The influence of zinc loadings on the selectivity control of bio-ethanol transformation over MgO-SiO2 catalysts

Article abstract of DOI:10.1016/j.apcata.2020.117565

Catalytic transformation of ethanol has been studied over a series of MgO-SiO₂ composite catalysts with different ZnO loadings. Vast differences in product selectivity are obtained by varying ZnO loadings in enabling the control over product distribution. Our results reveal that MgO-SiO₂ composite catalysts with low ZnO loadings tend to show an enhanced efficiency for C[sbnd]C bond coupling and exceptionally high selectivity to 1,3-butadiene whereas the high ZnO loadings favor the formation of acetaldehyde via dehydrogenation. Thorough analysis of characterization results via XRD, BET, IR, TPD, and ²⁹Si MAS NMR indicates that ZnO loading influences the extent of MgO and SiO₂ interaction during preparation, and the surface acid-base chemistry, which were both found to correlate with the catalytic performance. This study proposes that the Mg[sbnd]O[sbnd]Si interfacial structure formed by the strong MgO and SiO₂ interaction at low ZnO loadings is of prime importance for the formation of 1,3-butadiene, benefiting from the desirable properties of balanced dehydrogenation and C[sbnd]C bond coupling while excess ZnO loadings destroy the Mg[sbnd]O[sbnd]Si interfacial bonds over the MgO-SiO₂ composite catalysts, which are the key structures required for C[sbnd]C bond growth.

Full text of DOI:10.1016/j.apcata.2020.117565

AppliedCatalysisA,General598(2020)117565
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Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
The influence of zinc loadings on the selectivity control of bio-ethanol
transformation over MgO-SiO2 catalysts
Xuefei Wanga, Yong Mena,*, Jinguo Wanga, Shuang Liua, Qiaoling Songa, Mei Yangb,*
a College of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Shanghai, 201620, PR China
b Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Ethanol
1,3-Butadiene
Acetaldehyde
MgO-SiO2 catalyst
ZnO loading
Catalytic transformation of ethanol has been studied over a series of MgO-SiO2 composite catalysts with different
ZnO loadings. Vast differences in product selectivity are obtained by varying ZnO loadings in enabling the
control over product distribution. Our results reveal that MgO-SiO2 composite catalysts with low ZnO loadings
tend to show an enhanced efficiency for CeC bond coupling and exceptionally high selectivity to 1,3-butadiene
whereas the high ZnO loadings favor the formation of acetaldehyde via dehydrogenation. Thorough analysis of
characterization results via XRD, BET, IR, TPD, and 29Si MAS NMR indicates that ZnO loading influences the
extent of MgO and SiO2 interaction during preparation, and the surface acid-base chemistry, which were both
found to correlate with the catalytic performance. This study proposes that the MgeOeSi interfacial structure
formed by the strong MgO and SiO2 interaction at low ZnO loadings is of prime importance for the formation of
1,3-butadiene, benefiting from the desirable properties of balanced dehydrogenation and CeC bond coupling
while excess ZnO loadings destroy the MgeOeSi interfacial bonds over the MgO-SiO2 composite catalysts, which
are the key structures required for CeC bond growth.
MgeOeSi interfacial bonds
1. Introduction
butanol, acetaldehyde, diethyl ether, ethyl acetate and ethane [22].
González et al. [23] conducted the experiments where relevant inter-
Bio-ethanol produced from biomass has emerged as a promising raw
material for production of a number of valuable chemical products
[1–4], such as acetaldehyde [5], ethylene [6], ethyl acetate [7], and
1,3-butadiene(1,3-BD) [8], which are mainly obtained as by-products of
the naphtha cracking process. Bio-ethanol has been extensively studied
as a platform molecule for the production of ethylene by acid-catalyzed
ethanol dehydration over various catalysts such as alumina [9], zeolites
[10] and heteropoly acids [11]. Supported metal catalysts can also
selectively transform ethanol into acetaldehyde [12], acetic acid [13]
and ethyl acetate [14] depending on the active metals and supports as
well as the reaction conditions. Moreover, supported metal catalysts, in
particular Ce and Ni with the good ability for deep dehydrogenation
and CeC bond scission, have been demonstrated to be excellent cata-
lysts for hydrogen production through ethanol reforming [15–18].
