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

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

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

Study of the Ethanol Conversion Kinetics on

CsxH3-xPW12O40 Catalysts

NEW TRENDS AND STRATEGIES IN THE CHEMISTRY ... 3-4 NOV 2011 TIMISOARA

INTRODUCTION

In the last time much attention has been paid to biomass as alternative

resource to petroleum, since biomass is a renewable resource and its

combustion does not lead to increased of CO2 in the atmosphere.

The search for renewable resources has stimulated the production of

hydrocarbons and others organic intermediary compounds from ethanol over

different acidic solid catalysts as the ethanol is the one of the main biomass-

derived products.

The H3-xPW12O40 and its salts, especially with Cs, are effective

catalysts in ethanol conversion to ethylene and the reaction kinetics is

interesting to the catalysis theory and industrial application also.


EXPERIMENTAL

The FTIR absorption spectra (diffuse reflectance mode) were

recorded “in situ” with a Bio-Rad FTIR spectrometer

equipped with diffuse reflectance attachment (Thermo-electron

corporation) and BaF windows with a wave number accuracy

of ± 4 cm-1. Typically 32 scans were registered. The whole

optical path was purged by CO2 and H2O free air generated by a

Balston purge gas generator. The catalysts were pre-treated at 573 K for 30 min, and then the

ethanol was introduced by bubbling of Ar gas through the ethanol at 273 K for 120 min. After, the

cell was flushed with He stream for 30 min, respectively with O2 for 20 min and finally the catalyst

was heated linearly with a heating of 10 K/min from 573K to 723 K. The IR spectra were recorded

for each of the experiment steps on H3PW and CsxH3-xPW, x= 2.5 and 3.

FTIR measurements of ethanol adsorption-desorption


EXPERIMENTAL

The conversion of ethanol was studied on the H3PW and CsxH3-xPW, x= 2.5 catalysts by continuous flow of reactant technique-CFRT. A micro reactor with fixed catalyst bed of about 100 mg was connected to the heated six-port valve with a loop for sampling as it can be seen in Fig. 1. The temperatures inside the catalyst bed were between from 473 K to 623 K and the reactant mixtures (11vol % and 22 vol% of ethanol in nitrogen) were obtained in the evaporator (2) by introducing the liquid ethanol in the nitrogen flow of the 30 ccm/min with a Hamilton syringe and a device which pushes its piston.

The reaction products analysis was carried out with the GC-FID equipped with a stainless steel column of 3 m length 2 mm inner diameter, packed with Porapak QS 80-100 mesh. The N2 carrier gas with the flow of the 30 cm3/min was used. The splitting was of 1:6 volume ratios. For a better separation of reaction products, a programme of temperature for the column was set up: 5 min at 323 K, then heating from 323 K to 473 K with the heating rate of 10 K/min and the last, an isothermal heating at 473 K for 5 min. The total time for a complete analysis was 25 min.

Catalytic activity tests

Figure 1. The installation scheme for the catalytic activity measurement by CFRT:

1. Four port valve; 3. Evaporator of liquid sample; 3. Micro reactor heated by electric furnace; 4. Four port

valve, 5-6 Six-port valve with gas sample loop; 7. Gas-chromatograph; 8. Integrator.


Catalysts primary structure

.

RESULTS AND DISCUSSION

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.

Figure 2. The FTIR spectra: (1) H3PW•6-7H2O and (2) Cs2.5H0.5PW•6H2O.


Catalysts primary structure

.


Figure 3. The FTIR spectra: (1)H3PW•x1H2O (2) Cs2.5H0.5PW•x4H2O after heating at: (a) 573 K, 1 hour and (b) 873 K, 1 hour.


4000 3500 3000 2500 2000 1500 1000 500

T (%

)

Wavenumber (cm-1)

1

2

a) b)

Catalysts

.



1) EtOH/573 K/0,5 min, 2) EtOH/573 KC/1 min,3) EtOH/573 K/15 min, 4) EtOH/573 K /60 min,5) EtOH/573 K /120 min, 6) He/573 K /30 min.7) O2/573 K/0.5 min, 8) O2/573 K /20 min,9) O2/623 K, 10) O2/673 K, 11) O2/723 K.

