Toluene Disproportionation Research Articles

Transformations of alkyl aromatics over delaminated MCM-22 zeolites and their composites with mesoporous MCM-41 silicate

Small (S)- and large (L)-crystalline zeolite MCM-22 precursors with the same Si/Al F ratio of 17, were synthesized and either converted to zeolites MCM-22(S/L) or delaminated. From the delaminated zeolite MCM-22 precursors delaminated zeolites MCM-22 (dl-MCM-22(S/L)) were prepared. Delaminated zeolite precursors were admixed to the synthesis mixture of mesoporous silica MCM-41 to obtain the respective dl-MCM-22/MCM-41(S/L) composite materials. The preparations were characterized by XRD, N 2 adsorption measurements, scanning electron microscopy. FT-IR spectroscopic studies of pyridine (Py) and 2,4-dimethylquinoline (2,4-DMQ) adsorption were used for determination of total (Brønsted and Lewis acidity) and external Brønsted acidity, respectively. It was found that the crystallite size of the MCM-22 precursors strongly influenced the structural, acidic and catalytic properties of the delaminated and composite materials. The effectiveness of the delamination procedure for the zeolite MCM-22(S) precursor having smaller and thinner particles is higher. At same synthesis gel compositions, the amount of mesoporous fraction in the composite materials was different. In dl-MCM-22/MCM-41(S) composite the formation of the mesoporous component was suppressed. Moreover, delamination resulted, for both delaminated and composite materials, in a similar decrease in the Brønsted acidity. The reduction in toluene disproportionation activity confirmed the above structural and acidity changes. 2,4-DMQ adsorption results proved that both the dl-MCM-22 samples and the composites possess higher amount of external Brønsted acid sites than zeolite MCM-22. The enhanced external acidity and accessibility of the active sites of these preparations appeared favorable when more basic and bulkier reactant, such as 1,2,4-trimethylbenzene was transformed.

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 Bronsted 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.

Disproportionation and Methylation of Toluene with Methanol Over Zeolite Catalysts

Zeolites TNU-9, SSZ-33, mordenite (MOR) and ZSM-5 were evaluated for their activities in toluene disproportionation and methylation reaction of toluene with methanol. The medium-pore zeolite TNU-9 was found to possess the highest conversion in toluene disproportionation as compared with SSZ-33, mordenite and ZSM-5 based catalyst. Zeolite mordenite with the highest Si/Al ratio of 135.9 (the lowest concentration of active sites) exhibited the highest toluene conversion and maximum xylene yield in toluene methylation. On mordenite, the presence of channel intersections allows more reaction space for the formation of bulky intermediates and/or products and the 12-ring channels on the other hand, allow diffusion without trapping, since the channel diameter is large enough. In toluene methylation, xylene selectivity at the same conversion level follows the order: ZSM-5 > TNU-9 > MOR > SSZ-33, which implies that xylene selectivity is directly related to the size of channels from medium to large pore zeolites. The medium-pore zeolite ZSM-5, with 10-ring channels, shows the lowest reactivity for further alkylation of xylene, while mordenite with 12-ring channel shows the highest reactivity for further alkylation of xylene.

Numerical Simulation of p-Xylene Synthesis via Selective Toluene Disproportionation over Modified ZSM-5 Catalyst

Numerical modeling of selective toluene disproportionation reaction over modified ZSM-5 zeolites to produce p-Xylene was developed and validated. The results of parametric analysis using the authenticated model provide an optimum guideline for the selection of appropriate operating conditions to achieve maximum toluene conversion and p-Xylene selectivity and yield. The predicted data indicate that increasing feed temperature from 450 to 600°C at constant (WHSV)-1 enhances the toluene conversion by 150%. Low weight hourly space velocities ((WHSV)-1=100 g.h/mol) and feed temperature (450°C) provides 99% p-Xylene selectivity. Moreover, at constant feed temperatures of 450, 500, 550 and 600°C, the maximum p-Xylene yield (? 0.087) occurs at 1300, 650, 330, and 175 g.h/mol, respectively, which clearly demonstrates that operation at higher feed temperature reduces the needed reactor residence time. Despite of imposed adiabatic condition, the model prediction demonstrates that an almost isothermal behavior prevails inside the reactor (2°C decrease in reactor temperature) due to the low heat of reaction (<1kJ/mol).

