Catalytic Conversion of Xylose and Xylan into Furfural Over

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Cite This: Ind. Eng. Chem. Res. 2019, 58, 13013−13020

Catalytic Conversion of Xylose and Xylan into Furfural Over Cr3+/PSBA-15 Catalyst Derived from Spent Adsorbent Siquan Xu,† Donghui Pan,† Yuanfeng Wu,† Jingdeng Fan,† Ningxin Wu,† Lijing Gao,† Wenqi Li,‡ and Guomin Xiao*,† †

School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China Biosystems and Agricultural Engineering, University of Kentucky, Lexington, Kentucky United States

‡

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S Supporting Information *

ABSTRACT: The mesoporous SBA-15 has long been used as an adsorbent for the removal of toxic metal ions. Achieving secondary utilize of spent adsorbent is undoubtedly an advisible option for environmental protection and recycling economy. In the present study, phosphoric acid-modified SBA-15 (P-SBA-15) was found to be an available adsorbent, and the composites (Cr3+/P-SBA-15) formed after adsorption of Cr3+ ions have the ability to efficiently catalyze the conversion of xylose and xylan into furfural. The physicochemical properties of the Cr3+/P-SBA-15 composites were characterized, and the effects of various reaction parameters on their catalytic performance were also investigated. Among the Cr3+/P-SBA-15 composites with different Cr3+ contents, the (0.25)Cr3+/P-SBA-15 composite not only achieved excellent 91% and 58% furfural yields from xylose and xylan, but also had almost constant catalytic performance after five cycles. The findings herein provided a reference for achieving high yields of furfural and reuse of spent adsorbents.

1. INTRODUCTION Furfural, listed as one of the top 10 biobased products by the U.S. Department of Energy, is a versatile platform compound that can supplement petrochemical products for preparing resins, lubricants, adhesives, and plastics.1−3 In view of the increasing global demand for high-value chemicals, furfural has gradually become a star product in the fuel market. Xylose is the most common and effective starting material for furfural synthesis, and its catalytic conversion route has been widely recognized as being synergistically driven by Brønsted and Lewis acid. Specifically, xylose was first subjected to isomerization under the catalysis of a Lewis acid, and then the formed xylulose was dehydrated in the presence of Brønsted acid to form furfural (Scheme 1).4,5 SBA-15 is a 2D hexagonal ordered mesoporous silica material with tunable pores in 4−11 nm, which not only has the relatively large pore size, desired hydrothermal stability, and adsorption capacity, but also achieves adsorption equilibrium in a short time.6,7 Therefore, in recent years, SBA-15 and its derivatives have been frequently

employed as impactful adsorbents for the removal of heavy metal ions (Cu2+, Zn2+, Cr3+, Cr6+, Ni2+, Cd2+, Mn2+, Pb2+, As5+) from wastewater.7−9 It was generally accepted that the adsorption capacity of SBA-15 composites for metal ions can be enhanced by grafting Brønsted acidic functional group on its surface due to the strong complexing ability of acidic functional groups with metal ions.10,11 In our previous work,4 Cr3+ was proven to be a particularly effective Lewis acid site that helped promote the formation of furfural from xylose. Referring to the above background, an interesting strategy was put forward that if Brønsted acidic SBA-15 derived adsorbents adsorbed with toxic Cr3+ ions can be used as acidic bifunctional heterogeneous catalysts in the catalytic formation of furfural, it would be of great significance for the secondary utilization of spent adsorbents. In this study, the adsorption capacity of phosphoric acidmodified SBA-15 (P-SBA-15) for toxic Cr3+ ions was measured, and a series of composites (Cr3+/P-SBA-15) obtained after adsorption were also tried as heterogeneous catalysts in the synthesis of furfural from xylose and xylan. The results showed that up to 91% and 58% of furfural yields were obtained from xylose and xylan in a water/tetrahydrofuran biphasic system under the catalysis of the (0.25)Cr3+/P-SBA15 composite. It was comparable to the many representative heterogeneous catalysts for furfural production (Supporting

