Without experimental research, nothing can be said. The surface of the catalyst must be physicochemically examined before and after the process. Extensive kinetic studies must also be carried out with different polymer initial concentrations, different temperatures, different stirrer rpm, and different hydrogen pressures and flow rates. It is also worth examining the dependence of the overall rate of the process on the size of the catalyst particles.

If we say heterogeneous catalyst, then this means that it is in a physical state different from the reactants, e.g. A gas + B gas ------–solid cat.-----→ Product. In such case, the catalyst provides a favorable surface that activates the reacting molecules. These molecules are activated by a process which involves interaction of reactants with solid catalyst by chemical adsorption "chemisorption" which is different from physical adsorption. In chemisorption, a sigma covalent bond may be stretched & a C=C double bond may utilize the pi electrons. Here, the pores & their sizes come to play a role in the kinetics of the reaction.

Once the reacting molecules are activated, then they may react at once to give the product which leaves the surface by a process called "desorption".

This is a general answer about the mechanism taking into account that the question is NOT clear about the identities of the polymer & the product. Also, how there was judgement that the benzene rings were completely converted to cyclohexane rings knowing that it is rare in polymer analogous conversions (i.e. polymer X to polymer Y) to have 100% change.

Avinash Lende , Back to the question. The catalyst does not directly affect the polymer particles, as it cannot, due to the pore size. It seems, however, that it can influence the formation of H radicals and they may react directly with cyclohexane rings. I am curious about the rate of polymer hydrogenation without a catalyst compared to a catalyst.

Dear Prof. Nizar Matar and Mirosław Grzesik Thank you very much for your kind consideration and suggestions. Yes, your saying is almost true but to prove the mechanism is very difficult. I understand it is "On-water" mechanism in which hydrogen bonding plays an important role. I performed the experiments using polybutylene terephthalate (PBT) as a reactant and got the product after the complete hydrogenation was polyester poly(butylene-1,4-cyclohexane dicarboxylate) (PBC). I studied in detail including all the parameters such, reaction time, temperature, hydrogen pressure, solvent (water) amount, stirring speed, and the concentration of the reactant. We observed that all parameters affect the selectivity and the conversion of PBT to PBC.

For the confirmation of the conversion PBT to PBC, we used NMR techniques as the primary techniques. The aromatic peak completely disappeared from the product spectra and other peaks are consistent with the reactant spectra, additional peaks are observed in the aliphatic region for the product spectra. These spectra confirmed the complete and selective hydrogenation of PBT to PBC.

The hydrogenation rate was totally changed without catalyst when I performed the blank experiment (no catalyst) at the optimized conditions the conversion was almost zero. That means, it is not possible to convert polymer A to polymer B without a catalyst.

If there was chemisorption or the hydrogen bond stretching or the hydrogen radical formation, how we can prove it? Because of this phenomenon present at the time of the experiment only. Additionally, we analyzed the catalyst before and after the experiment using, ICP-OES, BET-surface area analyzer, and HR-TEM. After the first cycle, no metal leaching observed (ICP-OES), almost 1-5% agglomeration was observed (HR-TEM) and slightly reduced the surface area.

I will try to read more open literature, to understand the actual phenomenon and how I can conclude this concept. If you have suggestions please help me to get a better solution. I am always ready to reply to you and to share my outcomes.

Thank you very much for your discussion, guidence, and valuable time.

I appreciate your kind consideration and answers.

Sincerely,

Avinash

Just Three Notes:

• Since the polymeric reagent is poly(butylene terephthalate), then the entangled chains are not expected usually to allow complete conversion or total hydrogenation.

• It is difficult to accept reliance on only NMR techniques in this type of research. One or "better" two techniques is/are required to confirm this "rare" event of 100% polymer to polymer conversion, if the result is as such.

• The 2 polyesters (PBT & PBC) are expected to be water-insoluble and my advice is to correct the statement starting by " I studied in detail including all the parameters such…" into " I studied in detail including many parameters such…" and to change description from "solvent(water) amount" to " medium(water) amount".

Good luck.

Avinash Lende, Based on the information given in the question, I was able to guess for the most part. But I think I have given a plausible explanation, especially since I now know that without a catalyst, the hydrogenation rate is close to zero. The detailed explanation is of course yours. But, I am wondering how long the lifetime of the H radical must be for hydrogenation of cyclohexane rings to be possible. Presumably, the breakdown of H2 will occur at the inner pore surface of the catalyst, and hydrogenation will occur outside the catalyst. Moreover, for the given mean pore diameter, one would expect a relatively high mass transport resistance. Therefore, I am curious what is the particle size of the catalyst? It may turn out that with their small size and intensive mixing, the mass transport resistance (external and internal) is of no importance.

As for water, in my opinion, its role is primarily to create a suspension and allow the transport of hydrogen inside the pore and possible radicals to the outside. However, there is another problem related to the very low solubility of hydrogen in water. So I have another question, how was the hydrogen dissipated in the water and whether you tried to assess the hydrogen consumption. Regards,

Dear Prof. Nizar Matar

Thank you very much for your kind consideration and valuable time, I will follow your suggestions and try to double-check the analysis and confirm the PBT and hydrogenated product PBC. As you advised, I will take care of my writing.

Thank you once again,

Sincerely,

Avinash

Dear Prof. Mirosław Grzesik

Thank you very much for your valuable suggestions and time, the particle size of the catalyst is around 2 microns. I have not tried to measure the hydrogen solubility or consumption in water but I used 1000 psi pressure for hydrogen.

Now I am planning to use another type of catalysts such as colloidal and core-shell catalysts without the support and compare the data with the previous one. I hope I will get some informative results. My aim is to understand the "On-water" mechanism and to confirm the presence of the same phenomenon in this system. I will update you soon.

Thank you very much once again for your kind consideration and help.

Sincerely,

Avinash