Vacuum gas-solid reaction

Dehydrochlorination of a 1-chlorosilanamine by vacuum gas solid reaction (VGSR). The presence of the iminosilane was confirmed by direct analysis of the gaseous flow.7... 

CONTENTS Preface, Brian Hatton. Applications of Ring Strain in Natural Product Synthesis, James D. White and Nadine Chauyi Lee. Uses of the Vacuum Gas-Solid Reaction Procedure in Synthesis, W. E. Billups and Benny E. Amey, Jr. Polysubstituted Cubanes Towards Complete Systematic Substitution of the Cubane Nucleus, John Tsanaktsidis. A Personal Perspective on Norbomenes, Cyclobutenes, and Other Ring-Strained Dienophiles in Organic Synthesis, Ron N. Warrener. Synthetic Studies Related to Fullerenes and Fullerene Fragments, Goverdhan Mehta and H. Surya Prakash Rao. Index. 

When a reactive atmosphere is present [402,1249], the reaction undergone by a solid may be changed, and perhaps complicated, by the presence of a concurrent or consecutive gas—solid reaction. When one or more of the products of the reaction in vacuum is an oxidizable metal [60] or... 

One of the most important of these new experimental tools has been the development and application of the vacuum microbalance technique in which the sensitive microbalance operates directly in the vacuum or reaction system. The success of the method depends upon the coordination of a number of different experimental as well as theoretical disciplines. Thus, from an experimental point of view precise weighing techniques on properly prepared specimens must be coordinated with high vacuum techniques and the use of ceramic materials at high temperatures. From a theoretical viewpoint thermodynamic calculations must be made for all of the reactions involved and the results interpreted in terms of diffusion process for gas-solid reactions in which a film is formed or the gas diffuses into the solid, or in terms of the absolute reaction rate theory or its equivalent for gas reactions on solids including catalytic reactions and the combustion of fuels. 

The literature of catalysis is so vast that it is impossible to do full justice in one part of a chapter, even to the experimental methods that are in use. Most catalytic reactions are quite complex and a complete reaction mechanism can rarely be determined from a measurement of kinetics alone. It is safe to say that no catalytic gas-solid reaction has yet been fully characterised. Because of the many difficulties, work on catalytic reactions tends to veer towards two opposite poles. One of these can be described as the clean surface approach ", which uses the simplest reactions under high vacuum conditions and attempts to define the catalyst surface completely. The other approach is to study much more complex reactions in order to obtain overall kinetic information which describes the process. Where the mechanism is not understood, it is common to employ expressions similar to those used for homogeneous reactions, or even purely empirical equations. 

The reaction can be completed in a maintained vacuum, during which volatile products are continually evacuated from the vicinity of the substance undergoing chemical change (reactant environment). These conditions can be used to measure the sublimation rate of a solid or the evaporation rate of a liquid. Alternatively, reactions can be studied in the presence of a controlled (often constant) pressure of product or of a volatile reactant, a method for investigation of gas-solid reactions. 

Fig. 9. (a) Experimental device for the study of gas-solid reaction imder constant oxygen pressure using a pump-sensor device, G vacuum gauge, LN liquid nitrogen trap (b) Variation of the stoichiometry ratio of Ce02-x at 1273 K. 

As a consequence of the way in which matrices are deposited, the generation of reactive species can be carried out either (a) by photolysis or radiolysis of precursors already trapped in the matrices or (b) by gas-phase or surface (gas-solid) reactions prior to deposition. The second category can include flash vacuum pyrolysis, microwave excitation, or reactions of organic precursors with metals. After the reactants have passed through the region where the reaction takes place, the resulting product mixture is diluted with a large excess of the host gas and condensed as quickly as possible on the cold window as a matrix. 

Measurement of relative humidity depends on the system used. Systems employing vacuum are usually evacuated prior to introduction of water vapor [29]. For cases in which there is not a gas-forming reaction occurring, measurement of total pressure in the system can be used as a measure of water vapor pressure. Systems in which air is not evacuated require specific measurement of water vapor pressure. (For the latter type of system, caution should be taken to assure that the relative humidity source is in close proximity to the solid, since the diffusion of water vapor through air to the solid is required to maintain a constant relative humidity in the immediate vicinity of the solid.) A wide variety of pressure measuring instrumentation is commercially available with varying accuracy, precision, and cost. 

