Efficient conversion of ethanol to 1-butanol and C5-C9 alcohols over calcium carbide

Article abstract of DOI:10.1039/c9ra02568e

Production of 1-butanol or alcohols with 4-9 carbon atoms (C₄-C₉ alcohols) from widely available bio-ethanol has attracted much interest in recent years in academia and industry of renewable chemicals and liquid fuels. This work discloses for the first time that calcium carbide (CaC₂) has a superior catalytic activity in condensation of ethanol to C₄-C₉ alcohols at 275-300 °C. The 1-butanol yield reached up to 24.5% with ethanol conversion of 62.4% at the optimized conditions. The by-products are mainly alcohols with 5-9 carbons besides 2-butanol, and the total yield of all the alcohols reached up to 56.3%. The reaction route was investigated through controlled experiments and quantitative analysis of the products. Results indicated that two reaction routes, aldol-condensation and self-condensation, took place simultaneously. The aldol-condensation route involves coupling of ethanol with acetaldehyde (formed from ethanol dehydrogenation) to form 2-butenol, which is subsequently hydrogenated to 1-butanol. The alkynyl moiety in CaC₂ plays an important role in the catalytic pathways of both routes and affords the good activity of CaC₂. CaC₂ is converted to acetylene [C₂H₂] and calcium hydroxide [Ca(OH)₂] simultaneously by the H₂O that was generated from the condensation of alcohols.

Full text of DOI:10.1039/c9ra02568e

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Efficient conversion of ethanol to 1-butanol and
C –C alcohols over calcium carbide†
5
9
Cite this: RSC Adv., 2019, 9, 18941
Dong Wang, Zhenyu Liu and Qingya Liu
*
4 9
Production of 1-butanol or alcohols with 4–9 carbon atoms (C –C alcohols) from widely available bio-
ethanol has attracted much interest in recent years in academia and industry of renewable chemicals
2
and liquid fuels. This work discloses for the first time that calcium carbide (CaC ) has a superior catalytic
ꢀ
activity in condensation of ethanol to C
4
–C
9
alcohols at 275–300 C. The 1-butanol yield reached up to
24.5% with ethanol conversion of 62.4% at the optimized conditions. The by-products are mainly
alcohols with 5–9 carbons besides 2-butanol, and the total yield of all the alcohols reached up to 56.3%.
The reaction route was investigated through controlled experiments and quantitative analysis of the
products. Results indicated that two reaction routes, aldol-condensation and self-condensation, took
place simultaneously. The aldol-condensation route involves coupling of ethanol with acetaldehyde
Received 5th April 2019
Accepted 3rd June 2019
(
formed from ethanol dehydrogenation) to form 2-butenol, which is subsequently hydrogenated to 1-
butanol. The alkynyl moiety in CaC plays an important role in the catalytic pathways of both routes and
affords the good activity of CaC . CaC is converted to acetylene [C ] and calcium hydroxide
[Ca(OH) ] simultaneously by the H
2
DOI: 10.1039/c9ra02568e
2
2
2 2
H
rsc.li/rsc-advances
2
2
O that was generated from the condensation of alcohols.
homogeneous-heterogeneous catalysts (alkali metal hydroxides
1
. Introduction
11,12
or alkoxides in the presence of RANEY® nickel/copper),
13,14
With gradual depletion of fossil fuels, widespread concerns have homogeneous Ru and Ir complexes
and heterogeneous basic
been raised regarding production of fuels or chemicals from solids. The hybrid and homogeneous catalysts show high
renewable biomass such as plants, agriculture wastes and forest activities and excellent selectivity towards 1-butanol under mild
1,2
residues. Ethanol has been produced on industrial scales reaction conditions but suffer from corrosion or separation
3
15
through fermentation of lignocellulose and is one of the largest problems. Heterogeneous catalysts overcome these problems
volume bio-fuels. It is used directly as a fuel or blended in and have been widely studied in both ow and batch reactors,
4
,5
16–19
20,21
gasoline as an additive; however, a relatively low energy density, such as hydroxyapatites (HAP),
MgO,
Mg–Al oxides or
22–27
28
29
high hygroscopicity and corrosion to metal parts restrict its metal-doped ones,
Cu/CeO
2
31
,
Ni/Al
2
O
3
,
Na/ZrO (ref. 30)
2
6
application in the transportation sector. 1-Butanol is considered and alkali-exchanged zeolites. The relatively optimal ethanol
to be an excellent alternative to ethanol because it possesses an conversion and 1-butanol yield of these catalysts, along with the
energy density closer to gasoline, low solubility in water and less- reaction conditions, are summarized in Table S1.† On the
7,8
corrosive nature. 1-Butanol is also a bulk platform chemical for whole, studies with ow reactors were carried out at higher
synthesis of 1-butyl esters plasticizer, general extraction agent of temperatures than those with batch reactors. The maximum 1-
9
ꢀ
catalyst at 330 C in
perfumes and drugs, and paint solvent. Although fermentation butanol yield was 30.0% over Cu/CeO
of biomass to 1-butanol has been developed, economical a ow reactor, and the maximum space-time yield was 705 gpro
2
ꢁ1
ꢁ1
production on an industrial scale was reported to remain a chal- kgcat
h
over Cu10Ni10-porous metal oxides (hydrotalcites) at
lenge. Efficient conversion of ethanol to 1-butanol has been of 320 C with a batch reactor. Development of a highly efficient
great interest during the past decade.
heterogeneous catalyst at mild reaction conditions is still a hot
The Guerbet reaction is well-known and converts a primary topic for synthesis of 1-butanol or higher alcohols from ethanol.
aliphatic alcohol into its b-alkylated dimer alcohol by elimina-
The Guerbet reaction mechanism over heterogeneous cata-
tion of H
O without the need for hydrogen. Many catalysts have lysts has been widely discussed and are overviewed herein in
10
ꢀ
2
been developed for this transformation, including hybrid order to develop a novel catalyst. A number of authors have
proposed aldol-condensation route which involves several
32,33
consecutive steps as shown in Scheme 1.
