VO Supported on Functionalized CNTs for Oxidative Conversion of Furfural to Maleic Anhydride

Abstract

Commercial non-functionalized (CNTs) and functionalized carbon nanotubes (CNT-COOH and CNT-NH₂) were used as supports to synthesize vanadium-supported catalysts to be used in the gas phase partial oxidation of furfural towards maleic anhydride (MA). The CNTs and the VO₂-V₂O₅/CNTs, so-called VO/CNT catalysts, were characterized by AAS, TGA, XRD, N₂ adsorption isotherms at −196 °C, Raman, NH₃-TPD and XPS. The surface area values, TGA and XRD results indicate that the larger thermal stability and larger dispersion of vanadium species is reached for the VO/CNT-NH₂ catalyst. XPS indicates presence of surface VO₂ and V₂O₅ species for the non-functionalized (CNT) and functionalized (CNT-COOH and CNT-NH₂) catalysts, with a large interaction of the functional group with the surface vanadium species only for the VO/CNT-NH₂ catalyst. The catalytic activity, evaluated in the range 305 °C to 350 °C, indicates that CO, CO₂ and MA yield (%) and MA productivity are associated to the redox properties of the vanadium species, the oxygen exchange ability of the support and the vanadium–support interaction. For the reaction temperatures between 320 °C and 335 °C, the maximum MA yield (%) is found in the functionalized VO/CNT-COOH and VO/CNT-NH₂ catalysts. This behavior is attributed to a decreased oxidation capability of the CNT with the functionalization. In addition, VO/CNT-NH₂ is the more active and selective catalyst for MA productivity at 305 °C and 320 °C, which is related to the greater interaction of the surface vanadium species with the -NH₂ group, which enhances the redox properties and stabilization of the VO₂ and V₂O₅ surface active sites. Recycling at 350 °C resulted in 100% furfural conversion for all catalysts and a similar MA yield (%) compared to the fresh catalyst, indicating no loss of surface active sites.

1. Introduction

Furfural (FUR), a natural precursor of furan-based chemicals, plays an important role in the production of biofuels and biochemicals [1]. In particular, the development of new conversions and commercial technologies from FUR to a wider range of feedstocks supports the development of future biorefineries. Due to the presence of the aldehyde group and the conjugated double bond system in its structure, FUR is considered a platform molecule to produce by partial oxidation maleic acid or maleic anhydride (MA) [2]. In this context, MA is considered a bulk chemical and is one of the most important building blocks in the synthesis of various compounds such as unsaturated polyester resins, surface coatings, lubricants, additives, plasticizers, copolymers and agrochemicals [3,4]. Many efforts have been made to achieve active and stable catalytic systems for the conversion of FUR [5]. Due to the furfural conversion’s dependence on the reactor type, the main challenge is to improve the mechanical, hydrothermal and chemical stability of the catalysts [4,6,7,8]. It has been observed that in batch reactors operating under high oxygen pressure [9] or using hydrogen peroxide [10], leaching is the most common deactivation process [6], and in fixed bed reactors the main challenge is the formation of deactivating surface resins [4,9].

Vanadium in combination with other elements has shown promise in both bulk and supported catalysts for the oxidation of FUR [7,8,9,10,11,12,13]. The V^IV/V^V redox pair gives to vanadium unique properties in the conversion of organic molecules into oxygenated products [14]. This advantage is based on the ability of V to generate vacancies and reduced oxygen species of the O²⁻ type, allowing oxidation to take place with oxygen spillover by an MVK mechanism [15]. Gómez-Capiro et al. and Santander et al. [7,11] studied the effect of the support (SiO₂, γ-Al₂O₃, ZrO₂ and TiO₂) and the surface vanadium content on the partial catalytic oxidation in the gas phase of FUR to MA. They found that on ZrO₂ and TiO₂ a complete oxidation to CO₂ and CO was obtained, which was attributed to the ability of these supports to exchange oxygen atoms. Meanwhile, on Al₂O₃ and SiO₂, the activity is related to the dispersion of vanadium species. Alonso-Fagundez et al. [9] reported that the optimum gas phase experimental conditions of O₂ pressure and temperature for the FUR oxidation on vanadium oxide supported on Al₂O₃ were 320 °C and 5.7 kPa of O₂ pressure, to obtain the high yield of MA of 73%. Alonso-Fagundez et al. [13] studied the effect of temperature on the oxidation of furfural in gas phase on the V₂O₅/Al₂O₃ catalyst. The authors found that at low temperatures (240 °C), maleates and resins are deposited on the active phase of the catalyst which is rapidly and deeply deactivated. On the contrary, at high temperatures (between 255 and 315 °C), the deposition of maleates and resins decreases, and the MA yield is remarkably improved, also improved by increasing the O₂/furfural mole ratio. At higher temperatures, above 315 °C, MA is overoxidized to CO₂.

