本實驗利用傅立葉轉換紅外線光譜儀(FTIR)研究異丙叉丙酮(mesityl oxide, 4-methyl-3-penten-2-one)及其同分異構物──3-甲基-3-戊烯-2-酮(3-methyl-3-penten- 2-one)在二氧化鈦粉末表面上的吸附及不同環境下的反應。
異丙叉丙酮分子吸附在二氧化鈦粉末表面(35 °C)上就會開始反應，分解成兩個丙酮分子，是兩個丙酮分子進行縮合反應產生異丙叉丙酮分子的逆反應。真空環境熱反應與無氧環境熱反應類似，產生丙酮跟乙酸根。有氧環境熱反應在300 °C以下時，會產生丙酮、乙酸根及二氧化碳，當溫度超過300 °C時，會因高溫、氧化分解產生某個含C＝O的分子，而分解得到的丙酮也會因為高溫分解，再與氧氣作用產生水及二氧化碳。密閉有氧光催化反應時，會分解產生丙酮、二氧化碳及甲酸根、乙酸根或碳酸根，而分解得到的丙酮會受到光催化影響再度進行縮合及脫水反應產生異丙叉丙酮和水。當有氧環境光催化反應使用的氧由16O2變成18O2時，會產生氧的同位素置換，可證實氧氣會參與反應。
3-甲基-3-戊烯-2-酮分子吸附在二氧化鈦粉末表面(35 °C)上並不會開始產生反應。真空環境熱反應會斷位的C－H鍵產生共振結構，斷掉的H原子與表面上的O產生OH基。有氧環境熱反應300 °C以下與真空環境熱反應相似，會產生共振結構及OH基，另外還有乙酸根及二氧化碳，300 °C以上與異丙叉丙酮相同，會因高溫氧化分解產生某個含C＝O的分子。有氧環境光催化反應與熱反應相同。會產生共振結構及OH基，另外還有甲酸根、乙酸根或碳酸根及二氧化碳，也有可能C＝C會因光催化變成C＝O形成乙醯丙酮吸附在表面上或與表面產生錯合物。
由此可知同分異構物就算差別只有一個甲基的位置不同，但在相同環境下的反應結果可能也會不同。

SUMMARY
Fourier transform infrared spectroscopy was employed to study the thermal and photochemical reactions of mesityl oxide on powdered TiO2. Also tested was the adsorption of 3-methyl-3-penten-2-one to compare the reaction pathways of the two isomers. The thermal decomposition of mesityl oxide generated acetone and acetate absorbed on TiO2. In the presence of O2, the photocatalytic reaction of mesityl oxide produced acetone, CO2, formate, acetate and/or carbonate on the TiO2 surface. And then, the acetone produced from the mesityl oxide reaction would undergo self-condensation and dehydration to form mesityl oxide and water. The thermal decomposition of 3-methyl-3- penten-2-one occured by C-H bond cleavage, forming an intermediate with a resonance structure and surface OH groups. In the presence of O2, the intermediate and products from the photocatalytic reaction of 3-methyl-3-penten-2-one were similar to those from the thermal decomposition, forming an intermediate with a delocalized Ti system and surface groups of OH, formate, acetate and/or carbonate. But in the case of the photocatalytic reaction, the C=C might be transformed into C=O to form acetyl acetone. Our study showed an invesgating contrast in the the reaction pathways of the two isomers on TiO2.

Key words: TiO2, mesityl oxide, 3-methyl-3-penten-2-one, FTIR

INTRODUCTION
Mesityl oxide is a ,-unsaturated ketone with the formula CH3C(O)CH=C(CH3)2. This compound is a colorless, volatile liquid with a strong cat urine odor. It is prepared by the aldol condensation of acetone to give diacetone alcohol, which readily dehydrates to give this compound. Because many organic compounds can dissolve in mesityl oxide, it can use as a solvent. In industry, mesityl oxide is used as a precursor in the production of methyl isobutyl ketone. 3-Methyl-3-penten-2-one is a ,-unsaturated ketone, too, also an isomer of mesityl oxide. It is a precursor of 3-methyl-2-pentanone and is obtained by acid-catalyzed dehydration of 4-hydroxy-3-methyl-2-pentanone. On TiO2, mesityl oxide can be generated from aldol-condensation of aceton. However, the adsorption, thermal and photocatalytic reactions of mesityl oxide have not received much attention and need further investigation. In this paper, we presented the interesting, contrasting results of thermal and photochemical reaction of mesityl oxide and 3-methyl-3-penten-2-one on TiO2.

