Liquid phase studies

When liquid content of the feed is high, a condenser and a separator are needed. The liquid-to-gas ratio can be as high, so that even at reaction temperatures a liquid phase is present. The reactor still performs as a CSTR, however the response time for changes will be much longer than for vapor phase alone. Much lower RPM will be needed for liquid-phase studies (or liquid and gas phase experiments) since the density of the pumped fluid is an order-of-magnitude greater than for vapor phase alone. In this case a foamy mixture or a liquid saturated with gas is recirculated. 

Carlier fundamental studies of autoxidations of hydrocarbons have concentrated on liquid-phase oxidations below 100 °C., gas-phase oxidations above 200°C., and reactions of alkyl radicals with oxygen in the gas phase at 25°C. To investigate the transitions between these three regions, we have studied the oxidation of isobutane (2-methylpropane) between 50° and 155°C., emphasizing the kinetics and products. Isobutane was chosen because its oxidation has been studied in both the gas and liquid phases (9, 34, 36), and both the products and intermediate radicals are simple and known. Its physical properties make both gas- and liquid -phase studies feasible at 100°C. where primary oxidation products are stable and initiation and oxidation rates are convenient. 

Our own liquid-phase studies were carried out at 50°-100°C., where the products are stable (16, 17). We obtained more information on radical interactions and determined the effects of dilution with CC14. Our oxidations were carried out by heating known amounts of isobutane, initiator, and oxygen (sometimes with solvents) in sealed glass tubes as described above. Nearly all conversions of isobutane were kept below 1%. Experimental data are summarized in Table I. 

When acquiring gas phase 33S spectra, other problems must be considered. Spectrometers used for liquid phase studies can be employed for gas phase studies without any modification, but signals are generally much broader than in the liquid phase. Care must be taken in preparing samples. These should be no longer than 3-4 cm, in order to minimize the diffusion of molecules and temperature gradients,17 and should match exactly the active region of the probe. 

Probably, a combination of both i) digitally recording of LSC pulses plus their analysis by a neural network and ii) reducing the amount of reaction products entering the SISAK system with a pre-separator gives the most promising perspective. Various liquid-liquid extraction schemes for the transactinides have already been elaborated. An example is the separation procedure proposed for liquid-phase studies of element 107, bohrium (Bh) [62],... 

First of all, liquid-phase studies generally do not obtain data which allows static and dynamic solvent effects to be separated [96,97], Static solvent effects produce changes in activation barriers. Dynamic solvent effects induce barrier recrossing and can lead to modification of rate constants without changing the barrier height. Dynamic solvent effects are temperature and viscosity dependent. In some cases they can cause a breakdown in transition state theory [96]. 

In solid-state studies, ESR spectroscopy is the best detection method for studying radical intermediates in radiolysis. It is, however, difficult to apply to liquid-phase studies, and generally, optical methods are favoured. In solid-state work, radicals are trapped (matrix-isolated) and can be studied by any spectroscopic technique at leisure. However, for liquid-phase studies, time-resolved methods are often necessary because the intermediates are usually very short lived. In the technique of pulse radiolysis, short pulses of radiation, followed by pulses of light which explore the UV spectrum, are used. The spectra help to identify the species, but also their kinetic behaviour can be accurately monitored over very short time-scales (from picoseconds to milliseconds). This technique is discussed in Section 3.3. 

Detailed description of the wealth of techniques available for studying such systems is beyond the scope of this book. However, the authors have attempted to provide a broad overview of some of the approaches which can be undertaken for identification of radical damage which DNA may have experienced in intact cellular systems or for standard assessment of the ways in which DNA may be damaged by these reactive species. As well as the use of cell systems we also use liquid-phase studies on dilute solutions of DNA to illustrate the effects of OH radical damage, and solid-state studies to show the effects of electron-gain and -loss. 

Co and Cr have been found to be incorporated into the lattice of aluminiumphosphates in a well dispersed manner [159]. Both elements assume two oxidation states in the lattice depending upon the pretreatment procedures. While it seems certain that during synthesis incorporation can be achieved and that these tetrahedrally coordinated atoms are stable in gas phase reactions, conclusive evidence is lacking that leaching is not an important side reaction in liquid phase studies. Indeed, it seems that for several reactions the highly active complexes that are leached out of the lattice and homogeneously dissolved in the reactant/solvent mixture dominate the catalytic properties. 

Other photolysis studies have been reported for ethyl iodide -propyl iodide and isopropyl iodide in the gas phase, and also in liquid and solu-tion . Solution and liquid phase studies have also been reported for butyl ° , pentyl and cetyl iodides ". The primary process in the photolysis of alkyl iodides has been further discussed by Donovan and Husain in reference to the formation of excited halogen atoms. 

Recently, some of the methyltriptycene derivatives were re-investigated with use of the methodology of Ref. 51 (Section 4.2) to line shape analysis. In these liquid-phase studies, instead of the solid echo spectra, the Carr-PurceU (CP) echo spectra were... 

