Iodine(III) reagent (ABX—N3)-induced intermolecular anti-Markovnikov hydroazidation of unactivated alkenes

Article abstract of DOI:10.1007/s11426-019-9628-9

Anti-Markovnikov hydroazidation of unactivated alkenes using ABX2014;N₃ as an initiator has been developed at room temperature, wherein hydrogen azide (HN₃) acts as both hydrogen and azidating agent. Notably, the HN₃ reage

Full text of DOI:10.1007/s11426-019-9628-9

Research Article
pubs.acs.org/journal/ascecg
Cite This: ACS Sustainable Chem. Eng. 2019, 7, 16529−16538
Adsorption of Major Nitrogen-Containing Components in Microalgal
Bio-Oil by Activated Carbon: Equilibrium, Kinetics, and Ideal
Adsorbed Solution Theory (IAST) Model
Fanghua Li,†,‡,§ Lynn Katz,† and Zhiquan Hu*,§
†Department of Civil, Architectural and Environmental Engineering, The University of Texas at Austin, 301 E Dean Keeton Street
Stop C1786, Austin, Texas 78712-1173, United States
‡Department of Chemical Engineering, Monash University, Wellington Rd, Clayton VIC 3800, Australia
§School of Environmental Science and Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan
430074, China
S
* Supporting Information
ABSTRACT: This study provides a fundamental understanding
of the adsorption process for nitrogenous compound removal
from microalgal bio-oil over activated carbon (AC) and offers a
useful model based on the carbon adsorption isotherms and
kinetics. The results show that AC favors adsorption of
hexadacanamide and indole rather than valeramide and 3-
phenylpropionitrile. The surface oxygenous functional groups
and acidity of the AC play a vital role in the adsorptive
performance of AC. The AC presents a Freundlich isotherm in the
adsorption of hexadacanamide, indole, valeramide, and 3-phenyl-
propionitrile. Adsorption removal for the four nitrogen-containing
compounds follows pseudo-second-order kinetics. There exists
competitive adsorption among different nitrogen-containing compounds which can be simulated by the IAST model. The IAST
model can be used to predict the required amount of adsorbent and appropriate initial concentration of adsorbate for maximum
enhancement of the removal efficiency of nitrogen-containing compounds from microalgal bio-oil. It can also predict the effects
of the specific surface area of the adsorbents and reaction temperature on the carbon adsorption performance.
KEYWORDS: Adsorption, AC, Nitrogenous compounds, Equilibrium, Kinetics, IAST model
produced from Tetraselmis suecica has high calorific value and a
low concentration of oxygenous compounds.5 However, the
microalgal bio-oil has high nitrogen-containing functional
groups which can cause the emission of nitrogen-containing
gas during combustion. The nitrogenous compounds pre-
sented in petroleum-derived oils are able to be efficiently
removed by adsorption denitrogenation (ADN).7
Several adsorbents like ACs, activated aluminas, silica gels,
and metal-exchanged zeolites have been investigated for an
effective removal of nitrogen from liquid hydrocarbon fuels.8
AC as an adsorbent has relatively high surface area and total
pore volume. The surface chemistry and porous structure of
ACs are significant factors in the ADN processs.9 Studies have
shown that AC has a high adsorption performance toward the
reduction of nitrogen-containing compounds in a synthesized
diesel fuel,10 and it also played a positive role in the
nitrogenous compounds separation from shale oil and light
cycle oil.11 The large surface area and the surface oxygenous
INTRODUCTION
■
Pyrolysis technology has attracted considerable attention in the
design of a renewable bio-oil production process performed on
a larger scale for both chemical industries and fuels for
transport or aviation. Nur and co-workers (2017) put forward
that the catalytic pyrolysis process enhanced the quality of the
bio-oil produced by Chlorella vulgaris with low oxygen and
nitrogen content.1 Duan et al. (2019) pointed out that
pyrolysis technology has attracted increasing attention as an
effective pathway for biomass utilization.2 The waste resource
was converted into high-value hydrocarbons like jet fuel via
copyrolysis of soapstock and lignin. Li et al. (2019) mentioned
that pyrolysis is of great interest in bio-oil production via
microalgae biomass due to low carbon emission and energy
consumption.3 The pyrolysis of bio-oil is a promising source
for the generation of high-value chemicals or biofuels for
transport except in its use for power, heat, or combined power
and heat. It is clear that the use of the pyrolysis process to
convert microalgae to bio-oil is increasingly important in the
application of biomass resources. Tetraselmis suecica can be
used for the generation of biofuels, particularly as a renewable
microalgal source of pyrolysis conversion.4−6 The bio-oil
Received: July 3, 2019
Revised: September 6, 2019
Published: September 9, 2019
© 2019 American Chemical Society
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(b) The adsorption time was 0−24 h, 36 h, and 48 h, respectively,
while maintaining the volume of microalgal bio-oil and the
amount of AC at 10 mL and 0.1 g, respectively.
