Selective synthesis of octahydroacridines and diannelated pyridines over zinc-containing mesoporous aluminosilicate molecular sieve catalysts

Article abstract of DOI:10.1039/c9dt01196j

We demonstrate a very eco-friendly and single-step catalytic method for the highly selective synthesis of 1,2,3,4,5,6,7,8-octahydroacridine (OHA) by the vapour phase aminocyclization of cyclohexanone (CyO) with a mixture of formaldehyde (HCHO) and ammonia (NH₃) over mesoporous bimetallic ZnAlMCM-41 (ZnAl-41) molecular sieves as efficient catalysts, which were synthesised by a simple basic hydrothermal method. To find optimum parameters for the synthesis of OHA, different reaction parameters, such as temperature, time on stream (TOS), weight hourly space velocity (WHSV), and feed molar ratios of CyO:HCHO:NH₃, have been extensively studied. The used ZnAl-41 catalysts were treated by washing and calcination to recover the recyclable catalysts which were then reused in these reactions to study their catalytic abilities. To selectively synthesize a variety of pyridine compounds, the active mesoporous catalysts, namely, ZnAl-41(75) and recyclable ZnAl-41(75), with different reaction parameters, were extensively used in the vapour phase aminocyclization reaction with different aldehydes and cycloketones, and produced excellent product selectivities, e.g., 9-alkyl substituted octahydroacridines (9-ASOHAs) and diannelated pyridines (DAPs), with good ketone conversions. In this catalytic reaction, OHA, 9-ASOHAs and DAPs are the main products and are important as starting materials in the preparation of biologically active compounds, drugs, dyes and alkaloids. It is shown by our remarkable catalytic results that the ZnAl-41(75) catalyst, as an environmentally friendly heterogeneous catalyst, has outstanding catalytic activity in the production of OHA, 9-ASOHAs and DAPs by a single-step synthetic method.

Full text of DOI:10.1039/c9dt01196j

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Selective synthesis of octahydroacridines and
diannelated pyridines over zinc-containing
mesoporous aluminosilicate molecular sieve
catalysts†
Cite this: DOI: 10.1039/c9dt01196j
Manickam Selvaraj * and Mohammed A. Assiri
We demonstrate a very eco-friendly and single-step catalytic method for the highly selective synthesis of
1,2,3,4,5,6,7,8-octahydroacridine (OHA) by the vapour phase aminocyclization of cyclohexanone (CyvO)
with a mixture of formaldehyde (HCHO) and ammonia (NH3) over mesoporous bimetallic ZnAlMCM-41
(ZnAl-41) molecular sieves as efficient catalysts, which were synthesised by a simple basic hydrothermal
method. To find optimum parameters for the synthesis of OHA, different reaction parameters, such as
temperature, time on stream (TOS), weight hourly space velocity (WHSV), and feed molar ratios of
CyvO : HCHO : NH3, have been extensively studied. The used ZnAl-41 catalysts were treated by washing
and calcination to recover the recyclable catalysts which were then reused in these reactions to study
their catalytic abilities. To selectively synthesize a variety of pyridine compounds, the active mesoporous
catalysts, namely, ZnAl-41(75) and recyclable ZnAl-41(75), with different reaction parameters, were exten-
sively used in the vapour phase aminocyclization reaction with different aldehydes and cycloketones, and
produced excellent product selectivities, e.g., 9-alkyl substituted octahydroacridines (9-ASOHAs) and dia-
nnelated pyridines (DAPs), with good ketone conversions. In this catalytic reaction, OHA, 9-ASOHAs and
DAPs are the main products and are important as starting materials in the preparation of biologically
active compounds, drugs, dyes and alkaloids. It is shown by our remarkable catalytic results that the ZnAl-
41(75) catalyst, as an environmentally friendly heterogeneous catalyst, has outstanding catalytic activity in
the production of OHA, 9-ASOHAs and DAPs by a single-step synthetic method.
Received 20th March 2019,
Accepted 8th July 2019
DOI: 10.1039/c9dt01196j
rsc.li/dalton
Bayer research group reported that the Friedlander conden-
sation of cyclopentanone and cyclohexanone with β-aminoacrolein
1. Introduction
Aminocyclization of ketones with ammonia (NH3), which is a produces 2,3-fused pyridines.6 The synthesis of octahydrophe-
building block chemical, is an important reaction in synthetic nanthridines was also carried out using bicyclic aromatic and
organic chemistry and medicinal chemistry to produce pyri- aliphatic dienamides, which was a multi-step procedure,
dine-based heterocyclic compounds, such as octahydroacri- involved corrosive reagents and a homogeneous catalytic
dine and its derivatives, which are mainly used as the starting system.6 All the catalytic systems cause large amounts of waste
materials for the synthesis of biologically active compounds, production with a low amount of major product selectivity.
