Highly Efficient and Selective N-Alkylation of Amines with Alcohols Catalyzed by in Situ Rehydrated Titanium Hydroxide

Article abstract of DOI:10.1021/acscatal.9b05525

Catalytic N-alkylation of amines by alcohols to produce desired amines is an important catalytic reaction in industry. Various noble-metal-based homogeneous and heterogeneous catalysts have been reported for this process. The development of cheap non-noble-metal heterogeneous catalysts for the N-alkylation reaction would be highly desirable. Hereby, we propose the N-alkylation of amines by alcohols over a cheap and efficient heterogeneous catalyst-titanium hydroxide. This catalyst provides a selectivity higher than 90% to secondary amines for functionalized aromatic and aliphatic alcohols and amines with high catalytic activity and stability. Mild Br?nsted acidity formed by the continuous rehydration of Lewis acidity excludes the side reactions and deactivation by adsorbed species. The mechanism of the reaction involves dehydration of alcohols to ethers with subsequent C-O bond cleavage by amine with the formation of secondary amine and recovery of alcohol.

Full text of DOI:10.1021/acscatal.9b05525

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Article
Highly efficient and selective N-alkylation of amines with
alcohols catalyzed by in-situ rehydrated titanium hydroxide
Feng Niu, Qiyan Wang, Zhen Yan, Bright Kusema, Andrei Y Khodakov, and Vitaly V. Ordomsky
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b05525 • Publication Date (Web): 11 Feb 2020
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Highly Efficient and Selective N-alkylation of Amines with
Alcohols Catalyzed by in-situ Rehydrated Titanium
Hydroxide
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Feng Niua,b, Qiyan Wanga,b, Zhen Yanb, Bright Kusemab, Andrei Y. Khodakova* and
Vitaly V. Ordomskyb*
a Univ. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181 - UCCS - Unité
de Catalyse et Chimie du Solide, F-59000 Lille, France.
b E2P2L, UMI 3464 CNRS-Solvay, 3966 Jin Du Rd., 201108 Shanghai, China
*Corresponding authors: Andrei Khodakov E-mail:andrei.khodakov@univ-lille.fr;
Vitaly Ordormsky E-mail:vitaly.ordomsky-ext@solvay.com
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Abstract
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Catalytic N-alkylation of amines by alcohols to produce desired amines is an
important catalytic reaction in industry. Various noble metal-based homogeneous and
heterogeneous catalysts have been reported for this process. Development of cheap
non-noble metal heterogeneous catalysts for N-alkylation reaction would be highly
desirable. Hereby, we propose the N-alkylation of amines by alcohols over a cheap
and efficient heterogeneous catalyst - titanium hydroxide. The catalyst provides the
selectivity higher than 90% to secondary amines for functionalized aromatic, aliphatic
alcohols and amines at high catalytic activity and stability. Mild Brönsted acidity
formed by continuous rehydration of Lewis acidity excludes side reactions and
deactivation by adsorbed species. The mechanism of the reaction involves
dehydration of alcohols to ethers with subsequent C-O bond cleavage by amine with
formation of secondary amine and recovery of alcohol.
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Keywords: N-alkylation; Amines; Alcohols; Heterogeneous; Titanium hydroxide;
Alkoxides
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Introduction
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Complex amines are very important intermediates in chemical industry and life
science. They are widely used in the production of agrochemicals, pharmaceuticals,
organic dyes, detergents, fabric softeners, surfactants, corrosion inhibitors, lubricants,
polymers and so on [1,2,3]. Due to the widespread use of these products, a number of
synthesis routes have been developed based on classic nucleophilic substitution, like
Buchwald-Hartwig [4], Ullmann reactions [5], and hydroamination [6,7,8].
Conventionally, N-alkyl amines are synthesized using alkylating agents, such as alkyl
halides, which is environmentally unfriendly due to the toxic nature of halides and
production of large amount of inorganic salts as waste. The selectivity to desired
amines is generally low due to further over-alkylation [9,10,11].
