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Article

Controlling the Incorporation of Phosphorus Functionalities on Carbon Nanofibers: Effects on the Catalytic Performance of Fructose Dehydration

1
Dipartimento di Chimica, Università degli Studi di Milano, Milano 20133, Italy
2
Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Cardiff CF10 3AT, UK
*
Authors to whom correspondence should be addressed.
Received: 13 November 2017 / Revised: 20 December 2017 / Accepted: 23 January 2018 / Published: 27 January 2018
(This article belongs to the Special Issue Smart Carbon Materials in Catalysis)

Abstract

:
Phosphorylated carbons have been reported to be effective catalysts in dehydration reactions for biomass valorization. The amount and the nature of P groups are a key parameter affecting the catalytic performances of functionalized materials. Herein, we investigate the role of structural and surface properties of carbon-based materials, specifically carbon nanofibers, in determining the amount of P-functionalities. In order to incorporate P groups on carbon surfaces, various carbon nanofibers (CNFs) with different graphitization degrees have been functionalized through treatment with a H3PO4-HNO3 mixture at 150 °C. The pristine materials, as well as the functionalization protocol, were properly selected to achieve an effective functionalization without drastically altering the morphology of the samples. Surface and structural properties of the synthesized functionalized materials have been investigated by means of transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The catalytic behavior of phosphorylated carbon nanofibers has been evaluated in the selective dehydration of fructose to hydroxymethylfurfural (HMF) to elucidate structure-activity relationships.

Graphical Abstract

1. Introduction

The ideal material to be successfully applied to catalysis should offer high surface area, high porosity and accessibility, for limiting mass transfer, and at the same time assure chemical and mechanical stability [1]. Carbon materials comply with all the above-mentioned requirements [2,3]. Furthermore, their low cost and high versatility have made carbon-based materials the natural choice for many catalytic processes [4,5,6]. Physical and chemical properties of this class of materials is strongly affected by the hybridization and the structural organization at local and long-range level. According to the carbon atom arrangement in the lattice, a usual classification distinguishes between graphitic materials, characterized by uniform sp2 hybridization and a highly-ordered structure, and amorphous materials (e.g., activated carbon, charcoal), where disordered and graphitic components coexist [7]. Moreover, recent discoveries led the family of carbon materials to further expand so as to include the low-dimensional allotropes (fullerenes, carbon nanotubes, carbon nanofibers, graphene) [8,9,10]. The use of nanostructured carbonaceous materials in catalysis resulted in the introduction of some additional advantageous features, such as confinement [11,12,13], electronic effects [14,15,16], and unique surface reactivity [17,18,19], whose extent is strongly determined by the structural parameters (e.g., diameter, length, chirality, topological defects) [20]. The latter are not easily tunable during the synthesis; therefore, it is difficult to obtain carbon nanostructured materials with tailored properties. From this point of view, the functionalization represents an alternative tool for imparting novel properties and improving catalytic performances [21,22].
The introduction of heteroatoms (N, O, P, and S) has been reported to confer higher hydrophilicity, enhanced metal-support interactions, and additional acidic or basic sites [23,24,25,26,27,28,29,30,31,32]. Obviously, the distribution, the amount, the location, and the specific binding forms of the heteroatoms on the surface strictly depend on the nature of the carbonaceous substrate. A crucial role is played by the graphitization degree, which is an index of the order degree in carbon-based materials [3]. A low graphitization degree should imply the presence of surface impurities and defects acting as preferential sites for the anchoring of functional groups. However, the relationship between the graphitization degree and the extent and the characteristics of the functionalization process need to be further investigated, mainly relative to the effect on the catalytic performances.
In this work, we investigated the role of structural and surface properties of different carbon nanofibers in determining the amount of P-functionalities and their catalytic performance. Carbon nanofibers were selected instead of classical carbon supports, such as activated carbon, since they can be easily functionalized without undergoing morphological alterations. Several efficient routes have been proposed to incorporate phosphorus on carbon materials [33]. In any case, when mild functionalization conditions are used, P groups are essentially attached on carbon surface by the formation of C–O–P bonds. From this point of view, using a HNO3-H3PO4 mixture represents a suitable strategy to promote the phosphorylation in mild conditions without affecting the original morphology of the samples. Indeed, the co-presence of HNO3 results in the introduction of O-functionalities on the surface, acting as preferential anchoring sites for phosphate groups [31]. As known from the literature, P-containing materials are effective catalysts in the selective dehydration of fructose to 5-hydroxymethylfurfural (HMF) in water [34,35,36,37,38]. Indeed, grafting P groups on the surface allowed to tune the surface acidity and resulted in more selective catalysts compared to other solid acid catalysts [37]. Therefore, the catalytic performances of P-functionalized carbon nanofibers were evaluated in this reaction, whose relevance in the field of biomass valorization is well-established.

