Ikai Wang

Disproportionation and transalkylation of alkylbenzenes over zeolite catalysts

Disproportionation and transalkylation are important processes for the interconversion of mono-, di-, and tri-alkylbenzenes. In this review, we discuss the recent advances in process technology with special focus on improvements of para-isomer selectivity and catalyst stability. Extensive patent search and discussion on technology development are presented. The key criteria for process development are identified. The working principles of para-isomer selectivity improvements involve the reduction of diffusivity and the inactivation of external surface. In conjunction with the fundamental research, various practical modification aspects particularly the pre-coking and the silica deposition techniques, are extensively reviewed. The impact of para-isomer selective technology on process economics and product recovery strategy is discussed. Furthermore, perspective trends in related research and development are provided.

Nonoxidative dehydrogenation of ethylbenzene over TiO2ZrO2 catalysts: II. The effect of pretreatment on surface properties and catalytic activities

Catalytic activities for the nonoxidative dehydrogenation of ethylbenzene were determined for TiO2ZrO2 catalysts which had been calcined at temperatures between 550 and 1000 °C. The acidic and basic properties and surface areas of these catalysts were also measured. The change of crystal structure with calcination temperatures was studied by XRD analysis. The maximum of catalytic activity was obtained by calcination at about 650 °C with an equal-molar ratio of TiO2 and ZrO2. This high activity may be due to the formation of ZrTiO4 crystal. The activity of the catalyst was poisoned not only by K2O but also by H3BO3. Based on the effects of pretreatment with H2, O2, and steam, structures are proposed for the catalytically active sites. It is concluded that the formation of ZrTiO4 crystal as well as the amounts of acid and base sites are the major factors affecting the catalytic activity.

Nonoxidative dehydrogenation of ethylbenzene over TiO2ZrO2 catalysts: I. Effect of composition on surface properties and catalytic activities

The acid-base solid, TiO2ZrO2, was examined as a potential catalyst for nonoxidative dehydrogenation of ethylbenzene. The acidic and basic properties and surface area of these catalysts were measured. A good correlation was found between the activity for this reaction and the surface acid-base properties of the catalysts. The high catalytic activity occurred as the two components, TiO2 and ZrO2, are presented at compatible amount. The selectivity of styrene increased monotonically with the increase in TiO2 content. The activity was poisoned not only by K2O but also by H3BO3, indicating that the active sites consist of both acidic and basic sites. A concerted two-center mechanism was proposed.

Dehydrocyclization of C6C8 n-paraffins to aromatics over TiO2ZrO2 catalysts

This study examined the catalytic behavior of Ti02ZrO2 and the mechanism of n-paraffins dehydrocyclization. The results showed that TiO2-ZrO2 calcined at 823 K gave higher aromatic selectivity and activity than other catalysts calcined at higher temperatures. The activity can be poisoned by adding small amounts of K20 or B203 onto Ti02-ZrO2. The aromatization rate was correlated with the site density of the paired acid-base sites on the catalysts. The results indicated that the active centers consisted of both acid and base sites, and that only the paired acid-base sites played the primary role in the dehydrocyclization reaction. The results also showed that the apparent overall kinetics of n-heptane conversion was a first-order reaction. It was conclusively shown that the five-membered ring alkylcyclopentanes were not the intermediates in the transformation of n-paraffins into aromatics. The results showed that the n-paraffin was first adsorbed on the paired acid-base sites, and two hydrogen atoms were abstracted by the catalyst. Then the adsorbed molecules proceeded to cyclization through a direct six-membered ring closure without desorption, before dehydrogenation to aromatics.

Para-selectivity of dialkylbenzenes over modified HZSM-5 by vapour phase deposition of silica

Abstract HZSM-5 was modified by both the impregnation and vapour-phase deposition methods. The modified catalysts were then applied to various reactions to make p-dialkylbenzenes. The results show that catalysts modified by the vapour-phase deposition of silica are always superior to those modified by the impregnation method. From the results of the diffusivity measurement, β-naphthoquinoline (5,6-dibenzoquinoline) poisoning, steam effects, and the para-selectivity of different types of dialkylbenzenes, we conclude that the para-isomer might be the primary product within the zeolite pores and that the other isomers could only have been formed by re-isomerization on the external surface sites of the zeolite crystallites.

