Stability and reactivity of active sites for direct benzene oxidation to phenol in Fe/ZSM-5: A comprehensive periodic DFT study

Understanding Alkene Interaction with Metal-Modified Zeolites: Thermodynamics and Mechanism of Bonding in the π-Complex

Zeolites modified with metal cations are perspective catalysts for converting light alkenes to valuable chemicals. A crucial step of the transformation is an alkene interaction with zeolite to afford π-complex with metal cations. The mechanism of alkene bonding with cations is still unclear. To address this problem, propene adsorption on H+ (Bro̷nsted acid site), Na+, Ca2+, Zn2+, Co2+, Cu2+, Cu+, and Ag+ cationic sites in ZSM-5 zeolite has been studied by quantum chemical calculations in terms of adsorption enthalpy, νC═C frequency, and natural bond orbital (NBO) analysis together with natural energy decomposition analysis (NEDA). It is revealed that the conventional concept of σ- and π-bonding is only partially applicable to alkene interaction with metal cations in zeolites. The orbital interaction between an alkene molecule and a metal site is more complex. Several different bonding mechanisms have been identified depending on the nature and electron configuration of the metal cation. This finding explains the complex correlations observed for propene π-complex stability and νC═C frequency shift or charge transfer from the alkene molecule. The results provide the basis for further understanding the interactions between alkenes and inorganic solid Bro̷nsted and Lewis acids.

Preparation of sodium silicate/red mud-based ZSM-5 with glucose as a second template for catalytic cracking of waste plastics into useful chemicals

ZSM-5 was economically synthesized with red mud (RM) and industrial sodium silicate (ISS) in a tetrapropylammonium bromide (TPABr)-glucose dual-template system. The roles of glucose, Fe and Ca in RM on the formation of ZSM-5 were investigated. The catalytic performances of the resultant ZSM-5 were tested by cracking waste plastics. It was found that the formation of ZSM-5 was attributed to a synergistic effect between TPABr and glucose. The addition of glucose decreased the pH value in the crystallization solution and thus promoted the crystallization effect. Glucose acted as a hard template to generate mesopores. Fe atoms were partly distributed in the framework and partly adsorbed in the pores of ZSM-5, and helped to generate more Lewis acid sites. Ca atoms were mainly adsorbed in the pores of ZSM-5, and showed an inhibitory effect on the formation of zeolites. The synthesized ZSM-5 showed a weakly acidic and mesoporous structure and achieved an enhanced effect on producing gaseous products (gas yield: 85.3%), especially light olefins (C^{[double bond, length as m-dash]} _2-4) (selectivity: 77.1%) from cracking of low density polyethylene at 500 °C. The long-term cracking experiment showed that the synthesized ZSM-5 is superior in converting waste plastics to light olefins (ethylene and propene) than the commercial ZSM-5.

Theoretical Analysis of the Catalytic Hydrolysis Mechanism of HCN over Cu-ZSM-5

HCN catalytic hydrolysis mechanism over Cu-ZSM-5 was investigated based on the density functional theory (DFT) with 6-31++g (d, p) basis set. Five paths (A, B, C, D, and E) were designed. For path A and path B, the first step is the nucleophilic attack of water molecule. Next, the hydrogen atom of H2O is transferred to the nitrogen atom first for path A, while in path B, the hydrogen atom of the HCN is first transferred to the nitrogen atom. In path C, HCN isomerizes to HNC initially, and the remaining steps are similar to that of path A. The H atom of HCN shifts to Cu-ZSM-5 initially in path D, and the H atom is transferred to N atom subsequently. The last step is the attack on water molecule. The first step for path E is similar to that of path D. The next step is the attack on water molecule, in which the H atom of water molecule shifts to N atom, and the H on Cu-ZSM-5 shifts to the N atom. Meanwhile, the H atom of oxygen atom is transferred to the N atom. The results show that path C is the most favorable path, with the lowest free energy barrier (35.45 kcal/mol). The results indicate that the Cu-ZSM-5 strongly reduces the energy barrier of HCN and isomerizes to HNC, making it an effective catalyst for HCN hydrolysis.

