Rhodium exchanged ETS-10 and ETS-4: Efficient heterogeneous catalyst for hydroaminomethylation

Article abstract of DOI:10.1016/j.apcata.2014.01.002

Rhodium exchanged titanosilicates (ETS-10 and ETS-4) were synthesized and found to be efficient and stable catalysts for hydroaminomethylation reaction. The catalyst was highly active and selective towards amine product within lower reaction time of 4 h. 1-Hexene and pyrrolidine were taken as representative substrates for parametric variations using Rh-ETS-10. The increase in pyrrolidine ratio gave some significant information showing a decrease in conversion with increased selectivity. The reason may be the competition between pyrrolidine and 1-hexene for coordination sites in rhodium and which prevents the beta-hydride elimination. The ratios of H₂ and CO have influenced hydroaminomethylation and the best performance was in the CO:H₂ ratio of 1:4 under the studied conditions. The initial rate of formation of amine was double than that of isomerization of 1-hexene. The Rh cluster like complex was formed on treating the catalyst with syn gas. A homogenous- heterogeneous dual nature of Rh was found during hydroaminomethylation. The catalyst was effectively recycled for three times without much loss in its activity and selectivity.

Full text of DOI:10.1016/j.apcata.2014.01.002

Applied Catalysis A: General 473 (2014) 116–124
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Rhodium exchanged ETS-10 and ETS-4: Efficient heterogeneous
catalyst for hydroaminomethylation
N. Sudheesh, Ram S. Shukla∗
Discipline of Inorganic Materials and Catalysis, Council of Scientific and Industrial Research (CSIR), Central Salt and Marine Chemicals Research Institute, G.
B. Marg, Bhavnagar, Gujarat 364 002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 June 2013
Received in revised form 3 January 2014
Accepted 5 January 2014
Available online 13 January 2014
Rhodium exchanged titanosilicates (ETS-10 and ETS-4) were synthesized and found to be efficient and
stable catalysts for hydroaminomethylation reaction. The catalyst was highly active and selective towards
amine product within lower reaction time of 4 h. 1-Hexene and pyrrolidine were taken as representative
substrates for parametric variations using Rh-ETS-10. The increase in pyrrolidine ratio gave some signif-
icant information showing a decrease in conversion with increased selectivity. The reason may be the
competition between pyrrolidine and 1-hexene for coordination sites in rhodium and which prevents
the beta-hydride elimination. The ratios of H2 and CO have influenced hydroaminomethylation and the
best performance was in the CO:H2 ratio of 1:4 under the studied conditions. The initial rate of formation
of amine was double than that of isomerization of 1-hexene. The Rh cluster like complex was formed on
treating the catalyst with syn gas. A homogenous–heterogeneous dual nature of Rh was found during
hydroaminomethylation. The catalyst was effectively recycled for three times without much loss in its
activity and selectivity.
Keywords:
Hydroaminomethylation
Titanosilicate
Heterogeneous catalyst
Rhodium
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
solvents (polar and non-polar) is better choice for fair hydroformy-
lation especially for n/iso ratio and hydrogenation of enamine or
Amines are industrially important compounds and for that
research are focused on the development of various routes capable
enough to have efficient and atom economic synthesis of amines.
Hydroaminomethylation is highly atom economic and straight
forward route for the synthesis of amines [1–10]. The reaction
proceeds via hydroformylation followed by reductive amination
(Scheme 1). The products obtained are n- and iso-enamines as
intermediates (1 and 3) and it gets transformed to n- and iso-
amines (2 and 4). If a primary amine is used as substrate then the
intermediates are n- and/or iso-imines.
Various striking results are available in literature by using vari-
ety of metals, ligands and complexes to obtain simple to complex
amines [11–17]. These amines find various applications in bulk as
well as in fine chemicals. Rhodium complexes are the main attrac-
tive catalysts but the complexes of Ru, Pt, Ir etc. are also gaining
interest [14,18]. From these studies there have been notable conclu-
sions like the effect of temperature, ligand, complex and solvents.
The main conclusions are: temperature should be high enough
to hydrogenate the enamine or imine intermediate, using mixed
imine intermediate, and the use of cationic precursor or addition of
acetic or tetrafluoroboric acid enhanced the yield of amine [1]. In
absence of an acidic species hydroaminomethylation runs slowly
and may take long time of about 24–72 h [19,20]. It is well known
that the presence of acidic species can catalyze the formation of
enamine or imine from an aldehyde and amine. It has been also
found that acid makes the Rh species cationic, which is active for
hydrogenation of imine and/or enamine [21–23]. However, use
of organic acids in hydroformylation has possibility to generate
Rh–carboxylate complexes which are inactive for hydroformyla-
tion [24]. Hence the regeneration of this deactivated catalyst for
successive catalytic recycles may be a problem. Like hydroformy-
lation, hydroaminomethylation was also tried in homogenous and
biphasic conditions [11,25–28]. The use of heterogeneous catalyst
helps in easy separation and recycling of the catalyst.
Considering the core observations and conclusions from pre-
vious studies on hydroaminomethylation, here we have used
Rh exchanged titanosilicates [29–31], ETS-4 and ETS-10, for
hydroaminomethylation reactions. Titanosilicates possesses acidic
sites, formed due to strong electric field of cationic sites on X-ETS-
n (X = cation, n = 4 or 10), which could act as a Lewis acid [32].