Transformation of bio-ethanol to acetaldehyde and its derivatives
C4-olefins has been recognized as one of the most important strategic
reactions for the production of high value-added chemicals [19–21].
Direct conversion of bio-ethanol into 1,3-BD (Lebedev process, ETB) is
quite complex due to the nature of cascade reaction involving many
unwanted competitive reactions that produce many by-products such as
mediate reaction products were individually fed or co-fed in different
ratios with ethanol, and showed that acetaldehyde is a stable inter-
mediate with an important role in 1,3-BD production. MgO-SiO2 mixed
metal oxide is the classic bi-functional catalyst effective for Lebedev
reaction. It is generally accepted that the appropriate balance of the
surface acidic and basic sites of the catalysts dictate the reaction
pathway and the selectivity of the final products [24]. For example,
previous studies revealed that MgO-SiO2 composites prepared by wet-
kneading method exhibited the superior catalytic performance [25]. It
was illustrated that the optimum of surface acidity/basicity can be
obtained to achieve a high 1,3-BD yield by variation of MgO:SiO2 ratios
in wet-kneaded samples [26]. Our recent work highlighted the impact
of morphology control on the enhancement of 1,3-BD yield [27]. The
unique layered flower-like architectures may facilitate the formation of
interfacial Mg-O-Si chemical bond and highly enhanced surface basicity
in binary MgO-SiO2 composite catalysts which are mainly responsible
for the superior activity.
It is also a common method that the performance of these catalysts
can be greatly enhanced by the addition of a third component, typically
a transition metal or metal oxide [28,29]. The choice of the third
⁎ Corresponding authors.
E-mail addresses: men@sues.edu.cn (Y. Men), yangmei@dicp.ac.cn (M. Yang).
https://doi.org/10.1016/j.apcata.2020.117565
Received 14 January 2020; Received in revised form 10 April 2020; Accepted 13 April 2020
Availableonline18April2020
0926-860X/©2020ElsevierB.V.Allrightsreserved.

X. Wang, et al.
AppliedCatalysisA,General598(2020)117565
component used as a promoter is determined by its dehydrogenation
capability. Cu and Ag have been used as metallic promoters to increase
the yield of 1,3-BD. Angelici et al. [30] used 1 wt.% of copper loading in
Mg:Si = 1:1(mole ratio) catalyst to get 40 % ethanol conversion and 53
% 1,3-BD selectivity. Dagle et al. [31] showed that a 1 % Ag/4 % ZrO2/
SiO2-SBA-16 catalyst leading to 99 % conversion and 71 % 1,3-BD se-
lectivity. In other studies, metal oxides such as ZnO and Na2O were also
applied to increase the catalytic activity and selectivity for 1,3-BD.
Baerdemaeker et al. [32] reported that due to the suppression effect of
ethanol dehydration by Zn2+, the combination of Zn and Hf in bime-
tallic Zn and Hf in silica-supported catalyst resulted in a stable, active,
and selective catalyst for butadiene production from ethanol, while
Wang et al. [33] found that 2000 ppm Na doped Zn1Zr10Oz catalyst
gave 47 % selectivity to 1,3-BD at 97 % ethanol conversion. Despite the
considerable progress achieved in enhancement of 1,3-BD yield due to
metal or metal oxide promoters, the principle for the controllable
production of acetaldehyde or desirable 1,3-BD during Lebedev process
still remained largely unexplored. Our recent study represented the first
report that uncovers the principles for tailoring the selectivity of acet-
aldehyde or 1,3-BD by tuning the size of nano-gold Au/ZnZr10Ox cat-
alyst [34].
2.3. Catalyst testing
The conversions of ethanol to 1,3-BD were conducted in a fixed bed
quartz tube reactor at atmospheric pressure. In a typical experiment,
100 mg catalyst was loaded in the middle of 5-mm internal diameter
quartz beds. A K-type thermocouple was placed in the middle of the
catalyst bed to monitor the reaction temperature. The carrier gas
(20 mL/min) carried ethanol to pass over the catalyst to on-line gas
chromatography through an evaporator kept at constant temperature
(20 °C). The reaction temperature was within the range of 300−500 °C.
