Wet Aerobic Oxidation of Lignin into Aromatic Aldehydes Catalysed by a Perovskite-type Oxide: LaFe1-xCuxO3 (x=0, 0.1, 0.2)

The perovskite-type oxide catalyst LaFe1-xCuxO3 (x=0, 0.1, 0.2) was prepared by the sol–gel method, and tested as a catalyst in the wet aerobic oxidation (WAO) of lignin into aromatic aldehydes. The lignin conversion and the yield of each aromatic aldehyde were significantly enhanced in the catalytic process, compared with the non-catalyzed process. Moreover, it was shown that the stability of activity and structure of LaFe1-xCuxO3 (x=0, 0.1, 0.2) remained nearly unchanged after a series of successive recyclings of the catalytic reactions, indicating it was an efficient and recyclable heterogeneous catalyst for the conversion of lignin into aromatic aldehydes in the WAO process.


Introduction
With the gradual diminishing of fossil fuel reserves and growing concerns about global warming, finding ways of exploiting feasible pathways for the replacement of the petroleum-based chemicals is highly desirable [1]. To meet the growing demand for this, biomass can serve as a sustainable source of renewable fuels and chemicals. It was estimated that a sustainable production of 1.3 billion dry OPEN ACCESS tonnes of biomass per year can be achieved without significant changes in agricultural practices and food supplies, as reported by the U.S. Department of Agriculture (USDA) and U.S. Department of Energy (DOE) recently [2][3]. However, lignin, one of the main components of biomass in the biorefining process, is usually discarded as non-cellulosic waste. Lignin is an extremely complex threedimensional macromolecule with irregular structure, which results from random dehydrogenative polymerization of phenylpropane building units, including coniferylic, sinapylic and р-coumarylic alcohols (Figure 1), in the presence of peroxidase enzymes [4], and it can be converted into aromatic aldehydes such as p-hydroxybenzaldehyde, vanillaldehyde and syringaldehyde ( Figure 2). These aromatic aldehydes have wide applications in flavoring, as chemical intermediates for pharmaceutical drugs and agricultural pesticides [5][6]. So the study of the conversion of aromatic aldehydes from lignin is of significant importance.  A routine way to convert lignin into aromatic aldehydes is Wet Aerobic Oxidation (WAO) process, which often uses oxygen and catalysts to increase the yields of aldehydes. Some noble metals have been used as catalysts in the lignin WAO process [6][7][8], but they are expensive, which is an impediment for their commercial applications. Alternatively, some inexpensive metal ions such as iron, copper, and cobalt have also shown activity in the lignin oxidation process [9][10][11][12][13]. Nevertheless, these homogeneous catalysts can lead to secondary pollution, result in high recycling costs, and restricting their industrial utilization.
It is well known that perovskite-type oxides of general formula ABO 3 have high activity and stability in catalytic hydrocarbon oxidation processes, and they are a promising alternative to noble metal catalysts for the wet hydrocarbon catalytic oxidation process [14][15][16][17]. Nevertheless, the activity of perovskite-type oxides as catalysts in the WAO reaction has rarely been studied. To the best of our knowledge, only three recent studies deal with the activity of perovskite-type oxides La 1-x A x BO 3 (A=Sr, Ce; B=Co, Mn) for the WAO reaction [18][19][20], all claiming that this perovskite-type oxide presented high activity for this reaction. However, metal ions such as Sr, Ce, Co and Mn can cause a secondary pollution problem to a certain extent. For this reason, the WAO reaction of lignin is one process where the demand for green chemistry and sustainable technology is stimulating the replacement of these ''toxic" metal ions by alternative "non-toxic" metal ions.
Iron (Fe) is the only transition metal which can be considered "non-toxic" to nature, so the study of LaFeO 3 as a catalyst in the catalytic WAO of lignin is an inviting prospect. However, in the past decades, the effect of iron (Fe) on the hydrocarbon catalytic oxidation has rarely been studied due to its poor oxidation ability. In this paper, Cu ion was loaded into LaFeO 3 to partially replace the Fe ion because the Cu ion has higher oxidation reaction activity [21][22][23], and a new catalyst: LaFe 1-x Cu x O 3 (x=0, 0.1, 0.2) was thus obtained. The activity and stability of this species as a catalyst were tested in the catalytic WAO of lignin into aromatic aldehydes, and LaFe 1-x Cu x O 3 (x=0, 0.1, 0.2) was also characterized by XRD, TPR and XPS. ), respectively, according to the prior literature [24][25]. From the integral of O 1s spectrum, it can be found that the content of oxygen increased as the x value of LaFe 1-x Cu x O 3 increased (x=0, 0.1, 0.2), indicating that the absorption ability of oxygen increases as the content of copper ion increases, a result similar to that obtained by Zhang et al. [26]. Moreover, in Figure 3(b) the appearance of two signals can be observed, which belong to Fe 2p3/2 and Fe 2p1/2, respectively, indicating that there are Fe 3+ ions in the catalyst, as indicated in [27]. Furthermore, two signals at BE=930-935 and BE=940-955 ev can be also observed from Figure 3(c), which belong to Cu 2+ according to the literature [28]. All these indicated that the LaFe 1-x Cu x O 3 (x=0, 0.1, 0.2) catalyst was a typical perovskite-type oxide.

