(41h) Urea-Functionalized Silica on Top of Graded Structured Gamma-Alumina Membranes

A functionalized silica membrane is synthesized on top of γ-alumina intermediate layer which has graded structure by controlling particle size of boehmite sol. Three successive layers of γ-alumina with decreasing particle sizes are coated on an α-alumina substrate and functionalized by co-condensation of 1-[3-(Trimethoxysilyl) propyl]urea (TMPU) and TEOS with different ratio and is used for selective separation of CO₂.

The graded γ-alumina membrane was characterized as the following: DLS from boehmite sol, BET from unsupported γ-alumina membrane, permporometry and SEM from γ-alumina membrane were performed. The urea functionalized membranes was characterized by gas separation measurements, Si NMR, C NMR and TG analysis. The γ-alumina intermediate layers were made by dip-coating and calcination of boehmite (AlOOH) sols with different particle size mixed with PVA solution on top of a macroporous α-alumina support sequentially to give a graded structure substantially free of defects.

The size of the sol particles was tuned carefully by controlling synthesis parameters including quantity of acid and hydrolysis time.  Different hydrolysis time for aluminum isopropoxide in presence of water and the subsequent acid peptization using different quantity of acetic acid was used.The size distribution of boehmite sols was measured by Dynamic Light Scattering (DLS). The boehmite sizes were controlled by adjusting H⁺/alkoxide molar ratios of 0.04, 0.07 and 0.15 and hydrolysis times of 24, 24 and 0.5 h, respectively. The boehmite average sizes obtained were 160, 60 and 6 nm, correspondingly. The DLS results showed that higher acid concentration and shorter hydrolysis time led to smaller particle size. In accordance with the DLS results, BET surface area for the unsupported γ-alumina of 160, 60, 6 nm boehmite average sizes were measured to be 214, 236 and 326 m²/g, respectively.

n-hexane/helium permporometry experiment for the γ-alumina membrane after several coating steps with boehmite sols were carried out. The permporometry data of γ-alumina with graded structure showed that the most permeance contribution of pores are less than Kelvin diameter of 4 nm while the dry He permeance for γ-alumina was about 1.4 ×10⁻⁵ mol.m⁻² Pa⁻¹ s⁻¹. Cross-sectional scanning electron microscopy images of the intermediate layers of γ-alumina were recorded. The images showed that we could prepare a membrane with a graded structure. The final thickness was about 6 μm. No infiltration of γ-alumina particles into the α-alumina macroporous support was indicated due to the first coating of larger boehmite particles. It was also observed that the top γ-alumina layer had a finer structure as compared to the layers underneath.

Urea-functionalized silica sols were synthesized from X TMPU and 1 TEOS in 22 EtOH, 5 H₂O and 0.4 HNO₃, in which three X’s of 0.05, 0.2 and 0.4 were used for the co-condensation. 10% CO₂ in either CH₄ or N₂gas mixture was separated on the membranes at 100°C and feed pressure of 150 kPa. CO₂/N₂ and CO₂/CH₄ measurements were obtained for the γ-alumina and the functionalized silica membrane. As the γ-alumina is functionalized with urea group, the CO₂ separation factor increases up to 4 times. The separation factor increases with the quantity of the functional groups, indicating a proper interaction between the urea groups and CO₂molecules. CO₂/CH₄ separation factor of 2.5, and CO₂/N₂ separation factor of 3 were observed for the urea functionalized membrane, as compared to the γ-alumina membrane with separation factor of 0.6 and 0.8 for  CO₂/CH₄  and CO₂/N₂, respectively, according to Knudsen separation factor.  CO₂ permeance decreased from 10×10⁻⁸  to 0.7 ×10⁻⁸ mol.m⁻² Pa⁻¹ s⁻¹ where X increased from 0.05 to 0.4. This decrease in the permeance may be attributed to an increase in amount of the adsorbed water.

The successful incorporation of functional groups in the membrane structure was confirmed by FTIR methodology, Si MAS NMR and C MAS NMR. The FTIR spectra of the functionalized  silica are mainly characterized by bands at 3150–3600 cm⁻¹ (N–H stretching vibrations), 2800–3000 cm⁻¹ (C–H asymmetric and symmetric stretching vibrations), 1600–1800 cm⁻¹ (C=O vibrations), 1400-1500 cm⁻¹ (NH₂ bending vibration),1200  cm⁻¹ (Si-C vibrations), 400,800,1078 cm⁻¹(Si-O-Si vibrations).

Covalent bond between organic functional groups and the silica network was confirmed by assigning T site in the Si MAS NMR results for the functionalized silica.  In the functionalized samples, the resonances at around −64 ppm represent T3(RSi(OSi)3) silicon site,  where R represents the functional groups. To find a semi quantitative analysis for functional group incorporation in silica network, curve deconvolution of Si MAS NMR spectra of functionalized samples were performed to determine the relative concentration of T and Q resonances.  The relative concentration of T site to the total sites was 16 and 28% for X=0.2 and 0.4, respectively. This result indicates that almost all of the functional groups are covalently attached to the silica network and the degree of functional group incorporation in the silica network is in accordance with the amount of functional groups in the starting silica sol as it increases from X=0.2 to 0.4.

The carbon atoms present in the functional groups were detected in C MAS NMR; especially carbonyl group were observed as a strong signal at 160 ppm.  For measuring the functional groups thermal stability as well as real degree of functionality for the functionalized silica, thermal gravimetric analysis was conducted under Argon flow. The TG results showed that the functional groups were stable under 200°C. The real functionality of the functionalized silica for X= 0.2 and 0.4 was found to be 20 and 26% respectively, which is in accordance with the weight percent of the functional groups in the starting sol.