Vanadium-oxo immobilized onto Schiff base modified graphene oxide for efficient catalytic oxidation of 5-hydroxymethylfurfural and furfural into maleic anhydride

Abstract

Graphene oxide (GO) sheets are emerging as a new class of carbocatalyst, and also a perfect platform for molecular engineering. The hydroxyl groups on either side of GO sheets can function as anchors by employing them as scaffolds linking organometallic nodes and vanadium-oxo was homogeneously immobilized on a Schiff base modified GO support via covalent bonding. The developed VO–NH₂-GO was shown to be an efficient and recyclable heterogeneous catalyst for the aerobic oxidation of 5-hydroxymethylfurfural (HMF) into maleic anhydride. Up to 95.3% yield of maleic anhydride from HMF and 62.4% from furfural were achieved under optimized reaction conditions. The immobilized vanadium oxo was identified as the active sites, while the residual oxygen-containing groups worked synergistically to adsorb HMF to maintain a high reactant concentration around the catalyst. The STY value was enhanced significantly over VO–NH₂-GO, compared with homogeneous or heterogeneous traditional supported V based catalyst.

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Introduction

With the declining fossil resources, significant effort has been devoted into converting renewable biomass into fuels,^1–3 and valuable chemicals.^4–6 Replacement of petroleum-derived chemicals with those from biomass will play a key role in sustaining the growth of the chemical industry.^7–11 5-Hydroxymethylfurfural (HMF), derived from C6-based carbohydrates and containing an aldehyde group, a hydroxymethyl group and a furan ring, has been regarded as one of the most promising platform chemicals and used as a versatile precursor for the production of fine chemicals, plastics, pharmaceuticals, polymers and liquid fuels.^10,12–15 Selective oxidation of HMF is one of the most pivotal transformations in biorefinery. The oxidation of HMF can generate several kinds of oxidation products such as 2,5-diformylfuran (DFF), 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), 5-formyl-2-furancarboxylic acid (FFCA), and 2,5-furandicarboxylic acid (FDCA),^16–21 as depicted in Scheme 1. Besides, C5-based furfural is an analog of HMF and comes from rich agricultural materials like corncobs, oat, wheat bran, and sawdust, and they are not competitive with food of human beings. Particularly, unlike HMF, which is currently synthesized on a lab scale, furfural production is an on-going industrial process; therefore, it is considered that furfural is a better starting material than HMF for MA synthesis from biomass.^22,23

Scheme 1 Schematic illustration of the potential oxidation products during the oxidation of HMF.

Maleic anhydride (MA) and its derivatives are manufactured with a large capacity, which are widely employed to synthesize unsaturated polyester resins, agricultural chemicals, food additives, lubricating oil additives, pharmaceuticals, and so forth.²⁴ In industry, MA is mainly produced via oxidation of petroleum-derived chemicals such as n-butane and benzene.²⁴ Regarding the diminishing of the fossil resources, developing new chemical process to transform biomass into MA other than fossil feedstock is greatly attractive. MA can be produced from HMF in an entirely different route with DFF, HMFCA, FFCA and FDCA. Yin et al.²⁴ and Xu et al.²⁵ have demonstrated experimentally and theoretically that HMF can be oxidized into MA over vanadium-based catalyst by the C–C bond (adjacent to the hydroxymethyl group) cleavage. The results indicate that only HMF and HMFCA containing hydroxymethyl group can convert into MA easily over vanadium-based catalyst, as depicted in Scheme 1.

Graphene oxide and its derivatives (GO) have attracted significant attention for their outstanding electrical, optical, electrochemical and mechanical properties.^26,27 The generally accepted models of GO structure depict hydroxyl and epoxide groups as dominant functionalities residing mainly on the basal plane. Hole defects are found through of the scaffold, which accommodate groups like carboxylic acids, ketones, phenols, lactols, and lactones.²⁸ The Hummers method made GO also have some sulfonic groups linked on the edge of graphene plane. The oxygen containing functionalities imparted GO the function as an acid catalyst,^29,30 redox catalyst^31,32 et al. in chemical synthesis reaction. Beside, using GO as a building block in supramolecular chemistry, the scope of functionalities in GO hybrids can be extended. The presence of epoxy and hydroxyl functional groups on either side of the GO sheet imparts bifunctional properties that allow it to act as a structural node within metal–organic frameworks. For example, homogeneous molecular catalyst can be anchored on the GO surfaces by employing them as scaffolds linking organometallic nodes.^28,33 Rhenium-oxo,³⁴ vanadium-oxo^35,36 and Fe²⁺, Co²⁺, Cu²⁺,³⁷ have been immobilized onto graphene oxide via covalent bonding and the composites can be used as efficient and recyclable catalyst in various oxidation reactions.

