From Conventional Lewis Acids to Heterogeneous Montmorillonite K10: Eco-Friendly Plant-Based Catalysts Used as Green Lewis Acids

Solid acids as cocatalysts in the chelation-assisted hydroacylation of alkenes and alkynes

The use of homogeneous Brønsted acid cocatalysts (such as benzoic acid) in hydroacylation reactions via imine intermediates has been extensively studied. However, the use of heterogeneous cocatalysts has been limited to montmorillonite K10. Thus, we can use other solid acids to increase the efficiency of the reaction. In this study, we describe the effects of sulfated zirconia, Al-MCM-41 or superacid modified montmorillonite on the hydroacylation of alkenes and alkynes with aldehydes via imine intermediates and in the presence of the Wilkinson complex. Furthermore, we addressed the dual role of montmorillonite, a redox reagent in the presence of TEMPO and an acid solid, allowing the direct use of benzyl alcohols as substrates to generate saturated or α,β-unsaturated ketones.

Clay based heterogeneous catalysts for carbon-nitrogen bond formation: a review

Clay and modified clay-based catalysts are widely used in organic transformation. Owing to the interlayer ions and good ion exchange capacity of clay, replacement with another ion and incorporation of different nanomaterials can be done. Due to these significant properties of clay, it can be utilized in the synthesis of various organic compounds. Carbon-nitrogen bonded compounds possess diverse applications in different fields. These compounds are prepared using different solid acid heterogeneous catalysts. This review presents a detailed discussion on clay used for the carbon-nitrogen bond formation reaction, such as the Biginelli reaction and A3 and KA2 coupling reactions. Additionally, other C-N bond formation reactions using various clay-based catalysts such as bentonite, montmorillonite, hydrotalcite and halloysite clay with various metals, metal oxides, Kegging type heteropoly acid and various nanomaterial incorporated clay heterogeneous catalysts are discussed.

Acid-catalysed reactions of amines with dimethyl carbonate

Highly effective acid-catalysed reactions of amines with dimethyl carbonate (DMC) have been conducted with significant yields and selectivity of carboxymethylation or methylation products. Lewis acids (FeCl₃, ZnCl₂, and AlCl₃·6H₂O), Brønsted acids (PTSA, acetic, and formic acids), and acids supported on silica (silica sulfuric and silica perchlorate) resulted in carboxymethylation of primary aliphatic amines with high conversions. It was found that the Lewis acid FeCl₃ also promoted carboxymethylation of primary aromatic amines and secondary amines. At both 90 °C or an elevated temperature of 150 °C under pressure, AlCl₃·6H₂O demonstrated highly selective monomethylation of aromatic amines. In addition, both silica sulfuric acid and silica perchlorate at 90 °C exhibited no conversion for secondary amines but enhanced carboxymethylation with high conversions of 80.7-87.5% and selectivity of >99.00% at 150 °C in a pressure reactor. At 1.0 equivalent, both promoted excellent conversion and selectivity of primary aliphatic amines at 90 °C. In addition, they were easily recovered and reused for at least four additional reactions without significant loss of efficiency with consistent conversions and selectivity. Green metrics evaluation for the silica sulfuric acid-catalysed reaction highlighted the sustainability features of the process. Silica-supported catalysts are highly stable, making them ideal alternative catalysts for the methylation and carbonylation of various amines with DMC. Acid-catalysed DMC reactions of amines may expand the substrate scope and offer new opportunities for developing sustainable organic synthetic methodologies.

Preparation of Pd Nanoparticles Stabilized by Modified Montmorillonite for Efficient Hydrodeoxygenation of Lignin-Derived Phenolic Compounds in Water

Developing a new and efficient catalytic route for the production of alkanes by upgrading the aqueous phenolic biofuels still remains a challenge. Here, we designed and synthesized a bifunctional catalyst that uses natural montmorillonite (MMT) as support and combines metal active sites and Brӧnsted acid sites in the MMT via ion exchange and reduction roasting process. The catalytic activity of the as-synthesized Pd-MMT (H⁺) was evaluated by the hydrodeoxygenation (HDO) of a series of lignin-derived phenolic compounds in water. Our model reaction study reveals that the HDO of phenol undergoes an initial hydrogenation of aromatic rings to produce cyclohexanol and cyclohexanone, followed by the dehydration of cyclohexanol to provide intermediate cyclohexene and a final hydrogenation of cyclohexene to create a cyclohexane product. The combination of high metal catalytic activity and Brӧnsted acidity in Pd-MMT (H⁺) synergistically accelerated the HDO of phenol. Furthermore, good catalytic activity and recycling ability were also observed for other lignin-derived phenolic compounds.

