Recyclable Supported Carbene Catalysts for High-Yielding Self-Condensation of Furaldehydes into C10 and C12 Furoins

Catalytic Upgrading of Acetaldehyde to Acetoin Using a Supported N-Heterocyclic Carbene Catalyst

We report the catalytic synthesis of 3-hydroxy-2-butanon (acetoin) from acetaldehyde as a key step in the synthesis of C4-molecules from ethanol. Facile C-C bond formation at the α-carbon of the C2 building block is achieved using an N-heterocyclic carbene (NHC) catalyst. The immobilization of the catalyst on a Merrifield's peptide resin and its spectroscopic characterisation using solid-state Nuclear Magnetic Resonance (NMR) is described herein. The immobilization of the NHC catalyst allows for process intensification steps and the reported catalytic system was subjected to batch recycling as well as continuous flow experiments. The robustness of the catalytic system was shown over a maximum of 10 h time-on-stream. Overall, high selectivity S>90 % was observed. The observed deactivation of the catalyst with increasing time-on-stream is explained by ex-situ 1H solution-state, as well as 13C and 15N solid-state NMR spectra allowing us to develop a deeper understanding of the underlying decomposition mechanism of the catalyst.

Production of Renewable Hydrocarbon Biofuels with Lignocellulose and Its Derivatives over Heterogeneous Catalysts

In recent years, the issues of global warming and CO2 emission reduction have garnered increasing global attention. In the 21st Conference of the Parties (convened in Paris in 2015), 179 nations and the European Union signed a pivotal agreement to limit the global temperature increase of this century to well below 2 K above preindustrial levels. To fulfill this objective, extensive research has been conducted to use renewable energy sources as potential replacements for traditional fossil fuels. Among them, the production of hydrocarbon transportation fuels from CO2-neutral and renewable biomass has proven to be a particularly promising solution due to its compatibility with existing infrastructure. This review systematically summarizes research progress in the synthesis of liquid hydrocarbon biofuels from lignocellulose during the past two decades. Based on the chemical structure (including n-paraffins, iso-paraffins, aromatics, and cycloalkanes) of hydrocarbon transportation fuels, the synthesis pathways of these biofuels are discussed in four separate sections. Furthermore, this review proposes three guiding principles for the design of practical hydrocarbon biofuels, providing insights into future directions for the development of viable biomass-derived liquid fuels.

Silane-Functionalized Metal-Organic Frameworks for Stimuli-Responsive Drug Delivery Systems: A New Universal Strategy

A new universal strategy for silane functionalization of metal-organic frameworks (MOFs) was developed. It was demonstrated that silanes were coupled both with terminal hydroxyl (OH) groups and with bridging OH groups of metal-oxo clusters of MOFs through condensation reactions between the silanols of hydrolyzed silanes and the terminal/bridging OH groups to form metal-O-Si bonds. A wide variety of functionalization of MOFs with conventional silanes can be realized by combining synthesis reactions in the solution phase and chemical modifications on the surface. Multivalent supramolecular nanovalves based on the host-guest chemistry of cyclodextrin polymer (CDP) and benzimidazole stalks silanized on the nanoscale MOF (NMOF) surface were successfully constructed. The CDP-valved NMOFs showed the excellent performance of low pH- and α-amylase-responsive controlled drug release. In vitro and in vivo results demonstrated that the CDP-valved NMOFs had a significant inhibitory effect on tumor growth and almost no damage/toxicity to normal tissues. The silanization strategy is universal and opens up a new way for the functionalization of MOFs, which are endowed with a wide variety of applications spanning gas storage, chemical sensing, adsorption and separation, heterogeneous catalysis, and drug delivery.

Influence of Pore Size in Benzoin Condensation of Furfural Using Heterogenized Benzimidazole Organocatalysts

A designed N-heterocyclic carbene (NHC) catalyst was covalently anchored on a range of mesoporous and hierarchical supports, to study the influence of pore size in the benzoin condensation of furfural. The structural and spectroscopic characteristics of the anchored catalysts were investigated, also with the help of molecular dynamics simulations, in order to rationalize the degree of stability and recyclability of the heterogenized organocatalysts. Quantitative yields (99 %) and complete recyclability were maintained after several cycles, vindicating the design rationale.

