Chaofeng Zhang,Tonglu Zhang,Jing ZhangJiandong ZhangRuifeng Li,

1 College of Biomedical Engineering,Taiyuan University of Technology,Taiyuan 030024,China

2 College of Chemistry and Chemical Engineering,Taiyuan University of Technology,Taiyuan 030024,China

Keywords:SBA-16 Polyoxymethylene dimethyl ethers Ionic liquids Heterogeneous catalyst Diesel fuel additives

ABSTRACT The promising combustion and emission properties of polyoxymethylene dimethyl ethers(PODEn)are of significant interest.However,the synthesis of PODEn products with desired chain lengths is still a problem facing synthetic PODEn.Herein,a series of unique IL@SBA-16-Cx solid catalysts are prepared by encapsulation of ionic liquids(ILs)within the nanocage of SBA-16 through a silylation method.The structure of the encapsulated catalyst was characterized by UV–vis spectra,Fourier transform infrared(FT-IR),N2 adsorption–desorption isotherms,Powder X-ray diffraction (XRD),Transmission electron microscopy(TEM)and Elemental analysis.The encapsulated catalysts show similar catalytic activity to the homogeneous counterparts and display higher selectivity to the targeted PODE3–5 products than their homogeneous counterparts in the synthesis of PODEn from methanol (MeOH) and trioxymethylene (TOM).The encapsulated catalysts exhibit a superior PODE3–5 selectivity and could be the promising catalysts for PODEn synthetic reaction.?2021 The Chemical Industry and Engineering Society of China,and Chemical Industry Press Co.,Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-ncnd/4.0/).

1.Introduction

In 2018,China’s crude oil imports reached a new high of 462 million tons,and its dependence on foreign countries reached 70.8%.At the same time,the extensive development model of high energy consumption and high emissions has not been fundamentally changed [1].Both the sustainable development of energy and environment promote technique innovations to improve the efficiency of oil combustion[2,3].Because of their suitable physical and chemical properties,polyoxymethylene dimethyl ethers(PODEn) are a promising substitute diesel fuel.PODEnrefers to compounds with the formula CH3O(CH2O)nCH3,in which n is an integer equal to or greater than 1 (usually smaller than 8).These compounds contain nearly 50% oxygen and have a high cetane number.As diesel additives,they can improve the thermal efficiency by supporting burning of diesel in an engine,leading to the reduction of emissions of particular materials,COx and NOx[4].Among the PODEnoligomers,short-chain PODEnwith n <3(PODE1–2) does not fulfill the security criterion due to its lower vapor pressure and low flash point [2,3].Long-chain PODEnwith n >5 (PODEn>5) precipitate at temperatures below 291 K leading to a risk of blocking the fuel filter[5].The PODE3–5are thus suitable fuel additives to use in diesel engines with only slight modifications of the fuel supply system [2,6].

In the last five years,studies on the synthesis [7–13],reaction kinetics [14–17] and application [18,19] of PODEnhave received more and more attention all over the world.However,besides the targeted PODE3–5compounds,a significant amount of shortchain PODE1-2compounds and long-chain PODEn>5compounds were also produced in the reported literatures [7–19] and a series of patents [20–23].Therefore,to control the undesired PODEn(n<3 or n>5)and obtain PODE3–5compounds is a challenge facing the synthetic production of PODEn.Burger et al.[2,24]proposed to feed the unconverted reactants,dimethoxymethane and trioxymethylene,and the PODEnof undesired chain lengths back into the reactor in the PODEnproduction process.However,a significant amount of long-chain PODEncompounds,even PODEn>10,were formed.Zheng et al.[25] studied the molecular size reforming of PODE1–2and PODEn>4by two different reactions of self-reforming and reacting with dimethoxymethane over an acidic ion exchange resin.The former resulted in a high concentration of formaldehyde,and shifted the distribution to longer chain PODEn.The latter gave the desired reactions.Although the current production scale of PODE3–5is more than 240,000 t﹒a-1in China[18],it is still a serious challenge to obtain the target products in industrial industries with large-scale.The catalyst design and process design may be vital for the production of desired PODEn.

