Predicting Molecular Weight Characteristics of Reductively Depolymerized Lignins by ATR-FTIR and Chemometrics.

Recent scientific advances in the valorization of lignin, through e.g., (partial-)catalytic depolymerization, require equally state-of-the-art approaches for the analysis of the obtained depolymerized lignins (DLs) or lignin bio-oils. The use of chemometrics in combination with infrared (IR) spectroscopy is one avenue to provide rapid access to pertinent lignin parameters, such as molecular weight (MW) characteristics, which typically require analysis via time-consuming size-exclusion methods, or diffusion-ordered NMR spectroscopy. Importantly, MW serves as a marker for the degree of depolymerization (or recondensation) that the lignin has undergone, and thus probing this parameter is essential for the optimization of depolymerization conditions to achieve DLs with desired properties. Here, we show that our ATR-IR-based chemometrics approach used previously for technical lignin analysis can be extended to analyze these more processed, lignin-derived samples as well. Remarkably, also at this lower end of the MW scale, the use of partial least-squares (PLS) regression models well-predicted the MW parameters for a sample set of 57 depolymerized lignins, with relative errors of 9.9-11.2%. Furthermore, principal component analysis (PCA) showed good correspondence with features in the regression vectors for each of the biomass classes (hardwood, herbaceous/grass, and softwood) obtained from PLS-discriminant analysis (PLS-DA). Overall, we show that the IR spectra of DLs are still amenable to chemometric analysis and specifically to rapid, predictive characterization of their MW, circumventing the time-consuming, tedious, and not generally accessible methods typically employed.

Chitosan as Support Material for Metal-Organic Framework based Catalysts.

Turning waste into valuable products is one of the main challenges of the chemical industry. In this work, chitosan (CS), an abundant, low-cost, and non-toxic biopolymer derived from chitin, was reshaped into beads of ~3 mm. Their suitability as a support material for active phase catalyst materials was tested for a zirconium-based Metal-Organic Framework (MOF) with incorporated Pt, namely UiO-67-Pt. Its incorporation was investigated via two procedures: a one-pot synthesis (OPS) and a post-synthetic functionalization (PSF) synthesis method. Scanning electron microscopy (SEM) images show good UiO-67-Pt dispersion throughout the CS beads for the one-pot synthesized material (UiO-67-Pt-OPS@CS). However, this uniform dispersion was not observed for the post-synthetically functionalized material (UiO-67-Pt-PSF@CS). The success of the implementation of UiO-67-Pt was evaluated with ultraviolet-visible and infrared spectroscopy for both composite materials. Thermogravimetric analysis (TGA) reveals higher thermal stabilities for UiO-67-Pt-OPS@CS composite beads in comparison to pure CS beads, but not for UiO-67-Pt-PSF@CS. The study provides valuable insights into the potential of chitosan as a green, bead-shaped support material for MOFs, offering flexibility in their incorporation through different synthesis routes. It further contributes to the broader goal of the sustainable and eco-friendly design of a new generation of catalysts made from waste materials, which will be the topic of future studies.

X-ray standing wave characterization of the strong metal-support interaction in Co/TiOx model catalysts.

The strong metal-support interaction (SMSI) is a phenomenon observed in supported metal catalyst systems in which reducible metal oxide supports can form overlayers over the surface of active metal nanoparticles (NPs) under a hydrogen (H2) environment at elevated temperatures. SMSI has been shown to affect catalyst performance in many reactions by changing the type and number of active sites on the catalyst surface. Laboratory methods for the analysis of SMSI at the nanoparticle-ensemble level are lacking and mostly based on indirect evidence, such as gas chemisorption. Here, we demonstrate the possibility to detect and characterize SMSIs in Co/TiOx model catalysts using the laboratory X-ray standing wave (XSW) technique for a large ensemble of NPs at the bulk scale. We designed a thermally stable MoNx/SiNx periodic multilayer to retain XSW generation after reduction with H2 gas at 600°C. The model catalyst system was synthesized here by deposition of a thin TiOx layer on top of the periodic multilayer, followed by Co NP deposition via spare ablation. A partial encapsulation of Co NPs by TiOx was identified by analyzing the change in Ti atomic distribution. This novel methodological approach can be extended to observe surface restructuring of model catalysts in situ at high temperature (up to 1000°C) and pressure (≤3 mbar), and can also be relevant for fundamental studies in the thermal stability of membranes, as well as metallurgy.

Correction: Unravelling potential reaction intermediates during catalytic pyrolysis of polypropylene with microscopy and spectroscopy.

