Synthesis and Characterization of Tin, Tin-Aluminum, and Tin-Boron Containing MFI Zeolites

Graphical Abstract:

Zeolites are an
intellectually fascinating and
technologically important class of materials used
in the
chemical conversion and separation of small molecules. Their ability to
achieve these unit operations is a
result of the crystalline microporous structures enabling highly size-selective separations.[1] Coupling
that with the ability to impart acidity to the zeolites through the introduction of trivalent elements such as aluminum and boron into the framework has led to zeolites finding a wide range of applications in the chemical and
petrochemical industry.[2] Over the last 20-30
years beyond aluminum and boron,
it
has been
shown that other elements including titanium[3], tin[4] and germanium[5] can
be introduced into the zeolite
lattice with varying
levels of success. This expansion of the zeolite framework composition space has led
to new uses for zeolites
in
a range of catalytic chemistries.

The synthesis and characterization of tin, tin/aluminum, and tin/boron containing MFI zeolites will be presented. It is shown that tin uptake is high (near 100%) and decreases with
the introduction of a second
heteroatom. Also, the boron uptake was found to be low for tin-boron-MFI samples, consistent with our
prior work on germanium-boron-MFI[6]. XRD and
FE-SEM show that higher tin contents and higher aluminum and boron contents in the synthesis gel lead to smaller and less faceted crystals. Diffuse reflectance UV-Vis spectroscopy confirms the tetrahedral framework coordination of tin within the zeolite
and
no polymeric SnO2 can be detected, given the absence of a strong signal at 280
nm. XPS,
in
consistent
with UV-Vis, indicates the absence of an additional phase in the surface of these particles. In-Situ IR
spectroscopy, using methanol as a probe molecule, shows that the nature of number of acid
sites in
these materials changes in going from pure tin-MFI (very weakly acidic) to tin-aluminum-MFI (mildly acidic).
The
Lewis acid nature of the tin
centers
was confirmed using
IR
of d-acetonitrile treated samples.

Keywords: Tin-MFI, synthesis, probe molecule binding, mixed heteroatom zeolites

Reference:

(1)
 R.M.
Barrer, Hydrothermal Chemistry of Zeolites, Academic Press Inc., London, 1982.


(2)
 G.T. Kokotailo, S.L. Lawton, D.H. Olson,
W.M.
Meier, Structure of Synthetic Zeolite ZSM- 5,
Nature,
272

(1978) 437-438.

(3)
 M. Taramasso, U.S.,
1983.

(4)  A. Corma, L.T. Nemeth, M. Renz, S. Valencia, Sn-zeolite beta as a heterogeneous chemoselective catalyst for Baeyer-Villiger oxidations, Nature, 412 (2001) 423-425.


(5)  Z. Gabelica, J.L. Guth, A Silicogermanate with a Si/Ge Ratio Greater Than or Equal to
2. Ann MFI Zeolite of Novel Composition,
Angewandte Chemie-International Edition in English,
28 (1989) 81-83.

(6)  Vargas,
Nataly Garcia, Scott Stevenson,
and Daniel F. Shantz. "Simultaneous isomorphous incorporation of boron
and germanium in
MFI zeolites." Microporous and
Mesoporous Materials 170 (2013): 131-140.