Research Article pubs.acs.org/journal/ascecg
Effective NbOx‑Modified Ir/SiO2 Catalyst for Selective Gas-Phase Hydrogenation of Crotonaldehyde to Crotyl Alcohol Masazumi Tamura,* Kensuke Tokonami, Yoshinao Nakagawa, and Keiichi Tomishige* Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan S Supporting Information *
ABSTRACT: NbOx-modified Ir/SiO2 catalyst (Ir-NbOx/SiO2) showed high activity and selectivity for gas-phase hydrogenation of crotonaldehyde, and Ir-NbOx/SiO2 with Nb/Ir molar ratio of 0.5 (Ir-NbOx/SiO2 (Nb/Ir = 0.5)) provided high crotyl alcohol yield of 87%. This is the highest crotyl alcohol yield among those in gas-phase over reported heterogeneous catalysts. The activities (TOF per surface Ir atom) at low conversion level (40%, W/F = 0.37 gcat·h·mol−1) and high conversion level (97%, W/F = 2.2 gcat·h·mol−1) were calculated to be 0.21 and 0.08 s−1, which are more than 3 times higher than those over reported heterogeneous catalysts. High activity and high yield are derived from the combination of Ir metal with NbOx species. Based on the spectroscopic and kinetic studies, the high activity and high selectivity are attributed to strong adsorption of crotonaldehyde on NbOx species by the high Lewis acidity, and suppression of over-hydrogenation of the produced crotyl alcohol at high conversion level. It is suggested that the formation of hydride species at the interface between Ir metal and NbOx species can promote the selective hydrogenation considering the reaction order with respect to hydrogen pressure. High yield and high activity of Ir-NbOx/SiO2 can contribute to energy saving, cost reduction, and waste reduction, which are directly connected to realization of sustainability. KEYWORDS: Selective hydrogenation, Iridium, Unsaturated alcohol, Niobium oxide
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INTRODUCTION To attain sustainable society, development of highly selective and active catalysts is essential, and heterogeneous catalysts are preferable to homogeneous catalysts because of their easy handling, reusability, durability, and easy separation of products and catalysts. In liquid-phase hydrogenation reactions, solvents and high-pressure H2 are required because the solubility of H2 in solvents is typically low. In contrast, gas-phase hydrogenation by a fixed bed reactor is more practical and eco-friendly than the liquid-phase hydrogenation due to high conversion per catalyst amount, low operating cost, continuous operation, and so on. In addition, low H2 pressure can be easily applied in gasphase reaction; however, the activity is generally decreased with the reaction order of 0.5−1, which requires development of highly active heterogeneous catalysts. Therefore, highly active heterogeneous catalysts with high selectivity are desired to be developed. Selective hydrogenation of α,β-unsaturated aldehydes to unsaturated alcohols is an important organic reaction in industry because α,β-unsaturated alcohols are useful chemicals as intermediates for fragrances, medicines, and pesticides.1−3 However, selective hydrogenation of α,β-unsaturated aldehydes to unsaturated alcohols is quite difficult because in general hydrogenation of CO bond in α,β-unsaturated aldehydes is thermodynamically and kinetically unfavorable compared with that of CC bond, and the produced unsaturated alcohols are subject to over-hydrogenation to saturated alcohols. Traditionally, metal hydrides such as NaBH4 and LiAlH4 are used for the © 2017 American Chemical Society
selective hydrogenation of carbonyl groups, and these methods are still used in laboratories and fine chemical industries. However, the metal hydrides are expensive, and a large amount of salt is produced by neutralization, in addition, in the case of large-scale hydrogenation reactions, handling of a large amount of metal hydrides is also a problem because at least equivalent amount of metal hydrides are generally required for the reactions. Therefore, metal hydrides are unfavorable for largescale hydrogenation reactions. However, there are many largescale hydrogenation reactions in chemical industry, therefore, in order to overcome these drawbacks, development of effective catalysts with gaseous H2 is desirable because utilization of H2 for large-scale hydrogenation reactions was established in industry as well as the low cost and high atom economy. Many effective homogeneous catalysts such as Ru or Fe complexes4−7 and heterogeneous ones such as Ag-, Au-, Pt3Sn-, and Irsupported ones8−13 for selective hydrogenation of α,βunsaturated aldehydes in liquid phase with gaseous H2 were reported, and high yield (>90%) and high activity (TOF > 10 h−1 based on total noble metal amount) were achieved in hydrogenation of crotonaldehyde as a model reaction. For Special Issue: Asia-Pacific Congress on Catalysis: Advances in Catalysis for Sustainable Development Received: December 15, 2016 Revised: February 22, 2017 Published: February 26, 2017 3685
