chemengineering

Article Oxidation of with Unactivated Electroless CuNiMoP Catalyst

Joel Sankar 1 ID , Edith N. Onyeozili 2 and Egwu Eric Kalu 3,*

1 Department of Civil & Environmental Engineering, Florida A&M University, Tallahassee, FL 32310, USA; [email protected] 2 Department of Chemistry, Florida A&M University, Tallahassee, FL 32307, USA; [email protected] 3 Department of Chemical & Biomedical Engineering, Florida A&M University, Tallahassee, FL 32310, USA * Correspondence: [email protected] or [email protected]; Tel.: +1-850-410-6327

Received: 10 September 2017; Accepted: 25 October 2017; Published: 27 October 2017

Abstract: Unannealed CuNiMoP electrocatalyst was found active in electrochemical oxidation of glycerol, providing over 60% conversion without optimization. Prompted by this result, the same catalyst was investigated for the thermochemical oxidation of glycerol. For the thermochemical oxidation of glycerol using the as-deposited electroless CuNiMoP catalyst, a 23 full factorial design of experiments (two level factorial experiment design with three factors) to assess the influence of temperature (A), reaction time (B) and pressure (C). The major reaction products detected by high performance liquid chromatography (HPLC) were glyceric, hydroxypyruvic, tartronic, oxalic and formic acids. The factors found to be most significant for the production of glyceric and tartronic acids were A, B, C, AB and BC. The highest percent conversion obtained for 30-min and 60-min catalysts was 10.6% and 9.4%, respectively. The presence of was observed only for the 60-min as-deposited electroless CuNiMoP/Al2O3 catalyst. The results suggest the feasibility of an inexpensive catalyst based on non-noble metals for the thermochemical oxidation of glycerol through the electroless deposition technique. Some differences exist between the thermochemical and electrochemical product selectivity of the CuNiMoP catalyst, and reasons are suggested for the observed differences.

Keywords: glycerol oxidation; electroless deposition; CuNiMoP catalyst; full factorial design; statistical analysis

1. Introduction The production and use of fossil fuels have long-standing negative impacts on the global climate, public health, local communities and ecosystem goods and services [1]. This motivates the development of renewable energy sources, which include the production of bio-ethanol, biogas and biodiesel [2]. With an increase in biodiesel production comes an increase in the production of crude glycerol, as it is a by-product of the transesterification of vegetable and animal fats/oils. Crude glycerol has little economic or industrial value due to the presence of methanol, inorganic salts, free fatty acids and unreacted . The purification process required to produce pure glycerol is very costly and involves many steps; thus, it would be beneficial to develop processes that make use of the unpurified compound directly [3–6]. Due to the presence of three hydroxyl groups in glycerol, its oxidation under certain specific conditions affords high-value chemicals such as , , and dihydroxyacetone. Tartronic acid is a very costly reagent (US$1564.00 per gram), as well as a high-value chemical in the pharmaceutical industry for obesity and osteoporosis treatment. It is also used as an anti-corrosive protective agent in boilers and high temperature applications and as an

ChemEngineering 2017, 1, 11; doi:10.3390/chemengineering1020011 www.mdpi.com/journal/chemengineering ChemEngineering 2017, 1, 11 2 of 14 scavenger in the food industry [2]. Glyceric acid is also very costly to produce; thus, 10 mg of it can cost as much as US$56.00. It is thus conceivable that developing inexpensive oxidation catalysts that can be applied to both pure and crude glycerol will potentially make these chemicals more widely available. There are several ways in which catalytic oxidation of glycerol occurs, with essential features of the mechanism being the simultaneous dehydrogenation of the hydroxyl group and oxidation of the formed intermediates. The dehydrogenation of the primary alcoholic functions on the metal surface is dependent on the presence of a base, and the reaction rate is directly proportional to the concentration of the base used [7]. The use of a basic medium enhances the selectivity of glyceric and tartronic acids as compared to the application of an acidic medium that yields dihydroxyacetone, hydroxypyruvic acid and mesoxalic acid [8]. The production of three- compounds from glycerol has been extensively investigated [9–13]. Research has been conducted where noble-metal nanoparticles such as palladium (Pd), platinum (Pt) and gold (Au) in a basic medium have been used in the oxidation of glycerol [14]. With the use of Pd and Pt as the catalysts, glyceric acid is the major product formed [15] and tartronic acid and as the major over-oxidation by-products [16]. Furthermore, the addition of bismuth to Pt catalyst resulted in the oxidation of the secondary hydroxyl group, improved catalytic performance, selectivity and stability [8]. The use of Pt/CeO2 catalysts was reported to promote the oxidation of the primary hydroxyl groups to produce a 40% tartronic acid yield [16]. The main drawback of the application of both Pt and Pd is their decrease in catalytic performance at prolonged reaction times. One percent Au supported on activated carbon and graphite afforded 100% selectivity of glyceric acid under mild conditions of 333 K and 0.3–0.6 MPa of oxygen [17]. The presence of Pt on the Au metal surface affected the selectivity of oxidation products; the use of the mole fraction of Pt between 0.25 and 0.33 increased the selectivity of dihydroxyacetone and decreased that of glyceric acid [18]. Not only is the selectivity of glycerol oxidation products affected by the nature of metal catalyst used, the use of crude glycerol is found to affect both the selectivity and reaction rates of glycerol oxidation [3]. Although limited thermochemical and electrochemical oxidation of crude glycerol has been conducted, one of the reported works shows that crude glycerol significantly decreased the reaction rates of Ag/Al2O3, Au/Al2O3, Pd/Al2O3 and Pt/Al2O3 and also affected the selectivity of formic acid and glyceric acid [3]. The reason for this is not unconnected with the impurities found in crude glycerol including methanol, ash (sodium sulfate), organic sulfur derivatives (OSD) and non-methanol organic matter derivatives (MONG-NM). The noble metal-based catalysts are prone to poison by sulfur; thus, interest in the development of non-noble metal-based catalysts [7] that can tolerate sulfur becomes attractive. Hence, catalytic systems developed for glycerol oxidation must have enhanced sulfur and MONG-NM resistance [3,19]. A drawback of the aforementioned approaches is the price of the noble metal used in the catalysts. A potential cost-effective approach would involve the use of non-noble metal-based catalyst for the thermochemical oxidation of glycerol. Elendu et al. [20] synthesized low-cost transition metal-based catalyst containing copper, nickel, molybdenum and phosphorus for the electrochemical oxidation of glycerol. The major electrochemical oxidation products of the electroless catalysts were not provided in [20], but were discussed extensively in [21]. Our interest in this work is to evaluate the as-deposited electroless CuNiMoP catalyst for its thermochemical oxidation capability for pure glycerol and compare its thermochemical oxidation products’ selectivity versus its electrochemical oxidation products’ selectivity for pure glycerol as reported in [21]. A qualitative comparison of the electrochemical and thermochemical oxidation products of the catalyst could provide insight into the mechanism that is operating at different electrochemical oxidation potentials. The effect of deposition time of CuNiMoP onto the catalyst support was also evaluated. A 23 full factorial design of experiments was performed to assess the influence of temperature, time and pressure on the oxidation products from pure glycerol.

