Influence of monovalent alkaline metal cations on binder-free nano-zeolite X in para-xylene separation

The adsorption process was studied for separating para-xylene from xylene mixture on modified nano-zeolite X in a breakthrough system. Nano-zeolitic adsorbent with different ratios of SiO2/Al2O3 was synthesized through hydrothermal process and ion-exchanged with alkaline metal cations like lithium, sodium and potassium. The product was characterized by X-ray diffraction, scanning electron microscopy (SEM), nitrogen adsorption, transform electron microscopy (TEM) and in situ Fourier transform infrared (FTIR) spectroscopy. The influence of nano-zeolite water content and desorbent type on the selectivity of para-xylene toward other C8 aromatic isomers was studied. The optimization of adsorption process was also investigated under variable operation conditions. The isotherm for each isomer of C8 aromatics and the desorbents possess the adsorption characteristics of Langmuir type. The selectivity factor of para-xylene relative to each of meta-xylene, ortho-xylene and ethylben-zene under the optimum conditions obtained to be 5.36, 2.43 and 3.22, in the order given.     

Adsorptive separation of meta-xylene from C8 aromatics

Industrial adsorptive separation process for liquids is most successful when the involved species have very close boiling points, making distillation expensive or are thermally sensitive at convenient distillation temperatures. The adsorption process was studied for separating meta-xylene from a feed mixture containing all C₈ aromatics on binder-free X and Y zeolites in the liquid phase. Zeolitic adsorbents with different SiO₂/Al₂O₃ were synthesized by the hydrothermal method and ion-exchanged with alkaline metal cations like lithium, sodium and potassium. The adsorption process was carried out in a breakthrough system at temperature of 110–160 °C and pressure of 6–8 atm. The influence of adsorbent moisture content on the separation process was studied. The optimization of adsorption process was also investigated by the changing operation conditions. The isotherms for each isomer of C₈ aromatics and the desorbent possess the adsorption characteristics of Langmuir type. The selectivity factor of meta-xylene and the saturation adsorption capacities of adsorbates were determined. It was observed that the selectivity of meta-xylene increased by sodium ion-exchanging of cationic sites in Y zeolite and the selectivity factor of meta-xylene/para-xylene, meta-xylene/ortho-xylene and meta-xylene/ethylbenzene in the optimum conditions was determined to be 2.62, 2.83 and 5.93, respectively.     

Separation of p-xylene from binary xylene mixture over silicalite-1 membrane: Experimental and modeling studies

The permeation of single component and binary mixture containing p-xylene and o-xylene through silicalite-1 membrane was studied experimentally in the temperature range of 150–250 °C at feed partial pressure of 0.26 kPa for p-xylene and 0.22 kPa for o-xylene. The model for single component flux based on the combination of dual-site Langmuir isotherm and Maxwell–Stefan formulation was derived. The adsorption parameters were estimated by minimizing the difference between the experimental flux and simulated flux. The heat of adsorption and entropy values obtained were in good agreement with the reported values. The effect of feed partial pressure in the range of 0.20–1.50 kPa on xylene flux was predicted using the adsorption parameters and compared with the experimental values. The Maxwell–Stefan diffusion model, in combination with the ideal adsorbed solution (IAS) theory and single-component adsorption parameters was used to predict the permeation flux of p-xylene and o-xylene for binary xylene mixture through the silicalite-1 membrane. The simulated results were in good agreement with the experimental data. The simulated adsorption isotherm in higher temperature range of 150–250 °C using the model and derived adsorption parameters could provide useful information for adsorption of xylene molecules on silicalite-1 membrane at a higher operating temperature.     

