.
Angewandte
Communications
DOI: 10.1002/anie.201109026
Zeolite Catalysis
X-ray Imaging of Zeolite Particles at the Nanoscale: Influence of
Steaming on the State of Aluminum and the Methanol-To-Olefin
Reaction**
Luis R. Aramburo, Emiel de Smit, Bjørnar Arstad, Matti M. van Schooneveld, Linn Sommer,
Amꢀlie Juhin, Tadahiro Yokosawa, Henny W. Zandbergen, Unni Olsbye, Frank M. F. de Groot,
and Bert M. Weckhuysen*
In view of the limited oil reserves the methanol-to-olefin
(MTO) process is an interesting catalytic route to provide raw
materials for chemical industries. In the last decades, a vast
number of studies have been devoted to increase our under-
standing of this important catalytic reaction leading to
a consensus concerning the mechanism.[1–4] Accordingly,
MTO is thought to proceed through the so-called “hydro-
carbon pool” (HCP) mechanism,[5,6] in which methanol is
added to an organic scaffold present within the zeolite
framework. This is followed by elimination of olefinic species
in a closed catalytic cycle. Microporous silicoaluminophos-
phates and aluminosilicates, such as SAPO-34 and ZSM-5, are
often used as MTO catalysts because of their unique acidic
and structural properties. In the case of ZSM-5 the formation
of ethene and propene is governed by two different catalytic
routes,[7,8] allowing in principle to control the ethene/propene
ratio. Unfortunately, throughout the MTO reaction undesired
carbon deposits are formed in the narrow micropore system
of ZSM-5, leading to severely restricted diffusion and there-
fore limited catalytic activity.[9] To overcome these limitations
efforts have been made to improve the pore accessibility
during synthesis,[10–12] and/or in post-synthetic steps,[13,14]
resulting in significant improvements in the diffusion proper-
ties of ZSM-5.
In this work, two commercial ZSM-5 zeolites with
dimensions of approximately 200–800 nm have been studied
by scanning transmission X-ray microscopy (STXM). The first
sample, denoted as ZSM-5-C, was calcined for 6 h at 5508C,
whereas the second sample, further labeled as ZSM-5-S, was
steamed for 3 h at 7008C. Details on the preparation and
characteristics of ZSM-5-C and ZSM-5-S can be found in the
Supporting Information (Figures S1–S13, Tables S1–S6). We
will show how STXM, in combination with bulk character-
ization techniques, allows investigating the physicochemical
properties of ZSM-5 zeolites in a novel way at the nano-
scale.[15,16] More specifically, detailed chemical maps, with
a spatial resolution of 70 nm, have been obtained of
aluminum, oxygen, and carbon, even under realistic reaction
conditions.[17–19] In this manner, the influence of steaming on
the state of aluminum, that is, the coordination and spatial
distribution, as well as on the MTO performance, has been
unraveled.
In a first set of experiments, STXM was applied to
characterize the X-ray absorption of ZSM-5-C and ZSM-5-S
at the O, Si, and Al K-edge. The obtained spectra are
presented in Figure 1, showing important changes in the Al
environment after steaming. The Al K-edge X-ray absorption
(XA) spectrum of ZSM-5-C showed a sharp white line located
at 1565.5 eV and a broad peak present at 1580 eV related to
medium range order.[20] In contrast, the Al K-edge XA
spectrum of ZSM-5-S disclosed the appearance of new
features at 1567 and 1570 eV characteristic of higher Al
coordination states.[21] As the intensity and shape of these
features are strongly influenced by the local electronic
structure around the absorber,[20–22] the comparison with
reference compounds is helpful.[23,24] To this end, the exper-
imental Al K-edge XA spectra of ZSM-5-C and ZSM-5-S
were fitted with a linear combination of the XA spectra of
three reference compounds, namely albite (four-fold Al
mineral), berlinite (four-fold Al mineral), and andalusite
(mineral with five- and six-fold Al). The obtained results,
along with details concerning the fitting procedure, are
presented in the Supporting Information. Overall, the spec-
tral fitting indicated that ZSM-5-C contained mainly four-fold
Al, next to minor amounts of five- and six-fold Al. In contrast
[*] L. R. Aramburo, Dr. E. de Smit, M. M. van Schooneveld, Dr. A. Juhin,
Prof. Dr. F. M. F. de Groot, Prof. Dr. B. M. Weckhuysen
Inorganic Chemistry and Catalysis Group
Debye Institute for Nanomaterials Science
Utrecht University, Universiteitslaan 99
3584 CG Utrecht (The Netherlands)
E-mail: b.m.weckhuysen@uu.nl
Dr. T. Yokosawa, Prof. Dr. H. W. Zandbergen
Kavli Institute of NanoScience
National Centre for High Resolution Electron Microscopy
Delft University of Technology
PO Box 5046, 2600 GA Delft (The Netherlands)
Dr. L. Sommer, Prof. Dr. U. Olsbye
Centre for Materials Science and Nanotechnology
Department of Chemistry, University of Oslo
0315 Oslo (Norway)
Dr. B. Arstad
Department of Hydrocarbon Process Chemistry
SINTEF Materials & Chemistry, 0314 Oslo (Norway)
[**] We thank the NRSC-C (B.M.W.), the NWO-CW Top (B.M.W.), and
the NWO-CW VICI (F.M.F.d.G.) for financial support and D. Cabaret
(IMPMC, Universitꢀ Pierre et Marie Curie) for providing the
aluminum references. T. Tyliszczak (Lawrence Berkeley National
Laboratory), J. Wang (Canadian Light Source), S. Svelle (University
of Oslo), and A. M. J. van der Eerden (Utrecht University) are kindly
thanked for their contributions.
Supporting information for this article is available on the WWW
3616
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 3616 –3619