1) EtOH /573 K /1 min, 2) EtOH /573 K /15 min,3) EtOH /573 K /60 min, 4) Et /573 K /120 min.5) He/573 K/30 min, 6) O2/573 K /1 min, 7) O2/ 573 K /20 min, 8) O2/673 K/677-683 K,9) O2/ 623 K/600-626 K, 10) O2/723 K/700-726 K.

Figure 4 a,b. The FTIR spectra registered in diffuse reflectance mode for the ethanol absorption/desorption on the H3PW12O40 (a) and Cs2.5H0.5PW12O40 (b).

a) b)

Ethanol Conversion Kinetics

The adsorption-desorption studies of ethanol by the FTIR in diffuse reflectance mode were carried out in purpose to observe the ethanol chemisorbed species and thus to clarify the reaction mechanism. In the FTIR spectra registered during the adsorption of ethanol at 573 K and desorption under He, respectively under O2, the main absorption bands could be assigned to: - CH3 stretch and C-H deformation vibrations which belong to chemisorbed ethanol (1490, 1385, 1365 and 1330 cm-1); - O=C=O asymmetric stretch vibration of the adsorbed CO2 at 2350 cm-1, -C=O from carbonyl between 1650 and 1780 cm-1,- OCO stretch vibration from carboxylates and carbonates at 1650 cm-1 (asymmetric), 1480 cm-1 (symmetric) and 1220 cm-1 (bending); -ethoxy group deformation vibrations in the range 3000-2800 cm-1; -intermolecular H-bonded alcohol species at about 3600 cm-1 and -OH stretch vibration from molecular ethanol bonded on surface of the hydrogen–bridge bonds which gives a large broad band centered at about 3400 cm-1.

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 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.



Reaction mechanism for ethanol conversion on Bronsted acid sites:

C2H5OH + H+ ... OUK2- C2H5OH2

+ ... OUK2- ... (1)

OUK2- ... C2H5OH2

+ [-OC2H5]UK ... H2O ... (2)

[-OC2H5]UK C2H4 + H+ ... OUK2- (3)

and/or

[-OC2H5]UK + C2H5OH (C2H5 )2O + H+ ... OUK2- (4)

[-OC2H5]UK could react also with C2H4 according to the next equation:

C2H4 + [-OC2H5]UK CH3- (CH2)3 – OUK2- ... (5)

CH3 – (CH2)3 – OUK2- (CH3)2 – CH=CH2 + OUK

2- ... (6)




Figure 5 a, b. The ethanol conversion on the H3PW12O40 for 11 vol% (a) and 22 vol% (b) ethanol in N2 at different temperatures.


a) b)


0

20

40

60

80

100

0 100 200 300 400 500 600 700

Time, min

Eth

ano

l co

nve

rsio

n, %

623 K

573 K

548 K

523 K

498 K

473 K

20

30

40

50

60

70

80

90

100

0 100 200 300 400 500 600 700

Time, minE

than

ol C

onv

ersi

on, %

623 K

573 K

598 K

523 K

498 K

473 K



Figure 6 a, b. The ethylene and diethyl ether selectivity on the H3PW12O40 for 11 vol% (a) and 22 vol% (b) ethanol in N2 at different temperatures.

b)a)


0

10

20

30

40

50

60

70

80

0 100 200 300 400 500 600 700

Time, min

Sel

ectiv

ity, %

Ethylene 623 K Diethyl ether 623 K

Ethylene 573 K Dietyl ether 573 K




Ethylene 473 K Diethyl ethe 473 Kr

0

10

20

30

40

50

60

70

80

90

0 100 200 300 400 500 600 700

Time, min.

