Catalyst Substrate Research Articles

Exploring the Mpemba effect: a universal ice pressing enables porous ceramics.

Piezoceramics with global porosity and local compaction are highly desired to exploit the combination of mechanical and electrical properties. However, achieving such a functional combination is challenging because of the lack of techniques for applying uniform pressure inside porous ceramic green parts. Nature provides many examples of generating strong forces inside the macro and micro channels via the state transformation of water. Inspired by these phenomena, we present a technique of "ice and fire", that is, water freezing (ice pressing) and high-temperature sintering (fire), to produce ideal porous piezoceramics. We introduce a new compaction method called the "ice pressing method", which manipulates liquid phase transition for compaction. This method has several advantages, including uniform pressure distribution, a wide pressure range, high effectiveness, and selective freezing. It can generate an ultrahigh pressure of up to 180 MPa on the piezoceramic green skeletons in minutes while retaining their functional pore structures. By exploiting the Mpemba phenomenon, we further accelerate the compaction procedure by 11%. The first ice-pressed and second fire-consolidated lead zirconate titanate (PZT) ceramics are highly densified and exhibit an outstanding piezoelectric response (d33 = 531 pC N-1), comparable to conventional pressed bulk counterparts and 10-20 times higher than those of unpressed materials. The novel ice pressing method breaks the limitation of lacking a compaction technique for porous ceramics. The versatile and effective ice pressing method is a green and low-cost route promoting applications in sensors, acoustics, water filtration, catalyst substrates, and energy harvesting.

Copper/ruthenium relay catalysis enables 1,6-double chiral inductions with stereodivergence.

The dual catalysis strategy is an efficient and powerful tool to fulfill the stereodivergent synthesis of stereoisomeric products from the same set of starting materials. Great attention has been given to the construction of chiral compounds with two contiguous stereocenters. However, the synthesis of two remote noncontiguous stereocenters is more challenging and is less developed, despite the high demand for synthetic tactics. We herein developed an unprecedented example of the stereodivergent preparation of synthetically useful and biologically important chiral ζ-hydroxy amino ester derivatives containing remote 1,6-noncontiguous stereocenters and a unique β,γ-unsaturation moiety. This cascade dehydrogenation/1,6-Michael addition/hydrogenation protocol between readily-available ketoimine esters and racemic branched dienyl carbinols was rationally realized with bimetallic copper/ruthenium relay catalysis. The key features of the process were atom economy, step economy, and redox-neutrality. All four stereoisomers of chiral ζ-hydroxy amino ester derivatives were easily achieved by the orthogonal permutations of a chiral copper catalyst and chiral ruthenium catalyst. Importantly, a much more challenging stereodivergent synthesis of all eight stereoisomers of chiral peptide products containing three remote stereocenters was accomplished with excellent results through the cooperation of two chiral catalyst pairs and substrate enantiomers.

Water co-catalysis in aerobic olefin epoxidation mediated by ruthenium oxo complexes.

We report the development of a versatile Ru-porphyrin catalyst system which performs the aerobic epoxidation of aromatic and aliphatic (internal) alkenes under mild conditions, with product yields of up to 95% and turnover numbers (TON) up to 300. Water is shown to play a crucial role in the reaction, significantly increasing catalyst efficiency and substrate scope. Detailed mechanistic investigations employing both computational studies and a range of experimental techniques revealed that water activates the RuVI di-oxo complex for alkene epoxidation via hydrogen bonding, stabilises the RuIV mono-oxo intermediate, and is involved in the regeneration of the RuVI di-oxo complex leading to oxygen atom exchange. Distinct kinetics are obtained in the presence of water, and side reactions involved in catalyst deactivation have been identified.

Development of a flow photochemical process for a π-Lewis acidic metal-catalyzed cyclization/radical addition sequence: in situ-generated 2-benzopyrylium as photoredox catalyst and reactive intermediate.

