Shape Selective Adsorption Property of Molecular Sieving Silica Overlayer Prepared by Chemical Vapor Deposition Using Organic Template on Tin Oxide Naonobu Katada, Shohei Yamakita and Miki Niwa 1. Concept of This Work SnO 2 etc. OO - Si(OCH 3 ) 4 Reaction Cavity Catalytic Function Sensing Function + Molecular Sieving Property O 2 NH 3 CHO SiO 2 layer Backgrounds Niwa et al. found: 1. Chemisorption of such an aldehyde as benzaldehyde on weakly basic metal oxide surface to form carboxylate anion. 2. CVD of Si(OCH 3 ) 4 forms a layer of silica with a homogeneous thickness on an atomic dimension. By combining these facts CO 2, H 2 O CN

2. Background Very early attempts using silica to construct a cavity Poljakov (1931), Dickey (1949) Organic template -- Sodium silicate Development of molecular imprinting on organic polymers Wulff (1972-) Molecular imprinting on inorganic surface Morihara (1988) Hydrolysis of SiO 2 gel surface after Al doping in the presence of organic template (Footprint catalyst) Si alkoxide deposition / organophosphoric acid-adsorbing silica -> P/SiO 2 Heilmann (1994) CVD of molecular-sieving SiO 2 layer using aldehyde template on SnO 2 Kodakari, Katada and Niwa (1995) CVD of SiO 2 on organophosphoric acid-adsorbing Al 2 O 3 Suzuki and Iwasawa et al. (2002) CVD of dense SiOSi network accelerated by CH 3 COOH Katada, Akazawa and Niwa, Adv. Mater., CVD, 10, 103 (2004) However, the observed selectivity in adsorption property or catalysis was not high, and therefore not clearly due to the shape of cavity. OO - OO - CHO Adsorption Capacity 0.50 nm nm nm nm -2

CHO 3. Aim of This Study CHOCl Cl Cl BA 1-NA CHO 4-CBA 3-CBA 2-CBA para meta ortho Chemisorption experiments How precisely does the silica layer detect the molecular shape?

GC 1. SnO 2 (20 m 2 g -1, 0.1 g) 2. Benzaldehyde at 423 K 3. Si(OCH 3 ) 4 | CH 3 COOH/H 2 O | Si(OCH 3 ) 4 at 473 K 4. NH 3 at 673 K, Benzonitrile was analyzed: No. of cavities 5. O 2 at 673 K 4. Experimental 1. SiO 2 /SnO g 2. Aldehyde at 573 K 3. NH 3 at 673 K 4. Formed nitrile was analyzed Preparation Measurements of Chemisorption Capacity

5. Results (1) Effect of Template Template Surface density / nm -2 of Si atoms BA 4-CBA 3-CBA 2-CBA1-NA Si atoms BA 4-CBA 3-CBA 2-CBA1-NA para meta ortho para meta ortho no 0 (pure SnO 2 ) no 5.7 (0.5 ML) no 34 (3 ML) All the tested aldehydes were well adsorbed on the SnO 2 surface. The aldehydes were not adsorbed on the SiO 2 surface. No selectivity was observed unless template was used. BA 5.7 (0.8 ML) NA 5.7 (1 ML) Cavities CO 2 - CHOCl Cl Cl CO 2 - CHO CO 2 -Cl Cl The selectivity was observed on SiO 2 /SnO 2 prepared using BA template; only one Cl atom at the ortho position was detected; the selectivity was 100%. All the tested aldehydes were adsorbed on SiO 2 /SnO 2 prepared using 1-NA template, showing that the selectivity was controlled by the molecular shape of template. CO 2 - CHO

(2) Thickness of Layer CHOCl Cl Cl 0 at 6 Si nm -2 0 at 17 Si nm -2 BA 4-CBApara 3-CBA meta 2-CBA ortho Template: BA not suppressed

BA Template, Si nm -2 BA, 4-CBA and 3-CBA ○ 2-CBA and 1-NA × at 6 nm -2 Adsorption of 3-CBA (meta) Adsorption of 2-CBA (ortho) SnO 2 SiO 2, 1 ML

BA Template, Si nm -2 Adsorption of 3-CBA (meta) BA, and 4-CBA ○ 3-CBA, 2-CBA and 1-NA × at 17 nm -2 SnO 2 SiO 2, 2 ML

6. Conclusion The selectivity was high (the adsorption of molecule larger than the template was not detectable). These were in good agreement with the molecular models. Selective adsorption precisely controlled by shape of template molecule and thickness of silica layer BA p-CBA m-CBAo-CBA1-NA BA p-CBA m-CBA o-CBA 1-NA SnO 2 ○ ○ ○ ○ ○ SiO 2 -monolayer/SnO 2 × × × × × BA-SiO 2 monolayer/SnO 2 ○ ○ ○ × × BA-SiO 2 doublelayer/SnO 2 ○ ○ × × × 1-NA-SiO 2 monolayer/SnO 2 ○ ○ ○ ○ ○