Montmorillonite-catalysed coupling reactions: a green overview

Coupling reactions are widely significant in organic synthesis, and within the green chemistry perspective the utilization of montmorillonite clay as a catalyst offers a viable alternative to traditional coupling protocols. Montmorillonite clay is an effective, eco-friendly, economic and recyclable catalyst, and its heterogeneous nature facilitates easy isolation and reusability. The majority of coupling reactions executed with montmorillonite catalysts offer notable advantages over their homogeneous counterparts, including enhanced efficiency, selectivity, operational simplicity, elimination of toxic solvents and ligands, maintenance of mild reaction conditions, and cost-effectiveness. This review gives a comprehensive summary of the literature to date on coupling reactions catalysed by montmorillonite clay, while also outlining future prospects in this area.

Design, synthesis and antiviral evaluation of novel conjugates of the 1,7,7-trimethylbicyclo[2.2.1]heptane scaffold and saturated N-heterocycles via 1,2,3-triazole linker

A new series of heterocyclic derivatives with a 1,7,7-trimethylbicyclo[2.2.1]heptane fragment was designed, synthesised and biologically evaluated. Synthesis of the target compounds was performed using the Cu(I) catalysed cycloaddition reaction. The key starting substances in the click reaction were an alkyne containing a 1,7,7-trimethylbicyclo[2.2.1]heptane fragment and a series of azides with saturated nitrogen-containing heterocycles. Some of the derivatives were found to exhibit strong antiviral activity against Marburg and Ebola pseudotype viruses. Lysosomal trapping assays revealed the derivatives to possess lysosomotropic properties. The molecular modelling study demonstrated the binding affinity between the compounds investigated and the possible active site to be mainly due to hydrophobic interactions. Thus, combining a natural hydrophobic structural fragment and a lysosome-targetable heterocycle may be an effective strategy for designing antiviral agents.

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.

Montmorillonite Membranes with Tunable Ion Transport by Controlling Interlayer Spacing

Membranes incorporating two-dimensional (2D) materials have shown great potential for water purification and energy storage and conversion applications. Their ordered interlayer galleries can be modified for their tunable chemical and structural properties. Montmorillonite (MMT) is an earth-abundant phyllosilicate mineral that can be exfoliated into 2D flakes and reassembled into membranes. However, the poor water stability and random interlayer spacing of MMT caused by weak interlamellar interactions pose challenges for practical membrane applications. Herein, we demonstrate a facile approach to fabricating 2D MMT membranes with alkanediamines as cross-linkers. The incorporation of diamine molecules of different lengths enables controllable interlayer spacing and strengthens interlamellar connections, leading to tunable ion transport properties and boosted membrane stability in aqueous environments.

Montmorillonite K10 Clay Catalyzed Synthesis of Novel β-Aminocarbonyl Compounds and Their Biological Evaluation

An efficient and reusable green catalyst for the synthesis of β-aminocarbonyl compounds has been developed. In this new and greener approach, β-aminocarbonyl compounds (1a-1r) were obtained by Montmorillonite K10 clay catalyzed reaction of aryl amines, aliphatic/aromatic aldehydes and β-ketoesters. Molecular docking investigations were performed for all compounds (1a-1r) with the proteins PDB ID: 1JIJ and 1KZN for S. aureus and E. coli, respectively. For all compounds good to strong interactions with the active sites were observed. The biological activities of β-aminocarbonyl compounds were further assessed for their antibacterial and antioxidant activities. The results confirmed that β-aminocarbonyl compounds could be further developed into new drugs with potent antibacterial and antioxidant activities.

