Specific Catalytic Effect Research Articles

Silica grain catalysis of methanol formation

The specific catalytic effect of a silica grain on the formation of methanol via the sequential addition of H atoms to CO adsorbed on the surface is investigated. A negatively charged defect on a siliceous edingtonite surface is found to reduce the gas phase barriers for the H + COads and H + H2C=Oads reactions by 770 and 399 K, respectively, when compared to the same reactions in the gas phase. The catalytic effect of negatively charged surface sites could also be applicable to the hydrogenation of other adsorbed unsaturated species. However, the activation energies on the surface defect are still too large (1150 and 2230 K) for CH3OH to form efficiently at 10–20 K in the interstellar medium via a classical mechanism. It is therefore suggested that quantum mechanical tunnelling through the activation barrier is required for these hydrogen addition reactions to proceed at such temperatures. The calculations show that because the adsorption energies of CO and H2C=O on the negatively charged defect are substantial, CH3OH may form efficiently during the warm-up period in star-forming regions.

Catalysis of a β-elimination applying membranes with incorporated molecularly imprinted polymer particles

Molecularly imprinted membranes are known to be applicable as recognition elements in separation, extraction and sensors, however, not as catalysts. In this paper, two different formats of molecularly imprinted membranes were generated and used as catalysts in a dehydrofluorination reaction. The first format was based on cellulose filter membranes which were coated with a polymer imprinted with a transition-state analogue of the chosen reaction. Alternatively, ground bulk polymers have been incorporated in polyvinyl alcohol (PVA) matrices on cellulose membranes. Both types of membranes were evaluated in membrane reactors. Either the substrate solution was pumped once only through the membrane, or the substrate solution was recirculated through the membrane. The coated membranes did not show any specific catalytic effect when comparing the molecularly imprinted with a non-imprinted control polymer membrane. Nevertheless, the PVA membranes containing the imprinted polymer particles showed specific catalytic effects, compared to PVA membranes with incorporated non-imprinted control polymer particles.

Catalyzing a cycloaddition with molecularly imprinted polymers obtained via immobilized templates

Polymeric catalysts to be applied in the Diels–Alder cycloaddition of hexachlorocyclopentadiene and maleic acid have been prepared via molecular imprinting with template molecules immobilized on silica particles. These enzyme mimicking polymers exhibit specific catalytic effects compared to non-imprinted control polymers or polymer-free solutions. It could be demonstrated that the activity of the molecularly imprinted material rises when increasing the temperature. By this means, the reduction of the activation energy (as expected for catalysts) from 63 to 55 kJ mol −1 could be observed. Furthermore, the reaction was characterized based on the Michaelis–Menten model. For the diene compound a Michaelis constant of K M=5.8 mmol l −1 and an effective reaction rate of r max,eff=0.4 μmol l −1 s −1, leading to a reaction rate constant k eff=1.1×10 −3 s −1, were determined.

Modeling biotransformation reactions by combined quantum mechanical/molecular mechanical approaches: from structure to activity.

An overview of the combined quantum mechanical/molecular mechanical (QM/MM) approach and its application to studies of biotransformation enzymes and drug metabolism is given. Theoretical methods to simulate enzymatic reactions have rapidly developed during the last decade. In particular, QM/MM methods provide detailed insights into enzyme catalyzed reactions, which can be extremely valuable in complementing experimental research. QM/MM methods allow the reacting groups in the active site of an enzyme to be studied at a quantum mechanical level, while the surrounding protein and solvent is included at a classical (and computationally less expensive) molecular mechanical level. Existing QM/MM implementations vary in the level of interaction between the QM and MM regions and in the way the partitioning into QM and MM regions is setup. Some general considerations concerning reaction modeling are discussed and a number of QM/MM studies related to drug metabolism are described. These studies illustrate that theoretical modeling of important metabolic reactions provides detailed insights into mechanisms of reaction and specific catalytic effects of enzyme residues as well as explaining variation in rates of conversion of different metabolites. Such information is essential in the development of methods to predict metabolism of drugs and to understand metabolic effects of genetic polymorphism in biotransformation enzymes.

