2000 Progress Report: 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 Period Covered by this Report: October 1, 1999 through September 30, 2000
Project Amount: $265,000
RFA: Technology for a Sustainable Environment (1998) RFA Text | Recipients Lists
Research Category: Sustainability , Pollution Prevention/Sustainable Development
Objective:The use of supercritical fluids (SCFs) as nontoxic solvent replacements in fine chemical synthesis is one strategy for waste minimization that is gaining increasing attention in research and industrial applications. 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 SCFs 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 SCFs. 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 SCFs, but its full potential has not yet been realized.
In this research program, we plan to: (1) develop Raman spectroscopy as a robust, in situ diagnostic technique to address these fundamental questions, and (2) provide predictive modeling tools to enhance commercial applications of SCF reaction chemistry. Raman spectroscopy will be used to probe local solvation effects on species dissolved in supercritical carbon dioxide and near-critical water. Using band shape analysis to determine local densities, we can quantify the effects of density on reaction rates and pathways. Local solvent densities measured in SCFs 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.
Progress Summary:In the second year of our project, significant progress was made towards better understanding the effects of solvation on reactions in supercritical fluids. Much of the work has focused on experiments and modeling of the reaction pathways of methyl tert-butyl ether (MTBE) hydrolysis in sub- and supercritical water. Continuing work is aimed at using CO2-water emulsions for synthetic chemistry, such as Diels-Alder cycloadditions.
Effect of Solvent Effects on Reactions in Near-Critical and Supercritical Water. Experiments were conducted to measure the rate of MTBE hydrolysis in water from 150 to 600?C and 250 bar. As the temperature is increased (at constant pressure), the rate of reaction reaches a local maximum just below the critical temperature followed by a significant drop at the critical point to a local minimum, and then increases again.
We initially hypothesized that the reaction proceeded by a unimolecular decomposition as is seen in the gas-phase, but with the activation energy reduced due to solvation of the transition state. Ab initio calculations were performed using Gaussian 98 to determine the structure of MTBE and the transition state. The solvent was modeled using the isodensity polarized continuum model and energy calculations were performed using density functional theory (B3LYP). The calculations did show the relative decrease of the activation energy with increasing dielectric strength and the local maximum below the critical point. However, the calculations also showed that the unimolecular decomposition pathway was insignificant for all temperatures less than 550? C.
An acid-catalyzed pathway was hypothesized for the hydrolysis reaction. The drop in reaction rate near the critical point was due to the reduced concentration of H+, as Kw decreases more than five orders of magnitude in the critical region. A rate expression that was first order in both MTBE and H+ was fit to the data and modeled the experimentally measured rate constant quantitatively over the entire temperature domain. Additional experiments were conducted in the presence of strong acid or base. These experiments verified that the acid-catalyzed pathway was dominant in both the subcritical and supercritical regions.
CO2-Water Emulsions for Synthetic Organic Chemistry. Supercritical carbon dioxide (scCO2) has been proposed as an alternative to organic solvents for chemical reactions. One of the primary obstacles that has prevented more widespread use of scCO2 in such a capacity is the extremely low solubility of many chemical compounds, including ionic compounds and proteins. Previous efforts to overcome this difficulty have focused on using expensive, environmentally incompatible fluorinated surfactants to solubilize small amounts of water in scCO2 as microemulsions. Our research eliminates the need for these fluorinated surfactants by using ultrasound to generate kinetically stable carbon dioxide/water emulsions.
Acoustically-generated carbon dioxide/water emulsions (temperature of 25?C, pressure of 60 bar, and acoustic frequency of 20 kHz) are stable for approximately 15 minutes in the absence of surfactants. Upon the addition of the common and inexpensive surfactant, sodium dodecylsulfate, in concentrations just above its critical micelle concentration (approximately 0.001 M), the acoustic emulsions are stable for several days. We have investigated the hydrolysis of benzoyl chloride to benzoic acid. Even when conducted at room temperature and moderate pressure (60 bar), the hydrolysis yielded the desired product in high yield, demonstrating the synthetic potential of acoustic carbon dioxide/water emulsions.
Our positive results for the hydrolysis of acid chlorides has motivated investigations of more useful chemistries. Diels-Alder cycloadditions represent one class of reactions that is particularly interesting, based on the extreme improvements in selectivity observed in aqueous environments for certain Diels-Alder reactions.1 (1Breslow R, Maitra U, Rideout D. Selective Diels-Alder reactions in aqueous reactions in solution and suspensions. Tetrahedron Letters 1983;24:1901-1904.) Quantitative interpretation of yields and selectivities in carbon dioxide/water emulsions will require a large amount of additional data, including reaction rates, equilibrium constants of reversible reactions, partition coefficients, and mass transfer or diffusion coefficients. The data published for Diels-Alder reactions are limited to rate constants based on the disappearance of reactants. This global approach is not sufficient for a mechanistic model of Diels-Alder reactions in carbon dioxide/water emulsions.
We propose measuring rate constants for the reaction of methyl vinyl ketone and cyclopentadiene (which has both an endo and an exo product) in both pure water and pure carbon dioxide. Two sets of experiments will be conducted for each experiment. First, the ratio of rate constants for the endo and exo products will be measured based on initial rate studies. Second, Raman spectroscopy will be used to study long-term behavior of the reaction network. Raman spectroscopy will be used to monitor disappearance of the cyclopentadiene double bond. Sampling of the reaction mixture and ultraviolet absorption spectroscopy also may be used to confirm the results obtained with Raman. Ab initio quantum calculations will be used to estimate equilibrium constants. The temperature range of interest in carbon dioxide is 0-100?C. For water, higher temperatures in the near-critical region (up to 350?C) will be investigated. This study will provide mechanistic insight into an important class of Diels-Alder reactions. The results of these spectroscopic studies will be used to interpret and analyze results obtained in carbon dioxide/water emulsions. Subsequent work will focus on the addition of surfactants and environmentally benign catalysts and the chemical effects of ultrasound.
Future Activities:A paper summarizing the computational work and the experiments conducted with MTBE in the presence of acid or base currently is being prepared and will be submitted for publication. Continuing work will focus on moving toward CO2-water systems for synthetic chemistry. Raman spectroscopy will be used to measure the kinetic of Diels-Alder reactions in both water and CO2.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
|Other project views:||All 11 publications||3 publications in selected types||All 1 journal articles|
||Taylor JD, Steinfeld JI, Tester JW. Experimental measurement of the rate of methyl tert-butyl ether hydrolysis in sub- and supercritical water. Industrial and Engineering Chemistry Research 2001;(1):67-74.||