Adsorption of Benzene Vapors

Few studies have been made of benzene chemisorption by the volumetric method. Zettlemoyer et al. (8) have examined the adsorption of benzene vapor at 0°C on powders of nickel and of copper. First, the monolayer coverage of argon (vm) A, was measured. The argon was then removed by pumping and the amount of benzene required to form a monolayer, (vmi) Bz, was measured. Weakly adsorbed benzene was then removed by pumping, after which further benzene adsorption provided the value (vm2) Bz. Some results are reproduced in Table I. On the assumption that the same extent of surface is accessible both for argon and for benzene adsorption, it is clear that complete monolayers of benzene were not achieved, that some (Ni) or all (Cu) of the benzene was adsorbed reversibly. It was considered that only the irreversibly adsorbed benzene was chemisorbed, the remainder being physically adsorbed. Thus chemisorption of benzene on copper appeared not to occur. The heat of adsorption of benzene on nickel at zero... 

Figure 1. Isotherms of adsorption of benzene vapor on Lr-zeolites (1) NaL (2) KL, (8) CsL,...

The replacement of sodium ions by ions of calcium and strontium leads to a decrease in the adsorption value per unit cell. A similar picture is observed in studies of adsorption of benzene vapor on calcium forms. 

Consideration of isotherms of benzene vapor adsorption on specimens of lanthanum zeolite X shows that up to 50% replacement of sodium ions leads to some increase of adsorption capacity, and that deeper exchange promotes decrease of adsorption capacity. Adsorption of benzene vapor is decreased on lanthanum and yttrium zeolites of type Y in comparison with sodium zeolites (Figure 3). Such a difference in... 

Suresh S., Vijayalakshmi G., Rajmohan B., Subbaramaiah V., Adsorption of benzene vapor onto activated biomass from cashew nut shell Batch and column study. Recent Patents Chem. Eng., 2012b 5(2) 116-133. 

Fig. 28. Change of the photoelectric yield at X = 265.5 mp. of a platinum foil (in vacuo on the adsorption of benzene molecules at T = SS K. At the points marked by arrows, benzene vapor of low pressure was admitted [according to (76)].

Equation 5 at n = 2 describes, to a good approximation, the adsorption of vapors on active carbons, for instance, the adsorption of benzene with a variation of the characteristic energy 2 from 3000 to 6000 cal/ mole. However, for active carbons with the finest micropores, when 2 of benzene substantially exceeds 6000 cal/mole—for example, for active carbon obtained from polyvinylidene chloride ( 2 = 7240 cal/mole)— Equation 5 with n = 2 is applicable only to 0.5 with the effective... 

The main porous structure characteristics (Table 2) were determined on the basis of benzene vapor adsorption isotherms using McBain-Baker sorption balances at 20°C (293 K), i.e., the specific BET surface area (5bht) [39], the surface area of mesopores (5 ,e), and the parameters of the Dubinin-Radushkevich equation (the volumes of the micropores and supermicropores. Woi and W 2, and the characteristic energies of adsorption, E, and o ) 136,37). In addition, the total micropore volume (ZVT, ) and geometric micropore surface area (5J 1168] were calcu-... 

The adsorption/desorption isotherms of benzene vapors were measured at 293K by gravimetric method using the McBain-Bakr balance. 

Experimental results [7,8] obtained in the case of the breakthrough curves of binary mixtiues imder constant pattern condition have been compared with the anal5ttical solution. Figure 16.1 compares the experimental breakthrough curves obtained in the case of the vapor phase adsorption of benzene and toluene carried by nitrogen through a bed of activated carbon [8] with the analytical solution calculated from the binary adsorption data and imder the assumption of constant pattern behavior [1,3]. The agreement achieved is excellent. 

