276573 Steam Reforming of Ethanol Over Ru/Alumina: Effect of Temperature On Activity, Selectivity and Carbon Laydown

Tuesday, October 30, 2012: 1:10 PM
321 (Convention Center )
Mohammad Bilal, University of Glasgow, Glasgow, United Kingdom and S. David Jackson, WestCHEM, Department of Chemistry, University of Glasgow, Glasgow, United Kingdom

Steam Reforming of Ethanol over Ru/alumina:

effect of temperature on activity, selectivity and carbon laydown.

Mohammad Bilal and S David Jackson

Centre for Catalysis Research, WestCHEM, School of Chemistry, University of Glasgow, Glasgow , G12 8QQ Scotland

David.Jackson@glasgow.ac.uk

Introduction

The production of hydrogen is a key enabler for much of the chemical industry, for example hydrogenation is a ubiquitous process used across the whole of chemistry from refinery to pharmaceuticals, and continued production from bio-renewables rather than hydrocarbons is important for future chemical developments.  The production of synthesis gas is also a key enabler for Fischer-Tropsch synthesis and methanol synthesis.  Currently hydrogen and syn-gas is produced principally by steam reforming of methane, hence we in this study we were interested to examine the steam reforming a simple bio-renewable, ethanol, examining the effect of temperature on activity, selectivity and catalyst deactivation.

Experimental

Steam reforming of ethanol was performed in a high pressure reactor (20 bar) at 873 K, 823 K and 773 K, over 0.2 % Ru/α-Al2O3, at 50,000 GHSV.  The catalyst (0.25 g) was reduced in flowing hydrogen (50 ml/min) at 873 K for 2 h.  After reduction the flow was switched to Ar and the temperature adjusted if necessary.  The flow of the carrier gas (argon) was controlled using Brooks 5805S while a water ethanol mixture (5:1) was passed through a Gilson pump at a rate of 0.412 ml/min.  The carrier gas and water/ethanol mixture were integrated in the vaporizer whose temperature was kept at 773 K.  After exiting the reactor tube the product gases entered a knockout pot (273 K) where high boiling point products were liquefied and collected and analyzed by GC using a Zebron column and FID detector.  Gaseous products were analyzed by on-line GC using a TCD detector and a carboxenTm1010 plot column.

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Results and Discussion

            The catalysts were run for around 100 h to ensure that the systems were at steady state.  In this study we have shown that a catalyst with 0.2 wt % loading of ruthenium is active for ESR between 773 K and 873 K.  The conversion of ethanol, especially at 873 K, can be described in terms of ethanol decomposition, ethanol dehydration and the WGS reaction resulting in a hydrogen yield per mole of ethanol consumed of 0.8.  Hydrogen was the major product in the steady state and at 773 K the H2:CO ratio was >10:1 but at 873 K the H2:CO ratio had reduced to give ~2:1.  Ethene was a significant product especially at 773 and 823 K.  The hydrogenation of ethene to ethane is significantly inhibited over Ru/Al2O3 sufficient for a sensible activation energy to be determined.  The dry gas selectivity at 773 K, 823 K and 873 K is shown in Table 1

The formation of acetaldehyde and other liquid products indicate that not only was ethanol dehydrated over the Ru/Al2O3 catalyst but it also underwent dehydrogenation.  The dehydrogenation reaction can take place over support and metal 1.  The presence of diethylether formed by acid catalysed reaction of ethanol confirmed the presence of acidic sites on the alumina.  Ethyl acetate was similarly be formed from ethanol and acetic acid, while 1,1-diethoxy ethane, which was also

Table 1.  Dry gas molar selectivity at 100 h time-on-stream

 

Temp (K)

H2

CO

CH4

CO2

C2H4

C2H6

773

44.3

3.6

5.7

7.5

33.5

5.4

823

42.3

5.5

8.5

6.7

29.8

7.3

873

35.1

14.7

19.7

3.9

16.8

9.8

formed, is the acetal formed by acid catalysed reaction between ethanol and acetaldehyde.  The formation of acetic acid was slightly surprising but can be viewed as an intermediate in the oxidation of ethanol to carbon dioxide.  The liquid products detected reveal the complexity of the reaction sequence with products formed requiring a cascade of reactions (e.g. acetone).  They also reveal that most of the non-decomposition reactions are acid or base catalysed and do not, in themselves, require a metal to be present.

Catalyst deactivation occurred over the first 30 H on stream and was shown to be due to carbon deposition.  After 30 h steady-state operation was observed.  At all temperatures the TPO profile shows an event at high temperatures indicative of graphitic species, this is supported by the Raman spectra. 

Figure 1.  Post reaction Raman spectra of reduced and post reaction of Ru/Al2O3 at 773 K, 823 K and 873 K.

SEM images (figure 2) show that on the 773 K post reaction sample, no filamentous type carbon was observed.  However, as the temperature was increased from 773 K to 873 K filamentous carbon was observed (figure 2).  On the catalyst run at 873 K, besides filamentous coke, multi-wall carbon nanotubes appeared; these are only seen in the sample that had been used for ESR at 873 K.At 873 K CNTs are formed, which appears to be the first example of CNT formation over ruthenium.  Although CNTs are present on the catalyst surface it is likely that they have little effect on catalyst activity, rather it is the other carbonaceous species that cause the reduction in activity.  The existence of such species can be seen from the limited temperature TPO, the Raman spectra having a different IG/ID ratio from those in the literature representative of CNTs and the full TPO is different from that found in pure CNT systems.  It is likely that ethene is the major coke precursor with contributions from carbon monoxide and methane.  However although filamentous carbon/CNTs may not directly affect activity they are a major issue in industrial application. 

Figure 2.  SEM images of Ru/Al2O3 after ESR at 773 K and 873 K.

In steam reforming of hydrocarbons over nickel catalysts it is important to avoid the formation of CNTs or whisker carbon 2.  The CNT/whisker is strong and, although the property is much desired in nanotechnology, it has the potential to cause significant damage in a steam reformer 3.  CNTs can easily fracture a catalyst pellet when they come into contact with a pore wall and in a reformer tube broken catalyst pellets and carbon can cause maldistribution of the feed gas leading to overheating of tubes and, in the limit, result in tube failure.  Therefore the presence of CNT/whisker species in steam reforming must be avoided.  The presence of these species in ESR over Ru/alumina is an issue that must be resolved before large scale application could be envisaged. 

References.

1.  M. Dobrovolszky, P. Tétényi, Z. Paál, J. Catal., 1982, 74, 31-43.

2.  S. Helveg, J. Sehested, J.R. Rostrup-Nielsen, Catal. Today, 2011, 178, 42–46.

3.  J.R. Rostrup-Nielsen, L.J. Christiansen, Concepts in Syngas Manufacture, Imperial College Press, London, 2011

 


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