Application of the Indium-containing mesoporous silicas catalysts in the alkylation of aromatics reaction

The benzylation of benzene and substituted benzenes reaction employing benzyl chloride as the alkylating agent over a series of Indium-containing mesoporous silicas with different In contents has been investigated. These materials (In-HMS-n) have been characterized by chemical analysis, N₂ adsorption/desorption isotherm, X-ray diffraction (XRD) and FTIR spectroscopy. The mesoporous Indium-containing materials showed both high activity and high selectivity for benzylation of benzene. The activity of these catalysts for the benzylation of different aromatic compounds is in the following order: benzene > toluene > p-xylene > anisole. More interesting is the observation that these catalysts are always active and selective for large molecules like naphthenic compounds such as methoxynaphthalene. Kinetics of the benzene benzylation over these catalysts have also been investigated.     

Permeation of xylene isomers through nitrile gloves

The physical parameters of the xylene isomers (the positional isomers o-, m-, and p-xylenes and the skeletal isomer ethyl benzene) responsible for the differing permeation behavior of the isomers through lined unsupported 0.41 mm thick nitrile glove material were investigated. An ASTM type permeation cell at 30°C, constant mixing conditions, hexane liquid collection, and capillary column gas chromatography/mass spectrometry of samples taken from the collection side every ten minutes allowed break through times t_b and steady-state sections to be defined. While pure isomers had distinct break through times t_b(m-xylene = p-xylene < ethyl benzene = o-xylene), steady-state permeation rates P_s(p-xylene > m-xylene > ethyl benzene = o-xylene), lag times t_l(m-xylene < p-xylene = ethyl benzene < o-xylene), and diffusion coefficients D_p(m-xylene < p-xylene = ethyl benzene < o-xylene), such behavior was lost in a equal volume mixture (t_b, t_l, P_s, and D_p were equivalent). The average P_s of the mixture isomers of equal volumes did not differ from that expected from the individual pure isomer P_s values. The results for the pure isomers were attributed to o-xylene and ethyl benzene being similarly sterically hindered, the p-xylene being the flattest and most symmetrical molecule and having no dipole moment, and m-xylene being intermediate in steric structure. The pure isomer t_l were directly related to viscosity divided by the log octanol-water coefficient, while their log P_s was inversely related to dipole moment times the logarithm of the capacity factor for water for a reversed-phase high-performance liquid chromatography column. In an equivolume mixture of the isomers, isomer interactions caused equivalence for all permeation kinetic parameters, indicating that the kinetics of mixture constituents is not predictable from the behavior of the pure constituents, although mass transfer appears additive. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1713–1721, 1997     

Homogeneous and two phase sulphonation of some aromatic hydrocarbons

The two phase sulphonation of benzene, toluene and p-xylene with concentrated sulphuric acid has been studied in a one litre batch reactor. The kinetic rate constants as a function of sulphuric acid concentration and the overall mass transfer coefficient times the interfacial area per unit volume of the acid phase as a function of agitation speed have been determined. The rates of sulphonation of toluene and p-xylene relative to that of benzene in the range 15.5–17 mol/litre sulphuric acid have been found to be 64 and 280 respectively. Finally, it has been shown that in aromatic sulphonation no linear relationship exists between the logarithms of the partial rate factors and the relative cation localisation energies. This is in contrast to the behaviour of some other electrophilic aromatic substitution such as halogenation, but is similar to aromatic nitration.     

Vapour liquid equilibrium with association in both phases: A model with the concentration-dependent liquid phase association constant applied to acetic acid—benzene, -toluene, -p- and -o-xylene

A model employing the correction factors of Marek and Standart, but using a concentration-dependent liquid phase association factor of Jenkins—Robinson, has been used to model vapour—liquid equilibrium data for mixtures of acetic acid with benzene, toluene, o-xylene and p-xylene. With the aim to use systems of acetic acid—benzene and acetic acid—toluene as the test mixtures for distillation columns, the examination of the systems of acetic acid—aromatic hydrocarbons was undertaken. The model promises to be useful in modelling isobaric and isothermal data of acetic acid—benzine, acetic acid—toluene, acetic acid-p-xylene and acetic acid-o-xylene systems. Deviation plots show that the isothermal and isobaric data are represented well.     

