TiO2 supported Pt based bimetallic

nanocatalysts for selective hydrogenation

of citral

S.A. Ananthan , R. Suresh, K. Giribabu, R. Manikandan and V. Narayanan Department of Inorganic Chemistry, University of Madras,

Guindy Maraimalai Campus, Chennai 600 025, India

NATIONAL CONFERENCE ON CHEMISTRY SOLUTIONS (NCCS-2013

The catalytic hydrogenation of α,β-unsaturated aldehydes to their corresponding alcohols are used in the fine chemical industries, such as for the production of pharmaceuticals, detergents, cosmetics, flavors and fragrances.

The nanoparticles for the liquid phase hydrogenation of citral to unsaturated alcohols are mostly based on supported noble metals.

The selectivity towards α,β-unsaturated alcohols (geraniol and nerol) could also be increased by nature of precious metals, supports and particle sizes.

In the case of these metals are deposited on reducible support as TiO2, the hydrogenation of the C=O bond can be promoted due to the presence of partially reduced species generated upon reduction at higher temperature , that is called (Strong Metal -Support Interactions ) SMSI effect.

TiO2 supported Pt based bimetallic nanocatalysts have been successfully used to selective hydrogenation of citral, and got a desirable product

Among other factors, the electronic effects of the support, the presence of a second metal, and the metal particle sizes and morphology, could enhance selectivity. Considering the better selectivity presented by Pt-Ru catalysts, these metals were used as active phases.

Introduction

Aim and Scope

The preparation of TiO2 supported Pt-Pd, Pt-Ru and Pt-Au Nano catalysts by the impregnation method, being reduced in two different temperatures, 375ºC and 575ºC.

Citral is an α,β-unsaturated aldehyde including conjugated C=C and C=O bonds as well as isolated C=C bond.

The catalysts are characterised by BET surface area,, XRD, SEM,. HRTEM and XPS techniques.

Their catalytic activity is studied for selective hydrogenation of citral towards α,β-unsaturated alcohols (geraniol and nerol).

Nano catalysts are eco-friendly with cost effective for industrial applications.

Reaction Scheme

H2PtCl6.6H2O (0.1992 g) and PdCl2 (0.125 g) + H2O (20 mL) Stirred 30 min

TiO2-Anatase (9.85 g) + H2O (50 mL) stirred 30 min

Mixed together and stirred for 4 h

Ice cooled aqueous solution of NaBH4 (0.4116 g in 10 mL water) was added

Pt-Pd/TiO2575 Pt-Pd/TiO2375

Stirred for 3 h

Heated at 90 °C for 24 h

Separated by filtration and washed with H2O at 60 °C and dried

Calcined at 400 °C for 8 h

As prepared Pt-Pd/TiO2

Thermal Reduction at 375 and 575 °C

for 4 h/N2

Preparation by Impregnation method

Characterisation of Nanocatalysts

Surface Area (BET)

Morphology

(SEM,TEM)

Nature of Phase (XRD,TEM,TPR)

Surface Composition (XPS, EDS)

Particle Size & Shape

(TEM, SEM, XRD)

Overall Composition (XRD, ICP)

Study of Nanocatalysts

(a) Pt-Pd/TiO2 (b) Pt-Ru/TiO2 and (c) Pt-Au/TiO2 bimetallic catalyst

(a) (b)

(c) A sharp adsorption and desorption steps followed by a level at high P/Po ,which is characteristic of capillary condensation and evaporation in the pores, are clearly observed.

N2 adsorption-desorption measurements for TiO2 supported bimetallic nanocatalysts

While a hysteresis loop close to H1-type is observed for the TiO2 support. The hysteresis loop is changing with the noble metals incorporation. Then, evolution from H1-type to H2-type is observed.

Continue…

aICP method bSpecific surface area deduced from the isothermal analysis at liquid nitrogen temperature in the relative pressure range from 0.05 to 1.00. cTotal pore volume. dPore diameter calculated using BJH method.

