Efficient one-pot production of 1,2-propanediol and ethylene glycol from microalgae (Chlorococcum sp.) in water

Article abstract of DOI:10.1039/c4gc02467b

The catalytic valorization of microalgae, a sustainable feedstock to alleviate dependence on fossil fuel and offset greenhouse gases emissions, is of great significance for production of biofuels and value-added chemicals from aquatic plants. Here, an interesting catalytic process is reported to convert microalgae (Chlorococcum sp.) into 1,2-propanediol (1,2-PDO) and ethylene glycol (EG) in water over nickel-based catalysts. The influences of reaction temperature, initial H₂ pressure and reaction time on the product distribution were systematically investigated by using a batch reactor. Under optimal reaction conditions (at 250 °C for 3 h with 6.0 MPa of H₂ pressure), microalgae were directly and efficiently converted over a Ni-MgO-ZnO catalyst and the total yield of polyols was up to 41.5%. The excellent catalytic activity was attributed to the smaller size and better dispersion of Ni particles on the MgO-ZnO supporter based on the characterization results as well as its tolerance to nitrogen-containing compounds. Besides, the reaction pathway was proposed based on the formation of reaction intermediates and the results of model compound conversion.

Full text of DOI:10.1039/c4gc02467b

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Efficient one-pot production of 1,2-propanediol
and ethylene glycol from microalgae
(Chlorococcum sp.) in water
Cite this: DOI: 10.1039/c4gc02467b
G. Miao,a C. C. Zhu,a J. J. Wang,a Z. C. Tan,a,b L. Wang,a J. L. Liu,a L. Z. Kong*a and
Y. H. Sun*a,c
The catalytic valorization of microalgae, a sustainable feedstock to alleviate dependence on fossil fuel and
offset greenhouse gases emissions, is of great significance for production of biofuels and value-added
chemicals from aquatic plants. Here, an interesting catalytic process is reported to convert microalgae
(Chlorococcum sp.) into 1,2-propanediol (1,2-PDO) and ethylene glycol (EG) in water over nickel-based
catalysts. The influences of reaction temperature, initial H2 pressure and reaction time on the product dis-
tribution were systematically investigated by using a batch reactor. Under optimal reaction conditions (at
250 °C for 3 h with 6.0 MPa of H2 pressure), microalgae were directly and efficiently converted over a
Ni–MgO–ZnO catalyst and the total yield of polyols was up to 41.5%. The excellent catalytic activity was
attributed to the smaller size and better dispersion of Ni particles on the MgO–ZnO supporter based on
the characterization results as well as its tolerance to nitrogen-containing compounds. Besides, the reac-
tion pathway was proposed based on the formation of reaction intermediates and the results of model
compound conversion.
Received 18th December 2014,
Accepted 16th February 2015
DOI: 10.1039/c4gc02467b
www.rsc.org/greenchem
of 1,2-PDO was achieved from JAT containing high percentage
of saccharides in it over a similar catalyst.12 However, in the
1. Introduction
With fossil fuel reserves declining and the environment dete- process of complex raw biomass conversion for polyols pro-
riorating, catalytic valorization of renewable energy sources for duction, pretreatment processes should be involved to reduce
the production of value-added chemicals or liquid biofuels the formation of unexpected byproducts.15,16 For example,
from these biomass resources has attracted increasing atten- starting from miscanthus, a base solvent should be first used
tion. As one of the existing solid organic carbon resources to remove epidermal protectors and lignin fractions, which
biomass has several advantages, such as ease of storage and could preferentially poison the Ni–W2C/AC catalyst due to the
transportation, wide distribution and short growth duration.1–4 formation of organic acids and unsaturated compounds.16
Among them, catalytic conversion of biomass to polyols (such Similarly, corn stalk was pretreated with 1,4-butanediol, NaOH,
as 1,2-PDO and EG) is particularly noteworthy because of their H2O2 and ammonia first, and much higher yields of ethylene
versatile uses as important bulk chemicals directly or precur- glycol and 1,2-propylene glycol could be achieved over the same
sors in the synthesis of fuels and value-added compounds.5–9
catalyst compared with that from untreated raw corn stalk. In
Sugars, pure cellulose and pretreated terrestrial raw view of higher cost of tungsten catalysts, Mu et al. reported a
biomass can be used as feedstock for the production of EG catalytic production process of 1,2-alkanediols from cellulose
and 1,2-PDO.10–17 In 2008, Zhang et al. reported a one-pot con- over Ni-supported catalysts, and that the total yield of glycols
version of pure cellulose to EG over Ni-promoted tungsten reached up to 70.4%.17 However, pure cellulose was only used
carbide catalyst.10 Such process opened a novel route for green as reaction substrate in this process, and the activities of such
EG production, and the EG yield could be enhanced to 75% catalyst for raw feedstock conversion were not reported.
