CHARACTERIZATION AND CATALYTIC PROPERTIES OF KEGGIN TYPE HETEROPOLYACIDS SUPPORTED ON DIFFERENT...

CHARACTERIZATION AND CATALYTIC PROPERTIES OF

KEGGIN TYPE HETEROPOLYACIDS SUPPORTED ON DIFFERENT

MOLECULAR SIEVES

A. POPA, V. Z. SASCA, O. VERDES, L. AVRAM

Institute of Chemistry Timişoara, Bl.Mihai Viteazul 24, Romania

• 1. coatings• 2. analytical chemistry• 3. processing radioactive waste• 4. separations• 5. sorbents of gases• 6. membranes• 7. sensors• 8. dyes/pigments• 9. electrooptics• 10. electrochemistry/electrodes

• 11. capacitors• 12. dopants in nonconductive

polymers• 12. dopants in conductive polymers• 13. dopants in sol-gel matrixes• 14. cation exchangers• 15. flammability control• 16. bleaching of paper pulp• 17. clinical analysis• 18. food chemistry

INTRODUCTION

The patent and referee literature survey indicates that the utilization of Keggin type polyoxometalates (POMs) continues to grow in many areas in addition to the traditional area of catalysis.

About 80-85% of patent and applied literature claims or investigates POMs for their catalytic activity.

The remaining 15-20% of the applications can be grouped in the following categories:

Categories of Applications for POMs (Derived from Patent Literature; Excluding Catalysis and Medicine)

• Heteropolyacids (HPA) with the Keggin structure are known to be highly active catalysts for both acid and oxidation reactions;

• Most frequently used are:

• 12-tungstophosphoric acid H3[PW12O40]xH2O (HPW),

• 12-molybdophosphoric acid H3[PMo12O40]xH2O (HPM) and

• 1-vanado-11-molybdophosphoric acid H4[PMo11VO40]yH2O (HPVM)

in consequence of their high catalytic activity in the selective oxidation of unsaturated aldehydes and the oxidation of low alcohols;

• One important drawback of Keggin-type HPA is their low specific surface areas, disadvantage which can be overcame by dispersing the HPA on high surface area supports;

INTRODUCTION

• The type of carrier and textural properties influence the thermal stability and the catalytic activity of Keggin-type heteropolyacids;

• In order to obtain highly dispersed heteropolyacids species, H3PMo12O40

and H4PVMo11O40 were supported

- by impregnation on high surface area molecular sieves supports

MCM-41 and SBA-15 types

• The catalytic properties of supported heteropolyacids were evaluated in reference to the bulk solid acids by a test reaction (ethanol conversion).

INTRODUCTION

EXPERIMENTAL

Preparation of pure and supported HPAs

• I. H4[PMo11VO40] (HPVM) was prepared by two methods: Tsigdinos method and hydrothermal method

Tsigdinos method - ether extraction of HPVM from solution

Hydrothermal method - HPVM was obtained from the constituent oxides (MoO3,

V2O5 şi H3PO4) at 80C

In both cases HPAs were crystallized slowly from aqueous solutions at room temperature. They are stable at room temperature with 12-14 H2O molecules.

H3[PMo12O40]·13H2O is a Merck commercial product.

• II. The HPA active phase deposition on the supports was performed by impregnation from water:ethanol=1:1 solution

The HPM and HPVM acids were deposited by impregnation in the concentration of 15 and 30 % loading.

In the present work five types of supports were used: Si-MCM-41, Fe-MCM41, Al-MCM41 and respectively SBA-15 and Ti-SBA15;

Methods

• Powder X-ray diffraction data were obtained with a XD 8 Advanced Bruker diffractometer using the Cu Kα radiation in the range 2θ = 0.5÷5 and 2θ = 5÷60;

• The I.R. absorption spectra were obtained with: Biorad FTS 60A spectrometer (spectral range 4000-600 cm-1) and Jasco 430 spectrometer in the 2000 - 400 cm-1 range, using KBr pellets.

