Hierarchical ZSM‐5 Zeolites in Shape‐Selective Xylene Isomerization: Role of Mesoporosity and Acid Site Speciation

The isomerization of o‐xylene, a prototypical example of shape‐selective catalysis by zeolites, was investigated on hierarchical porous ZSM‐5. Extensive intracrystalline mesoporosity in ZSM‐5 was introduced by controlled silicon leaching with NaOH. In addition to the development of secondary porosity, the treatment also induced substantial aluminum redistribution, increasing the density of Lewis acid sites located at the external surface of the crystals. However, the strength of the remaining Brønsted sites was not changed. The mesoporous zeolite displayed a higher o‐xylene conversion than its parent, owing to the reduced diffusion limitations. However, the selectivity to p‐xylene decreased, and fast deactivation due to coking occurred. This is mainly due to the deleterious effect of acidity at the substantially increased external surface and near the pore mouths. A consecutive mild HCl washing of the hierarchical zeolite proved effective to increase the p‐xylene selectivity and reduce the deactivation rate. The HCl‐washed hierarchical ZSM‐5 displayed an approximately twofold increase in p‐xylene yield compared to the purely microporous zeolite. The reaction was followed by operando infrared spectroscopy to simultaneously monitor the catalytic performance and the buildup of carbonaceous deposits on the surface. Our results show that the interplay between activity, selectivity, and stability in modified zeolites can be optimized by relatively simple post‐synthesis treatments, such as base leaching (introduction of mesoporosity) and acid washing (surface acidity modification).     

Catalytic Performance of Modified HMCM‐22 Catalysts in Xylene Isomerization

HMCM‐22 catalysts modified with La₂O₃ (5% La) and MgO (≈0.87% Mg) were prepared respectively by impregnation method, and were characterized by scanning electron microscopy, X‐ray diffraction, N₂ physical adsorption‐desorption and temperature‐programmed desorption of NH₃. The effect of supported metallic oxides (La₂O₃, MgO) on catalytic performance in xylene isomerization of C8 aromatics (ethylbenzene, m‐xylene and o‐xylene) was investigated in detail. The experimental results showed that 5% La/HMCM‐22 catalyst had higher isomerization activity and stronger shape‐selectivity than 0.87% Mg/HMCM‐22 catalyst, owing to its more acid sites and smaller pore size. And the loading amount of La was optimized to be about 7%. Moreover, supporting metal over 7% La/HMCM‐22, respectively with 0.3% Pt, 3% Ni and 3% Mo, was carried out to prepare bifunctional isomerization catalysts. In comparison, 3% Mo/7% La/HMCM‐22 showed the best catalytic performance with both high activity and high selectivity, with the low hydrocracking of m‐xylene and o‐xylene. Besides, the optimal reaction conditions were found: 340°C, 1.5 MPa H₂, WHSV 4 h^?1 and H₂/C8 4 mol/mol. Under the above conditions, ethylbenzene conversion was up to 20%, para‐selectivity was over 23% with low xylene loss of 2.9%.     

Chemical Composition and Reaction Mechanisms for Aged p‐Xylene Secondary Organic Aerosol in the Presence of Ammonia

A laboratory study was carried out to investigate the chemical composition of aged aromatic secondary organic aerosol (SOA) formed from the photoxidation of p‐xylene in the presence of ammonia (NH₃). The experiments were conducted by irradiating p‐xylene/CH₃ONO/NH₃ air mixtures without and with NO in a home‐made smog chamber. The particulate products of aged p‐ xylene SOA in the presence of NH₃ were measured by UV–vis spectrophotometry, attenuated total reflectance Fourier transform infrared (ATR‐FTIR) spectroscopy, and aerosol laser time‐of‐flight mass spectrometry (ALTOFMS) coupled with the fuzzy C‐means (FCM) clustering algorithm. The experimental results show that NH₃ does not alter the gas–particle partitioning in the photoxidation of p‐xylene without NO and that 2,5‐dimethylphenol is the predominant NH₃‐aged p‐xylene SOA without NO. However, NH₃ has a significant promotional effect on the formation of organonitrogen compounds in the OH‐initiated oxidation of p‐xylene with NO. Organic ammonium salts such as ammonium glyoxylate and p‐methyl ammonium benzoate, which are formed from NH₃ reactions with gaseous organic acids, were detected as the major particulate organonitrogen products of NH₃‐aged p‐xylene SOA with NO. 1H‐Imidazole, 4‐methyl‐1H‐imidazole, and other imidazole products of the heterogeneous reactions between NH₃ and dialdehydes of p‐xylene SOA were newly measured. The possible reaction mechanisms leading to these organonitrogen products are also discussed and proposed. The formation of imidazole products suggests that some ambient particles containing organonitrogen compounds may be the result of this mechanism. The results of this study may provide valuable information for discussing anthropogenic SOA aging mechanisms.     

