Chuanhui Li,Yuanzhong Li,Xiaoxiang Luo,Zhengyi Li,Heng Zhang,Hu Li,Song Yang

State Key Laboratory Breeding Base of Green Pesticide & Agricultural Bioengineering,Key Laboratory of Green Pesticide & Agricultural Bioengineering,Ministry of Education,State-Local Joint Laboratory for Comprehensive Utilization of Biomass,Center for R&D of Fine Chemicals,Guizhou University,Guiyang 550025,China

Keywords: Biomass Diesel Catalysis Biofuranic compound Acetylation-alkylation

ABSTRACT Lignocellulosic biomass is a promising feedstock for the synthesis of value-added chemicals and biofuels.However,one of the biggest challenges for producing high-quality diesel fuels is the lack of sufficient carbon-chain length in biomass derivatives.In this study,a C17 diesel precursor 1,1,1-tris(5-methyl-2-f uryl)ethane (TEMF) with a yield of ca.70% was synthesized from the cascade acetylation-hydroxyalkyla tion/alkylation of bio-based 2-methylfuran (MF) with acetic anhydride (AA) catalyzed by acid-treated montmorillonite with enhanced acidity and improved porosity.The catalytic mechanism of the cascade reaction process was investigated over different types of acid species(Br?nsted acid and Lewis acid),and the influence of in situ formed acetic acid was also examined.A synergistic effect was observed to enable the synthesis of TEMF from the trimerization of MF with AA,in which Lewis acid and weak Br?nsted acid species mainly catalyze the acetylation and hydroxyalkylation processes,while the subsequent alkylation step is mainly catalyzed by strong Br?nsted acid.

1.Introduction

With the depletion of fossil fuels and the deterioration of the ecological environment,modern society urgently needs to develop renewable energy resources [1,2].Biomass accounts for about 70%of total renewable energy like biomass energy,solar energy,geothermal energy,wind energy,hydrogen energy,and ocean energy [1,3].The utilization of non-food lignocellulosic biomass has received increasing attention due to its natural abundance,low cost,and sustainability.

In nature,lignocellulosic biomass mostly is produced through photosynthesis,which primarily contains cellulose,hemicellulose,and lignin[4].Lignocellulose biomass is the most abundant renewable resource and can be converted into biofuels and value-added chemicals,such as phenolic compounds,pentose,and hexose sugars that can be used in the production of aromatic and furanic products [5,6].One of the most important applications of the biomass derivatives is used as carbon resources for the production of biofuelsviahydrogenation or hydrodeoxygenation reaction [7].However,direct hydrodeoxygenation of sugar derivatives can only provide C5–C6candidates,which cannot meet the requirements of traditional petroleum-based fuels such as diesel (C10–C25) and jet fuel (C9–C13) [8].Therefore,the lengthening of the carbon chain is a major challenge for producing high-quality fuels from biomass derivatives.

Much effort has been focused on aldol condensation of biomass derivatives(e.g.,5-hydroxymethylfurfural or furfural with acetone or cyclopentanone) to produce bio-based diesels,and a base catalyst was found to be efficient for the overall reaction process(Fig.1a) [9].Catalytic self-coupling of furfural,5-methylfurfural,or other relevant bio-based aromatic aldehydes with ionic liquid[10],N-heterocyclic carbene [11]or metal (e.g.,Al,Mg,Mn) [12]as a catalyst was also developed to be efficient for increasing carbon-chain to directly produce high-quality fuel precursors(Fig.1b).In addition,hydroxyalkylation/alkylation of bio-based 2-methylfuran (MF) and furfural coupled with hydrogenation and deoxygenation is a promising way to synthesize high-quality diesel or jet fuel[13](Fig.1c).MF can be produced from furfural and has hydrophobic property,which can be separated from water at room temperature,and is highly reactive and selective toward hydroxyalkylation/alkylation reaction with carbonyl compounds over both homogeneous and heterogeneous acid catalysts (e.g.,H2SO4,para-toluenesulfonic acid (p-TosOH),ion exchange resins(Amberlyst-15,Nafion-212),lignin resins,zeolites,niobium phosphate,and sulfonic acid functionalized silica,ionic liquid,and graphene oxide) [4,14,15].However,these reaction processes with aldehydes or ketones only allow the dimerization of MF,which somehow restricts the carbon-chain length of the resulting fuel precursors.In addition,most solid acids suffer from high cost,difficulty in reusability,and complex synthesis procedures.Therefore,the facile and appropriate design of a solid acid catalyst with good catalytic activity and stability for the upgrading of biomass derivatives to desired high-quality diesel fuels is still of great challenge.

