Functional characterization of bimetallic CuPd_xnanoparticles in hydrothermal conversion of glycerol to lactic acid

Abstract

The crude glycerol from biomass represents an abundant and inexpensive resource which can be utilized in producing food additives such as lactic acid. The direct transformation of bioderived glycerol to lactic acid under the catalysis of bimetallic CuPdx nanoparticles as well as monometallic Cu and Pd was investigated in hydrothermal conditions. The properties of fresh and spent bimetallic CuPdx nanoparticles were characterized with various physicochemical techniques viz. XRD, TEM, HRTEM, XPS, and AAS measurements. Catalytic activity of the prepared CuPdx nanoparticles is superior to the monometallic ones due to the alloying trend and synergistic effects. At optimal experimental conditions (100 ml of glycerol and NaOH solution, catalyst/glycerol mass ratio 2:100, 220°C, and 2.0 hr), the desired lactic acid selectivity catalyzed by the bimetallic CuPd2, CuPd3, and CuPd4 catalysts reached 95.3%, 91.4%, and 90.9%, respectively. Practical applications Lactic acid, a widely used food additive, was traditionally produced by fermentation. However, due to the limitation such as time‐consuming and complex separation procedure, interest has been attracted in developing an alternative approach toward efficient production of lactic acid. An attempt was made in present study to use the biodiesel byproduct, glycerol, and chemical conversion to high‐valued lactic acid. Compared with traditional biological fermentation route, it was evidenced that glycerol selective transformation to lactic acid involves a new chemical reaction path for commodity lactic acid with a large availability and economic efficiency. This finding is significant for sustainable development of biodiesel industry and elimination of environmental issues arising from the abandoned crude glycerol.

A review of the catalytic conversion of glycerol to lactic acid in the presence of aqueous base

“…The highest glycerol conversion (99.6%) was obtained using CuPd 5 /PEG catalyst. 19 However, the highest selectivity (95.3%) to LA was achieved by CuPd 2 /PEG catalyst. 19 Increasing the reaction temperature caused an increase in the glycerol conversion but a decrease in the selectivity for LA.…”

“…19 However, the highest selectivity (95.3%) to LA was achieved by CuPd 2 /PEG catalyst. 19 Increasing the reaction temperature caused an increase in the glycerol conversion but a decrease in the selectivity for LA. A similar effect was also observed for increasing the reaction time: 100% conversion of glycerol was achieved upon extending the reaction time to four hours with the CuPd 5 /PEG catalyst but the selectivity for LA dropped to 92.8% for the reaction using the CuPd 2 /PEG.…”

“…Using the colloidal metal nanoparticles is an auspicious way of increasing the catalytic performance due to the high fraction of surface atoms. 26 A recent paper by Wang et al 19 reported the preparation of the PEG-stabilized bimetallic copper(0)–palladium(0) nanoparticles with various compositions as well as the monometallic copper(0) and palladium(0) nanoparticles, which have been tested as catalysts in the oxidation of glycerol in an aqueous solution of glycerol with a NaOH/glycerol molar ratio of 1.0 or 1.3, and a catalyst/glycerol mass ratio of 2 : 100 at 180–220 °C for 2.0 h. The bimetallic CuPd x /PEG nanoparticles have superior catalytic activity in glycerol oxidation as compared to the monometallic ones (Table 2). An increase in the palladium content of bimetallic nanoparticles resulted in the enhancement of the conversion of glycerol.…”

“…As hybrid catalysts, colloidal nanoparticles of transition metals are expected to play an important role in catalysing the glycerol conversion to LA. Unfortunately, there exists only one example of using colloidal metal nanoparticles as catalysts for glycerol conversion; 19 namely, the PEG-stabilized bimetallic copper(0)-palladium(0) nanoparticles are found to be highly active catalysts providing the highest glycerol conversion at 180 C. However, the use of colloidal metal(0) nanoparticles as catalysts in glycerol oxidation has not been further explored, likely due to their instability against agglomeration, particularly at high temperatures. Further research is needed to stabilize metal nanoparticles in basic solution at temperatures as high as 200 C for use as catalysts in the selective conversion of glycerol to LA.…”

“…A similar effect was also observed for increasing the reaction time: 100% conversion of glycerol was achieved upon extending the reaction time to four hours with the CuPd 5 /PEG catalyst but the selectivity for LA dropped to 92.8% for the reaction using the CuPd 2 /PEG. 19…”