Conversion of Carbohydrates Biomass Into Levulinate Esters Using Heterogeneous Catalysts

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Conversion of carbohydrates biomass into levulinate esters using heterogeneous catalysts

ARTICLE in APPLIED ENERGY · DECEMBER 2011 Impact Factor: 5.61 · DOI: 10.1016/j.apenergy.2011.05.049

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Lincai Peng Lu Lin Kunming University of Science and Technol… Dalian University of Technology

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Hui Li Qiulin Yang Kunming University of Science and Technol… Tianjin University of Science and Technology

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Applied Energy

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Conversion of carbohydrates biomass into levulinate esters using heterogeneous catalysts ⇑ Lincai Peng, Lu Lin , Hui Li, Qiulin Yang

State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, Guangdong, China article info abstract

Article history: The catalytic performances of common solid acids (ZSM-5(25), ZSM-5(36), NaY, H-mordenite, Zr3(PO4)4, Received 20 February 2011 2 2 SO4 /ZrO2,SO4 /TiO2, and TiO2) for the conversion of carbohydrates such as glucose to methyl levulinate Received in revised form 22 May 2011 in near-critical methanol were investigated to develop an environmentally benign catalyst with high Accepted 26 May 2011 activity. Among these catalysts employed, sulfated metal oxides (especially SO2/TiO ) were found to Available online 21 June 2011 4 2 be a type of potential catalysts for prospective utilization, which showed remarkably high selectivity and yield of methyl levulinate and had negligible undesired dimethyl ether formation from the dehydra- Keywords: tion of methanol. With SO2/TiO as the catalyst, methyl levulinate in ca. 43, 33 and 59 mol% yields could Carbohydrates 4 2 be obtained from sucrose, glucose and fructose, respectively, at 473 K for 2 h reaction time with a catalyst Catalysis 2 Methyl levulinate loading of 2.5 wt.%. The heterogeneous catalyst (SO4 /TiO2) was easily recovered by ﬁltration and exhib- Heterogeneous catalyst ited good catalytic activities after calcination in ﬁve cycles of reusing. The surface structure and acidity 2 2 SO4 /TiO2 variations of the fresh and recycled SO4 /TiO2 catalysts after calcination were characterized by XRD and NH3-TPD techniques. The results indicate that the catalyst crystallization structure was preserved after multiple cycles, the acid amount and acid strength of the catalyst reduced gradually as the increasing of recycling times. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Industrially, levulinate esters were mainly obtained through esterification of levulinic acid with alkyl alcohols in the presence Due to gradual diminishment of fossil fuel reserves and progres- of sulfuric acid that leads to a high yield of products [9]. Immobi- sive depravation of environmental quality, the development of lized lipases as the biocatalyst for this process can also be equally renewable biomass energy invites more and more concerns [1]. effective under milder reaction conditions [5,10]. However, levu- In recent decades, extensive research is being carried out world- linic acid as raw material for this purpose is of high cost with its wide to convert cellulosic biomass into liquid fuels and high- present production from the acid hydrolysis of cellulose and sug- quality chemicals and to develop economically feasible processes ars. Recently, Mascal and Nikitin [11] had developed a new and on an industrial scale [2–4], among which the preparation of levu- efficient procedure for the conversion of cellulosic biomass into linate esters have been one of the focuses under study. Levulinate levulinate esters in overall yields exceeding 80% through two reac- esters, like methyl levulinate, ethyl levulinate, and butyl levulinate, tion steps: biomass reacted with hydrochloric acid into 5-(chloro- are a kind of short chain fatty esters with their properties similar to methyl)furfural followed by the alcoholysis of resulting product the biodiesel fatty acid methyl esters (FAME) [5,6]. These esters are with alcohols. However, the intermediates obtained from biomass suitable to be used as additives for gasoline and diesel of transpor- must be isolated before the subsequent process for all above tation fuels, which have manifold excellent performances, such as routes, the isolation procedure which requires the use of some non-toxic, high lubricity, flashpoint stability and better flow prop- energy consuming technique (distillation) or of environmentally erties under cold condition [7]. On the other hand, levulinate esters non-friendly solvents. Direct production of levulinate esters from also can either be used in the flavoring and fragrance industries or biomass or biomass-based sugars is also possible by the acid-cata- as substrates for various kinds of condensation and addition reac- lyzed reaction with alcohols, which is economically more attrac- tions at the ester and keto groups in organic chemistry [8]. tive, has yet to be developed. The generally accepted reaction pathway for the direct conversion of cellulose to levulinate esters is schematically given in Fig. 1 [8,12]. Compared with the numerous reactions studied in aqueous ⇑ Corresponding author. Tel./fax: +86 20 22236719. solvent for the making of chemicals [13], the reaction system in E-mail address: [email protected] (L. Lin). alcohols media has some advantages, for example, minimized

