Periodic DFT calculations of the stability of Al/Si substitutions and extraframework Zn2+ cations in mordenite and reaction pathway for the dissociation of H2 and CH4

The local stability of Al atoms replacing Si in the zeolite framework is compared for all inequivalent tetrahedral (T) sites in mordenite. For Al/Si substitutions in two T sites the stable location of the compensating extraframework Zn(2+) cation forming a Lewis acid site is determined. In the most stable Zn-MOR structures Zn(2+) is located in a small ring (5MR, 6MR) containing two Al/Si substitutions. In less stable structures the Al atoms are placed at larger distances from each other and Zn(2+) interacts with only one Al site. The simulated adsorption of H(2) and CH(4) shows that adsorption strength decreases with increasing stability of the Zn(2+) Lewis site. A higher adsorption strength is observed for Zn(2+) deposited in the 5MR than for the 6MR. The reactivity of a series of stable Zn(2+) Lewis sites is tested via the dissociative adsorption of H(2) and CH(4). The heterolytic dissociation of the adsorbed molecule on the extraframework Zn(2+) cation produces a proton and an anion. The anion binds to Zn(2+) and proton goes to the zeolite framework, restoring a Br?nsted acid site. Because bonding of the anion to Zn(2+) is almost energetically equivalent for Zn(2+) in any of the extraframework positions the dissociation is governed by stabilizing bonding of the proton to the framework. Those structures which can exothermically accommodate the proton represent reaction pathways. Due to the repulsion between the proton and Zn(2+) the most favorable proton-accepting O sites are not those of the ring where Zn(2+) is deposited, but O sites close to the ring. Large differences are observed for neighboring positions in a- and b-directions and those oriented along the c-vector. Finally, among the stable Zn(2+) Lewis sites not all represent reaction pathways for dehydrogenation. For all of them the dissociation of H(2) is an exothermic process. In structures exhibiting the highest reactivity the Al/Si substitutions are placed at a large distance and the Zn(2+) cation interacts with O-atoms next to Al in the T4 site of the 5MR. This Lewis site is strong enough to break the C-H bond in the CH(4) molecule.     

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.     

A density functional theory study of molecular and dissociative adsorption of H2 on active sites in mordenite

Adsorption and chemisorption of H2 in mordenite is studied using ab initio density functional theory (DFT) calculations. The geometries of the adsorption complex, the adsorption energies, stretching frequencies, and the capacity to dissociate the adsorbed molecule are compared for different active sites. The active centers include a Br?nsted acid site, a three-coordinated surface Al site, and Lewis sites formed by extraframework cations: Na+, Cu+, Ag+, Zn2+, Cu2+, Ga3+, and Al3+. Adsorption properties of cations are compared for a location of the cation in the five-membered ring. This location differs from the location in the six-membered ring observed for hydrated cations. The five-membered ring, however, represents a stable location of the bare cation. In this position any cation exhibits higher reactivity compared with the location in the six-membered ring and is well accessible by molecules adsorbed in the main channel of the zeolite. Calculated adsorption energies range from 4 to 87 kJ/mol, depending on electronegativity and ionic radius of the cation and the stability of the cation-zeolite complex. The largest adsorption energy is observed for Cu+ and the lowest for Al3+ integrated into the interstitial site of the zeolite framework. A linear dependence is observed between the stretching frequency and the bond length of the adsorbed H2 molecule. The capacity of the metal-exchanged zeolite to dissociate the H2 molecule does not correlate with the adsorption energy. Dissociation is not possible on single Cu+ cation. The best performance is observed for the Ga3+, Zn2+, and Al3+ extraframework cations, in good agreement with experimental data.     

Modeling the local structure and energetics of protozeolitic nanoclusters in hydrothermally stable aluminosilicate mesostructures

The density functional theory (DFT) method is used to investigate the structure and bonding of silica and aluminosilicate nanoclusters containing five- and six-membered oxygen rings. The clusters, which are derived from the BEA zeolite structure, are considered as models of the protozeolitic clusters that are incorporated into the pore walls of steam stable aluminosilicate mesostructures assembled from zeolite seeds. Two locally different Br?nsted acid sites in the aluminosilicate structure are identified for the adsorption of a water molecule. The sterically more open acid site is favored for water binding. The stability of the aluminosilicate structure in the presence of H2O molecule is studied by breaking an Al-O bond and inserting a water molecule into the five-membered ring structure. We find that an excitation energy at least 18 times larger than the room-temperature thermal energy is needed to break the stable five-membered ring structure, implying a high hydrothermal stability and acidity for this aluminosilicate structure.     