Previously, many vanadium-based catalysts, such as VO(acac)₂,²⁵ V–Mo containing heteropolyacid,^24,38,39 supported or unsupported VO_x,^40,41 Mo–V metal oxide,²³ have been reported to catalyze the oxidation of HMF into MA. However, a low MA selectivity (50–80%) was often obtained under harsh reaction condition. Furthermore, the aggregation or leaching of VO_x component on traditional support (such as SiO₂, Al₂O₃) leads to low catalytic efficiency in reaction. In this paper, the ultrasonic exfoliated and freeze-dried GO was chosen as the high hydroxyl containing support for immobilizing the vanadium-oxo. The as-prepared composite exhibited superior activity and selectivity in aerobic oxidation of HMF and furfural into MA. The grafted vanadium-oxo worked as the active site, while the residual oxygen-containing groups in GO worked synergistically to adsorb HMF to maintain a high reactant concentration around the catalyst.

Catalyst characterization

The detailed procedure for preparing the vanadium-oxo immobilized and Schiff base modified graphene oxide (VO–NH₂-GO) is described in the ESI.† The fresh catalyst and recycled catalyst for 5 times in reaction are characterized and compared. BET analysis (Table S1†) shows that the obtained graphene oxide has a BET surface area of 379.0 m² g⁻¹ with an average pore diameter of 14.4 nm. After immobilization of vanadium oxo onto the Schiff base modified graphene oxide, the oxygen-containing functionalities in GO reacted with the introduced APTES and VO(acac)₂. After re-construction, the surface area decrease to 148.8 m² g⁻¹ with a little enlarged pore diameter of 14.6 nm. Fig. S1† shows the XRD patterns of the as-prepared materials. The interlayer distance obtained from the (002) peak was 3.38 Å (a, 2θ = 26.4°) in graphite. This was markedly expanded to 8.27 Å (b, 2θ = 10.8°) with oxidation to form GO. The large interlayer distance has been attributed to the formation of hydroxyl, epoxy, and the carboxyl groups. After amino functionalization (c) and immobilization of the vanadium-oxo procedure (d), the peak of the large interlayer distance in GO (10.8°) decreased radically and another broad diffraction at ca. 22° appeared, which is closer to the reduced graphene,⁴² indicating that the major oxygen-containing groups of GO have been successfully reduced or functionalized under hydro-thermal conditions.^35,36 The recycled catalyst (e) shows a similar XRD pattern to the fresh catalyst (d). Elemental analysis of the fresh and recycled VO–NH₂-GO is shown in the ESI Table S2.† The prepared GO has a 46.5 wt% of oxygen. After amino functionalization and immobilization of vanadium-oxo, the oxygen content decreased to 24.9 wt%, the graphene oxide was partially reduced and these results are corresponding with the XRD analysis. The loading of the amino propyl groups onto the GO support was found to be 2.4 mmol g⁻¹, as determined by the nitrogen content (3.4 wt% N). Elemental analysis shows 0.32 mmol g⁻¹ V was immobilized onto GO. The V/N atom ratio in as-prepared VO–NH₂-GO was ca. 0.13, much smaller than 1, indicating most of the introduced APTES did not participated into the further chemical functionalization with VO(acac)₂. It is deduced that the formation of VO–Schiff base complex is restricted on GO surface due to the steric hindrance, similar results were reported in literature.^35,36 FT-IR spectrum presented in Fig. S2† show the vibrational assessments of GO (a), NH₂-GO (b), VO–NH₂-GO (c), and recycled VO–NH₂-GO (d). The strong bands at 1717, 1254 and 1060 cm⁻¹ are attributed to the stretching modes of C=O, C–OH and C–O–C, respectively, revealing the presence of carboxyl, hydroxyl, and epoxy groups in pure GO. In addition, a broad peak at 3432 cm⁻¹ is attributed to the stretching mode of O–H bonds, revealing the presence of many hydroxyl groups. However, the O–H can be derived from GO material and also adsorbed water. Considering the XRD results and oxygen content in GO, functionalized NH₂-GO and VO–NH₂-GO, it is deduced that the peak at 3432 cm⁻¹ in GO (a) involves the hydroxyl groups in GO plane, while the peaks at 3432 cm⁻¹ in NH₂-GO (b) and VO–NH₂-GO (c) mainly was caused by adsorbed water from atmosphere during characterization. The peak at 1600 cm⁻¹ is attributed to the C=C stretching of sp²-bonded carbons in the GO.³⁶ The FTIR spectrum of NH₂-GO (b) exhibit an enhanced doublet at 2850 cm⁻¹ and 2917 cm⁻¹, corresponding to the symmetric and asymmetric CH₂ of the alkyl chains assigned to the silane moieties of APTES, and confirm the successful grafting of the aminosilane moieties onto the GO support. Moreover, the appearance of the bands at 1127 cm⁻¹, 1033 cm⁻¹, and 1577 cm⁻¹ due to Si–O–Si, Si–O–C, and C–N vibration respectively,³⁶ provide more evidence for the successful chemical functionalization.