Carbonaceous materials exhibit interesting properties such as insolubility in aqueous media, resistance to high temperature and pressure and excellent mechanical resistance to agitation and current flow [16,17]. Furthermore, advances in nanocarbon technology have enabled the controllable tailoring of the pore structure and surface functionalities of porous carbon, making CNTs supported catalysts currently of major trend in the development of integrated processes for the conversion of lignocellulosic derived compounds into value added products [18,19,20]. Liu et al. [21] studied the conversion of furfural at 130 °C, 40 bar of H₂ and ethanol as solvent, and reported high selectivity to tetrahydrofurfuryl alcohol (THFA) of 85% to 90% on Ni/CNTs and NiCu/CNTs catalysts. The higher conversion of furfural and improved selectivity to THFA of the Ni/CNTs and NiCu/CNTs catalysts with the counterparts supported on MgO, γ-Al₂O₃, TiO₂ and ZrO₂, respectively, are attributed to the favorable structure of CNTs, which promotes the confinement and activation of the metallic components and improves their catalytic performance. Ye et al. [22] studied the effect of CNT functionalization (O-CNT) with mineral acids (HNO₃ and H₂SO₄) on the activity of VW/Ti, VW/CNT/Ti and VW/O-CNT/Ti catalysts in NO_x reduction. The authors found that functionalized CNTs (O-CNT) inhibited agglomeration, improving the nano-dispersal of active metals and surface area and pore volume, which increased the efficiency (90%) compared to those of VW/Ti (88%) and VW/CNT/Ti (76%) in NO_x reduction. The activity of surface-oxidized Co/CNTs catalysts in the Fisher–Tropsch reaction was also reported [23]. The results showed that the high catalytic activity of Co/CNTs was attributed to a smaller particle size of Co confinement in the CNTs’ structure and to the hydrogen spillover effect by the quinone surface functional groups of the CNTs. This important result points to another advantage of CNTs in dispersing and stabilizing the active phase with prior functionalization. Ahmed et al. [24] reported that the presence of the -COOH and NH₂ functional groups allowed the effective immobilization of the Pt particles on the surface of the CNTs. Furthermore, Lee et al. [25], reported that the amino groups of the mesoporous MCM-41 solids were then used to immobilize (VO)²⁺ ions which were used to study the catalytic oxidation of benzene. The anchored vanadium complexes have better catalytic activities with higher stability than the framework-substituted V-MCM-41 materials. They showed that immobilization of catalytic metal ions on a functionalized surface has the advantages of better control of available reactive sites and site isolation. Figueiredo et al. [26] reported a very useful method to identify oxygenated functional groups in carbonaceous materials by XPS and temperature-programmed desorption techniques. In this sense, Xia et al. [27], showed that nitric acid is effective in incorporating oxygen into the anchored functional groups, including nitrogenous functional groups [28,29]. The nature and concentration of surface functional groups can also be modified by oxidizing agents [28,29,30,31,32].

According to the above information, considering the properties of the CNTs and the fact that there are few reports for vanadium oxides as active sites [22], in this work, vanadium supported on commercially available non-functionalized (CNTs) and functionalized carbon nanotubes (CNT-COOH and CNT-NH₂) were synthesized and characterized to be used in the partial oxidation of the gas phase of FUR to MA in a reaction temperature range from 305 to 350 °C. A recycle procedure was carried out at 350 °C to obtain a better catalyst.