Nanocatalysts Metal loading

(mol %)a SBET

(m2/g)b Dpore (nm)c

VPore (cm3/g)d

Pt Pd Ru Au

Pt-Pd/TiO2375 0.8 0.7 - - 46 18.1 0.27

Pt-Pd/TiO2575 0.9 0.6 - - 44 20.3 0.22

Pt-Ru/TiO2375 0.8 - 0.7 - 49 17.4 0.21

Pt-Ru/TiO2575 0.7 - 0.8 - 45 21.2 0.15

Pt-Au/TiO2375 0.8 - - 0.7 58 18.2 0.18

Pt-Au/TiO2575 0.9 - - 0.7 53 19.8 0.17

Pore size (Dp) and pore volume (Vp) are decreased with the increase in reduction temperature of the prepared nanocatalysts. Further, the nanocatalyst exhibit decrease in surface area at higher temperature reduction than at lower temperature. The decrease of the pore volume is more marked as shown.

(a) (b)

(a) Pt-Pd/TiO2 (b) Pt-Ru/TiO2 and (c) Pt-Au/TiO2 bimetallic catalyst (c)

A sharp adsorption and desorption steps followed by a level at high P/Po ,which is characteristic of capillary condensation and evaporation in the pores, are clearly observed.

N2 adsorption-desorption measurements for TiO2 supported bimetallic) nanocatalysts

While a hysteresis loop close to H1-type is observed for the TiO2 support. The hysteresis loop is changing with the noble metals incorporation. Then, evolution from H1-type to H2-type is observed.

XRD of TiO2 supported bimetallic nanocatalysts

Figure A shows that Pt-Pd/TiO2375 and Pt-Pd/TiO2575 nanocatalysts, which exhibit diffraction peaks at 25.5°, 37.7° and 48.2° that are consistent with the (101), (004) and (200) is characteristic planes (JCPDS no.: 84-1286) of anatase TiO2 also exhibit other peaks at 39.9°, 46.3° and 67.45° with corresponding planes of (111), (200) and (220) is characteristic of Pt (JCPDS 04-802) and the peaks at 40.1°, 46.7° and 68.1° with corresponding planes of (111), (200) and (220) of Pd (JCPDS 05-0681) face centered cubic phase.

Figure B shows that Pt-Ru/TiO2375 and Pt-Ru/TiO2575 nanocatalysts, the diffraction peaks at 38.3°, 42.2°,44.0°, 58.3°, 69.5°, and 78.4° with corresponding to (100), (002), (101), (102), (110) planes is due to (JCPDS 06-0663) of hexagonal close packed (hcp) Ru. Furthermore, the diffraction peaks corresponding to Pt and TiO2.

Figure C shows Pt-Au/TiO2375 and Pt-Au/TiO2575

nanocatalysts. The diffraction peaks 38.1°, 44.4°, 64.3°, and 77.5° corresponding to (111), (200), (220), (311) and (112) planes are due to fcc phase (JCPDS card no. 06-0663) of Au. Further, the diffraction peaks corresponding Pt and TiO2 is also found.

Morphology Image of supported Pt-Ru/TiO2nanocatalyst reduced at HTR(575°C)

(a) SEM

The shape and size of the particles studies were carried out using SEM and HR-TEM the images reveal that the catalysts reduced at HTR of Pt-Ru/TiO2

Length ~500 nm

Diameter ~50-100nm

Its show that the particles were spherical–like irregular particles with the average diameter is in 50-100 nm scale range. For the nanocatalysts, thermally reduced at higher temperature relatively larger particles are observed due to agglomeration at sintering process.

The EDAX spectrum shows the presence of Pt, Ru along with Ti and O.

HR TEM

(b) EDAX

TEM Image of supported Pt-Pd/TiO2 nanocatalysts

(a) TEM image (b) SAED pattern of Pt-Pd/TiO2375

(a) TEM image (b) SAED pattern of Pt-Pd/TiO2575

(a) (b)

(b) (a)

Increasing the reduction temperature from 375 to 575°C, the average particle sizes also increased.

SAED patterns show that the nanocatalysts are crystalline in nature.

Reduction at higher temperatures lead to significant increase in size and agglomeration of nanoparticles at the exterior surface compared to the catalysts reduced at lower temperature.

(a) (b)

XPS of TiO2 supported Pt-Pd Nanocatalysts

(a) Pt 4f core level spectra (b) Pd 3d core level spectra

Pt 4f shows two sets of peaks with binding energies of Pt 4f7/2 at 70.24 eV and 4f5/2 at 73.58 eV, suggest that the Pt is present in the zerovalent state and same peaks is observed in HTR (Pt 4f7/2 at 70.27 eV and 4f5/2 at 73.65eV) suggest the existence of Pt0 species on the surface.

