Adsorption/desorption methods

The competitive adsorption isotherms were determined experimentally for the separation of chiral epoxide enantiomers at 25 °C by the adsorption-desorption method [37]. A mass balance allows the knowledge of the concentration of each component retained in the particle, q, in equilibrium with the feed concentration, < In fact includes both the adsorbed phase concentration and the concentration in the fluid inside pores. This overall retained concentration is used to be consistent with the models presented for the SMB simulations based on homogeneous particles. The bed porosity was taken as = 0.4 since the total porosity was measured as Ej = 0.67 and the particle porosity of microcrystalline cellulose triacetate is p = 0.45 [38]. This procedure provides one point of the adsorption isotherm for each component (Cp q. The determination of the complete isotherm will require a set of experiments using different feed concentrations. To support the measured isotherms, a dynamic method of frontal chromatography is implemented based on the analysis of the response curves to a step change in feed concentration (adsorption) followed by the desorption of the column with pure eluent. It is well known that often the selectivity factor decreases with the increase of the concentration of chiral species and therefore the linear -i- Langmuir competitive isotherm was used ... 

For membranes with pore diameters smaller than 3.5 nm, the nitrogen adsorption/desorption method based on the widely used BET theory ean be employed. This measurement technique, however, is good only for pore diameters ranging from 1.5 nm to 100 nm ( = 0.1 micron). Typical data from this method are split into two portions adsorption and desorption. The nitrogen desorption curve is usually used to describe the pore size distribution and corresponds better to the mercury intrusion curve. Given in Figure... 

Fundamental studies on the adsorption of supercritical fluids at the gas-solid interface are rarely cited in the supercritical fluid extraction literature. This is most unfortunate since equilibrium shifts induced by gas phase non-ideality in multiphase systems can rarely be totally attributed to solute solubility in the supercritical fluid phase. The partitioning of an adsorbed specie between the interface and gaseous phase can be governed by a complex array of molecular interactions which depend on the relative intensity of the adsorbate-adsorbent interactions, adsorbate-adsorbate association, the sorption of the supercritical fluid at the solid interface, and the solubility of the sorbate in the critical fluid. As we shall demonstrate, competitive adsorption between the sorbate and the supercritical fluid at the gas-solid interface is a significant mechanism which should be considered in the proper design of adsorption/desorption methods which incorporate dense gases as one of the active phases. 

The adsorption apparatus, shown in Figure 1, uses a static volumetric method to measure pure gas adsorption, and an open-flow adsorption/desorption method for mixture... 

Nitrogen adsorption/desorption. One of the most common techniques used for analyzing size distributions of mesopores is the nitrogen adsorption/desorption method. The method is capable of describing pore diameters in the 1.5 nm to 100 nm (or 0.1 micron) range. Pore size distribution can be deteimined from either the adsorption or the desorption isotherm based on the Kelvin equation ... 

Figure 4.11 Differential pore size distribution of a Y-AI2O3 membrane by the nitrogen adsorption/desorption method [Anderson etal., 1988]...

This method has been applied to ceramic membranes (e.g., gamma-alumina membranes) and compared to other methods such as nitrogen adsorption/desorption and thermoporometry (to be discussed next) in Figure 4.13. It can be seen that the mean pore diameter measured by the three methods agrees quite well. The pore size distribution by permporometry, however, appears to be narrower than those by the other two techniques. Similar conclusions have been drawn regarding the comparison between permporometry and nitrogen adsorption/desorption methods applied to porous alumina membranes [Cao et al., 1993]. The broader pore size distribution obtained from nitrogen adsorption/desorption is attributable to the notion that the method includes the contribution of passive pores as well as active pores. Permporometry only accounts for active pores. 

FT-IR spectroscopy. Self-supporting wafers (4-8 mg/cm ) were analyzed by FT-IR (Fourier Transform Infe-Red) spectroscopy (mod. 2000, PERKIN-ELMER). All treatments were performed in situ in a pyrex cell with KBr windows and spectra were registered at 20°C. The following experiments were performed (i) acidity determination by pyridine adsorption-desorption method [11]. After evacuation (300°C, Ih, 10 mbar) the catalyst was contacted by 10 mbar pyridine vapor at 200°C for Ih desorptions were performed for Ih at 200 and 300°C in dynamic vacuum (ii) hydroxyl evolution after different in situ treatments (reduction, sulfidation, H2 adsorption). The wafer was heated at 300°C in a dynamic vacuum, contacted by a known amount of H2S/H2 mixture for 1 hr at the same temperature, reduced for 2 h at 300°C in H2 flow and cleaned in N2 flow at the same temperature. After this pretreatment, H2 chemisorption was performed at different temperatiues (20-100°C), followed by pyridine adsorption-desorption at 200°C. 

Adsorption-desorption method An initially unloaded (q = 0) column is equilibrated by a feed concentration, Cfeeti, which may be a single- or multi-component mixture. Equilibrium is achieved by pumping a sufficient quantity of feed through this column. The plant is then flushed without the column to remove the solute solution. Afterwards, all solute is eluted from the column, collected and analyzed to obtain the... 

Independent of the method applied, the grey area D in Fig. 6.18 has to be equal to the area A and results from the desorption step back to c1. Comparison of both values can be used as a consistency check. Another possibility to verify the result is to stop the flow after the plateau is reached and analyze the desorption front according to the adsorption-desorption method (Section 6.5.7.3). 

The strong acidity of the zeolite without and with silica binder were measured in terms of the pyridine chemisorbed at 400°C, GC adsorption/ desorption method [16. The zeolite without and with silica binder were also characterized for their acid function by carrying out over them two catalyzed model reactions - cracking of isooctane (at 400°C) for measuring external acidity and toluene disproportionation (at 500 C) for measuring the intracrystalline or overall acidity, using pulse micro-reactor (id 4.5 mm) made of quartz, connected to a gas chromatograph [11, 14, 17]. 