The primary
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical alcohol is rstly oxidized to aldehyde by dehydrogenation.
Technology, Beijing, 100029, China. E-mail: qyliu@mail.buct.edu.cn
Then, the b-C in one aldehyde molecule attacks another alde-
hyde molecule to form hydroxyl-aldehyde, followed by
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ra02568e
1
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 18941–18948 | 18941

View Article Online
RSC Advances
Paper
atmosphere. The alcohols including methanol, ethanol, 1-
propanol, isopropanol, 1-butanol and 2-butanol (Beijing
Chemical Works) were dehydrated with 3A molecular sieves.
The commercial reagents of calcium ethoxide (95%), calcium
hydroxide, calcium oxide, magnesia, HAP and hydrotalcites
(Mg/Al ¼ 3 : 1) were used as received.
2.2 Reaction experiment
Typically, ethanol (10.0 g, 217 mmol) and CaC (1.5 g, 23 mmol)
were added into a quartz tube in a stirred autoclave reactor (Parr
2
Scheme 1 Conventional aldol-condensation route of primary alcohol
to dimer one.
32,33
4
2
597, 50 mL). Then, the reactor was sealed, purged with Ar of
.0 MPa for 5 times to remove air and heated to a specied
dehydration to form olen-aldehyde. The olen-aldehyde is
nally reduced to dimer alcohol by hydrogenation. In this route,
temperature under stirring at a speed of 200 rpm. Aer the
indicated reaction time, the reactor was cooled to room
temperature with ice water. The gas product was collected
carefully with a gas collecting bag, and the solid and liquid
products were quantitatively transferred to a centrifuge tube
under nitrogen atmosphere for separation.

the acid sites of heterogeneous catalysts adsorb ethanol and
acetaldehyde, and the base sites catalyze the hydrogen transfer
1
6,19,22,34
reaction.
A direct condensation mechanism shown in
17,18,20
Scheme 2 has also been reported on various basic solids,
in
which the b-H in ethanol is activated by a catalyst and the
activated ethanol molecule subsequently condenses with
another ethanol molecule through dehydration to form 1-
butanol. In both routes, activation of b-H is considered to be
2
.3 Analysis of reaction products
The composition of gas product was analyzed by a gas chro-
matography (Agilent GC-7890B) equipped with a thermal
conductivity detector (TCD) and two packed columns of Agilent
Porapak Q (6  ꢂ 1/8 ꢂ 2.0 mm) and Mol Sieve 5A (6  ꢂ 1/8 ꢂ
20,35
a crucial step.
It is reported that the alkynyl moiety of calcium carbide
(
CaC ) is susceptible to nucleophilic attack of the acidic
2
ꢁ
1
36
2
.0 mm). He of 30 mL min was employed as the carrier gas.
terminal hydrogen, which makes it possible to promote b-H
activation and hydrogen transfer. CaC has been found to have
a superior activity in activating C–H bond of acetone during
synthesis of isophorone. Besides C–H activation, the Ca in
CaC , similar to that in HAP, may serve as an acidic site to
The temperatures of injection port, column and detector were
1
product was estimated according to its composition; and the
mass of gas product was estimated according to the average
density and the volume of gas product at the atmospheric
pressure (detailed calculation can be found in ESI†).
2
ꢀ
00, 70 and 200 C, respectively. The average density of gas
37
2+
2
adsorb alcohols to initiate its condensation. These analyses
indicate a potential catalytic application of CaC2 in ethanol
ꢀ
The solid product was dried at 120 C under vacuum for 12 h
condensation. If CaC takes a catalytic effect, it may convert in
2
and then weighed. X-ray diffraction (XRD) analysis of the solid
product was performed on a D8FOCUS Powder diffractometer
situ to C H due to formation of H O in ethanol condensation.
2
2
2
That is, this protocol may couple the catalytic activity and
reactivity of CaC . CaC has been reported to be produced from
bio-char at lower temperatures by auto-thermal heating,
which renders the idea to be more attractive. In the present
contribution, condensation of ethanol was evaluated over CaC
˚
using Cu Ka radiation (l ¼ 1.5432 A) at 40 kV and 40 mA. Step
2
2
ꢀ
38,39
scans were taken over a range of 2q from 5 to 80 at a speed of
ꢀ
ꢁ1
6
min . The organic carbon content of the solid product was
analyzed by a total organic carbon analyzer (SSM-5000A, Shi-
madzu, Japan).
2
in a batch reactor at different reaction conditions. The reaction
pathway was investigated through detailed analysis of the main
products and controlled experiments.
The mass of liquid product was obtained by subtracting the
mass of gas and solid products from the total mass of reactants.
