Tungsten incorporated mobil-type eleven zeolite membranes: Facile synthesis and tuneable wettability for highly efficient separation of oil/water mixtures

Hammad Saulat ,Jianhua Yang,2,*,Tao Yan ,Waseem Raza ,Wensen Song ,Gaohong He,2

1 State Key Laboratory of Fine Chemicals,Institute of Adsorption and Inorganic Membrane,Dalian University of Technology,Dalian 116024,China

2 Panjin Institute of Industrial Technology,Dalian University of Technology,Panjin 124221,China

3 Institute of Advance Study,Shenzhen University,Shenzhen 518060,China

Keywords:Corrosion Dodecyltrimethoxysilane Hexadecyltrimethoxysilane Membranes Oil/water separation Zeolite

ABSTRACT Tungsten (W) incorporated mobil-type eleven (MEL) zeolite membrane (referred to as W-MEL membrane) with high separation performance was firstly explored for the separation of oil/water mixtures under the influence of gravity.W-MEL membranes were grown on stainless steel (SS) meshes through in-situ hydrothermal growth method facilitated with (3-aminopropyl)triethoxysilane (APTES) modification of stainless steel meshes,which promote the heterogeneous nucleation and crystal growth of WMEL zeolites onto the mesh surface.W-MEL membranes were grown on different mesh size supports to investigate the effect of mesh size on the separation performance of the membrane.The assynthesized W-MEL membrane supported on 500 mesh (25 μm) (W-MEL-500) exhibit the hydrophilic nature with a water contact angle of 11.8°and delivers the best hexane/water mixture separation with a water flux and separation efficiency of 46247 L.m-2.h-1 and 99.5%,respectively.The wettability of W-MEL membranes was manipulated from hydrophilic to hydrophobic nature by chemically modifying with the fluorine-free compounds (hexadecyltrimethoxysilane (HDTMS) and dodecyltrimethoxysilane(DDTMS)) to achieve efficient oil-permselective separation of heavy oils from water.Among the hydrophobically modified W-MEL membranes,W-MEL-500-HDTMS having a water contact angle of 146.4°delivers the best separation performance for dichloromethane/water mixtures with a constant oil flux and separation efficiency of 61490 L.m-2.h-1 and 99.2%,respectively along with the stability tested up to 20 cycles.Both W-MEL-500-HDTMS and W-MEL-500-DDTMS membranes also exhibit similar separation performances for the separation of heavy oil from sea water along with a 20-fold lower corrosion rate in comparison with the bare stainless-steel mesh,indicating their excellent stability in seawater.Compared to the reported zeolite membranes for oil/water separation,the as-synthesized and hydrophobically modified W-MEL membranes shows competitive separation performances in terms of flux and separation efficiency,demonstrating the good potentiality for oil/water separation.

1.Introduction

Human society evolves from various revolutions with an evergrowing desire to improve their living standards [1].Among various revolutions,the industrial revolution played a pivotal role in the development of human society.However,the fossil fuelbased industrial revolution is associated with many environmental and ecological-related issues.These issues continuously increase with the growing global population,urbanization and industrialization,while posing severe threats to the surroundings due to the discharge of untreated effluent containing dyes and oils from various industrial units (textile,gas and oil sectors) [2-5].Among these rapidly growing issues,oil-water contamination has become one of the most concerning issues for researchers in remediating freshwater streams contaminated by foreign resources,such as the discharge of oily wastewater from household and industrial units,along with accidental oil spills that cause economic and environmental problems [6-8].

To overcome these issues,much research has been devoted to energy-efficient and environmentally friendly routes for separating oil from wastewater,seawater,and further utilizing the recovered oil to produce bio-plastics[9]and bio-lubricants[10].Several technologies have been developed so far,including in-situ burning[11],oil skimming [12],ultrasonic separation [13],demulsifier [14],microbial degradation [15],and centrifugation [16] to tackle oilwater contamination.Among these technologies,membranebased separation has become a desirable option to eradicate oil/water contamination with high separation efficiencies along with its desirable advantages,such as zero-carbon footprints,ease to scale up,and eco-friendly nature [17-21].Developing membranes with high separation performances especially in term of flux and separation efficiency is consider as a critical aspect for their effective and efficient utilization in oil/water separation.Besides that,the membrane material should also possess desirable anticorrosive properties along with excellent chemical,mechanical,and thermal stabilities due to the presence of oily wastewater in hot,acidic,alkaline,and saline conditions.So far,different types of membrane materials such as polymers [22,23],zeolites [24],metal organic frameworks (MOFs) [25,26],carbon [27],organicinorganic hybrids [28],and covalent organic frameworks (COFs)[29] have been developed and successfully applied to separate oil/water mixtures.Among all these membrane materials,the desirable features of zeolites such as tuneable surface nature,excellent thermal and chemical stabilities along with high corrosion resistance properties are attracting the attention of researchers for their effective utilization in the field of oil-water separation[30-32].Moreover,zeolite membranes are also extensively employed for the separation of various molecular mixtures present in both the liquid and gaseous phase [33-35].

