Two-Step Saccharification of Rice Straw Using Solid Acid Catalysts

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Two-Step Saccharification of Rice Straw Using Solid Acid Catalysts Luh Putu Pitrayani Sukma, Xiuhui Wang, Sen Li, Thanh Tung Nguyen, Jianglong Pu, and Eika W. Qian Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b06473 • Publication Date (Web): 14 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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Industrial & Engineering Chemistry Research

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Two-Step Saccharification of Rice Straw Using

2

Solid Acid Catalysts

3

Luh Putu Pitrayani Sukma1, Xiuhui Wang2, Sen Li3, Thanh Tung Nguyen2, Jianglong

4

Pu4 and Eika W. Qian2*.

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1

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Indonesia, 13880.

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2

8

Agriculture and Technology, Nakacho 2-24-16, Koganei, Tokyo 184-8588, Japan.

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3

Anglo Chinese School Jakarta, Jalan Bantar Jati, Kelurahan Setu Jakarta Timur–

Graduate School of Bio-Applications and Systems Engineering, Tokyo University of

Shanghai Research Institute of Chemical Industry Co. Ltd, No. 345, East Yunling

10

Road, Shanghai, 200062, China.

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4

12

Jiahang Road 118, Zhwjiang Province, 314001, China.

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Abstract: To establish an efficient bioethanol production system, a solid-acid catalyst-

14

based saccharification of rice straw in two steps was developed. It was rarely

College of Biological, Chemical Sciences and Engineering, Jiaxing University,

ACS Paragon Plus Environment

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

Page 2 of 59

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concerned so far that many biomass species contain a significant amount of

2

nonstructural carbohydrates (NSCs, free-sugars and starch). NSCs, including

3

hemicellulose, are readily to hydrolyze but also susceptible to overreaction with

4

byproducts. In the developed process, the first-step is intended to effectively hydrolyze

5

the significant sugar source of NSCs and HC in rice straw under a mild condition, and

6

the second-step is addressed to hydrolyze the cellulose in the residue obtained under

7

a harsh condition. For each step, several catalysts were screened and reaction

8

condition were investigated. For the first-step, an appropriate catalyst was Amberlyst

9

35 Dry, at 130 oC for 30 min in which high yields of C6 monosaccharide, 47.2 % and

10

C5 monosaccharide, 10.8 % were obtained. For the second-step, the most active

11

catalyst was a sulfonated mesoporous carbon that provided a maximum yield of C6

12

sugar (52.5 %) at 220 oC for 0 min. A test with the sequential two-step saccharification

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provided a 65 % yield of sugars. From the investigations, the most studied modified

14

pathways of dilute-acid catalyzed hydrolysis of HC and cellulose were also likely to

15

occur with a solid-acid catalyst with an additional pathway of direct hydrolysis of NSCs

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to C6. To support the observed phenomena, the characteristics of the RS, before and

17

after saccharifications were analyzed using SEM and XRD. Thus, this process


2

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represents a method to increase the cost-effectiveness of bioethanol production

2

system in an environmentally sound way.

3

1. Introduction

4

Saccharification of lignocellulosic biomass to produce monosaccharide is

5

considered as an intermediate for green energy because monosaccharide can be

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further fermented to bioethanol or biobutanol. For a long period of time, considerable

7

attention has been focused on utilization of homogenous acids and enzymes for

8

saccharification. Although homogenous acids show reasonable catalytic prices and

9

good catalytic performances, their practical applications are still difficult due to reactor

10

corrosion, catalyst recovery and acid-waste generation.1 On the other hand, enzymatic

11

saccharification is seen to have a reasonable prospect for a commercial scale

12

production in future since it offers the potentials for higher yields, higher selectivity,

13

lower energy costs, milder operating conditions, more environmentally friendly than

14

the chemical process. 2-4 However, the innate biomass recalcitrance is one of the main

15

barriers to the economic production of this process.3 The biomass recalcitrance is

16

related to the complex characteristics of lignocellulose due to cell wall structure,


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cellulose crystallinity, degree of polymerization, surface area, and close association

2

with lignin and hemicellulose.4 These characteristics restrain carbohydrates (cellulose

3

and hemicellulose) from degradation by enzymes.5-6 Therefore, pretreatment is

4

required to remove the barrier and make cellulose component more accessible to

5

enzymes by changing the chemical composition or physical structure of biomass.7

6

Lack of low-cost and high-activity hydrolytic enzymes is another barrier to

7

lignocellulosic ethanol production. It has been reported that enzyme production is the

8

most expensive step in the production of lignocellulosic ethanol, covering about 40%

9

of the total cost.5,8 In order to resolve these barriers, there has also been an emerging

10

trend to use extremophilic enzymes for biomass pretreatment and saccharification;

11

however, these enzymes have not perfected and needs further investigations.9 The

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application of solid acid catalysts for saccharification of cellulosic biomass has the

13

potential to overcome some the above barriers. The feature of this process is the ease

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of separating the catalyst, thereby enabling the possibility of recovering and reusing

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the catalyst, along with minimizing corrosion and suppressing waste generation.

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These advantages have encouraged aggressive research and development of

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recyclable solid acids catalyst in the saccharification of biomass.10-12


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Most previous researches in saccharification by solid acid catalysts used pure

2

cellulose as substrate. For practical application, it is important to investigate the

3

saccharification process for a real cellulosic biomass. The utilization of cellulosic

4

biomass such as rice straw has attracted much attention because it is an agricultural

5

waste, abundant availability, low-cost and tackles food competing fuels issues.13-14

6

Problems with the utilization of cellulosic biomass are carbohydrates (starch,

7

hemicellulose, and cellulose) present in it which have different reactivity that result in

8

different hydrolysis rates.15 Starch and hemicellulose are relatively easy to be

9

hydrolyzed16 under a mild condition compared to the cellulose. The efficient

10

saccharification of cellulose should be conducted under a harsher condition. Li et al.13

11

showed that in one-step process of saccharification of rice straw, most components of

12

glucan and xylan derived from hemicellulose are decomposed at 180 oC and below

13

but glucan derived from cellulose is mainly decomposed at 200 oC and above. Also,

14

Li et al.17 reported that degradation rates of xylose and glucose are different from each

15

other. Consequently, the utilization of one-step process to hydrolyze that complex

16

biomass may be not enough to generate a maximum monosaccharide yield. In

17

addition, in an effort to generate maximum conversion in the one-step process, xylose,


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the product of hydrolysis of hemicellulose, may undergo further reactions and form

2

furfural that can inhibit microbial growth and formation of bioethanol at the fermentation

3

step even though only contain at a very small dosage.18

4

In the present study, to improve the performance of saccharification process using

5

solid acid catalysts, including to increase total yield of monosaccharides and to

6

suppress conversion of monosaccharides into byproducts, a two-step saccharification

7

process of rice straw is proposed. In the first-step, the saccharification process is

8

intended to hydrolyze starch and hemicellulose in the rice straw and the reaction

9

process is carried out under a mild reaction condition. Subsequently, the second-step

10

saccharification process is intended to hydrolyze the remaining biomass residue that

11

might be largely composed of cellulose crystals, under a harsher reaction condition.

