Continuous flow synthesis of amines from the cascade reactions of nitriles and carbonyl-containing compounds promoted by Pt-modified titania catalysts

Article abstract of DOI:10.1039/c8gc03037e

The effective design of an active and stable catalytic system was performed by a simple modification of a commercial titania with a low platinum loading. The prepared material was fully characterized by XRD, XPS, N₂ adsorption-desorption measurements, ICP-MS, TEM and SEM analyses. Such techniques corroborated the successful incorporation of Pt onto the titania surface, without affecting its original structure, morphology and chemical nature. The obtained TiO₂-Pt catalyst was effectively applied in several continuous flow reactions between nitriles and carbonyl containing compounds for amine preparation. Remarkably, conversion of levulinic acid, a biomass derived molecule, was achieved with outstanding conversion (87%) and selectivity (80%) to 1-ethyl-5-methylpyrrolidin-2-one. The catalytic system demonstrated a high stability through 120 min of reaction. Moreover, the effect of the nitrile was investigated by performing the reaction with benzonitrile and ethylcyanoacetate. The TiO₂-Pt catalyst was also tested in the conversion of benzaldehyde, displaying remarkable results. The influence of substitution in the aromatic ring was investigated using p-nitro-benzaldehyde and p-chloro-benzaldehyde.

Full text of DOI:10.1039/c8gc03037e

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Continuous flow synthesis of amines from the
cascade reactions of nitriles and carbonyl-
Cite this: DOI: 10.1039/c8gc03037e
containing compounds promoted by Pt-modified
titania catalysts†
b
Cevher Altuğ,‡a,b Mario J. Muñoz-Batista, ‡b Daily Rodríguez-Padrón,
*
b,c
Alina M. Balu, b Antonio A. Romero b and Rafael Luque
*
The effective design of an active and stable catalytic system was performed by a simple modification of a
commercial titania with a low platinum loading. The prepared material was fully characterized by XRD,
XPS, N2 adsorption–desorption measurements, ICP-MS, TEM and SEM analyses. Such techniques corro-
borated the successful incorporation of Pt onto the titania surface, without affecting its original structure,
morphology and chemical nature. The obtained TiO2–Pt catalyst was effectively applied in several con-
tinuous flow reactions between nitriles and carbonyl containing compounds for amine preparation.
Remarkably, conversion of levulinic acid, a biomass derived molecule, was achieved with outstanding
conversion (87%) and selectivity (80%) to 1-ethyl-5-methylpyrrolidin-2-one. The catalytic system demon-
strated a high stability through 120 min of reaction. Moreover, the effect of the nitrile was investigated by
performing the reaction with benzonitrile and ethylcyanoacetate. The TiO2–Pt catalyst was also tested in
the conversion of benzaldehyde, displaying remarkable results. The influence of substitution in the aro-
matic ring was investigated using p-nitro-benzaldehyde and p-chloro-benzaldehyde.
Received 27th September 2018,
Accepted 29th November 2018
DOI: 10.1039/c8gc03037e
rsc.li/greenchem
ceutical, dye, plastic and agrochemical industries.7,8 A variety
of useful methods have been used for the conversion of carbo-
Introduction
Biomass transformation into useful organic molecules is one nyls to amines in the presence of transition metal catalysts
of the most promising methods to the use of petroleum- and ammonia as a nitrogen-containing molecule, under batch
derived products being one of the most abundant and renew- conditions for years.9 Besides nitrogen gas or ammonia deriva-
able carbon sources worldwide.1–3
tives, nitriles,10 amides,11 nitro derivatives,12 urea,13 sulfonyl-
Biomass valorization towards chemicals, added value com- hydrazines,14 azides15 methyl carbamates16 and isocyanate17
pounds and fuels represent an attractive and challenging functional groups have been investigated as a nitrogen source
option for the chemical industry.4,5 Carbonyl containing mole- towards amine synthesis.
