INVESTIGADORES
PADRO Cristina Liliana
congresos y reuniones científicas
Título:
Mechanism of Alkylation of phenol with methanol on acid zeolites
Autor/es:
MARIA EUGENIA SAD; CRISTINA PADRO; CARLOS APESTEGUIA
Lugar:
Genova, Ancient Harbor
Reunión:
Congreso; 6th world congress on Catalysis by Acids and Bases; 2009
Resumen:
The gas-phase alkylation of phenol with methanol has been widely studied
on acid catalysts but knowledge regarding the reaction mechanism and the
exact requirements of acid site density and strength to efficiently promote
the selective formation of cresols is still needed. A variety of products such
as cresols, anisole, xylenols and methylanisoles are formed from
methylation of phenol on acid catalysts [1,2]. Cresols are formed by both
direct C-alkylation of phenol with methanol and conversion of anisole
previously formed by O-methylation of phenol. But cresols can in turn
react with methanol on acid sites to produce dialkylated compounds
(xylenols, methylanisoles). In this work, we performed a detailed study on
the effect of the pore microstructure and the nature, density, and strength of
surface acid sites on product distribution for the methylation of phenol with
methanol on acid zeolites (zeolites HZSM5, HBEA and HMCM22). We
propose a reaction mechanism that interprets the primary and secondary
reaction pathways leading from phenol to mono- and dialkylated products
and also explains the unique high para-selectivity obtained on HMCM22.para-selectivity obtained on HMCM22.
Results and Discussion
The mechanism of methylation of phenol on zeolites HZSM5, HBEA and
HMCM22 was studied by: i) performing the reaction at increasing contact
times; ii) using as reactants the intermediate reaction products, namely,
pure anisole, anisole + CH3OH, anisole + phenol, o-cresol + CH3OH, and3OH, anisole + phenol, o-cresol + CH3OH, and
p-cresol + CH3OH.
Results of the methylation of phenol 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
amounts of anisole than HMCM22. 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 always formed. Phenol was
produced from anisole dealkylation, but also by anisole disproportionation.
Experiences feeding o-cresol and p-cresol with methanol showed that
zeolites HBEA and
HZSM5, which
exhibit similar
amounts of Brønsted
and Lewis acid sites,
form mainly 2,6- and
2,4-xylenols via
alkylation reactions.
In contrast, zeolite
HMCM22 that
contains
predominantly
Brønsted acid sites,
promotes also
isomerization
reactions, forming mcresol
by consecutive
isomerization of oand
by consecutive
isomerization of oand
zeolites HBEA and
HZSM5, which
exhibit similar
amounts of Brønsted
and Lewis acid sites,
form mainly 2,6- and
2,4-xylenols via
alkylation reactions.
In contrast, zeolite
HMCM22 that
contains
predominantly
Brønsted acid sites,
promotes also
isomerization
reactions, forming mcresol
by consecutive
isomerization of oand
by consecutive
isomerization of oand
Results of the methylation of phenol 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
amounts of anisole than HMCM22. 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 always formed. Phenol was
produced from anisole dealkylation, but also by anisole disproportionation.
Experiences feeding o-cresol and p-cresol with methanol showed that
zeolites HBEA and
HZSM5, which
exhibit similar
amounts of Brønsted
and Lewis acid sites,
form mainly 2,6- and
2,4-xylenols via
alkylation reactions.
In contrast, zeolite
HMCM22 that
contains
predominantly
Brønsted acid sites,
promotes also
isomerization
reactions, forming mcresol
by consecutive
isomerization of oand
by consecutive
isomerization of oand
zeolites HBEA and
HZSM5, which
exhibit similar
amounts of Brønsted
and Lewis acid sites,
form mainly 2,6- and
2,4-xylenols via
alkylation reactions.
In contrast, zeolite
HMCM22 that
contains
predominantly
Brønsted acid sites,
promotes also
isomerization
reactions, forming mcresol
by consecutive
isomerization of oand
by consecutive
isomerization of oand
-cresol + CH3OH.
