Synthesis、Characterization And Catalytic Performance Of Layered Double Hydroxides Based Oxidation Catalysts

Nowadays, the selective oxidation of less expensive alkylaromatics to high added-value chemicals （e.g. alcohols, aldehydes, ketones, epoxide and carboxylic acids） is attracting more and more attention due to its importance both in academic research and in a variety of industrial and fine chemical processes. Acetophenone （AP）, which is commonly produced by selective oxidation of ethylbenzene （EB）, is an important intermediate for the production of esters, dye, perfumes, pharmaceuticals, and resins. Conventionally, acetophenone is synthesized by Friedel-Crafts acylation of aromatic by acylhalides or acid anhydrides with Lewis acids or by oxidation of alkylarenes with stoichiometric inorganic oxidants such as permanganate or dichromate. These reagents are not only relatively expensive, but they also produce huge amounts of noxious and corrosive wastes. Usually, acetophenone is commercially produced from catalytic oxidation of ethylbenzene with molecular oxygen in the presence of homogeneous cobalt based catalyst and additives such as manganese and bromide species in an acetic acid solvent. However, the conditions are often harsh, the reagent mixture is corrosive （bromide is used as a promoter）, and the chemistry is rarely selective. From both economic and environmental viewpoints, efficient and environmentally benign heterogeneous oxidation catalysts have great potential economic benefits, social benefits and environmental benefits. Become one of the most challenging hot research topics in the field of catalysis and organic synthesis.Layered double hydroxides （LDHs） with the formula [M²⁺1-xM³⁺,（OH）₂][An-]x/n-wH₂O are a class of two-dimensional （2D） anionic clay materials. In LDHs, divalent (e.g. Mg²⁺, Ni²⁺, Zn²⁺, Cu²⁺, Co²⁺, Fe²⁺) and trivalent (e.g. Al³⁺, Fe³⁺, Cr³⁺, Ga³⁺, Mn³⁺) metal cations may prearrange orderly in the layer lattices at the atomic level, giving positively charged layers. Water and exchangeable inorganic or organic charge-compensating anions are present in the interlayer galleries. Cation-tunability of the brucite-like layers and anionic exchangeability gives LDHs great versatility in composition make these low-cost materials can be used widely as catalyst precursors, catalysts and supports. For example, Mn-or Cu-containing LDHs were found to be effective for the oxidation of EB to AP. In most cases, LDHs only showed moderate catalytic activity. In addition, calcined Cr-containing LDHs were active for the EB oxidation. However, the high toxicity of Cr⁶⁺ ion causes serious pollution to humans and environment. Therefore, how to further improve the catalytic performance of LDHs-based catalysts still remains a challenging task.In this thesis, we use these LDHs materials as catalyst precursors, catalysts and catalyst supports in design and fabrication various highly efficient oxidation catalysts:1) Hierarchical flower-like core-shell structured Co-based catalyst prepared by CoZnAl-LDH precursor on amorphous alumina microspheres. Compared with traditional catalysts have a larger specific surface area and total pore volume. The results revealed that Al₂O₃@CoZnAl-MMO catalysts exhibited high dispersion of cobalt species due to well-developed three-dimensional flower-like CoZnAl-MMO platelets as well as the separating effect of the resulting ZnO phase.2) Other kinds of doping metals （M= Mg, Ni, Mn, Zn, Ce, and Fe） were successfully incorporated into Co-containing hierarchical catalysts by controlling experimental conditions. We found the Cu-doped catalyst show the highest activity in the selective oxidation of ethylbenzene, a high conversion of 92.8% with an acetophenone selectivity of 89.4% was achieved over the Al₂O₃@CoCuAl-MMO catalyst with a Cu/（Cu+Co） molar ratio of 0.25. The characterization results revealed that the high catalytic efficiency was attributed mainly to the synergistic effect between highly dispersed active Co and Cu species.3) Hybrid composite of CoCuAl-LDH/G-x nanocatalysts. A series of characterizations revealed that graphene could stabilize CoCuAl-LDH nanoplatelets effectively in the nanocomposites, and in turn, highly dispersed CoCuAl-LDH could prevent the aggregation of the graphene nanosheets. By fine-tuning the mass ratio of graphene to CoCuAl-LDH, such nanocomposites offered a tunable catalytic oxidation performance. In particular, the nanocomposite with the graphene/CoCuAl-LDH mass ratio of 0.4:1 （LDH/G-0.4） exhibited a remarkable catalytic performance with a considerable conversion （96.8%） and selectivity to acetophenone （>95.0%）, which was mainly attributed to the metal-support synergism between the active CoCuAl-LDH component and the graphene matrix in the unique hetero-nanostructure. In fact, the LDH/G-0.4 is the highest activity non-noble metal catalyst in the liquid-phase selective oxidation of EB using TBHP as the oxidant in all published reports. Moreover, the as-assembled nanocomposite catalysts displayed good recyclability and were active for the selective oxidation of other alkylaromatics.4) ZnCr-LDH/CNT catalyst. We take the cheap and practical O₂ as the oxidant for the liquid-phase selective oxidation of ethylbenzene. We have studied three different types of Carbon materials as composite supporters, and we found the ZnCr-LDH/CNT catalyst show a better catalytic performance than ZnCr-LDH/AC and ZnCr-LDH/G. On one hand, the high catalytic performance attribute to the strongest metal-support interaction between carbon nanotubes and ZnCr-LDH, on the other hand, carbon nanotubes owns large specific surface area and abundant pore structure. The CNTs can not only as the supporter for in situ growth LDHs, but also due to the random orientation and mutual cross-linked of CNTs to form a network skeleton. This structure can restrict the LDH particles grew up, effectively prevent the agglomeration of LDHs, resulting exposed more active centers. The ZnCr-LDH/CNT-0.225 is the highest activity non-noble metal heterogeneous catalyst for the liquid-phase selective oxidation of ethylbenzene using O₂ as the oxidant in all published reports. The highest ethylbenzene conversion and acetophenone selectivity can be reached to 60.4% and 95.2% respectively, and the TON can reach up to 2532.5) The MgAl-LDH used as a catalyst support to prepare highly efficient bimetallic AuPd nanoalloy catalysts. We successfully introduce the AuPd into LDH via palladium ligand (PdCl₄^2-) and palladium ligand （AuCl₄^-）, the AuPd alloy particle can highly dispersed on the catalyst due to the confinement effect of the LDHs. Doping Ca in MgAl-LDH can effectively improve the alkaline of catalyst, and use a little La to replace Al in LDH can further improve the alkaline, but excessive introduction La would destroy the LDH structure. The selected AuPd-0.6/CaMgAl-LDH-0.5-La-5% catalyst was found to exhibit remarkable activity for FDCA synthesis via HMF oxidation in water without addition of homogeneous base. Both the HMF conversion and FDCA selectivity are above 99% under the optimum reaction conditions. The ultra-high catalytic activity comes from the synergism between AuPd-alloy particle and the basic sites in LDHs support.