Rhodium porphyrin molecule-based catalysts for the hydrogenation of biomass derived levulinic acid to biofuel additive γ-valerolactone

Article abstract of DOI:10.1039/d0nj01180k

Rhodium-meso-tetraphenylporphyrin (RhTPP) and rhodium-meso-tetrakis(4-carboxyphenyl)porphyrin (RhTCPP) complexes were synthesized and surface grafted on amine-functional mesoporous molecular sieves. The formation of ligands (TPP and TCPP) and their rhodium complexes (RhTPP and RhTCPP) was evident with the help of 1H NMR, 13C NMR, mass spectral and elemental (CHN) analysis. The absence of the pyrrole proton of the porphyrin units on RhTPP and RhTCPP confirmed the successful formation of metallo-macro-molecules. Further, RhTPP and RhTCPP were fixed on the surface of SBA-15 through amino-silane as the linker. The rhodium containing homogeneous (RhTPP and RhTCPP) and heterogeneous catalysts (RhTPP-SBA-AM and RhTCPP-SBA-AM) were used for biomass-derived levulinic acid hydrogenation. Further, the synthesized catalysts were utilized for the hydrogenation of various organic molecules having arene, carbonyl, and alkene functionality under moderate reaction conditions. Both homogeneous and heterogeneous catalysts showed more than 95% levulinic acid conversion even after four cycles. The homogeneous RhTPP and RhTCPP yielded γ-valerolactone (GVL) as a major product, whereas heterogeneous catalysts showed only comparable γ-valerolactone (GVL) selectivity, with improved reusability and recylability in the presence of relatively lower amounts of rhodium complexes. Among RhTPP and RhTCPP, RhTPP showed better selectivity for γ-valerolactone (GVL) compared to RhTCPP. Further, as the number of catalytic runs increased, there was a gradual increase in diol selectivity, which was predominant in the case of RhTCPP, possibly due to the presence of the hydrophilic carboxylate ion which facilitates the interaction of water by-products with γ-valerolactone and favours diol formation. The hydrophobic environment of the RhTPP catalyst facilitated the formation of γ-valerolactone as the major product even in the fifth run. This journal is

Full text of DOI:10.1039/d0nj01180k

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Rhodium porphyrin molecule-based catalyst
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hydrogenation of biomass derived levulinic acid to biofuel additive
γ-valerolactone
Received 00th January 20xx,
Accepted 00th January 20xx
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Kaiprathu Anjali , Naragalu J Venkatesha , Jayaraj Christopher , and Ayyamperumal Sakthivel
Rhodium-meso-tetraphenylporphyrin (RhTPP) and rhodium-meso-tetrakis(4-carboxyphenyl)porphyrin (RhTCPP) complexes
were synthesized and surface grafted on amine-functional mesoporous molecular sieves. The formation of ligands (TPP and
TCPP) and its rhodium complexes (RhTPP and RhTCPP) were evident with the help of H NMR, 13C NMR, mass spectral and
elemental (CHN) analysis. The absence of pyrrole proton of the porphyrin units on RhTPP and RhTCPP confirmed the successful
formation of metallo-macro-molecules. Further, RhTPP and RhTCPP were fixed on the surface of SBA-15 through amino-silane
as linker. The rhodium containing homogeneous (RhTPP and RhTCPP) and heterogenised catalysts (RhTPP−SBA-AM and RhTCPP-
SBA-AM) were used for biomass-derived levulinic acid hydrogenation. Further, the synthesized catalysts were utilized for the
hydrogenation of various organic molecules having arene, carbonyl, and alkene functionality under moderate reaction
conditions. Both the homogeneous and heterogeneous catalysts showed more than 95 % levulinic acid conversion even after
four cycles. The homogeneous RhTPP and RhTCPP yielded -valerolactone (GVL) as a major product, whereas the use of
heterogeneous catalyst gives only comparable -valerolactone (GVL) selectivity, with improved reusability, and recylability in
presence of relatively less rhodium complexes. Among the RhTPP and RhTCPP, the RhTPP shows better selectivity for -
valerolactone (GVL) compared to RhTCPP. Further, as the number of catalytic run increases, there is a gradual increase in diol
selectivity, which is predominant in RhTCPP, possibly due to the presence of hydrophilic carboxylate ion facilitate the interaction
of water by-products with -valerolactone and favour the diol formation. The hydrophobic environment of RhTPP catalyst
facilitates -valerolactone as the major product even in the fifth run.
1
well as increase in the demand on energy resource and fine
chemicals, there is an essential requirement of substitute and
renewable resources.
economic and sustainability aspects, a feedstock based on
biomass appears to be much attractive.
alternative and promising raw material for the preparation of
chemicals and fuels due to its abundance and inexpensive-
renewable sources.
biomass feedstock
valuable chemicals, viz., furfuryl derivative (HMF), levulinic acid
LA), 1,4-pentanediol, and -valerolactone (GVL).
these various processes, -valerolactone (GVL) produced from
LA plays an important part in viable biomass transformation. It
has various applications such as a potential fuel additive, fine
1. Introduction
1
2-14
Based on the current scenario of the
Hydrogenation of organic molecules is one of the most
important process in synthetic organic chemistry. It has wide
applications ranging from fine chemicals to pharmaceutical
synthesis.1 Furthermore, hydrogenation process is very
1
5,16
17
Biomass is an
-3
18
4
,5
important in petrochemical industries.
hydrogenation involves transfer of a dihydrogen molecule to
unsaturated moieties in molecule by using various
In general,
1
9,20
For example, cellulose is an important
that can be efficiently converted to
2
1,22
a
hydrogenation sources such as gaseous hydrogen, hydrazine
and formic acid. Often, catalysts play a major role in improving
reactivity, reaction rates and selectivity of hydrogenation
2
3,24
(
Among
6
process. Development of ideal and improved catalysts for the
hydrogenation processes is one of the most fascinating, but
challenging7 area in this field of research. In industries, the
need of hydrogenation process is highly demanding due to the
application in the production of fine chemicals such as value-
2
5
chemical intermediates, food ingredient, green solvent for
,8
2
3,26-29
lacquers, insecticide and adhesive.
