One-step hydroprocessing of fatty acids into renewable aromatic hydrocarbons over Ni/HZSM-5: Insights into the major reaction pathways

Article abstract of DOI:10.1039/c6cp06327f

For high caloricity and stability in bio-aviation fuels, a certain content of aromatic hydrocarbons (AHCs, 8-25 wt%) is crucial. Fatty acids, obtained from waste or inedible oils, are a renewable and economic feedstock for AHC production. Considerable amounts of AHCs, up to 64.61 wt%, were produced through the one-step hydroprocessing of fatty acids over Ni/HZSM-5 catalysts. Hydrogenation, hydrocracking, and aromatization constituted the principal AHC formation processes. At a lower temperature, fatty acids were first hydrosaturated and then hydrodeoxygenated at metal sites to form long-chain hydrocarbons. Alternatively, the unsaturated fatty acids could be directly deoxygenated at acid sites without first being saturated. The long-chain hydrocarbons were cracked into gases such as ethane, propane, and C₆-C₈ olefins over the catalysts' Br?nsted acid sites; these underwent Diels-Alder reactions on the catalysts' Lewis acid sites to form AHCs. C₆-C₈ olefins were determined as critical intermediates for AHC formation. As the Ni content in the catalyst increased, the Br?nsted-acid site density was reduced due to coverage by the metal nanoparticles. Good performance was achieved with a loading of 10 wt% Ni, where the Ni nanoparticles exhibited a polyhedral morphology which exposed more active sites for aromatization.

Full text of DOI:10.1039/c6cp06327f

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One-step hydroprocessing of fatty acids into renewable aromatic
hydrocarbons over Ni/HZSM-5: Insight into the major reaction
pathways
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Shiyou Xinga,b,c,d, Pengmei Lva,b,d,e*, Jiayan Wanga,f, Junying Fua,b,d, Pei Fana,b,c,d, Lingmei
Yanga,b,d, Gaixiu Yanga,b,d, Zhenhong Yuana,b,d,e*, Yong Chena,b,d,
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aGuangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China.
bKey Laboratory of Renewable Energy, Chinese Academy of Sciences, Guangzhou 510640, China.
cUniversity of Chinese Academy of Sciences, Beijing 100049, China.
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dGuangdong Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640,
China.
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eCollaborative Innovation Center of Biomass Energy, Henan Province, Zhengzhou 450002, China.
fNano Science and Technology Institute, University of Science & Technology of China, Suzhou 215123, China.
*Corresponding author.
Eꢀmail address: lvpm@ms.giec.ac.cn (P. M. Lv), yuanzh@ms.giec.ac.cn (Z. H. Yuan).
Abstract. For high caloricity and stability in bioꢀaviation fuels, a certain content of aromatic
hydrocarbons (AHCs, 8–25 wt%) is crucial. Fatty acids, obtained from waste or inedible oils, are a
renewable and economic feedstock for AHC production. Considerable amounts of AHCs, up to
64.61 wt%, were produced through the oneꢀstep hydroprocessing of fatty acids over Ni/HZSMꢀ5
catalysts. Hydrogenation, hydrocracking, and aromatization constituted the principal AHC
formation processes. At a lower temperature, fatty acids were first hydrosaturated and then
hydrodeoxygenated at metal sites to form longꢀchain hydrocarbons. Alternatively, the unsaturated
fatty acids could be directly deoxygenated at acid sites without first being saturated. The
longꢀchain hydrocarbons were cracked into gases such as ethane, propane, and C6–C8 olefins over
the catalysts’ Brønsted acid sites; these underwent DielsꢀAlder reactions on the catalysts’ Lewis
acid sites to form the AHCs. The C6–C8 olefins were determined as critical intermediates for AHC
formation. As the Ni content in the catalyst increased, the Brønstedꢀacid site density was reduced
due to coverage by the metal nanoparticles. Good performance was achieved with a loading of 10
wt% Ni, where the Ni nanoparticles exhibited a polyhedral morphology which exposed more
active sites for aromatization.
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1 Introduction
The rapid, steady consumption of commercial transportation fuels such as aviation fuel,
1
commonly produced by petroleum refining , has placed substantial burdens on the natural
environment through the excessive emission of greenhouse gases. The development of aviation
fuels derived from renewable and sustainable feedstocks has shown great potential because of
2
their ability to reduce the total carbon footprint by ~80% over their full lifeꢀcycle , and has
attracted increasing interest in recent years 3, 4. Specifically designed for use in aircraft, aviation
fuels must meet stringent specifications such as high caloricity and low boiling points for safe,
highꢀaltitude flight. Aromatic hydrocarbons (AHCs) are indispensable components of aviation
fuels because of their higher densities and volumetric heating values, in contrast to straightꢀchain
and isoꢀalkanes. More importantly, the presence of AHCs is necessary to ensure the shrinkage of
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aged elastomer seals and prevent the leakage of aviation fuels . Therefore, for safe application,
current aviation fuels must contain 8–25% AHCs.
Because of their plentiful supply, ready accessibility, and low cost, biomassꢀderived feedstocks
for AHC synthesis are primarily composed of raw lignocellulosic materials such as cellulose,
hemicellulose, and lignin, and lipidic materials such as lipids and fatty acids 6ꢀ9. Several
researchers have investigated the synthesis of AHCs from biomass derivatives such as furans (e.g.,
2,5ꢀdimethylfuran (DMF) or 2ꢀmethylfuran (MF)) and ethylene or propylene via DielsꢀAlder
cycloadditions and subsequent dehydration reactions 10ꢀ14. For example, toluene, a typical AHC,
could be synthesized through two principal steps: the DielsꢀAlder cycloaddition between MF and
ethylene, and the dehydrogenation of the cyclic intermediate to produce toluene 15. Unfortunately,
several side reactions also occurred during the DielsꢀAlder cycloaddition of MF and ethylene,
which made it difficult to achieve a high AHC yield and selectivity 10. Similarly, pꢀxylene could be
11,
successfully synthesized from DMF and ethylene
16. However, for the conversion of
lignocellulose into AHCs, several pretreatment steps are needed, including the hydrolysis of
hemicellulose to xylose; the dehydration of xylose to furfural; and the hydrodeoxygenation of
furfural to 2ꢀmethylfuran.
