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.     

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.     

Zeolite encapsulated Ru(III), Cu(II) and Zn(II) complexes of benzimidazole as reusable catalysts for the oxidation of organic substrates

Ru(III), Cu(II) and Zn(II) complexes of benzimidazole (BzlH) have been synthesized in the supercages of zeolite-Y by the flexible ligand method and were characterized by spectroscopic (IR, UV–Vis and ESR) studies, XRD and thermogravimetric analysis, surface area, and pore volume measurements. The zeolite encapsulated complexes catalyzed the oxidation of ethylbenzene, benzoin, and cyclohexanol. Various parameters, such as concentration of oxidant and catalyst, reaction time, temperature of the reaction and type of solvents have been optimized to obtain the maximum transformation of ethylbenzene to a mixture of acetophenone, benzaldehyde and styrene. Under the optimized reaction conditions, [Ru(BzlH)]-Y gave 80.4 % conversion of ethylbenzene in 1 h. All these zeolite encapsulated complexes were more selective towards acetophenone formation. Oxidation of benzoin catalyzed by [Cu(BzlH)]-Y, [Ru(BzlH)]-Y and [Zn(BzlH)]-Y encapsulated complexes resulted in 75.5, 78.7 and 59.9 % conversion respectively to give benzaldehyde as exclusive product. A maximum conversion of 39.1 % cyclohexanol with [Cu(BzlH)]-Y was achieved to give cyclohexanone. The activity of neat complexes towards these reactions was also carried out. The encapsulated catalysts were significantly more active than neat complexes and recyclable without much loss in catalytic activity.     

Encapsulation of bis(ethylenediamine) copper(II) complex in zeolite matrices

Bis(ethylenediamine) copper(II) complex was encapsulated in NaY, KL, Naβ and NaZSM-5 zeolite. The redox properties of metal complexes in neat and encapsulated state were studied by cyclic voltammetry. The redox potential of metal complex is altered towards negative value upon encapsulation in various zeolites. This may be due to axial interaction with various zeolite matrix. The redox properties of biological systems are also altered (cytochrome-C, and Vitamin-B₁₂) when they are immobilized on NaAlMCM-41 materials. The copper ethylenediamine complex in constrained environment shows higher activity for the oxidation of dimethyl sulfide compared to that of neat complex.     

Zeolite encapsulated Ru(III), Cu(II) and Zn(II) complexes as effective catalysts for the oxidation of ethylbenzene

Ru(III), Cu(II) and Zn(II) complexes of imidazole (ImzlH) have been synthesized in the supercages of zeolite-Y by flexible ligand method and characterized by spectroscopic (IR and UV?CVis) studies, XRD and thermogravimetric analysis, surface area, and pore volume measurements. These complexes were screened for their catalytic study towards the oxidation of ethylbenzene to a mixture of acetophenone, benzaldehyde and styrene using tert-butylhydroperoxide (TBHP) as an oxidant. A best-suited reaction condition has been optimized for these catalysts by varying the amount of the oxidant and catalyst, reaction time and volume of solvent for maximum transformation of ethylbenzene. Under the optimized reaction conditions, [Cu(ImzlH)]-Y gave 79.3% conversion after 1?h of reaction time. All these catalysts were more selective towards acetophenone formation. Among the prepared catalysts, zeolite encapsulated Cu(II) complex was found to be more active than the corresponding Ru(III) and Zn(II) complexes and all the complexes were stable enough to be reused. The catalytic activities of the neat complexes and metal exchanged zeolites were also compared with the zeolite encapsulated metal complexes.     

Zeolite-encapsulated Ru(III) tetrahydro-Schiff base complex: An efficient heterogeneous catalyst for the hydrogenation of benzene under mild conditions

A series of Ru(III) tetrahydro-Schiff base complexes (denoted as Ru[H₄]-Schiff base with Schiff base = salen, salpn and salcn, see Scheme 1) were encapsulated in the supercages of zeolite Y by flexible ligand method. The prepared catalysts were characterized by X-ray diffraction, diffuse reflectance UV–Vis spectroscopy, Infrared spectroscopy, elemental analysis, as well as N₂ adsorption techniques. It was shown that upon encapsulation in zeolite Y, Ru(III) tetrahydro-Schiff base complexes exhibited higher activity for the hydrogenation of benzene than the corresponding Ru(III)-Schiff base complexes. This indicates that hydrogenation of the CN bond of the Schiff base ligands led to a modification of the coordination environment of the central Ru(III) cations. The stability of the prepared catalysts has also been confirmed against leaching of the complex molecule from the zeolite cavities, as revealed by the result that no loss of catalytic activity was observed within three successive runs with regeneration.     