Results and discussion

Oxidation of HMF into maleic anhydride

In a typical experiment, 2 mmol HMF dissolved in 10 mL pure acetic acid was heated to 90 °C for 4 h under 20 bar oxygen pressure without any catalyst in a 100 mL Teflon-lined autoclave. Analysis of the products showed that 48.5% HMF disappeared, and only 6.7% MA yield was obtained, with DFF yield of 1.2% observed (Table 1, entry 1). (The detailed experimental procedure and analytical methods are provided in the ESI.†) Traditionally, vanadium oxide, such as V₂O₅ and VO(acac)₂ are applied as heterogeneous or homogeneous catalyst in aerobic oxidation of HMF into MA.^25,40 In this work, V₂O₅ (entry 2) and VO(acac)₂ (entry 3) are also tested under same reaction conditions above mentioned, and HMF conversion increased to 98.1% and 99.8%, with MA yield up to 44.0% and 59.0% achieved, implying the effectiveness of the vanadium oxide in catalytic oxidation of HMF into MA. Traditional catalyst support (such Al₂O₃, SiO₂) are often selected to immobilize and disperse the noble or transition metal component, enhancing the efficiency and recyclable of the metal catalyst. For comparison, 100 mg 10 wt% V₂O₅/SiO₂ prepared by incipient wetness impregnation method was tested, nearly full HMF conversion and an increased MA yield of 64.5% was observed (entry 3). It is reported that it is the high dispersion of V₂O₅ on SiO₂ at low V₂O₅ loading, and the interaction between V₂O₅ and SiO₂ induced the distortion of the V₂O₅ crystals as well as the changes in the valence and coordination states of surface V atoms that are responsible for the enhanced activity of SiO₂ supported V₂O₅.⁴⁰ When VO–NH₂-GO was applied into this reaction under the same reaction conditions, nearly full HMF conversion and up to 95.3% yield of MA was obtained. The site-time-yield (STY; expressed as the amount of MA (mmol) that converted from HMF catalyzed by V (mmol) in various catalyst per hour) was calculated and applied to compare the effectiveness of various V-based catalyst. V₂O₅ and VO(acac)₂ showed a lower STY values (entry 2 and 3), while STY increased from 2.0 h⁻¹ to 2.9 h⁻¹ after the V₂O₅ was supported on SiO₂. Remarkably, the STY for VO–NH₂-GO increased to a high value of 148.9 h⁻¹, demonstrating VO–NH₂-GO is a high effective catalyst for the aerobic oxidation of HMF into MA.

