Catalytic Implications Research Articles

Correlations of WO 3 species and structure with the catalytic performance of the selective oxidation of cyclopentene to glutaraldehyde on WO 3/TiO 2 catalysts

A series of WO 3/TiO 2 catalysts were synthesized by ultrasonic impregnation method using as-prepared TiO 2 microspheres as support as a function of tungsten oxide species loading. The crystalline structure, molecular structure and the interaction with the support of the supported tungsten oxide phase were characterized by various techniques (scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS) and specific surface areas (BET)), and their effects on the catalytic performance in selective oxidation of cyclopentene to glutaraldehyde were investigated. It was found that highly dispersed WO 3 species were obtained on the surface of TiO 2, and only W(VI) oxidation state was present on the support. The WO 3/TiO 2 catalysts showed high catalytic activity for selective oxidation of cyclopentene to glutaraldehyde. The catalytic activity increased with the WO 3 loading, and reached maximum on the catalysts with the WO 3 loading of 15–20 wt%. The molecular structures of calcinated tungsten oxide phase were determined to be tetrahedral surface tungsten oxide species with the WO 3 loading below 20 wt%, and both Brönsted acid sites and Lewis acid sites were present on the surface of the catalysts. The strong support effect on the dispersion and molecular structure of WO 3 as well as the Brönsted acid observed in the present work may have important catalytic implication. Further increasing the WO 3 loading resulted in the decrease of catalytic performance of the catalyst, meanwhile the crystalline WO 3 nanoparticles were present on the surface of the support. These reactivity trends showed the influence of molecular structure of WO 3 on the surface of TiO 2 support on the selective oxidation activity of cyclopentene to glutaraldehyde.

Malonate-bound structure of the glycerophosphodiesterase fromEnterobacter aerogenes(GpdQ) and characterization of the native Fe2+metal-ion preference

The structure of a malonate-bound form of the glycerophosphodiesterase from Enterobacter aerogenes, GpdQ, has been refined at a resolution of 2.2 A to a final R factor of 17.1%. The structure was originally solved to 2.9 A resolution using SAD phases from Zn2+ metal ions introduced into the active site of the apoenzyme [Jackson et al. (2007), J. Mol. Biol. 367, 1047-1062]. However, the 2.9 A resolution was insufficient to discern significant details of the architecture of the binuclear metal centre that constitutes the active site. Furthermore, kinetic analysis revealed that the enzyme lost a significant amount of activity in the presence of Zn2+, suggesting that it is unlikely to be a catalytically relevant metal ion. In this communication, a higher resolution structure of GpdQ is presented in which malonate is visibly coordinated in the active site and analysis of the native metal-ion preference is presented using atomic absorption spectroscopy and anomalous scattering. Catalytic implications of the structure and its Fe2+ metal-ion preference are discussed.

Bis(imino)aryl Complex of Nickel(II): N,C,N-Pincer Type Complex, (2,6-(2,6-Et2PhN=CH)2C6H3)NiBr

Thus the terdentate chelation in the complexesinhibits both dissociation of the phosphine (or nitrogen)groups and reductive elimination of the aryl group. Such aremarkably rigid framework provided by the pincer typeligands, therefore, enhances thermal stability of resultingcomplexes, and may have catalytic implication in a widerange of homogeneous catalysis.

Structural Roles of Monovalent Cations in the HDV Ribozyme

The hepatitis delta virus (HDV) ribozyme catalyzes viral RNA self-cleavage through general acid-base chemistry in which an active-site cytidine and at least one metal ion are involved. Monovalent metal ions support slow catalysis and were proposed to substitute for structural, but not catalytic, divalent metal ions in the RNA. To investigate the role of monovalent cations in ribozyme structure and function, we determined the crystal structure of the precursor HDV ribozyme in the presence of thallium ions (Tl(+)). Two Tl(+) ions can occupy a previously observed divalent metal ion hexahydrate-binding site located near the scissile phosphate, but are easily competed away by cobalt hexammine, a magnesium hexahydrate mimic and potent reaction inhibitor. Intriguingly, a third Tl(+) ion forms direct inner-sphere contacts with the ribose 2'-OH nucleophile and the pro-S(p) scissile phosphate oxygen. We discuss possible structural and catalytic implications of monovalent cation binding for the HDV ribozyme mechanism.