Compared to lignocellulosic biomass, fatty acids require less pretreatment. Lipidic feedstocks
are abundantly available in waste or inedible oils, and are significantly renewable and sustainable.
More importantly, fatty acids share structurally similar characteristics with petroleum substrates
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and should undergo hydroprocessing without additional pretreatments. Many studies have
extensively addressed the hydroprocessing of waste oils or fatty acids into renewable fuelꢀlike
hydrocarbons, but the products are mainly either isomerized C8–C16 alkanes, or dieselꢀlike
hydrocarbons, such as longꢀchain C17 and C18 alkanes, rather than AHCs 17ꢀ20. In our group, other
researchers have conducted the study of hydroꢀconverting fatty acid methyl esters into renewable
alkane fuels with considerable isomerized ratios in the presence of solvent hexane and the
conversion routines were investigated 21, 22. The main transformation routines included (i) the
hydrosaturation of unsaturated feedstocks, (ii) the hydrodeoxygenation of the saturated oils or
fatty acids into longꢀchain alkanes via dehydration, decarbonylation, and decarboxylation
reactions, and (iii) the partial hydrocracking and isomerization of longꢀchain alkanes into products
with the required carbon numbers and properties 9, 23. Due to the challenge of obtaining highly
isomerized products in the third step to improve lowꢀtemperature fluidity, a number of studies
have concentrated on tuning the fatty acid hydroconversion process by employing advanced
metalꢀsupported acidic zeolite catalysts 21, 23, 24
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However, these synthesized alkanes have not been able to meet the application specifications
and must be blended with conventional aviation fuel 25 due to the absence of AHCs. Some recent
works have reported the production of AHCs in the hydroprocessing of waste oils and fatty acids.
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Li T. et al. investigated the hydroprocessing of waste oil into jet biofuel using three types of
zeolites (mesoꢀY, SAPOꢀ34, and HY) loaded with nickel. Their results indicated that mesoꢀY
exhibited a high jetꢀrange alkane selectivity of 53% and an AHC selectivity of 13.4% in the liquid
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fuel products. Rabaev M. et al.
found that a certain level of polyunsaturated fatty acids is
required to produce relatively high aromatics contents. Despite having structural similarity with
petroleumꢀbased substrates, the hydroprocessing of fatty acids is considered to be quite different
from current petroleum refinery methods. Conventional methods for AHC synthesis employ the
catalytic reforming of oxygenꢀfree naphtha into AHCꢀenriched reformate. However, a new
catalytic reaction system is required for the hydroprocessing of these oxygenated lipidic
feedstocks, because more complex reactions (e.g., hydrodeoxygenation) are involved. By
inference, hydrodeoxygenation should play an important role throughout the whole transformation
process, but that conclusion remains unsubstantiated at present. Hence, it is of prime importance
to clarify the conversion routes and catalytic mechanisms for AHC formation during the
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hydroprocessing of fatty acids.
In this work, under solventꢀfree conditions, we employed the Niꢀimpregnated HZSMꢀ5 catalyst
for oneꢀstep hydroprocessing of fatty acids and successfully obtained considerable amounts of
AHCs. The main catalytic reaction pathways were analyzed in detail on the basis of detected
intermediates. The catalytic roles of the loaded Ni metal and catalyst acid sites were
wellꢀdetermined combining the catalyst characterization through scanning electron microscopy
(SEM), transmission electron microscopy (TEM), and pyridineꢀadsorbed infrared spectroscopy
(PyꢀIR), etc. The overall catalytic conversion routes for AHC formation were proposed and
discussed. The advantage of this work was that it provided a new facile, effective approach for the
mass production of renewable AHCs. Further, the major reaction pathways for AHCs formation
were first proposed and the catalytic mechanisms were investigated into deep as well.
2 Materials and methods
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2.1 Materials
The fatty acids feedstock was purchased from Tianjin Damao Chemical Reagent Factory, China,
and its specific composition is shown in Table 1. Nickel nitrate hexahydrate [Ni(NO3)2·6H2O]
(98%, Reagent Plus) was purchased from Tianjin Fuchen Chemical Factory. The HZSMꢀ5
supports with Si/Al ratios of 25, 80, 120, 150, and 200 were obtained from Nankai Catalysts
Industry, China. Cyclohexane (AR) was purchased from Aladdin, China. All materials were used
as received.
Table 1. Feedstock composition
Fatty acid
Palmitic acid
Stearic acid
Fatty acid structure
C16:0
Formula
C16H32O2
C18H36O2
C18H34O2
C18H32O2
C18H30O2
C20H38O2
Composition (wt%)
8.36
3.59
C18:0
Oleic acid
C18:1
67.20
13.67
2.87
Linoleic acid
Linolenic acid
Eicosenoic acid
C18:2
C18:3
C20:1
4.31
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2.2 Catalyst preparation and characterization
Ni/HZSMꢀ5 catalysts with different wt% nickel contents (5 wt%, 10 wt%, 12.5 wt% and 15
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wt%) were prepared by the impregnation of the HZSMꢀ5 supports in aqueous solutions of
Ni(NO3)2•6H2O. The Si/Al ratios of used HZSMꢀ5 were 25, 80, 120, 150 and 200. The mixture
was stirred at 150 rpm for 6 h at ambient temperature, dried at 110°C for 12 h, and calcined at a
rate of 10°C/min from ambient temperature to 550°C and maintained for 8 h. After reduction
under H2 atmosphere at 500°C for 2 h, Ni/HZSMꢀ5 was finally obtained. For comparison, a Ni
metal catalyst was prepared by the calcination of Ni(NO3)2•6H2O followed by H2 reduction under
similar conditions. XRD measurements were carried out using an X’Pert PRO MPD
diffractometer (PANalytical) operated at 40 kV and 40 mA with Cu Kα (λ = 0.15406 nm) radiation.
