Synthesis of 2,2,4-trimethyl-1,2-H-dihydroquinoline (TMQ) over selected organosulfonic acid silica catalysts: Selectivity aspects

Article abstract of DOI:10.1016/j.mcat.2018.05.016

This paper presents investigation on the synthesis of 2,2,4-trimethyl-1,2-H-dihydroquinoline (TMQ) as a result of the reaction of aniline and both acetone and mesityl oxide in the presence of selected sulfonic acid silica catalysts. Condensation of aniline with acetone is very complex process with the formation of significant number of side products, both desirable and undesirable considering the final product (TMQ). In acidic conditions and elevated reaction temperature the reactivity of main raw materials (aniline, acetone) is significantly high, what causes the formation of many side by-products lowering the selectivity of this reaction. In this paper the reaction of aniline with acetone in the presence of heterogeneous acidic silica catalysts were investigated in more detail and discussed. The results were confirmed by GC/MS analysis, that the presence of TMQ isomers and other by-products significantly affected the formation of final product. The formation of previously not described structural isomer of TMQ has been also demonstrated.

Full text of DOI:10.1016/j.mcat.2018.05.016

MolecularCatalysis454(2018)94–103
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Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Synthesis of 2,2,4-trimethyl-1,2-H-dihydroquinoline (TMQ) over selected
organosulfonic acid silica catalysts: Selectivity aspects
J. Nowickia,⁎, K. Jaroszewskab, E. Nowakowska-Bogdana, M. Szmatołaa, J. Iłowskaa
a
Institute of Heavy Organic Synthesis “Blachownia”, PL47-225 Kędzierzyn-Koźle, Poland
Wrocław University of Science and Technology, Department of Chemistry, PL50-344 Wrocław, Poland
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Skraup reaction
Aniline
Acetone
Mesityl oxide
TMQ
This paper presents investigation on the synthesis of 2,2,4-trimethyl-1,2-H-dihydroquinoline (TMQ) as a result of
the reaction of aniline and both acetone and mesityl oxide in the presence of selected sulfonic acid silica cat-
alysts. Condensation of aniline with acetone is very complex process with the formation of significant number of
side products, both desirable and undesirable considering the final product (TMQ). In acidic conditions and
elevated reaction temperature the reactivity of main raw materials (aniline, acetone) is significantly high, what
causes the formation of many side by-products lowering the selectivity of this reaction. In this paper the reaction
of aniline with acetone in the presence of heterogeneous acidic silica catalysts were investigated in more detail
and discussed. The results were confirmed by GC/MS analysis, that the presence of TMQ isomers and other by-
products significantly affected the formation of final product. The formation of previously not described
structural isomer of TMQ has been also demonstrated.
1. Introduction
presence of homogeneous catalysts. As catalysts most frequently I2 [1],
HCl [6–8], p-toluenesulfonic acid [9], fluorine acids [10,11] and Lewis
Alkyl derivatives of 1,2-H-dihydroquinoline (1,2-DHQ) belong to a
large group of quinoline derivatives of great practical importance. The
most important representative of them is 2,2,4-trimethyl-1,2-H-dihy-
droquinoline known in the market under the acronime TMQ. 2,2,4-
Trimethyl-1,2-dihydroquinoline, because of relatively low price and
great ease of application, is considered as a very important and effective
antooxidant in rubber technologies. The methods of the synthesis of
1,2-quinoline derivatives are based on cyclization reaction well known
as Skraup reaction. More detailed studies led to the development of
various Skraup-based modifications (Combes, Knorr, Doebner,
Friedlander).
The main raw material used in the synthesis of 1,2-dihydroquinoline
derivatives is aniline or its alkyl derivatives and detailed variants of the
synthesis depend only on the kind of carbonyl component used. As a
rule acetone is used, which as a result of sequence reaction condensed
with aniline with the formation of 1,2-H-dihydroquinoline derivative,
i.e. 2,2,4-trimethyl-1,2-H-dihydoquinoline (Scheme 1).
acids [12–17] are used. Condensation of aniline with carbonyl com-
pounds is also possible to perform in the presence of heterogeneous
catalysts. It should be noted, however, that this issue has not been
particularly more deeply examined, particularly with regard to the
analysis of reaction selectivity, which is reflected in the relatively small
number of reports on the subject. Shaikh described the study on con-
densation of aniline with carbonyl compounds with the use of various
special catalysts with mesoporous structure, prepared in the reaction of
thionyl chloride and mesoporous silicate [18]. At temperature of 80 °C,
in the presence of toluene as a solvent, he obtained corresponding
quinoline derivatives with the yields of 57 ÷ 62 wt%. Sidhe used the
same catalyst in the reaction of acetylaniline with various ketones [19].
The yields of corresponding quinolines were from 50 to 80 wt%. In turn,
Venkateswarlu as catalyst used Chloramine-T [20]. In the presence of
acetonitryle as a solvent, in temperature of 80 °C, the yield of quinoline
derivatives was 95 wt%. High yields of quinoline derivatives have been
achieved in the reaction catalysed by heteropolyacids. Heravi in
Friedlander modification of Skraup method (80 °C, 3 ÷ 5 h) obtained
quinolines with the yield of 91 wt% [21]. Heteropolyacids, in particular
phosphomolybdic acid, were used as catalysts in the study conducted
by Chaskar [22].
