Understanding the Role of Synthetic Parameters in the Defect Engineering of UiO-66: A Review and Meta-analysis

UiO-66 is one of the most significant and highly studied metal–organic frameworks to date due to its exceptional thermal and chemical stability and its demonstrated potential in a myriad of applications, such as the purification of contaminated waters, gas storage, and catalysis. Defective UiO-66, where either organic linkers or whole inorganic clusters are missing from the pristine framework structure, is of key interest as the unsaturated Zr sites left by these missed connections lead to enhanced catalytic and adsorptive performance, among other benefits such as increased surface area. A solvothermal approach with the inclusion of modulators is typically employed to synthesize defective UiO-66, but a complete understanding of the links between synthetic parameters and structural outcomes has yet to be reached. This review aims to address this gap by providing a thorough overview of the current literature and a detailed discussion of key aspects of the synthesis of UiO-66 and its resulting defectivity; this literature review is supported by a meta-analysis of representative experimental papers. The focus of the meta-analysis is to draw, where possible, quantitative connections between important synthetic parameters (namely, temperature, time, modulator acidity, and reagent concentration) and the structural features reported in the literature for defective UiO-66 (namely, quantitative defectivity, defect types, surface area, particle size, and pore volume). Despite significant limitations of the meta-analysis data set, we are able to determine some statistically significant relationships between parameters and provide recommendations for essential future experimental work. The research questions not conclusively answered by the meta-analysis are addressed by a thorough examination of individual papers discussing the various influencing factors. From this, we provide recommendations to aid the judicious choice of reagents and other synthetic parameters. Overall, this detailed analysis and review compiles critical experimental findings, provides key insights into optimizing defect formation in UiO-66, and highlights important research gaps in the current literature.