MOF@POP core–shell architecture as synergetic catalyst for high-efficient CO₂ fixation without cocatalyst under mild conditions

Highlights

• A MOF@POP core-shell catalyst (NH2-UiO-66(Hf)@CoTPy-CAP) is constructed to be firstly applied in the cycloaddition of CO₂.

• The catalyst can realize bifunctional and synergistic catalysis of the reaction without cocatalyst under mild conditions.

• The catalyst possesses well thermal stability, chemical stability and cyclic stability.

Abstract

A bifunctional MOF@POP core-shell catalyst (NH₂-UiO-66(Hf)@CoTPy-CAP) constructed by amino functionalized NH₂-UiO-66(Hf) and ionic porphyrin-based CoTPy-CAP was first reported to be applied in the cycloaddition of CO₂. Because NH₂-UiO-66(Hf)@CoTPy-CAP both possess abundant Lewis acidic sites and nucleophilic centers in one system, the catalyst can efficiently catalyze CO₂ fixation with epoxides without homogeneous cocalyst under mild conditions, achieving propylene oxide (PO) conversion of 98 % at 80 °C and 0.5 MPa. Furthermore, the bifunctional synergetic catalyst was easy to be recovered and recycled for five runs, and the reduction of activity was not obvious. This present work provides a viable direction for the fabrication of bifunctional synergetic catalysts for high-efficient CO₂ fixation without cocatalyst under mild conditions.

Introduction

Carbon dioxide (CO₂), one of the main causes of climate damage and global warming, is also an inexpensive, non-toxic and abundant C1 feed stock [[1], [2], [3], [4]]. Using CO₂ as raw material to prepare variety of value-based fine chemicals is an effective way to realize carbon reduction and reutilization [5,6]. Cycloaddition of CO₂ with epoxides to prepare cyclic carbonates (CCs), not only on account of its 100 % atom efficiency but also great industrial applications of CCs, such as electrolyte components in lithium batteries [7], intermediates for the production of plastics [8], pharmaceuticals [9], polar aprotic solvents [10] and other fine chemicals [[11], [12], [13], [14], [15], [16]]. However, the low reactivity of catalysts and the kinetic inertias of CO₂ limit the extensive industrial production of CCs. Therefore, to develop and find a high quality production technology and highly active catalyst is of important practical significance. Up to now, some homogeneous and heterogeneous catalysts have been received wide coverage for the preparation of CCs [[17], [18], [19]]. Heterogeneous catalysts, like functional polymers [20], metal-organic frameworks (MOFs) [21], zeolites [22], metal oxides [23] and graphite oxide [24], etc., have several advantages over homogenous catalysts due to their simple method of separation, sample treatment, and reusability. Among them, MOFs, due to excellent properties including large BET surface area, tunable pore architectures and diverse functionalities, make them extensively developed as ideal heterogeneous catalysts for CO₂ fixation [[25], [26], [27]]. MOFs possessing coordinatively unsaturated metal center as Lewis acidic sites have excellent activation ability for epoxides to result in the ring-opening of epoxides much easier [[28], [29], [30], [31]]. Further, MOFs with functional groups can activate the CO₂ molecules easily by a nucleophilic attack, such as amino-group [32,33]. Thus, MOFs are an appropriate candidate for catalyzing the cycloaddition of CO₂. Han group synthesized CCs for the first time using a MOF-5/n-Bu₄NBr catalytic system in 2009 [34]. Since then, Zhou group reported a Zr-porphyrin MOFs (PCN-224) and revealed superior catalytic activity for the cycloaddition of CO₂ [35]. However, cocatalysts, for example tetraalkyl ammonium halides, are needed in most MOFs catalytic system, which complicate the separation and purification process because of their homogeneous nature. To solve this problem, it is a rational method that nucleophilic functional groups and MOFs are combined to create the bifunctional materials for cycloaddition of CO₂, such as ionic liquid (IL) functionalized MOFs via post-synthesis modification [[36], [37], [38]].

Recent years, bifunctional materials, are composed with two active sites, have become research hotspots in catalysis [[39], [40], [41], [42], [43]]. The advisable combination of two different active functional sites can produce bifunctional catalysts that synergistically boost a lot of organic reactions that cannot be readily or efficiently catalyzed by catalysts with only a single functional site [[44], [45], [46]]. As a member of the bifunctional materials family, core-shell MOFs materials have attracted extensive research interest due to fascinating topologies and remarkable chemical performances. The core-shell MOFs materials can maintain the excellent properties of core and shell components through the synergistic effect, and effectively overcome the defects of single components, which can expand their application in diverse fields [[47], [48], [49], [50], [51]]. Compared with other reported bifunctional materials [[52], [53], [54]], the core-shell MOFs materials have better performances, such as the following: (1) Improve the stability of the frameworks. Using a material with strong stability as the shell to protect the poorly stable MOFs core can improve its stability to a certain extent [55]. (2) Promote selective gas separation and adsorption. The pore size of single MOFs materials is too large to effectively separate gas molecules of different sizes. If the shell components with appropriate pore size are synthesized on its surface, it can not only separate the small gas molecules from the shell, but also store the large gas molecules in the core, so as to achieve the dual purpose of separation and adsorption [56]. (3) Optimize the performance of nonporous materials. The core-shell structure composed of MOFs and nonporous materials, especially when MOFs materials are used as shell components, can generate uniform, ordered and porous core-shell structure by controlling the conditions, which can enhance the performance of the nonporous materials [57]. Importantly, as a single catalytic system, the core-shell MOFs materials are simple in preparation, increase the active sites and maintain their respective advantages [58]. A core-shell MOFs@COFs (NH₂-MIL-101(Fe)@NTU) material synthesized by Li group observably improved conversion and selectivity of the styrene oxidation reaction [59]. In our group’s previous research, a novel core-shell MOF@POP(UiO-66@SNW-1) with acid-base sites was constructed as bifunctional catalyst for tandem deacetalization-Knoevenagel reaction [58]. Every coin has a flip side, core-shell MOFs materials are immature in industrial application, and there are still several deficiencies. On the one hand, the synthesis of core-shell MOFs materials is usually complex and the reaction conditions are harsh, which put forward to strict demand for the stability of MOFs, on the other hand, the special construction characteristics make them difficult for large-size substrates to diffuse during the catalysis. Therefore, the value of scientific research on core-shell MOFs materials is worth investigating, and it will be a meaningful research to explore their application in the field of catalysis.

In the light of these observations and our previous works, we reported a porous MOF@POP core-shell architecture (NH₂-UiO-66(Hf)@CoTPy-CAP), which was prepared by introducing NH₂-UiO-66(Hf) in the synthesis of CoTPy-CAP (Fig. 1). So far, to be best of our knowledge, the ionic porphyrin-based POP and MOF are incorporated in a catalyst system to form core-shell architecture as the bifunctional catalyst for cycloaddition of CO₂ has not been reported. Coating on NH₂-UiO-66(Hf) with CoTPy-CAP shell to form core-shell MOF@POP architecture not only increases the exposure degree of active sites in CoTPy-CAP but also Hf clusters presented in

NH₂-UiO-66(Hf) can also increase Lewis acidic sites of the whole catalyst. Meanwhile, pyridyl quaternary ammonium bromides were introduced into the metal-porphyrin, thus leading to a bifunctional synergetic catalyst in one system. Under mild conditions, the resulting MOF@POP architecture revealed high catalytic activity, indicating that porphyrin Co(II), Hf clusters and quaternary ammonium bromides could synergistically act on the ring-opening of epoxides and succeeding CO₂ insertion.