Yuying Shu

Methane dehydro‐aromatization over Mo/MCM‐22 catalysts: a highly selective catalyst for the formation of benzene

A molybdenum‐modified MCM‐22 catalyst has been used for methane dehydro‐aromatization. The catalytic performance on this Mo/MCM‐22 catalyst is featured by a higher yield of benzene and a lesser yield of naphthalene in comparison with that on a Mo/HZSM‐5 catalyst under the same experimental conditions. Methane conversion of 10.0% and benzene selectivity of 80% over a 6Mo/MCM‐22 catalyst at 973 K was obtained. Based on the effect of contact time, it is suggested that the reaction is severely inhibited by the products, probably due to their strong adsorption and slow desorption. The Mo/MCM‐22 catalysts were characterized by XRD, NH3‐TPD and TPSR techniques. XRD patterns of the Mo/MCM‐22 catalysts confirmed that Mo species are highly dispersed on/in the MCM‐22 zeolite if the Mo loading is less than 10%. NH3‐TPD experiment shows that the MCM‐22 zeolite contains strong and exchangeable Brønsted acid sites. TPSR of methane revealed that there is an induction period during which the Mo species are reduced by methane and transformed probably into Mo2C or Mo2OxCy. It is concluded that the nature of the methane dehydro‐aromatization reaction over the Mo/MCM‐22 catalysts is similar to that on the Mo/HZSM‐5 catalysts. The unique pore systems, the proper acid strength of the MCM‐22 zeolite and the Mo species are factors important for methane dehydro‐aromatization over Mo/MCM‐22 catalysts.

MAS NMR, ESR and TPD studies of Mo/HZSM-5 catalysts: evidence for the migration of molybdenum species into the zeolitic channels

NH3‐TPD, MAS NMR and ESR spectroscopies were employed to investigate Mo‐modified HZSM‐5 catalysts prepared by impregnation. It was found that the modification of Mo ions results in a pronounced decrease in the intensity of 1H MAS NMR resonance originating from Brønsted acid sites in the zeolites and a distinct splitting of Mo5+ ESR signals, which is attributed to the interaction of Mo with the Al atom of the zeolite framework. This presents distinct evidence that Mo ions migrate from the external surface of the zeolite into the lattice channels during the impregnation and subsequent treatment. The remaining Brønsted acid sites associated with the migrated Mo ions form the bifunctional catalytic centers that may be responsible for the outstanding catalytic performance in methane aromatization.

Interaction between ammonium heptamolybdate and NH4ZSM-5 zeolite: the location of Mo species and the acidity of Mo/HZSM-5

Mo/HZSM-5 catalysts show high reactivity and selectivity in the activation of methane without using oxidants. Mo/HZSM-5 catalysts with Mo loading ranging from 0 to 10% were prepared by impregnation with an aqueous solution of ammonium heptamolybdate (AHM). The samples were dried at 393 K, and then calcined at different temperatures for 4 h. The interaction between Mo species and NH4ZSM-5 zeolite was characterized by FT-IR spectroscopy, differential thermal analysis (DTA) and temperature programmed decomposition (TPDE) and NH3-TPD at different stages of catalyst preparation. The results showed that if Mo/HZSM-5 catalysts were calcined at a proper temperature, the Mo species will interact with acid sites (mainly with BrØnsted acid sites) and part of the Mo species will move into the channel. The Mo species in the form of small MoO3 crystallites residing on the external surface and/or in the channel, and interacting with BrØnsted acid sites may be responsible for the methane activation. Strong interaction between Mo species and the skeleton of HZSM-5 will occur if the catalyst is calcined at 973 K. This may lead to the formation of MoO42− species, which is detrimental to methane activation.

Methane dehydro‐aromatization over a Mo/phosphoric rare earth‐containing penta‐sil type zeolite in the absence of oxygen

The dehydro‐aromatization of methane over a Mo‐modified penta‐sil type high‐silica zeolite containing phosphoric and rare earth oxide (abbreviated as Mo/HZRP‐1) was investigated. As a modification of HZSM‐5, HZRP‐1 is also a good support for the preparation of Mo‐based zeolite catalysts, and is active for methane dehydro‐aromatization. Mo/HZRP‐1 catalysts are more active at high Mo loadings compared with Mo/HZSM‐5 catalysts. 27Al MAS NMR spectra of Mo/HZRP‐1 reveal that there are two kinds of framework Al in HZRP‐1. It is suggested that only the tetrahedral coordinated Al atoms in the form of Al–O–Si species in the zeolite, in the proton forms, are responsible for the formation of aromatics.

Synthesis and characterization of galloaluminosilicate/gallosilicalite (MFI) and their evaluation in methane dehydro-aromatization

Abstract MFI-type gallosilicalite and galloaluminosilicate were synthesized by hydro-thermal method. For all the as-synthesized Ga-containing samples, 71 Ga MAS NMR spectra confirmed that the Ga 3+ cations are located in the zeolite framework. As for the case in galloaluminosilicate, Ga 3+ and Al 3+ cations can enter the zeolitic site at the same time, but there is a competition between the two cations to incorporate into the zeolite framework. Quantitative MAS NMR results suggest that the incorporation of the Ga 3+ cations is seriously inhibited by that of the Al 3+ cations if they existed together, while the reversed process is not observed. The catalytic performances of non-oxidative aromatization of methane on these zeolite catalysts, with or without loading of MoO 3 , have been investigated. The results show that zeolites containing framework Ga species, which are excellent catalysts for propane aromatization, are poor catalyst supports for methane dehydro-aromatization. The catalytic performance of the molybdenum-loaded H-gallosilicate (MFI) catalysts present an activity for methane activation and subsequent aromatization, however, it is not as good as that of the Mo-loaded H-galloaluminosilicate (MFI) catalysts. Bearing this in mind, it was suggested that zeolitic acidity that originated from the heteroatom substitution is essential for catalyzing methane aromatization. The weaker acidity of framework GaO 4 − tetrahedral species as compared to framework AlO 4 − tetrahedral species is suggested to be responsible for the inferior activity for methane aromatization reaction.