Morphological control of inverted MgO-SiO2 composite catalysts for efficient conversion of ethanol to 1,3-butadiene

Article abstract of DOI:10.1016/j.apcata.2019.03.007

In this work, three novel MgO-SiO₂ composite catalysts with inverted structure have been synthesized via a facile and scalable strategy by means of the incipient wetness impregnation of the silica sol onto MgO precursors with different morphologies, and evaluated for the one-step conversion of ethanol to 1,3-butadiene. The prepared flower-like MgO-SiO₂ composite catalysts exhibited highly enhanced catalytic activity than those catalysts synthesized from MgO precursors with nanodisks and nanosheets morphologies. Characterization results based on XRD, FT-IR, UV–vis, BET, CO₂-TPD, NH₃-TPD, ethanol-TPD, and ²⁹Si MAS NMR revealed that unique layered flower-like architectures may facilitate the formation of interfacial Mg-O-Si chemical bond in binary MgO-SiO₂ composite catalysts which are mainly responsible for the superior activity. This study presents a new strategy to design and develop the catalyst for efficient conversion of bio-ethanol to 1,3-butadiene by morphological control of MgO-SiO₂ bifunctional catalysts.

Full text of DOI:10.1016/j.apcata.2019.03.007

Accepted Manuscript
Title: Morphological Control of Inverted MgO-SiO2
Composite Catalysts for Efficient Conversion of Ethanol to
1
,3-Butadiene
Authors: Shenxiao Li, Yong Men, Jinguo Wang, Shuang Liu,
Xuefei Wang, Fei Ji, Shanshan Chai, Qiaoling Song
PII:
S0926-860X(19)30115-2
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2019.03.007
APCATA 17012
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
2 January 2019
12 March 2019
13 March 2019
Please cite this article as: Li S, Men Y, Wang J, Liu S, Wang X, Ji F, Chai S,
Song Q, Morphological Control of Inverted MgO-SiO Composite Catalysts for
2
Efficient Conversion of Ethanol to 1,3-Butadiene, Applied Catalysis A, General (2019),
https://doi.org/10.1016/j.apcata.2019.03.007
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Morphological Control of Inverted MgO-SiO
Catalysts for Efficient Conversion of Ethanol to
,3-Butadiene
2
Composite
1
Shenxiao Li, Yong Men*, Jinguo Wang, Shuang Liu, Xuefei Wang, Fei Ji,
Shanshan Chai, Qiaoling Song
College of Chemistry and Chemical Engineering, Shanghai University of Engineering
Science, Shanghai 201620, P. R. China
*
Authors to whom correspondence should be addressed. E-mail addresses:
men@sues.edu.cn, Fax: +86-21 6779 1215; Tel: +86-21 6787 4046
Graphical abstract

The effect of MgO precursor morphologies was explored for conversion of ethanol to
,3-BD. Extensive characterizations demonstrate that the unique layered flower-like
1
architectures may facilitate the formation of interfacial Mg-O-Si species and enhance
Lewis basicity on the bridged O in Mg-O-Si bond via the strong interaction between
MgO and SiO
2
in binary MgO-SiO composite catalysts which are favorable for the
2
superior activity.
Highlights:





Synthesis of three novel MgO-SiO
wetness impregnation.
2
composite catalysts by reverse incipient
MgO-SiO composite catalysts exhibited remarkable morphology-dependent
2
reactivity for Lebedev reaction.
The MgO precursors with different morphologies affect the physicochemical
properties of the catalyst surface.
The unique layered structure of the flower-like catalyst is more favorable for 1,
3
-butadiene formation.
The Mg-O-Si bond formed plays a crucial role in the formation of 1,

3
-butadiene.
Abstract:
In this work, three novel MgO-SiO composite catalysts with inverted structure
2
have been synthesized via a facile and scalable strategy by means of the incipient
wetness impregnation of the silica sol onto MgO precursors with different
morphologies, and evaluated for the one-step conversion of ethanol to 1,3-butadiene.
The prepared flower-like MgO-SiO composite catalysts exhibited highly enhanced
2
catalytic activity than those catalysts synthesized from MgO precursors with
nanodisks and nanosheets morphologies. Characterization results based on XRD,
FT-IR, UV–vis, BET, CO
revealed that unique layered flower-like architectures may facilitate the formation of
interfacial Mg-O-Si chemical bond in binary MgO-SiO composite catalysts which
2
-TPD, NH
-TPD, ethanol-TPD, and 29Si MAS NMR
3
2
are mainly responsible for the superior activity. This study presents a new strategy to
design and develop the catalyst for efficient conversion of bio-ethanol to
1
,3-butadiene by morphological control of MgO-SiO bifunctional catalysts.
2
Keywords: ethanol, 1,3-butadiene, MgO-SiO , Mg-O-Si bonds, flower-like,
2
morphological control

