FTIR adsorption studies of H2O and CH3OH in the isostructural H-SSZ-13 and H-SAPO-34: formation of H-bonded adducts and protonated clusters

The acidity of the isostructural H-SSZ-13 and H-SAPO-34 has been investigated by transmission FTIR spectroscopy using H2O and CH3OH as molecular probes. Interactions between the zeolitic samples and the probe molecules led to perturbations and proton transfers directly related to the acidity of the materials. The entire set of acidic sites in H-SSZ-13 interacts with H2O and CH3OH to give H-bonded adducts or protonated species. H3O+ is not formed in appreciable amounts upon H2O adsorption on H-SSZ-13, but at high coverages H2O generates clusters that have a proton affinity sufficiently high to abstract protons from the zeolite framework. Parallel experiments carried out for H-SAPO-34 showed that the H2O clusters abstract protons from Br?nsted sites only to a minor extent. Moving to CH3OH, even if it has a higher proton affinity than H2O and should expectingly experience an easier protonation, proton transfer is totally absent in H-SAPO-34 under our set of conditions. The clear evidence of methanol protonation in H-SSZ-13 definitely states the strong acidic character of this material. When irreversibly adsorbed CH3OH is present in H-SSZ-13, an appreciable amount of (CH3)2O is formed upon heating to 573 K. Compared to its SAPO analogue, the present set of data indisputably points to H-SSZ-13 as the strongest Br?nsted acidic material.     

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.     

Liquid hydrogen in protonic chabazite

Due to its fully reversible nature, H(2) storage by molecular adsorption could represent an advantage with respect to dissociative processes, where kinetic effects during the charging and discharging processes are present. A drawback of this strategy is represented by the extremely weak interactions that require low temperature and high pressure. High surface area materials hosting polarizing sites can represent a viable way toward more favorable working conditions. Of these, in this contribution, we have studied hydrogen adsorption in a series of zeolites using volumetric techniques and infrared spectroscopy at 15 K. We have found that in H-SSZ-13 zeolite the cooperative role played by high surface area, internal wall topology, and presence of high binding energy sites (protons) allows hydrogen to densify inside the nanopores at favorable temperature and pressure conditions.     

Formation of acetone enol on acidic zeolite ZSM-5 evidenced by H/D exchange

H/D exchange observed between bridging hydroxyl groups in acidic zeolite ZSM-5 and adsorbed acetone (acetone-d6/H-ZSM-5 and 13C-2-acetone/D-ZSM-5) indicates a concerted catalytic function of Br?nsted acid sites and neighbouring framework oxygen atoms acting as Lewis base sites.     

Stability of Zn(II) cations in chabazite studied by periodical density functional theory.

The location of the Zn(2+) cation in Zn-exchanged chabazite has been studied by the periodical density functional method. Chabazite was chosen as a zeolite model, because it contains three different types of rings commonly found in the zeolite structures: four-, six-, and eight-membered rings. Two aluminum atoms have been employed to substitute the silicon atoms in the same D6R unit cell of the zeolite framework. This leads to different arrangements for the Br?nsted site pair and the Zn(II) cation. The two Br?nsted sites are found to be more stable when placed in the small ring (4T ring) than in the other rings. This suggests that the most reactive Br?nsted sites are located in the large rings. Two Br?nsted sites are most stable when the O(H)-Al-O-Si-O(H)-Al sequence is followed in the same ring instead of being located in two different rings. This resembles the aluminum distribution in the small four-membered ring and agrees with bond order conservation rules. The cation stability is markedly influenced by the distortions of the framework. Other factors that also contribute to the stabilization are the aluminum content near the cation and the stability of the original Br?nsted sites. The Zn(2+) cation is more stable in the large rings than in the small ones, the six-membered one being the most stable configuration. In the small rings, the cation is, therefore, more reactive. Two different probe molecules have been used to study the interaction with the Zn(II) cation: water and methane. These probe molecules can extract the active center from its original position. For the water molecule, this effect is large and leads to a high framework relaxation. The value of the binding energy of this molecule to the active sites is influenced by these framework relaxations as well as by the cationic position environment. For weakly interacting methane, these effects are significantly less.     

