Time-Resolved In Situ Monitoring of Mechanochemical Reactions

Abstract

Mechanochemical transformations offer environmentally benign synthesis routes, whilst enhancing both the speed and selectivity of reactions. In this regard, mechanochemistry promises to transform the way in which chemistry is done in both academia and industry but is greatly hindered by a current lack of mechanistic understanding. The continued development and use of time-resolved in situ (TRIS) approaches to monitor mechanochemical reactions provides a new dimension to elucidate these fascinating transformations. We here discuss recent trends in method development that have pushed the boundaries of mechanochemical research. New features of mechanochemical reactions obtained by TRIS techniques are subsequently discussed, which sheds light on how different TRIS approaches have been used. Emphasis is placed on the strength of combining complementary techniques. Finally, we outline our views on the potential of TRIS methods in mechanochemical research, towards establishing a new, environmentally benign paradigm in the chemical sciences.

Keywords: Kinetics; Mechanism; Mechanochemistry; XRD; in Situ Analysis.

Figure 1

The application of ex situ and in situ methods over the years. Left inset: the number of papers in Web of science each year in which “in situ” and “mechanochemistry” are used in the title, abstract, or as keywords. a) First in situ XRD experiment including a commercial instrument adapted for use at the synchrotron; Copyright 2021 Nature Publishing Group. b) Customized milling container with a continuous “probing” ring for enhanced XRD data for neat grinding experiments; Copyright 2017 American Chemical Society. c) The recently optimized sample environment and measuring strategy for XRD data. Copyright 2021 Nature Publishing Group.

Figure 2

The influence of mechanical treatment on the energetics of solids. Mechanical treatment drives the solid away from equilibrium into an energetically “activated” state. Once mechanical treatment is stopped, this excess energy is allowed to relax, although the relaxed state may still be “activated” with respect to the nascent state.

Figure 3

General evolution of mechanochemical transformations and the TRIS methods available to study them. I) A schematic generic kinetic profile for a mechanochemical transformation, comprising an induction period, reaction period, and product period. II) Schematic representation of the macroscopic mechanism describing a generic mechanochemical transformation, comprising bulk mixing, molecular mixing, reaction and nucleation, and growth phases. III) Existing TRIS methods to characterize mechanochemical transformations, with indications of their strength to characterize different reactivity regimes.

Figure 4

Tandem TRIS‐XANES and TRIS‐PXRD setup for the investigation of the bottom‐up synthesis of gold nanoparticles. Figure adapted from Ref. with permission from the Royal Society of Chemistry.

Figure 5

A degree of control is required to achieve reproducible and scalable mechanochemical processes. This can only be achieved through detailed understanding of the underlying mechanisms, which is attainable through analytical investigations.

Figure 6

Applications of TRIS‐XRPD to follow ball‐milling syntheses. I) Mechanochemical formation of MOF‐74, characterized by TRIS‐XRPD. The time‐resolved diffractograms (left) show a complex interconversion of intermediates prior to formation of the final MOF‐74 phase. Figure modified from Ref.  with permission. Copyright 2021 American Chemical Society. II) Mechanochemical synthesis of ZIF‐8, as followed by TRIS‐XRD (left) and Raman spectroscopy (right). Yellow: reactants, orange: reactants and product ZIF‐ 8, red: product ZIF‐8, blue: product ZIF‐8 with not yet perfectly arranged 2‐methylimidazolate molecules within the crystal structure. Figures modified from Ref.  with permission. Copyright 2015, Wiley‐VCH.

Figure 7

Kinetic analysis of TRIS data reveals new insights into the physical mechanisms of mechanochemical transformations. I) Ball‐milling synthesis of 1‐chloro‐3‐ethyl‐5,5′‐dimethylhydantoin as followed by TRIS‐XRD. The phase fraction ${\alpha }_p\left(t\right)$ is shown as a function of the ball volume ($V_b$ ) and time $t$ . Figure adapted from Ref. . Copyright 2021, American Chemical Society. II) Ball‐milling synthesis of ZIF‐8 monitored by tandem TRIS‐thermometry (top) and TRIS‐XRD (bottom). Kinetic heat flow models were fitted to thermometric data. Reproduced from Ref.  with permission from the Royal Society of Chemistry.

Figure 8

Established and potential methods (dashed lines) for time‐resolved in situ investigations of mechanochemical reactions. I) a) Typical XRD measurement for a reaction between bis(2‐nitrophenyl)‐ and bis(4‐chlorophenyl)disulfide, Copyright 2021, Nature Publishing Group. b) Relative pressure during the ball milling of Zn and S₈, Copyright 2015, Wiley‐VCH. c) XANES data for the mechanochemical formation of gold nanoparticles. Figure adapted from Ref.  with permission from the Royal Society of Chemistry. d) Raman spectra of cocrystal formation from nicotinamide and suberic acid, Copyright 2014, Wiley‐VCH. e) Temperature development during the mechanochemical cocrystallization of theobromine and oxalic acid dihydrate. Figure adapted from Ref.  with permission from the Royal Society of Chemistry. f) ssNMR data showing product evolution as a function of time during the formation of zinc phenyl phosphonate. Copyright 2020, Elsevier. Dashed boxes: potential extensions of the range of TRIS methods: Terahertz time‐domain spectroscopy (THz‐TDS), small‐angle X‐ray scattering (SAXS), and pair distribution function (PDF) analyses. II) Schematic representation describing the pathway to more detailed chemical information from time‐resolved in situ (TRIS) studies.