Catalysts — Open Access Journal

The hematite photo-Fenton catalysis has attracted increasing attention because it offers strong oxidation of organic pollutants under visible light at neutral pH. In the present work, aqueous synthesis of hematite photo-Fenton catalysts with high activity is demonstrated. We compare photo-Fenton activity for hematite obtained by hydrolyzation at 60 °C or by a thermally induced transformation from iron-bearing nanoparticles, such as amorphous iron oxyhydroxide or goethite. A link between their structure and visible light photo-Fenton reactivity is established. The highest activity was observed for hematite obtained from goethite nanowires due to oblong platelet-like structure, high surface area and the presence of nanopores. Full article

The application and advantages of variable frequency microwaves (VFM; range, 5.85–6.65 GHz) are reported for the first time in microwave chemistry, particularly when carrying out reactions catalyzed by metallic conductive catalysts so as to avoid the formation of arc discharges, and especially when using a strong microwave absorber such as activated carbon (AC) particulates as supports of metal-based catalysts. Two model reactions performed in low boiling point nonpolar solvents are described wherein arc discharges easily occur under the more conventional fixed frequency microwave (FFM) approach: (i) the synthesis of 4-methylbiphenyl (4MBP) by the Suzuki-Miyaura cross-coupling process catalyzed by Pd particles supported on AC particulates (Pd/AC), and (ii) the synthesis of toluene via the dehydrogenation of methylcyclohexane (MCH) catalyzed by Pt particles dispersed on AC particulates (Pt/AC). Contrary to the usage of fixed frequency microwaves (5.85 GHz and 6.65 GHz), the use of VFM microwaves increased the chemical yields of 4MBP {49% versus 5–8% after 60 min} and toluene {89% versus 24% after 10 min} by suppressing the formation of discharges that otherwise occur on the catalyst/AC surface with FFM microwaves. Consequently, relative to the latter approach, the VFM technology is significantly advantageous, especially in reactions with solid conductive catalysts, not least of which are the reduction in power consumption, thus energy savings, and the prevention of potential mishaps. Full article

The reaction of copper(II) acetate with the 2-aminobenzothiazole (abt) heterocycle affords the new copper(II) complex of formula [Cu(abt)₂(OOCCH₃)₂] (1) in a straightforward manner. Compound 1 served as a precatalyst for azide/alkyne cycloaddition reactions (CuAAC) in water, leading to 1,4-disubstituted-1,2,3-triazole derivatives in a regioselective manner and with excellent yields at room temperature. The main advantages of the coordination of such a heterocyclic ligand in 1 are its strong σ-donating ability (N-Cu), nontoxicity and biological properties. In addition, the click chemistry reaction conditions using 1 allow the formation of a great variety of 1,2,3-triazole-based heterocyclic compounds that make this protocol potentially relevant from biological and sustainable viewpoints. A molecular electron density theory (MEDT) study was performed by using density functional theory (DFT) calculations at the B3LYP/6-31G(d,p) (LANL2DZ for Cu) level to understand the observed regioselectivity in the CuAAC reaction. The intramolecular nature of this reaction accounts for the regioselective formation of the 1,4-regioisomeric triazole derivatives. The ionic nature of the starting copper-acetylide precludes any type of covalent interaction throughout the reaction, as supported by the electron localization function (ELF) topological analysis, reaffirming the zwitterionic-type (zw-type) mechanism of the copper(I)/aminobenzothiazole-catalysed azide-alkyne cycloaddition reactions. Full article

Paper sludge is an attractive biomass feedstock for bioconversion to ethanol due to its low cost and the lack of pretreatment required for its bioprocessing. This study assessed the use of a recombinant cellulase cocktail (mono-components: S. cerevisiae-derived PcBGL1B (BGL), TeCel7A (CBHI), ClCel6A (CBHII) and TrCel5A (EGII) mono-component cellulase enzymes) for the efficient saccharification of softwood-derived paper sludge to produce fermentable sugars. The paper sludge mainly contained 74.3% moisture and 89.7% (per dry mass (DM)) glucan with a crystallinity index of 91.5%. The optimal protein ratio for paper sludge hydrolysis was observed at 9.4: 30.2: 30.2: 30.2% for BGL: CBHI: CBHII: EGII. At a protein loading of 7.5 mg/g DW paper sludge, the yield from hydrolysis was approximately 80%, based on glucan, with scanning electron microscopy micrographs indicating a significant alteration in the microfibril size (length reduced from ≥ 2 mm to 93 µm) of the paper sludge. The paper sludge hydrolysis potential of the Opt CelMix (formulated cellulase cocktail) was similar to the commercial Cellic CTec2^® and Celluclast^® 1.5 L cellulase preparations and better than Viscozyme^® L. Low enzyme loadings (15 mg/g paper sludge) of the Opt CelMix and solid loadings ranging between 1 to 10% (w/v) rendered over 80% glucan conversion. The high glucose yields attained on the paper sludge by the low enzyme loading of the Opt CelMix demonstrated the value of enzyme cocktail optimisation on specific substrates for efficient cellulose conversion to fermentable sugars. Full article

