A high‐resolution solid‐state NMR study on nano‐structured HZSM‐5 zeolite

Variations in the structure and acidity properties of HZSM‐5 zeolites with reduction in crystal sizes down to nanoscale (less than 100 nm) have been investigated by XRD, TEM and solid‐state NMR with a system capable of in situ sample pretreatment. As evidenced by a combination of ²⁷Al MAS NMR, ²⁹Si MAS, CP/MAS NMR and ¹H MAS NMR techniques, the downsize of the zeolite crystal leads to an obvious line broadening of the ²⁷Al, ²⁹Si MAS NMR spectrum, an increasing of the silanol concentration on the external surface, and a pronounced alteration of the acidity distribution between the external and internal surfaces of the zeolite. In a HZSM‐5 zeolite with an average size at about 70 nm, the nonacidic hydroxyl groups (silanols) are about 14% with respect to the total amount of Si, while only 4% of such hydroxyl groups exist in the same kind of zeolite at 1000 nm crystal size. The result of ¹H MAS NMR obtained using Fluorinert^® FC‐43 (perfluorotributyl amine) as a probe molecule demonstrates that most of the silanols are located on the external surface of the zeolite. Moreover, the concentration of Brønsted acid sites on the external surface of the nano‐structured zeolite appears to be distinctly higher than that of the microsized zeolite.     

Effect of water on the reduction of NOx with propane on Fe‐ZSM‐5. An FTIR mechanistic study

Adsorption of NO on Fe‐ZSM‐5 leads to formation of Feⁿ⁺–NO (n = 2 or 3) species (1880 cm^-1), Fe²⁺(NO)₂ complexes (1920 and 1835 cm^-1) and NO⁺ (2133 cm^-1). Water strongly suppresses the formation of NO⁺ and Feⁿ⁺(NO)₂ and more slightly the formation of Feⁿ⁺ –NO. Introduction of oxygen to NO converts the nitrosyls into surface nitrates (1620 and 1575 cm^-1) and this process is almost unaffected by water. The nitrates are thermally stable up to ca. 300^°C, but readily interact with propane at 200^°C, thus forming surface C–H–N–O deposit (bands in the 1700–1300 cm^-1 region). Here again, water does not hinder the process. The C–H–N–O deposit is relatively inert (it does not interact with NO or NO + O₂ at ambient temperature) but, at temperatures higher than 250 °C, it is decomposed to NCO^- species (bands at 2215 (Fe–NCO) and 2256 cm^-1 (Al–NCO)). In the presence of water, however, the Fe–NCO species only are formed. At ambient temperature the NCO^- species are inert towards NO and O₂, but easily react with a NO + O₂ mixture. The mechanism of the selective catalytic reduction of nitrogen oxides on Fe‐ZSM‐5 and the effect of water on the process are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of silver on the catalytic activity of Cu‐ZSM‐5 for NO SCR by propane. Effect of the presence of water and hydrothermal agings

The effect of silver on the activity and hydrothermal stability of Cu‐ZSM‐5 for NOχ reduction with C₃H₈ under oxidizing and wet conditions has been investigated. The addition of silver as a cocation to Cu‐ZSM‐5 catalysts decreases the reversible inhibition by water occurring under catalytic conditions when the temperature does not exceed 773 K. Furthermore, it suppresses the irreversible deactivation observed when the reaction is performed up to 873 K. In effect, the Cu–Ag‐ZSM‐5 solids are more resistant to the dealumination induced by the acids formed by reactions between NO, O₂ and H₂O than the Cu‐ZSM‐5 ones. The presence of silver provides also an higher hydrothermal stability under an (air + 10 vol% H₂O) mixture if the temperature does not exceed 973 K. The catalytic activity and the deactivation behaviour are dependent on the concentration of silver and copper and more especially on the preparation procedure. This revised version was published online in July 2006 with corrections to the Cover Date.     

Variable‐temperature IR spectroscopic studies of CO adsorbed on Na‐ZSM‐5 and Na‐Y zeolites

CO interacts with extraframework alkali metal cations (M⁺=) of zeolites to form both M⁺CO and M⁺OC species. By using variabletemperature FTIR spectroscopy, these Cbonded and Obonded species were found to be in a temperaturedependent equilibrium. For the same cation, the difference in interaction energy depends upon the zeolite framework. Thus, for the equilibrium process ZNa⁺=CO ZNa⁺OC, where Z represents the zeolite framework, H ⁰ was found to take the values 3.8 and 2.4 kJ mol^– for CO/NaZSM5 and CO/NaY, respectively. The Cbonded species show always the highest cation–CO interaction energy.     

