Ethanol Dehydration Research Articles

Computational Study of Dehydration and Dehydrogenation of Ethanol on (TiO2)n (n = 2–4) Nanoclusters

Dehydration and dehydrogenation of an ethanol molecule on (TiO2)n, n = 2-4, nanoclusters were studied at the correlated molecular orbital theory CCSD(T)/aug-cc-pVDZ(-PP(Ti)) level using density functional theory B3LYP/DZVP2-optimized geometries. Physisorption and chemisorption of ethanol at the bridge Ti site on the trimer and tetramer are thermodynamically preferred over these reactions at the Ti site with a terminal Ti═O. Two possible lowest energy reaction coordinates of dehydration were predicted for the dimer and trimer where the β hydrogen on ethanol transfers to the adjacent terminal oxygen, or to the adjacent bidentate oxygen. Only the latter reaction coordinate was predicted to be the lowest energy one for the tetramer. Removal of ethylene from the (TiO2)nOH2-C2H4 complex for n = 2-4 at 0 K requires 2-7 kcal/mol. For dehydrogenation, transfer of the α hydrogen to the adjacent Ti atom results in the lowest energy reaction coordinate following a proton-coupled electron-transfer (PCET) process. Removal of the acetaldehyde molecule requires 14-26 kcal/mol from the (TiO2)nH2-C2H4O complex. Loss of H2 from the (TiO2)nH2 complex requires 5-8 kcal/mol. Dehydration and dehydrogenation of one ethanol molecule occur below the reactant asymptote for (TiO2)n, n = 2-4, whereas for (WO3)3 and (MoO3)3, two ethanol molecules are required for this process to be below the reactant asymptote. Dehydration of ethanol is thermodynamically preferred over dehydrogenation on (TiO2)n, n = 2-4. There is an approximate linear correlation of metal Lewis acidity with physisorption of ethanol. A quadratic correlation is predicted between the chemisorption barrier of ethanol and the corresponding proton affinity of oxygen to which the proton is being transferred. There are linear correlations between the basicity of the oxygen site and the acidity of the OH group versus the energy to remove C2H4 from that site. The results for the nanoclusters for n = 3 and 4 are consistent with the experimental results for the reactivity of ethanol on Ti5c4+ rutile TiO2 (110) surface sites.

Dehydration of Aqueous Ethanol Mixtures By Extractive Distillation

This paper investigate the dehydration of ethanol by extractive distillation process with salt in solvent and also without salt. There are many sources to produce ethanol. It can be obtained from plants and agricultural wastes as well as olefins from cracking units of petroleum refining. Because the methods of ethanol production are the hydration method and fermentation, water molecules are present in ethanol. It has to be dehydrated to use as a fuel in automobile engines and other applications. There are many methods to separate water from ethanol to improved concentration. In this paper, the dehydration of ethanol is carried out by extractive distillation process and use ethylene glycol as a separating agent with calcium chloride as salt in order to improve the concentration of ethanol. Different ratios of ethanol-water and ethylene glycol such as (1:1:0, 1:1:0.25, 1:1:0.25 (20% of solvent)) were tested to obtain the best result of concentration percent. The maximum ethanol concentration was 92 vol% which obtained from 52 vol% ethanol in feed mixture.

Fabrication of a new composite membrane consisting of MXene/PES /PEI for biofuel dehydration via pervaporation

A novel two-dimensional transition metal carbide with great potential for diverse applications is MXenes, which are synthesized and characterized in this paper. The study focuses on developing polyethersulfone (PES) and polyetherimide (PEI) mixed membranes and their modification with MXene nanoparticles for ethanol dehydration. X-ray diffraction (XRD), and Fourier transform infrared (FTIR) techniques confirmed that MXene and MAX were appropriately synthesized. The morphology and characteristics of the membranes were investigated by FTIR, field emission scanning electron microscopy (FE-SEM) imaging, and EDX map analysis. the result of contact angle analysis shows that the hydrophilicity of the MXene membrane is modified. Also the membrane's pervaporation ability evaluated for ethanol dehydration. The modified membrane, containing 5 wt% MXene, demonstrates superior performance with a high flux rate (>81.7 g/m2h) and a reasonable separation coefficient (112.67).

