Polymer encapsulation of metallophthalocyanines: efficient catalysts for aerobic oxidation of alcohols

Metallophthalocynines (MPcs) of iron, cobalt and copper have been successfully encapsulated for the first time on polystyrene matrix, rendering them highly dispersible in common organic solvents. These catalysts were characterised by diffuse reflectance UV–Vis as well as FT-IR spectroscopy. The encapsulated metallophthalocyanines (MCMPcs) were found to be stable and more active than their unencapsulated counterparts. These encapsulated catalysts showed enhanced activity for aerobic oxidation of alcohols mimicking cytochrome P-450 dependent mono oxygenases. These catalysts not only have high turnover frequencies but could be recovered quantitatively by simple filtration and reused without loss of activity.     

Metallo Salicylidenetriazol Complexes Encapsulated in Zeolite-Y: Synthesis, Physicochemical Properties and Catalytic Studies

Chromium(III), zinc(II) and nickel(II) complexes of thio-Schiff base derived from salicylaldehyde and 4-amino-2,4-dihydro-1,2,4-triazole-5-thione have been encapsulated in the nanopores of zeolite-Y by a flexible ligand method. The prepared encapsulated metal complexes have been characterized by surface analysis (XRD and N₂ adsorption/desorption), spectroscopic methods, chemical and thermal analyses. The various techniques of characterization used demonstrated that these complexes were effectively encapsulated in the zeolite supercages without structural modification or loss of crystallinity of the zeolite framework. The encapsulated complexes were screened as heterogeneous catalysts for various oxidation reactions such as phenol, cyclohexene and styrene oxidation, using H₂O₂ as an oxidant. Under the optimized conditions, these catalysts exhibited high to moderate activity. After a few cycles these catalysts were found to be stable and could be reused after recovering without detectable catalyst leaching or significant loss of activity.     

Alkylguanidine-catalyzed heterogeneous transesterification of soybean oil

Transesterification of soybean oil with methanol has been carried out in the presence of heterogenized alkylguanidines as catalysts. The alkylguanidines were anchored to modified polystyrene or siliceous MCM-41, encapsulated in the supercages of zeolite Y, or entrapped in SiO₂ sol-gel matrices. The catalytic activity of these catalysts was compared with that of their homogeneous counterparts, showing that the yields of methyl esters obtained in the homogeneous phase can be obtained with the guanidines anchored to the supports after longer reaction times. The catalysts prepared by immobilization of alkylguanidines in microporous systems showed diffusion restrictions for the vegetable oil as well as the low stability of the inorganic framework.     

Effect of MgF2 and Al2O3 supports on the structure and catalytic activity of copper–manganese oxide catalysts

Structure and catalytic activity of double copper–manganese oxide catalysts supported on MgF₂ and Al₂O₃ have been studied. All samples were calcined at 400 °C and those supported on Al₂O₃ also at 550 and 950 °C. The properties of surface species have been characterized by low temperature adsorption of nitrogen, XRD and TPR-H₂. The catalytic activities have been tested in low-temperature CO oxidation and in NO reduction by propene. The supported oxides react with each other during calcination to form CuMn₂O₄ spinel. The spinel seems to be responsible for the catalytic activity of the double copper–manganese catalysts. The temperature of calcination changes the strength of interaction between the active phase and the supports influencing the catalytic activity.     

Role of defect structure of active oxides in oxidative coupling of methane

A new approach to the choice of catalysts for oxidative coupling of methane based on the determination of the defect degree is suggested. For the most active catalysts (oxides of rare earth elements (REE), alkaline earth metals (AEM), manganese, lead), it is shown that oxygen defectness is of great importance in this process.Based on the structural approach Bi₂O₃-containing catalytic systems have been suggested. They have C₂-hydrocarbon formation activity and selectivity to be comparable with those ones for most active catalysts as REE oxides.     

Polystyrene and silica gel supported AlCl3 as highly chemoselective heterogeneous Lewis acid catalysts for Friedel–Crafts sulfonylation of aromatic compounds

Crosslinked polystyrene supported AlCl₃ (Ps–AlCl₃) and silica gel supported AlCl₃ (SiO₂–AlCl₃) have shown to be mild and chemoselective heterogeneous Lewis acid catalysts for the sulfonylation of different kinds of aromatic compounds with p-toluene sulfonyl chloride (TsCl) and benzene sulfonyl chloride (PhSO₂Cl) as sulfonylating agents. These solid acid catalysts are stable (as bench top catalysts) and can be easily recovered and reused without appreciable change in their efficiency.     