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.     

MFI分子筛限域空间内Pd催化剂上甲烷燃烧

甲烷是一种重要的温室气体,其开发利用过程中不完全燃烧所残留的气体排放到大气中会造成严重的环境问题,因此提高甲烷燃烧效率显得尤为重要.与传统燃烧方式相比,催化燃烧在低温区表现出高的燃烧效率,成为甲烷燃烧理想的选择.在实际应用时,甲烷燃烧催化剂应在低温区具备高的催化活性,同时在过量水蒸气存在下具备好的稳定性.负载型Pd基催化剂是当前研究最多的甲烷燃烧催化剂,Pd粒子尺寸、载体类型、酸性位点以及金属与载体的相互作用是影响甲烷燃烧活性与稳定性的关键因素.本文设计了原位水热合成路线将孤立的Pd离子稳定封装于MFI分子筛孔道内(Pd@MFI),以期获得高活性、高稳定性的甲烷燃烧催化剂,并揭示其反应机理与构效关系.通过X射线粉末衍射、高分辨透射电子显微镜以及球差校正扫描透射电子显微镜分析了Pd@MFI催化剂的基本结构,并直接观测了Pd物种在分子筛晶体中的分布;进而利用氨气程序升温脱附、固体核磁共振、氢气程序升温还原、X射线光电子能谱(XPS)和CO吸附红外光谱等表征技术研究了催化剂的酸性以及Pd在分子筛中的存在状态.表征结果证实,通过原位水热合成方法可将Pd物种以pd2+和Pd(OH)+的形式封装在MFI分子筛孔道内,孤立的Pd离子与分子筛骨架之间存在着强相互作用,有效稳定Pd离子并实现贵金属Pd的最大化利用.在甲烷燃烧反应中,Pd@H-ZSM-5在高空速下表现出较好的催化活性与较低的表观活化能(70.7 kJ/mol).热稳定性及耐水性测试结果表明,Pd@H-ZSM-5在400℃下连续反应100 h后甲烷燃烧活性无明显下降,且反应后Pd物种在分子筛孔道内仍保持高度分散,说明该催化剂在甲烷燃烧过程中具备优异的稳定性和抗烧结性能.通过反应动力学、程序升温脱附以及原位红外光谱等技术手段研究了甲烷催化氧化机理,结果表明,Brφnsted酸性位点的存在有利于甲烷吸附并促进其在相邻Pd位点上活化,在MFI分子筛限域空间内形成Pd位点和Brφnsted酸性位点的有效协同.原位近常压XPS分析结果表明,Pd@H-ZSM-5催化的甲烷燃烧过程中存在着pd2+-pdn+-pd2+的可逆氧化还原循环.综合分析上述结果,最终可阐明Pd@H-ZSM-5模型催化剂上甲烷燃烧的反应机理.     

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。     

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.     

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。     

Quantitative measurement of the Brönsted acid sites in solid acids: toward a single-site design of Mo-modified ZSM-5 zeolite

On the basis of our previous H/D exchange studies devoted to the quantification of the number of Br?nsted acid sites in solid acids, we report here an innovative approach to determine both the amount and the localization of Mo atoms inside the Mo/ZSM-5 catalyst, commonly used for the methane dehydroaromatization reaction. The influence of Mo introduction in the MFI framework was studied by means of BET, X-ray diffraction, 27Al magic angle spinning NMR, NH3 temperature-programmed desorption, and H/D isotopic exchange techniques. A dependence was found between the decrease of acidic OH groups and the Mo content. Depending on the Si/Al ratio of the zeolite, i.e., the proximity of two Br?nsted acid sites, the Mo atoms substitute a different number of OH groups. Consequently, a chemical structure was proposed to describe the geometry of the Mo complex in the channels of the ZSM-5 zeolite.     

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.     

