1. Introduction
As consumption of energy, conventional oil resource decreases sharply1-2. In order to enhance the yield of oil, the most
common method is injecting water into the oil well 3.
Although this strategy can improve the output of crude oil, it meanwhile
caused high water content for crude oil and pollution for environment3~5. The crude oil with high water
content generally exists with stable emulsion 5,6,
which can corrode pipeline and refining equipment 3,5.
Moreover, emulsion can form from the wastewater containing oil with the
help of surfactant in water, which could scatter the sunlight, hamper
the input of energy for aquatic system and increase the (bio-) chemical
oxygen demand (COD/BOD)4,7. All above will cause local
aquatic system anoxic, and reduce the water quality. Emulsion also
raises the migratory risk of toxic and insoluble pollutants, and
enlarges the difficulty of remediation for pollution4,7,8. As a result, the remove of both oil and
emulsion in water is important for the environmental remediation.
Compared with the oil/water separation 9,10, the
removalof emulsion is a challenge because of its high stability.
Therefore, the demulsification has become one of the focuses for the
petroleum industry and environment.
Compared with physical and biological methods, the chemical
demulsification is effective, energy-saving and fast11-13. An ideal demulsifier has the characteristics of
large surface area (nm2·molecule-1)
and amphipathicity 6,14. The mechanism of
demulsification for conventional
demulsifier is competing with surfactant on the interfacial film to
decrease the interface tension (IFT) and accelerate the coalescence of
emulsion 6,13,15. The driving force of demulsification
for the conventional demulsifier is
supramolecular
interaction15, which is weak and make demulsification
performance influenced frequently by the practical conditions, such as
salinity, pH and so on 6,9,16. Typically, counter ions
can shrink the double electric layer along with an increase of hydration
for the interface of emulsion, enhancing the stability of emulsion and
decreasing demulsification efficiency of conventional demulsifier17. Additionally, the conventional demulsifier need
relatively long time to obtain satisfying performance18-20. Furthermore, the weak supramolecular
interaction limits the improvement of demulsification performance. Thus,
enhancing the interaction between demulsifier and emulsion could be a
potential strategy to conquer the shortcomings of those conventional
demulsifiers.
Most of conventional demulsifiers are soluble, such as ethylene
oxide/propylene oxide (EO/PO) segmented copolymer, dendritic polymer
with amine group, polyether and ionic liquid 17,21.
The soluble demulsifier is also a potential pollutant, which increases
the cost and burden of environment. Recently, heterogeneous demulsifier
has attracted much attention due to the merit of recyclability and
environment-friendly 22-24. Metal-organic frameworks
(MOFs) are functional crystal materials, which are consisted of polar
node and non-polar bridge. This structure ensures a special microdomain
with amphipathicity 25, meeting the design of
molecular structure of demulsifier. The node of MOFs is strong Lewis
acid or Brønsted acid site 26,27, which can react with
the species contained oxygen (O), nitrogen (N), sulfur (S) and so on by
coordination, ligand/ion exchange and electrostatic interaction28,29. It should be noted that the surfactant covered
on emulsion consists of hydrophobic alkyl chain and hydrophilic group,
which usually contained O, N or S. And the crude oil also contained
heteroatomic (O, S and N) at some degree 30.
Therefore, there is potential strong interaction between the node of
MOFs and emulsion. Meanwhile the non-polar bridge of MOFs can interact
with organic species by the supramolecular interaction, which could
promote the coalescence of oil phase when the polar node of MOFs
interacts with emulsion. What’s more, the pore of MOFs can regulate the
existing states of surfactant or oil phase molecule. All of these
properties of MOFs could bring some different features for the
demulsification, compared with that of the conventional demulsifier. To
the best of our knowledge, the MOFs do not gain due attention as an
intrinsic heterogeneous demulsifier.
Herein, we prepared the MIL-100(Fe), a classical MOFs, as an intrinsic
heterogeneous demulsifier. Then, the demulsification performances both
for model emulsion and crude oil emulsion were investigated by batch
experiments using MIL-100(Fe). The process of demulsification was
studied by the inverted microscope. Moreover, the interactional
characteristics between the MIL-100(Fe) and model emulsion were revealed
by the surface measurement and component analysis.