SEM was performed on a Hitachi Sꢀ4800 instrument operated at 10 kV. Before the SEM
experiment, samples were placed under an E1010 ionꢀsputtering instrument to improve
conductivity. TEM studies were carried out on a Hitachi Hꢀ7560 electron microscope with an
accelerating voltage of 100 kV. An Xꢀray fluorescence spectrometer (XRF, model: AxiosmAX
Petro, PANalytical B.V.) was used to identify the Ni contents of the prepared catalysts. The type of
acid sites on the catalysts was determined by recording infrared spectra of the substrates after the
adsorption of pyridine (PyꢀIR) using a Nicolet 6700 spectrometer.
2.3 Hydroprocessing of fatty acids and product analysis
The oneꢀstep hydroprocessing of fatty acids was conducted in a highꢀpressure autoclave with a
volume of 150 mL as shown in Figure S1. About 10 g feedstock and 0.6 g catalyst were added into
the reactor. After charging and discharging with hydrogen (99.999%) several times to remove
oxygen, the reaction system was pressurized to 4MPa with hydrogen. The H2/oil mol ratio was
calculated as 7.13. After 8 h reaction at a designated temperature (280℃, 320℃, 340℃, 360℃, and
380℃) with agitator stirring speed of 200 rpm, the reactor was immediately removed and cooled
with ambientꢀtemperature water. The reactor pressure during the hydroconversion process was
monitored with a paperless recorder (Model VX5304R, Hangzhou Pangu Ltd., China). Gaseous
products were collected into a gas sampling bag, and the liquid products were sampled after
filtration through a 0.22 µm organic nylon 66 membrane (Tianjing Jinteng Ltd., China).
The gaseous products (CH4, CO, CO2, and C2–C6 hydrocarbons) were quantified with an
Agilent 7890A gas chromatograph (GC) equipped with a capillary column (GSꢀGASPRO, 60 m ×
0.32 mm), a thermal conductivity detector (TCD), and a flameꢀionization detector (FID). The
composition of the liquid products was analyzed with a GCꢀmass spectrometry instrument
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(GC/MS, Agilent 7890Aꢀ5975C) equipped with an Agilent HPꢀ5MS column (5% phenyl methyl
silox, 30 m × 0.25 mm × 0.25 µm). Before being tested, the liquid products were diluted with
cyclohexane at a ratio of 1:100. The GCꢀMS test parameters were as follows. The diluted sample
(1 µL) was injected with a split ratio of 1:10 at 280°C. The initial column temperature of 60°C
was increased at 10°C/min to 300°C over 28.67 min. The major chemical constituents were
identified using the NIST 2014 Mass Spectral Library. The FID response was linear for each
component over the concentration ranges used, and the content analyses of the products generated
are summarized in terms of peak area %.
3 Results and discussion
3.1 Products distribution
The overall products distribution of the hydroprocessed fatty acids as a function of reaction time
is shown in Figure 1, and the yields of the produced AHCs are listed in Table 2. GC/MS analysis
of the liquid products shows that a large number of AHCs is produced, including toluene, oꢀxylene,
pꢀxylene, and propylbenzene (Figure 1(a)). Toluene, oꢀxylene, and 1ꢀethylꢀ2ꢀmethylbenzene are
the main AHC components. The total AHC content increases with reaction time, reaching 64.61
wt% after 12 h, as shown in Table 3. Other products such as alkanes, olefins, and cyclic
hydrocarbons are also formed, and their yields are shown in Table 3. After 2 h, the yields of
alkanes, olefins, cyclic hydrocarbons, and AHCs are 55.57, 7.48, 8.50, and 19.00 wt%,
respectively. With increasing reaction time, the alkane yield sharply decreases while that of the
AHCs greatly increases, suggesting that the AHCs are formed mainly from the transformation of
alkanes. Gases such as CO, CO2, CH4, C2H6, C3H8, and C3H6 are increasingly produced over time,
as illustrated in Figure 1(b), during the oneꢀstep hydroprocessing of the fatty acids.
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(b)
C3H6
C3H8
C2H6
CH4
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CO2
CO
3
15
2
10
1
5
0
0
2h
4h
8h
12h
Reaction time
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Figure 1. GC spectra for liquid products (a) and gas products distributions (b). The compounds in
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(a) were as follows: (1) toluene, (2) ethylbenzene, (3) oꢀxylene, (4) pꢀxylene, (5) propylbenzene,
(6) 1ꢀethylꢀ2ꢀmethylꢀbenzene. Reaction conditions: 360°C, 4 MPa H2, 10 g fatty acids, 0.6 g 10 wt%
Ni/HZSMꢀ5.
Table 2. AHC distribution as a function of reaction time (wt%)a
AHC
products
2 h
Propyl
benzene
0
1ꢀEthylꢀ2ꢀ
methylbenzene
3.09
Toluene
Ethylbenzene ꢀꢀXylene pꢀXylene
Otherb
6.53
9.90
2.20
3.07
4.51
5.38
5.12
7.10
0.75
0.77
2.84
2.59
1.31
4.63
9.85
4 h
0.87
1.48
1.47
4.31
8 h
15.61
16.87
12.17
13.81
7.50
12 h
9.18
15.31
b
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aReaction conditions: 10 g fatty acids, 0.6 g 10 wt% Ni/HZSMꢀ5, 360°C, 4 MPa H2. Other
products represent those containing one benzene ring and several methyl branches, as shown in
Figure S2.
Table 3. Liquid products distribution as a function of reaction time (wt%)a
Products
2 h
Alkanes
57.57
39.94
28.18
14.07
Olefins
7.48
Cyclic hydrocarbons
AHCs
19.00
30.65
53.96
64.61
Fatty acids
Otherb
6.55
4.30
0
6.50
5.21
5.06
4.59
5.16
1.26
0
4 h
14.98
7.33
8 h
12 h
4.33
0
0
b
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aReaction conditions: 10 g fatty acids, 0.6 g 10 wt% Ni/HZSMꢀ5, 360°C, 4 MPa H2. Other
products represent oxygenꢀcontaining compounds such as octadecanol and octadecanal.
3.2 Major reaction pathways in the one-step hydroprocessing of fatty acids
3.2.1 Hydrodeoxygenation
A variety of complex reactions, including hydrodeoxygenation, hydrocracking, and
aromatization, occur during the oneꢀstep hydroprocessing of fatty acids for AHC formation.