According to Scheme 1 TMQ is manufactured in technical scale.
Skraup synthesis of TMQ was described for the first time in 1921 [1].
Later study allow confirmation its chemical structure [2–5]. The
synthesis of 1,2-H-dihydroquinoline derivatives, according to Skraup
method or its various modifications, are carried out mainly in the
Studies described above concerned very particular cases of
⁎
Corresponding author.
E-mail address: nowicki.j@icso.com.pl (J. Nowicki).
https://doi.org/10.1016/j.mcat.2018.05.016
Received 13 February 2018; Received in revised form 10 May 2018; Accepted 11 May 2018
2468-8231/©2018ElsevierB.V.Allrightsreserved.

J. Nowicki et al.
MolecularCatalysis454(2018)94–103
Scheme 1. Synthesis pathway of 2,2,4-trimethyl-1,2-H-dihydrochinoline (TMQ) according to Skraup mechanism.
condensation of aniline with various carbonyl compounds leading to
corresponding quinoline derivatives. Among of the described results,
only a few of them describe the synthesis of 2,2,4-trimethyl-1,2-H-di-
hydroquinoline. Yadav described the synthesis of TMQ in the reaction
of aniline with acetone in the presence of catalyst based on sesquisi-
loxane framework, functionalized by acidic sulfonic group [23]. At
temperature of 80 °C he achieved quinoline with the yield of 90 wt%.
Hegedüs used as catalyst klinoptilolite modified by sulfuric acid [24].
The reaction of aniline with acetone was conducted in temperature of
110 °C in the presence of toluene as a solvent. The amount of used
catalyst was, however, very significant i.e. 4.3 g/g aniline. In above
reaction conditions TMQ was achieved with the yield of 95 wt%. Me-
thyl analog of TMQ was obtained in the reaction of 3-methylaniline
with acetone in the presence of Amberlyst A15 as a catalyst [25]. The
yield of MeTMQ was very high (93 wt%) but after long time of synthesis
(48 h).
TMQ is considered as commercial product of great practical im-
portance, but in none of the abovementioned papers on the synthesis of
TMQ, did not provide more detailed information about characteristic of
the post-synthesis mixture. These informations are useful for the as-
sessment of the course of this very complex synthesis, especially the
selectivity of the reaction of aniline with acetone catalysed by hetero-
geneous catalysts.
solution. The molar composition of the reactants used in these synthesis
was 1 SiO2: 0.017 Pluronic P123: 5.88 HCl:192 H2O. After stirring at
40 °C for 2 h, the reaction mixture was transferred into a Teflon bottle
and aged at 100 °C for 48 h under static conditions. The mixture was
then allowed to cool to room temperature, and the solid, white product
was filtered, dried at 110 °C for 12 h and calcined at 550 °C for 8 h
(temperature gradient 5 °C/min). Typical yield of pure SBA-15 was
96 wt% based on used SiO2.
For the synthesis of organosulfonic acid functionalized silicas was
applied post-synthesis method based on the work described by
Mrowiec-Białoń (SBA-15) [28] and Karimi (SiO2) [29]. First, pure si-
liceous materials (SBA-15) were contacted with water steam for 2.5 h
and subsequently calcined at 200 °C for 2 h. The nominal content of
organic groups was 1.0 mmol g−1. All the operations were made under
a nitrogen atmosphere. The synthesis pathway of the preparation of
organosulfonic acid functionalized SBA-15 is presented in Fig. 1.
In a preparation of the SBA-15–propyl-SO3H catalyst, a mixture of
corresponding silica and MPTMS in n-hexane was magnetically stirred
for 15 min at room temperature, and then refluxed for 24 h at 45 °C
(Fig. 1a); all the above operations were made under a nitrogen atmo-
sphere. Then the reaction mixture was cooled, and the solvent was
removed by evaporation and the product was dried at 60 °C for 24 h.
After that mercaptopropyl groups were oxidized with 30% aqueous
hydrogen peroxide solution under stirring at 40 °C for 24 h under ni-
trogen atmosphere. After filtration, the solid product was washed with
deionized water. The material was filtered and washed several times
with water and ethanol. The wet material (1.0 wt%) was suspended in
1 M H2SO4 for 2 h. Finally the material was filtered, washed with water
and ethanol, dried at 60 °C under vacuum overnight. The catalysts were
denoted as SBA-15–Pr-SO3H. Using amorphous silicas in the place of
SBA-15 corresponding organosulfonic acid modified silica catalysts
were obtained and denoted as SiO2–Pr-SO3H.
Owing to the relevance of TMQ and also its methyl analogues, we
described the novel approach to the synthesis of important 1,2-DHQ
derivatives with the use of selected sulfonic acid silica catalysts. The
detailed structural analysis of reaction products have been presented
for the first time.
2. Experimental
2.1. Materials
The SBA-15–phenyl-SO3H catalyst were prepared by grafting of PTES
over the silicas (Fig. 1b). The calcined SBA-15 material was refluxed
with excess quantity of PTES dissolved in n-hexane under nitrogen at-
mosphere overnight for 24 h at 45 °C. Then the reaction mixture was
cooled, then the solvent was removed by evaporation and the product
was dried at 60 °C for 24 h. The obtained material further reacted with
chlorosulfonic acid, which resulted in generating the phenyl sulfonic
acid groups. The material was degassed at 130 °C under vacuum for
12 h and then 30 mL of CH2Cl2 containing chlorosulfonic acid (1 mL)
was slowly added at room temperature under nitrogen atmosphere.
