Selective oxidation of 1,2-propanediol to lactic acid catalyzed by hydroxyapatite-supported Pd and Pd–Ag nanoparticles

Abstract

Selective catalytic oxidation of 1,2-propanediol is an alternative, effective, and environmentally benign method for producing lactic acid. Hydroxyapatite-supported Pd (Pd/HAP) and Pd-Ag (Pd-Ag/HAP) catalysts were prepared by the sol-immobilization method. The as-prepared catalysts were characterized by XRD, TEM, HRTEM, XPS, BET, and CO 2 -TPD techniques. The average particle sizes of Pd nanocubes in Pd/HAP catalysts were tuned from 6 to 18 nm by changing the amount of KBr and PVP, which significantly affected the 1,2-propanediol conversion and the lactic acid selectivity. Pd/HAP catalyst with cubic Pd nanoparticles was more selective to lactic acid than Pd/HAP catalyst with spherical Pd nanoparticles in the oxidation of 1,2-propanediol. The coalesced Pd and Ag nanoparticles in Pd-Ag/HAP bimetallic catalysts synergistically catalyzed the oxidation of 1,2-propanediol to lactic acid. The support HAP with high basicity promoted the oxidation of 1,2-propanediol to lactic acid. When the catalytic oxidation of 1,2-propanediol with O 2 was carried out at 100 C for 2 h in an alkaline solution, the lactic acid selectivity was 86.2% at the 1,2-propanediol conversion of 96.2% over Pd 2 /HAP-6 catalyst while the lactic acid selectivity was 88.8% at the 1,2-propanediol conversion of 86.3% over Pd 1 Ag 1 /HAP catalyst.The activation energies for the catalytic oxidation of 1,2-propanediol were in the order of E a (

Ag-Based Catalysts in Heterogeneous Selective Oxidation of Alcohols: A Review

“…Various catalytic materials were proposed for the selective oxidation of PG [101,138]. Xue et al [98,139,140] Kolobova et al [136] studied several silver-promoted titania catalysts with or without the addition of Ce, Fe and Mg oxides in selective oxidation of n-octanol and betulin. The highest conversion value of n-octanol of 12% after 6 h experiment was reached over the Ag/CeO 2 /TiO 2 system.…”

“…Xue et al [98,139,140] explored the PG oxidation over unsupported and hydroxyapatite-supported silver and silver-palladium NPs as catalysts and molecular oxygen as an oxidant. According to the literature, the PG oxidation proceeds through two parallel mechanisms: (1) oxidation of the primary hydroxyl group of PG to LAld, which further rapidly transforms into LA; (2) oxidation of the secondary hydroxyl group of PG to yield HA, which can be further oxidized into MeGO or converted into LAld through tautomeric equilibration in alkaline media.…”

“…According to the literature, the PG oxidation proceeds through two parallel mechanisms: (1) oxidation of the primary hydroxyl group of PG to LAld, which further rapidly transforms into LA; (2) oxidation of the secondary hydroxyl group of PG to yield HA, which can be further oxidized into MeGO or converted into LAld through tautomeric equilibration in alkaline media. The obtained MeGO is highly susceptible to C-C bond cleavage to yield acetic (AcA) and formic (FA) acids [139], CO and CO 2 [140] or can be converted to LA through the Canizzarro reaction in alkaline media. The possible ways of PG transformation under oxidative conditions are presented in Figure 18.…”

“…Carbon mass balance was 100% in all catalytic tests. Unsupported silver-palladium NPs and those supported on hydroxyapatite (HAP) as the catalyst of PG selective oxidation were investigated in by Xue et al [139] and Feng et al [140]. The unsupported samples were prepared by a wet chemical reduction method.…”

Nanostructured Metal Catalysts for Selective Hydrogenation and Oxidation of Cellulosic Biomass to Chemicals

“…This chemistry only involves catalytic activation of hydroxyl group at end carbon atom and good selectivity towards lactic acid can be achieved under mild conditions (T <100 °C). Au, Pd and Pt based nanocatalysts have shown remarkable performances in 1,2‐PDO oxidation (TOF ∼602 h −1 , X >99 %, S >99 %).…”