Lignocellulosic biomass is an abundant renewable feedstock resource for biorefineries to produce fuels and chemicals [1]. In biorefinery industry, fast pyrolysis is a thermochemical process which can produce up to 70 wt% bio-oils from raw biomass and is being deployed at pilot-scale [2, 3, 4, 5, 6, 7, 8]. However, due to the high oxygen contents (35-40 wt%), low heating values, instability, corrosiveness, and immiscibility with gasoline and diesel, the bio-oil products cannot be used as liquid transportation fuels without substantial upgrading [9, 10]. Hydrodeoxygenation (HDO) is extensively studied for upgrading pyrolysis oil [11, 12, 13]. However, the H2 consumption for upgrading pyrolysis oil is a major hurdle of making HDO cost-effective. Especially, due to the unselective nature of HDO, unnecessary saturation of aromatic molecules, which account for ~30 wt% of the total pyrolysis oil, causes excess H2 consumption and therefore makes HDO cost-prohibitive, considering the low profit margin of fuel products [14]. In contrast, selective oxidation of low-value lignin derived compounds into value-added commodity chemicals could improve the overall process economics of a biorefinery.
Guaiacol (2-methoxyphenol), one of the major lignin- derived bio-oil components, is widely used as the model feed for HDO upgrading [15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31]. However, the catalytic upgrading of guaiacol via oxidation methods is much less explored. Sasaki’s group reported that the main products of hydrothermal oxidation of guaiacol in sub- and super- critical water were catechol, phenol, and o-cresol [32, 33]. Suzuki et al. [34] found that guaiacol was unselectively oxidized to low-molecular weight carboxylic acids like acetic acid and formic acid with molecular oxygen in aqueous media at 300 °C. Although the mechanistic studies of guaiacol oxidation were scarce in literature, knowledge learned from extensive studies of mineralization of phenol, a common water pollutant, by catalytic wet oxidation can be leveraged since phenol is a possible intermediate during guaiacol oxidation. The main intermediate products detected in phenol mineralization were ring compounds (hydroquinone, catechol, benzoquinones, etc.), and short chain carboxylic acids including maleic, malonic, succinic, fumaric, formic, acetic, and oxalic acids. However the yields of organic acids, especially C4 diacids, were negligible [35, 36, 37, 38, 39, 40].
Among the biomass-derived dicarboxylic acids, maleic acid is an important raw material used in the manufacture of phthalic-type alkyd and unsaturated polyester resins, surface coatings, lubricant additives, plasticizers, copolymers, and so on [41]. Industrial processes of the production of maleic anhydride, a dehydrated form of maleic acid, use petroleum-based butane or benzene as the feedstock. Thus oxidation of lignin derived by-products would be an appealing process to produce green maleic acid. The challenge, however, is how to cost-effectively accomplish a high yield of the desired carboxylic acid product.
Our group recently found that levulinic acid was generated in a high production yield by the aqueous-phase partial oxidation of cellulose over the ZrO2 catalyst via a possible radical reaction pathway [42]. Herein, we expand our searching for heterogeneous oxidation catalysts for the selective conversion of lignin-derived compounds. Titanium silicalite-1 (TS-1), a synthetic zeolite in which a small number of Ti atoms substitute tetrahedral Si atoms in a purely siliceous framework with the MFI structure[43], is an active and selective catalyst for a number of low-temperature oxidation reactions with aqueous H2O2 as the oxidant. The TS-1/H2O2 reaction system has been one of the most actively studied green processes in the past decade [44, 45, 46]. In the present study, we demonstrate the feasibility of the selective oxidation of guaiacol to produce maleic acid with TS-1/H2O2 under mild conditions. In addition, the oxidative ring-opening reaction mechanism is also tentatively discussed.