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In the last few decades, three main strategies have been applied for amination of
alcohols: (i) “Borrowing Hydrogen” methodology using either noble metal based Ru-
[12], Pd- [13], Ir- [14], Rh- [15], and Pt- [16] complexes, and more recently
non-noble metal Mn- [17,18], Co- [19,20,21], Ni- [22,23,24,25] and Fe- [26]
complexes. Also various of heterogeneous catalysts [27,28,29,30,31,32,33,34] have
been used. (ii) Tsuji-Trost type reactions for allylic alcohols using Pd [35] or Ni [36]
complexes. (iii) Lewis-acid catalyzed reactions using a variety of salts and ligands
[37,38]. Among the best performing catalysts, different metal triflates and triflimides,
including the parent triflic acid (HOTf) were found active for synthesis of secondary
amines through N-alkylation of amines with alcohols based on SN1 or SN2
mechanisms [39]. However, these processes have several disadvantages due to use of
expensive non-recoverable catalysts, difficulties in catalyst-product separation and
indispensable use of additives or co-catalysts such as inorganic or organic bases and
stabilizing ligands [40]. Thus, development of easily recoverable and recyclable
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non-noble metal based heterogeneous catalysts for N-alkylation system is still a
challenging topic.
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Herein, we firstly report the selective N-alkylation of amines by alcohols in the
absence of hydrogen, bases and organic ligands under ambient pressure of N2 by an
easily prepared transition metal- and solid acid-based heterogeneous catalyst such as
titanium hydroxide. For comparison, a number of metal oxides and hydroxides have
been tested in the N-alkylation process. Interestingly, titanium hydroxide exhibits the
highest activity and selectivity towards the desired amine. In addition, the titanium
hydroxide catalyst provides high catalyst stability and good substrate tolerance for
amination of different alcohols and substituted amines under optimized conditions. To
the best of our knowledge, this new efficient heterogeneous catalyst provides a green
route for selective synthesis of amines.
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In terms of catalytic mechanism, it is generally accepted that substitution of
OH-groups with N nucleophiles proceeds via a SN1 or SN2 type mechanism, in the
presence of hard Lewis or Brönsted acids [41]. In this work, we show that
N-alkylation process of alcohols involves intermediate formation of ethers, which
could be a kinetic relevant step of the whole reaction. The high yield of secondary
amine can be attributed to the easier C-O bond cleavage in the ether by amines based
on effect of mild Brönsted acidity formed by adsorbed water over Lewis acid sites.
Experimental Section
Catalyst preparation
Titanium hydroxide was prepared through the hydrolysis of titanium isopropoxide
(99%, supplied by J&K) under neutral condition. 200 ml of distilled water was heated
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up to 60 C and 10 mL titanium isopropoxide was added dropwise using a peristaltic
pump (0.05 mL/min) under continuous stirring (500 rpm) for almost 10 h. Then the
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obtained white suspension was centrifuged and washed with ethanol and distilled
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water for several times followed by drying in vacuum at 80 C overnight. The
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obtained sample was denoted as TiOH. The hydrolysis of isopropoxide in hot water is
an easy and efficient method to produce amorphous titanium hydroxide with high
purity (no NH3 or Cl-) and surface area. This sample has been further calcined at 180
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oC and 400 C for 2 h to produce dehydrated samples denoted as TiOH-180 and
TiOH-400, respectively.
The hydroxides of Al (Al-OH) and Zr (Zr-OH) have been prepared in the similar
way using alkoxides of aluminium t-butoxide, aluminium isopropoxide, zirconium
butoxide and zirconium ethoxide. For comparison, different metal oxides and
commercial zeolite ZSM-5 were also used for this reaction as references. The
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hydrolyzed AlOH sample was calcined under 180 C, which was denoted as
AlOH-180.
Catalytic test
Typical procedure for N-alkylation of amines with alcohols was carried out in a
30 mL stainless steel autoclave geared with a pressure gauge and a safety rupture disk.
In a typical experiment, the reactor was charged with 4.62 mmol benzyl alcohol, 21.5
mmol aniline and 100 mg of catalyst. 2 g xylene, THF or water was used as a solvent.
Then, the reactor was purged with N2 several times to remove the air. Finally, the
reactor was placed on a hot plate equipped with a magnetic stirrer (500 rpm) at 50
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oC-180 C for 0.5-48 h. After the reaction, the reactor was cooled down to room
temperature, and the mixture was filtrated and analyzed using an Agilent 7890 GC
equipped with a HP-5 capillary column using biphenyl as the internal standard for
calculation of alcohol conversion and product yields. After each amination test, the
catalyst was washed with ethanol and separated by centrifugation several times and
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dried at 80 C for 10 h for further test of the recyclability. The used catalyst after
reaction was denoted as TiOH-80-AR. For the rehydration of the dehydrated catalysts
TiOH-180, water was added into the reactor before catalytic test. For characterization,
the catalyst was suspended in distilled water overnight followed by drying under
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vacuum at 80 C before test. The corresponding rehydrated samples were denoted as
TiOH-180-Re, TiOH-400-Re, TiO2-Re and AlOH-180-Re.