Catalytic implications of the higher plant ADP-glucose pyrophosphorylase large subunit

ADP-glucose pyrophosphorylase, a key regulatory enzyme of starch biosynthesis, is composed of a pair of catalytic small subunits (SSs) and a pair of catalytically disabled large subunits (LSs). The N-terminal region of the LS has been known to be essential for the allosteric regulatory properties of the heterotetrameric enzyme. To gain further insight on the role of this region and the LS itself in enzyme function, the six proline residues found in the N-terminal region of the potato tuber AGPase were subjected to scanning mutagenesis. The wildtype and various mutant heterotetramers were expressed using our newly developed host-vector system, purified, and their kinetic parameters assessed. While P 17L, P 26L, and P 55L mutations only moderately affected the kinetic properties, P 52L and P 66L gave rise to significant and contrasting changes in allosteric properties: P 66L enzyme displayed up-regulatory properties toward 3-PGA while the P 52L enzyme had down-regulatory properties. Unlike the other mutants, however, various mutations at P 44 led to only moderate changes in regulatory properties, but had severely impaired catalytic rates, apparent substrate affinities, and responsiveness to metabolic effectors, indicating Pro-44 or the LS is essential for optimal catalysis and activation of the AGPase heterotetramer. The catalytic importance of the LS is further supported by photoaffinity labeling studies, which revealed that the LS binds ATP at the same efficiency as the SS. These results indicate that the LS, although considered having no catalytic activity, may mimic many of the catalytic events undertaken by the SS and, thereby, influences net catalysis of the heterotetrameric enzyme.

Synthesis and Reactivity of New Complexes of Rhodium and Iridium with Bis(dichloroimidazolylidene) Ligands. Electronic and Catalytic Implications of the Introduction of the Chloro Substituents in the NHC Rings

The preparation of a new bis(dichloroimidazolylidene) ligand has provided chelate-N-heterocyclic complexes of Rh(I) and Ir(I), which have been fully characterized. The crystal structures of three of the new complexes are described. The study of the electronic properties of the new ligands was made on the basis of the ν(CO) stretching frequencies of the carbonyl derivatives, showing that the chloroimidazolylidene ligand is significantly less σ-donating than the related nonchlorinated analogue. This electronic modification of the ligand has important implications for the catalytic properties of the compounds obtained, as observed from enhanced activity shown in catalytic hydrosilylation of terminal acetylenes and cyclization of acetylenic carboxylic acids, compared to the complexes with the nonchlorinated NHC analogue ligand.

Crystal structures of ferrous and ferrous–NO forms of verdoheme in a complex with human heme oxygenase-1: catalytic implications for heme cleavage

Heme oxygenase oxidatively degrades heme to biliverdin resulting in the release of iron and CO through a process in which the heme participates both as a cofactor and substrate. One of the least understood steps in the heme degradation pathway is the conversion of verdoheme to biliverdin. In order to obtain a better understanding of this step we report the crystal structures of ferrous–verdoheme and, as a mimic for the oxy–verdoheme complex, ferrous–NO verdoheme in a complex with human HO-1 at 2.20 and 2.10 Å, respectively. In both structures the verdoheme occupies the same binding location as heme in heme–HO-1, but rather than being ruffled verdoheme in both sets of structures is flat. Both structures are similar to their heme counterparts except for the distal helix and heme pocket solvent structure. In the ferrous–verdoheme structure the distal helix moves closer to the verdoheme, thus tightening the active site. NO binds to verdoheme in a similar bent conformation to that found in heme–HO-1. The bend angle in the verodoheme–NO structure places the terminal NO oxygen 1 Å closer to the α- meso oxygen of verdoheme compared to the α- meso carbon on the heme–NO structure. A network of water molecules, which provide the required protons to activate the iron–oxy complex of heme–HO-1, is absent in both ferrous–verdoheme and the verdoheme–NO structure.

ITQ-15: the first ultralarge pore zeolite with a bi-directional pore system formed by intersecting 14- and 12-ring channels, and its catalytic implications.

The pore topology of ITQ-15 zeolite consists of an ultra-large 14-ring channel that is intersected perpendicularly by a 12-ring pore; acid sites have been introduced in its framework and this unique structure shows advantages over unidirectional ultralarge pore zeolites for diffusing and reacting large molecules.

