Mesoporosity definition

The name "monolith" stems from the Greek mono and lithos, single and stone. Every art and science has its own definition of the term monolith, and some confusion may arise. The Egyptian obelisks are monoliths, as are the menhirs of Asterix and Obelix, and the spectacular Ayers Rock in Australia can be considered as a monolith of nature. In separation technology, monoliths are used that are rods with well-defined macro- and mesoporosity. 

There is disagreement within the aerogel community about the definitions of aerogel materials. Some definitions include porosity restrictions such as mesoporosity requirements (2-20 nm), others require supercritical drying techniques, and some include density restrictions. Even measuring these properties can lead to uncertainties and controversy. 

The increase of the aging time after (DP) at 363 K induces an increase in the BET area, but the pore volume passes through a maximum after an aging time of l.Sh, there is an increase in the specific area and a more definite adsorbed volume at p/po= 1- For longer aging times, there is a progressive decrease in the pore volume, indicating a loss in mesoporosity. 

Throughout this book, reference is made to the characterizations or definitions used to describe this porosity. Thus, porosity in carbons is placed into four categories, not for convenience but out of necessity. The size of the pore determines how adsorption takes place in a pore be it narrow and wide microporosity <2.0 nm, mesoporosity 2.0-50 nm and macroporosity >50 nm. Chapter 1 introduces the reader through the sevaal areas of relevant expertise. 

Mesoporosity is defined as that porosity which has an entrance dimension >2nm and <50 nm. Such a definition is based around the way that nitrogen is adsorbed into porosity. The filling of the porosity, >2 <50 nm, is called capillary condensation. Adsorption occurs initially on the pore walls with the result that the pore is not only a narrower pore, but also it behaves as a narrower pore with a higher adsorption potential well in the center (Figure 4.2). This process continues with increasing relative pressure within pores of increasing diameter until the 50 nm limit (or thereabouts) is reached when the entrance dimension of the pore (gap between the pore walls) is so large that the porosity behaves as an open surface. 

These transitions in pore filling mechanisms are dependent on the size of the adsorbate molecule and so the above definition (>2 nm and <50 nm) is arbitrary being a function of the adsorbate used. However, as nitrogen dominantly is the characterizing adsorptive, these definitions must suffice. From an adsorption point of view, mesoporosity is not that all-important. Its role is as a transport pore (a means of passage), particularly for adsorptions from solutions, to permit rapid access to the retaining microporosity of the adsorbate of the system. 

It is questionable as to whether the various isotherms attributed to Dubinin and coworkers yield the surface area. They are definitely useful for finding the mesoporosity volume due to the clear linear extrapolation. According to Kaganer [16] the intercept of the DR equation is the mono-layer amount. This seems to have been empirically based upon the BET formulation. The modified DR equation, referred to as the DRK equation, for a flat surface is... 

The signals in the isotherm that indicate mesoporosity are the type IV and V isotherm or feature 3 in the standard plot designation, that is a positive curvature at pressure well above the threshold in the standard or x plot. This would be a practical definition for purposes of analysis. The lUPAC definition is... 

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