Bridging OH Groups Research Articles

Different Brønsted acidity of H-ZSM-5 and D-ZSM-5 zeolites revealed by the FTIR spectra of adsorbed CD3CN

The acidity of the bridging OH/OD groups in high-silica H-ZSM-5 and D-ZSM-5 zeolites was measured using CD 3 CN as an IR probe molecule. We found that the OH groups are more acidic because the ν(CN) modes of the CD 3 CN···HO complexes formed with bridging hydroxyls appeared at frequencies higher by 1.1–1.2 cm − 1 than the respective modes of CD 3 CN···DO complexes. This difference corresponds to ca. 0.2 units of the Hammett acidity function. Similar but less pronounced effect was established with the AlOH/AlOD groups (difference of 0.7 cm − 1 ), i.e., contrary to current opinions, the isotopic effect increases with the acidity strength. Adsorption of CD 3 CN clearly reveals the difference in acidity of the OH and OD groups in ZSM-5 zeolite. • Bridging OH groups in ZSM-5 are more acidic than the corresponding OD groups • The difference in acidity corresponds to 0.2 units of the Hammett acidity function • The isotopic effect decreases with the decrease of the acidity of the OH groups.

Structure and Reverse Hydrogen Spillover in Mononuclear Au0 and AuI Complexes Bonded to Faujasite Zeolite: A Density Functional Study

We have studied the structure of mononuclear gold supported on acidic form of faujasite zeolite in two oxidation states, namely, 0 and +1, using density functional theory. The binding of the gold monomer to the zeolite support is stronger in the oxidation state +1 than in the oxidation state 0. For the oxidation state 0, the hydrogenated clusters AuH/(2H)-FAU, AuH2/H-FAU generated by stepwise reverse hydrogen spillover from bridging OH groups of zeolite are energetically preferred over the Au/(3H)-FAU structure. Reverse hydrogen spillover of all the three acidic protons from the zeolite to the Au monomer did not lead to a stable structure. The calculated reverse hydrogen spillover energy per hydrogen atom for zeolite supported AuH and AuH2 clusters are −10.2 and −5.1 kJ/mol, respectively, in the oxidation state 0, while in the oxidation state +1 it is 20.9 kJ/mol for zeolite supported Au+H cluster.

Synthesis, structures, and thermolysis of new heterometallic cobalt, nickel, and copper complexes with the [Ru(NO)(NO2)4(OH)]2− anion as a ligand

Heterometallic complexes with pyridine-N-oxide (PyO), Ru(NO)(NO2)4(OH)Ni(PyO)2(H2O)] · CH3COCH3 (I), [{Ru(NO)(NO2)2(μ-NO2)2(μ-OH)Co}2(μ-PyO)] · H2O · CH3COCH (II), and [Ru(NO)(NO2)4(OH)Cu(PyO)2 (III), are isolated in the reactions of Na2[Ru(NO)(NO2)4(OH)] with nitrates of the corresponding metals in the presence of the organic ligand. The compounds synthesized are characterized by IR spectra, thermal analysis, and X-ray diffraction analysis. Depending on the M2+ cation, the ruthenium cation is coordinated through the bidentate (III, Cu2+) or tridentate (I, Ni2+ and II, CO2+) mode involving the bridging OH group and one or two NO2 groups. The thermal decomposition of complex II results in the formation of a Co0.5Ru0.5 solid solution, which is thermodynamically stable under the decomposition conditions. The thermolysis of complexes I and III in a hydrogen atmosphere leads to the formation of metastable solid solutions.