Tackling the problem of sulfur poisoning of perovskite catalysts for natural gas combustion

LaMn_0.5Mg_0.5O₃y MgO and LaCr_0.5-xMn_xMg_0.5O₃.y MgO catalysts (x=0-0.25, y=0-17) were prepared, characterized (XRD, BET, SEM-EDS, TEM, FTIR, chemical and atomic absorption analyses), tested for high-temperature methane combustion and aged in presence of SO₂ to investigate sulfur effect on catalytic activity. Different roles of manganese and chromium in the perovskite structure have been assessed: (i) manganese improves the activity of catalysts in the fresh state, whereas chromium worsens it; (ii) owing to its basic nature, manganese makes perovskites more prone to adsorb SO₂ and so less resistant to sulfur poisoning, whereas chromium, owing to its acidic nature, has opposite effects; (iii) the partial solubility of chromium oxide in basic media renders Cr-catalyst regeneration by NH₃ leaching less effective. MgO promotes catalytic activity of fresh LaCr_0.5Mg_0.5O₃.y MgO catalysts and slows down sulfur poisoning of LaMn_0.5Mg_0.5O₃.y MgO and LaCr_0.25Mn_0.25Mg_0.5O₃.17 MgO catalysts only.     

Study on Cu–Mn–Si catalysts for synthesis of cyclohexanone and 2-methylfuran through the coupling process

A series of Cu–Mn–Si catalysts for synthesis of cyclohexanone (CHN) and 2-methylfuran (2-MF) through the coupling of cyclohexanol (CHL) dehydrogenation and furfural (FFA) hydrogenation were prepared by co-precipitation method. The catalysts were characterized by using N₂ physisorption, X-ray diffraction (XRD), H₂ temperature programmed reduction (H₂-TPR), N₂O-chemisorption and ammonia temperature-programmed desorption (NH₃-TPD) methods. The results show that metal–silica/or metal–metal interactions are presented in the catalysts, and the contents of copper, manganese and silicon affect the BET surface area, acidity and copper dispersion. The results of the catalytic tests indicate that manganese increases the activity of CHL dehydrogenation and FFA hydrogenation as well as the selectivity of 2-MF. A Cu–Mn–Si catalyst including appropriate copper, manganese and silicon is found to be most active, selective and stable under reaction conditions.     

Synergistic effect of Pd in methane combustion PdMnOx/Al2O3 catalysts

The catalytic methane combustion was investigated over alumina-supported monometallic and bimetallic palladium and manganese oxide catalysts. The catalytic activity of these systems showed that palladium incorporation on MnO_x/Al₂O₃ catalyst leads to an enhancement in methane combustion. The higher catalytic activity of the PdMn/Al₂O₃ catalysts is related to a greater mobility of lattice oxygen in manganese oxide in the presence of palladium. These bimetallic catalysts also showed a significant improvement in catalysts stability with respect the monometallic ones. Surface analysis of the used catalysts revealed less amount of coke and Mn/Al and Pd/Al atomic ratios almost unchanged, which is indication of absence of active phase sintering.     

Influence of catalyst type on the curing process and network structure of alkyd coatings

Recent studies have shown that cobalt catalysts, used for curing of alkyd coatings, are potentially carcinogenic, and hence replacement by new environmental friendly catalysts is needed. The influence of different metal based catalysts on the oxidation process has been studied extensively in model systems, consisting of unsaturated oils. However, these results may not be representative for real coatings, since in these systems the oxygen diffusion is much lower than in model systems and therefore may have a large effect on the curing. In this paper, we will show how the curing of an alkyd coating depends on the type of catalyst (cobalt or manganese based). The curing process is studied using a high spatial resolution nuclear magnetic resonance (NMR) setup. The final network structure and cross-link density are found to be correlated with the catalyst used, i.e. a cobalt based catalyst and two manganese based catalysts. The difference in final network structure is investigated by NMR T₂ relaxation analysis and the glass transition temperature T_g measured using a differential scanning calorimeter (DSC). In case of the cobalt based catalyst a cross-linking front was observed and a high cross-link density was found, compared to the manganese based catalysts, in which case no sharp cross-linking front was observed. To interpret the observed NMR profiles in more detail, simulations based on a reaction-diffusion model were performed. From the results of these simulations estimates were obtained for the reaction constants and the diffusion of oxygen for the different catalysts.     

