The Effect of Ge Incorporation on the Brønsted Acidity of ZSM‐5

The effect of the isomorphous substitution of some of the Si atoms in ZSM‐5 by Ge atoms on the Brønsted acid strength has been investigated by i) DFT calculations on cluster models of the formula ((HO)₃SiO)₃‐Al‐O(H)‐T‐(OSi(OH)₃)₃, with T=Si or Ge, and ((HO)₃SiO)₃‐Al‐O(H)‐Si‐(OGe(OH)₃)(OSi(OH)₃)₂, ii) a ³¹P NMR study of zeolite samples contacted with trimethyl phosphine oxide probe molecules and iii) a X‐ray photoelectron spectroscopy (XPS) study of ZSM‐5 and Ge‐ZSM‐5 samples. The calculations reveal that the effect of Ge incorporation on the framework acidity strongly depends on the degree of substitution and on the exact T‐atom positions that are occupied by Ge. High Ge concentrations allow for enhanced stabilisation of the deprotonated Ge‐ZSM‐5 through structural relaxation, resulting in a slightly higher acidity as compared to ZSM‐5. This structural relaxation is not achievable in Ge‐ZSM‐5 with a low Ge content, which therefore has a slightly lower acidity than ZSM‐5. The NMR study indicates no difference between the Brønsted acidity of ZSM‐5(47) and Ge(0.09)ZSM‐5(36). Instead, evidence for the presence of a substantial amount of Ge? OH groups in the Ge‐containing samples was obtained from the NMR results, which is consistent with earlier FTIR studies. The XPS results do not point to an effect of Ge on the framework acidity of ZSM‐5(47), instead, the results can be best interpreted by assuming the presence of additional Ge? OH and Si? OH groups near the surface of the Ge(0.08)ZSM‐5(47) sample.     

Framework Stability and Brønsted Acidity of Isomorphously Substituted Interlayer‐Expanded Zeolite COE‐4: A Density Functional Theory Study

COE‐4 zeolites possess a unique two‐dimensional ten‐ring pore structure with the Si(OH)₂ hydroxyl groups attached to the linker position between the ferrierite‐type layers, which has been demonstrated through the interlayer‐expansion approach in our previous work (H. Gies et al. Chem. Mater.­ 2012 , 24, 1536). Herein, density functional theory is used to study the framework stability and Brønsted acidity of the zeolite T‐COE‐4, in which the tetravalent Si is isomorphously substituted by a trivalent Fe, B, Ga, or Al heteroatom at the linker position. The influences of substitution energy and equilibrium geometry parameters on the stability of T‐COE‐4 are investigated in detail. The relative acid strength of the linker position is revealed by the proton affinity, charge analysis, and NH₃ adsorption. It is found that the range of the 〈T‐O‐Si〉 angles is widened to maintain the stability of isomorphously substituted T‐COE‐4 zeolites. The smaller the 〈O1‐T‐O2〉 bond angle is, the more difficult is to form the regular tetrahedral unit. Thus, the substitution energies at the linker positions increase in the following sequence: Al‐COE‐4 < Ga‐COE‐4 < Fe‐COE‐4 < B‐COE‐4. The adsorption of NH₃ as a probe molecule indicates that the acidity can affect the hydrogen‐bonding interaction between (N?H???O2) and (N???H?O2). The relative Brønsted‐acid strength of the interlayer‐expanded T‐COE‐4 zeolite decreases in the order of Al‐COE‐4 > Ga‐COE‐4 > Fe‐COE‐4 > B‐COE‐4. These findings may be helpful for the structural design and functional modification of interlayer‐expanded zeolites.     