12

Our group had reported saccharification of rice straw using solid acid catalysts in one-

13

step process13,17 and successfully obtained total monosaccharide yields of 43.4 %

14

based on holocellulose content, structural carbohydrate that consist of glucan and

15

xylan, after saccharification at 150 oC for 1 h. The original concept of two-step

16

saccharification of biomass had been introduced by our group in Patent of Japan.19


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Meanwhile, Kobayashi et al.20 had also been reported briefly the saccharification of

2

bagasse using activated carbon in HCl solution and using one-pot reactor with two

3

step temperatures. However, the use of two-step reaction temperature in different

4

reactors, without the use of acid solutions, may have some advantages, such as in

5

suppressing formation of byproducts that can be detrimental to the following

6

bioethanol synthesis, avoiding neutralization step of the product, and minimizing

7

waste. Regarding this, to gain a deeper understanding of the process and mechanism

8

of the two-step saccharification of biomass, an extensive investigation on operating

9

conditions and analyses of morphology and chemical compositions of biomass

10

samples and residues of saccharification, are still required. In addition, screening of

11

solid acid catalysts and investigating optimum condition of reaction are also necessary

12

in order to improve the performance of two-step saccharification of biomass using solid

13

acid catalysts.

14

To construct the proposed two-step saccharification of rice straw, several solid acid

15

catalysts were screened for the first and the second-step saccharification. In each

16

step, effects of reaction conditions (temperature and time) were investigated to obtain


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high saccharification performance and to understand mechanism occurred during the

2

process. The contents of glucan and xylan of rice straw samples before and after

3

saccharifications were analyzed using the NREL analysis method. Morphological

4

changes and crystallinity pattern of rice straw samples were evaluated using SEM and

5

XRD analysis. Finally, a mechanism of two-step process was proposed.

6

2. Experimental section

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2.1 Raw materials. Rice straw (from forage cultivar Leaf Star, dried, ground under

8

14 mesh) was supplied by Tokyo University of Agriculture and Technology. Sucrose,

9

sulfuric acid (>98.0%), HCl, tetraethylorthosilicate (TEOS, >95 %), ethanol, ZrCl2O

10

8H2O (>99.0 %), AlCl3

6H2O, tetramethlammonium hydroxide and HPLC standard

11

samples such as glucose, xylose and organic acids were purchased from Wako Pure

12

Chemical Industries, 5-hydroxymethylfurfural (5-HMF) from Tokyo Chemical Industry

13

Co., and furfural from MERCK.

14

2.2 Preparation of Catalysts. All of the catalysts used in this study were prepared by

15

our laboratory except for Amberlyst 35 Dry (AD35), a kind of acidic ion exchange resin,

16

which was purchased from Rohm and Hass Co.


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ZrO2/SO42- Catalyst (ZS). ZS catalyst was prepared according to a procedure

2

reported before.21 10 g zirconia was added to 84 mL 1 M H2SO4 solution. Then, the

3

mixture was incubated for 15 min. The solid precursor was separated from H2SO4

4

solution by filtration, dried at 60 oC for 18 h, and calcined at 650 oC for 3 h.

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ZrO2-SBA-15/SO42- Catalyst (5ZSBAS and 10ZSBAS). SBA-15 coated ZrO2 with

6

ratio of ZrO2/SBA-15 was 5 % (w/w), abbreviated as 5ZSBAS, and 10 % (w/w),

7

abbreviated as 10ZSBAS, were prepared. SBA-15 as mesoporous silica template was

8

prepared according to a procedure reported before.22 For preparation of 10ZSBAS,

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2.59 g of ZrCl2O•8H2O was dissolved in 200 mL ethanol at 60 oC for 1 h. After that,

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7.41 g of SBA-15 was added to the solution. Then the water content of solid catalyst

11

was evaporated by drying at 60 oC and was continued by drying at 80 oC for 2 h. Then,

12

the catalyst was calcined at 600 oC for 10 h. The method for introducing the SO42-

13

functional group was the same as the preparation of ZrO2/SO42- catalyst (ZS).

14

ZrO2-SiO2/SO42- (5ZSiS). SiO2 coated ZrO2 with ratio of ZrO2/SiO2 of 5 % were

15

prepared. The procedure was the same as the preparation of ZrO2-SBA-15/SO42- but

16

the SBA-15 was replaced by SiO2.


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Al-SBA-15 Catalyst (ASBA). 120 g H2O, 16.1 g AlCl3•6H2O and 151.7 g of

2

tetramethylammonium hydroxide were mixed and incubated in an oil bath at 80 oC for

3

2 h. When the mixture was still incubated in the oil bath, 12 g SBA-15 was added to

4

the solution. The solid catalyst was washed and then dried at 105 oC for 2 h,

5

subsequently calcined at 550 oC for 4 h.

6

MC Catalyst. Sulfonated mesoporous carbon with intact silica template (MC

7

catalyst) was prepared according to the method in previous report.22 Silica, SBA-15,

8

was used as template for the preparation of mesoporous carbon structure.

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2.3 Characterization of Catalysts. N2 adsoprtion and desorption isotherms were

10

measured over the relative pressure (P/P0) with a range of 0.05-0.99 at 77 K using

11

Belsorp-mini II automated sorption system. The surface area and pore diameter of

12

samples were calculated using Brunauer-Emmett-Teller (BET) methods and Barret-

13

Joyner-Halenda (BJH), respectively. The acid amount of prepared catalysts was

14

measured using titration method according to our previous report.22 In this titration

15

method, 0.5 g of catalyst was added to 2 M NaCl (20 mL), and then the mixture was

16

stirred for 8 hours. The resulting solution was titrated using 0.1 M NaOH solution with


10

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1

an indicator of phenolphthalein solution. The acid amount and acid density of the

2

catalysts were calculated according to the procedure reported by Qian et al.22 and

3

Dalla Costa et al.23, respectively.

