A comprehensive investigation of the condensation of furanic platform molecules to C14–C15 fuel precursors over sulfonic acid functionalized silica supports

Mahlet N. Gebresillase , Raghavendra Shavi and Jeong Gil Seo *
Department of Energy Science Technology, Myongji University, Myongji-ro 116, Nam-dong, Cheoin-gu, Yongin-si, Gyeonggi-do 449-728, South Korea. E-mail: jgseo@mju.ac.kr

Received 22nd June 2018 , Accepted 25th September 2018

First published on 25th September 2018


Furfural has excellent potential for the production of versatile furanic platform molecules that can be upgraded to high carbon fuel precursors. 2-Methylfuran is one of the most important derivatives of furfural. The hydrogenation of furfural to 2-methylfuran results in the formation of by-products (n-butanal and 2-pentanone). Implementing all the primary and by-products from the hydrogenation reaction for the production of high carbon fuel precursors allows the complete utilization of the lignocellulosic xylose derived furfural. In this work, a self-condensation reaction of 2-methylfuran and its cross-condensation reactions with furfural, n-butanal and 2-pentanone have been implemented to produce C14 and C15 fuel precursors. Sulfonic acid based catalysts with and without alkyl chain linkers supported on silica nanoparticles (NP), MCM-41, SBA-15, and KCC-1 were synthesized to evaluate the effect of catalyst morphology on activity and selectivity. The correlation between the different support systems and corresponding activity was studied using SEM, TEM, BET, FTIR, and TGA before and after the reaction. Among the synthesized catalysts, sulfonic acid-functionalized KCC-1 (KCC-1SO3H) and 3-((3-(trimethoxysilyl) propyl) thio) propane-1-oxy-sulfonic acid-functionalized (KCC-1APSO3H) showed higher conversion and selectivity for the self-condensation and cross-condensation reactions, respectively. The effects of various parameters on the activity and selectivity, such as the reaction time and temperature, were studied. The catalysts have substantial hydrolytic stability in the presence of water and retain their acidity over multiple reaction cycles. The low cost, high activity, and pronounced stability of these fibrous nano silica-based catalysts indicate a promising future application in the biorefinery industries.


Introduction

Lignocellulosic biomass holds enormous potential for the production of renewable fuels and chemicals to provide a cleaner and sustainable energy.1–3 Although it is the most promising source of renewable fuels, its upgrading process to biofuels is complicated and expensive. This upgrading process uses two different approaches. The first one aims at the utilization of the total lignocellulosic biomass as in gasification and pyrolysis. The second method targets the production of valuable sugar molecules, which are consumed to produce the desired carbon range fuels.4,5

In recent years, considerable emphasis has been given to the transformation of sugar derived intermediates into higher carbon fuel precursors. The C–C coupling reaction of the platform chemicals is either through self-coupling or through cross-coupling.6–8 The hydrolysis and dehydration of xylose contained in lignocellulosic biomass produce furfural (FUR). FUR has received attention as a potential platform for the production of biofuels and biochemicals.9–11 One of the derivative chemicals of FUR is 2-methylfuran (2-MF). 2-MF is produced by selective hydrogenation of FUR via hydrogenolysis of the C–OH bond.12 2-MF can involve in hydroxyalkylation/alkylation with ketones and aldehydes13 or undergo trimerization itself to form a C15 trimer.2,14

The nucleophilic aromatic ring of 2-MF shows remarkable chemical properties. 2-MF can undergo selective electrophilic aromatic substitution in the presence of an electron-poor group (in this case, carbonyl compounds).14,15 Thus, in the trimerization reaction (hereafter referred to as the self-condensation reaction), the first step is the formation of an intermediate open-ring aldehyde in the presence of water. The aldehyde (4-oxopentanal) then acts as an electrophile for the reaction with the remaining 2 moles of 2-MF. The unreactive methyl group blocks one of the two reactive positions of the furan ring. This phenomenon represses further polymerization. The production of high carbon fuel precursors by the self-condensation of 2-MF over mineral acid catalysts was first reported by Corma et al.7 In response to the problems related to the use of homogeneous catalysts, Yati et al.16 studied the use of Amberlyst-15 (commercial acidic resin) as a catalyst. According to their findings, the use of water was necessary for the ring opening process in the production of 5,5-bis(5-methylfuran-2-yl) pentan-2-one (1a, Scheme 1). In their work, they did not achieve 100% selectivity to trimer even though they studied the relationship between the use of water and the selectivity to trimer. To the best of our knowledge, up to now, no studies were conducted on non-commercial catalysts for the synthesis of 1a, thus marking the need to explore more catalysts.


image file: c8gc01953c-s1.tif
Scheme 1 The reaction pathways for the self-condensation and cross-condensation reactions of 2-MF.

Another method for the production of C15 fuel precursors is the use of FUR and its hydrogenation by-products.17 The direct hydrogenation of FUR to 2-MF has been studied on Group VIIIB elements and Cu based catalysts.18 The Group VIIIB elements studied for this reaction, such as Pt, Ru, Pd, and Ni, interact actively with the furan ring, therefore attacking furfural through decarbonylation to furan, ring hydrogenation to tetrahydrofuran, and ring-opening products. In contrast, Cu-based catalysts are less oxophilic and interact weakly with the furan ring. As a result, they primarily enable selective hydrogenation of the carbonyl of furfural to form furfuryl alcohol (FOL), followed to a lesser extent by hydrodeoxygenation to 2-MF.19 FOL is an intermediate in the production of 2-MF, which cannot be directly utilized for the cross-condensation reaction. While a remarkable conversion on Cu-containing catalysts has been achieved, none of the Cu-based catalysts reported to date could produce 2-MF with competitive selectivity.18,20 Thus applying all the side products for the synthesis of high carbon fuel precursors plays a significant role in solving the problems associated with selectivity.14

For post-grafting, a suitable support material is critical, as it will influence the activity and performance of organo–silica hybrid materials. So, synthesizing a suitable support material for grafting is indispensable. NP, MCM-41, and SBA-15 supported sulfonic acid catalysts can play an essential role since these catalysts are simple to synthesize, are economically viable, have high activity and are environmentally friendly.21 Nonetheless, their surface accessibility is limited and is a problem for the condensation reactions. KCC-1, a new fibrous silica nanosphere, has been reported by Polshettiwar et al.22 The fibrous nature of this material increases the accessibility of active sites drastically. Although some individual applications of these support materials have been studied, research studies on how the silica types affect the synthesis and performance of catalysts for fuel precursor production are scarce. In general, the restrained conversion, yield, and selectivity related to the above reactions show opportunities to develop other efficient, reusable acid catalysts for the targeted furan condensation reactions.