dyes, drugs and alkaloids.1–3
Homogenous catalytic system causes many problems concern-
Many conventional methods are used for the synthesis of ing undesirable side products formed using high amounts of
octahydroacridines, as noted in literature.4–6 Several homo- raw chemicals, produces high corrosiveness and has several
geneous synthetic methods have not only several multi-step steps to recover the main product. In addition, in the chemical
synthesis procedures under expensive catalytic methods but industry, homogenous catalytic materials are unable to be
the long durations are also required for recovering the used recycled and thus are highly non-ecofriendly. Heterogeneous
catalysts after the completion of the reaction. For example, the catalytic chemistry can play a key role in achieving the major
goals of ‘green chemistry’: producing the main product with a
higher selectivity, reducing by-products to a trace number, pro-
moting the conversion of large amounts of starting materials
Department of Chemistry, King Khalid University, Faculty of Science, P.O. Box 9004,
Abha 61413, Saudi Arabia. E-mail: mselvaraj@kku.edu.sa; Fax: +966 17 241 7637
used in catalytic reactions, and replacing hazardous and stoi-
chiometric reagents by green catalytic materials which can be
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9dt01196j
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easily recovered from the final reaction mixture and used as of new drug chemicals in pharmaceutical industries, and
recyclable catalysts. Recent research has been highly focused potent anti-inflammatory agents, anti-bacterial agents, inhibi-
on traditional homogeneous catalysts being replaced by green tors of gastric acid secretion and calcium channel
heterogeneous mesoporous catalysts. Therefore, hetero- blockers.15–18 However, conventional methods require many
geneous catalytic materials were used in the production of steps for the synthesis of 9-ASOHAs and DAPs19,20 and
octahydroacridine and its derivatives. However, microporous produce low yields. Accordingly, we decided to use an eco-
zeolites used in the vapour phase cyclization reaction produce friendly catalytic system, which is highly desirable as a single-
several by-products along with the main product, and compli- step synthetic method, with green routes in the selective syn-
cated and expensive processes might be needed to separate the thesis of 9-ASOHAs and DAPs.
main product from the several by-products.7
Selvaraj et al., for the first time, produced the mesoporous
Generally, green catalytic reactions minimise environmental ZnAl-41 catalysts with the details of their physio-chemical
threats and maximise economic benefits. To supply the great characterization.21 The ZnAl-41 catalysts have been widely used
demand in the fine chemical industry, highly suitable solid as promising heterogeneous solid acid catalysts in
acid catalysts with a large number of catalytic active sites have alkylation,21a,22 cyclization23 and Prins condensation24 for the
been used under mild reaction conditions without employing selective synthesis of valuable fine chemicals by the routes of
toxic materials. The Kresge research group, in 1992, discovered green heterogeneous catalytic chemistry. To the best of our
well-ordered hexagonal mesoporous materials that have large knowledge, a single-step synthesis of OHA, 9-ASOHAs and
surface areas, large pore sizes and large pore volumes, which DAPs has not been carried out using the mesoporous ZnAl-
have been used in catalysis, adsorption, controlled drug deliv- 41 molecular sieve catalysts. Our present investigation presents
ery, energy and environmental applications.8 However, till a unique, eco-friendly and simple method for the selective syn-
date, pure mesoporous silica molecular sieves have not been thesis of OHA, 9-ASOHAs and DAPs using low cost reactants
majorly used as catalysts or catalyst supports in chemical under a single-step vapour phase method; the true hetero-
industry for bulk molecule conversion due to the limitations geneous catalysts, ZnAl-41, have been used for the first time in
of acidity and poor hydrothermal stability, because their thin this catalyst system. The catalytic results obtained over ZnAl-41
amorphous silica pore walls are easily destroyed in boiling catalysts for the selective synthesis of OHA, 9-ASOHAs and
water and steam through their fast hydrolysis reaction. Thus, DAPs were correlated and compared with those for other solid
enormous amounts of heteroatomic species incorporated into catalysts, such as AlMCM-41(Al-41), ZnMCM-41 (Zn-41), USY,
the mesoporous silica walls improve the surface acidity with Hβ, HZSM-5 and H-mordenite.
enhanced hydrothermal stability and produce abundant
superior catalytic active sites.9–11 Compared to zeolites, the
synthesised mesoporous catalysts have a lower content of acid
2. Experimental
sites, but their uniformly hexagonal pores enhance the pro-
duction of octahydroacridine derivatives.12,13
A variety of mesoporous catalysts, namely bimetallic ZnAl-
In 2004, Ratnamala et al. used microporous and meso- 41 mesoporous catalysts (nSi/(nZn + nAl) = 75, 151, 228, 304 and
porous catalysts for the synthesis of OHA by the vapour phase 380), monometallic mesoporous catalysts Al-41 (nSi/nAl = 21)
cyclization reaction of CyvO, HCHO and NH3 for the first and Zn-41 (nSi/nZn = 21), and mesoporous MCM-41 silica, were
time and reported that 3 wt% CeZSM-5 has higher OHA selecti- synthesized and characterized according to the procedure pre-
vity (92%) and CyvO conversion (100%) than other micro- viously published by Selvaraj et al.21 For comparison
porous and mesoporous catalysts, because the Ce3+ ions studies, Hβ (nSi/nAl = 20, Strem), HY (nSi/nAl = 2.9, PQ),
produce more acid sites uniformly dispersed on the surfaces H-ZSM-5 (nSi/nAl = 30, PQ) and H-mordenite (nSi/nAl = 20, PQ)
of pore channels.12 In 2007, liquid phase cyclization reactions were commercially purchased. Prior to performing the catalytic
over many molecular sieve catalysts, introduced by Krishna reactions, the purchased zeolite catalysts were heat-treated at
Mohan et al., produced OHA, with a lower selectivity because 473 K in air for 6 h to remove the moisture on the surfaces of
the liquid phase autoclavation system is only suitable for the active sites.
synthesis of 1,2,3,4,7,8,9,10-octahydrophenanthridine (OHP)
All catalytic experiments for the vapour phase aminocycliza-
and its derivatives, whereas high amounts of catalysts and a tion reaction were carried out in a fixed-bed vertical flow
toxic solvent (methanol) were used for the liquid phase reac- reactor (20 mm diameter) with a continuous down flow. The
tions.14 However, the selectivity for OHA and its derivatives catalyst bed temperature was measured with a thermocouple
increases on increasing the temperature from 423 to 473 K in placed in the middle of the catalyst bed. Typically, prior to the
the vapour phase cyclization reaction using mesoporous alu- reaction, the mesoporous solid acid catalytic material (2 g) was
minosilicate catalysts, as reported in literature.13 On the basis activated at 773 K in flowing air for 10 h, followed by cooling
of the above research accounts, we used ZnAl-41 catalysts in an to the reaction temperature (450 K) under nitrogen. After an
eco-friendly, single-step aminocyclization method for the selec- hour, the reactant mixture of CyvO, (>99.0% ACS reagent,
tive synthesis of OHA and its derivatives.
