Mechanochemistry: an overview*

The field of mechanochemistry is reviewed. A large number of mechanochemical reactions are described as well as industrial applications. INTRODUCTION At the beginning of this century, W. Nernst classified the different fields of chemistry according to the type of energy supplied to the system: thermochemistry, electrochemistry, photochemistry, etc. The name mechanochemistry was applied to the field of reactions caused by mechanical energy. A narrower field, tribochemistry, was used for reactions generated by friction during the milling of solid reagents [1–3]. A variety of processes takes place on mechanical grinding of solids such as: (a) Conminution of the particles to a very small size. (b) Generation of large new surfaces. (c) Formation of dislocations and point defects in the crystalline structure. (d) Phase transformations in polymorphic materials. (e) Chemical reactions: decomposition, ionic exchange, oxidation-reduction, complex and adduct formation, etc. The occurrence of these reactions was attributed to the heat generated in the milling process, favored by the large area of contact between the solids [1–3]. However, since the end of the last century, Carey Lee noticed that mechanochemical processes were different from thermal processes [1]. For example, heating of AgCl and HgCl leads to melting and subliming of these solids, while milling them produces their decomposition into Cl2 gas and metal. The role played by mechanical defects as high energy structures and its importance in chemical transformations was recognized later [3]. PECULIARITIES OF MECHANOCHEMICAL PROCESSES The grinding of two solid substances generates a complex series of transformations, the mechanical energy breaking the order of the crystalline structure, producing cracks, and new surfaces. At the point of collision of the edges the solids deform and even melt, forming hot points where the molecules can reach very high vibrational excitation leading to bond breaking. These stochastic processes occur in a period of 10 s. in which thermal equilibrium does not exist [1]. This period, called the plasma phase, is followed by a post plasma period of 10 s. or more in which relaxation processes dissipate the energy reaching the Maxwell–Boltzmann distribution [1]. This post plasmatic reactions are responsible for many of the products formed. Finally, the energy accumulated in the defects of the crystalline structure can lead to slower chemical processes. As we can see, mechanochemical reactions can be very complex processes. *Plenary lecture presented at the 3rd International Congress of the Cuban Chemical Society, Havana, Cuba, 1–4 December 1998, pp. 559–586. Correspondence: Fax: (537) 33 64 71, E-mail: [email protected] MECHANOCHEMICAL REACTORS Milling can be carried out in a variety of ways. The simplest is the laboratory mortar and pestle. This hand milling processes can provoke a large number of mechanochemical reactions which do not require surmounting a high energy barrier. Ball mills are used when higher energy is required and when the milling time involves hours or even days. Laboratory vibrators of the Wiggle-Bug type are very efficient in milling small samples. Very high energy vibrators such as high speed attritors or stainless steel ball mills of high impact (Spex type) are used for prolonged high energy milling as in mechanical alloying or amorphization of hard crystalline solids [1–3]. Ultrasonic can also be used mechanochemically [4,5]. MONITORING MECHANOCHEMICAL PROCESSES The best way to study mechanochemical transformations is to analyze in-situ the milled mixture using appropriate spectroscopic methods, since chemical handling can obscure the true nature of the initial products. Most common tools are IR and XRD techniques which normally allow the identification of the products. In organic reactions, solid state NMR can be very useful and with Fe and Sn compounds, Mossbauer spectroscopy is most valuable. Other techniques like HREM, EXFAX and X-ray cyclotron resonance have been used to study the new surfaces [5,6]. In sophisticated experiments like reactions produced during crack formation, MS enables the determination of the gaseous products as in the decomposition of nitrates and bromates [7].