In the oxygen system at approximately 50 mm. pressure (collision frequency — 109 sec. 1) half of the 02 " ions are stabilized before emission can take place (13). In the condensed phase, therefore, deactivation should compete to the exclusion of electron emission. The much higher probability of collisional deactivation in liquids may explain why compounds such as C02 and CH3C1, for which attachment is very inefficient in the gas phase, are often effective electron scavengers in liquid systems. One must be wary, therefore, of using even relative gas-phase electron attachment coefficients in liquid-phase studies. For molecules with very small electron affinities (< 0.1 e.v.) the reversibility of Reaction 3 may have to be considered even after the excitation energy of the negative ion has been removed by collision. 

Thus Szwarc and co-workers (see ref. 396) have carried out extensive studies of H abstraction and addition to unsaturates of methyl radicals in iso-octane solution. Most of the work involved acetyl peroxide as the methyl radical source and subsequent experiments with azomethane confirmed the original findings. The systems yielded rate coefficients at 338°K for attack of the methyl radicals on the substrate relative to attack on iso-octane. If the rate coefficient for attack of methyl radicals on iso-octane in the gas phase was known it would be possible to make direct comparisons of gas and liquid phase reactions of methyl radicals with some substrate molecules. Unfortunately, the only available rate coefficient for methyl attack on iso-octane was measured at 773°K and it is of doubtful validity [3]. Nevertheless, the liquid phase studies of methyl radicals yield relative rate coefficients for attack on primary, secondary and tertiary C—H bonds which are fully compatible with the gas phase values. 

Carbon-sulphur bond homolysis has been shown to be, in addition to sulphur-sulphur bond homolysis, a primary process in the photolysis of disulphides in solution. Sulphur-sulphur bond homolysis is, however, responsible for the establishment of an equilibrium between various alkyl disulphides on irradiation." Other liquid-phase studies of the photodecomposition of acyclic alkyl disulphides have been reported," -and the quantum yield for the formation of methyl ethyl disulphide from methyl disulphide and ethyl disulphide has been determined. ... 

Over the years, a number of studies have probed the interaction of xenon in numerous liquid environments. The xenon chemical shift is known to vary widely depending on the particular solvent. For example, the xenon chemical shift in methanol is 148 ppm while in methyl iodide it is 333 ppm, with more typical values around 180-220 ppm. The xenon chemical shift is very sensitive to its liquid environment such that it can detect the difference between protonated and deute-rated solvents, with changes of the order of l ppm seen for various solvents. " Most liquid-phase studies involving xenon now probe other solute species, as described below, however fundamental studies of xenon in various solvents are still reported. ... 

AAdiile the active catalyst species has not been fully defined, the results obtained with this system are similar to those found in liquid-phase studies (5,6). The catalyst (3% Rh/C) is effective at low temperatures (175-250°C) and low pressures (1-14 atm) with very high selectivity (>99%) in the presence of methyl iodide promoter (Table 7). Even at atmospheric pressure reasonable conversions of methanol are obtained. At higher pressures essentially total conversion of CH3OH is obtained at only 10-30% of CH3I level used at atmospheric pressure. Hydrogen iodide can be used in place of methyl iodide as the promoter. 

NO reacts with alkenes at room temperature by a free-radical mechanism [4,59-62], Gas-phase and liquid-phase studies revealed the formation of two types of products addition products and ally lie substitution products [62], The change in concentrations of NO leads to variation of the addition/substitution ratio. As the NO concentration is decreased, the relative amounts of substitution products increase. These observations were made with cyclohexene as substrate [61]. In principle, NO or its dimer may be involved in these reactions. One pathway is represented by the following scheme [4, 63-65] ... 

LIQUID PHASE STUDIES OF INDUCED MOMENTS 4.1 General Cons iderations... 

The subject matter was organized into four broad areas (a) Theories of Liquid Structures, (b) Ionic and Electronic Processes, (c) Interfacial Phenomena, and (d) Breakdown and Conduction. These four areas covered the bulk of the Institute. In addition, results of current research were presented in two Poster Sessions, and Future Research Directions and technological innovations derived from liquid-phase studies were discussed in a special session. 

This review is concerned mainly with solids, because this reflects the Reporter s interests. A small section is devoted to solutions. It is hoped that future volumes will contain a more comprehensive coverage of the subject and include the gas- and liquid-phase studies. 

For many gas-phase molecular processes with activation energies in the 5-20 kcal mol range, temperature- and pressure-dependent rate constants can be obtained from analysis of exchange-broadened NMR line shapes. Frequently, rate constants can be obtained with accuracy comparable to that obtained in the best liquid-phase studies. This technique can be applied successfully to gases that have at least 1 torr of vapour pressure at temperatures at which slow or intermediate exchange can be observed. Temperature-dependent gas-phase rate constants... 

Levina, A.B., Chomaja, S.S., Grigorjeva, I.A., Sergejava, O.N., Trusov, S.R., 1997. Nitrogen oxides catalyzed selective oxidation by oxygen in the liquid phase. Studies in Surface Science and Catalysis 110,585-591. 

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