A carbon-silica-zirconia ceramic membrane with CO2flow-switching behaviour promising versatile high-temperature H2/CO2separation

Article abstract of DOI:10.1039/d0ta07065c

Many researchers regard silica, silica-based and zeolite membranes as the agents that will accomplish H2 separation. These membranes are expected to be productive in various mixture systems and under very high temperatures. This work reports the successful fabrication of a composite carbon-SiO2-ZrO2 ceramic membrane with a unique pressure-induced switching of CO2 flows that allows versatile H2/CO2 separation at elevated temperatures. TG-MS, DTG-TGA, FT-IR, CP-MAS-13C-NMR, and TEM provide corroborative evidence of the carbonization of starting material SiO2-ZrO2-acetylacetonate into C-SiO2-ZrO2. The resultant C-SiO2-ZrO2 displayed significant hysteresis in the CO2 adsorption isotherm at a temperature well above the critical temperature of CO2 (31 °C), which indicates structural conformation. Furthermore, single-gas permeation measurements showing upstream pressures of 200 and 500 kPa reveal different permeation values for CO2 at 300 °C. In separating a H2/CO2 mixture at 50 and 300 °C under upstream pressures of 200 and 500 kPa, respectively, the flow of H2 permeance reduces as the concentration of CO2 increases in the feed side at 50 °C (1.14 × 10-8 down to 3.9 × 10-9 mol m-2 s-1 Pa-1 at 200 kPa). The pressure-induced surface flow of CO2 at 300 °C and 500 kPa, however, reduces the hindrance to H2 flow and results in H2/CO2 selectivity of ～20-30 for all CO2 concentrations, which is on a par with molecular sieving membranes. This novel C-SiO2-ZrO2 material shows promise for many interesting applications. This journal is

Full text of DOI:10.1039/d0ta07065c

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Materials Chemistry A
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A carbon–silica–zirconia ceramic membrane with
CO2 flow-switching behaviour promising versatile
high-temperature H2/CO2 separation†
Cite this: DOI: 10.1039/d0ta07065c
Sulaiman Oladipo Lawal,
and Masakoto Kanezashi
Liang Yu, Hiroki Nagasawa,
*
Toshinori Tsuru
Many researchers regard silica, silica-based and zeolite membranes as the agents that will accomplish H2
separation. These membranes are expected to be productive in various mixture systems and under very high
temperatures. This work reports the successful fabrication of a composite carbon–SiO2–ZrO2 ceramic
membrane with a unique pressure-induced switching of CO2 flows that allows versatile H2/CO2 separation at
elevated temperatures. TG-MS, DTG-TGA, FT-IR, CP-MAS-13C-NMR, and TEM provide corroborative evidence
of the carbonization of starting material SiO2–ZrO2-acetylacetonate into C–SiO2–ZrO2. The resultant C–
SiO2–ZrO2 displayed significant hysteresis in the CO2 adsorption isotherm at a temperature well above the
critical temperature of CO2 (31 ꢀC), which indicates structural conformation. Furthermore, single-gas
permeation measurements showing upstream pressures of 200 and 500 kPa reveal different permeation
values for CO2 at 300 ꢀC. In separating a H2/CO2 mixture at 50 and 300 ꢀC under upstream pressures of 200
and 500 kPa, respectively, the flow of H2 permeance reduces as the concentration of CO2 increases in the
feed side at 50 ꢀC (1.14 ꢁ 10ꢂ8 down to 3.9 ꢁ 10ꢂ9 mol mꢂ2 sꢂ1 Paꢂ1 at 200 kPa). The pressure-induced
surface flow of CO2 at 300 ꢀC and 500 kPa, however, reduces the hindrance to H2 flow and results in H2/
CO2 selectivity of ꢃ20–30 for all CO2 concentrations, which is on a par with molecular sieving membranes.
This novel C–SiO2–ZrO2 material shows promise for many interesting applications.
Received 20th July 2020
Accepted 30th October 2020
DOI: 10.1039/d0ta07065c
rsc.li/materials-a
of hydrogen separation membranes could be a two-pronged solu-
tion to the problems highlighted above.6 On the one hand,
Introduction
membrane separation could be an energy-efficient way to separate
H2 from different mixtures,2,7–10 while on the other hand the resul-
tant H2 could provide a sustainable and ‘cheap’ source of energy.11,12
Hydrogen production from the steam reforming of natural gas
currently produces levels of production efficiency that average
between 65 and 75%, which is the highest of all non-renewable
production sources.11 Currently, steam reforming of natural gas
also has the lowest cost, uses existing infrastructure,13 requires no
oxygen for processing, and operates at the lowest temperature of
all methods.14 Obviously, steam reforming is the most widely used
method for hydrogen production as it accounts for 80–85% of total
hydrogen production.15 As shown below, steam reforming of
natural gas consists mainly of methane reforming and is basically
accomplished in two steps:6,16
Separation processes have already made a signicant impact on the
world.1 The operating expenses of these processes, however, repre-
sent exorbitant investments of capital in industries such as petro-
leum, chemical, petrochemical, pharmaceutical, pulp, mineral and
others.2 Currently, about 80% of the world's energy demand is
satised using fossil fuels.3 These facts suggest that the world's
industrial energy demand (and by extension fossil fuel consump-
tion) derives from separation processes. By 2035, it is estimated that
8 countries (China, the USA, India, Brazil, Japan, South Korea,
Canada, and Mexico) will demand as much as 5.24 ꢁ 1017 kJ (4.97
ꢁ 1017 BTU) of energy,4 and 37% of that is estimated to come from
the industrial sector alone of which about 79% is foreseen to be
fossil fuel-derived by 2030.5 Therefore, two interwoven problems
must be solved: (1) reliance on fossil fuels; and, (2) energy
requirement for industrial separation processes. Fortunately, new
separation techniques such as membrane separation have been
developed over the years to tackle these problems. The development
CH4 + H2O(g) / CO + 3H2 (steam methane reforming step)
CO + H2O(g) / CO2 + H2 (water–gas shift step)
Department of Chemical Engineering, Graduate School of Engineering, Hiroshima
University, 1-4-1 Kagamiyama, Higashi-Hiroshima, 739-8527, Japan. E-mail:
