Abstract
5-hydroxymethyl furfural (5-HMF) and 2,5-furandicarboxylic acid (FDCA) are interesting
platform molecules that may be employed as intermediates in the manufacturing of biofuel
and biodegradable plastic. This study explored the direct conversion of biomass to 5-HMF
without chemical pretreatment and the oxidation of 5-HMF to FDCA without 5-HMF
extraction. Initially, biomass was converted into 5-HMF in an oil bath at 150 oC for 120
minutes using DMSO/H2O as a green solvent with SiO2/Al2O3 and [Bmin]HSO4 as cocatalysts. After the reaction was complete, the catalysts were removed from the 5-HMF
solvent for the oxidation of 5-HMF to FDCA. Under steady stirring, the oxidation process
was done at 70 oC for 12 hours in the presence of Cu-Mn2O4 as a catalyst, acetonitrile as a
mixed solvent, and tert-butyl hydroperoxide (tBuOOH) as the oxidant. The yields of 5-HMF
at 83.68% and FDCA at 87.37%, based on the quantity of 5-HMF synthesized, were
determined from jackfruit peel. More importantly, the Cu-Mn2O4 catalyst may be reused
repeatedly without losing its catalytic efficiency; it has been shown to be recyclable seven
times and beyond. Due to the non-noble metals and the absence of a base solvent, the
procedure was more cost-effective and ecologically friendly.
KEYWORDS: Agricultural wastes, 5-Hydroxymethylfurfural, 2, 5-Furandicarboxylic acid,
Sustainable chemistry.
1. Introduction
Agriculture, the principal industry in almost any country, has a huge effect on the overall
development of that nation (Viana et al., 2022). However, agricultural manufacturing
processes create vast amounts of residue every year (Shinde et al., 2022). Agricultural wastes
can be generated from different sources, including crop residues, agro-industries, and
livestock. The main components of crop residues and agro-industries are cellulose, lignin,
and hemicellulose, all of which are considered regarded as major natural-value carbon
sources (Pires et al., 2019). Improper management of them can lead to environmental
pollution and organic source prodigality (Shinde et al., 2022). In contrast, when the residues
are appropriately exploited and used, they not only bring effective economies but also
contribute to environmental protection and reduce greenhouse gas emissions (Smith et al.,
2008). Therefore, finding the optimal ways to utilize these sources is of great interest to both
the local authorities and contemporary scientists.
The recent fossil fuel crisis in the whole world has raised concern about energy security and
sparked the development of alternatives for these depleting resources (Al-Shetwi, 2022).
Chemical industries are turning their attention to more environmentally friendly processes,
such as converting plentiful biomass, particularly agricultural residues, into high-value
platform chemicals (Shinde et al., 2022). This biorefinery has the potential to address two
major difficult issues: agricultural residue pollution and reliance on fossil fuels. Furthermore,
the biorefinery also represents a unique feasible solution for humanity as a whole to achieve
net-zero carbon dioxide emissions (Wang et al., 2021) Among many chemicals, 5hydroxymethylfurfural and 2,5-furandicarboxylic acid, have received significant attention
due to their ability to replace petroleum-based chemicals in the manufacturing process of
plastic products (Chang et al., 2022). 5-HMF and FDCA were named to the important
building-block bio-chemical list by the Department of Energy in the United States (de Jong
et al., 2022).
Because it can be used as a starting point for the production of a wide variety of interesting
compounds, 5-Hydroxymethylfurfural (5-HMF) is regarded as an extremely potential
foundational substance (Pińkowska et al., 2020). 5-HMF can be converted into a variety of
useful chemicals using a variety of different methods. One of the methods includes either
introducing oxygen to the particle or removing hydrogen from it, both of which result in
oxidized
products,
such
as
2,5-diformylfuran
(DFF)
and
5-hydroxymethyl-2-
furancarboxylic acid (HMFCA) (Wei et al., 2019), 5-formyl-2-furancarboxylic acid (FFCA)
(Xu et al., 2019), 2,5-furandicarboxylic acid (FDCA) (Xia et al., 2018). Another approach
involves adding hydrogen to the molecule of 5-HMF, which ultimately results in the
production of hydrogenated products, such as 2,5-dimethylfuran (DMF) (Hoang et al., 2022),
2,5-dimethyltetrahydrofuran (DMTHF) (Chen et al., 2020). A variety of homogeneous and
heterogeneous catalysts have been explored to aid the tandem conversion of biomass-derived
feedstock. These catalysts encompass Bronsted acids, for example, HCl (Li et al., 2017),
H2SO4 (Sailer-Kronlachner et al., 2021), and Amberlyst (Qi et al., 2009), which stimulate
hydrolysis and dehydration reactions. Additionally, Lewis acids such as AlCl 3 (Guo et al.,
2020), CrCl3 (Choudhary et al., 2020), and Sn-beta (Saenluang et al., 2020) have been
evaluated for their ability to promote isomerization. Numerous solvents have been
investigated to aid in the conversion process, including water, polar aprotic solvents such as
dimethyl sulfoxide (Jia et al., 2014), tetrahydrofuran (Yu et al., 2017), and methyl isobutyl
ketone (Esmaeili et al., 2016), ionic liquids (Zunita et al., 2022), and DESs (Amesho et al.,
2022). These solvents have their own unique properties that can be utilized to facilitate the
conversion process. For instance, they can alter the polarity of the reaction mixture, control
the reaction rate, stabilize the catalysts, solubilize the reactants and products, and prevent
undesirable side reactions.
5-HMF is the precursor to FDCA, and FDCA has garnered considerable attention in recent
years as a result of its structural similarity to terephthalic acid (TPA), a chemical made from
fossils (Hameed et al., 2020). TPA is often used as a monomer to make polyethylene
terephthalate (PET), a popular chemical that is used in packaging manufacturing (Benyathiar
et al., 2022). The switch from FDCA to TPA can effectively reduce dependence on fossil
fuels and mitigate environmental concerns (Hameed et al., 2020).
