Search

Aromatization of light alkanes is pivotal for producing benzene, toluene, and xylene (BTX). However, achieving selective BTX production necessitates advancements in catalytic strategies. This study outlines a methodology to precisely control BTX species selectivity and distribution. The choice of active metal profoundly influences BTX product distribution over ZSM-5 catalysts. Mo/HZSM-5 facilitates n-butane dissociation to hydrocarbon intermediates, favoring benzene-rich products. In contrast, Ga/HZSM-5 enhances toluene and xylene production rates compared to Mo/HZSM-5. Intermediate composition on Mo/HZSM-5 significantly influences final selectivity, with paraffin-rich conditions and high methane conversion yielding benzene-rich products. Adjusting reaction conditions, such as methane recycle and temperature, further fine-tunes product distribution. [Display omitted] • Parameters affecting the selectivity of BTX were examined in n-butane aromatization. • Individual selectivity of BTX depends largely on the metal species. • In the case of Mo/HZSM-5, intermediate composition affects final BTX distribution. • The presence of methane and the reaction temperature also decides the BTX selectivity. [ABSTRACT FROM AUTHOR]

• New synthesis route for one-pot co-crystallization of BEA:MFI* aggregates with tunable BEA to MFI* ratios. • Nano-sized BEA and MFI* crystals lead to high external surface areas and mesopore volumes, reduced diffusion limitations. • Intimate proximity of BEA and MFI* nano-crystals results in a highly efficient catalyst for the direct conversion of an industrial Heavy Reformate fraction into xylenes. • The new bifunctional catalyst based on the co-crystallized BEA:MFI* aggregates presents high activity, high xylene selectivity and low deactivation rate. Upgrading of the heavy reformate fraction (HR), containing mainly C 9+ aromatics, is usually performed by dealkylation or by transalkylation with added benzene and/or toluene to obtain the more valuable xylenes. However, when the costs related to the use of benzene and toluene are considered, the one-step dealkylation/transalkylation of the C 9+ alkylaromatics to xylenes becomes economically attractive. Thus, in a first step, ethylmethylbenzenes (EMB) will have to be dealkylated to toluene, which will then react with the trimethylbenzenes (TMB) present in the HR feed to produce xylenes by transalkylation. Medium pore zeolites will favor dealkylation, whereas large pore zeolites will be more adequate for carrying out the transalkylation reaction. In this work, we present the one-pot synthesis of beta-pentasil aggregates with tunable ratios of the large pore beta to the medium pore component. We show that the close proximity of the beta and pentasil nanocrystals obtained by one-pot co-crystallization synthesis, results in a highly efficient catalyst for the consecutive dealkylation/transalkylation process. The bifunctional catalyst based on the co-crystallized aggregate is more active and selective to xylenes than a catalyst based on a physical mixture of equivalent beta and pentasil nanozeolites synthesized following an analogous procedure. The small crystallite sizes of the co-crystallized zeolites provide the additional advantage of a lower deactivation rate as compared to a reference benchmark catalyst. Results are shown on both, model molecules and industrial HR feed. [ABSTRACT FROM AUTHOR]

An experimental and parametric behavior study of disproportionation and transalkylation between C8 aromatics and toluene were carried out over mono-functional acid Beta zeolite with a SiO 2 /Al 2 O 3 ratio of 35 (BEA35). The experiments were performed in liquid phase under the following conditions: 453, 473, and 493 K and 2.1 MPa. Ethylbenzene disproportionation and ethylbenzene-toluene transalkylation were identified as the main side reactions; therefore, the kinetics of both reactions were estimated. Both reactions were verified to be second-order with no signs of competition for the adsorption sites under the studied conditions. Additionally, ethylbenzene disproportionation and ethylbenzene-toluene transalkylation, undergo through a deethylation-ethylation mechanism, the activation energies were 66 and 57 kJ/mol respectively. Furthermore, the existence of parallel reactions, as well as the feed concentration, showed no effect on the p -diethylbenzene selectivity. Selectivity towards the para -isomer was observed only at very low conversions. Finally, an analytical solution was developed for the system showing excellent agreement with the experimental data. [ABSTRACT FROM AUTHOR]