A flow photochemical reaction system for a π-Lewis acidic metal-catalyzed cyclization/radical addition sequence was developed, which utilizes in situ-generated 2-benzopyrylium intermediates as the photoredox catalyst and electrophilic substrates. The key 2-benzopyrylium intermediates were generated in the flow reaction system through the intramolecular cyclization of ortho-carbonyl alkynylbenzene derivatives by the π-Lewis acidic metal catalyst AgNTf2 and the subsequent proto-demetalation with trifluoroacetic acid. The 2-benzopyrylium intermediates underwent further photoreactions with benzyltrimethylsilane derivatives as the donor molecule in the flow photoreactor to provide 1H-isochromene derivatives in higher yields in most cases than the batch reaction system.

Reactor design via scan line patterning: An implicit approach to create scalable microstructured parts in selective laser melting

The adaptation of additive manufacturing for chemical flow reactors has recently gained momentum as the methods become more advanced and equipment is increasingly affordable. With the design of periodic open cellular structures (POCS) from metal selective laser melting (SLM), structured catalyst substrates can be realized in a much more ordered fashion than well-established metal foams with a random structure. Hence, tailored transport properties are achieved and flow profiles are homogenized, eliminating flow channeling and stagnant flow zones.However, with current SLM methods, the minimal achievable feature size and therewith the specific surface area is still limited. Moreover, the size of finely resolved STL files that define the structure grows exponentially with decreasing feature sizes, further limiting the scalability of methods with direct structure definition.In this work, we present a method for implicit structure design by metal SLM with specific surface areas and porosities that compete with established catalyst substrates. The definition of the microstructure is realized by control of the laser scan lines, where single scan lines create fine struts. The general applicability of our method to various metal materials is demonstrated by production from stainless steel and copper. In a numerical study for a model steam reforming application, we show that the POCS provide mass transfer coefficients and volumetric reaction rates that compete with benchmark substrates.Scan line patterning therefore provides a promising method to create highly ordered, well-scalable substrates for heterogeneous catalysis from a wide range of materials, e.g., for the application in highly efficient small-scale reactors.

An assessment of the application of catalyst preheating prior to engine cold start to improve the filtration performance of CDPF on particulate emissions

The catalytic diesel particulate filter (CDPF) is an effective technology for reducing particle emissions. However, during engine startup, especially in cold start conditions, notable particle emissions are observed at the CDPF outlet. These emissions are attributed to combustion deterioration during startup and the low temperature of the aftertreatment catalyst. Addressing this challenge, this study investigated the effectiveness of rapidly preheating the diesel oxidation catalyst (DOC) and CDPF substrate temperatures before engine start to enhance particle removal during cold starts. Experimental results indicated that CDPF filtration efficiency during cold start was lower compared to hot start conditions, with a clean CDPF exhibiting very low efficiency due to minimal particle deposition. Substrate temperature preheating of either the DOC or CDPF was found to improve CDPF filtration efficiency for particles during cold start, albeit with a concurrent increase in emissions of particles smaller than 50 nm. Notably, a combination of a 250 °C DOC with a 22 °C CDPF yielded the most significant reduction in particle emissions, particularly for particles under 50 nm. Given that the DOC shares the same material composition as the CDPF but has a smaller volume that requires less energy for heating, preheating the DOC emerges as a more energy-efficient strategy. These findings underscore the need for more comprehensive studies on catalyst preheating to further the commercialization of technologies that effectively reduce cold start particle emissions, aligning with increasingly stringent future emission regulations.

Dehydrogenative Conversions of Aldehydes and Amines to Amides Catalyzed by a Nickel(II) Pincer Complex

A C-N cross-coupling approach involving oxidative amidations of aromatic aldehydes in the presence of an amide-based nickel(II) pincer catalyst (2) is demonstrated. Upon optimization, quick reaction times (15 min) and an ideal temperature (25 °C) were established and implemented for the conversion of 33 different amide products using only 0.2 mol% of catalyst. Moderate to good turnover numbers (TONs) were obtained for secondary benzamide products, and moderate TONs were obtained for tertiary benzamide products, with the highest turnover number calculated for the 4-chloro-N-(3-phenylpropyl)benzamide product (4i, 309). Gas chromatographic–mass spectrometric (GC–MS) analysis also indicates the formation of alcohols in different reactions, indicating an oxidative amidation process. Kinetic studies were performed by varying the amount of catalyst, aldehyde, LiHMDS base, and amine substrate to determine the order of reaction for each component. Benzaldehyde and benzaldehyde-d6 were reacted with benzylamine, and the kH/kD ratio was determined to understand the rate-determining step. Isotope labeling further revealed that deuterium was being transferred to both the alcohol side product and the target amide product. With the help of kinetic data and UV–visible spectra, a mechanism for the amidation process via the catalyst (2) is proposed through a Ni(I)–Ni(III) pathway.