The formation of HCN during the reduction of NO by isobutane over Fe‐MFI made by solid‐state ion exchange

FTIR analysis of the gaseous products of the selective catalytic reduction of NO by isobutane over a Fe‐MFI catalyst made by solid‐state ion exchange shows, for the first time, that HCN can be a substantial product. Under dry conditions the amounts formed can exceed that of N₂ for temperatures up to 280°C and NO conversions of 25% with a maximum HCN concentration of∼150 ppm formed at 315°C. However, introduction of 0.7% water causes the subsequent conversion of about one‐half the HCN to N₂, probably through hydrolysis to NH₃ and the NH₃‐SCR reaction which is very rapid on Fe‐MFI. The steps through which HCN forms remain to be established. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of Anions on the Extraction of Lanthanides (III) by N,N′‐Dimethyl‐N,N′‐Diphenyl‐3‐Oxapentanediamide

Abstract

The extraction of microquantities of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y by N,N′‐dimethyl‐N,N′‐diphenyl‐3‐oxapentanediamide (DMDPhOPDA) in 1,2‐dichloroethane from aqueous media containing ClO₄ ^?, PF₆ ^?, (CF₃SO₂)₂N^? anions or by DMDPhOPDA in 1,2‐dichloroethane in the presence of 1‐butyl‐3‐methylimidazolium bis[(trifluoremethyl)sulfonyl]imide ([C₄mim][Tf₂N]) and 1‐butyl‐3‐methylimidazolium hexafluorophosphate ([C₄mim][PF₆]) from HNO₃ solutions has been studied. The effect of HNO₃ concentration in the aqueous phase and that of the extractant concentration in the organic phase on the extraction of metal ions is considered. The stoichiometry of the extracted complexes has been determined. The addition of HPF₆ and (CF₃SO₂)₂NH or their salts to the aqueous HNO₃ or HCl solutions leads to an enchancement of lanthanides (III) extraction by DMDPhOPDA. A considerable synergistic effect was observed in the presence of ionic liquids (IL) in the organic phase containing DMDPhOPDA. This effect is connected with the hydrophobic nature of the IL anion. The distribution of ILs between the equilibrium organic and aqueous phases can govern the extractability of lanthanides (III) in DMDPhOPDA‐IL systems.     

IR study of reaction of 2‐butene adsorbed on deuterated ZSM‐5 and mordenite

The dehydrogenation of ethylbenzene to styrene was studied over single-crystalline iron oxide model catalyst films grown epitaxially onto Pt(111) substrates. The role of the iron oxide stoichiometry and of atomic surface defects for the catalytic activity was investigated by preparing single-phased Fe₃O₄(111) and α-Fe₂O₃(0001) films with defined surface structures and varying concentrations of atomic surface defects. The structure and composition of the iron oxide films were controlled by low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES), the surface defect concentrations were determined from the diffuse background intensities in the LEED patterns. These ultrahigh vacuum experiments were combined with batch reactor experiments performed in water–ethylbenzene mixtures with a total gas pressure of 0.6 mbar. No styrene formation is observed on the Fe₃O₄ films. The α-Fe₂O₃ films are catalytically active, and the styrene formation rate increases with increasing surface defect concentration on these films. This reveals atomic surface defects as active sites for the ethylbenzene dehydrogenation over unpromoted α-Fe₂O₃. After 30 min reaction time, the films were deactivated by hydrocarbon surface deposits. The deactivation process was monitored by imaging the surface deposits with a photoelectron emission microscope (PEEM). It starts at extended defects and exhibits a pattern formation after further growth. This indicates that the deactivation is a site-selective process. Post-reaction LEED and AES analysis reveals partly reduced Fe₂O₃ films, which shows that a reduction process takes place during the reaction which also deactivates the Fe₂O₃ films.     

The effect of oxygen on the aromatization of methane over the Mo/HZSM‐5 catalyst

The aromatization of methane over a Mo/HZSM‐5 catalyst was carried out in the presence of oxygen. It is shown that the addition of a small amount of oxygen is beneficial to improve the durability of the catalyst. UV‐Raman spectra disclose that the carbonaceous deposits formed on the HZSM‐5 are mainly polyolefinic and aromatic, while that on the Mo/HZSM‐5 is mainly polyaromatic. The small amount of O₂ added may partly remove the coke deposits on the active sites and keep the catalyst as MoO_xC_y/HZSM‐5, thus resulting in an improvement of the catalytic performance of the Mo/HZSM‐5 catalyst. This revised version was published online in July 2006 with corrections to the Cover Date.     

