Investigation into 1,3-butadiene and other bulk chemicals' formation from bioethanol over Mg-Al catalysts

Article abstract of DOI:10.1039/c6ra27610e

Mg-Al catalysts were adopted in the direct synthesis of 1,3-butadiene and other bulk chemicals from bioethanol. The influence of MgO content and sample calcination temperature on the catalytic performance was investigated. The optimization of reaction conditions for 1,3-butadiene, ethylene and 1-butanol, including reaction temperatures and the feed rate of ethanol, was also carried out. The catalysts were characterized by X-ray diffraction, nitrogen sorption, SEM, NH₃-TPD, CO₂-TPD and FTIR of adsorbed pyridine. On the basis of these analysis results and catalytic performance of yielding 1,3-butadiene, both the MgO content and the calcination temperature play a great role on the nature of acid and basic sites, then the proper active sites for producing 1,3-butadiene and other important chemicals such as ethylene and 1-butanol have been elaborated. The balance of moderate acid-basic sites is crucial to production of 1,3-butadiene from ethanol. Ethylene is obtained on strong acid and strong basic sites. 1-Butanol tends to form on the strong basic sites. To further improve the target product selectivity, modifiers adjusting the balance of acidity-basicity need to be involved in the Mg-Al catalysts.

Full text of DOI:10.1039/c6ra27610e

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Investigation into 1,3-butadiene and other bulk
chemicals' formation from bioethanol over Mg–Al
Cite this: RSC Adv., 2017, 7, 11929
catalysts†
ab
Meixiang Gao,ab Minhua Zhangab and Yonghui Li
*
Mg–Al catalysts were adopted in the direct synthesis of 1,3-butadiene and other bulk chemicals from
bioethanol. The influence of MgO content and sample calcination temperature on the catalytic
performance was investigated. The optimization of reaction conditions for 1,3-butadiene, ethylene and
1-butanol, including reaction temperatures and the feed rate of ethanol, was also carried out. The
catalysts were characterized by X-ray diffraction, nitrogen sorption, SEM, NH3-TPD, CO2-TPD and
FTIR of adsorbed pyridine. On the basis of these analysis results and catalytic performance of yielding
1,3-butadiene, both the MgO content and the calcination temperature play a great role on the nature
of acid and basic sites, then the proper active sites for producing 1,3-butadiene and other important
chemicals such as ethylene and 1-butanol have been elaborated. The balance of moderate acid–basic
sites is crucial to production of 1,3-butadiene from ethanol. Ethylene is obtained on strong acid and
strong basic sites. 1-Butanol tends to form on the strong basic sites. To further improve the target
product selectivity, modifiers adjusting the balance of acidity–basicity need to be involved in the Mg–
Al catalysts.
Received 1st December 2016
Accepted 3rd February 2017
DOI: 10.1039/c6ra27610e
rsc.li/rsc-advances
1. Introduction
In the 21st century, the world is faced with the pressure of
energy sources and energy-saving emission reduction.1 Utili-
zation of reproducible resources effectively such as biomass,
solar and geothermal power will be an effective measure to
take to address those energy issue. Especially, large-scale
utilization of the biomass is becoming a more and more
important research topic.2 One of the most abundant practi-
cable and sustainable raw materials produced from biomass
is bioethanol,3–6 especially lots of researches have shied
toward non-grain biomass feedstock conversion to bioethanol
recently.7–9 Conversion of bioethanol to some bulk chemicals
is very important for several reasons, and maybe most
signicant of all, it helps to realize the reduction in nonre-
newable fuel consumption and reduce environment
pollution.10
Fig. 1 XRD patterns of Mg–Al catalysts with different MgO contents.
1,3-Butadiene (BD) is widely applied in synthesis of rubber
and polymers.11,12 With the development of economy, demand
for BD is growing. The use of ethylene as raw material for
Table 1 Structural properties of Mg–Al catalysts with different MgO
contents
BET surface
area (m2 gꢀ1
Average pore
diameter (A)
˚
MgO content
)
aKey Laboratory for Green Chemical Technology of Ministry of Education, R&D Center
for Petrochemical Technology, Tianjin University, Tianjin 300072, PR China. E-mail:
yhli@tju.edu.cn; liyh@tju.edu.cn; Fax: +86-22-27401826; Tel: +86-22-27401826
bCollaborative Innovation Center of Chemical Science and Engineering, Tianjin
300072, China
15%
20%
25%
30%
35%
40%
45%
271.2
188.7
176.7
164.6
155.8
151.3
133.2
54
44
51
45
43
46
47
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra27610e
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Fig. 2 SEM images of (a) 15% Mg–Al (b) 20% Mg–Al (c) 25% Mg–Al (d) 30% Mg–Al (e) 35% Mg–Al (f) 40% Mg–Al (g) 245% Mg–Al.
