V. Z. SASCA a, Orsina VERDES a, Livia AVRAM a, A. POPA a A. ERDÕHELYI b and A. OSZKO b a Institute of Chemistry Timisoara-Romanian Academy b Institute

V. Z. SASCAa, Orsina VERDESa, Livia AVRAMa, A. POPAa A. ERDÕHELYIb and A. OSZKOb

a Institute of Chemistry Timisoara-Romanian Academyb Institute of Physical Chemistry and Material Science-University of Szeged

ETHANOL CONVERSION ON CsxH3-xPW12O40 CATALYSTS AND

THEIR MICROSTRUCTURE

The PROJECT “CATBIOETAMETCON”, HURO 901/090/2.2.2,

SEMINAR, 24-25 March 2011, SZEGED

INTRODUCTION The heteropoly compounds-HPCs offer good opportunities for

catalysts design as result of: - the control of acidic and redox properties in a large scale of values by choosing the constituent elements of anion and countercations, - the pseudo-liquid behaviour, - the structure defined at molecular level and unique complex formation with various organic and organometallic compounds. Among HPCs, heteropoly acids (HPA) with Keggin anion structures have received considerable attention due to their simple preparation and strong acidity (Misono 1987; Kozhevnikov 1995). Specifically, 12-tungstophosphoric acid (H3PW12O40), denoted as H3PW hereinafter, is among the most extensively studied (Misono and Nojiri 1990; Corma 1995; Okuhara et al. 1996) since it possesses the highest Brönsted acidity (Misono et al. 1982).

This type of HPCs are effective catalysts in important catalytic reactions that use the renewable raw material as the bioethanol.

The microstructure for acidic salts of the H3PW, especially with Cs, are controversial in the literature: - some authors propose a model of catalysts particles with a core of Cs3PW12O40 coated of the H3PW layers; - the others sustain a homogeneous structure of the acid salts.

The goal of this study was to unravel the CsxH3-xPW microstructure by corroboration of structure, texture and surface catalysts composition data with catalytic properties for ethanol conversion and selectivity to products reactions taking into account the type of reactions, in bulk or surface

The PROJECT “CATBIOETAMETCON”, HURO 901/090/2.2.2, SEMINAR, 24-25 March,, 2011, SZEGED

EXPERIMENTAL

● The tungstophosphoric acid H3[PW12O40]xH2O (H3PW) was synthesized according to the methods described of J. C. Bailar(Inorg. Synth., 1, 1939, 132-133) and M. Misono et. al. (Bul. Chem. Soc. Jpn., 55, 1982, 400-406).Na2HPO4•H2O and Na2WO4•2H2O were dissolved in hot distilled water, under stirring and the solution was kept at 80oC, 2 hours. After, HCl 37% was added by dropwise. When the solution has cooled, ether was added until, after shaking, three layers remain. The lower layer was separated and it was washed with water a several times. After separation from water, the ether was evaporated by air bubbling and a precipitate was formed. When precipitate no longer smell of ether, it was dissolved in a small quantity of water. After slow water evaporation the H3PW crystals are obtained. 12WO4

2- + HPO42- + 23H+ [PW12O40] 3- + 12H2O

● The salts of H3PW were prepared by precipitation from an aqueous solution of the parent acid adding the required stoichiometric quantity of counter-ion salts as cesium nitrate under stirring. The pH was under 1.5 during the all syntheses. The precipitates were dried at 50oC under stirring until a paste was obtained. After, the CsxH3-xPW samples were heated at 250oC in air for nitrate anion total decomposition. The water content of all prepared heteropoly compounds was determined after their keeping in air at room temperature until constant weight was observed.

The PROJECT “CATBIOETAMETCON”, HURO 901/090/2.2.2, SEMINAR, 24-25 March 2011, SZEGED

SYNTHESIS

EXPERIMENTAL

● The thermal analyses were carried out on thermoanalyzer system Mettler TGA/SDTA 851/LF/1100. The measurements were conducted in dynamic atmosphere of synthetic air (50ml/min), using the alumina plates crucibles of 150 μl. Heating rates were 2.5-10 C min-1 and the mass samples were about 30 mg.

● The IR absorption spectra were recorded with a Jasco 430 spectrometer (spectral range 4000-400 cm-1 range, 256 scans, and resolution 2 cm-1) using KBr pellets.

● Powder X-ray diffraction data were obtained with a XR Fischer diffractometer using the Cu Kα radiation in the range 2θ = 5÷60.

