Highly Selective Production of Biobased p-Xylene from 2,5

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Highly selective production of bio-based p-xylene from 2,5-dimethylfuran over SiO2-SO3H catalysts Xinqiang Feng, Chun Shen, Chenchen Tian, and Tianwei Tan Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 04 May 2017 Downloaded from http://pubs.acs.org on May 8, 2017

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Industrial & Engineering Chemistry Research

Highly selective production of bio-based p-xylene from 2,5-dimethylfuran over SiO2−SO3H catalysts Xinqiang Feng, Chun Shen*, Chenchen Tian, and Tianwei Tan

Beijing Key Laboratory of Bioprocess, National Energy R&D Center for Biorefinery, 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

Corresponding author: Email address: [email protected]

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Abstract Aimed at highly selective production of p-xylene (PX) from biomass-derived 2,5-dimethylfuran with a high carbon balance, SiO2 supported with sulfonic acid was especially designed and

prepared. As one kind of the most widely used Brønsted acid, sulfonic acid groups were chosen as the active sites of probes. The density of sulfonic acid groups in the whole composite was highly adjustable, ranging from 4.7 to 540.6 µmol/g. The catalytic activity was strongly

related to the acid concentration and location: the one with active sites mainly on the exterior surface exhibited much better catalytic activity by facile mass transfer through mesopores. The suitable acid species and structures of the as-prepared catalyst contributed to the improved

activity: a selectivity of 89% and a carbon balance of 95% were achieved at 523 K. Overall, this new reusable catalyst provided an alternative for highly efficient production of bio-based PX.

Keywords: p-Xylene production; 2,5-Dimethylfuran; Sulfonic acid groups; Mass transfer; Catalytic

performance

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1. Introduction p-Xylene (PX), one of the most important bulk chemicals, is mainly used to produce terephthalic acid, which is the monomer for polyester and polyethylene terephthalate plastics (e.g., plastics, polyester chips, and synthetic fibers).1–3 Conventionally, PX is separated from aromatic mixtures produced by catalytic reforming of naphtha. With the increase in demand for energy and materials, there are ongoing efforts worldwide to develop alternative technologies from renewable biomass.4–14 Four routes have been investigated, and bio-based PX are synthesized from bio-ethanol, isobutanol, whole biomass, and biomass-derived 2,5-dimethylfuran (DMF). Among these, the synthesis route starting from DMF has attracted the most attention because of its economic feasibility.14 The reaction details are described in Figure 1. First, the cycloaddition of DMF and ethylene through Diels-Alder reaction (with a barrier of 24.7 kcal/mol) produces the cycloadduct intermediate, 1,4-dimethyl-7-oxabicyclo [2.2.1] hept-2-ene. Second, the rate-determining step, namely the dehydration of the cycloadduct intermediate (with a barrier of 58.0 kcal/mol), which could be catalyzed by Brønsted acid or Lewis acid takes place.15–19 In the meantime, a molecule of water is produced and then reacts with DMF to form 2,5-hexanedione (HDO), one kind of major byproducts. HDO will further condense intra-molecularly to form 3-methyl-2-cyclopentenone (MCP). Another kind of major byproducts belongs to alkylated aromatic compounds, such as 1-methyl-4-propylbenzene (MPB). These competing side reactions will greatly reduce the PX selectivity. So far, most of the catalysts used in this system are zeolite catalysts, among which H-Y, H-Beta exhibit the best catalytic performances.20–24 In the work of Williams et al.,21 a PX selectivity of 75% was obtained using H-Y zeolite catalyst; and a PX selectivity of 80% was obtained using H-Beta zeolite catalyst in Kim’s study.22 However, as the catalyst for PX production, zeolites still face serious challenges: (i) because of the deactivation caused by carbon deposition on the active sites, a sharp decrease in the PX production rate of 31% was observed just after one hour’s reaction.22 Active sites on the internal surface were covered with macromolecular byproducts and polymers. Similar results have also been reported by other researchers.25,26 (ii) The

