Direct Conversion of Cellulose to Levulinic Acid over Multifunctional

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Direct conversion of cellulose to levulinic acid over multifunctional sulfonated humins in sulfolane-water solution Kui Wang, Jian-Chun Jiang, Xinyu Liang, Huan Wu, and Junming Xu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03558 • Publication Date (Web): 26 Sep 2018 Downloaded from http://pubs.acs.org on October 2, 2018

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ACS Sustainable Chemistry & Engineering

Direct conversion of cellulose to levulinic acid over multifunctional sulfonated humins in sulfolane-water solution Kui Wang,† Jianchun Jiang,† Xinyu Liang,† Huan Wu,† Junming Xu*,† †

Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry，No. 16, Suojin 5 Village, Xuanwu District, Nanjing 210042, China *Email address: [email protected]

Abstract Levulinic acid (LA) is an ideal platform chemical that can be generated by acid-catalyzed

dehydration

and

hydrolysis

of

sugars

from

lignocellulosic biomass. Hereby, we reported an environmental benign process for selective conversion of cellulose and bamboo meal to produce LA efficiently. When multifunctional sulfonated humins (MSH) was used as the catalyst in 90 wt% of aqueous sulfolane solution, cellulose was converted to LA with a high yield of 65.9 mol%. The MSH exhibited outstanding performance both on the catalytic activity and retrievability due to its synergistic effect with sulfolane and water. High LA yield of 45.6 mol% was also obtained from real biomass (bamboo meal), which is efficiency and specificity in parallel to liquid acid catalyzed process. Furthermore, all the solvent, catalyst and byproducts (humins and furfural) in this process could be easily recycled and reused, which proposed a new strategy for high value-added utilization of agriculture and forestry lignocellulosic residues. Keywords:

Lignocellulosic biomass, Thermo-chemical conversion,

Cellulase-mimetic catalyst, Platform chemicals, Process flow assessment. 1

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Introduction Accompanying with exhaustion of petroleum resources and aggravation of global environmental pollution, the increasing energy consumption has promoted the development of clean energy and environment-friendly biomaterials all over the world.1 Lignocellulosic biomass is an abundant renewable resource which can be directly converted into liquid fuel and fine chemical, and is thus believed as an ideal substitute for fossil resources.2-4 Levulinic acid (LA), that could be produced through thermochemical conversion process of lignocellulosic biomass, has been listed as Top 12 platform chemicals from biomass by United States Department of Energy.5 Due to its outstanding reaction activity of carboxyl and carbonyl active functional groups in its molecular structure, LA is regarded as one of the most promising biomass- derived chemicals which can be used as an intermediate to prepare high value added products and has been widely applied in different fields.6 Although remarkable reaction activity provides LA with broad application prospect, it also leads to instability, difficult refining, and other problems during production.7 In order to solve these problems, the catalytic liquefaction processes using various catalysts with good selectivity have been conducted by researches, 8 which could improve the selectivity of the target products and prevent negative repolymerization and other side reactions. 2


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It is known that liquid acid including H2SO4, HCl and H3PO4, just to name a few, have been widely applied to catalytic conversion of cellulose to LA. 9 Owing to excellent accessibility between substrates and liquid acid, the yield of LA is extremely high. However, in spite of high yield rate, liquid acid still leads to undesirable side reactions and some problems such as difficult separation and high corrosion which increase equipment investment and operation costs invisibly.10 Due to its good selectivity, easy recycling, low corrosion and other advantages, solid acid catalysts have attracted broad attention from research scholars and have been used to catalyze cellulose to prepare LA directly through liquefaction.