● Specific surface and porosity measurement by BET method were calculated from the nitrogen adsorption-desorption isotherms, date using a Nova 1200 Quantachrome equipment. The sample was previously degassed to 10-5Pa at 250 0C for 2 h.

● The Bronsted acidity of catalysts were measured by temperature programmed desorption-TPD of n-butylamine (adsorption at 373 K and desorption by increasing the temperature from 373 K to 873 K).

● The surface morphology for synthesized compounds was observed by SEM method with a Jeol JSM 6460 LV instrument equipped with an EDS analyser. The samples were heated at 523 K, 1h and then were covered with a film of Au before the measurements..

Catalyst characterization and catalytic activity

PROJECT “CATBIOETAMETCON”, HURO 901/090/2.2.2, SEMINAR, 24-25 March 2011, SZEGED

EXPERIMENTAL

Catalytic activity for conversion of ethanol was studied by pulse reactant technique-PRT.Two micro reactors, with fixed bed of about 100 mg catalyst, connected directly to a GC with thermal conductivity detector-TCD for CO and CO2 detection, Ar carrier gas flow of 30 cm3·min-1, respectively, a GC with flame ionisation detector-FID, N2 carrier gas flow of 30 cm3·min-1, for detection of ethanol, ethylene, diethyl ether and other hydrocarbon fractions (C3-C6) were used. The reaction products were separated on the Porapak QS 80-100 mesh packed columns, 3 m length, at 453 K for TCD-GC and with a temperature programme from 323 K to 473 K for FID-GC. The standard experimental protocol consisted of “in situ” catalysts drying, at 523 K for 1 h under flow of carrier gas, followed of six successive pulses of 3 μl liquid ethanol at 523, 573 and 623 K in the catalyst bed. For each temperature a fresh sample of catalysts was used.

Catalyst characterization and catalytic activity

Fig. 1. The instalation scheme for catalytic activity measurement by PRT: 1. Six port valve ; 2. Gas sample loop; 3. Liquid sample evaporator; 4. Microreactor heated by electric furnace; 5. Gas-cromatograf; 6. – Integrator.


EXPERIMENTAL

The powder samples were pressed into tablets with about 1 cm diameter and a few tenth of mm thickness and placed into the load lock of the spectrometer. Sample treatments were carried out in a high pressure cell (catalysis chamber) directly attached to the main chamber and isolated from that with a gate valve.The samples were transferred from the analysis chamber into the high pressure cell in high vacuum,without the reach of air. The samples were pre-treated in the same way as described above. After the pre-treatment the samples were cooled to room temperature. Afterwards the high pressure cell was evacuated and the sample was taken back to the analysis chamber. XP spectra were taken with a SPECS instrument equipped with aPHOIBOS 150 MCD 9 hemispherical electron energy analyzer operated in the FAT mode. The excitation source was the K alfa radiation of a magnesium anode (h = 1253.6 eV). The X-ray gun was operated at 180 W power (12 kV, 15 mA). The pass energy was set to 20 eV, the step size was 25 meV. To compensate for possible charging effects the binding energies were normalized with respect to the position of C (1s), this value being assumed constant at 285.1 eV. Typically five scans were added to get a single spectrum. For data acquisition and evaluation both manufacturers' (SpecsLab2) and commercial (CasaXPS) software were used. Fig.2.SPECS instrument equipped with a PHOIBOS 150 MCD 9 hemispherical electron energy analyzer

XPS analysis


Keggin primary and secondary structure

Figure 4. Keggin units and water molecules arrangement in H3PW12O40 · 6H2O

RESULTS AND DISCUSSION

Figure 3. Keggin unit structure represented by triads of edge-linked MoO6 octahedra and central thetraedra PO4

and its hexahydrate (down).

The PROJECT “CATBIOETAMETCON”, HURO 901/090/2.2.2,

SEMINAR, 24-25 March 2011, SZEGED

The complete elimination of the crystallization water after isothermal heating for 1 h at the temperatures of 523-623 K without the loss of constitutional water was observed, see Figure 5.


Thermal stability, water content and Brőnsted acidity

The constitutional water content (the water formed from the protons and the oxygen of the [PW12O40]3-, denoted as Keggin unit-KU hereinafter) of the acid and all acidic salts was measured of the TG curves (segment 1-2). Close values to theoretical ones were found for all compounds.

Fig.5. The TG, DTA curves and the temperature heating program-T curve for H3PW.