Mobil-type eleven(MEL)zeolite framework belongs to the pentasil family of zeolites having similar pore sizes(0.54×0.53 nm)as mobil-type five (MFI) zeolite framework but it only possess intersecting straight channels with 10-membered ring openings as shown in Fig.1 [36].Pure silica MEL zeolite is generally termed as Silicalite-2,while their alumina-incorporated counterparts are referred to as zeolite socony mobil 11(ZSM-11).Straight channels,tuneable surface nature and the presence of desirable intrinsic features of zeolite frameworks make MEL zeolite framework a highly attractive membrane material for various separation applications.For membrane-based oil/water separation,the surface nature(hydrophobicity/hydrophilicity)of zeolite membranes plays a critical role in defining the separation of desirable component from oil/water mixtures [37,38].Generally,hydrophilic zeolite membranes synthesized on mesh supports having water contact angles less than 90° are employed to separate water from oil/water mixtures.On contrary,hydrophobic zeolite membranes having water contact angles usually higher than 120°are employed for the effective separation of oil from oil/water mixtures.In order to achieve efficient separation of oil/water mixtures,material engineering of zeolite membranes is highly desirable for effectively tuning their surface nature.Material engineering of zeolite membranes are carried out by manipulating the silica-alumina ratio (SAR) [39],postmodification with different chemicals [40] and incorporation of different heteroatoms into the zeolite frameworks [41].The introduction of heteroatoms into the zeolite framework not only tunes the surface properties but also imparts desirable features to the zeolite [42,43].Liuetal.[41] synthesised boron-incorporated MFI zeolite membrane supported on stainless steel mesh (referred to as B-MFI(Boron incorporated mobile-type five)membrane)to separate water from oil/water mixtures.The incorporation of boron into the MFI zeolite framework has shown a significant increase in the hydrophilicity of the fabricated B-MFI membranes while exhibiting a water flux and separation efficiency of ＞14000 L.m-2.h-1and ＞99%,respectively.Moreover,post-modification with various chemicals was also employed to modify the zeolite membranes to attain a hydrophobic surface for achieving effective separation of oils (especially heavy oils) from oil/water mixtures[31,44].

Fig.1.Schematic diagram of MEL pore structure and its atomic structures along ＜100＞and ＜001＞directions.

In this study,we firstly demonstrate the novel fabrication of tungsten (W) incorporated MEL zeolite (W-MEL) membrane supported on stainless steel mesh (referred to as W-MEL membrane)with tunable surface wettability for the efficient separation of heavy and light oil/water mixtures under the influence of gravity(Fig.2).The W-MEL membrane was obtained through anin-situgrowth method on (3-aminopropyl)triethoxysilane (APTES) modified stainless steel mesh.The APTES agent serves as a linker between silica source and stainless steel mesh to enhance the heterogeneous nucleation and crystal growth of W-MEL zeolite layer onto the mesh.The as-synthesized W-MEL membrane having hydrophilic nature was effectively utilized for the separation of water from light oil/water mixtures.Post-modification of W-MEL membranes were further carried out by utilizing fluorine-free chemicals like dodecyltrimethoxysilane (DDTMS) and hexadecyltrimethoxysilane(HDTMS)to effectively tune their surface wettability for attaining efficient separation of heavy oils.Hydrophobically modified W-MEL membranes exhibit high oil flux and selectivity over water due to higher water contact angles and display better anti-corrosion performance than the bare stainless steel mesh,demonstrating their stability in seawater.This study will hopefully open many doors for the effective utilization of various zeolite-based membranes and their material engineering aspect for achieving efficient separation of oil/water mixtures.