Silicalite-1 Shell Synthesized onto Cylinder-Shaped ZSM-5 Extrudate for Disproportionation of Toluene into Para-Xylene

The cylinder-shaped ZSM-5 extrudate with a diameter 2 mm and a length 4–5 mm was coated with a silicalite-1 shell at its external surface by a hydrothermal synthesis with TEOS (tetraethylorthosilicate) as a silica source and TPAOH (tetrapropylammonium hydroxide) as a structure-directing agent. Catalytic performance of the obtained core/shell zeolite composite was measured in disproportionation of toluene using a pressured fixed-bed reactor. Ratios of H2O/SiO2 and TPAOH/SiO2, crystallization time and temperature, and synthesis cycles were adjusted to get optimal synthesis conditions, at which the selectivity to para-xylene is enhanced from the thermodynamic equilibrium value 24% to nearly 80%. SEM and XRD results further confirm the formation of the ZSM-5/silicalite-1 core/shell structure, which is responsible for the evident shape-selective production of para-xylene. Silicalite-1 shell is hydrothermally synthesized over the external surface of cylinder-shaped ZSM-5 extrudate; the obtained core/shell composite exhibits a high shape-selectivity for para-xylene of nearly 80% in disproportionation of toluene

Carbon Deposition on a Toluene Disproportionation and Transalkylation Catalyst

The nature of carbon deposition on a toluene disproportionation and transalkylation catalyst was characterized using thermogravimetry and differential thermal analysis (TG-DTA), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) spectroscopy, N2 adsorption/desorption and NH3-temperature programmed desorption (NH3-TPD). Result showed that the kinetics of hard coke combustion approximately fitted a first order reaction with respect to the amount of coke and, therefore, most of the coke existed in a monolayer state and the corresponding apparent activation energy was about 110 kJ·mol-1. There were three kinds of carbon species on the surface of the Cat-1000 catalyst, which presented the catalyst after 1000 h reaction. The C atom fractions in the presence of C—O, C=O, and —C—C— were 22.7%, 9.1%, 68.1%, respectively. The formed carbon species on the Cat-1000 catalyst surface was mostly a graphite structure. The results showed that three quarters of the total carbon deposition on Cat-1000 was hard coke and the rest was soft coke. The specific surface area of the Cat-1000 catalyst decreased obviously compared with the catalyst without reaction (Cat-0). The Cat-1000 had almost unchanged acid intensity as well as high activity and stability. This shows that the cokes mainly deposit on the binder.

A catalytic ZSM-5 membrane sandwiched with silicalite-1 layers for highly selective toluene disproportionation

A catalytic zeolite membrane composed of silicalite-1, ZSM-5 and silicalite-1 layers supported on a porous stainless support was prepared by hydrothermal synthesis (silicalite-1/ZSM-5/silicalite-1 membrane). The permeance of hydrogen, argon, nitrogen, n-butane and isobutane for silicalite-1/ZSM-5/silicalite-1 membrane was measured from 353 to 513 K. The permeance of these gases were from 5 to 25 × 10 −8 mol/(m 2 s Pa) the same range as typical MFI-type membranes and the ratio of the permeance of n-butane to isobutane was 2 or less. The toluene disproportionation was examined at 673 K with silicalite-1/ZSM-5/silicalite-1 membrane as a membrane reactor as well as ZSM-5/silicalite-1 membrane and ZSM-5 powder. The products detected were benzene, p-, m- and o-xylene. The toluene conversion with two membrane reactors was less than 3%. At lower than 1% of toluene conversion, the selectivity of p-xylene for silicalite-1/ZSM-5/silicalite-1 was above 96% whereas that for ZSM-5/silicalite-1 membrane was below 89%. The silicalite-1 layers sandwiching the ZSM-5 layer probably suppressed the surface reaction on both ZSM-5 layers, which would achieve the significantly high selectivity of p-xylene in toluene disproportionation.