Fig.1.Synthetic approaches developed for producing bio-based diesel precursors.

Montmorillonite is a kind of silicate mineral,which has a low price,abundant reserve,unique layered structure,large specific surface area,and certain cation exchange capacity with unique acidity[16–18].Inspired by this,montmorillonite K-10 was hereby modified by hydrochloric acid solutions in different concentrations for one-pot cascade acetylation-hydroxyalkylation/alkylation of MF with acetic anhydride (AA),envisioning to obtain pronounced reactivity in trimerization of MF to furnish production of a C17diesel fuel precursor 1,1,1-tris(5-methyl-2-furyl)ethane (TEMF)(Fig.1d).The synergistic effects of Lewis and Br?nsted acid sites on the acetylation-alkylation multi-step conversion processes were investigated,and the catalyst recycling ability was also studied.

2.Methods

2.1.Materials

Montmorillonite K-10,acetic anhydride (AA,>99%),2-methylfuran (MF,99%),5-methyl-2-acetylfuran (MAF,>98%),dichloromethane (CH2Cl2,99.9%),n-hexane (99%),acetone (>99%),acetic acid (CH3COOH,≥99%) and naphthalene (99.6%) were bought from Beijing Innochem Sci.& Tech.Amberlyst-15 (hydrogen ion form,wet),AlCl3·6H2O (99%),silica (SiO2,0.5 mm particle size,pore size ca.2 nm),and γ-Al2O3were purchased from Sigma-Aldrich,Inc.

2.2.Catalyst preparation

The catalyst was preparedviaa simple treatment of K-10 with different concentrations of hydrochloric acid aqueous solution (1,2,3 mol·L-1).In a general synthetic procedure,5.0 g of K-10 was magnetically stirred in 50 ml of hydrochloric acid aqueous solution at 50 °C for 24 h.After the reaction was cooled to room temperature,the suspension was centrifuged and the precipitate was repeatedly washed with deionized water until the pH of the supernatant was neutral.Finally,the slurry was transferred to an oven and dried at 80°C for 12 h.The K-10 treated with 1,2,and 3 mol·L-1of hydrochloric acid aqueous solution was denoted as K-10-1,K-10-2,and K-10-3,respectively.

2.3.Catalyst characterization

The X-ray diffraction(XRD)analysis of K-10,K-10-1,K-10-2 and K-10-3 was carried out on a D/max-TTR III X-ray powder diffractometer (Rigaku International Corp.,Tokyo) using Cu Kα radiation source in the 2θ range of 5°-80°.The catalyst structure was determined on the NICOLET iS50 FT-IR spectrometer with a spectral resolution of 4 cm-1in the wavenumber range of 400–4000 cm-1.Nitrogen adsorption–desorption isotherms were recorded on a Micromeritics Tristar II 3020 at 77 K.Acidity of the samples was determined by ammonia-temperature programmed desorption(NH3-TPD) on AutoChem 2920 chemisorption analyzer,and were also analyzed by pyridine adsorption FT-IR spectroscopy in Bruker VERTEX V70v system.The inductively coupled plasma emission spectrometer (ICP-OES) was used to determine the content of silicon,aluminum,and magnesium with PerkinElmer’s Optima 5300 DV instrument.X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo VG scientific ESCA MultiLab-2000 spectrometer with a monochromatized Al Kα source(1486.6 eV).The thermal stability of the catalyst was analyzed by thermogravimetric analysis (TGA) using a TA Instruments Discovery TGA5500 over the temperature range of 20 °C to 700 °C.

2.4.Reaction procedures

In a general reaction procedure,1 mmol MF,1.5 mmol AA,0.05 g catalyst were added into an ACE pressure tube(15 ml).Then,the pressure tube was placed into an oil bath at 40°C with a magnetic stirrer(400 r·min-1)for 6 h.Upon completion,0.01 g of naphthalene used as the internal standard was added to the reaction mixture,followed by the addition of 5 ml CH2Cl2as diluent.Subsequently,the diluted reaction mixture solution was filtered through a 0.22 μm filter membrane and the obtained filtrate was directly sent for gas chromatography (GC) analysis.

