Metal-containing Catalysts Research Articles

P-Sulfonic acid calixarenes as efficient and reusable organocatalysts for the synthesis of 3,4-dihydropyrimidin-2(1H)-ones/-thiones

A new and efficient methodology is proposed for obtaining 3,4-dihydropyrimidin-2(1H)-ones/-thiones through Biginelli reactions. It is based on the use of less than the stoichiometric amount of p-sulfonic acid calixarenes as organocatalysts. A number of aromatic aldehydes as well as urea or thiourea can be employed for successfully synthesizing the corresponding Biginelli adducts. The described methodology is devoid of metal-containing catalysts, which in turn is very attractive for safely producing 3,4-dihydropyrimidin-2-(1H)-ones/-thiones of pharmacological interest. In addition, the catalyst efficiency is not compromised after its successive use in reactions. This is the first report about the application of calixarenes as catalysts in the multicomponent Biginelli reaction.

NOx storage and reduction on a SrTiCuO3 perovskite catalyst studied by operando DRIFTS

The behaviour of a perovskite SrTiCuO3 as NSR catalyst has been studied by simultaneous DRIFTS and activity measurement. The amount of NOx stored on the perovskite SrTiCuO3 (41μmol/m2 at 300°C) is much higher, when compared on a surface area basis, than on noble metal-containing reference catalysts (0.3–23μmol/m2). The perovskite SrTiCuO3 is able to perform both the NO oxidation to NO2 and the NOx storage, and copper enhances both the NO oxidation and NOx chemisorption capacities of copper-free SrTiO3 perovskite. During NOx chemisorption on SrTiCuO3, the following process have been identified (i) NO chemisorption and nitrite formation or NO oxidation to gas phase NO2 and further chemisorption of NO2 with nitrites or nitrates formation, (ii) nitrite oxidation to nitro and/or nitrates, and (iii) nitro and/or nitrate decomposition with release of NO2. NOx chemisorption at 300°C is more effective than chemisorption at 350°C, while regeneration at 350°C is more effective than regeneration at 300°C because: (i) decreased NOx slip (only for CO regeneration) and (ii) maintains the same chemisorption capacity of the perovskite after regeneration. H2 is a better reductant than CO, and the differences between both reducing gases are much higher at 300°C than at 350°C.

Catalytic Reduction of Dioxygen to Water with a Monomeric Manganese Complex at Room Temperature

There have been numerous efforts to incorporate dioxygen into chemical processes because of its economic and environmental benefits. The conversion of dioxygen to water is one such example, having importance in both biology and fuel cell technology. Metals or metal complexes are usually necessary to promote this type of reaction and several systems have been reported. However, mechanistic insights into this conversion are still lacking, especially the detection of intermediates. Reported herein is the first example of a monomeric manganese(II) complex that can catalytically convert dioxygen to water. The complex contains a tripodal ligand with two urea groups and one carboxyamidopyridyl unit; this ligand creates an intramolecular hydrogen-bonding network within the secondary coordination sphere that aids in the observed chemistry. The manganese(II) complex is five-coordinate with an N(4)O primary coordination sphere; the oxygen donor comes from the deprotonated carboxyamido moiety. Two key intermediates were detected and characterized: a peroxo-manganese(III) species and a hybrid oxo/hydroxo-manganese(III) species (1). The formulation of 1 was based on spectroscopic and analytical data, including an X-ray diffraction analysis. Reactivity studies showed dioxygen was catalytically converted to water in the presence of reductants, such as diphenylhydrazine and hydrazine. Water was confirmed as a product in greater than 90% yield. A mechanism was proposed that is consistent with the spectroscopy and product distribution, in which the carboxyamido group switches between a coordinated ligand and a basic site to scavenge protons produced during the catalytic cycle. These results highlight the importance of incorporating intramolecular functional groups within the secondary coordination sphere of metal-containing catalysts.