Advantages and limits to copper phytoextraction in vineyards

Copper (Cu) contamination of soils may alter the functioning and sustainability of vineyard ecosystems. Cultivating Cu-extracting plants in vineyard inter-rows, or phytoextraction, is one possible way currently under consideration in agroecology to reduce Cu contamination of vineyard topsoils. This option is rarely used, mainly because Cu phytoextraction yields are too low to significantly reduce contamination due to the relatively "low" phytoavailability of Cu in the soil (compared to other trace metals) and its preferential accumulation in the roots of most extracting plants. This article describes the main practices and associated constraints that could theoretically be used to maximize Cu phytoextraction at field scale, including the use of Cu-accumulating plants grown (i) with acidifying plants (e.g., leguminous plants), and/or (ii) in the presence of acidifying fertilizers (ammonium, elemental sulfur), or (iii) with soluble "biochelators" added to the soil such as natural humic substances or metabolites produced by rhizospheric bacteria such as siderophores, in the inter-rows. This discussion article also provides an overview of the possible ways to exploit Cu-enriched biomass, notably through ecocatalysis or biofortification of animal feed.

Montmorillonite-based Heterogeneous Catalysts for Efficient Organic Reactions

In this review, we give a brief overview of recently developed montmorillonite-based heterogeneous catalysts used for efficient organic reactions. Cation-exchanged montmorillonite catalysts, metal catalysts supported on montmorillonite, and an interlayer design used for selective catalysis are introduced and discussed. In traditional syntheses, homogeneous acids and metal salts were used as catalysts, but the difficulty in separation of catalysts from products was a bottleneck when considering industrialization. The use of solid heterogeneous catalysts is one of the major solutions to overcome this problem. Montmorillonite can be used as a heterogeneous catalyst and/or catalyst support. This clay material exhibits strong acidity and a stabilizing effect on active species, such as metal nanoparticles, due to its unique layered structure. These advantages have led to the development of montmorillonite-based heterogeneous catalysts. Acidic montmorillonite, such as proton-exchanged montmorillonite, exhibits a high catalytic activity for the activation of electrophiles, such as alcohols, alkenes, and even alkanes. The montmorillonite interlayer/surface also functions as a good support for various metal species used for oxidation and carbon-carbon bond forming reactions. The use of an interlayer structure enables selective reactions and the stabilization of catalytically active species.

The phytoextraction power of Cichorium intybus L. on metal-contaminated soil: Focus on time- and cultivar-depending accumulation and distribution of cadmium, lead and zinc

This study is focused on the evaluation of the accumulation of Cd, Pb and Zn in five cultivars of Cichorium intybus L. (chicory) which were produced on contaminated agricultural soil. Over a growth period of 211 days, the roots and leaves were collected in four stages and then analyzed by flame atomic absorption spectrometry to measure the concentration and distribution of the target metals in these two chicory organs considering the weather and the nature of the cultivar. For all cultivars, sharp decreases of Pb (from 165 mg kg-1 to 3 mg kg-1), Cd (from 11 mg kg-1 to 5 mg kg-1) and Zn concentrations (from 157 mg kg-1 to 40 mg kg-1) in the roots were highlighted over time. The data collected enabled the calculation of the variation of the bioconcentration factor, the biological absorption coefficient and the translocation factor for Cd, Pb and Zn. These parameters were then correlated with the distribution of the fresh biomass of leaves and roots and several indicators such as chlorophyll content, flavonols, anthocyanin and nitrogen balance index were measured. The study concludes with the discussion on the ability of chicory to clean up contaminated agricultural soil. The current investigation has shown: i) a translocation of Cd (and Zn to a lesser extent) from the roots to the leaves; ii) an increase in the level of anthocyanins with the increase of the metal trace elements concentration in the leaf, while the content of chlorophyll and the nitrogen balance index decrease, which could be linked to the phenomenon of senescence; iii) an ability of the chicory to reduce the bioavailable pool of the three metal trace elements studied, in particular for Cd.