The synergic effect between Mo species and acid sites in Mo/HMCM-22 catalysts for methane aromatization

The acid properties of Mo/HMCM-22 catalyst, which is the precursor form of the working catalyst for methane aromatization reaction, and the synergic effect between Mo species and acid sites were studied and characterized by various characterization techniques. It is concluded that Br?nsted and Lewis acidities of HMCM-22 are modified due to the introduction of molybdenum. We suggest a monomer of Mo species is formed by the exchange of Mo species with the Br?nsted acid sites. On the other hand, coordinate unsaturated sites (CUS) are suggested to be responsible for the formation of newly detected Lewis acid sites. Computer modelling is established and coupling with experimental results, it is then speculated that the effective activation of methane is properly accomplished on Mo species accommodated in the 12 MR supercages of MCM-22 zeolite whereas the Br?nsted acid sites in the same channel system play a key role for the formation of benzene. A much more pronounced volcano-typed reactivity curve of the Mo/HMCM-22 catalysts, as compared with that of the Mo/HZSM-5, with respect to Mo loading is found and this can be well understood due to the unique channel structure of MCM-22 zeolite and synergic effect between Mo species and acid sites.     

Spectroscopic and XRD characterisation of zeolite catalysts active for the oxidative methylation of benzene with methane

The benzene methylation with methane over zeolite catalysts was previously shown in our laboratory to require the presence of oxygen. Thus, a two-step mechanism involving the intermediate formation of methanol by partial oxidation of methane followed by the methylation of benzene with methanol in the second step, was postulated. This paper now reports the results of the characterisation of the zeolite catalysts used for the oxidative benzene methylation reaction in order to provide some information about their composition, structure, properties and their behaviour before and after the reaction. The catalysts were characterised by X-ray diffraction (XRD), inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray fluorescence (XRF), FT-IR and solid state NMR. XRD results indicate that the crystalline structures of all the ZSM-5 and H-beta catalysts remained unchanged after batch reaction of benzene with methane over the catalysts in agreement with the observation that the catalysts recovered from the reactor could be reused without loss of activity. Elemental analyses and FT-IR data show that as the level of metal ion exchange increases, the Br?nsted acid concentration decreases but this metal ion exchange does not totally remove Br?nsted acidity. FT-IR results further show that only a small amount of acid sites is actually necessary for a catalyst to be active since used catalysts containing highly reduced Br?nsted acidity are found to be reusable without any loss of their activity. 29Si and 27Al magic angle spinning (MAS) NMR together with FT-IR spectra also show that all the active zeolites catalysts contain some extra-framework octahedral aluminium in addition to the normal tetrahedral framework aluminium. The presence of this extra-lattice aluminium does not, however, have any adverse effect on the crystallinity of the catalysts both before and after oxidative benzene methylation reaction. There appears also to be no significant dealumination of the zeolite catalysts during reaction since their catalytic performance was retained after use.     

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。     

Solid-state MAS NMR studies of Brønsted acid sites in zeolite H-Mordenite

(17)O-(1)H double resonance NMR spectroscopy was used to study the local structure of zeolite H-Mordenite. Different contact times were used in cross-polarization magic angle spinning (CPMAS) NMR, CP rotational-echo double resonance (CP-REDOR) NMR, and heteronuclear correlation (HETCOR) NMR spectroscopy to distinguish between Br?nsted acid sites with different O-H distances. The accessibility of the various Br?nsted acid sites was quantified by adsorbing the basic probe molecule trimethylphosphine in known amounts. On the basis of these experiments, locations of different Br?nsted acid sites in H-Mordenite (H-MOR) were proposed. The use of (17)O chemical shift correlations to help assign sites is discussed.     

环己烯对噻吩在CuY分子筛上吸附的影响机制

A CuY zeolite prepared by liquid phase ion exchange was characterized by X-ray photoelectron spectroscopy, pyridine in situ Fourier transform infrared (in situ FTIR) spectroscopy, and ammonia temperature programmed desorption. The effect of cyclohexene on the adsorption of thiophene over the prepared CuY zeolite was explored by in situ FTIR. In particular, the role of the zeolite's Br?nsted acidity was investigated in the adsorption process. The results show that the percentage of Cu⁺ on the surface of the CuY zeolite can reach 77%. The surface acidity of the CuY zeolite mainly comprises medium and strong Br?nsted acidity and Lewis acidity. According to the adsorption results, cyclohexene negatively influences thiophene adsorption on the Br?nsted or Lewis acid sites in CuY by competitive adsorption. Although polymerization of thiophene and cyclohexene can occur easily on the HY or REY zeolites, the presence of Br?nsted acids in the CuY zeolite was not sufficient to polymerize either thiophene or cyclohexene. This difference may be caused by an anti-synergistic effect between the Cu ions of the CuY zeolite and neighboring Br?nsted acid sites, the result of which inhibits the polymerization of adsorbed thiophene and cyclohexene.     

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.     

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.