v. sasca, orsina verdes, livia avram, a. popa a comparison of cs x h 3-x pw 12 o 40 catalysts in the...

V. Sasca, Orsina Verdes, Livia Avram, A. Popa

A comparison of CsxH3-xPW12O40 catalysts

in the ethanol conversion

by the pulse and flow techniques

12th EDITION OF ACADEMIC DAYS TIMISOARA May 26-27, 2011 Timisoara, ROMANIA

Chemistry Institute of Romanian Academy-Timisoara, Bd. M. Viteazul 24, 300223

INTRODUCTION

• Ethanol conversion to ethylene, acetaldehyde, ethylic ether and other organic compounds used as starting materials for organic industry of synthesis is of major interest because the ethanol is obtained from regenerative starting material.

• The pulse reaction technique and the continuous flow technique experiments has been carried for ethanol conversion on CsxH3-xPW catalysts. The conversion of reactants and the yield of products were compared for the studied catalyst.

• The dehydration of ethanol to ethylene on strong acidic catalysts as CsxH3-xPW is accompanied of secondary reactions: oligomerization, aromatization, cracking and hydrogenation.


EXPERIMENTAL

The tungstophosphoric acid H3PW12O40xH2O (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.

12WO42- + HPO4

2- + 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 250 oC in air for nitrate anion total decomposition. The water content of all prepared heteropolycompounds was determined after their keeping in air at room temperature until constant weight was observed.

SYNTHESIS


• The thermal analysis 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 Brönsted 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


Catalytic activity test

1. Pulse reaction method:

The standard experimental protocol consisted of six successive pulses of 3 μl liquid injections at 523, 573 and 623 K and chromatographic analysis for each pulse separately.

2. Continuous flow method:

Liquid ethanol (99.8% Riedel de Haen) was pumped by a syringe pump at a flow rate of 1.2 ml/h into an evaporator where it is mixed with nitrogen to adjust the reactor feed composition to 20% alchool in the gas mixture.The syringe-type pump, Cole Parmer model 74900-00, -05 using the Hamilton-type syringe with a volume of 10 ml.The carrier gas flow of nitrogen was 30 ml/min.

GC Detection:

- FID detector;- column filled with Chromosorb 102, 80-100 mesh, 4

mm inner diameter, 2 m length;- Thermo Separation Products SP 4400 integrator; - carrier gas flow of nitrogen 30 ml/min;- Column temperature programming: initial

temperature 50 0C, final temperature 200 °C and a heating rate of 20 °C/min.

- ethanol concetration ≥ 99.8%;- microsyringe – 1 pulse of 3µL ethanol.

Figure 1. Installation for catalytic test by pulse reaction and continuous flow techniques

gas-chromatograph.


Keggin primary and secondary structure

Figure 3. Keggin units and water molecules arrangement in H3PW12O40 ·

6H2O

RESULTS AND DISCUSSION

Figure 2. H3PW12O40·6H2O molecule structure represented by triads of edge-linked MoO6

octahedra, the central thetraedra PO4 (yellow)

and its hydration water.


• thermal analysis • 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

Thermal stability

Figure 4. The TG, DTA curves and the temperature heating program-T curve for H3PW.


Catalysts primary structure

.

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

Figure 5. 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 and (6) Cs3PW•8-9H2O



.Figure 6. 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



.Figure 7. 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 600 oC, 1 hour .


Catalysts secondary structure

• H3PW·6-7H2O and CsxH3-xPW·5-8H2O have a cubic structure

.

Figure 8. The X-ray diffraction spectra for H3PW and its CsxH3-xPW salts.


Catalysts secondary structure

.

10 20 30 40 50 60

1

6

5

4

3

2

Inte

nsita

tea

(u.a

.)

2 Theta

Figure 9. The XRD spectra: (1)H3PW•x1H2O (2)CsH2PW•x2H2O, (3)Cs2HPW•x3H2O,

(4)Cs2.25H0.75PW•x3H2O, (5)Cs2.5H0.5PW•x4H2O and (6)Cs3PW•x5H2O after heating at 600 oC, 1 hour .


Figure 10. The X-ray diffraction spectra for

H3PW heated at 600 oC.

Catalytic selectivity and activity

• The reaction products detected by the pulse reactant and flow continuous techniques 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. 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.• As the H3PW to Cs 2.5 catalysts could be consider the limits of the series CsxH3-xPW, their comparison of catalytic activity will be presented. The other catalysts have showed almost similar catalytic properties in function of their kindred of compositions with the two before mentioned catalysts.


