Improved Activity for Cellulose Conversion to Levulinic Acid through

Research Article pubs.acs.org/journal/ascecg

Improved Activity for Cellulose Conversion to Levulinic Acid through Hierarchization of ETS-10 Zeolite Mei Xiang,† Jingna Liu,† Wenqian Fu,§ Tiandi Tang,§ and Dongfang Wu*,† †

Department of Chemical Engineering, School of Chemistry and Chemical Engineering, Southeast University, Jiangning District, Nanjing 211189, Jiangsu, China § Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, College of Chemistry and Chemical Engineering, Changzhou University, Changzhou 213164, Jiangsu, China

ABSTRACT: A hierarchically porous ETS-10-supported highly dispersed Ni catalyst was designed to selectively catalyze the conversion of cellulose to levulinic acid under mild reaction conditions. Full conversion of cellulose and remarkably high yield (91.0%) of levulinic acid are achieved due to the synergistic effect of the unique porous structures and Lewis acid sites in the presence of a hydrogen atmosphere. Moderate Lewis acidic centers of the heterogeneous catalyst are revealed to play a positive role in activating the reaction substrates and intermediates, resulting in prominently catalytic activity. Meanwhile, hierarchical pores centered at 16 nm in ETS-10 benefit the mass transfer and therefore further enhance its catalytic performance. Also, during the process of levulinic acid formation, introduction of hydrogen is responsible for converting cellulose thoroughly and keeping the metal Ni from oxidization by the oxy-compounds, which is distinctive from the widely reported pathway dominated only by cellulose hydrolysis. KEYWORDS: Cellulose, Levulinic acid, ETS-10, Mesopore, Lewis acid

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INTRODUCTION

from the separation of products and recovery of the catalyst during the catalytic hydrolysis processes. Noble metals-based solid Brønsted acids have been reported to be highly active for cellulose conversion, and they can overcome the disadvantages of mineral acids.11,12 However, the yield of LA was still limited due to not only carbon deposition and cracking but also detrimental contact. To prolong the life of catalysts and increase the exposure of active sites, a multitude of heterogeneous catalysts including S2O82−/ZrO2−SiO2− Sm2O3, HY zeolite, Ru−Al−SBA-15, AlNbOPO4, Fe-resin, CP-SO3H-1.69, and Amberlyst 70 have been employed to efficiently convert cellulose into LA,13−19 especially assisted by high-viscosity ionic liquids to yield as many total reducing sugars as possible without consideration of expensive cost. Recently, some reports showed that homogeneous Lewis acid catalysts displayed superior performances for the

Cellulose is an abundant and inedible raw material derived from natural biomass, which represents an immense potential for the substitution of fossil fuels to produce biobased platform chemicals.1,2 Hence, there is a significant need for developing a highly efficient and environmentally friendly catalytic system to take a leap forward into efficient conversion of biomass resources. Levulinic acid (LA), one of the most attractive 12 platform chemicals, has received significant attention with high chemical reactivity for fuel additives, resin, herbicides, pharmaceuticals, flavor substances, and chemical intermediates.3,4 However, due to the confinement of densely packed structure and insolubility of cellulose, conversion of cellulose directly into LA has not achieved great breakthrough with various catalytic systems attempted.5,6 Homogeneous mineral acids, such as H2SO4, HCl, and HBr, are known to effectively synthesize LA from cellulosic biomass feedstock,7−12 but they all inevitably bring about serious pollution and corrosion of apparatus and suffer © 2017 American Chemical Society