• Micro-Raman spectra were recorded on a DXR Raman Microscope (Termo Scientific). The laser spot diameter on the sample was 1µm. Thermo Scientific OMNICTM software was used for spectra collection and manipulation

• Solid-state NMR experiments were recorded on a Bruker Avance DRX 500 spectrometer equipped with a CP MAS probe. Spectra were referenced to external NH4H2PO4 (δ31P = 0 ppm).

EXPERIMENTAL

Methods

• Textural characteristics of the outgassed samples were obtained from nitrogen physisorption at 77K using a Micrometrics ASAP 2000 or a Quantachrome instrument, Nova 2000 series instrument. The specific surface area SBET, mean cylindrical pore diameters dp and adsorption pore volume VpN2 were determined.

• Microstructural characterization by SEM-EDS of the catalyst particles was carried out with a Jeol JSM 6460 LV instrument equipped with an EDS analyzer.

• Surface morphology and the nanostructure were examined by AFM instrument, AutoProbe CP Research, TM Microscope. All measurements were performed in noncontact mod (NCM) using a rectangular silicon cantilever. The samples were pressed in pellets and than micrographs were recorded. For each sample, images of 2 x 2 μm, 5 x 5 μm and 10 x 10 μm were recorded.

• Catalytic experiments were carried out using a pulse microreactor, in the temperature range 483-563K. Six pulses of 3 μl of C2H5OH (2,4∙10-3g ethanol corresponds to 58.25 μmol) were introduced within 20 min. The products were analyzed gas chromatographs with columns filled with Porapak QS.

EXPERIMENTAL

Table 1 Support types and loading of the supported HPA prepared by impregnation method

Support SBET, m2/g Pore size, Å HPA, wt %

Si-MCM41 1104 25 15-30

Fe-MCM41 917 26.8 15-30

Al-MCM41 559 28.8 15-30

SBA-15 725 60 15-30

Ti-SBA-15 615 53 15-30

Keggin type heteropolyacids structure

Figure 1 Keggin unit structure represented by atoms arrangement and bonds

Figure 2 Keggin unit structure represented by triads of edge-linked

MoO6 octahedra and central thetraedra PO4

Figure 3 Flat cluster of four triads of edge-linked MoO6 octahedra formed by opening of Keggin anion

• At low coverage, the system HPA-silica may be considered as composed of KU partially ‘immersed’in the bidimensional hydration layer at the silica surface;

• At silica surface hydrolysis of Mo-Oc-Mo bonds (corner-linking the triads of MoO6 ochtahedra) take place, resulting in the opening of the Keggin anion structure;

• The formation of a flat structure composed of 4 triads linked together through Mo-Oc-Mo bridges and linked to silica surface by PO4 tetrahedron and by some corner oxygen atoms of the triads.

Keggin unit• At low coverage, the system HPA-silica may be considered as

composed of KU partially ‘immersed’in the bidimensional hydration layer at the silica surface;

• At silica surface hydrolysis of Mo-Oc-Mo bonds (corner-linking the triads of MoO6 ochtahedra) take place, resulting in the opening of the Keggin anion structure;

• The formation of a flat structure composed of 4 triads linked together through Mo-Oc-Mo bridges and linked to silica surface by PO4 tetrahedron and by some corner oxygen atoms of the triads

• As a result of these chemical interactions between the heteropolyanion and the support (silica, molecular sieves) it could be expected that the acidity of the supported HPAs diminished and such systems will show mainly redox properties.

5.0 50.010.0 15.0 20.0 25.0 30.0 35.0 40.0 45.02 Theta

1000

2000

3000

4000

5000

6000Counts

Figure 4 X-ray diffraction of HPVM 12-14 H2O

1 2 3 4 5 6

(200)(110)

(100)

cb

a

(a) Si-MCM41 (b) 15HPM/Si-MCM41 (c) 15HPVM/Si-MCM41

Inte

nsi

ty, a

.u.