Furthermore, it can be
and also in the synthesis of carbon-
Recently, numerous studies and reviews
have been devoted to the production of GVL and 1, 4-
3
0,31
used as an energy carrier
23,27,32
based products.
9
10
added platform molecules and low-aromatic fuels.
Hydrogenation of organic compounds through green processes
for the production of gasoline, fine chemical intermediates and
value-added chemicals are important processes in the bio-
refining industry. On the other hand, depletion of fossil fuel as
33-35
pentanediol.
The production of GVL and 1,4-pentanediol
from LA began as early as the 1930s using various catalysts, such
as rhenium (Re) black , platinum oxide (PtO
(Zr)-based beta zeolites , Raney nickel
oxides. In most of the cases, better yields of -valerolactone was
obtained under severe conditions such as high temperature
2
e.g. 200-700 °C), pressure (e.g.50-200 bars of H ) and longer
reaction times (20-50 h). Often, the formation of polymerized
by-products was evident and the catalysts were not reusable.
Thus, it is necessary to develop efficient catalysts which are eco-
friendly, economically benign, less energy consuming and safe
during the process with mild reaction conditions.
metals such as rhodium (Rh), ruthenium (Ru), Platinum(Pt),
palladium (Pd), iridium (Ir) Rhenium (Re), nickel (Ni) and base
1
1
3
6
37,38
2
),
zirconium
and mixed metal-
2
4
39,40
aInorganic Materials & Heterogeneous Catalysis Laboratory, Department of
Chemistry, School of Physical Sciences, Central University of Kerala, Kasaragod–
71316, Kerala, India.
(
6
Email:sakthiveldu@gmail.com / sakthivelcuk@cukerala.ac.in
10
b
Department of Chemistry, Bangalore Institute of Technology, Bengaluru 560004,
India
c
Indian Oil Corporation Limited, R&D Centre, Faridabad 121007, India
41-44
Noble
†
Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
Please do not adjust margins

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catalysts on different supported materials such as aluminium
oxide (Al ), silicon dioxide (SiO ) and carbon were commonly
used for the hydrogenation reactions.
homogeneous and heterogeneous catalysts such as
View Article Online
DOI: 10.1039/D0NJ01180K
2
O
3
2
2
7,45,46
Several Ru-based
4
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RuCl −PPh
3
,
ruthenium
acetylacetonate-sulfophenyl
containing phosphine complex48, Ru/C
15,28
49
,
Ru/MOFs ,
50
51
Ru−Re/C, Ru−Sn/C and Ru−La/Al O , were developed, which
showed excellent catalytic activity for the hydrogenation rhodium based catalysts.
reactions. During the process, phosphine complex was used,
which is toxic, air and moisture-sensitive. As an alternative to
phosphine ligands, in recent years, studies have attempted to
mimic naturally occurring biocatalysts such as porphyrin and its
2 3
Scheme 1: Hydrogenation of levulinic acid to -valerolactone using
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. Experimental
2
.1. Catalysts Preparation:
All experiments were conducted using the Schlenk technique.
Prior to the experiments, all the solvents and starting materials
derivatives, as well as enzymes such as cytochromep450 and
chlorophyll.52 This type of biocatalysts provide the advantages
70-72
were dried through a standard procedure.
The detailed
such as high enantio- selectivity, environmentally benign
reaction conditions, less hazardous and the possibility of
catalyst recycling. In addition, these biocatalysts are an
important tool to design and develop new stereo-genic centers
as well as perfect green technique.
extremely versatile in regio- and stereo-specific oxidation of
non-activated substrates and can act as bio-mimicking catalysts
for the hydroxylation of various hydrocarbons. Metallo-
porphyrins and other bio-model molecules show a verity of
catalytic transformations such as photo-catalytic activity,
epoxidation, hydroxylation and oxidation of alcohol and
synthesis procedures for the preparation of rhodium-meso
tetraphenylporphyrin (RhTPP), rhodium-meso-tetrakis(4-
carboxyphenyl)porphyrin (RhTCPP), amino functionalized SBA
15 (SBA-AM), grafting of RhTPP and RhTCPP complex on the
53,54
Cytochrome p450s are
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0-
amino functionalized SBA-15 material are summarized below.
7
8
5
5
Synthesis of Rhodium-meso-tetraphenylporphyrin (RhTPP):
meso-Tetraphenylporphyrin(TPP) was prepared first by using
pyrrole and benzaldehyde in the presence of propionic acid
2
under N atmosphere. Afterwards, the preparation of RhTPP
70
5
6
was carried out as per the procedure described earlier .
Synthesis of Rhodium-meso-tetrakis-(4carboxyphenyl)porphyrin
(RhTCPP):
aldehyde . In addition, it also play a vital role in biological
function such as oxygen transport, photosynthesis, and redox
reactions.5
7,58
Various metallo-porphyrins having nearly forty
Firstly, the ligand meso-tetrakis-(4carboxyphenyl)porphyrin
(TCPP) was synthesized as follows. Equimolar portions of
pyrrole (0.2 mL, 0.193 g, 2.88 mmol) and terephthaladehydic
acid (0.338 g, 2.25 mmol) were refluxed with an excess of
propionic acid (100 mL, 99.3 g, 1.34 mol). The solution was
allowed to reflux for two hours. Once the reaction completed,
the solution was allowed to cool, then filtered and rinsed with
methanol. After being allowed to dry, the precipitate was
collected as a dark blue powder, which then recrystallized in 1:1
chloroform-acetone mixture, resulting in a 12% yield. Further,
metals have been reported and which have found several
applications in the field of photodynamic therapeutic agents,
biological and bio-mimicking reactions.
complexes are also active in several oxidation processes.
