Preparation of low-fouling reverse-osmosis membranes on an Al2O3 carrier for desalination exploratory research

1 Introduction

Semi-permeable membranes are key components in the seawater desalination industry.Inorganic membranes comprised of zeolitic molecular sieves are semipermeable.Compared to carbon-based membranes,inorganic membranes exhibit good thermal resistance,chemical stability at high pressures,resistance to microbial degradation,and strong cleaning state.Molecular sieve membranes are divided into the following two classes:selfsupporting zeolitic crystalline membrane films and zeolitic films on stable supports,also called carriers.Self-supporting films consist of a pure molecular sieve phase or molecular sieves that have been incorporated into an organic polymer.Supported zeolitic films are made by layering a molecular sieve membrane on a carrier.In 1983,Wemick et al.[1]and Paravar et al.[2]reported syntheses of carrier-free molecular sieve membranes.In 1987,Suzuki et al.[3]reported the first synthesis of an ultrathin (1 m) molecular sieve membrane on a porous support.With these advances,research involving zeolitic molecular sieve membranes [4-6]and their applications [7-9]was launched worldwide.Without stable supports,molecular sieve membranes are easily damaged and are not convenient for most applications.For this reason,molecular sieve membranes are often supported on stable substrates.These supporting materials can be broadly characterized as either porous or nonporous.Porous supports include porous alumina ceramics and porous stainless steel.Non-porous substrates are often layers of monocrystalline silicon quartz,LiTO3(LTO),or specialized glass.Many types of molecular sieves have been used to fabricate membranes on various supports.Molecular sieves include MFI-type zeolites(ZSM-5 or silicalite-1 [10-14]),A-type synthetic zeolites[15-18],faujasite zeolites (FAU X,Y),ferrierite (FER)zeolites,mordenite (MOR) zeolites,aluminophosphate(AlPO) zeolites [4-5],and silicoaluminophosphate zeolites(SAPO-5).

The carrier and the top layer of a molecular sieve film are usually coated with an intermediate layer.The fluid resistance of the carrier can be ignored,although this is not the case for the membrane layer.The initial particle sizes in the membrane layer and the support prior to synthesis must be significantly different from the pore size in the membrane following synthesis.This is because membrane particles have the potential to enter macropores in the support and block the pore channels.When this occurs,permeable flux in the support is reduced.To circumvent this problem,a single intermediate layer or multiple layers can be deposited on top of the support.Pores in the intermediate layer must be smaller than pores in the support and larger than pores in the top layer.

Electroless plating is a substrate processing technique,in which a substrate surface is treated with a metal-ion reducing agent.Metal reduction and deposition occur in a plating solution.Porous α-Al2O3 is a well-known porous alumina ceramic with a regular morphology and is easily manufactured.In this study,a porous α-Al2O3 support was uniformly coated with a layer of copper via electroless copper plating.This process metallized the ceramic surface,which reduced its roughness.Because the thin copper coating had little effect on the permeation flux of the porous α-Al2O3 carrier,it could serve as an intermediate layer between the carrier and the molecular sieve film.Following the electroless copper plating process,the conductivity of the substrate was sufficient to fabricate molecular sieve films.In addition,hydroxyl groups and other hydrophilic moieties were present on the metal surface.These promoted nucleation of the molecular sieves.Copper ions generated on the surface of the intermediate layer infiltrated the molecular sieves during synthesis of the membrane,thereby generating Cu-ZSM-5 molecular sieve films doped with Cu atoms.

