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Zeolites are one of the most successful catalyst materials of the 20th century and are anticipated to be crucial in the coming decades to transition towards a more sustainable and circular society. Traditional zeolite-based catalytic processes, such as hydrocarbon cracking and transalkylation involving fossil-based resources, are usually performed in the absence of water. With the development of renewable processes based on agricultural and municipal waste, oxygen-rich molecules must be converted, which involves the presence of water. Hence, the impact of water on zeolite-based catalytic performance becomes crucial. In this review, we discuss the current understanding of the role of water during zeolite catalysis and provide insights into mechanistic aspects of water–zeolite interactions. Special attention is paid to molecular modeling approaches. A synergy between experimental and theoretical approaches represents another major challenge in modern catalysis science as it provides routes towards the design of novel and more stable zeolite catalysts.

The methanol-to-hydrocarbons (MTH) process, commonly catalyzed by zeolites, is of great commercial interest and therefore widely studied both in industry and academia. However, zeolite-based catalyst materials are notoriously hard to study at the nano-scale. Atom probe tomography (APT) is uniquely positioned among the suite of characterization techniques, as it can provide 3D chemical information with sub-nm resolution. In this work, we have used APT to study the nano-scale coking behavior of zeolite SSZ-13 and its relation to bulk coke formation on the macro-/micro-scale studied with operando and in situ UV-vis spectroscopy and microscopy. Radial distribution function analysis (RDF) of the APT data revealed short carbon–carbon length scale affinities, consistent with the formation of larger aromatic molecules (coke species). Using nearest neighbor distribution (NND) analysis, an increase in the homogeneity of carbon was found with increasing time-on-stream. However, carbon clusters could not be isolated due to spatial noise and limited clustering. Therefore, it was found that the coke formation in zeolite SSZ-13 (CHA) is reasonably homogeneous on the nano-scale, and is rather similar to the silicoaluminophosphate analogue SAPO-34 (CHA) but different in nano-scale coking behavior compared to previously studied zeolite ZSM-5 (MFI).

In this review article, we discuss the latest advances and future perspectives of microfluidics for micro/nanoscale catalyst particle synthesis and analysis. In the first section, we present an overview of the different methods to synthesize catalysts making use of microfluidics and in the second section, we critically review catalyst particle characterization using microfluidics. The strengths and challenges of these approaches are highlighted with various showcases selected from the recent literature. In the third section, we give our opinion on the future perspectives of the combination of catalytic nanostructures and microfluidics. We anticipate that in the synthesis and analysis of individual catalyst particles, generation of higher throughput and better understanding of transport inside individual porous catalyst particles are some of the most important benefits of microfluidics for catalyst research.

The effect of oxidation of a carbon nanotube (CNT) support on the structure and catalytic performance in the Fischer-Tropsch (FT) synthesis of pre-synthesized cobalt nanocrystals (NC) has been investigated. 6 nm CoO-NC were prepared using colloidal techniques and impregnated on pristine and surface-oxidized CNT. The CoO-NC on pristine CNT were reduced completely to Co, while ∼50 % CoO reduction was observed on oxidized CNT (oxCNT), indicating a stronger Co-support interaction on oxCNT. In FT, the turnover frequency (TOF) after 100 h was exceptionally high on pristine CNT (150·10−3 s-1) against 63·10−3 s-1 on oxCNT. In the initial stage of FT, the TOF was 35 % lower on oxCNT than on pristine CNT, which matched with 32 % lower crystallinity of cobalt determined using in situ XRD. Furthermore, deactivation was observed mainly on oxCNT. These results demonstrate the large impact of the support on the structure and performance of nanocrystal-based catalysts.

Stability of metal nanoparticles under reaction conditions is crucial in many catalytic processes. Nickel-based catalysts often encounter severe particle growth in the presence of carbon monoxide due to the formation and migration of nickel carbonyl. In this research, we showed that the reduction temperature of nickel oxide supported on niobia (Nb2O5) influenced the stability of the resulting nickel catalyst during subsequent carbon monoxide hydrogenation. Low reduction temperatures resulted in high initial nickel-normalized activity towards long-chain hydrocarbons (C5+), but fast deactivation throughout the experiment. High reduction temperatures led to a shift in product distribution towards shorter hydrocarbons and a decreased initial nickel-normalized activity, while during the first hours of the experiment an increase in turnover frequency and nickel-normalized activity was observed, resulting eventually in a stable catalytic performance. Electron microscopy analysis revealed extensive particle growth after catalysis when the catalyst had been reduced at low temperatures and no significant changes in particle size when reduced at high temperatures. By use of in-situ FT-IR spectroscopy, nickel subcarbonyl species which are precursors of volatile nickel tetracarbonyl were detected on Ni/Nb2O5 after low temperature reduction and exposure to CO, but not after high temperature reduction. Hence, particle growth is explained by the formation and diffusion of nickel carbonyl and subsequent Ostwald ripening, that leads to larger nickel particles with concomitant decrease in nickel-normalized activity. The stability of the catalyst reduced at high temperature was linked to the formation of niobium suboxides and their partial coverage of the nickel particles limiting the formation of nickel carbonyl and slowing down particle growth.

