On the reactivity of Mo species for methane partial oxidation on Mo/HMCM-22 catalysts

By characterizing fresh and used Mo/HMCM-22 catalysts with ICP-AES, XRD, NH3-TPD technique, UV - Vis DRS and UV Raman spectroscopy, the reactivity of Mo species for methane partial oxidation into formaldehyde were directly studied with a new point of view. By comparing the fresh and used catalysts, it was found that the tetrahedral Mo species bonding chemically to the support surface were practically unchanged after the reaction, while the polymolybdate octahedral Mo species, which had a rather weak interaction with the MCM-22 zeolite, leached out during the reaction, especially when the Mo loading was high. Correspondingly, it was found from the time-on-stream reaction data that the HCHO yield remained unchanged, while COx decreased with the reaction time during the reaction. By combining the characterization results and the reaction data, it can be drawn that the isolated tetrahedral molybdenum oxo-species (T-d) is responsible for HCHO formation, while the octahedral polyoxomolybdate species (O-h) will lead to the total oxidation of methane.

The role of coke in the deactivation of Mo/MCM-22 catalyst for methane dehydroaromatization with CO2

The effect of space velocity on reaction performance and coke deposition over 6Mo/MCM-22 catalyst in methane dehydro-aromatization (MDA) with CO2 were studied. The characterization of catalysts reacted at different space velocity after the same amount of methane feed by TG, TPO and Benzene/NH3-TPD techniques suggested that the inert coke maybe responsible for the deactivation of catalyst because of its blockage effect for pore system.

Promotional effect of colloidal alumina on the activity of the In/HZSM-5 catalyst for the selective reduction of NO with methane

Colloidal alumina was used to improve the activity of an In/HZSM-5 catalyst for the selective reduction of NO with CH4 in the excess of oxygen. Compared with In/HZSM-5, the In/HZSM-5/Al2O3 catalyst showed higher activity in a wide range of reaction temperatures. It is visualized that a synergetic effect between In/HZSM-5 and Al2O3 enhances the conversion of NOx. The addition of Al2O3 improved the conversion of NO to NO2 and facilitated the activation of methane. An In/HZSM-5/Al2O3 pre-treated with steam for 15 h at 700 degreesC still showed a high activity for the removal of NOx with methane, while an In/HZSM-5 similarly pre-treated with steam showed a lower activity than the fresh sample. The activity of the In/HZSM-5/Al2O3 catalyst could be restored completely after water vapor was removed from the feed gas. Furthermore, it was found that the In/HZSM-5/Al2O3 remained fairly active under high GHSV and O-2 concentration conditions. It was also interesting to find that an increase in NO content could enhance the conversion of methane, and this illustrates that the existence of NO is beneficial for the activation of methane. (C) 2002 Elsevier Science B.V. All rights reserved.

Investigation on the catalytic roles of silver species in the selective catalytic reduction of NOx with methane

Silver impregnated H-ZSM-5 zeolite catalysts with silver loading from 3 to 15 wt.% were investigated for the selective catalytic reduction (SCR) of NOx with CH4 in the excess of oxygen. X-ray diffraction (XRD) and UV-Vis measurements established the structure of silver catalysts. A relationship between the structure of silver catalysts and their catalytic functions for the SCR of NOx by CH4 was clarified. The NO conversion to N-2 showed a S-shape dependence on the increase of Ag loading. No linear dependence of catalytic activity on the amount of silver ions in the zeolite cation sites was observed. Contrastively, the activity was significantly enhanced by the nano-sized silver particles formed on the higher Ag loading samples (greater than or equal to7 wt.%). Temperature programmed desorption (TPD) and temperature programmed reduction (TPR) studies showed that nano-silver particles provided much stronger adsorption centers for active intermediates NO3-(s) on which adsorbed NO3-(s) could be effectively reduced by the activated methane. Silver ions in the zeolite cation sites might catalyze the reaction through activation of CH4 at lower temperatures. Activated CH4 reacted with NO3-(s) adsorbed on nano-silver particles to produce N-2 and CO2. (C) 2003 Elsevier B.V. All rights reserved.

Combined single-pass conversion of methane via oxidative coupling and dehydroaromatization

A new reaction mode, i.e., the combined single-pass conversion of methane via oxidative coupling (OCM) over mixed metal oxide (SLC) catalysts and dehydroaromatization (MDA) over Mo/HZSM-5 catalysts, is reported. With the assistance of an OCM reaction over SLC catalysts in the top layer of the reactor, the deactivation resistance of Mo/HZSM-5 catalysts is remarkably enhanced. Under the selected reaction conditions, the CH(4) conversion decreased from similar to18 to similar to1% and the aromatics yield decreased from 12.8 to 0.1%, respectively, after running the reaction for 960 min on both 6Mo/HZSM-5 and SLC-6Mo/HZSM-5 catalyst system without O(2) in the feed. On the other hand, for the SLC-6Mo/HZSM-5 catalyst system with O(2) in the feed, the deactivation was improved greatly, and after 960 min onstream the CH(4) conversion and aromatics yield were still as high as 12.0 and 8.0%, respectively. The promotion effect mainly appears to be associated with in situ formation of CO(2) in the OCM layer, which reacts with coke via the reverse Boudouard reaction.

Direct conversion of methane under nonoxidative conditions

Direct conversion of methane into hydrogen and valuable chemicals under nonoxidative conditions is a process severely limited thermodynamically. However, the movement from the present era of fossil fuels into the coming hydrogen energy age makes it an interesting and important approach compared with the direct conversion of methane under the aid of oxidants. This paper gives a brief overview of the direct conversion of CH4 under nonoxidative conditions. At the same time, our understanding of methane dehydroaromatization over Mo/HZSM-5 catalysts for the simultaneous formation of hydrogen and light aromatics is discussed in general, while the bifunctionality of Mo/HZSM-5 catalysts and the role of carbonaceous deposits formed during the reaction are reviewed in more detail. A perspective of the topic from both academic points of view and potential industrial applications is also presented. (C) 2003 Elsevier Science (USA). All rights reserved.

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.