Catalyst characterization
The reducibility of LaFe 1-x Cu x O 3 (x=0, 0.1, 0.2) samples is depicted in Figure 4. An obvious LaFe 1-x Cu x O 3 (x=0, 0.1, 0.2) reducing peak can be observed as the x value decreases from 0.2 to 0, which is attributed to the reduction of Cu 2+ according to the literature cited [29][30][31], indicating that the Cu has attached to the LaFe 1-x Cu x O 3 (x=0, 0.1, 0.2) catalyst as the x value increased.

Catalytic oxidation lignin into aromatic aldehydes
The aromatic aldehydes obtained from lignin in the catalyzed WAO process can be further oxidized to aromatic acids, and even to carbon dioxide and water under certain conditions. In our experiment, the catalytic activity was evaluated with the rate of lignin conversion and the yield of aldehydes, which was shown in Figure 5. It can be seen from Figure 5(a) that the lignin conversion was 41.8% at 3.0 hours in the absence of a catalyst, and the conversion gradually increased when LaFeO 3 , LaFe 0.9 Cu 0.1 O 3 and LaFe 0.8 Cu 0.2 O 3 were added, respectively. The maximum lignin conversion (66.6%) could be reached after 3.0 hours when lignin oxidation was catalysed by LaFe 0.8 Cu 0.2 O 3 , which was 1.59 times the yield obtained in the non-catalytic process. Similar concentrations of LaCl 3 and FeCl 3 did not present any catalytic activity under these conditions. and syringaldehyde (d) with reaction time (the reaction conditions were as follows: NaOH (2 mol/L), 120 °C, 5 bar partial pressure of oxygen in 20 bar total pressure, C L0 =60.00 kg/m 3 , C catalyst =3 g/L and the lignin conversion=(C 0 -C t )/C 0 , where C 0 is the initial concentration of lignin, and C t is the concentration of lignin at any reaction time). The yields of p-hydroxybenzaldehyde, vanillaldehyde and syringaldehyde are shown in Figures  5(b-d). The same results were obtained when LaCl 3 , FeCl 3 , LaFeO 3 , LaFe 0.9 Cu 0.1 O 3 and LaFe 0.8 Cu 0.2 O 3 were added, respectively, and the yields increased gradually. The maximum yields of p-hydroxybenzaldehyde, vanillaldehyde and syringaldehyde were 2.49% (at 120 min), 4.56% (at 60 min) and 11.51% (at 30 min) in the LaFe 0.8 Cu 0.2 O 3 catalytic process, which were 1.66, 1.42 and 2.51 times those obtained in the non-catalytic process, respectively. All results indicate that the catalyst activity will be enhanced by an increase in Cu content.
Although it is premature to discuss the precise role of the perovskite oxide LaFe 1-x Cu x O 3 (x=0, 0.1, 0.2) in the catalytic mechanism of the lignin oxidation process at present, it is noteworthy that two aspects can account for the activity of LaFe 1-x Cu x O 3 (x=0, 0.1, 0.2) in the CWAO as reported [18]: On the one hand, an O 2 -Fe(surf) 3+ -lignin intermediate was generated on the surface of LaFe 1- x Cu x O 3 (x=0, 0.1, 0.2), which will enhance the oxygen contact of lignin and accelerate the generation of the intermediate of quinine methide radical: (3) or (3a); On the other hand, the oxygen space will be enhanced with the partial replacement of Fe 3+ by Cu 2+ , according to the report of Zhang [17], which would accelerate the oxygen surface absorption ability of the catalyst and the intermediate content of   The catalytic stability of LaFeO 3 was investigated by repeatedly using LaFeO 3 for the lignin conversion under the same reaction conditions; the lignin conversion and yield of aromatic aldehydes from the non-catalyzed process and five successive runs is shown in Figure 7, where it can be seen that the lignin conversion and the yield of each aromatic aldehyde were significantly higher in each of successive catalyst run than the non-catalyzed process. Furthermore, it was clearly demonstrated that the lignin conversion and the yield of aromatic aldehydes remained nearly the same in each run, which indicated that the catalyst has better recovery ability. The crystalline phases of the catalysts before and after use were also determined by X-ray diffraction, which is depicted in Figure 8. The XRD patterns of the LaFeO 3 , LaFe 0.9 Cu 0.1 O 3 , LaFe 0.8 Cu 0.2 O 3 and LaFeO 3 which were used five times all showed characteristic reflections for the perovskite-type oxide without other phases. No obvious changes can be seen in the structure of the LaFeO 3 catalyst used five times, which suggested that the LaFeO 3 catalyst is stable in the CWAO process.

