Base catalysis catalyst characterization

Spectroscopy, too, is by no means simple. Quick and easy experiments in catalyst characterization hardly exist. The correct interpretation of spectra requires experience based on practice and a sound theoretical background in spectroscopy, in physical chemistry and often in solid state physics as well. Intensive cooperation between spectroscopists and experts in catalysis is the best way to ensure meaningful and correctly interpreted results. 

Stevani and coworkers prepared and characterized a peracid intermediate, 4-chloro-pheny 1-0,0-hydrogen monoperoxalate (57) and found that no chemiluminescence was observed in the presence of activators (i.e. rubrene, perylene and DPA) and the absence of a base. Based on this result, the authors excluded 57 and similar peracid derivatives as HEI in the peroxyoxalate system. Moreover, 57 only emits light in the presence of an activator and a base with pK > 6, suggesting that a slow chemical transformation must still occur prior to the chemiexcitation step. Kinetic experiments with 57, using mainly imidazole, but also in the presence of other bases such as potassium 4-chlorophenolate, f-butoxide and l,8-bis(dimethylamino)naphthalene , showed that imidazole can act competitively as base and nucleophilic catalyst (Scheme 41). At low imidazole concentrations, base catalysis is the main pathway (steps 1 and 2) however, increasing the base concentration causes nucleophilic attack of imidazole catalyzed by imidazole to become the main pathway (steps la and 2a). Contrary to the proposal of Hohman and coworkers , the... 

In this area, recent unrelated efforts of the groups of Bhattacharya and Fife toward the development of new aggregate and polymer-based DAAP catalysts deserve mention. Bhattacharya and Snehalatha [22] report the micellar catalysis in mixtures of cetyl trimethyl ammonium bromide (CTAB) with synthetic anionic, cationic, nonionic, and zwitterionic 4,4 -(dialkylamino)pyridine functional surfactant systems, lb-c and 2a-b. Mixed micelles of these functional surfactants in CTAB effectively catalyze cleavage of various alkanoate and phosphotriester substrates. Interestingly these catalysts also conform to the Michaelis-Menten model often used to characterize the efficiency of natural enzymes. These systems also demonstrate superior catalytic activity as compared to the ones previously developed by Katritzky and co-workers (3 and 4). 

Ma Z, Zaera F Chiral, Modification of Catalytic Surfaces In Design of Heterogeneous Catalysts. In Ozkan US (ed) Design of heterogeneous catalysis New approaches based on synthesis, characterization, and modelling. WUey-VCH, New York. 

The use of nitromethane as a probe of basicity of zeolites (NaX, CsX, CsX 9Cs) and mixed oxides, Mg(Al)0, is discussed. Various species (physisorbed nitromethane, aci-anion nitromethane, and methazonate salt analogue) formed upon nitromethane adsorption were characterized by C MAS NMR spectroscopy. Heterogeneous base catalysis of the Michael addition of nitromethane on cyclohex-2-en-1-one was also studied. Low rates were obtained for catalysts showing only nitromethane physisorption. Formation of aci-anion nitromethane was observed for solids of medium efficiency correlation of the chemical shift with the initial rate was established. Finally, the decrease of Lewis acidity and concomitant increase of basicity led to methazonate formation and to the more efficient catalysts. 

Preparation and characterization of liposomes formed with natural phospholipids were well established. However, in using liposomes for simulation of enzymatic functions, especially in acid-base catalysis, difficulties would be encountered due to their chemicai and morphological instabilities. Thus, bilayer membranes composed of synthetic amphiphiles are more favorable candidates for enzyme mimics. For example, artificial vitamin Bg-dependent enzymes were constructed from catalytic bilayer membranes in combination with a bilayer-forming peptide lipid (10), a hydrophobic vitamin derivative (11), and metal ions (Fig. 5). The catalyst acts as an artificial aminotransferase, showing marked substrate specificity, high enantioselectivity, and turnover behavior for the transamination of a-amino acid with a-keto acids. In addition, the reaction fields provided by the catalytic bilayer membranes are suitable to establish multienzyme systems through functional ahgnments of artificial enzymes and natural ones in a sequential manner. 

ABSTRACT. Progress in molecular sieve science and technology is reviewed in the fields of synthesis, catalyst characterization, and catalysis. In synthesis, the molecular sieve crystal types and compositions have been greatly expanded from zeolites and pure silica materials to aluminophosphate-based molecular sieves representing more than two dozen crystal structures and many more chemical compositions, with up to six framework metals chosen from 13 elements. These new molecular sieves represent a variety of chemical properties, and they display weak to medium-strong acid catalytic activity. 

Characterization is an important field in catalysis. Spectroscopy, microscopy, diffraction and methods based on adsorption and desorption or bulk reactions (reduction, oxidation) all offer tools to investigate the nature of an active catalyst. With such knowledge we hope to understand catalysts better, so that we can improve them or even design new catalysts. 

This complex and structurally related molecules served as a functional homogeneous model system for commercially used heterogeneous catalysts based on chromium (e.g. Cp2Cr on silica - Union Carbide catalyst). The kinetics of the polymerization have been studied to elucidate mechanistic features of the catalysis and in order to characterize the potential energy surface of the catalytic reaction. 

The aim of this study is to develop model reaction for the characterization of the acidity and basicity of various transition aluminas, the experimental conditions being close to that for catalysis use. Among various model reactions, the transformation of cyclopentanol and cyclohexanone mixture was chosen for this work. Indeed, this reaction was well known for estimating simultaneously the acid-base properties of oxide catalysts [1], Two reactions take place the hydrogen transfer (HT) on basic sites and the alcohol dehydration (DEH) on acid sites. The global reaction scheme is shown in Figure 1. 

In non-electrochemical heterogeneous catalysis, the interface between the catalyst and the gas phase can often be characterized using a wide variety of spectroscopic probes. Differences between reaction conditions and the UHV conditions used in many studies have been probed extensively 8 as have differences between polycrystalline and single-crystalline materials. Nevertheless, the adsorbate-substrate interactions can often be characterized in the absence of pressure effects. Therefore, UHY based surface science techniques are able to elucidate the surface structures and energetics of the heterogeneous catalysis of gas phase molecules. 

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