This article reports the activity, reaction mechanism and reaction kinetics of 2-butene metathesis over tungsten oxide containing mesoporous silica (SBA-15) catalyst. Physicochemical characterisation of the catalyst indicates that the dispersion and nature of tungsten oxide species on the catalyst are mainly responsible for the metathesis activity. The ample availability of Brønsted acid sites created by tetrahedrally co-ordinated tungsten species also enhances the metathesis reaction. The product analysis indicates that lower temperature favours the butene isomerisation, while higher temperature is required for metathesis. The high temperature is also responsible for cracking reactions. A kinetics model is developed based on the experimental observations and the possible reactions including isomerisation, metathesis and cracking. The model parameters are estimated by fitting the experimental data implemented in MATLAB. The estimated apparent activation energy of 2-butene isomerisation reaction was found to be the lowest (39.4 kJ/mol) among the competing reactions. The activation energy of cross metathesis of 2-butene and 1-butene, self-metathesis of 1-butene and 2-butene cracking are 71.3, 176.9 and 73.1 kJ/mol, respectively. These results are consistent to the product selectivity and the proposed reaction scheme, which suggests that the isomerisation of 2-butene gives 1-butene and it further reacts (cross metathesis) with 2-butene to produce propylene.
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.
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