Introduction
1,3-Butadiene (BD) is a very
versatile organic raw material, a petrochemical-based VOC, used as a
monomer for the manufacture of synthetic rubber1. Due
to the presence of conjugated double bonds in its molecule, addition,
cyclization, substitution, and polymerization reactions can take place,
which makes this VOC a widely used compound in synthetic rubber and
organic synthesis2. Due to its high consumption for
the synthetic rubber industries, it became so essential to control it
from the tail gases of synthetic rubber plants. In addition, the
international agency for research on cancer has designated 1,3-Butadiene
as a possible carcinogenic material, while environmental protection
agency (EPA) has listed it as the 10th most
carcinogenic material3. Butadiene mainly mixed with
the nitrogen during its post-treatment, i.e., drying process, where
butadiene evaporates from the synthesized rubber and mixed with the tail
gas of the plant. At present, the catalytic oxidation method used to
remove the BD from tail gas before emitted to air, which completely
converted the VOCs into CO2 and H2O.
Although catalytic oxidation removed BD from tail gas of the synthetic
rubber plant, it is a destructive method to control the VOC emission so
that there must be some constructive method required, which can
effectively recover the BD from tail gas and can be reused as well.
Membrane-based technology is one of the best possible solutions for
complete VOC recovery. The membrane separation process developed over
the last decades and considered to be very attractive due to its ease of
operation, small footprint, low energy usage, easily scale up
production, and more significantly, it offers the ability to meet future
stricter environmental limits4.
In the recent works, MMMs showed
good results for many applications such as water separation, flue gas
purification, petrochemical separation, and natural gas
separation5–9. The synthesis of mixed matrix
membranes (MMMs) is the most promising approach for using both organic
and inorganic material properties at the same time, which also improves
gas transport properties and may surpass the Robeson upper bound
limit4,10. It involves the fabrication of composite
membranes between polymeric materials as a base, and inorganic material
as filler particles. Theoretically, by use of MMMs, benefits of both
polymer and inorganic content can be used at the same time. Moore et al.
identified the different non-ideal structures in MMMs such as rigidified
polymer layer around the inorganic fillers, interface voids or sieve
in-a-cage, and particle pore blockage11. In order to
surpass the Robeson upper bound, the MMMs structure must be defect-free
at the polymer/filler interface. For overcoming the poor adhesion, it
requires careful selection of the filler and polymer, which must have
good interaction with each other. However, the selection of fillers and
polymers is limited to form a defect-free interface on the basis of
these factors12. Polydimethylsiloxane
(PDMS)13–15, polyether block amide
(PEBA)5,16, matrimid17, polymers of
intrinsic micro porosity (PIM)18, polyethylene oxide
(PEO)19, polyvinyl alcohol (PVA)20,
and several polyimides21,22 are polymers that have
been commonly used in the fabrication of MMMs, while carbon molecular
sieves23, zeolites24, carbon
nanotubes13, graphene oxide
(GO)25,26, and metal-organic frameworks
(MOFs)27,28, are the most extensively used nano porous
materials as fillers in MMMs.
There is no data available for the separation of BD from its nitrogen
mixture through the membrane separation process in the literature. Baker
et al. stated that rubbery membranes are beneficial for the 99%
recovery of hydrocarbons with a suitable process
design29.
Polydimethylsiloxane (PDMS) is a
rubbery polymer and well-known membrane material due to its excellent
chain flexibility makes it very permeable even at high operating
temperature and low trans-membrane pressure. By the end of the last
century, PDMS predominated and widely studied polymer for hydrocarbon
separation. Past results have shown that the PDMS membrane has excellent
potential for recovering hydrocarbons from nitrogen mixed
streams30–32. Additionally, it is worth noticing that
the 1,3-butadiene permeation through rubbery polymers such as PDMS is
superior to its permeation through any glassy polymer since there exist
two to three orders of magnitude difference in
permeation33.
Zeolite imidazolate framework (ZIF) is a subfamily of MOFs, which is
coordinated between the transition metals such as zinc and cobalt and
imidazole linkers34. In recent years, ZIFs have
engrossed deep interest as a versatile crystalline porous material for
gas storage and gas separation applications. The highly crystalline and
nano-porous structures of ZIFs, make them an ideal candidate for the
separation of small kinetic diameters gas particles. The imidazolate
linkers in ZIFs frameworks made them more hydrophobic as compared with
any other filler types, which deliberates the excellent interfacial
compatibility between polymers and ZIFs35. Moreover,
the imidazolate linker property of structural flexibility makes its role
more essential and vital as it involves in gate opening effect of ZIFs
for large molecules than the pores of ZIFs36. ZIF-8 is
the most studied ZIF material among all of its types in MMMs for the gas
separation34,37,38. It is worth noticing that; besides
the extensively researched linker-substituted effect, metal-substitution
also influences the molecular sieving efficiency of the associated ZIF
crystals. By introducing a second metal ion in the MOF cluster, the
stability and affinity towards the target gases can be largely
improved39–41. Many recent works have reported in
which researchers have used mixed metal MOFs by adding another metal in
the ZIF-8 crystal for properties enhancement41. The
results were improved by the addition of another metal in the ZIF-8
cluster. To the best of our knowledge, the separation of low boiling
point hydrocarbons (gases at room temperature) from permanent gases,
using ZIF-based membranes, is very limited. Just one study reported by
Fang et al. in which they used ZIF-8/PDMS/PVDF MMMs to separate propane
from nitrogen mixture42. The
C3H8/N2 selectivity was
38% improved by using 10% ZIF-8-MMMs relative to the pure PDMS
membrane. Therefore, the ZIF-8 based MMMs should be further examined for
the hydrocarbon separation from permanent gases.
This work, for the first time, employed Zn/Ni-ZIF-8 as a filler for the
preparation of MMMs with PDMS polymer to improve the gas-separation
properties of the membranes for the separation of BD/N2.
Ni-ZIF-8 has the same zeolite nets-sodalite (SOD) topological structure
with ZIF-8, while Ni just partially substitutes the ZnN4in the backbone of the ZIF structure, and tetrahedrally connected with
the nitrogen, which makes it a very stable structure of four coordinated
with Ni centers. Mixed metal ZIF was used to enhance BD affinity, which
resulted in a sufficient adsorption capacity due to the presence of two
metals in its cluster. The difference of gas permeation results was
tested for single and mixed metal ZIF-8 in their MMMs.
The effects of Ni-ZIF-8 loading on
the microstructure of the membranes were investigated. The influence of
Ni-ZIF-8 nanoparticle loadings feed temperature, and feed pressure on
gas-permeation performance were also observed. Importantly, the gas
transport properties across these MMMs for 1,3-butadiene and
N2 were investigated on a preliminary basis by exploring
the effects of different MMMs. This study will introduce the permeation
and solution-diffusion properties of Ni-ZIF-8/PDMS MMMs for the removal
of BD from nitrogen mixture, which will also provide a scientific
contribution in the literature.