原位程序升温脱附(TPD)方法的建立及其应用实例

介绍了在ＶＧＥＳＣＡＬＡＢ２１０电子能谱仪上自己研制组装的用于研究单晶金属表面吸附层的原位ＴＰＤ装置。研制的单晶样台采用钽丝加热，最高加热速度可达３０Ｋ／ｓ，线性升温范围－１００－１００℃，最高可达１０００℃以上。     

迎头色谱程序升温脱附快速测定吸附热新方法

在Ｐｏｌａｎｙｉ吸附势理论基础上，结合程序升温脱附曲线的测定，建立了一个快速测定吸附热的新方法，详细讨论了该方法的原理，通过微机采样和数据处理，测定一条吸附热与覆盖度的曲线仅需１ｈ左右。     

Cyclization Triggered by Deprotonation: The Gas‐Phase Acidity of 1,8‐Chalcogen‐Bridged Naphthalenes

High‐level density functional theory computations have been used to estimate the gas‐phase (intrinsic) acidities of the complete series of 1,8‐chalcogen‐bridged naphthalene derivatives. The existence of a chalcogen? chalcogen bond in chalcogen‐bridged naphthalene derivatives plays a crucial role in the intrinsic acidity of the system. For 1,8‐naphthalenediylbis(oxy), where this bond does not exist, the para C? H group is the most acidic site, whereas for the remaining compounds, deprotonation of the ortho CH groups is the most favorable process. Deprotonation of the aromatic rings has a large effect on the strength of the bonds of the five‐membered ring. These effects depend on the nature of the heteroatoms forming the X? Y bridge, and modulate the acidity of the molecule. Also importantly, when one of the heteroatoms is oxygen, ortho and para deprotonation lead to cleavage of the X? Y bridge. This bond fission favors the formation of a CYC (Y=S, Se, Te) three‐membered ring that enhances the stability of the anion and, therefore, increases the acidity of these compounds. We have shown that, whereas this cyclization process is energetically favorable for oxygen‐containing compounds, it is not favorable for the remaining derivatives.     

Coverage‐ and Temperature‐Dependent Metalation and Dehydrogenation of Tetraphenylporphyrin on Cu(111)

Using temperature‐programmed desorption, supported by X‐ray photoelectron spectroscopy and scanning tunneling microscopy, a comprehensive overview of the main reactions of 5,10,15,20‐tetraphenyl‐21H,23H‐porphyrin (2HTPP) on Cu(111) as a function of coverage and temperature is obtained. Three reactions were identified: metalation with Cu substrate atoms, stepwise partial dehydrogenation, and finally complete dehydrogenation. At low coverage the reactions are independent of coverage, but at higher coverage metalation becomes faster and partial dehydrogenation slower. This behavior is explained by a weaker interaction between the iminic nitrogen atoms and the Cu(111) surface in the high‐coverage checkerboard structure, leading to faster metalation, and the stabilizing effect of T‐type interactions in the CuTPP islands formed at high coverage after metalation, leading to slower dehydrogenation. Based on the amount of hydrogen released and the appearance in STM, a structure of the partially dehydrogenated molecule is suggested.     

Structural,Electronic, and Bonding Properties of Zeolite Sn‐Beta: A Periodic Density Functional Theory Study

The structural, electronic, and the bonding properties of the zeolite Sn‐beta (Sn‐BEA) have been investigated by using the periodic density functional theory. Each of the nine different T‐sites in BEA were substituted by Sn atoms and all the nine geometries were completely optimized by using the plane‐wave basis set in conjunction with the ultra‐soft pseudopotential. On the basis of the structural and the electronic properties, it has been demonstrated that the substitution of Sn atoms in the BEA framework is an endothermic process and hence the incorporation of Sn in the BEA is limited. The lowest unoccupied molecular orbitals (LUMO) energies have been used to characterize the Lewis acidity of each T‐site. On the basis of the relative cohesive energy and the LUMO energy, the T2 site is shown to be the most favorable site for the substitution Sn atoms in the BEA framework.     

Mechanistic Studies on the Transformation of Ethanol into Ethene over Fe‐ZSM‐5 Zeolite

Ethanol, through the utilization of bioethanol as a chemical resource, has received considerable industrial attention as it provides an alternative route to produce more valuable hydrocarbons. Using a density functional theory approach incorporating the M06‐L functional, which includes dispersion interactions, a large 34T nanocluster model of Fe‐ZSM‐5 zeolite in which T is a Si or Al atom is employed to examine both the stepwise and concerted mechanisms of the transformation of ethanol into ethene. For the stepwise mechanism, ethanol dehydration commences from the first hydrogen abstraction of the ethanol OH group to form the ethoxide‐hydroxide intermediate with a low activation energy of 17.7 kcal mol^?1. Consequently, the ethoxide‐hydroxide intermediate is decomposed into ethene through hydrogen abstraction from the ethoxide methyl carbon to either the OH group of hydroxide or the oxygen of the ethoxide group with high activation energies of 64.8 and 63.5 kcal mol^?1, respectively. For the concerted mechanism, ethanol transformation into the ethene product occurs in a single step without intermediate formation, with an activation energy of 32.9 kcal mol^?1.     