2. Materials and Methods

All chemicals were used as received without any further purification. CuSO4·5H2O(≥98%), NiSO4·6H2O(≥98%), H2NaO2P·H2O(≥99%), Na2MoO4·2H2O(≥99%) and HCOOH (ACS, 37 wt% ChemEngineering 2017, 1, 11 3 of 14

in H2O, contains 10–15% methanol as a stabilizer solution) were purchased from Sigma-Aldrich. Sodium citrate dihydrate (99%), gluconic acid potassium (99%) and potassium sodium L-tartrate tetrahydrate (ACS, 99%–102.0%) were purchased from Alfa Aesar. Degassed deionized water was used throughout the experiment.

2.1. Catalyst Preparation via Electroless Plating Based on studies conducted by Elendu et al. [20], a CuNiMoP bath was prepared using the reagents shown in Table1. The plating bath was allowed to reach a temperature of 80 ◦C, and a few drops of concentrated NaOH were added to bring the pH to 11. The catalyst support, Pd catalyzed alumina, was then introduced, and the mixture was continuously stirred. Plating was conducted for 30 and 60 min. At the end of each plating time, the plating mixture was placed in an ice bath for approximately 2 min. The reaction mixture was then centrifuged for 15 min, and the solution was pipetted out, leaving the solid catalyst, which was subsequently washed with distilled water and 2-propanol and dried for 24 h at 60 ◦C. Subsequently, the catalyst was subjected to physical characterization.

Table 1. Chemical constituents used in the production of the CuNiMoP bath.

Reagent Molecular Weight (g/mol) Mass (g) Volume (mL) Density (g/mL)

CuSO4·5H2O 249.68 0.25 - - NiSO4·6H2O 262.86 3.36 - - Na2MoO4·2H2O 241.95 1 - - KNaC4H4O6·4H2O 282.22 2.5 - - Gluconic acid potassium salt, C6H11KO7 234.25 4.75 - - NaO2H2P·H2O 105.99 2.5 - - Sodium citrate 294.10 2 - - HCOOH 30.03 - 10 1.09 H2O 18 - 250 1.00

2.2. Characterization of CuNiMoP/Al2O3 The prepared catalyst was characterized by three methods: scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). Zeiss 1540 XB Cross Beam SEM was conducted to determine the change in morphology of the alumina surface after electroless deposition. XRD was performed to determine the crystallinity of the deposited catalyst on the alumina surface, while EDS was done to identify the elemental composition of the catalyst.

2.3. Oxidation Experiments The glycerol oxidations were carried out in a 250-mL high-pressure chemical reactor equipped with pressure, temperature and speed controllers (Yudian 508/509). The 23 full factorial experiments were designed to investigate the influence of temperature, time and pressure on glycerol oxidation as shown in Table2. Each reaction was conducted twice at the prescribed conditions. The reactions were performed with 100 mL of an aqueous glycerol/NaOH (0.3/1.2 M) solution, 0.1 g of the respective catalyst and a stirring rate of 205 rpm. Samples were taken after each experiment and subjected to analysis via high performance liquid chromatography (HPLC).

Table 2. The 23 full factorial experiment design for glycerol oxidation.

Variables Order Temperature (◦C) Time (min) Pressure (MPa) 1 30 30 0.1 2 80 30 0.1 3 30 150 0.1 4 80 150 0.1 5 30 30 0.8 6 80 30 0.8 7 30 150 0.8 8 80 150 0.8 ChemEngineering 2017, 1, 11 4 of 14

2.4. Product Analysis ChemEngineering 2017, 1, 11 4 of 14 The quantitative analysis of each reaction mixture was conducted by HPLC. The chromatograph (Schimadzu2.4. Product Technologies) Analysis was equipped with a Hi-plex column (7.7 mm × 300 mm, Agilent), a refractive index and a UV (190 nm) detector. Dilute H2SO4 (5 mM) was used as the eluent. The quantitative analysis of each reaction mixture was conducted by HPLC. The chromatograph Three hundred microliters of each sample were diluted with distilled water with a dilution factor (Schimadzu Technologies) was equipped with a Hi-plex column (7.7 mm × 300 mm, Agilent), a of 5/3. An injection volume of 10 µL, a measuring time of 20 min and a flow rate of 0.7 mL min−1 refractive index and a UV (190 nm) detector. Dilute H2SO4 (5 mM) was used as the eluent. Three werehundred adjusted. microliters of each sample were diluted with distilled water with a dilution factor of 5/3. An injection volume of 10 μL, a measuring time of 20 min and a flow rate of 0.7 mL min−1 were adjusted. 3. Results and Discussion 3. Results and Discussion 3.1. Catalyst Characterization The3.1. Catalyst electroless Characterization CuNiMoP deposited on alumina was characterized using SEM, XRD and EDS. The changeThe in electroless the surface CuNiMoP morphology deposited from on the alumina alumina was to thecharacterized deposited using samples SEM, is evidentXRD and in EDS. the SEM images.The Figure change1 ain shows the surface that themorphology alumina from surface the isalumina flat and to free the ofdeposited any particles samples before is evident any electrolessin the deposition.SEM images. In Figure Figure1b,c, 1a changesshows that can the be alumina observed surface on theis flat surface and free of theof any catalyst particles after before 30 min any and electroless deposition. In Figure 1b,c, changes can be observed on the surface of the catalyst after 30 with the increase in plating time to 60 min. The particle density on the substrates varies also with min and with the increase in plating time to 60 min. The particle density on the substrates varies also deposition time. For both the 30-min and 60-min samples, clusters of particles on the substrate edges with deposition time. For both the 30-min and 60-min samples, clusters of particles on the substrate result in thicker particle deposition on the edges. The size of the Al O substrate on which the catalyst edges result in thicker particle deposition on the edges. The size of 2the3 Al2O3 substrate on which the was depositedcatalyst was ranges deposited from ranges 1.5–2.0 fromµm, 1.5–2.0 while μm, the while average the average catalyst catalyst particle particle size is size 63 is nm 63 andnm and 83.3 nm for the83.3 30-min nm for and the 60-min30-min and deposition 60-min deposition times, respectively. times, respectively.

