Glyceric Acid Selectivity Research Articles

Insight into the basic strength-dependent catalytic performance in aqueous phase oxidation of glycerol to glyceric acid

Fundamental understanding of basic site of supported metal catalysts is highly desirable for aqueous phase oxidation of alcohols without adding extra alkali. Herein, nanoscale alkaline earth metals doped Pt/M−MCM−41 catalysts (M = Mg, Ca, Sr and Ba) were prepared for the aqueous phase oxidation of glycerol to glyceric acid in base-free medium. By establishing the correlation between basic active site and catalytic performance, it is found that catalytic activity increases almost linearly with the strength of basic sites, and excessively strong basic sites are not conducive to the improvement of glyceric acid selectivity. Multi-characterizations revealed that strong basic sites generated by alkaline earth metal oxides (especially MgO and CaO) could induce the enhanced electronic transfer ability, the modified reduction properties of Pt and abundant surface oxygen vacancies in the Pt/M−MCM−41 catalysts. Moreover, reaction kinetics demonstrated that the addition of alkaline earth metal oxides (e.g. CaO) could promote the activation of primary hydroxyl group and C-H bond, resulting in the excellent catalytic performance. In addition, as the representative of the optimal solid base catalysts, the Pt/Ca-MCM-41 (70) catalyst also exhibits record high turnover frequency (TOF) values for the oxidation of glucose (9623.0 h−1) and xylose (4561.0 h−1) to corresponding carboxylic acids.

Catalytic Vapor Phase Oxidation of Glycerol to Glyceric Acid Over Activated Carbon Supported Gold Nanocatalysts

A series of activated carbon (AC) supported Au nanocatalysts with different loadings of Au were prepared by using the homogeneous deposition–precipitation (HDP) method. The samples were characterised with myriad techniques such as X-ray diffraction (XRD), CO-chemisorption, N2 adsorption–desorption measurements, transmission electron microscopy (TEM), inductively coupled plasma-optical emission spectrometer (ICP-OES) and X-ray photoelectron spectroscopy (XPS) to understand the structural and textural properties in detail. The catalysts were tested for the vapour phase oxidation of glycerol to glyceric acid under base-free medium in an aerobic condition at normal atmospheric pressure. The Au/AC nanocatalysts with smaller size Au particles ([Formula: see text][Formula: see text]nm) showed higher glycerol conversion and selectivity for glyceric acid, and also a longer catalyst life. While the larger Au particles ([Formula: see text][Formula: see text]nm) showed less activity and selectivity. Among all the nanocatalysts tested, the 1.0[Formula: see text]wt.% Au/AC sample having smaller particle size of Au showed the best catalytic performance in terms of glycerol conversion and glyceric acid selectivity. These results suggest that the oxidation activities of Au/AC nanocatalysts are strongly influenced by the size of Au nanoparticle, nature of the support material and through a metal-support interaction.

Electronically Coupled PtCo/MgAl Hydrotalcite Catalysts Display Tunable Selectivity Toward Glyceric Acid and Lactic Acid for Glycerol Conversion

Selective oxidation of glycerol is one of the most promising routes to produce value-added fine chemicals from biomass. However, achieving good selectivity towards glyceric and lactic acids still remains a grand challenge in this field. A series of unique electronically coupled PtCo catalysts, supported on layered double hydroxides via co-precipitation method, was reported in this work, as they display tunable selectivity toward glyceric acid and lactic acid during glycerol conversion. It is found that the unusual electronic transfer from MgAl hydrotalcite support to cobalt species, leading to high electron binding energy of PtCo sites on catalyst surface. The presence of cobalt components contributes to synergistic enhancement of oxidation activity at electron deficient Pt sites and more critically, weakened Pt–O strength during glycerol conversion. Such unique electronic reconfiguration, as characterized by XRD and XPS spectroscopy, induces two alternative reaction pathways under different reaction temperatures. While Pt–O bonding strength is significantly weakened by strong Co–O affinity at relatively higher reaction temperature (100 °C), such effect is almost negligible at 50 °C. As a result, lactic acid selectivity of 67% was obtained at 100 °C, while glyceric acid selectivity of 65% was achieved at 50 °C, over the bifunctional PtCo/MgAl hydrotalcite catalysts. The design principles reported in this work offer alternative approaches for manufacturing cost-effective catalytic materials for glycerol conversion to various value-added carboxylic acids and derivatives.

