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

Lactic acid is a high-value-added chemical with large production, which is used in many industries including the production of pyruvic and acrylic acids. Lactic acid is largely obtained from the oxidation of glycerol, which is a prevalent by-product of biodiesel production. However, the oxidation of glycerol to lactic acid requires harsh reaction conditions such as high temperature and pressure as well as the use of a hefty strong base. In the presence of suitable catalysts, the production of lactic acid from glycerol can be achieved under mild conditions with 1 equivalent base per mole of glycerol. Herein, we review the reports of the catalytic conversion of glycerol to lactic acid in an aqueous alkaline medium considering the reaction conditions, catalytic activity for glycerol conversion and selectivity for lactic acid. We start first with the reports on the use of homogeneous catalysts that have high catalytic activity but miserable recovery. Next, we discuss the employment of colloidal metal(0) nanoparticles as catalysts in glycerol oxidation. The papers on the use of supported metal(0) nanoparticles are reviewed according to the type of support. We then review the polymetallic and metal/metal oxide nanocatalysts used for the conversion of glycerol to lactic acid in an alkaline medium. The catalysts tested for glycerol conversion to lactic acid without any additional bases are also discussed to emphasize the importance of a strong base for catalytic performance. The proposed mechanisms of glycerol oxidation to lactic acid in the presence or absence of catalysts as well as for the formation of side products are discussed. The available experimental kinetics data are shown to fit the mechanism with the formation of glyceraldehyde from glycerol alkoxide as the rate-determining step.

Magnetically Isolable Pt0/Co3O4 Nanocatalysts: Outstanding Catalytic Activity and High Reusability in Hydrolytic Dehydrogenation of Ammonia Borane

The development of a new platinum nanocatalyst to maximize the catalytic efficiency of the precious noble metal catalyst in releasing hydrogen from ammonia borane (AB) is reported. Platinum(0) nanoparticles are impregnated on a reducible cobalt(II,III) oxide surface, forming magnetically isolable Pt⁰/Co₃O₄ nanocatalysts, which have (i) superb catalytic activity providing a record turnover frequency (TOF) of 4366 min^-1 for hydrogen evolution from the hydrolysis of AB at room temperature and (ii) excellent reusability, retaining the complete catalytic activity even after the 10th run of hydrolysis reaction. The outstanding activity and stability of the catalyst can be ascribed to the strong interaction between the platinum(0) nanoparticles and reducible cobalt oxide, which is supported by the results of XPS analysis. Pt⁰/Co₃O₄ exhibits the highest TOF among the reported platinum-nanocatalysts developed for hydrogen generation from the hydrolysis of AB.

A review on platinum(0) nanocatalysts for hydrogen generation from the hydrolysis of ammonia borane

This review reports a survey on the progress in developing highly efficient platinum nanocatalysts for the hydrolytic dehydrogenation of ammonia borane (AB). After a short prelude emphasizing the importance of increasing the atom efficiency of high cost, precious platinum nanoparticles (NPs) which are known to be one of the highest activity catalysts for hydrogen generation from the hydrolysis of AB, this article reviews all the available reports on the use of platinum-based catalysts for this hydrolysis reaction covering (i) early tested platinum catalysts, (ii) platinum(0) NPs supported on oxides, (iii) platinum(0) NPs supported on carbonaceous materials, (iv) supported platinum single-atom catalysts, (v) bimetallic- and (vi) multimetallic-platinum NP nanocatalysts, and (vii) magnetically separable platinum-based catalysts. All the reported results are tabulated along with the important parameters used in the platinum-catalyzed hydrolysis of AB. In the section "Concluding remarks and a look towards the future" a discussion is devoted to the approaches for making high cost, precious platinum catalysts as efficient as possible, ultimately lowering the cost, including the suggestions for the future research in this field.