Among the various metal-based porphyrins, Rh porphyrins have
been actively investigated over the past 30 years, leading to
important discoveries from catalytic enantio-selective cyclo-
propanation reactions to the more recent discovery of carbon-
hydrogen and carbon-carbon bond activation. The Rh(I)
porphyrin anion was first developed in 1986 via the reduction
of (por)RhX (X = I, Cl) using NaBH4. The basic porphyrin
consists of tetrapyrrole units with non-planar geometry, which
is highly stable, due to the high degree of conjugation. Each
pyrrole nitrogen faces inward to offer the coordination center.
59–61
Metallo-porphyrin
6
2
6
3
6
4
the
(RhTCPP) complex was prepared by dissolving the metal salt
(RhCl ·XH O; 1 mmol) in 5 mL of dry acetone, followed by
introducing a solution containing 1 mmol of TCPP in 45 mL of
dry acetone. The final mixture was refluxed under N
rhodium-meso-tetrakis(4-carboxyphenyl)porphyrin
3
2
2
Rh will coordinate easily with pyrrolic nitrogen and form almost
planar structure.6
5-67
atmosphere for 24 h. After the completion of the reaction, the
solvent was removed and washed with pentane to remove
The resulting complex acts as catalyst for
several organic conversions. Most of the Rh porphyrin complex
shows square pyramidal and square planar geometries with
7
0-72
impurities, after which the product was isolated.
Synthesis of amino functionalized SBA 15 (SBA-AM):
Mesoporous SBA-15 molecular sieve was prepared as per the
vacant axial sites which will be utilized for the coordination of
donor ligands.6
8,69
The diversity of Rh porphyrin-mediated
73-77
standard procedure
using the pluronic P123 triblock co-
transformations for the development of innovative materials
are extensive. Therefore, we synthesized Rh-based porphyrin
complexes (RhTPP and RhTCPP) and explored their catalytic
properties on conversion of levulinic acid hydrogenation to -
polymer as a surfactant and tetraethyl orthosilicate as silica
source. Subsequently, the amine functionalized SBA-15 (SBA-
AM) was obtained through treating calcined SBA-15 (6.5 g) with
a pre-determined amount (3 mmol per gram) of 3-aminopropyl-
v
alerolactone. Further, both the complexes were heterogenised
7
6,77
trimethoxysilane.
to improve the reusability and recyclability of the catalysts in
various organic transformations. Inspiring by the naturally
existing porphyrin-based biochemical processes, we have made
attempts to develop catalysts for the hydrogenation reactions,
which constitutes a step forward in the design of sustainable
biomass hydrogenation processes (scheme 1) under
comparatively mild reaction conditions catalyzed by Rh-based
porphyrin systems.
Grafting of the RhTPP and RhTCPP complex on the amino
functionalized SBA-15 material: The homogeneous macro-
molecules (RhTPP and RhTCPP) grafted on the surface of
functionalized material was achieved as follows. Firstly,
introduced 0.05 mmol of RhTPP and RhTCPP in 50 mL of dry
methanol suspended with 1 g of the support material (SBA-AM),
78
followed by refluxing for 24 h under N
2
atmosphere. The
metallo-porphyrin RhTPP and RhTCPP grafted materials were
2
| J. Name., 2012, 00, 1-3
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represented as RhTPP−SBA-AM and RhTCPP-SBA-AM. The electron microscopy (SEM) images on an EVOViMewAA1rti5cle ZOenliisnes
lyI: der
amount of RhTPP and RhTCPP on the support is about 3.7 and microscope operated at 10−20 kV. Fin
D
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s
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/D
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p
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w
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K
4
. 8 wt. % respectively.
samples were “dusted” on a double-sided carbon tape and
mounted over the sample holder. Gold sputtering of the
materials was carried out on a gold sputter coater JEC 300, to
avoid charging and to make the surface conducting. Finally, the
specimens were analysed in a secondary electron imaging mode
at different magnifications and desired voltages. Transmission
electron microscopy (TEM micrographs) images obtained on a
FEI-TECNAIG2-20 TWIN with LaB6 filament.
2
.2. Catalytic Studies
In a typical reaction (Scheme 1), 5 mmol of levulinic acid (LA) in
isopropyl alcohol (IPA) was introduced into a 25 mL autoclave
containing 0.01 g of the homogeneous catalyst (RhTPP,
RhTCPP). Subsequently, the autoclave was charged with
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required H
temperature for 12 h. Afterwards, the reactor was cooled to
room temperature and excess H was vented out. The products
were confirmed by gas chromatography (GC) by using authentic
samples obtained commercially -valerolactone from TCI
2
pressure. The reaction was performed at required
3
. Results and Discussion
The RhTPP and RhTCPP molecules prepared and heterogenized
on the surface of amine functionalized SBA-15 materials. The
homogeneous and heterogeneous catalysts were explored for
the first time on hydrogenation of levulinic acid.
2
[
chemicals and 1, 4-pentane diol from sigma Aldrich). The results
were analysed quantitatively by gas chromatography using an
FID detector equipped with an HP 88 column (Mayura Analytical
model 2100) in temperature program mode (heating from 100
3
.1. Characterization of the catalysts
The ligands (TPP and TCPP), Rh-incorporated porphyrin complexes
(RhTPP and RhTCPP), heterogeneous RhTPP-SBA-AM and RhTCPP-
SBA-AM were characterized by various analytical and spectroscopic
techniques. Rh-based porphyrin systems have served as potential
catalysts for hydrogenation reactions under moderate reaction
conditions using molar hydrogen as hydrogen source. Fourier
transform infrared (FT-IR) spectra (Figure 1a and 1b) of TPP, TCPP,
and the corresponding Rh complex (RhTPP, RhTCPP) show
o
o
o
to 240 C, in 10 /minute, then keeping 240 C for 10 minutes).
Similarly, the reaction was carried out by using a heterogeneous
catalyst in which 5 mmol of levulinic acid (LA) was introduced
into a 25 mL autoclave containing 0.1 g of the heterogeneous
catalyst (RhTPP−SBA-AM/RhTCPP−SBA-AM) and the autoclave
was charged with H
2
pressure. The reaction was performed at
72
1
00 °C for 12 h, with the required H pressure of 10 bar.