Crystal Structures of the Ferric, Ferrous, and Ferrous–NO Forms of the Asp140Ala Mutant of Human Heme Oxygenase-1: Catalytic Implications

Site-directed mutagenesis studies have shown that Asp140 in both human and rat heme oxygenase-1 is critical for enzyme activity. Here, we report the D140A mutant crystal structure in the Fe(III) and Fe(II) redox states as well as the Fe(II)–NO complex as a model for the Fe(II)–oxy complex. These structures are compared to the corresponding wild-type structures. The mutant and wild-type structures are very similar, except for the distal heme pocket solvent structure. In the Fe(III) D140A mutant one water molecule takes the place of the missing Asp140 carboxylate side-chain and a second water molecule, novel to the mutant, binds in the distal pocket. Upon reduction to the Fe(II) state, the distal helix running along one face of the heme moves closer to the heme in both the wild-type and mutant structures thus tightening the active site. NO binds to both the wild-type and mutant in a bent conformation that orients the NO O atom toward the α- meso heme carbon atom. A network of water molecules provides a H-bonded network to the NO ligand, suggesting a possible proton shuttle pathway required to activate dioxygen for catalysis. In the wild-type structure, Asp140 exhibits two conformations, suggesting a dynamic role for Asp140 in shuttling protons from bulk solvent via the water network to the iron-linked oxy complex. On the basis of these structures, we consider why the D140A mutant is inactive as a heme oxygenase but active as a peroxidase.

Unexpected reactivity of two-coordinate palladium–carbene complexes; synthetic and catalytic implications

Ligand exchange reactions reveal unexpected lability of the carbene ligands in two coordinate palladium(0) N-heterocyclic carbene complexes; the latter are found to be very effective catalysts for amination of aryl chlorides.

Behavior of Palladium–Copper Catalysts for CO and NO Elimination

The behavior of a series of palladium and palladium–copper catalysts supported on ceria/alumina for the CO+NO+O2 reaction has been analyzed by a combination of electron transmission microscopy, infrared, and electron paramagnetic spectroscopies and catalytic test studies. In both systems, the catalytic behavior is dominated by the properties of the metal–ceria interface. The addition of copper to a palladium system leads to beneficial effects related to alloy formation in which both CO and NO elimination are enhanced. The chemical consequences and catalytic implications of Pd-Cu alloying are discussed.

Ultra fast and efficient synthesis of Ti-ITQ-7 and positive catalytic implications

Incorporation of germanium into a zeolite framework allows the synthesis of Ti-Ge-ITQ-7 without seeding in <12 h, instead of 12 days when seeding or >40 days without seeding for the Ge-free ITQ-7; the synthesis reported also increases the efficiency of Ti incorporation and allows the preparation of almost nanocrystaline Ti-ITQ-7 materials with a corresponding beneficial effect on the final catalytic activity.

Raman and infrared spectroscopies as in situ probes of catalytic adsorbate chemistry at electrochemical and related metal–gas interfaces: some perspectives and prospects

Some recent progress in the utilization of infrared and especially Raman spectroscopies for the in situ vibrational characterization of adsorbates at electrochemical interfaces having relevance to catalytic chemistry is briefly outlined, and illustrated by means of examples culled chiefly from our laboratory. The primary factors responsible for the differences as well as similarities in the experimental strategies pursued for metal–solution interfaces as compared with metal surfaces in gas‐phase and ultrahigh vacuum (UHV) environments are discussed, and the distinct virtues of surface‐enhanced Raman scattering (SERS) and infrared reflection‐absorption spectroscopy (IRAS) for scrutinizing the first interfacial type are assessed. The detailed influences of the electrochemical double layer on chemisorbate vibrational properties at ordered metal–solution interfaces as gleaned by in situ IRAS data in comparison with spectra for analogous “model electrochemical” interfaces in UHV are described, and briefly illustrated for carbon monoxide on Pt(111) and Ir(111). The significance of the surface potential φ in controlling chemisorbate properties on metal surfaces in gaseous and UHV as well as electrochemical environments is pointed out. Evidence for the occurrence of “redox pinning” of φ by gaseous species in ambient‐pressure systems is outlined, along with possible catalytic implications. The burgeoning prospects for utilizing SERS as a versatile as well as uniquely sensitive vibrational probe of catalytically significant, especially transition‐metal, interfaces in both electrochemical and gas‐phase environments are delineated. Emphasis is placed on the typically richer vibrational spectra attainable for SERS compared to IRAS, arising from differing surface selection rules along with the greater sensitivity and wider wavenumber ranges accessible to the former method, and exemplified by benzene adsorption on rhodium and palladium electrodes. The anticipated power of SERS for assessing the reactivity as well as identity of adsorbed intermediates in ambient catalytic systems by means of transient in situ spectral measurements is noted, and illustrated briefly for ambient‐pressure methanol oxidation on rhodium.

