Direct Synthesis Of H₂O₂ By Fe-Pd/SBA-15 And In-situ Oxidation Of Methane To Methano

Methane is the main component of fossil energy sources such as natural gas and shale gas resources.If it is only used for direct combustion,it not only makes a large number of resources wasted but also brings about the greenhouse effect,leading to global warming and causing great pressure on the environment.Therefore,the conversion of methane into high-value-added oxygenated compounds（methanol and formaldehyde）has become a hot research topic.Methanol is an energy-intensive fuel that is widely used as a raw material in the chemical and energy industries and is in increasing demand today.Therefore,the conversion of methane to methanol is the subject of this paper.The current industrial synthesis of methanol is widely used by the indirect syngas-to-methanol method,the Fischer-Tropsch process,in which methane is reformed to syngas（CO,H₂）and then catalyzed at high temperature to methanol.However,this process needs to be carried out at high temperatures of 800～1000°C,which has the disadvantages of high energy consumption,complex process flow,high investment in equipment,and the need for large-scale production,thus prompting the development of an alternative low-temperature one-step process that can be used to be able to directly oxidize methane to methanol.The current mainstream method for the direct oxidation of methane to methanol at low temperatures is performed in a liquid-solid phase reaction system,and most researchers are using the green and environmentally friendly H₂O₂ as the oxidant because it gives the best catalytic activity.However,commercial H₂O₂ has the drawbacks of being expensive,decomposing a lot during transportation,and having low practical utilization.Therefore,in this paper,the direct synthesis of H₂O₂ using hydrogen and oxygen for the next step of the coupling reaction of methane oxidation makes the whole process more economical.Currently,the best-recognized activity in DSHP is the Pd-based catalyst,therefore,we consider adding Pd to the reaction for in situ synthesis of H₂O₂.In addition,most researchers also point out that monomeric Fe species play a crucial role in methane activation.Therefore,we considered adding Fe to the reaction again to promote the activation of methane.In the selection of the carrier,ordered mesoporous silica SBA-15 was chosen as the carrier for the experiment.Because it has the following characteristics:（1）Large specific surface area（600-1600m²/g）.（2）High porosity and thick pore walls.（3）Ordered and adjustable pore size（3.6-30nm）.（4）Good hydrothermal stability.（5）Ordered mesoporous pores prevent the growth and agglomeration of metal NPs.（6）The spatial restriction is beneficial to the retention of small molecules such as H₂O₂ to allow sufficient reaction with the active sites of Fe.In summary,we prepared Fe-Pd/SBA-15 loaded catalysts for the in-situ synthesis of H₂O₂ oxidation of methane to methanol.High concentration of H₂O₂ is beneficial to improve the yield of CH₃OH.Therefore,we used ammonia functionalization to improve the concentration of H₂O₂,and specifically studied the influence of ammonia functionalized Pd/SBA-15 on the catalytic performance of direct synthesis of H₂O₂.Thus,we prepared five ordered mesoporous silicas with different morphological structures（SBA-15）and compared the catalytic performance of the loaded Pd for the direct synthesis of H₂O₂ from hydrogen and oxygen in the atmospheric pressure system,and the results showed that SBA-15-3loaded Pd has the highest H₂O₂ yield,which is attributed to its large pore volume and specific surface area,and even after ammonia functionalization,the H₂O₂ yield can reach 4888mmol/g _Pd·h^-1 in 1h.Using characterization,it was found that the ammonia functionalization resulted in highly dispersed Pd particles within the pore channel,where the average particle size of Pd NPs of Pd/SBA-15-3-NH2-3 was as small as 1 nm,exposing more active sites favorable for H₂ conversion and a large amount of H₂O₂generation.XPS showed that the addition of amino groups led to more Pd⁰ production,and the selectivity of H₂O₂ was generally higher than 80%after ammonia functionalization.The catalysts maintained good activity and stability even after five cycles,and these phenomena were once explained by strong metal-molecular carrier interactions（SMSI）.We obtained the best H₂O₂ yield on Pd/SBA-15-3-NH2-3,which paved the way for the next step of methane to methanol.So,we continued to introduce Fe and Pd metals on SBA-15-3 and investigated the effect of multifunctional modification of amino and ionic liquids on the in-situ synthesis of H₂O₂ oxidation of methane to methanol under high-pressure conditions.It was found that Fe-Pd/SIL-NH2 modified by both amino and ionic liquids could make the methanol yield as high as 146mmol/g _Pd·h^-1 in 0.5h,and in addition the CH₄ conversion reached 5.4%,which was attributed to（I）the strong electron exchange between Fe and Pd metals,which made the electron transfer from Pd to Fe,and the increase of Fe²⁺accelerated the generation of OH radicals in the Fenton reaction.（II）The synergistic effect between the amine and ionic liquid forms a unique molecular sieve structure within the pore channel of SBA-15,which restricts the flow of small molecules.（III）The regular molecular sieve structure effectively prevents the ionic liquid from causing pore plugging.（Ⅳ）The ionic liquid forms a liquid film protective layer inside the pore channel,which improves the accessibility of CH₄ and H₂.TEM shows that Fe-Pd and Pd exist mainly in the form of alloy,and the metal particles of Fe-Pd/SIL-NH2 are uniformly distributed at 10 nm.These unique phenomena promote the formation of methanol in large quantities,which brings great reference significance and research value for the in-situ synthesis of H₂O₂oxidation of methane to methanol.