22
L. Martins et al. / Journal of Catalysis 258 (2008) 14–24
Table 4
◦
Me1-Y and Me1-X catalysts used
50 mg of catalyst)
4
times in Knoevenagel condensation (60 C,
Uses
Me1-Y (IED 71%)
Me1-X (IED 71%)
Benzaldehyde
conversion (%)
Selectivity
(%)
Benzaldehyde
conversion (%)
Selectivity
(%)
Scheme 2. Reactions taking place during propylene oxide methanolysis.
1
2
3
4
38.4
39.8
39.0
36.0
99.2
100
100
83.1
83.4
82.5
84.6
95.8
98.1
98.8
98.8
100
pronouncedly increase reactants consumption, and a possible ex-
planation could be the low basic strength. Concerning the selec-
tivity among the two CH3O(PrO)H isomers (Scheme 1c), without
catalyst 1-methoxy-2-propanol (1M2P) and 2-methoxy-1-propanol
(2M1P) molar ratio was 1.4, i.e. almost equivalent amounts of
the 1M2P and 2M1P are formed. Interestingly, selectivity toward
1M2P or 2M1P showed to be strongly dependent on catalyst (see
Fig. 9B). Then, these results can be interpreted in terms of the
relative strength of the conjugated acid–base ion pairs that are
present in zeolites: the more active catalysts are also the more se-
lective to the 1M2P isomer. Over sample Me1-X, the isomer 1M2P
was selectively produced (molar ratio = 11.0), as a consequence of
4.4.1. Catalysts recycling in Knoevenagel condensation
Catalyst recycling experiments are important in determining
the long term viability of the catalysts. These were performed
+
in Knoevenagel condensation reaction, in a series of Me1 -Y and
-X catalyst recycling experiments. After each cycle, catalysts were
thoroughly washed with acetone to remove adsorbed products and
dried at room temperature. The results of catalyst recycling ex-
periments, depicted in Table 4, demonstrate the reproducibility
of the catalytic results: upon repeated use, almost constant con-
versions of the benzaldehyde were observed (Me1-Y was close
to 39% and Me1-X was close to 83%). Selectivity to condensa-
tion product is also practically unchanged during recycles. So,
alkylammonium-FAU zeolites seem to be an appropriate catalyst
for condensation reactions, because they show reasonable activi-
ties, very high selectivity and they are stable. Generally, the ba-
sic strength and catalytic activity of alkali-exchanged zeolites can
be greatly increased by occlusion of alkali metal oxide clusters
via impregnation and decomposition of alkali metal compounds
[36]. However, although these basic catalysts show great poten-
tial in many reactions, their major disadvantage is the reduced
stability of the alkali metal oxides to impurities, mostly due to
CO2 poisoning [32]. On the other hand, reactions, which demand
+
cation Me1 low acidity, which does not contribute significantly
to 2M1P isomer formation. Sample Na-Y showed a 1M2P/2M1P
ratio of 3.4 and sample Na-X of 7.6. Despite both samples com-
prise only sodium as cation in ion exchange centers, sample Na-X
showed higher selectivity towards 1M2P because it encloses higher
oxygen basicity, and therefore higher proton-abstracting capacity.
As a general result, the catalytic activity (PrO conversion) and iso-
mers selectivity cannot be correlated only with the oxygen anion
basicity, but also cation acidity should be considered. Controlling
selective production of 1M2P and 2M1P is a very important step
in obtaining high-quality glycol ethers. Glycol ethers are versa-
tile molecules and extremely important from an industrial point of
view. Catalysts Mei-FAU showed excellent results, in providing dif-
ferent acid–basic sites, for obtaining the desired selectivity towards
1M2P and 2M1P isomers. In order to confirm this trend, an acidic
sample comprising zeolite Y (denoted as H-Y, with 74% IED) was
prepared by thermal decomposition of a NH4-Y zeolite and tested
in propylene oxide alcoholysis with methanol. The H-Y zeolite was
the most active, presenting a conversion of 98.6% of propylene ox-
ide, but the less selective to the 1M2P isomer: a 1M2P/2M1P ratio
of 0.9 was observed (Fig. 9B).
◦
temperatures higher than 190 C will probably limit the applica-
tion of alkylammonium exchanged zeolites, as TGA results showed
+
that CH3NH3 cations in Me1-X zeolite start to decompose close to
◦
190 C.
4.5. Propylene oxide alcoholysis with methanol
Commonly, propylene oxide (PrO) methanolysis can be de-
scribed by equations depicted in Scheme 2. The most preferable
conditions would be obtained for the highest content of mono-
propylene glycol methyl ethers (MPGME, or CH3O(PrO)H, first re-
action in Scheme 1c), which is a mixture of isomers 1-methoxy-
2-propanol (1M2P) and 2-methoxy-1-propanol (2M1P). In the case
of this work the GC analysis gave that the oligomerization degree,
n (Scheme 2) was always ꢀ2, probably limited by the zeolite’s
pore and cavity dimensions, which selectively hinder the forma-
tion of more voluminous compounds. Some samples were run for
60 min in GC analysis (the last peak—25 min of retention time—
was assigned to CH3O(PrO)2H) to confirm the absence of higher
degree of oligomerization (n > 2). However, the sum of the selec-
tivity to all products did not sum 100%, suggesting that part of
the propylene oxide is retained inside the zeolite cavities, probably
as higher oligomers. Further reactions with oligomerization degree
n > 2 would give a very complex mixture of isomers having differ-
ent lengths of oligooxypropylene chain.
Generally, during propylene oxide methanolysis, dipropylene
glycol methyl ether mixture of isomers (DPGME, or CH3O(PrO)2H
in Scheme 2), is also formed and its content increases with ad-
vancement of the reaction. However, on zeolites Mei-FAU good
selectivity towards CH3O(PrO)H was obtained and the formation
of bulkier products was suppressed (Figs. 9C and 9D). Actually,
the aforementioned micropore volume results (Table 1) suggested
pore narrowing caused by ion exchange with more voluminous
cations (Me1 and Me4 ) and allow explaining the high selec-
tivity towards monopropylene glycol methyl ether isomers (1M2P
and 2M1P). Particularly on zeolites Me1-FAU, whose ion exchange
degree was of 71%, high selectivity towards the propylene oxide
monomer (CH3O(PrO)H) was obtained.
+
+
Propylene oxide ring is opened as a consequence of the attach-
−
ment of a methoxy anion (CH3O ), which is formed on the surface
of the zeolites. To verify if leaching of the organic cation occurs
−
+
and if the anion is formed in the reaction system as Mei CH3O
,
◦
Me1-Y and Me1-X catalysts were suspended in methanol at 140 C
for 5 h. Then the resulting methanol was analyzed and no basicity
was found in the liquid phase (5 mL from the methanol was di-
luted in 5 mL from deionized water, and 3 drops of the phenolph-
thalein indicator was added to this test solution and no change in
color was observed). This indicates that Mei CH3O is not present
or the concentration of methoxy anion is not high enough to cause
the color of the indicator to change. Me1-Y and Me1-X catalysts
When compared to condensation reactions, the addition reac-
tion of propylene oxide (PrO) with methanol had similar behavior
on Mei-FAU zeolites (see Fig. 9A). For instance, Me1-FAU was found
to be also more active than Cs-FAU for the conversion of propy-
lene oxide. Without catalyst, this reaction gave 34.7% propylene
oxide conversion, which is roughly close to the result obtained
with Na-Y sample (39.4%), indicating that Na-FAU zeolite did not
+
−