Decolorization needs to use decolorization agent. Decolorizing agent according to the principle of decolorizing can be divided into three categories: flocculation decolorizing agent, oxidation decolorizing agent and adsorption decolorizing agent. Most of the flocculant decolorization agents are quaternary amine type cationic polymer compounds. Oxidative decolorizing agent is the use of such as: potassium permanganate, ozone and other oxidizing compounds will be colored groups oxidative destruction and color; Adsorptive decolorization agent for activated carbon, clay, silica gel or adsorption resin can be directly used to remove impurities by filtering.
The selection of decolorizing agent in organic synthesis is generally based on adsorption decolorizing agent, among which activated carbon and silica gel are the most frequently used.
First of all, to be clear: activated carbon and silica gel decolorization are dependent on the intermolecular force, is a physical adsorption decolorization process, which does not involve chemical reaction; And activated carbon and silica gel properties are different, for different properties of pigments have different effects, For example, silica gel is a polar adsorbent, its hydrophilic pigment adsorption effect is much higher than that of lipophilic pigment; Activated carbon is a non-polar adsorbent, which has a much higher adsorption effect on lipophilic pigment than hydrophilic pigment. There is also a point that we must accurately understand the specific process of decolorization, decolorization is a physical adsorption – elution process, which adsorption is not only pigment adsorption, but also organic adsorption; Elution is not only pigment elution, but also organic elution: of course, if the color removal effect is good and the yield is high, is the adsorption of pigment in the process of adsorption, and less organic adsorption: and in the elution process, the pigment was not eluted, and the organic matter was eluted.
Actived carbon decoloration
Activated carbon according to its shape can be divided into powdered activated carbon, granular activated carbon and fibrous activated carbon. Activated carbon is a kind of carbon material with great specific surface area and strong adsorption and decolorization ability. It was used in the 19th century to decolor, taste and purify sugar, wine and water. Because most pigments have a conjugated structure, it is easy to adsorb, and activated carbon has a large specific surface area and strong adsorption capacity, so it can effectively remove many kinds of pigments. At work, often see some people do decolorization, add a little activated carbon to stir casually, and then filter, the effect is not good, this belongs to the “look at the sky”, the effect is not good most of the time. In order to decolorize organic matter well, it is a systematic process, of course, there are also routine, the basic considerations are: solvent, solvent amount, stirring temperature, stirring time, the amount of activated carbon, and a very important factor is the model of activated carbon.
solvent: Generally solvents capable of completely dissolving compounds
Amount of solvent: Generally, 1g compound about 1ml is the best, of course, more or less is not a big problem;
Mixing temperature: Generally 60-80 degrees Celsius is appropriate, the same, high and low points are also possible:
Churning time: Generally 1-2 hours is appropriate, of course, stirring overnight is also acceptable;
Amount of activated carbon: Generally with five to ten thousandths is appropriate, of course, if our compound is not adsorbed by activated carbon, so a little more is just as well;
Activated carbon model: This should be especially emphasized, there are many activated carbon models, choose the model must leave more heart, choose the model of activated carbon suitable for their own compounds.
Have two point to emphasize that activated carbon is a kind of non-polar adsorbent, it in the water of the decolorization effect is strongest, decoloring effect can also be in big polar solvent, decoloring effect is the worst in nonpolar solvent: the second point is to join the timing of the activated carbon, it is best to our compounds had been heated before completely dissolved, it is best not to add from the start, so you don’t know whether our compounds have completely dissolved: do not add active carbon decoloring liquid at high temperature, this time with bumping rushed material easily.
The silicone decoloring
The chemical properties of silica gel are relatively stable, insoluble in water and do not react with water. It is an acid oxide and does not react with general acids. Unlike activated carbon, it does not have as large specific surface area, and its decolorization principle is somewhat similar to that of a post. The use of silica gel decolorization, of course, there are routines, the basic considerations are: solvent, solvent amount, stirring temperature, stirring time, the amount of silica gel (these basic considerations are similar, in fact, many reaction optimization is also similar conditions to consider). Sometimes, activated carbon decolorization is not good, you can try to use silica gel decolorization; Sometimes the two can be used in combination