Catalyzing Cascade Production of Methyl Levulinate from

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Catalyzing cascade production of methyl levulinate from polysaccharides using heteropolyacids HnPW11MO39 with Brønsted/Lewis acidic sites Xueyan Zhang, Yue Li, Lifang Xue, Shengtian Wang, Xiaohong Wang, and Zijiang Jiang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02042 • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on November 5, 2017

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ACS Sustainable Chemistry & Engineering

Catalyzing cascade production of methyl levulinate from polysaccharides using heteropolyacids HnPW11MO39 with Brønsted/Lewis acidic sites Xueyan Zhang,[a] † Yue Li, [a] † Lifang Xue, [a] † Shengtian Wang, [a] † Xiaohong Wang,[a]*† Zijiang Jiang,[b]** † [a]

† Key Lab of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry,

Northeast Normal University, No. 5268, Renmin street, Changchun 130024 (P. R. China number 5268), E-mail address: [email protected], [a]


Northeast Normal University, No. 5268, Renmin street, Changchun 130024 (P. R. China), Email address: [email protected] [a]


Northeast Normal University, No. 5268, Renmin street, Changchun 130024 (P. R. China), Email address: [email protected] [a]

† Center of analysis and measurement, Northeast Normal University, No. 5268, Renmin street, Changchun 130024 (P. R. China), E-mail address: [email protected]

ACS Paragon Plus Environment

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[a]

* † Key Lab of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry,

Northeast Normal University, No. 5268, Renmin street, Changchun 130024 (P. R. China), Email address: [email protected] [b]

** † Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, National

Analytical Research Center of Electrochemistry and Spectroscopy, No. 5625, Renmin street, Changchun 130024 (P. R. China) Tel.: + 86- 431-85262452; E-mail address: [email protected]

KEYWORDS: Heteropolyacids, Double acidic sites, Catalytic methanolysis, Microwaveassistance, Cellulose

ABSTRACT: A series of Lewis acid metals mono-substituted phosphotungstic acids HnPW11MO39 (HPWM, M = CuII, ZnII, CrⅢ, FeⅢ, SnⅣ, TiⅣ, and ZrⅣ; for Ti and Zr, the number of oxygen is 40) was evaluated in direct production of methyl levulinate (ML) from cellulosic biomass in cascade reaction. One of the solid catalysts H5PW11TiO40 (HPWTi) was found to be highly efficiency for generation of ML from mono- or polysaccharides, reaching 51.3 % ML yield directly from cellulose. And under microwave-assistance, the efficiency could be improved to 62.6 % ML yield within 2 h, which was almost the best result so far among reported solid catalysts. Identification of the reaction intermediates and the products provided some insight into the reaction mechanism and showed the requirement of certain Brønsted/Lewis acid ratio as 2.84/1 for HPWM. Moreover, the different metals in catalysts profoundly affected the Lewis or total acidity, and therefore the catalytic activity and selectivity to ML or methyl glucosides (MG).


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HPWTi acted as a heterogeneous catalyst after being calcinated at 200 ºC and showed high recyclability with minor loss of performance.

INTRODUCTION Biomass is recognized as a sustainable and possible resource for the production of chemicals and fuels, which the conversions have received much attention in recent years due to the depletion of fossil fuel resources.1 Among various chemicals derived from cellulose, levulinic acid (LA) or levulinate is regarded as one of the most promising building blocks for the biomass refinery in fuel additives, polyacrylates, biodegradable herbicides, photo-synthesis promoters,2 and oxygenate additives for gasoline and diesel fuels.3-8 First report concerning alkyl levulinates was originated from the 19th century using levulinic acid and alcohol as feedstocks catalyzed by HCl.9 Due to the recent interest for biomass transformation and the discovery of new applications for levulinates or 5-hydroxymethylfurfural, their production has considerably attracted much attention during the last five years.4,10-18 The most important process for levulinate production involves the treatment of cellulosic feedstocks with alcohol in the presence of acid catalysts. However it is most difficult to synthesize levulinates from cellulose in good yields because of the strong β-1, 4-glycosidic bonds between glucose subunits. Therefore, high temperatures (190 - 250 ºC) and high pressure were required when using mineral acids as homogeneous catalysts,12 which are well-known drawbacks such as catalyst separation, reactor corrosion, and recyclability. The use of solid catalysts is relatively less developed and only several solid catalysts had been used including M(OTf)3 combined with 2-naphtanelesulfonic acid, metal oxides, sulfated zirconia, sulfonated carbons, heteropolyacids, and resins.4,19-27 However, transferring difficulty between cellulose and the solid catalysts hinders its application in production of alkyl levulinates directly from cellulose. Another drawback is the formation of


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alkyl glucosides, 5-hydroxymethylfurfural (HMF) ether, and humin accompanied with the production of alkyl levulinates.28 In addition, elevated temperatures and acidic media might promote dehydration of alcohol to give ether.19,29 Therefore, the best result reported so far was 56.0 % yield of ML catalyzed by heterogeneous niobium-based phosphate at 180 ºC for 24 h directly from cellulose,30 which the higher efficiency was attributed to the mixed-acids consisting of both Brønsted and Lewis acids. Among the heterogeneous catalysts, heteropolyacids (HPAs) have been regarded as potential candidates for the conversions of biomass due to their fascinating architectures and excellent physicochemical properties such as strong Brønsted acidity, high proton mobility and good stability.31 Deng et al. investigated the efficiency of soluble H3PW12O40 (HPW) and H4SiW12O40 (HSiW) in treatment of cellulose with methanol and ethanol at 180 ºC for 0.5 h, which mainly produced alkyl glucosides with 43.0 and 48.0 % yields, respectively.32 The ML was obtained only as a by-product, but its yield increased to 20.0 % with time and temperature at the expense of the methyl glucosides confirming a consecutive reaction pathway. N. Essayem reported the use of insoluble Cs2.5H0.5PW12O40 in supercritical methanol-water (300 ºC) to give up to 20.0 % yield of ML after 1 min.33 Liu et al. used Fe-exchanged HPW as a homogeneous catalyst for alcoholysis of cellulose to ML with 14.0 % yield at 220 ºC and Ar 20 bar for 2 h.34 More recently, Chang’s group reported K-exchanged phosphotungstic salts KH2PW12O40 as a solid catalyst in saccharides conversion in alcohol, finding the yields of ML varying from 64.6, 14.5, 35.4, 52.3 and 14.8 % corresponding to substrates as fructose, glucose, sucrose, inulin and cellulose, respectively,35 under 150 °C for 2 h. The reason why such low yields of ML from other saccharides than fructose was mainly attributed to only existence of single Brønsted acidic sites for KH2PW12O40, which did not favor for the sequent reaction to ML. Recently, we found that


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production of levulinic acid directly from cellulose needed HPA catalysts not only with strong Brønsted acidity and also certain Lewis acidity.36 A double acidic H5AlW12O40, free proton for Brønsted and Al3+ for Lewis acidity, was used as a catalyst in cascade conversion of cellulose to LA with 74.8 % yield at 98.9 % conversion of cellulose. However, during the synthetic procedure of H5AlW12O40, a lower yield was found due to the hydrolysis of AlCl3 in aqueous solution. In our previous report,37 double acids HPWM (M represented Lewis acid metal, M = CuII, ZnII, CrⅢ, FeⅢ, SnⅣ, TiⅣ, and ZrⅣ) had been evaluated for their acidic properties and the influence of different acid sites on esterification and transesterification reaction. It was found that HPWM (M = CuII, ZnII, CrⅢ, FeⅢ, SnⅣ, TiⅣ, and ZrⅣ) was most active in esterification of glycerol and alcohol. In order to develop highly efficient HPA catalysts on conversion of cellulosic feedstocks into chemicals, Lewis acid metals mono-substituted HPWM had been evaluated for production of ML from cellulose or other saccharides. The conversion of polysaccharide by HPWM has a great value for the advance of HPA catalysis and the biomass transformation. Ⅲ Ⅲ Ⅳ Ⅳ Ⅳ Herein, a series of HPWM (M = CuII, ZnII, Cr , Fe , Sn , Ti , and Zr ) was evaluated in

methanolysis of polysaccharides in order to seek the most active HPA species. To the best of our knowledge, this is the first study which underlines the influence of the nature of the acidity on double acidic HPAs in the field of alcoholysis of cellulose. Through the study, the influence of B/L ratio of the acidic sites on conversion of cellulose and ML yield could be established for HPA catalysis, which is available for further designation of HPA acidic catalysts. And it was of great value to clarify the pathways for cellulose conversion step-by-step to ML being catalyzed by various active sites with different strength, which could guide the tailoring of B/L ratio for other acid catalysts. Then, the microwave as an effective assistant method has been used in biomass conversion widely.38-40 Thus, in order to obtain a highly efficient reaction procedure,


