Nitrogen adsorption at 77 K on mesoporous silica materials (MPS) with varying synthesis time and treatment conditions was investigated. Scanning electron microscope (SEM) and X-ray diffraction (XRD) were also used to characterize the mesoporous materials. This study was performed at 6, 24 and 72-h synthesis times. It is shown that 6-h is not enough for complete formation of the MPS material and at least 24-h is necessary. The pore structure starts decaying for the 72-h synthesis time. The three-after-synthesis treatment conditions used were 1) washed, 2) washed and calcined and 3) directly calcined after synthesis. Ethanol/HCl mixtures were used for washing and calcinations were performed at 550°C. Among these samples, directly washed sample yields the lowest adsorption capacity while washed and calcined sample yields the highest adsorption capacity. Hence, it is concluded that washing stabilizes the structure before high temperature treatment

Ertan, Asli

Kodumuri, Pradeep

Talu, Orhan

Tewari, Surendra N.

English

EngagedScholarship @ Cleveland State University

Cleveland State University
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Chemical & Biomedical Engineering Faculty
Publications Chemical & Biomedical Engineering Department
2-2009
Effect of Synthesis Time and Treatment on Porosity
of Mesoporous Silica Materials
Asli Ertan
Cleveland State University, a.ertan@csuohio.edu
Pradeep Kodumuri
Cleveland State University
Orhan Talu
Cleveland State University, o.talu@csuohio.edu
Surendra N. Tewari
Cleveland State University, s.tewari@csuohio.edu
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Ertan, Asli; Kodumuri, Pradeep; Talu, Orhan; and Tewari, Surendra N., "Effect of Synthesis Time and Treatment on Porosity of Mesoporous Silica
Materials" (2009). Chemical & Biomedical Engineering Faculty Publications. 81.
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Original Citation
Ertan, A., Kodumuri, P., Talu, O., , & Tewari, S. N. (2009). Effect of synthesis time and treatment on porosity of mesoporous silica
materials. Adsorption, 15(1), 81 - 86. doi:10.1007/s10450-009-9153-1
Effect of synthesis time and treatment on porosity of mesoporous  
silica materials 
Asli Ertan· Pradeep Kodumuri . Orhan Talu . 
Surcnctra N. Tewari 
Abstract Nitrogen adsorption at 77 K on mcsoporous sil-
ica materials (MPS) wilh vary ing synthesis time and treat-
ment conditions was investigated. Scan ning electron micro-
scope (S EM) and X-ray diffraction (XRD) were also used 
to characterize the rnesoporous materials. Thi s study was 
performed al 6, 24 and n-h synthesis limes. It is shown 
that 6-h is not enough for complete formation of the MPS 
material and al least 24-h is necessary. The pore struc-
ture starts decaying for the n-h synthesis time. The three-
after-synthesis treatment conditions used wcrc I) washed. 
2) washed and calcined and 3) directly calcined after synthe-
sis. Et hanollHCl mixtures were used for washing and calci-
nations were performed at 550°C. Among these samples, di-
rectly washed sample yie lds the lowest adsorption capacity 
whi le washed and calcined sample yields the highest adsorp-
tion capacity. Hence, it is concluded that washing stabilizes 
the structu re before high temperature treatment. 
Keywords Mesoporous silica synthesis · Nitrogen 
adsorption· Treatment · Synthesis time 
I Introduction 
Mesoporous silicas (M PS) have attracted considerable at-
tention due to thei r numerous potential applications. MPS 
materials with hexagonal, cubic, or lamellar structures were 
invented by scienti sts at Mobil Corporation in 1992 (Inagaki 
et a l. 1996; Zhao et al. 1998). These matcrial s have ordered 
cages or channels and they possess high surface areas with 
pore di ameters rangi ng from 1.5 to 10 nm (Kruk et al. 1997; 
Mou and Li n 2()(H) . MPS materials are normally in pow-
der form when synthesized. Recently, mesoporous silica 
thin films are investigated for various applications such as 
me mbrane separations, chemical sensors, optical devices 
etc. (Mi yata et al. 2004; Lin et al. 2005). The porosity and 
surface area of MPS materials playa crucial role in appli-
cations using powder or thi n film morphology (M iyata and 
Kuroda 1999). Extensive research has been performed to 
identify the physicochem ical properties o f these novel mate-
rials, thc impurities they possess, as well as thc consistency 
of thei r characteristics (Hloff et al. 1999). 
Physical gas adsorpt ion of nitrogen at 77 K and at sub-
atmospheric pressures is a common ly used and relatively 
simple method for characterization of surface and struc-
tural properties of porous solid materials (Wloch et al. 2002 ; 
Jaroniec et al. 1999; Kruk and Jaron iec 1997). The informa-
tion obtai ned from ni trogen isotherms are used to determi ne 
the surface area, pore volume, pore size di stribution in the 
micro- (0.5- 2 nm), meso- (2- 20 nm), and macro-pore range 
(> 20 nm). In addi tion, the technique is re lati vely fa st and 