Characterization methods
The powder X-ray diffraction (XRD) patterns were obtained using an X-ray
diffractometer (D5000, Siemens) using Cu Kα radiation (λ = 0.15418 nm). Scans
were recorded in the 2θ range of 10-80o using a step size of 0.02o and a step time of 5
s.
The BET specific surface areas and pore size were measured from the N2
adsorption-desorption isotherms at 77 K on an automated gas sorption analyzer
Micromeritics ASAP 2010.
Thermogravimetric analysis (TGA) was carried out in air flow at the temperature
range of 20-900 C with a heating rate of 5 C·min-1 on a Mettler Toledo
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SMP/PF7458/MET/600W instrument.
The FTIR (Perkin-Elmer 1720) spectra of the titanium hydroxide catalysts were
measured in the range from 4000 to 1200 cm-1 to trace the variation of surface
hydroxyl group of the catalysts. To investigate the nature of acid sites in the catalyst,
the pyridine adsorption FTIR technique was employed and spectra were recorded in
the range of 1700-1400 cm-1. The self-supported wafers of all the catalysts were
prepared by a pellet press instrument and the samples were degassed at 100 °C for 30
minutes followed by saturation with pyridine as reported elsewhere [42]. To explain
the mechanism of alcohol dehydration over titanium hydroxide, in-situ FTIR was
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performed through heating the pellet from 25 to 180 oC (step 25 oC) in benzyl alcohol
vapors (20 Torr).
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The NH3 temperature programmed desorption (NH3-TPD) was carried out on a
Micromeritics AutoChem 2920 with a thermal conductivity detector (TCD). In all the
experiments, ~50 mg of sample was calcined at 100 oC for 1 h in helium gas flow (20
mL (STP)/min) and then cooled down to 40 °C followed by saturation with 10%
ammonia-helium for 1 h. Then the sample was flushed with helium for 1 h to remove
the surface physisorbed ammonia. Finally, the desorption profiles were recorded in
the temperature range of 40 oC-800 oC with a heating rate of 5 °C min-1.
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Results and Discussion
Catalysis over oxides and hydroxides
The additive-free N-alkylation reaction of benzyl alcohol (1) with aniline (2) to
give corresponding secondary amine N-phenylbenzylamine (3) has been used as a
model reaction over different non-noble metal hydroxide, oxide and zeolite catalysts
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(Table 1). The product has been identified by GC-MS, H-NMR and 13C-NMR
analysis (Figure S1, S2, S3, SI). The blank test without catalyst did not show
formation of desired amine (entry 1). The titanium hydroxide obtained by hydrolysis
of titanium isopropoxide showed the highest catalytic activity and gave the desired
secondary amine N-phenylbenzylamine (3) with the 99.6% yield (entry 2) without
formation of secondary imine. Only traces of dibenzyl ether have been detected as
byproduct. The product yield was slightly lower for the reaction performed without
solvent due to easier formation of tertiary amine through condensation reaction (entry
3). It is interesting to note that the reaction can be performed in aqueous phase (entry
5), however, organic solvents provide the highest yield of the product (entry 4). The
reaction in air atmosphere provides a bit lower yield than that under N2 mainly due to
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the partial oxidation of benzyl alcohol to benzoic acid (entry 6). Hydroxides of Al and
Zr prepared by hydrolysis of different types of alkoxides could afford the maximum
yield of N-phenylbenzylamine of only 16.9% (entry 7-10). Additionally, different
metal oxides were also tested for N-alkylation reaction. The yield was lower than 2%
for TiO2, CeO2, ZrO2, MgO, SiO2 and Al2O3 (entry 11-16). The alkylation yield for
commercial zeolite ZSM-5 was only 25% (entry 17).