4

2.4 Analysis of Samples. The chemical compositions of fresh and post-

5

saccharification of rice straw were analyzed according to the procedure described in

6

the NREL Chemical Analysis & Testing Procedure.13 Fraction of hemicellulose and

7

cellulose in rice straw was analyzed following the procedure described in the Manual

8

of Wood Science Experiment published by the Japan Wood Research Society.24

9

Samples of rice straw before and after saccharification were also characterized by

10

using XRD and SEM. XRD measurements were conducted using X-ray diffractometer

11

(RIGAKU RINT 2000/PC) with Cu (M radiation. Crstallinity index (CrI) was calculated

12

according to the method described by Segal et al.25 The microscopic features of

13

samples were observed by Scanning Electron Microscopy (SEM, JSM-6510, JEOL).

14

2.5 Saccharification Tests. The saccharification of feedstocks (fresh rice straw or

15

residues of rice straw) was carried out in an autoclave (120 mL, Taiatsu Techno Co.)

16

with the stirring rate of 400 rpm. The amount of prepared catalyst, rice straw, and


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distilled water used was 1:3:30 (w/w/w), respectively. After the reaction, the liquid

2

products and solid residues were separated by filtration using membrane filter (0.2

3

µm, Sartorius Stedim Biotech). The solid residues were dried in a vacuum oven at 65

4

oC.

5

Monosaccharides and 5-hydroxymethylfurfural (5-HMF) were analyzed using a

6

HPLC with Sugar SP0810 column (Shodex, Japan) at 80 oC. The eluent used was

7

distilled water at a flow rate of 1.0 cm3/min and refractive index detector (RI-8020,

8

Tosoh, Japan) was applied. Organic acids were analyzed using a HPLC with a TSKgel

9

OApak-A column (Tosoh Co.) at 40 oC and 0.075 mol/L sulfuric acid solution as the

10

eluent at a flow rate of 0.7 cm3/min with a conductivity detector (CDD-10AVP,

11

Shimadzu, Japan).22 The analyses were performed duplicates.

12

13

Liquefaction rate, conversion, and yield of component i in product were calculated

using these following equations.

(WR/WF)]

14

Liquefaction Rate [%] = [1

15

Conversion [%] = [1

16

Yield of component i, Yi [%] = (Wi/WHC

(WHC

R/WHC, F)]

100

100

F)

100


12

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WF and WR were the weight of feedstock charged and residue after saccharification

2

reaction. WHC, F and WHC, R were the weight of holocellulose contained in the feedstock

3

and residue, respectively. Wi was the weight of component i in the product, where i

4

was oligosaccharides, C6, C5 monosaccharides, or components of byproducts (5-

5

HMF, furfural, and organic acids).

6

3. Results and discussion

7

A promising green approach to improve effectiveness of downstream processes

8

from biomass energy sources is saccharification based on a solid acid catalyst.

9

However, from our previous work, an application of saccharification in a single step for

10

lignocellulosic biomass suffers with problems, particularly lower conversion to

11

monosaccharides and relatively high production of inhibitor compounds to the

12

fermentation process, because a harsh reaction condition is required to hydrolyze rigid

13

cellulose at a significant level.13,17 Meanwhile, some initial monosaccharides produced

14

from less rigid carbohydrates, such as nonstructural carbohydrates (NSCs) and

15

hemicellulose (HC) are degraded to produce the inhibitory compounds. In an

16

established process based on homogeneous catalysts, a strategy of applying a


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sequential pretreatment in two-stages or more is essential to improve efficiency of

2

saccharification. In general, the sequential pretreatment is intended to break down the

3

rigid structure of lignocellulose, dissolve HC and/or lignin, decrease crystallinity, and

4

increase availability of surface area and pore volume.26 Analogous with this strategy,

5

a sequential two-step saccharification based on solid acid catalyst has been

6

developed with a schematic process diagram shown in Scheme 1. The first-step

7

reaction under a mild condition is addressed to be in line with the above pretreatment

8

objectives and is expected to effectively hydrolyze NSCs and HC to monosaccharides.

9

The hydrolysate, residue and catalyst from this first-step reactor can be separated

10

through a series of separators. The resulting residue is intended for the second-step

11

saccharification in the presence of a solid acid catalyst under a harsher condition,

12

followed by separating products with the same way as the first step. The hydrolysates

13

obtained from both reactors are cooled and then transferred to the fermentation

14

process. Catalysts used are expected to be regenerated and reused for several times.


14

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2

3

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Table 1. Composition of rice straw

Component

wt%, Dry

Nonstructural Carbohydrates:

16.1

Free Sugar

3.0

Starch

13.1

Cell Wall Component:

61.0

Cellulose

23.3

Hemicellulose:

20.2

Glucan

8.5

Xylan

11.7

Lignin

17.5

Ash

13.9

Extractive (by difference)

9.0

Holocellulose:

59.6

Glucan

47.9

Xylan

11.7

5

6

HC and cellulose with a total of 43.5 % are main sources of monosaccharides in the

7

hydrolysis process, and in the RS, they are present in comparable amount. Due to


16

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1

its branched structure, HC is amorphous and well-known to be much more

2

susceptible to acid hydrolysis than cellulose. In fact, hemicellulose can be almost

3

completely hydrolyzed with limited effect on cellulose.27 The investigation of RS

4

components has characterized the presence of NSCs in a significant amount (16.1

5

%), contributing 27 % of potentially fermentable carbohydrates in the RS. It is known

6

that NSCs are directly fermentable without strong pretreatment.28 Furthermore, the

7

content of monosaccharides in liquid hydrolysate after dilute acid pretreatment is

8

influenced by NSCs content in biomass.28-29 However, both some free sugars which

9

originally contain in NSCs and which are formed from the pretreatment are highly

10

susceptible to degrade during reaction to produce byproducts.28 The existence of

11

NSCs in some species of biomass is significant as identified by some studies as

12

shown at “Table S1. Variation in composition of carbohydrates of biomass, based on total

13

carbohydrates’’ at Supporting Information for Publication. Their content can reach more

14

than 30% for some biomass species derived from rice, sorghum, and sugarcane

15

plants. While for the popular herbaceous energy crops, the NSCs content is lower

16

but their HC content, especially reed canary grass and switchgrass are much higher

17

compared to the above biomass species. Given the significant amount of easily


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hydrolysable carbohydrates of NSCs and HC in many biomass species and their

2

behavior in the reaction, it may be important to consider in selection and processing

3

biomass feedstock. We hypothesized that the strategy to apply the first-step

4

saccharification under a mild condition in the presence of an appropriate solid acid

5

catalyst may be able to efficiently solubilize the NSCs and HC into monosaccharides

6

and suppress the inhibitor formation.