After 4 h of stirring, the suspension was slowly added to the mixture of
water and ice (0 °C). After filtration, the solid product was washed with
deionized water. Finally the powder product was dried at 60 °C under
vacuum overnight. To improve the hydrophobicity of the catalyst’s
surface, methyl groups (nominal content of methyl groups was
Aniline for synth. (99%), acetone puriss. p.a. (> 99.5%) and mesityl
oxide for synth. were supplied by Aldrich (Poland). SBA-15 silicas were
synthesized using a hydrothermal procedure. As the precursors were
used: tetraethylorthosilicate (TEOS, 98%) supplied by Aldrich (Poland)
as a silica source, nonionic triblock co-polymer Pluronic P123 [poly
(ethylene oxide)-block-(poly(propylene oxide)-block-poly(ethylene
oxide)], molecular weight = 5800, supplied by Aldrich (Poland) as a
template, chlorotrimethylsilane puriss. (CTMS, 99%), phenyltriethox-
ysilane (PTES) for synth. and chlorosulfonic acid supplied by Aldrich
(Poland) as functional modifiers. Distilled water, hydrochloric acid and
dichloromethane puriss. p.a. were supplied by POCh (Poland).
2.2. Catalysts preparation
Mesoporous molecular sieves of SBA-15 were synthesized according
to the method described in the literature with some modifications
[26,27]. In a typical synthesis, P123 (4 g) was dissolved in an aqueous
solution of HCl (1.6 M HCl, 150 g) at a room temperature, under vig-
orous stirring. Subsequently, a 8.5 g of TEOS was added to the above
1 mmol g−1
(CTMS). The catalyst was denoted as SBA-15–Ph-SO3H(-CH3) (Fig. 1c).
) were introduced by grafting chlorotrimethylsilane
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Fig. 1. Scheme of the preparative pathway of organosulfonic acid modified SBA-15 catalysts: (a) SBA-15–Pr-SO3H and (b) SBA-15–Ph-SO3H(-CH3).
2.3. General method for the synthesis of 1,2-dihydroquinoline derivatives
2.5. Analysis of the reaction products
For the synthesis of TMQ 100 mL standard glass reactor equipped
with mechanic stirrer, electronic temperature controller was used. The
reactor was also equipped with acetone dispensing system connected
with precise piston pump (Ismatec MCP-CPF IF60) and glass distillation
kit contains short path distillation column (c.a. 10 cm length) connected
with thermostatic mantle and cold water condenser for collecting of
acetone vapors, released continuously during the synthesis.
In a typical synthesis 60 g (0.02 mol) aniline was placed to a 100 mL
glass reactor containing 6 g of corresponding catalyst. The resulting
suspension was agitated intensively (700 rpm), and after heating to
desired temperature, acetone dosing was started. Dosing rate of acetone
was set at a level, which allowed constant temperature of a reaction
mixture. The dosing of acetone was complete after 5 h and the resulting
suspension was cooled down to room temperature, filtered and ob-
tained filtrate was analysed.
The composition of reaction products and identification of compo-
nents was performed using GC/MS method on gas chromatograph
7890A (Agilent) equipped with mass detector Model 7000 (Agilent).
The carrier gas was helium with flow rate maintained at 1 mL ml−1
.
Inlet temperature was kept at 300 °C. The oven temperature was pro-
grammed from 70 °C (1 min) to 280 °C (30 min) with temperature
progress 10 °C min−1. Injection sample was 0.5 μL. Quantitative ana-
lysis of TMQ in post-synthesis mixtures was based on external calibra-
tion (calibration curve) (Fig. S4, Supplementary information). For the
preparation of calibration curve purified TMQ sample was used. For this
purpose TMQ fraction was first separated by distillation and obtained
raw TMQ was purified by double crystallization (for details see
Supplementary information). The structure of the obtained TMQ ana-
lytical sample was confirmed by
H NMR spectroscopy (Fig. S5,
Supplementary information). Recorded H NMR spectrum was identical
to the TMQ spectrum published in the literature [30].
2.4. Supports and catalysts characterization
3. Results and discussion
Texture: Nitrogen adsorption and desorption isotherms were mea-
sured at 77 K with the Autosorb-1C Quantachrom sorption analyser.
Specific surface area was calculated using the BET model. The BJH
method was applied for determining the pore diameter.
3.1. Characterisation of SBA-15
Textural properties of pure siliceous SBA-15 material were char-
acterised by the low temperature N2 adsorption–desorption, XRD and
TEM techniques. According to the IUPAC classification, the SBA-15
material presents the adsorption–desorption isotherm of type IV
(Fig. 2a), showing a sharp inflection (steep hysteresis of type H1) at a
relative pressure around p/p0 = 0.6 ÷ 0.8 and narrow pore size dis-
tribution, which is characteristic of well-ordered mesoporous materials.