A highly shape-selective H[Zn,Al]ZSM-5/SiO 2 composite catalyst was prepared by direct incorporating Zn species via isomorphous substitution into the zeolite framework and subsequent SiO 2 deposition on zeolite surface. Its catalytic performance in methanol to aromatics (MTA) was compared with Zn/HZSM-5/SiO 2 prepared by impregnation of Zn species and subsequent SiO 2 deposition. The textural properties and acidic properties as well as the subsequent catalytic performances of the resultant catalysts in MTA reaction were obviously influenced by the introduction method of Zn species. H[Zn,Al]ZSM-5/SiO 2 prepared by isomorphous substitution contained more inter-crystalline voids, which provided much space to resist deposition of coke and avoided quick blockage of micropore entrances. In contrast, Zn/HZSM-5/SiO 2 prepared by impregnation contained sub-nanometric ZnO clusters at the pore entrances, which might restrict large molecule products diffusion and deteriorated the catalyst deactivation. Moreover, the sum of strong and medium acid sites in H[Zn,Al]ZSM-5/SiO 2 was more than that in Zn/HZSM-5/SiO 2 , which meant there were more active sites for cyclization and aromatization steps in MTA reaction in the former. As a result, the lifetime of H[Zn,Al]ZSM-5/SiO 2 was almost twice that of Zn/HZSM-5/SiO 2 . In addition, the enhancement in catalytic activity by Zn introduction and the improvement in para-selectivity in xylene by SiO 2 deposition were also studied. Finally, nearly 100% methanol conversion, 95.6% para-selectivity in xylene and 18.2% para-xylene yield were simultaneously obtained over the stable H[Zn,Al]ZSM-5/SiO 2 (with 24 h lifetime) in MTA reaction. [ABSTRACT FROM AUTHOR]

Cross-linked poly acidic ionic liquids (MPM-C 6 V-SO 3 CF 3 -IL) with mesoporous and macroporous network structure have been synthesized in different solvents. The novel solid acid catalyst with special network structure has a large surface area (103.83 m 2 /g), large pore volume (0.72 cm 3 /g) and abundant mesopores and macropores, which help to improve the contact between active site and reactants. Catalytic performance of catalyst was investigated through alkylation of o -xylene and styrene. Different reaction parameters specifically solvent on obtained catalyst and production of PXE were systematically investigated. Under optimal reaction conditions (reaction time was 3 h, reaction temperature was 120 °C, catalyst amount was 0.17 g (0.5 wt%), and o -xylene/styrene mass ratio was 7.5:1), a high conversion of styrene (100%) and 1,2-diphenylethane (PXE) yield of 99.67% was obtained, which is superior to commercial acid in liquid and even previous acids synthesized by our own group. Moreover, catalyst could keep relatively high thermostability under reaction and are easy to be separated and recycled from the solution, which are critical for heterogeneous solid catalysts. Thus, this novel catalyst can be potentially applied in other acidic reactions. [ABSTRACT FROM AUTHOR]

Aluminas with different textural characteristics were synthesized by carbonate , citrate and urea sol–gel methods. All supports were impregnated with an aqueous solution of ammonium metavanadate in order to obtain the same loading of vanadia (7% wt V 2 O 5 ), and then characterized by XRD, N 2 physisorption, Raman spectroscopy and NH 3 -TPD. In this way it was possible to tune the characteristics of the supported vanadia as it has also been demonstrated from the catalytic behavior of these materials in the cross-coupling of p -xylene to 2,2′,5,5′-tetramethyl 1,1′ biphenyl (biaryl) in the presence of O 2 . Alumina prepared via the carbonate method and calcined at 500 °C proved to be the most suitable support affording, for a level of conversion of 8%, selectivities to biaryl of 96%. It corresponded to an enough high dispersion of vanadium to allow the cleavage of the C H bonds. Recycling experiments confirmed the stability of these catalysts. [ABSTRACT FROM AUTHOR]

Highlights: [•] Zeolite MCM-22 samples were studied in the transformation of EB-m-xylene feed. [•] Acidity was modified by steam-dealumination/acid treatment and B substitution. [•] B-containing sample shows EB conversion of 40% at less than 5% xylene loss. [•] It has also high selectivity to p-xylene production at 723K. [•] Catalysts dealuminated by steaming and acid treatment were not effective. [ABSTRACT FROM AUTHOR]