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microwave-assistance was also used in the methanolysis of polysaccharides catalyzed by HPWM. To the best of our knowledge, there is seldom studying of the role of the double acidity for obtaining the production of levulinates esters from cellulosic substrates systematically. EXPERIMENTAL SECTION Materials Microcrystalline cellulose (white, average particle size 50 mm) was obtained from J&K Chemical Ltd. (Beijing, China) and the XRD of cellulose was shown in Fig. S1. 5Hydroxymethyl furfural, Levulinic acid, methyl levulinate and methyl lactate (MLA) was obtained from Aladdin Chemistry Co., Ltd. and was of > 99.0 % grade. 5Methoxymethylfurfural were obtained from Biochemistry and were of 98.0 > % grade. Other reagents and chemicals were all of analytical grade from Sinopharm Chemical Reagent Co., Ltd. and used without further purification or treatment. HPW, HSiW, H5BW12O40 (HBW) were prepared according to the literature method.41 HPWTi, H5PW11CuO39 (HPWCu), H3PW11SnO39 (HPWSn), H4PW11CrO39 (HPWCr), H5PW11ZrO40 (HPWZr), H5PW11ZnO39 (HPWZn) and H4PW11FeO39 (HPWFe) were prepared according to the literature method.35 K5PW11TiO40 was prepared on the basis of the previous literature.42 The K5PW11TiO40 was prepared by the following steps: Ti(SO4)2 solution (6 mmol in 2 M H2SO4) was added to the aqueous solution of Na7PW11O39 (6 mmol) and the PH of the mixed solution was adjusted to 5.6 by NaHCO3. The white precipitate (K5PW11TiO40) was formed after adding the solid KCl (2.24 g) to the above mixed solution and the precipitate was filtrated and recrystallized with water for three times. The other catalyst was prepared in the procedure of 29.4 g (100 mmol) Na2WO4 and 1.29 g (9.1 mmol) Na2HPO4 were added to 80 mL deionized water at ambient temperature and the metal salts aqueous solution (12 mmol, 20 mL, chlorides for Sn; nitrates for Fe, Cr and Zn; sulfate for


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Al, Cu and ZrO2·xH2O) was added to the above solution under vigorous stirring. Then the 100 mL deionized water was added former solution and the PH of the mixed solution was adjusted to 5.6 by HNO3. The KCl (3.39 g) was added to the mixed solution for the precipitated (KPWM, M= TiIV, CuII, AlIII, SnIV, FeIII, CrIII, ZrIV and ZnII) and recrystallized with water for three times. The potassium cations of KPWM (M= TiIV, CuII, AlIII, SnIV, FeIII, CrIII, ZrIV and ZnII) were replaced by H+ using strong-acid cation exchange resins (Type 732, 20 g) for several times to obtain HPWM, until no K+ can be detected by ICP analysis. The solution of HPWM was rotary evaporated at 50 °C to obtain the product and then the HPWM were calcinated for 4 h at 200 °C to obtain insoluble products. The components, structure and acidity were characterized by IR spectroscopy, XRD, 31P NMR, elementary analysis and acidic strength. IR spectra (4000-500 cm−1) was recorded in KBr discs on a Agilent Cary630 IR spectrometer. The IR spectra of adsorbed pyridine (Py-IR) were depicted by subtracting the spectra before and after exposure to pyridine. The 31P NMR spectra of the catalysts were achieved with a Bruker AM 400 spectrometer at 161.9 MHz. X-ray diffraction (XRD) patterns of the sample were collected on a Japan Rigaku Dmax 2000 X-ray diffractometer with Cu Kα radiation (λ = 0.154178 nm). Elemental analysis was carried out using a Leeman Plasma Spec (I) ICP-ES and a PE 2400 CHN elemental analyzer. From the results of the elemental analyses of the HPWTi, the P, W and Ti contents in catalyst were P, 1.14; W, 73.60 and Ti, 1.70 %, respectively. Compared with the calculated values of P, 1.13; W, 73.64 and Ti, 1.71 %, the results suggest that the molar ratio of P : W : Ti elements by the actual measured was 1 : 11 : 1 and thus it can be confirm that the molecular formula of catalyst was the H5PW11TiO40.


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The determination of the acidic properties was according to the literature method37 (results shown in Table 1). The total acid content of the solid catalysts was obtained by the titration.43 0.05 g HPWM suspended in 45 mL acetonitrile and then the mixture was stirred for 3 h. The density of acid sites in the catalysts was measured by titration with a solution of n-butylamine in acetonitrile (0.05 M) using the indicator anthraquinone (pKa= −8.2). The IR spectra of adsorbed pyridine (Py-IR) helped to measure the acid content and distinguish the properties of acid sites (Lewis or Brønsted). The samples were exposed to the pyridine vapor for 12 h under vacuum (10-3 Pa) at 60 °C for Py-IR. The amount of Brønsted and Lewis acid sites was estimated from the integrated area of the adsorption bands at ca. 1540 and 1450 cm−1, respectively.44 Catalytic procedure The reaction was performed in batch conditions using 10 mL stainless steel with Teflon liner autoclave. The air inside was flushed with N2 for 30 min. Cellulose (0.15 g) and catalyst (0.08 mmol) were introduced with 8 mL of methanol in an autoclave that was heated up to 160 ºC (the process of the heated to 160 ºC needs about 20 min and then began to record the reaction time) for 7 h with stirring (800 rpm). The reaction was stopped by rapidly cooling in ice-bath to room temperature. After the reaction, unreacted cellulose, solid catalyst and formed humin were separated through centrifugation then dried and weighted. A yellow liquid was obtained containing a mixture of organic products. The residue powder was firstly treated by hot H2SO4 solution (0.5 M, 160 ºC for 6 h) to remove unreacted cellulose because it could be dissolved quickly and completely, whereas the catalyst and humin did not dissolve. Then the catalyst was separated to dryness for reuse without further treatment. After four recycles, the rest of the powder after removal of cellulose was dried and weighted for catalyst and humin. Then the


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humin was dissolved in methanol at 100 ºC for 12 h to be quantified.45,46 The left solid was catalyst to be weighted and dried for the next further run. The reaction was also carried out in the XH-800C computer microwave apparatus. The system was equipped with sealed Teflon reaction vessels, an internal temperature sensor and a pressure sensor. The reaction temperature was set at 160 ºC and the reaction time was set from 50 min to 150 min. The reaction mixture consisted of 0.15 g cellulose, 8 mL of methanol and 0.08 mmol of catalyst. After the reaction, the vessels stopped and cooled to room temperature. The amount of ML, 5-methoxymethylfurfural (5-MMF), methyl lactate and dimethyl ether were analyzed by gas chromatography (Agilent 6890) equipped with an HP-5 capillary column and flame ionization detector. The glucose, fructose, methyl glucosides and methyl fructoside were measured by high-performance liquid chromatography (HPLC) equipped with a refractive index detector (Shimadzu LC-10A, HPX-87H column). The contents of all above mention products after the reaction solution were determined by standard curve of the standard samples solution of known concentration. A TOC (TOC was the total oxygen carbon) analyzer (TOC-L CPH, Shimadzu, Japan) were used to monitor TOC values before and after reaction. RESULTS AND DISCUSSION The performance of the catalysts under conventional conditions Catalyst screening The comparison between different catalysts on the methanolysis of cellulose had been done under conventional conditions, including H2SO4, HPW, HSiW, HBW, HPWTi, HPWZr, HPWZn, HPWCu, HPWCr, HPWFe, and HPWSn (Table 1) under the reaction conditions as 0.08 mmol of catalyst, 0.15 g of cellulose, and 8 mL of methanol at 160 ºC for 7 h. No ML was detected without any catalysts, showing the essentials of Brønsted acid catalysts for methanolysis of


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cellulose. The conversions were in range of HPWTi > HPW > HSiW ~ HPWCu > HPWSn ~ HBW > HPWZr ~ HPWZn > HPWFe > H2SO4 > HPWCr. For Brønsted homogeneous acids, HPW presented highest activity being opposite to the number of protons of HPAs as HBW > HSiW > HPW. The different performance was attributed to the differential in dissociation constants in alcohol for three HPAs.48,49 In polar solvents, HPW is completely dissociated at the three steps, while HSiW and HBW are partially dissociated from the third step. This resulted in higher Brønsted acidity for HPW and therefore higher conversion of cellulose than other HPAs. Table 1 For metal substituted HPWM (M = CuII, ZnII, CrⅢ, FeⅢ, SnⅣ, TiⅣ, and ZrⅣ), homogeneous HPWTi (without being calcinated at 200 °C for 4 h) could improve cellulose conversion to 97.8 % for 5 h reaction, which much higher than HPW did (93.7 % for 7 h). This could be attributed to the higher amount of Brønsted acidic sites and also introduction of Lewis acidic sites as well.44 Solid HPWTi also cold promote the cellulose methanolyze with conversion of 94.7 % for 7 h. And the Lewis acid metals also influenced the conversion in range of Ti > Cu > Sn > Zr > Zn > Fe > Cr. From this it can be seen that the number of protons is one of the key factors for contribution to Brønsted acidity and the cellulose conversion as well. For Ti, Zr, Zn and Cu HPAs, there are five protons in one unit and are the main contribution to their Brønsted acidity. But Sn HPAs is the exception, which owns three protons and three negative charges for heteropolyanions. The probable reason for Sn might be that its large electronegativity allows the strong attraction of electron pair of oxygen atom, which weakens the O-H bond resulting in proton being easily dissociated (Scheme 1). Therefore, the dissociation constant for HPWSn could decrease, hence Brønsted acidity increases. Scheme 1


