Investigation on Water Vapor Adsorption of Silica-Phosphonium Ionic Liquids Hybrid Material - MDPI
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materials
Article
Investigation on Water Vapor Adsorption of
Silica-Phosphonium Ionic Liquids Hybrid Material
Cancan Li, Jiamei Zhu *, Min Zhou, Shuangquan Zhang and Xiaodong He
School of Chemical Engineering and Technology, China University of Mining & Technology,
Xuzhou 221116, China; 17851145012@163.com (C.L.); zmcumt@126.com (M.Z.); cumtzsq@126.com (S.Z.);
researchcumt2017@126.com (X.H.)
* Correspondence: zhujiamei1991@cumt.edu.cn; Tel.: +86-516-83591109
Received: 7 May 2019; Accepted: 30 May 2019; Published: 1 June 2019
Abstract: Adsorption and diffusion of water vapor in phosphonium ionic liquid modified silica gel
were studied, aiming to reduce the loading of water vapor in porous materials. The modified silica
gel was prepared through a grafting method and characterized by FTIR, thermal gravity analysis and
X-ray photoelectron spectroscopy. N2 sorption isotherms at −196 ◦ C and CO2 sorption isotherms
at 0 ◦ C were also measured to analyzee the porosity. Water vapor adsorption equilibriums at 25 ◦ C
up to 30 mbar were tested. The results indicate that the ionic liquids (ILs) phase acts as a protecting
film which decreases water vapor adsorption. The improvement of water-resistant performance is
also attributed to the decrease of micro-porosity and silanol groups on the silica surface. Diffusion
behavior of water vapor on modified silica was determined on the basis of the adsorption equilibrium.
The effective diffusivity of water vapor in modified silica is almost the same as in bare silica and
decreases with the increasing of water vapor loading.
Keywords: ionic liquids; water vapor; modified silica; adsorption
1. Introduction
Ionic liquids (ILs) are salts with a melting point below 100 ◦ C, which are commonly composed
of organic cations and inorganic or organic anions, typically. ILs have been considered as promising
solvents in separation [1–3], chemical reaction, or catalytics, due to their unique properties, such as
low volatility, chemical stability, high gas selectivity, and tunable properties [4,5]. A lot of research into
imidazolium-based ILs have been reported [6–8], while phosphonium-based ILs are currently appear
more attractive, owing to their unique advantages in many aspects. Phosphonium salts are generally
more thermally stable and have lower densities than nitrogen-based ILs [9,10]. Moreover, compared
with imidazolium and pyridinium ILs, phosphonium ILs have no acidic proton or aromatic ring,
which makes them stable towards nucleophilic and basic conditions [11,12]. In addition, the kinetics
of phosphonium salt formations are much faster than those of nitrogen-based salts, implying higher
productivity and less expense than their equivalent imidazolium ILs [13–15]. Based on the characteristics
of ILs, there have been many studies on IL-modified porous materials, such as activated silica, carbon,
and alumina for CO2 adsorption. Zhang [16] showed that CO2 capture capacity increased markedly
for grafting amino-functionalized phosphonium ILs ([aP4443 ]AA) onto silica gel than in a bulk liquid
and bare support. Balsamo [17] reported that supported [Emim][Gly] ionic liquid on mesoporous
gamma-Al2 O3 at different loadings had better CO2 sorption capacity. These experiments reveal
that CO2 capture performance is greatly enhanced owing to the fact that ILs were supported on the
porous support.
A noticeable influence of water on the physical characteristics of adsorbents has been also reported,
due to the presence of a small amount of water vapor (8–15%) in the power plant flue gas, which tends
Materials 2019, 12, 1782; doi:10.3390/ma12111782 www.mdpi.com/journal/materialsMaterials 2019, 12, 1782 2 of 13
to significantly reduce CO2 adsorption capacity [18–20]. Because of the competition adsorption of
CO2 and H2 O, the adsorbed water occupies a part of adsorption sites, which is unfavorable for
CO2 adsorption. Li [21] studied that CO2 adsorption of activated alumina strongly decreased with
increasing water content in the vapor-gas. Xu [22] investigated the effect of water on CO2 adsorption
properties in a commercial activated carbon, and cyclic adsorption experiments with wet feed gas
showed that the CO2 adsorption capacity of activated carbon was negatively affected by the presence
of water. Uehara [23] studied CO2 sorption behaviors of supported amino acid ionic liquid [EMIM]AA
sorbents on poly(methyl methacrylate) (PMMA) under a humidified gas flow condition (2.3% CO2 ,
2.1% H2 O and balance N2 ), and the results showed that CO2 sorption capacities of [EMIM][Gly]- and
[EMIM][Lys]-PMMA decreased in the presence of water vapor as the adsorbed water inhibited their
reaction with CO2 .
Meanwhile, the water vapor adsorption performance of the pores adsorbent, especially silica gel,
has been investigated extensively. Alcañiz-Monge [24] reported that water adsorption at low relative
pressures occurred mainly in the micro-porosity of silica. Saliba [25] also showed that the adsorption
of water vapor on porous silica depended upon the concentration of surface hydroxyl groups (silanol)
and the porosity. At present, it has been reported that the water vapor adsorption capacity was affected
by the pore size and surface hydroxyl groups, but little research has been done to evaluate the surface
properties and pore structure of silica gel modified by ILs and different pretreated methods for the
supporter, which shows further influence of water vapor adsorption and its water vapor adsorption
performance. Thus, we considered surface-modifying and regulating the pore size of silica to be an
effective way of controlling the amount of adsorbed water vapor in competitive adsorption.
The water hold-up needs to be addressed in adsorption processes under humid conditions, as this
can reduce the adsorption capacity for CO2 . Based on this starting point, in this work, phosphonium
ILs, especially hydrophobic ionic liquids, were chosen to be supported on pretreated silica to prevent
the water vapor entering effectively. According to the literature [18–23], the surface performance and
porosity of porous materials are main effects for competitive adsorption of H2 O and CO2 . We assume
that pore structure and surface hydroxyl content of silica gels functionalized with ILs will be changed,
which can effectively play the advantages to reduce water vapor sorption capacity of porous materials,
aiming to prepare a new adsorbent with better solid supported ILs which achieve better water-resistance
performance for being more suitable for post-combustion CO2 capture applications. Simultaneously,
the diffusion of water vapor in IL-modified silica was investigated in order to characterize the influence
of the carrier surface and porosity.
