Production of liquid fuel intermediates from furfural via aldol condensation over potassium-promoted Sn-MFI catalyst

Fuel 237 (2019) 1281–1290

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Full Length Article

Production of liquid fuel intermediates from furfural via aldol condensation over potassium-promoted Sn-MFI catalyst

T

⁎

Wenzhi Lia, Mingxue Sua, , Tingwei Zhanga, Qiaozhi Maa, Wei Fanb a

Laboratory of Basic Research in Biomass Conversion and Utilization, Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Hefei 230026, China b Chemical Engineering Department, University of Massachusetts Amherst, 686 N. Pleasant Street, Amherst MA01003, USA

A R T I C LE I N FO

A B S T R A C T

Keywords: Aldol condensation Furfural Lewis acid zeolite Potassium promoted

Liquid fuel intermediates can be produced via aldol condensation reactions between furfural and acetone. It was found that potassium-promoted Lewis acid zeolite Sn-MFI performed superior catalytic performance for CeC bond coupling by aldol condensation reactions of furfural with acetone. The presence of K in Sn-MFI catalyst improves the catalytic activity of the catalyst likely due to increased basicity. The product distribution of aldol condensation was also changed remarkably with the addition of K. Besides 4-(2-furyl)-3-buten-2-one (FAc), 1, 5di-2-furanyl-1, 4-pentadien-3-one (F2Ac) was also formed in the existence of K compared with pure Sn-MFI. It was also found that the K-promoted Sn-MFI catalyst is more resistant to water and exhibits enhanced selectivity to F2Ac.

1. Introduction

be converted into high carbon chain precursors. For example, precursors for C8 or C13 liquid alkane can be produced through aldol condensation between furfural and acetone [17]. NaOH and Ca(OH)2 were initially used to catalyze this aldol condensation reaction step [18–23], but these homogeneous base catalysts present several critical problems involving corrosion, impelling self-condensation, difficult recovery and high cost to neutralize generated waste water stream. In addition, poor product adaptability and uncontrollable condensation process limited the applications of these homogeneous base catalysts [24]. Therefore, it was preferable to develop stable heterogeneous base catalysts to overcome these disadvantages. O’ Neill et al. reported that natural dolomitic rocks can be applied to aldol condensation reaction, showing about 72% yield of aldol product between furfural and acetone at 150 °C after 1 h [25]. In addition, basic mixed oxides including MgAl, Mg-Zr, Ca-Zr mixed metal oxides and anion-exchange resins, were also applied to aldol condensation reactions according to previous studies [26–35]. For instance, Huber et al. reported that HMF and furfural can undergo aldol condensation with acetone catalyzed by MgZr mixed oxides in a methanol-water biphasic system and the yield of FAc and F2Ac was about 54% at 120 °C after 24 h [36]. Besides basic catalysts mentioned above, metal chlorides, like VCl3, were also used in furfural condensation with acetone in 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) solvent with 94.7% yield of aldol products (C8 and C13) [37]. However, [BMIM]Cl solvent and metal chlorides were nonrecyclable and expensive to use.

The excessive consumption of fossil fuel and the growing concern about environmental sustainability such as global warming have accelerated the development of renewable liquid fuel production based on sustainable resources. Although there was a large amount of renewable sources, biomass was the only carbon-neutral, sustainable and abundant energy resource [1–3]. Lignocellulosic biomass could be transformed via different pathways including biological, thermal and chemical processes [4]. Chemical methods were the most flexible because it was possible to utilize the lignocellulosic biomass as raw materials to produce a large variety of chemicals and materials [5–9]. Generally, polysaccharides in lignocellulose can be hydrolyzed into their constituent sugar monomers, which can be then hydrolyzed into two important furanic platform chemicals (furfural and 5-hydroxymethylfurfural) by acid catalysts [10–13]. There were two main pathways to manufacture liquid hydrocarbon from the furfurals: direct hydrodeoxygenation (HDO) of furfurals to linear C5–C6 alkane that performed poorly as fuels [14,15] and production of C7–C15 alkane with high octane number values through sequential steps involving aldol condensation, hydrogenation and HDO.[16]. Through these chemical methods, biomass feedstock can be converted into high valueadded aviation kerosene. Aldol condensation reaction was a decisive step to manufacture jet fuels from lignocellulose because low carbon molecules like furfural can ⁎

Corresponding author. E-mail address: [email protected] (M. Su).

https://doi.org/10.1016/j.fuel.2018.10.094 Received 23 August 2018; Received in revised form 11 October 2018; Accepted 16 October 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.

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(Quantachrome). SEM images for samples were determined with ZEISS GeminiSEM 500 scanning electron microscope. TEM images for samples were determined with JEM-2100F transmission electron microscope. The carbon dioxide temperature programmed desorption (CO2-TPD) for samples were measured by ChemStar TPx chemisorption analyzer (Quantachrome Instrument). The functional groups of samples were determined by Fourier transform infrared spectroscopy (FTIR) on, using the KBr disks.

