Study On Preparation Of Ni-based Molecure Sieve Catalysts With Low Temperature Activity And Catalytic Performance For Carbon Dioxide Methanation

Owing to the dramatic increaseing emissions of CO₂ in the atmosphere,rusluting climate change and global warming,which have been affected human life and the ecological emvironment seriously.CO₂ as a recycling carbon source has become a hot spot for scientists to focus on the use of CO₂.The product methane generated by the hydromethanation reaction of CO₂ is a very important industrial fuel and chemical raw material.Therefore,converting CO₂ to methane and harming it will have strategic significance for the environment and energy,in the long turm.At present,non-noble metal Ni has good activity and selectivity in CO₂ methanation reaction,and has low cost and extensive research applications.However,the CO₂ methanation reaction is a strongly exothermic reaction,the conventional Ni-based catalyst is easily inactivated by sintering.The purpose of this paper is to design and synthesize Ni-based molecular sieve catalysts,to increase the dispersion of the active component Ni and to strengthen the interaction between the active component Ni and the carrier,in order to increase the low-temperature activity and sintering resistance of the catalyst.Therefore,in this paper,a high specific surface area mesoporous molecular sieve MCM-41 was used as a carrier,and Ni-based molecular sieve catalysts were synthesized by impregnation,sol-gel method,hydrothermal synthesis,precipitation deposition and other methods.XRD,Low temperature Liquif nitrogen?BET?,H₂-TPR,CO₂-TPD,and FT-IR characterization techniques were used to characterize the structure and physicochemical properties of the catalysts.The specific research content is as follows:?1?Optimization of Preparation Conditions and Reaction Process Parameters of Ni/MCM-41 Molecular Sieve Catalyst.First optimize the preparation conditions including Ni loading,calcination temperature.The catalytic performance evaluation results showed that the best Ni loading was 10%and the calcination temperature was 500?.XRD characterization results showed that the calcination temperature and Ni content have a great influence on the particle size and dispersion of the catalytically active Ni.BET characterization results showed that Ni/MCM-41 zeolite catalyst still maintain mesoporous characteristics.Subsequently,the reaction conditions were optimized including reduction temperature and space velocity.The catalytic performance evaluation results showed that the optimal reduction temperature was 500°C and the space velocity was 15000 ml?g.h?^-1.The moderate reduction temperature and space velocity are more conducive to the reduction of the active Ni and the progress of the reaction.The optimum technological conditions after optimization are:Ni content 10%,calcination temperature 500°C,reduction temperature 500?,space velocity 15000 ml?g.h?^-1.?2?Effect of promoter on Catalytic Performance of 10%Ni/MCM-41 catalyst.additiving Mn,Co,La,Ce,Fe,Zr into 10%Ni/MCM-41 Catalyst.the catalytic performance evaluation showed that the addition of Mn and Co did not increase the activity of the catalyst.The addition of Fe,Ce,La,and Zr could increase the catalytic activity of the catalyst.The optimum addition amounts of Fe,Ce,La,and Zr after optimization were 1%,4%,1%,and 3%respectively.At the reaction temperature of 300?,10%Ni-1%Fe/MCM-41,10%Ni-4%Ce/MCM-41,10%Ni-1%La/MCM-41 and 10%Ni-3%Zr.The CO₂ conversion rate of the/MCM-41 catalyst relative to the 10%Ni/MCM-41catalyst was 25.45%,24.5%,21.49%,and 22.77%respectively.XRD characterization results showed that the effective additives added have good dispersion in the catalyst;SEM characterization results showed that the addition of effective promoters can increase the degree of dispersion of Ni and reduce the particle size of Ni.BET characterization results showed that the mesoporous structure of the catalyst does not change after the addition of effective promoters,and the additives were beneficial to increase the specific surface area,pore volume and pore size of the catalyst,and increase the dispersion of Ni.of H₂-TPR characterization results showed that the addition of Ce and La can promote the reduction of Ni,and the introduction of Fe and Zr can increase the interaction between catalysts.The most effective additive was 10%Ni-4%Ce/MCM-41 catalyst.Subsequently,Zr auxiliaries were further studied and it was found that different Zr species had a significant influence on the catalytic performance of the catalyst.The order of influence was as follows:Nitrate Oxide Zr>Oxychlorinated Zr>Nitrate Zr.?3?Effect of different synthetic methods on 10%Ni-4%Ce/MCM-41 Catalyst.Firstly,the effects of different impregnation sequences on the catalyst were compared.The results of catalytic performance showed that the co-impregnation method showed better catalytic activity by fractional impregnation method.At a reaction temperature of 450?,10%Ni-4%Ce prepared by co-impregnation was used.The