Interactions between Nanoclay, CTAB and Linear/Star Shaped Polymers

The influence of star-shaped (PAA-SS) and linear polyacrylic acid (PAA) with different molecular weights (high—PAA-HMW and low—PAA-LMW) on the structure of the adsorption layer, adsorption amount, electrokinetic and stabilizing properties of the PAA/CTAB/nanoclay suspensions was studied. The properties of the systems containing one of these polymers, the cationic surfactant—hexadecyltrimethylammonium bromide (CTAB) and the surface-modified nanoclay (N-SM) were analyzed using the following techniques: BET, CHN, FT-IR, ED-XRF, XRD, HRTEM, UV-Vis, tensiometry and zeta potential measurements. It was proved that PAA could be used as an effective stabilizer of N-SM. Moreover, the addition of CTAB caused a significant increase in the stability of the systems but decreased the adsorption of PAA on the N-SM surface and changed the structure of the adsorption layers. The largest stability was observed in the PAA-HMW/CTAB system. The PAA polymers and PAA/CTAB complexes adsorbed, especially on the clay surface, influenced the primary distribution of the layered sheets but kept the same basal d-spacing. The adsorption of PAA and the PAA/CTAB complexes took place mainly at the plate edges and on the contact space between the sheets. The obtained results will be used for the preparation of the PAA/CTAB/nanoclay composite for water purification.

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.

Research Trends on Pillared Interlayered Clays (PILCs) Used as Catalysts in Environmental and Chemical Processes: Bibliometric Analysis

Over the last four decades, a large number of studies have been published on pillared interlayered clays (PILCs) used as adsorbent materials and catalysts or supports for transition metals in heterogeneous catalysis. Particularly, PILCs have been used for water treatment through advanced oxidation processes (AOPs) to remove organic pollutants. They have also been studied in various chemical and environmental processes. Because of the growing interest in PILCs, this article is focused on analyzing scientific publications such as research/review articles and book chapters from the last four decades (from 1980 to 2019) through a bibliometric analysis (BA) to visualize and describe research trends on PILCs. By narrowing the bibliographic search to titles, keywords, and abstracts of publications related to PILCs, using Scopus and Web of Science (WoS) (the two scientific databases), a total of 3425 documents have been retrieved. The bibliometric dataset was analyzed by VantagePoint®. The main research trends identified in the last four decades were the use of PILCs in environmental processes (34.4% of total publications) along with chemical processes (petrochemical reactions 17.5%, SCR NOx 10.8%, and decomposition 8.2%). In environmental processes, PILCs have been used in photo-oxidation (32%), CWPO (21.1%), and heterogeneous catalysis (19.4%). Phenols, dyes, and VOCs have been the main pollutants studied using PILCs as catalysts. Fe, Ti, Zr, Cu, and Co are the most supported active phases in PILCs. Other research trends grouped by characterization techniques, countries, research areas, institutes, scientific journals that have published the most on this topic, number of publications per 5-year period, and most frequently used keywords through the last four decades have been identified. It was determined that the number of publications on PILCs has increased since 1980 and the countries with the highest number of publications are China, Spain, and The United States of America.

Clay-Supported Metal Oxide Nanoparticles in Catalytic Advanced Oxidation Processes: A Review

Advanced oxidation processes (AOPs) utilizing heterogeneous catalysts have attracted great attention in the last decade. The use of solid catalysts, including metal and metal oxide nanoparticle support materials, exhibited better performance compared with the use of homogeneous catalysts, which is mainly related to their stability in hostile environments and recyclability and reusability. Various solid supports have been reported to enhance the performance of metal and metal oxide catalysts for AOPs; undoubtedly, the utilization of clay as a support is the priority under consideration and has received intensive interest. This review provides up-to-date progress on the synthesis, features, and future perspectives of clay-supported metal and metal oxide for AOPs. The methods and characteristics of metal and metal oxide incorporated into the clay structure are strongly influenced by various factors in the synthesis, including the kind of clay mineral. In addition, the benefits of nanomaterials from a green chemistry perspective are key aspects for their further considerations in various applications. Special emphasis is given to the basic schemes for clay modifications and role of clay supports for the enhanced mechanism of AOPs. The scaling-up issue is suggested for being studied to further applications at industrial scale.