Metal complexes of 2-methylimidazole encapsulated in zeolite-Y as efficient and reusable catalysts for oxidation of phenol and benzyl alcohol

Copper(II), ruthenium(III) and zinc(II) complexes of 2-methylimidazole (2-MeImzlH) encapsulated in the supercages of zeolite-Y have been synthesized and characterized by various physicochemical measurements. The catalytic potential of these complexes were tested for the oxidation of phenol and benzyl alcohol using 30 % H₂O₂ as an oxidant. Various parameters, such as concentration of oxidant and catalyst, reaction time, temperature of the reaction, volume of solvent and type of solvents have been optimized to obtain the maximum transformation of phenol to catechol and hydroquinone. The catalytic activity of zeolite encapsulated complexes followed the order: [Cu(2-MeImzlH)]-Y (72.5 %) > [Ru(2-MeImzlH)]-Y (57.8 %) > [Zn(2-MeImzlH)]-Y (43.2 %) after 5 h of reaction time. Oxidation of benzyl alcohol catalyzed by these encapsulated complexes gave only benzaldehyde as the product. The zeolite-encapsulated complexes could be easily separated after the reaction and reused. The neat complexes gave low conversions as compared to the encapsulated catalysts and decomposed. The catalytic activity of zeolite encapsulated complexes was found to be better than their respective non-encapsulated complexes and metal exchanged zeolites.     

Role of chlorine in improving selectivity in the oxidative coupling of methane to ethylene

Samarium, magnesium and manganese oxide and alkali-promoted oxide catalysts have been prepared and tested for the oxidative coupling of methane. The results show that alkali-promoted oxides inhibit total oxidation and have a higher selectivity for the formation of C₂ products than the undoped metal oxides. These catalysts have been promoted by injecting pulses of gaseous chlorinated compounds (dichloromethane and chloroform) during the reaction. It has been found that these chlorinated compounds markedly increase the selectivity for the formation of C₂ products for all the MnO₂-based catalysts and for lithium-doped MgO and Sm₂O₃ catalysts. The effect is greatest in MnO₂-based catalysts. When dichloromethane is added to a pure, unpromoted MnO₂ catalyst the selectivity for the formation of carbon dioxide decreases from 82.6% to 4.1% and the selectivity for the formation of C₂H₄ increases from virtually zero to 56.3%. The highest C₂ selectivity observed after promotion of pure MnO₂ by dichloromethane is about 93%. Promotion of these pure oxide catalysts by gaseous chlorinated compounds provides an alternative to alkali promotion as a method of inhibiting total oxidation and of increasing ethylene production.     

The catalytic activity of immobilized on modified silica metalloporphyrins bearing antioxidative 2,6-di-tert-butylphenol pendants

A comparative study of the catalytic activity of supported manganese(III) and iron(III) chlorides of meso-tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl)porphyrin (R₄PMnCl, R₄PFeCl) and meso-tetraphenylporphyrin (TPPMnCl, TPPFeCl) is reported. The metalloporphyrins have been immobilized via coordination bond on the surface of two series of imidazole modified silica, imidazole propyl silica (IPS) and imidazole 3-(glycidyloxypropyl) silica (IGOPS). The heterogenised catalysts have been evaluated for hydrocarbon oxidation by sodium periodate. The critical role of 2,6-di-tert-butylphenol groups on the periphery of porphyrin ring in their catalytic activity has been evaluated and pertinent structural and mechanistic aspects are discussed.     

A recoverable catalyst Co(salen) in zeolite Y for the synthesis of methyl N-phenylcarbamate by oxidative carbonylation of aniline

A green process for the synthesis of methyl N-phenylcarbamate (MPC) by the oxidative carbonylation of aniline was studied. In this process, Co(salen) complexes were successfully encapsulated in zeolite Y by a flexible ligand method. The heterogeneous catalysts were characterized by AAS, BET, IR, TGA, XRD and XPS. The catalytic activities of the encapsulated catalysts and their homogeneous analogues were examined in the oxidative carbonylation of aniline to MPC. Under the conditions of aniline (11 mmol), encapsulated catalyst (0.5 g), KI (0.365 g), CO/O₂ ratio 9:1, 4 MPa, 170 °C, 3 h, Co(salophen)(OH)₂–Y catalyst shows the highest activity with the conversion of 67.1% and the selectivity of 77.3%. Co(salophen)(OH)₂–Y could be recycled at least five times and no significant loss of catalytic activity was observed.     