Sel

ectiv

ity, % Ethylene 623 K Diethyl ether 623 K

Ethylene 573 K Diethyl ether 573 KEthylene 548 K Diethyl ether 548 KEthylene 523 K Diethyl ether 523 KEthylene 498 K Diethyl ether 498 KEthylene 473 K Diethyl ether 473 K



Figure 7 a, b. The C4 hydrocarbons selectivity on the H3PW12O40 for 11 vol% (a) and 22 vol% (b) ethanol in N2 at different temperatures.

b)a)


0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

0,45

0,5

0 100 200 300 400 500 600 700

Time, min

C4

Hyd

roca

bon

Sel

ectiv

ity, %

623 K

573 K

548 K

523 K

498 K

473 K

35;4,26770;1,7

0

0,2

0,4

0,6

0,8

1

0 100 200 300 400 500 600 700

Time, min.C

4 H

yd

rocarb

on

Sele

cti

vit

y, %

623 K

573 K

548 K

523 K

498 K

473 K

35;3,9435:3,53



Figure 8 a, b. The ethanol conversion on the Cs2.5H0.5PW12O40 for 11 vol% (a) and 22 vol% (b) ethanol in N2 at different temperatures.

b)a)


40

50

60

70

80

90

100

0 100 200 300 400 500 600 700

Time, min

Eth

an

ol

co

nv

ers

ion

, %

623 K

573 KC

548 K

523 K

498 K

473 K

30

40

50

60

70

80

90

100

0 100 200 300 400 500 600 700

Time, minE

than

ol C

on

vers

ion

, %

623 K

573 K

548 K

523 K

498 K

483 K

473 K



Figure 9 a, b. The ethylene and diethyl ether selectivity on the Cs2.5H0.5PW12O40 for 11 vol% (a) and 22 vol% (b) ethanol in N2 at different temperatures.

b)a)


0

10

20

30

40

50

60

70

80

0 100 200 300 400 500 600 700

Time, min

Sel

ectiv

ity, %

Ethylene 623 K Diethyl Ether 623 KEthylene 573 K Diethyl Ether 573 KEthylene 548 K Diethyl Ether 548 KEthylene 523 K Diethyl Ether 523 KEthylene 498 K Diethyl Ether 498 KEthylene 473 K Diethyl Ether 473 K

0

10

20

30

40

50

60

70

80

90

0 100 200 300 400 500 600 700

Time, minS

elec

tivity

, %

Ethylene 623 K Ethylene 573 K Ethylene 523 KEthylene 498 K Ethylene 483 K Ethylene 473 KDiethyl ether 623 K Diethyl ether 573 K Diethyl ether 523 KDiethyl ether 498 K K Diethyl ether 483 K Diethyl ether 473 K



Figure 10 a, b. The C4 hydrocarbons selectivity on the Cs2.5H0.5PW12O40 for 11 vol% (a) and 22 vol% (b) ethanol in N2 at different temperatures.

b)a)


0,00

0,10

0,20

0,30

0,40

0,50

0 100 200 300 400 500 600 700

Time, min

C4

Sel

ecti

vity

, %

623 K 573 K

548 K 573 K

498 K 473 K

35;3%

0,00

0,10

0,20

0,30

0,40

0,50

0 100 200 300 400 500 600 700

Time, min

C4

Sel

ecti

vity

,%

623 KC 573 K

548 K 523 K

498 K 483 K

473 K

35;6.8235;6.23




Ethanol Conversion KineticsThe measurements of catalytic activity at the temperatures of the 473

K,

498 K and 523 K for 11 mol% and 22 mol% ethanol in reaction mixture were

used for kinetic parameters calculation based on the reaction rate equations:

r=k[pi]n (7), k=A*e-Ea/RT (8)

where k=reaction constant rate and pi = ethanol partial pressure (kPa).

The kinetic parameters of the ethanol transformation to reaction

products

on the Cs2.5H0.5PW12O40 were calculated from the logarithmic form of the reaction

rate equation (7) and reaction constant rate (8), for three temperatures at each of

the two ethanol partial pressures:

lnr= lnA-Ea/RT+nlnpi or lnr=-(Ea/R)*1/T+lnA+nlnpi (9)