Highly Selective,High‐Capacity Separation of o‐Xylene from C8 Aromatics by a Switching Adsorbent Layered Material

Purification of the C₈ aromatics (xylenes and ethylbenzene) is particularly challenging because of their similar physical properties. It is also relevant because of their industrial utility. Physisorptive separation of C₈ aromatics has long been suggested as an energy efficient solution but no physisorbent has yet combined high selectivity (>5) with high adsorption capacity (>50 wt %). Now a counterintuitive approach to the adsorptive separation of o‐xylene from other C₈ aromatics involves the study of a known nonporous layered material, [Co(bipy)₂(NCS)₂]_n ( sql‐1‐Co‐NCS ), which can reversibly switch to C₈ aromatics loaded phases with different switching pressures and kinetics, manifesting benchmark o‐xylene selectivity (S_OX/EB≈60) and high saturation capacity (>80 wt %). Structural insight into the observed selectivity and capacity is gained by analysis of the crystal structures of C₈ aromatics loaded phases.     

Highly Selective,High‐Capacity Separation of o‐Xylene from C8 Aromatics by a Switching Adsorbent Layered Material

Purification of the C₈ aromatics (xylenes and ethylbenzene) is particularly challenging because of their similar physical properties. It is also relevant because of their industrial utility. Physisorptive separation of C₈ aromatics has long been suggested as an energy efficient solution but no physisorbent has yet combined high selectivity (>5) with high adsorption capacity (>50 wt %). Now a counterintuitive approach to the adsorptive separation of o‐xylene from other C₈ aromatics involves the study of a known nonporous layered material, [Co(bipy)₂(NCS)₂]_n ( sql‐1‐Co‐NCS ), which can reversibly switch to C₈ aromatics loaded phases with different switching pressures and kinetics, manifesting benchmark o‐xylene selectivity (S_OX/EB≈60) and high saturation capacity (>80 wt %). Structural insight into the observed selectivity and capacity is gained by analysis of the crystal structures of C₈ aromatics loaded phases.     

Separation of p‐Divinylbenzene by Selective Room‐Temperature Adsorption Inside Mg‐CUK‐1 Prepared by Aqueous Microwave Synthesis

A new Mg^II‐based version of the porous coordination polymer CUK‐1 with one‐dimensional pore structure was prepared by microwave synthesis in water. Mg‐CUK‐1 is moisture‐stable, thermally stable up to 500 °C, and shows unusual reversible soft‐crystal behavior: dehydrated single crystals of the material selectively adsorb a range of organic molecules at ambient temperature and pressure. Both polar and apolar aromatic compounds, including pyridine, benzene, p‐xylene, and p‐divinylbenzene (p‐DVB), are all readily adsorbed, while other isomers from complex mixtures of xylenes or DVBs are selectively excluded. The solvent‐loaded structures have been studied by single‐crystal X‐ray diffraction. Time‐dependent liquid sorption experiments using commercially available DVB demonstrate a high and rapid selective adsorption of p‐DVB and exclusion of m‐DVB and ethylvinylbenzene isomers.     

