Texture specific surface area

One of the most important characteristics of a catalyst is its porous texture (specific surface area, pore volume, pore size and size distribution) which must allow good reactant and product circulations in the catalyst bulk. According to its use, it is necessary to give to a catalyst a tailor-made texture. As they present many advantages, silica-aluminas are widely used as matrices (for cracking catalysts) or supports (supported metals) of catalytic phases. 

Our work shows that, by the impregnation method, active and selective catalysts can be obtained for the oxidation of isobutene to methacrolein. The optimization of the properties of the active phase of the supports (textural specific surface area, chemical dopes, etc.) and of the supported phases (surface area, ability for oxygen activation, etc.) coupled with an adequate strategy in the impregnation procedure, can allow to optimize the cooperative effects between the phases to get very perfOTmant catalysts. 

Soaking a siUca gel in dilute ammonium hydroxide solution at 50—85°C can result in significant coarsening of the gel texture (5). Aging and thermal treatments result in a one-way process, ie, loss of specific surface area and in increase in pore size. The pore size can also be enlarged by dissolution of some of the siUca. Treating a siUca gel with O.S-N KOH or dilute HF can enlarge the pores from 0.7 to 3.7 nm (3). 

The XRD patterns demonstrated that the MCM-22 zeolites were well crystallized and pillars have been created in the MCM-36 sample, respectively. Thus, the last material exhibited a typical intense peak at 29 2°, corresponding to a Aspacing of 4 nm. The textural properties of solids (Table 1) indicated that the pillaring in MCM-36 resulted in increases in BET specific surface area and external surface area compared with the MCM-22 zeolite. 

Measuring the specific surface area, A, related to the mass of PS does not require a textural model (a morpho-independent parameter, i.e., one can apply an approach of partitioning and, correspondingly, the second statement of texturology, as we have already done for volume-related parameters). Let us consider the most widespread adsorption method based on proportionality of adsorption, Q. and the specific surface area in the absence of volumetric effects (capillaiy condensation, micropore filling, etc.) ... 

Chemical composition of fresh HTs was determined in a Perkin Elmer Mod. OPTIMA 3200 Dual Vision by inductively coupled plasma atomic emission spectrometry (ICP-AES). The crystalline structure of the solids was studied by X-ray diffraction (XRD) using a Siemens D-500 diffractometer equipped with a CuKa radiation source. The average crystal sizes were calculated from the (003) and (110) reflections employing the Debye-Scherrer equation. Textural properties of calcined HTs (at 500°C/4h) were analyzed by N2 adsorption-desorption isotherms on an AUTOSORB-I, prior to analysis the samples were outgassed in vacuum (10 Torr) at 300°C for 5 h. The specific surface areas were calculated by using the Brunauer-... 

Figure 5. Change in the textural properties of the materials with the temperature (a) specific surface area and (b) average pore size.

The properties of these new materials as catalyst support were tested on Fischer-Tropsch process (CO-H2 reaction) in a fixed bed differential reactor. Three materials were tested a) CON, a conventional activated carbon b) SC-155 (G40.60) and c) C-155 (G20.20). All of them were previously iron doped until 5% metallic iron wt/wt was reached. The test conditions were Reaction temperature =270°C H2/CO ratio=3, pressure = latm. The main properties of the tested catalyst supports and their performance in the first hour test are shown in Table 2. SC-155 (G40.60) and C-155 (G20.20) were selected for this test in order to compare materials with near the same specific surface area but with different structural composition, and CON was selected because it is of common use and has very different texture characteristics respect to the other two materials. 

The characterization of a solid must be considered at different levels. The first one is the sum of its textural characteristics which corresponds to the classical identification of the solid in terms of its granulometry, specific surface area, pore volume, porosity and density. 

Quantitative determinations of the degree of polycondensation in systematic studies have demonstrated that kinetic parameters such as the temperature have little effect on the level of polycondensation in xerogels 40 and 73, and the solvent has a more important effect149,151,152. Variations of 5 to 20% are observed like those presented in Table 3. From these data and those shown in Table 4, it is evident that the degree of polycondensation and the textural properties of the xerogels are generally not related for these precursors. A variation of 5% of the degree of polycondensation is measured while the specific surface area is increased 10 fold. 