Ni–Co oxide catalysts with lattice distortions for enhanced oxidation of glycerol to glyceric acid

Developing cheap and highly efficient catalysts for the selective oxidation of glycerol to value-added carboxylic acids still remains a challenge. Here, non-noble-metal Ni–Co oxide catalysts with lattice distortions are reported for the first time as highly efficient and stable catalysts for the oxidation of glycerol to glyceric acid. The Ni1Co1Ox catalyst, prepared by a modified hard template method, exhibits superior activity (1.5 × 10−3 s−1), excellent glyceric acid selectivity (66.5%), and good stability under mild conditions (80 °C, 1 MPa O2). Multiple characterizations demonstrated that the nanosized Ni1Co1Ox catalyst (2.3 nm) contains lattice distortion Ni–Co structures with nickel-rich surfaces. This morphology induces electron transfer from Co cations (mainly Co3+) to Ni cations (mainly Ni3+), resulting in the generation of surface oxygen vacancies. Density functional theory calculations and catalytic kinetics demonstrated that oxygen vacancies on Co species could improve glyceric acid selectivity by preventing the cleavage of CC bond, and oxygen vacancies on the Ni species could promote the activation of CH bonds, which results in the exceptional catalytic performance of Ni1Co1Ox. The novel Ni–Co oxide catalyst with lattice distortions synthesized here may provide some guidance for the rational design of inexpensive and highly efficient catalysts for the catalytic oxidation of biopolyols.

Influence of Lewis Acid on the Activity and Selectivity of Pt/MCM-41 (Al) Catalysts for Oxidation of C3 Polyols in Base-Free Medium

The nature of a solid support is crucial for the metal–support interaction, leading to tunable activity and selectivity on supported catalysts. We prepared the Lewis acidic MCM-41 (Al)-supported Pt catalysts for selective oxidation of C3 polyols in base-free medium. Catalyst characterizations (e.g., high-angle annular dark-field scanning transmission electron microscopy and X-ray photoelectron spectroscopy) and density functional theory calculations confirmed that the electron transfer from Pt to MCM-41 (Al) weakens the adsorption of reactants and products, and the enhanced metal–support interaction between Pt and Lewis acid of MCM-41 (Al) promotes the selectivity of carboxylic acids by preventing the cleavage of the C–C bond of polyols. The conversion of glycerol and the selectivity of glyceric acid were 72.2 and 77.9%, respectively, over 3 wt % Pt/MCM-41catalyst (Si/Al ratio, 70) at 60 °C and 1 MPa O2. In addition, the location and number of hydroxyl groups also affect the catalytic oxidation activity. The insights may provide new suggestions on design of the bifunctional Pt–Lewis catalyst for the selective oxidation of alcohols.

Synergistic effects of bimetallic PtRu/MCM-41 nanocatalysts for glycerol oxidation in base-free medium: Structure and electronic coupling dependent activity

Bimetallic PtRu catalyst was first reported for oxidation of glycerol to glyceric acid in base-free medium using O2 as the oxidant. A combination of density functional theory (DFT) calculations and multi-characterizations (e.g., H2-TPR, HAADF-STEM and XPS) revealed that the strong interaction between Pt and Ru could promote the dispersion of PtRu alloy nanoparticles and enhance the electronic coupling effect on the metal surface. Meanwhile, compared to the monometallic Pt catalyst, the introduction of Ru contributes to the direct dissociation of molecular oxygen and water to hydroxyl group, leading to the excellent catalytic activity. A volcanic-shaped relationship between Ru/Pt ratio, catalytic performance, structure-sensitivity and electronic coupling effect was systematically established. Furthermore, the role of structure-sensitivity and electronic coupling effect for the enhanced catalytic activity of PtRu catalysts with different Ru/Pt ratios are distinguished in detail. Finally, the Pt0.8Ru0.8/MCM-41 catalyst showed excellent catalytic activity (TOF: 823.9 h−1), glyceric acid selectivity (80.1%) and stability (recycling for 5th) under the optimized conditions (80 °C, 1 MPa O2 and 12 h). The insights and methodology reported here may pave the way to the rational design of bimetallic catalysts for efficient conversion of bio-derived substrates under mild conditions.