Efficient photothermal alcohol dehydration over a plasmonic W18O49 nanostructure under visible-to-near-infrared irradiation

Developing new techniques toward low-cost, high-efficiency, and environmental-friendly transformation of alcohol to hydrocarbon compounds via carbon–carbon coupling has been paid a lot of research attention. In this study, we report for the first time on photothermal alcohol dehydration over a plasmonic W18O49 nanostructure under visible-to-near-infrared (Vis−NIR) irradiation. In the case of methanol dehydration, dimethyl ether and ethylene are generated as the major products with a 75% of methanol conversion efficiency. The light source, especially the NIR part, plays an important role in activation of the target methanol molecules, favoring the photothermal catalysis for both the methanol conversion and ethylene selectivity. The methanol dehydration activity is also greatly improved with increasing the content of W5+ in the surface of W18O49. More importantly, the non-stoichiometric W18O49 catalyst can be self-remediated via the reduction of surface W6+ to W5+ by photoelectron and methanol. In addition to methanol dehydration, the W18O49 catalyst also exhibits an extremely high catalytic activity for photothermal ethanol dehydration with nearly 98% ethanol conversion and 100% ethylene selectivity under Vis−NIR irradiation. This work provides a new insight into the photothermal catalytic alcohol dehydration over plasmonic semiconductors.

Fine‐Tuning Texture of Highly Acidic HZSM‐5 Zeolite for Efficient Ethanol Dehydration

AbstractAchieving high and stable ethylene yield from (bio)ethanol dehydration over highly active acidic zeolite remains challenging due to undesired side‐reactions. To overcome this issue, the fine‐tuned textural property of the HZSM‐5 catalyst with a hierarchical structure is crucial to overcome diffusion restrictions and limit undesired side‐reactions. Herein, the texture of high acid catalysts obtained via hydrothermal synthesis was simply tuned in the presence of tetrabutylammonium hydroxide as a meso‐ and micropore directing agent and the controlled molar ratio of NaF‐to‐Al2O3. The hierarchically designed HZSM‐5 with the small nanosheet size of 6.5 nm exhibits a high external surface area and mesopores, largely enhancing the catalytic performance of ethanol dehydration up to 95 % ethylene yield as well as preventing the formation of heavy hydrocarbons. To gain insights into the mechanistic points of view, the in situ DRIFTS study revealed that ethylene could form through ethoxy‐mediated mechanism or decomposition of diethyl ether (DEE). The catalyst deactivation caused by polyaromatics obtained from side‐reactions is the reason for low ethanol conversion and high DEE selectivity. Reducing the crystal size of highly acidic zeolite to ultra‐thin nanosheet can shorten the residence time of ethanol, intermediates, and products in porous structures, substantially suppressing the transformation of coke precursors into heavy hydrocarbons to achieve high and stable ethylene yield.

SUZ-4 zeolite interlayer enhanced thin-film composite pervaporation membrane for ethanol dehydration

SUZ-4 zeolite is a needle-like porous material with potential in separation. By incorporating a SUZ-4 zeolite interlayer on the macroporous ceramic hollow fibers to remedy the defects on the surface, thin-film composite (TFC) membranes with better pervaporation (PV) performance were expected to be prepared by interfacial polymerization (IP) processes. Through relevant experiments and characterization, high performance SUZ-4 zeolite interlayer enhanced TFC membranes were obtained by incorporating a zeolite interlayer and undergoing two-pass IP process. In addition, the TFC membrane had a special structure in which the inner layer was a nesting of polyamide and zeolite crystals, while the outer layer was a polyamide layer wrapping the inner layer. The structure of the inner layer similar to the mixed matrix membrane allowed the properties of the SUZ-4 zeolite to be utilized, whose performance was significantly better than the TFC membrane without the zeolite interlayer. After further optimization, the SUZ-4 enhanced TFC membrane exhibited a flux of 3.18 ± 0.3 kg/(m2·h) with a separation factor of 1056 ± 150 when applied to the pervaporation of 90 wt% ethanol dehydration at 60 °C. Moreover, the developed novel TFC membranes maintained good stability, which indicated these membranes possess great potential for pervaporation dehydration.

Photodehydration of Ethanol Mediated by CuCl2-Ethanol Complex.

