József L. Margitfalvi

Reactions of acetylene during hydrogenation on Pd black catalyst

Deuteration of trace amounts of acetylene in the presence and absence of ethylene has been studied on Pd black catalyst in a continuous flow reactor. At nonsteady state the Pd black catalysts showed very low selectivities, whereas higher selectivities were obtained at steady state. Pretreatment with air and H2 yielded a catalyst with higher selectivity than the same catalyst treated only with H2. The order in acetylene pressure passed from positive to negative with increasing pC2H2 both in the presence and absence of ethylene. The deuterium distribution of the ethylene formed showed that hydrogen originating from acetylene also took part in the hydrogenation reaction; thus, part of the acetylene dissociatively adsorbed on the surface. Using [14C]C2H2 it was demonstrated that at low acetylene partial pressure the main route of acetylene hydrogenation was the formation of ethane, ethylene, and C4 hydrocarbons. Using [14C]C2H4 it was also shown that at certain level of acetylene partial pressure the formation of ethane from ethylene was completely terminated. The observed kinetic data are discussed and it is suggested that different surface species such as dissociatively and associatively adsorbed acetylene as well as ethylidyne species are present on the palladium surface. Experimental conditions, such as catalyst pretreatment and partial pressure of the reactants, can influence the relative concentrations of these surface species and can also change the routes of surface reactions, leading to different reaction products from acetylene. The reaction mechanism proposed is discussed based on different organometallic and spectroscopic evidence published recently.

Mössbauer spectroscopy studies of Sn-Pt/Al2O3 catalysts prepared by controlled surface reactions

Abstract Mossbauer spectroscopy was used to study the chemical state of tin in a new type of Sn-Pt/Al2O3 catalysts. Controlled surface reaction of tin tetraethyl with hydrogen preadsorbed on platinum resulted in the exclusive formation of a Pt-SnEt3 surface complex, which decomposed in hydrogen at 773 K to PtxSn alloy (3

Formation of multilayered tin organometallic surface species. Preparation of new type of supported Sn–Pt catalysts

Abstract In this study, a new aspect of anchoring of tin organic moieties onto platinum is described and discussed. The new approach resulted in substantial increase of the Sn/Pt ratio in tin modified Pt/SiO2 catalysts. In the earlier approach, adsorbed hydrogen was exclusively used for tin anchoring, resulting in a monolayer of tin organic moieties at the top of platinum. In the new approach, a large excess of either tin tetraethyl or hydrogen was used in the tin-anchoring reaction. The presence of coadsorbents, such as oxygen, also led to a substantial increase of the amount of tin anchored. When the primary formed –SnR3 surface entities were partially decomposed, the coordinatively formed unsaturated –SnR2 and –SnR surface species provided additional anchoring sites for the next layer of SnR4. In the presence of adsorbed oxygen, additional new types of landing sites were created to anchor SnR4 in the neighborhood of platinum. The above approach resulted in Sn–Pt/SiO2 catalysts with exclusive tin–platinum interaction and an Sn/Pt ratio ca. 2. Results obtained in this study also reveal that the formation of surface organometallic moieties takes place in a stepwise way, e.g. the buildup of tin organic moieties occurs layer-by-layer. The supported Sn–Pt bimetallic entities formed showed both high activity and selectivity in the hydrogenation of crotonaldehyde to crotylalcohol.

Enantioselective hydrogenation of α-keto esters over cinchona-PtAl2O3 catalyst. Kinetic evidence for the substrate-modifier interaction in the liquid phase

Abstract The hydrogenation of ethyl pyruvate was studied over cinchonidine- Pt Al 2 O 3 catalyst. Contrary to earlier results it has been found that the initial enantiomeric excess extrapolated to zero conversion is close to zero. Based on kinetic analysis the results are considered as indirect evidence for the substrate-modifier interaction taking place in the liquid phase. The above interaction leads to the formation of a weak substrate-modifier complex. The formation of the complex in the liquid phase is the key step to control both the rate acceleration and the induction of enantio-differentiation in the hydrogenation of α-keto esters in the presence of cinchona- Pt Al 2 O 3 catalysts. The character of interactions in the substrate-modifier complex is discussed. By using molecular modelling the possible form of the complex is also given.

The role of cinchona alkaloids in enantioselective hydrogenation reactions: Are they modifiers or hosts involved in supramolecular heterogeneous catalysis?

Abstract In this study experimental evidences are summarized supporting the modifier–substrate interaction taking place in the liquid phase in the enantioselective hydrogenation of α-keto esters and related compounds. The results indicate that the catalytic system cinchona alkaloids-supported platinum (or palladium) can effectively be used in enantioselective hydrogenation for prochiral substrates, in which the prochiral group is part of a conjugated double bond system. It is considered that the above catalytic system is the first example of a new class of heterogeneous catalytic reactions with the involvement of supramolecular catalysis.

