DISCRIMINATION OF ACIDIC SITES IN ANION MODIFIED METAL OXIDES – A DFT STUDY

K. JOSEPH ANTONY RAJ AND B. VISWANATHAN

FEB 16-18, 2010

INDO-HUNGARIAN WORKSHOP ON CATALYSIS

INTRODUCTION SYNTHESIS OF SULFATED Fe2O3-TiO2

ACIDITY MEASUREMENT FOR SFT, ST, PT BY NH3-TPD.

DETERMINATION OF ACIDIC SITES BY IR MEASUREMENT FOR PYRIDNE ADSORBED SAMPLES.

WILL DISCUSS THE OPTIMISATION OF PYRIDINE SULFATED METAL OXIDE STRUCTURES BY DFTTO UNDERSTAND THE ADSORPTION OF PYRIDINE

ON Fe / Ti / S / PTO UNDERSTAND WATER OR HYDROXYL GROUPS

ARE PRESENT IN THE STRUCTURE.

INTRODUCTION

Sulfated metal oxides were reported as effective catalysts for many catalytic reactions such as..

Esterification of oleic acid with glycerol Preparation of dioctyl phthalate Synthesis of butyl phenols Liquefaction of coal Isomerisation of n-butane, cyclopropane Polymerization of alkyl vinyl ethers

All these reactions were found to take place on sulfated metal oxides at a relatively lower temperature than over other catalysts (zeolites / mesoporous materials).

Therefore to understand the nature of acidic sites present on the surface of the catalyst and thereby to understand where the reactants are anticipated to get adsorbed on the catalyst before the formation of products.

CATALYTIC APPLICATIONS OF SULFATED Fe2O3 – TiO2

We have studied this catalyst (SFT) for reactions such as… – trans-esterification of vegetable oil– tert-butylation of phenol– Alkylation of C10-C13 olefins with benzene– Photo degradation of para-chlorophenol

SFT can be reused by treating with ammonium sulfate.

INTRODUCTION

Lee et al. J. Catal. 1989, 120, 46.

Saur et al. reported the generation of BrØnsted acid sites on sulfated Fe2O3, due to the presence ofmoisture in the sample.

Yamaguchi et al. showed the high catalytic activity of sulfated Fe2O3 is due to (i) the strong IR absorption band at 1375 cm-1, (ii) the presence of S(IV) in SO bonds, and (iii) covalent characteristics of SO bonds Tanabe et al. showed that the generation of strong acidity in sulfated metal oxides was due to the existence of covalent S=O bonds on the surface of metal oxides.

Therefore it is still an assumption that pyridine or reactants can adsorb on Fe / Ti / or S.

Jin et al. J. Phys. Chem. 1986, 90, 4194.

Saur et al. J. Catal. 1986, 99, 104; Yamaguchi et al. J. Phys. Chem. 1986, 90, 3148; Tanabe et al. Chem. Com. 1981, 602.

Hue et al. reported the enhancement in catalytic activity of sulfated TiO2 / ZrO2 / Fe2O3 on modification with Al2O3. The enhancement is essentially due to the formation Al-O-Zr and Al-O-Ti. These kind of linkages leads to greater activity and thermal stability of the catalysts.

Das et al. reported the preparation of sulfated TiO2-ZrO2 and its catalytic activity for isopropylation of benzene. The greater activity of the catalyst is attributed to enhanced acidity and surface area.

With this background we synthesized sulfated Fe2O3-TiO2 using ilmenite and sulfuric acid as the starting material.

??? WHY SULFATED MIXED OXIDES ???

Hua et al. J. Catal. 2000, 196, 104. Das et al. Appl. Catal. 2003, 243, 271.

SYNTHESIS OF SULPHATED Fe2O3-TiO2

10 g of ball milled ilmenite is taken in a 250 ml beaker

20 g H2SO4

Aged for 2 h at R.T.

10 g of water is added to initiate the rxn.

The rxn. mass is stirred for one hour

Washed with water to remove all the free sulphates

Dried at 100 oC for 12 h & calcined at various temperatures SULPHATED Fe2O3-TiO2

Composition of ilmenite: 55 wt.% of TiO2,

42 wt.% of Fe3O4, 2.9 wt.% of SiO2 & traces ofAl2O3, ZrO2, V2O5 & Cr2O3

TiOSO4 was used as the precursor for producing sulphated titania and phosphated titania.