kanezashi@hiroshima-u.ac.jp
The overall reaction becomes
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ta07065c
CH4 + 2H2O(g) / CO2 + 4H2
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The nal step in steam methane reforming (SMR) is sepa-
Composite SiO2–ZrO2 ceramic membranes have exhibited
ration of the effluent stream, which is comprised mostly of H2, desirable levels of hydrothermal stability for H2 separation, but
H2O and CO2, the composition of which depends on factors expensive molecular sieving is required.29 Hence, our group
such as the reformer operating temperature and pressure and recently reported the successful chemical modication of
the steam-to-carbon ratio.6,16 Techniques utilized for separation a composite SiO2–ZrO2 ceramic membrane capable of molec-
of H2 from CO2 and steam (for >99% pure H2 stream) have ular sieving.30 This membrane in its ligand-modied form
traditionally consisted of venerable energy intensive offered no promise for H2/CO2 separation. A successful modi-
approaches such as the pressure swing adsorption (PSA) of H2 cation with a chelating ligand of acetylacetone, however, has
or CO2, cryogenic distillation,2,6,16,17 and the more recent process opened the possibility for in situ carbonization (pyrolysis of the
of energy conservative membrane separation.6,16
organic ligand). If the deposited free carbon during pyrolysis
The composition of the nal effluent gas stream is dependent approximates the anisotropic characteristics of graphite,31 then
on variable conditions such as temperature, pressure, and the the resultant van der Waals forces between the p–p stacks of
steam-to-carbon ratio, which means that the separation technique carbon layers could serve as ‘interaction devices’ and endow
of choice must be versatile enough to deal with these composition new ‘dynamic’ characteristics into the SiO2–ZrO2 matrix. Van
variations. This poses a challenge for the development of der Waals interactions could also develop between the carbon
membrane separation techniques, and researchers must develop particles and the metal centers due to shis in the electron
robust and versatile materials that simultaneously deliver the core densities. Moreover, a recent report of the carbonization of
targets of viable H2 separation. Integrated gasication combined a molecular sieve silica membrane has introduced interesting
cycle applications (IGCC) also utilize the water–gas shi (WGS), properties to ordinary molecular sieve silica.32,33
and candidate materials must achieve levels of H2 recovery and H2/
Herein, we describe how SiO2–ZrO2-acetylacetonate was
CO2 selectivity of 70–90% and 21–62, respectively, under high successfully pyrolyzed to form a carbonized form of SiO2–ZrO2
temperatures and pressures,18 although SMR/WGS targets in (hereaer called C–SiO2–ZrO2). Fig. 1 presents the expected
industrial sources of H2 production have more stringent require- pyrolysis and network formation pathway. The in situ carbon-
ments for H2 stream purity at >99.99%.6,16,19 In addition to these ization was evaluated by characterization methods. C–SiO2–
targets, we believe taking H2 separation membranes a step further ZrO2 interaction with CO2 at various temperatures was investi-
would entail versatility in order to deal with H2/CO2 mixtures of gated via sorption experiments. Single and binary gas mixture
widely varying compositions.
experiments at 50 and 300 ꢀC were performed to determine the
Several H2 separation membranes that are applicable in a wide permeation characteristics of H2 and CO2 as well as the sepa-
variety of temperature ranges have been developed over the years. ration performance of an alumina-supported C–SiO2–ZrO2
The developers of these membranes have aspired to meet the set membrane. Finally, we discuss the place of a C–SiO2–ZrO2
targets, and beyond, with varying degrees of success, which is membrane in the scheme of H2 separation in industrial H2
associated with the material make-up of the membranes. Various production systems and postulate as to the possibility that
H2 separating membrane materials and processes have been a versatile H2 separation can be achieved. In the present study,
reviewed.2,6,20 Overall, H2 separating membranes have yet to meet we believe that we achieved a rational design for selective
the desired potential, and as of recently, only two industrial dynamism in a porous ceramic membrane.
applications of H2 separation membranes are known to exist
(recovery of H2 from off-gases in the ammonia industry and
Experimental
Preparation and pyrolysis of SiO2–ZrO2-acac powder and C–
production of pure H2 in the electronic industry).20 Thus, the
principal driver of success in this quest is conned to creativity in
materials research. An interesting group of materials that have
received much attention in recent times are exible metal–organic
frameworks (FL-MOFs),21–28 specically because of their ability to
structurally respond to external stimuli such as pressure, temper-
ature, light and electric elds.26 Such ‘dynamic’ responses are
exploitable in separation processes. For example, a uorinated
MOF derived from a self-assembly of 2,20-bis(4-carboxyphenyl)
hexauoro propane and zinc nitrate hexahydrate has shown
high selectivity of CO2 over N2 because peruorinated compounds
exhibit electrostatic interactions with CO2.23 According to Kitagawa
and Uemura,21 a construction of ‘dynamic’ porous coordination
polymers is rational via the placement of weak ‘interaction devices’
such as van der Waals force, p–p stacks and hydrogen bonds
between stiff 1-D, 2-D and 3-D building blocks. This strategy offers
a viable design for new porous materials by utilizing existing
ceramic materials that have shown promise for H2 separation.
Thus, the ‘interaction devices’ employed must selectively engage
with the mixture to be separated.
SiO2–ZrO2 membrane fabrication
The preparation of SiO2–ZrO2-acac sol was described in
a previous work.30 Briey, a 5 wt% SiO2–ZrO2-acac polymeric sol
Fig.
1 Illustration of expected pyrolysis and network formation
pathway of SiO2–ZrO2-acac to C–SiO2–ZrO2.
