Study on Bifunctionality of Mo/HZSM-5 Catalysts for Methane Dehydro-Aromatization under Non-oxidative Condition

The optimum Mo/[H^+] ratio per unit cell of the active precursors in Mo/HZSM-5 catalysts for methane dehydro-aromatization, measured by1H MAS NMR, was found to be about 1 when adjusting the acid sites by altering either the SiO2/Al2O3 ratios or the Mo loading. This implies that a concerted interaction between the Mo species and the Bronsted acid sites probably features the bifunctionality of the Mo/HZSM-5 catalyst. On the other hand, it was found that the driving force for Mo species to move into the HSZM-5 zeolite channels and the interaction between the Mo species and the Bronsted acid sites are closely and proportionably related with the amount of Bronsted acid sites per unit cell.

Effect of silicon carbide-based iron catalyst on reactor optimization for non-oxidative direct conversion of methane

The conversion of methane to olefins,aromatics,and hydrogen(MTOAH)can be used to stably obtain hydrocarbons when the effect of the catalytic surface is optimized from the reaction engineering perspective.In this study,Fe/Si C catalysts were packed into a quartz tube reactor.The catalytic surfaces of Si C and the impregnated Fe species decreased the apparent activation energies(E_a)of methane consumption in the blank reactor between 965 and 1020℃.Consequently,the hydrocarbon yield increased by 2.4times at 1020℃.Based on the model reactions of ethane,ethylene,and acetylene mixed with hydrogen in the range of 500-1020℃,an excess amount of Fe in the reactor favored the C-C coupling reaction over the selective hydrogenation of acetylene;consequently,coke formation was favored over the hydrogenation reaction.The gas-phase reactions and catalyst properties were optimized to increase hydrocarbon yields while reducing coke selectivity.The 0.2Fe catalyst-packed reactor(0.26 wt%Fe)resulted in a hydrocarbon yield of 7.1%and a coke selectivity of<2%when the ratio of the void space of the postcatalyst zone to the catalyst space was adjusted to be≥2.Based on these findings,the facile approach of decoupling the reaction zone between the catalyst surface and the gas-phase reaction can provide insights into catalytic reactor design,thereby facilitating the scale-up from the laboratory to the commercial scale.

Direct experimental detection of hydrogen radicals in non-oxidative methane catalytic reaction

Non-oxidative conversion of methane to olefins,aromatics and hydrogen(MTOAH) has been reported recently over metal single sites such as iron and platinum.The reaction was proposed to involve catalytic activation of methane followed by gas phase C-C coupling of methyl radicals.This study using H atom Rydberg Tagging time-of-flight technique provides direct experimental evidence for the formation of hydrogen radicals during MTOAH reaction over a catalytic quartz wall reactor containing embedded iron species(denoted as Fe-reactor).Fe-reactor gives 7.3% methane conversion at 1273 K with 41.2% selectivity toward C2(ethane,ethylene and acetylene) and 31.8% toward BTX(benzene,toluene and xylene),respectively.The enhancing effects of hydrogen radicals on overall MTOAH performance are validated by cofeeding hydrogen donor benzene,which provides an additional route of methane activation apart from catalytic activation.

Study on attrition of spherical-shaped Mo/HZSM-5 catalyst for methane dehydro-aromatization in a gas–solid fluidized bed

As a potential methane efficient conversion process,non-oxidative aromatization of methane in fluidized bed requires a catalyst with good attrition resistance,especially in the states of high temperature,longtime rapid movement and chemical reaction.Existing evaluation methods for attrition resistance,such as ASTM D5757 and Jet Cup test,are targeted for fresh catalysts at ambient temperature,which cannot well reflect the real process.In this study,spherical-shaped Mo/HZSM-5 catalyst prepared by dipping and spray drying was placed in a self-made apparatus for attrition testing,in which the catalyst attrition under different system temperatures,running time and process factors was investigated with percent mass loss(PML),particle size-mass distribution(PSMD)and scanning electron microscope(SEM).Carbon deposition on the catalyst before and after activation,aromatization and regeneration was analyzed by thermogravimetry(TG),and the attrited catalysts were evaluated for methane dehydro-aromatization(MDA).The results show that the surface abrasion and body breakage of catalyst particles occur continuously,with the increase of system temperature and running time,and make the PML rise gradually.The process factors of activation,aromatization and regeneration can cause the catalyst attrition and carbon deposits,which broaden the PSMD in varying degrees,and the carbon-substances on catalysts greatly improve their attrition resistance at high temperature.Catalyst attrition has a certain influence on its catalytic performance,and the main reasons point to particle breakage and fine powder escape.

Methane dehydro-aromatization over Mo/HZSM-5 catalysts in the absence of oxygen:Effect of steam-treatment on catalyst stability

The effect of steam-treatment to HZSM-5 zeolite and Mo/HZSM-5 with a steaming time range of 0.5-1 h on the catalytic performance of methane dehydro-aromatization （MDA） over Mo/HZSM-5 catalyst prepared with impregnation has been studied in detail in combination with the characterization of 1H MAS NMR technique. Both the deactivation rate constant （kd） and the Brtnsted acid sites per unit cell were calculated to quantitatively evaluate the stability of Mo/HZSM-5 catalysts treated with steam at 813 K before and after impregnation of molybdenum species, and the corresponding variation of their Brtnsted acid sites. The results reveal that a V-shape relationship between kd and the number of B 1 acid sites per unit cell is presented on Mo/HZSM-5 catalyst under the tested steam-treatment and reaction conditions.