over these tungsten-based catalysts, while 38.5% yield (calcu-
Microalgae, a kind of aquatic plants with high efficiency in
lated based on saccharides in Jerusalem Artichole Tubers, JAT) capturing light, were one of the fastest growing light-driven
cell factories. They were considered as appropriate sources for
hydrocarbons due to their higher percentage of carbohydrates
aCAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai
and lipid components.18,19 Recently, research on microalgae
Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210,
mainly focused on the biofuel production, eutrophic waste-
PR China. E-mail: konglz@sari.ac.cn, sunyh@sari.ac.cn
water purification and usage on food industry.20–25 However,
bUniversity of Chinese Academy of Sciences, Beijing 100049, PR China
cShanghaiTech University, Shanghai 201210, PR China
the efficient conversion of microalgae to high value-added
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chemicals, such as alkanediols was rarely reported. Since the same temperature for 3–4 h. The suspension was poured out
percentage of moisture in microalgae is very high, microalgae of the reactor and separated by vacuum filtration. The filter
with its nutrient solution can be directly used as reaction cake was washed thoroughly and then transferred to the
system, avoiding the addition of other reaction solvents and drying vessel. The resultant dry cake was ground into a
complicated separation and purification processes compared powder, followed by calcination at 673 K under N2. The syn-
with terrestrial raw biomass. Therefore, selective conversion of thesized catalysts were reduced at 673 K for 300 min under a
microalgae into high value-added polyols, especially alkane- H2 flow. Prior to exposure to air, the catalysts were passivated
diols (e.g., EG and 1,2-PDO) was of great interest.
in a flow of 0.5% O2/N2 for 5 h at room temperature. Other
On the other hand, the percentage of nitrogen-containing catalysts (such as Ni–ZnO, Ni–MgO and MgO–ZnO) were pre-
components in microalgae is quite high, and it would poison pared using similar methods, just changing the types and
the hydrogenation or hydrogenolysis catalysts during the reac- molar ratios of metal nitrates.
tion process.26,27 Herein, a nickel-based catalyst, which was
2.3. Catalyst characterization
easily prepared via a traditional co-precipitation method in our
group, was used to convert microalgae (Chlorococcum sp.) into The X-ray powder diffraction (XRD) patterns of catalysts were
polyols directly. It was found that such as-synthesized nickel- performed on a Rigaku Ultima IV X-ray diffractometer with a
based catalyst exhibited excellent tolerance of nitrogen-con- CuKα Radiation. The CO-chemisorption was carried out at
taining components, and the total yields of polyols from room temperature on a Micromeritics ASAP 2020 instrument.
microalgae hydrogenation were approximately 41.5% under Prior to the measurements, the samples were degassed at
mild conditions. Meanwhile, the influences of reaction con- 350 °C for 300 min. The metal dispersion was obtained
ditions were systematically investigated and the reaction through difference calculation. The BET surface areas were
pathway of microalgae conversion was proposed. Because of determined by adsorption–desorption of nitrogen at liquid
the use of green feedstock and the process being free of nitrogen temperature, using Micromeritics TriStar II equip-
organic solvent, such catalytic utilization of microalgae for ment. Samples were degassed at 200 °C prior to acquiring the
polyols production would provide a potential route for the pro- adsorption isotherm. Element analysis of feedstock was per-
duction of valuable chemicals from renewable biomass.
formed using a CHNS Analyzer (Thermo scientific Flash 2000).
The total organic carbons (TOC) of liquid products were
achieved on Shimadzu TOC-L. The contents of each metal in
the synthesized catalyst were determined by ICP on an
Optimal Emission Spectrometer (Perkin Elmer Optima 8000).
