Chemical engineering was strongly related to high-volume production and large facilities, in the past. Here, the focus was mainly on efficient synthesis, high yields, and high quality at low costs, the best-suited reactors and other equipment for production. Besides these traditional aspects, the view on new developments in production of chemicals turned more to the aspect of diversification of products during the last decades. A large spectrum of special chemicals and material was developed and recently found entrance in industry and households. It is not difficult to predict that this trend will continue in the next years and decades. The challenges for the development of new approaches for the production of large numbers of different special chemicals and materials in low volumes get increasingly important in relation to the development of powerful production strategies for a limited number of large-scale products.
However, the vast majority of recently used nanomaterials must be counted in this group of low-volume products. Additives for coatings, pigments for printing industry, quantum dots, and plasmonic particles for biolabeling and optoelectronics, as well as organic particles for drug delivery are some examples for these developments. In particular, it has to be expected that the diversity of produced and applied nanoparticles will increase dramatically in the near future. This expectation is supported by two reasons: first, the fascinating new properties which can be realized by nanomaterials. They are neither classical solids nor molecules but integrate interesting properties of both: in nanoparticles, solid matter becomes mobile, flexible, and active on the one hand; on the other hand, molecules become constructions, architectures, sensors, and machines. The borderline between molecules and solid state and the borderline between material and device will be dissolved in the world of nanoparticles. The second reason is the unimaginable high variability in composition, properties, and functions of nanoparticles. A short survey over the publications of the last years on new nanomaterials gives already a rough idea about the enormous plurality of shapes, construction principles, and parameters of nano-objects. The combination of atoms of different chemical elements and molecular building units, of a hierarchy of classes of chemical bonds, the combination of movable and stiff units, of hardness and softness in electrical polarization, in controlling charge mobility and phonon transport, electronic excitation, exciton transfer, and other parameters span a multidimensional space of design parameters. The resulting basic diversity of nanomaterials is practically unlimited.
With respect to this potential of nanomaterials, chemical engineering is confronted with the urgent need for the development of new strategies that are able to support the development and the production of highly diverse chemicals and materials in small amounts and highest quality. New methods and devices for nanoparticle synthesis are demanded – in particular, for supplying nanomaterials in a similar quality as we know it from the synthesis of molecules. The future of nanomaterial production will challenge for well-defined compositions and arrangements of atoms in nanoparticles as we know them from the numbers and arrangements of atoms in molecules: exact precisions in atomar dimension, the arrangement, and connection accuracy should be comparable to the conditions in single crystals of a certain size and to pure isomers of well-defined molecules. These demands are not only addressing the design of nanomaterial, but also the materials engineering. They are challenging, in particular, for new strategies in material technology and, therefore, in chemical engineering. The traditional handling of large numbers of equal atoms, molecules, and particles has to be substituted by an operation of small particle ensembles, by directed molecular motions and by well-controlled frame conditions for spontaneous processes at the nanoscale. Probably, we cannot solve these problems by atomar handling using macroscopic machines, in general. By contrast, we require a stepwise improvement of chemical technologies for approaching the requirements of systems with reduced particle numbers, for respecting the particle individuality in the nanocosmos, but without neglecting the fundamentals and the potentials of thermodynamic and kinetic control, molecular interaction principles, and self-organization. The introduction of microreaction technology was an important step in this direction.
This special edition of Nanotechnology Reviews is devoted to the synthesis and development of nanoparticles in microreactors. It includes contributions bridging the world of nanomaterials and the world of miniaturization in chemical engineering, reviews the development in this field during the last years, and reports on exciting examples of nanomaterial development and applications by use of microfluidics and microreaction technology. The contributions demonstrate the power of microfluidic approaches for an efficient nanomaterial preparation, for important improvements of nanoparticle quality, for realizing new particle types, for developing new strategies for preparation and testing of particle properties, and for particle application. The use of nanoparticles in catalysis was chosen as a particularly important field of applications with high relevance for chemical production. It can be assumed that in the future, catalysis will strongly promote the ability of making molecular machines, nanodevices, and efficient miniaturized material conversion very strong. Thus, a positive feedback arises between the development of microreaction technology for nanomaterial preparation, the application of these materials in catalytic microreactors, the formation of new molecules, the synthesis, the surface modification, and arranging of nanoparticles and molecules for new nanomachines.