Oxidative functionalisation of alkanes: synthesis, molecular structure and catalytic implications of anionic vanadium(V) oxo and peroxo complexes containing bidentate N,O ligands †

A mixture of [NBu4][VO3] and pyrazine-2-carboxylic acid (Hpca) in acetonitrile catalysed smoothly the reaction of alkanes RH (R = CH3 or C6H11) with molecular oxygen (from air) and hydrogen peroxide to give the corresponding alkyl hydroperoxide ROOH as the primary product. The oxo and peroxo anions [VO2(pca)2]– and [VO(O2)(pca)2]–, isolated as the tetrabutylammonium or ammonium salts from acetonitrile solution and fully characterised by single crystal structure analyses, are assumed to be involved in the catalytic process. A screening of different N,O ligands showed Hpca to be the best co-catalyst, while anthranilic acid (Hana) proved to be almost inactive. The isolation and crystal structure analysis of the analogous oxo compound [NBu4][VO2(ana)2] showed a fundamental difference in the co-ordination of the bidentate N,O ligands which might explain the different catalytic activities: while in [VO2(pca)2]– the two pca ligands are bonded through a nitrogen and an oxygen atom (N,O co-ordination), in [VO2(ana)2]– the two ana ligands are co-ordinated via two oxygen atoms of the carboxylato group (O,O co-ordination).

Triruthenium–iridium clusters containing alkyne ligands: synthesis, structure, and catalytic implications of [(μ-H)IrRu3(CO)11(μ3-η2-PhCCPh)] and [IrRu3(CO)10(μ4-η2-PhCCPh)(μ-η2-PhCCHPh)

The mixed-metal cluster [HIrRu3(CO)13] 1 reacts with one equivalent of disubstituted alkynes RCCR to give [HIrRu3(CO)11(µ3-η2-RCCR)] (R = Ph 2; R = Me 3), with a second equivalent of the alkyne the clusters [IrRu3(CO)10(µ4-η2-RCCR)(µ-η2-RCCHR)] (R = Ph 4; R = Me 5) are obtained. The single-crystal X-ray structure analyses of 2 and 3 show these clusters to have a tetrahedral Ru3Ir framework containing the alkyne ligand coordinated in a parallel fashion over the Ru3 face of the metal skeleton. In contrast, the clusters 4 and 5 consist of a butterfly arrangement of the Ru3Ir framework with the alkyne ligand coordinated to all four metal atoms, giving an overall octahedral Ru3IrC2 skeleton, as demonstrated by the single-crystal structure analysis of 4. Cluster 1 is an excellent catalyst for the hydrogenation of diphenylacetylene to give stilbene (catalytic turnover number 990 within 15 min), clusters 2 and 4 are also catalytically active but seem to represent side-channels of the catalytic cycle.

Aluminophosphates Oxynitrides as Base Catalysts: Nature of the Base Sites and Their Catalytic Implications

By means of IR spectroscopy it has been found that Al–NH–P and P–NH2groups are formed during the high temperature NH3treatment of amorphous aluminophosphates (ALPONs). The amount of nitrogen incorporated increases with time of exposure to NH3, while also increasing the ratio of the more basic –NH– with respect to the less basic –NH2groups. This is reflected in the catalytic activities of these materials for the Knoevenagel condensation reaction using the activated methylenic groups of different pKavalues as reactants. ALPON materials have stronger basic sites than alkaline exchanged Y zeolites and weaker basic sites than MgO and Hydrotalcites. However, in the case of Knoevenagel condensation reactions the ALPONs present a very adequate basicity, especially when the competing Michael addition can also occur. In such cases the selectivities of the oxynitrides are better that those showed by MgO and hydrotalcites.