MATERIALS AND METHODS
The reagents used in this study were mesityl oxide (90%, Aldrich), 3-methyl-3-penten -2-one (>95%, TCI), 16O2 (99.998%, Matheson), 18O2 (99 atom%, Isotec) and TiO2 powder (Degussa P25). TiO2 dispersed in water/acetone was sprayed onto a tungsten mesh to prepare a TiO2-coated W mesh (denoted as TiO2/W) and then it was placed inside the infrared cell for evacuation and thermal treatment, which included resistive heating to 450 oC under vacuum and 350 oC in the presence of 16O2. As the temperature of TiO2/W was decreased to 35 oC, the cell was evacuated and an infrared spectrum was taken as a reference background. Mesityl oxide liquid was put into the sample tube and was purified by several cycles of freeze－pump－thaw before introducting its vapor into the IR cell. The IR cell with two KBr windows for IR transmission down to ~400 cm-1 was connected to a gas manifold maintained by a turbomolecular pump at a base pressure of ~1x10-7 torr. The pressure was monitored with a Baratron capacitance and an ion gauge. The TiO2 temperature was measured by a K-type thermalcouple spotwelded on the tungsten mesh. In the photochemical studies, IR and UV beam were set 45o to the normal of the TiO2/W. A Hg arc lamp (Oriel Crop.) operated at 500W was used as the light source, with a water filter and a band-pass filter (Oriel 51650) centered at ~320 nm with a bandwidth of ~100 nm. Transmission IR spectra were obtained at a 4 cm-1 resolution using a FTIR spectrometer (Bruker Vector 22) with MCT detector.

RESULTS AND DISCUSSION
Figure 1 shows the temperature-dependent infrared spectra of mesityl oxide on TiO2. At 35 oC, the absorption peaks appear at 2975, 2941, 2919, 2875, 1700, 1680, 1663, 1616, 1603, 1580, 1446, 1420, 1380, 1363, 1276, 1240, 1192 and 980 cm-1. In comparing with the reported infrared absorptions of mesityl oxide in various states as shown in Table 1, the peaks of 2975, 2941, 2919 and 2875 cm-1 can be assigned to C-H stretching; 1700, 1680 and 1663 cm-1 to C=O stretching; 1616, 1603 and 1580 cm-1 to C=C stretching; 1446, 1380 and 1363 cm-1 to C-H and/or CH3 bending; 1276, 1240 and 1192 cm-1 to C-C stretching; 980 cm-1 to CH3 rocking. In terms of the infrared absorption of acetone on TiO2(Table 1), it is found that the six peaks of acetone are similar to the set of the peaks at 2975, 2941, 1700, 1420, 1363 and 1240 cm-1 observed in Figure 1. It is concluded that the 1420 cm-1 peak belongs to acetone, not to mesityl oxide. After heating the surface to 150 oC, the peaks of 1700, 1420 1363, 1240 cm-1 grow, opposite to the decrease of the peaks of 1616, 1603 cm-1. At 250 oC, the main absorptions appear at 1576 and 1446 cm-1. These results show that mesityl oxide decompose on TiO2 to form acetone and acetate on the surface.

Figure 2 shows the time-dependent infrared spectra of mesityl oxide on TiO2 in the UV light illumination. At 180 min irradiation, acetone is formed with the increased intensities at 1700, 1420, 1353 amd 1245 cm-1. The enhanced absorptions at 1576, 1446, 1353 and 1326 cm-1 suggest the formation of formate, acetate and carbonate. In addition, adsorbed CO2 appears at 2357 cm-1. OH groups absorptions increase in the ranges of 3600-3800 cm-1 and 3200-3500 cm-1. A water shoulder peak appears at 1619 cm-1 in the spectrum obtained after evacuation.

