Surface Lewis complexes

The formation of surface Lewis complexes through a plain cr-coordination being fully reversible upon evacuation of pco, this process entirely fulfilled the requirements imposed by the method. Equation 1.26 was employed ... 

The bond strength of this surface coordination complex should depend on the Lewis acidity of the particular site (coordination number and ligand distribu-... 

More recently, Pereira and coworkers [44] found a close relationship between anionic dye adsorption and carbon surface basicity due to oxygen-free Lewis sites. Surface oxygen complexes of acid character inhibited anionic dye adsorption. However, the latter complexes had a positive effect on cationic dye adsorption. The removal of these groups by heat treatment in H2 at high temperature... 

Figure 4.2 Schematic of the pyridine complexes proposed for the four types of surface Lewis acid site in r -alumina. (Reprinted with permission from D. T. Lundie, A. R. Mclnroy, R. Marshall, J. M. Winfield, P. Jones, C. C. Dudman, S. F. Parker, C. Mitchell and D. Lennon, J. Phys. Chem., B, 109, 11592-11601 Copyright (2005) American Chemical Society.)...

However, it is known that, in the absence of processes other than plain surface coordination, CO acts as a weak Lewis base and can interact with the strongest surface Lewis acid sites. NO can also be employed either as a probe to identify Lewis acid sites or as a reducing agent. However, NO may disproportionate into N2O and oxygen and it is also very likely to form nitrosyl complexes in the presence of transition metal ions [60]. 

The enhancement of methyl migration to form a surface acyl complex upon reaction of, for example, Mn(C0)5(CH3) or Fe(Cp)(CO)2(CH3) with y-alumina has been explained by the known reactivity of these same molecular metal-alkyl complexes in the presence of soluble Lewis acids. 

The adsorbed complex interacts via the oxygen of the bridging CO group to a surface Lewis acid site. CO evolution proceeds at >40 C [38,41], and oxidation is evident (by H2 evolution) at temperatures above 100°C. In contrast, Fe3(CO)i2 adsorbed as such on silica at room... 

In contrast to the situation in the absence of catalytically active Lewis acids, micelles of Cu(DS)2 induce rate enhancements up to a factor 1.8710 compared to the uncatalysed reaction in acetonitrile. These enzyme-like accelerations result from a very efficient complexation of the dienophile to the catalytically active copper ions, both species being concentrated at the micellar surface. Moreover, the higher affinity of 5.2 for Cu(DS)2 compared to SDS and CTAB (Psj = 96 versus 61 and 68, respectively) will diminish the inhibitory effect due to spatial separation of 5.1 and 5.2 as observed for SDS and CTAB. 

The metal-ion complexmg properties of crown ethers are clearly evident m their effects on the solubility and reactivity of ionic compounds m nonpolar media Potassium fluoride (KF) is ionic and practically insoluble m benzene alone but dissolves m it when 18 crown 6 is present This happens because of the electron distribution of 18 crown 6 as shown m Figure 16 2a The electrostatic potential surface consists of essentially two regions an electron rich interior associated with the oxygens and a hydrocarbon like exterior associated with the CH2 groups When KF is added to a solution of 18 crown 6 m benzene potassium ion (K ) interacts with the oxygens of the crown ether to form a Lewis acid Lewis base complex As can be seen m the space filling model of this... 

Cosolvents ana Surfactants Many nonvolatile polar substances cannot be dissolved at moderate temperatures in nonpolar fluids such as CO9. Cosolvents (also called entrainers, modifiers, moderators) such as alcohols and acetone have been added to fluids to raise the solvent strength. The addition of only 2 mol % of the complexing agent tri-/i-butyl phosphate (TBP) to CO9 increases the solubility ofnydro-quinone by a factor of 250 due to Lewis acid-base interactions. Veiy recently, surfac tants have been used to form reverse micelles, microemulsions, and polymeric latexes in SCFs including CO9. These organized molecular assemblies can dissolve hydrophilic solutes and ionic species such as amino acids and even proteins. Examples of surfactant tails which interact favorably with CO9 include fluoroethers, fluoroacrylates, fluoroalkanes, propylene oxides, and siloxanes. 

Spectroscopy. In the methods discussed so far, the information obtained is essentially limited to the analysis of mass balances. In that re.spect they are blind methods, since they only yield macroscopic averaged information. It is also possible to study the spectrum of a suitable probe molecule adsorbed on a catalyst surface and to derive information on the type and nature of the surface sites from it. A good illustration is that of pyridine adsorbed on a zeolite containing both Lewis (L) and Brbnsted (B) acid sites. Figure 3.53 shows a typical IR ab.sorption spectrum of adsorbed pyridine. The spectrum exhibits four bands that can be assigned to adsorbed pyridine and pyridinium ions. Pyridine adsorbed on a Bronsted site forms a (protonated) pyridium ion whereas adsorption on a Lewis site only leads to the formation of a co-ordination complex. 

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