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(referred to as SZa4 sol in later sections) was prepared in two polarization/dipolar decoupling magic-angle spinning nuclear
stages. In the rst stage, a zirconium(IV) tert-butoxide solution magnetic resonance (CP/DD MAS-NMR), which was recorded
(ZrTB, purity 80% in 1-butanol, Aldrich) was modied in using a NMR spectrometer (Varian 600PS, Agilent Inc., USA).
ethanol (5 wt%) by coordinating with acetylacetone (Hacac, 13C-CP/DD MAS-NMR measurement was carried out at
purity 99%, Aldrich) for one hour at room temperature, and the a frequency of 150.87 Hz using both 13C–1H dipolar coupling
proportion of ZrTB/Hacac ¼ 1/4. In the second stage, a 5 wt% and decoupling modes. The spectra were acquired in 360 scans
solution of 98% pure tetraethoxysilane (TEOS, Aldrich) in each with a 90ꢀ pulse of 3.2 ms and recycle delays of 70 s. Hex-
ethanol was co-hydrolyzed and co-condensed with amethylbenzene was used as a reference against which the
acetylacetone-modied zirconium(IV) tert-butoxide using observed peaks were positioned, and a 6 mm zirconia rotor
deionized water and hydrochloric acid (37%, Nacalai Tesque) as spun at a magic angle of 57.4ꢀ at 7 kHz contained the sample.
a catalyst according to the following proportions: TEOS/ZrTB/
The physical evidence of the presence of carbon nano-
H2O/HCl ¼ 1/1/4/0.25. Hydrolysis and poly-condensation were particles was conrmed by obtaining the micrographs of the
carried out by stirring the mixture in a closed vial at 500 rpm for SZa4 and CSZ550 particles using transmission electron
more than 12 hours at room temperature.
microscopy. The samples to be resolved were prepared on ultra-
SZa4 (SiO2–ZrO2-acetylacetonate) powders were subsequently high-resolution carbon supports (STEM 100Cu Grids) by drop-
prepared from a SZa4 sol by slowly drying the sol under an ping approximately 10 ml of a 2 wt% dispersion of the ne
atmosphere controlled at 40–50 ꢀC. Carbon–SiO2–ZrO2 powders particles in butanol onto the grids. Prior to examination, the
ꢀ
were then prepared from SZa4 powders via calcination in a 3- prepared grids were vacuum-dried at 50 C for 48 hours.
zone furnace at 550 or 750 ꢀC under a N2 stream at 600 ml minꢂ1
The crystal/amorphous structure and lattice spacing values
for 20–25 min. For convenience the resultant pyrolyzed prod- of SZa4 and CSZ550 powders were obtained using X-ray
ucts will hereaer be referred to as CSZ550 or CSZ750 diffraction spectroscopy (D2 PHASER X-ray Diffractometer,
depending on the calcination temperature.
Bruker, Germany) with Cu Ka as the radiation source at
˚
To prepare CSZ550 membranes, it was necessary to fabricate a wavelength of 1.54 A. Furthermore, the sorption behaviors of
membrane supports. Details of this fabrication process have been CO2 on both SZa4 and CSZ550 powders were analyzed with CO2
previously reported.34 Principally, the membrane supports were adsorption/desorption experiments carried out at 25, 30 and
cylindrical a-alumina porous tubes (60% porosity, 1.2 mm pore 35 ꢀC. Prior to this measurement, samples of adsorbed gases
size, outer diameter 1 cm, and length 10 cm; Nikkato Corporation, and vapors were evacuated: SZa4 at 100 ꢀC and CSZ550 at 300 ꢀC
Japan) connected to closed and open non-porous supports at for at least 6 hours. Finally, temperature-programmed desorp-
either end. The membrane support was comprised of additional tion of N2 and CO2 was carried out using a custom-built
graded layers of 2–3 mm a-alumina particles and 0.2 mm a-alumina equipment setup tted with a mass spectrometer (Dymaxion
particles that were coated onto the support in that order. The SiO2– Dycor, Ametek Process Instruments, USA).
ZrO2 intermediate layer was prepared using colloidal SiO2–ZrO2
The cross-section morphology of the composite membrane
sol, and has been previously described.30 The coating of the SiO2– was examined using Field Emission-Scanning Electron
ZrO2 intermediate layer onto the a-Al2O3 particle layer was Microscopy (FE-SEM, Hitachi S-4800, Japan). Prior to examina-
repeated 6–8 times to cover large pores from the a-alumina particle tion, chipped samples of the membrane were attached to
layers. Following this, to form the active separation layer, 1 wt% sample holders via carbon tape, and these were vacuum-dried at
ꢀ
SZa4 sol in ethanol was wipe-coated onto the SiO2–ZrO2 interme- 50 C for 24 hours.
diate layer followed by calcination under a 600 ml minꢂ1 nitrogen
gas ow at 550 ꢀC for 20–25 min in a 3-zone continuous ow
furnace. This process was repeated 6–8 times to obtain the nal
Gas permeation measurement
defect-free CSZ550 membrane ready for single-gas and binary- A schematic representation of the gas permeation measurement
mixture permeation measurements. It should be noted that aer setup is shown in Fig. S1.† Prior to the gas permeation
fabrication the CSZ550 membrane was kept in the membrane measurement, a CSZ550 membrane was tted into its module
module under a He ow for 12 hours at 200 ꢀC.
immediately aer fabrication and placed inside the furnace in
the gas permeation measurement rig at 200 ꢀC under
a moderate helium ow of 100 ml minꢂ1 for about 12 hours to
remove the adsorbed vapor. Single-gas permeation tests of the
Characterization of materials
The pyrolysis route of the acetylacetonate ligand in SZa4 powders CSZ550 membranes were made using high purity gases (H2, He,
was analyzed and monitored using thermogravimetry and mass CO2, N2, CH4, CF4, SF6 in that order). Each gas was fed to the
spectroscopy (TGA-DTA-PIMS 410/S, Rigaku, Japan; DTG-60 Shi- upstream of the closed-end membrane module at 200–500 kPa
madzu Co., Japan) under nitrogen or He gas ows of 80 ml minꢂ1 of aꢀbsolute pressure under temperatures ranging from 50–
with a heating rate of 10 ꢀC minꢂ1. In a similar manner, the 300 C. Permeate side pressure was kept at atmospheric pres-
combustion test for free carbon under air was performed using sure and the permeate gas ow was measured using a bubble
thermogravimetry (DTG-60 Shimadzu Co., Japan).
lm ow meter (HORIBA-STEC, Japan).
