Synthesis of quinolines from aniline and propanol over modified USY zeolite: Catalytic performance and mechanism evaluated by: In situ Fourier transform infrared spectroscopy

Article abstract of DOI:10.1039/c7ra04526c

The reaction of aniline and propanol to quinolines was conducted in a fixed-bed flow-type reactor, using a series of modified USY zeolite catalysts. The structural, textural and acidic properties of the catalyst were characterized by XRD, N₂-physisorption, ²⁷Al MAS NMR, NH₃-TPD and pyridine-FTIR, while the mechanism for the reaction of aniline and propanol was investigated by in situ FTIR. It was identified that the reaction of aniline and propanol generated predominantly quinolines, including 2-ethyl-3-methylquinoline and other alkyl quinoline, N-alkyl aniline and other byproducts. Among others, the ZnCl₂/Ni-USY catalyst exhibited the best performance, providing a 96.4% conversion of aniline and a 78.3% total yield of quinolines with 81.2% total selectivity to quinolines and 60.1% selectivity to 2-ethyl-3-methylquinoline at 683 K. This was attributed to the larger concentration ratio of Lewis acid sites to Bronsted acid sites over the ZnCl₂/Ni-USY catalyst, relative to other catalysts. There were predominantly two possible routes for the formation of quinolines, which required predominantly Lewis acid sites and Bronsted acid sites, respectively. In both the routes, N-phenylpropan-1-imine was proposed as the key intermediate. Relative to that based on Bronsted acid sites, the route based on Lewis acid sites appeared to contribute much more in the generation of quinolines from the reaction of aniline and propanol.

Full text of DOI:10.1039/c7ra04526c

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Synthesis of quinolines from aniline and propanol
over modified USY zeolite: catalytic performance
and mechanism evaluated by in situ Fourier
transform infrared spectroscopy
Cite this: RSC Adv., 2017, 7, 24950
*
Chen Huang, An Li,
Li-Jun Li and Zi-Sheng Chao
The reaction of aniline and propanol to quinolines was conducted in a fixed-bed flow-type reactor, using
a series of modified USY zeolite catalysts. The structural, textural and acidic properties of the catalyst were
characterized by XRD, N2-physisorption, 27Al MAS NMR, NH3-TPD and pyridine-FTIR, while the mechanism
for the reaction of aniline and propanol was investigated by in situ FTIR. It was identified that the reaction of
aniline and propanol generated predominantly quinolines, including 2-ethyl-3-methylquinoline and other
alkyl quinoline, N-alkyl aniline and other byproducts. Among others, the ZnCl2/Ni-USY catalyst exhibited
the best performance, providing a 96.4% conversion of aniline and a 78.3% total yield of quinolines with
81.2% total selectivity to quinolines and 60.1% selectivity to 2-ethyl-3-methylquinoline at 683 K. This was
attributed to the larger concentration ratio of Lewis acid sites to Bronsted acid sites over the ZnCl2/Ni-
USY catalyst, relative to other catalysts. There were predominantly two possible routes for the formation
of quinolines, which required predominantly Lewis acid sites and Bronsted acid sites, respectively. In
both the routes, N-phenylpropan-1-imine was proposed as the key intermediate. Relative to that based
on Bronsted acid sites, the route based on Lewis acid sites appeared to contribute much more in the
generation of quinolines from the reaction of aniline and propanol.
Received 22nd April 2017
Accepted 28th April 2017
DOI: 10.1039/c7ra04526c
rsc.li/rsc-advances
containing a,b-unsaturated carbonyl or b-diketone unit, using
homogeneous Lewis or Bronsted acid catalyst, such as SnCl4,
1. Introduction
Sc(OTf)3, p-toluenesulfonic acid, perchloric acid and sulphuric
acid. The product can be quinoline, 2-alkylquinoline or 2,4-
dialkylquinoline, dependent on the carbonyl-containing reac-
tant. The reaction mechanism consists of the nucleophilic
attack of the amino group at the active carbon atom in either
carbonyl group or unsaturated bond, the cyclization occurs
between carbonyl group and ortho-position of aniline, and then,
the oxide hydrogenation to generate the quinoline structure.
The presence of an electron-attracting active group, such as
halogen atom, phenyl, carboxyl and alkoxy, in the reactant
promotes further the formation of the quinoline structure. The
Quinolines including quinoline and its derivatives, as a group of
most valuable heterocycle compounds, are widely found in
natural products.1–4 Many quinolines are reported to possess
biological activities, such as antimicrobial, antifungal, anti-
malarial and anti-neoplastic,5 and they have been broadly
applied in pharmaceuticals, insecticidal agents and functional
chemicals like dyestuffs and ligands in agriculture.6 Therefore,
the production of quinolines is of signicant importance.
Quinolines were rst produced via extraction from coal tar.
However, this process was associated with many problems, e.g.,
low production (ca. only 0.3–0.5 content of quinolines in coal
car), high energy input and large environmental pollution.
Therefore, the chemical synthesis of quinolines is receiving
more and more attention. Several traditional methods for the
synthesis of quinolines have been reported in the literature,7
and they can be classied predominantly into three groups. The
rst group is mainly composed of the Skraup, Combes, Doeb-
ner–Miller and Combes–Limpach methods.8 These methods
¨
second group is mainly composed of the Friedlander, Camps,
Niementowski and Ptzinger methods.9 These methods are
based on the reaction between an ortho-substituted aniline or
nitrobenzene, in which the substitute group can be acyl, formyl,
hydromethyl, halomethyl, trioromethyl, cyano, vinyl, acetylene
or allyl, and a carbonyl compound containing active a-methy-
lene group, generating multi-substituted quinolines. Homoge-
neous catalysts, such as, transition metal chlorides, inorganic/
organic acids and alkalines, organometal agents, noble metal
coordination complexes, are usually employed in these reac-
tions. The reaction mechanism comprises the nucleophilic
attack of amino at the carbonyl carbon in another reactant to
involve the reaction between aniline and
a compound
College of Chemistry and Chemical Engineering, Hunan University, Changsha,
410082, China. E-mail: chao_zs@aliyun.com; zschao@yahoo.com; Fax: +86-731-
88713257; Tel: +86-731-88713257
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generate imine structure, the nucleophilic attack of carbon in size, being comparable to the molecular sizes of quinolines so
imine structure at the ortho-carbonyl carbon in benzene cycle, as to provide a favorable shape-selective catalysis effect.
and then the dehydration to generate quinoline structure.
