In situ Raman spectroscopy for comparing ball milling and resonant acoustic mixing in organic mechanosynthesis

Vugrin, Leonarda; Chatzigiannis, Christos; Colacino, Evelina; Halasz, Ivan (2025) In situ Raman spectroscopy for comparing ball milling and resonant acoustic mixing in organic mechanosynthesis. RSC Mechanochemistry, 2 (3). pp. 482-487. ISSN 2976-8683

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Official URL: http://doi.org/10.1039/d5mr00016e

Abstract

By using in situ Raman spectroscopy for reaction monitoring, we compare ball milling (BM) and resonant acoustic mixing (RAM) in the preparation α,β-unsaturated ketones (chalcones). RAM achieves similar transformations to BM, though adjustments in reaction conditions may be necessary due to different mixing regimes. Graphical abstract: In situ Raman spectroscopy for comparing ball milling and resonant acoustic mixing in organic mechanosynthesis Mechanochemical processing techniques are becoming increasingly important in various industries for grinding, mixing, alloying and inducing chemical reactions among solid reagents and materials.1,2 The mechanical energy driving such transformations arises from collisional and frictional interactions between the milling media and reaction material.3 The former is most often achieved in ball milling (BM) devices, wherein the kinetic energy of milling balls activates and mixes powder materials to ultimately trigger and drive chemical reactions.4,5 While BM is versatile and straight-forward, particularly suitable for laboratory-scale experiments, it may present certain drawbacks such as excessive heating6 (mostly in the synthesis of hard materials), machine noise, and potential contamination due to the wear of the milling media7,8 (especially when grinding media made of stainless steel or tungsten carbide are used). These issues are less relevant when softer organic materials are processed and can be further mitigated by the use of less aggressive chemicals, if available, the use of liquid additives, the use of more appropriate milling media, such as zirconium oxide, PTFE, or agate, and conducting the process at a lower operating speed.9 Material contamination is a common concern in mechanochemical methods, as collisions and friction are two key mechanical actions that may cause abrasion and introduce metal impurities.10 Resonant acoustic mixing (RAM) has been recently proposed as a possible alternative to traditional ball-mills,11 the milling balls not being necessarily required for promoting mechanochemical stress, generally leading to an efficient mixing process (which however, will be dependent on the rheology of the mixture), by inducing bulk movements of the material inside the vessel.12 RAM operates by vertical oscillations at a frequency of 60 Hz, occurring at variable accelerations, expressed commonly as a multiple of the Earth's force of gravity (g). Though the use of milling balls is eliminated in RAM, friction between the reactants and the vial walls remains and may still cause abrasion.13,14 Oscillations in RAM contrasts the random and chaotic nature of energy transfer in BM and may allow for a simpler description of reaction kinetics and, possibly, more accurate prediction of reaction outcomes.15,16 The mechanistic and theoretical understanding of RAM-driven reactions remains unexplored and underdeveloped compared to those of BM.17 To date, limited studies have applied in situ Raman spectroscopy in RAM.16 Herein, the base-catalyzed Claisen–Schmidt condensation reaction between p-bromoacetophenone (1) and an array of different aromatic benzaldehydes (2a–g) was explored, with a particular focus on the reaction between 1 and p-nitrobenzaldehyde (2a), to compare the reaction rates and mechanisms of chalcone formation in BM and RAM by in situ Raman spectroscopy.

Item Type:

Article

Uncontrolled Keywords:

mechanochemistry; resonant-acoustic mixing; Raman spectroscopy; chalcone synthesis

Subjects:

NATURAL SCIENCES > Chemistry

Divisions:

Division of Physical Chemistry

Projects: