Progress 11/15/02 to 11/14/06
Outputs
This report summarizes work over the past four years on the conversion of crop-derived organic acids to value-added chemical intermediates and products. The focus of the work has been on unraveling the detailed chemical pathways involved in organic acid hydrogenation over heterogeneous transition metal catalysts in water. In concurrence with the project objectives related to organic acid hydrogenation, our research has focused on: 1) catalytic conversion rates of structurally related organic acids; 2) adsorption of organic acids onto catalyst metal surfaces; and 3) isotope exchange of organic and amino acids as a mechanistic probe of substrate-catalyst interactions. We have carried out an extensive study of the hydrogenation of numerous two- and three-carbon organic acids. Acetic (C2) acid and its substituted analogs investigated include acetic acid, propionic acid, glycolic acid, glycine, sarcosine (N-methyl-glycine), N,N-dimethyl glycine, betaine
(N,N,N-trimethylglycine chloride), and methoxyacetic acid. A similar set of three-carbon acids was investigated. Several results stand out. Acetic acid is much less reactive than nearly all of its substituted analogs - the order of reactivity with substituent group on the C2 carbon is -NH2 > -OH > -OCH3 > -NH(CH3) > N(CH3)2 > CH3 > H (acetic acid). Interestingly, the N-methyl substituted glycines show a sharp decline in conversion rate, about 30% per methyl group added. We have conducted a detailed kinetic analysis of the hydrogenation of the C3 acids lactic acid and propionic acid both individually and in mixtures; kinetic parameters from single acid experiments can be used to predict hydrogenation rates in acid mixtures. We have also carried out a detailed kinetic and mechanistic investigation of H-D isotope exchange in N-substituted glycines. The H-D exchange in a compound such as N-methyl glycine in D2O solution takes place in the presence only of hydrogen (or D2) and the metal
catalyst. The experimental extent of H-D exchange, determined by 1H-NMR spectroscopy, is correlated to a simple reaction model that predicts behavior with excellent accuracy and reliability, thus giving a clear picture of the surface chemistry and interactions taking place. The H-D exchange takes place both on the methyl group and on the C2 carbon, with the rate of exchange at C2 substantially faster than on the methyl group. A third major thrust is the adsorption of biomass-derived molecules from aqueous solution onto metal catalyst surfaces. We have developed a recirculating reactor system that allows rapid, sensitive measurement of quantity adsorbed on a metal surface. Many interesting findings have surfaced: for instance, we have shown that glycerol adsorbs onto ruthenium metal only if that surface is hydrogen free. This supports the initial step of glycerol conversion previously postulated in the literature - that of dehydrogenation of glycerol to form a surface aldehyde analog.
Surprisingly, the presence of hydrogen does not affect lactic acid adsorption, suggesting that the mode of acid hydrogenation is different. We have characterized both extent and strength of adsorption of several species.
Impacts
This project has significantly advanced our technical understanding of the chemical transformations of biomass-derived species in water. Understanding chemical transformations in water is a critical component in building the nation's biorefining capability, as most small molecules derived from biomass, particularly organic acids, are nonvolatile and soluble in water, thus making water the only logical environment for reaction. Further, the design of processes for the biorefinery, including choice of conditions, depends on a deep understanding of the chemistry taking place, and this work has advanced the understanding of organic acid hydrogenation. Because water has not been widely used as a reaction environment for catalysis, our work provides novel results and new interpretations of this novel reaction setting.
Publications
- Jere, F.T., Jackson, J.E., Miller, D.J. 2004. Kinetics of the Aqueous Phase Hydrogenation of L-Alanine to L-Alaninol. I&EC Research 43:3297-3304.
- Dalavoy, T.S., Jackson, J.E., Swain, G.M., Miller, D.J., Lei, J., Lipkowski, J. 2007. Mild electrocatalytic hydrogenation of lactic acid to lactaldehyde and propylene glycol. J. Catal. 246: 15-28.
- Peereboom, L., Koenigsknecht, B.,Hunter, M., Jackson, J.E., Miller, D.J. 2007. Aqueous Phase adsorption of glycerol and propylene glycol onto activated carbon. Carbon in press.
- Chen, Y., Jackson, J.E., Miller, D.J. 2007 Kinetics of Aqueous Phase Hydrogenation of Organic Acids and their Mixtures over Carbon-supported Ruthenium Catalyst. Ind. & Eng. Chem. Res. in revision.