Figure 3 shows the temperature-dependent infrared spectra of 3-methyl-3-penten-2-one on TiO2. At 35 oC, the absorption peaks appear at 2975, 2965, 2930, 2870, 1696, 1662, 1637, 1602, 1448, 1392, 1363, 1342, 1281 and 1208 cm-1. According to Table 1, the peaks of 2975, 2965, 2930 and 2870 cm-1 can be assigned to C-H stretching; 1696 and 1662 cm-1 to C=O stretching; 1637 and 1602 cm-1 to C=C stretching; 1448, 1392, 1363 and 1342 cm-1 to C-H and/or CH3 bending; 1281 and 1208 cm-1 to C-C stretching. After heating the surface to 150 oC, the 1662 cm-1 peak is shifted to 1668 cm-1 and becomes broader; the 1637 cm-1 peak is shifted to 1634 cm-1 and becomes smaller; the 1602 cm-1 peak is shifted to 1595 cm-1; the absorption at 1281 cm-1decreases; the absorptions in the range of 3600-3800 cm-1 increase. The results suggest that C-H bond cleavage of 3-methyl-3-penten-2-one on TiO2 occurs to form an intermediate with a π-resonance structure. Besides, surface OH groups increase due to the H loss.

Figure 4 shows the time-dependent infrared spectra obtained in the photocatalytic decomposition 3-methyl-3-penten-2-one on TiO2. The photoreaction after 180 min irradiation generates the products formate, acetate, carbonate and CO2 on TiO2. It is likely the C=C is transformed to C=O group.

CONCLUSION
The reaction of mesityl oxide on TiO2 forms acetone, however 3-methyl-3- penten-2-one may undergo C-H bond cleavage to form an intermediate with a delocalized π-structure on TiO2. These two isomers show different thermal reaction pathways on TiO2.

[1] M. Prutton, Introduction to Surface Physics, Oxford Science Publications Press 1994.
[2] W. Stillwell, An Introduction to Biological Membranes: From Bilayers to Rafts, Elsevier/Academic Press 2013.
[3] P. M. D. Collins, The Pivotal Role of Platinum in the Discovery of Catalysis, Platinum Metals Review, 30 (1986) 141-146.
[4] A. J. B. Robertson, The Early History of Catalysis, Platinum Metals Review, 19 (1975) 64-69.
[5] E. Cook, Peregrine Phillips, The Inventor of the Contact Process for Sulphuric acid, Nature, 117 (1926) 419-421.
[6] M. E. Davis, R. J. Davis, Fundamentals of Chemical Reaction Engineering, Dover Publications 2013.
[7] L. Lloyd, Handbook of Industrial Catalysts, Springer 2011.
[8] G. A. Somorjai, Y. Li, Introduction to Surface Chemistry and Catalysis, John Wiley & Sons 2010.
[9] Deacon, H. U.S. Patent 1656802 (1875).
[10] J. C. Vickerman, I. Gilmore, Surface Analysis: The Principal Techniques, Wiley 2011.
[11] L. B. Hunt, Sir Humphry Davy on Platinum, Platinum Metals Review, 23 (1979) 29-31.
[12] P. Atkins, J. de Paula, Atkins' Physical Chemistry, OUP Oxford 2010.
[13] M. Landmann, E. Rauls, W.G. Schmidt, The Electronic Structure and Optical Response of Rutile, Anatase and Brookite TiO2, Journal of Physics. Condensed Matter : An Institute of Physics Journal, 24 (2012) 195503.
[14] J. Augustynski, The Role of the Surface Intermediates in the Photoelectrochemical Behaviour of Anatase and Rutile TiO2, Electrochimica Acta, 38 (1993) 43-46.
[15] E. Stoyanov, F. Langenhorst, G. Steinle-Neumann, The Effect of Valence State and Site Geometry on Ti L3,2 and O K Electron Energy-loss Spectra of TixOy phases, American Mineralogist, 92 (2007) 577-586.