Cobalt ferrite supported platinum nanoparticles: Superb catalytic activity and outstanding reusability in hydrogen generation from the hydrolysis of ammonia borane

In this work, platinum(0) nanoparticles are deposited on the surface of magnetic cobalt ferrite forming magnetically separable Pt⁰/CoFe₂O₄ nanoparticles, which are efficient catalysts in H₂ generation from the hydrolysis of ammonia borane. Catalytic activity of Pt⁰/CoFe₂O₄ nanoparticles decreases with the increasing platinum loading, parallel to the average particle size. Pt⁰/CoFe₂O₄ (0.23% wt. Pt) nanoparticles have an average diameter of 2.30 ± 0.47 nm and show an extraordinary turnover frequency of 3628 min^-1 in releasing 3.0 equivalent H₂ per mole of ammonia borane from the hydrolysis at 25.0 °C. Moreover, the magnetically separable Pt⁰/CoFe₂O₄ nanoparticles possess high reusability retaining 100% of their initial catalytic activity even after ten runs of hydrolysis. The superb catalytic activity and outstanding reusability make the Pt⁰/CoFe₂O₄ nanoparticles very attractive catalysts for the hydrogen generation systems in portable and stationary fuel cell applications.

Dust Effects on Ir(0)n Nanoparticle Formation Nucleation and Growth Kinetics and Particle Size-Distributions: Analysis by and Insights from Mechanism-Enabled Population Balance Modeling

The effects of microfiltration removal of filterable dust on nanoparticle formation kinetics and particle-size distribution, in a polyoxometalate polyanion (P₂W₁₅Nb₃O₆₂^9-)-stabilized Ir(0)_n nanoparticle formation system, are analyzed by the newly developed method of Mechanism-Enabled Population Balance Modeling (ME-PBM). The [(Bu₄N)₅Na₃(1,5-COD)Ir·P₂W₁₅Nb₃O₆₂] precatalyst system produces on average Ir(0)_∼200 nanoparticles of 1.74 ± 0.33 nm and hence a particle-size distribution (PSD) of ±19% dispersion when the precatalyst is reduced under H₂ in unfiltered propylene carbonate solvent. But if the precatalyst is reduced in microfiltered solvent and microfiltered reagent solutions (where the filtered solvent is then also used to rinse dust from the glassware), then larger Ir(0)_∼300 1.96 ± 0.16 nm nanoparticles are produced with a remarkable, 2.4-fold lowered ±8% dispersion. The results and effects of the microfiltration reduction of dust are analyzed by the newly developed method of ME-PBM. More specifically, the studies reported herein address eight outstanding questions that are listed in the Introduction. Those questions include: how easy or difficult it is to fit PSD data? What is the ability of the recently discovered alternative termolecular nucleation and two size-dependent growth steps mechanism to account for the effects of dust on the PSD? What types and amount of PSD kinetics data are needed to deconvolute the PSD into the parameters of the ME-PBM? What is the reliability of the resulting rate constants? Additional questions addressed include: if the ME-PBM results offer insights into the remarkable 2.4-fold narrowing of the PSD post simple microfiltration lowering of the dust, and if the results are likely to be more general? The Summary and Conclusions section lists nine specific insights that include comments on needed future studies.

"Weakly Ligated, Labile Ligand" Nanoparticles: The Case of Ir(0) n ·(H+Cl-) m

It is of considerable interest to prepare weakly ligated, labile ligand (WLLL) nanoparticles for applications in areas such as chemical catalysis. WLLL nanoparticles can be defined as nanoparticles with sufficient, albeit minimal, surface ligands of moderate binding strength to meta-stabilize nanoparticles, initial stabilizer ligands that can be readily replaced by other, desired, more strongly coordinating ligands and removed completely when desired. Herein, we describe WLLL nanoparticles prepared from [Ir(1,5-COD)Cl]₂ reduction under H₂, in acetone. The results suggest that H⁺Cl^--stabilized Ir(0) _n nanoparticles, herein Ir(0) _n ·(H⁺Cl^-) _a , serve as a WLLL nanoparticle for the preparation of, as illustrative examples, five specific nanoparticle products: Ir(0) _n ·(Cl^-Bu₃NH⁺) _a , Ir(0) _n ·(Cl^-Dodec₃NH⁺) _a , Ir(0) _n ·(POct₃)_0.2n (Cl^-H⁺) _b , Ir(0) _n ·(POct₃)_0.2n , and the γ-Al₂O₃-supported heterogeneous catalyst, Ir(0) _n ·(γ-Al₂O₃) _a (Cl^-H⁺) _b . (where a and b vary for the differently ligated nanoparticles; in addition, solvent can be present as a nanoparticle surface ligand). With added POct₃ as a key, prototype example, an important feature is that a minimum, desired, experimentally determinable amount of ligand (e.g., just 0.2 equiv POct₃ per mole of Ir) can be added, which is shown to provide sufficient stabilization that the resultant Ir(0) _n ·(POct₃)_0.2n (Cl^-H⁺) _b is isolable. Additionally, the initial labile ligand stabilizer HCl can be removed to yield Ir(0) _n ·(POct₃)_0.2n that is >99% free of Cl^- by a AgCl precipitation test. The results provide strong support for the weakly ligated, labile ligand nanoparticle concept and specific support for Ir(0) _n ·(H⁺Cl^-) _a as a WLLL nanoparticle.

