INVESTIGADORES
BERTERO nicolas Maximiliano
congresos y reuniones científicas
Título:
Lewis/Brønsted ratio as a key factor in the selective liquid-phase dehydration of 1-phenylethanol
Autor/es:
NICOLÁS M. BERTERO; CARLOS R. APESTEGUÍA; ALBERTO J. MARCHI
Lugar:
Génova
Reunión:
Congreso; 6th World Congress on Catalysis by Acids and Bases; 2009
Resumen:
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.