(a) (b)

(c)

Figure 1. SEM images at magnification of 8.11 K for (a) Al2O3 support, (b) 30-min and (c) 60-min Figure 1. SEM images at magnification of 8.11 K for (a) Al2O3 support, (b) 30-min and (c) 60-min as-deposited electroless CuNiMoP/Al2O3. as-deposited electroless CuNiMoP/Al2O3. Characteristic peaks of the Cu, Ni, Mo, P or their alloys are not uniquely identifiable in Figure 2 Characteristicsince the XRD patterns peaks of for the the Cu, Al2 Ni,O3 substrate Mo, P or dominate their alloys the are30- and not 60-min uniquely as-deposited identifiable catalysts. in Figure 2 Chen et al. experienced the same difficulty for CuNiMo/CeO2 catalyst used for the hydrogenation of since the XRD patterns for the Al2O3 substrate dominate the 30- and 60-min as-deposited catalysts. CO since the CeO2 displayed four distinctive peaks dominantly present in all their samples, making Chen et al. experienced the same difficulty for CuNiMo/CeO2 catalyst used for the hydrogenation of it difficult to identify CuO in their catalyst [22]. However, EDS was successful in recognizing the CO since the CeO displayed four distinctive peaks dominantly present in all their samples, making elements present2 in the analyzed samples. it difficult to identify CuO in their catalyst [22]. However, EDS was successful in recognizing the elements present in the analyzed samples. ChemEngineering 2017, 1, 11 5 of 14

ChemEngineering 2017, 1, 11 5 of 14

Figure3 confirms the presence of the elements used in the CuNiMoP bath and the Al 2O3 ChemEngineeringFigure 3 confirms 2017, 1, 11 the presence of the elements used in the CuNiMoP bath and the Al2O3 support.5 of 14 support.With the With spectra the of spectra Cu and of Ni Cu shown and Niin shownthe figure in and the figuresupported and by supported their composition by their in composition Table 3, inthere Table 3is,Figure therethe obvious 3 is confirms the obvioussuggestion the presence suggestion that of morethe elem that activeents more usedcatalyst active in the materials catalyst CuNiMoP materialsare bath deposited and arethe Al deposited in2O 3the support. 60-min in the 60-minelectrolessWith electroless the catalyst spectra catalyst thanof Cu in and than the Ni 30-min in shown the 30-minsample. in the sample.figureIn addition, and In supported addition, the weight by the percentagestheir weight composition percentages in Table in Table 3 display in Table3, 3 displaythethere relationship the is relationship the obvious between betweensuggestion plating plating timethat andmore time the active and am the ountcatalyst amount of depositedmaterials of deposited arematerial. deposited material. Approximately, in Approximately,the 60-min the thecomposition compositionelectroless catalystof ofCu Cu and than and Ni in Ni thedeposited deposited 30-min sample.on on the the Incata catalystaddition,lyst support supportthe weight increased increased percentages by byabout in about Table 86% 86% 3 anddisplay and 21%, 21%, the relationship between plating time and the amount of deposited material. Approximately, the respectivelyrespectively as as the the plating plating time time increasedincreased fromfrom 30–6030–60 min. The The approximate approximate compositions compositions of ofthe the composition of Cu and Ni deposited on the catalyst support increased by about 86% and 21%, 30-30- and and 60-min 60-min samples samples were were Cu Cu2121NN1414Mo and Cu 39NiNi1717Mo,Mo, respectively. respectively. Phosphorus Phosphorus was was not not detected detected respectively as the plating time increased from 30–60 min. The approximate compositions of the inin the the catalyst. catalyst. 30- and 60-min samples were Cu21N14Mo and Cu39Ni17Mo, respectively. Phosphorus was not detected in the catalyst. 2000 Legend 2000 Alumina Support Legend60 minute deposition 30 minute deposition Alumina Support 1500 60 minute deposition 30 minute deposition 1500

Intensity 1000

Intensity 1000

500 500

0 10 20 30 40 50 60 70 80 0 10 20 30 402 theta 50 60 70 80 2 theta FigureFigure 2. 2.XRD XRD patterns patterns fromfrom thethe alumina support, 30-min 30-min and and 60-min 60-min deposition. deposition. Figure 2. XRD patterns from the alumina support, 30-min and 60-min deposition.

((aa))

(b) (b) Figure 3. EDS spectrums of the (a) 30-min and (b) 60-min as-deposited electroless CuNiMoP/Al2O3. Figure 3. EDS spectrums of the (a) 30-min and (b) 60-min as-deposited electroless CuNiMoP/Al2O3. Figure 3. EDS spectrums of the (a) 30-min and (b) 60-min as-deposited electroless CuNiMoP/Al2O3. ChemEngineering 2017, 1, 11 6 of 14

Table 3. Weight percentages and moles of Cu, Ni and Mo of the 30-min and 60-min electroless

as-deposited CuNiMoP/Al2O3 catalysts.

30-min Electroless as-Deposited Catalyst 60-min Electroless as-Deposited Catalyst Element wt (%) moles (mol) wt (%) moles (mol) Mo L 4.2 0.044 2.7 0.028 Ni K 36.7 0.625 27.4 0.467 Cu K 59.1 0.931 69.9 1.100

3.2. Products Formed from Thermochemical Oxidation of Glycerol Tables4 and5 show that both the 30-min and 60-min as-deposited electroless CuNiMoP/Al2O3-catalyzed oxidations afforded similar oxidative products such as glyceric, hydroxypyruvic, tartronic, oxalic and formic acids. In addition to these products, lactic acid was only observed with the use of the 60-min as-deposited electroless CuNiMoP/Al2O3 catalyst. Both tables show that the percent conversion of glycerol for both catalysts was low and that the increase in catalyst deposition time did not confer any significant effect on the catalytic performance. In over 16 experimental runs, the highest percent conversions of glycerol obtained were 10.6% and 9.4% for the 30-min and 60-min as-deposited electroless CuNiMoP/Al2O3, respectively. The slightly higher percent conversion of the 30-min sample was probably a consequence of it having more active sites than the 60-min sample. We speculate that the poor catalytic behavior may be due to the catalyst being used “as-deposited” without any subsequent reductive treatment with hydrogen under thermal or annealing conditions. It can be inferred that the as-deposited electroless catalyst may contain surface oxides. Thus, it can be postulated that the presence of copper and nickel oxides and amorphous deposits on the catalyst surface hindered CuNiMoP/Al2O3’s activity to further oxidize glycerol into the respective three-carbon products. The low conversion observed here for the catalyst was not the case when the catalyst was used for the electrochemical oxidation of glycerol.

Table 4. Glycerol conversion and amount of oxidative products formed for the 30-min as-deposited

electroless CuNiMoP/Al2O3 catalyzed oxidations.

Glycerol Glyceric Oxalic Tartronic Formic Hydroxypyruvic Order Conversion (%) Acid (g) Acid (g) Acid (g) Acid (g) Acid (g)

µGLC SD µGA µOA µTA µFA µHA 1 3.75 0.52 0.250 0.067 0.023 0.00 0.577 2 9.23 1.93 0.196 0.023 0.021 0.020 0.937 3 3.49 3.08 0.390 0.016 0.020 0.035 0.000 4 5.17 2.07 0.341 0.018 0.044 0.087 0.700 5 6.15 1.08 0.334 0.018 0.045 0.033 1.120 6 6.32 1.32 0.319 0.014 0.049 0.116 0.912 7 8.87 2.35 0.401 0.020 0.100 0.048 1.090 8 10.58 4.05 1.373 0.024 0.136 0.500 0.454

Table 5. Glycerol conversion and amount of oxidative products formed for the 60-min as-deposited

electroless CuNiMoP/Al2O3 catalyzed oxidations.

