Resumen de Ethyl octyl ether synthesis from 1-octanol and ethanol or diethyl carbonate on acidic ion-exchange resins
Jordi Guilera Sala
Ethyl octyl ether is a bioethanol-derived component that has excellent properties as diesel fuel. This work proved that ethyl octyl ether can be produced successfully in liquid-phase at the temperature range of 130-190ºC by using acidic ion-exchange resins, as suitable and economic catalysts. The use of two promising reactants that can be a renewable compound source, ethanol and diethyl carbonate, have been explored. Both reactants are able to ethylate 1-octanol and form the desired product. However, an identical industrial drawback is observed on both reactants, the loss of ethyl groups to form diethyl ether, which is not suitable as diesel compound. In order to minimize the diethyl ether formation, and in this way, to maximize the ethyl octyl ether production; several commercial acidic resins were tested, or else, prepared and subsequently tested. The best catalysts are those allowing 1-octanol to access to most sulfonic groups of the catalyst. Such desired properties can be achieved by decreasing the amount of crosslinking agent of resins, as a result, the resin has a high capacity to swell and at the same time a low gel-phase density. Another tailoring technique that lets 1-octanol to access to the vast majority of sulfonic groups is by locating them only in the least crosslinked domains of the gel-phase. Both tailoring techniques involve higher selectivity to ethyl octyl ether, which can be extrapolated to other bulky molecules. However, the former involves a reduction of the catalytic activity per volume unit of the catalyst bed, and the latter, per mass unit. Interestingly for the resin designers and exploiters, it is proved that the Inverse Steric Exclusion Chromatography characterization technique allows predicting the catalyst performance in polar environments with high accuracy. In such a manner that polymeric catalysts having high specific volume of the swollen gel-phase and predominant domains with low polymer density are desired to enhance selectivity and yield to ethyl octyl ether formation. The comparison between both ethylating agents, ethanol and diethyl carbonate, revealed that similar selectivity and yield can be potentially obtained over acidic resins. Nevertheless, diethyl carbonate is less competitive at shorter reaction times in a batch reactor, or at lower catalyst mass in continuous units, as a result of the slow decomposition of the required intermediate, ethyl octyl carbonate. On the other hand, the production of CO2 via diethyl carbonate and the availability of ethanol nowadays suggest that use of the alcohol to form ethyl octyl ether is preferred. Reaction rates to form ethyl octyl ether from ethanol and 1-octanol showed similar, or slightly higher, dependency on the temperature than that to form the main side product, diethyl ether. Thus, an enhancement of the reactor temperature clearly increases the feasibility of an ethyl octyl ether production unit. Accordingly, the use of chlorinated resins, which proved to be thermally stable up to 190ºC in the ethyl octyl ether production, is desired. Among the commercial ones, Amberlyst 70 is the most suitable catalyst in terms of selectivity to ethyl octyl ether due to its low polymer density in aqueous swollen state. Such polymeric expansion should be taken into account to not block the liquid flow when fixed-bed reactors are employed. That is to say, Amberlyst 70 must be loaded to the reactor in a swollen state. The relatively large values found of the thermodynamic equilibrium constant of ethyl octyl ether formation assure high conversion levels in an industrial etherification process. Interestingly, the equilibrium values of the formation of diethyl ether are around a half than those of ethyl octyl ether (150-190ºC). A comprehensive kinetic analysis enlightened that reaction rates to form ethyl octyl ether on Amberlyst 70 are strongly inhibited by the presence of water. Thus, reaction rates would be enhanced if most water is removed from bioethanol.
