Reduction of copper in porous matrixes. Stepwise and autocatalytic reduction routes

The reduction of Cu(II) oxide species in siliceous matrixes of different porosity (MFI, FAU, MCM-48) and in alumosilicate MFI was studied by temperature-programmed reduction in hydrogen (TPR), by X-ray absorption fine structure (after stationary hydrogen treatments), and by transmission electron microscopy. It was found that the reduction may proceed in one or in two reduction steps. The two-step scheme known for zeolites was observed also for Cu(II) in siliceous microporous matrixes, with similar temperature of Cu(II) reduction onset as for the alumosilicate MFI. Therefore, the two-step scheme cannot be explained by the stabilization of Cu ions by intra-zeolite electrical fields. CuOx clusters in MCM-48 were reduced in a one-step scheme (similar to bulk CuO) at high Cu content (6 wt %) but in a two-step scheme at low Cu content (1 wt %). The two reduction steps observed with most samples cannot be identified with the transitions of all Cu(II) to Cu(I) and of Cu(I) to Cu(0). Instead, Cu(0) nuclei were observed already at low reduction temperatures and were found to coexist with Cu ions over temperature ranges of different extension. This coexistence range was narrow in materials that favor aggregation of the Cu nuclei into particles: Cu-MCM-48 of low Cu content and Cu-ZSM-5. In the latter, metal segregation from the pore system was found to be accompanied by an autocatalytic initiation of the second reduction step. In the siliceous microporous matrixes, the Cu(0) nuclei were observed to coexist with Cu ions over wide temperature ranges (100 K for MFI) at temperatures far above that of Cu reduction in the bulk oxide. These observations suggest that oligomeric Cu metal nuclei which may have been formed, e.g., at the intersections of the MFI channel system, may be unable to activate hydrogen, which would be required for rapid reduction of the coexisting Cu ions.     

EPR spectroscopy of Cu(I)-NO adsorption complexes formed over Cu-ZSM-5 and Cu-MCM-22 zeolites

The Cu(I)-NO adsorption complexes were formed over copper exchanged and autoreduced high siliceous Cu-ZSM-5 and Cu-MCM-22 zeolites and studied by EPR spectroscopy at X-, Q-, and W-band frequencies. The spin Hamiltonian parameters of the Cu(I)-NO species are indicative of a nitrogen-centered radical complex with a bent geometry and a significant contribution of the Cu(I) 4s atomic orbital to the wave function of the unpaired electron. Two different Cu(I)-NO species were found in both zeolites. It has been confirmed by comparing the experimental data with the results of previous theoretical studies that the presence of two different species is due to the formation of Cu(I)-NO adsorption complexes from two different Cu(I) sites in the zeolite matrix with different numbers of oxygen coligands. The structure of the two sites in the Cu-ZSM-5 and Cu-MCM-22 zeolites must be similar as the spin Hamiltonian parameters are found to be almost independent of the zeolite matrix, where the Cu(I)-NO complex is formed. The EPR signal intensity of the Cu(I)-NO species was studied as a function of the NO loading, and the formation of diamagnetic Cu(I)-(NO)(2) species with rising NO pressure at the expense of paramagnetic Cu(I)-NO monomers could be demonstrated for both systems at low temperatures.     

Oxidation of cyclohexane catalyzed by metal-ion-exchanged zeolites

The ion-exchange rates and capacities of the zeolite NaY for the Cu(II), Co(II), and Pb(II) metal ions were investigated. Ion-exchange equilibria were achieved in approximately 72 h for all the metal ions. The maximum ion exchange of metal ions into the zeolite was found to be 120 mg Pb(II), 110 mg Cu(II), and 100 mg Co(II) per gram of zeolite NaY. It is observed that the exchange capacity of a zeolite varies with the exchanged metal ion and the amount of metal ions exchanged into zeolite decreases in the sequence Pb(II) > Cu(II) > Co(II). Application of the metal-ion-exchanged zeolites in oxidation of cyclohexane in liquid phase with visible light was examined and it is observed that the order of reactivity of the zeolites for the conversion of cyclohexane to cyclohexanone and cyclohexanol is CuY > CoY > PbY. It is found that conversion increases by increase of the empty active sites of a zeolite and the formation of cyclohexanol is favored initially, but the cyclohexanol is subsequently converted to cyclohexanone.     

