ZINC OXIDE XEROGEL DESULFURIZATION SORBENT FOR BIOGAS:
SYNTHESIS AND PROPERTIES
Kannasoot Kannokkanchana1,*, Sumittra Charojrochkul2, Waret Veerasai1,#
1
Physical Chemistry Program, Department of Chemistry, Faculty of Science,
Mahidol University, Bangkok 10400, Thailand
2
National Metal and Materials Technology Center, Pathumthani 12120, Thailand
*e-mail: kannasoot.bas@gmail.com, #e-mail: waret.vee@mahidol.ac.th
Abstract
This research aims to synthesize zinc oxide xerogel desulfurization sorbent for biogas by
using simple zinc nitrate salt and simple process called epoxide addition method. In the
preparation step, zinc nitrate precursor solution was simply mixed with propylene oxide
gelation initiator. Effect of precursor solution concentration and amount of gelation initiator
was also studied. After the gelation process was completed, xerogel was washed by acetone
and dried under atmospheric condition. As-prepared xerogel was investigated by powder Xray diffractometer (PXRD), scanning electron microscope (SEM), and thermogravimetric
analysis (TGA) to obtain phase, morphology and appropriate calcination temperature. After
characterization, the as prepared sorbent was calcined at 250°C to obtain zinc oxide xerogel
sorbent. The sorbent was characterized again by PXRD, SEM, and Brunauer-Emmett-Teller
nitrogen adsorption (BET) to confirm phase, morphology and surface area. Then, a
desulfurization performance of the sorbent was measured by using a glass reactor coupled
with micro GC. The removal efficiency of as-prepared zinc oxide xerogel sorbents are greater
than 99.98% resulting in the outlet H2S concentration <2ppmv from 10050 ppmv inlet gas,
which passed Department of Energy Business standard for NGV (H2S<23 ppmv). The
desulfurization capacities of as-prepared sorbents in the range of 6.67 – 9.43 mgS/g are
achieved at 30°C.
Keywords: zinc oxide, desulfurization, xerogel, biogas
Introduction
Thailand is an agricultural country. Tremendous amount of agricultural products are
produced each year, for example, rice, sugar cane, cassava, and livestock. After harvesting,
enormous amount of agricultural waste remains and causes environmental problems. One of
the most effective ways to eradicate human biowaste and serve energy demand together is
converting biowaste into biogas by anaerobic digestion process. By using this method,
biowaste can be eliminated effectively and converted to valuable gas fuel.
Biogas contains mostly methane (40-75%v/v) which can be used directly in internal
combustion engine vehicles, which can be upgraded to bio-natural gas for vehicles (NGV).
However, biogas is not simply pure methane. It contains various contaminants, i.e. carbon
dioxide (15-60%v/v), water (5-10%v/v), nitrogen (0-2%v/v), oxygen (0-1%v/v), hydrogen
sulfide (0.005-2%v/v), siloxane (0-0.02%v/v), ammonia (<1%v/v), and carbon monoxide
(<0.6%v/v) [1]. Their concentrations vary from raw materials and anaerobic digestion
condition. However, these contaminant gases need to be eliminated before feeding to the
combustion.
In this group of contaminant gases, hydrogen sulfide is the most undesirable, because
its corrosion effect on piping and engine parts, and hazardous for human and animals.
Furthermore, one of the combustion products of hydrogen sulfide is sulfur dioxide, which
causes many environmental problems.
There are many ways to remove hydrogen sulfide from biogas (this process is called
desulfurization) such as biological oxygen dosing, chemical adsorption in aqueous solution
by ferric/ferrous salts, absorption in water, amine scrubbing, and biological filter [1,2].
However, these techniques have low removal efficiency, and/or may require complex fluid
system. This limitation hinders the scale down of purification system for small size
applications, which needs low cost, small, but efficient desulfurization system. Other
technique such as rush steel wool, impregnated wood chip sorbent based on metal oxide
sorbent, are efficient enough to pass biomethane standard for biogas [1-3]. Unfortunately, the
reaction of rust steel wool sorbent depends strongly on moisture content, which causes two
major problems. First, it needs complex humidifier or humidity control system. Second, this
excessive moisture needs further steps to remove. Moreover, iron sulfide, which is the
product of sulfided iron oxide sorbent, is toxic and pyrophoric. These reasons are obstacles
that prevent the use of rust steel wool sorbent in a small scale system and motivate us to find
other alternative metal oxides, which can replace an iron oxide sorbent effectively with no
further moisture, pyrophoricity, and toxicity problems.
Zinc oxide is one of the best known desulfurization sorbent. It has been widely used
for desulfurization at moderate temperature (300-500°C) in coal-gas and syngas
desulfurization process [4]. Zinc oxide has very high removal efficiency, non toxic, and
inexpensive. Thermodynamics interpretation of zinc oxide sorbent by Westmoreland et al. [5]
suggested a possibility to use zinc oxide sorbent at room temperature. However, from
practical experiment by Garces et al. [6], zinc oxide was not meant to adsorb hydrogen
sulfide at 60°C, and adsorbed only 6 mgS/g sorbent even at 100°C. This result can be
explained based on low thermal diffusivity of sulfide intermediate to inner layer of zinc oxide
at room temperature, which causes only surface reaction to occur [7]. It can be obviously
indicated by the experimental results of Baird et al., which showed direct proportional
relation between surface area of zinc oxide and sulfur capacity [7]. These reasons motivate us
to find a high surface area and high porosity zinc oxide material to use in room temperature
biogas desulfurization applications.
Aerogel and xerogel are known as extraordinary property materials, which have high
surface area and porosity. In comparison, xerogel is easier to synthesize, because it can be
dried under normal atmospheric condition, and does not require a special supercritical fluid
extractor like aerogel. Unfortunately, both aerogel and xerogel are conventionally synthesized
by using metal alkoxide salts, which are scarce and expensive. This reason prevents widely
use of aerogel and xerogel materials. Recently, there are attempts to synthesize aerogel and
xerogel from metal nitrate salts via a process called epoxide addition method [8-9], which
activates our attention to synthesize zinc oxide xerogel sorbent to desulfurization in biogas at
room temperature. Hence, the study of zinc oxide xerogel synthesis by epoxide addition
method and performance of as-prepared xerogel material in desulfurization application at
room temperature have been carried out, which capable for use in a small scale biogas
desulfurization applications.
Experimental
Zinc oxide xerogel was synthesized by using epoxide addition route, which are
previously described by Gao et. al. [9]. In the preparation process, precursor solution was
prepared by dissolve zinc nitrate hexahydrate (Zn(NO3)2·6H2O, analytical reagent grade from
Alfa Aesar) in methanol (CH3OH, GC grade from Merck) in a 50cm3 plastic capped conical
tube. After the salt is dissolved, propylene oxide (C3H6O, analytical reagent grade from
Sigma Aldrich) was added to the solution, and then the tube was closed, vigorously stirred for
2 minutes by using a vortex shaker, and left for 5 days. The effect of precursor concentration
and amount of gelation initiator were studied. The synthesis conditions of all sorbents are
shown in table 1.
Table 1. Zinc oxide xerogel synthesis conditions
Sample
Z1
Z2
Z3
Z4
Z5
Condition
As described by Gao et. al. [9]
2 times amount of propylene oxide
3 times amount of propylene oxide
2 times precursor concentration
0.5 times precursor concentration
Zn(NO3)2·6H2O, g
2.856
2.856
2.856
2.856
2.856
CH3OH, cm3
15.0
15.0
15.0
7.5
30.0
C3H6O, g
5.58
11.16
16.74
5.58
5.58
After the completion of gelation process, the gels were added with 15cm3 acetone
and shaken overnight in a closed conical tube. Solvent exchanging was completed by
additional five time repeating of acetone exchange step. In details about solvent exchanging
process, the tube was vigorously shaken, until the gel became fine powder in the solution.
After that, the tube was centrifuged at 2,000 rpm for 3 minutes and rinsed off the old solution.
Fresh acetone was exchanged and left overnight. When the solvent was completely
exchanged, it was left to dry overnight in a fume hood.
The last step was drying and calcination. The gel was removed from conical tube,
pasted on petri dish and dried at 50°C overnight. Then, sample was weighted and calcined at
250°C for 1 hour. These samples were characterized using XRD (5-50°, scan speed
1.45°/min), TGA (40-600°C, heating rate 5°C/min), SEM, BET and desulfurization
performance test at 30°C.
In desulfurization test, 0.5 grams of sample was added to a glass reactor tube, of
inner diameter 9 mm. The bed length is controlled at 3 cm. The sample fed with 1.005 mol.%
H2S/N2, while the outlet gas was analyzed using Varian 490GC with PoraPlot-U column and
thermal conductivity detector (TCD). The temperature of the reactor was controlled at 30°C
during desulfurization measurement. The breakthrough capacity was calculated from
breakthrough time by using the following equation.