Type 1 isotherms exhibit prominent adsorption at low relative pressures p/po (the relative pressure p/po is defined as the equilibrium v or pressure divided by the saturation vapor pressure) and then level off. Type 1 isotherm is usually considered to be indicative of adsorption in micropores (e.g., adsorption of benzene on microporous active carbon) or monolayer adsorption due to the stror adsorbent-adsorbate interactions (which may be the case for chemisorption, which involves chemical bonding between adsorbate and the adsorbent surface, e.g., adsorption of hydrogen on iron). In the case of nonpolar gases commonly used for charactmzation of porous solids (nitrogen, argon) [10, 12, 13, 17, 56], chemisorption is unlikely and therefore e I reflects usually adsorption on microporous solids. However, type I isotherms may also be observed for mesoporous materials with pore size close to the micropore range. In particular, in the case of adsorption of N2 at 77 K or Ar at both 77 K and 87 K in cylindrical pores, a type I isotherm would have to level off below the relative pressure of about 0.1 for the material to be exclusively microporous, as inferred fi-om tile results of recent studies of siliceous and carbonaceous ordered mesoporous materials (OMM) [57-59]. Consequently, when a type 1 isotherm does not level off below the relative... 

Figure 2. Adsorption-desorption isotherms of benzene vapors by nanosized fibrous alumina annealed at temperatures 750 (1), 900 (2) and 1100 °C (3) - A and an image of alumina fiber (900 °C) - B.

Laser Raman Spectroscopy was used by Buechler and Turkevich (76) to study the nature of adsorption of benzene and water vapor on porous Vycor glass. Unfortunately photocatalysis producing fluorescent materials limited the usefulness of this approach to catalytlcally-active surfaces. As we shall see later C-13 NMR was found to be an incisive tool for the study of adsorption of hydrocarbons on catalytic surfaces. 

The influence of sulfur surface compounds on the adsorption of polar and nonpolar vapors of varying molecular dimensions was examined by Puri and Hazra. The adsorption of water vapors increased appreciably at relative pressures lower than 0.4 and decreased at higher relative pressures. The effect increased with increase in the amount of sulfur fixed and was attributed to the variation of the pore-size distribution caused by the fixation of sulfur along the pore walls. The adsorption isotherms of methanol and benzene vapors indicated that these larger molecules found smaller and smaller areas as more and more sulfur was being incorporated into the pores. Bansal et prepared carbon molecular sieves by blocking pores of PVDC charcoals by... 

FIGURE 4.8 Adsorption isotherms of benzene vapors on Saran charcoal before and after pore blocking with sulphur. (After Bansal, R.C., Bala, S., and Sharma, N., Indian J. TechnoL, 27, 206, 1989. With permission.)... 

Huang and coworkers studied the adsorption of methyl ethyl ketone and benzene vapors on activated carbon libers (ACF) prepared by activation in CO2 at different temperatures and for different intervals of time. The adsorption of both vapors increased with increased surface area of the ACF sample. 

Isirikyan AA, Kiselev AV. The absolute adsorption isotherms of vapors of nitrogen, benzene and n-hexane, and heats of adsorption of benzene and n-hexane on graphitized carbon blacks. I. Graphitized thermal blacks. J Phys Chem 1961 65 601-607. 

The adsorption of water vapor, ethanol, and benzene causes a decrease in strength of porous adsorbents and zeolites granulated with clay-based binders. However, it was demonstrated that these porous disperse materials act as catalysts under unfavorable conditions. This is the case because the principle of heterogeneous catalysis requires a decrease in the surface energy of a catalyst during the catalyzed reaction. An unfortunate result is the decrease in the durability of the catalyst, as will be discussed in more detail in Section 7.3. 

A quartz plate equipped with a gold electrode on each face can detect the adsorption of mercury vapor on the gold. A carbon monoxide sensor can therefore be constructed since this gas reacts with mercury oxide at 210 liberating mercury vapor [224] which is detectable with the piezoelectric sensor. Similarly, the deposition of copper complexes on a quartz crystal is used to detect organophosphates [225] such as diisopropylmethylphosphonate (DIMP), and modified cyclodextrins are us to detect benzene vapors [226]. The piezoelectnc sensor is also used in the liquid phase to monitor the viscosity of fluids and gelation, and deduce, for example, the concentration of fibrinogen [227]. 

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