The search for chemical processes to convert biogenic feedstocks to renewable chemicals and fuels has driven efforts to discover and develop new effective reactions that enable the selective refunctionalization of these polyoxygenate materials [1-5]. In this context, furfural (FAL) represents a promising biogenic building block and the direct conversion of FAL into furfuryl alcohol (FOL) by catalytic reduction has been identified as a reliable feedstock source to open new scenarios for sustainable chemical production [6-10]. Historically, the reduction of furfural under an aggressive hydrogen atmosphere has been the focus of extensive efforts. Recent progress has demonstrated the potential of this approach in setting up a biobased economy [11-17]. However, H2 is mainly produced from fossil fuels, which in turn make the process dependent on fossil carbon. Moreover, the necessity of special handling for the highly flammable H2 gas and the requirement of an organic solvent to achieve a high selectivity have diminished the appeal of these procedures [12-15]. Any strategy and conditions that can address these issues will be advantageous not only for FAL reduction but also for the reduction of biomass intermediates such as 5-hydroxymethylfurfural (HMF).
Due to the potential for the use of CO as a key intermediate in the chemical industry, mainly as a C1 building block, there is continuous interest in the development of transformations for CO valorization. The use of CO instead of hydrogen or other hydrogen donor agents is particularly interesting since CO is currently produced in multi-ton quantities as a byproduct of the steel industry [18] and thus represents an abundant source for chemical reduction [19, 20]. One conceptually straightforward way is to use a CO/H2O couple as a source of hydrogen, although the most ideal scenario would be CO as sole reductant without an external hydrogen source [19]. We have contributed to this field by discovering an excellent Au-catalyzed CO/H2O-driven strategy that allowed rapid and chemoselective reduction of substituted nitro and carbonyl groups under very mild conditions [20, 21]. Our subsequent studies revealed that the Au-CO-assisted reduction strategy could be used for reductive imination and carbonylative alkyne semireduction [22, 23].
Our continuing interest in the development of innovative CO-based catalytic reduction technologies led us to investigate the reduction efficiency of the Au-CO/H2O protocol for the transformation of biogenic platform chemicals [24]. We disclose here the versatile catalytic behavior of supported Au nanoparticles (NPs) in the reductive transformation of FAL to FOL without the use of H2. We demonstrate that adopting a suitable inorganic support such as single phase rutile TiO2 to anchor finely dispersed Au NPs enabled a highly selective and efficient transformation of FAL to FOL in water under a mild CO atmosphere (Scheme 1). As opposed to previous procedures, this Au-CO/H2O-mediated reduction system can deliver high selectivity in the reduction of aqueous FAL, affording exclusively the corresponding FOL in a mild, economical and green manner. This reduction using a simple and robust gold catalyst coupled with CO/H2O as the hydrogen source under mild conditions can make a significant contribution to the catalytic potential of supported Au NPs and to establish an approach for more efficient FAL reduction.
Anatase (TiO2-A, surface area of 154 m2/g), rutile (TiO2-R, surface area of 65 m2/g), Au/ZrO2 (ZrO2, surface area of 115 m2/g), Au/TiO2(P25, surface area of 45 m2/g; P90, surface area of 95 m2/g), Au/CeO2 (CeO2, surface area of 139 m2/g) and Au/HT (HT, surface area of 118 m2/g). All the supports and catalysts were prepared by using previously described methods [23-30].
The 0.6 wt% Au/TiO2-R sample was prepared by a routine deposition-precipitation (DP) method [30, 31]. In brief, an aqueous solution of HAuCl4 was added to 100 mL of deionized water at 80 ℃ under vigorous stirring. The pH was adjusted to 7.0 and 1.0 g TiO2-R was dispersed in the solution with the pH kept constant at 6.5-7.0. The mixture was stirred for 2 h at 80 ℃. After that, the suspension was cooled to 25 ℃. After thorough washing with deionised water, the samples were dried under vacuum at 25 ℃ for 12 h and then calcined in a muffle oven at 350 ℃ for 2 h. 0.6 wt% Au/TiO2-A, 0.6 wt% Au/TiO2-P25, and 0.6 wt% Au/TiO2-P90 were prepared by the same method.
0.6 wt% Ir/TiO2-R, 0.6 wt% Pd/TiO2-R, 0.6 wt% Pt/TiO2-R, 0.6 wt% Ru/TiO2-R catalysts were prepared by incipient wetness impregnation (IWI) of the support with aqueous solutions of H2IrCl6·6H2O, PdCl2, H2PtCl6·6H2O or RuCl3 precursors, respectively [31]. After mixing the corresponding slurries, the mixture was vigorously stirred at 80 ℃ for 4 h. Then samples were dried under vacuum at 25 ℃ for 12 h and then reduced in 5 vol% H2/Ar (80 mL/min) at 400 ℃ for 2 h.