2. Materials and Methods
2.1. Materials
Tri-n-octylphosphine (purity: 97%) was purchased from Stream Chemicals, Inc. (Cambridge, UK).
Bistrifluoromethanesulfonimide lithium (purity: 99%) was purchased from J&K Scientific (Beijing, China).
3-Chloropropyltriethoxysilane (purity: 97%) was purchased from Beijing Fengtuo Chem Co., Ltd., Beijing,
China. Toluene (dried by Na), acetone, and HCl were all analytical grade. All chemicals were purchased
from a commercial supplier and used as received. Silica gel with 0.5–1.5 mm in particle size was purchased
from Shanghai Silica Gel Factory, Shanghai, China. Silica was pretreated before use.
2.2. Synthesis of grafted silica gel
The pretreated methods of silica affect the pore structure and hydroxyls of the silica surface,
thus influencing water vapor sorption capacity. In this study, two different treatement methods were
used. Silica was calcined at 150 ◦ C for 3.5 h (coded as SiO2 -C) or acid treated with hydrochloric acid
(8 mol/g) at 70 ◦ C for 12 h (coded as SiO2 -H). It was then washed with distilled water until neural, and
dried at 353 K under vacuum for 8 h before further use. The pretreated silica is referred to as bare silica
in the following text.Materials 2019, 12, 1782 3 of 13
Materials 2019, 12, x FOR PEER REVIEW 3 of 14
The preparation of phosphonium IL-modified silica using grafted method was carried out,
accordingThe to apreparation of phosphonium
previous report [26]. In thisIL-modified silica using
work, another kindgrafted method
of silica with was carried
different out,
pretreated
according to a previous report [26]. In this work, another kind of silica with different pretreated
methods was selected for further controlled amounts of IL. The general procedure is as follows.
methods was selected for further controlled amounts of IL. The general procedure is as follows. Tri-
Tri-n-octylphosphine was added to 3-chloropropyltriethoxysilane and anhydrous toluene and stirred
n-octylphosphine was added to 3-chloropropyltriethoxysilane and anhydrous toluene and stirred
underunder
nitrogennitrogen 110at◦ C
gas atgas 110for°C40for
h. 40
Phosphonium
h. Phosphoniumchloride was was
chloride thenthen
stirred withwith
stirred an aqueous
an aqueoussolution
of bistrifluoromethanesulfonimide lithium atlithium
solution of bistrifluoromethanesulfonimide room at temperature
room temperaturefor 30 for
h. 30
The h. resulting
The resultingcrude IL was
crude
extracted
IL was extracted from the aqueous phase using dichloromethane. The dichloromethane phase was then
from the aqueous phase using dichloromethane. The dichloromethane phase was
washedthen with distilled
washed withwater forwater
distilled severalfortimes
severaluntil
timesnountil
residual chloride
no residual salt was
chloride saltdetected with the
was detected withuse of
AgNO the
3 . use
TheofILAgNO 3. The in
was dried IL vacuum
was driedatin333 vacuum
K for at 333The
8 h. K for 8 h. The pretreated
pretreated silica was silica
added was to added to
the prepared
the prepared
grafting agent in grafting
anhydrous agent in anhydrous
toluene under toluene under
a nitrogen a nitrogen atmosphere
atmosphere and stirredand stirred
at 90 at 90
◦ C for 24°C
h. The
for 24 h. The solid was collected and washed with acetonitrile and distilled
solid was collected and washed with acetonitrile and distilled water and then dried at 80 C overnightwater and then ◦dried at
80 °C overnight under vacuum to obtain an IL-modified silica. The product prepared by calcined
under vacuum to obtain an IL-modified silica. The product prepared by calcined silica was labeled as
silica was labeled as Si-P8TFSI/SiO2-C, when the molar ration of silica and IL is 1:1. The product
Si-P8 TFSI/SiO2 -C, when the molar ration of silica and IL is 1:1. The product prepared by acid-treatment
prepared by acid-treatment silica was coded as Si-P8TFSI/SiO2-H(I) and Si-P8TFSI/SiO2-H(II),
silicarespectively,
was codeddepending
as Si-P8 TFSI/SiO 2 -H(I) and
on the different molarSi-P 8 TFSI/SiO
ratio 2 -H(II),
of silica and respectively,
the grafting depending
agent (1:0.5 on the
and 1:1), as
different molar ratio of
illustrated in Figure 1. silica and the grafting agent (1:0.5 and 1:1), as illustrated in Figure 1.
Figure 1. Illustration
Figure of of
1. Illustration synthesizing
synthesizing(EtO)
(EtO)33Si[P
Si[P8883
8883 ]TFSI
]TFSI ionic
ionic liquid-modified
liquid-modified silica.silica.
2.3. Characterization
2.3. Characterization
2.3.1.2.3.1.
Chemical Analysis
Chemical Analysis
Fourier
Fourier transform
transform infrared
infrared (FTIR)spectra
(FTIR) spectra were
were recorded
recordedonona aThermoThermo Scientific Nicolet
Scientific 380 IR
Nicolet 380 IR
Spectrometer
Spectrometer (Thermo
(Thermo Scientific,
Scientific, Massachusetts, USA),
Massachusetts, USA), and
andthe thesamples,
samples, in in
powder
powderform, along
form, with with
along
KBr, were
KBr, were compressed
compressed intointo tablets.
tablets. TheThe spectra
spectra (32(32 scansfor
scans forthe
thepure
pureionic
ionic liquid
liquid and
and 128
128scans
scansforfor the
the IL-modified silica) were collected with a spectral resolution of 4 cm −1 −1.
IL-modified silica) were collected with a spectral resolution of 4 cm .