Pd 3d shows two sets of peaks with binding energies of Pd 3d5/2 at 335.22 eV and 3d3/2 at 340.57eV, suggest that the Pd is present in the zerovalent state same peaks were obtained at HTR (Pd 3d5/2 at 335.55 eV and 3d3/2 at 340.99 eV).

(a)

O1s

(b)

Ti 2p core level spectra O1s core level spectra

This evidenced the existence of partially reduced surface states (Ti3+) with the peak position at 457.74 eV, while the binding energy of Ti4+ is located at 458.52 eV. The formation of Ti3+ implies the occurrence of strong interaction between Pt species and TiO2 matrix during the reduction reaction

The spectra indicate the presence of O2- (530.11) and OH- (531.52). Regarding the Ti species, curve fitting indicated the presence of Ti3+ in addition to the dominant Ti4+ peak.

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Ru 3d

C 1s

Au 4f

(a) Ru 3d core level spectra of Pt-Ru/TiO2375 °C

(a) (b)

Au 4f7/2 : 82.35 eV Metallic Au(0) 4f5/2 : 85.92 eV

(b) Au 4f core level spectra of Pt-Au/TiO2375 °C

The core level spectrum of Ru 3d spectrum has been obscured by the C 1s (286.6 eV) spectrum, but the deconvoluted spectrum shows a doublet with peak binding energies of Ru 3d5/2 : 282.29 eV Metallic Ru(0) 3d3/2 : 286.1 eV

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FI-IR spectra of bimetallic nanocatalysts

(a) Pt-Pd/TiO2, (b) Pt-Ru/TiO2 and (c) Pt-Au/TiO2 nanocatalysts reduced at (I) 375 °C and (II) 575 °C

(I)

(II)

Ti-O-Ti symmetric stretching vibration is 750 cm-1 , Ti-OH bending vibration is 985 cm-1 . The band at 1114 cm-1 is due to Ti-O stretching vibration. The presence of band at 1633 cm-1 corresponding to OH shows that a large number of OH groups (H2O) exist on the surface of TiO2. It is observed that the narrow and more intense peaks at 1074, 964, and 816 cm−1 in Pt-Pd/TiO2 are due to stretching vibrations of Ti-O-Ti bond respectively. Similar features were observed for the other two bimetallic catalysts also. The nanocatalysts reduced at higher temperature shows less intense OH bands compared to catalysts reduced at lower temperature.

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M/s Shimadzu by Japan, it

contains atuosampler, FID

detector and chemsolution

software with newly updated

version library chemicals

and pharmaceutical.

GCMS-QP2010 Plus

Hydrogenator and GCMS equipment

Nanocatalysts testing

(Citral) in - (Citral ) out

Conversion (mole %) = x 100

(Citral) in

(Product) out

Yield of Desired product (mole %) = x 100

(Citral) in

(Product) out

Selectivity of Desired product (mole %) = x 100

(Citral)in - (Citral) out

Number of moles of citral

passed per second X fractional conversion

Turn over frequency = ------------------------------------------------------ Number of surface Pt sites

(1)

(4)

(3)

(2)

GCMS-QP2010 Plus

Citral is a terpenoid family and usually have two isomers cis and trans It is prepare by isoprene rout (Petrochemical industries) Commercially produced only two manufacturers in world (Kurray and BASF) It is key raw material for Vitamine

Peak identification by GCMS

Identification with products selectivity by GCMS

Geraniol and Nerol (GOL & NOL

α & β - Isomers of Citronellol (CNO)

Citronellol (CNO)

Di-methyl Octanol (DMO)

Nanocatalysts Conversion % Selectivity (%)

GOL &NOL

CNA CNO DMO ISOP Others

Pt-Pd/TiO2375 50 42 12 3 2 6 15

Pt-Pd/TiO2575 55 50 18 4 5 10 13

Pt-Ru/TiO2375 70 68 8 5 3 6 10

Pt-Ru/TiO2575 80 78 7 5 3 6 5

Pt-Au/TiO2375 72 38 13 10 8 17 14

Pt-Au/TiO2575 55 42 18 5 6 14 15

Hydrogenation of citral over TiO2 support various bimetallic nanocatalysts

** Conversion, selectivity towards unsaturated alcohols as a function of Pt based bimetallic nanocatalysts thermally reduced at two different temperatures 375 and 575 °C on 90 °C, 10 MPa, 750 rpm, IPA solvent, 1.5% metal content, and m= 250 mg.

