Characterization of acid properties of [Al]- and [Ga]-HZSM-5 zeolites by low temperature Fourier transform infrared spectroscopy of adsorbed carbon monoxide

[Al]- and [Ga]-HZSM-5 having closely similar acid site densities were prepared. Temperature-programmed desorption of ammonia (TPDA) proved the known acid strength sequence: [Al]>[Ga]. Frequency shifts of the OH stretching mode to lower frequencies were induced upon CO adsorption at 77 K due to OH CO H-bonding interactions. These frequency shifts are a measure of the H-bond donor strength of the OH groups and hence, of their acidic strength. The observed shifts of 313 and 282 cm^–1 for the [Al]- and [Ga]-silicate, respectively, clearly reflect their relative acid strength. [Ga]-HZSM-5 develops an acid strength similar to that found for HNaY at high exchange levels. Evidence is presented for an intrinsic heterogeneity of acid properties of the acidic hydroxyls probably caused by inhomogeneous distributions of the trivalent cations in the framework.On leave from Institute of Petroleum and Chemistry, M. Azizbekov, Baku, Azerbeidjan.     

NO Adsorption on Ex-Framework [Fe,X]MFI Catalysts: Novel IR Bands and Evaluation of Assignments

IR spectra of NO adsorbed on isomorphously substituted [Fe,Al]MFI, [Fe,Ga]MFI and [Fe]MFI after steaming at 873 K in 30 vol% H₂O are presented. On ex-[Fe,Al]MFI, NO adsorption leads to bands at 2133 cm^-1 and a doublet at 1886 and 1874 cm^-1. The 2133 cm^-1 band is assigned to NO⁺ occupying cationic positions in the zeolite structure. Of the doublet, the 1874 cm^-1 band is much more susceptible to reaction with O₂ than the 1886 cm^-1 band, yielding adsorbed NO₂ with an absorption frequency of 1635 cm^-1. After evaluation of the constitution of the catalyst and (sometimes contradictory) literature assignments, the 1886 cm^-1 band is assigned to NO adsorbed on Fe ions located in isolated positions, and/or (FeO)_n clusters inside the zeolite channels, whereas the 1874 cm^-1 band is proposed to be induced by 2 nm FeAlO_x nano-particles. The ex-[Fe,Ga]MFI catalyst showed a similar absorption pattern (doublet), which is shifted to lower wavenumbers (1881 and 1867 cm^-1), suggesting that both frequencies are affected by the vicinity of Ga (or Al) to the Fe site involved. The absence of bands at 1765 and 1835 cm^-1 suggests that the isolated sites causing these absorptions are in the Fe^III state in ex-[Fe,Al]MFI and ex-[Fe,Ga]MFI. For the ex-[Fe]MFI sample, which did not contain any 2 nm FeO_x nano-particles, an NO absorption band at 1854 cm^-1 is assigned to mono-nitrosyl on extra-framework oligonuclear (Fe^IIO)_n species in the zeolite channels.     

2-[1-(2,6-Dibenzhydryl-4-chlorophenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridyliron(II) dichlorides: Synthesis,characterization and ethylene polymerization behavior

A series of 2-[1-(2,6-dibenzhydryl-4-chlorophenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridine ligands (L1–L5) as well as the ligand 2,6-bis[1-(2,6-dibenzhydryl-4-chloro-phenylimino)ethyl]pyridine (L6) were synthesized and reacted with FeCl₂·4H₂O to afford the iron(II) dichloride complexes [LFeCl₂] (Fe1–Fe6). All new compounds were fully characterized by elemental and spectroscopic analysis, and the molecular structures of the complexes Fe1, Fe2 and Fe4 were determined by single-crystal X-ray diffraction, which revealed a pseudo-square-pyramidal geometry at iron. Upon activation with either MAO or MMAO, all iron pre-catalysts exhibited very high activity in ethylene polymerization with good thermal stability. To the best of our knowledge, the current system showed the highest activity amongst iron bis(imino)pyridine pre-catalysts reported to-date. The polymerization parameters were explored to determine the optimum conditions for catalytic activity, which were typically found to be 2500 eq. Al to Fe at 60 °C in the presence of MMAO, and 80 °C in the presence of MAO. The resultant polyethylene possessed a narrow molecular polydispersity index (PDI) consistent with the formation of single-site active species.     