In this work, self-condensation and cross-condensation reactions of 2-MF have been studied to produce C14 and C15 fuel precursors over sulfonic acid functionalized silica catalysts. Our objective was to develop catalysts with higher selectivity to 1a, thus avoiding the formation of 2,4,4-tris(5-methylfuran-2-yl) pentan-1-ol (1b, Scheme 1) in the case of self-condensation. As for the cross-condensation reactions, increased interaction with the hydrophobic reactants and stability over the in situ formed water to enhance the formation of C15 fuel precursors were the targets. The reaction time and temperature were optimized to achieve higher conversion of both 2-MF and co-reactants. The activity and selectivity of the synthesized catalysts were compared with different commercial homogeneous and heterogeneous catalysts.

Experimental section

Materials

2-MF (99%, stabilized), FUR (99%), n-butanal (99%) and 2-pentanone (99%, pure) were purchased from Across Organics, USA. 3-(Mercaptopropyl) trimethoxysilane (95%), cetyltrimethylammonium bromide (CTAB) (≥99%), silicon(IV) oxide powder (NP, 500 nm, 99.9%), 3-chloro-1-propanol (98%), and tetraethoxysilane (98%) were obtained from Sigma-Aldrich, USA. All HPLC-grade solvents, such as dichloromethane (≥99%), diethyl ether (≥99%), ethyl acetate (≥99%), methanol (≥99.9%), 1-propanol (99%, pure), cyclohexane (≥99%), and toluene (≥99.5%), were acquired from Across Organics, USA. Silica gel 60, 0.032–0.063 mm (230–450 mesh), was purchased from Alfa Aesar. Sulfuric acid and hydrochloric acid (36.5%) were procured from DaeJung Chemicals, South Korea, while chlorosulfonic acid and para-toluene sulfonic acid were purchased from Sigma-Aldrich, USA. The solid acid catalysts, Amberlyst-15 (4.7 meq g−1), Amberlyst-36 and Nafion-212, were obtained from Sigma-Aldrich, USA. A Nafion film (thickness: 51 μm), which was used in this work, was cut into about 2 × 5 mm pieces. All the purchased materials were used without further purification.

Catalyst synthesis and characterization

Synthesis of KCC-1. KCC-1 was synthesized based on a published procedure with some modifications.23,24 In the synthesis of KCC-1, cetyltrimethylammonium bromide (CTAB 0.75 g, 0.0020 mol) and urea (0.23 g, 0.0038 mol) were dissolved in deionized water (75 mL). The solution was stirred for 1 h until all of the CTAB dissolved. Then a solution of tetraethoxysilane (TEOS, 3.75 g, 0.018 mol), cyclohexane (75 mL), and pentanol (5 mL) was added dropwise to the above solution of CTAB and urea in water. The mixture was stirred at room temperature for another hour and transferred into a 200 mL Teflon lined hydrothermal reactor. The reactor was then placed in an oven at 120 °C for 6 h. The mixture was then allowed to cool to room temperature, and the silica gets isolated by centrifugation (30 min, 6000 rpm). After thoroughly washing the isolated solid with deionized water and ethanol, it was dried for 12 h at 40 °C. The as-synthesized silica was calcined at 550 °C with 5 °C ramping for 6 h in air.
Synthesis of MCM-41. A method previously developed by Raji et al. was used to synthesize MCM-41 with some modifications.25 2 g of CTAB was dissolved in 50 g of deionized water. A solution of 16.8 g of an ammonia solution (25 wt% in water) and 50 mL of ethanol was added. The solution was stirred for 15 min followed by the dropwise addition of 4.0 g of TEOS. The resulting solution was stirred for 2 h at 40 °C temperature. Then a solid product was obtained by filtration, which was then washed with deionized water and ethanol, dried in an oven at 80 °C for 8 h, and calcined at 550 °C at a heating rate of 1 °C min−1 for 4 h to remove the CTAB.
Synthesis of SBA-15. SBA-15 was synthesized using a method previously reported by Manukyan et al. with some modifications.26 A total of 4 g of pluronic P123 triblock copolymer was dissolved in 120 mL of a 2 M HCl aqueous solution under stirring at room temperature in a glass bottle. 30 mL of deionized water and 8.5 g of tetraethyl orthosilicate were added consecutively. Then the solution was placed in an oil bath for 18 h under stirring at 50 °C. When the color of the solution changed from transparent to milky white, the solution was heated at 150 °C for 24 h. The resulting materials were washed with water and ethanol followed by drying at 80 °C for 8 h. The template was removed by heating at 550 °C at 1 °C min−1 heating rate for 6 h under continuous air flow.
Synthesis of silica-bound SO3H. Silica-bound SO3H was synthesized by modifying a previously published procedure.23 silica (NP, MCM-41, SBA-15 or KCC-1) (2 g) was dispersed in dichloromethane (20 mL). Chlorosulfonic acid (0.016 mol) dissolved in dichloromethane (10 mL) was added dropwise to the suspension under stirring for 1 h at 0 °C. The mixture was then kept at room temperature under stirring for an additional hour. The solid acid catalyst was then collected by filtration and washed with diethyl ether (3 × 20 mL). The obtained solid acid catalyst was dried at room temperature for 12 h.
Synthesis of silica-bound PSO3H. Silica (NP, MCM-41, SBA-15 or KCC-1) (2 g) was activated by refluxing with HCl (50%) for 12 h. The obtained silica was then washed with deionized water and dried at 100 °C for 6 h. The activated silica was refluxed in a solution of 3-(mercaptopropyl) trimethoxysilane (3.5 g, 0.018 mol) and dry toluene (40 mL) for 18 h to obtain the surface-bound mercaptopropyl silica (MPS silica). The resulting solid was then filtered and washed with hot toluene and dried in air for 8 h. The thiol (SH) group was converted into the –SO3H group by oxidation with 30% H2O2 at 50 °C for 24 h. The obtained solid was then stirred in 0.2 M H2SO4 (20 mL) for 2 h at room temperature. The solid collected by filtration was dried at room temperature.
Synthesis of silica-bound APSO3H. Mercaptopropyl silica (MPS silica) (2 g) was stirred in toluene (10 mL) followed by the addition of 3-chloro-1-propanol (0.12 g, 0.0012 mol). The mixture was then refluxed for 18 h. The obtained solution was filtered and washed with ethanol (10 × 3 mL) and dried at 100 °C for 4 h. A solid powder, 3-(3-(silicapropylthio)-1-propanol, was obtained. It was then dispersed in chloroform (8 mL), to which chlorosulfonic acid (0.6 mL) was added dropwise at 0 °C for 2 h. The mixture was then stirred for 2 h. The solid was collected by filtering and washing with ethanol (3 × 10 mL). Silica-bound APSO3H was obtained after drying at room temperature.

