Rigid Arrangements of Ionic Charge in Zeolite Frameworks Conferred by Specific Aluminum Distributions Preferentially Stabilize Alkanol Dehydration Transition States

Zeolite reactivity depends on the solvating environments of their micropores and the proximity of their Brønsted acid sites. Turnover rates (per H⁺) for methanol and ethanol dehydration increase with the fraction of H⁺ sites sharing six-membered rings of chabazite (CHA) zeolites. Density functional theory (DFT) shows that activation barriers vary widely with the number and arrangement of Al (1–5 per 36 T-site unit cell), but cannot be described solely by Al–Al distance or density. Certain Al distributions yield rigid arrangements of anionic charge that stabilize cationic intermediates and transition states via H-bonding to decrease barriers. This is a key feature of acid catalysis in zeolite solvents, which lack the isotropy of liquid solvents. The sensitivity of polar transition states to specific arrangements of charge in their solvating environments and the ability to position such charges in zeolite lattices with increasing precision herald rich catalytic diversity among zeolites of varying Al arrangement.     

Efficient use of bifunctional acid-base properties for alkylammonium formation in amine-substituted zeolites

The formation of alkylammonium groups in amine-doped zeolites is studied using density functional theory on small clusters representing the chemically active site. The presence of both strong Lewis base and Br?nsted acid sites leads to a significant lowering of reaction barriers as opposed to alkoxide formation in full-oxygen zeolites. Furthermore, amine-substituted zeolites suggest novel reaction pathways that are not solely centralized around the aluminum substitution but in which two tetrahedral sites are involved, maximizing use of the zeolitic acid site and its surroundings. An investigation of the proton mobility in these yet to be synthesized materials demonstrates the need for minimizing the amount of Al-NH-Si bridges, as to prevent protonation of the amine group.     

Comparison of density functionals for reactions of sulfur ylides with aldehydes and olefins

The reactions of a model sulfur ylide with formaldehyde and 1,1-dicianoethylene, leading to the formation of an epoxyde and a cyclopropane, respectively, have been studied using different computational methods, and the results have been compared to those obtained with the CBS-QB3 method. The second step of these reactions presents transition states similar to that of an SN2 reaction. Depending on the degree of electron delocalization at the transition state, a different amount of exact exchange is necessary in the exchange functional to obtain accurate energy barriers. This amount is larger for the reaction of formaldehyde, in which the transition state is more delocalized, than for the reaction of 1,1-dicianoethylene. Similar results have been obtained for symmetric and non-symmetric SN2 reactions. The calculation of the reaction path has shown that the error relative to CBS-QB3 tends to increase when approaching the transition state. Among the different computational methods, PBE1PBE is the one to provide the most accurate energy barriers and reaction energies, whereas BB1K leads to the best results for the reaction path before the transition state.     

Ab initio and microcalorimetric investigations of alkene adsorption on phosphotungstic acid

The adsorption of ethene, propene, 1-butene, trans-2-butene, and isobutene on phosphotungstic acid has been characterized by density functional theory (DFT) calculations and microcalorimetric experiments. The DFT-calculated chemisorption energies to form the corresponding alkoxides for ethene, propene, 1-butene, trans-2-butene, and isobutene were -86.8, -90.3, -102.6, -79.9, and -91.4 kJ mol(-1), respectively (for their most-favorable binding modes). The relative chemisorption energies to form the alkoxides are dictated by the strength of interaction of the acidic proton with the carbon atom of the double bond that becomes protonated. The activation barrier for chemisorption was greatest for alkenes with primary (1 degrees) carbenium-like transition states followed by secondary (2 degrees) and tertiary (3 degrees) transition states. The adsorption enthalpy established from microcalorimetric experiments with propene and isobutene was approximately -100 kJ mol(-1), which is close to the DFT-calculated values. Chemisorption of ethene on phosphotungstic acid during microcalorimetric experiments was minimal, presumably because of the large activation barrier associated with a 1 degrees carbenium-like transition state. The results from this study are compared with those in the literature for the adsorption of alkenes on zeolites, which have a similar adsorption mechanism. Our results suggest that alkene adsorption is stronger on phosphotungstic acid than on zeolites, as supported by the more exothermic chemisorption energies. Additionally, activation barriers for alkene adsorption are lower over phosphotungstic acid than over zeolites.     