OH OH OH OH +ROH O OH O O HO HO O HO OH H2O HO HO O HO O HO OR OH O OH O H+ OH OH OH n Cellulose Alkyl glucoside + H 3H2O

O O +ROH +H O OHC O 2 OR H3C R + HCOOR O H+ Levulinate ester Formate ester 5-alkoxymethylfurfural

Fig. 1. Reaction pathway for the acid-catalyzed conversion of cellulose to levulinate esters. wastewater discharged and higher-grade products easily isolated 2. Experimental by fractionation. Process development for conversion of biomass into biofuels and high-value chemicals in sub- and supercritical 2.1. Catalysts preparation alcohols has been largely studied in recent years [14,15]. There 2 2 have also been several reports about the production of levulinate SO4 /ZrO2 and SO4 /TiO2 were prepared by precipitation and esters in near-critical alcohols from carbohydrates biomass, such impregnation method, and the detailed procedure was presented as sucrose, glucose, fructose and biomass feedstocks including elsewhere [32]. Other solid acid catalysts were obtained commer- wood, bagasse, wheat meal and agricultural wastes [8,12,16,17]. cially. As for zeolites, ZSM-5, H-mordenite and NaY were from the

For example, Le Van Mao et al. [18] most recently developed a Catalyst Plant of Nankai University. TiO2 was obtained from one-pot system for the direct catalytic conversion of cellulosic Aladdin Reagent. Zr3(PO4)4 was purchased from Xiamen Xindakang biomass into alkyl levulinates and other esters, and the product Inorganic Materials Co. Ltd. All catalysts were calcined at 773 K for extraction. In these studies, a homogeneous acid (H2SO4) was most 3 h in static air prior to use. commonly employed as the catalyst, since it is relatively cheap and also very active. However it suffers from several obvious draw- 2.2. Catalytic reaction procedure backs, such as equipment corrosion, side reaction for the inter- molecular dehydration of the massive alcohols to ether and the The experiments were carried out in a cylindrical stainless steel requirement of special processing for the neutralization of spent pressurized reactor with inner diameter 33 mm, depth 117 mm H SO [19,20]. Owing to the existence of these problems, it is ex- 2 4 and 100 mL total volume made by PARR instrument company, tremely important and necessary to develop an environmentally USA. The reactor was heated in an adjustable electric stove. The benign catalyst with high activity for the direct production of lev- temperature of the reactor contents was monitored by a thermo- ulinate esters. couple connected to the reactor. For each experiment, carbohy- In recent years, solid acid catalysts have attracted considerable drate (2.5 g), methanol (50 mL), and a given amount of solid acid interests as heterogeneous catalysts, which can overcome the above catalyst were mixed to form a suspension and were poured into mentioned some disadvantages of the inorganic acid in acid cataly- the reactor. The reactor was then brought to the desired tempera- sis and have been broadly applied to catalyze dehydration, alkyl- ture by external heating and shaken at 500 rpm. After certain reac- ation, cracking, isomerization, esterification, acylation, and so on tion time, the reactor was taken from the stove and quenched in an [21–26]. The conversion of cellulose in supercritical methanol ice cool water bath to terminate the reaction. The liquid substance (573 K/10 MPa) in the presence of solid acid catalysts has been re- and solid acid catalyst were separated by filtration, and then ana- ported, giving a 20% yield of methyl levulinate over Cs H PW O x 3x 12 40 lyzed. To test the catalyst stability, SO2/TiO catalyst was recov- and sulfated zirconia [27]. However, minimal attention has been gi- 4 2 ered and reused without any treatment in a new reaction cycle ven to the use of solid acid catalysts to replace conventional H SO 2 4 under the same reaction conditions described above. In another for the direct synthesis of levulinate esters in near-critical methanol. run, prior to each reuse, the recovered catalyst was calcined at Presently, commercially available heterogeneous acid catalysts 773 K for 3 h in static air. mainly include ion-exchange resin, zeolite, sulfated metal oxide, metal oxide, and inorganic metal salt. The long-term use of ion-exchange resin at high temperature is limited by its relatively poor 2.3. Products analysis thermal stability. Zeolite showed a potential use in dehydration reaction of carbohydrates and it was found that the molecular struc- Methyl levulinate was analyzed on a GC (Agilent 6890 instru- 2 ture exerts a significant influence on the reaction process [28].SO4 / ment) equipped with an HP-5 capillary column with dimensions 2 ZrO2 and SO4 /TiO2 are the representative catalysts of sulfated metal of 30.0 m 320 lm 0.25 lm and a flame ionization detector oxides that showed high catalytic activities for many reactions (FID) operating at 543 K. The amount of methyl levulinate in the [29,30]. Some metal oxides and salts also were performed well in reaction mixture was determined using calibration curves ob- the conversion process of biomass [31]. tained by analyzing its standard solutions with known amount.

