Alkoxy species

In the other mechanism, the catalytic cycle initiates through the insertion of CO into a Pd-alkoxy bond, with formation of a Pd-carboalkoxy intermediate, which inserts the olefin with formation of an alkylcarboalkoxy /i-chelate, which undergoes protonolysis by the alkanol through the intermediacy of its enolate isomer (see Sect. 2.3.1), yielding the ester and the Pd-alkoxy species, which then initiates a new catalytic cycle [122-125]. 

The disproportionation of tertiary and primary alkoxy species 4 has been reported. The NMR spectrum of an equivalent molar mixture of LiAlH3(OBu ) and LiAlH(OBu )3 was the same as that of LiAlH2(OBu )2, suggesting disproportionation of the nonsymmetrical compounds (37). Kinetics of the reduction of several aromatic ketones in ether with reagents formed by the reaction of LAH with Bu OH were consistent with partial disproportionation of species 4 (38) ... 

Many of the characterization techniques described in this chapter require ambient or vacuum conditions, which may or may not be translatable to operational conditions. In situ or in opemndo characterization avoids such issues and can provide insight and information under more realistic conditions. Such approaches are becoming more common in X-ray adsorption spectroscopy (XAS) methods ofXANES and EXAFS, in NMR and in transmission electron microscopy where environmental instruments and cells are becoming common. In situ MAS NMR has been used to characterize reaction intermediates, organic deposits, surface complexes and the nature of transition state and reaction pathways. The formation of alkoxy species on zeolites upon adsorption of olefins or alcohols have been observed by C in situ and ex situ NMR [253]. Sensitivity enhancement techniques play an important role in the progress of this area. In operando infrared and RAMAN is becoming more widely used. In situ RAMAN spectroscopy has been used to online monitor synthesis of zeolites in pressurized reactors [254]. Such techniques will become commonplace. 

A.l. Formation of Surface Alkoxy Species with Carbenium-Ion-Like Properties... 

So far, fewer than 10 types of carbenium ions have been reported to be persistent species formed upon adsorption of olefins or alcohols on acidic zeolites. Instead, surface alkoxy (alkoxide) species with carbenium-ion-like properties are suggested to act, most likely, as catalytic intermediates in reactions catalyzed by acidic zeolites. Various groups have observed that, upon adsorption of olefins or alcohols on acidic zeolites, alkoxy species are formed the observations are based on both in situ and ex situ A MAS NMR spectroscopy (49,50,71-80). 

In Table 2, the A chemical shifts of the carbon atoms of alkoxy species attached to zeolite framework oxygen atoms are summarized. In general, the spins of surface alkoxy species are characterized by relatively long T times (2-5 s), an efficient CP, and broad spinning sideband patterns. The adsorption and... 

MAS NMR Chemical Shifts of Surface Alkoxy Species Observed upon the Adsorption of Alkenes or Alcohols on Acidic Zeolites... 

Surface alkoxy species Chemical shift (ppm) Adsorbate Zeolite References... 

By characterizing various zeolite catalysts under the same reaction conditions, the authors found weaker MAS NMR signals of alkoxy species for the less active zeolites HY and HZSM-5 than for the more active zeolite H-beta (250). This observation suggests that the alkoxy species observed under steady-state conditions act as reactive surface species in the MTBE synthesis from isobutylene and methanol on acidic zeolite catalysts. 

Until now, the detailed mechanism involved in the MTG/MTO process has been a matter of debate. Two key aspects considered in mechanistic investigations are the following the first is the mechanism of the dehydration of methanol to DME. It has been a matter of discussion whether surface methoxy species formed from methanol at acidic bridging OH groups act as reactive intermediates in this conversion. The second is the initial C—C bond formation from the Ci reactants. More than 20 possible mechanistic proposals have been reported for the first C-C bond formation in the MTO process. Some of these are based on roles of surface-bound alkoxy species, oxonium ylides, carbenes, carbocations, or free radicals as intermediates (210). 

