Solid acids: Green alternatives for acid catalysis

Solving the Water Hypersensitive Challenge of Sulfated Solid Superacid in Acid-Catalyzed Reactions

In the past decades, water tolerance has always been the long-pending key issue of sulfated solid superacids (SO₄^2-/M _xO _y) toward industrial applications. Herein, we report a strategy for the facile coating of a thick tunable hydrophobic layer over SO₄^2-/M _xO _y, which can significantly improve water tolerance, with negligible inhibition on the catalytic performance of SO₄^2-/M _xO _y. Even after being directly immersed in water, the hydrophobic SO₄^2-/M _xO _y can still maintain above 90% of original catalytic activity, whereas pristine SO₄^2-/M _xO _y and control samples are almost completely deactivated. This strategy opens a new route to enhance the water tolerance of sulfated solid superacids.

Effect of Dilute Acid and Alkali Pretreatments on the Catalytic Performance of Bamboo-Derived Carbonaceous Magnetic Solid Acid

Lignocellulose is a widely used renewable energy source on the Earth that is rich in carbon skeletons. The catalytic hydrolysis of lignocellulose over magnetic solid acid is an efficient pathway for the conversion of biomass into fuels and chemicals. In this study, a bamboo-derived carbonaceous magnetic solid acid catalyst was synthesized by FeCl3 impregnation, followed by carbonization and –SO3H group functionalization. The prepared catalyst was further subjected as the solid acid catalyst for the catalytic conversion of corncob polysaccharides into reducing sugars. The results showed that the as-prepared magnetic solid acid contained –SO3H, –COOH, and polycyclic aromatic, and presented good catalytic performance for the hydrolysis of corncob in the aqueous phase. The concentration of H+ was in the range of 0.6487 to 2.3204 mmol/g. Dilute acid and alkali pretreatments of raw material can greatly improve the catalytic activity of bamboo-derived carbonaceous magnetic solid acid. Using the catalyst prepared by 0.25% H2SO4-pretreated bamboo, 6417.5 mg/L of reducing sugars corresponding to 37.17% carbohydrates conversion could be obtained under the reaction conditions of 120 °C for 30 min.

Permanent porous hydrogen-bonded frameworks with two types of Brønsted acid sites for heterogeneous asymmetric catalysis

The search for porous materials with strong Brønsted acid sites for challenging reactions has long been of significant interest, but it remains a formidable synthetic challenge. Here we demonstrate a cage extension strategy to construct chiral permanent porous hydrogen-bonded frameworks with strong Brønsted acid groups for heterogeneous asymmetric catalysis. We report the synthesis of two octahedral coordination cages using enantiopure 4,4’,6,6’-tetra(benzoate) ligand of 1,1’-spirobiindane-7,7’-phosphoric acid and Ni₄/Co₄-p-tert-butylsulfonylcalix[4]arene clusters. Intercage hydrogen-bonds and hydrophobic interactions between tert-butyl groups direct the hierarchical assembly of the cages into a permanent porous material. The chiral phosphoric acid-containing frameworks can be high efficient and recyclable heterogeneous Brønsted acid catalysts for asymmetric [3+2] coupling of indoles with quinone monoimine and Friedel-Crafts alkylations of indole with aryl aldimines. The afforded enantioselectivities (up to 99.9% ee) surpass those of the homogeneous counterparts and compare favorably with those of the most enantioselective homogeneous phosphoric acid catalysts reported to date.

The search for porous materials with strong Brønsted acid sites for challenging chemical reactions has been of significant interest, but remains challenging. Here the authors report a cage extension strategy to construct chiral permanent porous hydrogen-bonded frameworks with strong Brønsted acid groups for heterogeneous asymmetric catalysis.

