Synthesis of mesoporous alumina using polyvinyl alcohol template as porosity control additive

The effects of polyvinyl alcohol (PVA) template and calcinations temperatures on the characteristics of the alumina fi lms were investigated. The samples were prepared by sol-gel method using aluminium triisopropylate precursor. The variation of microstructure, pore size and pore volume, were determined by nitrogen adsorption/ desorption analysis and the macropore size distribution was determined using mercury porosimetry. TEM and SEM were used to observe the texture of these samples and the particle morphology. Experimental observation after drying and annealing shows that it is possible to produce crack free nanoporous alumina fi lms using polyvinyl alcohol template. The obtained alumina samples have macroporous microstructure (with the average pore diameter dav = 34.9 μm, for sample prepared with 42.5 wt% of PVA addition and annealed at 1000°C) with high portion of mesopores (with the average pore diameter Dav = 14.0 nm for the same sample).


I. Introduction
The sol-gel process is applied to prepare amor phous and poorly crystalline materials, as nanoporous ceramics with high surface areas and small pore sizes [1].Synthesis of ceramic materials with high specifi c surface area and controlled porosity is of great interest for application such as catalysis, catalyst support and porous membrane technology for separation processes [2,3].For use in separation processes, the mesoporous ceramic fi lms are deposited onto a macroporous ceramic support, made by traditional ceramic technology, which confers the required mechanical resistance for micro and ultrafi ltration processes, for high pressures industrial applications [4].
Different types of additives are used to improve the textural properties of the fi lms prepared by sol-gel route and deposited on a ceramic support.However, their infl uence on the process has not been clearly elucidateed yet [1].Among these the polyvinyl alcohol is by far the most studied.Using PVA, less critical but more controllable drying and calcination procedures are obtained [5].Polyvinyl alcohol is a semicrystalline polymer hav-ing hydroxyl groups, which give rise to inter-and intramolecular hydrogen bonds formation [6].Most PVA solutions including PVA/water solution are well known to form thermally reversible gels at low temperature.Additions of polyvinyl alcohol to the colloidal precursor solution prevent the defects formation in alumina fi lms [2].
During the thermal treatment, cracking and cru shing of the fi lm are the most frequently phenomena.If PVA is used as template the densifi cation process can be controlled, and nanoporous alumina fi lms with controlled porosity can be prepared [4].The thickness of the porous fi lms shrank to be about 60% of the initial one after a heat treatment at high temperatures, up to 950ºC, accompanied by an abrupt decrease in porosity [7].
This paper summarizes the study about production and textural characterization of alumina membranes with nanoporous structure, deposited on glass support and as unsupported thin fi lms.The alumina samples were prepared by the sol-gel method, based on hydrolysis and condensation reaction of the aluminium triisopropylate.The effects of polyvinyl alcohol template and calcinations temperatures on the characteristics of the prepared membranes were investigated.

II. Experimental Procedure
Alumina gel was synthesized by sol-gel method, using aluminium triisopropylate (Al (O i C 3 H 7 ) 3 Merck), water, nitric acid (Riedel-de Haën) and PVA -poly vinyl alcohol (Moviol 10-98).The sol was prepared under continuous stirring using an experimental ins tallation build with a glass fl ask having attached water cooled refrigerator, mixer and thermometer as follows: 1 mole aluminium triisopropylate (Al (O i C 3 H 7 ) 3 ) was diluted with 20 moles of isopropanol (Chimopar) and than hydrolyzed with 200 moles of water.As peptize tion agent, 1 mole of nitric acid was added dropwise to yield a stable sol.The temperature was kept constant at 80ºC, using a thermostated oil bath.The turbulent state of the reaction liquid was achieved using a blade mixer adjusted at 300 rpm.When the increase of the viscosity was observed due to the condensation pro cesses, different amount of polyvinyl alcohol (42.5-60 % wt) was added as aqueous solution.After PVA addition, the system was opened for the evaporation of the alcohol and water until increasing of viscosity is observed.The asprepared viscous sol was deposited as a thin fi lm into Petri dishes, where gelation took place.The formed fi lm has been dried 24 hours in normal conditions at room temperature, followed by 4 hours at 90ºC in a drying stove, than annealed at different temperature between 600-1000ºC.The used hydrolysis molar ratio, r = H 2 O : Al(O i C 3 H 7 ) 3 , pH and dilution agent ratio were determined in previous work [4].In order to study the infl uence of PVA on the fi lm porosity, samples were prepared with different PVA content in range of 1.85-3.75g disolved in 50 ml distilled water and added to 250 ml sol containing 10 g aluminium triisopropylate.
The samples macroporosity was determined by mercury porosimetry (Pascal 240, Porotec) and the textural properties were determined using transmission electron microscopy (PHILIPS EM 201) and scanning electron microscopy (Hitachi S4700).The pore size distribution, pore volume, average pore size and surface area were determined by nitrogen adsorption/ desorption porosimetry (NOVA 2200, Quantachrome).The samples were previously degassed to below 6.7 kPa at room temperature and analyses were performed, using liquid nitrogen.The equilibration interval was 5 s.Surface areas were calculated from the isotherm data using the Brunauer-Emmet-Teller (BET) method.Pore diameter distributions and cumulative pore volumes were determined with the Barret-Joyner-Halenda (BJH) method using the adsorption and desor ption data.The total pore volume, Vp, was derived from the amount of vapor adsorbed at a relative pressure close to unit, by assuming that pores fi lled subsequently with condensed adsorbate in the normal liquid state.