Catalyst characterization

The specific BET surface areas of the bare silica and the catalysts were measured by N2 adsorption–desorption using a BELSORP-miniII instrument (Bel Japan Inc., Japan) at 77 K. Preheat treatment, to remove physically adsorbed water and impurities, was conducted at 100 °C for 4 h in a vacuum. The morphology and particle size were studied by field emission-scanning electron microscopy (FE-SEM) and by field emission-transmission electron microscopy (FE-TEM) imaging. Images were recorded on a Helios 650 scanning electron microscope and a JEM-2100F (JEOL) microscope equipped with energy-dispersive X-ray (EDX) elemental mapping, respectively. Fourier transform infrared (FTIR) spectroscopy, using a Varian 2000 Fourier transforms infrared spectrophotometer, was used to record IR spectra with KBr pellets. The spectrometer was operated in the transmittance mode at a resolution of 4 cm−1. Acid–base titration was used to determine the H+ ion concentration of the catalysts. A NaOH solution (20 mL, 0.1 M) was added to the catalyst (100 mg) in an Erlenmeyer flask and stirred for 30 min. The addition of a HCl solution (0.1 M) neutralized the excess amount of base to the equivalence point of the titration.

Thermogravimetric analysis (TGA), using a TGA N 1000 (SCINCO) thermogravimetric analyzer, was used to study the thermal stability and degradation patterns of the catalysts. Elemental analysis was performed using a Flash EA 1112 elemental analyzer. The X-ray diffraction (XRD) patterns of fresh and recycled catalysts were recorded with a 2θ value of 5–80° by D-Max 2500-PC (Rigaku) measurements using Cu-Kα radiation (k = 1.541 Å) operated at 50 kV and 150 mA.

General procedure for the condensation reactions

Self-condensation of 2-MF. Self-condensation of 2-MF was carried out in a 100 mL round bottom flask. 2-MF (10 g, 0.12 mol), water (10 wt%), and catalyst (3 wt%) were mixed at room temperature. The mixture was added to the round bottom flask, which was then connected to a reflux condenser. The reaction temperature was set at 85 °C and controlled using an oil bath. The mixture was refluxed for 48 h. The catalyst was separated by filtration. Column chromatography was conducted using silica gel and a mixture of dichloromethane and hexane (95[thin space (1/6-em)]:[thin space (1/6-em)]5) to separate the products. The obtained products were concentrated by evaporating the solvents. The separated liquid product was then analyzed by 1H and 13C NMR and Waters high-performance liquid chromatograph (HPLC) that was equipped with a ZORBAX SB-C18 column (4.6 mm × 150 mm × 5 μm) and a refractive index detector for identification and quantification. A mixture of methanol and water (7[thin space (1/6-em)]:[thin space (1/6-em)]3) was used as a mobile phase.
Cross-condensation reactions. In the cross-condensation reaction, 2-MF (3.69 g, 0.045 mol) and FUR (1.93 g, 0.022 mol), 2-MF (5 g, 0.060 mol) and n-butanal (2.2 g, 0.030 mol), and 2-MF (5 g, 0.060 mol) and 2-pentanone (2.6 g, 0.030 mol), and catalyst (3 wt%) were mixed individually at room temperature. The individual mixtures in three separate round bottom flasks were connected to reflux condensers and oil baths at 70 °C, 50 °C and 85 °C correspondingly. The reactions were conducted for 2 h, 4 h, and 6 h, respectively. After the reaction was completed, the catalysts were recovered by filtration. The obtained liquid reaction mixture was analyzed by NMR (1H and 13C NMR on a Bruker Advanced II + 400 MHz instrument operated at a rate of 400 MHz with a spin rate of 13 kHz). The products were quantified using a Waters high-performance liquid chromatograph (HPLC) that was equipped with a ZORBAX SB-C18 column (4.6 mm × 150 mm × 5 μm) and a refractive index detector for identification and quantification. A mixture of methanol and water (9[thin space (1/6-em)]:[thin space (1/6-em)]1) was used as a mobile phase.

Results and discussion

Catalyst synthesis and characterization

KCC-1SO3H, KCC-1PSO3H, and KCC-1APSO3H were synthesized according to Scheme 2 and utilized for self-condensation and cross-condensation reactions. Transmission electron microscopy (TEM) was used to observe the structural features of KCC-1 and KCC-1 supported catalysts. The observation of TEM images of the material revealed the existence of dendrimeric fibers arranged to form uniform spheres (Fig. 1a–d). It was also possible to confirm that the morphology remains unchanged after functionalization. Scanning electron microscopy (SEM) was performed for obtaining details of the morphology and size of the KCC-1 nanospheres showed uniform spheres of ∼400 nm diameter. The SEM images for the functionalized catalysts (Fig. 1e–h) revealed that the fibrous structure of KCC-1 was maintained. In the same way, the morphologies of bare and functionalized MCM-41, SBA-15, and NP (Fig. S1) were studied. No significant changes were observed because of the functionalization.
image file: c8gc01953c-s2.tif
Scheme 2 Schematic representation of the synthesis of silica-bound SO3H, silica-bound PSO3H and silica-bound APSO3H.

image file: c8gc01953c-f1.tif
Fig. 1 TEM images of KCC-1, KCC-1SO3H, KCC-1PSO3H and KCC-1APSO3H (1a–1d), and SEM images KCC-1, KCC-1SO3H, KCC-1PSO3H and KCC-1APSO3H (1e–1h).