The TS-1 catalyst (Si/Ti ratio is 33) was synthesized using the modified method in literature [43]. Briefly, 50 g tetraethyl orthosilicate (TEOS) was added into 70 g aqueous solution of 20 wt% tetrapropylammonium hydroxide (TPAOH). To the resultant clear solution, a solution of 2.3 g titanium butoxide (Ti(OBu)4) in isopropyl alcohol (15 g) was added and then 60 g water was added. The mixture was crystallized at 160 °C for 12 h under autogeneous pressure. The as-synthesized samples were calcined at 540 °C for 5 h before use. All TS-1 precursors were bought from Sigma Aldrich. Other materials including Al2O3, and H-ZSM-5 were purchased from Alfa Aesar. The surface morphology of catalysts was characterized by a Hitachi S-4700 scanning electron microscopy (SEM). Raman spectra were obtained on Renishaw InVia Raman Microscope System with the excitation laser wavelength of 512 nm. Electron paramagnetic resorance (EPR) spectra were recorded at room temperature in air using a Bruker EMX Plus spectrometer operated in the X band with 100-kHz field modulation with 10 mW microwave power, 100 kHz modulation frequency, 1G modulation amplitude, 30 dB receiver gain, 0.01 ms time constant, 100 G scan range, 35 s scan time, and 8 number of scan.
The aqueous-phase oxidation reaction experiments were carried out in the leak-proof 30mL perfluoroalkoxy (PFA) vials (Thermo Scientific). In a typical experiment, 10 mL of aqueous solution of guaiacol (10 wt%), 0.5 mL H2O2 solution (35 wt%) and 0.1g solid catalyst were added into each PFA vial. Eight vials were fixed on the rotating vial holder and then placed in a preheated oven at the set temperature. The rotational speed was at 90 r/min to ensure a good mixing of catalysts in aqueous solutions. The schematic diagram of the reactor is shown in Fig. 1. After the reaction, the vials were quenched immediately in an ice water bath.
Aqueous samples collected from the oxidation experiments were diluted as needed and filtered through a 0.22-μm pore-size filter for analysis using total organic carbon (TOC) analyzer, high performance liquid chromatography (HPLC), electrospray ionization mass spectrometer (ESI-MS), and gas chromatography with mass spectrometer detector (GC/MS). TOC was measured by a Shimadzu Total Organic Carbon Analyzer (model TOC-V). HPLC analysis was performed using a Shimadzu HPLC system equipped with a dual UV-VIS Detector (Shimadzu SPD 10-AV) at 208 and 290 nm and a Refractive Index Detector (Shimadzu RID-10A). For the analysis of organic acids and reaction intermediates, the samples were separated in an Aminex 87-H column from Bio-Rad, with 5 mmol/L H2SO4 as the mobile phase (0.7 mL/min) and a column temperature of 55 °C. All samples for ESI-MS analysis were diluted with a base solution containing 0.1 wt% triethylamine and the analysis was performed using a Waters Micromass ZQ quadrupole mass spectrometer. GC/MS analysis was carried out using an Agilent 6890 series equipped with a DB5-MS column (30 m x 0.25 mm x 0.25 μm) and an Agilent 5973 Mass Selective Detector. The derivitization of aqueous-phase samples by silylation was performed for GC/MS analysis. Briefly, 100 µL aqueous-phase product was lyophilized overnight in a deactivated 1.5 mL vial. To the dried solids, 150 µL of acetonitrile was added and mixed to allow the solids to dissolve, and then 50 µL of pyridine and 150 µL of BSFTA with TMCS (99:1) were added. The capped vial was placed in a sand bath maintained at 65 °C for 2 h to allow a complete silylation. After the silylation, the sample was cooled and 5 µL silylation mixture was diluted with 1.5 mL of hexane for GC/MS analysis.
The SEM image shown in Fig. 2 displays that the as- synthesized TS-1 catalysts are porous spherical particles with an average size of ~100 nm. Table 1 shows the guaiacol conversion, the TOC yields, and the molar yields of major aqueous-phase products by reacting guaiacol over the TS-1 catalyst at different reaction conditions. The pH, the reaction temperature, and the H2O2 amount all affect the conversion of guaiacol and the yields of carboxylic acid products. The guaiacol oxidation could be carried out even at 25 °C. However, increasing temperature until 80 °C enhanced the guaiacol conversion, the yields of maleic acid, and the H2O2 utilization efficiency. Moreover, increasing the amount of H2O2 (35 wt%) from 0.25 to 1 g also enhanced the guaiacol conversion and the yield of maleic acid. Further increasing temperature and H2O2 amount led to the increase of guaiacol conversion but the decrease of maleic acid yields and H2O2 utilization efficiency. Under the optimum reaction conditions (80 °C, pH = 13.3, and 1 g H2O2), the guaiacol is almost fully converted and the highest yield of maleic acid, 27.7%, was achieved.
