Highly active and selective binary MgO-SiO2 catalysts for the production of 1,3-butadiene from ethanol

Article abstract of DOI:10.1039/c6cy02091g

A series of magnesia-silica binary composite catalysts with different molar ratios were prepared using various preparation methods and evaluated for the one-step production of 1,3-butadiene (BD) from bio-ethanol. The MgO-SiO₂ catalyst prepared by wet-kneading is distinguished from the samples prepared by other methods by its superior activity. An exceptionally high productivity of butadiene with the high ethanol conversion of 95% and BD selectivity of 77% was obtained over wet-kneaded hierarchical flower-like MgO nanostructures with the catalytic material made up of SiO₂ spheres (with a MgO:SiO₂ molar ratio of 65:35 at 450 °C and an WHSV of 4.1 h^-1), which translates into an unprecedented BD yield of 0.025 mol g_cat^-1 h^-1. The as-prepared catalysts were characterized by XRD, FESEM, BET, FT-IR, UV/vis, CO₂-TPD and NH₃-TPD. The Mg precursor used and the preparation method are found to influence the extent of MgO and SiO₂ interactions during preparation, as estimated by various techniques, which, in turn, was found to correlate with the catalytic performance. A subtle balance of surface acidity/basicity was found to be necessary to achieve a high butadiene yield which can be obtained by variation of MgO:SiO₂ ratios in wet-kneaded samples. In view of the difference in catalytic activity and the physicochemical properties of the MgO-SiO₂ composite catalysts, the Si-O-Mg chemical bond formed by the strong MgO and SiO₂ interaction is suggested to be essential for the balanced surface acid/base sites in relation to the strong interaction between MgO and SiO₂ and the consequent high catalytic performance.

Full text of DOI:10.1039/c6cy02091g

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Highly active and selective binary MgO-SiO2 catalysts for the
production of 1,3-butadiene from ethanol
Received 00th January 20xx,
Xiaoxiong Huang, Yong Men*, Jinguo Wang, Wei An, Yuanqiang Wang
A series of magnesia-silica binary composite catalysts with different molar ratios were prepared using various preparation
methods and evaluated for the one-step production of 1,3-butadiene (BD) from bio-ethanol. The MgO-SiO2 catalyst
prepared by wet-kneading is distinguished from the samples prepared by other methods by its superior activity. An
exceptionally high productivity of butadiene with the high ethanol conversion of 95% and BD selectivity of 77% was
obtained over wet-kneaded hierarchical flower-like MgO nanostructures with SiO2 spheres catalytic material with
MgO:SiO2 molar ratio of 65:35 at 450 °C with the WHSV of 4.1 h-1, which translates into an unprecedented BD yield of
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
0.025 mol gcat h-1. As-prepared catalysts were characterized by XRD, FESEM, BET, FT-IR, UV/Vis, CO2-TPD and NH3-TPD.
-1
The Mg precursor used and preparation method are found to influence the extent of MgO and SiO2 interaction during
preparation, as estimated by various techniques, which, in turn, was found to correlate with the catalytic performance. A
subtle balance of surface acidity/basicity was found to be necessary to achieve a high butadiene yield which can be
obtained by variation of MgO:SiO2 ratios in wet-kneaded samples. In view of the difference in catalytic activity and their
physicochemical properties of the MgO-SiO2 composite catalysts, the Si-O-Mg chemical bond formed by the strong MgO
and SiO2 interaction is suggested to be essential for the balanced surface acid/base sites in relation to the strong
interaction
between
MgO
and
SiO2
and
the
consequent
high
catalytic
performance.
direct catalytic conversion of ethanol into BD has been
recently renewed as a potential alternative renewable route
due to the high crude oil prices and world-wide abundant
supply of bio-ethanol. One-step process was developed by
1. Introduction
1,3-Butadiene (BD) is one of the most important lighter olefins
for being used as an intermediate in the production of a
variety of polymers such as styrene-butadiene rubber,
12
Lebedev using a variety of mixed metal oxide as catalysts.
acrylonitrile-butadiene-styrene
polymer,
acrylonitrile-
Several literatures had reported a consensus on a number of
1-5
butadiene-styrene (ABS) resins and polybutadiene.
Currently, BD is predominantly obtained as a by-product from
C4 fraction from naphtha steam crackers where ethylene is the
key steps for the cascade transformation of ethanol into BD
13-18
(see Scheme 1):
(1) ethanol dehydrogenates to
acetaldehyde; (2) the aldol condensation of acetaldehyde to
acetaldol; (3) dehydration of acetaldol to crotonaldehyde; (4)
6
main product. However, recent trends in shifting to lighter
feedstock for the steam crackers will decrease BD yields while
the increasing demand for BD on the market may lead to a
serious shortage in BD production. To develop alternative
Meerwein-Ponndorf-Verley
type
reduction
between
crotonaldehyde and ethanol to obtain crotyl alcohol and
acetaldehyde; (5) a final dehydration of crotyl alcohol to BD.
technologies for BD production,
a renewable and non-
petroleum rooted route is highly desirable. As one of the most
7, 8
perspective sources for butadiene production,
bio-ethanol
can be obtained via fermentation of carbohydrates (sugar and
corn etc.) or the cellulosic. 9-11
The direct production of BD from ethanol is an old
industrially proven process, which was used from the 1920s to
the early 1960s. This process was completely scrapped with
increasing oil production. However, the research interest on
College of Chemistry and Chemical Engineering, Shanghai University of Engineering
Science, Shanghai 201620, P. R. China. E-mail: men@sues.edu.cn; Fax: +86-21-
67791214
Scheme 1 Commonly proposed mechanism for the conversion
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See DOI:
10.1039/x0xx00000x
of ethanol into 1,3-butadiene.
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According to different literature sources,
Journal Name
19-33
various forming a plethora of by-products, including butanol, butene,
catalysts have been proposed to perform this reaction diethyl ether, ethyl acetate and ethane and so forth. Despite
involved different transition metal combinations. Magnesia- the efforts conducted over MgO-SiO2 catalyst for the ethanol
silica catalysts were intensively investigated by many research conversion to BD, the detailed mechanism has not yet been
groups, 13, 34 and demonstrated to be very efficient for the one- fully elucidated and still under debate. Based upon the
pot conversion of ethanol to BD, whose performance was also literature reports, a subtle balance of different surface active
34, 35
affected greatly by the preparation method.