Qualitative analysis of the liquid product was performed on an
Agilent GC-mass spectrometry (GC-MS) equipped with a HP-5
capillary column (30 m ꢂ 0.25 mm ꢂ 0.25 mm) and a quadru-
2. Experimental
pole analyzer system (5977B) with high efficiency ion source
2.1 Material
ꢀ
(
HES) GC/MSD operated at 300 C. The carrier gas was 1.0
CaC with a purity of 97%+ (Acros Organics) was ground and
2
ꢁ1
mL min He, the sample dosage was 1.0 mL and the split ratio
was 50 : 1. The injection temperature was 250 C, and the
temperature program of column was 35 C for 5 min,
0 C min to 230 C and 230 C for 1 min. The scan m/z of MS
screened to 80–100 mesh in a glove box under nitrogen
ꢀ
ꢀ
ꢀ ꢀ ꢀ
ꢁ1
1
ranged from 30 to 350.
Quantitative analysis of the liquid products of interest was
performed on an Agilent GC-7890B equipped with a HP-5
capillary column (30 m ꢂ 0.32 mm ꢂ 0.25 mm) and a ame
ionization detector (FID). Acetone was used as the internal
Scheme 2 Direct condensation route of primary alcohol to dimer
one.
17,18,20
standard and the mass ratio of acetone to sample was 1 : 10. N
2
18942 | RSC Adv., 2019, 9, 18941–18948
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
ꢁ
1
ꢀ
was used as the carrier gas at a ow rate of 1.0 mL min and the range, reaching up to 26.1% at 315 C. The selectivity of total
ꢀ
ꢀ
detector temperature was 250 C. The other conditions were the alcohols is as high as 84–85% at 275–300 C. The selectivity of
ꢀ
same as those of GC-MS.
total alcohols at 190 C is only 37.9% (10.1% plus 27.8%) due to
Based on the product analysis results, ethanol conversion, consumption of ethanol by CaC to form EVE and calcium
2
42
yields of various alcohols, carbon yields of gas products and ethoxide as shown in Re (1). The carbon balance, including the
solid residual, as well as carbon balance (% C) were determined carbon in the main liquid products, unreacted ethanol, gas
ꢀ
(For more details, see formulas (6)–(10) in ESI†).
products and solid residual, keeps at 95–96% at 190–300 C and
ꢀ
slightly decreases to 92.8% at 315 C (Table S3 in the ESI†).
Since the gas products and solid residual were quantied
precisely, the decreased carbon balance at 315 C suggests
3. Results and discussion
ꢀ
3.1 Qualitative analysis of the liquid product
formation of more trace liquid products.
ꢀ
Taking the reaction at 190 C for 8 h as an example, the
distribution of liquid product was determined by GC-MS. The
total ion chromatogram shown in Fig. S1† indicates that there
are twelve obvious peaks with some minor ones. MS spectra of
the twelve peaks are shown in Table S2 in the ESI.† Besides
ethanol, they are ethyl vinyl ether (termed EVE), 2-butanol, 1-
butanol, 2-pentanol, 3-hexanol, 2-ethyl-1-butanol, 1-hexanol, 4-
heptanol, 2-heptanol, 3-methylcyclohexanol and 4-nonanol. It is
noted that acetaldehyde, 3-hydroxybutanal, 2-butenal and
butyraldehyde frequently reported in literatures were not
detected in our reaction; and tiny ethyl acetate and 2-butenol
were observed (at retention time of 2.40 and 3.17 min in
Fig. S1†).
CaC
2
+ 3CH
3
CH
2
OH / Ca(OCH
CH
2 3 2
CH )
+
CH ]CHOCH
2
2
3
(1)
ꢀ
The effect of reaction time was investigated at 275 C in 1–
15 h and the results are summarized in entries 7–12 of Table 1.
ꢀ
At 275 C for 1 h, the ethanol conversion and 1-butanol yield
have been as high as 31.7% and 12.8%, respectively. With
increasing the reaction time, the ethanol conversion slightly
increases to 54.3% in 15 h; the 1-butanol yield slightly increases
to the maximum of 19.5% at 6 h and then decreases; the yield of
other alcohols gradually increases in 8 h and then keeps at 24–
26% in 8–12 h. The maximum yield of total alcohols of 43.1%
Obviously, all the main products except EVE are alcohols
with carbon numbers from 4 to 9. Although this work focuses on
the synthesis of 1-butanol, all the alcohols produced are
ethanol-derived condensation products and in principle suit-
able as fuels or as blending agents in gasoline. Formation of 2-
ethyl-1-butanol and 1-hexanol have been observed on many
(19.0% plus 24.1%) was achieved at 8 h with ethanol conversion
of 50.1%. The carbon balance is higher than 95% in 8 h and
slightly decreases aer then (Table S3†), which is similar to the
observation on the effect of temperature and suggests conver-
sion of major liquid products to minor ones aer 8 h. To obtain
40
a higher 1-butanol yield, the optimal reaction conditions are
19,41
heterogeneous catalysts,
–
which are formed by coupling of
OH in ethanol with b-H in 1-butanol and –OH in 1-butanol
ꢀ
275–300 C for 6–8 h, at which the yield of other alcohols also
reaches a higher value.
The space-time yield (STY) is usually used to evaluate activ-
ities of different catalysts or change/deactivation of a catalyst.
The STY in this work signicantly decreases from 687 to 58 gpro
kgcat
suggesting gradual change of catalytic component. To obtain an
optimal STY, the reaction time should be shorten to 1 h at the
expense of 1-butonal yield.
The catalysts frequently used in literatures, Mg–Al oxides,
HAP and MgO were evaluated in our batch reactor at 275 C for
with b-H in ethanol, respectively. Formation of 2-butanol indi-
cates coupling of –OH in ethanol with a-H in another ethanol;
and 3-hexanol may be formed by coupling of –OH in ethanol
with b-H in 2-butanol or a-H in 1-butanol. Formation of 2-
pentanol is really surprising because it is odd carbon number
alcohol (C ) and was seldom reported. Anyway, once the 2-
5
pentanol is formed, it may couple with ethanol or butanol to
form heptanols, 3-methylcyclohexanol and 4-nonanol (C and
ꢁ1 ꢁ1
h
with increasing the reaction time from 1 to 15 h,
7
C alcohols).