Instead of recycling undesired PODEncompounds,herein we present a new method to achieve controllable synthesis of PODEnfrom methanol (MeOH) and trioxymethylene (TOM) in the nanocages of SBA-16.The key to this approach is to design and prepare of heterogeneous catalyst IL@SBA-16-Cx (Fig.1).The reason of using mesoporous silica SBA-16 rather than other molecular sieves as a support is that SBA-16 has a tunable small pore entrance and isolated large cage size[26].Such a large cage can not only accommodate the ionic liquids (ILs),but also,to some extent,limit the growth of long-chain PODEnby the space steric hindrance.The reason of using acidic ILs rather than other inorganic acids,organic acids,and mixed acids as model catalysts is that acidic ILs not only show better performance,especially better selectivity in catalytic synthesis of PODEn[27],but also have a larger size than other acidic compounds and are easily trapped in the cages of SBA-16.After introducing of acidic ILs into the nanocage of SBA-16,the pore entrance size is reduced through a silylation reaction.When the catalytic reaction is going on,the entrapped acidic ILs can catalyze the reactants (MeOH and TOM) which have entered the nanocage through the smaller pore entrances of silylated SBA-16 to synthesis of PODEn.Subsequently,the short-chain PODEnproducts can be diffused through the pore entrances to the outside of the nanocage.However,the smaller pore entrances of silylated SBA-16 may inhibit the diffusion of long-chain PODEnproducts,thereby preventing the undesired polymerization.

2.Experimental

2.1.Preparation of SBA-16

Mesoporous cage-like material SBA-16 was synthesized according to a modified literature protocol [26,28].Pluronic F127 was used as the template and TEOS was used as the silica source under acidic synthesis conditions.Briefly,1.0 g Pluronic F127 and 40 ml hydrochloric acid (2 mol﹒L-1) were added in a 100 ml beaker.The mixture was magnetically stirred at 313 K for about 2 h.4 ml of TEOS was then dropwise added to the solution.After being stirred for 24 h,the resultant suspension was transferred into autoclave.The autoclave was placed in an oven at 373 K for 48 h.After the hydrothermal treatment,the precipitated solid was isolated by filtration,washed with distilled water,and dried at 373 K for 10 h,yielding white solid powders.The powder sample was then subjected to calcination at 823 K for 6 h and the mesoporous cagelike material SBA-16 was eventually obtained.All chemicals purchased were of reagent grade which were used without further purification.

2.2.Preparation of SBA-16-Cx

The pore entrance size of SBA-16 was tuned according to the method reported [29].15 ml of dichloromethane was added to 1.0 g of SBA-16(evacuated at 373 K for 6 h),followed by the addition of 5 mmol of silylating reagent(Cx).After stirring at refluxing temperature for 24 h under N2atmosphere,the resulting solid was isolated by a rapid filtration,washed thoroughly with dichloromethane and dried under vacuum.When the silylating reagents are propyltrimethoxysilane (C3),octyltrimethoxysilane (C8) and dodecyltrimethoxysilane (C12),the modified SBA-16 are denoted as SBA-16-C3,SBA-16-C8 and SBA-16-C12,respectively.

The pore entrance size of SBA-16/SBA-16-Cx was estimated by adsorption experiment:0.06 g of SBA-16/SBA-16-Cx was dispersed in 4–6 ml of methanol containing a measured amount of ILs.The mixture was stirred in a sealed tube for 4 h.After centrifugation,the supernatant was isolated by filtration and measured with a UV–vis spectrophotometer.

2.3.Preparation of IL@SBA-16-Cx

A typical synthesis procedure for the heterogeneous catalysts is as follows:First,1.0 g of SBA-16 was suspended in 30 ml of methanol absolute containing 0.5 g ionic liquid 1-(propyl-3-sulfonate)pyridinium hydrosulfate,1-(propyl-3-sulfonate)pyridinium methylsulfonate,1-(propyl-3-sulfonate) pyridinium trifluoro methanesulfonate ([Py-PS][TfO-]),1-(propyl-3-sulfonate) pyridinium p-toluenesulfonate ([Py-PS][TsO-]).The mixture was stirred and refluxed for 24 h under N2atmosphere.Methanol absolute was removed by evaporation.Second,15 ml of dichloromethane and 5 mmol of silylating reagent were added to the resultant solid.After refluxing for 24 h under N2atmosphere,the resulting solid was isolated by filtration and thoroughly washed with dichloromethane to remove any unreacted silylating reagent on the surface of the solid.Finally,the obtained product was dried under vacuum overnight.When the ILs are,[Py-PS][TfO-] and [Py-PS][TsO-],the resultant solid catalyst are designated as@SBA-16-Cx,[Py-PS]@SBA-16-Cx,[Py-PS][TfO-]@SBA-16-Cx and [Py-PS][TsO-]@SBA-16-Cx,respectively.