Table 2. The activity of solid-acid catalysts in the first-step saccharification of rice straw at 150 oC

for 30 min

Catalysts

Liquefaction Rate (%)

Yield (%), based on holocellulose content of feedstock charged Oligo-

C6-

C5-

saccharide

sugar

sugar

5-HMF

Furfural

Organic Acids

MC

66.7

39.9

20.8

7.7

1.2

0.3

2.3

AD35

58.8

6.3

38.7

14.8

8.4

1.2

5.9

ZS

51.2

35.6

15.9

0.6

0.6

0.1

2.0

10ZSBAS

47.7

47.2

17.6

1.2

0.8

0.1

1.5

5ZSiS

45.5

41.8

17.2

1.0

0.6

0.1

1.1

5ZSBAS

45.3

40.3

18.1

0.8

0.6

0.1

2.1

ASBA

28.8

34.8

20.0

0.6

0.8

0.0

2.1

7

3.2 The First-Step Saccharification of Rice Straw

8

3.2.1 Catalyst Selection for the 1st-Step Process. To obtain an appropriate catalyst

9

in the first-step saccharification of fresh RS, the catalyst activity of strong candidate


18

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1

of several solid acid catalysts was investigated in the reaction at 150 °C for 30 min

2

(Table 2). In this table, the catalysts were sorted according to order of catalyst

3

reactivity to solubilizing carbohydrates in the RS (called as liquefaction rate). The MC

4

catalyst was the most reactive, but the catalyst activity provided an unfavorable

5

product distribution with the highest yield of intermediate oligosaccharides (39.9 %),

6

relatively low yields of C6 monosaccharide (20.8 %) and C5 monosaccharide (7.7

7

%). The second highest catalyst reactivity was the AD35 catalyst. Interestingly,

8

compared to the MC catalyst, this catalyst activity remarkably enhanced the

9

production of C6- and C5-monosaccharide by nearly two-fold (yields of 38.7 % and

10

14.8 %, respectively), and remarkably suppressed the production of oligosaccharides

11

to a yield of 6.3 %. The other catalysts were observed to have relatively lower

12

reactivities compared to the two catalysts above and their activities followed the

13

propensity of MC catalyst activity to produce oligosaccharides as the dominant

14

product.

15

The effect of silica in the RS was examined in our previous work, in the

16

saccharification reaction of RS without catalysts at 180 oC for 1 h. 13 In this reaction,

17

both liquefaction rate and total monosaccharide yield were at very low levels (16%


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and 8%, respectively), indicating that silica contained in the feedstock did not have a

2

significant catalyst effect on the saccharification reaction.

3

To gain a better understanding of factors that affect the activity and the efficiency

4

of solid catalyst in the production of monosaccharides at a low temperature, textural

5

and acidity properties of the used catalysts were characterized (Table 3).

6

Furthermore, the catalyst properties were also correlated with the efficiency of the

7

catalysts to produce monosaccharides. Among the catalyst properties, acid properties

8

(i.e., acid amount and acid density) have a strong influence on the reaction efficiency (Figure

9

1). In general, the total monosaccharide yield increased significantly with increases in the acid

10

amount (Figure 1A) and the acid density (Figure 1B). The exception for the ZS catalyst, the

11

observed monosaccharides yield was the lowest whereas its acid density was significantly

12

high (7.1 6" 7"2) compared to the other zirconia and the ASBA catalysts (0.95 – 2.2

13

6" 7"2) (Figure 1B). This may be due to the lowest amount of acid loading between these

14

catalysts (Table 3). The type of acid site has been also shown to have an important role in

15

hydrolytic reaction activities and product selectivity.

16

(proton donor) promotes the efficient hydrolysis of lignocellulosic biomass and the

17

dehydration of monosaccharides 30-31, while Lewis acid site (electron-pair acceptor) can also

18

catalyze the hydrolysis but significant in the isomerization reaction of glucose.32 The catalyst

19

bearing Brønsted acid site are AD3533 and MC34, and both catalysts show the high

20

saccharification activity even at the low temperature, validating the significant role of

30-32

In general, Brønsted acid site


20

Page 21 of 59

1

Brønsted acid site. Furthermore, an increase in acid amount in the case of AD35 catalyst can

2

enhance significantly the efficiency of catalyst to produce much higher the monosaccharides

3

and much lower the oligosaccharides, and even promotes overreaction to generate the by-

4

products. As known, Brønsted acidity can also promote the direct conversion of glucose to 5-

5

HMF followed by the formation of organic acids such as levulinic acid and formic acid.35 On

6

the other hand, Zr-based (ZS, 10ZSBAS, 5ZSiS, and 5ZSBAS) and Al-containing SBA-15

7

(ASBA) catalysts contain Brønsted and Lewis acid sites.36-37 Although these catalysts have

8

significantly lower acid amount than the MC catalyst, in general, the catalyst activity for

9

producing the oligosaccharides and C6 monosaccharide are still comparable with the MC

10

catalyst (Table 2). It has been reported that the modification of surface of metal oxide

11

catalysts with sulfate group, such as ZrO2/SO42- catalyst, can strengthen the Brønsted acidity

12

of the catalyst and therefore enhancing the hydrolysis reaction.38 However, comparing the

13

yields of oligosaccharides and C6-monosaccharide obtained from the sulfated Zr-based with

14

non-sulfated ASBA catalysts (Tabel 2), the Brønsted acidity strengthening effect is not so

15

clear. Thus, in line with previous studies32,39, this should suggest that the Lewis acid site on

16

the Brønsted and Lewis acid catalyst can promote the hydrolysis of lignocellulose, with

17

respect to the low acid amount of the catalysts. Regarding the goal of first-step

18

saccharification, the solid acid of AD35 was determined as the appropriate catalyst for

19

saccharification of RS at mild condition.

