Abstract
Two solvent free methods of a one-to-one alcohol/acid mol ratio synthesis of benzyl esters of the formic, acetic, benzoic, salicylic, nicotinic, and oxalic acids are described. The stoichiometric reactions used 1.5 mol ratio solid NbCl5 as the reagent and required from two to three hours for completion at room temperature; for the catalytic processes, NbCl5 was grafted directly, at room temperature, onto a silica gel of specific area of 507 m2g−1, produced from construction sand and sodium carbonate, forming a 5.4% Nb w/w SiO2-Nb gel with a specific area of 412 m2g−1. At 10% w/w catalyst/alcohol ratio, this SiO2-Nb catalyst gave similarly very good yields but required from 6 to 9 hours at the reflux temperature of the slurry. The catalyst could be re-used three times.
Highlights
We have previously shown that a mixture composed of ZnCl2/SiO2 is a very powerful heterogeneous catalyst for the microwave-driven production of esters from aromatic or aliphatic carboxylic acids and alcohol mixtures, but we demonstrated that the esterification catalyst, as were all others to date, was ineffective if the alcohol participant bore an aromatic ring, such as benzyl alcohol [1]
In the search for an active catalyst for the latter transformations, we found that suspensions of NbCl5/ Al2O3 in carboxylic acid-alcohol mixtures did afford the corresponding esters from mixtures of benzyl alcohol with acetic acid but, more interestingly, from the aromatic ring bearing benzoic acid, all the reactions promoted by microwave irradiation
It must be pointed out that we have already developed an hydrophilic acidsulfonated silica catalyst, SiO2-SO3H, that is capable of producing benzyl benzoate from benzyl alcohol and benzoic acid in excellent yields (>93%) [4], and that to the best of our knowledge, this is the first report of the direct use of the dimeric NbCl5 in the synthesis of solid catalysts involving the niobium-silica grafting, Basset et all employed recently the monomeric niobium pentachloride etherate for the same purpose [5]
Summary
We have previously shown that a mixture composed of ZnCl2/SiO2 is a very powerful heterogeneous catalyst for the microwave-driven production of esters from aromatic or aliphatic carboxylic acids and alcohol mixtures, but we demonstrated that the esterification catalyst, as were all others to date, was ineffective if the alcohol participant bore an aromatic ring, such as benzyl alcohol [1]. It must be pointed out that we have already developed an hydrophilic acidsulfonated silica catalyst, SiO2-SO3H, that is capable of producing benzyl benzoate from benzyl alcohol and benzoic acid in excellent yields (>93%) [4], and that to the best of our knowledge, this is the first report of the direct use of the dimeric NbCl5 in the synthesis of solid catalysts involving the niobium-silica grafting, Basset et all employed recently the monomeric niobium pentachloride etherate for the same purpose [5]
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