Received: February 20, 2017 Revised: April 19, 2017 Published: May 26, 2017 5800

DOI: 10.1021/acssuschemeng.7b00529 ACS Sustainable Chem. Eng. 2017, 5, 5800−5809

Research Article

ACS Sustainable Chemistry & Engineering

overnight, and calcination at 450 °C in oxygen for 4 h, the process was repeated again to afford almost completely exchanged HETS-10, HMETS-10, and Ni-HMETS-10. Furthermore, the Ni catalysts supported on HMETS-10, HMZSM5, and HMBeta (Ni/HMETS-10, Ni/HMZSM-5, and Ni/HMBeta) were prepared by an incipient wetness impregnation method using a required amount of nickel nitrate solution, and the Ni loadings were all 5.0 wt %. After impregnation, the samples were dried under ambient condition for 12 h and then dried at 120 °C for 12 h without calcination. Mesoporous HZSM-5 and HBeta (HMZSM-5 and HMBeta) were synthesized with the methods reported by previous authors.28,29 Catalyst Characterization. Powder X-ray diffraction patterns (XRD) were obtained with a Rigaku powder X-ray diffractometer using Cu Kα radiation (λ = 0.1542 nm). The scan range is from 5° to 45°. Nitrogen physisorption was conducted at −196 °C on a Micromeritics ASAP 2020 M apparatus. The sample was degassed for 8 h at 300 °C before measurement. Specific surface area was calculated from the adsorption data using the Brunauer−Emmett− Teller (BET) equation. The pore size distribution was calculated according to the Barrett−Joyner−Halenda (BJH) model using adsorption data. Scanning electron microscopy (SEM) was performed using an FEI Inspect F50. Transmission electron microscope (TEM) images were collected using a JEM-2100F. The acidity of the catalysts was measured using temperature-programmed desorption of ammonia (NH3-TPD) on a Micromeritics ASAP2920 instrument. A 200 mg sample was placed in a quartz tube and pretreated in a helium stream at 450 °C for 2 h. After the sample was cooled to 100 °C, NH3−He mixed gas (10 vol % NH3) was passed over the sample for 30 min. After removal of the physically adsorbed NH3 by flowing helium for 2 h at 100 °C, the total flow rate of gas was fixed at 10 cm3/min, and the spectrum was recorded from 100 to 650 °C at a heating rate of 10 °C/ min. FT-IR spectra of pyridine adsorbed on the catalyst samples were recorded on a Thermo Fisher Nicolet 6700 spectrometer, equipped with a deuterium triglycine sulfate detector. The sample was compressed into a self-supporting wafer (10 mg, 13 mm in diameter) and subsequently placed into a quartz IR in situ cell equipped with CaF2 windows. The wafer was evacuated under vacuum (6 × 10−3 Pa) at 400 °C for 60 min and then cooled to room temperature, followed by exposure to pyridine vapor for 30 min. The IR spectra were recorded at 150, 200, 250, 300, and 350 °C, respectively, after subsequent evacuation at each temperature for 1 h. The Ni content and the Ti/Si ratio were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) with a PerkinElmer 3300DV emission spectrometer. Catalytic Test and Product Analysis. Before use, powdered catalyst was sieved to obtain 100 mesh particles and then reduced by H2 at 400 °C for 4 h. The hydrogenation of cellulose to levulinic acid was conducted in a 250 mL high-pressure stainless steel reactor with a magnetic stirrer. Initially, 1.0 g of microcrystalline cellulose was mixed with 0.5 g of reduced catalyst in 100 mL hot water and then pretreated by ultrasonic at 50 °C for 30 min. The resulting mixture was transferred into the reactor and stirred at 800 rpm for 10 min at room temperature. Then, hydrogen was introduced and maintained at a desired pressure after displacing air, and the reaction system was heated to a given temperature and kept for a certain period. The reaction temperature was measured with a thermometer, and the hydrogen pressure in the autoclave was recorded at the reaction temperature. After the reaction, the reactor was quenched immediately to room temperature in an ice water bath and then opened. After filtration of solid residues, the collected aqueous solution was further filtered with a 0.22 μm syringe filter prior to analysis. Liquid products mainly including glucose, HMF, and levulinic acid were quantified by high-performance liquid chromatography (HPLC; Shimadzu, LC-20AT) fitted with a Biorad Hypersil NH2 5 μm column and an RID detector using 6:4 v/v acetonitrile:water as mobile phase with a flow rate of 0.8 mL/min and a column temperature of 25 °C. The analysis of HMF was carried out on an Agilent 1010 with C18 column and UV detection at 283 nm. During this process, the column temperature remained constant at 30 °C, while the mobile phase was

hydrolysis of cellulose20,21 but failed to be separated and recycled. To overcome these disadvantages, some heterogeneous Lewis acid catalysts,22,23 such as Cs2SnPW12O40 and Sn0.75PW12O40,24,25 were employed to decompose cellulose and promote the yield of sugars (soluble oligosaccharides or total reducing sugars). Although the reusability of catalysts and the ease of their separation bring great advantages, the formation of undesired side products, e.g., soluble and insoluble humins deposited on the catalyst surface or inside the catalyst pores, bring great challenges for catalyst regeneration. Therefore, it is desirable to develop a new heterogeneous catalyst with an appropriate amount of Lewis acid and distinct superiorities for efficient cellulose conversion and LA production in aqueous media. We previously reported the synthesis of mesoporous zeolite ETS-10 via templating with a mesoscale silane surfactant,26 which is a facile and attractive synthesis strategy. The unique framework architecture and chemical composition of the resulting hierarchically porous zeolite material give rise to some peculiar properties, e.g., unique performances in adsorption, ion exchange, and shape-selective catalysis.27 Also, very few studies have focused on the catalytic utilization of zeolite ETS-10 without loading of any noble metal. In this study, a kind of nickel-based mesoporous ETS-10 was prepared and used to efficiently catalyze converting cellulose into LA in water through a simple hydrothermal system. An appropriate modification was performed on the mesoporous ETS-10 catalyst, and its effect on the catalytic efficiency was examined. Furthermore, the effect of mesoporous structures, Lewis acid, and reaction conditions on the yield of LA were investigated in detail.

