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.     

Adsorption of xylene isomers on ordered hexagonal mesoporous FDU-15 polymer and carbon materials

The oil industry has been facing the challenges of separation of xylene isomers, o-xylene, m-xylene and p-xylene or removing them from the environment. In our present work, we investigated the adsorption of the three isomers on two mesoporous materials, FDU-15-350 polymer and FDU-15-900 carbon materials. The isomer adsorption capacities are well correlated with their physical pore properties. It is found that the micropores are very crucial for the adsorption of these three isomers. The more micropore volume the adsorbent has, the better the adsorption capacity is. Henry’s constants were also calculated for the three isomers on the two adsorbents. Both on FDU-15-350 polymer and FDU-15-900, the Henry’s constants for the three isomers show the same trend o>m>p xylene which is coincidently in accordance with their polarity trend, indicating more polar adsorbate is preferred for adsorption on the two adsorbents. The isosteric heats of adsorption are correlated with the microporosity and the size of the adsorbate molecule. More microporosity and smaller molecules give higher heats of adsorption. Extracted information on pore properties of adsorbents by using the three isomers has very similar results as that resolved from nitrogen adsorption, indicating the feasibility of using the three isomers as adsorbates to extract pore information. This work is devoted to commemorating the 60th birthday of Professor Mieczyslaw (Mietek) Jaroniec.     

An Adaptive Hydrogen-Bonded Organic Framework for the Exclusive Recognition of p-Xylene

Separation of xylene isomers is one of the most important but most challenging and energy-intensive separation processes in the petrochemical industry. Here, we report an adaptive hydrogen-bonded organic framework (HOF-29) constructed from a porphyrin based organic building block 4,4′,4′′,4′′′-(porphyrin-5,10,15,20-tetrayl) tetrabenzonitrile (PTTBN), exhibiting the exclusive molecular recognition of p-xylene (pX) over its isomers of o-xylene (oX) and m-xylene (mX), as clearly demonstrated in the single crystal structure transformation and ¹H NMR studies. Single crystal structure studies show that single-crystal-to-single-crystal transformation from the as-synthesized HOF-29 to the pX exclusively included HOF-29⊃pX is triggered by the encapsulation of pX molecules, accompanied by sliding of the 2D layers and local distortion of the ligand, which provides multiple C−H⋅⋅⋅π interactions.     

Disproportionation of ethylbenzene in the presence of C8 aromatics

The selective synthesis of p-diethylbenzene (p-DEB) by disproportionation of ethylbenzene (EB) in the presence of aromatics like m-and p-xylene isomers has been studied over a pore size regulated HZSM-5 catalyst. The industrial feed having different compositions of ethylbenzene and xylene isomers was used for the experimentation. Hence, they were expected to hinder the movement of reactant molecules both on the external surface and within the zeolite channels. It was observed that irrespective of the different feed compositions the concentration of the xylene isomers was intact in the product. There is no other byproducts formation like para-ethylmethyl benzene. The effects of varying the concentration of aromatic compounds in the feed on ethylbenzene conversion and product distribution over the parent and modified H-ZSM-5 catalyst have been discussed. Ethylbenzene disproportionation reaction follows the pseudo first order reaction with an activation energy of 8.6 kcal/mol.     

Separation of xylenes by extractive crystallization with calixarenes

Several calixarenes1–5 and benzopinacolone6 were recrystallized from 1:1:1 mixtures of the three xylene isomers.p-Iso-propylcalix[4]arene 1 was shown to extractp-xylene with 86% selectivity.p-Iso-propylbishomooxacalix[4]arene5 extractso-xylene with 84% selectivity.     

Monte Carlo and energy minimization studies of binary xylene adsorption in AEL and AFI networks

Adsorption of binary xylene mixtures in AEL and AFI networks was investigated using normal and biased GCMC simulations. Preferential o-xylene adsorption was evidenced in the simulations, as previously reported in single-component experimental data. In contrast to the FAU and MFI sieves, the AEL and AFI networks exhibit surprising azeotropic behavior. The selectivity switches from o-xylene to p-xylene at a gas phase mole fraction of ca. 0.5. Energy minimization was performed in the AlPO₄-11 molecular sieve to determine the energy differences between the adsorption sites. The minimization study showed that AlPO₄-11 has small adsorption energy differences between sites. The azeotropic behavior of the AEL and AFI networks can be explained using the two patch model proposed by Do and Do (Adsorption 5:319–329, 1999).     