9
ꢀ
8
2
h and compared with CaC at the same conditions. As seen in
3
.2 Results of ethanol conversion and liquid product yield
Table 2, the blank test in the absence of a catalyst shows a lower
ethanol conversion and little formation of 1-butanol and other
alcohols. Compared to the blank test, all the catalysts improve
the ethanol conversion and 1-butanol yield as expected while
the efficiency of CaC2 is more obvious than those of other
The positive results were elaborated by quantifying ethanol
conversion and yields of various alcohols. Herein, 1-butanol is
discussed separately and the other nine alcohols are discussed
as a group. The by-product ethyl acetate and intermediate 2-
butenol are in trace amounts in all the runs and will not be
catalysts. Furthermore, the reaction over CaC yields the largest
2
2
amount of other alcohols. These results indicate that CaC is
discussed. The effect of reaction temperature was studied at
ꢀ
superior to Mg–Al oxide, HAP and MgO in converting ethanol to
190–315 C in 6 h and the results are summarized in entries 1–6
ꢀ
1
-butanol and other medium-chain alcohols at 275 C in the
of Table 1. It is clear that the ethanol conversion increases from
6.4% to 50.0% with increasing the temperature. The yield of 1-
butanol increases to the maximum of 19.5% with elevating
batch reactor. It is noted that 2-butenal was observed over HAP
and Mg–Al oxide catalysts while 2-butenol was observed over
2
ꢀ
ꢀ
CaC
2
and MgO, suggesting occurrence of different aldol-
temperature to 275 C and then decrease at 300 and 315 C,
which results in the maximum selectivity of 42.4% at 275 C.
The yield of other alcohols increase in the whole temperature
ꢀ
condensation reactions. Formation of 2-butenol over MgO
21
catalyst was also reported in literature.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 18941–18948 | 18943

View Article Online
RSC Advances
Paper
a
Table 1 Ethanol conversion and products' yields under different reaction conditions
b
Carbon yield
(
%)
Other alcohols
yield (%)
ꢀ
catꢁ1 ꢁ1
h )
Entry
Temp. ( C)
Time (h)
Ethanol conv. (%)
1-Butanol yield (%)
STY (gpro kg
Solid
Gas
c
c
1
2
3
4
5
6
7
8
9
190
235
255
275
300
315
275
275
275
275
275
275
6
6
6
6
6
6
1
3
6
8
12
15
26.4
35.8
40.9
46.0
48.9
50.0
31.7
38.9
46.0
50.1
52.5
54.3
2.7 (10.1)
7.3 (27.8)
24
109
148
174
166
119
687
284
174
127
78
8.7
7.4
7.0
5.4
4.6
2.6
8.9
7.5
5.4
5.1
4.9
4.7
2.8
3.0
3.9
4.3
5.2
7.0
4.2
4.3
4.3
4.4
5.2
7.2
12.2 (34.1)
16.6 (40.6)
19.5 (42.4)
18.6 (38.0)
13.4 (26.8)
12.8 (40.4)
15.9 (40.9)
19.5 (42.4)
19.0 (37.9)
17.6 (33.5)
16.2 (29.8)
12.6 (35.2)
15.8 (38.6)
19.3 (41.9)
22.9 (46.9)
26.1 (52.2)
9.9 (31.2)
14.7 (37.8)
19.3 (41.9)
24.1 (48.1)
25.0 (47.6)
25.5 (47.0)
1
1
1
0
1
2
58
a
b
The amounts of ethanol and CaC
2
are 217 and 23 mmol, respectively. The ratio of the amount of organic carbon in gas product or solid residual
c
to the total carbon fed into the reactor. Number in the parentheses is the selectivity.
3
.3 Evolution of CaC
The above discussion has shown the conversion of CaC
calcium ethoxide [Ca(OCH CH ]. To further understand
evolution of CaC at the optimized temperatures of 275–300 C,
XRD results of the solid residuals are illustrated in Fig. 1, along
with those of the raw material CaC and Ca(OCH CH for
2
during ethanol reaction
The XRD results clearly indicate that the main calcium
compound is CaC initially, CaC and Ca(OCH CH ) in 1–3 h,
2
2
2
3 2
2
to
Ca(OCH
catalytic activities of Ca(OCH
2
CH
3
)
2
in 3–6 h and Ca(OH)
CH
2
aer 6–8 h. To identify the
and Ca(OH) , ethanol
2
3 2
)
ꢀ
2
3
)
2
2
2
conversion over them and the impurity CaO were evaluated at
ꢀ
2
75 C and the results are summarized in Table 3. In compar-
2
2
3 2
)
ison with the blank test shown in Table 2, Ca(OCH
Ca(OH)
formation. However, the 1-butanol yields over Ca(OCH
2
CH
3
)
2
,
comparison. The typical diffraction peaks of CaC are located at
2
ꢀ
and CaO do promote ethanol conversion and alcohols
2
2q of 28.1 and 32.7 and those of Ca(OCH CH ) are located at
2
3 2
ꢀ
CH
2
3
)
2
2q of 10.7, 30.8 and 49.7 . As seen in Fig. 1, the raw material
and Ca(OH) for 6 h are only 6.0% and 3.8%, respectively, which
2
CaC
2
contains impurities of CaO and calcium hydroxide
ꢀ
are far smaller than the 1-butanol yield over CaC
3 in Table 3); the yields of other alcohols over Ca(OCH
and Ca(OH) are also far smaller than that of CaC . This result
2
(19.5%, entry
[Ca(OH)
2
] (2q ¼ 34.1, 18.2, 50.2 ). The solid residual obtained at
ꢀ
2
CH
3 2
)
275 C for 1 h shows obvious diffraction peaks of CaC
2
and
2
2
Ca(OCH
2
CH and a very weak diffraction peak of Ca(OH) .
