Mesoporosity nitrogen adsorption

Nitrogen adsorption/desorption isotherms on Zeolite and V-Mo-zeolite are very similar and close to a type I characteristic of microporous materials, although the V-Mo-catalysts show small hysterisis loop at higher partial pressures, which reveals some intergranular mesoporosity. Table 1 shows that BET surface area, microporous and porous volumes, decrease after the introduction of Molybdenum and vanadium in zeolite indicating a textural alteration probably because of pore blocking by vanadium or molybdenum species either dispersed in the channels or deposited at the outer surface of the zeolite. The effect is far less important for the catalysts issued from ZSM-5. 

The nitrogen adsorption / desorption isotherms (Fig. 2) are typical of well-defined porous frameworks that are characteristic of either supermicroporosity (MSU-1) or a small mesoporosity (MSU-4) without any textural porosity [14]. In these two compounds, the silica walls (deduced from x-ray diffraction and nitrogen isotherms) are quite thick (< 20 A) [5],... 

The porous nature of the Qo crystals has now been examined in more detail by Kaneko and his co-workers (Setoyama et al., 1996 Thess et al., 1996). The Cw powder was recrystallized from carbon disulfide and then studied before and after annealing by heat treatment The nitrogen adsorption measurements revealed that the recrystallized material was both microporous and mesoporous the mesoporosity was completely removed by heat treatment, but the sample remained microporous. Since the micropores and the molecules appeared to be of similar size, it was concluded that the micropores were largely in the form of molecular defects and lattice vacancies. 

Figure 1 presents the nitrogen adsorption considerable nitrogen uptake at low relative (according to the lUPAC classification), which reflects the presence of microporosity. On the other hand, nitrogen uptake at relative pressures P/Pq>0.2, approximately, indicates the existence of mesoporosity. The different shape of the isotherms indicates the different pore size distribution on this series of samples. 

Pore volumes have been obtained from water adsorption considering two different water densities. The density of water in solid phase (0.92 g/cc) has been used to estimate the micropore volume from the amount of water adsorbed until relative pressure around 0.6. In the other hand, the liquid water density has been used to calculate the water volume adsorbed on the relative pressure range around 0.6-0.95. The pore volumes obtained from water adsorption data are quite similar to the corresponding micropore and mesojxrre volumes obtained by nitrogen adsorption, which seems to corroborate that water adsorbs in the microporosity as solid ice, while it adsorbs in the mesoporosity as liquid. 

The progress in the determination of porosity of various types of materials has arisen over the past ten years from advances in application of new spectroscopy techniques. In the present paper the application of small angle X-ray scattering (SAXS), positronium annihilation lifetime spectroscopy (PALS) and low temperature nitrogen adsorption methods to the characterization of mesoporosity is reviewed using different types of silica gels with chemically modified surface. The results from the three methods are compared and discussed. 

Fig. 6 Nitrogen adsorption-desorption isotherms of A disordered wormhole-like sample, B highly organized CMI-1 compound and C sample prepared at 30 wt.% showing secondary mesoporosity.

The sorption values of n-DBA-VPI-5 sample for water, n-hexane and 1,3,5-triisopropylbenzene (kinetic diameters = 2.65, 4.30 and 8.50 A, respectively) at 298K (p/po = 0.5 for water and n-hexane and 0.8 for triisopropylbenzene) were obtained as 33.2%, 18.5% and 11.4%, respectively. The different adsorption capacities for the above molecules is probably related to the differences in packing of these molecules. In fact, in terms of volume, water and n-hexane sorption correspond to 33 ml and 28 ml per 100 g, respectively which are more than the micropore volume of 0.124 ml/g obtained from nitrogen adsorption suggesting the presence of mesoporosity. The sorption of the bigger molecule like triisopropylbenzene... 

Nitrogen adsorption-desorption isotherms measured for all samples were of type II with H3 hysteresis loops typical for materials with ill defined mesoporosity that extends into the macropore range. The use of AC as templating agent decreases the surface area of the Pt loaded samples due to the ash content of the AC which would remain in the sample after thermal treatment. When y-alumina or titania were included in the supports the surface areas of the resulting catalysts were those expected fi om the values measured for the raw materials heat treated at 500 °C. 

