Adsorption-assisted desorption

One of the major problems is that in many cases the resulting product interacts more strongly with the molecular sieve than the reactant. This leads to the situation that many reactions are desorption controlled and need either a reactant to desorb (adsorption assisted desorption) or a gaseous/liquid cocatalyst that also facilitates the desorption of the products without participating in the reaction. Note that for liquid phase reactions the solvent can take over the role of the cocatalyst. 

The reaction order in buta-1,3-diene is close to zero, indicating that the fraction of vacant sites is very low, and at the total consumption of buta-1,3-diene the mole fractions of butenes are not equal to zero. The assumption of equilibrium adsorption of the intermediate compound (but-l-ene) in the case of irreversible butadiene hydrogenation and but-l-ene isomerization and hydrogenation cannot explain the latter observation. Therefore, adsorption/desorption steps for buta-1,3-diene but-l-ene, but-2-ene are thought to be reversible and have an "adsorption-assisted desorption " nature. The desorption of butane step 15 is assumed to be irreversible and fast. For conformational isomerization (step 2) a quasi-equilibrium approximation will be used. 

Aluminas are commonly used as acid catalysts and as support material for catalysts. Here, we will investigate the influence of the alumina support on ammonia adsorption in the ammonia oxidation reaction over Pt/7-alumina. Ammonia appears to be involved in a phenomenon called adsorption assisted desorption (AAD). Adsorption assisted desorption (AAD) has become well-known in catalysis. We foimd that the rate of desorption of ammonia is increased by the partial ammonia pressure in the gas phase. 

Figure 23 further shows that after changing the flow to NHs/He, [ N]-NH3 desorbs and travels as a pulse through the reactor. This indicates that [13N]-NH3 exchanges rapidly with [ Nj-NHs. At first sight, this exchange process is very similar to the experiment shown in Figure 22. However, in this case, radio-labelled ammonia is not in full equilibrium on 7-almnina. After switching to imlabelled ammonia, first of all, the available Lewis sites are saturated. The time to satmate the 7-alumina bed with ammonia, measured with the mass spectrometer at the outlet of the reactor, is equal to the retention time of radiolabelled ammonia in the catalyst bed. Thus, the radiolabelled ammonia moves with the saturation front, where aimnonia adsorption/desorption is in quasi equilibrium. We conclude that gas phase anunonia clearly facilitates desorption of [ Nj-NHs it remains adsorbed at the same bed position without ammonia in the gas phase. This proves that Adsorption Assisted Desorption takes place for aimnonia desorption from 7-alumina.

The [ Nj-NHs/l NJ-NHs exchange experiments on 7-alumina, showed that adsorption assisted desorption of ammonia takes place on 7-alumina. 

The recognition that ANl-type photosorption proceeding via models similar to that in Fig. 1(b), represents the net outcome of opposing photo-assisted adsorption and desorption processes initially found expression in equations of the type... 

Upon the adsorption of the glucose molecule into the enzyme cavity, the cavity closes as observed in Fig. 7. lb. The cavity continues to change as the reaction proceeds, which helps to drive the reaction over the potential energy surface to the product state. Desorption of the product molecules requires reopening of the cavity in order to release them. This process can be aided by the coadsorption of an additional reactant molecule at a second peptide binding site (allosteric effect). This reduces the interaction between product molecules and cavity, assists desorption and decreases the tendency of the enzyme to become deactivated by product poisoning. 

The similarities between the nitrile hydrolysis and CO2 hydrolysis system relate to the heterolytic H20-assisted splitting of H2O and the importance of the adsorption-induced desorption of the reactant molecule. The primary difference between the enzyme and the zeolite relates to the specific interactions with the imidizole group in the histidine framework, which can simultaniously interact with the reaction center to aid bond cleavage and bond formation reactions. 

Table 9.14 Multiple technique polymer studies - PMR spectroscopy -assisted desorption adsorption ionisation ToF-MS laser...

FTIR spectroscopy - Laser assisted desorption, adsorption ionisation ToF-MS ... 

This interfacial convection is possible during a notable time only if some process of desorption occurs. Now, the desorption can be spontaneous with a short chain surfactant, which is easily desorbable. In this case, the kinetics of transfer, adsorption and desorption have to be considered simultaneous in order to account for some instability, that is the same molecule will be adsorbed and desorbed. When the surfactant is not or only slightly desorbable - that is if the chains are long - the desorption cannot be spontaneous but it can happen that it is assisted by an interfacial reaction which transforms a very adsorbable species into an easily desorbable one as by our counter ion exchange reaction. 

Korkut, O., Sayan, E., Lacin, O., Bayrak, B. Investigation of adsorption and ultrasound assisted desorption of lead (II) and copper (II) on local bentonite A modeling study. Desalination, 2010,259,243-248. 

Separation Techniques. Current methods for separating fatty acids are by solvent crystaUi2ation or by the hydrophili2ation process. Other methods that have been used in the past, or perhaps could be used in the future, are panning and pressing, solvent extraction, supercritical fluid extraction, the use of metal salts in assisting in separation, separations using urea complexes, and adsorption/desorption. 

The extent of equilibrium adsorption of a given chain, thus, would depend on the counteraction between the three enthalpic and entropic components, plus the negative entropic contribution of the chain restraint. The rate of adsorption should be fairly fast, in fact, accelerated after the first segment was held, while the rate of desorption requiring simultaneous multiple desorption steps should be very slow, so slow indeed as to be barely measurable except when assisted by a displacing species. 

This proposal describes the development of a new, systematic approach for qualitatively and quantitatively studying surface-biomolecule interactions by matrix-assisted laser desorption ionization (MALDl) mass spectrometry (MS). This methodology is being developed because of the profound importance that surface-biomolecule interactions play in applications where biomaterials come into contact with complex biological fluids, it can readily be shown that undesired reactions occurring in response to surface-biomolecule contact (protein adsorption, biofouling, immune response activation, etc.) lead to enormous economic and human costs. Thus, the development of analytical methodologies that allow for efficient assessment of the properties of new biomaterials and/or the study of detailed fundamental processes initiated upon surface-biomolecule contact are of critical value ... 

Several authors have proposed that CH4 combustion over PdO occurs via a redox mechanism [82-85]. Methane activation through assisted hydrogen extraction is generally regarded as the rate-determining step, although there is not a general consensus on the nature of the adsorption sites. Further, desorption of H2O by decomposition of surface hydroxyls has been reported to play a key role in reaction kinetics at temperatures below 450 °C [67, 86]. 

The shape of the hysteresis loop in the adsorption/desorption isotherms provides information about the nature of the pores. The loops have been classified according to shape as A, B and E (De Boer, 1958) or as HI - H4 by lUPAC (Sing et al, 1985). Ideally, the different loop shapes correspond to cylindrical, slit shaped and ink-bottle pores the loops in the isotherm IV and V of Figure 5.3 correspond to cylindrical pores. Wide loops indicate a broad pore size distribution (for an example see Fig. 14.9). The absence of such a loop may mean that the sample is either nonporous or microporous. These generalizations provide some initial assistance in assessing the porosity of a sample. In fact the adsorption/desorption isotherms are often more complicated than those shown in Figure 5.3 owing to a mixture of pore types and/or to a wide pore size distribution. 

---