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Progress 01/01/05 to 12/31/05
Outputs
Research efforts in 2005 focused on two fronts: continuing effect of alpha-substituent groups on the hydrogenation rate of organic acids in water, and the study of organic acid adsorption into catalyst support materials. Both lend key insight into the development of technologies for producing chemicals from renewable resources. The effects of vicinal substituents on organic acids reactivity and selectivity toward hydrogenation were studied by comparing reduction rates over a wide range of electron-withdrawing, hydrogen-bonding, or sterically vicinal substituents. Organic acids bearing different vicinal functional groups, X, (X=H, CH3, OCH3, OH, NH3+, NH2CH3+, NH(CH3)2+, N(CH3)3+) have revealed varying reactivity in aqueous catalytic hydrogenation using Ru/C as a catalyst. Specifically, protonated alanine (CH3CHNH3+COOH) and lactic acid (CH3CHOHCOOH) undergo aqueous-phase hydrogenation over a carbon-supported ruthenium catalyst ca. 10 and 2-3 times as fast as their
unsubstituted analogue, propanoic acid. The general trend in reactivity for the two- and three-carbon compounds was: +NH3 > OH > OCH3, +NH2CH3 > +NH(CH3)2 > CH3 > H > +N(CH3)3. Three-carbon organic acids showed slightly higher conversion and selectivity compared to the two-carbon analog. As methylation increases in the amino species, the rate is reduced. This series was studied at three different temperatures; reaction rate data was used to understand the electronic and steric controllers in the hydrogenation reaction. At 130 degrees C, the rate of hydrogenation for protonated glycine, protonated N-methylglycine, and protonated N,N-dimethylglycine was 1.056x10-4, 0.239x10-4, and 0.172x10-4 sec-1. The results also suggest organic acid hydrogenation with 5% Ru/C is a relatively clean reaction. High selectivity is achieved and carbon balances close with greater than 90%. 1H NMR studies in the presence of deuterium oxide and deuterium gas were used to monitor H/D exchange at the alpha
position of several organic acids. The kinetics of H/D exchange were monitored, particularly in relation to hydrogenation, to give insight into rate-limiting steps in the conversion. Results support our initial findings that activated methylenes promote both hydrogenation and H/D exchange with Ru/C. Selective adsorption of reactants and products into activated carbon catalysts in organic acid hydrogenation was studied in equilibrium experiments. We found that acid adsorption increased with decreasing hydrocarbon character of the acid; e.g. the preference of adsorption was propionic > acetic > lactic > glycolic. Thus, the local species concentrations in the pore is significantly different at reaction conditions than the surrounding bulk fluid, thus strongly affecting reaction kinetics and selectivity.
Impacts
Our work continues to produce valuable insight into unraveling the chemistry and physics of converting crop-based renewable substrates to useful chemical products currently made from petroleum. The work conducted here is at the forefront of catalytic conversions in water.
Publications
- No publications reported this period
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Progress 01/01/04 to 12/31/04
Outputs
During the past year we have made substantial progress in achieving project goals and objectives with regards to developing high value products from biomass-derived organic acids. Together, the results we have obtained have substantially deepened our understanding of the reactivity of organic acids in water and other media. Project Objective 1) focuses on the effect of organic acid substrate structure on reactivity and selectivity to desired products. We have conducted aqueous phase hydrogenation of a set of substituted organic acids in the presence of a ruthenium catalyst as a test measure of reactivity. Initially, we examined C2-substituent groups on the C3 alkanoic acid including H (propanoic acid), OH (lactic acid), NH2 (alanine), and OCH3 (methoxy-propanoic acid), along with the corresponding C2 substituted C2 acids. We also showed that solvents other than water reduce reactivity of organic acids, with pure ethanol solvent essentially stopping hydrogenation.