The surface areas and pore size distributions of the bare silica and the corresponding catalysts were obtained from N2 adsorption–desorption isotherms by using the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively. The obtained values are summarized in Table 1, including titration results. From the elemental analysis, it was possible to acquire the sulfur weight % of the KCC-1 based catalysts. The obtained values for KCC-1SO3H, KCC-1PSO3H, and KCC-1APSO3H (3.83 wt%, 2.86 wt%, and 5.52 wt%) show similar trends to the acid amount calculated from the titration value. Although KCC-1APSO3H has the lowest acid amount compared to the others, the highest sulfur amount was observed. The highest sulfur weight % is a result of the presence of the second sulfur atom in the functionalizing group (Table S1). As expected, the surface area and the pore volume of the catalysts are lower than those of bare supports. From Table 1, a decrease in the BET surface area and in the pore size after functionalization indicated that the employed synthesis method allowed the organo-sulfonic acid group to be incorporated both into and on the mesoporous structure and surface at the same time. For the synthesis of C14 and C15 fuel precursors, larger pore volume and pore diameter are critical.27 Although MCM-41 (1005 m2 g−1) has a much larger surface area than any of the other supports, the smaller pore diameter easily leads to pore blockage during functionalization, which resulted in drastic surface area and pore volume changes. KCC-1SO3H, KCC-1PSO3H, and KCC-1APSO3H showed the least percent changes in surface area and pore volume (28%, 33%, 58% and 32%, 41%, 53%, respectively) as compared to other silica supports (Fig. S2). The functionalization of the support was also confirmed by FT-IR spectroscopy (Fig. S3). The thermal stability of KCC-1 was verified by conducting a thermogravimetric analysis (Fig. S4) in the temperature range of 25–800 °C under nitrogen conditions with 20 °C ramping. The thermal stabilities of MCM-41, SBA-15, NP and their corresponding catalysts were also studied (Fig. S5).

Table 1 Physicochemical properties of the synthesised silica supports and the corresponding catalysts
  S BET (m2 g−1) V total (cm3 g−1) Acid amountb (mmol g−1) TON TOF (h−1)
Rxn1 Rxn2 Rxn1 Rxn2
a From the N2 adsorption–desorption isotherm by using the Brunauer–Emmett–Teller (BET) method. b Calculated from titration. Rxn1 – Self-condensation (10 g 2-MF and 3 wt% catalyst and 10 wt% water were condensed at 85 °C. To obtain <50% conversion the reaction was conducted for 24 h). Rxn2 – Cross-condensation 3.7 g 2-MF, 1.93 g FUR, and 3 wt% catalyst were condensed for 2 h at 70 °C. To obtain <50% conversion the reaction was conducted for 0.25 h). TON – Calculated from the moles of the target product and the acid amount obtained from titration. TOF (h−1) – Calculated from the TON and reaction time.
NP 36 0.05
MCM-41 1005 0.68
SBA-15 727 0.99
KCC-1 540 1.73
MCM-41SO3H 433 0.28 1.30 61 22 2.5 87
SBA-15SO3H 440 0.39 1.37 72 28 2.9 111
KCC-1SO 3 H 385 1.17 1.51 110 35 4.5 141
MCM-41PSO3H 265 0.21 0.99 62 37 2.6 150
SBA-15PSO3H 290 0.31 1.09 69 43 2.8 173
KCC-1PSO3H 345 0.95 1.39 100 58 4.1 234
MCM-41APSO3H 204 0.19 0.92 60 47 2.5 190
SBA-15APSO3H 214 0.29 0.98 67 64 2.7 255
KCC-1APSO 3 H 225 0.81 1.15 93 88 3.8 349


Self-condensation reaction

The reaction was conducted at a temperature of 85 °C for 48 h.

The first step in the self-condensation of 2-MF is the hydrolysis (ring-opening) reaction of 2-MF in the presence of H2O. As mentioned in previous studies2,16 the production of tetramer is suppressed by the addition of water as a promoter, hence leaving the α-carbocation during the protonation to be active, allowing it to undergo ring-opening to produce the intermediate aldehyde 4-oxopentanal.16,28,29 The intermediate aldehyde undergoes alkylation with the remaining two moles of 2-MF to form the C15 trimer 1a. From the analysis of the liquid product using NMR and HPLC (Fig. S6–S8), it was possible to identify 1a as the primary product. The products of 1a and 1b after HDO are C15 and C20 alkanes, respectively. Owing to the ease of the HDO process and carbon yield of diesel range fuel, it is preferential to produce C15 oxygenates selectively rather than the C20 oxygenates.16

The reaction was conducted over different solid acid catalysts (Fig. 2). Both Amberlyst-15 and Amberlyst-36 are active for this reaction but have lower conversion compared to Nafion-212. The higher activity of Nafion-212 over the Amberlyst resins is credited to its structure. Amberlyst resins are sulfonic acid functionalized cross-linked polystyrenes while Nafion-212 is perfluorinated sulfonic acid; thus the presence of fluorine increases the acidic strength of the –SO3H groups.10,28,30 Although Nafion-212 exhibited higher conversion of 2-MF, the selectivity to 1a was very limited. The superhydrophobic nature suppresses α-carbocation promoted by water for ring-opening, thus leaving β-carbocation active during the protonation. The β-carbocation pathway results in high selectivity towards 1b. The effect of water was confirmed by varying the amount of water in the reaction from 0 to 12 wt% (Fig. S9). The conversion of 2-MF decreased with the increase in the amount of water while a slight increase was observed in the selectivity to 1a. In contrast, Amberlyst-15 resins showed improved selectivity with the addition of water due to the surface wettability nature. 84% selectivity to 1a was achieved with 10 wt% water over Amberlyst-15. A further increase in the amount of water showed a slight increase in selectivity but resulted in reduced conversion due to the hindered accessibility of the active sites by the water molecules.


image file: c8gc01953c-f2.tif
Fig. 2 Results of the self-condensation reaction of 2-MF over various catalysts (reaction time = 48 h). 2-MF (10 g) and catalyst (3 wt%) were condensed with the addition of water (10 wt%) at 85 °C.

The catalysts, without the alkyl chain, show lower conversion and the highest selectivity among the synthesized catalysts supported on KCC-1. The low conversion can be explained by the low solubility of the catalysts with the hydrophobic reactant. The selectivity to the formation of 1a on KCC-1 based catalysts follows the reverse trend of hydrophobicity, KCC-1SO3H > KCC-1PSO3H > KCC-1APSO3H.

Having established that the catalyst with no alkyl chain possesses higher selectivity, we proceeded to further study the effect of the surface area and morphology of the support material on the activity. NP, MCM-41, and SBA-15 functionalized with the same functional groups were synthesized and used to compare with KCC-1SO3H. The surface areas of MCM-41SO3H (433 m2 g−1) and SBA-15SO3H (440 m2 g−1) supported catalysts were higher than those of KCC-1SO3H (385 m2 g−1). NP-SO3H has the lowest surface area and the smallest pore volume (15 m2 g−1 and 0.03 cm3 g−1, respectively). The effect of the pore volume can also be seen by comparing MCM-41SO3H and SBA-15SO3H (Fig. 3). For MCM-41 and SBA-15, the pore blocking impact impedes the diffusion of –SO3H toward the centre of the pore of MCM-41 and SBA-15, leaving the hydroxyl groups located inside the pores not functionalized. These limitations resulted in lower acid density compared to the fibrous KCC-1 support (Table 1). After functionalization, the pores of MCM-41 and SBA-15 were reduced to 0.28 cm3 g−1 and 0.39 cm3 g−1 from their initial values of 0.68 cm3 g−1 and 0.99 cm3 g−1, respectively.


image file: c8gc01953c-f3.tif
Fig. 3 TON and pore volume over different support systems (TON was calculated from the yield after 24 h reaction. 2-MF (10 g) and catalyst (3 wt%) were condensed with the addition of water (10 wt%) at 85 °C).