2. Results

2.1. Catalytic Fructose Dehydration

Commercial carbon nanofibers (CNFs) from Applied Science (Cedarville, OH, US) (average diameter of 100 ± 30 nm and a specific surface area of 40–45 m2/g), thermally processed at different severity degrees [39], were selected as carbonaceous substrates. In particular vapor-grown carbon nanofibers treated at 700 °C (Pyrolytically Striped Carbon Nanofibers, PS-CNFs), 1500 °C (Low-temperature Heat Treated Carbon Nanofibers, LHT-CNFs), and 3000 °C (High-temperature Heat Treated Carbon Nanofibers, HHT-CNFs) were used as pristine materials to study the influence of CNF structural properties on the incorporation of P groups. To introduce P- functionalities, carbon materials were treated with a H3PO4-HNO3 mixture at 150 °C following a modified procedure from [31]. The functionalized carbon materials, labelled as P-PS-CNFs, P-LHT-CNFs, and P-HHT-CNFs, were tested in the fructose dehydration reaction, in a batch reactor at 120 °C under diffusional control (Scheme 1). Reaction conditions were selected according to previous tests performed with phosphorylated ordered mesoporous carbon [37], in order to perform a proper comparison.
The catalytic activities of the samples are compared in Figure 1, reporting the fructose conversion as a function of the reaction time. For comparison, pristine materials (PS-CNFs, LHT-CNFs, and HHT-CNFs) were also tested with negligible activity (fructose (FRU) conversion <2% in all cases). Among the studied materials, P-HHT CNFs exhibited the best performances in terms of activity, reaching the 45% of conversion after 3 h and almost completely converting fructose after 12 h (94%). The remaining samples, P-PS and P-LHT, showed a lower conversion (17% and 20%, respectively.) Moreover, these two catalysts showed deactivation after 3 h of reaction.
Concerning the selectivity to HMF (Figure 1b), the desired product from fructose dehydration, entries in Table 1 compare the performances of all the samples at increasing reaction times (3, 6, and 12 h). P-PS and P-LHT CNFs showed a very similar behavior also in terms of selectivity (SHMF: 45% and 49%, respectively, after 12 h). Despite the high activity, a low selectivity to HMF was obtained for P-HHT (46% after 12 h), probably due to the consecutive transformation of HMF to side-products. It is known that the presence of secondary reactions, such as the formation of formic and levulinic acids or humins, is promoted by the strong acidity typical of homogenous acid catalysts [36,40,41,42,43]. When 0.1 M phosphoric acid solution (pH ≈ 2) is used as a homogeneous catalyst, fructose is almost completely converted after three hours (conv. 88%), but the selectivity to HMF is quite low (52%) and comparable to the values obtained with phosphorylated carbon nanofibers.
We ascribed, therefore, the decrease in the selectivity to HMF for P- CNFs catalysts to the possible partial leaching of phosphate groups. Indeed, filtering off the solid catalyst at the reaction temperature the reaction proceeded slowly and the selectivity to HMF decreased from 40–50% to 30%. To suppress the release of phosphate group into the solution, P-HHT CNFs, showing the best catalytic performances, were calcined at 400 °C in static air.
Indeed, a high-temperature thermal treatment of phosphorylated carbon materials results in the formation of more stable C–P bonds less prone to leaching compared to C–O–P bonds [44]. Calcined P-HHT CNFs were then tested in the fructose dehydration reaction. The calcined sample showed a lower activity compared to the uncalcined one (21 and 45%, respectively, after 3h) but with the conversion which linearly increased up to 74% after 12 h. However, a higher selectivity to HMF was obtained in this case, confirming our previous hypothesis (SHMF 81% at 75% of fructose conversion). This result demonstrates the feasibility of these materials as promising catalysts for selective dehydration processes. Indeed this result is better for example than the best results obtained using phosphorylated mesoporous carbon under the same reaction conditions (SHMF 53% at 78% of conversion) [37].
A thorough characterization of the surface and structural properties of the functionalized materials was performed to provide insight into the different trends observed in terms of activity and selectivity.