(c) The concentration of each nitrogen-containing compound was
0, 20, 40, 60, 80, and 100 ppm, respectively, while maintaining
the amount of AC and adsorption time at 0.1 g and 36 h,
respectively.
(d) The concentration of a mixed-adsorbate system (hexadacana-
mide, indole, and 3-phenylpropionitrile) was 20, 60, and 100
ppm, respectively, while maintaining the amount of AC and
adsorption time at 0.1 g and 36 h, respectively.
groups of AC play essential roles in improving the nitrogen
removal efficiency from real gas oil.12 Mochida also
investigated an interesting work on adsorption denitrogenation
of real gas oil over ACs with a surface area between 683 and
2972 m2/g.12,13 The results illustrated that the use of ACs can
be effective in the removal of nitrogen-containing compounds
from gas oil, mainly due to the surface properties of carbon
materials and the groups that can produce CO.
Jiang et al. reported that mesopore volume could enhance
the adsorption capacity of AC for nitrogen removal from fuel
oils.14 However, there is no work currently reported on the
removal of nitrogenous components from microalgal bio-oil
using AC. Therefore, the focus of this work is to study the
adsorption denitrogenation process of marine microalgal bio-
oil by AC. The adsorption capability of AC for nitrogenous
compounds in microalgal bio-oil and, more importantly, the
adsorption mechanism of nitrogenous compounds onto AC
were examined through batch adsorption experiments.
In this study, the adsorption performance of AC in the
denitrogenation processes of four different nitrogenous
compounds, hexadecanamide, indole, 3-phenylpropionitrile,
and valeramide was examined and the adsorption isotherm and
kinetics of each nitrogenous compound onto AC were also
investigated. On the basis of the results of the investigation
above, an IAST model was examined to further simulate the
competitive adsorption of the main nitrogenous compounds in
the microalgal bio-oil over AC. It is important to have a better
understanding of the removal of nitrogenous compounds by
adsorptive denitrogenation and to gain insightful information
about the control of nitrogen emission during the utilization of
microalgal bio-oil.
Characterization Methods. Gas Chromatography-Mass Spec-
trometry (GC-MS). The method for GC-MS analysis is based on our
previous work.6 Both the quantitative and qualitative analyses
(MassHunter Workstation Software) of the bio-oil were performed
using a 5977A GC-MS (Agilent Technologies Inc.) equipped with
HP-5 capillary column (30m × 0.25 mm × 0.25 mm). Helium was
used in this part as the carrier gas. The injector temperature was 250
°C at a 20:1 injector split ratio. Standard curves of each nitrogen-
containing compound (hexadecanamide, indole, 3-phenylpropioni-
trile, and valeramide, respectively) were built for quantitative analysis.
Surface Area and Porosity Analyzer. The BET surface area, pore
size, and pore volume of ACs were characterized using 3FLEX 3500.
A 120 mg portion of sample was degassed in a 3FLEX sample tube
under 150 °C for 8 h. The sample tube was then transferred to the
analysis port to examine the BET surface area, pore size distribution,
and pore volume under the condition of −196 °C with liquid
nitrogen.
X-ray Photoelectron Spectroscopy (XPS). XPS measurements were
achieved with a Kratos AXIS Ultra analysis instrument, using dual
anode X-ray gun (Al/Mg)+ focused monochromatic Al X-ray gun.
Magnesium Kα X-rays were applied as primary excitations and are
typically operated at 250W. The analytical area was 4 mm × 6 mm.
The XPS spectra were obtained in a fixed analyzer transmission mode
with an energy step of 0.1 eV and a pass energy of 40 eV. The
correction of the binding energy value was performed by designating
the main component of the aluminum 2p photoelectron peak
envelope to 75 eV to compensate for sample charging. The Shirley
fitting program was used to decompose the spectrum into Gauss-
Lorentz components after background subtraction.
Chemisorb 2720 (Micromeritics, U.S.A.) Analysis. Chemisorb
2720 was applied to examine the surface oxygen-containing functional
groups and acidity of AC. The decomposition of surface groups was
conducted to measure the surface oxygen-containing functional
groups of the AC. A 100 mg portion of AC sample was pretreated
under the conditions of 150 °C for 1 h with 50 mL/min helium gas
and cooled to ambient temperature. Subsequently, the sample was
heated to 1000 °C under the conditions of 10 °C/min and helium
flow gas of 50 mL/min. The gases evolved (CO and CO2) from the
AC were monitored by quadruple mass spectrometry. NH3-TPD
(temperature-programmed desorption) was conducted to measure the
acidity distribution of the AC. The sample was saturated in 10%
ammonia gas for an hour. Helium gas was then passed through to
remove the physisorbed ammonia for an hour. Subsequently, the
sample was heated to 700 °C under the conditions of 10 °C/min. In
this study, a mass spectrometer was connected with the NH3-TPD,
which only detected and retained the signal peak of NH3. The NH3
evolved was calibrated with the standard and quantified by integrating
related peak areas.