The H3PW and CsxH3-xPW thermal decomposition scheme


TPD for Brőnsted acidity measurements

Fig. 6. The TG and DTG curves for TPD of NBA in the temperature range of 100-600oC on (1) H3PW, (2) CsH2PW, (3) Cs2HPW, (4) Cs2.25H0.75PW, (5) Cs2.5H0.5PW and (6) Cs3PW


RESULTS AND DISCUSSIONTPD for Brőnsted acidity measurements

Sasca, L. Avram, O. Verdes, A. Popa, Appl. Surf. Sci., 256(2010) 5533-5538


The method is based on the alkylammonium ions formation when alkyl amines are protonated

by Brønsted sites and their decomposition to ammonia and olefins in a well-defined temperature: HRNH2 + ZOH HRNH3

+ ··· ZO- R + NH3 + ZOH (1)

Water content and Brőnsted acidity

Table 1. The physical adsorbed and crystallization water molecules per KU and proton content per KU for the synthesized compounds.


CatalystsPhysical adsorbed

water, mol Crystallization

water, molH+/KU

H3PW 1.0-1.5 5.5-6.0 2.85-2.90

Cs1H2PW 1.0-2.0 3.5-4.0 1.90-1,96

Cs2H1PW 2.0.-3.0 2.0-2.3 1.00-1,05

Cs2.25H0.75PW 4.0-5.0 1.3-1.5 0.72-0,75

Cs2.5H0.5PW 5.0-7.0 0.9-1.0 0.48-0,51

Cs3PW 8.0-9.0 - 0.00


Catalysts primary structure

.


The 3200-3400 cm-1 band is assigned to crystallization water-hydrogen bonded and to hydrogen-bond vibrations (hydrogen-bonds formed between neighbouring KUs). The 1715 cm-1 band is ascribed to hydroxonium ions, H3O+ or H5O2+, δ vibrations and the 1615 cm-1 band is assigned to δ vibrations of nonprotonated water molecules.

The specific absorbtion bands of the Keggin Unit - [PW12O40]

4-

are: νasP-Oi-W; 1060-1080; νasW-Ot, 960-1000; νasW-Oc-W, 840-910; νasW-Oe-W, 780-820 cm-1.


Fig. 8. The FTIR spectra: (1)H3PW•6-7H2O (2) CsH2PW•6-7H2O, (3) Cs2HPW•5-

6H2O, (4) Cs2.25H0.75PW•4-5H2O, (5) Cs2.5H0.5PW•6-8H2O şi (6) Cs3PW•8-9H2O


.



Fig. 9. The FTIR spectra: (1)H3PW•x1H2O (2) CsH2PW•x2, (3) Cs2HPW•x3H2O, (4) Cs2.25H0.75

PW•x3H2O, (5) Cs2.5H0.5PW•x4H2O and (6) Cs3PW•x5H2O after heating at 300 oC, 1 hour


.



Fig. 10. The FTIR spectra: (1)H3PW•x1H2O (2) CsH2PW•x2, (3) Cs2HPW•x3H2O, (4) Cs2.25H0.75

PW•x3H2O, (5) Cs2.5H0.5PW•x4H2O and (6) Cs3PW•x5H2O after heating at 600oC, 1 hour .

Catalysts secondary structure

The all synthesized compounds, H3PW•6-7H2O and CsxH3-xPW•5-8H2O, have a cubic structure and they show the XR-diffraction pattern in agreement with the literature data, as can be seen in Figure 11.

.


Fig. 11. The X-ray diffraction spectra for H3PW and its

CsxH3-xPW salts

The increase of width for the diffraction maxima with the Cs /KU ratio increasing was observed, which can be an effect of the crystallites size changes function of the chemical composition.

The PROJECT “CATBIOETAMETCON”, HURO 901/090/2.2.2, SEMINAR, , 24-25 March 2011, SZEGED


.



Fig. 12. The X-ray diffraction spectra for H3PW heated at 600oC


.



10 20 30 40 50 60

1

6

5

4

3

2

Inte

nsita

tea

(u.a

.)

2 Theta

Fig. 13. The XRD spectra: (1)H3PW•x1H2O (2)CsH2PW•x2, (3)Cs2HPW•x3H2O, (4)Cs2.25H0.75

PW•x3H2O, (5)Cs2.5H0.5PW•x4H2O and (6)Cs3PW•x5H2O after heating at 600oC, 1 hour .

Catalysts texture

λ– X-ray wavelength(CuKα) in angstroms (1.54Å)θ – diffraction angleβ – line width (radians)Β0 - instrumental line width (radians)

.

