Abstract
High-quality ZSM-5 zeolite membranes were successfully synthesized on
the outer surface of tubular α-alumina substrates by manipulating the
microstructure of the zeolite layer using a new different-sized seeding
(DSSM) in combination with variable-temperature/time (VTT) methodd. In
this method the microcracks/defects in the primary seed layer are filled
by smaller seeds and effectively eliminated using a proper
temperature/time profile, resulting in a significant increase in
selectivity without scarifying permeance. The effects of number and
combination of seed layers, seed size, and synthesis conditions on the
microstructure, N2/SF6 gas permeation,
and H2/CO2 separation performance of
membranes are evaluated in detail. The synthesized ZSM-5 membranes show
the microstructure of thin, fully inter-grown, and densified zeolite
layer that is responsible for high selectivity without any negative
effect on the permeance. The best synthesized defect-free membranes
exhibited a very low H2/CO2 selectivity
of 0.1 with CO2 permeance of 3×10-5mol m-2 s-1 Pa-1.
Keywords: ZSM-5, Zeolite membrane; Different-sized seeding;
Variable temperature/time; CO2 selective
Introduction
Accumulation of carbon dioxide in the atmosphere significantly threatens
human life due to its greenhouse effect [1] indicating the
importance of the development of efficient separation processes for
carbon dioxide removal [2]. MFI-type zeolite membranes due to their
high thermal, mechanical, and chemical stabilities, proper hydrophobic
properties, and high CO2 adsorption capacity are
promising to control CO2 emission [3]. Among
MFI-type zeolite membranes, its Al-containing analogue ZSM-5, if
supported on porous ceramic tubes are attract for large scale separation
of CO2 from gaseous mixtures involved in natural gas,
biogas, and flu gas industries [4] due to its unique pore structure,
which is similar to many industrially important molecules [5].
It is well-known that the performance of a tubular supported zeolite
membrane in a gas separation process depends on the quality of the
polycrystalline zeolite layer in terms of the number of
defects/non-zeolitic pores and the thickness of the zeolite membrane.
The former determines the membrane selectivity, while the latter
responsible for permeability [6, 7]. Therefore, recent works have
been focused on the elimination of the defects/non-zeolitic pores and
reducing the membrane thickness [8, 9]. The defects generally form
as a result of support defects, expansion mismatch problems,
high-temperature heating/cooling cycles, especially during the template
removal process, and a non-uniform seed layer [10].
Towards eliminate inter-crystalline gaps in the ZSM-5 zeolite framework,
Zhang et al. [11] found that seed size has important effects on the
formation of seed layers that determines the quality of regrown zeolite
membranes. They showed that the zeolite layer synthesized from small
seeds (100 nm) are more uniform and denser. An increase in seed size
(600 nm – 1.5 μm) makes the zeolite layer coarser and the intergrowth
of the membranes becomes poorer. Their results showed also that the
crystal intergrowth of the membrane improves by increasing synthesis
temperature reducing the voids and changing the orientation of crystal
growth. Xia et al. [12] systematically manipulated the
microstructure of the MFI-type zeolite membranes by tuning the seed size
and seed morphology. They showed that small seeds (< 200 nm)
tend to form c-oriented zeolite layer, whereas large seeds
(> 500 nm) tend to grow in different directions and form
more defective layer due to the low space limitation suppression under
the same synthesis condition. As a result, small seeds tend to form a
continuous, less defective, and dense zeolite layer, while larger seeds
usually form a defective porous layer [11]. Shan et al. [13]
found that the seeding procedure, as well as the quality/density of the
seed layer, greatly influence on the MFI-type zeolite membrane formation
and its separation performance. They showed that a continuous and dense
zeolite seed layer is necessary to obtain a ZSM-5 membrane with good
quality. However, their small seed size (70 nm) along with high seed
concentrations (> 10 wt. %) led to a thick zeolite layer
and caused the membrane to lose its permeability.
Looking from another perspective, Li et al. [14] tried to improve
the performance of ZSM-5 zeolite membranes using a two-stage
varying-temperature method. According to their method, membrane
synthesis begins at 130˚C for 4 h, synthesis temperature then increases
rapidly to 170˚C, and synthesis continues for another 4 h. They
demonstrated that compared with synthesis at the constant temperature,
this method is very effective for the preparation of high-quality
membranes via controlling the rates of nuclei formation and crystal
growth by changing the synthesis temperatures. The best ZSM-5 zeolite
membrane obtained by Li et al. showed hydrogen permeance of
2.4×10-6 mol m-2s-1 Pa-1 along with an
H2/n-butane selectivity of 129. The varying-temperature
method was applied also by Kong et al. [15] to prepare thin and
continuous MFI-type zeolite membranes on disc-shaped α-alumina supports.
They showed that the inter-crystalline regions can effectively be
eliminated using a proper synthesis temperature profile that leads to
higher selectivities. However, due to application on unseeded supports,
their selectivity (⁓22) for the same separation conditions was almost 6
times lower than that of Li et al. (⁓130). Considering all the
above-mentioned facts, to further improve the selectivity of tubular
ZSM-5 zeolite membranes, we need to minimize inter-crystalline gaps of
the membrane layer via modifying the seeding method and combine it with
an appropriate synthesis temperature/time profile. In this study, we
report a new secondary-growth synthesis method consisting of multi-layer
seeding in combination with the variable temperature/time profiles to
prepare high-quality ZSM-5 zeolite membranes on alumina tubular
supports. The schematic procedure of our proposed hybrid different-sized
seeding / varying-temperature method is shown in Fig. 1.
In this method, three different sized seed particles are coated, fixed,
and calcined to the support surface, in order of size. The hydrothermal
synthesis process then is carried out under different varying
temperature/time profiles. The as-synthesized zeolite membrane would
contain a well inter-grown, densified, and uniform top layer which leads
to higher selectivity, and a less inter-grown porous bottom layer that
leads to higher permeance. This novel hybrid method enhances the
selectivity of the tubular ZSM-5 zeolite membrane without sacrificing
its permeance via the elimination of the inter-crystalline gaps/defects
and decreasing the membrane thickness.