11-13

Zuo et al.12 used CP-SO3H-1.69 as

catalyst to catalyze cellulose at 170℃ which was subject to hydrolysis reaction for 10h with the yield of LA of 65 mol%. Peng et al. identified that the alkali metal halides play an important role on selective conversion of cellulose to produce LA.13 Optimum LA yield with 67 mol% could be obtained under 200℃ for 3h with CrCl3 as a catalyst. Obviously, the above mentioned solid acid catalysts exhibit remarkable catalytic effect with high LA yield. However, the strictly required condition that with long reaction duration or excessive reaction temperature has limited its application at industrial scale. 14 Moreover, the adsorption of LA by porous catalyst or the toxicity of metal catalyst also restricts the application scope of LA products.15 3



Recently, preparation of LA through hydrolysis by using ionic liquid and supercritical fluid as catalyst to catalyze cellulose has received attractive attention. 16-18 The conversion of cellulose into LA with sulfonated ionic liquid [C3SO3Hmim]HSO4 as a catalyst was performed by Ren et al.16, and the highest yield of LA of 70 mol %could be achieved at 170℃for 6h. Because of excellent thermal stability and low saturated vapor pressure, ionic liquid catalyzed hydrolysis proved to be an effect way for conversion of cellulose into LA in moderate conditions. However, higher process costs and difficult purification and separation of the target product impede its industrial application. 17 Similarly, Morais et al.18 used supercritical CO2 as the reaction system to prepare LA through cellulose liquefaction. Though the LA yield was remarkably higher than that in traditional process, unreasonable reaction pressure increased equipment costs and affected the industrial application prospect. Herein, based on our recent work on cation polarization effect of sulfolane19 and cellulase-mimetic solid acid catalyst20, we demonstrated a facile process to efficiently produce LA from cellulose under moderate condition with multifunctional sulfonated humins (MSH) as a catalyst in mixed solvents of sulfolane and water. A higher LA yield could be obtained in our process when compared with other solvents or catalysts. Furthermore, water, furfural (FF, the main volatile by-product) and LA could be easily isolated and recovered gradually by distillation according 4


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to its significant different boiling point. Besides, it was found that sulfolane and MSH could be easily retrieved and reused without observing distinct decrease of LA yield. Notably, the conceptual design for production of LA was also discussed in our study. Experimental Section Raw Materials All the standards (analytical pure, 99 %) for LA, furfural and HMF were obtained from Sigma Aldrich, US. Microcrystalline cellulose (chemical pure, 98%), sulfolane (chemical pure, 98%) and other chemicals (chemical pure, 98%) were purchased from China Sinopharm Co., Ltd. The bamboo meal was collected from Zhejiang Province in China and used after sifting by 120 meshes. Preparation of MSH The Multifunctional sulfonated humins (MSH) catalyst was synthesized with modified procedure20. In a typical procedure, 20g of humins (solid residues during hydrolysis of cellulose or biomass) was calcinated under nitrogen atmosphere for 5 h at 550℃to give black carbon materials in powder form, followed by cooking in 60 mL concentrated sulfuric acid (98%) at 120℃ for 10 h to introduce active SO3H group into the carbon materials. After cooling to moderate temperature under 80 ℃ , the suspension was filtered and washed repeatedly with hot deionized water until no SO4-2 could be detected in the filtrate by BaCl2 solution. The 5



black powder-like filter cake was dried at 110℃ for 4 h to give the target multifunctional sulfonated humins bearing -SO3H, -OH and -COOH groups (denoted as MSH). Degradation of cellulose to LA In a typical experiment21, 10 g cellulose and certain amounts of catalysts were first mixed in the complex solvents with 10 mL water and 90 mL sulfolane, and then the mixture was charged into a 250 mL stainless steel reactor. Inert N2 was applied here to displaced air inside the reactor and set initial pressure with 0.5 MPa, followed by programmed heating the reactor up to target temperature for a certain time. After reaction, the MSH was recycled by filtration and washing, dried under 105℃ for 1h. The liquid reactants were washed with 100 mL water to allow the humins precipitated from sulfolane/water solvent, following with filtration by a 0.45 µm membrane. Other liquids that collected during the process were also filtered by the 0.45 µm membrane and analyzed by external standard method for gas chromatograph (GC). After washing with deionized water and drying at 105℃ for 12 h, the obtained solid residues were weighted for FT- IR analysis. The water and sulfolane could be easily recycled by vacuum distillation and directly reused without further purification. Analytical methods Briefly, qualitative and quantitative analysis of target products was performed simultaneously by GC (Shimadzu, 2010) with Restek 6