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Titanium hydroxide (TiOH-80) was the most active catalyst. The reaction
temperature dependence of the secondary amine yield in the N-alkylation of benzyl
alcohol with aniline on this catalyst was further investigated (Figure S4, SI). Below
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100 C, there was nearly no activity in this reaction. The conversion gradually
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increases as the temperature increases from 100 to 180 C reaching full conversion.
The apparent activation energy of the reaction corresponds to 66.4 kJ/mol (Figure 1a).
The time course of the secondary amine synthesis over titanium hydroxide (TiOH-80)
at 180 oC was investigated (Figure 2). The reaction proceeded smoothly to afford the
corresponding secondary amine with 99% yield in 15 h. Dibenzyl ether was observed
at the initial stage and its yield slowly decreased with time, suggesting the dibenzyl
ether could be an intermediate. The reaction seems to proceed in two consecutive
steps: 1) transformation of benzyl alcohol to dibenzyl ether and 2) secondary amine
formation from dibenzyl ether with aniline.
Characterization
To identify, whether the high performance of secondary amine synthesis comes
from the suitable acidic sites formed from surface hydroxyl group of the catalyst,
titanium hydroxide obtained by hydrolysis was calcined at different temperatures. The
TGA analysis (Figure 3a) shows that titanium hydroxide exhibited weight loss in two
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main steps. The initial weight loss below 250 C can be assigned to desorption of
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surface water. The weight loss between 250 C and 450 C could be due to the
removal of surface hydroxyl groups through dehydroxylation process [43]. No
obvious weight loss was detected for TiOH-400 sample. This indicates that high
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crystalline TiO2 phase was formed after calcination at 400 C. The catalyst
TiOH-80-AR after reaction showed similar weight loss compared to the fresh catalyst.
This indicates similar content of hydroxyl groups and rehydration of the catalyst
during reaction. The state of the material during calcination at different temperatures
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has been studied by XRD. The parent sample and sample after calcination at 180 C
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are amorphous, the presence of anatase was detected after calcination at 400 C [44].
XRD showed that the catalyst was still amorphous after reaction (Figure 3b).
FTIR has provided further information about the concentration and acidity of
surface hydroxyl groups in titanium hydroxide. As shown in Figure 4a, the titanium
hydroxide without calcination exhibited a broad peak at 3600 cm-1 and a peak at 1626
cm-1 due to the stretching vibration of hydroxyl groups interacting with adsorbed
water and bending vibration of hydroxyl groups of adsorbed water, respectively [45].
As the calcination temperature increased, the intensity of the peak at 1626 cm-1
decreased and finally disappeared at 400 oC with significant narrowing of the peak at
3600 cm-1. It indicates desorption of water and significant decrease in the amount of
hydroxyl groups during dehydration of hydroxide and its transformation to anatase
phase.
To identify the acidic properties of the surface hydroxyl groups of the catalyst,
FTIR spectra of pyridine adsorbed on titanium hydroxide calcined at different
temperatures were recorded (Figure 4b). The interaction of pyridine with Lewis acid
sites gives rise to the specific bending vibration bands of pyridine ring at 1448 cm-1,
1575 cm-1 and 1605 cm-1 [42]. The peak located at 1540 cm-1 can be assigned to the
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interaction of pyridine with Brönsted acid sites. It can be observed that an increase in
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the calcination temperature of titanium hydroxide to 180 C leads to decrease in the
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intensity of all the peaks related to Lewis acid sites and nearly disappearance of
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Brönsted acid sites [43]. Further increase in the calcination temperature to 400 C
results in disappearance of all adsorbed pyridine species over the catalyst, which
means nearly no acidic sites left on the catalyst. It is interesting to note that the
process of dehydration is reversible and treatment of TiOH-180 in water leads to
recovery of adsorbed pyridine species corresponding to Brönsted and Lewis acid sites
(Figure 4b). However, rehydration process did not lead to acidity generation over the
crystalline sample TiOH-400, which could be explained by high stability of anatase
structure. Thus, defected labile nature of titanium hydroxide provides both Brönsted
and Lewis acid sites, which disappear during dehydration procedure due to the
process of healing of defected sites (Figure 5). Indeed, the formation of Brönsted and
Lewis acid sites can be proposed as reaction of protonation of hydroxyl groups with
subsequent dehydration. The easy rehydration process provide constant presence of
both types of acid sites in the catalyst during reaction (Figure 5).
