Flexibility Matters: Cooperative Active Sites in Covalent Organic Framework and Threaded Ionic Polymer

Abstract

The combination of two or more reactive centers working in concert on a substrate to facilitate the reaction is now considered state of the art in catalysis, yet there still remains a tremendous challenge. Few heterogeneous systems of this sort have been exploited, as the active sites spatially separated within the rigid framework are usually difficult to cooperate. It is now shown that this roadblock can be surpassed. The underlying principle of the strategy presented here is the integration of catalytic components with excellent flexibility and porous heterogeneous catalysts, as demonstrated by the placement of linear ionic polymers in close proximity to surface Lewis acid active sites anchored on the walls of a covalent organic framework (COF). Using the cycloaddition of the epoxides and CO as a model reaction, dramatic activity improvements have been achieved for the composite catalysts in relation to the individual catalytic component. Furthermore, they also clearly outperform the benchmark catalytic systems formed by the combination of the molecular organocatalysts and heterogeneous Lewis acid catalysts, while affording additional recyclability. The extraordinary flexibility and enriched concentration of the catalytically active moieties on linear polymers facilitate the concerted catalysis, thus leading to superior catalytic performance. This work therefore uncovers an entirely new strategy for designing bifunctional catalysts with double-activation behavior and opens a new avenue in the design of multicapable systems that mimic biocatalysis.

Opportunities of Covalent Organic Frameworks for Advanced Applications

“…[54] As a proof-ofconcept study, ionic polymers with halide anions were placed into a COF bearing Lewis acid sites via in situ polymerization (Figure 12). In an ideal scenario, the functional groups act cooperatively to enhance reactivity and/or selectivity.…”

“…In an ideal scenario, the functional groups act cooperatively to enhance reactivity and/or selectivity. [54] Copyright 2016, American Chemical Society. We proposed a general and effective strategy for concerted heterogeneous catalysis as demonstrated by encapsulating catalytically active linear polymers within the channels of a COF bearing another type of catalytic component.…”

Lower Activation Energy for Catalytic Reactions through Host–Guest Cooperation within Metal–Organic Frameworks

“…[6] Our group previously employed this strategy within ac ovalent organic framework (COF) to demonstrate the enhanced catalytic activity for the chemical fixation of carbon dioxide into epoxides to form cyclic carbonates. [7] This process uses one of the top greenhouse gases as ac arbon feedstock in a100 %atom-economical reaction;not only removing carbon dioxide but also providing it afunction, rather than traditional approaches of underground adsorption. [8] It was found that due to the absence of binding,t he linear ionic polymer threaded with high flexibility enables the catalytic component therein and the Lewis acid sites anchored on the COF wall worked in aconcerted manner, outperforming the individual components and many benchmark catalysts for this reaction.…”

“…[8] It was found that due to the absence of binding,t he linear ionic polymer threaded with high flexibility enables the catalytic component therein and the Lewis acid sites anchored on the COF wall worked in aconcerted manner, outperforming the individual components and many benchmark catalysts for this reaction. [7] Another class of advanced materials that have shown potential for cooperative use in catalysis is metal-organic frameworks (MOFs), [9] which are characterized by their diverse structures,t unable pore sizes,a nd high surface areas. [10] These properties have led to their use in ar ange of applications such as gas storage and separation, [11] optoelectronics, [12] and catalysis.…”

Porous Polyelectrolytes: The Interplay of Charge and Pores for New Functionalities

“…The hybrid porous polyelectrolyte showed significantly improved catalytic efficiency in the cycloaddition of epoxides, as a result of the synergy between the flexibility of the polymer chain and ionic COF–polyelectrolyte interactions (Figure 14). 98 Moreover, Liu and co‐workers discovered that porous polystyrene resins with an ultrahigh concentration of amino groups are effective in Friedel–Crafts alkylation and Beckmann rearrangement reactions. Aside from the properties of the ionic interfaces, the charges can stabilize ligands or metal ions and/or serve as the catalyst themselves under basic99 or acidic100 conditions.…”