[This corrects the article DOI: 10.1039/D3CY01473H.].

Unravelling potential reaction intermediates during catalytic pyrolysis of polypropylene with microscopy and spectroscopy.

While plastics-to-plastics recycling via melting and re-extrusion is often the preferred option due to a relatively low CO2 footprint, this technique requires a highly sorted waste stream and plastic properties can often not be maintained. Obtaining aromatics, such as benzene, toluene, and xylene (BTX), via catalytic pyrolysis of polyolefins, such as polypropylene and polyethylene, offers another attractive recycling technology. In this process, a discarded crude oil refinery catalyst (ECAT) was previously shown to lower the unwanted formation of deactivating coke species compared to a fresh crude oil refinery catalyst (FCC-cat), while yielding 20 wt% aromatics from polypropylene. In this work, we study the underlying reaction mechanism for this chemical recycling process over the fresh and used refinery catalyst as well as a model system, not containing any zeolite material, using a combination of microscopy and spectroscopy. More specifically, by using in situ fluorescence microscopy, in situ infrared spectroscopy, in situ ultraviolet-visible spectroscopy as well as ex situ solid-state nuclear magnetic resonance, we observe highly fluorescent methylated aromatic intermediates that differ for the three catalyst materials under study both in their fluorescence, IR, UV-vis, and NMR spectroscopy features. This detailed micro-spectroscopic comparison informs which potential reaction intermediates lead to increased coke formation. Our results suggests that a next generation of catalyst materials for this process would profit from a higher accessibility and a milder acidity compared to an FCC-cat and shows the great potential of using ECAT to reduce coking and obtain a BTX stream, which could be become the chemical building blocks for the manufacturing of e.g., plastics and coating materials.

Rapid Lignin Thermal Property Prediction through Attenuated Total Reflectance-Infrared Spectroscopy and Chemometrics.

To expedite the valorisation of lignin as a sustainable component in materials applications, rapid and generally available analytical methods are essential to overcome the bottleneck of lignin characterisation. Where features of a lignin's chemical structure have previously been found to be predicted by Partial Least Squares (PLS) regression models built on Infrared (IR) data, we now show for the first time that this approach can be extended to prediction of the glass transition temperature (Tg), a key physicochemical property. This methodology is shown to be convenient and more robust for prediction of Tg than prediction through empirically derived relationships (e. g., Flory-Fox). The chemometric analysis provided root mean squared errors of prediction (RMSEP) as low as 10.0 °C for a botanically, and a process-diverse set of lignins, and 6.2 °C for kraft-only samples. The PLS models could separately predict both the Tg as well as the degree of allylation (%allyl) for allylated lignin fractions, which were all derived from a single lignin source. The models performed exceptionally well, delivering RMSEP of 6.1 °C, and 5.4 %, respectively, despite the conflicting influences of increasing molecular weight and %allyl on Tg. Finally, the method provided accurate determinations of %allyl with RMSEP of 5.2 %.

Molecular Accessibility and Diffusion of Resorufin in Zeolite Crystals.

Invited for the cover of this issue is the group of Professor Bert Weckhuysen at Utrecht University. The image depicts the change in fluorescence color of a resorufin dye molecule when it is protonated and confined inside the micropores of zeolite-ß. Read the full text of the article at 10.1002/chem.202302553.

Visualizing the Structure, Composition and Activity of Single Catalyst Particles for Olefin Polymerization and Polyolefin Decomposition.

The structural and morphological characterization of individual catalyst particles for olefin polymerization, as well as for the reverse process of polyolefin decomposition, can provide an improved understanding for how these catalyst materials operate under relevant reaction conditions. In this review, we discuss an emerging analytical toolbox of 2D and 3D chemical imaging techniques that is suitable for investigating the chemistry and reactivity of related catalyst systems. While synchrotron-based X-ray microscopy still provides unparalleled spatial resolutions in 2D and 3D, a number of laboratory-based techniques, most notably focused ion beam-scanning electron microscopy, confocal fluorescence microscopy, infrared photoinduced force microscopy and laboratory-based X-ray nano-computed tomography, have helped to significantly expand the arsenal of analytical tools available to scientists in heterogeneous catalysis and polymer science. In terms of future research, the review outlines the role and impact of in situ and operando (spectro-)microscopy experiments, involving sophisticated reactors as well as online reactant and product analysis, to obtain real-time information on the formation, decomposition, and mobility of polymer phases within single catalyst particles. Furthermore, the potential of fluorescence microscopy, X-ray microscopy and optical microscopy is highlighted for the high-throughput characterization of olefin polymerization and polyolefin decomposition catalysts. By combining these chemical imaging techniques with, for example, chemical staining methodologies, selective probe molecules as well as particle sorting approaches, representative structure-activity relationships can be derived at the level of single catalyst particles.