The SBA-15 sample displays the BET specific area amounting to
710 m2 g−1, pore volume of 0.9 cm3 g−1 and mean pore diameter of
5.6 nm (Table S1, Supplementary information).
The external surface area of this material was found be to very small
(i.e. below 10 m2 g−1), suggesting that there is almost no macropores.
In the XRD pattern of SBA-15, three peaks are observed, i.e. an intense
peak at 0.96° and low-intensity peaks at 1.56° and 1.80° (Fig. 2b). These
peaks are indexable as (1 0 0), (1 1 0) and (2 0 0) reflections, which are
associated with p6mm hexagonal symmetry. Further evidence for a
highly ordered hexagonal structure is provided by the transmission
electron microscopy (TEM) micrograph, as shown in Fig. 2c.
X-ray powder diffraction (XRD) patterns of the SBA-15 materials
were collected on
a Siemens D5005 diffractometer using CuKα
(α = 0.154 nm) radiation.
NMR: The 29Si MAS NMR spectra were recorded with an AC200
Bruker spectrometer.
Transmission electron microscopy (TEM): Transmission electron mi-
croscope (TEM) images were generated using a Jeol Jem 1200 EX II
microscope operating at 80 kV. Prior to observation, the samples were
ultrasonically dispersed in butanol and transferred to a holeycarbon
coated copper grid (300 mesh).
Scanning electron microscope (SEM) images were generated using a
Zeiss EVO 40 microscope. Micrographs were obtained at 20 kV. The
samples were deposited on a sample holder with an adhesive carbon foil
and sputtered with gold using Balzers SCD 050 sputter deposition
equipment.
FT-IR spectroscopy: Infrared spectra (FT-IR) were recorded on a
BRUKER VECTOR 22 spectrophotometer. All of the compounds were
solid and solid state IR spectra were recorded using the KBr disk
technique.
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Fig. 2. Characterization of the SBA-15 sieve; (a) N2 adsorption (full dots) – desorption (open dots) isotherms, pore size distribution as well as (b) XRD pattern and (c)
TEM images.
Fig. 3. N2 (full dots) – desorption (open dots) isotherms and pore size distribution (inside) of (a) SBA-15–Pr-SO3H, (b) SBA-15–Ph-SO3H(-CH3), (c) SiO2-Pr–SO3H(1)
and (d) SiO2–Pr-SO3H(2).
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3.2. Catalysts characterization
3.2.1. Structure properties
Fig. 3 shows the N2 adsorption–desorption isotherms and pore size
distributions recorded for organosulfonic acid SBA-15 as well as SiO2
supported catalysts.
In the case of SBA-15 supported catalysts (Fig. 3a and b), the iso-
therms of type IV were obtained what confirmed the presence of or-
dered mesoporous structure. A steep capillary condensation step (p/
p0 = 0.6 ÷ 0.8) is observed for both SBA-15 based samples, suggesting
a presence of uniform-size pore system. The incorporation of sulphonyl
groups does not produce changes in the characteristic shape of the
isotherm with the exception of some reduction in the amount of ad-
sorbed N2 compared with pure SBA-15 carrier. As can be seen from
Fig. 3a and b, the SBA-15–Pr-SO3H and SBA-15–Ph-SO3H(-CH3) cata-
lysts have a narrow distribution of pores (dBJH) with peak positions at
5.6 an 4.9 nm, respectively. Texture parameters of the studied catalysts
are given in Table S1. There are some changes in the porous structure of
the functionalized SBA-15 support in respect to the SBA-15 mesoporous
material. The preparation of SBA-15–Pr-SO3H and SBA-15–Ph-SO3H
(-CH3) catalysts is accompanied by two-fold decrease in SBET as well as
in pore volume compared with pure SBA-15 carrier. Moreover, the
immobilization of sulphonic groups onto the SBA-15 surface causes the
increase of the average pore diameter by 0.4 and 0.7 nm for SBA-15–Pr-
SO3H and SBA-15–Ph-SO3H(-CH3) samples, respectively.
The N2 adsorption–desorption isotherms of SiO2 supported systems
are plotted in Fig. 3c and d. For both samples an isotherm of type I
(according to IUPAC) was obtained, which is typical of microporous
materials. The texture of SiO2–Pr-SO3H(1) catalyst and that of SiO2–Pr-
SO3H(2) differs only slightly (Table S1). The SiO2–Pr-SO3H(1) and
SiO2–Pr-SO3H(2) catalysts exhibit an SBET of 484 and 412 m2 g−1, re-
spectively. In the SiO2–Pr-SO3H(1) catalytic system, dominant are mi-
cropores with diameters of 2.6 nm (dDFT) and they are greater by about
1 nm as compared with the diameters of SiO2–Pr-SO3H(2)
(dDFT = 1.6 nm). However, both catalysts have an average pore dia-
meter of 2.3 nm. The pore volume of the catalysts ranges between 0.2
Fig. 4. TEM images of (a) SBA-15-Pr-SO3H and (b) SBA-15-Ph-SO3H(-CH3).
are a rod-like particles organized into the packages. The spherical-
shaped morphology of SBA-15 could be obtained inter alia by in-
corporating co-surfactant e.g. cetyltrimethyl ammonium bromide
(CTABr) [32] which lowers the local curvature energy and conduces to
the formation of a sphere shape. Also, alcohols like n-butanol [33,34]
and ethanol [35] were applied as co-solvents to synthesize spherical
SBA-15 particles. Tetraethyl orthosilicate was also selected as the silica
source to reduce the local energy of curvature; TEOS has slower con-
densation rates as compared to tetramethyl orthosilicate [36]. Ethanol
plays a very important role in determining the characteristics of the
SBA-15 particles because ethanol is one of the products formed during
the sol–gel synthesis from TEOS. We believe, that even though, the
synthesis of SBA-15 was conducted without any additional ethanol, the
local concentration of alcohol from TEOS was high enough to decrease
the local surface curvature energy and leads to the formation of curved
morphologies of SBA-15 particles.
and 0.3 cm3 g−1
.