Glycerol Glyceric Oxalic Tartronic Formic Lactic Hydroxypyruvic Order Conversion (%) Acid (g) Acid (g) Acid (g) Acid (g) Acid (g) Acid (g)

µGLC SD µGA µOA µTA µFA µLA µHA 1 4.93 3.24 0.066 0.025 0.000 0.000 0.000 0.860 2 3.96 3.03 0.067 0.000 0.000 0.017 0.005 0.000 3 9.45 1.06 0.586 0.036 0.157 0.021 0.020 0.000 4 2.74 1.42 0.417 0.016 0.116 0.066 0.033 0.450 5 7.32 4.52 0.367 0.000 0.003 0.045 0.012 0.000 6 4.27 0.14 0.522 0.033 0.086 0.257 0.033 0.531 7 6.27 0.82 0.268 0.026 0.056 0.076 0.005 1.590 8 7.98 0.18 1.060 0.024 1.210 0.055 0.100 0.054 ChemEngineering 2017, 1, 11 7 of 14

The observed low glycerol conversion can also be attributed to the low stirring rate applied in each experimental run. The slow stir speed of 205 rpm may not have dispersed the heavy catalyst particles uniformly throughout the glycerol solution mixture. Thus, not all the active sites for each catalyst molecule were exposed in an equivalent manner. In addition, due to inadequate stirring, some catalyst particles may have settled at the bottom of the reactor during each experimental run. It is suggested to anneal the catalyst under hydrogen after deposition and to perform each reaction at a stirring rate of approximately 500 rpm, in order to improve catalytic activity. As the 30-min as-deposited catalyst showed a slightly higher activity than the 60-min as-deposited catalyst, a shorter deposition time may also advance catalytic performance. The low conversions obtained in the oxidation of pure glycerol with both our catalysts led us to postulate negligible conversion for crude glycerol using the same catalysts. This will be in agreement with the case of noble metals in the oxidation of crude glycerol, in which reaction rates were decreased by at least half in comparison to the rates for oxidation of pure glycerol [3]. In order to extend this study to crude glycerol, we must first optimize reaction conditions for pure glycerol oxidation to make the process more practical. Unlike the thermochemical oxidation process, the electrochemical oxidation products reported in [21], in addition to organic acid byproducts, included partial oxidation products of glycerol such as aldehydes and ketones. Further, the products’ selectivity was found to be sensitive to the oxidation potential. The electrochemical oxidation was carried out in a batch reactor and the oxidation products sampled with time. The key by-products reported after 24-h batch oxidations were formic and tartronic acids at 0.5 V. As the oxidation potential increased to 0.7 V, the major products were , tartronic acid and formic acid. With the increase of potential to 0.9 V, the number of by-products found after 24 h included dihydroxyacetone, glyceraldehyde, tartronic acid, formic acid, mesoxalic acid and lactic acid. At a potential of 1.1 V for which 62% conversion was achieved, glyceraldehyde, mesoxalic and tartronic acids were the major oxidation products. A comparison of the batch thermochemical oxidation products with those formed from electrochemistry suggests that the electrochemical process is milder than the thermochemical oxidation. No intermediate oxidation products such as aldehydes and ketones were recovered from the thermochemical oxidation carried out within 2-h batch oxidation. This contrasts with the electrochemical process where even after 24 h under certain potentials, dihydroxyacetone and glyceraldehyde were produced in very large quantities. Although the electrochemical reaction mechanism for the catalyst may be similar to the thermochemical mechanism in some respects, the differences in the products’ distribution observed in the two systems suggest that some important differences exist. One such difference could be that while under electrochemical conditions, the unactivated catalyst (probably covered by surface oxide) allowed electron flow for glycerol oxidation; while under thermochemical conditions, such electron exchange for oxidation could be hindered. The existence of possible differences in the two mechanisms is currently being investigated in our labs.

3.3. Reaction Mechanism Scheme1 suggests that the oxidation of the primary hydroxyl functionality of glycerol can form glyceraldehyde, which is subsequently oxidized to glyceric acid. Given that glyceraldehyde was not detected by HPLC, it is hypothesized that it is very reactive under our thermochemical reaction conditions and is thus rapidly oxidized into glyceric acid by the electroless CuNiMoP/Al2O3 catalyst. Researchers using a similar starting solution of 0.3/1.2 M Gly/NaOH concluded that ◦ glyceraldehyde was unstable under the applied reaction conditions (60 C, O2 at a pressure of 5 bar) [23]. They substantiated this claim by the addition of 0.1 M glyceraldehyde solution to NaOH, which produced a number of derivatives as judged by HPLC [23]. The production of multiple derivatives was also observed during our calibration of pure glyceraldehyde solutions. Dihydroxyacetone (Scheme1) was also undetected by HPLC; however, it is hypothesized that a similar mechanism as speculated for glyceric acid could account for the presence of hydroxypyruvic acid, which is produced via ChemEngineering 2017, 1, 11 8 of 14 the oxidation of the secondary hydroxyl group of glycerol [23], without the observed presence of ChemEngineeringdihydroxyacetone. 2017, 1, Glyoxylic11 acid was also not detected by HPLC, but its absence can be explained8 of 14 through the Cannizzaro reaction, in which the intermediate glyoxylic acid is quickly converted into . This is in line with Skryńska et al. (2012), who attributed the absence of glyoxylic acid in oxalic acid. This is in line with Skry´nskaet al. (2012), who attributed the absence of glyoxylic acid in their reaction mixture to the Cannizzaro reaction [23]. their reaction mixture to the Cannizzaro reaction [23].

