Steam reforming of light oxygenates

Article abstract of DOI:10.1039/c3cy00635b

Steam reforming (SR) of ethanol, acetic acid, acetone, acetol, 1-propanol, and propanal has been investigated over Ni/MgAl₂O₄ at temperatures between 400 and 700 °C and at a steam-to-carbon-ratio (S/C) of 6. The yield of H₂ and conversion increased with temperature, while the yield of by-products decreased with temperature in the SR of the investigated compounds. The yield of H₂ approached the thermodynamic limit at the highest temperatures investigated. No significant differences in conversion as a function of temperature among the different model compounds were observed. However, the product distribution depended on the model compound, and C₃-oxygenates produced a larger fraction of by-products compared to C₂-oxygenates. Temperatures of 600 °C or above were generally needed to minimize the fraction of by-products and obtain a syngas containing mainly CO, CO₂, H₂, and H₂O with only traces of CH₄. Significant deactivation of the catalyst was observed for all of the compounds and was mainly due to carbon formation. The carbon formation was highest for alcohols due to a high formation of olefins, which are potent coke precursors. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cy00635b

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Steam reforming of light oxygenates
a
b
a
*
Rasmus Trane-Restrup, Daniel E. Resasco and Anker Degn Jensen
Cite this: Catal. Sci. Technol., 2013, 3,
3292
Steam reforming (SR) of ethanol, acetic acid, acetone, acetol, 1-propanol, and propanal has been investigated over
Ni/MgAl2O4 at temperatures between 400 and 700 °C and at a steam-to-carbon-ratio (S/C) of 6. The yield of
H2 and conversion increased with temperature, while the yield of by-products decreased with temperature in the
SR of the investigated compounds. The yield of H2 approached the thermodynamic limit at the highest temper-
atures investigated. No significant differences in conversion as a function of temperature among the different
model compounds were observed. However, the product distribution depended on the model compound, and
C3-oxygenates produced a larger fraction of by-products compared to C2-oxygenates. Temperatures of 600 °C or
above were generally needed to minimize the fraction of by-products and obtain a syngas containing mainly
CO, CO2, H2, and H2O with only traces of CH4. Significant deactivation of the catalyst was observed for all of the
compounds and was mainly due to carbon formation. The carbon formation was highest for alcohols due to a high
formation of olefins, which are potent coke precursors.
Received 26th August 2013,
Accepted 24th September 2013
DOI: 10.1039/c3cy00635b
www.rsc.org/catalysis
1. Introduction
can be combusted to provide energy for the pyrolysis process
or drying of the biomass.3,8,13,14 The bio-oil has an energy
Most of the energy converted in the world is based on fossil
fuels like oil, coal, or natural gas. However, the fossil
resources are limited and associated with the emission of
CO2, which is a greenhouse gas, leading to changes in the
environment. Biomass, like fossil fuels, contains carbon, and
furthermore, it is a renewable resource and has the potential
to be CO2-neutral. Therefore, it is interesting to use biomass
as an energy source, and for this purpose, different options
exist. Biomass can be combusted directly in boilers for heat
and power production, or it can be gasified to produce syn-
thesis gas.1–3 Synthesis gas can be combusted in a gas tur-
bine or utilized to synthesize fuels or chemicals.3,4 However,
both of these processes have problems associated with the
content of inorganics in the biomass, which can lead to ash
deposition on heat transfer surfaces or on downstream gas
cleaning catalysts, which leads to problems like reduced heat
transfer and catalyst deactivation.5,6 Furthermore, biomass
has a low energy density, which leads to high costs of trans-
porting the biomass to power plants or gasifiers.7
density, which can be up to ten times higher than untreated
biomass.7,8 Therefore, pyrolysis can be used to reduce some
of the problems associated with biomass utilization, as the
energy density can be increased and the content of inor-
ganics can be decreased.
Bio-oil is an acidic and viscous liquid with a high content
of oxygen, which induces a low heating value, and further-
more, the different oxygenates can polymerize during stor-
age.15–17 This leads to increased viscosity and, in severe
cases, also phase separation and gum formation.15–17 Fossil
fuels have only small amounts of oxygen, and if oxygen is
removed from the bio-oil, a crude oil-like product can be
obtained. For this purpose, hydrodeoxygenation (HDO) can be
used, which is a catalytic process carried out at 250–350 °C
and 50–100 bar of H2.18 The hydrogen needed for HDO can
be produced from the bio-oil through steam reforming
(SR).19 Furthermore, SR of bio-oil can be used to produce
hydrogen or synthesis gas for other processes like ammonia
synthesis or Fischer–Tropsch synthesis.
Another option for the utilization of biomass is to flash
pyrolyze the biomass, which produces gas, liquid, and solids.