Effect of Noble Metals on the Strength of Brønsted Acid Sites in Bifunctional Zeolites

Abstract1H magic angle spinning (MAS) NMR and FTIR spectroscopy with deuterated acetonitrile and pyridine as probe molecules, respectively, were used to study the strength of Brønsted acid sites in Pt‐ and Ir‐modified zeolites Y and Beta in comparison with the materials in their H,Na forms. For both types of zeolites, the following sequence of the acid strengths was found: zeolites in their H,Na form>Pt‐modified zeolites>Ir‐modified zeolites. As the 1H MAS NMR signals of the bridging OH groups (Si(OH)Al) in the noble‐metal‐modified zeolites have the same spectroscopic properties as those observed in the materials in the H,Na forms, no direct influence of the noble metals on the nature of the hydroxyl protons or on the local structure of Si(OH)Al groups is expected. However, it is suggested that noble metals in these bifunctional zeolite catalysts are involved in charge transfer with neighboring framework atoms, which affects the mean framework electronegativity of the zeolites under study. This effect causes a variation of the acid strength of the Si(OH)Al groups similar to that observed for zeolites with different framework Al contents.

The state of Cu2+ ions in concentrated aqueous ammonia solutions of copper nitrate

Stabilization of Cu2+ ions in concentrated aqueous ammonia solutions of copper nitrate in a wide range of ammonium ion concentrations has been studied by EPR and electronic absorption spectroscopy. Three types of Cu2+ associates with different types of orbital ordering have been identified. The ammonium ion concentration in a solution has a decisive effect on the type of orbital ordering of Cu2+ ions in associates. In all cases, Cu2+ ordering in associates is caused by the existence of bridging OH groups in the axial and equatorial positions of [Cu(NH3)n(H2O)6 − n]2+ complexes (n < 6). At a high concentration of ammonium ions, weakly bound associates of tetramminecopper with the \(d_{x^2 - y^2 }\) ground state are formed. In solutions with low ammonium concentrations, bulky associates with the \(d_{y^2 }\) and \(d_{x^2 - z^2 }\) ground states and associates of Cu2+ ions with the \(d_{x^2 - y^2 }\) ground state with hydroxyl groups in the equatorial plane and axial water molecules are formed.

In situ MAS NMR investigation of the hydrogenation of acrylonitrile on Pt- and Rh-containing zeolites Y

This work presents first ex situ and in situ MAS NMR spectroscopic studies of the effect of noble metal reduction on the hydroxyl coverage of zeolites 0.9[Pt]Na-Y and 0.4[Rh]Na-Y and the hydrogenation of a double-bond-containing organic reactant, acrylonitrile. By 1H MAS NMR spectroscopy, it was found that the number of Brønsted acidic bridging OH groups (SiOHAl) formed upon noble metal reduction under flowing hydrogen nearly agrees with the number of the initially introduced complexes of bivalent metal cations. Hence, about 50% of the bivalent metal cations formed two SiOHAl groups per metal site upon reduction. After adsorption of reactants on the reduced metal-containing catalysts, no effect of the noble metals on the chemical shieldings of 13C nuclei of these molecules occurred. On the other hand, the 1H MAS NMR signals of reactant molecules adsorbed on zeolite 0.9[Pt]Na-Y have shown a significantly larger residual line width compared with those on zeolite 0.4[Rh]Na-Y, which indicates a higher mobility of reactants adsorbed on the latter zeolite. By in situ1H MAS NMR spectroscopy of the acrylonitrile hydrogenation carried out under semi-batch conditions at 295K, a reaction velocity on zeolite 0.4[Rh]Na-Y (r=7.0×10−3mmol/s) was determined, which is more than a factor six higher in comparison with that on zeolite 0.9[Pt]Na-Y (r=1.1×10−3mmol/s). During the initial period (t=12–24s) of the acrylonitrile hydrogenation on zeolite 0.4[Rh]Na-Y, weak 1H MAS NMR signals of temporary intermediates were observed. These temporary signals may be due to half-hydrogenated intermediates with short lifetime.

Reactions of VX, GD, and HD with Zr(OH)4: Near Instantaneous Decontamination of VX

Zirconium hydroxide was evaluated for the ability to detoxify chemical warfare agents GD, HD, and VX. Observed half-lives were 8.7 min, 2.3 h, and 1 min, respectively. Owing to its extremely fast reaction rate, the mechanism for VX was further characterized. Zirconium hydroxide samples were calcined at temperatures ranging from 150 to 900 °C to investigate the effect of surface speciation on VX hydrolysis rates. NMR, TGA/DSC, TEM, and potentiometric tritration reveal the importance of the acidic, bridging OH groups of Zr(OH)4 which are proposed to protonate and catalytically hydrolyze VX in a manner similar to autocatalysis by EMPA in solution.