The first review (by A. Knauer and J. M. Koehler) gives an overview on the research on investigating and controlling of nanoparticle parameters. Beside composition and size, also functional properties are very important for the use of nanomaterials. Recent investigations address the controlling and tuning of nanoparticle composition and morphological parameters in order to control their physical properties. It is shown that microfluidic strategies offer a new and very efficient way for tuning nanoparticle properties and for screening and investigating nanoparticles in larger parameter spaces. Therefore, examples of special types of plasmonic nanoparticles are given, which demonstrate the large effect of controlled changing of the process parameters on the optical and electronic properties in this class of nanomaterials.
Besides inorganic nanomaterials, organic and polymer nanoparticles are of strong interest for future applications. The second review (by D. Boskovic and S. Loebbecke) deals with the example of synthesis of polymer particles and capsules by using microfluidics. The high reproducibility in the formation of droplets in microfluidic systems is the basis for generating polymer particles with a very narrow size distribution. The review gives an overview on these microfluidic techniques and demonstrates how different types of microfluidic devices can be used for combining the flows of different monomers or mixtures in order to generate well-defined types of more complex organized particles as, e.g., Janus particles, multiphase particles, core/shell-particles, and capsules.
Ch. V. Navin et al. review the development of chip-based microreactors for the synthesis of gold nanoparticles and their application in microcontinuous-flow catalysis. The application of chip technology for microreactor fabrication opens the potential of solid state technology and microlithography for a design of chemical devices with a larger spectrum of functions. The authors give an overview on the recent developments in using the potential of microsystem technology and microfluidics for the synthesis and use of high-quality noble metal nanoparticles.
The following review (by E. Shahbazali et al.) shows that this strategy could not only be successfully applied for gold nanoparticles but also for other catalytic active inorganic materials. Thus, they report about recent developments in the microfluidic synthesis and application of silver, palladium, platinum, and copper nanoparticles. The review shows that the use of microreactors for the syntheses of catalytic materials was extended to a larger spectrum of metals and could be applied successfully in different continuous-flow syntheses. The comparison between batch- and microflow-through catalytic processes proves that the transition to the miniaturized heterogeneous flow synthesis can lead to very important improvements in yield and process efficiency.
The last two articles are research highlights and report on two typical examples for the preparation, processing, and use of nanoparticle catalysts. The first of the two contributions (J. Baumgard et al.) deals with the preparation of particularly small and efficient palladium nanoparticles supported by titania microparticles and their application in a hydrogenation reaction. It can be demonstrated that the catalytic efficiency is strongly dependent on the particle size. Very small palladium particles (2.7, 3.3, and 5.7 nm) are much more active in the reduction of an aldehyde group than little larger particles (7.4 nm). The second report (H. Alex et al.) is devoted to the problem of application of Pd and Au/Pd nanoparticles in the selective aerobic oxidation of benzyl alcohol to benzyl aldehyde. They can show that these particles immobilized on a polymer support can lead to very high yields, which are otherwise only known from inorganically supported catalytic particles or a colloidal solution of gold nanoparticles. The key issue is the preparation of very small catalytic particles (about 2 nm). Both reports give an instructive insight into the strong interference between the nanoparticle quality and the application of nanomaterials in catalysis. The contributions clearly express the advantages of using microreactors in both the preparation of the catalytic nanoparticles and their application in highly efficient catalytic processes.
All contributions of this special issue show that microreaction technology is well established in research and development for new nanomaterials and their application as catalysts. They show that microfluidics and microreaction technology can be applied for the generation of a large spectrum of materials reaching from metals to polymers. The material development as well as the research on material properties and their application in continuous-flow syntheses benefit greatly if the advantages of microreaction technology are used.