The conversion and selectivity results over different support systems after the complete reaction time of 48 h are also compared. 28% conversion of 2-MF and 81% selectivity to the target trimer were obtained on MCM-41SO3H, while SBA-15SO3H gave a conversion of 49% and 91% selectivity to the target trimer (Fig. S11). In MCM-41SO3H and SBA-15SO3H, the decrease in selectivity could be a result of further polymerization of the reactant and reaction intermediates since large molecules cannot readily diffuse out from the long cylindrical pores. NPSO3H showed limited activity (5.5%) and selectivity (77%) for this reaction because the size of the pores limits the mass transfer and cannot accommodate the trimer of 2-MF.

To estimate the longevity of the catalyst systems, we calculated the TONs and TOFs of 2-MF to 1a over the different commercial and synthesized catalysts according to the carbon yields of 1a and the amounts of –SO3H groups on the surfaces of these catalysts which were measured by chemical titration (Fig. 3). By lowering the reaction time to 24 h, we were able to use the yields below 50% conversion to calculate the TON and TOF values of each catalyst system. The results (Fig. 3) show that KCC-1SO3H has the highest TON. This is mainly due to the substantial pore volume and fibrous nature of KCC-1SO3H; thus, the active site accessibility and diffusion of both reactants and products increased. The selectivity to the target trimer in KCC-1SO3H was increased by eliminating the unintended product deposition inside the pores and further polymerization, which in turn increased the yield of the target trimer. The TON and TOF values of KCC-1SO3H were also higher compared to some commercial catalysts (Fig. S11).

The time-course study of the reaction summarized in Table 2 provided more information about the catalytic activity of KCC-1SO3H. A sample was taken at the analysis time from the reaction system, filtered and analyzed by HPLC. Between the times of 3 h and 48 h, a significant increase in conversion of 2-MF (18–60%) and a slight change in selectivity to trimer 1a (99.1–100%) were observed. The recyclability and stability of the catalyst systems were further evaluated in the self-condensation reaction for Amberlyst-15 and the synthesized catalysts. To avoid disturbance of the physically adsorbed reactants, after filtration, the catalyst was washed (3×) with ethanol and dried at 80 °C under vacuum for 6 h. Significant catalyst deactivation was observed for Amberlyst-15, MCM-41SO3H, SBA-15SO3H, and NPSO3H just after 2 cycles. KCC-1SO3H was found to be recyclable for 4 cycles (Fig. 4).


image file: c8gc01953c-f4.tif
Fig. 4 Self-condensation of 2-MF (10 g) over the recycled catalyst KCC-1SO3H (3 wt%). Reaction conditions: 48 h at 85 °C.
Table 2 Conversion of 2-MF and selectivity to 1a and 1b at various reaction times and catalyst loadings
T (°C) Catalyst (wt%) Reaction time (h) Conversion Selectivity (%)
2-MF (%) 1a
Reaction: 2-MF (10 g) and catalyst (3 wt%) with the addition of water (10 wt%).
85 1 48 35.7 100
85 2 48 46.2 100
85 3 48 60.0 100
85 4 48 60.5 100
85 3 3 18.0 100
85 3 12 25.0 100
85 3 24 41.0 100
85 3 36 48.1 100
85 3 48 60.0 100


In the case of KCC-1SO3H, moderate deactivation was observed after the 4th cycle. The values of conversion and selectivity after 4 cycles on the different silica supports show the same pattern (Fig. S12). The delay in deactivation, despite similar reaction conditions for KCC-1SO3H, can be ascribed to the increased activity and selectivity towards 1a. The selective production of 1a suppresses the formation of oligomers that result in the poisoning.31 X-Ray diffraction (XRD) patterns of fresh and recycled catalyst samples were recorded (Fig. S13a). The characteristic peak of silica was observed at 2θ = 22.6 for KCC-1SO3H. In the case of recycled KCC-1SO3H, a decrease in the intensity and in the additional peaks were found. The characteristic peaks found in the XRD spectrum of the recycled catalyst are like the characteristic peak of pregraphite-like carbon, suggesting that it has a certain degree of crystallization. These were also supported by the TGA patterns of the recycled catalysts with an increased weight loss at around 200–400 °C and 400–700 °C. The deposition was observed to be severe in the case of self-condensation reaction due to high temperature, long reaction time and the presence of water.32 The EA shows an increased carbon composition of 10% on the surface of the catalyst after the 4th cycle.

From the TGA analysis of the deactivated catalysts, it was possible to see that a significant amount of carbon deposition was formed (Fig. S14). The commercial catalyst Amberlyst-15 also showed a significant weight loss difference after recycling (Fig. S14a). The weight loss can be the result of inhibition by water, poisoning from a long reaction time that results in the formation of oligomers and hydrolysis of the sulfonic groups in the case of Amberlyst-15 and NPSO3H since both these catalysts have active sites on the surface.31 In MCM-41SO3H and SBA-15SO3H, the deactivation is probably because the condensation products are large molecules that cannot readily diffuse out from the long cylindrical pores of MCM-41 and SBA-15, leading to a severe polymerization of the reactant and reaction intermediates. Further confirmations were done by N2 adsorption–desorption analysis and SEM imaging. As shown in Table S2, the BET surface area of the deactivated catalysts reduced drastically, except in the case of KCC-1SO3H, due to the significant amount of carbon deposition, which may block the pores of the MCM-41 and SBA-15 support. We, therefore, suspect that the carbon deposition may be the main reason for catalyst deactivation. The SEM images for the fresh Amberlyst-15 catalyst exhibited a smooth surface; in contrast, many cracks were observed on the surface of the recovered catalyst at low magnification (Fig. S15a). A change was also seen in the morphologies of NPSO3H, MCM-41SO3H, and SBA-15SO3H (Fig. S15b–d). Despite the moderate deactivation, the fibrous structure of KCC-1SO3H remained intact as observed from the SEM image (Fig. S15e).