Intrinsic kinetics of methane aromatization under non-oxidative conditions over modified Mo/HZSM-5 catalysts

The intrinsic reaction kinetics of methane aromatization under non-oxidative conditions over modified Mo/HZSM-5 catalysts was studied in the quartz pipe-reactor under ordinary pressure with the temperature ranging from 913.15 to 973.15 K and the space velocity from 700 to 2100 ml/（g·h）. The Langmuir-Hinshelwood model was chosen to describe the intrinsic kinetics while Levenberg-Marquardt method was selected to determine the parameters in the kinetic model. Statistical test and residual error distribution diagrams showed that experimental data were in good agreement with calculated data, and Langmuir-Hinshelwood model was suitable for the description of the intrinsic kinetics of methane aromatization under the reaction conditions discussed in this article.

Hydrogen Effect on Coke Removal and Catalytic Performance in Pre-Carburization and Methane Dehydro-Aromatization Reaction on Mo/HZSM-5

In this study, the effects of pre-carburization of catalyst, hydrogen addition to methane feed and the space velocity of methane on the catalytic performance in methane to benzene （MTB） reaction were discussed in detail over Mo/HZSM-5 catalyst at 1023 K and 0.3 MPa. Compared with the non-precarburized catalyst, the Mo catalyst pre-carburized under the flow of CHa＋4H2 at 973 K was found to have the higher activity and better stability. Further 6% H2 addition to the methane feed suppressed the aromatic type of coke formation effectively, and improved the stability of catalyst markedly, moreover gave a much longer reaction life of catalyst （53 h at 1023 K and 5400 ml/（g-h）） and much more formation amounts of benzene and hydrogen. With increase of methane space velocity, both the naphthalene formation selectivity and the coke formation selectivity were decreased by the shortened contact time; the benzene formation selectivity and total formation amount before the complete deactivation of catalyst were increased markedly, while the total naphthalene and coke formation amounts did not change much. At high methane space velocity （≥5400 ml/（g·h））, a new middle temperature coke derived from the high temperature aromatic coke was formed on the catalyst; all the coke formed could be burnt off at lower temperature in oxygen, compared with those obtained at low space velocity. Considering the benzene formation amount and catalyst stability together, 5400 ml/（g·h） was proved to be the most efficient methane space velocity for benzene production.

Energy-Efficient coaromatization of methane and propane

Development of highly effective catalysts for one-stage conversion of methane with high selectivity to valuable products and energy efficiency will provide an efficient way to utilize natural gas and oil-associated gases and to protect environment. In recent years, there have been many efforts on direct catalytic transformations of methane into higher hydrocarbons by feeding additives together with methane under non-oxidative conditions. This paper reviewed the advances in recent research on non-oxidative aromatization of methane in the presence of propane over different modified HZSM-5 catalysts. The thermodynamic consideration, the isotope verification and the mechanism of the activation of methane in the presence of propane are discussed in the paper in detail.

Oxygen vacancy self-doped single crystal-like TiO_(2) nanotube arrays for efficient light-driven methane non-oxidative coupling

Photocatalytic non-oxidative coupling of methane(PNOCM)is a mild and cost-effective method for the production of multicarbon compounds.However,the separation of photogenerated charges and activation of methane(CH4)are the main challenges for this reaction.Here,single crystal-like TiO_(2) nanotubes(VO-p-TNTs)with oxygen vacancies(VO)and preferential orientation were prepared and applied to PNOCM.The results demonstrate that the significantly enhanced photocatalytic performance is mainly related to the strong synergistic effect between preferential orientation and VO.The preferential orientation of VO-p-TNT along the[001]direction reduces the formation of complex centers at grain boundaries as the form of interfacial states and potential barriers,which improves the separation and transport of photogenerated carriers.Meanwhile,VO provides abundant coordination unsaturated sites for CH4 chemisorption and also acts as electron traps to hinder the recombination of electrons and holes,establishing an effective electron transfer channel between the adsorbed CH4 molecule and photocatalyst,thus weakening the C–H bond.In addition,the introduction of VO broadens the light absorption range.As a result,VO-p-TNT exhibits excellent PNOCM performance and provides new insights into catalyst design for CH4 conversion.

Non-oxidative dehydro-oligomerization of methane to higher molecular weight hydrocarbons at low temperatures

The non-oxidative dehydro-oligomerization of methane to higher molecular weight hydrocarbons such as aroma tics and C2 hydrocarbons in a low temperature range of 773-973 K with Mo/HZSM-5,Mo-Zr/HZSM-5 and Mo-W/HZSM-5 catalysts is studied.The means for enhancing the activity and stability of the Mo-containing catalysts under the reaction conditions is reported.Quite a stable methane conversion rate of over 10% with a high selectivity to the higher hydrocarbons has been obtained at a temperature of 973 K.Pure methane conversions of about 5.2% and 2.0% have been obtained at 923 and 873 K,respectively.In addition,accompanied by the C2-C3 mixture,tht- methane reaction can be initiated even at a lower temperature and the conversion rate of methane is enhanced by the presence of tne initiator of C2-C3 hydrocarbons.Compared with methane oxidative coupling to ethylene,the novel way for methane transformation is significant and reasonable for its lower reaction temperatures and high selectivity to the desired products.