This Lewis acidic site can catalyze the formation of enamine or
amine from the formed aldehyde and amine reactant. Rhodium
is exchanged on ETS-4 and ETS-10, thus the presence of Rh+ can
∗
Corresponding author. Tel.: +91 278 2567760; fax: +91 278 2566970.
E-mail addresses: sudhinarayanan@gmail.com (N. Sudheesh),
rshukla@csmcri.org, ramshukla55@yahoo.in (R.S. Shukla).
0926-860X/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2014.01.002

N. Sudheesh, R.S. Shukla / Applied Catalysis A: General 473 (2014) 116–124
117
R3
N
R2
R3
R1
N
R1
3
R2
1
CO+H2
H
N
R3
+
R1
R2
R3
Catalyst, T., P.
N
R3
R2
R1
N
R2
R1
4
2
Scheme 1. Hydroaminomethylation of olefin.
provide the effective and active cationic catalyst species for hydro-
formylation followed by hydrogenation of enamine or imine to the
desired amine product.
chloride with solid to liquid ratio of 1:80 at 80 ◦C for 4 h. The solid
was then filtered and washed with hot distilled water until the
filtrate was free from chloride ions and then dried at room temper-
ature. Similar procedure was followed for the synthesis of Rh-ETS-4.
2. Experimental
2.4. Characterization techniques
2.1. Materials
Powder X-ray diffraction (P-XRD) of the samples were recorded
with Phillips X’Pert MPD system equipped with XRK 900 reac-
tion chamber, using Ni-filtered Cu K␣ radiation (ꢀ = 1.54050 A).
Sodium silicate solution, titanium butoxide, titanium dioxide,
rhodium chloride, all the olefins and amines were purchased
from Sigma–Aldrich, USA. Sodium chloride, sodium hydroxide and
potassium chloride were procured from Sd-Fine Chemicals, Mum-
bai. H2 and CO (UHP grade) were procured from Ami Traders,
Bhavnagar, India.
˚
The FT-IR spectra were recorded from 400 cm−1 to 4000 cm−1
with a PerkinElmer Spectrum GX FT-IR. Inductively coupled plasma
atomic emission spectroscopy (ICP-AES) analysis (Optima 2000DV,
PerkinElmer instruments) was used to determine the rhodium
exchanged in the catalyst. The surface area analysis and pore size
distribution of the samples were measured by nitrogen adsorption
at 77.4 K using ASAP-2010, Micromeritics surface area and porosity
analyzer. All the samples were degassed at 80 ◦C for 4 h prior to the
measurements.
2.2. Synthesis of Na-ETS-10 and Na-ETS-4
Na-ETS-10 was synthesized by following the procedure reported
elsewhere [29]. In a typical synthesis procedure, sodium silicate
solution was treated with deionized water followed by the addi-
tion of NaCl, KF and KCl. The gel formed was vigorously stirred
and titanium dioxide was added. The gel had a composition of
5.3SiO2:1.0TiO2:1.6Na2O:7.2NaCl:1.0KF. The pH was maintained at
10.5. The stirring was continued for about 40 min at room tem-
perature and then transferred to teflon lined autoclave, which is
then heated at 200 ◦C for 42 h. The obtained crystalline product was
washed with deionized water and dried at 60 ◦C.
The synthesis of Na-ETS-4 was done by the procedure reported
elsewhere [30]. Titanium (IV) butoxide was added to the aque-
ous NaOH solution. The formed white precipitate was then
dissolved by adding an aqueous solution of H2O2, resulting in
a bright yellow solution. Sodium silicate was then dissolved
in this yellow solution under stirring. The initial composi-
tion of the solution used for preparing the ETS-4 powder
was Ti:Si:Na:H2O2:H2O = 0.9:10:14:8:675 on a molar basis. The
hydrothermal synthesis was performed at 180 ◦C for 72 h in an
autoclave. The powder was filtered, washed with deionized water
and then dried at 40 ◦C.
2.5. Hydroaminomethylation
Hydroaminomethylation reactions were carried out in a 100 mL
stainless steel autoclave reactor (Autoclave Engineers, USA).
Utmost care was taken for the use of carbon monoxide and hydro-
gen at high pressure and temperature. Reaction was performed in a
specially equipped high pressure laboratory. The weighed amount
of alkene and amine with catalyst was taken in the autoclave with
30 mL toluene and 20 mL methanol as solvent. The autoclave was
raised to reaction temperature and then CO and H2 were pres-
surized in the required ratio. Aliquots of product mixture were
withdrawn at regular time intervals for analysis and determination
of conversion and selectivity. The products were analyzed by GC
(Shimadzu GC-17A, Japan) and GC-MS (Shimadzu GCMS-QP2010,
Japan). To ensure reproducibility the experiments were repeated
under identical reaction conditions.
