Catalysis in the nm‐regime: manufacturing of supported model catalysts and theoretical studies of the reaction kinetics

We briefly review the methods employed to fabricate model supported nm catalysts, including wetness impregnation, vacuum vapor deposition, electron‐beam lithography, spin‐coating, and vesicle‐mediated deposition. Recent simulations of the kinetics of heterogeneous reactions occurring on supported catalyst particles are discussed as well. The attention is focused on such effects as reactant supply via the support, interplay of the reaction kinetics on different facets and edges, and adsorbate‐induced reshaping of catalyst particles. This revised version was published online in June 2006 with corrections to the Cover Date.     

Modifier–substrate interactions of various types in the Orito reaction: Reversal of the enantioselection in the hydrogenation of ketopantolactone on Pt modified by β-isocinchonine and O-phenylcinchonidine

The enantioselective hydrogenation of ketopantolactone (KPL) on Pt–alumina catalyst modified by β-isocinchonine (β-ICN) and O-phenylcinchonidine (PhOCD) in toluene, acetic acid and their mixtures under otherwise identical experimental conditions was studied. Reversal of the enantioselection was obtained dependent on the concentration of acetic acid (ee_max = 17% (S) on Pt–PhOCD and 50% (R) on Pt–β-ICN, respectively). The possible role in enantioselection of adducts forming in the reaction mixture and the stability of PhOCD under the conditions of the hydrogenation was investigated by ESI-MS. The results of the nonlinear phenomenon measurements on β-ICN + PhOCD mixtures suggest that the intermediate surface complexes β-ICN–KPL and PhOCD–KPL responsible for the opposite enantioselection include different types of interactions and the enantioselection is directed by the competition between these interactions.     

Zeolite effect in the enantioselective transhydrogenation over a Co‐salen “ship‐in‐the‐bottle” complex

Our investigations were focused on the transhydrogenation of carbonyl groups using Co(II)‐salen complexes entrapped in zeolites. The Co‐salen 1 complex was occluded in the zeolite as a “ship‐in‐the‐bottle” (SIB) catalyst, as could be demonstrated by means of DRIFT‐IR spectra, DTG, nitrogen adsorption isothermes and soxhlet extraction. The results of the test reactions show that an immobilization of the Co‐salen 1 complex in the zeolite as a “SIB” complex results in an enantiomeric excess as high as in the homogeneous case. The Co‐salen 1 USY SIB catalyst shows its best ee‐value at higher temperature (25°C) than the homogeneous one (-10°C). The reason could be the steric constraints of the Co‐salen complex in the zeolite framework. This revised version was published online in July 2006 with corrections to the Cover Date.     

Preparation and catalytic activity in ethanol combustion reaction of cobalt–iron spinel catalysts

Iron–cobalt spinel catalysts were prepared via the coprecipitation method. The effect of different parameters on textural, structural and catalytic properties, in ethanol combustion, was investigated. The CoFe₂O₄ phase was obtained at calcination temperatures as low as 500 °C and the usage of ammonia as precipitating agent, results in the formation of Fe₂O₃ in addition to the spinel phase. The catalyst prepared using nitrate salts, NaOH as precipitation agent and calcined at 600 °C had the best catalytic performance achieving ethanol complete oxidation at 271 °C.     

Heterogeneous enantioselective catalysts: can molecular simulation techniques aid the design of improved catalysts?

The purpose of this paper is to review the use of molecular simulation techniques in the study of enantioselective heterogeneous catalysts. In particular we focus our attention on the use of molecular simulation in two well studied systems; (a) enantioselective hydrogenation of ά-ketoesters using modified metal catalysts and (b) the enantioselective dehydration of alcohols using modified zeolites. This revised version was published online in June 2006 with corrections to the Cover Date.     

Acid function of Al‐MCM‐41 supported platinum catalysts in hydrogenation of benzene, toluene o‐xylene

A series of Al‐MCM‐41 mesoporous materials with different Si/Al ratios are synthesized. The acidities of catalysts are measured by the temperature‐programmed desorption of ammonia (NH₃‐TPD) and IR spectra of pyridine. Their catalytic activities for hydrogenation of benzene, toluene and o‐xylene are investigated on a pulse reactor system. NH₃‐TPD and IR results show that only weak and medium acid sites could be observed on the catalysts, and the number of total acid sites decreases obviously with the increase of the Si/Al ratio whereas the medium acid sites are somewhat constant. The introduction of platinum onto Al‐MCM‐41 material decreases the total acid number by a small amount. The hydrogenation activities of the 1% Pt/HAl‐MCM‐41 catalysts are found to correspond well with the ratio of medium acid sites to total acid sites in the employed catalysts. It is proposed that, in addition to metal sites, the acid sites in the metal–acid interfacial regions are also the active sites for hydrogenation and the medium acid sites play important roles in the reaction. This revised version was published online in July 2006 with corrections to the Cover Date.     

