Synthesis of Zeolitic Enwrapped Catalysts By Chemical Vapor Deposition

Synthesis of
Zeolitic Enwrapped Catalysts by Chemical Vapor Deposition

Yijia Sun*, Shoucheng
Du, Chunxiang Zhu, George M. Bollas

* Department of
Chemical & Biomolecular Engineering, University of Connecticut, Storrs, 191
Auditorium Road, Unit 3222, Storrs, CT, 06269-3222, USA.
Email:yijia.sun@uconn.edu

Abstract

Zeolites
have been widely used as molecular separation media [1] and in membrane reactors
[2] due to their unique pore channel structure and advantages
in spatial selectivity. They are also used in catalytic conversions in
petro-chemical industry [3]. Recently, novel zeolite enwrapped metal catalysts
attracted significant attention [4,5]. Zeolite enwrapped catalysts have
a core/shell structure, where the zeolite membrane (shell) and enwrapped
catalyst (core) provide independent catalytic functionalities. In these
catalytic structures, the reactants need to pass through the zeolite membrane
first and react on the core catalyst to form products.  In order to exit the catalyst,
the intermediate products must diffuse out through the pore channels, where they
are catalyzed and produce final products. Except for catalytic activity, the core
usually has high adsorption capacity and the shell possesses separation
properties. In the preparation of zeolite enwrapped catalysts, the zeolite
membrane is coated directly onto the core catalyst surface. The common method
of preparing these zeolite membranes is the so-called secondary growth method,
where a seeding layer is deposited on the support and then undergoes
crystallization in the presence of a structure directing agent (SDA) and silica
source. Hydrothermal synthesis is usually used in membrane preparation. The
schematic illustration is shown in Figure 1.

Figure 1. Schematic
illustration of zeolite membrane preparation by hydrothermal synthesis.

In
liquid-phase hydrothermal synthesis, zeolite composite membranes are prepared
by secondary growth in a Teflon-lined autoclave in the presence of template
solution [6]. However, hydrothermal synthesis only consumes a small amount of
template substrates in the synthesis solution, thus generating significant
amount of waste. The high alkalinity of the substrates causes corrosion on the
support and destroys the active sites. Moreover, it is hard to control the
reaction rate and crystalline thickness as hydrothermal synthesis is operated
in a closed system. In this work, we explore a novel method to prepare zeolite
capsuled catalysts: chemical vapor deposition. Chemical vapor deposition has
the capacity of producing dense and pure materials. Besides, it has the ability
to control the crystal structure and deposition rate. Moreover, the whole
process presents merits in preventing corrosion and reducing the seed loss. To
implement this strategy, briefly, two seeding techniques, hydrothermal
synthesis (HS) and sol-gel formation (GF), were applied to prepare different
types of zeolite seeds. Subsequently, γ-Al₂O₃
pellets coated with zeolite precursors from HS were subjected to chemical vapor
deposition of gaseous phase SDA, with which the zeolite seeds could react and
be transformed into zeolite membranes. γ-Al₂O₃ pellets
were also coated with sol-gel, after which the coated pellets were exposed to a
vapor phase silica source: tetraethyl orthosilicate (TEOS). This strategy incorporates
the benefits of chemical vapor deposition as well as avoids the disadvantages of
hydrothermal synthesis. Schematic illustrations of the two methods are
presented in Table 1. Detailed preparation procedures will be presented but are
not discussed here.

Table 1. Schematic
illustrations of the synthesis methods (HS-CVD and GF-CVD) used in this study.

Figure 2 shows the XRD patterns of the parent alumina
and zeolite coated materials synthesized by HS-CVD and GF-CVD methods. Due to the
thin and small quantity of the zeolitic membranes formed, the peaks that
represent alumina are still dominant in the XRD patterns of the coated
materials. Other than the peaks from alumina, the peaks of the zeolite from the
HS-CVD method are clear in
the ranges 2θ= 8-9 and 20-25°.
The
characteristic peaks of the zeolite in the GF-CVD synthesized material have
very low intensities. The formation of zeolite crystals on the alumina core is
further verified using Scanning Electron Microscopy (SEM). In Figure 3, unevenly
distributed zeolite seeds are observed after the seeding step in the GF-CVD
method. After CVD of TEOS on seeded supports, clear crystal coverage is observed.
The zeolite crystals are growing individually with random orientation and few
occurrences of intergrowth. In order for the membrane to possess high
separation performance, it is preferred that the crystals are oriented.
Membrane separation performance could be improved by varying the alkalinity and
SDA concentration in the precursor [7].

Figure
2.  XRD
patterns of (a) parent alumina, (b) material after HS-CVD, (c) material after GF-CVD.
Peak identification: (+) alumina, (*) zeolite crystals after HS-CVD, (o)
zeolite crystals after GF-CVD.

Figure 3. SEM images of materials
after seeding (left) and secondary growth (right) using the GF-CVD method.

The pore size distributions of the parent alumina
and the zeolite coated materials are shown in Figure 4a. A decrease in pore
diameters is observed in the coated materials synthesized by both methods. In the
HS-CVD material, a significant increase in pore volume is observed, due to the
increase in structured channels in the zeolite crystals. In the GF-CVD
material, a decrease in the mesopore volume in the pellets is observed, as shown
in Figure 4a and 4b. According to the SEM images of Figure 3, alumina pellets are
covered by a layer of zeolite crystals, possibly blocking the original pore
channels of the parent alumina. This can be potentially addressed by using a different
seeding technique, such as wetting the alumina support with a liquid agent
before rubbing the zeolite dry crystals [8].

Figure
4.
(a) Pore size distributions and (b) nitrogen adsorption/desorption
isotherm curves of parent alumina, material after HS-CVD and material after
GF-CVD.

In
summary, two methods (HS-CVD and GF-CVD) were investigated for synthesizing
zeolite enwrapped core/shell catalysts. The characterization results showed the
success and feasibility of using CVD in zeolite membrane synthesis. Future work
includes optimization of the microstructure to reduce randomness in the crystal
orientation and tuning of CVD parameters to control the membrane thickness.
Separation and pervaporation tests are required to test membrane performances.

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