Joachim Sauer

Bridging hydrodyl groups in zeolitic catalysts: a computer simulation of their structure, vibrational properties and acidity in protonated faujasites (HY zeolites)

Abstract The structure and properties of the four possible bridging hydroxyl groups in silicon-rich H-faujasite (zeolite Y) are studied by lattice-energy minimization carried out within the classical shell model. The most likely proton-accepting sites are the O1 and O3 oxygens. Their hydroxyl stretching frequencies are shown to be responsible for the characteristic high-frequency (HF) and low-frequency (LF) bands, respectively, in the infrared spectra of H-faujasites. The variation of the OH stretching frequencies for the four isolated sites is correlated with changes of local geometry such as the OH bond length and the SiO(H)Al and the SiOH angles. However, there appears to be no correlation with differences in the acid strength as evidenced by the values of the calculated deprotonation energy of the O(1)H and O(3)H hydroxyl groups for which only a minor difference is predicted.

Application of semiempirical long‐range dispersion corrections to periodic systems in density functional theory

Ewald summation is used to apply semiempirical long‐range dispersion corrections (Grimme, J Comput Chem 2006, 27, 1787; 2004, 25, 1463) to periodic systems in density functional theory. Using the parameters determined before for molecules and the Perdew‐Burke‐Ernzerhof functional, structure parameters and binding energies for solid methane, graphite, and vanadium pentoxide are determined in close agreement with observed values. For methane, a lattice constant a of 580 pm and a sublimation energy of 11 kJ mol−1 are calculated. For the layered solids graphite and vanadia, the interlayer distances are 320 pm and 450 pm, respectively, whereas the graphite interlayer energy is −5.5 kJ mol−1 per carbon atom and layer. Only when adding the semiempirical dispersion corrections, realistic values are obtained for the energies of adsorption of C4 alkenes in microporous silica (−66 to −73 kJ mol−1) and the adsorption and chemisorption (alkoxide formation) of isobutene on acidic sites in the micropores of zeolite ferrierite (−78 to −94 kJ mol−1). As expected, errors due to missing self‐interaction correction as in the energy for the proton transfer from the acidic site to the alkene forming a carbenium ion are not affected by the dispersion term. The adsorption and reaction energies are compared with the results from Møller‐Plesset second‐order perturbation theory with basis set extrapolation.

Combining quantum mechanics and interatomic potential functions in ab initio studies of extended systems

The errors made when large chemical systems are replaced by small models are discussed: interrupted charge transfer, missing structure constraints, neglected long‐range interactions. A combined quantum mechanics (QM)–interatomic potential function (Pot) approach is described. Characteristic features of the QM‐Pot approach include: (1) periodic boundary conditions, (2) consistent definition of forces in the presence of link atoms that terminate the QM cluster, (3) interatomic potential functions parametrized on ab initio data and accounting for polarization effects, (4) use of reaction force fields (EVB potentials) in combination with QM methods for efficient localization of transition structures in large systems, (5) implementation as a loose coupling of existing QM and Pot engines. Comparison is made with some other hybrid QM/MM methods. Applications of the combined QM‐Pot method for ab initio modeling of the structure and reactivity of zeolite catalysts are reviewed with both protons and transition metal cations as active species. Potential functions of the ion‐pair shell‐model type available for such studies are compiled. The reliability of the method is checked by comparison with periodic ab initio studies and by examining the convergence of the results with increasing size of the QM cluster. The problems tackled are: different types of Cu+ sites in the CuZSM‐5 catalyst and their properties, acidity differences between active sites in different zeolite framework structures (energies of deprotonation, NH3 adsorption energies), and proton mobility in acidic zeolites. The combined QM‐Pot approach made possible a full ab initio prediction of reaction rates for an elementary process on the surface of solid catalysts and of how these rates differ between different catalysts with the same active site.

Ab initio study of hydrogen adsorption in MOF-5.

Metal-organic frameworks (MOFs) are promising adsorbents for hydrogen storage. Density functional theory and second-order Møller-Plesset perturbation theory (MP2) are used to calculate the interaction energies between H(2) and individual structural elements of the MOF-5 framework. The strongest interaction, DeltaH(77) = -7.1 kJ/mol, is found for the alpha-site of the OZn(4)(O(2)Ph)(6) nodes. We show that dispersion interactions and zero-point vibrational energies must be taken into account. Comparison of calculations done under periodic boundary conditions for the complete structure with those done for finite models cut from the MOF-5 framework shows that the interactions with H(2) originate mainly from the local environment around the adsorption site. When used within a Multi-Langmuir model, the MP2 results reproduce measured adsorption isotherms (the predicted amount is 6 wt % at 77 K and 40 bar) if we assume that the H(2) molecules preserve their rotational degrees of freedom in the adsorbed state. This allows to discriminate between different isotherms measured for different MOF-5 samples and to reliably predict isotherms for new MOF structures.

