 15% Mo-HZSM-5-A is more immune to coking than $$\upbeta $$ - $$\hbox {Mo}_{2}\hbox {C}$$ phase over 15% Mo-HZSM-5-B and 15% Mo-HZSM-5-C. The active phases, $${\upalpha }$$ - $$\hbox {MoC}_{1{-}\mathrm{x}} $$ and $$\hbox {MoO}_{\mathrm{x}}\hbox {C}_{\mathrm{y}}$$ , associated with Brønsted acid sites in 15% Mo-HZSM-5-A is responsible for high activity in methane conversion ( $$\sim $$ 13%), excellent aromatic selectivity (38%), and high stability of the catalyst.