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Sending ethane over Cu-oxo sites in zeolites allows for the selective formation of ethylene in a stepwise reaction at mild conditions. Experimental results indicate that the reaction proceeds via the formation of ethanol intermediates.

In the pursuit of controlling the propensity of Cu-mordenite (MOR) for the selective oxidation of CH4, we take a closer look at intrinsic zeolite parameters. Via synthesis design, we vary the relative proportion of Al situated near the 8-rings and 12-rings of MOR zeolite. This is accomplished using different Al sources impacting the local degree of silica dissolution and zeolite formation as evidenced by crystallization times and morphological differences. Interrogating the crystalline system with steric probe molecules in conjunction with spectroscopic techniques such as H-1 magic angle spinning (MAS) NMR, infrared spectroscopy, as well as temperature-programmed desorption confirms discrete changes of the Al within the unit cell. The subsequent copper exchange allows for the generation of Cu-MOR materials of different inclinations for the activation of methane in the stepwise formation of MeOH. Here, an increasing degree of acid sites in more easily accessible locations (e.g., 12-ring) correlates with increasing maximum productivity toward MeOH at moderate exchange degrees. X-ray absorption spectroscopy supports this notion, finding a higher concentration of self-reduction-resistant framework-associated Cu2+ species, previously established as the active sites in the selective oxidation of CH4.

CeO2 surfaces play decisive roles in heterogeneous catalysis of important processes. Here, we investigate adsorption and dissociation of water and migration of protons on internal surfaces of nanoscopic porous CeO2. Sorption and thermogravimetry confirm literature suggestions that the surface is hydrogenated to Ce3+ ions and protons H+. The following chemisorption is dissociative, yet weak, and physisorption sets in only at the very highest relative humidities, reflecting hydrophobic behaviour. We link sample conductivities to surface protonic conductances via a brick layer model and show that behaviours at high, intermediate, and low temperatures with, respectively, positive, close to zero, and negative apparent activation energies and pH2O^(1/2), pH2O^1, and pH2O^(3/2) dependences, can be attributed to different models of migration all within the chemisorbed layer, without contribution from physisorbed water. While CeO2 may special in this respect due to the effect of the hydrogenated surface, we believe the extended models of transport in the chemisorbed layer may apply also to other oxides. Unsaturated chemisorption may play an important role for CeO2 as catalyst in that the surface is left available for reactant molecules, still with availability of dissociated and mobile protons in the chemisorbed layer and electronic defects by Ce3+ in the surface.

For the successful implementation of catalysts in industrial processes, three overall goals are optimized: activity, selectivity, and stability. This review will address the role of chemical descriptors in aiding and guiding the development of optimal zeolite catalyst designs with the above performance criteria in mind. It will focus on both gas‐phase and liquid‐phase catalysis with a special focus on methanol to hydrocarbon, and biomass valorization reactions. By way of preface, the research on constraint and spaciousness indices is discussed. These indices can be considered the original chemical descriptors of catalysis research.

Modular and distributed biomanufacturing requires continuous flow microreactors integrated with efficient separation units operating at comparable time scales: biphasic reactive extraction of 5-hydroxymethyl furfural (HMF) by fructose dehydration is an excellent example. The liquid–liquid extraction (LLE) and fast reaction kinetics in biphasic microchannels can immensely benefit from a downstream microseparator enabling separation of an HMF-rich organic extract and an aqueous raffinate. Here we demonstrate the successful implementation of an effective slit-shaped microseparator for eleven organic-water biphasic systems. The microseparator successfully separates six of these over reasonable flow rates. The ratio of capillary and hydraulic pressures qualitatively rationalizes the separation performance, while a transition to non-segmented flow patterns correlates with performance deterioration. Acids and salts, integral parts of the chemistry, significantly expand the flow rates for efficient separation enabling a broader slate of organic solvents. For the MIBK/water biphasic system, we demonstrate perfect separation performance over a 16-fold variation in the organic to aqueous flow ratio. Here we also integrate the microseparator and extractive microreactor into a modular system and achieve an HMF yield of up to 93% – the highest reported fractional HMF productivity of 27.9 min−1 – at an ultrashort residence time of 2 s. This unprecedented performance is maintained over a 50-fold fructose concentration range and is stable with time-on-stream. This microseparator exhibits a ten-fold reduction in separation time and substantial energy savings over conventional decanters. As such, it holds promise for continuous process intensification and modular biomanufacturing.