[16] J. K. Burdett, T. Hughbanks, G. J. Miller, J. W. Richardson, J. V. Smith, Structural-electronic Relationships in Inorganic Solids: Powder Neutron Diffraction Studies of the Rutile and Anatase Polymorphs of Titanium Dioxide at 15 and 295 K, Journal of the American Chemical Society, 109 (1987) 3639-3646.
[17] B. Ohtani, O. O. Prieto-Mahaney, D. Li, R. Abe, What is Degussa (Evonik) P25? Crystalline Composition Analysis, Reconstruction From Isolated Pure Particles and Photocatalytic Activity Test, Journal of Photochemistry and Photobiology A: Chemistry, 216 (2010) 179-182.
[18] T. Ohno, K. Sarukawa, K. Tokieda, M. Matsumura, Morphology of a TiO2 Photocatalyst (Degussa, P-25) Consisting of Anatase and Rutile Crystalline Phases, Journal of Catalysis, 203 (2001) 82-86.
[19] J. Pan, G. Liu, G. Q. Lu, H. M. Cheng, On the True Photoreactivity Order of {001}, {010}, and {101} Facets of Anatase TiO2 Crystals, Angewandte Chemie, 50 (2011) 2133-2137.
[20] Z. Zhang, C. C. Wang, R. Zakaria, J. Y. Ying, Role of Particle Size in Nanocrystalline TiO2-Based Photocatalysts, The Journal of Physical Chemistry B, 102 (1998) 10871-10878.
[21] D. Sarkar, C. K. Ghosh, K. K. Chattopadhyay, Morphology Control of Rutile TiO2 Hierarchical Architectures and Their Excellent Field Emission Properties, CrystEngComm, 14 (2012) 2683-2690.
[22] J.-Y. Liao, J.-W. He, H. Xu, D.-B. Kuang, C.-Y. Su, Effect of TiO2 Morphology on Photovoltaic Performance of Dye-sensitized Solar Cells: Nanoparticles, Nanofibers, Hierarchical Spheres and Ellipsoid Spheres, Journal of Materials Chemistry, 22 (2012) 7910-7918.
[23] W. Zhou, G. Du, P. Hu, G. Li, D. Wang, H. Liu, J. Wang, R. I. Boughton, D. Liu, H. Jiang, Nanoheterostructures on TiO2 Nanobelts Achieved by Acid Hydrothermal Method with Enhanced Photocatalytic and Gas Sensitive Performance, Journal of Materials Chemistry, 21 (2011) 7937-7945.
[24] M. Lazzeri, A. Vittadini, A. Selloni, Structure and Energetics of Stoichiometric TiO2 Anatase Surfaces, Physical Review B, 63 (2001) 155409.
[25] L. Ye, J. Mao, J. Liu, Z. Jiang, T. Peng, L. Zan, Synthesis of Anatase TiO2 Nanocrystals with {101}, {001} or {010} Single Facets of 90% Level Exposure and Liquid-phase Photocatalytic Reduction and Oxidation Activity Orders, Journal of Materials Chemistry A, 1 (2013) 10532-10537.
[26] J. Pan, X. Wu, L. Wang, G. Liu, G. Q. Lu, H.-M. Cheng, Synthesis of Anatase TiO2 Rods with Dominant Reactive {010} Facets for the Photoreduction of CO2 to CH4 and Use in Dye-sensitized Solar Cells, Chemical Communications, 47 (2011) 8361-8363.
[27] I. Jang, K. Song, J.-H. Park, S.-G. Oh, Enhancement of Dye Adsorption on TiO2 Surface through Hydroxylation Process for Dye-sensitized Solar Cells, Bulletin of the Korean Chemical Society, 34 (2013) 2883-2888.
[28] T. K. Le, D. Flahaut, H. Martinez, T. Pigot, H. K. H. Nguyen, T. K. X. Huynh, Surface Fluorination of Single-phase TiO2 by Thermal Shock Method for Enhanced UV and Visible Light Induced Photocatalytic Activity, Applied Catalysis B: Environmental, 144 (2014) 1-11.
[29] A. Fujishima, K. Honda, Electrochemical Photolysis of Water at a Semiconductor Electrode, Nature, 238 (1972) 37-38.