A major issue in the catalytic fast pyrolysis (CFP) of biomass is the rapid deactivation of the typically employed zeolite-based catalysts. Detailed understanding of the deactivation pathways and the type and location of coke deposits are essential for the further development of improved or new catalyst materials, including appropriate regeneration protocols. Such deactivation and regeneration studies focus almost invariably on small-scale CFP reactor units employing catalyst materials in powder form. In this study, we report the in-depth characterization of deactivated and regenerated ZrO2-promoted zeolite ZSM-5 catalyst extrudates after ex-situ CFP tests carried out in a bench scale reactor. The findings support that coking is the main reason for catalyst deactivation, i.e. for the observed decreased activity in cracking and deoxygenation. Post-mortem characterization by confocal fluorescence microscopy reveals an egg-shell spatial distribution of the coke deposits within the catalyst extrudates. These deposits are heavily poly-aromatic in nature. The majority of the coke build-up occurs in the first 20 min of the reaction and is formed on the strong Brønsted acid sites, which promote deep deoxygenation and cracking. With increasing time-on-stream, the coke deposition slows down, occurring now mainly on the external surface of the zeolite to generate a softer, i.e. more hydrogen-rich, coke on the ZrO2 domains. The catalyst is readily regenerated via thermal oxidation in air, with optimal regeneration at 500 °C. This temperature removes all coke deposits, with no detrimental effect on the catalyst's structural, textural and acid (type and strength) properties.

The Fischer–Tropsch synthesis (FTS) reaction is one of the most promising processes to convert alternative energy sources, such as natural gas, coal or biomass, into liquid fuels and other high-value products. Despite its commercial implementation, we still lack fundamental insights into the various deactivation processes taking place during FTS. In this work, a combination of three methods for studying single catalyst particles at different length scales has been developed and applied to study the deactivation of Co/TiO2 Fischer–Tropsch synthesis (FTS) catalysts. By combining transmission X-ray microscopy (TXM), scanning transmission X-ray microscopy (STXM) and scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) we visualized changes in the structure, aggregate size and distribution of supported Co nanoparticles that occur during FTS. At the microscale, Co nanoparticle aggregates are transported over several μm leading to a more homogeneous Co distribution, while at the nanoscale Co forms a thin layer of ∼1–2 nm around the TiO2 support. The formation of the Co layer is the opposite case to the “classical” strong metal–support interaction (SMSI) in which TiO2 surrounds the Co, and is possibly related to the surface oxidation of Co metal nanoparticles in combination with coke formation. In other words, the observed migration and formation of a thin CoOx layer are similar to a previously discussed reaction-induced spreading of metal oxides across a TiO2 surface.

A study on the influence of H2 and the degree of titanation of a Cr/SiO2 Phillips polymerisation catalyst on the selective oligomerisation of ethylene induced by pre-contacting the catalyst with triethylaluminium (TEAl) is presented. Ethylene oligomerisation reactions were performed at 373 K and 1 bar, inside a quartz reactor of a specially designed operando experimental setup, which allowed examination of the catalysts by UV-vis-NIR diffuse reflectance spectroscopy, while the gas phase was continuously monitored by on-line mass spectroscopy and subsequently analysed by gas chromatography. With this combination of techniques, it was possible to acquire detailed insight into the different distributions of produced oligomers, which were highly dependent on the catalyst structure. The addition of small amounts of Ti significantly changes the electronic environment of Cr oligomerisation sites by the formation of Cr–O–Ti–O–Si linkages, favouring β-H transfer and increasing the selectivity towards butene at the expense of 1-hexene. Moreover, we show that ethylene oligo-/polymerisation reactions follow at least two different pathways, i.e. metallacycle for olefinic species with a broken Schulz-Flory distribution, and Cossee–Arlman for other hydrocarbon species.