Site-Isolated Molecular Iridium Complex Catalyst Supported in the 1-Dimensional Channels of Zeolite HSSZ-53: Characterization by Spectroscopy and Aberration-Corrected Scanning Transmission Electron Microscopy

Zeolite HSSZ-53, which has 1-dimensional channels with 14-ring extra-large pores, was used as a support for a molecular iridium complex synthesized from Ir(C2H4)2(C5H7O2) and characterized with infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies and atomic-resolution aberration-corrected scanning transmission electron microscopy (STEM). The spectra show that Ir(C2H4)2(C5H7O2) reacted readily with the bridging OH groups of the zeolite, leading to the removal of C5H7O2 ligands and the formation of mononuclear Ir(C2H4)2 complexes bonded to the zeolite by Ir–O bonds at the framework aluminum sites. STEM images confirm the spectra, showing site-isolated iridium centers within the zeolite channels, with no evidence of iridium clusters. The samples constitute a highly uniform, well-defined array of essentially molecular catalytic species in a highly uniform, confined environment, allowing precise investigations of the chemistry of the iridium complex in the absence of solvents. IR s...

Fischer Tropsch synthesis: Deuterium isotopic study for the formation of oxygenates over CeO2 supported Pt–Co catalysts

The effect of deuterium on Fischer-Tropsch (FT) synthesis was investigated over a Pt promoted Co/CeO2 catalyst. CeO2 supported Pt–Co catalyst is more selective towards methane and oxygenates. H2–D2–H2 switching experiments suggest that the hydrogenation of CO is involved in the rate determining step (inverse isotope effect). The suppression of oxygenate formation with deuterium leads to the conclusion that common FT intermediates are responsible for the observed shift in the product spectrum. The deuterium isotopic study supports our previously proposed mechanism for the formation of alcohols in that the Co–CeO2 interface is involved in the catalysis, such that termination of chain growth occurs at the metal–oxide junction and involves bridging OH groups on partially reduced ceria.

Density functional investigation of reverse hydrogen spillover on zeolite supported Pd6 and Au6 clusters

The present density functional study investigates the stepwise reverse hydrogen spillover from bridging OH groups of the faujasite support to Pd6 and Au6 clusters. Calculations reveal that the process of proton transfer from the bridging OH groups results in hydrogenated clusters, M6Hn/FAU(3−n); M=Pd, Au and n=1–3, which are energetically preferable over faujasite supported bare clusters, M6/FAU(3H). The calculated reverse hydrogen spillover energy per transferred hydrogen atom for faujasite supported Pd6H3 and Au6H3 are −29.33kJmol−1 and −24.50kJmol−1, respectively. For both the metals, hydrogen migration results in partial oxidation of the metal atoms with concomitant reduction of H+ to H−.

Low Temperature Water Gas Shift: Evaluation of Pt/HfO2 and Correlation between Reaction Mechanism and Periodic Trends in Tetravalent (Ti, Zr, Hf, Ce, Th) Metal Oxides

It is well-known that catalysts containing small quantities of noble metals (e.g., Pt, Au) that are highly dispersed on Group IV and related tetravalent metal oxides (e.g., TiO2, ZrO2, CeO2, ThO2) display, once activated, high conversion for water-gas shift (WGS). The mechanism suggested for this by many authors is bimolecular in nature. In one view, adsorption of CO on the partially reducible oxide produces formates by reaction with active bridging OH groups, followed by water-assisted formate decomposition by dehydrogenation, with this second step being facilitated by metal particles at the metal–support interface. Considering this group of oxides, HfO2, as far as we are aware, had not yet been studied as a potential candidate for low temperature water-gas shift (LTWGS). Thus, in this contribution, catalysts composed of Pt supported on HfO2 were prepared and tested. Catalyst testing results suggest that this system is, like Pt/ZrO2 and Pt/CeO2, active for the WGS reaction, although additional studies ar...

