Catalytic decomposition of N2O over Fe-ZSM-11 catalysts prepared by different methods: Nature of active Fe species

Methane Oxidation to Methanol

The direct transformation of methane to methanol remains a significant challenge for operation at a larger scale. Central to this challenge is the low reactivity of methane at conditions that can facilitate product recovery. This review discusses the issue through examination of several promising routes to methanol and an evaluation of performance targets that are required to develop the process at scale. We explore the methods currently used, the emergence of active heterogeneous catalysts and their design and reaction mechanisms and provide a critical perspective on future operation. Initial experiments are discussed where identification of gas phase radical chemistry limited further development by this approach. Subsequently, a new class of catalytic materials based on natural systems such as iron or copper containing zeolites were explored at milder conditions. The key issues of these technologies are low methane conversion and often significant overoxidation of products. Despite this, interest remains high in this reaction and the wider appeal of an effective route to key products from C-H activation, particularly with the need to transition to net carbon zero with new routes from renewable methane sources is exciting.

Identifying key mononuclear Fe species for low-temperature methane oxidation

The direct functionalization of methane into platform chemicals is arguably one of the holy grails in chemistry. The actual active sites for methane activation are intensively debated. By correlating a wide variety of characterization results with catalytic performance data we have been able to identify mononuclear Fe species as the active site in the Fe/ZSM-5 zeolites for the mild oxidation of methane with H₂O₂ at 50 °C. The 0.1% Fe/ZSM-5 catalyst with dominant mononuclear Fe species possess an excellent turnover rate (TOR) of 66 mol_MeOH mol_Fe⁻¹ h⁻¹, approximately 4 times higher compared to the state-of-the-art dimer-containing Fe/ZSM-5 catalysts. Based on a series of advanced in situ spectroscopic studies and ¹H- and ¹³C- nuclear magnetic resonance (NMR), we found that methane activation initially proceeds on the Fe site of mononuclear Fe species. With the aid of adjacent Brønsted acid sites (BAS), methane can be first oxidized to CH₃OOH and CH₃OH, and then subsequently converted into HOCH₂OOH and consecutively into HCOOH. These findings will facilitate the search towards new metal-zeolite combinations for the activation of C–H bonds in various hydrocarbons, for light alkanes and beyond.

The monomeric Fe species in Fe/ZSM-5 have been identified as the intrinsic active sites for the low-temperature methane oxidation.

Tailoring the catalytic properties of alkylation using Cu- and Fe-containing mesoporous MEL zeolites

The catalytic performances of alkylation can be maximized by optimizing the redox properties and pore architectures of zeolites.

Direct observation of the formation and stabilization of metallic nanoparticles on carbon supports

Direct formation of ultra-small nanoparticles on carbon supports by rapid high temperature synthesis method offers new opportunities for scalable nanomanufacturing and the synthesis of stable multi-elemental nanoparticles. However, the underlying mechanisms affecting the dispersion and stability of nanoparticles on the supports during high temperature processing remain enigmatic. In this work, we report the observation of metallic nanoparticles formation and stabilization on carbon supports through in situ Joule heating method. We find that the formation of metallic nanoparticles is associated with the simultaneous phase transition of amorphous carbon to a highly defective turbostratic graphite (T-graphite). Molecular dynamic (MD) simulations suggest that the defective T-graphite provide numerous nucleation sites for the nanoparticles to form. Furthermore, the nanoparticles partially intercalate and take root on edge planes, leading to high binding energy on support. This interaction between nanoparticles and T-graphite substrate strengthens the anchoring and provides excellent thermal stability to the nanoparticles. These findings provide mechanistic understanding of rapid high temperature synthesis of metal nanoparticles on carbon supports and the origin of their stability.

Metal nanoparticle-decorated carbon supports are vital for many applications, ranging from energy storage and catalysis to filtration and environmental remedies. Here, using real-time electron microscopy of a single carbon nanofiber during Joule heating, the authors report atomistic mechanisms responsible for nucleation and stabilization of nanoparticles on amorphous carbon supports.