Specific micellar catalytic effects of the protolysis of ortho-Benzoic acid derivatives studied by the ultrasonic absorption method

Ultrasonic relaxation absorption has been observed in aqueous solutions of ortho-benzoic acid derivatives (OBAD) in the presence of a micelle of tetradecyltrimethylammonium bromide (TTAB). Relaxation absorptions were ascribed to the protolysis of acids on the surface of the micelle. The apparent dissociation constant (pKa=kb/γ2kf), rate constants (γ2kf, kb) and volume change (ΔV) for the reactions were determined. The dependence of rate constants on pKa for OBAD was compared with that for meta- and para-benzoic acid derivatives (BAD). It was especially noticeable that the dissociation of OBAD in the micellar solution is depressed more than that in aqueous solution. This can be explained as due to electrostatic effects, the polarity of solution and solvation in the TTAB micellar solution. The dependence of ΔV of OBAD on pKa was also compared with that of BAD. The result was considered to be due to differences in solute–solvent interactions and the situation around OBAD and BAD in the micellar solution.

Hydrolysis of di(p-nitrophenyl)methylphosphonate in the presence of cetyl- and decylamines

It was shown that long-chain amines have a specific catalytic effect on the hydrolysis of di(p-nitrophenyl)methylphosphonate. Long-chain amines have a high catalytic activity, which is associated with their aggregation in aqueous solutions.

Kinetics of silica polymerization

The polymerization of silicic acid in dilute aqueous solutions was studied experimentally and theoretically. Two basic processes were studied: the formation of colloidal particles by homogeneous nucleation, and their further growth by deposition of dissolved silica upon them. It was found that the state of ionization of the silica surface controls the rate of polymerization. The rate of deposition of dissolved silica on the surface of amorphous silica is proportional to the surface density of ionized silanol groups. The extent of surface ionization also determines the value of the surface tension, and thus also the rate of homogeneous nucleation. Added salts accelerate both molecular deposition and homogeneous nucleation by increasing the extent of surface ionization and decreasing the solubility of silica. Except for fluoride, salt ions were found not to have any specific catalytic effect. Fluoride was confirmed to be a powerful catalyst. Aluminum and boron were found to inhibit the reaction at pH 8. A successful quantitative theory of the homogeneous nucleation of colloidal silica particles was developed. The Lothe-Pound factor was shown to be about 3.34 × 10 25 (kg H 2O) −1. The nucleation model and results of our data analysis are incorporated in a computer code which models the homogeneous nucleation and growth of colloidal silica particles. This code is able to reproduce much of our data to within experimental error, and may be used to generate useful predictions for conditions that are typical of geothermal brines. The concentration of “adsorbed silica” on the surface of amorphous silica was determined, and was found to increase with the concentration of dissolved silica in the ambient solution. It is probably a reaction intermediate between dissolved silica and solid amorphous silica.

Pyridine-catalyzed halogenation of aromatic compounds. III. Chlorination and the relative stabilities of trichloride and tribromide in chloroform

The mechanism of catalysis of aromatic chlorination by pyridine was investigated by measuring the rates of chlorination of toluene in acetic acid with and without added pyridine, pyridinium nitrate, or lithium chloride. All three increase the rate of chlorination slightly and to about the same extent. The effect on the second-order rate constant for chlorination of p-chloranisole in chloroform was determined for pyridine, pyridinium chloride, N-ethylpiperidinium chloride, tetra-n-butylammonium chloride, and tetra-n-butylammonium perchlorate. Again, all additives increase the rate constant, with the increase being least for pyridine. It is concluded that, for chlorination in dilute solution, pyridine has no specific catalytic effect, but probably increases the rate by increasing the polarity of the medium. Tetra-n-butylammonium chloride was observed to have a decreasing accelerating effect on chlorination in chloroform as its concentration is increased. This was thought to be the result of complexation of chlorine as trichloride. The formation constant for tetra-n-butylammonium trichloride in chloroform was found by the Benesi–Hildebrand spectrophotometric method to be 17 ± 3 M−1. This, together with the previously determined value of 9 × 104 M−1 for the corresponding tribromide, shows that the order of stability of trihalides in nonaqueous solvents is [Formula: see text] contrary to an earlier report.