References:– Surface area, Pore Size, and Particle Size engineering of Titania with

Seeding Technique and Phosphate Modification. K.Joseph Antony Raj, A.V.Ramaswamy, and B.Viswanathan. J. Phy. Chem. C 113 (2009) 13750.

– Single-step synthesis and structural study of mesoporous sulphated titania nanopowder by controlled hydrolysis process. K. Joseph Antony Raj, and B. Viswanathan. Applied Mat. & Interfaces (Oct-2009).

SYNTHESIS OF SULPHATED TITANIA AND

PHOSPHATED TITANIA

XRD PATTERNS OF ST, PT AND SFT

1. The peaks appeared at 2θ of 27.5, 36.1, and 41.3 which is characteristicof rutile phase.

2. The peaks at 32.8, 34.9, and 48.2 are due to the formation of FeTiO3

3. The peak at a 2θ of 31.1 is dueto sulfated Fe2O3-TiO2

4. The XRD pattern confirms the absence Of FeO in the sample

5. The XRD pattern for SFT also shows the presence of 13-15% of anatase phase in the sample.

6. The peaks at 25.4, 37.9 and 48.1 are due to anatase phase in ST and PT.

N2 ADSORPTION-DESORPTION ISOTHERMS OF PHOSPHATED TITANIA, SULFATED TITANIA & SULFATED Fe2O3-TiO2 SAMPLES CALCINED AT 500 oC

310 m2/g0.35 ml/g

275 m2/g0.34 ml/g

18 m2/g0.07 ml/g

The adsorption isotherm is classified as type IV for PT and and ST. Both PT and ST showed adsorption up to 0.45 relative pressure region which is characteristic of mesoporous material. While SFT showed type II isotherm which is characteristic of non-porous material. The BET surface area measured for the samples showed 310 m2/g for PT, 275 m2/g for ST and 18 m2/g for SFT. Similar to surface area, pore volume also found to be lowerfor SFT.

ACIDITY METHODS

There are various methods by which acidity can be measured for the solid acid catalysts

– Hammet indicator (Colored samples can’t be measured)

– n-butylamine TPD (Measures the total acidity)

– Ammonia TPD (Can measure moderate & strong acid sites)

– IR spectra for pyridine adsorbed samples (Can discriminate BrØnsted & Lewis acid sites)

Hammet indicator method can be adopted for moisture free and colorless samples. Sulfated Fe2O3-TiO2 is brown in color and also moisture could not be completely removed from the sample.

We have adopted NH3/TPD and Pyridine-IR spectroscopy method.

NH3-TPD PROFILES OF SFT AND PT SAMPLES

The acidity profiles obtained by ammonia-TPD for the SFTsamples calcined at 500 and 700 oC are presented in Fig.

SFT-500 showed three desorption peaks at 100, 160 and 310 oC. While SFT-700 showed a desorption peak at 100 oC.

The acidity values for these peaks are 0.11, 0.2 and 0.37 mmol/g. The desorption pattern obtained for SFT-500 shows the Strong adsorption of ammonia on SFT-500 and presence of medium and strong acid sites in the sample.

PT showed two desorption bands at 135 and 359 oC. The acidity for these peaks correspond to 1.7 and 0.18 mmol/g.

Sulfated titania showed desoprtion bands similar to that of SFT.

DRIFT SPECTRA OF THE SULFATED TiO2 / Fe2O3- TiO2 AND PHOSPHATED TITANIA SAMPLES CALCINED AT 500 oC AND 700 oC

The characteristics of sulfate species on the surface of Fe2O3-TiO2 during calcination were examined using DRIFT spectroscopy.

Fig. shows the DRIFT spectra of the SFT samples calcinedat 500 and 700 oC, the range displayed is between 700 and 1300 cm-1.

A peak was observed at 1160 cm-1 for SFT-500 which is due to asymmetric stretching of sulfate vibrations.

Peaks were also detected at about 840, 940, 1030 and 1210 cm -1. These bands are assigned to S=O and S-O, symmetric and asymmetric stretching frequencies, respectively.

The SFT sample calcined at 700 oC showed broad bands at 840 and 1170 cm-1 and no other bands in the region of 700 -1300 cm-1. This is attributed to the removal of 92 wt.% of sulfates from the sample when the calcination temperature is increased to 700 oC.