DFT Study on the Catalytic Activity of ALD-Grown Diiron Oxide Nanoclusters for Partial Oxidation of Methane to Methanol

Using density functional theory (DFT), we studied the catalytic activity of iron oxide nanoclusters that mimic the structure of the active site in the soluble form of methane monooxygenase (sMMO) for the partial oxidation of methane to methanol. Using N₂O as the oxidant, we consider a radical-rebound mechanism and a concerted mechanism for the oxidation of methane on either a bridging oxygen (O_b) or a terminal oxygen (O_t) active site. We find that the radical-rebound pathway is preferred over the concerted pathway by 40-50 kJ/mol, but the desorption of methanol and the regeneration of the oxygen site are found to be the highest barriers for the direct conversion of methane to methanol with these catalysts. As demonstrated by a population analysis, the O_x (x = b or t) site behaves as an oxygen radical during the H abstraction, and the [Fe⁺-O_x^-] site behaves as a Lewis acid-base pair during the concerted C-H cleavage. Molecular orbital decomposition analysis further demonstrates electron transfer during the oxidation and reduction steps of the reaction. High-level multireference calculations were also carried out to further assess the DFT results. Understanding how these systems behave during the proposed reaction pathways provides new insights into how they can be tuned for methane partial oxidation.

A Supramolecular View on the Cooperative Role of Brønsted and Lewis Acid Sites in Zeolites for Methanol Conversion

A systematic molecular level and spectroscopic investigation is presented to show the cooperative role of Brønsted acid and Lewis acid sites in zeolites for the conversion of methanol. Extra-framework alkaline-earth metal containing species and aluminum species decrease the number of Brønsted acid sites, as protonated metal clusters are formed. A combined experimental and theoretical effort shows that postsynthetically modified ZSM-5 zeolites, by incorporation of extra-framework alkaline-earth metals or by demetalation with dealuminating agents, contain both mononuclear [MOH]⁺ and double protonated binuclear metal clusters [M(μ-OH)₂M]²⁺ (M = Mg, Ca, Sr, Ba, and HOAl). The metal in the extra-framework clusters has a Lewis acid character, which is confirmed experimentally and theoretically by IR spectra of adsorbed pyridine. The strength of the Lewis acid sites (Mg > Ca > Sr > Ba) was characterized by a blue shift of characteristic IR peaks, thus offering a tool to sample Lewis acidity experimentally. The incorporation of extra-framework Lewis acid sites has a substantial influence on the reactivity of propene and benzene methylations. Alkaline-earth Lewis acid sites yield increased benzene methylation barriers and destabilization of typical aromatic intermediates, whereas propene methylation routes are less affected. The effect on the catalytic function is especially induced by the double protonated binuclear species. Overall, the extra-framework metal clusters have a dual effect on the catalytic function. By reducing the number of Brønsted acid sites and suppressing typical catalytic reactions in which aromatics are involved, an optimal propene selectivity and increased lifetime for methanol conversion over zeolites is obtained. The combined experimental and theoretical approach gives a unique insight into the nature of the supramolecular zeolite catalyst for methanol conversion which can be meticulously tuned by subtle interplay of Brønsted and Lewis acid sites.

Determining the structures, acidity and adsorption properties of Al substituted HZSM-5

Determining the locations and distributions of Al substitution in zeolite-based catalysts and catalysis is always very challenging. Despite advanced experimental characterization techniques and improved theoretical models, this issue is not reasonably solved and this is because the locations and distributions of Al substitution in zeolites are more kinetically than thermodynamically controlled. In this work, we computed one Al substitution in the orthorhombic form of MFI (HZSM-5) which contains 12 distinct tetrahedral (T) centers on the basis of a periodic slab model containing 96 T centers including van der Waals dispersion correction (GGA-PBE-D3). For all 12 T centers, there are 48 acidic sites and each site can be considered for the adsorption of probe molecules. Thermodynamically, the energy span of the twelve most stable acidic sites is less than 15 kJ mol-1, and such a small energy difference enables all adjustable possibilities for the locations and distributions of Al substitution under suitable conditions. Excellent agreement between experiment and theory in the adsorption enthalpies of pyridine, methylamine, dimethylamine and trimethylamine shows that the location of Al substitution is most likely at T1, T3, T5, T7 and T11, while much less likely at the often used T12 site. These results provide the basis for identifying Al substitution in new synthesized HZSM-5 catalysts and for studying the acidic site-dependent catalytic activity of HZSM-5 in cracking and hydrogenation reactions.

