M. Hegedüs

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.

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.

Reactions of methylcyclopentane on Pt/Al2O3 and Sn-Pt/Al2O3 catalysts

Abstract Reaction of methylcyclopentane has been studied on different Sn-Pt/Al2O3 catalysts at 520 °C. Catalysts were prepared by using controlled surface reactions creating either tin-platinum or tin-alumina interactions exclusively. Based on the analysis of the yield-conversion dependences it was established that in addition to the consecutive routes for the formation of methylcyclopentadiene and benzene, direct routes should also exist. Formation of benzene can be strongly hindered by simultaneous introduction of metallic tin into platinum and ionic tin into alumina.

Enantio-differentiation over heterogeneous catalysts. The shielding effect model

This paper deals with the origin of enantio-differentiation over heterogeneous catalysts. A new model is proposed, in which the modifier provides a specific shielding effect. A prochiral molecule, due to the specific character of shielding, can adsorb onto the metal surface by its unshielded site resulting in enantio-differentiation. As emerges from computer modeling, quantum chemical and quantum mechanical calculations made on the catalytic system: cinchonidine—α-keto esters—Pt the shielding effect can be responsible both for the rate acceleration and induction of enantio-differentiation. This model is based on an earlier proposition, which suggest (i) the formation of a weak complex between the modifier and the substrate in the liquid phase and (ii) the hydrogenation of either the shielded or unshielded forms of α-keto ester over the Pt sites. The shielded form gives the optically active, while the unshielded one the racemic product. Further support for this model was obtained in kinetic experiments and kinetic modeling.

Role of rhenium in bimetallic reforming catalysts

The influence of rhenium on the behavior of a Pt/Al2O3 catalyst was studied in n-hexane reaction by means of dilution of the catalyst bed with Re/Al2O3. Parallel to activity, selectivity and ageing data obtained in slug pulse and continuous flow reactors, the amount of the surface carbon formed during the reaction was determined and its reactivity in hydrogen atmosphere was also studied. It is suggested that in the presence of rhenium due to the increase in the amount of hydrogen available for hydrogen consuming reactions including hydrodepolymerization of the carbonaceous surface overlayer, the routes of ageing processes are changed, resulting in lower carbon content on the catalyst and less pronounced catalyst deactivation.AbstractВлияние рения на поведение катализатора Pt/Al2O3 было исследовано в реакции н-гексана, с помощью разбавления катализаторной подложки Re/Al2O3. Параллельно с данными активности, селективности и старения в импульсном реакторе и в реакторе с проточной струей, было определено количество поверхностного углерода, образующегося в реакции, атакже его реактивность в атмосфере водорода. Полагается, гто в присутствии рения, вследствие увеличения количества водорода, доступного для реакций, потредляющих водород, включая реакцию гидродеполимерцзации углистой поверхности верхнего слоя, изменяются пути процессов старения, приводящих к пониженному содерзанию углерода на катализаторе и менее выраженной дезактивации катализатора.

New approaches to prepare supported Sn-Pt bimetallic catalysts

General principles of modification of supported Pt catalysts with tin tetraalkyls are summarized and a new approach to prepare supported Sn−Pt catalysts is described. The new approach leads to the formation of Multilayered Organometallic Complexes (MLOC) anchored onto supported platinum. The formed MLOC can be decomposed either in reductive or oxidative atmosphere with the formation of new type of supported Sn−Pt catalysts with high Sn/Pts ratios. The decomposition in a reductive atmosphere leads to the formation of alloy-type bimetallic phases, while the decomposition in the presence of oxygen provides Lewis-acid type surface species anchored to supported platinum.

Characterization of Trimetallic Pt-Pd-Au/CeO2 Catalysts Combinatorial Designed for Methane Total Oxidation

In the present work, the role and the effect of platinum and gold on the catalytic performance of ceria supported tri-metallic Pt-Pd-Au catalysts have been studied. The optimum composition of these tri-metallic supported catalysts has been discovered using methods and tools of combinatorial catalyst library design. Detailed catalytic, spectroscopic and physico-chemical characterization of catalysts in the vicinity of the optimum in the given compositional space has been performed. The temperature-programmed oxidation of methane revealed that the addition of Pt and Au to Pd/CeO2 catalyst resulted in higher conversion values in the whole investigated temperature range compared to the monometallic Pd catalyst. The time-on-stream experiments provided further evidence for the high-stability of tri-metallic catalysts compared to the monometallic one. Kinetic studies revealed the stronger adsorption of methane on Pt-Pd/CeO2 catalysts than over Pd/CeO2. XPS analysis showed that Pt and Au stabilize Pd in a more reduced form even under condition of methane oxidation. FTIR spectroscopy of adsorbed CO and hydrogen TPD measurements provided indirect evidences for alloying of Pt and Au with Pd. CO chemisorption data indicated that tri-metallic catalysts have increased accessible metallic surface area. It is suggested that advantageous catalytic properties of tri-metallic Pt-Au-Pd/CeO2 catalysts compared to the monometallic one can be attributed to (i) suppression of the formation of ionic forms of Pd(II), (ii) reaching an optimum ratio between Pd0 and PdO species, and (iii) stabilization of Pd in high dispersion. The results also indicate that Pd0 - PdO ensemble sites are required for methane activation.

