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Highly selective production of p-xylene from 2, 5dimethylfuran over hierarchical NbOx-based catalyst Jiabin Yin, Chun Shen, Xinqiang Feng, Kaiyue Ji, and Le Du ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03297 • Publication Date (Web): 05 Dec 2017 Downloaded from http://pubs.acs.org on December 10, 2017

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ACS Sustainable Chemistry & Engineering

Highly selective production of p-xylene from 2, 5-dimethylfuran over hierarchical NbOx-based catalyst Jiabin Yina,b, Chun Shena,*, Xinqiang Fenga, Kaiyue Jia, and Le Dub,*

a

Beijing Key Laboratory of Bioprocess, College of Life Science and Technology, Beijing

University of Chemical Technology, No. 15 of North Three-Ring East Road, Chaoyang District, Beijing 100029, P. R. China b

The State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of

Membrane Science and Technology, Beijing University of Chemical Technology, No. 15 of North Three-Ring East Road, Chaoyang District, Beijing 100029, P. R. China

Corresponding authors: Email address: [email protected] (Chun Shen), [email protected] (Le Du)

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ABSTRACT：：A hierarchical NbOx-based catalyst with both Brønsted acid and Lewis acid sites was synthesized in the absence of corrosive hydrofluoric acid, exhibiting a high catalytic activity for bio-based p-xylene (PX) production from 2,5-dimethylfuran (DMF). The as-prepared composite was composed of Nb2O5 and NbOPO4 crystals, and the densities of Brønsted acid and Lewis acid were determined to be 232.9 and 80.4 µmol/g, respectively. The well-balanced Brønsted/Lewis acidity and the hierarchical structure with small mesopores (3 nm) and large mesopores (48 nm) contributed to the high activity and stability: a conversion of 87.2% with the PX selectivity of 92.7%, and a carbon balance of 94.6% was achieved after 6 hours’ reaction at 523 K. In comparison with Sn-Beta, NbOx-based catalyst prepared in this work showed obvious advantages in suppressing carbon deposition: 90.9 and 54.7 mmol of PX were obtained over the NbOx-based catalyst and the Sn-Beta, respectively after 24 h. Spent catalysts were regenerated through calcination at high temperature and they proved to be recyclable: a decrease of 3.7% in DMF conversion and no loss in PX selectivity could be evidenced over five consecutive runs. Overall, NbOx-based catalyst which is synthesized through the green and sustainable approach is sufficiently stable, active, and regenerable, and provides an alternative candidate for efficient PX production.

Keywords: p-Xylene production; NbOx-based catalyst; Acid sites; Hierarchical structure; Catalytic performance.

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INTRODUCTION With concerns on environmental pollution and continued depletion of fossil fuels, great efforts

have been directed to the production of renewable chemicals and biofuels. p-Xylene (PX), as a widely used feedstock for the production of polyethylene terephthalate, has received extensive attention because of its increasing demand.1–11 Diels–Alder cycloaddition of biomass-derived 2,5-dimethylfuran (DMF) with subsequent dehydration has been demonstrated as one of the most potential approaches for producing renewable PX.12–16 As depicted in Figure 1, Lewis acid catalyzes Diels–Alder reaction between DMF and ethylene with a barrier of 24.7 kcal/mol,17 then PX could be obtained by dehydration of the cycloadduct intermediate. Both Lewis acid and Brønsted acid are reported to be capable for the latter step,18,19 and density functional theory (DFT) calculations have emphasized the significant role of Brønsted acid by reducing the barrier from 58–60 kcal/mol to 14–19 kcal/mol.20,21 In the meantime, Brønsted acid sites are also highly catalytic active for hydrolysis of DMF, resulting in the main side product, namely 2,5-hexanedione (HDO). Further aldol condensation of HDO producing oligomerized byproducts could be also carried out on Brønsted acid sites, leading to serious

carbon

deposition

and

loss.22,23

Additionally,

alkylation

of

PX

to

1-methyl-4-propylbenzene (MPB) is another type of side reaction. To data, the most active catalysts reported are zeolites with the weak and medium acid strengths, such as Y and Beta.22–25 More recently, Fan et al. have reported that the phosphorous containing zeolite Beta (P-Beta) exhibited the highest selectivity for the synthesis of PX.26 Although they have exhibited promising activity, their inherent micropores greatly reduced the diffusion rates of the reactants and products, leading to coke formation in the micropores and reduction in lifetime. It is reported by Kim and coworkers that an abrupt decrease of 31% in PX production rate was observed after 1 h,27 and Gorte reported that H-Beta was almost completely deactivated after 4 hours because the micropores were blocked by macromolecular polymers.23 As reported by Kim and coworkers,28 an enhanced catalytic activity was achieved by introducing mesopores into MFI-type zeolite: PX yields were 8.4% and 75.8% on the pristine and mesoporous ZSM-5, respectively. In our previous work,29 it was also demonstrated that mesoporous catalysts

