Aromatization catalyst deactivation

By decreasing the GHSV values, the selectivity dramatically decreased due to the presence of the side-reaction of C-alkylation on the aromatic ring, giving rise to relevant amounts of 3-MC and a not-fully-identified methyl-MDB derivative (however, the 3-methyl isomer is the most probable candidate). Lastly, the lowest GHSV value was conducive to the condensation of PYC, with a formation of heavy by-products, a dramatic decrease of C-balance, and resulting catalyst deactivation. 

Due to the relative stability of such 6-arene complexes there is a strong likelihood that in aromatic solvents only parts of the employed Rh-catalyst are available for catalysis. As seen earlier, substrates and products with aromatic substituents can also lead to catalyst deactivation. 

One carbon atom in a wrong interstice may block the C5 cyclization activity of several surrounding sites. Therefore, C5 cyclic reactions are suppressed first during catalyst deactivation, while aromatization activity lasts much longer 159). This again supports the reactive adsorption mechanism 154). A different type of deactivation was reported as being due to disordered and ordered surface carbonaceous deposits 138,148). 

A number of ex situ spectroscopic techniques, multinuclear NMR, IR, EXAFS, UV-vis, have contributed to rationalise the overall mechanism of the copolymerisation as well as specific aspects related to the nature of the unsaturated monomer (ethene, 1-alkenes, vinyl aromatics, cyclic alkenes, allenes). Valuable information on the initiation, propagation and termination steps has been provided by end-group analysis of the polyketone products, by labelling experiments of the catalyst precursors and solvents either with deuterated compounds or with easily identifiable functional groups, by X-ray diffraction analysis of precursors, model compounds and products, and by kinetic and thermodynamic studies of model reactions. The structure of some catalysis resting states and several catalyst deactivation paths have been traced. There is little doubt, however, that the most spectacular mechanistic breakthroughs have been obtained from in situ spectroscopic studies. 

Another effect of aromatics is increased carbon formation, which has long been recognized as the primary means of catalyst deactivation in the ATR of hydrocarbons. Using carbon-forming reactions (6)-(8), an equilibrium line for carbon formation as a function of O2/C and S/C ratios can be calculated. Figure 9 shows the results of this calculation for n-Ci4, along with the experimental results for two Ni-based commercial catalysts. 

Side reactions specific to one component play an important role in the reforming of a mixture. For example, aromatics are more prone to coking upon reforming, so their presence in a mixture can lower syngas yields over time due to catalyst deactivation. Also, the catalyst surface-component interactions may play an important role in the reforming of a mixture. For example, aromatics have an abundance of 71-electrons, so they may occupy active sites for a longer duration, due to 71-complexation between d-orbitals of the metal and 7i-elec-trons. Hence there will not be enough reactive sites available for the desired reaction to occur. 

Catalyst deactivation is primarily caused by the blockage of active sites due to the coke formed from these olefinic intermediates. Higher hydrogen pressures suppress the diolefin formation, making the selectivity between olefinic intermediates and liquid products (in contrast to coke products) more favorable. However, higher pressures reduce selectivity to aromatics in the desired liquid product. Thus, a rigorous model must accurately predict not only the rates of product formation, but also the formation of coke precursors... 

The presence of gas-phase water is generally beneficial to the photocatalytic oxidation of aromatic contaminants. In continuous photoreactors, humidity appears to prolong catalyst activity and delay or prevent catalyst deactivation. The effects of humidity on reaction rates, however, appear to vary, depending on the aromatic contaminant concentration and the humidity level. For example. Petal and Ollis [18] examined the continuous photocatalytic oxidation of m-xylene in a powder-layer photoreactor at several different relative humidity levels. The m-xylene photo-oxidation reaction rate was observed to increase for gas-phase water concentrations up to 1000 mg/m (—7% relative humidity). Increasing the humidity level further (up to 5500 mg/m ) produced a gradual decrease in the observed reaction rate, possibly due to increased adsorption-site competition between xylene and water. The reported xylene removal rate for a water concentration of 5500 mg/m was approximately half that seen at 1000 mg/m. ... 

Blount and Falconer [54] further examined the photocatalytic oxidation of toluene using TPH. During TPH analysis of used catalyst samples, the strongly bound intermediates observed by Larson and Falconer [43] were reported to be hydrogenated and desorbed predominantly as toluene, along with smaller quantities of benzene. This indicated that the intermediate species responsible for apparent catalyst deactivation during toluene photooxidation retained an aromatic ring structure. 

Mendez-Roman and Cardona-Martinez [55] examined titanium dioxide catalysts with FTIR spectroscopy during the photocatalytic oxidation of toluene. Reaction intermediates, believed to be benzaldehyde and benzoic acid, were reported to accumulate on catalyst samples. This accumulation of intermediates was found to be reduced in the presence of gas-phase water. Mendez-Roman and Cardona-Martinez concluded that toluene appeared to be converted to benzaldehyde, which was then oxidized further to form benzoic acid. They suggested that the accumulation of benzoic acid led to the observed apparent catalyst deactivation. Other researchers, however, have argued that benzoic acid is unlikely to be the compound responsible for apparent deactivation in the photocatalytic oxidation of aromatics. For example, Larson and Falconer [43] concluded, based on higher CO2 evolution rates for benzoic acid relative to toluene during photooxidation, that benzoic acid was not sufficiently recalcitrant to be responsible for the deactivation seen with aromatic contaminants. 

Photocatalytic oxidation over illuminated titanium dioxide has been demonstrated to be effective at removing low concentrations of a variety of hazardous aromatic contaminants from air at ambient temperatures. At low contaminant concentration levels and modest humidity levels, complete or nearly complete oxidation of aromatic contaminants can be obtained in photocatalytic systems. Although aromatic contaminants are less reactive than many other potential air pollutants, and apparent catalyst deactivation may occur in simations where recalcitrant reaction intermediates build up on the catalyst surface, several approaches have already been developed to counter these potential problems. The introduction of a chlorine source, either in the form of a reactive chloro-olefin cofeed or an HCl-pretreated catalyst, has been demonstrated to promote the photocatalytic oxidation of... 

Vapor-phase alkylation of benzene by ethene and propene over HY, LaY, and REHY has been studied in a tubular flow reactor. Transient data were obtained. The observed rate of reaction passes through a maximum with time, which results from build-up of product concentration in the zeolite pores coupled with catalyst deactivation. The rate decay is related to aromatic olefin ratio temperature, and olefin type. The observed rate fits a model involving desorption of product from the zeolite crystallites into the gas phase as a rate-limiting step. The activation energy for the desorption term is 16.5 heal/mole, approximately equivalent to the heat of adsorption of ethylbenzene. For low molecular weight alkylates intracrystalline diffusion limitations do not exist. 

The chromatogram obtained for the extract of the cumene-deactivated parent H-mordenite is shown in Figure 9. The temperature profile-product distribution of the chromatogram is similar to that obtained by Venuto et al. (8, 4) in their studies on REX catalyst deactivation. They established the presence of condensed polynuclear aromatics in the REX adsorbate. 

The type of solvent or diluent should be specified in reporting a Ziegler-Natta catalyst system. Alkene polymerisations are usually carried out in inert solvents, such as aliphatic or aromatic hydrocarbons (e.g. some gasoline fractions or toluene). The use of protic or aprotic polar solvents or diluents instead of the hydrocarbon polymerisation medium can drastically alter the reaction mechanism. This usually results in catalyst deactivation for alkene coordination polymerisation. Modern alkene polymerisation processes are carried out in a gas phase, using fluidised-bed catalysts, and in a liquid monomer as in the case of propylene polymerisation [28,37]. 

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