Pure and Palladium‐Loaded Co3O4 Hollow Hierarchical Nanostructures with Giant and Ultraselective Chemiresistivity to Xylene and Toluene

Pure and palladium‐loaded Co₃O₄ hollow hierarchical nanostructures consisting of nanosheets have been prepared by solvothermal self‐assembly. The nanostructures exhibited an ultrahigh response and selectivity towards p‐xylene and toluene. The responses (resistance ratio) of the palladium‐loaded Co₃O₄ hollow hierarchical nanostructures to 5 ppm of p‐xylene and toluene were as high as 361 and 305, respectively, whereas the selectivity values (response ratios) towards p‐xylene and toluene over interference from ethanol were 18.1 and 16.1, respectively. We attributed the giant response and unprecedented high selectivity towards methylbenzenes to the abundant adsorption of oxygen by Co₃O₄, the high chemiresistive variation in the Co₃O₄ nanosheets (thickness≈11 nm), and the catalytic promotion of the specific gas‐sensing reaction. The morphological design of the p‐type Co₃O₄ nanostructures and loading of the palladium catalyst have paved a new way to monitoring the most representative indoor air pollutants in a highly selective, sensitive, and reliable manner.     

Experimental and Computational Study of the Thermal Decomposition of 3‐Methyl‐3‐buten‐1‐ol in m‐Xylene Solution

An experimental study of the thermal decomposition of a β‐hydroxy alkene, 3‐methyl‐3‐buten‐1‐ol, in m‐xylene solution, has been carried out at five different temperatures in the range of 513.15–563.15 K. The temperature dependence of the rate constants for the decomposition of this compound in the corresponding Arrhenius equation is given by ln k (s^?1) = (25.65 ± 1.52) ? (17,944 ± 814) (kJ·mol^?1)·T^?1. A computational study has been carried out at the M05–2X/6–31+G(d,p) level of theory to calculate the rate constants and the activation parameters by the classical transition state theory. There is a good agreement between the experimental and calculated rate constants and activation Gibbs energies. The bonding characteristics of reactant, transition state, and products have been investigated by the natural bond orbital analysis, which provides the natural atomic charges and the Wiberg bond indices. Based on the results obtained, the mechanism proposed is a one‐step process proceeding through a six‐membered cyclic transition state, being a concerted and slightly asynchronous process. The results have been compared with those obtained previously by us (Struct Chem 2013, 24, 1811–1816) for the thermal decomposition of 3‐buten‐1‐ol, in m‐xylene solution. We can conclude that in the compound studied in this work, 3‐methyl‐3‐buten‐1‐ol, the effect of substitution at position 3 by a weakly activating CH₃ group is the stabilization of the transition state formed in the reaction and therefore a small increase in the rate of thermal decomposition.     

Creation of Well‐Defined “Mid‐Sized” Micropores in Carbon Molecular Sieve Membranes

Carbon molecular sieve (CMS) membranes are candidates for the separation of organic molecules due to their stability, ability to be scaled at practical form factors, and the avoidance of expensive supports or complex multi‐step fabrication processes. A critical challenge is the creation of “mid‐range” (e.g., 5–9 Å) microstructures that allow for facile permeation of organic solvents and selection between similarly‐sized guest molecules. Here, we create these microstructures via the pyrolysis of a microporous polymer (PIM‐1) under low concentrations of hydrogen gas. The introduction of H₂ inhibits aromatization of the decomposing polymer and ultimately results in the creation of a well‐defined bimodal pore network that exhibits an ultramicropore size of 5.1 Å. The H₂ assisted CMS dense membranes show a dramatic increase in p‐xylene ideal permeability (≈15 times), with little loss in p‐xylene/o‐xylene selectivity (18.8 vs. 25.0) when compared to PIM‐1 membranes pyrolyzed under a pure argon atmosphere. This approach is successfully extended to hollow fiber membranes operating in organic solvent reverse osmosis mode, highlighting the potential of this approach to be translated from the laboratory to the field.     

Three p‐xylene‐solvated pseudopolymorphs of bis[1,3‐bis(pentafluorophenyl)propane‐1,3‐dionato]copper(II)

The Cu²⁺ ions in the title compounds, namely bis[1,3‐bis(pentafluorophenyl)propane‐1,3‐dionato‐κ²O,O′]copper(II) p‐xylene n‐solvate, [Cu(C₁₅HF₁₀O₂)₂]·nC₈H₁₀, with n = 1, (I), n = 2, (II), and n = 4, (III), are coordinated by two 1,3‐bis(pentafluorophenyl)propane‐1,3‐dionate ligands. The coordination complexes of (I) and (II) have crystallographic inversion symmetry at the Cu atom and the p‐xylene molecule in (I) also lies across an inversion centre. The p‐xylene molecules in (I) and (II) interact with the pentafluorophenyl groups of the complex via arene–perfluoroarene interactions. In the crystal of (III), two of the p‐xylene molecules interact with the pentafluorophenyl groups via arene–perfluoroarene interactions. The other two p‐xylene molecules are located on the CuO₄ coordination plane, forming a uniform cavity produced by metal...π interactions.     