Selective Oxidation of Glycerol Using 3% H2O2 Catalyzed by Supported Nano-Au Catalysts

A series of transition metal oxides or mixed oxides supported nano-Au catalysts were prepared for the selective oxidation of glycerol to glyceric acid using 3% H2O2. It was found that the composition and structure of supports significantly influenced the catalytic performance of catalysts. The mesoporous trimetal mixed oxide (CuNiAlO) supported nano-Au catalysts were more active in comparison with the others. In the present catalytic system, the highest glycerol conversion was 90.5%, while the selectivity of glyceric acid could reach 72%. Moreover, the catalytic performance remained after 11 times of reaction.

Sustainable Conversion of Glycerol into Value‐Added Chemicals by Selective Electro‐Oxidation on Pt‐Based Catalysts

AbstractElectro‐oxidation of glycerol affords a totally green route to produce high value‐added chemicals. Herein, we report a study on glycerol electro‐oxidation over a series of graphene nanosheet supported Pt (Pt/GNS), PtNi (PtNi/GNS), PtRu (PtRu/GNS), PtRh (PtRh/GNS), PtRuNi (PtRuNi/GNS), and PtRhNi (PtRhNi/GNS) catalysts in alkaline solution. The activity of the catalysts was evaluated by cyclic voltammetry, linear sweep voltammetry, and chronoamperometric measurements. The PtRh/GNS and PtRhNi/GNS catalysts exhibited superior activity in terms of higher current densities and lower onset potentials. The products of glycerol oxidation formed at potentials of −0.4, −0.1, and 0.2 V, were systematically analyzed by high performance liquid chromatography (HPLC). Five compounds as products from glycerol electro‐oxidation catalyzed by the prepared materials were found, including glyceraldehyde, glycolic acid, tartronic acid, glyceric acid and oxalate acid. The product distribution at the different potentials was investigated for all catalysts. A maximum glycolic acid selectivity of 65.4 % was obtained for the Pt/GNS catalyst at 0.2 V while a maximum glyceric acid selectivity of 47.7 % was achieved using the PtNi/GNS catalyst at −0.1 V. It was found that the introduction of Ru facilitated the formation of C3 products while the addition of Rh was beneficial for the formation of C2 products. Based on HPLC results, the pathways of glycerol electro‐oxidation by the prepared catalysts were proposed.

Platinum nanoparticles supported on N-doped carbon nanotubes for the selective oxidation of glycerol to glyceric acid in a base-free aqueous solution

Pt/N-CNT out-performed Pt/CNT for glycerol oxidation with molecular oxygen under atmospheric pressure in a base-free aqueous solution, in terms of glycerol conversion and glyceric acid selectivity.

Synthesis and Characterization of Nitrogen-Doped Carbon Nanospheres Decorated with Au Nanoparticles for the Liquid-Phase Oxidation of Glycerol

Carbon and nitrogen-doped carbon nanospheres (CNS) were prepared by thermal pyrolysis of benzene (CNSB), aniline (CNSAN), and nitrobenzene (CNSNB) and used as supports for gold nanoparticles. Gold-based catalysts were prepared by the gold-sol method. The catalysts were checked in liquid-phase glycerol oxidation. The nature of the N-containing groups influenced both the acid–base properties of the supports and the Au particle deposition. As a consequence, an enhanced metal sintering by enriching the surface electron density of the support, essentially in the quaternary form, was observed. Both the glycerol conversion and glyceric acid selectivity increased with decreasing gold particle size. Moreover, catalyst Au/CNSB promoted the formation of glyceric acid, whereas Au/CNSAN and Au/CNSNB catalysts favored the oxidation of the secondary hydroxyl group of glycerol. Results clearly confirmed the influence of the support properties on the catalytic performance of gold in selective glycerol oxidation.