3. Results and discussion
2.3. Synthesis of rhodium exchanged ETS-4 and ETS-10
3.1. Catalyst characterization
The rhodium was exchanged with Na-ETS-4 and Na-ETS-10 by
a cation exchange method [31]. In a typical rhodium exchange,
Na-ETS-10 was treated with 0.05 M aqueous solution of rhodium
The powder XRD patterns of the Na-ETS-4, Na-ETS-10, Rh-ETS-4
and Rh-ETS-10 are shown in Fig. 1. The X-ray diffraction patterns of
the as synthesized Na-ETS-4 [33] and Na-ETS-10 [34,35] were found

118
N. Sudheesh, R.S. Shukla / Applied Catalysis A: General 473 (2014) 116–124
(202)
(020)
Rh-ETS-10
Rh-ETS-4
(116)
(204)
Q
Na-ETS-10
Rh-ETS-10
(105)
Q
(001)
3437
(200)
Na-ETS-4
676
555
1634
1647
Rh-ETS-4
1051
1500
3442
3500
690 562
500
1024
1000
10
20
30
40
50
2θ
4000
3000
2500
2000
-1
cm
Fig. 1. P-XRD patterns of the synthesized support and catalyst.
Fig. 2. FT-IR spectra of Rh-ETS-4 and Rh-ETS-10.
to be in good agreement with those reported in literature. The major
diffraction patterns of Na-ETS-10 are corresponding to the planes
1 0 5, 2 0 2, 1 1 6 and 2 0 4 at 2ꢁ values near 20.2◦, 24.7◦, 25.7◦ and
26.6◦ respectively [36]. The formation of quartz was observed and
corresponding plane was found at 2ꢁ of 20.8◦ and 27.4◦ [37]. But
no anatase phase of TiO2 was formed during the ETS-10 synthe-
sis. The diffraction pattern of Na-ETS-4 showed the major planes of
ETS-4 at 2ꢁ values of 7.6◦, 12.7◦ and 24.7◦ corresponding to 2 0 0,
0 0 1 and 0 2 0 planes respectively [38]. No formation of anatase
or quartz phase was observed due to the usage of titanium (IV)
butoxide as the precursor in case of ETS-4 whereas the TiO2 was
used in ETS-10 synthesis. The diffraction patterns of both Na-ETS-
10 and Na-ETS-4 confirm the formation of ETS framework. As the
rhodium was exchanged with the Na-ETS-4 and Na-ETS-10 the
diffraction pattern shows that there were similar peaks confirming
that the ETS framework is undisturbed during the ion-exchange
process. All the major planes were seen and there was some drop
in the intensities of the peaks showing a possible loss in crystallinity
after Rh exchange. The decrease in crystallinity during the rhodium
exchange may be due to the hydrated rhodium ions, which are
hydrolyzed within the titanosilicate channels leading to breakage
of framework [32]. The percentage of loading determined by ICP-
AES technique and the % crystallinity calculated from the PXRD are
given in Table 1.
surface area of Rh-ETS-4 was very low (10 m2 g−1) showing that
the N2 adsorption at 77 K was not taking the whole internal sur-
face related to the zeolitic cages. The narrow pore size of the ETS-4
at liquid nitrogen temperatures may be the reason for inaccessi-
bility of N2 molecule. The pore shrinkage at lower temperature
resulted to this low surface area for ETS-4 materials. Similar obser-
vations of very low surface area and abnormal isotherm for ETS-4
samples are reported [39]. The Langmuir surface area calculated by
Prasanth et al. using H2 adsorption into Langmuir isotherm model
showed a surface area of 207 m2 g−1 for Rh-ETS-4 [33]. That means
Rh-ETS-4 exists a surface area but the shrinkage of pore diameter
at 77 K results in inaccessibility of N2 molecules. To conclude this
CO2 adsorption was done over Rh-ETS-4 catalyst at 273 K consid-
˚
ering molecular diameter of CO2 as 3.3 A and saturation pressure
as 3.48 MPa. The BET surface area was found to be 234 m2 g−1
.
The SEM images of Rh-ETS-4 and Rh-ETS-10 are given in Fig. 3.
They showed the well-defined ETS-4 and ETS-10 crystals. These
were the characteristic crystal structure of ETS-4 and ETS-10 mate-
rials. Rh-ETS-4 showed a layered structure stacked on one another
whereas the Rh-ETS-10 showed a non-layered well defined square
structure. The size of each particle was in the range of 3–5 ␮m for
both the materials.
The EDX spectrum of Rh-ETS-10 is given in Fig. 4. This shows
the presence of rhodium in the synthesized catalyst. This is further
confirmed by the ICP-AES analysis of the catalysts showing that the
Rh-ETS-4 and Rh-ETS-10 contained 3.2 wt% and 4.8 wt% of Rh.
FT-IR spectra of Rh-ETS-4 and Rh-ETS-10 gave almost similar
transmittance, depicting similar kind of –Si–O–Ti– like bond-
ing. The FT-IR spectra of Rh-ETS-4 and Rh-ETS-10 are given in
Fig. 2. The FT-IR bands observed are in well agreement with that
reported in literature [37]. The broad band near 1020–1050 cm−1
and at 660–690 cm−1 corresponded to the Si–O and Ti–O stretch-
ing respectively. The band near 540–560 cm−1 corresponds to the
Si–O rocking and O–Ti–O bending vibrations. The broad peak near
∼3400–3500 cm−1 is for the surface hydroxyl groups and also for
the stretching of adsorbed water molecule in the ETS framework.