Table 1 HMF oxidation over homogeneous or heterogeneous vanadium-based catalyst^a

Entry	Cat.	PO2 (bar)	Conv. (%)	Yield		STY^c
				(MA + HMA)^b	DFF
a Reaction conditions: HMF 252.2 mg (2.0 mmol); acetic acid 10.0 mL; catalyst 10.0 mg; 90 °C; 4 h.b The yield includes maleic anhydride (MA) and hydrolyzed maleic anhydride (HMA) in reaction mixture. The data in the bracket indicates the ratio of maleic anhydride to maleic acid determined by NMR analysis.c The site-time-yield was calculated as the amount of MA (mmol) that converted from HMF catalyzed by V (mmol) in various catalyst per hour (h^−1, excluding the amount of MA produced without catalyst).d 100 mg 10% V2O5/SiO2 was used.
1	—	20	48.5	6.7	1.2	—
2	V2O5	20	99.1	48.3 (1 : 5.8)	0	2.2
3	VO(acac)2	20	99.8	55.2 (1 : 3.8)	2.3	7.3
4^d	(10%) V2O5/SiO2	20	99.8	64.5 (1 : 4.2)	1.6	2.9
5	VO–NH2-GO	20	99.8	95.3 (1 : 2.5)	2.4	148.9
6	VO–NH2-GO	15	89.5	71.5	2.6	—
7	VO–NH2-GO	10	75.1	52.9	5.0	—
8	VO–NH2-GO	5	60.3	45.3	2.1	—
9	VO–NH2-GO	1	41.8	16.2	3.8	—
10	VO–NH2-GO	N2	17.9	0	0	—
11	GO	20	46.2	9.5	2.0	—
12	NH2-GO	20	48.9	8.9	1.8	—

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To determine whether the residual oxygen groups in VO–NH₂-GO was directly oxidizing the HMF or functioning just as a catalyst with pressured oxygen as the terminal oxidant,^32,43 the aforementioned oxidant reaction was performed under an continually decreased pressure of oxygen. After 4 hours at 90 °C, analysis found that the HMF conversion and MA yield decreased continually with the oxygen pressure (entry 5–9) when the reaction was conducted under N₂ atmosphere at 90 °C for 4 h, analysis of the reaction mixture found the HMF conversion decreased to 17.9%, and no MA or DFF was observed (entry 10). For underscoring the importance of vanadium-oxo, the aforementioned reaction was conducted in the presence of GO or NH₂-GO, after 4 hours at 90 °C, the GO or NH₂-GO provide HMF conversion of 46.2% and 48.9%, respectively (entry 11 and 12), similar to the results without any catalyst (entry 1), and the MA yield also had no obvious increase compared with entry 1. During the catalyst tests, DFF was observed as the main by-products in a yield range from ca. 0–5%, and no HMFCA, FFCA or FDCA was observed. Formic acid was detected as the accompanied product by NMR while the cleavage of the C–C bond between the hydroxymethyl group and furan sketch of HMF over V-based catalyst (ESI 1.4†).²⁴ On another side, the analysis of the gas mixture after reaction by GC found about 0.5% CO and 2.0% CO₂ existed (not shown in Table 1), indicating the decarbonylation of the intermediates during the reaction.^22,24 Over different V-based catalyst, the distribution of maleic anhydride and maleic acid in reaction mixture was determined by NMR right after the reaction, and results found maleic acid was the dominant product. The ratio of maleic anhydride and maleic acid was around 1 : 2 to 1 : 6, due to the inevitable produced water in reaction or absorbed water from atmosphere during experiment (Table 1, entry 2–5, ESI 1.4†).