That is equivalent with: y=ax+b, y=lnr, x=1/T and b=lnA+nlnpi



y = -1,352x + 8,686

R2 = 0,9997

y = -1,435x + 8,827

R2 = 0,9999y = -1,476x + 8,90

R2 = 0,9999

y = -1,415x + 8,763

R2 = 0,9979

5,75

5,80

5,85

5,90

5,95

6,00

6,05

6,10

6,15

1,90 1,95 2,00 2,05 2,10

1000/T, 1/K

ln r

2nd Sample

3rd Sample

4th Sample

5th sample

y = -0,475x + 7,668

R2 = 0,9899

y = -0,472x + 7,649

R2 = 0,9597

y = -0,4405x + 7,573

R2 = 0,9917

y = -0,3949x + 7,467

R2 = 0,99936,62

6,66

6,70

6,74

6,78

1,90 1,95 2,00 2,05 2,10

1000/T, 1/Kln

r

2nd Sample

3rd Sample

4th Sample

5th Sample

Figure 11 a, b. The plotting of the logarithmic form of the ethanol conversion rate vs the inverse absolute temperature on the H3PW12O40 for 11 vol% (a) and 22 vol% (b) ethanol in N2.


a) b)

Figure 12 a, b. The plotting of the logarithmic form of the ethylene formation rate vs the inverse absolute temperature on the H3PW12O40 for 11 vol% (a) and 22 vol% (b) ethanol in N2.



y = -2,849x + 11,389

R2 = 0,9689

y = -3,072x + 11,786

R2 = 0,9707y = -3,114x + 11,848

R2 = 0,9685

y = -3,0649x + 11,7186

R2 = 0,9829

5,20

5,30

5,40

5,50

5,60

5,70

5,80

5,90

6,00

1,90 1,95 2,00 2,05 2,10 2,15

1000/T, 1/K

ln r

2nd Sample

3rd Sample

4th Sample

5th sample

y = -10,924x + 27,947

R2 = 0,9797

y = -10,154x + 26,08

R2 = 0,986

y = -11,353x + 28,621

R2 = 0,9545

y = -8,9189x + 23,386

R2 = 0,9922

4,40

4,90

5,40

5,90

6,40

6,90

1,90 1,95 2,00 2,05 2,101000/T

ln r

2nd Sample

3rd sample

4th Sample

5th sample


b)a)

Figure 13. The plotting of the logarithmic form of the ethanol conversion rate vs the inverse temperature on the Cs2.5H0.5PW12O40 for 11 vol% (a) and 22 vol% (b) ethanol in N2.



y = -0,8905x + 7,8132

R2 = 0,9988

y = -0,9207x + 8,5204

R2 = 0,9909

5,80

6,00

6,20

6,40

6,60

6,80

7,00

1,90 1,95 2,00 2,05 2,10

1000/T

ln r

11 vol.% EtOH

22 vol.% EtOH

y = -4,2169x + 14,007

R2 = 0,9929

y = -4,8521x + 15,856

R2 = 0,9899

5,00

5,30

5,60

5,90

6,20

6,50

6,80

1,90 1,95 2,00 2,05 2,10

1000/Tln

r

11 vol.% EtOH

22 vol,% EtOH

Figure 14. The plotting of the logarithmic form of the ethylene formation rate vs the inverse absolute temperature on the Cs2.5H0.5PW12O40 for 11 vol% (a) and 22 vol% (b) ethanol in N2 .



Catalysts: Cs2.5H0.5PW12O40

Process: Ethanol conversionEa-apparent activation energy=7.55±0.15 kJ/molA-preexponential factor= 22584±113 andn-reaction order=1.

Process: Ethylene formation:Ea-apparent activation energy=37.7±2.6 kJ/molA-preexponential factor= 3649132 n-reaction order=0.5.


CONCLUSIONS


The best catalyst is the Cs2.5H0.5PW12O40 with high catalytic

activity and selectivity to ethylene.

The values of thekinetical parameter: Ea-apparent activation

energy,

A-preexponential factor and n-reaction order were calculated for

the ethanol conversion and ethylene formation.

The elucidation of the reaction mechanism needs supplementary FTIR “in situ” investigation.

Acknowledgment: Financial support of this work by Cross Border Cooperation Programme 2007-2013, HURO 0901, is gratefully acknowledged.


Two countries, one goal, joint success!

The financial support of the Romanian Academy for the research

programme from Institute of ChemistryTimisoara which was the

basis of this cooperation project is also gratefully acknowledged.

Thank you for attention!


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

Documents

liquid ethanol

ethanol conversion kinetics

effective catalysts

reaction products analysis

reaction kinetics

nitrogen flow

heating rate

isothermal heating