Tunable catalytic properties of bi-functional mixed oxides in ethanol conversion to high value compounds

Article abstract of DOI:10.1016/j.cattod.2015.11.045

A highly versatile ethanol conversion process to selectively generate high value compounds is presented here. By changing the reaction temperature, ethanol can be selectively converted to >C₂ alcohols/oxygenates or phenolic compounds over hydrotalcite derived bi-functional MgO-Al₂O₃ catalyst via complex cascade mechanism. Reaction temperature plays a role in whether aldol condensation or the acetone formation is the path taken in changing the product composition. This article contains the catalytic activity comparison between the mono-functional and physical mixture counterpart to the hydrotalcite derived mixed oxides and the detailed discussion on the reaction mechanisms.

Full text of DOI:10.1016/j.cattod.2015.11.045

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Contents lists available at ScienceDirect
Catalysis Today
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Tunable catalytic properties of bi-functional mixed oxides in ethanol
conversion to high value compounds
Karthikeyan K. Ramasamya,∗, Michel Graya, Heather Joba, Colin Smitha, Yong Wanga,b,∗
a Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA 99354, USA
b The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99164-2710, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A highly versatile ethanol conversion process to selectively generate high value compounds is pre-
sented here. By changing the reaction temperature, ethanol can be selectively converted to >C2
alcohols/oxygenates or phenolic compounds over hydrotalcite derived bi-functional MgO–Al2O3 cata-
lyst via complex cascade mechanism. Reaction temperature plays a role in whether aldol condensation
or the acetone formation is the path taken in changing the product composition. This article contains
the catalytic activity comparison between the mono-functional and physical mixture counterpart to the
hydrotalcite derived mixed oxides and the detailed discussion on the reaction mechanisms.
© 2016 Published by Elsevier B.V.
Received 3 August 2015
Received in revised form 9 November 2015
Accepted 18 November 2015
Available online xxx
Keywords:
Ethanol
Butanol
Hydrotalcite
Mixed oxides
Guerbet
1. Introduction
hygroscopicity [11] and higher alcohols can be used as inter-
mediates to generate jet-fuel and diesel-range hydrocarbons [4].
The projected crude oil depletion along with increased green-
house gas emissions has created great interest in developing
technologies to produce fuels from renewable resources. In addi-
tion to fuels, a large fraction of chemicals are also produced
from crude oil resources. Thus, developing biomass-based renew-
able resources to supplement or replace the crude-oil-based fuels
and chemicals is very important for the sustainable future of
humankind. It has been proven that ethanol can be produced from
renewable resources via biochemical and thermochemical routes
in very efficient manner [1–3]. So developing technologies that uti-
lize ethanol as a building block to produce high value compounds
can advance us toward freedom from fossil based resources. There
are numerous literature articles on converting ethanol to higher
alcohols and other valuable compounds [4–10]. Here we show that
ethanol can be selectively converted to >C2 alcohols/oxygenates
or to phenolic compounds on MgO–Al2O3 catalyst derived from
hydrotalcite (HT) by changing the reaction temperature alone.
Alcohols with higher carbon number (C2+) offer advantages as
petrol substitutes because of their higher energy density and lower
Phenolic compounds are the key participant in the production of
many different commodities, e.g., insulating material, paint and
nylon fibers. Currently most phenolic compounds are generated
from the crude-oil derivative benzene [12].
Conversion of ethanol to 1-butanol over mixed oxides is a well-
known process [5] and the conversion of ethanol to acetone [13]
and isophorone [14] compounds are reported sporadically but to
our awareness we have not seen any publication on generating
ethanol to 1-butanol and phenolic compounds in single catalytic
step over the same catalyst. In this paper, the influence of reaction
temperature on the structured bi-functional mixed oxide catalyst
to convert ethanol to high value compounds was discussed.
2. Experimental
2.1. Materials
The
catalyst
of
interest
in
this
work,
HT
[Mg6Al2(CO3)(OH)16·4H2O, purchased from Sigma–Aldrich,
part# 652288] is made up of anionic clays in which divalent
cations [Mg2+] within brucite-like layers are replaced by trivalent
cations [Al3+] [15]. Calcination at high temperature decomposes
the HT via dehydration, dehydroxylation, and decarbonization to
MgO–Al2O3 having strong Lewis basic sites associated with the
Mn+O2− acid-base pair sites [16–18]. This gives the combination
∗
Corresponding authors at: Energy and Environment Directorate, Pacific North-
west National Laboratory, Richland, WA 99354, USA.