Highly Efficient Cleavage of Ether Bonds in Lignin Models by Transfer Hydrogenolysis over Dual-Functional Ruthenium/Montmorillonite

Cleavage of ether bonds is a crucial but challenging step for lignin valorization. To efficiently realize this transformation, the development of robust catalysts or catalytic systems is required. In this study, montmorillonite (MMT)-supported Ru (denoted as Ru/MMT) is fabricated as a dual-functional heterogeneous catalyst to cleave various types of ether bonds through transfer hydrogenolysis without using any additional acids or bases. The prepared Ru/MMT material is found to efficiently catalyze the cleavage of various lignin models and lignin-derived phenols; cyclohexanes (fuels) and cyclohexanols (key intermediates) are the main products. The synergistic effect between electron-enriched Ru and the acidic sites on MMT contributes to the excellent performance of Ru/MMT. Systematic studies reveal that the reaction proceeds through two possible reaction pathways, including the direct cleavage of ether bonds and the formation of intermediates with one hydrogenated benzene ring, for all examined types of ether bonds, namely, 4-O-5, α-O-4, and β-O-4.

Montmorillonite K10: An Efficient Organo-Heterogeneous Catalyst for Synthesis of Benzimidazole Derivatives

The use of toxic solvents, high energy consumption, the production of waste and the application of traditional processes that do not follow the principles of green chemistry are problems for the pharmaceutical industry. The organic synthesis of chemical structures that represent the starting point for obtaining active pharmacological compounds, such as benzimidazole derivatives, has become a focal point in chemistry. Benzimidazole derivatives have found very strong applications in medicine. Their synthesis is often based on methods that are not convenient and not very respectful of the environment. A simple montmorillonite K10 (MK10) catalyzed method for the synthesis of benzimidazole derivatives has been developed. The use of MK10 for heterogeneous catalysis provides various advantages: the reaction yields are decidedly high, the work-up procedures of the reaction are easy and suitable, there is an increase in selectivity and the possibility of recycling the catalyst without waste formation is demonstrated. The reactions were carried out in solvent-free conditions and in a short reaction time using inexpensive and environmentally friendly heterogeneous catalysis. It has been shown that the reaction process is applicable in the industrial field.

Direct Alkylation of Benzene at Lower Temperatures in the Liquid Phase: Catalysis by Montmorillonites as Noble-Metal-Free Solid Acids

Alkylated benzenes are widely used as raw materials for the production of a variety of chemical compounds. Conventionally, they are obtained by the Friedel-Crafts reaction between alkyl halides and benzene. In this study, the synthesis of halogen-free alkylated benzenes was made possible by the direct alkylation of benzene with alkanes using montmorillonites as noble-metal-free solid acid catalysts. The direct alkylation of benzene with n-heptane was performed at 150 °C. Aluminum-exchanged montmorillonite showed the highest yield of the target C-7 alkylated products (Ph-C7) compared with other homogeneous and heterogeneous acid catalysts: 1.8 % conversion of benzene with 58 % selectivity in 16 h. The montmorillonite catalyst system was applied to other linear and cyclic alkanes to give the corresponding alkylated products with good selectivities.

Substrate substitution effects in the Fries rearrangement of aryl esters over zeolite catalysts

The reaction scheme of the Fries rearrangement of aryl esters over zeolite catalysts strongly depends on the framework type, acidic properties, and substrate identity.

Effect of HCOOK/Ethanol on Fe/HUSY, Ni/HUSY, and Ni-Fe/HUSY Catalysts on Lignin Depolymerization to Benzyl Alcohols and Bioaromatics

We have investigated the production of benzyl alcohols and bioaromatics via the reductive lignin depolymerization process over Fe/H-style ultrastable Y (HUSY), Ni/HUSY, and Ni-Fe/HUSY catalysts using HCOOK/ETOH in air. Synergy effect between HCOOK and the catalysts improved the depolymerization process, resulting in a higher bio-oil recovery. HCOOK does not act solely as an in situ hydrogen source; it also interacts with lignin to enable its initial depolymerization via a base-catalyzed mechanism to low-molecular-weight fragments, and in tandem with the catalyst, the hydrogenolysis rate of the depolymerized lignin monomers was enhanced. Fe/HUSY displayed an excellent activity for the catalytic reductive step in contrast to Ni/HUSY and Ni-Fe/HUSY by facilitating methoxy group removal via hydrogenolysis, thereby contributing to the yield and stabilization of the low-molecular-weight aromatics [diethyl ether (DEE)-soluble products]. Fe/HUSY gave the highest DEE product yield of >99 wt % and a total benzyl alcohol yield of 16 wt % with a total selectivity of 47 wt % (60 wt % for aromatic alcohols). Fe/HUSY was reused for the lignin depolymerization reaction without much loss of its initial activity, giving 13 wt % yield of benzyl alcohols with a selectivity of 58 wt % (77 wt % for aromatic alcohols).