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showed higher activity for bio-based PX production by facile mass transfer. In the meantime, Lewis acid sites contained in H-Y or H-Beta zeolites are not stable, which would transfer into Brønsted acid sites in the presence of water molecular, and the absence of catalysts for the Diels–Alder reaction negatively impacts the overall reaction rate. In later studies, Lewis acid zeolites, such as Sn-Beta and Zr-Beta have been synthesized and explored for the production of bio-based PX.22,23,30 However, on account of the superior catalytic activity of Brønsted acid for dehydration reactions, H-Beta exhibited higher PX production rate compared with the Lewis acid zeolites.13,22 Therefore, from the standpoint of proving the overall reaction rate and lifetime, catalysts with water-tolerant Lewis acid, Brønsted acid, and high stability, are in need for the tandem reaction. As solid acids, both Nb2O5 and niobium phosphate which was prepared with hydrofluoric acid have been reported to be active and efficient for dehydration of carbohydrates (such as glucose and fructose) to 5-hydroxymethylfuran (HMF).31–38 Mesoporous NbOx-based catalysts are envisioned to be a promising alternative for PX production from DMF by virtue of their good water tolerance, Brønsted–Lewis acidic property, and high thermal stability. Herein, we report on the new preparation method for hierarchical NbOx-based catalysts with both mesopores and macropores, in which highly corrosive hydrofluoric acid was avoided, and explore the feasibility of their applications in PX production from DMF.

EXPERIMENTAL SECTION Materials and chemicals. Niobium oxalate was supplied by Shanghai Yuanye Biological

Technology Co. Ltd. Diammonium hydrogen phosphate was purchased from Fuchen Chemical Plant (Tianjin, China). Hexadecyltrimethylammonium bromide (CTAB) was provided by Sinopharm Chemical Reagent Co. Ltd. n-Heptane, ammonia solution (25%), phosphoric acid and concentrated nitric acid were purchased from Beijing Chemical Plant. DMF was obtained from Shandong Xiya Chemical Plant. n-Decane was purchased from Tianjin Guangfu Fine Chemical Research Institute. SnCl4·5H2O was purchased from Sigma Co. Ltd. Anhydrous grade isopropanol was purchased from Aladdin Reagents (Shanghai) Co. Ltd. All reagents were analytical grade and

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used as received without further purification. Al-Beta (SiO2/Al2O3=25) were purchased from Nankai University Catalyst Co. Ltd. Preparation of NbOx-based catalyst. 10.76 g of niobium oxalate was dissolved in 30 mL of H2O under stirring until a clear solution was obtained. Meanwhile, 2.64 g of diammonium hydrogen phosphate was dissolved in 10 mL of H2O, followed by the addition of 1.7 g of H3PO4, obtaining a mixed solution. Then, the mixture was added into the above-mentioned clear solution. Finally, 1 g of CTAB in 10 mL of H2O was added into the above-mentioned mixture and stirred for 1 h at 301 K. The obtained homogeneous mixture was transferred into a Teflon lined stainless steel autoclave and then heated at 433 K for 24 h. After cooling to room temperature, the solid product was filtered, washed with distilled water, and dried at 353 K for 12 h. After calcination at 823 K for 6 h to remove the organic template, the NbOx-based catalyst was obtained. Synthesis of NbOx-based catalysts at different pH values: the NbOx-based catalyst synthesized at different pH values was denoted as NbOx-pHy (y = 1, 2, and 3). All the steps were the same as described above, except that the pH value was adjusted by using phosphoric acid or ammonia solution (25%) before adding CTAB. Spent catalyst was separated, washed with anhydrous ethanol, dried at 353 K, and calcined at 823 K for 6 h. Preparation of Sn-Beta. Sn-Beta was prepared by insertion of Sn atoms into the framework of dealuminated Beta zeolites with vacancy defects via an isopropanol flux.39 Dealumination treatment of Al-Beta (SiO2/Al2O3=25) was performed using HNO3. 5.0 g of Al-Beta was treated by 100 mL of concentrated HNO3 (65 wt%) at 353 K for 20 h. Then the dealuminated zeolite Beta was collected by centrifugation, washed by water until the supernatant approached a neutral pH, and dried at 353 K overnight. Then 2.0 g of the dealuminated zeolite Beta was added to 54 mmol of SnCl4·5H2O in 200 mL of dried isopropanol and placed in a reflux setup under inert atmosphere. After 7 h, the product was filtered, washed with dry isopropanol, dried, and calcined at air (3 K/min to 473 K, dwell 6 h, 3 K/min to 823 K, dwell 6 h). Characterizations. X-ray diffraction (XRD) patterns were obtained with a RINT 2000 vertical goniometer using Cu Kα radiation (λ = 0.154 nm, 40 kV, 50 mA). Transmission Electron Microscope (TEM) images were acquired on a JEM-2100F high-resolution transmission electron