Study on δ‐Form Complex in a Syndiotactic Polystyrene/Organic Molecules System, 1. Preferential Complexing Behavior of Xylene Isomers

To investigate the preferential complexing behavior of isomeric xylenes, syndiotactic polystyrene (sPS) membranes are prepared using varying compositions of m‐ and p‐xylene. Complex formation between sPS and the xylenes was studied by means of thermogravimetric and FT‐IR analyses to determine the exact amounts of solvent molecules present per styrene repeating unit. A preferential complexing ability of p‐xylene was revealed due to its favorable interaction with sPS.     

Separation of Xylene Isomers in the Anion-Pillared Square Grid Material SIFSIX-1-Cu

Xylene isomer separation is considered one of the seven separation challenges that changed the world. In addition, the high-energy demand of xylene separation highlights the need for efficient novel adsorbents. Herein, the liquid-phase separation potential of the anion-pillared hybrid material SIFSIX-1-Cu was studied for preferential adsorption of o-xylene and m-xylene over p-xylene, which was inspired by a previous complexation crystallization method for separating m-xylene. We report detailed experimental liquid-phase adsorption experiments, yielding selectivities of 3.0 for o-xylene versus p-xylene and 2.6 for m-xylene versus p-xylene. Our theoretical calculations thus provide a reasonable explanation that the xylene adsorption selectivity is attributed to the C−H⋅⋅⋅F interaction, and the host–guest interaction order agrees with the adsorption priority: o-xylene > m-xylene > p-xylene.     

Hypercrosslinked Additives for Ageless Gas‐Separation Membranes

The loss of internal pores, a process known as physical aging, inhibits the long‐term use of the most promising gas‐separation polymers. Previously we reported that a porous aromatic framework (PAF‐1) could form a remarkable nanocomposite with gas‐separation polymers to stop aging. However, PAF‐1 synthesis is very onerous both from a reagent and reaction‐condition perspective, making it difficult to scale‐up. We now reveal a highly dispersible and scalable additive based on α,α′‐dichloro‐p‐xylene (p‐DCX), that inhibits aging more effectively, and crucially almost doubles gas‐transport selectivity. These synergistic effects are related to the intimately mixed nanocomposite that is formed though the high dispersibility of p‐DCX in the gas‐separation polymer. This reduces particle‐size effects and the internal free volume is almost unchanged over time. This study shows this inexpensive and scalable polymer additive delivers exceptional gas‐transport performance and selectivity.     

Intermolecular Interactions in Weak Donor–Acceptor Complexes from Symmetry‐Adapted Perturbation and Coupled‐Cluster Theory: Tetracyanoethylene–Benzene and Tetracyanoethylene–p‐Xylene

The interactions in the complexes of tetracyanothylene (TCNE) with benzene and p‐xylene, often classified as weak electron donor–acceptor (EDA) complexes, are investigated by a range of quantum chemical methods including intermolecular perturbation theory at the DFT‐SAPT (symmetry‐adapted perturbation theory combined with density functional theory) level and explicitly correlated coupled‐cluster theory at the CCSD(T)‐F12 level. The DFT‐SAPT interaction energies for TCNE–benzene and TCNE–p‐xylene are estimated to be ?35.7 and ?44.9 kJ mol^?1, respectively, at the complete basis set limit. The best estimates for the CCSD(T) interaction energy are ?37.5 and ?46.0 kJ mol^?1, respectively. It is shown that the second‐order dispersion term provides the most important attractive contribution to the interaction energy, followed by the first‐order electrostatic term. The sum of second‐ and higher‐order induction and exchange–induction energies is found to provide nearly 40 % of the total interaction energy. After addition of vibrational, rigid‐rotor, and translational contributions, the computed internal energy changes on complex formation approach results from gas‐phase spectrophotometry at elevated temperatures within experimental uncertainties, while the corresponding entropy changes differ substantially.     