3
)
2
2
accounts for the decreasing STY with increasing the reaction
time (Table 1). The catalytic activity of CaO is higher than those
of Ca(OCH CH ) and Ca(OH) but obviously lower than that of
With increasing the reaction time to 3 and 6 h, diffraction peaks
of CaC disappear, those of Ca(OCH CH ) are still obvious and
weak diffraction peaks of Ca(OH) are discernible. The solid
residuals obtained at 275 C for 8 h and 300 C for 6 h are
mainly Ca(OH) with little Ca(OCH CH . It should be pointed
out although little C-containing compounds were observed on
the last two residuals by XRD, a certain amount of organic
carbon was detected by TOC (entries 5 and 10 in Table 1) and
the residuals are black; this phenomenon is attributed to EVE
polymerization since it is recognized to be unstable, which is
conrmed by the carbon balance calculation on the carbon in
2
2
3 2
2
3 2
2
2
ꢀ
ꢀ
CaC . Since all the compounds are calcium-containing, the far
2
difference in their catalytic activities indicates an important role
of alkynyl group in efficient conversion of ethanol into higher
alcohols.
2
2
3 2
)
2 2
Since CaC is gradually converted, the effects of CaC loading
on ethanol conversion and yields of alcohol products were
ꢀ
43
studied at 275 C. The results shown in Table 3 indicate that the
yields of 1-butanol and other alcohols increase with the CaC2
loading (entries 3, 5 and 6) and they are roughly in proportion
2
CaC (Table S4 in ESI†).
ꢀ
Table 2 Results of ethanol conversion into alcohols over different catalysts at 275 C for 8 h
a
Entry
Catalyst
Ethanol conv. (%)
1-Butanol yield (%)
Other alcohols yield (%)
Yield of main by-product (%)
b
1
2
3
4
5
None
6.4
50.1
28.4
24.4
32.9
ND
ND
24.1
6.2
2.1
3.6
ND
CaC
2
19.0
11.8
10.4
9.9
1.0 (2-butenol)
2.0 (2-butenal)
1.2 (2-butenal)
1.4 (2-butenol)
HAP
Mg–Al oxide (Mg/Al ¼ 3)
MgO
a
b
The amounts of all the catalysts are 23 mmol. Not detected.
18944 | RSC Adv., 2019, 9, 18941–18948
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
Table 4 The amounts of each gas product and EVE produced at
a
different reaction conditions
Amount of products (mmol)
ꢀ
Entry Temp. ( C) Time (h)
C
2
H
2
C
2
H
4
CH
4
CO
H
2
EVE
1
2
3
4
5
6
7
275
275
275
275
275
275
300
1
3
6
8
12
15
6
9.0
8.0
7.4
6.9
6.6
6.2
8.0
0.9
1.9
2.5
2.8
4.0
6.7
2.8
0.2
0.5
0.6
0.9
1.9
4.7
1.7
0.1
0.3
5.9 0.9
7.2 1.1
0.5 10.5 1.2
0.7 14.9 1.3
1.8 16.8 1.4
4.1 20.2 1.4
1.5 16.0 0.8
a
The amounts of ethanol and CaC
2
are 217 and 23 mmol, respectively.
45
21–23
literatures and C
2 4
H indicates dehydration of ethanol;
C H undoubtedly results from CaC conversion.
2
2
2
To further conrm the aldol-condensation reaction over
CaC , reactions of ethanol with all the possible intermediates,
2
Fig. 1 XRD patterns of the solid residuals after ethanol reaction over
CaC , along with those of the raw material CaC and calcium ethoxide.
2 2
acetaldehyde, 2-butenal and 2-butenol, were carried out at
ꢀ
2
75 C for 6 h. For comparison, the total amount of carbon in all
the runs are the same. Results shown in Table 5 indicate that
CaC promotes reaction of ethanol with all the intermediates to
form 1-butanol and the 1-butanol yield follows the order: 2-
butenol 2-butenal acetaldehyde. Furthermore, trace
amounts of 2-butenal and 2-butenol are detected for ethanol +
acetaldehyde reaction and a quantity of 2-butenol is detected for
ethanol + 2-butenal reaction. These observations indicate that
aldol-condensation via 2-butenal intermediate may also occur
but that via 2-butenol intermediate proceeds more easily.
Hydrogenation of 2-butenol to form 1-butanol requires
hydrogen source. Although the hydrogen source is described as
2
(
2
2
Fig. S2 in ESI†). The STYs of 1-butanol at CaC loadings of 12,
catꢁ
1
ꢁ1
3 and 34 mmol are 203, 174 and 144 gpro kg
h
, respec-
with
>
>
tively, indicating a decreasing catalytic efficiency of CaC
increasing the loading.
