Preparation of high-permeance ceramic microfiltration membranes using a pore-sealing method

A pore-sealing method for preparation of high-permeance alumina microfiltration (MF) membranes free of any intermediate layers is presented. It involves sequential coating of a polyvinyl butyral (PVB) layer and an alumina membrane precursor on the surface of the macroporous alumina support. An alumina MF membrane with no intermediate layers can be obtained on the support after pyrolysis of the PVB interlayer. The interlayer-free membrane prepared by this method has an average pore diameter of 0.26 μm and a water permeance of 1468 ± 81 L m−2 h−1 bar−1 which is prominently higher than that of the ceramic membranes prepared with other techniques. The conspicuous increase of water permeance is speculated mainly due to the filtration resistance decrease of the interlayer-free ceramic membrane.


Introduction
Ceramic membranes with all sorts of insoluble metal oxides such as Al 2 O 3 , ZrO 2 , SiO 2 and TiO 2 (ref. 1) have shown interesting separation and processing properties. Usually, a porous ceramic membrane is characterized by a multi-layered asymmetric structure which is composed of a thicker (1-5 mm) support layer with relatively large pores (1-15 mm) to provide mechanical integrity for the membrane systems, an intermediate layer to reduce pore size to mesoporous dimensions and a much thinner (10-50 mm) top layer with small (2-500 nm) and selective pores for selective separation. 2,3 Typically, the fabrication of a porous membrane involves multiple steps where the coating of intermediate layers and the nal separation layer is normally performed on the prepared support layer. Multiple high-temperature sintering processes are normally required to combine these layers. In fact, the complexity and the expensive starting materials during membrane fabrication have concertedly resulted in the high production cost of ceramic membranes. [4][5][6] Membrane-forming particles of the ne separation layer are easy to penetrate into the large pores of the support layer under capillary force, which results in pore blockings. In this way, the intermediate layers which bridge the pore size differences between the support layer and the top separation layer seem necessary. 7 However, the intermediate layers lead to the rapid increase of membrane ltration resistance accompanied with the sharp decrease of water permeance.
In the literatures, Bayat et al. 8 reported that a g-Al 2 O 3 multilayer ultraltration (UF) membrane on an a-alumina (a-Al 2 O 3 ) substrate was successfully fabricated via the sol-gel processing method. The optimum permeate ux of the membrane was indentied as 112.7 L m À2 h À1 bar À1 . Zou et al. 9 designed one-step preparation of high-performance bi-layer aalumina ultraltration membranes supported on coarse tubular substrates by co-sintering process. In this approach, boehmite sol and alumina nanoparticles were mixed in different ratios for the fabrication of MF layer and the UF layer. The membrane thickness of the MF layer and the UF layer was controlled to be 40-50 mm and about 1 mm, respectively. Recently, a method called "precursor lm ring method" is proposed to improve the permeance of ceramic MF membranes by avoiding intermediate layers and dip-coating process, and efficiently control the thickness of the separation layer. 10 Moreover, a sacricial interlayer-based technique has been used to produce membranes without any intermediate layers. 7 Some different polymers such as polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP) and polyvinyl butyral (PVB) can be used as sacricial phases. [11][12][13] The use of these polymer binders can improve the drying of the wet lm and avoid cracks in the membrane precursor. In this study, we design a pore-sealing method for preparation of high-permeance alumina MF membrane free of any intermediate layers. This method involves sequential coatings of a PVB layer and an alumina membrane precursor on the surface of the macroporous alumina support. In this approach, a high-permeance alumina MF membrane with no intermediate layers can be prepared aer pyrolysis of the PVB interlayer.

