Facile synthesis of metal-organic frameworks embedded in interconnected macroporous polymer as a dual acid-base bifunctional catalyst for efficient conversion of cellulose to 5-hydroxymethylfurfural

Yanan Wei,Yunlei Zhang,*,Bing Li,Wen Guan,Changhao Yan,Xin Li,Yongsheng Yan,*

1 Institute of Green Chemistry and Chemical Technology,School of Chemistry and Chemical Engineering,Jiangsu University,Zhenjiang 212000,China

2 Research Center of Fluid Machinery Engineering and Technology,Jiangsu University,Zhenjiang 212000,China

Keywords:Catalyst Biomass Hydrolysis 5-Hydroxymethylfurfural Pickering high internal phase emulsions templated polymer Acid-base bifunctional site

ABSTRACT 5-Hydroxymethylfurfural(5-HMF),as a key platform compound for the conversion of biomass to various biomass-derived chemicals and biofuels,has been attracted extensive attention.In this research,using Pickering high internal phase emulsions (Pickering HIPEs) as template and functional metal-organic frameworks (MOFs,UiO-66-SO3H and UiO-66-NH2)/Tween 85 as co-stabilizers to synthesis the dual acid-base bifunctional macroporous polymer catalyst by one-pot process,which has excellent catalytic activity in the cascade reaction of converting cellulose to 5-HMF.The effects of the emulsion parameters including the amount of surfactant(ranging from 0.5%to 2.0%(mass)),the internal phase volume fraction(ranging from 75% to 90%) and the acid/base Pickering particles mass ratio (ranging from 0:6 to 6:0) on the morphology and catalytic performance of solid catalyst were systematically researched.The results of catalytic experiments suggested that the connected large pore size of catalyst can effectively improve the cellulose conversion,and the synergistic effect of acid and base active sites can effectively improve the 5-HMF yield.The highest 5-HMF yield,about 40.5%,can be obtained by using polymer/MOFs composite as catalyst(Poly-P12,the pore size of(53.3±11.3)μm,the acid density of 1.99 mmol?g-1 and the base density of 1.13 mol?g-1)under the optimal reaction conditions(130°C,3 h).Herein,the polymer/MOFs composite with open-cell structure was prepared by the Pickering HIPEs templating method,which provided a favorable experimental basis and theoretical reference for achieving efficient production of high addedvalue product from abundant biomass.

1.Introduction

Generation of available chemicals and valuable bio-fuels from renewable and rich biomass offers great potential to meet growing energy demand in a sustainable manner [1].5-Hydroxymethylfurfural(5-HMF)is a key intermediate for the conversion of biomass to various high added-value chemicals and biofuels due to its special chemical structure,which has attracted significant interest in the last decades[2,3].Therefore,a great deal of efforts has been devoted to the development of advanced technologies for 5-HMF production.Among these methods,chemical catalytic conversion is considered as an efficient 5-HMF preparation strategy from biomass due to its simple operation and highconversion efficiency [4,5].

In recent years,the catalytic conversion of cellulose to 5-HMF has attracted more interesting of researchers,because it has a wider range of sources,lower prices and more abundant storage than glucose and fructose [6–8].Numerous studies have proved that solid acid catalysts such as Lewis active sites modified ultrastable Y zeolite (Cr-USY,the 5-HMF yield of 34.3%),beat zeolite(Cr-Beta,the 5-HMF yield of 34.1%),and Br?nsted acid (-SO3H)functionalized mesoporous silica nanoparticles (the 5-HMF yield of 19.2%) have catalytic ability for the conversion of cellulose to 5-HMF[9–11].Besides single function catalysts with only Br?nsted or Lewis acid active sites,bifunctional solid catalysts also have been investigated for the conversion of cellulose to HMF,because the reaction can be better promoted by the synergistic action of Lewis acid and Br?nsted acid active sites [12–15].Zhanget al.reported that when using Br?nsted acid functionalized attapulgite(APT-SO3H) was used as catalyst,the yield of 5-HMF was about 24.7% in ionic liquid solvent.When the attapulgite functionalized with Br?nsted acid and Lewis acid with catalyst (APT-SO3H-CrIII)was as catalyst,the yield of 5-HMF increased to 31.2%in the same reaction system [16].Chenet al.reported the rapid of glucose to fructose in 20 min with high selectivity(84%)by solid base catalyst[17].Their works also proved that the conversion of cellulose to HMF is consisted by a complex series of cascade reaction pathways,including hydrolysis of cellulose by Br?nsted active sites,isomerization reaction of glucose to fructose required the Lewis acid or basic active sites,and then Br?nsted active sites promoted the fructose dehydration to 5-HMF.It can be seen that the construction of acid-base bifunctional catalyst is more conducive to catalytic conversion of cellulose to 5-HMF.In addition,Caoet al.studied the role of bifunctional mesoporous material(SBA-15)in the conversion of fructose to 5-HMF (the yield of 86%),and found that when the mesoporous channels are good and the specific surface area of catalyst is large,the reactants are more likely to adhere the catalyst surface and contact with the active sites,which can effectively improve the yield of 5-HMF [18].Unfortunately,the cellulose rarely comes into contact with the active sites inside the deep pores of mic/mesoporous catalysts due to the surface tension,slow mass transfer rate and so on,resulting in a low yield of 5-HMF[19,20].At the same time,common mesoporous materials such as SBA-15,molecular and zeolite have low acidity,so they are necessary to post-modify active sites before they used as catalyst materials.However,the pore structure of this kind of materials is likely to be destroyed in the modification process.These shortcomings seriously hinder the practical application of mesoporous materials in cellulose conversion field.Based on the above analysis,the design and construction of a multifunctional catalyst with macorporous structure is of great significance for the conversion of cellulose to 5-HMF.