Four silver phosphate-based materials were successfully synthesized, characterized, and evaluated, together with TiO₂, in the photodegradation of synthetic dyes (tartrazine, Orange II, rhodamine, and Brilliant Blue FCF) under two irradiation sources centered at 420 and 450 nm. Scanning Electron Microscopy (SEM) images showed different topologies of the synthesized materials, whereas diffuse reflectance spectra demonstrated that they display absorption up to 500 nm. Degradation experiments were performed in parallel with the silver materials and TiO₂. Upon irradiation centered at 420 nm, the abatement of the dyes was slightly more efficient in the case of TiO₂—except for Orange II. Nevertheless, upon irradiation centered at 450 nm, TiO₂ demonstrated complete inefficiency and silver phosphates accomplished the complete abatement of the dyes—except for Brilliant Blue FCF. A careful analysis of the achieved degradation of dyes revealed that the main reaction mechanism involves electron transfer to the photogenerated holes in the valence band of silver photocatalysts, together with the direct excitation of dyes and the subsequent formation of reactive species. The performance of TiO₂ was only comparable at the shorter wavelength when hydroxyl radicals could be formed; however, it could not compete under irradiation at 450 nm since the formed superoxide anion is not as reactive as hydroxyl radicals. Full article

Our planet urgently needs sustainable solutions to alleviate the anthropogenic global warming and climate change. Homogeneous catalysis has the potential to play a fundamental role in this process, providing novel, efficient, and at the same time eco-friendly routes for both chemicals and energy production. In particular, pincer-type ligation shows promising properties in terms of long-term stability and selectivity, as well as allowing for mild reaction conditions and low catalyst loading. Indeed, pincer complexes have been applied to a plethora of sustainable chemical processes, such as hydrogen release, CO₂ capture and conversion, N₂ fixation, and biomass valorization for the synthesis of high-value chemicals and fuels. In this work, we show the main advances of the last five years in the use of pincer transition metal complexes in key catalytic processes aiming for a more sustainable chemical and energy production. Full article

A new efficient polyoxometalate composite catalyst of hierarchical MIL-101 and phosphotungstic acid (PTA) was facilely prepared by the immersion method. The material was thoroughly characterized by powder x-ray diffraction (PXRD), scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX) and inductively coupled plasma‒optical emission spectrometry (ICP-OES). Compared to the pristine nonhierarchical MIL-101 composite, the hierarchical composite demonstrated much higher catalytic performance in methanolysis of styrene oxide, such as catalytic activity and reusability. Full article

Persistent organic pollutants have been increasingly detected in natural waters, and this represents a real challenge to the quality of this resource. To remove these species, advanced treatment technologies are required. Among these technologies, Fenton-like and photo-Fenton-like processes have been investigated for the removal of pollutants from water. Delicate aspects of photo-Fenton processes are that light-driven processes are energy intensive and require a fair amount of chemical inputs, which strongly affects their overall environmental burdens. At present, aside from determining the efficiency of the processes to remove pollutants of a particular technology, it becomes fundamental to assess also the environmental sustainability of the overall process. In this work, the methodology of the life cycle assessment (LCA) was applied to identify the hotspots of using magnetite particles covered with humic acid (Fe₃O₄/HA) as a heterogeneous photo-Fenton catalyst for water remediation. The sustainability of the overall process was considered, and a comparative LCA study was performed between H₂O₂ and persulfate activation at different pH. The addition of humic substances to the particles allows the effectiveness of the catalyst to improve without increasing the environmental impacts; these processes are strongly correlated with energy consumption and therefore with the efficiency of the process. For this reason, working at acidic pH allows us to contain the impacts. Full article