Material and methods
The commercial tubular a-Al 2 O 3 supports provided by Foshan Ceramics Research Institute Co., Ltd (Guangdong, China) were immersed in 5% HCl solution for 30 min, followed by heat treatment at 650 C for 60 min. The above pretreated supports were characterized by an open porosity of (48.6 AE 3.2)% and an average pore size of 2.71 mm (Fig. 1), along with a water permeance of 8625 AE 172 L m À2 h À1 bar À1 . The pretreated supports were soaked in the 5 wt% PVB/EtOH solution 14 by ultrasound for 30 min until no air bubbles were visible and the supports were completely lled with the PVB/EtOH solution, and then pulled out slowly. Aer volatilization of ethanol in the wet lm a PVB lm formed on the support surface. The surface pores of the support were sealed compactly with the PVB lm. Both ends of the support were sealed with masking tape, and dip coating was performed on the support with the membrane-forming suspensions which composed of 5 wt% PVB (15-35 mPa s, butyl aldehyde: 45-49%, Sinopharm Chemical Reagent Co., Ltd, China), 80 wt% ethanol absolute (AR, Tianjin Yongda Chemical Reagent Co., Ltd, China) and 15 wt% a-Al 2 O 3 (99.9% purity, d 50 ¼ 100 nm, Taimei Chemicals Co., Ltd Nagano-ken, Japan), and the support was immediately pulled out at the speed of 5 cm s À1 . 15 Based on our previous work, 15 the membrane precursor was dried slowly at ambient temperature and nally sintered at 1300 C for 2 h at the heating rate of 5 C min À1 , which was accompanied with natural cooling.
The porosities of the supports were measured according to the Archimedes method and the theoretical density of a-Al 2 O 3 was taken as 3.99 g cm À3 . Pore size of the support and membrane was determined by the gas bubble pressure method based on the ASTM Publication F316-03(2011). The support and membrane was saturated with deionized water (18 MU cm) before the pressure was applied so as to avoid non-stationary transient effects. Morphologies of the support and membrane were observed by scanning electron microscope (ZEISS EVO 18, Germany). A Fully Automated Fluid and Gas Handling System (OSMO Inspector 2.0, Convergence, Netherlands) was utilized to measure the water permeance of the support and membrane.

