Green Chemistry Cover Article: Critical Review
"Selective Oxidation of Alcohols and Aldehydes over Supported Metal Nanoparticles"
Hydrogenolysis of Glycerol
Biorenewable resources such as carbohydrates are alternative feedstocks for the production of oxygenated chemicals. In some processing schemes, conversion of carbohydrates involves the initial hydrogenation of a simple sugar, such as glucose, to the sugar alcohol sorbitol. The subsequent hydrogenolysis of sorbitol yields lower molecular weight polyols like glycerol, propylene glycol and ethylene glycol, along with organic acids such as lactic acid. Glycerol, which is also a byproduct from biodiesel production, reacts further to glycols and lactic acid. All of these reactions are typically conducted in the aqueous phase in the presence of a supported transition metal catalyst.
Our
group is investigating a variety of monometallic (Ru, Pt) and
bimetallic catalysts (Pt-Ru, Au-Ru, Pt-Re) for the hydrogenolysis of
glycerol. Interestingly, Re promotes the activity and shifts the
selectivity of Pt to produce some 1,3 propanediol, which is not observed
over monometallic catalysts. Extensive characterization of the metal
catalysts by microscopy, X-ray absorption spectroscopy and chemisorption
is a major component of this work.
Figure 1.
Propanediols were produced by hydrogenolysis of glycerol over nanometer
size carbon-supported PtRe particles at 443 K, whereas analogous
monometallic particles were inactive. X-ray absorption spectroscopy
above the Pt and Re LIII edges revealed that both metals were reduced by
H2 at 473 K to form bimetallic particles. The presence of oxophilic Re
in contact with Pt promoted the selective hydrogenolysis reaction [from
ChemCatChem, 2 (2010) 1107.]
Oxidation of Glycerol
While
the hydrogenolysis of glycerol is one option for the utilization of
biorenewable resources, the oxidation of glycerol also offers a
promising route for creating specialty chemicals and pharmaceutical
intermediates. Supported Au catalysts exhibit excellent rates and little
deactivation during the oxidation of alcohols to carboxylic acids
compared to both supported Pd and Pt catalysts. However, Au catalysts
suffer from lower selectivity. Investigation of Au catalysts in the
aqueous-phase oxidation of glycerol (303-333 K, 1-10 atm O2, base
required) indicated that the in-situ formation of H2O2 leads to the
formation of the C-C cleavage product, glycolic acid. Studies to limit
the formation of H2O2 and increase the selectivity to glyceric acid are
ongoing.
Figure 2. Proposed
pathway for the aqueous phase oxidation of glycerol over supported Au
catalysts in the presence of a strong base. Species within boxes are
proposed intermediates that were never directly isolated.
Transesterification for Biodiesel Synthesis
Biodiesel is an attractive biorenewable alternative to petroleum-based transportation fuels. It does not contribute significantly to the greenhouse effect, contains no aromatic species and is free of sulfur. The main components of vegetable oils and animal fats, triglycerides, are commonly used as feedstocks for the production of biodiesel. These triglycerides can undergo catalytic transesterification with short chain alcohols such as methanol and ethanol to form monoalkyl esters known as biodiesel.
Figure 3. Transesterification of triglycerides with methanol.
Currently, the most common way to produce biodiesel is by liquid-phase reaction involving a homogeneous alkali metal hydroxide or methoxide catalyst. This process has several drawbacks including the purification of byproduct glycerol and treatment of wastewater. These problems arise from the use of a homogeneous base catalyst that must be neutralized at the end of the reaction. To alleviate these problems, use of a solid base catalyst is proposed.
In this study,
solid bases derived from Mg-Al hydrotalcite are being explored as
transesterification catalysts. The activity of the catalysts depends on
the type and density of the base sites on the catalysts as well as the
overall level of hydration. Ongoing work seeks to understand how the
nature of the base sites influences the transesterification reaction and
the deactivation mechanism.
Hydrogenation of α,β-unsaturated ketones and aldehydes
Our
group is a member of the Center for Biorenewable Chemicals (CBiRC), an
NSF-sponsored Engineering Research Center (ERC) that focuses on the
production of chemicals from biorenewable feedstocks. One chemical
transformation of interest to the Center involves the selective
hydrogenation of the carbonyl group in α,β-unsaturated ketones and
aldehydes over a supported metal catalyst in aqueous solvent without
hydrogenating the C=C bond.
Our work in particular has studied
the selective hydrogenation of methyl vinyl ketone and benzalacetone
over a variety of supported metal catalysts, including gold. The effects
of C=O position in the molecule, steric hindrance of substituent
groups, metal type and support composition on the turnover frequency and
product selectivity are being investigated.
