Sorption capacity, nitrogen

The nitrogen-containing carbonaceous replicas of siliceous materials were prepared and studied with the nitrogen adsorption, TEM, TGA, XPS, and EDX methods. The carbons obtained using SBA-15 as a matrix exhibited well-developed and highly ordered porous structures. Those from the MLV material showed lower sorption capacities and 3-D structures less ordered as in the case of the SBA-15 replicas. 

The shape of the isotherms of nitrogen ad/desorption (type IV, IUPAC) for SBA-15 and its replicas indicates the presence of mesopores and a small fraction of micropores. The isotherms for all the N-doped replicas of SBA-15 (Fig. la) are of the shape similar to that of the isotherm for a typical CMK-3 material [4]. The mesopores are also similarly large (ca. 4 nm, Fig. lc). However, the sorption capacities differ notably, being 1.1 and 1.40-1.76 cm3/g for CMK-3 and CMK-3N, respectively. 

Both types of molecular sieves, MCM-36 and MCM-41, demonstrate large BET surface area and high static sorption capacity (see Table 2). Considerable qualitative differences are observed in N2 isotherms, which are shown in Figure 3. The nitrogen isotherm for MCM-41, prepared with cetyltrimethylammonium cation, is type IV [9] and shows the characteristic reversible steep capillary condensation at p/p0 = -0.4 corresponding to the pore opening -40 A [1]. MCM-36 also shows the type IV isotherm with almost linear and reversible uptake increase up to - p/p0 = 0.5, followed by a hysteresis loop. This profile of adsorption/desorption is typical for layered materials with slit-like porosity generated between layers [9],... 

The observed sorption capacity Q for nitrogen is shown as a function of pressure p, for the dehydrated zeolites in Figure 1. Here, and throughout this paper, sorption capacities are expressed as one-hundred times the mass sorbate x per unit mass of dehydrated sorbent m,... 

The observed variation of nitrogen sorption capacity at atmospheric pressure with content of presorbed water is shown in Figure 2. In all cases, the nitrogen capacity is seen to fall off regularly with increasing amounts of presorbed water, the rate of fall being highest in zeolite NaA and lowest in zeolite NaY. 

Table VII. Reported Sorption Capacities for Nitrogen on Zeolite NaA at 25°C and Atmospheric Pressure...

All of these features considerably improve the sorption capacity of carbons, and their capacity to remove contaminants and pollutants interacting with the surface of carbons in a dispersive way [177], In addition, active carbon contains heteroatoms such as oxygen, and, to a smaller degree, nitrogen and sulfur. These atoms are bound to the activated carbon surface in the form of functional groups, which are acidic or basic, giving the activated carbon surface an acidic or basic character, respectively [173,178], It is as well necessary to state that the chemical heterogeneity of the carbon surface is mostly the result of the presence of heteroatoms [175],... 

In addition, the large pore volume, pore size flexibility, and structural variety of MCM-41 can be extensively used for the selective adsorption of a diversity of gases and liquids [39,40], An extremely high sorption capacity for benzene has been demonstrated [40], Widespread work has been carried out on the sorption properties of some adsorbates, such as nitrogen, argon, oxygen, water, benzene, cyclopentane, toluene, and carbon tetrachloride, as well as certain lower hydrocarbons and alcohols on MCM-41 [122],... 

Many investigations have been made of the sorption of water vapour by various forms of montmorillonite and vermiculite. In a study of the effect of increasing the outgassing temperature of natural vermiculite on the sorption of water vapour, Gregg and Packer (1954) obtained a set of unusual stepwise Type I isotherms, all of which displayed low-pressure hysteresis. It is of interest that the location of the step riser, at about p/p° = 0.02, appeared to be almost independent of the outgassing temperature. The water sorption capacities were reported to be far greater than would be expected for adsorption on the external areas, as determined by the BET-nitrogen method (i.e. 1-2 m2 g" ). 

Basic nitrogen species present on the surface of activated carbons or carbon fibers, like in the case of H2S, were found to enhance the sulfur dioxide uptake. Polyacrylonitrile (PAN)-based activated carbon fibers are examples of good adsorbents for SO2 removal [82, 92]. Although role of nitrogen present in the carbon matrix was not emphasized by Lee and coworkers [93] in their studies of SO2 adsorption on PAN-based activated carbon fibers, [93] Kawabuchi and coworkers noticed a significant increase in the sorption capacity when activated carbon fibers were modified with pyridine and basic nitrogen functionalities were introduced to the surface [93]. Pyridine provided basic functionality, which increased catalytic removal of SO,. . 

During the study of a series of rare-earth-exchanged X and Y zeolites, it was observed that the sorption capacity for nitrogen (Shiralkar and Kulkami 1978) or hydrocarbons (Lee and Ma 1977) decreases when the rare-earth content of the zeolites or the size of adsorbed molecules is increased. 

The usefulness of nitrogen in increasing the sorption capacity toward anions [9] and the thermal stability of catalysts [28] are anphasized by the development of methods of preparation of nitrogen-enriched materials, mainly active carbons. This topic has received great attention since the 1990s, as briefly summarized in later text. 

The uptake values obtained for MEC adsorption are in good agreement with the results obtained for nitrogen sorption. As expected, the lower adsorption capacity belongs to SBA-15. After Al incorporation by direct-synthesis, the surface areas and pore volumes... 

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