Benefits of Ryegrass on Multicontaminated Soils Part 1: Effects of Fertilizers on Bioavailability and Accumulation of Metals

Effects of three phosphorus fertilizers on the shoot biomass and on the accumulation of alkali, alkaline earth, and transition metals in the shoots and roots of ryegrass were studied with two contaminated garden soils. Phosphates were added in sustainable quantities in order to reduce the environmental availability of carcinogenic metals (e.g., Cd and Pb) and to enhance the bioavailability of alkali and alkaline earth metals as well as micronutrients needed by plants. Addition of Ca(H2PO4)2 was the most convenient way to (i) limit the concentration of Cd and Pb, (ii) keep constant the transfer of macro- and micronutrient from the soil to the ryegrass shoots, (iii) decrease the availability of metals, and (iv) increase the ratio values between potential Lewis acids and Cd or Pb in order to produce biosourced catalysis. For instance, the real phytoavailability was reduced by 27%–57% and 64.2%–94.8% for Cd and Pb, respectively. Interestingly, the real phytoavailability of Zn was the highest in the least contaminated soils. Even if soils were highly contaminated, no visual toxicity symptoms were recorded in the growing ryegrasses. This indicates that ryegrass is suitable for the revegetation of contaminated gardens. To promote the sustainable ryegrass production on contaminated soils for production of new organic fragrance and drugs in green processes according to REACH (Registration, Evaluation, Authorisation, and Restriction of Chemicals) regulation, two processes should be recommended: assisted phytostabilization of the elements, and then assisted phytoextraction by using chelators.

Improved Carotenoid Processing with Sustainable Solvents Utilizing Z-Isomerization-Induced Alteration in Physicochemical Properties: A Review and Future Directions

Carotenoids—natural fat-soluble pigments—have attracted considerable attention because of their potential to prevent of various diseases, such as cancer and arteriosclerosis, and their strong antioxidant capacity. They have many geometric isomers due to the presence of numerous conjugated double bonds in the molecule. However, in plants, most carotenoids are present in the all-E-configuration. (all-E)-Carotenoids are characterized by high crystallinity as well as low solubility in safe and sustainable solvents, such as ethanol and supercritical CO₂ (SC-CO₂). Thus, these properties result in the decreased efficiency of carotenoid processing, such as extraction and emulsification, using such sustainable solvents. On the other hand, Z-isomerization of carotenoids induces alteration in physicochemical properties, i.e., the solubility of carotenoids dramatically improves and they change from a “crystalline state” to an “oily (amorphous) state”. For example, the solubility in ethanol of lycopene Z-isomers is more than 4000 times higher than the all-E-isomer. Recently, improvement of carotenoid processing efficiency utilizing these changes has attracted attention. Namely, it is possible to markedly improve carotenoid processing using safe and sustainable solvents, which had previously been difficult to put into practical use due to the low efficiency. The objective of this paper is to review the effect of Z-isomerization on the physicochemical properties of carotenoids and its application to carotenoid processing, such as extraction, micronization, and emulsification, using sustainable solvents. Moreover, aspects of Z-isomerization methods for carotenoids and functional difference, such as bioavailability and antioxidant capacity, between isomers are also included in this review.

The potential of ryegrass (Lolium perenne L.) to clean up multi-contaminated soils from labile and phytoavailable potentially toxic elements to contribute into a circular economy

Aided phytoremediation was studied for 48 weeks with the aim of reducing extractable and phytoavailable toxic elements and producing potential marketable biomass. In this sense, biomass of ryegrass was produced under greenhouse on two contaminated garden soils that have been amended with two successive additions of phosphates. After the first addition of phosphates, seeds of ryegrass were sown and shoots were harvested twice. A second seedling was performed after carefully mixing the roots from the first production (used as compost), soils and phosphates. Forty-eight weeks after starting the experiments, the concentrations of Cd, Pb, Zn, Cu, Fe, and Mn extracted using the rhizosphere-based method were generally lower than those measured before the addition of phosphates and cultivation (except for Pb and Fe in the most contaminated soil). The concentrations of metals in the shoots of ryegrass from the second production were lower than those from the first (except for Al). The best results were obtained with phosphates and were the most relevant in the lowest contaminated soil, demonstrating that the available metal concentrations have to be taken into account in the management of contaminated soils. In view of the concentration of metals defined as carcinogens, mutagens, and reprotoxics (e.g., Cd, Pb) and those capable to be transformed into Lewis acids (e.g., Zn, Fe), the utilization of ryegrass in the revegetation of contaminated soils and in risk management may be a new production of marketable biomass. The development of phytomanagement in combination with this type of biomass coincided with the view that contaminated soils can still represent a valuable resource that should be used sustainably.

