Supporting Information Protonated Titanate Nanotubes as Solid Acid Catalyst Masaaki Kitano, Kiyotaka Nakajima, Junko N. Kondo, Shigenobu Hayashi, and Michikazu Hara *,, П Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan, Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan, Research Institute of Instrumentation Frontier, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan, and Kanagawa Academy of Science and Technology, 3-2-1 Sakado, Takatsu-ku, kawasaki 213-0012, Japan S1
Experimental 1. Preparation of protonated titanate nanotubes The titanate nanotubes were synthesized by hydrothermal reaction of TiO 2 powder in a concentrated NaOH aqueous solution. 5.0 g of TiO 2 powder and 70 ml of 10 M NaOH aqueous solution were placed into a Teflonlined stainless autoclave. The autoclave was heated at 423 K for 20 h. After the reaction, a white precipitate (sodium titanate nanotubes) was filtered and washed with 0.1 M HNO 3 aqueous solution. The material was then washed repeatedly with distilled water until NO 3- was no longer detected, resulting in protonated titanate nanotubes. 2. Preparation of Layered titanate and titanate nanosheet Na 2 Ti 3 O 7 was prepared by heating a mixture of Na 2 CO 3 and TiO 2 in a molar ratio of 1.1:3 at 1073 K for 18 h, and then for another 20 h at 1073 K after grinding. The obtained polycrystalline material was converted to H 2 Ti 3 O 7 by immersion in 1 M HCl at 333 K for 1 week, with the acid solutions replaced with fresh solutions each day. Finally, the protonated layered titanate was obtained after filtration and washing with distilled water. Exfoliated titanate nanosheets were obtained by adding alkylamine solution to suspensions of the protonated layered titanate in distilled water. A suspension of H 2 Ti 3 O 7 in an aqueous solution of methylamine (40 wt%) was sealed in a Teflon-lined stainless autoclave and stirred at 333 K for 6 days. The resultant methylammonium/ti 3 O 7 intercalation compound was filtered and allowed to react in the autoclave with an aqueous solution of propylamine (50 vol%) at 333 K for 6 days, giving a propylammonium/ti 3 O 7 intercalation compound. The insertion of voluminous and hydrophilic propylammonium cations expands and hydrates the interlayer spaces, resulting in the exfoliation of individual titanate oxide sheets. The suspension was then centrifuged, and supernatant solution containing the nanosheets was collected. The addition of a 1.0 M HNO 3 aqueous solution into the nanosheet solution resulted in immediate aggregation of the nanosheets as a precipitate. The aggregated titanate nanosheets were rinsed 5-10 times with 100 ml of distilled water to remove HNO 3. 3. Acid catalyzed reaction The Friedel-Crafts alkylation of toluene with benzylchloride in the liquid phase was carried out in a 100 ml two-neck round-bottom flask with a reflux condenser at 373 and 300 K. The reactions were carried out using a mixture of 0.2 g of catalyst, toluene (0.1 mol) and benzylchloride (0.01 or 0.02 mol) under an Ar atmosphere. For comparison, ion-exchange resins (Amberlyst-15, Nafion NR50 and Nafion SAC13), protonated zeolites (H- ZSM5; JRC-Z-5-90H, SiO 2 /Al 2 O 3 = 90, Hβ; JRC-Z-B25, SiO 2 /Al 2 O 3 = 25), sulfated zirconia (SO 2-4 /ZrO 2, JRC- SZ-1), niobic acid (Nb 2 O 5 nh 2 O; HY-340, CBMM), and titanium dioxide (TiO 2, ST-01, Ishihara Sangyo) were used. The same amount of catalyst was used in all cases (0.2 g). All catalysts, except for the protonated zeolites, were evacuated at 423 K for 1 h prior to the reactions, while the protonated zeolites were evacuated at 673 K for 1 h. During the reactions, the liquid phase was monitored using gas chromatography (GC-17A, Shimadzu) with a DB-FFAP capillary column and a flame ionization detector. S2