Product distribution in the aromatization of dilute ethene over H-GaAlMFI zeolite: effect of space velocity

Influence of space velocity on the aromatization of dilute ethene (5 mol% in N₂) over H-GaAlMFI zeolite catalyst, having high acidity (0.46 mmol g⁻¹, measured in terms of the pyridine chemisorbed at 400 °C) and high concentration of non-framework Ga-oxide species (0.32 mmol g⁻¹), at atmospheric pressure covering a wide temperature range (300–500 °C) has been thoroughly investigated. The selectivity of aromatics, propene, propane and C₄ hydrocarbons and alkane/aromatics and H₂/aromatics mole ratios are strongly influenced by the space velocity. The results indicate that the aromatization involves H₂ transfer reactions predominantly at the lower temperatures and/or higher space velocities whereas dehydrogenation reactions become predominant at higher temperatures and/or lower space velocities. The distribution of aromatics and C₈-aromatic isomers depends strongly upon the amount (i.e. yield) of aromatics and C₈-aromatics, respectively, formed in the process. The primary aromatics produced in the process are found to be mainly p- and o-xylenes. The aromatics distribution is, however, controlled by the aromatics inter-transformation (viz. isomerization, alkylation/dealkylation and disproportionation) reactions. The p-xylene/m-xylene ratio is decreased as expected, but the p-xylene/o-xylene ratio is increased with increasing both the space velocity and temperature. The increase of p-xylene/o-xylene ratio is found to be unusual, much above the equilibrium value.     

Separation of a binary mixture of ethylene and ethane by adsorption on Na-ETS-10

Ethylene produced by steam cracking and thermal decomposition of ethane must be purified prior to use in the production of plastics, rubber and films. Ethane–ethylene separation is generally achieved by cryodistillation. The energy and equipment costs associated with ethylene purification could be significantly reduced by the development of alternative separation methods. Both modelling predictions for the binary adsorption of ethylene and ethane, and recent work investigating the adsorption of ethane and ethylene on the surface of Engelhard Titanosilicate-10 (ETS-10), a large-pored, mixed octahedral/tetrahedral titanium silicate molecular sieve, indicate that this molecular sieve might be an adsorbent capable of ethylene/ethane separations. However, the actual separation of a binary mixture of ethylene and ethane on ETS-10 has yet to be demonstrated. In this work, Na-ETS-10 was used to separate a mixture of ethylene and ethane (59% C₂H₄, 41% C₂H₆) from an industrial process stream with a measured binary bed selectivity for ethylene over ethane of approximately 5 at 25 °C and 1 atm. In a laboratory-scale demonstration, both high purity ethane and significantly enriched ethylene were produced, and the Na-ETS-10 adsorbent was regenerated for further separation cycles by both steam and microwave desorption, without degradation of the adsorbent or products.     

Oxidative competition between aliphatic and aromatic CH bonds in the N2O–Fe-ZSM-5 system

The peculiar electrophilic behaviour of active oxygen formed over Fe-ZSM-5 zeolites upon nitrous oxide decomposition has been investigated in toluene oxidation. Fe-ZSM-5 catalysts have been produced by hydrothermal and post-synthetic techniques, with particular attention given to preparation by chemical vapour deposition of anhydrous FeCl₃ into H-ZSM-5. Higher yields of hydroxylated product are possible over Fe-zeolites prepared by hydrothermal synthesis, with para-cresol being the predominant oxygenate formed, though the formation of tars and polycondensed aromatic hydrocarbons retards catalytic activity over time. Oxidation of para-xylene, isopropylbenzene, benzaldehyde, chlorobenzene and phenol demonstrate that the oxidative competition between aliphatic and aromatic CH bonds is influenced considerably by the steric and electronic nature of the substituent groups.     

Reduction of NOx over a NOx-Trap Catalyst and the Regeneration Behaviour of Adsorbed SO2

A conventional NO _x -trap catalyst containing platinum, rhodium, barium and lanthanum was conditioned with oxygen at 500°C, preloaded with NO under standard oxidising conditions and then subjected to regeneration with the reductants H₂, CO and C₃H₆, either alone or as a mixture. Hydrogen is the most efficient reductant in terms of NO _x conversion efficiency and reductant usage efficiency. There is a temperature optimum for CO between 300 and 400°C and a catalyst loading optimum (mols reductant added)/(mols NO _x adsorbed) between 1.5 and 3.0. The behaviour of the catalyst towards sulphur poisoning was examined in supplementary trials with the adsorption of SO₂ in the presence or absence of water vapour. When water is not present in both adsorption and reduction steps, very stable sulphates are formed, unattacked by reductants even at 1000°C. Sulfates are more easily reduced when water is present in the reductant mixture.     

