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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