Hexachloroplatinate(IV) anion-induced cucurbit[5]uril and cucurbit[8]uril supramolecular assemblies with linear channels

Two Q[n]-based porous compounds, Q[5]·[PtCl₆]^2 −·2H₃O·12H₂O and 2Q[8]·[PtCl₆]^2 −·2H₃O·74H₂O in cooperating the hexachloroplatinate(IV) anion ([PtCl₆]^2 −) as an inorganic structure directing agent are demonstrated. The driving forces for the formation of such novel Q[n]-based porous compounds are considered to be the outer surface interactions of Q[n]s, including dipole interactions between Q[n]s and ion-dipole interactions between [PtCl₆]^2 − anions and Q[n]s. Moreover, the Q[5]-based porous compound displays absorption distinctness for tetrachloromethane, whereas the Q[8]-based porous compound displays absorption distinctness for methanol.     

Tunable activity of [Ga]HZSM-5 with H2 treatment: Ethane dehydrogenation

Ethane dehydrogenation over [Ga]HZSM-5 catalyst were studied to probe activity of various gallium species (Ga₂O₃, GaO⁺, Ga⁺, and GaH₂⁺) in a pulsed reactor. H₂-TPR was used for characterizing the reducible Ga species. It was found that the activity of [Ga]HZSM-5 can be regulated by the H₂ treatment. Among the various Ga species, the H₂-incorporated species (i.e. GaH₂⁺) displays the highest dehydrogenation activity (~ 55% conversion) under the H₂ stream. However, this so-called GaH₂⁺ can readily decompose in the absence of H₂, presumably to Ga⁺ that give a lower ethane conversion (~ 30%). While the Ga oxide species, which may well be retained as isolated Ga₂O₃ and exchangeable GaO⁺, provide the lowest dehydrogenation activity (~ 25% conversion). The lower activity is observed with an increase in the Si/Al ratio of the host zeolite, suggesting that the active site is associated with the negative framework charge, presumably as a charge balancing cation (i.e. GaH₂⁺, Ga⁺, GaO⁺). In addition, treatment with hydrogen and steam can readily disperse occluded Ga₂O₃ into exchangeable Ga⁺ and GaO⁺, respectively.     

Crystallization of [Ga]-MFI Under Atmospheric Pressure

The crystallization of [Ga]-MFI was investigated as a function of synthesis time under atmospheric pressure. The molar composition of the reactants was 100SiO₂–Ga₂O₃–11Na₂O–11TPABr–3500H₂O. The crystallinity of [Ga]-MFI was examined using several analytical instruments, such as XRD, XPS, XRF, FT-IR, solid state MAS-NMR, and DTG/DTA. [Ga]-MFI was successfully synthesized under atmospheric pressure at 97°C in 72 h. It was found that the nucleation of [Ga]-MFI took quite a long time, but the crystallization took place very fast. It is supposed that the nucleation is the rate-controlling step in [Ga]-MFI synthesis under atmospheric pressure. Consequently, if the induction period of the nucleation can be shortened, it would be possible to synthesize [Ga]-MFI commercially under atmospheric pressure.     

Aromatization of pentane catalyzed over various metallosilicates

Various metallosilicates were synthesized using a hydrothermal method and characterized by SEM, XRD,²⁹Si MAS NMR, chemical analysis and surface area measurements. These results showed that they had a MFI structure. The pentane aromatization reaction was carried out over these metallosilicates in a continuous flow reactor at 550 °C, He/pentane=3, WHSV=1.5 h⁻¹ and 1 atm. Among the various metallosilicates, [Ga]ZSM-5(20) (52.3%) and [Zn]ZSM-5(40) (37.6%) showed higher aromatic selectivities for pentane aromatization. When [Al] ZSM-5(40) was ion-exchanged with gallium nitrate and zinc chloride, the selectivities for aromatics increased from 23.0% to 35.5% and to 32.7%, respectively. The Si/metal mole ratios of [Ga]ZSM-5 and [Al]ZSM-5 were changed from 20 to 250 and NH₃ temperature programmed desorption (TPD) was carried out. As the Si/metal ratio was changed from 250 to 20, the selectivities for aromatics were increased from 5.3 % to 52.3 % over [Ga]ZSM-5 and from 10.1% to 25.7% over [Al]ZSM-5. NH₃ TPD of [Ga]ZSM-5 indicated that the sites of medium acidity play an important role in the formation of aromatics. When H₂ and CO were added to the reactant of pentane, the production of methane and ethane increased and that of aromatics decreased.     