2 Experimental

2.1 Preparation of the porous α-Al2O3 carrier and copper plating of the support surface

α-Al2O3 powder and water were thoroughly mixed in a mortar in a mass ratio of 5:1.A 4 g portion of the mixed powder was pressed into a mold and placed on a tablet press.Pressure (2 MPa) was applied and held for 1 min.The pressed powder was removed from the press and placed back in the mortar.The finely ground granules were separated from the mixture with 40-mesh and 80-mesh sieves.Granules larger than 80-mesh and smaller than 40-mesh were set aside for later use.The sieved powder was weighed,and 2.3 g was placed in a mold.The powder was subjected to 10 MPa of pressure for 1 min.The pressed porous alumina was dried at room temperature for one day,then placed in a muffle furnace for calcination.The carrier was washed in boiling deionized water,then dried at 60 °C.The porous alumina carrier then underwent electroless copper plating.The carrier was first coarsened in 5%hydrofluoric acid and soaked for 5 min at room temperature.For sensitization,3 g stannous chloride was fully dissolved in 2 mL hydrochloric acid.Water was added to a final volume of 100 mL to dissolve the solid.The carrier was washed,then soaked in the solution at room temperature for 5 min and collected via filtration.The carrier was activated in a 2 g/L palladium chloride solution by soaking it for 5 min at room temperature.It was then washed and subjected to electroless plating.A mixture of copper sulfate (12 g/L),nickel sulfate (1.5 g/L),potassium sodium tartrate (40 g/L),formaldehyde (20 mL/L),sodium hydroxide (8 g/L),and sodium carbonate (2 g/L) was prepared.The carrier was immersed in the mixture and allowed to sit for 5-20 min at 40 °C.The carrier was then washed and dried.

2.2 Synthesis of ZSM- 5 zeolite membranes

A sol was prepared from aluminum,silicon,template material,an inorganic base,and water.The reagents were mixed in a molar ratio of 150 (SiO2):1 (A12O3):10 (TPABr):25 (Na2O):15,000 (H2O) and stirred for 24 h.The mixture was then aged overnight.The carrier was placed in a PTFE-lined stainless steel crystallization vessel,and the sol was added.The vessel was sealed and heated at 180 °C for 12-48 h to crystallize the mixture.The carrier was collected via filtration,washed with deionized water,then dried at 100 °C for 12 h.The ZSM-5 zeolite membrane was obtained as a final product.

2.3 Characterization

Scanning electron microscope (SEM) images of the samples were obtained with a JSM-6700F SEM (JEOL,Tokyo,Japan).The operating voltage was 8 kV,and the scanning distance was approximately 10 cm.Structural analysis was performed with a D/MAX-γA transfer target X-ray diffractometer (Rigaku,Spring TX,USA) equipped with a Cu K-alpha X-ray source (λ = 1.54178 Å).The tube current was 100 mA,and the tube voltage was 40 kV.Analysis was performed at a scan rate of 5°/min.

Fig.1 SEM images of the porous α-Al2O3 carrier at two magnifications:(a) × 3,000; (b) × 30,000

3 Results and Discussion

3.1 Effect of electroless copper plating on porous α-Al2O3 carrier surface topography

The porous α-Al2O3 carrier had a diameter of 19 mm and a thickness of 2 mm.The pore diameter of the carrier was 0.32 m.SEM images of the non-plated porous alumina carrier surface at different levels of magnification are shown in Fig.1.The surface of the carrier was rough and exhibited closely packed,irregularly shaped α-Al2O3 particles of varying size.

Following reduction,copper ions were deposited on the carrier surface.The reaction gradually expanded from the initial deposition site,eventually covering the entire substrate surface.SEM images of the porous α-Al2O3 carrier surface after electroless copper plating are shown in Fig.2.The surface was smoother after the plating process.The deposited copper layer was distributed in granular form,with particle diameters ranging from 500 nm to 1 μm.The spacing between the particles was approximately 300 nm.

Fig.2 SEM images of the copper-coated porous α-Al2O3 carrier at two magnifications:(a) × 2,000; (b) × 30,000

3.2 Effect of electroless copper plating time on carrier morphology

Fig.3 SEM images of copper-coated carrier surfaces after electroless plating for (a) 5 min,(b) 10 min,(c) 20 min

SEM images showing the surface morphology of the carrier following electroless copper plating for 5,10,and 30 min are shown in Fig.3.Plating times were based on the results of previous trial experiments.With longer plating times,the copper particles in the coating become more densely packed as the number of particles increased.The copper particles on the coating were 450 nm,550 nm,and 900 nm after plating for 5,10,and 30 min,respectively.

Fig.4 SEM images of copper-coated carrier cross sections after electroless plating for (a) 5 min,(b) 10 min,(c) 20 min

The thickness of the copper coating ranged from ～30 to 40 μm (Fig.4).These results indicated that the plating time affected the growth of the copper particles in the coating.The longer the plating time,the smoother the surface of the copper coating.However,plating time had little impact on the thickness of the coating layer.

During the plating process,copper ions in the plating solution were rapidly reduced on the surface of the substrate via autocatalytic activity.The copper particles were not dispersed evenly on the substrate and were covered by other deposited particles.Thus,as the plating time increased,the copper particles gradually became larger,while the thickness of the sedimentary layer did not change significantly.

