Performance of Fe-BEA catalysts for the selective hydroxylation of benzene with N2O

Steam Reforming of Bioethanol Using Metallic Catalysts on Zeolitic Supports: An Overview

Hydrogen is considered one of the energy carriers of the future due to its high mass-based calorific value. Hydrogen combustion generates only water, and it can be used directly as a fuel for electricity/heat generation. Nowadays, about 95% of the hydrogen is produced via conversion of fossil fuels. One of the future challenges is to find processes based on a renewable source to produce hydrogen in a sustainable way. Bioethanol is a promising candidate, since it can be obtained from the fermentation of biomasses, and easily converted into hydrogen via steam catalytic reforming. The correct design of catalysts and catalytic supports plays a crucial role in the optimization of this reaction. The best results have to date been achieved by noble metals, but their high costs make them unsuitable for industrial application. Very satisfactory results have also been achieved by using nickel and cobalt as active metals. Furthermore, it has been found that the support physical and chemical properties strongly affect the catalytic performance. In this review, zeolitic materials used for the ethanol steam reforming reaction are overviewed. We discuss thermodynamics, reaction mechanisms and the role of active metal, as well as the main noble and non-noble active compounds involved in ethanol steam reforming reaction. Finally, an overview of the zeolitic supports reported in the literature that can be profitably used to produce hydrogen through ethanol steam reforming is presented.

Recent Advances in Catalysis Based on Transition Metals Supported on Zeolites

This article reviews the current state and development of thermal catalytic processes using transition metals (TM) supported on zeolites (TM/Z), as well as the contribution of theoretical studies to understand the details of the catalytic processes. Structural features inherent to zeolites, and their corresponding properties such as ion exchange capacity, stable and very regular microporosity, the ability to create additional mesoporosity, as well as the potential chemical modification of their properties by isomorphic substitution of tetrahedral atoms in the crystal framework, make them unique catalyst carriers. New methods that modify zeolites, including sequential ion exchange, multiple isomorphic substitution, and the creation of hierarchically porous structures both during synthesis and in subsequent stages of post-synthetic processing, continue to be discovered. TM/Z catalysts can be applied to new processes such as CO₂ capture/conversion, methane activation/conversion, selective catalytic NO_x reduction (SCR-deNO_x), catalytic depolymerization, biomass conversion and H₂ production/storage.

The Effect of Zeolite Features on the Dehydration Reaction of Methanol to Dimethyl Ether: Catalytic Behaviour and Kinetics

The synthesis of dimethyl ether (DME) is an important step in the production of chemical intermediate because it is possible to prepare it by direct hydrogenation of CO₂. This paper reports the effect of different zeolitic frameworks (such as: BEA, EUO, FER, MFI, MOR, MTW, TON) on methanol conversion, DME selectivity and catalyst deactivation. The effect of crystal size, Si/Al ratio and acidity of the investigated catalysts have been also studied. Finally, the kinetic parameters (such as: ∆H, ∆S and ∆G) have been evaluated together with pre-exponential factor and activation energy for catalysts with FER and MFI structure topology.

The active site of low-temperature methane hydroxylation in iron-containing zeolites

An efficient catalytic process for converting methane into methanol could have far-reaching economic implications. Iron-containing zeolites (microporous aluminosilicate minerals) are noteworthy in this regard, having an outstanding ability to hydroxylate methane rapidly at room temperature to form methanol. Reactivity occurs at an extra-lattice active site called α-Fe(ii), which is activated by nitrous oxide to form the reactive intermediate α-O; however, despite nearly three decades of research, the nature of the active site and the factors determining its exceptional reactivity are unclear. The main difficulty is that the reactive species-α-Fe(ii) and α-O-are challenging to probe spectroscopically: data from bulk techniques such as X-ray absorption spectroscopy and magnetic susceptibility are complicated by contributions from inactive 'spectator' iron. Here we show that a site-selective spectroscopic method regularly used in bioinorganic chemistry can overcome this problem. Magnetic circular dichroism reveals α-Fe(ii) to be a mononuclear, high-spin, square planar Fe(ii) site, while the reactive intermediate, α-O, is a mononuclear, high-spin Fe(iv)=O species, whose exceptional reactivity derives from a constrained coordination geometry enforced by the zeolite lattice. These findings illustrate the value of our approach to exploring active sites in heterogeneous systems. The results also suggest that using matrix constraints to activate metal sites for function-producing what is known in the context of metalloenzymes as an 'entatic' state-might be a useful way to tune the activity of heterogeneous catalysts.

Structure of the Active Platinum Cluster and Reaction Pathway of the Selective Synthesis of Phenol from Benzene and Oxygen Regulated with Ammonia on a Platinum Cluster/β-Zeolite Catalyst Studied by DFT Calculations

DFT calculations were used to investigate the structure of the active Pt cluster and the catalytic reaction pathway for the selective synthesis of phenol from benzene and molecular oxygen regulated with ammonia on a Pt cluster/β-zeolite catalyst that was reported to be active for the selective hydroxylation of benzene only in the coexistence of ammonia. It was found that Pt5-Pt6 clusters were active for the direct synthesis of phenol, and they provided the reaction sites for bond rearrangements among ammonia, oxygen, and benzene; furthermore, the coexistence of ammonia was crucial for the selective oxidation of benzene to phenol, as it suppressed benzene combustion to CO2 and promoted the selective synthesis of phenol. It was further found that water coexisting in the system also played a significant role in desorbing phenol on the Pt cluster surface, which resulted in promotion of the overall selective synthesis of phenol. The energy diagram for the reaction sequences and the structures of the transition states were obtained, which indicated the origin of the Pt/β catalysis.