1
. Introduction
1
,3-Butadiene (BD) is an important monomer in the synthesis of polymer industry.
Self-polymerization of butadiene and polymerization with other olefin monomers has
shown great commercial value.1 BD mainly used for the synthesis of
styrene-butadiene rubber, polybutadiene and acrylonitrile-butadiene-styrene（ABS）
rubber, and also applied in the Diels-Alder reaction to form cycloalkanes and
cycloalkanes.2-5 At present, BD is mainly derived from the naphtha steam cracking
produces byproducts of the ethylene process. Isolation of butadiene from the other
6
products requires a number of expensive extractive distillation steps. However, the
predicted shift towards lighter feeds for steam cracking and the increased usage of
shale gas as an alternative source for ethylene have resulted in a supply shortage and
7
consequent price increase for BD. In order to meet the market demand of BD,
looking for an alternative solution to produce BD, a renewable and nonpetroleum
rooted route is highly desirable. The Lebedev reaction path using bioethanol to
convert to BD in one step (Scheme 1) has been extensively studied in the early
twentieth century by using mixed metal oxides as catalysts, typically MgO/SiO ,
2
because bio-ethanol can be obtained via fermentation of carbohydrates (sugar and
corn etc.) or the cellulosic.8-10

Scheme 1. Generally accepted reaction network for the Lebedev process.
At present, various types of catalysts have been reported to perform the reaction,
,11 ZrBEA zeolite, ZnZr/MgO-SiO
12
,13 and Hf-Zn/SiO
14
including Ag/ZrO
2
/SiO
2
2
2
catalysts. Magnesia-silica catalysts were intensively investigated by many research
groups,2, 4 and typically performed well in terms of butadiene production yield,
demonstrated to be very efficient for the direct conversion of ethanol to BD, whose
performance was also affected greatly by the preparation method. The preparation
method and compositions of MgO-SiO -based systems are considered to be crucial
2
factors for their performance.15 Among many methods for synthesizing catalysts,
including wet-kneading method,1, 16-20 sol-gel method,15, 21co-precipitation method,19,
2
2-23
incipient wetness impregnation method and physical mixing method,1, 17 etc., the
wet-kneading has been demonstrated to be a very efficient method for producing
MgO-SiO catalysts for the process of ethanol-to-butadiene since the first discovery
1
2
by Natta et al. 19, 24-26. In recent years, the sol-gel method has been reported as one of
the effective preparation methods. Ochoa and co-workers15 investigated that the
activity of different ratios of MgO-SiO
explained in terms of acid/basic catalyst requirements. However, the preparation of
MgO-SiO by incipient wetness impregnation is usually studied as a comparative
2
catalyst prepared by sol-gel method and
2

method, and it is rarely discussed as a major preparation method because the
selectivity of BD obtained by this method is generally low.
In a heterogeneously catalyzed multistep cascade reaction system, the catalytic
activity and selectivity are often enhanced by synergistic effects through which
different active phases or sites can function in a concerted manner to encompass all
the necessary catalytic functionalities.27-28 Bifunctional catalysts operate by utilizing
two different types of catalytic sites on the same solid. Extensive research revealed
that the basic function of MgO-SiO catalyzes dehydrogenation/condensation/MPV
2
reduction of ethanol and intermediates, whereas the acidic function of the composite
facilitates dehydration of 3-hydroxybutanal to crotonaldehyde and crotyl alcohol.16, 19,
2
9
Obviously, for such a complex reaction network step, the synergy of the catalyst
plays a crucial role in the reaction. It has been reported in the literatures that the
difference in the preparation methods and compositions of the catalysts leads to a
delicate balance of different active sites (acidic, basic, and redox) on the catalyst
surface that is indispensable for the Lebedev reaction process.30 Although a large
number of studies have been conducted on the MgO-SiO system, the detailed
2
structure-activity relationship has not yet been fully elucidated and needs further
study.
Studies have shown that the difference in catalyst structure due to the difference in
morphology has a great influence on the activity of the catalyst. In recent years, the
controlled synthesis of nano- or micro sized metals and metal oxides with different
morphologies has received great research interests in the use of catalysis.31-32 Recently,

there has been extensive research into morphology control of nano-catalyst in tuning
3
3
the catalytic properties of a nanocatalyst in structure-sensitive reactions. Therefore,
developing facile and template-free methods for morphology- and structure-controlled
synthesis is of particular interest.
In general, the MgO-SiO
incipient wetness impregnation method by impregnating magnesium salt precursor
solutions onto the relatively inert component SiO . Herein, to provide further insight
on the design principle of MgO-SiO catalysts, three catalysts with inverted structure
2
composite catalysts can be prepared via the conventional
2
2
and different morphologies of flower-like, nanodisks and nanosheets MgO precursors
were prepared by facile incipient wetness impregnation of silica sol onto the
respective carrier and evaluated for the direct conversion of ethanol to BD. A unique
layered structure can be formed by the method of incipient wetness impregnation with
the appropriate MgO precursor with the flower-like morphology. As demonstrated by
various physicochemical characterizations, the morphology of MgO-SiO
2
catalysts
which
was identified to play a crucial role in the extent of synergy of MgO and SiO
2
favors the formation of large amounts of Mg-O-Si interfacial bonds and dictates the
catalytic performance of the catalyst.
2
. Experimental Section
2
.1. Materials
Ethylene glycol (EG), Mg(CH
3
COO)
2
•4H O, ethanol，polyvinylpyrrolidone (PVP,
2
K30) MgCl
2
·6H O，ethylenediaminetetraacetic acid disodium salt，benzoic acid，
2