Y zeolite has a similar larger pore size as BEA zeolite,
Besides the aniline and carbonyl compounds involved in the however, the former is relatively cheaper than the latter.
above two group methods, other starting materials are also Compared to carbonyl-containing compounds, propanol is
employed in the synthesis of quinolines, constituting the third inexpensive, greener and more available. In this paper, we
group method for the synthesis of quinolines. For examples, an report for the rst time the synthesis of quinolines from the gas
o-acylaminoacetophenone was directly transformed into two phase reaction of aniline and propanol basing on modied
different hydroxyquinolines, using hydroxide ion as catalyst.10 ultra-stabilized Y (USY) zeolite catalyst. The effects of modier,
Cho et al.11 reported the synthesis of quinolines via the RuCl3- reaction temperature and space velocity on the catalytic
$nH2O/SnCl2$2H2O catalyzed reaction of aniline and trialkyl- performance are investigated, while the structural property of
amine, involving the formation of Schiff-base structure as a key catalyst is identied by means of XRD, BET, 27Al MAS NMR and
intermediate. All the above traditional methods have been FTIR. Particularly, the adsorptions of aniline and propanol and
based on the liquid phase reaction using homogeneous catalyst, the reaction of the two over the surface of catalyst are studied by
and they are generally suffered from many drawbacks, such as in situ FTIR, and this enables us to propose the mechanism for
the expensive or toxic feedstock, the anhydrous and tedious the reaction of aniline and propanol to quinolines over modi-
work-up procedure, the volatile organic solvent, the corrosive, ed USY zeolite catalyst.
costly and hard recyclable catalyst, the unsatised yield and
selectivity to aimed product, and the prolonged reaction time.
Contrastively, the synthesis of quinolines in gas phase reaction
2. Experimental
2.1 Chemicals
basing on heterogeneous catalyst can overcome most of the
above problems and thus is receiving more and more
attentions.8–14
All the chemicals for the synthesis of quinolines and the prep-
aration of catalyst were purchased from commercial companies
and had purities higher than 99%. These chemicals were used
as received, without further purication.
USY (Si/Al ¼ 10) zeolite powder (SINOPEC Catalyst Co. LTD.,
Changling Division) was calcined at 550 ꢀC for 8 h, before being
used.
A few heterogeneous catalysts for the gas phase synthesis of
quinolines have been reported in the literature, and they can be
classied mainly into three groups: (I) non-zeolitic solid acid
catalysts,9–14 such as amorphous Si–Al, inorganic acid-loaded
Kaolin, acidic metal salt or oxide-modied amorphous Si–Al
and Kaolin. They can catalyze the reaction between aniline and
aldehyde or glycol into quinoline and 2-alkylquinoline at ca. 40–
60% yield.13 Most of the reports indicate that both the Bronsted
and Lewis acid sites in the catalyst are favorable to the forma-
2.2 Preparation of catalyst
Ion-exchange method. USY zeolite powder was rst
tion of quinolines, with the Lewis acid site contributing rela- dispersed into a 0.1 M nickel nitrate aqueous solution,
tively less;8 however, some reports also show that the presence according to a weight ration solid/liquid ¼ 1/10, at room
of Lewis acid site retards the formation of quinolines.14 There- temperature and under stirring. Then, the mixture was heated
fore, the role of acid site is still uncertain; (II) mixed metal oxide to 373 K under reuxing and stirring, and it was remained at
catalysts,8,12,13 including mainly ZnO–Cr2O3, CuO–ZnO/Al2O3, that state for ca. 6 h before cooled to room temperature. Aer
MoO–V2O5/Al2O3 and NiO–MoO3/Al2O3. They can catalyze the that, solid was recovered by ltration, washing with deionized
reaction between aniline and glycerol in the presence of oxygen water, drying at 373 K and calcining at 823 K for 4 hours. These-
to generate quinoline, however, the yield of quinoline is rela- obtained was subjected the above whole procedure for three
tively low, usually below 40%, due to its deep oxidation by time, with fresh nickel nitrate aqueous solution being employed
oxygen.13 The acid property of catalyst is found to show each time. Finally, the nickel ion exchanged USY zeolite, namely
a promotion on the formation of quinoline.15 (III) Zeolite cata- Ni-USY, was obtained, and it was not only directly employed as
lysts, including mainly MOR, FER, MFI and BEA. They can catalyst but also further subjected to the subsequent modica-
catalyze the reaction of aniline and aldehyde to quinoline, 2- tion procedure. The nickel content in the Ni-USY catalyst was
and 4-methylquinolines at ca. 50–80% yield.12–14 Among these determined as 4 wt%.
zeolites, BEA is reported to exhibit the largest catalytic effect,
Impregnation method. The above-obtained Ni-USY was
and the addition of NH4F promotes further the yields of quin- dispersed into a ZnCl2 aqueous solution, according to a weight
olines.14 This can be ascribed to the delamination by NH4F to ratio ZnCl2/Ni-USY ¼ 1/9, and then the mixture was stirred at
generate the extra-framework Al species, increasing the ratio of room temperature for 24 h. Aer that, the mixture was heated at
Lewis/Bronsted acid sites and reducing the total acid concen- ca. 363 K to evaporate slowly water, followed by drying at 523 K
tration.14 It can be seen that, among all the above heterogeneous overnight. The thus-prepared catalyst was denoted as ZnCl2/Ni-
catalysts, zeolites have exhibited the largest activity for the USY, in which the ZnCl2 loading was determined as ca. 10 wt%.
generation of quinolines. This may be due to the facts that
Deposition–precipitation method. A superuous amount of
zeolites usually possess an adjustable and controllable acid urea was rst dissolved into a calculated amount of 0.1 M nickel
performance, which is pivotal to the formation of quinolines, nitrate aqueous solution, by which the nickel ions could be
while BEA among various zeolites has a relatively larger pore theoretically completely deposited in the case of heating, under
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stirring at room temperature. Then, the resultant clear solution reactant was pumped at a liquid hourly space velocity (LHSV) of
was added by USY zeolite powder, according to a weight ratio 0.6 to 1.9 hꢂ1, passing through an evaporator heated at 573 K,
and the mixture was heated to 363 K with vigorous stirring for into the reactor, while the ow of carrier gas was maintained.
4–6 h. Aer that, solid was recovered by ltration, washing with Aer the reaction had been run stably for 2 h, the effluent from
deionized water and drying at 393 K for 12 h, followed by the reactor was cooled by a condenser at ice-water temperature
calcining at 823 K for 4 h. The thus-prepared catalyst was to collect the products mixture.
denoted as Ni/USY, in which the nickel loading was determined
as ca. 4 wt%.
The analysis of products mixture was conducted over a Var-
ian CP-3800/Saturn 2200 gas chromatograph-mass spectrom-
eter (GC-MS). Two CP8944 capillary columns (30 m ꢁ 0.25 mm
ꢁ 0.25 mm) were respectively connected to mass detector and
ame ionization detector (FID) for the quantitative and quali-
tative analyses. Basing on the converted aniline, the conversion
of aniline (c), selectivity to component i (Si) and yield of quin-
olines (YQS) were respectively calculated as follows:
2.3 Characterization of catalyst
X-ray diffraction spectroscopy (XRD) was performed with
a Bruker D8-Advance X-ray diffractometer. The operation
˚
conditions were as follows: Cu target Ka ray (l ¼ 1.54187 A);
scanning voltage 40 kV, scanning current 40 mA; scanning
speed 0.2 s, scanning step 0.02ꢀ.
Naniline in feed ꢂ Naniline in products mixture
c ð%Þ ¼
ꢁ 100
Naniline in feed
N2-physisorption was performed at liquid nitrogen temper-
ature using a Quantachrome Autosorb-1 instrument. Before the
measurement, the specimen was degassed for 16 h at 573 K
under a vacuum of 4 ꢁ 10ꢂ4 Pa.
Ni
X
Si ð%Þ ¼
ꢁ 100
Ni
NH3 temperature programmed desorption (NH3-TPD) was
determined on a Micromeritics AutoChem II 2920 instrument
equipped with a thermal conductivity detector (TCD). The
specimen was rst degassed in a ow of helium with a ow rate
of 50 mL minꢂ1 at 773 K for 30 min, followed by cooling to 373
K. Then, NH3 was repeatedly pulse-injected until a saturation
adsorption over the specimen had been achieved. Aer that,
NH3 was desorbed by heating the specimen from 373 to 1023 K
at a rate of 15 K minꢂ1. During the adsorption and desorption of
NH3, the helium ow was retained and its ow rate was main-
YQS (%) ¼ c ꢁ SQS ꢁ 100,
where Naniline in feed and Naniline in products mixture referred to the
moles of aniline in the feed and in the products mixture, Ni to
the moles of component i in the products mixture and SQS to the
total selectivity to quinolines, respectively.
The carbon balance (Yc), basing on aniline, for the reaction
between aniline and propanol was approximately evaluated as
follows:
tained constant at 60 mL minꢂ1
.
The Ni and Zn contents of catalysts were determined over
a Varian 240AA atomic absorption spectrometer. The operation
conditions: sample aspiration rate ¼ 3.0 mL minꢂ1; lamp
current ¼ 3.0 mA; slit width ¼ 0.2 nm; air ow rate ¼ 0.8 mL
Ff
Yc ð%Þ ¼
ꢁ 100;
Fp
minꢂ1 and acetylene ow rate ¼ 5.0 mL minꢂ1
.
where Ff and Fp referred to the ow rates of feed and products
mixture, respectively.
27Al magic angle spinning Nuclear Magnetic Resonance
spectroscopy (27Al MAS NMR) was conducted over a Bruker
AVANCEIII (400 MHz) spectrometer at room temperature. The
operation conditions were as follows: sample spinning rate ¼ 12
kHz; pulse width ¼ 1.0 ms; recycling delay ¼ 500 ms; resonance
frequency ¼ 130.32 MHz.
2.5 In situ Fourier transform infrared (FTIR) spectroscopy
The in situ FTIR spectroscopic study was conducted on a set of
home-made reaction system, which was schematically illus-
trated in Fig. 1a. The reaction system consisted of feeding,
vacuum, reaction cell and FT-IR monitoring units. The reaction
cell (Varian company, model HVC-DRP), possessing a volume of
2.4 Evaluation of catalytic performance
Catalytic performance test was carried out in a xed-bed tubular ca. 4 mL, consisted of a cover and a pedestal (Fig. 1b), which
quartz reactor (i.d. 6 mm, total length 400 mm) at atmospheric were sealed together by silicone O-ring and mounting nuts. The
pressure. The catalyst (2.0 g) was located in the middle of cover contained two 10 ꢁ 10 ꢁ 2 mm CaF2 windows and two gas
reactor, with the upper space of catalyst bed in the reactor being ow vents. The pedestal possessed a recessed region for the
lled with quartz granulates (30 meshes). The reactor was loading of catalyst, and it could be heated by a temperature
electronically heated, and the temperature was monitored by controller. The reaction cell was mounted in the specimen
a thermocouple that contacted tightly with the outer wall of chamber of a Varian 3100 FTIR spectrometer equipped with
reactor in the zone containing catalyst. A homogeneous solu- a MCT detector and CeI beamsplitter. The feed, either pyridine
tion consisting of aniline and n-propanol (1 : 2 molar ratio) was or aniline and/or propanol, was charged into a pre-evacuated
employed as the reactant. At rst, the catalyst was in situ pre- vessel (10ꢂ3 Pa), of which the temperature (ca. 298–773 K) was
treated at a certain reaction temperature ranging from 623 to regulated by a preheater. At rst, a catalyst (ca. 0.1 g) was loaded
713 K for 1 hour in a ow of carrier gas (H2 or air) with a preset in the reaction cell and evacuated to (10ꢂ3 Pa) at 773 K for 4 h.
gas hourly space velocity of 300–900 hꢂ1 (GHSV). Then, the Aer that, the catalyst was cooled to room temperature, and
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Fig. 1 Schematic illustrations of in situ FTIR reaction devices (a) and reaction cell (b).
then, the vapor of feed was carried by a ow of nitrogen (ca. 200 byproducts other than the N-alkylaniline (NPA and DNPA) are
mL minꢂ1), regulated by needle valve and monitored by ow- also arbitrarily named as “others”, being listed in Table 1. These
meter, into the reaction cell. The reaction was thus carried out byproducts are generated respectively via the alkylation of
at various preset temperature points and for various periods of aniline by propanol on the amino group and benzene ring and
time, while FTIR spectra were recorded from 400 to 4000 cmꢂ1 the cyclization between aniline and a certain intermediate
at a scanning number of 32 and a resolution of 4 cmꢂ1
.
related to propanol. All the conversion of aniline, total selectivity
to quinolines and selectivity to 2E-3MQ as well as the total yield
of quinolines over various catalysts (entries 1–5 in Table 1) show
an order ZnCl2/Ni-USY > Ni-USY > Ni/USY > USY, while the
selectivity to NPA displays the reverse order to the above. This can
be a result of the variations in the textural and acidic properties
for these catalysts, which is discussed in the following section.
The above result also hints that NPA may have originated from
a certain important intermediate (N-phenylpropan-1-imine; see
in Section 3.4), which also leads to the formation of quinolines
during the reaction of aniline and propanol.11 Over the ZnCl2/Ni-
USY catalyst, the conversion of aniline, total selectivity to quin-
olines and selectivity to 2E-3MQ as well as the total yield of
quinolines are all increased but the selectivity to NPA decreased
with increasing the reaction temperature from 623 K to 683 K (cf.
entries 4 and 5 in Table 1). The further studies on the effect of
reaction temperature (Fig. 2) shows that, with increasing the
reaction temperature in the range of 623 K to 703 K, the
3. Results and discussion
3.1 Catalytic performance test
Table 1 shows the results for the reaction of aniline and n-
propanol over various catalysts. One can see that, over all the
catalysts, 2-ethyl-3-methylquinoline (2E-3MQ) as the predomi-
nant quinolines and 2,3-dimethylquinoline (2,3-DMQ), 2-ethyl-
quinoline (2-EQ) and 2-methyl quinoline (2-MQ) as the minor
ones are generated. Theoretically, one molecular 2E-3MQ can be
generated from tow molecular propanol. Accordingly, the other
quinolines apart from 2E-3MQ, including 1,3-DMQ, 2-EQ and 2-
MQ, should have been predominantly generated via the disso-
ciation of 2E-3MQ. Besides quinolines, the byproducts
including N-propylaniline (NPA) as the main component and
N,N-dipropylaniline (DNPA), 2-propylaniline, 4-propylaniline
and 3-methylindole as the minor ones are also identied. The
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Table 1 Results for the reaction of aniline and n-propanol over various catalystsa
Sic (%)
Temp (K) Carrier gas cb (%) 2E-3MQf 2,3-DMQg 2-EQh 2-MQi NPAj DNPAk Othersl SQSd (%) YQSe (%)
Entry Catalyst
1
2
3
4
5
6
USY
Ni/USY
Ni-USY
ZnCl2/Ni-USY 623
ZnCl2/Ni-USY 683
ZnCl2/Ni-USY 683
623
623
623
H2
H2
H2
H2
H2
Air
57.8
61.2
66.7
87.2
96.4
85.3
34.6
37.6
42.3
47.4
60.1
52.9
9.2
9.2
10.1
13.3
20.7
14.2
2.1
1.7
1.8
0.2
0.4
0
2.6
1.4
1.1
0
0
0
40.8 2.3
39.3 5.2
39.5 2.1
26.4 2.9
9.8 3.4
8.4
5.6
3.1
9.8
5.6
5.5
48.5
49.9
55.3
60.9
81.2
67.1
28.0
30.5
36.8
53.1
78.3
57.2
23.5 3.9
a
b
Catalyst weight ¼ 2.0 g; carrier gas GHSV ¼ 300 hꢂ1; feed LHSV ¼ 0.8 hꢂ1
.
c: conversion of aniline. c Si: selectivity to component i in products
d
e
f
mixture. SQS: total selectivity to quinolines in products mixture. YQS: total yield of quinolines in products mixture. 2E-3MQ: 2-ethyl-3-
g
h
i
j
k
methylquinoline. 2,3-DMQ: 2,3-dimethylquinoline. 2-EQ: 2-ethylquinoline. 2-MQ: 2-methyl quinoline. NPA: N-propylaniline. DNPA: N,N-
dipropylaniline. l Others: 2-propylaniline, 4-propylaniline and 3-methylindole.
increasing both the carrier gas GHSV and products mixture
LHSV, the conversion of aniline and yield of quinolines as well
as the selectivities to quinolines and “others” are all decreased,
however, both the selectivities to N-alkylaniline (NPA and
DNPA) and the carbon balance are increased. This is due to the
fact that the generation of quinolines and “others” can be
a result of the further conversion of the important intermediate
related to N-alkylaniline (N-phenylpropan-1-imine; see in
Section 3.4). The increase in the carrier gas GHSV and products
mixture LHSV removes quickly N-alkylaniline from the catalyst
bed and thus decreases largely the chance for the further
conversion of the important intermediate, being related to N-
alkylaniline, into quinolines and “others”, and it also reduces
the secondary reaction of various products and byproducts, e.g.,
pyrolysis, so as to increase the carbon balance.
Fig. 2 Effect of reaction temperature on the reaction of aniline and
propanol over ZnCl2/Ni-USY catalyst (2 g catalyst, H2 carrier gas GHSV
¼ 300 hꢂ1, feed LHSV ¼ 0.8 hꢂ1).
3.2 Catalyst characterization
Fig. 3 shows the XRD patterns for various catalysts. One can see
that all the XRD patterns of various catalysts exhibit the char-
acteristic diffraction peaks of USY zeolite, being very similar to
conversion of aniline increases all through, however, both the
total selectivity to quinolines and the yield of quinolines increase
rst, achieving their maxima at 683 K, and then decrease. At the
each other, while the diffraction peak intensities for various
optimized temperature (683 K), a 96.4% conversion of aniline
catalysts shows an order ZnCl2/Ni-USY < Ni-USY < USY. Besides,
with a 78.3% total yield of quinolines at 60.1% selectivity to 2E-
3MQ and 81.2% total selectivity to quinolines have been ach-
ieved over the ZnCl2/Ni-USY catalyst. When the carrier gas H2 is
replaced by air (cf. entries 5 and 6 in Table 1), the conversion of
the enlargement of XRD pattern shows that the diffraction
peaks have shied towards high angle direction over the
modied USY catalyst, particularly over the ZnCl2/Ni-USY cata-
lyst, relative to the USY catalyst. The above result indicates that,
aniline, total selectivity to quinolines and selectivity to 2E-3MQ as
while the basic structure of USY zeolite is remained, the partial
well as the total yield of quinolines are all decreased but the
dealumination of zeolite framework and thus its destruction to
selectivity to NPA increased. It shows that the employment of H2
some extent is also present during the modication of USY
zeolite.16 This would affect the textural and acid properties and
in turn the catalytic performance of modied USY catalyst.
Fig. 4 displays the 27Al MAS NMR spectra of USY and Ni-USY
zeolite catalysts. One can see that two peaks at ca. d ¼ 0 and
60 ppm are present over both the USY and Ni-USY catalysts,
which can be respectively ascribed to the 6-coordinate extra-
as carrier gas is more favorable over air carrier gas for the reac-
tion of aniline and propanol to quinolines. This is due to the fact
that the formation of quinolines involves the cyclization–
aromatization over the low-valent metal active site, which can be
generated and stabilized in the reductive atmosphere but not the
oxidative one. Therefore, H2 has been employed as the carrier gas
in the subsequent experiments.
