ARTICLE pubs.acs.org/JPCC
Selective Oxidation of Alkenes over Uranyl-Anchored Mesoporous MCM-41 Molecular Sieves Parasuraman Selvam,*,† Vilas M. Ravat,‡ and Vidya Krishna† † ‡
National Center for Catalysis Research, Department of Chemistry, Indian Institute of Technology-Madras, Chennai 600 036, India Solid State and Catalysis Laboratory, Department of Chemistry, Indian Institute of Technology-Bombay, Powai, Mumbai 400 076, India ABSTRACT: Uranyl-anchored MCM-41 (UO22þ/MCM-41) catalysts were prepared hydrothermally and systematically characterized employing various analytical and spectroscopic techniques: namely, X-ray diffraction (XRD), nitrogen sorption isotherms, transmission electron microscopy (TEM), electron diffraction (ED), and thermogravimetry-differential thermal analysis, inductively coupled plasma-atomic emission spectroscopy, diffuse reflectance ultraviolet-visible (DRUV-vis) spectroscopy and fluorescence spectroscopy, and Fourier transformintrared (FT-IR) spectroscopy. XRD confirms the incorporation of uranyl ions into the silicate matrix and that TEM and ED investigations corroborate the highly ordered structure of uranyl-incorporated MCM-41. Further, these findings were well supported by DRUV-vis, fluorescence, and FT-IR spectra, indicating the nature of uranyl ion species as well as their interaction with the silicate framework. Well-characterized, high-quality UO22þ/MCM-41 catalysts were employed for the liquid-phase allylic oxidation of R-pinene, β-pinene, and cyclohexene under moderate reaction conditions using various solvents and oxidants. Under the optimized experimental conditions, the catalysts showed high substrate conversion and excellent product selectivity. In addition, the influence of various other parameters (viz., temperature, time, recyclability, uranium content, etc.) were also performed.
’ INTRODUCTION Terpenes are known to be widely distributed in nature, and their epoxides often serve as starting materials for the synthesis of fragrances, flavors, and therapeutically active substances.1,2 Among a number of terpenes, R-pinene is an important substance in the manufacture of a variety of synthetic aroma chemicals, and its epoxide is isomerized to produce campholenic aldehyde, which is an intermediate for the sandalwood fragrance, santalol.3,4 In this context, oxidation of R-pinene and β-pinene, which occurs widely in the plant kingdom, is viewed as an important reaction because oxidation products find use as the starting materials for fragrance, flavor, and therapeutic agents and as key intermediates for the manufacture of various fine chemicals, including citral, menthol, taxol, and vitamins A and E.1-4 The allylic oxidation of olefins leads to R,β-unsaturated ketones, which have been widely employed as important transformations in natural products.5 In particular, the oxidation products of cyclohexene and their derivatives (viz. 2-cyclohexen-1-one, 1-methylcyclohex-1-en-3-one, etc.) are important in organic synthesis owing to the presence of a highly reactive carbonyl group, which is utilized in cycloaddition reactions.6-8 In the field of catalytic oxidation, great efforts have been made to convert homogeneous to selective heterogeneous processes, r 2011 American Chemical Society
including the use of clean and safe oxidants such as O2, H2O2, and alkylhydroperoxide.9,10 In view of this, a number of studies have utilized uranyl ions for homogeneous liquid phase photooxidation reactions of hydrocarbons,11-16 chlorophenols,17 and substituted phenols.18 Comparatively, fewer studies have been devoted to the catalytic activity of UO22þ ions in the heterogeneous reaction mode. Suib and co-workers employed uranylexchanged clays and zeolites for the photooxidation of ethanol, isopropyl alcohol, and diethyl ether to yield the corresponding aldehydes and ketones.19-21 In another study, the photocatalytic oxidation of ethanol solution by UO22þ -doped glass22 resulted in the formation of acetaldehyde. Uranium-based oxide catalysts have also been found to exhibit excellent activity in the oxidation and dehydration of various hydrocarbons.23,24 On the other hand, the mesoporous materials, designated as the M41S family, comprising hexagonal MCM-41 and cubic MCM-48 structures, are used as supports for a number of catalytically active metal oxides owing to their large pore diameters (2-20 nm), high surface areas (700-1500 m2 g-1), Received: July 29, 2010 Revised: December 7, 2010 Published: January 5, 2011 1922
dx.doi.org/10.1021/jp107121d | J. Phys. Chem. C 2011, 115, 1922–1931
The Journal of Physical Chemistry C
ARTICLE
Table 1. Physicochemical Properties of UO22þ/MCM-41 with Varying Si/U Ratio XRD data a0 (Å)b samplea
a
synthesized
calcined
Si/U = ¥
41.9
38.9
Si/U = 200
44.3
Si/U = 100
uranium content (wt %)c
N2 sorption data pore volume (cm3/g)
pore diameter (Å)
surface area (m2/g)
0.99
25
1108
0.96
23
1057
2.34
0.86
22
997
5.01
4.88
0.80
23
959
8.49
8.10
0.77
24
884
synthesized
calcined
41.3
1.50
1.43
43.3
42.0
2.44
Si/U = 50
43.4
42.4
Si/U = 25
43.6
42.6
Si/U molar ratio in synthesis gel. b Average unit cell parameter (a0) calculated using 1/d2 = 4/3(h2 þ hkþ h2)/a2. c ICP-AES data.