In recent year,particle stable Pickering high internal phase emulsions (Pickering HIPE) the templating,a simple and general method,has attracted considerable attention in the synthesis of various porous and macroporpus materials.Particularly,the pore structure and pore connectivity of polymer materials can be easily customized by simply changing the emulsion parameters[21–23].With the above advantages,the well-connected macroporous structural material prepared by Pickering HIPE template method.It can effectively improve the mass transfer efficiency of macromolecular,which is more conducive to the effective contact between macromolecules such as cellulose and the active site inside the catalyst,and overcomes the shortcomings of mic/mesoporous materials [24,25].In our previous study,the macroporous catalyst was prepared by Pickering HIPE template prepared for the effective conversion of cellulose to 5-HMF,and acid-base bifunctional active sites of catalyst were synthesized by functional polymer monomers in one step [26].However,Pickering particles(such as silica,titania and graphene oxide) used to stabilize emulsions generally have not any catalytic active sites,which reduces the exposure of active sites from the surface of polymer materials and weakens the catalytic activity of the catalyst [27–29].Consequently,it is of great importance to conduct researches on Pickering HIPEs porous catalyst prepared by the Pickering particles with catalytic active sites.

In recent years,metal-organic frameworks (MOFs) have been proposed as one of the most potential candidates for Pickering particles,because they have mid-range zeta potentials and tunable structures,which is very suitable for liquid-liquid interface assembly [30].Meanwhile,as a new type of porous material,MOFs can obtain acid or base functional catalysts by coordinate acid and base functional ligand,which have been widely researched in the catalytic reactions [31–34].Hence,it is expected to improve the efficiency of cellulose conversion to 5-HMF by using functional MOFs as Pickering particles and Pickering HIPE as the template to produce novel MOFs/polymer effective catalysts with more sites.

In this work,macroporous polymer catalyst with acid-base bifunctional connectivity was prepared by functional UiO-66-type MOFs (UiO-66-SO3H and UiO-66-NH2) stabilized Pickering HIPEs as template (Fig.1),which exhibited excellent catalytic activity in the conversion of cellulose to 5-HMF in a one pot manner.The effects of the volume fraction of the internal phase,surfactant amounts and acid-base strength parameters on the porous structure and catalytic performances were described in detail.In addition,the catalytic properties of the catalyst for fructose and glucose and the recyclability of the catalyst were discussed.

2.Materials and Method

2.1.Chemicals and reagents

1-Ethyl-3-methyl-imidazolium chloride ([EMIM]-Cl),cellulose(powder,ca.50 μm),fructose,glucose,zirconium chloride (ZrCl4),2-aminoterephthalic acid (BDC-NH2),Tween 85,acetic acid (Ac),acrylamide (AM),sodium 4-vinylbenzenesulfonate (SS),N,N’-met hylene-bis-(acrylamide) (MBA),dimethylsulfoxide (DMSO),pure 5-HMF and dopamine hydrochloride in analytical grade were purchased from Aladdin Industrial Inc.(Shanghai,China).HPLC-grade methanol,sulfuric acid(H2SO4),methanol,acetone,potassium peroxydisulfate (KPS) and paraffin liquid were purchased from Sinopharm Chemical (Shanghai,China).2-Sulfoterephtalic acid monosodium salt(BDC-SO3Na)was purchased from Tokyo Chemical Industry (TCI Shanghai,China).Deionized water was obtained through a Mili-Q water-purification system.

2.2.Catalyst preparation

2.2.1.UiO-66-SO3H/ UiO-66-NH2 preparation

Firstly,the UiO-66-SO3H was synthesized through the modulated hydrothermal (MHT) approach according to the literature[35].Briefly,ZrCl4(0.24 mmol) and BDC-SO3Na (0.24 mmol) were dispersed into 5.0 ml CH3COOH aqueous solution (VCH3COOH:Vwa-ter=2:3),and resulting mixture was treated with ultrasound for 0.5 h.Then,the mixture was placed in a water bath at 90 °C for 12.0 h.The synthesized sample was washed with deionized water for 3–4 times,centrifuged,and the obtained solid soaked in methanol for three days in order to remove objective molecules.Moreover,fresh anhydrous methanol was changed every 24.0 h.Finally,the UiO-66-SO3H was obtained after the dried process in a vacuum oven at 60 °C for 24.0 h.UiO-66-NH2was synthesized by a similar MHT process.Typically,0.24 mmol ZrCl4and 0.24 mmol BDC-NH2were added into 5.0 ml CH3COOH aqueous solution (VCH3COOH:Vwater=1:4),and the mixture was dispersed by ultrasound for 0.5 h.And the other steps in the preparation of UiO-66-NH2were exactly the same as those of UiO-66-SO3H.