cules play an important role, from simple living organisms to
From an environmental and industrial point of view, in situ
complex metabolites, and can be found in most biomass reduction of nitriles and further reaction with carbonyl com-
derivatives.6 The reduction of carbonyl compounds to amines pounds is a valuable synthetic transformation for amine prepa-
is an indispensable tool available to synthetic organic ration. Most of the nitrile reductions and transformations to
chemists. Amines have a pivotal role in synthetic organic substituted amines need harsh reaction conditions and noble
chemistry, widely present in compounds from the pharma- metal catalysts. In order to overcome difficulties of dealing
with systems which usually need high temperatures, pressures,
and constant external gas, the continuous flow synthetic
methodology, employing heterogeneous catalytic materials,
has become a noteworthy option to obtain organic molecules
through a “greener” pathway, with rather controllable reaction
parameters (temperature, mixing, volume, time, amount of
reagents, solvents and gases).18
Flow chemistry offers attractive advantages including
improved mass and heat transfer, enhanced micro-mixing
(improved reaction rates) as well as potential to scale-up under
aDepartment of Chemistry, Bolu Abant İzzet Baysal University, 14030 Bolu, Turkey
bDepartamento de Química Orgánica, Universidad de Córdoba, Campus de
Rabanales, Edificio Marie Curie (C-3), Ctra Nnal IV-A, Km 396, E14014 Cordoba,
Spain. E-mail: dailydggs@gmail.com, rafael-luque@uco.es
cPeoples Friendship University of Russia (RUDN University), 6 Miklukho-Maklaya
Str., 117198 Moscow, Russia
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8gc03037e
‡These authors contributed equally to this work.
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controllable reaction conditions.19–21 In addition, the utiliz-
N2 physisorption experiments were accomplished in the
ation of heterogeneous catalytic systems gave rise to more Micromeritics ASAP 2000 instrument. The sample was pre-
environmentally friendly catalyzed processes, representing a viously degassed for 24 h under vacuum (p < 10−2 Pa).
key priority to extend the horizons of green chemistry.22 Moreover, TEM images were recorded in the JEOL JEM 1400
Particularly, the cyclization of levulinic acid, which is a instrument, assembled with a charge coupling camera device.
biomass derived carbonyl compound, to pyrrolidones has Samples were previously suspended in ethanol and deposited
been reported in the presence of amines and using various on a copper grid. SEM-EDX micrographs were acquired in the
catalytic systems.23–25 However, there are limited examples of JEOL-SEM JSM-7800 LV scanning microscope.
tandem reductive amination of carbonyl molecules from
Catalytic experiments
readily available nitrile compounds and further cyclization to
pyrrolidone derivatives. These transformations can be achieved
utilizing a range of catalytic materials based on noble metals
such as Ir/SiO2, Ru/Al2O3 or Pd/C systems.18
In particular, titania-based catalysts possess great potential
due to their inherent good properties including low cost, wide
availability, and significant stability.26,27 In this work, a Pt-modi-
fied titania-based catalyst, with a low platinum loading, was pre-
pared employing a simple deposition strategy. The designed
system was successfully utilized in the synthesis of amines
employing several carbonyl containing molecules (such as
levulinic acid and benzaldehyde derivatives) and nitriles.
Catalytic experiments were conducted in the H-Cube Mini
Plus™ flow hydrogenation reactor. The TiO2–Pt catalyst was
packed (ca. 0.1 g of catalyst per cartridge) in 30 mm long
ThalesNano CatCarts. Firstly, the system was washed with (1)
methanol and (2) the corresponding nitrile (0.3 mL min−1
,
20 min for each solvent). 0.3 M solutions of the reactants were
subsequently pumped through, respectively for each reaction,
and the optimum reaction conditions were set (90 °C, 50 bar,
0.3 mL min−1). During the reaction, hydrogen was in situ gen-
erated by water electrolysis in the H-Cube equipment. The con-
tinuous flow process was followed for 120 min (ca. 4.2 min
residence time of the feed in the catalyst), collecting samples
every 10 min for further analysis by GC-MS. The system was
eventually washed with the nitrile employed in the reaction as
the solvent and subsequently methanol in order to recover the
employed catalyst.
Experimental
Synthesis of TiO2–Pt nanocatalysts
A simple protocol was used to deposit metallic Pt on the
surface of commercial TiO2 (P25, Evonik). Pt was introduced
using a chemical deposition method using a H2PtCl6 (Aldrich)
solution. The appropriate amount of H2PtCl6 was added to a pre-
viously prepared suspension of P25 in deionized water
(1 mg mL−1) to reach a theoretical 1 wt% Pt in the final material.