The presence and the chemical state of free carbon was
Binary mixture separation performance was evaluated using
ꢀ
conrmed using Fourier transform-infrared spectroscopy (FT- the setup shown in Fig. S1† at 50 and 300 C. In the measure-
IR, FTIR-4100, JASCO, Japan) and solid-state 13C cross ment of binary gas mixture separation, H2/CO2 (80/20, 50/50, 20/
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80) mixtures were fed to the upstream of the module while
maintaining the pressure by setting the retentate side needle
valve at 200 and 500 kPa with the downstream kept at atmo-
spheric pressure. The compositions of the feed, retentate and
permeate streams were analyzed using a pre-calibrated gas
chromatograph (GC-14B, Shimadzu, Japan) equipped with
a TCD detector (column: Porapak N (GL Science, Japan)). To
evaluate the transmembrane pressure-drop of each component,
the log mean pressure difference (DPi,lm) was applied (eqn (1)).
DPi;in ꢂ DPi;out
ꢀ
DPi;lm
¼
(1)
lnðDPi;in DPi;out
Þ
In eqn (1), DPi,in and DPi,out represent the difference in
partial pressures of component i between the retentate side and
the permeate side at the inlet and outlet of the module,
respectively.
Fig. 3 CP-MAS-13C-NMR spectra of fresh SiO2–ZrO2-acac powder
and that pyrolyzed at 550 and 750 ꢀC.
ꢀ
600 C with exothermal peaks in the DTA curve indicating the
combustion of deposited non-volatile carbon to CO and/or CO2.
The weight loss was calculated at 20%, which corresponded to
203 mg carbon/g of C–SiO2–ZrO2.
Results and discussion
Carbonization of SiO2–ZrO2-acac into C–SiO2–ZrO2 and
identication of the carbon state
It is important to determine the state of carbon in the SiO2–
ZrO2 matrix as this will aid in understanding the behavior of the
composite material. Fig. 3 shows the change in the CP-MAS-13C-
NMR (cross polarization) spectra of the SiO2–ZrO2-acac trans-
formation to C–SiO2–ZrO2 (pyrolyzed at 550 and 750 ꢀC). For the
spectrum of SiO2–ZrO2-acac, the different peaks indicate the
intensity of the different functional groups of the enol and keto
forms of the acacꢂ chelate resulting from 13C–1H dipolar
coupling. Acetylacetone in its keto tautomer form exists with
two carbonyl groups (C]O), and as such is not able to properly
chelate with transition metals. However, in its enol form, ace-
tylacetone presents with one carbonyl group (C]O) and one
enol group (C]C–OH) that more easily forms acetylacetonate,
thus facilitating chelation. The magnied chemical shis of
peaks 5 and 6 on the SiO2–ZrO2-acac spectra show very small
intensities indicating that the enol forms of acacꢂ were
adequately chelated to SiO2–ZrO2 by comparison with the
spectrum of pure acetylacetone.35 Furthermore, the spectra of
In Fig. 2(a), the decomposition products of acacꢂ (acetylaceto-
nate) were conrmed via TG-MS (thermogravimetry-mass
spectroscopy under an inert He atmosphere). In this gure
ꢀ
the nal decomposition temperature was around 500–550 C,
which corresponds to the prole for SiO2–ZrO2-acac thermal
decomposition under an inert N2 atmosphere, as presented in
Fig. 2(b), which shows the DTG-DTA proles for the thermal
decompositions of SiO2–ZrO2-acac and C–SiO2–ZrO2 under N2
and air, respectively. The nal decomposition temperature for
SiO2–ZrO2-acac under N2 was approximately 500–550 ꢀC with
the DTA curve showing no exothermal peaks, which indicated
no combustion reactions and possible deposition of non-
volatile decomposition products. Under air, C–SiO2–ZrO2
ꢀ
(prepared by pyrolyzing SiO2–ZrO2-acac under N2 at 550 C for
20 minutes, as discussed under the Experimental section)
showed thermal decomposition until the temperature reached
ꢀ
550 and 750 C-pyrolyzed C–SiO2–ZrO2 showed broad peaks in
the aromatic chemical shi region (ꢃ130 ppm) with two spin-
ning side bands. The spinning side bands are peaks due to the
chemical shi in anisotropy that is associated with sp2
hybridized C species such as aromatic and carbonyl C.36,37 This
indicates the transformation of acacꢂ into aggregated sp2
hybridized graphitic carbon. The DD-MAS-13C-NMR (dipolar
decoupling mode) that resolved the magnetic environment of
13C by decoupling 13C–1H revealed very sharp peaks (Fig. S2†).
This is reasonable since free carbon has no bonded 1H. The 550
and 750 ꢀC-pyrolyzed C–SiO2–ZrO2 samples showed similar
spectra, further indicating that pyrolysis up to 750 ꢀC results in
little or no change in the chemical and physical states of the
pyrolysis product. The aromatic C]C bonds were also
conrmed via FTIR (Fig. S3†).
Fig. 2 Observed TG-MS profile of SiO2–ZrO2-acac under He (a) and
TGA-DTA profiles of SiO2–ZrO2-acac under N2 and C–SiO2–ZrO2
under air (b) (ramping rate: 10 ꢀC minꢂ1).