2. Results

2.1. Thermal Properties and Chemical Composition

The thermogravimetric profiles of the commercial CNTs (CNT, CNT-COOH and CNT-NH₂) shown in Figure 1 were carried out to detect differences regarding their functionalization. As expected, the lowest mass loss up to 900 °C corresponds to CNTs (26%), followed by CNT-NH₂ (32%), with the highest mass loss for CNT-COOH (74%). For CNTs, the mass loss is attributed to anhydrides, lactones, hydroxides and phenol [33,34], and also presents in the functionalized CNT-NH₂ and -COOH. The high lability per water and the decarboxylation of the -COOH groups [29,33] explains the larger mass loss of CNT-COOH, and the decomposition of amines and amides [24] for CNT-NH₂. After the addition of vanadium, higher mass loss for VO/CNT and VO/CNT-COOH compared to VO/CNT-NH₂ catalyst is seen. This behavior suggests that the functional groups of CNT-NH₂ anchor the vanadium species in a higher interaction compared to VO/CNT and VO/CNT-COOH catalysts. The wt% of V obtained from atomic absorption spectroscopy (AAS) on CNT, CNT-COOH and CNT-NH₂ are shown

Table 1. AAS vanadium content, surface area, pore volume, I_D/I_G ratio, Raman bands and total acidity for CNTs and VO/CNTs catalysts.

	wt% V	SBET m2 g−1	Vp cm3; g−1	ID/IG	ID″/IG	Acidity mmol NH3 g−1
CNT	-	241	0.43	2.0	0.21	0.13
CNT-COOH	-	234	0.41	2.3	0.15	0.31
CNT-NH2	-	222	0.38	2.4	0.11	0.44
VO/CNT	5.2	160	0.29	2.2	0.20	12.0
VO/CNT-COOH	4.8	158	0.28	2.8	0.23	13.0
VO/CNT-NH2	4.5	118	0.20	2.3	0.19	11.0

, being similar for the three catalysts.

2.2. Textural Properties

The N₂ adsorption isotherms (Figure S1) of the CNTs and the VO/CNTs catalysts correspond to type IV with a hysteresis loop H1 that corresponds to mesoporous solids with uniform cylindrical pores [35]. The pore size distributions (Figure S2) display two peaks, one in the limit zone of microporosity and the other located ~30 nm. In Table 1, it can be seen similar S_BET for the CNTs, which decrease after the impregnation of vanadium. The large S_BET decrease for VO/CNT-NH₂ close to 47% compared to a 30% for VO/CNT and VO/CNT-COOH catalysts is attributed to the blocking of the pore structure by surface vanadium species or a change in the pore’s channels of the CNTs. Considering the previous TGA results it can be proposed a different interaction of the vanadium species with the surface -NH₂ groups. This behavior agrees with previous reports for NH₂ functional groups allowing an effective immobilization in Pt supported in CNTs [24] and (VO)²⁺ in mesoporous MCM-41 [25].

2.3. Structural Properties

The XRD patterns of the CNTs and the VO/CNT catalysts are shown in Figure 2. The commercial non-functionalized and functionalized CNTs do not show differences corresponding to graphene layers [36] with almost no differences in the structure with the functionalization process. The most intense peak at 2θ = 26.3° and 42° corresponds to the graphite hexagonal lattice [37]. Regarding the VO/CNT catalysts, bulk V₂O₅ (ICDD ID 01-089-0612) [38] is clearly detected at 2θ = 20°, for VO/CNT and VO/CNT-COOH catalysts. Unfortunately, the corresponding peak of VO₂ (PDF 01-073-0514) at 2θ = 26.3° and 33.9° [37,39] is masked with the diffraction peaks of the CNTs. The absence of new diffraction peaks of vanadium species for VO/CNT-NH₂ catalyst does not indicate absence of bulk vanadium species since it can be present in a larger dispersion degree and/or inserted in the graphene layers. In the Raman spectra (Figure S3) of the CNTs and the VO/CNT catalysts appears the characteristic D (~1350 cm⁻¹) and G band (~1580 cm⁻¹) [40,41]. The G band corresponds to the sp² vibration in the graphitic structure and the D band, the corresponding vibration out of the plane [42]. An increase in the intensity of the D band indicates structural imperfection and in the G band, a more ordered graphitic structure and the I_D″/I_G ratio are a descriptor of crystallinity and purity, respectively [41]. In Table 1, the almost similar I_D/I_G ratios for the CNTs and the VO/CNT catalysts indicate almost no changes in the graphitic structure by the functionalization and impregnation of vanadium. Choi et al. [41], report that I_D″/I_G values closer to 0.06 correspond to a pure and crystalline (100%) structure and I_D″/I_G values closer to 0.37, to a total loss of the crystalline structure. In Table 1, the I_D″/I_G values indicate that the crystallinity structure is maintained for the VO/CNT catalyst, whereas a decrease in the crystalline structure is obtained for the VO/CNT-COOH and VO/CNT-NH₂ catalysts. This result is indicative that after the incorporation of vanadium occurs a partial destruction of the CNT structure with the formation of amorphous carbon.