2.2. Characterization of P-Functionalized Carbon Nanofibers (CNFs)

A combination of complementary imaging techniques, namely field-emission gun scanning electron microscopy (FEG-SEM), energy dispersive X-ray diffraction (EDX), and high-resolution transmission electron microscopy (HRTEM), was used to assess the composition and the effect of the functionalization on the microstructure of treated materials.
HRTEM analysis (Figure 2) provided evidence that the overall structure of the CNFs has not been significantly altered after the functionalization [39]. Further examination of the surface area by Brunauer-Emmett-Teller (BET) single-point measurements confirmed that no significant changes in the surface area and porosity occurred after acid treatment (Table 2).
The elemental composition and the local distribution of O- and P- atoms were determined by energy-dispersive X-ray spectroscopy (EDS) in FEG-SEM. Figure 3 shows examples for the analysis of HHT CNFs. The EDS mappings confirmed that both O- and P- functionalities were successfully introduced in the carbon nanofibers. In all cases P groups were homogeneously dispersed on the surface of carbon nanofibers.
A quantification of O, P, and C species at carbon material surfaces was performed by X-ray photoelectron spectroscopy (Table 3 and Table 4).
XPS analysis of the different supports confirmed that P-functionalities were introduced on the CNF surfaces A similar amount of P was found on the surface of P-PS-CNFs and P-LHT-CNFs (0.23 and 0.24%, respectively (Table 3), whereas a higher amount of P species were observed on P-HHT-CNFs (1.19%). The calcination step decreased the amount of P from 1.19% to 0.98%. Table 3, and Figure 4, Figure 5 and Figure 6, summarize the chemical species observed on the surface, their concentration, and the overall elemental composition. Peak assignments were made according to phosphorylated carbons [45] and to oxidized carbon nanotubes [46] reported previously. In the C1s region we can identified four different components in the BE range of 283–295 eV. The signal at 284.5 eV can be associated to the graphitic carbon (C sp2), the signal at 285 to C sp3, the one at 288.5 eV with the C=O (i.e., alcohol, phenol,) and the peak at 291.1 with aromatic ring. Analyzing the O1s signal, three main oxygen groups were identified. BE of 531.6–532.5 eV can be assigned to a carbon-oxygen double bond and to P=O and P-O species, the signal around 533 eV to C–O, C–O–C, P–O–C, and the signal at 538.5 eV can be assigned to adsorbed water and/or oxygen [47].
The comparison of TEM images pristine and functionalized materials revealed slight differences in the graphitization degree, which needed to be investigated deeper in the detail. The graphitization degree is a key parameter for describing the structural order of carbon materials. Eventual changes in the graphitization degree provide clear evidence that structural modifications and/or an increase in the number of defects occurred. We used Raman spectroscopy to investigate the graphitization degree of carbon materials. Two characteristic bands at 1597–1599 cm−1 (G band) and 1288–1315 cm−1 (D band), respectively, can be usually observed in the Raman spectra of carbonaceous materials. The G band is generated by the allowed C=C stretching vibrations of E2g in the hexagonal graphite lattice and, therefore, it is related to structurally-ordered graphite domains [48]. On the contrary, the D band corresponds to the A1g mode that is forbidden according to the selection rules in graphite, but it is activated in the presence of structural defects or in plane substitutional heteroatoms. The appearance of an additional D band was then associated with the turbostratic and/or disordered carbonaceous structures [48]. Consequently, the ratio of the integral intensities of D and G bands, respectively, ID/IG is an index of the defectiveness of the graphite layers.
In all the Raman spectra of functionalized materials G and D bands have been observed. Comparing the graphitization degree of pristine and functionalized materials, no clear trends can be detected. Indeed, for PS-CNFs the functionalization seems to decrease the defectiveness of the materials (Table 5). This might appear as an unexpected result, since the introduction of functional chemical groups should entail the formation of sp3 carbon and defects, thus inducing an increase of the D band and, consequently, of the ID/IG ratio. Actually, as reported in the literature, acidic oxidation treatment produces the opening of the closed loop ends on the surface of CNFs, thus favoring the overall alignment of hexagonal planes and resulting in an enhanced graphitization [49,50,51]. Moreover, the functionalization can remove the amorphous carbon present on the external surface of PS-CNFs. This phenomenon could compensate for the increased defectiveness deriving from the functionalization. On the contrary, as expected the functionalization increases the defectiveness of the LHT CNFs, whereas no significant modifications were observed for HHT CNFs.
Clearer and rationalizable differences can be disclosed before and after the use of functionalized materials as catalysts in the fructose dehydration reaction. In all samples after the catalytic tests a significant increase in the defectiveness (ID/IG ratio) was observed. This evidence could be ascribed to the presence of reaction by-products adsorbing on the carbon nanofiber surface, thus providing a possible explanation to the deactivation observed in the catalytic tests.