Nanoporous materials as new engineered catalysts for the synthesis of green fuels

This review summarizes the importance of nanoporous materials and their fascinating structural properties with respect to the catalytic and photocatalytic reduction of CO₂ to methane, toward achieving a sustainable energy supply. The importance of catalysis as a bridge step for advanced energy systems and the associated environmental issues are stressed. A deep understanding of the fundamentals of these nanoporous solids is necessary to improve the design and efficiency of CO₂ methanation. The role of the support dominates the design in terms of developing an efficient methanation catalyst, specifically with respect to ensuring enhanced metal dispersion and a long catalyst lifetime. Nanoporous materials provide the best supports for Ni, Ru, Rh, Co, Fe particles because they can prevent sintering and deactivation through coking, which otherwise blocks the metal surface as carbon accumulates. This review concludes with the major challenges facing the CO₂ methanation by nanoporous materials for fuel applications.

Structure and nuclearity of active sites in Fe-zeolites: comparison with iron sites in enzymes and homogeneous catalysts

Fe-ZSM-5 and Fe-silicalite zeolites efficiently catalyse several oxidation reactions which find close analogues in the oxidation reactions catalyzed by homogeneous and enzymatic compounds. The iron centres are highly dispersed in the crystalline matrix and on highly diluted samples, mononuclear and dinuclear structures are expected to become predominant. The crystalline and robust character of the MFI framework has allowed to hypothesize that the catalytic sites are located in well defined crystallographic positions. For this reason these catalysts have been considered as the closest and best defined heterogeneous counterparts of heme and non heme iron complexes and of Fenton type Fe(2+) homogeneous counterparts. On this basis, an analogy with the methane monooxygenase has been advanced several times. In this review we have examined the abundant literature on the subject and summarized the most widely accepted views on the structure, nuclearity and catalytic activity of the iron species. By comparing the results obtained with the various characterization techniques, we conclude that Fe-ZSM-5 and Fe-silicalite are not the ideal samples conceived before and that many types of species are present, some active and some other silent from adsorptive and catalytic point of view. The relative concentration of these species changes with thermal treatments, preparation procedures and loading. Only at lowest loadings the catalytically active species become the dominant fraction of the iron species. On the basis of the spectroscopic titration of the active sites by using NO as a probe, we conclude that the active species on very diluted samples are isolated and highly coordinatively unsaturated Fe(2+) grafted to the crystalline matrix. Indication of the constant presence of a smaller fraction of Fe(2+) presumably located on small clusters is also obtained. The nitrosyl species formed upon dosing NO from the gas phase on activated Fe-ZSM-5 and Fe-silicalite, have been analyzed in detail and the similarities and differences with the cationic, heme and non heme homogeneous counterparts have been evidenced. The same has been done for the oxygen species formed by N(2)O decomposition on isolated sites, whose properties are more similar to those of the (FeO)(2+) in cationic complexes (included the [(H(2)O)(5)FeO](2+)"brown ring" complex active in Fenton reaction) than to those of ferryl groups in heme and non heme counterparts.

Behavior of Extraframework Fe Sites in MFI and MCM-22 Zeolites upon Interaction with N2O and NO

We report on the characterization of an isomorphously substituted Fe-MCM-22 sample containing both Fe and Al in framework positions (Si/Fe = 44, Si/Al = 25). XANES spectroscopy was used to study the evolution of Fe sites as a consequence of thermal activation at high temperature (1073 K) and subsequent oxidation with N2O. The results were compared to those obtained in the same conditions on a well-known Fe-silicalite sample (Si/Fe = 68, Si/Al = infinity). In both samples, thermal activation causes migration of a fraction of Fe ions from framework to extraframework positions, this migration being accompanied by a reduction of Fe3+ to Fe2+. Upon oxidation with N2O at 523 K, the two samples show a different behavior. While in Fe-silicalite practically all of the Fe2+ sites formed by thermal activation are reoxidized to Fe3+, in Fe-MCM-22 only a fraction of the extraframework iron sites is involved in the reoxidation process. The accessibility of the extraframework Fe sites was also investigated by using the NO molecule as a surface probe. Upon NO dosage on the sample, the modification of the pre-edge peak and of the edge position suggests an important charge release from the extraframework Fe2+ ions to the adsorbed molecules. This could be formalized with the formation of Fe3+(NO-) complexes, compatible (on the basis of the simple molecular orbital theory) with a bent NO geometry. The formation of a complex family of Fe2+ mono-, di-, and trinitrosyl complexes was also confirmed by FTIR spectroscopy. Similarly to what was observed in the oxidation experiments, the fraction of extraframework Fe sites able to interact with NO in Fe-MCM-22 sample is smaller than that in Fe-silicalite treated in the same conditions. This trend is explained with a major clustering of extraframework Fe sites in Fe-MCM-22 sample, as was also suggested by FTIR experiments. These results suggest that the dispersion of iron in zeolitic matrixes prepared by isomorphous substitution could also depend on the zeolitic structure.