The properties of clusters in the gas phase: ammonia about Bi+, Rb+, and K+

Thermodynamic data are reported for NH₃ clustered about Bi⁺, Rb⁺, and K⁺ in the gas phase. Unusually strong bondings of NH₃ to Bi⁺ suggests the probable importance of partial covalent bonding in stabilizing the first ligand cluster. Differences in relative bond strengths for NH₃ and H₂O about Rb⁺ andK⁺ are consistent with the results of extended Hückel calculations reported herein.     

Mixed Beds for Selective Removal of Ammonium Carbonate in Dialysate

We have studied experimentally and theoretically the exchange of NH₄ ⁺, Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺, and H⁺ ions in zeolite beds, weakly-acidic resin beds, and mixed beds of the zeolite and the resin. The zeolite is highly selective for NH₄ ⁺ and K⁺, whereas the resin with carboxyl functional groups is highly selective for Ca⁺⁺ and Mg⁺⁺. The effluent histories of single exchanger beds can be well predicted by the Multicomponent Chromatography Theory developed by Helfferich and Klein (1970). These histories are mainly the result of ion competition for ion exchanger sites; they can not be adjusted to meet the goal that NH₄ ⁺ ions are removed and simultaneously the pH and concentrations of all the major physiological cations are maintained at normal values. The effluent histories of mixed beds, on the other hand, can be adjusted. We have designed a mixed bed which can meet the goal except that for each equivalent of NH₄ ⁺ removed, 0.3 equivalents of Na⁺ are returned. The effluent histories of K⁺ and NH₄ ⁺ for the mixed beds are similar to those of the zeolite beds, whereas the Ca⁺⁺ and Mg⁺⁺ histories are similar to those of the resin beds.     

Thermoanalytical investigations on ion-exchanged Y-zeolites

NH₄Y and NH₄LaY-type zeolite catalysts were prepared by cyclic ion-exchange of a synthetic Linde Y-zeolite. The release of ammonia and water were followed by evolved gas analysis (automatic thermogastitrimetric equipment) as well as with a continuous selective water detector. The ion-exchangeability of NH ₄ ⁺ for La³⁺ on the zeolite was also investigated. The capacity of the NH₄Y-zeolite was found to be 3.60 mequiv./g calcined zeolite. After a three times repeated ion-exchange process, 88.9 % of the ammonia was replaced by lanthanum.     

The Cl−NH3, Cl−H2O,F−NH3 and F−H2O clusters and their photoelectron spectra

The geometries of the Cl^–NH₃, Cl^–H₂O, F^–NH₃ and F^–H₂O clusters are optimized using the coupled cluster method. The four lowest ionization potentials are then calculated, leading to the ground and low excited states of the neutral species. The first three IPs describe ionization from the externalp state of the halogen atom, whereas the fourth corresponds to ionization from the NH₃ or H₂O moiety, leading to charge transfer complexes. These complexes were recently observed in the photoelectron spectrum of Cl^–NH₃, in full accord with our calculations.Supported in part by the U.S.-Israel Binational Science Foundation     

Proton transfer in ammonia cluster cations: molecular dynamics in a self consistent field

A theoretical study of the energetics and intramolecular dynamics for ammonia cluster cations (NH₃) ₂ ⁺ and (NH₃) ₃ ⁺ is presented. The proton transfer mechanism after a vertical ionisation is followed in detail. Structural and energetic calculations are performed near the Hartree-Fock self consistent field (HF-SCF) limit; all open shell calculations are spin restricted (RHF). The calculations reconcile experimental results from thermochemical and photoionisation measurements: discrepancies in the energetics are shown to be due to the probing of different regions of the potential energy surface (PES). For the dynamics calculations, it is impractical to determine a large region of the multidimensional PES and then integrate Newton's equations. The calculations therefore incorporate a quantum mechanical determination of the electronic energy of the system after each time increment while nuclear degrees of freedom are handled classically. In this way, the classical reaction path across the Born-Oppenheimer surface is obtained.     