3. Discussion

The role of the graphitization degree in controlling and directing the introduction of P functionalities on the CNF surface has been investigated specifically for three different samples, PS-CNFs, LHT-CNFs, and HHT-CNFs. In all these cases, the use of a H3PO4-HNO3 mixture in relatively mild conditions (150 °C) resulted in being an effective strategy to functionalize the CNF surfaces without strongly altering the morphology of the fibers, as revealed by electron microscopies. Phosphorus atoms were present at the surface of CNFs in an atomic concentration ranging between 0.2 and 1%, according to the XPS analysis. Surprisingly, the highest content of phosphorus (1.19%) was observed for HHT CNFs, characterized by a high graphitization degree. This result might be explained by considering the high amount of surface free edges in graphitic CNFs, which could undergo easy functionalization. Indeed, as reported by Lim et al. [50], acidic oxidation results in the opening of the numerous closed loop end in graphitic CNFs and in the subsequent formation of free edges, which are more prone to surface functionalization. The analysis of O1s and C1s XPS signals revealed that, as expected, in all cases the insertion of P functionalities occurred preferentially by formation of C–O–P bonds. Different alterations in the graphitization degree, as calculated from the Raman spectra, were observed after the functionalization treatment for the different CNFs. This evidence could be essentially ascribed to three effects: the relatively low functionalization degree, the removal of amorphous carbon, and the graphitization induced by acidic treatment cutting off the closed-loop ends of CNFs.
The extent of the functionalization and the relative concentration of P functionalities on the CNF surface reflected on the catalytic performances in the fructose dehydration reaction. The selective dehydration of fructose to HMF is an acid-catalyzed reaction. Brønsted acid sites and, in particular, phosphate groups are known to be active catalysts for this reaction [34,35,36,37]. Actually, correlating the final conversion after 12 h and the P concentration on the surface, as determined by XPS, a clear correlation between activity and amount of P groups (Figure 7) appears. The dependence of the conversion on the P content reflected the acidity scale of the materials, as obtained by acid site titration (Table 6).
P-HHT CNFs, which possessed the highest P content and the highest number of acid sites, demonstrated to be the most active catalyst in fructose dehydration, reaching full conversion after 12 h. On the contrary, P-PS CNFs and P-LHT CNFs suffered from fast deactivation after 3 h, probably due to the leaching of phosphate groups, as suggested by the XPS analysis of used catalysts, revealing a remarkable decrease in the amount of P at the surface. This fact can be due to the leaching of P species into the solution as evidenced by the hot filtration test, where the reaction slowly proceeded even in the absence of the solid catalysts. In any case, we cannot exclude a concomitant blocking effect due to the deposit of solid carbonaceous by-products (e.g., humins) on the surface. The last hypothesis seems to be supported by the increase in the surface oxygen concentration (Table 3) and the decrease of graphitization degree (Table 5) observed in used catalysts.