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RTX-VMS capillary column and flame ionization detector. Helium gas was suggested as carrier gas with 0.43 mL min-1 column flow rate. The injector temperature were set at 240℃. The GC oven temperature was initially held at 40 ℃ for 5 min, then programmed to 240℃ at a rate of 7.5℃ min-1, and maintained at 240 ℃ for 15 min. Based on our previous study19, both of the yields of LA and FF were counted via following formula (1), supposed that the molecular weight of the constructed glucose unit of cellulose being 162: (%) =

[ ()× ]/() / !

(1)

In which M (i) were denoted as the molecule weight of LA and FF with values of 116 and 96, respectively. By the way, the concentration (i) of LA and FF and their retention time were determined by external standard method. Characterization of MSH The Fourier transform infrared spectroscopy (FT-IR, I80, NICOLET) of the components of bamboo powders, cellulose, humins and MSH were obtained at 0.4 cm-1 resolution from 400 to 4000 cm-1, respectively. The composition and structure of as- prepared MSH were tested by elemental analyzer (EA, Thermo Scientific Flash 2000), X-ray diffractometer (XRD, Bruker D8) and a Laser Micro- raman spectroscopy (RS, Thermo Scientific DXR, 532 nM), respectively. In order to better understand the characteristics of MSH formation, a DTG-FTIR test of humins was also 7



carried out here to identify the volatile components in real time. The temperature program was set up to first raised from room temperature to 550 ℃ at heating rate of 10 ℃/min under N2 flow, following with isothermal process at 550 ℃ for 5h which is the same as calcination process of MSH. Results and Discussion Characterization of SC catalysts The structure information of as-prepared MSH catalyst, cellulose, humins and char were characterized and presented as follows: (i) BET measurements confirmed that the surface area of MSH was estimated to be 2- 3 m2/g . In view of other reported solid acid catalysts, the small surface area of MSH could effectively get rid of unwanted adsorption of LA during the hydrolysis process. (ii) Except for the characteristic diffraction peaks of cellulose, the XRD patterns as shown in Fig 1(a) revealed that two weak and broad diffraction peaks at 25° and 44° 2θ were all observed and confirmed for MSH catalyst, char and humins, this might be interpreted by amorphous carbon which was comprised of aromatic carbon sheets directed in a significantly random fashion.22 As shown in Fig 1 (b), all the traces in RS spectrum reflected higher amount of disordered structure, which could be specified as morphing patterns of C-O-C and stretching vibrations in which notions of C5 and C6 atoms are forcefully concerned. The RS for all the above samples revealed that the 8


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intensity ratios of the D band (1350 cm-1) and G band (1580 cm-1) in MSH and char were to be around 0.9, indicating the average graphene size in the MSH catalyst were 1nm.23 As is well known, the 2D band detected at 2700 cm-1 implied the fused ring structure in MSH catalyst, indicating that the graphite-like layers number in MSH catalyst were around 2- 5.24 (iii) Fig. 1 c showed that the stretch vibration bands of O=S=O was detected at 1377 cm-1 in spectrum of as-prepared MSH, which implied that the existence of SO3H groups. The adsorption peaks around 1610 and 1710 cm-1 detected in MSH and humins could be assigned to vibration of hydroxyl and carbonyl, respectively. Notably, the bending vibration of the strong hydrogen bond linked between hydroxyl and carbonyl was also detected as a broad band appeared at 2300- 2700 cm-1 in the spectrum. The rupture of the hydrogen bond and C=O bands during the calcining process of humins are believed to be responsible for the disappear of bands in the trace of char at 2300-2700 cm-1 and 1715 cm-1, respectively. (iv) The so called humins observed during the hydrolysis process of cellulose and real biomass (bamboo meals) was denoted as humins 1 and humins 2, respectively. And TG-FTIR was applied to tell the difference of components and thermal degradation behaviors between humins 1 and humins 2, which will make comprehensive and profound sense of the catalytic mechanism of asprepared MSH catalysts. Fig 1(d) showed the TG curves of substrates and 9