We describe a two-step catalytic process for the selective and rapid conversion of biomass-derived acetylfurans into vinylfurans. This is accomplished by nearly quantitatively reducing acetylfurans...

The work reviewed here attempts to summarize the growing literature on zeolite stability in hot liquid water. This required to first establish the nature, structure and interactions of the active sites in Brønsted and Lewis acidic microporous materials in the presence of liquid water. This understanding was then transferred to zeolite stability and catalyst deactivation. While early results correctly established Si–O–Si hydrolysis as the dominant pathway compared to the Si–O–Al hydrolysis observed in steaming, it also focused extensively on the positive role of framework and extra-framework Al in stabilizing zeolites. However, stability was instead found to more directly correlate with the number of internal structural defects and intraporous water concentration. Stabilization protocols for Brønsted acidic zeolites are described in detail. In the case of Lewis acidic zeolites, their inherent hydrophobic behavior (Si/M >100) makes them more resistant towards water, however, issues such as reversible carbonaceous species formation as well as irreversible metal leaching and fouling remain. Finally, we summarize the most important factors in designing robust and efficient zeolite catalysts made to withstand hot liquid water.

Direct conversion of methane to methanol (MTM) over Cu-zeolites is a so-called “dream reaction” for the chemical industry. There is still a lot that can be done in order to optimize the reaction by e.g. achieving a deeper understanding of the reaction mechanism and the nature of the Cu-sites. In this study, we investigated a solid-state ion exchange method to incorporate CuI ions into zeolites (MOR, BEA, ZSM-5 and FAU), as a more scalable technique. The solid-state ion exchange led to a Cu/Al ration of about 0.8, however with a heterogeneous distribution of Cu. Regardless, Fourier transform-infrared spectroscopy still revealed that most Brønsted acid sites were exchanged in all four samples. Further, CH4-temperature programmed reaction experiments showed that some Cu-sites formed were reactive towards CH4, with CuI-MOR and CuI-FAU having the largest CH4 consumption. Ultimately, the CuI-zeolites were tested in the MTM reaction and proved capable of producing methanol, even without the presence of Brønsted sites. A MOR with lower Cu/Al ratio (0.30) was also tested for comparison, and as this sample obtained a much higher productivity than the CuI-MOR with high Cu-loading (0.10 vs. 0.03 molMeOH/molCu), it was demonstrated that some fine-tuning is necessary to obtain the active Cu sites for methane activation.

Model 0.3 wt% and 1 wt% Pd/Beta was synthesized with nano-sized Beta crystals (average size O2 in dry streams promotes NO storage via additional formation of nitrosyl (NO+) ions; it also leads to stabilization of significant amounts of N2O3 in BEA pores. In the presence of water, NO storage capacity is dramatically lowered. However, in the presence of CO (which is always present in exhaust gas) the performance improves. With the aid of spectroscopy and PNA measurements we show that in the presence of CO, stable mixed palladium(II) carbonyl nitrosyl complex [Pd(II)(CO)(NO)] is formed and responsible for enhanced NOx storage. This points to a general conclusion that the species responsible for NOx storage on various zeolites in the presence of water and CO, are in fact, Pd(II)(CO)(NO). Decreasing the Pd loading to 0.3 wt% improves Pd dispersion and NO/Pd storage ratio. We also demonstrate that varying crystal sizes of BEA leads to significant changes in the NOx release temperature as well as NO/Pd ratio. Larger, defect free, more hydrophobic BEA crystals store more NOx and release it at higher temperature. Furthermore, larger, defect free and more hydrophobic BEA crystals stay significantly more active for PNA after hydrothermal aging, losing little activity compared with defective nanocrystals. High-field 27Al NMR results show that larger, hydrophobic BEA crystals are more resistant to de-alumination upon hydrothermal aging (HTA), and thus are more attractive for PNA storage with Pd.

The presence of fluoride anions in the synthesis gel leads to zeolite BEA with high hydrothermal stability. This longer lifetime is caused by a lower concentration of internal silanol defects that are sites of the initial framework hydrolysis eventually destroying the lattice structure. The lifetime of these zeolites was the higher the lower the concentration of framework bridging hydroxyl groups that form hydronium ions was. While lattice hydrolysis was initiated at defect sites in all cases, the role of defects became less important as the concentration of hydronium ions increased. At higher concentration of hydrated hydronium ions, the presence of water associated with the hydrated hydronium ions limits the useful lifetime of zeolite catalysts in water. For such materials, the selective hydrophobization of the external surface extends the lifetime.

