The rate inhibiting effect of water as a product on reactions catalysed by cation exchange resins: formation of mesityl oxide from acetone as case study

Abstract

The rate inhibiting effect of water as a product on reactions catalysed by cation exchange resins: Formation of mesityl oxide from acetone as a case studyElizabeth Louisa du Toit U Un ni iv ve er rs si it ty y o of f P Pr re et to or ri ia a e et td d - It is known that when water is a product in reactions catalysed by cation exchange resins, it inhibits the reaction rate much more than predicted by the reverse reaction or dilution effects. In this work the inhibiting effect is ascribed to the preferential association of the catalytically active sites with water. In the derivation of the kinetic model, a Freundlich type adsorption isotherm was used to quantify the number of sites occupied by water. This is combined with a power law expression for the reaction rate. The resultant expression can accurately predict the reaction rate for various initial concentrations of water and mesityl oxide. Even when water was initially added to the reaction mixture, this model still gave an absolute average error of 6.5% compared to a 54.6% error when the same approach was followed but with the more popular Langmuir isotherm to describe site deactivation. The kinetic expression previously proposed for this reaction system by Klein and Banchero also failed when water was added to the reaction mixture and gave i U Un ni iv ve er rs si it ty y o of f P Pr re et to or ri ia a e et td d --D Du u T To oi it t, , E E. .L L. . ( (2 20 00 03 3) ) an average error of 71.1%. The procedure used to derive the model is therefore suggested for all cation exchange catalysed reactions where water is one of the products.

Kinetics of 1-hexanol etherification on Amberlyst 70

“…As found elsewhere in two different studies [22,28], a new approach was undertaken to model the effect of water on the reaction rate, considering that it blocks -SO 3 H groups.…”

“…The Freundlich isotherm approach expects that α decreases almost linearly with temperature and K W is non-dependent [22,29]. Moreover, α should be larger than 1.…”

“…Toit and Nicol's work [22], a Freundlich adsorption isotherm was used to consider the amount of adsorbed water, where n are the sites taking part in the rate-limiting step.…”

“…Such factor is analogous to those mostly used to describe catalyst deactivation by poisoning and, at first sight, it can be seen as the fraction of active centers free of water [20][21][22] …”

“…Frequently, some reactant or reaction product adsorbs preferably on the catalyst surface, e.g., polar species onto sulfonic styrene/divinylbenzene (S/DVB) resins, leading to a decrease of the reaction rate. This rate-inhibiting effect of polar species on S/DVB resins is sometimes used to control selectivity to intermediate products in series reactions [8][9][10][11], however this effect is undesirable when such polar species, in particular water, are reaction products [12][13][14][15][16][17][18][19][20][21][22]. To quantify the effect of reaction medium-catalyst interaction, empirical corrections are suggested in the open literature.…”