2. Experimental
2.1. Materials
2.4. Catalytic experiments and product analysis
Microalgae (Chlorococcum sp.) were self-cultured by our co-
workers. Typically, the cells of Chlorococcum sp. were cultured
in a 550 mL glass column (43 mm diameter, 390 mm height)
with a working volume of 400 mL and treated with compressed
air enriched with 1% CO2 (v/v) and BG11 nutrients. The tem-
The catalytic conversion of microalgae without any treatment
was carried out in a stainless steel autoclave (Parr, 50 mL) with
an initial H2 pressure of 2–6 MPa at 423–533 K for 60–240 min.
Typically, 0.25 g of microalgae (1.0 wt%) and 0.15 g of catalyst
and 25 mL of deionized water were placed in the autoclave and
stirred at a speed of 600 rpm. After the reaction step, the auto-
clave was cooled automatically, and the liquid solution was
separated from the solid mixture by centrifugation. The solid
catalyst was collected and washed with deionized water and
ethanol for several times, following with desiccation at 353 K
overnight.
The collected liquid solution was filtered through 0.22 μm
pore-size filters prior to analysis. The main products in the
resultant solution were identified based on the standard com-
pounds and the structures of them were further confirmed
by GCMS. To guarantee the carbohydrates in the microalgae
had been totally converted, the liquid products were also ana-
lyzed by HPLC (Shimadzu LC-20AD, Aminex HPX-87H Ion
perature was kept at 26
2 °C and the light intensity was
∼200 µmol photons m−2 s−1. The pH value was maintained
at the range of 7.0–8.0. The initial inoculum was about
0.1–0.2 g L−1. The microalgae cells were harvested by centrifu-
gation at 5000 rpm for 5 min at day 6 or day 7 when the
biomass concentration reached about 5.0 g L−1. The micro-
algae cells were washed three times with deionized water and
then lyophilized. The lyophilized microalgae were ground into
powders and stored at a temperature below 273 K.
All the other chemicals were purchased from Sinopharm
Chemical Reagent Co., Ltd, and used as received without
further purification.
2.2. Catalyst preparation
The Ni–MgO–ZnO catalyst was prepared by a co-precipitation Exclusion Column: 300 mm × 7.8 mm) with differential refrac-
method. In a typical process, the metal salt solution was pre- tion detector (RID-10A). These products were quantified by gas
pared by mixing nickel nitrates, zinc nitrates and magnesium chromatography (GC, Shimadzu GC2010 Plus, HP-INNOWax
nitrates with a certain molar ratio. Then, a solution of Na2CO3 column: 30 m × 0.25 mm; film thickness, 0.25 μm) with a
was added dropwise to the prepared solution under vigorous flame ionization detector (FID). The conversion of microalgae
stirring at 343 K and the pH was maintained at 8.0. The resul- was calculated based on TOC data using the equation: Conver-
tant precipitate was aged under continuous stirring at the sion = (moles of carbon in the resultant liquid determined by
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TOC) ÷ (moles of carbon in microalgae determined by a CHNS
analyzer) × 100%. The yield of polyols was calculated based on
carbon via the equations: Yield = (moles of carbon in the pro-
ducts determined) ÷ (mole of carbon in the carbohydrates
components of microalgae) × 100%. The yields of gas products
were too low to be quantified in this work.
3. Results and discussion
3.1. Chemical compositions of microalgae
The results of microalgae composition characterization are
shown in Table 1. It can be seen that the moisture content in
the freeze-dried microalgae was 4.7% and the ash content was
1.2%, indicating that the content of the organic components
was more than 90%. The percentages of carbon, hydrogen and
nitrogen component in microalgae were 48.4, 7.6 and 1.9%,
respectively, while some sulfur components can also be
detected. Different from the structure of lignocellulose biomass,
lignin was not detected in microalgae, and the main com-
ponents of microalgae were carbohydrate, lipids and protein.
The content of carbohydrates almost accounted for half of the
total mass as well as 32.6% of lipid and 11.9% of protein.
Generally, the major component of microalgae was protein,
and the contents of lipid and carbohydrate were quite low.28
However, the content of protein in our self-cultured microalgae
was only 11.9%, and the main component was carbohydrate,
indicating that the microalgae used in this study was a special
carbohydrate-rich species which was quite different from tra-
ditional ones. Therefore, the hydrothermal processes of these
cultured microalgae conversions for the production of polyols
were of great interest and systematically investigated in our
work.