21 22

60

(A) 50 40 30 20 10

Yield of Total Monosaccharides (%), based on holocellulose

20 Yield of Total Monosaccharides (%), based on holocellulose

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


60

(B) 50 40 30

20 ACS Paragon Plus Environment 10

21


Page 22 of 59

Table 3. Properties of prepared catalysts

Acid Site Pore Diameter Catalysts

Acid Amount

SBET (m2/g)

Density (nm)

(mmol/g) (µmol/m2)

AD35

50

30

5.3

106.0

MC

169

3.9

2.2

13.0

ASBA

916

4.6

1.4

1.5

10ZSBAS

535

4.6

1.2

2.2

5ZSiS

265

14

0.61

2.3

5ZSBAS

652

3.9

0.62

0.95


22

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ZS

56

17

0.4

7.1

1

2

3

4

5

6

7

8

3.2.2 Effect of Reaction Conditions for the 1st-step. In the subsequent work,

9

influence of reaction conditions (temperature and time) on the saccharification activity

10

of the selected AD35 catalyst was investigated.

11

Effect of Temperature. The effect of reaction temperature on the AD35 catalyzed

12

saccharification of fresh RS was investigated at a range of 120-150 °C for 30 min

13

(Figure 2(A)). The catalyst activity to solubilize the feedstock reached a significant rate

14

of 43.1 % at a temperature as low as 120 °C and increased steadily with rising


23


Page 24 of 59

1

temperature, to reach the rate of 58.8 % at 150 °C. There were three observable

2

phenomena in the catalyst activity to produce C6 monosaccharide: (1) a high C6 yield

3

of 25.9 % at the low temperature of 120 °C, which was consistent with the observed

4

liquefaction rate at this temperature; (2) a drastic increase in the yield to achieve a

5

maximum of 47.2 % with a temperature rise of only up to 130 °C; and (3) a decrease

6

of the yield to 38.7 % with rising temperature to 150 °C. On the other hand, the yield

7

of C5 monosaccharide was relatively low (4.7 %) at 120 °C, but it continuously

8

increased to 14.8 % at 150 °C. The catalyst activity for producing oligosaccharides

9

was observed at a high yield of 27.6 % at 120 °C, however the yield decreased

10

drastically along with the temperature rise, being only 6.3 % at 150 °C. The production

11

of byproducts of 5-HMF and organic acid was observed with a yield increase from

12

1.8% to 8.1 % and 2.6 to 5.9, respectively, with rising temperature from 120 °C to 150

13

°C. Whereas, furfural formation was observed at a significant yield of 1.2 % at a higher

14

temperature of 150 °C.

15

16


24

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Page 26 of 59

1

yield obtained at this temperature is equivalent to the conversion of NSCs of around

2

96 %. Considering the fast production rate of C6 monosaccharide even at the low

3

temperature, it appears that hydrolysis of starch contained in NSCs occurs directly

4

without going to an intermediate. The observed phenomenon (1) should also consider

5

the active role of catalyst. As it can be seen from Table 2, the saccharification activity

6

of AD35 catalyst remarkably enhances the formation of C6 monosaccharide compared

7

to the other catalysts. Due to the observed formation of C5 monosaccharide and

8

oligosaccharides (Figure 1), hydrolysis of a more rigid structure of HC may occur at

9

some degree to produce C6 monosaccharides. In fact, the only source of C5

10

monosaccharide in RS is xylan contained in HC (Table 1). Moreover, as it is known

11

that oligosaccharides are intermediate products of the two-step conversion

12

mechanism: first from glucan and xylan in a feedstock to oligosaccharides then to

13

respective C6 and C5 monosaccharide. In the phenomenon (2), the significant

14

increase of C6 monosaccharide production may indicate that there is additional

15

contribution from the hydrolysis of glucan contained in HC via the two-step conversion

16

mechanism, in addition to the dissolve of NSCs. This is supported by the observed

17

significant decrease in the oligosaccharide production with rising temperature to 130


26

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1

oC.

2

NSCs and glucan derived from HC, then the conversion of these carbohydrates

3

reaches higher than 100 % (i.e., 114 %). This, therefore, may show that the presence

4

of AD35 catalyst can effectively dissolve HC and hydrolyze glucan derived from HC to

5

C6 monosaccharide. Moreover, some cellulose may be hydrolyzed even at this low

6

temperature, by considering the observed high yield of oligosaccharides (17 %). With

7

a rise of temperature above 130 °C, the formation of C6 monosaccharide should still

8

be expected with respect to the significant decreases in the yield of oligosaccharides.

9

However, as it is observed in the phenomenon (3), it seems that an intensive

10

degradation of this product occurs, resulting in the significant decrease of its

11

production. A similar phenomenon is found by previous studies in the saccharification

12

of RS with solid catalysts, stating that monosaccharides formed in the reaction are

13

particularly susceptible to undergo over-hydrolysis to form byproducts13,17. The

14

increased production of byproducts with rising temperature (Figure 2(A)) could also

15

explain this phenomenon. In a different phenomenon with C6 monosaccharide, the

16

formation of C5 monosaccharide increases steadily with respect to temperature rise.

17

It seems that C5 monosaccharide is less susceptible to degradation compared to C6

If it is assumed that the yield level of C6 monosaccharide is obtained from both


27


Page 28 of 59

1

monosaccharide. Given the source of C5 monosaccharide is only from xylan contained

2

in HC, then its formation is primarily through the two-step conversion. For additional

3

information, the yield level obtained from 120 to 150 °C correspond to the conversion

4

level of xylan 24 %-82 %. With respect to the oligo saccharides formation, the yield

5

obtained at 120 °C can be expected primarily from hydrolysis of HC. Thus, the

6

presence of AD35 catalyst can effectively hydrolyze HC to oligosaccharides even at

7

this low temperature, and then with rising temperature, the rate of the second-step

8

hydrolysis

9

monosaccharides,

10

oligosaccharides.

of

the

formed

along

oligosaccharides

with

the

significant

increases

decrease

rapidly

of

to

the

produce

observed

11

Effect of Reaction Time. To investigate the effect of reaction time on the AD35

12

catalyst activity in the first-step saccharification of fresh RS, the reaction was

13

performed at 150 °C with variation in reaction time 5-30 min (Figure 2(B)). The

14

liquefaction rate of RS increased from 32.3 % to 46.0 % from 5 to 15 min, then slower

15

increased for the longer time to reach 58.8 % for 30 min. In general, the change pattern

16

in the yields of products, except the byproducts, followed its liquefaction rate. From 5


28

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1

to 15 min, the yield of oligosaccharides decreased from 13.4 % to 5.7 %, C6

2

monosaccharide also decreased from 35.8 % to 31.8 %, while C5 monosaccharide

3

increased slightly from 14.0 % to 16.4 %. The yields of the three products were almost

4

unchanged after 15 min. On the other hand, the formation of byproducts such as 5-

5

HMF, furfural, and organic acids increased slowly along with the time from 5 min to 30

6

min, i.e., from 2.8 % to 8.4 %, 0.5 % to 1.2 %, and 2.6 % to 5.9 %, respectively. To

7

determine the conversion rates of NSCs and HC, the residues obtained from the

8

reactions at 150 °C were analyzed using the NREL analysis procedure (Table 4). From

9

the total glucan content in RS (47.9 %), there was 28.9 % remaining in the residue

10

from the reaction for a short time of 5 min, equivalent to 39.7 % glucan conversion. By

11

prolonging the time, the glucan conversion increased slowly to reach 55.7 % for 25

12

min. Meanwhile, the xylan content in the residue for the short time 5 min decreased

13

drastically to 2.4 % compared to initial content in the RS (11.7 %), corresponding

14

conversion of 79 %. Furthermore, the xylan conversion reached 96 % and 100 % for

15

the times of 15 and 25 min, respectively.

