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.