2 Theta

Figure 5 a, b XRD patterns of Si-MCM41 (a) and Fe-MCM41(b) molecular sieve and supported HPAs

A. Popa et al., J. Optoelectron. Adv. Mater., 10(6), 1401 (2008)

0 2 4 6 8 10

(d)

(c)(b)

(a)

(a) Fe-MCM41 (b) 15HPVM/Fe-MCM41 (c) 30HPVM/Fe-MCM41 (d) 30HPM/Fe-MCM41

Inte

nsity

(a.

u.)

2 Theta

(100)

(110) (200)

• For Si-MCM41 and supported samples - the long-range order of the hexagonal mesostructure is mentained after impregnation of HPAs.

• The d100 diffraction peaks of calcined Fe-MCM41 are shifted towards lower values compared to its Si-MCM-41 analogue.

• It can be observed that the long – range order of Fe-MCM41 is decreased for loading of 30 wt. % HPAs.

1 2 3 4 5 6 7

(110)

(100)

c

b

a

(a) Al-MCM41 (b) 15HPM/Al-MCM41 (c) 15HPVM/Al-MCM41

Inte

nsi

ty, a

.u.

2 Theta

Figure 6, a, b XRD patterns of Al-MCM 41 and SBA 15 molecular sieves and supported HPAs


0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

(c)

(b)

(a)

(a) SBA-15 (b) 15HPM/SBA15 (c) 15HPVM/SBA15

Inte

ns

ity

(a

.u.)

2 Theta, ( )

(100)

(110) (200)

• The d100 diffraction peak of Al-MCM41 supported HPVM appear like a shoulder. • The long – range order of Al-MCM41 is decreased evidently even for loading of 15 wt. % HPAs.• The hexagonal structure of SBA 15 is confirmed by the presence of three diffraction peaks at low angles of 0.9º,

1.56º and 1.76º, corresponding to the Miller crystal planes (100), (110) and (200)• The d100 diffraction peak of SBA 15 supported HPMo and HPVMo are shifted towards lower values compared

to its SBA 15 analogue.

Infrared spectra of HPA and supported HPA samples

• A special stainless-steel IR cell, connected to a gas-flow system, was used for the „in-situ“ measurements. •The gas flow (Ar, CO, O2, H2) through the cell

was controlled using electronic flow controllers;•finely ground compounds were deposited as a film on KBr pellets.

•The specific absorbtion bands of the Keggin structure are: νas P-Oi 1060-1080 cm-1;

νas Mo-Ot, 960-1000 cm-1;

νas Mo-Oc-Mo, 840-910 cm-1;

νas Mo-Oe-Mo, 780-820 cm-1.

Figure 7 "In situ" IR spectra of HVPM calcined at different temperatures

Figure 8 IR spectra of Fe-MCM41 molecular sieve and supported HPAs


1400 1200 1000 800 600 400

467

562

799

870

960

1090

3

2

1

(1) Fe-MCM41 (2) 30HPM/Fe-MCM41 (3) 30HPVM/Fe-MCM41

Tra

nsm

ittan

ce

Wavenumber, cm-1

Figure 9 IR spectra of Al-MCM41 molecular sieve and supported HPAs


1400 1200 1000 800 600 400

467

799

900

956

1090

3

2

1

(1) Al-MCM41 (2) 15HPM/Al-MCM41 (3) 15HPVM/Al-MCM41

Tra

nsm

itta

nce

Wavenumber, cm-1

Figure 10 Infrared spectra of HPVM, Ti-SBA 15 molecular sieves and supported HPAs


1200 1000 800 600 400

780

865

960

1064 HPVM Ti-SBA15 30HPM/Ti-SBA15 30HPVM/Ti-SBA15

Tra

nsm

itta

nce

Wavenumber, cm-1

Figure 11 31P MAS NMR of parent HPM (a) and HPVM acids (b)

31P MAS NMR spectra of HPA and supported HPA samples

•The 31P MAS NMR spectra are very sensitive to to the Keggin structure symmetry and to its hydration state;The spectrum of crystalline HPM consists of two well-resolved peaks indicating the existence of two types of P: an intense and sharp peak at – 5.4 ppm (confirming the presence of the majority of phosphorus environment in hydrated structure of HPM acid) and a smaller one at – 5 ppm. The assignment of the smaller peak could be done to the regions of the sample having different degree of hydration.