The liquid fraction, bio-oil, can be obtained in yields up to
70 wt%.7–9 The inorganics from the biomass are mainly
trapped in the solid residue10 and can be used as fertilizer
and for enhancing soil properties,11,12 while the gas fraction
Steam reforming of bio-oil has not been investigated to a
large extent, but studies have shown rapid deactivation due
to carbon deposition.19–23 Therefore, several different options
for reducing carbon deposition, like the use of a fluid bed
reactor technology or adding oxygen to combust the carbon
deposition, have been investigated.24–31 Another option is to
do a fractionation of the bio-oil by adding water to it, which
results in a fraction with a high content of phenolics, the
organic fraction, and an aqueous fraction containing light
oxygenates like acetol and acetic acid, as well as small
amounts of phenols and furans.9,32 The aqueous fraction can
a Department of Chemical and Biochemical Engineering, Technical University of
Denmark, Søltofts Plads, bygn. 229, 2800 Kgs Lyngby, Denmark. E-mail: aj@kt.
dtu.dk; Fax: +45 4588 2258; Tel: +45 4525 2841
b School of Chemical, Biological & Materials Engineering, The University of
Oklahoma, 100 East Boyd Street, Oklahoma, USA
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be up to 50–60% of the organics in bio-oil, and it has a S/C
ratio of about 4 on a weight basis or 6–8 on a molar basis,
which is suitable for SR.24,28,33,34 It should be noted that it is
higher than typical values of S/C ratios used in the SR of
hydrocarbons, but this may be beneficial due to a higher
potential for forming coke in the SR of oxygenates.35 The
addition of water to the bio-oil gives the possibility to use
the aqueous fraction of bio-oil to produce H2 for upgrading
the organic fraction of the bio-oil to fuels through HDO.
Studies on SR of bio-oil have been conducted with the
entire oil fraction, the aqueous fraction, or with single model
compounds like ethanol or acetic acid.19,24,26,28,33,34,36–40
Other model compounds, which have been investigated to
a lesser extent compared with bio-oil, ethanol, and acetic
acid, are acetol,41–43 propanol,42,44–46 butanol,41,42,46 ethylene
glycol,24,46–50 acetone,42,46,47,51,52 ethyl acetate,47 phenol,53–57
and sugars like glucose and sorbitol.47,58,59
condensation, and ketonization, which lead to oxygen
containing compounds, have also been observed.45
One of the few studies comparing the SR of different oxy-
genates at similar conditions was conducted by Palmeri et al.42
A similar conversion was observed for C2–C4 alcohols, while a
decrease in conversion with increasing carbon number was
observed for C2–C4 ketones and aldehydes in this study.42
Furthermore, it was found that the CH4 selectivity increased
with chain length, while the H2 selectivity decreased for
alcohols, ketones, and aldehydes.42 A decrease in reactivity
and an increase in carbon deposition with chain length have
also been reported by Hu and Lu46 in the SR of both ketones
and alcohols.
Information on the SR of light bio-oil oxygenates is rather
scarce, and furthermore, there is a lack of comparison of
reactivity, reaction pathways, and deactivation among the dif-
ferent light oxygenates. The scope of this work is therefore to
investigate the SR of different light oxygenates: ethanol,
acetic acid, acetone, acetol, 1-propanol, and propanal to elu-
cidate differences in reactivity, reaction paths, and side reac-
tions. Furthermore, deactivation and carbon deposition
during the SR of light oxygenates will be investigated. Acetic
acid and acetol were chosen as they are present in bio-
oil,15,71 while the remaining model compounds were chosen
to elucidate general trends in the SR of oxygenates.
The thermodynamically favorable products in the SR of
both hydrocarbons and oxygen containing hydrocarbons are
CO, CO2, CH4, and H2, and the equilibrium can be described
by SR, reaction 1, the water–gas shift (WGS), reaction 2, and
the methanation reaction, reaction 3.
CnHmOp þ ðn−pÞH2O→nCO þ ðn−p þ m=2ÞH2
CO þ H2O⇌CO2 þ H2
ð1Þ
ð2Þ
ð3Þ
2. Experimental
2.1. Catalyst preparation
The catalyst was prepared by incipient wetness impregnation
with a Ni(NO3)2-solution on MgAl2O4 (Puralox MG30/150
from SASOL, 143 m2 g−1). Ni(NO3)2·6H2O (Sigma-Aldrich,
>97% pure) was dissolved in a volume of water correspond-
ing to the pore volume of the dry carrier (porosity: 0.9 mL
g−1). The solution was mixed with the dry carrier material
and stirred. The wet particles were dried at 110 °C overnight
in air and calcined at 800 °C (heating rate of 10 °C min−1) for
2 h in stagnant air.
CO þ 3H2O⇌CH4 þ H2O
The thermodynamic equilibrium predicts that at low tem-
peratures, below 500 °C, the main products are CO2 and CH4,
and as the temperature increases, the CO2 and CH4 fractions
decrease, while the fractions of H2 and CO increase. The
hydrogen production has a maximum at 600–800 °C because
at even higher temperatures the WGS will shift towards CO
and H2O.
In the SR of hydrocarbons, the main products are CH4,
CO2, CO, and H2, but small amounts of hydrocarbons larger
than C1 can also be formed.60–65 The suggested reaction
mechanism is that the hydrocarbon adsorbs on the surface
followed by sequential cleaving of the C–C-bonds.61,64 This
leads to C1-fragments which can desorb as CH4 or react fur-
ther forming CO, CO2, and H2 by breaking the C–H bonds
and reacting with O-species on the surface of the catalyst.