16


29


Page 30 of 59

Table 4 Composition of glucan and xylan in residues after saccharification and its conversion Composition (%)

Samples

Glc

Xylan

47.9

11.7

1st step, 150oC, 5 min

28.9

1st

15 min

Conversion (%) Glc

Xylan

2.4

39.7

79.5

23.8

0.5

50.3

96.0

1st step, 150oC, 25 min

21.2

0.0

55.7

100

2nd

2.4

0.0

94.9

100

Fresh rice straw Residue from:

step,

step,

150oC, 220oC,

0 min

1

2

Corresponding to the results shown in Figure 2(B) and Table 4, including the result

3

shown in Figure 2(A), it can generally be stated that the saccharification activity in the

4

presence of AD35 catalyst at 150 °C occurs significantly in an initial reaction stage,

5

during heating up the reactor to the reaction temperature for 5 min. Afterward, the

6

saccharification activity decreases drastically by prolonging the reaction time, as it is

7

observed from much less changes in the yields of the products and the conversion of

8

glucan and xylan. Furthermore, from the observed results, it clearly indicates that the

9

easiness of carbohydrate components in the RS to be hydrolyzed in the presence of

10

solid acid catalyst can be ordered from the easiest: NSCs > HC >> cellulose. This

11

order is in accordance with general finding in studies based on homogeneous catalysts

12

of mineral acids. Based on this order, it can be analyzed glucan of which


30

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1

carbohydrates which is converted during the reaction. The glucan conversion obtained

2

for 5 min (i.e., 39.7 % based on total glucan content) is equivalent to 118% conversion

3

based on NSCs content, reflecting the conversion of all existing NSCs plus some

4

glucan contained in HC. In the same way, the conversion levels for 15 and 25 min

5

reflect that the whole of the NSCs and glucan in the HC can be hydrolyzed. Since

6

xylan contains only in HC, its conversion levels should reflect the hydrolysis level of

7

the HC and this is in accordance with the analysis above. Oligosaccharides formed at

8

150 °C should also be derived primarily from HC. To note, the observed yield at this

9

temperature for 5 min is only about half compared to the observed yield at the low

10

temperature of 120 oC for 30 min, and it decreases slowly until there is no change after

11

15 min, leveled at 6.3 %. This can be explained that some oligosaccharides may

12

immediately be hydrolyzed to monosaccharides at the initial reaction stage, some of

13

the rest are hydrolyzed at a much slower rate. This evidence can be explained in

14

accordance with a HC hydrolysis model introduced by Kobayashi and Sakai40 involving

15

two types of HC, fast- and slow-hydrolyzing HC, each with its own kinetic constant41.

16

Nowadays, most of kinetic studies of HC are based on this hydrolysis model41. In the

17

present study using RS, the fast-hydrolyzing HC appears to be present and proceed


31


Page 32 of 59

1

a fast two-step hydrolysis, to oligosaccharides and then monosaccharides. The

2

observed oligosaccharides at lower temperatures (Figure (2A)) may represent to be

3

derived from the fast-hydrolyzing HC. At the higher temperature of 150 °C, the second

4

step hydrolysis seem to be enhanced at much faster than the first step, so that

5

oligosaccharides formation initially is not recognized. There is also an evidence that

6

shows the presence of isomerization reversible reaction of glucose, called as glucose

7

reversion,

8

oligosaccharides and insoluble cellulose.42-43 This reversible reaction seems to

9

prohibit the complete conversion of oligosaccharides to monosaccharides, as can be

10

observed from unchanged yield of oligosaccharides for reaction time more than 15

11

min. In the meantime, the degradation and reversion of C6 monosaccharide may result

12

in the observed yields at this high temperature for the reaction time range to be

13

significantly lower than the observed maximum yield at 130 °C. Furthermore, along

14

with increases of the formations of byproducts, the production decreases with time. In

15

the production of C5 monosaccharide, the yield change pattern is compatible to the

16

obtained xylan conversions, where the conversion reaches a high level of 80 % for 5

17

min and almost entirely for 15 min.

when

the

system

consists

of

monosaccharides

and

soluble


32

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Page 34 of 59

1

second-step employed were also the same as those for the first-step. A solid residue

2

obtained from the first-step saccharification of rice straw at 150 oC for 15 min was used

3

as the feedstock. The residue composed 23.8 % glucan and 0.5 % xylan (Table 4).

4

3.3.1 Catalyst Selection for the 2nd-Step Process. Strong candidates of solid-acid

5

catalysts for the second-step saccharification of residue were tested (Table 5). The

6

reaction was carried out at 230 °C for 0 min. Here, 0 min was defined as the

7

temperature when the reactor reached 230 °C then the reactor was rapidly cooled

8

down to the room temperature (rapid-heating cooling condition: heating from room

9

temperature to 230 °C took 17 min and cooling back to the room temperature took 2.5

10

min). As a note, the efficient AD35 catalyst in the first-step was not included the test

11

because this catalyst has already known to be insufficiently active in a high

12

temperature saccharification.47 The reactivity of the catalysts in liquefying

13

carbohydrates in the residue was in this following order: MC > 10 ZSBAS > ZS. The

14

catalyst activity of MC provided the highest yields in producing C6 monosaccharide

15

(29.9 %) and oligosaccharides (8.8 %). Unfortunately, this catalyst was intense in the

16

production of byproduct of organic acids (20.7 %). The second reactive catalyst,


34

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1

10ZSBAS, provided slightly lower yields of C6 monosaccharide (27.8 %) and

2

oligosaccharides (5.3 %), and a much lower yield of organic acids (8.2 %), compared

3

to the MC catalyst. Disadvantageous of using the 10ZSBAS catalyst was intense in

4

the production of 5-HMF. The third reactive, ZS catalyst, although providing the lowest

5

C6 monosaccharide and oligosaccharides yield among the catalysts, had the best

6

activity in suppressing the production of by-products.