Scheme 1. Catalytic Synthesis Pathway of Furfural Formation from Xylose

Received: Revised: Accepted: Published: © 2019 American Chemical Society

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April 3, 2019 June 26, 2019 July 2, 2019 July 2, 2019 DOI: 10.1021/acs.iecr.9b01821 Ind. Eng. Chem. Res. 2019, 58, 13013−13020

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Industrial & Engineering Chemistry Research Scheme 2. Preparation and Application of Cr3+/P-SBA-15 Composites

Information (SI) Table S1).4,12−17 Moreover, it was worth mentioning that the catalytic performance of the (0.25)Cr3+/PSBA-15 composite hardly decreased after five consecutive cycles. It was reasonable to believe that the results presented in this study provided a basis and prospect for realizing the high yield of furfural and the reuse of spent adsorbent.

product analysis are summarized in the Supporting Information.

3. RESULTS AND DISCUSSION 3.1. Evaluation of Adsorption Capacity of SBA-15 and P-SBA-15. The adsorption capacity of SBA-15 and P-SBA-15 was evaluated in 50 mL of a solution containing an adsorbent (1 g/L) and Cr3+ ions (100 mg/L). The adsorption environment was maintained at 30 °C, whereas the pH was adjusted to 4.0 by dilute phosphoric acid and 5 wt % sodium hydroxide solution. According to the concentration of Cr3+ ions in the solution after adsorption (determined by ICPAES), the adsorption capacity of SBA-15 and P-SBA-15 adsorbents was calculated, which were 46.7 and 63.6 mg·g−1, respectively. It meant that the Cr3+ ions removal efficiency in the corresponding solutions were 46.7% and 63.6%, respectively. The results confirmed that the adsorption capacity of SBA-15 was indeed improved by the modification of phosphoric acid. The explanation for the improvement of adsorption capacity probably comes down to two aspects: (1) The ability to complex with Cr3+ ions was enhanced due to the presence of the grafted acidic functional group;18 (2) The SBA-15 modified with phosphoric acid has a shorter rod length (SI Figure S1), which was conducive to the multiangle diffusion of Cr3+ ions into abundant channel structure of the adsorbent. 3.2. Texture Properties of the Cr3+/P-SBA-15 Composites. After P-SBA-15 was demonstrated to be a useful adsorbent, the texture properties of a series of Cr3+/P-SBA-15 composites obtained were also studied. The small and wideangle XRD patterns of parent SBA-15 and its derived composites are shown in Figure 1(a) and (b), respectively. It was found that in the range of 0.5−2°, one intense (2θ = 0.8°) and two weak (2θ = 1.4° and 1.6°) diffraction peaks appeared in all samples, which represented the (1 0 0), (1 1 0), and (2 0 0) hexagonal lattice planes of SBA-15, respectively.19 The results indicated that after undergoing the phosphoric acid modification and adsorption process, the channels of Cr3+/PSBA-15 composites were not destroyed and still maintained a highly ordered structure with typical hexagonal space group P6mm symmetry. Moreover, in the wide-angle XRD analysis results, only one broad peak at around 23° was observed, which was attributed to amorphous silica,20 meaning that crystalline chromium species were not formed during the preparation of Cr3+/P-SBA-15 composites. The 2D mesoporous structure of parent SBA-15 and its derivatives was observed by TEM. It can be clearly seen from