After 30 min under an inert gas atmosphere, the reduction was
carried out using a NaBH4 (Aldrich) aqueous solution (Pt/NaBH4
molar ratio 1/5). The solid was rinsed 2 times with deionized
water, filtered and dried at 80 °C for 12 h.
The catalytic performance, in terms of conversion, selecti-
vity and stability was investigated by gas chromatography (GC)
in an Agilent 6890N gas chromatograph (60 mL min−1 N2
carrier flow, 20 psi column top head pressure) using a flame
ionization detector (FID). The capillary column HP-5 (30 m ×
0.32 mm × 0.25 mm) was employed. In addition, the collected
liquid fractions were analyzed by GC-MS using the Agilent
7820A GC/5977B High Efficiency Source (HES) MSD, in order
to identify the obtained products.
Material characterization
Results and discussion
XRD patterns were recorded in the Bruker D8 Advance diffract-
ometer with the LynxEye detector. The XRD analysis was The successful functionalization of commercial TiO2, as well
carried out in a 2θ scan range from 10° to 80°. Bruker Diffrac- as the influence of the synthetic procedure on the original
plus Eva software, supported by Power Diffraction File data- titania structure were investigated by several techniques,
base, was employed for phase identification.
including XRD, XPS, N2 adsorption–desorption measurements,
XPS measurements were performed in an ultrahigh vacuum TEM and SEM analyses.
multipurpose surface analysis instrument SpecsTM. Prior to
The crystal structure and arrangement of the titania-based
the analysis, the sample was evacuated overnight under catalyst were investigated by XRD analysis. XRD results for the
vacuum (10−6 Torr). XPS spectra acquisition was carried out at TiO2–Pt material exhibited the typical pattern of P25 titania, as
room temperature employing a conventional X-ray source with can be observed in Fig. 1A. XRD patterns of both samples
a Phoibos 150-MCD energy detector. XPS results were analyzed possess a mixture of two phases, namely, anatase and rutile in
employing the XPS CASA software.
an 80/20 ratio, in good agreement with the reported values in
The metal content in the TiO2–Pt catalyst was determined the literature, obtained by the calculation method from Zhang
by ICP-MS in an Elan DRC-e (PerkinElmer SCIEX) spectro- and co-workers.28 Anatase and rutile sizes, determined using
meter. The sample (≈25 mg) was digested by an acid mixture the Williamson–Hall formalism for both samples were found
of HF/HNO3/HCl 1 : 1 : 1. MilliQ water (double distilled) was to be ca. 20 and 30 nm, respectively (Table 1). Importantly, the
−
employed for dilutions up to a maximum of 1% of HF2 in modification process did not influence the original crystal
acid solution.
structure of titania. The absence of platinum signals in the
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Fig. 1 (A) XRD patterns, (B) XPS spectra and (C) N2 adsorption–desorption isotherms of the pure TiO2 and the TiO2–Pt sample. (D) TEM view of the
TiO2–Pt. (E) SEM view of the TiO2–Pt and EDX elemental analysis for (F) Ti, (G) O and (H) Pt.
Table 1 XRD derived parameters, textural properties and elemental compositions of TiO2–Pt and TiO2 samples
XRD
N2 physisorption
ICP-MS EDX
XPS
Pt/Ti
Sample Anatase size (nm) Rutile size (nm) BET surface area (m2 g−1
)
Pore volume (cm3 g−1
)
Pore size (nm) Pt/Ti
Pt/Ti
TiO2
TiO2–Pt 20.6
20.1
29.7
30.9
49
48
0.19
0.15
11.3
11.2
—
0.005
—
—
0.007 0.15
XRD pattern can be understood by the low concentration of
this metal in the modified sample.