DOI: 10.1021/acssuschemeng.6b03060 ACS Sustainable Chem. Eng. 2017, 5, 3685−3697
Research Article
ACS Sustainable Chemistry & Engineering
Mo, Fe, and Nb) catalysts were prepared by impregnating the dried Ir/ SiO2 that was obtained after the drying procedure with aqueous solutions of NH 4 ReO 4 (Soekawa Chemical Co., Ltd.), (NH4)10W12O41·5H2O (Wako Pure Chemical Industries, Ltd.), (NH4)6Mo7O24·4H2O (Wako Pure Chemical Industries, Ltd.), Fe(NO3)3·9H2O (Wako Pure Chemical Industries, Ltd.), and (NH4)NbO(C2O4)2·xH2O (Sigma-Aldrich Inc.). After evaporating water and drying at 383 K for 12 h, the catalysts were calcined in air at 773 K for 3 h. The loading amount of Ir metal was 4 wt%, and that of additive was represented by the molar ratio of the additive to Ir metal. Nb2O5/SiO2 catalyst was prepared by impregnating SiO2 with an aqueous solution of (NH4)NbO(C2O4)2·xH2O. After evaporation of solvent and drying at 383 K for 12 h, the catalyst was calcined in air at 773 K for 3 h. The loading amount of Nb was 4 wt%. Typical Procedure for Gas-Phase Hydrogenation of Crotonaldehyde. Gas-phase hydrogenation of crotonaldehyde (>98%, Tokyo Chemical Industry Co., Ltd.) was carried out using a fixedbed flow reactor. The reactor was made of a quartz tube (4 mm i.d.). The reaction temperature was monitored by a thermocouple, which was inserted into the outlet of the catalyst bed. The catalyst weight was 50 mg, which provided the bed thickness of about 5 mm. Before the experiment, the catalysts were pre-reduced in flowing H2 flow (50 mL min−1) at 773 K for 0.5 h. After the reactor was cooled to reaction temperature, crotonaldehyde was supplied through the vaporizing chamber. In the vaporization chamber, crotonaldehyde was vaporized at 393 K, and H2 sweeps the vapor out to the catalyst bed. The gas line was heated above 373 K to avoid any condensation. The standard feeding rate of crotonaldehyde and carrier gas was crotonaldehyde/H2 = 1/185, and the total pressure was 0.1 MPa. The standard contact time in the hydrogenation of crotonaldehyde is calculated to be W/F = 0.37 gcat·h·mol−1, where F represents the total flow rate of crotonaldehyde gas and H2 and W represents the catalyst weight. The reaction temperature was 373 K. The effluent gas was sampled by using a hot syringe heated at 493 K. The partial pressures of crotonaldehyde and products were measured by FID-GC (TC-WAX). It took about 1.5 h to get constant and stable feeding rate of crotonaldehyde in our apparatus. Therefore, the catalytic activity was evaluated on the basis of the results at or after 2 h. XRD, FE-STEM, TPR, and CO Adsorption. X-ray diffraction (XRD) patterns were recorded by a diffractometer (MiniFlex600; Rigaku). Cu Kα (λ = 0.154 nm, 45 kV, 40 mA) radiation was used as an X-ray source. Average metal particle size was estimated using the Scherrer’s equation. Field emission scanning transmission electron microscope (FE-STEM) images and energy-dispersive X-ray (EDX) analysis were obtained on a Hitachi spherical aberration corrected STEM/SEM HD-2700 instrument operated at 200 kV. After the reduction, the samples were dispersed in ethanol and placed on Cu grids under air atmosphere. Average particle size was calculated by ∑nidi3/∑nidi2 (di, particle size; ni, number of particles with di). Temperature-programmed reduction (TPR) was carried out in a fixedbed reactor equipped with a thermal conductivity detector using 5% H2 diluted with Ar (30 mL/min). The amount of catalyst was 0.1 g, and the temperature was increased from room temperature to 1023 K at a heating rate of 10 K/min. The amount of CO chemisorption was measured in a high-vacuum system using a volumetric method. The catalyst (∼0.1 g) in the measurement cell was reduced with H2 at 773 K for 1 h and evacuated at 773 K for 1 h. After cooling, the adsorption amount of CO was measured at room temperature. Gas pressure at adsorption equilibrium was about 1.1 kPa. The dead volume of the apparatus was about 60 cm3. XPS Analysis. X-ray photoelectron spectroscopy (XPS) experiments were conducted with an AXIS-ULTRA DLD (Shimadzu Co., Ltd.) using monochromatic Al Kα X-ray radiation (hν = 1486.6 eV) operated at 20 mA and 15 kV at room temperature under 8−10 Pa. The binding energy was calibrated with C 1s (284.6 eV). The catalysts after the reaction or reduction were transported to the analysis chamber in nitrogen atmosphere to avoid any exposure to air. Analysis of XPS data was performed by using the computer program CasaXPS ver. 2.3.15 (Casa Software, Ltd.).