2. EXPERIMENTAL SECTION 2.1. Materials and Cr3+/P-SBA-15 Composite Preparation. Molecular sieve SBA-15, xylose, xylan (from corncob) and furfural were purchased by Aladdin (Shanghai, China). The other chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals used were of analytical grade and were not further purified. The (x)Cr3+/P-SBA-15 composites were obtained through two steps as shown in Scheme 2. First, the hydroxyl group on the surface of SBA-15 underwent a dehydration reaction with phosphoric acid to form a Brønsted acidic functional group, and the obtained sample was referred to as P-SBA-15 (step 1). Thereafter, the P-SBA-15 adsorbent was placed in a different concentration of an aqueous solution of CrCl3 for sufficient adsorption to give a target composite (step 2). For example, the preparation of the target composite (0.15)Cr3+/P-SBA-15 was as follows: 1.0 g of SBA-15 powder was placed in 20 mL of a 30 wt % aqueous phosphoric acid solution and refluxed for 12 h. After the temperature of the mixed suspension was lowered to room temperature, the obtained P-SBA-15 was successively centrifuged, washed with water and dried at 80 °C. Afterward, 1.0 g of P-SBA-15 and 0.15 g of CrCl3·6H2O were put into 20 mL of an aqueous solution having a pH of 4.0 (adjusted by dilute phosphoric acid and 5 wt % aqueous sodium hydroxide solution), followed by adsorption for 3 h. Finally, the obtained target composite was named as (0.15)Cr3+/P-SBA-15 after being centrifuged, washed with water, and dried at 80 °C. The other two composites (0.25)Cr3+/P-SBA-15 and (0.5)Cr3+/P-SBA-15 of this series were obtained in the identical manner as described above, except that the doses of CrCl3·6H2O were 0.25 and 0.5 g, respectively. The preparation of the (0.25)Cr3+/SBA-15 composite was in accordance with (0.25)Cr3+/P-SBA-15, except that the phosphoric acid modification step was removed. A series of Cr3+/P-SBA-15 composites were characterized using ICR-AES, XRD, TEM, XPS, FTIR, N2 adsorption− desorption, NH3-TPD and Py-FTIR techniques. Detailed characterization methods, catalytic performance tests, and 13014

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in Table 1. As shown in SI Figure S2, the isotherms depicted by all samples were assigned to the type IV isotherms with H1type hysteresis loop, reflecting the typical ordered mesoporous SBA-15 structure.19,21 The results were consistent with the above XRD and TEM analysis, further demonstrating that phosphoric acid modification and adsorption process have no much effect on the ordered mesoporous structure in parent SBA-15. From Table 1, it can be found that the specific surface area and pore volume of P-SBA-15 showed a slight decrease compared to SBA-15. It indicated that the grafting process of functional group proceeded not only on the surface of SBA-15 but also in the internal pores, while causing a certain degree of blocking.22,23 Moreover, as the concentration of Cr3+ ions in the adsorption environment continued to elevate, the specific surface area and pore volume of the derived Cr3+/P-SBA-15 composites were gradually reduced. The results reflected the fact that Cr3+ ions were adsorbed and diffused into the P-SBA15 channel as expected. 3.3. Surface Chemical State of the Cr3+/P-SBA-15 Composites. The element compositions and surface chemical state of the Cr3+/P-SBA-15 composites were investigated by the XPS. The atomic ratios of Cr/Si in (0.15)Cr3+/P-SBA-15, (0.25)Cr3+/P-SBA-15 and (0.5)Cr3+/P-SBA-15 composites were determined to be 1.9%, 2.3%, and 3.1%, respectively (Table 1). It can be seen from Figure 3a that two evident peaks appeared near the binding energies of 577.4 and 586.8 eV, which were assigned to Cr 3+ 2p 3/2 and Cr 3+ 2p 1/2 , respectively,24,25 whereas the signal attributed to Cr6+ was not detected in all the spectra. The results indicated that the chromium species adsorbed in the Cr3+/P-SBA-15 composites mainly existed in the form of trivalent chromium ions. The XPS spectra corresponding to the phosphorus elements in the composites are shown in Figure 3b. It was found that a single peak at approximately 133.4 eV was detected in the P 2p region of the Cr3+/P-SBA-15 composites, which belonged to pentavalent phosphorus-oxidation state (P5+) of P−O bond in PO43−, meaning that the phosphate group was grafted onto the surface of SBA-15.26,27 FT−IR analysis of Cr3+/P-SBA-15 composites is presented in Figure 4. All of the composites showed a distinct absorption band at 3445 cm−1, corresponding to the O−H stretching

Figure 1. Small-angle (a) and wide-angle XRD (b) patterns of Cr3+/ P-SBA-15 composites.

