i
Functionalization of Metal Organic Frameworks for Enhanced
Stability under Humid Environment for CO2 Capture Applications
A Thesis Presented
By
Yu Lei
to
The Department of Chemical Engineering
In partial fulfillment of the requirements
For the degree of
Master of Science
In the field of
Chemical Engineering
Northeastern University
Boston, Massachusetts
February 26, 2013
ii
ACKNOWLEDGEMENT
I would like to thank my advisor, Professor Choi for providing me with an
opportunity to work in his research lab, the Nanomaterials Laboratory for Advanced
Catalysis and Separations. He has been a constant source of encouragement in my
graduate career. His critical feedback and commitment to excellence made me a better
scientist. Working with him has been a great learning experience in my life.
During my graduate studies, I have been greatly inspired by and have benefited
from the help of my colleagues Dinara Andirova, Christopher Cogswell and Jiang
Hui.
My sincerest thanks also to Prof. Shashi Murthy and Prof. Philip
Larese-Casanova for providing us with the temporary lab space and instruments while
our own lab was under construction.
Life outside work has been enjoyable too during the last few years at
Northeastern University, thanks to my friends and my roommates. You are always
there for me during hard times. Thank you so much. I couldn't have done it without
your help.
Most importantly I would like to thank my mother. I could not have finished my
graduate studies without her invaluable support.
I would also like to thank everyone else who helped me during the last three years;
because of your support, I have finally achieved my goal.
iii
ABSTRACT
Global climate change has been subjected to widespread research interest in
recent decades while being attributed for its reason to CO2 anthropogenic
accumulation in the atmosphere.1 Metal-organic frameworks (MOFs) are an emerging
new class of nanoporous crystalline solids built of metal coordination sites linked by
organic molecules ,which have attracted increasing attention since their well-defined
porosity, high surface area and tunable functionalities.17,38
Among them, Mg/DOBDC,
also named as Mg-MOF-74 or CPO-27-Mg, is considered as a strong candidate for
CO2 capture in flue gas even in the ambient air,10
but its hydrophilic properties limit
its application since its CO2 uptake capacity dramatically decreases in humid
conditions.13
In this work, we try to impregnate ethylenediamine into the pores of
Mg/DOBDC to yield a new porous material ED-Mg/DOBDC that exhibits stability
after exposure to water vapor. To investigate structural changes and CO2 uptake
capacity in moist conditions, accelerated steam treatment of all samples were
performed. Pore characteristics and CO2 uptake capacity of Mg/DOBDC and
ED-Mg/DOBDC before and after steaming were measured. ED-Mg/DOBDC showed
higher CO2 uptake capacity for both the non-steamed and steamed samples. Also,
different ED loadings were suggested by our group`s previous work,23
and we further
investigated the effect of pore characteristics of different ED loadings in the pores of
ED-Mg/DOBDC to find the impact of ED molecules in ED-Mg/DOBDC for CO2
capture applications.
iv
TABLE OF CONTENTS
LIST OF FIGURES ................................................................................................... v
LIST OF TABLES…………………………………………………….………...……..x
1.0 INTRODUCTION…………………………………………………………………1
2.0 CRITICAL LITRATURE REVIEW……………………………………...……….3
3.0 EXPERIMENTAL………………………………………………………………....8
3.1. Scale Up of Mg/DOBDC Synthesis…………………………………………..8
3.2. Accelerated Steam Treatment Experiments………………………..………….8
3.3. Functionalization…….…………………………..………….……..….....…..9
3.4. Characterization……...…….………………………………….………....…..9
3.5. CO2 Adsorption……………………………………………………………...10
3.6. ED Loadings…………………………………………………………………10
4.0 RESULTS AND DISCUSSION………………………………………….………11
4.1. Structure Changes of Functionalization……………………………………..11
4.2. CO2 adsorption at room temperature…………………………………..……12
4.3. Effect of ED loading………….………..…….…………………………...13
5.0 CONCLUSION…………………………………………………………………..15
6.0 RECOMMEDATIONS…………………………………………………………...16
7.0 NOMENCLATURE……………………………………………………….……..18
8.0 REFERENCES………………………………………………………………..….19
v
LIST OF FIGURES
Figure 1: Unit cell structures of Mg/DOBDC with (a) 0 (bare), (b) 1, (c) 3, (d) 6,
and (e) 18 EDs per unit cell
vi
Figure 2: XRD patterns of Mg/DOBDC and ED-Mg/DOBDC before and after
steaming
Figure 3: N2 isotherms of Mg/DOBDC and ED-Mg/DOBDC before and after
steaming. Based on these isotherms we can calculate the pore
characteristics differences as shown in Table 2
0
10000
20000
30000
40000
50000
60000
70000
0 10 20 30 40 50 60
Inte
nsi
ty (
a.u
.)