Most of the generated vapour is condensed in spray condensers which are equipped with circulation pumps and an EG cooler. The vapour that is still uncondensed is withdrawn from the gas phase with the help of a vapour jet which is located down-stream behind the spray condenser and generates the necessary vacuum in the reaction zone. The most critical part of the spray condenser system is the end of the pipe leading the vapour from the prepolycondensation reactors and the finishers into the spray condenser. The transition from a hot to a cold environment causes deposition of solid material onto the cold walls which has to be removed manually or by means of a mechanical scraper. 

A very old gas-solid bromination of tyrosine (280) [97] has been revisited and it gave a quantitative yield for the reaction of rac-280 [22]. The doubly bromi-nated hydrobromide rac-281 is spectroscopically pure after removal of included gases at 50 °C in a vacuum. Quite spectacular is the specific and quantitative waste-free gas-solid tetrabromination of tetraphenylethylene (282), which shows some signs of autocatalysis and requires rotation of the flask around a horizontal axis at room temperature for 12 h as the reactant and product gases require mixing [60]. The isomer-free tetrabromide 283 is an attractive starting point for dendrimer syntheses and inclusion studies (Scheme 40). Also 4-bro-mo antipyrin hydrobromide is quantitatively obtained from antipyrin(hydro-bromide) and bromine vapor [22]. 

A systematic study to identify solid oxide catalysts for the oxidation of methane to methanol resulted in the development of a Ga203—M0O3 mixed metal oxide catalyst showing an increased methanol yield compared with the homogeneous gas-phase reaction.1080,1081 Fe-ZSM-5 after proper activation (pretreatment under vacuum at 800-900°C and activation with N20 at 250°C) shows high activity in the formation of methanol at 20°C.1082 Density functional theory studies were conducted for the reaction pathway of the methane to methanol conversion by first-row transition-metal monoxide cations (MO+).1083 These are key to the mechanistic aspects in methane hydroxylation, and CuO+ was found to be a likely excellent mediator for the reaction. A mixture of vanadate ions and pyrazine-2-carboxylic acid efficiently catalyzes the oxidation of methane with 02 and H202 to give methyl hydroperoxide and, as consecutive products, methanol and formaldehyde.1084 1085... 

V-Vinylpyrrolidone 1 (1.00 g, 9.0 mmol) was spread on cylindric glass rings (Raschig coils) in a 500-mL flask and crystallized by cooling to -40 °C in a vacuum. HBr gas (1 bar) was applied for 2h at that temperature. Excess gas was pumped off to a recipient at -196 °C for further use and the solid reaction product 2 was left at room temperature with repeated evacuation for removal of the liberated HBr. The racemic product 3 was recrystallized from acetone to yield 650 mg (65%) of the pure compound (mp 70-73 °C) that was spectroscopically characterized. 

Adsorption of a high-boiling solvent onto a high-surface-area microporous solid yields a supported liquid phase that can be removed from the sohd only by extraction with a second solvent or by distillation at high temperature under vacuum. Under typical reaction conditions, a solid that contains a supported liquid phase looks and behaves as a solid, yet it can dissolve small quantities of a metal complex into the supported phase. One of the first examples of this arrangement was achieved with the immobilization of Rh(CO)(PPh3)2Cl in benzyl butyl phthalate on silica. The supported complex was successfiilly used to effect the gas-phase hydroformylation of propene. 

A system that links microcalorimetry to the volumetric measurement of quantities of adsorbed reactants makes it possible to study gas-solid interactions and catalyhc reactions. This system works under stahc vacuum. The admission of gases into the calorimeter can be performed either in a discontinuous way (by successive doses) by means of a valve, or in a continuous manner by means of a capillary. The classical technique of adsorption calorimetry by doses is the most appropriate way to measure the energy of interaction between the adsorbed species and the catalyst. If the surface can be a priori considered as heterogeneous, the heat of adsorption, the amount adsorbed and the kinetics of adsorption must be measured for very small successive doses of the adsorbate so as to obtain accu-... 

Molecular beams are limited to reactions that are carried out in vacuum, where well-defined beams of reactant molecules can be prepared. This limits their application to gas-phase reactions and to reactions of gaseous molecules with solid surfaces. Molecular beam methods cannot be used to study kinetics in liquid solvents. The detailed information they provide for gas-gas and gas-surface reactions allows precise testing of models and theories for the dynamics of these classes of reactions. 

---