In situ Diagnostic Techniques for Probing Solvation Effects in Supercritical Fluid Reaction Media for Synthetic Organic ChemistryEPA Grant Number: R826738
Title: In situ Diagnostic Techniques for Probing Solvation Effects in Supercritical Fluid Reaction Media for Synthetic Organic Chemistry
Investigators: Steinfeld, Jeffrey I. , Tester, Jefferson W.
Institution: Massachusetts Institute of Technology
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1998 through September 30, 2001
Project Amount: $265,000
RFA: Technology for a Sustainable Environment (1998) RFA Text | Recipients Lists
Research Category: Sustainability , Pollution Prevention/Sustainable Development
Description:Chemical industry has traditionally developed processes in a manner where economic considerations have been the primary driving force, and waste treatment, when it was considered at all, was dealt with only to the extent required by regulations. Government and industry have recently realized that developing chemical processes with pollution prevention as a primary goal can be both environmentally friendly and economically beneficial in the long run. This view has led to processes which minimize waste production and utilize solvents that are less toxic and less detrimental to the environment. The use of supercritical fluids (SCF's) as non-toxic solvent replacements in fine chemical synthesis is one such strategy that is gaining increasing attention.
Because SCF properties change significantly with relatively small changes in pressure or density in the critical region, parameters such as solubilities, reaction rates, and selectivities may be "tunable", making SCF's particularly versatile and desirable reaction media. However, basic data and theoretical models are lacking for solvation of reactants and the influence of solvent density on reaction pathways in SCF's. This is due in part to the difficulty of making measurements in these high-pressure, sometimes high-temperature fluids. Raman spectroscopy has been demonstrated as a feasible in situ, non-invasive measurement technique in SCF's, but its full potential has not yet been realized.
In this research program, we plan to (i) develop Raman spectroscopy as a robust, in situ diagnostic technique to address these fundamental questions and (ii) provide predictive modeling tools to enhance commercial applications of SCF reaction chemistry. Raman spectroscopy will be used to probe local solvation structures of species dissolved in supercritical carbon dioxide and near-critical water. Using band shape analysis to measure local densities, we can quantify the effects of density on reaction rates and pathways. Local solvent densities measured in SCF's will be used to determine empirically correct intermolecular potential functions, which will enable molecular modeling tools to predict solubilities and reaction dynamics in these fluids. This molecular-level understanding is critical to designing chemical processes using SCF solvents.
Approach:Among the model systems we plan to study are (i) Kolbe-Schmitt transformations in supercritical CO2; (ii) Model halocarbon hydrolysis in near- and supercritical water; (iii) Diels-Alder reactions in supercritical CO2-water mixtures; (iv) Hydrocarbon fuel combustion reactions in supercritical water; and (v) Methyl-tert-butyl-ether (MTBE) hydrolysis and oxidation in supercritical water. All of these systems involve reactants, products, and intermediates which may be readily detectable by Raman spectroscopy. Additionally, we plan to investigate the applicability of several advanced Raman-based techniques for use in SCF's. The feasibility of using UV Resonance Enhanced Raman Spectroscopy for detecting intermediates and products at low concentrations will be investigated. Raman imaging will also be evaluated as a means for determining phase partitioning of species in two-phase reaction systems. Such a probe of component solubilities in SCF's may help explain some of the anomalous selectivities reported for reactions near the critical point.
The expected results of this program will be:
- Development of in situ diagnostic methods for evaluating and optimizing the effectiveness of supercritical fluids as replacement solvents in the chemical syntheses of commercial organic compounds under conditions that materially reduce environmental pollution and waste burdens;
- spectroscopic information on molecular interactions between reactions, products, and solvents in supercritical fluids;
- new information on solubility and partitioning in multiphase reaction media;
- practical design parameters for chemical processing in such media.
- deeper fundamental understanding of reaction dynamics in such media, which will aid in rational design and selection of reaction conditions for optimum yield and minimum waste;
- training students in the variety of disciplines needed to address the challenge of redesigning chemical processes in order to meet the simultaneous goals of economic viability and environmentally sustainable production.