MONOLAYER-PROTECTED PLATINUM NANOPARTICLES AS HETEROGENEOUS
CATALYSTS FOR ORGANIC TRANSFORMATIONS
Rahul Nagarkar, Alkesh Ahire, Anirban Ghosh,* Sudhir Dapurkar, K. S. Nagabhushana, Nawal Kishor
Mal,* Rajiv Kumar
Tata Chemicals Ltd – Innovation Center, Pune 412108, Maharashtra
E-mail: anghosh@tatachemicals.com, nmmal@tatachemicals.com
Phone: +91-20-66549700; Fax: +91-20-66549735

Abstract
A novel heterogeneous catalyst was developed, comprising of platinum nanoparticles functionalized with sulfonic acidligands and supported on carbon. The nanocomposite catalyst system was characterized by XRD, TEM, XPS and TGDTA, and was further employed in the hydrogenation of D-xylose to xylitol with excellent activity and selectivity.

Keywords: platinum nanoparticles; xylose; hydrogenation; xylitol

1. Introduction
Metallic and semiconductor nanoparticles [NP]exhibit interesting size and shape-dependent
physicochemical properties. Incorporation of organic andbiomolecular moieties having intrinsic functionalities on
the surface of NPs will result in organic–inorganic hybrid
systems with novel characteristics, including (but notlimited to) catalytic efficiency. In recent years, monolayer
protected gold and platinum NPs have been emerging as
attractive candidates for catalysis [1,2]. The primaryreason is the facile separation of the NPs from theorganic reaction products by simple precipitation. The
chemisorbed alkanethiol monolayer can also be tailoredfor desired functionalities, e.g., sulfonic acid head groups
which can act as acid catalysts or proton conductors.
D-Xylose is a five-carbon monosaccharide which
can be metabolized into useful products, such as xylitol,using a number of microorganisms [3]. As an alternative
sweetener, xylitol is recommended for diabetics and forthe prevention of dental caries. Xylitol can also beproduced chemically from xylose using suitablehydrogenation catalysts [4]. In this report, Pt NPs
functionalized with sulfonic acid ligands and supported
on carbon were employed as solid catalysts in thechemical transformation of xylose to xylitol. A range ofkinetic parameters were obtained by varying the reaction
temperature, reaction time, reactant ratio and catalyst
weight. The reaction mixtures were analyzed by GC andHPLC. About 36% conversion and 99% selectivity
towards desired product were obtained using the novel
catalyst system. The activity and selectivity of thecatalyst was compared with those of an industrial Raney
Nickel catalyst.

2. Materials and Methods
Potassium hexachloroplatinate (K2PtCl6) and 3mercapto-1-propanesulfonic acid sodium salt (MPSA-Na+) were purchased from Sigma-Aldrich. Sodium
borohydride (NaBH4) and concentrated H2SO4 waspurchased from SD Fine Chem, India. Raney Nickel(assay 50% Ni and 50% Al) was obtained from Sisco
Research Lab, India. All chemicals were reagent grade
and were used as received.
Sulfonic acid ligand coated Pt NPs weresynthesized using K2PtCl6, MPSA and NaBH4 following amethod reported by us elsewhere [2]. To disperse the Pt
NPs on carbon support (40 wt% Pt/C), the colloidal NP
solution was treated with Vulcan XC72 at 60 °C for 1 h.
The terminal SO3– head groups were protonated bytreatment with 10 mL of 1 M H2SO4 (per 100 mg of thesolid catalyst) for 4 h at room temperature. Without thisprotonation step, no significant activity of the Pt NPs wasobserved.
Transmission
electronmicroscopy(TEM)
experiments were carried out on a JEOL 200CX
instrument used at 200 kV. The core sizes of the NPs
were determined using TEM images. The diameter ofapproximately 1000 monodispersed NPs per sample wasestimated and averaged using Image J software. The
size distribution was calculated using Origin Pro 8.1
software. Thermogravimetric analyses (TGA) were
performed on a TGA Q50 (TA Instruments) with a
Thermostar Mass Spectrometer (Pfeiffer Vacuum). Thesamples were equilibrated at 25 °C; then kept at 100 °C
for 2 h; then ramped to 600 °C at 30°/min; and finallydwelled at 600 °C for 1 h. X-ray photoelectron
spectroscopic (XPS) experiments were performed on aKratos Axis Ultra X-ray photoelectron spectrometer
employing a monochromatic Al Kα source (1486.7 eV)
and an electron takeoff angle of 90° relative to thesample plane. The survey scan was conducted at 80 eV
pass energy in the binding energy (BE) range of 0–1100
eV. The high-resolution scans of O 1s, C 1s, S 2p, and Pt
4f were conducted at 10 eV pass energy. The core level
spectra were background-corrected using the Shirley

