Separation of Xylene Isomers: A Review of Recent ... - ACS Publications

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Cite This: Ind. Eng. Chem. Res. 2017, 56, 14725−14753

Separation of Xylene Isomers: A Review of Recent Advances in Materials Yuxi Yang, Peng Bai, and Xianghai Guo*

Ind. Eng. Chem. Res. 2017.56:14725-14753. Downloaded from pubs.acs.org by UNIV OF FLORIDA on 01/10/19. For personal use only.

Department of Pharmaceutical Engineering, School of Chemical Engineering and Technology, and Key Laboratory of Systems Bioengineering, Ministry of Education, Tianjin University, Tianjin 300350, China ABSTRACT: The xylene isomers p-xylene, o-xylene, and m-xylene are aromatic hydrocarbons comprising with two methyl groups located at different positions on a benzene ring. The mixture originates from catalytic reforming of crude oil, and each individual isomer acts as a valuable intermediate; however, similar physicochemical properties make their separation difficult. This Review focuses on materials employed for their separation, such as metal−organic frameworks, molecular sieves, organics, and graphene quantum dots. Recent advances in separation of xylene isomers are summarized, including adsorption, membrane, and chromatographic separation techniques, and adsorption capacity and selectivity combined with mechanisms of separation are discussed.

1. INTRODUCTION Xylene isomers, i.e., p-xylene (PX), o-xylene (OX), and m-xylene (MX), are mainly derived from catalytic reforming of crude oil. Among the isomers, PX is the most valuable intermediate, being an indispensable raw material for synthesizing polyethylene terephthalate (PET) and polybutylene terephthalate (PBT). OX is mainly used in the production of phthalic anhydride, and MX is mostly applied to produce isophthalic acid.1 Due to the value of the individual isomers, efficient separation of xylene isomers is a critical focus. Though ethylbenzene (EB), benzene (B), and toluene (T) are also byproducts of catalytic reforming processes, pure benzene and pure toluene are easily isolated during the second separation of aromatic complexes, which converts reformates into basic petrochemical intermediates, i.e., benzene, toluene, and a mixture of ethylbenzene and xylenes, called the BTEX group.2 Since EB usually exists as an impurity of xylene isomers, referred to collectively as C8 aromatics, separation of EB and xylene isomers will be the focus of this Review. Separation of xylene isomers has been thought of as one of seven world-changing separations,3 highly challenging as a result of the similar physicochemical properties of these isomers (Table 1). Distillation, the conventional separation method, is not applicable here, for the number of theoretical plates needed to separate o-xylene to commercial specifications is more than 150; isolating m-xylene and p-xylene requires as many as 360 theoretical plates because of their close boiling points.4 Fractional crystallization5 and selective adsorption are used industrially as alternatives for the separation. Crystallization is based on the difference in freezing points among the xylenes. PX recovery is merely 60−70% due to the eutectic point accompanied with the high energy requirement to operate at around 220 K. As a result, crystallization has only been applied for separation of PX-rich feedstock (above 80%) since the 1990s and © 2017 American Chemical Society

Table 1. Physical Properties of C8 Aromatics kinetic diameter (Å)a boiling point (K)b freezing point (K)b dipole moment (D)c polarizability (cm3)c density at 298 K (g cm−3)c a

PX

MX

OX

EB

6.7 411.5 286.4 0 13.7 0.858

7.1 412.3 222.5 0.36 (liquid) 14.2 0.861

7.4 417.6 248.0 0.62 (gas) 14.9 0.876

6.7 409.3 178.2 0.59 14.2 0.867

Data from ref 12. bData from ref 13. cData from ref 14.

accounts for 25% of the PX separation worldwide currently.2,6 Adsorption is the process of material enrichment or fluid density elevation adjacent to an interface.7 It is the dominant method and works for the remaining 75% of xylenes separation due to its higher efficiency and lower energy consumption, although the desorption process squanders most of the energy. Industrially, selective adsorption is carried out on simulated moving beds (SMBs) developed by Universal Oil Products (UOP) in the 1960s.8 UOP’s Parex, Toray’s Aromax, and IFP’s Eluxyl are currently used for separation of xylene isomers.9,10 The industrial SMB process operates at a temperature of about 180 °C and a pressure of about 9 bar in tghe liquid phase under pore saturation conditions of adsorbents, with PX recovery of 97−99% and purity of 99.7−99.9%.2 The factors determining the performance of a SMB are process conditions and adsorbent selection, in which the adsorbent should possess simultaneously high selectivity, high capacity, high diffusivity, and chemical, thermal, and mechanic stabilities.11 Received: Revised: Accepted: Published: 14725

July 28, 2017 November 11, 2017 November 21, 2017 November 21, 2017 DOI: 10.1021/acs.iecr.7b03127 Ind. Eng. Chem. Res. 2017, 56, 14725−14753

Review

Industrial & Engineering Chemistry Research

177 °C.53 PX capacity as well as selectivity declined along with temperature elevation, pointing to higher PX productivity at lower temperatures. When balanced with mass transfer rate, 150 °C was selected as a favorable temperature. Despite many advantages, there were significant mass transfer limitations on conventional X zeolite below 177 °C. Small-crystallite-size X zeolite, exhibiting an average size of 1.4 μm, smaller than conventional X zeolite of 1.8 μm or more, manifested a considerably greater transfer rate in SMB, especially at temperatures below 175 °C. This originates from the shortened mass transfer path and could benefit the separation of C8 aromatics. Similar nanosize zeolite (50−300 nm) was synthesized and evaluated. Experiments separating MX from C8 aromatics employing NaY zeolite showed better capacity and selectivity on nanosize zeolite compared with micrometer-size zeolite (Figure 1). All trials demonstrated Langmuir-type adsorption isotherms. The best operating conditions were a temperature of 130 °C, pressure of 8 atm, toluene as desorbent, flow rate of 2.5 mL min−1, and SiO2/Al2O3 ratio of 4.5 for microcrystalline zeolite or 4.8 for nanocrystalline zeolite. Water content had little influence at these operating conditions. Saturation capacities on nanosize zeolite were 110, 45, 42, and 11 mg g−1 for MX, PX, OX, and EB (Figure 1b), and optimized selectivity values were 6.88, 3.16, and 2.90 for MX/EB, MX/OX, and MX/PX, respectively.54 Nanosize X zeolite crystals (around 100 nm) also manifested larger surface areas and shorter diffusion lengths. Rasouli et al. studied the influence of several factors on xylenes separation through liquid-phase breakthrough experiments. Results demonstrated excellent PX separation on Ba2+ ion-exchanged X zeolite. When the SiO2/Al2O3 ratio of BaX was increased from 1.1 to 1.7, selectivity dropped, implying a best value of 1.1. A study on water content depicted little influence. Compared with indan, toluene as desorbent achieved more effective desorption. All adsorption isotherms fitted to a Langmuir-type isotherm with saturation adsorption capacities of 103.4, 49, 31.5, and 20.2 mg g−1 for PX, OX, EB, and MX, respectively. Further experiments optimized the operating conditions at a temperature of 150 °C, pressure of 6 atm, and flow rate of 2 mL min−1. Selectivities were 7.19, 3.75, and 2.82 for PX/MX, PX/EB, and PX/OX, respectively.55 Binder-free nanosize zeolite X was then studied. KX zeolite exhibited the highest selectivity on PX separation compared to HX, LiX, and NaX zeolite. The optimal SiO2/Al2O3 ratio was 1.1. The best operating conditions were a temperature of 150 °C, pressure of 0.7 MPa, and flow rate of 2 mL min−1. Liquid-phase experiments showed saturation capacities of 101, 42, 32, and 18 mg g−1 for PX, OX, EB, and MX, respectively. Optimized selectivity values were 5.36 for PX/MX, 3.22 for PX/EB, and 2.43 for PX/OX.56 Trends for water content and desorbent were similar to those seen with nanosize BaX zeolite.55 The foregoing articles indicated that water content had a negligible influence on separation, but this may result from the small range investigated. Water molecules adsorbed on zeolite are polarizable because of the strong electrostatic field between the alumina framework and exchanged cations. As a result, water molecules elevate the acidic properties of Brönsted acids in the zeolite. So, water polarization not only causes the variation of interactions between adsorbent and adsorbate but also changes the selectivity of adsorbent and mass-transfer rate of adsorbate.57 Lahot et al.58 arrived at the conclusion that only when zeolite was kept at optimal water content could it exhibit the best separation. For BaY, decreasing water content led to increased adsorption capacity, and the optimum water content for PX adsorption at 20 °C was 4−5.5 wt%.

Typical adsorbents applied in industrial SMB processes are cation-exchanged faujasite (FAU)-type zeolites X and Y. On separating xylenes, K+-exchanged Y zeolite put forward selectivity values of 5.25 for PX/MX, 1.94 for PX/EB, 4.63 for PX/OX, and 1.0 for MX/OX in batch experiments in n-octane at 293 K with a loading capacity of 1.75 mmol g−1, estimated from the fitted Langmuir model.15 (K+,Ba2+)-exchanged and K+exchanged Y zeolite exhibited selectivity values of 4 and 4.5 for PX/MX, respectively.16 Ba2+-exchanged X zeolite presented selectivities of 3.2−4.2 for PX/MX at 175 and 130 °C in binary competitive experiments.17 Nearly all typical cation-exchanged FAU-type zeolites manifested a saturation capacity of 0.8−1.8 mmol g−1.15,17−20 Besides X and Y zeolites, various adsorbents have been evaluated as separation media, such as carbon materials,21 other types of zeolites,22,23 polymers,21,24 silica gel,25 and metal− organic frameworks (MOFs).26−30 Recently, experiments have been combined with simulations in some adsorbent studies.31,32 A specific material having potential application in xylenes separation is mainly assessed on the basis of adsorption isotherm experiments, competitive batch experiments, pulse chromatography, or breakthrough experiments. In addition to adsorption, membrane and chromatographic separation also showed great potential in acquiring individual xylene isomers in recent years. Membrane separation is environmentally benign with low energy intensity, but it has yet to find its way into industrial separation processes. The exquisite preparation, fair life span, and difficulties involved in mass production of applicable membranes set up barriers against practical applications. Although a review was published on separating xylenes with zeolites in 2011,33 and some results of adsorptive separation of xylene isomers with MOFs were included in several early reviews,13,34−40 no recent review has concentrated on the isolation of xylene isomers on the whole. This contribution aims to provide a comprehensive overview on recent advances in separation of valuable xylene isomers, over the period of 2011−2017. The content is mainly organized in terms of different materials employed in the separation of xylene isomers, with a further category focused on different separation processes, such as adsorption, chromatography, and membrane. Capacities and selectivity values combined with experiment conditions, mechanisms leading to the separation, and factors influencing the separation are presented, which it hoped to provide an exhaustive summary of xylenes separation.

2. MOLECULAR SIEVES Molecular sieves are inorganic, crystalline, and porous materials with high internal surface areas. Their name originated from their function as a sieve operating on a molecular scale. Molecular sieves exhibit highly ordered channels, high surface areas, exchangeable cations, adjustable pore sizes, stabilities, and other beneficial properties. For these reasons, molecular sieves are widely used in adsorption41−43 and catalysis.44−49 They also play an important role in separation of xylene isomers because of their identity as the only adsorbent applied industrially.50−52 2.1. Adsorption Study for Simulated Moving Bed Process. Based on the industrial SMB process for isolation of xylene isomers, we will deal with liquid-phase separations relevant for the SMB process in this part. The capacities of the adsorbents in this Review are mostly derived from the adsorption isotherms, except for the ones with special descriptions. Cheng et al. carried out separation tests with conventional Ba2+ and K+ ion-exchanged X zeolite at temperatures between 122 and 14726

DOI: 10.1021/acs.iecr.7b03127 Ind. Eng. Chem. Res. 2017, 56, 14725−14753

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Figure 1. Scanning electron microscopy images (top) and xylenes adsorption isotherms (bottom) for (a) microcrystalline and (b) nanocrystalline NaY zeolite. Adapted with permission from ref 54, Figures 3, 5, and 6.