E-mail addresses: karthi@pnnl.gov (K.K. Ramasamy), yong.wang@pnnl.gov
(Y. Wang).
http://dx.doi.org/10.1016/j.cattod.2015.11.045
0920-5861/© 2016 Published by Elsevier B.V.
Please cite this article in press as: K.K. Ramasamy, et al., Tunable catalytic properties of bi-functional mixed oxides in ethanol conversion
to high value compounds, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2015.11.045

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Table 1
2
of acid-base (bi-functional) properties required for the complex
sequence of reaction mechanisms to convert the ethanol to high
molecular weight oxygenates [19–21]. Reactions over HT derived
MgO–Al2O3 catalyst have been shown to generate 1-butanol
selectively via Guerbet reaction, and this catalyst is also used
in many other research areas such as transesterification and
alcohol dehydrogenation [22,23]. To show the uniqueness of the
HT derived MgO–Al2O3, magnesium oxide (MgO), aluminium
oxide (Al2O3) and the physical mixture of MgO and Al2O3 with
3–1 ratio were also tested at similar operating conditions and the
results were presented here. The MgO and Al2O3 materials were
purchased from Sigma–Aldrich. The ethanol (200 proof) used in
the experiments was purchased from Decon Labs, Inc.
Conversion of ethanol, and carbon selectivity to primary compounds at 350 ◦C and
450 ◦C over various catalysts.
Catalyst
TempEthanol
Carbon selectivity (%)
(◦C) Conversion (%)EthyleneDEEAcetaldehyde1-ButanolPhenols
MgO
MgO
Al2O3
Al2O3
350 20
450 84
350 85
450 99
7
9
1.5 39
1.5 14
<20 0.1
18
11
0
0
0.5
0
0
0
0
0
0
0
<1
35
70
90
41
58
17
47
<1
34
0
2
MgO–Al2O3a 350 88
MgO–Al2O3a 450 98
MgO–Al2O3b 350 62
MgO–Al2O3b 450 98
1.7 0
4
0
5
0
42
<1
a
MgO–Al2O3 physical mix 3–1 ratio.
MgO–Al2O3 derived from HT with 3–1 ratio.
b
2.2. Catalyst testing
Over Al2O3 at 350 ◦C the conversion of ethanol is around 85% and
The catalyst testing experiments were conducted on a down
flow gas-phase reactor arrangement. The catalyst of interest was
placed in the middle of the reactor tube (isothermal zone) and
heated using a tube furnace. Ethanol and carrier gas nitrogen (N2)
was fed from the top of the reactor. The liquid product samples
were collected in the bottom of the reactor in a cold trap (ice bath)
arrangement. Later the collected liquid products were analysed in
the gas chromatography-flame ionisation detector and by gas chro-
matography/mass spectrometry. The non-condensable gases from
the cold trap passed through the flow meter and were analysed in
the gas chromatography-thermal conductivity detector. Before use,
the catalyst was pressed and sieved to 60–100 mesh and calcined
at 500 ◦C in air atmosphere. 2 g of catalyst loading was used in the
experiments with 0.002 ml/min of ethanol feed rate. The gas sam-
ples were collected every one hour interval and the liquid samples
were collected every 24 h.
the conversion of ethanol reached approximately 100% at 450 ◦C.
Ethylene was identified as the primary compound at both temper-
atures. At lower temperatures high level of DEE also generated via
inter molecular dehydration of ethanol. The spent catalyst collected
after the 450 ◦C experiment was very dark in color which shows
the high levels of coking at the elevated temperature. At 350 ◦C
over physical mixture of MgO–Al2O3 the ethanol conversion was
around 88% and it showed activity for both intra molecular dehy-
dration to ethylene and inter molecular dehydration to DEE and at
450 ◦C only ethylene was identified as the primary compound. For
both temperatures almost negligible levels of products derived via
coupling chemistry were identified on the physical mix catalyst. At
350 ◦C over MgO–Al2O3 derived from HT with 3–1 ratio generated
products with high selectivity towards butanol with other minor
by-products and at 450 ◦C the selective for phenolic compounds
was around 35% and high levels of ethylene was also detected. Only
the major compounds identified were listed in the Table 1. Par-
ticularly for the MgO and physically mixed MgO–Al2O3 at 450 ◦C
experiments acetone, higher alkenes, unsaturated aldehydes and
some levels of higher alcohols due to cross condensation were
identified in the product stream.