Catalytic selectivity and activity – Pulse reaction technique

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1 2 3 4 5 6

Pulses number

Sel

ectiv

ity, m

ol%

C3-523K

C4-523K

C3-573K

C4-573K

C3-623K

C4-623K

84

86

88

90

92

94

96

98

100

1 2 3 4 5 6

Pulses number

Co

nvers

ion

/Sele

cti

vit

y, m

ola

r%

Conv-523K

Sel-523K

Conv-573K

Sel-573K

Conv-623K

Sel-623K


Figure 11. The ethanol conversion and the selectivity

to ethylene at 523, 573, 623 K on H3PW.

0.00

0.10

0.20

0.30

0.40

0.50

1 2 3 4 5 6

Pulses number

Sel

ectiv

ity, m

ol%

ETHER-523K

ETHER-573K

ETHER-623K

Figure 13. The selectivities to ether at 523, 573 and 623 K on H3PW.

Figure 12. The selectivities to C3, C4 at 523, 573 and 623 K on H3PW.

- the results were an average of the pulses 4-6, as the quantity of reaction products for the pulses 1-3 varies significantly-the average of values for three pulses has reduced the error of liquid sample introduction

Catalytic selectivity and activity – Pulse reaction technique

50

60

70

80

90

100

1 2 3 4 5 6

Pulses number

Co

nv

ers

ion

/Se

lec

tiv

ity

, m

ola

r%

Conv-523K

Conv-573K

Conv-623

Sel-523K

Sel-573K

Sel-623K

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

5

1 2 3 4 5 6

Pulses number

Se

lec

tiv

ity

, %

C3-523K

C3-573K

C3-623K

C4-523K

C4-573K

C4-623K


Figure 14. The ethanol conversion and the selectivity to ethylene at 523, 573, 623 K on Cs2.5H0.5PW.

Figure 15. The selectivities to C3, C4 at 523, 573 and 623 K on Cs2.5H0.5PW.

.

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

1 1,5 2 2,5 3 3,5 4 4,5 5 5,5 6

Pulses number

Sel

ecti

vity

, %

Ether-523K

Ether-573K

Ether-623K

Figure 16. The selectivities to ether at 523, 573 and 623 K on Cs2.5H0.5PW.


Catalytic selectivity and activity – Continuous flow technique

0

10

20

30

40

50

60

70

80

90

100

35 135 235 335 435 535 635 735

Time, min

Co

nv

ers

ion

, %

Conv-523K

Conv-573K

Conv-623K

0

10

20

30

40

50

60

70

80

90

35 135 235 335 435 535 635 735

Time, min

Se

lec

tiv

ity

, %

Ethylene-523K

Ethylene-573

Ethylene-623K

Ether-523K

Ether-573K

Ether-623K

Figure 17. The ethanol conversion at 523, 573, 623 K on H3PW.

Figure 18. The selectivities to ethylene and ether at 523, 573 and 623 K on H3PW.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

35 135 235 335 435 535 635 735

Time, min

Sele

cti

vit

y, %

C4-523K

C4-573K

C4-623K

Figure 19. The selectivities to C4 at 523, 573 and 623 K on H3PW.


Catalytic selectivity and activity – Continuous flow technique

40

50

60

70

80

90

100

35 135 235 335 435 535 635 735

Time, minC

on

ve

rsio

n,

%

Conv-523K

Conv-573K

Conv-623K

Figure 20. The ethanol conversion at 523, 573, 623 K on Cs2.5H0.5PW.

Figure 21. The selectivities to ethylene and ether at 523, 573 and 623 K on Cs2.5H0.5PW.

.

0

10

20

30

40

50

60

70

80

90

35 135 235 335 435 535 635 735

Time, min

Sele

cti

vit

y, %

Ethylene-523K

Ethylene-573K

Ethylene-623K

Ether-523K

Ether-573K

Ether-623K

Figure 22. The selectivities to C4 at 523, 573 and 623 K on Cs2.5H0.5PW.

0

0,2

0,4

0,6

0,8

1

1,2

35 135 235 335 435 535 635 735

Time, min

Se

lec

tiv

ity

, %

C4-523K

C4-573K

C4-623K

CONCLUSIONS• The same reaction products were observed in ethanol

conversion on CsxH3-xPW catalysts by the pulse reaction technique and by the continuous flow technique.

• The highest ethanol conversion, over 95%, was observed for pulse reaction test on the CsxH3-xPW.

• For all catalysts the selectivity to ethylene decreases with increasing of the selectivity to C4 hydrocarbons fraction and diethyl ether.

• For the continuous flow tests the deactivation which was observed on H3PW could be owing to the blocking of catalytic active site from the strong chemisorption species. The deactivation on Cs2.5H0.5PW was observed at 573 and 623 K and it is due to organic deposit which was determined by thermal analysis.


Thank you for attention!

Acknowledgment: Financial support of this work by Cross Border Cooperation Programme2007-2013, HURO 0901,

is gratefully acknowledged.


v. sasca, orsina verdes, livia avram, a. popa a comparison of cs x h 3-x pw 12 o 40 catalysts in the...

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