more sustainable processes, heterogeneous catalysts in gasphase hydrogenation by a fixed bed reactor have been also developed. Noble metals have hydrogenation activity, and various kinds of noble metals have been applied to the selective hydrogenation of unsaturated aldehydes in the gas phase. Ptbased catalysts such as Pt/Ga2O3, Pt/CeO2, and Pt-Sn/ SiO214−16 and Ru-based catalysts such as Ru/ZnO,17 Ru-Ir/ ZnO,18 and so on19 were reported. It should be noted that Ir metal is more effective component than other noble metals on the basis of the previous reports such as Ir/Ga2O3, Ir-FeOx/ SiO2, Ir-CrOx/SiO2, and so on.20−22 Various kinds of catalysts were proposed and developed; however, high-yield synthesis of crotyl alcohol in gas-phase reaction seems to be difficult in comparison with the liquid-phase reaction, and the reported highest yield was 81%, which was obtained by using bimetallic Ru-Ir/ZnO catalyst18 (Table S1). Moreover, the catalytic activities are low (formation rate of crotyl alcohol typically 90%), providing 90% yield of crotyl alcohol.29 Herein, we applied the metal-oxide-modified Ir metal catalysts to the gas-phase hydrogenation of crotonaldehyde and found that Ir-NbOx/SiO2 (Nb/Ir = 0.5) was a preferable heterogeneous catalyst for the reaction. This catalyst provided high selectivity to crotyl alcohol at high conversion level and achieved high crotyl alcohol yield of 87%, which is higher than the yields over the reported heterogeneous catalysts in gasphase reaction. The formation rate of crotyl alcohol over IrNbOx/SiO2 was calculated to be 45 μmol·gmetal−1·s−1 at W/F = 0.37 gcat ·h·mol −1 , and the TOF per surface Ir atom (TOFsurface Ir) was estimated to be 0.21 s−1, which is much higher than those over reported heterogeneous catalysts (Table S1, ≤0.024 s −1). Based on the characterization of Ir-NbOx/ SiO2 (Nb/Ir = 0.5) by, e.g., XRD, TEM, TPR, XPS, XAFS, FTIR, and kinetic studies, the catalyst structure and reaction mechanism over Ir-NbOx/SiO2 are also proposed.
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EXPERIMENTAL SECTION
Catalyst Preparation. Ir/SiO 2 catalyst was prepared by impregnating SiO2 (Fuji Silysia Chemical Ltd., G-6, BET surface area 535 m2/g) with an aqueous solution of H2IrCl6 (Furuya Metals Co., Ltd.). After evaporating solvent and drying at 383 K for 12 h, the catalyst was calcined in air at 773 K for 3 h. Ir-MOx/SiO2 (M = Re, W, 3686
DOI: 10.1021/acssuschemeng.6b03060 ACS Sustainable Chem. Eng. 2017, 5, 3685−3697
Research Article
ACS Sustainable Chemistry & Engineering
Table 1. Comparison of Metal-Oxide-Modified Ir/SiO2 Catalysts and Nb2O5/SiO2 in Gas-Phase Hydrogenation of Crotonaldehydea
selectivity/% entry
catalyst
conversion/%
crotyl alcohol
butanal
1-butanol
n-butane
1 2 3 4 5 6 7
Ir-ReOx/SiO2 (Re/Ir = 1) Ir-MoOx/SiO2(Mo/Ir = 1) Ir-FeOx/SiO2(Fe/Ir = 1) Ir-NbOx/SiO2 (Nb/Ir = 1) Ir-WOx/SiO2(W/Ir = 1) Ir/SiO2 Nb2O5/SiO2
45.2 37.4 23.2 20.4 10.6 7.3
