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 -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

A green approach to heterogeneous catalysis with metal-loaded cross-linked polycyclodextrins

In this work, we describe the application of a new series of solid cross-linked cyclodextrin (α-, ß-, and γ-CD) based catalysts that are obtained by reticulation with hexamethylene diisocyanate (HDI) in solutions containing Pd(II) or Cu(I) cations. Diisocyanates are efficient cross-linking agents for CDs because of their high reactivity towards hydroxyl groups. Metal content in the polyurethane-bridged CD polymers has been analyzed using ICP-MS. Pd(II) based catalysts have been successfully used in C-C couplings (Heck and Suzuki reactions), and a Cu(I) based system has been used in alkyne/azide [3+2] cycloadditions. Because metal leaching is negligible, which allows it to be recycled, and preparation is facile, this protocol is truly sustainable. As confirmed by ICP-MS analysis, the crude products do not require further purification for metal contamination. This versatile catalyst, due to its polar structure, is particularly suitable for microwave (MW)-assisted reactions that, in this study, gave the best yields in shorter reaction times. Due to their polar structure, both native CDs and cross-linked derivatives are very sensitive to dielectric heating; this effect is enhanced by the embedded cations. The design of sustainable synthetic protocols can be efficiently accomplished using a multi-faceted strategy that combines innovative solid catalysts and suitable enabling technologies

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