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The relationship between TOF and B/L acid ratio was given in Fig. 1a. It can be seen that the cellulose conversion increased as the increasing of B/L ratio from Cr to Ti, then decreased from Cu to Zn as the increasing of B/L ratios. The maximum value of conversion was obtained by HPWTi with B/L ratio of 2.84, showing that only those with moderate Lewis acidity and stronger Brønsted acidity could give higher conversion of cellulose. From the above result, it could be concluded that the coordination of Brønsted with Lewis acidic sites can enhance the reaction rate of cellulose in methanol, while Brønsted acidic sites played a main role on cellulose conversion. HPWFe was exception, which might be attributed to the further generation of Brønsted acidity during reaction under higher temperature in presence of trace amount of water: Fe3++ cellulose + H2O → Fe2+ + celluloseox + H+. This hypothesis was determined through methanolysis of cellulose in presence of small amount of water (7.6 mL MeOH + 0.4 mL H2O) giving 88.5 % conversion and 37.4 % ML yield. And the pH value also changed during the reaction, which also confirmed this hypothesis. The production of oxidation celluloseox also promoted the cleavage of β-1, 4-glycosidic bonds. The above two synergistic effects of enhanced Brønsted acidity and redox ability promoted the conversion of cellulose, which permitted HPWFe exhibit almost the same activity as Zn and Zr-HPAs with strong Brønsted acidity. Fig. 1 The production distribution varied from the acidic nature of HPWM. Without any catalyst, no ML and methyl glucosides were tested. Addition of Brønsted acidic catalysts, the main product was methyl glucosides,51 while the selectivities to ML were almost the same for HPW (20.5 %), HSiW (20.7 %) and HBW (20.3 %) (Table 1). This indicated that single Brønsted acidity did not favor for the generation of ML. When solids containing Lewis acid sites were added, the yields of ML were increased to the range of 26.0 ~ 52.0 % and hence the yields of methyl glucosides


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were decreased. From this, it can be concluded that the yields of ML largely depended on double acid sites than on single acid site catalysts, showing the essential of Lewis acid sites in solid catalysts in alcoholysis of cellulose to ML. The yields of ML were in order of HPWTi > HPWCu > HPWSn > HPWZr > HPWZn > HPWFe > HPWCr > HPW > HSiW > HBW > H2SO4. And the selectivities of ML changed from the different Lewis acid metals as Ti (B/L acid ratio 2.84/1, selectivity to ML 54.2 %) > Cu (3.71/1, 53.5 %) > Sn (3.47/1, 51.1 %) > Zr (4.37/1, 38.6 %) > Zn (5.33/1, 37.3 %) > Fe (0.98/1, 36.2 %) > Cr (1.28/1, 32.1 %) (Table 1, Fig. 1b). It can be seen that not very high or low B/L acid ratio gave rise to higher selectivity to ML. Therefore, it can be concluded that a mixed Brønsted/Lewis HPA catalyst with B/L ratio of 2.84/1 had the highest selectivity for ML, compared to pure Brønsted HPAs, and mixed HPAs with higher B/L acid ratio than 3.71 to 5.33. In previous report, 0.02 mmol of M(OTf)3 (M = In and some Lewis acid metals) and 0.10 mmol of Brønsted acids 2-naphalenesulfonic acid were used as combined catalysts for production of ML with 75.0 % yield at 180 ºC for 5 h, which B/L ratio of 5 : 1 gave the highest selectivity to ML.19 Our result was not total aligned with Tominaga’ report. But our result was similar to that by X. H. Liu, which 56.0 % yield of ML was obtained by solid niobium-based phosphate with B/L acid ratio of 2.15/1 at 180 °C for 24 h.30 HPWFe and HPWCr with lower B/L acid ratios and lower Brønsted acid contents gave lower selectivity to ML, showing that Brønsted acid contents were first parameter for the production of methyl levulinate. Methyl glucosides was an intermediate product during the alcoholysis of cellulose and the selectivities to methyl glucosides in our tested catalysts were almost in the opposite order to ML as H2SO4 > HPW > HSiW > HBW > HPWZn > HPWSn > HPWCu > HPWZr > HPWTi > HPWFe > HPWCr. In the previous report, methyl glucosides were main product catalyzed by


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HSiW or HPW26 with single acidic sites. Mixture acidity of combining the Brønsted acid sites with Lewis acid sites might enhance the generation of ML.19 Therefore, the acid type and strength should be taken into high consideration as the best approach for obtaining the desired products. The presence of relatively weak Brønsted acidity yielded more ML, and 51.3 % of ML was obtained via catalysis by HPWTi. Strong Brønsted acidity in the catalysts such as HPW, HSiW or HBW in Table 1 was beneficial for selectivities to methyl glucosides, but detrimental for selectivities to ML. Hence, the formation of ML should be summed up according to the above result as the two steps: solvolysis of cellulose to sugars catalyzed by Brønsted acid sites and then converting of above sugars to ML mainly catalyzed by Lewis acid sites. HPWTi could perform the conversion of cellulose to ML with 51.3 % yield at 160 ºC for 7 h. At this stage, there is no doubt that the understanding of the catalytic effect of HPWTi on the conversion of cellulose required further investigation, as attempted in the following: In order to determine the effect of Brønsted or Lewis acidic sites on the methylosis of cellulose, fructose and glucose had been firstly used as feedstocks. Catalyzing fructose was carried out under the reaction conditions as 0.15 g of fructose, 0.08 mmol of catalyst, 8 mL of methanol at 80 o C for 4 h (Fig. 2). It can be seen that HPWTi and HPW gave almost the same conversion of fructose and yields of ML under the same reaction conditions. Meanwhile, HPWZr and HPWZn presented lower activity in fructose alcoholysis. This might be attributed to the higher Brønsted acidity for HPWTi and HPW than for Zn- and Zr-HPAs. The products of methyl fructoside, 5methoxymethylfurfural, and ML had been found during the reaction, showing that production of ML from fructose catalyzed by HPWM probably underwent fructose → methyl fructoside → 5methoxymethylfurfural → ML. These tandom reactions were mainly controlled by Brønsted acidic sites. Methyl fructoside was the main product in the early time of the reaction, showing


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that methyl fructoside might be formed before HMF and 5-methoxymethylfurfural. And no methyl lactate was found during the reaction using pure Brønsted acid HPW, while there were few methyl lactate (< 2 %) being generated in HPWTi system due to the presence of Lewis acid promoting the retro-alkol reaction of methyl fructoside. The yield of methyl lactate could be enhanced as increasing reaction temperature and time under HPWTi. And the generation of methyl lactate was prevented by relative stronger Brønsted acid of HPWTi, showing that higher B/L ratio favored for the high selectively production of ML from fructose. Further, dimethyl ether was generated catalyzed by HPW due to its higher contends of free acid,34,52,53 while was negligible for HPWTi being attributed to its relative weak Brønsted acidity and its solid form. This showed that the formation of Me2O was limited using HPAs with mixed Lewis and Brønsted acidity, which is benefit for separation of dimethyl ether from the product. Therefore, it can be concluded that HPWTi with 1.59 mmol/g Brønsted acidity was enough to promote fructose alcoholysize into ML with highest yield of 67.0 %, while the formation of dimethyl ether was limited. And HPWZr and HPWZn did not give higher ML selectivity than HPWTi and HPW did, showing that HPAs with higher B/L ratio but low Brønsted acidity available for generation of ML. Previous studies showed that different acidic catalysts gave a 57.0 - 80.0 % yield of levulinate esters from fructose.24,34,52,54-57 HPWTi presented the top efficiency in production of ML from fructose by now among the reported results. Some studies showed that acid catalysts with Brønsted acidity often catalyzed fructose convert into 5-ethoxymethylfurfural (5-EMF) in ethanol.58-61 In our study, HPWTi catalyzed fructose convert to ML rather than 5methoxymethylfurfural, which was suggested that Lewis acidity might be attributed to sequent conversion of 5-methoxymethylfurfural to ML. Therefore, 5-methoxymethylfurfural was used as a substrate to clarify the effect of Lewis acidity (Fig. 3). It can be seen that 5-


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methoxymethylfurfural was rehydrated to ML in the presence of acid catalysts, while Lewis acid HPWTi was more favorable than single Brønsted acid HPW. Fig. 2 Fig. 3 The alcoholysis of glucose was done under the reaction conditions as 0.15 g of glucose, 0.08 mmol of catalyst, 8 mL of methanol at 130 o C for 4 h (Fig. 4). It did not show a great difference in conversion compared to fructose using four HPA catalysts, while the difference was noticeable concerning ML yields that were significantly lower from glucose. For different HPAs, the conversion of glucose showed no significant difference ranging from 83.3 % to 91.7 %, while the ML yield depended on the nature of the catalysts. And methyl glucosides was detected in the beginning of the reaction due to the existence of Brønsted acids for the four HPAs, showing that Brønsted acidity is key factor for glucose conversion. For pure Brønsted acid of HPW, the reaction mainly stopped at the methyl glucosides step for almost full conversion with only 25.3 % ML yield. Under such reaction conditions, most likely glucoside was not able to be isomerized into fruoctoside. Adding Lewis acid sites, glucoside was then isomerized into fructoside, which is a difficult step compared to dehydration to 5-methoxymethylfurfural and is suggested to be rate-limiting.55,62 HPWTi possessing certain Lewis acidity and mediate B/L acid ratio gave highest ML yield of 62.4 % compared to HPWZn and HPWFe with lower or higher B/L acid ratio. This suggested that the presence of either Lewis or Brønsted acidity to an extreme level, too high or too low B/L acid ratio, played an adverse effect on the ML formation. For HPWTi, the alcoholysis of glucose was also done by adding extra of 5 wt % water to check the effect of water on the conversion and yield. The result presented that the ML yield was improved to 95.4 % conversion and 65.7 % yield when using methanol and water mixture (as volume ratio of 15: 1).


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And by now the ML yield had been obtained as 13.0 - 64.0 % from glucose using various solid catalysts at 130 - 180 °C in 2 - 24 h.52,55,63-65 HPWTi gave almost the best ML yield among solid catalysts. As results, HPAs with certain ratio of Brønsted and Lewis acids could give the highest efficiency in alcoholysis of glucose to ML. And extra adding water could promote glucose isomerize to fructose then promote glucose convert being catalyzed by Lewis acids, and also prevented glucose convert to methyl glucosides. Fig. 4 HPWTi catalyzed cellulose via methanolysis to obtain ML with a maximum activity of 0.253 g/mmol·h and 51.3 % yield of ML. Side products methyl glucosides, 5-methoxymethylfurfural, methyl lactate had been detected by GC-MS and HPLC. It can be further concluded from the results of GC-MS and HPLC that conversion of cellulose in methanol catalyzed by HPAs with Brønsted acidity underwent the following main pathway (Scheme 2, route (i) ): cellulose reacts with methanol to generate methyl glucosides, then to 5-methoxymethylfurfural, further to ML. And for HPWTi, the methyl glucosides mgiht be transformed through different two ways (Scheme 2 (i) and (ii)): (i) directly dehydrates into 5-methoxymethylfurfural then hydrates to ML totally catalyzed by Brønsted acid combined with Lewis acid; and (ii) isomerizes to methyl fructoside by Lewis acid, and dehydrates to 5-methoxymethylfurfural and then to ML by Brønsted acid and Lewis acid. Therefore, the ML yield obtained by HPWTi was improved compared to HPW due to its mixed acidic sites. There are also some side-reaction in cellulose alcoholysis to ML including (iii) generation of methyl lactate and humins in the presence of Lewis acid of HPWTi. Scheme 2


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Because of optimum level of Brønsted and Lewis acidity for HPWTi, an appreciable level of ML yield could be obtained from a variety of monosaccharides analogous to glucose and fructose including cellobiose (a dimer of glucose), sucrose(one fructose and one glucose unit), and starch (a polysaccharide consisting of hundreds of glucose units) (Fig. 5). 94.5 % conversion and 57.5 % yield were obtained from sucrose under 100 °C for 5 h, which contains a dimer of fructose and glucose units. There was also 23.8 % of methyl glucosides existence, showing that the glucose unit of sucrose was likely to form methyl glucosides, which is difficult to be isomerize to methyl fructoside. Compared to sucrose, the alcoholysis of cellobiose was more difficult due to this dimer unit of glucose with 93.7 % conversion and 51.0 % yield under 120 °C for 5 h. And almost 99.0 % conversion and 47.0 % yield for starch had been obtained under 130 °C for 5 h, respectively. In the previous reports, 43.0 - 49.0 % yield from sucrose and 44.0 % yield from cellobiose had been achieved.54 Therefore, our HPWTi presented almost the best results in production of ML through such polysaccharides by far. Fig. 5 In our previous study,37 we only focused on the influence of calcintation temperature on HPA solubility in water and did not concern about their acidity. Therefore, the influence of the calcination treatment for HPWTi at different temperature on acidity had been tested and the influence on cellulose conversion and selectivity to ML were also checked (Fig. 6). It can be seen that the ratio of B/L increased as increasing the treatment temperature. It had been reported that protons exists in two ways in HPA secondary structure (Scheme S1), they are delocalized H3O+ and protons attached to terminal oxygen. When being calcinated at a certain temperature, the loss of crystal water might lead to decreasing trend for hydrogen bond between proton and water, hence increasing proton dissociating. Therefore, increasing calcination temperature to


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400 °C might increase the Brønsted acidity for HPWTi, but further enhancement temperature might cause its decomposition to loss its Brønsted acidity. Therefore, calcinations treatment of HPWTi above 400 °C gave rise to decreasing of the ratio of B/L. The conversion of cellulose varied in range of 94.0 % to 94.7 % corresponding to treatment of from 150 o C to 200 o C, while ML yields changed from 50.0 % to 51.3 % accordingly. The conversion of cellulose varied in range of 94.7 % to 90.3 % corresponding to treatment of from 200

o

C to 400

o

C, while ML

yields changed from 51.3 % to 47.2 % accordingly. This results also confirmed that the conversion of cellulose as well as ML yield mainly depended on the different Brønsted acidity, while HPWTi samples exhibited same Lewis acidity being calcinated at the different temperature and leading to the different ratio of B/L. Fig. 6 Optimism of the reaction conditions catalyzed by HPWTi Main parameters affecting the methanolysis reaction including temperature, reaction time, catalyst dosage, and usage of cellulose were investigated (Fig. 7). When the reaction temperature was increased from 120 to 160 ºC, the yields of ML were improved gradually from 3.0 % to 51.3 %. The optimum temperature was 160 ºC, while further increase in temperature led to a formation of an insoluble dark powder. And the carbon balance of the cellulose conversion before and after the reaction was about 96.0 %. The reaction time played a positive role on the conversions of cellulose, while the maximum yield of ML was obtained at 7 h and the decrease trend was found to prolong time to 9 h for generation of methyl lactate or humins. Increase in usage of catalyst could enhance the conversions of cellulose, but decreased the yields of ML and methyl glucosides. The increase of insoluble powder was found as increasing of catalyst concentrations. The optimum amount of catalyst was 0.08 mmol. It can also be seen that the


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trend decreased when the cellulose content increased from 0.15 g to 0.40 g. Therefore, increasing of reaction temperature, time and the usage of catalyst gave the negative effects on the yields of ML or methyl glucosides due to the polymerization to insoluble by-products. Thus it can be concluded that in order to obtain a high yield of ML, 160 ºC, 7 h, 0.08 mmol of catalyst, 0.15 g of cellulose are needed. Fig. 7 The production of ML from lignocelluloses had been done including corn straw, pinewood and husk of xanthoceras. The best ML yield as 36.4 %, 20.5 % and 28.8 % had been achieved by HPWTi under the reaction conditions as the optimal conditions of cellulose alcoholysis at 160 ºC. In comparison of the high yields of levulinate from mono-, di- and polysaccharides, the levulinate yields derived from lignocellulose in methanol remained low. In the previous reports, about 40.0 - 50.0 % yields of levulinate were obtained by H2SO4 under harsh reactions at 180 200 °C.6,66,67 Our results showed potentials and environment benign for production of ML directly from lignocellulose under relative mild conditions. The performance of the catalysts under microwave-assistance Microwave has been utilized in radar and telecommunication as well for heating ovens in domestic kitchens. The unique heating properties of microwave have been effectively applied to organic and inorganic chemical reactions.38-40 Today, combination of microwave and heteropolyacids are used in the treatment of cellulose or starch into glucose due to the dielectric property of phosphotungstic acid, which could conserve microwave energy. In order to obtain high efficiency, the methanolysis of cellulose were done under microwave-assistance catalyzed by HPAs catalysts (Table 2). According to the above conventional result, the optimized parameters were selected as 160 ºC, 0.08 mmol of catalyst, and 0.15 g of cellulose, respectively.


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Table 2 It can be concluded that the maximum conversion of cellulose and yield of ML were achieved by HPWTi with 95.8 and 62.6 %, respectively, at 160 ºC, under microwave-assistance for 2 h. The reaction time dependence of this reaction under microwave-assistance was also studied (Fig. 8). When the reaction time was increased from 50 min to 120 min, the yields of ML increased to the highest yield of 62.6 %. Further increasing the reaction time did not give the increase of ML yield but decrease trends for ML and methyl glucosides, which was attributed to the sidereaction occurrence. Compared to convenient heat, the heat transfer at the catalyst surface was improved and hence speeded the reaction rates under the assistance of microwave irradiation. In addition, the side-reactions could be escaped under microwave to give higher yield of ML. The previous reports had pointed that insoluble Cs2.5H0.5PW12O40 could give up to 20.0 % yield of ML after 1 min in supercritical methanol-water (300 ºC) and Fe-exchanged HPW catalyst gave 14.0 % yield of ML at 220 ºC and Ar 20 bar for 2 h.32,33 The catalytic activity was enhanced by combination of microwave-assistance and use of HPWTi, which was almost the best result catalyzed by HPAs so far. Fig. 8 The conversions of other saccharides including fructose, glucose, sucrose, cellobiose and starch had also been done under microwave-assistance catalyzed by HPWTi (Fig. 9). It also could be found that the catalytic activity of HPWTi was improved under microwave-assistance, meanwhile the reaction time was shortened more than four time than that under conventional conditions. The optimum reaction time was different for different saccharides by the catalyst of HPWTi. Then the reaction for different saccharides has been done at different time and could obtain the different conversion according to the literature.26 The conversions and yields to ML


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were respectively: 93.9 and 70.0 % for fructose, 90.0 and 63.7 % for glucose, 93.3 and 63.2 % for sucrose, 90.2 and 54.1 % for cellobiose, 99.0 and 50.8 % for starch. It is known that ethyl levulinate is another fuel additive, whether HPWTi is available in ethanolysis of cellulose or not is also interesting. Then the ethanolysis of cellulose by HPWTi was done under the reaction conditions as o.1 g of cellulose, 0.08 mmol of HPWTi, 8 mL of ethanol, 160 °C, 7 h. HPWTi also showed better performance with 95.0 % conversion of cellulose and 49.7 % yield of ethyl levulinate. Further study was undergoing. Fig. 9 Recycling of HPWTi Reusability of the catalyst is very important in practical applications. In order to determine the solubility of the catalyst, HPWTi was calcined at 200 °C for different time and determined its UV-vis spectroscopy after being dispersed in methanol (Fig. S2). It can be seen that the solubility of HPWTi changed according to the different calcined time, as being treated at 200 °C for 4 h gave rise to its lowest solubility in methanol because the dissolving amount was proportional to the UV-Vis absorption and the peak strength. This confirmed that HPWTi is insoluble after being calcined at 200 °C for 4 h and acted as a heterogeneous catalyst in cellulose methanolysis with benefits for recovery and reuse. The leaching of HPWTi was also done in the following way: 0.08 mmol of catalyst and 0.15 g of cellulose were added in 8 mL methanol to react 4 h under conventional heating at 160 °C then was centrifuged to determine IR and UV-Vis spectroscopy of the solution (Fig. S3). The reaction mixture after the reaction was centrifuged to separate solution containing products and leaching catalyst and solid containing unreacted cellulose and HPWTi. The solution was measured for the IR and UV-Vis spectroscope. It can be seen that there were no characteristic peaks belonging to HPWTi either in IR or in UV spectra,


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indicating no leaching of the catalysts into the reaction mixture during the reaction. What’s more, no P, W and Ti elements were detected from the results of the elemental analyses of the solution. In addition, the solution after centrifugation was further heated for 2 h to determine the product distribution, which showed that the yields of ML and methyl glucosides did not change significantly from 24.3 and 24.9 % to 26.8 and 25.0 %, respectively. Compared to the results catalyzed by HPWTi at 6 h (45.0 and 31.6 %), the yields of ML and methyl glucosides changed little indicating no leaching of the catalyst into the mixture during the reaction. After four cycles under conversional heating, there was fast decrease in efficiency not only for cellulose conversion but also for ML yield (Fig. 10). It can be seen that the efficiency decreased by ca. 2.0 and 2.5 % (from 94.7 and 51.3 % to 92.7 and 49.8 %, respectively). This might to be attributed to the formation of humin in the conversion of sugar and furfural species, which covered the surface of the solid HPWTi. This prevented the access of substrates to catalytic sites, hence lowering the catalytic activity. From the results of the elemental analyses of the HPWTi after four cycles, the C, P, W and Ti contents in catalyst were C, 2.22; P, 1.11; W, 72.00 and Ti, 1.70 %, respectively. Compared with the calculated values of P, 1.13; W, 73.64 and Ti, 1.71 %, the results suggest that there was slight humin attached to the surface of HPWTi during the reaction. After removal of humin by being treated in methanol at 100 ºC for 12 h, the solid HPAs could be regenerated for reuse and the catalytic activity was refreshed (Fifth and sixth cycles). From the results of the elemental analyses of the HPWTi after fifth and sixth cycles, the P, W and Ti contents in catalyst were P, 1.14; W, 73.55 and Ti, 1.69 %, respectively. The result of elemental analyses was coincidence with the calculated values of HPWTi. For being treated recycle HPWTi after fourth cycles, the catalsyts could be used for about 12 times with no obviously decrease. From the IR spectra of the catalyst before and after the reaction (Fig. S4), it


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can be seen that four characteristic peaks corresponding to HPWTi were observed in range of 790 to 1100 cm-1, which demonstrated that HPWTi was stable during the reaction. Moreover, its stability was also determined by 31P MAS NMR (Fig. S5). Compared to the 31P MAS NMR for fresh one (-14.594 ppm), the regenerated HPWTi also gave the sharp signal at -14.594 ppm. This indicated that the structure of HPWTi did not change during the reaction under such conditions, showing its higher stability. Fig. 10 After six reactions cycles under microwave conditions, there was some little change in the catalytic activity (Fig. 11) compared to conventional heating. It can be seen that the conversion of cellulose and the yield of ML were almost no significant changes for first four cycles with only 1.1 % and 1.5 % decreases, respectively. For fifth and sixth time, the conversion and yield decreased to 92.8 and 61.0 %, 91.0 and 59.9 %, respectively. From the results of the elemental analyses of the HPWTi after sixth cycles in microwave, the C, P, W and Ti contents in catalyst were C, 0.45; P, 1.12; W, 73.31 and Ti, 1.73 %, respectively. For twelve time, the conversion and yield decreased to 89.1 and 58.5 %. This result determined that microwave was more benefits for conversion of cellulose in methanol than conventional heating, which was contributed to the smaller reaction time and little formation of humin during the reaction (the carbon balance was about 98.0 wt %). Fig. 11 CONCLUSIONS This study presented an excellent catalytic process for the production of ML through cellulose Ⅲ Ⅲ Ⅳ Ⅳ methanolysis over a series of HPA catalysts (HPWM, M = CuII, ZnII, Cr , Fe , Sn , Ti , and Zr

Ⅳ

), which as high as 51.3 and 62.6 % ML yields were achieved under conventional and


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microwave conditions by HPWTi. This might be the highest one over solid acid catalysts until now using litter amount of catalyst (0.08 mmol) within short time (7 h and 2 h). It was found that Brønsted and Lewis double acid sites, and proper B/L acid ratio for HPAs were main contribution to the high efficiency with high selectivity to ML. The formation of ML from cellulose in methanol undergoes the following steps: cellulose → methyl glucosides catalyzed by highest Brønsted acid sites and co-existence of Lewis acid sites, methyl glucosides → methyl fructoside

by

Lewis

acid

sites,

methyl

glucosides

or

methyl

fructoside

→

5-

methoxymethylfurfural by Brønsted acid sites, 5-methoxymethylfurfural → ML by Brønsted acid sites combined with Lewis acid. And single Brønsted HPAs such as HPW only favored for the conversion of cellulose and production of methyl glucosides owing to no isomerization process for generation more ML. Strong Lewis acid can promote C-C bond cleavage and prevent 5-methoxymethylfurfural hydration so that HPWFe gave low ML yield compared to HPWTi. Above all, controlling B/L acid ratio gave rise to different conversion and selectivities to ML, thus making HPWTi with certain B/L acid ratio with 2.84/1 an excellent catalyst in this report. Moreover, HPWTi exhibited wide tolerant to different feedstocks including fructose, glucose, sucrose, cellobiose, and starch even lignocellulose at low temperatures ranging from 80 to 160 °C. In view of environment, HPWTi was treated through calcinations at 200 °C for 4 h and was insoluble in methanol absolutely. HPWTi was an environmental friendly catalyst which could be used for more than four times without any treatment. It was also available for production of ML from lignocelluloses. ASSOCIATED CONTENT Supporting Information


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The content of the supporting information including the solubility, IR, UV-Vis, 31P MAS NMR of the catalyst, the XRD of the cellulose and the probable pathway for catalyst loss its crystal water. The following files are available free of charge. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. (X.H. Wang) Present Addresses Key Lab of Polyoxometalate Science of Ministry of Education, Northeast Normal University, Changchun, 130024, P. R. China. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (20871026 and 51578119). And it was support by the major projects of Jilin Provincial Science and Technology Department (20086035, and 20140204085GX). REFERENCES (1) Luterbacher, J. S.; Alonso, D. M.; Dumesic, J. A. Targeted chemical upgrading of lignocellulosic biomass to platform molecules. Green Chem. 2014, 16, 4816-4838, DOI: 10.1039/c4gc01160k.


