Open gel reactor

In Fig. 1.8 we show schematically the set-np in use in our experiment, for details please see [7]. The set-up adopted for our experiments has as central element an open gel-reactor, which allows to maintain constant non-equilibrium conditions during the measurements, see Fig. 1.8, With it, and by means of computer-based spectrophotometry [37], we aiialyze w ave activity in the BZ medium with sufficiently liigh spatial and temporal resolution. 

H. Brandtstadter, M. Braune, I. Schebesch, and H. Engel. Experimental study of the dynamics of spiral pairs in light-sensitive Belousov-Zhabotinskii media using an open-gel reactor. Chem. Phys. Lett., 322.-145—154, 2000. 

The light-sensitive BZ reaction demonstrates all basic features of excitable media of quite different nature and represents a very suitable experimental system to study controlled motion of spiral waves. In the experiments reported below an open gel reactor is used [29-31]. The catalyst is immobilized in a silicahydrogel layer of 0.5 mm thickness prepared on a plate... 

While until recently the investigation of the intrinsic dynamics of the reaction was in the main focus of interest, more emphasis will be given in the future to the experimentation under conditions of external forcing and control of the patterns. Initial results were summarized in Section 6, and this first series of efforts should be continued, preferentially in open gel reactors. Efficient ways to control some of the system properties are available, for instance the electric field and the photosensitive systems, that make future research in this direction a worthwhile endeavor. 

The chemical system used for our study is a chlorite-iodide-malonic acid (CIMA) reaction in an acidic (sulfuric acid) aqueous solution. The CIMA reaction exhibits a rich variety of phenomena oscillations in a batch reactor or in a CSTR [26], transient target waves in a closed Petri dish [26], bistability in a CSTR [26, 27], front structures in a Couette reactor [27-30], and Turing patterns in open gel reactors [7-10]. In our two-side-fed reactor. Figure lb, components of the reaction are distributed in the two compartments in such a way that neither compartment is separately reactive. Chlorite is only in compartment A , and malonic acid is only in compartment B thus there are opposing chemical concentration gradients in the direction normal to the plane of the gel. The other chemical species are contained in equal amounts in both reservoirs, except for sulfuric acid, which is more concentrated in compartment B than in compartment A. Note that chlorite and iodide in compartment A are at a low acid concentration they would react rapidly at high acid conditions. 

Those increasingly complex dynamics are thus very likely to be observed in the active region of the open gel reactors which is confined to the front, e.g. in a narrow linear (linear gel reactors) or circular (annular gel reactors) strip. The understanding of these dynamics provides a very exciting theoretical and... 

Since they are open systems that can exchange chemical species with their surrounding solvent, gels can also play the role of chemical reactors. In this framework, the design of open spatial gel reactors has allowed well controlled experimental studies of chemical patterns such as chemical waves or Turing structures (2). They are made of a thin film of gel in contact with one or two continuous stirred tank reactors that sustain controlled nonequilibrium conditions. 

In the modified open gel-ring reactor, a gel ring (Fig. 10.4b) separates the two electrolyte solutions, which are fed continuously at the same flow rates. These reactors... 

Open-tube reactor The conditions of analysis using an open-tube wall reactor with an enzyme immobilzed at the inner wall are very similar to assays with soluble enzymes in flow systems. A monomolecu-lar enzyme layer covalently bound to the etched inner wall of polystyrene or nylon tubes [365] has a rather low activity requiring a long tube to provide a sufficient amount of reaction product for detection. For this reason it is more advantageous to bind enzyme(s) to a polycarboxylic gel layer attached to the inner wall of a small-bore nylon tube providing a thick porous enzymatic annulus in a tubular envelope [366]. 

Fig. 1. Schematic diagrams of open spatial reactors (a) the reaction medium, a thin gel disk (b) reaction system for two-side-fed reactor and (c) reaction system for one-side-fed reactor, ((a) and (b) from [10], (c) from [20])...

In 1990, De Kepper and colleagues in Bordeaux, working with an open unstirred gel reactor, observed the first experimental evidence for Turing structures in a chemical system, the chlorite-iodide-malonic acid (CIMA) reaction [4]. Since then this work has been verified and extended both by the Bordeaux group [11-13], and by Ouyang and Swinney [14] in Texas, using several different reactor configurations. 

To enhance the visibility of the oscillations and spatial structures in the CIMA reaction, starch is used as a color indicator of triiodide in most experiments. In the open spatial gel reactors developed in Bordeaux and Texas polyacrylamide gel (PAA) was used as the reaction medium to prevent convective motion. The starch is introduced into the acrylamide monomer solution before poly-... 