Table 2. Surface area, pores volume and estimated crystallites size of the CsxH3-

xPW catalysts with Scherrer equation

Catalyst Surface area,m2/g

Pores volume,

cc/g

Size of crystallites(D),

Ǻ

H3PW 3.1 0.0025 575

CsH2PW 1.8 0.0067 175

Cs2HPW 6.6 0.022 145

Cs2,25H0.75PW 54.2 0.0489 145

Cs2,5H0.5PW 130.3 0.119 117

Cs3PW 137.8 0.118 117


cos9.0

0

D

Scherrer equation for crystallites size estimation based on XRD spectra:

, (1)


Figure 15. HRSEM micrographs of bulk Cs2.5H0.5PW12O40 salt taken at x150 000 magnification.

Microp.Mes.Mater. 80 (2005) 43-55

Catalysts texture


Figure 14. SEM micrographs of bulk Cs2.5H0.5PW12O40 salt taken at x50 000 magnification.


Correlation of catalytic activity with Bronsted acidity

The dehydration of ethanol in the presence of strong Brönsted acid sites could involve a complex series of reactions, including oligomerization, aromatization, cracking and hydrogenation. The reaction products detected by the pulse reactant technique on CsxH3-xPW catalysts were: methane, C2 fraction (ethylene, ethane), C3 (propene, propane), C4 (butane, butene), C5 (pentane, pentene), C6 (hexane, hexene) and diethyl ether. The aromatic compounds, as benzene, toluene and xylene, could be also present but in undetectable quantities. In plus, very small quantities of H2 and CO were detected with GC-TCD. For the hydrocarbons formation, the selectivity to C4 fraction has the higher values and its variation function of the number of pulses show the same trends for the all hydrocarbons fractions, so, from the point of view of the our research goal the conversion of ethanol and selectivity to ethylene as main reaction product, respectively the selectivity to C4 hydrocarbon fraction will be analyzed.

All the results were an average of the pulses 4-6, as the quantity of reaction products for the pulses 1-3 varies significantly and for the next pulses only the small changes were observed from one to the other. The average of values for three pulses has reduced the error of liquid sample introduction also. Typical examples of conversion and selectivities variation function of pulse number can be seen in Figure 10a,b,c.




Fig. 16a,b,c. Ethanol conversion (a), selectivity to ethylene (b) and selectivity to C4 hydrocarbons fraction (c) at 573 K on Cs2.25H0.75PW12O40.


95

96

97

98

99

100

1 2 3 4 5 6Pulse number

Co

nve

rsio

n, m

ol %

Ethanol conversion 523 K

Ethanol conversion 573 K

Ethanol conversion 623 K60

65

70

75

80

85

90

95

100


Sel

ecti

vity

, mo

l %

Ehylene Selectivity 523 K



0

1

2

3

4

5

6


Sel

ecti

vity

, mo

l %

C4 Fraction Selectivity 523 K



a)

c)

b)




Figure 17a, b, c. The ethanol conversion and selectivity on the CsxH3-xPW catalysts function of Brőnsted acidity: 0-Cs3PW, 0.5-Cs2.5H0.5PW, 0.75-Cs2.25H0.75PW, 1-Cs2H1PW, 2-Cs1H2PW12, 3-H3PW: a)523 K; b) 573 K; c) 623 K


In purpose to explain the catalytic behaviour, a model of catalysts particles as cubes or spheres of Cs3PW12O40 coated of the H3PW12O40 layers was built on the basis of:

- The highest catalytic selectivity for C4 hydrocarbon fraction on Cs2.25H0.75PW, respectively on Cs2.5H0.5PW, a surface reaction type;

- The equivalence of strenth for all Bronsted acidic sites; - Higher specific surfaces for content of Cs>2 and pores

volume values increase for higher content of Cs;- The size of microcrystallites calculated with the Scherrer

relation of X-ray diffraction spectra and the size of aggregates estimated from SEM data.