Investigation of optimized spraying process for directly coated electrode in polymer electrolyte membrane fuel cell

Membrane electrode assembly (MEA) is a core component in polymer electrolyte membrane fuel cells (PEMFCs). A well-constructed catalyst layer is essential for achieving high-performance and reliable fuel cells. Among MEA fabrication processes, direct spray coating has advantages regarding cost-efficiency, processability, and controllability compared to direct sputtering and decal transfer method. In the spray process, the morphology of the catalyst layer depends on conditions in which ink droplets are sprayed and evaporated. Although quantitatively optimizing the spray process is crucial, there is still a lack of well-defined research for optimized spray process. Herein, we propose k-value that can indicate uniform coverage, expressed as a ratio of the area where ink droplets spread compared to the sprayed area. The k-1.0 MEA is fabricated under k-value conditions of one by adjusting nozzle moving speed, which is expected to have the most uniform coverage. Compared to k-0.3 MEA and k-3.0 MEA, the power density improves by 22.3 % and 11.1 %, respectively. This is attributed to the reduced ion transport resistance of the catalyst layer being reduced by 23.7 % and 8.5 % and the increased electrochemical active surface area by 16.1 % and 9.4 %, respectively. Furthermore, the morphology of catalyst layers and substrate temperature effect are investigated.

An Efficient and Recyclable Acid Catalyst (PolyE‐IL) for Production of Pectin from Citrus Peel Waste

AbstractCitrus fruit waste is a potential source for valorisation into pectin. In present manuscript a PolyE‐IL‐1 as an efficient acid catalyst has been integrated for fractionation of pectin from citrus fruit waste. The use of PolyE‐IL‐1 catalyst has specific advantages in term of high yield, ease of catalyst separation, recycle and reuse etc. The process optimization study was conducted to check the influence of catalyst concentration, temperature, time and substrate concentration etc., to obtain maximum yield of pectin. The intensified process yields 30 % pectin at 95 °C in 90 min in batch mode of operation. Moreover, batch reactor, ultrasound batch reactor and microwave batch reactor were compared to observe the influence of the mode of reactor operation. Various citrus waste feedstock (Orange, Sweet lime and Lemon) was tested and maximum yield of (32.52 %) pectin was obtained from Lemon peels. The catalyst recyclability study showed consistency in pectin yield (30 %) for five consecutive recycle runs. Thus, the use of an easily recyclable PolyE‐IL‐1 catalyst provides an efficient route to produce pectin from citrus fruit peel waste.

Substrate‐Driven Catalyst Reducibility for Oxygen Evolution and Its Effect on the Operation of Proton Exchange Membrane Water Electrolyzers

To increase the efficiency of hydrogen production by proton exchange membrane water electrolyzers (PEMWEs), the relationships between the specific activity and stability of the membrane–electrode assembly (MEA) must be clarified. Ir oxide electrodeposited on Ti substrate is used as an oxygen electrode, and its electronic properties and electrochemical behavior in PEMWE operation are observed. The electrode, fabricated through a facile strategy based on annealing in the air atmosphere, enhances the specific oxygen evolution reaction (OER) activity and stability in PEMWE operation. Furthermore, the electronic catalyst–substrate interactions associated with different reducibilities in the heteroatom system are studied. The morphology, electronic properties, and chemical state of the oxygen electrode are investigated through X‐ray spectroscopy, microscopy techniques, and computational analyses. Thermal treatment of the catalyst‐coated substrate decreases the bulk oxidation state of Ir and increases the surface oxidation state. According to the electrochemical and physical behavior analyses of PEMWEs, the oxygen content in the Ir oxide structure, which defines the OER activity and its stability, is influenced by the crystalline structure and formation of a stable interface between the catalyst and substrate. The outcomes can facilitate the development of strategies for enhancing the performance of PEMWEs and designing rational MEAs.