Sigma-Aldrich) and HCHO (37 wt% in H2O, Sigma-Aldrich) at
The 9-ASOHAs and DAPs are very important drug inter- the desired feed molar ratio and WHSV (obtained by dividing
mediates, and valuable synthetic templates for the preparation the mass flow of the reactant by the mass of catalyst) were fed
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into the reactor using
a
dual syringe pump (Harvard perties, namely d-spacing value (d100) and unit cell parameter
Apparatus). A desired amount of ammonia gas was also fed (ao), was also investigated, as shown in Table 1S.† Fig. 2S†
through a manometer (calibrated with flow meter) to the cata- shows that large amounts of Zn and Al ions, which can be con-
lytic reactor. The condensed liquid products obtained from the firmed by the intensity of the corresponding FTIR absorption
gaseous products were collected at different times, analysed by peaks, are highly incorporated into mesoporous silica walls. In
gas chromatography using 10% SE-30 columns, and confirmed addition, the intensity of the FTIR adsorption peaks with the
using a combined gas chromatography-mass spectrometry shift in wavenumbers shows that the bimetallic ZnAl-41 cata-
system (GC-MS, Hewlett G1800A). The products were purified lysts have higher antisymmetric Si–O–Si vibration bands
by column chromatography over
a )
suitable silica gel (1096 cm−1) than the monometallic catalysts Al-41 (1083 cm−1
(100–200 mesh) using 2–10% ethyl acetate and hexane as the and Zn-41 (1085 cm−1). The incorporation of the larger ionic
eluent. The vapour phase aminocyclization reaction was radii of Zn and Al leads to an increase in wall thickness, as
further carried out using a variety of aldehydes, namely, acet- described in literature.21 ICP-AES results confirm that the Zn
aldehyde (≥99.5%), propionaldehyde (97%), butyraldehyde and Al ion contents on the surface of inner pore walls are
(≥99.5%), isobutyraldehyde (≥99%) and valeraldehyde (97%), mostly constant before and after the synthesis of OHA,
and different cycloketones, namely, cyclopentanone (≥99%) 9-ASOHAs and DAPs, as shown in Table 1S.† Although the tex-
and cycloheptanone (99%). The aldehydes, cycloketones and tural properties of ZnAl-41(75) are smaller than those of the
other chemicals used were purchased from Sigma-Aldrich Inc. other ZnAl-41 catalysts (Table 1S†), the number of acid sites of
(USA). All the products were identified by 1H NMR and 13C ZnAl-41(75) are higher than that in other mesoporous catalysts,
NMR spectra and the spectral details are presented in ESI.†
as shown in Table 2S and Fig. 3S,† due to the increase in the
The incorporation of Zn and Al ions in ZnAl-41(75) is number of zinc ions.21 All the calcined ZnAl-41 catalysts, as
higher and lower in ZnAl-41(380). To find stability, both bi- confirmed by SEM studies,21 have the micellar rod-like hexag-
metallic mesoporous catalysts were reused in the single-step onal shape (Fig. 4S†). The hexagonal symmetry and uniform
vapour phase synthesis of OHA, 9-ASOHAs and DAPs. In a pore channels were clearly identified by the TEM studies, as
typical reaction process, from the reaction mixture, the used shown in the TEM image (Fig. 5S†) of ZnAl-41(75), indicating
ZnAl-41(75) catalyst was recovered by washing with acetone that the ZnAl-41 catalysts have uniform mesostructures that
several times, and then dried at 393 K. Finally, the ZnAl-41(75) have been also confirmed by the results of XRD patterns.21
catalyst was calcined at 773 K for 6 h in air for the removal of
the formed organics (as coke) and unreacted molecules. The
recovered ZnAl-41(75) catalyst was reused in this reaction. The
recyclable bimetallic mesoporous catalysts, ZnAl-41(75) with
rich Al and Zn ion content and ZnAl-41(380) with low Zn and
Al ion content, were regenerated before applying to the cata-
lytic reactions. A similar reaction procedure was used for the
recyclable ZnAl-41(75) catalyst for the synthesis of 9-ASOHAs
and DAPs. After completion of the reaction, the percentage of
Zn and Al ion content of the used catalyst was analyzed by
ICP-AES, and the analysis results of GC and GC-MS calculated
using standard formulas were used to find the conversions
and selectivities.