The textural properties of the synthesized catalytic systems
were investigated by N2 physisorption analysis. No consider-
In order to confirm the presence of Pt in the obtained cata- able changes were found in the Pt-functionalized samples in
lyst, XPS measurements were performed (Fig. 1B). A typical comparison with the unmodified titania. Remarkably, surface
band at 70.9 eV associated with Pt 4f was found after back- areas, pore sizes and pore volumes remained unaltered within
ground subtraction of the Ti 3s plasmon (Fig. 1B inset).29,30 the experimental error. N2 adsorption–desorption isotherms,
Although the low concentration and high dispersity of plati- which are presented in Fig. 1C, resulted to be type IV, consist-
num nanoparticles on the titania surface, as well as the over- ent with mesoporous structures (pore sizes: ca. 11 nm) and
lapping with the Ti 3s plasmon, made the analysis difficult, it low surface area materials, typical of commercial P25. Some of
was possible to corroborate the presence of Pt and identify its the small Pt nanoparticles may be occluded at the pore struc-
oxidation state, which predominantly corresponds to Pt(0).31 ture of titania, being thus not fully accessible for the catalytic
Colour changes in titania materials also support these results process. In any case, both structural and morphological
(Fig. 1C, inset). In addition, quantification analysis was carried characterization results indicated that it may not be critical to
out, obtaining a superficial Pt/Ti ratio of 0.15. Elemental ana- maintain the accessible active sites of Pt.
lysis was additionally performed by ICP-MS, obtaining a Pt/Ti
Fig. 1D shows the TEM micrograph of the TiO2–Pt catalyst.
ratio of 0.005, corresponding to a ca. 1 wt% Pt in the material. The morphology of the aforementioned sample revealed a
This value was in accordance with the EDX elemental analysis homogeneous nanoparticle distribution with the typical quasi-
as can be seen in Table 1. This marked difference is expected spherical shape of P25 titania.34 Although it was not possible
and is associated with the synthetic protocol. As previously to observe platinum nanoparticles by TEM analysis, due to
reported examples,32,33 the chemical deposition method pro- their small size and low concentration, SEM-mapping results
vides higher superficial metal/support ratios in comparison strongly confirmed the effective and homogeneous platinum
with the bulk counterpart.
modification of the titania surface (Fig. 1E–H).
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The catalytic performance and versatility of the prepared
TiO2–Pt catalyst were evaluated in several hydrogenation reac-
tions, including the conversion of levulinic acid, benz-
aldehyde, p-chloro-benzaldehyde and p-nitro-benzaldehyde to
nitrogen-containing compounds under continuous flow con-
ditions. Blank reactions were performed, using P25 titania and
in the absence of the catalyst, revealing the critical role of an
effective catalytic system for the successful reaction progress in
all the cases.
Fig. 2 presents the detailed information about the continu-
ous flow conversion of levulinic acid, employing the TiO2–Pt
catalytic system. Acetonitrile was employed as both, solvent
and reagent, giving rise to ethylamine, ethanimine and
ammonia by hydrogenation.
The redistribution mechanism of ethylamine to ethanimine
and ammonia in the presence of a Ru complex has been
explained in the literature.35 In particular, for this work,
similar pathways can be proposed, considering the obtained
products.
The performed cascade reaction mainly proceeds through
the condensation of levulinic acid, a 1,4-dicarbonyl com-
pound, with in situ generated ethyl amine and ammonia to
give rise to 1-ethyl-5-methylpyrrolidin-2-one (9) and 5-methyl-
pyrrolidin-2-one (8), respectively (Fig. 3). The latest reaction
product constitutes, to the best of our knowledge, the first lit-
erature example of ammonia (formed from acetonitrile) cap-
tured by levulinic acid.
Fig. 3 Proposed mechanism of the N-containing compound formation.
taking into account their fragmentation patterns in the MS
Additionally, 1,4-bis(ethylamino)pentan-1-ol (10) was spectra. Particularly, products 8 and 9 presented similar
obtained as a product of the reaction by the nucleophilic attack patterns corresponding to their common skeleton.
of ethylamine on both carbonyl and carboxyl carbons in the
The reaction progress was investigated by taking samples
levulinic acid molecule. The reaction products were identified every 10 min, which were analyzed by GC (Fig. 2). The
by GC-MS, revealing the presence of C7H13NO (127.10 g mol−1), employed catalytic system displayed outstanding conversion
C5H9NO (99.07 g mol−1) and C9H22N2O (174.28 g mol−1
)
values of 87% and selectivity to 1-ethyl-5-methylpyrrolidin-2-
(Fig. S1†). The structures of these products were determined one (9) of 80%. Remarkably, the inherent stability of the
catalyst was evidenced by maintaining 94% of the initial
conversion after 2 h of reaction (4.2 min residence time). In
addition, changes in the selectivity values were negligible
during the studied reaction time, corroborating as well the
high catalyst stability.
