A novel approach to vapor-phase synthesis of 2- and 4-methylquinoline from lactic acid and aniline

Article abstract of DOI:10.1016/j.catcom.2017.02.005

A novel and green route for vapor-phase synthesis of 2- and 4-methylquinoline was provided in this work, in which lactic acid as one of the reactants was for the first time employed. Various influencing factors, including types of catalysts, reaction temperature and stability of catalyst were investigated systematically. The results showed that a 67.6% total yield of quinolines was obtained over the HBeta catalyst. The characterization by using BET, NH₃-TPD and pyridine-IR techniques revealed that strong Br?nsted acid sites are favorable for generation of 2- and 4-methylquinoline whereas Lewis acid sites could increase the proportion of 4-methylquinoline in target products. Besides, a feasible reaction pathway to synthesize 2- and 4-methylquinoline was proposed on the basis of the reaction products.

Full text of DOI:10.1016/j.catcom.2017.02.005

Accepted Manuscript
A novel approach to vapor-phase synthesis of 2- and
4
-methylquinoline from lactic acid and aniline
An Li, Chen Huang, Cai-Wu Luo, Li-Jun Li, Wen-Jun Yi, Tian-
Wei Liu, Zi-Sheng Chao
PII:
S1566-7367(17)30050-X
doi: 10.1016/j.catcom.2017.02.005
CATCOM 4933
DOI:
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
26 October 2016
3 February 2017
6 February 2017
Please cite this article as: An Li, Chen Huang, Cai-Wu Luo, Li-Jun Li, Wen-Jun Yi,
Tian-Wei Liu, Zi-Sheng Chao , A novel approach to vapor-phase synthesis of 2- and
4
-methylquinoline from lactic acid and aniline. The address for the corresponding author
was captured as affiliation for all authors. Please check if appropriate. Catcom(2017), doi:
0.1016/j.catcom.2017.02.005
1
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ACCEPTED MANUSCRIPT
A Novel Approach to Vapor-phase Synthesis of 2- and 4-methylquinoline from Lactic Acid and
Aniline
†
†
†
†
†
†
*,†
An Li , Chen Huang , Cai-Wu Luo , Li-Jun LI , Wen-Jun Yi , Tian-Wei Liu , Zi-Sheng Chao
†
College of Chemistry and Chemical Engineering, Key Laboratory of Chemometrics & Chemical
Biological Sensing Technologies, Ministry of Education, Hunan University, Changsha, 410082,
China
*Corresponding author. Tel & Fax: +86-731-88713257; Email: zschao@yahoo.com
1

ACCEPTED MANUSCRIPT
ABSTRACT
A novel and green route for vapor-phase synthesis of 2- and
4
-methylquinoline was provided in this work, in which lactic acid as one
of the reactants was for the first time employed. Various influencing
factors, including types of catalysts, reaction temperature and stability of
catalyst were investigated systematically. The results showed that a
67.6% total yield of quinolines was obtained over the HBeta catalyst. The
characterization by using BET, NH -TPD and pyridine-IR techniques
3
revealed that strong Brønsted acid sites are favorable for generation of 2-
and 4-methylquinoline whereas Lewis acid sites could increase the
proportion of 4-methylquinoline in target products. Besides, a feasible
reaction pathway to synthesize 2- and 4-methylquinoline was proposed
on the basis of the reaction products.
Keywords: vapor-phase, lactic acid, 2- and 4-methylquinoline, HBeta
zeolite
2