In recent years, zeolites have been used to aldol condensation reactions because of their tunable catalytic sites and easy regeneration [38,39]. Yuriy et al. studied some aldehydes or ketones such as HMF, formaldehyde, ethyl pyruvate cross- or self-aldol condensation using Lewis acid zeolites involving Sn-, Hf-, Zr-Beta and the results were largely different with different zeolites [40–42]. Kikhtyanin et al. reported the catalytic performance of BEA and MWW materials in furfural-acetone aldol condensation and about 54% yield of aldol product was obtaining by MCM-22 zeolite [43,44]. Sn-MFI was also used to catalyze furfural condensation with acetone and 60% yield was achieved [45]. Although these studies highlighted the activity of zeolites in aldol condensation reactions, their resistance to water and product selectivity still need to be improved. Herein, potassium was used to promote Lewis acid zeolite, Sn-MFI, and the aldol condensation between furfural and acetone was systematically studied using this potassium-promoted Sn-MFI. The effect of potassium on this Lewis acid zeolite catalyst or aldol condensation reaction was explored. Different product selectivity was found over potassium-promoted Sn-MFI. The catalytic performance of this potassiumpromoted Sn-MFI in the existence of water in reaction system was studied. Effects of other reaction parameters and the regeneration of the catalysts were also investigated.

2.4. Catalytic reaction and analysis methods

2. Experimental

Aldol condensation reaction between furfural and acetone was performed in Synthware thick-walled pressure bottle. In a typical run, 0.24 g of furfural and 1.42 g of acetone (furfural/acetone = 1/10 by moles) were put into the reactor with 0.2 g catalyst (furfural/catalyst = 1.2 by weight). The reactor was put into an oil bath preheated at 100, 120, 140 and 160 °C under stirring for 0–1 h. After reaction, the reactor was rapidly cool down to room temperature by an ice bath. Then, the catalyst was recovered by centrifugation. Finally, the rest solution was diverted into a GC vial and the analysis was performed by Shimadzu GC-2010 equipped with FID. The catalyst performance was evaluated by furfural conversion, FAc and F2Ac yield and the selectivity to aldol products. The equation was defined as follow [36]:

2.1. Materials preparation

Furfural conversion(%) =

Acetone (AR), tin (IV) chloride pentahydrate (SnCl4·5H2O, AR), potassium nitrate (KNO3, AR), aluminum nitrate nonahydrate (Al (NO3)3·9H2O, AR) and magnesium nitrate hexahydrate (Mg (NO3)2·6H2O, AR) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Furfural (AR, 99%), tetrapropylammonium hydroxide solution (TPAOH, 40%) and tetraethyl orthosilicate (TEOS, 98%) were purchased from Aladdin Industrial, Inc. (Shanghai, China). All reagents were used without further purification.

FAc yield(%) =

F2 Ac yield (%) =

2.2. Catalyst preparation Synthesis of Sn-MFI. Sn-MFI was prepared according to previous literature [46]. TPAOH (5.5 g) was mixed with SnCl4·5H2O (0.07 g) under stirring, and then TEOS (5.2 g) was added. After stirring at room temperature for 30 min, DI water (6.33 g) was added into this mixture. A transparent homogeneous solution with a composition of 1 SiO2: 0.008 SnO2: 0.43 TPAOH: 22.2 H2O was obtained after stirring for one day at room temperature. The synthesis solution was transferred into Teflon-lined autoclave and crystal at 170 °C for 2 days. The obtained white solid product was recovered by filtration, washed with DI water several times and dried at 100 °C. Finally, the powders were calcined at 550 °C in air for 12 h. Synthesis of K-promoted Sn-MFI. K-promoted Sn-MFI was prepared by impregnation method. In details, 0.05 g KNO3 was dissolved in 15 ml DI water firstly, and then 1 g Sn-MFI was added, followed by stirring for 24 h at room temperature. The DI water in the mixture was evaporated at 80 °C and the obtained powder was dried at 100 °C overnight. Finally, the powder was calcined at 550 °C in air for 4 h. The as-prepared catalyst was denominated as nK/Sn-MFI, where n represented KNO3 loading. For instance, 0.05K/Sn-MFI denoted 5 wt% KNO3 supported on Sn-MFI. Other catalysts, such as 0.01K/Sn-MFI, 0.1K/Sn-MFI, 0.05Mg/ Sn-MFI and 0.05Al/Sn-MFI were synthesized by the same method with different KNO3 loading or different metal salts.

