Gas phase concomitant dehydrations of ethanol and 1-butanol to alkenes mixture useful in a successive metathesis reaction to produce propylene
Résumé
In the future, a strong increase in the demand for propylene is expected because of the growing markets for polypropylene and propylene oxide. In the other hand next generation of steam crackers will use shale gas ethane as raw material and less by-products will be available for propylene production. There is thus a need to develop bio-propylene. The later could be produce by dehydration of 1 or 2-propanol but contrarily to ethanol and 1-butanol, there are not large-scale industrial processes to produce them. One way to overcome this problem would be to produce propylene by metathesis from ethylene and 2-butene. The process would be economically attractive if 2-butene is produced by dehydration of 1-butanol, which is the isomer predominantly formed by fermentation and the number of reaction steps minimized. This can be done by running the dehydration of ethanol and 1-butanol in one step without water pre-separation. In this study, we report on the catalytic properties of rare earth phosphates tested for the dehydration of alcohols mixtures. The study of the catalytic properties of these solids has been extended to ABE (acetone-butanol-ethanol) mixtures produced by bacterial fermentation. Nd, Gd and Sm phosphates with Rhabdophane structure have been prepared by co-precipitation [1]. The acid-base properties of the catalysts have been studied by NH3- and CO2-TPD and Lutidine adsorption followed by IRTF to correlate these properties to the catalytic properties [2]. The phosphates have been tested in the dehydration of ethanol and 1-butanol mixtures (Fig.1). Activity in dehydration of ethanol was in the order: Gd>Sm>Nd but inverse for activity in 1-butanol and 2-butene selectivity. The results were from acid-base properties. Gd is more active for ethanol dehydration because it has more strong Br?nsted acid sites compared to Sm and Nd. But Sm and Nd have more weak and moderate acids sites, which make them more active to dehydrate 1-butanol and more selective to 2-butene. In any case, only 1-butene is formed besides 2-butene when 1-butanol is dehydrated. Since ethanol dehydration takes preferentially place on stronger acid sites, there is little competition between the reactions and total conversion of the 2 alcohols is observed. The catalysts were also shown to be very efficient to dehydrate ABE mixture (acetone, butanol, ethanol in 3-6-1 ratio).The conclusion of the work is that rare earth phosphates can dehydrate selectively alcohol mixtures or ABE mixtures produced by industrial fermentation processes. The catalysts are very selective for all alcohols in the same time and stable with time on stream. 1-butanol is preferentially dehydrated to 2-butene, which can be fruitfully used to transform ethanol 1-butanol mixtures and subsequently performed metathesis reaction to produce propene. Industrially they open the way to build a large-scale process to produce propene from alcohols minimizing purification/separation steps energetically costly.