The physical evidence of the presence of free carbon was
obtained by transmission electron microscopy. Fig. 4(a)–(d)
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CO2/C–SiO2–ZrO2 interaction. In Fig. 5(a), the adsorption
isotherms of SiO2–ZrO2-acac and C–SiO2–ZrO2 powders at 25
and 35 ꢀC are shown. Both samples show gentle Langmuir-like
(almost Henry) type I adsorption isotherms in the pressure
range observed. The adsorption–desorption isotherms of SiO2–
ZrO2-acac at both 25 and 35 ꢀC follow reversible paths indi-
cating almost no special effect of CO2 adsorption on neither the
SiO2–ZrO2-acac structure nor the surface. In contrast, aer
carbonization of SiO2–ZrO2-acac resulted in C–SiO2–ZrO2, the
adsorption–desorption isotherms of CO2 showed signicant
hysteresis despite the smaller amount of adsorbed CO2 indi-
cating a structural conformation of the C–SiO2–ZrO2 to allow
CO2 adsorption. The existence of this hysteresis at a tempera-
ture above the critical temperature of CO2 (31 ꢀC) rules out the
possibility of capillary condensation, and instead suggests this
is the result of a structural transformation.22 Culp et al.22
proposed that when host–guest interaction can energetically
compensate for the shape transformation of the host lattice, the
host structure could conform to the shape of the guest
molecules.
Fig. 4 TEM images of SiO2–ZrO2-acac: (a) original; (b) high-contrast
and 550 ꢀC-derived C–SiO2–ZrO2: (c) original; (d) high-contrast.
Insets: electron diffraction images.
Fig. 5(b) presents the isosteric heat of the adsorption of CO2
onto SiO2–ZrO2-acac and C–SiO2–ZrO2 calculated using the
adsorption isotherms at 25, 30 and 35 ꢀC and the Clausius–
Clapeyron equation. The isosteric heat of CO2 adsorption for C–
SiO2–ZrO2 shows an average value of ꢃ170 kJ molꢂ1 compared
with ꢃ34 kJ molꢂ1 for SiO2–ZrO2-acac. Lin et al.39 calculated the
theoretical potential energy surface (PES) barrier required for
CO2 chemisorption on a pristine graphite to be ꢃ350 kJ molꢂ1
(ꢃ84 kcal molꢂ1). With CO2 adsorption on C–SiO2–ZrO2
releasing a maximum of ꢃ290 kJ molꢂ1, this suggests that CO2
adsorption onto C–SiO2–ZrO2 can produce enough energy to
deform the lattice structure to allow adsorption. The drastic
decrease in the Qst of C–SiO2–ZrO2 indicates an energetic
heterogeneity of the C–SiO2–ZrO2 surface.
compare the electron diffraction patterns in the TEM micro-
graphs of 50 nm-scale SiO2–ZrO2-acac (Fig. 4(a) and (b)) and C–
SiO2–ZrO2 (Fig. 4(c) and (d)) that were used to detect the pres-
ence of carbon nanoparticles. Fig. 4(b) and (d) are high-contrast
conversions of Fig. 4(a) and (c). The images show the edges of
the particles. The micrographs reveal dark patches of several
nanometers dispersed in the C–SiO2–ZrO2 matrix. Similar TEM
images have been reported for uorescent carbon nanoparticles
(CNPs).38 The inset images of both SiO2–ZrO2-acac and C–SiO2–
ZrO2 show halo electron diffraction patterns indicating that the
carbonization process had little effect on the amorphous
structure of the SiO2–ZrO2 matrix.
Fig. 6 shows the observed XRD patterns and lattice spacing
values calculated from the centers of the amorphous peaks of
SiO2–ZrO2-acac, C–SiO2–ZrO2, and pure SiO2–ZrO2 powders
(with and without ring) measured at room temperature. The d-
spacing value was reduced when SiO2–ZrO2-acac (0.356 nm) was
Unique permeation properties of CO2 in C–SiO2–ZrO2
membranes
Understanding the interaction between CO2 and C–SiO2–
ZrO2. Fig. 5(a) and (b) illustrate the observed evolution of the
Fig. 5 (a) Adsorption–desorption isotherms of CO2 onto SiO2–ZrO2-acac and C–SiO2–ZrO2 powders measured at 25 and 35 ꢀC; (b) calculated
isosteric heats of adsorption of CO2 onto SiO2–ZrO2-acac and C–SiO2–ZrO2 powders using temperatures of 25, 30 and 35 ꢀC.
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van der Waals forces produced by delocalized p-orbital elec-
trons (p–p stacking) and sp2-hybridized C–C s-bonding in the
horizontal direction.40 These delocalized electrons can induce
dipole moments on approaching CO2 molecules specically
because of the known vibrational modes of CO2 which make it
susceptible to dipole induction. The induced CO2 molecules
become attracted and electronically adsorbed onto the graphitic
carbon layers thereby undergoing structural changes because
the weak van der Waals forces allow the lattice to conform to the
functionality of CO2 guest molecules.22
Pressure-induced transition of CO2 ow in C–SiO2–ZrO2
membranes. Preferential adsorption of CO2 over other gases
(such as N2) onto C–SiO2–ZrO2 was established from N2/CO2-
TPD measurements (Fig. S4†). A desorption curve can be
ꢀ
observed at a temperature as high as 300 C. Fig. 8(a) and (b)
Fig. 6 Observed XRD patterns and calculated d-spacing values of
SiO2–ZrO2-acac, C–SiO2–ZrO2 and pure SiO2–ZrO2 powders
measured at room temperature.
show the time courses for the single-gas permeance of He, H2
and CO2 at 50 and 300 ꢀC, respectively, through an alumina-
supported C–SiO2–ZrO2 membrane, as measured at an
upstream pressure of 200 kPa. In both cases, a dynamic
permeation trend is observed for CO2 whereby the permeance
reduces drastically with time before reaching a steady state.