25


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(2) Werpy, T.; Petersen, G. R. Top Value Added Chemicals from Biomass. Volume I. Results of Screening for Potential Candidates from Sugars and Synthesis Gas, U.S.D. Energy, 2004. (3) Joshi, H.; Moser, B. R.; Toler, J.; Smith, W. F.; Walker, T. Ethyl levulinate: A potential bio-based diluent for biodiesel which improves cold flow properties. Biomass Bioenergy 2011, 35, 3262-3266, DOI: 10.1016/j.biombioe.2011.04.020. (4) Climent, M. J.; Corma, A.; Iborra, S. Conversion of biomass platform molecules into fuel additives

and

liquid

hydrocarbon

fuels.

Green

Chem.

2014,

16,

516-547,

DOI:

10.1039/c3gc41492b. (5) Lange, J. P.; Price, R.; Ayoub, P. M.; Louis, J.; Petrus, L.; Clarke, L.; Gosselink, H. Valeric Biofuels: A Platform of Cellulosic Transportation Fuels. Angew. Chem. Int. Ed. 2010, 49, 44794483, DOI: 10.1002/anie.201000655. (6) Mao, R. L. V.; Zhao, Q.; Dima, G.; Petrac cone, D. New Process for the Acid-Catalyzed Conversion of Cellulosic Biomass (AC3B) into Alkyl Levulinates and Other Esters Using a Unique One-Pot System of Reaction and Product Extraction. Catal. Lett. 2011, 141, 271-276, DOI: 10.1007/s10562-010-0493-y. (7) Lake, M. A.; Burton, S. W. Diesel fuel compositions containing levulinate. US Patent, 20100313467, 2010. (8) Hayes, D. J. An examination of biorefining processes, catalysts and challenges. Catal. Today 2009, 145, 138-151, DOI: 10.1007/s10562-010-0493-y. (9) Sah, P. P. T.; Ma, S. Y. Levulinic acid and its esters. J. Am. Chem. Soc. 1930, 52, 48804883, DOI: 10.1021/ja01375a033.


26

Page 27 of 51

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(10) Hayes, D. J.; Fitzpatrick, S.; Hayes, M. H. B.; Ross, J. R. H. Biorefineries-Industrial Processes and Product, WILEY-VCH Verlag GmbH & Co., Weinheim, 2006, pp.139-164. (11) Zhang, J.; Wu, S.; Li, B.; Zhang, H. D. Advances in the Catalytic Production of Valuable Levulinic Acid Derivatives. ChemCatChem. 2012, 4, 1230-1237, DOI: 10.1002/cctc.201200113. (12) Démolis, A.; Essayem, N.; Rataboul, F. Synthesis and Applications of Alkyl Levulinates. ACS Sustain. Chem. Eng. 2014, 2, 1338-1352, DOI: 10.1021/sc500082n. (13) Girisuta, B.; Janssen, L. P. B. M.; Heeres, H. J. Kinetic Study on the Acid-Catalyzed Hydrolysis of Cellulose to Levulinic Acid. Ind. Eng. Chem. Res. 2007, 46, 1696-1708, DOI: 10.1021/ie061186z. (14) Shen, F.; Smith, R. L.; Li, Jr.; L. Y.; Yan, L. L.; Qi, X. H. Eco-friendly Method for Efficient Conversion of Cellulose into Levulinic Acid in Pure Water with Cellulase-Mimetic Solid

Acid

Catalyst.

ACS

Sustain.

Chem.

Eng.

2017,

5,

2421-2427,

DOI:

10.1021/acssuschemeng.6b02765. (15) Fu, ; Xu, X. X.; Lu, X. L.; Lu, X. Y. Hydrothermal Decomposition of Carbohydrates to Levulinic Acid with Catalysis by Ionic Liquids. Ind. Eng. Chem. Res. 2016, 55, 11044-11051, DOI: 10.1021/acs.iecr.6b02478. (16) Zhang, X. Y.; Zhang, D.; Sun, Z.; Xue, L. F.; Wang, X. H.; Jiang, Z. J. Highly efficient preparation of HMF from cellulose using temperature-responsive heteropolyacid catalysts in cascade reaction. Appl. Catal. B: Environ. 2016, 196, 50-56, DOI: 10.1016/j.apcatb.2016.05.019. (17) Silveira, M. H. L.; Morais, A. R. C.; Costa Lopes, A. M. da; Olekszyszen, D. N.; BogelŁukasik, R.; Andreaus, J.; Ramos, L. P. Current Pretreatment Technologies for the Development


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 51

of Cellulosic Ethanol and Biorefineries. ChemSusChem. 2015, 8, 3366-3390, DOI: 10.1002/cssc.201500282. (18) Zakrzewska, M. E.; Bogel-Łukasik, E.; Bogel-Łukasik, R. Ionic Liquid-Mediated Formation of 5-HydroxymethylfurfuralsA Promising Biomass-Derived Building Block. Chem. Rev. 2011, 111, 397-417, DOI: 10.1021/cr100171a. (19) Tominaga, K.; Mori, A.; Fukushima, Y.; Shimada, S.; Sato, K. Mixed-acid systems for the catalytic synthesis of methyl levulinate from cellulose. Green Chem. 2011, 13, 810-812, DOI: 10.1039/c0gc00715c. (20) Yadav, G. D.; Yadav, A. R. Synthesis of ethyl levulinate as fuel additives using heterogeneous solid superacidic catalysts: Efficacy and kinetic modeling. Chem. Eng. J. 2014, 243, 556-563, DOI: 10.1016/j.cej.2014.01.013. (21) Chen, B.; Li, F.; Huang, Z.; Lu, T.; Yuan, Y.; Yuan, G. Integrated Catalytic Process to Directly Convert Furfural to Levulinate Ester with High Selectivity. ChemSusChem. 2014, 7, 202-209, DOI: 10.1002/cssc.201300542. (22) Hu, X.; Song, Y.; Wu, L.; Gholizadeh, M.; Li, C. Z. One-Pot Synthesis of Levulinic Acid/Ester from C5 Carbohydrates in a Methanol Medium. ACS Sustain. Chem. Eng. 2013, 1, 1593-1599, DOI: 10.1021/sc400229w. (23) Lange, J. P.; Graaf, W. D.; Haan, R. J. Conversion of Furfuryl Alcohol into Ethyl Levulinate

using

Solid

Acid

Catalysts.

ChemSusChem.

2009,

2,

437-441,

DOI:

10.1002/cssc.200800216.


28

Page 29 of 51

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(24) Kuo, C. H.; Poyraz, A. S.; Jin, L.; Meng, Y.; Pahalagedara, L.; Chen, S. Y.; Kriz, D. A.; Guild, C.; Gudz, A.; Suib, S. L. Heterogeneous acidic TiO2 nanoparticles for efficient conversion of biomass derived carbohydrates. Green Chem. 2014, 16, 785-791, DOI: 10.1039/c3gc40909k . (25) Ahmad, E.; Alam, M. I.; Pant, K. K.; Haider, M. A. Catalytic and mechanistic insights into the production of ethyl levulinate from biorenewable feedstocks. Green Chem. 2016, 18, 4804-4823, DOI: 10.1039/c6gc01523a. (26) Song, C. H.; Liu, S. J.; Peng, X. W.; Long, J. X.; Lou, W. Y.; Li, X. H. Catalytic Conversion of Carbohydrates to Levulinate Ester over Heteropolyanion-Based Ionic Liquids. ChemSusChem. 2016, 9, 3307-3316, DOI: 10.1002/cssc.201601080. (27) Ogasawara, Y.; Itagaki, S.; Yamaguchi, K.; Mizuno, N. Saccharification of Natural Lignocellulose Biomass and Polysaccharides by Highly Negatively Charged Heteropolyacids in Concentrated

Aqueous

Solution.

ChemSusChem.