Mechanisms of Hydrolysis–Oligomerization of Aluminum Alkoxide Al(OC3H7)3

As one of the representative superinsulating materials, the aluminum trioxypropyl Al(OC(3)H(7))(3) aerogel may be applied in launch vehicles and manned spacecrafts. In this study, the structures and hydrolysis mechanisms of the monomer, dimers, and trimers of Al(OC(3)H(7))(3) in neutral and alkaline environments were studied at the B3LYP/6-31G(d,p) level by using the CPCM solvation model to understand the fundamental chemistry of Al(OC(3)H(7))(3) hydrolysis and oligomerization. Our calculation shows that the first-order hydrolyses of the monomer and oligomers are energetically favorable in both alkaline and neutral solutions. In alkaline solutions, they are more apt to oligomerize than to hydrolyze due to high energy barriers and large binding energies in the formation of anionic species. For the oligomers under neutral condition (1) Al(OC(3)H(7))(3) is linked by four-membered Al-O rings with pentacoordinated bridging and tetracoordinated Al atoms, (2) the hydrolyzed propoxy groups will be expelled by solvent molecules, and (3) partly hydrolyzed species can condense to oligomers with bridging OH groups or O atoms.

Interaction of benzene with hydroxyl groups in zeolites: A FTIR study of C 6H 6 and C 6D 6 adsorption on H-ZSM-5 and D-ZSM-5

Adsorption of benzene on H-ZSM-5 leads, at low coverages, mainly to formation of 1:1 adducts with the zeolite bridging hydroxyl groups (3612 cm −1). As a result, the O–H modes are shifted to two discrete bands with maxima at ca. 3350 and 3250 cm −1. The appearance of two bands is attributed to Fermi resonance of the shifted O–H stretchings with the 3δ(OH) modes. Small amounts of benzene molecules interacting simultaneously with two hydroxyls are also observed. At high benzene coverages ordered structures are formed. As a result, the shifted band appears at 3175 cm −1, a value misleading to overestimated acidity. In addition, a part of benzene with time turns bonded to bridging OH groups via hydrogen atoms thus producing another shifted band at ca. 3500 cm −1. Small amount of Al–OH species were also present (band at 3663 cm −1). It was concluded that the O–H stretchings of these groups were shifted by ca. 225 cm −1 upon benzene adsorption. Silanol groups (3745–3700 cm -1) are heterogeneous and produce two shifted bands at 3625 and 3585 cm −1. Adsorption of C 6D 6 leads to similar results. The shifted O–H bands coincide in position with those produced by adsorption of C 6H 6. In this case, however, D–H exchange of the bridging OH groups easily occurs. The C 6D 6-induced shift of the O–D modes of the bridging deuteroxyls is lower than expected taking into account the H–D isotopic shift factor. In addition, the ratio between the intensities of the two bands and the split value also differ. These results confirm the occurrence of Fermi resonance.

H-Bonding of Zeolite Hydroxyls with Weak Bases: FTIR Study of CO and N2Adsorption on H-D-ZSM-5