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EXPERIMENTAL SECTION

Materials. Microcrystalline cellulose (average particle size of 50 μm), sorbitol (AR, 98%), glycerol (ACS, 99.5%), and levulinic acid (AR, 99%) were purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). D-Glucose (AR, 99%), ethylene glycol (AR, 99%), and propylene glycol (AR, 99%) were obtained from the Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). HMF (GC, 95%) was supplied by Aladdin Chemistry Co., Ltd. All other reagents were of analytical grade and were used as-received without further purification. Catalyst Preparation. Ni-based mesoporous ETS-10 (Ni-METS10) was prepared by a hydrothermal approach similar to that reported elsewhere,26 in which a molar composition of 4.4Na2O/1.9K2O/ 1.0TiO 2 /7.1SiO 2 /0.65TPOAB/0.4NiO/163.0H 2 O was used (TPOAB: mesoscale template of N,N-dimethyl-N-octadecyl-N-(3triethoxysilylpropyl) ammonium bromide). In a typical run, 3.0 mL aqueous NaOH (6.25 mol/L) was mixed with 4.84 mL aqueous waterglass solution (SiO2/Na2O molar ratio: 3.67) under vigorous stirring, and the obtained solution denoted as solution A. 0.4 g nickel(II) acetylacetonate was added to 3.5 g of TiCl3 solution (17 wt % in HCl) and stirred for 20 min, followed by slow dropwise addition to solution A under stirring for 60 min. Then, 4.0 mL KF aqueous solution (3.75 mol/L) was added, followed by addition of TPOAB (1.5 mL). After further stirring for 2 h, the obtained gel was transferred into a Teflon-coated stainless-steel autoclave for crystallization at 230 °C for 72 h. The resulting product was filtered, washed, dried at 100 °C overnight, and calcined in air at 450 °C for 5 h. For comparison, mesoporous ETS-10 (METS-10) was prepared in the same procedure without addition of nickel species. The mesopore-free ETS-10 (ETS10) was also synthesized in the absence of TPOAB. The H-type zeolites (Ni-HMETS-10, HMETS-10, and HETS-10) were obtained by NH4+ ion exchange. Na, K-type ETS-10, METS-10, and Ni-METS-10 were treated with 1.0 mol/L NH4Cl aqueous solution at 80 °C for 4 h under stirring, in a ratio of 1 g of solid sample to 10 mL NH4Cl solution. After filtration, washing, drying at 120 °C 5801


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Figure 1. (a) XRD patterns of the ETS-10, METS-10, Ni-METS-10, Ni-HMETS-10, and Ni/HMETS-10 samples. (b) N2 adsorption/desorption isotherms and pore size distributions (inset, calculated using desorption branch) of the METS-10 and Ni-METS-10 samples. (c) SEM image of NiMETS-10. (d) TEM images of the thin-sectioned Ni-METS-10 sample (light dots are mesopores). 2:8 v/v acetonitrile:water at a flow rate of 0.6 mL/min, and the injection volume was 25 μL. An external standard calibration method was used for quantification of various products. The cellulose conversion and levulinic acid yield were calculated as follows:

Table 1. Textural Parameters of Catalyst Samples

Cellulose conversion (%) = (1 − mass of unconverted cellulose/mass of initial cellulose) × 100% Yield of LA (%)

■

= (moles of carbon in LA)/(moles of carbon in initial cellulose) × 100%

sample

SBET (m2/g)a

Smic (m2/g)b

Sext (m2/g)c

Vmicro (cm3/g)d

Vmeso (cm3/g)e

ETS-10 HETS-10 METS-10 HMETS-10 Ni/HMETS-10 Ni-METS-10 Ni-HMETS-10

318 302 310 295 280 272 271

304 290 245 245 233 186 188

14 12 64 50 48 87 83

0.12 0.11 0.10 0.08 0.08 0.08 0.08

0.01 0.02 0.13 0.11 0.09 0.21 0.20

a

BET surface area. bMicroporous surface area. cExternal surface area, obtained from t-plot method. dMicroporous pore volume, obtained from t-plot method. eMesoporous pore volume, obtained from BJH adsorption cumulative volume of pores between 1.7 and 300 nm in diameter.