It was sites (acidic, basic, and redox) resulted from the morphological
reported that the wet-kneaded material, containing of and structural differences observed for the various catalysts
Mg(OH)2 prepared by hydrolysis of Mg(NO3)2 with NH3 and are essential for the Lebedev process. For key biomass-related
SiO2 prepared from hydrolysis of Si(OEt)4 with HNO3, EtOH and reaction such as ethanol to BD, studies of MgO-SiO2 binary
3
NH3, is performing best catalytic activity. The term “wet- catalysts with
a proper-tuned surface composition and
kneading”, coined by Natta et al., 36 was the first to be used to property may provide an important insight into the structure-
prepare a MgO-SiO2 catalyst from magnesium hydroxide and property relationship, allowing us to better understand the key
colloidal silica for the ethanol-to-butadiene process. The effect factors necessary to make better catalysts.
of the preparation method was further studied by Kvisle et al.,
In the present work, the hierarchical structured flowerlike
who showed that the MgO-SiO2 catalyst prepared by wet- MgO precursor was synthesized by an ethylene-glycol-
kneading performed better than the mechanical mixture mediated (EG) self-assembly process at the presence of
components, highlighting the synergic effect between the poly(vinylpyrrolidone) (PVP) and used for the wet-kneading
34
solids during the preparation. Angelici et al. also observed with SiO2 for direct conversion of ethanol to yield BD.
that MgO-SiO2 catalyst prepared by wet-kneading with 1 wt % Subsequently, the different preparation methods and
of CuO as promoter exhibited the higher butadiene yields and MgO:SiO2 molar ratios were employed with the purpose to
selectivities than the physical mixture or catalysts prepared by develop high performance on which MgO-SiO2 binary catalysts
35
37
co-precipitation.
Very recently, Chung and co-workers
for direct conversion of ethanol into BD. The physicochemical
investigated that the variation of the Mg precursor and of MgO properties of these catalysts were examined by means of X-ray
content determines the extent of magnesium silicates diffractions (XRD), physisorption (BET method), Fourier
formation during wet-kneading pointing out the amounts of transform infrared spectroscopy (FT-IR), UV/Vis diffuse
layered magnesium silicates were found to correlate with the absorption
spectroscopy, CO2-temperature-programmed
(CO2-TPD), NH3-temperature-programmed
catalyst performance in the Lebedev process. Weckhuysen and desorption
38
co-workers showed that the equimolar MgO-SiO2 catalysts desorption (NH3-TPD) and field emission-scanning electron
prepared by wet-kneading was performing better and microscopy (FESEM), and correlated with their catalytic
concluded that the best performing catalysts are due to the performance.
appropriate balance among a small amount of strong basic
sites, combined with an intermediate amount of acidic sites
2. Experimental Section
and weak basic ones. On the other hand, the optimal Mg/Si
ratio is also known to be an important parameter strongly
influencing catalyst performance. For instance, Makshina et al.
2.1. Materials
5
Ethylene glycol (EG),
Mg(CH3COO)2•4H2O, ethanol,
reported an optimal content of 0.66 MgO to achieve 55% BD
polyvinylpyrrolidone (PVP, K30), NH3•H2O (25 wt.%), tetraethyl
orthosilicate (TEOS) and (MgCO3)4Mg(OH)2•5H2O were
purchased from Aladdin Chemistry Co. Ltd. (Shanghai, China).
All of the chemicals were used as received without further
purification.
selectivity with complete ethanol conversion. Additionally,
many catalytic systems were devoted to synthesis of BD from
ethanol involved amphoteric oxides with different transition
28
26
metal combinations, including Hf-Zn/SiO2, ZrBEA zeolite,
29
30
Ag/ZrBEA,
ZnZr/MgO-SiO2,
catalysts. Furthermore,
35
considerable improvements were realized by supporting Cu
39
2.2. Catalyst preparation
or Ag
to facilitate the dehydrogenation, showing almost
complete ethanol conversion, and butadiene yields higher
than 50%. Despite these advances, the MgO-SiO2 catalysts still
The MgO was prepared by the procedure reported by Cui et al.
40
showed butadiene yields ranging from 9 to 42 % and
13, 34, 35
Typically,
1.712
g
of
magnesium
acetate
selectivities between 30 and 84 %.
Therefore, the
(Mg(CH3COO)2•4H2O) and 1.328 g PVP were dissolved in 40 mL
of EG. After stirring with a magnetic stir rotor for 30 min, a
clear solution was obtained. The clear solution was then
attainment of a high selectivity to 1,3-BD together with a high
conversion of ethanol in a one-step process still remains to be
a challenging task, primarily because the parameters of
preparation method, structural properties and Mg/Si ratio and
so on would influence the catalytic performance of MgO-SiO2
catalysts.
transferred into
a
100-mL Teflon-lined stainless-steel
autoclave, which was then kept in an oven at 180 °C for 5 h.
After natural cooling to room temperature, the solid products
were collected by centrifugation, thoroughly washed with
o
The direct conversion of ethanol to BD constitutes to be a
very complex reaction network, and many undesired reactions
may occur competitively with main reaction pathway by
ethanol for several times, and then dried in an oven at 100 C
for 12 h to obtain magnesium oxide precursor. Finally, the
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MgO precursor was calcined at 500 oC for 2 h to obtain the Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) models
crystalline MgO. To compare the catalytic performance of our on the desorption branches. Before the analysis was
obtained materials, commercial MgO (MgO-C) (synthesized by conducted, the sample was outgassed at 200 oC for 6 h in a
calcining commercial (MgCO3)4Mg(OH)2•5H2O in air at 450 °C degassing station.
for 16 h) were also used.