Fig.1.Encapsulation approach to design the heterogeneous catalyst for the synthesis of PODEn compounds and regulate their molecular size distribution.

2.4.Catalysts characterization

Powder X-ray diffraction (XRD) patterns were recorded on a Simadzu XRD-6000 diffractometer (CuKα,40 kV,30 mA).N2adsorption–desorption isotherms were measured at liquid N2temperature (-196 °C) using Quantachrome Nova 1200e volumetric adsorption analyzer.Before the adsorption measurements,the samples were degassed at 393 K for 5 h under high vacumm.Transmission electron microscopy(TEM)analysis was performed using a Tecnai G2 S-TWIN TMP instrument with an accelerating voltage of 200 kV.Fourier transform infrared (FT-IR) spectra were recorded on Shimadzu FTIR-8400 s infrared spectrometer.UV–vis spectra were recorded on a TU-1901 UV–vis spectrophotometer using methanol as the reference.Elemental analysis(EA)was performed on an Elementar Vario EL cube analyzer (Elementar Analysensysteme GmbH,Germany).X-ray photoelectron spectroscopy (XPS)was taken on an AXIS ULTRA DLD spectrometer(Kratos,Shimadzu)with AlKα radiation.

2.5.Catalytic activity tests

Catalytic tests were conducted in a 100 ml stainless-steel reactor.TOM,MeOH and catalyst were added quantitatively into the reactor successively.The reaction conditions were set as follows:383 K for temperature,2.0 MPa for pressure,2:1 for the MeOH/TOM molar ratio,3 h for reaction time,3.0%(mass) for catalyst loading,and 90 r﹒min-1for stirring speed.After the reaction was completed,the resultant was cooled to room temperature and filtered to separate the catalyst.The products were analyzed by GC-4000A gas chromatograph with a Agilent DB-1 capillary column(60 m × 0.25 mm × 0.25 μm),which connected to a flame ionization detector (FID).Undecane was used as the internal standard compound for the quantitative analysis of TOM and PODEnin the system.Analytical conditions were as follows:injection port temperature,533 K;FID temperature,543 K;oven temperature program,initially the temperature was held at 353 K for 5 min,then ramped to 528 K at a rate of 298 K﹒min-1and held for 10 min;carrier gas,nitrogen of 0.4 MPa;reagent gases,air of 0.3 MPa,hydrogen of 0.3 MPa.The conversion of TOM (xTOM) and the mass selectivity to PODEn(sPODEn) were determined by the following expressions:

where mTOM,feedand mTOM,productare the mass of TOM in the feedstock and product,respectively,and miis the mass of species i in the product.

3.Results and Discussion

3.1.Adjustment of the pore entrance of SBA-16

Silylation was employed to adjust the pore entrance size of SBA-16 and propyltrimethoxysilane (C3),octyltrimethoxysilane (C8)and dodecyltrimethoxysilane (C12) were used as silylating reagents.UV–vis spectroscopy was used to estimate the pore entrance size of SBA-16/silylated SBA-16 and the molecular size of ILs,because all pyridine ILs exhibit characteristic band at 260 nm in methanol solution (Fig.2).If the pore entrance size of SBA-16/silylated SBA-16 is larger than the molecular size of ILs,the ILs will diffuse into the cage of SBA-16,resulting in a decrease of ILs concentration in methanol and vice versa.The UV–vis spectra of ILs in methanol before and after treatment with SBA-16,SBA-16-C3,SBA-16-C8,and SBA-16-C12 are presented in Fig.2.After treatment with SBA-16 and SBA-16-C3,the intensities of the UV–vis bands decreased sharply compared with those of the original solution,suggesting that the pore entrance size of SBA-16 and SBA-16-C3 is large enough to allow the diffusion of ILs into the nanocage of SBA-16.When the solution was treated with SBA-16-C8 and SBA-16-C12,the intensities of the UV–vis bands hardly changed,indicating that the pore entrance size of SBA-16-C8 and SBA-16-C12 is small enough to prevent ILs from diffusing into the cage of SBA-16.The absorption experiment clearly demonstrates that the pore entrance size of SBA-16 silylated by C8 or C12 can effectively confine ILs within the nanocage of SBA-16.