Aromatic Transformations Over Mesoporous ZSM-5: Advantages and Disadvantages

Catalytic behavior of mesoporous ZSM-5 was investigated in toluene disproportionation, toluene alkylation with isopropyl alcohol, and p-xylene alkylation with isopropyl alcohol to understand the effect of the presence of mesopores. Three ZSM-5 zeolites (conventional one and two mesoporous differing in the mesopore volume) having similar Si/Al ratio were synthesized and characterized as for their acidity (internal and external) as well as their micropore/mesopore volume. No substantial differences among three samples were observed as for the type and concentration of Brønsted and Lewis acid sites as well as their location in zeolite channels or on external surface of zeolite crystals. Conversions of toluene and p-xylene increased with increasing volume of mesopores in ZSM-5 zeolite while the selectivity to individual products depended on the type of reaction. In general, selectivity to sum of xylenes in toluene disproportionation, sum of isopropyltoluenes in toluene alkylation and to 1-isopropyl-2,5-dimethylbenzene in p-xylene alkylation increased due to a shorter contact time molecules spent in mesoporous ZSM-5 catalysts. In contrast, para-selectivity decreased as diffusion pathways were shorten due to the presence of mesopores.     

Mechanism of benzene diffusion in MOF-5: A molecular dynamics investigation

In this contribution, the diffusion of benzene in the porous metal organic framework MOF-5 is investigated by molecular dynamics simulations. Previously, we have shown that by using a first principles derived fully flexible force field the experimentally determined self-diffusion coefficients D_self could be well reproduced [S. Amirjalayer, M. Tafipolsky, R. Schmid, Angew. Chem. Int. Ed. 46 (2007) 463]. Here, we use the same methodology to determine the loading dependence on the diffusion. It is found that diffusivity, which is in the range of liquid benzene, slightly increases up to a load of 32 molecules per unit cell and then falls off at higher load. Free energy maps reveal that additional sites appear at higher load due to attractive guest–guest interactions. The topology of these sites is very close to the experimentally determined locations of ferrocene molecules in MOF-5, which corroborates that attractive π–π interactions govern these systems. The site–site and site-phenylene distances are very similar to the first solvation radius of liquid benzene. For the very open MOF-5, the main barrier for diffusive transport is to overcome the attractive interaction in the binding pockets, which is in contrast to zeolitic microporous systems, where the barrier for diffusion is the hindrance of the pore window. Spatial free energy maps are used to investigate the diffusion pathway on a molecular level and the load dependence of the free energy barriers for these transport processes.     

Adsorption and desorption kinetics of hydrocarbons in FCC catalysts studied using a tapered element oscillating microbalance (TEOM). Part 2: numerical simulations

The factors affecting the adsorption and desorption kinetics in a TEOM are reviewed in detail with particular attention given to the assumptions required to obtain physical transport parameters from the data. Two models are presented to simulate TEOM adsorption data in the case that concentration differences down the catalyst bed can be neglected, as is appropriate when the amount of catalyst used is small, the carrier gas flowrate is large, and/or the adsorbate partial pressure is low. In the first model, the effective diffusion coefficient, D_e, is taken to be constant. In the second model, the effective diffusion coefficient is assumed to obey the Darken equation, D_e=D₀/(1−θ). The TEOM results obtained on n-hexane, n heptane, n-octane, toluene and p-xylene on a commercial FCC catalyst and on pure rare-earth exchanged zeolite Y under non-reacting conditions (373-) are analysed in detail. It is found that intracrystalline diffusion is not the limiting factor affecting the overall rates of adsorption and desorption for the systems studied. Instead, it is the transport of molecules between the adsorbed and vapour phases at the edge of zeolite crystallites that is the limiting transport step affecting the overall kinetics. For the FCC catalyst, the limiting step is the transport of molecules at the zeolite-matrix interface rather than, say, the matrix-vapour interface. Local rate constants for the desorption of the hydrocarbons at the rate-controlling interface have been obtained.     