In order to further investigate the pore structure of the SBA-15
supported catalysts, they are also evaluated by transmission electron
microscopy.
TEM images presented in Fig. 4a and b demonstrate the structure of
the SBA-15–Pr-SO3H and SBA-15–Ph-SO3H(-CH3) catalysts, respec-
tively. TEM images of both catalysts clearly show well-ordered hex-
agonal arrays with one-dimensional channels as well as parallel fringes
corresponding to side-on projections of the hexagonal mesostructured.
These results confirmed that the samples possess two-dimensional
hexagonal mesoporous structure which is in good agreement with the
N2 sorption observations.
The SEM images of the SBA-15–Pr-SO3H catalysts represented that
after the active groups immobilization, the morphological structure and
the particle shapes did not change considerably. Only, slight morpho-
logical differences can be detected for the SBA-15 supported catalysts
(Fig. 5b and c). It is found that both of the SBA-15 supported catalysts
have partially spherical-shaped and partially a cubic-like morphology.
Some irregular small and large particles shaped are also present and
some defects on the surface are detected. Average grain size dominating
in the SBA-15–Pr-SO3H catalyst approached 5 μm.
The morphology of SiO2 based catalysts are illustrated in Fig. 6.
Presented SEM images revealed that both SiO2-Pr-SO3H(1) and (b)
SiO2-Pr-SO3H(2) samples are consisted of units with irregular shapes. It
is clear that, the silica supported catalysts exhibit amorphous nature of
the particles.
3.2.2. Morphology characterisation (SEM)
The morphological characterization of the supports and the cata-
lysts were performed using scanning electron microscopy (Figs. 5 and
6).
The SEM micrographs show that the particles of pure siliceous SBA-
15 are mainly spherical-shaped (Fig. 5a). These particles have mostly a
smooth surface, only few visible defects on the surface are observed. It
also shows that micrometer-sized sphere particles had a relatively
uniform size with a diameter of about ranges from 2 to 7 μm and some
of them are tended to adhere together.
As it was shown in Zhao et al. work the morphology of the SBA-15
can to be controlled by different synthesis conditions, i.e. condensation
rate of silica species (pH and silica source), stirring rate, micelle shape,
and inorganic species present [31]. The morphology of these materials
is highly dependent on the energy of local curvature and the attainment
of a lower local energy of curvature will result in a more curved mor-
phology. The most common macroscopic structures of siliceous SBA-15
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MolecularCatalysis454(2018)94–103
Fig. 5. SEM images of (a) SBA-15, (b) SBA-15–Pr-SO3H and (c) SBA-15–Ph-SO3H(-CH3).
3.2.3. Characterization of sulfonic acid-functionalized SBA-15 and SiO2
catalysts
stretching and bending vibrations of condensed silica network [37–39].
Non-condensed SieOH groups were formed on the surface of all the
samples as evidenced by the absorption bands around 960 cm−1. While
the broad peak around 3450 cm−1 and the strong one around
1640 cm−1 are due to the stretching and bending vibrations of ad-
sorbed H2O. The FT-IR spectra for the SBA-15–Ph-SO3H(-CH3) catalyst
(as an representative example) and for the SiO2–Pr-SO3H(2) catalyst (as
an representative example) are shown in Figs. S1b and S2, respectively.
Qualitative identification of the active moieties in the materials was
performed by FT-IR and spectroscopy (for details see Supplementary
information). Fig. S1 illustrates the FT-IR spectra of the functionalized
SBA-15 and SiO2 materials in comparison to that of pure silica SBA-15
(Fig. S1a). Common to all samples are the bands at a frequency of 1220,
1070, 800, and 450 cm−1 which represent the typical SieOeSi
Fig. 6. SEM images (a) SiO2–Pr-SO3H(1) and (b) SiO2–Pr-SO3H(2).
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Scheme 2. Reaction pathway of the reaction of aniline with acetone.