The pore structure of SBA-16 and silylated SBA-16 were further estimated using N2adsorption–desorption isotherms.When C12 was used as the silylating reagent,the pore entrance of SBA-16 was completely blocked showing only a small amount of adsorption of N2[30].Therefore,C8 was used as the silylating reagent.Similar to the parent SBA-16,SBA-16-C8 also exhibit a typical type IV isotherm pattern with H2 hysteresis loops,indicating that the mesoporous cage-like structure was maintained after silylating(Fig.3).Compared with the parent SBA-16,the hysteresis loops of SBA-16-C8 shifts to lower relative pressure,suggesting the pore size is reduced by silylation.Pore size distribution (Fig.4) shows that SBA-16 has a monomodal distribution with the maximum located at 6.3 nm.By comparison,SBA-16-C8 presents a monomodal distribution with the maximum located at 5.3 nm.The physical parameters of SBA-16 and silylated SBA-16 are listed in Table 1.The surface area and pore volume of the SBA-16-C8 were apparently decreased.The surface area decreased by 298 m2﹒g-1and the pore volume decreased by 0.13 cm3﹒g-1.This further confirms that the SBA-16 was successfully silylated by C8 and SBA-16-C8 can be used as appropriate support to entrap ILs.

3.2.Encapsulation of ionic liquids in the nanocages of SBA-16

The solid catalysts [Py-PS][TfO-]@SBA-16-C8,@SBA-16-C8,@SBA-16-C8 and@SBA-16-C8 were prepared by encapsulation of [Py-PS][TfO-],[Py-PS][TsO-],andin the cages of SBA-16 with C8 silylation,respectively.The process for preparing these catalysts was monitored by N2sorption.The N2sorption isotherms and the pore size distribution of the corresponding catalysts are also displayed in Figs.3 and 4,respectively.The textural parameters determined by N2sorption are summarized in Table 1.Similar to the parent SBA-16 with cage-like pore structure,the N2sorption of these solid catalysts also showed a typical type IV isotherm pattern with H2 hysteresis loops.It is worthwhile to note that the surface area,pore volume and cage size of SBA-16,SBA-16-C8 and IL@SBA-16-C8 were continuously decreased.The synthetic SBA-16 mesoporous material has a surface area of 715 m2﹒g-1,a pore volume of 0.75 cm3﹒g-1,and a cavity cage of 6.3 nm.After silylation with C8,the surface area and pore volume showed a significant decrease,and the cage size decreased by 1.0 nm.After further encapsulating of ILs,the surface area and pore volume further decreased,and the cage size decreased again by 0.4 nm,0.8 nm,0.9 nm,and 1.9 nm for,[Py-PS][TfO-],[Py-PS][TsO-] and,respectively,confirm-ing that the ILs were introduced into the cavity cage of SBA-16.However,the mesoporous cagelike structure for these solid catalysts was still maintained,as evidenced by its type-IV isotherm with an H2 hysteresis loop.The ordered mesoporous structure of these solid catalysts was further confirmed by the XRD pattern,which is similar to that of the parent material SBA-16 (Fig.5).

Fig.2.UV–vis spectra of ILs in methanol after absorption with SBA-16-Cx samples.0.06 g of SBA-16 or SBA-16-Cx was dispersed in (a) 4.2 ml of methanol containing 3.77×10-7 mol[Py-PS][CH3],(b)5.5 ml of methanol containing 4.94×10-7 mol[Py-PS][TsO-],(c)5 ml of methanol containing 6.66×10-7 mol[Py-PS][TfO-],(d)4 ml of methanol containing 1.87 × 10-7 mol [Py-PS][].The mixture was stirred in a sealed tube for 4 h.After centrifugation,the solution was measured with UV–vis spectroscopy.