2 theta (degree)
Bare Mg/DOBDC
Steamed Mg/DOBDC
ED-Mg/DOBDC
Steamed ED-Mg/DOBDC
0
50
100
150
200
250
300
350
0 0.2 0.4 0.6 0.8 1
CO
2 U
pat
ke C
apac
ity
(cc/
g, S
TP)
P/P0
Bare Mg/DOBDC
Steamed Mg/DOBDC
ED-Mg/DOBDC
Steamed ED-Mg/DOBDC
vii
Figure 4: CO2 adsorption isotherms of Mg/DOBDC and ED-Mg/DOBDC before
and after steaming in room temperature
Figure 5: CO2 adsorption capacity of Mg/DOBDC and ED-Mg/DOBDC before
and after steaming in room temperature by TGA and NOVA 2200e
0
20
40
60
80
100
120
140
160
180
200
0.00E+00 2.00E-01 4.00E-01 6.00E-01 8.00E-01 1.00E+00
CO
2 U
pta
ke C
apac
ity
cc/g
, STP
P/P0
Bare Mg/DOBDC
Steamed Mg/DOBDC
ED-Mg/DOBDC
Steamed ED-Mg/DOBDC
0
1
2
3
4
5
6
7
8
9
CO
2 A
dso
rpti
on
Cap
acit
y, m
mo
l/g
TGA Physisorption
Bare Mg/DOBDC
Steamed Mg/DOBDC
ED-Mg/DOBDC
Steamed ED-Mg/DOBDC
viii
Figure 6: Weight changes of Mg/DOBDC and ED-Mg/DOBDC before and after
steaming by TGA
Figure 7: Weight changes of different ED loadings of ED-Mg/DOBDC by TGA
while heating from 25 °C to 600 °C. Based on this graph, we can
calculate the ED loadings of Batch 1 are ~6.3% and Batch 2 is 5.5%
60
65
70
75
80
85
90
95
100
0 100 200 300 400 500 600
We
igh
t %
Temperature °C
Bare Mg/DOBDC
Steamed Mg/DOBDC
ED-Mg/DOBDC
Steamed ED-Mg/DOBDC
Bare Mg/DOBDC
Steamed Mg/DOBDC
ED-Mg/DOBDC Steamed ED-Mg/DOBDC
60
65
70
75
80
85
90
95
100
0 100 200 300 400 500 600
We
igh
t %
Temperature °C
Batch 2
Batch 1
ix
LIST OF TABLES
Table 1: Pore characteristics of Mg/DOBDC and its derivatives
BET surface area
(m2/g)
Total Pore Volume
(cc/g)
Pore Diameter
(Å)
Mg/DOBDC 1082 0.49 13.6
Steamed
Mg/DOBDC
28 0.24 8
ED-Mg/DOBDC 381 0.38 12
Steamed
ED-Mg/DOBDC
121 0.25 10
1
1.0 INTRODUCTION
According to the Stern Review on the Economics of Climate Change,1 rising
global carbon dioxide levels in the atmosphere are one of the major contributors to
global climate change.2 Therefore it is important to establish efficient,
environmentally friendly, and economically efficient methods to capture carbon
dioxide. Capture of carbon dioxide from flue gas or ambient air is a promising method
to reduce CO2 emissions into the environment and decrease heightened CO2
concentration in the air.26
One of the promising approaches to reduce CO2 emissions
is to design an efficient technology for carbon capture and sequestration (CCS) where
CO2 could be captured from the exhaust gas (flue gas) of coal burning power plants
and then transported and buried underground or in the deep sea waters.39
Alternatively,
captured carbon dioxide could be used in other useful applications.48
To design an
effective method for carbon dioxide capture from gas streams, the conditions of the
stream of interest should be considered. A typical flue gas contains not only CO2 but
also other gases including N2, H2O, SOx, NOx, etc. In general, composition ratios of
the main gases present are approximately 6.5:1:1 (N2:H2O:CO2) by weight.3 For flue
gas or ambient air capture, pressure is around atmospheric. With the low CO2
concentrations in these gases separation of CO2 has become one of the main
challenges for effective capture and has been intensely studied by many researchers in
recent years.
