Mesoporosity pore volume distribution

FIGURE 50 Pore volume distribution of Cr/silica-titania catalysts dried by various methods and then activated at 800 °C. The mesoporosity of the catalyst influences its activity and the polymer melt index (tested at 105 °C with 1.5 mol C2H4 L ). 

The use of clays as supports for hydroprocessing has been reported and summarized [9-11], Dibenzothiophene (DBT) diluted with hexadecane (0.75 wt% S) was the liquid feed for HDS tests. The pore diameter of the MSC catalysts is seen to have a strong effect on both the HDS activity and selectivity (Figure 4). A commercial catalyst (Crosfield 465, Co/Mo alumina) was also measured under these conditions where it gave 77% DBT conversion and 61% BP selectivity. In a previous study [12], other synthetic hectorites were compared using these conditions except that a 1 wt% S feed was utilized. One sample was a control made without template that consisted of only micropores. The DBT conversion and BP selectivity were very low for this microporous material. The Crosfield material has significant macroporosity (42% of the pore volume) in addition to a broad distribution of mesoporosity, and has clearly been optimized to perform well under these HDS conditions. 

From these results it can be said that the shape of water isotherm up to relative pressures around 0.6 is mainly related to the micropore size distribution. On the other hand, the adsorption at higher P/Po is due to the presence of mesoporosity. Depending on the pore size distribution and the contribution of the different pore volumes, the shape of water isotherms can be more similar to the sample 1A or 3B. 

From Fig. 3, it appears that the specific surface area of the disordered DWM materials remains very high till 800°C. Beyond this tenperature, for sample B for instance, its value decreases from 800 to 20 mVg if calcination ten rature is raised from 800 to 1000°C. Whatever the hydrothermal treatment conditions of the synthesis are, the thermal resistance of the materials is similar. Even at 1000°C, the recovered materials exhibit a type IV isotherm, characteristic of mesoporous compounds. A part of mesoporosity is thus maintained with quite a narrow pore size distribution, but the maximum adsorbed volume is sharply reduced. This is in accordance with the very broken appearance of the particles observed by SEM. Thermal stability of these disordered materials is, however, very superior to MCM-41, whose structure does not resist beyond 600°C. This behaviour can be related to the different preparation method that affords compounds with a different structure and also thicker walls. 

On dry gels, standard characterization techniques for porous media are used, several of which have been described in Volume 2 of this series helium pycnometry for pore volume determination (Section 6.3.1.2) as well as nitrogen adsorption at 77 K for surface area (Section 6.3.2.2, BET method), for microporosity (Section 6.3.3.2, Dubinin-Radushkevich method), for pore size distribution (Section 6.3.3.3, BJFl method), and for total pore volume (Section 6.3.3.4). When characterizing gels by nitrogen adsorption, other methods are also used for data interpretation, for example, the t-plot method for microporosity (Lippens and de Boer, 1965) and the Dollimore-Heal method (Dollimore and Heal, 1964) or Broekhoff-de Boer theory for mesoporosity (Lecloux, 1981). 

The model, so generated, shows a heterogeneity of density of carbon atoms there appear to be volume elements of mesoporosity, even macroporosity. Is this model indicating how mesoporosity may exist within an activated carbon Mesoporosity cannot all be associated with cone-shaped (wedge-shaped) porosity at surfaces of particles. Although this model approximates the structures in carbon networks, it does not predict pore-size distributions, that is the molecular space networks, a matter of some importance to adsorption studies. Nevertheless, the possibility of doing this seems to be realistic. 

Carbons prepared from sucrose have surface areas up to 1500 m g , and pore volumes up to 1 cm g (Table 1). These carbons also have micropore volumes of around 0.210 cm g, which is likely related to the intrinsic microporosity of the carbon precursor. For both samples a capillary condensation occurs in the p/po range between 0.1-0.4 and 0.3-0.6, which clearly indicates that the structural porosity of the silica framework is replicated in the carbon. The pore size distributions for these carbons are very narrow with peak maxima at 1.2 nm and 1 nm, for T30 SUC and T60 SUC, respectively. Additionally, a slight development of complementary mesoporosity (estimated from the amount of nitrogen adsorbed at p/po > 0.7) can be seen, accounting for 6 % and 7 % of the total pore volume. 

As suggested before, a correlation could exist between oc and the evolution of the pore size. It has already been shown that, when the TMOS content increases, the average pore size decreases and the pore size distribution curve becomes narrower. The total porous volume in a gel consists of the macroporosity (/ > 50 nm), the mesoporosity... 

The BET specific surface area, mesopore volumes, and pore wall thickness of the calcined and water-treated samples are given in Table 3. BET surface area of the samples prepared with Cm surfactants were found to be less affected by hydrothermal treatment. When the samples synthesized without TPA+ subjected to hydrothermal treatment the sharp inflection in the isotherm became very broad indicating wide distribution of pores. In contrast, the mesopore distribution of the samples prepared with TPA was found to be less affected by hydrothermal treatment. For the samples prepared without TPA, the mesopore volume was found to decrease sharply and the pore diameter was broadened over a large range indicating loss of the mesopore structure. Addition of TPA was found to minimize the structural collapse and thereby helps to preserve the mesoporosity. 

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