RESULTS AND DISCUSSION Structural Characteristics of Catalysts. The XRD patterns of the catalysts reveal well-resolved peaks in the range of 5°−45° associated with the ETS structure for ETS-10, METS-10, Ni-METS-10, Ni-HMETS-10, and Ni/HMETS-10 samples (Figure 1a). In Figure 1b, the nitrogen adsorption/ desorption isotherms of METS-10 and Ni-METS-10 are plotted. It can be seen that a typical step at a relative pressure of 0.45−0.95 appears in both isotherms, indicating the presence of mesopores. Correspondingly, the pore size distributions are centered at 10 and 16 nm, respectively (insert, Figure 1b). Sample textural parameters are presented in Table 1. Apparently, all the H-form samples present not only smaller pore volumes but also lower special surface areas compared with their initial forms except the as-prepared Ni-HMETS-10 sample, which is essentially attributed to the descending crystallinities shown by the XRD results. Moreover, the metal Ni loading on HMETS-10 leads to a further decrease in the BET specific surface area and the blocking of the pore-channel structure of METS-10. A representative bipyramidal truncated morphology of Ni-METS-10 is shown in the SEM image

(Figure 1c). The TEM image of the thin-section sample NiHMETS-10 further confirms the existence of abundant hierarchical mesopores (Figure 1d). The size range of the mesopores is in good agreement with the N2 sorption. It is worth noting that the Ni particles are not observed in both XRD patterns and the TEM images, while the ICP analysis shows that the Ni content in the Ni-HMETS-10 sample is 4.3 wt %, indicating that the Ni species could be highly dispersed in Ni-HMETS-10. The acid properties of HETS-10, Ni/HMETS-10, and NiHMETS-10 were investigated by both NH3-TPD and pyridineIR methods. Figure 2a shows that only one desorption peak appears at about 150 °C for HETS-10 and that the peak shifts slightly to high temperature for Ni/HMETS-10 and NiHMETS-10, indicating that the acidities of both Ni/HMETS10 and Ni-HMETS-10 become slightly stronger. Furthermore, 5802


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Figure 2. (a) NH3-TPD profiles of the HETS-10, Ni/HMETS-10, and Ni-HMETS-10 samples. (b) Pyridine-IR spectra of Ni-HMETS-10.

a very weak desorption peak is observed at a higher temperature of 481 °C for Ni-HMETS-10. All of these prove that the acid properties of all samples are rather modest. Pyridine-IR spectra are shown in Figure 2b. Five pyridine desorption temperatures were chosen to make a detailed study of the acid properties of Ni-HMETS-10. The bands at 1442 and 1491 cm−1 can be assigned to Lpy species (formed as coordinately bound pyridine through its lone pair of electrons on nitrogen atom to Lewis acid site), and the bands at 1596 cm−1 to hydrogen-bonded pyridine. No obvious bands are observed at 1540 cm−1, indicating that there are no Brönsted acid sites on the Ni-HMETS-10 surface or they are not strong enough to react with pyridine. Thus, it can be seen that the protonic acid of Ni-HMETS-10 is fairly weak and Ni-HMETS10 is a characteristic Lewis acid, which agrees well with earlier statements in the literature.30,31 Furthermore, Figure 2b shows that all the bands become weak with increasing the desorption temperature. The amounts of acid sites were, therefore, calculated as a function of temperature,27 and the results are shown in Table 2. As expected, the amount of L acid sites

Figure 3. Pyridine-IR spectra of Ni-HMETS-10 and Ni/HMETS-10 at 150 °C.

“hexitols”). Compared with the blank run, most of the catalytic runs show promising activities toward cellulose conversion and LA formation. In addition, accompanied by LA and 5-HMF, a trace of formic acid (FA) and furfural come into being and make up the main byproducts. Particularly, a certain amount of hexitols and their smaller derivatives such as glycerol and ethylene glycol resulting from hydrogenation/hydrogenolysis of cellulose always exist whatever the condition and catalyst are, which is completely different from the conventional method of the conversion of cellulose to LA.4−7 Compared with ETS-10 and HETS-10, both Ni/HMETS-10 and Ni-HMETS-10 give not only higher cellulose conversions (>90%) but also higher LA yields (>60%). It can also be found that the order of the catalytic activities of these catalysts is in good agreement with the order of their acidities, i.e., NiHMETS-10 > Ni/HMETS-10 > HETS-10 > METS-10 ≈ ETS10. Notably, although METS-10 has larger pore diameters making for more molecule transfers than ETS-10, the reactivity exhibits no distinct superiority. This can be demonstrated by the fact that the existence of more mesopores promotes exposure of more inherent basic sites in METS-10, which is not beneficial for advancing the established reaction under present conditions. Meanwhile, according to the comparison between HETS-10 and HMETS-10, the increased cellulose conversion and LA yield over HMETS-10 give direct proof of the superior performance resulting from the presence of mesopores. Additionally, the incorporation of transition metal nickel has

Table 2. Lewis Acidity of Ni-HMETS-10 and Ni/HMETS-10 L acid (μmol/g) temperature (°C)

