Chemistry Reference
In-Depth Information
7.3 ALD for Catalysis: Case Studies
7.3.1 Case 1: Introducing Acid Sites in Ordered Mesoporous
Materials
Zeolites are microporous crystalline oxide materials that are widely used as
industrial catalysts, in particular in the oil refining and petrochemical in-
dustry. The isomorphic substitution of Si
41
with a trivalent element such as
Al
31
in the tetrahedral oxide framework of zeolites introduces negative
framework charges that are compensated with protons (or exchangeable
cations), thus giving these materials high Brønsted acidity (and ion exchange
capacity).
97
The zeolite micropores offer excellent shape selectivity prop-
erties. However, in specific applications such as hydrocracking, the narrow
pores cause diffusional limitations due to the similarities in size of the
zeolite micropores and the hydrocarbons involved in the reaction. Therefore,
acidic materials with pores in the mesoporous regime are of great interest
for cracking molecules that are too bulky to fit within the narrow
zeolite pores.
A promising class of materials in this respect are the ordered mesoporous
silica materials characterized by large surface areas and regular pore ar-
rangements. Their use in acid catalysis requires the generation of Brønsted
sites, often attempted by the incorporation of Al in the silica pore walls. The
introduction of Al directly during the synthesis of ordered mesoporous silicas
has proven to be dicult.
98,99
The acid strength of the Al sites is typically very
low and the order in the material becomes less well defined than for the pure-
silica form. Another problem is the relatively low thermal stability at the
elevated temperatures used in catalytic processes. Therefore, several methods
have been proposed to introduce Al atoms in a post-synthesis operation,
99-105
e.g., by treatment of the silicate framework with Al(NO
3
)
3
,
102,103
anhydrous
AlCl
3
,
104
or aluminium chlorohydrate
101
solutions.
This case study presents an alternative ALD-based method for post-
synthesis alumination of mesoporous silicate materials.
11,96
Incorporation
of Al species in Zeotile-4, a silica material containing micropores as well as a
3D ordered network of mesopores,
40,41
was achieved in an elegant way by
exposing the powder to 10 cycles of the TMA/H
2
O ALD process.
The amount and coordination of the Al species added by the ALD process
were studied using
27
Al magic angle spinning nuclear magnetic resonance
(MAS NMR) spectroscopy. The as-synthesized Zeotile-4 material is a pure
silica and did not show any signal in
27
Al MAS NMR. The spectrummeasured
after the ALD treatment is shown in Figure 7.15 and presents three signals
with intensity maxima around 54, 30 and 0 ppm. The largest signal around
0 pmm is due to octahedral Al species located on the surface of the pore
walls. The signals around 54 and 30 ppm are ascribed to tetrahedral Al
atoms incorporated in the Zeotile-4 pore walls and distorted tetrahedral or
pentacoordinated Al atoms, respectively.
106
The total Al content estimated
from the signal intensities was found to be 1.815 mol kg
1
.
d
n
9
r
4
n
g
|
7
.
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