This points to a strong permeation hindrance to CO2 ow due to
blocking by adsorbed immobilized CO2. At 50 ꢀC, He and H2
recovered about 65 and 79% of their initial values, respectively,
aer the feeding of CO2, which effectively blocked certain pores
due to adsorption by the C–SiO2–ZrO2 pores. This shows that
adsorbed CO2 has a blocking effect on the permeation paths of
He and H2 and that He, H2 and CO2 share the same permeation
paths. At 300 ꢀC the values for He and H2 permeance are
recovered at rates of 80 and 83%, respectively, and it would be
safe to assume that the recovery ratio increases as temperature
increases so that adsorption would be stronger at 50 ꢀC than at
pyrolyzed to C–SiO2–ZrO2 (0.295 nm) at 550 ꢀC under N2. On the
other hand, pure SiO2–ZrO2 displayed a d-spacing shi from
0.314 nm to 0.291 nm aer calcination at 550 ꢀC. The resultant
d-spacing of C–SiO2–ZrO2 was similar but somewhat higher in
value compared with that of pure SiO2–ZrO2 (0.291 nm). The
similar d-spacing values of C–SiO2–ZrO2 and pure SiO2–ZrO2
may suggest that free carbon layers are well integrated into the
SiO2–ZrO2 lattice and do not form separate structures. The
spacing between the successive layers of carbon can thus be
safely assumed to be less than 0.295 nm. In Fig. 7, a scheme
illustrating the interaction between CO2 and C–SiO2–ZrO2 is
shown. As established from characterization methods, free
carbon is integrated in graphitic form in the C–SiO2–ZrO2
lattice. Graphitic carbon is anisotropic so that the layers of
carbon are closely bound together in the vertical axis by weak
ꢀ
300 C given the continued occurrence of adsorption.
Fig. 9(a) shows the single-gas permeance for a C–SiO2–ZrO2
membrane as a function of the kinetic diameter of different
Fig. 7 Scheme illustrating the mechanism of the interaction between CO2 and C–SiO2–ZrO2.
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Fig. 8 Time courses for single-gas (He, H2 and CO2) permeation at 50 (a) and 300 ꢀC (b).
Fig. 9 (a) Single-gas permeance at 300 ꢀC as a function of kinetic diameter at 200 and 500 kPa upstream pressures; (b) CO2 permeance as
a function of upstream pressure measured at 300 ꢀC; (c) time course for the single-gas permeance of H2, CO2 and N2 at 300 ꢀC measured at
upstream pressures of 200 and 500 kPa (closed symbols: 200 kPa; open symbols: 500 kPa).
gases (He (0.26 nm), H2 (0.289 nm), CO2 (0.33 nm), N2 (0.364 that values for gas permeance at 200 kPa remained either
nm), CH4 (0.38 nm), CF4 (0.48 nm), and SF6 (0.55 nm)) approximately the same (He, H2, CH4, CF4, SF6) at 500 kPa or
ꢀ
measured at 300 C under 200 and 500 kPa of upstream pres- somewhat less (N2), but CO2 permeance at 500 kPa (1.5 ꢁ
sure. In evaluating the departure of a membrane's performance 10ꢂ9 mol mꢂ2 sꢂ1 Paꢂ1) was increased by almost one order
from Knudsen permeance to molecular sieving, the theoretical compared with that at 200 kPa (1.7 ꢁ 10ꢂ10 mol mꢂ2 sꢂ1 Paꢂ1).
permeance based on SF6 permeance via Knudsen diffusion is To further investigate this CO2-specic trend, the pressure-
shown in Fig. 9(a), as indicated by the broken lines. The dependence of CO2 permeance was investigated using a fresh
experimentally obtained values for He and H2 permeance far C–SiO2–ZrO2 membrane calcined at 550 ꢀC, as shown in
exceeded those calculated via the Knudsen mechanism, which Fig. 9(b). The detailed time course of this experiment appears in
suggests the existence of pores so small that only H2 and He Fig. S5.† The measurement was made multiple times to ensure
could permeate them, which indicates a bimodal pore distri- steady-state values and to verify accuracy. The data presented in
bution in C–SiO2–ZrO2 membranes. It should be noted that Fig. S5† are data recorded as soon as the CO2 reached a steady
pressures are presented in absolute values. For the measure- state at 200 kPa. At subsequent pressures, the system response
ment made at 200 kPa of upstream pressure, the gas permeance was quick, and a steady state was reached in a short time. In the
followed a trend such that permeance was decreased with an initial feed of CO2 gas to the upstream of the membrane at 200
increase in the kinetic diameter, which generally indicates kPa and 300 ꢀC, a wait of approximately 2 hours was required for
a molecular sieving property. CO2 is the exception, and deviates a steady state to be reached before recording the results as
from the trend with a permeance lower than other gases irre- shown in Fig. S6.† The results obtained reveal that CO2 per-
spective of the kinetic diameter. At 500 kPa of upstream pres- meance showed almost no dependence on upstream pressures
sure, however, gas permeance follows a trend whereby it that reached approximately 350 kPa. At 400 kPa and above,
decreases with an increase in the kinetic diameter without however, a positive slope in the permeance of CO2 was almost
exception as opposed to gas permeance at 200 kPa. It was noted linear with the slope of further increases in the pressure.
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This is consistent with the kinetic diameter dependence of
single gas permeance measurements made at 200 and 500 kPa.
