Setrak K. Tanielyan

Effect of catalyst pretreatment on the oxidation of carbon monoxide over coprecipitated gold catalysts

Au/Co3O4, Au/Fe2O3 and Au/CuO and the corresponding silver containing catalysts were prepared by coprecipitation and subjected to various heat and oxygen pretreatments prior to being used to pro-mote carbon monoxide oxidations. The most active catalysts were those which were heated in oxygen before use. The relative activities of these catalysts depended on the degree of heating and oxygen exposure before use. Reduction of the gold containing precipitates with formaldehyde gave species with markedly diminished activities. The unreduced gold containing catalysts were more active than the silver species. These catalysts were also subjected to alternate pulses of O2, CO and O2 at varying temperatures to determine the type of interaction taking place on the catalyst surface. These data indicate that carbon monoxide oxidation takes place primarily by the interaction of carbon monoxide with adsorbed oxygen and to a lesser extent by the reverse.

Anchored homogeneous catalysts: the role of the heteropoly acid anchoring agent

Heteropoly acids (HPAs) are used to anchor catalytically active homogeneous complexes to supports by an interaction between the HPA and the metal atom of the complex. In an attempt to determine the nature of the HPA/metal bonding several substrates were hydrogenated using chiral and achiral complexes anchored to alumina by the commercially available Keggin HPAs, phosphotungstic acid (PTA), phosphomolybdic acid (PMA), silicotungstic acid (STA) and silicomolybdic acid (SMA). It was found that the nature of the HPA can have a significant influence on both the activity and selectivity of reactions run using these HPA anchored homogeneous catalysts. The results obtained indicate that there is a direct interaction between the HPA and the metal atom of the active organometallic complex. While these data do not unequivocally establish that there is a bond between the metal atom and an oxygen on the surface of the HPA, the extent to which the activity and selectivity change when different HPAs are used as anchors is more like what would be expected when one uses a series of ligands attached to the metal atom rather than changing the counterions associated with a charged organometallic species.

Enantioselective heterogeneous catalysis. 3 . Effect of oxygen on catalyst activity and selectivity in the enantioselective hydrogenation of pyruvates

Data are presented which are in agreement with previous reports concerning the enhanced activity and enantioselectivity of freshly re-reduced platinum catalysts when used for the chiral hydrogenation of α-ketoesters. Catalyst deactivation was observed on heating the platinum catalyst in either hydrogen or helium at 350°C for two hours and then using it without exposure to oxygen. The presence or absence of cinchonidine had no effect on the rates of pyruvate hydrogenations run using these catalysts. These data establish that the deactivation of the catalyst was not caused by a strong adsorption of the alkaloid on the platinum surface. The activity and enantioselectivity of these catalysts were increased on addition of air to the reaction mixture. The reported enhanced activity and enantioselectivity shown by freshly re-reduced catalysts depend, then, not only on the re-reduction step but also on a brief exposure of this catalyst to air. It appears from published experimental details that the oxygen present in the solvent used in the hydrogenation is probably sufficient for this purpose.

A new technique for anchoring homogeneous catalysts

Heteropoly acids have been found to serve as anchoring agents between a support material and the metal atom of a homogeneous catalytic complex.

Acetoxylation of toluene catalyzed by supported Pd-Sn catalysts

Abstract The toluene acetoxylation run in the presence of Pd(OAc)2, Sn(OH)2 and KOAc was found to proceed through three characteristic stages of the oxygen absorption. In the first stage, the system produces a Pd(II) [Sn(II)]2(OAc)6 complex which remains homogeneous until the end of that phase. In the second region of the oxygen uptake curve there is an absorption of one equivalent of oxygen after which a heterogeneous palladium-tin species precipitates on the surface of the silica carrier present in the system. In the third region the oxygen adsorption is related only to the oxidation of the toluene to benzyl acetate and benzylidene diacetate which are formed in a 6.7/1 ratio. The turnover frequencies determined on the basis of the reaction rates were 3–4 times higher than observed with the reference system which used Sn(OAc)2 as the tin precursor. An additional triple increase in the rate was observed for the same optimized system when the oxidation was begun in the presence of ICN 60 silica. The activity of these in-situ supported Pd/Sn species depends strongly on the porosity of the silica carriers. The optimal average pore diameter was found to be near 20–30 A. The samples with variable Sn/Pd ratios and with variable loading exhibit strongly suppressed hydrogen chemisorption and single turnover (STO) surface reactivity which is thought to result from an SMSI state of the metallic palladium caused by the co-adsorbed tin.