The role of the zeolite channel architecture and acidity on the activity and selectivity in aromatic transformations: The effect of zeolite cages in SSZ-35 zeolite

A series of zeolites differing in the channel architecture and acidity was investigated in toluene disproportionation, together with toluene and p-xylene alkylation with isopropyl alcohol. Zeolites with one- to three-dimensional 10-ring and 12-ring channels with and without cages, and those having 12–12–10 and 12–10–10-ring channel systems were studied. It was shown that general relationship of increasing zeolite activity with increasing pore diameter and pore connectivity is not valid as the size of some 12-ring channels (Beta, MCM-68) is comparable with 10-ring channels (ZSM-5, SSZ-35). In addition, the presence of cages in the structure of SSZ-35 and MCM-58 attributes the unusual catalytic behavior of these zeolites. SSZ-35 and MCM-58 zeolites behave in both toluene reactions such as three-dimensional large-pore zeolites. Subtle differences between zeolites of similar pore sizes and dimensionality can be usually explained based on the differences in the acidity of the individual zeolites. In p-xylene alkylation SSZ-35 exhibited high conversion with the highest selectivity to 1-isopropyl-2,5-dimethyl benzene and a low rate of deactivation. The presence of 18-ring cages in the channels of 10-ring zeolite SSZ-35 (STF) gives rise to an unusual catalytic behavior of this zeolite by comparison to other 10-ring zeolites. SSZ-35, possessing channels of 0.54 × 0.57 nm in diameter, exhibits catalytic activity in transformation of aromatic hydrocarbons that is similar to large-pore zeolites. 18-Ring cages enable the formation of relatively bulkier transition states while the diffusion of the product molecules out of the 10-ring channel system is not slowed down due to 10-ring windows. In addition, the channel system of SSZ-35 prevents the formation of coke precursors.

Mechanism for selective para-disproportionation of toluene on pentasil-based catalysts

We propose a bimolecular mechanism for disproportionation of toluene to form benzene and para-xylene via protonation of the toluene molecule at the methyl group, reaction of the carbocation formed with the para position of the next toluene molecule, and α-scission of the complex obtained. The mechanism presumes formation of methyl diphenyl methane as a byproduct of disproportionation.

Alkaline Modification of MCM-22 to a 3D Interconnected Pore System and its Application in Toluene Disproportionation and Alkylation

Modification of HMCM-22 zeolite by alkaline treatment was investigated by various characterization techniques and in toluene disproportionation and alkylation with isopropyl alcohol. This ‘desilication’ process led for mild alkaline concentrations (~0.10–0.20 M NaOH at 323 K for 45 min) to the partial destruction of the zeolite framework, but also to the formation of additional mesoporosity. Furthermore, the accessibility/availability of Lewis acid sites, investigated by d 3-acetonitrile and pyridine adsorption using FTIR spectroscopy, increased for these mild alkaline treatments, while the Bronsted acidity decreased. Higher alkaline concentrations (up to 0.50 M NaOH) led to a too severe framework and pore destruction and a decrease of both the Lewis and Bronsted acid site concentration. Decomposition and deconvolution of 29Si MAS-NMR spectra confirmed the Si extraction and partial framework destruction, since more Q3 SiOH groups were formed at the expense of the Q4 T-atoms in the framework. Furthermore, the T6 and T7 Si-atoms were preferentially extracted, which would indicate that an interconnection between the intralayer and the interlayer and/or outer surface is formed. The toluene conversion in its disproportionation reaction increased for the mildly treated sample, while the selectivity to xylene isomers (and cymene and n-propyltoluene isomers in the alkylation reaction with isopropyl alcohol) was similar to the thermodynamic equilibrium, suggesting that the reaction primarily occurs at outer surface cups of the HMCM-22 zeolite.

Selectivity Enhancement to the Exclusive Formation of Ethyltoluenes and Hydrogen During Dehydroalkylation of Toluene with Ethane

The dehydroalkylation of toluene with ethane to the isomeric ethyltoluenes was studied on 0.4Pt/H-ZSM-5 at varying contact times (1/WHSV). At a high contact time of 1.0 h, toluene disproportionation and hydrogenolysis reactions dominate, resulting in low selectivity to the desired ethyltoluenes via the alkylation reaction. However, at a low contact time of 0.12 h side reactions are eliminated, resulting in maximum selectivities to the kinetically favored ethyltoluenes and hydrogen. Results at high selectivities to ethyltoluenes provide significant insight into reaction pathways.