Modification of Montmorillonite with Polyethylene Oxide and Its Use as Support for Pd0 Nanoparticle Catalysts

In this study, montmorillonite (MMT) was modified by intercalating polyethylene oxide (PEO) macromolecules between the interlayer spaces in an MMT-water suspension system. X-ray diffraction results revealed that the galleries of MMT were expanded significantly after intercalation of different loading of PEO. MMT/PEO 80/20 composite was chosen as the support platform for immobilization of Pd species in preparing novel heterogeneous catalysts. After immobilization of Pd species, the interlayer spacing of MMT/PEO (80/20) (1.52 nm) was further increased to 1.72 nm (Pd²⁺@MMT/PEO) and 1.73 nm (Pd⁰@MMT/PEO), confirming the well-immobilization of the Pd species in the interlayer spaces of PEO-modified MMT. High-resolution transmission electron microscopy (HR-TEM) observation results confirmed that Pd nanoparticles were confined inside the interlayer space of MMT and/or dispersed well on the outer surface of MMT. The conversion of Pd²⁺ to Pd⁰ species was evidenced by binding energy characterization with X-ray photo electron spectroscopy (XPS). The microstructure variation caused by the Pd immobilization was sensitively detected by positron annihilation lifetime spectroscopy (PALS) studies. The prepared Pd⁰@MMT/PEO (0.2/80/20) catalytic composite exhibits good thermal stability up to around 200 °C, and it showed high activities for Heck reactions between aryl iodides and butyl acrylates and could be recycled for five times. The correlations between the microstructure and properties of the Pd@MMT/PEO catalytic composites were discussed.

Montmorillonite K10-Catalyzed Solvent-Free Conversion of Furfural into Cyclopentenones

A simple and eco-friendly montmorillonite K10 (MK10)-catalyzed method for the synthesis of cyclopentenone derivatives from biomass-produced furfural has been developed. The versatility of this protocol is that the reactions were performed under solvent-free conditions and in a short reaction time under heterogeneous catalysis. Montmorillonite K10 is mostly explored as a heterogeneous catalyst since it is inexpensive and environmentally friendly.

A Highly Effective Route to Si-O-Si Moieties through O-Silylation of Silanols and Polyhedral Oligomeric Silsesquioxane Silanols with Disilazanes

A simple and highly practical catalyst-free O-silylation of silanols with commercially available disilazanes has been developed under mild conditions. In the case of polyhedral oligomeric silsesquioxane (POSS) silanols and some other silanols, it was necessary to use catalytic amounts of inexpensive Bi(OTf)₃ as additional catalyst. This efficient chlorine-free protocol involves the synthesis of a wide range of important organosilicon derivatives such as unsymmetrical disiloxanes and functionalized silsesquioxanes.

Organic Synthesis Using Environmentally Benign Acid Catalysis

Recent advances in the application of environmentally benign acid catalysts in organic synthesis are reviewed. The work includes three main parts; (i) description of environmentally benign acid catalysts, (ii) synthesis with heterogeneous and (iii) homogeneous catalysts. The first part provides a brief overview of acid catalysts, both solid acids (metal oxides, zeolites, clays, ion-exchange resins, metal-organic framework based catalysts) and those that are soluble in green solvents (water, alcohols) and at the same time could be regenerated after reactions (metal triflates, heteropoly acids, acidic organocatalysts etc.). The synthesis sections review a broad array of the most common and practical reactions such as Friedel-Crafts and related reactions (acylation, alkylations, hydroxyalkylations, halogenations, nitrations etc.), multicomponent reactions, rearrangements and ring transformations (cyclizations, ring opening). Both the heterogeneous and homogeneous catalytic synthesis parts include an overview of asymmetric acid catalysis with chiral Lewis and Brønsted acids. Although a broad array of catalytic processes are discussed, emphasis is placed on applications with commercially available catalysts as well as those of sustainable nature; thus individual examples are critically reviewed regarding their contribution to sustainable synthesis.