H-MCM-22沸石分子筛中Brnsted/Lewis酸协同效应的~1H 和~(27)Al双量子魔角旋转固体核磁共振研究

采用各种固体核磁共振 (NMR) 技术详细研究了 H-MCM-22 分子筛中 Brnsted/Lewis 酸的协同效应. 二维 1H 双量子魔角旋转 (DQ-MAS) NMR 结果表明, 在脱铝 H-MCM-22 分子筛中 Brnsted 酸位 (骨架桥式羟基) 和 Lewis 酸位 (非骨架铝羟基) 之间是空间邻近的, 暗示着可能存在 B/L 酸协同效应. 二维 27Al DQ-MAS NMR 结果揭示了各种铝物种之间的空间邻近性, 表明 B/L 酸协同效应优先发生在 H-MCM-22 分子筛超笼中的骨架 T6 位铝和非骨架铝物种之间. 2-13C-丙酮探针分子实验发现, 因 B/L 酸协同效应而导致脱铝 H-MCM-22 分子筛酸性明显增强, 氘代吡啶探针分子实验也证实在 H-MCM-22 分子筛的超笼中发生了 B/L 酸协同效应. 上述结果将有助于我们理解在脱铝 H-MCM-22 分子筛上发生的多相催化机理.     

Acidity of mesoporous MoO(x)/ZrO2 and WO(x)/ZrO2 materials: a combined solid-state NMR and theoretical calculation study

The acidity of mesoporous MoO(x)/ZrO2 and WO(x)/ZrO2 materials was studied in detail by multinuclear solid-state NMR techniques as well as DFT quantum chemical calculations. The 1H MAS NMR experiments clearly revealed the presence of two different types of strong Br?nsted acid sites on both MoO(x)/ZrO2 and WO(x)/ZrO2 mesoporous materials, which were able to prontonate adsorbed pyrine-d5 (resulting in 1H NMR signals at chemical shifts in the range 16-19 ppm) as well as adsorbed trimethylphosphine (giving rise to 31P NMR signal at ca. 0 ppm). The 13C NMR of adsorbed 2-(13)C-acetone indicated that the average Br?nsted acid strength of the two mesoporous materials was stronger than that of zeolite HZSM-5 but still weaker than that of 100% H2SO4, which was in good agreement with theoretical predictions. The quantum chemical calculations revealed the detailed structures of the two distinct types of Br?nsted acid sites formed on the mesoporous MoO(x)/ZrO2 and WO(x)/ZrO2. The existence of both monomer and oligomer Mo (or W) species containing a Mo-OH-Zr (or W-OH-Zr) bridging OH group was confirmed with the former having an acid strength close to zeolite HZSM-5, with the latter having an acid strength similar to sulfated zirconia. On the basis of our NMR experimental and theoretical calculation results, a possible mechanism was proposed for the formation of acid sites on these mesoporous materials.     

Sn-Al-β分子筛酸性在葡萄糖转化反应中作用的固体NMR研究

制备了一系列具有不同酸性质的β分子筛催化剂, 通过固体核磁共振(NMR)探针分子技术对其酸性质进行了表征, 并考察了其催化葡萄糖转化为乙酰丙酸甲酯的性能. 吸附三甲基磷的³¹P NMR实验结果表明, 含有骨架Sn以及Al原子的Sn-Al-β催化剂同时具有Br?nsted与Lewis酸性. 通过2-¹³C-丙酮探针分子区分出 3种酸强度的Br?nsted酸位, 其中一种酸强度接近“超强酸”, 可能是由于空间邻近的Br?nsted酸位和Lewis酸位发生协同作用产生的. 葡萄糖转化为乙酰丙酸甲酯的催化反应结果表明, 相比于分别只含有Lewis酸位和Br?nsted酸位的Sn-β和Al-β样品以及两者的物理混合样品, Sn-Al-β分子筛催化剂具有高催化活性与产物选择性, 这主要是由于Br?nsted酸位和Lewis酸位的协同作用产生了强Br?nsted酸位, 这种强Br?nsted酸位进一步导致了更高的催化活性.     

