Catalyst Retention Research Articles

Floating Photothermal Hydrogen Production.

Solar-to-hydrogen (STH) is emerging as a promising approach for energy storage and conversion to contribute to carbon neutrality. The lack of efficient catalysts and sustainable reaction systems is stimulating the fast development of photothermal hydrogen production based on floating carriers to achieve unprecedented STH efficiency. This technology involves three major components: floating carriers with hierarchically porous structures, photothermal materials for solar-to-heat conversion and photocatalysts for hydrogen production. Under solar irradiation, the floating photothermal system realizes steam generation which quickly diffuses to the active site for sustainable hydrogen generation with the assistance of a hierarchically porous structure. Additionally, this technology is endowed with advantages in the high utilization of solar energy and catalyst retention, making it suitable for various scenarios, including domestic water supply, wastewater treatment, and desalination. A comprehensive overview of the photothermal hydrogen production system is present due to the economic feasibility for industrial application. The in-depth mechanism of a floating photothermal system, including the solar-to-heat effect, steam diffusion, and triple-phase interaction are highlighted by elucidating the logical relationship among buoyant carriers, photothermal materials, and catalysts for hydrogen production. Finally, the challenges and new opportunities facing current photothermal catalytic hydrogen production systems are analyzed.

Biopolymer-supramolecular polymer hybrids for photocatalytic hydrogen production.

Solar generation of H2 is a promising strategy for dense energy storage. Supramolecular polymers composed of chromophore amphiphile monomers containing perylene monoimide (PMI) have been reported as crystalline light-harvesting assemblies for aqueous H2-evolving catalysts. Gelation of these supramolecular polymers with multivalent ions creates hydrogels with high diffusivity but insufficient mechanical stability and catalyst retention for reusability. We report here on using sodium alginate (SA) biopolymer to both induce supramolecular polymerization of PMI and co-immobilize them with catalysts in a robust hydrogel with high diffusivity that can also be 3D-printed. Faster mass transfer was achieved by controlling the material macrostructure by reducing gel diameter and microstructure by reducing biopolymer loading. Optimized gels produce H2 at rates rivaling solution-based PMI and generate H2 for up to 6 days. The PMI assemblies in the SA matrix create a percolation network capable of bulk-electron transfer under illumination. These PMI-SA materials were then 3D-printed on conductive substrates to create 3D hydrogel photoelectrodes with optimized porosity. The design of these versatile hybrid materials was bioinspired by the soft matter environment of natural photosynthetic systems and opens the opportunity to carry out light-to-fuel conversion within soft matter with arbitrary shapes and particular local environments.

The rational design of high-performance graphene-based single-atom electrocatalysts for the ORR using machine learning.

In this work, high-performance two-dimensional (2D) graphene-based single-atom electrocatalysts (ZZ/ZA-MNxCy) for the oxygen reduction reaction (ORR) were screened out using machine learning (ML). A model was built for the fast prediction of electrocatalysts and two descriptors valence electron correction (VEc) and degree of construction differences (DC) were proposed to improve the accuracy of the model prediction. Two evaluation criteria, high-performance catalyst retention rate rR and high-performance catalyst occupancy rate rO, were proposed to evaluate the accuracy of ML models in high-performance catalyst screening. The addition of VEc and DC in the model could change the mean absolute error (MAEtest) of the test set, the coefficient of determination (R2test) of the test set, rO, and rR from 0.334 V, 0.683, 0.222, and 0.360 to 0.271 V, 0.774, 0.421, and 0.671, respectively. The partially screened potential high-performance ORR electrocatalysts such as ZZ-CoN4 and ZZ-CoN3C1 were also further investigated using a Density Functional Theory (DFT) method, which confirmed the accuracy of the ML model (MAE = 0.157 V, R2 = 0.821).

From Inert to Catalytically Active Milling Media: Galvanostatic Coating for Direct Mechanocatalysis.

The inert milling balls, commonly utilized in mechanochemical reactions, were coated with a layer of Pd and utilized as catalyst in the direct mechanocatalytic Suzuki reaction. With high yields (>80 %), the milling balls can be recycled multiple times in the absence of any solvents, ligands, catalyst-molecules and -powders, while utilizing as little as 0.8 mg of Pd per coated milling ball. The coating sequence, the support material, and the layer thickness were examined towards archiving high catalyst retention, low abrasion and high conversion. The approach was transferred to the coating of milling vessels revealing the interplay between catalytically available surface area and the mechanical energy impact in direct mechanocatalysis.