2
−1
characteristic modes of vibration in the range of 450 to 970 cm
which is typical for in- and out of plane vibrational mode of porphyrin
−1
moiety. The vibrational bands appeared around 3340, 1590, 985 cm
2.3. Characterization Techniques
are characteristics of symmetric stretching and bending modes of
N−H plane of the pyrrole unit of the porphyrin. Upon incorporation
of Rh into TPP, these bands were disappeared, which support the
incorporation of rhodium ion in the cavity of TPP by covalent bonding
with nitrogen. In addition, a weak shoulder band was detected at
58 cm after Rh incorporation, (Figure 1a), which is characteristics
Furthermore, there are shifts in vibrational
bands from 1470 cm (TPP) to 1476 cm (RhTPP); possibly due to
the coordination of Rh to the TPP center. In the case of TCPP, the
The functional group present in the ligands (TPP, TCPP) and
metal complexes (RhTPP, RhTCPP) were obtained by Fourier
Transfer-Infra-red spectra, which were measured using
PerkinElmer spectrum-2 FT-IR spectrometer in the range of
71
−1
7
−
1
4
00−4000 cm by KBr method. The spectral data were gathered
7
0–72
to the Rh−N bond.
−1
with 4 cm resolutions and 120 scans. The coordination nature
and oxidation state of homogeneous and heterogenised
complexes were obtained using UV−Vis spectra using Shimadzu
−1
−1
-1
-1
vibrational bands at 1496 cm and 1231 cm corresponds to
aromatic C=C and C–N stretching respectively, which yielded a shift
in lower vibrational frequency range after Rh ion introduction.
1
13
UV2600 spectrometer. The H and C nuclear magnetic
resonance (NMR) spectra of the TPP, TCPP, RhTPP and RhTCPP
molecules were collected using CDCl -d and DMF- d as the
3 6 7
–1
Additionally, a new vibrational band was appeared at 662 cm which
is characteristic vibration to the Rh–N bond. The vibrational bands of
solvent for ligand and metallo-porphyrins molecules
respectively. The tetramethylsilane was used as standard on a
Brücker AVANCE III 400 instrument. Electrospray ionization
time-of flight mass spectra were recorded on a Bruckerma Xis
mass spectrometer using methanol as the solvent. Elemental
analyses were performed on an Elementarvario MICRO cube by
grinding the sample into a fine powder and packing in an
aluminium foil. The textural properties such as
–
1
C=O, C=C and C-O were observed at 1682 cm (carboxyl
functionality), 1600 cm (aromatic) and 1170 cm (Figure 1b)
respectively, which are the characteristic bands of the meso-benzoic
–1
-1
72
group present in the TCPP and RhTCPP.
1
H NMR studies support RhTPP and RhTCPP formation (Table 1). In
the spectra, typical peak was appeared around highly shielded region
(−2.74 ppm, corresponding to the N−H protons of pyrrole unit
present in TPP) and it was disappeared upon Rh incorporation into
RhTPP (Supporting Information Figure S1 in ESI*). Thus, it can be
confirmed that Rh was connected to the nitrogen center of the TPP
ligand. In the case of RhTCPP complex, the absence of signal at highly
shielded region i.e.; -2.79 ppm, which is due to the N–H protons in
TCPP [79], confirming that the Rh ion was bonded to the “N” center
of TCPP. The other NMR signals of RhTPP and RhTCPP, being
characteristic of the TPP and TCPP frameworks, remained identical
to that of the pure ligands (TPP and TCPP) (Supporting Information
[
Brunauer−Emmett−Teller
Barrett−Joyner−Halenda (BJH) average pore volume] of the
heterogenized samples were followed by sorption
(BET)
surface
area
and
N
2
measurements at −196 °C using an automatic micropore
physisorption analyser (Quanta Chrome Nova 2200e). The
samples were degassed at 110 °C for 10 h under a pressure of
−3
1
0 Torr prior to each run. Powder X-ray diffraction patterns of
all the heterogenized materials were collected on a Shimadzu
MAXima_X XRD-700 X-ray diffractometer with Cu Kα radiation
(α = 1.5418), in the 2θ range of 0.5−10° with a scan speed and
1
3
Figure S2 in ESI*). The C NMR spectra, shows characteristic peaks
at 120 ppm corresponds to meso-carbon, 127, 126, and 142 ppm for
phenyl ring and 134 ppm for pyrrole unit, and are similar to both
step size of 0.5°/min and 0.02°, respectively. The morphology
and particle size of materials were obtained from the scanning
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DOI: 10.1039/D0NJ01180K
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Figure 1: Fourier-transform infrared (FT-IR) spectra of (a) tetraphenylporphyrin (TPP), rhodium TPP (RhTPP) and RhTPP-SBA-AM
(b) tetrakis(4-carboxyphenyl)porphyrin (TCPP), RhTCPP, and RhTCPP-SBA-AM.
tetraphenylporphyrin (TPP) and rhodium TPP (RhTPP). On the other
1
3
hand, C NMR of TCPP and RhTCPP showed signals in the region of Complexes showed parent ion peak appeared at 751.1098
1
20-140 ppm and 175 ppm respectively, that are characteristics of [C44
H
28ClN
4
Rh], (theoretical m/z value = 750.1058), and at 961.0531
RhCl ] (theoretical m/z value = 961.0339] respectively for
the phenyl, meso, and carboxylic moiety of the TCPP. The peaks [C48
appeared at 126 and 105 ppm indicate pyrrolic and meso carbons, RhTPP and RhTCPP molecules.
respectively (Supporting Information Figure S3 and S4). The
H
28
N
4
O
8
2
formation of RhTPP and RhTCPP were further confirmed from mass
spectral analysis. The mass spectral analysis of RhTPP and RhTCPP
1
13
Table 1: H and C NMR of tetraphenylporphyrin (TPP), rhodium TPP (RhTPP), tetracarboxyphenylporphyrin (TCPP), and RhTCPP.