Clays and the Origin of Life: The Experiments

There are three groups of scientists dominating the search for the origin of life: the organic chemists (the Soup), the molecular biologists (RNA world), and the inorganic chemists (metabolism and transient-state metal ions), all of which have experimental adjuncts. It is time for Clays and the Origin of Life to have its experimental adjunct. The clay data coming from Mars and carbonaceous chondrites have necessitated a review of the role that clays played in the origin of life on Earth. The data from Mars have suggested that Fe-clays such as nontronite, ferrous saponites, and several other clays were formed on early Mars when it had sufficient water. This raised the question of the possible role that these clays may have played in the origin of life on Mars. This has put clays front and center in the studies on the origin of life not only on Mars but also here on Earth. One of the major questions is: What was the catalytic role of Fe-clays in the origin and development of metabolism here on Earth? First, there is the recent finding of a chiral amino acid (isovaline) that formed on the surface of a clay mineral on several carbonaceous chondrites. This points to the formation of amino acids on the surface of clay minerals on carbonaceous chondrites from simpler molecules, e.g., CO2, NH3, and HCN. Additionally, there is the catalytic role of small organic molecules, such as dicarboxylic acids and amino acids found on carbonaceous chondrites, in the formation of Fe-clays themselves. Amino acids and nucleotides adsorb on clay surfaces on Earth and subsequently polymerize. All of these observations and more must be subjected to strict experimental analysis. This review provides an overview of what has happened and is now happening in the experimental clay world related to the origin of life. The emphasis is on smectite-group clay minerals, such as montmorillonite and nontronite.

Emission enhancement in twisted pyridyl salt using Montmorillonite nanoclay by intercalation and surface-fixation process

Boosting the fluorescence (FL) intensity for a weak/nonemissive molecule is a challenge. This work describes the improvement of FL intensity for a nonemissive conformationally twisted π-conjugated pyridyl salt. This pyridyl...

Supported metals on porous solids as heterogeneous catalysts for the synthesis of propargylamines

This perspective summarizes recent developments in the synthesis of propargylamines using porous solids (zeolites, MOFs and carbon) as supports/catalysts.

Montmorillonite clay-based heterogenous catalyst for the synthesis of nitrogen heterocycle organic moieties: a review

The use of montmorillonite clay as solid catalyst has grabbed much attention in the liquid phase reactions for organic synthesis. In recent years, there has been a lot of interest in organic synthesis using montmorillonite-based composites, especially in the synthesis of heterogeneous nanoparticles. Due to the robust and green nature of montmorillonite-based nanocatalysts, it has been widely used in N-heterocyclic reactions. In this review, we have concentrated on the reports pertaining the use of montmorillonite-based nanocatalyst in the synthesis of N-heterocycles, a category of organic compounds with excellent biological properties. This manuscript is arranged by the types of N-containing heterocycles synthesized using montmorillonite-based composite as catalysts including polycyclic spirooxindoles, heterocyclic propargylamine, indole-based heterocycles, quinoline and its derivatives, six-membered N-heterocyclic-based compounds and five-membered N-heterocyclic-based compounds. Special attention was given to the structural stability under experimental parameters of the montmorillonite-based composite with the incidence of metal leaching and reusability. Finally, along with recent developments, new findings in heterogeneous montmorillonite (Mt)-based catalysis have also been addressed.

Silica gel and microwave-promoted synthesis of dihydropyrrolizines and tetrahydroindolizines from enaminones

A wide range of N-(ethoxycarbonylmethyl)enaminones, prepared by the Eschenmoser sulfide contraction between N-(ethoxycarbonylmethyl)pyrrolidine-2-thione and various bromomethyl aryl and heteroaryl ketones, underwent cyclization in the presence of silica gel to give ethyl 6-(hetero)aryl-2,3-dihydro-1H-pyrrolizine-5-carboxylates within minutes upon microwave heating in xylene at 150 °C. Instead of functioning as a nucleophile, the enaminone acted as an electrophile at its carbonyl group during the cyclization. Yields of the bicyclic products were generally above 75%. The analogous microwave-assisted reaction to produce ethyl 2-aryl-5,6,7,8-tetrahydroindolizine-3-carboxylates from (E)-ethyl 2-[2-(2-oxo-2-arylethylidene)piperidin-1-yl]acetates failed in nonpolar solvents, but occurred in ethanol at lower temperature and microwave power, although requiring much longer time. A possible mechanism for the cyclization is presented, and further functionalization of the newly created pyrrole ring in the dihydropyrrolizine core is described.

Recent Progress in Low-Cost Catalysts for Pyrolysis of Plastic Waste to Fuels

The catalytic and thermal decomposition of plastic waste to fuels over low-cost catalysts like zeolite, clay, and bimetallic material is highlighted. In this paper, several relevant studies are examined, specifically the effects of each type of catalyst used on the characteristics and product distribution of the produced products. The type of catalyst plays an important role in the decomposition of plastic waste and the characteristics of the oil yields and quality. In addition, the quality and yield of the oil products depend on several factors such as (i) the operating temperature, (ii) the ratio of plastic waste and catalyst, and (iii) the type of reactor. The development of low-cost catalysts is revisited for designing better and effective materials for plastic solid waste (PSW) conversion to oil/bio-oil products.