Methods of determining in situ the electrochemically active surface area of bulk noble metal electrodes (e.g., underpotential deposition of a strongly interacting sorbent followed by columbic stripping) may not be appropriate for certain base metals supported on electroactive supports, such as carbon, because of the difficulty of finding a metal-specific titrant and because the support itself can contribute a very large background current. For example, the coulometric stripping of metal-selective sorbates, like carbon monoxide , works for the in situ characterization of certain electrode-supported metals (e.g. Pt, Pd) if applied carefully, but not for base metals, like Cu that do not strongly adsorb the titrant. Moreover, in scoping studies, we found that underpotential deposition of Tl on carbon-supported Cu failed to be selective for the copper. Here we present the use of gentle oxidation of the metal domains using N2 O as the oxidant, followed by cathodic stripping of the surface oxide. The method compares approximately with XRD line broadening for characterizing ∼10 nm particles of Cu supported on graphite felt. More significantly, we show that the method usefully normalizes the rates of two electrochemical hydrogenation reactions: conversion of nitrobenzene to aniline and conversion of benzaldehyde to benzyl alcohol.

Alkali-exchanged SSZ-13 adsorbents were investigated for their applicability in separating N2 from CO2 in flue gas streams using a dynamic breakthrough method. In contrast to IAST calculations based on equilibrium isotherms, K+ exchanged SSZ-13 was found to yield the best N2 productivity, comparable to Ni-MOF-74, under dynamic conditions where diffusion properties play a significant role. This was attributed to the selective, partial blockage of access to the chabazite cavities, enhancing the separation potential in a 15/85 CO2/N2 binary gas mixture.

Ex situ and in situ spectroscopy was used to identify the kinetics of processes during the formation of the faujasite (FAU) zeolite lattice from a hydrous gel. Using solid-state 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR), the autocatalytic transformation from the amorphous gel into the crystalline material was monitored. Al X-ray absorption near-edge structure shows that most Al already adopts a tetrahedral coordination in the X-ray-amorphous aluminosilicate at the beginning of the induction period, which hardly changes throughout the rest of the synthesis. Using 23Na NMR spectroscopy, environments in the growing zeolite crystal were identified and used to define the processes in the stepwise formation of the zeolite lattice. The end of the induction period was accompanied by a narrowing of the 27Al and 23Na MAS NMR peak widths, indicating the increased level of long-range order. The experiments show conclusively that the formation of faujasite occurs via the continuous formation and...

The hydrothermal stability of Cu/SSZ-13 SCR catalysts has been extensively studied, yet atomic-level understanding of changes to the zeolite support and the Cu active sites during hydrothermal aging are still lacking. In this work, via the utilization of spectroscopic methods including solid-state 27Al and 29Si NMR, EPR, DRIFTS, and XPS, together with imaging and elemental mapping using STEM, detailed kinetic analyses, and theoretical calculations with DFT, various Cu species, including two types of isolated active sites and CuOx clusters, were precisely quantified for samples hydrothermally aged under varying conditions. This quantification convincingly confirms the exceptional hydrothermal stability of isolated Cu2+-2Z sites and the gradual conversion of [Cu(OH)]+-Z to CuOx clusters with increasing aging severity. This stability difference is rationalized from the hydrolysis activation barrier difference between the two isolated sites via DFT. Discussions are provided on the nature of the CuOx clusters ...

Strategies to understand and mitigate the corrosive interactions of zeolites in aqueous phase under reaction conditions have been explored using zeolite BEA as an example. The states of Si and Al atoms after chemical modification and during gradual degradation were followed by cross-polarization enhanced 29Si MAS NMR and 27Al MAS NMR as well as IR spectroscopy. The key to stabilizing a zeolite for aqueous phase catalysis is to reduce the pore concentration of water in the presence of reacting substrates. The concentration of tetrahedral aluminum, which is charge balanced by hydrated hydronium ions, is the most important parameter determining the concentration of water in the zeolite pores. Lower intraporous water concentrations, largely independent of ubiquitous defects, led to longer zeolite lifetimes during cyclohexanol dehydration. The concentration of intraporous water was directly related to the rate of hydrolysis of Si4+ from the zeolite lattice and its removal from the crystal. Dissolution of Si4+ ...

