Dear Przemysław P. Michalski, please find attached a reference in which the citrate sol-gel method is described in some detail. In order to find such explanations, one has to look for the oldest references. In more recent paper the knowledge about the process is often taken for granted. I'm not an expert in sol-gel processes, but I think that no "esterification" will take place in aqueous solutions. A major factor is the formation of soluble metal citrate complexes, which is aided by the addition of a base (e.g. ammonia).

Dear Frank T. Edelmann , thank you for the answer and source. However, I see that Authors used ethylene glycol for polymerisation. This case is quite clear for me. Unfortunately, it is not my case - I do not use glycol or other substrates which would cause polymerisation. Despite that, I obtain a viscous gel-like amorphous material. I am curious what is happening in my case.

Best regards.

Przemysław P. Michalski

Your assumptions are somewhat true. In addition, during hydrolysis in the medium, decavanadate is formed, which gels the solution. There is a competition of two processes. This competition must be clarified by physicochemical methods.

Dear Przemysław P. Michalski, I think Yuri Mirgorod added a good point. The problem is, that your reaction system is rather complex. Did you check previous questions, discussions and research items on RG for the keywords "citrate sol-gel method"? This is often very helpful.

Dear Przemysław P. Michalski, as an interesting side-aspect of your question I just found an article in which it is described that citric acid reacts with ammonium vanadate to give a well-defined vanadyl citrate comples. The paper is entitled "Synthesis and crystal structure of sodium ammonium dimeric (citrato)dioxovanadium(V) Na2(NH4)4[VO2(cit)]2·6H2O". Unfortunately I don't have access to the full text (Chinese Science Bulletin 1995, 40, 749-52)

Dear Yuri Mirgorod and Frank T. Edelmann, thank you very much for your further clarifications and additional sources. I did not know that decavanadate also can influence gelation process. I checked on previous questions on RG, also studied the literature, but unfortunately all I found was a hypothesis (that was not confirmed) about citric acid self-estrification. I will do some further research about vanadates. However, I do not think that it would be the main factor in my case, as the vanadium amount used is small - it is only 5% mol dopant to a final product.

Dear Przemysław P. Michalski, the self-esterification of citric acid has been previously discussed on RG:

https://www.researchgate.net/post/Why_is_there_no_Citric_acid_self-esterification

You should also take into consideration that boron oxide and ammonium metavanate could react under formation of a borovanadate....

Dear Frank T. Edelmann - that was exactly the discussion I was thinking about when mentioning self-estrification :) It led me to my second thought about estrification between citric acid and other carboxylic acids present in solution. Thank you for other ideas.

Dear Przemysław P. Michalski, I just found another very interesting publication in this context. The paper is available on RG. It shows that esterification of citric aid with n-butanol can be catalyzed by heteropolymetalates. Such species (borovanadate) could also be present in your system.

Article Heteropolyanion-Based Ionic Liquids: Reaction-Induced Self-S...