Acidic OH Groups Research Articles

FT-IR study of NO + O2 co-adsorption on H-ZSM-5: re-assignment of the 2133 cm-1 band to NO+ species

Whereas NO adsorption at room temperature on activated H-ZSM-5 (Si/Al = 29) caused only negligible changes in its IR spectrum, addition on O2 to NO led to the appearance of bands at 2133 and 977 cm-1. Concomitantly, the number of acidic zeolite OH groups decreased while H2O hydrogen-bonded to zeolite OH groups developed. Introduction of small amounts of 18O2 did not change the 2133 cm-1 band wavenumber, nor the use of a partly deuteroxylated D–H-ZSM-5 sample. In such a case, HOD formation was detected. The results obtained evidence that the 2133 cm-1 band, generally considered as characterizing NO+2 species, is, in fact, due to NO+ species occupying cationic positions in the zeolite. The 977 cm-1 band is attributed to the Olattice–NO+ vibration. A scheme of the NO+ formation, involving NO2 molecules as NO oxidizing agent, is proposed.

1,4,5,8-naphthalenetetracarboxylic acid cyclic 1,8-anhydride bis(dimethyl sulfoxide) solvate and 1,4,5,8-naphthalenetetracarboxylic 1,8:4,5-dianhydride.

1,4,5,8-Naphthalenetetracarboxylic acid cyclic 1,8-anhydride crystallized from dimethyl sulfoxide (DMSO) as the solvate, C14H6O7.2C2H6OS, in the centrosymmetric space group P1. Two O-H...O hydrogen bonds with O...O distances of 2.592 (3) and 2.598 (3) A are formed, with the two carboxylic acid OH groups as donors and the O atoms of the two inequivalent DMSO molecules as acceptors. The carboxylic H atoms are ordered, as are the carboxylic O atoms. In addition to the conventional hydrogen bonds, there are numerous C-H...O interactions consistent with the large number of potential CH donors and O-atom acceptors. 1,4,5,8-Naphthalenetetracarboxylic 1,8:4,5-dianhydride, C14H4O6, crystallized in the centrosymmetric space group P2(1)/c with half the molecule as the asymmetric unit. Each molecule is involved in four significant C-H...O interactions as a donor and in an additional four as an acceptor. These eight interactions link each molecule to six neighboring molecules, forming a three-dimensional network. Geometric parameters of both substances are in general agreement with analogous parameters for naphthalic anhydride, monosodium 1,4,5,8-naphthalenetetracarboxylic acid cyclic 1,8-anhydride monohydrate and 1,4,5,8-naphthalenetetracarboxylic 1,8:4,5-dianhydride, as reported previously.

On the Basicity of Ozone

Abstract Comparison of IR spectra of species formed from adsorption of different bases on the acidic OH groups of zeolites (HY, MFI) indicates that O3 basicity is close to that of CO. Calculations confirm such a result ; in particular they involve that the terminal oxygen atoms of ozone are the basic centers.

The role of the density of active sites in skeletal isomerisation of n-butene

Acidity of the catalysts plays a crucial role in the skeletal isomerisation of n-butene. Very weakly acidic OH groups are not active, very strongly acidic groups are active but they induce a too high extent of the undesired side reactions. This paper addresses the question to which extent and in which way the selectivity can be manipulated by changes on the catalyst. One can either vary the OH acid strength, by using different three-valent ions in the skeleton of, e.g., ZSM-5, or to change the total number of OH groups, by variation in the Si Al (Si M)-ratio . Both ways are explored here and it is concluded that when a selectivity change is achieved, the dominating effect is that of the density of the active sites. This is not a primary mechanistic effect but an effect of variations in the apparent contact time on the extent of side reactions.

IR study of adsorption and reaction of 1-butene on H-ZSM-5

Adsorption and reaction of 1-butene on H-ZSM-5 were studied by FT-IR spectroscopy. The adsorbed 1-butenes were converted to cis- and trans-2-butenesbelow 250 K under evacuation, and dimerization of the adsorbed 2-butenes was observed at room temperature. The produced dimer was tentatively assigned to3,4-dimethyl-3-hexene which hydrogen-bonded to the acidic OH groups with its alkyl chains. Therefore, the reaction terminated at the dimerization since proton transfer from H-ZSM-5 to the dimer did not occur.