The dehydration of glucose into HMF using acid catalysts was performed in a Pyrex glass reactor at 393 K by reacting 0.05 g of catalyst, 0.05 g of glucose, and 5.0 ml of distilled water. After the reaction, an aliquot of the supernatant solution, readily obtained by decantation, was analyzed by liquid chromatography (LC-2000 Plus, JASCO) using an ion exclusion column (Aminex HPX-87H, Bio-Rad). 4. FT-IR FT-IR (Jasco FT/IR-6100) spectra were measured with a spectrometer equipped with an MCT detector at a resolution of 4 cm -1. Samples were pressed into self-supported disks (2 cm 2 area, ca. 10 mg cm -2 ). The disk was placed in a quartz cell equipped with NaCl windows and connected to a closed gas-circulation system allowing thermal adsorption-desorption experiments. The FT-IR spectrum of a clean disk was measured after pretreatment at 423 K for 1 h to obtain a background spectrum. Adsorption of pyridine was carried out at room temperature (298 K). The FT-IR spectra shown in this paper have background spectra subtracted from those of the adsorbed species. To estimate the amounts of Lewis and Brønsted acid sites on the protonated titanate nanotubes, FT-IR spectra of pyridine adsorbed on Na + -exchanged titanate nanotubes and protonated titanate nanotubes were measured at 298 K. Na + -exchanged titanate nanotubes only with Lewis acid sites (without Brønsted acid sites) was prepared by cation-exchange of protonated titanate nanotubes in an aqueous NaOH solution. The FT-IR spectra for Na + - exchanged titanate nanotubes that adsorbed the small amounts (10-30 µmol g -1 ) of pyridine were measured, and the band intensity at ca. 1450 cm -1 (pyridine coordinatively bound to Lewis acid sites) was calibrated by the amount of pyridine adsorbed on the Lewis acid sites. The band at ca. 1440 cm -1 due to physisorbed pyridine was not observed under the experimental conditions. Next, the intensities of the bands at 1450 cm -1 (pyridine coordinatively bound to Lewis acid sites) and 1540 cm -1 (pyridinium ions formed by Brønsted acid sites) were plotted against the amounts of pyridine adsorbed on the Lewis and Brønsted acid sites of protonated titanate nanotubes, respectively. The amount of pyridine on the Brønsted acid sites was obtained by subtracting the amount of pyridine on the Lewis acid sites from the total amount of adsorbed pyridine in the case without physisorbed pyridine. Both band intensities increase in proportion to the amounts of chemisorbed pyridine species, reaching plateaus with the appearance of the band due to physisorbed pyridine (1440 cm -1 ). While the band at 1440 cm -1 disappeared after vacuum evacuation at room temperature, there was no significant difference in intensity of the bands at 1450 and 1540 cm -1 before and after evacuation, suggesting that these bands can be attributed to the Lewis and Brønsted acid sites to chemisorb the saturated amount of pyridine. The amounts of the Brønsted and Lewis acid sites on protonated titanate nanotubes were therefore estimated to be 0.10 and 0.25 mmol g -1, respectively, from the band intensities after vacuum evacuation of excess pyridine-adsorbed titanate nanotubes. The ion exchange capacity (IEC) was also estimated to be 0.2 mmol g -1 from Na + -exchange in aqueous NaCl solution. This value is larger than the amount of the Brønsted acid sites (0.1 mmol g -1 ) obtained by the FT-IR measurements. This can be attributed to the layer structure in titanate nanotubes. Although H + in the interlayer spaces can be exchanged for Na + cations in the solution, pyridine is not intercalated into the interlayer spaces S3