Ni-HMETS-10

Ni/HMETS-10

150 200 250 300 350

276 157 81 33 12

190 83 45 21 9

decreases rapidly with the increase in desorption temperature. Also, a comparison of the pyridine-IR spectra between NiHMETS-10 and Ni/HMETS-10 at 150 °C is given in Figure 3, which shows only peak intensity difference. Combined with Table 2, the lower peak intensity of Ni/HMETS-10 than that of Ni-HMETS-10 better elucidates that less mesoporosity is not beneficial for the exposition of L acid sites, which further leads to the difference in acid strength between the two samples. Catalytic Activity. Six catalysts (ETS-10, METS-10, HETS10, HMETS-10, Ni/HMETS-10m and Ni-HMETS-10) were tested in this study, and the results are listed in Table 3. A blank experiment without any catalyst was also conducted. The results show that cellulose is barely converted to LA and HMF, and the main products concentrate on sugars (cellobiose and fructose), hexitols, and smaller derivatives (here all denoted as 5803


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ACS Sustainable Chemistry & Engineering Table 3. Cellulose Conversion with Different Catalystsa yield (%)

a

catalyst

conversion (%)

sugars

furfural

hexitols

HMF

LA

FA

Ni-HMETS-10 Ni/HMETS-10 HMETS-10 HETS-10 METS-10 ETS-10 Blank

100 90.1 87.3 67.7 36.4 28.8 23.4

1.6 2.5 0.9 1.4 13.3 15.7 18.5

2.8 3.1 3.4 1.0 0 0.2 0.3

1.7 3.4 1.2 1.2 23.1 4.6 3.4

2.7 11.4 15.2 12 0 4.6 1.2

91 64.6 61.6 52.1 0 3.5 0

0.2 5.1 5.0 2.8 0 0.2 0

Reaction conditions: 1.0 g cellulose, 0.5 g catalyst, 100 g H2O, 200 °C, 6 MPa H2, 6 h.

Table 4. Conversion of Cellulose under Different Conditionsa yield (%)

a

run

t/h

T/°C

P/MPa

conversion (%)

sugars

furfural

hexitols

HMF

LA

FA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

0.5 1 4 6 8 12 6 6 6 6 6 6 6 6 6 6 6

200 200 200 200 200 200 150 180 200 230 245 200 200 200 200 200 200

6 6 6 6 6 6 6 6 6 6 6 0 2 3 4 5 6

95 94 95 100 100 100 69.4 87.7 100 100 100 40.5 54.6 79.1 100 100 98.8

23.5 20.1 7.8 2.7 2.3 0.8 3.4 2.0 1.9 1.7 1.4 13.8 1.2 2.0 0.9 1.7 1.3

3.1 0.7 0.9 0.4 0.1 0.1 0.3 0.1 0.4 0.1 0.2 9.2 0 0.3 0.2 0.3 0.1

3.7 3.5 10.2 9 16.2 35.1 0.5 0.5 6.0 18.5 35.6 0 0.2 3.7 5.9 3.4 6.2

20.9 15.9 4.3 0 0 0 21.4 10.5 1.4 0.92 0.34 17.0 34.4 40.4 11.5 4.7 0.34

42.8 51.4 70.2 87.4 81.0 63.7 42.2 74.6 90.2 78.6 62.2 0.56 18.8 32.3 80.2 89.7 90.7

2.2 2.7 1.6 0.5 0.4 0.3 1.6 0.1 0.1 0.2 0.3 0 0 0.4 1.3 0.2 0.2

Reaction conditions: 1.0 g cellulose, 0.5 g catalyst, 100 g H2O.

played a positive role in conversion growth, which proves to be a quite delightful and feasible strategy to take place of those widely used noble metals with promising activity. It is notable that there is still a significant difference in the catalytic performances between Ni/HMETS-10 and Ni-HMETS-10, although they have no acidic disparity and similar Ni loadings, which could be attributed to the more mesoporosity of NiHMETS-10, greatly facilitating the mass transfer and thus making contributions to the improvement of the catalytic activity. Compared to Ni-HMETS-10, the limiting pore aperture and the narrow pore size distribution in Ni/ HMETS-10 retard the mass transfer rate of reactants and/or products and correspondingly suppress the overall hydrogenation reaction. Accordingly, in this paper, Ni-HMETS-10 is chosen to be the best catalyst to explore the following process of cellulose degradation. Table 4 shows the cellulose conversions and LA yields over Ni-HMETS-10 as functions of the reaction time, temperature, and hydrogen pressure. It can be seen that the cellulose conversion and LA yield vary during the reaction course. The LA yield increases with the reaction time from 0.5 to 6 h and then decreases afterward (run 4), which demonstrates the favorable influence of prolonging the reaction time on LA formation. On the other hand, HMF, a product frequently reported to our knowledge, is formed in the yield of 20.9% at 0.5 h and converted totally after 6 h. For this reason, the