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microscope at an operating voltage of 200 kV. Scanning electron microscopy (SEM) measurements were performed on a Hitachi SU-8010 microscope. The nitrogen isotherms at 77 K were measured on a Micromeritics ASAP 2046. All samples were outgassed at 453 K for 12 h under vacuum. The surface area was determined by the Brunauer–Emmett–Teller (BET) method. The Fourier transform infrared (FTIR) absorption spectra and Infrared spectra of pyridine adsorption were recorded on a PerkinElmer Spectrum 400. The pyridine adsorption spectra were recorded at 373, 473, 573 and 673 K, respectively. The B/L acid ratio was calculated from the recorded pyridine adsorption integral curve. The amounts of total acid sites were calculated from the results of temperature-programmed desorption of NH3 (NH3-TPD). The amount of Brønsted acid sites and Lewis acid sites were calculated based on the results of pyridine adsorption spectra and NH3-TPD. NH3-TPD analysis was carried out on a Micromeritics ChemiSorb 2720. The catalyst (100 mg) was heated to 773 K with a heating rate of 10 K/min in He. When the temperature decreased to 373 K, NH3 (5 vol% NH3 in He) was injected for 0.5 h. X-ray photoelectron spectroscopy (XPS) was recorded using a Thermo Scientific Escalab 250Xi with Al Kα radiation at θ = 90° for the X-ray source. Thermogravimetric analysis (TGA) was carried out on a Discovery TGA 55. Air was used as the purge gas and the samples were heated to 473 K (remained for 0.5 h to remove adsorbed water) at a rate of 10 K/min, and then heated to 1073 K at the same heating rate. The amount of deposited carbon on the spent catalyst was calculated according to the data from 473 to 773K. The contents of Nb and P elements were determined by Inductively Coupled Plasma atomic emission spectrometer (ICP). Activity tests. Information regarding details of the activity tests is available in the Supporting Information. The DMF conversion, PX selectivity, and carbon balance were calculated by Equations (1), (2), and (3), respectively, as described in the literature.22

Conversion =

Selectivity =

n0 − n1 ×100% n0

nPX ×100% ∑n

Carbon balance =

(1)

(2)

∑ n + n ×100% 1

n0

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(3)

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where n0 is the initial mole number of DMF, n1 is the mole number of left DMF after the reaction,

∑n

is the mole number of all of the detected products, and nPX is the mole number

of PX after the reaction.

RESULTS AND DISCUSSION Characteristics of NbOx-based catalysts. Figure 2 shows the XRD pattern of the

as-synthesized NbOx-based materials. There are two sharp peaks located at 19.8° and 28.4°, indicating the existence of crystallized NbOPO4 (JCPDS card 37-0377) and Nb2O5 (JCPDS card 27-1312), respectively. Moreover, the two broad peaks in the 2θ ranges of 15–40° and 40–60°, indicating that the pore walls are amorphous, and similar results were also reported by other works.33,40 Figure 3 shows the FTIR spectra of the prepared NbOx-based catalyst. A strong band is found at 630 cm−1, which could be assigned to Nb–O–Nb stretching modes, directly indicating the existence of Nb2O5 in the prepared composite. The band at 750 cm−1 is attributed to the symmetrical stretching vibration and asymmetrical stretching vibration of P–O–P. Another important band is found at 1025 cm−1, resulting from the asymmetric stretching vibration of the phosphate ion. The band located around 1400 cm−1 could be assigned to the P–O–H deformation mode. In addition, four broad bands in the region of 1500 to 3800 cm−1 are also observed, resulting from the vibration of the bonded O–H groups or the adsorbed H–O–H groups.31,41,42 Combined with the XRD pattern, the results indicate that the as-prepared catalyst is composed of both NbOPO4 and Nb2O5 crystals. The surface morphology of the catalyst is shown in Figure 4. Figures 4a and 4b show that the catalyst is composed of a large number of sheet-like nanostructures, and the multilayer lamellar structures are assembled into a flower-shaped morphology. TEM images of the catalyst are presented in Figures 4c, and 4d, in which the mesopores distributed on the surface of the nanosheet could be clearly observed. As shown in Figure 5a, two peaks around 3 and 48 nm could be observed. The smaller pores could result from CTAB, and the larger pores which are just in the boundary of mesopores and

