CO₂ Oxidative Dehydrogenation Of Propane To Propylene Over GaN/Zeolite Catalyst

The CO2 oxidative dehydrogenation of propane to propylene(CO2-ODHP),one of the typical propylene production routes in recent years,not only can make up for the demand gap of propylene but also exhibits a certain CO2 emission reduction potential,which caught the attention of academia and industry.Compared with routes of the direct dehydrogenation of propane(DDHP)and the O2 oxidative dehydrogenation of propane to propylene(O2-ODHP),introducing soft oxidant of CO2 into propane dehydrogenation reaction can avoid excessive oxidation of the reactants and products to increase propylene yield,and maintain a high oxidation state of the redox-active sites to improve the stability of catalyst.In addition,the presence of CO2 enhances propane conversion via coupling dehydrogenation of propane(DHP)with reverse water gas reaction(RWGS),and inhibits the coke formation on catalyst surface through the Boudouard reaction to prolong the catalyst life.However,an efficient catalyst of CO2-ODHP not only should possess the abilities of selective activation of C-H bond in propane and prohibition of C-C cleavage but also be efficient for activation of C=O bonds in CO2 to improve catalytic performance.Therefore,the design of efficient catalysts for CO2-ODHP has become a hotspot and difficulty.Nowadays,the main catalysts in CO2-ODHP are concentrated on Cr-,V-,Ga-,In-,and Ce-based systems.However,most of them exhibit various defects such as poor activity,low propylene selectivity,or poor thermal stability.The previous research showed that GaN is an active species of the GaN/Q-3 catalyst(Q-3:a kind of SiO2 material),which is responsible for the dehydrogenation with higher performance than that of traditional Ga2O3-based catalyst in CO2-ODHP reaction.Unfortunately,the GaN/Q-3 catalyst has a long induction period and a hard texture which limit industrial application and catalyst molding.Herein,the GaN-based MFI-type zeolite bifunctional catalysts in which the GaN is active species and MFI-type zeolite is support,are designed and synthesized,and then applied in the CO2-ODHP reaction.The controllable modulations of the dispersion of GaN active species and the acid-base type/intensity of the catalyst surface are achieved via adjusting the type of micromesoporous channels,morphology,the amount of silanol groups,and the framework atom of zeolite to obtain highly dispersed/active GaN-based MFI-type zeolite bifunctional catalysts.And the influence mechanisms of silanol groups/acid-based type on the GaN active species/grain size/electronic effect are elucidated to obtain the correlation between active species structure of the catalyst and the reaction performance.Meanwhile,the influence mechanisms of support types on species/dispersion of the GaN active species,and the reaction-diffusion behavior of the catalyst are also clarified with the combination of various characterization techniques and DFT theoretical calculations.Based on these results,the catalytic reaction kinetics was obtained and the CO2-ODHP reaction mechanism was proposed.The main contents and conclusion of this dissertation are as follows:(1)Mesoporous(SBA-15,MCF,and KIT-6)and microporous(Silicalite-1)siliceous supports with different pore structure and surface properties were prepared by hydrothermal method,and then were supported GaN via initial wet impregnation method.The prepared catalysts were applied in the CO2-ODHP to explore the relationship between catalytic texture/support surface properties and catalytic performance.The mesoporous structure/long-range channel type of 5GaN/SBA-15,5GaN/MCF,5GaN/KIT-6 and the microporous structure/MFI crystal type of 5GaN/Silicalite-1 were verified by N2 adsorption/desorption isotherm,XRD,SEM,and TEM results.The FT-IR,29Si MAS NMR,and XPS results indicated that there was a large amount of silicon nest on the surface of Silicalite-1 which possessed strong interaction with GaN,resulting in more active sites for 5GaN/Silicalite-1 than the mesoporous catalysts.The internal and external diffusion limitations of the catalysts were investigated and found that the 5GaN/KIT-6 catalyst with interconnected 3D mesoporous pore structure can significantly improve the mass transfer to eliminate the internal and external diffusion limitations,resulting in an improvement of the propane conversion.Although the microporous structure of 5GaN/Silicalite-1 increases the mass