2002 Progress Report: Biocatalytic Polyester SynthesisEPA Grant Number: R828131
Title: Biocatalytic Polyester Synthesis
Investigators: Russell, Alan J. , Beckman, Eric J.
Institution: University of Pittsburgh
EPA Project Officer: Karn, Barbara
Project Period: April 1, 2000 through March 31, 2003 (Extended to September 30, 2004)
Project Period Covered by this Report: April 1, 2001 through March 31, 2002
Project Amount: $375,000
RFA: Technology for a Sustainable Environment (1999) RFA Text | Recipients Lists
Research Category: Nanotechnology , Sustainability , Pollution Prevention/Sustainable Development
The overall objective of this research project is directed towards enhancing our knowledge of biocatalytic polyester synthesis. Specific objectives are to:
(1) Test the general hypothesis that mass transfer is responsible for limiting the molecular weight of the polymer in solvent-free biocatalytic polymerization. In examining this hypothesis, we are investigating the roles of both internal and external diffusion by varying: (a) catalyst porosity, (b) catalyst support, (c) enzyme particle size, (d) reactor configurations, (e) agitation speed, and (f) viscosity. We speculate that pore diffusion only contributes to mass transfer resistance up to the critical molecular weight of the polymer.
(2) Demonstrate that a reduction in viscosity through the use of liquid or supercritical carbon dioxide (CO2) can reduce the external mass transfer limitations during the polymerization. Further, because CO2 is used only as a viscosity modifier, full solubility of substrates is not necessary. To investigate the importance of compatibility of the monomers with CO2, we will measure the impact of using fluorinated components in the monomers.
(3) Use the information acquired after studying Objectives (1) and (2) to focus on the molecular weight optimization of polymers that are difficult to synthesize using chemical means, such as polyesters with pendant hydroxyl groups. The use of enzymes will allow us to capitalize on their inherent specificity to accomplish the synthesis. Once we have developed an understanding of the breadth of enzyme specificity in biocatalytic polytransesterification, we will initiate a search for ester substrates that are productive without releasing acetaldehyde as a byproduct.
(4) Select a series of model polymerizations and, using the knowledge generated from Objectives (1)-(3) to guide our studies, perform biocatalytic polyesterifications in ionic liquids. We will study the stability, activity, and specificity of the enzymes in these novel solvents, and we will attempt to develop structure-function-environment relationships for systems that demonstrate significant activity.
In the past year, we have made significant advances in Objectives (3) and (4). For Objective (3), the compounds disopropenyl adipate (DIPA) and diethoxyvinyl adipate (DEVA) were synthesized and characterized. Substituting the monomer divinyl adipate (DVA) in enzyme catalyzed polymerizations with DIPA or DEVA will produce acetone or ethyl acetate as a byproduct rather than acetalyhyde. To determine the best monomer for enzymatic polytransesterifications, DIPA, DEVA, and DVA were reacted with 1,4-butanediol (BD) at 50°C using Novozym® 435 in a solvent-based (tetrahydrofuran) environment. After 24 hours, polymer was produced using DIPA (806 Mw) and DVA (1115 Mw), but no polymer was formed with DEVA as a monomer. These preliminary results suggest that DIPA may be a viable substitute for DVA in enzyme-catalyzed transesterifications.
Regarding Objective (4), our studies on enzymatic catalysis in ionic liquids have been extended to include a variety of ionic liquids, the vast majority of which are hydrophilic in nature. Lipase activity and stability was investigated in 1-butyl-3-methylimidazolium and pyrrolidinium-based ionic liquids with an assortment of anions, including hexafluorophosphate, acetate, nitrate, methanesulfonate, trifluoroacetate, and trifluoromethylsulfonate. The lipase-catalyzed transesterification of methylmethacrylate and 2-ethylhexanol was selected as a model reaction to determine the activity of enzymes as a function of ionic liquid physical properties. All ionic liquids were supplied by SACHEM Inc. (Austin, TX) as part of a joint research partnership. Our results indicated that in 1-butyl-3-methylimidazolium hexafluorophosphate, free Candida rugosa lipase catalyzed the transesterification at an initial rate of 6.75 µM/hr/mg-enzyme, 1.5 times faster than the reaction in hexane (3.90 µM/hr/mg-enzyme). However, free C. rugosa lipase was inactive in all of the other ionic liquids investigated. Polyethylene glycol (PEG) modification, adsorption onto an acrylic support, and immobilization into polyurethane foams proved ineffective in preventing lipase deactivation in the ionic liquids. Stability studies indicated that reversible inactivation of lipase is observed when the enzyme is incubated in [bmim][CH3CO2], [mmep][CH3CO2], and [mmep][CH3SO3], while incubation in [bmim][NO3] and [mmep][NO3] irreversibly inactivates lipase.
To develop a structure-function-environment relationship for enzymatic catalysis in ionic liquids, we determined the polarity and hydrophobicity of the ionic liquids by measuring the solvatochromatic parameters and octanol-water partition coefficients. Our results suggest that ionic liquids are more hydrophilic than many of the organic solvents conventionally used in non-aqueous biocatalysis such as hexane, acetonitrile, and tetrahydrofuran. However, because all of the ionic liquids studies (including [bmim][PF6]) were hydrophilic, no clear relationship between ionic liquid structure and enzyme function could be developed.