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macropores may result from accumulation of sheet-like nanostructures. The enormous peak around 48 nm indicates the existence of plenty of pores which result from accumulation of sheet-like nanostructures. The catalyst prepared in this work is composed of a large number of sheet-like nanostructures which are assembled into a flower-shaped morphology, leading to the abundance of large pores around 50 nm. The result agrees well with the SEM images. The N2 sorption isotherm is shown as Figure 5b, indicating the porous structure of the catalyst. The result is consistent with the TEM images. Correspondingly, the hierarchical structure with small mesopores (3 nm) and large mesopores (48 nm) will intensify the reaction by facile mass transfer. Other BET results are summarized in Table 1. The acidic property of the catalysts was investigated by NH3-TPD, as shown in Figure 6a. A broad peak in the range of 473–573 K could be observed, confirming the presence of both weak and medium strength acid sites. In order to investigate the different acid types of the catalyst, pyridine-FTIR (Py-FTIR) spectra were recorded. As for the NbOx-based catalyst, Lewis acid sites are generated from coordinatively unsaturated Nb5+ ions, which are related to the tetrahedral structure of NbO4. The terminal P–OH and Nb–OH function as Brønsted acid sites.40,43 As expected, three sharp bands are observed in Figure 6b. The band at 1490 cm–1 should be attributed to the adsorption of pyridine on both Brønsted acid and Lewis acid sites. Meanwhile, the band at 1450 cm–1 corresponds to the adsorption of pyridine at the Lewis acid sites and the band at 1540 cm–1 is the characteristic peak caused by Brønsted acid sites. It confirms the co-existence of Brønsted acid and Lewis acid on the NbOx-based catalysts. The acid amounts are summarized in Table 2. Surface composition of the catalyst was investigated by XPS, which shows a P/Nb molar ratio of 1.44. As shown in Figure 7, the spectra of Nb shows two signals at 211.0 and 208.3 eV, which could be assigned to Nb5+ 3d5/2 and Nb5+ 3d3/2, respectively. The results indicated that the Nb valence was mainly +5.44,45 The result strongly suggests that the Lewis acid sites are generated from coordinatively unsaturated Nb5+ ions. The XPS spectra of P element is shown in Figure S2 in the supporting information.The molar ratio was determined three times by ICP with the average value of 2.18, indicating the molar ratio of Nb2O5/NbOPO4 in the prepared catalyst was 0.59.