framework
aluminum
and
4-coordinate
framework
aluminum.17,18 The intensities for the two peaks decrease obvi-
ously over the Ni-USY catalyst, relative to the USY catalyst. It
indicates that the nickel ion exchange leads to not only the
Table 2 shows the effect of carrier gas GHSV and products
mixture LHSV on the reaction of aniline and propanol over
various catalysts. One can see that, over all the catalysts, with
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Table 2 Effect of carrier gas GHSV and feed flow rate on the reaction of aniline and propanol over various catalystsa
Sie (%)
GHSV (hꢂ1
)
Ffb (hꢂ1
)
Fp (hꢂ1
)
c
d (%) 2E-3MQi 2,3-DMQj 2-EQk 2-MQl NPAm DNPAn Otherso SQS (%) YQSg (%) Ych (%)
c
f
Catalyst
USY
300
600
900
300
600
900
0.673
1.26
1.79
0.631
1
1.81
0.676
1.31
1.88
0.558
1.09
1.60
0.525
1.07
1.66
0.599
1.21
1.80
57.8
56.4
53.2
66.7
64.3
63.1
87.2
85.3
82.6
34.6
33.7
32.5
42.3
39.5
37.2
47.4
45.6
43.3
9.2
8.7
7.7
10.1
9.7
9.6
13.3
12.5
11.9
2.1
1.9
1.8
1.8
1.6
1.5
0.2
0
2.6
2.1
1.8
1.1
1.1
1.0
0
40.8
43.2
46.1
39.5
42.2
44.9
26.4
29.7
32.6
2.3
2.4
2.7
2.1
2.4
3.0
2.9
3.1
3.4
8.4
8
48.5
46.4
43.8
53.3
51.9
49.3
60.9
58.1
55.2
28.0
26.2
23.3
35.6
33.4
31.1
53.1
49.6
45.6
82.9
86.7
89.7
83.3
86.9
91.6
88.6
92.5
95.5
7.4
3.1
3.5
2.8
9.8
9.1
8.8
Ni-USY
Zn/Ni-USY 300
600
900
0
0
0
a
Catalyst weight ¼ 2.0 g; reaction temperature ¼ 623 K; carrier gas ¼ H2. b Ff: the ow rate of feed (LHSV). c Fp: the ow rate of products mixture
(LHSV). d c: conversion of aniline. e Si: selectivity to component i in products mixture. f SQS: total selectivity to quinolines in products mixture. g YQS
:
total yield of quinolines in products mixture. h Yc: carbon balance. i 2E-3MQ: 2-ethyl-3-methylquinoline. j 2,3-DMQ: 2,3-dimethylquinoline. k 2-EQ:
2-ethylquinoline. l 2-MQ: 2-methyl quinoline. m NPA: N-propylaniline. n DNPA: N,N-dipropylaniline. o Others: 2-propylaniline, 4-propylaniline and
3-methylindole.
Fig. 3 XRD patterns for various catalysts. The bottom patterns marked by (1)–(3) are the enlargements of the areas marked in the upper patterns.
(a) USY; (b) Ni-USY; (c) ZnCl2/Ni-USY.
removal of a proportion of extra-framework aluminum in zeolite may be due to the fact that, during the preparation of Ni/USY
but also the partial dealumination of zeolite framework.
catalyst via the deposition–precipitation method, a proportion
Table 3 shows the textural properties determined by N2- of nickel species are deposited in the micropores of USY zeolite,
physisorption over various catalysts. One can see that the decreasing the microporosity of Ni/USY catalyst. Compared to
specic surface area (SBET), micropore surface area (Smicro), the USY catalyst, both the SBET and Smicro decrease slightly and
micropore volume (Vmicro) and total pore volume (Vtotal) are all both the Vmicro and Vtotal increase over the Ni-USY catalyst. It
smaller over the Ni/USY catalyst than over the USY catalyst. This may be due to the fact that a certain amount of extra-framework
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Fig. 5 In situ FTIR spectra for the pyridine adsorption on various
catalysts. (a) USY, (b) Ni-USY and (c) ZnCl2/Ni-USY.
Fig. 4 27Al MAS NMR spectra for various catalysts. (a) USY; (b) Ni-USY.
Table 3 Textural properties for various catalystsa
charge-compensating cations Mn+ like transition metal ions.21
The area ratio of the peak at 1450 cmꢂ1 to that at 1538 cmꢂ1, i.e.,
the concentration ratio of Lewis acid sites to Bronsted acid sites,
is calculated to be 0.79, 0.95 and 1.03 for the USY, Ni-USY and
ZnCl2/Ni-USY catalysts, respectively, indicating the sequential
increase in the concentration of Lewis acid sites for the modi-
cation of USY by nickel ion exchange and ZnCl2 loading. This
can be due to the fact that, for the Ni-USY catalyst, the nickel ion
exchange leads to not only the removal of extra-framework
aluminum but also the partial dealumination of zeolite frame-
work, therefore decreasing both the concentrations of Lewis
and Bronsted acid sites; however, the nickel ions exchanged
onto the USY zeolite can behave as Lewis acid sites, and this
enables the concentration ratio of Lewis acid sites to Bronsted
acid sites to be higher over the Ni-USY catalyst than over the
USY catalyst. For the ZnCl2/Ni-USY, the introduction of zinc ions
provides additional Lewis acid sites and thus increases further
the concentration ratio of Lewis acid sites to Bronsted acid sites,
relative to Ni-USY. In fact, it had been reported that the ZnCl2
modied zeolite catalysts employed in the Friedel–Cras
alkylation reactions could provide more Lewis acid sites than
the transition metal modied zeolite catalysts.22,23
Fig. 6 shows the NH3-TPD proles for various catalysts. The
temperature at the maximum (Tm,i) and integral area (Ai) of
desorption peak, which correspond respectively to the strength
and concentration of acid sites, are summarized in Table 4. The
peaks over these desorption proles can be approximately
classied into three groups, i.e., the low-temperature (<500 K;
Tm,1), medium-temperature (500–700 K; Tm,2 and Tm,3) and
high-temperature (>700 K; Tm,4) peaks, which corresponds to
the weak, moderate and strong acid sites, respectively. The Tm,1
peak has a desorption temperature centered at ca. 420–433 K
over all the catalysts, and its area decreases slightly aer nickel
exchange and ZnCl2 loading. This peak can be ascribed to the
weak acidic terminal silanol group.24 Over the USY catalyst, the
SBET
Smicro
Vmicro
Vtotal
Catalyst
(m2 gꢂ1
)
(m2 gꢂ1
)
(cm3 gꢂ1
)
(cm3 gꢂ1
)
USY
Ni/USY
Ni-USY
718
554
705
635
476
623
0.32
0.25
0.34
0.41
0.38
0.47
a
Note: SBET and Smicro refer to specic surface area and micropore
surface area and Vmicro and Vtotal to micropore pore volume and total
pore volume, respectively.
aluminum species has been enwrapped in the micropores of USY
catalyst during its preparation; besides, an additional amount of
extra-framework aluminum species is also generated via the
partial dealumination of zeolite framework during the nickel ion
exchange, resulting in, to some extent, the destruction of zeolite
structure.19 Both the above extra-framework aluminum species
have been removed by water washing during the preparation of
Ni-USY catalyst. This enables the Ni-USY catalyst to possess
a slightly lower surface area but higher pore volume. Compared
to the Ni/USY catalyst, the SBET, Smicro, Vmicro and Vtotal are all
larger over the USY catalyst. It indicates that the nickel ions
possesses a higher dispersion degree in the Ni-USY catalyst than
in the Ni/USY catalyst.
Fig. 5 shows the in situ FTIR spectra for the pyridine
adsorption over various catalysts. Three peak at ca. 1450, 1490
and 1538 cmꢂ1, respectively, are identied over all the catalysts.