Hybrid porous solids are considered strong candidates for CO2 capture.35
Among
those, metal organic frameworks, (MOFs), an emerging class of nanoporous
2
crystalline solids built of metal coordination sites linked by organic molecules, show
promising properties for gas capture applications. These materials offer well-defined
porosity, high surface area, and tunable chemical functionalities demonstrating
applications in catalysis19,20
, separations14,15
, gas storage17,18
(such as methane6,
hydrogen8,16
, acetylene18
), biomedicine,21
and exhibit luminescent properties leading
to their being considered for sensors.33
In the past decades, They are also attracted a
great attention as CO2 capture materials.17,38
3
2.0 Critical Literature Review
Currently one of the benchmark implemented solutions for carbon dioxide
capture is absorptive capture by alkylamine-containing liquids such as mono- and
triethanolamines.4,5
However, the use of these materials has a number of drawbacks.
Regeneration of such absorbents is only possible at high temperatures which require a
high energy input, something that should be avoided for design of an environmentally
friendly CO2 capture material. In addition, inhibitors for corrosion control need to be
used with aqueous amine materials, and amine vapors could cause contamination of
the gas streams being treated.17
These limitations call for an improved method to
capture CO2 for industrial applications. Adsorption processes onto porous solid media
could be a better option to capture carbon dioxide, since those materials operate
through physisorptive and chemisorptive processes and potentially could be easier to
regenerate for cyclic operations.
In particular, zeolites have attracted attention as solid adsorbents for carbon
dioxide capture.38
Compared to aqueous alkanolamine absorbents, zeolites require
significantly less energy input for adsorbent regeneration, 43
but their hydrophilic
properties44,45
limit their applications in this area. Activated carbon is another solid
adsorbent which is considered a strong candidate carbon dioxide adsorbent. It requires
less energy for regeneration40
and its hydrophobic properties lead to better
performance under moisture conditions compared to zeolites.41
While the high surface
area of activated carbon contributed to much higher carbon dioxide capture capacities
at high pressures,42
it does not perform very well at low pressure range due to its low
4
enthalpy of adsorption.40
Therefore, solid adsorbents for carbon dioxide capture still
need further improvement, and in recent years another class of porous material,
metal-organic frameworks (MOFs) have attracted attention for their unique properties
and potential applications in carbon dioxide capture.