robust with recently developed automated equi pment. Probe 
gases other than N2, such as Ar, 0 2 and C02 are sometimes 
used as adsorpti ves depend ing on the nature of the adsorbent 
and the informat ion required (Groen et al. 2003). 
This study concentrates on the characterization of a series 
of mesoporous silicas with varying synthesis time and treat-
ment conditions using nitrogen adsorption, scanning elec-
tron microscopy (SEM) and X-ray diffraction (XRD) to gai n 
infonnation about the MPS structure. 
2 Experimental 
The mesoporous silica samples were synthesized using 
sodium silicate, CTMABr (cetyl trimethyl ammonium bro-
mide) and distilled water. During a typical synthesis, 150 cc 
distilled water was mixed with 6.57 grams of CTMABr and 
was stirred for 10 minutes. Then, 5.54 grams of aqueous so-
lution of sodium silicate (14% NaOH, 27% SiO2) was added 
drop by drop while mixing with an ultrasonic wand at room 
temperature for 1 hour. The mixture was placed in an auto-
clave and heated to 120°C for varying time intervals. After 
heating, the mixtures were cooled to room temperature and 
the pH of the mixture was adjusted to 4.83 by adding acetic 
acid (CH3COOH). 
Some of the samples were washed with HCl and ethanol 
solution to remove the surfactant template. Alternatively, 
the surfactants were removed by calcination at 550°C for 
6 h with a heating rate of 50°C/15 min. Scanning electron 
Table 1 Properties of the treated mesoporous silica samples 
2 3Treatment Exp # SBET (m g−1) Vt (cm g−1) BJH avg. des. pore 
diameter (nm) 
only calcined 6 h 678 0.29 3.1 
24 h 739 0.30 4.2 
72 h 43 0.02 10.8 
only washed 6 h 431 0.18 3.4 
24 h 537 0.22 4.0 
72 h 98 0.04 5.8 
washed and 6 h 911 0.39 3.6 
calcined 24 h 1284 0.52 4.6 
72 h 373 0.16 5.2 
microscopy (Amray 1820) and X-ray diffraction (Scintag 
X-1) were performed on part of the dried samples. Nitrogen 
Fig. 1 Synthesis procedure and 
treatment conditions of MPS 
materials 
adsorption isotherms were measured at 77 K in the rela-
tive pressure range from 10−5 to about 1 atm with a Mi-
cromeritics 2010 volumetric adsorption analyzer. Before the 
adsorption analysis, the calcined samples were outgased at 
250°C under vacuum of about 10−6 atm. During the acti-
vation process, complete outgassing was assured by closing 
the vacuum and observing the pressure for any increase. 
3 Results and discussion 
Mesoporous solids reveal adsorption isotherms showing 
monolayer and multilayer formation, followed by capil-
lary condensation (types IV and V). Initially, the adsorption 
process is similar to that occurring on microporous solids; 
but, at higher relative pressures, the adsorption amount rises 
steeply due to the filling of the mesopores. The capillary 
condensation and evaporation takes place at different rela-
tive pressures which leads to the formation of a hysteresis 
loop on the adsorption- desorption isotherm. Normally, hys-
teresis due to capillary condensation is not expected to occur 
in well-structured homogeneous MPS materials since meso-
pore diameters are much smaller to support a liquid menis-
cus (Choma et al. 2003). 
In this study, one of the parameters investigated was op-
timum synthesis time using 6 h, 24 h and 72 h. In addition 
to time, the effect of washing and/or calcination was investi-
gated. The three treatment conditions used were; 1) washed, 
2) washed and calcined and 3) directly calcined after syn-
thesis. 
Adsorption isotherms for a 6-h synthesis sample are 
shown in Fig.  2. As can be seen from the figure, the washed 
sample yields the lowest adsorption capacity while washed 
and calcined sample yields the highest adsorption capacity. 
The directly calcined sample is intermediate between the 
two. It is not clear why washing before high temperature 
calcination increases the porosity. Table 1 represents a sum-
mary of various properties of the samples under study. The 
same trend was observed for all samples regardless of syn-
thesis time. One mechanism may be that ethanol/HCl wash-
ing stabilizes the structure before high temperature treat-
ment. BET (Brunauer-Emmett-Teller) specific surface areas 
are 431, 678, and 911 m2/g (Table 1). Total pore volume of 
the samples listed in Table 1 follows the same trend. Cal-
cined sample with 24 h synthesis time has a surface area of 
739 m2/g which is consistent with the previously reported 
data (Araujo et al. 2005). 
Sharp steps in 77 K nitrogen adsorption isotherms 
of MPS materials around 0.3 P /P o are commonly ob-
served (Kruk et al. 1997). These steps are attributed to mul-
tilayer formation in well-structured homogeneous wall MPS 
materials. Our materials include some heterogeneity (either 
as mesopore size distribution and/or as surface groups) as 
evidenced by gradual increase in the amount adsorbed in 
the same region as indicated in Fig. 2. These steps become 
steeper with increasing synthesis time. Hysteresis loops due 
to capillary condensation are not observed in this study. The 