Treating dispersion effects in extended systems by hybrid MP2:DFT calculations—protonation of isobutene in zeolite ferrierite

We propose use of a hybrid method to study problems that involve both bond rearrangements and van-der-Waals interactions. The method combines second-order Møller-Plesset perturbation theory (MP2) calculations for the reaction site with density functional theory (DFT) calculations for a large system under periodic boundary conditions. Hybrid MP2:DFT structure optimisation for a cluster embedded in the periodic model is the first of three steps in a multi-level approach. The second step is extrapolation of the MP2 energy to the complete basis set limit. The third step is extrapolating the high-level (MP2) correction to the limiting case of the full periodic structure. This is done by calculating the MP2 correction for a series of cluster models of increasing size, fitting an analytic expression to these energy corrections, and applying the fitted expression to the full periodic structure. We assume that, up to a constant, the high-level correction is described by a damped dispersion expression. Combining the results of all three steps yields an estimate of the MP2 reaction energy for the full periodic system at the complete basis set level. The method is designed for a reaction between a small or medium sized substrate molecule and a very large chemical system. For adsorption of isobutene in zeolite H-ferrierite, the energies obtained for the formation of different structures, the pi-complex, the isobutoxide, the tert-butoxide, and the tert-butyl carbenium ion, are -78, -73, -48, and -21 kJ mol(-1), respectively. This corresponds to corrections of the pure DFT (PBE functional) results by -62, -70, -67, and -29 kJ mol(-1), respectively. Hence, the MP2 corrections are substantial and, perhaps more importantly, not the same for the different hydrocarbon species in the zeolite. Coupled-cluster (CCSD(T)) calculations change the MP2 energies by -4 kJ mol(-1) (tert-butyl cation) or less (below +/-1 kJ mol(-1) for the other species).

Role of ceria in oxidative dehydrogenation on supported vanadia catalysts

The effect of the suppport on oxidative dehydrogenation activity for vanadia/ceria systems is examined for the oxidation of methanol to formaldehyde by use of well-defined VO(x)/CeO(2)(111) model catalysts. Temperature-programmed desorption at low vanadia loadings revealed reactivity at much lower temperature (370 K) as compared to pure ceria and vanadia on inert supports such as silica. Density functional theory is applied and the energies of hydrogenation and oxygen vacancy formation also predict an enhanced reactivity of the vanadia/ceria system. At the origin of this support effect is the ability of ceria to stabilize reduced states by accommodating electrons in localized f-states.

Coordination and siting of Cu+ ions in ZSM-5: A combined quantum mechanics/interatomic potential function study

Siting and coordination of Cu+ ions in zeolite ZSM-5 have been studied by a combined quantum mechanics/interatomic potential function technique. A new Cu(I)–O interaction potential has been parameterized based on abinitio data which is compatible with abinitio-parametrized shell model potentials for zeolites. Several different sites of Cu+ inside ZSM-5 have been found. The structure of the site and the coordination of the Cu+ ion depend on the T-site where the Si atom is replaced by an Al atom. If Al is at the edge of the main and sinusoidal channels the Cu+ ion prefers to occupy thc open space in the channel intersection and it is coordinated to two oxygen atoms of the AlO4 tetrahedron. The largest binding energy of Cu+ with ZSM-5 was found for Cu+ located inside a six-membered ring on the wall of the sinusoidal channel, where it can coordinate to three or four oxygen atoms of the zeolite framework. The Cu+ sites predicted are in accord with available experimental results.

Combining ab initio techniques with analytical potential functions for structure predictions of large systems: Method and application to crystalline silica polymorphs

A computational scheme is presented which combines quantum mechanical ab initio techniques with methods using analytical potential functions. The scheme is designed for use in structure optimizations and is also applicable to molecular dynamics simulations. The implementation covers both molecular and periodic systems. The problem of the link atoms is solved by a subtraction scheme which is easily implemented for any combination of methods. As a first application dense and microporous silica polymorphs are studied. The Hartree‐Fock method is combined with both a force field and an ion pair shell model potential. Comparison is made with lattice energy minimizations which use the force field or the shell model potential alone as well as with free cluster optimizations and optimizations in which the outer part of the cluster is kept fixed.

The Atomic Structure of a Metal-Supported Vitreous Thin Silica Film**

Clear as glass: The atomic structure of a metal-supported vitreous thin silica film was resolved using low-temperature scanning tunneling microscopy (STM). Based on the STM image, a model was constructed and the atomic arrangement of the thin silica glass determined (see picture). The total pair correlation function of the structural model shows good agreement with diffraction experiments performed on vitreous silica.

Pyrazolate-Based Cobalt(II)-Containing Metal–Organic Frameworks in Heterogeneous Catalytic Oxidation Reactions: Elucidating the Role of Entatic States for Biomimetic Oxidation Processes

Crystal structures of two metal-organic frameworks (MFU-1 and MFU-2) are presented, both of which contain redox-active Co(II) centres coordinated by linear 1,4-bis[(3,5-dimethyl)pyrazol-4-yl] ligands. In contrast to many MOFs reported previously, these compounds show excellent stability against hydrolytic decomposition. Catalytic turnover is achieved in oxidation reactions by employing tert-butyl hydroperoxide and the solid catalysts are easily recovered from the reaction mixture. Whereas heterogeneous catalysis is unambiguously demonstrated for MFU-1, MFU-2 shows catalytic activity due to slow metal leaching, emphasising the need for a deeper understanding of structure-reactivity relationships in the future design of redox-active metal-organic frameworks. Mechanistic details for oxidation reactions employing tert-butyl hydroperoxide are studied by UV/Vis and IR spectroscopy and XRPD measurements. The catalytic process accompanying changes of redox states and structural changes were investigated by means of cobalt K-edge X-ray absorption spectroscopy. To probe the putative binding modes of molecular oxygen, the isosteric heats of adsorption of O(2) were determined and compared with models from DFT calculations. The stabilities of the frameworks in an oxygen atmosphere as a reactive gas were examined by temperature-programmed oxidation (TPO). Solution impregnation of MFU-1 with a co-catalyst (N-hydroxyphthalimide) led to NHPI@MFU-1, which oxidised a range of organic substrates under ambient conditions by employing molecular oxygen from air. The catalytic reaction involved a biomimetic reaction cascade based on free radicals. The concept of an entatic state of the cobalt centres is proposed and its relevance for sustained catalytic activity is briefly discussed.