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1
. Introduction
Quinolines and related compounds constitute an important category of
heterocyclic compounds. Among them, 2- and 4-methylquinoline are
widely used in medical, agriculture and industries.[1-5] For instance,
4-methylquinoline is one of the most importance intermediates of
photosensitive material used for colour cinefilm as sensitizer. In the early
years, the traditional liquid-phase synthesis routes,[6-10] including
Skraup, Doebner-Miller and Friedlander synthesis, were widely
employed to synthesize quinolines. However, they suffer from many
disadvantages, e.g., expensive, toxic feed and hard recycling of catalysts.
There is no doubt that the development of a gas-phase synthesis is of
great interest, as it has numerous advantages, including continuous
production, simplified product recovery, catalyst regenerability and
absence of acid waste stream, relative to the liquid-phase method. [11-13]
In the vapor-phase route, many good approaches have been applied
for the synthesis of methylquinolines. For instance, one synthesis from
quinoline and methanol (molar ratio of quinoline/methanol was 1:3) to
2-methylquinoline was performed. However, these reagents are either
expensive or toxicity.[14] Besides, acetaldehyde was employed to
synthesize 2- and 4-methylquiniline with aniline over HBeta zeolite
catalyst.[13] Unfortunately, acetaldehyde is toxic and derived from oil
resources. Moreover, acetaldehyde is prone to polymerization due to its
3

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strong activity, which results in the serious deactivation of catalyst.
Therefore, it is highly desired to exploit a green, sustainable, and low-cost
route for the synthesis of 2- and 4-methylquinoline. Lactic acid, as an
important product of industrial importance, can currently large-scaled
manufacture via fermentation approach.[15-18] Moreover, it is cheap,
environment-friendly and renewable materials.[19-21] Thus, efficient
utilization of abundant lactic acid is highly desired.
In this paper, we reported the vapor-phase synthesis of 2- and
4-methylquinoline, for the first time, utilizing lactic acid as one of the
reactants. Zeolite-based catalysts were employed in this reaction because
zeolites possess large surface area, high hydrothermal stability as well as
flexible adjustable acidity. Many factors like catalysts and reaction
temperature are investigated in detail. BET, NH -TPD and pyridine-IR
3
techniques are used to reveal the relationship between the structure of
catalysts and catalytic performance.
2
. Experimental
2.1. Catalysts Preparation.
The Beta, ZSM-5, Y and USY zeolites were supplied by Nankai
University Catalyst Factory. The modified-HBeta catalysts were prepared
by wet impregnation method. The typical procedures were as follows: the
5.0 g Beta powder was added into a 0.1 M ferric nitrate aqueous solution,
according to a calculated liquid / solid ratio, and then stirred vigorously at
4

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room temperature for 24 h. After that, the mixture was dried at 120 °C for
12 h and calcined at 550 °C for 4 h. The thus-obtained catalyst was
denoted as Fe/Beta. The similar procedure was also carried out for the
preparations of other metal-loaded Beta zeolite catalysts, which were
denoted as metal/Beta, where M = Fe, Co, Mn, Cr, Ni. In the above
prepared catalysts, the contents of metal component were all 5 wt%.
2.2. Catalysts Characterization
N -physisorption was conducted on a Quantachrome Autosorb-1
2
instrument at liquid-N temperature. Before measurement, the specimen
2
was in situ outgassed in the instrument at 300 °C for 12 h under a vacuum
-
8
of 10 Torr. The specific surface area was calculated by multipoint BET
equation, and the total pore volume was calculated at relative pressure of
P/P =0.99.
0
Temperature programmed desorption of NH (NH -TPD) was
3
3
determined on a Micromeritics Autochem II 2920 instrument equipped
with a thermal conductivity detector (TCD). The catalyst was first
pretreated at 400 °C for 30 min in a flow of helium (99.99%) with a flow
rate of 60ml/min, followed by cooling to 100 °C. Then, ammonia was
repeatedly pulse-injected in a stream of 10 % NH /He with a flow rate of
3
50 ml/min until its saturation adsorption over the catalyst had been
achieved. After purging with helium at 100 °C for 1.0 h, ammonia was
desorbed by heated the catalyst from 100 °C to 800 °C at a rate of
5