In this paper, zeolites with various structures and SiO2/Al2O3 Methyl levulinate with an optimal yield in the reaction mixture 2 2 molar ratios, SO4 /ZrO2,SO4 /TiO2,Zr3(PO4)4, and TiO2 were se- was purified by distillation [18]. From this solution, the excess lected as heterogeneous acid catalysts for the conversion of carbo- methanol and low boiling products were first removed at 353 K. hydrates to methyl levulinate in near-critical methanol. Based on After n-dodecane was added to help distil the heavy products, this, effects of different process parameters including reaction the residue was heated up to 433 K using a vacuum distillation time, reaction temperature, catalyst loading and carbohydrate type while the products were collected and methyl levulinate was on as well as catalyst recycling over a fine solid acid catalyst towards the lower. The residual glucose and methyl glucosides were deter- the reaction performance were conducted. mined by ion chromatography (IC, DIONEX ICS-3000) equipped 4592 L. Peng et al. / Applied Energy 88 (2011) 4590–4596 with a CarboPac PA1 (2 mm 250 mm) analytical column and an higher temperature of desorption, the stronger the acid strength. electrochemical detector. A solution of sodium hydroxide As can be seen from Fig. 2, the profile feature of ZSM-5(25) was (80 mmol/L) was used as the eluent with a volumetric flow rate in harmony with ZSM-5(36), which both showed two distinct of 0.35 mL/min at 303 K. The concentration of 5-methoxymethyl- peaks appearing from 400 to 600 K and 650 to 850 K, attributing furfural was determined using a HPLC system (Agilent 1100 Series) to weak and strong acid sites, respectively. A single major peak with a Waters Nova-pak C18 column and a UV detector. Methanol was observed for H-mordenite and NaY, signifying a single type was used as the mobile phase with a volumetric flow rate of 1 mL/ of acid site. The acid amount of the zeolites was greater than min. The column was operated at 303 K. Products yield on a molar 1 mmol/g, but the majority of acid sites had the weak acid 2 2 base according to reaction stoichiometry given in Fig. 1 were calcu- strength. The acid amounts of SO4 /ZrO2 and SO4 /TiO2 were lated using the following equation: 0.57 and 0.63 mmol/g, respectively. There were two peaks on the 2 profile of SO4 /ZrO2, while there were three clear peaks on the Products yield ðmol%Þ¼Ci M0 100=ðC0 MiÞ 2 2 spectrum of SO4 /TiO2. The highest peak of SO4 /ZrO2 was around 2 where C denotes the initial concentration of carbohydrates used. C 839 K, and the main peaks of SO4 /TiO2 were observed at 558 and 0 i 2 is the concentration of products obtained by acid-catalyzed meth- 806 K. Hence, the SO4 /ZrO2 mainly had strong acid sites, and the 2 SO /TiO2 had middle and strong acid sites. A small curve over anolysis. The terms M0 and Mi represent the molecular weight of 4 carbohydrates and products, respectively. the temperature range from 400 to 600 K was detected for TiO2 and Zr3(PO4)4, suggesting that they had weak acidity. It is well known that the acid amount and acid strength are two important 2.4. Catalysts characterization parameters of the solid acid catalysts, which may greatly influence the reaction activity in acid-catalyzed alcoholysis. Temperature-programmed desorption of ammonia (NH3-TPD) and BET surface area were performed on a Micromeritics Auto- 3.2. Catalytic performance for the conversion of glucose chem II 2920 instrument. For NH3-TPD measurements, a thermal conductivity detector was used for continuous monitoring of the The catalysts were employed in the catalytic conversion of glu- desorbed ammonia and the areas under the peaks were integrated cose to methyl levulinate in near-critical methanol at 473 K and 2 h to estimate the amount of acid in the catalysts. About 0.2 g of sam- with a catalyst loading of 2.5 wt.