Oxidative Dehydrogenation of Propane over a VO2‐Exchanged MCM‐22 Zeolite: A DFT Study

The adsorption and the mechanism of the oxidative dehydrogenation (ODH) of propane over VO₂‐exchanged MCM‐22 are investigated by DFT calculations using the M06‐L functional, which takes into account dispersion contributions to the energy. The adsorption energies of propane are in good agreement with those from computationally much more demanding MP2 calculations and with experimental results. In contrast, B3LYP binding energies are too small. The reaction begins with the movement of a methylene hydrogen atom to the oxygen atom of the VO₂ group, which leads to an isopropyl radical bound to a HO? V? O intermediate. This step is rate determining with the apparent activation energy of 30.9 kcal mol^?1, a value within the range of experimental results for ODH over other silica supports. In the propene formation step, the hydroxyl group is the more reactive group requiring an apparent activation energy of 27.7 kcal mol^?1 compared to that of the oxy group of 40.8 kcal mol^?1. To take the effect of the extended framework into account, single‐point calculations on 120T structures at the same level of theory are performed. The apparent activation energy is reduced to 28.5 kcal mol^?1 by a stabilizing effect caused by the framework. Reoxidation of the catalyst is found to be important for the product release at the end of the reaction.     

Computational and Variable‐Temperature Infrared Spectroscopic Studies on Carbon Monoxide Adsorption on Zeolite Ca‐A

Bridged Ca ²⁺ ???CO???Ca ²⁺ complexes are formed on dual‐cation sites, constituted by a pair of nearby Ca²⁺ cations, when CO is adsorbed on zeolite Ca‐A. Two types of such species can be formed, designated S2–S1 and S1–S1 (see picture). Ca²⁺???CO monocarbonyl species are also identified, and at a relatively high CO equilibrium pressure, dicarbonyl complexes can also form.

     

On the Origin of the Enhanced Acidity of Chalcocyclopentadienes (Cyclopentadiene Chalcogenols) in the Gas Phase

The intrinsic acidity of chalcocyclopentadienes (CpXH; X=O, S, Se, Te) is investigated by high‐level G3B3 and G2 ab initio as well as B3LYP DFT calculations, which show that, independent of the nature of the heteroatom, all chalcocyclopentadienes are stronger acids in the gas phase than cyclopentadiene. However the acidity does not increase regularly down the group, and the acidity enhancement for Te derivatives is five times larger than for O derivatives, but only twice that of S‐containing compounds. The most favorable deprotonation process corresponds to loss of the proton attached to the heteroatom, with the sole exception of the 5‐substituted 1,3‐cyclopentadienes, for which the O and S derivatives are predicted to behave as carbon acids. No matter the nature of the heteroatom, the 1‐substituted 1,3‐cyclopentadienes are the strongest acids. The intrinsic acidity of all isomers, namely, 1‐substituted, 2‐substituted, and 5‐substituted 1,3‐cyclopentadienes, increases with increasing aromaticity of the anion formed on deprotonation, and therefore the Te compound is the strongest acid for the three series. However, the intrinsic acidity of chalcocyclopentadienes is not dictated by aromaticity, so that, in general, the most stable deprotonated species do not coincide with the most aromatic ones.     

A DFT Study of Tungsten–Methylidene Formation on a W/ZSM‐5 Zeolite: The Metathesis Active Site

Tungsten–methylidene formation from ethene on either the W^IV, W^V, or W^VI active sites of a W/ZSM‐5 zeolite is investigated by using the M06‐L functional. The reaction is assumed to proceed in two steps; the first step is the [2+2] cycloaddition between ethene and the W?O active site to form an oxametallacycle intermediate. The intermediate is then decomposed to produce the W–methylidene active site from the metathesis reaction. The overall activation barrier of the reaction on W^VI (27.3 kcal mol^?1) is considerably lower than the ones for W^IV and W^V (69.4 and 37.1 kcal mol^?1, respectively). Moreover, the reaction involving the W^VI site also stabilizes intermediates and products to a larger extent than the ones on the W^IV and W^V sites. As a result, we have demonstrated that the reaction of the W–methylidene metathesis active site is both kinetically and thermodynamically favored to occur on the W^VI active site of the zeolite.     