ammonium hydroxide and NaOH were purchased from Aladdin Chemistry Co. Ltd.
(
Shanghai, China),Silica Sol was purchased from Jiyida Silicone Reagent Factory
Qingdao, China). All of the chemicals used as received without further purification.
(
2
.2. Catalyst Preparation
The MgO precursors were synthesized by a hydrothermal process with the aid of
surfactants.34-35 Typically, for flower-like MgO precursor, 3.424 g magnesium acetate
Mg(CH COO) •4H O) dissolved in 80 mL ethylene glycol, while adding 2.656 g
(
3
2
2
PVP, stirring at room temperature for 30 min, then ultrasonic for 30 min, and finally
the resulting clear transparent solution was transferred into a 100 mL Teflon-lined
stainless-steel autoclave, the oven was maintained at 180 °C for 4 h, after natural
cooling to room temperature, the solid was collected, washed and centrifuged with
anhydrous ethanol for several times, and then dried in an oven at 100 °C for 12 h to
obtain magnesium oxide precursor. In the case of nanodisks MgO precursor, the
ethylenediaminetetraacetic acid disodium salt was used as the surfactant. 2 g of
MgCl
2
·6H O and 0.1 g of surfactant were dissolved in 70 mL of deionized water,
2
stirred at room temperature for 20 min, and then 10 mL of aqueous ammonia was
added dropwise to form a white precipitate. For the synthesis of nanosheets MgO
precursor, 2 g of MgCl
2
·6H O and 0.12 g of benzoic acid were dissolved in 60 mL of
2
deionized water, stirred at room temperature for 10 min, and then 20 mL (2 M) of
sodium hydroxide solution was added dropwise to form a white precipitate. For each
sample, the suspension was transferred to a 100 mL stainless steel autoclave and held

at 180 °C for 24 h. After cooling to room temperature, filter, wash with water, dry at
8
0 °C for 12 h, MgO precursor were obtained.
The catalyst with molar ratio (Mg/Si) of 1.34 was made by incipient wetness
impregnation method. During the preparation of the catalyst, we impregnated the
silica sol as a solution onto the magnesium oxide precursor, then allowed to stand at
room temperature for 12 h, dried at 80 °C for 12 h, and finally calcined at 500 °C for
4
h in an air atmosphere.
2
.3. Catalyst characterization
X-ray diffraction (XRD) patterns were using a diffraction meter (D/Max-rB) with
Cu-Kα radiation (λ=1.54056 Å) at room temperature. The scanning rate was 4°/min
and the 2θ was from 10° to 80°.
The specific surface area of the material was determined by applying Brunauer-
Emmett-Teller (BET) model on a Micromeritics ASAP 2460 apparatus, and the
samples were pretreated for 6 h in nitrogen at 200 °C before analysis. The
morphology of the material was determined by field emission scanning electron
microscope (FESEM).
The Fourier transform infrared spectroscopy (FT-IR) signal of the MgO-SiO
2
catalyst was recorded on a Nicolet 380 infrared spectrometer (USA). FT-IR spectral
-1
signals in the wave number range of 4000-400 cm were recorded by diluting the
sample in KBr and pressing the sample into the IR chamber.

The spectrum of UV-Visible diffuse reflectance spectroscopy (UV-Vis) was
measured on a UV-2450 spectrophotometer (Shimadzu, Japan) with BaSO
background, and the absorption intensity between 200-800 nm was tested.
4
as the
The acidity and basicity of the catalyst were determined by
CO -temperature-programmed desorption (CO -TPD) and
NH -temperature-programmed desorption (NH -TPD) on a Micromeritics Autochem
2
2
3
3
II 2920 apparatus. The test procedure was performed using a 0.05 g catalyst through a
U-shaped fixed-bed quartz micro reactor. First pre-treatment under argon at 500 °C
for 1 h, then cooled to 100 °C, pre-adsorption for 1 h at 10% CO
switching to He gas, and finally raising the temperature to 900 °C (10 °C /min) to
induce desorption of CO (NH ).
2
(NH ) /He,
3
2
3
To further investigated the reaction course of 1,3-butadiene production, the
temperature-programmed desorption of ethanol (ethanol-TPD) was tested. 0.05 g of
the catalyst was placed on a U-shaped quartz reactor and pretreated in a 3% O /He (20
2
ml/min) atmosphere at 500 °C for 1 h. Then cooled to 50 °C, Inert gas He (20 ml/min)
is purged for 30 min to remove water, impurities, etc. adsorbed on the surface of the
material. Then bubbling the ethanol into the U-shaped quartz tube for pulse
adsorption, when the ethanol adsorption saturation, in a He (30 ml/min) gas
atmosphere, raise the temperature to 900 °C at 10 °C/min, at the same time during the
heating process, the exhaust gas was connected to the HPR20 online mass
spectrometer to monitor the product in real time. In the on-line mass spectrometer,
different mass-to-charge ratios (m/e) were set to be collected to further distinguish

