I have synthesized a catalyst and I am applying it in catalysing biomass pyrolysis reaction to obtain bio-oil. The catalyst shows best performance when it is used at a concentration of 7% giving the maximum amount of bio-oil. We do not see any further increase in the amount of bio-oil even after increasing the catalyst concentration beyond 7%. Why is it so?
This is a pretty difficult question to answer since there are so many factors to consider. One suggestion that comes to mind most readily is that whatever material on which your catalyst acts is used up completely and you have reached the maximum possible yield. Can you give any additional details of your system?
Many homogeneous catalysts decompose by pathways that involve multiple catalytic centers coming together. Whether it is formation of a stable off-cycle dimer, disproportionation, comproportionation, or aggregation/precipitation, these reactions will be 2nd order or higher in catalyst, while the desired reaction is usually 1st order in catalyst. Therefore, there is a catalyst concentration above which the reaction will suffer.
Another possibility is that there is a byproduct of catalyst activation that can interfere with the reaction. I once ran a reaction using a Pd2dba3 precatalyst where the displaced dba caused problems if I used too much...
The answer is it depends. Can you please give more details of the experiment? Is it batch or continuous? After the 7% does the yield get stable or keep going down? If it gets stable this means that the number of active site is not the limitation factor and you have reach your maximum yield at that particular condition. If it keeps going down, which means you have a competition of processes going opposite each other. For example, for biomass conversion over an acidic catalyst, you can have a competition between the cracking of biomass polymers leading to higher yield of the lighter fraction (i.e. bio-oil). Imagine if this is a non-catalytic reaction those heavybiomass polymers will probably end up as char. However, the cracking also forms coke on catalysts and gas, which reduce the bio-oil's yield. For that reason, there should be an "optimum" concentration of catalyst (or active sites) which is just enough to break down biomass polymers into bio-oil but not too much so that coke and gas formation do not dominate. Maybe 7% is that "optimum" concentration in your situation. Also, mass and heat transfer can play an independent role. You have to design your experiment carefully to see what is really happening.
Hi
Can you please provide us with the type of the reactor (tubular or batch) and the reaction condition as there are different factors controlling the catalytic performance
Thanks
Dear Kashmiri,
Adding on to the above answers by other respected colleagues, there is certainly no straight answer to your question. Many factors such as your process type and scale, your feedstock composition, and your catalyst characteristics and so on will effect the yield and quality of your final products.
As I understood your question, you are trying to increase bio-oil yield by using your synthesised catalyst. However, please remember that the catalyst will not only effect the product yield but more importantly its quality.
In fact, we have noticed that the catalyst actually reduces the liquid oil yields (but improved oil quality) in our case of oil from waste plastic using a small batch pyrolysis process.
For more details on catalyst effect on pyrolysis products yields and quality and mechanisms, you may find our recently published review article interesting and informative for your study.
Catalytic pyrolysis of plastic waste: A review
http://www.sciencedirect.com/science/article/pii/S0957582016301082
Wish you all the best.
Rehan