SchemeScheme 1. TheThe glycerol glycerol oxidation mechanismmechanism ononCuNiMoP. CuNiMoP. The The compounds compounds in in red red were were not not detected detected by byHPLC. HPLC. The compoundThe compound in blue wasin onlyblue producedwas only with produced the application with ofthe 60-min application as-deposited of catalyst.60-min as-deposited catalyst. Glyceric acid is further oxidized via two separate routes into either tartronic acid or oxalic acid (SchemeGlyceric1). The acid first is pathwayfurther oxidized shows thevia oxidationtwo separate of the routes primary into hydroxyleither tartronic group acid of glyceric or oxalic acid acid to (Schemeform tartronic 1). The acid, first while pathway the other shows pathway the oxidation shows theof the C-C primary cleavage hydroxyl of glyceric group acid of to glyceric produce acid formic to formand oxalic tartronic acids. acid, In the while course the of other this experiment,pathway shows it was the observed C-C cleavage that glycerol of glyceric oxidation acid on to the produce 60-min formic and oxalic acids. In the course of this experiment, it was observed that glycerol oxidation on CuNiMoP/Al2O3 yielded lactic acid as suggested in Scheme1. According to [ 24], glyceraldehyde theis dehydrated 60-min CuNiMoP/Al to produce2O 2-hydroxypropenal,3 yielded lactic acid which as suggested undergoes in keto-enol Scheme tautomerization 1. According to to [24], give glyceraldehydepyruvaldehyde, which,is dehydrated through benzylicto produce acid arrangement,2-hydroxypropenal, is converted which into lacticundergoes acid [24 keto-enol]. tautomerization to give pyruvaldehyde, which, through benzylic acid arrangement, is converted into lactic3.4. Conditions acid [24]. that Affect Glycerol Oxidation to High Value Chemicals Glyceric and tartronic acids are two high-value chemicals of interest resulting from the 3.4. Conditions that Affect Glycerol Oxidation to High Value Chemicals thermochemical oxidation of glycerol on CuNiMoP/Al2O3. As shown in Table6, the CuNiMoP catalystsGlyceric displayed and atartronic much higher acids selectivity are two forhigh-value glyceric acidchemicals versus tartronicof interest acid resulting production. from For the the thermochemical30-min as-deposited oxidation catalysts, of theglycerol highest on selectivities CuNiMoP/Al for2 bothO3. As glyceric shown (8.11) in Table and tartronic 6, the acidsCuNiMoP (0.43) catalystswere obtained displayed for Run a much 3. However, higher selectivity for the 60-min for glyceric as-deposited acid versus catalysts, tartronic the highest acid production. selectivities For for theglyceric 30-min (14.84) as-deposited and tartronic catalysts, acids (5.45)the highest were obtained selectivities for Runsfor both 2 and glyceric 8, respectively. (8.11) and Simple tartronic statistical acids (0.43) were obtained for Run 3. However, for the 60-min as-deposited catalysts, the highest selectivities for glyceric (14.84) and tartronic acids (5.45) were obtained for Runs 2 and 8, respectively. Simple statistical analysis of the experimental results was undertaken for further elucidation of the conditions that favored the production of these by-products. ChemEngineering 2017, 1, 11 9 of 14 analysis of the experimental results was undertaken for further elucidation of the conditions that ChemEngineering 2017, 1, 11 9 of 14 favored the production of these by-products. Table 6. Selectivities of glyceric and tartronic acids for both catalysts. Table 6. Selectivities of glyceric and tartronic acids for both catalysts. 30-min As-Deposited Electroless 60-min As-Deposited Electroless Variables 30-minCuNiMoP/Al As-Deposited2O Electroless3 60-min As-DepositedCuNiMoP/Al Electroless2O3 CuNiMoP/Al O CuNiMoP/Al O Order Variables 2 Average3 Selectivity 2 3 Order Temp Time Pressure Average Selectivity Glyceric Acid Tartronic Acid Glyceric Acid Tartronic Acid (°C)Temp (min)Time (MPa)Pressure Glyceric Acid Tartronic Acid Glyceric Acid Tartronic Acid 1 30(◦ C) 30(min) 0.1(MPa) 1.25 0.04 0.42 0.00 21 80 30 30 30 0.1 0.1 0.20 1.25 0.04 0.04 0.42 14.84 0.00 0.00 32 30 80150 300.1 0.18.11 0.20 0.43 0.04 14.847.71 0.002.09 43 80 30150 1500.1 0.10.44 8.11 0.06 0.43 7.711.71 2.091.71 54 30 8030 1500.8 0.10.29 0.44 0.04 0.06 1.719.87 1.710.11 5 30 30 0.8 0.29 0.04 9.87 0.11 66 80 8030 300.8 0.80.31 0.31 0.05 0.05 1.001.00 0.170.17 77 30 30150 1500.8 0.80.35 0.35 0.08 0.08 0.160.16 0.040.04 88 80 80150 1500.8 0.82.07 2.07 0.22 0.22 4.914.91 5.455.45

3.4.1.3.4.1. Glyceric Glyceric Acid Acid

AA half-normal half-normal plot plot for bothfor theboth 30- the and 30- 60-min and as-deposited60-min as-deposited electroless CuNiMoP/Alelectroless CuNiMoP/Al2O3 reactions2O3 asreactions in Figure as4 showsin Figure that 4 allshows the factorsthat all andthe factors several and of their several interactions of their interactions are positioned are topositioned the right to sidethe ofright the normalside of line.the normal This implies line. thatThis theseimplies factors that andthese their factors interactions, and their except interactions, the time-pressure except the interaction,time-pressure are significantinteraction, for are glyceric significant acid formation.for glyceric Time acid (B),formation. pressure Time (C) and(B), thepressure interaction (C) and (BC) the contributeinteraction the (BC) most contribute to the conversion the most of to glycerol the conversion into glyceric of glycerol acid. As into the glyceric plots in Figureacid. As4 indicate, the plots B, in CFigure and BC 4 contributeindicate, B, at C least and 65%BC contribute to glyceric at acid least formation. 65% to glyceric acid formation.

(a) (b)

Figure 4. Half-normal plot for glyceric acid production for (a) the 30-min and (b) the 60-min Figure 4. Half-normal plot for glyceric acid production for (a) the 30-min and (b) the 60-min as-deposited electroless CuNiMoP/Al2O3 oxidation reactions. as-deposited electroless CuNiMoP/Al2O3 oxidation reactions. Using ANOVA analysis, the main factors and their interactions were tested for significance for glycericUsing acid ANOVA production analysis, at the the 95% main confidence factors and level. their Table interactions 7 shows werethat for tested 30-min for significanceas-deposited forelectroless glyceric acidCuNiMoP/Al production2O at3 theoxidation 95% confidence reactions, level. low Tablep-values7 shows were that obtained for 30-min for as-deposited temperature electroless(A: 0.026), CuNiMoP/Al pressure (C:2O 0.038)3 oxidation and all reactions, the interactions low p-values (AC: were 0.017, obtained BC: 0.035 for temperature and AB: 0.017). (A: 0.026), With pressurerespect (C:to the 0.038) 60-min and as-deposited all the interactions electroless (AC: CuNiMoP/Al 0.017, BC: 0.0352O3 oxidation and AB: 0.017).reactions, With a p respect-value of to 0.004 the 60-minwas obtained as-deposited for the electroless AC interactions. CuNiMoP/Al These2O 3respectiveoxidation reactions,p-values agree a p-value with of the 0.004 half-normal was obtained and forinteraction the AC interactions. plot, and thus, These the respective factors arep-values significant agree for with the the formation half-normal of glyceric and interaction acid. plot, and thus, theA simple factors regression are significant analysis for the for formationglyceric acid of glycericproduction acid. is represented by Equations (1) and (2) for Athe simple 30- and regression 60-min as-dep analysisosited for electroless glyceric acid CuNiMoP/Al production2O is3 representedcatalysts, respectively. by Equations The (1) R-squared and (2) forvalues the 30- for and both 60-min models as-deposited were 87.4%, electroless indicating CuNiMoP/Al that the equations2O3 catalysts, can be respectively. used to predict The R-squaredthe amount of glyceric acid produced at any given temperature, time and pressure.