Investigations on the effect of Mn ions on the local structure and photocatalytic activity of Cu(I)-ZSM-5 catalysts

The introduction of Mn ions into Cu(I)-ZSM-5 was found to lead to an enhancement of the photocatalytic activity for the direct decomposition of N2O into N2 and O2 at 298 K. Various in-situ techniques such as ESR, photoluminescence, XAFS as well as a combination of CO-FT-IR and CO-TPD measurements revealed that the accommodation of Mn ions within ZSM-5 zeolite cavities significantly affects the location sites of the ion-exchanged Cu(II) ions as well as the local structure of the Cu(I) ion species formed by evacuation at high temperatures. Moreover, the introduction of Mn ions into ZSM-5 led to an increase in the amount of 3-coordinated Cu(I) species at the main channel of the zeolite, playing a major role as the active species for the photocatalytic decomposition of N2O into N2 and O2.     

Organic‐Free Synthesis of a Highly Siliceous Faujasite Zeolite with Spatially Biased Q4(nAl) Si Speciation

We report the most siliceous FAU‐type zeolite, HOU‐3, prepared via a one‐step organic‐free synthesis route. Computational studies indicate that it is thermodynamically feasible to synthesize FAU with SAR=2–7, though kinetic factors seemingly impose a more restricted upper limit for HOU‐3 (SAR≈3). Our findings suggest that a slow rate of crystallization and/or low concentration of Na⁺ ions in HOU‐3 growth mixtures facilitate Si incorporation into the framework. Interestingly, Q⁴(nAl) Si speciation measured by solid‐state NMR can only be modeled with a few combinations of Al positioning at tetrahedral sites in the crystal unit cell, indicating the distribution of Si(‐O‐Si)_4−n(‐O‐Al)_n species is spatially biased as opposed to being random. Achieving higher SAR is desirable for improved zeolite (hydro)thermal stability and enhanced catalytic performance, which we demonstrate in benchmark tests that show HOU‐3 is superior to commercial zeolite Y.     

Theoretical studies of Cu(I) sites in faujasite and their interaction with carbon monoxide

Sitting, coordination, and properties of Cu(I) cations in zeolite faujasite are investigated using a combined quantum mechanics-interatomic potential function method. The coordination of Cu(I) ions depends on their location within the zeolite lattice. Cu(I) located inside the hexagonal prisms (site I') and in the plane of six-membered aluminosilicate rings on the walls of sodalite units (site II) is threefold coordinated, whereas Cu(I) located in the supercages (site III) is twofold coordinated. In agreement with available experimental data Cu(I) appears to be more strongly bound in sites I' and II than in site III. The binding energy of site II Cu(I) ions increases with the number of Al atoms, but only closest Al atoms have a substantial influence. The CO molecule binds more strongly onto sites with weaker bound cations and lower coordination. We assign the two CO stretching IR bands observed for Cu(I)-Y zeolites to sites II with one Al (2157-2161 cm(-1)) and two Al atoms (2140-2148 cm(-1)) in the six-membered aluminosilicate ring. For Cu(I)-X we tentatively assign the high frequency band to site III (2156-2168 cm(-1)) and the low-frequency band to site II with three Al atoms in the six-membered ring (2136-2138 cm(-1)).     

Investigation of Metal State in Metal-containing Zeolite Catalysts by DTA and ESR Methods

Thermal stability and its influence on the catalytic activity in CO oxidation of Cu, Pd and Pd-Cu zeolite systems were investigated. The increasing of catalytic activity in the first cycle of reaction is connected with the thermal decomposition of complexions and consequently with the changing of metal state in catalyst in the case of Cu/ZSM-5catalyst. This activity does not relate to initial zeolite with complex ions, but to the metal ions with the decreasing ligands number in the coordination sphere of metal ion. According to the EPR spectrum the copper ions form clusters in zeolite channels due to the spin changed interaction. It was established ESR method that 1.8% Cu/ZSM-5 catalyst in a reduced form has copper (I and II) ions by. The Pd/ZSM-5 catalysts with different metal content have high catalytic activity below the temperature decomposition in contrary to Cu-containing zeolites. On the one hand, it may be connected with the partial reduction of Pd ions during CO oxidation and, on the other hand, with the ability of Pd ions to form the respective label complexes with reagents as additional ligands. The same character of relation between thermal stability and catalytic activity for Pd-Cu/ZSM-5 catalyst was observed. This revised version was published online in August 2006 with corrections to the Cover Date.     