The effluent gas products were heated above 200 °C to avoid the con-
densation of condensable species and quantified by an online Shimadzu
2014 Gas Chromatography (GC). The product was analyzed with mo-
lecular sieves C13X, Al2O3 column (50 m, 0.53 mm ID, 10 μm), Rt-Q-
BOND PLOT column (30 m, 0.32 mm ID, 10 μm) by one TCD and two
FID detectors. The catalysts were firstly pre-treated in N2/ethanol
(20 mL/min) at 300 °C for 0.5 h before reaction. Nitrogen was used as
the internal standard for calibration and calculation of GC results. The
kinetic study was carried out by the Weisz-Prater criterion to discard
any mass transfer limitation [40–43].
Zinc is a special transition metal, as it was reported to improve
availability of Lewis base sites [35,36], and has a good track record on
ethanol dehydrogenation [37]. In this work, a series of MgO-SiO2
(65:35) catalysts with different ZnO loadings were prepared by a simple
impregnation method and evaluated for selective ethanol transforma-
tion. We showed that how selectivity of this reaction can be tailored
towards specific cascade formation of either acetaldehyde or 1,3-BD as
major products by controlling amounts of ZnO addition into MgO-SiO2
(65:35) substrate. The purpose of this work is not only to investigate the
effect of different ZnO loadings on the selectivity control of desirable
products on Zn-promoted MgO-SiO2 catalyst, but also to improve the
fundamental understandings about the interfacial effect of mixed metal
oxide catalysts on ethanol conversion.
2.4. Catalyst characterization
The crystalline phases of the samples were identified by X-ray
powder diffractions (XRD) using a diffraction meter (D/Max-rB) with
Cu-Kα radiation (λ = 1.54056 Å) at room temperature. The scanning
rate was 4°/min, the 2θ was from 10° to 80°.
The specific surface area of the material was determined by ap-
plying Brunauer- Emmett-Teller (BET) model on
a Micromeritics
ASAP2460 apparatus. The samples were pretreated for 6 h in nitrogen
at 200 °C before analysis.
The Fourier transform infrared spectroscopy (FT-IR) signal of the
catalyst was recorded on a Nicolet 380 infrared spectrometer (USA).
The sample was pressed into a thin self-supported wafer in KBr and
placed in the IR cell. The FT-IR spectra of the catalysts were recorded at
room temperature against an air background. FT-IR spectral signals in
2. Experimental section
the wave number range of 4000−400 cm−1
.
2.1. Materials
The acidity and basicity of the catalyst were investigated by CO2-
temperature-programmed desorption (CO2-TPD) and NH3-temperature-
programmed desorption (NH3-TPD) on a Micromeritics Autochem II
2920 apparatus. 0.05 g of catalyst was installed in a U-shaped fixed-bed
quartz micro reactor. After pretreatment under argon at 500 °C for 1 h,
the catalyst was cooled to 100 °C, then a mixture gas 5 % CO2(NH3)/He
(30 mL/min) were applied, followed by flowing in He (30 mL/min)
until the base line was stabilized, and heating up to 900 °C at 10 °C/min
to induce desorption of CO2(NH3).
Magnesium hydroxide, tetraethyl orthosilicate (TEOS) and ammo-
nium hydroxide were purchased from Aladdin Chemistry Co. Ltd.
(Shanghai, China), ethanol and zinc nitrate hexahydrate were pur-
chased from Sinopharm group chemical reagent co. LTD (Shanghai,
China). All chemicals are used directly without further purification.
2.2. Catalyst preparation
The temperature-programmed desorption of ethanol (ethanol-TPD)
was used to study the ethanol to 1,3-BD reaction process. 50 mg catalyst
in a U-shaped fixed-bed quartz microreactor treated at 500 °C (ramping
rate 10 °C/min) for 0.5 h under flowing 3 % O2/He (20 mL/min). Then
the sample was cooled to 50 °C, switching to He gas (20 mL/min) for
30 min remove water and impurities from the surface of the material.