Abstract: The samples of Cs2HPW12O40 salt were precipitated with CsCl, CsBr or CsI reagent as well as with commonly used Cs2CO3. The use of cesium halides resulted in the Cs2HPW12O40 samples of mesoporous structure composed of relatively loosely aggregated primary particles. It was observed that the type of halogen ion influenced textural properties of the Cs2HPW12O40 samples. As the atomic size of halogen ion increased (from Cl to I), the specific surface area and microporosity decreased. The so-obtained samples exhibited textural and morphological features similar to those of Cs2.5H0.5PW12O40 salt. In the transformation of m-xylene, the pore-size sensitive reaction, the catalytic activity of the Cs2HPW12O40 samples prepared with CsBr and CsI reagents was about two-fold higher than that of Cs2.5H0.5PW12O40 salt. All these samples exhibited similar strength of acid sites. Therefore, high catalytic activity of the samples prepared with CsBr and CsI could be ascribed to their open pore structure, which allowed the accessibility of almost all active sites for m-xylene molecules. [Copyright &y& Elsevier]

Abstract: Vanadium incorporated molybdophosphoric acid (MPAV) was used as a precursor to prepare Moh name="sbnd" />P oxide supported on alumina catalysts. The catalysts were prepared by impregnation of MPAV on alumina and subjecting them to calcination at 450°C. The catalysts were characterized by X-ray diffraction, Fourier transform infrared, temperature programmed reduction and X-ray photo electron spectroscopy to study the nature of interaction of MPAV with Al2O3. The characterization results revealed the decomposition of MPAV Keggin ion on alumina and the formation of its constituent oxides during calcination. The nature of metal oxide species formed on the surface of alumina depended on MPAV loading and the V content. The crystalline nature of metal oxides derived from the decomposition of MPAV x increased with increase in vanadium content. The activity of the catalysts during the ammoxidation of m-xylene also depended on MPAV loading and the content of V, as they were responsible for the dispersion of the constituent metal oxides on alumina. [Copyright &y& Elsevier]

Abstract: The Single-Event Microkinetic (SEMK) methodology has been applied towards “ethylbenzene dealkylation/xylene isomerization” under industrially relevant conditions. This includes the isomerization of xylenes towards thermodynamic equilibrium, the dealkylation of ethylbenzene as well as a limited amount of xylene transalkylation into toluene and trimethylbenzenes. By accounting for symmetry effects through the calculation of the number of single events and defining elementary reaction families rather than applying product lumping, a huge reduction in the number of adjustable parameters can be achieved without the loss of the molecular detail in the reaction network. In the kinetic model, 37 components, 78 intermediates and a total of 327 elementary reaction steps, classified in families such as (de-)protonation, alkyl shift, dealkylation, transalkylation and hydrogenation, are considered. Only reactant protonation enthalpies and the activation energies of the considered reaction families, are estimated by model regression to experimental data acquired on a bifunctional Pt/H-ZSM-5 catalyst, while the remaining parameters are determined from first principles or retrieved from literature information. The experimental data are adequately described with physically significant parameter values. Dealkylation is found to be energetically most demanding with an activation energy amounting to 198kJmol−1, while alkyl shift and transalkylation reactions are having the lowest activation energies. Entropic effects result in the lowest rate coefficient for transalkylation, however, and rate coefficients of a similar order of magnitude for alkyl shift and dealkylation. The investigated catalyst is shown to have an adequate acid strength for establishing the thermodynamic equilibrium between the xylene isomers. [Copyright &y& Elsevier]

Abstract: MOR zeolites were modified via desilication treatments with NaOH, under conventional and microwave heating. The samples were characterized by powder X-ray diffraction, 27Al and 29Si NMR spectroscopy, TEM and N2 adsorption at −196°C. The acidity of the samples and the space available inside the pores were evaluated through a catalytic model reaction, the isomerization of m-xylene, for which the profiles of the coke thermal decomposition were also analyzed. Powder X-ray diffraction and 29Si and 27Al MNR results show that in comparison with conventional heating, microwave irradiation (a less time consuming process) leads to identical amount of Si extraction from the zeolite framework. With this treatment, in addition to the customary mesopores development promoted by conventional heating, a partial conversion of the zeolite microporosity into larger micropores, is observed. The microwave irradiated and conventionally heated samples show different catalytic behavior in the m-xylene isomerization model reaction. It was observed that, by controlling the experimental conditions, it is possible to obtain samples with catalytic properties closer to the parent material, which is also confirmed by the respective coke analysis. [Copyright &y& Elsevier]