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10 °C/min.
Fourier transform infrared (FT-IR) spectroscopy of pyridine
absorption was recorded on a Varian 3100 spectrometer equipped with a
liquid nitrogen cooled MCT detector and a Cel beamsplitter. The catalyst
was first mixed with KBr powders by thoroughly grounding, and then,
the mixture was pressed into a disc and pre-heated at 300 °C for 4 h under
N atmosphere. After that, the catalyst was cooled to room temperature
2
and exposed to pyridine vapor for 90 min. Finally, the catalyst was heated
-
5
at 250 °C for 90 min in N atmosphere, followed by exposing to a 10
2
Torr vacuum at room temperature for 3 h, removing the gaseous and
physisorbed pyridine molecules. The thus-treated catalyst was subjected
to the FT-IR determination.
2.3. Catalytic Performance Evaluation
Catalytic performance test was carried out in a fixed-bed stainless
steel reactor. Aniline and lactic acid aqueous solution (concentration of
-
1
Lactic acid = 40 wt%, LHSV of lactic acid=0.5 h , molar ratio of
aniline/lactic acid =1:1) were initially vapored, respectively, and then fed
into the reactor in the presence of N (20 mL/min). The products mixture
2
was analyzed by a Varian Saturn 2200/ CP-3800 gas
chromatography-mass spectrometry equipped with two CP8944 capillary
columns (VF-5, 30 m  0.25 mm  0.25 m). The yield of products was
6

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calculated as follow based on the converted lactic acid, where the product
refers to quinoline, 2- and 4-methylquinoline.
ꢈꢀꢉꢃꢅꢇꢀꢌꢇꢉꢍꢎꢏꢆꢎꢇꢍꢎꢆꢐꢇꢎꢀꢁꢂꢃꢄꢏꢃꢐ
ꢀ
ꢁꢂꢃꢄꢅꢆꢀꢁꢇ(ꢈꢀꢉꢊ)ꢋ
ꢇꢓꢊ
ꢈꢀꢉꢃꢅꢇꢀꢌꢇꢉꢍꢎꢏꢆꢎꢇꢍꢎꢆꢐꢇꢆꢁꢑꢒꢏ
ꢖꢓꢈꢀꢉꢃꢅꢇꢀꢌꢇꢑꢄꢀꢐꢒꢎꢏ
ꢔꢃꢉꢃꢎꢏꢆꢂꢆꢏꢕꢇ(ꢈꢀꢉꢊ)ꢋ
ꢇꢓꢊ
ꢈꢀꢉꢃꢅꢇꢀꢌꢇꢉꢍꢎꢏꢆꢎꢇꢍꢎꢆꢐꢇꢎꢀꢁꢂꢃꢄꢏꢃꢐ
ꢗꢆꢃꢉꢐꢇ(ꢈꢀꢉꢊ)ꢋ ꢀꢁꢂꢃꢄꢅꢆꢀꢁꢇꢓꢔꢃꢉꢃꢎꢏꢆꢂꢆꢏꢕꢇꢓꢊ
. Resultand discussion
.1. Catalysts Characterization
The detailed textural properties of the pure and modified HBeta
zeolite catalysts are listed in Table 1. One can see than the surface area
S , S , S ), pore volume (V , V ) and micropore size (D ) over
3
3
(
BET
ext
mic
total
mic
mic
the NaBeta catalyst change little, relative to the pure HBeta one,
indicating that weak influence on textural properties of Beta zeolite
during Na-ion exchange process. However, the Fe/Beta catalyst has the
smaller surface area (S , S , S ), pore volume (V , V ) as well as
BET
ext
mic
total
mic
micropore size (D ) than the pure HBeta one. The decrease in the
mic
microporosity of the Fe/Beta catalyst may be due to the fact that its
micropores are occupied by active components (Fe) during the
preparation of catalysts.
The acidity property of the pure and modified HBeta zeolite
catalysts by NH -TPD (left) and FT-IR spectra of pyridine adsorption
3
(
right) are shown in Figure 1. For the NH -TPD characterization, the T
3
1
7

