Production of 1,2-Propanediol from Renewable Glycerol Over Highly Stable and Efficient Cu–Zn(4:1)/MgO Catalyst

Heterogeneous Catalysts for Glycerol Biorefineries: Hydrogenolysis to 1,2-Propylene Glycol

Research on the use of biomass resources for the generation of energy and chemical compounds is of great interest worldwide. The development and growth of the biodiesel industry has led to a parallel market for the supply of glycerol, its main by-product. Its wide availability and relatively low cost as a raw material make glycerol a basic component for obtaining various chemical products and allows for the development of a biorefinery around biodiesel plants, through the technological integration of different production processes. This work proposes a review of one of the reactions of interest in the biorefinery environment: the hydrogenolysis of glycerol to 1,2-propylene glycol. The article reviews more than 300 references, covering literature from about 20 years, focusing on the heterogeneous catalysts used for the production of glycol. In this sense, from about 175 catalysts, between bulk and supported ones, were revised and discussed critically, based on noble metals, such as Ru, Pt, Pd, and non-noble metals as Cu, Ni, Co, both in liquid (2-10 MPa, 120-260 °C) and vapor phase (0.1 MPa, 200-300 °C). Then, the effect of the main operational and decision variables, such as temperature, pressure, catalyst/glycerol mass ratio, space velocity, and H₂ flow, are discussed, depending on the reactors employed. Finally, the formulation of several kinetic models and stability studies are presented, discussing the main deactivation mechanisms of the catalytic systems such as coking, leaching, and sintering, and the presence of impurities in the glycerol feed. It is expected that this work will serve as a tool for the development of more efficient catalytic materials and processes towards the future projection of glycerol biorefineries.

Tuning of MgO's base characteristics by blending it with amphoteric ZnO facilitating the selective glucose isomerization to fructose for bioenergy development

Fructose serves as an important intermediate in the preparation of liquid fuel compounds. Herein, we report its selective production via a chemical catalysis method over ZnO/MgO nanocomposite. The blending of an amphoteric ZnO with MgO reduced the latter's unfavorable moderate/strong basic sites that can influence the side reactions in the sugar interconversion, reducing fructose productivity. Of all the ZnO/MgO combinations, a 1 : 1 ratio of ZnO and MgO showed a 20% reduction in moderate/strong basic sites in MgO with ∼2-2.5 times increase in weak basic sites (overall), which is favorable for the reaction. The analytical characterizations affirmed that MgO settles on the surface of ZnO by blocking the pores. The amphoteric ZnO undertakes the neutralization of the strong basic sites and improves the weak basic sites (cumulative) by the Zn-MgO alloy formation. Therefore, the composite afforded as high as 36% fructose yield and 90% selectivity at 90 °C; especially, the improved selectivity can be accounted for by the effect of both basic and acidic sites. The favorable action of acidic sites in controlling the unwanted side reactions was maximum when an aqueous medium contained 1/5^th methanol. However, ZnO's presence regulated the glucose's degradation rate by up to 40% compared to the kinetics of pristine MgO. From the isotopic labelling experiments, the proton transfer pathway (or LdB-AvE mechanism by the formation of 1,2-enediolate) is dominant in the glucose-to-fructose transformation. The composite exhibited a long-lasting ability based on the good recycling efficiency of up to 5 cycles. The insights into the fine-tuning of the physicochemical characteristics of widely available metal oxides would help develop a robust catalyst for sustainable fructose production for biofuel production (via a cascade approach).

Effective Production of Selected Dioxolanes by Sequential Bio- and Chemocatalysis Enabled by Adapted Solvent Switching

Most combinations of chemo- and biocatalysis take place in aqueous media or require a solvent change with complex intermediate processing. Using enzymes in the same organic solvent as the chemocatalyst eliminates this need. Here, it was shown that a complete chemoenzymatic cascade to form dioxolanes could be carried out in a purely organic environment. The result, including downstream processing, was compared with a classical mode, shifting solvent. First, a two-step enzyme cascade starting from aliphatic aldehydes to chiral diols (3,4-hexanediol and 4,5-octanediol) was run either in an aqueous buffer or in the potentially biobased solvent cyclopentyl methyl ether. Subsequently, a ruthenium molecular catalyst enabled the conversion to dioxolanes [e. g., (4S,5S)-dipropyl-1,3-dioxolane]. Importantly, the total synthesis of this product was not only highly stereoselective but also based on the combination of biomass, CO₂ , and hydrogen, thus providing an important example of a bio-hybrid chemical.