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carbon loss caused by condensation reactions of HDO was also serious.23 (iii) On account of the microporous structure of zeolites, the internal diffusion of reactants has severely adverse effects on the apparent kinetic. It has also been reported that small micropores decreasing the PX production rate is not favorable for the reaction, and introduction of mesopores would greatly help to improve the coke-tolerance of the catalysts and carbon balance.22, 24–26 Other solid acids such as niobic acid, TiO2, γ-Al2O3, WOx–ZrO2, and H2SO4/ZrO2 have also been explored.27,28 In the work of Wang et al.,28 TiO2 and γ-Al2O3, which possessed mainly Lewis acid sites, resulted in low PX selectivities of 17% and 10%, respectively. Catalysts possessing both Brønsted acid and Lewis acid sites exhibited better performance, and the highest PX selectivity of 77% was obtained on WOx–ZrO2, demonstrating the great importance of Brønsted acid for PX production. Otherwise, excessively strong acid might accelerate the hydrolysis of DMF and result in a low PX selectivity.23 Therefore, Brønsted acid sites with suitable strength are necessary to achieve a high selectivity for PX. On this basis, it is of great significance to develop new catalysts offering high catalytic activity and stability. In this work, we specially designed mesoporous aerosil supported with sulfonic acid groups (denoted as SiO2–SO3H) as the catalyst for highly selective production of PX from DMF, and investigated the effect of active site location on catalytic performance. To the best of our knowledge, the utilization of supported sulfonic acid groups and the effect of active sites on catalytic performance during the production of PX from DMF have not been studied yet. There may be three possible advantages for the design: first, the mesoporous structure may help to improve carbon balance caused by carbon deposition; second, as one kind of Brønsted acid sites,29–33 sulfonic acid group shows proper acid strength, which would be benefit for the promotion of PX selectivity; third, the sulfonic acid groups are highly dispersed on the external surface, in other words, the active sites are directly exposed to the reactants, which would accelerate the reaction by enhancing the mass transfer efficiency of reactants. A solution of DMF dissolved in n-heptane was used as the model reaction system. The effects of active site concentration and location, reaction duration, reaction temperature, and initial concentration of DMF have been systematically studied in relation to the DMF conversion and PX selectivity.

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2. Experimental Section 2.1. Materials and chemicals Hydrogen peroxide (30 wt%) aqueous solution and hydrochloric acid were all analytically pure and purchased from Beijing Chemical Plant. Analytical grade DMF was purchased from Xiya Chemical Plant in Shandong, China. Chromatographically pure n-heptane was purchased from Fisher Scientific. Analytical grade n-decane was purchased from Guangfu Fine Chemical Research Institute in Tianjin, China. 3-(Mercaptopropyl)-trimethoxysilane (MPTMS) was purchased from Aladdin Industrial Corporation. Ethylene (99.5%) and nitrogen (99.999%) were purchased from Haipu Gas Corporation in Beijing, China. Aerosil 200 was purchased from Degussa Company. Monodispersed silica microspheres with core-shell mesoporous structure were purchased from Nanjing Dongjian Biological Technology Corporation in China. Silica microspheres with microporous structure (the average diameter was 10.6 µm) were purchased from Sphere Scientific Corporation in Wuhan, China. All chemicals were used as received without further treatment. 2.2. Preparation and characteristic of SiO2–SO3H Preparation of SiO2–SO3H: As shown in Figure 2, 0.8 g of aerosil 200 was dispersed in 80 g of 2.0 M HCl at room temperature. Then 1.2 g of hydrogen peroxide aqueous solution (30 wt%) and a certain amount of MPTMS were added. The molar ratio of MPTMS to SiO2 was 0.00038, 0.0019, 0.0038, 0.0076, 0.019, 0.038, and 0.25, respectively. The resultant solution was stirred for 12 h at room temperature. Then it was transferred into an autoclave and treated at 373 K for 10 h. Finally, the solid product was obtained after centrifugation, washing with deionized water and ethanol, and drying at 353 K. Catalyst with different amount of MPTMS was denoted as SiO2–SO3H (x), and x represented the molar ratio of MPTMS to SiO2. Preparation of sulfonyl modified silica microsphere: Silica microspheres with core-shell mesoporous and microporous structures were modified by MPTMS (The molar ratio of MPTMS to SiO2 was 0.25) in the same way as described above. Regeneration of spent catalyst: The used catalyst was dispersed in ethanol solution (with 2.0 M HCl) and washed with an ultrasonic cleaner to wash off the organic impurities for three times. 5