含铜MFI分子筛的H2-TPT和O2-TPD研究

 采用H2－TPR和O2－TPD手段考察了不同金属离子交换的ZSM－5分子筛催化剂上的氢还原性能和氧脱附性能．发现Cu－ZSM－5催化剂的储氧能力及氧脱附性能优于Co－ZSM－5和Fe－ZSM－5催化剂;储氧能力强、低温下氧脱附性能好的催化剂,对NO分解反应的催化活性就高．铜离子是反应的活性中心．添加Ag和Ce可使Cu－ZSM－5催化剂上氧的脱附温度大大降低．Cu－ZSM－5／堇青石整体式催化剂上Cu的存在形式与单纯的Cu－ZSM－5有差异,整体式催化剂上的一价铜数量少,但较稳定．     

Ligand Reprogramming in Dinuclear Helicate Complexes: A Consequence of Allosteric or Electrostatic Effects?

The ditopic ligand 6,6'-bis(4-methylthiazol-2-yl)-3,3'-([18]crown-6)-2,2'-bipyridine (L(1)) contains both a potentially tetradentate pyridyl-thiazole (py-tz) N-donor chain and an additional "external" crown ether binding site which spans the central 2,2'-bipyridine unit. In polar solvents (MeCN, MeNO(2)) this ligand forms complexes with Zn(II), Cd(II), Hg(II) and Cu(I) ions via coordination of the N donors to the metal ion. Reaction with both Hg(II) and Cu(I) ions results in the self-assembly of dinuclear double-stranded helicate complexes. The ligands are partitioned by rotation about the central py--py bond, such that each can coordinate to both metals as a bis-bidentate donor ligand. With Zn(II) ions a single-stranded mononuclear species is formed in which one ligand coordinates the metal ion in a planar tetradentate fashion. Reaction with Cd(II) ions gives rise to an equilibrium between both the dinuclear double-stranded helicate and the mononuclear species. These complexes can further coordinate s-block metal cations via the remote crown ether O-donor domains; a consequence of which are some remarkable changes in the binding modes of the N-donor domains. Reaction of the Hg(II)- or Cd(II)-containing helicate with either Ba(2+) or Sr(2+) ions effectively reprogrammes the ligand to form only the single-stranded heterobinuclear complexes [MM'(L(1))](4+) (M=Hg(II), Cd(II); M'=Ba(2+), Sr(2+)), where the transition and s-block cations reside in the N- and O-donor sites, respectively. In contrast, the same ions have only a minor structural impact on the Zn(II) species, which already exists as a single-stranded mononuclear complex. Similar reactions with the Cd(II) system result in a shift in equilibrium towards the single-stranded species, the extent of which depends on the size and charge of the s-block cation in question. Reaction of the dicopper(I) double-stranded helicate with Ba(2+) shows that the dinuclear structure still remains intact but the pitch length is significantly increased.     

Role of clay-based membrane for removal of copper from aqueous solution

The present study focuses on the fabrication of polysulfone (PSf)-clay minerals impregnated hybrid membrane for treatment of Cu (II) ions. Blending and phase inversion methods have been employed to develop clay-based membranes by the mixing of bentonite, sepiolite and zeolite in the matrix of PSf. Moreover, characterization of fabricated membranes was carried out using SEM, FTIR, zeta potential, pore size and water contact angle measurement. Adsorption and filtration experiments were conducted to evaluate the permeation performance of the membrane. Obtained results of permeation study reveal that the presence of clay minerals in the matrix of modified membrane not only increases the adsorption and rejection efficiency for Cu (II) but, it also improves the flux of pure water. Among all developed membranes, the membrane prepared by the mixing of zeolite demonstrates the highest adsorption (2.82 mg/g) and rejection value (97%) towards Cu (II) at low pressure (0.5 bar). Regeneration performance results confirm the reusability of membrane up to 3–5 cycles along with 82.5–90% metal recovery. Based on significant metal recovery, clay-based low-cost zeolite/PSf membrane could be used to remove Cu (II) from water at low pressure to replace current conventional membrane.     

高硅FAU沸石与甲胺吸附物的相互作用

合成高硅沸石中所用的模板剂胺类分子与沸石骨架 Si—O基团间的相互作用机理尚不清楚 .迄今为止 ,沸石与吸附胺之间的相互作用的研究还只限于测定沸石酸性质 [1,2 ]、表面羟基活性位或了解模板分子在沸石骨架中的位置和状态 [3,4 ] .甲胺、乙胺在骨架完美的高硅 FAU(Y型 )沸石上的亲和性指数 AT 值 ,即被吸附有机物脱附峰温与该有机物的沸点之差分别高达 1 60与 1 5 0℃[5] .而在高硅 MFI(Silicalite-1 )沸石上为 1 60与 1 2 7℃ [6 ] .与大多数有机物不同 ,被吸附的胺类脱附时的吸热效应十分明显 .上述现象表明胺与高硅沸石骨架 O2 -…     