0.4% SO42-

5.1% SO42-

DRIFT SPECTRA OF THE SULFATED TiO2 / Fe2O3- TiO2 AND PHOSPHATED TITANIA SAMPLES CALCINED AT 500 oC AND 700 oC

The sulfated titania (ST-500)calcined at 500 oC for 2 hours showed a sulfate content of 5.3 wt.%.

ST-500 showed five bands in the region of 930 and 1200 cm -

1

An absorption band was observed at 1148 cm-1 which is due to asymmetric stretching of sulphate vibrations. Four more bands were detected at 940, 980, 1060, and 1105 cm-1. These bands are due to S=O and S-O, symmetric and asymmetric stretching frequencies. PT-500 showed IR bands at 840, 910, 960, 1040, 1130 and1190 cm-1 due to the interaction of P-O groups with the titaniumions to form P-O-Ti linkage.

0.4% SO42-

5.1% SO42-

5.3% SO42-

4.9% P2O5

DECYLATION OF BENZENE OVER SULFATE AND PHOSPHATE MODIFIED SAMPLES

CATALYSTBET-

SURFACE AREA, m2/g

ACIDITY, mmol/g

(NH3-TPD)

CONVERSION OF DECENE, wt.%

CONVERSION OF DECENE, wt.%

(WITH ~ 500 PPM WATER)

SULFATED TITANIA 275 0.60 5 8

SULFATED Fe2O3-TiO2

18 0.68 16 19

PHOSPHATED TITANIA

310 1.88 1.4 2.1

REACTION WAS PERFORMED AT 150 oC and 5 kg/cm2

1. Most of the sulfate is on the surface of SFT.2. Despite the non-porous characteristics of SFT showed greater conversion than the

porous ST and PT.3. Comparing the mesoporous ST and PT, ST showed higher conversion than PT, also the Leaching out of sulfate is faster than phosphates.4. The injection of 500 ppm of water was found to enhance the rate of the rxn. The leaching order of anions: SFT(58%) > ST(34%) > PT(12%)

DENSITY FUNCTIONAL THEORY

The geometries of the proposed structures were optimized by Density Functional Theory using Gaussian 03 software.

Becke’s three parameter exchange functional with Lee-Yang-Parr gradient-corrected correlation functional (B3LYP) in conjunction with the Los Alamos ECP plus DZ basis sets (Lanl2DZ).

The vibrational frequencies for the optimized structures and the Mulliken charges for the elements were determined.

PROPOSED STRUCTURES OF PYRIDINE ADSORBED ON SO4 / Fe2O3 -TiO2

a b c d

e f g h

1490 cm-1 1490 & 1510 cm-1 1480 & 1520 cm-1 1470 & 1530 cm-1

1480 & 1540 cm-1 1480 & 1540 cm-1 g = c h = a

IR FREQUENCIES OF THE PYRIDINE ADSORBED SFT STRUCTURES (a) – (f) OBTAINED BY DFT STUDIES AND DRIFT SPECTRA MEASURED FOR PYRIDINE

ADSORBED SFT (SFT-500 & SFT-700)

1. The SFT samples calcined at 500 and 700 oC showed sulfate contents of 5.1 and 0.4%

2. The DRIFT spectra obtained for SFT-700 revealed theabsence of IR absorption at 1480 & 1540 cm-1. This suggests that the presence of sulfates in the sample is essential for producing acidic sites in the sample.

3. Structure-a showed only Lewis acid sites at 1480 cm-1. While structures ‘b’ and ‘c’ showed major peakat about 1480 cm-1 due to Lewis acid sites. A shoulder and small peak is appeared at 1510 and 1520 cm-1 is due to the generation of small qty’s of Bronsted acidity.

4. Structure-d with water molecule adsorbed on itshowed a band at 1530 cm-1 due to Bronstedacidity and small peak at 1470 cm-1 due to Lewisacidity.

5. The IR frequencies obtained for structures ‘e’ & ‘f’ showed almost equal qty’s of Bronsted & Lewis acid Sites. However structure-f showed moderatelyHigher Bronsted sites and exactly matching IR patternWith that of the DRIFT spectra obtained for SFT-500.

IR FREQUENCIES OF THE PYRIDINE ADSORBED SFT STRUCTURES (a) – (f) OBTAINED BY DFT STUDIES AND DRIFT SPECTRA MEASURED FOR

PYRIDINE ADSORBED SFT (SFT-500 & SFT-700)

1. The sulfated metal oxides is anticipated to give IR absorption band at 1380 cm-1 due to the asymmetric vibration of the S=Obond which is used as a fingerprint for moisture free sulfated metal oxide.However, the DRIFT spectra obtained for the SFT samples showed no

peaks in the region of 1300 – 1380 cm-1 indicated the non removal of all the adsorbed hydroxyl groups or water molecules in SFT samples consequent to the vigorous calcination at 500 for 2 h in air.

