Ultrafiltration Mixed Matrix Membranes Based on Mesoporous Silica (MCM-41, HMS)Embedded in Polysulfone

Three composite membranes (M1-M3), with mesoporous silica (MCM-41 or HMS-C12/C16-type) embedded in polysulfone (Psf) were obtained by phase-inversion method and their performances were tested for use in ultrafiltration membrane processes. The structures of M (Psf 12%, reference membrane), M1 (Psf 12% + MCM-41), M2 (Psf 12% + HMS-C12), M3 (Psf 12% + HMS-C16) have been assessed by FTIR, TG-DSC and SEM-EDAX and the morphology and their hydrodynamic performances have been evaluated by contact angle measurements, dead-end and cross-flow filtration experiments.

Compared to solid silica particles, mesoporous silicas have special properties such as higher specific surface area, porosity, tunable pore structures with inner surface easy to functionalize, and the porous nature of the inorganic fillers gives them a high water permeability through mixed matrix membrane filler-polymer. Mesoporous MCM-41 materials have been incorporated in polyethersulfone (PES) [20] and the resulted membranes (with 2% wt MCM-41) exhibited excellent hydrophilicity, water permeability and * email: florina.dumitru@upb.ro good antifouling performance, efficient in raw water purification experiments [14,20]; in polysulfone (Psf) to enhance the gas permeability (N 2 , O 2 , CH 4 and CO 2 ) [21].

Experimental part Techniques and materials
All the raw materials were commercially available and used as received. Polysulfone (BASF, ULTRASON® -S-2010, white powder, 1.24 g/cm 3 , with low viscosity in organic solvents, 50mL/g, 25°C, and an average molecular weight of 40000 Da) has been used as polymer support.
FTIR spectra were recorded with a Bruker Tensor 27 spectrophotometer, with the ATR sampling unit, in the wavenumbers range of 500-4000 cm -1 .
Thermal analysis TG-DSC was carried out with a Netzsch 449C STA Jupiter. Samples were placed in open Al 2 O 3 crucible and heated with 10 degrees·min -1 from room temperature to 900°C, under the flow of 20 mL·min -1 dried air. An empty Al 2 O 3 crucible was used as reference.
SEM images were recorded on a SEM/EDAX High Resolution Scanning Electron Microscope, Quanta Inspect F FEG (resolution 1.4nm) with EDAX (133 eV resolution at MnKá) -FEI Company.
Contact angle measurements were carried out with Contact Angle Meter -KSV Instruments CAM 100. Each contact angle value represents the average of a minimum of 5 measurements.
Dead-end and cross-flow filtration experiments were carried out to characterize the performance of the prepared membranes. The ultrafiltration experiments were conducted using a laboratory-scale dead-end (DE) and cross-flow (CF) filtration system (equipped with a variable speed driven centrifugal pump: Q= 40 L/min, n=287 rpm, and H max = 3 bar) at temperature of 25ºC and pressures of 0.5, 1, 2, 2.5 and 3 bar. For each type of membrane, the water flux was determined by measuring the collecting time for the volume of 100 ml of permeate. The membranes diameter was 36 mm. The permeate volume was determined on steady flow conditions. The flux ( ), defined as the flow rate of water passing through the membrane, per unit area of membrane, was calculated using the MCM-41 mesoporous silica has been prepared by using cetyltrimethylammonium bromide (CTAB) as structuredirecting agent and tetraethoxysilane (TEOS) as sol precursor to form silica mesopores, according to adapted literature procedures [27,29,30]. The reaction mixture was composed of 1.0 CTAB: 9.21 TEOS: 2.55 NaOH: 4857 H 2 O (molar ratios).

Results and discussions
Mesoporous silicas: MCM-41, HMS-C12, HMS-C16 were synthesized and completely characterized to assess their porosity, pore size, ordered structure and thermal stability as reported in our previous paper [29].

FTIR spectra
In the FTIR spectra of M, M1-M3 membranes (Fig. 1), the absorption maxima attributed to the structure of polysulfone: 1108 cm -1 (C-O), 1150 cm -1 (R(SO 2 )-R), 1247 cm -1 (C-O), 1489, 1587 cm -1 (C=C aromatic), and 2968 cm -1 (CH aromatic), respectively, are present. The stretching vibration of Si-O-Si bond appear at ~1100 cm -1 but it is obscured by polysulfone vibration bands, the major compound in the composite membranes.  The increasing of decomposition temperatures (T d ) is attributed to the blending of mesoporous silicas with polysulfone solutions and to the subsequent intermolecular interactions established between SiO 2 and polymeric chains that lead to an enhanced rigidity of polymeric films and, hence, to enhanced thermal stability of mixed matrix membranes.

Dead-end and cross-flow experiments
The influence of mesoporous silicas addition into composite membranes upon pure water permeability has been evaluated in ultrafiltration (UF) experiments: deadend (DE) and cross-flow (CF) filtration modes. Water permeability measurements showed a decrease of the of the pure water flux through M1 (Psf 12%+MCM-41), M2 (Psf 12%+HMS-C12), M3 (Psf12%+HMS-C16) membranes, both in dead-end and cross-flow filtration modes, in comparison to the M (Psf 12%) membrane used as reference (table 1, fig. 5). The decrease of the flow rate is attributed to the silica particles barrier effect against to water transport, even though the hydrophilicities of M1 (Psf 12%+MCM-41), M2 (Psf 12%+HMS-C12), M3 (Psf 12%+HMS-C16) membranes are higher than that of M (Psf 12%) as evidenced by contact angle measurements.
The relative large water flux measured in filtration experiments and the improved hydrophilicities for M2-M3 membranes make these membranes appropriate for ultrafiltration processes.