2011,

4,

519-525,

DOI:

10.1002/cssc.201100025. (28) Delidovich, I.; Leonhardb, K.; Palkovits, R. Cellulose and hemicellulose valorisation: an integrated challenge of catalysis and reaction engineering. Energy Environ. Sci. 2014, 7, 28032830, DOI: 10.1039/c4ee01067a. (29) Mascal, M.; Nikitin, E. B. Comment on Processes for the Direct Conversion of Cellulose or Cellulosic Biomass into Levulinate Esters. ChemSusChem. 2010, 3, 1349-1351, DOI: 10.1002/cssc.201000326.


29


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(30) Ding, D. Q.; Xi, J. X.; Wang, J. J.; Liu, X. H.; Lu, G. Z.; Wang, Y. Q. Mixed-acid systems for the catalytic synthesis of methyl levulinate from cellulose. Green Chem. 2015, 17, 4037-4044, DOI: 10.1039/c5gc00440c. (31) Peng, W.; Zhang, Q.; Wang, Y. Polyoxometalates as efficient catalysts for transformations of cellulose into platform chemicals. Dalton Trans. 2012, 41, 9817-9831, DOI: 10.1039/c2dt30637a. (32) Deng, W.; Liu, M.; Zhang, Q.; Wang, Y. Direct transformation of cellulose into methyl and ethyl glucosides in methanol and ethanol media catalyzed by heteropolyacids. Catal. Today 2011, 164, 461-466, DOI: 10.1016/j.cattod.2010.10.055. (33) Rataboul, F.; Essayem, N. Cellulose Reactivity in Supercritical Methanol in the Presence of Solid Acid Catalysts: Direct Synthesis of Methyl-levulinate. Ind. Eng. Chem. Res. 2011, 50, 799-805, DOI: 10.1021/ie101616e. (34) Liu, Y.; Liu, C. L.; Wu, H. Z.; Dong, W. S. An Efficient Catalyst for the Conversion of Fructose into Methyl Levulinate. Catal. Lett. 2013, 143, 1346-1353, DOI: 10.1007/s10562-0131094-3. (35) Zhao, S. Q.; Xu, G. Z.; Chang, C.; Fang, S. Q.; Liu, Z.; Du, F. G. Direct Conversion of Carbohydrates into Ethyl Levulinate with Potassium Phosphotungstate as an Efficient Catalyst. Catalysts 2015, 5, 1897-1910, DOI: 10.3390/catal5041897. (36) Sun, Z.; Xue, L. F.; Wang, S. T.; Wang, X. H.; Shi, J. Y. Single step conversion of cellulose to levulinic acid using temperature-responsive dodeca-aluminotungstic acid catalysts. Green Chem. 2016, 18, 742-752, DOI: 10.1039/c5gc01730k.


30

Page 31 of 51

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(37) Tao, M. L.; Xue, L. F.; Sun, Z.; Wang, S. T.; Wang, X. H.; Shi, J. Y. Tailoring the Synergistic Bronsted-Lewis acidic effects in Heteropolyacid catalysts: Applied in Esterification and Transesterification Reactions. Sci. rep. 2015, 5, 13764 (1-10), DOI: 10.1038/srep13764. (38) Ching, T. W.; Haritos, V.; Tanksale, A.

Microwave assisted conversion of

microcrystalline cellulose into value added chemicals using dilute acid catalyst. Carbohydrate Polymers 2017, 157, 1794-1800, DOI: 10.1016/j.carbpol.2016.11.066. (39) Xiao, W. H.; Chen, X. L.; Zhang, Y. X.; Qu, T. J.; Han, L. J. Product Analysis for Microwave-Assisted Methanolysis of Lignocellulose. Energy Fuel 2016, 30, 8246-8251, DOI: 10.1021/acs.energyfuels.6b01186. (40) Bhanja, P.; Modak, A.; Chatterjee, S.; Bhaumik, A. Bifunctionalized Mesoporous SBA-15: A New Heterogeneous Catalyst for the Facile Synthesis of 5-Hydroxymethylfurfural. ACS Sustain. Chem. Eng. 2017, 5, 2763-2773, DOI: 10.1021/acssuschemeng.6b03100. (41) Kappe, O.C. Microwave dielectric heating in synthetic organic chemistry. Chem. Soc. Rev. 2008, 37, 1127-1139, DOI: 10.1016/j.apcatb.2011.07.021. (42) Troncea, S. B.; Wuttke, S.; Kemnitz, E.; Coman, S. M.; Parvulescu, V. I. Hydroxylated magnesium fluorides as environmentally friendly catalysts for glycerol acetylation. Appl. Catal. B: Environ. 2011, 107, 260-267, DOI: 10.1016/j.apcatb.2011.07.021. (43) Pizzio, L. R.; Blanco, M. N. A contribution to the physicochemical characterization of nonstoichiometric salts of tungstosilicic acid. Microporous Mesoporous Mater. 2007, 103, 40-47, DOI: 10.1016/j.micromeso.2007.01.036.


31


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 51

(44) Emeis, C. A. Determination of Integrated Molar Extinction Coefficients for Infrared Absorption Bands of Pyridine Adsorbed on Solid Acid Catalysts. J. Catal. 1993, 141, 347-354, DOI: 10.1006/jcat.1993.1145. (45) Patil, S. K. R.; Heltzel, J.; Lund, C. R. F. Comparison of Structural Features of Humins Formed Catalytically from Glucose, Fructose, and 5-Hydroxymethylfurfuraldehyde. Energy Fuel 2012, 26, 5281-5293, DOI: 10.1021/ef3007454. (46) Hoang, T. M. C.; van Eck, E. R. H.; Bula, W. P.; Gardeniers, J. G. E.; Leffertsa, L.; Seshan, K. Humin based by-products from biomass processing as a potential carbonaceous source for synthesis gas production. Green Chem. 2015, 17, 959-972, DOI: 10.1039/c4gc01324g. (47) Li, H.; Zhao, W. F.; Riisager, A.; Saravanamurugan, S.; Wang, Z. W.; Fang, Z.; Yang, S. A Pd-Catalyzed in situ domino process for mild and quantitative production of 2,5-dimethylfuran directly from carbohydrates. Green Chem. 2017, 19, 2101-2106, DOI: 10.1039/c7gc00580f. (48) Timofeeva, M. N. Acid catalysis by heteropoly acids. Appl. Catal. A: Gen. 2003, 256, 1935, DOI: 10.1016/S0926-860X(03)00386-7. (49) Kulikov, S. M.; Kozhevnikov, I. V. Izv. Akad. Nauk SSSR Ser. Kjim, 1980, 2213. (50) Chambon, F.; Rataboul, F.; Pinel, C.; Cabiac, A.; Guillon, E.; Essayem, N. Cellulose hydrothermal conversion promoted by heterogeneous Brønsted and Lewis acids: Remarkable efficiency of solid Lewis acids to produce lactic acid. Appl. Catal. B: Environ. 2011, 105, 171181, DOI: 10.1016/j.apcatb.2011.04.009.


32

Page 33 of 51

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(51) Deng, W. P.; Liu, M.; Zhang, Q. H.; Tan, X. S.; Wang, Y. Acid-catalysed direct transformation of cellulose into methyl glucosides in methanol at moderate temperatures. Chem. Commun. 2010, 46, 2668-2670, DOI: 10.1039/b925723c. (52) Peng, L. C.; Lin, L.; Li, H.; Yang, Q. L. Conversion of carbohydrates biomass into levulinate esters using heterogeneous catalysts. Appl. Energy 2011, 88, 4590-4596, DOI: 10.1016/j.apenergy.2011.05.049. (53) Peng, L. C.; Lin, L.; Zhang, J. H.; Shi, J. B.; Liu, S. J. Solid acid catalyzed glucose conversion to ethyl levulinate. Appl. Catal. A: Gen. 2011, 397, 259-265, DOI: 10.1016/j.apcata.2011.03.008. (54) Saravanamurugan, S.; Nguyen van Buu, O.; Riisager, A. Conversion of Mono- and Disaccharides to Ethyl Levulinate and Ethyl Pyranoside with Sulfonic Acid-Functionalized Ionic Liquids ChemSusChem. 2011, 4, 723-726, DOI: 10.1002/cssc.201100137. (55) Saravanamurugan, S.; Riisager, A. Zeolite Catalyzed Transformation of Carbohydrates to Alkyl Levulinates. ChemCatChem. 2013, 5, 1754-1757, DOI: 10.1002/cctc.201300006. (56) Balakrishnan, M.; Sacia, E. R.; Bell, A. T. Etherification and reductive etherification of 5(hydroxymethyl)furfural: 5-(alkoxymethyl)furfurals and 2,5-bis(alkoxymethyl)furans as potential bio-diesel candidates. Green Chem. 2012, 14, 1626-1634, DOI: 10.1039/c2gc35102a. (57) Tominaga, K. Preparation of levulinates from carbohydrates and alcohols with nonvolatile catalysts. Patent JP2006206579, 2006.