Ruthenium Nanoparticles Intercalated in Montmorillonite (nano-Ru@MMT) Is Highly Efficient Catalyst for the Selective Hydrogenation of 2-Furaldehyde in Benign Aqueous Medium

Chemoselective hydrogenation of 2-furaldehyde to furfuryl alcohol using green solvents is an important research area to get eco-friendly fuels and fine chemicals. Herein, we report ruthenium nanoparticles (~1.8 nm) intercalated in montmorillonite as an efficient catalytic system, which can selectively hydrogenate 2-furaldehyde in a benign aqueous medium. The complete conversion was observed at 40 °C with 1 MPa H2, the selectivity of furfuryl alcohol being >99%, and turnover number 1165. After a catalytic run, the montmorillonite-supported ruthenium nanoparticles can be recycled and reused without losing their activity and selectivity.

A Sustainable Palladium-Intercalated Montmorillonite Clay Catalytic System for Imine Hydrogenation under Mild Conditions

A series of palladium nanoparticles (Pd NPs) intercalated montmorillonite clay catalysts is reported for hydrogenation of 3-diphenyl prop-2-en-1-imine under mild reaction conditions. Pd/clay catalyst was prepared by a simple wet-impregnation method, and the physicochemical properties were characterized extensively by various techniques including N₂ adsorption, XRD, TEM, XPS and TPR etc., which showed the intercalation of active Pd NPs between the clay layers. The effect of reaction conditions such as catalyst loading, reaction time, temperature and H₂ pressure is explored, and thereby a plausible mechanism is proposed. The optimum amount of 6 wt % Pd/clay catalyst showed significant catalytic activity to yield 3-phenyl propyl aniline with 100 % conversion and selectivity under 5 bar pressure and a shorter reaction period of 3.5 h at 100 °C. The developed catalytic system unveiled excellent reusability over five cycles and hence paved the way for industrial applications.

Pristine and modified chitosan as solid catalysts for catalysis and biodiesel production: A minireview

Chitosan is one of the readily available polymers with relatively high abundance, biodegradable and sustainable materials with divergent functional groups that are employed in broad range of applications. Chitosan is widely used in many fields like adsorption, drug carrier for therapeutic activity, environmental remediation, drug formulation and among others. One of the unique features of chitosan is that it can be transformed to other forms like beads, films, flakes, sponges and fibres depending upon the applications. This review is aimed at showing the potential applications of chitosan and its modified solids in organic transformations. The number of existing articles is organized based on the nature of materials and subsequently with the types of reactions. After a brief description on the structural features of chitosan, properties, characterization methods including various analytical/microscopic techniques and some of the best practices to be followed in catalysis are also discussed. The next section of this review describes the catalytic activity of native chitosan without any modifications while the subsequent sections provide the catalytic activity of chitosan derivatives, chitosan covalently modified with metal complexes/salts through linkers and chitosan as support for metal nanoparticles (NPs). These sections discuss number of organic reactions that include Knoevenagel condensation, oxidation, reduction, heterocycles synthesis, cross-coupling reactions and pollutant degradation among others. A separate section provides the catalytic applications of chitosan and its modified forms for the production of fatty acid methyl esters (FAME) through esterification/transesterification reactions. The final section summarizes our views on the future directions of this field in the coming years.

Synthesis and search for 3β,3'β-disteryl ethers after high-temperature treatment of sterol-rich samples

It has been proven that at increased temperature, sterols can undergo various chemical reactions e.g., oxidation, dehydrogenation, dehydration and polymerisation. The objectives of this study are to prove the existence of dimers and to quantitatively analyse the dimers (3β,3'β-disteryl ethers). Sterol-rich samples were heated at 180 °C, 200 °C and 220 °C for 1 to 5 h. Quantitative analyses of the 3β,3'β-disteryl ethers were conducted using liquid extraction, solid-phase extraction and gas chromatography coupled with mass spectrometry. Additionally, for the analyses, suitable standards were synthetized from native sterols. To identify the mechanism of 3β,3'β-disteryl ether formation at high temperatures, an attempt was made to use the proposed synthesis method. Additionally, due to the association of sterols and sterol derivatives with atherosclerosis, preliminary studies with synthetized 3β,3'β-disteryl ethers on endothelial cells were conducted.