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Then it was washed with water to remove absorbed HCl for several times, and finally calcined at 473 K for 2 h. H-Beta (25) and H-Beta (100) were hydrothermally synthesized as described in the literature.34 TEM images were taken with a JEOL JEM-2011 high-resolution transmission electron microscope at an operating voltage of 300 kV. SEM images were acquired on HITACHI SU1510, HITACHI Ltd., Japan. The amount of sulfur element in SiO2–SO3H was detected by ICP (ICP, IRIS Intrepid II XSP from ThermoFisher Corp., USA). The acid amount was determined by titration with 0.01 M NaOH aqueous solution in the same way as described in the literature.33 In a typical experiment, 0.20 g of catalyst was dispersed in 20 g deionized water and the suspension was stirred for 30 min. Phenolphthalein was chosen as the indicator during the course of titration. Nitrogen adsorption–desorption isotherms were measured at 77 K on a Quantachrome Autosorb-1-C Chemisorption-Physisorption Analyzer. The specific surface area was calculated from the adsorption branches in the relative pressure range of 0.05 to 0.30. The pore diameter and pore

size

distribution

were

calculated

from

the

desorption

branches

using

the

Barrett−Joyner−Halenda (BJH) method. The total pore volume was evaluated at a relative pressure of approximately 0.99. Fourier transform infrared (FTIR) spectra were recorded on FTIS (Bruker Optics, Tensor 27). Before the FTIR examination, samples of certain mass were tableted to be transparent, and heated up to 523 K in vacuum status (remained at 523 K for 1 h to remove adsorbed water). 2.3. Activity test The reaction between DMF and ethylene to produce PX was performed in a high-pressure reactor (E50, manufactured by Beijing Century Senlong Experimental Apparatus Company) with a volume of 80 mL. 30 mL of the solution (DMF dissolved in n-heptane with the initial concentration of 0.35 ± 0.01 M), 0.43 ± 0.01 g of n-decane (internal standard), and 0.25 ± 0.01 g of catalysts were mixed in the reactor. Then the reactor was purged with nitrogen for four times and subsequently pressurized with 15 bar of ethylene gas. The mixture was stirred at 500 rpm by a gas entrainment impeller. During the reaction, the reaction temperature was kept at 523 K, and the total pressure was maintained at 45 bar. The composition of the liquid product was analyzed on a

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Shimadzu 2010 Gas Chromatograph (GC) equipped with a DB-WAX (30 m × 0.320 mm, df=0.25 µm) capillary column and a flame ionization detector. n-Decane was used as an internal standard for quantitative GC analysis. DMF conversion and product selectivity were calculated as described in the literature.20 As shown in Figure S1 in the Supporting Information, the products (DMF, PX, HDO) were detected and quantified using the internal standard method with standard chemicals. The contents of other products including 1-methyl-4-propylbenzene (MPB), meta-xylene (MX), and oxanorbornene cycloadduct were estimated using the response factor (RF) of PX; likewise, the content of 3-methyl-2-cyclopentenone (MCP) was estimated using the RF of HDO. The DMF conversion, selectivity for product i, and carbon balance were defined as Equations (1), (2), and (3), respectively.

X=

Si =

CDMF ,t0 − CDMF ,t

(1)

CDMF ,t0

Ci ∑ i Ci

Carbon balance =

(2)

CDMF ,t + ∑ i Ci CDMF ,t0

(3)

where CDMF ,t0 is the initial concentration of DMF, CDMF ,t is the concentration of DMF at the certain reaction duration time of t, and Ci is the concentration of product i.

