(544cm) Preparation of a SBA-15/Cordierite Monolith Support for Intensified Catalytic Reactions

Introduction

In 2005, a very elucidating presentation from Prof.
Smalley¹ showed that energy and environment are in the top ten
concerns of the century. The speaker foresaw that vital solutions would emerge
from innovations in several fields, including nanotechnology and science
materials. Nanotechnology studies applied for energy and environmental science
has been increasing substantially, from light harvesting, solar photovoltaics,
solar water splitting, fuel cells, photocatalysis and heterogeneous catalysis². The catalytic system that is most widely applied in
environmental applications is the monolithic reactor (MR), mostly in automotive
industry as three-way catalytic converters³. Such material are based mainly in cordierite
monoliths, which does not present an expressive surface area. Many strategies
have been developed to improve this feature, such as using it as a primary
support for a secondary support anchoring⁴. Several studies were performed in order to
successfully anchor metal oxides, such as alumina, zirconia or ceria, and to
determine the most relevant factors that impacts its preparation⁵. Ordered mesoporous silica (OMS) materials, such as
SBA-15, have drawn attention in the last few years due to its proven and
potential applications, such as air pollutant removal⁶, adsorption of volatile organic compounds⁷ or support for catalysts⁸. Its relevant features are high surface area,
hydrothermal stability, and uniform, tunable morphology⁸. Employing SBA-15 as support provides enhanced active
phase dispersions and possibility of surface functionalization, while
indirectly fine tuning nanoparticle size and controlling diffusion of
gas/liquids inside the channels, which in turn may affect the catalyst activity
and selectivity. The preparation of this support system relies on several
parameters that impacts the coating quality, requiring a systematic approach
for secondary support anchoring. Ideally, support anchoring requires
concomitantly a minimum of preparation steps to achieve the desired mass load
and an acceptable adherence of the coated layer. This study aims to develop a
method for cordierite monolith anchoring with SBA-15 as secondary support. For
coating quality and SBA-15 synthesis assessments, TEM, SEM stability test and
adherence test analyses were performed.

Methods

The pure mesoporous silica SBA-15 was synthesized by
hydrothermal method. Cordierite monolith was granted by Degussa. The SBA-15
preparation assessment was performed by TEM technique. The SBA-15 suspensions
were obtained using different methods, as shown in Table 1. PREP1 and PREP2 suspensions were prepared using
magnetic stirring of the aqueous solution (Aerosil or ethylene glycol,
respectively) with slow addition of SBA-15 solid at 400 rpm for 1h, while PREP2
was stirred at 600 rpm for 3h. In PREP3 and PREP4 the aqueous SBA-15 solutions
(for PREP4 the colloidal silica was added first) was stirred at 400 rpm for 3 h
prior to homogenization at ~17,000 rpm for 15 minutes. Polyvinyl alcohol (PVA)
was added to the stirred solutions at 400 rpm for 1 h at room temperature prior
heating at 363 K for 3 h. The final solution was stirred overnight.

Table 1 -
Suspensions prepared for monolith coating.

The SBA-15 was coated over the piece of monolith using
two different routes. PREP1 was prepared by the dip coating for 1 minute,
followed by air blowing and drying at 393 K for 30 min. The dip was repeated 10
times and the material was characterized. PREP2 was prepared by washcoating,
with air blowing, drying at 348 K for 2h and calcination at 823 K for 4 h, at 2
K/min at each step, until a mass increase about 10% was obtained. PREP3 and
PREP4 suspension quality was assessed through stability analysis.

Results

SBA-15 preparation assessment result is showed in
Figure 1.

Figure 1 -Prepared SBA-15 by transmission electron
microscopy technique with a resolution of (A) 5 μm; (B) 200 nm; and
(C),(D) 100nm.

In
Figure 1 (A), it was possible to
estimate both the mean length of 1.523±0.082 μm and mean width of
0.489±0.054 μm of a given SBA-15 particle. Moreover, it is clear that the
SBA-15 particles agglomerated and formed macrostructures. Figure 1 (B), (C) and (D)
denotes the morphology of the well-defined mesopores of the prepared OMS.

PREP1 coating SEM characterization result is showed in
Figure 2.

Figure 2 - SEM
images of the SBA-15+Aerosil/Cordierite monolith system: (A),(B) Channel view
in different positions; (C),(D) coating material over the monolith surface under different magnifications.

At Figure 1 (A) and (C) it is possible to visualize
the uncoated monolith channels, while Figure 1 (B) and (D) shows the coated materials.
It is possible to visualize in its surface different morphologies: the coating
process has lead to the addition of needle-like structures > 50 µm. This is not desirable, since it may result in poor adhesion quality.
In order to improve such feature, PREP2 preparation proceeded. The washcoating
of the monolith piece and thermal treatments were performed in a total amount
of seven to produce a structured support with a > 10 wt.% SBA-15 load. The
mass increase in each coating step is displayed in Figure 3.

Figure 3 -
SBA-15 support load (%) per dip using the washcoating technique.

This data shows that the overall mass increase per
coating is nearly linear. After the monolithic support preparation, the
material was submitted to an adherence test. The test for the prepared material
has resulted in a weight loss of 66 wt.%, which is very relevant.

PREP3 and PREP4 preparation was performed using
specific reagents and methods. Polyvinyl alcohol (PVA) is known to serve as a
stabilizer agent by steric repulsion⁹, while the colloidal silica added to PREP4 serves as
a binder for SBA-15 particles and the monolith¹⁰. Moreover, the use of stirring using Ultra Turrax for
particle deagglomeration was performed. Figure 4 and Figure 5 displays the stabilization analysis for the PREP3 and
PREP4 suspensions, respectively.

Figure 4 - Stabilization analysis based on the
backscattering (BS%) per cell height (mm) for the PREP3 suspension. The scale
on the right is related to the run time (total time: ~8h)

Figure 5 - Stabilization analysis based on the
backscattering (BS%) per cell height (mm) for the PREP4 suspension. The scale
on the right is related to the run time (total time: ~7h)

The
stabilization analysis of each sample represents dynamically the occurring
phenomena in the suspension. The suspensions displays concomitant
sedimentation/clarification and flocculation phenomena. The flocculation
instability is related to an increase in the size of the particles, affecting its
diameter in the suspension and leading to sedimentation. The differences in
initial BS between samples may be due to the origin of the silica sources that
constitutes the suspensions. The formation of flocs and further settlement due
to gravity effects are explained by means of the Brownian motion of particles
in a suspension and its agglomeration due to Van der Waals attractive forces¹¹. It is known that
steric or electrostatic repulsion builds a barrier in the floc formation,
preventing flocculation. In this way, PVA was used as a steric repulsive agent.
However, the stability analysis has shown that it was not very effective.

Conclusion

The preparation of four different samples for the
preparation of a SBA-15/Cordierite Monolith was performed. It was determined by
PREP1 that aqueous solutions leads to the formation of needle-like SBA-15
agglomerates over the monolith surface. Moreover, PREP2 showed that the mass
increase per dip presents a linear behavior. The preparation of more elaborated
solutions, such as PREP3 and PREP4, with an steric repulsion agent and high
shear stress for particle deagglomeration presented characteristic
instabilities along time. The stability analysis showed the formation of
flocculation, precipitation and clarification phenomena for the suspensions,
even in the presence of PVA. Overall, another strategy for suspension stability must be applied for the
preparation of the desired support.

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