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Effect of reaction duration on DMF conversion and PX selectivity. To quantify the effect of reaction duration on catalytic performance, products collected at different intervals were analyzed. As shown in Figure 8, both the DMF conversion and PX selectivity increase gradually with the increase of reaction duration. While, the carbon balance decreases because of the side-reaction, namely HDO condensation, in which macromolecular polymers (undetectable by GC) were produced. As revealed in Figure 8b, HDO selectivity shows a decreasing trend when the reaction duration increases. As one of the best catalysts for PX production from DMF, Sn-Beta was also prepared and characterized in this work. The XRD pattern of Sn-Beta catalyst is shown in Figure S3 in the Supporting Information, indicating the successful preparation of Sn-Beta. The NH3-TPD profiles and Pyridine-FTIR spectra of Sn-Beta are depicted in Figures S4 and S5, respectively in the Supporting Information. The result shows that the acid sites contained in Sn-Beta mainly exist in the form of Lewis acid (B/L acid ratio was 0.05). Comparison in catalytic activity is shown in Figure 8c, and the effects of reaction duration on DMF conversion, product selectivity and carbon balance over Sn-Beta are depicted in Figures S6a, and S6b in the supporting information. As expected, with the reaction proceeding, the formation rate of PX decreases gradually, indicating a loss in catalytic activity which may be caused by carbon deposition in the micropores of Sn-Beta. While, it exhibits a linear relationship between PX production amount versus reaction duration over the NbOx-based catalyst during the first 12 h, and 90.9 mmol PX was produced after 24 h. The carbon balance over NbOx-based catalyst varied in the range of 90.9% to 97.4%, while the value over Sn-Beta decreased from 92.0% to 82.2% during the whole reaction. The spent NbOx-based catalyst and Sn-Beta were characterized by TGA. As shown in Figure S7, only a carbon deposition of 0.16 wt% occurred on the spent NbOx-based catalyst, while it was 7.69 wt% on the used Sn-Beta. These results confirmed the advantages of the as-prepared catalyst in stability and in resistance to carbon deposition. Effect of acid amount. Figure 9a shows the effect of acid amount on catalytic performance (B/L acid ratio remained at 2.9). The conversion of DMF increases from 19.9% to 87.2% when the acid amount increases from 0.094 to 0.376 mmol. When the acid amount further increases to 0.564 mmol, only a slight increase of 5.5% in DMF conversion could be observed. The PX selectivity increases gradually from 81.7% to 94.2% with the acid amount increasing from 0.094

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mmol to 0.564 mmol, while the carbon balance decreases gradually from 97.5% to 89.0%. Figure 9b shows the relation between the rate of PX production and the molar ratio of acid sites to DMF. With the increase in the molar ratio of acid sites to DMF, PX production rate increases linearly and then remains at the maximum value of 16.04 mmol/h. Before the molar ratio of acid sites to DMF reaches 0.0038, PX production rate increases linearly because dehydration of the cycloadduct was the rate-controlling step at a low acid concentration.21–23 The selectivities for other byproducts are shown as Figure S8 in the Supporting Information. The effect of acid amount on the rate of PX production by Sn-Beta is shown in Figure S9 in the Supporting Information. With the increase in the molar ratio of acid sites to DMF, PX production rate increases gradually at first and then remains at the maximum value of 9.78 mmol/h, which is lower than that of NbOx-based catalyst. Besides, at least 0.0056 equivalents of acid sites were required for the platform region using Sn-Beta as the catalyst, indicating the better catalytic activity of the NbOx-based catalyst. Effect of B/L acid ratio. As previously mentioned, both Brønsted and Lewis acid sites are necessary in highly efficient catalyst, and the B/L acid ratio also plays a significant role in determining the catalytic activity. Herein, another three NbOx-based catalysts with different B/L acid ratios were synthesized by adjusting the pH value. N2 adsorption–desorption isotherms and pore size distributions of the catalysts are given in Figure S10 in the supporting information. Surface area, average pore size, and pore volume are summarized in Table S1 in the supporting information. NH3-TPD profiles of the catalysts are shown in Figure S11, and acid amounts are depicted in Table S2. The B/L acid ratio decreased with the increase of pH value during the synthetic process. Their pyridine-FTIR spectra are presented in Figure S12. XPS spectra of the three catalysts are shown in Figure S13 in the supporting information. Molar ratios of P/Nb are summarized in Table S3. It is well accepted that the Diels–Alder reaction is catalyzed by Lewis acid, and the second step in the tandem reaction, which is dehydration of the produced cycloadduct, could take place over both Brønsted acid and Lewis acid (Brønsted acid is more active than Lewis acid).19–23 Therefore, it is anticipated that the dehydration would be enhanced with the increase in B/L molar ratio, leading to a decrease in cycloadduct selectivity and an increase in DMF conversion. As shown in Figure 10a, both the DMF conversion and PX selectivity increase gradually with the