by converting binder material to zeolite X. Quaternary breakthrough experiments at 150 °C manifested selectivity values of 1.8, 5.4, and 4.8 for PX/EB, PX/MX, and PX/OX, respectively, with 0.18 cm3/cm3 adsorbent for capacity.65 Binderless zeolite BaKX contained inert clay binder-converted zeolite and zeolite X. Selectivity values were 1.84 for PX/EB, 5.25 for PX/MX, and 4.75 for PX/OX, with adsorption capacity index about 11.0. Binderless BaKX zeolite put forward high selectivity, high capacity, good mass transfer property, and mechanical strength.66 A preparation method and application in xylenes separation of binderless X zeolite were also reported in another paper, though without specific values.67 Silva et al. assessed Ba2+exchanged FAU zeolite through breakthrough experiments under conditions of the Parex process (T = 177 °C, P = 9 bar, and iso-octane as tracer). Results pointed to selective adsorption of PX over the other isomers, with selectivity values of 3.02 for PX/MX, 2.84 for PX/OX, and 2.27 for PX/EB.68 A core−shell material, SiO2/silicalite-1, containing a mesoporous SiO2 core and a thin silicalite-1 shell, was assessed in the vapor-phase separation of PX and OX. Adsorption isotherms fitted to a Langmuir isotherm with saturation capacities of 286.78 mol m−3 for PX and 154.76 mol m−3 for OX. Binary breakthrough experiments at 423 K showed a high selectivity of 15 for PX/OX. Further research concluded that PX/OX selectivity arose solely from a molecular sieving effect of the silicalite-1 shell, while the core possessed no selectivity.69 A ZSM-11/silicalite-2 core−shell molecular sieve with a compact silicalite-2 shell also proved effective for the separation of xylenes.70 Dehkordi et al. studied adsorption of C8 isomers on Beta zeolite, which exhibited a high ratio of Si/Al. Adsorption isotherms in liquid phase on Na-Beta zeolite at 15, 25, and 35 °C best fitted to a Langmuir isotherm model and indicated PX selectivity. Adsorption capacities at 25 °C were 143, 105, 83, and 68 mg g−1 for PX, EB, MX, and OX, respectively, and capacities declined with increasing temperature. Water content manifested

Breakthrough experiments with conventional Y zeolites ionexchanged with Li+, Na+, and K+ pointed to maximum selectivity on NaY zeolite. Adsorption capacities were 99.6, 51.3, 41.5, and 19.5 mg g−1 for MX, PX, OX, and EB, respectively.57 The best operating conditions were the same as above,54 and optimized selectivity values were 5.93 for MX/EB, 2.83 for MX/OX, and 2.62 for MX/PX.57 Another kind of adsorbent contained mostly Na+- and Ag+-exchanged Y zeolite and a little amount of binder. Separation experiments acquired maximum selectivities of 3.86 for MX/EB, 1.89 for MX/PX, and 1.83 for MX/OX at 160 °C. Selectivity climbed with decreasing water content, but when the water content was extremely low, it brought about difficulty in desorption, so 2.0−3.2% was the most favorable.59 Y zeolite exchanged with Na+ or Li+ put forth a selectivity of 1.70 for MX/OX in a binary mixture at 140 °C.60 As for conventional X zeolite, Laroche et al. synthesized BaX with high surface area.61 Liquid-phase breakthrough experiments on separating PX from MX showed a capacity of 0.191 cm3 g−1 and PX/MX selectivity of 3.35 at 175 °C, while similar tests at 160 °C presented a capacity of 0.189 cm3 g−1 and selectivity of 3.72.62 The capacity of this conventional BaX zeolite55 is higher than that on nanosize BaX zeolite, which is attributable to the experiments being carried out by different researchers using different forms of zeolite under different conditions. This could not overturn the conclusion of higher capacity and selectivity on nanosize zeolites proved by tests under the same conditions. BaX-type zeolite granules manifested well-formed pores, excellent separation, and mechanical strength that could be useful in a SMB. Experiments at 177 °C provided a capacity of 0.098 g g−1 for PX.63 Ba2+- and K+-exchanged low silica X (LSX) zeolite adsorbent were also evaluated on PX separation from C8 aromatics. Binary experiments at 175 °C with Ba2+-exchanged LSX zeolite depicted capacity of 0.181 cm3 g−1 and selectivity of 2.53 for PX/EB.64 A new binder-converted X zeolite contained two kinds of zeolite and a little unconverted binder: the first zeolite was conventional zeolite X, and the second was prepared 14727


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Industrial & Engineering Chemistry Research a little influence on the selectivity value.71 Adsorption isotherms of microporous Beta zeolite were fitted by a single-site Toth model, while isotherms on mesoporous Beta zeolite were fitted by a dual-site Toth model. Capacities on both zeolites followed OX > PX > MX, arising from a combination of molecular constitution and electrostatic effects. Diffusivities were in the sequence PX > MX > OX, related to PX being the smallest molecule as well as having the weakest adsorbate−adsorbent interaction. Accretion in mesoporosity led to enhanced mass transfer, higher adsorption capacity, and reduction in Henry’s constant for xylenes.72 Breakthrough tests at 398 K with Beta zeolite showed capacities of 0.72 and 1.31 mmol cm−3 for OX and PX, with a selectivity of 2 for PX/OX.73 Vapor-phase adsorption isotherms at 303 K on SAPO-5 manifested saturation adsorption capacities of 0.782, 0.516, and 0.520 mmol g−1 for OX, PX, and MX, respectively. Temperature increase caused the capacities to decrease. Adsorption kinetics and energy further confirmed OX selectivity.74,75 High silica zeolite ZSM-5 was studied for separation of xylene isomers at different conditions. Breakthrough experiments manifested a decline in the Si/Al ratio accompanied with a decrease in selectivity. Among five desorbentstoluene, benzene, p-diethylbenzene (PDEB), p-dichlorobenzene (PDCB), and n-butylbenzene (n-BB)PDEB presented the most effective desorption. Further experiments confirmed little influence of water content on selectivity. All adsorption isotherms illustrated a Langmuirtype isotherm. Optimized saturation adsorption capacities were 142.2, 60.3, 30.5, and 12.4 mg g−1 for PX, EB, OX, and MX, with maximum selectivity values of 24.99, 16.78, and 6.76 for PX/MX, PX/OX, and PX/EB, respectively, at a flow rate of 2.5 mL min−1 and Si/Al ratio over 450.76 Hasan et al. applied a new computational framework to select zeolites and optimize the SMB process for PX separation. It was a multiscale approach, and calculations illustrated that MWW followed by MEL zeolites were better than current adsorbents (MWW and MEL zeolites had no framework aluminum and associated cations). The SMB process could be optimized by employing multiple zeolites, i.e., different columns using different zeolites11 (Figure 2). Tables 2 and 3 summarize chromatographic separation of xylene isomers applying molecular sieves with preference for PX and MX, respectively. The capacity of the adsorbent originates mostly from the quantity adsorbed of its most selective isomer. 2.2. Membrane Separation. Membrane separation takes advantage of selective permeability for different molecules in the membrane and was used in mixture separation, purification, and enrichment. It had already found applications in the field of food science, medicine, environmental protection, chemistry, metallurgy, water treatment, and many others. In the separation of xylenes, it has become more and more important due to its outstanding merits of less energy consumption and continuous operation.79−81 Free-standing carbon molecular sieve (CMS) hollow fiber films contained slit-like transport paths and resulted in high productivity (Figure 3a,b). These films were applied in separating liquid feed xylenes at high pressures via reverse osmosis. PX permeance calculated from single-component permeation results decreased with pyrolysis temperature increasing from 450 to 550 °C. This implied that higher pyrolysis temperature contributed to formation of smaller but more selective channels79 (Figure 3c), and this phenomenon was also observed in gas separation tests.80 Experiments with feed mixtures containing 50:50 and 90:10 (mol mol−1) for PX/OX at room

Figure 2. (a) 24-column SMB with multiple zeolites (columns 1 and 2 using Y zeolite, columns 3−5 using MEL, columns 6−9 using Y zeolite, columns 10−21 using MWW, and columns 22−24 using Y zeolite). Adapted with permission from ref 11, Figure 8.

temperature, 50−120 bar pressure, and pyrolysis at 550 °C obtained PX/OX permselectivity about 100, PX permeance around 10−9 mol m−2 s−1 Pa−1, and PX flux above 10−3 mol m−2 s−1; the PX flux was 10 times higher than that of the state-of-the-art zeolite membrane (Figure 3c). These films put forth outstanding mechanical resistance to over 100 bar pressure and took advantage of energy savings derived from the reverse osmosis process.79 MFI zeolite membranes were also applied for xylenes separation. Lee et al. synthesized c-oriented MFI membranes by rapid thermal processing (RTP) with a shorter time for secondary growth81 (from 2 to 1 d), in which RTP could reduce grain boundary defects in MFI films.82 In binary xylenes separation experiments with different films by different calcination modes, membrane calcined by RTP mode, which was named RS1 (Figure 4a,b), showed maximum PX permeance of 3 × 10−8 mol m−2 s−1 Pa−1 and separation factor of 88 for PX/OX at 125 °C, while c-oriented MFI membrane F1, fast calcined at 480 °C with heating ramp rates of 30 °C min−1 exhibited separation factor of 3 for PX/OX (Figure 4c). Fluorescence confocal optical microscopy (FCOM) illustrated extremely reduced grain boundary defects in RS film, though there were cracks throughout surface. The much lower diffusion coefficient of OX than PX in membrane RS contributed to high selectivity of PX/OX. Lee and co-workers had revealed in previous experiments the poor selectivity was attributed to extrazeolitic paths including defects.81 B-MFI (B-MFI-50:Si/B = 50 and B-MFI-100:Si/B = 100) and Al-MFI hollow-fiber films with nanocomposite structure achieved separation of C8 aromatics. B-MFI-50 acquired separation of PX and EB, which was the first time these two compounds were separated through MFI films. Permeance of PX was significantly the highest in all prepared films, with maximum values of 30 nmol m−2 s−1 Pa−1 at 523 K for B-MFI-50 and 8 nmol m−2 s−1 Pa−1 at 473 K for B-MFI-100. PX/MX separation factors were above 40 for all films below total aromatic pressure 14728


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Industrial & Engineering Chemistry Research Table 2. Molecular Sieves with PX Preference Applied in Separation of Xylene Isomers by Adsorption selectivity molecular sieve

−1

a

KX BaXa BaX

Xb BaKXb BaXc FAUd H/ZSM-5 Na-Beta Beta SiO2/silicalite-1 a

capacity 0.95 mmol g 0.97 mmol g−1 1.29 mmol g−1 1.4 mmol g−1 0.92 mmol g−1 1.3 mmol g−1 1.25 mmol cm−3 − 1.22 mmol g−1 − 1.34 mmol g−1 1.35 mmol g−1 1.31 mmol cm−3 0.287 mmol cm−3

temp (K)

PX/MX

PX/OX

PX/EB

ref

423 423 433 323 450 343 423 421−450 448 450 403−443 298 398 423

5.36 7.19 3.72 1.6 − 2.6 5.4 5.25 − 3.02 24.99 − − −

2.43 2.82 − 1.4 − 2.4 4.8 4.75 − 2.84 16.78 − 2 15

3.22 3.75 − 1.9 − 2.5 1.8 1.84 2.53 2.27 6.76 − − −

56 55 62 77 63 78 65 66 64 68 76 71 73 69

Zeolite in nanosize. bBinder-converted or binderless zeolite. cLSX-type zeolite. dBarium-exchanged zeolite.

380 h, and the separation factor arrived at a steady state of 1100 after 50 h. The thinner film arrived at a steady state quickly, indicating no channel blocking by OX due to the shorter channel length. It was concluded that the thinner film with higher separation factor and permeance was much more favorable.85 Agrawal et al. reported preparation of silicalite-1 membranes with b-out-of-plane orientation as thin as 100 nm via gel-less secondary growth on both porous Stöber silica supports and novel sintered silica fiber (SSF) supports, the latter exhibiting high permeance and strong mechanical properties. Films grown on porous Stöber silica supports presented a maximum separation factor of 830 and permeance of 2.0 × 10 −7−3.0 × 10 −7 mol m−2 s−1 Pa−1 for PX. Nevertheless, Stöber silica supports were not strong enough and often broke during coating, so SSF support was invented. Zeolite nanosheet seeding on SSF showed maximum separation factor of 185 for PX/OX, with improved PX permeance ranging between 1.7 × 10−7 and 5.1 × 10−7 mol m−2 s−1 Pa−1 without degradation detected during 1 month of testing.86 Separation of PX and OX applying another silicalite-1 membrane revealed a maximum PX flux of 6.33 × 10−6 mol m−2 s−1 at 200 °C in binary tests at feed partial pressure of 0.26 kPa for PX and 0.22 kPa for OX, while the flux of OX kept decreasing along with temperature increasing from 150 to 250 °C. Maximum separation factor was 29 for PX/OX at 200 °C. Simulation depicted flux and separation factor ascending with feed partial pressure elevation and reaching 50 × 10−6 mol m−2 s−1 and 42, respectively, at 1 kPa and 200 °C. Intracrystalline zeolitic diffusion dominated transport of PX and OX as well as their separation.87

Table 3. Molecular Sieves with MX Preference Applied in Separation of Xylene Isomers by Adsorption selectivity

a

molecular sieve

capacity (mg g−1)

temp (K)

MX/PX

MX/OX

MX/EB

ref

NaYa NaY NaAgY NaY/LiY

110 99.6 − −

403 403 433 413

2.90 2.62 1.89 −

3.16 2.83 1.83 1.70

6.88 5.93 3.86 −

54 57 59 60

Zeolite in nanosize.