Preparation of new type of Sn-Pt/SiO2 catalysts for carbonyl activation

Abstract The paper is aimed to demonstrate how the activity and selectivity of a Pt/SiO2 catalyst can be tailored by introduction of tin either to hydrogenate the aldehyde group in α,β-unsaturated aldehydes (UA) or oxidize carbon monoxide at room temperature. In this paper general aspects of catalyst design based on the use of tin tetraalkyls are described in detail. Results summarized in this work show that upon using new approaches for anchoring tin into platinum high Snanch/Pts can be achieved. The experimental evidences indicate also that the hydrogenation of the aldehyde group in α,β-unsaturated aldehydes and oxidation of CO requires in situ formed “Sn4+–Pt ensemble” sites to activate either the carbonyl group or the CO molecule.

Reaction kinetic approach to study activity, selectivity and deactivation of Pt/Al2O3 in n-hexane conversion

Abstract In this work factors influencing the activity and selectivity of the Pt/Al2O3 catalyst were investigated in n-hexane conversion. Based on steady state and transient reaction kinetic data it was concluded that the main factors controlling the selectivity of this catalyst in n-hexane conversion are: (i) the amount of substrate (n-hexane) chemisorbed, i.e. the substrate coverage and (ii) the mobility of all surface species including chemisorbed hydrogen. These two factors strongly influence both the extent of dehydrogenation and the geometry of the surface species originated from the hydrocarbon. It is suggested that on the Pt sites there are at least two types of surface intermediates involved in the formation of different reaction products. Surface carbon, responsible for the catalyst deactivation, appeared to be a selective poison, which strongly suppressed the formation of benzene.

Enantioselective hydrogenation of α-keto esters over cinchona-Pt/ Al2O3 catalysts: New interpretation of the rate acceleration and the induction of enantio-differentiation

Abstract The hydrogenation of ethyl pyruvate over the cinchonidine-Pt/Al2O3 catalyst system has been investigated in different solvents. Experimental variables used were as follows: (i) substrate and modifier concentration.,(ii) presence of acetic acid, (iii) presence of (R)-ethyl lactate, (iv) mode of introduction of the interacting components. A completely different kinetic pattern was observed in ethanol and toluene. This difference is attributed to the lack of half-ketal formation in toluene. The optical yield vs. conversion dependencies show a monotone increase at low conversion, i.e. the optical yield extrapolated to zero conversion is close to zero. It is suggested that the rate acceleration is due to the enhanced reactivity of the substrate induced by substrate-modifier interaction. It is proposed that the induction of enantio-differentiation is attributed to the following interactions: (i) conformational changes of the modifier induced by substrate or acetic acid, (ii) formation of a substrate-modifier complex, (iii) shielding effect induced by the quinoline ring of the modifier.

Preparation of alumina supported tin-platinum catalysts by reaction of tin tetraethyl with hydrogen adsorbed on platinum. Study of the formation and decomposition of the primary surface complex

In this work surface reactions involved in the preparation of alumina supported tin-platinum catalysts with metal-metal interaction were studied. The preparation of these catalysts is based on surface reaction between tin tetra-alkyls and hydrogen adsorbed on platinum. In the above reaction a Primary Surface Complex (PSC (I)) is formed, which has a twodimensional layer of -SnR3 moieties at the platinum surface. PSC (I) can be decomposed in hydrogen atmosphere with formation of bimetallic surface entities. The fate of the PSC (I) strongly depended on (i) the presence or absence of gas phase hydrogen, (ii) the dispersion of platinum, (iii) the time interval between its formation and decomposition. In addition to the main reactions involved in this preparation side reactions with partial dehydrogenation of the alkyl groups of the -SnR3 moieties were also evidenced. Upon using this approach to prepare tailor made supported bimetallic catalysts these side reactions should be avoided as they are responsible for the formation of surface carbon on the platinum sites.

Natural alkaloids and synthetic relatives as chiral templates of the Orito's reaction.

The enantioselective hydrogenation of methyl or ethyl pyruvate over cinchona-platinum catalyst system (Oritos reaction) is one of the most intensively studied heterogeneous catalytic asymmetric hydrogenation reactions. Studies aiming at systematic changes of the chiral template have played a crucial role in creating hypotheses for the mechanism of Oritos reaction. It is very important to clarify which structural unit of the alkaloid takes part in the enantiodifferentiation, and learn about the role of the different structural units of chiral templates. In this article, we made an attempt to describe the behavior of natural alkaloids, their synthetic derivatives, and analogues as chiral templates in the heterogeneous catalytic asymmetric hydrogenation of activated ketones.