Synthetic zeolites play an important role in industrial applications and researchers in academia and industry exert significant effort in creating new zeolite structures and understanding the material’s properties. While there are an extensive number of reviews available focused on specific aspects of zeolites, this chapter will instead paint a broad picture of the most important features concerning zeolites. After briefly giving an overview of zeolite nomenclature and structural characteristics, we then discuss the formation of zeolites by highlighting important factors such as organic structure-directing agents, gel composition, and alkalinity. The chapter concludes with an exploration of the characterization techniques available to the zeolite chemist. This ranges from, diffraction techniques, porosity measurements with gas-phase adsorption, to spectroscopies used to probe active sites, such as IR and solid-state NMR.

For the first time, sub-micron Cu/SSZ-13, obtained by modifying an existing synthesis procedure, was shown to be an effective and stable catalyst for selective catalytic reduction of NO. Characterization of the materials with X-ray diffraction, N2-physisorption and 27Al MAS NMR shows that hydrothermal aging, which simulates SCR reaction conditions, is more destructive for smaller particles in a sodium form. After Cu exchange, however, the catalytic performance and hydrothermal stability for Cu/SSZ-13 is independent of the particle size. In particular, a clear positive correlation is found between remaining tetrahedral framework Al and isolated Cu-ion concentrations in aged Cu/SSZ-13 catalysts of comparable Al and Cu contents. This indicates that (1) isolated Cu-ion and paired framework Al configurations display remarkable hydrothermal stabilities; and (2) paired-Al contents can be varied via modifying the synthesis procedures, which appear to have a more critical influence on stabilizing isolated Cu-ions during harsh hydrothermal aging than the particle size. This study is of high interest for applications in vehicular DeNOx technologies where high loadings of active species on wash coats can be achieved by using sub-micron Cu/SSZ-13.

Alkali-exchanged SSZ-13 adsorbents were investigated for their applicability in separating N

Chabazite SSZ-13 samples with varying silica content (Si/Al from 5 to ∼20) were synthesized under both stirring and static conditions to obtain materials with changing particle size and morphology and thoroughly analysed using various characterization techniques. The role of particle size and chemical compositions in CO2 and N2 adsorption measurements was investigated. The Si/Al ratio played a major role in CO2 adsorption; Al-rich SSZ-13 demonstrated a higher CO2 uptake than an Al-poor material. This was attributed to the high density of active charged species in the chabazite cage. The particle size also played a role in the sorption capacities; smaller particles, obtained under stirring conditions, showed enhanced CO2 uptake compared to larger particles of similar chemical composition. This was associated with a higher contribution of micropores containing active sites for CO2 adsorption.

The formation of carbonaceous deposits (coke) in zeolite pores during catalysis leads to temporary deactivation of catalyst, necessitating regeneration steps, affecting throughput, and resulting in partial permanent loss of catalytic efficiency. Yet, even to date, the coke molecule distribution is quite challenging to study with high spatial resolution from surface to bulk of the catalyst particles at a single particle level. To address this challenge we investigated the coke molecules in HZSM-5 catalyst after ethanol conversion treatment by a combination of C K-edge X-ray absorption spectroscopy (XAS), 13C Cross polarization-magic angle spinning nuclear magnetic resonance (CP-MAS NMR) spectroscopy, and atom probe tomography (APT). XAS and NMR highlighted the aromatic character of coke molecules. APT permitted the imaging of the spatial distribution of hydrocarbon molecules located within the pores of spent HZSM-5 catalyst from surface to bulk at a single particle level. 27Al NMR results and APT results indicated association of coke molecules with Al enriched regions within the spent HZSM-5 catalyst particles. The experimental results were additionally validated by a level-set–based APT field evaporation model. These results provide a new approach to investigate catalytic deactivation due to hydrocarbon coking or poisoning of zeolites at an unprecedented spatial resolution.

Missing silicon-oxygen bonds in zeolites are shown to be the cause for structural instability of zeolites in hot liquid water. Their selective removal drastically improved their structural stability as demonstrated using zeolite beta as example. The defects in the siloxy bonds were capped by reaction with trimethylchlorosilane, and Si-O-Si bonds were eventually formed. Hydrolysis of Si-O-Si bonds of the parent materials and dissolution of silica-oxygen tetrahedra in water causing a decrease in sorption capacity by reprecipitation of dissolved silica and pore blocking was largely mitigated by the treatment. The stability of the modified molecular sieves was monitored by (29)Si-MAS NMR, transmission electron micrographs, X-ray diffraction, and adsorption isotherms. The microporosity, sorption capacity, and long-range order of the stabilized material were fully retained even after prolonged exposure to hot liquid water.