%. After the reaction, all corre- ple was heated at a rate of 15 K/min up to 873 K and kept for 0.5 h sponding products anticipated in Fig. 1 were detected in this study, in a flow of He gas (20 mL/min) to remove adsorbed species on the indicating that the reaction process over heterogeneous acid catal- surface. The sample cooled down to 373 K in a flow of He gas, then ysis followed the generally reaction pathway given in Fig. 1. Other followed by adsorption of NH in 10% NH gas flow (balance He, 3 3 than the products checked, there were probably several other com- 20 mL/min) for 1 h. After flushed with He (20 mL/min) for 1 h to re- pounds in the liquid phase. Here, the identification of all the other move the physically adsorbed NH , the TPD data was recorded 3 chemicals and the determination of their yields did not presented. from 373 to 873 with a ramp of 15 K/min. BET surface area of The amounts of the residual reactant (glucose) and the target prod- the samples was measured by N adsorption–desorption isotherms 2 uct (methyl levulinate) as well as the main intermediates (methyl at liquid nitrogen temperature. Prior to analysis, each sample was glucosides and 5-methoxymethylfurfural) with and without the pre-treated at 573 K for 1 h to remove any adsorbed species on the addition of a solid acid catalyst during the conversion of glucose surface. X-ray diffraction (XRD) pattern of the catalysts was carried in near-critical methanol are shown in Fig. 3. The results indicate out using a BRUKER D8 Advance X-ray diffractometer with Cu Ka that the distribution of products was appreciably different for radiation source operated at 40 kV and 40 mA. Data was collected various solid acid catalysts used. The desired methyl levulinate from 2h between 20° and 60° with a step of 0.02° at a scanning was obtained in various amounts over the catalysts besides H- speed of 3°/min. The phases were identified using the power dif- mordenite, while no methyl levulinate was detected in the absence fraction file (PDF) database (JCPDS, International Centre for Diffrac- of the catalyst. Thus, the choice of catalysts is crucial for the formation Data). The crystallite size was calculated by XRD-line tion and yield of methyl levulinate. Among those solid catalysts broadening using the Scherrer equation [33]. Sulfur content of tested, it is interesting that sulfated metal oxides (especially the samples was measured by means of high temperature combus- 2 SO /TiO ) showed high activity and remarkably high selectivity tion method. The sample was decomposed by combustion and all 4 2 for methyl levulinate production from glucose. Other catalysts the sulfur species were converted into SO and SO which were ab- 3 2 were little effective for the formation of methyl levulinate, even sorbed by neutral hydrogen peroxide solution and the sulfure the H-mordenite, NaY, ZSM-5(25) and ZSM-5(36) with a larger amount was determined by sodium hydroxide titration. amount of acid sites. These findings suggest that the acid strength of the catalysts is a key factor for the formation of methyl levuli- 3. Results and discussion nate, that is to say, the stronger acid site of the catalysts gives rise to higher yield of product. Fig. 3 also indicates that the higher con-

