Improved benzene production from methane dehydroaromatization over Mo/HZSM-5 catalysts via hydrogen-permselective palladium membrane reactors

Abstract

The effectiveness of hydrogen-permselective palladium membrane reactors for non-oxidative methane dehydroaromatization (MDA) over 4 wt% Mo/HZSM-5 catalysts was investigated as a function of weight hourly space velocity (WHSV) at 700 °C and atmospheric pressure. CH4 conversion and aromatic product yield decrease with increasing WHSV from 750 to 9000 cm 3 gcat -1 h -1 . C6H6 is the main C-containing product at and below 3000 cm 3 gcat -1 h -1 whereas C2H4 dominates the C-product distribution at higher WHSVs. Due to selective removal of H2 from the reaction products in catalytic membrane reactors, C6H6 yield is significantly improved over the whole WHSV range compared to those obtained in fixed-bed reactors. H2 recovery is strongly influenced by WHSV as it decreases from 48.3% at 750 cm 3 gcat -1 h -1 to 6.8% at 9000 cm 3 gcat -1 h -1 . There exists a trade-off between catalytic activity and H2 recovery, which results in the maximum enhancement (~360%) in C6H6 yield at 3000 cm 3 gcat -1 h -1 . At this intermediate space velocity, the largest concentration of H2 is found in the retentate stream and helps alleviate coke accumulation particularly on HZSM-5. Carbon is deposited on the inner surface of the membrane reactor portion in contact with the catalyst bed and transports to the outer surface, thus causing H2 permeability to decrease over the 15-h reaction period.Palladium membrane reactors significantly improve C 6 H 6 yield from CH 4 dehydroaromatization on Mo/HZSM-5 catalysts over a wide range of space velocity.

In Situ Formation of Metal Carbide Catalysts

“…The catalyst formed by direct carburization showed low activity throughout the reaction profile, but did not deactivate as quickly as the other catalysts. Most commonly, MoO 3 was reduced to MoO 2 followed by carburization via CH 4 to form MoC x species, which varied depending on reaction conditions . Iron has also been used for methane dehydroaromatization, and its incorporation into a silica matrix allowed for activation of methane and formation of methyl radicals that increased the selectivity towards ethylene.…”

“…Most commonly,M oO 3 was reduced to MoO 2 followed by carburization via CH 4 to form MoC x species,w hich varied depending on reaction conditions. [31] Iron has also been used for methane dehydroaromatization, and its incorporation into as ilica matrix allowed for activation of methanea nd formation of methyl radicals that increased the selectivity towards ethylene. Additionally, this catalyst wasa ble to convert up to 48 %o ft he methane reactant gas with ah ydrocarbon selectivity (benzene, ethylene and naphthalene) of 99 %a t1 363K and 21 400 space velocity mL g À1 h À1 .H owever,m ost of the methanew as converted to ethylene or naphthalene, with benzene yields remaining fairly low.…”

“…As a result, much of the research in this area has been focused on the catalyst and finding ways to limit coke formation while increasing benzene selectivity. Attempts have been made to improve methane conversion by incorporating membranes that remove hydrogen as it is produced, but these reactions still suffer from high naphthalene formation rates and coking . Many different transition metals have been studied for MDA including W, Re, Ga and Co, however; the best performing metal is Mo, and it is most commonly studied for this chemistry .…”

Modeling and Design Optimization of Multifunctional Membrane Reactors for Direct Methane Aromatization

“…Membrane reactors have emerged as a potential solution to these reaction challenges. There have been many efforts involving hydrogen-selective membranes used to overcome the equilibrium barriers, leading to higher methane conversions [ 26 , 27 , 28 ]. However, it has been proven difficult to replicate predictive models in experimental work due to the lack of sufficient hydrogen permeation fluxes through the membrane and of sufficient catalyst resistances to coking, which has been accelerated due to hydrogen removal [ 28 , 29 ].…”

Modeling and Design Optimization of Multifunctional Membrane Reactors for Direct Methane Aromatization

“…Membrane reactors have emerged as a potential solution to these reaction challenges. There have been many efforts involving hydrogen-selective membranes used to overcome the equilibrium barriers, leading to higher methane conversions [15,21,24]. However, it has been proven difficult to replicate predictive models in experimental work due to the lack of sufficient hydrogen permeation fluxes through the membrane and of sufficient catalyst resistances to coking, which has been accelerated due to hydrogen removal [11,24].…”