Low-Temperature Continuous-Flow Dehydration of Xylose Over Water-Tolerant Niobia-Titania Heterogeneous Catalysts

The sustainable conversion of vegetable biomass-derived feeds to useful chemicals requires innovative routes meeting environmental and economical criteria. The approach herein pursued is the synthesis of water-tolerant, unconventional solid acid monolithic catalysts based on a mixed niobia-titania skeleton building up a hierarchical open-cell network of meso- and macropores, and tailored for use under continuous-flow conditions. The materials were characterized by spectroscopic, microscopy, and diffraction techniques, showing a reproducible isotropic structure and an increasing Lewis/Brønsted acid sites ratio with increasing Nb content. The catalytic dehydration reaction of xylose to furfural was investigated as a representative application. The efficiency of the catalyst was found to be dramatically affected by the niobia content in the titania lattice. The presence of as low as 2 wt % niobium resulted in the highest furfural yield at 140 °C under continuous-flow conditions, by using H₂ O/γ-valerolactone as a safe monophasic solvent system. The interception of a transient 2,5-anhydroxylose species suggested the dehydration process occurs via a cyclic intermediates mechanism. The catalytic activity and the formation of the anhydro intermediate were related to the Lewis acid sites (LAS)/Brønsted acid sites (BAS) ratio and indicated a significant contribution of xylose-xylulose isomerization. No significant catalyst deactivation was observed over 4 days usage.

Active Sites in Heterogeneous Catalytic Reaction on Metal and Metal Oxide: Theory and Practice

Active sites play an essential role in heterogeneous catalysis and largely determine the reaction properties. Yet identification and study of the active sites remain challenging owing to their dynamic behaviors during catalysis process and issues with current characterization techniques. This article provides a short review of research progresses in active sites of metal and metal oxide catalysts, which covers the past achievements, current research status, and perspectives in this research field. In particular, the concepts and theories of active sites are introduced. Major experimental and computational approaches that are used in active site study are summarized, with their applications and limitations being discussed. An outlook of future research direction in both experimental and computational catalysis research is provided.

An efficient ordered mesoporous molybdate-zirconium oxophosphate solid acid catalyst with homogeneously dispersed active sites: Synthesis, characterization and application

Ordered mesoporous molybdate-zirconium oxophosphate (M-ZrPMo) solid acid catalysts with controllable molybdenum contents (0-20%) are designed and synthesized through a one-pot evaporation-induced self-assembly strategy. Afterwards, ordered mesostructure and molybdenum species in the materials are systematically researched by a variety of means. The results show that M-ZrPMo has highly ordered mesoporous structure with large specific surface area (∼200 m²·g^-1), big pore volume (∼0.30 cm³·g^-1) and pore size (∼6.5 nm). Additionally, ordered mesoporous structure of M-ZrPMo can be efficiently preserved even treated at 700 °C, presenting an outstanding thermal stability. Meanwhile, the molybdenum species are introduced as designed and homogeneously dispersed in mesoporous framework even at molybdenum content up to 20%. More importantly, the Brønsted and Lewis acidic properties of these materials are successfully enhanced with the introduction of molybdenum species. Meantime, the M-ZrPMo is employed as a solid acid catalyst for alkylation of aromatic compounds and esterification of levulinic acid with 1-butanol. The effect of molybdenum contents and calcination temperature on catalytic performance is thoroughly discussed. The excellent activity and reusability suggested that M-ZrPMo is a promising solid acid catalyst.

Heterogeneous acidic catalysts for the tetrahydropyranylation of alcohols and phenols in green ethereal solvents

The application of heterogeneous catalysis and green solvents to the set up of widely employed reactions is a challenge in contemporary organic chemistry. We applied such an approach to the synthesis and further conversion of tetrahydropyranyl ethers, an important class of compounds widely employed in multistep syntheses. Several alcohols and phenols were almost quantitatively converted into the corresponding tetrahydropyranyl ethers in cyclopentyl methyl ether or 2-methyltetrahydrofuran employing NH₄HSO₄ supported on SiO₂ as a recyclable acidic catalyst. Easy work up of the reaction mixtures and the versatility of the solvents allowed further conversion of the reaction products under one-pot reaction conditions.