First d-block metal-based perovskite oxides (FDMPOs) have garnered significant attention in research for their utilization in the water oxidation reaction due to their low cost, earth abundance, and promising activities. Recently, FDMPOs are being applied in electrocatalysis for the hydrogen evolution reaction (HER) and overall water splitting reaction. Numerous promising FDMPO-based water splitting electrocatalysts have been reported, along with new catalytic mechanisms. Therefore, an in-time summary of the current progress of FDMPO-based water splitting electrocatalysts is now considered imperative. However, few reviews have focused on this particular subject thus far. In this contribution, we review the most recent advances (mainly within the years 2014–2020) of FDMPO electrocatalysts for alkaline water splitting, which is widely considered to be the most promising next-generation technology for future large-scale hydrogen production. This review begins with an introduction describing the fundamentals of alkaline water electrolysis and perovskite oxides. We then carefully elaborate on the various design strategies used for the preparation of FDMPO electrocatalysts applied in the alkaline water splitting reaction, including defecting engineering, strain tuning, nanostructuring, and hybridization. Finally, we discuss the current advances of various FDMPO-based water splitting electrocatalysts, including those based on Co, Ni, Fe, Mn, and other first d-block metal-based catalysts. By conveying various methods, developments, perspectives, and challenges, this review will contribute toward the understanding and development of FDMPO electrocatalysts for alkaline water splitting. Full article

Solar-light-driven N-doped 3C–SiC powder was synthesized via a simple one-step combustion route. SiC–N₂ photocatalysts exhibited 205.3 μL/(g·h) hydrogen evolution rate, nearly 2 times that of SiC–Ar(120.1 μL/(g·h)), and was much higher than that of SiC nanowires (83.9 μL/(g·h)), SiC nanoparticles (82.8 μL/(g·h)) as well as the B-doped SiC photocatalysts(166 μL/(g·h)). In cyclic tests, N-doped SiC also performed excellent photocatalytic durability and good structural stability. It can be concluded that the influence of N-doping introduced defects into the SiC photocatalyst by occupation and mixed phase structure, transformed the band structure into the direct band gap, and formed a shallow donor level for trapping holes. Consequently, higher photocatalytic activities and lower recombination was achieved. Furthermore, the carbon on the photocatalyst which was yielded from the substitution of N or which remained after combustion would build constructed efficient interfacial contact with SiC for the quickening of light-driven electron transfer to the surface, and simultaneously strengthen the adsorption capacity and light-harvesting potential. Full article

ZrO₂ and Ce_0.8Zr_0.2O₂ mixed oxides were prepared and tested in the oxidation of carbon soot at different oxygen partial pressures and degrees of catalyst/soot contact to investigate their activity under typical gasoline direct injection (GDI) operating conditions. Under reductive atmospheres, generation of oxygen vacancies occurs in Ce_0.8Zr_0.2O₂, while no reduction is observed on ZrO₂. Both materials can oxidize carbon under high oxygen partial pressures; however, at low oxygen partial pressures, the presence of carbon can contribute to the reduction of the catalyst and formation of oxygen vacancies, which can then be used for soot oxidation, increasing the overall performance. This mechanism is more efficient in Ce_0.8Zr_0.2O₂ than ZrO₂, and depends heavily on the interaction and the degree of contact between soot and catalyst. Thus, the ability to form oxygen vacancies at lower temperatures is particularly helpful to oxidize soot at low oxygen partial pressures, and with higher CO₂ selectivity under conditions typically found in GDI engine exhaust gases. Full article

This study reports a new strategy for enzyme immobilization based on passive immobilization in neat and magnetically responsive polyamide 4 (PA4) highly porous particles. The microsized particulate supports were synthesized by low-temperature activated anionic ring-opening polymerization. The enzyme of choice was laccase from Trametes versicolor and was immobilized by either adsorption on prefabricated PA4 microparticles ([email protected]) or by physical in situ entrapment during the PA4 synthesis ([email protected]). The surface topography of all PA4 particulate supports and laccase conjugates, as well as their chemical and physical structure, were studied by microscopic, spectral, thermal, and synchrotron WAXS/SAXS methods. The laccase content and activity in each conjugate were determined by complementary spectral and enzyme activity measurements. [email protected] samples displayed >93% enzyme retention after five incubation cycles in an aqueous medium, and the [email protected] series retained ca. 60% of the laccase. The newly synthesized PA4-laccase complexes were successfully used in dyestuff decolorization aiming at potential applications in effluent remediation. All of them displayed excellent decolorization of positively charged dyestuffs reaching ~100% in 15 min. With negative dyes after 24 h the decolorization reached 55% for [email protected] and 85% for [email protected] A second consecutive decolorization test revealed only a 5–10% decrease in effectiveness indicating the reusability potential of the laccase-PA4 conjugates. Full article

