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

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

<i>In Situ</i> Investigation of Multicomponent MOF Crystallization during Rapid Continuous Flow Synthesis

Access to the potential applications of metal–organic frameworks (MOFs) depends on rapid fabrication. While there have been advances in the large-scale production of single-component MOFs, rapid synthesis of multicomponent MOFs presents greater challenges. Multicomponent systems subjected to rapid synthesis conditions have the opportunity to form separate kinetic phases that are each built up using just one linker. We sought to investigate whether continuous flow chemistry could be adapted to the rapid formation of multicomponent MOFs, exploring the UMCM-1 and MUF-77 series. Surprisingly, phase pure, highly crystalline multicomponent materials emerge under these conditions. To explore this, in situ WAXS was undertaken to gain an understanding of the formation mechanisms at play during flow synthesis. Key differences were found between the ternary UMCM-1 and the quaternary MUF-7, and key details about how the MOFs form were then uncovered. Counterintuitively, despite consisting of just two ligands UMCM-1 proceeds via MOF-5, whereas MUF-7 consists of three ligands but is generated directly from the reaction mixture. By taking advantage of the scalable high-quality materials produced, C6 separations were achieved in breakthrough settings

Advances in adsorptive separation of benzene and cyclohexane by metal-organic framework adsorbents

The chemical industry represents ca. 7% of the global GDP and 40% of its immense energy footprint stems from the separation/purification processes of commodity chemicals, particularly downstream processing of hydrocarbons. Of critical importance is the separation of C6 cyclic hydrocarbons benzene (C6H6) and cyclohexane (C6H12). Supplanting thermally driven distillation protocols such as azeotropic and extractive distillation methods by recyclable adsorbents, such as metal-organic framework (MOF) physisorbents, holds great promise for the reduction of this energy footprint. Whilst MOFs have come of age as physisorbents, they have been studied as benzene or cyclohexane selective adsorbents only rarely. Thanks to their amenability to crystal engineering, intensive research efforts have enabled metal-organic chemists to offer tunable coordination nanospaces in MOF sorbents in an adsorbate-specific manner, including aromatic benzene or aliphatic cyclohexane molecules. Despite the ever-expanding library of MOFs that often features families or isoreticular platforms of high surface-area materials with electron-rich or electron-deficient local pore environments, this research topic is underexplored and represents a niche area with a high upside potential. This review captures the progress made in MOF adsorbents to accomplish adsorption selectivity guided separation of the foregoing pair of C6 azeotropic hydrocarbons, which is crucial to the production of high-grade cyclohexane and benzene -important feedstock chemicals for further conversion into more useable commodity products, or as liquid organic hydrogen carriers. We also critically interrogate these examples to understand key structural and compositional approaches in order to efficiently design MOFs to extract benchmark selectivities and consequent high separation performances. </p

Lowering the Energetic Landscape for Negative Thermal Expansion in 3D-Linker Metal–Organic Frameworks

Tuning the coefficient of thermal expansion (CTE) of functional materials is paramount for their practical implementation. The multicomponent nature of metal–organic frameworks (MOFs) offers an opportunity to finely adjust negative thermal expansion (NTE) properties by varying the metal ions and linkers used. We describe a new strategy to adjust the NTE by using organic linkers that include additional rotational degrees of freedom. Specifically, we employ cubane-1,4-dicarboxylate and bicyclo[1.1.1]pentane-1,3-dicarboxylate to form the MOFs CUB-5 and 3DL-MOF-1, respectively, where each linker has low torsional energy barriers. The core of these nonconjugated linkers is decoupled from the carboxylate functionalities, which frees the relative movement of these components. This results in enhanced NTE compared to the analogous, conjugated system; VT-PXRD results were used to calculate the CTE for 3DL-MOF-1 (αL = −13.9(2) × 10–6 K–1), and CUB-5 (αL = −14.7(3) × 10–6 K–1), which is greater than the NTE of MOF-5 (αL = −13.1(1) × 10–6 K–1). These results identify a new route to enhanced NTE behaviors in IRMOF materials influenced by low energy molecular torsion of the linker