GC/MS spectra showing the intermediate product distributions during the heating period are
presented in Figure 2. Note that the fatty acids feedstock consists mostly of monounsaturated oleic
acid and polyunsaturated linoleic acid. Clearly, the yield of stearic acid increases as the reaction
temperature is elevated from 200 to 260°C. This reveals that unsaturated acids such as oleic acid
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undergo hydrosaturation to form stearic acid, as shown by reaction ꢀ in Figure 3. Jovanovic
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and Gabrovska employed various nickel catalysts for the hydrogenation of unsaturated oils to
produce vegetable tallows and fats in the temperature range of 145–190°C. They reported high
selectivity in the hydrogenation of C18:3 (linolenic) into C18:2 (linoleic), C18:2 (linoleic) into
C18:1 (oleic), and C18:1 (oleic) into C18:0 (stearic), respectively, but did not observe stearic acid
as a major product. In this study, the further increase in the reaction temperature apparently
promotes the hydrogenation of all the unsaturated C18 acids to stearic acid 30, 31
.
Stearic acid
Oleic acid
8-Heptadecene
Palmitic acid
340oC
320oC
300oC
280oC
260oC
240oC
200oC
13
14
15
16
17
18
19
20
21
Retention time (min)
191
192
Figure 2. GC/MS spectra obtained after heating at various temperatures. Reaction conditions: 10 g
fatty acids, 0.6 g 10 wt% Ni/HZSMꢀ5 (Si/Al = 25), 4 MPa H2, 10°C/min heating rate.
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Figure 3. Pathways for the hydrosaturation and hydrodeoxygenation of fatty acids, using oleic acid
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as an example and including reactions ꢀ–ꢁ.
Subsequently, the saturated stearic acid is converted into longꢀchain hydrocarbons by
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dehydration, decarbonylation, and decarboxylation reactions. Figure 4 shows the intermediate
products distribution after the fatty acids are hydroprocessed for various reaction times at 320°C.
Stearic acid is mostly transformed with increasing reaction time. Octadecane (C18H38) is found
among the liquid products, suggesting the dehydration of stearic acid via reaction ꢂ in Figure 3.
The formation of heptadecane, CO, and CO2 products (Figure 1(b)) indicates the dehydration,
decarbonylation, and decarboxylation reactions of stearic acid, as depicted in reaction ꢃ of Figure
3. Pentadecane (C15H34) is produced from palmitic acid via reaction ꢃ. The partial enlargement of
Figure 4(a) shows the presence of 8ꢀheptadecene after 1 and 2 h reaction, suggesting that oleic
acid can be directly deoxygenated without first being saturated, via reaction ꢃ. After 4 h, the
8ꢀheptadecene is nearly completely consumed, most likely via hydrogenation to heptadecane
(reaction ꢁ). The relative percentages of C15 and C17 hydrocarbons after 1 h and 12 h reaction are
both higher than those of C16 and C18 hydrocarbons, as shown in Figure 4(b), which suggested that
the decarbonylation and decarboxylation reactions proceeded preferentially than the dehydration
reaction during the whole process 32
.
C15H32
(a)
C18H38
C17H36
Stearic acid
12h
4h
8h
8-Heptadecene
2h
6h
4h
2h
1h
1h
13.5
13.8
4
6
12
14
16
18
20
Retention Time (min)
212
(b)
1 h
12 h
80
60
40
20
0
15
16
17
18
Carbon number
213
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Figure 4. (a) GC/MS spectra of fatty acid products after hydroprocessing for various reaction
times. (b) Relative percentages of C15–C18 hydrocarbons after 1 h and 12 h reaction. Reaction
conditions: 10 g fatty acids, 0.6 g 10 wt% Ni/HZSMꢀ5 (Si/Al = 25), 320°C, 4 MPa H2.
3.2.2 Hydrocracking
Longꢀchain hydrocarbons produced in the hydrodeoxygenation stage can be further cracked into
C6–C8 olefins and gases such as CH4, C2H6, and C3H8 through a series of hydrocracking reactions.
Figure 5(a) shows the results after 8 h at temperatures of 280–380°C. The conversion ratio is
36.90 wt% at 280°C, and reaches 100 wt% at 340°C, demonstrating the significance of the
reaction temperature for the high conversion of fatty acids. The yield of alkanes is 15.90 wt% at
280°C, increases to a maximum value of 52.08 wt% at 340°C, and sharply drops to 25.18 wt% at
360°C. Although only minor amounts of olefins are produced at 320°C, this fraction increases to
9.5 wt% at 340°C and remains stable even at 380°C. The yields of cracked lowꢀmolecularꢀweight
gases increase at higher reaction temperatures, as shown in Figure 5(b). C2H6, and C3H8 are
mainly produced by thermal cracking and catalytic cracking processes 33ꢀ35, respectively. However,
the yield of C3H8 is about three times higher than that of C2H6, suggesting that catalytic
hydrocracking is the main cracking process.
Figure 5(c) shows the hydroprocessing results with various reaction times at 360°C. The fatty
acids are mostly converted during the initial 2 h reaction period. At this stage, the alkanes yield is
the highest, up to 55.57 wt%, and the olefins yield is low (7.48 wt%). After 4 h, the yield of
alkanes decreases to 39.94 wt%, while the amount of olefins increases to 14.98 wt%. Details of
the alkane and shortꢀchain olefin (C6–C8) structures are shown in Table 4. Straightꢀchain alkanes
with different carbon numbers are present in the liquid products, and a variety of C6–C8 olefins
such as 2ꢀmethylꢀ1ꢀpentene, (Z)ꢀ3ꢀmethylꢀ2ꢀpentene, and 2,4ꢀdimethylꢀ1ꢀpentene are produced.
36, 37
Katikaneni and coꢀworkers
conducted the catalytic conversion of canola oil over
potassiumꢀimpregnated HZSMꢀ5, and obtained shortꢀchain olefins. They believed that the
amorphous, nonꢀshapeꢀselective characteristics of the catalyst and severe thermal effects at
reaction temperatures of 400–500°C enhanced the subsequent secondary cracking of longꢀchain
hydrocarbons into lighter molecules.