Many MOFs have been reported in the past few decades and some exhibit
exceptional CO2 capacities over large pressure ranges (up to 42 bar) at room
temperature,7 overcoming other physisorbents such as zeolites and activated
carbons.36,37
Nevertheless, in the pressure range of interest (low pressure), not many
metal organic frameworks perform well. A series of MOFs, M/DOBDC are currently
reported to have the best capture capacity in low pressure range and are considered to
be the strongest candidate for carbon dioxide capture. Yaghi et.al.7
reported the
M/DOBDC series of MOFs (M=Ni, Zn, Co), where the organic linker was formed by
2,5-dioxido-1,4-benzendicarboxylate(DOBDC), connecting metal ions like
magnesium, nickel, zinc and cobalt as metal sites.10
Mg/DOBDC is a MOF composed of 2, 5-Dihydroxyterephthalic acid linkers
coordinated to magnesium centers.10
This material was successfully synthesized by
the solvothermal method9,12,13
and was reported to have high CO2 adsorption capacity
and CH4 selectivity over CO2.10,11
At low CO2 partial pressure a competitive
adsorption capacity of 0.38 g CO2/g sorbent was observed.23
There are, however,
some difficulties encountered with this adsorbent. For instance, a high energy input is
required to regenerate the material which is not cost-effective or energy-effective
when scaled up for industrial applications.10
In addition, the application of this MOF
5
to CO2 capture demands partial or complete drying of the gas stream due to its
hydrophilic metal-ligand bonds.17
Previous experiments have shown that in humid
conditions, the BET surface area of Mg/DOBDC dropped from 1406 to 38 m2/g, and
the CO2 adsorption capacity is also reduced by more than half.13
It was also reported
that even under dry conditions, after long-term storage in sealed containers for one
year, the BET surface area declined by about 70% and CO2 adsorption capacity
decreased as well. Kizzie et al. developed a breakthrough experiment of N2/CO2 for
M/DOBDC series to show the hydrophilic properties of these materials.22
In their
experiment, M/DOBDC series, especially Mg/DOBDC showed a novel capacity for
CO2 capture in dry gas mixtures but its capacity dramatically decreased when
N2/CO2/H2O breakthrough experiments were performed. It was found that with an
increase in relative humidity, carbon dioxide capture capacity decreases drastically
even after regeneration.
Amine functionalization of pore surfaces within various materials has been
performed by several groups. 23,27-32,34
These modifications are known to be effective
in carbon dioxide capture due to amines high affinity towards CO2 gas molecules. In
Arstad`s report, 27
three different amine-functionalized MOFs were tested for carbon
dioxide capture. The results showed that even if surface areas and pore volumes
decreased to some extent after functionalization, carbon dioxide capacities showed
better results than for the non-functionalized original structures of tested MOFs.
In order to accurately access the performance of the material under humid
conditions, material stability was tested via accelerated steam treatment. Accelerated
6
steam treatment would provide an exaggerated humid environment to test for the
materials stability, and can be taken as a worst case scenario, as in the real application
relative humidity would be much lower. Steam treatment could also be considered as
a possible method for material regeneration. Generally, large amounts of unused or
waste steam is available at industrial plants and its use for regeneration of CO2
capture adsorbent could be easily implemented. It would be, therefore, interesting to
see whether or not steam treatment could be used as a new adsorbent regeneration
process and simultaneously test for the materials stability.
Currently, Mg/DOBDC is made in a very small amount, on the milligram scale,
in laboratory settings9,12
but few reports discuss scale-up of this material. 22
Another
goal of this work is to find an efficient, one-pot synthetic route to generate
Mg/DOBDC on a larger scale.
The same technique was used to functionalize Mg/DOBDC with ethylenediamine
(ED) by our group. As shown in Figure 1, ethylenediamine was impregnated on the
surface of the pores where one amine group coordinated directly to a metal site of Mg
while the other amine group was free within the pore and available to interact with
guest molecules.23
It was found that such modification of the material increase the
CO2 adsorption capacity and lowers the energy requirement for regeneration of the
adsorbent. Besides, it increased its structure stability, bare Mg/DOBDC showed
clearly degradation of CO2 adsorption capacity after four cycles of adsorption of CO2
by TGA. While for ethylenediamine functionalized Mg/DOBDC, even after four
cycles of adsorption and desorption under dry conditions, CO2 capacity stayed
7
constant at about 1.51 mmol/g. But the CO2 capture ability in humid conditions was
not tested previously despite the fact that these conditions are more similar to those
which will be encountered in industrial applications. The carbon dioxide capture
ability under humid conditions of ED functionalized Mg/DOBDC was tested and will
be presented in this work.
