Increasing the ion-exchange capacity of MFI zeolites by introducing Zn to aluminosilicate frameworks

MFI zeolites exchanged with various cations have gained a great deal of attention as catalysts. Increase in the ion-exchange capacity of zeolites can improve their catalytic properties by introducing more active sites; however, the ion-exchange capacity of MFI zeolites is limited by maximum aluminum content in the structure. To improve the ion-exchange capability of the MFI zeolites beyond the upper limit of the aluminosilicate MFI zeolites, we propose herein an approach to incorporate Zn(ii) in the zeolitic framework, because Zn in the framework sites generates two negative charges per atom. Using zincoaluminosilicate gels prepared via co-precipitation, organic-free synthesis of zincoaluminosilicate MFI zeolites was achieved. The obtained zincoaluminosilicate MFI zeolites had high Zn contents comparable to those in the initial zincoaluminosilicate gels with both Zn and Al in the zeolite framework. In contrast, the use of conventional sources of Si, Al, and Zn resulted in zeolites with extra-framework zinc oxide species. The obtained Zn-substituted MFI zeolites were shown to possess higher ion-exchange capacity compared to aluminosilicate MFI zeolites. It was also revealed that the zincoaluminosilicate MFI zeolites have high affinity for the divalent cation compared to the aluminosilicate analog, likely due to the two negative charges in close proximity. Because of these higher ion-exchange efficiencies, especially for divalent cations, the obtained zincoaluminosilicate MFI zeolites are expected to be efficient platforms for several important catalytic reactions.

Small-Pore Zeolites: Synthesis and Catalysis

In the past decade or so, small-pore zeolites have received greater attention than large- and medium-pore molecular sieves that have historically dominated the literature. This is primarily due to the commercialization of two major catalytic processes, NOx exhaust removal and methanol conversion to light olefins, that take advantage of the properties of these materials with smaller apertures. Small-pore zeolites possess pores that are constructed of eight tetrahedral atoms (Si⁴⁺ and Al³⁺), each time linked by a shared oxygen These eight-member ring pores (8MR) provide small molecules access to the intracrystalline void space, e.g., to NOx during car exhaust cleaning (NOx removal) or to methanol en route to its conversion into light olefins, while restricting larger molecule entrance and departure that is critical to overall catalyst performance. In total, there are forty-four structurally different small-pore zeolites. Forty-one of these zeolites can be synthesized, and the first synthetic zeolite (KFI, 1948) was in fact a small-pore material. Although the field of 8MR zeolite chemistry has expanded in many directions, the progress in synthesis is framework-specific, leaving insights and generalizations difficult to realize. This review first focuses on the relevant synthesis details of all 8MR zeolites and provides some generalized findings and related insights. Next, catalytic applications where 8MR zeolites either have been commercialized or have dominated investigations are presented, with the aim of providing structure-activity relationships. The review ends with a summary that discusses (i) both synthetic and catalytic progress, (ii) a list of opportunities in the 8MR zeolite field, and (iii) a brief future outlook.

Fe2 O+ Cation Mediated Propane Oxidation by Dioxygen in the Gas Phase

The mass-selected Fe₂ O⁺ cation mediated propane oxidation by O₂ was investigated by mass spectrometry and density functional theory calculations. In the reaction of Fe₂ O⁺ with C₃ H₈ , H₂ was liberated by C-H bond activation to give Fe₂ OC₃ H₆⁺ . Interestingly, when a mixture of C₃ H₈ /O₂ was introduced into the reactor, an intense signal that corresponded to the Fe₂ O₂⁺ cation was present; the experiments indicated that O₂ was activated in its reaction with Fe₂ O(C₃ H₆ )⁺ to give Fe₂ O₂⁺ and C₃ H₆ O (acetone or propanal). A Langmuir-Hinshelwood-like mechanism was adopted in the propane oxidation reaction by O₂ on gas-phase Fe₂ O⁺ cations. In comparison with the absence of Fe₂ O₂⁺ in the reaction of Fe₂ O⁺ with O₂ , the ligand effect of C₃ H₆ on Fe₂ OC₃ H₆⁺ is important in the oxygen activation reaction. The theoretical results are consistent with the experimental observations. The propane oxidation by O₂ in the presence of Fe₂ O⁺ might be applied as a model for alkane and O₂ activations over iron oxide catalysts, and the mechanisms and kinetic data are useful for understanding corresponding heterogeneous reactions.

Single rhodium atoms anchored in micropores for efficient transformation of methane under mild conditions

Catalytic transformation of CH₄ under a mild condition is significant for efficient utilization of shale gas under the circumstance of switching raw materials of chemical industries to shale gas. Here, we report the transformation of CH₄ to acetic acid and methanol through coupling of CH₄, CO and O₂ on single-site Rh₁O₅ anchored in microporous aluminosilicates in solution at ≤150 °C. The activity of these singly dispersed precious metal sites for production of organic oxygenates can reach about 0.10 acetic acid molecules on a Rh₁O₅ site per second at 150 °C with a selectivity of ~70% for production of acetic acid. It is higher than the activity of free Rh cations by >1000 times. Computational studies suggest that the first C-H bond of CH₄ is activated by Rh₁O₅ anchored on the wall of micropores of ZSM-5; the formed CH₃ then couples with CO and OH, to produce acetic acid over a low activation barrier.