Fig.2.Schematic diagram for the fabrication of W-MEL and hydrophobically modified W-MEL membranes.

2.Experimental

2.1.Materials

Ethanol (EtOH,99.7%),isopropyl alcohol (IPA,99.7%),dichloromethane (DCM,99.5%),trichloromethane (TCM,99.5%),hexane(99.5%),and tetraethyl orthosilicate (TEOS,98%) were purchased from Tianjin Kermel Chemical Reagent company.(3-Aminopropyl)triethoxysilane (APTES,99%),tetrabutylammonium hydroxide(TBAOH,25% (mass)),hexadecyltrimethoxysilane (HDTMS,＞85%),and dodecyltrimethoxysilane (DDTMS,95%) were purchased from Macklin Inc.Sodium tungstate dihydrate (Na2WO4.2H2O,99.5%)was purchased from Aladdin Inc.Stainless steel meshes of different sizes (300,500,1000 and 2000 mesh,corresponding to 53,25,13,6.5 μm)were purchased from Shanghai filter factory,China.

2.2.APTES modification of stainless steel meshes

Stainless steel meshes of different mesh sizes were cut into square dimensions of 4 cm×4 cm and subjected to ultrasonication with 1 mol.L-1NaOH solution,deionized water,and ethanol for 15 min each to remove any unwanted material present on their surfaces.The cleaned stainless steel meshes were further dried in an electric oven at 80°C for overnight.The cleaned and dried stainless steel meshes were then modified with APTES solution having a volume ratio of 0.08 APTES:0.12 IPA:0.8 H2O at 70°C for 3 h.The APTES-modified stainless steel meshes were washed three times with deionized water and dried at 60°C for 3 h.The modified stainless steel meshes were then stored in air-tight bags for later use.

2.3.Fabrication of W-MEL membranes

W-MEL zeolite was grown on APTES-modified stainless steel meshes using thein-situgrowth method.APTES-modified stainless steel meshes were placed in a Teflon-lined autoclave for hydrothermal synthesis with a synthesis solution having a molar ratio of 1 TEOS:0.35 TBAOH:0.12 Na2WO4.2H2O: 120 H2O at 130°C for 24 h.After hydrothermal synthesis,W-MEL membranes were washed with ample amount of deionized water and dried at 60 °C for overnight.Later,the synthesized W-MEL membranes were subjected to calcination at 500 °C for 4 h with a heating/cooling rate of 1 °C.min-1.

2.4.Hydrophobic post-modification of W-MEL membranes

The synthesized W-MEL membranes were hydrophobically modified using fluorine-free compounds like DDTMS and HDTMS.For DDTMS and HDTMS based hydrophobic modification,synthesized solutions with a volume ratio of 6.5 DDTMS:81.9 C2H5OH:4.3 H2O:7.3 CH3COOH (1 mol.L-1) and 1 HDTMS: 89 C2H5OH: 10 H2O were utilized,respectively.Initially,both solutions were hydrolyzed for 1 h at 65 °C to obtain the required hydrophobic solution for post-modification.The fabricated WMEL membranes were immersed in the synthesized hydrophobic solution at 30 °C.After 3 h,the hydrophobically modified W-MEL membranes were taken out,washed with deionized water and dried at 120 °C for 2 h.

2.5.Characterization

Scanning electron microscopy(FEI,QUANTA 450)with an acceleration voltage of 20 kV was used to observe the morphology of the synthesized meshes that were sputter coated with gold.The elemental composition was analyzed by using energy-dispersive spectroscopy (EDS).X-ray diffraction analysis (XRD) was used to analyze the crystalline structure by using Rigaku Smart Lab diffractometer (9KW0303050201) in the range of 2θ=5°-50° with step size and scanning rate of 0.02 and 15 (°).min-1using Cu Kα radiation.Contact angles of the fabricated zeolite membranes were determined by using the water contact angle measurement system(Dataphysics,OCA 25) at room temperature.Fourier transform infrared (FT-IR) spectroscopy was performed by Thermo-Nicolet 6700 FT-IR machine within a spectrum range of 4000-500 cm-1to detect the functional groups.ATR-FTIR analysis of unmodified and APTES-modified meshes was carried out using a Thermo-Nicolet IS50 machine.

