Differentiation of xylene isomers remains as one of the most important challenges in the chemical industry, mainly due to the similar molecular sizes and boiling points of the three xylene isomers. Fluorescence-based chemical sensors have attracted wide attention due to their high sensitivity and versatile applications. Here, we report a novel fluorescent metal–organic framework named NUS-40, which is able to selectively detect and discriminate o-xylene from other xylene isomers. Suspension of NUS-40 in o-xylene produces a distinct red shift in the fluorescence emission compared to that in either m-xylene or p-xylene. Moreover, the extent of peak shift is dependent on the concentration of o-xylene in xylene isomer mixtures, and the observed linear correlation between fluorescence intensity and o-xylene concentration is beneficial for quantitative detection. The possible mechanism of such responsive fluorescence behavior was investigated by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance, and vapor sorption experiments. In addition, facile metalation of the porphyrin centers with metal ions provides an additional route to fine-tune the sensing properties.
The development of energy-efficient processes for selective separation of p-xylene from mixtures with its isomers is of vital importance in the petrochemical industries. Current industrial practice uses BaX zeolite that has high adsorption selectivity for p-xylene. Finding para-selective structures is challenging. With state-of-the-art simulation methodologies we systematically screened a wide variety of zeolites and metal-organic frameworks ( MOFs). Our investigations highlight the crucial importance of the channel dimension on the separation. MAF-X8 is particularly noteworthy because the channel dimensions and geometry allow "commensurate stacking" which we exploit as a separation mechanism at saturation conditions. Due to a significantly improved capacity compared to BaX, the cycle times for p-xylene with MAF-X8 are found to be about a factor of 4.5 longer. This is expected to result in significant process improvements.
It's time for application: Chromium terephthalate MIL‐101, a metal–organic framework (MOF) with coordinatively unsaturated sites, was utilized as the stationary phase to fabricate the first MOF‐coated capillary column for high‐resolution gas‐chromatographic separation of xylene isomers and ethylbenzene with excellent selectivity.
An ambipolar poly(diketopyrrolopyrrole‐terthiophene)‐based field‐effect transistor (FET) sensitively detects xylene isomers at low ppm levels with multiple sensing features. Combined with pattern‐recognition algorithms, a sole ambipolar FET sensor, rather than arrays of sensors, can discriminate highly similar xylene structural isomers from one another.
Purification of the C8 alkylaromatics o-xylene, m-xylene, p-xylene, and ethylbenzene remains among the most challenging industrial separations, due to the similar shapes, boiling points, and polarities of these molecules. Herein, we report the evaluation of the metal–organic frameworks Co2(dobdc) (dobdc4– = 2,5-dioxido-1,4-benzenedicarboxylate) and Co2(m-dobdc) (m-dobdc4– = 4,6-dioxido-1,3-benzenedicarboxylate) for the separation of xylene isomers using single-component adsorption isotherms and multicomponent breakthrough measurements. Remarkably, Co2(dobdc) distinguishes among all four molecules, with binding affinities that follow the trend o-xylene > ethylbenzene > m-xylene > p-xylene. Multicomponent liquid-phase adsorption measurements further demonstrate that Co2(dobdc) maintains this selectivity over a wide range of concentrations. Structural characterization by single-crystal X-ray diffraction reveals that both frameworks facilitate the separation through the extent of interaction between each C8 guest molecule with two adjacent cobalt(II) centers, as well as the ability of each isomer to pack within the framework pores. Moreover, counter to the presumed rigidity of the M2(dobdc) structure, Co2(dobdc) exhibits an unexpected structural distortion in the presence of either o-xylene or ethylbenzene that enables the accommodation of additional guest molecules.
A breakthrough result: The microporous metal–organic framework MIL‐47 is an excellent adsorbent for the separation of C8 alkylaromatic compounds, such as ethylbenzene, meta‐xylene, and para‐xylene. The potential of MIL‐47, with its high uptake capacity and its hydrophobic nature, for real separations of the C8 alkylaromatic compounds was demonstrated by breakthrough and chromatographic experiments (see picture).
