Heterogeneous Catalysis in Supercritical Media,Part 3: Other Media

For technical applications, those reactions in supercritical fluids are of particular interest which have significant advantages over conventional methods or which are impossible in classical solvents or in the gas phase. This last part of the trilogy deals with heterogeneously catalyzed reactions in supercritical fluids other than carbon dioxide or water.     

Heterogeneous Catalysis in Supercritical Media: 1. Carbon Dioxide

Over the past few years, supercritical fluids have received considerable interest as reaction media. Supercritical fluids combine properties of gases and liquids. Densities are lower than in the liquid phase, but significantly higher than in the gas phase. This makes supercritical fluids excellent solvents for a variety of substances. The good dissolution capacity exists only in or near the supercritical range. Reaction products can be separated, for instance, simply by decreasing temperature and pressure. This may be used to intensify the process as complex separation processes, like distillation, are avoided.     

The Partial Oxidation of Isobutene in Sub‐ and Supercritical Water

The task of this work was to investigate which kind of reaction pathway takes place under supercritical water conditions in the oxidation of isobutene with molecular oxygen. The reaction was examined at temperatures of 350–20 °C, pressures of 25–35 MPa, uncatalyzed and under platinum‐catalyses. Isobutyraldehyde was the main product. Tert‐butanol was used as a source for isobutene, which dehydrates to isobutene quantitatively under the conditions used.     

Influence of Operating Parameters on Biomass Conversion under Sub‐ and Supercritical Water Conditions

The influence of reaction pressure, reaction time, reaction temperature, and biomass‐to‐water mass ratio R on the conversion of miscanthus biomass to biofuels under sub‐ and supercritical water (SCW) conditions was investigated. The highest total conversion was obtained under SCW conditions and the heating value increased under subcritical and SCW conditions. The findings herein show that near‐supercritical and supercritical water at 400 °C and high pressure can be an effective reaction medium for converting biomass to oils with relatively low oxygen contents in high yields.     

New Paradigms and Future Critical Directions in Heterogeneous Catalysis and Multifunctional Reactors

Heterogeneous catalysis is a key pillar of the global industrial chemical and petrochemical sector, and 85% of all chemical products are produced with at least one catalytic step. Indeed, catalysis and catalytic reactors are a critical underpinning science for energy, environmental, and economic security. This paper reviews some future critical directions for research in catalysis science, toward a greener and more sustainable future. We believe that even a relatively mature field as heterogeneous catalysis and nanomaterials can be vitalized and spurred by major discoveries, but an outside-the-box thinking and a focused effort in a large plurality of disciplines is necessary. Thus, critical research needs in several areas, including heterogeneous and homogeneous catalysis, biocatalysis, photocatalysis, electrochemical conversions, and computational catalysis, are reviewed. The research needs of the future lie at the intersection of synthesis of novel nanostructured materials with tunable pore size distribution, controlled porosity, and high surface area; development of new catalytic applications for such materials; and the science of advanced characterization including in situ spatiotemporal analysis. In the area of computational catalysis, we believe that the future lies in the development of hybrid methods (parallel and serial) which can model the typical multiscale phenomena that are typically encountered in protein translocation and signal transduction, charge transport, enzymatic catalysis, surface chemistry, and self-assembly in complex fluids. As we promulgate the new directions to the catalysis fraternity, some prior research areas will unfortunately need to be relegated to obsolescence, to maintain a healthy balance on the research forefront.     

超临界水中苯酚的氧化分解

超临界水中苯酚的氧化分解林春绵金耀门潘志彦（浙江工业大学职教学院,杭州３１００１４）关键词超临界水苯酚氧化１前言开发有效的废水处理技术,强化废水的处理,实现水的循环使用,对减轻环境污染问题和水资源危机都有非常重要的意义。超临界氧化法处理工业有机废水...     