The peaks 1450 and 1538 cmꢂ1 are ascribed to the pyridine
adsorptions on the Lewis and Bronsted acid sites, respectively,
while the peak at 1490 cmꢂ1 is usually related to the cooperative
adsorption of pyridine over the Lewis and Bronsted acid sites.20
It is known that the Bronsted acid sites are related to the
protons in bridging Si–OH–Al groups and the Lewis acid sites to
the unsaturated surface cations (extra-framework aluminum) or
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a proportion of extra-framework aluminum, as has been proved
by the above characterizations of 27Al MAS NMR and N2-phys-
isorption, and this should have decreased the area of Tm,2 peak;
however, nickel ions possess also the Lewis acid property and
their exchange onto the USY zeolite provides an additional
amount of Lewis acid site, and this counteracts the above loss of
Lewis acid sites via the removal of extra-framework aluminum
and therefore increases slightly the area of Tm,2 peak. Besides,
as has been pointed out above, the area of Tm,1 peak is
decreased aer the nickel exchange. It is deduced that
a proportion of nickel ions may have interacted with some
terminal silanol groups, and under the inducement of nickel
ions, the silanol groups are activated to exhibit, to some extent,
a characteristic of moderate Bronsted acid site and the nickel
ions themselves behave as Lewis acid sites. Nevertheless, this
process contributes less to the relative concentrations of
Bronsted and Lewis acid sites over the Ni-USY and USY cata-
lysts. Compared to the USY catalyst, a new peak with desorption
temperature centered at ca. 800 K, namely Tm,4, also appears
over the Ni-USY catalyst. This can be due to the fact that the
nickel ions possess a strong electric eld so as to induce an
abnormally high acid strength of adjacent Si–OH–Al groups.
Fig. 6 NH3-TPD profiles for various catalysts. (a) USY; (b) Ni-USY; (c) Therefore, the Tm,4 peak is attributed to the strong Bronsted
ZnCl2/Ni-USY.
acid sites originating from some un-exchanged Si–OH–Al
groups with nickel ions in the vicinity.27 Compared to the Ni-
USY catalyst, the area of Tm,2 peak increases and that of Tm,3
peak decreases, while the Tm,4 peak disappears over the ZnCl2/
Table 4 NH3-TPD results over various catalysts
Ni-USY catalyst. It was reported that ZnCl2 could reacted with
Tm,ia (K) and Aib (mmol gꢂ1) for various desorption
the surface hydroxyl groups of Mont-K 10 clay to form the Lewis
peaks
acid sites (–O–Zn–Cl) by the thermal activation.13 Accordingly,
P
Catalyst
Tm,1 A1
Tm,2 A2
Tm,3 A3
Tm,4 A4
Ai
the surface bridged hydroxyl groups (Si–OH–Al) over the Ni-USY-
acid catalyst can also react with ZnCl2, generating the –O–Zn–Cl
Lewis acid sites. This accounts for the changes for the Tm,2 to
Tm,4 peaks over the over the ZnCl2/Ni-USY catalyst, relative to
the Ni-USY catalyst. Because of the lower electronegativity of
Zn2+ than Al3+, the –O–Zn–Cl Lewis acid sites are expected to
USY
Ni-USY
420 2.62 533 0.59 600 0.79
—
4.00
422 2.50 533 0.67 600 0.18 800 0.54 3.89
ZnCl2/Ni-USY 433 2.56 533 0.80 600 0.16
—
3.52
a
Tm,i refers to the temperature at the maximum of desorption peak i.
Ai refers to the integral area of desorption peak i, and it also
b
corresponds to the concentration of acid site manifested by the possess a smaller acid strength, i.e., lower desorption temper-
desorption peak.
ature, than the Lewis acid sites associated the extra-framework
Al. Therefore, the Tm,2 peak over the ZnCl2/Ni-USY catalyst is
related to the –O–Zn–Cl weak Lewis acid sites. As a whole, the
modications of USY via nickel ion exchange and ZnCl2
loading, decreases both the total concentration of acid sites and
Bronsted acid sites but increases the concentration of Lewis
desorption temperature and integral area for the Tm,2 peak are
determined as ca. 533 K and 0.59 and those for the Tm,3 peak as
ca. 600 K and 0.79, respectively. It is known that the extra-
framework Al species behave as Lewis acid sites and possess
acid sites sequentially.
a weaker strength than the bridged hydroxyl groups (Si–OH–Al)
as Bronsted acid sites in Y zeolite.25,26 Therefore, the Tm,2 peak
3.3 In situ FTIR studies on the reaction of aniline and
can be attributed to the moderate Lewis acid sites associated
with the extra-framework Al species and the Tm,3 peak to the
moderate Bronsted acid sites associated with the Si–OH–Al
groups. Compared to the USY catalyst, the area of Tm,2 peak
increases slightly and that of Tm,3 decreases obviously over the
Ni-USY catalyst. This is due to the facts that, during the nickel
ion exchange, the protons of Si–OH–Al groups are replaced with
nickel ions, and also, there exist the partial dealumination of
framework, decreasing the concentration of Bronsted acid sites.
The nickel ion exchange also leads to the removal of
propanol
Fig. 7 shows the FTIR spectra in the range of 1000–3800 cmꢂ1
for the adsorbed aniline on the Ni-USY catalyst and the free
aniline (NIST standard reference data). The Ni-USY catalyst is
rst heated at 773 K for 4 h under 2 ꢁ 10ꢂ4 Pa, followed by
cooling to 293 K, and then, exposed to a ow of aniline vapor
and nitrogen at 293 K. The free aniline exhibits mainly the
peaks for the N–H stretching vibrations at ca. 3475 and 3391
cmꢂ1, the ]C–H stretching vibration of benzene ring at ca. 3040
cmꢂ1, the N–H bending vibration at ca. 1614 cmꢂ1, the benzene
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Fig. 7 FT-IR spectra for the free aniline and the adsorbed aniline on
Ni-USY catalyst. (a) Free aniline (NIST Standard Reference Data, http://
webbook.nist.gov/chemistry); (b) adsorbed aniline (Ni-USY is first
heated at 773 K for 4 h under 2 ꢁ 10ꢂ4 Pa, followed by cooling to 293
K, and then, exposed to a flow of aniline vapor and nitrogen at 293 K for
3 min).
