I think that the answer can be found in the functional groups present on the surface (or in the pores). Activated carbon can have a number of functional groups, most of them oxygen containing, like carbonyls, hydoxyls, carboxyl groups. In some reactions these groups can be a part of the catalytic reactions.

The activated carbons can also be functionalized by surface modifications that increase the number of specific functional groups.

Actually, porous carbon is more widely used as a support for noble metal catalyst in industry. And lots of active sites can be put inside the framework of porous silica. Surface bonding on silica is more versatile than carbon. Thus, silica is more widely used as a catalyst or support compared to carbon.

Porous catalysts are used in order to increase the degree of reagents meeting on their surface. If the specific surface of the catalyst is the same, then in the first approximation the extent of the meeting will be the same. Next, it is necessary to compare the specificity of adsorption. Benzene will be better adsorbed on the carbon adsorbent. Therefore, its hydrogenation on the carbon adsorbent to cyclohexane will be better. Sulfur dioxide will adsorb better on mesoporous silica and therefore it will be better to oxidize to sulfuric anhydride.

The answers by Zijie Wang and Yuri Mirgorod are good answers but they are a little bit out of the scope from the initial question "why mesoporous carbon can be used as catalysts while mesoporous silica can only be used as catalyst support"

Henrik Romar

Hi Henrik,

Thanks for your useful input! I know carbon like graphite usually has hydroxyls on the surface. where does the carboxyl come from? Can you tell me more?

Also, I also know that silica also got a lot of silanol groups (Si-OH) on the surface, so I wonder any special reason why carbon is so better off over silica?

Regards,

Kenny

Zijie Wang

Hi ZiJie, I am a bit confused from what you mentioned. It seems that both carbon and silica are used as catalyst support and yet carbon is better?

I wonder why carbon can be catalyst while silica has been rarely used as catalyst.

Please tell me more~

Thanks

Yuri Mirgorod

Hi Yuri,

Thanks so much for your input! I wonder from your experience, can porous silica be used as catalyst to facilitate redox reaction of metals? lets say it is a case where metal reacts with some reductants in solution.

Best,

kenny

I believe that the copper ion will be better restored with hydrazine on porous silica than without it.

Yuri Mirgorod

Thank you Yuri. Your input will be a tremendous help for me!

Dr. S. Kalidhasan

Hi Kalidhasan,

Thanks for your input! Honestly I am not quite good at analyst chemistry. As your suggestion, I might really need to go back and read more related theories about this. Thanks again!

Source of carbon and types of treatments used govern the property of carbon based materials. Activated carbon contains various functionalities like ketones, esters, lactones, acids, etc. At the same time few carbons depending upon source contains more nitrogen based functionalities (like acridines). Moreover carbon materials also contain variety of metals and metal containing species.

The FGs on carbon as well as metal may behave as catalysts during the course of the reaction.

Activated carbons are further classified on the basis of treatment given. Some of the commercial used carbons are L2S, L3S, Norit, etc.

Porous nature of carbon provides higher surface area which further enhances the activity of the material in catalytic process.

I hope this information will be useful.

@ Anand S. Burange. I doubt that the substances with functional groups listed by you remain after calcination in activated carbon. "Activated carbon contains various functionalities like ketones, esters, lactones, acids, etc. At the same time few carbons depending upon source contains more nitrogen based functionalities (like acridines)." It would be good to give a link to an article on this issue.

Yuri Mirgorod. You are right that there are much less functional groups in ACs than in chars. But I don't think they are completely removed. By XPS one can see some functionalities even if the XPS "reads" a rather small area, is more a surface than a bulk method and the results are to some extent dependent on the pre-treatment of the sample.

@Prof.Mirgorod

Activated carbon does contain these functionalities.

Graphitization removes the FGs from the carbon where carbon is heated at very high temperature in absence of oxygen.

The carbon supports mentioned are commercial one and contains variety of FGs. Therefore we first check the pH of the carbon slurry.

It is quite difficult to handle carbon without FGs as it becomes hydrophobic in nature and difficult to do further modifications.

Mesoporous carbon will be more active due to accessibility of surface area compared to microporous ones.

I agree that everything depends on the processing of the sample of activated carbon. But activated carbon is heated to red heat.

If it is heated at very high temperature for long time then it will be no longer mesoporous.

Zhuoran Wu

The use of mesoporous carbon as a catalyst is based on its good mechanical and thermal stabilities, high specific surface area and tunable surface chemistry which allows the introduction of heteroatoms, and consequently, the anchoring of metal complexes. On the other hand, mesoporous silica material mainly consists of silica dioxide (SiO2) with simple surface chemistry property and poor stabilities under acid and base conditions.

Milad Mousazadeh

Hi Milad,

Thanks for your response. I agree with you that silica is not stable in acidic/basic condition. But when you stated "tuneable surface chemistry", I know silica also can be fine tuned by surface modification. Any more details you can add to that?