Figure 12 31P MAS NMR of HPM supported on Si-MCM41 (a) and HPM supported on Al-MCM41 (b) • The two 31P NMR peaks are preserved but broadened slightly and shifted to low field;An additional resonance at – 1.7 ppm was evidenced in the spectrum of Si-MCM41 supported HPM. This peak is shifted to down-field indicating the existence of HPM crystallites with different numbers of water molecules lost during impregnation.Supporting the HPM on Al-MCM41 results in a slightly broadened of the main 31P NMR peak, but resonance was preserved in the spectrum at -5.2 ppm. The broadening of the 31P NMR peaks of supported samples can be attributed to a distortion of the HPAs symmetry compared with that in the crystalline form.

Figure 16 Nitrogen adsorption-desorption isotherms at 77 K (a) and pore size distribution (b) of HPA supported on Fe-MCM41

Texture of supported HPA determined by BET and BJH methods

0

100

200

300

400

500

600

0 0.2 0.4 0.6 0.8 1

relative pressure, p/po

vo

lum

e a

ds

orb

ed

, cc

/g adsorption

desorption

• For 30HPA/ Fe-MCM-41 one observe a type IV isotherm with a type H1 hysteresis loop in the high range of relative pressure. For the values of relative pressure higher than 0.8, condensation take place giving a sharp adsorption volume increase.

• This behavior indicates that this sample has a mesoporous character. Hysteresis loop type shows that HPVMo/ Fe-MCM-41 sample consists of clusters or compacts of approximately uniform spheres in fairly regular array.

• The pore size distributions were calculated by Barret-Joyner-Halenda (BJH) method applied to the desorption branches of the isotherms.

• A diminution of the pore size of molecular sieves was observed from 2.68 nm for Fe-MCM41 molecular sieve to 2.42 nm for 30% HPAs/Fe-MCM41 samples.

Figure 17 Nitrogen adsorption-desorption isotherms at 77 K (a) and pore size distribution (b) of HPA supported on SBA15

0

100

200

300

400

500

600

700

0 0.2 0.4 0.6 0.8 1

relative pressure, p/po

volu

me

adso

rbed

, cc/

g adsorption

desorption


• For SBA 15 molecular sieves and HPAs supported on SBA 15 one observe a type IV isotherm with a type H1 hysteresis loop in the high range of relative pressure;

• samples have a mesoporous character.

• The pore size distribution Barret-Joyner-Halenda (BJH) method applied to the desorption branches of the isotherms.

• The pore size distribution curves of SBA 15 support and HPMo and HPVMo supported on SBA 15 have narrow pore size distribution within mesopore range with a maximum at aproximatively 60 Å.

• The average pore size does not significantly change with HPAs impregnation only slightly variation of the values around the SBA-15 sample being observed.

Table 2 Structure and texture properties of MCM41 molecular sieves and supported HPAs

Sample XRD d100 d-

spacing (Å)

Unit cell, ao

(nm)

Surface area (m2/g)

Pore size BJHDes (Å)

Pore volume BJHDes

(cc/g)

Si-MCM41 37.40 4.32 1104 25.02 1.06

15% HPM/Si-MCM41

35.88 4.14 860 25.01 0.776

15% HPVM/Si-MCM41

36.03 4.16 888 25.07 0.770

Fe-MCM41 36.78 4.25 917 26.8 0.968

15% HPVM/Fe-MCM41

36.48 4.21 769.5 24.8 0.649

30% HPVM/Fe-MCM41

36.18 4.18 655 24.2 0.685

30% HPM/Fe-MCM41

36.18 4.18 439 24.2 0.562

Al-MCM41 42.58 4.92 559.3 28.8 0.82

15% HPM/Al-MCM41

41.25 4.76 408.5 26.9 0.56

15% HPVM/Al-MCM41

42.04 4.85 364.3 26.9 0.44

A. Popa et al., Applied Surface Science, 255 (2008) 1830-1835

Table 3 Structure and texture properties of SBA15 molecular sieves and supported HPAs

