An Unexplored O₂‐Involved Pathway for the Decarboxylation of Saturated Carboxylic Acids by TiO₂ Photocatalysis: An Isotopic Probe Study

Abstract

The aerobic decarboxylation of saturated carboxylic acids (from C(2) to C(5)) in water by TiO(2) photocatalysis was systematically investigated in this work. It was found that the split of C(1)-C(2) bond of the acids to release CO(2) proceeds sequentially (that is, a C(5) acid sequentially forms C(4) products, then C(3) and so forth). As a model reaction, the decarboxylation of propionic acid to produce acetic acid was tracked by using isotopic-labeled H(2)(18)O. As much as ≈42% of oxygen atoms of the produced acetic acids were from dioxygen ((16)O(2)). Through diffuse reflectance FTIR measurements (DRIFTS), we confirmed that an intermediate pyruvic acid was generated prior to the cut-off of the initial carboxyl group; this intermediate was evidenced by the appearance of an absorption peak at 1772 cm(-1) (attributed to C=O stretch of α-keto group of pyruvic acid) and the shift of this peak to 1726 cm(-1) when H(2)(16)O was replaced by H(2)(18)O. Consequently, pyruvic acid was chosen as another model molecule to observe how its decarboxylation occurs in H(2)(16)O under an atmosphere of (18)O(2). With the α-keto oxygen of pyruvic acid preserved in the carboxyl group of acetic acid, ≈24% new oxygen atoms of the produced acetic acid were from molecular oxygen at near 100% conversion of pyruvic acid. The other ≈76% oxygen atoms were provided by H(2)O through hole/OH radical oxidation. In the presence of conduction band electrons, O(2) can independently accomplish such C(1)-C(2) bond cleavage of pyruvic acid to generate acetic acid with ≈100% selectivity, as confirmed by an electrochemical experiment carried out in the dark. More importantly, the ratio of O(2) participation in decarboxylation increased along with the increase of pyruvic acid conversion, indicating the differences between non-substituted acids and α-keto acids. This also suggests that the O(2)-dependent decarboxylation competes with hole/OH-radical-promoted decarboxylation and depends on TiO(2) surface defects at which Ti(4c) sites are available for the simultaneous coordination of substrates and O(2).

Understanding the oxidative dehydrogenation of ethyl lactate to ethyl pyruvate over vanadia/titania

“…The C=O stretching of ethyl lactate was weakened (1730 cm -1 and 1667 cm -1 ), accompanying two new shoulder peaks at 1780 cm -1 and 1651 cm -1 . These bands were assigned Please do not adjust margins Please do not adjust margins to the carbonyl stretching of the α-keto group of the pyruvate, 57 supporting the experimental results (Fig. S5), the surface chemisorbed oxygen of V 2 O 5 /TiO 2 could participate in the oxidation of lactate without molecular oxygen.…”

“…[55][56] Those bands decreased in intensity with rising temperature, indicating the degradation of ethyl lactate. 57 This process was also reflected by the changes of the carbonyl and carboxyl on the V 2 O 5 /TiO 2 surface (four peaks at 1730 cm -1 ,1667 cm -1 , 1566 cm -1 and 1420 cm -1 , assigned to ν (C=O), ν (C=O•••M + ), ν s (COO) and ν as (COO) vibrations, respectively). 55,[58][59] Increasing the temperature broadens those carbonyl and carboxyl peaks, where the signals of ν s (COO) and ν as (COO) were blue shifted to 1540 cm -1 and 1445 cm -1 over 250 °C.…”

“…11). Upon increasing temperature to 150 °C, we observed the characteristic bands of α-keto in pyruvate (1795 cm -1 and 1651 cm -1 ), 57] together with a red-shifted COO stretch at 1597 cm -1 . 55,62 The in situ DRIFTS were also reflected the adsorption and dissociation of ethyl lactate on V 2 O 5 /TiO 2 surface in the presence of oxygen.…”

In situ photodeposited nanoCu on TiO2 as a catalyst for hydrogen production under UV/visible radiation

“…After reaction 9, under deaerated conditions, carbon centered radicals react with photogenerated electron (reaction 10), thus preventing the formation of hydrogen [36,37], or give at a lower extent a cross termination (reaction 11):…”

Surface-Modified Photocatalysts

“…To add some complication, often the net result of a primary step of a chemical reaction could be the result of both oxidative and reductive processes. For example, it was recently showed through isotopic-labeling examinations combined with DRIFTS and electrochemical experiments that, contrary to the radical Kolbe decarboxylation, the decarboxylation of saturated carboxylic acids is possible by a concerted mechanism [69]. At first step, the pristine acid is oxidized to α-keto acid by holes or (adsorbed) OH radicals without loss of carbon atoms, and then, the intermediate α-keto acid is decarboxylated to shorter-chain acid through an e CB /O 2 process, in which an oxygen atom of O 2 is selectively incorporated into product.…”