The effects of fullerene (C60) crystal structure on its electrochemical capacitance

The electrochemical properties of different types of fullerene crystals (x-C₆₀), having hollow-long cylindrical (C-C₆₀), hollow-long square (S-C₆₀), and thick solid-short hexagonal (H-C₆₀) shapes were investigated. The prepared x-C₆₀ samples had specific dimensions with respect to aspect ratio (12.0, 6.7, and 1.5) and lattice spacing distance (1.12, 1.04, and 1.00 nm). Interestingly, it was possible to control the aspect ratio and lattice spacing distance by adjusting the molecular ratio of C₆₀ to the aromatic solvent (m-xylene) used in the preparation. In addition, the number of m-xylene molecule in the x-C₆₀ crystal structure increased with decreasing ratio of x-C₆₀ to m-xylene in the solution, corresponding to ca. C₆₀·0.83m-xylene (for C-C₆₀), C₆₀·0.39m-xylene (for S-C₆₀), and C₆₀·0.36m-xylene (for H-C₆₀). A more ordered arrangement of m-xylene molecules resulted in an improved electrochemical capacitance of x-C₆₀. Importantly, in the case of the regular structure (C-, S-, H-C₆₀), when m-xylene was assembled in the x-C₆₀ structure, the large lattice spacing distance increased. This explains why the C-C₆₀ sample had the largest electrochemical capacitance, compared to the S- and H-C₆₀ samples. Such a configuration would allow for a high charge accumulation and the formation of a donor-acceptor complex, which would permit an easier charge transfer.     

Selection of ionic liquids for the extraction of aromatic hydrocarbons from aromatic/aliphatic mixtures

The separation of aromatic hydrocarbons (benzene, toluene, ethyl benzene and xylenes) from C₄ to C₁₀ aliphatic hydrocarbon mixtures is challenging since these hydrocarbons have boiling points in a close range and several combinations form azeotropes. In this work, we investigated the separation of toluene from heptane by extraction with ionic liquids.Several ionic liquids are suitable for extraction of toluene from toluene/heptane mixtures. The toluene/heptane selectivities at 40 °C and 75 °C with several ionic liquids, [mebupy]BF₄, [mebupy]CH₃SO₄, [bmim]BF₄ (40 °C) and [emim] tosylate (75 °C), are a factor of 1.5–2.5 higher compared to those obtained with sulfolane (S_tol/hept = 30.9, D_tol = 0.31 at 40 °C), which is the most industrially used solvent for the extraction of aromatic hydrocarbons from a mixed aromatic/aliphatic hydrocarbon stream. From these five ionic liquids, [mebupy]BF₄ appeared to be the most suitable, because of a combination of a high toluene distribution coefficient (D_tol = 0.44) and a high toluene/heptane selectivity (S_tol/hept = 53.6). Therefore, with [mebupy]BF₄ also extraction experiments with other aromatic/aliphatic combinations (benzene/n-hexane, ethylbenzene/n-octane and m-xylene/n-octane) were carried out. The aromatic/aliphatic selectivities were all in the same range, from which it can be concluded that the toluene/heptane mixture is a representative model system for the aromatic/aliphatic separation.     

