Preparation And Characterization Of Metakaolin-based Geopolymer Hydrogenation Catalysts

The resource utilization of carbon dioxide is a major problem which attracts the cconcerns of researchers are in the 21st century.In this study,metakaolin was used as the precursor raw material,and Na OH solution was used as the alkali activator,and the geopolymer microsphere catalyst carrier was prepared by the suspension dispersion solidification method,and the spherical catalyst carrier was loaded with active metal ions to make the microsphere catalyst.The physical and chemical properties of geopolymer microsphere supports and catalysts were characterized by X-ray diffraction analyzer（XRD）,specific surface area and pore size distribution tester（BET）,scanning electron microscope（SEM）,transmission electron microscope（TEM）,X-ray photoelectron spectroscopy（XPS）,hydrogen temperature programmed reduction（H₂-TPR）,thermogravimetric analyzer（TG）,and In situ FT-IR（In situ DRIFTS）.The effect of catalysts on the conversion and selectivity of catalytic hydrogenation reduction of carbon dioxide was investigated,and obtained the following research results:（1）Na OH-excited metakaolin was used to prepare geopolymer-based Na A molecular sieve microspheres（GNa A）:using metakaolin as the main raw material,the high-territorial polymer microspheres were prepared by the suspension dispersion and solidification method.The sphericity and particle size distribution of the microspheres were controlled by the viscosity and temperature of the silicone oil.Optical microscope and scanning electron microscope were used to determine the optimal process parameters of geopolymer microspheres:rotational speed（r）=2000 r/min,temperature of silicone oil（T_c）=60℃,viscosity of silicone oil=2000 cs,n（H₂O）:n（Na₂O）=19,n（Na₂O）:n（Al₂O₃）=1.0.Furthermore,the geopolymeric microspheres were transformed into molecular sieve microspheres GNa A by changing the curing temperature and curing time of the geopolymeric microspheres.The optimal transformation parameters of geopolymer microspheres were determined by X-ray diffraction analyzer and scanning electron microscope:curing temperature T=80℃,curing time t=24h.（2）The GNaA catalyst carrier obtained under the optimum process conditions was loaded with single metal,and the GNa A prepared under this process condition had adsorption effect on Ni²⁺,Pd²⁺and Pt⁴⁺metal ions.Under the metal loadings of 1 wt.%and 5 wt.%,the metal loadings of M-GNa A（M=Ni1,Ni5,Pd1,Pd5,Pt1,Pt5）all were above 90%.The performance test of GNa A catalytic carbon dioxide hydrogenation was tested in a fixed bed.The experimental results showed that different types of metals and metal particles with different particle sizes would affect the products of carbon dioxide hydrogenation.Pt⁴⁺had good dispersion on Pt5-GNa A.After the reduction and activation of Pt5-GNa A,the apparent morphology was highly dispersed Pt nanoparticles,and the particle size distribution of Pt was mainly 6～9 nm.Compared with the Ni and Pd-based catalysts,highly dispersed nanoparticle catalysts can be more easily prepare by the Pt-based catalysts.The containing of a large amount of Na⁺in the geopolymer had a positive effect on the dispersion of active metals.At normal pressure and 550℃,X（CO₂）was 52.97%,Y（CO）was 51.73%,S（CO）was97.96%and S（CH₄）was 2.04%.Pt5-GNa A can maintain a high CO₂ selectivity under a high CO₂ conversion rate due to the inhibitation of CH₄ production derived from the existing of Na⁺.These findings demonstrated that it was feasible to rationally regulate the catalyst activity and selectivity of CO₂ hydrogenation with using geopolymer microspheres supported nanoclusters.（3）The MGM catalyst carrier obtained under the optimal process conditions was loaded with bimetals.The MGM prepared under this process condition had certain adsorption effect towards the three metal ions including Ni²⁺,Pd²⁺and Cu²⁺.Under the total metal loading of 1 wt.%,the loadings of X-MGM（X=Ni1,Cu1,Pd1,Pd0.5Ni0.5,Pd0.5Cu0.5）were all above 95%.The performance test of MGM catalytic carbon dioxide hydrogenation was tested in a fixed bed.The experimental results showed that different kinds of metals were loaded on MGM in two combinations.Although the total metal loading was 1 wt.%,the bimetallic catalyst had a better effect on the activity of CO₂ hydrogenation.The Ni,Cu or Pd metals adsorbed on the MGM were highly dispersed nanoparticles under the action of alkali metals.Many Pd and Ni nanoparticles size below 2 nm were contained in the Na A molecular sieve layer on the surface of Pd0.5Ni0.5-MGM.Under normal pressure conditions,the CO₂ conversion rate of Pd0.5Ni0.5-MGM at 500℃was 47.12%,the selectivity to CH₄ can be maintained at 46.55%,which were better than those obtained with other monometallic and bimetallic catalysts prepared in this paper.Under the premise of the same total metal loading,the catalytic activity of CO₂ methanation can be significantly improved by properly adjusting the ratio of bimetal ions in the adsorbed microspheres.At the same time,the presence of Na⁺in the geopolymer can adjust the size of nano-metal particles and inhibit the large-scale agglomeration of metals.These findings demonstrated that it was feasible to use geopolymer microsphere supports to adsorb bimetallic ions to prepare bimetallic catalysts and apply them to carbon dioxide methanation.