Porous metallosilicates for heterogeneous, liquid-phase catalysis: perspectives and pertaining challenges

Porous silicates containing dilute amounts of tri-, tetra- and penta-valent metal sites, such as TS-1, Sn-β and Fe-ZSM-5, have recently emerged as state of the art catalysts for a variety of sustainable chemical transformations. In contrast with their aluminosilicate cousins, which are widely employed throughout the refinery industry for gas-phase catalytic transformations, such metallosilicates have exhibited unprecedented levels of performance for a variety of liquid-phase catalytic processes, including the conversion of biomass to chemicals, and sustainable oxidation technologies with H2O2. However, despite their unique levels of performance for these new types of chemical transformations, increased utilization of these promising materials is complicated by several factors. For example, their utilization in a liquid, and often polar, medium hinders process intensification (scale-up, catalyst deactivation). Moreover, such materials do not generally exhibit the active-site homogeneity of conventional aluminosilicates, and they typically possess a wide variety of active-site ensembles, only some of which may be directly involved in the catalytic chemistry of interest. Consequently, mechanistic understanding of these catalysts remains relatively low, and competitive reactions are commonly observed. Accordingly, unified approaches towards developing more active, selective and stable porous metallosilicates have not yet been achieved. Drawing on some of the most recent literature in the field, the purpose of this mini review is both to highlight the breakthroughs made with regard to the use of porous metallosilicates as heterogeneous catalysts for liquid-phase processing, and to highlight the pertaining challenges that we, and others, aim to overcome during the forthcoming years.

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.

Low-temperature activation of methane on doped single atoms: descriptor and prediction

We predict that Pt and several other single atoms on rutile TiO₂(110) can chemisorb and activate methane at low temperatures.

Effect of Al-distribution on oxygen activation over Cu–CHA

The Al-distribution affects the formation of Cu(NH₃)₂⁺-pairs during NH₃-SCR over Cu–CHA and oxygen dissociates with low barriers over such pairs.

Investigating the influence of acid sites in continuous methane oxidation with N2O over Fe/MFI zeolites

Methane oxidation using N₂O was carried out with Fe–MFI zeolite catalysts at 300 °C.

Cation-exchanged zeolites for the selective oxidation of methane to methanol

Development of an ideal methane activation catalyst presents a trade-off between stability and reactivity of the active site that can be achieved by tuning the transition metal cation, active site motif and the zeolite topology.

Investigating the Influence of Fe Speciation on N2O Decomposition Over Fe-ZSM-5 Catalysts

The influence of Fe speciation on the decomposition rates of N₂O over Fe-ZSM-5 catalysts prepared by Chemical Vapour Impregnation were investigated. Various weight loadings of Fe-ZSM-5 catalysts were prepared from the parent zeolite H-ZSM-5 with a Si:Al ratio of 23 or 30. The effect of Si:Al ratio and Fe weight loading was initially investigated before focussing on a single weight loading and the effects of acid washing on catalyst activity and iron speciation. UV/Vis spectroscopy, surface area analysis, XPS and ICP-OES of the acid washed catalysts indicated a reduction of ca. 60% of Fe loading when compared to the parent catalyst with a 0.4 wt% Fe loading. The TOF of N₂O decomposition at 600 °C improved to 3.99 × 10³ s^-1 over the acid washed catalyst which had a weight loading of 0.16%, in contrast, the parent catalyst had a TOF of 1.60 × 10³ s^-1. Propane was added to the gas stream to act as a reductant and remove any inhibiting oxygen species that remain on the surface of the catalyst. Comparison of catalysts with relatively high and low Fe loadings achieved comparable levels of N₂O decomposition when propane is present. When only N₂O is present, low metal loading Fe-ZSM-5 catalysts are not capable of achieving high conversions due to the low proximity of active framework Fe³⁺ ions and extra-framework ɑ-Fe species, which limits oxygen desorption. Acid washing extracts Fe from these active sites and deposits it on the surface of the catalyst as Fe_xO_y, leading to a drop in activity. The Fe species present in the catalyst were identified using UV/Vis spectroscopy and speculate on the active species. We consider high loadings of Fe do not lead to an active catalyst when propane is present due to the formation of Fe_xO_y nanoparticles and clusters during catalyst preparation. These are inactive species which lead to a decrease in overall efficiency of the Fe ions and consequentially a lower TOF.

