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10.1002/anie.202009230
Angewandte Chemie International Edition
COMMUNICATION
Control of surface barriers in mass transfer to modulate
methanol-to-olefins reaction over SAPO-34 zeolites
Shichao Peng, [a], [b] Mingbin Gao, [a], [b] Hua Li, * [a] Miao Yang, [a] Mao Ye, * [a] and Zhongmin Liu[a]
[a]
S. Peng, M. Gao, Dr. M. Yang, Dr. H. Li, Prof. Dr. M. Ye, Prof. Dr. Z. Liu
National Engineering Laboratory for Methanol-to-Olefins, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences
Dalian 116023 (P. R. China)
E-mail: lihua@dicp.ac.cn; maoye@dicp.ac.cn
S. Peng, M. Gao
[b]
University of Chinese Academy of Sciences
Beijing 100049 (P. R. China)
Supporting information for this article is given via a link at the end of the document.
Abstract: Mass transfer of guest molecules has great impact on the
applications of nanoporous crystalline materials and particularly the
shape selective catalysis over zeolites. Control of mass transfer to
alter reaction over zeolites, however, remains an open challenge.
Recent studies show that, in addition to intracrystalline diffusion,
surface barriers represent another transport mechanism which may
dominate the overall mass transport rate in zeolites. Here, we
demonstrate the methanol-to-olefins (MTO) reaction can be
modulated by regulating surface permeability in SAPO-34 zeolites
with improved chemical liquid deposition and acid etching. Our
results explicitly show the reduction of surface barriers can prolong
catalyst lifetime and promote light olefins selectivity, which opens a
potential avenue for improving reaction performance by controlling
the mass transport of guest molecules in zeolite catalysis.
which heavy aromatic species are readily generated and
accumulated within cavities during olefins formation. This can
result in the pronounced mass transport limitation for the large
molecules and thus the enhanced light olefins selectivity.[7] It is
shown by IFM that the dominating transport mechanism of
methanol in SAPO-34 zeolites is surface barriers.[4b] Based on
the distribution of coke species (i.e. heavy aromatics) observed
by confocal fluorescence microscopy (CFM), the researchers
further speculated that surface barriers may have significant
impact on MTO reaction.[8] This provides important and practical
implications on the possibility of altering MTO reaction via
regulating the mass transport. In this work, for the first time, we
demonstrate the modulation of MTO reaction over SAPO-34
zeolites by controlling surface permeability of guest molecules.
The external surface of zeolite crystals can be decorated by
post-synthesis treatments,[9] including chemical liquid/vapor
deposition (CLD/CVD) and HF etching. However, modifying
zeolite surfaces by these conventional methods would cause the
suppression of acid density[10] and damage of internal structure
of crystals[11], unavoidably changing intracrystalline diffusivities
of guest molecules. To achieve the modulation of surface
barriers, a prerequisite is that the modification is only made for
external surface of crystals while the change of interiors is
negligible. In doing so, SAPO-34 zeolite samples were first
synthesized hydrothermally[12] and denoted as SAPO-34-B.
Then part of SAPO-34-B samples were decorated by CLD of
tetraethoxysilane (TEOS), denoted as SAPO-34-L. In the CLD
treatment, although the SiO2 on the outer surface may increase
the surface sticking probability, multiple deposits likely cause
pore blockage.[2b] As a consequence, it is more difficult for
molecules to enter the crystals, resulting in the pronounced
surface barriers or low surface permeability in SAPO-34-L. As
the large molecules of TEOS could hardly pass the nanopores,
the decoration was expected to occur on the external surface of
SAPO-34-L. Parts of SAPO-34-B samples were treated by
etching with acetic acidic solution, removing the structural
defects and opening the orifices.[3b] These samples would have
reduced surface barriers, denoted as SAPO-34-H. As organic
template could effectively prevent acetic acid from entering the
interior, a minor change of internal structures of SAPO-34-H is
also expected. Furthermore, a decrease of acid strength for
etching was achieved to avoid the destruction of crystals and
assure the stability of samples. The details of synthesis and
modifications are described in Supporting Information.
Mass transfer of guest molecules in nanoporous crystalline
materials is of fundamental significance in processes spanning
heterogenous catalysis and gas separation.[1] One of particularly
notable applications is zeolite catalysts widely utilized for
producing liquid fuel and chemicals, in favor of the unique shape
selectivity.[2] Routinely limitation of mass transfer, especially
molecular diffusion, which is related to topological structures of
zeolites and steric dimension of molecules, is considered
governing shape selective catalysis. The mechanism underlying
molecular diffusion in zeolites, however, is quite involved and
not yet fully understood.
It is recently found that, despite the well-acknowledged
intracrystalline diffusion that is intrinsic to molecular properties
and material structures, surface barriers represent another
important transport mechanism.[3] The origins of surface barriers,
though not fully understood, are closely related to the non-
ideality of zeolite crystals which can be the consequences of,
among others, surface modification, defects formation, and
pores decoration.[4] As revealed by interference microscopy
(IFM) and infrared microscopy (IRM),[4b, 5] surface barriers may
dominate the overall mass transport rate in zeolite catalysts.
Despite the importance of surface resistance in mass transfer
over zeolite crystals being qualitatively identified, controlling the
performance of catalytic reaction by directed modulation of
surface barriers remains an open challenge.
Methanol-to-olefins (MTO), first commercialized in 2010, has
gained considerable interests for effectively producing ethylene
and propylene from alternative resources such as natural gas,
coal, and biomass.[6] SAPO-34 zeolites have been accepted well
suitable for MTO owing to the special CHA-type structure, in
1
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