Mechanism of selective benzene hydroxylation catalyzed by iron-containing zeolites

A direct, catalytic conversion of benzene to phenol would have wide-reaching economic impacts. Fe zeolites exhibit a remarkable combination of high activity and selectivity in this conversion, leading to their past implementation at the pilot plant level. There were, however, issues related to catalyst deactivation for this process. Mechanistic insight could resolve these issues, and also provide a blueprint for achieving high performance in selective oxidation catalysis. Recently, we demonstrated that the active site of selective hydrocarbon oxidation in Fe zeolites, named α-O, is an unusually reactive Fe(IV)=O species. Here, we apply advanced spectroscopic techniques to determine that the reaction of this Fe(IV)=O intermediate with benzene in fact regenerates the reduced Fe(II) active site, enabling catalytic turnover. At the same time, a small fraction of Fe(III)-phenolate poisoned active sites form, defining a mechanism for catalyst deactivation. Density-functional theory calculations provide further insight into the experimentally defined mechanism. The extreme reactivity of α-O significantly tunes down (eliminates) the rate-limiting barrier for aromatic hydroxylation, leading to a diffusion-limited reaction coordinate. This favors hydroxylation of the rapidly diffusing benzene substrate over the slowly diffusing (but more reactive) oxygenated product, thereby enhancing selectivity. This defines a mechanism to simultaneously attain high activity (conversion) and selectivity, enabling the efficient oxidative upgrading of inert hydrocarbon substrates.

Unraveling reaction networks behind the catalytic oxidation of methane with H2O2 over a mixed-metal MIL-53(Al,Fe) MOF catalyst

Reaction paths underlying the catalytic oxidation of methane with H2O2 over an Fe containing MIL-53(Al) metal-organic framework were studied by periodic DFT calculations. Not only the activation of methane, but the full reaction network was considered, which includes the formation of the active site, the overoxidation of methane to CO2 and the decomposition of H2O2 to H2O and O2. Calculations indicate that the activation barrier for the initial activation of the Fe sites upon reaction with H2O2 is comparable to that of the subsequent C-H activation and also of the reaction steps involved in the undesirable overoxidation processes. The pronounced selectivity of the oxidation reaction over MIL-53(Al,Fe) towards the target mono-oxygenated CH3OH and CH3OOH products is attributed to the limited coordination freedom of the Fe species encapsulated in the extended octahedral [AlO6] structure-forming chains, which effectively prevents the direct overoxidation paths prior to product desorption from the active sites. Importantly, our computational analysis reveals that the active sites for the desired methane oxidation are able to much more efficiently promote the direct catalytic H2O2 decomposition reaction, rendering thus the current combination of the active site and the reactants undesirable for the prospective methane valorization process.

Mechanistic Complexity of Methane Oxidation with H2O2 by Single-Site Fe/ZSM-5 Catalyst

Periodic density functional theory (DFT) calculations were carried out to investigate the mechanism of methane oxidation with H2O2 over the defined Fe sites in Fe/ZSM-5 zeolite. The initial Fe site is modeled as a [(H2O)2-Fe(III)-(μO)2-Fe(III)-(H2O)2]2+ extraframework cluster deposited in the zeolite pore and charge-compensated by two anionic lattice sites. The activation of this cluster with H2O2 gives rise to the formation of a variety of Fe(III)-oxo and Fe(IV)-oxo complexes potentially reactive toward methane dissociation. These sites are all able to promote the first C-H bond cleavage in methane by following three possible reaction mechanisms: namely, (a) heterolytic and (b) homolytic methane dissociation as well as (c) Fenton-type reaction involving free OH radicals as the catalytic species. The C-H activation step is followed by formation of MeOH and MeOOH and regeneration of the active site. The Fenton-type path is found to proceed with the lowest activation barrier. Although the barriers for the alternative heterolytic and homolytic pathways are found to be somewhat higher, they are still quite favorable and are expected to be feasible under reaction conditions, resulting ultimately in MeOH and MeOOH products. H2O2 oxidant competes with CH4 substrate for the same sites. Since the oxidation of H2O2 to O2 and two [H+] is energetically more favorable than the C-H oxofunctionalization, the overall efficiency of the latter target process remains low.