Cyclodextrin polymer-valved MoS2-embedded mesoporous silica nanopesticides toward hierarchical targets via multidimensional stimuli of biological and natural environments

Targeted delivery of pesticides towards pests and pathogens can significantly improve the bioavailability and efficacy of pesticides and minimize the impact on the environment. Cyclodextrin polymer (CDP)-valved, benzimidazole functionalized, MoS₂-embedded mesoporous silica (MoS₂@MSN@CDP) nanopesticides were constructed toward hierarchical biological targets of pests, pathogens, and foliage. The splash and bounce of the aqueous droplets containing MoS₂@MSN@CDP nanoparticles in the presence of Aersosol OT on superhydrophobic surfaces were well inhibited available for excellent wetting to prevent pesticides from losing to the environment. The multivalent supramolecular nanovalves between CDP and the functionalized benzimidazole moieties could be activated for the controlled release of pesticides in the cases of low pH and α-amylase. It is the first time to report the foliage-triggered controlled release of pesticides, owing to the competitive binding of epicuticular wax components to CDP. Furthermore, thermogenic MoS₂ cores triggered the controlled release of pesticides under irradiation of near infrared light. The fungicidal efficacies of the stimuli-responsive nanopesticides against pathogenic fungi Rhizoctonia solani and Fusarium graminearum were demonstrated. It is clear that the smart nanopesticides could realize the controlled release of pesticides toward hierarchical biological targets for enhanced pesticide bioavailability and efficacy via the multidimensional stimuli of pH, α-amylase, epicuticular waxes, and sunlight.

Biomass-derived chemical substitutes for bisphenol A: recent advancements in catalytic synthesis

Bisphenol A is an oil-derived, large market volume chemical with a wide spectrum of applications in plastics, adhesives and thermal papers. However, bisphenol A is not considered safe due to its endocrine disrupting properties and reproductive toxicity. Several functional substitutes of bisphenol A have been proposed in the literature, produced from plant biomass. Unless otherwise specified, the present review covers the most significant contributions that appeared in the time span January 2015-August 2019, describing the sustainable catalytic synthesis of rigid diols from biomass derivatives. The focus is thereupon on heterogeneous catalysis, use of green solvents and mild conditions, cascade processes in one-pot, and continuous flow setups. More than 500 up-to-date references describe the various substitutes proposed and the catalytic methods for their manufacture, broken down according to the main biomass types from which they originate.

Recyclable polyetheretherketone fiber-supported N-heterocyclic carbene catalysts for nucleophilic acylation of fluorobenzenes

We report for the first time a novel support of polyetheretherketone fiber for the synthesis of recyclable N-heterocyclic carbene (NHC) catalysts. The fiber catalysts were verified in nucleophilic acylation of fluorobenzenes with superior catalytic activities, and successfully recycled by a tiny pair of tweezers over 21 cycles with minimal loss of performance.

Imidazolium-Catalyzed Synthesis of an Imidazolium Catalyst

The chemistry of imidazolium-catalyzed imidazolium synthesis was studied as part of an effort to develop a plausible prebiotic synthesis of a small catalytic molecule capable of catalyzing its own synthesis. Specifically, we investigated the one-pot 1-ethyl-3-methylimidazolium acetate (EMIM-Ac) catalyzed synthesis of 1,3-dibutyl-4,5-difuryl-imidazolium acetate (DBDFIM-Ac) from furfural, n-butylamine, formaldehyde, and acetic acid at 80 °C. Liu et al. (2012) had previously demonstrated the first reaction of the synthetic process, the EMIM-Ac catalyzed benzoin condensation of furfural that yields furoin. Our early studies established the second reaction of the synthetic process, the multicomponent reaction of furoin, n-butylamine, formaldehyde, and acetic acid that yields the imidazolium salt, DBDFIM-Ac. Studies of the complete two-reaction process that uses furfural for the synthesis of DBDFIM-Ac showed that the highest yield of DBDFIM-Ac was obtained when the mole ratio of n-butylamine, formaldehyde, and acetic acid relative to furfural was respectively (0.5:0.25:0.25:1.0-furfural), or one-half of the stoichiometric ratio (1.0:0.5:0.5:1.0-furfural). A time course study of the process showed transient formation of furoin, the obligatory reaction intermediate. DBDFIM-Ac and the imidazolium side product, 1,3-dibutyl-4,5-trifuryl-imidazolium acetate (DBTFIM-Ac), were stable under the reaction conditions. Imidazolium products (DBDFIM and DBTFIM) and the furoin intermediate were not formed in control reactions (80 °C, 24 h) in which EMIM catalyst was either absent or replaced with an equal volume of acetonitrile or DMF. The imidazolium product, DBDFIM-Ac, was shown to catalyze the synthesis of structurally similar 1,3-dipentyl-4,5-difuryl-imidazolium acetate (DPDFIM-Ac) from furfural, n-pentylamine, formaldehyde, and acetic acid at 80 °C.