Hydrogenation of acetonitrile was also carried out employ-
ing the obtained TiO2–Pt catalyst under identical reaction con-
ditions in the absence of levulinic acid. GC analysis revealed a
32% conversion of acetonitrile, with the selective formation of
ethylamine (84%). Similar conversion values of acetonitrile
were obtained when the reaction was performed in the pres-
ence of LA. Such results are comparable to other studies
reported in the literature for these kinds of reactions.36,37
In order to understand the scope and limitations of the
catalyst with various nitrile-containing molecules, benzonitrile
and ethylcyanoacetate were employed instead of acetonitrile
under the same reaction conditions. Interestingly, the
reduction of benzonitrile and ethylcyanoacetate to benzyl amine
and ethyl 3-iminopropanoate (Schiff base) was respectively
observed (Fig. S2†). However, the obtained products did not
Fig. 2 Catalytic performance of TiO2–Pt for the conversion of levulinic
acid to nitrogen-containing compounds. Reaction conditions: 0.3 M
react with levulinic acid, most likely due to the electron with-
drawing properties of the ester group in ethylcyanoacetate,
levulinic acid solution in acetonitrile, 0.1 g of catalyst, T = 90 °C, P = 50
bar, flow = 0.3 mL min−1
.
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which might reduce the nucleophilicity of the nitrogen in order
to further attack the carbonyl group, and to the possible inter-
action of platinum with the phenyl group from benzonitrile,
apart from the formation of a sterically hindered benzyl amine.
Another carbonyl containing molecule, namely benz-
aldehyde, was also studied with the aim to explore the catalytic
activity of the prepared material within other types of carbonyl
functionalized molecules. Additionally, the influence of the
substitution in the aromatic ring has been explored by using
p-nitrobenzaldehyde and p-chlorobenzaldehyde (Scheme 1).
Fig. 4A and B show the details of the continuous flow reac-
tion of benzaldehyde and its derivatives to nitrogen containing
compounds, in terms of conversion and selectivity. TiO2–Pt
exhibited remarkable conversion values ca. 99% for the reac-
tions employing benzaldehyde and p-nitrobenzaldehyde, while
the conversion of p-chlorobenzaldehyde was just 22%.
Conversion of benzaldehyde gave rise to benzyl alcohol and to
the secondary and tertiary amine (Fig. S3†). Similarly, reaction
of p-nitro-benzaldehyde resulted in the formation of p-methyl-
aniline and to the corresponding secondary and tertiary amine
(Fig. S4†). Remarkably, TiO2–Pt resulted to be an effective cata-
lyst for other reducible groups, such as NO2 towards amine for-
mation. Therefore, such material could be a potential candi-
date for the catalytic reduction of NO2 groups. In turn, conver-
sion of p-chlorobenzaldehyde just gave rise to the formation of
p-chlorobenzyl alcohol and the secondary amine, without the
formation of the tertiary amine (Fig. S5†). The low conversion
obtained in the latest reaction could be related most likely to
the interaction of chloride with platinum nanoparticles on the
TiO2 surface, preventing the catalytic effect of the active
centres for the desired reaction. Furthermore, a mesomeric
effect of the chloride group could also be influencing the
electrophilicity of carbonyl carbon in p-chlorobenzaldehyde.
Fig. 4 (A) Catalytic performance of the Pt/P25 nanocatalyst in the con-
version of benzaldehyde, p-chlorobenzaldehyde and p-nitrobenzalde-
hyde to nitrogen-containing compounds. Reaction conditions: 0.3 M
reactant solution in acetonitrile, 0.1 g of catalyst, T = 90 °C, P = 50 bar,
flow = 0.3 mL min−1. (B) Catalytic activity of the Pt/P25 catalyst in the
conversion of benzaldehyde during 90 min of reaction.