each product by the law of mass cracking, The ratio of mass to charge was set to:
hydrogen=2, water=18, helium=4, ethanol=46, acetaldehyde=44, butadiene=54 ,
crotonaldehyde=70, crotyl alcohol=57, aldol=42, ethylene=28.
Magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra were
2
9
recorded on a Bruker spectrometer. At this field, the resonance frequency of Si is
9.6 MHz; the recycle time is 30 s, and the pulse length is 5.0 μs. The spinning
frequency of the rotor is 5 kHz. The catalysts were packed in 4 mm Zirconia rotors.
5
2
9
Tetramethylsilane was used as chemical shift reference. The field of Si CP MAS
2
9
NMR spectrum，the resonance frequency of Si is 79.5 MHz. 17,000 scans were
accumulated with a recycle delay of 10 s. The CP contact time was 4.0 ms, the
spinning frequency of the rotor was 10 kHz. For both measurements the samples were
packed in 4 mm Zirconia rotors.
A Bruker Tensor 27 FTIR spectrometer was used to collect FTIR-pyridine spectra.
About 20 mg of the catalyst sample was pretreated at 450 °C for 1 h. After being
−
1
cooled down to 50 °C at 50 STP mL min He, a background spectra was recorded.
Pyridine adsorption took place at 50 °C through a bubbler for 5–10 min while He
−
1
flowed at 10 STP mL min . A pyridine purge took place at 50 °C under a He flow of
−
1
5
0 STP mL min for 30 min. Spectra were taken during adsorption to ensure pyridine
saturation. Spectra were recorded at 50 °C after 30 min of He purging to remove
physisorbed pyridine. The sample temperature was ramped at 10 °C/min to 200 °C
and ramped down to 50 °C for spectra collection.
2
.4. Catalyst Testing

The conversion of ethanol to 1,3-butadiene (BD) was performed at atmospheric
pressure, the reaction reactor was a fixed bed quartz reaction tube having an inner
diameter of about 5 mm. Typically, 0.1 g (40-60 mesh particles) of catalyst was
placed in the middle of the fixed bed. Nitrogen (50 ml/min) as carrier gas, carrying
ethanol (temperature maintained at 20 °C) was passed over the catalyst to on-line gas
chromatography(Shimadzu 2014 Gas Chromatography (GC)), and the product was
analyzed by one TCD and two FID detectors. The reaction temperature is 300-500 °C,
one interval for every 50 °C. Prior to the reaction, the catalyst was pretreated for 30
min at 300 °C.
Kinetic test：The kinetic study was performed in the reaction temperature range of
3
40-370 °C with the temperatature interval of 10 °C, in order to ensure that the
conversion rate of ethanol is less than 12%. The other test conditions were the same as
the conditions used for catalytic.
In this paper, the conversion of ethanol and the selectivity of the product are
calculated by the following formula, and the carbon balance of the reaction is greater
than 95%:

x
i
is the mole fraction of products (i); n
z
is the number of carbon atoms in
carbon-containing products (z).
3
. Results and Discussion
3
.1. Catalyst characterization
The XRD patterns of all the catalysts samples were demonstrated in Fig. S1. The
diffraction pattern of the MgO in the composites exclusively reveals the cubic
1
structure of the percales phase (PDF#45-0946), while the broad peak between
1
5°-30° is attributed to amorphous phase of SiO
SiO maintain their individual crystal phase, and in addition to the diffraction peaks of
MgO and SiO , we also observed diffraction peaks of the proto-enstatite phase on the
2
. It can be seen that both MgO and
2
2
nanodisks and nanosheets.
The specific surface area, pore volume and pore size distribution of MgO-SiO
2
composites were determined by N adsorption-desorption (Fig. S2) isotherms and
2
summarized in Table 1. The flower-like sample obviously shows the smallest specific
surface area, the largest pore volume and pore size than the other two catalysts.Type
IV N adsorption-desorption isotherms with H3 hysteresis loop were obtained in all
2
3
0
three cases, revealing the presence of mesoporous structure. The large pore size and
large pore volume of the flower-like catalyst contribute to the entry of reactant
molecules and combine with the active sites on the catalyst surface to promote the
occurrence of catalytic reactions.

Table 1 Textural properties of MgO-SiO
2
mixed oxides determined by N adsorption.
2
S
BET
2
V
p
D
p
Samples
3
(
m /g)
(cm /g)
(nm)
Flower-like
Nanodisks
Nanosheets
151
155
161
0.59
0.37
0.38
12.7
7.60
7.80
Fig. 1 was a morphological analysis of the catalysts.It can be clearly seen that the
flower-like MgO precursor is in the form by ultrathin sheets self-assembly while the
nanodisk and nanosheet displayed hexagonal and circular top view with a certain
thickness in Fig. 1b and Fig. 1c. Compared with the MgO precursor, the surface of the
composite catalyst after loading the SiO
2
became rougher after annealing process. The
in Fig. 1d, e, and f show that the final
SEM images of the as-prepared MgO-SiO
2
products successfully inherited the architectures of the MgO precursors, presenting
the flower-like, nanodisk and nanosheet morphology in Fig. 1a, b and c respectively.
a
b
c
d
e
f