and substantial amounts of silanol (defect sites) groups (30-40%).25 In this study, we present a detailed investigation of the synthesis and characterization of hexavalent uranium substituted in hexagonal mesoporous MCM-41 molecular sieves. The performance of the catalyst was evaluated for the oxidation reaction of R-pinene, β-pinene, and cyclohexene under mild reaction conditions.
2. EXPERIMENTAL SECTION 2.1. Starting Materials. Fumed silica (SiO2; Aldrich), cetyltrimethylammonium bromide (CTAB; Aldrich; 99%), tetramethylammonium hydroxide (TMAOH; Lancaster), sodium hydroxide (NaOH; Loba Chemie; 98%), and uranyl nitrate hexahydrate [UO2(NO2)3 3 6H2O; Analytical] were used as sources for silicon, template alkali, and uranium salt, respectively. Cyclohexene (S.D. Fine; 99%), R-pinene and β-pinene, (Aldrich), tert-butyl hydrogen peroxide (TBHP; t-BuOOH; Lancaster; 70% aq), and hydrogen peroxide (H2O2; S.D.Fine; 50% aq) were employed for the organic reactions. 2.2. Synthesis. In the synthesis of uranyl-anchored ordered mesoporous silica (UO22þ/MCM-41), at first, TMAOH was diluted in distilled water and stirred for 10 min. To this, fumed silica was added slowly, and a homogeneous “solution A” was obtained. A “solution B” was prepared by adding CTAB slowly into distilled water and stirring for 1 h. A solution of NaOH in distilled water was added to the CTAB solution and was stirred for ∼30 min. The uranyl nitrate diluted in distilled water was added dropwise with constant stirring for 1 h. Now, both solution A and solution B were mixed together slowly under constant stirring for 1 h. The resulting gel was stirred further for 2 h for homogenization. The typical gel (molar) composition was as follows: SiO2, 0.27; CTAB, 0.26; TMAOH, 0.136; and Na2O, x UO2, 60 H2O, where x = 0.0025, 0.005, 0.01, 0.02, 0.04 (Si/U = 500, 200, 100, 50, 25). The pH of the gel was adjusted to 11.5 by adding either dilute H2SO4 or aqueous NaOH and was placed in an air oven at 373 K for 10 days in Teflon-lined stainless steel autoclaves. The solid products obtained were washed with distilled water, filtered, and dried at 353 K. The as-prepared UO22þ/MCM-41 were then calcined in a tubular furnace at 823 K in a flow of N2 for 1 or 2 h, followed by 6 h in air with a flow rate of 50 mL min-1 and a heating rate of 1 C min-1 2.3. Characterization. All the as-synthesized and calcined samples were characterized using several analytical and spectroscopic techniques. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku-Miniflex diffractometer using a nickel-filtered Cu KR radiation (λ = 1:5418 Å) with a step size of 0.02. Transmission electron micrograph (TEM) was recorded on a Philips 200 microscope operated at 160 kV. The sample (in powder form) was dispersed in methanol with sonication
Figure 1. N2 adsorption-desorption isotherm of UO22þ/MCM-41: (a) Si/U = , (b) Si/U = 200, and (c) Si/U = 25.
(Oscar ultrasonics), and a drop of it was placed on a carbon-coated copper grid (300 mesh; Sigma-Aldrich). Surface area analysis was measured at 77 K using a Micromeritics ASAP 2020. Before nitrogen adsorption, the samples were treated at 250 C under vacuum (
