Preparation Of Catalyst With High Selectivity And High Stability For Para-Xylene

Para-xylene (PX) is an essential organic material for producing terephthalic acid (PTA). At the present time, principal sources of para-xylene in industry include pyrolysis gasoline, catalytic reforming and byproduct in coking; In addition, toluene disproportionation (TDP) or selective disproportionation of toluene (STDP), transalkylation, isomerization of mixed xylenes and adsorption separation are important processes in increasing yield of para-xylene. However, mixed aromatics of C8A are produced in processes above. PX accounts for only 4.2% in pyrolysis gasoline,7.5% in reformed gasoline and 22-26% in mixed aromatics of C8A. Therefore, energy consumption in separation is high. The dwindling of petroleum resource, however, will seriously affect the process of producing PX from petroleum. With spring up of domestic coal chemistry industry, the supplies of methanol tends to excessiveness. Thus, the technology of alkylation of toluene with methanol oriented to PX attracts great attention. Under conditions of high para-selectivity, however, fast deactivation of the catalyst has been the crux of the issue, which is the core problem should be solved if the process is expected to achieve industrialization.The zeolite of nano-scale H-ZSM-5(nH-ZSM-5) was chosen as precursor. Firstly, high para-selectivity(>98%) was achieved over nH-ZSM-5 modified with 6%SiO2,5%P2O5 and 3%MgO. The stability of catalyst in activity was not very good, however, when methanol as alkylating agentthe. Under the thesis's point of view, one of the essential reasons, perhaps the most important, was intermolecular dehydration of methanol and generation of low carbon olefins which were very active in chemical properties, especially ethylene and propylene. The succedent reactions, such as oligomerization, cyclic action and multi-alkylation, which will lead to deactivation by coke deposition. That overcoming the problem became to be the key of industrial application for the alkylation technics. Based on high para-selective nH-ZSM-5 catalyst, a hydrogenation metal (Pt, Pd, Co, Ni, etc.) was added. As a result, in situ hydrogenation of olefins into alkanes could be generated, the way of coke formation was blocked, it was possible to achieve high selectivity and high stability for the synthesis of PX.Under the optimized reaction conditions of atmospheric pressure,460℃, the mole ratio of toluene/methanol was eight (nT/nM=8:1), WHSV of feed(toluene and methanol) was two per hour (WHSV=2h-1), the mole ratio of carrier gas to feed was eight (n carrier gas/n(T+M) =8:1), the mole ratio of water to feed was eight (nH2O/n(T+M)=8:1), the conversion of toluene (CT) was 10-12% and para-selectivity (SPX) stablized at 98%, no signs of deactivation was found during the reaction of 1000 h on stream. Under the severe reaction conditions, that is, nT/nM=2:1, and the others the same as above. Conversion of toluene was 22-25%, selectivity of para-xylene stablized at 98% and the activity was still stability after 500 h on stream. The catalysts had excellent regeneration properties.