7

Each of the three catalysts tested has advantages and disadvantages in its

8

saccharification activity. The acid properties of catalyst seem also to influence the

9

production of monosaccharides, but the influences are as strong as in the first-step

10

process (Figure 1). The presence of Brønsted acidity only in the MC catalyst, on the

11

one hand promotes the solubilization of carbohydrates in the feedstock, but on the

12

other hand also significantly increases the by-products, especially organic acids.

13

Lewis acidity in the Brønsted and Lewis acid catalysts (10ZSBAS and ZS) in the

14

reaction at the higher temperature may also promote the hydrolysis reaction, with

15

respect to the low acid amount in the catalysts. Furthermore, a reduce in the acid

16

amount of catalyst gains benefit form a decrease in the formation of by-products but


35


Page 36 of 59

1

must be paid to the decreased production of C6 monosaccharide. Regarding the

2

features of each catalysts described above, we have selected the MC catalyst to

3

catalyze the second-step saccharification.

Table 5. The activity of solid acid catalyst candidates in the 2nd-step saccharification of residue at 230 oC

for 0 min

Catalysts

Liquefaction Rate (%)

Yield (%), based on holocellulose content of feedstock charged OligoOrganic C6-sugar C5-sugar 5-HMF Furfural saccharide Acids

MC

56.7

8.8

29.9

0.5

8.2

6.6

20.7

10ZSBAS

48.3

5.3

27.8

1.0

11.1

5.5

8.2

ZS

37.6

5.3

23.7

2.6

4.6

3.4

8.1

4

5

3.3.2 Effect of Reaction Conditions for the 2nd-Step

6

Effect of Reaction Temperature. In the investigation of the effect of reaction

7

temperature in the saccharification activity of residue in the presence of MC catalyst,

8

the reaction was carried out at a temperature range of 210 – 230 oC for 0 min (Figure

9

3(A)). The catalyst activity for liquefying the residue increased continuously from 38.6

10

% at 210 oC to 56.7 % at 230 oC. A similar trend to the first-step saccharification, the

11

yield of C6 monosaccharide increased rapidly from 37.8 % at 210 oC to reach a

12

maximum of 56.7 % at 220 oC, and then decreased more rapidly until reach 29.8 % at


36

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1

230 oC. The C5 monosaccharide had relatively low yield of 2.1 % at 210 oC and

2

decreased to undetectable concentration at 230 oC. For oligosaccharides, the yield

3

almost did not change within the temperature range, which was about 8.9 – 8.2 %. In

4

general, the formation of byproducts was observed in relativly small quantities at

5

210 °C and increased significantly with rising temperature. Yields of 5-HMF, furfural,

6

and organic acids at 230 oC were 8.2 %, 6.7 %, and 20.7 %, respectively.

7

The above result also demonstrates the saccharification activity of active solid-acid

8

MC catalyst in promoting the hydrolysis of the remaining carbohydrates in the residue

9

to produce C6 monosaccharide at lower temperatures (210-220 °C) and degradation

10

of the product leading to a drastic reduction in its production at a higher temperature.

11

The remaining carbohydrates are primarily cellulose (Table 4). As it is known, the

12

cellulose molecules have a strong tendency to form intramolecular and intermolecular

13

hydrogen bonds which can lead to a variety of ordered crystalline arrangements.48

14

Therefore, there are two regions in cellulose, one region is bound laterally hydrogen

15

bonds arranged highly ordered to form a crystalline structure, and one region is less

16

ordered which is called amorphous cellulose.49 It has been shown that the amorphous


37


Page 38 of 59

1

portion is more susceptible to be hydrolyzed while the crystal portion is more resistant

2

to hydrolysis.50-51 From this knowledge, it may be considered that the presence of the

3

amorphous portion is one of factors affecting an enhanced production of C6

4

monosaccharide at 210 °C, in addition to the active role of MC catalyst. It is also

5

desirable that degree of crystallinity and degree of polymerization in the residue

6

decrease by the previous process of the first-step saccharification. Based on the NREL

7

analysis of the residue at 220 °C (Table 4), the glucan conversion reaches 95 %. From

8

the amount of converted glucan, however, only about half of it can be hydrolyzed to

9

C6 monosaccharide. An intensive degradation of this product seems to occur at this

10

temperature and becoming more intensive with a higher temperature, corresponding

11

to the observed significant formation of byproducts, especially organic acids. This

12

enhanced byproduct production is also a specific activity of the MC catalyst (Table 5).

13

The increase of the production of C6 monosaccharide from 210 °C to 220 °C is not

14

followed by a change in the obtained oligosaccharide. Again, this may show that a

15

much faster hydrolysis of oligosaccharide formed compared to its formation results in

16

the formation is not recognized, in accordance with the finding by Abatzoglov et al.52

17

In addition, there are other evidences that prohibit the hydrolysis of oligosaccharides


38

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1

derived from cellulose to C6 monosaccharide, although at the temperature as high as

2

230 °C as observed. The evidence of glucose - oligosaccharides reversible reaction

3

has been reported, and this reaction is much faster than irreversible glucose

4

decomposition and equilibrium is achieved at early phase of the reaction.43 Another

5

important fact is both the chemical modification of solid cellulose and the formation of

6

non-reactive oligosaccharides occur at high temperatures, thus those may lower the

7

C6 monosaccharide production and prohibit the conversion of oligosaccharides to the

8

monosaccharide.53-54 This evidence leads to addition of parasitic pathway that

9

compete with hydrolysis catalyzed by acids.41 Meanwhile, due to the xylan content is

10

very small in the raw material, the observed C5-monosaccharide production is also

11

small. For a note, the yield level at 210 °C (2.1 %) is equivalent to the whole existing

12

xylan in the residue is hydrolyzed to the monosaccharide. Furthermore, the

13

degradation of this monosaccharide occurs at a higher temperature until its production

14

is undetectable at 230 °C.

15

Effect of Reaction Time. In the inspection of effect of reaction time, the reaction was

16

carried out at 220 oC with a time variation of 0-10 min, in the presence of MC catalyst.


