Angewandte
Chemie
The observed difference between the various Beta
zeolites is consistent with previous studies, which indicated
that catalytic activity is dependent on the polarizability of the
metal atom in the active site and the Brønsted basicity of the
associated oxygen atom.[13] The d-block transition metals Hf
and Zr possess similar electronic structures when inserted in
the zeolite framework and can be expected to show similar
catalytic activity, while the p-block element Sn exhibits
different bonding characteristics and charge distribution
through the site because of its antibonding s* LUMO.[13,15]
We next extended our study to include several BA derivatives
(Table 1, entries 2–4). The observed trend in catalytic activity
of the metal centers is similar to the one seen for reactions
with BA: Hf ꢁ Zr> Sn. Selectivity toward the desired single
cross-aldol condensation product remained over 90% when
using Hf- and Zr-Beta. Sn-Beta, in contrast, only reached 86
and 78% selectivity when starting from 4-nitrobenzaldehyde
and 4-chlorobenzaldehyde, respectively.
A comparison
between BA derivatives shows a decrease in activity with
the addition of both electron-withdrawing and electron-
donating groups, a trend not solely characteristic of strong
base or acid catalysts.[16] This mixed trend is indicative of
a cooperative effect between weak acid and base sites in the
zeolite. Additionally, steric limitations in the pores of zeolite
Beta may impede transition-state formation for benzaldehyde
analogues with larger substituents at the 4-position. The aldol
reaction between 5-(hydroxymethyl)furfural (HMF) and
acetone was also examined (Table 1, entry 5), as this is
a potential pathway to produce liquid alkanes from bio-
mass.[2a] Reactions with both Hf- and Zr-Beta generated the
single cross-aldol product with selectivities of 99 and 87%,
respectively, at similar conversions. Surprisingly, reactions
catalyzed by Sn-Beta only generated undesired polymeri-
zation products.
The stability of Hf-Beta for the aldol condensation of BA
and acetone was probed by reusing the catalyst with fresh
solution for five consecutive batch reactions (see Figure S8).
Conversion decreased by 18% between the first and second
runs, and it remained constant for subsequent runs. In all
cases, selectivity for 2 remained above 98%. Calcination
between the fourth and fifth runs did not alter activity.
Thermogravimetric analysis showed a 2% weight loss, which
we ascribe to the presence of organic components within the
zeolite pores after reaction (see Figure S8). These results are
consistent with the observed deactivation behavior of Hf-
Beta during catalytic transfer hydrogenation reactions.[17] Hot
filtration tests confirmed the heterogeneous nature of the
catalytic system (see Figure S9).
Figure 2. Effects of a) acetic acid and b) water on the aldol condensa-
tion of BA and acetone. Reaction conditions: same as those used in
Table 1. MgO loaded at 2 wt% for (a), and acetone used as a solvent
for (b). Acetic acid concentrations of 0.06 and 0.11 wt% correspond to
molar ratios of acetic acid/BA/Hf=5:50:1 and 10:50:1, respectively.
Water concentrations of 1, 5, and 10 wt% correspond to molar ratios
of water/BA/Hf=300:50:1, 1500:50:1, and 3000:50:1, respectively. The
entry marked 1* in (b) had both water and acetic acid impurities with
molar ratios of water/acetic acid/BA/Hf=300:5:50:1. Legend: Yields
^
of 1 (&), 2 (&), and 3 (&). =conversion.
achieved 42% selectivity for 2 under acid-free conditions,
primarily because of the production of 3. Figure 2b shows that
Hf-Beta is capable of catalyzing the aldol condensation with
as much as 10 wt% water, although conversions decreased by
50% when compared to the dry reaction. Note that acetone
was used as the solvent for the water tests because of the
immiscibility of water with toluene. In pure acetone, reaction
rates were lower compared to those obtained in toluene. We
hypothesize that high acetone concentrations hinder BA
adsorption because of competitive interaction with the Lewis
acidic sites. Indeed, self-aldol reaction of acetone to form
mesityl oxide and diacetone alcohol was observed in small
quantities (< 2%). Reactions in acetone also displayed lower
selectivity for 2 because of reduced dehydration of the aldol
addition product 1. As expected, the selectivity for 1 increased
with higher water concentrations, as water is a by-product of
Organic acid impurities can deactivate strong base
catalysts, while water may react undesirably with homoge-
neous Lewis acids. Thus, Hf-Beta was tested in the presence of
these components for the aldol condensation of BA and
acetone (Figure 2). Unlike traditional base catalysts, Hf-Beta
was not quenched by the presence of acetic acid. Although
conversion decreased by 35% upon adding 0.11 wt% acetic
acid (i.e., a molar ratio of 1:5 acetic acid/BA), the zeolite
generated the desired product with a selectivity of 93%.
Conversely, the solid base catalyst MgO showed almost no
activity in the presence of acetic acid. Furthermore, MgO only
Angew. Chem. Int. Ed. 2015, 54, 9835 –9838
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9837