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11 pages, 2241 KiB

Open AccessArticle

Batch and Flow Synthesis of CeO₂ Nanomaterials Using Solid-State Microwave Generators

by Cristina Rodríguez-Carrillo, Juan Torres García, Miriam Benítez, Jamal El Haskouri, Pedro Amorós and Jose V. Ros-Lis

Molecules 2022, 27(9), 2712; https://doi.org/10.3390/molecules27092712 - 22 Apr 2022

Cited by 4 | Viewed by 1466

Abstract

Microwave-assisted synthesis in combination with flow synthesis offers an interesting approach to develop faster and more sustainable procedures for the preparation of homogeneous nanomaterials. Recently, solid-state generators of microwaves appeared as a tool with improved control over power and frequency. Cerium oxide, despite [...] Read more.

Microwave-assisted synthesis in combination with flow synthesis offers an interesting approach to develop faster and more sustainable procedures for the preparation of homogeneous nanomaterials. Recently, solid-state generators of microwaves appeared as a tool with improved control over power and frequency. Cerium oxide, despite its excellent catalytic activity, has not been prepared before using solid-state generators or microwave-assisted flow chemistry. We report a procedure for the preparation of nanoparticulated CeO₂ (around 4 nm) under 2.45 GHz microwaves in only 30 s. The materials are further calcined at 800 °C to increase particle size, with a better defined particle size and crystallinity. The procedure was tested in batch at pH 11 and 12 and diverse potencies, and the products were characterized by TEM, XRD, DLS, and N₂ adsorption–desorption isotherms. The materials were similar at the diverse pH values and potencies. XRD confirms the crystallinity of the CeO₂ material with a fluorite-like structure. They are composed of particles around 40 nm that aggregate as structures of around 100 nm. The procedure was successfully adapted to flow synthesis, obtaining materials with structure and properties equivalent to batch synthesis. The batch and flow materials offer peroxidase properties, opening the door for their use as ROS scavengers. Full article

(This article belongs to the Special Issue Exclusive Feature Papers in Microwave Chemistry)

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14 pages, 7610 KiB

Open AccessArticle

Microwave-Assisted Ionic Liquid-Catalyzed Selective Monoesterification of Alkylphosphonic Acids—An Experimental and a Theoretical Study

by Nikoletta Harsági, Réka Henyecz, Péter Ábrányi-Balogh, László Drahos and György Keglevich

Molecules 2021, 26(17), 5303; https://doi.org/10.3390/molecules26175303 - 31 Aug 2021

Cited by 6 | Viewed by 1657

Abstract

It is well-known that the P-acids including phosphonic acids resist undergoing direct esterification. However, it was found that a series of alkylphoshonic acids could be involved in monoesterification with C₂–C₄ alcohols under microwave (MW) irradiation in the presence of [bmim][BF [...] Read more.

It is well-known that the P-acids including phosphonic acids resist undergoing direct esterification. However, it was found that a series of alkylphoshonic acids could be involved in monoesterification with C₂–C₄ alcohols under microwave (MW) irradiation in the presence of [bmim][BF₄] as an additive. The selectivity amounted to 80–98%, while the isolated yields fell in the range of 61–79%. The method developed is a green method for P-acid esterification. DFT calculations at the M062X/6–311+G (d,p) level of theory (performed considering the solvent effect of the corresponding alcohol) explored the three-step mechanism, and justified a higher enthalpy of activation (160.6–194.1 kJ·mol⁻¹) that may be overcome only by MW irradiation. The major role of the [bmim][BF₄] additive is to increase the absorption of MW energy. The specific chemical role of the [BF₄] anion of the ionic liquid in an alternative mechanism was also raised by the computations. Full article

(This article belongs to the Special Issue Exclusive Feature Papers in Microwave Chemistry)

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13 pages, 4652 KiB

Open AccessArticle

Microwave-Assisted Defibrillation of Microalgae

by Frederik L. Zitzmann, Ewan Ward, Xiangju Meng and Avtar S. Matharu

Molecules 2021, 26(16), 4972; https://doi.org/10.3390/molecules26164972 - 17 Aug 2021

Cited by 5 | Viewed by 2269

Abstract

The first production of defibrillated celluloses from microalgal biomass using acid-free, TEMPO-free and bleach-free hydrothermal microwave processing is reported. Two routes were explored: i. direct microwave process of native microalgae (“standard”), and ii. scCO₂ pre-treatment followed by microwave processing. ScCO₂ was [...] Read more.

The first production of defibrillated celluloses from microalgal biomass using acid-free, TEMPO-free and bleach-free hydrothermal microwave processing is reported. Two routes were explored: i. direct microwave process of native microalgae (“standard”), and ii. scCO₂ pre-treatment followed by microwave processing. ScCO₂ was investigated as it is commonly used to extract lipids and generates considerable quantities of spent algal biomass. Defibrillation was evidenced in both cases to afford cellulosic strands, which progressively decreased in their width and length as the microwave processing temperature increased from 160 °C to 220 °C. Lower temperatures revealed aspect ratios similar to microfibrillated cellulose whilst at the highest temperature (220 °C), a mixture of microfibrillated cellulose and nanocrystals were evidenced. XRD studies showed similar patterns to cellulose I but also some unresolved peaks. The crystallinity index (CrI), determined by XRD, increased with increasing microwave processing temperature. The water holding capacity (WHC) of all materials was approximately 4.5 g H₂O/g sample. The materials were able to form partially stable hydrogels, but only with those processed above 200 °C and at a concentration of 3 wt% in water. This unique work provides a new set of materials with potential applications in the packaging, food, pharmaceutical and cosmetic industries. Full article

(This article belongs to the Special Issue Exclusive Feature Papers in Microwave Chemistry)

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