The conversion at a given temperature is lowest for CH4 and
in the same range for hydrocarbons with longer chains.60
The reaction mechanisms for ethanol and acetic acid sug-
gest that the first step is adsorption through the oxygen
followed by breaking of the C–C-bonds, which mainly pro-
duces C1-species.37,52,66–69
2.2. Experimental setup
The setup consisted of four gas lines with a corresponding
mass flow controller (MFC) connected to a vaporizer where
injection of a liquid was possible. The vaporizer was a 6.35 mm
stainless steel tube heated to 300 °C by heating tape, and the
feed mixture of water and oxygenate was delivered by a syringe
pump and injected into the vaporizer through a septum. In
the vaporizer, the feed was mixed with the carrier gas. The
reactant mixture then entered a quartz reactor placed in a fur-
nace. The temperature was measured by a thermocouple on
the outside of the reactor tube. The reactor effluent was
connected to a Varian 3800 gas chromatograph (GC) equipped
with a Restek Rxi-5Sil mass spectrometry (MS) column, a
flame ionization detector (FID), and a thermal conductivity
detector (TCD) by a heated line kept at 300 °C. This GC was
used to determine the concentrations of oxygenates and
C3-hydrocarbons. After the GC, there was a cold trap with ice
The main products reported in the SR of oxygen con-
taining hydrocarbons are carbon oxides, hydrogen, and hydro-
carbons of equal or shorter chain length than the original
compounds.41,42,46,51,70 Side reactions like dehydrogenation,
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to condense water, products, and unconverted oxygenates.
The gases from the cold trap were collected in a gas bag and
analyzed by a GC/TCD (Carle AGC 400) to determine the con-
centrations of CO, CO2, H2, CH4, C2H4, C2H6, and N2. The
schematics of the setup are shown in Fig. 1.
2.5. Catalyst characterization
The BET surface area of the prepared catalyst was measured
by N2 adsorption at its boiling point using multipoint BET
theory with seven points in the p/p0 = 0.05–0.3 range using a
Quantachrome iQ2.
The XRD-spectra of the calcined and unreduced catalyst
powders were recorded by a PANalytical X'Pert PRO diffrac-
tometer, which had a rotating sample holder, a rotating copper
anode X-ray source, a nickel filter, and automatic anti-scatter
and divergence slits. The particle size of the NiO-crystals was
determined by the Scherrer equation, eqn (4), from the NiO
reflection peak at 2θ = 62.8°:72
2.3. Catalytic test
The catalyst, with a particle size of dp = 250–425 μm, was
loaded in a quartz tube with an inner diameter of 4.35 mm
and a length of 343 mm. The catalyst was placed on a con-
striction in the middle of the quartz tube between two layers
of quartz wool. The catalyst was heated to 600 °C in a flow of
N2 of 40 N mL min−1. At 600 °C, the flow was changed to a
50/50 mixture of N2 and H2 for 1 h to reduce the NiO in the
catalyst to Ni. After the reduction, the catalyst was flushed
with N2 for 5 min. The experiment was started hereafter by
adjusting the N2 flow, starting the syringe pump, and
inserting the needle through the septum in the vaporizer.
The feed gas had a S/C ratio of 6, and the mole fractions of
the oxygenates and H2O were 2–3% and 36%, respectively,
with N2 as balance.
K 
 cos()
dp 
ð4Þ
K is a shape factor set to 0.9,72 λ is the X-ray wavelength, β is
the full width at half maximum corrected for the instrumental
line broadening, while θ is the Bragg angle.
2.6. Calculations
W/F is defined as mass of catalyst per molar flow of oxygen-
The temperature dependence experiments were conducted
without changing the catalyst sample and by staying for 2 h
at each temperature from 700 °C to 400 °C to minimize the
influence of carbon deposition which is more significant at
low temperatures, 400 and 500 °C.35
ates, FOxy,m
:
mCat
W / F 
(g h mol1)
ð5Þ
FEth,m
The conversion is calculated on a carbon basis as:
2.4. Determination of carbon deposition
nC,i  ni
ð6Þ
X 
100%
The amount of carbon on the spent catalysts was determined
by temperature programmed oxidation (TPO). 2–5 mg of the
spent catalyst was placed between two layers of quartz wool
in a quartz reactor with an inner diameter of 4.35 mm and a
length of 394 mm. The sample was heated from room tem-
perature to 800 °C (heating rate of 10 °C min−1) in a flow of
5% O2 in He. The released carbon oxides were converted to
CH4 over a methanation catalyst operated at 400 °C, and the
amount of CH4 formed was measured by a FID.
nC,Oxy  nOxy
where nC,i is the number of carbon atoms in component i,
and ni is the number of moles of compound i produced.
The yield of products is calculated using the definition
in:73
nC,i  ni
100%
Y  nC,i  ni
ð7Þ
i
The yield of H2 is defined as:
nH
2
YH

100%
ð8Þ
2
n
,max  X  nOxy
H2
The factor of nH ,max is the maximum number of H2 moles,
2
which can be produced from 1 mole of oxygenate, including
full shift of CO to CO2 and H2. nH ,max is 6 for ethanol, 4 for
2
acetic acid, 8 for acetone, 8 for propanal, 9 for 1-propanol,
and 7 for acetol.
Carbon deposition is reported as the amount of carbon
deposited per amount of carbon in the feed, mmol C mol
CFeed−1, and as the mass of carbon deposited per mass of
Fig. 1 Flow sheet of the experimental setup. CT: cold trap, GC: gas chromatograph,
catalyst and time, mg C gCat h−1.