moles of furfural reacted × 100 moles of starting furfural

moles of FAc × 100 moles of starting furfural

moles of F2 Ac × 100 moles of starting furfural

FAc selectivity(%) =

moles of FAc × 100 moles of reacted furfural

F2 Ac selectivity (%) =

2 × moles of F2 Ac × 100 moles of reacted furfural

3. Results and discussion 3.1. Catalysts characterization In order to determine the phases and chemical compositions of these catalysts, the XRD patterns of Sn-MFI and K-promoted Sn-MFI with different K contents prepared in this study were shown in Fig. 1. Clearly, characteristic peaks according with the MFI topology were observed for all prepared catalysts, indicating that MFI zeolites were synthesized successfully, and no unassigned diffraction lines from Tin oxide was obtained for all samples (Fig. 1a) [47]. In addition, no distinct signals of other crystalline phases such as XRD-visible potassium clusters (KNO3 or potassium oxides) due to K loading were detected. This was probably ascribed to the low concentration of K loading and the high dispersion degree of K on Sn-MFI surface [48]. The XRD patterns in the 2 theta region 7–10° and 22–25° were obtained and shown in Fig. 1b and c. The corresponding peaks of Kpromoted Sn-MFI shift clearly to higher diffraction angles with increasing K loading compared with pure Sn-MFI. It may be attributed to the doping effect of K into Sn-MFI lattice or expansion of the framework. The biggest peak transition appeared in the sample doped with 5 wt% KNO3, but with further increasing K loading (10 wt% KNO3 loading) no peak shift was observed [49]. The textural properties of K-doped Sn-MFI with different K loading were characterized by N2 adsorption-desorption isotherms at 77 K and the results were shown in Table 1. Compared with pure Sn-MFI, multipoint BET surface area, micropore volume and total pore volume of all

2.3. Catalyst characterization Powder X-ray diffraction (XRD) patterns of samples were measured by a Smartlab X-ray diffractometer equipped with a Cu-Kα source. The textual properties of samples were determined by an Autosorb Q 1282

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Fig. 1. The XRD patterns of Sn-MFI and K-promoted Sn-MFI with different potassium loading: (a) 5–65°, (b) 7–10° and (c) 22–25°.

Table 1 The textural properties of K/Sn-MFI. catalysts

BET surface area (m2g−1)a

Total pore volume (cm3g−1)b

Micropore volume (cm3g−1)c

Sn-MFI 0.01K/SnMFI 0.05K/SnMFI 0.1K/Sn-MFI

385 331

0.22 0.2

0.14 0.09

246

0.16

0.08

92

0.06

0.03

a b c

Multipoint BET. Determined from the amount adsorbed at P/P0 = 0.95. Determined by the t-plot method.

K-promoted Sn-MFI decreased in different extent. Particularly, when K loading increased to 10 wt%, multipoint BET surface area, micropore volume and total pore volume rapidly decreased to 112 m2g−1, 0.04 cm3g−1 and 0.07 cm3g−1, respectively. It may be due to the block of pores by increased K species. These results indicated that some K species exist in the pores of Sn-MFI via doping. The SEM images of 0.05K/Sn-MFI were performed to investigate the morphology and shown in Fig. 2. As visualized by SEM, it was obviously noted that 0.05K/Sn-MFI catalyst consisted of agglomerated spherical crystals with a particle diameter of ca.350 nm, similar to pure Sn-MFI [46]. The elemental mapping images for K, Sn, Si and O of 0.05K/SnMFI indicated some K species exist on the outer surface of the catalyst (Fig. S2). The TEM image and the elemental mapping images for K, Sn, Si and O of 0.05K/Sn-MFI were shown in Fig. 3. From the elemental mapping image of K, it was distinctly seen that K specie was highly dispersed in the Sn-MFI. The FT-IR was also employed to clarify the structure of the K promoted Sn-MFI zeolites (Fig. 4). All of the catalysts showed vibration

Fig. 2. The SEM image of 0.05K/Sn-MFI.

bands at 800 and 1100 cm−1, corresponding to silicon-oxygen-silicon symmetric stretching (outer SiO4 tetrahedron) and silicon-oxygen-silicon assymmetric stretching (inner SiO4 tetrahedron) of a concentrated silica network, respectively. The absorption peaks at around 550 cm−1 was typically attributed to the stretching vibration of the MFI zeolite [50–52]. The FT-IR spectra of Sn-MFI and 0.05K/Sn-MFI after adsorption of pyridine were showed in Fig. 5. The spectra of two samples were acquired after exposure to pyridine vapor at room temperature and removal of physically adsorbed pyridine under vacuum. It was obviously that the peak intensity of 0.05K/Sn-MFI at around 1450 cm−1 is enhanced compared with Sn-MFI. However, the specific reason was not clear and it might be due to either the interaction between K and Sn or the ion-dipole interaction of K [53]. The CO2-TPD was performed to investigate the basic strength 1283

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Fig. 3. The TEM and element mapping images of 0.05K/Sn-MFI.

Fig. 4. The FT-IR spectra of Sn-MFI and K-promoted Sn-MFI with different potassium loading.