Results of the methylation of phenol 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
amounts of anisole than HMCM22. 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 always formed. Phenol was
produced from anisole dealkylation, but also by anisole disproportionation.
Experiences feeding o-cresol and p-cresol with methanol showed that
zeolites HBEA and
HZSM5, which
exhibit similar
amounts of Brønsted
and Lewis acid sites,
form mainly 2,6- and
2,4-xylenols via
alkylation reactions.
In contrast, zeolite
HMCM22 that
contains
predominantly
Brønsted acid sites,
promotes also
isomerization
reactions, forming mcresol
by consecutive
isomerization of oand
by consecutive
isomerization of oand
zeolites HBEA and
HZSM5, which
exhibit similar
amounts of Brønsted
and Lewis acid sites,
form mainly 2,6- and
2,4-xylenols via
alkylation reactions.
In contrast, zeolite
HMCM22 that
contains
predominantly
Brønsted acid sites,
promotes also
isomerization
reactions, forming mcresol
by consecutive
isomerization of oand
by consecutive
isomerization of oand
o-cresol and p-cresol with methanol showed that
zeolites HBEA and
HZSM5, which
exhibit similar
amounts of Brønsted
and Lewis acid sites,
form mainly 2,6- and
2,4-xylenols via
alkylation reactions.
In contrast, zeolite
HMCM22 that
contains
predominantly
Brønsted acid sites,
promotes also
isomerization
reactions, forming mcresol
by consecutive
isomerization of oand
by consecutive
isomerization of oand
mcresol
by consecutive
isomerization of oandoand
p-cresol isomers.
Our catalytic results are well explained by considering that the alkylation of
phenol with methanol on acid zeolites occurs according to the complex
reaction network proposed in Figure 1. Cresol formation rates are better
promoted on wide pore zeolites containing strong surface Lewis and
Brønsted acid sites, such as zeolite HBEA. Zeolite HZSM5 forms more
anisole and less xylenols than HBEA because its narrow pore structure
hampers the formation of bulky intermediates involved in both the cresol
alkylation to xylenols and the anisole disproportionation. Zeolite HMCM22
suppresses the formation of anisole, decreases the o-cresol formation rate
and forms p-cresol at high rates, thereby increasing substantially the paraselectivity
and forms p-cresol at high rates, thereby increasing substantially the paraselectivity
Our catalytic results are well explained by considering that the alkylation of
phenol with methanol on acid zeolites occurs according to the complex
reaction network proposed in Figure 1. Cresol formation rates are better
promoted on wide pore zeolites containing strong surface Lewis and
Brønsted acid sites, such as zeolite HBEA. Zeolite HZSM5 forms more
anisole and less xylenols than HBEA because its narrow pore structure
hampers the formation of bulky intermediates involved in both the cresol
alkylation to xylenols and the anisole disproportionation. Zeolite HMCM22
suppresses the formation of anisole, decreases the o-cresol formation rate
and forms p-cresol at high rates, thereby increasing substantially the paraselectivity
and forms p-cresol at high rates, thereby increasing substantially the paraselectivity
-cresol isomers.
Our catalytic results are well explained by considering that the alkylation of
phenol with methanol on acid zeolites occurs according to the complex
reaction network proposed in Figure 1. Cresol formation rates are better
promoted on wide pore zeolites containing strong surface Lewis and
Brønsted acid sites, such as zeolite HBEA. Zeolite HZSM5 forms more
anisole and less xylenols than HBEA because its narrow pore structure
hampers the formation of bulky intermediates involved in both the cresol
alkylation to xylenols and the anisole disproportionation. Zeolite HMCM22
suppresses the formation of anisole, decreases the o-cresol formation rate
and forms p-cresol at high rates, thereby increasing substantially the paraselectivity
and forms p-cresol at high rates, thereby increasing substantially the paraselectivity
o-cresol formation rate
and forms p-cresol at high rates, thereby increasing substantially the paraselectivityp-cresol at high rates, thereby increasing substantially the paraselectivity