Sample XRD d100 d-

spacing (Å)

Unit cell, ao

(Å)

Surface area (m2/g)

Average pore diameter

BJHDes (Å)

Pore volume BJHDes

(cc/g)

SBA-15 98.1 113.3 725.6 60.1 1.19

15% HPMo/ SBA-15

92.0 106.2 534.3 59.7 0.92

15% HPVMo/ SBA-15

90.1 104.0 550.9 60.1 0.91

Ti-SBA-15 94.9 109.6 615.1 53.0 1.19

30% HPMo/ Ti-SBA-15

90.1 104.1 267.8 35.9 0.39

30% HPVMo/ Ti-SBA-15

91.9 106.1 345.3 34.4 0.42

Figure 18 SEM micrografs of : a) Si-MCM41 (x20000); b) 15HPM/Si-MCM4 (x10000)

Texture of supported HPA determined by SEM and EDS


Figure 19 SEM micrografs of (a) SBA15 (x20000) and (b) 15HPVM/SBA15 (x20000);


Figure 20 SEM micrografs of: a) Ti-SBA15 (x10000) and (b) 15HPVM/Ti-SBA15 (x10000);


Figure 21 Microanalytical data of a 10x10 μm area and quantitative results of 15HPMo/SBA-15

Quantitative results

Wei

ght%

0

10

20

30

40

50

60

O Si P Mo

Figure 22 Microanalytical data of a 10x10 μm area and quantitative results of

15HPVMo/ SBA-15 (b)

Quantitative results

Wei

ght%

0

10

20

30

40

50

60

O Si P V Mo

Table 4 . Elemental analysis (wt.%) by EDS method of HPAs supported on molecular sieves

Sample

Elemental analysis (wt.%)

Mo P V

exp. stoich. exp. stoich. exp. stoich.

15HPM /SBA-15 10.4 9.5 0.31 0.25 - -

15HPVM /SBA-15 9.4 8.9 0.28 0.26 0.39 0.42

30HPM /Ti-SBA-15 17.1 18.9 0.37 0.50 - -

30HPVM /Ti-SBA-15 17.3 17.8 0.49 0.52 0.85 0.86

15HPM/Si-MCM41 9.3 9.5 0.29 0.25 - -

15HPVM/Si-MCM41 9.2 8.9 0.24 0.26 0.45 0.42

15HPM/FeMCM41 9.1 9.5 0.21 0.25 - -

15HPVM/FeMCM41 8.4 8.9 0.32 0.26 0.25 0.42

30HPM/FeMCM41 17.8 18.9 0.42 0.50 - -

30HPVM/FeMCM41 16.8 17.8 0.45 0.52 0.79 0.86

15HPM/Al-MCM41 8.8 9.5 0.28 0.25 - -

15HPVM/Al-MCM41 9.1 8.9 0.28 0.26 0.35 0.42


Texture of supported HPA determined by AFM

Figure 23 3D AFM images (2 x 2 μm) of Si-MCM41 (a), 15HPM/Si-MCM41 (b)


AFM features show that nanoclusters of HPA-molecular sieves compositesof various heights were formed on the surface.

The morphology of the pure molecular sieves and HPA-molecular sieves composites surface, respectively, has an island-like structure.

Figure 24 3D AFM images (2 x 2 μm) of SBA-15 (a) and 15 HPVM/SBA-15 composites (b)

Figure 25 2D AFM images (2 x 2 μm) of Fe-MCM41 (a), and SBA-15 composites (b)

The roughness average Ra is the arithmetic average of the absolute distances of the surface points from the mean plane.

It is the most frequently used surface roughness parameter giving information about the statistical average properties.

Table 5. Roughness amplitude parameters of molecular sieves and HPAs supported on molecular sieves.