Dealumination of zeolites Part VIIIAcidity and catalytic properties of HEMT zeolites dealuminated by steaming

S. Morin, A. Berreghis, P. Ayrault, N. S. Gnep and M. Guisnet, J. Chem. Soc., Faraday Trans., 1997, 93, 3269 DOI: 10.1039/A702538F

Zeolite Silylation for the Enhancement of para-Selectivity in Toluene Alkylation with Ethylene

The effect of zeolite silylation using tetraethyl and tetramethyl orthosilicate on the enhancement of para-selectivity and of yields of p-ethyltoluene in toluene alkylation with ethylene was investigated for different Si loadings. It was found that zeolite silylation caused a substantial increase in the selectivity and yield of p-ethyltoluene. Data on changes in sorption capacity, in the number of surface and inner strong acid OH groups, in the rate of uptake of p-ethyltoluene together with the catalytic activity of zeolite and its deactivation indicate that the silylation is restricted to surface and subsurface zeolite layers. The enhancement of para-selectivity and yield of p-ethyltoluene are controlled predominantly by changes in geometry of pore openings and thus by transport rates of ethyltoluenes.

Interaction between CD3OH and a H-mordenite studied by 1H NMR, broad-line at 4 K and MAS at 300 K

The simulation of 4 K 1H broad-line NMR spectra of CD3OH interacting with the acidic OH groups of H-mordenite zeolite shows no CD3OH2 + ions but hydrogen-bonded complexes. For high CD3OH concentrations, clusters of hydrogen-bonded methanol molecules are formed, also hydrogen-bonded to Bronsted acid sites of the zeolite. The 300 K 1H MAS spectra prove that the desorbed H-mordenite sample contains also silanol groups in electrostatic interaction with the framework; these silanols do not interact with methanol under our experimental conditions.

The Adsorption of NO and Reaction of NO with O2on H-, NaH-, CuH-, and Cu-ZSM-5: Anin SituFTIR Investigation

The adsorption of NO and the reaction of NO with O2on H-, NaH-, CuH-, and Cu-ZSM-5 zeolites were studied at 300 K usingin situFourier transform infrared spectroscopy (FTIR). At this temperature, NO readily adsorbs on the Cu+sites of CuH- and Cu-ZSM-5 catalysts and decomposition of NO is observed for all catalysts, although the rate of decomposition is vastly different on these materials. In comparison, this reaction is negligible over the H- and NaH-ZSM-5 samples. The time evolution of several nitrogen-containing molecules after controlled O2exposure to the NO/ZSM-5 systems has allowed the spectral correlation of these species. These nitrogen-containing species can interact with either the protonic sites of bridging hydroxyls forming hydrogen bonding complexes or the metal cations producing primarily surface nitrates and nitrites. The hydrogen bonded NxOycomplexes were characterized with their IR absorption features: (a) NO2, 2133 cm−1; (b) N2O3, 1875 and 1587 cm−1; and (c) N2O4, 2185 and 1745 cm−1. The stretching vibrational frequency of the acidic OH groups of ZSM-5 red-shifts due to the interaction with nitrogen-containing molecules and forms “ABC” band structures characteristic of medium and strong hydrogen bonding complexes. Although the adsorbed NxOyspecies (N2O3, N2O4) interacting with the Brønsted protons exhibit characteristics of a strong Lewis base, adsorption enthalpies are sufficiently weak that their existence is not observed even after brief evacuation. Nitric oxide, oxygen coadsorption produces metal cation (Na+and Cun+) bonded surface species possessing IR absorption bands between 1400 and 1650 cm−1characteristic of nitrite and nitrate species. The thermal stability of the nitrite and nitrate adsorbed species is much greater than the NxOyspecies nucleated at free Brønsted acid sites.

