(265b) Steam Reforming of Ethanol Over Ru/Alumina: Effect of Temperature On Activity, Selectivity and Carbon Laydown

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.

.

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

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 I_G/I_D
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.

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3.  J.R. Rostrup-Nielsen,
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