2.6.Oil/water separation

The oil/water separation of the synthesized membranes was carried out under the influence of gravity by using 50:50 (volume ratio)oil/water as a feed mixture.Oil was coloured with dye to give pink colour for the ease of distinguishing it from water.The synthesized membranes were cut into circular shapes with an area of 7.21 cm2and then clamped between the glass tubes for the separation test.The feed mixture was poured onto the surface of the zeolite membrane and the permeate was collected at the bottom of the beaker.The filtration fluxF(L.m-2.h-1) of the mesh under the influence of gravity was calculated by Eq.(1),and the separation efficiency (η) was calculated using Eq.(2).

For Eq.(1),m(kg)is the mass of the permeate collected;ρ(kg.m-3)is the density of oil;S(m-2)is the effective area of the synthesized membrane exposed to the oil/water mixture andt(h) is the time duration in which the oil or water from oil/water feed mixture come entirely towards the permeate side.For Eq.(2),m0(g)is the mass of oil or water in oil/water feed mixture,andm1(g)is the mass of oil or water collected as permeate.

2.7.Anticorrosion test

Corrosion is one of the major problems that can limit the application of membranes supported on mesh substrates,especially in saline conditions.To evaluate the anti-corrosion ability,the hydrophobically modified MEL membrane was placed in a threeelectrode cell system(CHI 650E,CH Instruments,China)with a saline solution (sea water) at 25 °C.All three electrodes are placed vertically in the saline solution with an equal distance among all the electrodes for the measurements.The synthesized membrane with a defined area acts as a working electrode,while an Ag/AgCl electrode and platinum plate act as a reference and auxiliary electrodes,respectively.Tafel test measurements were carried out to examine the anti-corrosion ability of hydrophobically modified zeolite membranes as compared with the bare stainless steel mesh.Tafel test was carried out at a scan rate of 0.01 V.S-1,and the polarization resistance (Rp) was calculated from the Tafel plot by using the Stern-Geary equation as shown in Eq.(3),and the corrosion rate (CR) was calculated by using Eq.(4).

Here,Icorr(A.cm-2) is the corrosion current density obtained from the extrapolation of Tafel plots.βa(V.dec-1) and βc(V.dec-1) are the anodic and cathodic slopes of Tafel plots.Mand ρ (g.cm-3)are the equivalent mass and density of the stainless steel mesh,respectively,whileDrepresents the valence.

3.Results and Discussion

3.1.Characterization of bare and APTES-modified stainless steel mesh support

Fig.S1(in Supplementary Material) exhibits the morphology of bare stainless steel meshes having different mesh sizes (2000,1000,500 and 300 mesh,corresponding to 6.5,13,25,53 μm)viaSEM images at lower and higher magnifications.The average aperture sizes/mesh openings of 2000,1000,500 and 300 meshes are approximately 6.7 μm(knitted mesh openings),11.5 μm(knitted mesh opening),30 μm×34 μm (rectangular holes) and 140 μm×114 μm (rectangular holes),respectively.The surface of bare stainless steel meshes was not so uniform with no observable growth of any other crystalline material.The XRD patterns also confirm that no other crystalline structure was present on the meshes while exhibiting a characteristic peak of bare stainless steel meshes at 2θ value of 43.1° (Fig.S2).Bare stainless steel meshes were modified with APTES to achieve promoted heterogeneous growth of W-MEL zeolite layer during thein-situhydrothermal synthesis.APTES modification of stainless steel meshes was confirmed through the ATR-FTIR analysis (Fig.3).The spectrum of APTES-modified stainless steel mesh exhibit a band at 3350 cm-1that is assigned to N—H stretching mode,and an additional absorbance band was observed at 1573 cm-1representing the bending vibration of N—H from the secondary amine group while these bands were not observed in the spectra of unmodified stainless steel mesh [45-49].Based on these results,the stainless steel meshes were successfully modified by APTES,and the morphology of un-modified and APTES-modified stainless steel meshes was almost similar (Fig.S3).

Fig.3.ATR-FTIR analysis of un-modified and APTES-modified stainless steel mesh.