Table 2 compares the different catalysts and shows that, when the reactant solutions were neutral (pH = 6.8), all solid catalysts including TS-1, ZrO2, TiO2, Al2O3, and ZSM-5, enhanced conversion of guaiacol compared to that without a solid catalyst. Figure 3 shows that the guaiacol conversion without a solid catalyst was only ~20% in the range of pH = 1.0-10.0, while increasing pH led to the increase of guaiacol conversion, reaching ~80% at pH = 14.3. On the other hand, the highest conversion of guaiacol, ~88%, was achieved with the presence of the TS-1 catalyst in the range of pH = 13.3-14.3. The starting pH of the reactant solution also significantly affected the product distribution, indicating that both Brönsted base (OH-) and Brönsted acid (H+) exhibit the strong homogeneously catalytic effects. The TOC yields, which indicated its content in the aqueous-phase products, maintained at 80%-90%. However, the TOC decreased slightly when the starting pH of the reactant solutions increased from 1.0 to 13.3. The TIC yields were less than 0.5% in the acidic and neutral solutions but were increased to ~5%-15% at pH = 12.3-14.3. The increase of TIC is due to the formation of NaHCO3 or Na2CO3 in the basic solutions by reacting CO2 with excess NaOH. The high guaiacol conversion and TOC yields, as well as the low TIC yields, suggested that the carbon loss to CO2 due to complete oxidation of guaiacol with aqueous H2O2 was low over the TS-1 catalyst. Such a selective oxidation is different from catalytic wet oxidation, in which CO2 is the dominant product [47, 48, 49, 50, 51, 52, 53, 54, 55, 56].
The aqueous-phase products of guaiacol oxidation with TS-1/H2O2 were in three categories: (1) carboxylic acids such as maleic acid, fumaric acid, malic acid, acrylic acid, acetic acid, formic acid, and oxalic acid which were the aromatic ring opening products detected by ESI-MS (Fig. 4) and HPLC (Fig. 5); (2) aromatic compounds including catechol, o-benzoquinone, and 1,2,4-benzenetriol, which were the intermediate products without breaking the benzene ring detected by GC/MS (Fig. 6) and HPLC (Fig. 5). Catechol is the hydrolysis product of guaiacol, while o-benzoquinone and 1,2,4-benzenetriol are the oxidation and hydroxylation products of catechol, respectively; and (3) unidentified di-cyclic or tri-cyclic colored aromatic compounds or deep colored pigments which were observed from the color change of the aqueous products during the oxidation reaction. These colored compounds evolved complicated structures via the radical induced reaction pathway [57, 58, 59]. It was found that Brönsted base promoted the formation of carboxylic acids and combining TS-1 with the base catalyst created synergy that significantly increased the selectivity to maleic and oxalic acids.
