Diego Carnaroglio

Microwave-assisted flash conversion of non-edible polysaccharides and post-harvest tomato plant waste to levulinic acid

A microwave-assisted protocol for the conversion of non-edible polysaccharides and tomato plant waste to levulinic acid has been developed. Full conversion has been achieved in all cases at 2 min irradiation and clean levulinic acid was obtained without any purification in high yields (63–95%).

Microwave, ultrasound and ball mill procedures for bio-waste valorisation

Bio-waste valorisation has been one of the hottest research topics worldwide over the last decade. Following the biorefinery concept, the main goals have been the search for inexpensive renewable resources for the production of chemicals, materials and energy, the transformation of bio-waste into useful by-products and the development of new technologies for process intensification to make all these conversions economically profitable. Bio-waste is generally a negative-cost feedstock for the potential production of high value-added chemicals and bioenergy. The present review has the aim of describing existing studies and applications of non-conventional energy sources such as microwaves, ultrasound and ball milling within this field. We have focused our attention on suitable protocols for the conversion of biomass into fine chemicals. Comparisons with classic procedures are a good means with which to highlight the huge advantages and potential scalability of these so-called “enabling technologies”.

Solvent-free chemoselective oxidation of thioethers and thiophenes by mechanical milling

Organosulphur compounds can be easily and selectively oxidized to sulfones using a small excess of Oxone(®) (1.6 eq.) under solventless mechanical milling conditions. This green procedure has been efficiently applied to a series of model compounds and to the desulphurization of medium/high sulphur content paraffins (up to 3000 mg kg(-1)).

A Microwave-Based Chemical Factory in the Lab: From Milligram to Multigram Preparations

Microwave technology is changing the way we design and optimize synthetic protocols and their scaling up to multigram production levels. The latest generation of dedicated microwave reactors enables operators to quickly screen reaction conditions by means of parallel tests and select the best catalyst, solvent, and conditions. Pilot scale synthetic procedures require flow-through conditions in microwave flow reactors which can be obtained by adapting classic batch protocols. Microwave-assisted chemical processes play a pivotal role in the design of sustainable multigram preparations which address the double requirement of process intensification and competitive production costs. Although most researchers are likely to be acquainted with the great potential of dielectric heating, the advantages and disadvantages of a particular device or the conditions needed to maximize efficiency and functionality are often overlooked. The double aims of the present review are to provide a panoramic snapshot of commercially available lab microwave reactors and their features as well as highlighting a few selected applications of microwave chemistry of particular relevance.

Microwave-Assisted γ-Valerolactone Production for Biomass Lignin Extraction: A Cascade Protocol

The general need to slow the depletion of fossil resources and reduce carbon footprints has led to tremendous effort being invested in creating “greener” industrial processes and developing alternative means to produce fuels and synthesize platform chemicals. This work aims to design a microwave-assisted cascade process for a full biomass valorisation cycle. GVL (γ-valerolactone), a renewable green solvent, has been used in aqueous acidic solution to achieve complete biomass lignin extraction. After lignin precipitation, the levulinic acid (LA)-rich organic fraction was hydrogenated, which regenerated the starting solvent for further biomass delignification. This process does not requires a purification step because GVL plays the dual role of solvent and product, while the reagent (LA) is a product of biomass delignification. In summary, this bio-refinery approach to lignin extraction is a cascade protocol in which the solvent loss is integrated into the conversion cycle, leading to simplified methods for biomass valorisation.

Simple sonochemical protocols for fast and reproducible Grignard reactions

The Grignard reaction is one of the most common synthetic pathways in organic chemistry and has widely been used in the preparation of fine chemicals and pharmaceutical intermediates. The reactions reproducibility can be affected by several factors (metal purity, degree of association in solution, Schlenk equilibrium) and its activation with chemical, mechanical, sonochemical or electromagnetic treatments is practically essential. In this study we demonstrate that ultrasound-assisted Grignard reactions are safe, efficient and reproducible. The activation of the Mg surface, the single electron transfer (SET) from the metal to organic halide, and the subsequent nucleophilic addition of ArMgX to carbonyl substrates, fall into the larger domain of sonochemistry. As expected for a radical mechanism the best sonochemical conditions were observed at 300 kHz (cup horn). An efficient one-pot protocol with poorly-reactive aryl halides and the subsequent quenching with a hindered ketone are reported.