Pd/Sn catalyst for toluene acetoxylation

Silica supported PdSn catalysts were prepared by controlled surface precipitation of SnCl2 with subsequent redox—deposition of Pd over the tin hydroxide layer. The variables studied were the pH level attained after the tin hydroxide precipitation, the effect of the Pd:Sn ratio, the palladium loading and the temperature of the final He heat treatment. The samples were characterized by DRFI-IR, physisorption, hydrogen, oxygen and CO chemisorption, single turnover reaction procedure and acetoxylation activity measurements. The results of this study suggest that the initial precipitation of the tin hydroxide takes place without significant interaction between the partially hydrolyzed SnCl2 and the surface silanol groups. The final pH in this step controls the strength of attachment and the distribution of the tin hydroxide species on the silica surface and apparently the size of the immobilized palladium particles. The DRFT-IR spectra of the samples taken at various stages of preparation suggest that the PdSnx coverage is completely formed in the drying stage and subsequent sintering in He at 300°C only increases the number of the bridging SnOSi bonds. The optimized system requires a pH level of 4.2–4.7 in the tin precipitation step, a Pd:Sn ratio of 0.5, a Pd loading of 1–1.25% by weight and a helium sintering temperature of 250–270°C. The ex-situ prepared catalyst displayed higher acetoxylation activity than did the in-situ formed catalysts which were prepared from Sn(OH)2, Pd(OAc)2 and KOAc.

Transition Metal Free Catalytic Aerobic Oxidation of Methyl-α-d-Glucopyranoside Under Mild Conditions Using Stable Nitroxyl Free Radicals

The selective oxidation of alcohols to the corresponding aldehydes, ketones or acids is an essential transformation in organic synthesis and a large number of reagents have been developed for this reaction (Kirk-Othmer, Encyclopedia of chemical technology, 1992; Hudlicky, Oxidations in organic chemistry, 1990; Sheldon and Kochi, Metal-catalyzed oxidations of organic compounds, 1981). Many of these well established procedures require the use of a toxic stoichiometric oxidant, transition metal catalyst and/or halogenated solvents. In this respect, the TEMPO (2,2,6,6,-tetramethylpiperidine-N-oxyl) based compositions have emerged as highly selective catalyst systems for the oxidation of primary alcohols to the corresponding aldehydes or acids (Holum, J Org Chem 26:4814, 1961; Lee and Spitzer, J Org Chem 35:3589, 1970). Here, we report on a new catalyst composition for the aerobic oxidation of methyl-α-d-glucopyranoside (1) to the corresponding methyl-α-d-glucuronic acid (2). The catalyst system is based on 4-acetamido-2,2,6,6-tetramethylpiperidine N-oxyl (AA-TEMPO, 3b) and a nitrate source. It utilizes ecologically friendly solvents and does not require any transition metal co-catalyst. It has been shown that the described process represents an efficient oxidation protocol that can easily and safely be scaled up to commercial scale. The influence of the most critical reaction parameters such as the nature of the substituent on the TEMPO structure, the nitrate source and the temperature–pressure matrix have been studied. By a detailed analysis of the effect of each of the reaction variables on the oxidation rate, a reaction pathway including a multistep cascade is presented.

Correlation between the single turnover (STO) site densities and the rates of 4-tert-butyl-1-methylcyclohexene hydrogenation over silica supported palladium catalysts

Abstract A systematic study of a set of silica supported palladium catalysts was undertaken. The catalysts were prepared by impregnation. Some of the impregnated samples were reduced isothermally, others by thermoprogramed reduction (TPR). The percentage of the exposed metal was determined both by hydrogen and CO chemisorption with the values ranging from 6% to 39% over the series. The catalysts prepared by TPR had higher dispersions and their activities were less sensitive to the final reduction temperature used in their preparation. A set of these catalysts having different reaction site densities as determined by the single turnover (STO) characterization procedure were used to catalyze the hydrogenation of 1-methyl-4-tert-butylcyclohexene (1). The correlation between these rate data and the STO saturation site densities for each catalyst provided the values for single site reactivities. The hydrogenation of 1 on palladium was found to proceed with a higher trans/cis ratio than was observed using a platinum catalyst. This was apparently due to the formation of a π-allyl absorbed intermediate in the palladium catalyzed reaction. Substrate self-inhibition behavior and non-competitive reversible inhibition by phosphine additives on the surface reactivity and stereoselectivity were also observed.

Hydrogenation of Succinimide to 2-Pyrrolidone Over Solid Catalysts

Abstract2-Pyrrolidone (P-Done) is an important chemical which is widely used both as a solvent and as an intermediate in industrial syntheses. The preparation of P-Done by the catalytic hydrogenation of succinimide has attracted little attention. Previous attempts to carry out this reaction resulted in low reaction rates and poor selectivity towards the desired product. Here, we present the results of the selective catalytic hydrogenation of succinimide to P-Done in the presence of skeletal nickel catalysts in aqueous and non-aqueous solvents.

A new type of anchored homogeneous catalyst

Almost all previous attempts to design a “heterogenized homogeneous catalyst” involved first attaching the ligand to a solid support material and then preparing the heterogeneous catalyst using this solid ligand. In contrast, we have succeeded in attaching preformed homogeneous complexes to a solid support using a heteropoly acid as the anchoring agent. In this way, the heteropoly acid is first attached to the support material and then interacts with a homogeneous catalyst through the metal atom of the complex. We have now used such anchored rhodium and ruthenium complexes for a number of chiral and achiral hydrogenations. We have established that the amount of metal lost in these reactions is less than 1 ppm. Details of these reactions will be presented along with the data used to determine the nature of the interaction between the complex and the heteropoly acid.