1H NMR chemical shift (ppm)
13C NMR chemical shift (ppm)
Sample Code
TPP
Pyrrolic
8.87
NH
ortho- / meta- & para-
Pyrrolic
134
meso-
Phenyl
COOH
--
8
.25 &
.75
8.27 &
.80
.34 &
.38
.44 &
8.49
-2.74
120
127, 126, 142
7
RhTPP
TCPP
8.89
8.86
8.90
---
-2.79
--
134
134
134
120
119
119
127, 126, 142
128, 130, 145
128, 130, 145
--
175
175
7
8
8
8
RhTCPP
Table 2: CHN of tetraphenylporphyrin (TPP), rhodium TPP (RhTPP),
tetracarboxyphenylporphyrin (TCPP), and RhTCPP
CHN Analysis (%)
Sample Code
TPP
C
H
N
85.9 (85.9)
70.6 (70.3)
71.2 (72.9)
59.7 (59.8)
4.1 (4.9)
4.1 (3.7)
4.6 (3.8)
2.7 (2.9)
9.8 (9.1)
7.3 (7.4)
6.2 (7.0)
5.8 (5.8)
RhTPP
TCPP
RhTCPP
Figure 2: Ultraviolet-visible (UV-Vis) spectra of TPP, RhTPP,
TCPP, and RhTCPP.
4
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Table 3: Textural properties of SBA-AM, RhTPP-SBA-AM, and RhTCPP-SBA-AM.
View Article Online
DOI: 10.1039/D0NJ01180K
BET surface area
BJH Pore volume
BJH Pore size
2
-1
3
-1
Sample code
SBA- AM
(m g )
243
(cm g )
0.44
(nm)
6.7
RhTPP-SBA-AM
RhTCPP-SBA-AM
U-RhTCPP-SBA-AM
135
0.18
3.7
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126
0.18
5.2
101
0.14
5.2
ligands and complexes are well match with theoretically estimated
values (Table 2). In the ultraviolet-visible (UV-Vis) spectrum (Figure
2
) of TPP complex observed peak at 419 nm corresponds to soret
band and Q bands were evident at 513, 547, and 652 nm. These are
characteristic peaks of porphyrin macromolecular structure. Upon
Rh ion introduction, the intensities of the Q bands at 513 and 547 nm
were decreased. The peak at 652 nm was disappeared due the
decrease in energy gap of the rhodium containing
tetraphenylporphyrin complex compared to pure ligand.70,80,81
A
similar trend was observed in the TCPP and RhTCPP complex. UV-
Visible spectrum of TCPP displays bands at 412 nm which is
characteristic for soret band; the quartet band appeared at 512, 549,
5
93, and 645 nm for Q bands. The intensity of the Q bands at 512 nm
decreased and shifted to 516 nm and the remaining Q bands were
disappeared, which supports the incorporation of rhodium in the
cavities of porphyrin unit. After the detailed characterization and
confirmation of RhTPP and RhTCPP structure, the complexes were
surface immobilized on the amino-functionalized molecular sieve
and were thoroughly characterized further. The successful anchoring
of the RhTPP on the amino silane modified SBA-15 material (SBA-AM)
was observed and confirmed through FT-IR study. In FT-IR spectra
Figure 3: X-ray diffraction (XRD) patterns of SBA-15,
RhTPP-SBA-AM, RhTCPP-SBA-AM, and U-RhTCPP-SBA-AM.
characteristics to the (1 0 0), (1 1 0), and (2 0 0) of the SBA-15
−1
(Figure 1), the vibrational bands appeared around 2934 cm , 1651
7
3-75
framework.
Due to the introduction of RhTPP into amino silane
−1
-1
cm and 1489 cm are characteristics of aliphatic hydrocarbon unit
and amine functionality of the silane linker present on the surface of
functionalized SBA-15. The porphyrin units are connected on the
surface of mesoporous SBA-15 through amino silane linker. The
amine functionalities present on the linker will be coordinating on
RhTPP complexes through axial coordinating sites. The FT-IR
spectrum of heterogeneous catalyst also showed the characteristic
feature of TPP and RhTPP. However, the peaks are merged with
modified SBA-15, a slight shift in 2θ values was observed. This shift is
characteristic to the (1 0 0) plane at 0.95 ° for RhTPP−SBA-AM, which
supports that RhTPP was successfully encapsulated on the surface of
amine functionalized SBA-15. Similar trend was observed on RhTCPP-
SBA-AM samples.
The nitrogen adsorption-desorption isotherms (Figure 4) of
RhTPP and RhTCPP encapsulated amino functionalized SBA-15 have
7
3-75
exhibited type-IV isotherm as per IUPAC representation.
A
broad frame work vibration. RhTCPP-SBA-AM showed
a
distinct, sharp uptake in the p/p range of 0.6 − 0.8 was observed,
o
-1
characteristic band at 1659 cm , which is characteristic of amide
linkage between the RhTCPP complexes and amino functionalized
SBA-15. Both functionalized SBA-15 support and Rh-containing
macromolecular grafted materials have displayed the typical band
around 1080–1200 cm , which was characteristics of the as-
symmetric Si-O-Si stretching vibrational modes of the SBA-15
framework. The molecular framework was retained even after the
heterogenization of bulk RhTPP and RhTCPP complexes. Further, the
incorporation of bulk RhTPP and RhTCPP on the functional molecular
sieve was confirmed based on the shift in X-ray diffraction (XRD)
which is due to the capillary condensation of nitrogen inside the
primary mesopores of the synthesized samples. RhTPP-SBA-AM and
RhTCPP-SBA-AM have shown a significant reduction in BET surface
area and BJH pore volume (Table 3) compared to the parent sample
SBA-AM), which supports that the RhTPP and RhTCPP were grafted
into the mesoporous channels. Furthermore, the pore size
distribution was observed in the range of 3 to 7 nm, which shows the
-1
(
71-75
mesoporous intactness of the materials.