Nature of carbonaceous deposits on the alumina supported transition metal oxide catalysts in the wet air oxidation of phenol

Al₂O₃ supported transition metal (Mn, Fe, Co, Ni, and Cu) oxide catalysts were prepared and tested for the wet oxidation of phenol. The supported copper oxide catalysts showed the highest catalytic activity due to their highest surface reducibility. There was carbonaceous deposits on the used catalysts for the wet oxidation of phenol and the supported manganese oxide catalysts showed the highest amount of carbonaceous deposits. These carbonaceous deposits must have their own micropores which resulted in the decrease of the pore volume and the increase of the surface area. The NMR and FTIR spectroscopy showed that the carbonaceous deposits were mostly of aromatic nature and contained some oxygen-bearing groups such as carboxylic acids and alcohols.     

Synthesis and Catalytic Activity of Cu(II), Fe(III) and Bi(III) Complexes of Thio-Schiff Base Encapsulated in Zeolite-Y for Hydroxylation of Phenol

Coordination of 4-{[(1E)-(2-hydroxyphenyl)methylene]amino}-2,4-dihydro-3H-1,2,4-triazole-3-thione, [sal(thiotriazol)], with M-exchanged zeolite-Y (M = Cu(II), Fe(III) and Bi(III)) leads to the encapsulation of the metal complexes in the supercages of zeolite-Y by flexible ligand method. The prepared encapsulated metal complexes have been characterized by physico-chemical techniques, which indicated that the complexes were effectively encapsulated inside the supercages of Na–Y, without any modification of the morphology and structure of the zeolite. 3D model structure generated for these complexes suggests that zeolite-Y can accommodate these complexes in the FAU supercages without any strain. The catalytic activity of all the catalysts towards the hydroxylation of phenol was evaluated under heterogeneous conditions using hydrogen peroxide as an oxidant. Under the optimized conditions, these catalysts show moderate activity with excellent selectivity (>95%) towards catechol. These catalysts were stable in hydroxylation of phenol and have been reused a further three times after recovering. The results reflect the reusability of the catalysts, as no significant loss in their catalytic activity was noticed.     

Polymer‐Bound Pyridine‐Bis(oxazoline). Preparation through Click Chemistry and Evaluation in Asymmetric Catalysis

A pyridine‐bis(oxazoline) ligand was efficiently immobilized by copper(I)‐catalyzed azide‐alkyne cycloaddition onto a polystyrene resin. The so obtained click‐pybox resin 1a was associated with various metal salts (YbCl₃, LuCl₃, CuOTf) and the resulting resin‐bound catalysts were explored in ring‐opening of cyclohexene oxide, silylcyanation of benzaldehyde and alkynylation of imines. These new polymer‐supported catalysts exhibit good to excellent performances in terms of catalytic activity, enantioselectivity and recyclability.     

A Comparative Study on the Catalytic Activity of Mn Containing MCM-41 Molecular Sieves for Oxidation of p-Cymene

Mesoporous silica molecular sieves SiMCM-41, AlMCM-41 with Si/Al ratio equal to 158 and MnMCM-41 with Si/Mn ratio equal to 29 were synthesized by hydrothermal method. Manganese oxide impregnated on SiMCM-41 and on AlMCM-41 catalysts were also prepared by wet impregnation method. These catalysts referred as Mn/SiMCM-41 and Mn/AlMCM-41 (158). Both the manganese incorporated and impregnated catalysts structure was elucidated by XRD. Nitrogen adsorption isotherm was used to determine specific surface area, pore volume, and pore size distribution. The thermal stability of the as-synthesized catalyst was studied using TG-DTA. The presence of Mn²⁺ was evident through DR UV–VIS and ESR spectroscopy. Through ESR studies, MnMCM-41 (29) was assigned to have framework Mn(II) sites. Extra-framework manganese species with well-resolved sextet, centered at g = 2.00 had octahedral symmetry was observed in Mn/SiMCM-41 and Mn/AlMCM-41 (158). Mn retained its +2 oxidation state (ESR active) even upon calcination at 550 °C in presence of air. The catalytic activity of the above-mentioned manganese containing MCM-41 catalysts were tested by vapor phase oxidation of p-cymene with CO₂-free air in the temperature range from 200 to 400 °C in steps of 50 °C. The major products were 4-methylacetophenone (4-MAP), 4-isopropyl benzaldehyde (4-IPB), 1,2-epoxyisopropyl benzaldehyde or 1,2-epoxycumenaldehyde (1,2-ECA) and 4-methyl styrene (4-MS). Among the product selectivity, (4-MAP) was found to be higher than that of others. The order of the activity of catalysts followed Mn/SiMCM-41 > Mn/AlMCM-41 (158) > MnMCM-41 (29) > Mn/SiO₂.     

