Study On Catalysts For Dehydrogenation Of Low-carbon Alkanes To High Value-added Chemicals

Low-carbon alkanes（C1-C4）are the hydrocarbon components with the highest H content.It is a promising technical route for the processing and utilization of low-carbon alkanes to dehydrogenate them into olefins,aromatics and other chemicals,and produce a part of hydrogen.Starting from the conversion of low-carbon alkanes,this paper studies the related problems of methane dry reforming,methane carbon dioxide conversion of aromatics,liquefied petroleum gas（LPG）dehydrogenation to olefins,LPG dehydrogenation to aromatics.The synergistic effect of Ni-Fe bimetallic and the introduction of active lattice oxygen carrier calcium aluminate(C12A7-O^2-,the composite carrier containing MCM-41 and calcium aluminate is abbreviated as MC12A7)into MCM-41 carrier were studied.The regulation of methane dry reforming reaction and the effect of alleviating carbon deposition on the catalyst were studied,and the mechanism of water on the reaction was deeply analyzed.The interaction of CH₄and CO₂ with active species on the catalyst surface was studied by isotope ¹³CH₄ tracer technique.The detection of¹³CO and ¹²CO by mass spectrometry showed that CO mainly came from CHx oxidation reaction and a small amount from CO₂ dissociation.The Ni-Fe/MC12A7catalyst has a lower reaction temperature starting point due to the presence of active lattice oxygen（XPS proof）.The monitoring of the reaction by temperature programmed mass spectrometry（TPSR-MS）proves that a certain amount of H₂O is generated in the initial stage of the reforming reaction（mainly from the reaction of CH₄ with lattice oxygen）.In further work,the effect of H₂O on the MC12A7 catalyst system in the methane reforming reaction was investigated by mass spectrometry.It was found that when a small amount of water entered the reforming reaction system,it could accelerate the adsorption of CO₂on the MC12A7 carrier and promote the dissociation of methane.The H₂ signal appeared later than CO,indicating that the oxidation of CH_x occurred after the dissociation of CO₂.The reaction path of water in the reaction of methane and carbon dioxide was studied by isotope D₂O tracer technique.The experiment showed that DH in the product came from the reaction of CH_x and D₂O to produce CO and DH,which provided direct evidence for water to promote methane conversion.This work found that another role of H₂O is to provide more paths to eliminate carbon deposition.The dissociation reaction of H₂O will inhibit the formation of C*,and H₂O will consume Cαon the catalyst in time during the reaction.When there is no water in the reaction system,the main reaction of carbon removal is C*+O*→CO*.Therefore,another role of water is to change the main path of C*conversion and increase the path of eliminating carbon deposition.Methane and carbon dioxide are thermodynamically and chemically stable molecules,which are difficult to be directly converted into aromatic hydrocarbons.Therefore,in the conversion of methane and carbon dioxide to aromatics,the dry reforming of methane and the conversion of syngas to aromatics（DRM-STA）were studied by using a series reactor,and the conversion of methane and carbon dioxide to aromatics was explored.Through ¹³CH₄ isotope labeling experiments,it is proved that methane enters aromatic hydrocarbons.Combined with online mass spectrometry analysis of product distribution,it is found that zinc ferrite without strong acid center can perform C-O bond activation,C-C coupling and aromatization without zeolite.DFT method was used to calculate that hydrogen pretreatment of zinc ferrite catalyst would produce more oxygen vacancies.XPS experimental characterization showed that the oxygen vacancy concentration of zinc ferrite catalyst after hydrogen treatment for 2 h at773 K was 41.49%.Due to oxygen vacancies,the C-O bond in CO is activated and CHx is formed in the hydrogen atmosphere,and the C-C bond is coupled to form aromatics on the ZnFeO_x catalyst.It is different from the concept of CO hydrogenation to aromatics in the bifunctional catalyst of metal oxide and zeolite（OX-ZEO）established in the existing knowledge system.The metal oxide activates CO to form intermediates,and the intermediates further migrate to the molecular sieve to achieve C-C bond coupling.The reason why the role of ZnFeOx catalyst used in this paper is different from that of OX-ZEO bifunctional catalyst is that the tandem reaction changes the product after dry reforming of methane to the subsequent syngas to aromatics reaction path,realizing the production of aromatics without molecular sieve.We also found that when CO/CO₂/H₂ was fed,the reaction did not produce aromatics in a single reactor.When CH₄ entered the reactor,a strong signal of m/z=84 appeared in the mass spectrum.This indicates that CH₄ or CH_x will change the FTS pathway and break the Anderson-Schulz-Flory（ASF）product distribution model.At the same time,the reaction products were analyzed by mass spectrometry in a single reactor with CO₂/CH₄as raw materials.It was found that the abundance of m/z=12 was greater than m/z=13,which provided conversion conditions for the subsequent aromatization reaction.The main components of LPG are propane and butane.Thermodynamic analysis of propane dehydrogenation reaction performance,propane dehydrogenation reaction is a strong endothermic and volume increase reaction,high temperature and low pressure is conducive to propane dehydrogenation forward,in order to improve the yield of propylene,propane direct dehydrogenation reaction temperature needs to be higher than823 K.At the same time,considering the effect of adding methanol on propane dehydrogenation,the functional relationship between temperature,pressure,molar ratio of propane to methanol and the molar fraction of propylene in the product was established by response surface analysis,and the optimal process range was analyzed by regression equation.Thermodynamics shows that with the increase of reaction temperature,the mass fraction of equilibrium product propylene increases first and then decreases.The mass fraction of propylene in the equilibrium product increases with the increase of the molar ratio of propane to methanol.The catalytic kinetic experiments show that the addition of methanol can reduce the activation energy barrier of propane dehydrogenation conversion and make the reaction easier.Finally,the reaction temperature,space velocity and catalyst regeneration in the process of liquefied gas dehydrogenation were preliminarily explored in a fixed fluidized bed reactor.Finally,the process conditions of dehydrogenation of liquefied gas to naphthalene were investigated in a fixed fluidized bed.The dehydrogenation aromatization of LPG is an endothermic reaction.The conversion of propane increases with the increase of reaction temperature,but the yield of aromatics decreases slightly.This is because liquefied gas is prone to cracking reaction under high temperature conditions to produce small molecule methane.The reaction temperature should not exceed 903 K.The aromatization process of liquefied gas also has a great relationship with the reaction time.The yield of aromatic hydrocarbons increases first and then decreases with the reaction time,showing a volcanic curve trend.This shows that there is an induction period in the aromatization reaction of liquefied gas.At the beginning of the reaction,the activity is low.After the formation of aromatic hydrocarbons,the activity begins to increase by orders of magnitude.After a certain formation,more polycyclic aromatic hydrocarbons are generated in the channel of the molecular sieve,which will hinder the further reaction of alkanes or olefins into the channel of the molecular sieve,so the yield begins to decrease.The reason for the rapid deactivation of the catalyst is that polycyclic aromatic hydrocarbons are easily polymerized to form graphite carbon deposition.The aromatization mechanism of liquefied gas is similar to the hydrocarbon pool mechanism,and there are olefin cycle and aromatic cycle.