In all cases HMF was the main product, even if the selectivity to this target molecule was always lower than 50%. The low selectivity to HMF might be attributed again to the leaching of phosphate groups, as confirmed by the hot filtration test. According to the literature, phosphate groups eventually dissolved in the solution would act as homogenous catalysts promoting several secondary reactions and the subsequent transformation of HMF into by-products [36,40,41,42,43]. This hypothesis was supported by the results obtained by performing the reaction under the same conditions using phosphoric acid as the homogeneous catalyst (see Table 1). Despite the high activity, fructose was not selectively converted to HMF in the presence of H3PO4. Based on these experimental evidences, we decided to test P-HHT CNFs after calcination. According to previous works, the thermal treatment led to the transformation of C–O–P bonds into stronger and more stable C–P bonds [44]. This assumption was confirmed in our experiments. Indeed, observing the O1s signal, calc. P-HHT-CNFs showed a lower contribution of P-O-C species at 533.2 eV (33.2%) compared to the uncalcined P-HHT-CNFs sample (64.7%). On the contrary the contribution of C-P signal at 285.2 eV (from XPS results) increased during the calcination from 2.4 to 2.9% (Table 4). Minimizing phosphate leaching allowed to obtain active and selective catalysts in fructose dehydration. The better resistance against deactivation of calc. P-HHT CNFs stability tests were performed. Recycling experiments were carried out by filtering and using the catalyst in the next run without any further purification. The catalyst first showed a gradual loss in the activity (15%) along the first two cycles, maintaining almost the same activity and selectivity during successive the four cycles (Figure 8).

4. Materials and Methods

4.1. Material Functionalization

Commercial CNFs PR24-PS from Applied Science (average diameter of 100 ± 30 nm and a specific surface area of 45 m2/g), CNFs PR24-LHT from Applied Science (average diameter of 100 ± 30 nm and a specific surface area of 43 m2/g) and CNFs PR24-HHT from Applied Science (average diameter of 100 ± 30 nm and a specific surface area of 41 m2/g) were used as pristine carbon materials. For the functionalization 2 g of CNF sample were suspended in 250 mL of a HNO3-H3PO4 1:1 v/v mixture, stirred and heated at 150 °C (2 h). As-functionalized carbon samples were then filtered, thoroughly washed with distilled water until neutrality, and finally dried at 100 °C overnight.

4.2. Catalytic Tests

Fructose (from Sigma Aldrich, >99%) 5 wt % and the catalyst (substrate/catalyst = 2/1 w/w) were mixed in distilled water (total volume 10 mL). Reactions were carried out in a 30 mL glass reactor equipped with a thermostat and an electronically-controlled magnetic stirrer connected to a 5000 mL reservoir charged with nitrogen (300 kPa). The reactor was pressurized at 300 kPa of nitrogen and set to 120 °C. Once this temperature was reached, the monitoring of the reaction started. The reaction was initiated by stirring. Samples were removed periodically and analyzed by high-performance liquid chromatography (HPLC) using a column (Alltech OA-10308, Fisher Scientific, Hampton, NH, USA, 300 mm 7.8 mm) with UV and refractive index (RI) detection to analyze the mixture of the samples. Aqueous H3PO4 solution (0.1 wt %) was used as the eluent. Products were identified by comparison with the original samples.
Recycling test: each run was carried out under the same conditions (Fructose 5 wt %; substrate/catalyst = 2/1 w/w, 120 °C, 3 bar N2). The catalyst was recycled in the subsequent run after filtration without any further treatment.