[30] A. J. Bard, Photoelectrochemistry, Science, 207 (1980) 139-144.
[31] B. O'Regan, M. Gratzel, A Low-cost, High-efficiency Solar Cell Based on Dye-sensitized Colloidal TiO2 Films, Nature, 353 (1991) 737-740.
[32] K. Tennakone, G. R. R. A. Kumara, A. R. Kumarasinghe, K. G. U. Wijayantha, P. M. Sirimanne, A Dye-sensitized Nano-porous Solid-state Photovoltaic Cell, Semiconductor Science and Technology, 10 (1995) 1689.
[33] A. Fujishima, X. Zhang, D. A. Tryk, TiO2 Photocatalysis and Related Surface Phenomena, Surface Science Reports, 63 (2008) 515-582.
[34] A. L. Linsebigler, G. Lu, J. T. Yates, Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results, Chemical Reviews, 95 (1995) 735-758.
[35] Ü. Özgür, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Doğan, V. Avrutin, S.-J. Cho, H. Morkoç, A Comprehensive Review of ZnO Materials and Devices, Journal of Applied Physics, 98 (2005) 041301.
[36] J. Nanda, B. A. Kuruvilla, D. D. Sarma, Photoelectron Spectroscopic Study of CdS Nanocrystallites, Physical Review B, 59 (1999) 7473-7479.
[37] O. Niitsoo, S. K. Sarkar, C. Pejoux, S. Rühle, D. Cahen, G. Hodes, Chemical Bath Deposited CdS/CdSe-sensitized Porous TiO2 Solar Cells, Journal of Photochemistry and Photobiology A: Chemistry, 181 (2006) 306-313.
[38] J. Zhang, H. Zhu, S. Zheng, F. Pan, T. Wang, TiO2 Film/Cu2O Microgrid Heterojunction with Photocatalytic Activity under Solar Light Irradiation, Applied Materials & Interfaces, 1 (2009) 2111-2114.
[39] J.-L. Lin, Principles and Applications of Photocatalysis on TiO2, Journal of the Chinese Chemical Society, 60 (2002) 457-461.
[40] M. A. Fox, M. T. Dulay, Heterogeneous Photocatalysis, Chemical Review, 83 (1995) 341-357.
[41] M. Maazawi, A. N. Finken, A. B. Nair, V. H. Grassian, Adsorption and Photocatalytic Oxidation of Acetone on TiO2: An in Situ Transmission FT-IR Study, Journal of Catalysis, 191 (2000) 138-146.
[42] K. Vinodgopal, D. E. Wynkoop, P. V. Kamat, Environmental Photochemistry on Semiconductor Surfaces: Photosensitized Degradation of a Textile Azo Dye, Acid Orange 7, on TiO2 Particles Using Visible Light, Environmental Science & Technology, 30 (1996) 1660-1666.
[43] R. Daghrir, P. Drogui, D. Robert, Modified TiO2 for Environmental Photocatalytic Applications: A Review, Industrial & Engineering Chemistry Research, 52 (2013) 3581-3599.
[44] J. Fan, J. T. Yates, Infrared Study of the Oxidation of Hexafluoropropene on TiO2, The Journal of Physical Chemistry, 98 (1994) 10621-10627.
[45] V. M. Shinde, S. M. Khopkar, Liquid-Liquid Extraction of Rhenium(VII) with Mesityl Oxide, Analytical Chemistry, 43 (1971) 473-474.
[46] W. K. O’Keefe, M. Jiang, F. T. T. Ng, G. L. Rempel, Liquid Phase Kinetics for the Selective Hydrogenation of Mesityl Oxide to Methyl Isobutyl Ketone in Acetone over a Pd/Al2O3 Catalyst, Chemical Engineering Science, 60 (2005) 4131 – 4140.
[47] F. G. Klein, J. T. Banchero, Condensation of Acetone to Mesityl Oxide, Industrial & Engineering Chemistry, 48 (1956) 1278-1286.
[48] A. Panov, J. J. Fripiat, An Infrared Spectroscopic Study of Acetone and Mesityl Oxide Adsorption on Acid Catalyst, Langmuir, 14 (1998) 3788-3796.