Fast TiO2-catalyzed direct amidation of neat carboxylic acids under mild dielectric heating

The development of green protocols for amide bond formation is a major socioeconomic goal for chemical and pharmaceutical industries and an important challenge for academic research. We herein report a protocol for the quantitative conversion of carboxylic acids and amines to form amides at 100 °C in the presence of a TiO2 powder catalyst, under monomodal microwave irradiation. The sustainability of the process appears to be augmented by the ease with which the catalyst is recycled.

From Lignocellulosic Biomass to Lactic‐ and Glycolic‐Acid Oligomers: A Gram‐Scale Microwave‐Assisted Protocol

The conversion of lignocellulosic biomass into platform chemicals is the key step in the valorization of agricultural waste. Of the biomass-derived platform chemicals currently produced, lactic acid plays a particularly pivotal role in modern biorefineries as it is a versatile commodity chemical and building block for the synthesis of biodegradable polymers. Microwave-assisted processes that furnish lactic acid avoid harsh depolymerization conditions while cutting down reaction time and energy consumption. We herein report a flash catalytic conversion (2 min) of lignocellulosic biomass into lactic and glycolic acids under microwave irradiation. The batch procedure was successfully adapted to a microwave-assisted flow process (35 mL min(-1) ), with the aim of designing a scalable process with higher productivity. The C2 and C4 units recovered from the depolymerization were directly used as the starting material for a solvent and catalyst-free microwave-assisted polycondensation that afforded oligomers in good yields.

Heterogeneous Phase Microwave-Assisted Reactions under CO₂ or CO Pressure.

The present review deals with the recent achievements and impressive potential applications of microwave (MW) heating to promote heterogeneous reactions under gas pressure. The high versatility of the latest generation of professional reactors combines extreme reaction conditions with safer and more efficient protocols. The double aims of this survey are to provide a panoramic snapshot of MW-assisted organic reactions with gaseous reagents, in particular CO and CO2, and outline future applications. Stubborn and time-consuming carbonylation-like heterogeneous reactions, which have not yet been studied under dielectric heating, may well find an outstanding ally in the present protocol.

Sonochemical preparation of alumina-spheres loaded with Pd nanoparticles for 2-butyne-1,4-diol semi-hydrogenation in a continuous flow microwave reactor

A novel protocol for microwave-assisted alkyne semi-hydrogenation under heterogeneous catalysis in a continuous flow reactor is reported herein. This challenging task has been accomplished using a multifaceted strategy which includes the ultrasound-assisted preparation of Pd nanoparticles (average O 3.0 ± 0.5 nm) that were synthesized on the μ-metric pores of sintered alumina spheres (O 0.8 mm) and a continuous flow reaction under H2 (flow rate 7.5 mL min−1) in a microwave reactor (counter-pressure 4.5 bar). The semi-hydrogenation of 2-butyne-1,4-diol in ethanol was chosen as a model reaction for the purposes of optimization. The high catalyst efficiency of the process, in spite of the low Pd loading (Pd content 111.15 mg kg−1 from ICP-MS), is due to the pivotal role of ultrasound in generating a regular distribution of Pd nanoparticles across the entire support surface. Ultrasound promotes the nucleation, rather than the growth, of crystalline Pd nanoparticles and does so within a particularly narrow Gaussian size distribution. High conversion (>90.5%) and selectivity to (Z)-2-butene-1,4-diol (95.20%) have been achieved at an alkyne solution flow rate of 10 mL min−1. The lead-free, alumina-stabilized Pd catalyst was fully characterized by TEM, HR-TEM, EDX, IR, XRPD and AAS. Highly dispersed Pd nanoparticles have proven themselves to be stable under the reaction conditions employed. The application of the method is subject to the dielectric properties of substrates and solvents, and is therefore hardly applicable to apolar alkynes. Considering the small volume of the reaction chamber, microwave-assisted flow hydrogenation has proven itself to be a safe procedure and one that is suitable for further scaling up to industrial application.