Acoustomicrofluidic Assembly of Oriented and Simultaneously Activated Metal Organic Frameworks

BET Surface area for HKUST-1 at 0, 1.5, 4.9, and 9 Vrms vs bulkXRD under different acoustic powers for HKUST-1XRD under 9 Vrms for MIL-88BCif files under different strain parameters</div

CUB-5: A Contoured Aliphatic Pore Environment in a Cubic Framework with Potential for Benzene Separation Applications

One prominent aspect of metal organic frameworks (MOFs) is the ability to tune the size, shape, and chemical characteristics of their pores. MOF-5, with its open cubic connectivity of Zn4O clusters joined by two-dimensional, terephthalate linkers, is the archetypal example: both functionalized and elongated linkers produce isoreticular frameworks that define pores with new shapes and chemical environments. The recent scalable synthesis of cubane-1,4-dicarboxylic acid (1,4-H2cdc) allows the first opportunity to explore its application in leading reticular architectures. Herein we describe the use of 1,4-H2cdc to construct [Zn4O­(1,4-cdc)3], referred to as CUB-5. Isoreticular with MOF-5, CUB-5 adopts a cubic architecture but features aliphatic, rather than aromatic, pore surfaces. Methine units point directly into the pores, delivering new and unconventional adsorption locations. Our results show that CUB-5 is capable of selectively adsorbing high amounts of benzene at low partial pressures, promising for future investigations into the industrial separation of benzene from gasoline using aliphatic MOF materials. These results present an effective design strategy for the generation of new MOF materials with aliphatic pore environments and properties previously unattainable in conventional frameworks

CUB-5: A Contoured Aliphatic Pore Environment in a Cubic Framework with Potential for Benzene Separation Applications

One prominent aspect of metal organic frameworks (MOFs) is the ability to tune the size, shape, and chemical characteristics of their pores. MOF-5, with its open cubic connectivity of Zn4O clusters joined by two-dimensional, terephthalate linkers, is the archetypal example: both functionalized and elongated linkers produce isoreticular frameworks that define pores with new shapes and chemical environments. The recent scalable synthesis of cubane-1,4-dicarboxylic acid (1,4-H2cdc) allows the first opportunity to explore its application in leading reticular architectures. Herein we describe the use of 1,4-H2cdc to construct [Zn4O­(1,4-cdc)3], referred to as CUB-5. Isoreticular with MOF-5, CUB-5 adopts a cubic architecture but features aliphatic, rather than aromatic, pore surfaces. Methine units point directly into the pores, delivering new and unconventional adsorption locations. Our results show that CUB-5 is capable of selectively adsorbing high amounts of benzene at low partial pressures, promising for future investigations into the industrial separation of benzene from gasoline using aliphatic MOF materials. These results present an effective design strategy for the generation of new MOF materials with aliphatic pore environments and properties previously unattainable in conventional frameworks

Robust Carborane-Based Metal–Organic Frameworks for Hexane Separation

Hexane isomers play a vital role as feedstocks and fuel additives in the petrochemical industry. However, their similar physical and chemical properties lead to significant challenges in the separation process. Traditional thermal separation techniques are energy-intensive and lead to significant carbon footprint penalties. As such, there is a growing demand for the development of less energy-intensive nonthermal separation methods. Adsorption-based separation methods, such as using solid sorbents or membranes, have emerged as promising alternatives to distillation. Here, we report the successful synthesis of two novel metal–organic frameworks (MOFs), NU-2004 and NU-2005, by incorporating a carborane-based three-dimensional (3D) linker and using aluminum and vanadium nodes, respectively. These MOFs exhibit exceptional thermal stability and structural rigidity compared to other MIL-53 analogues, which is further corroborated using synchrotron studies. Furthermore, the inclusion of the quasi-spherical 3D linker in NU-2004 demonstrates significant advancements in the separation of hexane isomers compared to other MIL MOFs containing two-dimensional (2D) and aliphatic 3D linkers