Catalytic reduction of NO to N2O by a designed heme copper center in myoglobin: implications for the role of metal ions

The effects of metal ions on the reduction of nitric oxide (NO) with a designed heme copper center in myoglobin (F43H/L29H sperm whale Mb, CuBMb) were investigated under reducing anaerobic conditions using UV-vis and EPR spectroscopic techniques as well as GC/MS. In the presence of Cu(I), catalytic reduction of NO to N2O by CuBMb was observed with turnover number of 2 mol NO.mol CuBMb-1.min-1, close to 3 mol NO.mol enzyme-1.min-1 reported for the ba3 oxidases from T. thermophilus. Formation of a His-heme-NO species was detected by UV-vis and EPR spectroscopy. In comparison to the EPR spectra of ferrous-CuBMb-NO in the absence of metal ions, the EPR spectra of ferrous-CuBMb-NO in the presence of Cu(I) showed less-resolved hyperfine splitting from the proximal histidine, probably due to weakening of the proximal His-heme bond. In the presence of Zn(II), formation of a five-coordinate ferrous-CuBMb-NO species, resulting from cleavage of the proximal heme Fe-His bond, was shown by UV-vis and EPR spectroscopic studies. The reduction of NO to N2O was not observed in the presence of Zn(II). Control experiments using wild-type myoglobin indicated no reduction of NO in the presence of either Cu(I) or Zn(II). These results suggest that both the identity and the oxidation state of the metal ion in the CuB center are important for NO reduction. A redox-active metal ion is required to deliver electrons, and a higher oxidation state is preferred to weaken the heme iron-proximal histidine toward a five-coordinate key intermediate in NO reduction.     

TPR investigations on the reducibility of Cu supported on Al2O3, zeolite Y and SAPO-5

Reducibility of Cu supported on Al₂O₃, zeolite Y and silicoaluminophosphate SAPO-5 has been investigated in dependence on the Cu content using a method combining conventional temperature programmed reduction (TPR) by hydrogen with reoxidation in N₂O followed by a second the so-called surface-TPR (s-TPR). The method enables discrimination and a quantitative estimation of the Cu oxidation states +2, +1 and 0. The quantitative results show that the initial oxidation state of Cu after calcination in air at 400 °C, independent on the nature of the support, is predominantly +2. Cu²⁺ supported on Al₂O₃ is quantitatively reduced by hydrogen to metallic Cu⁰. Comparing the TPR of the samples calcined in air and that of samples additionally pre-treated in argon reveals that in zeolite Y and SAPO-5 Cu²⁺ cations are stabilized as weakly and strongly forms. In both systems, strongly stabilized Cu²⁺ ions are not auto-reduced by pre-treatment in argon at 650 °C, but are reduced in hydrogen to form Cu⁺. The weakly stabilized Cu²⁺ ions, in contrast, may be auto-reduced by pre-treatment in argon at 650 °C forming Cu⁺ but are reduced in hydrogen to metallic Cu⁰.     

Accumulation and voltammetric determination of complexed metal ions at zeolite-modified sensor electrodes

Chemically modified electrodes based on zeolite-containing graphite pastes were constructed and evaluated as sensor electrodes for the voltammetric determination of traces of metallic species in solution. Zeolite molecular sieves with pore sizes of 3, 4, 5, and 10 A were all suitable for chemical deposition and subsequent voltammetric quantitation of traces of Cu(II), Cd(II), and Zn(II). The highest sensitivity was obtained using the zeolite with the 10 A pore size. The detection limit obtained for Cu(II) was 0.3 muM following a 2 min chemical deposition. The detection limits for Cd(II) and Zn(II) following a 4 min chemical deposition were 87 nM and 145 nM, respectively. The effects on the zinc signal of coexisting metallic species in the ammonia deposition medium were studied. While the addition of Hg(II) gave rise to increasing zinc signals, the addition of Ag(I), Cu(II), Cd(II), Ni(II), and Co(II) resulted in decreasing zinc signal amplitudes. Most notably, the magnitude of the interference from these latter metal ions correlated well with the coordination numbers of their ammonia complexes. Thus the electrode acted as a device which was sensitive to the size and shape of the interfering metal complex.     

An IR Study on Adsorption of CO and NO on Copper Ion-exchanged Zeolite Beta

Thee adsorption of CO and NO on copper ion-exchanged zeolite Beta was investigated using IR method.It was found that the thermalvacuum pretreatment procedure could result in the reduction of Cu2 ions in zeolite Beta.The adsorption of CO on Cu sites in zeolite Beta closely follows Langmuir isotherm.Another Cu species may form during the reaction between water and CO.The catalytic decomposition of NO on the zeolite was observed at room temperature,indicating that the decomposition reaction may occur between two coordinated NO ligands of the same dinitrosyhc complex.Furthermore,the appearance of two series of NO adsorption bands reveals that copper ions existing at different cation sites may have different effect on the adsorption and decomposition of NO molecules.     