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peak is present for all catalysts, which is attributed to the weak acid sites
relating to the terminal acid silanol groups.[22, 23] The T peak is
2
identified evidently over the HBeta catalyst but virtually absent over the
other catalysts, which can be ascribed to strong Brønsted acid sites
relating to the bridging Si-(OH)-Al groups. The T peak appears distinctly
3
over the Fe/Beta catalyst, which is attributed to strong Lewis acid sites
deriving from Fe attached to Beta zeolite with certain interactions. In the
FT-IR spectra of pyridine-absorption, two bands at ca. 1540 and 1450
-
1
cm are assigned to the Brønsted and Lewis acid sites, respectively.
-
1
Compared to the NaBeta catalyst, the intensity of peak at 1540 cm is
-
1
larger over the HBeta one but the intensity of peak at 1450 cm is larger
over the Fe/Beta one. Combining the NH -TPD result, it is well
3
concluded that the HBeta catalyst possesses a larger concentration of
strong Brønsted acid sites while the Fe/Beta catalyst presents a larger
concentration of strong Lewis acid sites. On the contrary, the NaBeta
catalyst exhibits a less concentration of both strong Brønsted and Lewis
acid sites.
3.2. Catalysts Performances
The catalytic activities of catalysts are presented in Table 1. In this
reaction, lactic acid is fully converted at high reaction temperature, and
2-/4-methylquinoline and quinoline are successfully generated. Besides,
other products, such as, ethylquinoline, 3-methylindol, 1,2-dimethylindol,
8

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acetaldehyde, acrylic acid, crotonaldehyde, N-ethylaniline, are also
identified in the product mixtures via Varian GC-MS analysis. One can
see that the total yield over different type of zeolite catalysts shows an
order HBeta > HZSM-5 > HUSY > HY. This is most probably due to the
fact that the USY and Y zeolites exist supercages in which the deep
polymerization of intermediates and/or products could be accelerated
while the HZSM-5 zeolite exhibits small intersected pore channel (pore
sizes: 0.53 nm × 0.56 nm and 0.51 nm × 0.55 nm) which could imposes
diffusion limitation of bulky products;[24, 25] on the contrary, the Beta
zeolite possesses large pore channel (pore size: 0.66 nm × 0.67 nm and
0.56 nm × 0.56 nm) and its pore size matches well with quinolines
molecule size.[26] Thus, this unique property for Beta is favor to conduct
the shape selectivity in this reaction and therefore HBeta zeolite catalyst
exhibits the best catalytic activity among various zeolites in this work.
When metal-modified Beta zeolites are tested, respectively, the total yield
decreases in different degrees and the ratio of 4MeQ/2MeQ increases
distinctly. It indicates that the addition of metal on Beta zeolite affects the
total yield and products distribution, to some extent. Combining the
acidity property of catalysts as shown in Figure 1, it shows that HBeta
with the larger concentration of strong Brønsted acid sites is favorable for
synthesizing 2- and 4-methylquinoline; and the Fe/Beta catalyst with the
larger concentration of strong Lewis acid sites could increase the
9

ACCEPTED MANUSCRIPT
proportion of 4-methylquinoline in the target products. As shown in
Scheme 1, strong Brønsted acid sites promote the decarbonylation of
lactic acid to acetaldehyde and subsequent condensation of acetaldehyde
to crotonaldehyde, whereas strong Lewis acid sites could favor the former
process butrestrain the latter one.
Table 2 shows the effect of reaction temperature for vapor-phase
synthesis of quinolines. One can see that with the increase of reaction
temperature, the conversion of lactic acid increases and reaches 100% at
440 °C. Total yield increases with the elevation of temperature, achieving
67.6% as its maximums at 440 °C, and then decrease. Besides, the yield
of quinoline (Q) exhibits an increasing trend with the increase of
temperature, possibly due to dealkylation of alkylquinolines at high
temperature. Accordingly, the optimal reaction temperature is 440 °C.
Figure 2 shows the stability of the HBeta catalyst. The yield of
2-methylquinoline decreases with nearly constant rate, whereas that of
4-methylquinoline decreases slightly before 10 h and distinctly after 10 h.
The ratio of 4MeQ/2MeQ increases obviously, arriving ca. 1 as its
maximum at reaction time of 9 h, and then decreases distinctly. The
results indicate that 2-methylquinoline is more favorable to generate on
active sites, while its pathway is restrained more easily because of
catalyst deactivation. The total yield decreases at almost nearly speed and
still ca. 39% total yield can be obtained at 9 h, indicating that still more
10

