Generalized Approach for Rapid Aqueous MOF Synthesis by Controlling Solution pH

Abstract

Metal−organic frameworks (MOFs) are a promising class of functional materials with applications in catalysis, separations, electronics, and drug delivery, among others. Despite a range of techniques utilized for MOF synthesis, a generalizable and scalable approach has yet to be developed for producing MOFs without using environmentally damaging organic solvents. Here, we look at MOF synthesis as a reaction in an aqueous medium and propose new methods of measuring conversion and selectivity. We show that controlling reactant speciation via pH is a generalizable approach to producing the prototypical MOFs UiO-66, UiO-66-NH 2 , ZIF-L, and HKUST-1 with space−time yields (STY) of over 2250 kg m −3 day −1 , which is a 1 order of magnitude improvement for zirconium-based MOFs. We show that UiO-66-NH 2 crystallization is complete in 5 min at room temperature, with 70% of the extent of reaction completed by 30 s. Finally, we apply the rapid synthesis approach to coating cotton fabric with up to 20 wt % UiO-66-NH 2 using a sequential dip-coating (SQD) technique and demonstrate particulate matter (PM 1−4 ) filtration up to 85%. This work shows a greenchemistry-based, generalizable pathway to rapid synthesis for multiple MOFs and demonstrates its utility for filtration applications. The ability to produce alternative filtration materials is especially relevant under pandemic conditions, where SQD offers a rapid and high-throughput manner of providing air filtration by modifying commonly available textile materials.

Comparison Between Conventional Solvothermal and Aqueous Solution-based Production of UiO-66-NH2: Life Cycle Assessment, Techno-economic Assessment, and Implications for CO2 Capture and Storage

“…Brief block flow diagram of the Zr source production (top box), the 2-ATA linker production (middle box), and the UiO-66-NH2 production (bottom box). These processes are either well-established in industry or representative at lab-scale [33,[60][61][62][63][64][65][66]. In brief, ZrCl4 is fabricated from zircon via chlorination, ZrOCl2•8H2O is obtained from ZrCl4 via hydrolysis, 2-ATA is synthesized from TPA via nitration and hydrogenation, and UiO-66-NH2 is produced using either the combination of ZrCl4 and ATA in DMF (solvothermal system) or the combination of ZrOCl2•8H2O and 2-NTA-Na2 in water (aqueous solutionbased system) [33,[60][61][62][63][64][65][66].…”

“…The obtained crude UiO-66-NH2 is washed with methanol or water to remove any residual DMF and unreacted linker/metal components. In the aqueous solution-based system, the zirconium-oxo-cluster solution (referred to as the metal solution for brevity) and the linker solution are prepared separately [33]. The metal solution is prepared by heating the ZrOCl2•8H2O in an acetic acid-water mixture at elevated temperature for a designated time.…”

“…These challenges mainly arise from two aspects, the process economic feasibility and the process environmental impacts [30][31][32][33]. The process economic feasibility has two significant measures [34,35], the production cost (e.g., capital cost and operating cost) and the process space-time yield (STY) [33,36,37]. The first measure, production cost, is an indicator for the product market price that makes the process breakeven.…”

“…As the process STY increases, the same production line produces more products in its lifetime, so the effective capital cost decreases while the total product sales increases. Hence, a process with lower cost and higher STY is desired for the MOFs production [33,36,37]. The process environmental impacts, on the other hand, measures the environmental burdens (e.g., greenhouse gas emissions and water pollution) the synthesis generates and is an indicator for the process sustainability [38][39][40].…”

“…In our recent work, Huelsenbeck et al reported a generalized aqueous solution-based approach for the rapid synthesis of several MOFs (e.g., synthesis of UiO-66(-NH2), ZIF-L and HKUST-1 within 10 min) and systematically studied the effects of solution chemistry on both the process STY and the product quality (e.g., surface area and defects) [33]. For all MOFs studied, the aqueous solution-based process has higher STY than the conventional organic solution-based process, and the quality of some MOFs (e.g., UiO-66-NH2 and HKUST-1) obtained from the aqueous solution-based is comparable to their counterpart obtained from the organic solution-based process [33]. These findings encourage us to further investigate the environmental and economic benefits of using the aqueous solution-based process as an alternative for the organic solution-based process in the MOFs production, and we will quantify those benefits via life-cycle analysis (LCA) and techno-economic assessment (TEA).…”