3.2. Selective synthesis of OHA
The characterized mesoporous solid acid catalysts, namely bi-
metallic ZnAl-41 catalysts and monometallic mesoporous cata-
lysts Al-41(21) and Zn-41(21), were used in the vapour phase
aminocyclization of CyvO with a mixture of HCHO and NH3
in a feed molar ratio of 2 : 1 : 3 (CyvO : HCHO : NH3) at 1 h−1
WHSV at a temperature of 613 K for 1 h TOS, as shown in
Table 1. The purchased microporous catalysts, after removal of
moisture by calcination, were also used in this reaction under
Table 1 Single step synthesis of OHA over various types of catalystsa
Selectivity (%)
Pore
Conversion
3. Results and discussion
Catalysts
size (Å)
of CyvO (%) OHA DCA MOHA
3.1. Characterization of ZnAl-41
ZnAl-41(75)
ZnAl-41(151)
ZnAl-41(228)
ZnAl-41(304)
ZnAl-41(380)
HY (2.6)
22.8
25.6
27.0
29.3
32.2
7.4
7.6 × 5.4 81.0
7.1
5.4
98.0
81.3
73.4
65.6
57.6
72.4
95.0 3.0
87.4 7.0
2.0
5.6
The simple basic hydrothermal method is an eco-friendly
method that has been used for the synthesis of a variety of
mesoporous catalysts, such as bimetallic mesoporous ZnAl-41
catalysts, monometallic mesoporous catalysts like Al-41(21)
and Zn-41(21), and pure mesoporous MCM-41. Relevant soph-
isticated instrumental techniques were used for the character-
ization of the synthesised catalysts, according to the procedure
published by Selvaraj et al.21 The XRD results (Fig. 1S†) of
several mesoporous catalysts, namely bimetallic ZnAl-41 cata-
lysts, monometallic catalysts such as Al-41(21) and Zn-41(21),
and MCM-41, confirm their mesoporous nature. When the
metal-ion content is increased, the decrease in structural pro-
66.7 26.1 7.2
58.9 19.6 3.8 (17.7)b
49.5 12.5 2.5(35.5)b
63.4 9.5
68.9 7.4
49.8 4.5
73.2 7.2
6.2 (20.9)b
15.3 (8.4)b
3.0 (42.7)b
11.5 (8.1)b
Hβ (20)
H-mordenite (20)
HZSM-5(15)
Al-41(21)
55.0
83.4
79.6
63.5
9.2
27.5
27.3
29.3
65.6 17.9 7.8 (8.7)b
26.4 65.6 2.1 (8.0)b
Zn-41(21)
MCM-41
7.5 4.0
1.0 (87.5)c
a Reaction conditions:
2
g
of catalyst, feed molar ratio of
CyvO : HCHO : NH3 = 2 : 1 : 3, T = 613 K, WHSV = 1 h−1, TOS = 2 h.
b Trace amounts of cyclohexylamine and aminooctahydroacridines
with other unidentified products. c Unreacted reactants.
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Scheme 1 The plausible reaction mechanism for the formation of OHA over ZnAl-41 catalysts.
the same reaction conditions noted in Table 1. In this amino- in mesoporous aluminosilicates are highly favourable catalysts
cyclization reaction, OHA is primarily produced by an intra- in cyclization, condensation, hydrogenation and dehydrogena-
molecular aromatization of the primary product intermediate, tion, as reported in literature.21–23 The OHA selectivity
1,2,3,4,5,6,7,8,9,10-decahydroacridine, as shown in Scheme 1. increases for the series of catalysts as follows: ZnAl-41(75) >
In addition to the byproduct 9-methyl-1,2,3,4,5,6,7,8,-octahy- ZnAl-41(151) > HZSM-5(15) > Hβ(20) ZnAl-41(228) > Al-41(21) >
droacridine (MOHA) produced by acid catalyzed methylation HY(2.6) > ZnAl-41(304) > H-mordenite(20) > ZnAl-41(380) > Zn-
of OHA, dicyclohexylamine (DCA) is also formed by direct 41(21) > MCM-41. The bimetallic ZnAl-41(75) catalyst produces
amination and condensation of cyclohexanones (CyvOs) with 95% OHA selectivity with 98% CyvO conversion. Compared to
NH3, while OHP forms with low selectivity at low reaction other ZnAl-41 catalysts, ZnAl-41(75) gives higher catalytic
temperature (513 K) because vapour phase aminocyclization activity that increases OHA production with no diffusional con-
was not favoured. Trace amounts of cyclohexylamine, amino- straint, while the incorporation of abundant Zn ions increases
octahydroacridines and other unidentified products were also the number of acid sites on the inner pore walls of the ZnAl-41
observed.
(75) catalyst.21 In addition, the OHA selectivity of ZnAl-41(75)
is higher than that of Al-41 due to the higher catalytic activity
promoted by the abundant Zn ions incorporated on the inner
3.3. Selectivity of OHA
As shown in literature, the mesoporous aluminosilicate cata- pore walls of Al-41.21 However, the HZSM-15(15) catalyst pro-
lysts Al-MCM-41(15) and Al-MCM-41(30) have been used as duces higher OHA selectivity than other microporous and
convenient solid acid catalysts for syntheses of 1,2,3,4,5,6,7,8- mesoporous catalysts, except ZnA-41(75) and ZnAl-41(151), as
octahydroacridin-9-amine (OHAA) and OHA by the vapor shown in Table 1, because its acid sites on the pore channels
phase cyclization method.12,13 The mesoporous Al-MCM-41 (pore size 5.4 Å) are better adapted for the acid-catalysed ami-
(30) has 52% OHA selectivity with byproduct selectivities of nocyclization of CyvO compared to those of other catalysts. In
5% MOHA and 14% DCA.12 In contrast, Al-MCM-41(15), in the contrast, the Zn-41(21) has less OHA selectivity compared to
synthesis of OHAA, has lower OHA selectivity (17%) than Al- other microporous and mesoporous catalysts used in this cata-
MCM-41(30) because OHA in this case is formed by the acid lytic reaction, but increases the DCA yield with a higher selecti-
catalysed deamination of OHAA.13 From the above literature vity because its acid sites lead to an increase in direct amino-
results, we found that the mesoporous aluminosilicate cata- condensation between two CyvOs with NH3, as shown in
lysts produce OHA with less selectivity; however, this catalyst literature.12,13,22 It was subsequently noted that HZSM-5 has a
system leads to a direct condensation product, DCA, with 14% higher selectivity of MOHA compared to all the microporous
selectivity because the acid sites could not be enhanced to and mesoporous catalysts. Overall, the structural and textural
achieve higher selectivity of OHA and its derivatives. We thus properties of ZnAl-41(75) increase the acid strength, causing
introduced bimetallic mesoporous ZnAl-41 catalysts to improve high chemisorption between the reactants and active surface
the selectivity of OHA and its derivatives by a single-step vapor of the catalyst to achieve higher OHA selectivity. It is signifi-
phase aminocyclization reaction, because Zn ions incorporated cantly observed that ZnAl-41(75) has higher OHA selectivity
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than CeZSM-15,12 because it has higher surface area and more sion of CyvO as well as OHA selectivity. The active sites of
active sites than CeZSM-15. On the basis of the results ZnA-41(75) at 613 K for 2 h TOS at 1 h−1 WHSV might not be
obtained using different molecular sieve acid catalysts, ZnAl-41 reversible, and could be confirmed by the studies of the acidity
(75) is found to be a promising heterogeneous catalyst in the measurements and hydrothermal stability.21
vapour phase aminocyclization of CyvO for the highly selec-
3.5. Effect of time on stream (TOS)
tive synthesis of OHA.