There are now three primary approaches to FDCA synthesis that have been reported in the
published research. The earliest and most well approach is 5-HMF oxidation. The second
approach includes converting various furanic compounds, such as 5-acetoxymethylfurfural
(5-AMF) or furoic acid (FA). The third way is based on carbohydrate dehydration, such as
fructose or glucose dehydration, with 5-HMF as an important intermediate. The oxidation of
5-HMF to produce FDCA requires multiple reaction steps, and the efficiency of the process
is determined by the catalyst chemicals utilized and the transformed methods (Xia et al.,
2018). The conversion of 5-HMF to FDCA typically involves a multi-step process with
various intermediates and pathways. The overall process can be broken down into three main
steps. In the first step, either Route A or Route B can be used. Route A involves the oxidation
of the aldehyde functional group of 5-HMF, resulting in the formation of 5-hydroxymethyl2-furancorboxylic acid (HFCA). On the other hand, Route B involves the oxidation of the
alcoholic group of 5-HMF to form 2,5-diformylfuran (DFF) as an intermediate. The second
step of the process involves the oxidation of the intermediates produced in Step 1 to generate
5-formyl-2-furancarboxylic acid (FFCA). Finally, in the third step, FDCA is synthesized by
oxidizing FFCA (Cong et al., 2021).
Fig 1: The route of the 5-HMF oxidation process to FDCA
Undergoing heating in an alkaline aqueous solution of pH 13 or higher, and temperatures
between 30 and 130 °C was the traditional method employed to transform 5-HMF into
FDCA. For this procedure, noble metals like platinum (Pt), palladium (Pd), ruthenium (Ru),
and gold (Au) were employed as catalysts in conjunction with high-pressure air or oxygen
(O2) to accomplish the process (Antonietti and Oschatz, 2018, Yi et al., 2016, Wu et al., 2014,
You et al., 2016, Siyo et al., 2014, Park et al., 2020). The use of noble catalysts can
significantly augment the efficacy and selectivity of the reaction, consequently resulting in
elevated FDCA yields. Despite its advantages, this approach is hindered by its expensive
catalysts and energy-demanding technical requirements, such as temperature and pressure
levels that are well above average. In recent years, researchers have been investigating
different means of changing 5-HMF to FDCA, such as through the use of organic catalysts
or non-noble metal catalysts in neutral solvents. The utilization of these novel approaches
could potentially bring about benefits, including increased environmental sustainability and
reduced expenses. Despite this, traditional techniques continue to be a viable option for the
manufacture of high-quality FDCA.
In fact, it is impractical producing FDCA from pure 5-HMF due to the expense of this
starting material (Triebl et al., 2013). Potential alternatives to 5-HMF include carbohydrates
like fructose and glucose due to their lower costs and greater availability. (Wang et al., 2015,
Rao et al., 2021). The first step in converting carbohydrates to FDCA is dehydration to 5HMF, followed by oxidation to FDCA. It is crucial for this approach to extract and purify 5HMF from the reaction media. However, this task is really challenging (Chen et al., 2018).
Numerous attempts were being made to find a way to turn carbohydrates into FDCA without
collecting 5-HMF from the reaction medium (Yi et al., 2015, Yan et al., 2018, Chen et al.,
2018). The direct conversion of sugars to FDCA has several benefits, including reduced CO2
emissions, reduced chemical consumption, and reduced manufacturing costs (Deshan et al.,
2020).
Directly converting carbohydrates to FDCA in a cost-effective and energy-efficient way is a
challenging yet appealing strategy for increasing the competitiveness of this bio-based
chemical in the market. While some breakthroughs have been made in FDCA production
using fructose, these methods still face challenges due to the high cost of materials and long
reaction times. For instance, one-pot conversion of fructose into FDCA has been successfully
demonstrated by Martin Kröger et al. and Marcelo L. Ribeiro, achieving yields of up to 72%
and 59.8%, respectively, using cheap metal catalysts with a reaction time of up to 15 hours
(Martin Kr¨oger et al., 2000, Ribeiro and Schuchardt, 2003). Another approach by Chen et
al. involved a two-step process to synthesize FDCA directly from fructose with a 91% yield
using noble-metal catalysts, and a reaction time to reach an optimal yield of only 10 hours
(Chen et al., 2018). In order to surmount the difficulties and make the process more
economically viable and environmentally friendly, additional research and development are
still required.
A practical method for FDCA synthesis was introduced in this study by converting
agricultural waste with inexpensive catalysts and solvents. The first step was to convert
biomass into 5-HMF in an oil bath at 150 oC for 120 minutes with DMSO as a green solvent
and co-catalyst of Al2O3-SiO2 and [Bmin]HSO4. Following the completion of the reaction,
the catalysts were removed from the 5-HMF solvent mixture for the oxidation of 5-HMF to
FDCA. The oxidation reaction was carried out at 70 oC for 12 hours with constant stirring in
the presence of Cu-Mn2O4 as a catalyst, acetonitrile as a mixed solvent, and tert-butyl
hydroperoxide (tBuOOH) as an oxidant.
2.1. Materials:
The biomass was collected, cleaned, and air-dried for 24 hours at 60 °C in an oven.
Consequently, the samples were milled into a uniform particle size under 40 mesh (0.42
mm) and kept in airtight containers at 25 °C.
Unless otherwise specified, all chemicals utilized in this study were commercially available
and were not purified further. Manganese(II) sulfate monohydrate (MnSO4.H2O, 99.5%),
Copper nitrate trihydrate (Cu(NO3)2.3H2O, 99.0%), Nitric acid (HNO3 65.0%–68.0%), Tertbutyl hydroperoxide (tBuOOH, 65.0%), Acetic acid (CH₃COOH, ≥99.5%), Tert-butanol
(C₄H₁₀O, 98.0%), Ethylene Glycol (CH3OH, 98%) were purchased from J.T.Baker
Chemical Company (Allentown, U.S), Sulfuric acid (H2SO4, ≥99.5%) and Acetonitrile
(C₂H₃N, ≥99.9%) were obtained from Honeywell company (Charlotte, North Carolina, US),
5-Hydroxymethylfurfural (C6H6O3, 97%), 2,5-furandiformaldehyde (C6H4O3, 98%), 5Formyl-2-furancarboxylic Acid (C6H4O4, 98%), 5-Hydroxymethyl-2-furancarboxylic acid
(C6H6O4, 98%), 2,5-Furandicarboxylic acid (C6H4O5, 99%) were purchased from Alfa
Aesar (Massachusetts, US). Dimethyl sulfoxide (C2H6OS, 99%), Sodium met silicate
(Na2SiO3, 99%), Aluminum sulfate (Al2(SO4)3, 99%) and Methanol (CH3OH, 99,9%) were
purchased from Thermos Scientific (Belgium, China), Sodium hydroxide (NaOH, 99%)
was obtained from Nippon Shinyaku Co., Ltd (Kyoto, Japan).
