Solvent-free Transformation Research Articles

Modeling Mechanochemical Depolymerization of PET in Ball-Mill Reactors Using DEM Simulations.

Developing efficient and sustainable chemical recycling pathways for consumer plastics is critical for mitigating the negative environmental implications associated with their end-of-life management. Mechanochemical depolymerization reactions have recently garnered great attention, as they are recognized as a promising solution for solvent-free transformation of polymers to monomers in the solid state. To this end, physics-based models that accurately describe the phenomena within ball mills are necessary to facilitate the exploration of operating conditions that would lead to optimal performance. Motivated by this, in this paper we develop a mathematical model that couples results from discrete element method (DEM) simulations and experiments to study mechanically-induced depolymerization. The DEM model was calibrated and validated via video experimental data and computer vision algorithms. A systematic study on the influence of the ball-mill operating parameters revealed a direct relationship between the operating conditions of the vibrating milling vessel and the total energy supplied to the system. Moreover, we propose a linear correlation between the high-fidelity DEM simulation results and experimental monomer yield data for poly(ethylene terephthalate) depolymerization, linking mechanical and energetic variables. Finally, we train a reduced-order model to address the high computational cost associated with DEM simulations. The predicted working variables are used as inputs to the proposed mathematical expression which allows for the fast estimation of monomer yields.

Solvent-free transformation of protic ionic liquids into zwitterions.

We synthesized several protic ionic liquids (ILs) composed of onium cations and the (trifluoromethylsulfonyl)(vinylsulfonyl)amide anion. The addition of a base catalytically facilitated their transformation into zwitterions (ZIs) under solvent-free conditions, which is a convenient method for synthesizing ZIs from ILs.

Stable Engineered Trimetallic Oxide Scaffold as a Catalyst for Enhanced Solvent-Free Conversion of CO2 into Value-Added Products

This work demonstrates the development of mesoporous trimetallic oxide scaffolds (TOSs) as heterogeneous catalysts for the solvent-free transformation of CO2 into various value-added products. The TOS catalyst was prepared using a solution-combustion protocol as a time- and energy-saving method using cobalt, magnesium, and cerium metal nitrate salts as precursors and ethylene glycol as the fuel system and compared with their monometallic counterparts. Characterization suggested strong metal–metal and metal–oxygen interactions in the 3D-interconnected hierarchical porous network, which resulted in substantial alteration in the electronic, structural, and physicochemical properties. This resulted in an appreciable surface area, acid–base cooperative sites, and a larger pore volume in the catalyst. Thereafter, the CoMgCe-TOS catalyst was first used for the solvent-free cyclization of o-phenylenediamines and CO2 to produce benzimidazoles. In the presence of dimethylamine borane as a reductant, benzimidazoles were obtained in 94% yield at 100 °C under pressurized conditions, along with good recyclability for 12 cycles. It was established that selectivity toward benzimidazole improved upon the incorporation of basic metals (Mg and Ce), with Co aiding in the formation of cooperative Lewis acid–base sites. A plausible mechanism was also predicted, wherein the overall conversion of reactants for the N-formylation step was influenced by the number and strength of basic sites, and the cyclization step for selectively obtaining benzimidazole was affected by the acidic site strength. Besides the production of benzimidazole, the catalyst was also highly active for the synthesis of cyclic carbonates using epoxides and CO2 under optimized reaction conditions. This work therefore provides a new and greener route for the synthesis of benzimidazoles and organic carbonates, which can be easily adapted for scale-up applications.

Solvent-free transformation of levulinic acid into valeric acid and its esters using the nickel phosphine complex and metal triflate co-catalytic system

Neat levulinic acid is directly converted to valeric acid under mild conditions. Inexpensive and environmentally friendly non-noble metal catalysts are used.

Phase Behavior That Enables Solvent-Free Carbonate-Promoted Furoate Carboxylation.

Solvent-free chemistry has been used to streamline synthesis, reduce waste, and access novel reactivity, but the physical nature of the reaction medium in the absence of solvent is often poorly understood. Here we reveal the phase behavior that enables the solvent-free carboxylation reaction in which carbonate, furan-2-carboxylate (furoate), and CO2 react to form furan-2,5-dicarboxylate (FDCA2-). This transformation has no solution-phase analogue and can be applied to convert lignocellulose into performance-advantaged plastics. Using operando powder X-ray diffraction and thermal analysis to elucidate the temperature- and conversion-dependent phase composition, we find that the reaction medium is a heterogeneous mixture of a ternary eutectic molten phase, solid Cs2CO3, and solid Cs2FDCA. During the reaction, the amounts of molten phase and solid Cs2CO3 diminish as solid Cs2FDCA accumulates. These insights are critical for increasing the scale of furoate carboxylation and provide a framework for guiding the development of other solvent-free transformations.