FTIR Studies on the Acidity of Sulfated Zirconia Prepared by Thermolysis of Zirconium Sulfate

Sulfated zirconia having a BET surface area of 90 m2g−1and a temperature-resistant mesoporous texture was prepared by thermolysis (at 1000 K) of zirconium sulfate. Infrared studies of surface sulfates, CO adsorption at 77 K, and room temperature adsorption of pyridine showed close similarity to sulfated zirconias prepared by impregnation or doping from the gas phase. Four main families of Lewis acid centers were found, which gave CO adducts characterized by stretching frequencies of 2212, 2202, 2196, and 2188 cm−1. Interaction of CO (at liquid nitrogen temperature) with surface hydroxyls (in partially hydroxylated samples) was found to shift the O–H stretching frequency from 3650 to 3510 cm−1, due to formation of hydrogen-bonded OH···CO complexes. This downward shift, ΔνOH=140 cm−1, is significantly larger than the corresponding value for pure zirconia (ΔνOH=90 cm−1), which strongly suggests enhancement of the Brønsted acidity. Samples showing the acidic OH group at 3650 cm−1were found to contain also disulfate groups and traces of molecular water. Surface hydroxyls in sulfated zirconia still appear, however, to be weaker Brønsted acid sites than are bridging OH groups in zeolites.

Study of H-ZSM-5 acidity by 1H NMR: broad-line at 4 K and high resolution MAS at 300 K; Brønsted acidity scale and comparison with other zeolites

The interaction of water molecules with the acidic OH groups of two H-ZSM-5 zeolites, with Si Al values of 23 and 39, is quantitatively studied using 1H NMR, broad-line at 4 K and magic angle spinning (MAS) at 300 K. The H-ZSM-5 are ranked on a scale of Brønsted acidity based on the “acidity coefficient”, α a, which is equal to the hydroxonium ion concentration per Brønsted acid site when one water molecule interacts with each acidic OH group. The results are compared with those obtained for H-Y and H-mordenite zeolites.

FRAGMENTATION OF A PHENYLPHOSPHONAMIDIC ACID AS A NEW TECHNIQUE FOR THE GENERATION OF PHENYL DIOXOPHOSPHORANE

Abstract N-(1-Adamantyl) phenylphosphonamidic acid (3), an easily prepared solid, on heating in toluene or 1,2-dichloroethane gave the crystalline adamantylamine salt of the anhydride PhP(O)(NHAd)[sbnd]O[sbnd]PPh(O)(OH). The mechanism is proposed to involve first the fragmentation of 3 to form phenyl dioxophosphorane, PhPO2, which then acts as a phosphorylating agent to unreacted 3. This mechanism was supported by the observation of first-order kinetics for the consumption of 3. When the phosphonamidic acid was fragmented in the presence of an alcohol, the intermediate PhPO2 was trapped as the monoalkyl phenylphosphonate. The OH groups on the surface of silica gel were also phosphonylated by PhPO2. The acidic OH group of a phosphoric acid monoester (thymyl phosphate) was phosphonylated to give Thy[sbnd]O[sbnd]P(OXOH)[sbnd]O[sbnd]PPh(O)(OH). It is concluded from this study that N-(1-adamantyl) phenylphosphonamidic acid is a useful precursor of phenyl dioxophosphorane, which can perform valuable phosphonyla...

Evaluation of interaction between aromatic penetrants and acidic OH groups of solvent-swollen coals by inverse liquid chromatography

Evaluation of interaction between aromatic penetrants and acidic OH groups of solvent-swollen coals by inverse liquid chromatography

Long-Range Interaction of Alkali Cations with the Acidic OHGroups in H-ZSM-5

The effect of alkali cations on the properties of the acidic OHgroups of H-ZSM-5 was studied by different methods. Thetemperature dependence of the line-width of the1H MAS NMRsignal attributed to acidic protons is greatly affected bysubstituting only 1% of Na+or NH4+ions for protons. Theline-width broadens significantly upon raising the temperatureof measurement from room temperature to 373 K. Theline-broadening was almost completely suppressed when only 1% ofprotons was replaced by Na+or NH4+ions. The rate of H–Dexchange between acidic OH groups and D2molecules in the gasphase is also greatly suppressed by a small introduction of Na+ions by ion exchange, though the whole of the protons behavehomogeneously for the exchange. These results indicate thepresence of a long-range interaction between alkali metalcations and the remaining acidic OH groups. The catalyticactivities for the acid-catalyzed reactions of cyclopropaneisomerization and hexane cracking are reduced greatly when asmall percentage of protons is replaced by alkali cations. K+ions are a more effective poison than Na+ions. These resultsfurther support the view that alkali cations in H-ZSM-5 exert along-range inductive effect on the activity of the acidic OH groups.