Diffusion of organic penetrants through low density polyethylene (LDPE) films: Effect of size and shape of the penetrant molecules

The diffusion coefficient at zero penetrant concentration D₀ of dichloromethane, chloroform, carbon tetrachloride, cyclohexane, benzene, o-xylene, m-xylene, and p-xylene, and n-hexane in LDPE were measured at 25°C, using the desorption method. The D₀ values obtained in this way are correlated with the size, shape, and chemical nature of the penetrant molecules. The temperature dependence of the diffusion coefficients of toluene and n-hexane in LDPE are also reported in the limited temperature range of 25–45°C. It indicates that, in spite of a size larger than that of toluene, n-hexane has a lower activation energy of diffusion.     

A comparison of rates of disproportionation and transalkylation of m-xylene

The relative rates of toluene transalkylation, disproportionation and benzene transalkylation of m-xylene over a catalyst consisting of 72% HY zeolite, 18% β-AlF₃ and 10% (all w/w) Cu have been studied at a temperature of 723K, reciprocal space velocity of 352 g h^?1 mol^?1 and at total pressures varying between 101 and 911.93 kPa. The catalysed disproportionation of m-xylene is fastest and this has been explained in terms of the greater basicity of m-xylene over either benzene or toluene, whereas the catalyst is acidic. Thus m-xylene would be expected to adsorb more strongly on the catalyst.     

Alkylation of toluene with ethene over H-ZSM-5 zeolites

It is shown that the alkylation of toluene with ethylene to give p-ethyltoluene can be conducted selectively on small crystals ( 1μ) of unmodified H-ZSM-5 zeolites to a level higher than that corresponding to a thermodynamic equilibrium composition. A comparison of the ethyltoluene isomers formed with H-Y and H-ZSM-5 zeolites at various conversions and temperatures indicates that p-ethyltoluene is preferentially formed in the channel intersections of the H-ZSM-5 zeolite, even during the initial alkylation step. The final ethyltoluene isomer composition is a result of competition between the rates of alkylation and isomerization and subsequent diffusional transport of individual isomers through the zeolite channels. Zeolite coking causes an apparent increase in the p-ethyltoluene selectivity; however, a more dramatic decrease in toluene conversion results in the formation of a considerably lower amount of p-ethyltoluene. This is caused by a partial blocking of the zeolite channel system.     

AROMATIZATION OF PROPYLENE OVER HZSM-5: A DESIGN OF EXPERIMENTS (DOE) APPROACH

Aromatization of propylene was performed in a continuous reactor over HZSM-5 catalysts. A full-factorial design of experiments (DOE) methodology identified the effects of temperature (400°–500°C), Si:Al ratio (50–80), propylene feed concentration (8.9–12.5 mol.%), and catalyst amount (0.2–1.0 g) on propylene conversion as well as the yields of benzene, toluene, p-xylene, o-xylene (BTX), and total BTX. The Si:Al ratio and amount of the HZSM-5 catalyst influenced all of the responses, while temperature affected all the responses except the yield of p-xylene. An increase in feed concentration significantly increased the yields of benzene, toluene, and total BTX. An interaction between propylene feed concentration and catalyst amount influenced the yields of benzene, toluene, and total BTX. This interaction indicated that a higher feed concentration promotes aromatization at higher catalyst concentrations. By contrast, the interaction of Si:Al ratio with propylene feed concentration was found significant for p-xylene and o-xylene yields, but not for benzene and toluene, suggesting that xylenes are synthesized on different sites than those for benzene and toluene. These interaction effects demonstrate how the use of DOE can uncover significant information generally missed using traditional experimental strategies.     