optimum reaction time is determined to be 6 h, leading to 87.4% yield of levulinic acid. Subsequently, reactions were performed at 150−245 °C for 6 h over Ni-HMETS-10 to explore the hydrogenation of cellulose. As presented in Table 4, when the temperature changes from 150 to 200 °C, the yield of LA increases from 42.2% to 90.2% (runs 6 and 9), further increasing the temperature leading to side reactions with low amounts of byproducts (organic acids, humins). Simultaneously, the yield of HMF decreases correspondingly, which could be attributed to the fact that the increasing temperature accelerates the mass transfer rate and then facilitates the reaction rate. Furthermore, it has been reported that glucose is usually formed at rather low temperatures and cannot be detected above 140 °C,32 as it is in this paper. The results of cellulose conversion and products distribution under different hydrogen atmospheres are also depicted in Table 4. When done without H2, it leads to a cellulose conversion of 43.5% with no selective formation of products (run 12). Instead, after introduction of 2 MPa hydrogen, a LA yield of 18.8% is obtained with a cellulose conversion of 54.6% (run 13). With the pressure going up continually, the cellulose converts almost completely. Here, 89.7% LA yield and 4.7% HMF yield are reached under 5 MPa hydrogen (run 16). When given higher pressures, it seems that there is not a considerable rise in the yield of LA. As a result, it is better to operate the catalytic system under 5 MPa hydrogen from an economic 5804


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decreases slightly with the experimental run, no significant difference exists among the cellulose conversion data obtained by the four runs, suggesting the excellent retention of the catalytic performance. Subsequently, the hydrothermal stability test of the selected Ni-HMETS-10 catalyst was carried out under the abovementioned catalytic test conditions without the addition of cellulose. The XRD results (Figure 4a) are in conformity with that in Figure 1a, which reveal typical peaks in the range of 5°− 45° associated with the ETS structure after hydrothermal treatment. Thus, the decrease in peak intensity is in the rational range and insufficient to destroy the structure of Ni-HMETS10. The IR spectrum in Figure 4b shows the peak positions and the type of the modes for ETS-10 before and after hydrothermal treatment. It is seen that the spectra frequency for both materials are both dominated by Si−O bond stretching modes (∼1068 cm−1), Ti−O bond stretching modes (∼670 and ∼783 cm−1), and Si−O and O−Ti−O bond rocking modes (∼565 cm −1 ). Also, the low frequency of ∼450 cm −1 contributes to interactions between Ti and O atoms, as well as Si−O bond bending. Furthermore, according to nitrogen adsorption/desorption results (Figure 4c), there seems to be no distinct physicochemical changes of Ni-HMETS-10 during treatment in hot water at 200 °C (Table 6). SEM images reveal the characteristic morphologies of Ni-HMETS-10 with particle sizes of 16−19 μm (Figure 4d). As a result, the hydrothermal

point of view. Finally, these results highlight that hydrogen atmosphere has a remarkable promoting effect on the rate and on the selectivity of cellulose reaction. Catalyst Stability. As is well known, it is necessary to separate the product and reuse the catalyst in industry; the stability and reusability of catalysts are, therefore, extremely important. In this study, catalyst recycling experiments were carried out. After each reaction, the catalyst was separated by centrifugation and then filtered. The obtained solid was washed with deionized water and dried at 100 °C overnight, followed by calcination at 450 °C in air for 4 h to remove the deposited carbon. The catalytic reaction was tested again with the recovered catalyst. Four experimental runs were performed, and the results are shown in Table 5. Although the yield of LA Table 5. Cellulose Conversion and LA Yield Obtained upon Recycling of Ni-HMETS-10 Catalysta yield (%) run

conversion (%)

sugars

furfural

hexitols

HMF

LA

FA

1 2 3 4

100 100 100 100

1.7 1.1 5.3 6.6

0.3 1.2 1.2 0.4

3.4 12.1 7.5 1.2

4.7 1.2 4.7 13.8

89.7 84.1 80.2 77.0

0.2 0.3 1.1 1.0

a

Reaction conditions: 1.0 g cellulose, 0.5 g catalyst, 100 g H2O, 200 °C, 5 MPa H2, 6 h.