The structure of the obtained products was deduced from
the observed fragmentation pattern in the MS spectra
(Fig. S3–5†). The conversion and selectivity values remained
stable after 60 min of reaction, achieving a pseudo-stationary
state. This conclusion can be observed in Fig. 4B, particularly
for benzaldehyde conversion.
Elemental analysis of the prepared catalyst after the reac-
tion was performed by SEM-EDX. Such analysis confirmed the
high stability of the catalytic material. SEM-EDX measure-
ments exhibited negligible variations in comparison with the
obtained results for the fresh catalyst (Fig. S6†). Moreover,
Pt leaching was investigated by ICP-MS, displaying 15 µg L−1
in the 30 min of reaction. Afterwards (measured at 40, 60 and
80 min of reaction), no leaching was observed. In addition to
the stable conversion and selectivity values in the performed
reactions (Fig. 2 and 4), both aforementioned techniques
(EDX and ICP-MS) confirmed the high stability of the Pt/P25
catalyst.
Scheme 1 Continuous flow reaction of (A) benzaldehyde, (B) p-nitro-
benzaldehyde and (C) p-chlorobenzaldehyde, with acetonitrile, to
produce nitrogen containing compounds, in the presence of the TiO2–
Pt catalyst.
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3 L. Filiciotto, A. M. Balu, J. C. Van der Waal and R. Luque,
Catal. Today, 2018, 302, 2–15.
Conclusions
An active and stable catalytic system based on platinum-
modified titania was prepared by a simple deposition method
from commercial P25 titania. Effective incorporation of plati-
num without compromising TiO2 original characteristics was
achieved for a 1 wt% Pt loading material. The prepared TiO2–
Pt catalyst exhibited high conversion (87%) and outstanding
selectivity (>80%) in the conversion of levulinic acid with ethyl-
amine, formed in situ from acetonitrile, towards 1-ethyl-5-
methylpyrrolidin-2-one. The use of different nitriles and carbo-
nyl compounds was further explored in order to gain more
insight into these types of reactions, as well as into the poten-
tial of the TiO2–Pt catalyst. Benzonitrile and ethylcyanoacetate
were tested as nitriles, revealing that the electron withdrawing
features of esters, as well as the interactions between platinum
and phenyl groups, considerably influence the reaction rates.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
We gratefully acknowledge the support of TUBİTAK (The
Scientific and Technological Research Council of Turkey) 2219 19 M. J. Muñoz-Batista, D. Rodríguez-Padrón, A. R. Puente-
Fellowship program for financial support of CA. Mario
J. Munoz-Batista gratefully acknowledges MINECO for a JdC
Santiago, A. Kubacka, R. Luque and M. Fernández-García,
ChemPhotoChem, 2018, 2, 870–977.
contract (Ref. FJCI-2016-29014). Rafael Luque gratefully 20 H. P. L. Gemoets, Y. Su, M. Shang, V. Hessel, R. Luque and
acknowledges support from MINECO under project CTQ2016- T. Noël, Chem. Soc. Rev., 2016, 45, 83–117.
78289-P, co-financed with FEDER funds. Daily Rodriguez- 21 J. M. Bermudez, J. A. Menéndez, A. A. Romero, E. Serrano,
Padrón also acknowledges MINECO for providing a research
contract associated to the CTQ2016-78289-P project. This
J. Garcia-Martinez and R. Luque, Green Chem., 2013, 15,
2786–2792.
publication has been prepared with support from RUDN 22 M. J. Muñoz Batista, D. Rodríguez-Padrón, A. R. Puente
University Program 5-100.
Santiago and R. Luque, ACS Sustainable Chem. Eng., 2018,
6, 9530–9544.
23 Y.-B. Huang, J.-J. Dai, X.-J. Deng, Y.-C. Qu, Q.-X. Guo and
Y. Fu, ChemSusChem, 2011, 4, 1578–1581.
24 X.-L. Du, L. He, S. Zhao, Y.-M. Liu, Y. Cao, H.-Y. He and
K.-N. Fan, Angew. Chem., Int. Ed., 2011, 123, 7961–7965.
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