Hydrogen storage in Chabazite zeolite frameworks

We have recently highlighted that H-SSZ-13, a highly siliceous zeolite (Si/Al = 11.6) with a chabazitic framework, is the most efficient zeolitic material for hydrogen storage [A. Zecchina, S. Bordiga, J. G. Vitillo, G. Ricchiardi, C. Lamberti, G. Spoto, M. Bj?rgen and K. P. Lillerud, J. Am. Chem. Soc., 2005, 127, 6361]. The aim of this new study is thus to clarify both the role played by the acidic strength and by the density of the polarizing centers hosted in the same framework topology in the increase of the adsorptive capabilities of the chabazitic materials towards H2. To achieve this goal, the volumetric experiments of H2 uptake (performed at 77 K) and the transmission IR experiment of H2 adsorption at 15 K have been performed on H-SSZ-13, H-SAPO-34 (the isostructural silico-aluminophosphate material with the same Br?nsted site density) and H-CHA (the standard chabazite zeolite: Si/Al = 2.1) materials. We have found that a H2 uptake improvement has been obtained by increasing the acidic strength of the Br?nsted sites (moving from H-SAPO-34 to H-SSZ-13). Conversely, the important increase of the Br?nsted sites density (moving from H-SSZ-13 to H-CHA) has played a negative role. This unexpected behavior has been explained as follows. The additional Br?nsted sites are in mutual interaction via H-bonds inside the small cages of the chabazitic framework and for most of them the energetic cost needed to displace the adjacent OH ligand is higher than the adsorption enthalpy of the OH...H2 adduct. From our work it can be concluded that proton exchanged chabazitic frameworks represent, among zeolites, the most efficient materials for hydrogen storage. We have shown that a proper balance between available space (volume accessible to hydrogen), high contact surface, and specific interaction with strong and isolated polarizing centers are the necessary characteristics requested to design better materials for molecular H2 storage.     

The crystallographic structure of a Lewis acid-assisted chiral Brønsted acid as an enantioselective protonation reagent for silyl enol ethers

It is difficult to control the enantioselectivity in the protonation of silyl enol ethers with simple chiral Br?nsted acids, mainly due to bond flexibility between the proton and its chiral counterion, the orientational flexibility of the proton, and the fact that the proton sources available are limited to acidic compounds such as chiral carboxylic acids. To overcome these difficulties, we have developed a Lewis acid-assisted chiral Br?nsted acid (LBA) system. The coordination of Lewis acids with Br?nsted acids restricts the orientation of protons and increases their acidity. Optically active binaphthol (BINOL) derivative.SnCl4 complexes are very effective as enantioselective protonation reagents for silyl enol ethers. However, their exact structures have not yet been determined. We describe here optically active 1,2-diarylethane-1,2-diol derivative.SnCl4 as a new type of LBA for the enantioselective protonation as well as its crystallographic structure. A variety of optically active 1,2-diarylethane-1,2-diols could be readily prepared by asymmetric syn-dihydroxylation. This is a great advantage over BINOL for the flexible design of a new LBA. The most significant finding is that we were able to specify the conformational direction of the H-O bond of LBA, which has some asymmetric inductivity, by X-ray diffraction analysis. The stereochemical course in the enantioselective protonation of silyl enol ethers using LBA would be controlled by a linear OH/pi interaction with an initial step. The absolute stereopreference in enantioselective reactions using BINOL.SnCl4 can also be explained in terms of this uniformly mechanistic interpretation.     

Bulk gas-phase acidity

The capability of a gaseous Br?nsted acid HB to deliver protons to a base is usually described by the gas-phase acidity (GA) value of the acid. However, GA values are standard Gibbs energy differences and refer to individual gas pressures of 1?bar for acid HB, base B(-), and proton H(+). We show that the GA value is not suited to describe the bulk acidity of a gaseous acid. Here the pressure dependence of the activities of HB, H(HB)(n)(+), and B(HB)(m)(-) that result from gaseous autoprotolysis have to be considered. In this work, the pressure-dependent absolute chemical potential of the proton in the representative gaseous proton acids CH(4), NH(3), H(2)O, HF, and HCl was worked out and the general theory to describe bulk gas phase acidity--that can directly be compared with solution acidity--was developed.     