8
3.0 EXPERIMENTAL
3.1. Scale Up of Mg/DOBDC Synthesis
The scale up of eight times the original recipe for Mg/DOBDC synthesis9,12
was
performed based on previously reported procedures. 2, 5-dihydroxyterephthalic acid
(H4DOBDC; 0.888 g, Sigma-Aldrich) and Mg (NO3)2·6H2O (3.8 g, Fisher) were
mixed together in a 250 ml Pyrex flask. A solvent solution of dimethylformamide
(352 ml, Fisher), ethanol (24 ml, Fisher) and DI water (24ml) was then added to the
flask. The resulting solution was sonicated for 1 hour to ensure mixing and then
placed into the oven for 20 hours at 125 °C. Once the heating was done, the flask was
removed from the oven and immediately the mother liquor was decanted and replaced
with methanol. The solution was set to cool down for 5-6 hours. Methanol
replacement was then performed every 12 hours over the next two days. The mixture
was then evaporated for three days to result in a yellow microcrystalline powder. The
resulting material was then activated by heating under vacuum at 250 C for five hours
to yield ~0.95 mg of Mg/DOBDC.
3.2. Accelerated Steam Treatment Experiments
Steam treatment procedures were borrowed from previous reports of similar
experiments performed on amine-functionalized mesocellular foam (MCF) materials
24 and on mesoporous alumina-supported amines.
25 About 200 mg of material was put
into glass test tubes which were then placed into an 80-ml Teflon lined Parr reactor.
The Teflon liner was carefully filled with about 20 mL of DI water. It should be noted
that water in the liner did not touch the solid samples in the test tubes. The whole set
9
up prior to steam stripping was maintained in a glove box, establishing an inert
nitrogen environment. Once the Parr reactor was sealed, it was transferred into the
oven. The Parr reactor was heated at 110 °C for 48 hours. The steam pressure in the
reactor was autogenous. Once heating was finished, the reactor was let to cool down
to room temperature. No liquid was observed in the test tubes indicating that solid
material only interacted with water vapor. Test tubes were then taken out and the solid
samples were dried under vacuum for 80 °C overnight. Dried powder samples were
then ready for characterization.
3.3. Functionalization
As mentioned before, some MOFs have already been amine-functionalized.
4-6,27-32,34 Functionalization with ethylenediamine was performed based on those
reports and on the recent paper by Choi et.al.23
About 40 ml anhydrous toluene and
1.0 g ethylenediamine (Fisher) was added into the flask with 200 mg of solid dry
Mg/DOBDC sample. The contents of the flask were stirred at 400 rpm under reflux.
After 12 hours, the mixture was washed with 100 ml DI water and then 100 ml
ethanol. The filtered material was dried at room temperature for 14 hours to yield
ethylenediamine functionalized Mg/DOBDC which was named as ED-Mg/DOBDC.
3.4. Characterization
The pore characterization of the synthesized and activated material was
performed by using a NOVA 2200e surface area and pore volume analyzer. This
instrument provided nitrogen adsorption-desorption isotherms, where samples were
degassed under vacuum flow at 250 °C for 16 hours prior to the analysis. X-ray
10
diffraction (XRD) patterns of material were obtained by a Rigaku Ultima IV X-RAY
Diffractometer (CuKα) over the range of 5-50° 2θ with a stepsize of 0.1° 2θ and a one
second integration time per step.
3.5. CO2 Adsorption
The CO2 adsorption capacity was measured by NOVA 2200e. This instrument
was used to detect the CO2 (UHP grade 100%, Airgas) adsorption isotherms in room
temperature. We degassed samples under vacuum at 80 °C for 16 hours before
analysis, and the CO2 adsorption/desorption isotherms were collected at room
temperature.