3. Results and discussion 3.1. Characteristics of SiO2−SO3H The TEM image of the as-prepared SiO2−SO3H (0.25) catalyst is shown as Figure 3a. The mesoporous structure formed by the accumulation of SiO2 nanoparticles could be clearly observed. Figure 3b shows the pore size distribution of the SiO2−SO3H (0.25) catalyst. The sharp peak in the high-pressure region (0.85 < P/P0 < 1.0) indicates the presence of mesopores. Other BET results are listed in Table S1 in the Supporting Information.

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FTIR spectra of the as-prepared catalyst and the support are shown as Figure 3c. The peak at 2900 cm−1 corresponding to C−H stretch vibration (of the propyl chain) could be clearly observed, confirming the successful immobilization of organic groups on the surface of SiO2 support. SiO2−SO3H catalysts with different contents of sulfonic propyl group have been prepared by varying the molar ratio of MPTMS to SiO2 (0.00038−0.25). The amount of sulfonic propyl group in the whole composite was determined by ICP, ranging from 4.7 to 540.6 µmol/g. 3.2. Catalytic activity for production of biomass-based PX The amount of sulfonic propyl groups that serve as the catalytic active sites plays a crucial role in determining the catalytic activity. It could be adjusted by varying the density of sulfonic propyl group or the catalyst amount. In order to gain an insight into the effect of active site concentration, two groups of experiment were designed. In the first series of experiment, a series of SiO2−SO3H catalysts with different densities of sulfonic propyl groups have been prepared. The acid density was evaluated in two ways, the titration method as described by Yang et al.33 and the ICP determination. Detailed results are given in Table S2 in the Supporting Information, and their catalytic performance is shown in Figure S2. When the density of sulfonic propyl groups was lower than 131 µmol/g, the conversion of DMF exhibited a liner dependence on the density of sulfonic propyl groups as shown in Figure S2a. DMF reacted with ethylene at a constant rate without Lewis acid. It was because SiO2−SO3H only served as Bronsted acid and catalyzed the dehydration of cycloadduct intermediate. Therefore, the dehydration of cycloadduct intermediate was the rate-limiting step when the acid density was low. With the increase in the density of sulfonic propyl groups, the selectivity for PX increased first and then remained around 87%. The carbon balance values were higher than 90% for all the runs with different acid densities. The effect of acid density on selectivity for other byproducts such as MX, MPB, oxanorbornene cycloadduct, and MCP is shown as Figure S2b, in which decreasing trends were apparently observed when the acid density increased. Meanwhile, the selectivity for HDO decreased first and then increased. The decreases in selectivity for byproducts resulted from the promotion of the main reaction. However, excessive acid sites might accelerate the hydrolysis of DMF and cause the increase in HDO selectivity with further increase in acid density. 8


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In the second series of experiment, the changes in catalytic performance were investigated related to catalyst amounts. Similarly, with the catalyst amount increased from 0.05 to 0.25 g, the conversion of DMF increased steadily from 17.4% to 76.8% (Figure S3a), which resulted from the increase of active sites concentration. As for the PX selectivity, it increased from 55.4% to 88.8% when the catalyst amount increased from 0.05 to 0.1 g, and then it remained at 88% with further increase in catalyst amount. The selectivities for other byproducts are shown in Figure S3b. The DMF conversion and product selectivity showed similar trends in the two ways. Figure 4 shows the PX production rate versus the molar ratio of acid sites to substrate (DMF). It was found that with increasing molar ratio of acid sites to substrate, the PX production rate was accordingly accelerated. There appears to be a linear relationship between them until the molar ratio of acid sites to substrate reached 0.0045, after which the PX production rate almost remained at the maximum value of 0.95 mmol/h. Figure 5 shows the changes in DMF conversion and PX selectivity as the reaction proceeded. The DMF conversion and PX selectivity both increased steadily to 92.6% after 12 h. The selectivity for HDO kept decreasing as a function of reaction duration. It resulted from the further condensation reactions of HDO.23 H-Beta(25) and H-Beta(100) were also tested for PX production in this work. As shown in Table 1, both of them showed low carbon balance with the values of 64% and 71%, respectively. It indicated that the H-Beta catalysts suffered from severe the by-reaction, namely the condensation of HDO. Production of PX from the reaction between DMF and ethylene has also been conducted in other studies.20,22,27,28 The comparison of catalytic performances with these studies is summarized in Table 1. As expected, the as-prepared SiO2−SO3H catalyst exhibited a high catalytic selectivity (89%) for PX with the highest carbon balance of 95%, which should be attributed to its low activity for hydrolysis of DMF and condensation of HDO. The promotion in catalytic activity was mainly attributed to three reasons. Besides the proper acid type and strength, the location of sulfonic propyl group also showed great influence.

