A robust strategy of homogeneously hybridizing silica and Cu3(BTC)2 to in situ synthesize highly dispersed copper catalyst for furfural hydrogenation

https://doi.org/10.1016/j.apcata.2020.117518Get rights and content

Highlights

  • A robust strategy of hybridizing the silica and Cu3(BTC)2 homogeneously is proposed.

  • Calcination in situ yields highly dispersed copper loaded on the mesoporous silica.

  • A balance between the metal dispersion and reducibility was obtained for sample with Cu/Si molar ratio of 0.23.

  • A high activity as 9.2 molFF molCu−1 h−1 with nearly 100 % selectivity of furfural alcohol was achieved.

Abstract

Catalyst deactivation caused by the metal sintering/leaching is still a great challenge confronted by the scientists. Herein, we report a robust strategy to in situ synthesize mesoporous silica supported highly dispersed copper catalyst by homogeneously hybridizing the silica and metal-organic framework (MOF) copper benzene-1,3,5-tricarboxylate (Cu3(BTC)2). We find the introducing silica is the key to the formation of homogeneous hybridization by regulating the growth of Cu-BTC crystallite, yielding various metal dispersion and reducibility during calcination; while the added BTC ligands acts as porous initiator, promoting the accessibility of copper centers for reactants. The optimized CuO#SiO2s1 with molar ratio Cu/Si of 0.23, exhibits excellent catalytic performance towards the furfural hydrogenation, showing a high activity as 9.2 molFF molCu−1 h−1, nearly 100 % furfural alcohol selectivity and minimized activity decline (<5%) after 5 repeated run. This work could open up a new avenue for the synthesis of mesoporous silica supported highly dispersed metal catalyst for hydrogenation.

Introduction

Over the past decades, extensive research effort has been put in the catalytic hydrogenation due to its great industrial and scientific significance in the important fields of petrochemical refining, fine chemical production and environ-contamination control [1]. Developing highly efficient catalysts not only can promote the progress of chemical industrial but also can alleviate the stress raised by the environmental pollution. There have been many reported metal catalysts applied for hydrogenation, including noble metals of Pt [2], Pd [3], Au [4] and Ru [5] with intriguing performance. However, the scarcity and high cost hinder their practical applications.

Even though scientists have paid great effort on developing free-noble transition metal catalysts, such as iron, cobalt [6], nickel [7], copper [8] and etc., it is unfortunate that the reactions catalyzed by the cheap transition metals often require high temperature or high pressure to activate the catalytic centers, which inevitably results in the metal aggregation or leaching [9], and severely deactivation consequently. To overcome these limitations, many modified strategies have been adopted, such as support modification [10], metal alloy [11], ligand anchor [12], space entrapment [[13], [14], [15]] and etc. For example, Dumesic et al. have engineered a mesoporous aluminum overcoat to stabilize a copper metal catalyst by atomic layer deposition route [16]. Zhao et al. have prepared a highly efficient CZA-oa@H-ZSM-5 capsule-structured bi-functional catalyst by coating an H-ZSM-5 shell on millimeter-sized copper–zinc–aluminum oxalate, which exhibited high activity for DME production from syngas [17]. Although great progress has been achieved, the metal sintering and leaching are still a great challenge for the free noble metal catalysts, therefore it is still highly desirable to develop an effectively method to synthesize highly dispersed and robust catalysts.

In the conventional preparation route for the heterogeneous catalysts, generally the active metal components are loaded on the selected support via ex situ manner, e.g., impregnation or co-precipitation method [[18], [19], [20]], and then transformed into metallic/oxide active phase by calcination. Unfortunately, the obtained active components are susceptible to aggregation owing to the relatively weak metal-support interactions. This mainly originates from two factors: i) the metal ions precursors are mixed with the support solids in the form of heterogeneous phase; ii) calcination of metal ions on the pre-formed support unavoidably leads to the phase aggregation and weakened metal-support interaction. To solve these issues, the approaches used in [21] or [22] are effective: both metal ions and support precursors are homogeneously mixed into a gel or other hybridized form, and in situ transformed into the active metal phase and support solids simultaneously, the phase aggregation and leaching would be controllably minimized due to the intensified metal-support interactions [21]. Faro Jr and co-workers have reported a copper-manganese catalyst by in situ calcination of layered-double hydroxides based on copper, manganese, and aluminum for methanol synthesis. They credited the smaller Cu crystallites and larger copper surface area to the larger surface area of its oxide precursor [22].Additionally, porosity initiators can be easily introduced into the hybridized composites together with the metal ions, leading to produce the meso/macropores, which are beneficial to mass transportation and reaction accesses to the active centers [23].

