Alumina pore size distribution

The amount or the kind of TAA-OH plays a key role on textural properties of amorphous silica-alumina pore size distribution can be modulated from meso (MSA) to microporous... 

The large majority of activated alumina products are derived from activation of aluminum hydroxide, rehydrated alumina, or pseudoboehmite gel. Other commerical methods to produce specialty activated aluminas are roasting of aluminum chloride [7446-70-0], AIQ calcination of precursors such as ammonium alum [7784-25-0], AlH2NOgS2. Processing is tailored to optimize one or more of the product properties such as surface area, purity, pore size distribution, particle size, shape, or strength. 

Rehydration Bonded Alumina. Rehydration bonded aluminas are agglomerates of activated alumina, which derive their strength from the rehydration bonding mechanism. Because more processing steps are involved in the manufacture, they are generally more expensive than activated aluminum hydroxides. On the other hand, rehydration bonded aluminas can be produced in a wider range of particle shape, surface area, and pore size distribution. 

Transition aluminas are good catalyst supports because they are inexpensive and have good physical properties. They are mechanically stable, stable at relatively high temperatures even under hydrothermal conditions, ie, in the presence of steam, and easily formed in processes such as extmsion into shapes that have good physical strength such as cylinders. Transition aluminas can be prepared with a wide range of surface areas, pore volumes, and pore size distributions. 

The advantage of sol-gel technology is the ability to produce a highly pure y-alumina and zirconia membrane at medium temperatures, about 700 °C, with a uniform pore size distribution in a thin film. However, the membrane is sensitive to heat treatment, resulting in cracking on the film layer. A successful crack-free product was produced, but it needed special care and time for suitable heat curing. Only y-alumina membrane have the disadvantage of poor chemical and thermal stability. 

Fig. 3.23 shows pore volume distributions of some commercially important porous materials. Note that zeolites and activated carbon consist predominantly of micropores, whereas alumina and silica have pores mainly in the me.sopore range. Zeolites and active carbons have a sharp peak in pore size distribution, but in the case of the activated carbon also larger pores are present. The wide-pore silica is prepared specially to facilitate internal mass-transfer. 

Pore. size and surface area distribution. Pore sizes and pore volume distributions may be calculated from the relative pressures at which pores are filled (in the adsorption mode) or emptied (in the desorption mode). Fig. 3.45 shows the pore size distribution of a commercial y-alumina. The distribution is very broad both meso- and macropores are present. In practice this is usually a desired situation a texture consisting of a network of large pores (main roads) and small pores (side roads) is ideal. 

Fig. 3.45. Pore size distribution of a commercial y-alumina calculated using the Kelvin equation.

Manufacturer Product Type of Alumina Pore size (nm) Particle Size Distribution (pm)... 

This value is considerably higher than the experimental value (0.17) obtained from rate measurements on different size particles, but several factors may be invoked to explain the inconsistency. There will be a distribution of both pore radii and pore lengths present in the actual catalyst rather than uniquely specified values. Alumina catalysts often have a bimodal pore-size distribution. Our estimate of an apparent first-order rate constant using the method outlined above will be somewhat in error. The catalyst surface may not be equally active throughout if selective deactivation has taken place and the peripheral region is less active than the catalyst core. Other sources of error are the... 

For regeneration to be technically viable, it must be able to remove deposited vanadium and nickel quantitatively as well as the carbonaceous coke which was co-deposited. The catalyti-cally active metals should remain unaffected in amount, chemistry, and state of dispersion. The alumina support should remain intact, with the surface area, pore-size distribution and crush strength after treatment comparable to that of the original. To be economically viable, the process should be accomplished in a minimum of steps at nearly ambient temperatures and preferably in aqueous solution. The ultimate proof of any such scheme is for the catalytic activity of the regenerated catalyst to be equal to that of a fresh one. 

Figure 17.12. Pore size distribution of an activated alumina calculated from the isotherm by the method of...

Figure 2B. Typical examples of a pore size distribution for (a) y-alumina membranes desorption branch (b) anatase titania membranes desorption branch.

Figure 3.7. Pore size distribution by nitrogen desorption of an unsupported experimental alumina membrane film.