An x-ray photoelectron spectroscopy measurement was carried out on an X-Ray Photoelectron
An x-ray photoelectron spectroscopy measurement was carried out on an X-Ray Photoelectron
Spectrometer ESCALAB 250Xi (Thermo Fisher Co, Massachusetts, USA). All binding energies were
Spectrometer
referencedESCALAB 250Xi
to the neutral (Thermo
C1 speak Fisher
at 284.8 eV toCo., Franklin,
compensate forMA, USA). All binding
the surface-charging energies were
effects.
referencedThermogravimetric
to the neutral C1analysis
speak at(TGA)284.8waseV to compensate for the surface-charging
performed at a heating rate of 10 °C/min in the range effects.
Thermogravimetric analysis (TGA) was performed at a heatingusingrate of ◦ C/min in the range from
from room temperature to 700 °C under nitrogen atmosphere, a 10
STA409C DTA/DSC-TG
room(Netzsch,Selb,
temperature to ◦
700 C under
Germany). The nitrogen
hydroxylatmosphere,
group contentsusingwerea STA409C
estimated DTA/DSC-TG
by assuming(Netzsch,
that the Selb,
condensation
Germany). occurredgroup
The hydroxyl based on the reaction
contents wereof two hydroxyl
estimated groups onthat
by assuming the silica surface and can
the condensation be
occurred
calculated as follows in Equation (1) [27]. Moreover, according to the thermogravimetric
based on the reaction of two hydroxyl groups on the silica surface and can be calculated as follows in curve, the
total (1)
Equation IL load
[27].on the surfaceaccording
Moreover, of silica geltoisthe
determined by using Equation
thermogravimetric curve, the (2). total IL load on the surface
of silica gel is determined by using Equation (2).2[W(T0 )-W(Tfinal )]
nOH (SiO2 ) = (1)
100MH2O
2[W(T0 ) − W(Tfinal )]
nOH (SiO2 ) = (1)
100M
W(T0 )-W(T H)2 O
final
GR(%) = ×100 (2)
W(Tfinal )
W(T0 ) − W(Tfinal )
Where n (SiO ) indicates the GR (%)of=hydroxyl groups on the×silica
moles 100 particles W(T ) and W(T ) (2)
W(Tfinal )
are the weight of the silica particles (g) at the temperatures 200 °C and 700 °C, respectively, MH2O is
wherethe
nOH (SiO2 ) weight
molecular indicates the moles
of water and GR of (wt%)
hydroxyl
is thegroups onofthe
total load ILssilica
on theparticles (T0 ) gel.
Wsilica
surface of and W(Tfinal )
are the weight of the silica particles (g) at the temperatures 200 ◦ C and 700 ◦ C, respectively, MH2O is
the molecular weight of water and GR (wt%) is the total load of ILs on the surface of silica gel.Materials 2019, 12, x FOR PEER REVIEW 4 of 14
Materials 2019, 12, 1782 4 of 13
2.3.2. Pore Structure Characteristics
2.3.2.
ThePore Structure
adsorption Characteristics
isotherm of N2 at −196 °C and CO2 at 0 °C were measured by an Autosorb-1-MP
(Quantachrome,
The adsorption isotherm ofan
Florida, USA), N2automatic
at −196 ◦ Csurface
and COarea and
◦ pore size analyzer. The porous
2 at 0 C were measured by an Autosorb-1-MP
particles were driedFlorida,
(Quantachrome, and degassed
USA), an atautomatic
100 °C under higharea
surface vacuum
and porefor 5size
h toanalyzer.
remove moisture
The porous or particles
other
volatile contaminants. As for N
were dried and degassed at 100 C under high vacuum for 5 h to remove moisture or other 0.001
2 adsorption,
◦ the relative pressure (P/P 0 ) range was set between volatile
and 0.995, and theAsCO
contaminants. for2 adsorption isotherms
N2 adsorption, were pressure
the relative collected(P/P
within the relative pressure (P/P0) range
0 ) range was set between 0.001 and 0.995,
of and
0.0001~0.03
the CO2 for restricting
adsorption pores narrower
isotherms than within
were collected 2 nm. A thedensity
relativefunctional theory
pressure (P/P (DFT) analysis
0 ) range of 0.0001~0.03
wasfor restricting pores narrower than 2 nm. A density functional theory (DFT) analysis wasusing
used to calculate the pore size distribution (PSD) and the pore structure parameters used to
Quantachrome ASiQwin
calculate the pore software (Quantachrome,
size distribution (PSD) and the poreFlorida,
structureUSA). based on
parameters usingtheQuantachrome
adsorption
isotherms.
ASiQwin software (Quantachrome, Florida, USA). based on the adsorption isotherms.
2.4.2.4.
Adsorption Studies
Adsorption Studies
Adsorption
Adsorptionand anddiffusivity watervapor
diffusivity of water vapor were
were measured
measured on anonIntelligent
an Intelligent Gravimetric
Gravimetric Analyzer
Analyzer (IGA-003, Hiden Isochema Ltd., Manchester, UK). Detailed information on the
(IGA-003, Hiden Isochema Ltd., Manchester, UK). Detailed information on the apparatus and analyzing apparatus
and analyzing
methods methods
have have beenreported
been previously previously reported
[28,29]. Water[28,29].
vaporWater
uptakevapor uptake was
was measured in measured in
the Analyzer’s
theHumidifier
Analyzer’sconfiguration,
Humidifier configuration,
using similarusing similar
methods at 25 ◦ C. Theat
methods 25 °C. The
samples weresamples
dried bywere dried bydry
circulating
circulating
nitrogen dry
untilnitrogen until
a constant a constant
weight weight was
was obtained. obtained.
Vapor pressureVapor
was pressure was then
then controlled by controlled
circulating a
by mixture
circulating a mixture
of dry of dry
nitrogen andnitrogen and vapor-saturated
vapor-saturated nitrogen
nitrogen to reach the to reach the
required requiredAt
humidity. humidity.
a required
At humidity,
a requiredthe
humidity,
mass gainthewasmass gain was
measured as ameasured
function ofastime
a function of timeconstant.
until reaching until reaching constant.
More details have
More
beendetails haveinbeen
reported reported
references in references [30,31].
[30,31].