Comparison of Activity and Selectivity of SSZ-33 Based Catalyst with other Zeolites in Toluene Disproportionation

SSZ-33 based-catalyst, after modification with Mo and alumina binder, was evaluated in long-run tests in the toluene disproportionation in the presence of hydrogen as a carrier gas. The activity and selectivity of this catalyst were compared with those of ZSM-5 and mesoporous ZSM-5 prepared with the same concentration of Mo and alumina. Formation of the final catalysts decreased the void volume of micropores in order SSZ-33 > ZSM-5 > ZSM-5/Meso. Simultaneously, the concentration of Lewis acid sites increased due to the addition of alumina to the catalyst. The highest toluene conversion was achieved with SSZ-33 catalyst comprising 12-12-10-ring channels, which is the result of high acidity of this zeolite together with increased mass transport through large pores. ZSM-5 zeolite exhibited lower toluene conversion while only a low activity was found for mesoporous ZSM-5 probably due to the limitations of the access to the zeolite channels after modification with alumina.

Synthesis of ZSM-5 with intracrystal or intercrystal mesopores by polyvinyl butyral templating method

Three facile routes were utilized to synthesize ZSM-5 materials with intracrystal or intercrystal mesopores, where the polyvinyl butyral gel served as mesopore directing template. The three routes were divided into two synthesis strategies: the hydrothermal treatment of silica/PVB composite and re-crystallization of preformed zeolite precursor with the assistance of PVB gel. The fabrication of silica/PVB composite was accomplished by two routes including sol–gel process and impregnation method. The resulting composite was undergone hydrothermal treatment. During the crystallization PVB was occluded in the ZSM-5 crystal, creating intracrystal mesopores in the zeolite. The last route for the synthesis of mesoporous ZSM-5 was realized by re-crystallization of preformed ZSM-5 zeolite in the presence of PVB. This route involved the pre-crystallization of the amorphous aluminosilicate to produce the pre-formed ZSM-5 precursor. Upon further crystallization of the mixture of PVB gel and pre-formed ZSM-5, the ZSM-5 precursor was transformed into ZSM-5 aggregate of nanocrystals, while the PVB gel was occluded in the ZSM-5 particles. Removal of the template generated the typical microporosity associated with ZSM-5 structure along with intercrystal mesoporosity produced from the PVB. The mesoporous ZSM-5 exhibited enhanced catalytic activity in the toluene disproportionation and transalkylation with C 9 and C 10 aromatics.

Selective ion-exchange in MAPO-36 and its application in toluene disproportionation

MAPO-36 was synthesized hydrothermally by isomorphic substitution of Mg2+ in the framework of AlPO-36 and ion-exchanged with Fe3+, Zn2+, La3+ and Ce3+ by wet method. The materials were characterized by XRD, TGA, TPD (ammonia) and SEM-EDX. XRD revealed absence of structural degradation after ion-exchange. TPD (ammonia) showed selective ion-exchange of strong acid sites in ion-exchanged MAPO-36. The weight loss around 550 °C in TGA for Fe, La and CeMAPO-36 suggested conversion of M(OH) 2 + to MO+. Toluene disproportionation was carried over all catalysts in which diphenyl methane derivative was suggested to be the principle intermediate in the formation of p-xylene and other products. The time on stream study showed exclusive formation of p-xylene after 6 h.

Specifics of C-C bond formation and rearrangement in transformations of hydrocarbons on modified zeolites

Using high-performance catalysts prepared from zeolites Y, M, and ZSM-5, the dependence on the acid and micropore characteristics of modified zeolites was studied for the yields of products of dehydrocycloaromatization of C2+ alkanes, the isomerization of n-pentane, and the disproportionation of toluene. On the basis of the obtained experimental results and published data, it was shown that the initial activation of reactant molecules occurs on acid sites regardless of their acid strength and that the formation and rearrangement of C-C bonds are controlled by the molecular-sieve parameters of a zeolite and can be varied via the modification of the virgin zeolites used to prepare the catalysts.