2
3
2
.4 Reaction pathway of ethanol to 1-butanol over CaC at
2
ꢀ
75–300 C
As said above, formation of 2-butenol suggests that the ethanol
reaction over CaC may involve the aldol-condensation route
2
H in Scheme 1, it is really activated hydrogen atoms derived
2
with 2-butenol as intermediate. This route starts from the
formation of acetaldehyde by ethanol dehydrogenation, but
little acetaldehyde was detected in this work. To nd evidence
for the formation of acetaldehyde, the amounts of various gas
products are shown in Table 4. It is clear that a certain amount
16
from ethanol dehydrogenation. These hydrogen atoms are
consumed completely by olen-aldehyde or enol species
according to the stoichiometry. However, a certain amount of
2
H was released in this work, as observed over HAP at 350–
ꢀ
18
4
10 C, suggesting that other hydrogen sources are involved in
2
of H was produced, which conrms occurrence of ethanol
the hydrogenation reaction. Ethanol was reported to be an
dehydrogenation to form acetaldehyde. Little acetaldehyde in
the product suggests that its aldol reaction is very fast, although
aldol reaction is considered to the limiting step in many
reports.
observed in the order of C
and CO indicates decomposition of acetaldehyde as reported in
18,32
alternative and the reaction was said to be Re (2).
In fact, the
Re (2) is really hydrogenation of 2-butenol by activated hydrogen
of ethanol, which also results in the by-product acetaldehyde.
The absence of acetaldehyde in this work suggests that hydro-
genation of 2-butenol may not be as Re (2) even if ethanol
16,32,33,44
It is noted that C
2
H
2
, C
2
H
4
, CH
4
and CO are also
2
H
2
> C
2
H
4
> CH z CO. Minor CH
4
4
ꢀ
a
Table 3 Effects of different calcium compounds and CaC loading on ethanol reaction at 275 C for 6 h
2
Entry
Ca-compound
Dosage (mmol)
Ethanol conv. (%)
1-Butanol yield (%)
Other alcohols' yield (%)
1
2
3
4
5
6
Ca(OCH
Ca(OH)
CaC
CaO
2
CH
3
)
2
23
23
23
23
12
34
14.3
12.6
46.0
32.9
30.6
62.4
6.0
3.8
19.5
10.3
12.2
24.5
5.4
3.0
19.3
8.1
11.2
31.8
2
2
CaC
CaC
2
2
a
The amounts of ethanol in all the cases are 217 mmol.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 18941–18948 | 18945

View Article Online
RSC Advances
Paper
a
2
Table 5 Distribution of the liquid products from the reaction of ethanol with intermediates over CaC
Yield (%)
Ethanol
Intermediate
Entry
Reactants
conversion (%)
conversion (%)
1-Butanol
2-Butenal
2-Butenol
Other alcohols
1
2
3
4
Ethanol/acetaldehyde (1 : 1)
Ethanol/2-butenal (2 : 1)
Ethanol/2-butenol (2 : 1)
Ethanol
33.2
40.9
37.8
46.0
64.8
66.2
70.4
—
10.7
11.9
14.1
19.5
1.2
—
0.3
0.0
1.9
6.4
—
7.3
8.8
9.7
19.3
0.6
a
ꢀ
Reaction condition: 275 C, 6 h, 23 mmol CaC
2
; the total amounts of reactants in all the runs are 217 mmol.
provides hydrogen or there are other undiscovered hydrogen b-H of ethanol are adsorbed on the alkynyl moiety of CaC
2
(base
sources. sites) and the O in the hydroxyl group and a-H of ethanol are
adsorbed on the Ca atom (Lewis acid sites). Next, the aldehyde
species react with the neighboring adsorbed ethanol molecule
CH
3
CH]CHCH
2 2 5 3 2 2 2
OH + C H OH / CH CH CH CH OH
+
CH CHO
3
(2)
via nucleophilic addition to form unsaturated C
4
alcohol
species like 2-butenol (b). Finally, the adsorbed 2-butenol is
hydrogenated to form 1-butanol by the proton-like hydrogen (c).
In this process, one CaC molecule is converted to calcium
2
It should be noted that the 1-butanol yield of ethanol itself
reaction is obviously higher than those of ethanol with 2-bute-
nol, 2-butenal or acetaldehyde (Table 5). That is to say, substi-
tution of a certain amount of ethanol with intermediates is
unbenecial to the formation of higher alcohols. This infor-
mation suggests that in addition to the aldol-condensation, self-
acetylide [HO–Ca–C^CH]. The [HO–Ca–C^CH] could continue
to catalyze ethanol condensation while itself is nally converted
to Ca(OH) and C H .
2
2
2
Self-condensation of ethanol should also involve b-H acti-
vation and a plausible catalytic pathway is shown in Scheme 4.
The b-H in one ethanol molecule is adsorbed on the alkynyl
condensation of ethanol over CaC
2
may also take place.
However, it is difficult to quantify the proportions of these two
routes.
moiety of CaC
molecule is adsorbed on the Ca atom of the same CaC
cule. Then the corresponding b-C combine with the a-C to form
-butanol. In this process, CaC is converted to [HO–Ca–
2
and the hydroxyl group of another ethanol
2
mole-
Although the hydrogen source is still vague, aldol-
condensation does take place during ethanol coupling reac-
1
2
ꢀ
tion over CaC
2
at 275–300 C. As addressed above, the alkynyl
C^CH], which continues to activate b-H and hydroxyl group in
different ethanol molecules, resulting in formation of another
2
moiety plays an important role in the catalytic activity of CaC .
