1. Introduction
Due to environmental concerns, sustainability issues, and the high cost of crude-oil based goods, industrial and academic research on biomass- based energy production and utilization has been actively explored for past several decades[1,2]. The use of biomass-based feedstock, which is abundant and sustainable source of energy, is of a tremendous advantage in minimizing the exhaustion of petrol resources[ 3]. Sustainable carbon-containing compounds[4] can be made from lignocellulosic biomass which is a distinguishable source of carbohydrates, but they have a few shortcomings, including (a) incompatibility with high-temperature chemical processes in industries, (b) structural intricacy, and (c) attachment of a large number of functional groups[5]. If the number of functional groups on complicated carbohydrate moieties[6-9] can be controlled, stable and practical bio-based bifunctional building blocks can be produced; isohexides made from various carbohydrates like starch or cellulose are a well-known example. In this view, isosorbide (1,4:3,6-dianhydro-D-sorbitol, 1a, Figure 1) may be easily made from sorbitol (a sugar-based hexitol) using a double dehydration procedure (Figure 2), and its stereoisomers, isoidide (1b) and isomannide (1c) can also be interconverted via catalytic isomerization[10].
Isosorbide has a V-shaped geometry owing to the cis-bonding of two flattened tetrahydrofuranic ring. These two five-membered rings form a nearly 120° angle. Isosorbide was first reported in 1927[11], and its structure was conformed in 1946[12] when its crystalline form was synthesized in 1940[13]. The position of –OH groups in isosorbide and two of its structural isomers differs, with isoidide showing exo conformation and isomannide revealing endo conformation. Isosorbide structure has both exo and endo conformations, making it a one-of-a-kind molecule. Exo configuration is shown by the –OH group at position-2 in isosorbide, whereas endo is seen by the one at position- 5. The difference in the physical properties of these three compounds, such as solubility, reactivity, and melting temperatures, is related to the different positions of the –OH functional group[14].
Isosorbide has been used as a platform material in a variety of applications during the last few decades, including catalysis[15], polymers[ 9], liquid crystalline composites[16], cosmetics and pharmaceutical additives[17], and ionic liquids[18]. This diverse use of isosorbide is attributed to its non-toxic nature, structural rigidity, relative stability, and chirality[10]. In comparison to isoidide and isomannide, the participation of the endo position -O atom of isosorbide in hydrogen bonding generates an increased nucleophilic ability, which sequentially amplifies the chemical reactivity of isosorbide. The steric hindrance[14] is the key factor which makes the –OH group at exo position less reactive towards nucleophilic attack. Alcohol oxidation reactions are a major focus of organic chemistry research since the resulting compounds have a wide scope of applications in chemical syntheses. Isosorbide is classified as a cyclic secondary alcohol, however there is relatively little literature on its oxidation processes. Various oxidizing agents and systems, such as chromic acid[1], TEMPO/laccase[19], Pt on carbon[20], sodium borohydride[21], and the TEMPO/NaNO2/NaNO3 system[22], have been employed in previous papers to depict the oxidation process. Isosorbide is oxidized to an isosorbide-diketone (2,6-dioxabicyclo(3,3,0)octan-4,8-one), which is a crucial precursor for the further derivatization of isosorbide into various efficient products such as isosorbide-diimine, isosorbide-diamine, or isosorbide-dialcohol. Use of isosorbide mono-nitrates and di-nitrates in medicine[23] and isosorbide dimethyl ether in pharmaceutical[24] solvent are worth mentioning. The previously reported procedures contain deficiencies in several reaction steps or the stoichiometric usage of expensive and hazardous reagents like bromine sources and organic solvents, which are a hurdle to the industrial scale preparation of isosorbide-diketone[22]. The use of catalysis under benign conditions is an economically promising and advantageous technique to solve most of the aforementioned challenges.
Therefore, we report the fabrication of a Ruthenium/TEMPO/nitrate catalytic system for efficient oxidation of bicyclic dialcohol, and it is successfully used for the oxidation of the hydroxyl group for the isosorbide in a halogen-free procedure under mild reaction conditions. At 50 °C, the procedure is carried out in the presence of a natural oxidant, O2, as well as a nitroxyl radical such as TEMPO and NO source from nitrate or nitrite. Remarkably, the as-synthesized catalyst is capable of oxidizing both of hydroxyl groups at 2 and 5 position of isosorbide alcohol to give a isosorbide-diketone under benign conditions in Figure 3. In addition, as shown in Figure 4, we have suggested a plausible catalytic reaction mechanism for the two hydroxyl groups with different reactivity.
2. Experimental
2.1. Catalyst preparation
The following is how the supported Ruthenium catalyst was produced: Powdered MnO2 was pretreated at 300 °C for 5 h, then rapidly mixed with aqueous RuCl3 (8.3 mM, 60 mL) at room temperature. While stirring, pH of the mixture was adjusted to 13 by adding 1 M of sodium hydroxide. The resulting slurry was left to agitate for 24 hours. It was then dried at room temperature to obtain a fully dried powder of supported Ruthenium catalyst. The Ruthenium content was 2.1-2.2 wt% by ICP-AES (inductively coupled plasma atomic emission spectroscopy) analysis.