Recycling and Separation of Homogeneous Catalyst from Aqueous Multicomponent Mixture by Organic Solvent Nanofiltration.

Organic solvent nanofiltration (OSN) has evolved to an established recycling method for homogeneous catalysts. However, commercial availability has not circumvented the need for classification and the scoping of possible applications for specific solvent mixtures. Therefore, Evonik’s DuraMem® 300 was assessed for the recycling of magnesium triflate at two transmembrane pressures from a mixture of ethanol, ethyl acetate and water. Catalyst retention up to 98% and permeability of up to 4.44·10−1∙L∙bar−1∙m−2∙h−1 were possible when less than 25% ethyl acetate was in the mixture. The retention of some of the components in the ternary mixture was observed while others were enriched, making the membrane also suitable for fractioning thereof.

Characterization of Selected Polymeric Membranes Used in the Separation and Recovery of Palladium-Based Catalyst Systems.

Membrane separation processes tender a capable option for energy-demanding separation processes. Nanofiltration (NF) and reverse osmosis (RO) membranes are among the most explored, with a latent use in the chemical industry. In this study, four commercial membranes (NF90, NF270, BW30, and XLE) were investigated for their applicability based on the key structural performance characteristics in the recycling of Pd-based catalysts from Heck coupling post-reaction mixture. Pure water and organic solvent permeabilities, uncharged solute permeability, swelling, and catalyst rejection studies of the membranes were conducted as well as the morphological characterization using Fourier transform infrared, field emission gun scanning electron microscopy, and atomic force microscopy. Characterization results showed trends consistent with the manufactures’ specifications. Pure water and organic solvent fluxes generally followed the trend NF270 > NF90 > BW30 > XLE, with the solvent choice playing a major role in the separation process. Pd(PPh3)2Cl2 was well rejected by almost all membranes in 2-propanol; however, XLE rejects Pd(OAc)2 better at high pressure in acetonitrile. Our study, therefore, revealed that the separation and reuse of the two catalysts by NF90 at 10 bar resulted in 97% and 49% product yields with 52% and 10% catalyst retention for Pd(OAc)2 while Pd(PPh3)2Cl2. gave 87% and 6% yields with 58% and 36% catalyst retention in the first and second cycles, respectively. Considering, the influence of membrane–solute interactions in Pd-catalyst rejection, a careful selection of the polymeric membrane and solvent, a satisfactory separation, and recovery can be achieved.

Catalyst-Free Partially Bio-Based Polyester Vitrimers

Most of the vitrimers based on ester linkages reported so far contain a Lewis acid or a strong organic base as the transesterification catalyst. The recyclability and reusability of these vitrimers are dependent on the catalyst retention, stability, and sintering issues. Herewith, a set of β-activated ester-based vitrimers are reported that can be thermally reprocessed at ∼150 °C under catalyst-free conditions. The relaxation temperature decreases to 110 °C in the presence of Sn(Oct)2. Importantly, the precursor of these vitrimers, malonic ester, is a cost-effective naturally occurring ester and can be extracted from various fruit juices. As a proof of concept, poly(hydroxyethyl methacrylate) is used as the hydroxyl precursor for the synthesis of vitrimers. These vitrimers display an adequate tensile strength (11.3–33.0 MPa), elongation (80–290%), and resilience. The materials can be effectively self-healed and reprocessed in the presence of heat without sacrificing the tensile properties. The vitrimers b...

Feasibility of a heterogeneous Fenton membrane reactor containing a Fe-ZSM5 catalyst for pharmaceuticals degradation: Membrane fouling control and long-term stability

Fenton oxidation is one of the promising advanced oxidation processes for the efficient elimination of pharmaceuticals from wastewater. The strong oxidation ability of this process is attributed to the generation of highly reactive hydroxyl radicals (OH) in the solution. In the present study, heterogeneous micro-sized zeolite catalysts that contain iron have been used in the Fenton-like process. This process enables operation in a wide pH range and facilitates the reuse of catalysts. Indeed, the coupling of the heterogeneous Fenton reaction with membrane filtration will ensure catalyst retention in the effluent compartment during the continuous water treatment. This study then investigated the fouling control strategies and membrane long-term stability in the heterogeneous Fenton reactor. During the filtration of the zeolite catalyst suspension, the critical flux for irreversible fouling was determined. One of the strategies to control membrane fouling can then be to choose an operating flux below this critical flux. In the case where a flux value above the critical flux is chosen, the results demonstrated total efficiency of hydrodynamic backwashing to eliminate hydraulically reversible fouling. Concerning the question of polymeric membrane long-term stability, it has been demonstrated that due to contact with the Fenton medium, membrane material undergoes oxidation and polymeric chain scissions. This latter is strongly linked to the decline in the mechanical resistance of membranes. In the tested conditions, despite the degradation to membrane material, the critical flux for irreversible fouling remained unchanged on aged membranes.