Acid and basic properties of catalysts were investigated by
41
Bare spherical silica was prepared by Stöber route
as NH3- and CO2-temperature-programmed desorption (NH3-TPD
follows: TEOS was hydrolyzed using an ethanol-water- and CO2-TPD) on a Micromeritics Autochem II 2920 apparatus,
ammonia solution (25:15:3.14 vol./vol./vol.) in a closed vessel equipped with a TCD. With regard to the acidity by NH3-TPD,
by stirring at room temperature for
4 h, followed by 50 mg catalyst in a U-shaped fixed-bed quartz microreactor
centrifugation and thorough washing with ethanol. Then the treated at 500 oC (ramping rate 10 oC min-1) for 0.5 h under
solid products were dried at 100 oC for 12 h.
flowing He (30 mL (STP) min-1), then subsequently the sample
was cooled to 100 oC. After that, a 5% NH3/He (30 mL (STP)
min-1) gas mixture were applied, followed by flowing in He (30
mL (STP) min-1) until the base line was stabilized. Then the
sample was heated to 900 oC with a ramping rate of 10 oC min-
Four MgO-SiO2 catalysts were prepared with a 65:35 molar
ratio (based on the above results) of the two components to
investigate the effect of the different preparation methods on
catalytic performance. The physical mixture MgO-SiO2 (I) was
made by mixing (5 min) the two calcined individual
components, prepared as detailed above, in a mortar. Finally,
the dried solid was calcined at 500 °C for 5 h in air. MgO-SiO2
(II) was prepared by sol-gel method 42. In a typical procedure,
1
to induce desorption of NH3. In the case of CO2-TPD, a
procedure similar to the one described above for NH3-TPD was
employed.
Fourier-transform infrared (FT-IR) spectra at room
0.6 mL NH3•H2O (25 wt.%) was first added to the 8 mL mixture temperature were recorded on a Nicolet 380 spectrometer
of ethanol and water (25:15 vol./vol.). After stirring for 0.5 h, (USA) infrared spectrometer. The sample was pressed into a
1.4 g Mg(CH3COO)2•4H2O precursor was dispersed in the thin self-supported wafer and placed in the IR cell. The IR
solution, followed by the addition of 0.7 mL TEOS. Then the spectra of the catalysts were recorded in air under the
mixture was stirred for a few hours until dryness, then dried at condition without evacuation. Diffuse reflectance (DR) spectra
120 °C overnight and calcined at 500 °C for 5 h in air. MgO-SiO2 in the UV-Vis region were recorded in the absorbance function
(III) was prepared by incipient wetness impregnation: 0.1 mL of mode at room temperature using BaSO4 as a background on a
a 4.03 M solution of Mg(CH3COO)2•4H2O in water was added UV-2450 spectrophotometer (Shimadzu, Japan). The
to the support material (SiO2) and then the sample was left for morphology of catalysts was probed by a field emission-
o
24 h to equilibrate, then dried at 100 C for 12 h. Finally, the scanning electron microscopy (FESEM, Hitachi S4800).
dried solid was calcined at 500 °C for 5 h in air. The sample
2.4. Catalyst testing
MgO-SiO2 (IV) was synthesized by wet-kneading. MgO-SiO2 (IV)
was prepared by a wet-kneading procedure similar to the
previous reports by Kvisle et al by using the appropriate molar
The conversions of ethanol to 1,3-butadiene were conducted
in a fixed bed quartz tube reactor with an internal diameter of
5 mm at atmospheric pressure. In a typical experiment, the
catalyst (100 mg) was loaded in the middle of quartz beds. A K-
type thermocouple was placed in the middle of the catalyst
bed to monitor the reaction temperature. Ethanol was
supplied by the carrier gas (50 mL (STP) min-1) through an
evaporator kept at constant temperature (20 °C). Prior to the
reaction, catalysts were firstly pre-treated in N2/ethanol (50 ml
34
ratio of MgO and SiO2. A mixture containing the MgO and
SiO2 were mixed at 50 oC for 5 h in water. Then the suspension
was stirred constantly and heated at about 100 oC to
evaporate water until dryness. Finally, the dried solid catalyst
was calcined at 500 oC for 5 h in air. Materials with various
MgO-SiO2 molar ratios equal to 50:50, 65:35, 75:25, 85:15 and
95:5 were prepared by wet-kneading procedure described
above. Samples were labelled as MgO-SiO2 (x), where x
denotes the preparation methods.
o
(STP) min-1) at 300 C for 0.5 h. The reaction temperature was
within 300-550 °C. The effluent gas products being heated
above 200 °C to avoid the condensation of condensable
species were measured by an online Shimadzu 2014 Gas
Chromatography (GC) with molecular sieves C13X, Al2O3
column (50m, 0.53mm, 10um), Rt-Q-BOND PLOT column (30m,
0.32mm ID, 10um) and equipped with a thermal conductivity
detector (TCD) and two flame ionization detectors (FID).
Nitrogen was used as the internal standard for calibration and
calculation of GC results. Furthermore, for comparison, the
pure oxides SiO2 and MgO were also tested.
2.3. Catalyst characterization
The crystalline phases of the samples were identified by X-ray
powder diffractions (XRD) using a diffraction meter (D /Max-r
B) with Cu- Kα radiation (
λ = 1.54056 Å) at room temperature.
The wideangle data were obtained by the step of scanning
with a rate of 4°/min from 2θ=10° to 2θ=80°. Nitrogen
adsorption-desorption isotherms were measured at -196 oC on
a Micromeritics ASAP 2460 apparatus by applying Brunauer-
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In present work, the ethanol conversion and product The X-ray diffraction (XRD) patterns of the MgO-SiO2 catalysts
selectivity were calculated on a per carbon basis and defined with varying molar ratios (Fig. 1(a)) and by different
by three following equations:
preparation methods (Fig. 1(b)) in Fig.
1 identified the
presence of crystalline MgO phase in all MgO-SiO2 composite
o
o
o
catalysts. The diffraction peaks at 2θ of 36.9 , 42.9 , 62.3 ,
74.7 o and 78.6 o were indexed as the (111), (200), (220), (311)
and (222) diffraction planes of the periclase phase of cubic
MgO (JCPDS 45-946), respectively. 5 A halo was observed on all
o
o
catalysts in the region at 2θ from 20 to 40 due to the
presence of amorphous SiO2. The XRD analyses also show that
MgO and SiO2 still maintain their native morphology in the
MgO-SiO2. Clearly, the MgO-SiO2 catalysts with higher Si content
were mainly amorphous with only weak reflections corresponding
to MgO. Notably, in the case of the MgO-SiO2 (molar ratio was
65:35), the XRD patterns contain diffraction peaks
corresponding to the crystalline MgO phase, and their
intensity is significantly lower than those for the other molar
ratios (including MgO-C-SiO2) suggests significant chemical
changes in the MgO component and more abundant crystal
defects creating more corners, edges, and steps on the MgO
surface formed by well dispersed phase over SiO2 surface .