Results this year focus on a set of N-substituted C2 amino acids including glycine, N-methyl glycine, N,N-dimethyl glycine, and betaine, the trimethyl chloride salt of glycine. Addition of each methyl group to the amine functionality reduces reactivity by about a factor of two, with betaine being entirely unreactive. This indicates the extent to which the nature of the C2 substituent affects organic acid reactivity in hydrogen, and essentially completes our work on Objective 1. For Objective 2), we are preparing to conduct adsorption of selected C3 organic acids and solvents onto pure ruthenium metal now that we have completed Objective 1). We have already conducted detailed adsorption studies on the products of organic acid hydrogenation, particularly ethylene glycol and propylene glycol. The results correspond closely to the reactivity of these species at hydrogenation conditions. For Objective 3), to examine the mechanism of activation and stereoretention, we are currently
characterizing the extent of H-D exchange and degree of stereoretention in N-substituted glycine. Combining these results with those on reactivity will give specific insights into the interaction of organic acids with metal surfaces and link together structure and reactivity with C-H bond activation at C2. Finally, in regard to Objective 4), we have continued to find success in the electrocatalytic hydrogenation of phenyl glyoxylic acid and lactic acid to their alcohol products. Electrocatalytic hydrogenation is attractive and a potential breakthrough technology, because hydrogen is generated directly at the catalyst surface so that reaction occurs very readily at low temperature and low pressure. This year, we have progressed from exceptionally dilute lactic acid concentrations (10-5 M) to higher values (0.1 M), and have achieved high conversion and selectivity to propylene glycol. We are currently examining high surface area electrodes to facilitate even higher reaction rates. Our
efforts this year have been highlighted by several technical presentations at national meetings. We have several papers in preparation for publication that should be submitted in the next several months.
Impacts
Organic acids constitute an important class of crop-derived chemical feedstocks. This project will provide a much deeper understanding of the chemistry of converting these feedstocks to value-added products than has been previously reported. This will in turn facilitate novel processing routes to chemicals that will make the biorefinery a much more profitable endeavor.
Publications
- Jere, F.T., Jackson, J.E., and Miller, D.J. 2004. Kinetics of the aqueous phase hydrogenation of L-alanine to L-alaninol. I&EC Research 43:3297.
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Progress 01/01/03 to 12/31/03
Outputs
The overall goal of this project is to gain a detailed chemical understanding of the aqueous phase hydrogenation of biomass-derived organic acids to value-added products. More specifically, project objectives involve 1) examining the dependence of reaction rate and product selectivity on substrate structure in the aqueous phase hydrogenation of crop-derived organic acids to value-added products, 2) characterizing the adsorption of organic acids and their hydrogenation products onto the hydrogenation catalysts, and 3) to examine novel means for activating hydrogenation and related reactions of organic acids. In the past year, we have completed an experimental study that characterizes hydrogenation rate of alpha-substituted C2- and C3- alkanoic acids as a function of the substituent group. The acids (substituent in parentheses) investigated include acetic acid (-H), glycolic acid (-OH), glycine (-NH2), and N-methyl glycine (-NHCH3) as C2 acids, and propanoic acid (-H),
lactic acid (-OH), alanine (-NH2), and methoxy-propanoic acid (-OCH3)as C3 acids. Lactic acid and glycolic acid have the highest conversion rates, and the alkanoic acids (acetic and propanoic) have the lowest conversion rates. Selectivity of over 80% to propylene glycol and ethylene glycol, respectively, were observed from lactic acid and propanoic acid. The low reactivity of propanoic acid and the low selectivity to n-propanol were somewhat striking. In addition to the substituent study, we also examined the effect of different solvents on the rate of hydrogenation. Surprisingly, the conversion rate of glycolic acid in dry ethanol or in dry tetrahydrofuran is much slower than in water; this is despite a much higher hydrogen solubility in the organic media than in water. This suggests that either water plays a key role in the activation of the catalyst or substrate for catalytic hydrogenation, or that these solvents preferentially adsorb on the catalyst surface and block the organic
acid from access to surface hydrogen required for hydrogenation. Objective 2) of the project will focus on unraveling the reasons behind the solvent effects. Finally (Objective 3), we have made an exciting discovery this past year: we observe direct electrocatalytic hydrogenation of an organic acid, phenyl glyoxylic acid, in water at room temperature. The electrocatalytic hydrogenation involves the application of a small potential across a platinum electrode in water; molecular hydrogen is generated directly from water at the catalyst surface and becomes immediately available for reaction. This discovery is a potential breakthrough for the conversion of bio-based substrates, because hydrogen is produced at the catalyst surface and does not have to be introduced externally. Indeed, hydrogen transport to the catalyst surface is normally the rate-limiting step in liquid-phase hydrogenation. Thus, a successful electrocatalytic process would facilitate greatly enhanced reaction rates,
lower processing costs because no hydrogen purchase would be necessary, and would facilitate high product selectivity with retention of stereochemistry because of the low temperatures amenable to reaction.
Impacts
Organic acids constitute an important class of crop-derived chemical feedstocks. This project will provide a much deeper understanding of the chemistry of converting these feedstocks to value-added products than has been previously reported. This will in turn facilitate novel processing routes to chemicals that will make the biorefinery a much more profitable endeavor.
Publications
- No publications reported this period
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