3. Results
3. Results andand Discussion
Discussion
3.1.3.1. Characterization
Characterization of the
of the Samples
Samples
3.1.1. FTIR Analysis
3.1.1. FTIR Analysis
The FTIR spectra of bare and grafted silica are shown in Figure 2. The typical peaks for SiO2 -H
The FTIR spectra of bare and grafted
−1 for silica are shown in Figure 2. The typical peaks for SiO2-H
and SiO2 -C were found at 1098 cm asymmetric stretching vibration and 800 cm−1 for symmetric
and SiO2-C were found at 1098 cm-1 for asymmetric stretching vibration and 800 cm-1 for symmetric
stretching vibration of Si-O-Si. The vibration bands of OH groups are located at different frequencies,
stretching vibration of Si-O-Si. The vibration bands of OH groups are located at different frequencies,
which depend on the bonding configuration. The broad band between 3800 cm-1−1 and 3050 cm −1 is
which depend on the bonding configuration. The broad band between 3800 cm and 3050 cm-1 is
attributed to the stretching of silanol associated with surface silanols, including isolated silanol, vicinal,
attributed to the stretching of silanol associated with surface silanols, including isolated silanol,
and geminal groups [32]. The shoulder at 956 cm−1 indicates the existence of the Si-OH asymmetric
vicinal, and geminal groups [32]. The shoulder at 956 cm-1 indicates the existence of the Si-OH
stretching band [33]. Based on the comparison of the intensity of the characteristic peaks of Si-OH in
asymmetric stretching band [33]. Based on the comparison of the intensity of the characteristic peaks
Figure 2a,b, Figure 2 reviews that hydroxyl content on the surface of SiO2 -H treated by hydrochloric
of Si-OH in Figure 2a,b, Figure 2 reviews that hydroxyl content on the surface of SiO2-H treated by
acid and SiO2 -C treated by calcined are a little different. The hydroxyl content was calculated in the
hydrochloric acid and SiO2-C treated by calcined are a little different. The hydroxyl content was
scetion of thermogravimetry analysis.
calculated in the scetion of thermogravimetry analysis.
(a)
(b)
%Transmittance
(c)
(d)
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber(cm-1)
Figure
Figure 2. FTIR
2. FTIR spectra
spectra of SiO
of SiO -H (a), SiO -C (b), Si-P8 TFSI/SiO
2-H2 (a), SiO2-C2(b), Si-P8TFSI/SiO 2 -H(I)
2-H(I) (c), (c),
andand
neatneat ionic
ionic liquid
liquid (IL)(IL)
(d).(d).
As shown in Figure 2, the infrared spectrum of Si-P8 TFSI/SiO2 -H(I) shows the appearance of the
As shown in Figure 2, the infrared spectrum of Si-P8TFSI/SiO2-H(І) shows the appearance of the
bands at 2924 cm−1 and 2856 cm−1 due to C-H aliphatic stretching vibration, and the peak at 1465
bands at 2924 cm-1 and 2856 cm-1 due to C-H aliphatic stretching vibration, and the peak at 1465 cm-1Materials 2019, 12, x FOR PEER REVIEW 5 of 14
Materials 2019, 12, 1782 5 of 13
belongs to P-C stretching vibration. Figure 2 appears to show that broad peak characteristic bands
cm−1 belongs
between 1024~1246
to P-Ccm-1, due to Si-O-Si and S=O, and the peak at 664 cm-1 belongs to S-C stretching
stretching vibration. Figure 2 appears to show that broad peak characteristic
vibration. The success
bands between of the
1024~1246 cm −1 , due
neat phosphonium IL grafted
to Si-O-Si and ontothe
S=O, and thepeak
silicaatsurface −1 belongs
664 cmhas been assessed
to S-C
based on the appearance of the characteristic bands for neat IL at 2926, 2855, 1466, and
stretching vibration. The success of the neat phosphonium IL grafted onto the silica surface has1227~1060,663
cm −1.
been assessed based on the appearance of the characteristic bands for neat IL at 2926, 2855, 1466,
and 1227~1060,663 cm−1 .
3.1.2. XPS Analysis
3.1.2. XPS Analysis
The XPS spectrum of SiO2-H, neat [Si-P3888][TFSI], and Si-P8TFSI/ SiO2-H(І) is shown in Figure 3.
The XPS
The peaks spectrum
of C 1s, O 1s,ofand SiOSi2 -H,
2p neat [Si-P
appear 3888 ][TFSI],
only and Si-P
in spectrum 8 TFSI/
(a) of SiO2 silica.
the bare -H(I) isItshown
can bein Figure
seen from3.
The peaks
Figure of Ca1s,
3b that O 1s, and
binding energy Si 2pof appear
168.4 eV,only in spectrum
397.4 eV, and 688.9(a) ofeV
the
is bare silica. to
attributed It Scan
2p,beNseen from
1s, and F
Figure
1s 3b neat
for the that aIL,binding energyinof[TFSI]
respectively, 168.4 anion
eV, 397.4 eV,
[34]. Inand 688.9 eV
addition, is attributed
there is a P 2p peakto S 2p,
withNa1s, and F
binding
1s for the
energy of neat
131.0IL,
eV,respectively,
which further in [TFSI]
provesanion [34]. In addition,
the successful synthesisthere is aionic
of the P 2pliquid
peak [Si-P
with3888
a binding
][TFSI].
energy of 131.0
Compared eV, which
to spectrum (a),further
the C proves
1s peaktheinsuccessful
spectrum synthesis of the ionic
(c) is significantly liquid [Si-P
enhanced, and3888two][TFSI].
new
Compared
peaks to spectrum
attributable (a),(binding
to F 1s the C 1senergy
peak in689.0
spectrum
eV) and(c) P
is 2p
significantly enhanced,
(binding energy 131.1andeV)two new in
appear peaks
the
attributable to F 1s (binding energy 689.0 eV) and P 2p (binding energy 131.1
XPS spectrum. This shows that the ionic liquid has been successfully immobilized on the surface of eV) appear in the XPS
spectrum. This shows that the ionic liquid has been successfully immobilized on the surface of silica.
silica.
O1s
Counts/s
(a) Si2s Si2p
C1s
F1s O1s C1s P2p
(b) N1s S2p Si2p
O1s
P2p
C1s
(c) F1s
Si2s Si2p
1200 1000 800 600 400 200 0
Binding Energy(ev)
Figure 3. Full-range X-ray photoelectron spectroscopy spectra of SiO -H (a), neat IL (b), and Si-P TFSI/
Figure 3. Full-range X-ray photoelectron spectroscopy spectra of SiO22-H (a), neat IL (b), and Si-P88TFSI/
SiO -H(I) (c).