Nanoceria-Supported Ruthenium(0) Nanoparticles: Highly Active and Stable Catalysts for Hydrogen Evolution from Water

Ruthenium(0) nanoparticles supported on nanoceria (Ru⁰/CeO₂) were prepared by reduction of Ru³⁺ ions on the surface of ceria using aqueous solution of NaBH₄. The Ru⁰/CeO₂ samples were characterized by advanced analytical tools and employed as electrocatalysts on the glassy carbon electrode (GCE) in hydrogen evolution from water. The GCE, modified by Ru⁰/CeO₂ (1.86 wt % Ru), provides an incredible electrocatalytic activity with a high exchange current density of 0.67 mA·cm^-2, low overpotential of 47 mV at j = 10 mA·cm^-2, and small Tafel slope of 41 mV·dec^-1. Moreover, this modified GCE provides an unprecedented long-term stability without changing the onset potential (33 mV) even after 10 000 scans in acidic water splitting at room temperature. The hydrogen gas, evolved during the water splitting using the Ru⁰/CeO₂ (1.86 wt % Ru) electrocatalyst, was also collected. The amount of the evolved H₂ gas matches well with the calculated value, which indicates the achievement of nearly 100% Faradaic efficiency.

Dust Effects on Nucleation Kinetics and Nanoparticle Product Size Distributions: Illustrative Case Study of a Prototype Ir(0)n Transition-Metal Nanoparticle Formation System

The question is addressed if dust is kinetically important in the nucleation and growth of Ir(0)_n nanoparticles formed from [Bu₄N]₅Na₃(1,5-COD)Ir^I·P₂W₁₅Nb₃O₆₂ (hereafter [(COD)Ir·POM]^8-), reduced by H₂ in propylene carbonate solvent. Following a concise review of the (often-neglected) literature addressing dust in nucleation phenomena dating back to the late 1800s, the nucleation and growth kinetics of the [(COD)Ir·POM]^8- precatalyst system are examined for the effects of 0.2 μm microfiltration of the solvent and precatalyst solution, of rinsing the glassware with that microfiltered solvent, of silanizing the glass reaction vessel, for the addition of <0.2 μm γ-Al₂O₃ (inorganic) dust, for the addition of flame-made carbon-based (organic) dust, and as a function of the starting, microfiltered [(COD)Ir·POM^8-] concentration. Efforts to detect dust and its removal by dynamic light scattering and by optical microscopy are also reported. The results yield a list of eight important conclusions, the four most noteworthy of which are (i) that the nucleation apparent rate "constant" k_1obs(bimol) is shown to be slowed by a factor of ∼5 to ∼7.6, depending on the precise experiment and its conditions, just by the filtration of the precatalyst solution using a 0.20 μm filter and rinsing the glassware surface with 0.20 μm filtered propylene carbonate solvent; (ii) that simply employing a 0.20 μm filtration step narrows the size distribution of the resulting Ir(0)_n nanoparticles by a factor of 2.4 from ±19 to ±8%, a remarkable result; (iii) that the narrower size distribution can be accounted for by the slowed nucleation rate constant, k_1obs(bimol), and by the unchanged autocatalytic growth rate constant, k_2obs(bimol), that is, by the increased ratio of k_2obs(bimol)/k_1obs(bimol) that further separates nucleation from growth in time for filtered vs unfiltered solutions; and (iv) that five lines of evidence indicate that the filterable component of the solution, which has nucleation rate-enhancing and size-dispersion broadening effects, is dust.