By taking this into account and referencing the catalytic
mechanism of the heterogeneous catalysts, adsorption and
activation of ethanol as well as condensation of acetaldehyde
1-butanol molecule. In comparison to the above aldol-
condensation route, two ethanol molecules are activated
simultaneously in the self-condensation route, which seems
simpler and more feasible. Anyway, the strong hydrogen
abstraction ability of alkynyl moiety is of importance in both
routes.
with ethanol over CaC are speculated and illustrated in Scheme
2
3. Firstly, ethanol is hydrogen-dissociatively adsorbed as alde-
hyde or enol species (a) where the H in the hydroxyl group and
2
Scheme 3 A plausible CaC -catalyzed aldol-condensation of ethanol to 1-butanol. (a) dehydrogenation of ethanol; (b) aldol condensation; (c)
hydrogenation.
18946 | RSC Adv., 2019, 9, 18941–18948
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
atoms. For example, the condensation product of 2-butanol is
mainly 5-methyl-l,3-heptanol (entry 6 in Table 6) rather than 3,4-
dimethyl-l,2-hexanol. In short, CaC2 is effective in catalyzing
condensation of a wide range of aliphatic alcohols with b-C,
affording the products in moderate to good yields.
4. Conclusions
2
This work rstly indicates that the commercial CaC could be
used directly to catalyze conversion of widely available ethanol
to 1-butanol and other C –C alcohols (liquid fuels) with satis-
5
9
factory yields. The yield of 1-butanol reached up to 20–25% and
that of total alcohols reached up to 39–56% with ethanol
conversions of 46–64% at 275 C and molar ratios of ethanol to
Scheme 4 A plausible CaC
to 1-butanol.
2
-catalyzed self-condensation of ethanol
ꢀ
CaC
condensation route are involved in the ethanol coupling over
CaC . The alkynyl moiety in CaC plays an important role in the
2
of 9.4–6.4. Both aldol-condensation route and self-
a
Table 6 Substrate scope of aliphatic alcohols
2
2
catalytic pathways of both routes, which could be attributed to
its strong hydrogen abstraction ability. CaC is also effective in
2
catalyzing condensation of a wide range of aliphatic alcohols
with b-C, featuring excellent substrate tolerance. This nding
opens up the catalytic application of CaC
involving activation of C–H bond. In this process, CaC
gradually converted to C and Ca(OH) . Development of
2
in organic synthesis
2
is
2
H
2
2
Entry Substrate Alcohols conversion (%) Main product Yield (%)
2 2
a novel reactor for efficient in situ conversion of C H to ethyl
1
2
15.7
46.0
0.0
vinyl ether is prospective.
19.5
3
24.7
11.4
Conflicts of interest
The authors declare no competing nancial interest.
4
5
20.1
49.8
9.6
Acknowledgements
32.6
This work was supported by the Major State Basic Research
Project (No. 2011CB201306).
6
17.5
8.8
a
The amounts of alcohol and CaC
2
are 217 and 23 mmol, respectively.
References
1
B. R. Caes, R. E. Teixeira, K. G. Knapp and R. T. Raines, ACS
Sustainable Chem. Eng., 2015, 3, 2591–2605.
3
.5 Exploration of substrate scope
2 O. D. Mante, T. A. Butcher, G. Wei, R. Trojanowski and
V. Sanchez, Energy Fuels, 2015, 29, 6536–6543.
3 Q. Zhu, C. Shen, J. Wang and T. Tan, ACS Sustainable Chem.
Eng., 2017, 5, 8181–8191.
Reactions of aliphatic alcohols other than ethanol were tested
in the presence of CaC2 at 275 C in order to understand
ꢀ
tolerance of this protocol. The yields of main alcohol products
are shown in Table 6. It is clear that methanol fails to yield
ethanol while other alcohols, either straight-chain or branched-
chain, succeed in generating b-alkylated dimer alcohols. The
yield of 2-ethyl-1-hexanol coupled by 1-butanol is as high as
4
M. A. Qubeissi, N. A. Esawi, S. S. Sazhin and M. Ghaleeh,
Energy Fuels, 2018, 32, 6498–6506.
5
R. M. Balabin, R. Z. Syunyaev and S. A. Karpov, Energy Fuels,
2007, 21, 2460–2465.
6
7
8
B. Zhang and S. M. Sarathy, Appl. Energy, 2016, 181, 38–53.
Y. Xu and C. T. Avedisian, Energy Fuels, 2015, 29, 3467–3475.
B. G. Harvey and H. A. Meylemans, J. Chem. Technol.
Biotechnol., 2011, 86, 2–9.
32.6%. These results indicate the importance of b-H in
condensation of alcohols over CaC . The yield of dimer alcohols
generated from straight-chain primary alcohols is much higher
than that of generated from the branched-chain due to a lower
steric hindrance. For example, 1-propanol and 1-butanol yield
more dimer alcohols than isopropanol and 2-butanol (entries 3–
2
46
9 M. Uyttebroek, W. V. Hecke and K. Vanbroekhoven, Catal.
Today, 2015, 239, 7–10.
10 L. A. Hazelwood, J. M. Daran, A. J. V. Maris, J. T. Pronk and
6
in Table 6). If an alcohol contains more than one b-C, the
J. R. Dickinson, Appl. Environ. Microbiol., 2008, 74, 2259–
condensation reaction occurs at the b-C with the most hydrogen
2266.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 18941–18948 | 18947

View Article Online
RSC Advances
Paper
11 C. Carlini, M. D. Girolamo, A. Macinai, M. Marchionna, 28 J. H. Earley, R. A. Bourne, M. J. Watson and M. Poliakoff,
M. Noviello, A. M. R. Galletti and G. Sbrana, J. Mol. Catal.
A: Chem., 2003, 200, 137–146.
2 R. Cano, M. Yus and D. J. Ramon, Chem. Commun., 2012, 48,
Green Chem., 2015, 17, 3018–3025.