The adsorbed water molecule gave H–O–H bending vibration near
3.2. Hydroaminomethylation
The results of hydroaminomethylation of 1-hexene with vari-
ous amines using catalysts based on Rh exchanged ETS are shown
in Table 2. All the reactions gave 100% conversion in 4 h reac-
tion time. Both the catalysts gave very high selectivity to product
amine. Obviously this higher conversion and selectivity to amine
was attributed to the presence of cationic rhodium species and
the presence of surface acidity of the support. The same observa-
tions were obtained in the homogeneous hydroaminomethylation
where a cationic rhodium species is created via the use of cationic
rhodium precursor [1].
1634 cm−1
.
The BET surface areas for Na-ETS-10 and Rh-ETS-10 were
278 m2 g−1 and 248 m2 g−1 respectively. The decrease in surface
area after Rh exchange may be due to the loss in crystallinity. The
From the initial studies using Rh-ETS-4 and Rh-ETS-10,
showed that the support was having significant effect on
hydroaminomethylation. From the reported studies it is known
that the presence of acid in hydroaminomethylation can enhance
the activity for hydrogenation step [40–42,22,27,43–45]. Hence
the acidic nature of the titanosilicate (ETS-4 and ETS-10) may
be responsible for the higher activity of Rh-ETS-4 and Rh-ETS-
10. A small amount of formyl product of the substrate amines
Table 1
Crystallinity and wt% of Rh exchanged.
Sample
wt% of rhodium exchanged
% Crystallinity
Na-ETS-4
Rh-ETS-4
Na-ETS-10
Rh-ETS-10
–
3.2
–
100
83
100
80
4.8

N. Sudheesh, R.S. Shukla / Applied Catalysis A: General 473 (2014) 116–124
119
Fig. 3. SEM images of Rh-ETS-4 and Rh-ETS-10.
and diethyl amine, was more active due to the higher stability of
the enamine formed by morpholine. The n/iso ratio of the final
amine product was in the range ∼0.8–1.0, which resulted due to
the use of unmodified Rh in absence of a ligand. It is known that
the regioselectivity of hydroaminomethylation is determined at
the first step of reaction, i.e., hydroformylation. Hydroformylation
needs a ligand which could direct the regioselective synthesis of
aldehydes.
Rh-ETS-10 was subjected for the detail investigations for
parametric variations using 1-hexene and pyrrolidine as a rep-
resentative alkene and amine. The reaction pathway for the
hydroaminomethylation reaction of 1-hexene with pyrrolidine is
shown in Scheme 2. The reaction of the hydroaminomethylation
of 1-hexene with pyrrolidine was studied as the function of the
various reaction parameters like, catalyst amount, temperature,
pressure and the ratio of 1-hexene:pyrrolidine.
Fig. 4. EDX spectrum of Rh-ETS-10.
were also observed as by-product, which are possible during
hydroaminomethylation under the reaction conditions. But inter-
estingly no hydrogenation of olefin was observed for any of the
studied substrates.
The use of primary amine, hexylamine, gave higher amount
of imine. Similarly, the reaction with diethylamine also gave
lower selectivity to amine and about 12% and 13% of enamine
were obtained for Rh-ETS-4 and Rh-ETS-10 respectively. This may
be due to the lower basicity and nucleophilic nature of hexy-
lamine and diethylamine and also may be due to lower stability
of imine or enamine, compared to the other similar amine sub-
strates [46,47]. Morpholine, though lower basic than hexylamine
The hydroaminomethylation of 1-hexene with pyrrolidine pro-
ceeds through the pathways shown in Scheme 2. 1-Hexene first
undergoes hydroformylation to form n- and iso-heptanal (2-methyl
hexanal). Also the side reaction, isomerization of 1-hexene to 2-
and 3-hexene takes place. This formed iso-hexene can again get
hydroformylated to aldehyde (n- and iso-heptanal). The formed
aldehyde undergoes condensation with pyrrolidine to form the
enamine. The n-enamine formed from n-heptanal and pyrroli-
dine is 1-(hept-1-en-1-yl) pyrrolidine. The iso-enamine formed
from iso-heptanal and pyrrolidine is 1-(2-methylhex-1-en-1-yl)
pyrrolidine. These n- and iso-enamine get hydrogenated to 1-
heptylpyrrolidine (n-amine) and 1-(2-methylhexyl) pyrrolidine
Table 2
Hydroaminomethylation using Rh exchanged titanosilicates.
% Conv.
% Selectivity
A
Catalyst
Olefin
Amine
B
C
D
E
1-Hexene
1-Hexene
1-Hexene
1-Hexene
Cyclohexene
1-Hexene
1-Hexene
1-Hexene
1-Hexene
1-Hexene
1-Hexene
Cyclohexene
1-Hexene
1-Hexene
Morpholine
Hexylamine
Cyclohexylamine
Pyrrolidine
Morpholine
Diethylamine
Piperidine
Morpholine
Hexylamine
Cyclohexylamine
Pyrrolidine
Morpholine
Diethylamine
Piperidine
100
100
100
100
100
100
100
100
100
100
100
100
100
100
47
42
49
48
99
43
48
44
38
46
45
95
40
48
–
9
–
–
1
5
–
–
12
–
–
4
3
52
41
52
52
–
41
49
54
39
52
52
–
–
8
–
–
–
8
–
–
10
1
1
–
–
–
–
3
3
2
1
1
3
1
3
2
Rh-ETS-10
Rh-ETS-4
–
–
9
2
45
47
1
Reaction conditions: olefin = 11.9 mmol, amine = 11.9 mmol, catalyst = 10 mg, temp. = 100 ◦C, pCO = 13.5 bar, pH2 = 54 bar, methanol/toluene = 20/30 mL and time = 4 h. A = n-
amine, B = n-enamine/imine, C = iso-amine, D = iso-enamine/imine, E = formylated product of amine substrate.