Transition metal-containing mixed oxides catalysts derived from LDH precursors for short-chain hydrocarbons oxidation

MMgAlO mixed oxide catalysts (M = Mn, Fe, Co, Ni, Cu, Zn, Ag and Pd) obtained from layered double hydroxide (LDH) precursors calcined at 1023 K were used in the complete oxidation of methane and the oxidative dehydrogenation of propane. The catalysts were characterized by XRD, N 2 adsorption, EDX and H 2-TPR experiments. In the catalytic complete oxidation of methane, the Pd-based catalyst has been proved the most active, but the less stable catalyst. Other mixed oxides containing transition metals have been tested and their catalytic activity followed the order: MgAlO ≈ FeMgAlO < NiMgAlO < ZnMgAlO < MnMgAlO < CoMgAlO < AgMgAlO < CuMgAlO. Total conversion was achieved at 858 K with CuMgAlO the most active and highly stable catalyst also able to perform the complete oxidation of ethane, propane and propene. It has been shown that highly reducible metal oxide species play an important role in the catalytic combustion of methane. In the oxidative dehydrogenation of propane in the temperature range from 723 K to 873 K the propene selectivity passed through a maximum for the Mn- and Fe-containing catalysts and decreased continuously for the other transition metal-containing catalysts to the benefit of CO x for CuMgAlO and of cracking products for CoMgAlO, NiMgAlO and ZnMgAlO. Considering the propene yield, the catalysts can be ranked in the following order: CoMgAlO > MnMgAlO > NiMgAlO > ZnMgAlO > FeMgAlO > CuMgAlO. No straight correlation between the H 2-TPR reducibility and the catalytic performances of the samples was found in the oxidative dehydrogenation of propane into propene. The effects of the contact time and of the propane-to-oxygen molar ratio on the catalytic performances of the most active CoMgAlO, MnMgAlO and NiMgAlO mixed oxides have been investigated.

Photooxidative Coupling of Thiophenol Derivatives to Disulfides

Disulfide bonds play an important role in determining the structure and stability of proteins and nanoparticles. Despite extensive studies on the oxidation of thiols for the synthesis of disulfides, little is known about the photooxidation of thiols, which may be a clean, safe, and economical alternative to the use of harmful and expensive metal-containing oxidants and catalysts. In this paper, we report the photooxidative coupling of thiophenol derivatives to disulfides. Para-substituted thiophenol derivatives, p-SHC(6)H(4)X (X = NO(2), COOH, Cl, and OCH(3)), are irradiated, and disulfides, X(2)(C(6)H(4))(2)S(2), are identified as the major photoproducts using Raman, UV-vis, IR, and NMR spectroscopies. For p-nitrothiophenol (pNTP), 4,4'-dinitrodiphenyldisulfide (DNDPDS) is produced in 81% yield. The product yield changes with pH, being the highest at pH ≈ 5, suggesting that both neutral thiol and anionic thiolate forms of pNTP are required for the photoreaction to occur. Excitation at 455 nm, at which the thiolate form of pNTP absorbs strongly, leads to the largest yield of DNDPDS, whereas very little DNDPDS is formed by excitation of the thiol form of pNTP at 325 nm. Our observations suggest that the photooxidation occurs via collisions of the electronically excited thiolate form of pNTP with the surrounding neutral thiol forms of pNTP. The photooxidation reaction happens regardless of the electron-withdrawing or electron-donating properties of the substituents if the pH and excitation wavelengths are properly chosen. The versatility of light and generality of the photooxidative coupling reaction of thiophenol derivatives may open new possibilities for selective and site-specific photocontrol of disulfide bond formation in biology and nanomaterial science as well as in synthetic chemistry.

Dimethyl disulfide conversion into dimethyl sulfide in the presence of sulfidized catalysts

The conversion of dimethyl disulfide in the presence of various supported sulfidized metal-containing catalysts at atmospheric pressure and T = 150−350°C was studied. Sulfidized transition metals supported onto aluminum oxide were more active than catalysts based on a carbon support, silicon dioxide, amorphous aluminosilicate, and zeolite ZSM-5. The most active catalyst was 10% Co/Al2O3 prepared with the use of cobalt acetate as an active component precursor and treated with a mixture of hydrogen sulfide with hydrogen at T = 400°C. From kinetic data, it follows that all of the reaction products were formed simultaneously at a temperature of <200°C, whereas a consecutive reaction scheme took place at higher temperatures. In the presence of a sulfidized alumina-cobalt catalyst, the output of dimethyl sulfide was higher than that reached with the use of other well-known catalysts.