2.3. Preparation of catalysts:
2.3.1. The catalyst for converting biomass to 5-HMF was adapted from Xiaohan Wang's
methodology (Wang et al., 2022):
To synthesize SiO2/Al2O3, two mixtures were prepared. Mixture A consisted of Na2SiO3,
NaOH, and DI water, which had a combined molar ratio of approximately 1:1.22:6.77 and
was stirred overnight at room temperature. For Mixture B, AlCl3.6H2O was dissolved in DI
water and HCl with molar ratio of approximately 1:8.39:15.5, and the solution was stirred
for 15 minutes. Then, Mixture B was slowly added to Mixture A while stirring, and the
reaction mixture was stirred for an additional 2 hours at room temperature. Next, the mixture
was transferred to a high-pressure autoclave and subjected to crystallization at 120 °C for 6
hours. Once the crystals had formed, they were washed, dried, and calcined in air at 550 °C
for 5 hours to remove the templates.
2.3.2. The method for modifying the catalyst used in the 5-HMF oxidation to FDCA was
based on Yun Lang's original method (Lang et al., 2020):
The Cu-Mn2O4 catalysts were prepared by the following method: Cu(NO3)2.3H2O,
MnSO4.H2O, and citric acid powder were mixed in a molar ratio of 1:2:3. Then methanol
and polyethylene glycol were added to the mixture in a volume ratio of 1:2.6. The resulting
solution was stirred at 25 °C for 8 hours until the color of the mixture changed from blue to
dark green. The solution was then transferred to a 100 ml ceramic cup for final calcination.
The calcination step was performed in a high-temperature furnace, first at 200 °C for 1 hour,
followed by raising the temperature to 400 °C for 2 hours and finally to 700 °C for 7 hours.
2.4. The conversion process
The conversion of biomass into FDCA involves a two-step process. Firstly, a mixture
containing 0.2 grams of biomass, 0.121 milliliters of [Bmin]HSO4, 0.121 grams of
SiO2/Al2O3, 7.5 milliliters of dimethyl sulfoxide (DMSO), and 2.5 milliliters of deionized
(DI) water is heated at 150°C for 2 hours in a 25 mL glass bottle placed in a preheated oil
bath. The catalysts and biomass residues are then separated by filtration to obtain a solution
of 5-HMF.
In the second step, the resulting 5-HMF solution (2.5 ml), which contains 0.0126 grams of
5-HMF, is mixed with a specific amount of Cu-Mn2O4 catalysts and tBuOOH oxidant along
with 2.5 milliliters of different mixed solvents in a 20 mL glass flask. The mixture is stirred
and heated at 70°C in an oil bath with a reflux system. After the reaction, the catalyst is
separated by filtration to obtain the final product.
2.5. Product Determination
The products obtained, including 5-HMF and FDCA, were determined using highperformance liquid chromatography (HPLC-Hitachi-CM5000-Japan) with specific
conditions. The 5-HMF was analyzed using an HPLC system with a Myghtysil RP-18 GP 5
µm 250 x 4.6mm column, UV detector set at a wavelength of 284 nm, and a mobile phase
of pure water/acetonitrile (80:20) flowing at a rate of 0.6 mL/min. The FDCA and its byproducts were analyzed using an HPLC system with a Diamonsil C18 (2) 3 µm 150 x 2.1mm
column, UV detector set at a wavelength of 254 nm, and a mobile phase of pure
water/acetonitrile (95:5) flowing at a rate of 0.6 mL/min. The molar conversion of 5-HMF
and the yield of 5-HMF, FDCA, and its by-products were calculated based on the respective
molar ratios:
The yield of 5-HMF (mol%)：
5-HMF yield =
CHMF * V* 162.14*10-6
x100%
cellulose of biomass(%)
126.11*m*
100
(1)
The yield of furfural (mol%)：
Furfural yield =
CFurfural * V* 150.13 *10−6
x100%
hemicellulose of biomass(%)
96.08 * m *
100
(2)
Conversion of 5-HMF (mol%)：
5-HMF conversion = (1-
moles of unreacted 5-HMF
moles of loading 5-HMF
The yield of FDCA and its by-products (mol%)：
FDCA or by-products yield =
) x100%
moles of produced FDCA/by-products
moles of loading 5-HMF
(3)
x100%
(4)
2.5. Statistical analysis
In this work, the experimental data were analyzed statistically by using the mean and
standard deviation provided by Design-Expert 13 software. One-way analysis of variance
was used to assess the statistical significance of differences between the groups in the
samples (ANOVA). The cutoff for determining statistical significance for differences in
means was established at 5%.
