Progress 09/01/98 to 08/31/03
Outputs
This project focused on hydrogenation of crop-derived organic acids to value-added alcohols. The project objectives were 1) develop catalysts and processing conditions for lactic acid (LA) conversion to propylene glycol (PG); 2) examine amino acid conversion to optically active amino alcohols; 3) investigate direct conversion of polylactic acid and polyhydroxybutyric acids to alcohols as a means of adding value in recycling; and 4) develop a fundamental, mechanistic understanding of organic acid hydrogenation that forms the basis for further catalyst development. Objective 1: Hydrogenation of optically pure L-lactic acid (LA, ee = 98%) as a 10 wt% solution in water at 135oC and 10.2 MPa H2 pressure over Ru/C catalyst gave 75% LA conversion after 4 hr with 96% selectivity to PG. The enantiomeric purity (ee) of the product PG and unreacted LA stay above 90% over the course of reaction. Reaction in D2/D2O at these conditions show that product PG is deuterated at the
primary (C1) carbon and to a lesser extent at the secondary (C2) carbon. This suggests that the primary mode of hydrogenation involves C1 only, e.g., the carboxylic acid carbon of LA. Unreacted LA also shows deuteration at the C2 carbon. Surprisingly, the H-D exchange at C2 in both PG and LA takes place without loss of optical purity at low temperature (<110oC). Since the intermediate in H-D exchange at C2 is a deprotonated, planar form of LA, one would expect that subsequent deuteration would take place on either face of the intermediate to give a racemic deuterated product. The fact that this does not happen indicates that, at low temperature, the adsorbed intermediate has only one face available for deuteration/rehydrogenation. Objective 2: Hydrogenation of amino acids has been examined in a manner analogous to LA with similar results. A stoichiometric quantity of mineral acid must be added to a reacting amino acid solution to maintain the carboxylic acid functionality in the
protonated, reactive form; conversion rate, product selectivity, and enantiomeric retention are all slightly better than for lactic acid. Objective 3: We have successfully demonstrated the direct hydrogenation of polylactic acid (PLA) polymers to PG. Interestingly, we observe that formation of PG from PLA during hydrogenation is more rapid than the rate of monomer LA liberation via hydrolysis. We postulate that this occurs because hydrogenation of the ester linkages on PLA is more rapid than simple PLA hydrolysis. Thus, the polymer chains are cleaved by hydrogenation as they unfold from bulk PLA and transferred into solution, where they more readily undergo hydrolysis and further hydrogenation. Objective 4: Using H-D incorporation, computational molecular modeling, and molecular spectroscopy, we have completed a mechanistic picture for LA hydrogenation to PG. The reaction proceeds first via hydrogenation at C1, followed by dehydration to form lactaldehyde, which then undergoes a
second hydrogenation to PG. An alternative pathway involving C2 is postulated as an initial keto-enol tautomerization of LA, followed by hydrogenation and then dehydration to lactaldehyde which is hydrogenated to PG.
Impacts
Organic acids are an important class of crop-derived intermediates. The ability to efficiently and economically transform them into higher value alcohols will greatly enhance the viability of a chemicals-from-crops industry in the U.S. This project has led to the development of two reaction systems for producing chiral organic alcohols, propylene glycol and amino alcohols. The project has also advanced the general understanding of organic acid hydrogenation in water, a broad contribution with application across the biobased product industry.
Publications
- Jere, F. T., Miller, D.J., Jackson, J.E. 2003. Surprisingly facile, stereoretentive aqueous phase catalytic hydrogenation of amino acids to amino alcohols. Org. Lett. 5:527-530.