Above considerations promoted us to propose a robust strategy of homogeneously hybridizing the silica with metal-organic framework (MOF) composite to obtain a highly dispersed and stable metal catalyst. The copper benzene-1,3,5-tricarboxylate (Cu3(BTC)2), as known as HKUST-1 [24], can be selected as both mesopore initiator and Cu ion precursor, and the silica played as inorganic skeleton to disperse and stabilize the copper species. Yao and co-authors have once used the preformed HKUST-1 as copper precursor to prepare a Cu/SiO2 catalyst [25]. They disclosed that the remove of BTC can make the copper effectively escape from silica coating and attain excellent performance for dimethyl oxalate hydrogenation. However, there still lacks study on the correlation between the metal and silica underlying the robust stability. Herein, we developed a facile one pot synthesized route by fine control of the hydrolysis of silica and coordination between the Cu ions and 1, 3, 5-benzenetricarboxylic acid (BTC), a Cu-BTC#SiO2 was simultaneously hybridized, and further calcination in situ yielded the mesoporous silica supported highly dispersed copper oxide nanoparticles catalyst. We found that the introduced silica content influenced remarkably the hybridized form by controlling the growth of Cu3(BTC)2 crystallite size, and the homogeneously hybridization caused an intensified metal-support interaction, which can regulate both the copper particle size and reducibility. We further optimized the silica contents to elucidate the optimized balance between the metal dispersion and reducible properties for catalyst design.

Section snippets

Catalyst preparation

All chemicals were purchased commercially (Aladdin, China) and used without further purification. In a typical synthetic process for the hybrid composite Cu-BTC#SiO2, firstly 0.25 g 1, 3, 5-benzenetricarboxylic acid (BTC) and 1 g tetraethyl orthosilicate (TEOS) were dissolved in 10 ml ethanol. Then, 0.25 g cupric nitrate hydrate (Cu(NO3)2•3H2O) was dissolved in 10 ml deionized water and added into the mixed solution. The final mixture contained components with molar ratio of Cu: BTC: TEOS:

Hybridization of Cu3(BTC)2 and silica

The simultaneous hybridization route of Cu3(BTC)2 and silica is depicted in Fig. 1. Firstly, the BTC ligand, Cu(NO3)2 and TEOS are mixed together in a mixed solvent (ethanol and deionized water with volume ratio 1:1). During the sol-gel process, the BTC coordinates with Cu ions to form Cu3(BTC)2 crystallite, while the TEOS simultaneously give rise to the silicate framework by hydrolysis and condensation to entrap the Cu3(BTC)2 particles. To accelerate the formation rate of hybridized gel, the

Conclusions

By fine control of the hydrolysis of TEOS and coordination of Cu ions with BTC, a homogeneously hybridized Cu-BTC#SiO2 composite was obtained successfully, and further calcination in situ yielded a mesoporous silica supported highly dispersed copper nanoparticles catalyst, CuO#SiO2. The optimized CuO#SiO2-s1 catalyst, showed excellent catalytic activity and stability towards the FF hydrogenation, a high mass activity as 9.2 molFF molCu−1 h−1 with nearly 100 % selectivity and minimized activity

CRediT authorship contribution statement

Xu Yang: Conceptualization, Formal analysis, Methodology, Writing - original draft, Writing - review & editing. Wu Liu: Investigation, Validation, Formal analysis. Fenghua Tan: Investigation, Methodology. Zhaoxia Zhang: Data curation, Formal analysis. Xindi Chen: Investigation. Tengda Liang: Investigation, Validation. Chuande Wu: Conceptualization, Funding acquisition, Project administration, Resource, Writing - review & editing.

Declaration of Competing Interest

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “A robust strategy of homogeneously hybridizing silica and Cu3(BTC)2 to in situ synthesize highly dispersed copper catalyst for

Acknowledgement

This work was supported by Natural Science Foundation of Guangdong Province (No. 2018A0303130212).

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