3. Results and discussion
3.1. The process of biomass conversion to 5-HMF:
In the process of converting biomass to FDCA, where 5-HMF is oxidized directly without
separation, attaining a high 5-HMF yield while producing as few by-products as possible
during the biomass conversion stage is crucial. Because a high concentration of by-products
in the 5-HMF solvent, particularly humins, will always deactivate the oxidation catalyst in
the next step (Chen et al., 2018). Therefore, a method that can promote 5-HMF synthesis
and prevent 5-HMF from breaking down as well as minimum by-product generation was
chosen. Dimethyl sulfoxide (DMSO) discovered to be an adequate and stable solvent (Yu et
al., 2017) was employed together with a co-catalyst of SiO2/Al2O3 activated as a Lewis acid
catalyst and [Bmin]HSO4 activated as a Bronsted acid catalyst. Typically, the production of
5-HMF from biomass usually involves hydrolyzing the biomass to glucose, isomerizing it to
fructose, and catalyzing its dehydration to 5-HMF through Bronsted acid, which are complex
and challenging processes (Amesho et al., 2022). From an eco-sustainable and financial
standpoint, it could be required to discover an effective method of converting biomass
directly to 5-HMF, which would integrate all phases into one process and prevent the
consumption of additional chemicals and energy (Gajula et al., 2017). Ionic liquid
[Bmin]HSO4 is known as a green multifunction chemical used as a catalyst or solvent for
the conversion of biomass and carbohydrates into 5-HMF (Marullo and D'Anna, 2022). In
this study, Ionic liquid [Bmin]HSO4 was used as a catalyst and its role is not only in
enhancing hydrolysis of cellulose and hemicelluloses, which is one of the essential steps for
increasing the SiO2/Al2O3 catalytic reaction's effectiveness in the isomerization reaction but
is also important play in the 5-HMF formation from fructose in the downstream process.
Four kinds of agricultural residues, including jackfruit peel, sugarcane bagasse, rice straw
and pineapple stem, were chosen as substrates for 5-HMF synthesis because of their
abundance. The reaction temperature was 150 °C, and the reaction time was 120 minutes.
The yield of the products was recorded and assessed.
As can be observed from Fig. 2, the platform chemicals obtained from the process are 5HMF and furfural, and their yields have significantly different among the four kinds of
biomass. The jackfruit peel produced the greatest yield of 5-HMF at 71.89%, followed by
the pineapple stem at 46.06%, sugarcane bagasse at 19.79%, and rice straw at 5.17%. In
contrast, the furfural yields were as follows: pineapple stem > sugarcane bagasse > rice straw
> jackfruit peel. The difference in the yields can be attributed to the difference in the
chemical structures of the biomass.
Fig 2: The results of 5-HMF and furfural from four kinds of biomass (Reaction condition:
0.2 g biomass, 2.5 ml H2O, 7.5 ml DMSO, 0.121 g SiO2/Al2O3, 0.121 ml [Bmin]HSO4, 2
hours, and 150 °C)
In Table 1, jackfruit peel has much less hemicellulose, which contains xyloses and arabinoses
as its primary components (Ismiyarto et al., 2017), than sugarcane bagasse, rice straw, and
pineapple when compared to the other agricultural biomass. Therefore, the yield of furfural
that could be produced from jackfruit peel was negligible. The dehydration of cellulose and
hemicellulose occurs concurrently in the process. In cases where the concentration of
hemicellulose in the biomass is too modest, the catalysts exclusively engage in the
dehydration of cellulose to 5-HMF. This could explain why the jackfruit peel and pineapple
stem reaction yields more 5-HMF than sugarcane bagasse and rice straw.
Table 1: The components of different types of biomass
Cellulose
Hemicellulose
Lignin
(%)
(%)
(%)
Rice straw
24.0 ± 0.6
27.8 ± 0.2
12.4 ± 0.2
Jack fruit peel
31.59 ± 1.31
2.46 ± 0.48
26.57 ± 0.09
Pineapple stem
47.07 ± 1.61
14.35 ± 1.22
13.90 ± 4.6
Sugar bagasse
35.2 ± 0.9
24.5 ± 0.6
22.2 ± 0.1
Biomass
According to the findings of the aforementioned experiment, jackfruit peel was in producing
the highest yield of 5-HMF and it can satisfy the requirements of the least amount of byproduct production. As a consequence, the substrate for the next studies was decided to be
jackfruit peel.
The present study employed the response surface method (RSM) using a Box-Behnken
design. It utilized four variables, namely reaction time, reaction temperature, Levis acid
catalyst mass, and Bronsted acid catalyst dosage. In total, 29 experiments were conducted to
evaluate the correlation between the process parameters and the optimal generation of 5HMF from biomass. The yield of 5-HMF was chosen as the response. The respective
variable values have been tabulated in Table 2. To avoid systematic errors in the variables,
the trials were conducted randomly.
Table 2: Experimental factors and levels used in Box–Behnken design
Level
Independent variable units code
Mass of Levis acid catalyst
Units
Code
-1
0
1
g
A
0,121
0,118
0,242
Reaction time
min
B
60
200
340
Reaction temperature
°C
C
100
105
150
Dosage of Bronsted acid catalyst
ml
D
0,121
0,181
0,242
The experimental design results on the different reaction conditions are shown in Table 3.
The results were analyzed using multiple regression analysis functions of the RSM
methodology. A second-order polynomial equation was created using the coded numbers to
show the quantitative connection between the test variables and the 5-HMF production.
Yield = 47.39-3.07A+5.27B+22.92C+2.71D+0.3788AB+0.1121AC-1.05AD-0.7643BC0.1479BD+01.5CD+1.56A2 -5.59B2 -0.1756C2-0.3311D2
(5)
Yield is the 5-HMF yield, and A, B, C and D are the test variables of amount of Levis acid
catalyst, time, temperature, dosage of Bronsted acid catalyst, respectively.