In concluding this part, three main points emerge from the summary of these results. First, the difficulty in achieving the preparation of these solids in a reproducible way can be solved only if a precision in the experimental parameters similar to that employed for physical or analytical chemistry measurements is used. This is a clear demonstration of the second point, which states that the textural parameters of the materials (porosity, specific surface area and surface composition) are under kinetic control. Temperature, solvent, catalyst, water/precursor ratio and concentration of reagents are the main parameters which, beside the nature of the organic subunit R, control the texture of the final material. The third point is the difficulty in rationalizing the effect of these parameters due to the numerous mechanisms involved in the sol-gel process and their interconnections. However, it must be kept in mind that all these parameters are also powerful tools that can be very useful for the development of further applications, because they allow one to tune the texture of the materials. 

Absorption of harmful organic compounds by activated carbons from gas and liquid media is of interest and importance for human and environmental protection purposes.1"21 The influence of the texture of carbon granules (size and volume of pores, specific surface area, granule size d, and carbon bed depth V), gas stream humidity and velocity, and amounts of pre-adsorbed water are investigated on adsorption of organics in different media.1 21... 

It is clearly evident that the texture of the support is different from that of bare cordierite (Figure 23A), and the typical layered structure of bare cordierite is no longer visible (Figure 23B), either in the channels or in the pores inside the channel walls, indicating a complete coverage of the monolithic structure. If the thickness of the coating is calculated from the specific surface area of the cordierite (0.7 m /g) and the washcoat characteristics (loading of 10 wt% with a density of 1600 kg/m ), a... 

As evidenced by curves on figures 4 and 5, changes in texture characteristics are quasi-linearly related to increasing carbon content. It also appears that calculated surface areas strongly depend on the physisorption conditions, while measured adsorbate volumes are less affected. This observation is in favor of erroneous assumptions on nitrogen molecule area for the calculations of specific surface area [10]. But, as mentioned above, nitrogen molecule could penetrate in smaller pores, increasing measured micropore volume and surface areas. 

Textural characterization was performed by N2 adsorption-desorption at 77 K using a Micromeritics ASAP 2010 analyzer. The samples were preheated under vacuum in three steps of Ih at 423 K, Ih at 513 K, and finally 4 h at 623 K. BET specific surface area, Sbet, was calculated using adsorption data in the relative pressure range, P/Po, from 0.05 to 0.2. Total pore volume, Vp , was estimated by Gurvitsch rule on the basis of the amount adsorbed at P/Po of about 0.95. The primary mesopore diameter, Dp, was evaluated using the BJH method from the desorption data of the isotherm. The primary mesopore volume, Vp, and the external surface area, Sext were determined using the t-plot method with the statistical film thickness curve of a macroporous silica gel [5]. 

Textural Parameters. Adsorption-desorption isotherms of N2 at 77K were determined in a Micromeritics ASAP 2010 with a micropore system. Prior to measurement, the samples were outgassed at 140 C for at least 16 h. The specific surface area was determined by the BET method, assuming that the area of a nitrogen molecule is 0.162 nm [12]. Micropore volume was calculated by the t-plot method using the Harkins and Jura [13] thickness. We used model isotherms calculated from density functional theory (DFT) to determine the pore size distributions and cumulative pore volume of the pillared samples by taking the adsorption branch of the experimental nitrogen isotherm, assuming slit-like pores [14]. 

The overall performance of a catalyst is known to depend not only on the inherent catalytic activity of the active phase but also on the textural properties of the solid. The ability to control the specific surface area and the pore size distribution during the synthesis of amorphous silica-aluminas has been described for both surfactant micelle templated syntheses (M41-S (1), FSM-16 (2), HMS (3), SBA (4), MSU (5), KIT-1 (6)) and cluster templated sol-gel syntheses (MSA (7), ERS-8 (8)). 

The influence of phosphorus on catalyst textural parameters, such as specific surface area (SSA), pore diameter (PD), and pore volume, has been thoroughly investigated (25, 30, 38, 60, 62, 68, 69, 72, 73). The SSA decreases with phosphorus loading, irrespective of the preparation procedures. In particular, it was reported that NiMoP catalysts obtained by coimpregnation have greater SSA decreases than those prepared by sequential impregnation 74). 

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