Modulation of chalcogen‐functionalized MXenes Ti2CT2 (T = O, S, Se, and Te) as catalysts for hydrogen evolution reaction

AbstractExploration and exploitation of efficient and nonprecious catalysts for hydrogen evolution reaction (HER) is one of the key solutions to energy problems. Two‐dimensional (2D) MXenes have attracted extensive attention as potential catalysts and supports due to their unique structure and customizable properties. In this work, we have systematically investigated the structural stability and electronic properties of chalcogen‐functionalized MXenes Ti2CT2 (T = O, S, Se, and Te) and Pt‐anchored Ti2CT2 MXenes (Pt‐Ti2CT2). Density functional theory calculations have discovered that all Ti2CT2 exhibit metallic electronic structures except Ti2CO2 with an indirect bandgap. Additionally, the electronic densities of states at Fermi level for Pt‐Ti2CT2 are greatly enhanced after anchoring Pt atoms. The reduction of chalcogen electronegativity is recognized to mainly contribute to weaker bond strength but greater electronic activity. Moreover, Pt‐Ti2CTe2 is predicted to exhibit near‐zero Gibbs free energy of hydrogen adsorption especially at high hydrogen coverage, indicating its significant potential as HER catalysts. It is also revealed that Ti2CT2 and Pt‐Ti2CT2 MXenes are not only ideal candidates of catalyst materials but also could serve as good catalyst substrates. This work provides insights into the structural and chemical complexity of MXenes for catalyst applications but also demonstrates a new strategy to modulate high‐efficiency and low‐cost catalyst and support materials.

An efficient and practical approach for the continuous‐flow synthesis of 5‐hydroxymethylfurfural from carbohydrates using an acidic ionic liquid

AbstractBACKGROUNDFlow chemistry is becoming an essential technique in organic synthesis. This trend is expected to continue and broaden in scope and complexity. This miniaturized protocol that includes microreactors enables excellent heat transfer, low solvent wastage, shorter reaction times, higher productivity and higher space–time yields.RESULTSIn this study, we synthesized four ionic liquids and applied them as catalysts for the selective preparation of 5‐hydroxymethylfurfural (5‐HMF) from fructose using the continuous flow system. The effects of reaction time and temperature, catalyst loading, solvent and substrates for carbohydrate conversion to 5‐HMF have been carefully investigated, and 5‐HMF yields were determined by high‐performance liquid chromatography.CONCLUSIONAs a result, 1‐(4‐sulfobutyl)pyridinium iron(III) chloride gave 71% yield of 5‐HMF for 30 min. The results of this research demonstrate that this approach is not only straightforward in design but also rapid and convenient in operation, and it has the potential to be carried out on a massive scale. © 2023 Society of Chemical Industry.

Manufacturing and Thermal Shock Resistance of 3D-Printed Porous Black Zirconia for Concentrated Solar Applications

A novel approach for manufacturing porous materials, foreseen as solar receivers for concentrated sun radiation, used in the power tower technology is presented. In such applications, materials are subjected to steep thermal gradients and thousands of cycles. Yet, materials consisting of honeycombs and ceramic foams showed insufficient thermal performance. By using the fused filament fabrication process, one can design printed parts meeting the requirements for solar receivers, namely dark color and high solar absorptance. This exploratory study unveils data on the retained crushing strength of newly developed 3D-printed porous Black Zirconia cubes after thermal cycling under similar conditions to those experienced by volumetric receivers and catalyst substrates for solar fuels (H2 and/or CO) production via the thermochemical cycle. Unlike dense ceramics, the resistance to thermal shock of 3D-printed cubes underwent a gradual decrease with the increase in the thermal gradient. The thermal shock cycles were performed between 800 °C and 1100, 1200, and 1300 °C, corresponding to a ΔT of 300, 400, and 500 K, respectively. Additionally, water quenching tests were performed at ΔT = 300 K up to 400 K. Crushing strength measurements carried out to evaluate the retained mechanical strength after exposure up to 100 cycles showed that the Black Zirconia cubes can withstand thermal gradients up to at least 400 K.

Catalytic Asymmetric Hydrogen Atom Transfer: Enantioselective Hydroamination of Alkenes.