X-ray diffraction (XRD) analysis was carried out on a German Bruker D8Advance X-ray diffractometer using nickel filtered Cu Kα radiation with a scanning angle (2θ) of 10°-80°, a scanning speed of 2°/min, and a voltage and current of 40 kV and 20 mA. Transmission electron microscope (TEM) images were taken with a JEOL 2011 electron microscope operating at 200 kV. Before being transferred into the TEM chamber, the samples were dispersed in ethanol and deposited onto a carbon-coated copper grid and then quickly moved into the vacuum evaporator. The size distribution of the metal particles was determined by measuring about 200 random particles in the images. CO temperature programmed reduction (CO-TPR) experiments were carried out on a homemade apparatus described elsewhere [32]. X-ray photoelectron spectroscopy (XPS) analysis was performed using a Perkin Elmer PHI 5000C system equipped with a hemispherical electron energy analyzer. The Mg Κα (hν=1253.6 eV) was operated at 15 kV and 20 mA. The energy scale was internally calibrated by setting the C 1s peak at 284.6 eV. The solution after reaction was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a Thermo Electron IRIS Intrepid II XSP spectrometer.
A 25-mL Hastelloy-C high pressure Parr reactor was used to carry out the liquid phase reduction reaction. A mixture of FAL (2.6 mmol, fresh distilled), supported metal catalysts, water (20 mL) were loaded into the reactor. The reactor was stirred at a rate of 800 r/min under 4 MPa CO (syngas, or H2) for a specific reaction time at a given temperature. After reaction, the CO atmosphere was removed and the resultant product mixture was transferred with 5 mL of ethanol. An amount of N, N-dimethylformamide (DMF) was then added as an internal standard substance. The samples were analyzed on a Agilent GC-6820 gas chromatograph equipped with a capillary column DB-Wax (30 m × 0.25 mm) and FID detector. The identification of the products was performed by using a GC-MS spectrometer.
At first, the reaction was evaluated using a diluted aqueous solution of freshly distilled FAL (0.13 mol/L) and Au NPs (average diameter of ca. 2.2 nm) deposited on some common inorganic oxides with a substrate-to-gold (nFAL/nAu) ratio of 200:1 under mild conditions (4 MPa of CO pressure in neat water at 90 ℃). We initially focused our attention on Au/CeO2 because of the successful application of this Au-based catalyst for the chemoselective reduction of a range of a, β-unsaturated aldehydes, e.g., crotonaldehyde and cinnamaldehyde, to their allyl alcohols in the presence of CO and H2O [21]. However, this material afforded only modest activity for the CO/H2O-mediated FAL to FOL reduction under the screening conditions employed (Table 1, entry 1). We then turned our attention to the selection of other commonly used Au-support combinations that have proven effective in the Au-CO/H2O-mediated reductive transformation, including Au/TiO2-P25, Au/Al2O3, Au/ZrO2 and Au/Mg-Al hydrotalcite (Table 1, entries 2-5). While none were found to be particularly active for FAL reduction, the modest success achieved with Au/TiO2-P25 prompted a more detailed survey of structurally related Au-TiO2-based systems.
We subsequently optimized the Au-TiO2 catalysts by altering the TiO2 phase during the preparation (Table 1, entry 1, 6-8). Eventually, the optimized Au/TiO2-R comprising gold deposited on single phase rutile TiO2 (Au/TiO2-R) was the most efficient, which accomplished quantitative FAL-to-FOL conversion within only 4 h at 90 ℃ (Table 1, entry 9). A plot of the time dependent conversion of FAL (Fig. 1) revealed that the reaction proceeded smoothly and FOL was the only product, that is, we did not detect any other product during the whole reaction. Furthermore, in a series of studies examining the effect of pressure, the reaction time was shortened from 4 to 2.5 h as the pCO was raised from 4 to 6 MPa (Table 1, entries 9 and 11). Further experiments on the effect of temperature indicated that the Au/TiO2-R catalyst exhibited rather low activity at 30 ℃ (Table 1, entry 13). At 120 ℃, quantitative formation of FOL was attained at 4 MPa within 2 h (Table 1, entry 12).