SiO22-H(І) (c).
3.1.3. Thermogravimetric Analysis
3.1.3. Thermogravimetric Analysis
Thermal gravimetric analysis (TGA) is performed to determine the quantitative information of
Thermalofgravimetric
the contents silanol andanalysis (TGA)
ionic liquids onisthe
performed
surface oftothe
determine the quantitative
porous silica information
products. TGA patterns of
the contents of silanol and ionic liquids on the surface of the porous silica products. TGA
bare silica and modified silicas are depicted in Figure 4. It is found that there are two distinct mass-loss patterns of
bare
stepssilica
in theand modified silicas are
thermogravimetric curvesdepicted in Figureespecially
of all samples, 4. It is found
the that
abrupt there are two
change distinct
of the mass-
first step at
loss steps in the
temperatures belowthermogravimetric
200 ◦ C for SiO2 -Hcurves
and SiO of 2all
-C.samples, especially
The removal the abrupt change
of the physiosorbed surfaceofadsorbed
the first
step
wateratand
temperatures
gas are lostbelow
before200
200°C◦ C.
forThe
SiOcondensation
2-H and SiO2-C. of The removal of
the hydroxyl the physiosorbed
group and the loaded surface
ionic
Materials 2019, 12, x FOR PEER REVIEW 6 of 14
adsorbed
liquid on thewater and
silica gas are
surface arelost
lost before 200 °C. The ranging
at the temperature condensation
from 200of the
◦ C tohydroxyl
700 C. group and the
◦
loaded ionic liquid on the silica surface are lost at the temperature ranging from 200 °C to 700 °C.
100
(e)
96
Weight retention(%)
(d)
92 (c)
(a)
88
(b)
84
80
76
0 100 200 300 400 500 600 700
Temperature(°C)
Figure4.4. Thermogravimetric
Figure curvesofof
Thermogravimetric curves SiO
SiO2-H2 -H
(a),(a),
SiO2SiO 2 -C Si-P
-C (b), (b),8TFSI/SiO
Si-P8 TFSI/SiO 2 -H(I)
2-H(I) (c), Si-P(c), Si-P8 TFSI/
8TFSI/SiO 2-
SiO -H(II) (d), and Si-P
2 (d), and Si-P8TFSI/SiO
H(II) 8 TFSI/SiO
2-C (e).
2 -C (e).
Silanol group content for silica and the total load of phosphonium ILs on the surface for modified
samples determined by TGA techniques are listed in Table 1. The condensation of all kinds of silanol
groups occurs by the reaction of two silanol groups on the silica surface, resulting in the release of
one molecule of water and the formation of one siloxane group. It is assumed that no other groupsMaterials 2019, 12, 1782 6 of 13
Silanol group content for silica and the total load of phosphonium ILs on the surface for modified
samples determined by TGA techniques are listed in Table 1. The condensation of all kinds of silanol
groups occurs by the reaction of two silanol groups on the silica surface, resulting in the release of one
molecule of water and the formation of one siloxane group. It is assumed that no other groups than
water were released from the sample. As shown in Table 1, for SiO2 -H and SiO2 -C, TGA shows that
the hydroxyl content of SiO2 -C is significantly higher than that of SiO2 -H, which is probably because
the H+ in the hydrochloric acid activating solution destroys the hydroxyl groups on the surface of
the silica gel [35]. Moreover, IL loading calculated for the Si-P8 TFSI/SiO2 -H(I), Si-P8 TFSI/SiO2 -H(II),
and Si-P8 TFSI/SiO2 -C was 4.92 wt%, 10.62 wt%, and 10.79 wt%, respectively, whereby the ionic liquid
loading depends on the limited number of active silanol groups on the supporter surface.
Table 1. Silanol group content for bare silica and the total load of phosphonium ILs onto silica surface
determined by TGA techniques.
Sample OH Group Content(mmol/g) Amount of Grafting (wt%)
SiO2 -H 3.867 -
SiO2 -C 5.479 -
Si-P8 TFSI/SiO2 -H(I) - 4.92
Si-P8 TFSI/SiO2 -H(II) - 10.62
Si-P8 TFSI/SiO2 -C - 10.79
3.1.4. Pore size distribution
There is a certain correlation between pore structure and water adsorption. The use of both
adsorbates, N2 and CO2 , provides complementary information about the porous texture of the
samples. Isotherms measured with N2 often lead to an underestimation of the microporosity due to
the lower mobility of N2 at −196 ◦ C and whose diffusion in small size pores is inhibited. To access the
microporosity, isotherms can be measured under CO2 at 0 ◦ C, since the CO2 molecules are more mobile.
Based on the N2 and CO2 adsorption isotherms at −196 ◦ C and 0 ◦ C, respectively, CO2 adsorption
capacity at 0 ◦ C and the data to evaluate their surface areas and pore structures of the samples were
obtained using the DFT method [36] in Table 2. The DFT model is based on the molecular statistical
thermodynamics equation that calculates the specific adsorption amount in an individual pore of a
given adsorbate–adsorbent system at a given experimental temperature and pressure by solving the
function of the grand thermodynamic potential in terms of the distribution of gas density in a specific
pore space.
Table 2. Pore structure parameters of samples based on N2 sorption isotherms at −196 ◦ C and CO2
sorption isotherms at 0 ◦ C.
N2 Sorption Isotherms CO2 Sorption Isotherms V (CO2 )
Sample
(wt%)
CPV × 10−1 (cm3 ·g−1 ) CSA (m2 ·g−1 ) CPV × 10−1 (cm3 ·g−1 ) CSA (m2 ·g−1 )
SiO2 -H 4.001 525.3 1.058 306.8 5.51
SiO2 -C 4.149 635.6 1.241 362.3 6.47
Si-P8 TFSI/SiO2- H(I) 3.303 425.8 0.860 263.2 4.78
Si-P8 TFSI/SiO2 -H(II) 3.263 373.1 0.801 238.7 4.27
Si-P8 TFSI/SiO2 -C 3.347 455.4 0.980 283.9 4.96
The cumulative pore volume (CPV) and cumulative surface area (CSA) of SiO2 -C shown in Table 2
are considerably higher than those of SiO2 -H, and the surface modified silica presents a significantly
lower pore volume and specific surface area as compared to the bare silica, which leads to a decrease
in the adsorption of CO2 . On the basis of structural characteristics, silica impregnated with ILs results
in the blockage of pores. Not surprisingly, Si-P8 TFSI/SiO2 -C prepared by grafting ILs on calcined
supporter has a higher pore volume and specific surface area than Si-P8 TFSI/SiO2 -H(II).higher IL loading and less micropores should exhibit lower water adsorption capacity, which will be
discussed later.