2.3. Catalyst characterization
X-ray powder diffraction (XRD) patterns of all the catalyst tested
were recorded on a Phillips X-Pert (50 kV and 40 mA) diffractome-
ter using Cu K␣ radiation (ꢀ¼ 1.5437 Å). Each sample was scanned
in the range between 20◦ and 80◦. Ammonia (NH3) and carbon
dioxide (CO2) Temperature Programmed Desorption (TPD) anal-
yses were performed on the catalysts tested for the acid and base
measurements. For the NH3–TPD measurement, sample material
was saturated with NH3 at room temperature in a flow of 15.7%
NH3 in helium (He) and for the CO2–TPD measurement, sample
material was saturated with CO2 at room temperature in a flow
of 5% CO2 in He. After saturation, the weakly bound NH3/CO2 was
desorbed prior to the measurement at 50 ◦C for 3 h at a He flow
rate of 25 ml min−1. The desorption curve was then attained at
a heating ramp of 10 ◦C min−1 from 50 ◦C to 800 ◦C at a He flow
rate of 25 ml min−1. The off-gas was analyzed on a Micromeretics
Autochem II equipped with a PFEIFFER mass spectrometer.
Figs. 1 and 2 depict the liquid product chromatograms of the
ethanol conversion over MgO–Al2O3 derived from HT at 350 ◦C
and 450 ◦C. The compounds identified from the liquid products
produced from 350 ◦C experiments consisted primarily of C2+ alco-
hols up to C8. Small fractions of aldehyde and ether compounds up
to C10 were also identified at this temperature range. The com-
pounds identified from the liquid products produced from 450 ◦C
experiments consisted primarily of phenolic compounds (phenol,
methyl phenol and dimethyl phenol). Small fractions of C3+ ketones
were also identified at this temperature. The concentration of the
ketone compounds increased with temperature between 350 ◦C
and 450 ◦C.
Fig. 3 shows the XRD pattern of the MgO, Al2O3 and HT catalysts
that are calcined at 500 ◦C for 4 h. The acquired XRD pattern for the
MgO, Al2O3 materials resembles the literature information [24].
The HT derived material with 3–1 ratio between MgO and Al2O3
shows the predominance of MgO due to its highest presence. Fig. 4
shows the CO2 TPD profiles and the amount CO2 desorbed from the
MgO, Al2O3, physical mixture of MgO–Al2O3 with 3–1 ratio and
HT derived MgO–Al2O3 with 3–1 ratio. All the samples analysed
shows more than one kind of basic site. Al2O3 exhibits only weak
and medium strength basic sites [20,25], whereas weak, medium
and strong basic sites were observed in the case of MgO [20]. The
physical mix shows the peaks from the combination of both MgO
and Al2O3. The HT derived mixed oxide shows a small peak corre-
sponds to the weak base site followed by a broad peak observed
corresponding to the combined medium and strong basic sites. The
amount of basic sites was in the similar range between the MgO,
3. Results and discussion
The experiments were conducted between 350 ◦C and 450 ◦C at
atmospheric pressure condition. The operating conditions, ethanol
conversion and the carbon selectivity to the primary compounds
[ethylene, diethyl ether (DEE), acetaldehyde, 1-butanol and pheno-
lic compounds] are listed in Table 1. Except MgO catalyst, both the
ethanol conversion and the ethylene selectivity tend to increase for
all the catalyst tested with the increase in temperature. Over MgO
at 350 ◦C the conversion of ethanol is only 20% and the major prod-
ucts in the liquid composition were acetaldehyde and 1-butanol. At
450 ◦C the ethanol conversion increased to around 80% and the liq-
uid product contains aldehydes, ketones and alcohols up to carbon
number C10
.
Please cite this article in press as: K.K. Ramasamy, et al., Tunable catalytic properties of bi-functional mixed oxides in ethanol conversion
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Fig. 1. Liquid product chromatogram of ethanol conversion over MgO-Al2O3 derived from HT at 350 ◦C.
Fig. 2. Liquid product chromatogram of ethanol conversion over MgO-Al2O3 derived from HT at 450 ◦C.
Al2O3 and physical mix (ranges between 140 and 200 ␮mol g−1
)
oxide provide the required proximity between the basic and acidic
active sites that are required for the cascade chemistry such as
ethanol conversion to 1-butanol or phenolic compounds.
but the strengths varied in wide range between weak, medium and
strong basic sites. The total basic sites measured for the HT derived
material was more than two folds higher than the other catalysts
tested and around 90% of the total sites located on the medium and
the high strength basic site region.