Design and Development of Zeolite-Based Catalytic Processes for Aromatics Production

Processes for the production of xylenes, which occur in an integrated aromatic complex, are discussed. A brief overview of the work carried out at Indian Petrochemicals Corporation Limited for the development of zeolite-based catalytic processes for the production of aromatics is presented. This includes xylene isomerization, transalkylation and disproportionation of C₇ and C₉ aromatics for maximization of xylenes, selective disproportionation of toluene and selective alkylation of mono-alkylaromatics to p-dialkylbenzene. Achievements in the commercialization of zeolite-based catalysts and processes for isomerization of m-xylene to p- and o-xylene along with dealkylation of ethylbenzene, and for selective ethylation of ethylbenzene to produce p-diethylbenzene are highlighted.     

Kinetic modeling of the ethylbenzene/xylene isomerization reaction over HZSM-5 zeolites revisited

In this article, a binderless dealuminated HZSM-5 zeolite (Si/Al = 41.4) was used as a catalyst for the isomerization of a mixture of ethylbenzene and xylene. The experimental results indicated that at low residence times the catalyst is effective to isomerize the ethylbenzene into xylenes. A comprehensive kinetic model considering chemisorption, surface chemical reactions, and diffusional processes was developed for this reaction. The intrinsic activation energy (71.99 kJ mol⁻¹) for the surface reaction of ethylbenzene into m-xylene was calculated for the first time, and the corresponding intrinsic activation energies for o-xylene to m-xylene and m-xylene to p-xylene surface reactions were calculated to be 59.45 and 50.68 kJ mol⁻¹, respectively. Lower apparent values have been reported in the literature, and we rationalize that they correspond to multistep processes and intrinsically include a negative activation energy pertaining to chemisorption. The results also revealed that the ethylbenzene diffusion within the zeolite channels was four orders of magnitude smaller than p-xylene.     

A method for simulation of photochemical processes and calculation of quantum yields of reactions

The possibility of construction of a semiempirical method for simulation of photochemical processes and calculation of quantum yields of reactions has been studied. The practicability of the approach was demonstrated for the o-xylene → m-xylene, m-xylene → p-xylene, m-xylene → o-xylene, and o-diethylbenzene → m-diethylbenzene photoisomerization reactions as an example. The calculated quantum yields of the reactions are in qualitative agreement with experimental data.     

Formation of ring-retaining products from the OH radical-initated reactions of o-, m-, and p-xylene

The ring-retaining products formed from the OH radical-initiated reactions of o-, m-, and p-xylene in the presence of NO_x have been identified and their formation yields determined. Experiments were carried out at 298 ± 2 K and in the presence of 740 torr total pressure of air. The products observed, and their yields, were: from o-xylene, o-tolualdehyde, 0.0453; 2-methylbenzyl nitrate, (0.0135 + 5.5 × 10^?17 [NO₂]); 2,3-dimethylphenol, 0.097; 3,4-dimethyl-phenol, 0.064; 3-nitro-o-xylene, 0.0059; 4-nitro-o-xylene, (0.0111 + 9.9 × 10^?17 [NO₂]); from m-xylene, m-tolualdehyde, 0.0331; 3-methylbenzyl nitrate, 0.0061; 2,4-dimethylphenol, 0.099; 2,6-dimethylphenol, 0.111; 4-nitro-m-xylene, 0.0018; 5-nitro-m-xylene, (0.0032 + 1.6 × 10^?17 [NO₂]); from p-xylene, p-tolualdehyde, 0.0701; 4-methylbenzyl nitrate, 0.0082; 2,5-dimethylphenol, 0.188, 2-nitro-p-xylene, (0.0120 + 2.8 × 10^?17 [NO₂]), where the NO₂ concentration is in molecule cm^?3 units. The nitro-xylene data are consistent with our recent product study of the corresponding reactions of benzene and toluene and indicate that under the experimental conditions employed the dimethylhydroxycyclohexadienyl radicals reacted with NO₂ and not with O₂. When combined with literature ring-cleavage product yields, these data show that ca. 55–80% of the reaction pathways are accounted for.     