Fig. 5. FT-IR of Sn-MFI and 0.05K/Sn-MFI after adsorption of pyridine.

distribution as well as total basicity of the catalysts and the curve was showed in Fig. 6. The sample of pure Sn-MFI exhibited a faint CO2 desorption peak at around 150 °C, which was attributed to the inherent weak basic property of high silica MFI zeolites [54]. Compared with pure Sn-MFI, 0.01K-promoted Sn-MFI showed a stronger CO2 desorption peak at 150 °C, which represented the increasing basic strength of this weak basic site. When using 0.05K/Sn-MFI, a new CO2 desorption peak at around 550 °C appeared, which represented desorption of CO2 from the strong basic sites after higher content of potassium doping. Moreover, the CO2 desorption peak of 0.1K/Sn-MFI shifted to higher temperature compared with 0.05K/Sn-MFI, at around 170 °C and 600 °C, respectively, indicating the stronger basic sites. The quantitative data of CO2-TPD for pure Sn-MFI, 0.01K/Sn-MFI, 0.05K/Sn-MFI and 0.1K/Sn-MFI was shown in Table 2. The results clearly proved the existence of basic sites in catalysts after potassium doping. The CO2-TPD of Mg and Al-doped Sn-MFI was also performed and depicted in Fig. 7. As shown in Fig. 7, compared with 0.05K/Sn-MFI, the samples of 0.05 Mg or 0.05Al/Sn-MFI only exhibited a CO2 desorption peak at around 170 °C, which was attributed to the weak basic

sites. However, 0.05K/Sn-MFI exhibited two CO2 desorption peaks at around 150 °C and 550 °C, respectively, which was attributed to the weak basic sites and strong basic sites. These results indicated clearly that the basicity of Sn-MFI doped by magnesium and aluminum was weaker than doped by potassium. The wide XPS spectra for Sn-MFI and 0.05K/Sn-MFI sample were shown in Fig. 8A. The Sn-MFI surfaces comprise of Sn, Si and O. However, in the 0.05K/Sn-MFI, the K 2p peak could only be found in the zoomed-in spectra, which implied that K is modestly incorporated into Sn-MFI crystal lattices or the low K content in Sn-MFI structure [48]. Nevertheless, the C signal was also found in the spectra of both samples and this phenomenon might originate from the detecting instrument pollution. No N signal was seen in the 0.05K/Sn-MFI spectra, indicating the complete decomposition of NO3− after 550 °C calcination during catalyst synthesis process or the N concentration below the instrumental detection limitation. The Binding Energy of Sn 3d, O 1s and Si 2p of two samples were presented in Fig. 8B, C and D. The BE of Sn 3d, Si 2p and O 1s for 0.05K/Sn-MFI were 487.36, 103.69 and 532.93 eV, slightly higher than

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yl)-4-hydroxybutan-2-one, a C8 alcohol (FAc-OH) firstly. This intermediate was unstable and dehydrated spontaneously to form an α, βunsaturated ketone (monomer product, FAc). FAc can react with another furfural molecule by repeating above process to generate dimer product (F2Ac) (Scheme 1). Fig. 9 showed the catalytic performance of aldol condensation between furfural and acetone over Sn-MFI, 0.05K/Sn-MFI and 0.05K/MFI. Clearly, furfural was fully converted at 160 °C after only 1 h by 0.05K/ Sn-MFI, but pure Sn-MFI merely achieved 80% furfural conversion after relatively long reaction time, 6 h. The selectivity to aldol products obtained from the two catalysts was also different. Pure Sn-MFI only showed 75% selectivity to FAc due to its characteristic pore geometry. However, 0.05K/Sn-MFI not only produced FAc with the same 75% selectivity, but also formed F2Ac with 20% selectivity, and the total selectivity to aldol products reached 95%, much higher than pure SnMFI. It indicated that the catalytic performance of Sn-MFI was changed and improved with potassium doping, either catalytic activity or selectivity. In order to more clearly show the differences in selectivity of Sn-MFI with/without the presence of potassium, a plot of FAc and F2Ac selectivity versus furfural conversion over pure Sn-MFI and 0.05K/SnMFI was depicted in Table 3. As shown in Table 3, although the reaction time increased to 12 h, there were still no F2Ac formed with the furfural conversion increasing to 90% over pure Sn-MFI. However, at the almost same furfural, the selectivity to FAc and F2Ac were 77.4% and 19.3% over 0.05K/Sn-MFI, respectively, which was higher than pure Sn-MFI. This improvement can be attributed to two main reasons: the introduction of new basic catalytic sites, probable potassium oxide, which allowed the doped catalyst to own more active sites, and the enhancement of Sn catalytic site due to the interaction between Sn and K according to XPS results in pure Sn-MFI via K doping and FT-IR after adsorption of pyridine results. In addition, the new catalytic sites, K species, partly exist on the outer surface of catalysts, which makes some aldol products distribution not be influenced by the pore geometry of Sn-MFI according to previous study. It may lead to the difference in product selectivity of two catalysts. On the other hand, 0.05K/MFI showed poor catalytic performance (only 50.7% furfural conversion with 34.4% FAc yield and 3% F2Ac yield respectively) at 160 °C after 1 h compared with 0.05K/Sn-MFI, indicating the combined effect of Lewis acid and base in this K-promoted Sn-MFI catalyst.

