Monodisperse pore size

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). 

In summary, nanometer-sized mesoporous silica and alumina spheres with tunable diameters (80 - 900 nm) can be synthesized in organic solvent. Mesoporous silica spheres templated by cationic surfactant (CTAB) have hexagonal array with monodispersed pore size (-2.4 nm), high surface areas (-1020 m2/g), and pore volume (1.02 cm3/g). Mesoporous alumina spheres templated by amphiphilic triblock copolymer show a large disordered mesopore (10.0 nm) and high BET surface area (360 m2/g). 

The Parallel-pore Model Wheeler proposed a model, based on the first three of these properties, to represent the monodisperse pore-size distribution in a catalyst pellet. From p and Vg the porosity e is obtained from Eq. (8-16). Then a mean pore radius d is evaluated by writing equations for the total pore volume and total pore surface in a pellet. The result, developed as Eq. (8-26), is... 

The original semiempirical parallel-pore model represents a monodisperse pore-size distribution and makes use of the measurable physical properties, Sg, Vg, ps, and Gp. The complex particle with porosity Gp is replaced by an array of straight and parallel cylindrical pores of radius a, much like a honeycomb structure. The mean pore radius a is simply calculated by assuming that the sum of the inner surface areas of all the n pores in an array nlnaL) is equal to the total surface area Sg and the sum of all the pore volumes nna L) is equal to the experimental pore volume V [5] ... 

It appears that for porous solids with monodisperse pore-size distribution the MTPM mean-pore radii and transport-pore distributions agree with the information from standard textural analysis. For porous solids with bidisperse pore-size distribution the MTPM mean-pore radii and transport-pore distributions are close to large pore sizes fiom standard textural analysis. 

The pore size distribution of the caibonized membranes for polymer concentrations of 12, 10 and 8 wt% are shown in Fig. 8.41a-c, respectively. The mean pore size of the membranes was in the microfiltration range, and the pore size increases with the polymer concentration. These results are in agreement with the fiber diameter of the membranes, which were noted to increase with the polymer concentration. A monodispersed pore size distribution was observed for 10 and 12 wt% polymer concentration, whereas a bidispersed pore size distribution was observed for the 8 wt% polymer concentration. It is hypothesized that this is due to the presence of beads in the 8 wt% polymer concentration, hence there are pores that are blocked by the beads, which results in a bidispersion of the pore size distribution. 

More recently, Widawski et al. [90] have reported that the preparation of porous polymer membranes controlled both the size distribution and relative positions of pores. They have found a way to generate polymer films with an essentially monodisperse pore size, in which the pores are organized sponta-... 

Typical pore size distributions for these adsorbents have been given (see Adsorption). Only molecular sieve carbons and crystalline molecular sieves have large pore volumes in pores smaller than 1 nm. Only the crystalline molecular sieves have monodisperse pore diameters because of the regularity of their crystalline stmctures (41). 

A criterion for selecting a right pore size to separate a given polydisperse polymer is provided here. To quantify how much the MW distribution narrows for the initial fraction, an exponent a is introduced (2). The exponent is defined by [PDI(0)] = PDI(l), where PDI(O) and PDI(l) are the polydispersity indices of the original sample and the initial fraction, respectively. A smaller a denotes a better resolution. If a = 0, the separation would produce a perfectly monodisperse fraction. Figure 23.7 shows a plot of a as a function of 2RJd (2). Results... 

The columns used were dry packed using a packing apparatus (purchased from Mandle Scientific). The packing employed was CPG of pore sizes 1000, 2000 and 3000 X and 200-UoO mesh size. The colimins were calibrated using Dow and Polysciences monodispersed polystyrene latices. 

For a monodisperse system this result is in good agreement with the values obtained from pore size distribution measurements, but it can be significantly in error if one is dealing with a bimodal pore size distribution (see Section 6.4.2). 

