Mesoporous silica parameters

Sophisticated catalysts, such as ZSM-5 or HZSM-5 [22] and other zeolites are also suggested in numerous papers, e.g. KEY [23], HY and H-mordenite [24], Re-zeolite-based Engelhardt FCC commercial catalyst [25], and steamed commercial zeolite catalyst [26]. These investigations are mainly devoted to fundamental studies and the correlation between feed composition, catalyst properties, process parameters and efficiency connected with prodnct distribution. Iron supported on silica-alumina, mesoporous silica and active carbons serves as the next example of materials applied in the waste plastics cracking [27, 28]. On the other hand, according to some results [29] application of cracking catalysts such as Zn-13X, Fe-5A and CoMo-HY are ineffective in waste plastics cracking. 

The results of the characterisation of the monolithic silicas are summarised in Tab. 2. The investigated materials possess very similar mesopore structural parameters as the particulate silicas in Tab. 1. The a (BET) values are approximately 300 m g , the mesopore volumes are all about 0.9mlg , the average mesopore diameters scatter around 12 nm. The specific macropore volume is slightly increasing from PKlll to PK118 with the exception of PK114. 

Now, an important question is How, those parameters Sp, Vp, (Dmax/2c), c, and x are affected by the gradual narrowing of pores due to some kind of surface functionalization A second question is which of, and how, the parameters t, c and (Dmax/2o) are interrelated. The question becomes more interesting, and perhaps intriguing, since all the above quantities are calculated just from one kind of measurement, namely the N2 adsorption/desorption data. A partly answer to the above question was attempted in a previous work [10] in which sixteen mesoporous vanado-phoshoro-aluminates solids were tested and some relationships between c and (Dmax/2o) were established. A first target of this paper is to extend the search for such possible inter-relations to a class of mesoporous silicas, with a random pore size distribution whose porosity has been systematically and gradually modified by surface fiinctionalization... 

This paper describes a novel process for the preparation of spherical mesoporous silica spheres in the submicrometer and micrometer size range. Tetra-n-alkoxysilanes are hydrolysed and condensed in the presence of n-alkylamine as nonionic template and ammonia as catalyst. The porosity and the morphology parameters can be independently adjusted in wide ranges. These materials are promising adsorbents in separations techniques and valuable catalyst supports. 

The synthesis of ordered mesoporous silicas of the MCM-41 type serves as profound starting basis for the tailoring of pore structural parameters. The classical synthesis of MCM-41 is performed with a silica source and an ionic... 

F) Control the effect of counteranion. MCM-48-like mesoporous silica[130] has been synthesized by using cetyltriethylammonium bromide as template and HN03 as acid source at 0 °C for 1 d. In this synthesis, HN03 plays a key role and favors the formation of the I a 3d mesophase with a larger g parameter. 

The effect of temperature on the condensation of silica has been mentioned before (e.g., stabilization of mesoporous silica). The effect of the temperature on the formation of the mesostructure can be understood by considering the g parameter of quaternary ammonium surfactants. As the mixture gel is heated, the conformational disorder of the surfactant tail increases, increasing the surfactant molecular volume and as a result the g value. At the same time, the repulsion of the charged headgroups is also increased with heating, which leads to an increase in the headgroup area a0 value. 

For silica gels a number of parameters have been demonstrated to have a large effect on the evolution of porosity and subsequently on the resulting silica materials [1]. Almost dense, micro- or mesoporous silica materials can be obtained depending on the experimental conditions in which hydrolysis and condensation reactions of silicon alkoxides are carried out. This is not the case for transition metal alkoxides which are very sensitive to hydrolysis. They do not cillow the adaptation of sol-to-gel transition in order to obtain controlled porous textures. Some years ago special attention was paid to the utilization of amphiphilic systems as reactive media to control hydrolysis and condensation kinetics with transition metal alkoxides [37]. In a more recent work Ayral et al. 

Fig. 4 Critical packing parameters and characteristic structure typical to mesoporous silica films. The top shows a cross-section of a micelle, displaying the parameters used to calculate the CPP. The spherical head group for the surfactant represents the effective cross-sectional area that the head group occupies. (View this art in color at www.dekker.com.)...

Titanium has also been incorporated into ordered mesoporous silicas of the SBA-15 type (126). The influence of synthesis parameters on the properties of titanium-substituted SB A-15 silicas prepared by a direct one-step synthesis (cocondensation) was systematically investigated (127) through characterizing the products by N2 physisorption, XRD, diffuse reflectance UV-vis spectroscopy, and elemental analysis. The results showed that when a low titanium precursor concentration (i.e., less than 0.05 mol/1) was used in the initial synthesis gel, incorporation of titanium into the solid was not completed. Under these conditions, titanium ions were well dispersed in the silica framework and present mainly in tetrahedral coordination in the product. When the isolated titanium species present in the gel reached a critical concentration, an increase in titanium incorporation into the sofid was observed as a consequence of the formation of anatase clusters on the material s surface. The titanium loading at which anatase formation was observed was found to he strongly influenced by the synthesis conditions. Moreover, when the formation of anatase takes place, the amorphous titanium species... 

Recent breakthroughs in sol-gel synthesis have resulted in a novel methodology for preparing ordered me soporous inorganic materials (Fig. 4) with extremely high surface areas (> 1000 m g ) in which the pore size can be tuned from 1.2 to 35 nm by adjustment of the synthetic parameters [26-28]. The original method was developed by scientists at Mobil Oil Research and Development for synthesis of ordered mesoporous silica [26]. The essence of this new methodology is the use of... 

---