Polyhedral Oligomeric Silsesquioxane Modification of Metathesis Catalysts: Improved Recycling and Lifetime in Membrane Separation

We herein report the synthesis, application and recycling of stable, polyhedral oligomeric silsesquioxane (POSS)-modified ruthenium complexes. The efficiency of all catalysts was evaluated in the ring closing metathesis of a test system relative to the commercially available Grubbs–Hoveyda second generation catalyst. Recycling experiments were carried out with a ceramic membrane in a cat-in-a-cup setup, where the necessity of two POSS units, one on the N-heterocyclic carbene, as well as one on the benzylidene moiety, was shown. ICP analyses were carried out to prove the low levels of ruthenium contamination in the product solutions and thus the high retention of catalyst by the membrane.

Catalyst retention utilizing a novel fluorinated phosphonium ionic liquid in Heck reactions under fluorous biphasic conditions

A novel ionic liquid based on a highly fluorinated phosphonium cation was synthesized and its physicochemical properties were compared to its semi- and non-fluorinated analogue. The fluorinated ionic liquid was found to show a thermomorphic mixing behavior with organic solvents so that it could be applied as a substitute for volatile perfluorinated solvents in fluorous biphasic catalysis to achieve the recovery of perfluoro-tagged catalysts. Efficient immobilization of a perfluoro-tagged palladium catalyst in the fluorous ionic liquid phase was demonstrated for the Heck reaction as a model reaction of the widely applied Pd-catalysed CC coupling reactions. The reaction of iodobenzene and methyl acrylate resulted in 83% yield after 20 runs proofing the efficient immobilization in the fluorophilic ionic liquid.

Ruthenium-Catalyzed Methylation of Amines with Paraformaldehyde in Water under Mild Conditions.

Methylated amines are highly important for a variety of pharmaceutical and agrochemical applications. Existing routes for their formation result in the production of large amounts of waste or require high reaction temperatures, both of which impact the ecological and economical footprint of the methodologies. Herein, we report the ruthenium-catalyzed reductive methylation of a range of aliphatic amines, using paraformaldehyde as both substrate and hydrogen source, in combination with water. This reaction proceeds under mild aqueous reaction conditions. Additionally the use of a secondary phase for catalyst retention and recycling has been investigated with promising results.

Organic Solvent Nanofiltration‐Supported Purification of Organocatalysts

AbstractIn this study, the successful use of organic solvent nanofiltration for the purification of three highly enantioselective quinine‐based organocatalysts was demonstrated. Commercially available solvent‐resistant membranes (DuraMem series) were utilized to remove specific impurities that originated from their respective synthesis pathways. DuraMem 200 exhibited the highest catalyst retention of 99 %, which facilitated catalyst purification at the gram scale with a purity >99 %.

Electrocatalytic Dioxygen Reduction by Carbon Electrodes Noncovalently Modified with Iron Porphyrin Complexes: Enhancements from a Single Proton Relay.

Oxygen reduction in acidic aqueous solution mediated by a series of asymmetric iron (III)-tetra(aryl)porphyrins adsorbed to basal- and edge- plane graphite electrodes is investigated. The asymmetric iron porphyrin systems bear phenyl groups at three meso positions and either a 2-pyridyl, a 2-benzoic acid, or a 2-hydroxyphenyl group at the remaining meso position. The presence of the three unmodified phenyl groups makes the compounds insoluble in water, enabling catalyst retention during electrochemical experiments. Resonance Raman data demonstrate that catalyst layers are maintained, but can undergo modification after prolonged catalysis in the presence of O2 . The introduction of a single proton relay group at the fourth meso position makes the asymmetric iron porphyrins markedly more robust catalysts; these molecules support higher sustained current densities than the parent iron tetraphenylporphyrin. Iron porphyrins bearing a 2-pyridyl group are the most active catalysts and operate at stable current densities ≥1 mA cm(-2) for over 5 h. Comparative analysis of the catalysts with different proton relays also is reported.