CO2 uptake was also measured by thermogravimetric analysis, TA Q500. Each
sample was heated up to 110 °C initially to draw out any excess water, was let to
equilibrate back to room temperature, and then exposed to 100 % CO2 gas for two
hours. The total CO2 uptake was calculated based on the weight increase of the
sample from the point when CO2 was first introduced until the saturation point was
reached where no more weight increase was observed.
3.6. ED Loadings
Thermogravimetric analysis (TGA), TA Q500 was used to determine amine
loadings on the functionalized material. Each sample was heated at the rate of
10 °C/min from room temperature to 600 °C. For further research of ED loadings
effect on CO2 adsorption, we synthesized two different batches of ED-Mg/DOBDC
with different ethylenediamine loadings. Batch 1 was loaded with 1 g of
ethylenediamine, and batch 2 was loaded with 0.8 g.
11
4.0 RESULTS AND DISCUSSION
4.1. Structure Changes of Functionalization
Mg/DOBDC, a microporous material, exhibits unsaturated metal centers after the
activation step, which could further be functionalized by other organic molecules. 13,46
In the previous research of our group23
, it was claimed that structural changes can
improve its stability after several adsorption – desorption cycles in a dry gas mixture
of 400 ppm CO2. ED-Mg/DOBDC was regenerated under 110 C with Ar purge,
conditions which are milder than for the bare Mg/DOBDC which is regenerated under
250 C in vacuum. In another words, ED-Mg/DOBDC requires less energy input for
full regeneration.
Since amine sites are considered to be effective for CO2 capture in the presence of
moisture, 38
in this work the pore structure and CO2 adsorption capacities were
investigated for functionalized Mg/DOBDC MOF. For further industrial application
consideration, a scale up Mg/DOBDC synthesis on the gram scale was performed by
modification of the original recipe.9 XRD and N2 adsorption – desorption isotherms
for this material and its derivatives were also measured.
Figure 2 illustrates XRD patterns of Mg/DOBDC and its derivatives, with
characteristic peaks at 6.9 and 11.9° 2θ, which identify with the values which were
reported in the previous report.12,23
As we can see in the figure, after functionalization,
the peak locations did not change too much which indicates the basic crystal structure
of our material did not change during functionalization. However, in the ,meantime,
degradation of the characteristic peaks could be clearly seen for functionalized
12
samples indicating that amine functionalization affects the structure of Mg/DOBDC.
With respect to steaming experiment, we may observe the similar trend that the
crystal structure retained after steaming, but the characteristic peaks have shown
obviously degradation which illustrates the water vapor destroyed the framework of
Mg/DOBDC`s.
The nitrogen isotherm is shown in Figure 3 and the pore characteristics are
presented in Table 1 for comparison. As it can be seen from Figure 3, the original bare
Mg/DOBDC shows apparent behavior of type I isotherm which indicates its
microporous structure.47
Due to enhanced adsorption in micropores, the adsorption
increases rapidly in low pressure range and then becomes saturated. After steam
treatment, the pore structure dramatically degraded, and BET surface area decreased
from 1082 m2/g to 23m
2/g. After functionalization of the bare material, even the BET
surface area of ED-Mg/DOBDC dropped to 381 m2/g, but its structure showed better
hydrophobicity. After steaming treatment, the surface area of ED-Mg/DOBDC was
121 m2/g, which is much higher than the steamed bare Mg/DOBDC. Also, compared
to bare Mg/DOBDC, the total pore volume of the steam treated material decreased
from 0.49 cm3/g to 0.24 cm
3/g, while average estimated pore diameter (approximated
from NOVA Win software, Quantachrome) decreased from 13.6 Å to 8 Å. The same
trend was observed in total pore volume and average pore diameter parameters: while
both properties degraded for steamed bare material, they increased for steamed
functionalized material. These results show that after steam treatment functionalized
Mg/DOBDC didn’t degraded as dramatically as bare Mg/DOBDC did in terms of
13
structural pore characteristics, which could mean enhanced stability under humid
conditions for ED-Mg/DOBDC.