conversion rate of CO₂ to/MCM-41reached 72.4%.Subsequently,the preparation method of the catalyst was investigated by precipitation deposition method,hydrothermal synthesis method and sol-gel method.It was found that the catalyst prepared by sol-gel method showed the best low-temperature catalytic activity.The CO₂ conversion rate of the 10%Ni-4%Ce/MCM-41?Sol-gel method?reached 83.36%at 400?,which is lower than the maximum CO₂ conversion temperature of 10%Ni-4%Ce/MCM-41 catalyst50?,at the reaction temperature 300?,the catalyst increased 32.69%relative to 10%Ni-4Ce%/MCM-41?co-impregnation?,and the selectivity to methane was 99.93%.Subsequently,on the basis of sol-gel,ultrasonic and microwave enhanced treatment were used to prepare the catalyst.The low-temperature activity of 10%Ni-4%Ce/MCM-41?sol-gel?catalyst was significantly enhanced at a reaction temperature of 250?.At the same time,the CO₂ conversion rate of the ultrasonically treated catalyst10%Ni-4%Ce/MCM-41?ultrasonic sol-gel method?increased by 11.19%.10%Ni-4%Ce/MCM-41 microwave treated catalyst?microwave Sol-gel method?increased CO₂conversion by 8.33%.XRD characterization results showed that NiO in the catalyst prepared by sol-gel method was amorphous,increased dispersion of Ni.the dispersion of the catalyst after ultrasonic and microwave treatment was further improved;BET characterization showed that the structure of 10%Ni-4%Ce/MCM-41 catalyst prepared by Sol-gel method has not changed,and the surface area of the catalyst has been increased after ultrasonic and microwave treatment;H₂-TPR show that the sol-gel method is beneficial to improve the interaction between 10%Ni-4%Ce/MCM-41 catalyst.Improved interaction between catalysts after treatment of 10%Ni-4%Ce/MCM-41 catalyst with ultrasound and microwave.?4?Effect of surface modification on catalytic performance of 10%Ni/MCM-41Catalyst.The addition of glycerol?Gly?and?-cyclodextrin?CD?modified the surface of the catalyst.catalytic performance results showed that the addition of glycerol and?-cyclodextrin can improve the low temperature activity of the catalyst.the optimum amount of glycerin is Gly/Ni=1/1?molar ratio?,and the optimal amount of?-cyclodextrin?CD?is Ni/CD=1/30?molar ratio?.At reaction temperature350?,the CO₂ conversion rates of were 82.19%and 89.36%respectively.CH₄ selectivity were 95.86%and 93.93%respectively,comparing to the highest conversion?69.3%?of 10%Ni/MCM-41 catalyst,the temperature reduced 100?.XRD characterization results showed that the catalyst active component NiO was included in the pore structure of the catalyst after addition of glycerol and?-cyclodextrin;SEM characterization results showed that the dispersity of Ni increased after adding glycerol and?-cyclodextrin,and the particle size of Ni decreased;BET characterization results showed that the mesoporous structure of the catalyst remained after addition of glycerol and?-cyclodextriN,H₂-TPR characterization showed that the interaction between the surface-modified catalysts was enhanced,which was beneficial to improve the sinter resistance of the catalysts,so that the reduced Ni had a smaller particle size.CO₂-TPD characterization showed that the addition of glycerol can improve the weak basicity and strong alkalinity of the catalyst,the addition of?-cyclodextrin can improve the acidity and alkalinity of the catalyst surface,enhanced the strong alkalinity of the catalyst,increasing the adsorption capacity of CO₂,and promoting theprogress of reaction.?5?Effect of perovskite type LaNiO₃/MCM-41 catalysts on catalytic Performance.A perovskite type LaNiO₃/MCM-41 catalyst was prepared by a citric acid complex method,and the catalyst showed good low temperature activity,after Ce doping,the catalyst's low temperature activity was further improved.At La/Ce=0.3/0.7?molar ratio?,the catalyst showed the best catalytic activity.At reaction temperature 400?,the CO₂ conversion rate of La_0.3Ce_0.7NiO₃/MCM-41 catalyst was 81.01%,and the methane selectivity was 97.6%.At 300?,the catalytic activity of La_0.3Ce_0.7NiO₃/MCM-41 catalyst increased by 56.74%compared to 10%Ni/MCM-41 catalyst.XRD characterization showed that the metal components in the perovskite-structured catalyst were well dispersed and the NiO was amorphous;SEM characterization results showed that the perovskite-type catalyst had good dispersibility and smaller particle size of the active component;BET results showed that the specific surface area and pore volume of the perovskite-structured catalyst was greatly reduced,the pore size of the catalyst was increased.When the Ce was doped,the specific surface area of the catalyst increased and the pore size decreased;H₂-TPR characterization showed that the perovskite-type structure is beneficial to enhance the interaction between the active components of the catalyst-adjuvant-support,thereby increasing the dispersion of the catalyst,reducing the particle size of Ni,and increasing the low temperature activity of the catalyst.After adding Ce,the catalyst was more easily reduced.