RESUMEN
The strong metal-support interaction (SMSI) between the three components in Au/CeO2-Mg(OH)2 can be controlled by the relative composition of CeO2 and Mg(OH)2 and by the calcination temperature for the direct oxidative esterification of methacrolein (MACR) with methanol to methyl methacrylate (MMA). The composition ratio of CeO2 and Mg(OH)2 in the catalyst affects the catalytic performance dramatically. An Au/CeO2 catalyst without Mg(OH)2 esterified MACR to a hemiacetal species without MMA production, which confirmed that Mg(OH)2 is a prerequisite for successful oxidative esterification. When Au/Mg(OH)2 was used without CeO2, the direct oxidative esterification of MACR was successful and produced MMA, the desired product. However, the MMA selectivity was much lower (72.5%) than that with Au/CeO2-Mg(OH)2 catalysts, which have an MMA selectivity of 93.9-99.8%, depending on the relative composition of CeO2 and Mg(OH)2. In addition, depending on the calcination temperature, the crystallinity of the CeO2-Mg(OH)2 and the surface acidity/basicity can be remarkably changed. Consequently, the Au-nanoparticle-supported catalysts exhibited different MACR conversions and MMA selectivities. The catalytic behavior can be explained by the different metal-support interactions between the three components depending on the composition ratio of CeO2 and Mg(OH)2 and the calcination temperature. These differences were evidenced by X-ray diffraction, X-ray photoelectron spectroscopy, and CO2 temperature-programmed desorption. The present study provides new insights into the design of SMSI-induced supported metal catalysts for the development of multifunctional heterogeneous catalysts.
RESUMEN
Jet propulsion 10 (JP-10) droplets with and without aluminum nanoparticles in conjunction with HZSM-5 zeolite and surfactants were ultrasonically levitated, and their oxidation processes were explored to identify how the oxidation process of JP-10 is catalytically affected by the HZSM-5 zeolites and how the surfactant and Al NPs in the system impacted the key experimental parameters of the ignition such as ignition delay time, burn rate, and the maximum temperatures. Singly levitated droplets were ignited using a carbon dioxide laser under an oxygen-argon atmosphere. Pure JP-10 droplets and JP-10 droplets with silicon dioxide of an identical size distribution as the zeolite HZSM-5 did not ignite in strong contrast to HZSM-5-doped droplets. Acidic sites were found to be critical in the ignition of the JP-10. With the addition of the surfactant, the characteristic features of the JP-10 ignition were improved, so the ignition delay time of the zeolite-JP-10 samples were decreased by 2-3 ms and the burn rates were increased by 1.3 to 1.6 × 105 K s-1. The addition of Al NPs increased the maximum temperatures during the combustion of the systems by 300-400 K. Intermediates and end products of the JP-10 oxidation over HZSM-5 were characterized by UV-vis emission and Fourier-transform infrared transmission spectroscopies, revealing key reactive intermediates (OH, CH, C2, O2, and HCO) along with the H2O molecules in highly excited rovibrational states. Overall, this work revealed that acetic sites in HZSM-5 are critical in the catalytic ignition of JP-10 droplets with the addition of the surfactant and Al NPs, enhancing the oxidation process of JP-10 over HZSM-5 zeolites.