of 4.5 kPa, while the highest value for PX/EB was 5, gained in B-MFI-50 at around 450 K (Figure 5a). Introduction of EB decreased PX flux and PX/MX separation factor value, along with increase of EB feed concentration they both went down drastically. Separation factors were 20 for PX/OX, 5 for PX/EB, and 70 for PX/MX at EB concentration of 8 vol%83 (Figure 5b). Silicalite-1 is a well-known class of MFI membranes. Uniformly b-oriented silicalite-1 films with porous silica supports (Figure 6a) on PX and OX separation at 80 °C showed much higher permeance for PX and manifested separation factor over 1900 initially, but the permeance reduced continuously in the following 216 h and finally reached a steady state of 0.7 × 10−8 mol s−1 m−2 Pa−1 for PX and 0.0092 × 10−8 mol s−1 m−2 Pa−1 for OX with separation factor of 71. Experiments at 150 °C showed a similar decrease, permeance for PX dropping from 21.6 × 10−8 to 5 × 10−8 mol s−1 m−2 Pa−1; for OX, it varied from 0.0097 × 10−8 to 0.0068 × 10−8 mol s−1 m−2 Pa−1 during 400 h. However, from 20 to 370 h, separation factor remained about 1000 (Figure 6b). Blockage of channels in films by adsorption of OX resulted in the continuous decline in permeances.84 High-quality b-oriented silicalite-1 films with no cracks during calcination were prepared through novel gel-free secondary growth. Uniform films with thicknesses of 1000 and 200 nm were tested for separation of OX and PX. At 150 °C, the 1000 nm thick film had a permeance of 23 × 10−8 mol s−1 m−2 Pa−1 with separation factor over 3000 for PX/OX initially, which fell to 7.5 × 10−8 mol s−1 m−2 Pa−1 after 480 h because of channel blocking by OX; the separation factor decreased to 680 and arrived at a steady state after 375 h. The 200 nm thick film demonstrated permeances of 12 × 10−8 mol s−1 m−2 Pa−1 for PX and 0.009 × 10−8 mol s−1 m−2 Pa−1 for OX during a period of

3. METAL−ORGANIC FRAMEWORKS MOFs, having emerged as novel porous materials, are synthesized from inorganic building units and organic linkers.88,89 Advantages such as high surface area, tunable or tailored pore topologies, structural flexibility, and versatile postsynthesis functionalization of MOFs contribute to their various applications in gas storage,90−92 drug delivery and imaging,26−30,93−95 catalysis,96,97 and separation,98−102 in which xylene isomers separation is included. MOFs appear quite promising in this field, and some MOFs were already presented as good materials, for they behaved equally or more effectively than state-of-the-art molecular sieves in aspects of capacity and selectivity. 14729


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Figure 3. (a) SEM images of cross-linked PVDF hollow fiber precursor and asymmetric CMS fiber film. Inset image exhibited outer skin layer and porous support (left and middle). HeIM image of the CMS film formed from cross-linked PVDF. Inset image depicts bimodal micropore size distribution, and scale bar represents 5 nm (right). (b) Idealized schematic of slit-like CMS microstructure, presenting small micropores (apertures) and micropores (galleries). (c) Ideal selectivity of PX/OX in CMS film with different pyrolysis temperatures (Tp) of 450 (□), 500 (△), and 550 °C (○). Mixture selectivity of PX/OX with Tp of 550 °C and different feed compositions: 50:50 (blue circles) and 90:10 (red circles). Blue and red arrows present increase in transmembrane pressure. Adapted with permission from ref 79, Figures 1 and 2.

Figure 4. SEM images of membrane RS1: (a) top view and (b) cross-sectional view. (c) Permeances of PX (filled) and OX (open) as well as separation factor of PX/OX (semifilled) versus temperature for membrane F1 (left) and RS1 (right). Adapted with permission from ref 81, Figures 1 and 2.

simulated calculations < gas-phase breakthrough experiments < liquid-phase breakthrough experiments diluted with an eluent < bulk mixture liquid-phase breakthrough experiments. The data

Among various experiments applied in selecting materials, those found to be successful, ranked according to their results being close to those of reality in the SMB process, include IAST 14730


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Figure 5. Separation of C8 aromatics on B-MFI-50 hollow-fiber film as a function of (a) temperature (feed partial pressure: PX 0.44 kPa, EB 0.54 kPa, MX 0.60 kPa, and OX 0.57 kPa). (b) EB concentration (temperature 473 K, total feed vapor pressure 2.2 kPa, equimolar PX, MX, and OX concentration). Adapted with permission from ref 83, Figures 6 and 8.

Figure 6. (a) SEM images of b-oriented silicalite-1 films with porous silica supports via secondary growth (inset figure illustrates side view). (b,c) Permeances of PX (open circle) and OX (open square) as well as separation factor for PX/OX with time at temperature of (b) 80 °C and (c) 150 °C. Adapted with permission from ref 84, Figures S24 and 4.

void volume of 915.8 Å3) experienced considerable squeeze compared to 1⊃G (void volume of 3441.2 Å3). 1 manifested selective adsorption of PX over the other isomers. According to C8 aromatics adsorption isotherms on 1 at 298 K, the amount of adsorbed PX was 64 mL g−1, nearly 3 mmol g−1, while for MX, OX, and EB the capacities were 5, 1.15, and 9.4 mL g−1, respectively. Batch experiments also confirmed significant selective adsorption of PX.103 IAST calculations at 298 K revealed a positive correlation of fractional pore occupancy and total gas-phase pressure. When fractional pore occupancy was increased from 0 to 1.0, PX capacity climbed to 2.73 mmol g−1, while capacities of MX, OX, and EB first reached 0.015,

for a certain material analyzed via different experiments varied a bit due to the different environments for the adsorbent. For example, in batch tests there usually exists a solvent, and in breakthrough tests, the liquid mixture may merely be the C8 aromatics. 3.1. Adsorptive Separation. Zinc-based MOFs have made great progress in separation of xylenes, as reported in recent papers. {[Zn4O(L)3(DMF)2]·xG}n (1⊃G, L = 4,4′-{[4-(tertbutyl)-1,2-phenylene]bis(oxy)}dibenzoate, DMF = N,Ndimethylformamide, G = free disordered guest solvent molecules) is a new dynamic MOF with ether linkage as its flexible nodes. The desolvated form Zn4O(L)3 (1, Dyna MOF-100, Figure 7a, 14731


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Figure 7. (a) Overall structure of compound 1 along crystallographic α-axis (desolvated squeezed framework). (b) IAST calculations of the adsorption loading in equilibrium with equimolar gas-phase C8 isomers mixture on Dyna MOF-100 at 298 K. (c) IAST calculations for PX adsorption selectivity of C8 isomers mixture on Dyna MOF-100 (at 298 K), MAF-X8 (at 433 K), and BaX zeolite (at 393 and 453 K). (d) Overall structure of re-solvated framework on PX accommodation mediated breathing along a-axis (free PX molecules in the channels). (e) Schematic representation of selective guestresponsive framework flexibility. Adapted with permission from ref 104, Figures 1, S20, and S21; ref 103, Figure 6; and ref 37, Figure 27.

0.003, and 0.04 mmol g−1, respectively, and finally descended to 0 (Figure 7b). Overall PX selectivity rose to 1000 at fractional pore occupancy of 1.0 and 298 K (overall PX selectivity was defined as 3q3/(q1+q2+q4), where qi was loading of isomer i on adsorbent, 1 = OX, 2 = MX, 3 = PX, and 4 = EB), significantly higher than the values obtained with of MAF-X8 (MAF = metal azolate framework) and BaX104 (Figure 7c). MAF-X8 was a Zn(II) pyrazolate-carboxylate framework with one-dimensional (1D) channels of 10 Å. Adsorption isotherms at 433 K exhibited capacity of 2.1 mmol g−1 for PX, higher in comparison with OX, MX, and EB. Overall PX selectivity was estimated to be 5.3 (Figure 8a). Breakthrough simulations confirmed the strong PX affinity with high selectivity and high loading of MAF-X8 (Figure 8b), which were not affected by the presence of benzene and toluene.6 In seeking mechanisms of selective adsorption, powder X-ray diffraction (PXRD) analysis revealed that PX molecules could mediate dynamic transformation from 1 to 1⊃G analogues, which had PXRD profiles similar to that of 1⊃G and were adsorbed inside the flexible framework. In contrast, MX and OX molecules could not change the framework and did not get adsorbed (Figure 7d,e).103 In the case of MAF-X8, PX selectivity resulted from efficient and commensurate packing of PX molecules in

all three directions which fitted perfectly, forming two layers6 (Figure 8c). Compared with packing effects, the response of a flexible framework toward specific isomers showed significantly higher selectivity. Further breakthrough experiments were required, and higher PX selectivity combined with higher capacity compared with BaX may make Zn4O(L)3 and MAF-X8 better adsorbents in xylenes separation and lead to considerable process improvements in industry. Another Zn-MOF, [Zn(μ4-L)]n (μ4-L = dicarboxylate ligand), also showed overwhelming PX selectivity among xylenes. [Zn(μ4-L)]n possessed a rare ptr topology, binodal 4,4-connected nets, and high stability. Both liquid-phase and vapor-phase competition experiments at ambient temperature proved selective PX adsorption,105 though specific values were not reported. In contrast, Zn-MOF Zn(BDC)(Dabco)0.5 (Dabco = 1,4-diazabicyclo[2.2.2]octane), with two energetically different adsorption sites, revealed a different OX selectivity.106 Adsorption isotherms provided capacities about 3.3 mmol g−1 at 398 K and 2.45 mmol g−1 at 448 K.107 Quaternary breakthrough experiments of gas-phase xylenes at 398 K showed selectivity values of 1.83, 1.77, and 1.64 for OX/PX, OX/EB, and OX/MX, respectively. Dipole moments and structures contributed to the different OX selectivity.106 14732


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Figure 8. Xylenes separation applying MAF-X8 at 433 K. (a) Pure component isotherms of equimolar mixture and IAST prediction. (b) Simulated step breakthrough at partial pressure of 25 kPa. (c) Stacking of PX in MAF-X8 framework. Adapted with permission from ref 6, Figures 3 and 4.

Materials of Institut Lavoisier (MIL) series adsorbents, especially MIL-47 and MIL-53, were evaluated on xylenes separation. MIL-47 and MIL-53 are isostructural analogues built from MO4(OH)2 octahedra with 1D channels of 8.5 Å. The metal atom in MIL-47 was V, and in MIL-53 it could be Al, Cr, Fe, or Ga. When calcined, V3+ was oxidized to V4+, causing cornersharing μ2-oxo groups to take the place previously occupied by μ2-OH groups, while the corner-sharing μ2-OH groups and the cations remained unchanged in MIL-53. High flexibility of the MIL-53 framework was due to the unchanged corner-sharing hydroxyl groups,108 which were absent in MIL-47 and resulted in no “breathing behavior”. For flexible MIL-53 with different metal atoms such as Al and Fe, the breathing effects caused by adsorbing guest molecules at different temperatures or pressures were also different109 (Figure 9). The breathing effect of MIL-53(Al) was proved in vapor-phase xylenes adsorption with two-step isotherms. This was ascribed to the “pore-opening” pressure, below which pores contracted so that nearly no molecules could be adsorbed, and displayed no selectivity. When pressure is above the “pore-opening” pressure, pores opened, and manifested large adsorption capacity accompanied with high selectivity of 6.4 for OX/EB.110 Yet in liquid-phase adsorption experiments, there was no breathing effect, and all C8 isomers pointed to typical type I isotherms, for pores in liquid-phase were filled with solvent molecules, which prevented framework deformation.111 Adsorption isotherms of vapor-phase xylenes on MIL-53(Al) put forth capacities of 3.4 molecules per unit cell (m uc−1) for OX and 3.3 m uc−1 for PX.112 Alaerts et al. explored adsorption performance in diluted liquid phase. Binary batch experiments at 298 K in hexane with MIL-53(Al) powder showed selectivity values of 2.7 for OX/MX, 3.5 for OX/PX, and 10.9 for OX/EB, while MX and PX could not be discriminated. Diluted binary breakthrough curves provided calculated selectivities of 11.0 for

Figure 9. Different breathing behavior in MIL-47(V), MIL-53(Al), and MIL-53(Fe). MIL-47(V) had no breathing effect (left). MIL-53(Al) converted from a closed phase to open phase with temperature increase (middle). MIL-53(Fe) converted from hydrous form to anhydrous form with temperature increase through an intermediately anhydrous framework containing two distinct channel types (V, green; Al, purple; Fe, orange; O, red; C, gray; H, white). Adapted with permission from ref 34, Figure 9.