Selection of ionic liquids for the extraction of aromatic hydrocarbons from aromatic/aliphatic mixtures

The separation of aromatic hydrocarbons (benzene, toluene, ethyl benzene and xylenes) from C₄ to C₁₀ aliphatic hydrocarbon mixtures is challenging since these hydrocarbons have boiling points in a close range and several combinations form azeotropes. In this work, we investigated the separation of toluene from heptane by extraction with ionic liquids.Several ionic liquids are suitable for extraction of toluene from toluene/heptane mixtures. The toluene/heptane selectivities at 40 °C and 75 °C with several ionic liquids, [mebupy]BF₄, [mebupy]CH₃SO₄, [bmim]BF₄ (40 °C) and [emim] tosylate (75 °C), are a factor of 1.5–2.5 higher compared to those obtained with sulfolane (S_tol/hept = 30.9, D_tol = 0.31 at 40 °C), which is the most industrially used solvent for the extraction of aromatic hydrocarbons from a mixed aromatic/aliphatic hydrocarbon stream. From these five ionic liquids, [mebupy]BF₄ appeared to be the most suitable, because of a combination of a high toluene distribution coefficient (D_tol = 0.44) and a high toluene/heptane selectivity (S_tol/hept = 53.6). Therefore, with [mebupy]BF₄ also extraction experiments with other aromatic/aliphatic combinations (benzene/n-hexane, ethylbenzene/n-octane and m-xylene/n-octane) were carried out. The aromatic/aliphatic selectivities were all in the same range, from which it can be concluded that the toluene/heptane mixture is a representative model system for the aromatic/aliphatic separation.     

The performances of higher alcohol synthesis over nickel modified K2CO3/MoS2 catalyst

Ni modified K₂CO₃/MoS₂ catalyst was prepared and the performance of higher alcohol synthesis catalyst was investigated under the conditions: T = 280–340 °C, H₂/CO (molar radio) = 2.0, GHSV = 3000 h^− 1, and P = 10.0 MPa. Compared with conventional K₂CO₃/MoS₂ catalyst, Ni/K₂CO₃/MoS₂ catalyst showed higher activity and higher selectivity to C₂⁺OH. The optimum temperature range was 320–340 °C and the maximum space-time yield (STY) of alcohol 0.30 g/ml h was obtained at 320 °C. The selectivity to hydrocarbons over Ni/K₂CO₃/MoS₂ was higher, however, it was close to that of K₂CO₃/MoS₂ catalyst as the temperature increased. The results indicated that nickel was an efficient promoter to improve the activity and selectivity of K₂CO₃/MoS₂ catalyst.     

Separation of CO2/CH4 mixtures with the MIL-53(Al) metal–organic framework

Adsorptive separation of CH₄/CO₂ mixtures was studied using a fixed-bed packed with MIL-53(Al) MOF pellets. Such pellets of MIL-53(Al) were produced using a polyvinyl alcohol binder. As revealed by N₂ adsorption isotherms, the use of polyvinyl alcohol as binder results in a loss in overall capacity of 32%. Separations of binary mixtures in breakthrough experiments were successfully performed at pressures varying between 1 and 8 bar and different mixture compositions. The binary adsorption isotherms reveal a preferential adsorption of CO₂ compared to CH₄ over the whole pressure and concentration range. The separation selectivity was affected by total pressure; below 5 bar, a constant selectivity, with an average separation factor of about 7 was observed. Above 5 bar, the average separation factor decreases to about 4. The adsorption selectivity is affected by breathing of the framework and specific interaction of CO₂ with framework hydroxyl groups. CO₂ desorption can be realised by mild thermal treatment.     

Synthesis of 4-Nitro-o-Xylene by Selective Nitration of o-Xylene

Nitration of o-xylene in the presence of fuming nitric acid and a combination of 115?% polyphosphoric acid, nitrobenzene, and H-Y zeolite catalyst provides a selectivity of 4-nitro-o-xylene of 71 at 85?mol% conversion of o-xylene. The present process has the potential to be an environmentally-friendly, sustainable, safe, and selective aromatic nitration process compared to the incumbent process.     

