Solid acidic catalysts for the production of bio-substitutes for petrochemical intermediates.

摘要

Light olefins such as ethylene and propylene are the most important chemical monomers used for the production of plastics and synthetic fibres. They are usually obtained by cracking (steam cracking or fluid catalytic cracking) of hydrocarbons from petroleum gas oils, and are generally called petrochemical intermediates. As we know, ethylene can also be produced through dehydration of ethanol obtained by fermentation of biomass materials (bioethanol). This may be an alternative way for obtaining the petrochemical intermediates.;In the present work, catalytic dehydration of aqueous ethanol (ethanol in aqueous solution) over some solid acidic catalysts, such as dealuminated zeolite X(also called silica nanoboxes), silica nanoboxes acidified by incorporation of triflic acid, zeolite Y, zeolite USY (Ultra Stable zeolite Y), zeolite ZSM-5 etc. (all in acidic form) has been studied. Dealuminated zeolite X has been obtained by AHFS (ammonium hexafluorosilicate) dealumination technique and has been fully characterized. Influences of experimental conditions on the textural properties of the porous products in dealumination of NaA, NaX, NH4Y zeolites and Na mordenite by AHFS treatment have been investigated. It has been found that the decreasing order of the resistance to AHFS dealuminating action is as follows: Na mordenite > NH4Y zeolites > NaX zeolite > NaA zeolite; and only NaA and NaX zeolites can be used as starting materials for preparation of mesoporous aluminosilicates by pore enlargement technique of AHFS dealumination. Mesoporous materials obtained from X zeolite(also called silica nanoboxes) show a high hydrothermal stability and a quite interesting ion-exchange capacity.;Zeolite ZSM-5/50 (SiO2/Al2O3 ratio is 50) shows the highest catalytic activity mostly when compared to the faujasite type zeolites. The order of catalytic activity for the catalysts is explained by their surface acidity (density and strength). Therefore, the second part of this study was based on the zeolite ZSM-5. For the ZSM-5 series, an increase of SiO2/Al2O3 ratio leads to the decrease in catalytic activity because of a decrease in their surface acidity.;The effects of reaction temperature, the WHSV (weight hourly space velocity) and the concentration of ethanol in water on the total conversion of bioethanol and on selectivity to ethylene and propylene over zeolite ZSM-5/50 are also investigated. It has been found that with increasing reaction temperature, total conversion of bioethanol increases, and nearly reaches 100% at 250°C. In temperature range of 200--250°C, the main product is ethylene. When the reaction temperature is higher than 250°C, the selectivity to other light olefins such as propylene and BTX aromatics significantly increases due to the further conversion of ethylene on strong acid sites. At 275°C, the yield of ethylene plus propylene reaches a maximum value.;The results also show that the yield of ethylene decreases when the WHSV increases and the yield of ethylene slightly increases when the concentration of ethanol in water increases.