Efficient gas chromatographic separation of xylene and other aromatic isomers by using pillar[6]arene-based stationary phase

High-resolution performance of pillar[6]arene functionalized with imidazolium ionic liquids for gas chromatography

Pillar[n]arenes (P[n]A, n = 5-10) have attracted much attention because of their highly symmetric pillar-shaped architecture with π-electron rich cavity. Nevertheless, the use of ionic liquid functionalized P[n]A in chromatography has not been reported up to data. This work reports the investigation of the imidazolium ionic liquids functionalized pillar[6]arene (P6A-C10-IM-C8[NTf2]) as the stationary phase for gas chromatography (GC). The statically coated P6A-C10-IM-C8[NTf2] column (0.25 mm i.d.) showed moderate polarity and high column efficiency of 4733 plates/m determined by n-dodecane at 120 °C (k = 2.29). Owing to its unique amphiphilic conformation, the P6A-C10-IM-C8[NTf2] showed good column inertness and resolving capability for a wide range of analytes and isomers. Particularly, the P6A-C10-IM-C8[NTf2] column exhibited distinctly advantageous performance for the challenging isomers of halogenated benzenes, benzaldehydes, phenols and anilines over the common commercial columns, namely 5% phenyl methyl polysiloxane (HP-5) and 35% phenyl methyl polysiloxane (HP-35). In addition, it exhibited good column repeatability and reproducibility with RSD values on the retention times less than 0.05% for run-to-run, 0.38% for day-to-day and 2.94% for column-to-column, respectively. This work demonstrates the promising future of ionic liquid P[n]A stationary phases for chromatographic separations.

Preparation and evaluation of two chiral stationary phases based on imidazolyl-functionalized bromoethoxy pillar[5]arene-bonded silica

Chiral pillar[5]arene-based mesoporous silica, an emerging class of chiral structure, possesses excellent characteristics such as abundant chiral active sites, encapsulated cavity and excellent chiral modification, which make them a promising candidate as new chiral stationary phases (CSPs) in enantioseparation. In this study, two imidazole-containing (S)-1-(4-phenyl-1H-imidazol-2-yl)ethanamine and (S)-Histidinol were respectively modified to bromoethoxy pillar[5]arene-bonded silica to construct new chiral stationary phases (sPIE-BP5-Sil and sHol-BP5-Sil) for the separation and analysis of enantiomers. The separation conditions such as mobile phase composition, flow rate and temperature were optimized. Under optimal conditions, both sPIE-BP5-Sil and sHol-BP5-Sil showed good separation performance for different types of enantiomers. Interestingly, sPIE-BP5-Sil and sHol-BP5-Sil showed better enantioselectivity for chiral aromatic compounds and chiral aliphatic compounds, respectively. This enantioseparation result was closely related to the presence of additional aromatic rings and abundant hydroxyl groups in the side chains of the two chiral groups. In addition, the enantioseparation process was further studied by molecular docking simulation. Therefore, this work provided a new strategy for the preparation and application of imidazolyl-derived pillar[5]arene-based chiral stationary phases, which can be efficiently used for screening and separating enantiomers.

High Selectivity of A Novel Pillar[5]arene with Ester Units as a Gas Chromatographic Stationary Phase toward Aromatic Isomers

This work reports the first example of employing ester-functionalized pillar[5]arene (P5A-C10-OAc) stationary phase for gas chromatography (GC) separations. The as-fabricated P5A-C10-OAc column achieved improved column efficiency of 4270 plates/m and separation performance in contrast to the P5-C10-Br column. The P5A-C10-OAc column showed good separation performance for a wide range of analytes such as alkanes, bromoalkanes, ketones, fatty acid methyl esters, aldehydes, alcohols, halobenzenes, anilines, phenols, naphthalenes, and showed sharp and symmetrical peak shapes for analytes that are liable to peak-tailing in GC analysis. As testified by the challenging isomer mixtures (bromonitrobenzene, chloronitrobenzene, bromobenzaldehyde, chlorobenzaldehyde, nitrobenzaldehyde), the P5A-C10-OAc column exhibited comprehensively higher separation capability than the P5A-C10-Br, P5A-C10 and commercial HP-35 columns. This work demonstrates the great potential of pillararene-based stationary phases as a new type of stationary phases for GC separations.

Multipurpose new gas chromatography column based on pillararenes functionalized with imidazolium ionic liquids

BACKGROUND

Gas chromatography is worldwide recognized as one of the most important analytical techniques, due to its high versatility and reliability. The heart of a gas chromatograph is the column, that allows analyte peak separations and, consequently, accurate qualitative and qualitative analyses. New and more efficient columns are always requested to satisfy new and challenging analytical needs.