FT-IR Studies of the Interaction between Zeolitic Hydroxyl Groups and Small Molecules. 3. Adsorption of Oxygen, Argon, Nitrogen, and Xenon on H−ZSM-5 at Low Temperatures

Adsorption of O2, Ar, N2, and Xe on H−ZSM-5 at low temperatures has been studied by transmission Fourier transform infrared spectroscopy. These molecules form 1:1 hydrogen-bonded complexes with the acidic OH groups in H−ZSM-5, which is characterized by the downward shift and the broadening of the ν(OH) band due to the acidic OH groups. The accompanying behavior of the adsorbed species has been examined simultaneously for the case of the O2 and N2 adsorption; their absorption bands have been observed at 1550 cm-1 for the dioxygen species adsorbed on Brønsted acid sites (BAS) and at 2334 and 2352 cm-1 for the dinitrogen species on BAS and Lewis acid sites (LAS), respectively. The broadening of the ν(OH) band caused by Xe is much larger than those by the other gases, which is tentatively associated with the large polarizability of Xe. The degrees of the shift of the acidic-OH band caused by the adsorption of these molecules were systematically in the order of H−ZSM-5 > H−MOR > H−Y. Using these data, the correlation between the shift and the acidity of the zeolites has been discussed.

FT-IR and Quantum Chemical Studies of the Interaction between Dimethyl Ether and HZSM-5 Zeolite

Adsorption of dimethyl ether (DME) on the protonated ZSM-5 zeolite was investigated by FT-IR spectroscopy and density functional theory (DFT) calculation. The coverage and temperature dependencies of characteristic bands and the adsorption on the partially deuterated zeolite were studied by FT-IR. Three bands observed at ca. 2900, 2400, and 1600 cm-1 were ascribed to the bands associated with the Brønsted acidic OH groups hydrogen-bonded to DME, and the band at ca. 3300 cm-1 was assigned to the OH stretching mode of the silanol OH groups hydrogen-bonded to DME. Both the IR and DFT results indicated that DME molecules adsorbed on OH groups by a hydrogen bonding irrespective of the acidity of the OH groups and that the oxonium ions of DME were not produced on the studied surface. The Clausius−Clapeyron plot of the OH stretching band gave the value of −82.4 kJ/mol as ΔH, the energy of adsorption at the acidic site.

Computation of the Infrared Spectrum of an Acidic Zeolite Proton Interacting with Acetonitrile

The influence of acetonitrile adsorption on the infrared spectrum of an acidic OH group inside a zeolite is studied by theoretical calculations. The zeolite is modeled by a cluster molecule. Potential energy and dipole surfaces of the stretch and two bending coordinates of the acidic H atom, and for the complex with acetonitrile, of an additional acetonitrile stretch coordinate, are computed employing Hartree−Fock as well as density functional methods. Infrared frequencies as well as absorption intensities are computed taking into account mechanical as well as electric anharmonicities up to fourth order. Fermi resonance proposed as the cause of the splitting of the OH stretch absorption bands in infrared spectra is explicitly considered.

Niobium oxide acidity studied by proton broad-line NMR at 4 K and MAS NMR at room temperature

The quantitative study of the Bronsted acidity of niobic acid (Nb2O5·xH2O) using broad-line1H NMR at 4 K has been performed by interacting niobic acid, pretreated at 573 K under vacuum, with water molecules. The number of oxyprotonated species (H3O+ and H2O...HO species formed, unreacted acidic OH groups or excess H2O molecules) deduced from the simulations of the broad-line1H NMR spectra shows a continuous increase in the number of H3O+ species with adsorbed water molecules. This increase may be due to a classical dilution effect or to a synergistic interaction between Bronsted and Lewis acid sites. These results are compared with those of some HY zeolites with or without framework defects.