Multifunctional Chiral Phosphine Organocatalysts in Catalytic Asymmetric Morita−Baylis−Hillman and Related Reactions

Catalytic asymmetric synthesis has received considerable attention over the past few decades, becoming a highly dynamic area of chemical research with significant contributions to the field of organic synthesis. In the development of new catalysts, the concept of multifunctional catalysis described by Shibasaki and co-workers, namely, the combination of more than one functional group within a single molecule to activate the transformation, has proved a powerful strategy in the design of efficient transition metal-containing catalysts. A variety of reactions have since been addressed with multifunctional organocatalysts. One example is the Morita-Baylis-Hillman (MBH) reaction, in which a carbon-carbon bond is created between the alpha-position of an activated double-bond compound and a carbon electrophile. The seminal report on this reaction in 1972 described the prototypical couplings of (i) ethyl acrylate with acetaldehyde and (ii) acrylonitrile with acetaldehyde; the reaction is promoted by the conjugate addition of a nucleophilic catalyst to the alpha,beta-unsaturated aldehyde. Many variations of the MBH reaction have been reported, such as the aza-MBH reaction, in which an N-tosyl imine stands in for acetaldehyde. Recent innovations include the development of chiral molecules that catalyze the production of asymmetric products. In this Account, we describe the refinement of catalysts for the MBH and related reactions, highlighting a series of multifunctional chiral phosphines that we have developed and synthesized over the past decade. We also review similar catalysts developed by other groups. These multifunctional chiral phosphines, which contain Lewis basic and Brønsted acidic sites within one molecule, provide good-to-excellent reactivities and stereoselectivities in the asymmetric aza-MBH reaction, the MBH reaction, and other related reactions. We demonstrate that the reactivities and enantioselectivies of these multifunctional chiral phosphines can be adjusted by enhancing the reactive center's nucleophilicity, which can be finely tuned by varying nearby hydrogen-bonding donors. Artificial catalysts now provide highly economic access to many desirable compounds, but the general adaptability and reactivity of these platforms remain problematic, particularly in comparison to nature's catalysts, enzymes. The multifunctional organocatalysts described in this Account represent another positive step in the synthetic chemist's efforts to profitably mimic nature's catalytic platform, helping develop small-molecule catalysts with enzyme-like reactivities and selectivities.

Comparing Al-SBA-15 Support and Pt/Al-SBA-15 Catalyst: Changes in Al Speciation and Acidic Properties Induced by the Introduction of Pt via Aqueous Medium

Differences between the acidic properties of silicoaluminic Pt-containing catalysts and those assessed on their parent supports have been reported in the literature and attributed to the presence of the metal nanoparticles and to their influence on the acid sites. It is shown here that for mesoporous materials containing various types of Al species, an alternative explanation can be proposed. 27Al NMR spectroscopy, FTIR of adsorbed CO and probe catalytic tests suggest the redistribution of aluminium atoms upon contact of the parent support with the aqueous solution containing the Pt precursor. Upon contact with water and thermal treatment, strong and mild Bronsted sites (Si–O(H)–Al) transform into strong Lewis sites (isolated tetracoordinated Al atoms). As a consequence, it may not be straightforward to deduce the acidic properties of metal-containing catalysts supported on Al-containing mesoporous materials from those of the bare support, because the surface species may differ significantly.

Photochromic Diarylethene-Containing Ionic Liquids and N-Heterocyclic Carbenes

The design and synthesis of a series of photochromic diarylethene-containing Arduengo-type N-heterocyclic carbenes (NHCs) of gold(I), silver(I) and palladium(II) and their imidazolium salt precursors are reported. Reversible photochromic behavior was observed. The open form and the closed form also show different luminescence properties. The electronic and conformational/steric changes associated with the photocyclization and photocycloreversion reactions may open up new opportunities for the discovery of new classes of photochromic ligands and metal-containing materials and catalysts with photoswitchable functions and activities and regio-/enantioselectivities.