To further identify the amount and strength of the acid sites, temperature
programmed desorption of ammonia (NH3-TPD) has been measured as shown in
Figure 6. The highest amount of acid sites is observed over pristine titanium
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hydroxide TiOH-80 with the broad desorption peak at 168 C and narrow intensive
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peak at 312 C corresponding to weak and intermediate acid sites, respectively [46].
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Calcination of titanium hydroxide at 180 C leads to nearly disappearance of
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intermediate acidity with the weak acidity peak shift to lower temperatures (100 C).
Combined with the results of Py-FTIR, it indicates that high temperature peak
(intermediate acidity) can be assigned to desorption of pyridine from Brönsted acidic
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sites and low temperature peak (weak acidity) to desorption of pyridine from Lewis
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acidic sites. The sample further calcined at 400 C did not show any obvious NH3
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desorption.
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The NH3-TPD results for titanium hydroxide were compared with those obtained
for aluminum hydroxide and zeolites. Aluminum hydroxide also shows the presence
of weak and intermediate acid sites. The broad desorption peaks at 220 oC and 420 oC
for ZSM-5 can be respectively assigned to intermediate and strong acidities. The
amount of acid sites for all the samples calculated through integration and calibration
is listed in Table S1, SI.
The samples pre-treated at different temperatures have been additionally tested in
the N-alkylation reaction. The initial reaction rate for different samples is shown in
Figure 7. The initial reaction rate for the sample TiOH-80 (Figure 1b, Figure 7) was
12.8 mmol/g·h, which was nearly twice higher than that of dehydrated sample
TiOH-180 (6.84 mmol/g·h). Calcination at 400 oC almost totally deactivates the TiOH
catalyst in N-alkylation reaction. It means that anatase structure of TiO2 without any
significant amount of acid sites is not active in N-alkylation of benzyl alcohol with
aniline. The catalytic test over commercial TiO2 catalyst further confirms this
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conclusion (Table 1). It is interesting to note that the sample calcined at 180 C with
low content of acid sites (Table S1, SI) still provides significant activity in amination
reaction. This result can be explained by rehydration of amorphous titanium oxide by
water, which produces new acid sites for the reaction. Indeed, the structure of
amorphous oxide is very labile and interaction of Lewis acid sites with water formed
during reaction should result in regeneration of Brönsted acidity
Crystalline anatase TiO2 cannot be rehydrated to form acidity during reaction.
This suggestion has been further proven by addition of extra water before amination
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test with the dehydrated catalysts (Figure 7). The reaction rate (12.55 mmol/g·h) for
amorphous titanium hydroxide (TiOH-180) was almost recovered to the initial state
after addition of water. We could propose reversible process of dehydration and
rehydration for titanium hydroxide during reaction, which provides permanent acidity
for amination reaction [47,48] (Figure 5).
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To investigate the structure-performance correlation, BET analysis of all catalysts
has been performed. There is no obvious relationship between reaction activity and
surface area or pore structure of the catalysts (Table S2, SI). Indeed, high surface area
of hydroxides AlOH and ZrOH or oxides Al2O3 and SiO2 does not provide high
activity of these catalysts in amination.
Analysis of the structure-catalytic performance relationship shows good
correlation between intermediate acidity measured by NH3 desorption in the range
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200-400 C and reaction rate in amination over different catalysts (Figure S5, SI). It
is obvious that the reaction rate increases with increase in the intermediate acidity.
However, high content of intermediate acid sites over Al-OH does not lead to high
activity in amination. The possible explanation of this effect could be low rehydration
o
of Al-OH at the reaction conditions. Dehydration of Al-OH at 180 C leads to
significantly lower activity in comparison with the parent catalyst Al-OH (Figure S6,
SI). Rehydration of AlOH-180 does not show significant increase of the activity in
comparison with the rehydration of TiOH-180. This fact could be explained by less
labile structure of Al hydroxide in terms of reversible hydration-dehydration process.
The activity recovery for the rehydrated sample TiOH-180 was mainly attributed to
the continuous regeneration of intermediate Brönsted and Lewis acidic sites, (Figure
5). The lower activity for ZSM-5 was partially because of quasi-irreversible
adsorption of aniline on the zeolite strong acid sites under the reaction conditions.
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Consequently, we could assume that weak and strong acidities would not play a
determining role in amination reaction. Titanium hydroxide with optimum
intermediate acidity and able to be rehydrated shows an exceptional catalytic
performance for the N-alkylation reactions.