In conclusion, the above catalytic results confirm that the
well-ordered mesoporosity with higher surface area and suit-
able acid sites highly promoted by the abundant Zn and Al
ions plays a significant catalytic role in the synthesis of OHA
with higher selectivity.
For finding the effects of CyvO conversion and product
selectivity, the vapour phase aminocyclization of CyvO with
different times on stream (TOSs) in a feed molar ratio of
2 : 1 : 3 (CyvO : HCHO : NH3) at 613 K over ZnAl-41(75) is dis-
played in Fig. 2. When the stream time (TOS) is increased from
0.5 to 2 h, the CyvO conversion and OHA selectivity increase
with the decrease in the selectivities for by-products, DCA and
MOHA. The low TOS (<1.5 h) is favourable for the acid-cata-
lysed methylation with OHA to form MOHA and causes a
direct condensation of CyvOs with NH3 to produce DCA. After
2 h TOS, the CyvO conversion and product selectivity gradu-
ally decrease due to the formation of heavy coke, which
screens the active sites, on the pore walls of the catalyst caused
by increasing the stream time of the reactor. The catalysis
results obtained under the reaction conditions remarkably
found that the best TOS is 2 h, which leads to 98% CyvO con-
version and 95% OHA selectivity, as shown in Fig. 2.
3.4. Effect of reaction temperature
The vapour phase aminocyclization of CyvO was carried out
at different reaction temperatures with a feed molar ratio of
2 : 1 : 3 (CyvO : HCHO : NH3) at 1 h−1 WHSV for 2 h TOS over
ZnAl-41(75), as shown in Fig. 1, to explore the effects of CyvO
conversion and the product selectivity. On increasing the
temperature from 513 to 613 K, the CyvO conversion and
OHA selectivity increase, whereas the selectivities for by-pro-
ducts OHP, DCA and MOHA decrease. Particularly, for the syn-
thesis of OHP with higher selectivity, the liquid phase autocla-
vation method is highly suitable, but a vapour phase catalytic
system is not favoured, as explained clearly in literature.14 The
CyvO conversion and OHA selectivity decrease when the reac-
tion temperature further increases from 613 to 653 K because
a few active sites on the inner pore walls of the catalysts are
screened by coke formation from unreacted reactants, which
cause sintering and poisoning, resulting in deactivation, as
described in literature.25 At low reaction temperature (<613 K),
the chemisorption between the reactants and the Brønsted
acid sites of catalysts decreases. In this case, the byproduct for-
mation of OHP, DCA and MOHA increases. Interestingly, we
found from the catalysis results that the best reaction tempera-
ture is 613 K, which is highly suitable to achieve higher conver-
3.6. Effect of CyvO, HCHO and NH3 ratio
To investigate the best feed molar ratio of CyvO : HCHO : NH3
for the highly selective synthesis of OHA, the vapour phase
aminocyclization of CyvO was conducted with a variety of
feed molar ratios of CyvO : HCHO : NH3 at 613 K for 2 h TOS
at 1 h−1 WHSV over ZnAl-41(75), as shown in Fig. 3. When this
reaction is carried out with a feed molar ratio of 2 : 1 : 3
(CyvO : HCHO : NH3), outstandingly, 98% CyvO conversion
and 95% OHA selectivity are observed. Moreover, the CyvO
conversion and OHA selectivity at this feed molar ratio are
higher than that of other feed molar ratios because of the
greater chemisorption that makes an equilibrium between the
Fig. 1 Effect of reaction temperature over ZnAl-41(75). Reaction con-
ditions: 2 g of catalyst, feed molar ratio of CyvO : HCHO : NH3 = 2 : 1 : 3, Fig. 2 Effect of TOS over ZnAl-41(75). Reaction conditions: 2 g of catalyst,
WHSV = 1 h−1, TOS = 2 h.
feed molar ratio of CyvO : HCHO : NH3 = 2 : 1 : 3, WHSV = 1 h−1, T = 613 K.
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Fig. 4 Effect of WHSVs over ZnAl-41(75). Reaction conditions: 2 g of
catalyst, feed molar ratio of CyvO : HCHO : NH3 = 2 : 1 : 3, TOS = 2 h,
T = 613 K.
Fig. 3 Effect of the feed molar ratios of CyvO : HCHO : NH3 over ZnAl-
41(75). Reaction conditions: 2 g of catalyst, WHSV = 1 h−1, T = 613 K,
TOS = 2 h.
increase in CyvO conversion. Following this, the CyvO con-
version and OHA selectivity gradually decreased until 2 h−1
WHSV, whereas the formation of byproducts such as DCA and
MOHA with ∼10–20% selectivity can also be observed at
0.5 h−1 WHSV because the contact times (before and after
1 h−1 WHSV) were not suitable to lead to further reactions,
such as cyclization and aromatization. After 2 h−1 WHSV, the
OHA decomposes with the formation of heavy coke because of
contact time. Based on the effect of the space velocities
(WHSV), we clearly find that the best contact time is 1 h−1
WHSV for the selective synthesis of OHA.
amounts of reactant and the acid sites created on the surfaces
of pore walls by the active species of Zn and Al of ZnAl-41(75).