Acid hydrolysis of carboxylatopentaamminecobalt(III) complexes Kinetics and mechanism of aquation of malonato, succinato, o-phthalato and o-methoxy benzoato pentaamminecobalt(III) complexes

The kinetics of aquation of malonato, succinato, o-phthalato and o-methoxybenzoato pentaamminecobalt(III) complexes have been studied in aqueoys acidic media adjusted to 0·3 M ionic strength. At [H +] = 0·005 − 0·3 M and 55 − 70°C, the observed pseudofirst-order rate constants are described by the relationship: k obs = k 1 + k 2 [H +] where k 1 and k 2 stand for the rate constants of spontaneous and acid catalysed equation of (NH 3) 5CoCO 2(CH 2) n CO 2H 2+ ( n = 1,2), (NH 3) 5CoCO 2C 6H 4CO 2H 2+ and (NH 3) 5CoCO 2C 6H 4OCH 3 2+ species respectively. The rate and the activation parameters for the k 1 and k 2 paths have been determined. The results indicate that the unbound carboxyl group does not exert a specific catalytic effect on the spontaneous and acid catalysed paths of aquation of the cobalt(III) substrates derived from the dicarboxylic acids. CoO bond fission appears to be rate limiting in both k 1 and k 2 paths of all complexes. The possibility of CO bond breaking, however, can not be ruled out in the k 2 path of the o-phthalato complex.

DECOMPOSITION OF SODIUM HYPOCHLORITE: THE UNCATALYZED REACTION

The decomposition of sodium hypochlorite has been re-examined. The results show that Foerster and Dolch’s mechanism of the decomposition to chlorate and chloride is correct; they postulated a slow bimolecular reaction to chlorite and chloride, followed by a faster reaction of chlorite with more hypochlorite. Values of the rate constants of both steps are reported; they make the activation energies 24.8 kcal./gm-molecule for the first step and 20.8 kcal./gm-molecule for the second. The rates are such that at 40 °C. a solution of sodium hypochlorite will contain about 1% as much chlorite as hypochlorite. The rate is strongly affected by changing ionic strength; at low ionic strengths it is nearly constant or falls slightly; above about 0.8, the rate rises and at high ionic strengths the rise is quite rapid. No signs of specific catalytic effects of sodium chloride, hydroxide, or carbonate could be observed, and it seems probable that earlier reports of this were due to variations in ionic strength. The decomposition to chloride and oxygen has been measured and is a unimolecular reaction, which is possibly, but not certainly, uncatalyzed. Values of its rate constant are reported; they also are much altered by changing the ionic strength.

抗菌性ピロン誘導体に関する研究 (第1報)

Reëxamination was carried out on the synthesis of dehydroacetic acid by the liquid condensation of ethyl acetoacetate, and it was found that silver oxide, barium oxide, and magnesium oxide catalyst also gave a good yield besides the usual catalyst of sodium bicarbonate. From the pyrolytic decomposition of dehydroacetic acid, especially in the case of using sodium bicarbonate, it was also observed that the dehydroacetic acid formed underwent pyrolytic decomposition at a reaction temperature above 190°. When the reaction temperature was kept below 190°, the theoretical yield against the consumed ester was increased although there was an increase in the recovery of the ester. This tendency was also observed in the case of magnesium oxide. Use of ion exchange resin, such as Diaion K (sodium form), Diaion A (hydroxyl form), and Amberlite IR-120 (sodium form), as a catalyst resulted in milder reactions and theoretical yield reached approximately 50%. Other catalysts give a large amount of tarry residue so that the ion exchange resins may possibly have specific catalytic effect.