International Forum for Green Asia 2020

Metal Organic Frameworks for Atmospheric Water Harvester
Tahmid Hasan Rupam1,2, M. L. Palash1,2, Bidyut Baran Saha2,3
1Kyushu University Program for Leading Graduate School, Green Asia Education Center
Kyushu University, Kasuga-koen 6-1, Kasuga-shi, Fukuoka 816-8580, Japan
2
International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka,
Nishi-ku, Fukuoka 819-0395, Japan3
Mechanical Engineering Department, Kyushu University, 744 Motooka, Nishi-ku,Fukuoka 819-0395, Japan
E-mail: tahmidrupam33@gmail.comOverview
Scarcity of pure drinkable water has made many researchers think of new alternativetechniques for harvesting water from the environment. In the quest for new sources of purewater, researchers have invented many technologies involving seawater sweetening and water
cultivation from the air. Desalination of seawater is expensive and also there is an issue ofsludge management whereas, water harvesting from moist air is less expensive and it has noissues of sludge [1]. Among many technologies, the environment-friendly adsorption based
water harvesting system stands out as an emerging future technology due to its low power
consumption[2]. Metal organic frameworks (MOFs) are thought to be the next generation
adsorbents for water based adsorption systems due their regular structure, tunable porosity andmost importantly their water adsorption isotherm’s shapes[3]. This study focuses on thesynthesis and characterization of two porous metal organic frameworks- MOF 801 and MIL
100 (Fe) intended for the application of adsorption based water harvester.
Synthesis of metal organic frameworks
MOF 801 is a microporous 3D framework having zirconium (Zr6) nodes bridged with fumarate
linkers[4]. To synthesize this ZrOCl2·8H2O and fumaric acid (having the ratio of 1:0.36 in termsof gm) was dissolved in a solvent made of DMF and formic acid (1: 0.35 in terms of liter).
Then the mixture was heated at 130 ºC for 6 hours and MOF 801 precipitated. Then thesynthesized MOF 801 was separated, washed, dried and then activated by heating at 150 ºC
inside the vacuum.
In case of MIL 100 (Fe), the molar ratio Fe : BTC : HNO3 : H2O of 1: 0.67 : 0.60 : 166 wasused for the hydrothermal synthesis[5]. All reactants were mixed and loaded into a Teflon
autoclave which was heated at 150 °C for 12 h. The resulting MIL-100(Fe) solid was collected
and washed for several times using distilled water. Subsequently, the MIL-100(Fe) solid waspurified by soaking in water at 80 °C for 1 h, followed by ethanol at 60 °C for 3 h. The purified
MIL-100(Fe) was dried in a 60 °C oven and heat-activated using a vacuum oven for 2 h at120 °C.

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Material characterization
N2 adsorption was done on both the samples at 77K to investigate about their BET surface
area and pore size distribution. The results are furnished in Table 1. FESEM was done on boththe samples to examine their morphological depiction. The water uptake was also measured
for both the MOFs using thermos-gravimetric analyzer.
Results and discussion
Table 1 shows that MIL 100 (Fe) has significantly higher surface area and pore volume whencompared with MOF 801. The pore size distribution of the two assorted MOFs are shown inFig 1 (a) and (b) respectively. We can conclude that both the samples are highly microporous.
FESEM images of the two MOFs are depicted in Fig 1 (c) and (d) respectively. MOF 801 hasregular circular shaped grains where for MIL 100 (Fe) the grains were irregular in shape. Thewater uptake for both the samples are exhibited in Fig 2 (a) and (b) at 303 K. It is evident thatMIL 100 (Fe) has higher water uptake than MOF 801. However, MOF 801 has high uptake inthe lower relative pressure region (less than 30 % RH).

(b)(a)

0.25

dV/dW Pore Volume [cm³/g·Å]dV/dW Pore Volume [cm³/g·Å]

0.25

0.2

0.15

0.1

0.05

0.15

0.1

0.05

000(c)

0.2

51015
Pore Width [Å]2025030(d)51015
Pore Width [Å]202530Fig. 1 Pore size distribution of (a) MOF 801, (b) MIL 100 (Fe); FESEM image of (c) MOF 801, (d) MIL 100 (Fe)20

International Forum for Green Asia 2020

Table 1. porous properties of MOF 801 and MIL 100 (Fe)Metal organic

BET surface area

Micro-pore

Micro-pore area

framework[m2g-1]volume[m2g-1][cm3 g-1]
MOF 801865

0.32758

MIL 100(Fe)1735

0.471132(a) MOF 801(b) MIL 100 (Fe)

0.45

0.8

0.4

0.7
0.6

Water uptake [g/g]Water uptake [g/g]

0.35
0.3
0.25
0.2
0.15

0.5
0.4
0.3
0.2

0.1

0.1

0.05
0

00%

10%

20%

30%

40% 50% 60% 70%
Relative humidity [-]80%

90% 100%

0%

10%

20%

30%

40% 50% 60% 70%
Relative humidity [-]80%

90% 100%Fig 2. Water adsorption isotherms on (a) MOF 801 and (b) MIL 100 (Fe) at 303 KConclusions
MIL 100 (Fe) MOF had higher surface area and pore volume than MOF 801. Hence MIL 100(Fe) higher water uptake in saturation pressure when compared with MOF 801. However, MOF
801 has steeper water uptake in low pressure region ensuring its applicability in water
harvesting in the desert areas of the world while MIL 100 (Fe) can be used in the tropicalregions. Again using MOF 801 an multiple cycle adsorption based atmospheric water harvester
is possible where as in case of MIL 100 Fe there will be only one cycle as the high humidityof 35% or more can only be achieved in the night times. On the other hand MIL 100 offers anadvantage over MOF 801 in case of regeneration temperatures. MOF 801 requires to beregenerated until it bed reaches a vapor pressure of 10% RH or less where for MIL 100 Fe therequirement is only until 20%-25% RH.

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International Forum for Green Asia 2020

Reference[1]

A. A. Salehi, et, al. Renew. Sustain. Energy Rev. 2020, 120, 109627.

[2]

M. J. Kalmutzki, C. S. Diercks, O. M. Yaghi, Adv. Mater. 2018, 30, 1–26.

[3]

M. L. Palash, I. Jahan, T. Hasan Rupam, S. Harish, B. Baran Saha, Inorganica Chim.
Acta 2019, 119313.

[4]

F. Ke, C. Peng, T. Zhang, M. Zhang, C. Zhou, H. Cai, J. Zhu, X. Wan, Sci. Rep. 2018,8, 1–11.

[5]

M. A. Simon, et, al. Sci. Rep. 2019, 9, 1–11.

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