Sample Roughness amplitude parameters (2 x 2 μm)

Ra [nm] RMS [nm] Avg. height [nm]

Max. range [nm]

SBA-15 30.8 37.4 123.5 249.2

Si-MCM41 5.4 7.11 40.6 80.8

Fe-MCM41 5.8 7.5 44.3 67.0

Al-MCM41 6.7 8.6 33.3 73.7

15HPVM/SBA-15 12.9 16.2 65.4 114.9

15HPM/Si-MCM41 16.3 20.3 63.6 150.0

15HPVM/FeMCM41 18.4 23.0 83.9 159.7

15HPM/Al-MCM41 11.8 15.2 52.8 114.1

Ra - The roughness average

RMS - root mean square

INTERMEDIATE CONCLUSIONS

• Solid heteropolyacids with Keggin structure deposited on different supports have been prepared by impregnation from water:ethanol solution. The concentration of active phase is varying from 10 to 30 wt.%. as a function of texture properties of support;

• HPAs anions preserved their Keggin structure on the MCM41 and SBA-15 surface and forms finely dispersed HPAs species.

• A relatively uniform distribution of HPA on the support surface is observed for all compositions of active phase. No separate crystallites of the bulk phase of HPA were found in the SEM images;

• The Fe-MCM41 host material suffers structural distortions at higher loadings, which leads to a loss in long range order;

• All Si-MCM41, Fe-MCM41 and Al-MCM41 supported HPAs exhibit differential pore size distribution in the mesoporosity range. The surface area and pore size diameter of supported HPAs decreases with increase in HPAs loading due to the partial blockage of the mesopores of the molecular sieve with particles of active phases;

Catalytic properties of supported heteropolyacids evaluated from ethanol conversion by pulse reaction technique

Catalytic experiments were carried out using a pulse microreactor in the following conditions:

• 300-500 mg catalyst;

• temperature range: 483-563K

• six pulses of 3 μl (2,4∙10-3g ethanol corresponds to 58.25 μmol) were introduced within 20 min;

• TCD and FID detectors with Ar and N2 respectively, as carrier gases;

• 2 gas chromatographs Perkin Elmer 900 type with columns filled with Porapak QS (80-100 mesh)

• catalysts reoxidation with air stream

Figure 26 Fix-bed microreactor : 1. Stainless steel microreactor; 2. termocuples;

3. tightness devices; 4. input of reactants; 5.output of reaction products

Ethanol conversion reactions

Selectivities of reaction products

Figure 27 Quantities of acethaldehyde for ethanol conversion on HPVM and Fe-MCM41-supported HPVM

Acethaldehyde quantities on HPVM

0

2

4

6

8

10

12

1 2 3 4 5 6

Pulse number

Mic

rom

ole

s

483K

503K

523K

543K

563K

Acethaldehyde quantities on HPVM/Fe-MCM41

0

5

10

15

20

1 2 3 4 5 6

Pulse number

Mic

rom

ole

s

503K

523K

543K

As a result of these chemical interactions between the heteropolyanion and the support (silica, molecular sieves) it could be expected that the acidity of the supported HPAs diminished and such systems will show mainly redox properties.

The Kinetics of reaction products formation

• Reaction rate of the main reaction products (acetaldehyde, ethylene and diethyl ether) was calculated by:

where: rPR - reaction rates; nPR – average of produts micromoles obtained during pulses 3 and 6 at temperature Tt; mcat – weight of catalyst, g; τc – reaction time, s;

• According to Mars-van-Krevelen model for oxidation reactions, the reaction rate for acetaldehyde formation is: :

rALAC = k · PETOL (1-θ)

where: k- reaction rate constant for oxidation of Et-OH to acetaldehyde; PETOL - partial pressure of Et-OH; θ - active sites in reduced state.