In line with several other CO/H2O-mediated reduction [20-23], we found that Au was as active for the reduction of FAL with CO/H2O as traditional noble metals (Table 1, entries 17-20). These results confirmed that the combination of Au NPs with a suitable TiO2 phase could achieve a high catalytic activity for the selective reduction of FAL into FOL. After the reduction of FAL, Au/TiO2-R can be separated from the reaction mixture and reused without loss of catalytic efficiency (Table 1, entries 16). To gain insight into the origin of the enhanced CO/H2O-mediated reduction activity achieved by using rutile TiO2 as the support, CO-TPR was performed for Au deposited on different supports (Fig. 2). This revealed that the lower performance observed with Au/TiO2-A, Au/Al2O3 and Au/CeO2 were due to that the H2 formation rates catalyzed by these catalysts were significantly lower than that over the Au/TiO2-R sample. This was reinforced by the observation that the low temperature H2 production rate over Au/TiO2-R also occurred at a much higher rate than that over TiO2-P25 supported Au. Taken together, these results demonstrated that the Au-catalyzed CO/H2O-mediated FAL reduction proceeded by a sequential water-gas-shift/FAL-reduction pathway (Scheme 2), in which the generation of a transient Au0-H species formed by CO-induced H2O activation was the main step.
These results showed that the Au/rutile-based catalyst facilitated efficient FAL-to-FOL conversion without an external H2 supply. By investigating the physicochemical properties of the Au/TiO2-R catalyst, the following information was collected. From the XPS results, it was seen that there was only metallic Au on the Au/rutile sample (Fig. 3(a)). ICP-AES data revealed that there was no Au species in the solution after the reaction, which indicated the absence of Au leaching during the reaction. XRD analysis displayed that both the fresh and used Au/TiO2-R catalysts showed the same crystal phase, with no obvious Au feature being identified in these two samples, which suggested that the particle sizes of Au were quite small (Fig. 3(b)). TEM confirmed that the average diameter of the Au particles was 2.2 nm in the fresh and used Au/TiO2-R catalysts, verifying there was no Au NPs aggregation during the reaction (Fig. 3(c) and (d)). These results accounted for the remarkable stability of Au/TiO2-R in the recycling tests.
As a further illustration of the effectiveness of the above reduction, full conversion of FAL was readily obtained at a nFAL/nAu ratio of 2000 (Table 1, entry 14). This Au-CO/H2O-mediated reduction still occurred with only 0.02 mol% of the Au/TiO2-R, albeit an extended reaction time of 61 h was required (Table 1, entry 15). Under these conditions, remarkable values of the turnover number (TON=5000) and average turnover frequency (TOF=91 h-1) were calculated (Table 1, entry 15). Perhaps the most notable finding was the ability of the Au-CO/H2O-based protocol to convert aged distillates of FAL to value added material. Along these lines, we demonstrated that the present Au-based technology could be used to convert FAL that was set aside for more than a month into FOL with high efficiency (Table 1, entry 21). These results were extremely encouraging because they showed that crude FOL could be used directly as a feedstock for industrially relevant transformations [33, 34]. Furthermore, this Au-CO/H2O- mediated reduction process was not limited to FAL. When HMF, another key intermediate in biomass conversion, was subjected to the reduction in the presence of the Au/TiO2-R sample, the reaction also proceeded efficiently. In this case, 2, 5-bis-(hydroxymethyl) furan (BHMF), an important monomer for industrial processes, was exclusively obtained under the standard reaction conditions (Scheme 3).
Given the fact that producing CO and H2 (syngas) from biomass is a crucial step in the production of most second generation biofuels, we explored the possibility to produce FOL by aqueous FAL reduction using simulated syngas with a varying H2/CO feed ratio. Three different volumetric compositions of syngas were examined. As shown in Table 2, in all cases, FAL was quantitatively converted into FOL but different reaction times were required. The reaction involving the H2-rich syngas proceeded much more rapidly than the reaction with the CO-rich syngas. This was in agreement with that the reaction proceeded at a much faster rate with H2 as the sole reductant. More important, this revealed that cheap, renewable and easily accessible CO2-rich bio-syngas could be successfully used for FAL reduction. These results are particularly relevant since flexible and versatile syngas can be used as a direct hydrogen source for selective biomass conversion. This would open the possibility to develop cost effective technologies for the production of bio-renewable chemicals based on CO-driven reduction.
TiO2 supported Au catalysts, in particular the Au/TiO2-R system, were very promising for the selective reduction of bio-derived furfural using CO as a convenient and cost competitive hydrogen source. This Au-based catalytic protocol constitutes the first furfural reduction system that directly used syngas as a hydrogen source and would furnish a new application of syngas. As controlled hydrogenation and reductive transformation is a very general issue in the conversion of bio-derived feedstocks, the procedure here is expected to be of broad applicability in the utilization of biomass.
2016,
Vol. 37 Issue (10): 1669-1675 
PDF
2016,
Vol. 37 Issue (10): 1669-1675 PDF