Aromatic Transformation in Fluidized Bed Reactors

In this work three important aromatic transformations, namely: toluene disproportionation, toluene methylation and m-xylene isomerization, were investigated in a riser simulator which closely mimics the operation of commercial fluidized bed reactors. The transformations were studied over a ZSM-5 based catalyst with medium acidity of 0.23 mmol/g and a series of Y zeolites of acidities between 0.55 and 0.03 mmol/g. For pure toluene feed, it was observed that conversion over the ZSM-5 based catalyst and the weakly acidic Y zeolite (USY-1) was very low. However, with the highly acidic Y zeolite (H-Y), significant toluene conversion was observed with paring reaction more prominent than disproportionation. On the other hand, when toluene was alkylated with methanol, higher toluene conversions were achieved over both the ZSM-5 based and the weakly acidic USY-1 catalysts as compared to when pure toluene feed was used. In addition, p-xylene/o-xylene (P/O) ratios higher than the equilibrium values were obtained in the reaction product over both catalysts. Finally, for m-xylene isomerization it was found that m-xylene conversion increased initially as the acidity of the catalyst increased up to 0.1 mmol/g beyond which any further increase in acidity resulted in a slight decrease in the m-xylene conversion.

Effect of Reaction Pressure and Carrier Gas on Toluene Disproportionation over Molybdenum−ZSM-5 Catalyst

This study was aimed at evaluating the effects of reaction pressure and temperature on toluene disproportionation using a ZSM-5-based catalyst impregnated with molybdenum. The results of the study showed that both toluene conversion and xylene selectivity were strongly dependent on temperature, pressure, and the kind of carrier gas used. Toluene conversion compared at the same temperature and pressure was found to be higher when nitrogen instead of hydrogen was used as the carrier gas. For both carrier gases, it was found that toluene conversion increased with both pressure and temperature. Furthermore, it was observed that catalyst deactivation was more rapid and more severe with nitrogen as the carrier gas than with hydrogen.

Activity and selectivity of zeolite MCM-22 catalysts in the disproportionation of toluene

Zeolite precursor MCM-22 samples were prepared by hydrothermal synthesis. The H-zeolites, obtained from the precursors, were characterized by scanning electron microscopy (SEM), Fourier-transform infrared (FT-IR) spectroscopy, and by their activity in the title reaction. Despite of the lower Bronsted acid site concentration of the zeolite having smaller crystallites, showed higher activity, while the zeolite, having larger crystals were more selective for p-xylene formation.

Economic Assessment of the Toluene Disproportionation Process Using an External Reactor - Separator Configuration

A model has been developed to investigate the operation of a distillation column with an external side reactor coupled to it (ESR). This system is compared to two more conventional process systems, namely a reactor followed by a single distillation column with recycle (R-D), and a reactor followed by two distillation columns in series with recycle (R-D-D). The toluene disproportionation reaction is used in the comparison. It is found that the reactor followed by a single column is an unattractive system as the overall conversion is low (14%) compared with 56% for the R-D-D system and 40% for the ESR system. An economic comparison of the two attractive processes was performed and the results show that the side reactor system requires similar capital costs, but reduced annual utility costs. The difference is approximately 12.5% which is significant, but below the 25-50% required to make a change in established technology viable. The side reactor system does not necessarily use new technology, but it does use the old technology in a new way. Therefore, it is still worth investigating these systems further and should be a possibility when designing a new plant.

Comparison of catalytic performances on nanoscale HZSM-5 and microscale HZSM-5

Nanoscale HZSM-5 and microscale HZSM-5 were characterized by NH 3–TPD, XRD, SEM and N 2 adsorption. The catalytic performance of the disproportionation of toluene, the alkylation of toluene with methanol and the cracking reaction of 1,3,5-TMB and 1,2,4-TMB were investigated. The similar reactivity of the disproportionation of toluene and the alkylation of toluene with methanol was obtained over nanoscale HZSM-5 and microscale HZSM-5. The reactivity of 1,3,5-TMB and 1,2,4-TMB over microscale HZSM-5 and nanoscale HZSM-5 was different because the different accessible acid sites and reactant molecule. The reason for the high proportion of external acid sites on ZSM-5 was also discussed.