Figure 2 that all composites exhibited well-ordered hexagonal arrays of mesopores run along the length direction of the particles. Meanwhile, the mesoporous cavities were observed to be dispersed on their outer surfaces in a highly uniform manner, which confirmed that the inner surfaces of the Cr3+/PSBA-15 composites were highly connected. The N2 adsorption−desorption isotherms and pore size distributions of Cr3+/P-SBA-15 composites are presented in SI Figure S2, and the corresponding texture data are summarized

Figure 2. TEM images of Cr3+/P-SBA-15 composites. 13015

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Industrial & Engineering Chemistry Research Table 1. Physicochemical Properties of Cr3+/P-SBA-15 Composites samples SBA-15 P-SBA-15 (0.15)Cr3+/PSBA-15 (0.25)Cr3+/PSBA-15 (0.5)Cr3+/P-SBA15

surface area (m2/g)a

pore volume (cm3/g)b

pore size (nm)c

total acidity (μmol/g)d

lewis acidity (μmol/g)d

Brønsted acidity (μmol/g)d

atomic ratio of Cr/Si (%)e

735 701 682

1.32 1.20 1.09

10.34 10.22 9.83

78.7 214.8

4.4 121.0

74.3 93.8

1.9%

646

1.09

9.61

337.1

260.5

76.6

2.3%

627

1.03

9.54

400.1

318.4

81.7

3.1%

a

BET surface area was determined from N2 adsorption isotherm. bVolume of pore was determined from single point desorption method. cAverage pore size was determined from the desorption average pore diameter (4 V/A by BET). dThe amount of acid sites was determined by Py-FTIR at 150 °C. eDetermined by XPS.

vibration of adsorbed water and surface hydroxyl groups.22 The other two visible absorption bands at around 797 and 1095 cm−1 were assigned to the symmetric stretching vibration and asymmetrical stretching vibration of Si−O−Si, respectively, whereas the one at 1621 cm−1 belonged to the deformation vibration of O−H.22,28 Since the absorption bands located in the range of 600−700 cm−1 were difficult to distinguish, we have amplified this section and embedded it in Figure 4. It was found that two new bands in the P-SBA-15 and Cr3+/P-SBA15 samples appeared near 616 and 667 cm−1 compared to parent SBA-15. They were attributed to phosphate network bending vibration and symmetric stretching of P−O−P, respectively.29,30 The results were consistent with the XPS analysis and further confirming the presence of phosphate groups. 3.4. Acidity of the Cr3+/P-SBA-15 Composites. The presence of Cr3+ ions (Lewis acid site) and phosphate groups (Brønsted acid site) gave the Cr3+/P-SBA-15 composites the potential to be an acidic bifunctional catalyst. Therefore, the NH3-TPD was first engaged to evaluate the acid nature of the Cr3+/P-SBA-15 composites. As shown in Figure 5, an ammonia

Figure 3. XPS spectra of Cr3+/P-SBA-15 composites; (a) Cr 2p, (b) P 2p.

Figure 5. NH3-TPD profiles of Cr3+/P-SBA-15 composites.

desorption peak attributed to a strong acid site in the desorption temperature range of 500−650 °C was clearly observed in the P-SBA-15 and Cr3+/P-SBA-15 composites compared to the neutral parent SBA-15. Moreover, as the Cr3+ content increased, the desorption peaks in the P-SBA-15 and Cr3+/P-SBA-15 composites gradually shifted toward a higher desorption temperature. It indicated that the phosphoric acid modification and Cr3+ ions incorporation caused some stronger acid sites to be formed in the composites.31 The type of acidic

Figure 4. FT−IR spectra of Cr3+/P-SBA-15 composites.