7th International Symposium on Feedstock Recycling of Polymeric Materials (7th ISFR 2013)
New Delhi, India, 23-26 October 2013

algorithm and the core level BE was aligned with respect
to the Pt 4f BE of 71 eV.
The hydrogenation reactions were carried out in a
100 mL high pressure reactor manufactured by Amar
Equipments, India. In a typical experiment, 2.5–10 g ofxylose was dissolved in 50–100 mL of deionized water,and 50–800 mg of the solid catalyst was directly taken
into the reaction vessel. The reaction mixture wascontinuously stirred during the reaction at temperatures
ranging from 55–150 °C and H2 pressure ranging from
34–65 bars, for 3–8 h. The reaction products wereanalyzed at room temperature by HPLC. The
experiments were run on Waters HPLC equipped with aWaters pump (515), UV-Vis (2487) and RI (2414)
detectors using a Aminex HPX-87H column (300 × 7.8mm). The mobile phase used was 0.008 N H2SO4 with aflow rate of 0.6 mL/min. The elution was carried out witha total run time of 60 min.

4. Conclusions
In conclusion, the conversion of D-xylose to xylitol
has been successfully carried out with sulfonic acidfunctionalized Pt NPs. This work signifies the effect ofmorphology of the ligand shell on catalytic behavior ofmetallic NPs.

References[1] A. Ghosh, S. Basak, B. H. Wunsch, R. Kumar and F.
Stellacci. Angew. Chem. Int. Ed. 50 (2011) 7900–7905.
[2] A. Ghosh, F. Stellacci and R. Kumar. Catal. Today
198 (2012) 77–84.
[3] H. Cheng, B. Wang, J. Lv, M. Jiang, S. Lin and Z.
Deng. Microb Cell Fact. 10 (2011) 5.
[4] M. Yadav, D. K. Mishra and J.-S. Hwang. Appl. Catal.
A: Gen. 425–426 (2012) 110–116.

3. Results and Discussion
TEM was used for high resolution imaging of thenanocomposite samples and analysis of their particle
size distribution. The TEM images of all thenanocomposites reveal that the chosen synthesis
procedures produce similar core sizes of the NPs (4–5nm).
TGA was used to quantify the average amount ofligand present in the NP samples. The representative
TGA scan for a Pt-MPSA/C sample revealed a two stage
weight loss; the first at 200–300 °C, and the second at
300–450 °C. The approximate amount of organics was12–15 wt% for all the samples.
XPS was used to determine the electronic structureof the surface species accurately. The representative S
2p core level spectrum shows two components at binding
energies 163 and 168 eV, which can be attributed to two
different oxidation states of sulfur (–2 and +5,respectively) present in the sample. The quantification ofthe S5+ component from the S 2p spectra was done toestimate the number of catalytically active sites. As
expected, the Pt 4f spectrum has two components,
namely 4f5/2 and 4f7/2 at binding energies 74 and 71 eV,respectively.
The conversion of D-xylose to xylitol is a first order
reaction with respect to the concentration of substrate. In
all the experiments, the selectivity towards the desired
product was >99%. Some representative reaction results
were shown in Table 1. The maximum conversion ofxylose obtained was 36%. The same using an industrial
Raney Nickel catalyst under similar condition was <10.
Table 1. Transformation of D-xylose to xylitol using PtMPSA nanocatalyst.
XyloseCatalyst
Temp.
TimeConv.
:H2O(wt%)
(°C)
(h)
(%)
50.8
140
536
10
1.614052520
1.6140510

7th International Symposium on Feedstock Recycling of Polymeric Materials (7th ISFR 2013)
New Delhi, India, 23-26 October 2013