The loading amounts of the trapped ILs were determined by elemental analysis.The ILs contain nitrogen,while the SBA-16 contains no nitrogen.So the loading amounts of the ILs can be calculated according to the content of nitrogen in the solid catalysts.From elemental analysis,the ILs mass contents in the solid catalysts are 20.14%,21.46%,23.86% and 24.23% for [Py-PS]@SBA-16-C8,[Py-PS][TfO-]@SBA-16-C8,[Py-PS][TsO-]@SBA-16-C8 and@SBA-16-C8,respectively(Table 1).

The solid catalysts were further characterized with FT-IR(Fig.6).In Fig.6a,the FT-IR spectrum of SBA-16 exhibits a broad band in the range of 3200–3700 cm-1and a strong broad band in the range of 1000–1300 cm-1.The broad absorbance band in the range of 3200–3700 cm-1is ascribed to the vibration of the hydrogen-bonded silanol groups [29].The strong broad band around 1000–1300 cm-1is due to asymmetric stretching vibrations of Si—O—Si (υas,Si—O—Si) bonds of SBA-16 [31].Two more bands at 800 cm-1(υs,Si—O—Si) and 460 cm-1(δ,Si—O—Si) can be ascribed to symmetric stretching and bending vibrations of Si—O—Si,respectively [31].In the FT-IR spectrum of 6b–6e,the peaks in the range of 2800–3000 cm-1can be assigned to the C—H stretching vibrations of the CH2groups of octyl groups.Also,the peak at 1460 cm-1can be assigned to C—H bending vibration[29],confirming the successful silylation of the pore entrance of SBA-16.The new peaks at 1494 and 1632 cm-1corresponded to the ring vibration of the pyridine [9].These FT-IR spectroscopy results further confirm that pyridine ILs are immobilized inside the nanocages of SBA-16 by C8 successfully.

The TEM images provided more detailed structural information about the samples.It can be seen from the TEM images(Fig.7)that the SBA-16 sample exhibits a typical cage-like mesopores with body-centered cubic structure (space group,Im3m).Large pores(～6.0 nm)can clearly be seen in the TEM images,which is in accordance with the N2adsorption–desorption analysis.The darker regions in the TEM images indicate ILs were wrapped and well dispersed in the nanocages of SBA-16.Meanwhile,a highly ordered mesoporous structure and a narrow pore distribution can be also observed,indicateing that the mesoporous structure of SBA-16 is maintained after the encapsulation of ILs.

3.3.Catalytic performance tests

Fig.3.N2 sorption isotherms of SBA-16,SBA-16-C8 and solid catalysts.(a) SBA-16,offset vertically by 300,(b) SBA-16-C8,by 300,(c) [Py-PS][TfO-]@SBA-16-C8,by 300,(d) [Py-PS][TsO-] @SBA-16-C8,by 240,(e) [Py-PS][]@SBA-16-C8,by150,(f) [Py-PS][CH3]@SBA-16-C8.

Fig.4.Pore size distribution of SBA-16,SBA-16-C8 and solid catalysts.(a) SBA-16,(b) SBA-16-C8,(c) [Py-PS][TfO-]@SBA-16-C8,offset vertically by 0.01,(d) [Py-PS][TsO-]@SBA-16-C8,by 0.005,(e) [Py-PS][] @SBA-16-C8,(f) [Py-PS][CH3SO3-]@SBA-16-C8.

Fig.5.XRD patterns of:(a)SBA-16,(b)[Py-PS][TsO-]@SBA-16-C8,(c)[Py-PS][TfO-]@SBA-16-C8,(d) [Py-PS] []@SBA-16-C8,(e) [Py-PS][CH3]@SBA-16-C8.

Fig.6.FT-IR spectra of:(a) SBA-16,(b) [Py-PS][]@SBA-16-C8,(c) [Py-PS][TsO-]@SBA-16-C8,(d) [Py-PS] [TfO-]@SBA-16-C8,(e) [Py-PS][CH3]@SBA-16-C8.

