Kinetics butyl alcohol dehydration

Note that the rate coefficients k determined by our kinetic studies with the static FTIR reactor for all four butyl alcohols are the true rate coefficients for the forward step of stage II of Scheme 1, i. e., k = k+//. But under the steady-state conditions of the flow microreactor, the observed reation rate, Wbuoh, of butyl alcohol dehydration is less than or equal to the product (k+//N) of the rate... 

It is interesting that, for reaction mixtures consisting of molecules with dimensions close to the cross-sections of the catalyst pores (as in the case of butyl alcohol dehydration in HZSM-5), the reacting mixture may be envisaged as a liquid with dimensions less than three. This, in turn, introduces additional factors with respect to the unanalyzed peculiarities of mass-transfer kinetics in the catalyst pores. 

The latter observation is confirmed by the studies of Gates et al. for various sulphonic acid resins and a number of alcohols. A kinetic study of t-butyl alcohol dehydration in the liquid phase between 35 and 77 °C for the -SO3H form revealed that (i) at low catalyst concentrations the reaction rate was first order in resin concentration and (ii) at high catalyst concentrations the order in resin concentration was four or five. A combined rate expression was written as (equation 5) ... 

Although theoretical and computational advances now afford powerful insights into the mechanisms of heterogeneous catalysis, especially on acidic, zeolitic solids (6a-d), experimental studies (7, 8) still hold sway. This we hope to demonstrate here by reference to the wide range of techniques—spectroscopic, kinetic, and analytical—that we have brought to bear in our studies of the catalytic dehydration of butyl alcohols. 

Fig. 3. Kinetics of n-butyl alcohol consumption (a) and dehydration (b) in HZSM-5 (flow microreactor, sample I, 399 K) (A) Water ( ) di-n-butyl ether, n-butene (X) unreacted n-butyl alcohol.

The kinetics of adsorption and dehydration of the butyl alcohol were measured in situ via the time-dependences of the line intensities at 1460-1470 cm-1 (CH deformation vibrations) and 1640 cm 1 (deformation vibrations of adsorbed H20), respectively. 

For all four alcohols in the zeolitic catalysts with small enough crystallite sizes—when diffusion limitations also disappear—dehydration kinetics are well approximated by the exponental function, a fact that is explicable in terms of the unimolecular decay of molecules of butyl alcohol adsorbed on identical active sites. With isobutyl alcohol, for example, the rate coefficient k may be written... 

Transient kinetic phenomena of another type were observed in the so-called purging experiments, whereby we switched from feeding the flow reactor with a helium-butyl alcohol mixture to one with pure helium and then back to the previous helium-butyl alcohol. A typical response of a catalyst to such purging is given in Fig. 6, referring to the dehydration of sec- and isobutyl alcohols over HZSM-5. For sec-butyl alcohol, the rate of butene formation initially increases by a factor of about 10 upon purging and then drops to zero. Return (8k) to the... 

All steady-state and transient kinetic data for the dehydration of all butyl alcohols over HZSM-5 and AAS catalysts may be rationalized in terms of the reaction mechanism presented in Scheme 1. 

Thus, reaction intermediate r/// for dehydration of butyl alcohols can exist in two forms, i.e., butyl silyl ether (BSE) and adsorbed butyl carbenium ion (BC1). Our NMR and kinetic data imply the existence of reversible transformations between BSE, BCI, and adsorbed butene (BuadJ that ar shown in Scheme... 

Typical acid-catalyzed reactions like the dehydration of alcohols and double bond shifts in olefins have been mentioned occasionally as reactions catalyzed by organic heterogeneous catalysts. An extensive kinetic study of the dehydration of tertiary butyl alcohol over pyrolized polyacrylonitrile has been describ-... 

Cyclic reaction intermediates were proposed both for s-butyl alcohol and isobutyl alcohol dehydration and this is supported both by the i.r. spectra and earlier kinetic data for t-butyl alcohol.It was concluded that the concerted mechanism is more efficient than that involving a carbonium ion, reactions involving the latter being slower. 

Water present in the catalytic systems shows versatile effects on reaction rates. Heath and Gates found the induction period in the dehydration of t-butyl alcohol and also noted that water addition reduced the induction time. This effect of water was attributed to the swelling of the resin network. The swelling reduces intraparticle resistance to mass transport, and makes an increasing fraction of the catalytic sites accessible to the reactant. Though water accelerated the reaction initially, it also inhibited the reaction. The retardation with water was observed also in estrification of salicic acid with methanol and benzene propylation. The retarding effect of water was explained by a kinetic expression based on the Langmuir-Hinshelwood model, in which the competitive chemisorption of water and a reactant (alcohol or acid) is assumed. 

Another kinetic approach which has been employed for a large number of amides and oxygen bases involves comparison of the strengths of bases through their ability to retard an acid-catalyzed reaction. This has been applied to the p-toluenesulfonic acid catalyzed self-etherification of benzhydrol and the dehydration of terin butyl alcohol in benzene solution (280,281). 

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