Lewis Acids: From Conventional Homogeneous to Green Homogeneous and Heterogeneous Catalysis

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 interm...

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

Common mechanistic aspects of liquid and solid acid catalyzed alkylation of isobutane with n-butene

Adsorption/desorption and pore-transport are key parameters influencing the activity and product selectivity in porous catalysts. With conventional reaction media (gas or liquid phase), one of these parameters is generally favorable while the other is not. For instance, while desorption of heavy hydrocarbons from the catalyst is usually the rate-limiting step in gas-phase reactions, transport of the reactants/products is the limiting step in liquid-phase reaction media. With conventional media, it is difficult to achieve the desired combination of fluid properties for optimum system performance. In contrast, density and transport properties can be continuously pressure-tuned in the near-critical region to obtain unique fluid properties (e.g. gas-like transport properties, liquid-like solvent power and heat capacities), that have been exploited in several ways such as (a) the in situ extraction of heavy hydrocarbons (i.e. coke precursors) from the catalyst surface and their transport out of the pores before they are transformed to consolidated coke; (b) complete miscibility of reactants such as hydrogen in the reaction mixture and enhanced pore-transport of these reactants to the catalyst surface, thereby promoting desired reaction pathways; and (c) control of temperature rise in exothermic reactions. Experimental and theoretical investigations are presented to demonstrate beneficial pressure-tuning effects on catalyst activity and product selectivity during continuous processing of a variety of reactions such as these: geometric isomerization and alkylation on solid acid catalysts; Fischer–Tropsch (FT) synthesis on supported Fe catalysts; and fixed-bed hydrogenation on supported catalysts. The possibility to perform solid acid catalysis with extended activity (an environmentally safer alternative to liquid acid processes) and fixed-bed hydrogenations with tunable selectivity and controlled temperature rise (preferred over slurry phase operation) makes supercritical reaction media particularly appealing alternatives to conventional reactor operation.

Enhancing the stability of porous catalysts with supercritical reaction media

Environmental concerns stemming from the use of conventional solvents and from hazardous waste generation have propelled research efforts aimed at developing benign chemical processing techniques that either eliminate or significantly mitigate pollution at the source. This paper provides an overview of heterogeneous and homogeneous catalysis in dense phase CO 2 , considered a green solvent. In addition to solvent replacement, the demonstrated advantages of using dense phase CO 2 include the enhanced miscibility of reactants, such as O 2 and H 2 which eliminate interphase transport limitations, and the chemical inertness of CO 2 . Further, the physicochemical properties of CO 2 -based reaction media can be pressure-tuned to obtain unique fluid properties (e.g. gas-like transport properties, liquid-like solvent power and heat capacities). The advantages of CO 2 -based reaction media for optimizing catalyst activity and product selectivity are highlighted for a variety of reactions including alkylation on solid-acid catalysts, hydrogenation on supported noble metal catalysts and a broad range of homogeneous oxidations with transition metal catalysts and dioxygen as an oxidant. Through these examples, the need is emphasized for a systematic approach to research and development of supercritical carbon dioxide based processes, taking into account conventional multiphase reaction engineering principles, catalytic chemistry and phase behavior.

Environmentally benign multiphase catalysis with dense phase carbon dioxide

Alkylation reaction on a zeolite catalyst was conducted in supercritical phase isobutane or isopentane, which was a reactant as well. Compared to the reactions conducted in a liquid phase or gas ph...

Supercritical-Phase Alkylation Reaction on Solid Acid Catalysts: Mechanistic Study and Catalyst Development

Ethylene oligomerization reaction on H-ZSM-5 zeolite catalyst was conducted in supercritical phase n-pentane. Compared with the reactions conducted in the gas phase, the supercritical phase ethylene oligomerization reaction exhibited higher activity, along with longer lifetime. High-molecular-weight olefins formed through oligomerization and cracking reactions readily deactivated the active sites, as in the gas-phase reaction. Supercritical n-pentane was able to extract high-molecular-weight olefins in situ to extend catalyst life successively.

Ethylene Oligomerization on H-ZSM-5 Zeolite Catalysts in Supercritical-Phase n-Pentane Mixtures

Alkylation reaction on zeolite catalysts was conducted in supercritical-phase 2-methyl-propane which was reactant as well. Compared to the reactions conducted in liquid phase or gas phase, the supercritical phase reaction exhibited higher activity, along with obviously longer lifetime, on Y-type zeolite catalyst. Supercritical fluid was able to extract in situ and transport high-molecular-weight olefins to extend catalyst life successively.

Shintaro Ishida

Papers

Supercritical-Phase Alkylation Reaction on Solid Acid Catalysts: Mechanistic Study and Catalyst Development

Ethylene Oligomerization on H-ZSM-5 Zeolite Catalysts in Supercritical-Phase n-Pentane Mixtures

Supercritical phase alkylation reaction on solid acid catalysts