Catalytic Dehydration of 1,2‐Butanediol to n‐Butyraldehyde in Sub‐ and Supercritical Water

The change of the raw material basis to alternative sources, especially biomass, is advisable because of both the continuous shortage of fossile resources and the advancing change in world climate. Carbohydrates have the highest share in biomass. However, as they are overfunctionalized with chemically almost equal hydroxy groups, the high‐scale industrial use of carbohydrates is restrained. In addition, the diversity and the high amount of functional groups of carbohydrates complicate the prediction of possible reaction pathways. This is the reason why monoalcohols and polyols are used as model compounds. One interesting reaction is the dehydration of 1,2‐butanediol to n‐butyraldehyde, an important chemical intermediate. Supercritical water is an appropriate reaction medium for dehydrations because of its special physical and chemical properties. The addition of acids enhances the reaction rate but at the same time intensifies corrosion. Sulfate salts, especially zinc sulfate, can have similar positive effects without increasing the corrosion potential. The experimental results of the catalytic influence of different metal sulfate salts on the dehydration of 1,2‐butanediol to n‐butyraldehyde are presented in this paper.     

Nano-Particles in Heterogeneous Catalysis

The introduction of in situ techniques has had a vast impact on research and development in the area of heterogeneous catalysis as emphasized in many reviews and monographs. Recently, the number of in situ techniques that can give information at the atomic scale has increased significantly and new possibilities exist for making the measurements under industrially relevant conditions. In order to fully exploit the results from the in situ and operando studies, it has also become increasingly gainful to combine the experimental studies with theoretical methodologies based on, for example, Density Functional Theory (DFT). This has allowed one to extract more detailed atomic-scale information from the measurements and it has also allowed the establishment of detailed structure-activity relationships. Furthermore, the interplay between in situ techniques and theory has helped bridging the pressure gap such that in situ information obtained at conditions far from industrial ones may be used in a more relevant manner. Here, we will illustrate how microscopy-, spectroscopy- and X-ray-based techniques in combination with experimental and theoretical surface science methods can aid industrial catalyst developments. We will do this by presenting examples of our current understanding and latest developments in the areas of heterogeneous nano-particle catalysts for methanol synthesis, steam reforming and hydrotreating.     

Space-Resolved Profiling Relevant in Heterogeneous Catalysis

Knowledge of gradients involved in chemical processes, such as heterogeneously catalyzed reactions, on molecular as well as reactor scale is of paramount importance for understanding and optimizing such processes. This review highlights and discusses recent advances in spatially resolved methods for the detection of chemical (structural) and temperature gradients, with particular focus on in situ methods.     

超临界水氧化处理含酚废水试验研究

在超临界水氧化系统中，以过氧化氢为氧化剂，研究了含酚废水的氧化作用及效果。在试验条件下，废水中有机物能够被显著降解。过氧化氢用量、体系的温度、压力、反应停留时间对有机物去除率有不同程度的影响。随着过氧化氢用量增加、体系温度、压力的升高，反应停留时间的延长，有机物去除率有明显的增加。当实际加入的过氧化氢量与理论需要量的比值K=2．0，系统压力为30MPa，温度超过450℃，反应停留时间超过190s时，COD去除率可接近100％。     

Hydrogenation of Levulinic Acid Using Formic Acid as a Hydrogen Source over Ni/SiO2 Catalysts

Several Ni/SiO₂ catalysts were developed for the hydrogenation of levulinic acid using formic acid as the hydrogen source. The catalysts were prepared by a variety of methods including impregnation, co‐precipitation, deposition‐precipitation, and citric acid assisted impregnation combustion. The morphological properties were investigated by XRD, N₂ sorption, HRTEM, and H₂ pulse chemisorption measurements. XRD patterns of the calcined material revealed the presence of NiO particles, while calcination in an inert atmosphere produced Ni particles through in situ reduction of NiO. The reaction proceeded without external H₂ flow using formic acid as hydrogen source. The Ni/SiO₂ catalyst prepared by the citric acid assisted method and calcined in inert gas flow was the most efficient for the hydrogenation of levulinic acid without external H₂ flow. The high catalytic performance was attributed to the high dispersion of cheap and earth‐abundant Ni nanoparticles and optimal porosity.     

Gasification of Microalgae Using Supercritical Water and the Potential of Effluent Recycling

The gasification of microalgae in supercritical water was investigated in this work. The product gas contained mainly H₂, CO₂, CH₄, and C₂H₆. Operation at high temperatures and lower biomass concentrations resulted in the highest carbon gasification efficiency and the lowest total organic carbon levels in the residual water. Due to its content of inorganic nutrients, the residual water was applied as cultivation medium for microalgae. However, algal growth in the untreated residual water was inhibited by the existence of potentially toxic substances evolved from gasification. Upon treatment by activated carbon filtration and ultraviolet light degradation, these substances were eliminated and cultivation in the residual water was possible. The major fraction of inorganic residues from gasification was recovered by means of water purging, increasing the potential of nutrient recycling for cultivation.     

