Microscopic reactors

One of the most promising applications of enzyme-immobilized mesoporous materials is as microscopic reactors. Galameau et al. investigated the effect of mesoporous silica structures and their surface natures on the activity of immobilized lipases [199]. Too hydrophilic (pure silica) or too hydrophobic (butyl-grafted silica) supports are not appropriate for the development of high activity for lipases. An adequate hydrophobic/hydrophilic balance of the support, such as a supported-micelle, provides the best route to enhance lipase activity. They also encapsulated the lipases in sponge mesoporous silicates, a new procedure based on the addition of a mixture of lecithin and amines to a sol-gel synthesis to provide pore-size control. 

Remarkable effects of nuclear spin on the photochemistry of dibenzyl ketone and its derivatives (see Special Topic 6.11) have been reported and substantial isotopic enrichment of 13C in specific products has been achieved under favourable conditions.434 The influence of magnetic fields and of microscopic reactors (micelles, zeolites, etc.) on these reactions was investigated in detail.435... 

Turro, N. J., Photochemistry of Organic Molecules in Microscopic Reactors, PureAppl. Chem. 1986, 58, 1219 1228. 

The remarkable capability of micelles to solubilize hydrophobic substances in water and locally to concentrate ions has a decisive influence on the course of elementary photochemical processes. In this respect micelles are sometimes referred to as supercages and as microscopic reactors which favor bimolecular processes (17,18). Also the term micellar catalysis is used to describe the higher rate at which bimolecular photochemical reactions proceed. Moreover, ionic micelles, being surrounded by an electric double layer, may promote or inhibit unimolecular processes involving ionization as well as electron transfer. 

Tao, F., Tang, D., Salmeron, M., and Somorjai, G.A. (2008) A new scanning tunneling microscope reactor used for high-pressure and high-temperature catalysis studies. Rev. Sci. Instrum., 79, 084101. 

Mcintyre B J, Saimeron M and Somor]ai G A 1994 Nanocataiysis by the tip of a scanning tunneiiing microscope operating inside a reactor ceii Sc/e/ ce 265 1415... 

The microscopic understanding of tire chemical reactivity of surfaces is of fundamental interest in chemical physics and important for heterogeneous catalysis. Cluster science provides a new approach for tire study of tire microscopic mechanisms of surface chemical reactivity [48]. Surfaces of small clusters possess a very rich variation of chemisoriDtion sites and are ideal models for bulk surfaces. Chemical reactivity of many transition-metal clusters has been investigated [49]. Transition-metal clusters are produced using laser vaporization, and tire chemical reactivity studies are carried out typically in a flow tube reactor in which tire clusters interact witli a reactant gas at a given temperature and pressure for a fixed period of time. Reaction products are measured at various pressures or temperatures and reaction rates are derived. It has been found tliat tire reactivity of small transition-metal clusters witli simple molecules such as H2 and NH can vary dramatically witli cluster size and stmcture [48, 49, M and 52]. 

The concept of macroscopic kinetics avoids the difficulties of microscopic kinetics [46, 47] This method allows a very compact description of different non-thennal plasma chemical reactors working with continuous gas flows or closed reactor systems. The state of the plasma chemical reaction is investigated, not in the active plasma zone, but... 

Emulsion Process. The emulsion polymerization process utilizes water as a continuous phase with the reactants suspended as microscopic particles. This low viscosity system allows facile mixing and heat transfer for control purposes. An emulsifier is generally employed to stabilize the water insoluble monomers and other reactants, and to prevent reactor fouling. With SAN the system is composed of water, monomers, chain-transfer agents for molecular weight control, emulsifiers, and initiators. Both batch and semibatch processes are employed. Copolymerization is normally carried out at 60 to 100°C to conversions of - 97%. Lower temperature polymerization can be achieved with redox-initiator systems (51). 

Clump A collection of molecules small enough to occupy no more than a microscopic volume in a reactor, but large enough for the concept of concentration to be meaningful. 

As the flow of a reacting fluid through a reactor is a very complex process, idealized chemical engineering models are useful in simplifying the interaction of the flow pattern with the chemical reaction. These interactions take place on different scales, ranging from the macroscopic scale (macromixing) to the microscopic scale (micromixing). 

A strategic structure for reactor development is illustrated in Figure 8.33. To design a commercial reactor, knowledge of the fluid dynamics should be combined with the kinetics of microscopic phenomena, viz. chemical reaction. 

Figure 4.66 Photoq anation of pyrene (PyH) to the corresponding cyanated product (PyCN) in dicyanobenzene (DCB). Left schematic of the flow inside the micro reactor and a microscope image of the chip micro channels. Right GC and mass spectra of samples from micro flow processing [29].

Gas-particle flows in fluidized beds and riser reactors are inherently unstable and they manifest inhomogeneous structures over a wide range of length and time scales. There is a substantial body of literature where researchers have sought to capture these fluctuations through numerical simulation of microscopic TFM equations, and it is now clear that TFMs for such flows do reveal unstable modes whose length scale is as small as ten particle diameters (e.g., see Agrawal et al., 2001 Andrews et al., 2005). 

When applying CFD to model a chemical reactor, we are interested in knowing how the basic quantities (density, velocity, concentrations, etc.) vary with the spatial location in the reactor at a given time instant. The starting point for developing a CFD model is the microscopic balance equation, described in detail in standard textbooks on transport phenomena (Bird et al., 2002). Letting O denote a quantity of interest, the general form of its microscopic balance... 

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