Effective Low-Temperature Arematization of Ethane over H-Galloaluminosilicate(MFI) Zeolites in the Presence of Higher Alkanes or Olefms

摘要

Over the last seven to eight years, aromatization of lower alkanes (C_3 and C_4) over Ga-modified zeolites of type ZSM-5 has been widely investigated, and commercial processes for aromatizing propane and higher alkanes have been developed. Natural gascontains an appreciable amount of ethane (upto about 10 mol %). In addition, ethane can be produced in large quantities by oxidative coupling of methane, but, because of its low concentration, separation of ethane from the product stream by conventionalmeans is difficult or uneconomical. Ethane is also formed to an appreciable extent in the Cy-clar LPG aromatization process as an undesirable byproduct. Its conversion into arenes would, therefore, be of great practical importance. However, because of its very low reactivity and high ther'modynamic barrier to aromatization, ethane can be converted and yields of arenes that are of practical significance obtained only at high temperatures (>=600°C). Conversion of ethane into arenes with high yield and selectivity at lower temperatures would not only be of scientific interest but also have a great impact on the natural gas conversion technology. Here we show that the therrnodynamic barrier can be overcome and that the conversion and yield of arenes in the aromatization of ethane over H-galloalluminosili-cate of ZSM-5 type (H-GaAlMFI) zeolite at lower temperatures (400-500 °C) can be increased manyfold (4-80 x) by adding olefins or higher alkanes to the feed. Furthermore, we show that the ethane-ethylene conversion mechanism is changed in the presence of olefins or higher alkanes; the large enhancement in the reactivity of ethane results from a hydrogen-transfer reaction between ethane and higher (C_(3+)) oiefins.