3.2.SEM,XRD and FT-IR analysis of as-synthesized and hydrophobically modified W-MEL membranes

Fig.4 shows the morphology of W-MEL zeolite layer grown on APTES-modified stainless steel meshes with different aperture sizesviaSEM images with low and high magnifications.W-MEL zeolite grown on the stainless steel meshes exhibits a rice-like morphology while forming a dense and crack-free zeolite layer(referred to as W-MEL membrane).From now on,the W-MEL membrane synthesized on 2000,1000,500,and 300 size meshes are written as W-MEL-2000,W-MEL-1000,W-MEL-500,and W-MEL-300 membranes,respectively.The XRD patterns of the four membranes also confirm the formation of MEL zeolite structure on stainless steel meshes in comparison with the simulated XRD pattern of the MEL zeolite framework (Fig.5(a)).The characteristic XRD peaks intensity of W-MEL membranes also increases with the increasing mesh size of stainless steel supports in the following order: W-MEI-300 ＜W-MEI-500 ＜W-MEI-1000 ＜W-MEI-2000.Moreover,the XRD peak intensity of stainless steel meshes also increases with the increasing mesh size.The increase in characteristic peak intensity of W-MEL membranes were probably attributed to the better growth of W-MEL zeolite on small aperture size of stainless steel mesh supports while increase in the intensity of stainless steel meshes peaks were attributed to the decrease in aperture size and the increase in mesh density of the stainless mesh support.The incorporation of W into the MEL zeolite framework was also evident by the shifting of XRD peaks towards the lower 2θ angles and the splitting of W-MEL zeolite powder XRD peaks (collected from the bottom of the autoclave) in comparison with the simulated XRD pattern of MEL zeolite framework(Fig.S4(b)).Moreover,the EDX analysis also exhibits the presence of W in the W-MEL membrane(Fig.S5).The attachment of W-MEL zeolite membrane with stainless steel mesh support was also tested through ultrasonication test.The membrane layer remains firmly attached with the surface of stainless steel mesh support after being subjected to ultrasonication for 30 min and no detachment of zeolite layer was observed (Fig.S6).

Fig.4.SEM images of W-MEL zeolite grown on different aperture size of stainless steel meshes with lower to higher magnifications(a,b,c)300 mesh;(d,e,f)500 mesh;(g,h,i)1000 mesh;(j,k,l) 2000 mesh.

Fig.5.(a)XRD patterns of W-MEL membrane grown on stainless steel meshes with different mesh sizes and the(b)FT-IR spectra of W-MEL zeolites modified with HDTMS and DDTMS.

The surface wettability of W-MEL membranes was further tuned by using fluorine-free hydrophobic compounds (DDTMS and HDTMS),and FT-IR analysis was conducted to confirm their successful surface modification.FT-IR spectra of W-MEL zeolite exhibit the peaks at 3467 cm-1,1637 cm-1,1095 cm-1and 803 cm-1representing the O—H,H—O—H,Si—O—Si,and Si—O bonds,respectively as shown in Fig.5(b).After modification of W-MEL zeolite with DDTMS and HDTMS,four new peaks appeared in the FT-IR spectra at 2922 cm-1,2856 cm-1,1467 cm-1,and 720 cm-1representing the stretching vibration of —CH2,—CH3,C—O bonds [50] and the bending vibration of —(CH2)n— bond[51] while confirming the successful modification of W-MEL zeolite with fluorine-free compounds (HDTMS and DDTMS).

3.3.Contact angle measurements

Contact angle measurements play a pivotal role in characterizing and investigating the surface wettability of the materials [52].Surface wettability assists in identifying the surface nature (hydrophobic/hydrophilic) of the synthesized and functionally modified materials.Static water contact angles of the as-synthesized and hydrophobically modified W-MEL membranes grown on 500 mesh size (25 μm) were carried out due to their excellent separation performance.Static water contact angle measurement of WMEL-500 membrane exhibits a water contact angle of 11.8° representing its hydrophilic nature (Fig.6(a)).W-MEL-2000,W-MEL-1000,and W-MEL-300 membranes also exhibit hydrophilic surface nature,as evident by the low water contact angles.W-MEL-2000,W-MEL-1000,and W-MEL-300 membranes exhibit water contact angles of 52.5°,20.5°,and 0°,respectively;at 1 s and further the water contact angles decrease with time to 0° as shown in Fig.S7.W-MEL-2000 and W-MEL-1000 membrane exhibits slightly higher water contact angles at 1 s as compared to W-MEL-500 and W-MEL-300 membranes,possibly because of the much denser mesh supports.Surface nature and the density of mesh supports together plays a pivotal role in attaining high separation membrane.The as-synthesized W-MEL-500 membrane was further modified with DDTMS and HDTMS exhibiting an increase in the water contact angle from 11.8° to 146.2° and 146.4°,respectively,due to the decrease in surface wettability caused by the reduction in surface energy,demonstrating successful surface modification(Fig.6(c,d)).The higher water contact angle of W-MEL-500 membranes modified with DDTMS and HDTMS relates well with the FTIR spectra(Fig.5(b)),exhibiting the presence of low energy groups(—CH3and —CH2) along with the C—O bond.These low-energy groups(—CH3and—CH2)help in reducing the surface energy while increasing the water contact angles.Moreover,the presence of large amounts of hydroxyl groups on the surface of the W-MEL-500 membrane plays a pivotal role in better surface modification[44,50,53].