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Mechanism
Among various strategies for amines synthesis, amination of alcohols using a
borrowing hydrogen methodology (or hydrogen auto-transfer process) has been
regarded as a highly atom-efficiency choice [49,50,51]. In this approach (Figure 8 (a))
alcohol was first dehydrogenated to the corresponding aldehyde with assistance of
noble metals like Ru, Pt, Pd, Ir [52,53,54,55]. Then the aldehyde was nucleophilically
attacked by an amine to form secondary imine with the eliminating of water as the
only byproduct. Finally, imine was hydrogenated to the desired secondary amine.
During the whole process, additional hydrogen is usually added for inhibition of the
catalyst deactivation and fast hydrogenation step to suppress secondary processes.
In our case, the non-metallic acidic heterogeneous catalyst such as titanium
hydroxide without dehydrogenation/ hydrogenation functions was found to be very
efficient for the direct N-alkylation of benzyl alcohol with aniline to give desired
secondary amine. A possible reaction mechanism in this case can be based on SN2
mechanism of direct nucleophilic attack of α-carbon atom by N of aniline [56,57].The
effect of the ratio of aniline to benzyl alcohol from 0 to 10 on the reaction selectivity
is displayed in Figure 9a. It is interesting and obvious to note the presence of
dibenzyl ether, which might be produced by dehydration of benzyl alcohol. The
intermolecular dehydration of benzyl alcohol catalyzed by acid sites of metal oxides
like alumina has been observed earlier [58,59,60]. Surprisingly, in the presence of
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larger amounts of aniline (amine to alcohol ratio >1), the dehydration of benzyl
alcohol is completely suppressed. There are two possible explanations of this effect:
1) acid sites of titanium hydroxide are responsible for dehydration of alcohol and
amination of alcohol. The presence of excess of aniline suppresses the first
reaction;
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2) ether is the primary product of the reaction. The ammonolysis of the ether by
aniline proceeds much faster than that of the relevant alcohol.
To provide experimental evidence for the second hypothesis, the catalytic
reaction of dibenzyl ether with aniline over titanium hydroxide was investigated
(Figure 9 (b)). No product was formed without catalyst. TiOH-80 clearly catalyzed
the ammonolysis reaction of dibenzyl ether with high initial reaction rate (80
mmol/g·h), which was about 6 times higher than that of the whole N-alkylation
reaction (12.8 mmol/g·h) (Figure 1b, Figure 6). It demonstrates that the
ammonolysis of dibenzyl ether through C-O bond cleavage [61,62,63,64] to the
desired secondary amine proceeds much faster than the relevant reaction of benzyl
alcohol. This indicates that dehydration of alcohol could be the limiting step of the
whole N-alkylation reaction (Figure 8b). Water was the only byproduct during the
whole reaction process. The product yield for the ammonolysis reaction dramatically
decreased from 86 % to nearly 0 as the pretreatment temperature increased from 180
o
to 400 C (inset of Figure 9b). It indicates that the ammonolysis of dibenzyl ether
requires acid sites as well, which is consistent with the previous report [65]. However,
the reaction is less sensitive to amount of acid sites in comparison with dehydration of
alcohol to ether.
To investigate the alcohol dehydration process on the acid sites of titanium
hydroxide, in-situ FTIR experiments were performed (Figure 10, Figure S7, SI).
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Adsorption of benzyl alcohol leads to appearance of the bands at 1230, 1380, 1360,
1447, 1470 and 1494 cm-1. The bands at 1447 and 1494 cm-1 can be assigned to the
in-plane C-O-H bending modes [66]. The bands at 1230 and 1470 cm−1 are associated
usually with methylene group [67]. The bands at 1360 and 1380 cm-1 can be assigned
to aromatic skeletal vibrations [68].
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As the temperature increased from room temperature to N-alkylation temperature,
the additional bands appears at 1600 cm-1 and 1575 cm-1 (Figure 10, Figure S7, SI)
These bands can be assigned to bending vibrations of strongly adsorbed water over
Lewis acid sites and C=C stretching vibrations in benzyl ether [69]. Additionally the
band at 3510 cm-1 of OH stretching vibration increases at higher temperature (Figure
S7, SI). This indicates dehydration of alcohol with formation of water and ether.