An in situ infrared study of NO reduction by C3H8 over Fe‐ZSM‐5

The interactions of NO, O₂ and NO₂ with Fe‐ZSM‐5, as well as the reduction of NO by C₃H₈ in the presence of O₂, have been investigated using in situ infrared spectroscopy. The sample of Fe‐ZSM‐5 (Fe/Al =0.56) was prepared by solid‐state ion exchange. NO adsorption in the absence of O₂ produces only mono‐ and dinitrosyl species associated with Fe²⁺ cations. Adsorbed NO₂/NO₃ species are formed via the reaction of adsorbed O₂ with gas‐phase NO or by the adsorption of gas‐phase NO₂. The reduction of NO in the presence of O₂ begins with the reaction of gas‐phase C₃H₈ with adsorbed NO₂/NO₃ species to form a nitrogen‐containing polymeric species. A reaction pathway is proposed for the catalyzed reduction of NO by C₃H₈ in the presence of O₂. This revised version was published online in July 2006 with corrections to the Cover Date.     

Studying the miscibility and thermal behavior of polybenzoxazine/poly(ε-caprolactone) blends using DSC, DMA, and solid state C NMR spectroscopy

Polymer blends of polybenzoxazine (PBZ) and poly(ε-caprolactone) (PCL) were prepared by solution blending of PCL and benzoxazine monomer (B-m), followed by thermal curing of B-m. The miscibility and thermal behavior of these PBZ/PCL blends were investigated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), Fourier transform infrared spectroscopy (FTIR), and solid state ¹³C nuclear magnetic resonance (NMR) spectroscopy. The FTIR spectra indicated that hydrogen bonding interactions occur between the carbonyl groups of PCL and the hydroxyl groups of PBZ upon curing. The DSC results revealed that this PBZ/PCL blend system has a single glass transition temperature over the entire range of compositions that we investigated. The DMA results indicated that the values of T_g of the PBZ/PCL blends were higher than those of the pure polymers. In addition, at higher PCL concentrations we observed two glass transitions for the PBZ/PCL blends: One, for the PCL component, occurred in the low-temperature region and the other, for the PBZ component, in the high-temperature region; this finding indicates that PCL and PBZ are partially miscible in the amorphous phase. The most pronounced effect of the addition of PCL was to broaden the glass transition region, judging from the E″ peaks and the decrease in the value of the loss tangent (tan δ) in the transition region upon increasing the PCL content. We have also studied the ¹H spin-lattice relaxation times in the laboratory frame, , and in the rotating frame, , as a function of the blend composition. The results are in good agreement with those from the DSC analysis; i.e. the blends are completely homogeneous on the scale of 40-70 nm. The values of indicate that the PCL present in the blends exists in both crystalline and amorphous phases; the mobility difference between these PCL phases is clearly visible from the data. In addition, the amorphous phase of PCL is not miscible with PBZ; i.e. it is larger than 2-4 nm.     

Effect of zinc concentration on the microstructure and relaxation frequency of Mn–Zn ferrites synthesized by solid state reaction

Mn3Zn polycrystalline ferrites with Mn_1−xZn_xFe₂O₄ stoichiometry (x=0.59, 0.61, 0.65) were prepared by solid state reaction. These ferrites were heated at different temperatures. The cubic structure with space group Fd3m (O_h⁷) No. 227 was confirmed by the refinement of x-ray diffraction (XRD) powders through Rietveld´s method using fullprof. Scanning electron microscopy (SEM) results revealed for all compounds a non-homogeneous grain size and shape distribution, with a mean grain size of 9 μm. The Curie temperature T_c was found to decrease as the Zn concentration increases. The magnetic domain relaxation was investigated by inductance spectroscopy (IS). The relaxation frequency f_r shows an increase with the increase of the grain size while the initial permeability µ_i decreased. We propose an R_pL_p parallel arm equivalent circuit to model the IS results. The theoretical approximation is in agreement with the experimental results. We found that Mn–Zn ferrites with zinc concentration at x=0.59 and heated to 1300 °C during 6 h show a slight improvement of the homogeneous microstructure and a relatively higher relaxation frequency without the abrupt degradation of their permeability. This result suggests that ferrites treated in the manner presented in this paper are good candidates for high frequency applications.     