Lignin and catalyst preparation
Lignin was obtained through the process of enzymatic hydrolysis of steam-explosion cornstalks [32]. The LaFe 1-x Cu x O 3 (x=0, 0.1, 0.2) catalyst was prepared from citrate precursors [18,20,33]: a concentrated solution of metal nitrates was mixed with an aqueous solution of citric acid, and the molar ratio of citric acid to metal cations was fixed at 1.5. Water was evaporated from the solution at 80 °C until a viscous gel was obtained. The gel was kept at 100 °C overnight, and then calcined at 700 °C for 6 h. [20] X-ray diffraction (XRD) measurements were performed on a Rigaku powder diffractometer (Rigaku, Japan) with CuK α radiation. The tube voltage was 45 kV, and the current was 40 mA. The XRD diffraction patterns were taken over 2 h in a range of 20° to 80° at a scan speed of 2°/min. Temperature programmed reduction (TPR) of H 2 was performed in a Micromeritics Autochem 2910 equipped with a thermal conductivity detector (TCD). The sample (50 mg) was pretreated in 10% O 2 /He flow at 800 °C for 30 min, and then it was reduced with 5 % H 2 /Ar (30 cm 3 /min) heating 5 °C /min from room temperature to 850 °C.

Catalyst characterization
The X-ray photoelectron spectroscopy (XPS) analysis was performed on a Kratos Axis Ultra system with 0.1 eV per step for detail scan and the binding energies for each spectrum were calibrated with a C1s spectrum of 284.6 eV. The core levels of O 1s, Fe 2p and Cu 2p species were recorded and their relative intensities determined. [20] The non-catalyzed WAO and catalyzed WAO processes of lignin were carried out in a highpressure SS-316 Parr slurry reactor (model 4843) at 120 ± 1 °C. A 500 mL alkaline solution (NaOH, 2 mol/L) of lignin dissolved at a concentration of 60 g/L was introduced into the reactor and a certain quantity of catalysts was added to the solution if necessary. The heating program was started under a slight nitrogen pressure. When the solution in the reactor reached the desired temperature, nitrogen was added until a total pressure of 15 bars was attained, and then, the oxygen was added until the total pressure attained 20 bars. The pressure in the reactor was kept at 20 bars by continuous flushing of oxygen as a supplement because of its consumption during the reaction. Time was recorded from zero. During each reaction, sampling was conducted from the reactor and filtrated, and the filtration was acidified to pH 2-3 with a dilute HCl solution. The resulting products were extracted with chloroform until the chloroform layer appeared colorless. The residual lignin was obtained by centrifugation of the suspensions after the extraction of resulted products.

Catalytic experiments
The contents of p-hydroxybenzaldehyde, vanillaldehyde and syringaldehyde in the chloroform extracts were analyzed by high performance liquid chromatography with a Hypersil ODS 2 column (4.6 mm × 250 mm) and a UV detector set at 280 nm, and a mixture of acetonitrile (10%), deionized water (90%) and acetic acid (1.5%) as the mobile phase. The non-converted lignin was diluted in a sodium hydroxide solution to dissolve the lignin and a UV spectrophotometer used to determine the content of the non-converted lignin at a wavelength of 280 nm [34].
The experiment to test the recycling ability of LaFeO 3 was performed as follows. Fresh catalyst and lignin solution were added into the reactor and the process was performed under the same conditions as mentioned above for 30 min. After the catalyzed WAO reaction, the reactor temperature was quickly cooled to room temperature, and the resulting products were carefully poured out and filtered. The catalyst was left in the reactor, and fresh lignin solution was added, then the process was performed again under the same conditions for 30 minutes. This procedure was repeated for four times, and the contents of p-hydroxybenzaldehyde, vanillaldehyde, syringaldehyde and lignin were analyzed using the above methods.

Conclusions
The perovskite-type oxide catalyst of LaFe 1-x Cu x O 3 (x=0, 0.1, 0.2) prepared by the sol-gel method exhibited high activity in the catalyzed WAO of lignin. Compared with the non-catalyzed process, the lignin conversion and yield of each aromatic aldehyde were improved significantly in the catalyzed process. It was shown that the perovskite-type oxide catalyst of LaFe 1-x Cu x O 3 (x=0, 0.1, 0.2) also possesses distinctive stability of activity and structure in the catalyzed WAO of lignin, and is thus an efficient and recyclable heterogeneous catalyst for the conversion of lignin in the catalyzed WAO process of lignin.