Abstract: The oxidation of o-xylene (OX) to phthalic anhydride (PA) is one of the important industrial processes based on selective oxidation reactions. However, the fundamental understanding of the by-product formation is still an open task. By using a sample port pilot plant, a detailed investigation was conducted for the first time of the by-product formation at different operation conditions. Several hitherto unknown intermediates could unambiguously be identified. The combination of process conditions and by-product formation enables the discussion of a new improved reaction scheme for the catalytic oxidation of o-xylene. The reaction path from OX to PA is commonly described by a rake mechanism consisting of a number of parallel and serial reactions. Tolualdehyde and phthalide are seen as the main intermediates. The most important by-products are maleic anhydride (MA), CO and CO2. The reaction paths towards these by-products are widely unknown. Several gas phase components, such as toluquinone and 2,3-dimethyl-p-benzoquinone not reported yet in the current literature, were observed for the first time in this study. Most of these previously unknown components are reaction intermediates, which later do not form the desired reaction product, PA. [Copyright &y& Elsevier]

Abstract: Catalytic transformation of three methyl benzenes (toluene, m-xylene, and 1,2,4-trimethyl benzene) has been investigated over ZSM-5, TNU-9, mordenite and SSZ-33 catalysts in a novel riser simulator at different operating conditions. Catalytic experiments were carried out in the temperature range of 300–400°C to understand the transformation of these alkyl benzenes over large pore (mordenite and SSZ-33) in contrast to medium-pore (ZSM-5 and TNU-9) zeolite-based catalysts. The effect of reaction conditions on the isomerization to disproportionation product ratio, distribution of trimethylbenzene (TMB) isomers, and p-xylene/o-xylene ratios are reported. The sequence of reactivity of the three alkyl benzenes depends upon the pore structure of zeolites. The zeolite structure controls primarily the diffusion of reactants and products while the acidity of these zeolites is of a secondary importance. In the case of medium pore zeolites, the order of conversion was m-xylene>1,2,4-TMB>toluene. Over large pore zeolites the order of reactivity was 1,2,4-TMB> m-xylene>toluene for SSZ-33 catalyst, and m-xylene∼1,2,4-TMB>toluene over mordenite. Significant effect of pore size between ZSM-5 and TNU-9 was observed; although TNU-9 is also 3D 10-ring channel system, its slightly larger pores compared with ZSM-5 provide sufficient reaction space to behave like large-pore zeolites in transformation of aromatic hydrocarbons. We have also carried out kinetic studies for these reactions and activation energies for all three reactants over all zeolite catalysts under study have been calculated. [Copyright &y& Elsevier]

Abstract: Transalkylation of heavy reformate (96% A9 and 4% A10) without addition of toluene was conducted over four bifunctional mordenite-based catalysts at various process conditions using a flow reactor. The effects of (i) molybdenum content (3 vs. 1wt.%); mordenite content (80 vs. 67wt.%); and (iii) metal type (Re vs. Mo) were assessed. The composition of products and their trends was scrutinized to determine the influence of the aforesaid parameters on dealkylation–transalkylation reactions. Higher molybdenum content enhanced the methylethylbenzene (MEB) dealkylation as well as trimethylbenzene (TMB) transalkylation, resulting in increased toluene and xylene formation. Higher mordenite content had little effect on either MEB or TMB conversion. Compared to molybdenum-containing catalysts, rhenium-containing catalyst not only exhibited higher MEB dealkylation and TMB transalkylation, but also resulted in higher hydrogen consumption and methane formation. A reaction scheme for the transalkylation of heavy reformate without the addition of toluene is proposed. Among several reactions that occur during this type of a process, de-ethylation of MEBs is kinetically controlled and probable rate-determining step in the overall reaction scheme. A linear correlation between MEB conversion and xylene yield was observed, which also supports the above rationale. [ABSTRACT FROM AUTHOR]