Hydroalkylation of benzene and ethylbenzene over metal-containing zeolite catalysts

The hydroalkylation reaction of benzene and ethylbenzene over BEA zeolites with a Si/Al ratio of 9–190, MOR with Si/Al = 48, and MFI with Si/Al = 25 containing ruthenium, rhodium, platinum, or palladium was studied, as well as over the Ru/BEA zeolites with Si/Al = 42 doped with a second metal: nickel, cobalt, or rhodium. The catalytic experiments were conducted under flow conditions in the temperature range 130–190°C, a pressure of 1 MPa, a feed weight hourly space velocity of 2–64 h−1, and a stoichiometric reactant ratio. It was shown that the main reaction routes are the complete hydrogenation of benzene and ethylbenzene yielding cyclohexane and ethylcyclohexane, respectively; hydroalkylation yielding cyclohexylbenzene, para- and meta-ethylcyclohexylbenzenes, and diethylcyclohexylbenzenes; and alkylation resulting in dicyclohexylbenzenes and ethyldicyclohexylbenzenes. The ruthenium-promoted (1 wt %) zeolite BEA with Si/Al = 42 displayed the highest activity and selectivity in the benzene and ethylbenzene hydroalkylation reactions. Doping of the catalyst with cobalt and rhodium did not improve its catalytic properties, presumably, owing to the fact that the dopant metals largely occur in the cationic form according to the IR data for adsorbed CO. An admixture of nickel (0.5 wt %) to the catalyst increases the catalyst operation stability without reducing the yield of ethylcy-clohexy lbenzenes.

Removal of Soluble Palladium Complexes from Reaction Mixtures by Fixed-Bed Adsorption

The increasing use of metal-containing catalysts in producing pharmaceutical intermediates and active pharmaceutical ingredients, in conjunction with requirements of low metal content in a drug substance, has motivated the development of efficient separations processes for metals removal. In this paper, fixed-bed adsorption, an attractive alternative to batch adsorption, was investigated using a reaction mixture from a Heck coupling. Adsorption isotherm determinations using three candidate adsorbents revealed that QuadraPure TU had the greatest affinity and identified the optimal adsorption temperature. The bed residence time was key in obtaining good adsorption efficiency in subsequent fixed-bed experiments. Methods for palladium detection by HPLC were developed to monitor adsorption column performance in quasi real-time. A preliminary design methodology was developed in which actual breakthrough time is estimated from the experimentally determined tradeoff between bed efficiency and residence time. Even at the realized 55% bed efficiency, fixed-bed adsorption requires less than one-fourth the adsorbent needed for a single-stage batch adsorption process.

Quaternary onium tribromide catalyzed cyclic carbonate synthesis from carbon dioxide and epoxides

Novel and efficient organic catalysts of quaternary onium tribromide and the related metal-containing catalysts of SalenCoX/PTAT were developed to catalyze the coupling reaction of carbon dioxide and epoxides generating relevant cyclic carbonates including chiral propylene carbonate under extremely mild condition.

Evaluation of various types of Al-MCM-41 materials as catalysts in biomass pyrolysis for the production of bio-fuels and chemicals

MCM-41, is one of the latest members of the mesoporous family of materials. They possess a hexagonal array of uniform mesopores (1.4–10 nm), high surface areas (>1000 m 2/g) and moderate acidity. Due to these properties the MCM-41 materials are currently under study in a variety of processes as catalysts or catalyst supports. The objective of this study was to evaluate different types of MCM-41 materials as potential catalysts in the catalytic biomass pyrolysis process. We expected that the very high pore size and the mild acidity of these materials could be beneficial to reformulate the high molecular weight primary molecules from biomass pyrolysis producing useful chemical (and especially phenolic compounds) and lighter bio-oil with less heavy molecules. Three different samples of Al-MCM-41 materials (with different Si/Al ratio) and three metal containing mesoporous samples (Cu–Al-MCM-41, Fe–Al-MCM-41 and Zn–Al-MCM-41) have been synthesised, characterized and tested as catalysts in the biomass catalytic pyrolysis process using a fixed bed pyrolysis combined with a fixed catalytic reactor and two different types of biomass feeds. Compared to conventional (non-catalytic) pyrolysis, it was found that the presence of the MCM-41 material alters significantly the quality of the pyrolysis products. All catalysts were found to increase the amount of phenolic compounds, which are very important in the chemical (adhesives) industry. A low Si/Al ratio was found to have a positive effect on product yields and composition. Fe–Al-MCM-41 and Cu–Al-MCM-41 are the best metal-containing catalysts in terms of phenols production. The presence of the Al-MCM-41 material was also found to decrease the fraction of undesirable oxygenated compounds in the bio-oil produced, which is an indication that the bio-oil produced is more stable.

