The effect of coke deposition on the activity and selectivity of the HZSM-5 zeolite during ethylbenzene alkylation reaction in the presence of ethanol

摘要

The alkylation of ethylbenzene (EB) with ethanol over HZSM-5 zeolite catalysts was carried out using a riser simulator reactor at different reaction temperatures and contact times. It was observed that the amount of coke deposited over the zeolite has a great influence on the reaction pathway. At higher temperature (400 °C), the spent catalyst was found to exhibit a much higher activity towards the alkylation products (DEB) as compared to the fresh catalyst. This difference between spent and fresh samples became less significant at lower temperature (250 °C). The highest yield of DEB products over the spent catalyst was obtained for intermediate temperatures (300-350 °C). The coke deposition was further analyzed using terahertz time-domain spectroscopy (THz-TDS), ~(27)Al MAS NMR spectroscopy, TPO measurements and pyridine-FTIR. THz-TDS and TPO results revealed that the structure of coke formed on all catalysts is essentially the same, while pyridine-FTIR studies revealed that coke leads to a reduction in the acidity of the catalyst. ~(27)Al MAS NMR results of spent samples suggested a relationship between alkylation activity and extra-framework aluminium species, which is possibly associated with the formation of very active Lewis sites. This work shows that an understanding and control of different types of catalytic sites over zeolite surfaces may improve and optimize reaction performances during alkylation of aromatics.