As shown in Fig. 1(b), the order of crystallinitys estimated by the
intensity of the periclase reflections (MgO-SiO2 (III)> MgO-SiO2 (I)>
MgO-SiO2 (IV)> MgO-SiO2 (II)) does not correlate with catalytic
performance (as shown below). MgO-SiO2 (II) exhibited the lowest
crystallinity likely due to the increased surface imperfection of the
magnesium oxide crystals, indicating the decrease in the size of the
MgO crystals and an increase in the crystal defects, both affected by
3. Results and Discussion
3.1. Properties of the as-prepared catalysts
39
the enhanced interaction of MgO with SiO2. For MgO-SiO2 (III)
and MgO-SiO2 (I), the high crystallinity of the periclase is evidently
retained. On the other hand, some minor phases other than MgO
and SiO2 were identified over MgO-SiO2 sample prepared by sol-gel
method which was assignable to be forsterite-like phase (Mg2SiO4)
in the Fig. 1(b). Similar XRD peaks have been observed in
previous reports. 42
Fig. 1 (a) X-ray diffractograms of the as prepared catalysts with
various molar ratios of MgO:SiO2: a) pure MgO; b)
MgO:SiO2=95:5; c) MgO:SiO2=85:15; d) MgO:SiO2=75:25; e)
MgO:SiO2=65:35; f) MgO-C:SiO2=65:35; g) MgO:SiO2=50:50; h)
pure SiO2. (b) diffraction patterns of the various MgO:SiO2
catalysts (all with molar ratio 65:35) by different preparation
methods: a) pure MgO; b) MgO-SiO2 (I); c) MgO-SiO2 (III); d)
MgO-SiO2 (II); e) MgO-SiO2 (IV); f) pure SiO2.
Fig. 2 FESEM images of (a and b) hierarchical flower-like MgO,
(c) SiO2 spheres, (d) MgO-SiO2 (IV).
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The morphology of MgO, SiO2 and MgO-SiO2 (IV) catalysts wide mesopore size distribution, which further confirmed the
was examined by the SEM images in Fig. 2(a-d). Fig. 2(a) and (b) above conclusion.
shows that MgO synthesized by using an EG-mediated self-
Fig. 3(c-d) shows the nitrogen isotherm and pore size distribution
assembly process gives a uniform flower-like structures with curve of the catalysts prepared by four different preparation
an average diameter of approximately 600 nm after calcination methods. The isotherm of the four MgO-SiO2 catalysts displays a
at 500 °C for 2 h. As seen in Fig. 1(b), the dense core of each type IV isotherm with a capillary condensation step. In contrast to
flower-like MgO structure consists of well separated spherical MgO-SiO2 (I) and MgO-SiO2 (III), the isotherms of MgO-SiO2 (II) and
particles, which indicates that the building blocks are MgO-SiO2 (IV) pronounced type IV characteristics with H3 hysteresis
connected to each other at the center and not aggregated at loop, which reveals the high ordering of the mesopore structure, as
the outside realm of the sphere. Such unique morphology may depicted in Fig. 3(d). The enhanced volume adsorbed by the MgO-
enbow the MgO materials with some distinct features on SiO2 (II) and MgO-SiO2 (IV) catalysts clearly indicates an increase of
strong mechanical strength and facile mass transportation on the BET surface area.
its surface. Fig. 2(c) shows the as-formed SiO2 sample consists
of well separated spherical particles with an average size of Table 1 Textural properties of silica-magnesia mixed oxides
300 nm and a narrow size distribution. As displayed in Fig. 2(d), determined by N2 adsorption and XRD.
the MgO-SiO2 (IV) wet-kneaded composite has a relatively
Samplesa
rugged surface, while the overall morphology of the MgO in
SBET
Vp
Dp
(nm)
Crystallite size
(nm)c
(m2/g)
(cm3/g)
binary wet-kneading catalyst remains the same original flower-
like nanostructure as sole MgO.
111
0.44
14.0
24
MgO
MgO:SiO2=95:5b
MgO:SiO2=85:15b
MgO:SiO2=75:25b
MgO:SiO2=65:35b
MgO-C:SiO2=65:35b
MgO:SiO2=50:50b
SiO2
142
171
165
204
185
70
0.89
0.56
0.55
0.60
0.46
0.28
0.04
0.29
0.30
0.11
18.1
11.0
10.9
9.4
23
22
22
21
21
24
/
6.6
11.2
20.4
19
11
MgO-SiO2 (I)
56
30
36
42
MgO-SiO2 (II)
147
29
7.2
MgO-SiO2 (III)
12.0
a MgO-SiO2 catalysts with various molar ratios of MgO:SiO2 prepared by different
methods. b Prepared by wet-kneading. c MgO crystallite size.
Fig. 3 Nitrogen sorption isotherms and pore size distributions
of (a), (b) the as prepared catalysts with various molar
ratios,and (c), (d) the various MgO:SiO2 catalysts (all with
molar ratio 65:35) by different preparation methods.
Table
1 lists the most important textural parameters
calculated from the isotherms and XRD patterns of the as-
prepared nanomaterials. In Table 1 magnesia-silica mixed
oxides have surface areas (BET area) in the range of 29-204
m2/g. However, the MgO-SiO2 mixed oxides have higher
surface areas than bare silica. The specific surface area of pure
MgO (111 m2/g) is also larger than the commercial MgO (95
m2/g). From the Table 1, one can see the different preparation
methods and compositions have a remarkable effect on the
specific surface area in binary MgO-SiO2 and the samples
prepared by wet-kneading method generally possess higher
surface area. The observed volcano-like behavior in surface
area may be associated with varying extent of the interaction
between the SiO2 and MgO phases. Notably, the catalysts with
various molar ratios prepared by the wet-kneading displayed a
volcano trend, giving the highest specific surface at
MgO:SiO2=65:35. The decrease in the surface area and pore
volume is also explained by a partial covering of the external
The isotherms and pore size distribution curves of the as-
prepared MgO-SiO2 binary oxide catalysts are illustrated in Fig.
3. As can be seen in Fig. 3(a), the nitrogen physisorption
isotherms for the five different molar ratios of the MgO:SiO2,
MgO-C:SiO2 and MgO samples show a type IV isotherm with
H3 hysteresis loop in the relative pressure (P/PO) range from
43
0.4 to 1.0,
which could be caused by a capillary
condensation step according to BDDT classification, indicating
the mesoporous pore structure of the catalysts. By contrary,
the bare silica prepared by the Stöber route displays a type II
isotherm, which is characteristic of non-porous or
44
macroporous materials.