Effect of Nickel as Dopant in Mn/TiO<Subscript>2</Subscript> Catalysts for the Low-Temperature Selective Reduction of NO with NH<Subscript>3</Subscript>

Nickel doped manganese oxide supported on titania materials were investigated for the low-temperature NH₃-SCR. For this purpose, a series of Ni modified Mn/TiO₂ catalysts were prepared and evaluated for the low-temperature SCR of NO with ammonia in the presence of excess oxygen. The catalytic performance of these materials was compared with respect to the nickel weight percentage in order to examine the correlation between physicochemical characteristics and reactivity of optimized materials. It was found that the 5% Mn–2% Ni/TiO₂ catalyst showed the highest activity and yielded 100% NO conversion at 200 °C. XRD results reveal highly dispersed manganese–nickel species on TiO₂ support for the Mn–Ni/TiO₂ catalysts. Our TPR data results suggested an increase in reducibility of manganese species in Mn–Ni/TiO₂ catalysts. The absence of the high-temperature (736 K) peak indicates that the dominant phase is MnO₂. This increase of reducibility and dominant MnO₂ phase seems to be the reason for the enhanced activity and time on stream patterns of nickel-promoted titania-supported manganese catalysts. BET results illustrate that the high NO conversion is strongly dependant on the specific surface area and pore volume of this catalyst. All the physicochemical techniques we used suggested that the composition of manganese and nickel oxides on the support surface is playing an important role for the enhancement of NO conversion and the prominent time on stream stability.     

Host (nanopores of zeolite Y)-guest (oxovanadium(IV) tetradentate schiff-base complexes) nanocomposite materials: synthesis,characterization and liquid phase hydroxylation of phenol with hydrogen peroxide

Oxovanadium(IV) tetradentate Schiff-base complexes; [VO(X₂-haacac)] (X = H, Cl, CH₃ and NO₂), X₂-haacac = substituted bis(2-hydroxyanil)acetylacetone; and encapsulated in the nanopores of zeolite NaY; [VO(X₂-haacac)]-NaY; have been synthesized and characterized. The host-guest nanocomposite materials; [VO(X₂-haacac)]-NaY; was characterized by chemical analysis and spectroscopic methods (FT-IR, UV/VIS, XRD, BET and DRS). The analytical data indicated a composition corresponding to the mononuclear complex of Schiff-base ligand. The characterization data showed the absence of extraneous complexes, retention of zeolite crystalline structure and encapsulation in the nanopores. Substitution of the aromatic hydrogen atoms of the Schiff-base ligand by electron withdrawing groups like −Cl, and −NO₂ has two major effects: (1) retention and concentration of the oxovanadium(IV) complex in the zeolite cavities is enhanced (due to the larger size of the substituents) and (2) the electronic and spectral properties of the encapsulated complex are modified. Liquid-phase selective hydroxylation of phenol with H₂O₂ to a mixture of catechol and hydroquinone in CH₃CN has been reported using oxovanadium(IV) Schiff-base complexes encapsulated in zeolite-Y as catalysts. Reaction conditions have been optimized by considering the concentration of substrate and oxidant, amount of catalyst, effect of time, volume of solvent and temperature. Under the optimized reaction conditions, [VO((NO₂)₂-haacac)]-NaY has shown the highest conversion of 42.3% after 6 h. All these catalysts are more selective toward catechol formation. Encapsulated oxovanadium(IV) complex is catalytically very efficient as compared to other neat complexes for the hydroxylation of phenol and is stable to be recycled without much deterioration.