Fourier Transform Infrared Spectroscopy (FTIR). Fresh and spent
ACs were studied under ambient conditions to investigate the
evolution of the surface functional groups through FTIR analysis. The
analysis was conducted in situ by the infrared microscopy beamline.
The method for FTIR analysis is based on our previous work.6
Ideal Adsorbed Solution Theory (IAST) Model. The IAST model
used in this study was based on a new solution algorithm introduced
by Mark M. Benjamin.15 The new solution algorithm allows the use of
simple spreadsheet analysis to solve the model equations where only
two independent factors are required to be estimated. This approach
has been primarily used for competitive sorption of only three
MATERIALS AND METHODS
■
Materials. In this study, the AC derived from the source of
coconut was an adsorptive and catalytic carbon, activated by chemical
method and supplied by Centaur-Calgon Carbon Corporation, U.S.A..
The particle size of the AC was 350 μm. All the other chemicals like
hexadacanamide, indole, 3-phenylpropionitrile, valeramide, and ethyl
acetate were purchased from Sigma-Aldrich, U.S.A..
The composition of microalgal bio-oil is demonstrated in
Supporting Information. It shows that there are mainly nitrogen-
containing compounds, hydrocarbons, and oxygen-containing com-
pounds in microalgal bio-oil. The dominant nitrogen-containing
compounds are hexadacanamide (17.78%), indole (7.29%), valer-
amide (2.09%), and 3-phenylpropionitrile (1.69%).
Batch Adsorption. The batch experiment was conducted in a
timber shaker. The shaker rotated once every two seconds at a speed
of 22.6 m/min (0.24 m in diameter). All experiments were conducted
in capped glass vials with continuous shaking at 25 °C in a
thermostatic chamber. The four nitrogen-containing compounds were
dissolved in ethyl acetate. The working volume of each vial was 10
mL, filled with AC of 0.1 g. The concentrations of nitrogenous
compounds are 0, 20, 40, 60, 80, and 100 ppm, respectively. The
samples were taken at 10−60 min with an interval of 10 min, 2−24 h
with an interval of 1h, 36 h, and 48 h until the adsorption equilibrium
was reached. Each experiment was carried out in triplicate. At the
preset adsorption time, the corresponding samples were brought out
and deposited for 15 min. The nitrogen-containing compounds were
isolated from reaction mixture by filtration (0.25 μm, filter) to remove
the AC from the solution. Subsequently, the quantity of the nitrogen-
containing compounds was analyzed by GC-MS.
The following experiments were conducted:
(a) The adsorption time was 0−24 h, 36 h, and 48 h, respectively,
while maintaining the concentration of each nitrogen-
containing compound and the amount of AC at 100 ppm
and 0.1 g, respectively.
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adsorbates with the assumption of a trace level presence of the fourth
adsorbate.
can lead to the fluctuations of the concentrations in adsorption
curves before the adsorption equilibrium is reached.
Due to a low concentration of these nitrogen-containing
components in microalgal bio-oil, the concentration of the
adsorbate was presented by area in this part. The
hexadacanamide and indole can be completely removed by
0.1 g carbon adsorption from the microalgal bio-oil as shown
in Figure 1b. In addition, the AC can remove the other two
compounds effectively as well. The results indicate that there
are some differences between the nitrogen removals for single
nitrogenous compounds and microalgal bio-oils. The other
molecules, partial surface pressure, or competitive adsorption
can promote the removal of these four nitrogenous compounds
from the microalgal bio-oil.16−18 As for the microalgal bio-oil,
the adsorption of hexadacanamide and indole both presented a
third stage. The three stages are defined as exponential phase,
declining relative adsorption phase, and stationary phase.
Consequently, the undesirable smell and yellow color were also
removed from the bio-oil by carbon adsorption.
RESULTS AND DISCUSSION
■
This part mainly focuses on five key aspects: (1) the
adsorption characteristics of AC for the four different
nitrogen-containing compounds and marine microalgal bio-
oil, (2) characterization of fresh and spent AC, (3) adsorption
isotherm, (4) adsorption kinetics, and (5) the ideal adsorbed
solution theory (IAST) model.