Removal of residual palm oil-based biodiesel catalyst using membrane ultra-filtration technique: An optimization study

In this research work, residual potassium hydroxide catalyst was removed from palm oil-based alkyl esters (biodiesel) using membrane separative technique, with the aim of achieving high-quality biodiesel that meets international standard specifications. Further, Central Composite Design (CCD) coupled with Response Surface Methodology (RSM) was employed to study the effects of the system variables such as flow rate, temperature and transmembrane pressure (TMP) on the retention of potassium. At the optimum conditions, the coefficient of retention (%R) of the catalyst was 93.642, and the content of the potassium was reduced from 8.328mg/L to 0.312mg/L; a value well below the one specified by both EN 14214 and ASTM D6751 standards. In addition, the comparison between predicted and experimental values for the catalyst retention offers a reasonable percentage error of 0.081%. Therefore, this study has proven that membrane technique can be used to post treat crude biodiesel; in order to achieve high-quality biodiesel fuel that can be efficiently used on diesel engines.

Electro‐Fenton decolourization of dyes in batch mode by the use of catalytic activity of iron loaded hydrogels

AbstractBACKGROUNDElectro‐Fenton technology has demonstrated great potential for the degradation of recalcitrant organic contaminants in polluted wastewaters. In this technology, the decomposition of hydrogen peroxide is catalysed by the presence of iron, and powerful oxidants, hydroxyl radicals, are generated. However, there is concern over the reduction of iron released in the treated effluent. Therefore, retention of catalyst is required, and advances in electro‐Fenton heterogeneous catalysis are necessary. In this study, the use of polyacrylamide hydrogel was evaluated as a support for the iron, and the performance of the catalyst (Fe‐PH) developed was assayed in the electro‐Fenton degradation of two dyes, Reactive Black 5 and Blue Sella Solid.RESULTSFirst, the synthesis of Fe‐PH catalyst was studied. The results showed that the loaded hydrogel with a mixture of Fe(II) and Fe(III) in 1:2 molar ratio reached the highest decolourization efficiency. The performance of the optimized Fe‐PH catalyst was evaluated in the decolourization of the selected dyes. These studies demonstrated the satisfactory performance of electro‐Fenton using Fe‐PH catalyst as well as its reusability.CONCLUSIONIn summary, the Fe‐PH catalyst developed gives the promise of wastewater treatment by means of an electro‐Fenton process. © 2014 Society of Chemical Industry

Reforming of Ethanol with Exhaust Heat at Automotive Scale

A low-temperature ethanol-reforming pathway, catalyzed by copper–nickel powder catalysts, transforms ethanol into a mixture of H2, CO, and CH4 at temperatures between 300 and 350 °C. Blending 25–50% of this gas mixture, known as “low-temperature ethanol reformate”, with ethanol or E85 fuels, enables dilute engine operation, resulting in substantial improvements in fuel economy and emissions. It is thermodynamically feasible to drive low-temperature reforming with exhaust heat, but this requires an onboard reformer providing adequate heat exchange between exhaust and fuel while retaining the catalyst. Three low-temperature ethanol reformer architectures (representing a design evolution) were developed and tested at automotive scale with exhaust from a V8 engine. The best catalyst retention and heat-transfer properties were achieved by embedding the catalyst in fibrous metal media with a density gradient. Longitudinal shell-and-tube and finned tube reformers achieved effective heat transfer and adequate initial conversion but proved inadequate for vehicular applications because of high thermal mass, catalyst settling, and unacceptable pressure build. A transverse shell-and-tube design in which banks of parallel, vertical catalyst tubes extended through a transverse stack of exhaust-side heat-exchange plates exhibited sustained high conversion with low and stable fuel-side pressure throughout a 500 h test period. This design has relatively low thermal mass and can be readily packaged on a vehicle. Thus, onboard reforming of ethanol- or methanol-rich fuels appears to be a feasible pathway to improve fuel economy and emissions in light-duty vehicles.

Hydrogenation of acrylonitrile-butadiene rubber latex using in situ synthesized RhCl(PPh3)3 catalyst

Catalytic hydrogenation of acrylonitrile-butadiene rubber (NBR) latex was achieved by using in situ synthesized RhCl(PPh3)3 catalyst. Water-soluble rhodium salt (RhCl3) was used as the catalyst precursor which was reacted in situ with triphenylphosphine (PPh3) to form RhCl(PPh3)3. Compared with using solid RhCl(PPh3)3, hydrogenation of NBR latex using the in situ RhCl(PPh3)3 showed a faster hydrogenation reaction. Based on the retention of catalyst in the polymer, it was revealed that the efficiency of in situ synthesis and the diffusion of the catalyst from the aqueous phase into the polymer particles were crucial in achieving the required high degree of hydrogenation. As a result, it was discovered that by introducing a small amount of alcohol (e.g. RhCl3=0.52mmol/L, PPh3=18mmol/L, ethanol/total NBR latex=1/10 volume ratio) to the feed latex, the in situ synthesized catalyst could be used in the hydrogenation reaction very efficiently.