The FTIR spectrum of the catalyst after reaction and intensive washing in
xylene shows the peaks at 1380 and 1360 cm-1 similar to FTIR spectra after benzyl
alcohol adsorption (Figure 11). In order to verify if these species could be
intermediates during catalytic reaction, the catalyst after reaction has been treated at
the reaction conditions in pure xylene. The liquid phase after reaction contained 0.33
mmol (3.3 mmol ether/gp) of dibenzyl ether which was the only product of the
reaction (Figure S8, SI). The amount of the product corresponds well to the amount
of acidic active sites determined by TPD of NH3 (Table S1, SI). FTIR analysis shows
that surface species have disappeared after reaction (Figure 11). In order to identify
the localization of these species, Py has been adsorbed over the catalyst before and
after treatment in solvent (Figure 11). It is interesting to note that the peak attributed
to adsorption of Py over Lewis acid sites decreased significantly after reaction but has
been totally recovered after treatment in xylene. The possible explanation of this
effect could be formation of surface alkoxide species by interaction of alcohol with
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Lewis acid sites (Figure 11). The presence of intermediate surface alkoxides have
been observed earlier over alumina [70]. FTIR analysis of low wavenumber region
(Figure 11) of the catalyst after reaction (TiOH-80-AR) shows intensive bands at
1126, 1070 and 1041 cm-1, which has been earlier assigned to vibration modes of
Ti-O-C species in alkoxides [71].
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The kinetic analysis of the reaction of benzyl alcohol dehydration to benzyl ether
shows that it corresponds to the second order similar to the whole N-alkylation
reaction (Figure S9, SI).
Based on the catalytic and characterization results discussed above, a possible
reaction mechanism of ether formation on acidic titanium hydroxide as key
intermediate for further ammonolysis to N-alkylation product was proposed in Figure
12. The alcohol reacts with Lewis acid sites over the TiOH catalyst with formation of
surface alkoxides. Protonation of oxygen of alkoxide species by hydroxyl group with
SN2 attack of α-carbon by oxygen of another alkoxide results in formation of ether
and partial hydration of the catalyst. Subsequent dehydration leads to regeneration of
the active sites. Thus, synergetic effect of mild Brönsted and Lewis acid sites over
titanium hydroxide for activation of alcohol able to transform it to ether by
dehydration is necessary. These properties make titanium hydroxide unique for this
reaction.
Adsorption of pyridine over other catalysts confirms this assumption. Oxides
containing mainly Lewis acid sites and zeolite with Brönsted acidity provide low
activity in N-alkylation reaction (Table S3, SI). Presence of both Brönsted and Lewis
acid sites can be observed also over other hydroxides like Al-OH, however, catalyst
cannot be rehydrated in-situ during reaction loosing activity in time (Figure S6, SI).
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Thus, amount, nature and regenerability of acid sites are the key factors affecting
activity of the catalyst in N-alkylation reaction.
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To investigate the stability and reusability of titanium hydroxide, the used
catalyst was washed with ethanol and separated from the liquid phase by
centrifugation for several cycles and dried at 80 oC for 10 h. As shown in Figure 13,
the titanium hydroxide could be used for at least three catalytic cycles without
obvious change in reaction rate and amine yield, indicating good stability and
recyclability for titanium hydroxide.
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Substrate scope
The scope of the present titanium hydroxide system with regard to alkylation of
various amines and alcohols was also examined. Table 2 shows that the catalyst can
be used for selective conversion with aniline of structurally diverse aromatic and
biomass based alcohols to the secondary amines (entry 1-5). The o-methyl benzyl
alcohol (entry 4) was less reactive, which could be due to the steric hindrance effect
of the methyl group at the o-position [34]. Aniline derivatives bearing an
electron-donating or electron-withdrawing group also reacted with benzyl alcohol to
afford the corresponding secondary amines with the yields higher than 90 % (entry
6-9).
Aliphatic amines (butylamine and dibutylamine) can be also used for the
synthesis of secondary and tertiary amines by reaction with benzyl alcohol,
respectively (entry 10 and 11). However, the yield of the product in this case is lower
than in the case of aromatic amines (<50 %) with significant contribution of ethers in
the products due to the lower reactivity of aliphatic amine. The reaction conditions
have been modified: the time of the test, amount of the catalyst were increased, while
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the loading of reagents were decreased (entry 12 and 13). Higher yield of the products
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comparable to the aromatic amines (≈ 90%) was obtained.
