Design of hypercrosslinked poly(ionic liquid)s for efficiently catalyzing high-selective hydrogenation of phenylacetylene under ambient conditions
Graphical abstract
Introduction
Styrene is an important monomer for the production of various commodities, such as polystyrene, ion exchange resin, and synthetic rubber. At present, ethylbenzene dehydrogenation is the dominant method to manufacture styrene in industry [[1], [2], [3], [4], [5]]. In this process, a small amount of phenylacetylene will be generated. However, even if there are trace amounts of phenylacetylene present in styrene, it is easy to poison the catalyst and affect the degree of polymerization [6,7,4] Therefore, the removal of phenylacetylene from crude styrene is the critical step in the production of high-purity styrene monomer. In practical production, alkynes are often hydrogenated to remove phenylacetylene from styrene. The process is divided into two reaction steps, including the hydrogenation of phenylacetylene to styrene and the hydrogenation of styrene to ethylbenzene. During the process of deep hydrogenation of phenylacetylene, it is inevitable to produce ethylbenzene [[8], [9], [10]]. Hence, how to improve the selectivity to styrene, especially in the complete conversion of phenylacetylene, is still a challenge [11].
The core to solving the problem is selecting an appropriate catalyst, which needs to design the active site and the attachment of the active site to the catalyst support [12,13]. Noble metal heterogeneous catalysts have attracted extensive attention in hydrogenation reaction due to their mild reaction conditions, convenient separation, and recovery [14,15]. Amongst which palladium (Pd) is widely used as the active center of heterogeneous catalysts because of its special adsorption and dissociation ability to hydrogen [[16], [17], [18]]. Meanwhile, its activity and selectivity rely on the arrangement of atoms on the surface of the support [[19], [20], [21]]. Zhou et al. [22] prepared a series of Pd/α-Al2O3 catalysts via impregnation and applied them to catalyze the selective hydrogenation of phenylacetylene. In this process, polyvinylpyrrolidone was employed as a protective agent to stable palladium nanoparticles (Pd NPs) on the surface of α-Al2O3, thus affecting the activity and the selectivity to styrene. Kaneda and co-workers [23] fabricated functionalized core-shell nanocomposite catalysts in which Pd NPs were covered with DMSO-like matrix on the surface of SiO2, promoting the selective hydrogenation of alkynes. The Yamashita group [24] reported a novel yolk-shell nanostructured composite where Pd NPs were confined in hollow silica spheres using poly(ethyleneimine) as a macroligand. Bai and co-workers [25] demonstrated a facial method to prepare Pd NPs anchored on amine-functionalized silica nanotubes using as efficient catalysts. These catalysts were fabricated using polymer or amine-functionalized materials to prevent the aggregation of Pd NPs. Therefore, to obtain stable NPs, it is necessary to choose suitable support. Recently, poly(ionic liquid)s (PILs) comprised of a polymeric backbone as well as an ionic liquids (ILs) species have become excellent supports and stabilizers for noble NPs, due to its strong complexing ability to bind metal ions onto the surface of PILs [[26], [27], [28], [29], [30], [31], [32]]. However, problems such as small specific surface area and disordered structure limit the catalytic performance of these catalysts. This has promoted the search for ways to control its morphology and textural property via tuning PILs composition. In recent years, it has been found that hypercrosslinked poly(ionic liquid)s (HCPILs) have the advantages of structural adjustability, large specific surface area, and good thermal stability [33]. Therefore, HCPILs can be employed as ideal support materials to confine the size of NPs. Besides, most of the NPs are generated by NaBF4 or H2 before they are used as catalysts [34,35]. By contrast, the in situ strategy offers a promising solution due to the reinforcing particles formed in situ enhance thermal stabilization, bond more firmly at the matrix interface, have a finer size, and are more evenly distributed in the matrix [36,37]. Though several HCPILs have been reported in CO2 capture and fixation, rare of HCPILs without reduction is directly employed as catalysts to catalyze the selective hydrogenation reactions [38].
Herein, we contribute an effective strategy to fabricate heterogeneous catalysts with large specific surface areas and highly dispersed active sites. To construct HCPILs, imidazolium tetrachloropalladate, Pd2+ from the single active site of, was selected as the ionic liquid (IL) moieties and Pd donors, dimethoxymethane was chosen as the crosslinker, and toluene was employed as the co-crosslinker. The obtained HCPILs combine the advantages of porous materials, polymers, ILs, and in situ reductions, thus endowing them with versatile functions. They are directly carried out for the hydrogenation of phenylacetylene at 1 atm H2 pressure to evaluate their performance. In the process, Pd2+ can be reduced in situ to Pd° at room temperature, verifying by the results of XPS. Besides, it is not easy to aggregate into large Pd NPs since the interaction with imidazolium and the constraint of the micro-mesoporous structure in the framework, which is confirmed by the results of TEM. As for the catalytic performance of HCPILs, when the conversion of phenylacetylene is close to 100 %, wherein styrene can maintain over 95 % selectivity. Meanwhile, the HCPILs exhibit high efficiency and recyclable catalytic activity under mild conditions. These findings provide a new design concept for achieving catalysts with highly dispersed active sites.
Section snippets
Material
1-(Trimethylsity)imidazole (97 %), benzyl chloride, phenylacetylene (GC), styrene (GC), ethylbenzene (GC), dimethoxymethane (DMM) and PdCl2 (Pd 59–60 %) were obtained from Aladdin Biochemical Technology Corporation Ltd. of China. Tetrahydrofuran (THF), acetonitrile, methanol, 1,2-dichloroethane (DCE), toluene and ether were obtained from the National Medicines Corporation Ltd. of China, all of which were of analytical grade and used as received.
Synthesis of [Bbmim][Cl] and [(Bbmim)2][PdCl4] (Scheme 1)
In this paper, [Bbmim][Cl] and [(Bbmim)2][PdCl4]
Composition and structure of HCPILs
Scheme 1, Scheme 2 show the synthesis process of the catalysts. In the first step, [Bbmim][Cl] was synthesized using benzyl chloride and 1-(trimethylsity)imidazole as raw materials. In the second step, [(Bbmim)2][PdCl4] was synthesized by reacting [Bbmim][Cl] with PdCl2 at room temperature. Lastly, DMM was used as a cross-linker to form HCPILs via Friedel-Crafts alkylation. In the process of polymerization, when DMM was the only crosslinker, the yield of the product (sample of a-1) from [(Bbmim)
Conclusion
In summary, a series of Pd-based HCPILs with high specific surface areas and micro-mesoporous structures were successfully fabricated via Friedel-Crafts alkylation. The morphology, pore size, and the number of ionic sites of the HCPILs could be well adjusted through the precise tuning of the synthesis parameters. Moreover, these catalysts were reduced in situ during the hydrogenation of phenylacetylene. Due to the constraint of microporous structure and the charge effect on moieties of IL in
CRediT authorship contribution statement
Hongbing Song: Conceptualization, Methodology. Yule Liu: Data curation, Writing - original draft. Yongjie Wang: Investigation, Visualization, Software. Bingxiao Feng: Software. Xin Jin: Formal analysis. Tingting Huang: Writing - review & editing. Meng Xiao: Writing - review & editing. Hengjun Gai: 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.
Acknowledgments
We thank the support provided by the National Natural Science Foundation of China (Nos. 21878164 and 21978143).
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