Table 3: The actual experiment data
Levis acid
(A)
Time
(B)
Temperature Bronsted acid 5-HMF
(C)
(D)
Yield
g
min
°C
ml
%
1
2
3
4
5
6
7
8
0.181(0)
0.241(1)
0.181(0)
0.181(0)
0.181(0)
0.241(1)
0.121(-1)
0.121(-1)
60(-1)
200(0)
60(-1)
200(0)
340(1)
60(-1)
200(0)
200(0)
125(0)
150(1)
150(1)
100(-1)
125(0)
125(0)
100(-1)
150(1)
0.121(-1)
0.181(0)
0.181(0)
0.121(-1)
0.242(1)
0.181(0)
0.181(0)
0.181(0)
37,01%
67,57%
62,70%
22,95%
47,32%
34,99%
31,89%
74,53%
9
10
11
12
13
14
15
16
0.121(-1)
0.112(-1)
0.181(0)
0.181(0)
0.242(1)
0.181(0)
0.121(-1)
0.181(0)
60(-1)
200(0)
200(0)
60(-1)
340(1)
200(0)
200(0)
200(0)
125(0)
125(0)
125(0)
100(-1)
125(0)
125(0)
125(0)
125(0)
0.181(0)
0.121(-1)
0.181(0)
0.181(0)
0.181(0)
0.181(0)
0.242(1)
0.181(0)
42,79%
44,35%
46,75%
8,27%
47,59%
48,73%
52,45%
46,91%
Entry
17
0.181(0)
200(0)
125(0)
0.181(0)
46,93%
18
19
20
21
22
23
24
25
26
27
0.181(0)
0.181(0)
0.242(1)
0.181(0)
0.18(0)
0.24(1)
0.181(0)
0.181(0)
0.181(0)
0.181(0)
200(0)
200(0)
200(0)
200(0)
60(-1)
200(0)
340(1)
340(1)
340(1)
200(0)
100(-1)
125(0)
125(0)
150(1)
125(0)
125(0)
125(0)
150(1)
100(-1)
150(1)
0.242(1)
0.181(0)
0.241(1)
0.242(1)
0.24(1)
0.121(-1)
0.121(-1)
0.181(0)
0.181(0)
0.121(-1)
31,97%
47,62%
46,17%
76,73%
35,52%
42,26%
49,40%
68,83%
17,46%
61,69%
28
29
0.121(-1)
0.242(1)
340(1)
200(0)
125(0)
100(-1)
0.181(0)
0.181(0)
53,88%
24,48%
The results of the multiple regression analysis and ANOVA are presented in Table 4. The Fvalue of 27.58 and extremely low P-value (< 0.0001) indicate that the model is highly
significant and has excellent predictability. Additionally, the R-squared value of 0.9650 and
the reasonably consistent predicted R-squared value of 0.8000 and adjusted R-squared value
of 0.9300 suggest that the model is a good fit for the data. The adequate precision value of
20.896 indicates that the signal-to-noise ratio is adequate for each response, allowing for the
prediction and analysis of the actual experiment conditions.
Table 4: The values of the regression coefficients and the analysis of variance
Sum of
Source
Mean
df
Squares
F-value
p-value
Significance
significant
Square
Model
8838.22
14
631.30
27.58
< 0.0001
A-Lewis acid catalyst
140.63
1
140.63
6.14
0.0265
B-Reaction time
413.95
1
413.95
18.09
0.0008
C-Reaction temperature
7841.81
1
7841.81 342.61
D-Bronsted acid catalyst
109.57
1
109.57
AB
0.7140
1
0.7140 0.0312
0.8623
AC
0.0625
1
0.0625 0.0027
0.9591
AD
5.45
1
5.45
4.79
0.2382
< 0.0001
0.0461
0.6331
BC
2.91
1
BD
0.1089
CD
0.1270
0.7269
1
0.1089 0.0048
0.9460
11.26
1
11.26
0.4918
0.4946
A²
19.60
1
19.60
0.8562
0.3705
B²
252.11
1
252.11
11.01
0.0051
C²
0.2513
1
0.2513 0.0110
0.9180
D²
0.8848
1
0.8848 0.0387
0.8470
Residual
320.44
14
22.89
Lack of Fit
317.07
10
31.71
Pure Error
3.36
4
0.8407
Cor Total
9158.66
28
R²
0.9650
Adjusted R²
0.9300
Predicted R²
0.8000
Adeq Precision
2.91
37.72
0.0016
significant
20.0896
The results revealed that all linear (A, B, C, and D) factors significantly impacted the
development of 5-HMF, as evidenced by their low P-values (< 0.05). Based on their relative
impacts on the response variables, the ANOVA showed that among the tested factors,
reaction temperature had the greatest impact on 5-HMF yield, followed by reaction time,
Lewis acid catalyst mass, and Bronsted acid catalyst mass.
Figure 3 serves to illustrate the interplay between the experimental variables and to estimate
the optimal values for each variable that result in maximum 5-HMF yield levels.
When the mass of the Lewis acid catalyst was low, the yield of 5-HMF increased with the
increasing reaction temperature and the dosage of the Bronsted acid catalyst, but when the
reaction time was longer, the 5-HMF yield decreased with the increased Lewis acid catalyst.
At the same time, it was also observed that the reaction temperature needs to rise to 150 °C
to reach the maximum value for 5-HMF because high temperatures make the dehydration
process more effective (Thoma et al., 2020).
(a)
(c)
(b)
(d)
(e)
(f)
Fig 3: Response surface (3-D) and contour (2-D) plots about the interaction of four factors
in the 5-HMF yield: (a)-Reaction time and Lewis acid catalyst; (b)- Reaction temperature
and Lewis acid catalyst; (c)- Bronsted acid catalyst and Lewis acid catalyst; (d)- Reaction
temperature and reaction time; (e)- Bronsted acid catalyst and Reaction time; (f)- Bronsted
acid catalyst and Reaction temperature.
By utilizing equation (5) and employing a numerical optimization function within a software
program, the optimal conditions for the synthesis of 5-HMF from biomass were determined.
The predicted maximum yield of 5-HMF was 80.28%, which was achieved by using a Lewis
acid catalyst with a mass of 0.121g, a Bronsted acid catalyst with a volume of 0.242 ml, a
reaction time of 250 minutes, and a reaction temperature of 150°C. In order to verify the
accuracy of the prediction, three experiments were conducted under these optimal
conditions, and the resulting 5-HMF yields were recorded in Table 5. The experimental data
showed a close agreement with the predicted values, indicating that the model used for the
synthesis of 5-HMF was satisfactory.