3.1. Catalyst characterization tent of Al2O3 in the zeolites, the lower average methyl levulinate yield. Extrapolation of the experimental data shows that the exis-

The characterization data for different catalysts are summarized tence of Al2O3 in the zeolites has some inhibitory action on the for- in Table 1, in which the values given in parentheses for the zeolites mation of methyl levulinate. For this reason, a conﬁrmation 2 are the SiO2/Al2O3 molar ratios. The BET surface area of the zeolites experiment was also carried out, where Al2O3 and SO4 /TiO2 with was higher than 300 m2/g, except that of H-mordenite was mass ratio of 1:9 was used as the catalyst. The result shows that 2 2 2 71.1 m /g. SO4 /TiO2 and SO4 /ZrO2 had more or less the same the yield of methyl levulinate decreased to 20 from 33 mol% for 2 2 BET surface area, of that 128.2 and 121.8 m /g, respectively. The SO4 /TiO2 catalyst. A certain number of residual glucose (9–14%) TPD proﬁles of desorbed ammonia (NH3) on various samples of cat- was detected over zeolites with relatively higher SiO2/Al2O3 molar alyst are shown in Fig. 2 and the corresponding results of acid ratio such as ZSM-5(25), ZSM-5(36), and H-mordenite. In contrast, amount as well as peak temperatures are listed in Table 1. The acid glucose was found in very small amounts or even completely amount corresponding to the amount of absorbed NH3 was esti- eliminated over other solid catalysts and no catalyst. For most mated by integrating the areas under the peaks. The desorption catalysts, an intermediate (methyl glucosides) was detected with temperature indicates the acid strength of the catalyst, i.e., the a larger amount, and a small quantity of 5-methoxymethylfurfural L. Peng et al. / Applied Energy 88 (2011) 4590–4596 4593

Table 1 concern is the formation of undesired dimethyl ether due to the Characterization data for different catalysts. acid-catalyzed inter-molecular dehydration of methanol. It is a Sample SiO2/Al2O3 BET surface Acid amount Peak found that the massive dimethyl ether (ca. 50% of methanol to di- molar ratio area (m2/g) (mmol/g) temperaturea (K) methyl ether) measured by collecting the gas after reaction was

TiO2 – 96.2 0.33 474 generated for ZSM-5(25) and ZSM-5(36). As for other catalysts, it 2 – 128.2 0.63 481, 558, 806 SO4 /TiO2 was obtained in extremely small quantity (less than 5% of metha- 2 – 121.8 0.57 453, 839 SO4 /ZrO2 nol to dimethyl ether). The conversion rate of methanol to di- Zr (PO ) – 64.1 0.30 443 2 3 4 4 methyl ether was about 2% when SO4 /TiO2 catalyst was selected H-mordenite 10 71.1 1.88 481 as the catalyst. It is very helpful to reduce dimethyl ether produc- NaY 5 531.0 1.04 471 tion for recycling of methanol, and security concern because of its ZSM-5(25) 25 309.3 1.44 501, 726 ZSM-5(36) 36 318.7 1.19 492, 710 low-down boiling point (248 K). With all these observations con- 2 sidered together, SO /TiO2 was chosen as the most suitable cata- a Acid amount and peak temperature were determined by NH -TPD 4 3 lyst for this purpose and was used in the subsequent exploration. measurement.

3.3. Inﬂuence of reaction temperature on yield of methyl levulinate

The influence of reaction temperature on the yield of methyl levulinate was investigated with reaction time to find the optimum conditions to increase the methyl levulinate yield. It can be seen from Fig. 4 that the reaction temperature played an important role in the reaction process. Normally, elevated temperature can con- tribute to the enhancement of reaction rate and conversion effi- ciency. When the temperature increased from 453 to 473 K, the yield of methyl levulinate grew significantly after the same reaction time. Then it was observed slightly to rise over 473 K. How- ever, it must be pointed out that methyl levulinate will be decomposed to some extent at higher temperature. Additionally, more humins and side-reactions may occur during the reaction. Therefore, elevation of temperature is unfavorable for the formation of methyl levulinate. At the temperature of 473 and 483 K, methyl levulinate yield grew very fast at the beginning of the reaction and then remained nearly at a constant level as the reactants had reached its near-equilibrium conversion. The optimum yield was obtained at the reaction time of 2 h. At 453 K, the yield of methyl levulinate increased smoothly with the prolonging of reac- Fig. 2. NH -TPD profiles of different catalysts. (a) TiO ; (b) SO2/TiO ; (c) SO2/ 3 2 4 2 4 tion time, which may need longer reaction time to reach the equi- ZrO2; (d) Zr3(PO4)4; (e) H-mordenite; (f) NaY; (g) ZSM-5(25); (h) ZSM-5(36). librium conversion.