Versatile design and synthesis of mesoporous sulfonic acid catalysts

Mesoporous sulfonic acid catalysts (MSAC) are widely used in acid-catalyzed reactions, including biomass conversions with plenty of polar solvents and precursors. The catalytic efficiency of MSAC is greatly affected by the microenvironment around the sulfonic acid sites. In this review, the progress on modification of microenvironment of MSAC is reviewed over the past decade. Hydrophobic modification allows MSAC prevent the adhesion of water molecules onto sulfonic acid sites, to abate the risk of reduced acid strength and catalytic efficiency. In comparison, hydrophilic properties can bring positive effect on acid-catalyzed reactions with the aid of hydrophilic interaction between polar functional groups on MSAC and hydrophilic groups of specific substrates. Amphiphilic MSAC with tunable wettability for specific substrates and solvents tend to improve the efficiency in certain reactions with mixed solvents or reactants of different polarity, especially for biphasic systems of immiscible liquids. Furthermore, much attention has been attracted on modification of surface to simulate the microenvironment of homogeneous solvents and enzyme biocatalysts in recent research. New trends of this field are also highlighted.

Acid-Catalyzed Conversion of Carbohydrates into Value-Added Small Molecules in Aqueous Media and Ionic Liquids

Biomass is the only realistic major alternative source (to crude oil) of hydrocarbon substrates for the commercial synthesis of bulk and fine chemicals. Within biomass, terrestrial sources are the most accessible, and therein lignocellulosic materials are most abundant. Although lignin shows promise for the delivery of certain types of organic molecules, cellulose is a biopolymer with significant potential for conversion into high-volume and high-value chemicals. This review covers the acid-catalyzed conversion of lower value (poly)carbohydrates into valorized organic building-block chemicals (platform molecules). It focuses on those conversions performed in aqueous media or ionic liquids to provide the reader with a perspective on what can be considered a best case scenario, that is, that the overall process is as sustainable as possible.

Novel nano-size and crab-like biological-based glycoluril with sulfonic acid tags as a reusable catalyst: its application to the synthesis of new mono- and bis-spiropyrans and theirin vitrobiological studies

Herein novel crab-like biological-based glycoluril with sulfonic acid tags was prepared in nano-size form and fully characterizedviaseveral techniques.

Microwave irradiation: a green approach for the synthesis of functionalizedN-methyl-1,4-dihydropyridines

An eco-friendly and cost-effective, microwave-assisted green approach has been developed for the synthesis of diverse functionalized N-methyl-1,4-dihydropyridines (1,4-DHPs). This pseudo three-component reaction was carried out between two equivalents of (E)-N-methyl-1-(methylthio)-2-nitroethenamine (NMSM) and one equivalent of aromatic aldehydes under microwave irradiation at 100 °C without catalyst and solvent. Short reaction times, avoidance of toxic solvents or expensive, metallic and corrosive catalysts and no need for column chromatographic purification are among the valuable features of the presented method. Moreover, the “greenness” of the method was evaluated within the ambits of the defined green metrics such as atom economy, carbon efficiency, E-factor, reaction mass efficiency, overall efficiency, process mass intensity and solvent intensity and the method exhibited a good to excellent score.

Microwave-assisted green synthesis of N-methyl-1,4-dihydropyridines under eco-friendly conditions.

Green chemistry and sustainable development: approaches to chemical footprint analysis

The methods to monitor the distribution of chemicals in the biosphere and to estimate the impact of chemicals on the biosphere are necessary to reach Sustainable Development Goals (SDGs). The paper presents the examples of methods to measure the concentration of heavy metals (including rare earth elements) and to rank them by the level of hazard to human health on different scales. The megacity scale presents the investigation of the impact of heavy metals on the small water bodies using water contamination index (WCI); and the investigation of snow contamination to estimate the level of short-term seasonal emission of heavy metals and rare earth elements. The 2nd part of the paper presents approaches to mitigate the exposure to mercury on the regional scale: the estimation of the current concentrations of mercury in atmospheric air, natural soils, and fresh waters using UNEP/SETAC USEtox model, as well as the estimations of the variations in the concentrations of mercury for the year 2045 in the federal districts of the Russian Federation, based on representative concentration pathways (RCPs) scenario and Minamata Convention scenario.