Table 5: The result of 5-HMF yield before and after applying the prediction formula
Before
Substrate
After
5-HMF yield
Furfural yield
5-HMF yield
Furfural yield
(%)
(%)
(%)
(%)
10.82% ± 3.08%
83.68% ± 1.79%
13.95% ± 0.28%
Pineapple stem 46.07% ± 2.80% 42.91% ± 0.04%
54.42% ± 0.16%
45.12% ± 0.04%
19,79% ± 2.92%
36.16% ± 4.42%
40.32% ± 2.15%
Jackfruit peel
Bagasse
71.89% ± 2.49%
36,69% ± 3.17%
Rice Straw
5,17% ± 3.02%
29.83% ± 1.08%
12.82% ± 4.23%
40.57% ± 1.07%
3.2. The oxidation of 5-HMF to FDCA:
Prior to utilizing the 5-HMF solution derived from sugarcane bagasse, pineapple stem, and
rice straw for the oxidation reaction, an oxidation reaction was conducted using the 5-HMF
solution from jackfruit peel as the starting material. The findings revealed that intermediates,
namely DFF and FFCA, were detected during the oxidation reaction, whereas HMFCA was
not observed. This outcome aligns with earlier studies that have reported DFF and FFCA as
intermediate products of the oxidation process with the pH of the reaction media close to 7
(Yi et al., 2016, Lai et al., 2021, Cheng et al., 2021) as opposed to HMFCA and FFCA which
are detected in reaction environment with pH typically above 7 (Nguyen et al., 2016,
Chadderdon et al., 2014).
3.2.1. The solvent effectivity in the oxidation process:
DMSO has been shown to effectively facilitate the conversion of biomass to 5-HMF due to
its strong binding affinity to 5-HMF, which reduces its susceptibility to nucleophilic attack
and suppresses the formation of undesired by-products such as humins (Istasse and Richel,
2020). Nonetheless, this attribute can become an obstacle in the oxidation of 5-HMF to
FDCA. Chen et al. observed a low conversion rate of only 13.2% after a six-hour reaction
using pure DMSO as a solvent, without any detectable formation of FDCA (Chen et al.,
2018). In a similar study, although Fe3O4CoOx in pure DMSO proved effective in achieving
a high yield of FDCA of 68.6% in the second phase of the fructose to FDCA conversion
process, the oxidation reaction required a lengthy period of 15 hours (Wang et al., 2015).
Furthermore, Zhang et al. utilized Fe2O3@HAP-Pd as a catalyst in pure DMSO for the
oxidation process of 5-HMF to FDCA, but only 7.2% of FDCA was identified after six hours.
(Zhang et al., 2015). In an effort to remedy this issue, alkaline aqueous solutions were
employed in place of DMSO as a solvent for 5-HMF oxidation. However, oxidizing 5-HMF
to FDCA in alkaline solutions is exceedingly challenging because 5-HMF presents low
stability and simplicity in transferring into humins in alkaline solutions (Liu et al., 2019). To
find out the suitable solvent to mix with in-situ solvent for the process, various experiments
were conducted in different solvents, including water, acetonitrile (MeCN), ethyl acetate
(ETAC), and tert-butanol (TBA), and the experimental results are shown in Fig. 6.
Fig 4: The efficacy of mixing solvents in the 5-HMF oxidation process (Reaction condition:
2.5 ml of 5-HMF solvent (0.1 mmol), 2.5 ml of mixed solvent, 0.05 g of catalyst, 0.9 ml of
oxidant, 12 hours, and 70 °C)
The reactions were conducted at 70 °C, using 0.05 g Cu-Mn2O4 catalysts, 0.9 ml tBuOOH
as oxidant, 2.5 ml in situ 5-HMF solvent (0.1 mmol), and 2.5 ml mixing solvent. The FDCA
yields for each solvent are rather different. This is because of the varying viscosity, polarity,
and dissolving capacity of the solvents. H2O is the best option in terms of cost and
environmental impact, but in this situation, the yield of FDCA was only 36.24%. The FDCA
yield increased to 45.77% when ETAC, a common ester, was present. In TBA solvent, the
simplest tertiary alcohol, production of FDCA increased to 57.29%. The maximum FDCA
yield of 87.73% was achieved when ACN was employed as the mixed solvent, suggesting
that ACN is preferable for the oxidation of 5-HMF with the Cu-Mn2O4 catalyst.
The efficacy of ACN was attributed to the fact that normally, Cu+ is unstable in aqueous
solutions because of spontaneous disproportionation into Cu2+ and Cu. However, due to its
Lewis basicity, it can be stabilized by interaction with the N-containing group of ACN and
therefore enhance the oxidation of 5-HMF to DFF throughout the process (Wei et al., 2019).
3.2.3. The temperature reaction effectivity in the oxidation process:
According to the findings of the aforementioned experiment, acetonitrile solvent is highly
effective when mixed with DMSO to develop a co-solvent system for 5-HMF oxidation.
This combination can boost oxidation efficiency and create an environmentally friendly
reaction. Therefore, for the next experiments, co-solvent DMSO-acetonitrile is considered
to use in the tests. In this experiment, the temperature effect was studied from 60°C to 90°C
in the presence of 0.05g catalyst, 5 ml co-solvent 5-HMF (0.1mmol), and 0.9 ml tBuOOH
for 12 hours. The results as shown in figure 7. The findings demonstrate that even at
temperatures as low as 60 °C, the rate of 5-HMF conversion approached 100%. At 70 °C,
the optimum FDCA yield of 87.73% was obtained. The oxidation of FFCA to FDCA is the
rate-limiting step after the fast transformation of 5-HMF to DFF and DFF to FFCA.
Following the results, the mass balance was usually always more than 85%, showing that
FFCA and FDCA were extremely stable under the reaction conditions used. Increased the
reaction temperature to 90 °C resulted in a drop in mass balance, indicating that more humins
produced at the higher temperature.
Fig 5: The impact of temperature in the 5-HMF oxidation process (Reaction condition: 5 ml
of 5-HMF co-solvent (0.1 mmol), 0.05 g of catalyst, 0.9 ml of tBuOOH, and 12 hours)
3.2.4. The effect of the oxidant dosage:
The quantity of tBuOOH is an important parameter since it is used as an oxidant in the 5HMF-to-FDCA pathway, where differing quantities might potentially result in varied
catalytic performances. Hence, to get detailed understanding of the effect of tBuOOH on the
oxidation of 5-HMF into FDCA over Cu-Mn2O4, several tBuOOH doses were studied. Fig
6 displays the observed variation in 5-HMF conversion and associated product yields toward
various oxidation products as a function of oxidant dose. Increasing the tBuOOH dose from
0.5 to 0.9 ml resulted in a significant increase in FDCA production, from 78.64% to 86.72%,
suggesting that tBuOOH was essential for the oxidation of 5-HMF. However, upon
increasing to 1.2 ml, the yield of FDCA reached 86.94%, only a small variation was noticed.