Meanwhile, their corresponding
pore size distribution curves in Fig. 3(b) demonstrated the
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Journal Name
MgO surface by SiO2 particles resulting in a partial pore transfer from four-coordinated (edge) and three-coordinated
blockage. It is important to note that the average crystallite (corner) O2- ions to Mg2+ based on the study of several MgO
sizes of the MgO calculated by Scherrer’s equation based on samples. The change in the relative intensity of the two bands
the peak of (200) followed with a similar but reversed volcano- at 230 and 280 nm are so pronounced, indicating that the
like behavior by variation of magnesia/silica content in the formation of a Si-O-Mg species assignable at 260 nm have to
magnesia-silica materials, agreeing well with the BET area be taken into account via MgO and SiO2 interaction. 34,39
results. As can also be seen in Table 1, the wet-kneading
synthesized catalyst shows the highest surface area and pore
volume. Overall, the enhanced large specific surface area and
unique mesoporous structure would be more favorable for
adsorption and mass transport, which is beneficial for high
catalytic performance.
Fig. 5 FTIR spectra of the as-obtained MgO-SiO2 samples.
The FT-IR transmission spectra of the various samples were
investigated in order to collect information on the structural
features of the magnesia-silica samples and to assess the
nature of the interaction of the SiO2 with MgO. Fig. 5
Fig. 4 Diffuse-reflectance UV-vis spectra of the as-prepared MgO- represents the FT-IR spectra of magnesia-silica samples in the
SiO2 samples.
region of 4,000-400 cm-1. Fig. 5(a) depicts the FT-IR
transmission spectra of bare MgO, SiO2, the samples with
different MgO-SiO2 molar ratios and MgO-C-SiO2. The FT-IR
spectra of bare MgO and SiO2 samples were similar to that
The UV-Vis diffuse reflectance spectra of MgO, the MgO-
SiO2 samples with different molar ratios and the MgO-SiO2
catalysts prepared by different method are shown in Fig. 4. As
shown in Fig. 4(a), the spectra of MgO-SiO2 catalysts reveal
two characteristic bands at 230 and 280 nm. According to
46
reported in the literatures. The band at 1636 cm-1 can be
assigned to the bending vibration of the water molecules. The
broad band at 3400 cm-1 belongs to the stretching vibrations
of the hydroxyl groups of water physically adsorbed on the
surface. 47,48,51 The bulk MgO has an IR band at 3700 cm-1 that
45
Coluccia et al., the observed absorption bands at 230 and
274 nm of MgO were assignable to the ligand to metal charge
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attributes to vibration of Mg(OH)2, which was formed by MgO behavior, NH3- and CO2-TPD measurements were performed
and water vapour reacting in air. 46 The small feature at around to quantitatively determine the distribution of surface acidity
3745 cm-1 can be assigned to the surface silanol groups, which and basicity and the number of acidic and basic sites of MgO-
appear to interact weakly with adsorbed water over silica SiO2 catalysts. All the results were summarised in Table 2 and
surface, according to the literatures. 49-51 Three obvious sharp Table 3. As shown in Fig. 6(a), bare MgO displayed three
absorption bands at 1100 cm-1, 1010 cm-1 and 796 cm-1 can be desorption peaks of NH3 centered at 130, 250, and 630oC,
assigned to the asymmetric Si-O-Si stretching mode, 44 and the respectively, corresponding to weak, moderate, and strong
52
Mg-O stretching vibration at 680 cm-1,
respectively. acidic sites, while the acidic site with medium strength was
1100 cm-1 diminished by SiO2 addition. The NH3-TPD profile of MgO-SiO2
Kneading MgO with SiO2 broadens the band at
～
and 1010 cm-1 due to the decrease of the silica content in composites with different molar ratios show two broad peaks
MgO-SiO2 compared to the parent silica, accompanying with of NH3 desorption at maximum desorption temperature below
the down-shift of Si-O-Si bands to lower wave number, 200 oC and above 600 oC for all catalysts. The maximum
indicative of the strengthened interaction between MgO and desorption temperature of NH3 over MgO-SiO2 at low
53
SiO2, preferentially generating Si-O-Mg linkage.
It is temperatures shifted from 132 up to 147, 153, 168, and 158 °C
worthwhile to note that the absorption band located at 470 with increasing MgO content, respectively, indicating the weak
cm-1 characteristic for the stretching vibration of Si-O-Mg acidities vary with MgO addition. Similar results have been
54-57
bonds
was found over the wet-kneaded composite observed in the previous reports. 42
samples. From the Fig. 5(a), one can also see that the increase
of the magnesium oxide content of the magnesia-silica Table 2 Amount of acidic sites for the different MgO-SiO2
composite catalysts result in a decreased intensity of the catalysts calculated via NH3-TPD analysis.
surface silanol groups, as compared to the parent silica.
Number of acidic sites (cm3/g)
All catalysts prepared via different methods show the similar
Samplesa
Total sites
(Ta)
Moderate
sites
ND*
Weak sites
48.3
Strong sites
66.5
bands as discussed above, albeit with different relative
intensities (Fig. 5(b)). The presence of the band at 470 cm-1
was observed on spectra of sol-gel and wet-kneaded materials,
whereas the catalysts prepared by the physical mixture and
incipient wetness impregnation exhibited the absence of this
fingerprint of Si-O-Mg vibrations. Using magnesium acetate as
precursor for MgO-SiO2 catalysts lead to the preferential
formation of Si-O-Mg chemical bond during the heating
process (compared with the MgO-C-SiO2) and thereby
reinforce the interaction of MgO and SiO2. 53
MgO:SiO2=95:5b
MgO:SiO2=85:15b
MgO:SiO2=75:25b
MgO:SiO2=65:35b
MgO-C:SiO2=65:35b
MgO:SiO2=50:50b
MgO-SiO2 (I)
114.8
121.6
122.2
133.8
105.3
95.1
60.4
63.9
76.9
52.5
49.6
39.3
79.6
50.6
ND*
ND*
ND*
ND*
ND*
ND*
ND*
ND*
61.2
58.3
56.9
52.8
45.5
47.5
69.0
56.8
86.8
MgO-SiO2 (II)
148.6
102.4
MgO-SiO2 (III)
a MgO-SiO2 catalysts with various molar ratios of MgO:SiO2 prepared by different methods. b
Prepared by wet-kneading. * ND not detected.
The amount of acidic sites of the various materials prepared
via different methods was also investigated by NH3-TPD in Fig.