A novel catalytic method for the alkylation of benzene to diphenylmethane over H‐ZSM‐5 zeolite catalysts

Vapour‐phase catalytic alkylation of benzene to diphenylmethane in high selectivity (92.5%) at 30.7 wt% conversion of benzene is reported for the first time using dichloromethane (DCM) as an alkylating agent and H‐ZSM‐5 as the catalyst. This revised version was published online in July 2006 with corrections to the Cover Date.     

A preliminary study of the n‐heptane conversion over supported platinum acid–base ZrPON catalysts

The catalytic behaviour of zirconium phosphate oxynitrides ZrPON and Pt supported ZrPON catalysts in n‐heptane conversion is examined. The results show that the distribution of the products depends on the reaction temperature. In addition, it has been established that the acid–base properties of ZrPON supports greatly influence the n‐heptane reforming reactions since the increase of ZrPON basicity enhances both n‐heptane conversion and aromatics selectivity. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comparison of the catalytic properties of Al‐ZSM‐22 and Fe‐ZSM‐22 in the skeletal isomerization of 1‐butene

The catalytic activities of Al‐ZSM‐22 (Al in the framework) and Fe‐ZSM‐22 (Fe in the framework) were compared in the skeletal isomerization of 1‐butene to isobutene. The catalysts synthesized in the laboratory were characterized by means of XRD, FTIR spectroscopy, SEM and surface area measurements. The activity of the zeolites was investigated using a fixed‐bed microreactor system. Al‐ZSM‐22 demonstrated higher activity in 1‐butene transformation compared to Fe‐ZSM‐22, while the selectivity to isobutene, on the other hand, was higher over Fe‐ZSM‐22. Coke formation was monitored using a microbalance and the results showed that the weight gain of Fe‐ZSM‐22 was slightly higher compared to Al‐ZSM‐22. This revised version was published online in July 2006 with corrections to the Cover Date.     

Enantioselective hydrogenation of activated ketones in the presence of Pt–cinchona catalysts. Is the proton transfer concept valid?

Experimental evidences related to the proton transfer in the catalytic system Pt–cinchona alkaloids for enantioselective hydrogenation of activated ketones were collected and analyzed. Both new and earlier results indicate that in aprotic media direct transfer of proton from platinum to the substrate with the involvement of quinuclidine nitrogen as a general rule can be questioned.     

Effects of the structure of CeCu catalysts on the catalytic combustion of toluene in air

CeCu composite oxide catalysts were prepared by a hard-template method (CeCu-HT) and a complex method (CeCu-CA). The prepared CeCu composite oxide catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and Brunauer–Emmett–Teller (BET) analyses. The catalytic properties of the prepared CeCu composite oxide catalysts were also investigated by the catalytic combustion of toluene in air. XRD results showed that the synthesized CeCu composite oxide catalysts had different phase components and crystallinities but similar CeO₂CuO solid solution phases. Low-angle XRD, TEM, and BET results indicated that the prepared CeCu-HT catalyst had a developed ordered mesoporous structure and a large specific surface area of 206.1 m² g^?1. Toluene catalytic combustion results indicated that the CeCu-HT catalyst had higher toluene catalytic combustion activity in air than the CeCu-CA catalyst. The minimum reaction temperature at which toluene conversion exceeded 90% for toluene catalytic combustion on the CeCu-HT catalyst was 225 °C. The toluene catalytic combustion conversion on the CeCu-HT catalyst at 240 °C exceeded 99.3% with decreased toluene concentration in air to below 70 ppm. On the other hand, the toluene catalytic combustion conversion on the CeCu-CA catalyst was only 92% even when the reaction temperature reached 280 °C. The differences between the toluene catalytic combustion performances of the CeCu composite oxide catalysts prepared by different methods can be attributed to their discrepant compositions and structures.     