Where BTcapacity is the breakthrough capacity of the sorbent (gS/g), mH2S(ads) is
mass of hydrogen sulfide adsorbed by the sorbent (g), mZnO is mass of the sorbent (g), χ0,H2S is
mole fraction of hydrogen sulfide in inlet gas (0.01005), Q is volumetric flow rate at standard
temperature and pressure (STP) (cm3/minute), MH2S is molar mass of hydrogen sulfide
(34.081 g/mol), Vm,N2 is molar volume of nitrogen at STP (22.393×103 cm3/mol), and
tbreakthrough is breakthrough time (minute).
Results and Discussion
As-prepared xerogel : morphology and crystal structure
Scanning electron microscopy (SEM) images of as-prepared xerogel were captured
using Hitachi FE-SEM S-4800 at 1,000 and 10,000 times magnification as shown in figure 1.
(a)
(b)
(d)
(c)
(e)
Figure 1 SEM images of as-prepared xerogel synthesized by different conditions;
(a) Z1, (b) Z2, (c) Z3, (d) Z4, and (e) Z5
From figure 1, the morphology of as-prepared xerogels were a flake like structure
with diameter of less than 5 micrometer for all conditions. These flake-like particles were
compacted in a small spherical particles in diameter 20 to 50 micrometers. The SEM images
clearly showed the void spaces between these flake-like particles that explained the very lowdensity property of as-prepared xerogel solid (ca. 0.1 g/cm3 for all conditions). Due to very
close microscopic morphology of the sorbent prepared from different conditions, these
images implied less effect of reagent composition to the gel properties.
Powder x-ray diffractometer (PXRD) was used to study phases of the as prepared
sorbents. All samples were weighted 300mg and measured by PXRD from 2θ in the range of
5-50°. The powder X-ray diffraction patterns are displayed in figure 2.
From figure 2, it was clearly seen that the sorbent synthesized at various conditions
gave the same phase of xerogel, which was zinc hydroxide nitrate hydrate
Zn5(OH)8(NO3)22H2O [JCPDS 00-024-1460]. Since the zinc oxide phase was desired,
further calcination process was required for this purpose. Thermogravimetric analysis (TGA)
technique is used to determine a suitable calcination temperature.
Determination of calcination temperature : thermogravimetric analysis (TGA)
Xerogel Z1 was selected as a representative to study the calcination process by
thermogravimetric analysis technique. The sample was measured in three different
environments, i.e. nitrogen, air zero, and oxygen, to study effect of oxygen in the calcination
process. Temperature range was 40-600°C at heating rate 5°C/minute. TGA results for each
ambient condition were shown in figure 3.
The thermograms in figure 3 showed the first mass loss region started from 75-135°C.
It could be explained by losing two water hydrate molecules [10]. The latter three peaks at
approximately 135-178°C, 178-215°C and 215-245°C may be related to the decomposition of
the remaining salt [10]. This process could be divided in to four regions as shown in figure 3,
and explained by four fundamental processes as shown by the following reactions [10].
Figure 2 Powder X-ray diffraction pattern of as-prepared xerogel prepared by various conditions
Figure 3 TGA and derivative of as-prepared xerogel prepared by Gao et. al. [9] (Z1) condition measured
in different ambient; a) nitrogen, b) air zero, and c) oxygen at a heating rate of 5°C/minute
Region I
Zn5(OH)8(NO3)22H2O
Zn5(OH)8(NO3)2 + 2H2O
Region II
Zn5(OH)8(NO3)2
Zn3(OH)4(NO3)2 + 2ZnO + 2H2O
Region III
4Zn3(OH)4(NO3)2
9ZnO + 3Zn(NO3)2 + 7H2O + 2HNO3
Region IV
Zn(NO3)2
ZnO + NO2 + NO + O2
Table 2 Mass loss of xerogel sample Z1 in nitrogen, air zero, and oxygen atmospheres compared to the
theoretical value for each decomposition steps [10]
Step
I (75-135°C)
II (135-178°C)
III (178-215°C)
IV (215-245°C)
Total
Nitrogen
6.28
8.88
12.67
6.34
34.17
Mass loss, %
Air zero
5.47
8.57
12.61
4.16
30.81
Theoretical mass loss, %
Oxygen
5.62
8.63
13.29
5.43
32.97
5.8
5.8
10.1
13.0
34.7
Calcination study : morphology and crystal structure
In this part, all the as-prepared xerogels were calcined at 250°C for 1 hour. Calcined
samples were characterized using SEM, PXRD, and BET to determine the morphology,
crystal phase and surface area.
