Metal-Free Reduction Creates Highly Active TiO₂ Surfaces for Fatty Acid Ketonization

Abstract

TiO2 is one of the most promising catalysts for liquid-phase ketonization of carboxylic acids. As recently reported, its use significantly contributes to the overall environmental impact of the fatty acid ketonization process. Therefore, postsynthetic modifications such as metal addition and thermal treatment under inert and reducing atmospheres were studied to enhance the catalytic activity of anatase TiO2 for the ketonization of lauric acid. Although more active surfaces can be generated by the additional metal through hydrogen spillover effects, its presence led to substantial decarboxylation rather than ketonization. Interestingly, metal-free anatase TiO2 is also substantially activated after thermal treatment in nitrogen or hydrogen gas, while maintaining its full ketonization selectivity. Ascertained by X-ray analysis, this was not related to polymorphism but due to a higher content of Lewis acid sites, of which the hydrogen-treated catalysts possessed the largest density. A quadratic relationship appears between the ketonization rate and surface acid site density, a defining characteristic of rate-determining bimolecular C–C coupling in the ketonization mechanism. In contrast to studies on short-chain carboxylic acids, this liquid-phase study renders hydrogenated TiO2 as the most promising catalyst, while metal addition is less suitable for selectivity reasons. Although less essential for bio-oil upgrading, high selectivity is of utmost importance for on-purpose fatty acid ketonization.

Decarboxylative Ketonization of Aliphatic Carboxylic Acids in a Continuous Flow Reactor Catalysed by Manganese Oxide on Silica

“…When attempting to transport hexanoic acid into the reactor with the help of a carrier gas, [7d,16,22a,35] low and varying substrate flow rates were observed. In contrast, consistent, well controllable flow rates were achieved by pumping the hexanoic acid into the hot reactor in liquid form with an HPLC pump.…”

“…Other groups such as Ponec, [13] Yamada, [14] Ignatchenko, [15] Sels [11c,16] and Bell [6a,d] reported that CeO 2 , [17] MnO 2 , [18] PbO, [19] ThO 2 , [20] ZrO, [21] MgO, [12,22] and Fe 2 O 3 [11a,12] performed particularly well in their specific transformations, which mainly targeted the synthesis of short‐chain ketones. Branched acids seem to react particularly well with sodium, [23] potassium [24] or zinc [25] catalysts, as reported by Renz and others.…”

Crystal facet dependence of the ketonization of propionic acid on anatase TiO2

“…This result indicates that the acid and base sites influence different elementary steps, and their effects may cancel each other. 13 Previous work showed that the presence of the oxygen vacancy can improve the ketonization activity 17,23,85 by stabilizing the reaction intermediates and reducing the activation energy. 31 However, no linear correlation between the concentration of the surface oxygen vacancy and ketonization rate can be identified (Fig.…”

“…Among these oxides, TiO 2 catalysts have been widely studied due to the low price and excellent activity. 14,[21][22][23][24][25][26] Although various mechanisms (such as bulk, ketene and anhydride) have been proposed for ketonization, 27,28 the β-ketoacid mechanism is now generally accepted. 14,29,30 As briefly shown in Scheme 1, carboxylic acid adsorbs dissociatively on one or two Ti-O pairs to form monodentate or bidentate carboxylates.…”

“…Among these oxides, TiO 2 catalysts have been widely studied due to the low price and excellent activity. 14,21–26…”