2.2.2.Pickering HIPEs preparation (P-x,x=1–13 and H)

A series of Pickering HIPEs with surfactant amounts of 0.5%,1%,1.5%and 2.0%(mass)were synthesized with the internal phase volume fraction of 85%and the UiO-66-SO3H/UiO-66-NH2mass ratio of 1:1,and the obtained Pickering HIPEs were denoted as P-1,P-2,P-3 and P-4,respectively(see Table 1 for details).Taking P-2 as an example:a certain amount of AM(0.40 g),SS(1.20 g),MBA(0.43 g)and KPS (0.04 g) were dissolved into 4.0 ml water by ultrasound.Then 0.3 g UiO-66-SO3H and 0.3 g UiO-66-NH2were dispersed into the above solution by mechanical stirring,and 26.66 ml paraffin liquid was slowly added dropwise to the mixed solution.The stable oil in water (O/W) Pickering HIPEs were obtained.

Table 1 Composition of Pickering HIPEs and the parameters of the corresponding polymer (Pickering HIPEs)

Fig.1.Schematic illustration for the preparation of a dual acid-base bifunctional macroporous polymer catalyst.

Then,a series of Pickering HIPEs were prepared with 75%,80%and 90% internal phase volume fraction,the surfactant amount of 1.5% (mass) and the UiO-66-SO3H/UiO-66-NH2mass ratio of 1:1.These obtained Pickering HIPEs were denoted as P-5,P-6 and P7(see Table 1 for details),respectively.Moreover,different mass ratio of UiO-66-SO3H/UiO-66-NH2(ranging from 0:6 to 6:0)monomers were also employed to prepare various of Pickering HIPEs using the same internal phase volume fraction of 80%and the surfactant amount of 1.5% (mass),which named as P-8,P-9,P-10,P-11,P-12,P-13 and P-H(UiO-66-SO3H/UiO-66-NH2=0),(see Table 1 for details).

2.2.3.Polymer prepared by polymerized Pickering HIPEs (Poly-Px,x=1–13 and H)

Macroporous polymers(Poly-Px,x=1–13 and H)were prepared by polymerizing the synthesized Pickering HIPEs at 70 °C for 24.0 h.Taking Poly-P3 as an example,free radical polymerization of P3 continuous phase was initiated by KPS under the condition of water bath to synthesize solid polymer.Then the polymerized monolith was washed in Soxhlet apparatus with acetone for 48.0 h to remove Tween 85 and other impurities,and dried 70 °C in vacuum for 24.0 h.The obtained polymer was paired with an acid solution for ion exchange.Typically,1.0 g polymer was dispersed to 100.0 ml sulfuric acid solution under continuous stirring.Then,the sample washed with water until neutral,and Poly-P3 can be obtained after the centrifuged and dried processes.In addition,other samples taken the same synthetic approach,named samples were listed in Table 1.

2.3.Characterization

The physical and chemical properties of obtain samples were characterized by various characterization technologies.The morphology of samples was observed by scanning electron microscope(SEM,S-4800 ⅡFESEM),which was carried out with an electron microscope equipped with an energy dispersive spectroscope.Fourier transform infrared spectrometer (FT-IR) was recorded on a Nicolet NEXUS-470 instrument.Powdered X-ray diffraction(XRD) was performed using a Bruker D8 diffractometer.Diffractograms were recorded in reflection mode using Ni-filtered Cu K α radiation (λ=0.154184 nm).X-ray photoelectron spectroscopy(XPS) spectra was recorded using a Thermo ESCALAB 250 with an accuracy of 0.3 eV.The Al Kα (y=901) radiation was used as the radiation sources,and the binding energies were calibrated using the C1s peak at 284.6 eV.Thermogravimetric-differential scanning calorimetry(TG-DSC)analysis of samples was performed by a Diamond TG/TGA instrument heated over a range of temperature from ambient temperature to 800 °C with heating rates 5 °C?min-1,under the nitrogen atmosphere.The acidic and basic features of samples were measured by temperature-programmed desorption(TPD).NH3-TPD and CO2-TPD were carried out on an AUTOCHEM Ⅱ2910 (Micromeritics Instrument Corp) instrument with a thermal conductivity detector (TCD).

2.4.Catalytic reactions

The cellulose conversion process included two main steps of pre-treatment and catalytic reaction.Recent studies have shown that ionic liquids (ILs) such as [EMIM]-Cl can effectively destroy the hydrogen bonding of cellulose and they have good solubility to cellulose [36].In this paper,[EMIM]-Cl (2.0 g) was used as the reaction solvent,cellulose(100.0 mg)was added into reaction flask and heated to 120 °C until the cellulose dissolved.For the typical catalytic reaction step,a certain amount of catalyst was added into the above solution when the reaction system reached the required temperature and stirring rate (600 r?min-1).After the reaction,cold-water quenching was applied to the reactor to stop the reaction.All catalytic reaction steps were repeated three times and the average yield of 5-HMF was calculated.The process of glucose and fructose catalytic experiments were similar to the cellulose conversion experiments,and DMSO-water(VDMSO:Vwater=1:1,a total volume of 5.0 ml) was used as solvent.