SEM images of calcined xerogels for each preparation condition were exhibited in
figure 4 (calcined samples were denoted by C ending).
(a)
(b)
(d)
(c)
(e)
Figure 4 SEM images of xerogels calcined at 250°C for 1 hour synthesized by different conditions;
(a) Z1C, (b) Z2C, (c) Z3C, (d) Z4C, and (e) Z5C
Figure 4 indicate the change in morphology changing after calcination. When
comparing the texture of xerogel particles, figure 4 Z1C and Z5C, which referred to the
reference and 3 times propylene oxide added condition, are significantly more porous than
those in other conditions. These two conditions might be suitable for use as a sorbent.
Samples Z2C, Z3C, and Z4C showed dense texture, which might be inappropriate. However,
only SEM investigation is not enough to distinguish the best condition from others. The
properties of calcined xerogel needed further characterization step to select the most
appropriate condition.
Calcined samples were characterized by powder X-ray diffractometer. All samples
were weighted 300mg and measured by PXRD from 2θ 5-50°. The powder X-ray diffraction
patterns are showed in figure 5.
From figure 5, all calcined samples indicated the same PXRD patterns including peak
position and relative intensity of ZnO. This means there are no significant difference of
calcined sample phase and degree of crystallinity. The results supported that the starting
chemical condition had less effect to sample property after calcination.
Figure 5 Powder X-ray diffraction pattern of xerogel prepared by various conditions after calcination
Desulfurization study of calcined sorbents
The breakthrough curves of all calcined sorbents at 30°C were shown in figure 6. In
breakthrough capacity measurement, 0.500g sorbent was used. Reactive gas was 1.005mol.%
H2S/N2 and volumetric flow rate was 10scc/minute. The breakthrough capacity and surface
area are summarized as shown in table 3
The removal efficiency of all sorbent were indifferent as displayed in figure 6. Before
reaching the breakthrough point, all sorbents reduced the hydrogen sulfide concentration
from 10050 ppmv to undetected. As the detection limit of the gas chromatography
instruments was 2 ppmv. It meant the removal efficiency higher than 99.98% was achieved
for all sorbents.
From the BET surface area measurement, it was found that the surface area of sorbent
prepared in different conditions were not significantly different. This made the sulfur
capacity of each sorbent as good as one another. However, from the breakthrough capacity
calculation, Z1C sample showed the highest capacity at 9.43 mgS/g. Although Z2C had
comparable capacity to Z1C (9.41 mgS/g compared to 9.43 mgS/g), it was not considered
from the economic reason since a larger amount of gelation initiator was used.
Figure 6 Breakthrough curve of calcined sorbents at different preparation conditions
Table 1. Breakthrough capacity and surface area of sorbents prepared by different conditions
Sample
Z1C
Z2C
Z3C
Z4C
Z5C
Breakthrough capacity (mgS/g)
9.43
9.41
8.61
6.73
6.67
Surface area (m2/g)
9.25
8.41
11.01
9.17
8.34
Conclusion
Zinc oxide xerogels prepared by simple epoxide addition method had been
synthesized and characterized for use as desulfurization sorbent at room temperature. Results
from PXRD, SEM, and BET indicated less effect of preparation condition for all calcined
sorbent properties. Desulfurization measurements indicated all samples could adsorb
hydrogen sulfide from 10050 ppmv inlet concentration to less than 2 ppmv resulting in the
removal efficiency >99.98%, which passed the Department of Energy Business standard for
NGV (23 ppmv).
The breakthrough capacity measurements indicated that Z1C had the greatest sulfur
capacity of 9.43mgS/g. Although Z2C had comparable capacity to Z1C (9.41 mgS/g
compared to 9.43 mgS/g), it was not considered due to economic reason, since a larger
amount of gelation initiator was used. This suggested that Z1C was the best sorbent in this
study both in performance and economy.
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Acknowledgements: This research was supported by Thailand Graduate Institute of Science
and Technology Scholarship (TGIST) under the grant No.TG-33-14-54-041M from National
Science and Technology Development Agency (NSTDA), National Metal and Materials
Technology Center (MTEC), and Faculty of Science, Mahidol University.