N. Sudheesh et al. / Applied Catalysis A: General 415–416 (2012) 124–131
125
investigations in these lines. Studies on kinetics and diffusion can
significantly contribute towards scaling up the reaction and reactor
design.
and Rh-complex, can act as a nanophase reactor. Further more, the
surfactant present can improve the solubility of the olefin inside the
pores by which a higher contact with metal center can be achieved.
In the present work, heterogenized HRh(CO)(PPh3)3 (Rh-
complex) in the mesopores of HMS has been investigated for the
hydroformylation of propene. Rh-complex was encapsulated in situ
into the dodecyl amine reverse micelle in side the pores of HMS at
room temperature. The encapsulation was elegant and the catalyst
was used as such without removing the surfactant. The catalyst as
a whole uses the properties of micelles present inside the pores to
improve the stability of the complex. Even though there is no direct
linkage between silica surface and rhodium metal center or the lig-
and, the Rh-complex was entrapped in the micelles present. The
catalyst has been investigated in detail to study the effect of various
parameters like, temperature, concentration of the catalyst, partial
pressure of the gases CO and H2 and weight ratio of Rh-complex
to tetraethylorthosilicate (Rh-complex: TEOS) on the conversion,
selectivity and rates of the hydroformylation of propene. The stud-
ies on mass transfer effects were performed based on Carberry
number and Wheeler–Weisz criterion.
2.3. Characterization techniques
The FT-IR spectra of the samples were recorded from 400 to
4000 cm−1 with a Perkin–Elmer Spectrum GX FT-IR system using
KBr pellets. Powder X-ray diffraction (PXRD) patterns of the cat-
alyst samples were done by Phillips X’Pert MPD system equipped
with XRK 900 reaction chamber, using◦Ni-filtered Cu-K␣ radiati−o1n
◦
˚
(ꢀ = 1.54050 A) over a 2ꢁ range of 1–10 at a step time of 0.05 s
.
The surface area and pore size distribution of the samples were
measured by nitrogen adsorption at 77.4 K using a Sorptometer
(ASAP-2010, Micromeritics). All the samples were degassed at 80 ◦C
for 4 h prior to the measurements. Morphology of the catalyst was
measured using a TEM, 200KV (JEOL. JEM 2100) equipped with EDX
facility. Rhodium elemental analysis was carried out with ICP-AES
(Perkin–Elmer, Optima 2000).
2.4. Hydroformylation reaction
2. Experimental
The hydroformylation experiments were carried out in a 100 ml
autoclave equipped with a mechanical stirrer at a stirring speed of
950 rpm. In the typical experiments, desired amount of Rh–HMS
catalyst in toluene and decane as GC internal standard were added
into the autoclave. After sealing, the autoclave was flushed twice
with N2. After flushing, propene, carbon monoxide and hydrogen
gases were charged at desired pressures. The reactor was then
brought to desired reaction temperature. After the reaction, the
reactor was cooled down by supplying water inside the coil and
reaction vessel was placed in the ice bath. The product analysis was
carried out using Gas Chromatography (GC) (Shimadzu 17A, Japan)
and GC–MS (Schimadzu QP-2010, Japan). To ensure the repro-
ducibility of the reaction, repeated experiments were carried out
under identical reaction conditions. The results obtained, includ-
ing conversion and selectivity was found to be in the range of 5%
variation. Hydroformylation reactions were done in the specially
made high-pressure laboratory having safety precautions for using
carbon monoxide and conducting reactions at high pressure and
temperature. A carbon monoxide gas detector system equipped
with alarm, sensing for human tolerance limit of CO, is kept in the
laboratory to avoid CO inhaling.
2.1. Materials
Carbon monoxide (CO, 99.8%), hydrogen (H2, 99.98%) and
propene (C3H6, 99.8%) were obtained from Alchemie Gases and
Chemicals Private Limited, India. The rhodium metal precur-
sors RhCl3·3H2O, triphenylphosphine (PPh3), sodium borohydride
(NaBH4, 99.98%) and formaldehyde (HCHO, 34%) were pur-
chased from Sigma–Aldrich, USA for the synthesis of Rh-complex.
Tetraethylorthosilicate (TEOS), dodecyl amine were purchased
from Sigma–Aldrich, USA. Solvents were purchased from Quali-
gens chemicals, India. All chemicals were used without any further
purification. The double distilled milli-pore de-ionized water was
2.2. Synthesis of Rh-complex–HMS (Rh–HMS) catalyst
The Rh-complex [22] and HMS [23] have been prepared by
reported methods. In a typical synthesis of HMS, 0.0027 mol of
dodecyl amine was dissolved in a mixture of 0.0909 mol ethanol
and 0.296 mol deionized water. In our previous report we had used
hexadecyl amine as template [20] which is replaced here by dode-
cyl amine due to its easy miscibility with water and ethanol. The
solution was stirred at room temperature (RT) on a magnetic stirrer.
To this solution 0.01 mol TEOS was added dropwise with vigorous
stirring and mixture was stirred for 1 h at room temperature. After
1 h, the white gel precipitate formed was kept at RT for 18 h of aging.
The material was filtered and dried in vacuum at RT.
3. Results and discussion
3.1. Characterization of the catalyst
The FT-IR spectra of Rh–HMS and HMS are given in Fig. 1. The FT-
IR spectrum of HMS showed the characteristic band at 1073 cm−1
for asymmetric stretching of Si
O
Si and 798 cm−1 for tetrahedral
For the synthesis of Rh–HMS, the in situ encapsulation of the
Rh-complex into the HMS pores was done, in which 0.0027 mol of
dodecyl amine was dissolved in a mixture of 0.0909 mol of ethanol
and 0.296 mol of deionized water. The solution was stirred on a
magnetic stirrer and in that 0.07 mmol of the Rh-complex was
added. To this suspension 0.01 mol of TEOS was added dropwise
with vigorous stirring. The stirring was continued for 1 h and a pale
yellow precipitate was formed, which was kept for 18 h for aging
at RT. The yellow precipitate was filtered and dried in vacuum at
room temperature.
The role of the surfactant is to heterogenize Rh-complex inside
the pores thereby avoiding the need of functionalization of the
support. In this heterogenized catalyst system, the Rh-complex
entrapped in the micelles can also act as a homogeneous catalyst
inside the pores. Each pore, containing surfactant reverse micelles
SiO4 structural units. The bands at 2926, 2856 and 1470 cm−1 are
and NH2 scissor respectively, of the organic template of dodecyl
tinguishable ꢂRh–CO and ꢂRh–P bands at 1920 cm−1 and 693 cm−1
respectively [24] evidencing the encapsulation of HRh(CO)(PPh3)3
in to the pore of HMS. The band at 3438 cm−1 corresponds to the
surface hydroxyl group and NH2 stretching vibrations.
As shown in Fig. 2, the powder X-ray diffraction patterns for HMS
and Rh–HMS are nearly identical and all materials showed a single
diffraction peak ∼2.3◦ of 2ꢁ corresponding to d100 spacing [23] of
3.5 and 3.6 nm, respectively. The diffuse scattering was observed at
∼5◦ and is attributed due to hkl reflections that are broadened as a
result of small crystalline domain effects. The observed similar kind
of PXRD pattern for Rh–HMS with that of HMS showed that the HMS