Nanoceria supported cobalt(0) nanoparticles: a magnetically separable and reusable catalyst in hydrogen generation from the hydrolysis of ammonia borane

Co⁰/CeO₂ is a magnetically separable and highly reusable catalyst in the hydrolysis of ammonia borane.

Palladium(0) Nanoparticle Formation, Stabilization, and Mechanistic Studies: Pd(acac)₂ as a Preferred Precursor, [Bu₄N]₂HPO₄ Stabilizer, plus the Stoichiometry, Kinetics, and Minimal, Four-Step Mechanism of the Palladium Nanoparticle Formation and Subsequent Agglomeration Reactions

Palladium(0) nanoparticles continue to be important in the field of catalysis. However, and despite the many prior reports of Pd(0)n nanoparticles, missing is a study that reports the kinetically controlled formation of Pd(0)n nanoparticles with the simple stabilizer [Bu4N]2HPO4 in an established, balanced formation reaction where the kinetics and mechanism of the nanoparticle-formation reaction are also provided. It is just such studies that are the focus of the present work. Specifically, the present studies reveal that Pd(acac)2, in the presence of 1 equiv of [Bu4N]2HPO4 as stabilizer in propylene carbonate, serves as a preferred precatalyst for the kinetically controlled nucleation following reduction under 40 ± 1 psig initial H2 pressure at 22.0 ± 0.1 °C to yield 7 ± 2 nm palladium(0) nanoparticles. Studies of the balanced stoichiometry of the Pd(0)n nanoparticle-formation reaction shows that 1.0 Pd(acac)2 consumes 1.0 equiv of H2 and produces 1.0 equiv of Pd(0)n while also releasing 2.0 ± 0.2 equiv of acetylacetone. The inexpensive, readily available HPO4(2-) also proved to be as effective a Pd(0)n nanoparticle stabilizer as the more anionic, sterically larger, "Gold Standard" stabilizer P2W15Nb3O62(9-). The kinetics and associated minimal mechanism of formation of the [Bu4N]2HPO4-stabilized Pd(0)n nanoparticles are also provided, arguably the most novel part of the present studies, specifically the four-step mechanism of nucleation (A → B, rate constant k1), autocatalytic surface growth (A + B → 2B, rate constant k2), bimolecular agglomeration (B + B → C, rate constant k3), and secondary autocatalytic surface growth (A + C → 1.5C, rate constant k4), where A is Pd(acac)2, B represents the growing, smaller Pd(0)n nanoparticles, and C represents the larger, most catalytically active Pd(0)n nanoparticles. Additional details on the mechanism and catalytic properties of the resultant Pd(0)n·HPO4(2-) nanoparticles are provided in this work.

Palladium(0) nanoparticles supported on polydopamine coated Fe3O4 as magnetically isolable, highly active and reusable catalysts for hydrolytic dehydrogenation of ammonia borane

Palladium(0) nanoparticles supported on polydopamine coated magnetic ferrite nanopowders are highly active and reusable catalyst in hydrogen generation from the hydrolysis of ammonia borane with a turnover frequency of 14.5 min⁻¹ at 25.0 ± 0.1 °C.

Ceria-supported ruthenium nanoparticles as highly active and long-lived catalysts in hydrogen generation from the hydrolysis of ammonia borane

Ruthenium(0) nanoparticles supported on ceria (Ru⁰/CeO₂) were in situ generated from the reduction of ruthenium(iii) ions impregnated on ceria during the hydrolysis of ammonia borane.

Immobilization of a molybdenum complex on the surface of magnetic nanoparticles for the catalytic epoxidation of olefins

A molybdenum complex was immobilized on Fe₃O₄ nanoparticles modified with chloropropyl by two routes and used as a nanocatalyst for the oxidation of alkenes.