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Progress 01/01/02 to 12/31/02
Outputs
Highlights of progress during 2002 include describing kinetics and stereoretention in amino acid hydrogenation, characterizing the effect of substituent groups on reactivity of alpha-substituted propionic acid, and examining the reactivity of organic acid oligomers and polymers. For amino acid hydrogenation in aqueous solution acidified by phosphoric acid, we have described the reaction mechanism by a Langmuir-Hinshelwood kinetic model that includes protonated alanine and phosphoric acid competing for one type of catalyst surface site and hydrogen dissociatively adsorbing on a different surface site. The novelty of the model rests in the explicit inclusion of ionic species concentrations into the rate expression; this is necessary to account for the zwitterionic nature of amino acids and the requirement that the amino acid be in a protonated state in order for hydrogenation to take place. Model parameters have been optimized via random walk minimization to give an
average error of less than 7% between experimental data and predicted reaction trajectories. In addition to reaction kinetic studies, our observation of H-D exchange at the chiral C2 carbon in alanine and alaninol during hydrogenation, without loss of stereochemistry, has fostered development of a simple and efficient manner for D labeling chiral amines with full retention of stereochemistry. We conduct the exchange in deuterated water at temperatures as low as 30 C under 30 psi H2; these are readily accessible conditions in any well-equipped chemistry laboratory. Since H2 and D2O readily exchange protons under these conditions, D2O becomes the deuterium source for the exchange. This method provides a very cheap alternative to purchasing labeled chiral amines costing $200/gram or more. We have verified that the exchange also readily occurs in cysteine, proline, valine, lysine, and 2-aminobutane. In amino acids with other functional groups, D exchange also occurs at positions adjacent
to electron withdrawing groups such as thiol and hydroxyl. In another aspect of the project, we have completed an initial set of experiments examining the effect of substituent groups in propanoic acid on hydrogenation rate. Results show yields to the alcohol from the acid are in the order 2-chloro-propionic > lactic > 2-acetoxypropionic > 2-methoxypropionic > isobutyric > propionic acids, with relative reaction rates spanning an 8-fold range. These results demonstrate the strong influence of substituent on rate. Finally, we have initiated studies of direct hydrogenation of bio-based organic acid oligomers to the monomer alcohols. Polyhydroxybutyric acid (PHB) was examined at typical reaction conditions, but gave no greater than 15% conversion to 1,3-butaniediol over a 12-hour reaction period. We are currently examining the reaction characteristics of additional biobased oligomers. U.S. Patent 6,403,844, Condensed Phase Catalytic Hydrogenation of Lactic Acid to Propylene Glycol, based
partially on the work completed earlier in this grant, issued June 11, 2002
Impacts
Aqueous phase catalysis is rapidly gaining recognition as a critical component of the biorefinery for a renewable and sustainable chemical industry. However, the scientific understanding of catalytic chemistry in the aqueous phase is in its infancy, both in terms of fundamental scientific understanding and in technology development. The work completed in this project describes a new technology for producing a class of high-value products, chiral amino alcohols, and more generally provides significant insight into conversion of organic acids, a key class of bio-derived feedstocks, to their corresponding alcohols. The understanding of catalysts and substrates gained here will spur development of highly efficient, environmentally friendly processes for bio-based chemicals production, and will thus strengthen the overall economic state of U.S. agriculture.
Publications
- Zhang, Z., Jackson, J.E., Miller, D.J. 2002. Kinetics of Aqueous Phase Hydrogenation of Lactic Acid to Propylene Glycol. I&EC Research 41:691
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Progress 01/01/01 to 12/31/01
Outputs
Progress during 2001 focused on two aspects of organic acid conversion hydrogenation: 1) retention of stereochemistry in hydrogenation, and 2) investigation of substituent effect on hydrogenation rate. With enantiomerically pure lactic acid (LA) (ee = 98%) and four-hour reaction in a batch autoclave with 10 wt% LA in water as the feed at 135 C and 10.2 MPa H2 pressure, 75% of LA is converted to PG with 96% selectivity. The enantiomeric purity (ee) of the product PG is 98% after one hour reaction time, and decreases to 88% after four hours. At the same time, the enantiomeric excess of unreacted LA remains high at 95-97%. H-D isotopic labelling experiments with lactic acid and product PG show show deuteration at the secondary (C2) carbon but to a lower extent than the primary (C1) carbon. This suggests that the primary mode of hydrogenation involves C1 only, e.g., the carboxylic acid carbon of lactic acid. The H-D exchange at C2 is an indication of a secondary
interaction of LA and PG with the catalyst in which only the exchange takes place. The combination of LA and PG retaining their sterochemistry along with deuterium incorporation at the chiral C2 carbon, shows that lactic acid is able to interact in a preferential way with the catalyst surface. Since the intermediate in H-D exchange is a deprotonated, planar form of lactic acid, one would expect that subsequent deuteration would take place on either face of the intermediate to give a racemic deuterated product. The fact that this does not happen indicates that, at low temperature, the intermediate is adsorbed and oriented on the catalyst in such a fashion that only one face is available for deuteration (or rehydrogenation). Study of alanine (2-aminopropanoic acid) hydrogenation to alaninol (2-amino-1-propanol) show that temperature has the greatest influence on optimization of the reaction conditions. Increasing temperature from 100 C to 150 C greatly increases reaction rate, however
it causes the product alaninol to racemize and decompose into other side products. A stoichiometric quantity of acid, usually phosphoric acid, is required to maintain the amino acid in its protonated form. As with lactic acid, mechanistic studies have been performed using H-D isotopic exchange and NMR to determine the extent of exchange at the various positions in alanine. In the presence of deuterium, complete H-D exchange at the chiral center occurs at 100 C in both alanine and alaninol without racemization, giving retention of stereochemistry in the same fashion as with lactic acid. In the past year, we have initiated reactions with substituted organic acids in addition to lactic acid and alanine; these include propanoic acid (H-substituted), chloropropanoic acid (Cl-substituted), 2-methoxypropanoic acid (O-CH3-substituted), and methylpropanoic acid (CH3-substituted). These have widely different electronic properties and significantly different reaction rates. We are currently
further studying these substituted organic acids to characterize their reacivity. In addition to one publication, we have one other accepted for publication and two more submitted for publication. We also have one patent pending.
Impacts
Our ability to understand and control the hydrogenation of organic acids facilitates the development of a broad-based chemicals-from-organic acids industry. These organic acids, formed via fermentation of crop-based carbohydrates such as corn starch, are key intermediates that can substitute for petroleum-based feedstocks. The development of bio-based chemicals, based in part on these organic acids and our hydrogenation technologies, will both enhance the value of U.S. agriculture products and reduce our dependence on foreign oil.