Performance of metal‐oxide‐promoted LiCl/sulfated‐zirconia catalysts in the ethane oxidative dehydrogenation into ethene

The effects of some transition‐ and lanthanide‐metal oxides in LiCl/sulfated‐zirconia (SZ) catalysts on catalytic behavior in the oxidative dehydrogenation of ethane were investigated. It is found that modification of LiCl/SZ by metal oxides significantly improves the catalytic activity and ethene yield. Among those additives, Ni and Nd oxides show the best promoting effect in terms of ethane conversion and ethene yield. 93% ethane conversion with 83% selectivity to ethene has been achieved over the Nd₂O₃–LiCl/SZ catalyst at 650°C. In addition, those oxide‐promoted LiCl/SZ catalysts are also found to exhibit a longer stability in catalytic performance. Metal‐oxide additives change the chemical structure and surface redox properties, which accounts for the enhancement of activity. This revised version was published online in July 2006 with corrections to the Cover Date.     

Pt/CeO2 catalysts in selective hydrogenation of crotonaldehyde: high performance of chlorine‐free catalysts

The hydrogenation of crotonaldehyde was conducted in gaseous phase, at atmospheric pressure, on Pt/CeO₂ catalysts prepared from metal precursors containing or not chlorine. The activities and selectivities were studied, at 253 K, as a function of the reduction temperature of the catalyst (473–993 K). The Pt/CeO₂ catalyst, prepared from tetraammineplatinum nitrate, led to 5–20% crotyl alcohol selectivity when the catalyst was reduced at low temperature (473–673 K), while increasing the reduction temperature up to 973 K, the crotyl alcohol selectivity reached more than 80%. Repeating a series of experiments after a re‐calcination treatment at 673 K, the selectivity decreased to only 40% after 473 K reduction to reach again more than 80% after 673 K reduction temperature. A phase transformation of Pt to CePt₅ was observed by XRD analysis after 973 K reduction treatment. Differently on Pt/CeO₂ catalysts containing chlorine, prepared from either chloroplatinic acid or tetraammineplatinum chloride, the crotyl alcohol selectivity never exceeded 30% and did not form alloy up to 973 K reduction temperature. The main results are interpreted considering that the activity of CePt₅ for C=C hydrogenation is low compared to unmodified platinum catalyst and the activation of the carbonyl bond is induced by the presence of oxygen vacancies sites located at the interface between ceria and the metallic particles. The results are in good accordance with the information known at the present time on the metal–support interactions in Pt deposited on CeO₂. This revised version was published online in August 2006 with corrections to the Cover Date.     

Reaction pathways for the oxygenates formation from propane and oxygen over potassium‐modified Fe/SiO2 catalysts

The partial oxidation of propane has been compared on Fe/SiO₂ and alkali‐modified Fe/SiO₂ catalysts. Addition of K⁺ to the catalyst can appreciably enhance the conversion of propane along with a high selectivity toward oxygenates. Adjacency of Fe and K on the silica surface seems to be important for the high oxygenate yield. Through the study on the reaction pathways, two types of intermediates, one derived from propene and the other from propan‐2‐ol, were postulated. The former is related to the acrolein formation and the latter to the acetone formation. Contribution of alkali‐catalyzed aldol condensation is also discussed for the formation of oxygenates with higher carbon numbers. This revised version was published online in July 2006 with corrections to the Cover Date.     

Triply‐promoted ethene epoxidation: NOx promotion of the Ag‐catalysed reaction in the presence of alkali and chlorine under electrochemical control

Potassium, electrochemically supplied from K β"‐alumina to a silver thin film catalyst in the presence of ppm levels of NO_x, strongly promotes the selectivity of ethene epoxidation. However, in the absence of gaseous NO_x, alkali catastrophically suppresses both activity and selectivity. Addition of surface chlorine via ppm levels of ethylene dichloride further enhances the promotional effect of alkali + NO yielding the highest selectivity of all. The minimum necessary and sufficient conditions for the appearance of NO_x promotion are submonolayer quantities of alkali on the metal surface, and ppm levels of gaseous NO_x. This revised version was published online in August 2006 with corrections to the Cover Date.     

SnO2‐supported palladium catalysts: activity in deNOx at low temperature

High surface area tin dioxide (174 m²/g) has been synthetised and characterised by XRD, TEM and UV‐visible DRS. DRS gives evidence for the formation of oxygen vacancies (donor levels) under reducing conditions. CO adsorption gives rise to terminal carbonyl species linked to Sn⁴⁺ and Sn²⁺. Palladium–tin oxide catalysts have been prepared from various precursors (Pd(acac)₂ and Pd(NO₃)₂) and by different preparation methods (grafting, photodeposition); they are active in deNOx reactions at low temperature (180 °C) in the presence of stoichiometric CO–NO–O₂ mixtures. A mechanism involving palladium and oxygen vacancies is proposed. This revised version was published online in July 2006 with corrections to the Cover Date.     

