Base catalysis alcohol dehydration

The aim of this study is to develop model reaction for the characterization of the acidity and basicity of various transition aluminas, the experimental conditions being close to that for catalysis use. Among various model reactions, the transformation of cyclopentanol and cyclohexanone mixture was chosen for this work. Indeed, this reaction was well known for estimating simultaneously the acid-base properties of oxide catalysts [1], Two reactions take place the hydrogen transfer (HT) on basic sites and the alcohol dehydration (DEH) on acid sites. The global reaction scheme is shown in Figure 1. 

The conversion of acetone to methyl isobutyl ketone (MIBK) also uses a combination of base catalysis with a hydrogenation catalyst [35], The base component converts the acetone to diacetone alcohol (DAA) via an aldol reaction, which is then dehydrated by the silica to give mesityl oxide (MO). The final step is the hydrogenation of the MO to MIBK over the metal component. The action of the base catalyst in the absence of the hydrogenating metal has been studied [36]. As well as the aldol condensation reactions shown below, the cesium oxide also hydrogenated MO to MIBK, albeit at a low level (Scheme 21.3). 

As is evident relatively little research has been devoted to catalysis with carbon molecular sieves. This is especially surprising in view of the amount of recent interest there has been in novel catalytic materials. The latter two reports dealing with alcohol dehydration over In-CMS and Diels-Alder dimerizations with CMS materials are very interesting. Although carbon-based molecular sieves have had considerable impact upon the science and technology of small molecule separations, they have been much less important in catalysis. 

Formation of the tetrahedral intermediate carbinolamine and subsequent elimination of water are amenable to acid-base catalysis and do not require a metal surface. The relative rates of adduct formation and subsequent dehydration to imine or enamine depend on the structure of alcohol and amine, and on the nature and strength of acidic and basic sites on the catalyst surface. It must be stressed that several side-reactions (e. g. dimerization and oligomerization, dehydration) are also acid or base-catalyzed, and good selectivity for the desired product requires proper tuning of the redox and acid-base properties of the catalyst. This is crucial in catalyst development when choosing a suitable support, additive, or modifier. Even traces of impurities remaining on the surface from the catalyst precursor can strongly influence product distribution [10]. 

Typical base-catalysed reactions that occur over alkali metal-exchanged zeolites include dehydrogenations, double bond isomerisations, side-chain alkylation of aromatics, conversion of methyl halides and a range of condensations. The reaction of alcohols over zeolites can be used to determine whether acid or base catalysis predominates. Whereas acid forms of zeolites catalyse dehydrations, leading to alkenes and the products of their subsequent reactions, basic sites catalyse dehydrogenations, leading to aldehydes and ketones. 

The point was made earlier (Section 5 9) that alcohols require acid catalysis in order to undergo dehydration to alkenes Thus it may seem strange that aldol addition products can be dehydrated in base This is another example of the way in which the enhanced acidity of protons at the a carbon atom affects the reactions of carbonyl com pounds Elimination may take place in a concerted E2 fashion or it may be stepwise and proceed through an enolate ion... 

Consequently, degradation of halogenated compounds in gaseous effluents, especially in diluted effluents, is often problematic, dehalogenation being the limiting step. In order to overcome this problem, a new process, based on solid/gas catalysis with whole dehydrated cells as the catalyst, has been proposed [14]. The aim is to convert halogenated compounds into alcohols. 

From the previous discussion, it follows that the intracrystalline volume in zeolites is accessible only to those molecules whose size and shape permits sorption through the entry pores thus, a highly selective form of catalysis, based on sieving effects, is possible. Weisz and coworkers 7) have conclusively established that the locus of catalytic activity is within the intracrystalline pores when Linde 5A sieve ( 5 A pore diameter) was used, selective cracking of linear paraffins, but not branched paraffins, was observed. Furthermore, isoparaffin products were essentially absent. With the same catalyst, -butanol, but not isobutanol, was smoothly dehydrated at 230-260°. At very high temperatures, slight conversion of the excluded branched alcohol was observed, suggesting catalysis by a small number of active sites located at the exterior surface. Similar selectivity between adsorption of n-paraffins and branched-chain or aromatic hydrocarbons is shown by chabazite and erionite (18). 

ZrOa and Th02 show similar catali ic behaviors. Although their basic properties are not so strong, they possess weakly acidic properties, and, therefore, show the acid-base bifunctional catalysis. ZrOa and Th02 as well as rare earth oxides showed dehydration activity for alcohols. 

Polyvinyl alcohol (PVA) has been studied in a polyamide 66 based on the possibility of high-temperature acid-eatalysed dehydration. The catalysis can be provided by the acid products of... 

---