−1
MFC: mass flow controller, P: pressure gauge, SP: syringe pump.
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3. Results and discussion
3.1. Characterization
Blank experiments in the SR of ethanol showed conversion
of less than 15% at 700 °C and less than 2% at 500 °C, so the
contribution from homogeneous reactions is considered to
be insignificant. Similar degrees of conversion have been
reported in the SR of ethanol41,74–76 and SR of acetic acid77,78
in a blank reactor. Higher degrees of conversion have been
reported in blank experiments in the literature,20,79,80 but it
is usually over quartz or SiC particles and therefore not com-
parable to our experiments as we do not dilute our catalyst.
The effect of the support material, MgAl2O4, has been
tested in the SR of ethanol at 600 °C and only showed dehy-
dration activity with a conversion of 30%, so the production
of carbon oxides and H2 is mainly related to metal particles.
A conversion of 40% has been reported in the SR of acetic
acid over CaAl2O4 at 750 °C mainly to decomposition prod-
ucts, CH4 and CO2, and similar products were reported in
the SR of acetone over the same support material, although
the conversion was 80%.51 Basagiannis and Verykios77
reported close to 0% conversion of acetic acid over Al2O3
and La2O3 at 700 °C. Therefore, MgAl2O4 may induce dehy-
dration, but formation of CH4, as well as CO, CO2, and H2, is
mainly related to Ni and interactions between Ni and
MgAl2O4. Ketonization and other coupling reactions may also
be catalyzed by MgAl2O4 as oxides have shown activity for
these reactions.81–83
The surface area and NiO particle size were determined for
the Ni/MgAl2O4 catalyst used for these experiments. The char-
acteristics of the support and the catalyst can be seen in
Table 1.
The catalyst had a surface area of roughly 115 m2 g−1, and
the NiO particles were approximately 5 nm in diameter. XRD
analysis of the catalyst showed MgAl2O4 and NiO phases and
did not indicate that Ni was incorporated into the spinel
structure, see Fig. 2.
3.2. General trends
The effects of temperature and time-on-stream on the SR of
several light oxygenates have been investigated at a S/C of 6
and mole fractions of oxygenates and H2O of 2–3% and 36%,
respectively. The total flow rate and molar flow of C were sim-
ilar in each experiment.
In the SR of different model compounds, it was found that
the trends in the yields of CO and CO2 with temperature were
in accordance with the equilibrium of the WGS at tempera-
tures of 500 °C or above. This means that the yield of CO
increased with temperature, while the yield of CO2 decreased.
The yield of H2 increased with temperature in all of the cases,
and a maximum yield of H2 of roughly 70–80% was observed
at 700 °C, showing that H2-production was not significant at
400 and 500 °C. The conversion of the model compounds
increased with temperature from roughly 10% at 400 °C to
full conversion at 700 °C.
3.3. Steam reforming of ethanol
The conversion of ethanol and the product yields of CO, CO2,
H2, CH4, and C2H4 as functions of temperature in the SR of
ethanol are summarized in Fig. 3. The yield of CO2 had a
maximum at 600 °C, while the yield of CO increased with
temperature from 500 to 700 °C. This is consistent with an
increased activity for the SR reactions and a shift in the WGS
with temperature as predicted by thermodynamic equilib-
rium. The yield of H2 increased with temperature from 6% at
400 °C to 74% at 700 °C. The yield of methane was high at
low temperatures (29% at 400 °C) but decreased with
Table 1 Characterization results of the fresh catalysts prepared by calcination
in stagnant air at 800 °C. The NiO peak at 2θ
= 62.8° was used for
determination of particle size.
Catalyst
Ni loading (wt%) Surface area (m2 g−1
)
dp,NiO (nm)
MgAl2O4
Ni/MgAl2O4
—
8.2
143
115
—
5
Fig. 3 Conversion and yield of CO, CO2, H2, CH4, and C2H4 as functions of
temperature in the SR of ethanol. 8.0 wt% Ni/MgAl2O4, S/C = 6.0, mCat = 50 mg,
flow = 230 N mL min−1, yEth = 3%, yH2O = 36%, N2 as balance.
Fig. 2 XRD spectrum for Ni/MgAl2O4.
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temperature and ended at 0% at 700 °C, while the yield of
C2H4 had a maximum of 3% at 500 °C, although only slightly
higher compared with the yield at 400 °C, and decreased to
0% at 600 °C. Ethene is most likely formed through dehydra-
tion of ethanol and is known as a potent precursor to carbon
formation.84,85 Note that with the chosen definition of yield, it
is possible to have a high yield even if the conversion is low.
These results suggest that ethanol adsorbs on the surface
and then either dehydrates to ethene or breaks the C–C-bond
leading to the formation of two C1-species, which can, de-
pending on the temperature, react and form either CH4 or COx.
Ethene might also react further to form CH2˙ by breaking the
C–C-bond. Further information on the SR of ethanol can be
found in ref. 35, 37, and 86–88.
ZrO2.81 The lower yield of CH4 could be due to ketonization,
which forms CO2 and acetone and thereby removes two poten-
tial CH3-species, which could be converted to CH2. Further-
more, the oxygen containing part of ethanol might also be
easier to convert to methane as only one C–O bond needs to
be broken before a CHx-fraction is formed. For acetic acid,
two C–O bonds need to be broken.