33


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 51

(58) Kraus, G. A.; Guney, T. A direct synthesis of 5-alkoxymethylfurfural ethers from fructose via sulfonic acid-functionalized ionic liquids. Green chem. 2012, 14, 1593-1596, DOI: 10.1039/c2gc35175g. (59) Liu, B.; Zhang, Z. H. One-pot conversion of carbohydrates into 5-ethoxymethylfurfural and ethyl D-glucopyranoside in ethanol catalyzed by a silica supported sulfonic acid catalyst. RSC Adv. 2013, 3, 12313-12319, DOI: 10.1039/c3ra41043a. (60) Wang, H. L.; Deng, T. S.; Wang, Y. X.; Cui, X. J.; Qi, Y. Q.; Mu, X. D.; Hou, X. G.; Zhu, Y. L. Graphene oxide as a facile acid catalyst for the one-pot conversion of carbohydrates into 5ethoxymethylfurfural. Green Chem. 2013, 15, 2379-2383, DOI: 10.1039/c3gc41109e. (61) Li, H.; Saravanamurugan, S.; Yang, S.; Riisager, A. Direct transformation of carbohydrates to the biofuel 5-ethoxymethylfurfural by solid acid catalysts. Green Chem. 2016, 18, 726-734, DOI: 10.1039/c5gc01043h. (62) Li, H.; Saravanamurugan, S.; Yang, S.; Riisager, A. Cover. Green Chem. 2015, 17, 1-8. (63) Peng, L.; Lin, L.; Li, H. Extremely low sulfuric acid catalyst system for synthesis of methyl

levulinate

from

glucose.

Ind.

Crops

Prod.

2012,

40,

136-144,

DOI:

10.1016/j.indcrop.2012.03.007. (64) Hu, X.; Li, C. Z. Levulinic esters from the acid-catalysed reactions of sugars and alcohols as part of a bio-refinery. Green Chem. 2011, 13, 1676-1679, DOI: 10.1039/c1gc15272f. (65) Howard, S. J.; Sanborn, A.J.; Microwave Assisted Synthesis of Dehydrated Sugar Derivatives Hydroxymethylfurfural, Levulinic Acid, Anhydro-Sugar Alcohols, and Ethers thereof. Patent WO2012015616A1, 2012.


34

Page 35 of 51

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(66) Chang, C.; Xu, G.; Jiang, X. Production of ethyl levulinate by direct conversion of wheat straw

in

ethanol

media.

Bioresource

Technol.

2012,

121,

93-99,

DOI:

10.1016/j.biortech.2012.06.105. (67) Chang, C.; Xu, G.; Zhu, W.; Bai, J.; Fang, S. One-pot production of a liquid biofuel candidate—Ethyl levulinate from glucose and furfural residues using a combination of extremely low sulfuric acid and zeolite USY. Fuel 2015, 140, 365-370, DOI: 10.1016/j.fuel.2014.09.102.


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Fig. 1 The correlation between B/L ratio and efficiency for cellulose methanolysis by HPAs catalysts.


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Fig. 2 Catalyzing fructose was carried out under the reaction conditions as 0.15 g of fructose, 0.08 mmol of catalyst, 8 mL of methanol at 80 o C for 4 h.


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Fig. 3 The 5-MMF conversion verse reaction time catalyzed by two HPAs of HPW and HPWTi.


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Fig. 4 The alcoholysis of glucose was done under the reaction conditions as 0.15 g of glucose, 0.08 mmol of catalyst, 8 mL of methanol at 130 o C for 4 h.


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Fig. 5 The conversions of other saccharides including sucrose, cellobiose, starch, and cellulose were catalyzed by HPWTi. Reaction conditions: 0.15 g saccharides, 0.08 mmol, 8 mL methanol, under conventional heating for 5 h, 5 h, 5 h and 7 h corresponding to sucrose, cellobiose, starch, and cellulose, respectively.


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Fig. 6 The influence of calcination treatment for HPWTi at different temperature on its acidity and the conversion of cellulose.


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Fig. 7 Main parameters affecting the methanolysis reaction catalyzed by HPWTi.


42

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Fig. 8 Effect of reaction time on catalytic performances of HPWTi for the conversions of cellulose in methanol under microwave-assistance. Reaction conditions: catalyst, 0.08 mmol; cellulose, 0.15 g; methanol, 8 mL; temperature, 160 ºC.


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Fig. 9 The conversions of other saccharides including fructose, glucose, sucrose, and starch had also been done catalyzed by HPWTi. Reaction conditions: 0.15 g saccharides, 0.08 mmol HPWTi, 8 mL methanol, under microwave-assistance for 1 h, 1 h, 1.5 h, 1.5 h, 1.5 h and 2 h corresponding to fructose, glucose, sucrose, cellobiose, starch, and cellulose, respectively.


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Fig. 10 Repeated uses of HPWTi for cellulose conversions in methanol. Reaction conditions: 0.15 g cellulose, 0.08 mmol HPWTi, 8 mL methanol, 160 ºC, under conversional heating for 7 h.


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Fig. 11 Repeated uses of HPWTi for cellulose conversions in methanol. Reaction conditions: 0.15 g cellulose, 0.08 mmol HPWTi, 8 mL methanol, 160 ºC, under microwave-assistance for 2 h.


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Scheme 1 The probable explanation for HPWSn generating strong Brønsted acidity (red, blue, pink and green balls represent oxygen, tungsten, phosphorus and Lewis metal Sn atoms, respectively.)


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Scheme 2 Reaction pathways for the HPWM-catalyzed conversion of cellulose to methyl levulinate and other side products in methanol.


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Table 1 Catalytic performance of various catalysts for the conversions of cellulose in methanol. Acidity Total Lewis

Brønsted

site

site

Catalysts

ML

MG

yields (%)

acidity (mmol/g)

TOFb,47

yields

ML selectivity

(%)

(%)

(%)

(g/mmol·h)

Con.

(mmol/g) (mmol/g)

The ratio of B/L

none

-

-

-

8.6

0

0

-

-

-

H2SO4

-

2.00

2.00

84.0

12.0

62.0

14.3

0.225

-

HPW

0.03

1.75

1.78

93.7

19.2

61.2

20.5

0.251

-

HSiW

0.03

2.29

2.32

90.2

18.7

56.4

20.7

0.242

-

HBW

0.03

2.15

2.17

88.8

18.0

50.1

20.3

0.238

-

HPWTi

0.56

1.59

2.15

94.7

51.3

25.0

54.2

0.253

2.84/1

HPWTic

0.56

1.66

2.22

90.3

48.2

26.0

53.4

0.242

2.96/1

HPWCu

0.41

1.52

1.93

89.8

48.0

25.9

53.5

0.241

3.71/1

HPWSn

0.32

1.22

1.54

88.7

45.3

28.3

51.1

0.238

3.81/1

HPWZr

0.25

1.14

1.39

86.8

33.5

25.5

38.6

0.233

4.56/1

HPWZn

0.21

1.12

1.33

86.1

32.1

33.4

37.3

0.231

5.33/1

HPWFe

0.58

0.57

1.15

85.3

30.9

19.0

36.2

0.228

0.98/1

HPWCr

0.40

0.54

0.94

79.4

25.5

18.5

32.1

0.213

1.35/1

a

Reaction conditions: temperature, 160 ºC; catalyst, 0.08 mmol; cellulose, 0.15 g; methanol, 8 mL; under conventional heating for 7 h. b

TOF=[0.15 (g) ×Conversion (%)]/[Catalyst (mmol) × Reaction time (h)] c

HPWTi was treated under 400 ºC to loss its crystal water.


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Table 2 Catalytic performances of various acid catalysts for the conversion of cellulose to ML in methanol under microwave-assistance.

Con.

ML yields

MG yields

ML selectivity

TOF

(%)

(%)

(%)

(%)

(g/mmol·h)

none

9.8

0

0

-

---

H2SO4

84.6

15.7

57.5

18.6

0.241

HPW

91.2

25.1

55.8

27.5

0.855

HSiW

90.3

23.2

49.9

25.7

0.847

HBW

89.9

23.0

43.3

25.6

0.842

HPWTi

95.8

62.6

24.9

65.3

0.898

HPWCu

92.5

54.0

25.5

58.4

0.867

HPWSn

91.0

48.0

27.0

52.7

0.853

HPWZr

88.2

45.8

25.2

51.9

0.827

HPWZn

87.3

39.1

32.4

44.8

0.818

HPWFe

75.5

33.2

19.3

44.0

0.708

HPWCr

69.7

29.7

17.1

42.6

0.653

Catalysts

a

Reaction conditions: temperature, 160 ºC; catalyst, 0.08 mmol, cellulose, 0.15 g, methanol, 8 mL, under microwave-assistance for 2 h.


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Synopisis： Our study obtained 51.3 % ML yield controlled by HPWTi (B/L acid=2.84:1) and ML is a sustainable resource from renewable biomass.


Catalyzing Cascade Production of Methyl Levulinate from

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