because of the narrow interlayer spaces: H + in the interlayer spaces cannot react with pyridine. As a result, the amounts of the effective acids sites for the reactions in this study are expected to be those estimated by the FT-IR experiment. S4
Table S1 Friedel-Crafts alkylation of toluene with benzylchloride over various solid acid catalysts. a Catalyst Yield [%] Selectivity of benzyltoluene [%] o -benzyltoluene m -benzyltoluene p -benzyltoluene Titanate nanotube 97.2 45.3 0.0 54.7 TiO 2 96.5 46.2 0.0 53.8 Nb 2 O 5 nh 2 O 92.0 45.6 0.0 54.4 SO 2-4 /ZrO 2 87.8 47.2 0.0 52.8 Hβ 87.8 38.4 0.0 61.6 H-ZSM-5 52.6 50.2 0.0 49.8 Amberlyst 15 8.8 44.1 0.0 55.9 Nafion NR50 32.5 47.4 0.0 52.6 Nafion SAC13 13.3 45.6 0.0 54.4 a Reaction conditions: catalyst (0.2 g), toluene (0.1 mol), benzylchloride (0.01 mol), reaction temperature 373 K. S5
Table S2 Friedel-Crafts alkylation of toluene with benzylchloride over titanate nanotubes. a Catalyst Yield of benzyltoluene [%] 373 K b 300 K b Protonated titanate nanotube 97.2 92.0 Na + -exchanged titanate nanotube 95.0 0.8 a Reaction conditions: catalyst (0.2 g), toluene (0.1 mol), benzylchloride (0.01 mol); b Reaction temperature. S6
Volume adsorbed (cm 3 g -1 ) 600 500 400 300 200 100 (a) 0 0 0.2 0.4 0.6 0.8 1 Relative pressure (P/P 0 ) Volume / cm 3 g -1 nm -1 0.14 0.12 0.1 0.08 0.06 0.04 0.02 (b) 0 0 10 20 30 40 50 Pore diameter / nm Figure S1. (a) Nitrogen adsorption-desorption isotherms and (b) pore size distribution for protonated titanate nanotubes. S7
Titanate nanotube Yield of benzyltoluene / % 100 80 60 40 20 0 TiO 2 Nb 2 O 5 nh 2 O Hβ SO 2-4 /ZrO 2 H-ZSM5 Nafion NR50 Nafion SAC13 Amberlyst-15 0 50 100 150 200 250 300 350 Time / min Figure S2. Time courses of benzyltoluene formation using various solid acid catalysts. Reaction conditions: catalyst (0.2 g), toluene (0.1 mol), benzylchloride (0.01 mol), reaction temperature 373 K. S8
100 Yield of benzyltoluene / % 80 60 40 20 run 1 run 2 run 3 0 0 100 200 300 Time / min Figure S3. Catalytic activity of protonated titanate nanotubes for benzyltoluene formation with catalyst reuse. Reaction conditions: catalyst (0.2 g), toluene (0.1 mol), benzylchloride (0.02 mol), reaction temperature 300 K. After each reaction, the recovered material was rinsed 3 times in 300 cm 3 of acetone by vigorous stirring for 10 min. The sample was collected by centrifugation and dry at 373 K prior to subsequent reaction. S9
Yield of benzyltoluene / % 0.6 0.5 0.4 0.3 0.2 0.1 0.0 H 2 SO 4 HCl 0 100 200 300 400 Time / min Figure S4. Time courses of benzyltoluene formation using HCl and H 2 SO 4. Reaction conditions: catalyst (0.2 g), toluene (0.1 mol), benzylchloride (0.02 mol), reaction temperature 300 K. S10
1446 1490 1640 1545 Titanate nanotube TiO 2 1800 1700 1600 1500 1400 1300 Wavenumber / cm -1 Figure S5. FT-IR spectra of pyridine adsorbed on titanate nanotube and TiO 2 at 298 K. S11
L B L+B (a) (b) 1700 1600 1500 1400 Wavenumber/ cm -1 1300 Figure S6. Difference FT-IR spectra of pyridine adsorbed on (a) protonated titanate nanotube and (b) Na + - exchanged titanate nanotube at 298 K. (B = Brønsted acid site; L = Lewis acid site). Although there is no significant difference in density of the effective Lewis acid sites between titanate nanotubes and Na + -exchanged tatanate nanotubes, a distinct peak due to the Brønsted acid sites (1540 cm -1 ) is not observed in the latter. S12
Blank Nafion NR50 Amberlyst-15 HMF formation from glucose HMF formation from fructose (a) (b) H-ZSM5 SO 2-4 /ZrO 2 Hβ Titanate nanotube Titanate nanosheet Layered titanate 2 4 TiO 2 0 16 6 8 10 Yield of HMF / % 12 14 18 Figure S7. HMF formation from glucose and fructose by various solid acid catalysts. Reaction conditions: (a) catalyst (0.05 g), glucose (0.05 g), water (5.0 ml), 393K, 3 h, and (b) catalyst (0.05 g), fructose (0.05 g), water (5.0 ml), 393K, 0.5 h. S13