Copper complex of isatin Schiff base encapsulated in zeolite as active heterogeneous catalyst: an efficient protocol for the acetylation reaction

Copper (II) complex of 3-phenylimino-1,3-dihydro-indol-2-one encapsulated in the super cages of zeolite-Y has been synthesized by flexible ligand method and characterized by various physicochemical measurements. The catalytic activity of cationic exchanged zeolite, copper complex of ligand and complex encapsulated inside the zeolite was investigated for the decomposition of H₂O₂ and for the acetylation of p-cresol. All catalysts show good to excellent yield. The results showed that conversion of p-cresol varies in the order homogeneous complex < Na-Y-Zeolite < Cu-Y-Zeolite < heterogeneous complex.     

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.     

Synthesis,characterization and catalytic activity of metallosalens and metallocalixpyrroles encapsulated in Y and MCM-41 molecular sieves

The in situ synthesis and encapsulation of metallo-octamethylcalix(4)pyrrole inside the pores of HMCM-41 catalyst is presented. The catalytic performance of the synthesized material was tested by the liquid phase oxidation of cyclohexene at room temperature using tert-butyl hydroperoxide as an oxidant. Similarly we have synthesized and encapsulated metallosalens in NaY zeolite and carried out the liquid phase oxidation of p-xylene at 100 °C using H₂O₂ as an oxidant. The central transition metal ions present in these two types of complexes were varied. The catalysts were characterized by XRD, IR spectra and thermal analysis.     

Synthesis, characterization and catalytic oxidation of ethylbenzene over “neat” and host (nanocavity of zeolite-Y)/guest (tetraza tetraone macrocyclic copper(II) complexes) nanocomposite materials (HGNM)

Copper(II) complexes of [12]aneN₄: 1,4,7,10-tetraazacyclododecane-2,3,8,9-tetraone; [14]aneN₄: 1,4,8,11-tetraazacyclotetradecane-2,3,9,10-tetraone; Bzo₂[12]aneN₄: dibenzo-1,4,7,10-tetraazacyclododecane-2,3,8,9-tetraone and Bzo₂[14]aneN₄: dibenzo-1,4,8,11-tetraazacyclotetradecane-2,3,9,10-tetraone have been encapsulated in the nanopores of zeolite-Y by the in situ one pot template condensation reaction. Copper(II) complexes with azamacrocyclic ligand were entrapped in the nanocavity of zeolite-Y by a two-step process in the liquid phase: (i) adsorption of [bis(diamine)copper(II)]; [Cu(N-N)₂]-NaY; in the supercages of the zeolite, and (ii) in situ condensation of the copper(II) precursor complex with diethyloxalate. The new host guest nanocomposite materials (HGNM) have been characterized by FTIR, DRS and UV–Vis spectroscopic techniques, XRD and elemental analysis, as well as nitrogen adsorption. The “neat” and encapsulated complexes exhibited good catalytic activity in the oxidation of ethylbenzene at 333 K, using tert-butyl hydroperoxide (TBHP) as the oxidant. Acetophenone was the major product though small amounts of o- and p-hydroxyacetophenones were also formed revealing that C–H bond activation takes place both at benzylic and aromatic ring carbon atoms. Ring hydroxylation was more over the “neat” complexes than over the encapsulated complexes.     

Host (nanocavity of zeolite-Y)/guest (Mn(II), Co(II), Ni(II) and Cu(II) complexes of pentadendate Schiff-base ligand) nanocomposite materials (HGNM): application in the heterogeneous oxidation of cyclohexene

Transition metal (M = Mn(II), Co(II), Ni(II) and Cu(II)) complexes with pentadendate Schiff-base ligand; N,N′-bis(salicylidene)-2,6-pyridinediaminato, H₂ [sal-2,6-py]; was entrapped in the nanocavity of zeolite-Y by a two-step process in the liquid phase: (i) adsorption of bis(salicylaldiminato)metal(II); [M(sal)₂]-NaY; in the supercages of the zeolite, and (ii) in situ Schiff condensation of the metal(II) precursor complex with the corresponding 2,6-pyridinediamine; [M(sal-2,6-py)]-NaY. The new materials were characterised by several techniques: chemical analysis, spectroscopic methods (DRS, BET, FTIR and UV/Vis), conductometric and magnetic measurements. Analysis of the data indicates that the M(II) complexes are encapsulated in the nanodimensional pores of zeolite-Y and exhibit different from those of the free complexes, which can arise from distortions caused by steric effects due to the presence of sodium cations, or from interactions with the zeolite matrix. The Host–Guest Nanocomposite Materials (HGNM); [M(sal-2,6-py)]-NaY; catalyzes the oxidation of cyclohexene with tert-butylhydroperoxide (TBHP). Oxidation of cyclohexene with HGNM gave 2-cyclohexene-1-one, 2-cyclohexene-1-ol and 1-(tert-butylperoxy)-2-cyclohexene. [Mn(sal-2,6-py)]-NaY shows significantly higher catalytic activity than other catalysts.     