Fig.1. FESEM images of the MgO precursors with three different morphologies (a.
Flower-like b. Nanodisks c. Nanosheets) and corresponding MgO-SiO
catalyst (d. Flower-like e. Nanodisks f. Nanosheets).
2
composite
The FT-IR spectra of three MgO-SiO
2
composites were collected to provide
-
1
information on the structural features of the samples between 4000-400 cm as shown
-1
in Fig. S3. The band at 3400 cm can be assigned to the hydroxyl stretching vibration
of the water molecules on the catalyst surface,36-37 and the absorption at 3700 cm-1
was attributed to vibration of Mg(OH) formed bythe interaction of MgO and water
2
3
8
-1
vapor in the air. The peak at 3745 cm can be attributed to the silanol group formed
by the weak interaction of SiO
2
with water adsorbed on its surface.37, 39 The peak at
-1
40
6
80 cm was the stretching vibration of the Mg-O bond. The absorption peaks
around 1100 cm-1 and 796 cm-1 can be assigned to the symmetrical stretching
vibration of Si-O-Si bonds whose signal is dependent on the morphology of MgO
4
1
precursor. From the FT-IR spectra, one can observe that the absorption strength at
-1
7
96 cm followed the order of nanosheets > nanodisks > flower-like MgO
−
1
morphology. It is noted that the absorption band located at 470 cm characteristic for
the stretching vibration of Mg-O-Si bonds was found over three composite
samples.42-43 The characteristic peak of the Mg-O-Si species exhibited the greatest
intensity at the expense of Si-O-Si bonds, indicative of the strengthened interaction
between flower-like MgO and SiO
2
, preferentially generating a Mg-O-Si linkage.44
composites with
The UV-visible diffuse reflectance spectra of three MgO-SiO
2
different morphology are shown in Fig.2. Notably, the flower-like sample shows an
intense absorption peak near 260 nm whereas the nanodisks and nanosheets materials

exhibit much weaker absorption bands. According to the previously reported literature,
the absorption at 260 nm can be attributed to the Mg-O-Si bond. 17, 26 Therefore, the
morphology-dependent UV absorption can be related to the interaction of MgO and
SiO
may be explained by the large amount of Mg-O-Si chemical bond formation resulted
from the strong interaction of MgO and SiO , demonstrating the readily formation of
2
in the composites. The intense absorption at 260 nm over flower-like sample
2
special properties of Mg-O-Si chemical bonds by using flower-like MgO precursor,
which is in good agreement with the results of FT-IR characterizations.
2
63
Nanosheets
Nanodisks
Flower-like
2
00
300
400
500
600
700
800
Wavelength(nm)
Fig.2. UV-vis diffuse-reflectance spectra of the catalysts with three different
morphologies.
CO
2
/NH
3
temperature-programmed desorption (CO
2
/NH -TPD) were used to probe
3
the surface acidic and basic properties of the catalysts to understand better their
intrinsic acid/base functionalities, as shown in Fig. 3 and quantitative data were
summarized in Table 2 and 3. In Fig. 3a and 3b, two broad desorption peaks of NH
3

and CO
2
were resolved near 150 °C and 250 °C for all catalysts, which were
and CO chemisorbed on weak and medium acidic/basic sites,
attributed to NH
3
2
respectively. According to data presented in Table 2 and Table 3, flower-like
catalyst possesses the most abundant basic sites, and nanosheets show the most acidic
sites over the catalyst surface. The order of relative strength of surface base/acid sites
is flower-like (0.97) > nanodisks (0.80) > nanosheets (0.63), suggesting the highly
enhanced basicity by using flower-like MgO precursor.

a
Nanosheets
Nanodisks
Flower-like
1
00
200
300
400
500
T/℃
b
Nanosheets
Nanodisks
Flower-like
1
00
200
300
400
500
T/℃
Fig.3. NH
with different morphologies.
Table 2 Summary of the amount of acidic sites over the different MgO-SiO
3
-TPD (a) profiles and CO
2
-TPD (b) profiles for the MgO-SiO
2
catalysts
catalysts
2
calculated via NH
3
-TPD analysis.
Number of acidic sites(mmol/g)
Samples
Total sites
Weak
sitesa
Moderate
sitesb
(T
a
)
Flower-like
0.40
0.28
0.12

Nanodisks
Nanosheets
0.45
0.48
0.30
0.31
0.15
0.17
a
The temperature range of weak acidic sites is 100~250 °C.
The temperature range of moderate acidic sites is 250~500 °C.
b
Table 3 Summary of the amount of basic sites over the different MgO-SiO
2
catalysts
calculated via CO
2
-TPD analysis.
Number of basic sites(mmol/g)
Samples
Total sites
Weak
sitesa
Moderate
sitesb
T
b
/T
a
b
(T )
Flower-like
Nanodisks
0.39
0.36
0.30
0.27
0.26
0.23
0.12
0.97
0.10
0.07
0.80
0.63
Nanosheets
a
The temperature range of weak basic sites is 100~250 °C.
The temperature range of moderate basic sites is 250~500 °C.
b
In order to further understand the influence of morphology on the sruface acidity of
the catalysts, pyridine adsorption was also examined by the FTIR to discriminate the
surface Lewis acid sites and Brønsted acid sites. As shown in Fig. 4, the characteristic
-1
peaks of the acidic site adsorbed by pyridine appears in 1448 cm for strong Lewis
-
1
acid sites (SL), 1490 cm for a combination of Lewis and Brønsted acid sites (B+L),
-
1
-1
1
1
537 cm for Brønsted acid sites (B), 1577 cm for weak Lewis acid sites (WL),
-1
26
607 cm for strong Lewis acid sites (SL). It can be seen from the infrared spectra
-1
-1
-1
that a strong absorption peaks can be observed at 1448 cm , 1490 cm , 1577 cm ,
-1
and 1607 cm , indicating that all three materials contain Lewis acid sites. No
-1
characteristic absorption peak of Brønsted acid was observed at 1537 cm , suggesting
that Brønsted acid sites on related to SiO have been considerably passivated by the
strong interaction of SiO with MgO and Lewis acid sites prevailed over the surface of
all three samples. Moerover, the absorption peak intensity of Lewis acid sites
2
2