Figure 4. Selective hydrogenation of α,β-unsaturated ketone.
Deoxygenation of Fatty Acids
Fatty acids found in natural fats and oils are potential feedstocks for biorenewable fuels and chemicals. However, the presence of oxygen in fatty acids decreases its quality and stability, especially for fuel. Therefore, removal of oxygen from biorenewable fatty acids is needed to enhance fuel enthalpies, decrease viscosity, and enhance thermal stability. In this work, we are studying deoxygenation reactions in which the oxygen from the carboxylic acid is removed via production of CO2 and/or CO, thus producing a linear hydrocarbon, as shown below. As part of our participation in the Center for Biorenewable Chemicals (CBiRC), we are exploring the roles of metal-support composition and reaction conditions on the activity and selectivity of transition metal catalyst for fatty acid deoxygenation reactions.
Figure 5. Reaction pathway for deoxygenation of fatty acids
Recent Publications
E.P. Maris, W.C. Ketchie, V. Oleshko and R.J. Davis, "Metal Particle Growth During Glucose Hydrogenation over Ru/SiO2 Evaluated by X-Ray Absorption Spectroscopy and Electron Microscopy," J. Phys. Chem. B, 110 (2006) 7869-7876.
W.C. Ketchie, M. Murayama and R.J. Davis, "Promotional Effect of Hydroxyl on the Aqueous Phase Oxidation of Carbon Monoxide and Glycerol over Supported Au Catalysts," Topics in Catal. 44 (2007) 307-317.
W.C. Ketchie, E.P. Maris and R.J. Davis, “In-situ X-ray Absorption Spectroscopy of Supported Ru Catalysts in the Aqueous Phase,” Chem. Mater. 19 (2007) 3406-3411.
E.P. Maris and R.J. Davis, “Hydrogenolysis of Glycerol over Carbon-Supported Ru and Pt Catalysts,” J. Catal. 249 (2007) 328-337.
W.C. Ketchie, Y.-L. Fang, M.S. Wong, M. Murayama and R.J. Davis, “Influence of Gold Particle Size on the Aqueous-phase Oxidation of Carbon Monoxide and Glycerol,” J. Catal. 250 (2007) 94-101.
W.C. Ketchie, M. Murayama and R.J. Davis, “Selective Oxidation of Glycerol over Carbon-Supported AuPd Catalysts,” J. Catal. 250 (2007) 264-273.
E.P. Maris, W.C. Ketchie, M. Murayama and R.J. Davis, “Glycerol Hydrogenolysis on Carbon-Supported PtRu and AuRu Bimetallic Catalysts,” J. Catal. 251 (2007) 281-294.
Y. Xi and R.J. Davis, “Influence of Water on the Activity and Stability of Activated Mg-Al Hydrotalcites for the Transesterification of Tributyrin with Methanol” J. Catal. 254 (2008) 190-197.
O.M. Daniel, A. DeLaRiva, E.L. Kunkes, A.K. Datye, J.A. Dumesic and R.J. Davis, “X-ray Absorption Spectroscopy of Bimetallic PtRe Catalysts for Hydrogenolysis of Glycerol to Propanediols” ChemCatChem 2 (2010) 1107-1114.
S.E. Davis, L.R. Houk, E.C. Tamargo, A.K. Datye, and R.J. Davis, “Oxidation of 5-Hydroxymethylfurfural over Supported Pt, Pd and Au Catalysts,” Catal. Today 160 (2011) 55-60.
B.N. Zope and R.J. Davis, “Inhibition of Gold and Platinum Catalysts by Reactive Intermediates Produced in the Selective Oxidation of Alcohols in Liquid Water,” Green Chem. 13 (2011) 3484-3491.
M. Chia, Y.J. Pagán-Torres, D. Hibbitts, Q. Tan, H.N. Pham, A.K. Datye, M. Neurock, R.J. Davis, and J.A. Dumesic, “Selective hydrogenolysis of polyols and cyclic ethers over bifunctional surface sites on rhodium-rhenium catalysts,” J. Am. Chem. Soc. 133 (2011) 12675-12689.
S.E. Davis, B.N. Zope and R.J. Davis “On the mechanism of selective oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid over supported Pt and Au catalysts” Green Chem. 15 (2013) 17-45.
S.E. Davis, M.S. Ide and R.J. Davis “ Selective oxidation of alcohols and aldehydes over supported metal nanoparticles ” Green Chem. 14 (2012) 143-147.