For the alkane cracking process, two mechanisms have been previously proposed: a
monomolecular mechanism and a bimolecular mechanism. In the former, an acidic catalyst such
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as a zeolite protonates an alkane to form a fiveꢀcoordinateꢀcarbon transition state 38, 39. This
carbonium ion may undergo cracking to produce an alkane and an alkene, regenerating the catalyst
by the loss of protons 40. The cracking products include dihydrogen, methane, and ethane. In the
bimolecular mechanism, an alkane is activated by hydride transfer between the alkane and an
adsorbed alkoxide species 18, 41. This reaction may be followed by βꢀscission or isomerization. Due
to dimerization, oligomerization, isomerization, and βꢀscission reactions, the resulting product
distribution is complicated. The sophisticated cracking products obtained in this study, which
include methane, ethane, propane, and C6–C8 olefins, suggest that both these mechanisms are
operative in the transformation of the longꢀchain alkanes. As the reaction time increases, the yields
of longꢀchain alkanes decrease, indicating that hydrocracking reactions proceed concurrently.
Moreover, the olefin yields also decrease after longer reaction times, suggesting that the produced
C6–C8 olefins are not accumulated, but rather, are quickly converted.
4
100
80
60
40
20
0
C3H6
C3H8
C2H6
CH4
CO2
CO
(a)80
Long chain alkanes
C6-C8 Olefins
AHCs
(b)
20
3
2
1
0
60
40
20
0
16
12
8
4
0
280oC
360oC
320oC
Reaction temperature
340oC
380oC
280oC
360oC
380oC
320oC
340oC
Reaction temperature
256
100
(c)80
Long chain alkanes
C6-C8 Olefins
AHCs
80
60
40
20
60
40
20
0
0
2h
4h
8h
12h
Reaction time
257
258
Figure 5. Conversion ratios and yields of longꢀchain alkanes, C6–C8 olefins, and AHCs as a
function of reaction temperature (a) and time (c). Gas yields at various reaction temperatures (b).
Reaction conditions: (a) and (b) 10 g fatty acids, 0.6 g 10 wt% Ni/HZSMꢀ5 (Si/Al = 25), 4 MPa
H2, 8 h, and (c) 10 g fatty acids, 0.6 g 10 wt% Ni/HZSMꢀ5 (Si/Al = 25), 360°C, 4 MPa H2.
259
260
261
262
263
Table 4. Structures of alkanes, shortꢀchain olefins (C6–C8), and cyclic hydrocarbons after 4 h at
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360°C
Retention
time(min)
Retention
Compound name
Structure
Compound name
Structure
time(min)
Alkanes
1.34
Butane
3ꢀMethylhexane
Heptane
9.19
Dodecane
Pentadecane
Hexadecane
Heptadecane
Octadecane
5
6
1.97
2.1
13.26
15.01
15.61
16.70
7
7
2.67
4.42
2ꢀMethylheptane
Decane
4
8
Short-chain olefins (C6–C8)
(E)ꢀ4ꢀMethylꢀ
1.62
2ꢀMethylꢀ1ꢀpentene
2.16
2.20
2ꢀhexene
1.67
(Z)ꢀ3ꢀMethylꢀ2ꢀpentene
3ꢀEthylꢀ2ꢀpentene
3,4ꢀDimethylꢀ
1.72
3ꢀMethylꢀ2ꢀpentene
2.23
(Z)ꢀ2ꢀpentene
2,4ꢀDimethylꢀ
1.77
1.87
2.03
2ꢀMethylꢀ2ꢀpentene
(Z),(Z)ꢀ2,4ꢀHexadiene
2,4ꢀDimethylꢀ1ꢀpentene
2.48
2.58
3.03
3ꢀhexene (c,t)
5ꢀMethylꢀ1,4ꢀhexadiene
(E)ꢀ4ꢀOctene
Cyclic hydrocarbons
2.12
1.5ꢀDimethylcyclopentene
2.54
2.84
1,2ꢀDimethylcyclohexene
1,3ꢀDimethylcyclohexane
2.24
3,5ꢀDimethylcyclopentene
1ꢀMethylcyclohexene
2.30
2.41
3.06
3.19
1,3ꢀDimethylꢀ1ꢀcyclohexene
1,2,3ꢀTrimethyl
cyclopentene
1ꢀEthylcyclopentene
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3.2.3 Aromatization
The produced C6–C8 olefins and small alkane molecules can cyclize and, by means of
aromatization, be converted into various types of AHCs. As shown in Figure 5(a), more AHCs are
produced at higher reaction temperatures, whereas the olefin yields remain stable at 9.5 wt%
without accumulation. Figure 5(c) shows a decreasing yield of the C6–C8 olefins with reaction
time, which suggests that these compounds are crucial intermediates for AHC formation. A variety
of cyclic hydrocarbons, amounting to ~5 wt%, were identified in the liquid product mixture, as
shown in Table 4. The cyclic hydrocarbons are mainly fiveꢀ and sixꢀmembered ring compounds
such as 1,5ꢀdimethylcyclopentene and 1ꢀmethylcyclohexene, respectively.
At a rather high temperature (380°C), substantial changes in the reaction pressure are observed
(see Figure S3). The distributions of the gaseous products CH4, C2H6, and C3H8 obtained from
alkane hydrocracking in this process are shown in Figure 6. The slope (k) of each line segment in
Figure 6 signifies the change in a product’s formation rate. Higher k values in Table 5 indicate that
more alkanes are cracked into gases, and vice versa. For better comparison, it was assumed that
the increased yield of every type of produced gas was proportional to the decreased amount of the
total alkanes. To characterize the relative changes, parameter n (defined as the absolute value of
the ratio of k (produced gas) vs. k (alkanes)) was used to eliminate the effect of the decreasing
alkane content on the equilibrium of the cracking reactions. The lower n value (see Table 5) from
n2ꢀ4 to n4ꢀ8 for C3H8 signifies that C3H8 is consumed during the 4–8 h reaction time. In comparison,
the higher n values of CH4 and C2H6 suggest that these products are generated via other routes in
addition to alkane hydrocracking. H2 is consumed in the hydrodeoxygenation and alkane
hydrocracking processes. The n value for H2 is lower during the 4–8 h reaction period compared
to 2–4 h, suggesting that there are additional sources of H2 production in this process. Here, it is
considered that the consumed C3H8 is transformed to AHCs, and in this process, CH4, C2H6, and
H2 are produced simultaneously.