We report a highly enantioselective radical-based hydroamination of enol esters with sulfonamides jointly catalyzed by an Ir photocatalyst, Brønsted base, and tetrapeptide thiol. This method is demonstrated for the formation of 23 protected β-amino-alcohol products, achieving selectivities up to 97:3 er. The stereochemistry of the product is set through selective hydrogen atom transfer from the chiral thiol catalyst to a prochiral C-centered radical. Structure-selectivity relationships derived from structural variation of both the peptide catalyst and olefin substrate provide key insights into the development of an optimal catalyst. Experimental and computational mechanistic studies indicate that hydrogen-bonding, π-π stacking, and London dispersion interactions are contributing factors for substrate recognition and enantioinduction. These findings further the development of radical-based asymmetric catalysis and contribute to the understanding of the noncovalent interactions relevant to such transformations.

Sulfur-functionalized porphyrin-based covalent organic framework as a metal-free dual-functional catalyst for photodegradation of organophosphorus pesticides under visible-LED-light

Parathion and diazinon are two significant organophosphorus pesticides broadly used in agriculture. However, these compounds are toxic and can enter into the environment and atmosphere via various processes. Herein, we synthesized and post-functionalized a porphyrinic covalent organic framework (COF), COF-366, with elemental sulfur under solvent-free conditions to give polysulfide-functionalized COF-366, namely PS@COF. The resulting material consisting of porphyrin sensitizer and sulfur nucleophilic sites was used as a dual-functional heterogeneous catalyst for the degradation of these organic compounds using visible-LED-light. Accordingly, the effects of several pertinent parameters such as pH (3–9), the catalyst dosage (5–30 mg), time (up to 80 min), and substrate concentration (10–50 mg L−1) were studied in detail and optimized. The post-modified COF showed excellent photocatalytic activity (>97%) in the detoxification of diazinon and parathion for 60 min at pH 5.5. Kinetic studies indicated a fast degradation rate with pseudo-second order model for 20 mg L−1 of diazinon and parathion. The total organic carbon detection and gas chromatography-mass spectrometry (GC-MS) confirmed the organic intermediates and byproducts formed during the process. PS@COF displayed good recyclability and high reusable efficiency for six cycles without a noteworthy lose in its catalytic activity, owing to its robust structure.

Unexpected Kinetic Solvent Effects Enhance Activity and Selectivity in Biphasic Systems

Biphasic dehydration of fructose to 5-hydroxymethylfurfural (HMF) has shown unprecedented increases in productivity, but a mechanistic understanding is lacking. Herein, we couple fast experimental reaction kinetics, multiscale modeling (phase behavior, classical molecular dynamics(MD), and quantum mechanics/molecular mechanics MD), in situ sampling, and IR and 13C-NMR spectroscopy to elucidate the complex effects of nonpolar extracting organic solvents on the kinetics of fructose dehydration. We show that these organic solvents can reach significant mutual solubility with water at reaction temperatures, enabling the partition of the sugar and catalyst into the extracting phase. In the organic-rich environment, the dehydration of fructose proceeds faster and more selectively than in water due to increased relative abundance of the reactive furanose isomer, enhanced water–catalyst–substrate interactions driven by nanophase separation, and higher product stability stemming from preferential solvation. We demonstrate that these solvent effects impact other critical biphasic reactions in biomass upgrading and provide qualitative principles for solvent selection.

CoFePBA Nanosheets on Carbon Nanotubes Coupled with Nickel-Encapsulated Carbon Tubules for Efficient and Highly Stable Overall Seawater Electrolysis

The generation of H2 by seawater electrolysis is limited by anodic chlorine corrosion and sluggish cathodic hydrogen evolution reaction (HER). Here, we report a simple approach for developing cobalt iron (CoFe)-based Prussian blue analogue with a nitrogen-doped carbon nanotube composite (CoFePBA/NCNT) and nickel-encapsulated carbon tubules (Ni/NCT) as oxygen evolution reaction (OER) and HER catalysts for overall seawater electrolysis utilizing graphite felt or carbon paper as the catalyst substrate, thus avoiding chlorine corrosion of the metal substrate. Instead of a commonly known hydrothermal method, a simple brush coating method was employed for catalyst coating which is feasible for large-scale electrolyzer fabrication. Our developed electrolyzer has shown a low overall seawater splitting voltage of 1.71 V and a good stability of 50 h in seawater with excellent selectivity for OER over hypochlorite formation at room temperature (26 °C). High OER activity is attributed to abundant catalytic sites provided by interconnected CoFePBA nanosheets and high intrinsic catalytic activity. Moreover, suppressed hypochlorite formation and corrosion resistance resulting from the synergistic effect of CoFePBA and NCNT make CoFePBA/NCNT||Ni/NCT a potential candidate for seawater electrolyzers. Our work paves a way to demonstrate seawater electrolyzer performance with nonprecious catalysts on a nonmetallic support.

