The papers show long range orderedness as a vital feature for most porous materials. Sure it will increase the amount of active surface area. However, it's known that pore-openings or pore mouths will decide the activity and selectivity of the entering reactant molecule.
Then how long range ordered feature is a boon to catalysis, despite increasing secondary interactions and hence fouling of catalyst.
I am not sure that long-range orderness is a so vital feature. What is required before all is a high surface area, allowing the existence of as many reactive sites as possible, and a high availability of these sites. Therefore, provided that the porosity is sufficiently open and interconnected, it will be good, I don't see why it should be also ordered in addition to this. For example, a hierarchical structure in which narrow pores are progressively accessed through wider pores at different scales will be perfect, even if disordered.
Alain
The following information may be useful for your research
Abstract:The influence of long-range residue interactions on defining secondary structure in a protein has long been discussed and is often cited as the current limitation to accurate secondary structure prediction. There are several experimental examples where a local sequence alone is not sufficient to determine its secondary structure, but a comprehensive survey on a large data set has not yet been done. Interestingly, some earlier studies denied the negative effect of long-range interactions on secondary structure prediction accuracy. Here, we have introduced the residue contact order (RCO), which directly indicates the separation of contacting residues in terms of the position in the sequence, and examined the relationship between the RCO and the prediction accuracy
Ref: The effect of long-range interactions on the secondary structure formation of proteins by Daisuke Kihara
www.ncbi.nlm.nih.gov › NCBI › Literature › PubMed Central (PMC)
Thank you Prof. Alain Celzard for the information. Hierarchical zeolites reminds me of famous 'canal system in sponges'. Since i'm interested in porous materials, I've seen articles emphasizing the this term long-range orderedness for better activity/selectivity. I think in the case amorphous materials where comparitively weak acid sites and wide pores are present. reactants may not react at the entrance but through out the pore. Yet its a guess. I want a paper/book or so for understanding its actual significance in various fields apart from catalysis.
In ordered porous materials (amorphous): long-range orderliness will generate more active sites and will have more activity; because of their open pores, their won't be any diffusion limitations and catalyst will remain active for long time. But they are not active as hierarchical crystalline materials.
where as in amorphous dis-ordered materials: their closed and dis-ordered pores will cause coking and deactivate the catalyst faster.
Therefore, as Prof. Alain Celzard said, hierarchical crystalline materials will be a good option or you can try 3D ordered porous materials.
A desired catalyst should possess 1. good activity for high conversions, 2. high stabilities (thermal/mechanical) for long catalytic life. then comes the diffusion.
amorphous materials are poor at both. However as you said, the increase in surface area might boost up the reactions where grafted materials can be studied
Mr. Parsapur, please note that catalyst used to produce millions of tons of polyethylene are based on amorphous silica.
Dr. Syriac Palackal, Thanks for the note sir. I meant 'ordered mesoporous silicates' as amorphous silicas where expensive surfactants are of concern and also bulk synthesis of the material by maintaining the inherent properties unchanged can also be an issue. Its good to know that amorphous silicas are supports in such large scale synthesis. I have also read that silica-aluminas were used to be catalysts in petroleum cracking before the zeolites replaced them.
Dear Rajesh
Two points are important which are described by Prof. Alan and Prof. Krishna
(1) High Surface Area and Hierarchical Structure
(2) Amoropous Porous Materials- More Activesite- More Activity
It all depens of each case i.e. of the catalyst-surface interaction. Sometimes long-range ordered porosity will be favorable, sometimes not. Porosity is generally desired, but a specific kind of porosity could be detrimental in several reactions...
May facilitate formation of conductive channels through which charge carriers can migrate across hence better activity
Humphrey
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