Vapour–liquid equilibria of butyl acetate with aromatic hydrocarbons at 298.15 K

Vapour pressures of butyl acetate?+?benzene or toluene or o- or m- or p-xylene were measured by static method at 298.15?±?0.01?K over the entire composition range. The activity coefficients and excess molar Gibb's free energies of mixing (G ^E) for these binary mixtures were calculated by fitting vapour pressure data to the Redlich–Kister equation using Barker's method of minimizing the residual pressure. The G ^E values for the binary mixtures containing benzene are positive; while these are negative for toluene, ortho, meta and para xylene system over the whole composition range. The G ^E values of an equimolar mixture for these systems vary in the order: benzene?>?m-xylene?>?o-xylene?>?p-xylene?>?toluene     

A theoretical study of the interactions between N,N-dimethylformamide and xylenes

The ab initio and density functional (DFT) methods were performed on binary systems of N,N-dimethylformamide (DMF) with xylenes (o-, or m-, or p-xylene), and seven stable configurations were obtained with no imaginary frequencies. To obtain the interaction energies of these complexes, single-point energy calculations with basis set superposition error (BSSE) correction were carried out at B3LYP/6-31G* and MP2/6-31G* levels. The structures, Chelpg (charges from electrostatic potentials using a grid-based method) charge distribution and bond characteristics of the mentioned complexes were calculated. The results indicated the presence of double C–H···O hydrogen bonds between DMF and xylenes in these complexes and the interaction energies of hydrogen bonding between DMF and xylene systems decreased in the following sequence: DMF–o-xylene: a1 > DMF–m-xylene: b1 > DMF–p-xylene: c1.     

Theoretical investigation on the detailed mechanism of the OH‐initiated atmospheric photooxidation of o‐xylene

The reaction mechanism for o‐xylene with OH radical and O₂ was studied by density functional theory (DFT) method. The geometries of the reactants, intermediates, transition states, and products were optimized at B3LYP/6‐31G(d,p) level. The corresponding vibration frequencies were calculated at the same level. The single‐point calculations for all the stationary points were carried out at the B3LYP/6‐311++G(2df,2pd) level using the B3LYP/6‐31G(d,p) optimized geometries. Reaction energies for the formation of the aromatic intermediate radicals have been obtained to determine their relative stability and reversibility, and their activation barriers have been analyzed to assess the energetically favorable pathways to propagate the o‐xylene oxidation. The results of the theoretical study indicate that OH addition to o‐xylene forms ipso, meta, and para isomers of o‐xylene‐OH adducts, and the ipso o‐xylene adduct is the most stable among these isomers. Oxygen is expected to add to the o‐xylene‐OH adducts forming o‐xylene peroxy radicals. And subsequent ring closure of the peroxyl radicals to form bicyclic radicals. With relatively low barriers, isomerization of the o‐xylene bicyclic radicals to more stable epoxide radicals likely occurs, competing with O₂ addition to form bicyclic peroxy radicals. The study provides thermochemical data for assessment of the photochemical production potential of ozone and formation of toxic products and secondary organic aerosol from o‐xylene photooxidation. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

A Monte-Carlo simulation of diffusion and reaction in zeolites

Investigations of two-dimensional models to simulate diffusion and reaction in zeolites with a Monte-Carlo method are presented herein. New results of the simulation of single component diffusion and binary diffusion and a possible method to rescale the obtained diffusion coefficients in real units is presented. The estimation of an accurate activation energy was found to be the most important factor, the other parameters were estimated by simple assumptions.Complex reactions such as a consecutive reaction and xylene isomerisation were simulated with a new model. With this model the selective behaviour of the two reactions was investigated. A Type-III selectivity according to Wheeler was found for the consecutive reaction. For xylene isomerisation, reaction paths were simulated and compared with experimental reaction paths. The composition of the thermodynamic equilibrium was obtained for high transition probabilities and the relative rate constants from the literature could be confirmed. According to the results of the simulations one can conclude that a ratio of the diffusion coefficients of o-, m-, and p-xylene=1:1:1000 is too large to explain the experimentally observed distribution of the xylene isomers and a difference of only one order of magnitude was found.     