Kinetic Study of Epoxy Resin Decomposition in Near‐Critical Water

A diglycidyl ether type epoxy resin from bisphenol A, E‐51, was cured by methylhexahydrophthalic anhydride (MeHHPA) and then decomposed in near‐critical water without any additives. The effects of the experimental parameters such as the reaction temperature, reaction time, pressure, and feedstock ratio on the percentage of decomposition were investigated to obtain optimized reaction conditions. The results revealed that the percentage of decomposition can be enhanced by increasing either temperature or reaction time. At 260 °C, it initially increased with higher pressure and then decreased dramatically when the pressure further was reduced to the saturated water vapor pressure. The kinetics study of the epoxy decomposition was also carried out by monitoring the glass transition temperature of the solid product using differential scanning calorimetry. The decomposition equation was established and the activation energy was calculated to be 123.5 kJ mol^–1.     

Kinetics Analysis of Heterogeneous Oxidation of Coal Particles in Supercritical Water

Little research has been conducted on the heterogeneous kinetics of coal oxidation in supercritical water (SCW). Therefore, the oxidation of coal particles in SCW coupled with heat and mass transfer was investigated. The real coal particle temperature was estimated in terms of a simplified heat balance. With increasing particle temperature, the rate‐limiting step of the oxidation reaction gradually changed from the surface reaction to the mass transfer of oxygen. According to this result, the reaction rate model was built. The model could be used to predict the quantitative relationship among the particle size, the particle temperature, and the time to complete oxidation.     

Decomposition of Epoxy Model Compounds in Near‐Critical Water

Two tpyes of epoxy resin model compounds, one containing an ether bond and bisphenol‐A structure (compound I) and the other comprising ether and tertiary amine bonds (compound II), were prepared and then decomposed in near‐critical water (NCW). In the case of model compound I, at low temperatures the water molecules behave as nucleophilic reagent reacting with the terminal ether group of the model compound. When the temperature is increased, the middle ether bond of the model compound molecule can be broken down. As to model compound II, at lower temperatures, cleavage of the ether bond happens more easily than that of the tertiary amine bond because of higher positive charge density centered at the carbon atom in the ether group. At higher temperatures, the energy required to break down the ether bond is reduced dramatically after being protonated by H⁺, and therefore, the bond can be cleaved more readily than the tertiary amine bond. The decomposition products in both model compounds were found unstable and could react with each other to generate other compounds. Decomposition mechanisms were also proposed based on the decomposition products for the model compounds.     

Supercritical Fluids in Catalysis: Opportunities of In Situ Spectroscopic Studies and Monitoring Phase Behavior

A number of homogeneous and heterogeneous catalytic reactions have been successfully performed in supercritical fluids (SCFs). An overview on recent developments in the areas of alkylation, isomerization, hydrogenation, partial oxidation, amination, and CO₂-fixation using heterogeneous catalysts and supercritical fluids is given. Additionally, strategies towards a more fundamental understanding of catalytic reactions in supercritical fluids are outlined. One aspect is the identification of phase behavior in such multicomponent systems. Their complexity and the input of in situ monitoring is discussed. It is proposed that binary fluid mixtures are an ideal guide for simplifying and understanding the phase behavior in reaction mixtures. In order to strengthen the future use of this knowledge, e.g., for optimization of reactions in SCFs, an overview on the different topologies of binary mixtures is given. Another aspect is in situ characterization of the catalytic reaction and their intermediates, the intermolecular interactions in the fluid, the heterogeneous catalyst phase, and the solid/fluid interphase. The opportunities of various available spectroscopic tools, applicable in situ, are also reviewed by referring to examples from homogeneous catalysis or low-pressure studies.     

Amphiphilic Block Polypeptide‐Type Ligands for Micellar Catalysis in Water

Novel amphiphilic block polypeptide ligands were synthesized and showed excellent behavior in the metal‐catalyzed organic transformations in pure water. The catalytic activity and/or recycling properties of the catalysts are the result of the micellar structure of the polymeric system in water.