Catalysts For Methane Dehydroaromatization Research Articles

Effect of thermal treatment on states of molybdenum in Mo/H?ZSM-5 catalyst for methane dehydroaromatization: ESR and UV?VIS study

Valence and coordination states of molybdenum ions formed upon thermal treatment of Mo/H–ZSM-5 catalyst for methane dehydroaromatization in Ar and Ar/CH4 media at 573–973 K have been studied by ESR and UV–VIS spectroscopy. For comparison, the characteristic ESR spectra of thermolyzed bulk ammonium heptamolybdate have been studied and analyzed in detail. The nature of earlier observed Mo5+ ions has been verified, and new paramagnetic states of molybdenum in Mo/H–ZSM-5 catalysts have been detected: Mo3+ ions, and Mo5+ ions in tetrahedral coordination with delocalization of unpaired electron to Al and H or Al and N atoms.

Effects of Rhodium Addition to Mo/HZSM-5 Catalyst for Methane Dehydroaromatization

Abstract Rhodium added 6 wt % Mo/HZSM-5 catalyst (Rh/Mo = 0.2) exhibited highly stable and active performances in the methane dehydroaromatization reaction at 1023 K under 0.3 MPa with 2700 mL g−1 h−1 of the methane feed with 6% hydrogen addition due to the effective suppression of the coke deposition on the Brønsted acid sites of HZSM-5.

Post-steam-treatment of Mo/HZSM-5 Catalysts: An Alternative and Effective Approach for Enhancing Their Catalytic Performances of Methane Dehydroaromatization

Post-steam-treatment is a facile and effective method for improving the catalytic performances of mo/hzsm-5 catalysts in methane dehydroaromatization under nonoxidative conditions. the treatment can enhance the stability of the catalyst and also give a higher methane conversion and a higher yield of light aromatics, as well as a decrease in the formation rate of carbonaceous deposits. (27)al, (29)si, and (1)h multinuclear magic angle spinning nuclear magnetic resonance, x-ray photoelectron spectroscopy, x-ray diffraction, x-ray fluorescence spectroscopy, and thermogravimetric analysis measurements as well as catalytic reaction evaluations were employed to conduct comparative studies on the properties of the catalysts before and after the post-steam-treatment. the results revealed that the number of free bronsted acid sites per unit cell decreased, while more mo species migrated into the hzsm-5 channels for the 6mo/hzsm-5 catalysts after the post-steam-treatment. in addition, the average pore diameter was also larger for the post-steam-treated catalysts, and this was advantageous for mass transport of the reaction products. however, a severe post-steam-treatment, i.e., with longer treating time, of the 6mo/hzsm-5 catalyst will lead to the formation of the al(2)(moo(4))(3) phases, which is detrimental to the reaction.

03/01627 Chemical characteristics of coke on Mo/HZSM-5 catalysts for methane dehydroaromatization under nonoxidative conditions by TP techniques: Liu, H.-M. et al. Gaodeng Xuexiao Huaxue Xuehao, 2002, 23, (8), 1556–1561. (In Chinese)

Over Mo/HZSM 5 catalysts, various carbonaceous depositions occurred during the course of methane dehydroaromatization(MDA). These species of surface carbon formed on HZSM 5 zeolite or Mo/HZSM 5 catalysts with different Mo loadings were investigated by means of TPH, TPCO 2 and TPO in combination with thermal gravimetric analysis(TG). The results of TG revealed that, the total amount of coke formed on 6Mo/MCM 22 during TPSR is larger than those formed on HZSM 5 zeolite, 2Mo/HZSM 5 and 10Mo/HZSM 5 catalysts. The TPO profiles recorded during the procedure of coke burning off show two different temperature peaks. The results of TPO and TG after TPH suggested that, the TPH experiments only had an effect on the coke burnt off at a high temperature, but didn′t result in the diminishing of the coke burnt off at low temperature peak. On the other hand, the result of the coked catalyst after succeeding TPCO 2 experiments exhibited an obvious reduction in the areas of both the high and low temperature peaks, particularly in the area of the low temperature peak. The study on the catalysts with different Mo loadings suggested that, the percentage of the coke burnt off at a low temperature turned larger with the increase of Mo loading, respectively. And the change trend of the coke burnt off at a high temperature was opposite exactly. By using the experiments of TPSR, TPH, TPCO 2 and TG, quantitative analysis of the coke and the kinetics of its burning off process have been done. The results show that, either TPH or TPCO 2 can diminish the starting temperature and decrease the activation energy, and there are some differences among the catalysts with various Mo loadings.