Conversion and selectivity towards unsaturated alcohols as a function of reduction temperature of catalyst (a) 375 and (b) 575 °C at 90 °C, 10 MPa, 750 rpm, IPA solvent, 1.5% metal content, and m= 250 mg.

Selectivity of α, β-unsaturated alcohols

.

In the Pt-Ru/TiO2 nanocatalyst the contact between Pt and Ru atoms favors the partial electronic transfer from Ru0 towards Pt0 due the difference in electronegativity.

In this way, there is +δ charge density in the Ru atoms and a –δ charge density in the Pt atoms.

Eventhough, Ru, Pd and Au have the same electronegativity, Ru is a hard acid, whereas Pd and Au are soft acids.

Considering a molecule of α, β-unsaturated aldehyde, it is accepted that the C=O group could be adsorbed on to the Ru metal surface due to partial negative charge (–δ) on the O atom. Which favours the hydrogenation of the C=O group resulting in the formation of unsaturated alcohols. The mode of adsorption of citral over Pt-Ru/TiO2 catalyst is shown in the Scheme.

Effects of Ruthenium promoter on Pt catalysts

(a) Schematic representation adsorption mode of citral

Conclusions

Nanocatalysts of Pt based bimetallic (Pt-Pd, Pt-Ru and Pt-Au) supported on TiO2 were prepared by impregnation and reduced either at 375 °C and at 575 °C .

The catalysts are characterized by TEM and XRD and the particles are well

dispersed; Pt,Pd and Au are having fcc and Ru is having hcp structure. XPS analysis revealed that the catalysts contain only zerovalent Pt and Au.

The bimetallic nanocatalysts are able to generate reactive TiO(2−x) species after

reduction at high temperature (575 °C), leading to a SMSI effect comparable or superior to that on bimetallic supported on bulk titania.

The SMSI effect shows that the catalysts reduced at higher temperature leads to an

increase in selectivity toward unsaturated alcohols (GOL & NOL) for Pt-Ru/TiO2 compared to Pt-Pd/TiO2 and Pt-Au/TiO2 nanocatalysts.

In addition a second metal (Ru) also leads to an increase of the GOL & NOL

selectivity during hydrogenation. The generated partially oxidized second metal species due to the difference in electronegativity, strongly binds the C=O group and also paves the way for the selective activation of the C=O bond. Metal support interaction and promoter active metal interaction increased the unsaturated alcohol formation.

Among these catalysts 80% of conversion and 78% of selectivity of α,β-unsaturated

alcohols (geraniol and nerol) is obtained with Pt-Ru/TiO2. These environment friendly nano catalysts can be recycled for more than 12 cycles

and the activity is retained upto 85%.

Reference: [1] P. Claus, Top. Catal. 5 (1998), 51 [2] G.V. Smith and F. Notheisz, Heterogenous Catalysis in Organic Chemistry, Academic Press, San Diego (1999). [3] P. Gallezot and D. Richard, Catal. Rev. Sci. Eng. 40 (1998), 81. [4] V. Ponec, Appl. Catal. A 149 (1997), 27. [5] B. Coq, F. Figueras, P. Geneste, C. Moreau, P. Moreau and M. Warawdekar, J. Mol. Catal. 78 (1993), 211 [6] P. Reyes, H. Rojas, G. Pecchi, JLG. Fierro, J. Mol. Catal A 179 (2002), 293 [7] S. Mukherjee, M. Albert Vannice, J. Catal. 243 (2006), 108 [8] J. Alvarez-Rodrıguez, A. Guerrero-Ruiz, I. Rodrıguez-Ramos, A. Arcoya-Martın, Microporous Mesoporous Mater. 97 (2006), 122 [9] IM. Vilella, I. Borbath, JL. Margitfalv, K. Lazar, Miguel de, OA. SR,Scelza, Appl. Catal. A 326 (2007), 37 [10] G. Neri, C. Milone, A, Donato, L. Mercadante, AM. Visco (1994) J. Chem. Tech. Biotechnol. 60 (1994), 83

Acknowledgement: We acknowledge the TEM and XPS facilities provided by the National Centre for Nanoscience and Nanotechnology, University of Madras. Also thanks to SAIF IIT Madras.