Fig. 6. The CO2-TPD profiles of Sn-MFI and K-promoted Sn-MFI with different potassium loading. Table 2 The quantitative data of CO2-TPD for K/Sn-MFI. Catalysts

Sn-MFI 0.01K/Sn-MFI 0.05K/Sn-MFI 0.1K/Sn-MFI

TPD-CO2 Weak basic site (μmmol/g)

Strong basic site (μmmol/g)

1.5 68.8 47.7 50.3

0 0 80.8 113.2

3.2.1. Effect of potassium loading on catalytic performance of catalyst As shown in Fig. 10, potassium loading would affect the catalytic performance of K-promoted Sn-MFI for aldol condensation of furfural. The furfural conversion was only 49.6% with 36.5% FAc yield and about 1% F2Ac yield over 0.01K/Sn-MFI at 160 °C after 1 h. When the K loading increased to 5 wt%, the furfural conversion reached 100% with 75% FAc yield and 10% F2Ac yield over 0.05K/Sn-MFI. However, when the K loading increased 10 wt%, there were no obvious changes in aldol products selectivity. It meant that in the low concentration of K loading, the catalytic performance of K-doped Sn-MFI would be improved with the increasing of K loading concentration but when K loading concentration increased to a certain extent, the doping would not affect the catalytic behavior continuously in the range of potassium doping we studied. In addition, considering that furfural had been completely converted when 0.05K/Sn-MFI was used, more furfural (0.48 g) was put into reaction system with the unaltered catalysts loading (0.2 g), the same molar ratio of furfural/acetone (1/10) and the same reaction temperature (160 °C) and time (1 h). As shown in Fig. 11, 82.1% of furfural conversion was achieved with 60.8% yield of FAc and 7.8% F2Ac by using 0.05K/Sn-MFI. When using 0.1K/Sn-MFI, furfural conversion was 81% with 60% yield of FAc and 8% yield of F2Ac. The results showed there was no significant catalytic difference between 0.05K/Sn-MFI and 0.1K/Sn-MFI to this aldol condensation. According to the results of CO2-TPD, the stronger alkaline exist in 0.1K/Sn-MFI compared with 0.05K/Sn-MFI. Combining with these results, the promoting effect on catalytic behavior of Sn-MFI to aldol condensation

Fig. 7. The CO2-TPD profiles of 0.05Mg, 0.05Al and 0.05K/Sn-MFI.

487.26, 103.61 and 532.88 eV for pure Sn-MFI, respectively. It implied that the chemical environment of Sn, Si and O was changed, which was corresponding to the results of XRD. In particular, the Sn 3d BE of 0.05K/Sn-MFI was 0.1 eV higher than pure Sn-MFI, suggesting the obvious interaction between Sn and K after potassium promoting.

3.2. Catalytic aldol condensation reaction over potassium-promoted Sn-MFI It has been well known that furfural undergo aldol condensation with acetone through the formation of a transient molecule, 4-(furan-21285

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Fig. 8. The XPS images of catalysts used in the study: (a) Sn-MFI and (b) 0.05K/Sn-MFI.

Scheme 1. The reaction pathway of aldol condensation between furfural and acetone over K-promoted Sn-MFI. Table 3 The selectivity to FAc and F2Ac versus furfural conversion over pure Sn-MFI and 0.05K/Sn-MFI. catalyst

Furfural conversion (%)

Selectivity to FAc (%)

Selectivity to F2Ac (%)

Sn-MFIa 0.05K/SnMFIb

90 93

66.7 77.4

0 19.3

a Reaction condition:0.24 g furfural, 1.42 g acetone, 0.2 g catalyst, 160 °C, 12 h. b Reaction condition:0.24 g furfural, 1.42 g acetone, 0.2 g catalyst, 160 °C, 0.75 h.

between furfural and acetone after potassium doping was owing to either the new basic catalytic sites or the interaction between Sn and K, and in the range of loading of potassium doping we studied, the effect of the interaction between Sn and K might be more important than introduction of new alkaline catalytic sites to the aldol condensation between furfural and acetone catalyzed by K-promoted Sn-MFI.

Fig. 9. Furfural conversion, FAc selectivity and F2Ac selectivity for aldol condensation of furfural with acetone over Sn-MFI, 0.05K/Sn-MFI and 0.05K/MFI. Reaction condition: atemperature: 160 °C, time: 6 h, btemperature: 160 °C, time: 1 h.