Theoretical study on reactions catalyzed by gallium-substituted zeolites

The dehydrogenation and cracking reactions of light alkanes in gallium-containing zeolites were studied using density functional theory. Gallium isomorphically substituted, generating Brønsted acid sites, was used in the computations. The following reactions were examined: dehydrogenation of methane, ethane, propane, isobutane and cracking of ethane, propane and isobutene, all catalyzed by the framework gallium species. The cracking reaction seems to be favored relative to the dehydrogenation when framework gallium species are used. This behavior is also observed in aluminum-containing zeolites (H-ZSM5). The geometries and energetics of the transition states found for the gallium zeolites were compared with theoretical data for the same transition states in aluminum zeolites. There seems to be no significant difference between framework gallium and framework aluminum species. Therefore the framework gallium should not be the species responsible for the catalytic enhancement observed in gallium-containing zeolites.From the Proceedings of the 28th Congreso de Químicos Teóricos de Expresión Latina (QUITEL 2002)     

Shifting the scaling relations of single-atom catalysts for facile methane activation by tuning the coordination number

We investigate oxidative methane activation on a wide range of single transition metal atom catalysts embedded on N-doped graphene derivatives using density functional theory calculations. An inverse scaling relationship between *O formation and its hydrogen affinity is observed, consistent with a previous report. However, we find that the latter scaling line can be shifted towards a more reactive region by tuning the coordination number (CN) of the active metal sites. Specifically, we find that lowering the CN plays an important role in increasing the reactivity for methane activation via a radical-like transition state by moving the scaling lines. Thus, in the new design strategy suggested here, different from the conventional efforts focusing mainly on breaking the scaling relations, one maintains the scaling relations but moves them towards more reactive regions by controlling the coordination number of the active sites. With this design principle, we suggest several single atom catalysts with lower C–H activation barriers than some of the most active methane activation catalysts in the literature such as Cu-based zeolites.

The scaling relationship of methane activation via a radical-like transition state shifts toward a more reactive region with decreasing coordination number of the active sites.     

A DFT study of the thermal, orbital symmetry forbidden, cyclophanediene to dihydropyrene electrocyclic reaction. Predictions to improve the dimethyldihydropyrene photoswitches

The orbital symmetry forbidden thermal electrocyclic equilibria between a series of cyclophanedienes and dimethyldihydropyrenes (CPD<==>DDPs) were studied using density functional theory (DFT). These reactions are important not only because of their fundamental interest but also in how they restrict the potential utility of the DDP photoswitches by limiting the thermal lifetime of the CPDs. The transition states (TSs) for these reactions could not be modeled using restricted DFT (RB3LYP) but were located using unrestricted DFT (UB3LYP). Each TS possesses significant biradical character as indicated by their spin contaminated wave functions, S2 not = 0. Specific substitution by nitrile or trifluoromethyl group(s) is predicted to strongly affect the magnitude of the activation barriers for these reactions. In particular, replacing the internal methyl groups of the CPDs/DDPs with nitrile groups is predicted to have the maximum effect and to raise the activation barriers and lifetimes of the CPDs considerably.     

Adsorption of nitrogen oxides on graphene and graphene oxides: insights from density functional calculations

The interactions of nitrogen oxides NO(x) (x = 1,2,3) and N(2)O(4) with graphene and graphene oxides (GOs) were studied by the density functional theory. Optimized geometries, binding energies, and electronic structures of the gas molecule-adsorbed graphene and GO were determined on the basis of first-principles calculations. The adsorption of nitrogen oxides on GO is generally stronger than that on graphene due to the presence of the active defect sites, such as the hydroxyl and carbonyl functional groups and the carbon atom near these groups. These active defect sites increase the binding energies and enhance charge transfers from nitrogen oxides to GO, eventually leading to the chemisorption of gas molecules and the doping character transition from acceptor to donor for NO(2) and NO. The interaction of nitrogen oxides with GO with various functional groups can result in the formation of hydrogen bonds OH???O (N) between -OH and nitrogen oxides and new weak covalent bonds C???N and C???O, as well as the H abstraction to form nitrous acid- and nitric acidlike moieties. The spin-polarized density of states reveals a strong hybridization of frontier orbitals of NO(2) and NO(3) with the electronic states around the Fermi level of GO, and gives rise to the strong acceptor doping by these molecules and remarkable charge transfers from molecules to GO, compared to NO and N(2)O(4) adsorptions on GO. The calculated results show good agreement with experimental observations.     