Adsorption Performance for Different Nitrogen-
Containing Compounds. The adsorption performances of
AC for different single nitrogen-containing compounds and
microalgal bio-oils are demonstrated in Figure 1a,b,
It is obvious that the adsorption performances of AC for
single nitrogen-containing compounds and marine microalgal
bio-oil are different. As for all the single nitrogen-containing
compounds, the adsorption reached equilibrium at around 25
h. As for the microalgal bio-oil, the valeramide molecules first
reached equilibrium at 21 h. The adsorption process was
continually performed until complete removal of hexadecana-
mide and indole. After the saturation of hexadecanamide,
indole, and valeramide, the carbon adsorption for 3-phenyl-
propionitrile remained in processing. The results showed that
the adsorption processes for microalgal bio-oil are relatively
complex and mechanisms may differ from single nitrogen-
containing compounds. In the model compounds test,
hexadacanamide and indole show similar adsorption behaviors
on the carbon sample used, while valeramide and 3-
phenylpropionitrile show similar adsorption behaviors; the
two groups exhibit very different adsorption behaviors.
However, in the test with real microalgal bio-oil, the patterns
of the adsorption curves are similar for all the four compounds.
Similar results have been reported by several authors who
found that AC had a high capacity for the adsorption of
nitrogen species.11,13 ACs with a high mesopore content are of
interest in the adsorption of organic contaminants with a high
molecular weight; a high micropore content suggests that the
carbon will be of interest in the removal of contaminants with a
relatively low molecular weight,19 which suggests that in this
study, the AC may have a high mesopore content because of a
high adsorption capacity for hexadacanamide. In a study on
AC adsorption of model nitrogenous compounds of indole,
quinolone, and carbazole, Wen and co-workers reported that
the AC favors adsorption of cyclic nitrogenous compounds
such as indole.11
Figure 1. Adsorption performance of AC for different nitrogen-
containing compounds. (a) Single nitrogen-containing compound of
100 ppm, AC of 0.10 g. (b) Microalgal bio-oil of 10 mL, AC of 0.10 g.
In a study on ADN for model fuel oil, it was found that ACs
containing lactone groups and carboxylic acid groups are of
interest for the adsorption of quinolone, and ACs containing
phenolic groups and carboxylic anhydride groups are of
interest for the adsorption of carbazole and indole.20 A study
found that ACs containing different numbers of surface oxygen
containing groups may follow different mechanisms. With high
oxygen content, hydrogen-bonding and acid−base attractions
play important roles in strong interactions with nitrogenous
compounds, while with low oxygen content, π−π interaction
plays an important role in a weak affinity to nitrogenous
compounds.21
respectively. As shown in Figure 1a, the concentration of the
adsorbate rapidly decreased during the first 30 min, after which
the concentration dropped slowly. As for 0.10 g of carbon
adsorption, the residual concentrations of hexadacanamide,
indole, 3-phenylpropionitrile, and valeramide were 15, 30, 70,
and 35 mg/L, respectively. The fluctuations of the concen-
trations of the adsorbates, especially for 3-phenylpropionitrile
and valeramide, occurred due to the desorption of related
molecules. It is evident that activated carbons have a relative
weak affinity to 3-phenylpropionitrile and valeramide, which
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In summary, it was found that the AC in this research has a
high adsorption capacity for hexadacanamide and indole. The
pore size, surface oxygen content, and acid−base interaction
may be the important factors in determining the removal of
nitrogenous compounds over AC.22,23 However, the adsorp-
tion capacities for 3-phenylpropionitrile and valeramide are
relative low. It has been reported that AC can be modified by
oxidant like 15% (NH4)2S2O8 to achieve an increased nitrogen
removal efficiency of 18% from light cycle oil.24 Consequently,
it is necessary to further study the modified AC with high
affinity toward 3-phenylpropionitrile and valeramide by
introducing specific functional groups such as oxygenated
functional groups.
Characterization of Fresh and Spent AC. The
adsorption performance of AC on single nitrogen-containing
compounds of hexadecanamide, indole, 3-phenylpropionitrile,
and valeramide were investigated in Adsorption Performance
for Different Nitrogen-Containing Compounds. The results
showed that different nitrogenous compounds may be
attracted to different active sites in AC. High oxygen content
and large surface area contribute to the high performance of
activated carbon.12,14 The carbon properties which may affect
the adsorption processes such as the surface area, particle size,
surface functional groups, surface polarity, and pore size
distribution were investigated in this section. The character-
izations of fresh and spent AC were investigated by surface
area and porosity analyses, XPS, TPD, FTIR spectra, and
scanning electron micrographs, and they are presented and
discussed in this section.