3.2. Water Vapor Adsorption
Materials 2019, 12, 1782 7 of 13
The mechanism of water adsorption on silica is very complex. It is known that the surface of
silica contains silanol or siloxane groups. The Si-OH shows a specific interaction with water
The pore size
(hydrophilic), anddistribution based
it is a typical on N
polar 2 sorption
group and isotherms
has some at −196 ◦properties
similar C and CO2 withsorption isotherms
water, so the
at 0 ◦ C are shown in Figure 5. According to the pore size distribution (PSD) of the micropore in
hydrogen bonds were formed by the reaction of water and the hydroxyl group [37,38]. It is reasonable
Figure
to think5b, thewater
that PSD of samples
vapor mainly concentrated
adsorption, depending oninthe theamount
range ofof0.4–1.0 nm. Thegroup
the hydroxyl proportion
on theof the
silica
micropores
surface and for theSi-P TFSI/SiO
adsorption
8 -H(II)
capacity,
2 is lower
increasedthan that
with theof Si-P TFSI/SiO
hydroxyl
8 content.
2 -H(I) and
Water Si-P
vapor
8 TFSI/SiO 2 -C,
adsorption
which is mainly due to the clogging of a part of the pores after the introduction of
isotherms, corresponding to all samples, are illustrated in Figure 6. SiO2-H has better water resistancethe ionic liquid.
Combined
than SiO2-C. with
SiOIL
2-Hloading
displaysobtained
a lowerby TGA data,
quantity it hydroxyl
of the is expected that the
group, Si-P8 TFSI/SiO2by
as demonstrated -H(II) with
the FTIR
higher
and TGA IL loading and less to
data. According micropores should exhibit
the pore structure lower water
parameters, SiO2-Hadsorption
also presents capacity,
lower which
CSA and willless
be
discussed
micropores, later.
which is unfavorable for the adsorption of water vapor.
0.03
(a) (b) SiO2-C
SiO2-C 0.10
SiO2-H SiO2-H
Si-P8TFSI/SiO2-H(Ⅰ
)
Si-P8TFSI/SiO2-H(Ⅰ) 0.08
0.02 Si-P8TFSI/SiO2-H(Ⅱ)
Si-P8TFSI/SiO2-H(Ⅱ)
Si-P8TFSI/SiO2-C
dvp/drp
dvp/drp
Si-P8TFSI/SiO2-C 0.06
0.04
0.01
0.02
0.00 0.00
1 10 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
rp(nm) rp(nm)
Figure 5. Pore size distribution of silica and modified silica based on: (a) N2 adsorption isotherms at
Figure 5. Pore size distribution of silica and modified silica based on: (a) N2 adsorption isotherms at
−196 ◦ C and (b) CO2 adsorption isotherms at 0 ◦ C.
−196 °C and (b) CO2 adsorption isotherms at 0 °C.
3.2. Water Vapor Adsorption
The mechanism of water adsorption on silica is very complex. It is known that the surface of silica
contains silanol or siloxane groups. The Si-OH shows a specific interaction with water (hydrophilic),
and it is a typical polar group and has some similar properties with water, so the hydrogen bonds
were formed by the reaction of water and the hydroxyl group [37,38]. It is reasonable to think that
water vapor adsorption, depending on the amount of the hydroxyl group on the silica surface and
the adsorption capacity, increased with the hydroxyl content. Water vapor adsorption isotherms,
corresponding to all samples, are illustrated in Figure 6. SiO2 -H has better water resistance than SiO2 -C.
SiO2 -H displays a lower quantity of the hydroxyl group, as demonstrated by the FTIR and TGA data.
According to the pore structure parameters, SiO2 -H also presents lower CSA and less micropores,
Materials
which is 2019, 12, x FOR PEER
unfavorable for REVIEW
the adsorption of water vapor. 8 of 14
30
SiO2-C
25 SiO2-H
Amount adsorbed (wt%)
Si-P8TFSI/SiO2-H(I)
20 Si-P8TFSI/SiO2-C
Si-P8TFSI/SiO2-H(II)
15
10
5
0
0 5 10 15 20 25 30 35
Pressure(mbar)
Watervapor
Figure6.6.Water vaporadsorption
adsorptionisotherms
isothermsat
at25 ◦ C on silica and modified silica.
25°C
Figure on silica and modified silica.
The results in Figure 6 also indicate that water adsorption of silica modified by ionic liquid reduces
The results in Figure 6 also indicate that water adsorption of silica modified by ionic liquid
significantly, and Si-P8 TFSI/SiO2 -H(II) with relatively higher IL content and a lower microporous
reduces significantly, and Si-P8TFSI/SiO2-H(II) with relatively higher IL content and a lower
structure shows better water resistance. Due to grafting the hydrophobic ionic liquid onto the silica
microporous structure shows better water resistance. Due to grafting the hydrophobic ionic liquid
onto the silica gel, there is an ionic liquid film formed by reacting with the silanol groups on the silica
surface, and the schematic is shown in Figure 7. According to the previous PSD data, the CPV,
especially of the micropores, and CSA of Si-P8TFSI/SiO2-H(II) are lower, resulting in a slight decrease
in water adsorption capacity.
The pore structure parameters and surface property show a correlation with water vaporwill be coadsorbed with CO2 at the high relative humidities than can be expected in post-combustion
applications.