Biomass ethanol is regarded as a renewable resource but it is not economically viable to transform it to high-value industrial chemicals at present. Herein, a simple, green, and low-cost CuCl2-ethanol complex is reported for ethanol dehydration to produce ethylene and acetal simultaneously with high selectivity under sunlight irradiation. Under N2 atmosphere, the generation rates of ethylene and acetal were 165 and 3672 μmol g-1 h-1, accounting for 100% in gas products and 97% in liquid products, respectively. An outstanding apparent quantum yield of 13.2% (365 nm) and the maximum conversion rate of 32% were achieved. The dehydration reactions start from the photoexcited CuCl2-ethanol complex, and then go through the energy transfer (EnT) and ligand to metal charge transfer (LMCT) mechanisms to produce ethylene and acetal, respectively. The formation energies of the CuCl2-ethanol complex and the key intermediate radicals (e.g., ·OH, CH3CH2·, and CH3CH2O·) were validated to clarify the mechanisms. Different from previous CuCl2-based oxidation and addition reactions, this work is anticipated to supply new insights into the dehydration reaction of ethanol to produce useful chemical feedstocks.

A Priori Design of Dual-Atom Alloy Sites and Experimental Demonstration of Ethanol Dehydrogenation and Dehydration on PtCrAg.

Single-atom catalysts have received significant attention for their ability to enable highly selective reactions. However, many reactions require more than one adjacent site to align reactants or break specific bonds. For example, breaking a C-O or O-H bond may be facilitated by a dual site containing an oxophilic element and a carbophilic or "hydrogenphilic" element that binds each molecular fragment. However, design of stable and well-defined dual-atom sites with desirable reactivity is difficult due to the complexity of multicomponent catalytic surfaces. Here, we describe a new type of dual-atom system, trimetallic dual-atom alloys, which were designed via computation of the alloying energetics. Through a broad computational screening we discovered that Pt-Cr dimers embedded in Ag(111) can be formed by virtue of the negative mixing enthalpy of Pt and Cr in Ag and the favorable interaction between Pt and Cr. These dual-atom alloy sites were then realized experimentally through surface science experiments that enabled the active sites to be imaged and their reactivity related to their atomic-scale structure. Specifically, Pt-Cr sites in Ag(111) can convert ethanol, whereas PtAg and CrAg are unreactive toward ethanol. Calculations show that the oxophilic Cr atom and the hydrogenphilic Pt atom act synergistically to break the O-H bond. Furthermore, ensembles with more than one Cr atom, present at higher dopant loadings, produce ethylene. Our calculations have identified many other thermodynamically favorable dual-atom alloy sites, and hence this work highlights a new class of materials that should offer new and useful chemical reactivity beyond the single-atom paradigm.

Hypergravity field induced self‐assembly of 2D MXene in polyvinyl alcohol membrane matrix and its improvement of alcohol/water pervaporation

AbstractHydrophilic nanomaterials were always used to improve the pervaporation (PV) performance of polyvinyl alcohol (PVA) membrane in the process of ethanol dehydration. In this work, the PVA/MXene composite membrane with high water flux and separation factor was fabricated with spin coating method. Induced by the hypergravity force, the MXene nanosheets were segregated and stacked layer‐by‐layer between the interface of the PVA/MXene layer and the support. The self‐assembly 2D MXene in the PVA not only improved the binding force between the separation layer and the support but also enhanced the PV performance. The water flux and separation factor of the PVA/SSA/MXene composite membrane reached 1.22 kg·m−2·h−1 and 1577, respectively. Both were higher than those of most reported PVA membranes. Nano indentation and scratch tests also revealed that due to the high modulus and hardness of the PVA/SSA/MXene composite membrane, 0.2822 and 0.019 GPa, strong binding force was existed between the separation layer and the support. Partly based on this reason, the structure of the PVA/SSA/MXene composite membrane remained stable during 300 hours of the PV test. It is believed that the prepared PVA/SSA/MXene composite membrane has a broad application prospect in the PV process for ethanol dehydration.

Aquivion® PFSA-based spray-freeze dried composite materials with SiO2 and TiO2 as hybrid catalysts for the gas phase dehydration of ethanol to ethylene in mild conditions.