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increase of the B/L acid ratio, and the selectivity for cycloadduct shows a decreasing trend as expected. The carbon balance decreases from 99.9% to 91.2%, and the total selectivity of HDO and MCP increases from 6.1% to 7.2%, which may both result from the higher catalytic activity for DMF hydrolysis and subsequent condensation of Brønsted acid than that of Lewis acid.19,20 Detailed catalytic performances of catalysts prepared under different pH values are listed in Table S4 in the supporting information. Production of bio-based PX has been conducted in other works, and the comparison in catalytic activity is summarized in Table 3. As expected, the as-prepared NbOx-based catalyst exhibits high catalytic performance for PX production, which may be attributed to the appropriate ratio of B/L acid and the hierarchical structure. Effect of reaction temperature. Reaction temperature significantly affects the catalytic performance, and the results are presented in Figure 11a. The tandem reaction was accelerated at high temperatures, and DMF conversion and PX selectivity increased with the increase of reaction temperature, while the selectivity of the main by-product, namely HDO decreased. It is reported that PX production from DMF would be described by pseudo-first order kinetics.23,46 As shown in Figure 11b, the value of PX production amount is plotted as a function of reaction duration at 473, 493, 513, and 523 K, respectively. The straight lines at different temperatures are mathematically fitted with high correlation coefficients, with the corresponding R2 values of 0.996, 0.998, 0.999, and 0.999, respectively. As shown in Figure 11c, according to the Arrhenius equation, the activation energy was calculated to be 37.7 kJ/mol with the R2 value of 0.993. Catalytic stability and regeneration performance. Given the environmentally friendly nature, the stability and recyclability of the catalyst are particularly important for practical applications. The recyclability of the catalyst was tested and the regeneration performance is presented in Figure 12. Compared to the fresh catalyst, no loss of PX selectivity could be evidenced even after five circulations, only the DMF conversion decreases by 3.7%. As shown in Figure S14, the regenerated catalyst remained white. The XRD pattern and the SEM image of the catalyst after five cycles are shown in Figure S15a, and S15b, respectively. The physical characterization results are approximately consistent with those of the regenerated catalyst,

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confirming the high stability of the as-prepared catalyst. The facility of regeneration and the high activity of the NbOx-based catalyst open up new opportunities for bio-based PX production.

CONCLUSIONS

In summary, we proposed a greener approach for the preparation of hierarchical NbOx-based catalyst avoiding the use of highly corrosive hydrofluoric acid. Hierarchical NbOx-based catalyst with small mesopores (3 nm) and large mesopores (48 nm) was synthesized and served as a highly efficient catalyst for the production of bio-based PX. NH3-TPD and Py-FTIR results identified the total acid amount was 313.3 µmol/g and the molar ratio of B/L acid was 2.9. Effects of the reaction duration, acid amount, B/L acid ratio, and reaction temperature on catalytic performance were systematically investigated. The PX selectivity and DMF conversion increased gradually with the increase of reaction duration, acid amount, and reaction temperature. The high catalytic performance for PX production may be attributed to the appropriate ratio of B/L acid and the hierarchical structure. The conversion of 87.2% with the PX selectivity of 92.7% and the carbon balance of 94.6% was achieved after 6 h at 523 K. In comparison with the Sn-Beta catalyst, the NbOx-based catalyst offered advantages in stability and in higher resistance to carbon deposition. Moreover, the NbOx-based catalyst was readily regenerated by calcination at 823 K: only a decrease of 3.7% in DMF conversion and no loss in PX selectivity were evidenced after five circulations.

ASSOCIATED CONTENT

Supporting information This information is available free of charge via the Internet at http://pubs.acs.org/. Detailed information about activity tests, GC chromatogram of products from the reaction of DMF and ethylene, XPS spectra of P 2p region, effect of reaction duration on DMF conversion, product selectivity and carbon balance over Sn-Beta, effect of reaction time on byproducts selectivity over Sn-Beta, TGA result of the spent NbOx-based catalyst and Sn-Beta, effect of the acid amount on byproducts selectivity, N2 adsorption–desorption isotherms and corresponding pore size distributions of catalysts synthesized at different pH values, BET results of catalysts synthesized at

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different pH values, NH3-TPD profiles of catalysts synthesized at different pH values, acidic properties of catalysts prepared at different pH values, pyridine-FTIR spectra of catalysts prepared at different pH values, XPS spectra, molar ratio of P/Nb of catalysts synthesized at different pH values, effect of B/L acid ratio on catalytic activity, photographs of fresh catalyst and spent catalyst after regeneration, XRD pattern and SEM image of the catalyst after regeneration (PDF)

Author Information Corresponding Authors *E-mail: [email protected], [email protected] ORCID Chun Shen: 0000-0001-6993-4336 Le Du: 0000-0002-4436-9649 Notes The authors declare no competing financial interest.

Acknowledgements We gratefully acknowledge the support of the National Nature Science Foundation of China (21606008, U1663227), the National Basic Research Foundation of China (973 program) (2013CB733600), the Fundamental Research Funds for the Central Universities (ZY1630, JD1617, buctrc201616), the State Key Laboratory of Chemical Engineering (SKL-ChE-16A01, SKL-ChE-17A02).

