(589a) Catalyst Design for Ethanol Selective Polymerization to Long Chain Alcohols and Aldehydes

Ethanol to jet fuel is a promising pathway for producing sulfur-free jet fuel from ethanol. Here we show the rational design of a highly selective bifunctional catalytic system to polymerize ethanol to higher alcohols and aldehydes which can be subsequently hydro-deoxygenated to C₆-C₁₆paraffins. This catalytic system consists of an amphoteric metal oxide promoted with transition metal nanoparticles (NP).

Diethyl ether is the main product from reactions of pure ethanol (1 to 10 kPa) on anatase titania (TiO₂) at 503 K, however, a significant selectivity shift towards C₄₊ aldehydes and alcohols was noticed when acetaldehyde (<0.5 kPa) was co-fed with ethanol  on the same material. The rate of formation of C₄₊ products reflects a second-order dependence on the aldehyde pressure, which suggests that the aldehyde is the reactive intermediate and C-C bond formation occurs through coupling of two adsorbed aldehydes via aldol condensation. Due to their mild strength, active sites on TiO₂are barely active for alcohol dehydrogenation to aldehyde, making this step the rate limiting when starting from ethanol. Interestingly, it was found out that alcohol has an inhibiting effect on the aldol reaction due to the competitive adsorption of the alkoxides on active sites.

A more acidic oxide such as gamma alumina was found to catalyze dehydration to ethers and hydrocarbon along with aldol condensation which lower the overall selectivity compared to that obtained on amphoteric (TiO₂).  A more basic oxide such as magnesium oxide selectively catalyzes aldol condensation but becomes rapidly poisoned by water and losses activity in few hours at reaction conditions.

From these finding, it is wise to maximize the ratio of the aldehyde to alcohol partial pressures in the reaction medium by promoting an amphoteric oxide with a dehydrogenation metallic function selected based on the metal surface electronic state and the resulting most stable aldehyde binding configuration. Despite their high dehydrogenation activity, Group 10 metals adsorb aldehydes in the η² mode¹ and decarbonylate these intermediates to produce undesirable shorter chain molecules. Group 11 metals, however, primarily bind aldehydes in the η¹ configuration¹, do not promote decarbonylation, but do produce small amounts of esters.

Esterification occurs with the greatest rates on highly coordinated metal terrace sites and with much lower rates at edges and corners.² We show the efforts made to synthesize highly dispersed particles with high edges and corners surface density utilizing ion exchange metal loading³ on supports that strongly stabilize the smaller NP. A comparison of the catalytic reactions of ethanol (1 to 10 kPa) and H₂(0 to 90 kPa) mixtures at 473 to 673 K on many different monometallic and bimetallic NP shows that small, highly reduced Cu NP give the greatest dehydrogenation selectivity.

The combination of these dehydrogenation and C-C bond forming functions allows highly selective (>95%) conversion of ethanol to C₄₊ alcohols and aldehydes, which continue to grow with longer residence times to longer chain aldehdyes and alcohols along with small amounts of termination products, such as  ethers, alkenes, and alkanes, which lack the carbonyl groups required for further coupling. This work shows how understanding the reaction mechanism along with the active sites properties can lead to the rational design of an effective catalyst.

References:

(1) M. Mavrikakis, M.A. Barteau. J. Mol. Catal. A: Chem. 1998, 131, 135

(2)  Sad, E. M.; Neurock, M.; Iglesia, A., J. Am. Chem. Soc. 2011, 133, 20384

(3)  Jiao, L.; Regalbuto, R.J.  J. Catal. 2008, 260, 329