A New Homogeneous Catalyst for the Dehydrogenation of Dimethylamine Borane Starting with Ruthenium(III) Acetylacetonate

The catalytic activity of ruthenium(III) acetylacetonate was investigated for the first time in the dehydrogenation of dimethylamine borane. During catalytic reaction, a new ruthenium(II) species is formed in situ from the reduction of ruthenium(III) and characterized using UV-Visible, Fourier transform infrared (FTIR), ¹H NMR, and mass spectroscopy. The most likely structure suggested for the ruthenium(II) species is mer-[Ru(N₂Me₄)₃(acac)H]. Mercury poisoning experiment indicates that the catalytic dehydrogenation of dimethylamine-borane is homogeneous catalysis. The kinetics of the catalytic dehydrogenation of dimethylamine borane starting with Ru(acac)₃ were studied depending on the catalyst concentration, substrate concentration and temperature. The hydrogen generation was found to be first-order with respect to catalyst concentration and zero-order regarding the substrate concentration. Evaluation of the kinetic data provides the activation parameters for the dehydrogenation reaction: the activation energy E_a = 85 ± 2 kJ·mol⁻¹, the enthalpy of activation ∆H^# = 82 ± 2 kJ·mol⁻¹ and the entropy of activation; ∆S^# = −85 ± 5 J·mol⁻¹·K⁻¹. The ruthenium(II) catalyst formed from the reduction of ruthenium(III) acetylacetonate provides 1700 turnovers over 100 hours in hydrogen generation from the dehydrogenation of dimethylamine borane before deactivation at 60 °C.

Notizen: 13C NMR Spectroscopic Study of Pentacarbonylacetonitrilemetal(0) Complexes of the Group 6 B Elements

The complexes M(CO)5(CH3CN) (M: Cr, Mo, W) were obtained from the substitution of THF in M(CO)5(THF) which has been generated by photolysis of M(CO)6 in THF and characterized by using IR, 1H NMR and 13C NMR spectroscopies. The acetonitrile ligand is found to be N-bonded to the M(CO)5-moiety with a local C4v-symmetry. The effect of the acetonitrile ligand on the metal-carbonyl bonding was discussed in terms of 13C NMR chemical shifts.

Synthesis and Spectroscopic Study of Pentacarbonyl(η2-tetracyanoethylene) Metal(0) Complexes of the Group 6 B Elements

Pentacarbonyl(η2-tetracyanoethylene) metal(0) complexes of chromium, molybdenum and tung­sten have been synthesized by the photochemical reaction of hexacarbonyl metal(o) with tetra- cyanoethylene in toluene at room temperature. The complexes were purified by chromatography and recrystallization, and characterized by UV- visible, IR and 13C NMR spectroscopy. Tetra- cyanoethylene is symmetrically bonded to the M(CO)5 unit through its carbon-carbon double bond as an η2-ligand. The spectral data are dis­cussed in terms of the metal → ligand π inter­action.

[μ-Bis(dialkylphosphino)alkane]-bis[pentacarbonylmetal(0)] Complexes of the Group 6 B Elements: Synthesis and Spectroscopic Study

[μ-Bis(dialkylphosphino)alkane]-bis[pentacarbonylmetal(0)] complexes (CO)5MR2P(CH2)nPR2M(CO)5, (M: Cr, Mo, W; n: 1, 2, 3; R: CH3, C2H5, C6H5) were synthesized from the reaction of M(CO)5(CH3CN) with the appropriate ligand and characterized by means of IR and NMR (1H , 13C, 31P) spectroscopy. Spectral data indicate that the complexes contain two identical M(CO)5 moieties linked to each other by the R2P(CH2)nPR2 ligand. Each metal atom has a pseudo-octahedral arrangement of the five CO ligands and one phosphorus atom .

Notizen: Photocatalytic 1,4-Hydrosilation of 1,3-Butadiene with Triethylsilane

Chromium and molybdenum carbonyl photocatalyzed hydrosilation of 1,3-butadiene by triethylsilane yields exclusively the 1,4-adduct, cis-l-triethylsilyl-2-butene which has been fully characterized. Mechanistic possibilities for photocatalytic hydrosilation reactions have been put forward.