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(23) Yu, J. Y.; Zhu, S. Y.; Dauenhauer, P. J.; Cho, H. J.; Fan, W.; Gorte, R. J. Adsorption and reaction properties of SnBEA, ZrBEA and H-BEA for the formation of p-xylene from DMF and ethylene. Catal. Sci. Technol. 2016, 6 (14), 5729–5736. (24) Do, P. T. M.; McAtee, J. R.; Watson, D. A.; Lobo, R. F. Elucidation of Diels–Alder reaction network of 2,5-dimethylfuran and ethylene on HY zeolite catalyst. ACS Catal. 2013, 3 (1), 41–46. (25) Chang, C. C.; Green, S. K.; Williams, C. L.; Dauenhauer, P. J.; Fan, W. Ultra-selective cycloaddition of dimethylfuran for renewable p-xylene with H-BEA. Green Chem. 2014, 16 (2), 585–588. (26) Cho, H. J.; Ren, L.; Vattipalli, V.; Yeh, Y. H.; Gould, N.; Xu, B.; Gorte, R. J.; Lobo, R.; Dauenhauer, P. J.; Tsapatsis, M.; Fan, W. Renewable p-xylene from 2,5-dimethylfuran and ethylene using phosphorus-containing zeolite catalysts. ChemCatChem. 2017, 9 (3), 398–402. (27) Kim, T. W.; Kim, S. Y.; Kim, J. C.; Kim, Y.; Ryoo, R.; Kim, C. U. Selective p-xylene production from biomass-derived dimethylfuran and ethylene over zeolite beta nanosponge catalysts. Appl. Catal. B-Environ. 2016, 185, 100–109. (28) Kim, J. C.; Kim, T. W.; Kim, Y.; Ryoo, R.; Jeong, S. Y.; Kim, C. U. Mesoporous MFI zeolites as high performance catalysts for Diels–Alder cycloaddition of bio-derived dimethylfuran and ethylene to renewable p-xylene. Appl. Catal. B-Environ. 2017, 206, 490–500. (29) Feng, X. Q.; Shen, C.; Tian, C. C.; Tan, T. W. Highly selective production of biobased p-xylene from 2,5-Dimethylfuran over SiO2-SO3H Catalysts. Ind. Eng. Chem. Res. 2017, 56 (20), 5852–5859. (30) Dijkmans, J.; Dusselier, M.; Gabriels, D.; Houthoofd, K.; Magusin, P. C. M. M.; Huang, S. G.; Pontikes, Y.; Trekels, M.; Vantomme, A.; Giebeler, L. Cooperative catalysis for multistep biomass conversion with Sn/Al Beta zeolite. ACS Catal. 2015, 5 (2), 928–940. (31) Mal, N. K.; Bhaumik, A.; Fujiwara, M.; Matsukata, M. Novel organic-inorganic hybrid and organic-free mesoporous niobium oxophosphate synthesized in the presence of an anionic surfactant. Micropor. Mesopor. Mat. 2006, 93 (1–3), 40–45. (32) Ordomsky, V. V.; Sushkevich, V. L.; Schouten, J. C.; van der Schaaf, J.; Nijhuis, T. A. Glucose dehydration to 5-hydroxymethylfurfural over phosphate catalysts. J. Catal. 2013, 300, 37–46. 16


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for fuel cells. J. Mater. Chem. 2012, 22 (42), 22452–22458. (43) Choi, Y.; Park, D. S.; Yun, H. J.; Bake, J.; Yun, D.; Yi, J. Mesoporous siliconiobium phosphate as a pure Bronsted acid catalyst with excellent performance for the dehydration of glycerol to acrolein. ChemSusChem 2012, 5 (12), 2460–2468. (44) Weng, W. H.; Davies, M.; Whiting, G.; Solsona, B.; Kiely, C. J.; Carley, A. F.; Taylor, S. H. Niobium phosphates as new highly selective catalysts for the oxidative dehydrogenation of ethane. Phys. Chem. Chem. Phys. 2011, 13 (38), 17395–17404. (45) Tamura, M.; Tokonami, K.; Nakagawa, Y.; Tomishige, K. Effective NbOx-modified Ir/SiO2 catalyst for selective gas-phase hydrogenation of crotonaldehyde to crotyl alcohol. ACS Sustan. Chem. Eng. 2017, 5 (5), 3685–3697. (46) Gupta, N. K.; Fukuoka, A.; Nakajima, K. Amorphous Nb2O5 as a selective and reusable catalyst for furfural production from xylose in biphasic water and toluene. ACS. Catal. 2017, 7 (4), 2430–2436.