OX/EB and 2.2 for OX/MX.111 Moreira et al. conducted breakthrough experiments at bulk concentrations of xylene mixture and 313 K in n-heptane. Their study pointed out selectivity of 2.0 for OX over both MX and PX, with capacity of 6.76 mmol g−1, 3 times higher than that of state-of-the-art KY zeolite.113 Moreira et al. then studied the impact of eluents on adsorption selectivity and capacity of MIL-53(Al) through a series of experiments at bulk concentrations. Ternary breakthrough experiments at 313 K demonstrated selectivity of 2.1 for OX/MX and OX/PX applying n-heptane as eluent. They became 2.1 and 1.9 when n-hexane was 14733


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Industrial & Engineering Chemistry Research used as eluent, and 1.4 and 1.3 in the presence of iso-octane.114 From the foregoing selectivity values, good separation of C8 isomers was achieved except for PX and MX. The little difference in selectivity values among the above experiments may derive from different xylene solution concentrations, eluents, and MIL-53(Al) forms, as the binary mixture in batch experiments was 0.028 mol L−1 with powder form adsorbent in hexane,111 while the ternary mixture was 2.7 mol L−1 in n-heptane and applied pelletized adsorbent with binder, which may block the pores.113,114 MIL-53(Fe), also exhibiting large swelling of framework when adsorbing guest molecules and varying temperature115 or pressure,116 remedied the difficulty in discriminating PX and MX on MIL-53(Al). Adsorption isotherms of MIL-53(Fe) in heptane depicted a preference for OX with saturation capacity about 40 wt%, yet uptakes of PX and MX were 25 wt%. Binary breakthrough experiments in diluted solution with heptane as eluent at 323 K pointed out selectivities of 2.53, 1.58, and 1.83 for OX/PX, OX/MX, and MX/PX, respectively.117 Different selectivities on similar structures were derived from totally different mechanisms. MIL-53(Al) loaded with xylenes underwent deformation of channels. This deformation led by OX was the strongest, for OX molecular interacted with carboxylate groups of the MIL-53(Al) framework through both methyl groups, while MX and PX molecules interacted with carboxylate groups through only one methyl group (Figure 10). The other

Binary breakthrough experiments in diluted mixture employing hexane as eluent at 298 K exhibited selectivities of 2.5 for PX/MX and 7.6 for PX/EB. Adsorption isotherms of PX and OX illustrated a plateau at 35−37 wt%.119 Activation could influence these selectivity values, as the presence of uncoordinated terephthalic acid in cavities increased the PX/MX selectivity value.120 The mechanism of selective adsorption of xylenes on MIL-47 was a bit like that on MIL-53(Fe), as MIL-47 loaded with xylenes pointed to pairwise packing in the framework, where the two PX molecules stayed perfectly parallel to each other with strong π−π interaction between adsorbates. For OX, packing was similar to that with PX molecules but was less effective. As for MIL-47[m-xylene], a steric interaction led to a tilt of MX molecules and resulted in less optimal interaction between adsorbates. In the case of EB, steric constraints prevented pair alignment. In the presence of a weak interaction between the methyl group in EB and ligands of MIL-47, there was no π−π interaction between adsorbates119 (Figure 11). Separation of C8 isomers except for PX and OX was a result of combined entropy and enthalpy effects,118 and quantum chemical calculations presented the decisive role of entropic differences.121 Simulations on adsorption behavior of MIL-47 and MIL-53 achieved great progress. Remy et al. developed the first simple methodology for simulating dynamic separation processes on MIL-47 and MIL-53(Al). Methodologies like this will help to predict separations both in the laboratory and in industry.122 In comparison with previous rigid structure simulations, Gee et al. took into account MIL-47 framework rotation induced by the adsorbate, and the results were in better agreement with test data.123 Besides simulation, partially fluorinated frameworks of MIL-47 and MIL-53(Al) were assessed. The results demonstrated that functionalization did not change adsorption of OX and PX but launched more hydrophobic frameworks.124 Functionalization could contribute to application of MIL-47 and MIL-53(Al) in xylenes separation processes. Since selectivities of vapor-phase adsorption on MIL-47 and MIL-53(Al) were both pressure and temperature dependent,110,125 the two materials may be applied in pressure swing adsorption (PSA) or thermal swing adsorption (TSA) processes. MIL-125(Ti) and MIL-125(Ti)-NH2, composed of large octahedral cages as well as small tetrahedral cages, both appeared to have PX selectivity among xylene isomers. The PX selectivity was concentration dependent, with higher values in diluted solution, and could be influenced by introduction of EB. Pulse experiments on MIL-125(Ti) using n-heptane as eluent at 313 K showed separation factor of 1.3 for PX/OX and PX/MX but could not distinguish OX and MX. Ternary breakthrough experiments at 313 K in xylenes solution diluted with n-heptane put forward selectivities of 1.5 and 1.6 for PX/MX and PX/OX, respectively. Nevertheless, these selectivities decreased to 1.0 as the concentration of the mixture increased to 3 mol L−1 from the original 0.1 mol L−1. Besides concentration, the presence of EB reduced selectivity for PX greatly, which may result from competitive adsorption of PX and EB. At a high EB concentration, selectivities were 0.8 for PX/MX as well as PX/OX and 0.7 for PX/EB in quaternary breakthrough experiments. Yet they became 1.5 for PX/MX and PX/OX, and no selectivity was found for PX/EB in diluted quaternary solutions.126 Concerning MIL-125(Ti)-NH2 (Figure 12a), simulations proposed selectivities of 3.4 for PX/MX and 2.8 for PX/OX in binary mixture at 1 kPa and 300 K. Subsequent breakthrough experiments in heptane at 298 K manifested selectivities of 3 for PX/MX, 2.2 for PX/OX, and 0.97 for OX/MX. In the presence

Figure 10. Rietveld refinements of MIL-53(Al) loaded with OX (left) and PX (right); green lines show interaction. Adapted with permission from ref 34, Figure 7.

methyl group as well as the C2 carbon atom of MX aromatic ring interacted with a host aromatic ring as well, but this interaction was much weaker.111 Adsorption selectivity resulted from both entropic and enthalpy effects, in which enthalpy played a dominate role. For PX and MX, which are hard to discriminate on MIL-53(Al), adsorption enthalpy favored MX, but this was balanced by more favorable adsorption entropy which arose from more adsorption sites on PX.118 As for MIL-53(Fe)[o-xylene], two OX molecules packed parallel to each other as well as the terephthalate linkers, with π−π interactions not only between the two molecules, the adsorbates, and linkers of MIL-53(Fe) but also OX and the adjacent wall’s linkers. MIL-53(Fe)[m-xylene] showed an arrangement similar to that of MIL-53(Fe)[o-xylene], though with weaker host−guest interactions. However, MIL-53(Fe)[p-xylene] put forward an evidently different arrangement with molecules arranged in a zigzag mode, which indicated different host−guest interactions. Efficient packing dominated OX preference. In the cases of MX over PX, besides packing effects, more negative apparent adsorption enthalpy of MX also contributed.117 Metal ions in the MIL-53 framework make a great influence in the separation of xylenes. In contrast to MIL-53, MIL-47 preferred PX over MX, with selectivity value of 2.9 in liquid-phase batch tests in hexane. 14734


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Figure 11. Structure refinements of MIL-47 loaded with (a) PX, (b) OX, (c) MX, and (d) EB. Adapted with permission from ref 119, Figure 5.

of EB, breakthrough experiments displayed selectivity of 1.6 for PX over EB.127 However, this preference depended on EB concentration. Quaternary breakthrough tests in n-heptane illustrated a preference for PX at low EB concentrations, while it showed EB preference with selectivity dropped from 1.4 to 0.8 at high EB concentrations. Selectivities of PX/OX and PX/MX were higher at low PX concentrations128 (Figure 12c). Higher selectivity on MIL-125(Ti)-NH2 in comparison with MIL125(Ti) was attributed to slightly smaller pores in MIL-125(Ti)NH2. This may also result in lower PX capacity of 1.2 mmol g−1 on MIL-125(Ti)-NH2 compared to 1.5 mmol g−1 on MIL125(Ti).6 Selective adsorption on these two materials was attributed to a combination of packing effects in large octahedral cages and molecular sieving effects in small tetrahedral cages only PX and EB could pack into tetrahedral cages (Figure 12b). EB possessed more efficient packing than PX at high EB concentrations.127,128 Gee et al. reported methodology for simulation combined with experiments to select proper MOFs with good stability, improved capacity, and selectivity in separating PX from C8 aromatics. Upon simulation, MOF-48, MIL-47, MIL-140B, and MIL-125-NH2 displayed good performance (Figure 13a). MOF-48 originated from dimethyl functionalization of BDC linkers in MIL-47. Breakthrough tests at 50 °C in bulk liquid quaternary mixture put forth PX/MX, PX/OX, and PX/EB selectivity values of 1.7, 1.7, and 1.5 on MOF-48; 1.6, 1.8, and 2.1 on MIL-140B; 1.5, 1.6, and 1.3 on MIL-125-NH2; and 1.1, 0.6, and 1.7 on MIL-47. Among the four adsorbents, selectivities on MOF-48 and MIL-140B exceeded those of the state-of-the-art adsorbent BaX, which gave values of 1.6, 1.4, and 1.9 for PX/MX, PX/OX, and PX/EB at 180 °C. Capacities were 0.95, 1.2, 1.25, 4.55, and 1.4 mmol g−1 on MOF-48, MIL-140B, MIL-125-NH2, MIL-47, and BaX, respectively (Figure 13b). Selectivity and capacity values were slightly different from other authors’ data, and this may result from different preparation methods and test conditions.77 Adsorption and breathing properties were also explored on another two Al-MOFs, which were structurally related to MIL53(Al). Al(OH)(O2C-C2H2-CO2)·3.5H2O was isostructural of MIL-53(Al) but had no flexibility,129 while Al(OH)(trans-CDC) (CAU-13, trans-CDC = trans-1,4-cyclohexanedicarboxylate) contained the flexible aliphatic linker trans-CDC.130 Pulse gas chromatography (GC) experiments at 210 °C with pelletized Al-fumarate MOF Al(OH)(O2C-C2H2-CO2)·3.5H2O acquired a baseline separation of xylene isomers with elution time increasing in sequence of OX < MX < PX. Retention time of PX was 3 times

Figure 12. (a) Large octahedral (left) and small tetrahedral (right) cages in MIL-125(Ti)-NH2 (blue spheres on behalf of effectively accessible volumes of the cages). (b) Molecular simulation of binary equimolar mixture of PX (blue) and OX (red) (left), or PX (blue) and MX (green) (right), in cages of MIL-125(Ti)-NH2 at 300 K and 1 kPa (octahedral cages are circled in dark-green, and tetrahedral cages are circled in pink). (c) Selectivity values versus concentrations: PX/MX vs CPX (left), PX/ OX vs CPX (right), and PX/EB vs CEB (bottom) on powder MIL-125(Ti)NH2 at 313 K in breakthrough tests with n-heptane as eluent. Adapted with permission from ref 127, Figures 1 and 2; and ref 128, Figure 6.

longer than that of OX.129 Al(OH)(trans-CDC) exhibited OX selectivity over the other isomers. Liquid-phase adsorption 14735


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values in ternary competitive adsorption experiments at 298 K.130 Compared with MIL-53(Al), Al-fumarate MOF showed higher selectivity toward xylenes because of its smaller pore size of about 6 Å, so van der Waals interactions dominated the selective adsorption.129 Concerning CAU-13, selectivity and adsorption capacity were lower, as bulkier aliphatic in CAU-13 was not likely to have shape-selective interactions. In the flexible framework of CAU-13, half of the linker molecules took a,a and the other half took e,e conformations in the empty pore form of CAU-13, while all CDC2− took e,e conformation in framework loaded with xylenes. This transformation led to increases in symmetry, pore opening, and cell volumes (Figure 14a). OX as well as MX molecules were loaded in a disorderly fashion in the CAU-13 framework, yet PX molecules were ordered, which may derive from linear shape or the smallest kinetic diameter130 (Figure 14b). Cu-MOF Cu(CDC) and Cu3(BTC)2 (HKUST-1, HKUST = Hong Kong University of Science and Technology, BTC = 1,3,5benzenetricarboxylate) presented selective adsorption on xylenes. As a hydrophobic material exhibiting high hydrolytic stability, Cu(CDC) put forth high selectivity values of 10 and 7 for PX/OX and PX/MX in ternary competitive batch experiments with mesitylene at bulk concentrations. In addition, for binary solution composed of 0.1 M per xylene, PX/MX selectivity was 9. In the presence of EB, PX/EB selectivity was 3.5−5 at low concentrations of binary mixture. Capacities at 298 K were 12, 5.5, and 1 wt% for PX, MX, and OX, respectively.131 In contrase, HKUST-1, a coordinately unsaturated MOF with open metal site copper atoms (Figure 15a), put forward selectivity values of 2.4, 1.4, and 1.4 for MX/OX, PX/OX, and EB/OX in batch experiments with hexane.119 Nevertheless, HKUST-1 pellets appeared to have a different preference in

Figure 13. (a) PX adsorption selectivity values versus total xylenes capacities for about 2500 MOFs, calculated by grand canonical Monte Carlo simulations applying a feed mixture of 0.33:1:2:1 for EB/OX/ MX/PX at 50 °C and 9 bar. (b) Comparison of experimental selectivity values versus capacities of selected MOFs and zeolite BaX applying the same feed mixture at 50 °C and 9 bar for the MOFs and 180 °C for BaX. Experimental results were derived from breakthrough results and model-fitted breakthrough curves. Adapted with permission from ref 77, Figures 1 and 5.

