Pore area very narrow

Surface area is one of the most important factors in determining throughput (amount of reactant converted per unit time per unit mass of catalyst). Many modem inorganic supports have surface areas of 100 to >1000 m g The vast majority of this area is due to the presence of internal pores these pores may be of very narrow size distribution to allow specific molecular sized species to enter or leave, or of a much broader size distribution. Materials with an average pore size of less than 1.5-2 nm are termed microporous whilst those with pore sizes above this are called mesoporous materials. Materials with very large pore sizes (>50 nm) are said to be macroporous, (see Box 4.1 for methods of determining surface area and pore size). 

Microporous, amorphous Mg-Si-O metallosilicates with a very narrow pore size distribution around 6 A diameter and a typical surface area of ca 350 m /g were obtained from the controlled calcination of compound 22. The resulting Mg-Si-O material was found to be very active in 1-butanol conversion even at 200 °C giving both dehydrogenation and dehydration. 

Although the surface area, pore diameter and total volume of AMM samples decreased (as a result of post-synthesis alumination), their pore size distributions are still very narrow. For example, Figure 2 shows that, even having an AI2O3 loading of 10 wt.%, AMM-10 has N2 adsorption-desorption isotherms similar to that of PSM material. The capillary condensation for mesopores is in a very narrow range of P/Po = 0.2-0.35. A sharp pore size distribution peak (25-30 A) is obtained from the isotherm. The results indicate that the uniform mesoporous structure of MCM-41 is still well maintained after post-synthesis alumination. 

To achieve a significant adsorptive capacity an adsorbent must have a high specific area, which implies a highly porous structure with very small micropores. Such microporous solids can be produced in several different ways. Adsorbents such as silica gel and activated alumina are made by precipitation of colloidal particles, followed by dehydration. Carbon adsorbents are prepared by controlled burn-out of carbonaceous materials such as coal, lignite, and coconut shells. The crystalline adsorbents (zeolite and zeolite analogues are different in that the dimensions of the micropores are determined by the crystal structure and there is therefore virtually no distribution of micropore size. Although structurally very different from the crystalline adsorbents, carbon molecular sieves also have a very narrow distribution of pore size. The adsorptive properties depend on the pore size and the pore size distribution as well as on the nature of the solid surface. 

The estimate of the surface area of chromatographic silica support is a complicated issue. It is usually performed via the BET method using low temperature nitrogen adsorption (N2.- sorptometry). The total surface area of the adsorbent is the product of the number of adsorbed molecules and the surface area per molecule. However, if the pore size distribution is not very narrow, an estimate of bonding density on the basis of carbon load and surface area may yield a large error because the smallest pores are not available for derivatization and the calculated bonding density is lower than the actual one. 

All the samples possess BET surface areas considerably smaller than those calculated fi om CO adsorption data, pointing to very narrow micropores or obstructions of pore entrances that restrict N diffusion. Thus, differences between values of Sbet and Sco2 suggest prevalence of micropores over meso and macropoies in the Ugnocellulosic wastes and chars. The relative contribution of micropores to fite samples structures may be additionally inferred from the Scot/Sset tatio. Char samples present smaller ratios than the wastes, indicating decreases in the relative proportions of micropores owing to the thermal treatment. 

The extent to which the total pore volumes can be utilised depends not only upon molecule sieving but also on the packing efficiency of the guest in the pores. In analcime oily small polar molecules (H2O or NH3) can enter because the interstices which make up the pore volume are too small and the windows are very narrow. Even for H2O and NH3 rather high temperatures are needed for equilibration in a reasonable time, and N2 areas calculated from water uptake are meaningless because N2 is not sorbed at -1958C. At high... 

These anatase sponge structures combine the convenient handling of larger spheres with a very high sur ce area and narrow pore size distribution and are expected to have potential in solar energy conversion, catalysis and optoelectronic... 

A small particle size of the reactant powders provides a high contact surface area for initiation of the solid state reaction diffusion paths are shorter, leading to more efficient completion of the reaction. Porosity is easily eliminated if the initial pores are very small. A narrow size... 

It cannot be said that one particular strategy - order versus disorder - is better than the other, and the choice will depend on the desired application. There are a number of potential advantages for crystalline MOPs. Structural uniformity makes it possible to design materials with very narrow pore size distributions and thus, conceivably, gain some of the advantages associated with zeolites [1] - for example, molecular specificity in catalysis. Crystalline MOPs can also be characterized structurally at the molecular level by X-ray diffraction [14,15, 20] in a manner that is not possible for amorphous materials. At the time of writing, the highest apparent BET surface areas reported for MOPs (up to 4,210 m g ) were found for ordered, crystalhne COFs [20]. 

---