Amine-promoted Ru1/Fe3O4 encapsulated in hollow periodic mesoporousorganosilica sphere as a highly selective and stable catalyst for aqueous levulinic acid hydrogenation

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Abstract

It is of great importance to develop selective and stable metal catalysts for the aqueous levulinic acid hydrogenation, yet challenging. Herein, we report a yolk-structured sing atom catalyst (SAC) with amine-modified Ru1/Fe3O4 core and periodic mesoporousorganosilica (PMO) shell, synthesized by a core–shell dual stabilization strategy. The Ru single atoms (0.76 wt%) are inserted into the oxygen vacancies of spheric Fe3O4, and stabilized by the amine groups from 1,6-hexanediamine. The hollow PMO sphere is hydrophobic, that affords a strong barrier for interior Ru1/Fe3O4 core, and the shell mesopore (4.2 nm) along with the cavity enhances the porosity of the resultant catalyst. As expected, the amine-promoted Ru1/Fe3O4 core in the hollow PMO shell (denoted as N-Ru1/Fe3O4@void@PMO), proves to be highly selective and stable for the aqueous levulinic acid (LA) hydrogenation under harsh conditions (pH ≈ 1), giving γ-valerolactone (GVL), a biomass-derived platform molecule with wide applications in the preparation of renewable chemicals and liquid transportation fuels. The elaborately fabricated catalyst is highly efficient, delivering 98.9% of selectivity to GVL and 99.0% of LA conversion in acidic water. And a high turnover frequency of 1084 h−1 is achieved and this catalyst can be cycled 7 times without apparent drop of GVL yield and LA conversion. The amine-stabilized Ru single sites, acid-resistant Fe3O4 circled by the hydrophobic shell, and the enhanced porosity of catalyst, are responsible for the excellent catalytic performance of N-Ru1/Fe3O4@void@PMO in acidic water.

Graphical abstract

A yolk-structured sing atom catalyst (SAC) with amine-modified Ru1/Fe3O4 core and periodic mesoporous organosilica (PMO) shell was synthesized by a core–shell dual stabilization strategy, and proves to be highly selective and robust for aqueous levulinic acid hydrogenation.

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Introduction

With the shrinking fossil resources as well as the increasing environmental concerns, the catalytic conversion of renewable biomass to fuels and chemicals has drawn much attention recently [1], [2]. Conversion of cellulose-derived levulinic acid (LA) toγ-valerolactone (GVL) offers tremendous flexibility in downstream applications and upgrading to high value-added products. GVL can serve directly as the gasoline blender, biorefining solvent, or subsequently be processed to yield energy-dense fuel additives and can be converted to chemical intermediates for the production of biobased polymers [3], [4], [5]. Conversion of LA to GVL goes through Cdouble bondO hydrogenation and the subsequent intramolecular dehydration, during which the presence of impurities (H2SO4) in LA feeds and the generation of H+ at elevated temperatures make the aqueous media very acidic (pH less than 1). Such a tough reaction environment raises a significant challenge for the catalyst selectivity and durability. Ru-based catalyst proves to be superior to other Pt-series metals owing to its lower price and higher activity, and is more stable than transition metals (Cu, Ni, Co, etc.) [6], [7], [8], [9]. However, the selectivity to GVL and stability of Ru catalysts involved in the aqueous LA hydrogenation are far from satisfactory. Particularly, Ru catalysts are easily deactivated as a result of metal sintering, leaching, support collapse or destruction in the acidic water, even at room temperature [10], [11]. Accordingly, these Ru catalysts are less selective owing to the diverse active sites, which render the adsorbed LA react with low selectivity, and thus the side reactions are largely promoted. Therefore, it is keenly desired to develop a highly selective and stable Ru catalyst for the aqueous LA hydrogenation.