chemical products is also being considered as an effective commonly used ethanol, 1-butanol might be a promising
measure to offer the market an alternative of commodity plas- gasoline replacement due to many advantages. Specically, 1-
tics with a wide range of applications, especially to produce butanol has a higher energy density than ethanol and is less
polyethylene and other polymers.13–16 Compared to the more corrosive.17
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The mechanism of ethanol conversion to BD is also open mm (20–40 mesh) particle size was packed in the middle of the
to debate, currently. But the following reaction route is quartz tube (R ¼ 22 mm). Before the reaction, the catalyst was
approved by most scholars: acetaldehyde formation from pretreated to the reaction temperature (673 K, heating rate ¼ 5.0
ethanol, acetaldehyde coupling to acetaldol, dehydration of K minꢀ1) carried by 40 ml minꢀ1 N2 ow. The reaction was then
acetaldol to crotonaldehyde, reaction between ethanol and performed with a preset weight hourly space velocity (WHSV) at
crotonaldehyde to crotyl alcohol and dehydration of crotyl preset reaction temperature. Products were detected online by
alcohol to BD. The process of 1-butanol from ethanol follows Agilent 7890A equipped with a Carbon-Plot column (0.535 mm
a similar reaction route with that of conversion of ethanol to id ꢁ 3 mm thickness ꢁ 30 m length) and Thermal Conductivity
BD, except that the nal step to be the hydrogenation of the Detector (TCD). The optimization reaction conditions included
acetaldol to 1-butanol. Ethylene is formed from ethanol reaction temperature of 623 K, 648 K, 673 K, 698 K and 723 K as
dehydration.
well as WHSV of 0.8 hꢀ1, 1.2 hꢀ1, 1.4 hꢀ1, 1.8 hꢀ1, 2.2 hꢀ1 and
The main challenge in BD and other bulk chemicals 2.4 hꢀ1
production from ethanol is the relationship between product
distribution and catalyst nature, although many classes of
.
3. Results and discussion
3.1 Properties of Mg–Al catalysts
catalysts studied in this process.18–24 Our previous work was
dedicated to improve new catalyst system and Mg–Al catalysts
were adopted in this process based on some literatures.25–29 So
the investigations on the inuence of MgO content and calci-
nation temperature on structure and properties of the catalysts,
BD selectivity, and the optimization of reaction conditions will
be focused in this study.
The structures of the Mg–Al catalysts with different MgO
contents were analyzed by powder X-ray diffraction (Fig. 1). The
XRD patterns of the Mg–Al samples with 15ꢂ40% show typical
peaks in 2q ¼ 37ꢃ, 45ꢃ and 65ꢃ, which corresponds to Al2O3
2. Experimental
2.1 Catalyst preparation
A balanced amount of 25% ammonia solution was slowly added
in the mixed Mg(NO3)2$6H2O and Al(NO3)3$9H2O aqueous
solutions respectively containing the 15 wt%, 20 wt%, 25 wt%,
30 wt%, 35 wt%, 40 wt% and 45% MgO at 333 K under stirring,
maintaining the pH 9–10. Repeat the above operation until
precipitate completely. All the precipitate samples were washed
several times until the pH of the solution was 7. Then these
samples were dried at 353 K for 6 h and calcined at 773 K for 5 h.
The same method was adopted in the synthesis of 35 wt% Mg–
Al but the samples were respectively calcined at 723 K, 773 K,
823 K, 873 K, 923 K and 973 K.
2.2 Catalyst characterization
Fig. 3 NH3-TPD on Mg–Al catalysts with different MgO content.
X-ray powdered diffraction patterns were obtained with
a Rigaku Multiplex instrument using Cu–K radiation operated
at 40 kV and 40 mA. Nitrogen adsorption isotherms were
determined at 77 K on a Micromeritics Tristar 3000 volu-
metric adsorption analyzer. Before the adsorption measure-
ments, all samples were preprocessed at 673 K in a degassing
station for 12 h. Scanning electron micrograph (SEM) images
were recorded on a Philips XL-30S FEG scanning electron
microscope. The nature of acid sites and basic sites were
measured by NH3-TPD and CO2-TPD using a Micromeritic
Autochem II 2920. Pyridine adsorbed FTIR was studied on
Thermo Scientic Nicolet 560 FTIR spectrometer with 64
scans at 4 cmꢀ1 resolution.
2.3 Catalytic test
The reaction of ethanol conversion to BD and other bulk
chemicals was performed in a xed bed reactor system at
atmospheric pressure. The 3.0 g catalyst sieved to 0.425–0.850 Fig. 4 CO2-TPD on Mg–Al catalysts with different MgO content.