Fluorescent-Probe Characterization for Pore-Space Mapping with Single-Particle Tracking.

Porous solids often contain complex pore networks with pores of various sizes. Tracking individual fluorescent probes as they diffuse through porous materials can be used to characterize pore networks at tens of nanometers resolution. However, understanding the motion behavior of fluorescent probes in confinement is crucial to reliably derive pore network properties. Here, we introduce well-defined lithography-made model pores developed to study probe behavior in confinement. We investigated the influence of probe-host interactions on diffusion and trapping of confined single-emitter quantum-dot probes. Using the pH-responsiveness of the probes, we were able to largely suppress trapping at the pore walls. This enabled us to define experimental conditions for mapping of the accessible pore space of a one-dimensional pore array as well as a real-life polymerization-catalyst-support particle.

Molecular Accessibility and Diffusion of Resorufin in Zeolite Crystals.

We have used confocal laser scanning microscopy on the small, fluorescent resorufin dye molecule to visualize molecular accessibility and diffusion in the hierarchical, anisotropic pore structure of large (~10 µm-sized) zeolite-ß crystals. The resorufin dye is widely used in life and materials science, but only in its deprotonated form because the protonated molecule is barely fluorescent in aqueous solution. In this work, we show that protonated resorufin is in fact strongly fluorescent when confined within zeolite micropores, thus enabling fluorescence microimaging experiments. We find that J-aggregation guest-guest interactions lead to a decrease in the measured fluorescence intensity that can be prevented by using non-fluorescent spacer molecules. We characterized the pore space by introducing resorufin from the outside solution and following its diffusion into zeolite-ß crystals. The eventual homogeneous distribution of resorufin molecules throughout the zeolite indicates a fully accessible pore network. This enables the quantification of the diffusion coefficient in the straight pores of zeolite-ß without the need for complex analysis, and we found a value of 3×10-15  m2  s-1 . Furthermore, we saw that diffusion through the straight pores of zeolite-ß is impeded when crossing the boundaries between zeolite subunits.

Correlated x-ray fluorescence and ptychographic nano-tomography on Rembrandt's The Night Watch reveals unknown lead "layer".

The Night Watch, one of the most famous masterpieces by Rembrandt, is the subject of a large research and conservation project. For the conservation treatment, it is of great importance to understand its current condition. Correlated nano-tomography using x-ray fluorescence and ptychography revealed a-so far unknown-lead-containing "layer", which likely acts as a protective impregnation layer applied on the canvas before the quartz-clay ground was applied. This layer might explain the presence of lead soap protrusions in areas where no other lead components are present. In addition to the three-dimensional elemental mapping, ptychography visualizes and quantifies components not detectable by hard x-ray fluorescence such as the organic fraction and quartz. The first-time use of this combination of synchrotron-based techniques on a historic paint micro-sample shows it to be an important tool to better interpret the results of noninvasive imaging techniques operating on the macroscale.

Expanding lignin thermal property space by fractionation and covalent modification.

To fully exploit kraft lignin's potential in material applications, we need to achieve tight control over those key physicochemical lignin parameters that ultimately determine, and serve as proxy for, the properties of lignin-derived materials. Here, we show that fractionation combined with systematic (incremental) modification provides a powerful strategy to expand and controllably tailor lignin property space. In particular, the glass transition temperature (Tg) of a typical kraft lignin could be tuned over a remarkable and unprecedented 213 °C. Remarkably, for all fractions the Tg proved to be highly linearly correlated with the degree of derivatisation by allylation, offering such tight control over the Tg of the lignin and ultimately the ability to 'dial-in' this key property. Importantly, such control over this proxy parameter indeed translated well to lignin-based thiol-ene thermosetting films, whose Tgs thus covered a range from 2-124 °C. This proof of concept suggests this approach to be a powerful and generalisable one, allowing a biorefinery or downstream operation to consciously and reliably tailor lignins to predictable specifications which fit their desired application.

Correction to "Correlating the Morphological Evolution of Individual Catalyst Particles to the Kinetic Behavior of Metallocene-Based Ethylene Polymerization Catalysts".

[This corrects the article DOI: 10.1021/jacsau.1c00324.].

Transport limitations in polyolefin cracking at the single catalyst particle level.

