Engineering frustrated lewis pair active sites in porous organic scaffolds for catalytic CO₂ hydrogenation

Frustrated Lewis pairs (FLPs), featuring reactive combinations of Lewis acids and Lewis bases, have been utilized for myriad metal-free homogeneous catalytic processes. Immobilizing the active Lewis sites to a solid support, especially to porous scaffolds, has shown great potential to ameliorate FLP catalysis by circumventing some of its inherent drawbacks, such as product separation and catalyst recyclability. Nevertheless, designing immobilized Lewis pair active sites (LPASs) is challenging due to the requirement of placing the donor and acceptor centers in appropriate geometric arrangements while maintaining the necessary chemical environment to perform catalysis, and clear design rules have not yet been established. In this work, we formulate simple guidelines to build highly active LPASs for direct catalytic hydrogenation of CO₂ through a large-scale screening of a diverse library of 25,000 immobilized FLPs.
The library is built by introducing boron-containing acidic sites in the vicinity of the existing basic nitrogen sites of the organic linkers of metal-organic frameworks collected in a "top-down" fashion from an experimental database. The chemical and geometrical appropriateness of these LPASs for CO₂ hydrogenation is determined by evaluating a series of simple descriptors representing the intrinsic strength (acidity and basicity) of the components and their spatial arrangement in the active sites. Analysis of the leading candidates enables the formulation of pragmatic and experimentally-relevant design principles and they constitute the starting point for further exploration of FLP-based catalysts for the reduction of CO₂.

受阻路易斯酸碱对（Frustrated Lewis pairs, FLPs）凭借路易斯酸与路易斯碱的活性组合，已被广泛应用于各类无金属均相催化过程。将活性路易斯位点负载至固相载体（尤其是多孔骨架）中，可通过规避该类体系的部分固有缺陷（如产物分离难题与催化剂可回收性问题），展现出改善FLP催化性能的极大潜力。然而，设计固定化路易斯酸碱对活性位点（immobilized Lewis pair active sites, LPASs）仍颇具挑战：需将给体与受体中心排布至合适的几何位置，同时维持催化所需的化学环境，且目前尚未形成明确的设计准则。
本研究通过对包含25000个固定化FLPs的多样化库开展大规模筛选，制定了用于二氧化碳直接催化加氢的高活性LPASs的简易构建指南。该库以“自上而下”的方式从实验数据库中提取金属有机框架的有机配体，在其原有碱性氮位点附近引入含硼酸性位点构建而成。我们通过评估一系列表征组分本征强度（酸性与碱性）及其在活性位点内空间排布的简单描述符，来判定这些LPASs用于二氧化碳加氢的化学与几何适配性。对最优候选体的分析可推导出兼具实用性与实验可行性的设计原则，这些原则可作为后续探索基于FLPs的二氧化碳还原催化剂的研究起点。