transfer resistance resulting in low propylene selectivity and coke formation on the surface of the catalyst,but more active sites on the surface improved the propane conversion.Subsequently,the catalytic performance of all the prepared catalysts was investigated for the CO2-ODHP in the optimal reaction conditions(T=600℃,WHSVpropane=1.77 h-1,the molar ratio of C3H8/CO2/N2=1:2:7,and mcat=0.2 g)to reveal that the pore structure/surface properties of support are critical for the catalytic activity.And the Ea of CO2-ODHP over 5GaN/SBA-15,5GaN/MCF,5GaN/KIT-6,and 5GaN/Silicalite-1 were calculated by the Arrhenius equations to be 79.0,67.8,66.9,and 55.7 kJ·mol-1,respectively.The 5GaN/Silicalite-1 exhibited the highest catalytic activity for CO2-ODHP with an initial C3H8 conversion of 30.8%,CO2 conversion of 7.5%,C3H6 selectivity of 80.8%,C3H6 yield of 24.9%,but a fastest deactivation rate.Finally,the TG,Raman,and O2-TPD results revealed that the graphitization degree/amount of coke were consistent with deactivation rate,namely,faster deactivation made more amount of coke on the catalyst surface and higher graphitization degree.The coke formation was the main reason for the catalyst deactivation.(2)To further improve the catalytic activity of GaN-based MFI-type zeolite bifunctional catalyst in CO2-ODHP,the NaZSM-5 was employed to support GaN to get GaN/NaZSM-5 catalysts and then the relationship between surface acidity of catalyst and activity was explored by adjusting the Si/Al molar ratio(300,360,470,and 5000).The characterization results showed that the surface acidity of NaZSM-5 can be adjusted by changing the Si/Al molar ratio and the higher Si/Al molar ratio made lower acidity while the strong acidic sites would be preferentially covered by GaN.Meanwhile,the C3H8 and C3H6 adsorption strength of the catalysts could also be affected by changing the Si/Al molar ratio and the catalyst with an Si/Al molar ratio of 470 exhibited the strongest adsorption to C3H8 and appropriate C3H6 adsorption strength.The effects of Si/Al molar ratio and GaN loadings on the catalytic activity were investigated under the optimal reaction conditions and it was found that the activity of catalyst increased first and then decreased with an increase of the Si/Al molar ratio,and gradually increased with an increase of the GaN loading.The 5GaN/NaZSM-5(470)exhibited highest catalytic activity for CO2-ODHP with an initial C3H8 conversion of 45.2%,C3H6 selectivity of 62.6%,and C3H6 yield of 28.3%.The element valence state of the spent 5GaN/NaZSM-5(470)catalyst was analyzed by XPS and it was found that the Si-O-GaN-H site which generated by the strong interaction between GaN and NaZSN-5 was involved in propane dehydrogenation.Afterward,the comparative experiment revealed that the activity of the GaN-based catalyst was higher than that of the Ga2O3-based catalyst.As compared with the DDHP and O2-ODHP,CO2 as a weak oxidant can avoid the over-oxidation of reactants and products in the CO2-ODHP.In addition,CO2 could also consume the H2 generated by the propane dehydrogenation via the RWGS reaction to force the reaction forward in the direction of C3H6,resulting in an increment of C3H8 conversion.Unfortunately,the high cracking ratio of the reaction results in low propylene selectivity,and coking still remains the main reason for the catalyst deactivation.(3)To reduce the cracking ratio of the reaction in the catalytic system to improve the propylene selectivity,the[Ga]MFI was prepared by hydrothermal method and then supported GaN to obtain GaN/[Ga]MFI catalysts.The relationship between acid type/strength of catalysts and catalytic performance was explored by comparing the acid-base/acid type of[Ga]MFI-100 and[Al]MFI-100 and adjusting the Si/Ga molar ratio(25,50,100,500,and 1000).Firstly,the optimal synthesis conditions of[Ga]MFI were obtained by the controlled variable method:the molar ratio of SiO2:TPAOH:H2O was 1:0.3:39,the crystallization temperature was 180℃,and the crystallization time was 3 days.The microporous structure with MFI topology of all synthesized[Ga]MFI with different Si/Ga molar ratios was confirmed.And the regularity of[Ga]MFI was gradually stronger with an increase of Si/Ga molar ratio.The NH3-TPD,CO2-TPD,and Py FT-IR characterization confirmed that the[Al]MFI-100 and[Ga]MFI-100 possessed similar acid-base content,but the Lewis/Br(?)nsted ratio of[Ga]MFI-100 was higher which could effectively avoid propane cracking to