Friedel–Crafts green alkylation of xylenes with tert-butanol over mesoporous superacid UDCaT-5

Friedel–Crafts green alkylation of xylenes with tert-butanol was investigated in the presence of mesoporous superacidic catalysts named as UDCaT-4, UDCaT-5 and UDCaT-6. The catalysts are modified versions of zirconia showing high catalytic activity, stability and reusability. The catalytic activity is in the order: UDCaT-5 (most active) > UDCaT-6 > UDCaT-4 > sulfated zirconia (least active). Synergistic effect of very high sulfur content present (9% (w/w) S) and preservation of tetragonal phase in UDCaT-5, in comparison with sulfated zirconia (4% (w/w) S), were responsible for higher catalytic activity. The performance of UDCaT-5 in alkylation of xylenes was studied with tert-butanol with reference to selectivity and stability. Alkylation of m-xylene over UDCaT-5 gives 96% conversion of tert-butanol with 82% selectivity towards 5-tert-butyl-m-xylene (5-TBMX) under optimum reaction conditions. The formation of products is correlated with the acidity of the catalyst. The reactions were conducted in liquid phase at relatively low reaction temperatures (130–160 °C). A systematic investigation of the effects of various operating parameters was done to describe the reaction pathway. The reaction was carried out without any solvent in order to make the process cleaner and greener. An overall second order kinetic equation was used to fit the experimental data, under the assumption that both xylene and tert-butanol are weakly adsorbed. An independent study of dehydration of tert-butanol (TBA) was also done. Alkylation of o-xylene and p-xylene with tert-butanol was also studied. The overall process is green and clean.     

Efficient adsorptive separation of propene over propane through a pillar-layer cobalt-based metal–organic framework

The purification of propene from the propene/propane binary mixture is one of the most important and challenging separation processes in the chemical industry. In this study, a cobalt-based pillar-layer metal–organic framework, Co(AIP)(BPY)_0.5, was synthesized. It exhibited excellent water and moisture stability and efficient C₃H₆/C₃H₈ separation performance. At 298 K and 100 kPa, the adsorption capacity of C₃H₆ was 1.99 mmol/g and the IAST adsorption selectivity was 21, which has exceeded most investigated MOFs adsorbents. The cyclic breakthrough experiments of C₃H₆/C₃H₈ binary mixture confirmed its efficient dynamic separation property and excellent recyclability. The molecular simulation showed that the C₃H₈ molecules with multiple binding sites (C─H) were restricted at the middle of the one-dimensional channel. Compared with C₃H₆, the large steric hindrance of C₃H₈ caused lower diffusion rate (with a C₃H₆/C₃H₈ kinetic selectivity of 29.7, at 303 K) in the narrow pore system of 1 , which has been confirmed by the kinetic adsorption experiments.     

Effects of HZSM-5 crystallite size on stability and alkyl-aromatics product distribution from conversion of propanal

The conversion of propanal on large (2–5 µm) and small (0.2–0.5 µm) crystallite HZSM-5 at 400 °C and atmospheric pressure has been studied. Improved catalyst stability was observed on small crystallites due to faster removal of products with the shorter diffusion path length, reducing the formation of coke precursors. As previously shown, C₉ aromatics are the initial aromatics produced from propanal via aldol condensation followed by cyclization and these have less opportunity to crack to lighter aromatics on the small crystallites. Thus, a higher ratio of C₉/(C₈+ C₇) aromatics was observed on the small crystallites. The main isomer of the C₈ aromatic products observed on small crystallites was the initial cracking product of the C₉ aromatics, and thermodynamically preferred, meta-xylene, while the shape-selective preferred para-xylene was the predominant product on large crystallites. The higher internal diffusion rate of the para isomer results in greater shape selectivity with the longer path of the large crystallite zeolite. It is concluded that the use of smaller crystallite HZSM-5 improves results for production of alkyl aromatics from light oxygenates at mild conditions that may prove useful for bio-oil upgrading.     