4.3. Catalyst Characterization

X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific K-alpha+ spectrometer (Waltham, MA, USA). Samples were analyzed using a monochromatic Al X-ray source (Waltham, MA, USA) operating at 72 W (6 mA × 12 kV), with the signal averaged over an oval-shaped area of approximately 600 × 400 microns. Data were recorded at pass energies of 150 eV for survey scans and 40 eV for high-resolution scans with a 1 and 0.1 eV step size, respectively. Charge neutralization of the sample was achieved using a combination of both low energy electrons and argon ions (less than 1 eV), which gave a C1s binding energy of 284.8 eV.
All data were analyzed using CasaXPS (v2.3.17 PR1.1, Teignmouth, UK) using Scofield sensitivity factors and an energy exponent of −0.6.
Samples for examination by transmission electron microscopy (TEM) were prepared by dispersing the catalyst powder in high-purity ethanol using ultra-sonication. Forty microliters of the suspension was dropped on to a holey carbon film supported by a 300 mesh copper TEM grid before the solvent was evaporated. The samples for TEM were then examined using a JEOL JEM 2100 TEM model (Peabody, MA, USA) operating at 200 kV. The morphology and composition was examined by scanning emission microscopy on a Tescan MAIA 3 field emission gun scanning electron microscope (FEG-SEM, Brno, Czech Republic) fitted with secondary and backscattered electron detection. Energy-dispersive X-ray (EDX) analysis and mapping was done using Oxford Instruments X-MaxN 80 detector (Abingdon-on-Thames, UK) and the data analyzed using Aztec software (Bangalore, India). Samples were mounted on 300 mesh copper grids coated holey carbon film and analyzed uncoated.
The acid sites on the CNF surface were determined through retro-titration. For this purpose, 0.2000 g of each material was mixed with 100.0 mL of sodium hydroxide 0.1 M solution for 2 h under N2 and constant magnetic stirring. The solid was separated by filtration and three aliquots of 20.0 mL of each solution were titrated with 0.1 M hydrochloric acid solution.
Nitrogen sorption measurements were performed using a Horiba SA-9601 instrument (Kyoto, Japan).

5. Conclusions

In summary, we demonstrated that highly graphitic carbon nanofibers can be easily phosphorylated by treatment with HNO3-H3PO4 mixture. This procedure results in the formation of C–O–P groups, which are active in the fructose dehydration, but not stable under the reaction conditions tending to be released with detrimental effects on the catalytic performances. High-temperature treatment represents a suitable strategy to create stronger C–P bonds, less susceptible to leaching and, consequently, to obtain more active and selective catalysts. These results represent an interesting starting point towards the design of tailored carbon-based materials for catalytic applications. To this aim, future studies will be devoted to the optimization of the functionalization procedure in order to control the density, the type, and the location of P-functionalities.

Acknowledgments

The authors acknowledge the Cardiff University for providing EM and XPS facilities, and thank David Morgan for his technical assistance. The authors are also immensely grateful to Marco Schiavoni (Università degli Studi di Milano), who provided insight and expertise that greatly assisted the research.