3.3 XRD analysis of copper coating

The XRD pattern of the electroless copper plating is shown in Fig.5.Three strong peaks attributed to copper were observed,which corresponded to the crystalline(111),(200),and (220) orientations.The other peaks were wave peaks of the α-Al2O3 carrier.No mixed phases were detected,indicating that a layer of elemental copper was deposited on the carrier.

Fig.5 XRD pattern of the plated copper coating

3.4 Effect of electroless copper plating time on carrier gas flux

The effect of plating time on the carrier oxygen gas flux is shown in Fig.6.Without copper plating,the oxygen gas flux of the carrier was 0.20 × 107 kg/m3 per hour.The oxygen gas flux of the carrier following copper plating for 20 min was 0.18 × 107 kg/m3 per hour.Thus,the copper plating had little effect on the gas flux of the porous carrier.This was likely because the copper coating was not very thick,so gaps were present between the copper particles following the coating process.

Fig.6 Influence of plating time on O2 gas flux

3.5 Surface morphology of ZSM-5 molecular sieve films following in-situ hydrothermal synthesis

Fig.7 SEM images of a zeolite membrane on copper-coated α-Al2O3 after 12 h hydrothermal synthesis.Magnification:(a) × 5,000; (b) × 30,000

Fig.8 SEM images of a zeolite membrane on copper-coated α-Al2O3 after 48 h hydrothermal synthesis.Magnification:(a) × 800; (b) × 3,000

SEM images of ZSM-5 molecular sieve membranes on the copper-plated α-Al2O3 carrier following hydrothermal synthesis for 12 h and 48 h are shown in Fig.7 and Fig.8,respectively.Following hydrothermal synthesis for 12 h,the crystals were approximately 3 μm in diameter.During hydrothermal synthesis for 48 h,the crystals continued to grow.The c-axis (length),the a-axis (width),and the b-axis(thickness) increased concurrently during crystal growth,but the length of c-axis increased more rapidly and reached between 10 and 20 μm.

3.6 XRD analysis of ZSM-5 zeolite membranes on different carriers

The XRD patterns of molecular sieve membranes prepared on a-Al2O3 and AAO supports are shown in Fig.9 and Fig.10,respectively.The pattern in Fig.9 contained five characteristic ZSM-5 peaks at (2θ) 7.90°,8.90°,23.30°,23.90 °,and 24.40 °.The crystallinity of ZSM-5 was very good,which meant that a ZSM-5 zeolite membrane was obtained.In addition to the characteristic ZSM-5 peaks,the three characteristic Cu peaks and carrier α-Al2O3 peaks were visible in the pattern.No mixed phases were indicated,meaning the molecular sieve membrane contained ZSM-5 as a single phase.

Fig.9 XRD pattern of a ZSM-5 zeolite membrane on a copper-coated α-Al2O3 support

Fig.10 XRD pattern of a ZSM-5 zeolite membrane on a AAO support

4 Conclusions

The electroless copper plating method was used to uniformly deposit a copper layer on a porous α-Al2O3 carrier,thereby metallizing the carrier surface.The smooth coating reduced the surface roughness of the ceramic layer.The copper particles in the coating became larger as the plating time was increased to attain a final coating thickness of approximately 30-40 μm.The coating had little effect on the carrier gas flux,indicating α-Al2O3 was a suitable carrier material for molecular sieve membrane preparation.

ZSM-5 molecular sieve membranes were then prepared on copper-plated porous α-Al2O3 via in-situ hydrothermal synthesis.The effects of varying hydrothermal synthesis times on molecular sieve grain size were investigated.The results showed that with increasing hydrothermal synthesis time,the zeolite crystals continued to grow.Following the same synthetic process with a different carrier,the resulting zeolite film exhibited different morphology.The ZSM-5 zeolite film prepared on anodic alumina contained large zeolite crystals in places,but small zeolite crystals were observed in other places on the carrier surface.The surface of the porous α-Al2O3 carrier following chemical copper plating was smoother than that of the uncoated surface,and the prepared molecular sieve membranes had better qualities with a more regular crystal arrangement.

In this study,semi-permeable zeolite membranes were prepared on a copper-coated α-Al2O3 carrier.The membranes can be installed in membrane desalination systems and used for sea water desalination in the future.