Model of catalysts particles



550 Ǻ

19.16 Ǻ

Figure 19. The Keggin units and water molecules arrangement

in H3PW12O40 · 6 H2O


Figure 18. The cubic model of Cs3PW conglomerate coated

of H3PW nanocrystals


G.M. Brown, M.-R. Noe-Spirlet, W.A. Busing and H.A. Levy, Acta cryst. B33 (1977) 1038



Table 3. The molar ratio of H3PW/Cs3PW from acidic Cs salts and Acid layers on the Cs3PWaggregates


The Caesium acidic salts of

H3PW

The acidic salts written down as mixture of H3PW and Cs3PW

Molar ratio of H3PW/ Cs3PW from

CsxH3-xPW as mixture

The number of H3PW layers on the

Cs3PW aggregates

Molar ratio of H3PW of outer layers/

Cs3PW of inner

aggregates

Cs2.5H0.5PW1H3PW+5Cs3PW=

6Cs2.5H0.5PW 1/5 1 1/5.1

Cs2.25H0.75PW1H3PW+3Cs3PW=

4( Cs2.25H0.75PW ) 1/3 1,5 1/3.3

Cs2H1PW1H3PW+2Cs3PW=

3( Cs2H1PW ) 1/2 2,3 1/2

Cs1.29H1.71PW*4H3PW+3Cs3PW=

7( Cs 1.29 H1.71PW ) 4/3 5 4/3.1

Cs1H2PW2H3PW+1Cs3PW=

3( Cs1H2PW ) 2/1 7 2/1


Concentration of constituent elements on the catalyts surface by XPS

RESULTS AND DISSCUTION

Fig. 20. Caesium,Oxygen, Phosphorus and Wolfram spectra on the catalysts

surface for: a) W12O, b)H3PW, c)Cs2 H1PW, d)Cs2.5H0.5 PW, e)Cs3PW.



Fig. 21. Oxygen and Wolfram concentration on the catalysts surface function of

Cesium ions/KU: 0 (H3PW) ; 2 (Cs2 H1PW); 2.5 (Cs2.5H0.5 PW); 3 (Cs3PW).

20

30

40

50

60

70

80

0 0,5 1 1,5 2 2,5 3

Cesium ions/KU

% a

t. c

on

c.

O1s-Exp

O1s-Theoret

W4f-Exp

W4f-Theoret




Fig. 16. Phosphor and Cesium concentration on the catalysts surface function of Cesium ions/KU : 0.H3PW ; 2. Cs2PW; 2.5. Cs2.5PW; 3.Cs3PW.

0

1

2

3

4

5

6

7

0 0,5 1 1,5 2 2,5 3

Cesium ions/KU

% a

t. c

on

c.

Cs 3d-Exp

Cs 3d-Theoret

P2p-Exp

P2p-Theoret

Linear (Cs 3d-Exp)

Linear (Cs 3d-Theoret)



Concentration of constituent elements on surface layers

Fig. 17. Phosphor and Cesium concentration on the catalysts surface for: 0.H3PW ; 2. Cs2PW; 2.5. Cs2.5PW; 3.Cs3PW.


y = 3,2261x - 3,7883R2 = 0,9989

y = 1,8x + 0,0015R2 = 1

0

1

2

3

4

5

6

0 0,5 1 1,5 2 2,5 3Cesium ions/KU

% a

t. c

on

c.P 2p-Experim

P 2p-Theoret

Cs 3d-Exper corr

Cs 3d-Theoret

Linear (Cs 3d-Theoret)


CONCLUSIONS

►The higher ethanol conversion, over 95%, was observed on the CsxH3-xPW catalysts where x = 0; 2.0; 2.25 and 2.5. The selectivity to ethylene decreases with increasing of the selectivity to C4 hydrocarbons fraction on the catalysts with Cs content of 2-2.5, especially on the Cs2.5H0.5PW catalyst

► A model of catalysts particles as cubes or spheres consist of Cs3PW coated of the H3PW layers is proposed on the basis of: - catalytic activity; -Bronsted acidity; -specific surfaces and pores volume values; - the size of microcrystallites calculated with the Scherrer relation of X-ray diffraction spectra and SEM data of literature.

► The model explains the abrupt increase of specific surface area for content of Cs>2 and the highest catalytic selectivity at ethanol conversion for C3 and C4 hydrocarbons fractions on Cs2.25H0.75PW, respectively on Cs2.5H0.5PW.

►The XPS measurements of Cs, W, P and O concentration in the surface layers for the H3PW, Cs2H1PW, Cs2.5H0.5PW and Cs3PW has confirmed the model, the Cs concentration for acid salts Cs1H2PW, Cs2H1PW being lower than theoretical, for surface layers. The higher concentrations of W and P as the theoretical ones could be explain as a result of the lower concentration of O in the surface layers


Thank you for attention!


Documents

V. Z. SASCA a, Orsina VERDES a, Livia AVRAM a, A. POPA a A. ERDÕHELYI b and A. OSZKO b a Institute of Chemistry Timisoara-Romanian Academy b Institute