3.2.2. Effect of other metal doping on catalytic performance of catalyst The catalytic performance of other metal-doped Sn-MFI (5 wt% Mg, Al) catalysts for aldol condensation reaction between furfural and acetone were also shown in Fig. 12. The furfural conversion were 56.2% 1286

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with 39% FAc yield and 1.8% F2Ac yield over 0.05 Mg/Sn-MFI and 50.8% with 37.4% FAc yield and about 1% F2Ac yield over 0.05Al/SnMFI at 160 °C after 1 h, respectively, much lower than 0.05K/Sn-MFI but slightly higher than Sn-MFI at the same reaction time. It may be attributed to the weaker basicity of magnesium and aluminum compared with potassium or weaker interaction between Sn and Mg or Al. Based on these results, in was an indication that potassium promoting would improve the catalytic performance of Sn-MFI for aldol condensation between furfural and acetone. 3.2.3. Effect of some experiment parameters on aldol condensation reaction Fig. 13 summarized the effects of some experiment parameters during aldol condensation reaction between furfural and acetone catalyzed by 0.05K/Sn-MFI. As shown in Fig. 13a, the reaction temperature was found to have a significant impact on both of furfural conversion and aldol products yield. A preliminary reaction between furfural and acetone only achieved 35.8% furfural conversion with 16.6% FAc yield and less than 1% F2Ac yield after 1 h at 100 °C. However, higher conversion and yield were obtained at higher temperatures. With the temperature increasing to 120 °C and 140 °C, the furfural conversion reached 57.8% with 40.6% FAc yield and 5% F2Ac yield and 81.1% with 61.2% FAc yield and 8.3% F2Ac yield, respectively. When temperature was increased to 160 °C, the aldol products yield was 75% for FAc and 10% for F2Ac at full conversion of furfural. As shown in Fig. 13b, both of furfural conversion and aldol products yield were discovered to increase with increasing reaction time. After 15 min the 62.1% FAc yield and 5% F2Ac yield were achieved at 77.2% furfural conversion. Upon increasing the reaction temperature from 30 min to 45 min, the furfural conversion increased from 86.4% to 93% and the FAc yield as well as F2Ac yield increased from 70% to 72% and 7% to 9%, respectively. The results showed the reaction reached a satisfactory aldol products yield (75% for FAc and 10% for F2Ac) and 100% furfural conversion after 60 min. Fig. 13c showed the effect of catalyst loading. The yield of aldol products was found to increase by increasing the loading of 0.05K/Sn-MFI from 0.05 g to 0.2 g. Using 0.1 g catalyst, the reaction produced 31.5% FAc and 3% F2Ac yield at 160 °C after 1 h. when the catalyst loading was increased to 0.2 g, higher furfural conversion (100%) and higher FAc as well as F2Ac yield (75% and 10%, respectively) were recorded. Under the same conditions, the reaction with lower catalyst loading (0.05 g) gave lower furfural conversion (19.3%) and FAc as well as F2Ac yield (9.7% and 0.2%). Fig. 13d indicated the molar ratio of furfural and acetone would significantly affect the selectivity to products. When the furfural/acetone was increased to 1:5, the furfural conversion decreased to 90.7%. However, the selectivity to F2Ac increased to 43% obviously with the selectivity to FAc decreasing to 62.5%. The results indicated lower molar ratio of furfural/acetone favored generation of F2Ac, which could be used to control the selectivity in aldol products, but lead to furfural conversion decreasing.

Fig. 10. Effect of different potassium loading on the catalytic performance of Kpromoted Sn-MFI for aldol condensation between furfural and acetone. Reaction condition: temperature: 160 °C, time: 1 h.

Fig. 11. The effect of potassium loading. Reaction condition: 0.48 g furfural, 2.84 g acetone, 0.2 g catalysts, 160 °C, 1 h.

3.2.4. Effect of water in reaction system It was reported that the present of water would affect significantly the catalytic behavior of acid or base solid catalysts [36,55]. Therefore, the effect of water on aldol condensation between furfural and acetone over this Lewis-base pair catalyst was studied by adding an amount of water into reaction system. Based on our previous study [45], water had an obviously negative impact on the catalytic performance of pure Lewis acid zeolite, Sn-MFI, during aldol condensation reaction, either furfural conversion or aldol products selectivity. However, Lewis-base pair catalyst, K-promoted Sn-MFI, performed excellent water resistance compared with pure Sn-MFI. As shown in Fig. 14, when water content was increased from 0 wt% to 50 wt%, although the furfural conversion still decreased from 49.6% to 38.4% by 0.01K/Sn-MFI, the reduction degree was less than pure Sn-MFI. The selectivity to FAc was also found to decrease from 73.5% to 59.8%, but interestingly, the selectivity to F2Ac was increased from 4% to 10%. This tendency of selectivity

Fig. 12. Effect of different metal doping on catalytic performance of Sn-MFI for aldol condensation between furfural and acetone. Raction condition: temperature: 160 °C, time: 1 h.