Theoretical and experimental study on the selectivity of dehydrogenation of α-limonene in ZSM-5 and zeolite-Y

Dehydrogenation of the natural terpene α-limonene to the industrially important p-cymene was studied over two zeolite-supported Pd catalysts, ZSM-5 and zeolite-Y. Reactor tests indicate that transalkylation of the p-cymene product can be avoided by employing the shape-selective properties of a medium-pore ZSM-5 catalyst. The diffusion properties of the three isomers of cymene were then calculated in the two zeolites using molecular mechanics. In case of ZSM-5, p-cymene was found to have the lowest barrier of the three isomers for diffusion through the straight, spherical channel of the zeolite. For zeolite-Y, differences in diffusivity for the three isomers were very small. Theory and experiments showed, in excellent agreement, that only ZSM-5 selectively produces p-cymene. This work shows that molecular mechanics is a powerful and reliable method for the screening of zeolites for performing shape-selective catalysis. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of guests and protonation on the conformation of N,N′-dipyridyl-bis-aza-18-crown-6

Crystallisation of the sodium perchlorate adduct of N,N^′-dipyridyl-bis-aza-18-crown-6 from acidic aqueous conditions yielded [(H₃O)(N,N^′-dipyridyl-bis-aza-18-crown-6)][ClO₄] and [Na·N,N^′-dipyridyl-bis-aza-18-crown-6]₂ [(H⁺)₂N,N^′-dipyridyl-bis-aza-18-crown-6][ClO₄]₄·2H₂O. The conformations of crown ethers were significantly influenced by incorporation of an H₃O⁺ or sodium ion, or protonation of the amino nitrogen atoms resulting in three different structures for the macrocycle.     

MAS NMR, ESR and TPD studies of Mo/HZSM‐5 catalysts: evidence for the migration of molybdenum species into the zeolitic channels

NH₃‐TPD, MAS NMR and ESR spectroscopies were employed to investigate Mo‐modified HZSM‐5 catalysts prepared by impregnation. It was found that the modification of Mo ions results in a pronounced decrease in the intensity of ¹H MAS NMR resonance originating from Brønsted acid sites in the zeolites and a distinct splitting of Mo⁵⁺ ESR signals, which is attributed to the interaction of Mo with the Al atom of the zeolite framework. This presents distinct evidence that Mo ions migrate from the external surface of the zeolite into the lattice channels during the impregnation and subsequent treatment. The remaining Brønsted acid sites associated with the migrated Mo ions form the bifunctional catalytic centers that may be responsible for the outstanding catalytic performance in methane aromatization.     

Influence of the Steric Hindrance,Ligand Hydrophobicity,and DN of solvents on Structure and Extraction of Cu(II) Complexes of 1‐Alkyl‐2‐Ethylimidazoles

Abstract

Formation of Cu(II) complexes of 1‐alkyl‐2‐ethylimidazoles (where alkyl=propyl, butyl, pentyl, hexyl, and octyl) has been studied by using the liquid‐liquid partition method, at 25°C and a fixed ionic strength of the aqueous phase (I=0.5; (HL)NO₃, KNO₃). The complexes were extracted with 2‐pentanone, 2‐butanol, isoamyl alcohol, 2‐ethyl‐1‐hexanol, dichloromethane, trichloromethane, and toluene. The length of the 1‐alkyl group and the nature of solvent have been shown to influence the extraction process. Extraction curves (log D_M vs. pH) are displaced towards lower pH's with increasing chain length of the 1‐alkyl substituent and donor number of the solvents. Stability constants of the complexes in aqueous solution were determined as well as their partition ratios between the aqueous and organic phase. The stability of the Cu(II) complexes increased with increasing 1‐alkyl chain length. The stability constants are comparable with β_n ones for the Cu(II) complexes of 1‐alkyl‐2‐methylimidazoles, but smaller than those of the Cu(II)–1‐alkylimidazole counterparts. The P_c‐partition ratios of the 1‐alkyl‐2‐ethylimidazole complexes with Cu(II) are high, and increased with increasing 1‐alkyl chain length and the donor number of the solvents. Both the strong steric effect of the ethyl substituent at position 2 and the bulkiness of the 1‐alkyl‐2‐ethylimidazole molecules as well as the strong electron‐donating properties of the solvent molecules have an effect on the change of the coordination number of Cu(II) from 6 to 4. The 4‐coordinate Cu(II) complexes (distorted tetrahedron) are more readily extractable by organic solvent than are the 6‐coordinate ones and for this reason their partition constants, P_c, are high. This finding offers the possibility of extraction of the Cu(II) ions from a mixture cations.     

