Neil G. Hamilton1, Andrew R. McFarlane1, David - Hiden Analytical

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The gas phase hydrogenation of 1,3pentadiene: an FTIR study. Neil G. Hamilton1, Andrew R. McFarlane1, David Siegel1, David T. Lundie2, S.F. Parker3 and David Lennon1. 1. Department of Chemistry, Joseph Black Building, The University of Glasgow, Glasgow, G12 8QQ, UK. 2. Hiden analytical Ltd., 420 Europa Boulevard, Warrington, WA5 7UN, U.K. 3. ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, UK.

1. Introduction:

5. Results:

Selective hydrogenation of mixtures of 1,3-dienes obtained from the C5 cracking fraction of a petrochemical complex could be one method of obtaining high-value stereo-specific mono-olefins. This study examines whether catalysts displaying distinct stereoselective hydrogenation characteristics (isomeristaion/hydrogenation) can be used to affect diene hydrogenation profiles.

Reaction profiles were constructed by examining the intensity of infrared bands uniquely diagnostic of each species present in the gas phase. A list of the diagnostic bands used to identify gaseous species is presented in the table below:

2. Catalyst characterisation: • Catalyst prepared by the wet impregnation technique. • Characterised using pulsed CO chemisorption followed by CO temperature programmed desorption (TPD) using Hiden mass spectrometer (HPR-20 Q/C).

Compound

Band position (cm-1)

IR intensity

Assignment

1,3-pentadiene

899

vs

ethylenic rock and C-C-C deformation

trans-2-pentene

969

s

in-phase trans-CH wag

cis-2-pentene

688

m

in-phase cis-CH wag

1,4-pentadiene

1642

s

out-of-phase C=C stretch

pentane

2965

vs

in-phase antisymmetric CH3 stretch

1-pentene

3087

S

antisymmetric ethylenic stretch

• Calculated metal dispersion = 13.4% (Pd:CO = 2:1). • Low temperature CO peak (Tmax 580 K) assigned to metal bound to Pd crystallites.

5.1 Mono-olefins

• High temperature peak (Tmax 650 K) assigned to decay of carboxy species associated with alumina support [1].

Hydrogenation of trans-2-pentene:

-6

4.0x10

4.0x10

CO -6

A 3.5x10

650 K

-6

3.0x10

3.0x10

2.0x10

-9

-9

-6

2.5x10

-6

-9

B1.0x10

0 mins 2 mins 10 mins 20 mins 35 mins

1.0

0.8 Absorbance [AU]

5.0x10

-6

0.6

0.4

0.2

-6

0.0

2.0x10

-7

4.0x10

-7

6.0x10

-7

8.0x10

-7

400

CO exposure [Moles]

500

600

700

800

2.0x10 900

-9

4000

3500

3000

2500

2000

1500

0.09

A0

0.0

1.0x10

C

-4

1000

8.0x10

-5

6.0x10

-5

0.08 0.07

trans-2-pentene pentane 4.0x10

-5

2.0x10

-5

0.06 0.05 0.04 0.03

0.0

500

pentane peak intensity

CO uptake [Moles/g]

6.0x10

-9

number of moles oftrans-2-pentene

580 K

Pressure / Torr

7.0x10

Here, trans-pent-2-ene is directly converted to pentane (Figure 4). Analysis of the mass balance showed 69% of the incident moles of trans-pent-2-ene had been converted to pentane, with 31% of the hydrocarbon retained at the catalyst surface.

H2

0.02

-1

wavenumber [cm ]

0

10

20

Temperature / K

30

40

50

60

time (mins)

Figure 4: (a) FTIR spectra of gaseous species acquired at selected reaction times, (b) reaction profile for the hydrogenation of trans-pent-2-ene and (c) associated reaction scheme.

Figure 1: (a) Pulsed chemisorption profile for 1%Pd/Al2O3 catalyst and (b) subsequent CO TPD profile.

3. Experimental:

Hydrogenation of cis- 2-pentene:

A modified Graseby-Specac 5661 heated infrared gas cell housed within a Nicolet Avatar FTIR spectrometer was employed as a batch reactor. Purified helium and hydrogen flows were regulated by dedicated mass flow controllers. The catalyst was mounted on a glass boat and loaded into the reactor as described in Figure 1. The glass boat is located out with the beam path such that only gaseous components are analysed as the reaction proceeds.