Abstract: This paper is a follow-up on our recent study on the applications of extractor-type zeolite catalytic membrane reactor (herewith referred to as e-ZCMR) for m-Xylene isomerization. In this paper, results of a preliminary investigation on the possibility of producing ultra-pure p-Xylene (PX) (purity>99%) via m-Xylene (MX) isomerization over Pt-HZSM-5 catalyst in an e-ZCMR with a “defect-free” nanocomposite MFI-alumina membrane tube as the separation unit is presented. Unlike “film-like” architectures, in nanocomposite architectures zeolite crystals are embedded within the pores of the supports. During m-Xylene isomerization conducted at a temperature range 473–573K, liquid meta-Xylene (99% purity) saturated in N2 gas was fed into the reactor and N2 gas was swept over the outer surface of the membrane on the shell side of the reactor. Analysis of results was based on permeate-only mode (products in permeate stream only) and combined mode (products in both permeate and retentate) operations. At 473K, e-ZCMR gave a maximum p-Xylene yield of 2.7% at permeate-only mode and 19.0% at combined mode. Throughout the temperatures investigated, the purity of PX approached 100% in the permeate and the membrane displayed 100% PX selectivity. These results indicate that there is a possibility of cutting down operational costs through a reduction in energy consumption during ultra-pure PX production and that this becomes feasible with the application of e-ZCMR having nanocomposite MFI-alumina membrane as separation unit. However, high flux defect-free nanocomposite MFI-alumina membranes are necessary to make this technology attractive and competitive with those currently in use. [ABSTRACT FROM AUTHOR]

Abstract: Zeolites Beta, mordenite and Y were evaluated for their activity in transalkylation reaction of toluene with trimethylbenzenes. Zeolite Beta was found to possess the highest conversion in toluene–trimethylbenzene transalkylation as well as a higher stability in time-on-stream compared with mordenite and zeolite Y. The effect of Si/Al ratio in zeolite Beta was evaluated and it was found that transalkylation activity and xylene yields increase with decreasing Si/Al ratio. Zeolite Beta with the lowest Si/Al ratio of 12.5 (the highest concentration of active sites) exhibited the highest 1,2,4-trimethylbenzene (124TMB) conversion and maximum xylene yield. The highest xylene yield was obtained at optimum equimolar ratio (1:1) of 124TMB to toluene. With increasing 124TMB concentration in the feed, the conversion of 124TMB and xylene yield decreased while toluene conversion simultaneously increased. The increase in the concentration of toluene in the feed resulted in the increase in the conversion of 124TMB. However, addition of higher concentrations of toluene led to a significant decrease in xylene yield. [Copyright &y& Elsevier]

Abstract: Co-oxidation of p-xylene (PX) and p-toluic acid (p-TA) to terephthalic acid (TA) in water with Mn acetate catalyst has been studied. Mn acetate showed better catalytic activity than Co acetate and no synergism was observed between two catalysts. During the reaction, PX was transformed to p-TA while p-TA was converted to TA by a radical reaction. In the co-oxidation, radicals generated from facile PX-to-p-TA conversion would assist more difficult p-TA to TA conversion. Addition of CO2 in the oxygen promoted the TA formation and suppressed the CO2 formation from the combustion of reactants. Additional transition metals such as Ni and Ti also promoted the catalytic activity of Mn catalyst. Furthermore, a synergism was observed between added CO2 and the added transition metal. Finally, the yield of terephthalic acid reached about 95% with Mn+(Ti, Ni or Cr) catalyst system and CO2 addition. [Copyright &y& Elsevier]

Abstract: Catalytic oxidation of xylene in air was performed under electron beam (EB) irradiation using pure TiO2 as well as TiO2 loaded with Ag, Pt, Au, or Mn to clarify the role of loaded metal in the enhancement of oxidation of xylene and its irradiation byproducts to CO2 in EB-induced non-thermal plasmas. EB irradiation experiments were performed with the catalyst bed placed in both irradiated and non-irradiated spaces. The highest conversion percentage of decomposed xylene to CO2 was obtained by irradiation/catalytic oxidation using an Ag/TiO2 bed placed in a non-irradiated space. The greater enhancement of CO2 production on an Ag/TiO2 pellet surface compared to that on other metal-loaded TiO2 pellet surfaces was due to the synergetic effect of strong adsorption of the byproducts on the Ag loaded on TiO2 and production of active oxygen from decomposition of O3 in the presence of Ag. [Copyright &y& Elsevier]