Cumene disproportionation over zeolite β. I: Comparison of catalytic performances and reaction mechanisms of zeolites

Abstract Cumene disproportionation has been demonstrated as an accurate probe reaction for zeolite structures. Over twelve-membered ring zeolites, cumene undergoes a bimolecular mechanism; but over ten-membered ring zeolites, it undergoes a monomolecular mechanism. Zeolite β, a highly siliceous twelve-membered ring zeolite, shows excellent activity, disproportionation selectivity and stability in cumene disproportionation and therefore has practical potential in the production of diisopropylbenzenes.

On the thermal stability of zeolite beta

Abstract The thermal stability of zeolite beta has been studied by 129Xe NMR and adsorption isotherm of adsorbed xenon in correlation with data from X-ray diffraction and 27Al magic-angle-spinning NMR experiments. Samples subject to different calcination and dehydration conditions were examined. Minor destruction of the crystalline framework resulting from dealumination processes has been found for samples treated at a temperature as small as 400°C. At higher temperatures, a severe process of dealumination involving the breaking of 12-membered rings leads to the conclusion that the maximum regeneration temperature of zeofite beta lies near 760°C.

Activity stability of a copper(II) oxide—zinc(II) oxide catalyst for oxidative dehydrogenation of cyclohexanol to cyclohexanone

Abstract Oxidative dehydrogenation of cyclohexanol to cyclohexanone over a commercial CuO ZnO catalyst was studied in a fixed-bed microreactor. Experimental results showed that the catalyst maintained constant activity at low oxygen: cyclohexanol mol ratios and its stability was dependent on the raction temperature. Both the activity and stability of the catalyst could be improved by using nitrous oxide instead of oxygen as the oxidant or by modifying the catalyst with palladium oxide or heteropoly acid (K3PMo12O40·3H2O). An analysis of the deposit on the spent catalyst showed the existence of oligomers of cyclohexanone. The accumulation of these higher molecular weight products on the catalyst is believed to be the major cause of the catalyst deactivation.

Catalytic hydrodesulfurization and hydrodenitrogenation over CoMo on TiO2ZrO2V2O5

Hydrodesulfurization (HDS) of dibenzothiophene and hydrodenitrogenation (HDN) of aniline over Co-Mo on TiO{sub 2}-ZrO{sub 2}-V{sub 2}O{sub 5} catalysts over Co-Mo on {gamma}-Al{sub 2}O{sub 3} catalysts were investigated and compared in a continuous-flow microreactor at 240-350{degree}C and 3.55 MPa. HDS and HDN activities on Ti/Zr/V-supported catalysts depended on the amounts and the order of impregnation of molybdenum and cobalt. It was found that Ti/Zr/V-supported catalysts has higher HDS and HDN activities than alumina-supported catalysts. The optimal amounts of CoO and MoO{sub 3} on TiO{sub 2}-ZrO{sub 2}-V{sub 2}O{sub 5} were much less than on alumina. Moreover, the mutual inhibition of HDS and HDN over Ti/Zr/V-supported catalysts was less pronounced than that over alumina-supported catalysts. The Ti/Zr/V-supported catalyst was also more active than the commercial catalyst, HR-306, in hydrodesulfurization of petroleum feedstocks.

Surface acidity and basicity of TiO2ZrO2V2O5: Dehydrogenation and isomerization of cyclohexane

The ternary oxides, TiO2ZrO2V2O5: with an equal molar ratio of TiO2 and ZrO2, were characterized by the dehydrogenation and isomerization reactions of cyclohexane. The effects of V2O5 molar ratio on the acidic and basic properties, crystal structure, VO species, and the reaction activities were investigated. The maximum activities of both reactions were observed at the V2O5 molar ratio of 0.1 when the calcination temperature was 600 °C. That correlated well with surface acidic sites. The crystalline separation of ternary oxides was observed at a V2O5 molar ratio higher than 0.25. The reaction of dehydrogenation was retarded by K2O but not significantly by B2O3, which indicates that the acidic sites play an important role in dehydrogenation reaction. A stepwise bifunctional mechanism was proposed.