Ethanol was bubbled in a U-shaped fixed-bed quartz micro reactor until
the saturated adsorption of ethanol. The sample was purged by flowing
30 mL/min He for 40 min till stabilization of the baseline. The tem-
perature was then raised to 900 °C (10 °C/min) in a He (30 mL/min) gas
atmosphere, and the exhaust gas was connected to the HPR20 online
mass spectrometer to determine composition at the same time during
the heating process. To analyze the composition, different mass-to-
charge ratios (m/e) in the mass spectrometry have been set to follow
the law of mass cracking: hydrogen = 2, acetaldehyde = 44, 1,3-buta-
diene = 54, ethylene = 27.
SiO2 was prepared by the procedure reported by Stöber et al. [38]:
TEOS was hydrolyzed using an ethanol-water ammonia solution
(25:15:3.14 vol./vol./vol.) in a closed vessel by stirring at room tem-
perature for 4 h, followed by centrifugation and thorough washing with
ethanol. Then the solid products were dried at 100 °C for 12 h, and
calcined at 500 °C for 5 h in air.
MgO-SiO2 (MgSi) catalysts were prepared with a 65:35 M ratio
(based on the results of our recent study) by the procedure reported by
Kvisle et al. [39]: Mg(OH)2 and SiO2 were mixed at 50 °C for 5 h in
water. Then the suspension was stirred constantly and heated at 100 °C
to remove excess H2O. Finally, the dried solid catalyst was calcined at
500 °C for 5 h in air.
The ZnO promoted MgO-SiO2 catalysts were prepared by incipient
wetness impregnation: the solution of Zn(NO3)2·6H2O (Solute mass was
determined by the load) was added to the support material and left to
equilibrate for 12 h, then dried at 100 °C for 12 h. Finally, the dried
solid was calcined at 500 °C for 5 h in air. The above methods were used
to prepare different MgSi catalysts loaded with ZnO of 0.1 wt.%, 0.2 wt.
%, 0.4 wt.%, 0.8 wt.%, 1 wt.%, 5 wt.%, as denoted as ZnMgSi-X.
In order to further study the acid-base properties of the catalysts,
the samples were characterized by isopropanol temperature-pro-
grammed desorption (IPA-TPD). A quartz U-tube reactor was loaded
with 100 mg of sample. The samples were pretreated in the presence of
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X. Wang, et al.
AppliedCatalysisA,General598(2020)117565
He with flow of 30 mL/min at 300 °C for 1 h. Subsequently, the sample
was cooled to 50 °C and pulse ethanol adsorption was conducted. After
saturated adsorption of isopropanol, the sample was then heated to
500 °C with a ramp of 10 °C/min, and the exhaust gas was analyzed by
the HPR20 online mass spectrometer. Mass spectrometry can distin-
guish different products by collecting different charge ratios. The
charge/mass ratios are set as follows: isopropanol (C3H8O) = 45,
acetone (C3H6O) = 43, propylene (C3H6) = 41.
A Bruker Avance400 spectrometer was used to collect Magic-angle
spinning nuclear magnetic resonance (MAS NMR) spectra. The re-
sonance frequency of 29Si is 79.5 MHz in the field of 29Si MAS NMR
spectrum. 17,000 scans were accumulated with a recycle delay of 10 s.
The spinning frequency of the rotor is 5 kHz. The catalysts were packed
in 4 mm zirconia rotors. Tetramethyl silane was used as chemical shift
reference.
Table 1
Textural properties of ZnMgSi-X mixed oxides determined by N2 adsorption and
XRD.
Samples
SBET (m2/g)
Vp (cm3/g)
Dp (nm)
Crystallite size(nm)
MgSi
87
75
48
34
31
0.53
0.33
0.32
0.24
0.24
17.9
21.1
27.8
28.6
29.9
12.4
12.3
11.4
11.0
10.5
ZnMgSi-0.1
ZnMgSi-0.2
ZnMgSi-0.8
ZnMgSi-5
(100), (002), (101) and (110) facets, respectively, in the bulk phase
ZnO (JCPDS Card No. 36-1451). No diffraction peaks of ZnO species
could be observed over the samples with ZnO loading lower than 5 wt.