DFT exploration of active site motifs in methane hydroxylation by Ni-ZSM-5 zeolite

DFT calculations suggest [Ni₂(μ-O)₂]²⁺ and [Ni₃(μ-O)₃]²⁺ species as two possible active sites in methane hydroxylation by Ni-ZSM-5 zeolite. Both of them are predicted to activate methane and desorb the formed methanol with low activation and desorption energies.

Electronic Structure of the [Cu3(μ-O)3]2+ Cluster in Mordenite Zeolite and Its Effects on the Methane to Methanol Oxidation

Identifying Cu-exchanged zeolites able to activate C-H bonds and selectively convert methane to methanol is a challenge in the field of biomimetic heterogeneous catalysis. Recent experiments point to the importance of trinuclear [Cu₃(μ-O)₃]²⁺ complexes inside the micropores of mordenite (MOR) zeolite for selective oxo-functionalization of methane. The electronic structures of these species, namely, the oxidation state of Cu ions and the reactive character of the oxygen centers, are not yet fully understood. In this study, we performed a detailed analysis of the electronic structure of the [Cu₃(μ-O)₃]²⁺ site using multiconfigurational wave-function-based methods and density functional theory. The calculations reveal that all Cu sites in the cluster are predominantly present in the Cu(II) formal oxidation state with a minor contribution from Cu(III), whereas two out of three oxygen anions possess a radical character. These electronic properties, along with the high accessibility of the out-of-plane oxygen center, make this oxygen the preferred site for the homolytic C-H activation of methane by [Cu₃(μ-O)₃]²⁺. These new insights aid in the construction of a theoretical framework for the design of novel catalysts for oxyfunctionalization of natural gas and suggest further spectroscopic examination.

Optimizing Open Iron Sites in Metal-Organic Frameworks for Ethane Oxidation: A First-Principles Study

Activation of the C-H bonds in ethane to form ethanol is a highly desirable, yet challenging, reaction. Metal-organic frameworks (MOFs) with open Fe sites are promising candidates for catalyzing this reaction. One advantage of MOFs is their modular construction from inorganic nodes and organic linkers, allowing for flexible design and detailed control of properties. In this work, we studied a series of single-metal atom Fe model systems with ligands that are commonly used as MOF linkers and tried to understand how one can design an optimal Fe catalyst. We found linear relationships between the binding enthalpy of oxygen to the Fe sites and common descriptors for catalytic reactions, such as the Fe 3d energy levels in different reaction intermediates. We further analyzed the three highest-barrier steps in the ethane oxidation cycle (including desorption of the product) with the Fe 3d energy levels. Volcano relationships are revealed with peaks toward higher Fe 3d energy and stronger electron-donating group functionalization of linkers. Furthermore, we found that the Fe 3d energy levels positively correlate with the electron-donating strength of functional groups on the linkers. Finally, we validated our hypotheses on larger models of MOF-74 iron sites. Compared with MOF-74, functionalizing the MOF-74 linkers with NH₂ groups lowers the enthalpic barrier for the most endothermic step in the reaction cycle. Our findings provide insight for catalyst optimization and point out directions for future experimental efforts.

Alkane Oxidation: Methane Monooxygenases, Related Enzymes, and Their Biomimetics

Methane monooxygenases (MMOs) mediate the facile conversion of methane into methanol in methanotrophic bacteria with high efficiency under ambient conditions. Because the selective oxidation of methane is extremely challenging, there is considerable interest in understanding how these enzymes carry out this difficult chemistry. The impetus of these efforts is to learn from the microbes to develop a biomimetic catalyst to accomplish the same chemical transformation. Here, we review the progress made over the past two to three decades toward delineating the structures and functions of the catalytic sites in two MMOs: soluble methane monooxygenase (sMMO) and particulate methane monooxygenase (pMMO). sMMO is a water-soluble three-component protein complex consisting of a hydroxylase with a nonheme diiron catalytic site; pMMO is a membrane-bound metalloenzyme with a unique tricopper cluster as the site of hydroxylation. The metal cluster in each of these MMOs harnesses O₂ to functionalize the C-H bond using different chemistry. We highlight some of the common basic principles that they share. Finally, the development of functional models of the catalytic sites of MMOs is described. These efforts have culminated in the first successful biomimetic catalyst capable of efficient methane oxidation without overoxidation at room temperature.

Understanding trends in C-H bond activation in heterogeneous catalysis

While the search for catalysts capable of directly converting methane to higher value commodity chemicals and liquid fuels has been active for over a century, a viable industrial process for selective methane activation has yet to be developed. Electronic structure calculations are playing an increasingly relevant role in this search, but large-scale materials screening efforts are hindered by computationally expensive transition state barrier calculations. The purpose of the present letter is twofold. First, we show that, for the wide range of catalysts that proceed via a radical intermediate, a unifying framework for predicting C-H activation barriers using a single universal descriptor can be established. Second, we combine this scaling approach with a thermodynamic analysis of active site formation to provide a map of methane activation rates. Our model successfully rationalizes the available empirical data and lays the foundation for future catalyst design strategies that transcend different catalyst classes.