In the following section the unique change in reaction mech-
anism with the temperature change on the ethanol conversion
over MgO–Al2O3 derived from HT will be discussed. Based on
the experimental results and the literature information, the reac-
tion scheme of the ethanol at 350 ◦C is shown in Scheme 1. At
low temperatures, ethanol generates ethylene via intramolecular
dehydration, and diethyl ether and C4+ ethers (cross-dehydration
between ethanol and higher alcohol products) via intermolecu-
lar dehydration. Butanol is primarily generated via a sequence of
complex reaction mechanisms. First ethanol dehydrogenates to
generate acetaldehyde, then acetaldehyde goes through aldola-
tion to form hydroxybutanal, followed by a condensation reaction
to give an alpha, beta unsaturated aldehyde compound (croton-
aldehyde), which then goes through a sequence of hydrogenation
reactions to form butanal and finally butanol. Butanol can also be
generated via direct condensation of ethanol, but the liquid prod-
uct composition identification proves that the indirect mechanism
via aldehyde is the major pathway for the higher alcohol genera-
tion [5,18–20,27,28]. Basic sites play the internal hydride transfer
role to generate the higher alcohol compounds [29,30]. Some liter-
ature information also points out that Meerwein–Ponndorf–Verley
Fig. 5 shows the NH3 TPD profiles of the MgO, Al2O3, physical
mix of MgO–Al2O3 with 3–1 ratio and HT derived MgO–Al2O3 with
3–1 ratio. The MgO sample does not show any acid sites. Physical
mixture shows the acidity with respect to the amount of Al2O3. HT
derived material contains more acidic sites than the pure alumina.
But compared with the total basic sites the measured acidic sites
were several folds lower and this result is in agreement with the
literature information [20,24]. For all the materials tested the total
measured acid sites from the NH3 TPD were below 5 ␮mol g−1
.
In cascade catalysis various chemical steps combine in one
step without separating the intermediates and this has been suc-
cessfully employed in the enzymatic catalysis using two essential
tools: site-isolation and substrate selectivity [26]. In the heteroge-
neous catalysis the concerted multi-functional pathway requires
the active sites co-exist in molecular distances so the proximity
between the active sites with different functionality and the chemi-
cal nature to complete the complex network of reaction mechanism
very efficiently [27]. The ordered structure of the HT derived mixed
Please cite this article in press as: K.K. Ramasamy, et al., Tunable catalytic properties of bi-functional mixed oxides in ethanol conversion
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Fig. 4. CO2 TPD profiles and the amount CO2 desorbed from the MgO, Al2O3, physical
mix of MgO-Al2O3 with 3 to 1 ratio and HT derived MgO-Al2O3 with 3 to 1 ratio.
experiments. Based on the experimental findings and the litera-
ture information, the reaction pathway of the ethanol to generate
phenolic compounds at 450 ◦C and higher temperature is shown
in Scheme 2. Based on the product liquid composition, it can be
concluded that acetone is an intermediate compound for the phe-
nolic compound formation [33,34]. The formation of acetone from
ethanol can occur via several mechanisms. For example, ethanol
dehydrogenate to form acetaldehyde followed by the aldolation
to form hydroxybutanal, then intermolecular hydride transfer to
form hydroxybutanone, which then decomposes to give acetone,
carbon monoxide and hydrogen [19,21]. In the gas-phase product
composition, no carbon monoxide was detected; only small lev-
els of carbon dioxide was detected. Due to the presence of water
and the carbon monoxide together potentially go through water-
gas shift reaction to form carbon dioxide but the nature of the HT
derived catalyst and temperature regime at which the experiment
was conducted does not promote the water-gas shift reaction so
Fig. 3. XRD profile of the MgO, Al2O3 and HT derived MgO-Al2O3 with 3 to 1 ratio.
reaction (MPV), in which the hydrogen from the feed ethanol
is directly transferred to the unsaturated aldol product to form
butanol and acetaldehyde, is responsible for the final hydro-
genation step [5,31,32]. The product distribution also contains
significant amounts of higher alcohols and aldehydes generated
from the cross-condensation of the product butanol and ethanol
to form C4+ compounds. In addition to the alcohols and the aldehy-
des, ethers with carbon number between C4 and C10 were identified
at minor levels in the liquid product composition.
At 450 ◦C and higher temperatures, ethanol primarily con-
verted to phenol and alkylated phenols. Ethylene generation is
higher in gaseous composition compared to the low temperature
Scheme 1. Proposed reaction mechanism for the conversion of ethanol at low temperature (350 ◦C) over MgO-Al2O3 derived from HT. Compounds shown in grey colour are
intermediates that are not identified in the product slate.