Above experiments demonstrate the grafted vanadium-oxo was the active sites in the aerobic oxidation of HMF. In our previous work, it is found the oxygen-containing groups in graphene oxide had an obvious affinity to polar reactant, such as fructose, HMF,²⁹ furfural alcohol and levulinic acid,³⁰ and thus facilitated the catalytic reaction by maintaining a high reactant concentration around the catalyst. It can be deduced that the residual oxygen-containing groups in VO–NH₂-GO are responsible for the high efficiency of the catalytic oxidation. To verify this hypothesis, affinities of different catalyst to HMF were evaluated by adsorption experiments (ESI Table S3†). It is found that the adsorption of HMF on GO was the highest, the HMF concentration in acetic acid decreased about 18.0% (R = 18%, Table S3,† entry 1 and 2), the Schiff base modification and vanadium-oxo immobilization make the GO partially reduced and the adsorption of HMF on the VO–NH₂-GO had a slight decrease (R = 14.5%, Table S3,† entry 2 and 3). However, the adsorption of HMF on graphene (thermal reduced graphene oxide, ca. 2 wt% oxygen content) decreased sharply (R = 6.2% Table S3,† entry 4), indicating the oxygen-containing groups in GO should account for the adsorption of HMF on the graphene based material. The V₂O₅/SiO₂ and V₂O₅ showed a relative low adsorption of HMF (R = 3.8% and 7.5% respectively, Table S3,† entry 4 and 5), compared with GO and VO–NH₂-GO. Clearly, it was reasonable to infer that the superior performance of VO–NH₂-GO in HMF catalytic oxidation could be ascribed to the synergistic effect of the grafted vanadium-oxo and residual oxygen-containing groups in modified GO material.

Effect of reaction solvents on HMF oxidation reaction

The reaction solvents affect the oxidation significantly, various solvents were tested and the results are summarized in Table 2. The highest MA yield was obtained in acetic acid (Table 2, entry 1). Acetonitrile gave a relative high HMF conversion and MA yield over homogeneous V-containing catalyst,^24,25 however, rather low HMF conversion and MA yield were achieved over current catalytic system (Table 2, entry 2). Similar results were obtained in ethanol and DMSO (Table 2, entry 3 and 4). Due to the strong acid properties of formic acid, HMF was fully decomposed, negligible MA or DFF was observed under current conditions in formic acid (Table 2, entry 5). The solvents, such as water, MIBK, and toluene gave nearly full HMF conversion and moderate MA yield around 25–35% with DFF yield around 3–7% (Table 2, entry 6–8). Above all, acetic acid showed significant different behavior in HMF aerobic oxidation over VO–NH₂-GO.

Table 2 HMF oxidation over VO–NH₂-GO in different solvents^a

Entry	Solvent	Conv. (%)	Yield (%)
			MA^b	DFF
a Reaction conditions: HMF 252.2 mg (2.0 mmol); acetic acid 10.0 mL; catalyst 10.0 mg; 90 °C; 4 h; O2, 20 bar.b The yield includes un-hydrolyzed maleic anhydride and hydrolyzed maleic anhydride in reaction mixture.
1	AcOH	99.8	95.3	4.4
2	CH3CN	5.0	1.4	0.2
3	EtOH	7.8	3.8	0
4	DMSO	9.9	2.0	0
5	Formic acid	100	0.4	0
6	H2O	99.8	29.2	4.2
7	MIBK	96.4	35.3	2.8
8	Toluene	97.7	23.8	7.3

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Kinetic analysis of catalytic oxidation of HMF over V-based catalyst

To explore the kinetic behavior of HMF oxidation, the product yield as a function of reaction time over VO–NH₂-GO is displayed in Fig. 1.

Fig. 1 Reaction kinetics of HMF oxidation over VO–NH₂-GO. Reaction condition: HMF 252.2 mg (2.0 mmol); acetic acid 10.0 mL; VO–NH₂-GO, 10 mg; 90 °C; O₂ 20 bar. The yield of MA in graph involves maleic anhydride and maleic acid.

The HMF converted quickly increased to above 90% after only 30 min. The MA yield also increased quickly to 80.6% in 30 min and DFF was observed as the only by-product during reaction. With reaction time prolonged, the HMF conversion increased to nearly 100% and the highest MA yield of 95.3% was obtained in 4 h. The DFF yield showed a relative low yield around 0–3% during the reaction. The kinetic analysis of HMF oxidation over V₂O₅ and VO(acac)₂ was provided in Fig. S4 and S5 in ESI.† In the presence of V₂O₅ (Fig. S4†), the HMF conversion and MA yield increased with reaction time prolonged, nearly full HMF conversion was achieved in 2 h and the highest MA yield of 48.3% was achieved in 6 h. No DFF was observed in V₂O₅ catalytic system. VO(acac)₂ provided the full HMF conversion in 2.5 h, and highest MA yield of 55.2% was achieved in 4 h. In VO(acac)₂ catalytic system, DFF was observed as the main by-products in a yield range of ca. 1.5–2.5%.