Sulfonic propyl groups were immobilized on the outer surface of aerosil, providing active sites for DMF and ethylene. The reaction was greatly enhanced by the facile mass transfer. In order to prove the hypothesis, another group of experiment was designed. SiO2 with different 9



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morphologies were chosen as the supports. Sulfonic propyl groups supported on egg-shell structured SiO2 nanoparticles that were composed of porous shell and compact core were denoted as catalyst A. The corresponding TEM image and the BET result are shown in Figure 6a and 6b, respectively. SiO2 microspheres with micropores inside immobilized with sulfonic propyl group were denoted as catalyst B, and their SEM image and BET result are given in Figure 6c and 6d, respectively. Obviously, the active sites were supported on the outer part of catalyst A and the internal part along the pores of catalyst B. Other detailed BET results of these two catalysts are listed in Table S3 in the Supporting Information. Surface areas of catalyst A and B are 842.8 and 531.8 m2/g, respectively, both of which are much higher than that of SiO2−SO3H catalyst (denoted as catalyst C). During the experiment, the amount of sulfonic propyl group was kept the same. Their catalytic performances are shown in Figure 7. As expected, catalyst A and catalyst C showed similar catalytic activity, while catalyst B showed much poorer performance with the DMF conversion of 12%. The result not only confirmed our hypothesis that the introduction of mesopores and direct exposure of active sites to the reactants would enhance the reaction by suppressing internal diffusion, but also guided the development of high-performance catalysts. 3.3. Effect of reaction temperature and initial concentration of DMF on conversion and selectivity The effect of reaction temperature was investigated, and the results are shown in Figure 8. Overall, the conversion of DMF and selectivity for PX increased with the increase of reaction temperature. Figure 8a shows that the selectivity for HDO decreased with the increase of temperature. Li et al.16 have reported that the apparent free-energy barrier of hydrolysis side reaction was much smaller than that of the reaction between DMF and ethylene to form PX. Therefore, the reaction temperature has a more signiﬁcant effect on the main reaction. The effect of reaction temperature on selectivities for other byproducts is shown in Figure 8b. Similarly, most of them showed decreasing trends, and it may be assumed that the main reaction featured the highest apparent free-energy barrier. The effects of initial concentration of DMF on DMF conversion and product selectivity are given in Table 2. The PX selectivity slightly varied when the initial concentration of DMF changed. 3.4. Regeneration performance 10


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Efficient catalysts must be regenerated for multiple cycles. Therefore, the regeneration of spent catalysts is necessary. The regeneration performance of SiO2−SO3H (0.25) catalyst was given in Figure 9. Compared with the fresh catalyst, the selectivity for PX slightly decreasedfrom 88.3% to 83.5%, while the conversion of DMF decreased from 67.9% to 50.5% after three cycles. The loss in catalytic activity was attributed to carbon deposition and leaching of −SO3H groups. The spent catalyst became darker as shown in Figure S4, demonstrating a serious carbon deposition on the surface. Because the used catalyst couldn’t be regenerated by calcination at high temperature, the acid sites were covered by impurities, resulting in the decrease in DMF conversion. The content of S element of the spent catalyst was determined to be 531.9 µmol/g by ICP after three cycles.