[49] M. I. Zaki, M. A. Hasan, F. A. Al-Sagheer, L. Pasupulety, Surface Chemistry of Acetone on Metal Oxides: IR Observation of Acetone Adsorption and Consequent Surface Reactions on Silica-Alumina versus Silica and Alumina, Langmuir, 16 (2000) 430-436.
[50] F. H. Stross, J. M. Monger, H. de V. Finch, The Isolation and Purification of Two Isomers of Mesityl Oxide, Journal of the American Chemical Society, 69 (1947) 1627-1628.
[51] N. C. Yang, D. M. Thap, Photochemical Reactions of Mesityl Oxide in 2-Propanol, The Journal of Organic Chemistry, 32 (1976) 2462-2465.
[52] J. Gebicki, A. Plonka, A. Krantz, Photochemical Generation and Detection of an Elusive Rotamer of Matrixisolated Mesityl Oxide. Dispersive Kinetics of the Thermal Isomerization: Twisted s-trans → s-cis Forms, Journal of the Chemical Society, Perkin Transaction 2, 12 (1990) 2051-2054.
[53] M. El-Maazawi, A. N. Finken, A. B. Nair, V. H. Grassiany, Adsorption and Photocatalytic Oxidation of Acetone on TiO2: An in Situ Transmission FT-IR Study, Journal of Catalysis, 191 (2000) 138-146.
[54] C. N. Rusu, J. T. Yates, Photochemistry of NO Chemisorbed on TiO2 (110) and TiO2 Powders, The Journal of Physical Chemistry B, 104 (2000) 1729-1737.
[55] J.-L. Lin, Y.-C. Lin, B.-C. Lin, P.-C. Lai, T.-E. Chien, S.-H. Li, Y.-F. Lin, Adsorption and Reactions on TiO2: Comparison of N,N-Dimethylformamide and Dimethylamine, The Journal of Physical Chemistry C, 118 (2014) 20291-20297.
[56] National Institute of Advanced Industrial Science and Technology, date of access), http://sdbs.db.aist.go.jp
[57] W. Xu, D. Raftery, J. S. Francisco, Effect of Irradiation Sources and Oxygen Concentration on the Photocatalytic Oxidation of 2-Propanol and Acetone Studied by in Situ FTIR, The Journal of Physical Chemistry B, 107 (2003) 4537-4544.
[58] W.-C. Wu, C.-C. Chuang, J.-L. Lin, Bonding Geometry and Reactivity of Methoxy and Ethoxy Groups Adsorbed on Powdered TiO2, The Journal of Physical Chemistry B, 104 (2000) 8719-8724.
[59] L.-F. Liao, C.-F. Lien, J.-L. Lin, FTIR Study of Adsorption and Photoreactions of Acetic Acid on TiO2, Physical Chemistry Chemical Physics, 3 (2001) 3831-3837.
[60] M. A. Henderson, Acetone Chemistry on Oxidized and Reduced TiO2(110), The Journal of Physical Chemistry B, 108 (2004) 18932-18941.
[61] J. C. S. Wong, A. Linsebigler, G. Lu, J. Fan, J. T. Yates, Photooxidation of CH3Cl on TiO2(110) Single Crystal and Powdered Ti02 Surfaces, The Journal of Physical Chemistry, 99 (1995) 335-344.
[62] L.-F. Liao, C.-F. Lien, D.-L. Shieh, M.-T. Chen, J.-L. Lin, FTIR Study of Adsorption and Photoassisted Oxygen Isotopic Exchange of Carbon Monoxide, Carbon Dioxide, Carbonate, and Formate on TiO2, The Journal of Physical Chemistry B, 106 (2002) 11240- 11245.
[63] P. A. Connor, K. D. Dobson, A. J. McQuillan, New Sol-Gel Attenuated Total Reflection Infrared Spectroscopic Method for Analysis of Adsorption at Metal Oxide Surfaces in Aqueous Solutions. Chelation of TiO2, ZrO2, and Al2O3 Surfaces by Catechol, 8-Quinolinol, and Acetylacetone, Langmuir, 11 (1995) 4193-4195.