In some cases, this type of trend can be associated with the
development of defects in a membrane.41,42 The cross-sectional
prole of the composite membrane taken with FE-SEM and
shown in Fig. S7† suggests a defect-free C–SiO2–ZrO2 membrane
layer with a thickness of about 20–40 nm, although the
boundary between the C–SiO2–ZrO2 membrane layer and the
SiO2–ZrO2 intermediate layer in not very clear. As shown in
Fig. 9(c), however, aer CO2 permeation at 200 and 500 kPa, the
permeance of H2 recovered as much as 99% of its levels before
CO2 ow (aer desorption of adsorbed CO2 in vacuum at 350
ꢀC). Furthermore, the values for N2 permeation at 200 and 500
kPa showed similar values following CO2 permeation, which is
a sign of the absence of any defects. This suggests that the
integrity of the membrane matrix was retained and was unaf-
fected by the ow of CO2. Therefore, it can be safely assumed
that CO2 only displayed a switching of ow regimes in the C–
SiO2–ZrO2 membrane with 400 kPa being the transition pres-
sure. Neither plasticization nor defects can be suggested in this
case. It should be noted that since the measurement tempera-
ture (300 ꢀC) was well above the critical temperature of CO2 (31
ꢀC), this phenomenon should not be ascribed to the ow of
capillary condensate. We therefore suggest a pressure-induced
multilayer diffusion mechanism. It has already been estab-
lished in this section that adsorbed and immobilized CO2
molecules prevent further permeation of gas-phase CO2 mole-
cules. This trend persists at low pressures. When the upstream
pressure increases and the concentration of CO2 exceeds
a certain threshold where adsorbent–adsorbate interaction
dominates, multi-layer adsorption may occur and thus lead to
a greater mobility of gas-phase CO2 molecules.43
Fig. 10 Pressure-induced switching cycles of CO2 flow represented
by the time course of CO2 and N2 permeance under sequential
pressurization at 200 and 500 kPa.
switch the ow of CO2 based on a pre-designed threshold
pressure. It is rational to assume that the threshold pressure for
the transition of CO2 ow would depend on the average pore
size of the C–SiO2–ZrO2 membrane. Fine-tuning the average
pore size to pre-design a threshold switching pressure, however,
is a subject for future investigation. To establish the recycla-
bility of CO2-ow switching and the reliability of a C–SiO2–ZrO2
membrane over time, 3 cycles of the sequential pressure
dependence of CO2 permeance at 200 and 500 kPa upstream
ꢀ
pressures were carried out at 300 C for 16 hours, as shown in
Fig. 10. The two regimes of CO2 ow before and aer transition
were perfectly repeatable over the 3 cycles without a signicant
difference in permeance. Also, N2 permeation was carried out to
further stress the fact that the ow-switching behavior is
specic to CO2. The permeance of N2 remained the same at 200
and 500 kPa, which indicated no pressure dependence for N2
permeation through the C–SiO2–ZrO2 membrane. Both experi-
ments further veried that no defects were formed during this
switching of the CO2 ow.
Table 1 lists the calculations of activation energy for the
temperature dependence of the gas permeation for different
gases (H2, CO2, N2 and SF6), as regressed against eqn (2) and
plotted in Fig. S8.† Based on these calculations, H2 exhibited an
activated diffusion mechanism with activation energy of
5.96 kJ molꢂ1, while other gases showed Knudsen permeation
with activation energies of less than 1 kJ molꢂ1. In Fig. S8,† it is
evident that the blocking effect of adsorbed CO2 greatly
contributes to a reduction in the apparent CO2 permeance
beyond the expected Knudsen value across the tested range of
temperatures.
Binary H2/CO2 mixture separation performance of a C–SiO2–
ZrO2-derived membrane
Effects of temperature, pressure and CO2 feed mole fraction
on the binary mixture separation performance. The detailed
time courses of single- and binary-gas permeance through a C–
SiO2–ZrO2 membrane are shown in Fig. S9.† Fig. 11(a) shows the
values for the permeance of H2 and CO2 through a C–SiO2–ZrO2
membrane as a function of the CO2 mole fraction in the feed
(and by extension CO2 upstream partial pressure) measured at
50 ꢀC. At 50 ꢀC and 200 kPa, H2 and CO2 permeance was
decreased as the CO2 mole fraction in the feed increased. This
trend resembles that of silica membranes where the permeance
of the adsorptive gas is expected to slightly increase as its mole
fraction in the feed decreases.44 The same trend was observed at
50 ꢀC and 500 kPa. Fig. 11(b) illustrates the separation of
ꢁ
ꢂ
k0
Ep;i
pffiffiffiffiffiffiffiffiffiffiffiffiffi
Pi ¼
exp ꢂ
(2)
RT
MiRT
Establishing the ow transition of CO2 presents an inter-
esting phenomenon whereby a C–SiO2–ZrO2 membrane could
Table 1 Calculated activation energy for the permeation of different
gaseous species through
against eqn (2)
a C–SiO2–ZrO2 membrane regressed
ꢀ
a binary gas mixture of H2 and CO2 at 300 C. At 200 kPa, the
Gas
H2
5.96
CO2
0.55
N2
ꢂ0.88
SF6
0.88
permeance of H2 maintained a high value of 7 ꢁ 10ꢂ8 mol mꢂ2
Activation energy [kJ molꢂ1
]
sꢂ1 Paꢂ1 up to a CO2 mole faction of 0.5 aer which the
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Fig. 11(a) and (b), however, were more substantial. In addition,
the permeance of the CO2 in a binary mixture through the
membrane was greater than that observed during single
permeation at the same feed pressure. This can be explained by
the fact that H2 has a much higher ux than the slower
permeating component CO2, which exerts a ‘sweeping’ effect,
and CO2 is ‘swept’ along in the permeation paths. The same
phenomenon was observed for a polypyrrolone membrane
where the much slower permeating CH4 molecules were ‘swept’
along by the much faster CO2 molecules.45
Such a ‘sweeping’ effect can be explained by considering the
co-existence of two gas species with different molecular sizes
and adsorptive tendencies moving through a common perme-
ation path. The molecules of the fast and less-adsorptive
permeating gas continually collide with the slow permeating
and more-adsorptive gas molecules and therefore displace them
from the adsorbent surface thereby increasing the gas phase
ow of the adsorbate gas molecules. Fig. 12 features a schematic
illustration of the effect of H2 presence on the adsorption-ux
balance of CO2 through C–SiO2–ZrO2 membrane pores. In the
case of a binary mixture of H2 and CO2 gases, faster H2 mole-
cules sweep slower CO2 molecules and therefore there is
a considerable contribution of CO2 to the overall bulk phase
transport of the gas mixture. The fraction of the bulk mass ux
contributed by each component in the gas mixture can be
calculated based on a set of equations presented by Kamar-
uddin and Koros,45 which compares to the gas-phase ux in
single-gas permeation systems. A detailed derivation of these
equations can be found in the ESI (eqn (S1)–(S5)).† When these
equations were applied to single and binary H2/CO2 systems at
300 ꢀC and 500 kPa, the results revealed that the fraction of the
bulk-phase ux contribution by CO2 is much higher in the
permeation of a gas mixture than in that of pure CO2 (Fig. S10†),
which was predicted by Kamaruddin and Koros.45
Fig. 11 Gas permeance as a function of CO2 feed mole fraction and
feed partial pressure at 50 ꢀC (a) and 300 ꢀC (b). Closed symbols: 200
kPa total upstream pressure; open symbols: 500 kPa total upstream
pressure.