2.5.Catalyst recycling study

After each cycle of reaction,the reaction mixture was cooled to room temperature,and 5 ml of methylene chloride was added,followed by centrifugal separation to obtain a brown solid catalyst,which was washed three times with acetone (3×5 ml) and dried at 80 °C for 6 h.The recovered catalyst was directly for the next run.

2.6.Product analysis

The structure of the product and intermediates was determined by gas chromatography-mass spectrometry(GC–MS:Agilent 6890-5973) with a HP-5MS capillary column.Substrate conversion and product yield were analyzed by GC (Agilent 6890B,FID,HP-5 capillary column).The equations used for calculating MF conversion(C1),and TEMF yield (Y1) are listed below:

3.Results and Discussion

3.1.Catalyst characterization

XRD measurement is a powerful tool for understanding the change of the clay microenvironment.The change of the interlayer spacing was investigated for pristine montmorillonite (K-10) and acid-treated K-10-1,K-10-2,and K-10-3 (Fig.2).The interlayer spacing was calculated following the Bragg’s law (2dsinθ=nλ),and thed001peak was used (n=1) [19].XRD patterns of all acidtreated montmorillonite samples were almost unchanged compared with that of K-10(Fig.2).The results showed that the crystal structure of acid-modified montmorillonite was almost not damaged.The diffraction bands at 5.7° (001),20° (110),26.5° (quart),35° (105),54° (210),and 62° (300) correspond to montmorillonite of hexagonal phase,which is in agreement with previous reports[20].Moreover,the intensity peak of K-10-1 increases significantly,indicating the enhanced crystallinity that may lead to an increase in active acidic sites.The interlayer distance of K-10 was calculated to be 1.549 nm by the Bragg equation.However,after the acid treatment,the characteristic peakd001was shifted to the right,which may be due to the exchange of hydrogen ions with interlayer aluminum ions and the release of interlayer water molecules,resulting in the decrease of interlayer spacing.

The nitrogen adsorption–desorption isotherms of K-10 and acid-treated K-10 were shown in Fig.3.It was found that all the curves are type-IV isotherm curves with an H3 hysteresis loop atP/P0of ~0.5–0.9,indicating that all the samples are of mesoporous structure [21].Notably,the specific surface area of montmorillonite after acid modification increased remarkably,which may be caused by the removal of some ions(e.g.,aluminum)from octahedral sites of the pristine montmorillonite(Table 1).From Table 1,it can be seen that the specific surface area and micropore volume of montmorillonite increase with the increase of hydrochloric acid concentration from 1 to 2 mol·L-1.However,when the hydrochloric acid concentration reaches 3 mol·L-1,the specific surface area and micropore volume of K-10-3 both decrease,possibly due to the partial decomposition of montmorillonite crystals by treatment with a relatively high acid concentration,in which some micropores are transformed into mesopores.

Fig.2.XRD patterns of K-10,K-10-1,K-10-2,and K-10-3.

Fig.3.N2 adsorption–desorption isotherms of K-10,K-10-1,K-10-2,and K-10-3.

FT-IR spectra of K-10 and acid-modified montmorillonite samples were collected in Fig.4.The band observed at near 3632 cm-1is resulted from the stretching vibration of the Al–OH bond,while the bands at 3433 and 1642 cm-1are caused by the stretching and bending vibration of the hydroxyl groups of the interlayer water molecules of montmorillonite.In addition,the bands at 1076 and 1039 cm-1are respectively caused by the Si–O vibration with a three-dimensional framework and the Si–O vibration of the tetrahedral sheet,and the peak at 792 cm-1is attributed to the stretching vibration of Si–O in the impurity quartz[22].Furthermore,914,842,624,520,and 465 cm-1are assigned to Al–Al–OH,Al–Mg–OH,Al–O,Al–O,and Si–O–Si bending vibrations of K-10,respectively.Upon modification of K-10 by acid(HCl),the absorption intensity of the peaks at 842 cm-1,625 cm-1,and 520 cm-1are significantly weakened.The results showed that the content of Al3+in K-10-1,K-10-2,and K-10-3 is reduced after acid treatment,which was confirmed by ICP-AES.The Al content in K-10 was found to be 5.083 g·kg-1by ICP-AES analysis,while the content of Al in K-10-1 was only 1.048 g·kg-1,implying that H+was exchanged with Al3+in the treatment process.The absorption peak of Si–OH bond at 914 cm-1proved the existence of Br?nsted acid on the surface of K-10 and acidtreated montmorillonite samples.The stretching vibration absorption peak of H–O–H bond in the acid-modified K-10 samples at 3433 cm-1tends to disappear,which again indicates that H+is successfully exchanged with Al3+to release interlamination water.This means that the weak acid in the modified K-10 is reduced,which is consistent with the results of NH3-TPD and Py-IR.

