(530f) The Effect of Water on the Reusability of Aminated Mesoporous Silica Catalysts for Aldol Condensations

Aldol condensations are important reactions which are
used in a wide range of processes to produce new carbon-carbon bonds, for
example in the pharmaceutical industry for the production of chalcones and in
the fine chemical industry for the production of 2-ethylhexanol for PVC
plasticizers [1]. A potential
novel application in the bio-based industry is the production of liquid fuels
from lignocellulosic biomass by upgrading smaller furanic molecules to heavier
hydrocarbons [2]. Currently,
homogeneous bases such as KOH or NaOH are typically used as aldol condensation
catalysts. Despite their excellent activity, these catalysts are dangerous to
handle, pose an environmental risk, and are difficult to separate from the final
product stream [1]. Hence,
innovative heterogeneous catalysts are pursued to establish a more sustainable
alternative.

Due to their large pore sizes, large surface areas and
the easy incorporation of different types of active sites, mesoporous silicas
are promising supports for many types of catalysts. By grafting amine groups on
a fraction of the weakly acidic surface silanols, an acid-base aldol
condensation catalyst can be obtained. Previously, it has been demonstrated that
the residual silanols enhance the catalytic activity of the amine active site
by promoting the attack of its nitrogen lone electron pair on the carbonyl
group of the reactants [3]. Previous
research on acid-base mesoporous silica catalysts has shown that the highest
catalytic activity can be achieved when a methyl substituted secondary amine is
attached with a propyl linker to the support, and is fully promoted by surface
silanols [4]. When the
amines are randomly grafted on mesoporous silica, full promotion is achieved
when at least 1.7 silanols are available per amine [5].

Although catalyst lifetime is an important factor for
the industrial application of a material, no systematic investigation of the
deactivation behavior of these cooperative acid-base catalysts has been
performed, yet. In this work, a detailed investigation is performed on the
factors which induce catalyst deactivation. Using this information, complete
catalyst reusability has been achieved.

First, a heterogeneous acid-base cooperative catalyst
has been synthesized by grafting of N‑methylaminopropyltrimethoxysilane
(MAPTMS) on a mesoporous silica gel. The amine density is kept low in order to
ensure full promotion by an excess of weakly acidic surface silanol groups. An
unpromoted amine catalyst is obtained by, subsequently, end-capping the
promoting silanol groups using 1,1,1,3,3,3-hexamethyldisilazane (HMDS). Both
materials are characterized by means of nitrogen adsorption-desorption
measurements and elemental (CHN) analysis. These catalysts were then employed
in the aldol condensation reaction of 4-nitrobenzaldehyde with acetone at 55 °C
in a batch-type reactor, using DMSO as a solvent and methyl-4-nitrobenzoate as
internal standard. The results of these experiments show that the reaction rate
remains constant throughout an experiment, i.e., the slope of the conversion
versus batch time graph remains constant. However, subsequent reuse of the
spent catalyst yields a lower, albeit also constant, reaction rate. Hence, it
seems that catalyst deactivation has occurred upon unloading the catalyst from
the reactor. The ratio of the turnover frequency in the second run to that of
the first run is used as a measure for the catalyst reusability. For the
cooperative acid-base catalyst this ratio amounts to 38 ± 10%, while with the unpromoted amine catalyst a
reusability of 77 ± 10% is achieved.

To investigate the nature
of the deactivation, the spent catalysts have been characterized via nitrogen
adsorption-desorption and CHN analysis. No evidence of pore-blocking was observed.
Interestingly, after reaction, the amount of nitrogen in the samples increased
with 60% for the cooperative catalyst and with 10% for the unpromoted amine
catalyst. This indicates the presence of reaction intermediates on the surface.
Raman spectroscopy confirms the presence of aromatic rings, conjugated with
double bonds and nitro groups, covalently attached to the active site.

Figure 1: catalytic reaction cycle of the aldol condensation of
acetone with 4-nitrobenzaldehyde, catalyzed by a secondary cooperative aminated
silica catalyst [3-5]

Possible intermediate
species that comply with the organic groups identified using Raman spectroscopy
are displayed in Figure 1 as species (VII), (V) and (VI).
Species (VII) is the iminium ion of a 4-nitrobenzaldehyde molecule. However,
this undesired side-reaction is suppressed by using a large excess of acetone.
The other potentially remaining species are the product iminium ion (V) and the
product carbinolamine (VI). This iminium ion can undergo hydration to form the
carbinolamine and subsequently desorb from the catalytic site. However, when
drying the catalyst after a catalytic run, iminium ions that are still present
on the surface could dehydrate and form a stable conjugated compound that acts
as a site-blocking species, as displayed in Figure 2.