Spectroscopic Monitoring of the Acidity of Water Films on Ru(0001): Orientation‐Specific Acidity of Adsorbed Water

We examined the acid–base properties of water films adsorbed onto a Ru(0001) substrate by using surface spectroscopic methods in vacuum environments. Ammonia adsorption experiments combined with low‐energy sputtering (LES), reactive ion scattering (RIS), reflection–absorption infrared spectroscopy (RAIRS) and temperature‐programmed desorption (TPD) measurements showed that the adsorbed water is acidic enough to transfer protons to ammonia. Only the water molecules in an intact water monolayer and water clusters larger than the hexamer exhibit such acidity, whereas small clusters, a thick ice film or a partially dissociated water monolayer that contains OH, H₂O and H species are not acidic. The observations indicate the orientation‐specific acidity of adsorbed water. The acidity stems from water molecules with H‐down adsorption geometry present in the monolayer. However, the dissociation of water into H and OH on the surface does not promote but rather suppresses the proton transfer to ammonia.     

Impact of Zeolite Framework Composition and Flexibility on Methanol-To-Olefins Selectivity: Confinement or Diffusion?

The methanol-to-olefins reaction catalyzed by small-pore cage-based acid zeolites and zeotypes produces a mixture of short chain olefins, whose selectivity to ethene, propene and butene varies with the cavity architecture and with the framework composition. The product distribution of aluminosilicates and silicoaluminophosphates with the CHA and AEI structures (H-SSZ-13, H-SAPO-34, H-SSZ-39 and H-SAPO-18) has been experimentally determined, and the impact of acidity and framework flexibility on the stability of the key cationic intermediates involved in the mechanism and on the diffusion of the olefin products through the 8r windows of the catalysts has been evaluated by means of periodic DFT calculations and ab initio molecular dynamics simulations. The preferential stabilization by confinement of fully methylated hydrocarbon pool intermediates favoring the paring pathway is the main factor controlling the final olefin product distribution.     

Production and collision-induced dissociation of gas-phase, water- and alcohol-coordinated uranyl complexes containing halide or perchlorate anions

Electrospray ionization was used to generate mono-positive gas-phase complexes of the general formula [UO2A(S)n]+ where A = OH, Cl, Br, I or ClO4, S = H2O, CH3OH or CH3CH2OH, and n = 1-3. The multiple-stage dissociation pathways of the complexes were then studied using ion-trap mass spectrometry. For H2O-coordinated cations, the dissociation reactions observed included the elimination of H2O ligands and the loss of HA (where A = Cl, Br or I). Only for the Br and ClO4 versions did collision-induced dissociation (CID) of the hydrated species generate the bare, uranyl-anion complexes. CID of the chloride and iodide versions led instead to the production of uranyl hydroxide and hydrated UO2+. Replacement of H2O ligands by alcohol increased the tendency to eliminate HA, consistent with the higher intrinsic acidity of the alcohols compared to water and potentially stronger UO2-O interactions within the alkoxide complexes compared to the hydroxide version.     