Notizen: Synthesis and NM R Study of (η6-1,4-diphenyl-1,3-butadiene)tricarbonylchromium(0)

(η6-1,4-Diphenyl-1,3-butadiene)tricarbonylchromium(0) has been synthesized in high yield by using an improved procedure, from tris(acetonitrile)chromium(0) and 1,4-diphenyl-1,3-butadiene. The IR and NMR spectroscopic data show that the organic ligand is bonded to the Cr(CO)3 moiety through only one of the two phenyl rings.

Photo-Induced Chromiumcarbonyl Catalyzed Hydrosilylation of Conjugated Dienes with Triethylsilane: The Solvent Effect

Photocatalytic hydrosilylation of conjugated dienes (1,3-butadiene, 2-methyl-1,3-butadiene, 2,3- dimethyl-1,3-butadiene, trans-1,3-pentadiene) with triethylsilane was studied by using Cr(CO)5L (L = CO, P(CH3)3, P(OCH3)3, P(C6H5)3, P(C6H11)3, NC5H5) in two very different solvents, toluene and tetrahydrofuran, for comparison with the results found in n-hexane. In toluene, the photocatalytic hydrosilylation yields the same products as those in n-hexane, with the exception of trans- 1,3-pentadiene which gives cis-1-triethylsilyl-2-pentene as the sole product. However, each of the precursor complexes shows different catalytic activities in toluene and n-hexane. The hydrosilylation of 1,3-butadiene in toluene is, in general, significantly faster than that in n-hexane. By using Cr(CO)6, Cr(CO)5[P(CH3)3] or Cr(CO)5[P(OCH3)3] in toluene, the conversion of triethylsilane increases almost linearly as the reaction proceeds, indicating the stability of the active catalyst throughout the reaction, similar to that in n-hexane. While no hydrosilylation of 1,3-butadiene could be achieved with Cr(CO)5[P(C6H5)3] or Cr(CO)5(NC5H5) in n-hexane, the same precursor complexes appear to be active in toluene, though the conversion occurs at much lower rate compared to that obtained using Cr(CO)5[P(CH3)3] or Cr(CO)5[P(OCH3)3]. The precursor complex Cr(CO)5[P(C6H11)3] shows catalytic activity neither in toluene nor in n-hexane. No photocatalytic hydrosilylation of 1,3-butadiene with triethylsilane was observed in tetrahydrofuran by using any of the precursor complexes. The relative reactivity of conjugated dienes in the hydrosilylation was investigated by using triethylsilane in the presence of Cr(CO)5[P(OCH3)3] as catalyst in toluene, and the same reactivity order was obtained as in n-hexane solution: 1,3-butadiene > 3-methyl-1,3-butadiene > 2,3-dimethyl-1,3-butadiene > trans-1,3-pentadiene. For all of the dienes, one obtains higher conversion to hydrosilylated product in toluene than in n-hexane.

Gehinderte Ligandenbewegungen in Übergangsmetallkomplexen, XII1 Tricarbonyl-η6-N-carbäthoxyazepin-chrom(0), -molybdän(0) und -wolfram(0) / Hindered Ligand Motions in Transition Metal Complexes, XII Tricarbonyl-η6-N-carbethoxyazepine-chromium(0), -molybdenum(0) and -tungsten(0)

Tricarbonyl-η6-N-carbethoxyazepine-metal(0) complexes of chromium, molybdenum and tungsten are formed, when tricarbonyl-trisacetonitrile-chromium(0), tricarbonyldiglyme-molybdenum(0) and tricarbonyl-trisacetonitrile-tungsten(0), respectively, are treated with N-carbethoxyazepine. According to the 1H and 13C NMR spectra of the N-carbethoxyazepine metal complexes two different ligand motions can be activated thermally. The carbethoxy group rotates around the C-N-bond (Δ G300 ≠ = 64.6 to 66.1 kJ • mole-1) and the azepine ligand itself moves against the tricarbonyl metal groups (Δ G250≠ 45.6-52.1 kJ • mole-1).

Rhodium(0) nanoparticles supported on nanotitania as highly active catalyst in hydrogen generation from the hydrolysis of ammonia borane

Rhodium(0) nanoparticles supported on nanotitania as a highly active catalyst in hydrogen generation from the hydrolysis of ammonia borane.