Methane Dehydrogenation And Oligomerization Over Oxide Catalysts Supported On Metallic Supports

Methane is the main component of natural gas,and the H:C ratio in its molecule is the highest among all fossil energy sources.Methane is currently the cleanest fossil fuel and an important raw material for hydrogen production in industry.Dehydrogenation is the first step of methane conversion.And the methane molecules after activation and dehydrogenation can further produce high value-added chemical products.However,the current research state-of-the-art on methane activation and conversion shows that,the main problems in the process of methane dehydro-oligomerization（DHA）to higher hydrocarbons are the low product yield limited by the thermodynamic equilibrium of the reaction at lower temperature and the carbon deposition on the catalyst surface caused by the deep dehydrogenation of methane at higher temperature.At present,in order to solve these problems,most researchers focus mainly on the preparation and modification of bifunctional catalysts composed of metalic components and Si O₂-Al₂O₃ acidic molecular sieves as the support.The purpose of those literature studies is to improve the catalytic performances by modifying or adjusting the surface morphology structure and acid-base properties of the catalysts.In order to overcome the thermodynamic equilibrium limitation and promote the conversion of methane to form C₂+hydrocarbons,literature studies have proved that the restriction of thermodynamic equilibrium can be overcome through effective separation of hydrogen from the reaction system by adding hydrogen absorbent to the catalyst bed or by using a membrane reactor.Regarding to the literature results,we proposed to use reactive metals as the bulk phase of the catalyst,and then build acidic centers on its surface,wishing to carry out methane C-H bond activation and transfer hydrogen away from the catalyst surface simultaneously,in order to increase/improve the conversion of methane.In this research,the DHA of methane is used as a model reaction to study the performances of the synthesized metallic materials supported/mixed acidic catalysts in a fixed bed reactor.The main research contents and experimental results are summarized as following:（1）As the first idea of using reaction coupling to promote CH₄ conversion to higher hydrocarbons,a coupled reaction system was constructed by combining the methane DHA with CO hydrogenation reaction.The influence of reaction conditions on the coupled system were analyzed based on thermodynamic calculation.The results show that the increase of reaction temperature is beneficial to the methane dehydrogenation reaction,but inhibits the CO hydrogenation reaction.Under standard atmosphere pressure,CO shows no obvious effect on the reaction system.Only when the reaction is carried out under higher pressure,the participation of CO in the reaction becomes possible,but the rate of carbon deposition increases obviously.The main products under low temperature（<600℃）conditions are ethane,propane.When the temperature is above 600℃,the system benefits to the production of ethylene,propylene and benzene.（2）In order to investigate the performances of oxide-supported metallic catalysts for methane DHA,two catalysts of oxides supported on metalloid/metal-based supports were prepared by the hydrothermal synthesis followed by high temperature carbonization method and the dopamine hydrochloride method.The catalytic activities of the catalysts in the methane DHA reaction were investigated regarding to the process conditions of the two preparation methods.The catalysts were characterized systematically using XRD,XPS,NH₃-TPD and other instrumental analysis methods.The results show that the oxide-supported catalyst with aluminum nitrate as the precursor exhibits the best methane conversion rate and the product benzene selectivity.The catalyst is characterized as Al₂O₃ supported on Mo₂C based on XRD and SEM results,and the morphology of the catalyst particles is spherical aggregates of different sizes.According to the pyridine adsorption infrared spectroscopy and NH₃-TPD analysis results,the acidic sites of the catalyst surface are mainly weak to medium strong,and the type is mainly Lewis acid.According to the results from the catalytic activity evaluation,for the catalyst samples being passivated for a long time,a induction period of time is needed for the catalyst reactivation through re-carbonization in the reaction gas stream.The maximum methane conversion was3.2%for catalyst synthesized using hydrothermal-high temperature carbonization method,and 3.9%for samples of hydrochloric acid dopamine method.The selectivity to the C₂+hydrocarbons and benzene of both catalysts were low.The main reason for the low activities and selectivities towards the C₂+products is probably due to lacking of a shape-selective effect,the lower amount,and lower strength of the surface acid sites.（3）In order to further explore the coupling effect of the methane DHA and the hydrogen removal process in the same reaction system.Composite catalysts were prepared by mechanical mixing 3wt%Mo/HZSM-5（Si/Al=38）with different hydrogen storage alloys.Their performances in the methane DHA were investigated,and the carbon deposition on the catalysts was analyzed using TG and H₂-TPR techniques.The influence of types and mixing amounts of hydrogen storage alloys on methane conversion under different reaction temperatures was also investigated.The experimental results show that the composite catalysts containing different hydrogen storage alloys can significantly increase the conversion of methane.Under the evaluated reaction conditions,the best performances were obtained when the mass ratio of Mo/HZSM-5 to the alloys is 1:1.The optimal mass ratio at 1:1 is mainly attributed to the influence of the rate of methane dehydrogenation on the Mo/HZSM-5catalyst and the rate of hydrogen absorption into the alloy.Increasing the reaction temperature will greatly increase the conversion of methane to a level well above the equilibrium limits when the alloy is not used.In summary,the metallic Mo₂C was synthesized and used as support for the acidic oxides for methane DHA reaction.All of the synthesized oxide-supported catalysts show certain activity towards the methane dehydrogenation reaction.Composite catalysts consisting of Mo/HZSM-5 and hydrogen storage alloys were prepared and investigated to verify the effects of coupling methane DHA reaction with the hydrogen removal reaction/process.The experimental results verified the effectiveness of reaction coupling in the methane activation and conversion to C2+hydrocarbons,and also provides a new idea for the design and preparation of the acidic-metallic catalysts for methane DHA reaction.