双金属Cr-In/H-SSZ-13催化剂上CH4选择催化还原NO

天然气 (主要成分为甲烷) 储量丰富,价格低,随着国内电厂煤改气的进行,甲烷作为还原剂选择催化还原氮氧化物成为电厂烟气脱硝的理想选择,受到广泛关注.In 改性分子筛催化剂在甲烷选择催化还原一氧化氮反应 (CH4-SCR) 中表现出一定催化性能,而 In 基双金属被认为是最有前景的 CH4-SCR 催化剂.本文通过湿浸渍法制备了分子筛双金属催化剂,并成功应用于CH4-SCR.我们首先研究了不同分子筛载体、助剂及含量、氧气浓度、甲烷浓度和空速对反应的影响,发现0.5%Cr-2%In/H-SSZ-13 在 CH4-SCR中表现出最佳的催化性能:NO 转化率 ＞ 90%,N2选择性 ＞ 99% (反应条件:550 ℃,6% H2O,空速 75000 /h).从透射电镜照片可以看出,0.5%Cr-2%In/H-SSZ-13催化剂上均匀分布纳米尺寸颗粒,扫描透射-高角环形暗场像元素分布分析表明 Cr 与 In 出现在同一个位置,即 Cr 与 In 紧密接触.X射线光电子能谱结果表明,与单金属催化剂相比,双金属催化剂中 In 的能谱发生了明显偏移,Cr 与 In 存在电子相互作用.CH4-SCR 反应活性实验发现,双金属催化剂活性远远超过单金属催化剂活性及其机械混合物,表明 Cr 的添加可以明显促进 In 的活性.换言之,双金属催化剂中,Cr 与 In 存在协同催化作用.Cr-In/H-SSZ-13在严苛反应条件下的活性长期保持不变,具有良好稳定性,这与沸石分子筛载体 H-SSZ-13 的高热稳定性分不开.结合稳态反应与程序升温表面反应结果,可以推测有氧条件下 CH4和 NO 分别在 In和 Cr 位点上活化.     

Kinetic study of proton-transfer reactions of phenylnitromethanes. Implication for the origin of nitroalkane anomaly

Measurements of rate constants and substituent effects for three important elementary steps of proton-transfer reactions of phenylnitromethane were reported. The Hammett ρ values for the deprotonation of ArCH(2)NO(2) with OH(-), protonation of ArCH═NO(2)(-) with H(2)O, and protonation of ArCH═NO(2)(-) with HCl were determined in aqueous MeOH at 25 °C. Comparison of these experimentally observed ρ values with those calculated at B3LYP/6-31G* revealed that aci-nitro species (ArCH═NO(2)H), which is formed on the O-protonation of ArCH═NO(2)(-), does not lie on the main route of the proton-transfer reaction. Analysis of the Br?nsted plot implies that the proton-transfer reaction of most XC(6)H(4)CH(2)NO(2) exhibits nitroalkane anomaly, but not for p-NO(2)C(6)H(4)CH(2)NO(2), and that the transition state charge imbalance is an origin of anomaly.     

Brønsted/Lewis acid synergy in dealuminated HY zeolite: a combined solid-state NMR and theoretical calculation study

The Br?nsted/Lewis acid synergy in dealuminated HY zeolite has been studied using solid-state NMR and density function theory (DFT) calculation. The 1H double quantum magic-angle spinning (DQ-MAS) NMR results have revealed, for the first time, the detailed spatial proximities of Lewis and Br?nsted acid sites. The results from 13C NMR of adsorbed acetone as well as DFT calculation demonstrated that the Br?nsted/Lewis acid synergy considerably enhanced the Br?nsted acid strength of dealuminated HY zeolite. Two types of Br?nsted acid sites (with enhanced acidity) in close proximity to extra-framework aluminum (EFAL) species were identified in the dealuminated HY zeolite. The NMR and DFT calculation results further revealed the detailed structures of EFAL species and the mechanism of Br?nsted/Lewis acid synergy. Extra-framework Al(OH)3 and Al(OH)2+ species in the supercage cage and Al(OH)2+ species in the sodalite cage are the preferred Lewis acid sites. Moreover, it is the coordination of the EFAL species to the oxygen atom nearest the framework aluminum that leads to the enhanced acidity of dealuminated HY zeolite though there is no direct interaction (such as the hydrogen-bonding) between the EFAL species and the Br?nsted acid sites. All these findings are expected to be important in understanding the roles of Lewis acid and its synergy with the Br?nsted acid in numerous zeolite-mediated hydrocarbon reactions.     