XPS analysis of the surface compositions and chemical valence of K-10 and K-10-1 was conducted and the results are shown in Fig.5.Al,Si,O,C,and Mg species were found to exist on the K-10 surface from the wide-scan XPS spectra (Fig.5a) and no new peak was found on the surface of K-10-1.It was reported that O 1s is decomposed into three peaks(Fig.S1),corresponding to metal oxide (M-O),metal-bound hydroxyl (M-OH),and interlayer water molecule at 531.28,531.99,and 532.68 eV,respectively [23].The binding energy values of the Si 2p,Al 2p,Mg 1s,C 1s,and O 1s in K-10 and K-10-1 are presented in Table S1.In addition,the compositions of the catalysts were detected by XPS and the results are collected in Table S2.It can be seen from Fig.5b that the peak intensity of aluminum was significantly reduced after acid treatment,showing that the acid treatment leaches aluminum ions from the montmorillonite structure.The change in the structural properties of montmorillonite after acid treatment can also find clues from the varying species composition(Table S2),and the aluminum content in montmorillonite K-10 decreased significantly,also indicating that hydrogen ions are exchanged with aluminum ions in montmorillonite during acid treatment.

Table 1Texture properties of K-10,K-10-1,K-10-2,and K-10-3

Typically,different peak integration areas and desorption temperatures in NH3-TPD profiles indicate different acidic properties.As shown in Fig.6,there are two desorption peaks in the NH3-TPD profile of K-10,indicating that K-10 has both weak acid sites(centered at 85 °C) and medium-strong acid sites (centered at 358°C),which could be weak Br?nsted acid and strong Lewis acidsites,respectively [24].Gratifyingly,two similar desorption peaks at about 70°C and 500°C with different concentrations were found for acid-treated montmorillonite (e.g.,K-10-1,K-10-2 and K-10-3;Fig.6),indicating that the medium-strong Lewis acid species of montmorillonite was changed to strong acid sites after acid modification.

Fig.4.FT-IR spectra of K-10,K-10-1,K-10-2 and K-10-3 at wavenumber of 400 cm-1 to 3800 cm-1 (a) and 400 cm-1 to 1200 cm-1 (b).

Fig.5.XPS spectra of wide scan (a) and Al 2p spectra (b) of K-10,K-10-1,and Re-K-10-1.

The types of acid sites in K-10 and the prepared catalysts were determined by the FT-IR spectra of the adsorbed pyridine (Fig.7).Obviously,after acid treatment,both Lewis and Br?nsted acid sites of the prepared catalysts increased significantly (Table S3).Three main types of Br?nsted acid were observed on the surface of montmorillonite:(1)the silanol(Si–OH)on the edge or end of the crystal layer,(2) the acidic bridging hydroxyl group formed by the substitution of aluminum on the edge of the crystal layer,and(3) the montmorillonite surface polarized water.There are two main sources of Lewis acid: (1) Al3+with incomplete coordination at the edge of the crystal layer,and (2) amorphous SiO2–Al2O3impurities.As illustrated in Fig.7,the absorption bands at 1442–1452 and 1594 cm-1can be assigned to Lewis acid sites,while the bands at 1544 and 1619 cm-1are assigned to Br?nsted acid sites [25].In addition,the band at 1490 cm-1indicates the presence of both Lewis acid and Br?nsted acid [26].Variable intensity of Lewis acid sites existed at~1442 cm-1in the broad band profile(Fig.7).With the increase of desorption temperature,the peak maximum value was transferred to 1452 cm-1due to the pyridine desorption from the weak acid sites,showing that there are weak Lewis acid sites in both K-10 and acid-modified K-10.