By encapsulation of ILs within the nanocages of SBA-16,we have prepared four heterogeneous catalysts for the synthesis of PODEnfrom MeOH and TOM.Two criteria were used to evaluate the performance of the encapsulated catalyst for synthesized of PODEn:its selectivity for desired products and the conversion of reactants [27].Table 2 summarizes the catalytic performance for the synthesis of PODEnwith different catalysts.Compared withthe corresponding homogeneous catalysts,all heterogeneous catalysts displayed higher selectivity for PODE3–5in the synthesis of PODEnfrom MeOH and TOM.We take the [Py-PS][TfO-]/[Py-PS][TfO-]@SBA-16-C8 catalysts as an example to explain the differences between the two types of catalysts.In Table 2,when [Py-PS][TfO-] was used,the conversion of TOM and the selectivity of PODE3–5were 93.98% and 40.71%,respectively.When the heterogeneous catalyst [Py-PS][TfO-]@SBA-16-C8 was used,the conversion of TOM and the selectivity of PODE3–5were 93.31% and 42.53%,respectively.While the conversion of the two catalysts was similar,the selectivity for the latter is improved.The other three heterogeneous catalysts@SBA-16-C8,[Py-PS][TsO-]@SBA-16-C8 and@SBA-16-C8 also has the same catalytic effects,and could catalyze the synthesis of PODEnin a similar way to that in the homogeneous catalysis process(Table 2).In addition,the [Py-PS][TfO-] content in the heterogeneous catalyst is only 21.46% (mass),which limits the associated high costs of the IL.The important is that [Py-PS][TfO-]@SBA-16-C8 can be reused several times without remarkable loss of activity and product selectivity (Fig.8).

Table 1 Physical properties of SBA-16,SBA-16-C8 and IL@SBA-16-C8

Table 2 Molecular size distribution of synthesis of PODEn with different catalysts①

Fig.7.TEM images of:(a) SBA-16-C8,(b) Py-PS][TfO-]@SBA-16-C8,(c) [Py-PS][TsO-]@SBA-16-C8,(d) [Py-PS][]@ SBA-16-C8,(e) [Py-PS][CH3]@SBA-16-C8.

Fig.8.Recyclability study of [Py-PS][TfO-]@SBA-16-C8 catalyst in the synthesis of PODEn.

As we can observe in Table 2,the conversion of TOM over the homogenous and heterogeneous catalysts was all higher than 93%.However,the PODEnproduct distributions of the two catalysts were significantly different.For [Py-PS][TfO-] catalyst,all of the PODE1–8compounds were produced and more than half of the product was short-chain PODE1–2.For[Py-PS][TfO-]@SBA-16-C8 catalyst,the production of PODE1–2and PODEn>5decreased and more PODE3–5was obtained.SBA-16 has a small pore entrance of about 2–4 nm and the produced PODEncan be diffused outward through the pore entrance.After the pore entrance size was reduced by C8,the diffusion of the PODEnproducts through the entrances of [Py-PS][TfO-]@SBA-16-C8 will be partially restricted.The slow diffusion of the products might result in a high polymerization and chain propagation.As shown in Table 2,compared with [Py-PS][TfO-],the [Py-PS][TfO-]@SBA-16-C8 catalyst showed a reduced selectivity for PODE1–2compounds but an increased selectivity for PODE3–5compounds.However,with the increase of the molecular size of PODEn,the diffusion resistance of the pore entrance increases correspondingly.It was difficult for [Py-PS][TfO-]@SBA-16-C8 to infinitely accommodate and diffuse PODEnproducts with large molecular size.As a result,the selectivity for long-chain PODEn>5decreased.It should be emphasized that the selectivity of the PODEnproducts decrease with the increase of polymerization degrees in the sequence of n=1 >n=2 >n=3 >n=4>n=5>n=6>n=7>n=8,indicating that the chain propagation proceeds through the insertion of an individual segment of CH2O one by one.It is unlikely to get a specific PODEnproduct in a single reaction step with a very high yield.A suitable nanoreactor[Py-PS][TfO-]@SBA-16-C8 with an appropriate pore entrance size is effective in suppressing the formation of long-chain products and meanwhile possesses high activity for the chain propagation forming targeted products PODE3–5.

Fig.9.Product distribution versus the n value in PODEn by using different catalysts.