As the emergence of concept of single atom catalysis, various single atom catalysts (SACs) have recently been fabricated by isolating homogeneous metal sites on the supporting matrix through a strong metal-support interaction [12], [13]. Such a strong metal-support interaction is the key to prevent aggregation of single atoms and leaching of active sites into solution. The single atoms having site homogeneity and tight attachment to the support could improve the selectivity and stability of Ru-based catalysts in the aqueous LA hydrogenation. Although never been achieved, several Ru SACs are fabricated by confinement of Ru single atoms into carbon nitride [14], [15], graphene [16], and metal–organic framework (zeolitic imidazole framework, UiO-66)-derived carbons [17], [18], [19], showing excellent catalytic performance in electrocatalytic H2/O2 evolution, O2 reduction and quinoline/vanillin hydrogenation. Nevertheless, the type of N-containing precursor and pyrolysis temperature are crucial to obtain atomically dispersed Ru catalysts. And the content, coordination configuration and thus the selectivity of anchored Ru single atoms are highly dependent on the support. Alternatively, inserting Ru single atoms into oxygen vacancies or substituting metal sites to form metal oxide/hydroxide supported Ru SACs seems to be more flexible [20], [21]. However, the porosity of these catalysts is very low, and the collapse of supports under acidic reaction media remains problematic, that would erode the stability of Ru SACs. Therefore, a desired Ru SAC for the aqueous LA hydrogenation should integrate the merits of mild/facile synthesis, large porosity and acid-resistance.

Inspired by the unique hierarchy of yolk-shell nanoarchitecture, Ru single atoms can be facilely anchored on a metal oxide core, that is encapsulated into a hollow, acid-resistant sphere, and thereby an ideal Ru SAC with both high selectivity and stability would be fabricated. Profit from the N dopants for Ru single atoms on carbons, the introduction of N-containing groups can further stabilize Ru single atoms on metal oxides, and the loading, coordination and thus the selectivity of Ru SAC could be optimized. All of these considerations are irrespective of metal oxide supports for their physicochemical properties such as porosity, acid-resistance and oxygen vacancy. To verify this hypothesis, herein we developed an amine-promoted Ru1/Fe3O4 core encapsulated in a hollow periodic mesoporous organosilica sphere, synthesized by a core–shell dual stabilization strategy. Starting from 1,6-hexanediamine modified Fe3O4 nanosphere, the loading and coordination configuration of anchored Ru single atoms are regulated, and the subsequent silica coating and organosilane-assisted etching are facilitated, to render a unique Ru SAC with a raspberry-like Ru1/Fe3O4 core and a hydrophobic periodic mesoporous organosilica shell. As expected, the elaborately fabricated N-Ru1/Fe3O4@void@PMO catalyst is highly selective and stable for the aqueous LA hydrogenation to GVL. A selectivity to GVL (98.9%) and a turnover frequency of 1084 h−1 are obtained, as well as a cycling stability up to 7 cycles is shown in the acidic water. The excellent catalytic performance of N-Ru1/Fe3O4@void@PMO can be ascribed to the joint effect of superior porosity, shell hydrophobicity and optimized single atom coordination, all of which are beneficial for the repeated LA adsorption, selective hydrogenation and GVL diffusion.

Section snippets

Synthesis

The schematic illustration of the synthetic procedure for N-Ru1/Fe3O4@void@PMO is shown in Scheme 1. Initially, water dispersible Fe3O nanosphere was prepared by reduction of FeCl3 in hot ethylene glycol, and then functionalized by 1,6-hexanediamine to improve the dispersion of subsequently anchored Ru species which were reduced by NaBH4. The resulting N-Ru1/Fe3O4 sphere was coated with silica through a stöber method to provide a core–shell structured N-Ru1/Fe3O4@SiO2. Through an

Microstructure and porous structure

The Fe3O4 nanosphere shows an average size around 300 nm based on the transmission electron microscopy (TEM) observation (Fig. S1a). Upon modified by amine groups and the subsequent Ru species, the resulting sphere, N-Ru1/Fe3O4, goes rough while the their size remins nearly unchanged (Fig. S1b). Coated with silica, the obtained N-Ru1/Fe3O4@SiO2 displays a silica layer thickness of ca. 80 nm and an increased average size of ca. 380 nm (Fig. S1c). The TEM image of N-Ru1/Fe3O4@void@PMO provides a

Conclusions

In summary, we have successfully developed a yolk-structured Ru catalyst with amine-promoted Ru1/Fe3O4 core and hollow PMO shell. Comparative investigations demonstrate that the amine-functionalization improves the microstructure/morphology, Ru content and coordination, which govern the porosity, surface property and active site structure, and thus the selectivity and stability of Ru SAC in this study. The advantages of this research can be summarized as follows: 1) A core–shell dual

CRediT authorship contribution statement

Ying Yang: Writing - original draft, Writing - review & editing, Supervision. Feng Yang: Investigation. Hai Wang: Methodology. Biao Zhou: Investigation. Shijie Hao: Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge financial support from the National Key Research and Development Program of China (2018YFB1105100), the National Natural Science Foundation of China (51471187, 51571211), and the funding from Science Foundation of China University of Petroleum, Beijing (24620188JC005).

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