Catalytic oxidation of cyclohexane to cyclohexanone and cyclohexanol by oxygen in a solvent-free system over metal-containing ZSM-5 catalysts

Heterogeneous oxidation of liquid cyclohexane by molecular oxygen was carried out over metal-containing ZSM-5 catalysts in a solvent-free system. Among those M-ZSM-5 and M/ZSM-5 catalysts tested, Co-containing ZSM-5 catalysts including Co/ZSM-5 (prepared by ion-exchange and calcination) and Co-ZSM-5 (prepared by ion-exchange and drying) showed the best activity for the oxidation of cyclohexane to cyclohexanone and cyclohexanol. Co/ZSM-5 had almost the same activity as Co-ZSM-5 for the cyclohexane oxidation; however, the leaching of cobalt from Co-ZSM-5 readily occurred. Co/ZSM-5 (calcined) catalyst could achieve about 10 mol% conversion of cyclohexane and 97% selectivity of KA-oil (the mixture of cyclohexanone and cyclohexanol) at 393 K under the pressure of 1.0 MPa O 2. In addition, other factors like the temperature, the pressure, the time, various metals, the tert-butylhydroperoxide (TBHP) addition, and the catalyst reuse were investigated carefully. For the titled oxidation, Co/ZSM-5 was recyclable, and could behave as a truly heterogeneous catalyst.

Infrared Emission and Theoretical Study of Carbon Monoxide Adsorbed on Alumina-Supported Rh, Ir, and Pt Catalysts

The infrared emission spectra of CO adsorbed on alumina-supported 1, 3, and 5 wt % Rh, Ir, and Pt metal-containing catalysts were studied at 423 and 473 K. While CO is adsorbed in dicarbonyl (dimer), linearly (on-top) bonded and bridged carbonyl forms on rhodium and platinum, the dimer form is dominant on iridium. The relative intensity of Rh-CO and Ir-CO linear bands decrease with increasing temperature compared to the intensity of the dicarbonyl bands; the corresponding bands on Pt behave the opposite way. Two dicarbonyl and two linear Pt-CO bands were identified in the infrared spectra of Pt/Al(2)O(3) catalysts. The surface structure (kinked or planar Pt atoms), the dispersity of the metal, the temperature, and the quantity of adsorbed CO on the surfaces all have an effect on the fine structure of the Pt-CO stretching bands. The metal-carbon and CO stretching force constants were calculated for surface dicarbonyl, linearly bonded CO, and bridged carbonyl species. The metal-carbon stretching wavenumbers and force constants were predicted and compared between surface species and metal carbonyl complexes. The iridium-carbon bonds were found always stronger than the Rh-C and Pt-C ones in all surface species. The observed stretching wavenumbers and force constants seem to support the idea that CO and metal-carbon bonds are always stronger in metal carbonyl complexes than in adsorbed surface species. The distribution and mode of CO adsorption on surface metal sites can be effectively studied by means of infrared emission spectroscopy.

Hydrodechlorination over Zeolite Supported Catalysts—Clarification of Reaction Mechanism—

Chlorinated compounds emitted into the atmosphere are responsible for the current thinning and shrinking of the ozone layer in the stratosphere. Two main potential methods have been suggested for their transformation: oxidative destruction to form environmentally safe products or hydrodechlorination to form potentially valuable chemical compounds. In this review, performances of halogenated compounds over the zeolites and the noble metal-containing zeolite catalysts are characterized by different spectroscopic methods (IR, NMR, etc.) to clarify the reaction mechanism of hydrodechlorination over the catalysts. NMR studies provided evidence that chlorofluorocarbon reacts more aggressively with zeolites at elevated temperatures. The extent and nature of reaction depended on the zeolite type and the exchanged cation. Infrared spectra of the gas phase showed that HCl, CO2 and COCl2 were detected as the reaction products in the hydrodechlorination reaction of CCl2F2 over HY. The phosgene intermediate is believed to be the key to the structural damage of the zeolite. Pt-containing zeolite catalyst was more active in the hydrodechlorination of carbon tetrachloride than the Pd-containing catalyst, and beside the main products (methane and HCl) chloroform as intermediate product was formed. On Pd-containing zeolite no chloroform, but high amounts of ethane as final products together with methane were formed. The activity and selectivity of the hydrodechlorination reaction of various chlorine-containing compounds (C1, C2, with and without fluorine content) on different metal-containing zeolites of various types is summarized.

