Construction, Characterization Of Bifunctional Cu-Based Catalysts And Their Catalytic Performance In Hydrogenation And Dehydrogenation

With the increasing concern about energy and environment issues originating from the shortage of fossil resources and increasing emission of greenhouse gases, the conversion from coal or sustainable and renewable raw biomass to useful fuel and chemicals is becoming more and more important. For example, Oxalic acid esters can be synthesized by carbon route; succinic acid esters, levulinic acid, levulinic acid esters and furfural can be produced by biomass fermentation. Moreover, the hydrogenation products of these carbonyl group-containing compounds are mostly alcohols and lactones with high added value. In this regard, the production of alcohols and lactones with high added value through the carbon and biomass conversion route is of great economic and environmental value. Therefore, design and development of catalysts with great performance in hydrogenation of carbonyl group-containing compounds is of great importance. Although, the noble based catalysts are very efficient in the hydrogenation of carbonyl group-containing compounds, the high cost of these catalysts limits their application in industry. In this regard, the industrial copper-chromium catalysts show excellent performance and has been widely used in the hydrogenation of carbonyl group-containing compounds. However, the highly toxic Cr6+ would lead to severe environmental pollution issues. Therefore, it is very necessary to design and preparation of environmentally-friendly Cr-free Cu-based catalysts with great catalytic performance in hydrogenation of carbonyl group-containing compounds. In addition, as important chemical intermediate, aldehydes and ketones play important role in the modern chemical production. In industry, aldehydes and ketones are mainly produced by oxidation of alcohols with Cr, Mn and high valence indine of inorganic salts, however, the high toxic and explosive strong oxidant would lead to environmental pollution and security issues. Thus, the anaerobic transfer dehydrogenation of alcohols using organics as hydrogen accepters was emerging. Moreover, because of the green environment-friendly reaction process and high selectivities of target produsts, the transfer dehydrogenation of alcohols has been widely acclaimed. However, supported catalysts especially copper-based catalysts competent to anaerobic transfer dehydrogenation of alcohols are lack. Thus, design and development of copper-based catalysts with high efficiency to the anaerobic transfer dehydrogenation of alcohols is of great practical significance and scientific value.The traditional Cu-based catalysts have same disadvantages:the poorly dispersed active constituent and the too single active constituent would easily lead to the low activity of these catalysts; the weak interaction betweent active Cu species and support would result in deactivation of catalysts in the reaction. To solve these problems, through homogeneous coprecipitation, single-resource layer doule hydroxide (LDH) precursor and two-step LDH precursor, the highly dispersed mixed metal oxides (MMO) were prepared repectively. After reducing these MMOs with hydrogen, Lewis acid, oxygen vacancies and Lewis base promoted Cu-based catalysts were obtained. In addition, a carbon-coated structure supported Cu@CN catalyst was prepared by pyrolyzation CuAl-LDH/melamine hybrid.(一) The gas-phase selective hydrogenation of a series of esters were conducted over highly dispersed Al-doped ZrO2-supported Cu-based catalysts, which were prepared through a homogenous coprecipitation route in the presence of CTAB. The systematic characterization revealed that the structure and catalytic performance of Cu/Al-ZrO2 catalysts were profoundly affected by the addition of Al. Compared with the Al-free catalyst, Al-doped catalyst has higher Cu surface area and smaller copper nanoparticles. Morovere, results confirmed that the incorporation of Al into ZrO2 framework could form tetrahedrally coordinated Al3+ species, leading to the improvement of metal dispersion and the formation of more surface Lewis acid sites. In the gas-phase selective hydrogenation of dimethyl oxalate (DMO) to ethylene glycol (EG),100% DMO conversion,97.1% EG selectivity, and a high turnover frequency of 16.9 h"1 were achieved over Cu/Al-ZrO2 catalyst with a Al/(Cu+Al+Zr) mass ratio of 0.1. The high efficiency of Cu/Al-ZrO2 catalyst in DMO hydrogenation was attributed mainly to the surface synergistic catalytic effect between highly dispersed metallic copper species and Lewis acid sites. The obtained catalysts displayed excellent catalytic performance in the gas-phase hydrogenation of a series of esters (dimthyl succinate, dimethyl maleate, dimethyl adipate, 1,4-cycolhexane dicarboxylate).(二) The new Mn-containing spinel-supported copper nanocata--lysts were prepared by single Cu-Mn-Al layered double hydroxide precursor route and employed in gas-phase selective hydrogenation of dimethyl succinate (DMS) to y-butyrolactone (GBL). As-formed copper-based nanocatalyst exhibited exceptional catalytic hydrogenation performance with an enduring complete conversion and more than 98% GBL selectivity up to 100 hours. A series of characterizations including XRD, STEM, TEM, EPR, H2-TPR, XPS, PL and in-situ FT-IR measurements revealed that the introduction of manganese into catalyst precursors led to the formation of Mn-containing spinel phases, thereby giving rise to highly dispersive Cu0 nanoparticles and a large amount of surface oxygen vacancies in reduced catalysts. The high catalytic efficiency of Cu-based nanocatalyst was reasonably attributed to the surface synergism between oxygen vacancy sites and Cu0 species, thereby greatly promoting the adsorption of DMS molecule followed by the activation of carbonyl group and improving the dissociation of hydrogen. Mostly importantly, such copper-based nanocatalyst displayed great application potential in the gas-phase hydrogenations of other biomass-derived compounds containing carbonyl group (e.g. acetol, levulinic acid, levulinic acid esters and furfural).(三) The CaAlO-supported copper catalysts with strong base sites were prepared via a two-step procedure based on CaAl-LDH precursor route. The resulting catalyst exhibited superior catalytic performance for gas-phase solvent-free coupling between dehydrogenation and hydrogenation without external hydrogen supply or recycle to simultaneously produce GBL and FOL:a good stability with a lasting 100 h for the high yields of GBL and FOL (>95.0%). The HRTEM, PAS, XPS, CO2-TPD, in situ IR of butanol adsorption confirmed the existence of defective Cu NPs, abundant SLB sites and Cu+species on the catalyst surface, which created a cooperative nanoenvironment for the highly efficient dehydrogenation of hydroxyl group in 1,4-BDO and hydrogenation of carbonyl group in FL(四) The new supported carbon-coated Cu nanoparticles@N-doped carbon (Cu@CN) catalysts, which were directly generated by a facile thermal decomposition of Cu-Al layered double hydroxide (CuAl-LDH) and melamine hybrid precursor, and applied in the liquid-phase transfer dehydrogenation of primary aliphatic alcohols. The strategy was found to be useful for the synthesis of a very thin in situ formed nitrogen-doped carbon layer (0.5-0.7 nm) over a metallic copper nanoparticle (9 nm). Compared with copper-based catalyst derived from pristine CuAl-LDH, as-fabricated Cu@CN nanocatalyst showed significantly enhanced catalytic performance in terms of the activity, the selectivity and stability. The high catalytic efficiency of Cu@CN nanocatalyst was attributed to the surface cooperative effect between strong Lewis base sites, highly dispersed Cu0 species and Cu+speceis, thereby greatly promoting the activation of hydroxyl group in primary aliphatic alcohols and the abstraction and transfer of hydrogen.