3.4. Effect of catalyst loading on yield of methyl levulinate

Fig. 5 illustrates the effect of catalyst loading ranging from 1.0 to 5.0 wt.% on the yield of methyl levulinate as a function of reaction time. The curves were similar to those in Fig. 4. The yield of methyl levulinate increased with the augment of catalyst loading

Fig. 3. Conversion of glucose in near-critical methanol over various solid acid catalysts. Reaction conditions: glucose, 2.5 g; catalyst, 1.25 g; methanol, 50 mL; temperature, 473 K; reaction time, 2 h. was detected to be produced. During all experiments, dark-brown insoluble substances known as humins were observed, which were adsorbed on the surface of solid catalysts. These are probably Fig. 4. Inﬂuence of reaction temperature on the yield of methyl levulinate with products of side-reaction of the acid-catalyzed decomposition of 2 reaction time over SO4 /TiO2 catalyst. Reaction conditions: glucose, 2.5 g; catalyst, the reactants and/or products. In this reaction process, another 1.25 g; methanol, 50 mL. 4594 L. Peng et al. / Applied Energy 88 (2011) 4590–4596

yield from the same substrate. The yield of methyl levulinate from cellulose was significantly lower than that from glucose. It is known that sugar units in the cellulose molecules bind strongly to each other and is relatively difficult to broken down compared to any other carbohydrate. In addition, cellulose is insoluble in near-critical methanol that causes poorer accessibility of the substrate to solid catalyst. Therefore, the cellulose conversion is hard to happen, and less methyl levulinate was detected to be produced in the reaction process. The yield of methyl levulinate from starch was almost similar to that from glucose, perhaps due to the solu- bility state of starch in near-critical methanol and during the methanolysis it is easily to be transferred into methyl glucosides as the intermediates by the action of an acid catalyst. With sucrose and fructose as the substrates, substantially higher yields of methyl levulinate were found as compared with glucose, particularly with fructose as the substrate. It can be concluded that the reaction pathway from glucose to methyl levulinate is involved the inter- Fig. 5. Effect of catalyst loading on the yield of methyl levulinate with reaction time 2 mediary formation of methyl glucosides, from which methyl fruc- over SO4 /TiO2 catalyst. Reaction conditions: glucose, 2.5 g; methanol, 50 mL; temperature, 473 K. toside is then formed via isomerization. For the sucrose, the experiment result with a medium yield between those from glucose and fructose is in line with above explanation. During the during the first 2 h of the reaction process. The equilibrium conver- reaction, the disaccharide sucrose is firstly converted in a serial sion for the formation of methyl levulinate was almost reached by mode to methyl glucoside and methyl fructoside by methanolysis. 1.5 and 2 h for the catalyst loading of 5.0 and 2.5 wt.%, respectively. To obtain a high methyl levulinate yield from glucose or any carbo- When the catalyst loading was 1.0 wt.%, the methyl levulinate hydrate containing glucose, it is a key step for the effective isomer- yield grew continually with the reaction time in 3 h, indicating that ization of methyl glucoside to methyl fructoside. an incomplete conversion of glucose into methyl levulinate hap- pened under these reaction conditions. A possible explanation for these appearances is that available acid sites of the catalyst might 3.6. Catalyst recycling undergo changes during the catalytic reaction. With increasing the catalyst loading, the increased total number of acid sites led to a Long-term stability and reusability of the heterogeneous cata- faster reaction rate to promote the conversion of glucose to methyl lyst are extremely important characteristics for future industrial levulinate. Similar phenomenon was also observed in many previ- application to reduce production cost substantially. After the reac- 2 ous reports [20,34]. tion was finished, the spent SO4 /TiO2 catalyst was separated from the liquid substance, and then reused in a new experiment for the reaction under the same reaction conditions. As seen from Fig. 7, 3.5. Conversion of various carbohydrates when the catalyst was reused without calcination, an obvious de- crease of the methyl levulinate yield from 33.2 to 14.6 mol% was To explore the application scope of the reaction system for the observed in the second run. After the third run, the yield of methyl production of methyl levulinate catalyzed by solid acids, other car- levulinate reduced to 5.6 mol%, which was approximately one- bohydrates including cellulose, starch, sucrose, and fructose were sixth of the yield produced with the fresh catalyst (33.2 mol%), selected as the substrates and the results are shown in Fig. 6.It indicating that the catalyst might be deactivated during the reac- can be noted that different amounts of methyl levulinate could tion process. Fortunately, the deactivated catalyst could be regen- be generated from various carbohydrates. The isolated yield (about erated to remove the deposited humins on the surface of the 95% purity determined by GC) by distillation was similar to the GC