Figure 2. XPS spectra for fresh AC and spent AC.
biochar, which were the active components that contributed to
the desulfurization process.26 The similar findings were also
described by Li et al. (2017) and Qiu et al. (2018).27,28 The
appearance of Cu and Zn may be from the chemical activation
process for ACs. Further study is required to confirm the role
of heavy metals in the denitrogenation performance of
activated carbon.
The decomposition of surface groups for fresh AC and NH3-
TPD profiles for fresh AC and spent AC after adsorption of
hexadecanamide, indole, 3-phenylpropionitrile, and valeramide
are shown in Figure 3. TPD is a tool for quantitative and
qualitative studies of solid surface using a chemisorption
technique. It can determine sample properties such as acidity,
acid strength, and surface functional groups.29
The evolution of CO and CO2 during a heat treatment
process up to 1000 °C was demonstrated in Figure 3a. It is
observed that all functional groups including oxygen
decompose into CO2 and CO during the temperature-
programmed desorption process.30−32 The examined total
quantity of CO2 and CO evolved were 0.66 and 2.88 mmol/g,
respectively, which showed that the quantity of CO was
relatively higher than the quantity of CO2. Besides, the CO2
evolution was initiated at a relatively low temperature of
around 200 °C and reached the first maximum peak at 250−
300 °C. In contrast, the CO evolution was initiated at a higher
temperature of around 400−450 °C and reached two resolved
peaks at 650−700 °C and 850−900 °C, respectively.
The ammonia gas (10%) that evolved was demonstrated in
Figure 3b from ambient conditions to 700 °C. NH3-TPD
profiles revealed that the acidity of activated carbon declined
after the adsorption process. The fresh AC exhibits the high
acidic concentration and strength. It is observed that the
acidity of AC decreased with the adsorption of large amounts
of hexadecanamide and indole. The acidity of the spent AC
decreased in the order of bio-oil < hexadecanamide < indole <
valeramide < 3-phenylpropionitrile. The results suggested that
the acidity of the activated carbon plays an important role in
the removal of nitrogen-containing functional groups.
In this study, the ACs have a surface area of 1717.66 m2/g,
mesopore volume of 1.35 mL/g, and the pore sizes of 2.12 nm.
Additionally, there are mainly three acidic groups in the ACs
including −OH, C=O, and −COOH with the numbers of 0.25,
0.15, and 0.08 mmol/g, respectively, established by the Boehm
method.25
The nitrogen adsorption and desorption isotherms for fresh
AC and spent AC after adsorption of HAM are presented in
Supporting Information. The curves present the type IV
adsorption isotherm demonstrating that the AC still retained
its mesoporous structure after the adsorption of HAM. Similar
results were found for the spent AC after the adsorption of
indole, 3-phenylpropionitrile, and valeramide.
The XPS analysis was performed to analyze the evolution of
surface elements and functional groups of the activated carbon
sample. The XPS spectra for fresh AC and spent AC after
adsorption of hexadecanamide, indole, 3-phenylpropionitrile,
and valeramide are shown in Figure 2. From the XPS spectra, it
can be seen that the XPS spectra is in agreement with the
adsorption performance. As for the pure nitrogen-containing
compounds, the peak areas for carbon, oxygen, and nitrogen
increased in the order of hexadecanamide, indole, valeramide,
and phenylpropionitrile after carbon adsorption. Five peaks
that appeared at 290.6, 288.4, 287.0, 284.6, and 284.1 eV
contributed to the C 1s spectra of ACs, which correspond to
the functional groups of −OC(=O)−O− (2.2%), −O−
C(=O)− (3.8%), −C−O− (4.0%), C-graphite* (18.2%,
sp3), and C-graphite (71.8%, sp2), respectively. The results
revealed that there are high concentrations of oxygen-
containing functional groups that appear on the carbon
surface, which play an important role in the removal of
nitrogen-containing functional groups. Song et al. (2017) also
found the presence of functional groups including oxygen
(−O−C(=O)− and −C−O−) on the surface of modified
The FTIR spectra for fresh ACs and spent ACs after the
adsorption of hexadecanamide, indole, 3-phenylpropionitrile,
valeramide, and bio-oil are exhibited in Figure 4. The FTIR
spectra of spent AC after the adsorption of single nitrogen-
containing compounds and microalgal bio-oil are different.
There are peaks observed mainly between 500 and 450 cm−1
for spent AC after the adsorption of microalgal bio-oil.
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numerous pores and steep edges exhibited on the surface of
the ACs. The physical structures of AC are complex based on
the SEM images, and thus the adsorption mechanism may be
relatively difficult to determine.38 However, there is no clear
trace on the surface of the ACs after the adsorption of the
nitrogen-containing compounds. No significant difference in
microstructure was noted on the spent ACs.
