Kinetics of catalytic isomerization of quadricyclane to norbornadiene using near infrared absorption spectroscopy: Conversion rate and diffusion motion in heterogeneous reaction

By use of Fourier transform near-infrared (NIR) absorption spectroscopy and the aid of a kinetic model, we have investigated the conversion of quadricyclane to norbornadiene catalyzed by anhydrous CuSO4 and SnCl2 in chloroform. The reaction mixture is not agitated so as to avoid the effect of sample heterogeneity. The NIR absorption spectra are acquired, at a position similar to2 mm above the catalyst surface, at 30-s intervals during 4 h. The concentrations of quadricyclane and norbornadiene are determined with the analysis of partial least squares. The isomerization of quadricyclane, as numerically solved from the model, is expected to describe its behavior more accurately in the catalytic system than that obtained previously. In addition to the isomerization rate, the kinetic model takes into account the contribution of diffusion. The diffusion coefficients of quadricyclane can be determined to be 3.8 x 10(-5) cm(2) s(-1) in chloroform and 1.14 x 10(-5) and 2.85 x 10(-6) cm(2) s(-1) inside the CuSO4 and SnCl2 stacks, respectively. Diffusion is slowed inside the solid stacks, and thus the molecular mechanism cannot be suitable for this system. Given the diffusion coefficients, the pseudo-first-order depletion rate constants are evaluated to be (3.7 +/- 0.1) x 10(-3) and (3.8 +/- 0.1) x 10(-3) s(-1) for CuSO4 and SnCl2, respectively. The corresponding second-order rate constants are determined to be (1.3 +/- 0.2) x 10(-5) and (2.0 +/- 0.1) x 10(-6) s(-1) A(-1) by considering the density and the size of the catalyst particles; A denotes the total catalyst surface area per unit effective volume of solvent. The rate constant with the CuSO4 catalyst is consistent with others obtained in a continuously stirred mixture. In the surface-mediated reaction, the catalytic isomerization is subject to one-site coordination (1:1 complex) between the reactant and the catalyst. Nevertheless, a two-site coordinated reaction cannot be excluded unless the interstitial size dependence of the depletion rate is known.