The results revealed that SBA-15 and SiO2 carriers were successfully
functionalized by anchoring the active groups to the surface. After SBA-
15 functionalization, the absorption peak were found at about 2942,
2938 and 2883 cm−1 which could be attributed to stretching vibrations
in CeH bonds of the alkyl groups. The spectrum of the SBA-15–Ph-SO3H
(-CH3) material has peaks at 594 and 702 cm−1 with medium intensity
which are assigned to the out-of-plane bending of the para di-sub-
stituted aromatic ring as well as the bending vibration of the sulfonic
acid group. In the case of SiO2–Pr-SO3H(2) sample the propyl group
attached to the surface of SiO2 in can be identified by the methylene
stretching bands also in the 2850–2950 cm−1 region (2943 and
2854 cm−1). The peaks corresponding to the sulfonate groups
stretching vibrations are normally observed in the range of
1000–1400 cm−1. These peaks cannot be resolved due to their over-
lapping with the absorbance of SieOeSi stretch in 1000–1130 cm−1
range and that of SieCH2eR stretch in 1200–1250 cm−1 range. How-
ever, in comparison to reference silica, the spectra of the sulfonic acid
functionalized materials i.e. SBA-15–Ph-SO3H(-CH3) and SiO2–Pr-SO3H
(2) have more pronounced peaks centered at about 1186 and
1099 cm−1 and 1184 and 1096 cm−1 respectively, which could be as-
signed to the symmetric and asymmetric stretching vibrations of the
sulfonate moieties, respectively.
key first stage of this process aniline react with protonated enol of
acetone with the formation of aniline Schiff base commonly called as
acetanil. In general, products of the reaction of aniline with acetone
conducted in elevated temperatures are very complex mixtures con-
tained and beside of starting raw materials (aniline, acetone) a sig-
nificant number of side products are formed. One of them is the product
of the reaction of one mole of aniline with one mole of acetone –
acetanil. This reaction is considered as a main side reaction compared
to self-condensation reaction of acetone into mesityl oxide.
More detailed analysis of the condensation of aniline with acetone
indicate that in the post-synthesis mixtures, beside of monomeric TMQ,
its oligomers are also present, mainly TMQ. From the application point
of view, the presence of TMQ oligomers should be considered bene-
ficial. The next step of the production of TMQ-based antioxidants is
oligomerization of TMQ. In this context, TMQ dimer may be considered
also as a one of desirable product of the reaction of aniline and acetone.
The reaction pathway of the reaction of aniline with acetone in the
presence of acid catalyst has been shown in Scheme 2. Table 1 re-
presents the composition of post-synthesis mixtures obtained at reac-
tion temperature of 180 °C.
In Table 1 only the main components were indicated, both aniline
and the key reaction products. Several others by-products were col-
lected and assigned as others. More detailed analysis of the reaction of
aniline and acetone will be described and discussed later. It was ob-
served that, in reaction temperature of 180 °C the conversion of aniline
depended on the catalyst used, which was in the range of 61.3 ÷ 71.7%
and selectivity to TMQ were 38.5 up to 57.2%. A little higher activity
characterized both SBA-based catalysts. Content of TMQ in post-
synthesis products were above 36 wt% and additionally noticeable
higher amounts (4.72 ÷ 6.31 wt%) of TMQ dimer were observed. In
Table 2 the compositions of post-synthesis mixtures obtained at 160 °C
were presented.
Therefore, the results above proved that SBA-15 and SiO2 carriers
were successfully functionalized with active groups. It is also worth
noting that FT-IR spectra of the catalysts well supported that all thiol
groups were converted to sulfonic groups through the lack of the peak
at 2580 cm−1 which is ascribed to the stretching vibrations in SeH
groups.
The SBA-15 surface modification process was further monitored by
29Si MAS NMR. The 29Si MAS NMR spectrum of SBA-15 sample (Fig.
S3a) shows three distinct resonance peaks around the shifts of
−92,−103 and −112 ppm. They are assigned to silicon groups, i.e. Qn
(Qn: Si(OSi)n(OH)4-n, n = 2 ÷ 4) in the following structures: geminal
silanol species (OSi)2Si(OH)2 (Q2), isolated silanol species (OSi)3SiOH
(Q3) and finally the most polymerized Si(OSi)4 (Q4) units, respectively
[40,41]. As a result of the introduction of active groups, the solid state
29Si MAS NMR spectrum of the SBA-15–Ph-SO3H(-CH3) shown in Fig.
S3b (as a representative example) displayed different signal patterns
when compared to the parent silica material. It shows a much higher
intensity of Q4 sites (−112 ppm) when compared to the parent SBA-15
silica (Fig. S3a). This indicates the positive effect of experimental
conditions on the local structures of the silicate framework. In addition,
new resonances at δ = −82 and −71 ppm are observed which are
corresponded to organosiloxane Tm species (Tm: ReSi(OSi)m(OH)3-
m, m = 1 ÷ 3), i.e. to T3 and T2 moieties, respectively. The appearance
of T3 and T2 peaks confirms the formation of SieC bonds in the SBA-15
materials.
Conversion of aniline depending on the used catalyst, was
60.6–68.7%. Comparison the results presented in Tables 1 and 2 shows,
that decreasing the reaction temperature resulted in a reduction of the
Table 1
The results of condensation of aniline with acetone at 180 °C. The composition
of post-synthesis mixtures (wt%).
SiO2–Pr-SO3H
SiO2–Pr-SO3H
SBA–Ph-SO3H
SBA–Pr-SO3H
(1)
(2)
(-CH3)
Sample
Aniline
Acetanil 18.42
TMQ
DiTMQ
Others
CA, %
1
2
3
4
37.93
33.13
22.66
24.82
0.90
18.49
65.5
24.32
17.61
36.92
6.30
14.85
71.6
35.80
9.45
36.65
4.71
13.39
62.9
33.02
1.36
9.27
61.2
STMQ, % 55.5
38.5
57.2
64.5
3.3. Synthesis of 2,2,4-trimethyl-1,2-H-dihydroquinoline (TMQ)
CA – conversion of aniline; STMQ – selectivity to TMQ (as a sum of TMQ and
The synthesis of TMQ is very complex and multistage process. In the
DiTMQ).