Fig.6.Water contact angle of (a) W-MEL-500;(b) W-MEL-500-DDTMS and (c) W-MEL-500-HDTMS membranes.

Floating test was also performed to verify the hydrophobic modification of the synthesized W-MEL-500 membranes through the naked eye.W-MEL-500 membrane without hydrophobic modifications was placed on the surface of the water in a beaker.The hydrophilic nature and low water contact angles of the membrane make it sink and settle at the bottom of the beaker(Fig.7(a)).However,hydrophobically modified W-MEL-500 membrane with DDTMS and HDTMS float on the water’s surface (Fig.7(b),(c)).These hydrophobically modified W-MEL membranes also continued to flounder over the surface of the water,when an O-ring(0.932 g) was placed over their surfaces (the picture of the Oring is shown in the inset of (Fig.7(b))).Floating of W-MEL-500 membranes modified with DDTMS and HDTMS confirms their successful hydrophobic modification through naked eye,which was also evident by the static water contact angle measurements and FTIR analysis.

Fig.7.Visual images of(a)W-MEL-500 membrane without hydrophobic modification and W-MEL-500 membrane modified with(b)DDTMS and(c)HDTMS placed inside a beaker filled with water.

3.4.Oil/water separation performance

3.4.1.AssynthesizedW-MELmembranesfortheseparationofwater fromlightoils

The as-synthesized W-MEL membranes grown on different aperture sizes of stainless steel meshes were evaluated for the separation of water from light oil (hexane)/water mixture (Fig.8(a)).Water flux of W-MEL membranes decreases with the increase in mesh size of the stainless steel meshes (decrease in aperture size)in the following order W-MEL-300 (91120 L.m-2.h-1) ＞W-MEL-500 (46247 L.m-2.h-1) ＞W-MEL-1000 (38414 L.m-2.h-1) ＞WMEL-2000(24799 L.m-2.h-1).Meanwhile,the separation efficiency of water over oil for the W-MEL-300 membrane is the lowest(76%)as compared to the other three W-MEL membranes having almost similar separation efficiency of ≥99.5%.The decreasing water flux is attributed to the decreasing aperture size(increasing mesh size)of the stainless steel mesh.Only the separation efficiency of the WMEL-300 membrane is low due to the presence of an extremely large aperture size of stainless steel mesh.Among all the W-MEL membranes synthesized on different mesh sizes,W-MEL-500 membrane exhibit the optimized separation performance with an average water flux and separation efficiency of 46247 L.m-2.h-1and 99.5%for the separation of water from hexane/water mixtures.The reason for better separation performance of water from light oil/water mixtures was attributed to the presence of low water contact angle (hydrophilic nature),suitable aperture size of stainless steel mesh and the presence of more hydroxyl groups that assist in the rapid penetration of water through these membranes.The as-synthesized W-MEL-500 membrane also exhibits long-term stability(tested up to 20 cycles)for separating water from hexane/water mixture (Fig.8(b)).W-MEL-500 membrane was also further subjected to the separation of water from other light oil/water mixtures (Fig.8(c)).In case of petroleum-ether/water mixture,W-MEL-500 membrane exhibits an average water flux and separation efficiency of 37462 L.m-2.h-1and 99.4%,respectively.W-MEL-500 membrane provides an effective solution for separating water from light oil/water mixtures.

Fig.8.(a) As-synthesized W-MEL membrane fabricated on different mesh sizes for the separation of hexane/water mixture;(b) stability of as-synthesized W-MEL-500 membrane for the separation of hexane/water mixture;(c) separation of different light oil/water mixtures by using as-synthesized W-MEL-500 membrane.