Investigation of acid properties of dealuminated H-mordenite zeolites by low-temperature diffuse reflectance FTIR

A series of dealuminated hydrogen mordenites (total Si/Al = 7.7, 11.6, 16.8, 29.0, 39.0) were investigated by diffuse reflectance IR spectroscopy with and without probe molecules (CO, H2). In the stretching vibration region of the pure samples bands were observed at 3613–3611, 3734, 3675 and 3510 cm–1 at room temperature and assigned to bridging OH groups [νAl(OH)Si], silanol groups (νSiOH), OH of Al-containing species partially coordinated to the framework and hydrogen-bonded SiOH species in highly dealuminated specimens, respectively. When the IR spectra were obtained at 77 K, both the bands of νAl(OH)Si and νSiOH shifted to higher wavenumbers, viz. 3619 and 3740 cm–1, respectively. In the region of combination modes, bands of the pure samples were detected at 4658–4650, 4565–4552 and ca. 4500 cm–1 which could be ascribed to νAl(OH)Si+δAl(OH)Si, νSiOH+δSiOH and νSiOH+νSiO, respectively. Upon interaction with H2 or CO at 77 K the following shifts were measured: ΔνAl(OH)Si/H2= 53–63 cm–1; ΔνSiOH/H2= 5 cm–1; Δ[νAl(OH)Si+δAl(OH)Si]/H2= 24–31 cm–1; Δ(νSiOH+δSiOH)/H2= 0; ΔνAl(OH)Si/CO = 359–389 cm–1 and ΔνSiOH/CO = 102 cm–1. The intensities of the bands due to νAl(OH)Si and νAl(OH)Si+δAl(OH)Si decreased with increasing dealumination as a result of removal of acidic bridging OH groups. By contrast, the shifts Δ[νAl(OH)Si+δAl(OH)Si]/H2 and ΔνAl(OH)Si/H2 continuously increased with the degree of dealumination which is proposed to indicate a corresponding increase in the strength of the Bronsted centres, Al(OH)Si, in line with the earlier TPD results obtained with the same samples. Bands, which appeard upon H2 interaction at 77 K around 4070 and 4020 cm–1, are discussed in view of findings reported in the literature for similar systems and tentatively assigned to ‘true’ Lewis-acid sites (removable by acid leaching) and to only threefold-coordinated Al and/or Si of the framework, respectively. Finally, the bands of H2 perturbed by silanol groups (4103 cm–1), of physisorbed CO (4250 cm–1), the effect of dealumination on the formation of defects and their interaction with the probe molecules as well as the problem of the influence of CO pressure on the band shift ΔνAl(OH)Si/CO are discussed in detail.

Evaluation of Interaction between Aromatic Penetrants and Acidic OH Groups of Solvent-Swollen Coals by Inverse Liquid Chromatography

The objective of the present study is to clarify the role of OH-π interaction in selective recognition of aromatic rings by Morwell brown coal. Low-volatile Pocahontas No. 3 bituminous coal was used as a reference. The microstructure of Morwell coal was modified by swelling in various polar and nonpolar solvents. The interaction between coal and aromatic penetrants was evaluated with an interaction index, S ar , that was determined by an inverse liquid chromatography technique. Swelling ratio and S ar for Morwell coal were strongly affected by solvents, while for Pocahontas coal they were influenced little. The lowest value of S ar for Morwell coal in n-hexane, which did not swell the coal (swelling ratio Q = 1.02), shows that aromatic molecules hardly penetrated into the micropores and therefore could not access self-associated OH groups. S ar showed a maximum in acetonitrile (Q = 1.25) and methanol (Q = 1.40), both of which formed moderate hydrogen bonds with acidic OH groups in the coal. Then the swelling induced by the disruption of hydrogen bonds was essential for the selective recognition of aromatic rings by Morwell coal. THF gave a higher swelling ratio Q (=2.0) than acetonitrile and methanol, but it lowered S ar . However, excessively strong hydrogen bond between OH groups and the solvent interfered with the interaction of aromatic molecules with OH groups. Pyridine added to acetonitrile reduced S ar significantly and obstructed the OH-π interaction by forming stable hydrogen bonds with acidic OH groups in the coal. Chemical removal of OH groups by O-butylation also greatly decreased S ar . These results indicate that selective recognition of aromatic rings by Morwell coal is ascribed to the formation of OH-π interaction between coal-OH and aromatic plane.