Lewis/Brønsted ratio as a key factor in the selective liquid-phase dehydration of 1-phenylethanol

NICOLÁS M. BERTERO; CARLOS R. APESTEGUÍA; ALBERTO J. MARCHI

Génova

Congreso; 6th World Congress on Catalysis by Acids and Bases; 2009

Dehydration of alcohols is a good method in chemical industry for obtaining olefins. In liquid phase, these reactions need strong mineral acid catalysts, such as H2SO4, KHSO4, H3PO4 or p-toluensulfonic acid [1], which are highly toxic, corrosive and lead to great material disposal [2]. Then, it becomes necessary to replace them by new active, selective and eco-friendly heterogeneous acid catalysts. About 15% of the worldwide styrene (S), widely used as raw material for the production of polymers, is produced from the dehydration of the 1-phenylethanol (PhE) [3]. Depending on the characteristics of the acid solid and operational conditions, other products can be formed during the dehydration: alpha-methylbenzyl ether (AME) and heavy byproducts (HP). PhE dehydration could also be of basic interest as a test reaction when the goal is the dehydration of more expensive but structural similar alcohols as, for instance, indanols and sustituted PhEs, used in Fine Chemistry [4]. The liquid-phase PhE dehydration was studied in previous works using solid acid catalysts at temperatures higher than 423 K [5]. However the influence of nature and strength of the acid sites was not investigated. In this work the liquid-phase dehydration of PhE to S was studied at 363 K, over several acid mixed oxides, and an explanation which relates catalytic activity results with the nature and strength of the acidity is given. Commercial samples used as catalyst were: SiO2-Al2O3 (Ketjen), HZSM-5 (Zeocat PZ), HBEA (Zeocat PB) and SiO2 (Grace G62). On the other hand, HPA(28%)/SiO2, ZnO(20%)/SiO2 and Al-MCM-41 were prepared in laboratory. Sample characterization was performed by employing XRD, N2 physisorption at 77 K, TPD of NH3, and IR of pyridine techniques. Liquid-phase dehydration of PhE was carried out in a 600 ml autoclave, at 363 K, using 0.5 g of catalyst and cyclohexane as solvent.   Initial S selectivity in the PhE dehydration for the samples are shown in Figure 1. The catalyst HPA/SiO2, which contains essentially strong Brønsted (B) acid sites, converted PhE at high rates forming initially mainly AME that was then converted to S and HP.  ZnO/SiO2, that contains only Lewis (L) acid sites, was poorly active but highly selective for the intermolecular dehydration reaction, yielding 99% AME. Samples containing mainly Lewis acid sites (SiO2-Al2O3, Al-MCM-41) formed also only AME as primary product that was then converted to S and HP, in particular on SiO2-Al2O3 because of its stronger acid strength. Zeolite HBEA, with similar density of Brønsted and Lewis acid sites, was the most active catalyst. The initial formation rates of S and AME were similar on HBEA, but then AME was converted to S. These results on HBEA suggest that the selective formation of S is promoted by the presence of similar amounts of strong Lewis and Brønsted acid sites. However, the yield to S was never higher than 70% on HBEA because S was consecutively converted to HP. HZSM-5, with similar Brønsted to Lewis acid sites ratio to HBEA, was also highly active and selective to S, and a 95% S yield was reached over this zeolite. This is because the narrow channels of HZSM-5 hinder the formation of bulky intermediates leading to AME and HP. Activity of HZSM-5 was lower than that of HBEA, probably due to diffusional constrains inside the narrower channels. In conclusion, liquid-phase dehydration of PhE into S takes place only over heterogeneous catalyst with a balanced Lewis and Brønsted acid density. In addition, shape selectivity can boost the olefin yield, avoiding the formation of the corresponding ether and HP.