The decrease in OHA selectivity is clearly observed in the feed
molar ratios of 2 : 1 : 2, 2 : 1 : 5 and 2 : 2 : 5, while at the same
feed molar ratios, the decrease in CyvO conversion is also
observed because of the increase in by-product selectivity. Due
to the increase in the concentrations of HCHO and NH3, the
formation of DCA with 18–25% selectivity is observed in the
feed molar ratios of 2 : 1 : 2 and 2 : 1 : 5, while MOHA selectivity
(10–30%) increases in the feed molar ratios of 2 : 1 : 2 and
2 : 2 : 5. The selectivity of byproducts, such as DCA and MOHA,
is higher compared to that obtained with the feed molar ratio
of 2 : 1 : 3 because the ratios of the reactants may be matched
with the suitable chemisorption of Brønsted acid sites on the
inner pore wall surface of the catalyst and could be highly favoured
for the direct condensation of CyvOs with NH3 to produce DCA
and for the acid-catalysed methylation of OHA in the preparation
of MOHA. The selectivity of all products such as OHA, DCA and
MOHA with CyvO conversion decreases in a feed molar ratio of
2 : 1 : 1 due to the uneven ratios of reactants that form coke on the
surfaces of the catalyst with a sintering effect, which screens the
active sites of the catalyst, resulting in deactivation.25 The effects
of the feed ratio display clearly that 2 : 1 : 3 feed molar ratio
(CyvO : HCHO : NH3) is an optimum ratio among all the ratios
tested for the highly selective synthesis of OHA.
3.8. Recyclability
Among the bimetallic ZnAl-41 catalysts evaluated for the
single-step aminocyclization, ZnAl-41(75) and ZnAl-41(380)
have correspondingly the highest and lowest OHA selectivities.
The evaluation of reusability for the bimetallic ZnAl-41 cata-
lysts was thus carried out under the reaction conditions noted
in Table 2 to determine their catalytic stabilities. Typically, the
catalysts used for this reaction are affected from the loss of
catalytic activity because most of the active sites are blocked by
unreacted organics, which is one of the factors contributing to
the sintering effect on the catalyst surface. Prior to reuse, the
catalysts were chemically treated and calcined for regeneration,
as explained in the procedure reported by Selvaraj et al.22 The
used catalysts were washed several times with acetone, dried
overnight at 393 K and further calcined at 773 K for 6 h in air
for the removal of organics and unreacted organic molecules.
3.7. Effect of weight hourly space velocity (WHSV)
A single-step synthesis of OHA was carried out by the The regenerated catalysts were recycled in this reaction, as
vapour phase aminocyclization of CyvO at various space described in Table 2. All the regenerated catalysts were recov-
velocities (WHSVs) with
a feed molar ratio of 2 : 1 : 3 ered using the above procedure before applying to each run. In
(CyvO : HCHO : NH3) at 613 K for 2 h TOS over ZnAl-41(75), as the first two runs, the increase in CyvO conversion and a
shown in Fig. 4, to investigate the effect of product selectivity slight increase in OHA selectivity were observed. The results
along with CyvO conversion. When the WHSV was increased of the first and second runs confirm clearly that small
from 0.5 to 1 h−1, the OHA selectivity increased with the numbers of active sites on the inner pore surfaces of ZnAl-41
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Table 2 Recycling of ZnAl-41(75) and ZnAl-41(380)a
and valeraldehyde, are used. At 613 K, the selectivities of
9-ASOHAs and conversions of CyvO increase compared to the
selectivities of 6-alkyl substituted octahydrophenanthridines
(6-ASOHPs). However, the selectivities of 9-ASOHAs and con-
versions of CyvO decrease at 513 K because the low reaction
temperature is highly suitable for the production of 6-ASOHPs,
as reported in the literature.14 The aminocyclization of cyclo-
hexanone with various aldehydes was carried out at 4 h TOS;
the selectivities of 9-ASOHAs and conversions of CyvO gradu-
ally decrease due to the formation of heavy coke by increasing
the stream time of reactor, and the coke screens the active sites
on the pore walls of the catalyst of ZnAl-41(75). When the ami-
Selectivity (%)
Pore
Conversion
Catalysts
size (Å) Run of CyvO (%) OHA DCA MOHA
ZnAl-41(75)
ZnAl-41(75)
ZnAl-41(75)
ZnAl-41(75)
ZnAl-41(380) 31.5
ZnAl-41(380) 31.5
ZnAl-41(380) 31.5
ZnAl-41(380) 31.5
22.8
22.9
22.9
22.9
1
2
3
4
1
2
3
4
98
99
96
97
2.0
2.0
2.0
1.0
1.0
1.0
100
100
57.6
57.7
57.7
57.7
97.5 1.5
97.5 1.5
49.5 12.5 3.0 (35)b
49.6 12.6 4.5 (33.3)b
49.6 12.6 4.6 (33.2)b
49.6 12.6 4.6 (33.2)b
a Reaction conditions:
2
g
of catalyst, feed molar ratio of
CyvO : HCHO : NH3 = 2 : 1 : 3, T = 613 K, WHSV = 1 h−1, TOS = 2 h. nocyclization reaction is carried out at 0.5 h−1 WHSV, the con-
b Trace amounts of cyclohexylamine and aminooctahydroacridines
versions of CyvO as well as selectivities of 9-ASOHAs decrease
because the contact time is not long enough to covert the
with other unidentified products.