It appears that acetic acid can follow two reaction pathways
after adsorption; it can either undergo ketonization to acetone
or cleavage of the C–C-bond leading to C1-species. The carbon
precursor, ethene, was not observed. DFT-calculations on the
SR of acetic acid over Co show that the most favorable
reaction pathway is adsorption through the carbonyl group,
followed by removal of one oxygen atom and subsequent
C–C-bond scission.66
3.4. Steam reforming of acetic acid
3.5. Steam reforming of acetone
The conversion of acetic acid and the product yields of
CO, CO2, H2, CH4, and acetone as functions of temperature
in the SR of acetic acid are summarized in Fig. 4. Here, it can
be seen that the main products were CO2 and CO at all
temperatures, while at low temperatures, CH4 and acetone
were formed as well. From 500 to 700 °C, the yield of CO2
slightly decreased, while the yield of CO increased, indicating
that the WGS shifted towards CO and H2O. The yield of
H2 increased with temperature from 5% at 400 °C to 80%
at 700 °C.
Ketonization of acetic acid to acetone occurred at 400 and
500 °C, and the yield of acetone was roughly 8%. The yield of
CH4 was less than 1% for all the temperatures and decreased
with temperature and was thus much lower in the SR of
acetic acid compared to the SR of ethanol. It might have been
expected that the yield would be similar to the yield of CH4
in the SR of ethanol as breaking the C–C bond in both cases
would form CH3- and CHxOy-species, where especially the
CH3-species are likely to react with H forming CH4 at low
temperatures. Ketonization of acetic acid is shown by DFT
and experiments to occur through the β-keto acid route on
The conversion of acetone and the product yields of CO, CO2,
H2, and CH4 as functions of temperature in the SR of acetone
can be seen in Fig. 5. CO, CO2, and CH4 were mainly observed
as carbon containing products. However, small amounts of
self-condensation products like 4-methyl-4-penten-2-one were
observed at 400 °C. The yield of CO increased with tempera-
ture, while the yields of CO2 and CH4 decreased with tempera-
ture. The formation of methane and carbon oxides as the only
products suggests that the acetone adsorbs through the car-
bonyl and then cleaves both of the C–C bonds. C2-compounds
like ethanol, acetaldehyde, or ethene might be expected in
the product gas, but these compounds were not observed.
This indicates that CH3CO˙ formed when removing one of
the methyl groups undergoes rapid transformation into
CO˙ and CH3˙.
3.6. Steam reforming of propanal
The conversion of propanal and the product yields of CO,
CO2, H2, CH4, and C2-species as functions of temperature in
the SR of propanal can be seen in Fig. 6. Ethene, ethane,
Fig. 4 Conversion and yield of CO, CO2, H2, CH4, and acetone as functions of
temperature in the SR of acetic acid. 8.0 wt% Ni/MgAl2O4, S/C = 6.0, mCat = 50
mg, flow = 210 N mL min−1, yAA = 3 vol%, yH2O = 36 vol%, N2 as balance.
Fig. 5 Conversion and yield of CO, CO2, H2, and CH4 as functions of temperature
in the SR of acetone. 8.0 wt% Ni/MgAl2O4, S/C = 6.0, mCat = 50 mg, flow = 210 N
mL min−1, yAce = 2 vol%, yH2O = 36 vol%, N2 as balance.
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Fig. 6 Conversion and yield of CO, CO2, H2, CH4, and C2H4 as functions of
temperature in the SR of propanal. 8.0 wt% Ni/MgAl2O4, S/C = 6.0, mCat = 50 mg,
flow = 210 N mL min−1, yPA = 2 vol%, yH2O = 36 vol%, N2 as balance.
Fig. 7 Conversion and yield of CO, CO2, H2, CH4, C2H4, C3H7, and propanal as
functions of temperature in the SR of 1-propanol. 8.0 wt% Ni/MgAl2O4, S/C = 6.0,
m
Cat = 50 mg, flow = 210 N mL min−1, yPO = 2 vol%, yH2O = 36 vol%, N2 as balance.
methane, and carbon oxides were observed as products. The
C2-fraction consisted of roughly 2/3 ethene and 1/3 ethane,
and the relative amounts of these species were not signifi-
cantly influenced by temperature. No ethane was observed
in the SR of ethanol. At 400 °C, there was a high yield of
C2-species and no formation of CH4. Methane was observed
in the SR of acetone, ethanol, and acetic acid at 400 °C, and
the difference could be that more C–C-bonds need to be bro-
ken to convert propanal to methane as compared with the
model compounds discussed so far. The yield of C2-species
decreased with temperature, while the yield of CH4 increased
with temperature until 600 °C and decreased thereafter. The
formation of C2-compounds could come from breaking the
C–C bond closest to the carbonyl group in propanal, and
methane might be produced by breaking the C–C bond of the
C2-species on the surface. This would also explain the
increasing level of methane in the off-gas, while the yield of
C2-species decreased.
might be formed from further reactions of C3- and C2-species
and is not a primary product.
The reaction path for conversion of 1-propanol appears to
consist of a sequential scission of the carbon backbone simi-
larly to what was observed in the SR of propanal. No methanol
or ethanol were observed in the SR of 1-propanol, indicating
that the C–C-bond scission starts from the hydroxyl end. Fur-
thermore, it appears that dehydration is favored over this cat-
alyst as large fractions of ethene and propene were observed
in the SR of ethanol and 1-propanol, respectively.
