Construction Of Ni And Cu Metal Catalysts Based On Layered Double Hydroxides And Their Catalytic Performance For Hydrogenation And Dehydrogenation

With the increasing consumption of fossil fuels and the serious damage of ecological environment, we are facing an unprecedented global energy crisis and environmental problems nowadays. It is urgent to develop renewable energy to replace traditional fossil fuels and to realize the requirements of green chemistry and atomic economy. The biomass provides opportunity to achieve this goal. Utilizing the conversion of biomass, we can obtain a series of important biomass-derived platform compounds like levulinic acid (LA) and 5-hydroxymethyl furfural (HMF)by means of green and environmentally friendly methods, and these compounds can be further converted into series of high value-added biodiesel fuel or fuel additive. Previous studies have shown that,represented by y-valerolactone (GVL) and 2,5-Dimethylfuran (DMF), this new type of biofuel has many excellent properties similar to gasoline or diesel, which has great application potential and research value. On the other hand, ?,?-unsaturated aldehyde or ketone is an important raw material and intermediate in traditional chemical industry. Represented by citral, its hydrodrogenation products like unsaturated alcohol and citronellol have important application value in the field of medicine, food and flavor. At present, the research on the transformation of biomass and the selective hydrogenation of citral is still mainly focused on the field of noble metal catalysts, although great contributions have been made, its limited reserves and expensive prices limit their large-scale applications unfortunately.It is of great practical significance to study the replacement of noble metal catalysts by the non-noble metal ones, and it has already been received extensive attention nowadays. However, restricted by the intrinsic low catalytic activity of non-noble metals and the traditional preparation methods, many researches on non-noble metal catalysts are still unable to achieve the goal of high efficiency conversion, and the choice of hydrogen donor is still not satisfactory, as well as the problems in the stability and lifetime of the catalysts. Therefore, the construction of high efficiency non-noble metal catalysts and the design of new reaction routes still have great research value. Layered double hydroxides (LDHs)material provides an opportunity for the design of highly dispersed catalysts. Thanks to the excellent properties of LDHs materials, such as the adjustable elements of the interlayer and the exchangeable of interlayer ions, different functional elements can be introduced into the LDH precursors according to the characteristics of different reactions,and the multifunctional catalysts with high activity, selectivity and stability can be easily constructed through the structural topological transformation.Based on the above viewpoints, in this thesis, we construct a series of Ni- or Cu-based non-noble metal catalysts with different structures via the LDH precursors. The structure-activity relationship of catalysts and the pathway of reactions were investigated systematically, and the main work is as follows:1. The construction of highly dispersed Ni- or Cu-based catalysts and their selective hydrogenation performance.Highly-dispersed Ni- and Cu-based nanocatalysts were designed based on LDHs precursors, and the effects of different active components on the selective hydrogenation of citral and the influence of the introduction of additives on the hydrogenation performance were investigated. It is found that the introduction of Fe can induce the formation of NiFe alloy, enhance the electron transfer between them, and weaken the adsorption of C=C bond of citral. At the same time, it can promote the hydrogen spillover effect and increase the hydrogenation activity of the catalyst. On the other hand, the introduction of Zn promoted the dispersion of Cu nanoparticles, and the strong metal-support interaction at the Cu-ZnO interface induced a large number of Cu+ species, formed abundant Lewis acid sites on the catalyst surface,and greatly promoted the activation of C=O bond of Citral. The synergistic catalytic effect of the active Cu0 center, the electrophilic Cu+species and the surface Lewis acid sites is the key to the high selectivity of unsaturated alcohols. The selectivity of unsaturated alcohols could reach above 75% under 80 ? and 1 MPa with a Zn/Cu molar ratio of 1:0.2. The construction of structured Ni-based catalysts and their performance in vapor-phase hydrogenation of biomass-derived levulinic acid.The preparation of traditional powdered metal-catalysts met several disadvantages such as the uneven dispersion of metal particles and the migration and growth of nanoparticles during the reaction. Based on this,we put forward the idea of building a novel Ni-based monolithic catalyst with a 3D multilevel structure by activating the nickel source on metal substrate and in situ growth of NiZrAl-LDH nanoarrays. The incorporation of Zr and the structured designing significantly improved the dispersion of the active metal, enhanced the surface acidity of the catalyst, and greatly improved the structural stability, thermal stability and recoverable of the catalyst. As-synthesized structured catalyst showed excellent catalytic performance in the vapor-phase solvent-free hydrogenation of biomass LA to produce GVL. Under 250 ? and atmospheric pressure conditions, the conversion of LA and the selectivity of GVL can reach 99% and 98%, respectively. It showed more favorable heat/mass transfer properties compared with the traditional catalysts, and can be widely applied to the vapor-phase hydrogenation of a series of biomass derivatives. The high performance of the catalyst can be attributed to its unique hierarchical structure which effectively promotes the exposure of active sites and the heat/mass transfer, as well as the facilitation of actonization of LA regulated by the surface acidic sites.3. The construction of bimetallic Ni-Cu alloy catalysts and their vapor-phase transfer hydrogenation performance.The conventional preparation route of DMF needs to consume a large number of non renewable molecules H2, and the current transfer hydrogenation has several drawbacks. Thus, a new method for the coproduction of phenol and HMF from CHL and DMF through vapor-phase coupling reaction (transfer hydrogenation) over Bimetallic Ni-Cu catalyst without external oxygen and hydrogen was proposed. The reaction pathway and catalytic mechanism were in-depth investigated,and it was found that the strong interaction between Ni-Cu alloy and the support plays an important role in controlling the coupling process. The alloying of Cu and Ni can promote the aromatization of CHL, which significantly improve the selectivity of phenol and DMF. High yields of phenol and DMF can be reached (>98%) under 240 ? and atmospheric pressure with a Ni/Cu ratio of 2:1. Moreover, this process can also be applied to the preparation of various substituted phenols.4. The construction of surface defects-promoted Ni-Mo catalysts and its selective hydrogenolysis performance.The study of GVL hydrogenolysis is faced with the bottleneck of harsh reaction conditions and low product selectivity at present. Thus, a new way to construct a multifunctional Ni-based catalyst with abundant surface oxygen defect derived from Mo7O246- intercalated LDH was proposed. It was found that the introduction of Mo and the regulation of reduction temperature could construct a large number of low valence unsaturated MoOx species and abundant surface oxygen vacancies on the catalyst surface, which promote the activation of C-O and C=O bond in GVL and thus lead to greatly increased yield of 1,4-PDO and 2-MTHF(>90%). Additionally, it was found that the reaction pressure will mainly have an effect on the conversion of GVL, whereas relatively high temperature will be conducive to the formation of 2-MTHF. When using H20 as a solvent, the production of 1,4-PDO can be effectively promoted.