The obtained 5-HMF was analyzed by 1200 Agilent HPLC equipped with Agilent TC-C18 (2) Column (4.6 mm × 250.0 mm,5.0 μm) and UV detector.Glucose and fructose were analyzed by 1200 Agilent HPLC fitted with a refractive index detector and Agilent Hi-Plex H Column (300.0 mm × 7.7 mm).The substrate conversion,and product yield were calculated based on the equation listed as follows:

moles of remaining cellulose=(the solid mass was obtained of after the reaction– the washed catalyst mass)/molar mass (base on glucose unit)

moles of initial cellulose=the initial cellulose mass/molar mass(base on glucose unit)

3.Results and Discussions

3.1.Morphology and porous structure of catalysts

Cellulose catalytic conversion into 5-HMF as a typical interfacial reaction,the surface structure of catalyst is much closely related to the conversion efficiency.Firstly,the morphology of prepared samples Poly-P1,Poly-P2,Poly-P3 and Poly-P4 were investigated with different amount of surfactant(0.5,1.0,1.5 and 2.0%(mass))and an internal phase volume fraction of 85%.Fig.2(a)–(d)exhibit all synthesized samples have macropores structures and the pores are connectedviapore throats,except for Poly-P1.The cell walls of synthesized samples are decorated with UiO-66-type MOFs particles(the inset in Fig.2(a)–(d)).It can also be seen from EDS image that the peaks including C,N,O,S and Zr of Poly-P3 are apparent,suggesting the cell walls are decorated with UiO-66-type MOFs(Fig.S1 in Supplementary Material ).Table 2 shows the specific pore structure of samples,including the average pore size and interconnecting pore size.It is can be seen that the degree of pore size and connectivity is dramatically decrease as the surfactant concentration increase.So,the pore size of Poly-P4 only is about(6.7 ± 9.9) μm.However,the Poly-P1 appeared obvious collapse phenomenon.It is possible that the emulsion composed of a small amount of Tween 85 and MOFs is unstable.The polymer synthesized by thermal polymerization of the emulsion is relatively brittle,and led to the polymer partially collapses during purification.Base on the above analysis,1.5% (mass) amount of surfactant was selected for further study.

Table 2 The effect of the internal phase volume fraction and the amount of surfactant on polymer pore structure

Fig.2.SEM images of Poly-P1 (a);Poly-P2 (b);Poly-P3 (c);and Poly-P4 (d),the amount of surfactant 0.5%,1.0%,1.5% and 2.0%,respectively.

Secondly,the different samples (Poly-P5,Poly-P6 and Poly-P7)with 1.5%surfactant dosage and the internal phase volume fraction range from 75% to 90% were synthesized,and the morphology is shown in Fig.3.All samples have macroporpus structure and channel connected by pore throat,except for Poly-P7.Obviously,with the increase of internal phase volume fraction,the formation of polymers pore size is apparently decrease.Meanwhile,the degree of connectivity between emulsion droplets also increases significantly with the increase of internal phase volume fraction(Table 2,entry 5–7).The average pore size of Poly-P7 only is about(6.7 ± 9.9) μm and the average interconnecting pore size only is about (5.0 ± 6.2)μm.When the internal phase volume fraction of Pickering HIPEs is only 75%,the polymer walls of Poly-P5 obviously collapses,as shown in Fig.3(a).A possible explanation for this effect is that the volume fraction of internal phase is too low,and the droplet thin film is more prone to rupture during the process of purification and drying.It can be seen from the inset of Fig.3(a)-(d) that the UiO-66-type MOFs particles distribute on the cell walls of polymers.Furthermore,the presence of C,N,O,S and Zr in the sample Poly-P6 is confirmed by elemental mapping(Fig.3(d)),which indirectly proved that MOFs are evenly embedded in polymer (Fig.3(d1)-(d5)).Base on the above statement,the 1.5% (mass) amount of surfactant and the 80% internal phase volume fraction were selected as emulsion parameters to regulate the mass ratio of acid-baes particles for further study.

3.2.Chemical composition catalysts

Fig.3.SEM images of Poly-P5 (a);Poly-P6 (b);and Poly-P7 (c);elemental mapping image of Poly-P6 (d),the internal phase volume fraction of 75%,80% and 90%.

Fig.4.EDS images of Poly-H (without Pickering particles) (a) and Poly-P6 (UiO-66-SO3H:UiO-66-NH2=1:1) (b);FT-IR spectra of Poly-H and Poly-P6 (c);and XRD of simulated UiO-66,synthesized UiO-66-NH2,synthesized UiO-66-SO3H,Poly-H and Poly-P6 (d).