Characterization of the acidic properties of zeolites by means of temperature-programmed desorption (TPD) of ammonia

Desorption energy distributions were calculated for temperature-programmed desorption (TPD) of ammonia from H zeolites of different type by means of regularization. This method does not require any limiting assumptions about the distribution function. It could be shown that the desorption energy distributions obtained are nearly independent of the experimental conditions and therefore they should represent a suitable measure for the distribution of the strength of acidic sites. The calculated desorption energy distributions for the ammonia desorption from the isolated bridging SiOHAl groups of H zeolites of different type significantly differ from each other in shape. The increase of the desorption energy of the main range of the distribution functions correlates well with the increase of the average acid strength of the SiOHAl groups with decreasing Al content of the zeolites.     

Identification of tert‐Butyl Cations in Zeolite H‐ZSM‐5: Evidence from NMR Spectroscopy and DFT Calculations

Experimental evidence for the presence of tert‐butyl cations, which are important intermediates in acid‐catalyzed heterogeneous reactions, on solid acids has still not been provided to date. By combining density functional theory (DFT) calculations with ¹H/¹³C magic‐angle‐spinning NMR spectroscopy, the tert‐butyl cation was successfully identified on zeolite H‐ZSM‐5 upon conversion of isobutene by capturing this intermediate with ammonia.     

Imidazoline‐N‐oxyl: A DFT Study of Its Protonation Reaction

Imidazoline‐based nitroxides are developed as pH probes. Their pK_a values vary over a wide range (from 1 to 11), depending on the substituents attached to the five‐membered cyclic nitroxide. Density functional calculations using the PBE1PBE method at the 6‐31+G(d,p) level, combined with natural bond orbital (NBO), frontier molecular orbital (FMO), geometry, Mulliken charge, and thermodynamic analyses, are carried out to disclose the effects involved in the changes in pK_a. The studies show that the decrease of seven pK_a units from pyrrolidine ( 11 ) to imidazoline‐N‐oxyl 8 is due to the inductive electron‐withdrawing capacity of the nitroxyl group. On the other hand, by combining both the inductive and mesomeric electron‐withdrawing capacities of the NO₂ group with delocalization of the lone pair on the amino N atom of the π system of the vinyl linker, the pK_a of 4.5 of 8 was increased by around three units to 7.8 for 1 / 2 .     

Comparison of Cu‐ZSM‐5 Zeolites and Cu‐MOF‐505 Metal‐Organic Frameworks as Heterogeneous Catalysts for the Mukaiyama Aldol Reaction: A DFT Mechanistic Study

The density functional theory (DFT) model ONIOM(M06L/6‐311++G(2df,2p):UFF was employed to reveal the catalytic activity of Cu^II in the paddle‐wheel unit of the metal‐organic framework (MOF)‐505 material in the Mukaiyama aldol reaction compared with the activity of Cu‐ZSM‐5 zeolites. The aldol reaction between a silyl enol ether and formaldehyde catalyzed by the Lewis acidic site of both materials takes place through a concerted pathway, in which the formation of the C? C bond and the transfer of the silyl group occurs in a single step. MOF‐505 and Cu‐ZSM‐5 are predicted to be efficient catalysts for this reaction as they strongly activate the formaldehyde carbonyl carbon electrophile, which leads to a considerably lower reaction barrier compared with the gas‐phase system. Both MOF‐505 and Cu‐ZSM‐5 catalysts stabilize the reacting species along the reaction coordinate, thereby lowering the activation energy, compared to the gas‐phase system. The activation barriers for the MOF‐505, Cu‐ZSM‐5, and gas‐phase system are 48, 21, and 61 kJ mol^?1, respectively. Our results show the importance of the enveloping framework by stabilizing the reacting species and promoting the reaction.