Elucidating Improvements to MIL‐101(Cr)’s Porosity and Particle Size Distributions based on Innovations and Fine‐Tuning in Synthesis Procedures

Abstract

Among the existing metal–organic frameworks (MOFs), MIL‐101(Cr) is renowned for its stability in air and water. As a result, MIL‐101(Cr) has numerous potential applications ranging from adsorptive cooling to catalysis. The industrial‐scale production of MIL‐101(Cr) is necessary before realizing these applications. Yet, there remain two main bottlenecks in preparing MIL‐101(Cr) in bulk: the toxicity of hydrofluoric acid (HF) used in conventional MIL‐101(Cr) synthesis and the challenge of ensuring that the as‐prepared MIL‐101(Cr) is highly porous with specific Brunauer–Emmett–Teller surface area (SBET) above 4000 m2 g−1. On the laboratory scale, MIL‐101(Cr) often presents SBET 2300–3500 m2 g−1. The synthesis and purification procedures often influence the yield, particle size, porosity, and other properties of MIL‐101(Cr). This critical review examines trends in MIL‐101(Cr) preparation procedures and the MOF's resulting properties to elucidate areas for improvement toward its real‐world applications. The purification processes for conventional HF‐based MIL‐101(Cr) whereby porosities vary despite the same synthesis approach are first investigated. Next, the reported additives for substituting HF and their influence on the resulting MIL‐101(Cr)’s porosity and particle size are discussed. The selection of additives may be application‐specific: exemplified in the examination of MIL‐101(Cr)’s preparation, its corresponding water sorption capacity, and desiccant‐related applications.

Enhanced moisture sorption through regulated MIL-101(Cr) synthesis and its integration onto heat exchangers

“…43,52 Alkalis for synthesising MIL-101(Cr) include tetramethylammonium hydroxide (TMAOH) and sodium hydroxide (NaOH). 55 Currently, NaOH-based MIL-101(Cr) syntheses still pend optimisation because the resulting MIL-101(Cr) has highly variable S BET from S BET = 1537, 66 1767 67 to 4065 m 2 g −1 . 56 On the other hand, Yang et al had previously optimised TMAOH-based MIL-101(Cr) synthesis (1Cr(NO 3 ) 3 : 1H 2 BDC : 0.25TMAOH : 280H 2 O) whereby lower TMAOH concentrations produce MIL-101(Cr) containing unreacted terephthalic acid and even traces of MIL-53(Cr) whereas higher concentrations yield amorphous solids instead of crystalline MIL-101(Cr).…”

“…43 Furthermore, uoride-free or fully hydroxylated MIL-101(Cr) oen have higher water sorption capacities. 55 Although additive-free syntheses are inherently less hazardous, 43 MIL-101(Cr) prepared through additive-free syntheses may be present as irregularly shaped particles 52,[56][57][58] and obtained in low yields (for example, 14% based on Cr). 59 Therefore, additive-based MIL-101(Cr) syntheses remain pertinent.…”

“…43,52 Alkalis for synthesising MIL-101(Cr) include tetramethylammonium hydroxide (TMAOH) and sodium hydroxide (NaOH). 55 of crystalline MIL-101(Cr). 52,68 TMAOH-MIL-101(Cr) has several advantages.…”

“…43,52 TMAOH also produces smaller MIL-101(Cr) particles by slowing crystal growth. 55,56 For example, Yang et al and Zhao et al both reported a narrower particle size distribution for TMAOH-MIL-101(Cr) versus HF-MIL-101(Cr). 52,56 Thirdly, TMAOH hinders the recrystallisation of terephthalic acid to enable higher yields (for example, 88% based on Cr).…”

“…The changes in water sorption capacity with RH also concur with those described elsewhere. 46,55,83 Briey, Fig. 7a shows TEAOH-MIL-101(Cr)'s water isotherms with increasing water sorption capacity from RH = 0-30%.…”