Origin of highly active metal–organic framework catalysts: defects? Defects!

Abstract

a This article provides a comprehensive review of the nature of catalytic sites in MOFs. In the last decade, a number of striking studies have reported outstanding catalytic activities of MOFs. In all cases, the authors were intrigued as it was unexpected from the ideal structure. We demonstrate here that (surface) defects are at the origin of the catalytic activities for the reported examples. The vacancy of ligands or linkers systematically generates (surface) terminations which can possibly show Lewis and/or Brønsted acido-basic features. The engineering of catalytic sites at the nodes by the creation of defects (on purpose) appears today as a rational approach for the design of active MOFs. Similarly to zeolite post-treatments, postmodifications of MOFs by linker or metal cation exchange appear to be methods of choice. Despite the mild acidity of defective MOFs, we can account for very active MOFs in a number of catalytic applications which show higher performances than zeolites or benchmark catalysts.

Chemical Reactions at Isolated Single-Sites Inside Metal–Organic Frameworks

“…Copyright 2013 American Chemical Society be sizeable, yielding different types of unsaturated sites which strongly affect the chemical and physical properties of these porous materials [105][106][107][108][109][110][111][112]. In this context, the intentional and controlled introduction of various defects into MOF frameworks is of great importance for rational design of MOF materials with desired specific properties [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48].…”

“…Furthermore, the approach of partial post-synthetic metal exchange was employed to produce mixed Al/Fe-MIL-53, Zr/Ti-UiO-66 as well as Zr/ Hf-UiO-66 MOFs featuring mixed metal nodes in the framework [141]. Overall, these different types of defects have been shown to account for the high reactivity of MOF catalysts for a number of catalytic reactions [40]. The controlled incorporation of MOFs with defects of different types and concentrations represents a novel approach for the predictive rational design of MOF-based single-site catalysts at the atomic level.…”

“…1) [33,34]. The defect engineered MOFs (DEMOFs), synthesized by tailoring of linkers and/or metal ions, have shown modified physical and chemical properties [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48]. Both local, isolated defects [modified CUS (mCUS)] and largescale defects (e.g.…”

Thermodynamic Insight in the High-Pressure Behavior of UiO-66: Effect of Linker Defects and Linker Expansion

“…44,45 Recently, Goodwin and co-workers showed that correlations between defects can be introduced and controlled, yielding nanoscale defect structures. 46 The presence of these defects may alter the catalytic properties, 47,48 thermal stability, 49 proton conductivity, 50,51 and adsorption behavior 43,52−55 of the host material. While recent work also indicates a decrease in bulk modulus and hence robustness upon the introduction of defects, 54,56 it remains to be investigated how the precise molecular nature of these defects alters the pressure-induced disorder in UiO-66-type materials.…”

Surface acid–base catalytic activity of ZIF-8 revealed by super-resolution fluorescence microscopy

“…This observed surfacelimited activity is in agreement with suggestion of Chizallet et al 7,27 Even more so, several other studies have indicated that active sites of many MOFs are most likely linked to defects in the crystalline structure. 28,29 Correspondingly, in some optical slices the activity at crystal defect is captured showing catalytic conversions also at the internal parts of the ZIF-8 crystal (Figure 3B-E). Some facets of the crystal are almost free of defects at every depth, while others are rich with defects.…”