Time- and Temperature-Varying Activation Energies: Isobutane Selective Oxidation to Methacrolein over Phosphomolybdic Acid and Copper(II) Phosphomolybdates

Abstract

Abstract:The selective oxidation energetics of isobutane to methacrolein over phosphomolybdic acid and copper(II) phosphomolybdates have been investigated using low-pressure, pseudo-steady-state and temperature-programming techniques. Time-varying flexible least squares methods were used to determine variations in oxidation activation energies as the temperature increases at 5 • C·min −1 . Catalyst activity stabilizes by the fourth consecutive temperature-programmed run. Rate parameters increase linearly with temperature in two sinusoidal, oscillating wave packets. For H 3 PMo 12 O 40 , three distinct reaction pathways are apparent in the fourth run with activation energies 76 ± 3, 93 ± 7 and 130 ± 3 kJ·mol −1 , and under these experimental conditions are observed at the optimum temperatures 704 ± 7 K, 667 ± 25 K and 745 ± 7 K, respectively. Over the copper-containing catalysts, two pathways are apparent: 76 ± 3 kJ·mol −1 at 665 ± 9 K and 130 ± 3 kJ·mol −1 at 706 ± 9 K. The three activation energies indicate either different reaction pathways leading to methacrolein or distinct active sites on the catalyst surface. The intermediate activation energy, 93 kJ·mol −1 , only observed over phosphomolybdic acid, may be linked to hydrogen bonding. Differences in optimum temperatures for the same activation energies for H 3 PMO 12 O 40 and for the copper catalysts indicate that compensating entropy changes are smaller over H 3 PMo 12 O 40 . The inclusion of copper enhances catalyst stability and activity.

Adsorption Properties from Pressure-Varying Langmuir Parameters: n-Butane and Isobutane on Activated Carbon

“…The pressure-varying parameters, n ∞, P and K P , in eq are determined using methodology developed by Kalaba and Tesfatsion, called flexible least squares for time-varying linear regression (FLS-PVLR, for the analogous pressure-varying regression) and have been applied to time-varying parameters for econometric processes as well as rate constants for gas–solid reactions. − For all of these processes, modeled parameters evolve slowly and smoothly over time. For isothermal adsorption, the parameters are assumed to vary slowly and smoothly with increasing pressure.…”