3.8. Steam reforming of acetol
The conversion of acetol and the product yields of CO, CO2,
H2, CH4, and other products as functions of temperature in
the SR of acetol can be seen in Fig. 8. The products from the
SR of acetol were different from the other oxygenates investi-
gated, especially at 400 and 500 °C. Here, a large fraction of
products was formed through different condensation
The results suggest that the propanal is converted by
sequential breaking of the carbon backbone from the car-
bonyl end as no oxygen containing by-products were formed.
3.7. Steam reforming of 1-propanol
The conversion of 1-propanol and the product yields of CO,
CO2, H2, CH4, C2-species, propene, and propanal as functions
of temperature in the SR of 1-propanol are summarized in
Fig. 7. The same products as in the SR of propanal with the
addition of propene and propanal, which can be formed
through dehydration and dehydrogenation of 1-propanol,
were observed. The C2 product fraction consisted of 75–85%
ethene and 15–25% ethane. The major products were
propene, CO2, propanal, and C2-species at 400 °C. The yields
of the by-products, propene, C2-species, and propanal, all
decreased with temperature and reached 0% at 600 °C. The
yield of CH4 increased with temperature until 600 °C and
then decreased. This behavior was similar to what was
observed in the SR of propanal and suggests that methane
Fig. 8 Conversion and yield of CO, CO2, H2, CH4, and condensation products
(others) as functions of temperature in the SR of acetol. 8.0 wt% Ni/MgAl2O4, S/C =
6.0, mCat = 50 mg, flow = 210 N mL min-1, yAT = 2 vol%, yH2O = 37 vol%, N2 as
balance.
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reactions forming butanediol, 2,5-hexanedione, substituted
cyclopentanedione, furandiones, and furanones (others in
Fig. 8) and constituted the major product group at both 400
and 500 °C. Furthermore, small amounts of ethanol and acet-
aldehyde were observed as products as well, which can be
formed by breaking one of the C–C-bonds in acetol. The yield
of CH4 had a maximum at 500 °C and then decreased to 0%
at 700 °C, while the yield of H2 increased from 0 at 400 °C to
76% at 700 °C. Acetol has two oxygen functionalities and
these seem to induce a high formation of condensation prod-
ucts especially at low temperatures. This might be explained
by the free oxygen end when acetol is adsorbed on the cata-
lyst, which can react with other compounds on the surface
and oligomerize.
acetone. These species are probably more readily converted
compared with ethene or C2-species formed by breaking the
first C–C-bond in propanal. DFT-calculations on the conver-
sion of acetic acid show that conversions of CH3CO˙ to CH3˙
and CO˙ are favorable with a decrease in energy of 38 kJ mol−1
and an activation energy of 78 kJ mol−1 66
.
Many by-products were produced at 400 and 500 °C, and a
comparison of the yield of oxygenates as a function of temper-
ature can be seen in Fig. 10, while a comparison of the yield
of hydrocarbons as a function of temperature can be seen in
Fig. 11. The yields of both types of by-products decreased with
temperature and were close to zero above 600 °C. For the alco-
hols, it can be seen that relatively high amounts of hydrocar-
bons were formed and a fraction of those were olefins, ethene
and propene, which might lead to carbon deposition.84,85,89
The yields of hydrocarbons and oxygenates were higher for
1-propanol compared with ethanol, which indicates that the
reactivity decreased slightly with increasing chain length. Sim-
ilar results have been reported by Palmeri et al.42 Acetic acid,
1-propanol, and acetol produced oxygen containing com-
pounds, as well as hydrocarbons. Acetol had the highest yield
of oxygenates of all the compounds.
From Fig. 10 and 11, it appears that temperatures above
600 °C are needed to avoid by-products in the off-gas. How-
ever, this might be due to either a higher degree of conver-
sion or a changed reaction path at higher temperature. To
investigate this, an experiment at a higher space velocity at
600 °C was carried out while keeping the overall conversion
close to that at 500 °C and the lower space velocity. A com-
parison of the yield of by-products under the two conditions
can be seen in Table 2. For most of the experiments at
600 °C, the yields of hydrocarbons and oxygenates were lower
compared to the experiment at 500 °C. It was especially pro-
nounced for acetol, where a decrease in the yield of oxygen-
ates from 51 to 5% was observed at similar conversion but
different temperatures. Furthermore, the by-products were
mainly acetone, ethanol, and acetaldehyde at 600 °C and
not condensation products as observed at 500 °C. Therefore,
Two main types of reaction pathways can take place in the
SR of acetol, either condensation reactions leading to larger
compounds or C–C-bond breaking leading to smaller com-
pounds like COx, H2, and CH4.
3.9. Comparison
The conversion of the investigated model compounds as a
function of temperature is summarized in Fig. 9. There was
mainly a difference in conversion at 500 °C, while the conver-
sions were similar at all other temperatures. The higher con-
version of acetol at 400 °C and 500 °C compared with the
other compounds was mainly as condensation products and
included a wide range of different oxygenates, as discussed
above. The conversion of ethanol was similar to the conver-
sion of 1-propanol at all temperatures, which indicates that
the chain length of the oxygenate did not influence the total
conversion in the SR of alcohols. Similar results were
obtained by Palmeri et al.42 for the SR of alcohols. Further-
more, the conversion of acetone was higher compared to the
conversion of propanal, indicating that the position of the
oxygen functionality in the molecule influences the reactivity.