ZSM-5分子筛催化1-己烯生成cis-2-己烯的理论研究

基于54T团簇模型, 采用ONIOM分层计算方法, 研究了1-己烯在ZSM-5分子筛上进行顺式双键异构的反应机理. 计算结果表明, 1-己烯的顺式双键异构反应通过只有分子筛Brønsted酸部分起作用的机理进行. 首先, 1-己烯与分子筛的Brønsted酸性位形成π配位复合物. 接着, 酸质子发生迁移使1-己烯的双键端基碳原子被质子化, 同时双键的另一碳原子与失去质子的Brønsted酸羟基的氧原子成键, 形成稳定的烷氧基中间体. 然后, 烷氧基中间体中的C―O共价键被打断, 同时Brønsted酸羟基的氧原子从C₆H₁₃基团提取一个氢原子还原分子筛的酸性位, 并且生成cis-2-己烯. 这一反应路径与借助于分子筛活性位的酸-碱双功能性质的反应路径是相互竞争的. 计算得到的表观活化能是59.37 kJ·mol^-1, 该值与实验值非常接近. 这一结果合理解释了双键异构过程中的能量特征, 并且扩展了对分子筛活性位本质的理解.     

Role of rare earth cations in Y zeolite for hydrocarbon cracking

Reaction kinetics data were collected for isobutane conversion over a series of ultra stable Y (USY) zeolite catalysts with and without rare earth cations and subjected to various extents of dealumination by steaming. We conducted these reaction studies at low temperatures (523-573 K) using isobutane feed streams containing known levels of isobutylene (100-400 ppm) so that the kinetics were controlled by bimolecular hydride transfer and oligomerization/beta-scission processes with little or no participation of monomolecular initiation reactions. These experimental conditions led to stable catalyst performance with the main products of isobutane conversion being propane, n-butane, and isopentane, with smaller amounts of propylene, trans-2-butene, and cis-2-butene. The rates of formation of these products per Br?nsted acid site (as counted by pyridine adsorption) depended exponentially on Br?nsted acid site density, regardless of whether the catalyst contained rare earth cations. Kinetic modeling showed an exponential dependence of hydride transfer and oligomerization/ beta-scission reaction rates on Br?nsted acid site density which translated into composite activation energies for these reactions having a linear relationship with site density. Based on results in the literature from theoretical calculations, we suggest that increasing Br?nsted acid site density in zeolite Y leads to larger zeolite elasticity, increased stabilization of cationic transition states, and lower composite activation barriers for hydride transfer and beta-scission steps. The role of rare earth cations, therefore, is to ensure the retention of high Br?nsted acid site density under hydrothermal conditions, such as in fluid catalytic cracking (FCC) regenerators, where steam would dealuminate the Y zeolite framework and reduce this site density. It is for this reason that hydride transfer reaction rates are high in the presence of rare earth cations and lead to higher yields of less olefinic gasoline during FCC.     

建筑形象那些事

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。     

In situ (1)H and (13)C MAS NMR kinetic study of the mechanism of H/D exchange for propane on zeolite H-ZSM-5