Superior acidic catalytic activity and stability of Fe-doped HTaWO6 nanotubes

Fe-doped HTaWO₆ (H_1-3xFe_xTaWO₆, x = 0.23) nanotubes as highly active solid acid catalysts were prepared via an exfoliation-scrolling-exchange process. The specific surface area and pore volume of undoped nanotubes (20.8 m² g^-1, 0.057 cm³ g^-1) were remarkably enhanced through Fe³⁺ ion-exchange (>100 m² g^-1, 0.547 cm³ g^-1). Doping Fe ions into the nanotubes endowed them with improved thermal stability due to the stronger interaction between the intercalated Fe³⁺ ions and the host layers. This interaction also facilitated the preservation of effective Brønsted acid sites and the generation of new acid sites. The integration of these functional roles resulted in Fe-doped nanotubes with high acidic catalytic activities in the Friedel-Crafts alkylation of anisole and the esterification of acetic acid. Facile accessibility to active sites, generation of effective Brønsted acid sites, high stability of the tubular structure and strong acid sites were found to synergistically contribute to the excellent acidic catalytic efficiency. Additionally, the activity of cycled nanocatalysts can be easily recovered through annealing treatment.

Synthesis of tricyanomethanesulfonic acid as a novel nanostructured and recyclable solid acid: application at the synthesis of biological henna-based chromenes

A novel and unique nanostructured solid acid, namely tricyanomethanesulfonic acid (TCMS), was designed, synthesized, and attained as an ecofriendly, reusable catalyst for the one-pot three-component synthesis of 2-amino-3-cyano-4-aryl-5,10-dioxo-5,10-dihydro-4H-benzo[g]chromene derivatives from the reaction between aromatic aldehydes, 2-hydroxynaphthalene-1,4-dione (henna), and malononitrile under solvent-free conditions at room temperature with good to excellent yields and a short reaction time. The TCMS catalyst was prepared and fully characterized by FTIR, 1H NMR, 13C NMR, mass, X-ray diffraction patterns (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal gravimetric (TG), and derivative thermal gravimetric (DTG) analyses. The nanostructured solid acid catalyst was simply recycled at least eight times without any loss of catalytic activity. This process is improved as a safe and appropriate procedure for the synthesis of 2-amino-3-cyano-4-aryl-5,10-dioxo-5,10-dihydro-4H-benzo[g]chromenes using an environmentally friendly and green recyclable catalyst. Thus, TCMS is a good candidate for the synthesis of new molecular organic frameworks and gelators.

Sulfonic acid functionalized silica nanoparticles as catalysts for the esterification of linoleic acid

Sulfonic acid functionalized SiO₂-nanoparticles showed excellent performance in linoleic acid esterification: 100% conversion, 2 h reaction time, TOF = 53–498 h⁻¹.

Direct Synthesis of Nitriles from Aldehydes and Hydroxylamine Hydrochloride Catalyzed by a HAP@AEPH2-SO3H Nanocatalyst

We describe an efficient method for the direct preparation of nitriles from aldehydes and hydroxylamine hydrochloride catalyzed by sulfonated nanohydroxyapatite functionalized by 2-aminoethyl dihydrogen phosphate (HAP@AEPH2-SO3H) as an eco-friendly and recyclable solid acid nanocatalyst. In this protocol the use of a solid acid nanocatalyst provides a green, useful, and rapid method for the preparation of nitriles in excellent yields. In addition, the notable feature of this methodolgy is that the synthesized nanocatalyst can be recovered and reused five times without any noticeable loss of efficiency.

Synthesis of the first magnetic nanoparticles with a thiourea dioxide-based sulfonic acid tag: application in the one-pot synthesis of 1,1,3-tri(1H-indol-3-yl) alkanes under mild and green conditions

Synthesis of the first thiourea dioxide-based silica-coated Fe₃O₄magnetic nanoparticles as an exquisite, extraordinary and effective catalyst for the synthesis of 1,1,3-tri(1H-indol-3-yl) alkane derivatives.

Nano-ZrO2 sulfuric acid: a heterogeneous solid acid nano catalyst for Biginelli reaction under solvent free conditions

Nano-ZrO₂ sulfuric acid [n-ZrO₂–SO₃H (n-ZrSA)] has been synthesized from the reaction of nano-ZrO₂ with chlorosulfonic acid as sulfonating agent.