29 T. L. Jordison, C. T. Lira and D. J. Miller, Ind. Eng. Chem. Res.,
2015, 54, 10991–11000.
1
1
1
7
628–7630.
30 J. T. Kozlowski and R. J. Davis, J. Energy Chem., 2013, 22, 58–
64.
31 C. Yang and Z. Meng, J. Catal., 1993, 142, 37–44.
3 K. N. T. Tseng, S. Lin, J. W. Kampf and N. K. Szymczak, Chem.
Commun., 2016, 52, 2901–2904.
4 G. Xu, T. Lammens, Q. Liu, X. Wang, L. Dong, A. Caiazzo, 32 C. R. Ho, S. Shylesh and A. T. Bell, ACS Catal., 2016, 6, 939–
N. Ashraf, J. Guan and X. Mu, Green Chem., 2014, 16, 3971–
977.
5 F. Su and Y. Guo, Green Chem., 2014, 16, 2934–2957.
948.
3
33 S. Hanspal, Z. D. Young, H. Shou and R. J. Davis, ACS Catal.,
2015, 5, 1737–1746.
1
1
6 S. Ogo, A. Onda, Y. Iwasa, K. Hara, A. Fukuoka and 34 X. Wu, G. Fang, Y. Tong, D. Jiang, Z. Liang, W. Leng, L. Liu,
K. Yanagisawa, J. Catal., 2012, 296, 24–30.
P. Tu, H. Wang, J. Ni and X. Li, ChemSusChem, 2018, 11, 71–
1
1
1
7 T. Tsuchida, S. Sakuma, T. Takeguchi and W. Ueda, Ind. Eng.
Chem. Res., 2006, 45, 8634–8642.
8 J. Scalbert, F. Thibault-Starzyk, R. Jacquot, D. Morvan and
F. Meunier, J. Catal., 2014, 311, 28–32.
9 L. Silvester, J. F. Lamonier, J. Faye, M. Capron, R. N. Vannier,
85.
35 A. Galadima and O. Muraza, Ind. Eng. Chem. Res., 2015, 54,
7181–7194.
36 R. Matake, Y. Niwa and H. Matsubara, Org. Lett., 2015, 17,
2354–2357.
C. Lamonier, J. L. Dubois, J. L. Couturier, C. Calais and 37 Y. Li, H. Meng, Y. Lu and C. Li, Ind. Eng. Chem. Res., 2016, 55,
F. Dumeignil, Catal. Sci. Technol., 2015, 5, 2994–3006. 5257–5262.
0 A. S. Ndou, N. Plint and N. J. Coville, Appl. Catal., A, 2003, 38 G. Li, Q. Liu, Z. Liu, Z. Zhang, C. Li and W. Wu, Angew.
51, 337–345. Chem., Int. Ed., 2010, 122, 8480–8483.
1 T. W. Birky, J. T. Kozlowski and R. J. Davis, J. Catal., 2013, 39 Z. Li, Z. Liu, R. Wang, X. Guo and Q. Liu, Chem. Eng. Sci.,
98, 130–137. 2018, 192, 516–525.
2
2
2
2
2
2 D. L. Carvalho, R. R. D. Avillez, M. T. Rodrigues, 40 M. Al-Hasan, Energy Convers. Manage., 2003, 44, 1547–1561.
L. E. P. Borges and L. G. Appel, Appl. Catal., A, 2012, 415– 41 T. Tsuchida, J. Kubo, T. Yoshioka, S. Sakuma, T. Takeguchi
4
16, 96–100.
3 M. Le ´o n, E. D ´ı az and S. Ord ´o nez, Catal. Today, 2011, 164, 42 Q. Liu, Q. Liu, R. Wang, T. Xu and Z. Liu, CIESC J., 2013, 64,
36–442. 2573–2579.
4 M. Le ´o n, E. D ´ı az, A. Vega, S. Ord ´o nez and A. Auroux, Appl. 43 Q. Michaudel, T. Chauvir ´e , V. Kottisch, M. J. Supej,
and W. Ueda, J. Catal., 2008, 259, 183–189.
2
2
2
4
Catal., B, 2011, 102, 590–599.
5 O. M. Perrone, F. Lobefaro, M. Aresta, F. Nocito, M. Boscolo
and A. Dibenedetto, Fuel Process. Technol., 2018, 177, 353–
K. J. Stawiasz, L. Shen, W. R. Zipfel, H. D. Abru n˜ a,
J. H. Freed and B. P. Fors, J. Am. Chem. Soc., 2017, 139,
15530–15538.
3
57.
44 J. I. D. Cosimo, C. R. Apesteguia, M. J. L. Gin ´e s and E. Iglesia,
J. Catal., 2000, 190, 261–275.
45 D. Gabri ¨e ls, W. Y. Hern ´a ndez, B. Sels, P. V. D. Voort and
A. Verberckmoes, Catal. Sci. Technol., 2015, 5, 3876–3902.
2
6 I. C. Marcu, D. Tichit, F. Fajula and N. Tanchoux, Catal.
Today, 2009, 147, 231–238.
7 Z. Sun, A. C. Vasconcelos, G. Bottari, M. C. A. Stuart,
2
G. Bonura, C. Cannilla, F. Frusteri and K. Barta, ACS 46 T. Matsu-ura, S. Sakaguchi, Y. Obora and Y. Ishii, J. Org.
Sustainable Chem. Eng., 2017, 5, 1738–1746. Chem., 2006, 71, 8306–8308.
18948 | RSC Adv., 2019, 9, 18941–18948
This journal is © The Royal Society of Chemistry 2019