In the past few years, however, there has been a rebirth of interest in the formation of dissipative structures in chemically reacting and diffusing systems. This interest has been mainly sparked by the developement of open spatial reactors by groups in Texas [21-28] and in Bordeaux [29-38]. Basically, two types of open reactors are currently operating (i) the two-dimensional continuously fed unstirred reactors where the transport process is essentially natural molecular diffusion and where the feeding is either uniform (continuously fed unstirred reactor [23,24]) or from the lateral boundaries (linear [34, 35], annular [21, 22, 38] or disc [25, 36] gel reactors) and (ii) the Couette... 

The synthesis of imidazoles is another reaction where the assistance of microwaves has been intensely investigated. Apart from the first synthesis described since 1995 [40-42], recently a combinatorial synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles has been described on inorganic solid support imder solvent-free conditions [43]. Different aldehydes and 1,2 dicarbonyl compounds 42 (mainly benzil and analogues) were reacted in the presence of ammonium acetate to give the trisubstituted ring 43. When a primary amine was added to the mixture, the tetrasubstituted imidazoles were obtained (Scheme 13). The reaction was done by adsorption of the reagent on a solid support, such as silica gel, alumina, montmorillonite KIO, bentonite or alumina followed by microwave irradiation for 20 min in an open vial (multimode reactor). The authors observed that when a non-acid support was used, addition of acetic acid was necessary to obtain good yields of the products. 

On the other hand, the mechanical properties of monolithic carbon gels are of importance when they are to be used as adsorbents and catalyst supports in fixed-bed reactors, since they must resist the weight of the bed and the stress produced by its vibrations or movements. A few smdies have been published on the mechanical properties of resorcinol-formaldehyde carbon gels under compression [7,36,37]. The compressive stress-strain curves of carbon aerogels are typical of brittle materials. The elastic modulus and compressive strength depend largely on the network connectivity and therefore on the bulk density, which in turn depends on the porosity, mainly the meso- and macroporosity. These mechanical properties show a power-law density dependence with an exponent close to 2, which is typical of open-cell foams. 

The reactions were conducted in an open continuously stirrer tank reactor (CSTR) at room temperature, with a peristaltic pump supplying the feed solutions of potassium bromate, sodium sulfite, potassium ferrocyanide and sulphuric acid and also pumping out bulk solution to keep the volume in the CSTR constant. Measurements of pH were made using a standard gel-filled probe, and the output from the meter monitored by a PC. A typical pH profile is shown in Figure 1. 

The reactor is open with respect to the flow of one reactant as the other electrolyte was taken in the gel. In this reactor, the concentration and level of the electrolyte in the empty space were always kept constant. The DPL reactor was further modified by Das and co-workers to make it open with respect to the flow of both the reactants [61]. 

If a normal chromatographic column, as shown in Fig. 4,is employed a uniform distribution of samples throughout the voids will take an unpractically long time. Two different solutions for this problem have been offered. The first of them employs a construction similar to the stirred batch enzyme reactors used in the analysis of immobilized enzymes (ref. 35,37). In that mode, an open chromatographic column containing appropriate gel slurry is stirred continuously (ref. 35). The apparent advantages of this method are its simple construction, short equilibration and lack of need of special sanpling port. Equilibration times of about 10 minutes are sufficient with the apparatus (ref. 

In addition to the intense internal current of liquid continuously penetrating the polymer support inside the spinning rotor, an external flow can be diverted since the mode of action in the vessel is comparable to a rotary pump. In this way external flow cells can be circulated with reaction solutions from the centrifugal reactor without any additional pump. Hereby, the alterations of concentrations in the circulating solutions caused by consumption of reagents or liberation of molecules cleaved from the gel support can be measured directly by flow photometry (UV, IR, conductivity, etc.) and recorded continuously. On opening an outlet valve on the bottom, the reactor is emptied rapidly and completely by spinning off. 

For example, alcohol dehydrogenase has been immobilized into nanotubes of Ti02, which were prepared by sol-gel template synthesis. This enzyme retained its ability to oxidize ethanol for more than 4 days (NAD" cofactor, phosphate bulfer, pH Since these Ti02 nanotubes were open at both ends, this configuration allowed their use as a flow reactor. Examples of boimd enzymes acting as bioreactor are large in number. They also involve covalent binding of antibodies to functionalize sol-gel films. ... 

In this chapter, we have considered the analytical techniques that can be used in monitoring chemical oscillations and waves, from standard spectrophotometry to spatial NMR methods. The most commonly used methods have been potentio-metric because of their low cost and ease of use. Of course, to measure something, you need a reaction in a vessel. We evaluated the types of reactors used for homogeneous reactions, of which the CSTR is the most common. Spatial phenomena can be studied in simple test tubes or Petri dishes, but careful work requires open systems using gel or membrane reactors. 

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