HFC-134a, one of the representative hydrofluorocarbons (HFCs) used as a coolant gas, is a known greenhouse gas with high global warming potential. Catalytic decomposition is considered a promising technology for the removal of fluorinated hydrocarbons. However, systematic studies on the catalytic decomposition of HFC-134a are rare compared to those for other fluorinated hydrocarbon gases. In this study, Ga-Al₂O₃ and S/Ga-Al₂O₃ catalysts were prepared and the change in their properties post-acid treatment was investigated by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), temperature-programmed desorption of ammonia (NH₃-TPD), in situ Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (SEM-EDS), and X-ray photoelectron spectroscopy (XPS). The S/Ga-Al₂O₃ catalyst achieved a much higher HFC-134a conversion than Ga-Al₂O₃, which was ascribed to the promotional effect of the sulfuric acid treatment on the Lewis acidity of the catalyst surface, as confirmed by NH₃-TPD. Furthermore, the effect of hydrogen fluoride (HF) gas produced by HFC-134a decomposition on the catalyst was investigated. The S/Ga-Al₂O₃ maintained a more stable and higher HFC-134a conversion than Ga-Al₂O₃. Combining the results of the stability test and characterization, it was established that the sulfuric acid treatment not only increased the acidity of the catalyst but also preserved the partially reduced Ga species. Full article

The thermocatalytic, photocatalytic and photothermo-catalytic oxidation of some volatile organic compounds (VOCs), 2-propanol, ethanol and toluene, was investigated over brookite TiO₂-CeO₂ composites. The multi-catalytic approach based on the synergistic effect between solar photocatalysis and thermocatalysis led to the considerable decrease in the conversion temperatures of the organic compounds. In particular, in the photothermo-catalytic runs, for the most active samples (TiO₂-3 wt% CeO₂ and TiO₂-5 wt% CeO₂), the temperature at which 90% of VOC conversion occurred was about 60 °C, 40 °C and 20 °C lower than in the thermocatalytic tests for 2-propanol, ethanol and toluene, respectively. Furthermore, the addition of cerium oxide to brookite TiO₂ favored the total oxidation to CO₂ already in the photocatalytic tests at room temperature. The presence of small amounts of cerium oxide allowed to obtain efficient brookite-based composites facilitating the space charge separation and increasing the lifetime of the photogenerated holes and electrons as confirmed by the characterization measurements. The possibility to concurrently utilize the photocatalytic properties of brookite and the redox properties of CeO₂, both activated in the photothermal tests, is an attractive approach easily applicable to purify air from VOCs. Full article

This work compared the effect of soaking and roasting Phoenix dactylifera L. seeds pretreatment methods on oil yield. The conversion of the Phoenix dactylifera L. seed oil to fatty acid methyl ester (FAME) was conducted via transesterification reaction using Ce-Zr/Al-MCM-41 monometallic and bimetallic catalysts. The reaction conditions were optimized using response surface methodology based on the central composite design (RSM-CCD). The result shows a quadratic model fitting with an R² value of ~0.98% from the analysis of variance. In addition, the optimum FAME yield of 93.83% was obtained at a reaction temperature of 60.5 °C, a reaction time of 3.8 h, a catalyst concentration of 4 wt.%, and a methanol to oil molar ratio of 6.2:1 mol/mol. The effect of the regenerated catalyst was significantly maintained for five cycles. The fuel properties of the produced FAME lie within the values reported in studies, ASTM D6751, and EN14214 standards. Full article