A detailed discussion of adsorption onto mesoporous solids is beyond the scope of this text, but certain features relevant to microporous solids should be described. Firstly, microporous solids can themselves contain mesoporosity. The most important example of this is observed in zeolites such as Y or mordenite that have been treated after synthesis to remove aluminium from the framework (Section 6.2.3). The migration of silica leaves mesopores that are evident from nitrogen adsorption isotherms and directly visible by electron microscopy. The presence of secondary mesopores enhances diffusion and catalytic properties. Conversely, mesoporous solids that are well ordered on the mesoscale can contain disordered micropores in their walls. The mesoporous channels of calcined SBA-15, for example, are connected by micropores that result from removal of block copolymer chains that run between the large channels in the as-synthesised material. This is observed from nitrogen... 

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). 

At high bum-offs, the DR-plots of nitrogen adsorption are curved and do not extrapolate. Micropore volumes are obtained from the Os-plofs. Hence, the nitrogen isotherm provides the total volume in micropores, and this, together with the volume in narrow micropores provides volumes of wider micropores. Information about mesoporosity can also be obtained (Section 4.8). 

Whether or not the adsorption of such internal mesoporosity would exhibit hysteresis effects on desorption is a matter of some uncertainty, the isotherm being of Type-IV. Gelb and Gubbins (1998) make the suggestion that hysteresis loops of nitrogen adsorption/desorp-tion can be attributed to the presence of thermodynamically metastable clusters of adsorbate molecules within the caves of the porous networks, and not to kinetic effects. That is, adsorption within the structures of Figure 4.55 would contribute to hysteresis effects. 

The data show that the water uptakes, calculated on the assumption of liquid density in the pores, tend to be slightly lower than the micropore volumes determined from nitrogen adsorption. However it has been reported that the density of water adsorbed in microporous carbons is less than the liquid value [5]. Taking this into account suggests that for dynamic testing of carbons pre-equilibrated to a relative humidity of 80%, the micropores will be filled with water. Study of the filling of the mesoporosity by water is limited by the experimental... 

Carbons containing controlled levels of micro- and mesoporosity have been prepared using a combination of templating and activation. Analysis by nitrogen adsorption shows fiiat the templated carbons are predominantly mesoporous. Subsequent activation of the templated carbons increases the micro mesopore ratio. 

Nitrogen adsorption indicates that in the course of activation of the chars the development of their microporous system is accompanied by successive creation of mesoporosity. 

Nitrogen physisorption measurements evidence the permanent mesoporosity of the NU-GeSi-A materials (Fig. 11). All the N2 adsorption-desorption isotherms... 

The adsorption branches of the isotherms (figures 1 and 2), at high relative pressures P/P°>0.2, remain parallel to each other after the consecutive thermal treatments, which indicates that the mesoporosity of the materials is not modified, affecting the decrease in the adsorption of nitrogen at low relative pressure values and therefore in the microporous zone. In addition, the adsorption branches of the Al-pillared material (Al-Wy), are seen to display lower values which decrease more quickly with the thermal treatment. 

The complete adsorption-desorption study of the nitrogen isotherm revealed that our sample of VPI-5 was microporous as well as mesoporous. The rapid rise in the volume within the relative pressure range less than 0.01 and upward trend of the rise in the volume thereafter are indicative of this phenomenon as can be seen in Figure 4. The desorption part of the isotherm traces the hysteresis loop similar to type A which is indicative of the presence of a mesoporosity created by a packing of the needle like crystals. 

GRT of particle sizes from 1 to 3 mm was treated by applying thermal, chemical, and combined thermal and chemical treatments to prepare carbonaceous adsorbents for removal of mercury in aqueous solution (Gupta et al., 2011). The adsorbents were prepared by heating the rubber at 400 or 900 C for 2 h in the nitrogen atmosphere and then chemically treating with sulfuric acid, nitric acid, or their mixer solutions for 24 h. The heat treatment of the rubber developed mainly the microporosity, particularly the mesoporosity. The chemical treatment provided the creation of macropores. In the combined heat and chemical treatments, the predominant effects on the porous structure were caused by the treatment that provided the first effect. The adsorption capacity of mercury was larger for the adsorbents of higher microporosity. 

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