their derived humins, which exhibited their thermal degradation behavior during the calcining process from room temperature to 500℃ under N2 flow. DTG curves disclosed that the thermal decomposition of humins1 could be divided into three stages, which was composed of dehydration (stage 1, 40-149℃), fast devolatilization (stage 2, 150-360℃), and carbonization stage (stage 3, 360-500℃). The dehydration stage was mainly resulted from the moisture evaporation (0.5 wt%). However, it was obvious that humins was depolymerized and generated to volatiles and char in the second stage, resulting in the occurrence of its major decomposition (60.5 wt %) via the devolatilization of carbohydrates, which is similar to the cellulose decomposition when reaction temperature ranged from 100 to 365℃. In the carbonization stage, the continuous decomposition of humins 1 (13 wt%) could be contributed to the devolatulization of bio-char which was defined as the main solid products in stage 3.25 In the curves of humins 2, similar phenomenon for weight loss was also observed, including dehydration (40- 150℃), fast devolatilization (150-375℃) and carbonization stage(375-500℃), could be found in Fig. 1(d), which is similar as curves of humins 1. However, the temperature range of fast devolatilization was wider than that of humins 1. Besides, it was noticed that the weight loss rate for humins 2 (0.5 wt%/℃) was lower than that of humins 1(1.3 wt%/℃). As the DTG curves shifted towards the side of higher temperature, this might be 10


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illustrated by the different components in humins. In carbonization stage of humins 2, a bigger part of humins 1 could be volatilized, which might be assigned to depolymerization of lignin. The lignin components in humins 2 could also be detected in 3D diagram of TG-FTIR.

Fig.1 The XRD(a), RS(b), FTIR(c) and DTG(d) analysis of as-prepared MSH catalyst and substrates.

In order to make better sense of the MSH formation, Fig. 2 illustrates the 3D FTIR diagram of Humins 1and Humins 2 under N2 atmosphere, accompanying with a 2D FTIR diagram from the real time data obtained at the peak of 320℃ and 347℃, respectively. It is known that some conventional gas such as H2O, CH4, CO2, and CO could be simply obtained in the FTIR diagram according to its characteristic absorption peaks. For instance, the intensified peak around 3584 cm-1 was observed and presented as the stretching vibration of O-H bond, indicating that 11



H2O came from moisture evaporation during the dehydration stage.25 Likewise, the stretching vibration adsorption peak at 2816 cm-1 was attributed to C-H bond in CH4 which was mainly derived from the breakdown of the functional group -OCH3, -CH3, and -CH2- during the devolatilization stage. Similarly, the CO2 could be confirmed by the C=O stretching vibration peaks at the band of 2350 cm-1 and 671 cm-1 which was believed to be generated via decarboxylation and decarbonylation reaction during devolatilization stage. Additionally, a relatively weak adsorption peak at 2174 cm-1closes to the peak of CO2 (2350 cm-1) was identified as the stretching vibration of C-O bond, indicating the existence of CO component which was presumably came from the decomposition of C-O-C and C=O bond. In addition to the conventional gases, other organic components could also be observed in the wave number ranged from 1000-2000cm-1. The stretching vibration adsorption peak around 1771 cm-1 was believed to deserved from the cleavage of the organic components which contained the C=O and -COOH. The characteristic absorbance band at 1177 cm-1 in fingerprint region was alkanes, alcohols, ethers and lipids with typical function groups, such as active carbon hydrogen bond and ether linkage in the carbon chain skeleton. It should be noted that, the peak at 1504 cm-1, along with absorbance peaks at 1580 cm-1 and 1600cm-1, was believed to be the C=C stretching vibration and benzene skeleton vibration which could only be 12


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observed in humins 2 instead of humins 1, suggesting the existence of lignin components in humins 2 from hydrolysis of biomass, which greatly corresponded to TG analysis results.