The major shortcoming to commercially implement direct hydrogenation of FUR to 2-MF is related to the selectivity since unwanted reactions, leading to side products, are unavoidable. These reactions include the decarbonylation of FUR to furan, which subsequently yields C4 products (e.g., n-butanal) via ring opening and further hydrogenation of 2-MF to 2-pentanone as shown in Scheme 3.33–36 Utilizing these side products together with 2-MF will maximize the possible pathways for the synthesis of high carbon fuel precursors.


image file: c8gc01953c-s3.tif
Scheme 3 Possible reaction pathways for the hydrogenation of furfural.

Reaction with aldehydes

The cross-condensation of 2-MF and FUR was conducted over a series of commercial and synthesized solid acid catalysts and a homogeneous catalyst, para-toluene sulfonic acid (p-TOSH). 2a was identified as the primary product of the NMR spectra and HPLC chromatogram (Fig. S16 and 17). Among the commercial acid catalysts, p-TOSH and Nafion-212 showed high conversion (83% and 72%, 2-MF, and 97% and 82%, FUR, respectively) with increased selectivity (98% and 93%) to 2a owing to the homogeneity of p-TOSH with the reactants (Fig. 5). Sulfonic acid resins, Amberlyst-15 and Amberlyst-36, were also active for this reaction. The reactivity follows p-TOSH > Nafion-212 > Amberlyst-36 > Amberlyst-15. The effect of the reactant molar ratio was studied using Amberlyst-15 (Fig. S18). At a stoichiometry ratio of 2-MF and FUR (2[thin space (1/6-em)]:[thin space (1/6-em)]1), 60% conversion of 2-MF and 71% conversion of FUR were obtained. With an increase in the moles of 2-MF to 2.5[thin space (1/6-em)]:[thin space (1/6-em)]1, the conversion of both reactants was enhanced to 80% and 94%. A further increase in the molar ratio to 2.75[thin space (1/6-em)]:[thin space (1/6-em)]1 resulted in the decrease of 2-MF conversion to 67% while the conversion of furfural increased to 100% and remained constant for the 3[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio. The decrease in the conversion of 2-MF is related to the slow kinetics of the self-condensation reaction related to the excess 2-MF and the in situ water.
image file: c8gc01953c-f5.tif
Fig. 5 Results of the cross-condensation reaction of 2-MF and FUR over various catalysts. 2-MF (3.7 g), FUR (1.93 g) and catalyst (3 wt% by mass of 2-MF) were condensed for 2 h at 70 °C.

All the fibrous silica supported catalysts showed higher conversion and selectivity. The activity sequence of these catalysts is found to be KCC-1APSO3H > KCC-1PSO3H > KCC-1SO3H. Although KCC-1SO3H possesses the highest surface area and acid density, catalysts with alkyl chains were found to have proficient activity. This might be because catalysts with alkyl chains pose reduced steric hindrance, increased hydrophobicity, and activity. The alkyl chain is also known to provide better solubility of the sulfonic acid in the hydrophobic reactants. The higher activity of KCC-1APSO3H over KCC-1PSO3H can be credited to the synergistic effect of increased hydrophobicity due to extended alkyl chain and increased acidic strength due to the electronic effect of additional S and O group.30,37 KCC-1SO3H showed the lowest selectivity to 2a.

The surface wettability and low hydrophobicity of KCC-1SO3H allowed the formation of 1a by the hydrolysis of 2-MF with the in situ formed water creating favourable conditions for the self-condensation.

Here again, NP, MCM-41 and SBA-15 supported sulfonic acid groups were used to study the effect of the surface area and morphology of the supporting material on cross-condensation reactions (Fig. S19). Compared to the extended, two-dimensional mass-transport limiting channels of SBA-15 and MCM-41, the unique structure of KCC-1 provides improved accessibility of the active site. For long-chain fuel precursor products, it is difficult to diffuse out of the 2D channels of SBA-15APSO3H and MCM-41APSO3H despite their mesoporous structure, and the restriction effect will be magnified with the increased bulkiness of the functional group from –SO3H to –APSO3H. The change in pore size of NP-SO3H was not as significant as the other supports indicating that most of the functionalization took place at the surface rather than in the pores. On MCM-41APSO3H, 63% conversion of FUR with 87% selectivity to 2a was achieved, which was much higher when compared to the values of NP-SO3H but significantly lower than that of SBA-15APSO3H.

The effect of pore volume on both the acid density and performance of the catalysts was shown by comparing KCC-1APSO3H, SBA-15APSO3H, and MCM-41APSO3H (Fig. 6). It is possible to see the decrease in the TON value with the decrease in pore volume, indicating the significance of sufficient reaction space. The TON and TOF were calculated according to the carbon yields of 2a and the amounts of –SO3H groups measured by chemical titration. Here also the reaction time was lowered to 0.25 h to give below 50% conversion from which the TON and TOF values of each catalyst system were calculated. From the results, it is possible to see the superior catalytic efficiency of KCC-1APSO3H in comparison with the other catalysts. The TON and TOF values of KCC-1APSO3H were also higher compared to some commercial catalysts (Fig. S20).


image file: c8gc01953c-f6.tif
Fig. 6 TON and pore volume over different support systems. (TON was calculated from the yield after 0.25 h reaction. 2-MF (3.7 g), FUR (1.93 g) and catalyst (5 wt% by mass of 2-MF) were condensed for 2 h at 70 °C.)

The influence of the reaction time on the conversion of both 2-MF and FUR and selectivity to 2a were investigated and summarized in Table 3. High conversions of 2-MF and FUR were observed in a short period for KCC-1APSO3H, confirming the increased activity due to the alkyl linker between the silica and the –SO3H group. The conversion of FUR remained the same after 2 h of reaction. However, a slight increase in the conversion of 2-MF was observed corresponding to the formation of 1a, thus decreasing the selectivity towards 2a. The performance of KCC-1APSO3H was further studied at different temperatures to find the optimum temperature for the highest conversion of reactants and selectivity. The conversion of 2-MF increased with the temperature. At higher temperature, the selectivity to 2a decreased due to the formation of 1a since the self-condensation reaction favours high temperature.