Ag doped TiO₂ photocatalysts ([email protected]₂) were prepared with an aim to extend the absorption range of TiO₂ into the visible region, for tentative application under solar irradiation. Photocatalyst synthesized by the novel method using chitosan for reduction of Ag⁺ to Ag⁰ nanoparticles was compared to similar catalysts previously reported. The photocatalytic activity of [email protected]₂ obtained by a simple novel method was evaluated based on degradation of salicylic acid as a model compound. The higher activity under visible irradiation can be attributed to the surface plasmon resonance and suppression of the electron-hole recombination when deposition of Ag nanoparticles on TiO₂ was achieved using chitosan. The photocatalysts were characterized by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), high resolution TEM (HRTEM), energy dispersive X-ray spectroscopy (EDXS), selected area diffraction (SAED), and diffuse reflectance spectroscopy (DRS). The photochromism of Ag was observed and explained. Full article

The first, recyclable protocol for the selective synthesis of (E)-alkenyl boronates via borylative coupling of olefins with vinylboronic acid pinacol ester in monophasic ([email protected]) or biphasic ([email protected]/scCO₂) systems is reported in this article. The efficient immobilization of [Ru(CO)Cl(H)(PCy₃)₂] (1 mol%) in [EMPyr][NTf₂] and [BMIm][OTf] with the subsequent extraction of products with n-heptane permitted multiple reuses of the catalyst without a significant decrease in its activity and stability (up to 7 runs). Utilization of scCO₂ as an extractant enabled a significant reduction in the amount of catalyst leaching during the separation process, compared to extraction with n-heptane. Such efficient catalyst immobilization allowed an intensification of the processes in terms of its productivity, which was indicated by high cumulative TON values (up to 956) in contrast to the traditional approach of applying volatile organic solvents (TON = ~50–100). The reaction was versatile to styrenes with electron-donating and withdrawing substituents and vinylcyclohexane, generating unsaturated organoboron compounds, of which synthetic utility was shown by the direct transformation of extracted products in iododeborylation and Suzuki coupling processes. All synthesized compounds were characterized using ¹H, ¹³C NMR and GC-MS, while leaching of the catalyst was detected with ICP-MS. Full article

A novel strategy for toluene abatement was investigated using a sequential adsorption-regeneration process. Commercial Hopcalite (CuMn₂O_x, Purelyst101MD), Ceria nanorods, and UiO-66-SO₃H, a metal–organic framework (MOF), were selected for this study. Toluene was first adsorbed on the material and a mild thermal activation was performed afterwards in order to oxidize toluene into CO₂ and H₂O. The materials were characterized by XRD, N₂ adsorption-desorption analysis, H₂-TPR and TGA/DSC. The best dynamic toluene adsorption capacity was observed for UiO-66-SO₃H due to its hierarchical porosity and high specific surface area. However, in terms of balance between storage and catalytic properties, Hopcalite stands out from others owing to its superior textural/chemical properties promoting irreversible toluene adsorption and outstanding redox properties, allowing a high activity and CO₂ selectivity in toluene oxidation. The high conversion of toluene into CO₂ which easily desorbs from the surface during heating treatment shows that the sequential adsorption-catalytic thermal oxidation can encompass a classical oxidation process in terms of efficiency, CO₂ yield, and energy-cost saving, providing that the bifunctional material displays a good stability in repetitive working conditions. Full article

The energy-saving glucose production process from starchy sources was developed by replacing high-temperature, liquid-phase by low-temperature, solid-phase. Therefore, the enzymatic hydrolysis under gelatinization temperature at very high gravity (≥300 g.L⁻¹) of starchy substrates presents as an emerging technology. This study focused on the hydrolysis kinetics of cassava flour affected by different pretreatment methods. Cassava flour (dried, milled) was prepared in acetate buffer (pH 4.2) with starch concentration ranging from 10–30% (w/w). The mash was then pre-treated by three different methods for 30 min using heating (30, 40, 50 °C), enzyme (Viscozyme L 0.1% w/w) and microwave (3 × 20 s at 800 W). The suspension was then hydrolyzed with Stargen 002 (0.2% w/w) at 30 °C for 48 h. The enzyme adsorption kinetics was described by the Langmuir isotherm equation. The pretreatments at 50 °C and with enzyme resulted in the highest efficiency with the hydrolysis yield ranging from 76–79% after 48 h. The hydrolysis yield decreased to 67% (using microwave), 66% (at 45 °C), 61% (at 40 °C) and 59% (at 30 °C). The linear relationship between enzyme adsorption and produced glucose was demonstrated. The kinetics of glucose production was fitted by an empirical equation (analogy with Michaelis-Menten model) and allowed predicting the maximum hydrolysis yield. Full article