Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso Highly efficient epoxidation of α-pinene with O 2 photocatalyzed by dioxoMo (VI) complex anchored on TiO 2 nanotubes Henry Martínez, Álvaro A. Amaya, Edgar A. Páez-Mozo, Fernando Martínez O. * Centro de Investigaciones en Catálisis-CICAT, Universidad Industrial de Santander, Escuela de Química, Km 2 vía El Refugio, Piedecuesta, Santander, Colombia ARTICLE INFO Keywords: Oxygen Atom Transfer Dioxo-molybdenum complexes TiO 2 nanotubes Hydroxyl groups density Surface functionalization Selective photo-oxidation ABSTRACT The effect of different surface OH's groups on the amount of anchored Mo complex: Mo (VI) Cl 2 O 2 Bipy (Bipy = 4,4′-dicaboxylato-2,2′-bipyridine) grafted on TiO 2 (Mo (VI) Cl 2 O 2 Bipy/TiO 2 ) and its effect on the photo- catalytic Oxygen Atom Transfer (OAT) activity to α-pinene with O 2 was studied. The amount of supported complex on TiO 2 nanotubes is 5 times the amount of the grafted complex on TiO 2 P-25 and OAT activity increase of 25%, may be associated to the physicochemical properties of TiO 2 nanotubes. High selectivity was preserved toward the epoxide formation (> 90%). The amount of Mo complex covalently anchored on TiO 2 depends on the concentration, distribution and accessibility of the Ti-OH surface groups, which depend on the TiO 2 preparation method. The different OH's groups, namely: Bridged, geminal and isolated were identified by FT-IR Photoacoustic Spectroscopy. Surface OH's concentration was determined by TGA methods, silylation reactions and XPS analysis. 1. Introduction Many organic oxygenated compounds are produced by catalytic oxidations using organic peroxides and other activated oxygen com- pounds like dimethyl sulfoxide, iodosylbenzene or H 2 O 2 as the oxygen source [1–6]. Today the green chemistry challenge is focused on the use of O 2 as the oxidant agent under mild conditions [7–9], accordingly partial oxidation of hydrocarbons and alcohols to aldehydes or epoxides has been studied using selective oxidation catalysis by metal complexes [10–14]. Olefin epoxidation with DioxoMo(VI) complexes in solution have been widely studied. However, despite of achieving high conver- sion and selectivity, there are problems associated with stability of the catalyst, by formation of an inactive μ-oxo dimer, which is thermo- dynamically stable [15]. In a previous work we have obtained a het- erogeneous system by covalently anchoring a dioxo-molybdenum complex on TiO 2 , to activate dioxygen to obtain oxygenated organic compounds, under UV–Vis light and O 2 as primary oxidant [16–21]. It has been observed that Titania Nanotubes (TNTs) with high specific surface area and with high number of reactive titanol sites (Ti-OH), favor the photo-assisted oxygen transfer [22–24]. To enhance the OAT activity is necessary to increase the amount of the anchored complex. In this work we have observed a relation be- tween the OAT activity with the amount of anchored Mo (VI) complex and the reactive OH's density. The Mo (VI) Cl 2 O 2 Bipy complex anchored on TiO 2 nanotubes show higher OAT activity than the complex supported on mesoporous and non-porous nano-TiO 2 . The amount of anchored Mo (VI) complex depends on the concentration, distribution and accessibility of the surface Ti-OH groups, which depend on the TiO 2 preparation method. 2. Experimental 2.1. Materials and methods All reagents were analytical grade and were used without further treatment. The TiO 2 supports were characterized by powder X-ray dif- fraction (XRD) using a Bruker AXS D8 Advance DaVinci geometry with monochromatized Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 30 mA. The diffraction patterns were recorded in the 2θ value range of 20–70° (with a step size of 0.01° and a step time of 0.4 s). The solid supports morphology was characterized by SEM (QuantaTM 650 FEG) operating at 20 kV. The Raman spectra were obtained using a Raman Confocal Microscope (LabRAM HR Evolution HORIBA Scientific), irradiated with a laser of wavelength 532 nm, 10 mW output power, 10X objective, integration time 2 s and 10 accumulations. The adsorption-desorption isotherms of N 2 at -196 °C were obtained using a Micromeritics 3Flex. Samples were degassed at 110 °C for 8 h before the adsorption mea- surements. The surface area and pore size distribution were determined from the adsorption-desorption isotherms of N 2 (BJH). The band gap energy was determined by UV–Vis diffuse reflectance spectroscopy https://doi.org/10.1016/j.micromeso.2018.02.005 Received 17 November 2017; Received in revised form 24 January 2018; Accepted 5 February 2018 * Corresponding author. E-mail address: fmartine@uis.edu.co (F. Martínez O.). Microporous and Mesoporous Materials 265 (2018) 202–210 Available online 11 February 2018 1387-1811/ © 2018 Elsevier Inc. All rights reserved. T