3.4.2.HydrophobicallymodifiedW-MELmembranesfortheseparation ofheavyoilsfromwater

In order to effectively separate the heavy oil from heavy oil/water mixtures,the surface wettability of the W-MEL membrane was tuned by using fluorine-free hydrophobic compounds like DDTMS and HDTMS.The separation performance of hydrophobically modified W-MEL membranes grown on different aperture sizes of stainless steel meshes were evaluated for the separation of heavy oil-like dichloromethane from a dichloromethane-water mixture(50%) (Fig.9 (a),(b)).The oil flux of W-MEL membranes modified with DDTMS and HDTMS increases with the increase in the aperture size of stainless steel meshes.Hydrophobically modified WMEL-300 membrane exhibit the highest oil flux but also presents low separation efficiency in the range of 97.8%to 98.4%.Hydrophobically modified W-MEL-1000,and W-MEL-2000 membranes exhibit low oil flux but enhanced separation efficiency due to the decreasing aperture size of the meshes as compared to the hydrophobically modified W-MEL-300 membrane.On the contrary,hydrophobically modified W-MEL-500 membrane exhibit the optimized separation performance in terms of separation efficiency and oil flux.Therefore,hydrophobically modified W-MEL-500 membranes were further selected for the separation performance analysis.The separation efficiency and oil flux of the WMEL-500 membrane modified with DDTMS (W-MEL-500-DDTMS)were low compared to the W-MEL-500 membrane modified with HDTMS (W-MEL-500-HDTMS).However,the water contact angle measurements of W-MEL-500-DDTMS and W-MEL-500-HDTMS membranes were approximately similar.Increase in the separation performance of W-MEL-500-HDTMS membrane would be attributed to the presence of long carbon chains of HDTMS and improved surface modification.Among the hydrophobically modified WMEL-500 membranes,W-MEL-500-HDTMS membrane exhibits the best separation performance with an oil flux and separation efficiency of 61490 L.m-2.h-1and 99.2%,respectively.Hydrophobically modified W-MEL-500 membranes were further subjected to the stability test for separating dichloromethane from the dichloromethane/water mixture(Fig.9(c)).The hydrophobically modified W-MEL-500 membranes exhibit an excellent stability for 20 cycles with a separation efficiency greater than 99% along with a minor variation in oil flux.The separation performance of these hydrophobically modified W-MEL membranes was also tested with different water sources for separating dichloromethane from the dichloromethane/water mixture.Deionized water,tap water,and seawater were selected as three different water sources to separate oil/water mixtures (50/50).The samples of deionized water and tap water were collected from the laboratory premises,while the seawater sample was collected from the Bohai sea,Dalian,China,and the collected samples were utilized without pretreatment.The basic water quality parameters of deionized water,tap water,and seawater are presented in Fig.S8.Deionized,tap,and seawater exhibit the conductivity of 10.71 μS.cm-1,228 μS.cm-1,and 50400 μS.cm-1,respectively.The oil/water separation performance using different water sources was almost similar,with no significant difference in separation efficiencies but a slight variation in oil flux (Fig.9(d)).

Membranes grown on mesh substrates usually face serious corrosion problems that limit their practical applications in saline conditions,especially seawater.The anti-corrosion properties of hydrophobically modified W-MEL-500 membranes were analysed by using Tafel analysis at a scan rate of 0.01 V.s-1in a seawater sample at 25°C.The collected seawater sample was utilized without any pre-treatment for Tafel analysis.The results of Tafel analysis shows that the corrosion potential (Ecorr) of hydrophobically modified W-MEL-500 membranes moves more towards the positive potential.In contrast,the corrosion current (Icorr) decreases as compared to the bare stainless steel mesh tested under the same conditions (Fig.10).The corrosion rate (CR) calculated from the Tafel analysis curves exhibits that the hydrophobically modified W-MEL-500 membranes have a low corrosion rate as compared to the bare stainless steel mesh in the same corrosive conditions at 25°C.These hydrophobically modified W-MEL-500 membranes exhibit a 20 times lower corrosion rate and 45 times higher polarization resistance (Rp) compared to bare stainless steel mesh(Table S1).

Fig.10.Tafel curves of(a)W-MEL-500-DDTMS and(b)W-MEL-500-HDTMS membranes along with bare SS mesh recorded at a scan rate of 0.01 V.s-1 in seawater at 25°C.