Fig. 1 Catalytic conversion of microalgae into polyols over various
catalysts. Reaction conditions: 250 °C, 3 h, 6 MPa of H2.
reached 30.7% as well as a little amount of monohydric alco-
hols (others-1, such as isobutanol, isoamylol, cyclopentanol
and 3-hexanol) and long-carbon-chain diols (others-2, such as
1,2-butanediol, 1,2-butyl glycol, 1,3-propanediol, 1,2-adipic
alcohol) at 250 °C for 3 h in water with 6 MPa of initial H2
pressure, indicating that aquatic microalgae could be directly
used as sustainable feedstock for the production of high value-
added alkanediols. Besides, using wet microalgae directly
obtained from nutrient solution without pretreatment as reac-
tion substrate, the total yields of polyols were also up to 40.5%
and the yields of 1,2-PDO and EG reached 29.8% as well, indi-
cating that complicated separation and purification processes
of microalgae from nutrient solution can be avoided and the
cost of alkanediols production would further decrease. The
results of TOC and CHNS Analyzer demonstrated that these
nitrogen-containing compounds stayed in the reaction solu-
tion and did not adsorb on the surface of catalyst.
3.2. Catalytic conversion of microalgae into polyols
The catalytic activities of microalgae (Chlorococcum sp.) conver-
sion for polyols production over these as-synthesized catalysts
were investigated under hydrothermal conditions in a hydro-
gen-rich atmosphere, and the distributions of products are
shown in Fig. 1. It can be seen that microalgae can be efficien-
tly converted in a one-pot process into polyols over Ni-based
catalyst under mild conditions, and the total yield of polyols
was up to 41.5%. Especially, the yields of 1,2-PDO and EG
Generally, nitrogen-containing compounds would poison
hydrogenation catalysts. Surprisingly, although the content of
nitrogen was up to 1.9%, such Ni–ZnO–MgO catalyst exhibited
excellent catalytic activities as well and showed a good toler-
ance for nitrogen. Adding a certain amount of glutamic acid
into the initial reaction mixture, the total yields of polyols and
alkanediols were consistent at about 43.7 and 32.1%, respect-
ively. Besides, increasing microalgae from 1.0 wt% to 4.0 wt%,
such catalyst also showed good performance in the conversion
of microalgae, and the yields of polyols and alkanediols were
about 41.5 and 30.8%, respectively. Therefore, this aqueous
catalytic process for alkanediols production from aquatic raw
biomass was much more promising since cheaper microalgae
can be directly used without any additional pretreatment for the
production of alkanediols and the as-synthesized catalyst had
an excellent tolerance for toxic components in microalgae cells.
To investigate the influence of the catalyst composition on
the yields of polyols, conversions of microalgae over the other
metal-based catalysts synthesized with a similar method were
also performed. From Fig. 1, it can be seen that the yield of
polyols was very low in the blank experiment without any
Table 1 Characterization of microalgae (Chlorococcum sp.)
Method
Component
Percentage (%)
Proximate analysis
Element analysis
Moisture
Ashes
C
H
4.7
1.2
48.4
7.6
N
S
1.9
0.1
Biochemical composition
Carbohydrate
Lipid
Protein
Lignin
49.6
32.6
11.9
N.D.
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addition of catalyst, where only a little amount of acetol was
detected in solution. However, the conversion of microalgae
was up to 41.5%, indicating that the structure of microalgae
can be destroyed at these conditions and converted into other
by-products undetected. Similarly, the MgO–ZnO catalyst did
not exhibit a satisfying catalytic activity for the conversion of
microalgae under the same conditions either, and only some
acetol was produced in the hydrogenolysis process. Besides,
some carbonaceous by-products were generated since black
solids gradually deposited at the bottom of the vessel after
being used for one or two days. However, the acetol yield
obtained over the MgO–ZnO catalyst was a little higher than
that from the blank experiment, indicating that MgO–ZnO
could promote the microalgae converting to liquid products or
reaction intermediates.
Fig. 2 XRD patterns of the as-synthesized catalysts.