A photo-catalysis and rotating nano-CaCO3 dynamic membrane system with Fe-ZnIn2S4 efficiently removes halogenated compounds in water

A photo-catalysis and dynamic membrane reactor system (PMR), with coated nanoCaCO3 layer on polypropylene non-woven fabric material, was studied for continuous photocatalysis and retention of catalysts in a single device, in removing halogenated compounds (2,4,6-Tribromophenol and 2,4-Dichlorophenol) in water. Both the batch and the continuous photocatalytic experiments were carried out to investigate the efficiency in COD removal, in removing halogenated compounds and in dehalogenation. A comparison of the separation performance of photocatalysts between bare (uncoated) and pre-coated nanoCaCO3 dynamic membranes was conducted. The pre-coated dynamic membrane can significantly enhance the filtration performance and improve the retention of photocatalysts. Particularly, nanoCaCO3 was found able to assist and improve debromination efficiency during UV irradiated photolysis. Tentative debromination pathway and dehalogenation mechanism were proposed. Due to the assisted additional reaction by the nanoCaCO3 dynamic membrane, the PMR system with nanoCaCO3-coating and Fe-ZnIn2S4 completely removed halogenated compounds, achieved highest COD removal (72% for 2,4,6-Tribromophenol and 75% for 2,4-Dichlorophenol), 85–90% debromination and 99% dechlorination efficiencies. The long-term continuous photocatalytic debromination experiments demonstrated that the dynamic membrane hybrid system was very steady in the long-term operation and the debromination maintained at about 85% during the 40h photocatalytic debromination reaction. The combination of dynamic membrane and photocatalysis provides a new alternative for practical application of photocatalysis.

Catalytic Biofilms on Structured Packing for the Production of Glycolic Acid

While structured packing modules are known to be efficient for surface wetting and gas-liquid exchange in abiotic surface catalysis, this model study explores structured packing as a growth surface for catalytic biofilms. Microbial biofilms have been proposed as self-immobilized and self-regenerating catalysts for the production of chemicals. A concern is that the complex and dynamic nature of biofilms may cause fluctuations in their catalytic performance over time or may affect process reproducibility. An aerated continuous trickle-bed biofilm reactor system was designed with a 3 L structured packing, liquid recycling and pH control. Pseudomonas diminuta established a biofilm on the stainless steel structured packing with a specific surface area of 500 m2 m-3 and catalyzed the oxidation of ethylene glycol to glycolic acid for over two months of continuous operation. A steady-state productivity of up to 1.6 gl-1h-1 was achieved at a dilution rate of 0.33 h-1. Process reproducibility between three independent runs was excellent, despite process interruptions and activity variations in cultures grown from biofilm effluent cells. The results demonstrate the robustness of a catalytic biofilm on structured packing, despite its dynamic nature. Implementation is recommended for whole-cell processes that require efficient gas-liquid exchange, catalyst retention for continuous operation, or improved catalyst stability.

Photocatalysis and Rotating Dynamic Membrane Hybrid System with Fe-ZnIn2S4 Efficiently Removes 2,4,6-Tribromophenol in Water: Effect of Dynamic Membrane

A photo-catalysis and dynamic membrane reactor system (PMR), with coated dynamic layer on polypropylene non- woven fabric material, was studied for continuous photocatalysis and retention of catalysts in a single device, in removing halogenated compound 2,4,6-Tribromophenol in water. Different dynamic layer materials such as graphite oxide, graphene oxide and nonoCaCO3 were applied to evaluate the efficiency in removing COD, 2,4,6- Tribromophenol and in debromination. The pre-coated dynamic membrane can significantly enhance the filtration performance and improve the debromination efficiency. The PMR system with dynamic layer and Fe-ZnIn2S4 photocatalyst completely removed 2,4,6-Tribromophenol when the retention time maintained 2.0h in the continuous mode. The hybrid system achieved 58%, 74% and 85% debromination when the dynamic layer materials were graphite oxide, graphene oxide and nanoCaCO3, respectively. And, the COD removal efficiencies were in the order of nanoCaCO3 (72%) > graphite oxide (51%) > graphene oxide (42%). NanoCaCO3 was found able to assist and improve debromination efficiency during UV irradiated photolysis. The combination of dynamic membrane and photocatalysis provides a new alternative for practical application of photocatalysis.