Heterogeneous catalytic methylation of catechol

Abstract

It has been shown that, after a partial deactivation, y-alumina ( y-A1203) is a good catalyst for selective methylation of 1,2-benzenediol in the temperature range of 260 -310 "C. The main products are the desired 3-methyl-I ,2-benzenediol and 2-methoxyphenol, which may be converted in another step into 3-methyl-I ,2-benzenediol, giving an overall selectivity for the desired products of 80 to 90%. The catechol forms a strongly chemisorbed surface species on y-alumina and its steric adsorption model correlates with the kinetic data.

Theoretical investigation of the adsorption of methanol on the (110) surface of γ -alumina

“…Isotopic studies have confirmed that the O-H bond was broken during adsorption of methanol on the alumina surface [1,36]. Consequently, methanol makes a covalent bond with the surface of g-alumina with the release of one water molecule, as shown by IR measurements [1,2,36]. Consequently, we assumed, in our calculation, that a previous deprotonation of methanol happens, leading to the formation of a water molecule, and this step is not considered here.…”

“…According to Busca et al [36], several chemisorbed species may be found, depending on temperature, one of them corresponding to a bidented species with the OH group of methanol bound to aluminum and oxygen surface atoms. Finally, in the experimental conditions reported by Porchet et al [2][3][4], methanol mainly adsorbs as a bidented species on g -alumina, i.e. corresponding to this species of Busca et al Methanol deprotonation by a basic hydroxyle of the surface involves water formation and Porchet et al have shown that this occurs during the adsorption.…”

“…It is industrially produced by homogeneous catalytic methylation of catechol in the presence of mineral acid as a catalyst [1]. Previously g -alumina was found to be an effective catalyst for the gas-phase methylation of catechol with methanol to selectively produce 3-methyl catechol [2][3][4]. This process is presented in Fig.…”

“…The selectivity towards the C-methylated products (formation of A 3 or A 4 ) is controlled by the orientation of adsorbed catechol on the surface. Porchet et al [2] and KiwiMinsker et al [3,4] have carried out a vibrational analysis of both adsorbed species with the aim to rationalize the type of adsorption taking place on catalyst surface. In view of the interest of the catalytic mechanism leading to the formation of 3-methyl-catechol, we decided to perform a quantum chemical modelling of the various steps of this reaction.…”

Catalyst development for the selective methylation of catechol

“…The formation rate of 4-methylcatechol is slow compared to the production rates of the other species which is favourable because it is an undesired stable product. On the other hand guaiacol is itself a methylation agent: it decomposes in a reverse reaction to catechol and methanol or can be transformed by rearrangement to the desired 3-methylcatechol (Porchet et al, 1994). Therefore the overall yield Y2 shows the typical reaction path of an intermediate product.…”