Nitrogen-containing molecules such as hexadacanamide,
indole, 3-phenylpropionitrile, and valeramide are of interest in
wide applications and competition in the chemical market. The
nitrogen-containing compounds adsorbed in AC can be easily
removed by thermal treatment or solvent washing. After
desorption, the nitrogen-containing compounds can be
recycled and reused as fine chemicals and precursors or main
ingredients of pharmaceuticals.16
Adsorption Isotherm in Single-Component System.
The adsorption isotherm was studied based on experimental
data from single-components of nitrogen-containing com-
pounds with different initial concentrations. The Freundlich
isotherm equation was selected to analyze the equilibrium
adsorption data. The isotherm equation is expressed as follows
Freundlich isotherm:.
q = KFCen
(1)
e
where KF (mg1−n Ln/g) and n correspond to the adsorptive
capacity and intensity, separately, and qe (mg/g) is the weight
of the nitrogen-containing adsorbate per weight of the
adsorbent. The linear form of the Freundlich equation is
expressed by eq 2
ln(qe) = ln(KF) + n ln(Ce)
(2)
The parameters of the Freundlich isotherm collected at 298 K
are presented in Table 1. Additionally, the isotherm
equilibriums for the nitrogen-containing compounds of
hexadecanamide, indole, 3-phenylpropionitrile, and
valeramide, respectively, are demonstrated in Figure 5. The
adsorption data of hexadacanamide, indole, 3-
phenylpropionitrile, and valeramide can be best-matched
with the Freundlich isotherm, with the correlation factors R2
values being 0.9994, 0.9992, 0.9990, and 0.9992, respectively.
The number of n < 1 normally suggests that the adsorption
capacity is constrained at relatively low equilibrium
concentrations. This isotherm predicts that AC shows a
favorably heterogeneous surface during the adsorption
processes of the four nitrogen-containing compounds.
Additionally, the saturation adsorption is not predicted by
the Freundlich model, and an unlimited surface coverage is
supposed to appear, which can result in a multilayer adsorption
of the nitrogen-containing components on the surface of AC.39
The isotherm results suggest that the AC shows a
heterogeneous surface for the adsorption of hexadecanamide,
indole, 3-phenylpropionitrile, and valeramide.
Adsorption Kinetics. The contact time is important in the
carbon adsorptive process for nitrogen-containing compound
removal from microalgal bio-oil. The results have been
presented in Figure 1. The nitrogen-containing compounds
were sustained and allowed to interact with the AC for 36 h.
The results indicate that the carbon adsorption was faster at
the beginning stage. Subsequently, the process became slower.
About 80% of hexadacanamide and indole were adsorbed at
the first 3 h. A steady-state adsorption process was reached
after a contact time of about 30 h.
Figure 3. TPD profiles. (a) Decomposition of surface groups for fresh
AC. (b) NH3 profiles for fresh AC and spent AC.
According to the literature, these peaks mainly refer to out-of-
plane vibrations of C−H functional groups in aromatic
structures.33 In brief, the FTIR spectra can partly reflect the
adsorption process due to the significant evolutions in the
surface functional groups of AC.34
The adsorption of AC is initially determined by its chemical
composition.35 The functional groups of the graphite structure
and the delocalized electrons establish the apparent chemical
properties of the AC.36 The presence of oxygen (O−H, C=O,
or C−O−C) and hydrogen (C−H) in the surface groups can
strongly affect the adsorption properties of AC. The character-
istics of these surface groups can be developed from the
activation process, the raw materials, or the introduction via
post-treatment. The oxygen surface groups are formed
predominantly via exposure to air or through special post-
treatment.37 The oxygen can exist in various forms such as
anhydride, carboxyl, phenol, lactone, aldehyde, ketone,
quinine, hydroquinone, or ether structures. These groups can
also interact with each other. Some groups such as carboxyl,
carbonyl, lactone groups, and phenolic hydroxyl are acidic.
Furthermore, the presence of quinone, pyrone, and chromene
structures can be postulated according to the basic character-
istics of the carbon surface.
The micrographs for fresh and spent AC are shown in
Supporting Information. It can be seen that there are
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Figure 4. FTIR spectra. (a) Fresh activated carbon. (b) Activated carbon after adsorption of hexadacanamide. (c) Activated carbon after adsorption
of indole. (d) Activated carbon after adsorption of 3-phenylpropionitrile. (e) Activated carbon after adsorption of valeramide. (f) Activated carbon
after adsorption of microalgal bio-oil.