New approach for determination of the influence of long-range order and selected ring oscillations on IR spectra in zeolites

Vibrational spectroscopy can be considered as one of the most important methods used for structural characterization of various porous aluminosilicate materials, including zeolites. On the other hand, vibrational spectra of zeolites are still difficult to interpret, particularly in the pseudolattice region, where bands related to ring oscillations can be observed. Using combination of theoretical and computational approach, a detailed analysis of these regions of spectra is possible; such analysis should be, however, carried out employing models with different level of complexity and simultaneously the same theory level. In this work, an attempt was made to identify ring oscillations in vibrational spectra of selected zeolite structures. A series of ab initio calculations focused on S4R, S6R, and as a novelty, 5-1 isolated clusters, as well as periodic siliceous frameworks built from those building units (ferrierite (FER), mordenite (MOR) and heulandite (HEU) type) have been carried out. Due to the hierarchical structure of zeolite frameworks it can be expected that the total envelope of the zeolite spectra should be with good accuracy a sum of the spectra of structural elements that build each zeolite framework. Based on the results of HF calculations, normal vibrations have been visualized and detailed analysis of pseudolattice range of resulting theoretical spectra have been carried out. Obtained results have been applied for interpretation of experimental spectra of selected zeolites.

Rationally designing mixed Cu–(μ-O)–M (M = Cu, Ag, Zn, Au) centers over zeolite materials with high catalytic activity towards methane activation

The direct conversion of methane to methanol on [Cu(μ-O)M]²⁺ (M = Cu, Ag, Zn, Au) bimetal centers in ZSM-5 zeolite is investigated using periodic DFT for the first time.

The challenge of catalyst prediction

New insights and successful use of computational catalysis is highlighted.

Stable Fe/ZSM-5 Nanosheet Zeolite Catalysts for the Oxidation of Benzene to Phenol

Fe/ZSM-5 nanosheet zeolites of varying thickness were synthesized with di- and tetraquaternary ammonium structure directing agents and extensively characterized for their textural, structural, and catalytic properties. Introduction of Fe3+ ions in the framework of nanosheet zeolites was slightly less effective than in bulk ZSM-5 zeolite. Steaming was necessary to activate all catalysts for N2O decomposition and benzene oxidation. The higher the Fe content, the higher the degree of Fe aggregation was after catalyst activation. The degree of Fe aggregation was lower when the crystal domain size of the zeolite or the Fe content was decreased. These two parameters had a substantial influence on the catalytic performance. Decreasing the number of Fe sites along the b-direction strongly suppressed secondary reactions of phenol and, accordingly, catalyst deactivation. This together with the absence of diffusional limitations in nanosheet zeolites explains the much higher phenol productivity obtainable with nanostructured Fe/ZSM-5. Steamed Fe/ZSM-5 zeolite nanosheet synthesized using C22-6-3·Br2 (domain size in b-direction ∼3 nm) and containing 0.24 wt % Fe exhibited the highest catalytic performance. During the first 24 h on stream, this catalyst produced 185 mmolphenol g-1. Calcination to remove the coke deposits completely restored the initial activity.

Selective Transformation of Various Nitrogen-Containing Exhaust Gases toward N2 over Zeolite Catalysts

In this review we focus on the catalytic removal of a series of N-containing exhaust gases with various valences, including nitriles (HCN, CH3CN, and C2H3CN), ammonia (NH3), nitrous oxide (N2O), and nitric oxides (NO(x)), which can cause some serious environmental problems, such as acid rain, haze weather, global warming, and even death. The zeolite catalysts with high internal surface areas, uniform pore systems, considerable ion-exchange capabilities, and satisfactory thermal stabilities are herein addressed for the corresponding depollution processes. The sources and toxicities of these pollutants are introduced. The important physicochemical properties of zeolite catalysts, including shape selectivity, surface area, acidity, and redox ability, are described in detail. The catalytic combustion of nitriles and ammonia, the direct catalytic decomposition of N2O, and the selective catalytic reduction and direct catalytic decomposition of NO are systematically discussed, involving the catalytic behaviors as well as mechanism studies based on spectroscopic and kinetic approaches and molecular simulations. Finally, concluding remarks and perspectives are given. In the present work, emphasis is placed on the structure-performance relationship with an aim to design an ideal zeolite-based catalyst for the effective elimination of harmful N-containing compounds.