Figure 9.25 Models of granules of monodisperse particles characteristic psds (pore size distributions) are given below (a) uniform packing (b) bidisperse packing of aggregates of particles of similar sizes (c) same as (b) but the size of aggregates vary in a wide range.

The idea of using membranes to filter molecules on the basis of size is not without precedent. Dialysis is used routinely to separate low molecular weight species from macromolecules [105]. In addition, nanofiltration membranes are known for certain small molecule separations (such as water purification), but such membranes typically combine both size and chemical transport selectivity and are particularly designed for the separation involved. Hence, in spite of the importance of the concept, synthetic membranes that contain a collection of monodisperse, molecule-sized pores that can be used as molecular filters to separate small molecules on the basis of size are currently not available. 

This work was done in collaboration with Professor Hiroshi Yoneyama of Osaka University [124], The procedure used to prepare the LiMu204 tubules is shown schematically in Fig. 21. A commercially available alumina filtration membrane (Anopore, Whatman) was used as the template. Alumina is especially suited for this application because of its high porosity, monodispersity of pore size, and the fact that it can be heated to high temperature without degradation. This membrane contains 200-nm-diameter pores, is 60 p,m thick, and has a porosity of 0.6. A 1.5 cm X 1.5 cm piece of this membrane was mounted on a Pt plate (2 cm X 2 cm) by applying a strip of plastic adhesive tape (also 2 cm X 2 cm NICHIBAN VT-19) across the upper face of the membrane. The Pt plate will serve as the current collector for the LiMn204 battery electrode material. The strip of tape, which will be subsequently removed, had a 1.0 cm circular hole punched in it, which defined the area of the membrane used for the template synthesis of the LiMn204. 

The work of Mallouk et al. (39) offers an interesting extension of the microemulsion sol-gel technique. In this case, microemulsion-derived silica nanoparticles were used as templates for preparing ordered mesoporous polymers with tailored pore sizes. Utilizing the Triton N-101/cyclohexane/hexanol/water/ammonia microemulsion, monodisperse silica nanoparticles were first synthesized. The silica product... 

The preparation of monodisperse hydrogel microspheres, such as poly-acrylam-ide-co-acrylic acid, poly(N-isopropylacrylamide-co-acrylic acid), has been performed for drug devices thanks to their biocompatibility [77, 79]. The average diameters of the microspheres were dependent on the pore sizes (from 0.33 to 1.70pm) of SPG membranes used in the preparation procedure. 

A trickle-bed reactor was used to study catalyst deactivation during hydrotreatment of a mixture of 30 wt% SRC and process solvent. The catalyst was Shell 324, NiMo/Al having monodispersed, medium pore diameters. The catalyst zones of the reactors were separated into five sections, and analyzed for pore sizes and coke content. A parallel fouling model is developed to represent the experimental observations. Both model predictions and experimental results consistently show that 1) the coking reactions are parallel to the main reactions, 2) hydrogenation and hydrodenitrogenation activities can be related to catalyst coke content with both time and space, and 3) the coke severely reduces the pore size and restricts the catalyst efficiency. The model is significant because it incorporates a variable diffusi-vity as a function of coke deposition, both time and space profiles for coke are predicted within pellet and reactor, activity is related to coke content, and the model is supported by experimental data. 

Unsteady state diffusion in monodisperse porous solids using a Wicke-Kallenbach cell have shown that non-equimolal diffusion fluxes can induce total pressure gradients which require a non-isobaric model to interpret the data. The values obtained from this analysis are then suitable for use in predicting effectiveness factors. There is evidence that adsorption of the non-tracer component can have a considerable influence on the diffusional flux of the tracer and hence on the estimation of the effective diffusion coefficient. For the simple porous structures used in these tests, it is shown that a consistent definition of the effective diffusion coefficient can be obtained which applies to both the steady and unsteady state and so can be used as a basis of examining the more complex bimodal pore size distributions found in many catalysts. 

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