Fig.2. 3D FTIR analysis of humins 1 and humins 2 thermal degradation under N2 atomosphere.

Degradation of cellulose in sulfolane and water with MSH1 as catalyst Table 1 depicts the conversion of cellulose and yield of LA and FF during cellulose hydrolysis with sulfonated char catalysts from different substrates including microcrystalline cellulose, xylan, milled wood lignin and humins. The comparison with control group using sucrose as precursor was also carried out in this paper for better understanding of catalytic efficiency. The conversion of cellulose catalyzed by varied sulfonated chars ranged from 92.1 wt% for sucrose to 99.3 wt% for humins. Besides variation due to experimental or analytical error, the 13



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difference of catalytic effects among sulfonated chars can be attributed to varied dosage of functional groups. The separate and total amount of functional groups could be calculated by the experimental results of elemental analyzer and cation exchange experiment, respectively. First of all, the SO3H group density of as-prepared catalyst could be simply observed according to the sulfur content detected by elemental analysis. Moreover, the COOH and OH group densities could be estimated from total contents of SO3H+ COOH and SO3H+ COOH+ OH which were directly determined by the Na+ exchange experiment with aqueous NaCl and NaOH solutions, respectively. The SO3H content of SCC, SCL and MSH were visibly higher than that on SCS and SCX, which was consistent with the acid site density detected by NH3-TPD. Total content of functional groups followed the sequence MSH >SCL≈SCC>SCX≈ SCS. Notably, the multifunctional groups endowed accessibility to polar substrates through hydrogen bond linkage, pushing enormous advance on catalytic efficiency, in spite of their small surface area. Table.1 The content, surface area and acid site density of multifunctional groups in as- prepared catalysts Element% SO3H Catalysts C

H

O

COOH

Total

Acid site

Content

density

OH

(mmol

(mmol

(mmol

g-1)

g-1)

g-1)

S

Surface Conversion

LA

FF

of Cellulose

Yield

Yield

area (mmol

(mmol

-1

-1

(m2g-1) g )

g )

(wt%)

(mol%)

(mol%)

SCS

71.4

2.6

24.6

2.4

0.75

0.42

1.56

2.73

1.39

2.1

92.3

60.8

11.6

SCC

63.0

2.9

31.0

3.1

0.97

0.45

1.63

3.05

2.01

2.0

94.3

63.5

12.9

SCX

66.8

2.9

28.1

2.2

0.69

0.46

1.64

2.79

1.55

1.9

93.0

61.4

12.0

SCL

61.4

2.7

32.0

3.9

1.22

0.38

1.47

3.07

1.98

2.0

94.7

63.6

13.0

MSH1

62.6

2.4

30.7

4.3

1.35

0.38

1.53

3.26

2.11

1.9

99.3

65.9

13.5

MSH2

61.2

2.6

31.8

4.4

1.38

0.33

1.56

3.27

2.09

1.9

99.6

65.9

13.8

14


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The effect of dosage of MSH, reaction time and temperature were also discussed in this study. Fig. 3a depicts the influence of the catalyst dosage on cellulose conversion and LA yield. The amount of MSH used was 0.1, 0.3, 0.5, 0.7, and 0.9 g, respectively, with 3g of cellulose substrate loaded. When 0.1 g of MSH was used, 21.3 wt% of cellulose was converted at 180 ℃ for 2 h with 15.2 mol % of LA yield. Accompanying with the dosage of MSH changed from0.1g to 0.5g, the conversion of cellulose and LA yield gradually increased to 99.3 wt% and 65.9 mol%, respectively. However, the conversion of cellulose slightly fluctuated with the increase of MSH dosage. According to the mild adsorption of LA by MSH catalyst during the filtration process, the LA yield curve likewise depicts same tendency as cellulose conversion. Fig. 3b and 3c summarize the effect of reaction time and temperature on cellulose conversion and LA yield. Results disclosed that both reaction temperature and time shown remarkable influence on cellulose conversion and LA yield. When reaction time ranged from 0.5 h to 2.0 h at 180 ℃, the conversion of cellulose increased from 33.7 wt% to 99.3 wt%, and LA yield increased correspondingly. Notably, slightly decrease could be achieved with increasing time and the optimum condition was obtained under 2.0 h with 99.3 wt% of cellulose conversion and 65.9 mol% of LA yield. In a parallel manner, when the temperature increased from 140 ℃ to 220 ℃ for 2 h, the tendency of temperature curve was similar to Fig.3b. The LA 15