Periodic model of an LTA framework

Zeolites are a group of microporous aluminosilicate frameworks with numerous applications in, for example, catalysis and ion-exchange and sorption processes. One of the most important tools for analyzing the properties of zeolite structures is vibrational spectroscopy. However, the complexity of these structures often leads to difficulties when attempting to interpret the resulting spectra, so an additional complementary tool is required: computational methods. The aim of this study was to formulate a simplified periodic model of an LTA framework containing alkali metal cations (either Li(+), Na(+), K(+), Rb(+), or Cs(+)) and to perform a set of ab initio calculations aimed at assessing the influence of these cations on the properties of the vibrational spectra of the LTA framework. Additionally, chemical bonding was analyzed by means of electron density topology analysis. Results obtained were compared with experimental spectra for alkali metal forms of zeolite A. It was found that the vibrational spectra obtained using the proposed model agree well with the corresponding experimentally derived spectra, meaning that the model can be used to analyze real spectra in detail.

Advances in theory and their application within the field of zeolite chemistry

Zeolites are versatile and fascinating materials which are vital for a wide range of industries, due to their unique structural and chemical properties, which are the basis of applications in gas separation, ion exchange and catalysis. Given their economic impact, there is a powerful incentive for smart design of new materials with enhanced functionalities to obtain the best material for a given application. Over the last decades, theoretical modeling has matured to a level that model guided design has become within reach. Major hurdles have been overcome to reach this point and almost all contemporary methods in computational materials chemistry are actively used in the field of modeling zeolite chemistry and applications. Integration of complementary modeling approaches is necessary to obtain reliable predictions and rationalizations from theory. A close synergy between experimentalists and theoreticians has led to a deep understanding of the complexity of the system at hand, but also allowed the identification of shortcomings in current theoretical approaches. Inspired by the importance of zeolite characterization which can now be performed at the single atom and single molecule level from experiment, computational spectroscopy has grown in importance in the last decade. In this review most of the currently available modeling tools are introduced and illustrated on the most challenging problems in zeolite science. Directions for future model developments will be given.

An efficient approach to explore the adsorption of benzene and phenol on nanostructured catalysts: a DFT analysis

The adsorption behavior of benzene and phenol on the zeolite is attributed to the differences in the strength of their interactions.

Direct oxidation of methane to methanol on Fe–O modified graphene

The reaction mechanisms of the partial oxidation of methane to methanol over FeO/graphene are unraveled using an advanced DFT approach.

A DFT study on direct benzene hydroxylation catalyzed by framework Fe and Al sites in zeolites

Framework Fe rather than Al Lewis acidic sites in zeolites are demonstrated to show superior catalytic activity for benzene hydroxylation.

Self-organization of extraframework cations in zeolites

Structural properties of a series of mordenite and ZSM-5 zeolites with different framework Al distribution modified with oxygenated extraframework Ga, Zn, Al, Cu and Fe complexes were investigated by means of periodic density functional theory calculations. It is demonstrated that mononuclear oxygenated and hydroxylated cationic metal complexes in high-silica zeolites tend to self-organize into binuclear complexes. In the cases of Ga- and Fe-modified zeolites, it is shown that the catalytic activity of the most stable binuclear extraframework cations is much higher than that of the hypothetical very reactive mononuclear counterparts. This is due to a weaker binding of reaction intermediates and easier regeneration of the initial active complexes in the course of the catalytic reaction. The formation of multiple-charged binuclear oxygenated metal species in zeolites is a general phenomenon. It does not require a specific distribution of the equivalent number of negative framework charges that compensate for the positive charge of the cationic complexes. The location and the stability of cationic complexes in zeolite micropores are mainly determined by the coordination properties of the metal centres.