glyceric acid (g) = 0.736 − 0.00998A − 0.00385B − 0.833C + 0.000083AB (1) + 0.01513AC + 0.00498BC ChemEngineering 2017, 1, 11 10 of 14 values for both models were 87.4%, indicating that the equations can be used to predict the amount of glycericChemEngineering acid produced 2017, 1, at11 any given temperature, time and pressure. 10 of 14

glyceric acid (g) glyceric acid30 min(g) = 0.736 − 0.00998A − 0.00385B − 0.833C + 0.000083AB + 0.01513AC + 0.00498BC (1) = 0.268 − 0.00677A + 0.00174B − 0.260C + 0.000039AB (2) + 0.01594AC − 0.00256BC glyceric acid60 min(g) = 0.268 − 0.00677A + 0.00174B − 0.260C + 0.000039AB + 0.01594AC − 0.00256BC (2) Equations (1) and (2) can also be represented by 3D surface plots as shown in Figure 5. The Equations (1) and (2) can also be represented by 3D surface plots as shown in Figure5. The factors factors chosen for the x- and y-axes in Figure 5 were based on the p-values determined by the ANOVA chosenanalysis. for the Using x- and Figure y-axes 5, very in Figurelow glyceric5 were acid based production on the is pestimated-values determinedto occur at 0.1 by MPa the and ANOVA ◦ analysis.30 °C Using (low Figurepressure5, veryand lowtemperature) glyceric acidfor both production the 30-min is estimated (Figure 5a) to and occur 60-min at 0.1 (Figure MPa and 5b) 30 C (low pressureas-deposited and catalyst’s temperature) reactions. for This both contrasts the 30-min with (Figurethe highest5a) production and 60-min rates (Figure estimated5b) as-deposited to occur catalyst’sat about reactions. 0.55 MPa Thisand 80 contrasts °C for the with 30-min the as-deposited highest production catalyst. This rates is also estimated within the to range occur for atthe about 0.55 MPa60-min and sample 80 ◦C as for shown the 30-minin Figure as-deposited 5b. catalyst. This is also within the range for the 60-min sample as shown in Figure5b. Table 7. ANOVA results for glyceric acid production.

OxidationTable 7. ANOVA Source results for glyceric SS DFacid production. Mean Square F-value p-value Regression 1.76898 6 0.294839 10.35 0.001 Oxidation SourceA SS0.20305 DF1 Mean0.203052 Square 7.13 F-Value 0.026p-Value B 0.11422 1 0.114221 4.01 0.076 30-min as-deposited Regression 1.76898 6 0.294839 10.35 0.001 AC 0.203050.16831 11 0.168313 0.203052 5.91 7.13 0.038 0.026 CuNiMoP/Al2O3 catalyzed reactions 30-min as-deposited BAB 0.114220.24602 11 0.246016 0.114221 8.63 4.01 0.017 0.076 CuNiMoP/Al2O3 catalyzed C 0.16831 1 0.168313 5.91 0.038 reactions ABAC 0.246020.28037 11 0.280370 0.246016 9.84 8.63 0.012 0.017 ACBC 0.280370.17472 11 0.174724 0.280370 6.13 9.84 0.035 0.012 BC 0.17472 1 0.174724 6.13 0.035 Regression 1.28696 6 0.214494 10.38 0.001 RegressionA 1.286960.09353 61 0.093527 0.214494 4.52 10.38 0.062 0.001 A 0.09353 1 0.093527 4.52 0.062 B 0.02334 1 0.023344 1.13 0.316 60-min60-min as-deposited as-deposited B 0.02334 1 0.023344 1.13 0.316 CuNiMoP/Al2O3 catalyzed CC 0.016390.01639 11 0.016387 0.016387 0.79 0.79 0.396 0.396 CuNiMoP/Al2O3 catalyzed reactions reactions ABAB 0.054520.05452 11 0.054522 0.054522 2.64 2.64 0.139 0.139 AC 0.31136 1 0.311364 15.06 0.004 BCAC 0.046220.31136 11 0.311364 0.046225 15.06 2.24 0.004 0.169 BC 0.04622 1 0.046225 2.24 0.169 A, B and C represent temperature, time and pressure, respectively. A, B and C represent temperature, time and pressure, respectively.

(a)

(b)

Figure 5. 3D surface plots for glyceric acid production for (a) the 30-min and (b) the 60-min Figure 5. 3D surface plots for glyceric acid production for (a) the 30-min and (b) the 60-min as-deposited as-deposited electroless CuNiMoP/Al2O3 oxidation reactions. electroless CuNiMoP/Al2O3 oxidation reactions. ChemEngineering 2017, 1, 11 11 of 14

ChemEngineering 2017, 1, 11 11 of 14 3.4.2. Tartronic Acid 3.4.2. Tartronic Acid SimilarChemEngineering statistical 2017, 1 analysis, 11 done for glyceric acid was performed for tartronic acid, and11 the of 14 results are representedSimilar statistical in Figures analysis6 and done7. While for glyceric time (B) acid and was pressure performed (C) arefor tartronic given as acid, the onlyand the significant results 3.4.2. Tartronic Acid factorsare represented for tartronic in acidFigures production 6 and 7. forWhile the time 30-min (B) and sample pressure in Figure (C) are6a, given the result as the in only Figure significant6b shows thatfactors for the Similarfor 60-min tartronic statistical sample, acid analysis production all the done main for for factors theglyceric 30-min and acid sample their was performed interactions in Figure for 6a, aretartronic the significant result acid, in andFigure for the tartronic 6bresults shows acid are represented in Figures 6 and 7. While time (B) and pressure (C) are given as the only significant formation.that for the This 60-min difference sample, in all behavior the main for factors the two and catalyst their interactions samples is are further significant demonstrated for tartronic by theacid BC factors for tartronic acid production for the 30-min sample in Figure 6a, the result in Figure 6b shows interactionformation. results, This difference where for in the behavior 30-min for as-deposited the two catalyst electroless samples CuNiMoP/Al is further demonstratedO sample, by athep-value BC that for the 60-min sample, all the main factors and their interactions are significant2 for3 tartronic acid interaction results, where for the 30-min as-deposited electroless CuNiMoP/Al2O3 sample, a p-value of 0.055formation. was obtained This difference while thein behavior 60-min for as-deposited the two catalyst electroless samples CuNiMoP/Al is further demonstrated2O3 sample by the gave BC a low of 0.055 was obtained while the 60-min as-deposited electroless CuNiMoP/Al2O3 sample gave a low p-valueinteraction (<0.05) results, for all where factors for and the factor30-min interactions as-deposited exceptelectroless for CuNiMoP/Al time (B) (see2O Table3 sample,8). Thisa p-value analysis p-value (<0.05) for all factors and factor interactions except for time (B) (see Table 8). This analysis revealedof 0.055 that was the obtained temperature-pressure while the 60-min interactionas-deposited (0.009) electroless is the CuNiMoP/Al most statistically2O3 sample significant gave a low factor revealed that the temperature-pressure interaction (0.009) is the most statistically significant factor for tartronicp-value (<0.05) acid formation. for all factors Thus, and the factor half-normal interactions plots except and for the time ANOVA (B) (see analysis Table 8). agreeThis analysis that the AC for tartronic acid formation. Thus, the half-normal plots and the ANOVA analysis agree that the AC interactionrevealed is that significant the temperature-pressure for the formation interaction of tartronic (0.009) acid is the for most the 60-minstatistically as-deposited significant electrolessfactor interaction is significant for the formation of tartronic acid for the 60-min as-deposited electroless for tartronic acid formation. Thus, the half-normal plots and the ANOVA analysis agree that the AC CuNiMoP/Al2O3 reactions. CuNiMoP/Alinteraction 2isO significant3 reactions. for the formation of tartronic acid for the 60-min as-deposited electroless CuNiMoP/Al2O3 reactions.

