Surface ethoxy species

As an example. Fig. 18 shows CP/MAS NMR spectra recorded during the investigation of surface ethoxy species (7S) formed on acidic zeolite HY ( si/ Ai = 2.7) by a SF protocol. Figure 18a shows the CP/MAS NMR spectrum recorded after a continuous injection of C-1-enriched ethanol, CHI CHzOH, into the MAS NMR rotor reactor containing calcined zeolite HY. The ethanol was injected at room temperature for 10 min. Subsequently, the loaded zeolite was purged with dry nitrogen (200 mL/min) at room temperature for 2h. 

Surface ethoxy species were also formed on acidic zeolite HY ( si/ Ai — with the conventional vacuum technique, that is, with the adsorption of C-1-enriched ethanol, CH/ CH2OH, and subsequent evacuation at 453 K for 24 h (7,/ ). The further transformation of surface ethoxy species into hydrocarbons at elevated... 

Wang W, Jiao J, Jiang YJ, Ray SS, Hunger M. Formation and decomposition of surface ethoxy species on acidic zeolite Y. ChemPhysChem 2005 6 1467-9. 

In acidic media, the reactivity of ethanol on Au electrodes is much lower than in alkaline media. The main product of the oxidation of ethanol on Au in an acidic electrolyte was found to be acetaldehyde, with small amounts of acetic acid [Tremiliosi-FiUio et al., 1998]. The different reactivities and the product distributions in different media were explained by considering the interactions between the active sites on Au, ethanol, and active oxygen species absorbed on or near the electrode surface. In acidic media, surface hydroxide concentrations are low, leading to relatively slow dehydrogenation of ethanol to form acetaldehyde as the main oxidation pathway. In contrast, in alkaline media, ethanol, adsorbed as an ethoxy species, reacts with a surface hydroxide, forming adsorbed acetate, leading to acetate (acetic acid) as the main reaction product. 

An explanation for this behaviour can be deduced fi om Scheme 1. Ethanol decomposes at the surface of the catalyst, forming ethoxy then acyl species. This reaction is an equilibrium, the concentration of ethoxy and acyl species dependent on temperature and operating pressure. For ethyl ethanoate formation, an adsorbed acyl species reacts with an adsorbed ethoxy species to form an intermediate hemiacetal subsequent dehydrogenation forms ethyl ethanoate. By-products, such as ketones, aldehydes and alcohols are formed by... 

Additional chemical evidence for the assignment of the 58 ppm resonance to the methoxy species III was the observation that it also formed from methyl bromide and methyl chloride in relative yields consistent with the leaving group stability T > Br > Cl. Methyl iodide was adsorbed on several zeolites with different Si/Al ratios, and the intensity of the 58 ppm resonance correlated with the A1 content, as it must for a framework-bound alkoxy. The final example of chemical evidence for the assignment regards the expected chemistry of species such as III and VI upon exposure to moisture. The Si-O-C linkage is easily hydrolyzed on a silica gel surface to form alcohols and/or ether. As demonstrated in Fig. 16, the species assigned to III readily hydrolyzes to methanol and dimethyl ether, whereas the proposed ethoxy species formed from ethyl iodide- C hydrolyzed to ethanol upon exposure to atmospheric moisture. 

Fainerman and Miller [35] found that displacement of an initially adsorbed surfactant by a second, more surface-active species allowed measurement of the desorption rate of the former. For example, competitive adsorption of sodium decyl sulfate and the nonionic Triton X-165 gave a desorption rate constant for the former of 40 s". Mul-queen and coworkers [36] recently developed a diffusion-based model to describe the kinetics of surface adsorption in multicomponent systems, based upon the Ward-Tor-dai equation. Experimental work with a binary mixture of two nonionic alkyl ethoxy-late surfectants [37] showed good agreement with the model, demonstrating a similar temporal adsorption profile to that found by Diamant and Andehnan [34],... 

Mixing of an aminosilane with silica gel results in a fast adsorption, by hydrogen bonding of the amine to a surface hydroxyl group.2 After adsorption, the amine group can catalyze the condensation of the silicon side of the molecule with a surface silanol. Thus siloxane bonds with the surface may be formed in the absence of water.3,4 For other silanes the siloxane bond formation requires an initial hydrolysis of the ethoxy groups or the addition of an amine in the reaction mixture.5 This general reaction scheme has been presented in chapter 8. Here we will go into further detail on the types of interaction of the aminosilane with the silica surface and the characterization of the bonded silane species. 

Anchoring of metal complexes through interaction with surface hydroxyl groups of inorganic supports continues to be of interest. Studies with catalysts prepared with allyl, carbonyl, chloride, and ethoxy ligands have been reported. Kuznetsov and co-workers conclude that the precursors of metathesis-active centres of surface metal complexes, prepared by anchoring allyl and ethoxy compounds of Mo, W, and Re to silica, are co-ordinatively unsaturated metal ions with oxidation number +4. Metathesis activity of the surface species depends on the ligand environment of the metal ion. 

The two sharp desorption peaks in the m/e 44 trace above 400 K arise from methoxy and carbonate (formate) species which hardly produce formaldehyde. This finding strongly underlines the notion that methoxy and formate prepared in the surface science experiment as long-lived species [50] are different from methoxy and formate existing as reaction intermediates in the steady state conversion situation. It is speculated that minor differences in the chemical bonding of the chemically identical ad-species decide over their character as stable spectator species or unstable intermediate structures. Vibrational spectroscopic data [44] of methoxy and the analysis of the homologue ethoxy system [51] strongly support this view. 

---