yields first increased to 65.9 mol% and then decreased. Simultaneously, the conversion of cellulose changed from 19.3% to 99.3 wt% and then decreased to 95.3 wt%. In fact, the above mentioned unwanted decrease of cellulose conversion and LA yield could be attributed to the formation of humins under higher temperature and longer time. It should be noted that remarkable performance on repeatability was also observed and shown in Fig.3d. After 8 times reuse, no significant change could be obtained and the conversion of cellulose and LA yield was still higher than 96.0 wt% and 63.0 mol%. We measured the content of free SO3H in the reaction solvent during the retrieve ability experiment and found that trace of SO3H leached into the solution, which basically matched the experiment results.

Fig.3 The effect of Substrate species(a), Dosage (b) of MSH, reaction time (c) and temperature(d) on the cellulose conversion and LA yield.

16


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Degradation of bamboo meal in sulfolane and water with MSH2 as catalyst In order to evaluate the adaptability of our process on real biomass, bamboo meal was applied here as the substrate and catalyzed by various liquid acid catalysts, such as H2SO4, HCl and H3PO4. Three literatures reported cellulase-mimetic solid acid catalyst, including ZrO226, SA-SO3H27, and CP-SO3H-1.6912, were also listed here as the control group. Table 2. Degradation of bamboo meal and cellulose to LA with different catalysts Entry

1

a

Catalyst

Substrate

optimum

optimum

optimum mass ratio

temperature

time

of catalyst/substrate

(℃)

(h)

(g/g)

2

1/100

H2SO4

bamboo

180

Conversion

95.2

a

Yield of

Yield of

LA

FF

(mol%)

(mol%)

52.3

39.5

2

HCl

bamboo

180

2

1/100

93.6

49.2

30.2

3

H3PO4

bamboo

180

2

1/100

92.7

40.7

32.7

4

MSH

bamboo

180

2

1/6

92.9

45.6

27.1

5

MSH

cellulose

180

2

1/6

99.3

65.9

13.5

6

ZrO2

cellulose

180

3

1/1

100

53.9

—

7

SA-SO3H

cellulose

180

12

1/1

100

51.5

—

8

CP-SO3H-1.69

cellulose

170

10

3/1

100

65.5

—

Reaction conditions for entry1-5: using 10 g of sulfolane/H2O (90/10) per gram of substrate as solvent; The entry

6, 7 both adopted 20g of H2O per gram of cellulose as solvent26, 27; The entry8 selected 20g GVL/H2O per gram of cellulose as solvent12.

Due to the easy community between bamboo meal and homogeneous catalysts, H2SO4, HCl and H3PO4 all exhibited remarkable performance with optimum conditions observed in relatively shorter reaction time of 2 h and strikingly slight catalyst amount of 0.01g per gram of bamboo meal. As shown in Table 2, H2SO4 showed better catalytic performance in complex solvents of sulfolane and water when compared with HCl and 17



