A clue to exploration of the pathway of coke formation on Mo/HZSM-5 catalyst in the non-oxidative methane dehydroaromatization at 1073K

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

“…In terms of CH 4 conversion as well as outlet H 2 concentration, the catalyst in Ar/CH 4 shows a two-stage activity-decreasing feature: rapid decrease over the first 5 min followed by slow decrease thereafter. Not surprisingly, this deactivation pattern duplicated our previous work with similar Mo/HZSM-5 catalysts [19,26], and confirmed that a 5 min activation at 1073 K is needed for our precarburized catalyst to attain its maximum benzene formation activity. In this activation period, pyrolysis of CH 4 on the H 2 -reduced Mo surface and carburization of the residual MoO 3 to form active Mo 2 C are believed to dominate the coking process.…”

“…However, to our best knowledge, no other study than one of ours tended to reveal the dominant pathway [26]. 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.…”

“…On the other hand, the aim of the second H 2 treatment in the heating from 923 to 1073 K was to remove most of the coke formed during the carburization and ensure that the starting catalyst sample in every experiment has an identical low coke content, i.e., an identical initial activity. In this way, experimental errors were minimized and the standard deviation of the error between repeated activity evaluation tests in terms of CH 4 conversion (at 10 and 35 min) was confirmed to be within ±3% [26].…”

“…In a previous paper, we have demonstrated that a nonuniform coke distribution forms along a three-layer fixed bed of Mo/HZSM-5 in 10%Ar/CH 4 at 1073 K [26]. In the same study, by pursuing both benzene and C 2 H 4 selectivity over the catalyst lifetime and simultaneously quantifying the coke accumulated in the different layers of the bed over different reaction periods, we have also successfully revealed that the reaction intermediate C 2 H 4 is the dominant source of coke formation at the last stage of catalyst deactivation, where benzene formation selectivity shows a rapid decrease from about 65% to 18%.…”

Non-oxidative dehydroaromatization of methane: an effective reaction–regeneration cyclic operation for catalyst life extension

“…In two recent papers, Zhang et al 32,33 have suggested that although polycondensation of aromatics will be the main contribution to coking of the external surface in the first stages of the reaction, oligomerization and/or cracking of ethene, the main product at longer TOS when benzene formation is significantly reduced, will be responsible for the increasing coke formation rate.…”