The multifunction polymers were preparedviaa one-pot method and their elemental composition analysis were performed by EDS analysis.Fig.4(a) and (b) shows the specific EDS results of Poly-H (without Pickering particles) and Poly-P6 (UiO-66-SO3H:UiO-66-NH2=1:1).It can be seen that Poly-P6 (Fig.3(b)) has a new peak of Zr element compared with pure polymer Poly-H(without Pickering particles),which is attributed to the successful introduction of Zr based MOFs containing into the polymer (Fig.4(a)).The FT-IR spectra of Poly-H and Poly-P6 is shown in Fig.4(c).Obviously,the main characteristic absorption peaks of Poly-H and Poly-P6 are similar:the -OH stretching band at approximately 3439 cm-1,the stretching vibration -C=O is around at 1646 cm-1,the peaks at 1121 cm-1,1035 cm-1and 1000 cm-1corresponding to the-SO3H groups[37,38].The appearance of-SO3H characteristic peak indicates that H ions are successfully introduced in the ion exchange process.Additionally,the peak strength of MOFs/ polymer composite is enhanced.The above results suggest that UiO-66-type MOFs were successfully embedded in functional polymer.The powder XRD patterns of different samples shown in Fig.4(d),it is demonstrated that synthesized UiO-66-SO3H and UiO-66-NH2contained the characteristic peaks of simulated UiO-66 and no other phase is detected.The characteristic peaks at 12.2°,17.1°,22.2° and 25.8° corresponded to the (022),(004),(115) and (006) crystal planes,respectively [39,40].Compared with the Poly-H of pure polymer,the diffraction peaks of MOFs can be clearly seen from Poly-P6 of MOFs/polymer composite,suggesting that the polymerization and purification processes have no effect on UiO-66-type MOFs particles.The results further confirm the successful incorporation of MOFs into the polymer.

Fig.5.XPS survey spectrum in the binding energy range 0–1350 eV of Poly-H(without Pickering particles)and Poly-P6(UiO-66-SO3H:UiO-66-NH2=1:1)(a);and the highresolution Zr spectra of Poly-P6 (b).

Fig.6.TG and DSC curves of Poly-H(without Pickering particles)(a);Poly-P6(UiO-66-SO3H:UiO-66-NH2=1:1)(b);Poly-P8(with acid UiO-66-SO3H)(c);and Poly-P13(with base UiO-66-NH2) (d).

Fig.7.CO2-TPD curves of Poly-H (a);Poly-P8 (b);Poly-P9 (c);Poly-P10 (d);Poly-P6 (e);Poly-P11 (f);Poly-P12 (g);Poly-P13 (h);and UiO-66-NH2 (i).

Fig.5 shows the XPS survey spectrum in the binding energy range 0–1100 eV and the high-resolution of C 1s,O 1s,N 1s and S 2p spectra of Poly-H and Poly-P6.Fig.5(a)and Fig.S2 are exhibit that Poly-H and Poly-P6 are composed of C,O,N and S elements.In addition,Poly-P6 of MOFs/polymer composite has two clearly binding energy peaks at about 182.7 and 185.2 eV corresponded to the peak of Zr 3d3/2and Zr 3d5/2,which confirms the existence of Zr element(Fig.5(b))[41].It is showed that the walls of polymer have been modified by the MOFs,and this result is consistent with EDS analysis result.Fig.S2(a)–(b) show the high-resolution XPS spectra of C1s,uncovering four clearly defined peaks for each of Poly-H and Poly-P6.The peaks at 284.6 and 285.2 eV can be assigned to C-C and C=O [42].The band corresponding to C=C at around 286.0 eV is much more intense for Poly-P6 than Poly-H,which may be caused by the introduction of UiO-66-NH2and UiO-66-SO3H.Fig.S2(c)-(d) exhibit the spectra of N 1s region for the around band at 401.2 and 399.4 eV and corresponding to N-C and NH2peaks,respectively [43].Fig.S2(e)-(f) show the two peaks corresponding to the O=C (532.3 eV) and O-C(531.2 eV),respectively [44,45].Fig.S2(g)-(h) show the spectra of S 1s region for the around band at 168.5 eV and 168.5 eV,corresponding to S 2p1/2and S 2p3/2peaks,respectively.The peak intensity of S element in Poly-H and Poly-P6 are obviously different[46].It can be caused by that the binding energy of S 2p is sensitive to acid strength,while the successful introduction -SO3H group of UiO-66-SO3H is proved.

High-temperature stability of catalyst is an important criterion to investigate the performance of the catalyst.We characterized the stability of samples in detail,the TG-DSC curves of samples(Poly-H,Poly-P6,Poly-P8 and Poly-P13) is shown as in Fig.6.The TG analysis of all samples exhibits quality loss,and the results of DSC exhibits the initial and final temperature of weightlessness and temperature of maximum reaction rate.All the samples underwent two stages of weightlessness.The first stage demonstrated distinct weight loss at 23–200°C,which is associated with the loss of absorbed water.The second stage in temperature range of 230–500°C is related to the decomposition of the polymer at high temperature.After the introduction of MOFs,the weightlessness of the second stage decreased.For example,the weightlessness of Poly-H(without Pickering particles),Poly-P6 (UiO-66-SO3H:UiO-66-NH2=1:1),Poly-P8 (with acid UiO-66-SO3H) and Poly-P13 (with base UiO-66-NH2) are 55.3%,54.8%,54.0% and 53.2%,respectively(Fig.6(a),(b)).It is indicated that the UiO-66 type MOFs are conducive to the stability of the polymer skeleton,and UiO-66-NH2is superior to UiO-66-SO3H (Fig.6(c),(d)).However,due to the small number of Pickering particles,the stability improvement of compound is not obvious.After calcination at 800 °C,the carbonized polymer and the zirconium oxide have been remained.In general,the samples we prepared completely meet the requires catalytic condition of cellulose conversion into 5-HMF(T<200°C).