Mesoporous Cu-SSZ-13 was created by first synthesizing zeolite H-SSZ-13 and subsequently desilicating the material by base leaching using NaOH in different concentrations. The catalyst materials were prepared by ion exchanging the leached samples back to their acidic form using NH4NO3, and to their active Cu form by ion exchanging them with CuSO4. For comparison, H- and Cu-SSZ-13 were steamed using a wide variety of different conditions. Using a 0.10 M NaOH solution for base leaching, it was found that Cu-SSZ-13 becomes more active in the selective catalytic reduction of NOx with NH3 (NH3-SCR) over the entire temperature region but especially in the low temperature region (

A series of fluorescent probe molecules based on the commercially available trans-4-(4-(diethylamino)styryl)-N-methylpyridinium iodide (DAMPI) scaffold has been developed. The dynamic radii of these DAMPI-type probes covered a range of 5.8 to 10.1 Å and could be changed by varying the alkyl substituents on the amine donor group, with limited effect on the electronic properties. These probe molecules allow for the direct evaluation of the molecular accessibility into confined spaces, more specifically the micropore architecture of zeolite materials. Evaluation of industrially relevant zeolite materials with 8- (CHA), 10- (MFI) and 12-membered ring pores (FAU) showed that steric bulk influences the rate of adsorption, the amount of probe molecule taken up by the zeolite as well as the interaction of the probe molecule with the zeolite material. Furthermore, a positive linear correlation is found between the pore–probe size difference and total probe uptake by the zeolite. The absorption spectra of each probe molecule within the zeolites show that this DAMPI-type compound is chemically bound to the zeolite's acid sites. The new approach shows the general principle of determining size-accessibility relationships in microporous solids with a series of fluorescent probes of systematically tunable size.

A series of fluorescent probe molecules based on the commercially available trans-4-(4-(diethylamino)styryl)-N-methylpyridinium iodide (DAMPI) scaffold has been developed. The dynamic radii of these DAMPI-type probes covered a range of 5.8 to 10.1 Å and could be changed by varying the alkyl substituents on the amine donor group, with limited effect on the electronic properties. These probe molecules allow for the direct evaluation of the molecular accessibility into confined spaces, more specifically the micropore architecture of zeolite materials. Evaluation of industrially relevant zeolite materials with 8- (CHA), 10- (MFI) and 12-membered ring pores (FAU) showed that steric bulk influences the rate of adsorption, the amount of probe molecule taken up by the zeolite as well as the interaction of the probe molecule with the zeolite material. Furthermore, a positive linear correlation is found between the pore–probe size difference and total probe uptake by the zeolite. The absorption spectra of each probe molecule within the zeolites show that this DAMPI-type compound is chemically bound to the zeolite's acid sites. The new approach shows the general principle of determining size-accessibility relationships in microporous solids with a series of fluorescent probes of systematically tunable size.

NbOx/SiO2 with a very high catalytic activity for the gas-phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam, was investigated by different spectroscopic methods in order to obtain new insights in the formation and nature of the active sites. FT-IR spectroscopy in combination with pyridine adsorption measurements revealed that the catalyst material contains Lewis-acidic sites, most probably related to the Nb[double bond, length as m-dash]O groups of isolated tetrahedral NbO4 surface species, whereas no Brønsted-acidic sites were observed. Results from in situ Raman and complementary FT-IR measurements strongly suggest that Brønsted-acidic Nb-OH sites can be generated from Nb[double bond, length as m-dash]O groups by reaction with ethanol. This is in agreement with the observation that ethanol is essential for obtaining a very good catalyst performance. However, the Brønsted-acidic sites can be detected in significant amounts in particular in the presence of a Lewis-base, e.g. pyridine, most probably because the formation and/or the stability of these Brønsted-acidic sites are enhanced by a basic molecule. Assuming that cyclohexanone oxime, being a base, can play a similar role as pyridine, we propose on the basis of the spectroscopic findings obtained in this work and our kinetic results published recently, a reaction scheme for the formation of the active site at the Nb[double bond, length as m-dash]O group as well as for the recovery of the Nb[double bond, length as m-dash]O site during the final stage of the gas-phase Beckmann rearrangement.

Cu-exchanged zeolites are commercially being used in the ammonia selective catalytic reduction (NH3-SCR) of NOx, especially in the use for mobile sources. In particular Cu exchanged zeolites having the CHA structure (Cu-SSZ-13 and Cu-SAPO-34) show an excellent performance in the NH3-SCR of NOx. The materials have been characterized extensively in current literature, using the latest analysis techniques. Due to the large amount of research performed on this topic, it has resulted in some deviations in the conclusions about the active site, which could partly be explained by variations in the used catalyst systems, such as different Si/Al ratios, different Cu/Al ratios and different synthesis procedures to get an active Cu zeolite catalyst. This chapter will start with an introduction about into the emission control system in road transport vehicles, followed by shortly explaining the different catalysts which able to perform the NH3-SCR of NOx. The following sub chapters will focus on the Cu containing zeolites, and in particular the small pore exchanged Cu zeolites having the CHA structure. This is followed by an overview of the used Cu exchange and synthesis methods and their differences, and the location of the Cu inside the CHA framework. Lastly, deactivation mechanisms for Cu exchanged zeolites are being discussed.