Abstract: We investigated the alkylation of toluene with methanol at 250°C over MCM-22 and ITQ-2 zeolites possessing an MWW structure. Whereas ITQ-2, produced by the delamination of MCM-22 precursor, showed a poor p-xylene selectivity of 23% at 1.7% of the level of toluene conversion, a high p-xylene selectivity of 80% at 0.6% of the level of toluene conversion was obtained by poisoning the external surface of ITQ-2 with collidine. Similarly, MCM-22 treated with collidine showed a high p-xylene selectivity of 74% at 3.8% of the level of toluene conversion. We concluded that both interlayer and intralayer 10-membered ring (10MR) micropores in the MWW structure caused the shape-selective formation of p-xylene in the alkylation of toluene with methanol. [Copyright &y& Elsevier]

Abstract: Vapor-phase methylation of toluene with methanol and isopropylation of toluene with 2-propanol has been investigated in a down flow reactor under atmospheric conditions using N2 gas carrier over a series of surface modified and unmodified ZSM-5 (Si/Al=60–170) loaded with H3PO4, differing in the external surface treatment of the zeolites. The feed molar ratios of toluene/methanol and toluene/2-propanol were varied over a wide range (8–0.125), and the optimum feed ratio of toluene/alcohol was less than 0.5 in both cases. Space velocity employed in toluene methylation reported as WHSV (toluene)=1.2h−1, and the space velocity employed in toluene isopropylation reported as WHSV (toluene)=0.8h−1. The methylation reactions were carried out in the temperature range of 623–773K, and the isopropylation reactions were carried out in the temperature range of 483–583K. Atmospheric pressures was maintained in all runs. Catalysts containing 0–4.9wt.% P were prepared using modified and unmodified ZSM-5 zeolites, and their catalytic performance for vapor-phase alkylation of toluene with methanol and 2-propanol were investigated. The optimum phosphorous content for methylation was 2.1wt.% P which was greater than the optimum phosphorous loading for isopropylation (0.7wt.% P). [Copyright &y& Elsevier]

Abstract: (Al,Ga)EMT-type zeolites (Al:Ga=1:0, 0.75:0.25 and 0.5:0.5) were prepared and tested as catalysts in the gas-phase toluene–methanol alkylation and toluene–trimethylbenzene transalkylation. Various techniques including XRD, N2 sorption, MAS NMR and SEM were used to monitor the physico-chemical properties of these solids. The results revealed a high purity and crystallinity of the materials, and that the Ga atoms are entirely incorporated into the framework. The ammonia TPD measurements indicated that the isomorphous substitution of aluminium by gallium leads to a decrease of both acidic strength and acid sites concentration. The catalytic performances of the EMT-type solids in the conversion of methylaromatic hydrocarbons were correlated with their acid properties. Thus, the less acidic (Al,Ga)EMT samples exhibited a lower activity, but a higher selectivity towards the xylenes formation by comparison with (Al)EMT. [Copyright &y& Elsevier]

Abstract: Samples of MCM-22 and MCM-36 (the pillared MCM-22 analog), synthesized from the same precursors were characterized by various techniques: XRD, N2 adsorption, pyridine and 2,4-dimethylquinoline adsorption followed by IR spectroscopy, m-xylene transformation at 350°C in absence or in presence of 2,4-dimethylquinoline. The MCM-36 samples were shown to present sinusoidal channels, mesopores and/or supermicropores containing cups but also a small amount of undesired supercages. The method previously developed to estimate the activity and selectivity of the three MCM-22 pore systems (sinusoidal channels, supercages and external cups) was adapted to the characterization of the MCM-36 pore systems. Pillaring causes a large change in the distribution of the activity: approximately 15% of m-xylene transformation occurs in the inner micropores of the synthesized MCM-36 against more than 70% in the corresponding MCM-22 sample. Hence, 85% of the activity of MCM-36 (against 30% in MCM-22) is due to the protonic sites located in cups which are accessible to bulky molecules. [Copyright &y& Elsevier]