4. Conclusion In order to face the great increase in demand for PX, SiO2−SO3H was successfully prepared and served as a novel and highly efficient catalyst for production of bio-based PX. The mesoporous structured catalyst featured a high surface area of 216.6 m2/g and a mean pore size of 16.8 nm. The density of sulfonic propyl groups in the whole composite was highly adjustable, ranging from 4.7 to 540.6 µmol/g. The effects of active site concentration and location, reaction duration, reaction temperature, and initial concentration of DMF on catalytic performances were systematically investigated. It has been experimentally proved that the proper acid sites (type and strength) and facile mass transfer through the mesoporous structure contributed to the high catalytic activity of the as-prepared catalyst. A PX selectivity of 89% and a carbon balance of 95% were obtained. SiO2−SO3H catalysts are highly active and low-cost, and are thus proving highly promising for producing bio-based PX. To fully understand the catalytic mechanism, further research should be conducted such as the search for the transition state, the study of reaction kinetic and investigation of effects of catalyst wettability.

Associated content Supporting Information This information is available free of charge via the Internet at http://pubs.acs.org/. 11



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GC chromatogram of products from the reaction of DMF and ethylene, BET results of SiO2−SO3H (0.25) catalyst. SiO2 modified by MPTMS gradient and the quantification of element S and acidity, DMF conversion and product selectivity using catalyst with different sulfonic propyl group contents, effects of catalyst amount on DMF conversion and product selectivity, BET results of catalyst A and B, photograph of fresh catalyst and spent catalyst after three cycles (PDF)

Author information Corresponding author *Email: [email protected] ORCID Chun Shen: 0000-0001-6993-4336 Notes The authors declare no competing financial interest.

Acknowledgements We gratefully acknowledge the support of the National Nature Science Foundation of China (21606008, 21436002), the National Basic Research Foundation of China (973 program) (2013CB733600), the Fundamental Research Funds for the Central Universities (ZY1630, JD1617), and the Fundamental Research Funds for the Central Universities (buctrc201616, buctrc201617).

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(17) Patet, R.E.; Nikbin, N.; Williams, C.L.; Green, S.K.; Chang, C.-C.; Fan, W.; Caratzoulas, S.; Dauenhauer, P.J.; Vlachos, D.G. Kinetic regime change in the tandem dehydrative aromatization of furan Diels-Alder products. ACS Catal. 2015, 5, 2367–2375. (18) Nikbin, N.; Do, P.T.; Caratzoulas, S.; Lobo, R.F.; Dauenhauer, P.J.; Vlachos, D.G. A DFT study of the acid-catalyzed conversion of 2,5-dimethylfuran and ethylene to p-xylene. J. Catal. 2013, 297, 35–43. (19) Nikbin, N.; Feng, S.; Caratzoulas, S.; Vlachos, D.G. p-Xylene formation by dehydrative aromatization of a Diels-Alder product in Lewis and Brønsted acidic zeolites. J. Phys. Chem. C 2014, 118, 24415–24424. (20) 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, 585–588. (21) Williams, C.L.; Chang, C.-C.; Do, P.; Nikbin, N.; Caratzoulas, S.; Vlachos, D.G.; Lobo, R.F.; Fan, W.; Dauenhauer, P. Cycloaddition of Biomass-Derived Furans for Catalytic Production of Renewable p-Xylene. ACS Catal. 2012, 2, 935–939. (22) Kim, T.-W.; Kim, S.-Y.; Kim, J.-C.; Kim, Y.; Ryoo, R.; Kim, C.-U. Selective p-xylene production from biomass-derived dimethylfuranand ethylene over zeolite beta nanosponge catalysts. Appl. Catal. B: Environ. 2016, 185, 100–109. (23) Chang, C.-C.; Cho, H.J.; Yu, J.; Gorte, R.J.; Gulbinski, J.; Dauenhauerb, P.; Fan, W. Lewis acid zeolites for tandem Diels-Alder cycloaddition and dehydration of biomass-derived dimethylfuran and ethylene to renewable p-xylene. Green Chem. 2016, 18, 1368–1376. (24) 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. (25) Kim, W.; Kim, J.C.; Kim, J.; Seo, Y.; Ryoo, R. External surface catalytic sites of surfactant-tailored nanomorphic zeolites for benzene isopropylation to cumene. ACS Catal. 2013, 3, 192–195. (26) Kim, J.-C.; Cho, K.; Ryoo, R. High catalytic performance of surfactant-directed

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Table legends Table 1. Comparisons in catalytic activity with other works. a b

Enough ethylene was added before the reaction started. Ethylene was continuously added in the whole reaction and the total pressure was kept

unchanged. 15



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c

/ meant the data were not provided in the references.