Table 1. Freundlich Constants of AC under 298 K
Freundlich
Molecular formula
Molecular wt
n
KF( mg1−nLn/g)
R2
Hexadacanamide
Indole
3-Phenylpropionitrile
Valeramide
C16H33NO
C8H7N
C9H9N
255.446
117.151
131.174
101.150
0.6
0.5
0.2
0.4
8.77
9.11
0.37
0.05
0.9994
0.9992
0.9990
0.9992
C5H11NO
In this part, pseudo-first-order and pseudo-second-order
kinetic models were applied to analyze the kinetics of nitrogen-
containing compound adsorption onto the AC.40 The pseudo-
first-order model is described by the Lagergren equation
where qe and qt (mmol/g) are the adsorption capacities
expressed at equilibrium and at time t, respectively, and k1ad
(min−1) is the adsorption rate constant of the pseudo-first-
order model. By the integration of eq 3 and the application of
boundary conditions, where qt varies from 0 to qt and t varies
dqt/dt = k1ad(qe − qt)
(3)
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coefficients (R2) for all nitrogen-containing compounds are
more than 0.99 and the values for the pseudo-first-order model
are between 0.80 and 0.91. This suggests that the pseudo-
second-order kinetics can describe the adsorption process of
nitrogenous compounds onto AC. Indole shows relatively
higher constant k2ad and initial adsorption rate, h, than
hexadecanamide, 3-phenylpropionitrile, and valeramide. This
may be because indole is a nonbasic nitrogenous compound
with high numbers of C (sp2) + N, high ionization, and charge
on the nitrogen atom, which enhances the electronic potential
that contributes to attractive forces.10
It takes several steps for the nitrogenous compounds to
transfer from the liquid phase to the carbon surface. These
processes include external diffusion, pore diffusion, and surface
adsorption.41,42 However, there may be one or more dominant
process during the overall adsorption process. The possibility
of intraparticle diffusion on carbon adsorption is explored
using the Weber-Morris model,41 which can be demonstrated
by eq 8
Figure 5. Isotherm equilibrium for different nitrogen-containing
compounds.
q = kidt1/2 + C
(8)
t
The Weber-Morris curves of qt versus t0.5 for nitrogenous
compounds are shown in Figure 6. It is evident that zone 1 and
from 0 to t to the integrated form, eq 3 can be expressed as eq
4
log(qe − qt) = log qe − k1adt/2.303
(4)
It is evident that the values of log(qe − qt) are linearly
correlational with t. The curve of log(qe − qt) versus t should
describe a linear relationship where k1ad can be calculated from
the slope of the curve.
Additionally, the pseudo-second-order adsorption kinetic
rate equation can be expressed by eq 5
dq /dt = k2ad(q − q )2
(5)
t
e
t
where k2ad (mmol/mg/min) is the adsorption rate constant for
the pseudo-second-order model. By integration of eq 5 and the
application of boundary conditions, where qt varies from 0 to qt
and t varies from 0 to t to the integrated form, eq 5 can be
expressed as eq 6 by rearrangement
2
t/qt = 1/k2adqe + t/qe
(6)
Figure 6. Weber-Morris intraparticle diffusion curves for the ADN
over AC.
and
2
h = k2adqe
(7)
where h (mmol/g/min) is the initial adsorptive rate at the
contact time of approximately t = 0. The curve of (t/qt) and t
of eq 6 should describe a linear function from which k2ad is
calculated according to the curve intercept. The results from
matching experimental data with the theoretical models for the
adsorption of nitrogenous compounds via AC are illustrated in
Table 2. For the pseudo-second-order model, the correlation
zone 2 (two linear zones) are presented during the adsorptive
process, which suggests that at least two steps may control the
ADN process over AC. The adsorption of nitrogenous
molecules onto AC matches the pseudo-second-order kinetics.
The external diffusion does not significantly affect the
adsorption process for all the investigated nitrogenous
compounds. The fastest adsorption rate among the four
Table 2. Adsorption Rate Constants of Kinetic Models for Different Nitrogenous Molecules
Pseudo-first-order
Pseudo-second-order
a
Adsorbate
Rate constants k1ad (min−1
)
R2
Rate constants k2ad (mmol/g/min)
R2
h
qe
Hexadacanamide
Indole
3-Phenylpropionitrile
Valeramide
0.041
0.050
0.0030
0.0040
0.8028
0.9094
0.8879
0.8558
0.1350
0.2090
0.0400
0.0500
0.9990
0.9996
0.9962
0.9960
0.0106
0.0126
0.0000925
0.0000581
0.8873
0.7764
0.0080
0.0033
a
Calculated by pseudo-second-order model.