金属-有机骨架材料中甲烷吸附机理的密度泛函理论研究

采用密度泛函理论研究了甲烷在MOF-5中的吸附位置、吸附构型和吸附能. 结果表明: 吸附位置主要有四种, Zn₄O簇为最佳吸附位, 其吸附能为17.38 kJ•mol^－1, 高于沸石中的甲烷吸附能. 从吸附能与MOF-5的结构关系分析得出: 在苯环中引入给电子基团, 有利于增强甲烷与MOFs的吸附作用; 引入含氧等极性官能团, 将增加甲烷吸附位, 有利于提高吸附储存量.     

第一性原理研究镍改性ZSM-12分子筛的酸性

分子筛被用作工业催化剂时常需要过渡金属改性,镍是制备加氢/脱氢催化剂常用的过渡金属,本研究采用密度泛函理论研究镍改性的ZSM-12分子筛的结构和酸性。结果表明,分子筛的B酸质子可以被镍原子还原成氢分子,而Ni_2的团簇不能将B酸质子还原生成氢气分子。镍原子在分子筛内会被氧化,并形成Lewis酸性位,这会导致分子筛骨架铝的Lewis酸性变弱,镍改性后,分子筛吸附氢气的能力变强,被吸附的氢分子解离为氢原子,并带负电荷,不再具有B酸的功能。从计算的氨分子的吸附能来判断,由于吸附的氢会从镍原子得到电子,吸附的氢分子会增强镍原子的Lewis酸性。     

Temperature-dependent growth shapes of Ni nanoclusters on NiAl(110)

Scanning tunneling microscopy studies reveal that two-dimensional nanoscale Ni islands formed by deposition of Ni on NiAl(110) between 200-400 K exhibit far-from-equilibrium growth shapes which change systematically with temperature. Island structure reflects the two types of adsorption sites available for Ni adatoms, and island shapes are controlled by the details of adatom diffusion along island edges accounting for numerous local configurations. The temperature dependence of the island shapes is captured and elucidated by kinetic Monte Carlo simulation of a realistic atomistic-level multisite lattice-gas model incorporating precise diffusion barriers. These barriers are obtained by utilizing density functional theory to probe energetics not just at adsorption sites but also at transition states for diffusion. This success demonstrates a capability for predictive atomistic-level modeling of nanocluster formation and shape selection in systems that have a high level of energetic and kinetic complexity.     

Origin of the SN2 benzylic effect

The S N2 identity exchange reactions of the fluoride ion with benzyl fluoride and 10 para-substituted derivatives (RC6H 4CH 2F, R = CH3, OH, OCH 3, NH2, F, Cl, CCH, CN, COF, and NO2) have been investigated by both rigorous ab initio methods and carefully calibrated density functional theory. Groundbreaking focal-point computations were executed for the C6H5CH 2F + F (-) and C 6H 5CH2Cl + Cl (-) SN2 reactions at the highest possible levels of electronic structure theory, employing complete basis set (CBS) extrapolations of aug-cc-pV XZ (X = 2-5) Hartree-Fock and MP2 energies, and including higher-order electron correlation via CCSD/aug-cc-pVQZ and CCSD(T)/aug-cc-pVTZ coupled cluster wave functions. Strong linear dependences are found between the computed electrostatic potential at the reaction-center carbon atom and the effective SN2 activation energies within the series of para-substituted benzyl fluorides. An activation strain energy decomposition indicates that the SN2 reactivity of these benzylic compounds is governed by the intrinsic electrostatic interaction between the reacting fragments. The delocalization of nucleophilic charge into the aromatic ring in the SN2 transition states is quite limited and should not be considered the origin of benzylic acceleration of SN2 reactions. Our rigorous focal-point computations validate the benzylic effect by establishing SN2 barriers for (F (-), Cl (-)) identity exchange in (C6H5CH2F, C6H 5CH2Cl) that are lower than those of (CH3F, CH3Cl) by (3.8, 1.6) kcal mol (-1), in order.     