• Apparent activation energy, Ea and pre-exponential factor, A were calculated with Arrhenius equation:

ln k =ln A – E/R*T

P

1

273

T

m

nr

t

ccat

PRPR

Table 6 The kinetic data for acetaldehyde formation on supported and unsupported heteropolyacids

Sample Temperature

(K)

Fresh catalysts After reoxidation

rACA·106

(mol·g-1·s-1)

Ea

(kJ/mol)

A(s-1)

rACA·106

(mol·g-1·s-1)

Ea

(kJ/mol)

A(s-1)

HPM 483503523543563

2.373.566.19

13.6714.89

56.6 3.14·106

3.825.386.50

12.2314.11

38.2 5.15·104

HPVM 483503523543563

2.482.987.098.76

10.55

44.9 1.8·105

4.075.768.739.48

16.18

36.6 3.82·104

HPM/Fe-MCM41

503523543

50.1768.777.8

24.9 2.05·104

59.5070.7479.46

28.8 5.87·104

HPVM/ Fe-MCM41

503523543

60.3381.7295.24

25.8 3.09·104

74.8680.8899.25

15.8 3.29·103

HPM/Al-MCM41

503523543

47.663.1995.53

39.2 5.97·105

43.862.688.9

40.9 7.98·105

HPVM/Al-MCM41

503523543

41.7356.20

102.20

50.3 7.28·106

36.8958.75125.46

68.9 5.36·108

A. Popa et al., Applied Surface Science, 255 (2008) 1830-1835

• At 270ºC the reaction rates constants present the following decreasing values :

HPVM/Al-MCM41 > HPM/Al-MCM41 > HPVM/Fe-MCM41 > HPM/Fe-MCM41 > HPM > HPVM.

Figure 28 Correlation between lg A vs. activation energy for acetaldehyde formation; A-the dots correspond to: HPM, HPVM fresh and reoxidated,

B-the dots correspond to HPM/Fe-MCM41, HPVM/Fe-MCM41 fresh and reoxidated,C-the dots correspond to HPM/Al-MCM41, HPVM/Al-MCM41 fresh and reoxidated

y = 0.0961x + 1.0227

R2 = 0.9952

y = 0.0955x + 1.9959

R2 = 0.9938

y = 0.1001x + 1.8282

R2 = 0.9998

0

2

4

6

8

10

0 20 40 60 80

Ea, kJ/mol

Lg A

A

B

C

Linear(A)Linear(B)Linear(C)

CONCLUSIONS

• The ethanol conversion proceeds by two main pathways: an oxidehydrogenation reaction on redox catalytic centres, and a dehydration reaction on acidic centres, respectively. Al-MCM41 and Fe-MCM41-supported HPAs have showed mainly redox properties and therefore the reaction product obtained in more important yield is acetaldehyde

• The favorable effect of supports results from the examination of reaction rates of HPA reduction process by ethanol conversion. At 270ºC the reaction rates present the following decreasing values :HPVM/Al-MCM41 > HPM/Al-MCM41 > HPVM/Fe-MCM41 > HPM/Fe-MCM41 > HPM > HPVM.

• The favourable effect of HPA deposition on Fe-MCM41 support for oxidehydrogenation pathway to acetaldehyde results from the apparent activation energies and reaction rate values. An evidenced decreasing of Ea was obtained for acetaldehyde formation in the case of supported HPM (24.9 kJ/mol) comparatively with pure HPM (56.6 kJ/mol);

• Vanadium placed in the anion of HPA increased extent of oxidative dehydrogenation of Et-OH to acetaldehyde

• A "compensation effect" was evidenced from the relationship between pre-exponential factor A and apparent activation energies Ea for acetaldehyde, ethylene and ethyl ether formation.

Acknowledgements

• P. BarvinschiFaculty of Physics, West University of Timisoara

• J. Halasz University of Szeged, Dep.of Applied and

Environmental Chemistry

• Erne E. Kiss, Radmila Marinkovic-Neducin, Milos T.Bokorov

University of Novi Sad, Faculty of Technology

• Ivanka Holclajtner-AntunovićUniversity of Belgrade, Faculty of Physical

Chemistry

CHARACTERIZATION AND CATALYTIC PROPERTIES OF KEGGIN TYPE HETEROPOLYACIDS SUPPORTED ON DIFFERENT...

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