With the introduction of Ni particles onto the supporter
(such as MgO, ZnO and MgO–ZnO), it can be clearly seen that
microalgae can be almost completely converted. The highest
polyols yield (41.5%) can be achieved over the Ni–MgO–ZnO
catalyst compared with that of 37.0% and 37.5% catalyzed by
Ni–ZnO and Ni–MgO under the same conditions. The BET
surface area of Ni–MgO–ZnO, Ni–ZnO and Ni–MgO were
39.8 m2 g−1, 20.7 m2 g−1 and 10.2 m2 g−1 respectively. Ni–
MgO–ZnO with the largest surface area depicts the best cataly-
tic activity. Catalytic activity of Ni–MgO and Ni–ZnO are almost
equal, both showing quite excellent activity for polyols pro-
duction, but Ni–MgO cannot be used on a large scale for
polyols production since the amount of obtained catalyst was
much less than that of Ni–ZnO and Ni–MgO–ZnO under the
same preparation conditions, suffering from leaching in the
catalyst-synthesized process due to virtually no MgO precipi-
tation at the pH value of 7.5–8.0. The ICP analysis showed that
the actual content of Mg in Ni–MgO–ZnO catalyst (theoretical
molar content of Mg in Ni–MgO–ZnO was 10%) was only 2.0%,
and the actual elemental ratio of Ni and Mg in the synthesized
Ni–MgO catalyst was just 15 : 1 with the feed intake of Ni and
Mg in molar ratio of 4/6. These results indicated that most of
the Mg precursor was drained, leading to a higher content of
Ni than that in the Ni–ZnO catalyst. From the XRD spectra of
the synthesized catalyst shown in Fig. 2, the peaks of ZnO and
Ni can be observed clearly, in contrast to the weak peaks of
MgO in the Ni–MgO–ZnO catalyst. Similarly, the peaks of MgO
in the Ni–MgO catalyst were quite weak and it almost cannot
be distinguished in the spectrum of the MgO–ZnO catalyst,
consistent with the results of ICP analysis. Because of high Ni
content in the Ni–MgO catalyst, the activity of the Ni–MgO
catalyst had a little increase compared to that of the Ni–ZnO
catalyst. However, the high Ni loading in Ni–MgO could result
in particle agglomeration and decrease the amount of the
exposed active sites. The calculated particle sizes of nickel on
ZnO, MgO, and MgO–ZnO using the Scherrer equation were
10.03 nm, 11.25 nm and 7.32 nm, respectively. And the Ni dis-
persion on ZnO, MgO, and MgO–ZnO obtained by difference
calculation through CO-chemisorption characterization was
1.23%, 1.16% and 1.25%. Therefore, the high content of Ni
inferior activity of the Ni–MgO catalyst compared with the Ni–
MgO–ZnO catalyst. On the other hand, the addition of a small
amount of MgO into Ni–ZnO could enhance the yield of
polyols, which would be due to a better dispersion and smaller
size of Ni particles on the Ni–MgO–ZnO catalyst from the
broader peaks of Ni compared to the other two catalysts.
Besides, the main peak of Ni shifted slightly in the XRD
pattern of the ZnO-contained catalyst, probably resulting from
lattice distortion of Ni when added ZnO.29 For the Ni–MgO
catalyst, there was little chance for the competition of the
other atoms, since the main component was Ni particles.
To make uttermost use of the synthesized catalyst, the recy-
cling and regeneration of the Ni–MgO–ZnO catalyst were inves-
tigated. The main products of the recycling and regenerating
experiments were glycerols and acetols. The activity of the cata-
lysts decreased when recycled. Even through regenerated, the
catalytic activity of Ni–MgO–ZnO could not be recovered. For
exploring the reason of deactivation of the recycled and regen-
erated catalysts, XRD characterization of Ni–MgO–ZnO was
conducted as shown in Fig. 3. It can be seen that the ZnO and
Ni peaks of the catalysts after recycling and regenerating
Fig. 3 XRD patterns of the recycled and regenerated Ni–MgO–ZnO
cannot compensate for the agglomeration of Ni, resulting in catalysts.