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H2
CH4
C2H6
C3H8
Long chain alkanes
80
60
40
20
0
80
60
40
20
0
2
4
6
8
Reaction time(h)
291
292
Figure 6. Gas and alkane percentages after different reaction times at 380°C.
Table 5. Changes in product formation rate over time.
293
k
Alkanes
CH4
C2H6
C3H8
H2
a
k2ꢀ4
k4ꢀ8
n2ꢀ4
n4ꢀ8
−8.09
1.23
0.78
0.15
0.19
1.65
0.86
0.20
0.21
4.60
1.82
0.57
0.45
−8.62
−3.98
1.07
a
−4.08
b
b
ꢀ
ꢀ
0.98
294
295
a k was the slope value of line segment 2ꢀ4 and 4ꢀ8
b
n was defined as the absolute value of the ratio of k (produced gas) vs. k (alkanes). For
(ꢉꢊꢋꢇ
)
ꢌꢍ
ꢕ.ꢂꢖ
ꢇ
296
instance, (ꢁꢂꢃꢄ
)
ꢅꢆꢇ
= ꢈ(ꢉ
ꢈ = ꢈ
ꢈ = 0.15.
ꢃꢗ.ꢘꢙ
)
ꢊꢋꢇ ꢎꢏꢐꢑꢒꢓꢔ
297
298
299
300
301
302
303
304
305
306
In recent publications, aromatics were proposed to be formed by the DielsꢀAlder reactions of
43
olefins 10, 42. Depeyre et al. examined the steam cracking of nꢀhexadecane at 750–850°C in a
quartz flow reactor, with the aim of maximizing alkene production. They found that aromatics
were increasingly formed at the expense of liquid olefin compounds with increasing temperature
13
and reaction time. Cheng and Huber studied the coꢀfeeding of olefins (ethylene and propene)
with a series of furanoid compounds over a ZSMꢀ5 catalyst. They found that coꢀfeeding propene
with furan (a C4 diene) and 2ꢀmethylfuran (a C5 diene), respectively, increased the selectivity for
toluene and xylenes, which clearly suggested DielsꢀAlder cycloaddition reactions between the
44
olefins and furans. Similar cases were also reported by Williams and Nikbin 14. Ethylene has
11
also been frequently used as a substrate for aromatics production with biomassꢀderived furans
.
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As shown in Figure 6, many small alkanes such as ethane and propane are produced in this
process, and they can be directly dehydrogenated into ethylene and propene, respectively. In
addition, various types of C6–C8 olefins are produced. It is therefore suggested that DielsꢀAlder
reactions between these olefins can account for the produced AHCs.
Four pathways are presented in Figure 7 to illustrate the possible reactions involved in AHC
formation. First, a molecule of propane is dehydrogenated to propene, which can be combined
with a C6 olefin such as 3ꢀmethylꢀ2ꢀpentene to form a C7 molecule with a fiveꢀmembered ring.
Through ring expansion 45, 46, the fiveꢀmembered carbocycle is reformed to a sixꢀmembered ring.
Subsequently, after the removal of hydrogen, this is transformed into toluene, as shown in
pathway (1). In the second case,
a
sixꢀmembered hydrocarbon ring such as
1ꢀethylꢀ6ꢀmethylcyclohexene is directly formed through cyclization, as shown in pathway (2).
Dehydrogenation then affords 1ꢀethylꢀ6ꢀmethylbenzene. Alternatively, as shown in pathway (3),
the sixꢀmembered ring hydrocarbon, after demethanization, can be converted into
1,6ꢀdimethylcyclohexene, which forms oꢀxylene after dehydrogenation. C6–C8 olefins such as
5ꢀmethylꢀ1,4ꢀhexadiene, through selfꢀcyclization 47, can form fiveꢀ or sixꢀmembered ring
hydrocarbons, as shown in pathway (4). A small number of higherꢀmolecularꢀweight AHCs 48 with
9 or more carbon atoms (e.g., 1,2,4ꢀtrimethylbenzene, 1ꢀethylꢀ2,4ꢀdimethylbenzene, and
1ꢀmethylꢀ3ꢀpropylbenzene, as shown in the Figure S2) may be formed through direct cyclization
and dehydrogenation reactions between two C6–C8 olefins.
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337
338
339
340
341
342
343
344
Figure 7. Possible pathways for AHC formation.
3.3 Catalysis mechanisms
According to the foregoing results, conversion routes for the oneꢀstep hydroprocessing of fatty
19, 20, 31, 49, 50
acids are proposed in Figure 8; these mainly consist of hydrodeoxygenation
,
hydrocracking, and aromatization processes. The first step is the hydrodeoxygenation of the fatty
acids into longꢀchain hydrocarbons, including the hydrosaturation of oleic and linoleic acids into
stearic acid at a low reaction temperature 28, 29. Subsequently, the longꢀchain hydrocarbon products
are transformed into shortꢀchain compounds by thermal and catalytic hydrocracking reactions.
These cracking products include: the gases methane, ethane, and propane; abundant C6–C8 olefins
such as 2ꢀmethylꢀ1ꢀpentene, (Z)ꢀ3ꢀmethylꢀ2ꢀpentene, and (Z),(Z)ꢀ2,4ꢀhexadiene (with two C=C
bonds); a few types of shortꢀchain alkanes such as butane, 3ꢀmethylhexane, and heptane; and
longꢀchain alkanes, including pentadecane. After the C6–C8 olefins and propene undergo
DielsꢀAlder reactions, cyclic hydrocarbons with fiveꢀ or sixꢀmembered rings containing C=C
bond(s) are formed. The fiveꢀmembered hydrocarbon rings can be converted into sixꢀmembered
hydrocarbon rings through ringꢀexpansion reactions. Eventually, these sixꢀmembered ring
hydrocarbons are transformed into AHCs by dehydrogenation.