3.3. Water Diffusion Coefficient
The2019,
Materials diffusion
12, 1782 coefficient
calculation presented a great significance toward the evaluation of the
8 of 13
industrial application value of the adsorbent. The diffusion in porous solids is more complicated than
in liquids. For bare silica and silica-phosphonium ionic liquid hybrid material prepared with the
gel, theremethod,
grafting is an ionic liquid film
the particle formed byasreacting
is considered a uniformwith the silanol
adsorbent. groups of
Diffusion onwater
the silica
vaporsurface,
into a
and the schematic is shown in Figure 7. According to the previous
porous spherical particle can be described by Equation (3) [39]. PSD data, the CPV, especially of
the micropores, and CSA of Si-P8 TFSI/SiO2 -H(II) are lower, resulting in a slight decrease in water
2
adsorption capacity. (1 − ) + = ( + ) (3)
Figure 7.
Figure Schematic diagram
7. Schematic diagram of
of the
the ionic
ionic liquid
liquid membrane
membrane on
on the
the surface
surface of
of the
the carrier.
carrier.
The pore structure parameters and surface property show a correlation with water vapor
sorption capacity, suggesting that porosity and the IL film play an important role in reducing a
small amount of water vaporin adsorption of the power plant flue gas for the bare and modified
SiO2 , thereby resulting in decreased CO2 adsorption capacity. The maximum adsorption capacity
of water vapor on silica-phosphonium ionic liquid hybrid material is lower than that for bare silica,
with a higher pore volume and quantity of the hydroxyl group, which is advantageous, as less
water vapor will be coadsorbed with CO2 at the high relative humidities than can be expected in
post-combustion applications.
3.3. Water Diffusion Coefficient
The diffusion coefficient calculation presented a great significance toward the evaluation of the
industrial application value of the adsorbent. The diffusion in porous solids is more complicated than
in liquids. For bare silica and silica-phosphonium ionic liquid hybrid material prepared with the
grafting method, the particle is considered as a uniform adsorbent. Diffusion of water vapor into a
porous spherical particle can be described by Equation (3) [39].
∂q ∂C ∂2 C 2 ∂C
(1 − ε) +ε = εDe ( 2 + ) (3)
∂t ∂t ∂R R ∂R
where De is the effective porous diffusion coefficient (m2 /s), ε is the material porosity, q is the adsorbate
concentration in the solid phase (wt%), C is the adsorbate concentration in the gas phase (wt%), R is
the particle of radius (m), and t is the adsorption time(s).
Initial conditions (IC) and boundary conditions (BC):
C(Rp , 0) = C0 q(RP , 0) = q0 (4)
C(Rp , ∞) = C∞ q(RP , ∞) = q∞ (5)
∂C ∂q
= =0
∂R R=0 ∂R R=0
The solution of the equation is given by Equation (6) [40].
∞
m q−q0 6 P 1 −n2 π2 (εDe /(ε+(1−ε)K))t
m∞ = q∞ −q0 = 1− π2 n2
exp(
R2p
)
n=1 (6)
∞ −n2 π2 Dap t
= 1 − π62 1
P
n2
exp( )
R2p
n=1Materials 2019, 12, 1782 9 of 13
where
Dap = εDe /[ε + (1 − ε)K] (7)
Dap is the apparent diffusion coefficient, q is the quantity adsorbed at time t, and K is the slope of
adsorption isotherm.
A simplified equation for the diffusion is given by Equation (8):
r
m 2A Dap t √
≈ =S t (8)
m∞ V π
where A is the particle surface area (m2 ) and V is the particle volume (m3 ). In case of spherical particles:
2A/V = 3/R.
From equation (8) it can be obtained that:
s
6 Dap
S= √ (9)
π R2p
where S is the slope of the curve m/m∞ versus t0.5 . Further, from Equation (9), Dap can be obtained as:
π 2 2
Dap = R S (10)
36 p
Finally, the effective diffusion coefficient (De ) can be calculated with the relationship:
ε + (1 − ε)K
De = Dap (11)
ε
where,
Vp × ρsi
ε= (12)
1 + Vp × ρsi
Vp is the pore volume and ρsi is the apparent density.
Examples of the experimental data and fitting curves for SiO2 -H, SiO2 -C, and Si-P8 TFSI/SiO2 -H(I)
at different pressures and at 25 ◦ C are illustrated in Figure 8. A good agreement between the
experimental values and the fitting curves was obtained.
Diffusion coefficients obtained from the curve fitting are listed in Table 3. It can be obtained from
Table 3 that the calculated effect diffusion coefficients of silica gel are at a level of 10−7 and these values
are close to reports on silica [41–43]. Comparing two kinds of bare silica, the diffusion coefficient of
SiO2 -C is lower than that of SiO2 -H. The reason may be that the diffusion resistance of water vapor is
increased by an excessive amount of water adsorbed by SiO2 -C with a higher hydroxyl group, CPV,
and CSA. Moreover, diffusion coefficients in Si-P8 TFSI/SiO2 -H(I) are about 10−8 ~10−7 m2 /s and slightly
lower than those of SiO2 -H, which implies that the thin IL coating on the silica surface and a loss of
micro-porosity impedes water diffusion.
Figure 9 shows the relationship between water vapor uptake and De . Effective diffusion coefficients
of all the samples show a decreasing trend with the increase of water vapor adsorption, which, due to
the porosity available for water vapor transport, decreases at higher loading as adsorbed water vapor
occupies a part of porous sites.Materials 2019, 12, 1782 10 of 13
Table 3. The diffusion coefficient of water vapor sorption in samples at 25 ◦ C.
De × 10−7 (m2 /s) De × 10−7 (m2 /s) De × 10−7 (m2 /s)
Pressure (mbar)
SiO2 -C SiO2 -H Si-P8 TFSI/SiO2 vH(I)
5 PEER REVIEW
Materials 2019, 12, x FOR 4.76 5.75 4.41 10 of 14
8 4.25 5.62 4.32
10
Diffusion coefficients 4.03 the curve fitting5.46
obtained from are listed in Table 3. It4.11
can be obtained from
12 3.89 5.37 4.36-7
Table 3 that the calculated
15 effect diffusion
3.12 coefficients of silica
4.37 gel are at a level of3.82
10 and these values
are close to reports
18 on silica [41–43].2.38
Comparing two kinds 3.61 of bare silica, the diffusion
2.67 coefficient of
20 that of SiO2-H. The
SiO2-C is lower than 2.41 reason may be that
3.82the diffusion resistance
2.43 of water vapor is
22
increased by an excessive amount of1.94
water adsorbed by 3.51 2.71
SiO2-C with a higher hydroxyl group, CPV,
25 1.54 3.42 2.55
and CSA. Moreover,28
diffusion coefficients
1.44
in Si-P 8TFSI/SiO2-H(I) are about 10-8~10-7 m2/s and slightly
2.64 2.98
lower than those30 of SiO2-H, which implies
1.38 that the thin IL
2.13coating on the silica1.84
surface and a loss of
micro-porosity impedes water diffusion.