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Table 1. BET results of NbOx-based catalyst Surface area

Pore volume

Mean pore size

(m2/g)

(cm3/g)

(nm)

76.8

0.40

21.6

Table 2. Acid amounts of NbOx-based catalyst Brønsted acid

Lewis acid

Total acid

(µmol/g)

(µmol/g)

(µmol/g)

232.9

80.4

313.3

B/L acid ratio 2.9

Table 3. Comparison in catalytic performance Catalyst

DMF

Reaction

Conversion

PX

Carbon

concentration

duration

(%)

selectivity

balance

(M)

(h)

(%)

(%)

1.00

6

87.2

92.7

94.6

This work

Sn-Beta

1.00

24

82.3

74.3

82.2

This work

NSP-BEA

2.35

24

99.3

93.9

85.2

(27)

H-BEA

1.00

24

99.0

90.0

/

(25)

Zr-BEA

1.35

24

84.0

81.0

/

(22)

WOx-ZrO2

0.81

6

60.0

77.0

/

(13)

γ-Al2O3

2.35

24

46.5

52.8

42.2

(27)

HY

1.00

24

52.0

74.0

81.0

(25)

SAA

0.73

6

90.0

66.7

/

(14)

NbOx-based

References

catalyst

19



Figure 1. Reaction details of DMF and ethylene to PX.

Figure 2. XRD pattern of NbOx-based catalyst.

Figure 3. FTIR spectra of NbOx-based catalyst.

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Figure 4. (a) and (b): SEM images, (c) and (d): TEM images of NbOx-based catalyst.

Figure 5. (a) Pore size distribution and (b) N2 adsorption–desorption isotherm of NbOx-based catalyst.

21



Figure 6. (a) NH3-TPD profiles of NbOx-based catalyst; (b) Pyridine-FTIR spectra of NbOx-based catalyst obtained after evacuation at different temperatures of (I) 373 K, (II) 473 K, (III) 573 K, and (IV) 673 K.

Figure 7. XPS spectra of Nb 3d region.

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Figure 8. (a) Effect of reaction duration on DMF conversion, PX selectivity, and carbon balance; (b) effect of reaction duration on selectivity of byproducts; (c) comparison in PX production amount: (I) Sn-Beta, (II) NbOx-based catalyst. (Reaction conditions: catalyst amount: 0.6 g; initial concentration of DMF: 1.0 M; reaction pressure: 54 bar; reaction temperature: 523 K)

Figure 9. (a) Effect of acid amount on DMF conversion and, PX selectivity, and carbon balance; (b) Effect of acid amount on the rate of PX production. (Reaction conditions: initial concentration of DMF: 1.0 M; reaction pressure: 54 bar; reaction temperature: 523 K; reaction duration: 6 h)

Figure 10. Effect of B/L acid ratio on DMF conversion, PX selectivity, and crabon balance (a), 23



and byproducts selectivity (b). (Reaction conditions: catalyst amount1.2 g; initial concentration of DMF: 1.0 M; reaction pressure: 54 bar; reaction temperature: 523 K; reaction duration: 6 h)

Figure 11. (a) Effect of reaction temperature. (reaction conditions: catalyst amount: 1.2 g; initial concentration of DMF: 1.0 M; reaction pressure: 54 bar; reaction duration: 6 h); (b) time courses for PX formation during the initial stage of reaction at 473–523 K; (c) corresponding Arrhenius plots. (reaction conditions: catalyst amount: 0.6 g; initial concentration of DMF: 1.0 M; reaction pressure: 54 bar; reaction duration: 3 h; total volume of the solution: 100 mL)

Figure 12. Catalytic stability and regeneration performance. (Reaction conditions: catalyst: 24


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catalyst amount: 0.6 g; initial concentration of DMF: 1.0 M; reaction pressure: 54 bar; reaction temperature: 523 K; reaction duration: 6 h)

25



For Table of Contents Use Only

Hierarchical NbOx-based catalysts with both Brønsted acid and Lewis acid sites were synthesized and are active for the production of para-xylene (PX) from 2,5-dimethylfuran (DMF) in a green and sustainable approach.

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