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a
Table 3. Summary of the Conditions in the Mixed-Adsorbate and the Single-Adsorbate Systems
Summary
mg/L
mg/L
mg/g
mg/g
kJ/m2 or kPa-m
kJ/m2 or kPa-m
Hexadacanamide
Indole
3-Phenylpropionitrile
C_non-competitive
C_competitive
Gam_non-competitive
Gam_competitive
Pi_non-competitive
Pi_competitive
7.03 × 1001
1.02 × 1002
1.18 × 1002
1.33 × 1002
4.36 × 10−06
3.79 × 10−06
45.8
8.03 × 1001
1.35 × 1002
7.92 × 1001
8.90 × 1001
6.23 × 10−06
6.00 × 10−06
9.97 × 1001
1.08 × 1002
1.00 × 1000
8.08 × 1001
5.70 × 10−06
1.15 × 10−06
8.7
% Increase in C_eq
% Reduction in Gam
67.9
3.7
13.0
79.8
a
Note: C, dissolved adsorbate concentration; Gam, adsorption density; Pi, surface pressure; C_eq, the dissolved adsorbate concentration at
equilibrium.
dominant nitrogenous compounds in microalgal bio-oil was
that of indole.
predicted data from this adsorption model were examined by
experimental results. Combining the data from the single
nitrogenous compound, the IAST model was developed to
predict the competitive adsorption of mixed nitrogenous
compounds, and this model was validated accurately. The
IAST model was in good agreement with the experimental
results from a mixed-adsorbate system. The performance of the
IAST model is determined by the quality of the single isotherm
data from each species and is unrelated to the theories or
assumptions underpinning the isotherm model.44 The IAST
model has been put forward to enhance predictability in some
cases, but it greatly improves the computational effort and
relies on a protocol to fit adsorption energies between different
molecules on local adsorption sites.45−47 The results suggest
that the IAST model can be used to simulate the removal of
nitrogen-containing functional groups from microalgal bio-oil,
thus avoiding the need for time-consuming experimental
investigations.
IAST Modeling. This section aims to establish a modeling
methodology to economically produce accurate outputs for
carbon adsorption performance in a mixed-adsorbate system.
According to the analytical data in this study and previous
studies,15,43,44 particular attention is given to the IAST model,
as an equilibrium model applied in the subsequent analysis of
multicomponent adsorption system. As a result, the IAST
model was applied to simulate the competitive equilibrium
adsorption of nitrogen-containing compounds by AC. It can
contribute to the further investigation of unanswered questions
which persist in the application of adsorption technology to
nitrogen-free fuel production. Due to the low concentration
and low removal efficiency of valeramide by AC, we chose
three nitrogen-containing compounds for the modeling (triple
mixtures). The competitive adsorption of hexadecanamide,
indole, and 3-phenylpropionitrile was studied by an IAST
model based on the study of Mark M. Benjamin.15
A summary of the conditions in the mixed-adsorbate and the
single-adsorbate systems is shown in Table 3. It can be seen
that the competition in the mixed-adsorbate system increases
the equilibrium dissolved concentrations of hexadacanamide,
indole, and 3-phenylpropionitrile by approximately 45.8%,
67.9%, and 8.7%, respectively, and reduces the corresponding
adsorption densities by 13%, 3.7%, and 79.8%, respectively.
The IAST model for the mixed-adsorbate system and a fit
with the experimental data are shown in Figure 7. The
CONCLUSIONS
■
This study has investigated the adsorption performance of AC
by focusing on four key aspects: the characterization of fresh
and spent AC, adsorption isotherm, adsorption kinetics, and
the IAST model. The adsorption capacity of AC for
hexadecanamide and indole is very high, but it is low for 3-
phenylpropionitrile and valeramide. Combining the data from
each nitrogenous compound, we developed a model to predict
the competitive adsorption of these nitrogenous compounds,
and this model was validated accurately. Further studies are
needed to examine the modified ACs for a complete removal
of all the nitrogenous compounds and to investigate if the
IAST model can be applied to microalgal bio-oil and different
bio-oil samples.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acssusche-
meng.9b03804.
Main components identified in the microalgal bio-oil,
typical IR spectra, XRD spectra, nitrogen adsorption−
desorption isotherms, scanning electron micrographs of
AC (PDF)
AUTHOR INFORMATION
Corresponding Author
*Phone: +86-27-87557464. Fax: +86-27-87557464. E-mail:
huzq@hust.edu.cn.
■
Figure 7. IAST model for the mixed-adsorbate system and a fit with
the experimental data.
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ORCID
Fanghua Li: 0000-0002-7696-6106
Zhiquan Hu: 0000-0002-6017-9108
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
Support was provided by the National Natural Science
Foundation of China (No. 21676112, 21975089) and
Wuhan Science and Technology Project (No.
2018060402011259). The help from Prof. Sankar Bhattachar-
ya, Prof. Warren Batchelor, and Dr. Srikanth Chakravarthula
Srivatsa at Monash University is also gratefully acknowledged.
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