(a) (b) (a) (b) Figure 6. Half normal plots for tartronic acid production for (a) the 30-min and (b) the 60-min FigureFigure 6. Half 6. Half normal normal plots plots for for tartronic tartronic acidacid production for for (a) ( athe) the 30-min 30-min and and (b) the (b) 60-min the 60-min as-deposited electroless CuNiMoP/Al2O3 oxidation reactions. as-depositedas-deposited electroless electroless CuNiMoP/Al CuNiMoP/Al22OO33 oxidation reactions. reactions.

(a)

(b) (b) Figure 7. 3D surface plots for tartronic acid production for (a) the 30-min and (b) the 60-min Figure 7. 3D surface plots for tartronic acid production for (a) the 30-min and (b) the 60-min Figureas-deposited 7. 3D surface electroless plots CuNiMoP/Al for tartro2nicO3 oxidationacid production reactions. for (a) the 30-min and (b) the 60-min as-depositedas-deposited electroless electroless CuNiMoP/Al CuNiMoP/Al22O33 oxidationoxidation reactions. reactions. ChemEngineering 2017, 1, 11 12 of 14

Table 8. ANOVA results for tartronic acid production.

Oxidation Source SS DF Mean Square F-Value p-Value Regression 0.024657 6 0.004110 5.27 0.014 A 0.000127 1 0.000127 0.16 0.695 30-min as-deposited B 0.000495 1 0.000495 0.64 0.446 CuNiMoP/Al2O3 catalyzed C 0.00 1 0.00 0.00 0.991 reactions AB 0.000834 1 0.000834 1.07 0.328 AC 0.000084 1 0.000084 0.11 0.750 BC 0.003773 1 0.003773 4.84 0.055 Regression 2.03807 6 0.339679 9.38 0.002 A 0.20081 1 0.200805 5.54 0.043 60-min as-deposited B 0.12911 1 0.129108 3.57 0.092 CuNiMoP/Al2O3 catalyzed C 0.29216 1 0.292163 8.07 0.019 reactions AB 0.26266 1 0.262656 7.25 0.025 AC 0.40577 1 0.405769 11.20 0.009 BC 0.20250 1 0.202500 5.59 0.042 A, B and C represent temperature, time and pressure, respectively.

The regression for tartronic acid formation for the 30-min and 60-min as-deposited electroless catalysts, as given by Equations (3) and (4), gave R-squared values of 77.86% and 87.40%, respectively. Similar to glyceric acid, the equations can be used to predict the amount of tartronic acid produced at any prescribed temperature, time and pressure.

tartronic acid30 min(g) = 0.0317 − 0.00025A − 0.000254B − 0.0007C + 0.000005AB + 0.000262AC + 0.000731BC (3)

tartronic acid60 min (g) = 0.521 − 0.00992A − 0.00410B − 1.098C + 0.000085AB + 0.01820AC + 0.00536BC (4) According to the regression model, while the lowest tartronic acid production occurs at 0.3 MPa and 30-min reaction time for the 30-min catalyst sample, the highest production is estimated to occur at 0.8 MPa and 150 min. On the other hand, Figure7b suggests that for the 60-min catalyst sample, the minimum and maximum tartronic acid product will occur at conditions of 0.1 MPa, 30 ◦C and 0.5 MPa, 80 ◦C, respectively.

3.4.3. Significance of Time on Glyceric Acid and Tartronic Acid Production The p-values obtained for time (B) for both glyceric and tartronic acids productions (above 0.05) suggest that time (B) is not significant. Intuitively, one would have expected that short reaction times would favor glyceric and tartronic acids formations, while long reaction times favor further oxidation of glyceric and tartronic acids into oxalic and formic acids, respectively. Examination of the time p-values for the formation of formic and oxalic acids with the 30-min catalyst sample yielded 0.095 and 0.084, respectively. These p-values are higher than 0.05, suggesting that time is not significant for their formation. Relatively, it may be argued that time is more important for their formation than for tartronic acid with a p-value of 0.446. On the other hand, for the 60-min catalyst sample, the time p-values for formic and oxalic acids were 0.062 and 0.058, respectively. Though not exactly less than 0.05, one may argue that these p-values suggest that time is more significant for the production of formic and oxalic acids with the 60-min catalyst sample. In addition to subtle differences observed between the 30-min and 60-min catalyst samples, the time p-values observed for the formation of formic and oxalic acids on these samples suggest some minor behavioral differences between the two catalyst samples.

4. Conclusions

As-deposited electroless CuNiMoP/Al2O3 catalyst was successful synthesized, and its thermochemical oxidation ability for glycerol was tested and the comparison made between its thermochemical oxidation products and those of its electrochemical oxidation of glycerol. The SEM and EDS characterization results showed that the increase in plating time favored more active material ChemEngineering 2017, 1, 11 13 of 14 deposition of CuNiMoP. HPLC analysis showed that except some minor differences in product distributions, two catalysts deposited for different lengths of time produced the following oxidation by-products: glyceric acid, tartronic acid, hydroxypyruvic acid, oxalic acid and formic acid. Lactic acid was observed only for the 60-min as-deposited electroless sample. Our results indicate that this low-cost non-noble metal-based CuNiMoP/Al2O3 catalyst is able to oxidize glycerol. However, more research has to be conducted to optimize its catalytic capability. A comparison of the thermochemical oxidation products with those formed from electrochemistry shows that in addition to organic acid by-products, the electrochemical process resulted in the production of some intermediate glycerol oxidation products, such as aldehydes and ketones, thereby suggesting that the electrochemical process is milder than the thermochemical oxidation process. No intermediate oxidation products such as aldehydes and ketones were recovered from the thermochemical oxidation. The electrochemical reaction mechanism for the oxidation of glycerol using the synthesized electroless CuNiMoP catalyst is suggested to have some important difference between it and the thermochemical oxidation mechanism for glycerol. The thermochemical oxidation mechanism for glycerol oxidation on the CuNiMoP/Al2O3 catalyst is suggested. The reason for the observed low conversion of glycerol with the catalyst is explained as being due to the presence of oxides, the amorphous nature of the deposit and the lack of catalyst activation by annealing under a hydrogen reductive environment. While the presence of oxides on the surface of CuNiMo catalyst does not seem to hinder the electron flow during electrocatalytic oxidation of glycerol on CuNiMo, it appears not favorable for electron exchange for the thermochemical oxidation of glycerol on the CuNiMo catalyst. From the three-carbon compounds produced, the full factorial design demonstrated that the increase in each factor (A, B and C) enhances the formation of glyceric and tartronic acids. Additionally, the statistical analysis of the model emphasized the significance of temperature (A) and pressure (C) and the interactions (AC, BC and AB) on their production.