2.2. Instruments and materials
The substrates and products were analyzed using a Younglin GC-6500 instrument with a flame ionization detector (FID) and an HP-INNOWAX capillary column (internal diameter = 0.25 mm, length = 30 m). Mass spectra (GC/MS) were recorded on Hewlett Packard 6850 gas chromatograph system with 5973 MSD (Hewlett Packard, USA) at an ionization voltage of 70 eV equipped with a DB-5 capillary column (internal diameter = 0.25 mm, length = 30 m). Elemental analysis for transition metals was carried out an ICP-AES (Shimadzu ICPS-7510). Sigma-Aldrich provided the isosorbide and Ruthenium precursors, while Tokyo Chemical Industry Inc. offered the TEMPO. NaNO3 was obtained from ACROS Organics. All additional reagents and solvents were used as received, with no further purification.
2.3. Catalyst reaction procedures
All operations were carried into a Pyrex-glass vial in a tubular type reactor. A typical reaction procedure was conducted as follows. Ruthenium catalyst (10 mol%) was put into acetic acid (3 ml) in the vial. Isosorbide substrate (0.5 mmol), TEMPO (5 mol%), and NaNO3 (0.03 mmol) in order was added with cross-shaped magnetic bar. The reaction mixture was stirred (500 rpm) at 50 °C under 1 atm of molecular oxygen. After several hours, the catalyst and the product(s) were separated by the filtration or centrifugation. The residue solid was washed with water. Then, diphenyl as an internal standard was added to the combined organic solution and the solution was analyzed by Gas Chromatography (GC) and GC/MS.
3. Results and discussion
3.1. Characterization of the supported Ruthenium catalyst (Ru/MnO2)
The X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to characterize the prepared Ru/MnO2 catalyst. The analysis results are demonstrated in Figure 5 and Figure 6. The XRD patterns of supported Ruthenium catalysts were identical to those of the parent supports, with no signs of Ru metal clusters or RuO2 (Figure 5). These findings show that Ruthenium species are widely dispersed on the surface of supports. Figure 6 shows TEM images of Ru/MnO2 at different resolutions, including 10, 20, and 50 nm. All of the elements of the Ru/MnO2 catalyst, such as Ruthenium, manganese, and oxygen, might be clearly shown in the images.
3.2. Double oxidation of isosobide with Ruthenium/TEMPO/nitrate
The isosorbide oxidation reaction was carried out by catalyst system of supported Ruthenium/TEMPO/Nitrate with isosorbide substrate and acetic acid as a solvent in a tubular type reactor with a teflon-coated magnetic stir bar at 50°C. The reaction was performed at 1 atm molecular oxygen pressure during 10 h. Reaction conditions were optimized using various catalyst systems and solvents. Table 1 shows the summary of the results. The supported Ru in the presence of TEMPO and NaNO3 demonstrated a good catalytic activity with a conversion rate of 90% and excellent selectivity (>97%) of the desired product (isosorbide-diketone) in 10 h (Entry 1). In the presence of other solvents except acetic acid, the systems did not produce the corresponding diketones (Entry 2-4). The supported Ruthenium without TEMPO did not catalyze the transformation (Entry 5). Not having Ruthenium or TEMPO, the reactions were not proceeded regardless of calcination of support MnO2 (Entry 6-7). There was no reaction in the absence of the Ru/TEMPO/Nitrate catalyst system for the conversion of isosorbide (Entry 8). All product samples were analyzed using GC and GC/MS[19].
3.3. The proposed double oxidative catalytic mechanism
In the transformation of isosorbide, the exo-hydroxy ketone production from oxidative hydrogen transfer of isosorbed is requried higher energy than endo-hydroxy ketone production[19]. This can be explained by the unique V shape of isosorbide which has distinctive reactivities of hydroxyl groups in 2 and 5[14]. These two seconday alcohols should overcome remarkable energy barrier to be oxidized. The Ruthenium/TEMPO as catalyst sytem was reported by Sheldon research group for oxidation of considerably unreactive secondary alcohols[ 25]. The corresponding diketone was not obtained as the oxidation reaction of the secondary alcohols without Ruthenium/TEMPO/Nitrate and acetic acid as a solvent in Table 1. In addition, the profile of reaction for the double oxidation of isosorbide showed that exo-hydroxyl (or endo-) monoketone was initially formed followed by the formation of final product, diketone (Figure 7). Therefore, the transformation of isosorbide with Ruthenim/TEMPO/Nitrate system in acetic acid solvent potentially proceeds through cascade sequence reactions to yield the isosorbide diketone.
The potential double oxidative reaction mechanism is proposed in Figure 8. Initially, This mechanism can be divided into four steps. In the first step, an oxoammonium salt is generated into based on the catalytic cycle of TEMPO oxidation[26] (Step I). In second step, this salt may react with the respective alcohol to produce endo- or exo-hydroxyl monoketones as intermediates and a hydroxylamine (Step II). In third step, the hydroxylamine can regenerate the oxoamonium salt to continue the catalytic cycle (Step III). Finally, the monoketones experience the same oxidation process to formation of the diketones, desirable products (Step IV).
4. Conclusion
In conclusion, the supported Ruthenium/TEMPO/nitrate catalytic system showed excellent selectivity (>97%) and high yield (87%) in presence of acetic acid solvent under environmentally benign reaction conditions. The possible reaction mechanism of isorsorbide transformation to isosorbide-diketone was proposed by intermediates, endoor exo-hydroxyl monoketones during the reactions via reaction profiles and various catalyst systems. Based on these proposition, double oxidation and oxoamonium salt should play an important role in producing largly unreactive seconday alcohols such as isohexides.