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Fig. 13. Effect of experiment parameters on aldol condensation between furfural and acetone over 0.05K/Sn-MFI: (a) effect of reaction temperature, reaction condition: time: 1 h. (b) Effect of reaction time, reaction condition: temperature: 160 °C. (c) Effect of catalyst loading, reaction condition: temperature: 160 °C, time: 1 h. (d) Effect of the molar ratio of furfural/acetone, reaction condition: temperature: 160 °C, time: 1 h.

was considered to have a positive impact on basic catalytic sites for aldol condensation between furfural and acetone caused by lowering the energy barrier of carbon-hydrogen bond cleavage of acetone under interaction between water and base catalysts [56]. In addition, this Lewis-base pair catalyst, K-promoted Sn-MFI, preferred to produce F2Ac in the existence of water. Although the reason of this tendency was not clear, the changes of selectivity to aldol products in the present of water catalyzed by K-promoted Sn-MFI provided an additional method to control the distribution of aldol products between monomer and dimer. 3.2.5. Recyclability study The recyclability of 0.05K/Sn-MFI was also investigated. Fig. 15

Fig. 14. Effect of water in reaction system over K-promoted Sn-MFI on aldol condensation between furfural and acetone. Reaction condition: atemperature: 160 °C, time: 6 h, btemperature: 160 °C, time: 1h.

changes was corresponding to others study [36]. When 0.05K/Sn-MFI was used, the furfural conversion was still fully converted and the selectivity to FAc and F2Ac was decreased from 75% to 69% and increased from 20% to 24.6% respectively, in the same condition of water content increasing from 0% to 50%. The total selectivity to aldol products was still 93.6%, sightly lower than total selectivity without water. The results suggested the passive effect of water on catalytic behavior of pure Sn-MFI for aldol condensation of furfural and acetone was significantly offset by potassium doping. This improvement of water resistance of this catalyst was mainly owing to the introduction of basic sites to pure Lewis acid zeolite via potassium promoted because water

Fig. 15. The reusability study of 0.05K/Sn-MFI. Reaction condition: temperature: 160 °C, time: 1 h. 1288

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Acknowledgements This study was financially supported by the Transformational Technologies for Clean Energy and Demonstration, Strategic Priority Research Program of the Chinese Academy of Sciences, Grant No. XDA 21060101, the National Key Technology R&D Program of China (No. 2015BAD15B06) and the Science and Technological Fund of Anhui Province for Outstanding Youth (1508085J01). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2018.10.094. References [1] Stöcker M. Biofuels and biomass-to-liquid fuels in the biorefinery: catalytic conversion of lignocellulosic biomass using porous material. Angew Chem Int Ed 2008;47(48):9200–11. [2] Zinoviev S, Müller-Langer F, Das P, Bertero N, Fornasiero P, Kaltschmitt M, et al. Next-generation biofuels: survey of emerging technologies and sustainability issues. ChemSusChem 2010;3:1106–33. [3] Kubička D, Kubičková I, Čejka J. Application of molecular sieves in transformations of biomass and biomass-derived feedstocks. Catal Rev-Sci Eng 2013;55(1):1–78. [4] Huber GW, Corma A. Synergies between bio-and oil refineries for the production of fuels from biomass. Angew Chem Int Ed 2007;46:7184–201. [5] Sun Y, Cheng J. Hydrolysis of lignocellulosic materials for ethanol production: a review. Bioresour Technol 2002;83(1):1–11. [6] Shi N, Liu Q, Zhang Q, Wang T, Ma L. High yield production of 5-hydroxymethylfurfural from cellulose by high concentration of sulfates in biphasic system. Green Chem 2013;15(7):1967–74. [7] Liao Y, Liu Q, Wang T, Long J, Ma L, Zhang Q. Zirconium phosphate combined with Ru/C as a highly efficient catalyst for the direct transformation of cellulose to C 6 alditols. Green Chem 2014;16(6):3305–12. [8] Zhang X, Zhang Q, Wang T, Ma L, Yu Y, Chen L. Hydrodeoxygenation of ligninderived phenolic compounds to hydrocarbons over Ni/SiO2–ZrO2 catalysts. Bioresour Technol 2013;134:73–80. [9] Liu Y, Chen L, Wang T, Zhang Q, Wang C, Yan J, et al. One-pot catalytic conversion of raw lignocellulosic biomass into gasoline alkanes and chemicals over LiTaMoO6 and Ru/C in aqueous phosphoric acid. ACS Sustainable Chem Eng 2015;3(8):1745–55. [10] Bohre A, Saha B, Abu-Omar MM. Catalytic upgrading of 5-hydroxymethylfurfural to drop-in biofuels by solid base and bifunctional metal-acid catalysts. ChemSusChem 2015;8(23):4022–9. [11] Wang S, Zhao Y, Lin H, Chen J, Zhu L, Luo Z. Conversion of C5 carbohydrates into furfural catalyzed by a Lewis acidic ionic liquid in renewable γ-valerolactone. Green Chem 2017;19:3869–79. [12] Lin H, Xiong Q, Zhao Y, Chen J, Wang S. Conversion of carbohydrates into 5-hydroxymethylfurfural in a green reaction system of CO2-water-isopropanol. AIChE J 2017;63(1):257–65. [13] Lin H, Chen J, Zhao Y, Wang S. Conversion of C5 carbohydrates into furfural catalyzed by SO3H-functionalized ionic liquid in renewable γ-valerolactone. Energy Fuels 2017;31(4):3929–34. [14] de Beeck BO, Dusselier M, Geboers J, Holsbeek J, Morré E, Oswald S, et al. Direct catalytic conversion of cellulose to liquid straight-chain alkanes. Energy Environ Sci 2015;8(1):230–40. [15] Seemala B, Cai CM, Wyman CE, Christopher P. Support induced control of surface composition in Cu–Ni/TiO2 catalysts enables high yield co-conversion of HMF and furfural to methylated furans. ACS Catal 2017;7:4070–82. [16] Bohre A, Dutta S, Saha B, Abu-Omar MM. Upgrading furfurals to drop-in biofuels: an overview. ACS Sustainable Chem Eng 2015;3(7):1263–77. [17] Huber GW, Chheda JN, Barrett CJ, Dumesic JA. Production of liquid alkanes by aqueous-phase processing of biomass-derived carbohydrates. Science 2005;308(5727):1446–50. [18] Zapata PA, Faria J, Ruiz MP, Resasco DE. Condensation/hydrogenation of biomassderived oxygenates in water/oil emulsions stabilized by nanohybrid catalysts. Top Catal 2012;55(1–2):38–52. [19] Fakhfakh N, Cognet P, Cabassud M, Lucchese Y, Días de Los Ríos M. Stoichio-kinetic modeling and optimization of chemical synthesis: application to the aldolic condensation of furfural on acetone. Chem Eng Process 2008;47(3):349–62. [20] Ramirez-Barria C, Guerrero-Ruiz A, Castillejos-López E, Rodriguez-Ramos I, Durand J, Volkman J, et al. Surface properties of amphiphilic carbon nanotubes and study of their applicability as basic catalysts. RSC Adv 2016;6(59):54293–8. [21] Gutsche CD, Redmore D, Buriks RS, Nowotny K, Grassner H, Armbruster CW. Basecatalyzed triose condensations. J Am Chem Soc 1967;89(5):1235–45. [22] West RM, Liu ZY, Peter M, Gärtner CA, Dumesic JA. Carbon–carbon bond formation for biomass-derived furfurals and ketones by aldol condensation in a biphasic system. J Mol Catal A: Chem 2008;296(1):18–27. [23] Xing R, Subrahmanyam AV, Olcay H, Qi W, van Walsum GP, Pendse H, et al. Production of jet and diesel fuel range alkanes from waste hemicellulose-derived aqueous solutions. Green Chem 2010;12(11):1933–46.

