CH4 dehydroaromatization on Mo/H–ZSM-5: 1. Effects of co-processing H2 and CH3COOH

The distribution of coke formed over a multilayer Mo/HZSM-5 fixed bed in H2 co-fed methane aromatization at 1073 K: Exploration of the coking pathway

“…This is about one order of magnitude higher than the rate obtained by Iglesia and co-workers for their Mo/HZSM-5 catalysts (Mo/Al f = 0.4) at a lower temperature of 950 K [14]. If the CH 4 conversion rate depended only on the Mo sites of the catalyst and increased with temperature according to the Arrhenius equation, the rate recorded over the present catalyst at 1073 K should be three orders of magnitude higher than those on the catalysts used in the previous studies [14,24], but it is not. One possible reason may be that most of the Mo atoms in our microzeolite-based catalyst were dispersed on the Brønsted acid sites deep inside the zeolite channels, so that their accessibility to CH 4 in the reaction was inhibited to make them catalytically useless.…”

“…According to Iglesia and co-workers [47,48] and Bhan and co-workers [24,29], both steps are reversible and inhibited by increased H 2 partial pressures. However, as one can derive from the equations, 2 CH 4 M C 2 H 4 + 2 H 2 and C 2 H 4 M 1/3 C 6 H 6 + 1/2 H 2 , the dependences of the two steps on H 2 partial pressures are respectively represented by the terms 1/p 2 and p 0.5 , and are not identical.…”

“…First the backward rates of all reversible coke formation reactions, such as oligomerization of the intermediate C 2 H 4 and polycondensation of formed aromatics, will be promoted. Very recently, Bhan and co-workers [24,29] have shown ''that dehydroaromatization of CH 4 is governed by thermodynamic reversibility so that an increase in H 2 partial pressure causes a decrease in the net benzene formation rate but does not affect the forward synthesis rate.'' Second, the rate of the coke-removing reaction 2H 2 + C = CH 4 will be increased.…”

“…However, it suffers rapid deactivation due to serious coke formation under practically required severe conditions [5][6][7][8]. Various approaches have attempted to tackle this problem, which include promotion of mass transfer by using zeolites with mesoporous/microporous hierarchical structures [9,10], optimization of the Si/Al ratios of the zeolites [11][12][13], silane modification of the zeolite surfaces [14,15], modification of the active metal composition [16][17][18], and addition of CO 2 , CO, or H 2 into the CH 4 feed [19][20][21][22][23][24]. However, no satisfactory solution has been found as yet.…”

“…While no approach capable of providing direct evidence for distinguishing these proposed coking routes has been reported yet, quantifying the suppressive effect of H 2 on coke formation via each route might provide insight into the main coke-formation controlling factor. In fact, adding H 2 into CH 4 feed has been experimentally demonstrated to be very effective in suppressing overall coke formation over Mo/HZSM-5 catalysts, and consequently stabilizing their activity [21][22][23][24]. Thus, the nascent hydrogen formed in the inlet layer of an integral catalyst bed is expected to function to suppress coke formation in the rest of the bed [27][28][29], particularly in its outlet layer, where the H 2 concentration in the gas phase always reaches its highest level.…”