9-ASOHAs completely from the reactants whereas the bypro-
(75) were screened/blocked by the non-framework octahedral
aluminium and zinc oxides that are completely removed after
the washing and calcination processes. After the third and
fourth runs, the CyvO conversion and OHA selectivity
remain constant, indicating that the incorporated or substi-
tuted active species (Zn- and Al-ions) were not leached, in the
present reaction conditions, from the surface of the meso-
porous silica matrix, as shown by the ICP-AES results of ZnAl-
41(75) after the recycling reactions (Table 1S†). The CyvO con-
version and OHA selectivity in ZnAl-41(380) almost remain
constant with each cycle, because the low amount of active
species can be homogenously incorporated on the inner silica
pore walls. The amount of Zn and Al ions in ZnAl-41(380) was
also determined by ICP-ACS after the recycling reaction, as
shown in Table 1S.† This process is also found to be a true
heterogeneous catalytic process.26,27 This reaction was also
carried out using pure MCM-41 synthesized by the direct basic
hydrothermal method.21c In this case, CyvO (9.2%) forms
with a trace amount of product selectivity (Table 1). In con-
clusion, we are clearly note from all the catalytic results that
the bimetallic ZnAl-41(75) is a principal catalyst to obtain
higher OHA selectivity by a single-step vapour phase aminocy-
clization reaction.
ducts could not be also formed by cyclization and aromatiza-
tion. When the amounts of aldehydes and NH3 are increased, as
in a feed ratio of 2 : 2 : 5 (Table 3), the selectivities of 9-ASOHAs
decrease with the decrease in conversions of CyvO because an
uneven ratio of reactants produces heavy coke under the reac-
tion conditions that screens the active sites on the surface of the
catalyst, resulting in deactivation by the sintering effect.25 The
conversions of CyvO and selectivities of 9-ASOHAs are similar
in recycled ZnAl-41(75) and calcined ZnAl-41(75) because the
active sites on the surface of the catalyst did not leach when the
recyclable ZnAl-41(75) catalyst was used up to 4 runs. This is
confirmed as the active species, Zn and Al, in recyclable ZnAl-41
(75) are in irreversible manner, according to ICP-AES studies.
Overall, an important fact in the catalytic reactions is that the
conversions of CyvO and selectivities of 9-ASOHAs decrease
when the alkyl chain length in aldehydes is increased, as
reported in elsewhere.7,14
3.10. Aminocyclization with various cyclic ketones
A single-step vapour phase aminocyclization reaction was
carried out using different cyclic ketones over ZnAl-41(75)
under the reaction conditions noted in Table 4. For the highly
selective synthesis of 1,2,3,5,6,7-hexahydrodicyclopenta[b,e]
pyridine (HHDCPP) and 1,2,3,4,5,7,8,9,10,11-decahydrodicyclo-
hepta[b,e]pyridine (DHDCHP), the aminocyclization reactions
3.9. Aminocyclization of cyclohexanone with various aldehydes
To obtain the highly selective synthesis of 9-alkyl substituted were carried out using the corresponding ketones, namely,
OHA derivatives, the ZnAl-41(75) catalyst was used extensively cyclopentanone and cycloheptanone, using ZnAl-41(75) under
in the single-step vapour phase aminocyclization of cyclohexa- the reaction conditions noted in Table 4. At 613 K, the ketone
none with various aldehydes under the reaction conditions conversions and selectivities of HHDCPP and DHDCHP are
noted in Table 3. In these catalytic reactions, the major pro- higher compared to those of 1,2,3,6,7,8-hexahydrodicyclopenta
ducts
9-propyl-1,2,3,4,5,6,7,8-octahydroacridine, 9-isopropyl-1,2,3,4, hydrodicyclohepta[b,d]pyridine (DHDCH[b,d]P). However, com-
5,6,7,8-octahydroacridine and 9-butyl-1,2,3,4,5,6,7,8-octa- pared to HHDCPP and DHDCHP, the selectivities of HHDCP
hydroacridine) and byproducts (6-methyl-1,2,3,4,7,8,9,10-octa- [b,d]P and DHDCH[b,d]P increase at 513 K because the low
hydrophenanthridine, 6-ethyl-1,2,3,4,7,8,9,10-octahydro- reaction temperature is favourable for the synthesis of these
phenanthridine,
6-propyl-1,2,3,4,7,8,9,10-octahydrophenan- products, as indicated in previous literature.14 When the ami-
(MOHA,
9-ethyl-1,2,3,4,5,6,7,8-octahydroacridine, [b,d]pyridine (HHDCP[b,d]P) and 1,2,3,4,5,8,9,10,11,12-deca-
thridine, 6-isopropyl-1,2,3,4,7,8,9,10-octahydrophenanthridine nocyclization reactions with various ketones were carried out
and 6-butyl-1,2,3,4,7,8,9,10-octahydrophenanthridine) can be at 4 h TOS, the selectivities of HHDCPP and DHDCHP
produced when the corresponding aldehydes, viz. acet- decrease with the decrease in ketone conversions because
aldehyde, propionaldehyde, butyraldehyde, isobutyraldehyde heavy coke can be formed on the inner pore walls of ZnAl-41
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Table 3 Aminocyclization of cyclohexanone with various aldehydesa
Ketone
conversion (%)
S. no.
1
Ketone
Aldehyde
Amine
Product
T (K)
Selectivity (%)
513
60
95
55
70
52
97.5
61
73
47
61
43
73
52
61
41
49
33
61
73
98
66
78
83
100
67
89
51
65
71
90
60
85
43
57
59
86
613
613b
613c
613d
613e
513
2
3
613
613b
613c
613d
613e
513
613
613b
613c
613d
613e
4
5
6
513
41
52
33
37
28
53
29
41
22
26
20
42
53
80
35
43
48
81
40
75
28
31
37
75
613
613b
613c
613d
613e
513
613
613b
613c
613d
613e
513
15
32
16
19
17
32
33
60
19
22
28
61
613
613b
613c
613d
613e
a Reaction conditions: 2 g of ZnAl-41(75), feed molar ratio of CyvO : HCHO : NH3 = 2 : 1 : 3, WHSV = 1 h−1, TOS = 2 h. b TOS = 4. c WHSV = 0.5 h−1
.
d Feed ratio: 2 : 2 : 5. e The catalytic results received in 4th run.
