Study of Gas Phase m-Cresol Alkylation
1953
selectively synthetized on HMCM22 while on HZSM5, the
selectivity to p-cresol was not improved, even though DMP
formation was hampered on both catalysts by diffusional
constraints.
unselective forming 3-MA (C/O ratio of 1.9) and DMP in a
statistics ratio, as well as trimethylated phenols: TMP and
DMA. However, the selectivity can be tuned by modifying
the nature and principally the strength of the acid sites. In
fact, ZnY (strong Lewis acidity) promote selectively the
C-methylation (C/O alkylation ratio: 8.8) leading to for-
mation of 2,5, 2,3 and 3,4 DMP. In contrast, Al-MCM-41
(weak Lewis acidity) favors the O-alkylation reaching a
selectivity value to the ether (3-MA) higher than 57 %.
Highest selectivity to C-alkylated products (C/O alkylation
ratio: 13.3) was obtained on HMCM22. This catalyst has
strong acid sites mostly Brønsted (B/L = 3). On another
hand, also by shape selectivity the product distribution can
be modified. As a matter of fact, on HMCM22 and HZSM5
the production of TMP and DMA was hampered by dif-
fusional constraint. Additionally on HZSM5, selectivity
greater than 50 % to the smallest isomer of DMP, 2,5
DMP, could be explained by shape selectivity.
Additionally, we have investigated the effect of contact
time on the product distribution on ZnY and Al-MCM-41.
0
i
0
Yields (g Þ and m-cresol conversion (X
) at t = 0
mꢁCresol
are shown as a function of contact time in Fig. 5. Because
of catalyst deactivation observed during the reaction, to
obtain each data point on fresh catalyst, initial yields were
determined by extrapolating to initial time on stream. The
local slopes of the yield curves in Fig. 5 give the rate of
formation of each product at a specific reactant conversion
and contact time. The nonzero initial slopes of DMP and 3-
MA yields evidence that these are primary products,
formed directly by the attack of alkylating agent to m-
cresol while the initial zero slopes observed for TMP, and
DMA suggest that these compounds are secondary pro-
ducts formed by the alkylation of DMP and 3-MA. In fact,
0
All the catalysts employed in this work form coke
(between 2 and 12 %) and undergo significant losses of
activity on stream, except Al-MCM-41. Although methanol
participates in parallel reactions that lead to formation of
light hydrocarbon and aromatics, it was proved that the
simultaneous presence of both reactants is required for
coke formation.
on ZnY, g
curve reaches a maximum, indicating that
3
ꢁMA
-MA, which is formed as primary product, then is con-
3
verted to a secondary product.
In order to get more insight about the effect of the
strength of the superficial acid sites, on promoting the
different pathways involved in the reaction network
(
(
Scheme 1), we have calculated the initial rate of formation
0
Acknowledgments This work was supported by the Universidad
Nacional del Litoral (UNL), Consejo Nacional de Investigaciones
Cient ´ı ficas y T e´ cnicas (CONICET), and Agencia Nacional de Pro-
moci o´ n Cient ´ı fica y Tecnol o´ gica (ANPCyT), Argentina.
X
! 0Þ of 3-MA (O-alkylation) and DMP (C-
mꢁcresol
0
alkylation) from the slopes at W=F
and b. The values of r0
! 0 in Fig. 5a
mꢁcresol
calculated were 0.42 and
while the values of r0 were 0.18 and
3
ꢁMA
-
1
-1
g
-1
0
.37 mmol h
-
DMP
1
1
.9 mmol h
g
on Al-MCM-41 and ZnY, respectively.
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