Solvent-free transformation of spray coated ZnO layers to ZIF-8 membranes

Abstract ZIF-8 membranes were fabricated by a two-step procedure consisting of (1) a flexible and simple spray coating process to deposit uniform ZnO layers on different supports and (2) subsequent solvent-free transformation of the ZnO layers into dense ZIF-8 membranes by a vapor-solid reaction. The spray coating offered superior flexibility regarding the choice of support and yielded even and homogeneous ZnO layers with thicknesses of 5–10 μm on non-porous crude steel plates, porous α-Al2O3 and porous stainless steel discs. The texture of the ZnO layer did not depend on the texture of the underlying support. The conversion of the ZnO to ZIF-8 via vapor-solid reaction of gaseous 2-mIm and ZnO was performed without the use of a solvent and no further activation or seeding was applied. Except the spray coated ZnO, no further zinc species were added to synthesize the ZIF-8 layers. The particle size, morphology and the degree of intergrowth were tunable by adjusting the temperature of the vapor-solid reaction in a range of 25–130 °C. The variation of the reaction time for the fabrication of membranes on α-Al2O3 discs at 80 °C resulted in well-intergrown layers without visible defects. The ideal gas selectivities increased with increasing temperature and the best membrane showed a H2 permeance of 2.3·10−8 mol m−2 s−1 Pa−1 at 100 °C and ideal selectivities of H2/CO2, H2/N2 and H2/CH4 of 5.4, 7.1 and 9.5.

Highly selective and efficient solvent-free transformation of bio-derived levulinic acid to γ-valerolactone by Ru(II) arene catalyst precursors

The selective and efficient solvent-free hydrogenation of bio-based levulinic acid (LA) to γ-valerolactone (GVL) was achieved with new pyridylimine ruthenium(II) complexes as catalyst precursors. The hydrogenation reactions were performed in the presence of formic acid as hydrogen source using a catalyst loading as low as 0.1 mol% with potassium hydroxide or triethylamine (Et3N). 4-Hydroxyvaleric acid (HVA) was produced only when KOH was used, whereas reactions involving Et3N were selective to GVL. At 150 °C, >96% LA conversions were achieved with 100% GVL selectivity. Recyclability of catalyst precursors was demonstrated by running three consecutive reactions where 100% conversion and selectivity was maintained. In-situ NMR studies show that hydrogen gas is formed by the decomposition of formic acid to carbon dioxide and hydrogen. Ru-hydride species have been detected, by 1H NMR, and are believed to be the catalytically active species, and a mechanism of the reaction has been proposed.

On the development of kinetic models for solvent-free benzyl alcohol oxidation over a gold-palladium catalyst

Bimetallic Au-Pd nanoparticles supported on TiO2 show excellent catalytic activity and selectivity to benzaldehyde in the solvent-free transformation of benzyl alcohol to benzaldehyde, where toluene is the main observed by-product, together with smaller amounts of benzoic acid, benzyl benzoate and dibenzyl ether. However, despite the industrial relevance of this reaction and importance of tuning the selectivity to the desired benzaldehyde, only a few attempts have been made in the literature on modeling the reaction kinetics for a quantitative description of this reaction system. A kinetic model for the oxidation of benzyl alcohol over Au-Pd is proposed in this paper. The model assumes that hydrogenolysis, disproportionation and dehydrogenation reactions may occur in parallel, and it has been found satisfactory after a model discrimination procedure was applied to a number of simplified candidate models developed from microkinetic studies. Despite its relative simplicity, the proposed model is capable of representing the reactant conversion and distribution of products observed in experiments carried out at different temperature, pressure and catalyst mass in a stirred batch reactor. Major findings include the quantitative evaluation of the impact of hydrogenolysis and disproportionation pathways on benzaldehyde production. At low temperature the disproportionation reaction is the dominant route to toluene formation, while hydrogenolysis dominates at high temperature.

Efficient solvent-free cationization of alkylpolyglycoside based surfactant compositions using natural glycine betaine

Solvent-free transformation of alkylpolyglycosides into 100% bio-based cationic sugar surfactants proceeded efficiently using a natural glycine betaine-derived cationizing agent.