Author Contributions

Sebastiano Campisi and Alberto Villa designed the experiments and carried out the catalytic evaluation; Davide Motta carried out the XPS experiments and helped in the interpretation; Felipe Sanchez Trujillo and Thomas E. Davies carried out the Raman, TEM, and SEM experiments and helped in the interpretation; Sebastiano Campisi and Alberto Villa analyzed the data; and Sebastiano Campisi, Alberto Villa, Nikolaos Dimitratos were involved in the writing and editing the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Scheme 1. Reaction scheme for fructose dehydration.
Scheme 1. Reaction scheme for fructose dehydration.
Carbon 04 00009 sch001
Figure 1. Conversion profiles as a function of the reaction time; (reaction conditions: fructose 5 wt % in water, fructose/catalyst 2:1 w/w, T = 120 °C, p(N2) = 3 atm).
Figure 1. Conversion profiles as a function of the reaction time; (reaction conditions: fructose 5 wt % in water, fructose/catalyst 2:1 w/w, T = 120 °C, p(N2) = 3 atm).
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Figure 2. Microstructure characterization of carbon nanofibers (a) PS-CNFs; (b) P-PS-CNFs; (c) LHT-CNFs; (d) P-LHT-CNFs; (e) HHT-CNFs; and (f) P-HHT-CNFs.
Figure 2. Microstructure characterization of carbon nanofibers (a) PS-CNFs; (b) P-PS-CNFs; (c) LHT-CNFs; (d) P-LHT-CNFs; (e) HHT-CNFs; and (f) P-HHT-CNFs.
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Figure 3. FEG-SEM and EDX mapping images of functionalized (a) P-PS-CNFs, (b) P-LHT-CNFs, and (c) P-HHT-CNFs.
Figure 3. FEG-SEM and EDX mapping images of functionalized (a) P-PS-CNFs, (b) P-LHT-CNFs, and (c) P-HHT-CNFs.
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Figure 4. O1s analysis of P-functionalized samples before (on the left) and after (on the right) their use.
Figure 4. O1s analysis of P-functionalized samples before (on the left) and after (on the right) their use.
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Figure 5. C1s analysis of P-functionalized samples before (on the left) and after (on the right) their use.
Figure 5. C1s analysis of P-functionalized samples before (on the left) and after (on the right) their use.
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Figure 6. P2p analysis of P-functionalized samples before (on the left) and after (on the right) their use.
Figure 6. P2p analysis of P-functionalized samples before (on the left) and after (on the right) their use.
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Figure 7. Correlation between catalyst activity and surface P content.
Figure 7. Correlation between catalyst activity and surface P content.
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Figure 8. Stability tests on calc. P-HHT CNFs. Reaction time: 12 h.
Figure 8. Stability tests on calc. P-HHT CNFs. Reaction time: 12 h.
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Table 1. Catalytic performances of all the functionalized samples in the fructose dehydration reaction after 3, 6, and 12 h of reaction.
Table 1. Catalytic performances of all the functionalized samples in the fructose dehydration reaction after 3, 6, and 12 h of reaction.
CatalystTime of Reaction (h)Conversion (%)Selectivity
HMFLevulinic AcidFormic Acid
P-PS-CNFs3175427
6185059
121948711
P-LHT-CNFs3205319
6234515
12294923
P-HHT-CNFs3455022
6604964
129446107
Calc P-HHT-CNFs3218722
6388635
12748168