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J. Nowicki et al.
MolecularCatalysis454(2018)94–103
Table 2
Table 3
The results of condensation of aniline with acetone at 160 °C. The composition
of post-synthesis mixtures (wt%).
The results of condensation of aniline with mesityl oxide at 180 °C. The com-
position of post-synthesis mixtures (wt%).
SiO2–Pr-SO3H
(1)
SiO2–Pr-SO3H
(2)
SBA–Ph-SO3H
(-CH3)
SBA–Pr-SO3H
SiO2–Pr-SO3H
(1)
SiO2–Pr-SO3H
(2)
SBA–Ph-SO3H
(-CH3)
SBA–Pr-SO3H
Sample
Aniline
Acetanil 22.37
TMQ
DiTMQ
Others
CA, %
5
6
7
8
Sample
9
10
11
12
34.30
36.48
28.20
21.69
1.70
12.07
60,5
28,05
14,38
32.23
5.16
20.18
68,6
31.12
10.45
32.06
6.94
19.43
65,4
Mesityl oxide 4.02
4.91
0.20
25.30
2.52
30.89
13.59
27.50
65.0
0.81
24.36
7.00
30.00
13.05
24.77
65.3
Aniline
Acetanil
TMQ
DiTMQ
Others
CA, %
19.34
18.33
15.08
3.68
39.55
71.5
19.83
17.51
9.74
3.62
44.39
72.7
29.19
2.23
11.91
63,0
STMQ, % 47.9
36.8
52.0
56.7
STMQ, %
24.5
17.8
59.7
57.6
CA – conversion of aniline; STMQ – selectivity to TMQ (as a sum of TMQ and
DiTMQ).
CA – conversion of aniline; STMQ – selectivity to TMQ (as a sum of TMQ and
DiTMQ).
selectivity. Selectivity to TMQ were 36.8–56.7%. The content of the
main by-product (acetanil), depending on the used catalyst, decreased
to 10.44–28.06 wt%. In the temperature of 160 °C a little higher activity
characterized also both SBA-based catalysts. Content of TMQ in post-
synthesis products were above 32 wt% and additionally noticeable
higher amounts (5.17–6.95 wt%) of TMQ dimer were also observed.
Analysing the results presented in Tables 1 and 2, it can be noticed that
the key factor having a significant impact on the selectivity of the tested
reaction is the textural character of the catalysts used in the research.
Both types of catalysts (SBA and SiO2) are characterized by similar
surface area. The presence of active SO3H groups makes them char-
acterized by similar acidic strength. However, what distinguishes them
is the significant difference in pore size diameters. Since the reaction
product (TMQ) is a much larger molecule than started aniline, its for-
mation, especially the last stage of this reaction, is more convenient in
the presence of catalyst with a larger pore size containing acidic centres
on its surface. This makes the SBA-based catalysts have noticeably
higher selectivity to the final products. This is particularly evident in
the case of the TMQ dimer, which in the presence of SBA-based cata-
lysts is formed in an amount even three times higher as compared to
SiO2 based catalysts.
observed. Amorphous silica sulfonic catalysts are characterized be
higher content of unreacted mesityl oxide, higher amount of aniline
Schiff base (acetanil), but also significantly lower content of TMQ and
TMQ dimer compared to SBA-based catalysts. This experimental data
indicate lower catalytic activity of amorphous silica catalysts. Although
the conversion of aniline was higher than over SBA-based catalysts
(71.5 ÷ 72.7%), but the selectivity to desired 1,2-H-dihydrochinoline
derivatives was very low and reached only 17.8 ÷ 24.5%. The effect of
used catalyst on the selectivity is analogous to that of the reaction of
aniline with acetone. Selectivity to TMQ and also to TMQ dimer is
significantly higher for SBA-based catalysts. Analysing the results pre-
sented in Table 3, you can also see a another catalyst effect. The con-
densation of aniline with mesityl oxide in the presence of SiO2-based
catalysts is characterized by a slightly higher conversion of aniline
compared to SBA-based catalysts. However, the higher conversion
value is due to the greater share of the main side reaction, i.e. the de-
composition of mesityl oxide into acetone and the formation of acetanil.
This effect can also be linked to the pore size diameter, which in the
case of SiO2 catalysts are significantly smaller.
For the synthesis of TMQ also Doebner-Miller mechanism is postu-
lated [42]. The key step according to this mechanism is aldol con-
densation of acetone with the formation of mesityl oxide. In the next
step mesityl oxide reacts with protonated aniline and in the sequence of
reaction, TMQ molecule is formed (Scheme 3).
In order to investigate this possible method of the synthesis of TMQ,
the reaction of aniline with mesityl oxide as carbonyl donor was con-
ducted. The reaction was carried out in the temperature of 180 °C in
analogous procedure adopted for acetone as carbonyl donor with the
use of mesityl oxide in 20% molar excess. The results were presented in
Table 3.