The reaction profile here differs significantly from Figure 4. Instead of a direct hydrogenation, a consecutive process is observed. Firstly, cis-pent-2-ene is isomerised to trans-pent-2-ene, which is then hydrogenated to pentane.

Gas in

Septum for injection

On completion of reaction, the mass balance shows the catalyst to have retained 20% of the hydrocarbon. A

B

Gas out 8.0x10

A0

-5

Isomerisation

1.2

IR Gas phase reagent Catalyst

6.0x10

4.0x10

2.0x10

-5

1.0 0.9

cis-pent-2-ene trans-pent-2-ene pentane

-5

0.8 0.7

-5

0.6

peak intensity of pentane

number of moles of olefins

1.1

H2

0.0

Figure 2: Schematic diagram describing reactor configuration.

0.5 0

10

20

30

40

50

60

timefor (mins) Figure 5: (a) Reaction profile the hydrogenation of cis-pent-2-ene and (b) associated reaction scheme.

Reaction conditions: • 5 mg catalyst diluted in 100 mg of -Al2O3.

5.2 Di-olefins

• Temperature maintained at 298 K.

Hydrogenation of trans-1,3-pentadiene:

• Reactor charged with 5% H2/He to a pressure of 0.2 bar guage. • 10 L of hydrdocarbon was introduced via a septum. This represents A0. • [H2] > [hydrocarbon] >> [Pd(s)] Under these conditions the cell was operating as a batch reactor.

Here, firstly, the terminal double bond is hydrogenated to produce trans-pent-2-ene. The internal bond is subsequently hydrogenated to produce pentane. No cis-pent-2-ene is detected. However, on completion of reaction, inspection of the mass balance shows 56% retention of the hydrocarbon at the catalyst surface. It is possible that any cis-pent-2-ene that might form in the initial step is selectively retained by the catalyst. A

4. Density Functional Theory (DFT) calculations:

Calculated IR spectrum Measured IR spectrum

1.0x10

number of moles of olefins

1.0

1.2x10

-4

1.2

A0

-4

H2

1.0 8.0x10

6.0x10

4.0x10

-5

trans-1,3 pentadiene cis-2-pentene trans-2-pentene pentane

-5

-5

0.8

0.6

0.4 2.0x10

-5

pentane peak intensity

• Vibrational modes for all relevant C5 molecules were calculated using the Gaussian03 software package [2]. • B3LYP/cc-AUG-pVDZ was selected as the basis set. • Calculated spectra used to aid the assignment of bands in the recorded spectra. • The discrepancy between band positions for stretching modes in the calculated and recorded spectra is due to the harmonic approximation.

B

H2

0.2

0.8

Absorbance

0.0 0.0

0.6

0

10

20

30

40

50

time (mins)

Figure 6: (a)The reaction profile for the hydrogenation of trans-1,3-pentadiene and (b) the associated reaction scheme.

0.4

0.2

6. Conclusions:

0.0

Previous studies have shown first and second stage hydrogenation of 1,3-pentadiene on Pd/Al2O3 occurs at two distinct sites [3]. Work is currently underway to examine whether selective poisoning strategies can be used to block stage 2 hydrogenation, which would enable trans-1,3-pentadiene to be selectively hydrogenated to trans-pent-2-ene.

500

1000

1500

2000

2500

3000

3500

-1

Wavenumber [cm ]

Figure 3: Comparison of calculated and measure FTIR spectra of cis-2-pentene.

7. References: [1] T. Lear et al., Catal. Today, 2007, 126, p219.Carbonate in alumina reference [2] Gaussian 03, Revision C.02, M.J. Frisch et al., Gaussian, Inc., Wallingford CT, 2004. [4] E. Opara et al., Phys. Chem. Chem. Phys., 2004, 6, p5588.

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Neil G. Hamilton1, Andrew R. McFarlane1, David - Hiden Analytical

The gas phase hydrogenation of 1,3pentadiene: an FTIR study. Neil G. Hamilton1, Andrew R. McFarlane1, David Siegel1, David T. Lundie2, S.F. Parker3 an...

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