Synthesis and properties of (BaxPb1−x)(Zn1/3Nb2/3)O3 relaxor ferroelectric ceramics using a reaction-sintering process

(Ba_xPb_1−x)(Zn_1/3Nb_2/3)O₃ (BPZN; x = 0.06–0.1) relaxor ferroelectric ceramics produced using a reaction-sintering process were investigated. Without any calcination involved, the mixture of raw materials was pressed and sintered directly. BPZN ceramics of 100% perovskite phase were obtained. Highly dense BPZN ceramics with a density higher than 98.5% of theoretical density could be obtained. Maximum dielectric constant K_max 13,500 (at 75 °C), 19,600 (at 50 °C) and 14,800 (at 28 °C) at 1 kHz could be obtained in 6BPZN, 8BPZN and 10BPZN, respectively. Dielectric maximum temperature (T_max) in BPZN ceramics via reaction-sintering process is lower than BPZN ceramics prepared via B-site precursor route.     

Amine modified mesocellular siliceous foam (MCF) as a sorbent for CO2

Adsorption is considered a promising method for carbon capture. CO₂ adsorbents take a variety of forms - but one approach is to fill mesoporous substrates with a polymeric CO₂ selective sorbent. SBA-15 and mesocellular siliceous foam (MCF) are high pore volume, high surface area ordered mesoporous materials for which modification with amine should result in high capacity, highly selective adsorbents. SBA-15 and MCF were separately loaded with approximately one pore volume equivalent of linear polyethyleneimine (PEI) (M_w = 2500) or branched PEI (M_n = 1200). CO₂ adsorption/desorption isotherms under dry CO₂ were obtained at 75, 105 and 115 °C. The CO₂ adsorption/desorption kinetics were improved with temperature, though the CO₂ capacities generally decreased. The adsorption capacity for MCF loaded with branched PEI at 105 and 115 °C were 151 and 133 mg/g adsorbent, respectively (in 50% CO₂/Ar, 20 min adsorption time). These are significantly higher than the adsorption capacity observed for SBA-15 loaded with branched PEI under same conditions, which were 107 and 83 mg/g adsorbent, respectively. Thus the results indicate that, on a unit mass basis, amine modified MCF's are potentially better adsorbents than amine modified SBA-15 for CO₂ capture at modestly elevated temperature in a vacuum swing adsorption process.     

Theoretical study of noncovalent functionalization of BN nanotubes by various aromatic molecules

The unsatisfactory stability of CNT under high temperature or subject to strong oxidants has limited the potential use of p- and n-type field-effect transistors (FETs). Promisingly, boron nitride (BN) nanotubes are known to have the excellent resistance to oxidant and thermal stability at high temperature as well as the uniform electronic properties, which rends their possible application as alternative FET device. In this paper, through the theoretical computation of the noncovalent functionalization of BN nanotube by various aromatic molecules (including C₁₀H₈, C₁₄H₁₀, porphyrin, DDQ, and TCNQ molecules), we for the first time evaluate its possibility as a candidate of stable FET device. We find that: (i) these aromatic molecules can be stably adsorbed on the studied BN with the adsorption energy ranging from ? 0.22 (C₁₀H₈) to ? 0.42 eV (porphyrin); (ii) the adsorption of electrophilic molecules on the outer sidewall of BN nanotube realizes a p-type semiconductor with a smaller band gap (~ 0.90 eV); (iii) exoherdral adsorption of nucleophilic aromatic molecules leads to an n-type semiconductor. This novel BN nanotube-based material offers great promise for molecule electronics in terms of their good stability.     

CO2 recovery from flue gas by PSA process using activated carbon

An experimental study was performed for the recovery of CO₂ from flue gas of the electric power plant by pressure swing adsorption process. Activated carbon was used as an adsorbent. The equilibrium adsorption isotherms of pure component and breakthrough curves of their mixture (CO₂ : N₂ : O₂=17 : 79 : 4 vol%) were measured. Pressure equalization step and product purge step were added to basic 4-step PSA for the recovery of strong adsorbates. Through investigation of the effects of each step and total feed rate, highly concentrated CO₂ could be obtained by increasing the adsorption time, product purge time, and evacuation time simultaneously with full pressure-equalization. Based on the basic results, the 3-bed, 8-step PSA cycle with the pressure equalization and product purge step was organized. Maximum product purity of CO₂ was 99.8% and recovery was 34%.