120
N. Sudheesh, R.S. Shukla / Applied Catalysis A: General 473 (2014) 116–124
1-hexene
R1
CHO
CHO
2-hexene
2-methylhexanal
heptanal
P2
+
P3
NH
NH
3-hexene
P1
pyrrolidine
pyrrolidine
R2
R2
N
N
1-(2-methylhex-1-en-1-yl)pyrrolidine
1-(hept-1-en-1-yl)pyrrolidine
P5
P4
H2 Rh-ETS-10, Temp.
H2, Rh-ETS-10
Temp.
N
N
1-(2-methylhexyl)pyrrolidine
1-heptylpyrrolidine
P6
P7
iso-Hexene (P1)
Enamine (P4+P5) Amine (P6+P7)
Aldehyde (P2+P3)
Scheme 2. Hydroaminomethylation reaction scheme of 1-hexene with pyrrolidine.
(iso-amine) respectively. In the further text for simplicity it is pre-
ferred to mention, 2/3-hexene for 2- and 3-hexene, aldehyde for n-
and iso-heptanal, enamine for n- and iso-enamine and amine for n-
and iso-amine. The n/iso ratios are given with respect to the final
product amine.
amines are too fast without showing the intermediates. The n/iso
ratio of product amine was decreased on increasing the catalyst
amount. The n/iso ratio for 5 mg catalyst was 1.74, which decreased
to 0.94 for 100 mg catalyst.
100
3.3. Parametric variation studies
The effect of catalyst amount on conversion and selectivity for
hydroaminomethylation of 1-hexene with pyrrolidine was stud-
ied over a range of 5–100 mg of the catalyst and shown in Fig. 5.
Lower amount of the catalyst (5 mg) was found to be enough for
active hydroaminomethylation giving good conversion and selec-
tivity. An amount of 5 mg of the catalyst gave 70% conversion and
was increased to 100% for 10 mg catalyst. The selectivity to the
product amines (n and iso) was also increased on increasing the
catalyst amount. It can be seen that the selectivity of amine is
increased from 69% for 5 mg catalyst to 81% for 10 mg catalyst. This
is again increased to 91% and 99% with 50 mg and 100 mg of catalyst
respectively. It is interesting to note that there were only two kinds
of products, iso-hexene and amine. No intermediate aldehyde or
enamines were found throughout the catalyst variation. It means
that the formation of enamine and subsequent hydrogenation to
80
% conversion
% iso-hexene
% amine
60
40
20
0
0
20
40
60
80
100
Catalyst amount, mg
Fig. 5. Effect of catalyst amount on conversion and selectivity at 2 h.

N. Sudheesh, R.S. Shukla / Applied Catalysis A: General 473 (2014) 116–124
121
100
80
60
40
20
0
100
80
60
% conversion
% iso-hexene
% amine
% conversion
% iso-hexene
% amine
40
20
0
1:1
1:2
1:3
1:4
40
60
80
hexene: pyrrolidine ratio
Total pressure (H2+CO), bar
Fig. 6. Effect of 1-hexene:pyrrolidine ratio on conversion and selectivity at 2 h.
Fig. 8. Effect of total pressure on conversion and selectivity at 2 h.
The effect of the substrates ratio (olefin to amine) is a parameter
that must have an effect on hydroaminomethylation and is given in
Fig. 6. It is found that as the 1-hexene:pyrrolidine ratio is increased
from 1:1 to 1:3, the conversion got decreased. The conversion of
100% for 1-hexene:pyrrolidine ratio of 1:1 got decreased to 75%
for the ratio of 1:4. The reason may be the increased adsorption
of pyrrolidine on the ETS surface on increasing the concentra-
tion. This may cause the lower adsorption of 1-hexene on the
surface and thus decreases the conversion. But the selectivity to
amine was increased as the 1-hexene:pyrrolidine ratio is increased.
The selectivity to the product amine got increased from 81%
(1-hexene:pyrrolidine ratio of 1:1) to 92% (1-hexene:pyrrolidine
ratio 1:4). The intermediates aldehyde and enamines were not
observed in any of the varied ratios. The major reason for decrease
in conversion and increase in selectivity with an increase in
1-hexene/pyrrolidine ratio is that pyrrolidine competes with 1-
hexene for coordination sites in rhodium and also prevents the
beta-hydride elimination.
The effects of ratio of partial pressure of H2 and CO were deter-
mined by varying H2/CO ratio from 1:1 to 4:1 and the results are
given in Fig. 7. The conversion got increased on increasing the ratio
of the partial pressures of H2 and CO. At the H2/CO ratio 1:1, the con-
version was 30% which increased to 98% for the ratio 3:1. Further
increase in the ratio increased the conversion to 100%. The signifi-
cant effect of H2/CO ratio is more pronounced on the selectivity. The
selectivity increased on increasing the H2/CO ratio. Higher selec-
tivity is observed for iso-hexene towards the lower H2/CO ratio.