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Industrial & Engineering Chemistry Research sites in the Cr3+/P-SBA-15 composites was identified and quantified by Py-FTIR, and the corresponding results are summarized in Table 1. It can be seen that after undergoing the grafting of phosphate group and adsorption process of Cr3+ ions, the total acidity of the obtained series of (x)Cr3+/P-SBA15 composites was greatly improved. With the gradual introduction of Cr3+ ions, the total acidity of the formed Cr3+/P-SBA-15 composites increased from 78.7 to 400.1 μmol/g, and the rise of the Lewis acid component was particularly significant, rising from 4.4 to 318.4 μmol/g. Meanwhile, the ratio of Lewis to Brønsted acidity (L/B) of (0.15)Cr3+/P-SBA-15, (0.25)Cr3+/P-SBA-15 and (0.5)Cr3+/PSBA-15 were also calculated, being 1.29, 3.40 and 3.90, respectively. 3.5. Catalytic Performance of Cr3+/P-SBA-15 Composites. Catalytic conversion of xylose to furfural as a probe reaction was used to evaluate the catalytic performance of the Cr3+/P-SBA-15 composites. As shown in Figure 6, the use of

acidic (0.25)Cr3+/SBA-15, the achieved furfural yields were 48% and 56%, respectively, which were lower than those provided by the bifunctional acidic Cr3+/P-SBA-15 composites under the identical conditions. Therefore, the good catalytic performance of Cr3+/P-SBA-15 composites was ascribed to its bifunctional properties of both Brønsted and Lewis acid. Moreover, compared to the other two, the (0.25)Cr3+/P-SBA15 composite therein allowed an optimal 77% yield of furfural to be achieved. Because of their similar texture properties, the difference in catalytic performance can be attributed to the changes in the acid nature including the total acidity and acidity ratio. Compared with weaker acidic (0.15)Cr3+/SBA-15 (214.8 μmol/g) and more acidic (0.5)Cr3+/SBA-15 (400.1 μmol/g), (0.25)Cr3+/SBA-15 with proper acidic strength (337.1 μmol/g) not only guaranteed sufficient active sites for xylose conversion, but also made the optimal dynamic equilibrium between isomerization and dehydration steps due to its suitable L/B ratio (3.40), which slowed down the occurrence of side reactions and achieved the desired furfural yield.34 So as a result, in a series of (x)Cr3+/P-SBA-15, (0.25)Cr3+/P-SBA-15 was reasonably screened as an ideal catalytic composite. Once (0.25)Cr3+/P-SBA-15 composite was ascertained to help catalyze the conversion of xylose to furfural, the effect of different process parameters on its catalytic performance was also investigated. As shown in Figure 7, the reaction temperature and time play an important role in the (0.25)Cr3+/P-SBA-15-catalyzed furfural formation from xylose. When the reaction was proceeded at 150 °C for 30 min, only 19% of furfural was obtained, while the conversion of xylose was poor. As the reaction temperature and time were continuously increased, the furfural yield obtained was also increased correspondingly, and an optimal 79% furfural was supplied when the reaction conditions were specified at 170 °C for 90 min. Such a large change in furfural yield reflected the fact that sufficient reaction energy was necessary to achieve desired furfural yield from xylose catalyzed by (0.25)Cr3+/PSBA-15 composite. However, an excessively violent reaction environment would result in a decrease in the furfural yield, which was due to the fact that the sharply elevated reaction energy not only accelerated the rate of xylose dehydration, but also promoted the furfural degradation.35,36 Therefore, 170 °C and 90 min were recognized to be the optimal reaction

Figure 6. Catalytic performance of Cr3+/P-SBA-15 composites.

(0.15)Cr3+/P-SBA-15, (0.25)Cr3+/P-SBA-15, and (0.5)Cr3+/ P-SBA-15 composites resulted in a significant increase in the furfural yield compared a control experiment without catalyst addition, confirming the practical ability of Cr3+/P-SBA-15 composites as an effective catalyst for furfural production. Catalytic formation of furfural from xylose is a process coordinated by Brønsted and Lewis acids.32,33 For a single acidic composite, such as Brønsted acidic P-SBA-15 and Lewis

Figure 7. Effect of reaction temperature and time on the catalytic performance of (0.25)Cr3+/P-SBA-15 composite. 13017