2.4. Acid Properties

The NH₃-TPD desorption classifies the strength of the acid sites into weak acid sites that desorb from 150 °C to 300 °C, medium sites in the range 250 to 400 °C and strong acid sites sites at temperatures >400 °C [32]. Figure 3 shows NH₃-TPD profiles obtained and the total acidity calculated from the area under the desorption profiles is summarized in Table 1. The non-acid CNT is an expected result, and the slight increase for CNT-COOH and CNT-NH₂, a consequence of their functionalization process [31]. The large increases in the total acidity after vanadium impregnation, with almost no dependence with the functionalized group, indicates that the vanadium species have the responsibility of the acid properties of the VO/CNTs catalysts. In this context, Shylesh et al. [43] report a total acidity of 1.05 mmol NH₃ g⁻¹ for a 15% V₂O₅/Nd₂O₅ catalyst with a surface area of 7.2 m² g⁻¹ [43]. Therefore the large obtained values (>10 mmol NH₃ g⁻¹) of total acidity, shown in Table 1, indicate that the highly dispersed vanadium oxides species are responsible for the acid properties of the synthesized catalysts. However, the non-significant differences in the total acidity values between the catalysts suggest that oxidation of furfural in gas phase cannot be explained from acid properties of the catalysts [11].

2.5. XPS

The XPS measurements were carried out for CNTs and VO/CNT catalysts. The spectra for O 1s, N 1s and V 2p are presented in Figure 4, Figure 5 and Figure 6, respectively, and the calibration signals were determined with respect to C 1s at BE of 284.5 eV (Figure S4 and Table S2) [28]. The commercial CNTs display the six expected peaks for C 1s peaks: at 284.5 eV the C=C bond of the graphene sheets [31,44,45], at 285.4 eV the C-C bond, at 286.9 eV the C-O, C-H and C-N bonds [28,31,44], at 289.1 eV the C=O bond, at 290.9 eV the O-C=O bonds characteristic of carboxylic acids, esters and other organic functional groups [28,31,32] and at 292.9 eV the π-π* interaction [31,44,45]. The O 1s spectrum of the commercial CNTs (Figure 4) indicates the presence of two oxygen species: at 532.3 eV the O=C bond of quinones, ketones and aldehydes, and at 533.5 eV the O-C bond of ethers and phenols (

Table 2. BE (eV) and surface atomic V/C ratio for CNTs and VO/CNT catalysts.

	Binding Energy, eV			V/C Atomic Ratio
	O 1s	N 1s	V 2p3/2
CNT	532.2 (49) 533.7 (52)	-	-	-
CNT-COOH	532.2 (47) 533.6 (53)	-	-	-
CNT-NH2	532.2 (47) 533.6 (53)	398.8 (63.1) 400.6 (36.9)	-	-
VO/CNT	530.3 (22) 531.4 (22) 532.3 (37) 533.5 (19)	-	516.6 (53) 517.6 (47)	0.076
VO/CNT-COOH	530.3 (27) 531.5 (30) 532.6 (30) 533.7 (13)	-	516.5 (50) 517.6 (50)	0.115
VO/CNT-NH2	530.0 (24) 531.0 (20) 532.3 (37) 533.5 (19)	398.9 (48.2) 400.7 (39.4) 402.8 (12.4)	516.4 (55) 517.5 (45)	0.075

