Sandro Guidi

Upgrading of glycerol acetals by thermal catalyst-free transesterification of dialkyl carbonates under continuous-flow conditions

At 250–300 °C and 30–50 bar, a continuous-flow (CF) transesterification of different dialkyl and alkylene carbonates (dimethyl-, diethyl-, dibenzyl-, and propylene carbonate, respectively) with two glycerol derived acetals (glycerol formal and solketal) was investigated without any catalyst. An unprecedented result was obtained; not only the desired process occurred, but also the formation of the corresponding mono-transesterification products took place with an excellent selectivity (up to 98%) in all cases. Under isothermal conditions, a study on the effect of pressure allowed us to optimize the conversion of acetals (up to 95%) for the reactions of dimethyl- and diethyl-carbonate (DMC and DEC, respectively). This proved that an abrupt progress of the reaction occurred for very small increments of pressure. For example, at 250 °C, the thermal transesterification of DMC with glycerol formal showed a sharp increase of the conversion from 1–2% at 30 bar to ∼85% at 37 bar. The lower the temperature, the lower the pressure interval at which the onset of the reaction is achieved. The absence of catalysts allowed us to run CF-reactions virtually indefinitely and with a very high productivity (up to 68 mg min−1) compared to the capacity (1 mL) of the used CF-reactor. Products of the transesterification of DMC and DEC were isolated in good-to-almost quantitative yields. In the case of heavier carbonates, steric reasons were responsible for the considerably lower reactivity of propylene carbonate (PC) with respect to DMC and DEC, while the transesterification of dibenzyl carbonate (DBnC, solid at room temperature) with glycerol formal required the presence of acetone as an additional solvent/carrier. Although the reactions of both PC and DBnC were not optimized, results offered a proof-of-concept on the extension of thermal transesterification processes to higher homologues of linear and alkylene carbonates.

Continuous-flow alkene metathesis: the model reaction of 1-octene catalyzed by Re2O7/γ-Al2O3 with supercritical CO2 as a carrier

In the presence of Re2O7 supported on γ-Al2O3, the self-metathesis of 1-octene was conveniently carried out under continuous-flow (CF) conditions using supercritical CO2 (scCO2) as a carrier. This investigation allowed optimization of reaction parameters, the best values of which were found to be 100 °C and 90 bar, operating at flow rates of 0.05 and 1 mL min−1 for 1-octene and scCO2, respectively, the reaction proceeded with very good self-metathesis selectivity (>90%) and an average productivity of ∼0.24 mL tetradecene gRe−1 min−1. Although the catalyst was completely deactivated after the first 100–150 min of reaction, it could be recycled for (at least) five subsequent reactions without any loss of performance. The results provided incontrovertible evidence that for the investigated reaction, scCO2was a superior carrier with respect to conventional liquids, such as toluene or n-hexane.

Ionic liquids as transesterification catalysts: applications for the synthesis of linear and cyclic organic carbonates

Summary The use of ionic liquids (ILs) as organocatalysts is reviewed for transesterification reactions, specifically for the conversion of nontoxic compounds such as dialkyl carbonates to both linear mono-transesterification products or alkylene carbonates. An introductory survey compares pros and cons of classic catalysts based on both acidic and basic systems, to ionic liquids. Then, innovative green syntheses of task-specific ILs and their representative applications are introduced to detail the efficiency and highly selective outcome of ILs-catalyzed transesterification reactions. A mechanistic hypothesis is discussed by the concept of cooperative catalysis based on the dual (electrophilic/nucleophilic) activation of reactants.

Continuous niobium phosphate catalysed Skraup reaction for quinoline synthesis from solketal

Solketal is derived from the reaction of acetone with glycerol, a by-product of the biodiesel industry. We report here the continuous reaction of solketal with anilines over a solid acid niobium phosphate (NbP), for the continuous generation of quinolines in the well-established Skraup reaction. This study shows that NbP can catalyse all the stages of this multistep reaction at 250 °C and 10 MPa pressure, with a selectivity for quinoline of up to 60%. We found that the catalyst eventually deactivates, most probably via a combination of coking and reduction processes but nevertheless we show the promise of this approach. We demonstrate here the application of our approach to synthesize both mono- and bis-quinolines from the commodity chemical, 4,4′-methylenedianiline.

Towards a Rational Design of a Continuous-Flow Method for the Acetalization of Crude Glycerol: Scope and Limitations of Commercial Amberlyst 36 and AlF3·3H2O as Model Catalysts

The acetalization of six different types of glycerol including pure, wet, and crude-like grade compounds of compositions simulating those of crude glycerols produced by the biodiesel manufacture, was carried out with two model ketones such as acetone and 2-butanone. The reaction was investigated under continuous-flow (CF) conditions through a comparative analysis of an already known acetalization catalyst such as Amberlyst 36 (A36), and aluminum fluoride three hydrate (AlF3·3H2O, AF) whose use was never previously reported for the synthesis of acetals. At 10 bar and 25 °C, A36 was a highly active catalyst allowing good-to-excellent conversion (85%–97%) and selectivity (99%) when either pure or wet glycerol was used as a reagent. This catalyst however, proved unsuitable for the CF acetalization of crude-like glycerol (CG) since it severely and irreversibly deactivated in a few hours by the presence of low amounts of NaCl (2.5 wt %) which is a typical inorganic impurity of raw glycerol from the biorefinery. Higher temperature and pressure (up to 100 °C and 30 bar) were not successful to improve the outcome. By contrast, at 10 bar and 100 °C, AF catalyzed the acetalization of CG with both acetone and 2-butanone, yielding stable conversion and productivity up to 78% and 5.6 h−1, respectively. A XRD analysis of fresh and used catalysts proved that the active phase was a solid solution (SS) of formula Al2[F1-x(OH)x]6(H2O)y present as a component of the investigated commercial AF sample. A hypothesis to explain the role of such SS phase was then formulated based on the Brønsted acidity of OH groups of the solid framework. Overall, the AF catalyst allowed not only a straightforward upgrading of CG to acetals, but also a more cost-efficient protocol avoiding the expensive refining of raw glycerol itself.