Selectivity of 63% and 62% were observed towards iso-hexene at
lower H2/CO ratios of 1:1 and 2:1 respectively.
The lower ratios of H2/CO also gave significant amounts
of aldehyde and enamine showing the effect of ratio on
hydroaminomethylation. No aldehyde or enamine intermediates
were observed for higher ratios of 3:1 and above. The formation of
amine was very low at 1:1 ratio. Amine selectivity of 4% clearly indi-
cated the inefficient ratio which has no potential to hydrogenate the
enamine to form amines. The 23% of enamine at H2/CO ratio of 1:1
confirms this inefficient hydrogenation of enamine.
Also the smaller amounts of aldehyde found towards lower
H2/CO ratio may be due to this inefficient hydrogenation of enam-
ine. Because the formation of enamine from an aldehyde and amine
is a reversible reaction and hence the presence of water produced
from enamine formation can make the enamine to reverse the
reaction to form aldehyde again [48]. The hydroaminomethylation
is done in a closed high pressure reactor and hence the formed
water is not getting removed from the reaction medium. This water
reverses the enamine to aldehyde because the enamine was not
hydrogenated efficiently to the product amine. The n/iso ratio of
product amine was decreased as the ratio of H2/CO was increased.
It may be due to better hydrogenation of n-enamine than that of
iso-enamine at lower H2/CO ratio.
The effect of total pressure by keeping the H2/CO ratio at 4:1 was
studied and given in Fig. 8. The increase in total pressure had sig-
nificant effect towards selectivity to amine. The total pressure had
not much significant effect on conversion when the ratio of H2/CO
was kept constant. A conversion of 98% was obtained at 37.5 bar
and all other conversions were reached 100%. The selectivity to
amine was increased with pressure. Selectivity of 62% at 37.5 bar
got increased to 81% at 67.5 bar. The n/iso ratio was decreased on
increasing the total pressure. At lower pressure the selectivity to
iso-hexene was higher leading to formation of iso-aldehyde, and
subsequently iso-amines resulted in lower n/iso ratio.
The effect of temperature on hydroaminomethylation was stud-
ied and the results are plotted in Fig. 9. The conversion got increased
on increasing the temperature from 80 ◦C to 100 ◦C. From 100 ◦C
and above the conversion was 100%. The selectivity was influenced
on increasing the temperature. At lower temperature of 80 ◦C the
selectivity to amine was low. Significant amount of aldehyde and
also smaller amounts of enamine formation were observed. The
selectivity to amine was 41% only at 80 ◦C and increased to 81%
at 100 ◦C. The n/iso ratio was decreased on increasing the tem-
perature. There were significant amounts of aldehyde at lower
temperature due to inefficient catalysis at this temperature for the
formation of enamine and amine.
100
80
% conversion
% iso-hexene
60
% aldehyde
% amine
% enamine
40
20
0
1:1
2:1
3:1
4:1
--
H2:CO ratio
The catalyst Rh-ETS-10 was subjected to investigate the
reaction kinetics of the hydroaminomethylation reaction. The
Fig. 7. Effect of H2:CO ratio on conversion and selectivity at 2 h.

122
N. Sudheesh, R.S. Shukla / Applied Catalysis A: General 473 (2014) 116–124
The rates of consumption of 1-hexene and formation of 2/3-
100
80
60
40
20
0
hexene and amine were calculated from the slope of early linear
portions of the corresponding curves in the kinetic profile from the
following equations.
−d[1-hexene]
% conversion
% iso-hexene
% aldehyde
% amine
Rate of consumption of 1-hexene, v =
(1)
(2)
(3)
dt
d[2/3-hexene]
Rate of formation of 2/3-hexene, v1
=
dt
% enamine
d[amine]
Rate of formation of amine, v2
=
dt
The rates of reactions were calculated using Eqs. (1)–(3). The
rate of consumption of 1-hexene (v) or the overall rate of reac-
tion was found to be 17.14 × 10−2 M h−1. The rates of formations
of 2/3-hexene (v1) and amine (v2) were 5.65 × 10−2 M h−1 and
11.48 × 10−2 M h−1, respectively. This clearly showed that the ini-
tial rate of formation of amine was double than that of formation
of 2/3-hexene.
80
90
100
110
120
130
140
Temperature, ºC
Fig. 9. Effect of temperature on conversion and selectivity at 2 h.
The catalyst was recycled to ensure its heterogeneous charac-
teristic. The catalyst was recycled by filtration with subsequent
washing with methanol–toluene mixed solvent. The results of
recycling are shown in Table 3. The catalyst had shown good recy-
clability ensuring the stability of the catalyst. The ion-exchange
process is very stable under the employed reaction conditions. The
conversion of 100% was obtained for all the recycles at 4 h. A slight
decrease in conversion was obtained for higher recycles which was
pronounced at 2 h of reaction time. This slight decrease in conver-
sion may be attributed to the loss of some adsorbed Rh that is not
ion-exchanged. But no loss of Rh metal was observed by the ICP
technique which was analyzed from the reaction mixture. There-
fore the amounts of Rh that are simply adsorbed are considered
to be very low and are out of the ICP detection limits. The reac-
tion mixture after hydroaminomethylation was analyzed by mass
spectroscopy for any leaching of rhodium complexes. No Rh com-
plex was detected by mass spectroscopy of the crude product. The
selectivity of the product amine was unaffected by the repeated
usage of the catalyst. The n/iso ratio was also not much affected by
the catalyst‘s recycles ensuring the robust and stable nature of the
catalyst.
