Modulating Microenvironment Effects Of Metal-Loaded Hollow Carbon Nanoreactors For Hydrogenation Performance Enhancement Of Biomass-Derived Platform Molecules

Catalytic hydrogenation of biomass platform molecules is one of the most important ways for the valorization of renewable biomass resources.However,the existing heterogeneous hydrogenation systems are still faced with technical challenges from multiple aspects such as catalytic reactivity,catalytic selectivity and catalytic stability.In recent years,metal-loaded hollow carbon nanoreactors have shown great potentials in promoting the systematic improvement of catalytic performance owing to their adjustable catalytic microenvironment,which stimulates their application in the field of biomass catalytic conversion.Nevertheless,as a kind of relatively novel catalytic materials,the theoretical cognition and regulatory strategies on the microenvironment effects of metal-loaded hollow carbon nanoreactors are still relatively scarce,which makes it difficult to effectively deal with the arduous challenges of biomass platform molecule transformation system with complex catalytic purposes.In this context,this work aims to discuss the basic concept and the corresponding connotations of metal-loaded hollow carbon nanoreactors,reveal the forming mechanism of the typical complex microenvironment effects,and seek approaches to improve the performance of nanoreactor-driven biomass hydrogenation.Further,this paper mainly focuses on the following key issues:First,the determination of key structural elements of metal-loaded hollow carbon nanoreactors and the discrimination of their systematic correlations;Secondly,the interaction relationship between active metals and hollow carbon structures,as well as its influencing mechanism on the catalytic behavior of guest platform molecules;Third,the targeted regulating mechanism of microenvironment effects of nanoreactors driven by specific catalytic purposes of platform molecules.The specific contents of this study are as follows:（1）Based on the systematic perspective,the basic concept,key elements and systematic correlation of metal-loaded hollow carbon nanoreactors and the corresponding derived basic microenvironment effects were discussed in detail.The key elements including active metals,hollow carbon structures and reactant molecules and their internal systematic relationships were summarized,and the microenvironment effects triggered by the synergy of the above elements,including metal-support interaction effect,reactant enrichment effect and molecular sieving effect,were explained.The potential prospects and future challenges of metal-loaded hollow carbon nanoreactors in biomass hydrogenation were further put forward.（2）Taking the hydrogenation of levulinic acid as the probe reaction,the forming mechanism of typical complex microenvironment effect,i.e.,the void-confinement effect in metal-loaded hollow carbon nanoreactors was investigated.According to the basic logic of"hypothesis-deduction",several pairs Ru@HCSs nanoreactor analogues with specific key structural variables were designed and synthesized based on the traditional hard-template method for comparative catalytic experiments.The results shown that the structural factors of metal-loaded hollow carbon nanoreactors exhibited a strong influence on the void-confinement effect.The void-confinement effect was proved to essentially belongs to a comprehensive effect involving electronic metal-support interaction,reactant enrichment effect and reactant diffusion effect.Ru@HCSs nanoreactors could prompt the efficient conversion of levulinic acid under given reaction conditions and maintain the catalytic stability in the process of five cycles.The apparent activation energy of levulinic acid hydrogenation over Ru@HCSs-m was about 60.66 k J mol^-1,which was significantly lower than 68.10 k J mol^-1 of commercial Ru/C catalyst.（3）The regulation strategies of carbon channel geometry and electronic microenvironment of metal-loaded hollow carbon nanoreactors were explored for the highly selective conversion of furfural towards 2-methylfuran.A metal pre-chelating-assisted assembly strategy was developed to synthesize ultrafine sub-nano Ru clusters encapsulated with a high dispersion in the carbon shell mesoporous channels of the hollow carbon spheres nanoreactor（Ru@Shell-HCSs）.It was found that the promotion effect of Ru@Shell-HCSs on the tandem hydrogenation-hydrogenolysis process of furfural was mainly due to the synergistic strategy of"channel encapsulating-strengthened metal-support interfacial charge transfer"and"channel extension-induced molecular diffusion path growth".Under the given reaction conditions,Ru@Shell-HCSs-L could achieve a complete conversion of furfural and obtain up to 79.1%selectivity of 2-methylfuran.The apparent activation energy of Ru@Shell-HCSs-L for the rate-determining step of furfuryl alcohol hydrogenolysis to 2-methylfuran was52.81 k J mol^-1,which was significantly lower than that of 77 k J mol^-1 reported for commercial Ru/C catalysts.（4）The multi-nitrogen species-modulated channel microenvironment strategies of metal-loaded hollow carbon nanoreactors were explored for the efficient hydrogenation conversion of levulinic acid toγ-valerolactone.Based on the developed metal pre-chelating-assisted assembly strategy,ultrafine Ru clusters channel-encapsulated hollow carbon nitride spheres nanoreactor（Ru@Shell-HNCSs）with adjustable nitrogen doped channel microenvironment were synthesized.It was found that the promoting effect of Ru@Shell-HNCSs on the hydrogenation of levulinic acid was mainly due to the synergistic strategy of molecular enrichment effect promoted by the"acid-base"interaction of pyridine nitrogen,secondary adsorption enhancement of adjacent pyrrole nitrogen vacancy,and the relative electron rich states of coordinated metals modulated by graphite nitrogen.All the Ru@Shell-HNCSs nanoreactors could achieve the efficient conversion of levulinic acid under given reaction conditions and maintain catalytic stability during five cycles.The apparent activation energy of Ru@Shell-HNCSs-800 for the hydrogenation of levulinic acid was about 47.39 k J mol^-1,which was significantly lower than 60.66 k J mol^-1 of the previous Ru@HCSs-m nanoreactors.The main contributions of this study are as follows:Firstly,the basic concept,key elements and systematic correlation of metal-loaded hollow nanoreactors were put forward,and the forming mechanism of their typical microenvironment and the utilization strategy of their basic microenvironment were explored,The theory of nanoreactor-driven catalysis based on the regulation of catalytic microenvironment effect of metal-loaded hollow nanoreactors has been developed;Secondly,the important role of the basic catalytic microenvironment effect of metal-loaded hollow nanoreactors including metal-support interaction,molecular enrichment and molecular diffusion in improving the efficiency of heterogeneous hydrogenation was emphasized,which enriched the means for improving catalytic performance traditionally based on the catalytic structural parameters controlling;Thirdly,this work fully demonstrated the structural advantages of metal-loaded hollow nanoreactors in promoting a wide range of biomass-derived platform molecule hydrogenation,and greatly expanded the application scope of metal-loaded hollow nanoreactors in developing a more sustainable energy system;Finally,the key role of microenvironment regulation of metal-loaded hollow nanoreactors in improving the hydrogenation efficiency of biomass-derived platform molecules was highlighted.