(377s) A Comparison of Gas Adsorption Capacities of Isostructural Metal Organic Frameworks: [{Cu2(abtc)}3] Versus [{Cu2(reduced abtc)}3]

A
Comparison of Gas Adsorption Capacities of Isostructural
Metal Organic Frameworks: [{Cu₂(abtc)}₃]
versus [{Cu₂(reduced
abtc)}₃]

Prudhviraj Medikonda¹,
Sastri Chivukula²
and Sasidhar Gumma^1*

¹
Department of Chemical Engineering, Indian Institute of Technology
Guwahati, India

²
Department of Chemistry, Indian Institute of Technology Guwahati,
India

Corresponding
author’s e-mail:
s.gumma@iitg.ac.in

Metal-organic
frameworks (MOFs) can be defined as crystalline micro porous
materials made by coordinatively organic linkers using metal
clusters. Different
types of aromatic polycarboxylate organic linkers such as 1,3-
benzene dicarboxylate, 1,3,5- benzene tricarboxylate and
3,3′,5,5′-biphenyl tetracarboxylate have been proven useful
organic linkers for preparation of MOFs. In contrast,
3,3′,5,5′-azobenzene tetracarboxylate (H₄AzoBTC)
and 5,5’-(hydrazine-1,2-diyl)
diisophthalic acid (H₄
Reduced
AzoBTC),
one kind of tetra carboxylate linkers possess many advantages
including azo groups presence, photo-chromatic in nature and rigidity
in the structure inspired to study this work. The distinct difference
in the studied structures were functional groups (N=N and HN-NH) and
twisted nature presented in Reduced AzoBTC. The
adsorption affinity of the HN-NH group over N=N
and pore arrangement in the MOF can
be understood through this study

Scheme 1:
Adsorption capacities of structures H₄AzoBTC
(left) and H₄
Reduced
AzoBTC (right) contained MOFs compared in this work

The
gas
adsorption characteristics were evaluated by measuring adsorption
isotherms over a wide range of polarity and polarizability viz.
CO₂,
CO, CH₄,
N₂,
C₂H₆,
C₃H₈
and O₂. For
all studied gases Type-I isotherm was observed.
The
isotherms were modeled using modified virial equation for polar gases
(CO₂
and CO) and Langmuir model used for less polar gases (O_2,
N₂,
CH_4, C₂H₆
and C₃H₈).
Model parameters used to calculate the enthalpies of adsorption.
Ideal Adsorbed Solution Theory was used to predict selectivity of
binary mixtures. The CO₂
Selectivity over N₂
increases significantly with increase in pressure. However, the CO₂
selectivity over N₂
for H₄
Reduced
AzoBTC contained MOF
is more than H₄
AzoBTC
contained MOF
due to strong affinity of functional group and framework-adsorbate
interactions.

Keywords:
metal-organic
frameworks, adsorption