Figure 4. (a) XRD patterns and (b) FT-IR spectra of Ni-HMETS-10 before and after hydrothermal stability test. (c) N2 adsorption/desorption isotherms and pore size distributions (inset, calculated using desorption branch). (d) SEM image of Ni-HMETS-10 after hydrothermal stability test. 5805


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Lewis acid sites on the external surface of H-form mesoporous ETS-10. Because of the high hydrophobicity of the zeolite surface, both the hydration tolerance of the acid sites and their ability to adsorb b-glucans could be increased. Thus, the formed soluble cello-oligomers and glucose will be adsorbed into the zeolite pores fast, where the zeolite acid sites further depolymerize the oligomers to glucose that will be converted via mutarotation of the α-anomer to the β one through hydrogen bonding on more accessible metal and acid sites. After that, the α and β forms of glucopyranoside undergo isomerization to form unstable 5-HMF, which hydrated immediately into levulinic and formic acids. On the other hand, in the presence of H2, the obtained cello-oligomers can also be hydrogenated directly to cellobitol (3-ß-D-glucopyranosyl-D-glucitol) which will fast transform into glucose. This process becomes obvious especially under the condition of a rather moderate acid. Considering that cellulose is a biopolymer composed of glucose monomers, glucose is chosen and expected to get further insights into the mechanism of cellulose conversion into LA under designed conditions. The conversion of glucose and yield of relevant products over different catalysts are plotted in Figure 6. As shown, with nearly the same content of metal Ni, Ni/HMBeta gives poor performance, while Ni/HMZSM-5 and Ni-HMETS-10 achieve total conversion of glucose. It is interesting that the products distributions for the three samples are completely different under the same condition. Both the two widely used and conventional aluminosilicate catalysts (Ni/ HMZSM-5 and Ni/HMBeta) show evident inclination to hexitols and HMF and do not go directly to the desired product LA, which could be attributed to their strong Brønsted acid and common framework. Although there are also a certain amount of hexitols and HMF, Ni-HMETS-10 leads to much more LA formation than Ni/HMZSM-5 and Ni/HMBeta. Figure 7 gives the product distribution and corresponding reaction mechanism of glucose conversion on Ni-HMETS-10. Glucose can be easily isomerized to and coexist with fructose, both of which will suffer dehydration and hydrogenation together. During the dehydration reaction, glucose and fructose can directly lead to 5-HMF and then to LA with furfural and formic acid (FA) as main byproducts. Sorbitol and a handful of other hexitols as well as their further transformation products result from hydrogenation process. It is desirable that there is no significant change of products distribution under the synergistic action of both dehydration and hydrogenation.

Table 6. Physicochemical Changes of Ni-HMETS-10 after Treatment in Hot Water at 200 °C SBET Si/Ti (m2/g) beforea after a

7.1 5.9

271 235

Smicro (m2/g)

Smeso (m2/g)

Vmicro (cm3/g)

Vmeso (cm3/g)

pore size (nm)

188 158

83 77

0.08 0.06

0.2 0.19

16 15

Data is from Table 1.

stability test further confirms the catalytic potential of NiHMETS-10 under the designed conditions. Given the subtle influence of recycling tests on the catalytic selectivity, the XRD, FT-IR, and inductively coupled plasma (ICP) experiments were also carried out over the used NiHMETS-10 catalyst. As shown in Figure 5a, the XRD peak intensity of the used catalyst is obviously lower than that of the fresh catalyst, while both catalysts share similar FT-IR spectra (Figure 5b), which could be interpreted as the formation of surface carbon deposition and occurrence of small structural changes but not serious enough to destroy the framework architectures of the catalysts completely. Furthermore, according to the ICP analysis, a certain amount of Ni species leaching (0.9%, 1.6%, and 1.6% for the second, third, and fourth runs, respectively) should be mainly responsible for the decreased catalytic performance. Besides, sintering of the Ni particles can lead to reduced Ni dispersion and thus result in loss of reactivity in aqueous conditions even at much lower temperatures, which has been observed by previous studies.33 Thus, it is desirable in the future study to focus on stabilization of the catalyst through immobilization of active metal species. Mechanism Investigation. After Na/K-type ETS-10 undergoes ion exchange with ammonium chloride solution and calcination, the acid site can arise from the coordinately unsaturated Ti4+ ions formed by thermal dehydroxylation of Ti−OH groups and is able to interact with one or two more ligands with its large covalent radius and terminal silanols that are no longer tetrahedrally bonded after dehydration,34−39 which function as Lewis acid centers and interact with the reactant molecules to facilitate cellulose hydrolysis followed by glucose dehydration, similar to zirconia in hydroxide form, ionic liquids, and CrCl3.40−42 The detailed process is discussed below and shown in Scheme 1. Cellulose is first depolymerized by breakdown of the glycosidic linkage in hot water playing a hydrolytic role, which is simultaneously under the action of

Figure 5. (a) XRD patterns of fresh and used Ni-HMETS-10. (b) FT-IR spectra of fresh and used Ni-HMETS-10. 5806


Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Proposed Mechanism for Ni-HMETS-10 Catalyzed Conversion of Cellulose to LA

Figure 6. Products distribution and glucose conversion over different catalysts. Reaction conditions: 1.0 g of glucose, 0.5 g of catalyst, 100 g of H2O, 200 °C, 5 MPa H2, 6 h.