) [26,27,28]. For the VO/CNT catalysts it can be seen (Figure 4) almost no changes in the BE of the two peaks associated to the CNTs, and two new peaks appear at a lower BE, indicative of more electronic oxygen species associated to surface vanadium species. Therefore, the signal at 531.4 eV is associated with V^IV and at 530.3 eV with V^V [46]. For N 1s (Figure 5), there are two surface contributions for the CNTs at 399.0 eV to the amino (C-NH₂) and at 400.9 eV to the N-C=O group amide. For the VO/CNT-NH₂ catalyst appears a new N1s signal located at a high BE, indicative of a more cationic nitrogen species. The new signal at 402.8 eV has been associated with a more cationic -NH₂ group [47], supporting a close interaction between -NH₂ groups with the surface vanadium species in the VO/CNT-NH₂ catalyst. This interaction between N-groups and metallic particles has been reported by Gonçalves et al. [48], demonstrating that CNTs doped with N species provided anchoring sites which favor a strongly Ni–N interaction improving the Ni sites dispersion. Lee et al. [25] also report that the amino groups of the mesoporous MCM-41 allow the immobilization of (VO)²⁺catalytic metal ions on a functionalized surface, controlling available isolated reactive sites. For the V 2p_3/2 spectra (Figure 6), two signals appear at 516.4 eV for V^IV and 517.5 eV for V^V species [32], with almost no changes upon the functionalization of the CNTs. Table 2 also presents the V/C surface atomic ratio which slightly increases following the order: VO/CNT < VO/CNT-NH₂ < VO/CNT-COOH. The increases in the surface N (%), estimated from XPS from 0.54% in CNT-NH₂ to 1.7% in the VO/CNT-NH₂ catalyst, is in line with the reported surface interaction of V^IV and V^V species with the unpaired electrons of nitrogen [29,47].

2.6. Catalytic Properties

The furfural conversion in the gas phase was evaluated in a steady state (Figure S5) established in consecutive reaction cycles. It can be seen in

Table 3. Furfural conversion (X_F) and MA Yield (%) for VO/CNT catalysts.

	Temperature, °C
	XF (%)				MA Yield (%)
	305 °C	320 °C	335 °C	350 °C	Fresh (350 °C)	Recycle (350 °C)
VO/CNT	42	80	100	100	23	22
VO/CNT-COOH	41	74	99	100	26	27
VO/CNT-NH2	59	90	100	100	29	22

that the conversion of furfural increases with temperature, reaching 100% at 335 °C and 350 °C. At 305 °C and 320 °C, the highest furfural conversion was obtained on the VO/CNT-NH₂ catalyst, and the lowest values displayed on the VO/CNT-COOH catalyst. Considering the previous characterization results, the larger furfural conversion on VO/CNT-NH₂ can be correlated to the larger interaction of the surface vanadium species with the -NH₂ group enhancing the redox properties of vanadium and stabilization of the VO₂ and V₂O₅ surface active sites [24,25,47,48]. On the contrary, the lability of the -COOH groups from the CNT used as a support does not favor the stability of the surface vanadium oxide species, as was observed by thermal measurements. On the other hand, in Figure 7 are shown the yields (%) to MA, CO₂ and CO as functions of the reaction temperature on the VO/CNT catalysts.

The complete and incomplete combustions of furfural towards CO and CO₂ increase with reaction temperature on the three VO/CNT catalysts [11,13], whereas the yield (%) to MA differences in the functionalized catalysts are detected. At 320 °C, the maximum yield (%) to MA is achieve on the VO/CNT catalyst, at 335 °C on VO/CNT-COOH and for the VO/CNT-NH₂ catalyst, the maximum MA yield (%) is obtained between 320 °C and 335 °C. Even though there are observed differences in the yield (%) of MA upon varying reaction temperatures, the maximum of the three catalysts is around 40%. This behavior indicates that the functionalized CNT with -COOH and -NH₂ groups has an effect in the oxidation capability of the catalyst, decreasing the reaction rate of the further oxidation of MA towards consecutive products (CO and CO₂) at larger reaction temperatures. In this line, previous reports have shown that supports such as ZrO₂ and TiO₂ have enough of an exchange surface O atoms ability to participate in oxidation reactions [49,50]. Therefore, the functionalization of a CNT with -COOH and -NH₂ groups decreases the availability of the CNT to participate in the oxidation process, being detected for the functionalized VO/CNT-COOH and VO/CNT-NH₂ catalysts between 320 °C and 335 °C, a more remarkable oxidation effect of the VO species.

Due to the conversion of furfural being less than 100% at 305 °C and 320 °C, the productivity towards MA can be calculated expressed as g of MA per g of vanadium and time, shown in Figure 8. It can be seen that VO/CNT-NH₂ is the more active and selective catalyst to MA productivity, associated with the larger interaction of the VO₂ and V₂O₅ surface active sites with the -NH₂ group and enhancing the redox properties and stabilization of vanadium. Furthermore, because XPS indicates that the non-functionalized and functionalized vanadium catalysts display almost the same surface vanadium content, the observed differences in the productivity of MA confirm that the oxidation of furfural not only depends on the oxidizing capacity of VO, but also on the oxygen exchange ability of the support and the vanadium–support interaction.