6(b). The NH3-TPD profiles obtained over the samples
prepared by the physical mixture, incipient wetness
impregnation and wet-kneading have similar desorption
profile at low temperature, evidencing the weak acidic sites. In
addition, NH3 desorption peaks for the sol-gel sample displays
a
tailed peak on the higher temperature side, which
corresponds to a stronger acid site. This is also in good
agreement with the significantly increased ethylene and
diethyl ether by-products over the sol-gel catalyst, as
evidenced by a broadening of the NH3 desorption signal at
whole temperatures range. In fact, sol-gel method resulted in
the formation of either forsterite or silica domains and led to a
Fig.
6 NH3-TPD profiles for the wet-kneaded MgO-SiO2
catalysts (a) and the catalysts prepared by different methods
(b); CO2 -TPD profiles for the wet-kneaded MgO-SiO2 catalysts
(c) and the catalysts prepared by different methods (d).
considerably enhanced total surface acidity, which gave rise to
42
To better understand the intrinsic acid-base functionalities
and correlate the catalysts with their interesting catalytic
the preferred formation of ethylene.
It indicates that the
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optimal catalyst for the one-pot ethanol transformation into
Journal Name
butadiene process requires an intimate contact between the 3.2. Catalytic test for 1,3-butadiene production from ethanol
35
two components but not full mixing of them. The amounts
of different acid sites and total acid sites are calculated from
the NH3-TPD peak areas and illustrated in Table 2. The number
of surface acid sites decreased in the order: MgO-SiO2 (II)
MgO-SiO2 (IV) MgO-SiO2 (III) MgO:SiO2 (I).
＞
＞
＞
Table 3 Amount of basic sites for the different MgO-SiO2
catalysts calculated via CO2-TPD analysis.
Number of basic sites (cm3/g)
Total sites
(Tb)
Weak
sites
Moderate
sites
Strong
sites
Samplesa
Tb/Ta
MgO:SiO2=95:5b
MgO:SiO2=85:15b
MgO:SiO2=75:25b
MgO:SiO2=65:35b
MgO-C:SiO2=65:35b
MgO:SiO2=50:50b
MgO-SiO2 (I)
41.9
41.5
36.5
34.8
35.3
30.0
15.9
26.8
20.5
9.9
14.3
11.9
11.4
10.3
9.9
18.9
18.9
17.0
12.8
16.8
10.8
3.4
13.1
8.3
0.37
0.34
0.30
0.26
0.34
0.32
0.18
0.18
0.20
7.6
10.6
8.2
9.3
6.2
6.3
MgO-SiO2 (II)
9.9
3.9
13.0
7.3
MgO-SiO2 (III)
6.6
6.6
a MgO-SiO2 catalysts with various molar ratios of MgO:SiO2 prepared by different methods. b
Prepared by wet-kneading. Ta= Total acidic sites; Tb= Total basic sites.
The basicity measurements of MgO-SiO2 catalysts with
different MgO loadings were carried out by the temperature-
programmed desorption of CO2 (Fig. 6(c)). The TPD profile
contain three main CO2 desorption peaks, which can be
attributed to the weak, moderate, and strong basic surface
sites, respectively. As shown in Fig. 6(c), the temperature of
maximum CO2 desorption was generally shifted toward higher
temperatures, indicating that the proportion of stronger basic
sites on the surface increased gradually with decreasing MgO
contents. Fig. 6(d) illustrated the CO2-TPD of MgO-SiO2
catalysts prepared by different methods. Basicity of MgO-SiO2
catalysts and the basicity/acidity ratio calculated from CO2-TPD
and NH3-TPD peak area are also summarized in Table 3. The
sol-gel catalyst showed a greater acidity than others, in terms
of acid sites and strength. Clearly, the wet-kneaded MgO-SiO2
samples presented a remarkable higher basicity/acidity ratio
than the catalysts prepared by other methods, regardless of
the MgO:SiO2 ratios employed. The number of basic sites
Fig. 7 Effect of reaction temperature on catalytic performance
of MgO:SiO2=65:35 (a) and MgO-C:SiO2=65:35 (molar ratio)
catalysts (b). Conditions: 0.1 g of catalyst, ethanol (gas phase)
molar fraction 5.79%, nitrogen flow 50 mL min-1, respectively.
In order to understand the relationship between structure and
activity, the effect of reaction temperature on the products
distribution was investigated over MgO-SiO2 and MgO-C-SiO2
(molar ratio=65:35) composite catalysts. It can be seen from
Fig. 7a, the ethanol conversions increase with increasing
temperature over MgO-SiO2, while the product distributions
are found to be dependent on the reaction temperature. A
significant increase of the butadiene yield was obtained by
increasing the temperature from 300 to 450 oC, when
continuing to increase the reaction temperature above 450 oC,
butadiene yield started to decline. In contrast, the MgO-C-SiO2
with the same composition shows quite low activity at
relatively low temperatures (Fig. 7b), indicating the limited
ability to catalyze conversion of ethanol into BD reaction under
mild conditions. The acetaldehyde yield remains a very low
varies in the order: MgO-SiO2 (IV)
＞MgO-SiO2 (II)＞MgO-SiO2
(III) ＞MgO:SiO2 (I). It should be noted that the trend of
basicity and MgO:SiO2 ratios variation were almost consistent
with the trend of catalytic performance of MgO-SiO2 catalysts
(see below). The result indicates that wet-kneading method
may give samples, generating much more abundant basic sites
on the catalysts surface.
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level even in the temperature range investigated. The ethylene value when the molar ratio of MgO:SiO2 was 65:35. The
was found as the main by-product and its yield increased presence of MgO phase in the catalyst gives rise to an increase
dramatically as the reaction temperature increased, indicating in the basicity and the surface area of the catalyst (see above).
that higher temperatures facilitated the dehydration activity of This indicates that the MgO-SiO2 (molar ratio=65:35) catalyst
2, 39
this catalyst and resulted in a high ethylene yield.