To date, most of the work involving enzyme catalysis in ionic liquids has not been performed under controlled water activity. Because enzyme activity, specificity, and hydrolytic equilibria are all dependent on the thermodynamic water activity, it is ideal to control water content when comparing enzyme activity in different organic solvents. Salt hydrate pairs commonly are used to control water activity in organic solvents; however, because ionic liquids are salts, it generally has been assumed that salt hydrates will dissolve. We have shown that as long as the salt hydrates do not completely dissolve in the ionic liquid, the salt hydrate pairs behave essentially the same in ionic liquids as they do in organic solvents. We have shown that the immobilized Candida antarctica lipase catalyzed transesterification of methylmethacrylate and 2-ethylhexanol have similar reaction rate-aw profiles in hexane and [bmim][PF6]. The ability to easily control water activity in ionic liquids using salt hydrate pairs should help investigators maintain easy control of water activity and allow for improved comparison of enzyme activity/specificity data.
The progress we have made in our proposed research again helps to further convert traditional chemical processes, which can dramatically harm the environment, to ones that alleviate negative environmental impact. By replacing conventional catalysts with enzymes, the need for harsh conditions is eliminated because enzymes can function at ambient pressures and temperatures, which can help to lower energy utilization as well as waste generation. Additionally, replacing divinyl adipate with diisopropenyl adipate eliminates the generation of acetaldehyde as a polymerization byproduct, which is considered to be an environmentally unfriendly compound. The use of "green" solvents, such as ionic liquids and supercritical CO2, also contributes to this line of research because ionic liquids possess no measurable vapor pressure and supercritical CO2 is nontoxic. However, our recent studies show that enzymes are inactive in a wide range of ionic liquids, thus greatly reducing the utility of ionic liquids in the field of non-aqueous enzymology. Issues involving the toxicity of ionic liquids and the complication of removing products from ionic liquids still need to be addressed before ionic liquids find widespread use.
Future activities involve addressing remaining issues. For Objective (1), the variation of catalyst porosity still needs to be investigated. Novozym® 435 will be immobilized to acrylic resins of varying pore sizes, and the effects of internal diffusion will be assessed.
For Objective (2), the viscosity of polymers synthesized with and without the presence of supercritical CO2 needs to be measured to determine if CO2 is, in fact, acting as a viscosity-reducing agent. Additionally, the effect of fluorination on the process also will be studied to ascertain whether the introduction of a CO2-philic species can enhance any observed effects.
For Objective (3), we will focus on optimizing the reaction between DIPA and 1,4-butanediol to enhance polymer molecular weight because DIPA was successfully synthesized. Other monomer combinations will be studied, as well. Additionally, if it is found that there is no enzyme-monomer combination that can compare to that which generates acetaldehyde as a byproduct, we will investigate the prospect of performing a biocatalytic polymerization in a chemoenzymatic reactor, where the acetaldehyde can be used in a synthetic reaction as it is made.
For Objective (4), we will continue to determine the activity and stability of various enzymes in ionic liquids. We also will study the solubility of monomers of interest in each ionic liquid, as well as perform polymerizations in them. If the synthesized polymers do not precipitate out of these new ionic liquids, as they did with [bmim][PF6], we will then investigate methods for separating the polymer from the ionic liquids, because they have no measurable vapor pressures. These studies hopefully will enable us to generate a set of data that can correlate enzyme activity with the physical properties of a given ionic liquid.
Finally, we want to apply green biocatalytic polymer processing to the synthesis of a high value polymer with important application potential. The new team member will focus on the above completion steps along with the synthesis of such high value polymers that combine all we have learned to date.
Journal Articles on this Report : 6 Displayed | Download in RIS Format
|Other project views:||All 12 publications||6 publications in selected types||All 6 journal articles|
||Berberich JA, Kaar JL, Russell AJ. Use of salt hydrate pairs to control water activity for enzyme catalysis in ionic liquids. Biotechnology Progress 2003;19(3): 1029-1032.||
||Erbeldinger M, Mesiano AJ, Russell AJ. Enzymatic catalysis of formation of Z-Aspartame in ionic liquid: an alternative to enzymatic catalysis in organic solvents. Biotechnology Progress 2000;16(6):1129-1131.||
||Kaar JL, Jesionowski AM, Berberich JA, Moulton R, Russell AJ. The impact of ionic liquid physical properties on lipase activity and stability. Journal of the American Chemical Society. 2003, Volume: 125, Number: 14 (APR 9), Page: 4125-4131.||
||Kline BJ, Lele SS, Beckman EJ, Russell AJ. Role of diffusion in biocatalytic polytransesterification. AIChE Journal 2001;47(2):489-499.||
||Mesiano AJ, Beckman EJ, Russell AJ. Biocatalytic synthesis of fluorinated polyesters. Biotechnology Progress 2000;16(1):64-68.||
||Mesiano AJ, Enick RM, Beckman EJ, Russell AJ. The phase behavior of fluorinated diols, divinyl adipate and a fluorinated polyester in supercritical carbon dioxide. Fluid Phase Equilibria 2001;178(1-2):169-177||