Activation barriers for DNA alkylation by carcinogenic methane diazonium ions

Methylation reactions of the DNA bases with the methane diazonium ion, which is the reactive intermediate formed from several carcinogenic methylating agents, were examined. The SN2 transition states of the methylation reactions at N7, N3, and O6 of guanine; N7, N3, and N1 of adenine; N3 and O2 of cytosine; and O2 and O4 of thymine were calculated using the B3LYP density functional method. Solvation effects were examined using the conductor-like polarizable continuum method and the combined discrete/SCRF method. The transition states for reactions at guanine N3, adenine N7, and adenine N1 are influenced by steric interactions between the methane diazonium ion and exocyclic amino groups. Both in the gas phase and in aqueous solution, the methylation reactions at N atoms have transition states that are looser, and generally occur earlier along the reaction pathways than reactions at O atoms. The forming bonds in the transition states in water are 0.03 to 0.13 A shorter than those observed in the gas phase, and the activation energies are 13 to 35 kcal/mol higher. The combined discrete/SCRF solvation energy calculations using base-water complexes with three water molecules yield base solvation energies that are larger than those obtained from the CPCM continuum method, especially for cytosine. Reactivities calculated using barriers obtained with the discrete/SCRF method are consistent with the experimentally observed high reactivity at N7 of guanine.     

Potential energy surfaces of SimOn cluster formation and isomerization

The reaction paths for formation and isomerization of a set of silica SimOn (m = 2,3, n = 1-5) nanoclusters have been investigated using second-order perturbation theory (MP2) with the 6-31G(d) basis set. The MP2/6-31G(d) calculations have predicted singlet ground states for all clusters excluding Si3O2. The total energies of the most important points on the potential energy surfaces (PES) have been determined using the completely renormalized (CR) singles and doubles coupled cluster method including perturbative triples, CR-CCSD(T) with the cc-pVTZ basis set. Although transition states have been located for many isomerization reactions, only for Si3O3 and Si3O4 have some transition states been found for the formation of a cluster from the separated reactants. In all other cases, the process of formation of SimOn clusters appears to proceed without potential energy barriers.     

A theoretical study of imine-ene reaction influencing factors

Potential energy surfaces of the imine-ene reaction between methanimine and propene have been calculated using restricted and unrestricted density functional theory at the B3LYP level. The results show that a concerted mechanism with an exo configuration for lone pair electrons transition structure is more favourable for the intermolecular bare imine-ene reaction. In addition, the C-C bond formation and the migration of the propene α-hydrogen occur simultaneously. The promoting effect of seventeen Lewis acids was also studied. When a Lewis acid is used as a promoter, the corresponding activation and reaction energies decline greatly compared to those of the bare imine-ene reaction. A good correlation (R(2) > 0.9) was found between the activation barriers and the polar character of the transition states. Lewis acids and electron-withdrawing substituents on methanimine were proven to facilitate the imine-ene reaction, whereas electron-donating groups, conjugated groups or a bulky group hinders the reaction. The steric factor is the most unfavourable. The reactivity indices defined by the conceptual density functional theory were also studied and there is a good correlation between the activation barrier and electrophilicity. Frontier molecular orbital theory gives a good explanation for the above results.     

Study of the geometry and location of the bridging OH groups in aluminosilicate and silicoaluminophosphate type zeolites using 1H MAS NMR sideband analysis and CP/MAS NMR

A numerical analysis of the ¹H MAS NMR sideband pattern of the bridging OH groups in various zeolites of the aluminosilicate type (LTA, faujasite, erionite, mordenite, pentasil) and of the silicoaluminophosphate type (SAPO-5, −17, −34, −37) yields H-Al distances of the hydroxyl protons to the adjacent framework aluminium nucleus which cover a range between 0.234 and 0.252 nm. From the results, a relation between the H-Al distance and the size of the oxygen rings (6−, 8−, 10− and 12-membered oxygen rings) could be derived. Using ²⁹Si CP/MAS NMR it was shown that the bridging OH groups in zeolites of the aluminosilicate type are located at the Si(nAl) sites with the largest possible number n. While the second coordination sphere of the T positions of the bridging OH groups in zeolites of the silicoaluminophosphate type (Si/(Al + P + Si) < 0.12) is occupied by the same number of phosphorus and aluminium atoms, the bridging OH groups in zeolites of the aluminosilicate type with medium framework aluminium densities are located at the Si(nAl) sites with n = 3, 2 and 1.     