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Industrial & Engineering Chemistry Research temperature and time for (0.25)Cr3+/P-SBA-15-catalyzed xylose conversion to furfural. The effect of (0.25)Cr3+/P-SBA-15 dosage on furfural yield was carried out at 170 °C for 90 min. From Figure 8, it can be

was calculated according to the total acid sites concentration, which was 21.76 h−1. The yields of furfural achieved at the xylose initial concentrations of 10%, 20% and 30% were 79%, 63%, and 52%, respectively. The cause for the decrease in furfural yield was that the higher the concentration of xylose, the greater the frequency of molecular collision between furfural and xylose and between furfural itself, which would lead to a prominent increase in the probability of selfpolymerization of furfural and cross-polymerization between furfural and xylose, thus making the yield of furfural poor.15,38 In order to broaden the catalytic application of (0.25)Cr3+/ P-SBA-15 composite, we have tried to replace the substrate from simple C5 xylose into a more complex corncob-derived xylan. As described in Table 2, the (0.25)Cr3+/P-SBA-15 Table 2. Conversion of Xylan to Furfural Catalyzed by (0.25)Cr3+/P-SBA-15 Composite

Figure 8. Effect of catalyst dosage on furfural yield.

found that 10 wt % of (0.25)Cr3+/P-SBA-15 composite was responsible for 72% of furfural, and when the dosage was increased to 30 wt %, preferably 79% of furfural yield was obtained. However, further increasing the dosage of the (0.25)Cr3+ /P-SBA-15 composite was negative for the production of furfural, resulting in a downward trend in yield. When 50 and 70 wt % of (0.25)Cr3+/P-SBA-15 composite were put into use, the yield of furfural decreased to 68% and 59%, respectively. It was due to the fact that an excess of catalyst would cause a rapid rise in local acidic active sites, which favored the degradation of furfural, such as fragmentation, resinification, and condensation, leading to the generation of humins and other unwanted byproducts.16,37 With the aim of further improving the yield of furfural, the effect of the initial concentration of xylose on the furfural production was also investigated and the relevant results are shown in Figure 9. It can be observed that as the initial concentration of xylose continued to rise, the corresponding yield of furfural decreased significantly. A yield of up to 91% furfural was afforded when 5 wt % of xylose was catalyzed by (0.25)Cr3+/P-SBA-15 composite at 170 °C for 90 min. Under this optimal process parameter, the TOF (turnover frequency) value corresponding to the (0.25)Cr3+/P-SBA-15 composite

entry

raction temperature (°C)

raction time (min)

ctalyst dosage (wt %)

frfural yield (%)

1 2 3 4 5 6 7

170 180 190 180 180 180 180

120 120 120 90 150 120 120

30 30 30 30 30 40 50

49% 56% 53% 42% 50% 58% 52%

composite-catalyzed xylan conversion process provided a furfural yield ranging from 42% to 58% under different reaction conditions. Among them, the highest 58% of furfural was obtained when the xylan conversion was performed at 180 °C for 120 min with 40 wt % of the catalyst. The results indicated that xylan depolymerization, xylose isomerization and xylulose dehydration could be proceeded simultaneously in “one-pot” conversion under the catalysis of (0.25)Cr3+/PSBA-15 composite, reflecting the potential of (0.25)Cr3+/PSBA-15 composite for further application in the conversion of lignocellulose raw feedstocks. 3.6. Catalytic Stability Evaluation of the Cr3+/P-SBA15. The most valuable advantage of heterogeneous catalysts over homogeneous catalysts is that they can be separated and recycled. Therefore, the recyclability and hydrothermal stability of (0.25)Cr3+/P-SBA-15 composite were also investigated. After each cycle, the (0.25)Cr3+/P-SBA-15 composite was recovered by centrifugation and washed sequentially with water and methanol. After drying, it was put into the next use. As described in Figure 10, after five consecutive cycles of testing, the catalytic performance of the (0.25)Cr3+/P-SBA-15 composite for xylose conversion was hardly changed, and the yield of furfural obtained was reduced from 91% to 83%, only 8%. By analyzing the structure and composition properties of the recovered composite (SI Table S2), it can be found that the specific surface area and pore volume of the recycled (0.25)Cr3+/P-SBA-15 composite (SBET: 560 m2/g, Vpore: 0.87 cm3/g) were lowered than those of fresh one (SBET: 682 m2/g, Vpore: 1.09 cm3/g) due to the occurrence of carbon deposition (Figure 2). Meanwhile, the chromium and phosphorus elements decreased from 2.11 and 2.92 wt %, respectively, to 1.78 and 1.86 wt % (SI Table S2). However, carbon deposition and leaching did not appear to have a significant effect on the catalytic performance of the (0.25)Cr3+/P-SBA-15 composite. It was probably due to the