More will give the optimum acidic and basic properties required for
importantly, as a major desired product, an exceptional high the formation of butadiene from ethanol.
o
yield of 73% for 1, 3-butadiene were achieved at 450 C, while
To investigate the effects of the different preparation
highest yield of acetaldehyde was obtained, which was similar method on butadiene production, four different synthesis
to the previous work 58 (Fig. 7a). As shown in Fig. 7, the results method were employed for preparing MgO:SiO2 catalysts
revealed the catalytic activities obtained by MgO-SiO2 catalyst whose results are as shown in Fig. 9. The catalysts showed
are much better than that of the MgO-C-SiO2 catalyst in terms significant differences in butadiene yield decreasing in the
of BD yield, indicating that the catalyst using the order: MgO-SiO2 (IV)＞ MgO-SiO2 (III)＞ MgO-SiO2 (II)＞ MgO-
hydrothermal-derived MgO, which possesses more active sites SiO2 (I). The catalysts prepared by physical mixture performed
than the commercial MgO, and consequently higher catalytic poorly, giving the similar conversion of ethanol and the yield of
activity, is more favorable for BD production. The conversion butadiene to the bare MgO (results not shown). It might be
of the ethanol over MgO-SiO2 (MgO:SiO2 = 65:35) reaches the understood by the fact that the contact and interaction
ethanol conversion of 95% and the selectivity of BD is 77% at between MgO and SiO2 is critical for achievement of a high
o
450 C, which are considerably better than the results of the yield toward the butadiene. The sol-gel material showed the
MgO-C-SiO2 under the same reaction conditions. The reasons highest ethanol conversion and ethylene yield, suggesting that
for this could stem from not only an increase of the MgO the dehydration of ethanol to ethylene is more favorable on
surface area and an enhanced MgO and SiO2 interaction but the catalyst prepared by sol-gel method, which could be
also a proper balance in acid-base sites formed on the MgO- associated with its considerably enhanced total surface acidity,
SiO2 catalyst surface. In addition, hierarchical flower-like compared to the other catalysts, in agreement with the NH3-
structured MgO provides a significantly larger active surface TPD results.
area and the rugged surface can favor mass transport and
consequently higher catalytic performance. 59, 60
Fig. 9 Effect of catalyst preparation method on catalytic
Fig. 8 Effect of catalyst composition on catalytic performance
performance of MgO:SiO2. Conditions: 0.1 g of catalyst,
of MgO:SiO2. Conditions: 0.1 g of catalyst, ethanol (gas phase)
ethanol (gas phase) molar fraction 5.79%, nitrogen flow 50 mL
molar fraction 5.79%, nitrogen flow 50 mL min-1, T = 450 oC,
min-1, T = 450 oC, respectively.
respectively.
The catalytic activity over MgO:SiO2 (molar ratio=65:35)
Fig.
8 shows the performance of various MgO-SiO2
catalyst is shown as a function of weight hourly space velocity
(WHSV) in Fig. 10. As contact time decreases, the conversion of
ethanol progressively decreases, accompanying by the
decreased selectivity of ethylene and a concomitant increase
of selectivity of butadiene and acetaldehyde. Acetaldehyde is a
primary product of ethanol dehydrogenation over the basic
MgO sites. It also can be seen that the 1,3-BD productivity
values rapidly increase as the contact time decreases and an
exceptionally high space time butadiene yield of 0.025 mol
catalysts for the conversion of ethanol to butadiene.
Compared with the negligible ethanol conversion of bare SiO2
(results not shown), the addition of MgO improves the
catalytic performance significantly. With the increasing of the
MgO content, the conversion of ethanol rises from 78 to 100
%, except the sample of MgO:SiO2=95:5 (decrease to 73%).
Variation of the Mg/Si ratio in a series of wet-kneaded samples
showed a volcano-type dependence of the butadiene yield on
MgO content. The yield of butadiene showed the maximum
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-1
gcat h-1 was achieved at 450 °C with the WHSV of 4.1 h-1. To
the best of our knowledge, the highest BD yield of 0.71 gBD
-1
-1
gcat h−1 (0.013 mol gcat h-1) so far obtained from one-pot
conversion of bio-ethanol ever in the literature records was
achieved by Larina et al. with a BD selectivity of 60.2% and
complete ethanol conversion, over a quaternary Zn-La-Zr-Si
61
catalyst operating at 400 °C and a WHSV of 2 h−1. A very
recent publication by Lee et al. investigated for the use of
Cu/MCF and Zr/MCF were separately loaded into two fixed
bed reactors for conversion of ethanol to BD, achieving the BD
selectivity of 73% with ethanol conversion of 96% at 400 °C
-1
and a WHSV of 3.7 h-1 and the productivity of 1.4 gBD gcat h−1
-1
62
(0.025 mol gcat h-1). Clearly, our catalytic system performs
even better in terms of BD productivity compared with the
above results, suggesting that our as-prepared material is a
promising catalyst for 1,3-BD production.
Fig. 11 Stability test of the MgO:SiO2 (molar ratio=65:35)
catalyst. Conditions: 0.1 g of catalyst, ethanol (gas phase)
molar fraction 5.79%, nitrogen flow 50 mL min-1, T = 450 oC,
respectively.
As documented in the literatures, it was generally accepted
that balanced acidic/basic sites on the catalyst surface is
important to achieve a high yield of butadiene from ethanol.
Therefore, the effect of total basicity/total acidity and strong
acidity/total acidity on the butadiene yield is shown as a three-
dimensional contour plot in Fig. 12a. This plot is very
practically useful to understand the interaction and effects of
different factors on the response. As can be clearly seen from
Fig. 12a, a proper balance of surface acid/base sites is essential
to obtain high butadiene yield, based on the statistics of all
samples employed. Notably, a relatively high butadiene yield
were observed on MgO:SiO2 catalyst with moderate total
basicity/ total acidity ratio in the range of 0.24-0.3 and strong
acidity/total acidity ratio in the range of 0.46-0.5. This also
Fig. 10 Effect of WHSV on catalytic activity of MgO:SiO2 and
1,3-butadiene productivity. Conditions: 0.1 g of catalyst,
ethanol (gas phase) molar fraction 5.79%, T = 450 oC,
respectively.
indicates that
a small amount of basic sites with an
intermediate density of strong acid sites are the favorable
conditions to achieve a higher yield of butadiene. In the
literature, the preparation method of these MgO-SiO2
The catalytic stability of the MgO:SiO2 (molar ratio=65:35)
catalyst was investigated as a function of time on stream, with
butadiene as main product and ethylene and acetaldehyde as
major by-products, as depicted in Fig. 11. Once the stream
entered the reactor, MgO:SiO2 (molar ratio=65:35) catalyst
started to be activated and reach the maximum ethanol
conversion of 95%, followed by a slow deactivation thereafter,
and the selectivity for butadiene declined with decreasing
ethanol conversion. In addition to butadiene, the selectivity for
ethylene and acetaldehyde increased with the decreasing
butadiene yield, indicating that the mitigated butadiene yield
is due to the gradual covering of surface active sites by coke
deposition. After 42 hs of time on stream test, wet-kneaded
MgO:SiO2 (65:35) still exhibited 51% ethanol conversion, and
49% of butadiene selectivity.
composite catalysts have been noticed to be of pivotal
34
importance for catalytic performance.