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Titanium hydroxide provides high selectivity at reasonable conversions for the
reaction of octanol, tetrahydrofurfuryl alcohol and isopropanol with aniline (entry
14-16). There is still a significant contribution of ether but the yield of the target
product can be easily improved by increase of the contact time as it has been
demonstrated earlier (entry 12 and 13).
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Titanium hydroxide also provides reasonable yield of secondary aliphatic amine
through N-alkylation of aliphatic alcohols (primary and secondary) with aliphatic
amines (entry 17-20). The main difference in comparison with aromatic alcohols and
amines is higher contribution of ether in the case of aliphatic amines and alcohols
(Table 2). Note that variation of the product yields with time of aliphatic amine
remains consistent with the hypothesis about the role of ether as an intermediate of
alcohol ammonolysis. Indeed, N-alkylation of isopropanol with aniline demonstrate
continuous increase of the yield of the product and a volcano curve for the
di-isopropyl ether as a function of time (Figure S10, SI). Thus, unique structure of
titanium hydroxide provides efficient amination of aromatic and aliphatic alcohols by
aromatic and aliphatic amines toward secondary and tertiary amines.
Conclusions
In summary, we have demonstrate that a non-noble metal solid titanium hydroxide
acts as a stable, reusable and efficient heterogeneous catalyst for N-alkylation of
amines with alcohols to secondary amines without hydrogen, bases or organic ligands
promotion. The conversion and selectivity of titanium hydroxide were much higher
than other hydroxides, metal oxides and zeolite due to the availability of mild
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intermediate Brönsted and Lewis acidities formed by rehydration-dehydration
processes. The high yield for direct secondary amine synthesis can be attributed to the
acidic catalytic intermolecular dehydration of alcohol to form corresponding ether
followed by easier ether ammonolysis.
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Supporting Information
1
13
GC-MS, H NMR, C NMR, FTIR, reaction activity, catalytic reaction rate, kinetic
analysis, ether formation, acidity of the catalysts and BET analysis.
This information is available free of charge on the ACS Publications website.
Data availability
All data are available from the authors upon reasonable request.
Acknowledgements
The authors thank Solvay and University of Lille for financial support of this work.
Chevreul Institute (FR 2638), Ministère de l’Enseignement Supérieur, de la
Recherche et de l’Innovation, Hauts-de-France Region and FEDER are acknowledged
for supporting and funding partially this work.
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Table 1. Amine synthesis from benzyl alcohol and aniline over various non-noble
metal oxides and hydroxides (benzyl alcohol (4.62 mmol), aniline (21.5 mmol),
xylene 2 g, catalyst 0.1 g, reaction temperature 180 ◦C, 15 h, atmospheric N2 )
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NH2
180 oC, 15 h, cat
N
OH
H2O
H
xylene (2 g)
3
1
2
Entry
Catalyst
Yield of 3 (%)
Entry
Catalyst
Yield of 3 (%)
Hydroxides
—
Oxides
TiO2
1
2
0
11
12
13
14
15
16
1.4
<1
<1
<1
0
Ti-OH
99.6
92.4
99.2
97
MgO
3
aTi-OH
bTi-OH
cTi-OH
dTi-OH
eAl-OH
fAl-OH
gZr-OH
hZr-OH
SiO2
4
Al2O3
CeO2
5
6
89.8
16.9
10
ZrO2
<1
7
Zeolites
ZSM-5
8
17
25
9
8.1
10
12
aNo solvent used; under solvent THF; under water; under air; hydrolysis from aluminium
b
c
d
e
t-butoxide; hydrolysis from aluminium isopropoxide; ghydrolysis from zirconium butoxide;
f
hhydrolysis from zirconium ethoxide.
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Figure 1 The Arrhenius plot of secondary amine yield for N-alkylation from benzyl alcohol and
aniline over titanium hydroxide TiOH-80 (a) and time dependence for N-alkylation with catalysts
before and after dehydration (b)
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Figure 2 Time dependence on amine yield for N-alkylation from benzyl alcohol and aniline over
titanium hydroxide. (Benzyl alcohol 4.62 mmol, aniline 21.5 mmol, xylene 2 g, catalyst 100 mg,
180 oC, atmospheric N2.)
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Figure 3 Characterization of titanium hydroxide catalysts using TG analysis (a) and XRD (b)
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