Fig. 8 In situ FT-IR spectra for the adsorption of aniline on Ni-USY
catalyst. The catalyst is first heated at 773 K for 4 h under 2 ꢁ 10ꢂ4 Pa,
followed by cooling to 293 K, and then, exposed to a flow of aniline
vapor and nitrogen at 293 K (a), 323 K (b), 373 K (c) and 393 K (d) for 2 h,
respectively.
vibration (ca. 1600 cmꢂ1), while the latter two peaks are due to
the stretching vibrations of terminal silanol and T–O tetrahe-
dron (T ¼ Si and/or Al) in USY zeolite.31 As is known, the peak for
ring stretching vibrations at ca. 1600 and 1499 cmꢂ1, the C–N
stretching vibration at ca. 1272 cmꢂ1 and the ]C–H bending
vibrations of benzene ring at ca. 1170 cmꢂ1 28 Compared to the
.
the terminal silanol of zeolite usually appears at ca. 3740 cmꢂ1
.
free aniline, all the above peaks shi toward low frequencies
over the adsorbed aniline and they appear at ca. 3389 and 3320
cmꢂ1 for the N–H stretching vibrations, ca. 3008 cmꢂ1 for the ]
C–H stretching vibration of benzene ring, ca. 1507 cmꢂ1 for the
N–H bending vibration, ca. 1463 and 1393 cmꢂ1 for the benzene
ring stretching vibrations, and ca. 1260 cmꢂ1 for the C–N
stretching vibration and ca. 1054 cmꢂ1 for the ]C–H bending
vibrations of benzene ring, respectively. As is known, aniline is
an aromatic organic base, consisting of benzyl and amino
groups. This enables that aniline can either accept a proton
from Bronsted acid to form an ammonium (Ph-NH3+) or donor
a pair of electrons to Lewis acid (usually coordination-
Therefore, the slightly higher wavenumber of the terminal
silanol (3752 cmꢂ1) observed in this work may be due to the
interaction between the proton of amino and the oxygen of
terminal silanol group on zeolite, which strengthens the H–O
bond of silanol group.
Fig. 8 shows in situ FT-IR spectra for the adsorption of aniline
on the Ni-USY catalyst at various temperatures. Before deter-
mination, the Ni-USY is rst heated at 773 K for 4 h under 2 ꢁ
10ꢂ4 Pa, followed by cooling to 293 K, and then, exposed to
a ow of aniline vapor and nitrogen at 293–393 K. One can see
that the above peaks for the adsorbed aniline in Fig. 7 are all
present, and with increasing the temperature, they decrease in
the strength. In any way, this result indicates that the adsorp-
tion of aniline on the Ni-USY catalyst is appreciably stable,
which is of importance for the activation and thus conversion of
aniline to quinolines.
unsaturated metal ion Mn+) to form an adduct Ph-NH2
/
Mn+. It was also reported29 that aniline can interact with Lewis
acid through the p-electrons of aromatic ring, since the
coupling between the unshared electron pair of NH2 group and
the conjugated electrons of aromatic ring increases the electron
density of benzene ring but decreases that of nitrogen atom.30
Besides, aniline can interact with a hydroxyl group via hydrogen
bond to form an association complex Ph-H2NH/OH. All the
above interactions between aniline and acid weaken the related
chemical bonds in aniline, e.g., N–H, C–N and ]C–H, and thus
decrease the vibration frequencies of those chemicals bonds.31
Besides, the above peak at ca. 3008 cmꢂ1 and a new peak at ca.
1634 cmꢂ1 can be assigned to the N–H stretching and bending
vibrations of anilinic cation.30,31 It should be addressed that
there is also a peak at ca. 3215 cmꢂ1 for both the free and
absorbed aniline and the peaks at ca. 3752 cmꢂ1 and ca. 1185–
1212 cmꢂ1 for the adsorbed aniline. The former peak is
assigned to the overtone band of benzene ring stretching
Fig. 9 shows in situ FT-IR spectra for the adsorption of n-
propanol on Ni-USY catalyst. The catalyst is rst heated at 773 K
for 4 h under 2 ꢁ 10ꢂ4 Pa, followed by cooling to 293 K, and
then, exposed to a ow of aniline vapor and nitrogen at 293–393
K. All the spectra show the characteristic adsorption peaks,32
which are the stretching vibration of hydroxyl group at ca. 3506
cmꢂ1, the stretching vibrations of CHx group at 2890–2962
cmꢂ1, the bending vibrations of CHx group at ca. 1472 and 1244
cmꢂ1, the bending vibration of hydroxyl group at ca. 1392 cmꢂ1
and the stretching vibration of C–O group at ca. 1060 cmꢂ1
,
respectively. The peak at ca. 1220 cmꢂ1 can be assigned to
stretching vibration of T–O tetrahedron (T ¼ Si and/or Al) in
USY zeolite.28,33 With increasing the temperature, not only the
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under 2 ꢁ 10ꢂ4 Pa, followed by cooling to 293 K, and then,
exposed to a ow of aniline vapor and nitrogen at 293 K for 2 h.
Aer that, the Ni-USY is exposed to a ow of propanol vapor and
nitrogen at 373–413 K for 3 min and at 423 K for 3–20 min,
respectively. One can see that, compared to the aniline-
adsorbed Ni-USY (Fig. 10a), aer the adsorption of propanol
at 373 K for 3 min (Fig. 10b), the peaks at ca. 3389 and 3323
cmꢂ1 for the N–H stretching vibrations of aniline disappear and
those at ca. 3008 and 1054 cmꢂ1 for the stretching and bending
vibrations of ]C–H in benzene ring become obviously weaken
due to the presence of the adjacent strong peaks, while the
peaks at ca. 1463 and 1393 cmꢂ1 for the stretching vibration of
benzene ring remain. Besides, the characteristic peaks of
propanol at 2884–2962 cmꢂ1 for the stretching vibrations of
CHx group and 1064 cmꢂ1 for the stretching vibration of C–O
group as well as a peak at ca. 1698 cmꢂ1 for the vibration of
carbonyl32 appear, while those at 1472 and 1244 cmꢂ1 for the
bending vibrations of CHx group and 1392 cmꢂ1 for the bending
vibration of hydroxyl group may have been covered by the
adjacentꢂp1eaks for aniline. It is also noted that the peak at ca.
1634 cm for the N–H vibration of aniline anion shis to ca.