Hence, it is not surprising that in addition to hydrolysis under the action of Lewis acid, glucose is possible to experience

hydrogenation on metal sites and then to be rapidly converted into the intermediate essential to 5-HMF formation. The 5807


Research Article


Figure 7. Dehydration and hydrogenation reactions of glucose.

Fundamental Research Funds for the Central Universities (Grant 2242016K41015).

reaction pathways of cellulose conversion mentioned above are reasonable and contribute to the complete conversion of cellulose with excellent selectivity under the optimized reaction conditions, while there are still some deficiencies about the mechanism in this case. For example, it is possible that Lewis acidic sites can be transformed into Bronsted acid sites in hot liquid water, which should be further investigated in future work.

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(1) Ragauskas, A. J.; Williams, C. K.; Davison, B. H.; Britovsek, G.; Cairney, J.; Eckert, C. A.; Frederick, W. J., Jr.; Hallett, J. P.; Leak, D. J.; Liotta, C. L.; Mielenz, J. R.; Murphy, R.; Templer, R.; Tschaplinski, T. The path forward for biofuels and biomaterials. Science 2006, 311, 484−489. (2) Ruppert, A. M.; Weinberg, K.; Palkovits, R. Hydrogenolysis Goes Bio: From Carbohydrates and Sugar Alcohols to Platform Chemicals. Angew. Chem., Int. Ed. 2012, 51, 2564−2601. (3) Lange, J.-P.; Price, R.; Ayoub, P. M.; Louis, J.; Petrus, L.; Clarke, L. H.; Gosselink, H. Valeric Biofuels: A Platform of Cellulosic Transportation Fuels. Angew. Chem., Int. Ed. 2010, 49, 4479−4483. (4) Rackemann, D. W.; Doherty, W. O. The conversion of lignocellulosics to levulinic acid. Biofuels, Bioprod. Biorefin. 2011, 5, 198−214. (5) 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. (6) Szabolcs, A.; Molnar, M.; Dibo, G.; Mika, L. T. Microwaveassisted conversion of carbohydrates to levulinic acid: an essential step in biomass conversion. Green Chem. 2013, 15, 439−445. (7) Shen, J.; Wyman, C. E. Hydrochloric acid-catalyzed levulinic acid formation from cellulose: data and kinetic model to maximize yields. AIChE J. 2012, 58, 236−246. (8) Zhou, C. H.; Xia, X.; Lin, C. X.; Tong, D. S.; Beltramini, J. Catalytic conversion of lignocellulosic biomass to fine chemicals and fuels. Chem. Soc. Rev. 2011, 40, 5588−5617. (9) Peng, L.; Lin, L.; Zhang, J.; Zhuang, J.; Zhang, B.; Gong, Y. Catalytic Conversion of Cellulose to Levulinic Acid by Metal Chlorides. Molecules 2010, 15, 5258−5272. (10) Ramli, N. A. S.; Amin, N. A. S. Catalytic hydrolysis of cellulose and oil palm biomass in ionic liquid to reducing sugar for levulinic acid production. Fuel Process. Technol. 2014, 128, 490−498. (11) Gallezot, P. Conversion of biomass to selected chemical products. Chem. Soc. Rev. 2012, 41, 1538−1558. (12) Geboers, J. A.; Van de Vyver, S.; Ooms, R.; Op de Beeck, B.; Jacobs, P. A.; Sels, B. F. Chemocatalytic conversion of cellulose: opportunities, advances and pitfalls. Catal. Sci. Technol. 2011, 1, 714− 726. (13) Chen, H. Z.; Yu, B.; Jin, S. Y. Production of levulinic acid from steam exploded rice straw via solid superacid. Bioresour. Technol. 2011, 102, 3568−3570.

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CONCLUSION An approach with highly catalytic activity, excellent selectivity, and extraordinary stability to directly convert cellulose into LA was developed via a Ni-based mesoporous ETS-10 zeolite catalyst. It can be seen that catalyst surface Lewis acidity and mesoporosity were two crucial factors for the catalytic performance. Compared to microporous HETS-10 and Ni/ HMETS-10, direct introduction of Ni species and more mesoporosity significantly boosted the yield of LA. The catalyst Ni-HMETS-10 with a larger mesopore diameter of 16 nm afforded the highest LA yield of 91%. The catalyst is also stable and can be recycled several times. Furthermore, by introducing an appropriate amount of hydrogen, a more complete conversion of cellulose and enhanced LA yield was achieved, which brings in a distinctive reaction pathway of LA formation from cellulose.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*(D. F. Wu) E-mail: [email protected]. Tel: +8625520906206125. ORCID

Mei Xiang: 0000-0002-9199-6637 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21676055 and 21376050) and 5808


Research Article

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Improved Activity for Cellulose Conversion to Levulinic Acid through

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