Figure 9. Effect of xylose concentration on furfural yield. 13018

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Detailed instrumental analysis and product analysis methods (2.2−2.4). Comparison of the catalytic performance of (0.25)Cr3+/P-SBA-15 composite with other catalysts (Table S1). The structure and compositional properties of the composite after the cycle (Table S2). The physical properties of the Cr3+/P-SBA-15 composite and the HPLC analysis of furfural (FiguresS1−S4) PDF)

AUTHOR INFORMATION

Corresponding Author

*Phone+86-25-52090612.Fax: +86-25-52090612.E-mail: [email protected]. ORCID

Figure 10. Evaluation of the cycling ability of (0.25)Cr3+/P-SBA-15 composite.

Guomin Xiao: 0000-0001-8482-1276 Notes

The authors declare no competing financial interest.

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larger pore size (9.61 nm) of the (0.25)Cr3+/P-SBA-15 composite, which contained sufficient active sites, while the kinetic diameter of xylose was 6.8 Å,14 so even local carbon deposition would not overly hinder the xylose molecules from diffusing near the active center and contacting with it. 3.7. Possible Conversion Path. In the previous study,4,32 [Cr(H2O)5OH]2+, which was an active center produced by hydrolysis of Cr3+ ions in the aqueous phase, has the ability to catalytically isomerize xylose to xylulose. In the presence of a Brønsted acid site, the formed xylulose was dehydrated to further form furfural. Herein, Cr3+ ions and phosphate groups have been confirmed to be present in the (0.25)Cr3+/P-SBA15 composite and the Lewis and Brønsted acidic sites were provided accordingly. Therefore, combined with the above background, a possible conversion path was speculated. That was, the Cr3+ ions adsorbed in (0.25)Cr3+/P-SBA-15 composite were hydrolyzed first in the aqueous phase, and the resulting [Cr(H2O)5OH]2+ catalyzed the isomerization of xylose to xylulose. Thereafter, under the catalysis of the phosphate group grafted on the (0.25)Cr3+/P-SBA-15 composite, xylulose removed three water molecules, eventually forming furfural.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 21676054), Fundamental Research Funds for the Central Universities (No. 2242018K40041), Scientific Research Foundation of Graduate School of Southeast University (No. 3207049713).

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4. CONCLUSION The phosphoric acid modified mesoporous SBA-15 (P-SBA15) as an adsorbent exhibited considerable adsorption capacity for toxic chromium ions. After the adsorption process was completed, the spent adsorbent-derived Cr3+/P-SBA-15 composite can be utilized as a highly efficient heterogeneous catalyst in the production of furfural from biomass derived carbohydrates. Under optimal reaction conditions, (0.25)Cr3+/ P-SBA-15 composite has the ability to catalyze xylose and xylan to acquire 91% and 58% furfural, respectively, and its recyclability has also been demonstrated to be desirable. Therefore, the study presented in this study provided a valuable approach for the secondary utilization of spent adsorbents, which is to serve as an acidic bifunctional catalyst in the formation of furfural from biomass resources.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b01821. 13019

DOI: 10.1021/acs.iecr.9b01821 Ind. Eng. Chem. Res. 2019, 58, 13013−13020

Article

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DOI: 10.1021/acs.iecr.9b01821 Ind. Eng. Chem. Res. 2019, 58, 13013−13020