Regarding the recycles, it was found that after the catalytic run, the system was brought back to 350 °C (Figure S5), recovering for the three catalysts the 100% of furfural conversion. The MA yield (%) remains practically constant in the first cycle (Table 3), indicating that the active sites are not lost after the reaction cycle [7,9,11].

3. Materials and Methods

3.1. Synthesis

The commercial CNT, CNT-COOH and CNT-NH₂ were impregnated using a solution of vanadyl sulfate (VOSO₄·2H₂O), Sigma-Aldrich (Concepción, Chile) (97%), to obtain a 7 wt% nominal vanadium content. VOSO₄·2H₂O was dissolved in water at 65 °C stirring at 500 rpm and the obtained blue dissolution was added to the commercial CNTs, CNT-COOH and CNT-NH₂ (Table S1). The temperature of dispersion was 65 °C, maintained for 2 h and introduced into a vacuum oven at 60 mbar and 80 °C overnight. The resulting solids were calcined from room temperature to 300 °C at 1 °C min⁻¹ for 7 h and cooled at 1 °C min⁻¹ to room temperature. The remaining catalysts were labeled as VO/CNT, VO/CNT-COOH and VO/CNT-NH₂.

3.2. Characterization

The bulk composition of C, H and N was determined by elemental analysis in a Fisons-EA-1108 CHNS Element Analyzer (Thermo Scientific, Concepción, Chile), and the bulk composition of vanadium was determined by AAS in a Thermo Scientific instrument model iCE Series 3000. Thermogravimetric analyses were performed in a Netzsch brand Iris TG 209F1 thermal analyzer, in N₂ atmosphere (250 mL min⁻¹) from room temperature to 850 °C (10 °C min⁻¹). XRD patterns were recorded on a Bruker D4 Endeavor AXS diffractometer (Concepción, Chile) at 40 kV and 20 mA equipped with CuKα1 radiation (λ = 1.5418 Å). The values of 2θ were scanned from 10° to 80° at 0.02 per step. Nitrogen isotherms at −196 °C were performed in a Micromeritics TriStar II 3020 equipment using samples degassed under vacuum at 200 °C for 2 h, and the specific area was calculated using the BET equation. Acid properties were determined by NH₃-TPD in a Micromeritics AutoChem II instrument (Concepción, Chile) equipped with a TCD. A sample of 50 mg was placed in a U-shaped quartz reactor and pretreated in He flow at 50 mL min⁻¹ from room temperature to 110 °C (10 °C min⁻¹) for 30 min. The sample was cooled to 42 °C and saturated with NH₃ at a flow rate of 10 mL min⁻¹ of for 10 min. To remove physisorbed NH₃ the sample was purged with a flow rate of 50 mL min⁻¹ of Ar at 75 °C per 1h. The amount of NH₃ desorbed was calculated from the area under the curve using ammonia TPD analytical areas. The XP spectra were obtained in a SPECS brand equipment, Phoibos instrument (DLD, HSA3500, Concepción, Chile), with a step energy of 150 eV, work function (W.F.) of 4.48 and a step energy of 1.0 eV, while the spectra of the elements in the high-resolution windows were obtained with a step energy of 50 eV, using an aluminum source (1487 eV). The peaks were decomposed into several components assuming a Gaussian and Lorentzian shape. Raman spectra were performed in a HORIBA (Concepción, Chile) Scientific brand Raman spectrophotometer (laser: 633 nm); spectra were decomposed in five components assuming Lorentzian shape.