Liquid-Phase Sorption of Higher Alkanes and Alkenes in Zeolite NaZSM-5 at 10, 30, and 50°C∗

Abstract

Adsorption and diffusion of normal alkanes and alkenes having carbon numbers from C8 to C15 have been measured in the liquid phase from a nonadsorbing solvent (trimethyl benzene) in zeolite NaZSM-5 at 10. 30. and 50°C. The diffusion coefficients show a sharp decrease for molecules whose carbon chain length is larger than the channel length of the zeolite. The adsorption isomers of octane have also been studied to observe the effect of minor structural changes in molecular shape on adsorption properties in the zeolite. The observed adsorption capacity data have been explained in terms of molecular packing in zeolite straight and sinusoidal channels and channel intersections.

     

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.     

Cellulose nanofibres and cellulose nanowhiskers based natural rubber composites: Diffusion,sorption, and permeation of aromatic organic solvents

This article investigates the transport behavior of three aromatic organic solvents, viz. benzene, toluene, and p‐xylene in natural rubber nanocomposite membranes containing cellulose nanofibres (CNFs) and cellulose nanowhiskers (CNWs) isolated from bamboo pulp. The solvent molecules act as molecular probes to study the diffusion, sorption, and permeation through the nanocomposites, and provide information on the nanocomposite structure and matrix–filler interactions. Both the nanocelluloses were found to decrease the uptake of aromatic solvents in nanocomposite membranes, but the effect was more significant in the case on nanofibers compared to nanowhiskers. Furthermore, the uptake decreased with increased penetrant size; being the highest for benzene and the lowest for p‐xylene. Transport parameters such as diffusion coefficient, sorption coefficient, and permeation coefficient have been calculated. Comparison of the experimental values of equilibrium solvent uptake with the predicted values indicated that both the nanocelluloses have restricted the molecular mobility at the interphase and thereby decreased the transport of solvents through the materials; being more significant for nanofibers. The results showed that both the used cellulosic nanomaterials act as functional additives capable of manipulating and tailoring the transport of organic solvents through elastomeric membranes, even at concentrations as low as 2.5 wt %. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Theoretical and experimental study on the selectivity of dehydrogenation of α-limonene in ZSM-5 and zeolite-Y

Dehydrogenation of the natural terpene α-limonene to the industrially important p-cymene was studied over two zeolite-supported Pd catalysts, ZSM-5 and zeolite-Y. Reactor tests indicate that transalkylation of the p-cymene product can be avoided by employing the shape-selective properties of a medium-pore ZSM-5 catalyst. The diffusion properties of the three isomers of cymene were then calculated in the two zeolites using molecular mechanics. In case of ZSM-5, p-cymene was found to have the lowest barrier of the three isomers for diffusion through the straight, spherical channel of the zeolite. For zeolite-Y, differences in diffusivity for the three isomers were very small. Theory and experiments showed, in excellent agreement, that only ZSM-5 selectively produces p-cymene. This work shows that molecular mechanics is a powerful and reliable method for the screening of zeolites for performing shape-selective catalysis. This revised version was published online in July 2006 with corrections to the Cover Date.     

Acylation of p-xylene over zeolites

Zeolites differing in structure and acidity were tested in liquid phase acylation of p-xylene. Hexanoyl chloride, propionic anhydride and isobutyric anhydride were used as acylating agents. The highest conversions of acylating agent were achieved over large pore zeolites USY (66.3%) and Beta (58.2%). It was found that acylation of p-xylene proceeds only over large pore zeolites Beta and USY. Selectivities to monoacylated p-xylene obtained over USY zeolite decreased in the order: propionic anhydride 78.0% > hexanoyl chloride 67.2% > isobutyric anhydride 33.1%. Diacylated product was formed over zeolite USY with all acylating agents tested but only with hexanoyl chloride over zeolite Beta. It was found that the optimum Si/Al ratio of zeolite Beta for p-xylene acylation with propionic anhydride is 25, while for isobutyric anhydride is 19. Conversion of isobutyric anhydride decreased with increasing isobutyric anhydride concentration and increased with increasing amount of catalyst.     