The scanning electron
microscopy (SEM) image shows the (Figure 5a and 5b) hexagonal
morphology, which was remained intact even after the introduction
of RhTPP and RhTCPP on the surface of amino functionalized SBA-15.
The observed surface roughness may be due to the presence of
surface functionality and metallo-macromolecules.
patterns. In addition, N
on the observed reduction in BET surface area and BJH pore volume
Table 3). The low-angle diffraction patterns of RhTPP and RhTCPP
2
sorption studies also confirmed this based
(
grafted materials were presented in Figure 3. All of the
heterogenised catalysts display the characteristic reflections of
mesoporous SBA-15 framework with the line broadening and
reduction in relative intensities of X-ray diffraction. The X-ray
diffraction peaks at 2θ values of 0.85 °, 1.51 °, and 1.73 °,
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molecules. The uniform distribution of RhTPP and RhTViCewPPArtiincletoOntlhinee
mesoporous channels of SBA-15 is also evideDntOfIr:o10m.1f0i3g9u/rDe05NcJa0n11d850Kd
respectively.
3.2. Catalytic studies
The hydrogenation of levulinic acid was carried out with both Rh
porphyrin-based homogeneous and heterogeneous catalysts
Scheme 1). These reactions were carried out at different
temperatures and pressures using different solvents for optimization
of the reaction conditions on RhTPP complex.
(
0
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3
.3. Effects of H
The influence of H
hydrogenation of LA is depicted in figure 6a. It is observed that with
an increase in H pressure from 2 to 10 bar, the conversion of LA
2
pressure and temperature
2
pressure on the catalytic activity of Rh-TPP on
2
HR-
TEM
2
adsorption isotherm of (A) SBA-AM (B) RhTPP−SBA-
increases from 68% to 100%. Further, when the pressure was
increased to 10 bar, exclusive formation of -valerolactone was
evident. The comparable conversion and poor selectivity of GVL at
Figure 4: N
AM, and (C) RhTCPP−SBA-AM
low pressure may be due to low solubility of H
2
in the solvent
at the
micrograph of the heterogenised catalyst, viz., RhTPP-SBA-AM is
shown in Figure 5c and 5d. From the TEM image it is evident that the
heterogeneous catalysts RhTPP-SBA-AM and RhTCPP-SBA-AM have
shown the hexagonal honey-comb like structure with retention of
long-range order even after the encapsulation of bulk RhTPP and
RhTCPP
(
isopropyl alcohol) and the low concentration of active H
2
8
2
metal center. .
1
µm
1 µm
c
d
1
00 nm
100 nm
Figure 5: SEM images (a) RhTPP-SBA-15 and (b) RhTPP-SBA-AM, High resolution transmission electron microscopy (HR-TEM)
micrographs of (c) RhTPP-SBA-AM and (d) RhTCPP-SBA-AM
6
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View Article Online
DOI: 10.1039/D0NJ01180K
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Figure 6: (a) Effects of pressure at 100 °C, (b) effects of temperature at 10 bar H . Reaction Conditions: LA = 5mmol, Solvent = 3ml,
Catalyst RhTPP = 0.01g.
The increase in pressure of approximately 10 bar led to improved conversion and better -valerolactone selectivity. Isopropyl alcohol
conversion and GVL selectivity owing to the improved H solubility in certainly played a pivotal role in hydride generation that ultimately
As the hydrogen pressure increases, improved catalytic efficiency of both the RhTPP and RhTCPP
2
8
2,83,84
isopropyl alcohol.
8
2
concentration of hydrogen increases which facilitate more active catalysts. This is consistent with the literature findings. These
hydrogen and hydrophobic environment at the catalytic sites. Thus, catalysts facilitate cyclization and ring closure mechanism to form
an exclusive formation of -valerolactone was observed.
The influence of reaction temperature on LA transformation over conversion and yielded 1, 4-pentanediol as the major product. The
the RhTPP catalyst was studied in the temperature range 70- 100 °C, water molecules under the reaction conditions may facilitate further
and the results are shown in figure 6b. In a catalytic reaction, ring opening of GVL to form 1, 4-pentanediol. Relatively better
temperature can influence the reaction rate and catalytic efficiency solubility of homogeneous catalysts on dimethyl formamide (DMF)
by facilitating the hydride transfer process.
conversion was found increasing linearly with an increase in reaction hydrogenation of LA takes place irrespective of protic and aprotic
temperature and reached a maximum at 100 °C with exclusive solvents. Thus, further reactions were conducted using isopropyl
formation of -valerolactone evident.
6
,87,88
the GVL.
The use of water as a solvent resulted a decrease in the
8
9
85,86
As expected, LA also yielded good conversion and selectivity for GVL. It implies that
alcohol, which is a green solvent, considering factors such as
selectivity for GVL, green chemistry aspects and environmental
impact. Therefore, the evaluation of catalytic activity of RhTPP-SBA-
3
.4. Effects of Solvent
Generally, in the case of homogeneous catalyst solvents play a vital AM, RhTCPP, and RhTCPP-SBA-AM were carried out under the
role in the catalytic activity. Therefore, attempts were made to optimum reaction temperature (100 °C) and pressure (10 bar) using
understand the role of solvent on LA conversion over RhTPP and isopropyl alcohol as the solvent.
RhTCPP catalysts, and the results are summarized in Table 4. Among
the various solvents studied, isopropyl alcohol yielded the maximum
Table 4: Reaction carried out with different solvent using RhTPP and RhTCPP catalysts.