Acknowledgments: The authors acknowledge the support of the Engineering Research Center (ERC) program of the National Science Foundation under Award Number EEC-08212121. Author Contributions: Joel Sankar, Edith N. Onyeozili and Egwu Eric Kalu conceived of and designed the experiments. Joel Sankar performed the experiments and wrote the paper. Edith N. Onyeozili and Egwu Eric Kalu advised on the project. Joel Sankar, Edith N. Onyeozili and Egwu Eric Kalu analyzed the results. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript; nor in the decision to publish the results.

References

1. Höök, M.; Tang, X. Depletion of fossil fuels and anthropogenic climate change—A review. Energy Policy 2013, 52, 797–809. [CrossRef] 2. Katryniok, B.; Kimura, H.; Skrzy´nska,E.; Girardon, J.S.; Fongarland, P.; Capron, M.; Ducoulombier, R.; Mimura, N.; Paul, S.; Dumeignil, F. Selective Catalytic Oxidation of Glycerol: Perspectives for High Value Chemicals. Green Chem. 2011, 13, 1960–1979. [CrossRef] 3. Skrzy´nska,E.; Zaid, S.; Girardon, J.S.; Capron, M.; Dumeignil, F. Catalytic Behaviour of Four Different Supported Noble Metals in the Crude Glycerol Oxidation. Appl. Catal. A Gen. 2015, 499, 89–100. [CrossRef] 4. Pagliaro, M. Glycerol the Platform Biochemical of the Chemical Industry; Simplicissimus Book Farm: Milan, Italy, 2013; ISBN 9788863699647. 5. Pagliaro, M.; Rossi, M. The Future of Glycerol, 2nd ed.; RSC Green Chemistry Book Series: Cambridge, UK, 2010. 6. Skrzynská, E.; Wondołowska-Grabowska, A.; Capron, M.; Dumeignil, F. Crude glycerol as a raw material for the liquid phase oxidation reaction. Appl. Catal. A Gen. 2014, 482, 245–257. [CrossRef] 7. Demirel-Gülen, S.; Lucas, M.; Claus, P. Liquid Phase Oxidation of Glycerol over Carbon Supported Gold Catalysts. Catal. Today 2005, 102–103, 166–172. [CrossRef] 8. Gallezot, P. Selective oxidation with Air on Metal Catalysts. Catal. Today 1997, 37, 405–418. [CrossRef] ChemEngineering 2017, 1, 11 14 of 14

9. Jin, X.; Zhao, M.; Zeng, C.; Yan, W.; Song, Z.; Thapa, P.S.; Subramaniam, B.; Chaudhari, R.V. Oxidation of Glycerol to Dicarboxylic Acids Using Cobalt Catalysts. ACS Catal. 2017, 6, 4576–4583. [CrossRef] 10. Guerrero-Perez, M.O.; Rosas, J.M.; Bedia, J.; Rodriguez-Mirasol, J.; Cordero, T. Recent Inventions in Glycerol Transformations and Processing. Rec. Pat. Chem. Eng. 2009, 2, 11–21. [CrossRef] 11. Zheng, Y.; Chen, X.; Shen, Y. Commodity Chemicals derived from Glycerol, an important biorefinery feedstock. Chem. Rev. 2008, 108, 5253–5277. [CrossRef][PubMed]

12. Mimura, N.; Hiyoshi, N.; Fujitani, T.; Dumeignil, F. Microscope Analysis of Au-Pd/TiO2 Glycerol Oxidation Catalysts Prepared by Deposition-Precipitation Method. RSC Adv. 2014, 144, 2167–2175. [CrossRef] 13. Behr, A.; Eilting, J.; Irawadi, K.; Leschinki, J.; Lindner, F. Improved utilisation of renewable resources: New important derivatives of glycerol. Green Chem. 2008, 10, 13–30. [CrossRef] 14. Chun-Hui, Z.; Beltramini, J.N.; Fan, Y.X.; Lu, G.Q. Chemoselective Catalytic Conversion of Glycerol as a Biorenewable Source to Valuable Commodity Chemicals. Chem. Soc. Rev. 2008, 37, 527–549. 15. Dimitratos, N.; Porta, F.; Prati, L. Au, Pd (mono and bimetallic) Catalysts Supported on Graphite Using the Immobilization Method: Synthesis and Catalytic Testing for Liquid Phase Oxidation of Glycerol. Appl. Catal. A Gen. 2005, 291, 210–214. [CrossRef] 16. Kimura, H. Polyketomalonate by Catalytic Oxidation of Glycerol over a CeBiPt Catalystyt. II. Polym. Chem. 1996, 34, 3607–3614. [CrossRef] 17. Carrettin, S.; McMorn, P.; Johnston, P.; Griffin, K.; Kiely, C.J.; Hutchings, G.J. Oxidation of Glycerol Using Supported Pt, Pd and Au catalysts. Phys. Chem. 2003, 6, 1329–1336. [CrossRef] 18. Demirel, S.; Lehnert, K.; Lucas, M.; Claus, P. Use of Renewables for the Production of Chemicals: Glycerol Oxidation Over Carbon Supported Gold Catalysts. Appl. Catal. B Environ. 2007, 70, 637–643. [CrossRef] 19. Skrzynská, E.; Ftouni, J.; Mamede, A.-S.; Addad, A.; Trentesaux, M.; Girardon, J.-S.; Capron, M.; Dumeignil, F. Glycerol oxidation over gold supported catalysts—“Two faces” of sulphur based anchoring agent. J. Mol. Catal. A 2014, 382, 71–78. [CrossRef] 20. Elendu, O.; Modupe, O.; Yeboah, Y.; Kalu, E. Use of a Mixed Formaldehyde and Sodium Hypophosphite Reducing Agent Bath in the Electroless Synthesis of Cu-Ni-MoP Electro-catalyst Active for Glycerol Oxidation. Int. J. Electrochem. Sci. 2015, 10, 10792–10805. 21. Elendu, O. Glycerol Electro-Oxidation with CuNiMoP Electrocatalyst for Cogeneration of Energy and High Value Chemicals under Alkalne Conditions. Ph.D. Thesis, Florida State University, Tallahassee, FL, USA, 2017.

22. Chen, J.; Li, W.; Shen, R. CO Hydrogrnation to Higher Alcohols over Ni- and Mo-modified Cu/CeO2 catalyst. Korean J. Chem. Eng. 2016, 33, 500–506. [CrossRef] 23. Skrzy´nska,E.; Ftouni, J.; Girardon, J.S.; Capron, M.; Jalowiecki-Duhamel, L.; Paul, J.F.; Dumeignil, F. Quasi-Homogeneous Oxidation of Glycerol by Unsupported Gold Nanoparticles in the Liquid Phase. ChemSusChem 2012, 5, 2065–2078. [CrossRef][PubMed] 24. Roy, D.; Subramaniam, B.; Chaudhari, R. Cu-Based Catalysts Show Low Temperature Activity for Glycerol Conversion to Lactic Acid. ACS Catal. 2011, 5, 548–551. [CrossRef]

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