1630 cmꢂ1, which is assigned to the stretching vibration of
C]N group.34 The 1260 cmꢂ1 for the C–N stretching vibration of
aniline shis to ca. 1223 cmꢂ1, which can be assigned to C–N–C
group.28 The similar peak position for the C–N–C group and T–O
tetragon leads to the obviously higher intensity for the 1223
cmꢂ1 peak aer the adsorption of propanol over the aniline-
adsorbed Ni-USY. The peak at 3754 cmꢂ1 due to the interac-
tion between aniline and terminal silanol shis to ca. 3740
cmꢂ1, which is the characteristic peak for the free terminal
silanol, while a new peak at 3677 cmꢂ1, being assigned to the
bridged hydroxyl group (Si–OH–Al) of zeolite, appears. The
above results indicate that there is the reaction between the
propanol and adsorbed aniline molecule, and this leads to the
replacement of hydrogen in amino group via the formation of
C–N–C and C]N bonds, the releasement of the terminal silanol
and the bridged hydroxyl group and the disassociation of
aniline cation. It hints the formation of intermediates con-
taining the imine and N-alkyl structures. We would like to
address that the peak of carbonyl group (ca. 1698 cmꢂ1) has not
been identied for the adsorption over the clean Ni-USY cata-
lyst, however, it appears over the aniline-adsorbed Ni-USY aer
the adsorption of propanol (cf. Fig. 10a and b). This is due to the
fact that alkanol can be converted into either alkene via dehy-
dration over Bronsted site at relatively lower temperature or
aldehyde via dehydrogenation over Lewis acid site at relatively
higher temperature.35 In the case for the adsorption of propanol
over the clean Ni-USY catalyst, the rate for dehydration may
have exceeded over that for dehydrogenation, while in the case
for the adsorption of propanol over the aniline-adsorbed Ni-
USY, the Bronsted acid site can be “poisoned” by aniline and
thus the dehydrogenation is favorable but the dehydration
unfavorable. Therefore, propaldehyde is rst generated from
propanol and then subjected to the subsequent reaction for the
generation of quinoline from aniline and propanol over the Ni-
USY catalyst. Increasing the temperature from 293 K to 423 K
(Fig. 10b–g) and prolonging the adsorption time until 10 min at
Fig. 9 In situ FT-IR spectra for the adsorption of n-propanol on Ni-
USY catalyst. The catalyst is first heated at 773 K for 4 h under 2 ꢁ 10ꢂ4
Pa, followed by cooling to 293 K, and then, exposed to a flow of aniline
vapor and nitrogen at 293 K (a), 323 K (b), 373 K (c) and 393 K (d) for 2 h,
respectively.
Fig. 10 In situ FT-IR spectra for the adsorption of n-propanol after the
adsorption of aniline on Ni-USY catalyst. (a) The catalyst is first heated
at 773 K for 4 h under 2 ꢁ 10ꢂ4 Pa, followed by cooling to 293 K, and
then, exposed to a flow of aniline vapor and nitrogen at 293 K for 2 h;
(b)–(g), after (a), the catalyst is exposed to a flow of propanol vapor and
nitrogen at 373 K, 383 K, 393 K, 403 K, 413 K and 423 K for 3 min,
respectively; (h)–(k), after (a), the catalyst is exposed to a flow of
propanol vapor and nitrogen at 423 K for 5 min, 10 min, 15 min and
20 min, respectively.
wavenumber but also the intensity of the peaks is almost
unchanged. It shows that n-propanol has an only very weak
interaction with the Ni-USY catalyst.
Fig. 10 shows the in situ FT-IR spectra for the adsorption of n-
propanol aer the adsorption of aniline on Ni-USY catalyst.
Before determination, Ni-USY is rst heated at 773 K for 4 h
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293 K (Fig. 10g–i), a new peak at ca. 1493 cmꢂ1, being related to to quinolines, over USY-based catalyst, which is illustrated in
the adsorption of pyridine compound over Lewis acid site,20 Fig. 11.
appears and its intensity increases gradually, while the other
There can be two possible routes for the generation of
peaks change are almost unchanged. It is indicated that quin- quinolines from the reaction of aniline and propanol. In route
oline structure, containing pyridine sub-structure, have been one, aniline is rst adsorbed via the interaction between amino
generated. For the longer adsorption of propanol over the group and a Bronsted acid site to formed a Ph-NH3+ cation over
aniline-adsorbed Ni-USY catalyst (Fig. 10j and k),the peaks at the surface of catalyst. Then, the adsorbed aniline cation reacts
2962–2884 cmꢂ1 for the stretching vibrations of CHx group, ca. with the free or weakly adsorbed propanol to generate the N-
1698 cmꢂ1 for the stretching vibration of C–O group, ca. 1223 alkylaniline cations, which are either dissociated to release the
cmꢂ1 for the C–N–C group and ca. 1064 cmꢂ1 for the stretching byproducts NPA and/or DNPA, recovering the Bronsted acid
vibration of C–O group become very weak and the peaks at ca. sites, or subjected to the subsequent reaction of nucleophilic
1463 and 1393 cmꢂ1 for the stretching vibration of benzene ring attacking by aniline to generate N-phenylpropan-1-imine as
and 1630 cmꢂ1 for the vibration of C]N group remain, while intermediate.11 It is also possible that propanol attacks the 2-
the characteristic peaks for quinolines compound become and 4-sites of the adsorbed aniline cation, due to the conjuga-
stronger.36 It shows that more quinolines compound are tion of positive charge, resulting in 2-propylaniline and/or 4-
generated with prolonging the time for the adsorption of propylaniline as byproducts. The further reaction between two
propanol.
molecular imines leads to the formation of an intermediate (I),
of which the cyclization generates the product 2E-3MQ. In route
two, aniline is rst adsorbed via the p-interaction between
benzene ring and a Lewis acid site to form a Ph-NH2 / Nin+
adduct and propanol is adsorbed and dehydrogenated into
propaldehyde over the surface of catalyst. Then, the adsorbed
aniline reacts with the adsorbed propaldehyde to generate N-
phenylpropan-1-imine as intermediate. The further reaction
between the imine and another molecular propaldehyde leads
to the formation of an intermediate (II), which is rst dehy-
drated into another intermediate (III) and then subjected to the
cyclization reaction to generate the product 2E-3MQ. In both the
3.4 Mechanism for the reaction of aniline and propanol
Based on all the above results and discussions in Sections 3.1–
3.4, one can concluded that the increase in the concentration
ratio of Lewis acid sites to Bronsted acid sites via nickel ion
exchange and/or ZnCl2 loading over USY zeolite favors the
conversion of aniline and propanol to quinolines, particularly
2E-3MQ; the generation of quinolines involves the formation of
the important intermediate related to N-alkylaniline and the
step for the formation of propaldehyde from propanol. We have
proposed a mechanism for the reaction of aniline and propanol
Fig. 11 Mechanism for the reaction of aniline and propanol over modified USY catalyst.
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routes, the N-phenylpropan-1-imine intermediate is generated, very help for the design and development of the catalyst for the
and it may be further converted into 3-methylindole as synthesis of quinolines.
byproduct, being promoted by Bronsted acid site. Besides, the
main product 2E-3MQ can be also converted into the byprod-
ucts like 2,3-DMQ, 2-EQ and 2-MQ via pyrolysis.
Acknowledgements
In the above mechanism, the route one requires the presence This work was supported by the National Natural Science
of Bronsted acid site for the generation of aniline cation and then Foundation of China (Grant 21376068), Program for New
alkylaniline cations, however, the Bronsted acid site is unfavor- Century Excellent Talents in University, the Ministry of Educa-
able to the reaction between alkylaniline cations and aniline to tion of P. R. China, and the Program for Lotus Scholar in Hunan
generate the key intermediate N-phenylpropan-1-imine and also Province, P. R. China.
promotes the generation of byproduct 3-methylindole. The route
two requires the presence of Lewis acid site for the generations of
p-adsorbed aniline and propaldehyde, of which the reaction
References
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