Adsorption of CO (13CO) and 15N2 at 100 K on H-ZSM-5 samples has been followed by FTIR spectroscopy. For better interpretation of the spectra, experiments with H-D-ZSM-5 and D-ZSM-5 were also carried out. It was established that zeolite bridging hydroxyls displaying at 100 K a band at 3616 cm−1 formed 1:1 adducts with CO, and as a result, the band disappeared, and two new bands at 3306 and 3415 cm−1 emerged at its expense. These two bands changed in concert together with a carbonyl band at 2175 cm−1. Experiments with H-D-ZSM-5 and D-ZSM-5 indicated that the OD band due to bridging deuteroxyls (2667 cm−1) was shifted, upon CO adsorption, again to two IR bands (at 2460 and 2568 cm−1). However, the shift is smaller than expected from the OH → OD isotopic shift factor, which indicates a smaller acidity of OD groups as compared with OH. In addition, the difference in the positions of the two bands (calculated again on the basis of the isotopic shift factor), as well as their relative intensities, deviate from those observed for the OH bands. The results exclude the phenomenon to be due to heterogeneity of the bridging OH/OD groups and indicate a spectral origin. It was concluded that the appearance of two shifted bands was due to Fermi resonance with the second excitation mode of δ(OH). At high CO equilibrium pressure solvatation occurs, and the band at 3306 cm−1 is shifted to lower frequencies. Simultaneously, a shift of the carbonyl band to 2172 cm−1 was observed. At intermediate CO coverages, AlOH···CO complexes are also formed. It is clearly demonstrated that the CO-induced shift of the AlO−H band at 3667 cm−1 is ∼−190 cm−1, and the often reported higher values are due to confusion with the 3415 cm−1 band described above. The AlOD groups have been found to manifest a lower acidity than that of AlOH. Here again, solvatation occurred at high CO coverages. Another effect observed at high CO coverages is the interaction of CO with silanol groups. Two kinds of interaction are unambiguously distinguished: (i) the well documented CO-induced shift of isolated silanols (3745 cm−1) to 3650 cm−1 and (ii) a shift of silanols absorbing in the 3730−3710 cm−1 region to 3580 cm−1. No solvatation was evidenced in this case. Experiment on low temperature adsorption of 15N2 confirmed that the bridging OH(OD) groups of H-ZSM-5 were homogeneous. Upon 15N2 adsorption, the 3616 cm−1 OH band shifted by 118 cm−1, and the respective OD band at 2667 cm−1, by 81 cm−1. Solvatation was detected at high coverages. The 15N2 induced shift of AlO−H modes was found to be 77 cm−1, and for AlO−D, 51 cm−1. Here again, two kinds of interaction with two different silanols were established. A good correlation between the shifts produced by the two probe molecules of different OH and OD groups was found.

Dissociation of methanol on hydroxylated TiO 2-B (1 0 0) surface: Insights from first principle DFT calculation

The adsorption of methanol on hydroxylated TiO 2-B (1 0 0) surface with bridging and terminal hydroxyl groups has been studied by first principle calculations. On both clean and hydroxylated surfaces with bridging OH group (OH br), the O–H bond scission is the most favorable dissociation of methanol and the C–O bond scission is also feasible. This indicates the OH br has little influence on the adsorption of methanol. The terminal OH group (OH t) plays a major role in the C–H scission of methanol, which is important for the applications associated with the direct use of hydrogen, such as in situ hydrogenation, and hydrogen generation via the photocatalytic reaction. The dissociative adsorption of methanol via C–H scission, which is an endothermic adsorption on other TiO 2 surfaces, is identified as exothermic adsorption with adsorption energy in the range of −1.54 eV to −1.91 eV around OH t on TiO 2-B (1 0 0) surface. The lowest activation barrier for C–H scission is ∼0.80 eV, which is lower than the release heat of molecular adsorption. Moreover, the hydrogen atoms in methanol are easily transferred to the OH t and then move to nearby O 2c sites to regenerate the hydroxyl group. This proton migration process could result in extra stable chemi-sorption of methanol with an adsorption energy as low as −2.23 eV, which is above twice that of methanol molecularly adsorbed on the surface. Thus, the proton channel feature of OH t on the surface is borne out by our calculations.

Steam and CO2 reforming of ethanol over Rh/CeO2 catalyst

The performance of ceria supported Rh catalysts prepared with high and low surface areas for steam reforming and dry reforming of ethanol was investigated. According to results of diffuse reflectance infrared Fourier transform spectroscopy of both reactions, dissociative adsorption of ethanol over ceria gives rise to ethoxy species and bridging OH groups. Oxidation of ethoxy species proceeds by addition of O provided by the support from species such as Type II bridging OH groups (i.e., steam reforming) or O adatoms (e.g., CO2 reforming). Rh promotes demethanation of acetate to carbonate and the steam/dry reforming of CHx species.Catalyst deactivation occurs from the deposition of carbon on the catalysts during both reactions. However, no carbon formation is observed on the Rh/CeO2 high surface area catalyst during steam reforming due to a higher surface fugacity of O from species adsorbed on the support that react to remove carbon.