To further investigate the catalytic effect of the TS-1 catalyst, maleic acid concentration and guaiacol conversion versus reaction time were compared with and without TS-1, as shown in Fig. 7. Both the yields of maleic acid and the guaiacol conversions increased rapidly during the initial stage of reaction and reached a plateau after 6 h. However, with the TS-1 catalyst, the concentrations of maleic acid were significantly higher than those without the TS-1. After 6 h of reaction and starting from pH = 13.3, the maleic acid concentrations were 1800 and 800 ppm with and without TS-1, respectively, as shown in Fig. 7(a). In the initial period of reaction (within the first 6 h), the concentration of H2O2 was relatively high so that most of guaiacol were reacted in this period. As shown in Fig. 7(b), ~75% of guaiacol was converted with the TS-1 catalyst while only ~50% of guaiacol was reacted without a solid catalyst after reacting for 6 h. Therefore TS-1 can activate the H2O2 to promote the ring opening reactions, which would be a key step for the production of maleic acid from guaiacol. Without the TS-1 catalyst, most of H2O2 probably were consumed in the oligomerization reactions or decompose data at relative high temperature (80 °C). At the beginning of oxidation, highly colored quinone intermediate compounds such as p-benzoquinone (yellow) and o-benzoquinone (red) were generated at 30 min and 3 h, respectively. The color comes from their quinoidal structure, which contains chromophore groups substituted in benzene rings. The color level monitored during oxidation gradually became darker with increasing the reaction time, indicating that other intermediates might contribute. The inter-molecular interactions between quinones (yellow) and dihydroxylated rings (colorless) might form the highly colored quinhydrones species (purple to brown) [60] that might be generated during the oxidation treatment.
The formation of colored aromatic compounds is an undesirable side reaction to the carboxylic acid production. Herein, we use the ROR to define the efficiency of guaiacol ring opening over different catalysts. As shown in Table 2, the ROR was zero without a catalyst. No ring-opening products were observed over the solid Lewis (Al2O3) and Brönsted (ZSM-5) acid catalysts as well. The ROR over the TS-1 was 6.1% in the neutral solutions, indicating that the TS-1 catalyst catalyzed the ring opening reactions even without a base. Furthermore, the highest ROR, ~30%, was contributed by the synergetic effects of both homogeneous base catalysis (NaOH) and heterogeneous oxidation catalysis (TS-1/H2O2). The corresponding selectivity to maleic acid reached ~70% among the carboxylic acids generated via ring opening under the optimized reaction conditions.
Previous studies show that the formation of carboxylic acids from catalytic wet oxidation of phenol went through three steps: firstly, hydroxylation of phenol with ·OH radicals to dihydroxylated benzene such as hydroquinone or catechol; secondly, the oxidation of hydroquinone or catechol to para- or ortho- benzoquinone; thirdly, the ring opening reaction of benzoquinone to form maleic and oxalic acids. So far, most of the studies supported that ring opening reactions were originated from an ·OH radical oxidation mechanism [35, 36, 37, 38, 39, 40]. As shown in Fig. 8, we also found that ·OH was the only radical in the TS-1/H2O2 aqueous solutions based on the EPR spectra under the simulated reaction conditions without adding guaiacol. The intensity of the EPR signals increased in the order of H2O2 in pH neutral solution < H2O2/TS-1 in pH neutral solution < H2O2/TS-1 in alkaline solution (pH = 13.3). Obviously, the TS-1 catalyst promoted the decomposition of H2O2 in aqueous solutions to produce hydroxyl radicals, especially in the alkaline solution. We thus tentatively relate the guaiacol conversion and the yield of ring opening products to the concentration of ·OH radical.
For different biomass substrates, hydroxyl radicals may trigger the formation of various organic radicals, and thus lead to different reaction pathways and result in different final products. Interestingly, our results show that the main product of catechol oxidation is malic acid, while maleic acid is the dominant carboxylic acid product when guaiacol is the reactant substrate, as seen in Fig. 9. The formation of different organic acid products suggests that catechol is not necessarily the key intermediate of guaiacol ring-opening oxidation in the TS-1/H2O2 system. Therefore we propose that there are three parallel reaction pathways for the conversion of guaiacol (Scheme 1): (1) the demethylation of guaiacol to form catechol; (2) the oligomerization of guaiacol or its derivatives such as catechol and benzoquinone, which forms the complex macromolecular colored products following an unknown radical reaction pathway; (3) the oxidative ring-opening reaction to produce maleic acid and oxalic acid, followed by further reactions including hydration to malic acid, isomerization to fumaric acid, decarboxylation to acrylic acid, and oxidation to acetic acid, oxalic acid, or formic acid. We hypothesize that the oxidative ring-opening reaction of guaiacol is induced by ·OH radicals.