Hydroxylation of phenol with H2O2 over transition metal containing nano-sized hollow core mesoporous shell carbon catalyst

The catalytic performance of transition metal (Fe2+ or Cu2+) containing nano-sized hol low core mesoporous shell carbon (HCMSC) heterogeneous catalysts for the hydroxylation of phenol with hydrogen peroxide (H2O2) in water was investigated in a batch reactor. The metal-containing HCMSC catalyst showed higher activity than the same metal ion-exchanged zeolites. The nature of the metal and its content in the HCMSC had remarkable influence on the reaction results under the typical reaction conditions (PhOH/H2O2=3, reaction temperature=60 ‡C). Fe2+ containing HCMSC catalyst showed high catalytic activity with phenol conversion of 29%, selectivity to catechol (CAT) and hydroquinone (HQ) about 85%, H2O2 effective conversion about 70% and selectivity to benzoquinone (BQ) below 1% in the batch system.

Ammonia removal from effluent streams of wet oxidation under high pressure

The formation of ammonia is inevitable during industrial-scale wet oxidation of wastewater if nitrogen-containing compounds are present. This undesired side-reaction requires additional measures for disposal. Common routes are either the use of noble metal-containing catalysts in the first oxidation step or end-of-pipe treatment. Problems rise for example from the insufficient stability of solid catalysts against hydrothermal impact. As most of the wet oxidation processes run at elevated pressure and temperature, running the heterogeneously catalysed oxidation of ammonia in the gas phase in a downstream reactor could protect the catalysts mainly from leaching and offers an economic alternative by avoiding loss of unused oxygen after depressurisation. This work reports on the oxidation of ammonia with air in steam atmosphere using Cu,Cr-containing supported and bulk catalysts at 235–305 °C and 30–60 bar. A copper chromite catalyst gave best performance (86% conversion at 305 °C, 45 bar, contact time 1 s). The spinel-type phase CuCr2O4 seems to be the active phase and shows superior stability. The results indicate that phase behaviour of water strongly influences activity and lifetime of catalysts. Characterisation of the solids (BET, XRD, XPS, ICP) proved that deactivation is mainly caused by leaching of Cr(VI) species from catalysts when the reaction runs near to dew point of water and by loss of BET surface area of supported catalysts due to hydrothermal impact.

Elimination of Nitrogen (I) Oxide on Zeolite and Zirconium Oxide Catalysts

In results of testing a series of the metal-containing zeolite catalysts based on different structure types zeolites (Y, M, pentansil), Ме/zeolite, zirconia and (zeolite+ZrO2, Me = Ce, Co, Fe, Cr) in the reaction of nitrogen (I) oxide decomposition was found that the most active Fe-containing catalysts over the zeolitic base are characterized by the presence of strongly acidic sites on their surface, activity of Co-containing catalysts depends on quantity of a gated in cobalt. The temperature of nitrogen (I) oxide elimination reaction on Fe zeolite catalysts was found to decrease by 70–150°С in the presence of a reductant – light alkanes (C3–C4) or carbon monooxide. Upon oxygen excess, N2O conversion high degrees (90–94%) were attained at lower by 50–150°С temperatures if hydrocarbons used and the catalysts’ activity being in correlation with the presence of strong acidic B-sites on their surface.

Catalytic chemistry of chloro- and chlorofluorocarbon dehalogenation: from macroscopic observations to molecular level understanding

The macroscopic performance of monometallic Group VIII metal catalysts and bimetallic Group VIII metal-containing catalysts for dehalogenation reactions and the mechanistic aspects of the reactions are reviewed. Of primary importance is the dechlorination of multiply chlorinated C2–C3 hydrocarbons containing Cl atoms attached to adjacent carbons. The chemistry of geminal chloro- and chlorofluorocarbons (CFCs) dehalogenation catalyzed by supported noble metal-containing catalysts in the presence of H2 is also reviewed. The hydrodehalogenative oligomerization of C1 chloro and chlorofluorocarbons is considered with the greatest detailed; however, the hydrodechlorination chemistry is also addressed.