followed the order of flower-like < nanodisks < nanosheets, agreeing well the results
of NH -TPD.
3
SL
SL
B+L
H
WL
B
Nanosheets
Nanodisks
Flower-like
1700
1650
1600
1550
1500
1450
1400
-
1
Wavenumber(cm )
Fig.4. FTIR spectra for the MgO-SiO catalysts with different morphologies after
2
pyridine adsorption with the desorption temperature at 200 °C. The characteristic
absorption peaks of pyridine on acidic sites are the following: Brønsted (B), weak
Lewis (WL), strong Lewis (SL) and hydrogen bound pyridine (H).
3
.2. Catalyst activity
Fig.5 displays the catalytic performance of the flower-like catalyst for ethanol to
-1
BD at WHSV of 4.1 h . Acetaldehyde, 1,3-BD and ethylene were the only quantified
carbon-containing products, and no other compounds such as carbon dioxide, acetic
acid and ethyl acetate were quantified because their content were very low. As shown
in Fig. 5, ethanol conversion over flower-like catalyst increased with increasing
o
reaction temperature up to 500 C. With the increasing temperature, the selectivity for
the desired product of BD increased first and went through a maximum of 63% when
the temperature reach 400 °C, and as the temperature further increased, the selectivity
of BD started to decline. The continuous increase of ethylene selectivity at the

expense of the decreasing acetaldehyde selectivity implies that the formation of
ethylene and acetaldehyde follows a different parallel competitive reaction pathway.
The continuous decreased acetaldehyde and the peak selectivity of BD in the whole
temperature range strongly suggest that acetaldehyde and BD are the primary and
secondary products in the cascade reaction sequences, respectively. The activity of the
catalysts with two other morphologies was presented in FigureS4.
100
CH CH OH
3
2
CH CHO
3
80
60
40
20
0
C H4
2
C H6
4
300
350
400
450
500
T/℃
Fig.5. Temperature-dependent reactivity of the flower-like MgO−SiO sample.( TOS:
2
Product analysis taken by averaging two measurements within a TOS of 1.5 h at each
temperature.)
The effect of the MgO precursor morphologies on the catalytic behavior of the
MgO-SiO
50 °C. It is evident in Fig. 6 that the activities of the samples in terms of ethanol
conversion and BD selectivity decrease in the following order: flower-like
nanodisks>nanosheets. The superior activity of flower-like catalyst was correlated
with the morphology-dependent strong metal oxide-support interaction, as estimated
2
catalysts were investigated for ethanol to BD at reaction temperature of
4
>

by FT-IR, UV-Vis and chemisorption. Our activity data combined with the
characterization results presented in this study illustrates that the morphology of MgO
precursors is of prime importance on the interaction of MgO and SiO , and thus
2
determines the catalytic activity in BD production. This conclusion is actually in
consistent with the previous results which reported that the proper surface properties
and the existence of Mg-O-Si bond are vital factor for catalytic efficiency.30, 45
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
Flower-like
Nanodisks
Nanosheets
Conversion
Acetaldehyde
Ethylene
1,3-butadiene
Fig.6. Comparison of catalytic behavior over three MgO−SiO catalysts with different
2
morphologies at 450 °C.
To further elucidate the nature of the conversion of ethanol to 1,3-BD, kinetic
studies were performed to obtain the activation energy of the final product 1,3-BD.
Fig. 7 shows the kinetic curves of three catalysts with different morphologies. From
the data of the activation energy, it is easy to see that the activation energy of the
flower-like catalyst showed the minimum energy of 157.4 KJ/mol, which indicate that
the flower-like catalyst is the most active material to convert ethanol into 1,3-BD. By