Table 2. Effects of initial concentration of DMF on conversion and selectivity. (Reaction conditions: catalyst: SiO2−SO3H (0.25); catalyst amount: 0.25 g; reaction pressure: 45 bar; reaction duration: 6 h.)

Figure legends Figure 1. Reaction details for synthesis of PX from DMF and ethylene. Figure 2. Preparation course of mesoporous SiO2−SO3H catalyst (The support SiO2 can be aerosil and silica microsphere which possess abundant silanol group on the surface). Figure 3. (a) TEM image of SiO2−SO3H (0.25) catalyst; (b) Pore size distribution (corresponds to the desorption branch) of SiO2−SO3H (0.25) catalyst; (c) FTIR spectra of SiO2 support and SiO−SO3H (0.25) catalyst. Figure 4. Rate of PX production versus the molar ratio of acid sites to substrate (DMF). Figure 5. DMF conversions and product selectivities with different reaction duration. (a) conversion of DMF and selectivity for PX and HDO; (b) selectivity for other byproducts. (Reaction conditions: catalyst: SiO2-SO3H (0.25); catalyst amount: 0.25 g; initial concentration of DMF: 0.35 M; reaction pressure: 45 bar; reaction temperature: 523 K) Figure 6. (a) TEM image and (b) pore size distribution of catalyst A; (c) SEM image and (d) pore size distribution of catalyst B. (Pore size distributions correspond to the desorption branch of samples) Figure 7. Catalytic performances of catalysts with different acid location. (Reaction conditions: catalyst amount: 0.25 g; initial concentration of DMF: 0.35 M; reaction pressure: 45 bar; reaction duration: 6 h; reaction temperature: 523 K.) Figure 8. DMF conversions and product selectivities of SiO2−SO3H (0.25) under different temperatures. (a) conversion of DMF and selectivity for PX and HDO; (b) selectivity for other byproducts. (Reaction conditions: catalyst: SiO2−SO3H (0.25); catalyst amount: 0.25 g; initial concentration of DMF: 0.35 M; reaction pressure: 45 bar; reaction duration: 6 h) Figure 9. Regeneration performance of the catalyst. Reaction conditions: catalyst: SiO2−SO3H (0.25); catalyst amount: 0.25 g; initial concentration of DMF: 0.35 M; reaction pressure: 45 bar; traction temperature: 523 K; reaction duration: 6 h.

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List of tables Table 1 Catalyst

DMF

Continuously

Reaction

concentration

add ethylene

duration

balance

(h)

(%)

(M)

X (%)

SPX (%)

Carbon

References

SiO2–SO3H(0.25)

1.04

No a

6

67

89

95

This work

H-Beta(25)

1.04

No

6

99

82

64

This work

H-Beta(100)

1.04

No

6

58

72

71

This work

SAA

0.73

No

6

75

70

/c

27

H2SO4/ZrO2

0.73

No

4

12

17

/

27

WOx/ZrO2

0.81

No

6

60

77

/

28

γ-Al2O3

1

Yes b

24

52

58

78

20

H-Y

1

Yes

24

52

74

81

20

C-BEA

2.35

Yes

24

98

75

/

22

Table 2 Si (%)

Carbon Concentration

X balance

(M)

(%) (%)

PX

MX

MPB

Cycloadduct

HDO

MCP

0.35

76.8

91.1

88.8

1.1

1.2

1.1

6.8

1.0

0.52

69.9

90.3

85.0

1.8

1.1

1.2

9.5

1.4

1.04

66.7

94.7

88.5

0.7

0.6

0.7

8.5

1.0

2.08

50.6

95.8

88.6

0.4

0.6

0.6

9.1

0.7

List of figures

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Figure 1

Figure 2

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Figure 3

Figure 4

Figure 5

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Figure 6

Figure 7

Figure 8 20


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Figure 9

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For table of contents only

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Highly Selective Production of Biobased p-Xylene from 2,5

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