It is our great interest to understand how the radicals are formed on the TS-1 catalyst through the decomposition of H2O2 in an alkaline aqueous solution. According to previous studies, three reactive oxo-Ti species (η1 and η2-hydroperoxo-Ti, and superoxo-Ti) could be easily generated in the TS-1/H2O2 system [61, 62, 63, 64, 65]. Upon contacting H2O2, four types of superoxo-Ti radicals were generated on TS-1. They were different according to the EPR characterization. A-type: gz = 2.0260; B-type: gz = 2.0235; C-type: gz = 2.0220; and D-type: gz = 2.031. Both EPR and UV-Vis spectrometry techniques confirmed that these superoxo-Ti radicals were in equilibrium during the oxidation reaction and their concentration depended on temperature, pH, and solvent [66, 67, 68]. In a TS-1/H2O2/H2O system, TS-1 could easily activate H2O2 to D-type superoxo-Ti radicals especially at high pH values [68]. As shown in Fig. 10, the bands located at 380, 520, 975, and 1125 cm-1 are the Raman features of the TS-1. After in contact with a H2O2/H2O solution, disappearance of the bands at 975 and 1125 cm-1 and creation of two new peaks at 630 and 875 cm-1 were observed. The quenching of the bands at 975 and 1125 cm-1 implies the reaction of framework Ti on the TS-1 surface with H2O2 [5]. The strong Raman band at 875 cm-1 stands for the O-O stretching mode of the physisorbed H2O2 on the TS-1 surface. The new band at 630 cm-1 is assigned to the vibration mode of a side-on η2 Ti-OO complex, which is an indir ect evidence of superoxo-Ti radicals presented in the TS-1/H2O2/H2O system. We therefore hypothesize that the D-type superoxo-Ti radicals are the active species for the generation of free ·OH radicals.
Usually silicate materials are not stable in highly basic aqueous solutions. As shown in Fig. 11(a), the XRD peaks at 23.2°, 23.8°, and 24.3° are characteristic of the MFI structure of TS-1. We compared the XRD pattern of the fresh TS-1 catalyst with that of the spent catalyst after reacted for 24 h at 80 °C with the initial pH = 13.3, and found that both samples exhibited all three characteristic peaks with only slight difference in intensity. On the other hand, after treated with NaOH solution (pH = 13.3) at 80 oC for 24 h, the same TS-1 material was transformed from crystalline to amorphous, indicating that strong alkaline solvents could leach out silica and therefore eventually destroy the framework of TS-1. However, due to the neutralization of base with the carboxylic acid products, the pH value in the actual reaction media decreased quickly (Fig. 11(b)). As the reaction proceeded, the pH value decreased from 13.3 to 10.5 in 30 min, and further decreased to 8.7 and 7.8, after 1 and 3 h, respectively. The catalytic effect of the TS-1 catalyst was obvious in the weakly basic solutions. Furthermore, the short-term stability of the TS-1 catalyst was validated in the alkaline H2O2 aqueous media due to the self- neutralization by the carboxylic acid products.
In summary, we demonstrated a “green” alternative approach to produce maleic acid from selective oxidation of guaiacol at low temperatures with alkaline aqueous H2O2 over heterogeneous TS-1 catalyst. The yields of maleic acid were highly dependent on the catalyst property, the temperature, the initial pH value, and the H2O2 amount. At 80 °C and the initial pH = 13.3, the maximum yield of maleic acid (~28%) was achieved from the guaiacol oxidation with the TS-1 catalyst. The synergetic effect of the Brönsted base and the TS-1 with H2O2 as the oxidant leads to the aromatic-ring opening reactions of guaiacol, a key step to selectively produce maleic acid. The proposed reaction mechanism for guaiacol oxidation indicates that the oligomerization of guaiacol and its derivatives is the undesired side reactions for the production of maleic acid. Further research will be carried out toward in-depth investigations of how to inhibit oligomerization in order to further enhance the yield of maleic acid.
The authors thank Dr. Stephen Spain of the Department of Chemistry at the University of Nevada Reno for the help on chemical analysis.
2014, Vol. 35 Issue (5): 622-630
PDF (633KB)
2014, Vol. 35 Issue (5): 622-630
PDF (633KB)