Separation of p‐xylene (kinetic diameter ca. 0.58 nm) from its bulkier isomers (o‐xylene and m‐xylene, ca. 0.68 nm) is challenging, but important in the petrochemical industry. Herein, we developed a highly selective and stable metal–organic framework (MOF) MIL‐160 membrane for selective separation of p‐xylene from its isomers by pervaporation. The suitable pore size (0.5∼0.6 nm) of the MIL‐160 membrane selectively allows p‐xylene to pass through, while excluding the bulkier o‐xylene and m‐xylene. For the separation of equimolar binary p‐/o‐xylene mixtures at 75 °C, high p‐xylene flux of 467 g m−2 h−1 and p‐/o‐xylene selectivity of 38.5 could be achieved. The stability of MIL‐160, ensured the separation performance of the MIL‐160 membrane was unchanged over a 24 h measurement. The high separation performance combined with its high thermal and chemical stability makes the MIL‐160 membrane a promising candidate for the separation of xylene isomers. Come on xylene: Through chemical modification of the Al2O3 support by bio‐inspired polydopamine, a highly selective and stable MIL‐160 membrane was developed for separation of p‐xylene from its isomers by pervaporation. Attributed to its suitable pore size (0.5≈0.6 nm), the MIL‐160 membrane selectively allows p‐xylene to pass through, while excluding the bulkier o‐xylene and m‐xylene.
We report a study of the use of the porous metal–organic framework material MIL-53(Fe), FeIII(OH)0.8F0.2[O2C–C6H4–CO2], for the separation of BTEX mixtures (benzene, toluene, ethylbenzene, and the three xylene isomers). Crystal structures of the three host:guest materials MIL-53(Fe)[xylene], where xylene = the ortho, meta, or para isomer of dimethylbenzene, have been solved and refined from powder X-ray diffraction. Each exhibits a fully expanded form with a variety of host:guest and guest:guest interactions responsible for stabilizing the structure. While the ortho- and meta- isomers present a similar arrangement when occluded in the MIL-53 host, the para-xylene shows a distinctly different set of interactions with the host. Upon thermal treatment, xylenes are partially lost to give crystalline phases MIL-53(Fe)[xylene]0.5, the structures of which have also been solved. The kinetics of uptake of each xylene by MIL-53(Fe)[H2O], in which the water is replaced by the organic guest, have been studied using time-resolved energy-dispersive X-ray diffraction: this shows differences in kinetics of the adsorption of the three isomers. Under chromatographic conditions in heptane at 293 K, anhydrous MIL-53(Fe) is able to separate the three xylene isomers with elution of the para-xylene before the other two isomers, and at 323 K the host is able to resolve all components of the BTEX mixture.
Vapor-phase adsorption and separation of the C8 alkylaromatic components p-xylene, m-xylene, o-xylene, and ethylbenzene on the metal−organic framework MIL-47 have been studied. Low coverage Henry adsorption constants and adsorption enthalpies were determined using the pulse chromatographic technique at temperatures between 230 and 290 °C. The four C8 alkylaromatic components have comparable Henry constants and adsorption enthalpies. Adsorption isotherms of the pure components were determined using the gravimetric technique at 70, 110, and 150 °C. The adsorption capacity and steepness of the isotherms differs among the components and are strongly temperature dependent. Breakthrough experiments with several binary mixtures were performed at 70−150 °C and varying total hydrocarbon pressure from 0.0004 to 0.05 bar. Separation of the different isomers could be achieved. In general, it was found that the adsorption selectivity increases with increasing partial pressure or degree of pore filling. The separation at a high degree of pore filling in the vapor phase is a result of differences in packing modes of the C8 alkylaromatic components in the pores of MIL-47.