Microwave-assisted Cycloaddition Reactions in Carbo- and Heterocyclic Chemistry

Recent developments in the microwave-assisted synthesis of carbo- and heterocyclicsystems are reviewed with the focus on diversity-oriented cycloaddition reactions. Ineed, these reactions represent one of the most important endeavors in chemistry and are of considerable interest in the synthesis of carbo-and heterocyclic compounds from simple unsaturated systems, and allow the creation of more than two bonds in one step. This review summarizes recent literature (2006-2013) on advances regarding [3+2], [4+2] and [2+2] cycloadditions including their miscellaneous version. Advantages of microwave dielectric heating are highlighted by comparison with conventional thermal conditions. In particular, we describe here the microwave effect, reaction media including solvent-free transformations and more ecological processes in the context of green and environmentally benign chemistry. Keywords: [2+2] cycloaddition, 1, 3-dipolar cycloaddition, carbocycles, diels-alder, green catalysis, green chemistry, heterocycles, microwave synthesis.

Chemoselective flow hydrogenation approaches to isoindole-7-carboxylic acids and 7-oxa-bicyclio[2.2.1]heptanes

Two libraries of highly decorated norcantharidin analogues were accessed via a series of sequential chemoselective flow hydrogenations and solvent-free transformations. Utilising a 10% Pd/C catalyst, modifications to reaction parameters (H2 pressure, temperature and flow rate conditions) allowed facile access to effect selective direct reductive aminations and olefin reductions in the presence of furan, benzyl and nitrile moieties were established. The use of 20% Pd(OH)2/C; Pd tetrakis; 5% Pt/C (sulfided) gave mixtures of furan and olefin (both reduced) and olefin reduced products. RuO2; 0.5% Re/C and Re2O7 resulted in no reduction. Concurrent olefin and nitrile reduction was achieved in the presence of furan moieties by employing a RANEY® nickel catalyst. In total, 31 reaction conditions were examined using less than 200 mg of reagents allowing optimised conditions to be efficiently determined. These optimised hydrogenation conditions afforded desired analogues in near quantitative yields thus removing the requirements of reaction workup and chromatography.

Fast copper-, ligand- and solvent-free Sonogashira coupling in a ball mill

A solvent-free method for the Sonogashira coupling reaction was established under ball milling conditions without the use of copper or additional ligands. Pd(OAc)2 or Pd(PPh3)4 in combination with 1,4-diazabicyclo[2.2.2]octane (DABCO) were chosen as catalysts and base, respectively. The reaction was investigated using a variety of aryl halides and acetylenes and with different amounts of the Pd-catalyst and DABCO. Results indicated that the employment of Pd(OAc)2 in combination with SiO2 as a grinding auxiliary preferentially induces the transformation of aryl iodides to the corresponding Sonogashira coupling products. In contrast, both the substitution of SiO2 by Al2O3 or replacement of Pd(OAc)2 with Pd(PPh3)4 enable the reaction of aryl bromides with phenylacetylenes. The selective reaction of bis-ethynyl compounds to double-coupled products and the influence of further common bases on the reaction was also scrutinized, confirming the high reactivity of DABCO as a base for solvent-free transformations.

Solvent‐Free, Efficient Synthesis of 2,5‐Piperazinediones from Boc‐Protected Dipeptide Esters under Microwave Irradiation.

Microwave irradiation allows the efficient, solvent-free transformation of N-Boc dipeptide esters into 2,5-piperazinediones. The microwave-assisted conditions were found to be much better than traditional heating in terms of reaction time, yield and stereo- center integrity.

Solvent-Free, Efficient Synthesis of 2,5-Piperazinediones from Boc-Protected Dipeptide Esters under Microwave Irradiation

Microwave irradiation allows the efficient, solvent-free transformation of N-Boc dipeptide esters into 2,5-piperazinediones. The microwave-assisted conditions were found to be much better than traditional heating in terms of reaction time, yield and stereo- center integrity.

Microwave-induced, solvent-free transformations of benzoheteracyclanones by HTIB (Koser`s reagent)

The microwave-activated reactions of (hydroxy(tosyloxy)iodo)benzene (HTIB) with various chromanones, thiochromanones and dihydroquinolones under solvent-free conditions have been studied. In addition to the common dehydrogenation reaction, 2,3-migration also has been observed in the case of flavanone and 2,2-disubstituted chromanones. 3-Tosyloxychromanones were isolated from the reaction of chromanone and 2-methylchromanone for the first time. Substrates with nucleophilic heteroatoms such as thiochromanones and 2-phenyl-2,3-dihydro-4- quinolone reacted by electrophilic attack of the heteroatom.