Metal-organic frameworks in separations: A review.

It is possible to design high permeability and selective metal-organic frameworks (MOFs) with designable functionality. The large number of possible MOF structural permutations arise from the considerable number of possible metal nodes and the great variety of organic ligands used for these materials. Herein, we discuss the applications of MOFs in all manner of separations including gas adsorption/separations, membranes, gas chromatography (GC), liquid chromatography (LC), water harvesting and the computer/machine learning design of MOFs. Each application requires MOFs with specific structural motifs. Relevant properties include polarity, temperature stability, solvent stability, pore size, pore volume, surface area, etc. MOFs used for the adsorption and separation of gases can be quite different from those used in membrane technologies or as chromatographic stationary phases. In the area of chromatography, there are far more reports of GC separations than LC. Also, there has been considerable efforts at developing MOF chiral stationary phases. Hence additional chiral components are added to the MOF support, such as cyclodextrins and various amino acids. MOFs have been used as general adsorbents for both inorganic and organic molecules. A very unique MOF application involves water harvesting. It is shown that potable water can be made in arid environments by selectively adsorbing water vapor from air, even at low humidity. Such MOFs could have important analytical applications, as well. Finally, there is a new focus on automated design of MOFs with desired properties for specific tasks, using computational design and machine learning. This is briefly covered in the final section of this review.

Liquid chromatography enantiomeric separation of chiral ethanolamine substituted compounds.

Eleven racemic ethanolamine derivatives were prepared, and their enantiomers were separated using liquid chromatography with various chiral columns. These derivatives included chiral vicinal amino alcohols, ß-hydroxy ureas, ß-hydroxy thioureas, and ß-hydroxy guanidines, all of which are present in many active pharmaceutical ingredients. The screening study was performed with six chiral stationary phase containing columns, including four recently introduced superficially porous particles bonded with two macrocyclic glycopeptides, a cyclodextrin derivative and a cyclofructan derivative. The two remaining columns contained chiral stationary phases, based on either a cellulose derivative or derivatized amylose, both bonded to fully porous particles. The cyclodextrin and cellulose-based chiral stationary phases proved to be the most broadly effective selectors and were able to separate 8 and 7 of the 11 tested compounds, respectively. With respect to analyte structural features, marked differences in enantiorecognition were observed between compounds containing phenyl and cyclohexyl groups adjacent to the stereogenic center. Additionally, replacing a small electronegative oxygen atom by a larger and less electronegative sulfur atom induced a significant difference in chiral recognition by the cellulose derivative as well as by the vancomycin-based chiral selectors.

High efficiency functionalized hydrophilic cyclofructans as stationary phases in sub/supercritical fluid chromatography.

Packed column SFC has become very popular for preparative and analytical separations due to the low cost of CO2, its accessible critical temperature, and pressure, with the additional benefit of a low environmental burden. Currently, there is a shortage of new polar stationary phase chemistries for SFC. In this work, two new functionalized cyclofructan columns are introduced and evaluated for their performance in achiral SFC separations for the first time. Cyclofructan (CF6), a macrocyclic oligosaccharide, was covalently linked with benzoic acid (BCF6) and propyl sulfonic acid (SCF6) groups by ether bonds. Superficially porous particles (2.7 µm) bonded with modified CF6 showed markedly different selectivity than native CF6. In SFC, peak shapes of amines and basic compounds are often compromised. We show that small quantities (~5.7% v/v) of water added to the methanol modifier in CO2 improves peak symmetries of primary, secondary, and tertiary amines. Efficiencies as high as 200,000 plates/m (reduced plate height ~ 1.8) were observed for benzamide and amitriptyline on the BCF6 column. The relative standard deviations (RSDs) of retention times on BCF6 were about 1.4%, and on SCF6 were less than 1%. Amines on the SCF6 column showed plate counts as high as 170,000 plates/m. Tetramethylammonium acetate is examined as an alternative to water in MeOH. A run time of 36 min with methanol, trifluoroacetic acid, triethylamine mobile phase was reduced to <5 min with complete baseline resolution for a set of amines. The new stationary phases allow greener approaches towards solving separation problems.

Enhancing supercritical fluid chromatographic efficiency: Predicting effects of small aqueous additives.

Supercritical fluid chromatography is becoming more prevalent, particularly in industry. This is due to the inexpensive, and more importantly, environmentally benign carbon dioxide that is used as the major component of the mobile phase. Water is minimally miscible with carbon dioxide at temperatures and pressures commonly used in SFC. However, the introduction of a polar alcohol modifier component increases the solubility of water in carbon dioxide. Previously, the addition of small amounts of water in the mobile phase was shown to provide significant gains in efficiency in chiral supercritical fluid chromatography, especially with polar stationary phases. In this work, we report the effect of the addition of small amounts of water on efficiency and retention factor with four different SFC stationary phases used for achiral analysis namely FructoShell-N (native cyclofructan-6), SilicaShell (bare silica), PoroShell 120 EC C18 (octadecyl silica) and Xselect C18 SB. This is the first reported use of FructoShell-N, a cyclofructan derivatized phase for SFC applications. We devised a predictive test to determine which analytes show an increase in efficiency using their known chemical constants (logKow, pKa, PSA and Hsum). We also use discriminant analysis to elucidate the most important analyte parameters that contribute to "water enhanced" efficiency gains.