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become sharper than that of fresh catalyst. However, the MgO In the conversion process, carbohydrates in the microalgae
peak was weaker after once recycle, and even disappeared after were firstly hydrolyzed to glucose under hydrothermal con-
the regeneration process. This indicates that the crystal size of ditions. Then the generated glucose was hydrogenated over
ZnO and Ni increased after the recycling and regeneration pro- nickel-based catalysts. Transformation of glucose into short
cesses, and that MgO content was drained in the reaction, chain alcohols is due to cleavage of C–C bonds. One of the pro-
leading to a collapse of the original structure of Ni–MgO–ZnO.
posed paths for C–C bond cleavage was from glucose to glyco-
Based on the above results, it can be inferred that Ni par- laldehyde via a retro-aldol reaction, followed by the production
ticles were the main active sites for the hydrogenolysis of of ethylene glycol from glycolaldehyde by hydrogenation.30 On
microalgae, and the MgO–ZnO supporter had played an impor- the other hand, as the heating rate was very fast, the glucose
tant role in the catalytic activities of polyols production. was also transformed into levoglucosan in a short time
Although Ni particles dispersed on a single ZnO supporter through dehydration and no formation of sorbitol via a hydro-
exhibited a good activity as well, the addition of MgO could genation process was detected in the reaction solution.31 Then
decrease the size of Ni particles, leading to its superior activi- the generated levoglucosan underwent cleavage of C–C and
ties over the Ni–MgO–ZnO catalyst. However, the recycling and C–O bonds and was converted into methylglyoxal. In the H2
regeneration of the catalysts needs to be improved in future atmosphere, methylglyoxal was further hydrogenated into
studies.
acetol, which could be detected in the reaction system when
the feedstock did not hydrogenate thoroughly over MgO–ZnO
catalysts or without catalysts. In some literature, it has also
been reported that glucose would be isomerized into fructose,
then producing 1,3-dihydroxyacetone via a retro-aldol reac-
tion.4,11,16,17 1,3-Dihydroxyacetone is rather unstable and was
likely to be converted into acetol under our reaction condition.
In the presence of Ni–MgO–ZnO catalysts, acetol was comple-
tely hydrogenated into 1,2-PDO. Therefore, it could be inferred
that the main reaction pathway for microalgae hydrogenolysis
for polyols production was direct cleavage of C–C bonds in
glucose, as shown in Scheme 1.
3.3. Reaction pathway of microalgae conversion
The conversion for microalgae was conducted at a temperature
of 200–250 °C with 4–6 MPa of initial H2 pressure as shown in
Fig. 4. To balance the carbon of the raw material and products,
it was calculated that about 3–5% of raw materials were con-
versed to gaseous products, including CH4, CO2 and CO. The
solid products, in which the carbon and hydrogen contents
were 74% and 12%, respectively, it was thought that most of it
was released as lipids after the hydrothermal process. It was
found that this part of products was between 25% and 35%. In
this process, more than half of solid carbon in the raw
materials was turned to liquids. From GC-MS analysis, total
peak area of the alkanediols was more than 70%, and also
some little furans, ketones and organic acids.
Such reaction pathway can be confirmed by the results of
model compound conversion. As shown in Table 2, glucose
As mentioned above, the feedstock used in this work was a
special kind of microalgae with a high carbohydrates content.
Fig. 4 The reaction process for microalgae conversion and distribution
Scheme 1 Proposed pathway for the conversion of microalgae over
of various products.
nickel based catalysts.
Table 2 Conversion results of model compounds over Ni–MgO–ZnO catalyst
Yield (%)
Entry
Feedstock
Reaction condition
1,2-PDO
EG
Ethanol
Total
1
2
3
4
5
6
Glucose
Glucose
Levoglucosan
Acetol
Acetol
Acetol
220 °C, 3 h, 6 MPa
250 °C, 3 h, 6 MPa
250 °C, 3 h, 6 MPa
250 °C, 1 h, 6 MPa
250 °C, 2 h, 6 MPa
250 °C, 3 h, 6 MPa
26.0
47.2
25.4
70.6
60.4
28.8
12.8
11.0
13.0
0.0
0.0
0.0
1.9
10.3
6.7
22.3
31.4
53.9
40.7
68.5
45.1
92.9
91.8
82.7
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can be efficiently converted as well, and a higher polyols yield
of 68.5% was achieved under the same reaction condition,
indicating that glucose as one of the reaction intermediates
was firstly generated from the depolymerization of microalgae
cells with sequential hydrolysis of the carbohydrate com-
ponents. Meanwhile, the ratio of 1,2-PDO to EG from glucose
was higher than that from microalgae under the same con-
ditions, indicating that the depolymerisation process of micro-
algae was the rate-determining step and acetol was the main
reaction intermediate from C–C cleavage of glucose via dehy-
dration/hydrogenation reaction. When levoglucosan was used
as the feedstock for hydrogenolysis, the main products were
1,2-PDO with a yield of 25.4%, as well as 13.0% of EG. In the
process, the transformation between glucose and levoglucosan
can be reversible, leading to the production of EG except for
1,2-PDO. To further confirm acetol as reaction intermediate,
acetol was directly used as the substrate and the catalytic con-
version of it at different reaction time was investigated. From
Table 2 it can be seen that half of acetol was transformed to
ethanol at 250 °C for 3 h, due to the excessive cleavage of C–C
bonds in the intermediates or products. Shortening the reac-
tion time, the formation of ethanol was inhibited and the yield
of 1,2-PDO increased. After reaction for only 1 h, the yield of
1,2-PDO production reached up to 70.6%, implying that acetol
was the main reaction intermediate and it can be selectively
hydrogenated into 1,2-PDO in a short time, while prolonging
the reaction time would just lead to excessive C–C breakage.