Polystyrene encapsulation of manganese porphyrins: highly efficient catalysts for oxidation of olefins

Metalloporphyrins (MPs) of manganese have been successfully encapsulated for the first time in polystyrene matrix. The manganese porphyrins have been anchored onto polymer on the basis of physical envelopment by the polystyrene fibers, rendering them highly dispersible in common organic solvents. These polystyrene supported catalysts are characterized by UV–Vis and diffuse reflectance FT-IR spectroscopy. These encapsulated catalysts (MCMPs) exhibit enhanced activity for oxidation of alkenes in the presence of NaIO₄, KHSO₅ and NaOCl as oxidants. These catalysts not only possess high turnover frequencies but they are found to be quite stable and could be recovered quantitatively by simple filtration and reused without loss of activity.     

Vanadium-catalysed enantioselective sulfoxidations: rational design of biocatalytic and biomimetic systems

Approaches to the rational design of vanadium-based biocatalytic and biomimetic model systems as catalysts for enantioselective oxidations are reviewed. Incorporation of vanadate ion into the active site of phytase (E.C. 3.1.3.8), which in vivo mediates the hydrolysis of phosphate esters, afforded a relatively stable and inexpensive semi-synthetic peroxidase. It catalysed the enantioselective oxidation of prochiral sulfides with H₂O₂ affording the _S-sulfoxide, e.g., in 68% ee at 100% conversion for thioanisole. Amongst the transition metal oxoanions that are known to be potent inhibitors of phosphatases, only vanadate resulted in a semi-synthetic peroxidase, when incorporated into phytase. In a biomimetic approach, vanadium complexes of chiral Schiff's base complexes were encapsulated in the super cages of a hydrophobic zeolite Y. Unfortunately, these ship-in-a-bottle complexes afforded only racemic sulfoxide in the catalytic oxidation of thioanisole with H₂O₂. This revised version was published online in June 2006 with corrections to the Cover Date.     

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.     

Solvent-free oxidation of benzyl alcohol with oxygen using zeolite-supported Au and Au–Pd catalysts

The oxidation of benzyl alcohol to benzaldehyde has been investigated in the absence of solvent using zeolite-supported Au and Au–Pd catalysts. Three zeolites were investigated, ZSM-5, zeolite β and zeolite Y, and these were contrasted with the titanoslicalite TS-1 and TiO₂ as supports. For the Au catalysts the best results are obtained with zeolite β as the support and the conversions were comparable or better than those observed with TiO₂ in terms of turn over frequencies. However, the selectivities observed with the acidic zeolites were lower than the non-acidic TS-1 and TiO₂. This is due to the subsequent reaction of benzaldehyde via acid catalysed reactions to give benzyl benzoate and its dibenzyl acetal, and, in some cases dibenzylether. Initial catalysts were evaluated with a gold loading of 2 wt% and increasing this to 4 wt% showed the expected increase in activity, indicating that there is scope to improve the performance of these catalysts. The most active catalysts were prepared by impregnation and catalysts prepared by deposition precipitation were considerably less active. Introduction of Pd into the catalyst improved the activity without significantly affecting the selectivity.     

Cobalt(II), Copper(II) and Zinc (II) complexes of 2-methyl benzimidazole encapsulated in zeolite as catalysts for the hydroxylation of phenol

Cobalt(II), Copper(II) and Zinc (II) complexes of 2-methylbenzimidazole (Mebzlh) encapsulated in the super cages of zeolite-Y and ZSM-5 have been synthesized by flexible ligand method and characterized by various physico-chemical measurements. The catalytic activity of encapsulated complexes was investigated for the decomposition of H₂O₂ and for the hydroxylation of phenol using H₂O₂ as an oxidant. The hydroxylation of phenol yielded catechol and hydroquinone as the major products. All catalysts show good selectivity for diphenol products. The results showed that conversion of phenol varies in the order [Cu(Mebzlh)]-Y > [Cu(Mebzlh)]-ZSM-5 > [Zn(Mebzlh)]-Y > [Co(Mebzlh)]-Y > [Zn(Mebzlh)]-ZSM-5 > [Co(Mebzlh)]-ZSM-5 after 6 h of reaction time.     

