In view of the emission and control of volatile organic pollutants (VOCs), it is of great significance to explore the reaction process, mechanism and kinetic behavior of key industries and organic pollutants on high-efficiency catalytic materials. In this thesis work, based on the emission characteristics of typical oxygenated volatile organic pollutants (OVOCs) in the packaging and printing industry, a highly efficient manganese-based oxide catalyst system was designed and prepared, the catalytic oxidation properties of typical OVOCs, such as acetaldehyde, isopropanol, acetone and ethyl acetate, and their mixed components were investigated, and the structure-activity relationship between catalytic materials and OVOCs was revealed. Finally, the reaction mechanism and kinetic behavior of typical OVOCs catalytic oxidation were elaborated, and the universality rule of efficient removal of OVOCs over manganese-based oxide catalyst system was proposed. The main findings and conclusions are as follows:
(1) MnxAlO catalysts for acetone and acetaldehyde catalytic oxidation
The catalytic performance and reaction mechanism of acetone and acetaldehyde over a series of CoAlO, FeAlO, NiAlO and MnAlO hydrotalcite-derived oxides systems were studied, respectively. Detailed results showed that the excellent catalytic activity for acetaldehyde (T100<150°C) and acetone (T100<170°C), and high CO2 selectivity over MnAlO catalyst were observed. Superior low-temperature redox properties, abundant surface -□-Mn4+-O2- defects and acid structure sites can effectively promote the adsorption and activation of OVOCs and oxygen molecules. The water vapor experiments shown that negative effect of water vapor on distribution of by-product and catalytic activity was observed due to the competitive process.
The experimental results of different Mn/Al molar ratios show that Mn3AlO(Mn/Al=3) catalyst has abundant inherent and formative defects, which leads to the weakening of Mn-O bond in the structural unit [MnO6] and enhances the adsorption and dissociation of acetone and oxygen molecules, and promote the production of reactive oxygen species. Furthermore, in situ DRIFT and theoretical calculations was adopted to explore the reaction mechanism. The η1(O)(ads), CH2=C(CH3)=O(ads), O*, CH3CHO*, CH2O* and COO(ads) were considered as the main intermediate species and/or transient state during the reaction progress. It was revealed that the acetone and oxygen molecules was activated by the dehydrogenation step (α-H abstraction) and dissociation process, respectively, subsequently, the breaking of -C-C- bonds and CH2O decomposion was occurred, yielding H2O and CO2 via the COO- adsorbed species. Particularly, -C-C- bond breaking was the main limiting step for acetone oxidation.
(2) Catalytic performance and reaction mechanism of vinyl acetate over different crystal structures manganese dioxides
Based on the difference of [MnO6] structural unit in crystal structure of MnO2, the one-dimensional rod structure MnO2 (OMS-2, β-MnO2, γ-MnO2), two-dimensional lamellar structure (δ-MnO2) and non-crystal structure (AMO) were prepared, respectively. The sequence of catalytic activity OMS-2 (T90=151oC) > AMO (T90=164 oC) > δ-MnO2 (T90=182 oC), for the one-dimensional MnO2, OMS-2 (2×2) > γ- MnO2 (2×1, 1×1) > β-MnO2 (1×1), the difference in catalytic activity dervied from that the OMS-2 catalyst has more oxygen vacancies, available surface oxygen species, weakening Mn-O bond of the [MnO6] structural unit, and Mn3+/Mn4+ redox pair. The reaction mechanism indicates that the breaking of the ester bond at low temperature (T<60°C) was occurred, it can induce to the the formation of intermediate species including acetaldehyde, acetic acid, formic acid and enol tautomer. Wherein, the weakening of the edge-sharing Mn-O bond in structural unit [MnO6] is a key active site, which can effective to catalytic oxidation of other typical OVOCs (acetaldehyde, acetone, isopropanol and ethyl acetate).
(3) Studies on the reaction process and kinetic behavior of multicomponent OVOCs over manganese-based oxide catalyst
The catalytic oxidation process and kinetic behavior of typical OVOCs (acetaldehyde, acetone and ethyl acetate) on OMS-2 catalyst were investigated.Detailed results shown that the internal and external mass transfer process does not affect on the kinetic behavior, and the high concentration of pollutants has a greater impact on the reaction performance at the high conversion stage, the mutual inhibition was occurred for the three types of pollutants over OMS-2 catalyst. It is more obvious for mutual inhibition between acetaldehyde and acetone catalytic oxidation, the competitive adsorption between acetaldehyde and acetone is stronger than that of acetaldehyde and ethyl acetate. Both of the Power-rate law kinetic model and the Mars-van Krevelen kinetic model are suitable for the simulation of the reaction process, particularly, the Mars-van Krevelen kinetic model can better predict the catalytic oxidation process of ethyl acetate.