It is worthwhile to note that the absolute value of the PODE3–5selectivity for [Py-PS][TfO-]@SBA-16-C8 is about 1.82% higher than that for [Py-PS][TfO-] and the absolute value of the PODEn>5selectivity for [Py-PS][TfO-]@SBA-16-C8 is about 1.46% lower than that for [Py-PS][TfO-].This absolute value looks small at first glance,but reveals the essential difference between two kinds of catalysts.First,as shown in Table 2,up to eight PODEnproducts,including short-chain (PODE1–2) products,desired products (PODE3–5) and long-chain (PODEn>5) products appear in the mixture after the catalytic reaction in the sequence of PODE1>-PODE2>PODE3>PODE4>PODE5>PODE6>PODE7>PODE8.For a complex reaction with more than eight products,the change of selectivity is the result of multiple factors,and its absolute value is not as large as that of a simple reaction with only two or three products [32].Second,all the product distribution of PODEncurves versus the n value showed an exponential distribution (Fig.9).When n was in the range of 3–5,the selectivity for every single PODE3–5product over [Py-PS][TfO-]@SBA-16-C8 was higher than that over [Py-PS][TfO-],and when n was in the range of 6–8,the selectivity for every single PODE6–8 product over [Py-PS][TfO-]@SBA-16-C8 was lower than that over [Py-PS][TfO-].The same is true for the other three pairs of catalysts.These results indicated that the discrepancy in PODE3–5/PODEn>5selectivity over the two catalysts was not from experimental error.A similar example is that Fu et al.[32] used different porous materials as catalysts to investigate the effects of pore size on selectivity for PODEnsynthesis.With C10-AS-50(super-micropores),they managed to increase the PODE3–5selectivity by 1.42% and 2.52% compared to USY-3 (60) (micropores)and C16-Al-SBA-1 (mesopores) materials,respectively.The catalyst had a similar number of acid sites and comparable acid strength.They reasoned that the selectivity achieved is matching between the pore dimension and the molecule size of the different PODEnleading to a partially restricted diffusion into the super-microporous materials.Another example is that Xue et al.[33] used Al-SBA-15–10 with different pore sizes (Al-SBA-15(1)-10,4.68 nm;Al-SBA-15(2)-10,5.70 nm;Al-SBA-15(3)-10,7.95 nm) as catalysts to investigate the effects of pore sizes on selectivity for PODEnsynthesis.When Al-SBA-15(1)-10,Al-SBA-15(2)-10 and Al-SBA-15(3)-10 was used,the selectivity of PODE2–8was about 90.5%,91.5% and 92.5% (see Fig.11 of Ref.[33],which only has coordinate axes,and the numerical value can only be roughly estimated),respectively.The absolute value of the PODE2–8selectivity for Al-SBA-15(3)-10 is 1.00% higher than that for Al-SBA-15(2)-10,and 2.00% higher than that for Al-SBA-15(1)-10.The present results are in accordance with recent works [13] that the pore size and pore structure can influence PODEnsynthesis by regulating product selectivity and distribution to a certain extent.

4.Conclusions

In summary,four pyridine ILs[Py-PS][TfO-],[Py-PS][TsO-],[Py-PS][CH3] and [Py-PS][] were successfully confined in the nanocages of SBA-16 by reducing the pore entrance size through a silylation method.Since there are no covalent linkage between the ILs and the surface of the nanocage,the properties of the ILs can be largely kept by simply confining in the nanocage.More importantly,the encapsulated catalysts are efficient in the synthesis of PODEnfrom MeOH and TOM,and show higher selectivity for the targeted PODE3–5compounds than their homogeneous counterparts,which has a great economic value to the industrial production of green diesel fuel.The high selectivity for targeted products is attributed to the particular pore size of nanocages of SBA-16.This work provides a new opportunity for the design of efficient solid catalysts for the production of desired PODEn.

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 greatly acknowledge Prof.Tim Storr from Simon Fraser University for his help in English writing and improving of this paper.The work was supported by the Natural Science Foundation of Shanxi Province (201801D121062),the Shanxi Scholarship Council of China (2017-037),and the Foundation of Taiyuan University of Technology (2016MS03).