Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
10.1002/cctc.201700768
ChemCatChem
FULL PAPER
Comparative Study of Diverse Cu-Zeolites for Conversion of
Methane-to-Methanol
Min Bum Park,[a, b] Sang Hyun Ahn,[c] Marco Ranocchiari,[b] and Jeroen A. van Bokhoven*[a, b]
Abstract: The characterization and reactive properties of Cu-
zeolites with twelve framework topologies (MOR, EON, MAZ, MEI,
BPH, FAU, LTL, MFI, HEU, FER, SZR, and CHA) are compared in
the stepwise partial oxidation of methane-to-methanol. Cu2+ ion-
exchanged zeolite omega, a MAZ-type material, reveals the highest
yield (86 μmol g-cat.-1) among these materials after high-
temperature activation and liquid methanol extraction. The high yield
representative commercial medium- and large-pore zeolites,
respectively, stabilize binuclear[3] and trinuclear[4] oxide
compounds of iron and/or copper, which are structurally
analogous to those found in methane monooxygenases. Over
the past decade, several research groups have focused on
these reaction systems to evaluate the nature of active core
species by means of diverse spectroscopic characterization
is ascribed to the relatively high density of copper-oxo active species, tools as well as computational analyses.[3-20] Recently, Grundner
which form in its three-dimensional 8-membered (MB) ring channels.
In-situ UV-Vis shows that diverse copper species form in different
zeolites after high-temperature activation, suggesting that there are
no universally active species. Nonetheless, there are some dominant
factors required for achieving high methanol yields: i) highly
dispersed copper-oxo species; ii) large amount of exchanged copper
in small-pore zeolites; iii) moderately high temperature of activation;
iv) use of proton form zeolite precursors. Cu-omega and Cu-
mordenite, with the proton form of mordenite as the precursor, yield
methanol after activation in oxygen and reaction with methane at
only 200 °C, i.e., under isothermal conditions.
et al. reported that single-site trinuclear copper-oxo species can
be stabilized in mordenite and that Cu-mordenite showed the
highest total yield of methane oxidation products.[4a] In general,
when comparing iron- and copper-containing zeolite systems,
even though the Cu-zeolites have some advantages, i.e., lower
activation temperature, possible use of either nitrous oxide or
oxygen as an oxidant, etc., the major disadvantage of this
system is the considerably lower methanol yield than that of
iron-based systems.[1b,9]
There are many hurdles to overcome before these transition
metal zeolites can be implemented for the conversion of
methane-to-methanol. The intermediate, chemisorbed methoxy
species, does not readily desorb from the active sites formed in
zeolite pores under the continuous reaction conditions, and
there is no methanol at the reactor outlet. When the temperature
is above 200 °C in order to desorb the methanol it undergoes
deeper oxidation to CO and/or CO2.[12] Therefore, it is necessary
to extract the methanol in an additional step.[6,12] Exposing
oxygen-activated and methane-reacted Cu-zeolites to steam
releases methanol, enabling cyclic operation.[3a,16]
As well as the two best known zeolites, mordenite and ZSM-
5, other commercial zeolites are also known.[6,7,18] Cu2+ ion-
exchanged ferrierite (FER) and beta (*BEA) zeolites, medium-
and large-pore zeolites, respectively, resulted in comparable
amounts of extracted methanol as Cu-ZSM-5. Recently, some
small-pore copper-containing zeolite materials (SSZ-13 (CHA),
SSZ-16 (AFX), and SSZ-39 (AEI)) showed a good methane-to-
methanol performance.[16-18] The amount of produced methanol
from these small-pore zeolites is greater than those with
mordenite and ZSM-5 under identical conditions. However, there
are still relatively few published data on the other structure types
of zeolites in this reaction.
There are two large-pore zeolites with a structure similar to
that of mordenite, i.e., EON- and MAZ-type zeolites.[21,22] The
EON structure is composed of strictly alternating maz and mor
layers, from which the well-known MAZ and MOR structures are
built, connected by five-ring chains in a regular 1:1 stacking
sequence. The EON structure has one-dimensional (1D) 12-
membered (MB) ring channels interconnected by 8-MB ring
channels like those of MOR but the channel system is much
more complicated than those of MOR and MAZ (Table 1). The
MAZ structure is also composed of 1D and 3D 12- and 8-MB
ring channels, respectively. However, unlike MOR and EON, the
Introduction
One of the reasons why methane is still an underutilized
feedstock is because its transport from remote drill sites is
difficult. Furthermore, because of the increasing availability of
cheap natural gas, the conversion of methane into more easily
transportable liquids or into chemicals is a very desirable goal.[1]
Although the visible applicability for direct methane upgrading is
still a long way off, the low temperature partial oxidation of
methane-to-methanol over metal-containing zeolites mimics
enzymatic systems.[2] The pentasil zeolites, such as ZSM-5
(framework type MFI) and mordenite (MOR), which are
[a]
[b]
Prof. Dr. M. B. Park,[+] Prof. Dr. J. A. van Bokhoven
Institute for Chemical and Bioengineering
ETH Zürich
Zürich 8093 (Switzerland)
E-mail: jeroen.vanbokhoven@chem.ethz.ch
Prof. Dr. M. B. Park,[+] Dr. M. Ranocchiari, Prof. Dr. J. A. van
Bokhoven
Laboratory for Catalysis and Sustainable Chemistry
Paul Scherrer Institute
Villigen 5232 (Switzerland)
[c]
[+]
S. H. Ahn
School of Environmental Science and Engineering
POSTECH
Pohang 37673 (Korea)
Present address: Department of Energy and Chemical Engineering,
Incheon National University, Incheon 22012 (Korea)
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.