载体H+含量对CuY甲醇氧化羰基化反应性能影响

通过过量浸渍Cu(NO_3)_2溶液于Y分子筛载体上,制备出Cu负载量为6.4%的CuY催化剂,考察了甲醇氧化羰基化反应的催化性能,并采用X射线衍射(XRD)、H2程序升温还原(H2-TPR)、透射电子显微镜(TEM)和NH3程序升温脱附(NH3-TPD)等手段对催化剂表面微观结构进行了表征。研究表明,随Y分子筛载体H+含量的增加,可使更多Cu物种落位于分子筛微孔笼结构中,且高度分散,而笼内未交换的Na+能进一步促进铜物种更多落位于载体超笼结构中,形成更多甲醇氧化羰基化反应的Cu+活性中心。同时随铜物种引入,催化剂中产生了明显的中强酸,酸量随落位于载体笼结构中的Cu物种的增加而增加,催化剂总酸量随之增加,导致甲醇氧化羰基化产物分布发生改变,碳酸二甲酯(DMC)选择性明显降低。对比等体积浸渍法制备的92.3%的高DMC选择性CuY催化剂,以不含H+的NaY分子筛为载体,过量浸渍法制备的CuY催化剂酸量少、Cu物种活性中心多,在保持82.4%的高DMC选择性时,其DMC的时空收率(STY)也高达109.1mg·g~(-1)·h-1。     

Evaluation of Co/SSZ-13 Zeolite Catalysts Prepared by Solid-Phase Reaction for NO-SCR by Methane

Co/SSZ-13 zeolites were prepared by heating the finely dispersed mixture of NH₄-SSZ-13 and different cobalt salts up to 550 °C. Investigations by thermogravimetry – differential scanning calorimetry – mass spectrometry provided new insight into details of the solid-state reaction. Formation of Co carrying hydrate melt or volatile species was shown to proceed from chloride, nitrate, or acetylacetonate Co precursor salts upon thermal treatment. This phase change allows the transport of the Co species into the zeolite pores. The reaction of the NH₄⁺ or H⁺ zeolite cations and the mobile Co precursors generates vapor or gas products, readily leaving the zeolite pores, and cobalt ions in lattice positions suggesting that solid-state ion-exchange is the prevailing process. The obtained catalysts are of good activity and N₂ selectivity in the CH₄/NO-SCR reaction. The thermal treatment of acetate or formate salts give solid intermediates that are unable to get in contact and react with the cations in the zeolite micropores. These catalysts contain mainly Co-oxide clusters located on the outer surface of the zeolite crystallites and have poor catalytic performance.     

A periodic DFT study of intramolecular isomerization reactions of toluene and xylenes catalyzed by acidic mordenite

A periodic density functional theory (DFT) study of the isomerization reactions of toluene and xylene catalyzed by acidic mordenite is reported. Monomolecular isomerization reactions have been considered and analyzed. The different reaction pathways have been discussed in detail. The use of periodic structure calculations allows consideration and analysis of zeolite electrostatic contributions and steric constraints that occur within zeolite micropores. Major differences in the details of protonation reaction pathways are found when periodic structures are used rather than small cluster models of the Br?nsted acidic site. Complex relationships are found between zeolite topology and reaction pathways.     

Introduction of vanadium species in <Emphasis Type="Italic">β</Emphasis> zeolite by solid-state reaction: spectroscopic study of V speciation and molecular mechanism

V-containing β zeolites were prepared by solid-state reaction between V₂O₅ and β zeolite. The zeolite structure was analysed by XRD and N₂ physisorption. The V speciation was studied by chemical analysis and different spectroscopies (FT-IR, ²⁷Al-NMR, UV-Vis, EPR, photoluminescence). After calcination of V₂O₅-β zeolite mechanical mixtures at 500°C, three kinds of V species were identified: (i) oligomeric vanadates with octahedral V⁵⁺ easily removed by treatment with NH₄OAc, (ii) isolated vanadyl (V=O)²⁺ ions in axially distorted octahedral or square pyramidal environment, interacting with framework and/or extraframework Al nuclei and (iii) isolated V⁵⁺ in tetrahedral and octahedral environments, localized in framework defect sites. The amount of the latter species is higher when water vapor is present during calcination and when parent β zeolite contains a high concentration of defect sites generated by a strong acid pretreatment. Isolated V⁵⁺ are easily reduced to tetrahedral V⁴⁺ or to square pyramidal (V=O)²⁺. Possible models of the mechanism of formation of V species by solid-state reaction and further reduction are proposed.     

