A Bifunctional Tungstate Catalyst for Chemical Fixation of CO₂ at Atmospheric Pressure

Abstract

No pressure: A simple monomeric tungstate, [WO4]2−, serves as a highly efficient homogeneous catalyst for various transformations of CO2 at atmospheric pressure. The tungsten‐oxo moiety activates CO2 and the substrate simultaneously. The catalyst system is high yielding and applicable to a wide range of substrates such as amines (see scheme), 2‐aminobenzonitriles, and propargylic alcohols.

Base Catalysis by Mono- and Polyoxometalates

“…Thus, various types of POMs can be synthesized in both aqueous and organic media by control of the acidity, concentration, and temperature. Mizuno and co-workers were the first to focus on the high charge density of [WO4] 2− in comparison with the other polytungstates and applied [WO4] 2− to base-catalyzed reactions [38,[42][43][44][45]. Various tungstate structures including [WO4] 2− were optimized through density functional theory (DFT) calculations, and the basicities were estimated using the natural bond orbital (NBO) charges of the tungstate oxygen atoms.…”

“…Table 1. Despite the weaker basicity of [WO 4 ] 2− (pK a of conjugate acid in water: 3.5), TBA 2 [WO 4 ] exhibited much higher catalytic activity than the strong inorganic and organic bases for the chemical fixation of CO 2 with various nucleophiles due to its bifunctional action towards CO 2 and substrates [38,42,45]. The combination of TBA 2 [WO 4 ] with various transition metal species such as Ag + [43,44,48,54].…”

“…The solution-state 17 O NMR spectra of [V10O28] 6− (enriched by the addition of H2 17 O) show an up-field shift of the 17 O NMR signals assignable to OB and OC upon acidification with HCl [71,74]. It has been reported that the protonation of oxygen atoms in POMs [38,55,56]. …”

“…Therefore, the low-energy catalytic fixation of CO 2 is highly desirable, and several effective catalytic systems have been reported [110][111][112][113][114][115][116]. Mizuno and co-workers reported that a simple monomeric tungstate TBA 2 [WO 4 ] could act as a highly efficient homogeneous catalyst for the chemical fixation of CO 2 with amines, 2-aminobenzonitriles, and propargylic alcohols to urea derivatives, quinazoline-2,4(1H,3H)-diones, and cyclic carbonates, respectively (Schemes 1-3) [38,42,45]. For the reaction of 1,2-phenylenediamine with CO 2 at atmospheric pressure (0.1 MPa), the reaction rate of TBA 2 [WO 4 ] was more than 48 times larger than those of other catalysts including other POMs and typical strong inorganic and organic bases.…”

One-Pot Synthesis of N,N′-dialkylureas via Carbonylation of Amines with CO2 Applying Y0.08Zr0.92O1.96 Mixed Oxide (YSZ-8) as a Heterogeneous Catalyst

“…Either harsh conditions (200 °C, and CO2 pressures higher than 10 MPa) or the addition of stoichiometric amounts of dehydrating agents (such as dicyclohexylcarbodiimide, PCl5, POCl3, and so on) are usually needed to shift the equilibrium towards the desired product (Scheme 1) [14,15], which would inevitably lead to a series of side products. However, the adoption of a proper catalyst could be an ideal solution, such as Y2(C2O4)3 [9], K3PO4 [16], 1,5-diazabicyclo[4.3.0]non-5-ene (DBU) [17], guanidine [18], Cs2CO3 [19], CsOH/[bmim] OH [20], polymer-immobilized nanogold (Au@polymer) [21], KOH/polyethylene glycol [22], TBA2[WO4] (TBA = [(n-C4H9)4N] + ) [23], and cesium benzotriazolide [24]. It is of particular interest that the employment of Au@polymer as a heterogeneous catalyst resulted in an appreciable yield of N,N′-dialkylureas.…”