Selective toluene disproportionation over ZSM-5 zeolites

Selective toluene disproportionation to benzene and xylenes is investigated over HZSM-5 and modified HZSM-5 zeolites. The zeolites are modified with nickel, magnesium, phosphorus and boron, characterised by sorption, acidity measurement and X-ray photoelectron spectroscopy, and their activity towards toluene disproportionation is discussed in the light of their physico-chemical properties. The two types of active sites have been identified; one promoting disproportionation and the other cracking and dealkylation of toluene. It is found that the latter sites are suppressed due to incorporation of the modifiers. The modified zeolites are found to be para-selective which appears to be consistent with the commonly accepted view that modifiers like magnesium, phosphorus and boron reside in the channels and react with Brønsted acid sites, leading to partial blockages and thus favouring the formation of p-xylene. The effect of poisoning the zeolite with 1-methylisoquinoline is also investigated. In the case of poisoned catalyst, p-xylene is produced at levels up to 99%. The total elimination of other isomers from the products is suggested due to the selective poisoning of the non-selective sites (on the external crystal surface) and to partial blocking of the pore openings.     

The structure and dynamics of adsorbed molecules in microporous solids; a comparison between experiments and computer simulations

A critical evaluation is made of the results of computer simulations of adsorbed molecules in microporous inorganic materials. Structural, thermochemical and dynamic data are compared with the corresponding experimental findings. The systems examined include inert gases and simple alkanes in zeolites Na-Y and silicalite, benzene and pyridine in Na-Y and K-L, respectively, andp-xylene in silicalite. Future developments in the area are also discussed.     

Synthesis of ZSM-5 type zeolites with and without template and evaluation of physicochemical properties and aniline alkylation activity

ZSM-5 zeolite containing SiO₂/Al₂O₃ = 28 was synthesised by hydrothermal process with and without template. Characterisation of the zeolites was done using XRD, IR, NMR, SEM and BET surface area measurements. Acidity of the zeolites was measured by ammonia adsorption-desorption. Catalytic activity of HZSM-5 zeolites was evaluated by aniline alkylation, cumene dealkylation, alkylation of benzene with ethylene and propylene and isomerisation ofm-xylene. Catalytic activity is discussed from the point of view of zeolite acidity and morphology.     

Study on alkylation of benzene with propylene over MCM-22 zeolite catalyst by in situ IR

For the alkylation reaction of benzene with propylene over MCM-22 zeolite catalyst, two completely different results can be obtained for two different operating procedures. If the benzene is pumped into a reactor then followed by propylene (operation 1), the alkylation reaction proceeds normally, while the reaction can not occur if the propylene is introduced first followed by benzene (operation 2). In situ IR technology was used to investigate two modes designed to simulate the above operating processes. The mechanisms of the two operations are as follows: in operation 1, benzene molecules first fully were adsorbed on the acidic sites of MCM-22 zeolite, when propylene was introduced, propylene molecules seized the acidic sites by repelling benzene, at the same times propylene molecules were polarized and reacted with absorbed benzene molecules. This is synchronous reaction mechanism; in operation 2, propylene introduced was molecularly absorbed on acidic sites strongly resulting in producing carbonium ions of CH₃–C⁺H–CH₃, then the carbonium ions reacts with other propylene molecules further to form polymeric species. The polymers blocked the pores and covered the acidic sites so that the alkylation reaction with benzene can not take place. This process is carbonium ions reaction mechanism of propylene itself.