Table 4 Aminocyclization with different ketonesa
Ketone
conversion (%)
S. no.
1
Ketone
Aldehyde
Amine
Product
T (K)
Selectivity (%)
513
72
89
66
61
69
90
75
96
70
69
76
97
77
96
69
56
60
97
80
99
72
61
68
100
613
613b
613c
613d
613e
513
2
613
613b
613c
613d
613e
a Reaction conditions: 2 g of ZnAl-41(75), feed molar ratio of CyvO : HCHO : NH3 = 2 : 1 : 3, WHSV = 1 h−1, TOS = 2 h. b TOS = 4. c WHSV = 0.5 h−1
.
d Feed ratio: 2 : 2 : 5. e The catalytic results received in 4th run.
(75) on increasing the stream time of reactor and the active ketone conversions and selectivities of HHDCPP and DHDCHP
sites on the pore walls of the ZnAl-41(75) catalyst can be were low due to the low contact time, which could not convert
screened by the formation of cokes. At 0.5 h−1 WHSV, the the main products completely from the reactants, whereas the
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byproducts can be easily formed by cyclization and aromatiza- bimetallic ZnAl-41 catalysts with highly suitable active sites for
tion. As noted in Table 4, in a feed ratio of 2 : 2 : 5, the selectiv- the amination of CyvO with NH3, heterocyclization reaction
ities of HHDCPP and DHDCHP decrease with the decrease in of cyclohex-1-enamine with HCHO, and aromatization and
ketone conversions when the amounts of formaldehyde and dehydrocyclization of 1,2,3,4,5,6,7,8,9,10-decahydroacridine
NH3 are increased, because this ratio of reactants could not into OHA. Compared with other mesoporous and microporous
evenly react and causes heavy coke formation, which screens catalysts, ZnAl-41(75) has higher number of active acid sites on
the active sites on the surface of catalyst, resulting in de- the surface pore walls that can lead to higher catalytic activity
activation by the sintering effect.25 Prior to reuse ZnAl-41(75), in this catalyst system. The results of optimizing reaction para-
it was treated by washing and calcination processes, using meters revealed that the highest OHA selectivity and CyvO
the procedure previously published by Selvaraj et al.,25 to conversion are obtained at 613 K for 2 h TOS at contact time
remove the unreacted reactants and moisture on the active and
1
h−1 WHSV using a feed molar ratio of 2 : 1 : 3
sites of the surface of the catalysts. The results show that the (CyvO : HCHO : NH3). The calcined ZnAl-41(75) was success-
catalytic activity of both catalysts, calcined ZnAl-41(75) and fully used for the synthesis of 9-ASOHAs by a single-step ami-
recycled ZnAl-41(75), are similar, as shown in Table 4, because nocyclization of CyvO using different aldehydes and for the
the active sites of ZnAl-41(75) were not changed when the synthesis of DAPs products, HHDCPP and DHDCHP, by a
recycled ZnAl-41(75) catalyst was used up to 4 runs, as can be single-step aminocyclization reaction with a variety of cycloke-
confirmed by the contents of Zn and Al species in recycled tones. The catalytic activity of recyclable ZnAl-41(75) is similar
ZnAl-41(75) based on ICP-AES studies. Overall, an important to that of calcined ZnAl-41(75). On the basis of all the catalysis
insight in the catalytic reactions, as we found, is that the studies, it is clear that the ZnAl-41(75) catalyst is a highly
ketone conversions and selectivities of HHDCPP and DHDCHP active, recyclable and promising heterogeneous catalyst for the
increase when the number of carbons in the cycloketones selective synthesis of OHA, 9-ASOHAs and DAPs by the single-
increased, according to the catalysis results of aminocycliza- step vapour phase aminocyclization reaction.
tion of different ketones displayed in Table 4.
3.11. Reaction mechanism
Conflicts of interest
On the basis of the major product, OHA, and minor by-pro-
ducts, DCA and MOHA, a plausible reaction mechanism is pro-
There are no conflicts to declare.
posed to clearly explain how the acid sites of bimetallic ZnAl-
41 catalysts work in the single-step vapour phase aminocycliza-
tion reaction of CyvO with a mixture of HCHO and NH3, as
Acknowledgements
shown in Scheme 1. Initially, the carbonyl group of cyclohexa-
none is protonated by the Brønsted acid site of ZnAl-41 and
further reacts with NH3 to give 1-cyclohexanimine to give an
imine, cyclohex-1-enamine, by auto-isomerization caused by
the Brønsted acid sites of ZnAl-41. Next, deamination produces
1,2,3,4,5,6,7,8,9,10-decahydroacrdine after an aminocyclization
reaction occurs between formaldehyde and two moles of cyclo-
hex-1-enamine. Finally, the major product, OHA, forms by aro-
matization and dehydrocyclization. In a similar manner, OHP
forms at low reaction temperatures. In addition, the formation
of by-products DCA and MOHA could be clearly observed by a
direct condensation of CyvOs with NH3 and acid-catalysed
methylation of OHA, as shown in Scheme 1.
The authors extend their appreciation to the Deanship of
Scientific Research at King Khalid University for funding this
study through Large Research Group Project under grant
number R.G.P. 2/60/40.
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