Synthesis of the [Ga]-MFI under atmospheric pressure

The crystallization of the [Ga]-MFI was investigated as a function of synthesis time under atmospheric pressure. The molar composition of the reactants was 100SiO₂-Ga₂O₃-llNa₂O-llTPABr-3500H₂O. The crystallinity of the [Ga]-MFT was examined by using several analytical instruments, such as XRD, XPS, XRF, FT-IR, solid-statemas-NMR, DTG/DTA, and SEM. The [Ga]-MFI was successfully synthesized under atmospheric pressure at 97 ‡C in 72 h. It was found that the nucleation of the [Ga]-MFI took a quite long time, but the crystallization took place very fast. It is supposed that nucleation is the rate-controlling step in the [Ga]-MFI synthesis under atmospheric pressure. Consequently, if the induction period of the nucleation can be shortened, it would be possible to synthesize the [Ga]-MFI commercially under atmospheric pressure.     

Synthesis and structure of the cubane type [Fe2Sb(CO)5(η5-C5H5)]4 cluster

The [Fe₂Sb(CO)₅Cp]₄ (Cp=η⁵-C₅H₅) cluster has been obtained by the thermolysis of [Et₄N][{Fe(CO)₄}₂{Fe(CO)₂Cp}SbI]. The structure of the [Fe₂Sb(CO)₅Cp]₄·2toluene solvate has been determined by X-ray analysis. The cluster contains the cubane [Fe₄Sb₄] core. Each Sb serves as a μ₄-ligand between three iron atoms of the core and one of a terminal [Fe(CO)₂Cp] group.     

Protonierung und Solvation schwacher organischer Basen. VII. 1H-NMR-spektroskopische Bestimmung der Basizität von cyclischen Ketonen

Protonation and Solvation of Weak Organic Bases. VII. ¹H-N.M.R. Spectroscopic Determination of the Basicity of Cyclic Ketones From protonation ratios for cycloalkanones of ring size C₄-C₇ in the acid system H₂O H₂SO₄ HSO₃F, determined by ¹H-n.m.r. spectroscopy, basicity and solvation parameters are estimated by application of extrapolation procedures based on the Hammett acidity function H₀ or the excess acidity X. The frequency shifts of the n̈_OH stretching vibrations of phenol in hydrogen bonded complexes with cycloalkanones have been measured. The Δν_OH values are related to the basicity parameter (H₀)_1/2.     

Complexation and electrochemical study of 1,5-diselena[5]ferrocenophane: X-ray crystal structures of 1,5-diselena[5]ferrocenophane,and silver,mercury complexes

The silver(I) and mercury(II) complexes of 1,5-diselena[5]ferrocenophane (L) have been synthesized. The X-ray crystal structures of L, [AgL₂]PF₆ and [HgI₂L] are reported. The small cavity of the macrocyclic ligand means that the ring must undergo a conformational change to allow coordination, and there is no significant through-space interaction M?Fe in any of the complexes. Electrochemical studies showed that no electronic communication between ferrocenylene groups was observed in the silver complex where the through-bond Fe···Fe distances are in the range 13.10 (3)–13.27(3) Å.     

[Fe]-Hydrogenase,Cofactor Biosynthesis and Engineering

[Fe]-hydrogenase catalyzes the heterolytic cleavage of H₂ and reversible hydride transfer to methenyl-tetrahydromethanopterin. The iron-guanylylpyridinol (FeGP) cofactor is the prosthetic group of this enzyme, in which mononuclear Fe(II) is ligated with a pyridinol and two CO ligands. The pyridinol ligand fixes the iron by an acyl carbon and a pyridinol nitrogen. Biosynthetic proteins for this cofactor are encoded in the hmd co-occurring (hcg) genes. The function of HcgB, HcgC, HcgD, HcgE, and HcgF was studied by using structure-to-function analysis, which is based on the crystal structure of the proteins and subsequent enzyme assays. Recently, we reported the catalytic properties of HcgA and HcgG, novel radical S-adenosyl methionine enzymes, by using an in vitro biosynthesis assay. Here, we review the properties of [Fe]-hydrogenase and the FeGP cofactor, and the biosynthesis of the FeGP cofactor. Finally, we discuss the expected engineering of [Fe]-hydrogenase and the FeGP cofactor.     