5.9 (a) 13.1 (b)
Water vapour uptake(wt%)
Water vapour uptake(wt%)
5.8 13.0
12.9
5.7
P=22mbar
P=8mbar 12.8
5.6 Experimantal data
Experimantal data Theoretical curve
12.7
5.5 Theoretical curve
12.6
5.4
12.5
550 555 560 565 570 575 580 585 1645 1650 1655 1660 1665 1670 1675 1680
t（min） t(min)
8.0 (c) (d)
19.2
Water vapour uptake(wt%)
Water vapour uptake(wt%)
7.9 19.0
7.8 P=8mbar 18.8 P=22mbar
Experimantal data Experimantal data
7.7 Theoretical curve 18.6 Theoretical curve
7.6
18.4
7.5
18.2
130 135 140 145 150 155 160 165 640 645 650 655 660 665 670 675
t(min) t(min)
(e) (f)
3.45 8.65
Water vapour uptake(wt%)
Water vapour uptake(wt%)
3.40 8.60
3.35 P=8mbar 8.55 P=22mbar
Experimantal data Experimantal data
3.30 Theoretical curve Theoretical curve
8.50
3.25 8.45
370 375 380 385 390 395 400 1480 1485 1490 1495 1500 1505 1510 1515
t(min) t(min)
Theexperimental
experimentalcurves
curvesatat ◦ C and the theoretical curves, according to Equation (6),
Figure 8.
Figure 8. The 2525 °C and the theoretical curves, according to Equation (6), for
for 2SiO
SiO -H (a,b),
-H 2(a,b), SiO2SiO2 -C (c,d),
-C (c,d), and and
Si-P8Si-P 8 TFSI/SiO
TFSI/SiO 2 -H(I)
2-H(I) (e,f). (e,f).30 1.38 2.13 1.84
Figure 9 shows the relationship between water vapor uptake and De. Effective diffusion
coefficients of all the samples show a decreasing trend with the increase of water vapor adsorption,
which, due to the porosity available for water vapor transport, decreases at higher loading as
Materials 2019, 12, 1782 11 of 13
adsorbed water vapor occupies a part of porous sites.
9
8 SiO2-C
SiO2-H
7
Si-P8TFSI/SiO2-H(I)
6
De×10-7(m2/s)
5
4
3
2
1
0
0 5 10 15 20 25
water vapor (wt%)
Figure 9.
Figure Thecurve
9. The curveof
of the
the effective
effective diffusion
diffusion coefficients
coefficients D
Dee versus
versus water
water vapor
vapor uptake.
uptake.
4. Conclusions
4. Conclusions
Phosphonium-based ILs were successfully immobilized on bare silica pretreated by hydrochloric
Phosphonium-based ILs were successfully immobilized on bare silica pretreated by
acid or calcination. Adsorption and diffusion of water vapor on bare and modified silica were
hydrochloric acid or calcination. Adsorption and diffusion of water vapor on bare and modified silica
investigated. It can be seen from the analyses of FTIR, TG, and PSD that SiO2 -H has less hydroxyl
were investigated. It can be seen from the analyses of FTIR, TG, and PSD that SiO2-H has less
groups and micropore volumes, which is unfavorable for the adsorption of water vapor, thus it has
hydroxyl groups and micropore volumes, which is unfavorable for the adsorption of water vapor,
better water resistance. After the grafting of ILs on silica, the sorbents form a hydrophobic film on
thus it has better water resistance. After the grafting of ILs on silica, the sorbents form a hydrophobic
the surface to block the entry of water molecules and retain partial pore characteristics of the support.
film on the surface to block the entry of water molecules and retain partial pore characteristics of the
Therefore, Si-P8 TFSI/SiO2 -H(II), with relatively higher IL loading and lower micropore, increases the
support. Therefore, Si-P8TFSI/SiO2-H(II), with relatively higher IL loading and lower micropore,
water-resistance performance. Fitting the model with experimental data is beneficial and suggests
increases the water-resistance performance. Fitting the model with −7 experimental data is beneficial
that the effective diffusion coefficients of bare silica are at a level of 10 , while SiO2 -H, with relatively
and suggests that the effective diffusion coefficients of bare silica are at a level of 10-7, while SiO2-H,
rapid diffusion, as lower content of water vapor is benefit for diffusivity. The diffusion of immobilized
with relatively rapid diffusion, as lower content of water vapor is benefit for diffusivity. The diffusion
ionic liquids on silica did not change much and most of the values under different water vapor
of immobilized ionic liquids on silica did not change much and most of the values under different
pressure were also at a level of 10−7 , while a slight reduction of the diffusivity was observed due to
water vapor pressure were also at a level of 10-7, while a slight reduction of the diffusivity was
the partial impairing of the pore structure and the forming of a hydrophobic ionic liquid film on the
observed due to the partial impairing of the pore structure and the forming of a hydrophobic ionic
surface. Furthermore, with the increase of water vapor loading, effective diffusion coefficients give a
liquid film on the surface. Furthermore, with the increase of water vapor loading, effective diffusion
decreasing trend.
coefficients give a decreasing trend.
Author Contributions: C.L. and J.Z. conceived the research topic and wrote the manuscript. C.L. and X.H.
designed and carried out the experiments. M.Z. and S.Z. participated to discuss and analyze the data.
Funding: This research was funded by Fundamental Research Funds for the Central Universities,
No. 2019XKQYMS14.
Acknowledgments: The authors gratefully acknowledge the Australia-China Joint Collaboration Group Fund for
Clean Coal Technology Round 2.
Conflicts of Interest: The authors declare no conflict of interest.
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