Activity and durability of iron-exchanged mordenite-type zeolite catalyst for the reduction of NO by NH3

NO removal activity and the durability of iron-exchanged mordenite type zeolite catalyst (FeHM) have been examined in a continuous fixed bed flow reactor. The catalytic activity for NO reduction by NH₃ in the presence of oxygen was much higher than that in the absence of oxygen, and it was fully reversible with respect to the presence of oxygen in the feed gas stream. The oxidation ability of SCR catalysts including FeHM was critical for both reactions of NH₃ and SO₂ oxidation, thus for the NO removal activity and its sulfur tolerance. The maximum conversion of NO for FeHM catalyst with respect to the reaction temperature shifted to the higher temperature due to its mild oxidation ability. The deactivation behaviors such as the changes of the physicochemical properties of the catalyst and the loss of NO removal activity induced by SO₂ could not be distinguished, regardless of the metals exchanged in zeolite. However, the amount of deactivating agents deposited on the catalyst surface depended on the species of metals exchanged on the mordenite type zeolite, which was mainly attributed to the oxidation ability of metals for SO₂ conversion to SO₃.     

Low molecular weight, polymeric, and covalently bound cobalt(II)-phthalocyanines for the oxidation of mercaptans

Cobalt(II)-phthalocyanines in different environments are investigated as catalysts for the oxidation of thiols. Water-soluble low molecular weight 2,9,10,23-tetracarboxyphthalocyanine (1b) and polymeric phthalocyanine (2b) with carboxylic end groups are prepared. Compound1b is covalently bound at linear and cross-linked poly(chloromethylstyrene) in the presence of pyridine to obtain the water-soluble polymers (3a, b) and gel-type polymers (4a, b). Covalent binding of1b to surface modified silica was also realized. Low molecular weight and polymeric phthalocyanines (1a, 2a) are synthesized on silica, alumina, and charcoal. In addition,1a is encapsulated in the interior of NaX zeolite. All materials are efficient catalysts for the oxidation of 2-mercaptoethanol. The mechanism employing water-soluble catalysts is discussed in the direction of a mononuclear complex coordinating dioxygen and thiol. Heterogeneous catalysts containing1a and2a on the carriers show enhanced activity with increasing dispersion. The proposed mechanism considers different reaction sites for the coordination of O₂ and thiol.     

Efficient liquid phase oxidation of olefins and aromatic alcohol catalyzed by reusable polymer anchored Schiff base complexes

BACKGROUND: Heterogenization of homogeneous catalyst has become an interesting process for the catalytic oxidation of olefins and aromatic alcohol. This may provide a new kind of catalyst that is not only friendly to the environment but also exhibits higher thermal and chemical stabilities. RESULTS: Polymer anchored Schiff‐base complexes of iron(III), copper(II) and cobalt(II) have been synthesized and characterized. The catalytic potential of these complexes has been tested for the oxidation of cyclohexene. The effect of varying solvent, oxidant, substrate oxidant molar ratio, temperature and catalyst amount has been studied. Under optimized reaction conditions 91, 88 and 81% conversion of cyclohexene was obtained with Fe(III), Cu(II) and Co(II) catalysts, respectively. Moreover, the oxidation of other substrates, such as styrene, benzylalcohol, toluene and 1‐hexene were also efficiently carried out by these catalysts. CONCLUSION: The immobilized complexes showed excellent catalytic activity along with high selectivity for the oxidation of olefins and alcohols. The catalysts can be recycled more than five times without any noticeable loss of catalytic activity. Copyright © 2009 Society of Chemical Industry     

Synthesis,characterization and catalytic activities of ruthenium complexes containing triphenylphosphine/triphenylarsine and tetradentate Schiff bases

Ruthenium(II) complexes of the type [Ru(CO)(B)(L)] (B=AsPh₃, pyridine, piperidine or morpholine; L=dianion of tetradentate Schiff bases) have been synthesized and characterized by physico-chemical methods. These complexes are found to be effective catalysts in the oxidation of primary and secondary alcohols using N-methylmorpholine-N-oxide as oxidant. The catalytic activity of these triphenylarsine complexes have been compared with that of triphenylphosphine complexes and with similar ruthenium(III) complexes. The formation of high valent Ruⁿ⁺²O species as catalytic intermediate is proposed for the catalytic processes.