Assessing the Adsorptive and Photodegradative Efficiencies of ZSM-11 Synthesized from Rice Husk Ash

Rice husk was used to synthesize zeolite (ZSM-11). FTIR and X-ray diffraction methods were used to characterize the product. The synthesized zeolite was used to treat underground water from some communities in Cape Coast considering parameters such as total dissolved solids, total hardness, conductivity, nitrate, and phosphate. The percentage reduction in PO43− was 96.1% in Ebubonko and 92.5% in Apewosika. Similarly, the NO3− levels also decreased significantly in Kwaprow. The adsorption capability was also determined by using it to remove Pb2+ and Zn2+ from laboratory prepared solutions with varying masses. The percentage reduction recorded 90.57% and 86.61% for the 1.0 g whilst the 1.5 g showed 93.26% and 89.36%, respectively. It was also realized that the adsorption process followed a pseudo-first-order rather than the pseudo-second-order process with their R2 values of 0.9929 and 0.8503 for the pseudo-first-order and 0.9662 and 0.6912 for the second-order for Pb2+ and Zn2+, respectively. The adsorption capacity also favored the Freundlich isotherm with R2 values of 0.7578 and 0.642 rather than Langmuir isotherm with R2 values of 0.1742 and 0.3856 for Pb2+ and Zn2+, respectively. The photodegradation ability of the synthesized zeolite was analyzed using rhodamine blue (RhB) and methyl orange (MO). The process was realized to favor the pseudo-second-order with R2 values of 0.9986 and 0.0007 and a constant K2 of 0.035 and 0.021 for RhB and MO, respectively, whereas the pseudo-first-order showed an R2 value of 0.9376 and 0.9757 with K1 values of 0.03 and 0.02.

Silica and silica–titania intercalated MCM-36 modified with iron as catalysts for selective reduction of nitrogen oxides – the role of associated reactions

Fe-MCM-36 zeolites are effective catalysts for high-temperature NH₃-SCR – their activity is related to effective NO to NO₂ oxidation over Fe₂O₃ species, while high N₂-selectivity is attributed to dispersed Fe-species active in N₂O decomposition.

New trends in tailoring active sites in zeolite-based catalysts

This review discusses approaches for tailoring active sites in extra-large pore, nanocrystalline, and hierarchical zeolites and their performance in emerging catalytic applications.

Recovery of ammonium from aqueous solutions using ZSM-5

The demand of reactive nitrogen (N), such as ammonium (NH₄⁺) and nitrate (NO₃^-), continues to increase for fertilizer applications as the population grows, but the Haber Bosch (HB) process currently employed for industrial N fixation is challenged by low efficiency and high energy consumption. Here we report on the investigation of ZSM-5 as a superior sorbent for the recovery of ammonium from aqueous solutions. Fast capture and release of ammonium (NH₄⁺) have been achieved with >90% overall efficiency of recovery using synthetic solutions of NH₄Cl and NaCl, respectively. The ZSM-5 sorbent has also been found to be recyclable and sustain high recovery efficiencies after multiple capture-release cycles. The capture of N has been further studied systematically in dependence of the dose of sorbent and reaction temperature, based on which the mechanism, thermodynamics and kinetics of ion exchange are discussed. Compared to other ion-exchange materials, the ZSM-5 sorbent exhibits superior selectivity for capturing ammonium in the presence of competing cations (NH₄⁺ » Ca²⁺ > Mg²⁺ > K⁺ > Na⁺) and demonstrates high efficiency of ammonium recovery from real wastewater streams.

A novel synthetic strategy of Fe-ZSM-35 with pure framework Fe species and its formation mechanism

A novel two-step crystallization process of Fe-ZSM-35 zeolite was designed, and analyzed by UV resonance Raman spectroscopy.

Bimetallic Rh–Fe catalysts for N2O decomposition: effects of surface structures on catalytic activity

Well-homogenized RhFe alloy nanoparticles and core–shell structured Fe@Rh nanoparticles were highly dispersed on SBA-15 and then applied to N₂O catalytic conversion.

Catalytic decomposition of N2O over Rh/Zn–Al2O3 catalysts

Promoted oxygen adsorption–desorption properties of smaller Rh₂O₃ nanoparticles supported on Zn-modified γ-Al₂O₃ result in a superior activity for N₂O decomposition.