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Catalysts 2016, 6(9), 137; https://doi.org/10.3390/catal6090137

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

1
School of Science and Technology, University of New England, Armidale, NSW 2351, Australia
2
School of Science and Mathematics, Howard Payne University, Brownwood, TX 76801, USA
*
Author to whom correspondence should be addressed.
Academic Editors: Keith Hohn and Andreas Martin
Received: 20 June 2016 / Revised: 27 July 2016 / Accepted: 25 August 2016 / Published: 10 September 2016
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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 H3PMo12O40, 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 H3PMO12O40 and for the copper catalysts indicate that compensating entropy changes are smaller over H3PMo12O40. The inclusion of copper enhances catalyst stability and activity. View Full-Text
Keywords: variable activation energies; isobutane selective oxidation; methacrolein; phosphomolybdic acid; copper(II) phosphomolybdates variable activation energies; isobutane selective oxidation; methacrolein; phosphomolybdic acid; copper(II) phosphomolybdates
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Brown, T.C.; Miron, D.J.; Brown, S.L.; Kendell, S.M. Time- and Temperature-Varying Activation Energies: Isobutane Selective Oxidation to Methacrolein over Phosphomolybdic Acid and Copper(II) Phosphomolybdates. Catalysts 2016, 6, 137.

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