Fig. 16. The XRD patterns of the spent 0.05K/Sn-MFI before/after calcination.

summarized the recycle research performed with 0.05K/Sn-MFI. After the first run of reaction, trial was made to recycle the spent catalyst through exhaustively washed with acetone and dried at 100 °C. The color of as-treated spent catalyst was faint pale, suggesting tiny humins possibly formed and the furfural conversion and aldol products yield decreased using this spent catalyst. The furfural conversion was 90% with 68% FAc yield and 6% F2Ac yield. The results indicated the catalyst cannot be regenerated through merely washing with acetone. Regeneration of catalyst was achieved partly via calcination at 550oC in air after washing by acetone and drying. The XRD characterization of the spent catalyst before/after calcination was depicted in Fig. 16. As shown in Fig. 16, the intensity of the corresponding peaks in spent 0.05K/Sn-MFI before calcination decreased compared with fresh 0.05K/Sn-MFI, which could be attributed to the formation of humins in the catalyst during reaction and corresponded to the experimental phenomenon. After calcination, the XRD image of the spent 0.05K/SnMFI was similar to the fresh 0.05K/Sn-MFI. The changes of the XRD characterization of spent catalyst before/after calcination could partly explain the differences of the catalytic activity of spent catalyst before/ after calcination. The furfural conversion decreased continuously from 100% to 95.4% and aldol products yield decreased from 75% to 71.8% for FAc and from 10% to 8.8% for F2Ac respectively, after five runs, compared with fresh catalyst. This decrease was reasonably considered to potassium leaching during catalytic reaction or treatment of spent catalyst process. Through ICP measure, K ions were present in the reaction solution and the concentration of K ions was 1.459 μg/ml. The existence of K ions in the reaction solution was a demonstration of the decrease in catalytic performance due to the loss of potassium. However, the results of recyclability were acceptable.

4. Conclusions The Lewis acid-base zeolite catalyst, K-promoted Sn-MFI, performed excellent activity for aldol condensation reaction between furfural and acetone. This may be due to the introduction of new basic sites and the interaction between K and Sn. Furfural was fully converted after only 1 h by K-promoted Sn-MFI, much faster than Sn-MFI. The selectivity was also changed via potassium doping. Both of FAc and F2Ac were formed by K-promoted Sn-MFI from aldol condensation reaction between furfural and acetone, while pure Sn-MFI only produced FAc. It was found that the negative impact of water on Sn-MFI for aldol condensation of furfural with acetone was offset by potassium doping in this reaction system. In addition, K-promoted Sn-MFI had a tendency to produce F2Ac in the present of water. 1289

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