very small hysteresis loop above 0.95 P /P o in the isotherms 
is attributed to the artifact of inter-particle condensation 
rather than capillary condensation in the MPS particles. 
Fig. 2 Nitrogen adsorption-desorption isotherms at 77 K for 6 h synthesis time. Filled symbols denote desorption 
Fig. 3 Nitrogen adsorption-desorption isotherms at 77 K for 24 h synthesis time. Filled symbols denote desorption 
Fig. 4 Nitrogen adsorption-desorption isotherms at 77 K for 72 h synthesis time. Filled symbols denote desorption 
Nitrogen gas adsorption-desorption curves are presented 
for 24-h synthesized samples in Fig. 3. The adsorption ca-
pacities of the samples for all treatment conditions yield 
higher adsorption capacities in comparison to 6-h synthe-
sis indicating that complete formation of the structure re-
quires at least 24 hours. As with 6-h synthesized sample, 
washed and calcined sample yields the highest adsorption 
capacity and hence has the highest surface area and poros-
ity (Table 1). The highest BET specific surface area in 
this study was observed for this sample with approximately 
Fig. 5 Effect of EtOH/HCl 
washing on crystal morphology 
Fig. 6 X-ray powder diffraction 
pattern of the directly calcined 
and washed and calcined 
samples with 24 h synthesis 
time 
1284 m2/g. The total pore volume calculated using single 
point method at a relative pressure of 0.19 is 0.52 cm3/g. 
Once again, the capacity of directly calcined sample is lower 
than the washed and calcined sample indicating that wash-
ing somehow stabilizes the structure. Similar specific sur-
face area was reported for MCM-41 with 2-h synthesis time 
using silica gel as the silica source (Liu et al. 2008). 
As the synthesis time is extended to 72 h, the structure 
begins to become unstable. Figure 4 shows the 77 K N2 
isotherms. The directly calcined sample after 72-h synthesis 
is completely collapsed showing very little adsorption ca-
pacity. When the samples are only washed, or washed and 
calcined, they have low but easily measurable adsorption 
capacities. Notice the enlarged hysteresis loops for these 
samples starting at 0.5 P /P o which was not observed for 
shorter synthesis times. This clearly indicates capillary con-
densation, or formation of larger pores beyond mesopore 
range. Some instability in the structure must be involved 
which results in the formation of macropores causing capil-
lary condensation. The 72-h synthesis experiment when re-
peated several times yielded the same results, indicating that 
the instability observation is reproducible. In another study, 
5 day synthesis time was found to produce higher quality 
MCM-41 materials as compared to the previous studies per-
formed under analogous conditions where tetraethoxysilane 
was used as a silica source (Choma et al. 2002). 
The SEM micrographs for all mesoporous samples indi-
cate loose aggregate morphology consisting of irregularly 
shaped particles. Washing with ethanol/HCl mixture does 
not change the morphology although the adsorption capaci-
ties increase as discussed above (Fig. 5). 
XRD analysis was performed on selected samples. Fig-
ure 6 shows an example for 24-h synthesis. Both cal-
cined and washed and calcined samples show 100, 110 and 
200 XRD peaks indicating good structural ordering. 100 
peak is at a slightly higher 2θ for the directly calcined sam-
ple indicating smaller pores as listed in Table 2. The small 
but significant shift in inter-planar spacing is another indi-
cation that washing causes structural changes which most 
likely stabilize the material. 
Table 2 Structural properties of MPS with 24-h synthesis time 
2Treatment d100 (nm) Vt (cm3/g) SBET (m g−1) 
Calcined 3.55 0.30 739 
washed & calcined 3.60 0.52 1284 
4 Conclusions 
This study investigated the effect of treatment conditions as 
well as the synthesis time on the adsorption properties for 
MPS materials. It is shown that 6 h is not enough and at 
least 24-h synthesis time is required for the complete forma-
tion of the MPS structure. The structure becomes instable 
when synthesis time is extended to 72 hours. It is also shown 
that ethanol/HCl washing to remove the surfactant template 
causes slight but significant changes in interplanar spacing. 
This also increases the porosity of the MPS structure as well 
as stabilizing it to some extent. 
Acknowledgements We thank the department of energy for their 
partial financial support (DOE, contract number: DE-FC36-04ER14007). 
Nomenclature 
SBET BET specific surface area (m2/g) 
V t Total pore volume (cm3/g) 
d100 Interplanar spacing (nm) 
P /P o Relative pressure 
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Effect of Synthesis Time and Treatment on Porosity of Mesoporous Silica Materials

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Effect of Synthesis Time and Treatment on Porosity of Mesoporous Silica Materials

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