ACCEPTED MANUSCRIPT
than 50% residual catalytic activity on catalyst can be remained after 9 h.
Anyway, a good stability of catalyst can be obtained in this reaction.
3
.3. Reactionpathways
When the reaction employing single lactic acid without aniline as
reactant is carried out over HBeta catalyst under the same reaction
condition, the major proportion of acetaldehyde and crotonaldehyde are
detected in the products based on GC-MS. Meanwhile, Brosius et al. [13]
reported that acetaldehyde could react with aniline to form 2- and
4-methylquinoline readily over Beta zeolite via different reaction pathway.
Thus, the generation of methylquinoline from lactic acid and aniline
undergoes two steps, namely, involving decarbonylation of lactic acid to
acetaldehyde and subsequent condensation of acetaldehyde and aniline
over acid sites (Eq. (1),).
The specific reaction pathways are shown in Scheme 1. Lactic acid firstly
converts to acetaldehyde via decarbonylation over acid sites.
Subsequently, crotonaldehyde (generated from the condensation of
acetaldehyde) reacts with aniline via Michael addition and subsequent
ring-closing electrophilic
-methylquinoline (2MeQ). By contrast, acetaldehyde reacts directly with
aniline to form enamine, and then the enamine further reacts with another
aromatic
substitution
to
produce
2
11

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acetaldehyde and generates 4-methylquinoline (4MeQ) via intramolecular
electrophilic aromatic substitution.
4
．Conclusion
A new efficient and convenient route was provided for vapor-phase
synthesis of 2- and 4-mthylquinoline from readily available starting
materials of lactic acid and aniline in this work. The reaction process
consists of decarbonylation of lactic acid to acetaldehyde and subsequent
condensation of acetaldehyde and aniline to 2- and 4-methylquinoline.
The HBeta catalyst exhibits the best catalytic activity and good catalytic
life. The strength and type of acid sites on catalyst significantly affects
the total yield and products distribution. Strong Brønsted acid sites are
favorable for synthesis of 2- and 4-methylquinoline; and the Lewis acid
sites increase the proportion of 4-methylquinoline in target products. A
plausible reaction pathway for the vapor-phase synthesis of 2- and
4-methylquinoline from lactic acid and aniline was proposed.
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (Grant 21376068), Program for New Century
Excellent Talents in University, the Ministry of Education of P. R. China,
and the Program for Fu-Rong Scholar in Hunan Province, P. R. China.
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[9]
6
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Table 1. The detailed textural properties of the pure and modified HBeta zeolite catalysts
2
2
2
3
3
Catalysts SBET(m /g) Sext(m /g) Smic(m /g) Vtotal(cm /g) Vmic(cm /g) Dmic(nm)
HBeta
NaBeta
Fe/Beta
543.2
531.3
403.6
122.4
140.4
76.47
420.8
390.9
327.1
0.35
0.34
0.24
0.17
0.17
0.14
0.61
0.61
0.58
Note: SBET, Sext and Smic ꢄꢃꢌꢃꢄꢅꢇꢏꢀꢇꢅꢑꢃꢎꢆfiꢎꢇꢅꢒꢄꢌꢍꢎꢃꢇꢍꢄꢃꢍ,ꢇꢃxꢏꢃꢄꢁꢍꢉꢇꢅꢒꢄꢌꢍꢎꢃꢇꢍꢄꢃꢍꢇꢍꢁꢐꢇꢈꢆꢎꢄꢀꢑꢀꢄꢃꢇꢅꢒꢄꢌꢍꢎꢃꢇꢍꢄꢃꢍ,ꢇꢄꢃꢅꢑꢃꢎꢏꢆꢂꢃꢉꢕ,ꢇꢍꢁꢐꢇꢔBET = Sext + Smic;
Vtotal and Vmic refers to total pore volume and micropore volume, respectively. Dmic refers to micropore pore size calculated by SF method.
14