When pure Ni metal (see XRD patterns in Figure S4) was used as the catalyst for the oneꢀstep
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hydroprocessing of the fatty acids, the vast majority of the products (~93 wt%) consisted of
longꢀchain alkanes (Figure 9). Thus, Ni metal perfectly facilitates the hydrodeoxygenation
reactions but has almost no effect on the hydrocracking of longꢀchain alkanes. In contrast, as little
as 25.18 wt% alkanes remain when using the 10 wt% Ni/HZSMꢀ5 catalyst, indicating the
enhanced hydrocracking with HZSMꢀ5. This zeolite has abundant nanoꢀholes with size of 0.55 nm
22, which are much smaller than the size of the longꢀchain alkane molecules. This suggests that the
surface properties of the catalyst are quite critical for the hydroconversion process, particularly the
acid sites 51
.
353
354
Figure 8. Proposed conversion routes for AHC formation in the oneꢀstep hydroprocessing of fatty
acids.
355
356
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358
Figure 9. GC/MS spectra of products from the hydroprocessing of fatty acids using Ni metal
catalyst. Reaction conditions: 10 g fatty acids, 0.6 g Ni metal, 360°C, 4 MPa H2, 8h.
359
360
361
362
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365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
The Si/Al ratios of the catalyst have a significant impact on the catalytic performance and the
product distribution. It is remarkable that, with increasing Si/Al ratios, the yield of longꢀchain
alkanes begins to increase and they become the major products, affording as much as 85.04 wt%
at Si/Al = 200 (Figure 10(a)). This suggests the inhibition of the hydrocracking of the longꢀchain
hydrocarbons. Note that the olefin yield increases to 15.36 wt% at Si/Al = 120 compared to Si/Al
= 80, but then decreases to 5.61 wt% at Si/Al = 200. It was reported that the cracking and
dehydrogenation of alkanes to form olefins take place at the Brønsted acid sites 48. The PyꢀIR
results in Table 6 show the highest density of Brønsted acid sites (0.12 mmol/g) for Si/Al = 25, but
this decreases sharply to 0.01 mmol/g at Si/Al = 200. Obviously, the decrease in the density of
Brønsted acid sites results in fewer longꢀchain alkanes being cracked. Moreover, the increase in
the recovery of C6–C8 olefins in the Si/Al range of 25–120 signifies the high cracking degree of
longꢀchain alkanes when the density of Brønsted acid sites exceeds 0.06 mmol/g. Thus, a high
number of Brønsted acid sites in the catalyst can effectively facilitate the hydrocracking of
longꢀchain alkanes.
According to previous investigations, the DielsꢀAlder reaction relies, to a great extent, on the
Lewis acidity of the catalyst 52ꢀ54. Recently, it was shown that the Lewisꢀacidic catalyst plays a
significant role in the production of renewable aromatics by the tandem DielsꢀAlder cycloaddition
and dehydration reactions of biomassꢀderived dimethylfuran and ethylene 12, 15, 55. In those studies,
the high selectivity for toluene by the Lewisꢀacidic catalysts was mainly attributed to their ability
to accelerate the cycloaddition of dimethylfuran and ethylene and retard side reactions such as
oligomerization and alkylation. In the case of Brønstedꢀacidic zeolites, the side reactions were
significantly enhanced, resulting in lower selectivity for AHCs. As shown in Table 6, the highest
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398
yield of AHCs, up to 53.96 wt%, is obtained at Si/Al = 25, at which the catalyst has a high Lewis
acid content of 0.06 mmol/g. However, the yield decreases quickly to 4.39 wt% at Si/Al = 200,
with only 0.02 mmol/g Lewis acid sites, suggesting the restriction of the DielsꢀAlder reactions
necessary for AHC formation.
It is noteworthy that the yield of C6–C8 olefins over catalysts with different Si/Al ratios is
determined by the degrees of both the hydrocracking of longꢀchain alkanes at the Brønsted acid
sites and the DielsꢀAlder reactions for AHC formation at the Lewis acid sites. For Si/Al = 25, the
low yield (25.18 wt%) of longꢀchain alkanes suggests a high degree of the hydrocracking reaction
on the Brønsted acid sites to produce C6–C8 olefins. However, the yield of C6–C8 olefins could be
as low as 7.33 wt%, which signifies that these olefins are ultimately transformed into AHCs on the
Lewis acid sites. When the catalyst Si/Al ratio is 120, the hydrocracking of the longꢀchain alkanes
still occurs to a significant extent due to the relatively adequate amount of Brønsted acid sites.
However, the insufficient number of Lewis acid sites inhibits the DielsꢀAlder reactions for AHC
formation, which leads to the increased yields of C6–C8 olefins. For Si/Al = 200, the high yield of
longꢀchain alkanes indicates a reduction in the extent of hydrocracking, which directly leads to
decreased yields of both the C6–C8 olefins and AHCs. It can be thus concluded that the Brønsted
and Lewis acid sites of the catalyst both influence the formation of AHCs.
(b)
100
80
60
40
20
0
100
80
60
40
20
0
(a)
80
60
40
20
0
Long chain alkanes
C6-C8 Olefins
AHCs
Long chain alkanes
C6-C8 Olefins
AHCs
80
60
40
20
0
5 wt%
10 wt%
12.5 wt%
15 wt%
Ni loading
399
400
Figure 10. Conversion ratios and productions of alkanes, C6–C8 olefins, and AHC products for
various Si/Al ratios and different Ni loadings. Reaction conditions: (a) 10 g fatty acids, 0.6 g 10 wt%
Ni/HZSMꢀ5, 4 MPa H2, 8 h, 360°C and (b) 10 g fatty acids, 0.6 g Ni/HZSMꢀ5 (Si/Al = 25), 360°C,
4 MPa H2, 8 h.
401
402
403
404
405
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Table 6. Acid site densities of the prepared catalysts with different Si/Al ratios, determined by
PyꢀIR
Acid site contents (mmol/g)
AHC yield
(wt%)
Si/Al ratio
Ni content (wt%)
Brønsted
Lewis
Total
25
80
10 (9.89)a
10 (9.91)a
10 (9.94)a
10 (9.90)a
10 (9.96)a
0.12
0.08
0.06
0.03
0.01
0.06
0.04
0.03
0.03
0.02
0.18
0.12
0.09
0.06
0.03
53.96
41.99
29.05
5.47
120
150
200
4.39
408
409
410
aActual Ni loading content (in the bracket) determined by XRF.