para‐Xylene Ultra‐selective Zeolite MFI Membranes Fabricated from Nanosheet Monolayers at the Air–Water Interface

The control of membrane morphology and microstructure is crucial to improve the separation performance of molecular‐sieve membranes. This can be enabled by making thin, dense, and uniform seed‐crystal coatings, which are then intergrown into continuous membranes. Herein, we show a novel and simple floating particle coating method can give closely packed monolayers of zeolite nanosheets on nonporous or porous supports. The zeolite nanosheet monolayer is formed at the air–water interface in a conical Teflon trough. As the water in the trough is drained, the monolayer is deposited on a support placed below. Membranes prepared by gel‐free secondary growth of the nanosheets deposited by this method show unprecedented ultra‐selective performance for separation of para‐ from ortho‐xylene (separation factor >10 000).     

Capillary GC using pyridyl β‐cyclodextrin stationary phase

A new β‐CD derivative, heptakis [2,6‐di‐O‐pentyl‐3‐O‐(4′‐chloro‐5′‐pyridylmethyl)]‐β‐CD, was synthesized by the selective introduction of a pyridyl group on the 3‐positions of β‐CD. The chromatographic properties of the pyridyl β‐CD derivative were studied by using it as the stationary phase in capillary GC. The polarity of the prepared stationary phase was moderate, and the separation results demonstrated that the prepared stationary phase possessed excellent separation ability and chiral recognition for a wide range of analytes. Not only the aromatic positional isomers, such as o‐, m‐, p‐xylene and α‐, β‐naphthol isomers, but also some compounds with multi‐stereogenic centers, such as n‐(1‐methylpropyl)‐3‐(2,2‐dichloroethenyl)‐2,2‐dimethylcyclopropanecarboxamide and n‐(1‐methylpropyl)‐3‐(2‐chloro‐3,3,3‐trifluoropropenyl)‐2,2‐dimethylcyclopropanecarboxamide with three stereogenic centers including eight configurational isomers, were successfully separated. The results also indicated that the polarity of the β‐CD derivative, and the hydrogen bonding between the β‐CD derivative, and the analytes had a very important effect on separation.     

Polymer segmental dynamics and solvent thermal transitions in poly(ethyl acrylate)/p‐xylene mixtures

A poly(ethyl acrylate) polymer network was swollen with different concentrations of the nonpolar solvent p‐xylene, c_px, from xerogel until saturation (0 ≤ c_px ≤ 0.85). Differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC) techniques were employed to study the polymer segmental dynamics and the solvent thermal transitions in homogeneous (c_px < 0.20) and partially crystallized (c_px ≥ 0.20) PEA/p‐xylene mixtures. Our DSC measurements indicate that p‐xylene undergoes cold crystallization for intermediate solvent concentrations, 0.20 ≤ c_px ≤ 0.30 while for higher c_px values crystallization takes place during cooling. The results show that for c_px ≤ 0.30 the T_g decreases with increasing c_px (plasticization effect) obeying the respective Fox equation. For the same c_px range we found that both the dielectric strength and the heat capacity increment of the segmental (α) relaxation process increase gradually with c_px whereas the distribution of relaxation times for the underlying molecular relaxations does not change. For c_px > 0.30 the partially crystallized mixtures exhibit a constant T_g corresponding to the gel phase of PEA with an amount of p‐xylene which is not able to crystallize under any conditions. The concentration of this noncrystallized p‐xylene, c_UCpx, has been estimated to be between 0.12 and 0.15, independent of the total p‐xylene concentration in the mixtures. When a separate p‐xylene crystal phase is formed (for c_px > 0.30) the segmental dielectric strength and heat capacity increment decrease significantly exhibiting values significantly lower than those measured for the homogeneous gels. In addition, we found that the presence of p‐xylene crystals may induce marginal spatial heterogeneity of polymer (or p‐xylene) concentration within the gel phase affecting thus slightly the breath of the segmental relaxation of PEA. We attribute these results to restrictions of polymer segmental configurations due to constraints imposed by the p‐xylene crystals and/or to the immobilization of a part of the polymer chains. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Photoinduced Iron‐Based Water‐Induced Phase Separable Catalysis (WPSC) ICAR ATRP of Poly(ethylene glycol) Methyl Ether Methacrylate