Table 3 Conversion of 2-MF and FUR and selectivity to 2a and 1a at different reaction temperatures, times and catalyst loadings
T (°C) Catalyst (wt%) Reaction time (h) Conversion Selectivity (%)
FUR (%) 2a 1a
Reaction: 2-MF (3.7 g), FUR (1.93 g) and catalyst (3 wt%).
70 1 2 70 99.2
70 2 2 90 99.5
70 3 2 100 100
70 4 2 100 100
70 5 2 100 100
70 3 0.5 80 97
70 3 1 98 98
70 3 1.5 100 100
70 3 2 100 100
70 3 3 100 98 1.3
50 3 2 77 97
60 3 2 90 98
70 3 2 100 100
80 3 2 100 96 2.1
90 3 2 100 93 4.8


The other significant properties of KCC-1APSO3H are stability and recyclability. These properties were evaluated by using XRD, SEM, and TGA. Four cycles of reactions were performed with the recycled KCC-1APSO3H (Fig. 7). The same recovering and washing steps were used. No substantial decrease in the conversion of FUR or 2-MF or selectivity of 2a was observed as the effect of hydrolysis due to the in situ formed water on KCC-1APSO3H was insignificant. The presence of the alkyl chain allows the end group in KCC-1APSO3H to be the less likely leaving group compared to KCC-1SO3H, thus avoiding significant leaching (Table S1). The TGA analysis confirms carbon deposition because of further polymerization in the temperature range of 300–700 °C in all support systems except KCC-1APSO3H (Fig. S14b). Consistent with the reaction data, no change was observed in the morphology of the KCC-1APSO3H while surface deposition was observed for other supports (Fig. S22). No additional peaks were found in the XRD spectrum of KCC-1APSO3H (Fig. S13b) taken at the end of the 4th cycle. EA showed a slight increase in carbon content from 1.69% to 2.18%. The recyclability trend of the different silica supports also showed that KCC-1APSO3H has the maximum conversion and selectivity values after 4 cycles (Fig. S21).


image file: c8gc01953c-f7.tif
Fig. 7 Results of the cross-condensation reaction of 2-MF and FUR over recycled KCC-1APSO3H. 2-MF (3.7 g), FUR (1.93 g) and catalyst (3 wt% by mass of 2-MF) were condensed for 2 h at 70 °C.

The other aldehyde side product from the hydrogenation of FUR to 2-MF is n-butanal (Scheme 3). Corma et al. have demonstrated a technique for producing long-chain hydrocarbons from 2-MF and butanal with high activity.14 From the reactions of 2-MF and butanal conducted at a ratio of 2[thin space (1/6-em)]:[thin space (1/6-em)]1 with p-TOSH and Amberlyst-15, conversions of 85% and 72% were obtained, respectively, with a selectivity of 91% and 82% after 6 h and 22 h, respectively, at 50 °C. However, the drawback of this approach is that to improve the yield, increasing the ratio of the reactants to 3[thin space (1/6-em)]:[thin space (1/6-em)]1 and using a longer reaction time were necessary.14 Dutta et al. also reported that Improved Graphene Oxide demonstrates high activity for the coupling of 2-MF with butanal, enabling a maximum yield of 83% at 60 °C for 6 h.38 Different sulfonated resins, zeolites, and Nafion based catalysts were used for this reaction.

The highest conversion and selectivity were obtained by using Nafion-212 at 50 °C for 4 h. Although a conversion of 96% and a selectivity of 89% were achieved, the price of Nafion-212 hinders its application on a larger scale.39 Here KCC-1APSO3H is used as a cost-effective, highly active and selective catalyst at a stoichiometric ratio. The cross-condensation reaction with n-butanal was conducted at a temperature of 50 °C for 4 h. 5,5′-(Butane-1,1-diyl) bis(2-methylfuran) (3a, Scheme 1) was identified as the primary product of the NMR spectra and HPLC chromatogram (Fig. S23 and 24). The results of the reaction on commercial homogeneous and heterogeneous catalysts are summarized in Table 4. 100% conversion of n-butanal with 94% selectivity to the target fuel precursor was obtained on KCC-1SO3H, once more signifying the importance of the fibrous structure in KCC-1.

Table 4 Conversion of n-butanal and 2-pentanone and selectivity to the target trimer
T (°C) Catalyst Reaction time (h) Conversion of n-butanal Selectivity (%)
50a p-TOSH 4 95 87
50a Amberlyst-15 4 64 73
50a Amberlyst-36 4 66 75
50a Nafion-212 4 91 89
50 KCC-1APSO 3 H 4 100 94

T (°C) Catalyst Reaction time (h) Conversion of 2-pentanone Selectivity (%)
a 2-MF (5 g) and n-butanal (2.2 g) at 50 °C for 4 h. b 2-MF (5 g) and 2-pentanone (2.6 g) at 85 °C for 6 h.
85b 24% H2SO4 6 73 78
85b Amberlyst-15 6 51 68
85b Amberlyst-36 6 54 70
85b Nafion-212 6 70 81
85 KCC-1APSO 3 H 6 82 89


Here also, in comparison with the NP, MCM-41, and SBA-15 supported catalysts of the same functional groups, KCC-1APSO3H shows maximum activity and selectivity (Fig. 8). The recyclability and stability of catalyst systems were evaluated by separating the catalysts by filtration, washing (3×) with ethanol and drying at 80 °C under vacuum for 6 h. KCC-1APSO3H was found to be recyclable for 4 cycles (Fig. 9). Compared to the reaction with FUR, the recyclability efficiency of KCC-1APSO3H has slightly decreased for the reaction with n-butanal, which may be the result of extended exposure to in situ water. The recyclability of NPAPSO3H, MCM-41APSO3H, and SBA-15APSO3H was also studied (Fig. S25). The recyclability trend follows KCC-1APSO3H > SBA-15APSO3H > MCM-41 APSO3H > NPAPSO3H. The recyclability trend shows decreased activity for the silica supports where the reaction takes place in the pores. This is the result of in situ polymerization and carbon deposition in the pores even after washing several times, which was confirmed by post reaction TGA analysis (Fig. S14c).


image file: c8gc01953c-f8.tif
Fig. 8 Results of the cross-condensation reaction of 2-MF and n-butanal using NP, MCM-41, SBA-15 and KCC-1 supported catalysts. 2-MF (5 g), n-butanal (2.2 g) and catalyst (3 wt%) were condensed for 4 h at 50 °C.

image file: c8gc01953c-f9.tif
Fig. 9 Results of the cross-condensation reaction of 2-MF and n-butanal over recycled KCC-1APSO3H. 2-MF (5 g), n-butanal (2.2 g) and catalyst (3 wt%) were condensed for 4 h at 50 °C.

From the TGA analysis, it can be clearly seen that the severity of deactivation increases with the decrease in pore volume from SBA-15APSO3H to MCM-41APSO3H. The post reaction changes in morphology were studied by SEM imaging (Fig. S26).