H3PO4 0.1M3885278
Fructose 5 wt % substrate/catalyst = 2/1 w/w, 120 °C, 3 bar N2.
Table 2. Surface area analysis.
Table 2. Surface area analysis.
SampleSpecific Surface Area (m2 g−1)
PS-CNFs43 ± 2.3
P-PS-CNFs45 ± 1.8
LHT-CNFs38 ± 3.3
P-LHT-CNFs39 ± 4.2
HHT-CNFs37 ± 0.9
P-HHT-CNFs36 ± 2.1
Table 3. Surface atomic composition from X-ray photoelectron spectroscopy (XPS) analysis.
Table 3. Surface atomic composition from X-ray photoelectron spectroscopy (XPS) analysis.
SampleAtomic Ratio % C:O:P
P-PS-CNFs87.12:12.65:0.23
P-PS-CNFs-used85.08:14.85:0.07
P-LHT-CNFs96.06:3.70:0.24
P-LHT-CNFs-used92.74:7.25:0.01
P-HHT-CNFs90.89:7.92:1.19
P-HHT-CNFs-used81.84:18.11:0.05
calc P-HHT-CNFs91.65:7.37:0.98
calc P-HHT-CNFs-used80.32:19.56:0.12
Table 4. XPS analysis of P-functionalized samples before and after their use.
Table 4. XPS analysis of P-functionalized samples before and after their use.
Sample O1SP1SC1s
C=O,
P=O, P–O
C–O,
C–O–C, P–O–C
H2OC–O–PO3
C–P
sp2sp3
C–P
C=OC=C
P-PS-CNFsBE531.6533.3535.6133.7284.5285.0288.5291.1
Atom %(50.1)(44.5)(5.4)(100)(77.4)(14.4)(5.7)(2.5)
P-PS-CNFs-usedBE531.6533.3535.9133.7284.5285.1288.4291.1
Atom %(49.1)(44.2)(6.7)(100)(73.9)(15.9)(7.1)(3.1)
P-LHT-CNFsBE531.3532.8536.2133.8284.4284.9287.9290.1
Atom %(35.6)(58.2)(6.2)(100)(86.8)(7.4)(2.6)(3.2)
P-LHT-CNFs-usedBE531.6533.2535.6133.7284.5285.0287.6290.5
Atom %(20.0)(76.2)(3.8)(100)(85.5)(7.3)(4.8)(2.4)
P-HHT-CNFsBE532.5533.8536.1133.4284.5285.1287.7291.0
Atom %(31.9)(64.7)(3.3)(100)(94.5)(2.4)(1.1)(2.0)
P-HHT-CNFs-usedBE532.5533.8536.0133.4284.5285.2287.5290.9
Atom %(70.3)(26.8)(2.9)(100)(70.2)(19.0)(9.5)(1.3)
calc P-HHT-CNFsBE531.4533.0536.0134.2284.5285.0287.1290.9
Atom %(61.3)(33.2)(5.5)(100)(93.6)(2.9)(1.1)(2.4)
calc P-HHT-CNFs-usedBE531.9533.2536.2134.2284.5285.2287.3290.9
Atom %(23.4)(73.0)(3.6)(100)(64.6)(22.9)(9.0)(3.5)
Table 5. Graphitization degree from Raman spectra.
Table 5. Graphitization degree from Raman spectra.
Sample NameGraphitization Degree (ID/IG) 1
Pristine P-FunctionalisedAfter Reaction
PS-CNFs0.750.581.16
LHT CNFs0.711.011.11
HHT CNFs0.110.090.34
Calcined Funct-HHT CNFs0.110.360.38
1 The peak fitting analysis was performed based on Gaussian functions.
Table 6. Quantification of acid sites.
Table 6. Quantification of acid sites.
SampleAcid Site (mmol g−1)
P-PS-CNFs0.116 ± 0.093
P-LHT-CNFs0.343 ± 0.032
P-HHT-CNFs1.257 ± 0.009
Calc. P-HHT-CNFs0.797 ± 0.015

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Campisi, S.; Sanchez Trujillo, F.J.; Motta, D.; Davies, T.E.; Dimitratos, N.; Villa, A. Controlling the Incorporation of Phosphorus Functionalities on Carbon Nanofibers: Effects on the Catalytic Performance of Fructose Dehydration. C 2018, 4, 9. https://doi.org/10.3390/c4010009

AMA Style

Campisi S, Sanchez Trujillo FJ, Motta D, Davies TE, Dimitratos N, Villa A. Controlling the Incorporation of Phosphorus Functionalities on Carbon Nanofibers: Effects on the Catalytic Performance of Fructose Dehydration. C. 2018; 4(1):9. https://doi.org/10.3390/c4010009

Chicago/Turabian Style

Campisi, Sebastiano, Felipe J Sanchez Trujillo, Davide Motta, Thomas E. Davies, Nikolaos Dimitratos, and Alberto Villa. 2018. "Controlling the Incorporation of Phosphorus Functionalities on Carbon Nanofibers: Effects on the Catalytic Performance of Fructose Dehydration" C 4, no. 1: 9. https://doi.org/10.3390/c4010009

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