Destruction of chlorobenzenes by catalytic oxidation over transition metal containing ZSM-5 and Y(FAU) zeolites

A potential method for oxidative destruction of environmentally harmful chlorobenzenes has been studied in a fixed-bed laboratory reactor using several tran- sition metal containing ZSM-5 and Y(FAU) zeolite catalysts prepared by conventional (denoted L) and solid state (denoted S) ion-exchange procedures. The catalysts used for 10 hours in chlorobenzene (or dichlorobenzenes) oxidation have been destroyed in different degrees. The ZSM-5 structure proved to be more stable than that of Y(FAU). Considering the conversion and destruction efficiency of chlorobenzene oxidation at 450 °C as test reaction, activities of the ZSM-5 samples can be listed as follows: Na < CoL < CoS < MnS < MnL < FeL < FeS < CuS < H < CrL < CrS. In the side reactions polychlorinated derivatives (i.e. dioxines and PCB-s as undesired environmental pollution compounds) may be formed by the participation of elemental chlorine produced by the Deacon process. The activity series for the “coupling” reactions is as follows: CrS, CrL < H < FeL, FeS < CoS, CoL < MnS < MnL < CuS. Using CrZSM-5(S) catalyst at 550–600 °C with relatively high residence time and stoichiometric air, the conversion of chlorobenzene exceeds 90% with a decomposition efficiency (formation of HC1 and Cl2 from organic chlorine) of about 85–90%.

Investigation of isomerization of n-pentane over supported Pt catalyst modified by adsorbed Bi and Pd

Abstractγ-Al2O3-supported Bi−Pt and Pd−Pt bimetallic catalysts were prepared by electrochemical metal adsorption. Isomerization of n-pentane was studied over these catalysts in a pulse reactor. It can be concluded from the results that the properties of these catalysts differ substantially from the base Pt/γ-Al2O3 catalyst. On the modified catalysts conversion and the yield of i-pentane was found to be lower although selectivity was better. The ratio of cracking reactions was also lower than over the base catalyst.It is very probable that the adsorbed bismuth was slowly desorbing from the catalyst surface during the experiments.AbstractС помощью электрохимической адсорбции металлов были приготовлены биметаллические катализаторы Bi−Pt и Pd−Pt на подложие γ-Al2O3. На этих катализаторах изучалась изомеризация н-пентана, реакция проводилась в импульсном реакторе. Полученные результаты говорят о том, что свойства таких катализаторов существенно отличаются от свойств катализаторов Pt/γAl2O3. Как было найдено, конверсия и выход н-пентана ниже, хотя селективность лучше. Доля реакций крекинга также ниже, чем на других катализаторах. Весьма вероятно, что адсорбированный висмут медленио десорбируется с поверхности по мере протекнния реакции.

Hydrogenation of benzonitrile on Sn-Pt/SiO2 Catalysts prepared by introducing SnEt4 to Pt/SiO2: Role of TIN

Liquid phase hydrogenation of benzonitrile was studied over Sn-Pt/SiO2 catalysts prepared by introducing tetraethyl tin onto the 3 wt.% Pt/SiO2 catalyst. Tin content of the catalysts ranged from 0.05 to 0.63 wt.%, whereas Sn/Pt surface atomic ratios determined by chemisorption measurements were between 0.1 to 3.5. Dibenzylamine selectivity influenced to a small extent by the level of conversion and the Sn/Pt ratio wasca. 75 %. The addition of tin to Pt in the range of (Sn/Pt)surface = 0.50–1.25 led to an increase in the turnover frequency (TOF) by a factor of 2. TOF showed a maximum at a surface atomic ratio of Sn/Pt = 1. The enhancement of catalyst activity upon the addition of tin is explained by the formation of Snδ+-Pt ensemble sites on the surface of bimetallic nanoclusters. It is suggested that highly dispersed positively charged tin species, by polarizing the triple bond, enhance the reactivity of the -CN group. Calcination at 300°C followed by re-reduction of the catalysts resulted in a monotonic decrease of specific activity with increasing Sn/Pt ratio.