Kinetics of a Diels-Alder Reaction of Maleic Anhydride and Isoprene in Supercritical CO 2 Evgeni M. Glebov, Larisa G. Krishtopa, Victor Stepanov, and Lev N. Krasnoperov* Department of Chemical Engineering, Chemistry and EnVironmental Science, New Jersey Institute of Technology, UniVersity Heights, Newark, New Jersey 07102 ReceiVed: April 24, 2001; In Final Form: July 12, 2001 The kinetics of the reaction of maleic anhydride and isoprene in supercritical carbon dioxide was studied over an extended concentration range, from 5 × 10 -5 to 1.3 × 10 -2 mole fractions of both maleic anhydride and isoprene at 60 °C and pressure 100 atm. The apparent rate constant of the reaction was found to be concentration dependent, in contradiction with the literature data. The results are interpreted in terms of limited solubility of maleic anhydride in supercritical CO 2 . The solubility of maleic anhydride in supercritical carbon dioxide was directly measured by sampling and analyzing the supercritical phase. The observed sharp increase in the apparent rate constant of the reaction between maleic anhydride and isoprene correlates with the solubility of maleic anhydride in supercritical carbon dioxide. The interpretation was confirmed using visual observations of the phase behavior. The rate constant of the reaction measured under the true supercritical, single phase, conditions is k x ) (17.0 ( 1.3) hr -1 (mole fraction units, 60 °C, pressure 100 atm). Introduction During the past decade, supercritical fluids (SCF) received significant attention as prospective solvents for organic synthetic reactions. 1-29 Supercritical carbon dioxide is considered to be a prospective environmentally friendly replacement of organic solvents. 2,25 From the engineering point of view, supercritical fluids as reaction media can be advantageous due to the increase of the number of the process parameterssboth temperature and pressure can be used to control chemical reac- tions. 2,6,9,10,14,15,20,21,24-28 From the fundamental point of view, supercritical fluids represent new class of solvents with some properties intermediate between the gas and the liquid phases and with other properties that are found neither in the gas nor in the liquid phases. Because of the high isothermal compress- ibility in the vicinity of the critical point, some reactions exhibit very large activation volumes and, hence, strong pressure dependencies. Examination of the pressure effects allows better understanding of the solvent effect on the reaction rates. 16-24,28 Supercritical carbon dioxide is one of the most studied supercritical fluids. Carbon dioxide has low critical temperature (T c ) 304.2 K) and a moderate critical pressure (p c ) 73.8 atm) and represents a prospective medium for reactions involving nonpolar or slightly polar reactants and transition states. 2 Diels- Alder cycloaddition is one of the classes of organic synthetic reactions intensively studied in supercritical carbon diox- ide. 6-12,14,15,26-29 One of the model cycloaddition reactions, reaction of maleic anhydride with isoprene (reaction 1), was one of the first reactions studied in supercritical CO 2 . Paulaitis and Alexander performed extensive investigation of the kinetics of reaction 1 in supercritical carbon dioxide. 6,7 Effects of both temperature and pressure on the rate constant were investigated. Strong dependence of the rate constant on pressure at 35 °C (near the critical temperature of CO 2 , T c ) 31 °C) was observed (Figure 1, filled symbols) as expected based on the solvatochromic shift measurements. 18,19 Currently, reaction 1 is considered to be a classical example of a Diels-Alder reaction in supercritical carbon dioxide and is widely cited in the literature on the reactions in supercritical media. 2,13,24 In the course of our experiments, we used reaction of maleic anhydride with isoprene (reaction 1) as a model reaction with the main purpose to validate the experimental approach and the procedures. In our measurements of the rate constant of reaction * To whom correspondence should be addressed. E-mail: KRASNOPEROV@ADM.NJIT.EDU Figure 1. Rate constants of reaction 1 measured by Paulaitis and Alexander. 6,7 Filled symbols, rate constants reported in 6, 7, open symbols are the rate constants recalculated in this work based on the raw data and the formulas taken from 7 (see Appendix). The reactor temperature was 35 °C (squares), 45 °C (circles), and 60 °C (triangles). The mole fractions of maleic anhydride were (3.4-5.2) × 10 -3 at 35 °C, (3.3-3.6) × 10 -3 at 45 °C, and 2.5 × 10 -3 at 60 °C. Isoprene was in excess from 1.5 to 3 times. 9427 J. Phys. Chem. A 2001, 105, 9427-9435 10.1021/jp011548a CCC: $20.00 © 2001 American Chemical Society Published on Web 09/21/2001