Heterodinuclear metal arrangement in a flat macrocycle with two chemically-equivalent metal chelating sites

A phenanthroline-based macrocycle 1 has been newly developed which has two chemically equivalent metal chelating sites within the spatially restricted cavity for dinuclear metal arrangement. The macrocycle 1 reacts with Zn(CF(3)CO(2))(2) or ZnCl(2) to form homodinuclear Zn(II)-complexes. A single-crystal X-ray structural analysis of the resulting Zn(2)1(CF(3)CO(2))(4) determined the complex structure in which two Zn(II) ions are bound by two phenanthroline sites and two CF(3)CO(2)(-) ions bind to each Zn(II) ion in a tetrahedral geometry. Similarly, a homodinuclear Cu(I)-macrocycle was formed from 1 and Cu(CH(3)CN)(4)BF(4). Notably, from 1 and an equimolar mixture of Cu(CH(3)CN)(4)BF(4) and Zn(CF(3)CO(2))(2), a heterodinuclear Cu(I)-Zn(II)-macrocycle was exclusively formed in high yield (>90%) because of the relatively low stability of the dinuclear Cu(I)-macrocycle. A heterodinuclear Ag(I)-Zn(II)-macrocycle was similarly formed with fairly high selectivity from a mixture of Ag(I) and Zn(II) ions. Such selective heterodinuclear metal arrangement was not observed with other combinations of M-Zn(II) (M = Li(I), Mg(II), Pd(II), Hg(II), La(III), and Tb(III)).     

Preparation of metallosilicates with zeolite structure by atom-planting method

Atom-planting, a useful method to prepare some metallosilicates having zeolitic structure, was proposed. By treatment of highly siliceous zeolite with metal chloride vapor at selected temperature, metal cation could be introduced into the defect sites (hydroxyl nests) of zeolite framework. By the atom-planting method, some metallosilicates which are difficult to be synthesized by hydrothermal synthesis could be prepared. The vapor phase shape-selective alkylation of ethylbenzene with ethanol, and the liquid phase selective oxidation with hydrogen peroxide on the metallosilicates prepared by atom-planting method were reviewed.     

The effect of aspartic acid on the binding of transition metals to kaolinite

The effect of aspartic acid on the adsorption of Pb(II), Cu(II), Zn(II), Co(II), and Mn(II) on kaolinite at 25 degrees C in the presence of 5 mM KNO3 was investigated by means of potentiometric titrations and adsorption measurements over a range of pH and concentration. Data were modeled by extended constant capacitance models. Aspartic acid slightly enhanced the adsorption of Pb(II), Zn(II), and Co(II) at low pH, but inhibited the adsorption of all the metal ions at higher pH. Adsorption of Cu(II) and Co(II) was inhibited strongly. Because aspartic acid is adsorbed only weakly by kaolinite, inhibition of metal ion adsorption depends on the ability of aspartic acid to form complexes with the various metal ions together with the adsorption characteristics of these complexes. In particular suppression of adsorption at high pH arises from competition between surface sites and dissolved aspartate ions for the available metal ions. Cu(II) and Co(II) form complexes with aspartic acid more strongly than the other metals. As these complexes do not adsorb, Cu(II) and Co(II) suffer greater suppression from aspartic acid than the other metals. There was no evidence of adsorption of aspartic acid complexes to the permanently charged kaolinite faces.     

Affinity of Cu+ for the copper-binding domain of the amyloid-β peptide of Alzheimer's disease

The role of metal ions in Alzheimer's disease etiology is unresolved. For the redox-active metal ions iron and copper, the formation of reactive oxygen species by metal amyloid complexes has been proposed to contribute to Alzheimer's disease neurodegeneration. For copper, reactive oxygen species are generated by copper redox cycling between its 1+ and 2+ oxidation states. Thus, the AβCu(I) complex is potentially a critical reactant associated with Alzheimer's disease etiology. Through competitive chelation, we have measured the affinity of the soluble copper-binding domain of the amyloid-β peptide for Cu(I). The dissociation constants are in the femtomolar range for both wild-type and histidine-to-alanine mutants. These results indicate that Cu(I) binds more tightly to monomeric amyloid-β than Cu(II) does, which leads us to propose that Cu(I) is a relevant in vivo oxidation state.