1-hexene
2/3-hexene
amine
25
20
15
10
5
0
0.0
0.5
1.0
Time, h
1.5
2.0
Fig. 10. Kinetic profile for the consumption of 1-hexene and formation of products.
corresponding kinetic profile of the consumption of 1-hexene and
formation of products are given in Fig. 10. The 1-hexene was con-
sumed as the reaction was progressed with time with formation
of products and attained complete conversion in 2 h. The prod-
ucts were distributed between isomerized hexene, 2- and 3-hexene
(2/3-hexene), and the desired product, amine. The catalyst was
highly active and gave higher amount of amine. The formation of
2/3-hexene got increased on increasing the time and then started
decreasing as the 1-hexene got completely consumed and also the
iso-hexene started forming aldehyde and subsequently to amine.
No intermediate aldehyde or enamine was detected which indi-
cated that the reaction of these intermediates into the product
amine was faster.
The nature of Rh in Rh-ETS-10 was investigated by a series of
experiments. Carbon monoxide adsorption onto the support was
done by treating Rh-ETS-10 in 15 mL toluene and 10 mL methanol
mixture with CO and H2 at 15 bar. After stirring the mixture
for 1 h at 100 ◦C, the catalyst was separated, washed twice with
toluene and then analyzed by FT-IR (Fig. 11). A band ∼2003 cm−1
,
1893 cm−1 and 1731 cm−1 was newly formed, and is attributed
to the Rh–CO species. IR vibrations of typical hydrido carbonyl
rhodium complexes are expected to show bands near 2041 cm−1
,
2071 cm−1 and 2123 cm−1 for Rh–CO and at 2000 cm−1 for Rh–H
[49]. Fig. 11 shows bands near 2003 cm−1 and 1893 cm−1 which
Table 3
Recycling with Rh-ETS-10.
% Conv.
% Selectivity
Recycle
Time (h)
A
B
C
D
n/iso
2
4
2
4
2
4
2
4
100
100
100
100
99
100
98
100
19
5
18
3
19
4
20
5
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
81
95
82
97
81
96
80
95
1.27
1.01
1.30
1.04
1.30
1.00
1.29
1.02
Fresh catalyst
First recycle
Second recycle
Third recycle
Reaction conditions: 1-hexene = 11.9 mmol, pyrrolidine = 11.9 mmol, catalyst = 10 mg, temp. = 100 ◦C, pCO = 13.5 bar, pH2 = 54 bar and methanol/toluene = 20/30 mL. A = iso-
hexene, B = aldehyde, C = enamine, D = amine.

N. Sudheesh, R.S. Shukla / Applied Catalysis A: General 473 (2014) 116–124
123
Table 4
Hydroaminomethylation of 1-hexene with pyrrolidine with and without ligands.
% Conv.
Selectivity
A
Ligand
B
C
D
n/iso
–
100
100
100
100
100
100
5
40
7
17
32
28
–
–
9
8
7
5
8
95
27
19
38
24
32
1.0
7.6
4.8
4.4
10.5
14.6
Tris(2,4,6-trimethoxyphenyl)phosphine
Triphenylphosphine
Tris(4-trifluromethylphenyl) phosphine
BINAP
19
62
34
39
32
Xantphos
Reaction conditions: 1-hexene = 11.9 mmol, pyrrolidine = 11.9 mmol, Rh-ETS-10 = 10 mg, Rh:L = 1:6, temp. = 100 ◦C, pCO = 13.5, pH2 = 54 bar, methanol/toluene = 20/30 mL,
time = 4 h.
A = iso-hexene, B = aldehyde, C = enamine, D = amine
accounts for the terminal CO vibrations which are shifted to lower
cm−1 due to hindrance in the pores. The Rh–H vibrations have low
intensity and thus might not be visible in the spectrum or it may
be merged with Rh–CO vibrations. The band near 1731 cm−1 may
be attributed to the formation of CO bridge bond between two of
the exchanged Rh [49]. This means that rhodium carbonyl species
are formed with Rh–CO–Rh cluster like complexes when Rh-ETS-10
was treated with CO and H2 under hydroaminomethylation condi-
tions.
geometry and mobility. Combining the above results of FT-IR and
hydroformylation, it may be concluded that the reaction under-
goes as a homogeneous catalyst in restricted pore. The active metal
gets converted to a homogeneous RhH(CO)4 or similar cluster like
complex in the confined pore. During the catalytic cycle, there may
be a series of transformations around Rh, so that it forms active
hydridocarbonylrhodium without losing the attachment to the
surface. Hydroformylation may be strongly due to homogeneous
confined active species and the acid sites would convert this neu-
tral hydride homogenous complex to the cationic species of higher
hydrogenation activity. The studies of Okamoto et al. found a strong
correlation between Rh–Y zeolites and homogenous Rh-complex
regarding the active states of rhodium on alkene hydrogenation
[50]. The above studies confirm the homogeneous-heterogeneous
dual nature of the present catalyst during hydroaminomethylation.