NbOx/SiO2 catalyst materials prepared by the incipient wetness impregnation method were studied in the industrially relevant gas-phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam. The catalytic experiments were carried out in a fixed bed reactor system at atmospheric pressure. Results have been complemented with Raman and FT-IR spectroscopy as well as N2 physisorption measurements. Optimal catalytic results were observed for a catalyst calcination temperature of 600 °C and a niobia loading of 0.3 wt.%. Raman spectroscopy revealed that isolated tetrahedral mono-oxo NbO4 surface species most probably play a key role in the catalytic reaction. Very positive effects were achieved by applying a catalyst silylation post-treatment. Firstly, the catalytic long-term stability increased very substantially as a consequence of a decreased coke deposition on the catalyst surface. Secondly, the harmful effect of water on catalytic performance was strongly suppressed even to such an extent that water could be added to the feed to enhance catalytic long-term stability. Cyclohexanone oxime conversion and ε-caprolactam selectivity could be maintained at constant high values >99% and around 95% respectively, for 26 h time-on-stream. Finally, considering the Sumitomo gas-phase Beckmann rearrangement process as a benchmark, we made a rough comparison between the catalytic performances of our niobia/silica catalyst and the silicalite catalyst used by Sumitomo.

A study on the influence of H2 and the degree of titanation of a Cr/SiO2 Phillips polymerisation catalyst on the selective oligomerisation of ethylene induced by pre-contacting the catalyst with triethylaluminium (TEAl) is presented. Ethylene oligomerisation reactions were performed at 373 K and 1 bar, inside a quartz reactor of a specially designed operando experimental setup, which allowed examination of the catalysts by UV-vis-NIR diffuse reflectance spectroscopy, while the gas phase was continuously monitored by on-line mass spectroscopy and subsequently analysed by gas chromatography. With this combination of techniques, it was possible to acquire detailed insight into the different distributions of produced oligomers, which were highly dependent on the catalyst structure. The addition of small amounts of Ti significantly changes the electronic environment of Cr oligomerisation sites by the formation of Cr–O–Ti–O–Si linkages, favouring β-H transfer and increasing the selectivity towards butene at the expense of 1-hexene. Moreover, we show that ethylene oligo-/polymerisation reactions follow at least two different pathways, i.e. metallacycle for olefinic species with a broken Schulz-Flory distribution, and Cossee–Arlman for other hydrocarbon species.

Straightforward analysis of chemical processes on the nanoscale is difficult, as the measurement volume is linked to a discrete number of molecules, ruling out any ensemble averaging over rotation and diffusion processes. Raman spectroscopy is sufficiently selective for monitoring chemical changes, but is not sufficiently sensitive to be applied directly. Surface-enhanced Raman spectroscopy (SERS) can be applied for studying reaction kinetics, but adds additional variability in the signal as the enhancement factor is not the same for every location. A novel chemometric method described here separates reaction kinetics from short-term variability, based on the lack of fit in a principal-component analysis. We show that it is possible to study effects that occur on different time scales independently without data reduction using the photocatalytic reduction of p-nitrothiophenol as a showcase system. Using this approach a better description of the nanoscale reaction kinetics becomes available, while the short-term variations can be examined separately to examine reorientation and/or diffusion effects. It may even be possible to identify reaction intermediates through this approach. With only a limited number of reactive molecules in the studied volume, an intermediate on a SERS hot spot may temporarily dominate the spectrum. Now such events can be easily separated from the bulk conversion process by making use of this chemometric method.

The formation of hydrocarbon species during the methanol to olefins (MTO) reaction over zeolite H-SSZ-13 has been systematically studied at reaction temperatures between 573 and 723 K with a combination of operando UV-vis spectroscopy and online gas chromatography. It was found that the applied reaction temperature influences the rate and nature of coke formation as well as the catalyst stability. Correlation between all hydrocarbon species formed inside the catalyst pores with the activity and deactivation of H-SSZ-13 zeolite material was established. By using a multivariate analysis, we found that the nature of the active and deactivating species varies with the reaction temperature. The majority of active hydrocarbon pool species at low reaction temperatures (i.e., 573-598 K) are methylated benzene carbocations, while at high reaction temperatures (i.e., 623-723 K) methylated naphthalene carbocations become dominant. At low reaction temperatures the deactivation occurs because of the pore filling with methylated naphthalene carbocations. In contrast, at higher reaction temperatures the formation of phenanthrene, pyrene carbocations, and highly conjugated polyaromatics during the deactivation increases. This suggests that the formation of highly conjugated polyaromatics on the external surface can play a role in the deactivation of the material by pore blockage.

X-ray nanotomography of a complete FCC particle cluster reveals increased metal concentrations at the interface of agglutinated E-cat particles, which might play a crucial role E-cat particle clustering., Metal accumulation at the catalyst particle surface plays a role in particle agglutination during fluid catalytic cracking.