3.4. Influences of reaction conditions
Since the reaction temperature played an important role in the
conversion of microalgae via hydrolysis and hydrogenation
processes, the influence of reaction temperature on the main
product distribution was first investigated and the results are
shown in Fig. 5a. It can be seen that the yields of polyols
increased at a temperature of 140–250 °C. Further increasing
of the reaction temperature, decreased the yields of EG and
others-2 in contrast to the slight increase of EG and others-1.
Below 170 °C, the yields of the main products were quite low,
less than 4%. Above 170 °C, yields of EG and 1,2-PDO
increased rapidly and reached up to ca. 20% and 15% at
250 °C. At this temperature, the highest yield of polyols of
41.5% was obtained. The concentration of acetol was too low
to be detected in the conversion process of microalgae over the
Ni–MgO–ZnO catalyst, due to its rapid hydrogenation rate to
1,2-PDO. Since further increasing the reaction temperature to
260 °C led to the degradation of polyols, a reaction tempera-
ture of 250 °C was employed in the standard reaction con-
ditions hereafter.
Fig. 5b showed the polyols product distribution as a func-
tion of the reaction time at 250 °C. From the figure it can be
seen that yields of polyols changed slightly when the reaction
time was prolonged from 60 min to 240 min gradually. During
the first 120 min, the yields of 1,2-PDO and EG remained
almost constant, and then went up slightly until 180 min, and
finally kept steady during the last 60 min. During the whole
process the yields of products remained almost unchanged. In
Fig. 5 Product distributions for microalgae conversion over Ni–MgO–
■
●
□
represents
ZnO catalyst ( represents 1,2-PDO;
represents EG;
others-1, including isobutanol, isoamylol, cyclopentanol and 3-hexanol;
○
represents others-2, including 1,2-butanediol, 1,2-butyl glycol, 1,3-
▲
propanediol, 1,2-adipic alcohol;
represents the conversion.): depen-
dence on (a) reaction temperature at 6 MPa H2 and 180 min; (b) reaction
time at 250 °C and 6 MPa; and (c) initial H2 pressure at 250 °C and
180 min.
other words, the reaction time is not a significantly controlling
variable on the polyols products distribution. The influences
of initial H2 pressure on the product distribution in Fig. 5c
showed that a high initial H2 pressure was necessary for the
sufficient hydrogenolysis of microalgae in the hydrothermal
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conditions. The increase of the initial H2 pressure from 2 MPa
to 6 MPa enhanced the yields of the main products (1,2-PDO
and EG). The yields of the 1,2-PDO and EG depicted a rapid
growth in the range of 2–4 MPa and mild rise in the other
pressure range, whereas the yields of others-1 and others-2
changed slightly all the time.
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4. Conclusions
In summary, an interesting catalytic process to directly convert
aquatic microalgae into industrially attractive 1,2-PDO and EG 13 N. Ji, T. Zhang, M. Zheng, A. Wang, H. Wang, X. Wang,
in a one-pot procedure with high yields in water over Ni-based
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components as well as a smaller particle size and good dis-
persion of Ni on the MgO–ZnO supporter, and a 41.5% yield of 16 J. Pang, M. Zheng, A. Wang, R. Sun, H. Wang, Y. Jiang and
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a potential route for the production of valuable chemicals
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Acknowledgements
We acknowledge financial supports provided by the National
Natural Science Foundation of China (21406255 and
51208305).
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