Oxidation of furfural into maleic anhydride over VO–NH₂-GO

Compared with unsupported or traditional supported V based catalysts, the prepared VO–NH₂-GO shows remarkable improvement in catalytic transformation of HMF into MA. To explore the catalytic activity of VO–NH₂-GO, synthesis of MA with furfural as substrate under above mentioned reaction conditions was operated. As shown in Fig. 2, both of furfural conversion and MA yield increased with the extending of the reaction time. 82.1% furfural conversion with the highest MA yield of 62.4% was achieved at a reaction time of 8 h. Furfural shows lower reactivity than HMF in oxidation reaction into MA, much longer reaction time (18 h) is needed for the full furfural conversion. However, the MA yield decreased beyond 8 h and decreased to 60.9% when furfural transformed completely. No other by-products was detected, it is speculated that the furfural polymerization under oxygen atmosphere decreased the MA selectivity.²²

Fig. 2 Reaction kinetics of furfural oxidation over VO–NH₂-GO. Reaction condition: furfural 192.2 mg (2.0 mmol); acetic acid 10.0 mL; VO–NH₂-GO, 10 mg; 90 °C; O₂ 20 bar. The yield of MA in graph involves maleic anhydride and maleic acid.

Recyclability of VO–NH₂-GO in catalytic oxidation of HMF and furfural

The recyclability of VO–NH₂-GO in aerobic oxidation of HMF and furfural was tested (Fig. S6 and S7, ESI†). Considering the high catalytic efficiency of the catalyst in oxidation of HMF, that, nearly full HMF conversion and 95% MA yield were achieved in 1.5 h, the reaction time in each cycle in recyclability test was fixed at 2 h and 5 mg VO–NH₂-GO was used to avoid the excess of catalyst in long reaction time. Similarly, the reaction time was fixed at 2 h in catalyst cycle test in oxidation of furfural, with 10 mg VO–NH₂-GO was used.

As shown in Fig. S6,† 62.1% HMF conversion with 48.8% MA yield was achieved under the selected reaction conditions. In Fig. S7,† 54.4% furfural conversion with 35.2% MA yield was achieved in first cycle. Slight decrease was observed in first three cycles in catalyst recyclability test in catalytic oxidation of both HMF and furfural, however, the catalytic activity showed an obvious decrease in the fourth and fifth cycles. Elemental analysis shows 0.32 mmol g⁻¹ V was immobilized onto GO in fresh catalyst and the V content dropped to 0.30 mmol g⁻¹ V after 5 cycles in HMF oxidation, and 0.27 mmol g⁻¹ V after 5 cycles in furfural oxidation (Table S2†). Besides, catalyst deactivation from coking can not be excluded, especially in aerobic oxidation of furfural, which is not very stable because of its polymerization when exposed to oxygen.²²

Conclusions

Vanadium oxo has been homogeneously immobilized on a Schiff base modified graphene oxide support via covalent bonding. The developed heterogeneous catalyst is found to be efficient for the aerobic oxidation of HMF and furfural into MA, with high MA selectivity obtained. The high efficiency of VO–NH₂-GO was attributed to the synergistic effect of active sites of vanadium-oxo and residual oxygen-containing group adsorption of reactant onto the catalyst surface. Acetic acid was demonstrated to be an outstanding solvent for the preparation of MA from HMF and furfural oxidation reaction over VO–NH₂-GO. As a new developed and remarkable material, graphene oxide is showing more potential in catalyst utilization.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (NNSFC) (Grant 21403273), the National Key Basic Research Program of China (No. 2012CB215305), and Science Foundation of Shanxi Province (2013011010-6).

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Footnote

† Electronic supplementary information (ESI) available: The detailed methods for catalyst preparation, experimental procedure, analysis, catalyst characterization SEM, TEM, EDS, BET, XRD, FT-IR, elemental analysis. Affinities of different materials to HMF and effect of temperature on the reaction are also provided in the ESI. See DOI: 10.1039/c6ra21795h

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