Metal Identity Effects in the Pincer Complex-Catalyzed Dehydrogenative Coupling of Formamides with Alcohols to Form Carbamates

This manuscript describes the impact of metal (ruthenium versus iron versus manganese) on the PNP pincer complex-catalyzed dehydrogenative coupling of formamides with alcohols to yield carbamates. Studies are conducted to compare the structure and reactivity of key organometallic intermediates as a function of metal. These reveal that all three pincer catalysts undergo fast reaction with isocyanates (the initial organic products of formamide dehydrogenation) to form off-cycle metallacyclic species. However, the structures and reactivities of these intermediates vary as a function of metal. The N-bound isomer of the metallacycle is formed with Fe (literature) and Ru (this work), while both O-bound (kinetic) and N-bound (thermodynamic) metallacycles are observed with Mn (this work). At 100 °C, the O-bound Mn cycloadduct reacts rapidly with cyclohexanol to release the pincer catalyst along with the corresponding carbamate. In contrast, all three of the N-bound metallacycles show dramatically lower reactivity toward catalyst regeneration under analogous conditions. The reactivity of the pincer complexes toward formamide dehydrogenation also varies as a function of metal, with Ru and Fe ≫ Mn at 100 °C. Overall, the Mn pincer complex is an effective catalyst for the conversion of a variety of formamides and alcohols to carbamates at 150 °C with catalyst loadings as low as 1 mol %. The catalyst performance and substrate scope are comparable to those of the Fe analogue.

Organophotocatalytic Mechanisms: Simplicity or Naïvety? Diverting Reactive Pathways by Modifications of Catalyst Structure, Redox States and Substrate Preassemblies

AbstractPhotocatalysis is a powerful tool to assemble diverse chemical scaffolds, yet a bottleneck on its further development is the understanding of the multitude of possible pathways when practitioners rely only on oversimplified thermodynamic and optical factors. Recently, there is a growing number of studies in the field that exploit, inter alia, kinetic parameters and organophotocatalysts that are synthetically more programmable in terms of their redox states and opportunities for aggregation with a target substrate. Non‐covalent interactions play a key role that enables access to a new generation of reactivities such as those of open‐shell organophotocatalysts. In this review, we discuss how targeted structural and redox modifications influence the organophotocatalytic mechanisms together with their underlying principles. We also highlight the benefits of strategies such as preassembly and static quenching that overcome common reactivity issues (e. g., diffusion rate limits and energetic limits).

Efficient Transformation of Polymer Main Chain Catalyzed by Macrocycle Metal Complexes via Pseudorotaxane Intermediate

AbstractPost‐synthesis modification of polymers streamlines the synthesis of functionalized polymers, but is often incomplete due to the negative polymer effects. Developing efficient polymer reactions in artificial systems thus represents a long‐standing objective in the fields of polymer and material science. Here, we show unprecedented macrocycle‐metal‐complex‐catalyzed systems for efficient polymer reaction that result in 100 % transformation of the main chain functional groups presumably via a processive mode reaction. The complete polymer reactions were confirmed in not only intramolecular reaction (hydroamination) but also intermolecular reaction (hydrosilylation) by using Pd‐ and Pt‐macrocycle‐catalyzed systems. The most fascinating feature of the both reactions is that higher‐molecular‐weight polymers reach completion faster. Various studies suggested that the reactions occur in the catalyst cavity via the formation of a supramolecular complex between the macrocycle catalyst and polymer substrate like pseudorotaxane, which should be of characteristic of the efficient polymer reactions progressing in a processive mode.