Therefore, a tBuOOH dose of 0.9 ml was chosen for subsequent studies as being best for the
conversion of 5-HMF from both a cost-benefit and environmental-impact standpoint.
Fig 6: The efficacy of oxidant dosage in the 5-HMF oxidation process (Reaction condition:
5 ml of 5-HMF co-solvent (0.1 mmol), 0.05 g of catalyst,12 hours, and 70 °C)
3.2.4. The effectivity of the amount of catalyst:
To observe the impact of catalyst loading on the oxidation of 5-HMF, various amounts of
Cu-Mn2O4 were utilized while maintaining the other parameters in optimum conditions. 5HMF conversion and FDCA yield as assessment factors for catalyst quantity is presented in
Fig. 7. In all cases, the 5-HMF conversion reached its maximum of 100%. 82.86% yield of
FDCA was obtained by using a small amount of Cu-Mn2O4 (30 mg). These results indicated
the catalyst was highly effective in the oxidation of 5-HMF. With a further increase in
catalyst loading to 50 mg, FDCA yield reaches a maximum of 86.72%. The FDCA yield
exhibited a declining trend with continuous catalyst addition from 70 to 90 mg. The reason
for the reduced FDCA yield was due to the poor interaction between the catalyst and the
substrate when the excess catalyst was added (Liu et al., 2022). This suggests that a 50 mg
catalyst loading was sufficient for the conversion of 5-HMF to FDCA.
Fig 7: The efficacy of catalyst loading in the 5-HMF oxidation process (Reaction condition:
5 ml of 5-HMF co-solvent (0.1 mmol), 0.9 ml of tBuOOH, 12 hours, and 70 °C)
3.2.6. The reaction time of the oxidation process:
By monitoring the time-dependent changes in the conversion of HMF and the yields of
FFCA, DFF, and FDCA during the oxidation of in 5-HMF under optimum of the reaction
conditions. Specifically, we used 5 ml of 5-HMF co-solvent (0.1 mmol), 0.05 g of catalyst,
0.9 ml of oxidant, and 70 °C. The results showed that the yield of FDCA increased gradually
over 12 hours, with a value of 84.16% at 10 hours, followed by a slight increase to 87.35%
at 12 hours. Meanwhile, the conversion of 5-HMF to intermediates occurred rapidly, with
100% conversion achieved in less than 4 hours. During the first 2 hours, FFCA was produced
rapidly, reaching a maximum yield of 52.72%. Furthermore, 5-HMF conversion was nearly
80%, and it was completely converted before the first 4 hours had elapsed. Surprisingly, the
yield of DFF, another intermediate, remained consistently low throughout the reaction. These
findings suggest that the conversion of 5-HMF to DFF occurred rapidly, followed by a fast
conversion of DFF to FFCA. However, the conversion of FFCA to FDCA was much slower
and represented a rate-limiting step, as shown in Scheme 1. Therefore, while the conversion
of 5-HMF to FFCA was swift, taking approximately 3–4 hours, the conversion of FFCA to
FDCA was much slower, requiring over 12 hours to complete. Previous researchers have
found similar reaction trends (Yi et al., 2016, Lai et al., 2021, Cheng et al., 2021).
Fig 8: Time time-dependent changes of oxidation of 5-HMF (Reaction condition: 5 ml of 5HMF co-solvent (0.1 mmol), 0.05 g of catalyst, 0.9 ml of oxidant, and 70 °C).
3.2.6. The reaction on the different kinds of biomass:
In this experiment, 5-HMF solvents derived from other types of biomass were used for the
oxidation process. The reactions were conducted under the optimal conditions of 5 ml of 5HMF co-solvent (with 0.0122 g of 5-HMF loading from pineapple, 0.008 g of 5-HMF
loading from sugarcane bagasse, and 0.0025 g of 5-HMF loading from rice straw), a mass
ratio of 1:4 of catalyst and 5-HMF, 0.9 ml of oxidant, 70 °C, and 12 hours. Table 6
summarizes the results.
In reactions with pineapple stem, sugarcane bagasse, and rice straw, FDCA yields were not
substantially different. The FDCA yields obtained for these reactions were 68.50%, 62.20%,
and 69.38% for pineapple, sugarcane bagasse, and rice straw, respectively; nevertheless,
their results were lower than jackfruit peel, which yielded 83.37%. This may be because the
5-HMF solvent derived from jackfruit peel has fewer byproducts than the other biomass.
The results serve as a demonstration of the efficacy of the method; however, the 5-HMF
yield obtained from rice straw and sugarcane bagasse is so low, leading to the FDCA yield
obtained from them being only 8.98% and 21.27%, respectively (in the biomass loading
calculation).
Biomass is a mixture of different types of components. In where, the composition,
architecture, and structure of cell walls are very different between woody and herbaceous
feedstocks (Yan et al., 2020). Herbaceous biomass, such as sugarcane bagasse or rice straw,
contains more hemicellulose and extractives, whereas woody biomass, such as jackfruit peel
or pineapple stem, which is usually denser and less porous, has more glucan (Kirker et al.,
2013, Yan et al., 2020). These differences in chemical composition and physical structure
influence its properties, which greatly influence the effective deconstruction and conversion
in this process. Therefore, the procedure of hemicellulose removal from sugarcane bagasse
or rice straw is important prior to convert to 5-HMF in order to get the optimal yield of 5HMF and FDCA.