Aquivion PFSA resin, a perfluorinated ion-exchange polymer, has been used as a heterogeneous strong acid catalyst for a range of reactions; however, the activity of this material is limited due to the extremely low surface area of the polymer. In this paper we described the one-step synthesis of Aquivion® PFSA-based hybrid materials using heterocoagulation and spray-freeze-drying of sols containing the precursor of the active phases. The intimated encapsulation of different nano-oxides, such as TiO2 and SiO2 in the superacid resin matrix was easily obtained using this technique and compared with similar catalysts prepared by the impregnation conventional route. The approach led to the preparation of porous micro-granules characterised by a high homogeneity in the phase distribution and high surface area. The prepared materials were active and selective for the gas phase dehydration of ethanol to ethylene in mild conditions. The increase of the porosity improved the activity of the composites, compared to the pure Aquivion® PFSA, and allowed to reduce the amount of the superacid resin. Moreover, the type of encapsulated oxide, TiO2 or SiO2, modified the improved performance of the catalysts, having TiO2 the higher efficiency for ethanol conversion and selectivity in ethylene at very low temperature.

Modelling and control of a bio-ethylene production process

Ethylene is one of the most consumed products in the world, as it has many uses such as the production of nylon from its polymeric compound, the production of vinyl chloride, which is polymerized to polyvinyl chloride for the production of plastics, the production of ethylene oxide, used as a ripening agent for fruits, etc. The conventional method adopted in the production of ethylene is by steam cracking of naphtha at high temperatures. Naphtha is a hydrocarbon, so when cracked, it releases harmful carbon dioxide (CO2) into the atmosphere, and this brought about looking for alternative methods of producing ethylene. As discovered, ethylene can be produced by catalytic dehydration of ethanol, but the main limitation of this process is that the purity of ethylene produced by this approach may not be up to the desired polymer grade of 99.97%. As such, this work has been carried out to model, with the aid of Aspen Plus, and develop control techniques that would enable the process to meet up with the desired output maximum purity from the distillation column (100% ethylene at the top product of the distillation column). In line with that, P-only, PI, and PID controllers tuned with the Tyreus-Luyben technique, Zeiger-Nichols method, and a modified Tyreus-Luyben approach have been used for the control of this process. It was discovered that the PID controller tuned with the modified Tyreus-Luyben parameter had the lowest Integral Absolute Error (IAE) of 1.474 and Integral Time Absolute Error (ITAE) of 4.767 and, hence, it was found out that it could be adopted for proper control of this process, although limited to small upsets caused by disturbances in the system. It is, therefore, recommended that the PID control system developed should be applied on a physical set-up plant to study its real effect.

Novel Complex Titanium NASICON-Type Phosphates as Acidic Catalysts for Ethanol Dehydration

The conversion of ethanol towards ethylene and diethyl ether in the presence of catalysts requires special consideration from the perspective of green chemistry. Ethanol dehydration was studied on a complex titanium phosphate MAlTiP (M0.5(1+x)AlxTi2-x(PO4)3 with M = Ni, Mn (x = 0; 0.2)) catalysts, alongside a NASICON-type structure synthesized by the sol–gel method. The initial catalysts were characterized by N2 gas sorption, SEM, XRD and spectroscopic methods (Raman and DRIFT of adsorbed CO and C6H6). The results revealed that all catalysts exhibited high activity and selectivity at 300–420 °C. The conversion of ethanol increases with the reaction temperature, reaching 67–80% at 420 °C. The MnAlTiP exhibited the highest ethylene selectivity among other catalysts, with 87% at 420 °C. The aluminum modification improved the acid properties of the catalysts, due to the appearance of Lewis acid sites (LAS) and the strength moderate Brønsted acid sites (BAS). It was shown that the activity of complex phosphates in ethanol dehydration increases with the strength of the Brønsted acid sites (BAS).

Mechanisms of ethanol dehydration to ethylene on γ-Al2O3（100）and (110C): A combined DFT and KMC study

Periodic density function theory (DFT) and kinetic Monte Carlo (KMC) method are carried out to investigate ethanol dehydration to ethylene on γ-Al2O3 surface. Data on the adsorption energy, adsorption sites and heat of reaction of key species for the reaction of ethanol dehydration to ethylene on γ-Al2O3(100) , (110C) and (110D) surfaces were obtained by the DFT method. Subsequently, KMC simulations are performed with DFT results employed as database. The kinetics of ethanol dehydration from γ-Al2O3(100) surface to ethylene was studied by KMC. The effects of water-alcohol ratios on the reaction process were investigated. Under the process conditions of 750 K, 1 MPa, and water-ethanol ratio ratios in the range of 1 to 3, appropriate reducing the water-alcohol ratio can effectively inhibit carbon build-up without having a significant impact on the rate of ethylene production, providing a theoretical basis for extending catalyst life and reduce the industrial production cost.