氟硅酸铵改性纳米HZSM-5分子筛催化甲醇制丙烯

在静态条件下, 采用不同浓度的氟硅酸铵溶液对纳米ZSM-5分子筛进行了改性处理. 利用粉末X射线衍射(XRD)、²⁷Al 魔角旋转固体核磁共振(²⁷Al MAS NMR)、X射线荧光光谱(XRF)、X射线光电子能谱(XPS)、N₂ 吸附、透射电镜(TEM)、NH₃程序升温脱附(NH₃-TPD)、吡啶吸附红外光谱(Py-IR)等技术对改性前后纳米ZSM-5分子筛的骨架结构、织构性质、酸性质进行了表征. 并在常压、反应温度为450℃、甲醇质量空速(WHSV)为1 h^-1的条件下, 研究了改性前后纳米HZSM- 的甲醇制丙烯(MTP)催化性能. 结果表明, 合适浓度的氟硅酸铵处 理能够选择性地脱除纳米ZSM-5 分子筛的外表面铝, 从而使得HZSM-5 的酸密度降低, 比表面积和孔容增大, MTP催化性能显著提高. 氟硅酸铵改性后纳米HZSM-5 的丙烯选择性和丙烯/乙烯(P/E)质量比分别由原来的 28.8%和2.6提高到45.1%和8.0, 催化剂寿命增加了近2倍.     

Analysis of NH3-TPD Profiles for CuSSZ-13 SCR Catalyst of Controlled Al Distribution – Complexity Resolved by First Principles Thermodynamics of NH3 Desorption,IR and EPR Insight into Cu Speciation**

NH₃ temperature-programmed desorption (NH₃-TPD) is frequently used for probing the nature of the active sites in CuSSZ-13 zeolite for selective catalytic reduction (SCR) of NO_x. Herein, we propose an interpretation of NH₃-TPD results, which takes into account the temperature-induced dynamics of NH₃ interaction with the active centers. It is based on a comprehensive DFT/GGA+D and first-principles thermodynamic (FPT) modeling of NH₃ adsorption on single Cu²⁺, Cu⁺, [CuOH]⁺ centers, dimeric [Cu-O-Cu]²⁺, [Cu-O₂²⁻-Cu]² species, segregated CuO nanocrystals and Brønsted acid sites (BAS). Theoretical TPD profiles are compared with the experimental data measured for samples of various Si/Al ratios and distribution of Al within the zeolite framework. Copper reduction, its relocation, followed by the intrazeolite olation/oxolation processes, which are concomitant with NH₃ desorption, were revealed by electron paramagnetic resonance (EPR) and IR. DFT/FPT results show that the peaks in the desorption profiles cannot be assigned univocally to the particular Cu and BAS centers, since the observed low-, medium- and high-temperature desorption bands have contributions coming from several species, which dynamically change their speciation and redox states during NH₃-TPD experiment. Thus, a rigorous interpretation of the NH₃-TPD profiles of CuSSZ-13 in terms of the strength and concentration of the active centers of a particular type is problematic. Nonetheless, useful connections for molecular interpretation of TPD profiles can be established between the individual component peaks and the corresponding ensembles of the adsorption centers.     

Predicting adsorption enthalpies on silicalite and HZSM‐5: A benchmark study on DFT strategies addressing dispersion interactions

We evaluated the accuracy of periodic density functional calculations for adsorption enthalpies of water, alkanes, and alcohols in silicalite and HZSM‐5 zeolites using a gradient‐corrected density functional with empirical dispersion corrections (PBE‐D) as well as a nonlocal correlation functional (vdW‐DF2). Results of both approaches agree in acceptable fashion with experimental adsorption energies of alcohols in silicalite, but the adsorption energies for n‐alkanes in both zeolite models are overestimated, by 21?46 kJ mol^?1. For PBE‐D calculations, the adsorption of alkanes is exclusively determined by the empirical dispersion term, while the generalized gradient approximation‐DFT part is purely repulsive, preventing the molecule to come too close to the zeolite walls. The vdW‐DF2 results are comparable to those of PBE‐D calculations, but the latter values are slightly closer to the experiment in most cases. Thus, both computational approaches are unable to reproduce available experimental adsorption energies of alkanes in silicalite and HZSM‐5 zeolite with chemical accuracy. © 2014 Wiley Periodicals, Inc.     