Hybrid catalysts based on N-heterocyclic carbene anchored on hierarchical zeolites

Hybrid materials have been synthesized by anchoring a N-heterocyclic carbene (NHC) precursor on different inorganic zeolitic supports with hierarchical porosity, in particular hierarchical HZSM-5 and SAPO-5. Hierarchical porous inorganic supports have been obtained both by top-down and bottom-up approaches and the role of hierarchical porosity has been evaluated. A detailed physico-chemical characterization has been performed on the organic–inorganic hybrids using a multi-technique approach (XRD, volumetric and thermogravimetric analysis, ssNMR and FTIR) in order to establish a structure–property relationship. The hybrids were tested in the benzoin condensation reaction of furfural, a base catalyzed reaction.

N-heterocyclic carbene has been grafted on hierarchical HZSM-5 and SAPO-5 to obtain hybrid catalysts, that are active in the benzoin condensation reaction.

5-Hydroxymethylfurfural (HMF) in Organic Synthesis: A Review of its Recent Applications Towards Fine Chemicals

Background:

5-Hydroxymethylfurfural (5-HMF) is a biomass-derived platform chemical, which can be produced from carbohydrates. In the past decades, 5- HMF has received tremendous attention because of its wide applications in the production of various value-added chemicals, materials and biofuels. The manufacture and the catalytic conversion of 5-HMF to simple industrially-important bulk chemicals have been well reviewed. However, employing 5-HMF as a building block in organic synthesis has never been summarized exclusively, despite the rapid development in this area.

Objective:

The aim of this review is to bring a fresh perspective on the use of 5-HMF in organic synthesis, to the exclusion of already well documented conversion of 5-HMF towards relatively simple molecules such as 2,5-furandicarboxylic acid, 2,5-dimethylfuran and so on notably used as monomers or biofuels.

Conclusion:

As it has been shown throughout this review, 5-HMF has been the object of numerous studies on its use in fine chemical synthesis. Thanks to the presence of different functional groups on this platform chemical, it proved to be an excellent starting material for the preparation of various fine chemicals. The use of this C-6 synthon in novel synthetic routes is appealing, as it allows the incorporation of renewable carbonsources into the final targets.

Towards Improved Biorefinery Technologies: 5-Methylfurfural as a Versatile C6 Platform for Biofuels Development

Low chemical stability and high oxygen content limit utilization of the bio-based platform chemical 5-(hydroxymethyl)furfural (HMF) in biofuels development. In this work, Lewis-acid-catalyzed conversion of renewable 6-deoxy sugars leading to formation of more stable 5-methylfurfural (MF) is carried out with high selectivity. Besides its higher stability, MF is a deoxygenated analogue of HMF with increased C/O ratio. A highly selective synthesis of the innovative liquid biofuel 2,5-dimethylfuran starting from MF under mild conditions is described. The superior synthetic utility of MF against HMF in benzoin and aldol condensation reactions leading to long-chain alkane precursors is demonstrated.

Enzymatic synthesis of biobased aliphatic–aromatic oligoesters using 5,5′-bis(hydroxymethyl)furoin as a building block

5,5′-Dihydroxymethyl furoin (DHMF) is a novel biobased difuranic polyol scaffold, achievable from the benzoin condensation of 5-hydroxymethylfurfural (HMF), which has recently been employed as a monomer for the preparation of cross-linked polyesters and polyurethane. Its upgrading by means of enzymatic reactions has not yet been reported. Here we demonstrated that Candida antarctica lipase B (CALB) is a suitable biocatalyst for the selective esterification of the primary hydroxyl groups of DHMF. Exploiting this enzymatic activity, DHMF has been reacted with the diethyl esters of succinic and sebacic acids obtaining fully biobased linear oligoesters with number-average molecular weight around 1000 g mol⁻¹ and free hydroxyl groups on the polymer backbone. The structures of the DHMF-diacid ethyl ester dimers and of the oligomers were elucidated by NMR and MS analyses.

Fully bio-based linear oligoesters were obtained by the unprecedented enzymatic polymerization of 5,5′-bis(hydroxymethyl)furoin with succinic and sebacic acid diethyl esters.

Catalytic coupling of biomass-derived aldehydes into intermediates for biofuels and materials

Catalytic upgrading of biomass-based aldehydes into chain-extended intermediates for downstream applications in biofuels, fine chemicals, and renewable materials, is reviewed.

Bi-functional heterogeneous catalysts for carbon dioxide conversion: enhanced performances at low temperature

Novel bi-functional catalysts allowing to decrease the reaction temperature for the synthesis of cyclic carbonates below 150 °C were successfully synthesized.