Clusters of hydrated methane sulfonic acid CH3SO3H.(H2O)n (n = 1-5): a theoretical study

Ab initio and density functional methods have been used to examine the structures and energetics of the hydrated clusters of methane sulfonic acid (MSA), CH3SO3H.(H2O)n (n = 1-5). For small clusters with one or two water molecules, the most stable clusters have strong cyclic hydrogen bonds between the proton of OH group in MSA and the water molecules. With three or more water molecules, the proton transfer from MSA to water becomes possible, forming ion-pair structures between CH3SO3- and H3O+ moieties. For MSA.(H2O)3, the energy difference between the most stable ion pair and neutral structures are less than 1 kJ/mol, thus coexistence of neutral and ion-pair isomers are expected. For larger clusters with four and five water molecules, the ion-pair isomers are more stable (>10 kJ/mol) than the neutral ones; thus, proton transfer takes place. The ion-pair clusters can have direct hydrogen bond between CH3SO3- and H3O+ or indirect one through water molecule. For MSA.(H2O)5, the energy difference between ion pairs with direct and indirect hydrogen bonds are less than 1 kJ/mol; namely, the charge separation and acid ionization is energetically possible. The calculated IR spectra of stable isomers of MSA.(H2O)n clusters clearly demonstrate the significant red shift of OH stretching of MSA and hydrogen-bonded OH stretching of water molecules as the size of cluster increases.     

碳氟醇对阳离子表面活性剂表面活性及胶团反离子结合度的影响

用表面张力及电动势法研究了C₁₀H₂₁N(CH₃)3Br、C₁₂H₂₅N(CH₃)3Br与C₃F₇CH₂OH混合水溶液的表面与胶团性质。结果表明,对于阳离子表面活性剂,C₃F₇CH₂OH的加入一方面增加表面活性,另一方面降低胶团反离子结合度。后者不同于阴离子表面活性剂/C₃F₇CH₂OH混合体系,可归因于C₃F₇CH₂OH略有酸性,因而具备一些类似阴离子表面活性剂的性质。     

IR spectroscopy of N-methylpyrrole adsorbed on oxides--a probe of surface acidity

IR spectra of N-methylpyrrole (NMP) have been measured following adsorption on, and subsequent desorption from, SiO(2), TiO(2), ZrO(2), SiO(2)-Al(2)O(3), H-mordenite, and sepiolite. Three modes of adsorption have been observed: (i) hydrogen bonding to surface hydroxyl groups, (ii) electron transfer at Lewis acidic surface sites, and (iii) proton transfer at Br?nsted acidic surface sites. Protonation of NMP was detected only for adsorption on SiO(2)-Al(2)O(3) and H-mordenite, indicating the presence of Br?nsted acidic sites with pK(a) values 相似文献   

The complex story of a simple Brønsted acid: unusual speciation of HBr in an ionic liquid medium

Crystalline solids, co-existing in equilibrium with the 3-methyl-1H-imidazolium bromohydrogenates(I) ionic liquid, have been characterised by X-ray diffraction analysis. The Br?nsted acidic, homo-conjugate [H(2)Br(3)](-) anions presented are discussed in terms of their structure and reactivity, in efforts to advance the understanding of Br?nsted acidity in ionic liquid media.     

Silsesquioxane models for geminal silica surface silanol sites. A spectroscopic investigation of different types of silanols