An ab initio study of CO adsorption on ceria(1 1 0)

Hartree–Fock and DFT calculations are reported for the CO/CeO₂(1 1 0) surface system. The electron density, electrostatic potential, atomic charges and projected electronic density of states have been calculated from an ECP-and-point-charge-embedded cluster model and is compared with periodic calculations. The agreement between the two surface models is reasonably good. A number of weakly bonding CO adsorption sites were found, with E_ads (BSSE-corrected) ranging from 0.01 to 0.22 eV per adsorbed molecule. The two most favourable sites are found in the vicinity of surface cerium ions, with the CO molecule oriented in a tilted fashion, C-end down. The surface-induced CO stretching vibrational frequency shifts on these sites are a redshift of ≈−30 cm⁻¹ and a blueshift of ≈25 cm⁻¹, respectively.     

An ab initio calculation of 14n quadrupole coupling constants

Ab initio SCF-MO calculations of ¹⁴N quadrupole coupling constants are reported for HCN, HNC, CH₃CN, CH₃NC, NH₃, NH₂NH₂, FCN, N₂O, (CN)₂, BrCN, pyridine and pyrazine. There is excellent correlation between calculation and experiment yielding Q = 1.503 ± 0.159 × 10^?26 cm² for the ¹⁴N nuclear quadrupole moment. Dunning sp basis sets are more than adequate for such calculations, STO/4G basis sets yielding almost identical results for pyridine and pyrazine. Unsuccessful attempts were made to correlate coupling constants with electronic population analysis indices.     

The O + NH3 reaction: A review

Experimental data for the reaction of O atoms with NH₃ have been reviewed with particular attention to the possible effects of secondary reactions on the deduced rate coefficient. A reaction mechanism was assembled and computer calculations carried out to simulate several sets of experiments. The sensitivity of the calculations to uncertainties in the various rate coefficients was assessed. Transition-state theory calculations were carried out on the rate coefficient k₁ for the O + NH₃ → OH + NH₂ reaction. These studies suggest that the reaction stoichiometry is dependent on temperature, initial reagent ratios, and extent of reaction; that available data are not sufficient to determine whether the initial step is H-atom abstraction (producing OH and NH₂ radicals) or O-atom addition (producing an NH₃O* complex): and that the low temperature values of k₁ (T ? 400 K) are not consistent with values deduced at higher temperatures if the reaction proceeds by H-atom abstraction. Taking all the evidence into account, it is recommended that the expression 1.1 × 10³ T^2.1 exp(?2620/T) L mol^?1 s^?1 be used for k₁ until more definitive experimental measurements are made at low temperatures.     

Author index for volume 52

Dehydrated samples of zeolite Y containing alkali-metal cations have been reacted with alkali-metal vapor in sealed silica tubes, and the products studied by electron-spin resonance (ESR) spectroscopy. Two distinct species were detected following the reaction of sodium-exchanged zeolite Y with sodium, potassium, or rubidium vapor. Exposure to a low concentration of metal vapor resulted in brightly colored samples with ESR signals characteristic of a stable ionic cluster species Na³⁺₄, in which an unpaired electron is trapped on four equivalent sodium cations in the sodalite cages of the zeolite. Exposure to higher concentrations of metal vapor resulted in dark-colored samples with ESR signals characteristic of small metallic particles located inside the zeolite cavities. A similar ionic cluster species K³⁺₄ was detected following the reaction of potassium-exchanged zeolite Y with sodium or potassium vapor although the potassium cluster was less stable than its sodium counterpart and an ESR signal from small metallic particles was observed at the same time. The corresponding Rb³⁺₄ ionic cluster species was not detected following the reaction of rubidium-exchanged zeolite Y with rubidium vapor; only an ESR signal from small metallic particles was observed. The narrow linewidths of the ESR signals from the small metallic particles suggest an inhibition of the spin-relaxation mechanisms in the bulk metals.     