Thermogravimetric analysis is an effective method for determining the catalyst thermal stability,morphological change,and structural property.Thermal analyses were carried out for K-10 and the prepared catalysts (Fig.8).It can be seen from Fig.8 that the catalyst weight loss can be mainly divided into three stages.The first stage is the temperature increasing from 20 to 150 °C,and the weight loss is mainly caused by the physically adsorbed water on the K-10 surface[20].The second stage is the weight loss at 150–500 °C resulted from water desorption between the K-10 layers [27].The third stage is mainly caused by the loss of the structural hydroxyl group of montmorillonite at the temperature range of 500–800°C.The total weight loss of K-10 after acid modification was about 11%,which is relatively lower compared to that of unmodified K-10(about 16%),indicating that the acid-treatment can improve the thermodynamic stability of K-10.

Fig.6.NH3-TPD profiles of K-10,K-10-1,K-10-2 and K-10-3.

Fig.7.Pyridine-adsorbed FT-IR spectra of K-10,K-10-1,K-10-2 and K-10-3.L:Lewis acid sites,B: Br?nsted acid sites.

Fig.8.Thermogravimetric analysis of K-10,K-10-1,K-10-2,and K-10-3.

As demonstrated above,two types of acid sites (i.e.,Br?nsted and Lewis acid)on the surface of the original montmorillonite were disclosed by NH3-TPD and pyridine-adsorbed FT-IR,although its acidity was relatively low.However,after acid treatment of K-10,both the acid strength and density of the resulting catalysts (K-10-1,K-10-2,and K-10-3) increase,which was mainly attributed to the change of surface properties and structural states of montmorillonite after acid modification [28].The possible acidification mechanism of montmorillonite is shown in Fig.9.It can be seen that the acid-modified montmorillonite forms more Lewis and Br?nsted acid sites,which provide catalytically active centers for the acetylation-alkylation reaction.

3.2.Catalytic activity

3.2.1.Catalytic acetylation-alkylation of MF and AA with different catalysts

When K-10 was used as the catalyst for the acetylationalkylation reaction,MF conversion,and MAF and TEMF yield were quite low at 40°C for 6 h(Fig.10).Under identical reaction conditions,the hydrochloric acid concentration for activation of montmorillonite has a significant effect on the acetylation-alkylation reaction.Among them,K-10-1 was found to show superior catalytic activity.Although K-10-2 has the largest acid content(Table S3) and specific surface area (Table 1),its catalytic activity is inferior as compared with K-10-1.These results show that the catalyst activity is not only determined by the specific surface area and acid content,but also related to the acid site type and structure of the catalyst.In connection with this,the characterization results show that the crystal form of K-10-1 was beyond others (Fig.2),also with the largest pore volume (Table 1).FT-IR spectra of adsorbed pyridine illustrate that K-10-1 has more Br?nsted acid sites,with the molar ratio of Br?nsted to Lewis acid sites of 0.75 at 250 °C (Table S3),which is significantly higher than that of K-10.So K-10-1 was chosen as the best catalyst for further optimization of the reaction.In addition,the effect of the amount of catalyst was also explored on the acetylation-alkylation reaction (Fig.S2),and both MF conversion and TEMF yield increased with the increase of the catalyst amount.When the amount of catalyst was 0.05 g,the conversion of MF and the yield of TEMF were 95%and 69%,respectively.Nevertheless,when the amount of catalyst was 0.07 g,the MF conversion and TEMF yield both decreased sharply.It should be noted that the reaction is a solvent-free reaction,in which the reaction mixture would become more viscous with the increase of the catalyst amount,thus resulting in the decrease of conversion reactivity.

3.2.2.Effect of reaction temperature on the acetylation-alkylation reaction

Fig.9.Formation mechanism of Br?nsted acid(B)and Lewis acid(L)sites after acid modification of montmorillonite.a: Pristine montmorillonite,b: acid-modified montmorillonite.

Fig.10.The influence of different catalysts on the acetylation-alkylation reaction.Reaction conditions:1 mmol MF(0.0821 g),1.5 mmol AA(0.153 g),catalyst(0.05 g),time (6 h) and temperature (40 °C).

The effect of reaction temperature on the catalytic performance of K-10-1 catalyst in the acetylation-alkylation reaction was investigated at the temperature range of 25–80°C.The conversion of MF was found to be markedly influenced by reaction temperature(Fig.11).When the reaction temperature was raised from 25 to 40 °C,the MF conversion increased significantly,while further increasing the temperature to 60 or 80 °C,the MF conversion rate was barely changed.When the temperature was 40°C,the yield of TEMF reached the maximum value(ca.70%).However,the yield of TEMF decreased with the further increase of reaction temperature,while the yield of MAF continued to increase,indicating that relatively high temperature(>40°C)properly hindered the conversion of MAF to TEMF.Therefore,the reaction temperature of 40 °C was chosen for the following studies.