2 Fig. 7. Methyl levulinate yield as a function of the recycling times of SO4 /TiO2 Fig. 6. Conversion of various carbohydrates into methyl levulinate in near-critical catalyst, with and without calcination between each experiment. Reaction condi- 2 methanol over SO4 /TiO2 catalyst. Reaction conditions: substrate, 2.5 g; catalyst, tions: glucose, 2.5 g; catalyst, 1.25 g; methanol, 50 mL; temperature, 473 k; reaction 1.25 g; methanol, 50 mL; temperature, 473 K; reaction time, 2 h. time, 2 h. L. Peng et al. / Applied Energy 88 (2011) 4590–4596 4595

major peak with middle acid site was disappeared in the recycled 2 SO4 /TiO2 catalysts, and the desorption temperature of NH3 decreased with the strongest peak appeared at 806, 751, and 734 K 2 in the profiles of fresh, first and seventh recycled SO4 /TiO2 cata- 2 lysts. It indicates that the acid strength of SO4 /TiO2 decreased as more cycles of reusing. In addition, the total amounts of acid sites reduced significantly varying from 0.63 mmol/g for the fresh catalyst to 0.35 mmol/g for the seventh recycled catalyst. An explanation for this phenomenon is that the sulfur in the catalyst lost gradually by solvation during the alcoholysis reaction and by calcination after the reaction, decreasing the catalyst acidity. To verify the possible loss of the sulfur in the catalyst, the sulfur contents 2 of the fresh and recycled SO4 /TiO2 catalysts were measured. The results show that the sulfur content decreased from 2.6 wt.% in the fresh catalyst to 1.8 wt.% in the first recycled catalyst, and to 0.5 wt.% in the seventh recycled catalyst, indicating that most of sulfur in the catalyst would be lost after multiple cycles due to its poor stability. This is a predominant cause of decreasing of Fig. 8. XRD patterns and the corresponding results of crystallite size for the fresh the yield of methyl levulinate as the recycling times increased. and recycled SO2/TiO catalysts after calcination. 4 2 Accordingly, how to improve the long-term stability of the solid acid catalyst remained as one of the topics for future exploration. catalyst by calcination and a methyl levulinate yield of 24.3 mol% was regained in the second run. However, the catalytic activity of 4. Conclusions the catalyst had not completely recovered after calcination, which reached only 73% of that obtained for the fresh catalyst. This may The present research presented a new and environmental be due to partial loss of sulfur in the catalyst by solvation. In the friendly catalytic process for the feasibility of direct converting subsequent two runs, there were slight changes in the yield of abundant and inexpensive carbohydrates biomass into methyl lev- methyl levulinate. Since the fifth cycle, the yield was quickly ulinate as biofuels and/or high-value chemicals using recoverable dropped and it reached only 10.0 mol% in the seventh run. The re- heterogeneous catalysts. Among all solid acid catalysts studied 2 sults lead to the conclusion that the catalytic activity of the catalyst for this purpose, sulfated metal oxides (especially SO4 /TiO2) were got decayed after multiple cycles. found to be a type of potential catalysts, which exhibited remarkably high selectivity for methyl levulinate production and had negligible product of undesired dimethyl ether formed from the 3.7. Characterization for recovered catalyst inter-molecular dehydration of methanol. The heterogeneous catalyst (SO2/TiO ) was easily recovered by filtration and about The surface structure and acidity variations of the fresh and 4 2 five cycles of effective reusing with above 20 mol% yield of methyl recycled SO2/TiO catalysts after calcination were characterized 4 2 levulinate from glucose could be reached after calcination. by XRD and NH3-TPD techniques, respectively. XRD results indicate 2 from Fig. 8 that the SO /TiO2 crystallization structure was pre- 4 Acknowledgements served after multiple cycles, and the amount of anatase TiO2 phase 2 for the recycled SO /TiO2 catalysts increased slightly as compared 4 The authors are grateful to the financial support from Natural with the fresh catalyst. The corresponding crystallite size of the Science Foundation of China (U0733001), National Key R&D catalyst also increased with the recycling times rose, as given with Research Program (2007BAD34B01) and National Key Basic an inset in Fig. 8.NH3-TPD profiles and the corresponding results of Research Program (2010CB732201) from the Ministry of Science acid amount were shown in Fig. 9. It can be observed clearly that a and Technology of China.

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