The kinetics of hydrogen (H/D) exchange between Br?nsted acid sites of zeolite H-ZSM-5 and variously deuterated propanes (propane-d(8), propane-1,1,1,3,3,3-d(6), propane-2,2-d(2)) have been monitored in situ by (1)H MAS NMR spectroscopy within the temperature range of 503-556 K. The contribution of intramolecular hydrogen transfer to the H/D exchange in the adsorbed propane was estimated by monitoring the kinetics of (13)C-labeled carbon scrambling in propane-2-(13)C in situ with (13)C MAS NMR at 543-573 K. Possible mechanisms of the exchange have been verified on the basis of the analysis of the variation of protium concentration in both the methyl and the methylene groups of propane in dependence of the reaction time. The main route of the exchange consists of a direct exchange of the acidic OH groups of the zeolite with either the methyl groups or the methylene group presumably with a pentacoordinated carbonium ion intermediate. The assumption that the intramolecular H scrambling between the methyl groups and the methylene group of propane via carbenium-ion-type intermediates is the fastest process among the other possible routes does not account for the experimental kinetics of H/D exchange for propanes with different initial contents and locations of deuterium in a propane molecule. The rate constant (k(3)) for intramolecular H/D exchange between the methyl and the methylene groups is 4-5 times lower compared to those of the direct exchange of both the methyl (k(1)) and the methylene (k(2)) groups with Br?nsted acid sites of the zeolite, the k(1) being ca. 1.5 times higher than k(2). At lower temperature (473 K), the exchange is slower, and the expected difference between k(1) and k(2) is more essential, k(1) = 3k(2). This accounts for earlier observed regioselectivity of the exchange for propane on H-ZSM-5 at 473 K. Faster direct exchange with the methyl groups compared to that with the methylene groups was attributed to a possible, more spatial accessibility of the methyl groups for the exchange. Similar activation energies for H and C scramblings with a 2 times more rapid rate of H scrambling was rationalization by the proceeding of these two processes through an isopropyl cation intermediate, as in classical carbenium ion chemistry.     

MCM-22分子筛酸性的DFT理论计算研究

本文利用量子力学中的密度泛函理论(DFT)计算，研究了MCM-22分子筛上骨架Al在8个不同的T位的分布和Br?nsted酸的落位及强度。所有计算基于分子筛的8T簇模型 (H₃SiO)₃Si-O(H)-T(OSiH₃)₃(T=Si，Al)，采用DFT的BLYP方法，所有原子均应用DNP基组。通过计算(Al，H)/Si替代能和质子亲和势，得出推论：MCM-22分子筛中骨架Al的最有利落位在T1，T4，T3和T8位。而形成Br?nsted-酸的最可能的位置为Al1-O3-Si4，Al4-O3-Si1，Al3-O11-Si2和Al8-O10-Si2桥基。Al1-O3H-Si4和Al4-O3H-Si1位的酸性强度接近，Al3-O11H-Si2和Al8-O10H-Si2位的酸性分别略低于和略高于前两个酸位。通过计算模板剂分子六次甲基亚胺(HMI)与B-酸中心的相互作用，进一步探讨了HMI对分子筛中Al落位的靶向作用。     

Density functional theoretical investigations on various nanostructural zeolite surfaces

Through density functional calculations, the Br?nsted acidities on various nanostructural ZSM-5 zeolite surfaces were studied as well as the hydrogen exchanging processes with adsorbed H(2)O monomer or dimer. The Br?nsted acidities on the four nanostructural surfaces show differences, although slightly, with their strengths increasing as (100) < (210) < (410) < (001). For hydrogen exchanging processes with H(2)O monomer or dimer, the reaction rate increases in the order (210) < (100) < (001) < (410) or (210) < (410) < (001). No transition-state structure is present on H(2)O dimer/(100) surface system. The introduction of a second H(2)O molecule accelerates the hydrogen exchanging processes and meanwhile influences the nanostructural geometries such that they are more evident. Besides the activation barrier, the adsorption energy and reaction heat display differences from one surface to another, which results in the preference of catalytic reactions to a specific nanostructural zeolite surface, such as the hydrogen exchanging processes studied in this paper.     