New mechanistic insights into the role of water in the dehydration of ethanol into ethylene over ZSM-5 catalysts at low temperature

We highlight the role of OH-treated weakly acidic ZSM-5 in the ethanol-to-ethylene at low temperature. Ethylene forms via (i) primary dimer from ethanol on Brønsted sites, followed by diethyl ether cracking on Lewis sites, and (ii) secondary cleavage of C–OH in ethanol over Brønsted sites.

A multi-faceted structural, thermodynamic, and spectroscopic approach for investigating ethanol dehydration over transition phase aluminas.

Understanding the role that the surface of a material plays in the mediation of a chemical reaction at the atomic level is paramount to the optimization and improvement of catalytic materials. While this area of research has matured over several decades, few techniques are sensitive enough to directly examine and differentiate the behavior of molecular adsorbates during the course of the chemical reaction with a substrate. In this study, a combined approach which involves structural characterization techniques, volumetric adsorption, temperature programmed desorption, and inelastic neutron scattering (INS) was used to investigate the mechanism of ethanol dehydration on the surface of transition phase aluminas. The alumina samples employed were extensively characterized using X-ray diffraction, solid-state 27Al nuclear magnetic resonance spectroscopy, and thermogravimetric analysis with differential scanning calorimetry. A high-precision volumetric adsorption apparatus was used to characterize the surface area and to controllably dose ethanol onto the surface of the aluminas. A modified temperature programmed desorption (TPD) method which samples the molecular composition of the vapor at discrete temperatures in a closed cell is described. INS results were used to confirm adsorption of ethanol on γ- and θ-alumina and show the reaction of ethanol and subsequent formation of ethylene as a function of temperature. The TPD and INS results affirm that the dehydration reaction and subsequent formation of ethylene on both γ- and θ-aluminas occur rapidly at 300 °C, though ethanol is still observed on θ-alumina indicating fewer active sites. These results demonstrate the value of a multi-faceted characterization approach, featuring INS, towards providing a detailed understanding of the ethanol dehydration mechanism on θ-alumina and further provide the basis for extending this approach to other systems in heterogeneous catalysis and areas where molecule-substrate interactions are poorly understood.

DEPENDENCE OF THE ACTIVITY OF Ce-Zn-O CATALYSTS IN THE REACTION OF THE CONVERSION OF ETHANOL INTO ACETONE ON THE ACIDIC PROPERTIES OF THE SURFACE

The reaction of ethanol conversion to acetone on binary cerium-zinc oxide catalysts has been studied. It has been established that the reaction products of the conversion of ethyl alcohol on binary cerium-zinc oxide catalysts are ethylene, acetaldehyde, acetone, carbon dioxide and, at high temperatures, destructive decomposition products. The highest yield of acetone is observed on samples rich in zinc at temperatures of the order of 450-550°C, while catalysts rich in cerium are active in the reaction of dehydration of ethanol to ethylene. In this work, the acid properties of cerium-zinc oxide catalysts were evaluated by their activity in the isomerization of butene-1 to trans and cis butenes-2. It was found that cerium-rich catalysts are active in butene-1 isomerization reaction. The activities of cerium-zinc oxide catalysts in the reactions of the conversion of ethanol to acetone and the isomerization of butene-1 into trans and cis butenes-2 are compared. It was found that the reaction of the conversion of ethanol to acetone proceeds on the centers of the basic nature, and the formation of ethylene proceeds on the centers of the acidic nature.

Effect of alkali metal addition on catalytic performance of Ag/ZrO2/SBA-16 catalyst for single-step conversion of ethanol to butadiene

Alkali addition to a Ag/ZrO2/SBA-16 system for ethanol to butadiene affects a beneficial decrease in Lewis acid sites, mitigating ethanol dehydration. This results in remarkable butadiene selectivity and superior productivity over the lifetime study.