Publications
- Zhang, Z., Jackson, J.E. and Miller, D.J. 2001. Aqueous-phase hydrogenation of lactic acid to propylene glycol. Appl. Catal. A: General 219:89-98.
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Progress 09/01/99 to 08/31/00
Outputs
Progress during the second year of this project has come on three fronts: 1) further clarification of the mechanism of lactic acid hydrogenation to propylene glycol; 2) extensive evaluation of the hydrogenation of lactic acid to propylene glycol in a trickle-bed reactor; and 3) initial studies on hydrogenation of amino acids to their corresponding amino alcohols with retention of optical activity. First, we have completed the mechanistic study of lactic acid hydrogenation using H-D isotope exchange coupled with 13C-NMR spectroscopy as a probe of initial reaction steps. We have found that isotopic exchange takes place at C2 in lactic acid, indicating that deprotonation of C2 to form a planar intermediate is an initial step in the mechanism. Hydrogenation of the carboxylic acid group followed by dehydration leads to formation of the glycol. A manuscript is now in preparation describing the lactic acid mechanism; an oral presentation will be given at the April, 2001
American Chemical Society meeting. In the trickle bed reactor, we have examined several catalysts and identified reaction conditions to achieve propylene glycol yields in excess of 85% of theoretical at optimal conditions. We have also examined the effects of impurities on the performance of the catalyst used in the reaction, and have found it to be substantially resistant to residual impurities in the lactic acid feedstock. We are still awaiting a patent office action on the application filed in November, 1999. We are currently preparing three papers for publication, and recently presented a paper describing the work at the American Institute of Chemical Engineers Annual Meeting in November, 2000. The third area of work in the past year has focused on the hydrogenation of amino acids to amino alcohols over inorganic supported catalysts. Amino alcohols are chiral intermediates used in pharmaceutical and pesticide applications; they are thus high-value materials. We have discovered
that with addition of a proper quantity of mineral acid, such as phosphoric acid, hydrogenation to the amino alcohol occurs readily. Most importantly, the chirality of the amino acid precursor is transferred to the amino alcohol product. These results shed important insight into the more general behavior of organic acid hydrogenation that is the subject of the continuation of the current grant through 10/02.
Impacts
Organic acids made by fermentation of biomass materials are important starting materials for chemicals production. This research will develop new processes to convert these organic acids to higher value products. The resulting technology will lead to value-added applications for agricultural products and reduced dependence on foreign petroleum.
Publications
- Zhang, Z. 2000. Aqueous-phase hydrogenation of biomass-derived lactic acid to propylene glycol. Ph.D. Dissertation, Michigan State University.
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Progress 01/01/99 to 12/31/99
Outputs
The objectives of the project are to 1) identify catalysts and reaction conditions for converting lactic acid to propylene glycol (PG); 2) characterize the conversion of lactic acid to PG in a continuous trickle-bed reactor; and 3) conduct mechanistic studies to elucidate key reaction pathways and intermediates of lactic acid conversion to PG. During the first year of funding, we have completed Objectives 1) and 2) and have made significant progress on Objective 3). In November, 1999, we filed a U.S. patent application claiming the catalysts and conditions used in converting lactic acid to PG. In batch studies, PG yields as high as 91% of theoretical at essentially complete lactic acid conversion are obtained. We also showed in batch studies that it is possible to hydrogenate lactic acid containing significant quantities of impurities, e.g. residual solids or salts from an unpurified lactate fermentation broth. In continuous trickle bed reactor studies, selectivity to
lactic acid as high as 90% was achieved. Conversion of lactic acid and selectivity to PG were unchanged in extended (100 hr) experiments, illustrating catalyst stability. We have probed the hydrogenation pathway of lactic acid by using isotopic H/D exchange in combination with H and 13-C nuclear magnetic resonance (NMR) spectroscopy. First, we conducted a control experiment with no lactic acid or PG present and showed that at reaction conditions the catalyst gives rise to rapid H-D exchange between H2 and D2O. Thus, in the liquid phase where catalyst is present, both reactant gas and water are activated by the catalyst. We also observed incorporation of D into propylene glycol in the presence of deuterium gas and water. However, we observe no incorporation of D onto the C2 carbon of lactic acid in the presence of deuterium and deuterated water. Thus it is clear that the catalyst interacts with each species differently. Finally, we have initiated studies on the hydrogenation of amino
acids to amino alcohols. We observe high selectivity to amino alcohols under the proper conditions of temperature, hydrogen pressure, and pH.
Impacts
This project has resulted in a new route for producing low-cost propylene glycol from corn-derived lactic acid, thus enhancing the profitability of corn production. We have also made significant progress in elucidating the chemical mechanism of liquid-phase hydrogenation and on conversion of amino acids. Overall, this research provides a more thorough understanding of aqueous-phase chemical conversion of biomass-derived feedstocks. These conversions are critical to the concept of large-scale chemicals production from renewable resources.
Publications
- No publications reported this period
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