H3PO4. Furthermore, the lower conversion of 93.6 and 92.7 % which was obtained at 180℃ with HCl and H3PO4, respectively, was consistent with above results. Notably, the higher yield of FF could be attributed to the conversion of hemicelluloses in bamboo meal, when compared with the FF yield from conversion of cellulose. Owing to remarkable selectivity and outstanding retrievability, the as-prepared MSH were also implied in degradation of bamboo meal and compared with above mentioned homogeneous catalysts. As show in Table 2, MSH showed efficient catalytic and outstanding selective performance on target product from bamboo meal with LA yield of 45.6 % at 180℃ for 2 h in sulfolane/ water system, that was really in parallel to the compared results (52.3 %, 49.2 % and 40.7 %) with H2SO4, HCl and H3PO4 as catalysts, respectively. The hydrogen bridge linked among multifunctional groups of MSH and neutral OH groups and glycosidic bonds on lignocelluloses were believed to be responsible for remarkable catalytic performance, which bind substrates (lignocelluloses or cellulose) to MSH catalyst surface, which worked just like the cellulase. Such cellulase-mimetic solid acid catalyst design ideas were also conducted in ZrO226, SA-SO3H27 and CP-SO3H-1.6912 in reported literatures. Joshi26 applied ZrO2 on selective LA production from cellulose at 180℃ for 3h with 53.9 mol% of LA. The outstanding catalytic performance was believed owing to the linkage between free hydroxyl groups on cellulose and zirconium hydroxide by 18


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an oxo complex. Qi and his coworkers27 prepared a biomimetic solid acid catalyst from sucralose, tetraethyl orthosilicate and trimethoxysilane which was denoted as SA-SO3H with Cl as binding domain and SO3H as catalytic domain. More than 51mol% of LA from cellulose could be obtained with SA-SO3H as catalyst at 180℃ for 12 h. Zhang and her coworkers12 also explored a novel cellulose-mimetic heterogeneous catalyst through sulfonated process from commercial chloromethyl polystyrene resin. 65.5 mol% of LA could be obtained at 170 ℃ for 10 h in 90 wt% of aqueousγ-valerolactone solution. Nonetheless, in order to get reason LA yield for above mentioned catalysts, longer catalytic time or lager catalyst amount were implied when compared with MSH. Actually, the synergetic effect of multifunctional groups in the MSH and the strong solvation effect of sulfolane were proved to be responsible for MSH remarkable catalytic performance. The cellulose-binding domain (-COOH and –OH groups) first adsorbed cellulose via strong specific interactions, then the solvated hydrogen proton from cellulose-catalytic domain (-SO3H group) efficiently attacked the carbon cation in cellulose, glucose and HMF, following with the cleavage of C-O bond and dehydration process to finally obtained LA. Conceptual design for LA production from cellulose or bamboo meal A conceptual process for conversion of cellulose or real biomass to produce LA is established here and evaluated according to the above 19



mentioned results in this paper. The process flow diagram is visually presented in Fig.4, accompanying with detailed material balance given in Fig. 5, which was compared with other reported process. In our process, MSH (stream 1) and cellulose or bamboo meal (stream 2) are first mixed up to give a premixed mixture. Then stream 3, which defined as the mixture of stream 7 of recycle water and stream 14 of recycle sulfolane with the ratio of 10: 90, is fed to reactor and form a slurry solution containing 11wt% solids. After reaction under related conditions for a while, the cellulose or bamboo meal is hydrolyzed and decomposed to LA, FF and humins, following with the separation of the product stream (stream 4) to a liquid stream (stream 6) and a solid stream (stream 5) composed of MSH and unreacted cellulose. The solids could be directly added in catalyst stream (stream 1) without pre-activated process. The aqueous solutions are first washed with water, followed by precipitation to form liquefied product stream (stream 11) and water insoluble humins (stream 9) which could be used for production of MSH catalysts (stream 10) as depicted in the Experimental section. The water could be easily recycled here (stream 7) and reused in stream 3 and 6, respectively. According to the remarkable differences on physical and chemical properties of LA, FF and sulfolane, especially the boiling point and thermal stability, the liquefied product could be efficiently separated into LA, FF and sulfolane by distillation (stream 12 and 13) with water 20


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soluble humins as solid product. The sulfolane (stream 14) and water soluble humins (stream 15) could be both effectively recycled and reused in our research as mentioned above.