4.2. CO2 Adsorption at Room Temperature
Some amine functionalized MOFs have been proved has higher capacities than
parent MOFs.27
Because CO2 can react with amine group of the MOFs to form
carbamate species by forming zwitterionic intermediates. Also, in the humid
conditions, through the help of H20 and OH-, the potential CO2 capture capacities of
the amine group would increase.38
Figure 4 presents the CO2 isotherms in room temperature, based on these
isotherms, adsorption capacity of bare Mg/DOBDC was about 5.08 mmol/g assuming
ideal gas conditions (low pressure range), while steamed Mg/DOBDC only reached a
capacity of 1.20 mmol/g. The functionalized Mg/DOBDC, ED-Mg/DOBDC showed
the highest CO2 adsorption capacity as 7.81 mmol/g, and 2.9 mmol/g after steam
treatment which is higher than for the non-functionalized material. While under
different settings, a similar trend was observed during CO2 adsorption via
thermogravimetric analysis (TGA). Figure 5 compares CO2 capacity values of the
same materials analyzed by TGA and NOVA 2200e. According to TGA, total CO2
uptake reached the capacity of 3.73 mmol/g, steamed Mg/DOBDC has 0.42 mmol/g
while ED-Mg/DOBDC has 4.30 mmol/g and steamed ED-Mg/DOBDC has 1.66
mmol/g. In general, CO2 capacity values were lower for samples analyzed by TGA
which could be attributed to the way the machine operates. In TGA, gas flows over
the material on the pan and therefore not all of the samples make contact with the CO2
14
molecules, causing it not become fully adsorbed by the material.38
In conclusion; it
was found that ethylenediamine functionalized Mg/DOBDC performed better for CO2
capture capacities under both humid and dry conditions compared to
non-functionalized material.49
4.3. Effect of ED Loadings
Figure 6 shows the weight changes by heating the sample from 25 °C to 600 °C.
The result suggests that ED-Mg/DOBDC lost more weight than bare Mg/DOBDC in
the range of 100 °C to 300 °C. Since the ethylenediamine`s boiling point is around
120 °C, the previous report23
reported this change as being due to ED loss. Similar
result happened to steamed samples of Mg/DOBDC and ED-Mg/DOBDC, the
steamed ED-Mg/DOBDC showed more weight loss in the range of 100 °C to 300 °C.
While comparing the sample before and after steaming, we may observe that the
difference curvature between bare Mg/DOBDC and steamed bare Mg/DOBDC
between the range of 0 °C to 100 °C due to the difference of initial moisture
conditions. While after functionalization, steamed samples lost less weights than the
parent material, since amine does not strong covalently bond to solid supports so it
may leach from solid support at high temperature and humid conditions.38
Our
steaming experiment occurs in the harsh conditions with higher concentration and
higher temperature than the real-world flue gas capture applications so it may result
amine leach in some extend. Nevertheless, we still can observe the ED molecules
inside the pores after steaming experiment, so we may suggest that our materials have
potential to be used in industrial application.
15
As I mentioned before, our group have been proved that there is not only one
configuration for functionalized material ED-Mg/DOBDC. Figure 1 illustrated our
group`s previous results that the different structures of ED-Mg/DOBDC. For each unit
cell, there could be 1, 3, 6 or 18 ED molecules, which change the pore characteristics
of the resulting material.23
In our work, by adding different amount of
ethylenediamine during synthesis, we achieved two different batches of
ED-Mg/DOBDC as shown in Figure 7. Compare the two different samples; we can
see there is a different curvature in the range of 100 °C to 400°C which are considered
to be due to the contribution of different ED loadings. By calculation, the batch 1 has
~6.3% ED inside the pores while batch 2 has only ~5.5% ED inside. This difference
may result in different pore characteristics and it certainly would affect its CO2
capture capacity. Further research of our group would include an investigation of
different ED loadings on the materials structure and further CO2 capture performance.