3.2.3.Effect of the reaction time on acetylation-alkylation reaction

With the increase of reaction time,the substrate conversion and product yield were raised obviously over K-10-1,as shown in Fig.12.When the time was 6 h,the conversion rate of MF was 95%,which was hardly changed after the further extension of the reaction time.A maximum yield of TEMF (ca.70%) was obtained after reaction for 6 h.To our surprise,MAF did not change significantly during the entire test period,indicating that MAF was generated very quickly while relatively stable to be further converted to TEMF by succeeding hydroxyalkylation/alkylation reactions.This may be related to the acidity of the catalyst and the in situ formed acetic acid.Also,the feedstock ratio of the reaction was screened,and the obtained results are shown in Fig.S3.When the molar ratio of MF to AA was set as 1 to –1.5,the conversion of MF and the yield of TEMF both reached the maximum.

Fig.11.Effect of reaction temperature on acetylation-alkylation reaction.Reaction conditions:1 mmol MF(0.0821 g),1.5 mmol AA(0.153 g),K-10-1(0.05 g)and time(6 h).Error bars with average error of ≤2.6%.

Fig.12.Effect of reaction time on acetylation-alkylation reaction.Reaction conditions: 1 mmol MF (0.0821 g),1.5 mmol AA (0.153 g); K-10-1 (0.05 g) and temperature (40 °C).Error bars with average error (θ) of ≤2.9%.

3.3.Catalyst recycling study

In addition to the excellent catalytic activity,another important issue is the catalyst reusability.To study the reusability of K-10-1 catalyst,at the end of each reaction cycle,the catalyst was separated by centrifugation,washed thrice with acetone (3×5 ml),and dried at 80 °C for 6 h.Under the identical reaction conditions,the recovered catalyst was used for the next reaction.After repeated use of K-10-1 for three times,the yield of TEMF was around 66%with an almost 100%carbon balance(Fig.13).The reason for the slight deactivation of the K-10-1 catalyst could be due to the carbonaceous species formed during the reaction to cover the catalyst active surface.In order to confirm this hypothesis,XPS test was performed after one recycle,and the intensity of carbon(C 1s)of Re-K-10-1 catalyst was revealed to increase obviously(Fig.5a).The carbon content of Re-K-10-1 catalyst was determined to be 16.78%,which is 2.4 times higher than that of the fresh catalyst(6.75%)(Table S3).The organic species covered on the catalyst could be mainly acetic acid in the reaction process,resulting in a decrease in the number of available acid sites.The specific surface area of the K-10-1 catalyst was decreased from 128.8 m2·g-1to 45.9 m2·g-1after recycle,which can be ascribed to the block of micropores by the organic matter (Fig.S4).Notably,the catalytic activity of the catalyst was hardly affected,indicating that the reaction mainly occurred in the mesopores of the catalyst.Other characterization techniques including XRD and FT-IR showed that the structure of the reused(after one cycle)K-10-1 catalyst was almost unchanged compared with the fresh counterpart (Figs.S5 and S6).

Fig.13.Reusability of K-10-1 in the cascade acetylation-alkylation of MF and AA to TEMF.Reaction conditions: 1 mmol MF (0.0821 g),1.5 mmol AA (0.153 g),K-10-1(0.05 g),time (6 h) and temperature (40 °C).

Fig.14.Possible pathways involved in one-pot cascade acetylation-alkylation.