permeance was reduced drastically to 4 ꢁ 10ꢂ9 mol mꢂ2 sꢂ1
Paꢂ1 at a CO2 mole faction of 0.8. At 500 kPa, however, only
a slight decrease in H2 permeance occurred even up to a CO2
feed mole fraction of 0.8. This underscores the fact that at high
values of both temperature and pressure, less CO2 is adsorbed
and the pressure-induced multilayer diffusion of CO2 presents
less of a hindrance to the permeation of H2, and, thereby, high
H2/CO2 selectivity is maintained across all levels of CO2
concentration.
As mentioned earlier in this section, the permeance increase
as the concentration of CO2 in the feed decreased was similar to
that observed for silica membranes in a surface diffusion
mechanism. The increases in CO2 permeance shown in
Outlook on the practical application of a C–SiO2–ZrO2
membrane for H2/CO2 separation. There are reports of
membranes that separate H2 and CO2 by utilizing strong CO2
adsorption to create reverse CO2/H2 permselectivity at low
temperature.46 In this process, the more adsorptive CO2 mole-
cules permeate at the expense of blocked non-adsorptive H2
molecules. In these membranes, as permeation temperature
increases, molecular sieving properties dominate and achieve
Fig. 12 Schematic illustration of the H2 ‘sweeping’ effect on CO2 permeation.
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carbonization of SiO2–ZrO2-acac to C–SiO2–ZrO2 was conrmed
by TG-MS, DTG-TGA, FT-IR, CP/DD-MAS-13C-NMR, and TEM.
The carbon content in C–SiO2–ZrO2 promoted the adsorption of
CO2, which resulted in a maximum heat of adsorption at
290 kJ molꢂ1 and provided energy sufficient to allow the carbon
layers in C–SiO2–ZrO2 to conform to the CO2 molecules. This
development revealed that CO2 could be trapped. The CO2
permeance of the C–SiO2–ZrO2 membrane demonstrated
a signicant level of dependence on the feed pressure, but little
dependence on either temperature or the CO2 feed concentra-
tion under conditions of 300 ꢀC and 500 kPa, which allowed for
an interesting separation performance when used in binary H2/
CO2 systems. The C–SiO2–ZrO2 membrane achieved a H2/CO2
separation performance comparable to molecular sieving
membranes irrespective of the CO2 feed content. In this study,
Fig. 13 Trade–off plot of H2/CO2 mixture selectivity versus H2 per-
meance values for different high-temperature separation membranes.
we fabricated
a
two-parameter CO2-switchable ceramic
membrane. The switching parameters were the feed side pres-
sure and the use of a fast permeating gas component.
a level of H2 permselectivity that is suitable for high-
temperature applications such as steam methane reforming/
water–gas shi reactions (SMR/WGS) and the integrated gasi-
cation combined cycle (IGCC). In the present work, however,
the molecular sieving properties at 300 ꢀC with 200 kPa of
upstream pressure seemed inadequate to achieve sufficient H2
permselectivity across all CO2 concentrations due to persistent
CO2 blocking. This is a drawback for practical and versatile
high-temperature applications. This drawback is eliminated
with a high pressure of 500 kPa where high H2 permselectivity
can be achieved across various CO2 concentrations, as illus-
trated in Fig. 11(b). For practical high-temperature, high-
pressure H2/CO2 separation purposes where membrane versa-
tility may be of paramount importance as a result of uctuating
CO2 concentrations, a C–SiO2–ZrO2 membrane could be useful.
Fig. 13 compares the H2 permeance-H2/CO2 mixture selectivity
trade-off of high-temperature separation membranes (details in
Table S1†). Most of the high-temperature H2/CO2 separation
membranes include a range of versions from zeolitic to ceramic
that achieve separation by means of molecular sieving (size
exclusion of CO2) of a 1 : 1 binary H2–CO2 mixture. Thus, it
would be a novel occurrence to achieve comparable separation
(good H2 permeance-H2/CO2 selectivity trade-off) with a CO2
adsorptive-type membrane across several H2–CO2 mixture
ratios. Fig. 13 clearly shows that C–SiO2–ZrO2 membranes can
achieve a separation performance that is comparable to many
molecular sieving membranes at high temperatures even with
most CO2-concentrated feed stock. This ability is a result of the
pressure-induced transition of CO2 ow in C–SiO2–ZrO2
membranes (as previously established), which results in less
hindrance to the ow of H2 and promotes activated H2
permeation.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
S. O. L. appreciates the support of the Japanese Government
Ministry of Education, Culture, Science and Technology. In
addition, the authors would like to thank Dr Maeda of the
NBARD, Hiroshima University, for assisting with the TEM
measurements and Dr Nakaya of the Department of Chemical
Engineering and Applied Chemistry, Graduate School of Engi-
neering, Hiroshima University, for assisting with the NMR
measurements.
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