In this case, it might be explained by a more favorable conver-
sion of CH3CO˙ formed when breaking the first C–C-bond in
Fig. 9 Conversion as a function of temperature in the SR of ethanol, acetic acid,
acetone, 1-propanol, and propanal. 8.0 wt% Ni/MgAl2O4, S/C = 6.0, mCat = 50 mg,
flow = 210 N mL min−1, yOxy = 2–3 vol%, yH2O = 37 vol%, N2 as balance.
Fig. 10 Yield of oxygen containing hydrocarbons as a function of temperature in
the SR of light oxygenates. 8.0 wt% Ni/MgAl2O4, S/C = 6.0, mCat = 50 mg, flow =
230 N mL min−1, yOxy = 2–3 vol%, yH2O = 36 vol%, N2 as balance.
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Fig. 13 Proposed reaction pathway in the SR of 1-propanol.
oxygenates was slightly higher compared with C3 oxygenates.
The one exception was acetone, which might be because
breaking the C–C bond closest to the carbonyl group would
lead to CH3-species, which might be easier to convert as
discussed previously.
Fig. 11 Yield of hydrocarbons as a function of temperature in the SR of light
oxygenates. 8.0 wt% Ni/MgAl2O4, S/C = 6.0, mCat = 50 mg, flow = 230 N mL min−1
,
y
Oxy = 2–3 vol%, yH2O = 36 vol%, N2 as balance.
3.10. Reaction pathway
Table 2 Comparison of conversion, yield of hydrocarbons, and yield of oxygenates
at different temperatures and similar conversions. 8.0 wt% Ni/MgAl2O4,
S/C = 6.0, mCat = 50 mg, flow= 210 N mL min−1, yOxy = 2–3 vol%, yH2O = 36 vol%,
Steam reforming of light oxygenates over Ni/MgAl2O4 at low
temperatures produced a large fraction of by-products with
similar or shorter chain length than the parent model com-
pounds. For alcohols, dehydration to olefins or dehydrogena-
tion to aldehydes with the same chain length was observed,
while for the carbonyls, only decarbonylation products were
observed. This indicates that the first step in the reaction is
adsorption to the catalytic site through the oxygen functional-
ity. Adsorption through oxygen is shown to be preferred by
DFT-studies on adsorption of oxygenates on transition metal
surfaces.66,67,90,91 The products shifted from C3- and C2-spe-
cies towards C1-species with increasing conversion and tem-
perature, and furthermore, a build-up of CH4 was observed
as C3- and C2-species were converted. This indicates that the
conversion occurs through sequential breaking of the C–C-
bonds in the oxygenates. Furthermore, in the SR of acetone,
only C1-products were formed, which also fits the proposed
pathway of adsorption through the oxygen and breaking of
the C–C-bonds closest to the oxygen. The proposed reaction
pathway is shown for the SR of 1-propanol in Fig. 13.
N2 as balance
Compound
Temperature (°C)
X (%)
YC Hy (%)
YC HyOz (%)
x
x
Ethanol
Ethanol
Acetone
Acetone
Acetic acid
Acetic acid
Acetol
500
600
500
600
500
600
500
600
500
600
500
600
57
68
49
60
51
62
65
58
59
65
43
61
27
12
11
2
1
0
3
2
33
27
5
0
0
0
0
8
8
51
5
13
5
0
Acetol
1-Propanol
1-Propanol
Propanal
Propanal
6
0
it appears that operation at 600 °C or higher minimized the for-
mation of by-products and especially condensation products.
The desired compounds in the SR are carbon oxides and
hydrogen, and the conversion to carbon oxides (COx) can be
seen in Fig. 12. Here it can be seen that the conversion of C2
The formation of condensation products at 500 °C was sig-
nificantly higher for acetol which has two oxygen containing
Fig. 12 Conversion to carbon oxides as a function of temperature in the SR of
light oxygenates. 8.0 wt% Ni/MgAl2O4, S/C = 6.0, mCat = 50 mg, flow = 210 N mL
min−1, yOxy = 2–3 vol%, yH2O = 37 vol%, N2 as balance.
Fig. 14 Conversion as a function of time in the SR of light oxygenates at 600 °C.
8.0 wt% Ni/MgAl2O4, S/C = 6.0, mCat = 25 mg, flow = 210 N mL min−1, yOxy = 2–3
vol%, yH2O = 37 vol%, N2 as balance.
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Table 3 Conversion, product yields, and carbon depositions in the SR of light oxygenates at 600 °C after 4 h on stream. 8.0 wt% Ni/MgAl2O4, S/C= 6.0, mCat
25 mg, flow= 210 N mL min−1, yOxy = 2–3 vol%, yH2O = 37 vol%, N2 as balance
=
Carbon deposition
X
(%)
YC Hy
(%)
YC HyOz
(%)
x
x
Compound
(mg gCat h−1
)
(mmolC molCfeed
)
−1
−1
Ethanol
Acetic acid
Acetone
Acetol
1-Propanol
Propanal
68
62
60
58
65
61
12
0.4
2
7
28
6
0
8
0
2
5
0
41
40
32
33
42
29
2.5
2.4
1.9
2.0
2.5
1.7
functional groups. This could be due to a free oxygen func-
tionality, which can react with other surface species forming
larger chains.
