Q: The stability of the catalytic reaction is very important. The experiments we do in our laboratory have poor repeatability and often fail to be repeated. With the same catalyst and the same conditions, the results are quite different…
I don’t know how you keep your experiments stable, like the use of a catalyst, we’re using an electronic scale to one decimal place, adding a little bit more or adding a little bit more or not changing at all, I don’t know if it makes a big difference, right?
What’s more, we used a self-designed reactor, which is a quartz tube inserted in the heating jacket. I try to keep the same position of the catalyst bed every time, but I can’t guarantee that it will be the same. Do you have any good Suggestions?
There are a lot of effects in polyphasic reactions, and repetition is a real headache.
The amount of catalyst, the way of loading (there are clear requirements in engineering, at least the formation of a flat push flow, the ratio of length to diameter of the catalyst tube, the loading height of the catalyst), and the distribution of the strong exothermic reaction catalyst on the carrier all have an impact. But according to our results, these effects are not significant.
Generally, the larger ones that are difficult to repeat are the effects in preparation. For example, common impregnation, such as co-precipitation. Different feeding methods, even if we do not cover the beaker when a catalyst is aging, and roasting (especially if it is not programmed) have a decisive influence on the result.
The evaluation of catalysts generally follows the following principles in terms of experimental operation:
1. Experimental data should be obtained within the stable period. The service life of the catalyst is usually based on activity changes over time can be divided into induction period, stable period and current, although this is hard to strictly divided three interval, but think of it this way: induction period is a product to achieve the process of the relative balance of stabilization is a process of active and selective change smaller, and loss of current is the process of rapid decline, selectivity becomes poor. Usually, the induction period is not too long. In the small test evaluation experiment, the reaction liquid can flow through the catalyst bed for 1h before starting the sample connection analysis or online analysis. In comparison experiments and repeated experiments with fixed airspeed, the selection of initial sampling or online analysis should be as consistent as possible.
2. The particle size and loading conditions of the catalyst are relatively key. In heterogeneous catalytic reaction, the complete process including (1) ontology of reactant molecules from the reaction mixture liquid film diffusion to the catalyst surface layer, (2) the reactant molecules through the outer surface of the liquid membrane layer diffusion to the catalyst surface, (3) the active center of reactant molecules and catalyst in chemical adsorption, (4) under the action of reactant molecules in the center of the catalytic activity into transition state intermediate, transient intermediate (5) from the catalytic activity center over a product from the formation of molecules, (6) the molecules from the catalyst surface spread to liquid film layer, (7) The product molecules are diffused to the reaction mixture body through the liquid film layer. In the above 7 steps, (1), (2), (6) and (7) are the diffusion control factors, (3), (4) and (5) are the reaction kinetics control factors. Experimental operation should try to eliminate the step (1), (7) the influence of external diffusion control factors, did not respond at the same time avoid the reactants and directly from the space between the catalyst particles by, therefore in the continuous flow tubular fixed bed in heterogeneous catalytic reaction, catalyst bed height and the particle size of catalyst, reaction tube diameter, the reaction mixture viscosity need to meet certain relations. For the reaction mixture with low viscosity coefficient, the bed height of 20-40 catalysts filled with a reaction tube with diameter D = 1cm or above should generally meet at least h/D>5. When the catalyst is loaded, the bed should vibrate so that the catalyst particles are in close contact to ensure the consistent height of the bed.
3. The temperature control accuracy and temperature gradient of the catalyst bed are the key influencing factors. Therefore, the loading location and height of the catalyst are very important. The loading location should be fixed and located in the constant temperature zone of the reaction tube. The smaller the temperature gradient of the catalyst bed, the better. Note that the reaction temperature should be measured directly on the bed rather than outside the reaction tube.
As for the catalyst weighing problem, accurate weighing is related to your attitude and basic quality of learning, not to mention. To avoid the restriction of laboratory conditions, many difficulties can always be solved by self-thinking. I do not believe that you can not find a better platform scale or semi-automatic photoelectric analysis balance, not their own laboratory is not a neighboring laboratory is not. Accurate to the decimal point of the electronic scale is only suitable for weighing more than a few grams, unless your catalyst dosage is more than a few grams, otherwise such a large standard deviation is placed there, as long as you fall within the error range, the results of quite a lot of strange.
The reaction I did also had poor repeatability. The catalyst was prepared in the same way, under the same reaction conditions and in the same reaction time, but the product conversion rate varied greatly. I don’t know what’s the problem?
he factors you mentioned may affect the repeatability, but there are many factors that affect the repeatability. Preparation is a very important process, but it is also a process with poor repeatability. The storage time and conditions of some catalysts will also affect the activity. However, two consecutive evaluations of the same batch of catalysts should not differ much.