4.Mechanism Investigation Experiments

As discussed above,acid-modified montmorillonite catalysts contain both Br?nsted and Lewis acid sites.It was reported that acetylation and alkylation reactions were usually catalyzed by Lewis acid and Br?nsted acid,respectively[29].In order to investigate the effects of acid type on the acetylation-alkylation reaction,different catalysts including AlCl3·6H2O,γ-Al2O3,SiO2,and Amberlyst-15 instead of K-10-1 were employed under otherwise same reaction conditions (Table 2).In terms of different catalysts(i.e.,AlCl3·6H2O,γ-Al2O3,SiO2,and Amberlyst-15),the same amount of acid species was employed for the reaction,which was set as 15 μmol.It was observed that neither MAF nor TEMF was generated when AlCl3·6H2O,γ-Al2O3or SiO2was used as catalyst,but a small amount of dimer was generated by cascade acetylation and hydroxyalkylation reactions (Table 2),explicitly indicating that thein situgenerated MAF was completely converted to the dimer in the presence of Lewis acid or weak Br?nsted acid.However,MAF and TEMF were simultaneously formed when Amberlyst-15 was used as catalyst,indicating that strong Br?nsted acid can catalyze sequential acetylation,hydroxyalkylation,and alkylation reactions,in which the conversion of the dimer to TEMF is rapid.To further understand the effect of Br?nsted and Lewis acid on the acetylation-alkylation reaction,the pyridine-adsorbed FT-IR spectra of AlCl3·6H2O,γ-Al2O3,SiO2,and Amberlyst-15 were recorded (Fig.S7).It was revealed that Lewis acid and weak Br?nsted acid sites mainly catalyzed the acetylationhydroxyalkylation reaction,while strong Br?nsted acid species mainly promoted the alkylation reaction.It should be noted that pronounced catalytic efficiency of K-10-1 was observed for the trimerization of MF with AA,which could be ascribed to the syner-gistic effect of its Lewis and Br?nsted acid sites.The possible reaction mechanism and pathways of the acetylation-hydroxyalkyla tion/alkylation of MF with AA is illustrated in Fig.14.The reaction is mainly divided into three steps: (1) catalytic formation of MAF and(2)formation of the dimer with Lewis and Br?nsted acid sites,as well as (3) the generation of TEMF predominantly catalyzed by Br?nsted acid sites.

Table 2One-pot cascade acetylation-alkylation with different catalysts①

Fig.15.The activity of acetic acid or K-10-1 in the reaction of MAF and MF.Reaction conditions: 1 mmol MAF,1 mmol MF and 0.06 g CH3COOH or 0.05 g K-10-1.N.D:not detacted; Conv: conversion.

It should be noted that SiO2is neutral (or weak Br?nsted acid like –OH species at low desorption temperature),as illustrated by pyridine-adsorbed FT-IR (Fig.S7).In this case,it was proposed that water adsorbed on the surface of silica in a small amount would react with acetate to form acetic acid under heating conditions,in which thein situformed acetic acid will promote the formation of dimers from MAF and 2-MF.

It is not difficult to see that acetic acid was co-formed with MAF in the reaction process,which is a kind of Br?nsted acid.Here,the effect of acetic acid on the overall reaction processes was also explored.MAF and MF were selected as substrates with acetic acid or K-10-1 used as a catalyst,respectively.It was found that when acetic acid was used as a catalyst,the dimer was exclusively formed by hydroxyalkylation of MAF with MF,indicating that acetic acid has a catalytic role in the hydroxyalkylation reaction(Fig.15a).However,when K-10-1 was used as a catalyst,in addition to the dimer formation,the trimer TEMF was also formed(45%yield),indicating that the hydroxyalkylation reaction requires the participation of strong Br?nsted acid to take place(Fig.15b).In a brief summary,Lewis acid sites mainly catalyze the initial acetylation and hydroxyalkylation processes,while the final alkylation reaction is primarily catalyzed by strong Br?nsted acid sites.Particularly,the co-existence of Br?nsted and Lewis acid species is synergistically active for improving the reaction efficiency of the cascade conversion processes.

5.Conclusions

In summary,a series of acid-modified montmorillonite catalysts(K-10-1,K-10-2,and K-10-3) were prepared and found to exhibit excellent catalytic performance in the trimerization of MF with AA to TEMFviacascade acetylation-hydroxyalkylation/alkylation.Among the tested catalysts,K-10-1 was demonstrated to have the best catalytic activity,possibly due to its crystal integrity,improved porosity,and switchable acidity.A high TEMF yield of ca.70%was obtained over K-10-1 at 40°C for 6 h,and the superior catalytic efficiency is mainly the result of the synergistic effect of Lewis and Br?nsted acid sites.In addition,acetic acid played a promotional role in the hydroxyalkylation step toward TEMF.This study provides a green and budget protocol for the preparation of bio-based C17diesel fuel precursor,which has an important implication in the future high-quality biodiesel production.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors thank the financial support from the National Natural Science Foundation of China (21666008,21908033,21576059),Fok Ying-Tong Education Foundation (161030),Guizhou Science & Technology Foundation ([2018]1037),and Program of Introducing Talents of Discipline to Universities of China (111 Program,D20023).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.09.037.