Carbon deposits can be formed through the Boudouard
reaction, CO decomposition, polymerization of acetone, and
methane decomposition, as well as through polymerization
of ethene and propene depending on the temperature and
the S/C-ratio.
3.11. Deactivation
Steam reforming of the different model compounds was
investigated for 4 hours at 600 °C, a S/C-ratio of 6, and with a
conversion between 60 and 80%. The influence of time on
stream on the conversion can be seen in Fig. 14. The conver-
sion for all of the compounds decreased with time, and the
decrease was most pronounced for acetol and acetic acid,
where the conversion decreased by 15–20%-points, while for
ethanol, 1-propanol, and propanal, the decrease in conversion
was roughly 10%-points. The conversion of acetone was most
stable with a 7%-points decrease in conversion over time.
The final conversion (initial conversion can be seen in
Fig. 14), yield of by-products, and carbon deposition for a
4 hour experiment can be seen in Table 3. The two model
compounds which led to the highest carbon deposition were
the two alcohols, ethanol and 1-propanol. This was due to
the relatively high formation of ethene and propene, which
are likely to polymerize as coke on the catalyst.84,85,92 Acetic
acid also had a high carbon deposition, while the other three
model compounds showed a 25% lower carbon deposition. A
higher carbon deposition in the SR of alcohols compared to
ketones and aldehydes over Ni/Al2O3 at 700 °C and a S/C-ratio
of 4 was also reported by Palmeri et al.42 However, the differ-
ences was more pronounced in the study by Palmeri et al.42
as the SR of ethanol resulted in 33% higher carbon deposi-
tion compared with the SR of acetaldehyde, while the SR
of 2-butanol produced 6 times as much carbon as the SR of
2-butanone. Hu and Lu47 reported a higher carbon deposi-
tion in the SR of acetone compared with acetic acid over
Ni/Al2O3 at 600 °C and a stoichiometric S/C-ratio. However,
the opposite trend was observed in this study.
Fig. 15 Formation of methane as a function of temperature during TPO of spent
catalysts. msample = 2–5 mg, ΔT: 10 °C min−1, yO2 = 5 vol%, He as balance.
Methanation catalyst operated at 400 °C.
carbon oxidation peaks, a main peak at roughly the same
temperatures as the other model compounds, and a smaller
peak at lower temperatures, 500–525 °C. The low-temperature
peak might indicate that encapsulating carbon was formed
with these oxygen containing model compounds, as it would
have close contact with Ni and therefore oxidizes at a lower
temperature. Furthermore, acetol, acetic acid, and propanal
also showed a rather steep deactivation with time on stream,
which also could point to the coverage of the Ni-particles by
carbon, while whisker formation would not significantly
deactivate the catalyst.89,93
4. Conclusions
Steam reforming of ethanol, acetic acid, acetone, acetol,
1-propanol, and propanal has been investigated on Ni/MgAl2O4
at temperatures between 400 and 700 °C at a S/C-ratio of 6.
In all the cases, full conversion was achieved at 700 °C at
the applied space velocity, and the yield of H2 varied between
70 and 80%. The conversion as a function of temperature did
not vary significantly between the different model com-
pounds. However, the yield of by-products was higher for
C3-oxygenates compared to C2-oxygenates, which shows a
decrease in reactivity with chain length.
The carbon release during the TPO of the spent catalysts
is shown in Fig. 15. Ethanol, 1-propanol, and acetone all
show carbon oxidation in a single peak with a maximum at
roughly 600–615 °C Acetol, acetic acid, and propanal had two
The product gas consisted mainly of H2, CO, CO2, and
CH4 in various proportions and a fraction of by-products,
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which was most pronounced at temperatures of 500 °C and
below. Olefins and aldehydes were by-products in the SR of
alcohols, while the SR of propanal and acetone produced ole-
fins and hydrocarbons as by-products. Ketonization to ace-
tone was observed in the SR of acetic acid at 500 °C and
below. These results indicate that the SR of oxygenates pro-
ceeds through adsorption by the oxygen containing part
followed by sequential breakage of the C–C-bonds.
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Deactivation with time-on-stream was observed for all of
the model compounds and was most pronounced for acetol
and acetic acid. However, the carbon deposition was roughly
30% higher in the SR of alcohols and acetic acid compared
with the SR of the remaining model compounds. The carbon
deposition in the SR of alcohols was due to formation of a
large amount of olefins which are potent precursors for car-
bon deposition. The results further indicate that stable opera-
tion would require high S/C-ratios or oxidative reforming.
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Acknowledgements
This work is part of the CHEC (Combustion and Harmful
Emission Control) Research Center, EGSSE (European Gradu-
ate School of Sustainable Energy), and CASE (Catalysis for
Sustainable Energy) and conducted in collaboration with the
School of Chemical, Biological, and Materials Engineering at
the University of Oklahoma (OU). The experiments were
conducted at OU, and the help from graduate student Miguel
Ángel Borja González is greatly appreciated. The present work
is financed by the Technical University of Denmark and the
University of Oklahoma.
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