Identification and measurements of strong Brønsted acid site in ultrastable Y (USY) zeolite

By using the IRMS-TPD method in which IR (infrared) and MS (mass spectroscopy) worked together, acid sites of USY (ultrastable Y) zeolite were studied. A new band of OH playing a role of Br?nsted acid was clearly detected on Na2H2-ethylenediaminetetraacetic acid (EDTA)-treated USY at 3595 cm(-1) during an elevation in temperature after the adsorption of ammonia. MS-measured TPD (temperature-programmed desorption) of NH3 and IR-measured TPD of the NH4+ cation coincided well to show that this zeolite consisted of the Br?nsted acid sites. The MS-TPD profile at higher temperatures corresponded to the IR-TPD of the 3595-cm(-1) band, and therefore, this OH was identified as a strong acid site. From comparison between IR-TPD of OH and MS-TPD, numbers of three kinds of Br?nsted OH (i.e., those in super and sodalite cages of a Y zeolite structure) and created strong Br?nsted acid site were quantified. On the other hand, strength of the Br?nsted acid site DeltaH was determined individually by a simulation method, where the corrected IR-TPD of OH was simulated based on the proposed equation. Thus, a new strong Br?nsted acid site was identified in the EDTA-treated USY, and the amount and strength was measured quantitatively.     

Hydrogen storage in low silica type X zeolites

Low silica type X zeolites (LSX, Si/Al = 1) fully exchanged by alkali-metal cations (Li(+), Na(+), and K(+)) were studied for their hydrogen storage capacities. Hydrogen adsorption isotherms were measured separately at 77 K and <1 atm, and at 298 K and <10 MPa. It was found that the hydrogen adsorption capacity of LSX zeolite depended strongly on the cationic radius and the density of the cations that are located on the exposed sites. The interaction energies between H(2) and the cations follow the order Li(+) > Na(+) > K(+), as predicted based on the ionic radii. Oxygen anions on zeolite framework were minor adsorption sites. Li-LSX had an H(2) capacity of 1.5 wt % at 77 K and 1 atm, and a capacity of 0.6 wt % at 298 K and 10 MPa, among the highest of known sorbents. The hydrogen capacity in LSX zeolite by bridged hydrogen spillover was also investigated. A simple and effective technique was employed to build carbon bridges between the H(2) dissociation catalyst and the zeolite to facilitate spillover of hydrogen atoms. Thus, the hydrogen storage capacity of Li-LSX zeolite was enhanced to 1.6 wt % (by a factor of 2.6) at 298 K and 10 MPa. This is by far the highest hydrogen storage capacity obtained on a zeolite material at room temperature. Furthermore, the adsorption rates were fast, and the storages were shown to be fully reversible and rechargeable. Further optimization of the bridge building technique would lead to an additional enhancement of hydrogen storage.     

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。     

Brönsted and Lewis acidity of the BF3/gamma-Al2O3 alkylation catalyst as revealed by solid-state NMR spectroscopy and DFT quantum chemical calculations

Multinuclear solid-state NMR techniques and DFT quantum chemical calculations were employed to investigate the detailed structure of acid sites on the BF3/gamma-Al2O3 alkylation catalyst. The NMR experiment results indicate that gaseous BF3 is able to react with the hydroxyl groups present on the surface of gamma-Al2O3, leading to the formation of new Br?nsted and Lewis acid sites. The 1H/11B and 1H/27Al TRAPDOR (TRAnsfer of Population in DOuble Resonance) experiments suggest that the 3.7 ppm signal in 1H NMR spectra of the BF3/gamma-Al2O3 catalyst is due to a bridging B-OH-Al group that acts as a Br?nsted acid site of the catalyst. On the other hand, a Lewis acid site on the surface of the catalysts, as revealed by 31P MAS and 31P/27Al TRAPDOR NMR of adsorbed trimethylphosphine, is associated with three-coordinate -OBF2 species. 13C NMR of adsorbed 2-13C-acetone indicates that the Br?nsted acid strength of the catalyst is slightly stronger than that of zeolite HZSM-5 but still weaker than that of 100% H2SO4, which is in good agreement with theoretical prediction. In addition, DFT calculations also reveal the detailed structure of various acid sites formed on the BF3/gamma-Al2O3 catalyst and the interaction of probe molecules with these sites.