No significant change was observed in the morphology of KCC-1APSO3H while the most significant change was observed over NPAPSO3H. The SEM images of NPAPSO3H from the reaction with FUR and n-butanal clearly show that with the extension of the reaction time, the damage to the catalyst becomes more significant.

Reaction with ketones

For this study, depending on the results of the cross-condensation reaction of 2-MF with FUR and n-butanal, we chose KCC-1APSO3H as a catalyst to study the reaction with 2-pentanone. The temperature was increased to 85 °C for the reaction of 2-MF and 2-pentanone for 6 h. Compared to the reactions of 2-MF with aldehydes, this reaction is less active. FUR and n-butanal are more reactive towards nucleophilic substitutions than 2-pentanone because of both the steric hindrance and electronic effects.40,41 The reaction of 2-MF and 2-pentanone (2.5[thin space (1/6-em)]:[thin space (1/6-em)]1) was reported by Corma et al. over p-TOSH at 100 °C for 22 h with a conversion of 63% and a selectivity to 4a of 76%.14 When H2SO4 (11.4 wt%) was used as a catalyst, 71% conversion and 55% selectivity were obtained. Even though the C15 fuel precursor was produced from this reaction, the use of mineral acid and selectivity to the desired product are still issues.

Unlike previous reports, the catalyst used in this work has shown higher catalytic activity with high selectivity to 5,5′-(pentane-2,2-diyl) bis(2-methylfuran) (4a, Scheme 1).

The superior activity and endurance to the effect of in situ generated water are the result of the hydrophobic alkyl chain. The results from the reaction conducted at 85 °C for the reaction of 2-MF and 2-pentanone for 6 h on different commercial catalysts and KCC-1APSO3H are summarized in Table 4. 4a was identified as the primary product of the NMR spectra and HPLC chromatograms (Fig. S28–29). It is possible to see that the selectivity to 4a was very low when Amberlyst-15 was used as a catalyst. This can be explained by the formation of more 1a in the presence of in situ water. This can be explained by the surface wettability of Amberlyst-15 and the high reaction temperature, which led to the self-condensation of 2-MF. Although p-TOSH had high activity and selectivity among the compared commercial catalysts, it was still lower than that of KCC-1APSO3H. 82% conversion of 2-pentanone with 89% selectivity to the target fuel precursor was obtained on KCC-1APSO3H. After applying NPAPSO3H, MCM-41APSO3H, SBA-15APSO3H, and KCC-1APSO3H to the reaction with 2-pentanone, we were able to confirm that the reactivity follows the same trend as that of their pore size, accessibility of the active site and acid density (Fig. 10). Here also, the recyclability and stability of catalyst systems were evaluated by separating the catalysts by filtration, washing (3×) with ethanol and drying at 80 °C under vacuum for 6 h.


image file: c8gc01953c-f10.tif
Fig. 10 Results of the cross-condensation reaction of 2-MF and 2-pentanone using NP, MCM-41, SBA-15 and KCC-1 supported catalysts. 2-MF (5 g), 2-pentanone (2.6 g) and catalyst (3 wt%) were condensed for 6 h at 85 °C.

The recyclability test was conducted for 4 cycles on KCC-1APSO3H (Fig. 11). The conversion and selectivity after 4 cycles on KCC-1APSO3H show a slight decrease from 82% and 89% to 80% and 86%. This can be ascribed to the difficulty of the reaction with a ketone, the extended reaction time and the higher reaction temperature, resulting in minor catalyst deactivation, which may be the result of extended exposure to in situ water. A pronounced activity loss was observed for the non-fibrous silica supports, leading to the conclusion that pore blockage and product polymerization are evident with small pore long-channel catalysts (Fig. S29). The increased weight loss associated with polymerization and carbon deposition after recycling can be seen from the post reaction TGA analysis (Fig. S14d). The weight loss observed after recycling NP-APSO3H, MCM-41APSO3H, SBA-15APSO3H, and KCC-1APSO3H is higher than that seen in the reaction of 2-MF with FUR. This can be attributed to the lower activity of 2-pentanone compared to aldehydes, lower selectivity to the target trimer and extended reaction time, leading to unwanted polymerization. After four cycles, KCC-1APSO3H still maintained the same morphology (Fig. S30).


image file: c8gc01953c-f11.tif
Fig. 11 Results of the cross-condensation reaction of 2-MF and 2-pentanone over recycled KCC-1APSO3H. 2-MF (5 g), 2-pentanone (2.6 g) and catalyst (3 wt%) were condensed for 6 h at 85 °C.

Conclusion

In summary, we have demonstrated the synthesis of NP, MCM-41, SBA-15, and KCC-1 supported sulfonic acid-based catalysts by the post-grafting method and their application in the condensation of lignocellulosic furfural and its hydrogenation products to fuel precursors. 2-methylfuran is the primary product of the hydrogenation reaction; it was used in both the self-condensation and cross-condensation reactions. From the synthesized catalysts KCC-1SO3H showed excellent conversion and selectivity to C15 trimer (1a) in the self-condensation reaction of 2-MF. The activity of KCC-1SO3H was higher than that of the other silica supports due to its high acid density and active site accessibility. The commercial catalysts Amberlyst-15 and Nafion-212 were found to be active with lower conversion and selectivity in comparison with the KCC-1 supported catalysts. KCC-1APSO3H presented the highest activity for all the cross-condensation reactions. Although all the silica supported APSO3H catalysts showed activity for the cross-condensation reactions, KCC-1APSO3H was superior. Complete utilization of all the hydrogenation products was possible over the KCC-1APSO3H catalyst. The activity can be attributed to the synergistic effect of the high acidity through the additional electronegative atoms and increased solubility of the sulfonic functionality in hydrophobic reactants due to the alkyl linker. The excellent performance of the KCC-1supported catalysts compared to NP, MCM-41 and SBA-15 supported catalysts was a result of the unique fibrous morphology of KCC-1 and the accessibility of the active sites, which were retained even after functionalization. The results showed the role of the morphology of the silica support, along with the advantages of the fibrous morphology of KCC-1 compared to that of porous materials, in the development of efficient catalysts for fuel precursor production. Also, the ease of synthesis and recyclability trend of these catalysts show a promising future for large-scale application.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Energy Efficiency & Resources Program (no. 20163010092210) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry & Energy. This work was also supported by Nano-Material Fundamental Technology Development (2016M3A7B4909370) through National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning

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Footnote

Electronic supplementary information (ESI) available: Detailed information about the characterization, textural properties and elemental analysis of catalysts; the effect of the support and reactant mole ratio, HPLC chromatograms of the liquid products and NMR spectra of the products. See DOI: 10.1039/c8gc01953c

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