Table 7. Acid site densities of the prepared catalysts with different Ni contents, determined by
PyꢀIR
Acid site contents (mmol/g)
AHC yield
(wt%)
Ni content
(wt%)
Si/Al ratio
Brønsted
Lewis
Total
25
25
25
25
ꢀ
5 (4.92)a
10 (9.91)a
12.5 (12.43)a
15 (14.95)a
100b
0.14
0.12
0.11
0.10
0
0.08
0.06
0.06
0.06
0.005
0.24
0.18
0.17
0.16
0.005
b
48.09
53.96
45.73
38.43
0
411
412
413
414
415
416
417
418
419
420
421
422
423
424
aActual Ni loading content (in the bracket) determined by XRF. Ni metal catalyst without
HZSMꢀ5.
As mentioned above, pure Ni metal has almost no acid sites. We attempted to tune the catalyst
acid properties through the coverage of its acid sites by loading different amounts of Ni metal (5,
10, 12.5, and 15 wt%). The PyꢀIR results (Table 7) support the successful reduction in the
densities of Brønstedꢀacid sites with increasing Ni loads, and accordingly, the alkane yields were
projected to increase and the AHC yields to decrease. However, the phenomena observed in
Figure 10(b) do not agree with these conjectures. Even the AHC yield exhibits a tendency to
decrease, on the whole, with the increasing Ni contents. However, the highest AHC yield of 53.96
wt% was obtained for a Ni content of 10 wt%. Note that the olefin yields obtained with 10 and
12.5 wt% Ni are lower than those achieved for 5 and 15 wt% Ni, as shown in Figure 9(b).
In view of these results, these four catalysts were characterized in detail by XRD, SEM, and
TEM. The catalyst XRD patterns in Figure S5 in the SI show that Ni metal was successfully
loaded on the HZSMꢀ5. The SEM and TEM results illustrated in Figure 11 reveal that the Ni
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426
427
428
429
430
431
432
433
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435
436
437
438
particle size increases with increased Ni loading. Moreover, it is interesting to note that the
morphology of the loaded Ni nanoparticles exhibits polyhedral shapes for the 10 wt% Ni content,
such as the tetrahedron example shown in Figure 11(b), whereas round shapes are observed for 5
and 15 wt% Ni. When the Ni content is 12.5 wt%, both polyhedral and round Ni nanoparticles are
evident (Figure 11(c)). Notably, more round than polyhedral particles are observed, suggesting the
disappearance of the polyhedrons. Previous studies have demonstrated that the morphology of a
metal catalyst substantially influences its catalytic activity 25, 56ꢀ58, and irregularly shaped metal
centers provided abundant surface defects as active sites, which exhibit high catalytic activity 59, 60
.
61, 62
Recently, Qiao and Yang
successfully obtained a singleꢀatom catalyst which exhibited
extremely high catalytic activity, suggesting that catalysts based on singleꢀatom dispersions would
be highly desirable to maximize atom efficiency. Based on these results, the polyhedral
morphology of the Ni nanoparticles in our catalysts may expose more surface defects, and the Ni
loading of 10 wt% may increase the number of accessible surface metal atoms, which would both
facilitate the DielsꢀAlder reactions and lead to higher AHC production.
439
440
441
442
Figure 11. Microstructural images of the prepared catalysts (Si/Al = 25): SEM images of HZSMꢀ5
with different Ni loadings: (a) 5.0, (b) 10, (c) 12.5, and (d) 15 wt%; (e) TEM and (f) HRTEM
images of HZSMꢀ5 with 10 wt% Ni loading. The loaded Ni particles showed round morphology in
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467
468
469
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471
472
(a), (c) and (d) whereas polyhedron morphology in (b).
4 Conclusions
The oneꢀstep hydroprocessing of fatty acids for the sustainable production of aromatic
hydrocarbons was conducted over Ni/HZSMꢀ5 catalysts. A yield of ~64% AHCs was obtained by
hydroprocessing 10 g fatty acids in the presence of 0.6 g catalyst comprising 10 wt% Ni/HZSMꢀ5
with a Si/Al ratio of 25 for 12 h at 360°C and 4 MPa H2. The major reaction pathways involved in
the oneꢀstep transformation consisted of hydrodeoxygenation, hydrocracking, and aromatization
reactions. During the hydrodeoxygenation process, the unsaturated oleic and linoleic acids in the
feedstock were hydrosaturated to stearic acid at a low reaction temperature. Subsequently, the
stearic acid was transformed into the longꢀchain hydrocarbons octadecane and heptadecane
through a series of dehydration, decarbonylation, and decarboxylation reactions. Conversely,
unsaturated acids could be directly hydrodeoxygenated to longꢀchain olefins, as in the conversion
of oleic acid to 8ꢀheptadecene. These pathways could be well performed on metal sites. However,
the hydrocracking of the longꢀchain hydrocarbons was performed on the catalyst acid sites rather
than metal sites. Through thermal and catalytic cracking reactions, the longꢀchain hydrocarbons
were transformed into gaseous products such as methane, ethane, propane, C6–C8 olefins, and a
fraction of alkanes. Subsequently, these cracked small molecules cyclized to form cyclic
hydrocarbons with fiveꢀ or sixꢀmembered rings that could be transformed into the AHCs after
ringꢀexpansion and dehydrogenation reactions. The C6–C8 olefins were determined to be crucial
intermediates for AHC formation. Lower numbers of catalytic Brønsted and Lewis acid sites
restricted the hydrocracking and aromatization reactions, respectively, and both led to a decrease
in AHC production. Additional Ni loading led to a decrease in the Brønsted acid site contents. A
polyhedral morphology was found for the loaded Ni nanoparticles at a Ni content of 10 wt%,
which may have exposed more metal defects and accessible surface metal atoms to promote AHC
formation. This work provides a simple, effective approach for the mass production of renewable
AHCs and will be helpful in ultilizating such oxygenousꢀrich substrates in large scales for the
effective synthesis of renewable transportation fuels to release the atmosphere warming
deterioration.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (No.
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21576260), the Natural Science Foundation of Guangdong Province (No. 2016A030308004), the
Special Funds of Applied Science and Technology Research of Guangdong Province (No.
2015B020241002) and the Natural Science Foundation of Guangdong Province (No.
2015A030313720).
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