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diffraction by planes (111), (100) and (110), respectively. While
The textural characteristics of Mg–Al catalysts with different
the MgO content increases to 45%, the typical diffraction MgO contents are presented in Table 1. The BET surface areas
patterns of MgO gradually appears. gradually decreases as the MgO content increases from 15% to
Fig. 5 FTIR spectra of (a) 15% Mg–Al (b) 20% Mg–Al (c) 25% Mg–Al (d) 30% Mg–Al (e) 35% Mg–Al (f) 40% Mg–Al (g) 45% Mg–Al after pyridine
adsorption.
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45%. However, the change of the average pore diameters of all that Lewis acid sites could not be detected when the MgO
˚
Mg–Al samples, which uctuate in the range of 40–50 A, shows content was 45%.
no pattern with the increase of the MgO content, from 15% to
45%.
Based on the following catalyst activity data, Mg–Al catalysts
with 35% MgO content were optimized. Now, calcination
SEM images of the Mg–Al samples with different MgO temperatures would be a more important factor needed to
contents are shown in Fig. 2. As a whole, all the samples present consider. XRD patterns of 35% Mg–Al catalysts with different
irregularly blocky shapes. But as the MgO content increases, it calcination temperatures are shown in Fig. 6. Catalysts calcined
exhibits a more spongy but distribution–intensive morphology. at the temperatures from 723 K to 973 K only show typical
This may be explained by the fact that catalyst morphology features of crystalline Al2O3 (111), (100) and (110). No peaks due
shows MgO image when the MgO content increases to a certain to crystalline MgO or any other crystalline impurity phases are
extent. Moreover, this is agreed with the trends of the BET discerned. But diffraction peak becomes sharper with the
surfaces.
calcination temperature increasing, which indicates the crys-
Acidic properties of Mg–Al catalysts with different MgO tallinity has improved.
contents were studied by NH3-TPD (Fig. 3). NH3-TPD proles of
Table 2 shows the structural properties of catalysts with
all samples show two intensive peaks on ꢂ423 K and ꢂ923 K, different calcination temperatures. Both the BET surface areas
which suggests catalysts have weak and strong acid sites. The and the average pore diameters uctuate within the scope of
relative acid amounts of the catalyst estimated from the peak 165–185 m2 gꢀ1 and 40–60 A, respectively. But there are still no
˚
areas don't have much regulation as the MgO content increases. patterns as the calcination temperature increasing, which can
But the Mg–Al sample with 35% MgO content has the most be conrmed by SEM images.
amount of acid sites on the surface.
SEM images of the samples with different calcination
Similarly, the basic properties of the catalysts were inves- temperatures are shown in Fig. 7. All the samples, as a whole,
tigated by CO2-TPD and the results are presented in Fig. 4. present irregularly blocky shapes. There is no apparent
Two peaks of all samples appear at ꢂ373 K and ꢂ823 K, agglomeration body as the calcination temperature goes up,
respectively. The peak at low temperature corresponds to which is agreed with BET measurement. This also illustrates
weak basic sites and the high temperature peak is attributed that calcination temperature has little inuence on the catalyst
to strong basic sites. The relative basic amounts of the morphology at the temperature range of 723–973 K.
catalyst estimated from the peak areas are still not regular
As shown in Fig. 8, the acid site nature of catalysts with
with the varying MgO content. There are also more basic different calcination temperatures was explored by IR spec-
sites on the surface of the Mg–Al sample with 35% MgO troscopy of adsorbed pyridine. For samples (a), (b) and (c), there
content.
are two bands that can be detected at 1590 cmꢀ1 and 1450 cmꢀ1
,
As shown in Fig. 5, the nature of acid sites was detected by which are due to H-bonded pyridine and pyridine adsorbed on
IR spectroscopy of adsorbed pyridine. For samples with the Lewis acid sites, respectively. However, no band can be
15ꢂ40% MgO content, two main bands detected at 1590 cmꢀ1 observed for the samples (d), (e) and (f). That is, calcination
and 1450 cmꢀ1 are attributed to H-bonded pyridine and temperature has inuence on the acid site nature and higher
pyridine adsorbed on the Lewis acid sites, respectively. More calcination temperature makes Lewis acid sites disappear from
interesting, adsorption strength of pyridine on catalyst the surface of the catalysts.
surfaces slowly weakened as the MgO content increased so
3.2 Catalytic test for ethanol conversion
The selectivity and product distribution of catalysts with
different MgO contents are shown in Fig. 9(a). The results show
that the reaction products were mainly BD, ethylene, ether,
acetaldehyde, 1-butene and 1-butanol. The BD selectivity
increases rst, and then decreases with the increase of MgO
content. However, there is no linear relationship between other
Table 2 Structural properties of catalysts with different calcination
temperatures
Calcined
temperature (K)
BET surface
area (m2 gꢀ1
Average pore
diameter (A)
˚
)
723
773
823
873
923
973
166.3
185.0
170.1
175.9
179.4
169.1
44
41
56
48
60
49
Fig. 6 XRD patterns of 35% Mg–Al catalysts with different calcination
temperatures.