Iron‐mediated atom transfer radical polymerization (ATRP) has gained extensive attention because of the superiority of iron catalysts, such as low toxicity, abundant reserves, and good biocompatibility. Herein, a practical iron catalyst recycling system, photoinduced iron‐based water‐induced phase separable catalysis ATRP with initiators for continuous activator regeneration, at room temperature is developed for the first time. In this polymerization system, the polymerization is conducted in homogenous solvents consisting of p‐xylene and ethanol, using commercially available 5,10,15,20‐tetraphenyl‐21H,23H‐porphine iron(III) chloride as the iron catalyst, ethyl 2‐bromophenylacetate as the ATRP initiator, 2,4,6‐trimethylbenzoyl diphenylphosphine oxide as the photoinitiator, and poly(ethylene glycol) methyl ether methacrylate as the model hydrophilic monomer. After polymerization, a certain amount of water is added to induce the phase separation so that the catalyst can be separated and recycled in p‐xylene phase with very low residual metal complexes (<12 ppm) in the resultant polymers even after six times recycle experiments.

     

Ultrathin polymer gate buffer layer for air‐stable,low‐voltage,n‐channel organic thin‐film transistors

An ultrathin poly(methyl methacrylate) (PMMA) buffer layer was developed to improve the performance of n‐channel organic thin‐film transistors (OTFTs). The 8 nm‐thick PMMA film, prepared by spin‐coating, provided a very smooth surface and a uniform coverage on SiO₂ surface reproducibly, which was confirmed by X‐ray reflectivity (XR) measurement. Then, we fabricated N,N′‐ditridecyl‐3,4,9,10‐perylenetetracarboxylic diimide (PTCDI‐C13) thin‐film transistors with and without this 8 nm‐thick PMMA insulating layer on SiO₂ gate insulators and achieved one‐order increase of field‐effect mobility (up to 0.11 cm²/(Vs) in a vacuum), one‐half decrease of threshold voltage, and reduction of current hysteresis with the PMMA layer. Only TFTs with the PMMA layer displayed n‐channel operation in air and showed field‐effect mobility of 0.10 cm²/(Vs). We consider that electrical characteristics of n‐channel OTFTs were considerably improved because the ultrathin PMMA film could effectively passivate the SiO₂ insulator surface and decrease interfacial electron traps. This result suggests the importance of the ultrathin PMMA layer for controlling the interfacial state at the semiconductor/insulator interface and the device characteristics of OTFTs. Copyright © 2009 John Wiley & Sons, Ltd.     

Metal‐organic framework ZIF‐8 nanocrystals as pseudostationary phase for capillary electrokinetic chromatography

The outstanding properties such as large surface area, diverse structure, and accessible tunnels and cages make metal organic frameworks (MOFs) attractive as novel separation media in separation sciences. However, the utilization of MOFs in EKC has not been reported before. Here we show the exploration of zeolitic imidazolate framework‐8 (ZIF‐8), one of famous MOFs, as the pseudostationary phase (PSP) in EKC. ZIF‐8 nanocrystals were used as the PSP through dispersing in the running buffer (20 mM phosphate solution containing a 1% v/v methanol (pH 9.2)) to enhance the separation of the phenolic isomers (p‐benzenediol, m‐benzenediol, o‐benzenediol, m‐nitrophenol, p‐nitrophenol, and o‐nitrophenol). ZIF‐8 nanocrystals in the running buffer were negatively charged, and interacted with the phenolic hydroxyl groups of the analytes, and thus greatly improved the separation of the phenolic isomers. Inclusion of 200 mg L?¹ ZIF‐8 in the running buffer as the background electrolyte gave a baseline separation of the phenolic isomers within 4 min. The relative standard deviations for five replicate separations of the phenolic isomers were 0.2–1.1% for migration time and 4.5–9.7% for peak area. The limits of detection varied from 0.44 to 2.0 mg L?¹. The results show that nanosized MOFs are promising for application in EKC.