CHAPTER ONE
INTRODUCTION
1.1 Background of Study
With the rate at which population is
increasing and the usage of fossil fuels, the need for alternative and
sustainable energy development has become a necessity. There is
therefore need for energy saving materials that are affordable and that
pose no threat to our environment. Areas of interests such as fuel
cells, batteries, capacitors, solar cells, etc. afford much opportunity
for improvement and research focus. (Lacey et al., 2014)
Detection of hazardous materials is a
significant concern for homeland security, and portable sensors are
potentially desirable to avoid the complexity of such materials, such as
explosives. Luminescent metal organic frameworks (MOFs) are potentially
useful as chemically-selective sensors.The study of luminescence
behaviour in metal organic frameworks is an active area of research in
inorganic chemistry. A large numberof 3d element have been employed to
investigate the ligand luminescence. The most commonly reported metal
organic framework structures are based on Zn2+ and Cd2+
ions, which have filled d orbitals and thus d–d transitions are not
possible. In these compounds, intra-ligand and/or ligand metal charge
transfer effects have been observed. The photoluminescence studies are
important to understand the charge-transfer pathways. The studies on the
luminescence behavior in MOFs clearly indicate that the energy transfer
from the organic ligands to the metal center is easy. The life-time
studies also suggest that the excited state possess considerable
stability and allows for exploitation in many important applications.
(Zhou, X et al., 2009; Konar et al., 2013)
Metal-organic frameworks (MOFs), also
known as metal-organic networks, or coordination polymers, represent a
new class of compounds containing metal ions linked by organic bridging
ligands. A metal-organic framework (MOF) material can be thought of as
the composition of two major components: a metal ion (or cluster of
metal ions) and an organic molecule called a linker (or bridging
ligands).
Metal ions + Organic linkers??? Coordination polymers/ MOFs
The organic ligands or linkers are
groups that can donate multiple lone pairs of electrons (polydendate) to
the metal ions, whereas the metal ions are made up of vacant orbital
shells that can accept these lone pairs of electrons to form a
metal-organic framework material.
Metal-organic framework materials are
well-defined, adjustable and highly porous materials, with spatial
confinement, often crystalline, sensitive to air and resistant to
structural collapse upon heating. (Zang et al., 2006)
1.1.1 Metal-Organic Frameworks as Functional Materials
Early research in MOFs tended to
concentrate on synthesis of a wide variety of new compounds with novel
topologies and compositions. Although the design of new structures
remains highly topical and a significant amount of research efforts are
still devoted to the discovery of unprecedented network topologies, the
primary motivation in crystal engineering of MOFs has shifted toward
constructing materials that possess specific physical properties and
supramolecular functions, such as catalysis, gas storage, luminescence,
nonlinear optical properties (NLO), and magnetism. Photoluminescence is a
form of luminescence. Phosphors are luminescent materials that emit
light when excited by radiation, and are usually microcrystalline
powders or thin-films designed to provide visible color emission. It is a
light emission from any form of matter after the absorption of photons.
It is a process whereby molecules absorb, stores and emits light and is
initiated by photo excitation. (Shinde et al., 2013)
1.2 Synthesis of metal organic framework
Two methods are adopted, namely: (i) conventional (ii) unconventional methods.
1. Conventional synthesis is
frequently performed by solvothermal methods; i.e. heating a mixture of
organic linker and metal salt in a solvent system. The materials
produced from this method are thermally unstable or reactive to solvent
used, and this could lead to breaking of bonds or creation of metal
sites for guest species such as gases to gain access into the
framework’s micropores. In some cases, assisted microwave-solvothermal
synthesis can be used, to monitor or regulate the temperature whilst the
reaction is in progress. Precipitation and crystallization follows
immediately after heating a mixture of a chosen metal ion and organic
linker in the presence of a solvent. Thereafter, filtration and drying
follows. Slow evaporation method is a conventional method and is
advantageous because the crystals assemble themselves though it is time
consuming. (Tella et al., 2012)
2. Unconventional Synthesis is
frequently performed by grinding a mixture of organic linker and metal
salt in an agate mortar and pestle or in a ball mill and in the absence
of solvent. This method is known as mechanochemical method. The metal
sites are exposed when the mixture is gently heated, thus allowing gases
such as hydrogen to bind at these sites. The mechanochemically
initiated reactions are comparable to those of solvent reactions. The
method is known to be environmentally friendly and can possibly give
high yield of products.
(Alex et al., 2005; Tella et al., 2012)
1.2.1 Chemistry of 8-Hydroxyquinoline
8-hydroxyquinoline is an organic compound with the formula C9H7NO.
It is a derivative of the heterocyclic quinoline by placement of an OH
group on carbon number 8. It is a monoprotic bidentate chelating agent.
8-hydroxyquinoline has a hydrogen atom that is replaceable by a metal,
and a heterocyclic nitrogen atom, which forms with these metals a five
membered ring. It forms stable complexes with several metal ions.(Sharet al., 2005)
1.2.2 Chemistry of Benzoic acid
Benzoic acid (C6H5COOH)
is a colourless crystalline solid and a simple aromatic carboxylic
acid. Benzoic acid occurs naturally in many plants and it serves as an
intermediate in the biosynthesis of many secondary metabolites. Benzoic
acid is an important precursor for the industrial synthesis of many
other organic substances. Benzoic acid is mainly consumed in the
production of phenol by oxidative decarboxylation at 300-400 0C. It is one of the ligands used in this research work. (Jarad et al., 2011)