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.

Separation of Xylene Isomers: A Review of Recent Advances in Materials

We examine mixture separations with microporous adsorbents such as zeolites, metal–organic frameworks (MOFs) and zeolitic imidazolate frameworks (ZIFs), operating under conditions close to pore saturation. Pore saturation is realized, for example, when separating bulk liquid phase mixtures of polar compounds such as water, alcohols and ketones. For the operating conditions used in industrial practice, pore saturation is also attained in separations of hydrocarbon mixtures such as xylene isomers and hexane isomers. Separations under pore saturation conditions are strongly influenced by differences in the saturation capacities of the constituent species; the adsorption is often in favor of the component with the higher saturation capacity. Effective separations are achieved by exploiting differences in the efficiency with which molecules pack within the ordered crystalline porous materials. For mixtures of chain alcohols, the shorter alcohol can be preferentially adsorbed because of its higher saturation capacity. With hydrophilic adsorbents, water can be selectively adsorbed from water–alcohol mixtures. For separations of o-xylene–m-xylene–p-xylene mixtures, the pore dimensions of MOFs can be tailored in such a manner as to allow optimal packing of the isomer that needs to be adsorbed preferentially. Subtle configurational differences between linear and branched alkane isomers result in significantly different packing efficiencies within the pore topology of MFI, AFI, ATS, and CFI zeolites. A common characteristic feature of most separations that are reliant on molecular packing effects is that adsorption and intra-crystalline diffusion are synergistic; this enhances the separation efficiencies in fixed bed adsorbers.

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Separating mixtures by exploiting molecular packing effects in microporous materials

Herein, we report the synthesis of hierarchical AlPO4-5 and SAPO-5 molecular sieves that exhibit varying degrees of mesoporous interconnectivity by controlling the amount of triethylamine (TEA) (used as the structure directing agent) and the Si-to-Al ratio (Si/Al); the gel mixture had a composition of (1 − y) Al2O3:y SiO2:1.3 P2O5:x TEA:200 H2O. The molecular sieves were characterized by X-Ray powder Diffraction (XRD) and Scanning Electron Microscopy/Energy Dispersive X-Ray Spectroscopy (SEM/EDX), and their sorption properties were studied by N2 adsorption-desorption at 77 K and water adsorption at 316 K. In the synthesis gel with x = 1.6 ((1 − y) Al2O3:y SiO2:1.3 P2O5:1.6 TEA:200 H2O), SAPO-5 and AlPO4-5 exhibit high Brunauer-Emmett-Teller specific surface areas of ~380 m2 g−1 and 325 m2 g−1, respectively. SAPO-5, having a composition of Si0.46Al0.6P0.804, exhibits significant water uptake at low relative pressures (3.4 mmol/g at P/P0 = 0.03 vs. 0.2 mmol/g for pure AlPO4-5) attributed to the contribution of hydrophilic Bronsted acid sites, arising from the charge imbalance created by the isomorphic substitution of Si into the aluminophosphate structure predominantly via the SM2 mechanism. Furthermore, SAPO-5 exhibited a hierarchical pore structure having distinct pore size distributions centred on 0.7 nm and 15.8 nm, attributed to the AFI framework and the induced mesoporosity, respectively. At higher relative pressures, SAPO-5 exhibited the highest total water adsorption capacity of 14.1 mmol/g at a relative pressure of 0.38, compared to 11 mmol/g for AlPO4-5. The higher overall capacity of SAPO-5 is attributed to capillary condensation into the relatively large mesopore volume.

Hierarchical AlPO4-5 and SAPO-5 microporous molecular sieves with mesoporous connectivity for water sorption applications

Confined water presents unusual properties in comparison with other sorbate species. First of all, the sorption isotherm is of type III, even in the microporous confinement range (O < 20 A). Whatever the pore diameter, water sorption phenomenon looks like the so-called capillary condensation phase transition. Our results clearly valid such an expected behaviour in the mesoporous confinement range (20 A < O < 40 A). The water confined phase is a liquid phase characterized by a short range order and a high translational molecular mobility. The confinement induces a strong displacement towards the low temperature of the water confined liquid solidification Tsol. (for instance, Tsol. = 230 K for D2O confined liquid in MCM-41 (O = 24 A). We have determined the structure of the water confined solid phase observed below Tsol.. It looks like those of the cubic ice structure affected by strong quasi-isotropic finite size effects induced by the confinement. Such a quasi-(1d) solid appears as a polycrystalline column rather than a single crystalline nanofiber. Concerning water confinement in the microporous range (as for example, AlPO4-5 zeolite (O = 7.3 A)), our results are more surprising. Type III sorption isotherm is the signature of a crystallization phenomenon at room temperature (T = 300 K). The confined water crystallizes in two helices that are commensurate with the AlPO4-5 micropore structure. The confined ice has a density of 1.2 g⋅ cm− 3.

Confined Water in Mesoporous MCM-41 and Nanoporous AlPO4-5: Structure and Dynamics

Green SAPO-5 supported NiO nanoparticles as a novel adsorbent for removal of petroleum asphaltenes: Financial assessment

The adsorption properties of an aluminophosphate molecular sieve, SAPO-5, were measured for a number of gases and vapors, including N2, water, isopropanol, and xylenes. The data showed that SAPO-5 is quite hydrophobic and has a strong selectivity of o-xylene over its isomers m- and p-xylene.

Adsorption Properties of the SAPO-5 Molecular Sieve

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Adsorption Properties of the SAPO-5 Molecular Sieve