3.3.The acid and base features of catalysts

Fig.8.NH3-TPD curves of Poly-H (a);Poly-P8 (b);Poly-P9 (c);Poly-P10 (d);Poly-P6 (e);Poly-P11 (f);Poly-P12 (g);Poly-P13 (h);and UiO-66-SO3H (i).

The quantification of the total acid and base density of samples were evaluated by NH3-TPD and CO2-TPD.According to the previous analysis of TG results,the samples experience significant weight loss at high temperature.NH3and CO2desorption experiments were carried out at a heating rate of 10°C?min-1from room temperature to 500 °C.Figs.7 and 8 show the desorption peaks of CO2-TPD and NH3-TPD for different samples,respectively.As the previous reports show that the peaks appeared at around 150 °C corresponds to the physical adsorption of NH3or CO2.However,the peak appearing in the higher temperature region corresponds to the chemisorption of NH3or CO2at acidic or alkaline sites on the surface of the samples.The highest peak of CO2-TPD and NH3-TPD curve of the pure polymer (Poly-H) appears at 303 °C and 306 °C.The highest peak value of UiO-66-SO3H and UiO-66-NH2is greater than 400°C(Fig.7(a),(i)and Fig.8(a),(i)).When the polymers were compounded with MOFs,the highest peak value removed to the high temperature region,indicating that the acid or base density of the polymer/MOFs were improved to some extent.Table 3 exhibits the resultant acidic and basic features of the prepared catalysts in detail.The results showed that the introduction of UiO-66-NH2and UiO-66-SO3H increased the acid and base density of catalysts,such as Poly-H (without Pickering particles) acidity 2.03 mmol?g-1,Poly-P8 (with UiO-66-SO3H) acidity 2.73 mmol?g-1,Poly-H alkalinity 0.94 mmol?g-1and Poly-P13(with UiO-66-NH2)alkalinity 1.43 mmol?g-1.It can be proved that the synthesized dual acid-base catalysts have more active sites.Meantime,the acid/base density of prepared catalysts can be controlled by adjusting the number of stable particles,which canenhance the synergistic effect of acid and base sites for the conversion of cellulose to 5-HMF.

Table 3 The acid and base densities of samples

3.4.Catalytic performance on cellulose conversion to 5-HMF

Catalytic transformation of cellulose to 5-HMF was first tested at different reaction temperature and different time.As shown in Fig.9(a),with Poly-P6 as the catalyst,the overall trend of the catalytic performance tends to be better with the increase of reaction time and temperature.Here,after the catalytic experiment reacted at 110°C for 6.0 h,the catalytic conversion efficiency of cellulose to 5-HMF can be calculated as about 26.5%.In particular,the 5-HMF yield of 35.5% is obtained at 130 °C for 3.0 h.When the reaction time is further extended to 6 h,only 28.8% yield can be obtained.At the same time,the yield of 5-HMF decreased from 31.1% to 9.8% after increasing the reaction temperature to 170 °C and the reaction time from 0.5 to 6.0 h.The results suggest that an excessive increase in reaction time or temperature may led to the formation of some undesired by-products such as soluble polymers and insoluble humin oligomers,as shown in Fig.S3.Although it is difficult to test the type and the conversion rate of by-product,the color of the reaction mixture deepens as the reaction progresses,which consist with the results of the researchers,as shown in Fig.S4 [21,26,47].Taking the energy efficiency into consideration,130 °C is recognized as the optimum temperature (reaction time 3 h) for further study.

Fig.9.Effects of reaction time and temperature on 5-HMF yield from cellulose conversion,reaction conditions:2.0 g of[EMIM]-Cl,100 mg cellulose and 50 mg Poly-P6 as the catalyst(a);effects of the catalyst loading on 5-HMF yield and cellulose conversion for cellulose-to-5-HMF,reaction conditions:2.0 g of[EMIM]-Cl,100 mg cellulose,130°C,3 h and Poly-P6 as catalyst (b);influence of catalyst structure on the 5-HMF yield and cellulose conversion for cellulose-to-5-HMF,reaction conditions:2.0 g of [EMIM]-Cl,100 mg cellulose,130 °C,3 h (c).