Stabilizer effects on the synthesis of gold-containing microparticles. Application to the liquid phase oxidation of glycerol

Gold-containing poly(urea-formaldehyde) microparticles were prepared by the in situ polymerization method using a series of stabilization agents with different chemical nature. The effects of cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), polyvinyl pyrrolidone (PVP), polyvinyl alcohol (PVA) and tetrakis(hydroxymethyl)phosphonium chloride (THPC) on the morphology, the particle size of encapsulated gold, the gold particle size distribution and the encapsulation efficiency were investigated by using scanning electron microscopy, X-ray diffraction and atomic absorption analyses. The chemical nature of stabilizer had a marked influence on both the encapsulated gold particle size and the encapsulation efficiency. Both gold particle size and gold encapsulation efficiency increased when decreasing the stabilizer polarity number. All the microparticles here prepared were tested in the liquid phase selective oxidation of glycerol. The glycerol conversion increased and the glyceric acid selectivity decreased when decreasing gold particle sizes. Results showed that use of stabilizers with hydrophobic surfaces enhanced the selectivity to C3 products in the resulting catalysts. On the other hand, the use of stabilizers with hydrophilic surfaces increased the C–C bond cleavage products in the resulting catalysts.

Selective Glycerol Oxidation by Electrocatalytic Dehydrogenation

This study demonstrates that an electrochemical dehydrogenation process can be used to oxidize glycerol to glyceraldehyde and glyceric acid even without using stoichiometric chemical oxidants. A glyceric acid selectivity of 87.0 % at 91.8 % glycerol conversion was obtained in an electrocatalytic batch reactor. A continuous-flow electrocatalytic reactor had over an 80 % high glyceric acid selectivity at 10 % glycerol conversion, as well as greater reaction rates than either an electrocatalytic or a conventional catalytic batch reactor.

Effect of the operation conditions on the selective oxidation of glycerol with catalysts based on Au supported on carbonaceous materials

The liquid-phase oxidation of glycerol has been studied on gold nanoparticles supported on different carbon materials: activated carbon (AC), graphite (G) and carbon ribbon-type nanofibers (CNF-R). Catalysts were prepared by two different methods: impregnation (-IMP) and gold-sol (-SGT). The influence of the operating conditions (e.g. pressure, temperature, NaOH/glycerol molar ratio and glycerol/Au molar ratio) on the catalytic activity and product selectivity was studied in detail. The oxygen pressure was found to have a very small influence on the reaction rates, with the reaction being zero order with respect to oxygen. The selectivity to glyceric acid was significantly enhanced on increasing both the oxygen pressure and reaction temperature, which simultaneously led to a decrease in both glycolic and tartronic acid selectivities. Glycerol conversion and glyceric acid selectivity increased on increasing the NaOH/glycerol molar ratio in the range from 1 to 2. At higher values of this molar ratio, these two parameters remained approximately constant. A linear correlation was observed between glycerol conversion and the amount of catalyst (up to 0.5 g) and this indicates that the reaction was kinetically controlled. However, along with the increase in the catalytic activity with the amount of catalyst, a substantial decrease in the selectivity to glyceric acid was also observed. Finally, a kinetic model to describe the reaction mechanism was proposed and good agreement between the experimental and predicted values was achieved.

Selective oxidation of biorenewable glycerol with molecular oxygen over Cu-containing layered double hydroxide-based catalysts

To convert glycerol, a by-product of biodiesel, into value-added fine chemicals is of great industrial and economic importance and represents a great challenge. This paper reports the influence of various metal elements (M = Ni, Zn, Cu, Co, etc.) incorporated into MgAl layered double hydroxide (LDH) materials used as catalysts for the liquid phase oxidation of glycerol to fine chemicals. The catalytic behaviour was also studied and discussed in terms of structure and texture of the synthesized materials that were determined using PXRD, FE-SEM, TEM, TG-DTA, FTIR, XPS and nitrogen adsorption–desorption isotherms. The results revealed that CuAlMg LDH materials are active in converting glycerol to glyceric acid (GLYAC) and formic acid, while yielding oxalic and hydroxyehthanoic acid. The selectivity to glyceric acid remarkably increased over Cu-containing catalysts, compared with those over similar materials with the addition of Ni, Zn, Co metal elements. The increase of Cu content not only affects the catalyst activity but also the selectivity. Moreover, an improvement of activity and selectivity was found on calcined CuAlMg samples. The calcined catalysts, with a Cu:Al:Mg molar ratio of 20:100:200, exhibited superior performances with a 97.3% conversion of glycerol and a 70% glyceric acid selectivity. It was concluded that the nature of the metal added to the LDH structure, such as Cu as active sites, in combination with the chemical state and microenvironment in calcined and uncalcined materials significantly affects the catalytic performance of the synthesized catalysts in the liquid phase oxidation of glycerol in terms of product activity and distribution.