Esterification of glycerol and solketal by oxidative NHC-catalysis under heterogeneous batch and flow conditions

Heterogeneous NHC-catalysis in batch and flow modes is an effective synthetic platform for the production of monoacylglycerols.

Synthesis of Renewable Lubricant Alkanes from Biomass-Derived Platform Chemicals

The catalytic synthesis of liquid alkanes from renewable biomass has received tremendous attention in recent years. However, bio-based platform chemicals have not to date been exploited for the synthesis of highly branched lubricant alkanes, which are currently produced by hydrocracking and hydroisomerization of long-chain n-paraffins. A selective catalytic synthetic route has been developed for the production of highly branched C₂₃ alkanes as lubricant base oil components from biomass-derived furfural and acetone through a sequential four-step process, including aldol condensation of furfural with acetone to produce a C₁₃ double adduct, selective hydrogenation of the adduct to a C₁₃ ketone, followed by a second condensation of the C₁₃ ketone with furfural to generate a C₂₃ aldol adduct, and finally hydrodeoxygenation to give highly branched C₂₃ alkanes in 50.6 % overall yield from furfural. This work opens a general strategy for the synthesis of high-quality lubricant alkanes from renewable biomass.

Catalytic Hydrodeoxygenation of High Carbon Furylmethanes to Renewable Jet-fuel Ranged Alkanes over a Rhenium-Modified Iridium Catalyst

Renewable jet-fuel-range alkanes are synthesized by hydrodeoxygenation of lignocellulose-derived high-carbon furylmethanes over ReO_x -modified Ir/SiO₂ catalysts under mild reaction conditions. Ir-ReO_x /SiO₂ with a Re/Ir molar ratio of 2:1 exhibits the best performance, achieving a combined alkanes yield of 82-99 % from C₁₂ -C₁₅ furylmethanes. The catalyst can be regenerated in three consecutive cycles with only about 12 % loss in the combined alkanes yield. Mechanistically, the furan moieties of furylmethanes undergo simultaneous ring saturation and ring opening to form a mixture of complex oxygenates consisting of saturated furan rings, mono-keto groups, and mono-hydroxy groups. Then, these oxygenates undergo a cascade of hydrogenolysis reactions to alkanes. The high activity of Ir-ReO_x /SiO₂ arises from a synergy between Ir and ReO_x , whereby the acidic sites of partially reduced ReO_x activate the C-O bonds of the saturated furans and alcoholic groups while the Ir sites are responsible for hydrogenation with H₂ .

Electron-transfer-initiated benzoin- and Stetter-like reactions in packed-bed reactors for process intensification

A convenient heterogeneous continuous-flow procedure for the polarity reversal of aromatic α-diketones is presented. Propaedeutic batch experiments have been initially performed to select the optimal supported base capable to initiate the two electron-transfer process from the carbamoyl anion of the N,N-dimethylformamide (DMF) solvent to the α-diketone and generate the corresponding enediolate active species. After having identified the 2-tert-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorine on polystyrene (PS-BEMP) as the suitable base, packed-bed microreactors (pressure-resistant stainless-steel columns) have been fabricated and operated to accomplish the chemoselective synthesis of aroylated α-hydroxy ketones and 2-benzoyl-1,4-diones (benzoin- and Stetter-like products, respectively) with a good level of efficiency and with a long-term stability of the packing material (up to five days).

Advances in Organic and Organic-Inorganic Hybrid Polymeric Supports for Catalytic Applications

In this review, the most recent advances (2014-2016) on the synthesis of new polymer-supported catalysts are reported, focusing the attention on the synthetic strategies developed for their preparation. The polymer-supported catalysts examined will be organic-based polymers and organic-inorganic hybrids and will include, among others, polystyrenes, poly-ionic liquids, chiral ionic polymers, dendrimers, carbon nanotubes, as well as silica and halloysite-based catalysts. Selected examples will show the synthesis and application in the field of organocatalysis and metal-based catalysis both for non-asymmetric and asymmetric transformations.

Recyclable montmorillonite-supported thiazolium ionic liquids for high-yielding and solvent-free upgrading of furfural and 5-hydroxymethylfurfural to C10 and C12 furoins

Six Na⁺/montmorillonite (MMT) – supported thiazolium (TM) ionic liquids (ILs) have been synthesized and employed to catalyze the highly selective condensation of furfural (FF) and 5-hydroxymethylfurfural (HMF) into C₁₀ and C₁₂ furoins, respectively.