Catalytic cracking is a promising approach to chemically recycle polyolefins by converting them into smaller hydrocarbons like naphtha, and important precursors of various platform chemicals, such as aromatics. Cracking catalysts, commonly used in the modern refinery and petrochemical industry, are tailored to process gaseous or liquid feedstock. Polyolefins, however, are very large macromolecules that form highly viscous melts at the temperatures required to break their backbone C-C bonds. Therefore, mass transport is expected to limit the performance of traditional cracking catalysts when applied to the conversion of polymers. In this work, we study these effects during the cracking of polypropylene (PP) over catalysts utilized in the fluid catalytic cracking (FCC) process. Thermogravimetric experiments using PP of varying molecular weight (Mw) and catalysts of varying accessibility showed that low Mw model polymers can be cracked below 275 °C, while PP of higher Mw required a 150 °C higher temperature. We propose that this difference is linked to different degrees of mass transport limitations and investigated this at length scales ranging from milli- to nanometers, utilizing in situ optical microscopy and electron microscopy to inspect cut open catalyst-polymer composites. We identified the main cause of transport limitations as the significantly higher melt viscosity of high Mw polymers, which prohibits efficient catalyst-polymer contact. Additionally, the high Mw polymer does not enter the inner pore system of the catalyst particles, severely limiting utilization of the active sites located there. Our results demonstrate that utilizing low Mw polymers can lead to a significant overestimation of catalyst activity, and suggest that polyolefins might need to undergo a viscosity reducing pre-treatment in order to be cracked efficiently.

Spatiotemporal Mapping of Local Heterogeneities during Electrochemical Carbon Dioxide Reduction.

The activity and selectivity of a copper electrocatalyst during the electrochemical CO2 reduction reaction (eCO2RR) are largely dominated by the interplay between local reaction environment, the catalyst surface, and the adsorbed intermediates. In situ characterization studies have revealed many aspects of this intimate relationship between surface reactivity and adsorbed species, but these investigations are often limited by the spatial and temporal resolution of the analytical technique of choice. Here, Raman spectroscopy with both space and time resolution was used to reveal the distribution of adsorbed species and potential reaction intermediates on a copper electrode during eCO2RR. Principal component analysis (PCA) of the in situ Raman spectra revealed that a working electrocatalyst exhibits spatial heterogeneities in adsorbed species, and that the electrode surface can be divided into CO-dominant (mainly located at dendrite structures) and C-C dominant regions (mainly located at the roughened electrode surface). Our spectral evaluation further showed that in the CO-dominant regions, linear CO was observed (as characterized by a band at ∼2090 cm-1), accompanied by the more classical Cu-CO bending and stretching vibrations located at ∼280 and ∼360 cm-1, respectively. In contrast, in the C-C directing region, these three Raman bands are suppressed, while at the same time a band at ∼495 cm-1 and a broad Cu-CO band at ∼2050 cm-1 dominate the Raman spectra. Furthermore, PCA revealed that anodization creates more C-C dominant regions, and labeling experiments confirmed that the 495 cm-1 band originates from the presence of a Cu-C intermediate. These results indicate that a copper electrode at work is very dynamic, thereby clearly displaying spatiotemporal heterogeneities, and that in situ micro-spectroscopic techniques are crucial for understanding the eCO2RR mechanism of working electrocatalyst materials.

Two Pathways for the Degradation of Orpiment Pigment (As2S3) Found in Paintings.

Paintings are complex objects containing many different chemical compounds that can react over time. The degradation of arsenic sulfide pigments causes optical changes in paintings. The main degradation product was thought to be white arsenolite (As2O3), but previous research also showed the abundant presence of As(V) species. In this study, we investigate the influence of the presence of a medium on the degradation mechanism of orpiment (As2S3) using synchrotron radiation (SR)-based tomographic transmission X-ray microscopy, SR-based micro-X-ray fluorescence, and X-ray absorption near edge structure spectroscopy. Upon direct illumination of dry orpiment powder using UV-visible light, only the formation of As2O3 was observed. When As2S3 was surrounded by a medium and illuminated, As2O3 was only observed in the area directly exposed to light, while As(V) degradation species were found elsewhere in the medium. Without accelerated artificial light aging, As(V)(aq) species are formed and migrate throughout the medium within weeks after preparation. In both scenarios, the As(V) species form via intermediate As(III)(aq) species and the presence of a medium is necessary. As(V)(aq) species can react with available cations to form insoluble metal arsenates, which induces stress within the paint layers (leading to, e.g., cracks and delamination) or can lead to a visual change of the image of the painting.