2. Structures ‘a’, ‘b’ and ‘c’ anticipated to show a band at 1380 cm -1 due to the anhydrous nature however not showed peak in the region of 1200 – 1300 cm-1. This is not consistent with the reports on the peakat 1380 cm-1 for the anhydrous sulfated metal oxide samples.

3. As anticipated structure-d showed a peak at 1300 cm -1 due to adsorption of water on the structure. This peak was not shifted further to < 1300 could Be due to the presence of S=O in the structure.

4. Structures ‘e’ and ‘f’ with adsorbed hydroxyl groups showed peaks at1220 cm-1 due to the absence of S=O.

5. Although H2O is adsorbed on the structures ‘e’ and ‘f’, the presence of hydroxyl groups and the absence of S=O groups are essential in shifting the band to lower frequency.

Kayo et al. J. Catal. 1983, 83, 99; Lee et al. J. Catal. 1989, 120, 46.

TG-DTG PROFILE OF SFT, ST AND PT

SFT ST

PT 1. SFT showed a weight loss of 5.9% up to 220 oC due to the removal of water molecules.2. A weight loss of 1.5% was obtained between 385 and 475 oCwhich may be due to the removal of sulfates and hydroxyl groups. 3. The final weight loss was observed as two stages at 540 and 590 oC and the weight loss is 4.9% and 2.8%, respectively. 4. Therefore SFT showed four stages of weight loss which explains the strong adsorption of hydroxyl and sulfate groups on its surface. 5. The evaluation of structure-d and ‘f’ ( by DFT) shows that similar to sulfates water is also adsorbed on iron oxide, subsequently, sulfates tend to lose their covalent character due to the generation of hydroxyl groups on sulfur. These hydroxyl groups are the cause for the generation of equal qty’s of Brønsted and Lewis acid sites on SFT sample.

17% loss15% loss

2% loss

5.9%

1.5%

4.9%

2.8%

13.5%

3.5%

MULLIKEN CHARGES

There are reports which showed the adsorption of pyridine on either sulfur or Fe.

The IR frequencies obtained for various structures revealed the possibility of adsorption of pyridine on sulfur of the sulfated Fe2O3.

In addition, the Mulliken charges obtained for Fe (0.33), Ti (1.15), S (1.23), and N (-0.3) of the pyridine adsorbed sulfated Fe2O3 suggests a more strong interaction between S and pyridine than Fe and pyridine.

The charge on S and Ti is almost same that in sulfated titania both types of interactions may be anticipated.

f

PROPOSED STRUCTURES OF PYRIDINE ADSORBED ON PHOSPHATED TiO2 a b

c d

1470 & 1520 cm-1 1470 & 1520 cm-1

1470 & 1520 cm-1 1470 & 1530 cm-1

The various possible PT structures showed more intense Lewis acid bands than BrØnsted acid bands. The DRIFT spectra measured for the pyridine adsorbed PT samplesalso showed more intense Lewis acid bands than BrØnsted acid bands.

CONCLUSION

The pyridine adsorption-desorption studies revealed the presence of nearly same quantities of BrØnsted and Lewis acid sites for the samples calcined at < 500 oC.

The TG/DTA revealed the greater water and hydroxyl affinity for the SFT sample. The combination of Fe2O3 and TiO2 present in the catalyst strongly holds the sulphate

on its surface with temperature facilitating greater catalytic activity than the materials possessed higher surface area.

The DFT study on SFT showed that the adsorption of water on Fe can lead to the formation of BrØnsted acid sites. Therefore the presence of water is significant for enhancing the BrØnsted acid sites.

The generation of BrØnsted acid sites also suggests the lowering of covalent characteristics of SO bonds.

The theoretical studies shows the adsorption of H2O on Fe and adsorption of –OH and pyridine on S.

The comparison of catalytic activity for ST, PT & SFT showed that the presence of Fe enhanced the catalytic activity although the sulfate and phosphate contents are same.

The DFT study on the structures of pyridine adsorbed PT and DRIFT spectra study on PT samples showed more intense Lewis acid bands than the BrØnsted acid bands.

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