Protection by different agents against inactivation of lipoxygenase by hydrogen peroxide

H₂O₂ is a potent inactivator of lipoxygenase. In his paper, the ability of different agents [mannitol, oleic, stearic and linoleic acid,n-butanol, and hydroperoxy octadecadienoic acid (HPOD)] to prevent the inactivation of tomato lipoxygenase by hydrogen peroxide has been studied. The involvement of OH^· in the inactivation process is suggested by the ability of mannitol to prevent the loss of activity. This radical would be produced by reaction of H₂O₂ with the Fe(II) lipoxygenase. The most effective protection was displayed by HPOD, the product of the reaction of lipoxygenase with linoleic acid. This result could be explained by the conversion of the native enzyme into the Fe(III) lipoxygenase in the presence of HPOD; the Fe(III) enzyme is not able to react with H₂O₂, and no OH^· will be produced. The protective effect obtained with oleic and stearic acid could be explained by an occupation of the active center by these inhibitors. The enzyme would not transform them, but their presence would hamper the conversion of H₂O₂ in OH^· and limit the damage in the active center.     

Mechanism and Application of the Catalytic Reaction of [NiFe] Hydrogenase: Recent Developments

Hydrogenases (H₂ase) catalyze the oxidation of dihydrogen and the reduction of protons with remarkable efficiency, thereby attracting considerable attention in the energy field due to their biotechnological potential. For this simple reaction, [NiFe] H₂ase has developed a sophisticated but intricate mechanism with the heterolytic cleavage of dihydrogen, where its Ni−Fe active site exhibits various redox states. Recently, new spectroscopic and crystal structure studies of [NiFe] H₂ases have been reported, providing significant insights into the catalytic reaction mechanism, hydrophobic gas-access tunnel, proton-transfer pathway, and electron-transfer pathway of [NiFe] H₂ases. In addition, [NiFe] H₂ases have been shown to play an important role in biofuel cell and solar dihydrogen production. This concept provides an overview of the biocatalytic reaction mechanism and biochemical application of [NiFe] H₂ases based on the new findings.     

Syntheses,crystal structures and properties of self-assembly supramolecular compound based on cucurbit[6]uril

A new cucurbit [6]uril based compound {[Cd₂Cl₂(INA)₂(HCOO)₂(H₂O)₂](CB [6])·2(HCOOH)·5H₂O}(CCUT-101) was synthesized by the solvothermal reaction of cadmium chloride, cucurbit[6]uril (CB[6]), and isonicotinic acid (HINA). X-ray diffraction analysis indicated that coordination of INA⁻ with the Cd^2 + resulted in the formation of some one-dimensional wave-like chains, which connected with CB[6] by π ⋯ π stacking and hydrogen bonding, and further extended to a 2D layer. The luminescence behaviors and sensing properties of CCUT-101 in different solvents were carried out.     

Pyryliumverbindungen. XXVII. Spezifisch deuterierte Carbo- und Heterocyclen via 2,4,6-Triaryl-[3,5-2H2]pyryliumsalze

Pyrylium Compounds. XXVIII. Specifically Deuterated Carbo- and heterocycles via 2,4,6-Triaryl[3,5-²H₂]pyrylium Salts On heating with catalytic amounts of bases(e.g. triethyl amine) in deuterated alcohols such as methan[²H]ol or ethan[²H]ol pseudobases of 2, 4, 6-triarylpyrylium salts 1 undergo fast¹H/²H isotopic exchange reaction affording 1, 3, 5-triaryl[2, 4, 4-²H₃]pent-2-ene-1,5-diones which with [²H] perchloric acid give highly deuterated 2, 4, 6-triaryl-[3, 5-²H] pyrylium perchlorates 8 . These salts are obtainable also directly from 1 through a one-pot procedure by ring opening of the latter with deuterium oxide under the above-mentioned¹H/²H isotopic exchange conditions followed by recyclization of the formed 1, 3, 5-triaryl[2, 4, 4-²H₃]pent-2-ene-1, 5-diones with [²H]ClO₄. Ring transformations of 2, 4, 6-triphenyl[3, 5-²H²]pyrylium perchlorate (8a) to 2, 4, 6-triphenyl[3, 5-²H₂]nitrobenzene (9) 2, 4, 6-triphenyl[3, 5-²H₂]pyridine (10) , 1, 2, 4, 6-tetraphenyl[3, 5-²H₂]pyrydinium perchlorate (11) , 2, 4, 6-triphencyl[3, 5-²H₂]thiopyrylium perchlorate (12) , 2-benzoyl-3, 5-diphenyl[4-²H]furan (13) , and 3, 5, 7-triphenyl-[4, 4, 6-²H₃]4H-1, 2-diazepin (14) demonstrate the usability of pyrylium salts of type 8 as starting materials for syntheses of specifically deuterated carbo- and hetrocycles.     