S. No.
Catalyst: RhTPP
Conv. (%)
Catalyst: RhTCPP
Conv. (%) Selectivity (%)
Solvent
IPA
Selectivity (%)
GVL
Diol
---
GVL
Diol
1
2
3
4
100
97
100
1
9
9
6
00
2
78
22
13
11
92
DMF
98
02
08
88
87
89
8
Butanol
Water
93
92
7
87
12
3
2
Reaction Conditions: LA = 5mmol, Solvent = 3mL, Pressure = 10 bar H , time = 12 hrs, Temperature = 100 °C, Catalyst= 0.01 g
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.5. Recyclability Study
shows improved selectivity of -valerolactone. This coVuielwdAbrteicldeuOenlitnoe
the by-product water which was formed duDrOinI:g10t.h10e3r9e/Dac0tNioJ0n1.1a8n0Kd
The catalytic efficiency of both homogeneous (RhTPP, RhTCPP) and
heterogeneous catalysts (RhTPP-SBA-AM, RhTCPP-SBA-AM) was
further evaluated under optimum reaction condition for several
runs. In the case of homogeneous catalysts, after each reaction cycle,
may be partially adsorbed on the SBA-15 support. Thus, -
valerolactone selectivity was increased considerably. The rhodium
complex on the used catalyst (U-RhTCPP-SBA-AM) was remained
intact after the reaction, which was evident from the surface area
data (Table 3) and low angle XRD pattern (Figure 3). In addition, the
HR-TEM image of the used catalysts (Used RhTPP-SBA-AM) also
5
mmol of reactant (LA) was again introduced into the reactor and the
reaction was continued under identical experimental conditions.
0
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shows (Figure S5 and S6 in ESI) that the structure was remained intact
even after the catalytic reaction.
After each run, the heterogeneous catalysts and the products were
isolated by filtration. These catalysts were washed with isopropyl
alcohol, dried under vacuum, and then reused. For both the RhTPP
and RhTCPP catalysts, the reaction was repeated by introducing the
substrate up to five cycles. The results are shown in figure 7a and b.
It was observed that the use of the RhTCPP catalyst yields consistent
conversion up to five runs, but the selectivity of 1, 4-pentanediol
increases gradually. This finding may due to the carboxylate group
present on RhTCPP, which is relatively hydrophilic in nature. Thus,
the water formed during reaction can influence more diol formation
over consecutive cycles. In the case of the RhTPP catalyst, the
Further, to check the metal leaching from the heterogenised
catalysts, the hot filtration test was carried out as per the procedure
described below. Firstly, the catalyst was treated with reactant at
1
00 °C under atmospheric pressure for 2hrs. Subsequently, the
catalyst was filtered and the reaction was pursued using filtrate
solution under 10 bar hydrogen pressure. The results are
summarized in table S1 (See ESI*). The catalysts RhTPP-SBA-AM and
RhTCPP-SBA-AM showed about 1.4 and 1.6 % conversion on the
filtrate solution and confirmed that there is no leaching of RhTPP and
RhTCPP complex from the heterogeneous catalysts.
conversion decreased with an increase in the number of cycles and 3.6. Mechanism of levulinic acid hydrogenation
Figure 7: Number of runs of the reaction by (a) RhTPP (b) RhTCPP, and recyclability of (c) RhTPP-SBA-AM (d) RhTCPP-SBA-AM catalyst.
shows ~54% conversion after the fifth cycle. The major product was
GVL, whereas the selectivity of 1,4-pentanediol was increased
slightly in subsequent runs. The heterogeneous catalysts RhTPP-SBA-
AM and RhTCPP-SBA-AM have retained their activities even after
four runs. By using RhTPP-SBA-AM, the selectivity of GVL was almost
consistent in each cycle, whereas in the case of RhTCPP-SBA-AM, the
GVL selectivity was slightly decreased (Figure 7c & d), similar to that
of homogeneous catalysts. However, in comparison with
homogeneous RhTCPP, the heterogeneous RhTCPP-SBA-AM catalyst
Based on the results on both homogeneous and heterogeneous
RhTPP and RhTCPP catalyst, it is proposed that the reaction may
proceed via the following pathway as depicted in scheme 2. It
involves the replacement of axially coordinated ligand by the hydride
91,92
ion at the rhodium center.
Subsequently, hydride ions undergo
migratory insertion (intramolecular hydride transfer) into the
carbonyl group present in the levulinic acid and form intermediates,
9,10,37,90,91,92
4
-hydroxyvaleric acid or/and 4-oxopentanal
based on the
hydrogen pressure.
8
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Scheme 2: Schematic representation of reaction pathway for the levulinic acid hydrogenation over RhTPP catalyst
View Article Online
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SBA-AM, -valerolactone is the major product (60-80 %), which may
be due to removal of water formed during the reaction by the SBA-
5 surface and facilitate the reaction to follow path 1 mechanism. In
addition, the formed 4-hydroxyvaleric acid may interact with the
hydrophilic heterogeneous support, i.e. amino-functionalised SBA-
These intermediates are unstable and give further products. At low
hydrogen pressure (2-8 bar), the poor hydrogen solubility under the
reaction conditions83,84 may facilitate the formation of 4-
1
9
,10,37,90
oxopentanal as intermediate
at the catalytic site (Path 2). The
1
5 and undergo further hydrogenation, to form 1,4-pentanediol as
unstable intermediate undergoes dehydration at the catalytic site
and facilitate the formation of 1, 4-pentanediol. The increase in
hydrogen pressure to 10 bar results an increase in hydrogen-to-
substrate ratio, which further improve the hydrogen solubility under
the reaction conditions.
environment facilitate the formation of the intermediate 4-
hydroxyvaleric acid
released from the catalytic center to solvent medium and
subsequently undergo cyclization
at high pressure and temperature, which gives -valerolactone as the
product (See reaction mechanism, Scheme 2, path 1). In addition, the
presence of by-product water facilitates the formation of 1,4-
pentandiol. Among the homogeneous catalysts, RhTPP gives 100 %
selectivity for -valerolactone whereas RhTCPP gives almost 80 % -
valerolactone and the remaining was 1, 4 pentanediol. As the
number of runs increases, there is a gradual increase in diol
formation and the diol selectivity reached to almost 85 % in the fifth
run on RhTCPP catalyst. This, possibly, may be due to the presence
of carboxylate ion, which increases the hydrophilic environment and
interact with water by-product and facilitate the diol formation. In
addition, the water molecule produced as the by-product may
further facilitate the diol formation in consecutive runs. In the case
of RhTPP catalyst, the presence of hydrophobhic environment
facilitate about 70 % GVL selectivity even after several runs.