% owing to the low Zn loading or its presence in form of highly dis-
persed state on the catalyst surface. Meanwhile, the broad peaks be-
tween 20°–40° found in all samples were attributed to the presence of
amorphous silica. No other phases were identified by XRD patterns, in
accordance with Angelici et al. [30]. Similar results have also been
observed by Larina [45] and Taifan [37].
Table 1 collected the physical structure parameters of the catalysts
obtained from XRD patterns and nitrogen adsorption isotherms (Fig.
S2). As illustrated in Fig. S2, all the samples displayed the typical type
IV isotherms with H3 hysteresis loops in the relative pressure (P/P0)
range from 0.4 to 1.0, which is associated to narrow slit pores. The
corresponding pore size distribution curves in Fig. S2 indicated that the
micropore structure of MgSi sample. The loading of ZnO into MgSi
matrix eliminates considerably the micropore due to the interaction of
Zn species and MgO within the micropores MgSi and creates meso-
porous structure. As can also be seen in Table 1, BET surface areas and
pore volumes of composite oxide catalysts varied significantly in the
range of 87–31 m2/g and 0.53–0.24 cm3/g, respectively, indicating a
strong influence of the ZnO loadings on the textual properties. It was
found that the specific surface area of all the catalyst decreased with the
increasing ZnO loadings along with the decreased crystal size of MgO.
Loading ZnO into MgSi leads to the loss of crystallization and the mi-
croporous structure due to the decreased crystal sizes, thus decreasing
the surface area. The incorporation of Zn into the MgO lattice could be
responsible for the collapse of micropore structure leading to a decrease
in surface area. Meanwhile, the pore volumes of all ZnMgSi catalyst
decreased with the increase of ZnO loadings, probably due to the en-
hanced pore blockage caused by excessive ZnO loading.
In this study, the following formula was used to calculate the con-
version rate of ethanol and the selectivity of the products, and the
carbon balance of all catalysts is greater than 95 %:
ethanol in − ethanolout
Conversion =
Selectivity =
× 100%
ethanolin
(1)
xi ni
× 100%
∑ xz nz
(2)
(3)
z
Yield = conversion × selectivity × 100%
xi is the mole fraction of products (i), nz is the number of carbon atoms
in carbon-containing products (z).
3. Results and discussion
3.1. Catalyst characterization
Aiming to detect the interaction between ZnO and MgSi catalysts,
the XRD patterns of MgSi catalysts with varying loadings of ZnO were
recorded. (Fig. 1). The diffraction peaks at 2θ of 36.9°, 42.9°, 62.3°,
74.7° and 78.6° was detected, corresponding to diffraction planes of the
periclase phase of cubic MgO (111), (200), (220), (311) and (222) [44].
With the addition of ZnO, the XRD intensity of MgO decreased and
broadening was observed, indicating a reduction of crystallite size and
crystallinity. The diffraction peak of MgO at 43.5° shifts toward lower
value indicated that the Zn-promoted catalysts may form a solid solu-
tion during the preparation due to Zn2+ incorporation into the Mg2+
lattice [37]. The ZnMgSi-10 catalysts displayed well-resolved diffrac-
tion peaks around 2θ of 31.7°, 34.4°, 36°, 56° (Fig. S1), indicative of the
Fig. 2 shows FT-IR spectra for the various samples in the region of
4000−400 cm−1. The FT-IR spectra of MgSi catalysts were similar to
that reported in the literatures [30]. The ZnMgSi catalysts show similar
Fig. 1. XRD patterns of ZnMgSi-X catalysts.
Fig. 2. FT-IR spectra of the ZnMgSi-X catalysts.
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X. Wang, et al.
AppliedCatalysisA,General598(2020)117565
Fig. 4. Product distribution of ethanol conversion over ZnMgSi-X catalysts.
Fig. 3. Temperature-dependent reactivity of the ZnMgSi-0.2 catalyst.
of ZnO, demonstrating pronounced effects of varying ZnO loadings on
the selectivity control of ethanol transformation in this catalyst system.