W/HZSM-5 catalyst for methane dehydroaromatization: a multinuclear MAS NMR study

Multinuclear solid-state NMR techniques were employed to investigate the interaction between tungsten species and zeolite in W/HZSM-5 catalysts prepared by controlled impregnation. Al-27, Si-29 NMR experiments suggest that introduction of W species leads to the formation of distorted four-coordinate framework and six-coordinate extra-framework aluminum species due to the interaction of the W species and framework aluminum, and the former can be further transformed into the latter, especially at high W loadings. H-1 MAS NMR indicates that some of the W species disperse on the external surface and some of them diffuse into the internal channels of the zeolite, which causes a decrease of the Bronsted acid sites and silanols as well as an increase of the extra-framework AI-OH species in the W/HZSM-5 catalysts. Cooperation of the W species and the remaining Bronsted acid sites is responsible for the activity of the W/HZAM-5 catalysts for methane dehydroaromatization. As revealed by C-13 NMR experiments, an induction period exists for methane dehydroaromatization reaction and W6+ oxide on/in the zeolite is likely to be converted into W4+ oxide during the reaction. It is W4+ oxide rather than carbide tungsten that acts as active metal sites for initial activation of methane on the W/HZSM-5 catalysts. (C) 2003 Elsevier Science B.V. All rights reserved.

Study of the carbonaceous deposits formed on a Mo/HZSM-5 catalyst in methane dehydro-aromatization by using TG and temperature-programmed techniques

Carbonaceous deposits formed during the temperature-programmed surface reaction (TPSR) of methane dehydro-aromatization (MDA) over Mo/HZSM-5 catalysts have been investigated by TPH, TPCO 2 and TPO, in combination with thermal gravimetric analysis (TG). The TPO profiles of the coked catalyst after TPSR of MDA show two temperature peaks: one is at about 776 K and the other at about 865 K. The succeeding TPH experiments only resulted in the diminishing of the area of the high-temperature peak, and had no effect on the area of the low-temperature peak. On the other hand, the TPO profiles of the coked catalyst after succeeding TPCO 2 experiments exhibited obvious reduction in the areas of both the high- and low-temperature peaks, particularly in the area of the low-temperature peak. On the basis of TPSR, TPR and TPCO 2 experiments and the corresponding TG analysis, quantitative analysis of the coke and the kinetics of its burning-off process have been studied.

Mo/HMCM-22 Catalysts for Methane Dehydroaromatization: A Multinuclear MAS NMR Study

Detailed NMR investigations on fresh and coked Mo/HMCM-22 catalysts for methane dehydro-aromatization were presented. 27Al MAS and 29Si MAS as well as corresponding CP/MAS NMR experiments have proved that an interaction between molybdenum and the zeolite lattice occurs during the impregnation and calcination processes of the catalysts. With Bronsted acid sites serving as a powerful trap, molybdenum migrates into the internal channels of the zeolite and reacts preferentially with bridging hydroxyls groups. Thus, molybdenum is anchored to the framework aluminum through an oxygen bridge, which, in turn, modifies the acidic properties of the HMCM-22 and leads to a high dispersion of the molybdenum. If this kind of interaction becomes stronger, expelling of aluminum from the zeolite lattice would occur, with the forming of both octahedral nonframework aluminum and Al2(MoO4)3 crystallites. The latter can be hydrated to [Al(OH)n(H2O)6-n]n(MoO4) (n = 1 or 2) in a water-saturated desiccator and can give a line at ...