The incompletely condensed monosilylated silsesquioxanes (c-C5H9)7Si7O9(OSiRR'2)(OH)2 (SiRR'2 = SiMe3, SiMe2C(H)CH2, SiMePh2) were reacted with SiCl(4) in the presence of an amine which yielded the dichloro compounds (c-C5H9)7Si7O9(OSiRR'2)O2SiCl2 (1-3). These compounds could be hydrolyzed into the corresponding silsesquioxanes containing geminal silanols, (c-C5H9)7Si7O9(OSiRR'2)O2Si(OH)2 (4-6). At elevated temperatures, the geminal silsesquioxanes 4 and 5 undergo condensation reactions and form the closed-cage silsesquioxane monosilanol, (c-C5H9)7Si8O12(OH). The more sterically hindered geminal silsesquioxane 6 undergoes in solution intermolecular dehydroxylation, yielding the thermodynamically stable dimeric disilanol, [(c-C5H9)7Si7O9(OSiMePh2)(O2Si(OH)-)]2-(mu-O) (7). NMR and FT-IR studies show that the two silanols of the geminal silsesquioxanes 4-6 are different from each other with respect to hydrogen bonding, both in solution and in the solid state. Hydrogen bonding of the geminal silanol-containing silsesquioxanes was examined and compared to hydrogen bonding in silsesquioxanes possessing vicinal or isolated silanol groups. The relative Br?nsted acidity of the geminal silanols was determined using pK(ip) (ion-pair acidity) measurements in THF with UV-vis. These acidities were compared with those of other silsesquioxanes containing silanol groups. Acidities of 4-6 were found to be among the lowest known for silsesquioxanes.     

Dynamics and mechanism of structural diffusion in linear hydrogen bond

Dynamics and mechanism of proton transfer in a protonated hydrogen bond (H-bond) chain were studied, using the CH(3)OH(2)(+)(CH(3)OH)(n) complexes, n = 1-4, as model systems. The present investigations used B3LYP/TZVP calculations and Born-Oppenheimer MD (BOMD) simulations at 350 K to obtain characteristic H-bond structures, energetic and IR spectra of the transferring protons in the gas phase and continuum liquid. The static and dynamic results were compared with the H(3)O(+)(H(2)O)(n) and CH(3)OH(2)(+)(H(2)O)(n) complexes, n = 1-4. It was found that the H-bond chains with n = 1 and 3 represent the most active intermediate states and the CH(3)OH(2)(+)(CH(3)OH)(n) complexes possess the lowest threshold frequency of proton transfer. The IR spectra obtained from BOMD simulations revealed that the thermal energy fluctuation and dynamics help promote proton transfer in the shared-proton structure with n = 3 by lowering the vibrational energy for the interconversion between the oscillatory shuttling and structural diffusion motions, leading to a higher population of the structural diffusion motion than in the shared-proton structure with n = 1. Additional explanation on the previously proposed mechanisms was introduced, with the emphases on the energetic of the transferring proton, the fluctuation of the number of the CH(3)OH molecules in the H-bond chain, and the quasi-dynamic equilibriums between the shared-proton structure (n = 3) and the close-contact structures (n ≥ 4). The latter prohibits proton transfer reaction in the H-bond chain from being concerted, since the rate of the structural diffusion depends upon the lifetime of the shared-proton intermediate state.     

Ammonia IRMS-TPD measurements on Brønsted acidity of proton-formed SAPO-34

By utilizing the advantages of a combined method of IRMS-TPD of ammonia and DFT calculations, the solid acidity of HSAPO-34 was studied. The number, strength and structure of the Br?nsted OH were measured experimentally. The quantitative measurements and DFT calculations supported the identification of Br?nsted OH to account for the generation model of the Br?nsted OH primarily located in the edge of the Si domain (island). The acid strength of SAPO-34 was slightly weaker than that of chabazite, a zeolite with the same structure. Thus, some important insights were obtained to understand the acid site generation of SAPO-34.     

Theoretical and experimental study of the acetohydroxamic acid protonation: the solvent effect

