Zeolite-embedded Ni@Silicalite-1 Catalysts For Partial Oxidation Of Methane To Syngas

The indirect conversion of methane to syngas shows the advantage of highly technical maturity,but steam reforming is not suitable for Fischer-Tropsch synthesis and methanol synthesis due to the high H₂/CO ratio of 3.In addition,the steam reforming process is high energy-consumption and high-cost.Partial oxidation of methane to syngas(POM)has attracted much attention because of high efficiency,low cost and suitable H₂/CO ratio(2/1)for downstream process.Ni-based catalysts exhibit promising POM activity/selectivity.However,the development of carbon-/sintering-resistant Ni-based catalysts still faces challenges.Metal@zeolite catalysts show the high activity,carbon-/sintering-resistance in many reaction processes due to the special structure,strong interaction between metal and zeolite and synergistic catalysis.Herein,Ni nanoparticles(NPs)of 3-7 nm are successfully embedded in the crystals of silicalite-1(S-1)zeolite via one-pot hydrothermal synthesis.As-obtained Ni@S-1 catalysts were examined in the catalytic partial oxidation of methane(CPOM),being active and selective with high carbon-/sintering resistance but not stable yet.Thus,the catalysts before and after CPOM reaction were systematically characterized to gain insight into the deactivation of such catalysts.The main research content and conclusions are given below:(1)Synthesis of zeolite-embedded Ni@S-1 catalysts and their application for catalytic partial oxidation of methane to syngasA series of Ni@S-1 catalysts were prepared via one-pot hydrothermal synthesis method,followed by calcination and reduction in H₂.Effects of synthesis conditions,including the raw material formulation(Ni/Si O₂,TPAOH/Si O₂,H₂O/Si O₂),crystallization conditions(crystallization temperature,crystallization time)and H₂reduction temperature,as well as reaction conditions on the catalytic performance of Ni@S-1 were systematically investigated.According to the reaction results,the Ni@S-1 catalysts with different H₂O/Si O₂ ratio(from 9 to 141)are highly active and selective for the CPOM reaction.Volcano-shape evolution of conversion and selectivity against the H₂O/Si O₂ ratio is observed,showing a climax at each temperature point over the Ni@S-1-W₂₁(H₂O/Si O₂=21).For example,the Ni@S-1-W₂₁ can achieve CH₄conversion of 87.8%and CO/H₂ selectivity of 94.5%/95.6%at the conditions of 750^oC,a gas hourly space velocity(GHSV)of 72 L g^-1_cat.h^-1 and a feed gas with CH₄/O₂molar ratio of 2.Combined with the characterization results of the fresh catalysts,it was found that the Ni loading,particle size and location of Ni NPs,as well as the textural properties of as-made Ni@S-1-W_x(x is the H₂O/Si O₂ratio)catalysts,are affected by the H₂O/Si O₂ ratio.The outstanding performance of Ni@S-1-W₂₁ is in line with the characterization results,which is attributed to its highest Ni content,the smallest Ni NPs size and the highest amount of easily reducible Ni₃(Si₂O₅)(OH)₄.Moreover,the catalytic performance of Ni@S-1-W₂₁,being comparable to that(750 ^oC:86.5%conversion with 93.6%/95.4%selectivity to CO/H₂)of the Rh@S-1,is better than those of the noble metal(Ru,Pd,Pt)/transition metal(Co)@S-1,Fe-S-1 catalysts,which are synthesized via the same procedure as Ni@S-1-W₂₁,and supported Ni/S-1catalysts.(2)Insight into deactivation of the zeolite-embedded Ni@S-1 catalyst for catalytic partial oxidation of methane to syngasIn the 50-h stability test,the activity of Ni@S-1-W_x catalysts,with high carbon-/sintering-resistance,decreased or even deactivated fast.Deactivation mechanism of the Ni@S-1 catalysts is as follows:the surface of Ni NPs involved in the reaction is oxidized to form intermediate Ni O species,which may react with the circumambient Si O₂(from S-1)in the presence of water vapor(by-product and/or intermediate-product of CPOM reaction)to form a dense Ni₃(Si₂O₅)₂(OH)₂ layer around Ni NPs.The formed Ni₃(Si₂O₅)₂(OH)₂ compounds are hard to be reduced(even in H₂ at 800 ^oC)while completely blocking the Ni~0 sites from contacting with reactants,which is the main cause for the Ni@S-1 deactivation.Not surprisingly,such deactivation problem can be solved easily by replacing Ni with highly anti-oxidative metals such as Rh.