Technical Implications of Organic Syntheses in Ball Mills - Ball Milling Towards Green Synthesis: Applications, Projects, Challenges - page 245

10.2 Ball Mills as Reactors in Organic Synthesis
The prerequisite of ecient mixing and particle refinement for a successful
mechanochemical reaction has been pointed out above. A technical solution
that combines these two tasks in one apparatus are ball mills, whose ap-
plication as reactor in organic synthesis has attracted attention within the
last decade, as manifested in numerous review articles dedicated to the field
in general. 10,27,32-37 With respect to process intensification, the utilization of
ball mills seemed a promising way to improve the material and energy in-
tensity of a chemical reaction. Beside the combination of mixing, particle
refinement and energy entry in one apparatus, the most obvious advantage
is the omission of solvents during the reaction. Therefore, reactions carried
out in ball mills are generally considered as solvent-free. 38 However, the
terms solvent-free or solvent-less are not defined by IUPAC, but definitions
disclosed by Rothenberg et al. or Welton account for processes that are
carried out in the absence of solvents in general. 38,39
With regard to reactions in ball mills and the fact that solids interact with
each other, i.e. a heterogeneous disperse reaction system is present, they can
be classified as solvent-free. 26,28,38 In some cases the addition of small
amounts of solvents (liquid-assisted grinding ¼ LAG) 40 or the processing of
liquids is reported in ball mills. In any event, compared to the amount of
solid materials present in most cases the reaction mixtures appeared as free
flowing powders that are not agglomerated.
Generally, processes in ball mills are characterized by different par-
ameters, regardless of the type of process, i.e. a mechanochemical or a
mechanically-assisted reaction or particle comminution, and the type of ball
mill. In contrast to particle refinement solely, the balance volume for re-
actions in ball mills has to be enlarged taking into account chemical par-
ameters of the specific reactions. 9,10,41 The large number of influencing
variables (Scheme 10.1) can be categorized into:

i. chemical parameters,
ii. technological parameters,
iii. process parameters.

Chemical parameters summarize all those variables that are directly linked
to the chemical transformations taking place in the chamber of the ball mill,
which include (i) the type of chemical reaction, (ii) the presence of catalysts

catalyst(s) / reagent(s)
milling auxiliary

educt(s) product(s)

type of ball mill
material for beaker / balls
time; frequency

Scheme 10.1 Generic reaction scheme for a synthesis carried out in a ball mill.