Structure sensitivity in gas sorption and conversion on metal-organic frameworks.

Many catalytic processes depend on the sorption and conversion of gaseous molecules on the surface of (porous) functional materials. These events often preferentially occur on specific, undercoordinated, external surface sites. Here we show the combination of in situ Photo-induced Force Microscopy (PiFM) with Density Functional Theory (DFT) calculations to study the site-specific sorption and conversion of formaldehyde on the external surfaces of well-defined faceted ZIF-8 microcrystals with nanoscale resolution. We observed preferential adsorption of formaldehyde on high index planes. Moreover, in situ PiFM allowed us to visualize unsaturated nanodomains within extended external crystal planes, showing enhanced sorption behavior on the nanoscale. Additionally, on defective ZIF-8 crystals, structure sensitive conversion of formaldehyde through a methoxy- and a formate mechanism mediated by Lewis acidity was found. Strikingly, sorption and conversion were influenced more by the external surface termination than by the concentration of defects. DFT calculations showed that this is due to the presence of specific atomic arrangements on high-index crystal surfaces. With this research, we showcase the high potential of in situ PiFM for structure sensitivity studies on porous functional materials.

Spectro-Microscopic Techniques for Studying Nanoplastics in the Environment and in Organisms.

Nanoplastics (NPs), small (<1 µm) polymer particles formed from bulk plastics, are a potential threat to human health and the environment. Orders of magnitude smaller than microplastics (MPs), they might behave differently due to their larger surface area and small size, which allows them to diffuse through organic barriers. However, detecting NPs in the environment and organic matrices has proven to be difficult, as their chemical nature is similar to these matrices. Furthermore, as their size is smaller than the (spatial) detection limit of common analytical tools, they are hard to find and quantify. We highlight different micro-spectroscopic techniques utilized for NP detection and argue that an analysis procedure should involve both particle imaging and correlative or direct chemical characterization of the same particles or samples. Finally, we highlight methods that can do both simultaneously, but with the downside that large particle numbers and statistics cannot be obtained.

Advancing the Compositional Analysis of Olefin Polymerization Catalysts with High-Throughput Fluorescence Microscopy.

To optimize the performance of supported olefin polymerization catalysts, novel methodologies are required to evaluate the composition, structure, and morphology of both pristine and prepolymerized samples in a resource-efficient, high-throughput manner. Here, we report on a unique combination of laboratory-based confocal fluorescence microscopy and advanced image processing that allowed us to quantitatively assess support fragmentation in a large number of autofluorescent metallocene-based catalyst particles. Using this approach, significant inter- and intraparticle heterogeneities were detected and quantified in a representative number of prepolymerized catalyst particles (2D: ≥135, 3D: 40). The heterogeneity that was observed over several stages of slurry-phase ethylene polymerization (10 bar) is primarily attributed to the catalyst particles' diverse support structures and to the inhomogeneities in the metallocene distribution. From a mechanistic point of view, the 2D and 3D analyses revealed extensive contributions from a layer-by-layer fragmentation mechanism in synergy with a less pronounced sectioning mechanism. A significant number of catalyst particles were also found to display limited support fragmentation at the onset of the reaction (i.e., at lower polymer yields). This delay in activity or "dormancy" is believed to contribute to a broadening of the particle size distribution during the early stages of polymerization. 2D and 3D catalyst screening via confocal fluorescence microscopy represents an accessible and fast approach to characterize the structure of heterogeneous catalysts and assess the distribution of their fluorescent components and reaction products. The automation of both image segmentation and postprocessing with machine learning can yield a powerful diagnostic tool for future research as well as quality control on industrial catalysts.

Operando Laboratory-Based Multi-Edge X-Ray Absorption Near-Edge Spectroscopy of Solid Catalysts.

Laboratory-based X-ray absorption spectroscopy (XAS) and especially X-ray absorption near-edge structure (XANES) offers new opportunities in catalyst characterization and presents not only an alternative, but also a complementary approach to precious beamtime at synchrotron facilities. We successfully designed a laboratory-based setup for performing operando, quasi-simultaneous XANES analysis at multiple K-edges, more specifically, operando XANES of mono-, bi-, and trimetallic CO2 hydrogenation catalysts containing Ni, Fe, and Cu. Detailed operando XANES studies of the multielement solid catalysts revealed metal-dependent differences in the reducibility and re-oxidation behavior and their influence on the catalytic performance in CO2 hydrogenation. The applicability of operando laboratory-based XANES at multiple K-edges paves the way for advanced multielement catalyst characterization complementing detailed studies at synchrotron facilities.