Condensed-Phase Ethanol Conversion to Higher Alcohols

Abstract

Higher alcohols (C 4 +) can be formed from ethanol via condensation pathways collectively known as Guerbet reactions. Most prior Guerbet reaction studies involve vapor phase reactions, with n-butanol yields typically no higher than 30% of theoretical. We report here condensed-phase Guerbet reactions of ethanol over Ni/γ-Al 2 O 3 catalysts modified by La 2 O 3 .Higher alcohol selectivities in excess of 80% at 230 o C and autogeneous pressures are obtained in batch autoclave reactions. At these conditions, which are near the critical temperature of ethanol, the liquid phase is significantly expanded, byproduct gases (CH 4 and CO 2 ) are significantly dissolved in the liquid phase, and the vapor phase contains significant quantities of alcohols. To accurately compute ethanol conversion and product yields, both composition and quantity of each phase present at reaction conditions must be determined. To do this, the S-R Polar equation of state is combined with chromatographic analysis of liquid phase samples taken during reaction to model the phase equilibrium in the reactor at reaction conditions. Composition, density, and total number of moles of the vapor and liquid phases in the reactor are determined from the model and analysis, and are used to calculate more accurate values of conversion and product yield than those calculated by liquid phase samples alone.

Catalytic Upgrading of Ethanol to n‐Butanol: Progress in Catalyst Development

“…Moreover,o ther unwanted byproducts, such as diethyl ether and ethyl acetate, were also collected, mainly due to the presence of Lewis and Brønsted acid sites on the surface of aluminum oxide.T he mechanisms for the formation of diethyl ether [44] and ethyl acetate [45] are shown in Schemes 7a nd 8, respectively.W ith attempts to reduce the formation of diethyl ether and ethyl acetate, and increaset he selectivity towards the products througha ldol condensation, La 2 O 3 -modified 8% Ni/Al 2 O 3 was appliedb yJ ordison and co-workers. [46] As ar esult, a3 9% yield of highera lcohols (C 4 +)a nd 55 %e thanol conversion were obtained, in comparison to 26 %y ield of highera lcohols and 46 %c onversion of ethanol over pure Ni/Al 2 O 3 catalyst;t his indicates that the number of basic sites is crucial for improving the catalytic performance. Based on the product distribution over Ni catalysts, the dominantr eaction route for this ethanol catalytic coupling also follows the Guerbet pathway.…”

“…The mechanisms for the formation of diethyl ether and ethyl acetate are shown in Schemes and , respectively. With attempts to reduce the formation of diethyl ether and ethyl acetate, and increase the selectivity towards the products through aldol condensation, La 2 O 3 ‐modified 8 % Ni/Al 2 O 3 was applied by Jordison and co‐workers . As a result, a 39 % yield of higher alcohols (C 4 +) and 55 % ethanol conversion were obtained, in comparison to 26 % yield of higher alcohols and 46 % conversion of ethanol over pure Ni/Al 2 O 3 catalyst; this indicates that the number of basic sites is crucial for improving the catalytic performance.…”

“…Based on a previous study, the aldol condensation of acetaldehyde is considered to be the rate‐determining step, whereas the adjacent Lewis acid–base pairs are responsible for this key step. Thus, interest in exploring rare‐earth‐metal oxides (La 2 O 3 and CeO 2 ) as basic modifiers of catalysts for the conversion of ethanol into n ‐butanol has increased considerably in recent years . However, attention should also be paid to the development of suitable Lewis acid sites because the requirement for the presence of strong base sites can be alleviated, if Lewis acid centers are present.…”

“…As mentioned above, although Ni metal is active for ethanol dehydrogenation, it is followed by the formation of a considerable amount of gaseous carbon products (through ethanol cracking, decarbonylation of acetaldehyde, etc. ), such as CO, CO 2 , and CH 4 , due to the strong cracking capability of the C−C bond of ethanol over Ni metal . Concerning this point, Cu‐based catalysts are more preferable in the upgrading of ethanol to n ‐butanol because the cleavage of C−C bonds is not favorable over Cu metal .…”

“…Thus, interesti ne xploring rare-earth-metal oxides( La 2 O 3 and CeO 2 )a sb asic modifiers of catalysts for the conversion of ethanol into n-butanol has increased considerably in recent years. [30,46,49,53,55,56] However,a ttention should also be paid to the development of suitable Lewis acid sites because the requirementfor the presence of strong base sites can be alleviated, if Lewis acid centers are present. Moreover,t he Lewis acidbase pairs, if introduced by the modifierors upport in the catalysts, would be affected by the properties of the metal loaded.…”