isotherms at 298 K in mesitylene depicted OX capacity of 0.34 mol mol−1. In binary competitive experiments with hexane, selectivities of OX/MX, OX/PX, and PX/MX were 1.9, 1.5, and 1.3, respectively, with 2.1, 1.7, and 1.3 as the corresponding

Figure 14. (a) Conformation change of xylene-loaded CAU-13. (b) Crystal structures of CAU-13 loaded with MX (left) and arrangement of the xylenes in CAU-13 framework (right) (MX molecules (top), OX molecules (middle), and PX molecules (bottom); carbon atoms of cyclohexane rings were omitted for perspicuity). Adapted with permission from ref 130, abstract and Figure 11. 14736


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Figure 15. Polyhedra representations of (a) HKUST-1 and (b) CPO-27-Ni. Breakthrough curves of binary mixture of xylenes on (c) HKUST-1 and (d) CPO-27-Ni at 398 K. Adapted with permission from ref 132, Figures 1, 5, and 6.

possessed extraordinary flexibility133−136 and excellent hydrolysis stability. PX molecules could be adsorbed on ZIF-8 through transitory tilt of ligands. Isotherms put forward adsorption of C8 isomers and 1,2,4-trimethylbenzene (124-TMB), despite extreme slow diffusion, pushing the “limiting” aperture size of ZIF-8 over 7.6 Å.73,133 Vapor-phase breakthrough experiments at 398 K displayed a baseline separation of xylenes except for OX and MX. Selectivities were 3.9 for PX/OX and 1.6 for PX/MX. Liquid-phase bulk quaternary breakthrough experiments were conducted in the presence of C8 aromatics. Selectivity values descended to 1.5 for PX/OX, 1.15 for PX/MX, and 1.1 for PX/ EB; therefore, PX and EB could hardly be discriminated.14 Peralta et al. reported in a patent that total separation of PX and EB could be obtained in a liquid-phase test at 70 °C with selectivity values of 3.1, 8.8, and more than 10 for PX/MX, PX/ OX, and PX/EB, respectively, larger than that of BaX, for which corresponding values were 2.6, 2.4, and 2.5. Selectivity values were 1.6 and 4.2 for PX/MX and PX/OX, respectively, in gasphase experiments at 125 °C.78 Different selectivities may derive from adsorbent preparation. As for capacity, ZIF-8 demonstrated 3 mmol g−1 for PX and EB at 293 K,137 2.1 mmol g−1 for PX in liquid-phase tests at 70 °C,78 and 1.5 mmol g−1 for PX at 650 Pa and 398 K,14 higher than 1.3 mmol g−1 on BaX.78 ZIF-8 could distinguish PX and EB over MX and OX because of shape selectivity.137 ZIF-68, comprising a two-pore system accessible for xylenes, presented capacity about 24 m uc−1 for OX at 323 K, followed by MX and PX owing to the highest packing efficiency of OX.138 Further competitive adsorption experiments were required to determine selectivity. PX capacity on ZIF-7 at 293 K was 0.6 mmol g−1, extraordinarily lower compared with 1.5 mmol g−1 for OX, MX, and EB. XRD revealed structure conversion from small pore to large pore upon loading with OX, MX, and EB on

vapor-phase breakthrough experiments at 398 K. Affinity was in the order of PX < MX < OX, with low selectivity values between 1.08 and 1.21 (Figure 15c). Adsorption isotherms illustrated 2.4 mmol g−1 for MX and 2.8 mmol g−1 for PX and OX at temperature of 398 K and partial pressure of 600 Pa.132 In the case of Cu(CDC), guest-loaded frameworks depicted a PX perfectly matched structure, where rotation of CDC linkers led to pore diameter increasing from 5.4 to 6 Å, which contributed to a higher loading and higher selectivity value. Molecular sieving effect resulted in selective adsorption of PX over OX, as PX fitted perfectly with the channel while OX could not go into intraporous space. Separation of PX and EB resulted from more efficient packing of PX.131 Yet in HKUST-1, electrostatic interactions favoring OX were counterbalanced by the PX preference of its cage system.132 Different preferences and selectivities on HKUST-1 from test to test may be attributed to this intrinsic similar affinity between adsorbates and framework or experiments in different phases, preparation methods, and operating conditions. Another unsaturated MOF, Ni2(dobdc) (CPO-27-Ni, dobdc = 2,5-dioxido-1,4-benzenedicarboxylate, Figure 15b), was assessed through the same experiments as those used for HKUST-1. Adsorption isotherms exhibited 2.1 mmol g−1 for MX, 2.0 mmol g−1 for PX, and 1.9 mmol g−1 for OX. Affinity in breakthrough tests for xylenes followed the same sequence as on HKUST-1, while the selectivity values of 2.0 for MX/PX, 1.7 for OX/MX, and 3.3 for OX/PX were significantly higher (Figure 15d). This resulted from combination of the inherent OX preference of the structure and electrostatic effects. The unsaturated metal sites may become potential adsorptive sites and contribute to selective adsorption.132 Zeolitic imidazolate frameworks (ZIFs) are a kind of MOFs featuring high thermal stability and chemical stability. ZIF-8 14737


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Figure 16. (a) Solid-state structure of CD-MOF-1: (left) viewed along the [1 0 0] axis, presenting the extended framework of the body-centered cubic packing arrangement (C, gray; O, red; K, purple); (right) viewed along the [1 1 1] axis, presenting the triangular windows (large cavities were filled will yellow spheres). (b) Molecular simulation snapshots of xylenes in CD-MOF-2, viewed down the [1 0 0] axis: equimolar binary mixture snapshots of MX/PX (left), OX/MX (middle), and OX/PX (right) (PX, black; MX, green; OX, blue; their corresponding methyl groups are shown in yellow). Adapted with permission from ref 147, Figures 1 and 8.

both evidenced reverse shape selectivity with elution time increasing in the sequence PX < MX < OX.12,144 Laboratoryscale liquid-phase breakthrough experiments in diluted mixtures at 313 K, applying n-heptane as eluent, obtained selectivity values of 1.8 for OX/MX and 2.4 for OX/PX; the values came down to 1.1 and 1.2 at high concentrations. In addition, selectivity values of tablet-form adsorbent were 1.7 and 2.3 for OX/MX and OX/ PX, respectively, with adsorption capacities of 2.32 mmol g−1 for both OX and PX in pilot scale. Binary breakthrough curves revealed that agglomerate and tablet forms of UiO-66 underwent a loss in capacity but not in selectivity compared with powder form. The loss may result from scale-up of synthesis or pelletizing.145 Discrimination arose from complete encapsulation of OX molecules in the cavities, with different diffusion mechanisms in the crystal for the isomers further contributing to separation. Structural movement of adsorbent played a crucial role in facilitating diffusion of xylenes.146 Cyclodextrin MOFs (CD-MOFs) represent a class of materials exhibiting green nature, high selectivity, and kilogram-scale synthesis.147 It comprised a porous net of γ-CDs and alkali metal cations (Figure 16a). Liquid-phase chromatographic experiments using column loaded with CD-MOFs showed high selectivity toward xylene isomers, as CD-MOF-1, made from KOH through bottom-up synthesis, provided separation factors of 17.9, 2.67, and 6.73 for OX/PX, MX/PX, and OX/MX, respectively, at 298 K with hexane as eluent. Elution time rose in order of PX < MX < OX. When EB was present, separation factor of 4.75 for OX/EB was achieved. CD-MOF-2, made from RbOH through top-down synthesis, presented capacity of 2.7 mmol g−1,

ZIF-7, while the structure converted to an unknown phase with pore volume between small and large pore when loaded with PX. ZIF-9 put forward similar behavior as ZIF-7, yet ZIF-65-Zn showed no selectivity due to its large pores.137 ZIF-76 had capacities of 0.26 and 0.18 mmol cm−3 for OX and PX, with selectivity of 1.1 for OX/PX at 398 K.73 MOFs related to cobalt assessed on xylenes separation put forth capacity of 1.5 mmol g−1 for PX, 1.2 mmol g−1 for OX, and 0.95 mmol g−1 for MX at 50 °C on Mg-CUK-1. This was a new, Mg ion replaced version of CUK-1139 (Cambridge University KRICT, CUK-1 = Co3(2,4-pdc)2(μ3-OH)2·9H2O, 2,4-pdc = pyridine-2,4-dicarboxylic acid dianion). Another material, Co2L2(AzoD)2140 (H2AzoD = azobenzene-3,3′-dicarboxylic acid, L = N1,N4-di(pyridin-4-yl)terephthalamide), showed capacity of 265.15 mg g−1 (about 2.5 mmol g−1) for PX, 101.25 mg g−1 for OX, and little MX at 298 K. These two MOFs both exhibited PX-selective adsorption, followed by OX and MX, while PX capacity on Co2L2(AzoD)2 was significantly larger. This may result from flexibility of the Co2L2(AzoD)2 framework, which together with size and shape effects lead to selective adsorption.140 As for Mg-CUK-1, preference for PX was due to their linear packing in the undulated 1D channels, with guest− guest π−π interactions dominating sorption force.139 UiO-66 (UiO = University of Oslo), manifesting a cubic 3D framework, contained 11 Å microporous octahedral and 8 Å tetrahedral cavities. UiO-66 had no framework flexibility or breathing effects, but structural movement occurred through rotational motion or flip of 1,4-benzenedicarboxylate (BDC) linkers.141−143 Breakthrough experiments and molecular simulations 14738


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Figure 17. (a) Crystal structure of [Cu(L)2(H2O)]·(ClO4)2: (top) 1D chain with twisted Cu2L2 macrocycles; (middle) two adjacent 1D chains interdigitated into each other and generating a 2D layered structure; and (bottom) β-sheet hydrogen-bonding interactions between the two interdigitated 1D chains. (b) Crystal structures of [Cu(L)2(H2O)2]·(ClO4)2·(OX)2: (top) 2D layer loaded with OX molecules (pink) and ClO4− (green) (offset packing of the 2D layers in α-axes and b-axes; alternate layers are shown); (middle) four equal channels loaded with OX molecules and ClO4−; and (bottom) β-sheet hydrogen-bonding interactions between the 2D layers. Adapted with permission from ref 151, Figures 1−4.

binary tests, and quaternary tests demonstrated 25:10:47:18 for EB:PX:MX:OX. Selectivity was ascribed to the most intimate packing of MX in crystal.150 Compared with this, the mechanism for Ni(NCS)2(ppp)4 seemed more complex. Rearrangement took place upon guest adsorption, and the framework varied from nonporous phase to porous phase. Factors allowing clathrates to adopt the most relaxed construction led to higher selectivity. Furthermore, PX, MX, and OX could be released at 46.1, 72.5, and 74.8 °C, respectively,4 which may enable it to be applied in industry, especially in the TSA procedure. Selective inclusion through a self-assembly process, as a class of adsorption, could also achieve separation of xylenes. Nath et al. carried out competitive crystallization experiments with [Cu(L)2(H2O)]·(ClO4)2 (L = bis(pyridylcarboxamide), which could include xylenes when self-assembling through β-sheet hydrogen bonds (Figure 17a). OX was selectively included in the presence of all three isomers and formed [Cu(L)2H2O)2]· (ClO4)2·(OX)2; various techniques proved the adsorption of only OX molecules (Figure 17b). As for MX and PX mixture, MX was selectively included, while the framework could only include PX in the absence of MX and OX. Tests indicated favor toward OX in the material, followed by MX and PX.151 Concerning separation of xylenes applying Ag4(O2CCF3)4(phenazine)3, experiments put forth inclusion of OX and MX only when one kind of arene (arene = PX, toluene, or benzene) was present. Binary competition tests revealed selectivities of 9.13 for PX/OX and 14.2 for PX/MX.152 Tables 4, 5, and 6 summarize adsorptive separation of xylene isomers applying MOFs with MX, PX, and OX preference, respectively. Figures 18 and 19 are comparisons of PX/MX and PX/OX selectivity versus capacity on representative MOFs and molecular sieves. The data in these figures are all derived from bulk liquid-phase breakthrough experiments, and selectivity between 0 and 1 indicates reverse preference.