New insight into selective catalytic reduction of nitrogen oxides by ammonia over H-form zeolites: a theoretical study

Density functional theory calculations were carried out to investigate the reaction mechanism of selective catalytic reduction of nitrogen oxides by ammonia in the presence of oxygen at the Br?nsted acid sites of H-form zeolites. The Br?nsted acid site of H-form zeolites was modeled by an aluminosilicate cluster containing five tetrahedral (Al, Si) atoms. A low-activation-energy pathway for the catalytic reduction of NO was proposed. It consists of two successive stages: first NH(2)NO is formed in gas phase, and then is decomposed into N(2) and H(2)O over H-form zeolites. In the first stage, the formation of NH(2)NO may occur via two routes: (1) NO is directly oxidized by O(2) to NO(2), and then NO(2) combines with NO to form N(2)O(3), which reacts with NH(3) to produce NH(2)NO; (2) when NO(2) exceeds NO in the content, NO(2) associates with itself to form N(2)O(4), and then N(2)O(4) reacts with NH(3) to produce NH(2)NO. The second stage was suggested to proceed with low activation energy via a series of synergic proton transfer steps catalyzed by H-form zeolites. The rate-determining step for the whole reduction of NO(x) is identified as the oxidation of NO to NO(2) with an activation barrier of 15.6 kcal mol(-1). This mechanism was found to account for many known experimental facts related to selective catalytic reduction of nitrogen oxides by ammonia over H-form zeolites.     

Zeolite-catalyzed hydrogenation of carbon dioxide and ethene

Ab initio molecular orbital theory and density functional theory calculations have been used to study the three-stage zeolite-catalyzed hydrogenation of CO2 to methanol and the hydrogenation of C2H 4 to ethane, with the aim of designing an effective zeolite catalyst for these reactions. Both Br?nsted acid (XH) and alkali metal (XM) sites in model zeolites (-X-Al-XH- or -X-Al-XM-) have been examined. It is found that appropriately designed zeolites can provide excellent catalysis for these reactions, particularly for the hydrogenation of CO2, HCO2H and CH2O, with uncatalyzed barriers of more than 300 kJ mol(-1) being reduced to as little as 17 kJ mol(-1) (in the case of CH2O). The reaction barrier depends on the acidity of the XH moiety or the nature of the metal cation M in the XM moiety, and the basicity of the adjacent X group in the catalyst. For a catalyst based on alkali metal zeolites (XM), the catalytic activity is relatively insensitive to the nature of X in the XM group. As a result, the catalytic activity for these types of zeolites increases as X becomes more basic. We propose that alkali metal zeolites with Ge and N incorporated into the framework could be very effective catalysts for hydrogenation processes.     

Reactivity of alkanes on zeolites: a computational study of propane conversion reactions

In this work, quantum chemical methods were used to study propane conversion reactions on zeolites; these reactions included protolytic cracking, primary hydrogen exchange, secondary hydrogen exchange, and dehydrogenation reactions. The reactants, products, and transition-state structures were optimized at the B3LYP/6-31G level and the energies were calculated with CBS-QB3, a complete basis set composite energy method. The computed activation barriers were 62.1 and 62.6 kcal/mol for protolytic cracking through two different transition states, 30.4 kcal/mol for primary hydrogen exchange, 29.8 kcal/mol for secondary hydrogen exchange, and 76.7 kcal/mol for dehydrogenation reactions. The effects of basis set for the geometry optimization and zeolite acidity on the reaction barriers were also investigated. Adding extra polarization and diffuse functions for the geometry optimization did not affect the activation barriers obtained with the composite energy method. The largest difference in calculated activation barriers is within 1 kcal/mol. Reaction activation barriers do change as zeolite acidity changes, however. Linear relationships were found between activation barriers and zeolite deprotonation energies. Analytical expressions for each reaction were proposed so that accurate activation barriers can be obtained when using different zeolites as catalysts, as long as the deprotonation energies are first acquired.