Application of a Synthetic Ferredoxin-Inspired [4Fe4S]-Peptide Maquette as the Redox Partner for an [FeFe]-Hydrogenase

‘Bacterial-type’ ferredoxins host a cubane [4Fe4S]^2+/+ cluster that enables these proteins to mediate electron transfer and facilitate a broad range of biological processes. Peptide maquettes based on the conserved cluster-forming motif have previously been reported and used to model the ferredoxins. Herein we explore the integration of a [4Fe4S]-peptide maquette into a H₂-powered electron transport chain. While routinely formed under anaerobic conditions, we illustrate by electron paramagnetic resonance (EPR) analysis that these maquettes can be reconstituted under aerobic conditions by using photoactivated NADH to reduce the cluster at 240 K. Attempts to tune the redox properties of the iron-sulfur cluster by introducing an Fe-coordinating selenocysteine residue were also explored. To demonstrate the integration of these artificial metalloproteins into a semi-synthetic electron transport chain, we utilize a ferredoxin-inspired [4Fe4S]-peptide maquette as the redox partner in the hydrogenase-mediated oxidation of H₂.     

N2O decomposition over [Fe]-ZSM-5 and Fe-HZSM-5 zeolites

The N₂O decomposition over an [Fe]-ZSM-5 and an Fe-HZSM-5 zeolite was studied. We found that framework incorporated iron species were much more active than Fe(III) introduced as framework charge countercations by ion exchange (TOF at 0.1 vol% N₂O:1.47 × 10^–4 at 280°C for [Fe]-ZSM-5 vs. 2.58 × 10^–4 at 468°C for Fe-HZSM-5). The higher activity of [Fe]-ZSM-5 was attributed to the uniqueness of framework iron species. Both [Fe]-ZSM-5 and Fe-HZSM-5 zeolites showed enhanced activity in the presence of excess oxygen. This is in sharp contrast to ruthenium exchanged zeolites which showed strong oxygen inhibiting effect on the rate of N₂O decomposition.     

A density functional study of CO adsorption on three- and five-coordinate Al in oxide systems

Conventional cluster models of strong and medium strength Lewis acid sites in alumina and zeolites, three-coordinate [Al(OH)₃] and five-coordinate [Al(OH)₃(OH₂)₂] respectively, are studied with the help of a density functional method. A constraint space orbital variation analysis reveals that the charge transfer from probe CO molecules adsorbed on cationic Al centre and the CO polarization comprise essential contributions to the adsorption energyD _e. An analysis of the adsorption-induced C-O frequency shift(C-O) is also provided. Structural modifications of the Lewis acid sites are considered with respect to their influence onD _e and(C-O). A comparison of the measured and calculated C-O frequency shifts supports a hypothesis on the existence of Lewis acid sites in oxides in the form of four-coordinate Al cations.     

The [4Fe-4S]-Cluster of HydF is not Required for the Binding and Transfer of the Diiron Site of [FeFe]-Hydrogenases

The active site of [FeFe]-hydrogenases contains a cubane [4Fe-4S]-cluster and a unique diiron cluster with biologically unusual CO and CN⁻ ligands. The biogenesis of this diiron site, termed [2Fe_H], requires the maturation proteins HydE, HydF and HydG. During the maturation process HydF serves as a scaffold protein for the final assembly steps and the subsequent transfer of the [2Fe_H] precursor, termed [2Fe_P], to the [FeFe]-hydrogenase. The binding site of [2Fe_P] in HydF has not been elucidated, however, the [4Fe-4S]-cluster of HydF was considered as a possible binding partner of [2Fe_P]. By targeting individual amino acids in HydF from Thermosipho melanesiensis using site directed mutagenesis, we examined the postulated binding mechanism as well as the importance and putative involvement of the [4Fe-4S]-cluster for binding and transferring [2Fe_P]. Surprisingly, our results suggest that binding or transfer of [2Fe_P] does not involve the proposed binding mechanism or the presence of a [4Fe-4S]-cluster at all.