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Table 2. Catalytic activity of various catalysts
Selectivity (%)
e
Catalyst
Conversion (%)
total yield
4MeQ/2MeQ
a
b
c
d
Q
2MeQ
2.1
4MeQ
11.2
11.2
11.5
21.8
25
Other
85.6
83.4
55.7
32.4
43.3
50.2
56.4
56.2
64.1
65.4
42.0
60.9
HY
100.0
100.0
100.0
100.0
100.0
99.3
1.1
0.7
10.4
5.1
5.4
5.2
3.7
4.7
3.4
4.8
4.9
6.7
14.4
16.6
44.3
67.6
56.7
49.5
43.2
43.6
35.8
34.6
57.3
38.9
5.33
2.38
0.51
0.54
0.95
0.47
0.75
0.99
0.78
0.90
0.52
0.54
HUSY
HZSM-5
HBeta
4.7
22.4
40.7
26.3
30.3
22.8
19.6
18.2
15.7
35.0
21.0
Fe/Beta
NaBeta
Co/Beta
Mn/Beta
Cr/Beta
Ni/Beta
F/Beta
14.3
17.1
19.5
14.2
14.1
18.1
11.4
99.0
99.5
99.8
100.0
98.8
P/Beta
99.4
Reaction temperature: 440 °C.
a~ c
corresponds respectively to quinoline, 2- and 4-methylquinoline.
d
other quinoline base products (1,2,3,4-tetrahymethyldroquinoline, ethylquinoline, butylquinoline), indol base products (indol, 3-methylindol,
,2-dimethylindol, N-methy-2,3-dimethylindol), acetaldehyde, acrylic acid, crotonaldehyde, N-ethylaniline, coking substances and etc.
total yield of Q, 2MeQ and 4MeQ.
1
e
15

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Table 3. Effect of reaction temperature over HBeta catalyst.
Selectivity (%)
o
e
Temperature ( C)
Conversion (%)
total yield (%)
a
b
c
d
Q
2MeQ 4MeQ Other
3
4
4
4
80
10
40
70
88.1
94.2
100.0
100.0
0.9
8.6
15.8
25.3
21.8
22.9
74.7
37.0
32.4
37.4
22.3
59.4
67.6
62.6
1.7 36.0
5.1 40.7
5.7 34.0
a~e
see the table 1.
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T1
c
1450
1540
b
a
T3
T2
c
b
a
1
00 200 300 400 500 600 700
1550
1500
1450
-
1
Temperature (C)
Wavenumber (cm )
Figure 1. The acidity property of the pure and modified HBeta zeolite catalysts by
NH3-TPD (left) and FT-IR spectra of pyridine adsorption (right). (a) HBeta; (b)
NaBeta; (c) Fe/Beta.
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1
00
1
.4
conversion of lactic acid (%)
yield of 4MeQ (%)
yield of 2MeQ (%)
total yields (%)
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
1.2
ratio of 4MeQ/2MeQ
1
0
0
0
0
.0
.8
.6
.4
.2
0.0
2
4
6
8
10 12
Time on stream/h
Figure 2. Stability of HBeta catalyst.
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Scheme 1. Proposed reaction pathways for synthesis of 2- and 4-quinoline from lactic
acid and aniline
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Highlights




2- and 4-methylquinoline was synthesized with lactic acid as one of reactants.
As high as 67.6% totalyield of quinolines were achieved over HBeta catalyst.
The strength and type of acid sites on catalyst affected the total yields.
The addition of metalon catalyst influenced the products distribution.
20