Results and discussion
A schematic diagram of a pore-sealing method for preparation of high-permeance ceramic microltration membrane is shown in Fig. 2. Ultrasonic immersion of the support in 5 wt% PVB/ EtOH solution aims at making a defect-free PVB lm to seal the surface pores of the support completely. The PVB molecule has both butyral (hydrophobic) group and hydroxyl (hydrophilic) group. It is as easy as a pie for the PVB/EtOH solution to saturate the support. The cured PVB lm is embedded in the surface pores of the support. The intrinsic viscosity and hydrophilicity of the PVB molecules facilitate the tight adhesion of a smooth PVB lm on the hydrophilic surface of the support (Fig. 3A). Aer dip coating in the Al 2 O 3 /PVB/EtOH suspensions, an alumina membrane precursor is formed on the PVB lm. The expanded conformation of the PVB molecules in the solvents is effective for dispersion of the ne alumina particles. 16 All the gaps among the membrane-forming particles are completely lled with the cured PVB (Fig. 3B). The crack-free membrane precursor combines compactly with the PVB interlayer due to the amphipathic property of PVB (Fig. 3D). For the as-deposited layers, the thickness of the PVB interlayer and the membrane precursor is about 18 mm and 13 mm, respectively.
The three-tier structure shows that a PVB interlayer bridges the membrane precursor and the support, and stops the ne alumina particles from penetrating into the pores of the support. A defect-free alumina membrane combines tightly with the support (Fig. 3E) aer the burning-off of the PVB interlayer at 1300 C for 2 h. The thickness of the sintered membrane is around 12 mm. During the continuous melting and pyrolysis of the PVB interlayer that acts as a pore former, the top membrane precursor is facilitated to conglutinate the support surface and a great deal of pores are formed evenly in the prepared membrane (Fig. 3C).
The pore size of the sintered membrane is measured at 25 C according to ASTM Publication F316-03(2011). When the transmembrane pressure difference reaches 1.5 bar, the rst bubble point is detected, correspsonding to the largest pore size of 0.58 mm (Fig. 4A). Nonlinear growth of the gas ow is observed with the increase of transmembrane pressure This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 5560-5565 | 5561 difference, which indicates more and more wet pores are opened. Sharp increase of the gas ow at the transmembrane pressure difference of 3.3 bar signies that the pore size of the membrane is centered at 0.26 mm. The gas ow of wet pores nally turns to increase linearly with transmembrane pressure difference aer the rest of the wet pores are completely opened, which accords with the Hagen-Poiseuille equation. Correspondingly, the membrane is endowed with a water permeance of 1468 AE 81 L m À2 h À1 bar À1 (Fig. 4C) which is remarkably higher than that of the inorganic membranes prepared by other techniques [17][18][19][20][21][22] (Table 1). The possible reasons for the high water permeance are that the PVB interlayer restrains the ne membrane-forming particles from inltrating into the support and blocking the pores, and that the absence of intermediate layers leads to the sharp decrease of membrane ltration resistance. But for intermediate layers, membrane-forming particles with very small size are very easy to inltrate into the pores of the membrane support under capillary force during dip coating. The pore-blockings of the support makes the ltering performance decrease sharply. Although intermediate layers can bridge the pore size differences between the support layer and the top separation layer and apparently reduce the inltration of membrane particles into the support, they will increase signicantly the thickness of the effective separation layers. The thicker the effective separation layers are, the higher the ltration resistance will be. In this way, the ltration resistance of the membrane will increase dramatically with the augment of the separation layers thickness. 23,24   The weight percentage of a-Al 2 O 3 in the membrane forming suspension has an inuence on the permeance of membrane. In the range from 5 wt% to 20 wt%, the permeance of membrane degrades from 2325 AE 75 L m À2 h À1 bar À1 to 578 AE 79 L m À2 h À1 bar À1 and the reduction rate accelerates with the increase of a-Al 2 O 3 content (Fig. 5A). This is probably because increment of solid content of the suspension would cause the increase of membrane thickness and pore length, which can greatly result in the enlargement of membrane resistance. The effect of PVB content on the permeance of membrane cannot be neglected yet. Fig. 5C shows when the weight percent of PVB was 1%, the permeance of membrane is as low as 489 AE 88 L m À2 h À1 bar À1 . One function of the PVB is to increase the viscosity of the suspension. The viscosity of the suspension with 1% PVB is so low that the particles in the suspension would easily inltrate into the support under capillary force. And this results in plugging part of the pores in the support and increasing the resistance. With the augment of PVB content the viscosity of the suspension increases. Due to the formation of giant network structure in the suspension, the particles are impeded to inltrate into the support. Meanwhile, PVB as a pore former can also increase the porosity and average pore size of the membrane (Fig. 5B). However, it doesn't mean the higher content of PVB, the better permeation performance of the membrane. As Fig. 5C shows, the increase rate of permeance gradually slows down with increasing PVB content whereas the viscosity of suspension is enhanced. The reason is may be that an excess of PVB may simultaneously increase the viscosity of the suspension and further enlarge the thickness of membrane. When PVB content is above 5 wt%, there is a turning point in the curve of permeance vs. PVB content, and permeance of the membrane descend due to the sharp ascent of resistance. This can be explained that the effects of pore length increment and pore connectivity decline surpass the effect of porosity and pore size increase. Control of PVB content is substantially effective to adjust the suspension viscosity and further regulate thickness of the precursor membrane.

Conclusions
A high-permeance alumina microltration membrane free of any intermediate layers has been prepared by a pore-sealing method. The tubular support for membrane is rstly covered with a PVB lm as a barrier that seals the surface pores completely then dip-coated in the Al 2 O 3 /PVB/EtOH suspensions, which is followed by calcination at 1300 C for 2 h and natural cooling. The PVB interlayer prevents the ne membrane-forming particles from inltrating into the support and blocking the pores. The absence of intermediate layers facilitates the membrane resistance to decrease sharply. These two reasons may account for the high water permeance of the membrane. The interlayer-free alumina membrane prepared by this pore-sealing method has an average pore size of 0.26 mm and a water permeance of 1468 AE 81 L m À2 h À1 bar À1 which is remarkably higher than that of the inorganic membranes prepared by other techniques.

Conflicts of interest
There are no conicts to declare.