High activity and stability of the Rh‐free Co‐based ex‐hydrotalcite containing Pd in the catalytic decomposition of N2O

The catalytic decomposition of nitrous oxide to nitrogen and oxygen has been studied over calcined hydrotalcite‐like compounds containing different combinations of bivalent (Co, Pd, Mg) and trivalent (Al, La, Rh) cations with carbonate as interlayer anion. The precursors were prepared by co‐precipitation under low supersaturation conditions and characterized by XRD and TG/DSC. The mixed oxides derived after calcination at 723 K were characterized by XRD, N₂ adsorption at 77 K, and XRF. The presence of Rh, La, or Pd in the Co‐based HTlc's improves considerably the catalytic activity. Co–Rh,Al‐HTlc (Co/Rh/Al==3/0.02/1) proved to be a very active catalyst, although the presence of the noble metal Pd in this catalyst ex‐Co,Pd–La,Al‐HT (Co/Pd/La/Al=3/1/1/1) produces a similar catalytic activity to that of Rh‐containing catalyst, both in a N₂O‐containing stream and in one containing also SO₂ and O₂, but with a better performance in stability tests. PdO phase has been identified by XRD as being responsible for the considerable improvement in the activity. The presence of Mg as spinel structure exerts a stabilizing effect in the more active catalysts when mixtures of SO₂ and O₂ are considered. This revised version was published online in July 2006 with corrections to the Cover Date.     

High catalytic activity in CO PROX reaction of low cobalt-oxide loading catalysts supported on nano-particulate CeO2–ZrO2 oxides

Co₃O₄/NP-ZrO₂, Co₃O₄/NP-CeO₂ and Co₃O₄/NP-Ce_0.8Zr_0.2O₂ catalysts were prepared via a reverse microemulsion/incipient wetness impregnation (RM–IWI) method. The catalytic properties for CO preferential oxidation (CO PROX) reaction in H₂-rich stream were investigated. The Co₃O₄/NP-Ce_0.8Zr_0.2O₂ catalyst with 1.8 wt.% Co₃O₄ loading has exhibited higher catalytic activity than that of the other two catalysts. The higher catalytic activity might be attributed to the combination effect of the highly dispersed cobalt oxide, the improvement in CeO₂ reducibility due to ZrO₂ incorporation in CeO₂ structures, and the strong cobalt oxide-support interaction.     

On the formation of cobalt–molybdenum sulfides in silica‐supported hydrotreating model catalysts

Model catalysts, consisting of a conducting substrate with a thin SiO₂ layer on top of which the active catalytic phase is deposited by spincoating impregnation, were applied to study the formation of the active CoMoS phase in HDS catalysts. The catalysts thus prepared showed representative activity in the hydrodesulfurization of thiophene, confirming that these models of HDS catalysts are realistic. Combination of the sulfidation behaviour of Co and Mo studied by XPS and activity measurements shows that the key in the formation of the CoMoS phase is the retardation of the sulfidation of Co. Complexing Co to nitrilotriacetic acid complexes retarded the Co sulfidation, resulting in the most active catalyst. Due to the retardation of Co in these catalysts, the sulfidation of Mo precedes that of Co, thereby creating the ideal conditions for CoMoS formation. In the CoMo catalyst without NTA the sulfidation of Co is also retarded due to a Co–Mo interaction. However, the sulfidation of Mo still lags behind that of Co, resulting in less active phase and a lower activity in thiophene HDS. This revised version was published online in June 2006 with corrections to the Cover Date.     

Cobalt Schiff base complexes: Synthesis characterization and catalytic application in Suzuki–Miyaura reaction

3-Methoxysalicylaldehyde was condensed with the amines 4-aminoacetophenone and 2-amino-5-bromopyridine to obtain Schiff base ligands, 1 and 2, which were coordinated to cobalt salts as complex 1 and complex 2, respectively. The synthesized ligands and complexes were characterized by spectroscopic (FT-IR, UV-Vis, ¹H-NMR and mass spectrometry), thermal (TGA) and elemental analysis. The structures of the complexes were verified by evaluating their magnetic susceptibility and spectroscopic evidences. Synthesized complexes were studied for their catalytic activity in the Suzuki-Miyaura cross-coupling of aryl halides with phenylboronic acid. Optimized reaction yields 90% of the cyanobiphenyl for complex 1 and 91% for complex 2 with 0.1 mmol of catalyst loading thereby substantiating the C-C coupling efficiency of the synthesized complexes, 1 and 2.