Study of the Glycerol Hydrogenolysis Reaction on Cu, Cu-Zn, and Cu-ZnO Clusters

Quantum chemistry calculations have been performed to access the efficacy of Cu-based catalysts in various mechanistic steps of the glycerol hydrogenolysis reaction. Calculations are first performed for reactants in the gas phase (noncatalyzed system) and reactants in the gas phase with a 3-atom Cu cluster (catalyzed system). We demonstrate that the glycerol to ethylene glycol conversion is preferred in the noncatalyzed system but glycerol conversion to 1,2-propanediol via the 2-acetol intermediate is preferred in the catalyzed system. We next analyze the adsorption energies of the reactant and product species involved in the glycerol to 1,2-PDO reaction on an 8-atom Cu cluster and Cu cluster doped with a Zn atom or a ZnO molecule. Finally, we study the effects of Zn or ZnO doping on the activation barriers of the two steps of the glycerol to 1,2-PDO reaction.

Kinetic modelling for the hydrogenolysis of bio-glycerol in the presence of a highly selective Cu–Ni–Al2O3 catalyst in a slurry reactor

A kinetic study on the liquid phase hydrogenolysis of glycerol was carried out in a slurry reactor in the presence of a highly selective Cu–Ni–Al₂O₃ catalyst at different reaction temperatures (180–220 °C) and pressures (3–6 MPa), and very high selectivity was achieved (∼95%) towards 1,2-propanediol (1,2-PDO).

Production of propylene glycol (1,2-propanediol) by the hydrogenolysis of glycerol in a fixed-bed downflow tubular reactor over a highly effective Cu–Zn bifunctional catalyst: effect of an acidic/basic support

Very high glycerol conversion of 98.5% with ∼89% selectivity to propylene glycol was achieved at a very low pressure (0.72 MPa) over Cu–Zn/MgO catalyst for vapor phase hydrogenolysis of glycerol. Uniformly distributed partially reduced copper species (Cu₂O, CuO, and Cu⁰) and higher basicity of the catalyst were the governing factors for higher selectivity to propylene glycol.

Re-evaluation of propane-1,2-diol (E 1520) as a food additive

The EFSA Panel on Food Additives and Nutrient Sources added to Food (ANS) provides a scientific opinion re-evaluating the safety of propane-1,2-diol (E 1520) when used as a food additive. In 1996, the Scientific Committee on Food (SCF) established an acceptable daily intake (ADI) of 25 mg/kg body weight (bw) per day for propane-1,2-diol. Propane-1,2-diol is readily absorbed from the gastrointestinal and is expected to be widely distributed to organs and tissues. The major route of metabolism is oxidation to lactic acid and pyruvic acid. At high concentrations, free propane-1,2-diol is excreted in the urine. No treatment-related effects were observed in subchronic toxicity studies. The available data did not raise concern with respect to genotoxicity. Haematological changes suggestive of an increased red blood cell destruction with a compensatory increased rate of haematopoiesis were observed at the highest dose level (5,000 mg/kg bw per day) in a 2-year study in dogs. No adverse effects were reported in a 2-year chronic study in rats with propane-1,2-diol (up to 2,500 mg/kg bw per day). The SCF used this study to derive the ADI. No adverse effects were observed in the available reproductive and developmental toxicity studies. Propane-1,2-diol (E 1520) is authorised according to Annex III in some food additives, food flavourings, enzymes and nutrients and it is then carried over to the final food. Dietary exposure to E 1520 was assessed based on the use levels and analytical data. The Panel considered that for the food categories for which information was available, the exposure was likely to be overestimated. Considering the toxicity database, the Panel concluded that there was no reason to revise the current ADI of 25 mg/kg bw per day. The Panel also concluded that the mean and the high exposure levels (P95) of the brand-loyal refined exposure scenario did not exceed the ADI in any of the population groups from the use of propane-1,2-diol (E 1520) at the reported use levels and analytical results.