Table 6: the results of FDCA yield from different kinds of biomass
FDCA yield
Substrates
5-HMF yield
Calculate the 5-HMF
Calculate the biomass
loading
loading
87.37% ± 2.56%
73.41% ± 2.14%
68.50% ± 3.82%
37.28% ± 2.08%
Sugarcane Bagasse
83.68% ± 1.79%
54.42% ± 0.16%
36.16% ± 4.42%
62.20% ± 3.37%
21.27% ± 1.22%
Rice Straw
12.82% ± 4.23%
69.38% ± 0.36%
8.98% ± 0.05%
Jackfruit peel
Pineapple stem
3.2.8. Possible reaction mechanism:
Based on our experiments and published research results, a possible reaction mechanism was
proposed for the oxidation of 5-HMF to FDCA over the Cu-Mn2O4 catalyst. The reaction
process consists of three distinct steps. In the first step, Cu2+ is combined with acetonitrile
to synthesize Cu(CH3CN)42+, and 5-HMF is oxidized by Cu(CH3CN)42+ to quickly produce
DFF. In this procedure, 5-HMF's hydroxyl group is efficiently adsorbed to the catalyst's
surface. tBuOOH, when heated, breaks down to provide the oxygen for the process, and
forms tert-butyl radical ((CH3)3CO-) and –OH (Cheng et al., 2021). The -OH binds to the
hydroxyl group on the 5-HMF side chain, attacking it and capturing the proton to produce
water. Due to proton loss, 5-HMF's hydroxyl side chain is transformed into a free radical.
The gem-diol intermediate 1 is formed when the O atom of the hydroxyl group of 5-HMF
donates its lone pair of electrons to the adjacent defector C atom. O2 from the decomposed
tBuOOH was adsorbed on the catalyst surface, dissociating into chemisorbed oxygen species.
It then reacts with the adsorbed hydrogen to form water (H2O) and cleans the catalyst's active
sites for the creation of DFF (Yang et al., 2019).
Second, Mn4+ is required for the oxidation of DFF to FFCA via reduction to Mn3+. Cu2+
oxidizes Mn3+ and then reduces to Cu+. tBuOOH then oxidizes Cu+ to Cu2+. When tBuOOH
degrades, it emits -OH, which is added to the CHO group nucleophilic to make gem-diol
intermediate 2, and dehydrogenation of gem-diol intermediate 2 gives off a COOH group.
In the end, FFCA is converted into FDCA using the same method (Yang et al., 2019, Cheng
et al., 2021).
Fig 9: Possible reaction mechanism
3.3. The reusability of the catalysts:
3.3.1. The reusability of the catalysts for 5-HMF synthesis:
In this work, we aimed to show that the solvent, Al2O3/SiO2, and [Bmim]HSO4 could all be
recycled. The separation of the biomass from the mixture's solvent once the reaction is
complete is crucial to achieving this aim. To keep the biomass isolated, we utilized a small
filter fabric bag with the size of 5x5 cm. The biomass bag worked as a prevention layer to
keep the biomass from mixing with the catalysts and was simple to put out when each
completed reaction. The experiments used the high-pressure bottle and were repeated in
many times without adding more solvent or catalysts until the 5-HMF yield remained
constant among the experiments.
Fig 10: The reusability of solvent and catalysts in the conversion of biomass to 5-HMF
As illustrated in Fig 10, the 5-HMF yield was obtained at 77.85% in the first reaction,
whereas less than 82.64% was produced in the experiment following the ideal condition. The
issue is caused by a lack of interaction between biomass and catalysts, which was brought
on by the use of biomass bag. The findings show that the catalysts are quite efficient after
three cycles of reaction; however, the 5-HMF yield reduced to 49.21% in the fourth cycle.
This shows that one of the two catalysts has lost activity. Consider the following observations:
Because Al2O3/SiO2 is still present in the mixture solvent, we ascribe the cause to the melting
of ionic liquid [Bmim]HSO4. In the fifth run, we only added 0.242 mL of ionic liquid, and
the 5-HMF yield was obtained at 73.43%. This result demonstrates the outstanding
recyclability of solvent and both catalysts, Al2O3/SiO2 and [Bmim]HSO4, in the production
of 5-HMF from biomass.
3.3.2. The reusability of the catalysts for FDCA synthesis:
After each reaction, a high-speed centrifuge was used to isolate the catalyst. The catalyst
was then dried at 100 °C for 5 hours. After centrifugation and drying, only a small amount
(0–4 mg) of catalyst was lost, and the loss was offset by the fresh catalyst. After seven cycles,
the yield of FDCA and the conversion of 5-HMF basically remained unchanged. This
demonstrates the excellent reuse ability of the Cu-Mn2O4 catalyst in the oxidation of 5-HMF
to FDCA
Fig 10: The reusability of the catalysts in the oxidation of 5-HMF to FDCA
4. Conclusions Lai
This research study effectively showcases the synthesis of FDCA from agricultural residues
without any chemical pretreatment or 5-HMF separation, using a two-step process under
benign conditions. Firstly, the reaction with jackfruit peel, pineapple stem, sugarcane
bagasse, and rice straw produced the best yield results of 5-HMF, which were 83.68%,
54.42%, 36.16%, and 12.8%, respectively, confirming the efficacy of the method. In the
second step, the formation of 5-HMF was oxidized at a temperature of 70 oC for a duration
of 12 hours, using Cu-Mn2O4 as a catalyst, acetonitrile as a mixed solvent, and tert-butyl
hydroperoxide (tBuOOH) as the oxidant to produce FDCA. The best yield results of FDCA
were obtained in the reaction with jackfruit peel, pineapple stem, sugarcane bagasse, and
rice straw, at 87.37%, 68.66%, 59.31%, and 65.60%, respectively, while total amount
produced of FDCA from these materials was found to be 73.11%, 37.37%, 21.44%, and
8.44%. The oxidation reaction of 5-HMF showcased rapid conversion to DFF and DFF to
FFCA, but the conversion of FFCA to FDCA was comparatively slow and rate-limiting. The
cost-effective and eco-friendly Cu-Mn2O4 catalyst replaces noble metal catalysts, as it is
easy to produce, inexpensive, and harmless to the environment. Remarkably, the catalyst
was still effective after being used seven times, in the absence of base solvents.
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