Electrochemical study of the stability of the electron-donor—acceptor (EDA) complex between N,N,N′,N′-tetramethyl-p-phenylendiamine and m-dinitrobenzene in acetonitrile

The stability constant of the EDA complex between N,N,N′,N′-tetramethyl-p-phenylendiamine and m-dinitrobenzene in acetonitrile has been studied by cyclic voltammetry and potentiometric titration at constant finite current. For comparison, spectral measurements were made. The spectroscopic techniques allowed us to assume the formation of a 1:1 EDA complex of which stability in acetonitrile is lower than that previously reported in chloroform. This solvent effect is expected for such weak complexes. The electrochemical techniques give a higher value of the stability constant in similar conditions. The possible causes of the discrepancies are discussed. However, the enthalpy and entropy values for the process closely agree in both electrochemical and spectroscopic methods. This is good evidence that both techniques are accounting for the same type of interaction. The electrochemical techniques, and particularly cyclic voltammetry, seem to be fast and sensible methods to study EDA complex interactions in polar aprotic solvents.     

Conversion of methanol to hydrocarbons on zeolite HZSM‐5 investigated by in situ MAS NMR spectroscopy under flow conditions and on‐line gas chromatography

In situ MAS NMR spectroscopy under flow conditions and on‐line gas chromatography have been applied to study the onset of the conversion of methanol on zeolite HZSM‐5 at temperatures between 373 and 573 K. In the steady states of methanol conversion at T ⩾ 523 K, by on‐line gas chromatography mainly the formation of ethene and propene was observed. Simultaneously recorded in situ ¹³C MAS NMR spectra show signals at 12–25 ppm and at ca. 125–131 ppm indicating the presence of adsorbed C₄–C₆ olefins. The observation of these adsorbates on a working catalyst supports the “hydrocarbon pool” mechanism previously proposed for the methanol‐to‐hydrocarbon conversion on acidic zeolites. Methanol conversion at 473 and 573 K and subsequent purging of the catalyst with dry nitrogen at 293 K led to a ¹³C MAS NMR signal at 59 ppm due to methoxy groups. No hints to the presence of ethoxy, propoxy or butoxy groups and the formation of alkyl oxonium ions were found by in situ ¹³C MAS NMR spectroscopy under flow conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

A new route for the synthesis of functionalized benzothiadiazine 1,1-dioxide derivatives via intramolecular C–H activation reactions of N,N′,N′′-trisubstituted guanidines and benzenesulfonylchloride

ABSTRACT

In this study, a simple and appropriate procedure for the synthesis of functionalized benzothiadiazine 1,1-dioxide with good yields via the Cu-catalyzed intramolecular C–H activation reaction from benzenesulfonylchloride and N,N′,N′′-trisubstitutedguanidines, generated by copper(II) oxide-catalyzed hydroamination of carbodiimides, is reported.     

Silylation of Ti‐MCM‐41 by trimethylsilyl‐imidazole and its effect on the olefin epoxidation with aqueous H2O2

Ti‐MCM‐41 catalyst was silylated with a new silylating agent, N‐trimethylsilyl‐imidazole (TMSI). TMSI was found to be more effective to improve the hydrophobicity and the catalytic performance of Ti‐MCM‐41 than other silylating agents. Different levels of silylation were obtained by varying the silylation time. A linear relationship was observed between the degree of silylation and each of the intensity ratios I₁₂₆₀/I₁₀₈₃,I₈₄₅/I₁₀₈₃ and I₇₆₀/I₁₀₈₃ of SiMe₃ spectra bands detected by FT‐IR or the intensity ratio SiMe₃/SiO₂ of the ²⁹Si‐MAS‐NMR signals. High level of silylation degree significantly enhances both the catalytic activity and the selectivity to epoxide. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comment on the individual ion activities of Na+ and Cl− by Arce,Wilczek-Vera and Vera

Conventional values of activity coefficients of ionic species, in this case sodium and chloride ions in aqueous NaCl, have been published, as if these were the experimental values of individual ion activity coefficients, although no experimental method is really able to provide these separate quantities—but only the mean activity coefficients. A dangerous consequence is the possibility that scientists concerned with ionic solution theory may rely on such data, and derive wrong conclusions about the effectiveness of their theoretical work. This paper shows the reasons why the data in question have no relation to the ionic activity coefficients, which remain unknown, and are a mere artifact of the method adopted.