39



Page 40 of 59

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Page 42 of 59

1

step saccharification were observed by using SEM (Figure 4). Structures of stomata

2

and epidermal cells of the fresh RS were clearly observed as shown at spot (1) and

3

(2) (Figure 4(a)). By contrast, after the first-step saccharification (Figure 4(b)), the

4

surface of RS became destroyed, and the microfibrils of cellulose were exposed as

5

shown at spot (3). Probably in this step, the amorphous portion of NSCs and HC of

6

RS was hydrolyzed, and the integrated lignin was partially broken so that it was

7

exposed and accessible for further reaction. After the second-step saccharification

8

(Figure 4(c)), the fibers of cellulose were hardly observed as shown at spot (4).

9

Additionally, there were many agglomerates and disordered structures, which were

10

probably lignin and ash.

11

The XRD patterns of the RS before, after the first-step, and after the second-step

12

saccharification are shown in Figure 5. The intensities of peaks were corresponding

13

to (101), > -Z? and (002) lattice planes. Regarding the RS before saccharification (a)

14

and after the first-step (b), those three peaks attributed into crystal cellulose can still

15

be observed. This suggested that during the first- step saccharification, the crystalline

16

structure of RS was affected but in a limited way. However, after the second-step


42

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Page 44 of 59

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


1

the first-step saccharification of dried feedstock at 150 °C for 15 min in the presence

2

of AD35 catalyst and the second-step saccharification of residue obtained from the

3

first-step, at 220 °C for 0 min in the presence of MC catalyst. Based on weight of the

4

feedstock fed, recoverable products from the first-step reactor consisted of residue 60

5

%, oligosaccharides 3.4 %, monosaccharides 28.8 % (C6 monosaccharide 19 %, C5

6

monosaccharide 9.8 %), and byproducts 4.5 %. The residue obtained was then

7

reacted in the second-step reactor. The entire product recovered by the sequential

8

two-steps was residue 35 %, oligosaccharides 4.7 %, monosaccharides 36.4 % (C6

9

monosaccharide, 26.6 %; C5 monosaccharide, 9.9 %), and byproducts 7.5 %. There

10

was a difference in the material balance of 15.7 % which may be due to extractive

11

organics and/or lignin dissolving. Furthermore, carbohydrates fraction in the residue

12

left only 2 % glucan, which meant that more than 95 % carbohydrates in the feedstock

13

were converted.

14

The production of total C6 and C5 monosaccharides achieves a yield of around 65

15

%, based on the holocellulose content in the feedstock, which increases significantly

16

compared to one-step process (with a yield of 43.4 % on the same base) using the


45


Page 46 of 59

1

same feedstock, as reported in the previous work of our group. Referring to the

2

investigation on the effect of the reaction conditions on the catalyst activity for each

3

saccharification step (Figures 2 and 3), higher monosaccharide yields for the two steps

4

should still be expected by exploring more intensive reaction conditions. Thus, this

5

process can be expected as a method to increase cost-effectiveness of the bioethanol

6

production system in an environmentally sound way.

7

Based on the results of NREL analysis, SEM, and XRD, a hypothetical mechanism

8

of two-steps saccharification process which involves the presence of solid acid catalyst

9

is shown in Scheme 4. For feedstocks, such as rice straw, it should consider that there

10

are four major components, i.e., nonstructural carbohydrates, hemicellulose, cellulose

11

and lignin. Cellulose microfibrils are rigid structure and are protected by lignin and

12

hemicellulose as shown in (i). Then, in the first-step saccharification, RS is attacked

13

by the solid acid catalyst, as a result, the amorphous structure of the RS including

14

hemicellulose, starch and free sugar is starting to be hydrolyzed (ii). Hydrolysis

15

process continues to occur, then at the end of first-step process, the surface of RS is

16

destroyed, the amorphous structure of RS is hydrolyzed and microfibrils of cellulose


46

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Page 48 of 59

1

11.4% at second-recycle, respectively. The reusability of MC catalyst has been published in

2

our previous publication22 using cellulose as substrate at 180 oC for 3 h. The liquefaction rate

3

of MC catalyst decreased from 43% for fresh catalyst to 30% for first recycle and then to 23%

4

and 18%, respectively for second and third recycle. For fresh catalyst, the yield of C6

5

monosaccharide decreased from 8.1% to 5.4% for first recycle and continue to decrease to

6

1.62% at third and fourth recycle. The decreasing of catalyst activity during recycle test may

7

be caused by these reasons: leaching of SO42- functional groups, poisoning of the catalyst active

8

sites or structure of catalysts were collapsed.

9

4. Conclusions

10

The results represented in this study show that the developed sequential

11

saccharification in two-steps with a solid acid catalyst is effective for the hydrolysis of

12

carbohydrates fraction in RS to monosaccharides. In fact, many biomass species

13

contain a significant amount of NSCs. They, including HC, are easily hydrolyzed but

14

also susceptible to undergo overreaction generating inhibitor compounds to the

15

fermentation process. In the first-step saccharification at mild conditions, the presence

16

of an appropriate catalyst of the AD35 catalyst can effectively hydrolyze these

17

components, whereby high yields of C6 monosaccharide (47.2 %) and C5

18

monosaccharide (10.8 %) were obtained at temperature as low as 130 °C for 30 min.

19

The objective of maximizing monosaccharide production by further hydrolyzing the


48

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1

rest of HC in the RS and the oligosaccharides formed, at higher temperatures, was

2

hampered by overreaction of some C6 monosaccharide which was initially formed to

3

byproducts. The second-step saccharification of remaining cellulose in the residue

4

using the most active catalyst of MC catalyst provided a maximum yield of C6

5

monosaccharide (52.5 %) at 220 °C for 0 min. The most studied modified pathways of

6

HC and cellulose catalyzed by dilute-acid were also observed to occur with a solid-

7

acid catalyst. An additional pathway founded in this study is a direct hydrolysis of NSCs

8

to C6 monosaccharides. Some of the reactions involved in the mechanism have

9

prohibited a full conversion of cellulose and oligosaccharides to monosaccharides.

10

Observed phenomena were supported by the characterization of the RS, before and

11

after the reactions, using SEM, XRD, and the NREL analysis procedure. A test in the

12

sequential two-steps saccharification of RS under specific process conditions gave a

13

significant increase in the yield of total monosaccharides (65 %) compared with one-

14

step process (43.4 %). Therefore, the proposed process can represent a method to

15

increase the cost-effectiveness of bioethanol production system in an environmentally

16

sound way.


49


1

[ Author information

2

Corresponding Author

3

* E-mail: [email protected].

4

Notes

5

The authors declare no competing financial interest.

6

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Two-Step Saccharification of Rice Straw Using Solid Acid Catalysts

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