16
5.0 CONCLUSIONS
In conclusion, Mg/DOBDC MOF was successfully functionalized with amine
groups for applications in carbon dioxide capture. Such functionalization increased
the hydrophobicity in a humid environment which is characteristic of both flue gas
and ambient air conditions. Steam treatment was performed to evaluate the structural
degradation and CO2 adsorption capacity in ultra-high moisture conditions.24,25
As a result, Mg/DOBDC showed significant degradation after exposure to steam
but less degradation was observed for amine- functionalized Mg/DOBDC. As for CO2
uptake, ED-Mg/DOBDC showed higher capacity at room temperature even though its
surface area and pore volume decreased. These results suggest that ethylenediamine
functionalization of Mg/DOBDC increased its structural stability against water vapor,
which further led to better performance in CO2 capture.
According to the previous report of our group,23
there are several possibilities for
different loadings of one ED molecule. For those different structures, the CO2 uptake
capacities are not the same. From our research, we found that by increasing the ED
loadings, the surface area decreases as presented in Figures 7 and 8. Further studies
are required in this area to figure out the best ED loadings to design the most efficient
CO2 adsorbent.
17
6.0 RECOMMENDATIONS
For short term plans of this project, evaluation of different ED loadings should be
addressed. Based on the estimates of Choi et al.23
, there are several possibilities for
ethylenediamine loading in the Mg/DOBDC MOF. Fourier Transform Infrared
Spectroscopy (FT-IR) and Transmission Electron Microscopy (TEM) might be used to
help find the structural differences between all of the synthesized materials with
various loadings.
For long-term consideration and industrial applications despite the fact that bare
Mg/DOBDC exhibits an exceptional capacity for carbon dioxide capture under dry
conditions, it needs to be optimized for stable and efficient capture in a humid
environment. Amine-functionalized Mg/DOBDC showed better structural stability
and working capacity in the presence of moisture. Nevertheless, there are many other
factors that should be accounted for in this process, such as the materials performance
under a real environment of flue gas or ambient air. Moreover, even though the scale
up the Mg/DOBDC to around 1 gram per batch was performed without any observed
structural changes, there is still the potential for structural degradation and pore
distribution variation on the larger scale, such as in kilograms and higher orders of
weight magnitudes. The steam treatment performed in this work mimics conditions
with the largest relative humidities possible and tests whether the material can
withstand the stress. While it might be a good test for stability, the material still needs
to be tested on its adsorption in the presence of water and corrosive gases such as SOx,
NOx, and HCl which are commonly present in flue gas. In parallel, we can investigate
18
some other hydrophilic MOFs such as MIL-53,28,32
USO-1,27
CuBTTri,29
UMCM-130
and HKUST-1,31
which have high carbon dioxide capacity to find out if
functionalization would enhance their performance as well.
In addition, carbon dioxide capture from ambient air is another topic that requires
attention. In comparison with capture from flue gas generated at coal fired power
plants, air capture is more challenging since CO2 gas is more dilute in the ambient air
(about 400 ppm).1 In the near future, there is a need to build a large carbon dioxide
capture system which would be energy efficient and economically viable.
Carbon dioxide capture by Mg/DOBDC only accounts for a small portion of
possible research and further studies need to be done on the implementation of such
adsorption technology in an industrial setting.
19
7.0 NOMENCLATURE
MOF Metal-Organic Framework
Mg/DOBDC Also named as Mg-MOF-74, CPO-27-Mg
ED-Mg/DOBDC Ethylenediamine Functionalized Mg/DOBDC
DMF Dimethylformamide
ED Ethylenediamine
H4DOBDC 2,5-Dihydroxyterephthalic acid
XRD X-ray diffraction
NOVA 2200e Physisorption Analyzer by Quantachrome
TA Q500 Thermogravimetric analysis
FTIR Fourier Transform Infrared Spectroscopy
TEM Transmission electron microscopy
20
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