W-MEL-500-HDTMS and W-MEL-500-DDTMS membranes were further subjected for the separation of other heavy and light oils,such as trichloromethane,hexane,and petroleum-ether from their binary mixtures with water(50/50)(Fig.11).In case of heavy oil (trichloromethane)/water separation,W-MEL-500-HDTMS and W-MEL-500-DDTMS membranes exhibit an oil flux and separation efficiency greater than 45988 L.m-2.h-1and ≥99.3%,respectively.For the separation of light oil/water mixtures under the influence of gravity,water covers the membrane surface first owing to its high density as compared to light oils while creating hindrance in the smooth separation.In order to overcome this challenge,the separation modular mounted with the hydrophobically modified W-MEL-500 membrane was placed in a tilted position to carry out the smooth separation of light oils.However,the low separation performance of light oil/water mixtures was observed especially in term of flux (＞1250 L.m-2.h-1) due to the lesser contact of light oils with the membrane surface.Moreover,the hydrophobically modified and the as-synthesized (unmodified) W-MEL-500 membrane exhibit comparable separation performances with other zeolite membranes for the separation of oil/water mixtures,as shown in Table 1.

Table 1 Separation performance comparison of as-synthesized W-MEL-500 and hydrophobically modified W-MEL-500 membranes with other zeolite membranes supported on stainless steel meshes for oil/water separation

Fig.11.W-MEL-500-DDTMS and W-MEL-500-HDTMS membranes for the separation of various oil/water mixtures.

4.Conclusions

In conclusion,W-MEL membranes were grown on APTESmodified stainless steel meshes of different aperture sizes by using the in-situ growth method to separate heavy and light oils/water mixtures under the influence of gravity.Due to the hydrophilic nature evident by low water contact angle and its growth on suitable aperture size mesh,the as-synthesized W-MEL-500 membrane delivers the best separation performance among the WMEL membranes (W-MEL-300,W-MEL-1000,W-MEL-2000) supported on various aperture size of stainless steel meshes for the separation of water from light oil/water mixture (hexane/water)with a water flux and separation efficiency of 46247 L.m-2.h-1and 99.5%,respectively.In the case of petroleum-ether/water mixture separation,the as-synthesized W-MEL-500 membrane exhibit a water flux and separation efficiency of 37462 L.m-2.h-1and 99.3%,respectively.The as-synthesized W-MEL membranes were further hydrophobically modified from fluorine-free compounds such as hexadecyltrimethoxysilane (HDTMS) and dodecyltrimethoxysilane (DDTMS) by one-pot method for their effective and efficient utilization in the separation of heavy oils from their binary mixtures with water.Hydrophobically modified WMEL-500-DDTMS membrane exhibits an oil flux of 50233 L.m-2.h-1and a separation efficiency of 99.2%,while W-MEL-500-HDTMS membrane exhibits the best separation performance with an oil flux and separation efficiency of 61490 L.m-2.h-1and 99.2%,respectively for the separation of dichloromethane/water mixture.Both the hydrophobically modified W-MEL membranes were stable up to 20 cycles with a separation efficiency greater than 99%along with a minor variation in oil flux.Hydrophobically modified W-MEL-500 membrane also exhibits almost stable oil/water separation performance under saline conditions with a corrosion rate 20 times lower than the bare stainless steel mesh,indicating excellent corrosion resistance under saline conditions.The hydrophobically modified W-MEL-500 membranes were also used to separate light oil/water mixtures.However,hydrophobically modified W-MEL-500 membranes exhibit low oil flux for the separation of light oil/water mixtures under the influence of gravity due to the lesser contact of light oils with the membrane surface.This study will hopefully pave many paths for the effective utilization of new zeolite materials in the field of water treatment,especially for oil/water separation.

Data Availability

Data will be made available on request.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Financial support from the Science Fund for Creative Research Groups of the National Science Foundation of China (22021005),the National Natural Science Foundation of China (21776032),and the Innovation Team of Dalian University of Technology(DUT2017TB01) are greatly acknowledged.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2023.02.022.

Nomenclature

CR corrosion rate,mm.a-1

Dvalence

Ecorrcorrosion potential,V

Fflux,L.m-2.h-1

Icorrcorrosion current density,A.cm-2

Rppolarization resistance,Ω.cm2

βaanodic slope,V.dec-1

βacathodic slope,V.dec-1

η separation efficiency,%

ρ density,g.cm-3