This is indeed also
observed in the case of our study. From Fig. 12a, it is found
that the catalyst prepared by sol-gel method, incipient
wetness impregnation, and physical mixing show much
stronger surface acidity than the samples prepared by wet-
kneading, as evidenced by their significantly lower values of
surface basicity/acidity ratio, and lower catalytic activity than
the wet-kneaded materials. The reason for this phenomenon
may be due to the basic sites provided by MgO favors the
dehydrogenation of ethanol and leads to the predominant
22
formation of acetaldehyde
whereas the acid sites on the
catalyst surface contribute mainly for ethanol dehydration to
ethylene. In this sense, a subtle balance of surface basicity and
acidity is necessary to achieve the first base-catalyzed
dehydrogenation of ethanol to acetaldehyde and aldol
condensation, and a follow-up acid-catalyzed dehydration. As
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a major by-product, ethylene yield was also as a three- perpared by the methods other than wet-kneading and sol-gel
dimensional contour plotted in Fig. 12b. As shown in the Fig. method, indicating that the wet-kneading and sol-gel method
12b, the increased ratio of the weak acidity and the total process may well promote the interaction of the components
acidity were beneficial to catalytic dehydration of ethanol to and lead to formation of Si-O-Mg chemical bonds. In a very
37
ethylene, indicating that ethanol dehydration is more sensitive recent study, Weckhuysen et al.
highlighted the role of
to the weak acidic sites. This interpretation is consistent with magnesium silicate in wet-kneaded silica-magnesia catalyst
some recent literatures reports. 63-65
and proposed that the formation of magnesium silicate phase
to the key for ethanol to butadiene. Nevertheless, the sol-gel
catalyst having magnesium silicate phase exhibited rather poor
activity in our study, implying that the formation of this
magnesium silicate phase, prepared by our work, may not be
the decisive structural characteristic to achieve high catalytic
performance. In view of the difference in catalytic activity and
their physicochemical properties of the MgO-SiO2 composite
catalysts, the catalytic activity is associated with the balanced
surface acid/base sites in relation to the interfacial Si-O-Mg
link caused by the strong interaction between MgO and SiO2.
The explanation is expected to be useful implication for
designing new acid-base multifunctional catalyst for ethanol to
butadiene process. However, how exactly the Si-O-Mg linkage
affects the catalytic performance remains an open question for
further investigations.
4. Conclusions
In the direct conversion of ethanol to butadiene over binary
MgO-SiO2 composite catalysts, the activity in terms of ethanol
conversion and butadiene selectivity was found to depend
significantly on the preparation methods and catalyst
compositions. The catalyst materials prepared by wet-
kneading proved to be more active and selective on ethanol
conversion to butadiene, which points at the importance of
the hierarchical flower-like structured MgO interacting with
the as-prepared SiO2 spheres. The sol-gel method containing
the high surface acidity led to the large amounts of by-
products (ethylene etc.) formed by ethanol dehydration. At
the optimal reaction conditions, the best-performing wet-
kneaded MgO:SiO2 = 65:35 (molar ratio) at 450 °C catalyst
exhibited ethanol conversion of 95% and BD selectivity of 77%
at a high WHSV of 4.1 h-1 and an unprecedented productivity
-1
of 0.025 mol gcat h-1 compared to commercial MgO, the
hierarchical structured MgO was synthesized by using an
ethylene-glycol-mediated self-assembly process, which
provided a large surface area and better access to the catalytic
sites. Extensive characterization shows the greater ethanol
conversion and butadiene selectivity were obtained by
combining proper balance of surface total basicity/total acidity
(total basicity/total acidity=0.24~0.3) with a small amount of
strong acid sites (strong acidity/total acidity=0.46~0.5), which
are critical in catalyzing the dehydrogenation of ethanol,
suppressing the dehydration and the aldol condensation,
respectively, and consequently enhancing the butadiene
selectivity. Together with the results presented above, the
crucial factor to butadiene formation requires a subtle balance
in acid-base sites in relation to the strong interaction between
MgO and SiO2 leading to the formation of interfacial Si-O-Mg
Fig. 12 Contour plot for variations between a pair of acidic and
basic variables on predicted optimization of relative amount of
total basicity/ total acidity and strong acidity/total acidity for
butadiene (a) and ethylene (b) yield.
According to XRD and physisorption, the best performing
catalyst consisting of MgO:SiO2 ratio of 65:35 has the smallest
particle size and largest surface area, but a general correlation
between activity and specific surface area was not established.
FT-IR spectroscopy provided valuable information on the
interaction of MgO and SiO2 on the as-prepared MgO-SiO2
composite catalysts. It is important to note the formation of Si-
O-Mg bond over samples prepared by wet-kneading and sol-
gel method, and the absence of such linkage over the catalysts
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Journal Name
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Acknowledgements
This work is supported by the Program for Professor of Special
Appointment (Eastern Scholar) at Shanghai Institutions of
Higher Learning, National Natural Science Foundation of China
(Grant 21503133, 21673137), Natural Science Foundation of
Shanghai City (15ZR1419100, 16ZR1413900), the Scientific
Research Foundation for the Returned Overseas Chinese
Scholars from State Education Ministry, Municipal Education of
Shanghai (ZZGCD15031) and Shanghai University of
Engineering Science Innovation Fund for Graduate Students
(E3-0903-16-01037-15KY0408).
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DOI: 10.1039/C6CY02091G
Highly Active and Selective Binary MgO-SiO2 Catalysts for the
Production of 1,3-Butadiene from Ethanol
Xiaoxiong Huang, Yong Men*, Jinguo Wang, Wei An, Yuanqiang Wang
Graphic abstract
Wet-kneaded binary MgO-SiO2 composite catalysts gave exceptionally high
productivity of butadiene from ethanol governed by a subtle balance in
acid-base sites in relation to the strong interaction between MgO and SiO2
leading to the formation of interfacial Si-O-Mg linkage and by varying
preparation methods and MgO to SiO2 ratio.