RESULTS

In this work, imidazolium ionic liquids functionalized pillar [5] arenes have been used for the first time as gas chromatographic stationary phases, considering their highly symmetric pillar-shaped architecture with cavities rich in π-electrons. Four imidazolium ionic liquids functionalized pillar [5] arenes have been tested as stationary phases with numerous analytes and isomers. In particular, one of these showed superior performances if compared to commercial columns, enabling challenging isomeric separations of halogenated benzenes, aromatic aldehydes, and aromatic anilines.

SIGNIFICANCE AND NOVELTY

To our knowledge, this is the first report on the use of the ionic liquid P[n]A as a stationary phase in chromatography, either in GC or liquid chromatography (LC) separations. This work demonstrates the promising potential of ionic liquid P[n]A stationary phases for chromatographic separations.

Entropy driven separation of xylene isomers on graphitic carbon adsorbents

Separation of xylene isomers remains one of the most important and challenging applications of adsorption-based separations in petrochemical industry. Despite the sustainable success of zeolite-based separations a search for efficient adsorbents selective for xylenes, especially para-xylene, is constantly ongoing. In this work, a potentially scalable chromatographic separation of all three xylenes was achieved on graphitic carbon sorbents, including a self-packed sorbent based on an oligo-graphene. A curious feature of this separation is stronger retention of para-xylene than meta- and, in some conditions, even than ortho-xylene. Noticeably, separation selectivity between para- and meta-isomers does not depend on temperature. Apparently, lower entropy of para-xylene in solution due to its higher molecular symmetry leads to a lesser adsorption entropy loss, which makes its adsorption statistically more likely. The concept of using carbon adsorbents for entropy driven chromatography separations may be useful for the isolation of xylenes from their mixture and, possibly, for other positional isomers separation.

Hydroxyl-functionalized pillar[5]arene with high separation performance for gas chromatography

Here we report the first example of employing hydroxyl-functionalized pillar[5]arene (P5A-C10-OH) as stationary phase for capillary gas chromatographic (GC) separations. The statically coated P5A-C10-OH capillary column possessed moderate polarity and column efficiency of 3233 plates per m determined by n-dodecane. As a result, the P5A-C10-OH column exhibited high-resolution capability for the mixture of 17 analytes from apolar to polar nature. Importantly, it exhibited advantageous performance for high resolution of the challenging isomers of bromonitrobenzene, chloroaniline, bromoaniline, iodoaniline and dimethylaniline with good peak shapes over the P5A-C10 and commercial HP-35 columns. In addition, eight cis-/trans-isomers with diverse types were baseline separated on the P5A-C10-OH column. And the application of detecting isomeric impurities in real samples gave strong evidence of its potential and feasibility for the viable GC analysis.

Ester-functionalized pillar[6]arene as the gas chromatographic stationary phase with high-resolution performance towards the challenging isomers of xylenes, diethylbenzenes, and ethyltoluenes

This work presents the first example of the utilization of polar ester group functionalized pillar[6]arene (P6A-C10-OAc) as a stationary phase for capillary gas chromatographic (GC) separations. The statically coated P6A-C10-OAc column showed a high column efficiency of 5393 plates/m and moderate polar nature. Its resolving capability and retention behaviors were investigated for a mixture of 20 analytes and more than a dozen isomers from apolar to polar in nature. As evidenced, the P6A-C10-OAc column achieved high-resolution separations of all the analytes and good inertness. Importantly, it exhibited distinctly advantageous performance for high resolution of the challenging isomers of xylenes, diethylbenzenes, ethyltoluenes, and halobenzenes over the commercial HP-5 (5% phenyl dimethyl polysiloxane), HP-35 (25% phenyl dimethyl polysiloxane), and PEG-20M (polyethylene glycol) columns.