3.4. Condensation of aniline with acetone – mechanistic study
Comparing the use of acetone and mesityl oxide as a carbonyl do-
nors it may be concluded that in adopted reaction conditions the use of
acetone is more beneficial. From mechanistic point of view, higher
selectivity and the absence of mesityl oxide in post-synthesis mixtures
indicates classical Skraup mechanism of the synthesis. On the other
hand the presence of significant amount of acetanil in post-synthesis
mixtures obtained with use of mesityl oxide indicates the course of
mesityl oxide decomposition with release of acetone.
The results presented in Tables 1–3 indicate that the reaction of
aniline with acetone in temperature range of 160 ÷ 180 °C is more
complex than it was depicted in Scheme 1. In this paper the analysis of
the reaction of aniline with acetone in more detail have been presented
for the first time. This analysis was performed with the support of
GC–MS (Fig. 7) spectrometry. Sample 7 was used as a reference sample
of post-synthesis mixture obtained at 160 °C with the use of SBA–Ph-
SO3H(-CH3) as a catalyst.
For both used SiO2 catalysts, a considerable differences were
Most of the scientific literature presenting the study on Doebner i
Doebner-Miller condensation processes, in particular concerning the
synthesis of TMQ describe the course of this reaction in a simplified
way. One of the main assumptions of this work is the analysis of the
course of the synthesis of TMQ catalysed by heterogeneous silica sul-
fonic acid catalysts in more detail. GC chromatograms of post-synthesis
products show that the reaction is more complex and, besides of the
main and desired products, leads to many by-products. The key com-
ponents, important for mechanistic analysis of studied reaction, were
numbered and indicated in this chromatogram (Fig. 7). For this com-
ponents MS spectra were recorded for identification purposes.
Scheme 3. Synthesis pathway of 2,2,4-trimethyl-1,2-H-dihydrochinoline
(TMQ) according to Doebner-Miller mechanism.
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MolecularCatalysis454(2018)94–103
Fig. 7. GC chromatogram of Sample 7.
Scheme 5. Formation of TMQ structural isomers.
Scheme 4. Formation of by-products 2 and 3 in the reaction of aniline and
acetone.
was recorded. The MS spectrum of this component is very similar to MS
spectrum of TMQ. The analysis of possible course of studied reaction
suggested that it may be a previously not described structural isomer of
TMQ (isoTMQ). The formation of this isomer is presented in Scheme 5.
From the chemical point of view dehydration of 4-hydroxy by-
product leads mainly to TMQ 4, but formation of isoTMQ is also pos-
sible and this isomer is present in post-synthesis mixtures. In the ap-
plication point of view, the presence of structural isomer of TMQ is
beneficial. Analogously to TMQ it also forms the desired 1,2-dihy-
drochinoline oligomers.
Components marked as 6–8 are the result of subsequent reactions
based on acetanil, TMQ and acetone. The formation of components 6
and 7 is presented in Scheme 6.
Component 6 is formed in the reaction of acetanil 2 with an excess
of acetone. In turn, component 7 is formed in synchronous reaction of
acetanil with aniline. Relatively higher amounts of component 6,
compared to compound 7, in post-synthesis mixtures proves that the
The main by-product of the reaction of aniline with acetone ac-
cording to Skraup (Doebner) mechanism is aniline Schiff base (acet-
anil). In fact, this component was found in all obtained in this study
post-synthesis products. In Scheme 1 acetanil is marked as component 2
(compound 1 – aniline). The presence of characteristic master ion
133 m/e and fragmentation ions at 118 and 103 m/e in MS spectrum
confirms the presence of acetanil in post-synthesis products (see Sup-
plementary informations). Component marked as 3 is formed from
acetanil in an excess of acetone. The formation of components 1–3 is
presented in Scheme 4.
Formation of component 3, however, should be considered as un-
favourable because it does not participate in the formation of TMQ.
Component marked as 4 has been identified as the main product –
TMQ. In the MS spectrum is presented the characteristic peak of 173 m/
e, assigned to master ion, which confirm the presence of TMQ in post-
synthesis products. Very close to compound 4 compound marked as 5
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MolecularCatalysis454(2018)94–103
high, what causes the formation of many side by-products lowering the
selectivity of this reaction. This reaction, leading to important 1,2-H-
dihydochinoline derivative (TMQ), was then studied paying particular
attention to formation of key side products, both desirable and un-
desirable considering the final product. Above detailed analysis of the
selectivity of this reaction was made for the first time. Moreover the
formation of not previously not described structural isomer of TMQ has
been also demonstrated and analytically confirmed.
Acknowledgment
This study was supported by The National Centre for Research and
Development, Poland, project “Quinoline antioxidants for plastics”
POIG.01.03.01-00-006/12.
Scheme 6. Formation of components 6 and 7.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.mcat.2018.05.016.
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In this paper, we investigated the reaction of aniline with acetone
and mesityl oxide as a model carbonyl donor in acid catalysed Skraup-
type condensation. As catalysts two type of organosulfonic modified
silicas (amorphous and well-ordered) were applied. The reaction of
aniline with acetone in the presence of heterogeneous acidic silica
catalysts were investigated in more detail and discussed. We've found
out that in acidic reaction conditions and elevated reaction temperature
the reactivity of main raw materials (aniline, acetone) is significantly
103