Alumina, adsorption isotherms

The adsorption of molybdates in the presence of phosphorus oxo-species in solution (or those already deposited on an alumina support) or vice versa (i.e., adsorption of phosphate on a Al—Mo support) has also been explored. For example, Fig. 20 shows the isothermal adsorption on alumina supports of solutions containing different concentrations of ammonium heptamolybdate (AHM). From Fig. 20a, which corresponds to the sequential impregnation steps whereby phosphorus is adsorbed first (before molybdenum), it is clear that molybdenum is adsorbed in lower amounts on the Al—P support than on the phosphorus-free alumina. However, the amount of molybdenum adsorbed on the Al—P support also depends on the pH of the AHM solution (Fig. 20b). Molybdenum oxo-species tend to adsorb in greater amounts on the Al—P support under acidic condition. 

Empirical adsorption equilibrium models like Langmuir and Freundlich can generally be used to describe type I pure water vapour adsorption isotherms on aluminas [6] ... 

Fickian Diffusion and Linear Driving Force models are generally used to describe the transport of water vapour into the alumina particles. For isothermal adsorption of water vapour from a constant partial pressure (P ) batch adsorption system on a spherical adsorbent particle of radius Rp, the uptake profiles are given by [13] ... 

The resistance to mass transport for adsorption of water into alumina particles can be governed by diffusion of water molecules through the liquid filled pores as well as by surface diffusion of adsorbed water molecules on the pore walls. A surface excess linear driving force model [SELDF] has been successfully used to describe the adsorption of water from liquid mixtures [27]. For isothermal adsorption of water from a bulk liquid mixture from a constant water composition (xj) batch adsorption system, the uptake profile is given by ... 

Measurement of adsorption isotherm Activated alumina is activated in a stream of N2 gas at 200°C for 4 hrs. About 1.0 g of sample is exactly weighed and taken in a 50 ml conical flask with a airtight stopper. Teflon tape is wound round the stopper to avoid loss of material due to evaporation. To this is added styrene of known TBC concentration and weighed. The conical flask is kept in a thermostated waterbath (Julabo SW 1) with constant stirring. The temperature is maintained at 15°C with an accuracy of + 0.1 C. After an equilibration period of 24 hrs. TBC in the solution phase is measured and the amount of TBC adsorbed is calculated by following relation... 

Fig. 2.8 The adsorption of nitrogen at 77 K on nonporous samples of silica and alumina, ranging from 2-6 to 11-5 m g for silica and from 58 to 153m g for alumina. n/n is plotted against pjp°. (A) O, silica , alumina. (B) BET isotherm (Equation 2.12) with c values of 100-2(X).

Fig. 3.4 Compaction of alumina powder. Isotherms of nitrogen at 77 K, on (A) the uncompacted powder, and (B) on the powder compacted at a pressure of 1480 GN (96 ton in" ). Open symbols, adsorption solid symbols, desorption.

Fig. 3.28 The Kiselev method for calculation of specific surface from the Type IV isotherm of a compact of alumina powder prepared at 64 ton in". (a) Plot of log, (p7p) against n (showing the upper (n,) and lower (n,) limits of the hysteresis loop) for (i) the desorption branch, and (ii) the adsorption branch of the loop. Values of. 4(des) and /4(ads) are obtained from the area under curves (i) or (ii) respectively, between the limits II, and n,. (6) The relevant part of the isotherm.

A detailed study of the physical and chemical adsorption of water on three xerogels, ferric oxide, alumina and titania, as well as on silica (cf. p. 272) has been carried out by Morimoto and his co-workers. Each sample was outgassed at 600°C for 4 hours, the water isotherm determined at or near 20°C, and a repeat isotherm measured after an outgassing at 30 C. The procedure was repeated on the same sample after it had been evacuated at a... 

Desiccants. A soHd desiccant is simply an adsorbent which has a high affinity and capacity for adsorption of moisture so that it can be used for selective adsorption of moisture from a gas (or Hquid) stream. The main requkements for an efficient desiccant are therefore a highly polar surface and a high specific area (small pores). The most widely used desiccants (qv) are siHca gel, activated alumina, and the aluminum rich zeoHtes (4A or 13X). The equiHbrium adsorption isotherms for moisture on these materials have characteristically different shapes (Fig. 3), making them suitable for different appHcations. 

Example 4 Application of Isotherms Thomas [Ann. N.Y. Acad. Sci., 49, 161 (1948)] provides the following Langmuir isotherm for the adsorption of anthracene from cyclohexane onto alumina ... 

Figure 3.42. Nitrogen adsorption/desorption isotherm for a commercial y-alumina. The arrows denote the mode of changing the pressure, viz. increasing or decreasing pressure.

Physical adsorption of nitrogen was carried out on an ASAP 2400 Micromeritics apparatus. Before measurements, samples were evacuated overnight at 350 °C at vacuum of 2 Pa. For all samples the same adsorption data table was used. Collected adsorption data were treated by BET-isotherm in the range 0.05 < P/micropore volume and mesopore + external surface, t-plot method, with master isotherm of nonporous alumina (Harkins-Jura) was used, t-plot was linearized in the range of 0.35 < t < 0.6 nm. 

The adsorption isotherm of sodium dodecyl sulfate (SDS) on alumina at pH = 6.5 in 0.1 M NaCI (Fig. 4.11a) is characteristic of anionic surfactant adsorption onto a positively charged oxide. As shown by Somasundaran and Fuerstenau (1966) and by Chandar et al. (1987), the isotherm can be divided into four regions. These authors give the following explanation for the adsorption mechanism ... 

In some systems, three stages of adsorption may be discerned. In the activated alumina-air-water vapour system at normal temperature, the isotherm is found to be of Type IV. This consists of two regions which are concave to the gas concentration axis separated by a region which is convex. The concave region that occurs at low gas concentrations is usually associated with the formation of a single layer of adsorbate molecules over the... 

Data taken from the adsorption leg of the isotherm of Figure 17.11 are listed in the first two columns of the following table. Test the applicability of the following equilibrium theories (a) Langmuir (b) infinite BET and (c) Harkins and Jura. From (a) and (b) obtain estimates of the surface area of the adsorbent and compare the values with that obtained by the point B method. One molecule of nitrogen adsorbed on alumina occupies 0.162 nm2. 

The adsorption isotherm obtained for dodecylsulfonate (CDS) on alumina is given in Figure 2. This isotherm is similar to that obtained in the past for sulfonate/alumina systems (4). This isotherm behaves in an s-shaped manner (6) revealing its four characteristic regions of adsorption 1) Electrostatic interaction 2) Lateral association (hemimicellization), 3) Electrostatic hindrance and 4) micellization. 

Fig. 5 (a) shows the nitrogen adsorption isotherms of aluminum hydroxy pillared clays after heat-treatment at 300-500°C. These are of the typical Langmuir type isotherm for microporous crystals. Fig, 5 (b) shows the water adsorption isotherms on the same Al-hydroxy pillared clays [27]. Unlike the water adsorption isotherms for hydrophilic zeolites, such as zeolites X and A, apparently these isotherms cannot be explained by Langmuir nor BET adsorption equations the water adsorption in the early stages is greatly suppressed, and shows hydrophobicity. Water adsorption isotherms for several microporous crystals [20] are compared with that of the alumina pillared clay in Fig. 6. Zeolites NaX and 4A have very steep Langmuir type adsorption isotherms, while new microporous crystals such as silicalite and AlPO -S having no cations in the...

Fig. 6. Comparison of the water adsorption isotherms of the alumina pillared clay and various micropor-ous crystals.

---