Please cite this article in press as: K.K. Ramasamy, et al., Tunable catalytic properties of bi-functional mixed oxides in ethanol conversion
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ketonization to form acetone from acetic acid, can also poten-
tially play a role in generating acetone [37,38]. Very small levels
of acetic acid presence in the product stream from the ethanol
conversion experiments and the close to 100% conversion levels
of acetic acid to ketone compounds when the acetic acid was co-
fed with ethanol supports later stated pathway that forms acetic
acid as an intermediate is the potential reaction mechanism for
the acetone formation. Ethanol conversion and product selectiv-
ities to various compounds were the same when acetic acid was
co-fed with ethanol. More in situ experiments are necessary to
positively confirm exact pathway that leads to the formation of
acetone from ethanol. Once the acetone is formed, it goes through
sequence of condensation reactions to form mesityl oxide followed
by phorones [14]. As per literature information, phorones undergo
1, 6-internal Michael cyclisation to give isophorone. Then the arom-
atization of isophorone occurs via dienone to form trimethyl phenol
and hydrogen. Finally the methyl phenols and phenol are formed
by the dealkylation of the dimethyl phenol [39,40].
The combination of the acid-base properties and the active
site location of the MgO–Al2O3 derived from HT plays a key role
in the complex reaction mechanism involved in converting the
ethanol to higher alcohol/oxygenate or the phenolic compounds
in a one-step process. Modifications of Mg and Al allow changing
the acid-base properties of the catalysts and subsequently improv-
ing the selectivity and the yield of the target compounds. At higher
temperatures, selectivity to ethylene increases due to the increased
dehydration activity. Irrespective of the temperature, the butene
selectivity did not change. This shows that the dehydration activ-
ity is very low for the higher alcohols that are generated from
the reaction. In addition, conversion of ethanol to acetaldehyde is
Fig. 5. NH3 TPD profiles of the MgO, Al2O3, physical mix of MgO-Al2O3 with 3 to 1
ratio and HT derived MgO-Al2O3 with 3 to 1 ratio.
it is difficult to believe this pathway is responsible for the acetone
formation.
The other pathways, such as Tishchenko reaction, dimerisation
of acetaldehyde to form esters then hydrolysis to form acetic acid
and ethanol followed by acetic acid ketonisation to form acetone
[35,36], and/or the oxidation of aldehyde to form acetic acid and
OH
Ethanol
Ethylene
-H2O
OH
2,4,6-Trimethylphenol
1,3,5-Trimethylbenzene
O
Acetaldehyde
O
+
O
O
O
OH
O
3,5,5-Trimethyl-2-cyclohexen-1-one
Ethyl acetate
4,6-Dimethyl hepta 3,5 dien-2-one
Isopropanol
O
OH
O
O
O
O
O
O
+
+
-CO2,-H2
-H2O
-H2O
OH
Acetic acid
2,6-Dimethyl-2,5-heptadien-4-one
4-Methylpent-3-en-2-one
Acetone
Scheme 2. Proposed reaction mechanism for the conversion of ethanol at high temperature (450 ◦C) over MgO-Al2O3 derived from HT. Compounds shown in grey colour
are intermediates that are not identified in the product slate.
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considered to be the rate-determining step in generating the higher
oxygenates [32,41]. Based on this phenomenon, ethanol can be
dehydrogenated to acetaldehyde over a catalyst such as copper and
then exposed to the HT derived catalyst at a desired temperature
to improve the overall ethanol conversion and carbon selectivity
toward the desired product class.
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methanol using Mg–Al hydrotalcite derived catalysts, Appl. Catal. A 331
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4. Conclusion
In conclusion, at temperatures around 350 ◦C ethanol can be
converted over bi-functional MgO–Al2O3 derived from HT catalyst
to C2+ oxygenates containing alcohols and aldehydes, which can
easily be mild-hydrogenated (aldehydes to alcohols) to generate a
mixture of higher alcohols (C4–C10). These higher alcohols can be
used in the gasoline blend stock, as a building block for the hydro-
carbon fuels and as a building block for chemical commodities. At
temperatures higher than 450 ◦C, the product composition shifts to
a mixture of phenolic compounds. These phenolic compounds can
be used in the production of many different commodities. More
research toward understanding the chemistry and identifying the
catalyst role will play a major role in developing this process to
commercial scale.
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Acknowledgements
The Pacific Northwest National Laboratory is operated by Bat-
telle Memorial Institute for the U. S. Department of Energy under
Contract No. DE-AC05-76RL01830. This work was supported by the
U.S. Department of Energy Bioenergy Technology Office.
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Please cite this article in press as: K.K. Ramasamy, et al., Tunable catalytic properties of bi-functional mixed oxides in ethanol conversion
to high value compounds, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2015.11.045