A new organic soluble cucurbit[7]uril-truxene derivative as stationary phase for gas chromatographic separation of some challenging isomers

The insolubility of parent cucurbit[n]uril or substituted-cucurbit[n]uril derivatives in organic solvents has always been a key factor restricting the column efficiency and selectivity of capillary gas chromatography column based on cucurbit[n]uril. In this work, a novel organic soluble cucurbit [7]uril-truxene derivative (Q [7]-Tr) was synthesized by Heck coupling reaction between the mono-iodinated hexahexyl-truxene and 4-vinylbenzyloxy-Q [7]. A capillary gas chromatographic column was prepared by static method with this derivative as stationary phase. The column exhibited moderate polarity and an efficiency of 5857 plates/m using n-dodecane as analyte at 120 °C, indicating the highest column efficiency of all the based-cucurbit[n]uril columns published. The Q [7]-Tr column showed high resolutions for a wide range of analytes with different polarities and better separated selectivity towards tough isomers such as aromatic amines isomers and xylene isomers compared to commercial HP-5, DB-35, Stabilwax and Q [7]-based columns. Moreover, the new column exhibited high thermal stability up to 320 °C and excellent repeatability, demonstrating a great potential as a new stationary phase in capillary gas chromatography.

A novel pillar[3]trianglimine macrocycle with a deep cavity used as a chiral selector to prepare a chiral stationary phase by thiol-ene click reaction for enantioseparation in high-performance liquid chromatography

A chiral pillar[3]trianglimine (C60 H72 N6 O6 ) with a deep cavity has been developed as a chiral selector and bonded to thiolated silica by thiol-ene click reaction to fabricate a novel chiral stationary phase for enantioseparation in high-performance liquid chromatography. The enantioseparation performance of the fabricated chiral stationary phase has been evaluated by separating various racemic compounds, including alcohols, esters, amines, ketones, amino acids, and epoxides, in both normal-phase and reversed-phase elution modes. In total, 14 and 17 racemates have been effectively separated in these two separation modes, respectively. In comparison with two widely used chiral columns (Chiralcel OD-H and Chiralpak AD-H), our novel chiral stationary phase offered good chiral separation complementarity, separating some of the tested racemates that could not be separated or were only partially separated on these two commercial columns. The influences of analyte mass, mobile phase composition, and column temperature on chiral separation have been investigated. Good repeatability, stability, and column-to-column reproducibility of the chiral stationary phase for enantioseparation have been observed. After the fabricated column had been eluted up to 400 times, the relative standard deviations (n = 5) of resolution (Rs) and retention time of the separated analytes were < 0.39% and < 0.20%, respectively. The relative standard deviations (n = 3) of Rs and retention time for column-to-column reproducibility were < 4.6% and < 5.2%, respectively. This study demonstrated that the new chiral stationary phase has great prospects for chiral separation in high-performance liquid chromatography.

One-pot synthesis of pillar[5]quinone-amine polymer coated silica as stationary phase for high-performance liquid chromatography

To expand the application of pillararene in chromatographic separation, we designed and fabricated a pillar[5]quinone-amine polymer coated silica through quinone-amine reaction by facile one-pot synthesis method, which was applied as a stationary phase for high-performance liquid chromatography. Separation of hydrophobic compounds, hydrophilic compounds, halogenated aromatic compounds, and 11 aromatic positional isomers was achieved successfully in this stationary phase. Reverse-phase separation mode and hydrophilic-interaction separation mode were proved to exist, indicating the potential application of the mix-mode stationary phase. Studies of chromatographic retention behavior and molecular simulation showed that multiple interactions might play an important role in the separation process, including hydrophobic interaction, hydrogen-bonding interaction, aromatic π-π interaction, electron donor-acceptor interaction, and host-guest interaction. Column repeatability and stability were tested, which showed relative standard deviations of retention time less than 0.2% for continuous 11 injections, and the durability relative standard deviations of retention time were less than 0.91% after 90 days. This novel design strategy would broaden the application of pillararene-based covalent organic polymer in chromatography and separation science.

High-Resolution Performance of Polycaprolactone Functionalized with Guanidinium Ionic Liquid for Gas Chromatography

This work firstly reported a new polycaprolactone based material functionalized with guanidinium ionic liquid (PCL-GIL) as the stationary phase with high resolution performance for capillary gas chromatography (GC). It is composed of polycaprolactone (PCL) and guanidinium ionic liquid (GIL) with amphiphilic conformation. The PCL-GIL capillary column coated by static method exhibited high column efficiency of 3942 plates/m and moderate polarity. As a result, the PCL-GIL column exhibited high-resolution capability. For a mixture of 27 analytes with a wide ranging polarity and outperformed the PCL-2OH and HP-35 columns, showing its advantageous separation capability for analytes of diverse types. Moreover, the PCL-GIL column showed high resolving capability for various positional isomers and cis-/trans-isomers, including alkylbenzenes, chlorobenzenes, naphthalenes, bromonitrobenzenes, chloronitrobenzenes, benzaldehydes, phenols, alcohols, respectively. In a word, PCL derivatized by GIL units as a new type of stationary phase has a promising future in GC separations.