Hydroformylation with catalyst separated from reaction mass
after hydroaminomethylation was conducted for observing any
leaching of the metal. There had been a conversion of 18% of 1-
hexene with 92% selectivity to 2/3-hexenes and 8% to aldehydes
with n/iso ratio of 1.1 at time of 4 h. This means that only a
minute amount of Rh was leached. To know further about het-
erogeneous nature of the catalyst, hydroaminomethylation under
homogeneous conditions was performed using RhCl3 with a ratio
equivalent to Rh-ETS-10 and reaction conditions as in recycle stud-
ies. The results showed that the reaction gave low conversion to
amine compared to that of Rh-ETS-10. Total conversion of 1-hexene
was 80% in 4 h with a selectivity of 70% to respective enamine
with a n/iso ratio of 1.05. The rest were 26% of aldehyde and
4% 2/3-hexene. The decrease in conversion and selectivity with
respect to Rh-ETS-10 may be because of the homogenous condi-
tions of the reaction. Under homogenous conditions, pyrrolidine
substrate has more affinity towards rhodium and gets coordinated.
1-Hexene competes for the active metal center and thus will drop
the conversion of hydroaminomethylation. In the case of Rh-ETS-
10, the micropores of ETS will not accommodate higher number
of pyrrolidine and thus may easily get reacted with 1-hexene to
yield aldehyde and then to subsequent amination. In the pores of
titanosilicates, cations of rhodium may get hydrocarbonylated to
form the active catalyst and get trapped in the micropores. Thus
the catalyst may act as a homogenous system with constraints in
3.4. Ligand effect on hydroaminomethylation
The catalyst Rh-ETS-10 was active for hydroaminomethylation
without any added ligand. Hydroaminomethylation was conducted
using various ligands with Rh-ETS-10 and the results are given in
Table 4. The reaction was conducted for 4 h and in the absence of
any ligand the catalyst gave 100% conversion with 95% selectiv-
ity to amine with n/iso ratio of one. The reaction was performed
with addition of ligand in Rh:L ratio of 1:6. The ligand was varied
from sigma-donor (Tris(2,4,6-trimethoxyphenyl)phosphine) to pi-
acceptor (Tris(4-trifluromethylphenyl)phosphine) and bidentate
(BINAP, Xantphos) ligands. The conversion was 100% for all the
ligands, but the selectivity and n/iso ratio was found to vary. The
product distribution was more towards isomerization and alde-
hyde.
The addition of ligand reduced the activity of the Rh which
may be due to the congestion of the ligand around metal centre
in the micropores of ETS-10. Also the use of triphenyl phosphine
catalyzed the hydroformylation step effectively but failed in effec-
tive hydrogenation of enamine to amine. The pi-acceptor ligand
Tris(4-trifluromethylphenyl)phosphine is having an intermedi-
ate effect on both hydroformylation and hydrogenation activity
with comparable n/iso ratio with that of triphenyl phosphine.
Sigma donor ligand Tris(2,4,6-trimethoxyphenyl)phosphine is not
so active for hydroformylation step showing the higher selectivity
to iso-hexene. But it could yield amines with higher n/iso selectivity
of 7.6. The use of BINAP and Xantphos, bidentate ligands, gave very
high n/iso ratio of 10.5 and 14.6 respectively. But the formation of
iso-hexene and slow hydrogenation of enamine to amine were due
to the bulky and bidentate nature of ligand leading to low reaction
rates.
28
2003
24
20
16
1893
2360
1731
1645
3400
4000
3500
3000
2500
2000
1500
4. Conclusion
cm-1
The rhodium exchanged titanosilicates (ETS-10 and ETS-4) were
synthesized and characterized. About 4.8 wt% and 3.2 wt% of Rh was
Fig. 11. FT-IR of catalyst treated with CO and H2.

124
N. Sudheesh, R.S. Shukla / Applied Catalysis A: General 473 (2014) 116–124
exchanged in Rh-RTS-10 and Rh-ETS-4 catalysts. These rhodium
exchanged titanosilicates were found to be efficient and stable cat-
alysts for hydroaminomethylation reaction. Catalyst was highly
active and selective towards product amine within lower reac-
tion time of 4 h. The presence of rhodium cluster like carbonyl
complexes was confirmed and may be the catalyst precursor for
hydroaminomethylation. 1-Hexene and pyrrolidine are taken as
model substrates for parametric variations using Rh-ETS-10 cata-
lyst. The increase in catalyst amount increased both the conversion
and selectivity. The increase in pyrrolidine ratio gave some note-
worthy information of decrease in conversion with increase in
selectivity. The reason may be the competitive coordination of
pyrrolidine with 1-hexene on rhodium and hindering the beta-
hydride elimination in the hydroformylation step. The ratios of H2
and CO have highly influenced the hydroaminomethylation and
gave that a ratio of 1:4 is the best under the studied reaction con-
ditions. Also a higher total pressure gave best results than the
lower pressures. Increase in temperature gave higher conversion
and selectivity. As a whole the identification of intermediate alde-
hyde and amine was negligible showing the high activity of the
catalyst towards hydroaminomethylation. The kinetics of the reac-
tion showed that the initial rate of formation of amine was double
than that of the 1-hexene isomerization. The catalyst is recycled
effectively without much loss in its activity and selectivity.
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