Vapor-liquid-solid catalysis

The scale of components in complex condensed matter often results in structures having a high surface-area-to-volume ratio. In these systems, interfacial effects can be very important. The interfaces between vapor and condensed phases and between two condensed phases have been well studied over the past four decades. These studies have contributed to technologies from electronic materials and devices, to corrosion passivation, to heterogeneous catalysis. In recent years, the focus has broadened to include the interfaces between vapors, liquids, or solids and self-assembled structures of organic, biological, and polymeric nature. 

Understanding chemical reactivity at liquid interfaces is important because in many systems the interesting and relevant chemistry occurs at the interface between two immiscible liquids, at the liquid/solid interface and at the free liquid (liquid/vapor) interface. Examples are reactions of atmospheric pollutants at the surface of water droplets[6], phase transfer catalysis[7] at the organic liquid/water interface, electrochemical electron and ion transfer reactions at liquidAiquid interfaces[8] and liquid/metal and liquid/semiconductor Interfaces. Interfacial chemical reactions give rise to changes in the concentration of surface species, but so do adsorption and desorption. Thus, understanding the dynamics and thermodynamics of adsorption and desorption is an important subject as well. 

Solid-vapor Solid-liquid Liquid-vapor Liquid-liquid Adsorption, catalysis, contamination, gas-liquid chromatography Cleaning and detergency, adhesion, lubrication, colloids Coating, wetting, foams Emulsions, detergency, tertiary oil recovery... 

Many phenomena of interest in science and technology take place at the interface between a liquid and a second phase. Corrosion, the operation of solar cells, and the water splitting reaction are examples of chemical processes that take place at the liquid/solid interface. Electron transfer, ion transfer, and proton transfer reactions at the interface between two immiscible liquids are important for understanding processes such as ion extraction, " phase transfer catalysis, drug delivery, and ion channel dynamics in membrane biophysics. The study of reactions at the water liquid/vapor interface is of crucial importance in atmospheric chemistry. Understanding the behavior of solute molecules adsorbed at these interfaces and their reactivity is also of fundamental theoretical interest. The surface region is an inhomogeneous environment where the asymmetry in the intermolecular forces may produce unique behavior. 

There has been an enormous technological interest in tertfa/j-butanol (tBA) dehydration during the past thirty years, first as a primary route to methyl te/f-butyl ether (MTBE) (1) and more recently for the production of isooctane and polyisobutylene (2). A number of commercializable processes have been developed for isobutylene manufacture (eq 1) in both the USA and Japan (3,4). These processes typically involve either vapor-phase tBA dehydration over a silica-alumina catalyst at 260-370°C, or liquid-phase processing utilizing either homogenous (sulfonic acid), or solid acid catalysis (e.g. acidic cationic resins). More recently, tBA dehydration has been examined using silica-supported heteropoly acids (5), montmorillonite clays (6), titanosilicates (7), as well as the use of compressed liquid water (8). 

Adsorption may be either physical or chemical. The theory of physical adsorption assumes that the adsorbed phase is a condensed liquid-phase layer of molecules of the vapor on the solid surface. Chemical adsorption is the chemical combination of a vapor molecule with a portion of catalyst surface. This portion of surface is called an active center. In the application of adsorption to catalysis, chemical adsorption is of major impO rtance. 

Gas-liquid phase-transfer catalysis (GL-PTC) relies on the use of thermally stable PT catalysts adsorbed onto a solid support, which can also act as a source of the desired nucleophile. Reactions are carried out at a temperature that ensures that the catalyst is in a molten state and that reagents are in the vapor phase, and that the chemical transformation occurs in the organic microphase of molten catalyst. The products are recovered after condensation outside the reaction vessel. Only catalysts having melting points lower then the process temperature <180°C are active, but despite this limitation, GL-PTC is a versatile technique that has been applied to a number of chemical transformations. 

Reactions between a gas and a liquid catalyzed by a solid are frequently encountered in chemical processes of great economical significance. Very often, it is technically impossible to operate with just one fluid phase, gas or liquid, in the reactor. The occurence of two fluid phases mainly depends on the temperature range in which the reaction can occur. It is well known that the low temperature limit depends on kinetics and catalysis whereas the highest possible temperature is fixed either by thermodynamics either by the product and the catalyst heat sensitivity. Nevertheless, some reactants, even at rather low temperature, will never been condensed or soluble enough to eliminate the gas phase. On the other hand, other reactants will never been completely vaporized whatever the temperature. Therefore, three phase catalytic processes will always occur when the volatility of two reactants is very different. This broad class of reactions is very important in chemical industry and, as we shall see, will be more frequent in the future. 

Ionic Hquids (ILs) are low melting salts (<100 °C) and represent a promising solvent class, for example, for homogeneous two-phase catalysis [7-9] and extractions [10-12]. These and other appHcations of ILs have been reviewed in a number of papers [7, 13-16]. One of the main reasons that ILs have gained interest both in academia and in industry is that they have an extremely low vapor pressure [17, 18], which makes them attractive as alternative reaction media for homogeneous (two-phase) and heterogeneous catalysis [19, 20]. This paper focuses on the concept of a solid catalyst with ionic liquid layer (SCILL) as a novel method to improve the selectivity of heterogeneous catalysts. 

The porous materials are known to be of importance in many different industrial processes e.g., catalysis, oil recovery, soil pollution, chromatography and separation. In all these systems, the pore structure is known to determine the physico-chemical characteristics. The pore shape and form is not easily determined. Microsporous material is not easily analyzed using electron microscope or diffraction methods, when the mean pore-radius is 2 -50 fjm. One generally uses mercury porosimetry for larger pores, which is based on a capillary phenomena. Other methods have also been used, which are based upon the effect of the curvature of a liquid on its solid - liquid phase transition equilibria, i.e. freezing point depression, vapor pressure or heat of evaporation. 

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