comparing the specific surface area and activation energy data of the three catalysts,
the flower-like catalyst has the smallest specific surface area and also the lowest
activation energy, but has the highest activity, which excludes the factor that the
catalyst activity is dominated by the specific surface area. These results indicate that
the special structure formed by the catalyst itself and the proper acid-basic properties
on the catalyst surface are the key factors affecting the catalytic activity.
-
9.5
Flower-
like:E
a=157.4K
-
-
-
-
-
-
-
10.0
10.5
11.0
11.5
12.0
12.5
13.0
J/mol
Nanodis
ks:Ea=175.9KJ
/
mol
Nanos
hee
ts:Ea=190.1KJ
/
mol
1.60
1
.54
1.56
1.58
1.62
1.64
-
1
1
000/T(K )
Fig.7. Arrhenius-type plots of 1,3-butadiene production rate over the catalyst with
three different morphologies.
The stability of the flower-like catalyst was studied in Fig. S5. With the time on
stream, the conversion rate of ethanol and the selectivity of butadiene decreased
gradually, which may be due to the carbon deposition on the catalyst surface,
resulting in the covering of active sites on the surface. After 40 hours of reaction, the
conversion rate of ethanol decreased from 71.6% to 52.9% while the selectivity of
1
,3-BD still maintained above 54.5%. The respective selectivity of the main

byproducts for ethylene and acetaldehyde did not increase significantly after 40 h of
time on stream. From these data, the MgO-SiO
reaction conditions.
2
catalyst is relatively stable under the
3
.3. The implication of morphology effect on ethanol conversion to BD
It is well known that MgO-SiO composite catalysts have been regarded as a
2
superior candidate in catalyzing the conversion of ethanol to BD. Numerous factors
are closely relevant to catalytic activity, such as BET surface areas, surface acid-base
properties, the existence of active phase, and morphology. Based on the
aforementioned catalytic data and characterization results, the morphology-dependent
surface properties contribute a momentous portion to the highly enhanced
performance of flower-like MgO-SiO
properties of the catalysts were also confirmed by CO
of the ratio of surface acidic and basic sites obtained from CO
2
catalyst. The differences in surface acid-base
and NH -TPD. A comparison
/NH -TPD showed
2
3
2
3
increased basicity over the flower-like sample. The enhanced surface basicity may
facilitate the key reaction step of ethanol dehydrogenation and sequential aldol
condensation, which occurs favorably over basic sites of the multifunctional catalysts.
To further understand the interaction of ethanol and catalyst surface, ethanol-TPD
was used to investigate the dependence of the reaction pathways on the surface
acid-base properties of the catalysts synthesized by different MgO morphologies. As
shown in scheme 1,46 dehydrogenation and dehydration occur competitively in
parallel over the surface of the MgO-SiO
2
catalysts, depending on the nature of the
surface acid-base properties. Figure 8 displays TPD data obtained following ethanol

pre-adsorption over catalyst surface. The main desorbed species were identified as
CH CHO, H , C (1,3-BD), C and CH CH OH. Notably, three CH CHO and H
peaks are observed over these MgO-SiO catalysts in Fig. 8a and Fig. 8b. The first
CH
CHO peak at low temperature below 200 oC was attributed to the
dehydrogenation of adsorbed ethanol on catalyst surface, which was further
confirmed by the presence of H in the same temperature range. The absence of the
second CH CHO peak in the intermediate temperature range over flower-like sample
may be explained by the highly enhanced surface reaction of ethanol at the expense of
CH CHO, as evidenced by the intense peak for C and H in the same temperature
range. The simultaneous desorption of CH CHO and H
the inhibited aldol condensation of CH
3
2
4
H
6
2
H
4
3
2
3
2
2
3
2
3
3
4
H
6
2
3
2
at high temperatures is due to
o
3
CHO molecules at temperatures above 400 C.
Based on the above analysis, combined with the desorption profiles of ethanol in
Figure 8e, it was found that the peak intensity of ethanol desorption followed the
order of flower-like < nanodisks < nanosheets, in reversed order of activity data. This
result is consistent with the results of the catalytic activity in Section 3.2. Moreover,
the ratio of basic sites/acid sites varies in the order: flower-like (0.97) > nanodisk
(
0.80) > nanosheet (0.63). This is in the same order of intensity ratio of CH
formed over these MgO-SiO catalysts. In this work, the strong interaction
between MgO and SiO can passivate the strong Brønsted acidic sites of SiO by
forming a Mg-O-Si bond, and generate new Lewis acid-base site pairs with enhanced
electron cloud density on the bridged O (i.e., ），which not only facilitates
the dehydrogenation of ethanol to acetaldehyde, but also boosts the reactivity of
3
CHO and
C
4
H
6
2
2
2

acetaldehyde to aldol condensation, thereby increasing the 1,3-butadiene productivity.
desorption peaks display higher intensity over nanosheet sample than over
flower-like and nanodisk catalysts.Correlating the NH -TPD spectra and the data
listed in Table 2, a relationship between the population of surface acidic sites and
generation can be well established, agreeing well with the literature reports.20, 22,
C
2
H
4
3
C
2
H
4
4
7
a
CH CHO b
H c
C H
d
C H
e
CH CH OH
3
2
4
6
2
4
3
2
200
400
200
400
200
400
200
400
200
400
T/℃
Fig.8. Desorption profiles of major products from ethanol-TPD over MgO-SiO
catalysts ( Flower-like; Nanodisks; Nanosheets)
2
The 29Si MAS NMR was used to investigate the changes in the chemical
environment around Si and provided a basis for a better understanding of the impact
of interfacial Mg-O-Si linkage on the catalytic efficiency. The 29Si MAS NMR
spectra of MgO-SiO
2
catalyst with different morphologies was presented in Fig. 9. It
2
9
is known that the signal intensity in these Si MAS NMR spectra depends on the
1
29
1
29
density of H around the Si core and the distance of H- Si in these species. The