with lower separation factors of 16.37, 3.44, and 4.76 for OX/PX, MX/PX, and OX/MX, respectively. Molecular simulation revealed that OX selectivity was derived from interactions between OX and adsorbent as well as the highest packing efficiency. OX took optimal slipped geometry and interacted with γ-CD rings through both its methyl groups, while methyl groups in MX and PX prevented similar positioning. Simulation snapshots of binary OX/MX and OX/PX mixtures manifested nearly exclusive packing of OX in the pores148 (Figure 16b). Ce(HTCPB) (H4TCPB = 1,2,4,5-tetrakis(4-carboxyphenyl)benzene), exhibiting a flexible structure, appeared to have affinity for PX over the other C8 aromatics. Liquid batch tests depicted selectivities of 4.5 for PX/MX, 5.6 for PX/OX, and 2.4 for PX/EB. Upon adsorbing PX and MX, the flexible framework responded differently, and this resulted in selective adsorption. A synergic feedback from host distortion and enhanced fitting toward PX molecules was observed, while producing a compromise MX molecular site and unused void room far away from guest molecules brought about a negative feedback. Selectivities were 6.33, 6.15, and 6.08 to the maximum for PX/MX applying Ce(HTCPB) analogues Nd(HTCPB), Pr(HTCPB), and Sm(HTCPB), respectively.149 Peralta et al. conducted breakthrough tests on xylenes separation with polar material rho-ZMOF (ZMOF = zeolitelike metal−organic frameworks). Experiments at 398 K depicted capacities of 0.92 and 0.63 mmol cm−3 for OX and PX, with selectivity of 1.3 for OX/PX.73 Werner complex and its Werner clathrates were also studied on xylenes separation. Ni(NCS)2(ppp)4 (ppp = p-phenylpyridine) performed excellent separation with preference for OX in competitive experiments; selectivity values were 34.2 for OX/ MX, 40.5 for OX/PX, and 12.7 for MX/PX.4 Another Werner complex, Ni(NCS)2(isoquinoline)2(4-phenylpyridine)2, showed a MX preference, followed by EB, OX, and PX. Selectivity values were 2.8 for MX/OX, 9.1 for MX/PX, and 1.0 for OX/PX in 14739


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Industrial & Engineering Chemistry Research Table 4. MOFs with MX Preference Applied in Separation of Xylene Isomers by Adsorption selectivity MOF

capacity

solvent

temp (K)

MX/PX

MX/OX

MX/EB

OX/PX

ref

Ni(NCS)2(isoquinoline)2(4-phenylpyridine)2 HKUST-1

− −

− hexane

321 298

4.7 1.1

2.6 2.4

1.9 1.4

1.8 0.7

150 119

Table 5. MOFs with PX Preference Applied in Separation of Xylene Isomers by Adsorption selectivity MOF Zn4O(L)3 MAF-X8 JUC-77 CoBDP MIL-125(Ti) MIL-125(Ti)-NH2 Cu(CDC) ZIF-8 MOF-48 MIL-140B MIL-47 Ce(HTCPB) Nd(HTCPB) Mg-CUK-1 Co2L2(AzoD)2 Ag4(O2CCF3)4(phenazine)3 a

capacity

solvent

temp (K)

2.73 mmol g−1 2.1 mmol g−1 0.55 mmol g−1 1.4 mmol g−1 1.5 mmol g−1 1.2 mmol g−1 1.25 mmol g−1 12 wt% 1.5 mmol g−1 2.1 mmol g−1 0.95 mmol g−1 1.2 mmol g−1 4.55 mmol g−1 − 1.62 mmol g−1 2.21 mmol g−1 1.5 mmol g−1 2.5 mmol g−1 −

− − − − n-heptane heptane − mesitylene − − − − − hexane − − − − −

298 433 433 433 313 298 323 298 398 343 323 323 323 298 383 383 298 298 −

PX/MX

PX/OX

PX/EB

MX/OX

ref

b 1.03 b 3.0 − − b b − − 1.22 − − − −

103, 104 6 6 6 6, 126 6, 127, 128 77 131 14 78 77 77 77 119 149 149 139 140 152

1000a 5.3a 2.25a 1.4a 1.5 3.0 1.5 7.0 1.15 3.1 1.7 1.6 1.1 2.5 4.5 6.33 − − 14.2

1.6 2.2 1.6 10 1.5 8.8 1.7 1.8 0.6 b 5.6 − − − 9.13

− 1.6 1.3 3.5−5 1.1 10 1.5 2.1 1.7 7.6 2.4 − − − −

Overall selectivity. bTwo isomers could not be separated.

Table 6. MOFs with OX Preference Applied in Separation of Xylene Isomers by Adsorption selectivity MOF

capacity

solvent

temp (K)

OX/PX

OX/MX

MX/PX

OX/EB

UiO-66 CAU-13 Zn(BDC)(Dabco)0.5 HKUST-1 CPO-27-Ni CD-MOF MIL-53(Al)

2.32 mmol g−1 2.0 mmol g−1 3.3 mmol g−1 2.8 mmol g−1 2.1 mmol g−1 2.7 mmol g−1 3.37 mmol g−1 6.76 mmol g−1 40 wt% 0.92 mmol cm−3 0.26 mmol cm−3 −

n-heptane hexane − − − hexane hexane n-heptane heptane − − −

313 298 398 398 398 298 298 313 323 398 398 295

2.4 1.7 1.83 1.08−1.21 3.3 17.9 3.5 2.1 2.53 1.3 1.1 40.5

1.8 2.1 1.64 − 1.7 6.73 2.7 2.1 1.58 − − 34.2

− 0.8 a 132 2 2.67 a a 1.83 − − 12.7

− − 1.77

145 130 107

− 4.75 10.9 − − − − −

132 147 111 113, 114 117 73 73 4

MIL-53(Fe) rho-ZMOF ZIF-76 Ni(NCS)2(ppp)4 a

ref

Two isomers could not be separated.

3.2. Chromatographic Separation. Due to the similarity in properties of xylenes, it is really challenging for nonpolar commercial chromatography columns to separate PX from MX. For example, GC with HP-5MS column (nonpolar, 30 m long × 0.25 mm i.d.) could not isolate PX and MX at all under a temperature program from 50 to 200 °C, a rate of 10 °C min−1, and a He flow rate of 1.2 m L min−1 153 (Figure 20a). The experiment in nonpolar high-performance liquid chromatography (HPLC) had similar issues. HPLC with a C18 column (nonpolar, 25 cm long × 4.0 mm i.d., 5 μm) separated the C8 isomers with poor resolution. MX and PX came out at the same retention time when a mobile phase of CH3CN/H2O (50:50) was applied at a flow rate of 1.0 mL min−1 154 (Figure 20b).

Recently, significant advances in both GC and HPLC for separation of C8 isomers have been achieved. 3.2.1. Gas Chromatography Separation. Lin et al. prepared a novel metal carboxylate framework, [Zn(Hpidba)]·2.6DMF· H2O (MCF-50, 2·g), whose activated guest-free form 2 featured large 1D channels with flexible and small apertures interconnected (Figure 21a). When used as stationary phase for GC, xylene isomers were baseline separated with high resolution of 1.54−1.65 for MX/PX within 4 min under a program. Elution time increased in the order of OX < MX < PX153 (Figure 21b). On the other hand, Zn-MOF rho-[Zn(eim)2] (MAF-6, Heim = 2-ethylimidazole) manifested higher resolution on the same separation. MAF-6 exhibited high hydrophobicity not only in 14740


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Figure 18. Comparison of selectivity for PX/MX versus capacity on representative MOFs and molecular sieves.

Figure 21. (a) 3D network (top) and coordination/pore structures of 2. Quadrilateral aperture was presented in space-filling form in order to highlight its effective aperture size and shape (middle and bottom). (b) 2-coated GC employed in xylene isomers separation with a temperature program of 180 to 200 °C at a rate of 5 °C min−1 and under a N2 flow rate of 14 mL min−1. Adapted with permission from ref 153, Figures 1 and 2.

Figure 19. Comparison of selectivity for PX/OX versus capacity on representative MOFs and molecular sieves.

Figure 22. (a) Perspective view and pore surface structures of MAF-6 (pore surface, yellow/gray; unit-cell edges, black; Zn, green; C, gray; N, blue; H, white). (b) Use of MAF-6-coated GC capillary in xylene isomers separation. Adapted with permission from ref 155, Figures 1 and 7.

employing a MAF-6-coated column put forward a well-resolved separation of xylene isomers within 3 min. Elution time increased following the order MX < PX < OX (Figure 22b). Resolutions at 90 °C were 10, 6.7, and 3.2 for OX/MX, OX/PX, and PX/MX, respectively.155 Another Zn-MOF, [HZn3(OH)(BTC)2(2H2O)(DMF)]·H2O (MOF-CJ3, Figure 23a), achieved separation within 13.5 min and followed an elution order of EB < PX < MX < OX, consistent with their boiling point and polarity values. Resolutions at 35 °C were 0.42, 1.35, and 3.85 for EB/PX, PX/MX, and MX/OX, respectively156 (Figure 23b). Pulse breakthrough simulations also proved similar elution times for EB and PX.6 In the case of MCF-50, the flexible and small aperture acted as an adsorption site, while the large channel was the path of fast diffusion. PX and OX completely inserted into the small aperture, and the width of the aperture was slightly reduced for the former but significantly expanded for the latter. In contrast, only one methyl group of MX inserted a little into the aperture, accompanied with a little expansion in width. Combining host−guest binding and host−framework distortion energies, the simulation result of total adsorption heats was completely consistent with the trend for GC. Accumulation of fast diffusion through the large 1D channel led to different host−guest interactions in the small aperture.153 The lowest resolution on MOF-CJ3 was due to

Figure 20. Chromatographic separation of C8 aromatics/xylenes. (a) Gas chromatography on commercial HP-5MS capillary column (30 m long × 0.25 mm i.d.) using a temperature program of 50 to 200 °C with a rate of 10 °C min−1, under a He flow rate of 1.2 m L min−1 and a split ratio of 15:1, performed on an Agilent 5975C system equipped with a mass spectrometry detector. (b) HPLC chromatogram on a C18 column (25 cm long × 4.0 mm i.d., 5 μm) using CH3CN/H2O (50:50) as the mobile phase, performed on an Agilent 1100 HPLC system equipped with a G1314 variable-wavelength detector. Adapted with permission from ref 153, Figure S10; and ref 154, Figure S6.

cavities but also on crystal surface (Figure 22a). Though it could hardly be wetted, MAF-6 could adsorb organic molecules. GC 14741


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Figure 23. (a) MOF-CJ3 structure viewed along the [0 0 1] direction (Zn, purple; O, red; C, gray; H, white; N, blue). (b) GC separation of C8 aromatics with MOF-CJ3-coated capillary column under a N2 flow and at 35 °C. Adapted with permission from ref 158, Figure 3; and ref 156, Figure 3.

the ineffective interactions arising from large pores. Potential CH···π interactions between two methyl groups in OX and benzene rings in adsorbent accounted for the longest retention time for OX.156,157 The mechanism of separation for MAF-6 still needs further research. UiO-66-coated capillary column had excellent chemical stability. Tests carried out in this GC column gained a baseline separation with elution order of PX < MX < EB < OX within 4 min, 600 plates m−1 for theoretical plate value of OX (Figure 24). Figure 25. (a) SEM image of ZIF-8@PDMS core−shell microsphere and particle size distribution. (b) Xylenes separation in GC with stationary phase of ZIF-8@PDMS under isothermal conditions. Adapted with permission from ref 161, Figures 2 and 7.

Temperature and mobile phase were influencing factors. The time required for a well-resolved separation on MIL-53(Al)packed column fell from 100 to 20 min with temperature elevated from 20 to 50 °C, and remained nearly unchanged from 50 to 70 °C. Elution followed the order EB < PX < OX for all temperature conditions with mobile phase of 75:25 acetonitrile/ 1‑propanol, while the pressure drop increased as temperature was varied from 20 to 50 °C, dominated by breathing-led variation of particle size and porosity in the column. When the temperature went up to 70 °C, the pressure drop went down, which resulted from decreased viscosity induced by temperature exaltation. Maximum separation factors were 17, 6, and 3 for OX/EB, PX/EB, and OX/PX, respectively, obtained at mobile phase of 25:75 for acetonitrile/1‑propanol. The values were reduced slightly with temperature elevation, except for OX/PX. Higher temperature was favored, taking into account shorter separation time and not much decrease in separation factors.162 In the case of MIL-101(Cr)-packed HPLC, C8 aromatics attained a baseline separation with elution order following EB < PX < MX < OX within 8 min, exhibiting high column efficiency of 20 000 plates m−1 represented by EB. Selectivities for PX/EB, MX/PX, and OX/MX were 1.67, 1.49, and 1.84 with mobile phase of 95:5 for hexane/dichloromethane (DCM) at 20 °C, and the values went down with as temperature was varied from 20 to 35 °C (Figure 26a). Making full use of its advantage of exceptionally high OX selectivity, pure DCM as mobile phase achieved fast OX separation within 3 min (Figure 26b). The longest retention time for OX was derived from the same mechanism as MOF-CJ3.157 Molecular simulation proved the elution sequence and ascribed the separation to combination of solvent−solute and framework−solute interactions, as the most

Figure 24. GC separation of C8 aromatics with UiO-66-coated capillary column at a N2 flow rate of 1 mL min−1 and temperature program of 140 °C for 1 min, and then increasing to 180 °C at 10 °C min−1. Adapted with permission from ref 159, Figure 2.