Hydrocondensation of Carbon Dioxide with Methanol Catalyzed by Anionic Ruthenium Complexes: Isolation, Structural Characterization, and Catalytic Implications of the Dinuclear Anion [Ru2(CO)4(μ2-η2-CO2CH3)2(μ2-OCH3)Cl2]-

The anionic ruthenium complex [Ru(CO)3Cl3]- (1) was found to catalyze the formation of methyl formate from CO2, H2, and CH3OH in the presence of a strong base. From the basic methanol solution, the dinuclear anion [Ru2(CO)4(μ2-η2-CO2CH3)2(μ2-OCH3)Cl2]- (2) has been isolated as the bis(triphenylphosphoranylidene)ammonium salt and characterized by a single-crystal X-ray diffraction analysis. In anion 2, two Ru(CO)2Cl units are held together by a μ2-OCH3 ligand and by two μ2-η2-OCOCH3 bridges; apart from the three bridging ligands there is no direct Ru−Ru interaction (distance 3.380 A). Complex 2 was found to react with 2 equiv of [OCH3]- to give two reactive intermediates 3 and 4, the latter one being considered as the catalytically active species in the hydrocondensation reaction. On the basis of the spectroscopic data and the isolation of aromatic analogues from the reaction of 2 with various phenolates, the constitution of [Ru2(CO)6(OCH3)4(μ2-OCH3)]- is proposed for the active species 4. The elimination ...

Thermal Stability and Reducibility of ZnO and Cu/ZnO Catalysts

The decomposition of N2O has been widely employed by catalytic scientists to selectively measure Cu surface areas in supported Cu catalysts. The present studies on binary Cu/ZnO catalysts show, however, that under highly reducing conditions the decomposition of N2O on the reduced surface of the ZnO component of the catalyst can contribute significantly to the overall N2O decomposition, leading to high apparent Cu surface areas. Furthermore, it is clear that the susceptibility of the oxide to reduction in such catalysts is substantially enhanced by the presence of the metal and that CO is a much more powerful reducing agent in this respect than H2. The origin and catalytic implications of these observations are discussed.

Acid softness and hardness in large-pore zeolites as a determinant parameter to control selectivity in orbital-controlled reactions

It has been found by means of quantum chemical calculations that by modifying the framework composition, i.e., changing the Si/Al ratio, and substitution of Al by other trivalent atoms such as Ga and B, in a given zeolite, the energy of the frontier orbitals of the zeolite changes and therefore the softness of the acid site changes. The softness increases when increasing the Si/Al ratio and also when going from B to Ga and to Al in the framework. the acid softness can play a determinant role specially in orbital-controlled reactions, the catalytic implications of the changes in the acidity softness-hardness of zeolites have been shown by carrying out the alkylation of toluene by methanol and looking into the para-ortho selectivity changes obtained when using HY zeolites with different framework Si/Al ratios and H-β zeolites with Al, Al+Ga, Ga, and B as trivalent framework elements

Catalytic Implications for Keggin and Dawson Ions: A Theoretical Study of Stability Factors of Heteropolyoxoanions

ABSTRACTHetreropolyoxoanions(HPAs) have received wide spread attention due to their unique oxidation-reduction properties in catalysis. Two main structural motifs groups of HPAs have been described by Keggin and Dawson. The first is composed of twelve distorted, octahedrallycoordinated metal atoms forming trimeric and tetrameric subunits connected such that a closed shell results. The second is formed by fusing two Keggin units. The most common Keggin ions(XM12O40n−,) with Td symmetry and the Dawson ions(X2M18O62n−) with D3h symmetry are the α-isomers; while the β-isomers are generated by rotating one trimer 60° along its C3 axis, thus producing the Cs, symmetry for β-Keggin ions and D34 symmetry for β-Dawson ions. Here, M is the framework metal atom, usually Mo, W, and X is the central heteroatom which can be the metal or nonmetal. From our previous studies, the instability of β-isomers caused by the “rotated” trimer can not be compensated by other coordination effects within the cluster and therefore defect states are retained between the HOMO-LUMO gap relative to the α-isomer. This enhances catalytic properties and causes thermal/photo lability. The catalytic function of a-Keggin ions are well studied; however, related research reports about Dawson ions are rare. The α-Dawson ion possesses stronger oxidizing ability and higher activities in oxidative dehydrogenation and oxygen addition reactions than the α-Keggin ions; the β-isomers have a greater tendency to be reduced than the α-isomers. Most of the characterization methods applied have not yielded the electronic or the structural requisites for the catalytic properties of the HPAs. In this work, the theoretical analysis is done through the application of crystal/tight binding theory coupled with the extended HMckel methodology. The stability trends and the redox behaviors provided by this work reliably trace the experimental data. In addition, the intrinsic instability and the redox effects in the β-isomers with respect to the α-isomers is assessed and a rationale for the observed catalytic and electronic develpoment of structrual function is also offered.