Then,the effect of the catalyst amount is also examined in details.Fig.9(b) show that the conversion efficiency of cellulose increased from 50.5% to 77.5% as the catalyst amount increased from 30.0 mg to 60.0 mg,and the 5-HMF yield increased from 20.3% to 36.9%.When the catalyst amount is further increased to 70.0 mg,the cellulose conversion is enhanced to 79.5%,but the yield of 5-HMF is only 35.6%.The previous repot shows that this phenomenon might be explained by the higher concentration acid site leading to further decomposition of 5-HMF to levulinic acid(Fig.S3)[48,49].Hence,catalyst loading should be 60 mg,the standard amount for cellulose conversion with large 5-HMF yield.

We have investigated the influence of pore structure during reaction on the product yieldviathe cellulose conversion,which is given in Fig.9(c).It can be seen that prepared catalysts with different average pore size and average interconnecting pore size exhibit different catalytic performance for cellulose conversion to HMF.It is notified that the catalytic performance of Poly-P1 and Poly-P5 is lower than that of the other samples may be caused by the partial collapse of their pore structure.The catalytic performance of Poly-P4 and Poly-P7 is lower than that Poly-P6 which may be due to the fact that the small pore sizes on the surface of Poly-P4 and Poly-P7 limits the contact of the cellulose to the active sites inside of catalysts.Gaoet al.reported that the pore size was closely related to catalytic performance,and suitable macroporous structure on the surface of catalyst is beneficial to improve the contact frequency between cellulose and catalytic active sites.[50].Wanget al.also studied that the micropores were not conducive to the migration of 5-HMF and fructose in the pores,the substrates mainly reacted with the surface-active sites of the catalyst.It is resulted that a low fructose conversion,which was instead conducive to side reaction,such as the generation of humin [51].As seen from Table 2,Poly-P6 with the largest pore size (53.3 μm)and connection pore size (10.5 μm) has the highest catalytic performance.The cellulose conversion is up to 77.5%,and the 5-HMF yield of 36.9% is obtained.

As the previous report show that the regulation of acid and base strength of catalysts is the key to the conversion of cellulose to 5-HMF[49].In our subsequent work,the emulsification condition of Poly-P6 was taken as the standard to investigate the influence of the acid/base mass ratio on catalytic performance.The catalytic performance of the obtained catalysts (Poly-P8,Poly-P9,Poly-P10,Poly-P6,Poly-P11,Poly-P12 and Poly-P13)were studied using fructose,glucose and cellulose as raw materials,respectively.The conversion of fructose and glucose to 5-HMF was tested after for 3 h catalytic reaction in DMSO/water (V/V=1:1) solvent at 130 °C.As shown in Fig.10(a),the fructose conversion is kept at a high level,in which Poly-H (without Pickering particles) acted as catalyst could obtain a 5-HMF yield of 79.2%.Wanget al.reported that the -SO3H group can be used as the Br?nsted active site to effectively convert fructose to 5-HMF,and the amount of -SO3H was related to the reaction orders [51].The successful introduction of-SO3H can be demonstrated by the FT-IR analysis of Poly-H in Fig.4(c).When Poly-P8 was used as catalyst,the highest fructose conversion (96.3%) and HMF yield (88.5%) can be obtained.This phenomenon may be caused by the enhanced acid strength of the catalyst after the introduction of acidic UiO-66-SO3H particles,which consisted with the results of CO2-TPD analysis.It can be clearly seen that with the decrease of mass of acidic particles embedded in the catalyst,the fructose conversion decreases,and the yield of 5-HMF is also decreased.

Fig.10.Effects of catalysts with different acid/base strengths in fructose and glucose to conversions,reaction conditions:10 ml DMSO/water (v:v=1:1),100 mg substrate,130°C,3 h and 60 mg of Poly-P6 as catalyst(a,b);effects of catalysts with different acid/base strengths on 5-HMF yield from cellulose conversion,reaction conditions:2.0 g of[EMIM]-Cl,100 mg cellulose,130 °C,3 h and 60 mg of Poly-P6 as catalyst (c).

The yield of 5-HMF and glucose conversion in the presence of prepared dual acid-base bifunctional catalysts are shown in Fig.10(b).Different from the conversion process of fructose,the catalytic conversion of glucose to HMF requires two steps:glucose isomerization to fructose,which needs base active sites,and then fructose dehydration to produce 5-HMF [17,52].The results of EDS and XPS prove that the prepared catalysts have a lot of basic group of -NH2.The 5-HMF yield is observed as 48% with Poly-P8(without functional particles) as catalyst.When Poly-P8 prepared by UiO-66-SO3H was introduced as catalyst,only 46.2% 5-HMF yield and 89.3% glucose conversion can be obtained.When the Poly-P13 prepared by prepared by UiO-66-SO3H was introduced as catalyst,the 5-HMF yield was about 47.4%.It is clearly seen that the yield of 5-HMF(56.7%)was highest when Poly-P11was used as catalyst.This result proves that the synergistic effect of suitable acid-base active sites can effectively enhance the efficiency of glucose conversion to 5-HMF [53].