The overall elution sequence was attributed to the closest proximity to framework walls of OX molecules, derived from the largest size, and this resulted in the strongest van der Waals interaction. Hydrogen bonding induced by dipoles also contributed to the longest retention time. The influence of temperature pointed to a decrease of selectivity along with temperature increase.159 Separation tests employing HKUST-1-coated GC columns also obtained a well-resolved separation. Elution time increased in the order of EB < OX < PX < MX.160 ZIF-8@PDMS (PDMS = poly(dimethylsiloxane)) core−shell microspheres also acted as stationary phase of GC (Figure 25a). The core−shell structure solved issues related to high pressure drop and low column efficiency of GC packed with conventional MOFs, resulting from wide distribution of particle size and broad shape distribution. Experiments without temperature programming demonstrated a baseline separation within 6 min, and the elution sequence was OX < MX < PX161 (Figure 25b). 3.2.2. Liquid Chromatography Separation. HPLC packed with MIL series MOFs acquired a baseline separation of xylenes. 14742


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Figure 26. Xylene isomers separation with HPLC packed with MIL-101(Cr): (a) at room temperature with hexane/DCM (95:5) as mobile phase and at a flow rate of 1 mL min−1 and (b) fast separation of OX at room temperature with pure DCM as mobile phase and at a flow rate of 0.5 mL min−1. (c) Interaction energies between xylene isomers and MIL-101/hexane. Adapted with permission from ref 157, Figures 2 and 3; and ref 163, Figure 7.

limitations that arose from inhomogeneous packing, widely distributed particle size, and broad shape distribution. In order to overcome these drawbacks, uniform silica-MOF composite was prepared.154,169,170 SEM images of silica-UiO-66 manifested core−shell microparticles (Figure 28). Yan et al. assessed xylenes separation in NP-HPLC with stationary phase of silica-UiO-66 composite, synthesized by a hydrothermal method. At mobile phase of 98:2 for hexane/DCM, a baseline separation within 4 min illustrated elution sequence following EB < PX < MX < OX. Resolutions were 2.15, 1.85, and 1.73 for MX/PX, PX/EB, and OX/MX, respectively, and column efficiency was 8780 plates m−1 represented by EB154 (Figure 27c). Zhang et al. fabricated UiO-66 on aminosilica by conventional solvothermal synthesis over 24 h and synthesized UiO-66@SiO2 core−shell microparticles. The well-resolved separation presented an elution sequence of MX < EB < PX < OX within 11 min in RP-HPLC169 (Figure 27d), slightly different from the order of PX < MX < OX acquired previously.168 This was due to the much higher basicity of MX and basic amino groups on the silica, resulting in the weakest retention of MX, and the greater retention of PX compared with EB of similar size was related to stronger hydrophobicity.169 Arrua et al. prepared similar UiO-66@SiO2 via microwaveassisted solvothermal synthesis with a deposition step in less than 2 h,170 significantly shorter than the preparation process reported by Zhang et al.169 NP-HPLC experiments acquired separation within 6 min and the same order of PX < MX < OX as found by Yan et al.,154 but with a much higher column efficiency of 15 270−32 440 plates m−1. RP-HPLC experiments gained sequence of MX < PX < OX, the same as that found by Zhang et al.,169 but within a shorter time of 5 min170 (Figure 27e). Column efficiency was greatly improved in these silica-MOFpacked HPLC columns in comparison with those HPLC columns directly packed with MOF particles. Similar sphere-on-sphere (SOS) silica particles as stationary phase also achieved low column backpressure and high column

polar compound OX interacted with MIL-101 most strongly, while it interacted with mobile-phase hexane most weakly163 (Figure 26c). Adsorption isotherms on MIL-101(Cr) at 313 K put forth capacity of 1405, 1320, and 1315 mg g−1 for PX, MX, and OX,164 respectively, and about 10.9 mmol g−1 for PX at 288 K and 6 mbar,165 similar to the data reported by Yang et al.166 Stacking effect of molecules in adsorbent cavities led to capacity difference for isomers.164 Concerning the stationary phase of MIL-125(Ti), separation within 5 min obtained elution order of MX < OX < PX with selectivity of 2.13 and resolution of 2.38 for PX/OX, while MX and OX could not be discriminated with slight selectivity for OX.167 Zhao et al. investigated separation of xylenes with UiO-66 as stationary phase in both reverse-phase (RP) and normal-phase (NP) HPLC. In RP-HPLC, a well-resolved separation within 20 min put forth resolution above 1.5 with 80:20 for MeOH/H2O (v/v) as mobile phase. Elution sequence followed PX < MX < OX with 1358 plates m−1 for MX. Mobile-phase composition had a great influence on separation, as poor resolutions, broad peaks, and long retention times were seen at a ratio of 75:25 for MeOH/H2O (v/v). When the MeOH fraction (v/v) was above 85, ineffective and even coelution separations were observed168 (Figure 27a). The strongest van der Waals interaction accounts for the OX elution sequence as explained above.159 Furthermore, geometric properties allowed its methyl groups to interact with carboxylate groups in the framework. Efficient packing of PX led to strong π−π interactions among themselves and weak interactions with the framework, which combined with dipole movement resulted in the first elution. NP-HPLC acquired superior separation within 12 min and the same order of elution at 95:5 for n-hexane/DCM (v/v), with lower backpressure, higher resolution, and greater column efficiency of 2154 plates m−1 for MX (Figure 27b). Mobilephase composition also had a significant influence on NP-HPLC. HPLC directly packed with MOF particles often exhibited low column efficiency and high column backpressure, related to 14743


Review


Figure 27. HPLC separation of xylenes/C8 aromatics. (a) RP-HPLC with stationary phase of UiO-66 using different mobile phase compositions of MeOH/H2O (v/v) at a flow rate of 0.5 mL min−1. (b) NP-HPLC with stationary phase of UiO-66 using different mobile phase compositions of n-hexane/DCM (v/v) at a flow rate of 1.0 mL min−1. (c) NP-HPLC with stationary phase of silica-UiO-66 using mobile phase of hexane/DCM (98:2) at a flow rate of 0.6 mL min−1. (d) RP-HPLC with stationary phase of [email protected] using mobile phase of 30% ACN at a flow rate of 1.0 mL min−1 (0.16 was concentration of precursors). (e) RP-HPLC with stationary phase of UiO-66(1x)@SiO2(5) using mobile phase of 40% ACN and 60% water at a flow rate of 0.2 mL min−1 (SiO2(5) = 5 μm SiO2; 1x = one deposition of UiO-66 crystals). Adapted with permission from ref 168, Figures 2 and 4; ref 154, Figure 5; ref 169, Figure 6; and ref 170, Figure 6.

2094 g m−2 h−1, with MX/PX selectivity varied from 1.10 to 1.93 and OX/PX selectivity elevated from 1.34 to 1.62. However, the membrane was unstable at 175 °C and could not separate OX and MX well.173

efficiency in HPLC separation.171 Silica SOS@HKUST-1 composite microspheres, with SOS silica particles functionalized by −COOH groups, were applied in separating xylenes (Figure 29a). The baseline separation within 5 min depicted high resolution in a column conditioned by DCM for 24 h. The elution sequence was OX < PX < MX, and the column efficiency was 8960 plates m−1, with PX overall selectivity of 2.0. When the column was conditioned with toluene, HPLC showed the same elution order but higher column efficiency of 17 789 plates m−1 for MX (Figure 29b). Pore size and interactions between xylene and framework as well as between guests all contributed to the separation. Mobile-phase selection and column conditioning with DCM or toluene were critical in achieving this well-resolved separation.172 3.3. Membrane Separation. Kang et al. prepared Zn(BDC)(Dabco)0.5 membrane on (3-aminopropyl)triethoxysilane-modified SiO2 substrate by a second growth approach. Experiments exhibited better permeability for OX and MX, originating from selective adsorption of Zn(BDC)(Dabco)0.5. Influences of membrane thickness and separation temperature on liquid separation selectivity were investigated in binary mixture systems. Increases in thickness by more growth cycles led to an increase in selectivity and decrease in permeability. Upon four cycles at 150 °C, selectivities rose to 1.93 and 1.62 for MX/PX and OX/PX, respectively, resulting from a decrease of defects and increase of membrane thickness. When temperature varied from 25 to 150 °C, MX permeance enhanced from 826 to

4. ORGANICS 4.1. Adsorptive Separation. 9,9′-Bianthryl, 9,9′-spirobifluorene, and trans-2,3-dibenzoylspiro[cyclopropane-1,9′fluorene] achieved xylenes separation through enclathration (Figure 30a). Among xylene isomers, 9,9′-bianthryl preferred OX over PX, while it did not enclathrate MX (Figure 30b); 9,9′-spirobifluorene merely conducted enclathration of PX, and trans-2,3-dibenzoylspiro[cyclopropane-1,9′-fluorene] only enclathrated OX. Preference for OX over PX in 9,9′-bianthryl arose from shorter dominant H···H interactions in 9,9′-bianthryl· 0.5OX structure.174 Unlike these materials, host material (R,R)(−)-2,3-dimethoxy-1,1,4,4-tetraphenylbutane-1,4-diol could complex with every C8 aromatic. Binary tests gave selectivity values of 70:30 for PX/OX, 61:40 for PX/EB, 75:25 for PX/MX, 60:40 for OX/MX, 54:47 for EB/OX, and 62:38 for EB/MX. Selective complexing was in sequence of PX > EB > OX > MX, consistent with their complex thermal stabilities.175 Xylenes precipitated in the presence of β-cyclodextrin (β-CD) in DMF by forming an inclusion complex based on supramolecular interactions. The complex could precipitate along with 14744


Review


Figure 28. SEM images of (a) UiO-66 and (b) silica-UiO-66 prepared by Yan and co-workers; (c) NH2-SiO2 (A,B), [email protected] (C,D), [email protected] (E,F), and [email protected] (G,H) prepared by Zhang and co-workers (0.16, 0.32, and 0.64 were concentration of precursors); (d) SiO2(5)-COOH (top), UiO-66(1x)@SiO2(5) (middle), and UiO-66(2x)@SiO2(5) (bottom) particles (SiO2(5) = 5 μm SiO2; 1x, 2x = one and two depositions of UiO-66 crystals). Adapted with permission from ref 154 Figure 3; ref 169, Figure 2; and ref 170, Figure 1.

Figure 29. (a) SEM images of HKUST-1@SOS-COOH (A,B) and HKUST-1 crystals (C). (b) Separation of xylene isomers using HPLC with stationary phase of SOS-COOH@HKUST‑1 at a flow rate of 0.35 cm3 min−1 and heptane mobile phase. Column conditioned with DCM for 24 h (A) and conditioned with toluene for 24 h (B), 1-OX, 2-PX, and 3-MX. Adapted with permission from ref 172, Figures 2 and 7.

temperature up to a specific temperature, Tt, and dissolve again upon cooling. Tt was different for different organic molecules.

This was used for xylenes separation. Binary separation experiments showed selectivities of 15.37 and 18.43 for OX/MX and 14745


Review


Figure 30. (a) Schematic graph of 9,9′-bianthryl (H1), 9,9′-spirobifluorene (H2), and trans-2,3-dibenzoylspiro[cyclopropane-1,9′-fluorene] (H3). (b) Packing figure of H1·0.5OX (left) and H1·0.5PX (right). Adapted with permission from ref 174, Scheme 1 and Figures 1 and 2. Figure 31. (a) SEM images of cross section of KAPs-1-coated capillary column. (b) GC coated with KAPs-1 used in separation of benzene, toluene, and C8 isomers at 170 °C and N2 flow rate of 0.8 mL min−1. Adapted with permission from ref 182, Figures 2 and 3.