As mentioned above, a variety of complexes of type (10) can easily be prepared by alcohol exchange and, in this way, compounds with R = Pr", Bun, Pen , Oct" have been isolated and characterized.221 However, when secondary alcohols have been used for exchange with (10), mixed alkoxy species were obtained. NMR showed that the bridging ethoxide ligands had not been replaced, a result that was confirmed by the crystal structure determination of the mixed ethoxide-isopropoxide and ethoxide-sec-pentoxide derivatives. 

While our proposed mechanism was interesting, it left some unanswered questions. What was the nature of the catalyst complex and more importantly, why was this not behaving like a classic acid catalysis Boe [11, 12] had postulated protonation of the alkoxy species followed by SN2 attack by the alcohol nucleophile. This is consistent with the negative value that he found for p. This does not agree though with the idea of a catalyst complex, nor does it agree with the findings reported here of a positive value of p. 

In the case of Brpnsted acidity, two reaction mechanisms have been suggested the formation of carbenium ion intermediate species (Scheme 6.2), which lead to the formation of branched oligomers[20] and the formation of a surface alkoxy structure intermediate, which leads to the formation of linear oligomers1191 (Scheme 6.3). In this case, the formation of alkoxy species has been recently... 

In THF, the surface films comprised Ca(OH)2 (trace water) and alkoxy species. 

The electrochemical behavior of lithium electrodes in a variety of polymeric electrolyte systems was studied extensively by a number of groups, including Scrosati et al. [390-392], Panero et al. [393], Abraham et al. [394-396], Osaka et al. [397-398], Watanabe et al. [399-401], Peled et al. [402], It is clear that there are surface reactions between the lithium and all of the polymeric systems mentioned above. It has already been clearly shown that the ether linkage is attacked by lithium, resulting in the formation of Li alkoxy species [149], Hence, it is expected the PEO-based polymers also react with Li surfaces. Spec-troelectrochemical studies of the Li-PEO system by Scherson et al. [177] provide some evidence for this possibility. Besides the polymers, the polymeric electrolyte systems contain salts with anions such as Aslv,, S03CF3, NlSOTTO),, ... 

A further Raman investigation of the effect of the antimony-to-vanadium atomic ratio, gave evidence that the formation of VSbCh resulted in higher yields of acrylonitrile when surface vanadium oxide species were also present (Banares et al., 2002 Guerrero-Perez and Banares, 2004). The presence of surface alkoxy species was not observed in the absence of dispersed surface vanadium oxide species. 

Figure 4.54 shows schematically the species observed upon adsorption of alcohols (alkoxy species), aldehydes, ketones and carboxylic acids. When alcohols are adsorbed, the methoxy group is always formed very easily with all metal (the evidence for it is from D2 exchange reactions [107] and various spectroscopic techniques for the latter see the references below). However, only on Cu and Ag does the stability of this intermediate allow a comfortable study. With the transi-... 

Another possible route into the carboalkoxy species which may be considered is the formal insertion of CO into a preformed Pd-alkoxy species. Such a mechanism was considered a viable possibility in the catalytic nonphosphine system of Fenton and Steinwand (102). Given that in the absence of added CO but under otherwise identical reaction conditions alcohols are oxidized to the corresponding aldehyde or ketone (117,118), this may be reasonable. Presumably this latter reactivity can be attributed to initial alkoxy formation, followed by /f-hydride elimination ... 

This reaction has been the subject of many experimental, " and theoretical studies. The reaction, which initiates from a propylene physisorbed to the acidic proton, leads to the formation of a more stable chemisorbed propylene, or alkoxy species. Such reaction has been shown experimentally to occur readily at room temperature within an acidic zeolite. Whereas this reaction in principle can produce two different alkoxy species (viz. a primary and a secondary alkoxy species), experiment reports that only the secondary alkoxy species can be formed. This is explained by the fact that the formation of a transient primary carbenium ion is energetically more demanding than the formation of a secondary carbenium ion. " As already... 

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