1
29
shorter H- Si distance usually leads to a greater density, and therefore results in a
highly enhanced signal intensity of 29Si.NMR spectra of the MgO-SiO
catalysts
show chemical shift signals appearing around -110 ppm, -93 ppm, -90 ppm, -86 ppm,
2
-
83 ppm, and -76 ppm. As reported in the previously literature,26, 45, 48 the signal at
110 ppm is attributed to a Si atom containing four siloxane bonds (Si*(OSi) ), and
) during
formation, -90 ppm corresponds a Si atom having two siloxane bonds and two
hydroxyl groups (Si*(OSi) (OH) ),-83 ppm is due to an enstatite phase intermediate,
86 ppm and -76 ppm are attributed to Mg-O-Si bond in the form of
-
4
the signal at -93 ppm should be the intermediate phase (Si*(OH)(OSi)
3
2
2
-
Si*(OMg)(OSi)
2
(OH) and Si*(OMg)
2
(OSi) . It can be seen from NMR spectra of
2
samples with three different morphologies that the spectra at nanodisks and
nanosheets show the distinct signals located at -83 ppm and -110 ppm, and the very
weak signals at -86 ppm and -76 ppm, indicating the weak interaction between MgO
and SiO during the course of the formation of the nanodisk and nanosheet composite
2
2
9
catalysts. By contrast, the Si MAS NMR spectra of the flower-like catalyst display
distinct signals of chemical shift at -93 ppm, -90 ppm, -86 ppm, and -76 ppm. The
strong signal at around -86 and -76 ppm suggest that the use of silica sol during the
preparation of the catalyst resulted in the formation of an amorphous surface
magnesium silicate phase26, 45, and generated much more intense peaks corresponding
to Mg-O-Si bonds in the form of Si*(OMg)(OSi)
Meanwhile, the strong intensity at -90 ppm and -93.9 ppm also suggests that SiO
partially retained its original phase during the preparation of the flower-like catalyst.
2
(OH) and Si*(OMg)
2
(OSi)
2
.
2
still

These results indicate that the use of MgO with different morphologies during the
impregnation followed by the subsequent calcination leads to the formation of
interfacial Mg-O-Si chemical bond. Similar results were observed previously by Sels
and co-workers for Ag/MgO-SiO
2
catalysts prepared by impregnation.26 Such
behavior is not unexpected taking into account the recent work revealed that the
surface acid-base change with the generation of new Lewis acid-base pairs with
enhanced Lewis basicity on the bridged O in Zr-O-Zn pair, which in turn affect the
reactivity in cascade conversion of ethanol to lower olefins. 49-50
-
110.8
-
76.9
-
83.4
86.8
-
90.3
-93.9
-
Nanosheets
Nanodisks
Flower-like
-40
-50
-60
-70
-80
-90 -100 -110 -120 -130 -140
2
9
Si Chemical Shift (ppm)
Fig.9. 29Si MAS NMR patterns of the MgO−SiO
catalysts with different
2
morphologies.
As documented in the literature, the key factors determining the activity and
selectivity of mixed oxides prepared are assumed to be the morphology (surface area
and pore size) and the relative proportions of M1-O-M to M-O-M bonds.51 For
instance, the catalytic activity of olefins epoxidation on TiO
2
-SiO mixed oxides has
2

been strongly correlated with the Ti-O-Si connectivity that is characteristic of Ti
52
dispersion in the silica matrix. The formation of Ti-O-Si bonds strongly modifies the
electronic structure of the bridge oxygen and titanium atoms at the interface of
TiO
2
/SiO
multilayers. Wachs et al.53-54 suggest that strong interactions between
2
oxides tend to form bridge oxygen bonds, which in turn alter the electronic properties
between the elements, increase the electron cloud density of the O atoms, resulting in
an enhanced bacisity and therefore the boosted performance of methanol oxidation
4
9
50
over the mixed oxide catalysts. The recent reports by Wang et al. and us
also
suggested that the strong interaction between ZnO and ZrO favors the formation of
2
Zn-O-Zr bond with enhanced Lewis-basicity of the bridged O, which further promotes
the reactivity of the aldol condensation reaction involved in the formation of
5
5
isobutene. Recently, Toshihide et al. proposed that the electron cloud density of the
O atom on the bridge oxygen bond may play a crucial role in the reaction of ethanol
to 1,3-BD. The O atom in the structure of Mg-O-Ge was found to have the strongest
electron cloud density and the lowest BE, as confirmed by XPS and DFT calculation.
Therefore, the increase of electron density and the reductive character of the
Mg-O-Ge structure can promote the crotonaldehyde formation from acetaldehyde and
the reaction of crotonaldehyde reduction to crotyl alcohol due to an increase in
basicity. In this work, results from FT-IR, UV-vis, and 29Si MAS NMR data
evidenced that the reinforced interaction between MgO and SiO
formation of new acid-base active structure by modulating the underlying MgO
morphologies. The strong interaction between MgO and SiO can passivate the strong
2
promotes the
2