Fig.4 Process flow diagram for conversion of cellulose or bamboo meal to produce LA

Due to extensive profit in LA adhibition, a quantity of feasible processes for commercial production of LA has been realized from cellulose and lignocellulosic biomass. As we know, the input-output analysis could be applied to compare and evaluate the economic viability of different process. As shown in Fig. 5, three different processes for LA production are investigated here and analyzed by the input-output analysis method. Notably, all the results are calculated on account of 100kg cellulose feedstock. First of all, the Biofine process28 exhibited in Fig. 5 (a) has been developed and applied since 1988 using a two- step method to generate LA, furfural and other products from lignocellulosic biomass. The optimal LA yield could be obtained with extremely high yield of 76 21



mol% with sulfuric acid as catalyst. However, it is hard to separate and purify of LA from the acidic reacted system containing sulfuric acid and other acidic byproducts, which invisibly increase the production cost. Furthermore, the acidic aqueous stream with mineral acid always accompanied with tough environmental problem and additional expense for purified process. Meanwhile, inevitable equipment cost is required for reactor and piping due to the corrosivity of liquid acid. As Fig.5 (b) depicts, another input-out analysis was also conducted according to the optimum conditions reported by George W. Huber et al.29 to produce LA from cellulose using ZrP as the catalyst. As mentioned in their study, a maximum yield of 28% LA could be obtained in aqueous solution without any additional organic solvent and homogeneous catalysts, which greatly simplifies the downstream separation and purification of LA. However, except for the relatively low of LA yield, 145kg of slurry stream, including 82kg water and 63kg humins also need be purge and recycled, due to the poor dissolving capacity of humins in water.30 The process introduced in our study adopt sulfolane, water and heterogeneous acid catalyst from humins to produce LA from cellulose with relatively high yield of 66 mol%, as shown in Fig.5(c). Recycle loops are utilized to treat catalyst, solvent and unreacted feedstock, thereby maximizing of economical efficiency of our process while maintaining a high yield of LA. The extremely high thermal-chemical stability and remarkable 22


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dissolving capacity on humins of sulfolane facilitates catalyst recycle and initial products separation in the downstream process. Only 36 kg of humins (including unreacted cellulose) are obtained in our process which could be easily regenerated to MSH catalyst through typical reported sulfonation process. Consequently, the downstream separation steps would require less energy and process unit cost. In order to develop a viable and effective process to produce LA from lignocellulisc biomass, it is essential to optimize our process parameters in pilot scale, and a kinetic study on cellulose decomposition and the life cycle assessment (LCA) will also be involved in our future research for better understanding the whole process.

Fig.5 Input- output analysis of LA production from cellulose for (a) the Biofine process; (b) developed by Geogre W. Huber et al. (c) our study. All unwanted decomposition products including unreacted cellulose were referred to as humins. 23



Conclusions We presented an environmental benign process with multifunctional sulfonated humins as catalyst to produce LA from cellulose and bamboo meal under mild reaction conditions. 65.9 mol% of LA yield was observed under 90 wt% of aqueous sulfolane solution, accompanying with 99.3 wt% of cellulose conversion. Due to the synergistic effect among multifunctional groups, the MSH catalyst exhibited outstanding performance both on the catalytic activity and retrievability. We also illustrated a conceptual design of our process, comparing with reported processes for the production of LA from cellulose. The reusability of solvent and catalyst in our process were proved to be efficient, which exhibited remarkable potential for industrial application. Our forward research would focus on the kinetic study and life cycle assessment of process for production of LA from lignocellulosic biomass.

Acknowledgments The authors would like to thank the National Key Research and Development Program (2017YFD0600805) and National Natural Science Foundation of China (31600590) for financial support.

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27



Abstract Graphic An environmental benign process for efficiently production of levulinic acid from lignocellulosic biomass is presented in our study with prospect of industrial application. MSH

activation

Humins

recycle Biomass / cellulose

liquefaction

Reaction mixture

filtration

filtration

Water

recycle

Liquid products vacuum distillation

Furfural

LA

Products

Sulfolane

vacuum distilltion

28


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Direct Conversion of Cellulose to Levulinic Acid over Multifunctional

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