Figure 3A Pore size distribution of a four-layered alumina membrane (Hsieh, Bhave and Fleming 1988).

Hgure 3.9. Dynamic pore size distributions of (a) two sol-gel alumina membranes and (b) two anodized alumina membranes (Fain 1990). 

The most commonly employed crystalline materials for liquid adsorptive separations are zeolite-based structured materials. Depending on the specific components and their structural framework, crystalline materials can be zeoUtes (silica, alumina), silicalite (silica) or AlPO-based molecular sieves (alumina, phosphoms oxide). Faujasites (X, Y) and other zeolites (A, ZSM-5, beta, mordenite, etc.) are the most popular materials. This is due to their narrow pore size distribution and the ability to tune or adjust their physicochemical properties, particularly their acidic-basic properties, by the ion exchange of cations, changing the Si02/Al203 ratio and varying the water content. These techniques are described and discussed in Chapter 2. By adjusting the properties almost an infinite number of zeolite materials and desorbent combinations can be studied. 

Unfortunately there are no routine methodologies for evaluating the morphology of solvent wet resins at least in terms of generating quantitative data on surface area and pore size distribution. It is possible to use the adsorption of a suitable molecule from the solvent, assume monolayer coverage and an molecular area for the molecule, and hence compute a surface area. This technique has been used in assessing the surface area of e.g. sihca and alumina but has not proved valuable in the case of resins. 

The importance of aluminas is due to their availability in large quantities and in high purity presenting high thermal stability and surface areas (in the 199-259 mVg range and even more). Their pore volumes can be controlled during fabrication and bimodal pore size distributions can be achieved. However, besides these textural aspects, the surface chemical properties of aluminas play a major role, since these are involved in the formation and stabilization of catalytically active components supported on their surfaces. Despite the widespread interest in catalytic aluminas there is still only a limited understanding about the real nature of the alumina surface [44,89,99]. 

For the detailed study of reaction-transport interactions in the porous catalytic layer, the spatially 3D model computer-reconstructed washcoat section can be employed (Koci et al., 2006, 2007a). The structure of porous catalyst support is controlled in the course of washcoat preparation on two levels (i) the level of macropores, influenced by mixing of wet supporting material particles with different sizes followed by specific thermal treatment and (ii) the level of meso-/ micropores, determined by the internal nanostructure of the used materials (e.g. alumina, zeolites) and sizes of noble metal crystallites. Information about the porous structure (pore size distribution, typical sizes of particles, etc.) on the micro- and nanoscale levels can be obtained from scanning electron microscopy (SEM), transmission electron microscopy ( ), or other high-resolution imaging techniques in combination with mercury porosimetry and BET adsorption isotherm data. This information can be used in computer reconstruction of porous catalytic medium. In the reconstructed catalyst, transport (diffusion, permeation, heat conduction) and combined reaction-transport processes can be simulated on detailed level (Kosek et al., 2005). 

In this paper, we report the synthesis of mesoporous silica and alumina spheres with nanometer size (80 to 900 nm) in the present of organic solvent with aqueous ammonia as the morphological catalyst to control the hydrolysis of tetraethyl orthosilicate (TEOS) and aluminum tri-sec-butoxide.1181 Mesoporous silica spheres show hexagonal arranged pores with monodispersed pore sizes ( 2.4 nm) and high surface areas ( 1020 m2/g) similar to MCM-41. A large pore ( 10 nm) mesoporous alumina sphere templated by triblock copolymer is thermally stable. Calcined alumina sphere shows disordered mesoporous arrays with relatively uniformed pore size distribution and high surface areas ( 360 m2/g). 

Figure 4. Nitrogen adsorption and desorption isotherm curves and pore size distribution curve (inset) from the adsorption branch of (a) calcined mesoporous silica sphere and (b) calcined mesoporous alumina sphere.

Nitrogen adsorption/desorption isotherm for calcined alumina sphere (Figure 4b) is a type IV with a large hysteresis. A steep increasing occurs in the isothem curve at a relative pressure 0.55

narrow pore size distribution at the mean value of 10.0 nm. Calcined alumina sphere has BET surface area of 360 m2/g and pore volume of 0.62 cm3/g. 

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. 

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