The behavior of aluminum sites in H-[Al]-RUB-18 catalysts: A theoretical-experimental investigation

Functional acidic sites can be generated in the structure of layered silicates by the isomorphic substitution of silicon by aluminum atoms, leading to suitable heterogeneous catalysts. H-[Al]-RUB-18 samples were synthesized with Si/Al molar ratios of 15, 30, and 60. Experimental characterizations showed that two different aluminum sites were obtained: one tetrahedrally coordinated (framework-Al), very similar to those found in microporous zeolites; and the other octahedrally coordinated, analogous to silanol groups (hydrated aluminol sites). Computational simulations based on Density Functional Theory (DFT) were used to test the atomic positions of these aluminum sites and confirmed, in terms of electronic energies, the tetrahedral and octahedral positions already predicted in the experimental characterizations. In addition, ethanol dehydration reaction was used to probe the acidic sites and evaluate the performance of the different [Al]-RUB-18 samples. Although a widely known reaction for zeolites and other porous solids, the chemical analysis in this work indicated that the Al-substituted layered silicates are promising candidates to be used in heterogeneous catalysis. The experimental catalytic data showed that the presence of aluminum atoms improved the reaction conversion. Moreover, MEP calculations also presented, in each step of ethanol dehydration, Alframework sites with the lowest electronic energy barriers.

Pore density effect on separations of water/ethanol and methanol/ethanol through graphene oxide membranes: A theoretical study

Pore density (PPI) plays an important role in membrane performance; however, its effect on separation factor is always ignored and its quantitative relationship to membrane flux has not been established so far. Herein, for the first time, we explored the effect of PPI on the separations of water/ethanol and methanol/ethanol mixtures through graphene oxide (GO) membranes by molecular dynamical simulations. The fluxes of water (Jw) and methanol (Jm) are found to linearly increase with PPI owing to the enhanced diffusivity and the reduced permeation barrier. Their mathematical relationships are quantified as Jw= 805.2 × PPI + 10.7 and Jm= 263.8 × PPI - 7.8 (PPI ≥ 0.11 nm−2). While the corresponding separation factors are sacrificed by large PPIs because the decreasing per-pore coverage of the pre-selected molecules vanishes the ethanol-resistant region at the membrane interface. Therefore, to achieve good separation performance, the moderate PPIs of 0.22 nm−2 and 0.11 nm−2 are well-judged for that GO membrane with 2.4 Å sized apertures to maintain not only considerable membrane fluxes but also a 100 wt% water content in permeate for water/ethanol separation and a high methanol separation factor of 18.0 for methanol/ethanol separation, respectively. This computational study quantitatively uncovers the crucial role of PPI in membrane separations and provides a theoretical basis to design high-performance GO membranes for ethanol dehydration and methanol recovery.

Graphene Oxide-Carbon Nanotube (GO-CNT) Hybrid Mixed Matrix Membrane for Pervaporative Dehydration of Ethanol.

The pervaporation process is an energy-conservative and environmentally sustainable way for dehydration studies. It efficiently separates close boiling point and azeotrope mixtures unlike the distillation process. The separation of ethanol and water is challenging as ethanol and water form an azeotrope at 95.6 wt.% of ethanol. In the last few decades, various polymers have been used as candidates in membrane preparation for pervaporation (PV) application, which are currently used in the preparation of mixed matrix membranes (MMMs) for ethanol recovery and ethanol dehydration but have not been able to achieve an enhanced performance both in terms of flux and selectivity. Composite membranes comprising of poly (vinyl alcohol) (PVA) incorporated with carboxylated carbon nanotubes (CNT-COOH), graphene oxide (GO) and GO-CNT-COOH mixtures were fabricated for the dehydration of ethanol by pervaporation (PV). The membranes were characterized with Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), Raman spectroscopy, Raman imaging, contact angle measurement, and water sorption to determine the effects of various nanocarbons on the intermolecular interactions, surface hydrophilicity, and degrees of swelling. The effects of feed water concentration and temperature on the dehydration performance were investigated. The incorporation of nanocarbons led to an increase in the permeation flux and separation factor. At a feed water concentration of 10 wt.%, a permeation flux of 0.87 kg/m2.h and a separation factor of 523 were achieved at 23 °C using a PVA-GO-CNT-COOH hybrid membrane.