Interestingly, addition of ZnO to MgSi catalyst suppressed the formation
of undesired byproduct ethylene, suggesting that Zn sites are re-
sponsible for the inhibition of ethylene formation via dehydration.
When MgSi is doped with 0.1 wt.% ZnO, significantly enhanced 1,3-BD
selectivity is observed along with the enhanced ethanol conversion rate.
An increase in the ZnO loading to 0.2 wt.% resulted in the highest
catalytic activity for C4 olefin, i.e. the 80 % ethanol conversion and 58
% 1,3-BD selectivity with the lowest acetaldehyde of 16 %. Notably,
1,3-BD selectivity started to decrease with further increase of ZnO
loading while acetaldehyde selectivity increased continuously, in-
dicating that the excess ZnO loadings disable the catalytic functionality
of the key structure required for CeC bond growth.
The stability of ZnMgSi-0.2 catalyst was studied, and the results are
shown in Fig. 5. The conversion rate of ethanol decreases gradually
within 50 h from 80 % to 55 % and 1,3-BD selectivity declined slowly
from 58 % to 55 %, while the selectivity of acetaldehyde and ethylene
retained almost unchanged. Based on literatures [33,37,46] and our
thermogravimetric results performed over spent sample (Fig. S3), the
deactivation of ZnMgSi catalyst may be mainly associated with the
surface basic sites for secondary acetaldehyde to 1,3-butadiene.
To shed further light on the origin of Zn-loading dependence on the
spectral features to these in the MgSi catalysts, illustrating two obvious
sharp absorption bands at 1110 cm−1 and 3450 cm−1, respectively,
while the former is the stretching vibrations of the hydroxyl groups of
water physically adsorbed on the surface and the latter can be assigned
to the asymmetric SieOeSi stretching mode. Moreover, depending on
the ZnO loadings, some obvious changes have been noted on the ab-
sorption feature located at 470 cm−1 belonging to the stretching vi-
bration of MgeOeSi bonds [25]. In particular, this feature weakened
due to the introduction of ZnO and almost vanished at the high ZnO
loading of 5 wt.%. In our previous work, MgeOeSi bonds have been
proposed to be the crucial active structure for the selective production
of 1,3-BD from ethanol [26]. The observed decrease in magnesium si-
licates catalyst by Taifan et al. [37] has been ascribed to broken
MgeOeSi linkages due to the interaction of Zn with MgeOeSi upon Zn
addition. Our result suggests that Zn interacts strongly with the OH
group and plays a key role in change of the Mg-O-Si structure.
3.2. Catalytic activity
The effect of reaction temperature on ETB reactivity is investigated
over ZnMgSi-X catalysts. Fig. 3 displays the catalytic performance of the
ZnMgSi-0.2 catalyst for ethanol to 1,3-BD at WHSV of 1.63 h−1, and
ethylene, acetaldehyde and 1,3-BD are detected as the main products in
the catalytic process. As the temperature increases, the ethanol con-
version rate increases gradually, while the product distribution is found
to be dependent on the reaction temperature. The selectivity of acet-
aldehyde decreased gradually, and that of 1,3-BD increased. The in-
crease of ethylene selectivity at the expense of decreasing acetaldehyde
selectivity observed in the investigated temperature range suggests that
ethanol dehydration and dehydrogenation are two parallel competitive
reactions. Moreover, volcano-like 1,3-BD selectivity along with the
continuously decreased acetaldehyde selectivity indicates that the
acetaldehyde formed via ethanol dehydrogenation is the primary pro-
duct of ethanol transformation, whereas 1,3-BD is a secondary product
in the cascade reaction sequences.
In order to understand the impact of catalyst composition on the
activity, the effect of Zn addition was investigated over the ternary
ZnMgSi catalysts with various Zn loadings. Fig. 4 compares the product
distribution for ethanol conversion over catalysts with different ZnO
loadings at 450 °C. Ethylene, acetaldehyde and 1,3-BD are detected as
the main products in the catalytic process over MgSi substrate. More
details about the selectivity of all detectable products over the catalysts
can be found in Table S1. On one hand, the rate of ethanol conversion is
found to increase with increasing ZnO loading. On the other hand, the
product selectivity of the reaction was highly dependent on the loading
Fig. 5. Stability test of the ZnMgSi-0.2 catalyst. Conditions: 0.1 g of catalyst,
ethanol (gas phase) molar fraction 5.79 vol.%, N2 flow rate = 20 mL/min,
T = 450 °C, respectively.