Figure 2: the iminium intermediate species (a) from the catalytic
cycle is in equilibrium with its enamine form (b) and can further dehydrate
into a stable conjugated species (c) which acts as a site blocker to the amine
active site

Adding extra water to the reactor could
increase the water-assisted desorption step of the iminium intermediate, as
displayed in Figure 1, and
thereby reduce its surface coverage upon unloading the catalyst from the
reactor. In this work, 0.5 wt% to 3 wt% water is co-fed to the batch reactor. The
first and second run turnover frequencies of both the acid-base catalyst, as
well as the unpromoted amine catalyst, as a function of the water amount in the
reactor, are displayed in Figure 3. When
adding at least 0.5 wt% water, the unpromoted amine catalysts are completely
reusable, exhibiting a turnover frequency ratio of 92 ± 10%. The maximum
reusability achieved with the cooperative acid-base catalysts amounted to 72 ±
10%. This indicates that the promotional ability of the silanol groups
irreversibly decreases due to the presence of water, while the activity of the
amine sites can be fully recovered.

Figure 3: turnover frequency (s^-1) for the aldol
condensation of 4-nitrobenzaldehyde with acetone as a function of water in the
reactor. Aminated silicas promoted by silanols (red) and unpromoted (blue),
empty bars are a first run, full bars are a second run. Error bars indicate a
95% confidence interval. Karl-Fischer titrations have been performed to correct
for water in the organic solvents.

Additionally, Figure 3 shows
that a small amount of water has a beneficial effect on the reaction rate. In
contrast, a high amount of water has a negative effect on the reaction rate. This
is directly related to the effect of the water concentration in the catalytic
cycle, as shown in Figure 1.
Firstly, the enamine (species IV) can react with water leading to the
desorption of acetone, which decreases the overall reaction rate. Secondly, by
adding water, the rate of the water-assisted desorption step of the iminium ion
(V) increases, thereby freeing up catalytic sites and increasing the overall
reaction rate.

In this work it has, thus, been shown that the catalyst
reusability can be drastically increased with the addition of water, causing a
reduction in surface coverage of iminium species that could otherwise dehydrate
to a stable site blocking species when the catalyst is dried. For the
unpromoted amine catalyst, 0.5 wt% additional water already causes the catalyst
to be fully reusable. However the cooperative acid-base catalysts cannot be
made fully reusable due to a loss of the promotional ability of the silanol
groups. The applicability of aminated mesoporous silica catalysts for continuous
aldol condensation processes has, hence, been confirmed in this work by proving
that a stable catalyst regime can be obtained by co-feeding a small amount of
water. Further testing of these catalysts in a continuous-flow reactor will
yield more information about the deactivation behavior of the catalyst with
time-on-stream.

[1]          G. Kelly, F. King, and M. Kett, "Waste elimination in condensation
reactions of industrial importance," Green Chemistry, vol. 4, no.
4, pp. 392-399, 2002.

[2]          Q.
Deng et al., "Efficient synthesis of high-density aviation biofuel
via solvent-free aldol condensation of cyclic ketones and furanic
aldehydes," Fuel Processing Technology, vol. 148, pp. 361-366,
2016.

[3]          N.
A. Brunelli, K. Venkatasubbaiah, and C. W. Jones, "Cooperative catalysis
with acid–base bifunctional mesoporous silica: impact of grafting and
co-condensation synthesis methods on material structure and catalytic
properties," Chemistry of Materials, vol. 24, no. 13, pp.
2433-2442, 2012.

[4]          J.
Lauwaert, E. De Canck, D. Esquivel, P. Van Der Voort, J. W. Thybaut, and G. B.
Marin, "Effects of amine structure and base strength on acid–base
cooperative aldol condensation," Catalysis Today, vol. 246, pp.
35-45, 2015.

[5]          J.
Lauwaert, E. De Canck, D. Esquivel, J. W. Thybaut, P. Van Der Voort, and G. B.
Marin, "Silanol‐Assisted Aldol Condensation on Aminated Silica:
Understanding the Arrangement of Functional Groups," ChemCatChem, vol.
6, no. 1, pp. 255-264, 2014.