The mechanism of the protonation of acetohydroxamic acid is investigated comparing experimental results and ab initio calculations. Experimentally, the UV spectral curves were recorded at different temperatures, at constant dioxane/water concentration, and at very high concentrations of strong mineral acids. The process is followed by monitoring the changes in the UV curves with increasing acid concentration. The molecular structures and the solvation energies were calculated with the RHF, B3LYP, and MP2 methods. The solvent is treated as a continuum of uniform dielectric constant. The isolated molecule of acetohydroxamic acid exhibits two protonation sites, the carbonyl oxygen and the nitrogen atom. In dioxane/water mixture, the RHF calculations predict the existence of a third cation of low stability, where the proton is bonded to the OH oxygen. With the MP2 ab initio calculations, the free energies of the formation processes in solution of the two most stable cations, CH3COH-NHOH+ (O3H+) and CH3CO-NH2OH+ have been evaluated to be -160.2 kcalmol(-1) and -157.6 kcal mol(-1). The carbonyl site is the most active center in solution and in the gas phase. The carbonyl site is also the most active center in the UV measurements. Experimentally, the ionization constant was found to be pK(O3H+) = -2.21 at 298.15 K, after the elimination of the medium effects using the Cox-Yates equation for hight acidity levels. Experiments and ab initio calculations indicate that K(O3H+) decreases with the temperature.     

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含铜的SSZ-39分子筛（AEI拓扑结构）在机动车尾气氨气选择性催化还原(NH3-SCR)反应中性能优异,其中SSZ-39分子筛的骨架铝分布与对应的Br?nsted酸性质对反应性能影响至关重要。本文通过密度泛函理论计算同时结合固体核磁共振谱学实验探究了高硅和富铝SSZ-39分子筛骨架Al位置以及与相应Br?nsted酸强度之间的关系。通过比较骨架Al在不同位置的替代能发现,高硅H-SSZ-39分子筛的骨架铝主要以孤立Al形式存在,同晶取代后落位在T3位上,其相应的Br?nsted酸质子与O7结合时最稳定。而富铝SSZ-39分子筛的骨架铝主要以NNNN与NNN序列的2Al形式存在,当两个骨架铝原子分别位于六元环和四元环对位的T3位上时体系能量最低,此时两个Br?nsted酸质子指向分子筛的超笼和八元环孔道。在最优构型下计算质子亲核势、NH3吸附态微观结构与脱附能以及吸附氘代乙腈后1H NMR化学位移来表征Br?nsted酸性,发现随着SSZ-39分子筛铝含量增加相应的Br?nsted酸含量增加,而Br?nsted酸强度趋于减弱。这些理论计算结果与NH3-TPD及吸附氘代乙腈的1H MAS NMR实验结果一致。本文为调控SSZ-39分子筛酸性以及合理设计高效催化剂提供了依据。     

DFT计算结合固体NMR研究富铝SSZ-13的铝分布和Br?nsted酸性

具有菱沸石(CHA)结构的SSZ-13分子筛在甲醇制烯烃(MTO)及柴油机车尾气氨选择性催化还原(NH_3-SCR)反应中具有重要的应用,采用富铝SSZ-13可以调节MTO反应的烯烃选择性和提升NH_3-SCR的低温脱硝活性,因此SSZ-13中的铝含量和分布与对应的酸性决定了其催化性能。本文采用密度泛函理论结合固体核磁共振实验研究了富铝和富硅HSSZ-13的Al位置与Br?nsted酸强度的内在关系。通过计算取代能发现,对于孤立Al位,质子位于Al周围4个不同O位时能量差异较小,最稳定的B酸位点是O(1)―H。对于富铝SSZ-13,两个Al原子位于同一六元环的对位是Al-Si-Si-Al (NNNN)序列中最稳定的结构,而Al-Si-Al (NNN)序列中能量最优的Al分布是两个铝原子排布于六棱柱上下不同的六元环上。通过计算最稳定构型下的质子亲和势、NH3脱附能和吸附氘代乙腈后的1H NMR化学位移,发现富铝SSZ-13中含有Si(2Al)分布的NNN序列导致了其Br?nsted酸强度弱于高硅的分子筛。分峰拟合29Si魔角旋转核磁共振(MASNMR)谱图表明富铝SSZ-13中Si(2Al)的含量在43%以上,而吸附氘代乙腈后的1H MAS NMR实验显示富铝SSZ-13的桥羟基化学位移向低场移动,进一步证明富铝SSZ-13具有较弱的Br?nsted酸强度。