One-pot construction of carbohydrate scaffolds mediated by metal catalysts

Owing to the environmental concern worldwide and also due to cost, time and labour issues, use of one-pot reactions [domino/cascade/tandem/multi-component (MC) or sequential] has gained much attention among the scientific and industrial communities for the generation of compound libraries having different scaffolds. Inclusion of sugars in such compounds is expected to increase the pharmacological efficacy because of the possibility of better interactions with the receptors of such unnatural glycoconjugates. In many of the one-pot transformations, the presence of a metal salt/complex can improve the reaction/change the course of reaction with remarkable increase in chemo-/regio-/stereo-selectivity. On the other hand because of the importance of natural polymeric glycoconjugates in life processes, the development and efficient synthesis of related oligosaccharides, particularly utilising one-pot MC-glycosylation techniques are necessary. The present review is an endeavour to discuss one-pot transformations involving carbohydrates catalysed by a metal salt/complex.

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.

Chitosan modified Ti-PILC supported PdOx catalysts for coupling reactions of aryl halides with terminal alkynes

Biopolymer of chitosan (CS) and titanium pillared clays (Ti-PILCs) have been combined in a hybrid as advanced supports for immobilization of PdOx=0,1 species to prepare novel PdOx=0,1@Ti-PILC/CS nano-composite catalysts. The Ti-PILC materials showed high specific surface areas and abundant meso-porous structure with many irregular pore channels caused by collapses of layered structure of clay during Ti pillaring process. Both CS chains and sub-nano sized PdOx particles were successfully incorporated into the pore channels of Ti-PILC, resulting in a decrease in both the specific surface areas and uniform distribution of pore size. Besides conventional methods characterizations, the strong interactions between PdOx species and Ti-PILC/CS support were further evidenced with positron annihilation lifetime spectroscopy studies. The resultant PdOx@Ti-PILC/CS catalyst was highly active for the coupling reactions of aryl halides with phenyl acetylenes. It was recyclable and gave excellent yield up to 13 runs with low leaching of Pd species.

Organically Modified Montmorillonite as Nanoreactor to Improve the Grafting Degree of Maleic Anhydride onto Polypropylene

The application of an organically modified montmorillonite nanoreactor in the reactive extrusion process of the free radical grafting of maleic anhydride onto polypropylene (PP) can increase the MAH grafting degree on the PP. The mechanism of grafting was studied by using transmission electron microscopy and high temperature gel permeation chromatography. It was found that both the strong interactions between MAH and MMT surface and the encapsulation effect of active species confined in o-MMT improved the grafting degree.

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.

Intermolecular Carbonyl-olefin Metathesis with Vinyl Ethers Catalyzed by Homogeneous and Solid Acids in Flow

The carbonyl-olefin metathesis reaction has experienced significant advances in the last seven years with new catalysts and reaction protocols. However, most of these procedures involve soluble catalysts for intramolecular reactions in batch. Herein, we show that recoverable, inexpensive, easy to handle, non-toxic, and widely available simple solid acids, such as the aluminosilicate montmorillonite, can catalyze the intermolecular carbonyl-olefin metathesis of aromatic ketones and aldehydes with vinyl ethers in-flow, to give alkenes with complete trans stereoselectivity on multi-gram scale and high yields. Experimental and computational data support a mechanism based on a carbocation-induced Grob fragmentation. These results open the way for the industrial implementation of carbonyl-olefin metathesis over solid catalysts in continuous mode, which is still the origin and main application of the parent alkene-alkene cross-metathesis.

Distinctive stability of a free-standing monolayer clay mineral nanosheet via transmission electron microscopy

The distinctive stability of the monolayer clay mineral demonstrated by electron diffraction.

Synthesis, photophysical and electrochemical properties of donor–acceptor type hydrazinyl thiazolyl coumarins

A practical green MCR strategy has been developed for the synthesis of D–A type hydrazinyl thiazolyl coumarins catalyzed by an environmentally compatible MMT K10 clay in water at RT.