III. Results and Discussion
The morphology of the alumina membrane pre pared with 60 wt% of PVA and annealed at 1000ºC with thicknesses of about 20-40 μm is presented in Fig. 1.The SEM images of the sample show worm hole-like macrosized and relatively ordered pores, with diameters in range of 5-20 μm.The macroporous structure of the alumina membranes obtained using different amounts of PVA has been confi rmed by mercury porosimetry.For the sample with 42.5 wt% of PVA addition, the pore size distribution chart (Fig. 2a) shows relatively wide range of pore sizes with the average pore diameter of 34.9 μm and 31.0 % of total macroporosity.For the sample with 60 wt% of PVA addition the pore size distribution chart (Fig. 2b) shows also relatively wide range of pore sizes with the average pore diameter of 31.3 μm and higher total macroporosity of about 56.7 %.
In order to evidence the presence and quantity of mesopores in the fi lms, N 2 adsorption and desorption hysteresis curves were obtained.The samples present type IV isotherm (defi nition by IUPAC), which is characteristic for mesoporous materials (Figs. 3 and 4).The physisorption measurements reveal a high BET surface area and a narrow pore size distribution (inset Figs. 3  and 4).The experimental results show that the specific surface area is infl uenced by the annealing temperature while the mesopore size and the total pore volume are infl uenced both by the amount of PVA and the annealing temperature (Table 1).The average me sopore diameter and the total pore volume decreased with increasing amount of PVA.Increasing the sintering temperature the BET specifi c surface area and the total pore volume were decreasing rapidly due to the decay of the small pores during the sintering process.This lead to increase of the average mesopore diameter of the samples obtained at higher temperatu re.The pore diameter, cal-   culated from the desorption branch of nitrogen adsorption-desorption isotherms, of sample sintered at 1000°C is larger than that of sample sintered at 800°C.Transmission electron microscopy image of the mesoporous alumina membranes prepared using 42.5 wt% of PVA and annealed at 800°C (Fig. 5) confi rms the presence of nanosized alumina particles with diameter in the range of 10-20 nm and shows no signifi cant order in pore arrangement.The electron diffra ction pattern (inset in Fig. 5) and XRD diffraction pattern (Fig. 6) of the same sample correspond to cubic γ-Al 2 O 3 (a=7.939Å).

IV. Conclusions
The obtained results prove the possibility of producing mesoporous crack-free Al 2 O 3 ceramic fi lms by sol-gel synthesis using aluminum triisopropylate precursor.The obtained alumina membranes had macroporous texture, with the average macropore diameter in the range of 31.3-34.9μm for temperatu res in the range of 800-1000°C (confi rmed by mercury porosimetry and SEM).The total macroporosity was in the range of 31-57 % and increased with the incre ase of the amount of PVA.
The mesoporosity of obtained membranes was confi rmed by nitrogen adsorption/desorption isother ms.The average pore diameter and specifi c surface area were in the range of 5.3-22.8nm and 46-168 m 2 /g, respectively, for the samples annealed at temperature in the range of 800-1000°C.
The TEM images of the mesoporous alumina membranes showed that they consisted of alumina particles with the average diameter in the range of 10-20 nm.
The process of inducing nanometrical pore sizes in the crack-free alumina fi lms requires a strict control of processing conditions.Textural characteristics and the nanometrical pore dimensions enable applicability of these membranes in ultrafi ltration processes.

Figure 1 .
Figure 1.SEM images (at different magnifi cations) of the alumina fi lm prepared with 60 wt% of PVA and annealed at 1000°C

Figure 2 .Figure 3 .Figure 4 .
Figure 2. Pore size distribution of samples annealed at 1000°C: a) sample prepared with addition of 42.5 wt% PVA and b) sample prepared with addition of 60 wt% PVA.a) b)

Figure 5 .
Figure 5. TEM image of the alumina fi lm prepared with 42.5 wt% of PVA and annealed at 800°C and the electron diffraction pattern of a region (inset)

Figure 6 .
Figure 6.XRD pattern of the alumina fi lm prepared with 42.5 wt% of PVA and annealed at 800°C