Oil/water separation is an important field, not only for scientific research but also for practical applications aiming to resolve industrial oily wastewater and oil-spill pollution, as well as environmental protection. Recently, research into the role of special wettability for oil/water separation has attracted much attention. In this review we summarize the design, fabrication, applications and recent developments of special wettable materials for oil/water separation. Based on the different types of separation, we organize this review into three parts: “oil-removing” type materials with superhydrophobicity and superoleophilicity (that selectively filter or absorb oil from oil/water mixtures), “water-removing” type materials with superhydrophilicity and superoleophobicity (that selectively separate water from oil/water mixtures), and smart controllable separation materials. In each section, we present in detail the representative work, introduce the design idea, outline their fabrication methods, and discuss the role of special wettability on the separation. Finally, the challenges and outlook for the future of this subject are discussed.

Special wettable materials for oil/water separation

This paper reviews the synthesis and the absorbing properties of the wide variety of porous sorbent materials that have been studied for application in the removal of organics, particularly in the area of oil spill cleanup. The discussion is especially focused on hydrophobic silica aerogels, zeolites, organoclays and natural sorbents many of which have been demonstrated to exhibit (or show potential to exhibit) excellent oil absorption properties. The areas for further development of some of these materials are identified.

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Porous Materials for Oil Spill Cleanup: A Review of Synthesis and Absorbing Properties

IR study of polycrystalline ceria properties in oxidised and reduced states

Infrared spectrometric studies of the surface basicity of metal oxides and zeolites using adsorbed probe molecules

The review is a summary and analysis of the data characterizing CO adsorption on surface cationic sites of oxides including supported materials and microporous and mesoporous materials. The contributions of various types of CO bonding to the IR frequency shifts of carbon-bonded molecules are analyzed, namely, the increase of the CO stretching frequency in cases of electrostatic and σ bonding and the decrease of the frequency with π bonding. Polycarbonyls, bridging CO, oxygen-bonded CO, and tilted CO are also considered. The main part of the review is a collection of the experimental results characterizing carbonyls of individual metal ions. The spectral behavior of CO bonded to metal atoms is also assessed in the cases when the metal ions are easily reduced to metal (Cu, Ag, Au, Pd, or Pt) or cationic carbonyls are produced after CO adsorption on supported metals (Ru, Rh, Ir, and Os). The interaction of CO with surface OH groups is also considered. It is demonstrated that IR spectroscopy of adsorbed CO is an efficient methodology to characterize cationic surface sites in terms of their nature, oxidation states, coordination environment and coordinative unsaturation, and location at faces, edges or corners of microcrystallites. When applied to materials with surface hydroxyl groups CO undergoes hydrogen bonding and information can be collected on the proton acid strength.

Characterization of oxide surfaces and zeolites by carbon monoxide as an IR probe molecule

The basicity of alkali-metal cation X, Y, mordenite and ZSM-5 can be characterized by the infrared NH stretching frequency of chemisorbed pyrrole. It was shown that the basic sites detected by pyrrole chemisorption in zeolites are Lewis bases. This is in agreement with the view that the basic sites in zeolites are the framework oxygens adjacent to the cations. Moreover the coexistence of two NH-stretching bands in the spectrum of pyrrole chemisorbed in a zeolite containing two kinds of cations indicates that the basicity of these sites is determined mainly by the local environment rather than by the bulk zeolite composition. The basicity in higher Si zeolites becomes weaker both in basic strength and the density of basic sites, as indicated by the high NH-stretching frequency and the weak band intensity. A pyrrole species associated with acid sites is also detected.

Zeolite basicity characterized by pyrrole chemisorption: an infrared study

Characterization of basicity in alkaline cation faujasite zeolites—An XPS study using pyrrole as a probe molecule

The catalytic properties of Y zeolites, dealuminated by steam stabilization, for the transalkylation of toluene with trimethylbenzene were investigated in a pulse microreactor. Some factors influencing this reaction were taken into account: the reaction temperature, the degree of dealumination, the nature of acid sites and the deposited coke. The transalkylation reaction proceeds with good yields on USY. At bulk silica to alumina ratio's around 5, a higher activity is observed for the transalkylation of toluene than for the disproportionation of trimethylbenzenes. Both HNaY and USY samples were characterized using FTIR and XPS of chemisorbed pyridine. These techniques allowed to detect the presence of the three kinds of acidic sites: the Bronsted acid sites, the Na+ cations and the extra framework AlO+ species. The effect of these three sites on catalytic results are discussed as well as the reactivity of alkylaromatic hydrocarbons in the reaction network and the reaction mechanism.

Transalkylation of the alkylaromatic hydrocarbons in the presence of ultrastable Y zeolites Transalkylation of toluene with trimethylbenzenes

The basicity of alkali-exchanged zeolites has been characterized by chloroform adsorption using FTIR spectroscopy. The chloroform is adsorbed on the basic oxygen site through its hydrogen atom, while a weak interaction also exists between the adjacent cation and the chlorine atom of chloroform. The charge on the oxygen atoms (basic sites) was calculated using the electronegativity equivalence method. The CH stretching frequency of chloroform adsorbed on basic sites is well correlated with this oxygen charge.

M. Huang

Papers

Zeolite basicity characterized by pyrrole chemisorption: an infrared study

Characterization of basicity in alkaline cation faujasite zeolites—An XPS study using pyrrole as a probe molecule

Transalkylation of the alkylaromatic hydrocarbons in the presence of ultrastable Y zeolites Transalkylation of toluene with trimethylbenzenes

Zeolite basicity characterized by chloroform chemisorption. An infrared study

Crystallinity dependence of acid site distribution in HA, HX and HY zeolites