3.3. Catalytic Activity

The catalytic oxidation of furfural starts with a pretreatment of the catalyst in synthetic air (Air Liquide, Concepción, Chile, 99.999%) at a flow of 20 mL min⁻¹ with a heating ramp of 10 °C min⁻¹ for 30 min. The feed gases were controlled by mass flow controllers (Kofloc D8500, Concepción, Chile) and recalibrated to volumetric flow (bubble flowmeter). Furfural (Sigma-Aldrich, Concepción, Chile, 99%) was acquired in vapor-phase saturates (Teflon saturator in bath at 25 °C (XMTD-204, HH-1, Concepción, Chile), a flow of N₂ (Air Liquide. 99.999%) of 19 mL min⁻¹). The total flow consists of a stream of synthetic air (Air Liquide. 99.999%) of 4.6 mL min⁻¹, and the stream of N₂ saturated with furfural and a stream of N₂ as carrier gas of 34.62 mL min⁻¹. Once the flow is established and without leakages, the temperature of the controller is increased to 350 °C at 10 °C min⁻¹ for 4 h, then decreased to 335 °C for 4 h, 320 °C for 4 h, 305 °C for 4 h and increased to 350 °C 10 °C min⁻¹ for 4 h. Reagents and reaction products were analyzed in-line using a GC (SRI 8610C, Concepción, Chile) with one capillary column (30 m × 0.53 mm I.D. 1.0U MXT-WAX, Restek 70 655-273, Concepción, Chile) and three packed columns (6′ × 1/8″ molecular sieve 5A, 18″ × 1/8″ Hayesep D and 6′ × 1/8″ Hayesep D, Concepción, Chile). The system was equipped with a TCD, an FID and an FID detector equipped with a methanizer.

The furfural conversion was determined according to Equation (1).

$$X_A={\displaystyle \frac{(F_{A_0}-F_A)}{F_{A_0}}}\times 100\%$$

(1)

where $X_A$ is the conversion of furfural. $F_{A_0}$ is the input flow. $F_A$ is output flow.

The MA yield was determined according to Equation (2).

$$Y_D={\displaystyle \frac{F_D/d}{F_{A_0}/a}}\times 100\%$$

(2)

where $Y_D$ is the yield to a particular product. $F_D$ is the output flow of the product, $a$ and $d$ are stoichiometric indices of the reaction under study.

The CO₂ yield was determined according to Equation (3), equivalent to the CO yield.

$$Y_{{\mathrm{C}\mathrm{O}}_2}={\displaystyle \frac{{{CO}_2}_{out}-{AM}_{out}}{{5\times Fur}_{in}}}\times 100\%$$

(3)

where $Y_{{\mathrm{C}\mathrm{O}}_2}$ is the CO₂ yield. ${{CO}_2}_{out}$ is the CO₂ concentration at the reactor outlet, ${AM}_{out}$ the concentration of maleic anhydride at the reactor outlet, ${Fur}_{in}$ the concentration of furfural at the reactor inlet and 5 the stoichiometric index of furfural in the reaction.

The selectivity to maleic anhydride was determined according to Equation (4).

$$S_{\mathrm{M}\mathrm{A}}={\displaystyle \frac{F_D/d}{(F_{A_0}-F_A)/a}}\times 100\%$$

(4)

where $S_{\mathrm{M}\mathrm{A}}$ is the selectivity of MA.

The productivity towards MA was determined according to Equation (5).

$$P_{\mathrm{M}\mathrm{A}}={\displaystyle \frac{g_{\mathrm{M}\mathrm{A}}}{g_V\times h}}$$

(5)

where $g_{\mathrm{M}\mathrm{A}}$ is the formation of MA in grams, $g_V$ is the vanadium mass per gram of catalyst and $h$ is the reaction time.

4. Conclusions

The characterization of the VO/CNT, VO/CNT-COOH and VO/CNT-NH₂ catalysts shows differences in their thermal, textural, surface and structural properties. The lower weight-loss upon temperature and the larger decrease in surface area and porosity of VO/CNT-NH₂ compared to the corresponding CNT-NH₂ indicate a stronger interaction between the VO₂ and V₂O₅ species with the -NH₂ groups. In addition, the loss of crystallinity indicates insertion of vanadium in the graphitic structure and XPS interaction of the vanadium species with the -NH₂ groups for the VO/CNT-NH₂ catalyst. The catalytic activity, evaluated in the range 305 to 350 °C indicates that CO, CO₂ and MA yield (%) and MA productivity are associated to the redox properties of the vanadium species, the oxygen exchange ability of the support and the vanadium–support interaction. The functionalization of the CNT with -COOH and -NH₂ groups reduces the oxidation capacity, decreasing the complete oxidation to CO₂ and CO at 320 °C and 335 °C for VO/CNT-COOH and VO/CNT-NH₂ catalysts. The largest furfural conversion, yield (%) of MA and MA productivity for the VO/CNT-NH₂ catalyst is attributed to the larger interaction of the surface vanadium species with the -NH₂ group, enhancing the redox properties and stabilization of the surface VO₂ and V₂O₅ active sites. Furthermore, at 350 °C, a similar yield to MA (%) in the recycling indicates that the active site is not lost.