Based on the above discussion,the Br?nsted acid active site of catalysts plays a key role in the preparation process of 5-HMF from fructose dehydration,and the base active sites is conducive to the isomerization of glucose to fructose.In addition,the Br?nsted acid active site of catalysts can promote the decomposition process for cellulose to glucose [54].As shown in Fig.10(c),all the catalysts with both Br?nsted acid and base active sites exhibit excellent production efficiency of 5-HMF from cellulose under the optimal experimental conditions (60.0 mg catalyst and 130 °C for 3.0 h).When the Poly-H (without functional particles) was used as catalyst,the cellulose conversion and the 5-HMF yield can be obtained as 70.6% and 31.5%,respectively.The cellulose conversion efficiency is increased with the acid density of catalyst.When Poly-P8(with UiO-66-SO3H as Pickering particles)was used as catalyst,the cellulose conversion efficiency reached the maximum level(82.1%) but the yield of 5-HMF is only 30.5%.It is indicated that acid sites facilitate the conversion of cellulose,while over acidity lead to the decomposition of 5-HMF.Meanwhile,when the Poly-P13 (with UiO-66-NH2as Pickering particles) with highest base density was used as catalyst,the conversion rate of cellulose was about 75.0% and the 5-HMF yield was about 34.3%.It is concluded that the base sites are beneficial to product 5-HMF.Therefore,[EMIM]-Cl can break the hydrogen bond network of cellulose and depolymerize to disaccharide.The disaccharide can be depolymerized to glucose by Br?nsted acid active sites.Then,glucose can be converted to 5-HMFviafructose under the synergistic effect of Br?nsted acid and base active sites,as shown in Fig.11.Finally,76.8% cellulose conversion efficiency and the highest 5-HMF yield (40.5%) can be obtained by using Poly-P12=1:5) as catalyst.

Fig.11.The plausible catalytic scheme for the conversion of cellulose to 5-HMFover polymer catalyst in the [BMIM]-Cl.

Fig.12.5-HMF yield obtained from the catalytic conversion of cellulose by Poly-P12 during 5 times of repeated use(reaction conditions:2.0 g of[EMIM]-Cl,100 mg cellulose,130 °C,3 h and 60 mg catalyst) (a);and FT-IR (b).

We compared the catalytic performance of different commercial products with our prepared catalysts in the conversion process of cellulose to 5-HMF,and the specific performances of these samples are shown in Table S1.It is obviously that the high 5-HMF yield is obtained when H2SO4as catalyst.However,the homogeneous catalyst is difficult to recycle and corrode the reactor,which limits its further application [55].Whether acidic UiO-66-SO3H or UiO-66-NH2is used for cellulose conversion of 5-HMF,the HMF yield is low.Although the MOFs material has a large specific surface area [56],the small pore size is not conducive to the mass transfer of cellulose,resulting in a low 5-HMF yield.The 5-HMF yield increased slightly when they were together as catalysts in the catalytic process,because the acid-base active sites synergistically promoted the conversion of cellulose to 5-HMF.Amberlyst-15 is a good candidate as catalyst for conversion of cellulose to 5-HMF due to the strong acid preformation on it.Meantime,benefiting from its macroporous structure,the yield of 5-HMF obtained is higher than that of H-ZSM-5 with mesoporous structure.Base on the above analysis,we prepared catalyst(Poly-P12)has the highest 5-HMF yield(40.5%)than the other samples,which can be attributed to its macroporous structure and the synergistic effect between the acid-base active sites.

3.5.Catalytic stability test

The recyclability of catalyst in the conversion process of cellulose to 5-HMF was evaluated.Five recycling tests of Poly-P12 polymer catalyst at the optimum condition were studied.As shown as Fig.11(a),the 5-HMF yield decreases from 40.5%to 37.2%after five cycles.It may be caused by some oligomeric products absorbing on the surface of catalyst,hindering the connection between the active sites and cellulose [57].As shown in FT-IR image (Fig.12(b)),the replaced catalyst shows the same characteristic peaks to the fresh one,which illustrates that the overall composition of the recovered catalyst is not changed.The slight reduce of catalytic performance after the cyclic experiments may be also caused by the covering or shedding of -SO3H of acid active sites and -NH2of the alkali active center on the polymer skeleton [58].

4.Conclusions

In summary,the acid-base bifunctional macroporous polymer catalyst was prepared by using the MOFs (UiO-66-SO3H and UiO-66-NH2)/Tween 85 co-stabilized Pickering HIPEs as template.Effects of the volume fraction of the internal phase,surfactant concentration and the mass ratio of acid/base particles on the catalytic activity were discussed in detail.A series of characterization and experimental results proved that the MOFs/polymer composite has great catalytic ability for conversion of cellulose into 5-HMF process,due to the synergistic effect between the large pore size and acid-base active sites.Under the optimized condition,the highest 5-HMF yield (40.5%) can be achieved by using the Poly-P12 as catalyst in the[EMIM]-Cl solution.Moreover,the cyclic catalytic experiments show that this catalyst has great recyclability.This study can provide a good experimental reference for the application of polymer catalysts in the fields of environment-friendly chemical materials and the transformation of high value-added chemicals.

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

This work was financially supported by the National Natural Science Foundation of China (No.21606100),the Natural Science Foundation of Jiangsu Province(No.BK20180850),the China Postdoctoral Science Foundation (Nos. 2019M651740 and 2019T120397) and the Young Talent Cultivate Programme of Jiangsu University.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.06.019.