INVESTIGADORES
SAD Maria Eugenia
congresos y reuniones científicas
Título:
Study of the mechanism of methylation of phenol over acid zeolites
Autor/es:
MARÍA EUGENIA SAD; CRISTINA LILIANA PADRÓ; CARLOS RODOLFO APESTEGUÍA
Lugar:
San Francisco, California
Reunión:
Congreso; 21st North American Catalysis Society Meeting; 2009
Institución organizadora:
North American Catalysis Society
Resumen:
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Introduction
Gas phase alkylation of phenol (P) with methanol (M) has been widely
studied for many years but the mechanism over solid acids remains still
unresolved. A variety of products such as cresols, anisole, xylenols (X) and
methylanisoles (MA) are formed when acid catalysts are used. In general,
cresols can be formed both by direct C-alkylation of phenol with methanol and
by conversion of the anisole formed by O-methylation of phenol. In a previous
work [1], we have investigated this reaction on acid catalysts with the goal of
promoting the selective synthesis of p-cresol. The para-selectivity was
dramatically increased by using zeolite HMCM22, revealing that the narrow
channels of this zeolite are particularly suitable for improving by shape
selectivity the formation of p-cresol. In this work, we performed a detailed
study of the gas-phase alkylation of phenol with methanol on samples containing
both Lewis and Brönsted acid sites and different porous structure (zeolites HZSM5,
HBEA and HMCM22) in order to better understand the mechanism of cresols
formation. Results will show that the primary and secondary reaction pathways
leading from phenol to cresols depend on both the pore microstructure and the
nature, density, and strength of surface acid sites and also explain the
unusual high para-selectivity obtained over HMCM22 [1].
Materials and Methods
Commercial zeolites HZSM5
(Zeocat PZ-2/54, Si/Al=20) and HBEA (Zeocat PB, Si/Al=12.5) were used. Zeolite HMCM22
(Si/Al = 15) was synthesized according to [2].
Sample acidity was determined by TPD of NH3 preadsorbed at
373 K. The nature of surface acid sites was determined by infrared spectroscopy
using pyridine as probe molecule. Gas phase reactions (alkylation of phenol
with methanol, alkylation of anisole with methanol, alkylation of phenol with anisol,
methylation of cresols, etc) were carried out in a fixed bed reactor at 473 K
and 101.3 kPa in continuous flow of N2. The
exit gases were analyzed on-line using an Agilent 6850 chromatograph that was equipped
with a flame ionization detector and a 30 m Innowax column (inner diameter:
0.32 mm, film thickness: 0.5 mm).
Results and Discussion
Results of the methylation of phenol on zeolites
HZSM5, HBEA and HMCM22 showed that the anisole yield increases with contact
time and then reaches a maximum indicating that anisole is converted to
secondary products. HBEA and HZSM5 formed higher amount of anisole than HMCM22.
Essentially, zeolite HMCM22 suppressed the formation of anisole and also
significantly decreased the o-cresol formation rate, but formed p-cresol at
high rates. Our results using pure anisole and anisole with phenol as reactants
suggested that on HBEA, cresols are mainly formed by alkylation of phenol with
anisole, whereas on HZSM5 and HMCM22 phenol does not promote the cresols
formation. All the experiences
feeding anisole (pure and with phenol or methanol) showed that significant amounts
of phenol were
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always formed. Phenol was mainly formed from anisole dealkylation, but
it was also produced by anisole disproportionation. Experiences feeding
o-cresol and methanol showed that
zeolites HBEA and HZSM5, which exhibit similar amounts of Brönsted and Lewis
acid sites, form mainly xylenols via alkylation reactions. In contrast, zeolite
HMCM22 that contains predominantly Brönsted acid sites, promotes isomerization
reactions, forming preferentially p-cresol but also m-cresol by consecutive isomerization of orto and para-isomers.
Our catalytic results on zeolites HBEA, HZSM5 and HMCM22 are well explained by considering
that the alkylation of phenol with methanol occurs according to the reaction
network proposed in Figure 1.
Figure 1. Reaction network for methylation of phenol on HBEA,
HZSM5 and HMCM22.
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Significance
Catalytic tests feeding intermediate reaction products allowed us to
clarify the complex reaction network of the methylation of phenol on acid
zeolites. We also concluded that both the particular porous structure and the
high Brönsted/Lewis acid sites ratio of HMCM22 are responsible for the
preferential formation of p-cresol on this zeolite.
References
1.
Maria E. Sad, Cristina L. Padro,
C. Apesteguia, Appl. Catal. A:Gen 342 (2008) 40.
2.
Rubin, M.A., and Chu, P. US Patent 4,954,325, 1990.