Novel purification method of commercial o- and m-xylenes by shape selective adsorption on HZSM-5

Liquid-phase competitive adsorption of three xylene isomers on ZSM-5 zeolites was studied. HZSM-5 zeolites exhibited an extremely high para-selectivity, this para-selectivity decreased with increasing adsorption temperatures. The introduction of Na⁺ enhanced the para-selectivity. It is apparent that such a high para-selectivity is caused solely by thermodynamic shape selectivity. The complete removal of impurities with small molecular dimensions, such as p-xylene and benzene, from commercial o- and m- xylenes could be attained by the repetition of the shape-selective adsorption on HZSM-5 zeolites.     

Catalytic Conversion of 1,2,3-Trimethylbenzene Over HY Zeolite

The reaction of 1,2,3-trimethylbenzene (1,2,3-TMB) over HY zeolite was investigated in a fixed-bed flow reactor at 200-300 °C under atmospheric pressure. The reaction products include toluene, pentamethylbenzene and isomers of xylene, 1,2,3-TMB and tetramethylbenzene. Based on the time-on-stream theory, the types and initial selectivities of these products were determined from plots of product selectivity. 1,2,4-TMB is initially produced from 1,2,3-TMB via isomerization whereas o-xylene, m-xylene, 1,2,3,4- and 1,2,3,5-tetramethylbenzene were primarily formed by disproportionation of 1,2,3-TMB. Isomerization and disproportionation obeyed first- and second-order kinetics, respectively; both reactions proceeded via a carbonium ion mechanism with the former occurring by methyl transfer on the benzene ring whereas the latter proceeded through the diphenylmethane transition state. The activation energies are 31.6 and 37.2 kJ mol^?1 for isomerizaion and disproportionation, respectively.     

m-Xylene Isomerization in SAPO-11/HZSM-5 Mixed Catalyst

The catalytic isomerization of m-xylene was studied over a solid acid silicoaluminophosphate type SAPO-11, mixed to HZSM-5 zeolite. The reaction was processed varying the temperature and weight hourly space velocity, using a fixed bed continuous flow reactor. The m-xylene suffers isomerization to p-xylene and o-xylene by molecular displacement of methyl groups. The mixed catalyst was selective to p-xylene at 623 K and 2.5 h⁻¹ with a maximum p/o ratio of 2.05. The ethylbenzene formation was not observed in the products. In this process an apparent activation energy of the order of 13.9 kJ mol⁻¹ was obtained. This revised version was published online in June 2006 with corrections to the Cover Date.     

Speed of Sound and Isentropic Compressibilities of N-Methylcyclohexylamine with Benzene and Substituted Benzenes at 303.15 K

Abstract

New experimental sound velocity and density data for binary mixtures of N-methyl-cyclohexylamine with benzene, toluene, o-xylene, m-xylene, p-xylene, chlorobenzene, bromobenzene and nitrobenzene at 303.15K have been reported. The sound velocity data were also used to compute the isentropic compressibilities (K_s ). The deviation in isentropic compressibilities (ΔK_s ) from ideal behaviour suggests that the existence of weak dipole-induced dipole and dipole-dipole interactions between unlike molecules.     

Isothermal Phase Equilibrium Studies on the Binary Mixtures of some Alkylbenzenes

Abstract

The gas chromatographic method proposed by us for simple and accurate measurement of isothermal phase equilibria has been applied to the binary mixtures formed by alkylbenzenes amongst themselves. Results on the binary mixtures of: benzene - toluene, toluene + o-xylene, toluene + p-xylene, toluene + ethylbenzene, ethylbenzene + o-xylene and ethylbenzene + p-xylene are presented in this paper. The present measurements on benzene + toluene system at 40°C are in good agreement with the isothermal phase equilibrium data available in the literature.     

MFI zeolite membranes from a- and randomly oriented monolayers

c-Oriented columnar MFI films made by secondary growth of randomly oriented seed monolayers, deposited using sonication-assisted covalent attachment, exhibit n-hexane/2,2-dimethylbutane separation factor of up to 10⁴, n-/i-butane separation factor of up to 50, and p-/o-xylene separation factor of up to 2. A MFI film from a-oriented seed layer shows lower separation factors for the linear vs. branched isomers but higher separation factor for p-/o-xylene.