Mesoporosity

A number of attempts have been made to demonstrate the connection between mesoporosity and the Type IV isotherm by comparing the isotherm of a vapour on a nonporous powder before and after it has been formed into a compact. The process of compaction produces pores in the form of interstices between the particles of the original powder such pores will tend to have dimensions of the same order as those of the constituent particles, and it can be arranged that these shall fall within the mesopore range of size. 

More often, however, microporosity is associated with an appreciable external surface, or with mesoporosity, or with both. The effect of microporosity on the isotherm will be seen from Fig. 4.11(a) and Fig. 4.12(a). In Fig. 4.11(a) curve (i) refers to a powder made up of nonporous particles and curve (ii) to a solid which is wholly microporous. However, if the particles of the powder are microporous (the total micropore volume being given by the plateau of curve (ii)), the isotherm will assume the form of curve (iii), obtained by summing curves (i) and (ii). Like isotherm (i), the composite isotherm is of Type II, but because of the contribution from the Type 1 isotherm, it has a steep initial portion the relative enhancement of adsorption in the low-pressure region will be reflected in a significantly increased value of the BET c-constant and a shortened linear branch of the BET plot. 

Thus, whilst a powder composed of nonporous particles gives rise to an isotherm of Type II, the converse is not necessarily true if a solid yields a Type II isotherm, it is not necessarily free of micropores. Similarly, though a Type IV isotherm signifies the presence of mesoporosity, it does not prove the absence of microporosity. - ... 

GAC (or other ultrafiltration medium) and ACF operating in series [101], or the development of ACF with controlled mesoporosity. 

Most of the papers available on desilication are devoted to the alkaline treatment of ZSM-5 zeolites. Since a large number of zeolites consist of aluminosilicate frameworks and framework Al plays a crucial role in the mesoporosity development during desilication, this methodology should be suitable for extrapolation to other... 

The aim of the newly introduced mesoporosity is to enhance the utilization of the microporous network by improved accessibihty of the active sites that are mostly present in the micropores. Although numerous papers have reported on the improved catalytic performance of desihcated zeolites in catalysis (details in Section 2.4.5), only few works are available that reaUy tackle the hierarchical nature of the desihcated zeohtes and demonstrate that selective sihcon removal leads to an enhanced physical transport in the zeohte crystals. 

Tunable Intracrystalline Mesoporosity by Partial Detemplation-Desilication... 

Figure 2.7 Schematic representation of the partial detem-plation and desilication treatment to tailor mesoporosity development in zeolite crystals.

The TS-l/MCM-41 catalysts were synthesized in two steps [8]. The first step was involved with the preparation of TPAOH impregnate mesoporous materials and the second stq) was the DGC process. The TPAOH impregnated H-MCM-41 was prepare with calcine Ti-MCM-41, TPAOH (1 M solution of water) and ethanol under stirring by impregnation method. The parent gels were prepared with a TPAOH/Ti-MCM-41 ratio of 1/3 by weight. After 4 h, ethanol and water were removed in a rotary evaporator at room temperature and solid products were dried in a convention oven at 373 K for 48 h. The DGC process was carried out at 448 K for 3 h to obtain TS-1/MCM-41-A and for 6 h to obtain TS-1/MCM-41-B. However, the mesoporosity of Ti-MCM-41 was lost when the DGC process was carried out for 9 h. 

The catalytic activitira of synfliesized catalysts are given in Table 1. The TS-1 catalyst exhibited the highest epoxide yield and the best catalytic performance for the epoxidation of 1-hexene. The convasion of cyclohexene, however, is the lowest over TS-1. In case of TS-1/MCM-41-A and TS-1/MCM-41-B, the selectivity to epoxide is much hi er than that of Ti-MCM-41. Moreover, the conversion of 1-hexene as well as cyclohexene is found larger on the TS-l/MCM-41-Aand TS-1/MCM-41-B than on other catalysts. While the epoxide yield from 1-hexene is nearly equivalent to that of TS-1, the yield from cyclohexene is much larger than those of the otiier two catalysts. Th e results of olefins epoxidation demonstrate that the TS-l/MCM-41-Aand TS-1/MCM-41-B possess the surface properties of TS-1 and mesoporosity of a typical mesoporous material, which were evidently brou in by the DGC process. 

Activity in n-butane isomerization reaction of various alkaline salts of H3PW12O40 and H4SiWi2O40 was shown to be strongly dependent on the strength and number of accessible protons whereas the stability with time on stream was correlated to the presence of mesoporosity. For the liquid iC4/C4 continuous alkylation reaction, the strength and the number of acid sites appeared less important than the existence of mesoporosity indicating that the diffusion of the reactants and of the products plays an important role in this reaction. 

The results are discussed in terms of strength and number of protonic sites and the presence of mesoporosity is shown to be important for the production of trimethylpentanes in the alkylation reaction. 

The BET surface area values are also reported with the distribution of porosity between microporosity (pore diameter <1.8 nm) deduced from N2 adsorption isotherms (t-curves) and mesoporosity (pore diameter > 1.8 nm). The following trend is observed for high atomic M/HPA ratio used for the precipitation, the precipitates exhibited high surface area mainly due to microporosity. However, depending on the nature of the coxmter cation and also of the previous ratio values, the textural characteristics were not similar. In particular, it is interesting to note the presence of mesopores for (NH4)2.4P, CS2.9P, CS2.7P and Cs2.4Si samples. 

It is to be noted that among the most active samples, (NH4)2.4P was the most stable with time on stream. That may be due to its lower proton content but more probably to a lower poisoning effect of coke deposits due to the presence of mesoporosity instead of microporosity. 

The most active samples for n-C4 isomerization, (NH4)2.4P and Csi.gP, showed opposite reactivities in liquid alkylation. The first one gave rise to a high production of TMP while the second one was only initially slightly active. The main difference between these two samples concerned their porosity (NH4)2.4P was mesoporous while Csi.gP was mainly microporous. Then, one may suggest that the presence of mesoporosity is essential for the accessibility of the reactants to the acid sites and the desorption of the products. As a consequence the catalytic activity seems more governed by the textural features than by the acidity. As a general trend, the samples which were, at the same time, active and stable for the alkylation reaction, exhibited a mesoporosity equivalent to about 40 m. g-i. 

By changing the ultrasound power, changes in the mesoporosity of ZnO nanoparticles (average pore sizes from 2.5 to 14.3 nm) have been observed. In addition to the changes in mesoporosity, changes in the morphology have also been noted [13]. Recently, Jia et al. [14] have used sonochemistry and prepared hollow ZnO microspheres with diameter 500 nm assembled by nanoparticles using carbon spheres as template. Such specific structure of hollow spheres has applications in nanoelectronics, nanophotonics and nanomedicine. 

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