The bulk and framework compositions of the H-GaAlMFI and Ga/H-ZSM-5 zeolites are given in Table 1. The zeolites were prepared by the procedures described elsewhere (6,9). The MFI structure of the zeolites was confirmed by XRD. The incorporation of Ga in the framework of GaAlMFI zeolite was confirmed by the Ga MAS NMR peak at +156 ppm. The framework (FW) Si/Ga and Si/Al ratios of the zeolites were obtained from their 29gi mAS NMR peaks, Si(0Ga or OAl) at about -112 ppm and Si(lGa or lAl) at about -104 ppm by the method described earlier (10). The crystal size and morphology of the zeolites was studied by SEM. The zeolite bulk chemical composition was determined by the chemical analysis of Ga, Al, Si and Na. Strong acid sites on the zeolites were measured in terms of the pyridine chemisorbed at 400 C, using the GC adsorption/desorption method (11). 

In technical applications zeolite molecular sieves and catalysts are generally used under conditions of multicomponent diffusion. Selective diffusion measurements of the individual components are therefore of immediate practical relevance. In the conventional adsorption/desorption method such measurements are complicated, however, by the requirement of maintaining well-defined initial and boundary conditions for any of the components involved. Being applied at equilibrium, such difficulties do not exist for PFG NMR. The traditional way to perform such experiments is to use deuterated compounds or compounds without hydrogen, thereby leaving only one proton-containing component, which then yields the H NMR signal [163-165]. 

Comparative investigations between the conventional adsorption/desorption method and PFG NMR have been carried out with aromatics in zeolite NaX. It was pointed out in Table 2 that there is still some divergence between the data obtained by both methods on intracrystalline diffusion. Table 3 compares the values for Tjnira and Tjn,ra determined by the NMR methods [143,175,176]. H PFG NMR measurements of these systems are complicated by the rather short transverse nuclear magnetic relaxation times, which range over milliseconds and lead to mean errors up to 50%. However, as with the n-paraffins in NaX, there is no indication of a significant enhancement of Tjn,ra 0 comparison with Tin,ra° " as... 

Ti02-coated ACF prepared by adsorption of TiCLj vapor over ACF a molecular adsorption - desorption method... 

Solid-phase microextraction (SPME) was developed as an alternative to many other sample preparation methods because it uses virtually no solvents or complicated equipment. It is an adsorption/desorption method where the compounds of interest are adsorbed onto a coated fused silica fiber. After a given time, the fiber is placed in a gas chromatography (GC), where the compounds are thermally desorbed. SPME has recently been adapted for use in HPLC, where compounds that are adsorbed are desorbed using an appropriate solvent. 

Interests in the nature of active phase in carbon-supported catalysts resulted in the use of a great number of spectroscopic techniques. Frequently, these evaluations were complemented by adsorption/desorption methods as well as by determination of catalyst activity. 

From calorimetric data and equation 6.13, the pore size and its distribution can be obtained. The pore size distribution of a ceramic membrane determined by DSC is in good agreement with data obtained by the gas adsorption-desorption method. 

Choudhary, V.R. Mayadevi, S., and Kamble, K..R., Adsorption of oxygen and nitrogen on AIPO4-5 and SaPO-5 at moderate pressures using a novel adsorption/desorption method, Ind. Eng. Chem. Res., 33(5), 1319-1323 (1994),... 

The electrochemical active surface area (EASA) of fuel cell Pt-based catalysts could be measured by the electrochemical hydrogen adsorption/desorption method. For carbon supported Pt, Pt alloy, and other noble metals catalysts, the real surface area can be measured by the cyclic voltammetry method [55-59], which is based on the formation of a hydrogen monolayer electrochemically adsorbed on the catalyst s surface. Generally, the electrode for measurement is prepared by dropping catalyst ink on the surface of smooth platinum or glassy carbon substrate (e.g, a glassy carbon disk electrode or platinum disk electrode), followed by drying to form a catalyst film on the substrate. The catalyst ink is composed of catalyst powder, adhesive material (e.g., Nafion solution), and solvent. 

Adsorption-Desorption Method Barett-Joyner-Halenda (BJH)... 

The adsorption-desorption method is a popular and commonly used method for characterization of surface and structural properties of porous materials, allowing the determination of their surface area, pore-size distribution, pore volume and adsorption energy distribution. Nitrogen is often used for the adsorbent gas but other adsorbent gases such as argon can also be used. According to this method, adsorption-isotherm (amount of adsorbed gas versus relative pressure [pressure/saturation vapor pressure of the adsorbent]) is drawn and the data are analyzed by assuming capillary condensation. 

Adsorption-desorption methods [20] allow analysis of pore size distribution. 

Techniques where there is a phase change in the fluid which penetrates die pores. In adsorption-desorption methods die amount of an inert gas which condenses into the pores is measured versus pressure. In permporometry die flux of a noncondensable gas is measured while other condensable gas re... 

The gas adsorption-desorption method involves the analysis of adsorption-desorption isotherms of an inert gas as a function of partial pressure (Table 15.3f) [175]. 

Because the gas adsorption-desorption method measures both dead-end and interconnecting pores, another variation of this method, namely, permporometry, is also introduced (Table 15.3g) [177]. Instead of measuring the volume of gas adsorbed/desorbed, the gas flux through open pores is measured in permporometry. Evapoporometry, a relatively new technique involving the evaporation of volatile liquid from membrane pores, is also developed based on the Kelvin equation (Table 15.3h) [176]. 

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