OX/PX, respectively. Ternary tests exhibited selectivity value of 2.44 for OX/MX, with PX left in filtrate.176 Table 7 summarizes adsorptive separation of xylene isomers applying organics. 4.2. Chromatographic Separation. KAPs-1 is a novel microporous organic polymer (MOP). MOPs are framed by linking of polymerizable monomers or post-cross-linking of preformed polymer chains, presenting diverse combinations and structures.177 MOPs are widely used in gas separation and storage,178 catalysis,179 sensing,180 and separation.181 GC column coated had KAPs-1 showed good selectivity and short retention time on xylenes separation. Elution time rose in order of EB < MX < PX < OX182 (Figure 31), slightly different from their boiling points, which may arise from more efficient packing of PX molecules in cavities.125 When poly(caprolactone)diol (PCL-Diol) acted as stationary phase in GC, a baseline separation of xylenes within 5 min was obtained at temperatures 50 to 80 °C with programming of 5 °C min−1. Elution time went up in sequence of benzene < toluene < EB < PX < MX < OX. Resolution values of MX/PX and PX/EB were 1.61 and 4.08, respectively. Different polarizabilities and dipole moments of xylenes account for the high resolutions.183 4.3. Membrane Separation. Zhang et al. prepared β-cyclodextrin−ethylene glycol diglycidyl ether/poly(vinyl alcohol) (β-CD-EGDE/PVA) blend membranes for pervaporation separation of PX over the other xylenes, based on molecular recognition of β-CD. When the EGDE content increased from 36.9% to 53.9%, separation factor decreased from 1.34 to 1.17, while flux increased from 58 to 73 g m−2 h−1 in binary separation experiments of PX and MX at 30 °C. As for separation of PX and OX, corresponding separation factor declined from 1.72 to 1.04, yet flux rose from 16 to 100 g m−2 h−1. PX was preferred in pervaporation process of the blend membranes.184

Molecularly imprinted polymeric membranes (MIPMs), prepared from print molecule 1,2-dihydroxybenzene and crosslinking monomer cellulose, were also evaluated for xylenes separation. Cellulose could form 3D polymer networks entrapping an imprint molecule; subsequent removal of the imprint molecule formed a cavity, which acted as a molecular recognition site (Figure 32a). Zheng et al. carried out solubility tests because of the dominant status of solubility in molecular imprinting. MIPM-10 (imprinting ratio = 1.0) appeared to have an OX preference, with solubility selectivities of 7.15 and 4.24 for OX/MX and OX/PX, respectively, at OX weight fraction of 0.1, The values were higher than those of MIPM-5 (imprinting ratio = 0.5). These membranes displayed no solubility selectivity at high OX concentrations, which may arise from microscopic swelling of MIPMs. Pervaporation experiments assessing MIPM-10 at 0.50 kPa and 40 °C manifested separation factors of 19.3 and 8.94 for OX/MX and OX/PX, respectively, at OX weight fraction of 0.1 in solution, but permselectivity values went down to 0.3 and 0.06 when OX fraction grew to 0.9 (Figure 32b,c). MIPMs also showed higher flux than non-imprinted control membranes.185 Table 8 summarizes membrane separation of xylene isomers, in which silicalite-1 membranes showed superior property.

5. OTHER MATERIAL Xylenes separation employed GC with stationary phase of graphene quantum dots (GQDs) for the first time,186 as GQDs were mainly applied in bioimaging, biosensing, drug/gene

Table 7. Organics Applied in Xylene Isomers Adsorptive Separation material 9,9′-bianthryl 9,9′-spirobifluorene trans-2,3-dibenzoylspiro[cyclopropane-1,9′-fluorene] (R,R)-(−)-2,3-dimethoxy-1,1,4,4-tetraphenylbutane1,4-diol β-CD

sequence of preference only OX > PX only PX only OX PX > EB > OX > MX OX > MX > PX

selectivity

ref

− − − 2.33 for PX/OX, 1.53 for PX/EB, 3.00 for PX/MX, 1.50 for OX/MX, 1.15 for EB/OX, and 1.63 for EB/MX 15.37 for OX/MX and 18.43 for OX/PX

174 174 174 175

14746

176


Review


delivery, and anticancer agents187 conventionally. Experiments obtained a baseline separation of EB, PX, and OX with elution time increasing in sequence of EB < PX < OX within 5 min at 50 °C, consistent with their boiling point, while PX and MX shared the same retention time. Selectivity values were 1.32 and 1.15 for PX/OX and EB/PX, respectively. Weaker van der Waals interaction between EB and stationary phase resulted in separation of EB and PX, and mostly was retained of OX due to a combination of stronger van der Waals interaction, hydrogenbonding, and higher polarity of OX.186 Table 9 summarizes chromatographic separation of xylene isomers. MAF-6, HKUST-1, PCL-Diol, UiO-66@SiO2, and MIL-53(Al) manifest excellent separation performances.

6. CONCLUSION This Review summarizes separation of valuable xylene isomers through adsorptive, chromatographic, and membrane processes. Adsorptive materials include MOFs, molecular sieves, organics, and GQDs, of which molecular sieves have already been applied

Figure 32. (a) Fabric of molecular recognition site toward OX and recognition of OX. Influence of OX concentration on solubility selectivity (SS(o/m)), separation factor (αo/m), and diffusivity selectivity (SD(o/m)) on (b) MIPM-10 and (c) MIPM-5. Adapted with permission from ref 185, Figures 3, 6, and 7.

Table 8. Membrane Separation of Xylene Isomers membrane Zn(BDC)(Dabco)0.5 a

MFI silicalite-1b silicalite-1d silicalite-1 B-MFI CMS β-CD-EGDE/PVA MIPMs

temp (K) 423 398 423 423 423 423 473 523 rt 303 313

separation factor

permeance

1.93 for MX/PX, 1.62 for OX/PX, and could not separate OX and MX 88 for PX/OX 1000 for PX/OXc 1100 for PX/OX 185 for PX/OX 830 for PX/OX 42 for PX/OX 20 for PX/OX, 5 for PX/EB, and 70 for PX/MX 100 for PX/OX 1.34 for PX/MX, and 1.72 for PX/OX 19.3 for OX/MX, and 8.94 for OX/PX

0.33 mol m

ref

−2 −1

s

173

3 × 10−8 mol m−2 s−1 Pa−1 declined from 21.6 × 10−8 to 5 × 10−8 mol s−1 m−2 Pa−1 during 400 h 12 × 10−8 mol s−1 m−2 Pa−1 (1.7−5.1) × 10−7 mol m−2 s−1 Pa−1 (2.0−3.0) × 10 −7 mol m−2 s−1 Pa−1 5 × 10−8 mol m−2 s−1 Pa−1 3 × 10−8 mol m−2 s−1 Pa−1 10−9 mol m−2 s−1 Pa−1 100 g m−2 h−1 −

81 84 85 86 86 87 83 79 184 185

a

c-Oriented membrane. bb-Oriented membrane. cSeparation factor for PX/OX remained 1000 during 20−370 h. db-Out-of-plane orientation membrane.

Table 9. MOFs and GQDs Applied in Separation of Xylene Isomers by Chromatography resolution stationary phase MCF-50 MOF-CJ3 MAF-6 UiO-66 HKUST-1 KAPs-1 PCL-Diol GQDs ZIF-8@PDMS UiO-66 silica-UiO-66 UiO-66@SiO2

SOS@HKUST-1 MIL-101(Cr) MIL-53(Al) MIL-125(Ti) a

elution sequence

chromatography

MX/PX

MX/EB

PX /EB

MX/OX

OX/EB

OX/PX

ref

OX < MX < PX EB < PX < MX < OX MX < PX < OX PX < MX < EB < OX EB < OX < PX < MX EB < MX < PX < OX EB < PX < MX < OX EB < PX < OX OX < MX < PX PX < MX < OX EB < PX < MX < OX EB < PX < MX < OX MX < EB < PX < OX PX < MX < OX MX < PX < OX OX < PX < MX EB < PX < MX < OX EB < PX < OX MX < OX < PX

GC GC GC GC GC GC GC GC GC RP-HPLC NP-HPLC NP-HPLC RP-HPLC NP-HPLC RP-HPLC HLPC HPLC HPLC HPLC

1.54−1.65 1.35 3.2 1.44a 2.55a 1.24a 1.61 b 1.45a 1.90a 1.88a 2.15 − 2.00a 1.64a 1.57a 1.49 b −

− − − 2.68a − 2.60a − − − − − − 2.99a − − −

− 0.42 − − − − 4.08 1.15 − − 1.66 1.85 1.51a − − − 1.67 6 −

1.9a 3.85 10 − − − 9.81a − 1.23a 1.34a 1.64a 1.73 − 5.02a − − 1.84 − b

− − − 5.05a 3.6a − − − − − − − − − − − − 17 −

− − 6.7 − 6.33a 1.94a − 1.32 − − − − 3.23a − 2.88a 1.88a − 3 2.38

153 156 155 159 160 182 183 186 161 168 168 154 169 170 170 172 157 162 167

− −

Resolution calculated from 2(t2 − t1)/(w1 + w2), where ti and wi are retention time and peak width of i. bTwo isomers had similar retention behavior. 14747


Review


Foundation of Tianjin University (2017XZY-0052), and Natural Science Foundation of Tianjin (16JCYBJC20300).

in industry through SMB processes. In the past decade, over 20 000 MOFs have been discovered,188 some of which have been assessed as adsorbents in the field of xylene isomers separation. Several MOFs exhibit selectivity as well as capacity equal to or even more effective than that of state-of-the-art molecular sieves. MOFs as a class of promising adsorbents may improve separation efficiency greatly. However, stability, accessibility, cost, and many other aspects of these materials need further investigations in order to determine the applicable ones. Driven by this, future directions should not only focus on development of new MOF materials but also industrialize those outstanding materials in practical applications. In terms of molecular sieve adsorbents, present efforts are focused on developing new adsorbents, such as zeolite nanoparticles, binderless forms, and novel core−shell structures, and optimizing operating conditions to improve the efficiency of the current SMB process. To sum it up, MOFs, including flexible Zn4O(L)3 and MIL-53, Cu(CDC) with structural movement, and CD-MOFs, all manifested outstanding performance in these fundamental researches. In the case of molecular sieves, H/ZSM-5 and nanosize BaX are excellent representatives in separating xylenes. The one most likely to replace state-of-the-art zeolite in the short term is the nanosize zeolite, featuring larger surface area and shorter diffusion length in comparison with conventional zeolites. As a result, it performs better in terms of both capacity and selectivity. However, greater pressure drop in SMB may result from a smaller crystal size, and this should be taken into consideration for industrial applications. On the other hand, breakthrough tests should be strongly emphasized for their value in evaluating practical ability in adsorptive separation of xylenes, especially under SMB conditions. For example, Zn4O(L)3 and MAF-X8 behaved well in singlecompound adsorption isotherms or molecular simulations, but further breakthrough experiments for evaluation of their industrial application are highly demanded. Besides experiments, computational modeling not only plays an important role in selecting materials from the present arsenal but also provides guidelines for designing new materials for use in xylenes separation. Membrane separation is an emerging field, which has developed rapidly in recent years due to the high separation factor and energy efficiency. For separation of xylene isomers, high separation factor and permeance were obtained with thin-film membranes, which manifest great potential for industrial applications. Besides problems of reproducibility in meeting industrial and commercial requirements, stability in long-term and multicomponent mixtures operation, cost of production, and difficulties in scale-up are still main challenges and will remain the focus of studies on application of membrane separation in xylene isomers separation.

■

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86-22-27406186. E-mail: [email protected]. ORCID

Yuxi Yang: 0000-0003-2042-2641 Xianghai Guo: 0000-0002-1519-4050 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No.21202116), Independent Innovation 14748

ABBREVIATIONS 1, Zn4O(L)3/Dyna MOF-100 1D, one-dimensional 1⊃G, {[Zn4O(L)3(DMF)2]·xG}n 2,4-pdc, pyridine-2,4-dicarboxylic acid dianion 2·G, [Zn(Hpidba)]·2.6DMF·H2O/MCF-50 β-CD, β-cyclodextrin BDC, 1,4-benzenedicarboxylate BTC, 1,3,5-benzenetricarboxylate CAU-13, Al(OH)(trans-CDC) CDC, 1,4-cyclohexanedicarboxylate CD-MOF, cyclodextrin metal−organic framework CMS, carbon molecular sieve CPO-27-Ni, Ni2(dobdc) CUK, Cambridge University−KRICT CUK-1, Co3(2,4-pdc)2(μ3-OH)2·9H2O Dabco, 1,4-diazabicyclo[2.2.2]octane DCM, dichloromethane DMF, N,N-dimethylformamide dobdc, 2,5-dioxido-1,4-benzenedicarboxylate EB, ethylbenzene EGDE/PVA, ethylene glycol diglycidyl ether/poly(vinyl alcohol) FAU, faujasite FCOM, fluorescence confocal optical microscopy G, guest solvent molecules GC, gas chromatography GQDs, graphene quantum dots H2AzoD, azobenzene-3,3′-dicarboxylic acid H4TCPB, 1,2,4,5-tetrakis(4-carboxyphenyl)benzene Heim, 2-ethylimidazole HKUST-1, Hong Kong University of Science and Technology/Cu3(BTC)2 HPLC, high-performance liquid chromatography IAST, ideal adsorbed solution theory MAF, metal−azolate framework MAF-6, rho-[Zn(eim)2] MFI, Mobile Five MIL, Materials of Institut Lavoisier MIPM, molecularly imprinted polymeric membrane MOF, metal−organic framework MOF-CJ3, [HZn3(OH)(BTC)2(2H2O)(DMF)]·H2O MOP, microporous organic polymer MX, m-xylene NP, normal-phase OX, o-xylene PBT, polybutylene terephthalate PCL-diol, poly(caprolactone)diol PDMS, poly(dimethylsiloxane) PET, polyethylene terephthalate ppp, p-phenylpyridine RP, reverse-phase PSA, pressure swing adsorption PX, p-xylene PXRD, powder X-ray diffraction SEM, scanning electron microscopy SMB, simulated moving bed SOS, sphere-on-sphere SSF, sintered silica fiber TSA, thermal swing adsorption DOI: 10.1021/acs.iecr.7b03127 Ind. Eng. Chem. Res. 2017, 56, 14725−14753

Review


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UiO, University of Oslo UOP, Universal Oil Products ZIF, zeolitic imidazolate framework ZMOF, zeolite-like metal−organic framework

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