Zeolite ZSM-5 containing copper ions: The effect of the copper salt anion and NH<Subscript>4</Subscript>OH/Cu<Superscript>2+</Superscript> ratio on the state of the copper ions and on the reactivity of the zeolite in DeNO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

Isotherms of copper cation sorption by H-ZSM-5 zeolite from aqueous and aqueous ammonia solutions of copper acetate, chloride, nitrate, and sulfate are considered in terms of Langmuir’s monomolecular adsorption model. Using UV-Vis diffuse reflectance spectroscopy, IR spectroscopy, and temperatureprogrammed reduction with hydrogen and carbon monoxide, it has been demonstrated that the electronic state of the copper ions is determined by the ion exchange and heat treatment conditions. The state of the copper ions has an effect on the redox properties and reactivity of the Cu-ZSM-5 catalysts in the selective catalytic reduction (SCR) of NO with propane and in N₂O decomposition. The amount of Cu²⁺ that is sorbed by zeolite H-ZSM-5 from aqueous solution and is stabilized as isolated Cu²⁺ cations in cationexchange sites of the zeolite depends largely on the copper salt anion. The quantity of Cu(II) cations sorbed from aqueous solutions of copper salts of strong acids is smaller than the quantity of the same cations sorbed from the copper acetate solution. When copper chloride or sulfate is used, the zeolite is modified by the chloride or sulfate anion. Because of the presence of these anions, the redox properties and nitrogen oxides removal (DeNO _x ) efficiency of the Cu-ZSM-5 catalysts prepared using the copper salts of strong acids are worse than the same characteristics of the sample prepared using the copper acetate solution. The addition of ammonia to the aqueous solutions of copper salts diminishes the copper salt anion effect on the amount of Cu(II) sorbed from these solutions and hampers the nonspecific sorption of anions on the zeolite surface. As a consequence, the redox and DeNO _x properties of Cu-ZSM-5 depend considerably on the NH₄OH/Cu²⁺ ratio in the solution used in ion exchange. The aqueous ammonia solutions of the copper salts with NH₄OH/Cu²⁺ = 6–10 stabilize, in the Cu-ZSM-5 structure, Cu²⁺ ions bonded with extraframework oxygen, which are more active in DeNO _x than isolated Cu²⁺ ions (which form at NH4OH/Cu2+ = 30) or nanosized CuO particles (which form at NH₄OH/Cu²⁺ = 3).     

NH3‐Driven Benzene C−H Activation with O2 that Opens a New Way for Selective Phenol Synthesis

Catalytic benzene C?H activation toward selective phenol synthesis with O₂ remains a stimulating challenge to be tackled. Phenol is currently produced industrially by the three‐steps cumene process in liquid phase, which is energy‐intensive and not environmentally friendly. Hence, there is a strong demand for an alternative gas‐phase single‐path reaction process. This account documents the pivotal confined single metal ion site platform with a sufficiently large coordination sphere in β zeolite pores, which promotes the unprecedented catalysis for the selective benzene hydroxylation with O₂ under coexisting NH₃ by the new inter‐ligand concerted mechanism. Among alkali and alkaline‐earth metal ions and transition and precious metal ions, single Cs⁺ and Rb⁺ sites with ion diameters >0.300 nm in the β pores exhibited good performances for the direct phenol synthesis in a gas‐phase single‐path reaction process. The single Cs⁺ and Rb⁺ sites that possess neither significant Lewis acidic?basic property nor redox property, cannot activate benzene, O₂, and NH₃, respectively, whereas when they coadsorbed together, the reaction of the inter‐coadsorbates on the single alkali‐metal ion site proceeds concertedly (the inter‐ligand concerted mechanism), bringing about the benzene C?H activation toward phenol synthesis. The NH₃‐driven benzene C?H activation with O₂ was compared to the switchover of the reaction pathways from the deep oxidation to selective oxidation of benzene by coexisting NH₃ on Pt₆ metallic cluster/β and Ni₄O₄ oxide cluster/β. The NH₃‐driven selective oxidation mechanism observed with the Cs⁺/β and Rb⁺/β differs from the traditional redox catalysis (Mars‐van Krevelen) mechanism, simple Langmuir‐Hinshelwood mechanism, and acid?base catalysis mechanism involving clearly defined interaction modes. The present catalysis concept opens a new way for catalytic selective oxidation processes involving direct phenol synthesis.