A tapered element oscillating microbalance (TEOM) was used to measure the rates of adsorption and desorption of p-xylene from FCC catalyst at 473K. The original aim of the work was to investigate whether coke in the catalyst, formed by an earlier reaction with isopropanol at 773K, influenced the adsorption kinetics. It was actually found that p-xylene reacted in the coked catalyst, even though no reaction occurred in the fresh catalyst at this temperature. This is a rare direct observation of coke causing chemical reaction in a system in which no reaction occurs in the absence of coke. [Copyright &y& Elsevier]

The present contribution is a brief account of investigations carried out on the ammoxidation of o-xylene over a few VPO and V2O5 based catalysts. The desired reaction product is phthalonitrile, however, phthalimide is the main product in most cases besides carbon oxides. The product distribution between nitrile and imide mainly depends on the reaction conditions and the nature of catalyst used. Influence of various reaction parameters such as effects of (i) reaction temperature, (ii) water vapour addition in the feed gas, (iii) NH3/o-xylene mole ratio and (iv) space velocity were studied. It is interesting to note that the removal of water vapour from the feed gas has a highly pronounced promotional effect on the selectivity of phthalonitrile. The nitrile selectivity increased from 2.1 to 34% at the expense of phthalimide (decreased from 53 to 9%) with the complete removal of water vapour in the reactant feed mixture. This observation gives an indication that phthalonitrile being formed in the reaction is further getting hydrolysed to phthalimide via amide intermediate in presence of water vapour. Another interesting aspect of the present investigation is that the titania supported catalysts always gave significantly higher selectivities of phthalimide compared to phthalonitrile. Phthalimide yields up to 80% were reached on a VPO/TiO2 catalyst. [Copyright &y& Elsevier]

A systematic study on the influence of reaction conditions (temperature, time, and reactant type) on the selectivity of xylene transformation has been carried out over USY zeolite. Initial product selectivity revealed that both isomerization and disproportionation of xylenes are primary reactions. Higher conversion was observed with p-xylene reactant as compared to m- and o-xylene. The “reactant converted” deactivation model was tested with xylenes as model compounds, using effective, mechanistic kinetic models, developed from the transformation of each xylene isomers. The parameters optimized to the experimental data gave a good prediction of the overall reaction kinetics for the xylene transformations. The results provide significant evidence that the “reactant converted” decay model can be adequately utilized to account for catalyst deactivation of the xylene transformation. [Copyright &y& Elsevier]

The role played in the transformation of the xylene isomers by each of the pore systems of a MCM-22 (MWW) zeolite was determined by selectively deactivating the protonic sites of the supercages and poisoning those of the external hemicages with 2,4-dimethylquinoline. In the large supercages (7.1 Å Ø, 18.2 Å height) with narrow apertures (4.0 Å×5.5 Å), the three isomers undergo isomerization I and disproportionation D, the first reaction being largely predominant (D/I ratio of 0.3 from the three isomers). The desorption of trimethylbenzene D products being strongly limited, a large part of them undergo secondary transformations leading to less bulky products and to “coke”. The isomer distribution is that expected from a product shape-selective process. Furthermore, within the narrow sinusoidal channels (4.1 Å×5.1 Å), only isomerization of meta- to para-xylene can occur; the ortho-xylene molecules are unable to enter (or to desorb from) the channels, which was confirmed by adsorption followed by IR spectroscopy. Lastly, in the large hemicages, the main reaction is non-shape-selective isomerization. The distribution of protonic sites within the three pore systems was estimated from the results of o-xylene adsorption and 2,4-DMQ poisoning experiments. The comparison of this distribution with the respective catalytic role of the different pore systems shows that the external hemicage protonic sites are 2–3 times more active than those of the other pore systems. This observation can be related to the complete absence of diffusion limitations in xylene transformation within the external hemicages. [Copyright &y& Elsevier]

The gas-phase transformation of mixtures of benzene and xylenes has been studied on several zeolites. Transalkylation between benzene and xylenes and disproportionation and isomerization of xylenes are the main reactions in this process. Under the reaction conditions used, medium pore zeolites (FER and ZSM-5) were inactive, whereas large pore zeolites (USY, BETA and MOR) yielded toluene and trimethylbenzenes on a different extension. The catalytic activity has been related to the acid properties and so MOR(20) exhibited the highest overall conversion. The relative contributions to the transalkylation and disproportionation reactions were found to be influenced by the feed composition (i.e. the benzene to xylene ratio), the strength of the acid sites and the pore size and geometry of the catalysts. All zeolites underwent a substantial deactivation by coke deposition, particularly when o-xylene was used as reactant. Pt-supported zeolites were used in order to improve the stability of the catalysts. [Copyright &y& Elsevier]

