The catalyst you used for the reaction might be only the precursor and it have to be converted to the reactive species in the reaction course, sometimes it's only a few amount (few % or even ppm) but actuallt catalyze the reaction. In the spent catalyst, there is higher chance for existence of these species then the reaction will be faster than fresh one.

Dear Le Phuc Thien , Can you share any references?

Thank you in advance

Le Phuc Thien Thank you so much for this information. Could you please suggest any refrence?

I'm not sure what type of your catalyst is, but there are lot of examples that initial cat is not the real 'catalytic species'. My group has reported a similar example in JACS 2015, in which they found that the active species only formed in ppm order, but have an incredible reactivity and mainly catalyzed the asymmetric hydrogenation.

Article Mechanism of Asymmetric Hydrogenation of Aromatic Ketones Ca...

Dear Noor Aljammal many thanks for posting this very interesting technical question on RG. Please note that we are synthetic inorganic chemists (organolanthanide chemistry), so that I am not a proven expert in either organic synthesis or heterogeneous catalysis. However, to the best of my knowledge, aldol condensation reactions are often heterogeneously catalyzed by different metal oxides such as MgO or ZrO2. Since you are talking about a spent catalyst, it appears clear that you are using a heterogeneous one. However, unfortunately you did not further specify in more detail the nature of your particular catalyst. As you certainly know, the success of heterogeneous catalysis largely depends on many different factors such as preparation and pre-treatment of the catalyst, particle and pore sizes, surface area, pore size distribution, and of course other reaction conditions such as solvent and temperature. Thus all I can do right now is to suggest to you two potentially useful literature references in which the acitivties of freshly prepared and spend aldol condensation catalysts has been studied in detail. One possible reason for the higher activity of the spent catalyst could be the presence of more OH– ions on the surface after the first reaction. Thus please havve a look at the two articles in the attachment.

I hope this helps answering your question. Good luck with your work and best wishes, Frank Edelmann

Frank T. Edelmann Many thanks, prof. Frank for the useful information.

Actually, I am using two types of catalysts one of them is Aminated poly(ethylene glycol) methacrylate (PEGMA) and the other one is one type of MOFs (UiO-66).

I have the same observation for both catalysts but for MOF-based catalysts, I explained that by the fact that with multiple reactions the MOFs become free from the solvent stayed in its pore while preparation as after the fourth iteration I noticed the activity decrease, While for the other type, aminated poly(ethylene glycol) methacrylate, the activity of the spent catalyst is higher than the fresh one even after 4 cycles.

Unfortunately for PEGMA we couldn't perform BET analysis to check the pore volume as I care about the active site loading. Moreover, the swelling rate of this material truly affects the catalyst performance.

I will have a deep look at the suggested articles by you. Thank you so much, it is highly appreciated.

Le Phuc Thien Many thanks for your help and the suggested article

Dear Noor Aljammal , I do not know your reaction, and the type of catalyst you used. Anyway, it can be happen that a catalyst could be more active in the "aged" status compared to the fresh obne. It depends on the nature of the active sites, and the transformation the fresh catalyst undergoes during the reaction. You should perform a post-mortem analysis of the used catalyst, and compare it with the fresh one for a better understanding. Could be some species hidden in the support, that can emerge if the support is corroded, for example, or modification of the active sites because of the adsorption of reactive species during reaction.

Good luck!

Very interesting technical question Madam..

Increase in catalytic activity with spent catalyst... Are you recovering and re-using the catalyst ? If yes.. What treatment (s) or purification (s) are you carrying out to recover it? I'm not sure, if you are using any mineral acids to wash and reusing it in the next cycle. Most of the aldol condensation reactions are acid catalysed and so if traces of acid present, may cause increase in reactivity. Or else, what Prof. F. T. Edelman sir explained due to increasing functional active sites of catalysts during first or successive reactions is considerable. And even, in some of the cases, the pre-catalysts are being converted to active species slowly during the reaction and enhance the catalytic activity as described by L. P. Thien.

Stefania Specchia Thank you so much

M Venkata Krishna Reddy Many thanks for your answer. I use the catalyst without any regeneration. Only drying .

I agree with Stefania Specchia and recomendations. This phemonen has been known for over 60 years. I remember to read it in a book during my graduation.

Dear Noor Aljammal, I was looking at some literature on ageing of catalysts I know quite well, you can read these pulications to have more info on ageing mechanisms, and how you can try to improve the resiance of your materials. Mauìybe not focusing on your topics, but I hope these articles can be helpful, and inspire you.

On methane combustion:

Article Sulphur poisoning of LaCr0.5−xMnxMg0.5O3·yMgO catalysts for ...

Article Aging of Premixed Metal Fiber Burners for Natural Gas Combus...

Article Effect of S-compounds on Pd over LaMnO3 center dot 2ZrO(2) a...

And oxygen reduction in PEM fuel cells:

Article Mapping Transition Metal-Nitrogen-Carbon Catalysts Performan...

with the references articles used for this review:

Article Achilles' heel of iron-based catalysts during oxygen reducti...

Article Recent progress in the durability of Fe-N-C oxygen reduction...

Stefania Specchia Thank you so much Prof. Stefania.

If your catalysts need some activation procedure in start up, the incomplete activation may cause such problem. Performing the short-time activity test in lab is another source for observing this kind of problem.

Akbar Irandoukht yes indeed, I have to thermally activate it under vacuum. Thank you for the hint :)

The catalyst may undergo various changes, including structural changes, during a chemical reaction. And this may affect its activity. Of course, preparation is important, but also pre-treatment. One should also not forget about the presence of undesirable substances in the reaction mixture and the possible side reactions.