Whereas, the heterogeneous catalysts, RhTPP-SBA-AM and RhTCPP-
the minor product. Therefore, in the case of heterogeneous
catalysts, both the products are formed, but the selectivity is more
for -valerolactone. Moreover, this catalyst follows the reaction
pathway similar to that of Wilkinson’s catalyst. During the
8
2,83,84
In addition, the hydrophobic
recyclability study, the heterogenised catalysts have given almost
th
9
6
0% conversion even in the 4 run with GVL as the major product (~
9,10,37,90
(Path 1). Further, the intermediate gets
th
0%). The product selectivity is also consistent even in the 4 cycle.
In the case of homogenous RhTCPP, the carboxylic group present in
the catalyst increases the hydrophilicity which favors the formation
of 1, 4-Pentanediol. Further, the catalytic activity was compared
through levulinic acid hydrogenation using iridium and ruthenium
based molecules (IrTPP and RuTPP) and the results are shown in table
S2 in ESI. The results show that compared to ruthenium and iridium-
based porphyrin catalysts, rhodium gives good conversion with
exclusive formation of -valerolactone in isopropyl alcohol, which is
a green solvent. There are many reports in which ruthenium,
rhodium and iridium based catalysts with phosphine ligands yield
good conversion with excellent selectivity of -valerolactone, which
9
,10,37,90,91,92
by the removal of water
1
0,12,14,25,27,93,94,95
is shown in table S3 in ESI.
The key feature of the
present work is the catalytic studies are carried out under the
phosphine ligands free conditions. Further, our catalyst showed
comparable activity with respect to the literature known catalysts
Table S3) under moderate reaction conditions. In addition, the
present catalyst is eco-friendly and is easy to heterogenise on the
(
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surface of amino functionalized SBA-15 (SBA-AM). Further, the
catalysts were explored for the hydrogenation of various substrates
such as arene, carbonyl and alkene functionalities (1-octene, cyclo-
octene, benzaldehyde, furfural, eugenol, nitrobenzene and
cinnamaldehyde. The results are summarized in Table S4 in ESI*.
From the results, it is clear that irrespective of the functional groups,
catalysts are efficient for the olefin hydrogenation. In particular, the
RhTCPP- based catalysts yielded better conversion for hydrogenation
of various functionalities.
2. Figure S2: H NMR spectrum of RhTCPP (DMF-d , 400MHz).
7
View Article Online
DOI: 10.1039/D0NJ01180K
13
3
. Figure S3: C NMR spectrum of RhTPP.
4. Figure S4: 13C NMR spectrum of RhTCPP.
5. Figure S5: High resolution transmission electron microscopy
(HR-TEM) micrographs of RhTPP-SBA-AM
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(
. Figure S6: High resolution transmission electron microscopy
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HR-TEM) micrographs RhTCPP-SBA-AM
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. Table S1: Quenching studies by filtering the solution (Hot
filtration study).
Conclusion
8. Table S2: Levulinic acid hydrogenation using ruthenium
porphyrin and iridium porphyrin complex.
In summary, Rh-containing porphyrin complexes (RhTPP, RhTCPP)
were prepared and grafted on the amino silane modified SBA-15
molecular sieve. The structure was thoroughly illustrated via H
9
. Table S3: Comparison of levulinic acid hydrogenation reaction
1
by various reported catalysts.
NMR, FT-IR, CHN, and mass spectral analyses. RhTPP and RhTCPP
grafting on functional SBA-15 was confirmed by XRD, NMR,
adsorption-desorption and SEM− EDAX analyses. The resulting
homogeneous (RhTPP, RhTCPP) and heterogeneous RhTPP-SBA-AM
and RhTCPP-SBA-AM were used as catalysts under moderate
reaction conditions for the hydrogenation of biomass-derived LA in
10. Table S4: Conversion of various substrates using the RhTPP,
RhTPP−SBA-AM, RhTCPP and RhTCPP−SBA-AM Catalysts‡.
addition to various substrates. Homogeneous catalysts (RhTPP and References
RhTCPP) have yielded 100 % conversion of LA with selective
production of GVL as the major component. The heterogeneous
catalysts have facilitated formation of 1, 4-pentanediol. The 1. L. Cervený, Catalytic hydrogenation, Elsevier, 1986.
homogeneous RhTPP and RhTCPP yielded -valerolactone (GVL) as a
major product, whereas using heterogeneous catalyst gives
comparable -valerolactone (GVL) selectivity. However, the
heterogeneous catalyst could improve the reusability and
recyclability. The hydrophobic environment of RhTPP facilitate better
2
3
. J. G. d. Vries, The handbook of homogeneous hydrogenation.
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carboxylate ion in RhTCPP catalyst increases the hydrophilic
environment and facilitates the diol formation, whereas, RhTPP
shows -valerolactone as the major product even in the fifth run.
Thus, the rhodium porphyrin-based catalysts can act as promising
catalysts for the hydrogenation of biomass derived levulinic acid to
biofuel additive -valerolactone under moderate reaction conditions.
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2. J. M. Tukacs, D. Király, A. Strádi, G. Novodarszki, Z. Eke, G. Dibó,
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Acknowledgement
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Authors are thankful to CSIR for the financial support through the 14, 1260-1263.
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1. Figure S1: H NMR spectrum of RhTPP (CDCl
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, 400MHz).
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View Article Online
DOI: 10.1039/D0NJ01180K
Rhodium porphyrin molecule-based catalysts for the
hydrogenation of biomass derived levulinic acid to biofuel additive
γ-valerolactone
Kaiprathu Anjali , Naragalu J Venkatesha , Jayaraj Christopher , and Ayyamperumal
Sakthivel [a*]
[
a]
[b]
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
RhTPP and RhTCPP were prepared, utilized for
the conversion of levulinic acid to -valerolactone
and the reaction mechanism was proposed.