The effects of alkali metals on the oxidation of p-xylene (p-X) have been studied. The oxidation reaction is eventually promoted due to the alkali metal itself, even though the reaction is decelerated a bit in the beginning of the reaction. The promoting effect of alkali metal is not related to the basicity or to the anion. The effect increases as the concentration of Mn or Br decreases. The acceleration effect generally increases with the degree of deceleration in the beginning of the reaction. Through the analysis of products, one may understand that the alkali metal does not change the reaction mechanism. [Copyright &y& Elsevier]

The MC-type O2 oxidation of xylenes to the corresponding dicarboxylic acids with Co/Mn/HBr in the liquid phase was remarkably improved in the co-presence of carbon dioxide in O2 feed stream. This striking phenomenon became even more prominent when the catalyst was modified with an alkali/transition metal additive. A synergistic interaction of CO2 and O2 molecules generated an active cyclic peroxocarbonate complex. This complex appears to play a key role in promoting hydrogen abstraction, which has been reported to be the rate-determining step for the kinetic oxidation pathway. In short, the introduction of CO2 into the O2 feed stream brought about an advanced oxidation process, which resulted in an increase in the yield and selectivity towards the acid product, a reduction of partially oxidized intermediates, and an enhancement of the product quality over the conventional MC-type O2 oxidation process. [Copyright &y& Elsevier]

• The catalytic behaviour of zeolites is strongly affected by the pore structure and the type and strength of acidity. • Over zeolite Beta, the Si/Al ratio of 12.5 exhibited the lowest deactivation behavior and highest xylenes yield. • Over zeolite Y, the mild dealumination (Si/Al = 6.0) improves the feed conversion and enhances the TMB diffusion. Effects of pore structure and acidity over Beta and Y zeolites with a range of Brønsted and Lewis acid sites on their catalytic behaviour in the transalkylation reaction were investigated. The tests were conducted for a time-on-stream (TOS) of 50 h in a fixed-bed reactor at 400 °C, 10 bar, H 2 /HC = 4, and WHSV=5 h−1 with a 50:50 wt. % toluene:124-TMB mixture as feedstock. While initial (TOS ≤3 h) TMBs conversion was similar, wide differences were evident as the effects of partial deactivation became apparent with TOS. Zeolite Beta (Si/Al = 12.5) exhibited highest conversion and xylenes yield. As the Si/Al ratio of zeolite Beta is increased, the performance dropped whereas an opposite trend was observed for Y zeolites underscoring the importance of optimum level of dealumination depending on the zeolite structure. Isomerization of 124-TMB was observed over Beta which was not sustained by Y. The existence of separate active sites for TMB isomerization and its transalkylation with toluene in Beta is suggested. [ABSTRACT FROM AUTHOR]

• Both Zn and Cr improved the formation of aromatics compared with HZSM-5. • Cr loading improved active centers number and tuned the acidity distribution. • Cr loading further enhanced n-hexane conversion, aromatics yield and stability. • Coaromatization of n-hexane and methanol over [Zn,Cr]/HZSM-5 was investigated. • Methanol addition enhanced the selectivity and yield of xylene due to methylation. The catalytic performance and properties of [Zn,Cr]/HZSM-5 were determined using XRD, py-FTIR, ICP, TG, and XPS and compared with those of Zn/HZSM-5 and HZSM-5 for the aromatization of n-hexane. A distinct role of Cr in enhancing n-hexane conversion, aromatic yield, and stability was observed. The catalytic performance of [Zn,Cr]/HZSM-5 and the effects of feed ratio, reaction temperature, and contact time on the coaromatization of n-hexane and methanol were investigated. Methanol addition increased the xylene yield due to methylation. Cofeeding provided high stability arising from the slow carbon deposition rate. At a feed ratio (n-hexane/methanol) of 3:7, reaction temperature 450 °C, and WHSV 2 h-1, the aromatic yield was 40.1%, with xylene selectivity of 40.4%. [ABSTRACT FROM AUTHOR]