improve the propylene selectivity and catalytic stability in subsequent activity tests.Subsequently,it was found that the C3H8 conversion decreased with an increase of the Si/Ga molar ratio and showed a trend of first increasing and then unchanged with an increase of the GaN loading.The C3H8 conversion and C3H6 selectivity exhibited a "lever" effect,namely,the higher C3H8 conversion with lower C3H6 selectivity.The 5GaN/[Ga]MFI-100 possessed the highest catalytic activity with an initial C3H8 conversion of 64.1%,CO2 conversion of 15.6%,C3H6 selectivity of 71.7%,and C3H6 yield of 46.0%.The deactivation constant of the catalyst was small and the C3H6 yield remained above 40%in the continuous 6 h reaction.But the anti-coking performance of the catalyst still needed to be improved.(4)To further improve the anti-coking performance of the 5GaN/[Ga]MFI100 catalyst,the urea was employed to inhibit the growth of zeolite crystals along the b-axis direction to obtain flake[Ga]MFI-100 with more amount of straight channel and then supported GaN to get 5GaN/[Ga]MFI-100.The size of the[Ga]MFI-100 along the a-c plane was controlled by adjusting the amount of urea to explore the relationship between the length along the a-c plane and catalytic performance.The characterization results indicated that the introduction of urea in synthetic mother liquor of[Ga]MFI-100 cannot destroy the crystal phase of MFI topology and micropore structure.The lengths of[Ga]MFI-100 along the c-axis direction were 1050,2250,and 3025 nm under the SiO2/urea molar ratio of 0.5,1.0,and 2.0,respectively.The TPD(NH3,CO2,C3H8,and C3H6)results revealed that the length of[Ga]MFI-100 along the a-c plane could not affect the surface acid-base and C3H8 adsorption strength of zeolite/catalysts,but the C3H6 adsorption strength of the catalyst was weaker with an increase of the size of 5GaN/[Ga]MFI-100 along the a-c plane.Subsequently,the activity of the 5GaN/flake[Ga]MFI-100 with different lengths along the a-c plane for CO2ODHP was investigated,and found that the catalyst with longer length along the a-c plane exhibited a lower C3H8 conversion,but also possessed a lower ratio of C3H8 cracking and higher C3H6 selectivity,which indicates that the flaky structure of 5GaN/[Ga]MFI-100 catalyst could inhibit the side reaction of CO2-ODHP,resulting in improvement of the C3H6 selectivity.The 5GaN/flake[Ga]MFI-100 with a length of 3025 nm along the c-axis direction showed the highest activity with an initial C3H8 conversion of 54.3%,CO2 conversion of 13.8%,C3H6 selectivity of 84%,and C3H6 yield of 45.6%.The catalyst exhibited excellent stability whose C3H8 conversion and C3H6 yield were maintained above 35%and 30%in a continuous 60 h reaction,which was significantly better than the common 5GaN/[Ga]MFI-100.Compared with commercial CrOx-based catalyst,the catalyst possessed higher C3H8 conversion and stability.However,it is necessary to further improve propylene selectivity to more than 95%to increase the advantages of its industrial application.Meanwhile,the mechanical stability of the catalyst with flake morphology also needs to be ameliorated.(5)To explore the mechanism of CO2-ODHP over the GaN-based MFI-type zeolite bifunctional catalyst and the reason for the differences in the catalytic activity of different zeolites(Silicalites-1,[Al]MFI-100,and[Ga]MFI-100),the DFT was employed to calculate the adsorption strengths of feed gas/products on each catalyst and the reaction pathway over 5GaN/[Ga]MFI-100 catalyst.It was found that the adsorption energies of active sites Si(M)>O-GaN-H(M=Si,Al,Ga)for C3H8,CO2 and C3H6 were different.The adsorption strength of C3H8,CO2,and C3H6 can be enhanced after the introduction of various metals into the molecular sieve framework that could increase the C3H8 conversion and C3H6 yield.Then these three potential reaction pathways of CO2-ODHP over the structure model of the active site of Si(Ga)>O-GaN-H in 5GaN/[Ga]MFI-100 were calculated and compared.It was found that the reaction pathway of CO2ODHP preferred to propane dehydrogenation coupling with RWGS reaction,namely,the site activates the C-H bond in C3H8 to form the intermediate of Si-OGa(*C3H7)N-H(*H),which further dehydrogenates and desorbs to form TS of SiO-Ga(*H)N-H(*H)and C3H6,then the TS reacts with adsorbed CO2 to form*COOH,which is finally hydrogenated by the neighbor*H and decomposes to get CO and H2O.The rate-determining step is*COOH+*H→CO+H2O,whose ΔGa value is 3.03 eV.