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Fig. 7 SEM images of (a) Mg–Al-723 K (b) Mg–Al-773 K (c) Mg–Al-823 K (d) Mg–Al-873 K (e) Mg–Al-923 K (f) Mg–Al-973 K.
products including ethylene, ether, acetaldehyde, 1-butene as spectroscopy of adsorbed pyridine, the Lewis acid sites are
well as 1-butanol and the MgO content.
appropriate for BD formation.
Similarly, the selectivity and product distribution of catalysts
To investigate the inuence of reaction conditions to bulk
with different calcination temperatures are shown in Fig. 9(b). chemicals including reaction temperatures and the feed rate of
The selectivity of both BD and ethylene increases rstly and ethanol, the following experiment was also carried out. The
then decreases as calcination temperature goes up. The selec- variation trend and specic data are summarized in Fig. 10. The
tivity of diethyl ether increases with the calcination tempera- suitable conditions for BD and ethylene: temperature 698 K and
ture. Likewise, other products such as acetaldehyde, 1-butylene WHSV 1.4 hꢀ1. The suitable conditions for 1-butanol: temper-
and 1-butanol change irregularly as the calcination temperature ature 713 K and WHSV 1.6 hꢀ1, and for higher reaction
varies.
temperatures, the production of 1-butanol becomes much
The possible reasons of the above mentioned results are that easier. The selectivity of both ethylene and 1-butanol uctuates
the acid–basic sites suitable for BD also increase rstly and then widely as the feed rate varies. However, the trends of products
decrease with the increase the MgO content and calcination such as BD, acetaldehyde diethyl ether and 1-butylene become
temperature, but the other acid or basic sites vary irregularly more moderate with the feed rate increasing. This may be due to
with the MgO content and calcination temperature based on the former is less demanding in terms of active sites while the
our NH3-TPD and CO2-TPD studies. On the basis of results of IR latter has high requirement to the proper active sites.
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Fig. 8 FTIR spectra of (a) Mg–Al-723 K (b) Mg–Al-773 K (c) Mg–Al-823 K (d) Mg–Al-873 K (e) Mg–Al-923 K (f) Mg–Al-973 K after pyridine
adsorption.
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Fig. 10 The optimization of reaction conditions.
Fig. 9 The selectivity and products distribution of catalysts with
different MgO contents (a), different calcination temperatures (b).
areas as the calcination temperature increasing. Both the BET
surface areas and the average pore diameters uctuate within
Reaction temperature 673 K, WHSV 1.8 hꢀ1
.
the scope of 165–185 m2 gꢀ1 and 40–60 A, respectively. The
˚
acid–basic sites suitable for BD also increase rstly and then
decrease with the increase the calcination temperature, but the
other acid or basic sites vary irregularly with calcination
temperature based on our NH3-TPD and CO2-TPD studies. On
the basis of results of IR spectroscopy of adsorbed pyridine, the
Lewis acid sites are appropriate for BD formation.
35% Mg–Al catalysts calcined at 773 K show good perfor-
mance for BD and ethylene when ethanol reacts at 698 K with
the feed rate of 1.4 hꢀ1. The suitable conditions for 1-butanol:
temperature 713 K and feed rate 1.6 hꢀ1, higher reaction
temperatures, the production of 1-butanol is much easier. To
further improve the product selectivity, modiers adjusting the
balance of acid–basic also need to be included in the Mg–Al
catalysts.
4. Conclusions
Conversion ethanol to BD and other bulk chemicals over Mg–Al
catalysts has been investigated. The reaction products were
mainly BD, ethylene, ether, acetaldehyde, 1-butene and 1-
butanol.
The inuence of MgO content has been systematic studied.
The MgO content has a great impact on the catalyst structure
and acid–basic properties. The BET surface areas gradually
decreases as the MgO content increases from 15% to 45%.
When MgO content is under 40%, MgO and Al2O3 components
can be uniformly distributed in the catalyst so as to raise the
activity of BD. There is no simple linear relationship between
the acid as well as basic sites and MgO content. So the products
selectivities based on acid, basic and acid–basic sites change
irregularly.
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