ChemInform Abstract: Nanosheets as Highly Active Solid Acid Catalysts for Green Chemical Syntheses

AbstractReview: 163 refs.

Fischer–Tropsch synthesis: Metal–support interfacial contact governs oxygenates selectivity over CeO 2 supported Pt–Co catalysts

The catalytic performance during Fischer–Tropsch synthesis (FTS) of Pt promoted Co/CeO 2 catalysts at two different cobalt metal loadings was investigated using a slurry-phase reactor. CeO 2 supported Pt–Co catalysts exhibit higher selectivities to oxygenates and especially linear alcohols (14.8%) at lower cobalt loadings. A plausible pathway for the formation of alcohols was proposed based on Co–CeO 2 interfacial catalysis, in which termination of chain growth occurs at the metal-oxide junction and involves bridging OH groups on partially reduced ceria. As cobalt content increases, the selectivity to oxygenates decreases. The resulting increase in cobalt metal particle size accelerates the mechanism involving normal FT termination, since the on-top surface area of cobalt increase at the expense of the interfacial area at the junction with ceria. Compared to γ-Al 2O 3, CeO 2 supported Pt–Co catalysts exhibit significantly higher oxygenates at similar CO conversions. Selectivity control is thus achieved by controlling the surface fugacity of bridging OH groups (i.e., located on ceria) in contact with cobalt sites on a per Co surface atom basis.

The effect of support reducibility on the stability of Co/CeO 2 for the oxidative steam reforming of ethanol

Ceria plays an active catalytic role in removing carbon from the catalyst by a support-mediated cleaning mechanism. Increasing the support surface area using a novel preparation method led to improved catalyst stability and a lower coking rate. DRIFTS of adsorbed ethanol shows that oxygen from the support facilitates the formation of acetate intermediate species, thus demonstrating the ability of the support to donate oxygen. This oxygen may come from either O adatoms by adsorption of O 2 at vacancies, whereby the cerium atoms involved are Ce 4+. Or, it may come from vacancy-associated Type II bridging OH groups, where the cerium atoms involved are Ce 3+. The higher oxygen/OH group mobility of high ceria surface area promotes the mechanism of carbon removal, which in turn contributes to the high stability of Co/CeO 2 catalyst.

ChemInform Abstract: High-Temperature Interaction of Solid Cu Chlorides and Cu Oxides in Mixtures with H-Forms of ZSM-5 and Y Zeolites.

A high-temperature solid-state ion exchange between separate phases of solid Cu chlorides and hydrogen forms of ZSM-5 and Y zeolites occurs above 570 K and reaches substantial degrees at 770 and 670 K, respectively, i.e. at temperatures at which considerable self-dehydroxylation of the particular zeolite does not take place. The process has been followed using IR, EPR and MS techniques through the evolution of hydrochloric acid into the gaseous phase, the consumption of the bridging OH groups in the zeolite as well as by the formation of isolated Cu ions that exhibit strong electron-accepting properties. The exchange rate is very fast in the initial stage, being proportional to the amount of copper salt in the mixture, and levels off to zero after several hours. Owing to the more difficult deaggregation of Cu oxides compared to Cu chlorides, the interaction of Cu oxides with H-ZSM-5 and H-Y at 770 K and 670 K, respectively, results in a very limited consumption of the bridging OH groups as well as in a decreased formation of isolated Cu ions and electron-acceptor sites compared with the corresponding Cu chloride mixtures. Besides the ability of the copper phases (chlorides or oxides) to deaggregate, a further striking factor affecting the exchange reaction is the mobility of the zeolite protons and exchanged cations, which is controlled by the structure type of the zeolite and by the temperature. This is clearly reflected by the positive effect of the temperature on the exchange process and the higher degree of exchange (at a given temperature) found for H-Y zeolites compared with H-ZSM-5.