2.1. Preparation and Permeation Measurements of Silica Membranes Derived from TMOS or HMDSO
Two kinds of porous α-alumina substrate tube (diameter of 6 mm, length of 100 mm.) were purchased from Noritake Co. (Nagoya, Japan), one is a symmetric tube with 150 nm pores (hydrogen permeance; 5.12 × 10−6
, nitrogen permeance.: 1.67 × 10−6
at 303 K) and another is an asymmetric one with 150 nm/700 nm pores (hydrogen permeance; 3.50 × 10−5
, nitrogen permeance: 1.20 × 10−5
at 303 K). A membrane was prepared at the center of substrate (70 mm), and the other parts were glazed with sealant. A γ-alumina layer was applied to the substrate surface to reduce the pore size, according to the report by Yoshino et al. [20
]. The outer surface of the effective area of the substrate was dipped in a boehmite sol (γ-AlOOH) for 5 seconds and then dried for 1 h in the air and calcined at 873 K for 3 h. The coating process was repeated three times. The γ-alumina layer was uniformly coated on the substrate as shown in Figure 2
and its pore size was determined to be around 4 nm by nanopermporometry measurements as shown in Figure 3
An amorphous silica layer was deposited on the γ-alumina layer by counter-diffusion chemical vapor deposition (CVD). A schematic diagram of the experimental apparatus for preparing TMOS or HMDSO-derived silica membranes and evaluating their permeation performance is shown in Figure 4
TMOS or HMDSO (Shin-Etsu Chemical Co. Ltd., Tokyo, Japan) vapor was supplied by a bubbler or a syringe pump (KDS-410, KD Scientific, Tokyo, Japan) in a nitrogen (200 mL min−1) carrier gas, from the outside of the γ-alumina-coated membrane substrate, and O2 (20 mL min−1) was supplied from the inside and CVD was conducted at the reaction temperature of 873 K.
To examine the influence of silica precursors, silica membranes were deposited to γ-alumina coated symmetric substrates with the precursor concentration around 0.85 mol m−3 (0.886 mol m−3 for HMDSO and 0.804 mol m−3 for TMOS) for 5 min as CVD time.
An asymmetric substrate coated with γ-alumina was deposited with 0.885 mol m−3 HMDSO to examine the effect of substrate.
Regarding the influence of precursor concentration and CVD time of HMDSO, first, CVD was conducted in which the precursor concentration was lowered from 0.885 mol m−3 to 0.584 mol m−3. Next, the CVD time was shortened from 5 min to 3 min with a precursor concentration of 0.624 mol m−3.
Membrane gas permeation performance was evaluated at 773, 673, and 573 K using single-component gases of hydrogen and nitrogen. Their permeation was measured with a bubble flow meter (SF 1U, Horiba Co., Kyoto, Japan).
2.2. Steam Durability Evaluation of Silica Membranes Derived from HMDSO
Steam durability evaluation of silica membranes derived from HMDSO was conducted with using the apparatus for the water gas shift reaction as shown in Figure 5
The evaluated HMDSO-derived silica membrane was prepared with precursor vapor concentration of 0.73 mol m-3 and CVD time of 5 min. Its hydrogen permeance was 1.1 × 10−6 mol m−2 s−1 Pa−1 and selectivity of H2/N2 = 159 at 573 K at just after preparation. A mixture of nitrogen and water was supplied into the inside of the silica membrane tube, and nitrogen gas was carried outside the silica membrane. Gas flow rates of nitrogen inside, water, and nitrogen for sweep were 54, 20, and 50 mL min−1 respectively. The feed gas pressure was 305 kPa, and the permeate pressure was 100 kPa and temperature was fixed at 573 K.
Membrane gas permeation performance was evaluated at 773, 673, and 573 K using single-component gases of hydrogen, nitrogen by a bubble flow meter (SF 1U, Horiba Co., Kyoto, Japan).
2.3. Water Gas Shift Reaction with Membrane Reactors Installed Developed Silica Membranes
shows a schematic diagram of a membrane reactor for the water gas shift reaction. Employed HMDSO-derived silica membrane had a hydrogen permeance of 0.98 × 10−6
and selectivity of H2
= 212 at 573 K. Cu/ZnO/Al2
granules (C18-7, Sud Chemie Catalysts, Munich, Germany) were employed as the catalyst. A catalyst (1.05 g) was put into the HMDSO-derived silica membrane tube, and model gas of BFG, which contained 52% of N2
, 22% of CO, 22% of CO2
, and 4% of H2
was fed into the tube.
The reaction was conducted at the temperature of 573 K, and the feed and permeate pressure were 305 kPa and 100 kPa respectively. On the permeation side of the membrane, 49.4 mL min−1 of argon as a sweep gas was flowed. The CO conversion was determined from the concentration of unreacted CO at the outlet of the feed side. The hydrogen recovery rate was calculated using the ratio of the amount of hydrogen at the outlet, based on the sum of hydrogen and carbon monoxide at the inlet as hydrogen. Hydrogen purity was calculated by the ratio of detected concentration by gas chromatography (7820A, Agilent Technologies, Santa Clara, CA, USA) excluding argon and hydrogen.
TMOS-derived silica membrane was also employed for the water gas shift reaction in the same manner.