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AppliedCatalysisA,General598(2020)117565
Fig. 6. Arrhenius-type plots of production rate over the MgO-SiO2 catalysts with different ZnO loadings (a, acetaldehyde b, 1,3-butadiene c, ethylene d, ethanol).
selectivity control of ethanol transformation, the respective activation
energy of the key products of acetaldehyde and 1,3-BD were obtained
by performing the reaction under the kinetically controlled regime. As
shown in Fig. 6, our kinetic study indicates that the difference of the
activation energy is the key selectivity descriptor, in consistent with
previous studies [34,47]. Over the catalyst without Zn, the activation
energies of acetaldehyde and 1,3-BD were determined to be 37 and
108 kJ/mol, respectively. Over the catalysts with low ZnO loading of
0.2 wt.%, the activation energy of acetaldehyde was slightly lower than
that without ZnO, however, the activation energy of 1,3-BD formation
became significantly lower than that on the binary catalyst. A further
increase of ZnO loading to 5 wt.% resulted in a dramatic increase of
activation energy for 1,3-BD formation along with a considerable de-
crease in activation energy for acetaldehyde generation, leading to
preference for the acetaldehyde formation. It was also observed in the
Arrhenius-type plots of ethylene in Fig. 6c that a continuous increase of
ZnO loading to 5 wt.% resulted in an increase of activation energy for
ethylene formation along with a considerable decrease in activation
energy for acetaldehyde generation, meanwhile the activation energy of
ethanol conversion rate (Fig. 6d) decreased significantly with the in-
crease of ZnO loadings, leading to a highly enhanced reaction rate of
ethanol conversion. As a consequence, the product selectivity can be
controlled by tuning the ZnO loadings over MgSi.
factor affecting the catalytic activity [24]. In our previous works
[21,26,48], the proper balance of acid-base sites has been thought to be
critical to determine the reaction pathway of ethanol conversion and
the activity of the catalyst. The acid-base distribution and the number
of acid-base sites can be quantitatively analyzed by NH3- and CO2-TPD
over a temperature range of 100–500 ℃ (Fig. 7). All results desorption
results are shown in the Table 2. The binary MgSi catalyst displayed one
broad desorption peak of NH3 centered at 210 °C, assignable to mod-
erate acidic sites. With the addition of ZnO, this desorption peak de-
creased and shifted toward lower temperature, indicating the appear-
ance of new weak acid sites. The fewer acid sites derived from the
reduced peak strength can be related to the generation of byproduct
ethylene because ethylene formation are more favorable over the acid
site of the catalyst [49]. A further increase of ZnO loading to 5 wt.%
resulted in a dramatic decrease of weak acidic site with a considerably
lowered moderate acidic sites. Obviously, the addition of Zn sig-
nificantly can passivate the acidic site of the catalyst surface. By the
contrary, there is no new desorption peak in CO2-TPD absorption with
the ZnO addition, and only one weak base desorption peak and one
moderate base absorption peak. The acid-base ratio presents an inter-
esting parameter. Apparently, the addition of zinc oxide increased
considerably the available surface basic sites and thus decreased the
relative ratio of acid-base sites, suggesting the increased surface basi-
city. The changes on the surface acid-base properties can be further
correlated with the variation of products selectivity. Previous study
[50] also reported the similar observation, suggesting that acidic sites
are important factors in the conversion of ethanol to ethylene, while the
3.3. Structure-activity relationship of catalysts
The inherent acid-base function of the catalysts is an important
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AppliedCatalysisA,General598(2020)117565
catalysts depended on the presence of Zn on the catalyst surface. Two
distinct H2 peaks were observed over MgSi catalyst, and centered at low
temperature of 150 °C and high temperature of 350 °C, respectively.
These two H2 peaks were attributed to the dehydrogenation of adsorbed
ethanol on the catalyst surface, as evidenced by the CH3CHO desorption
peaks in the same temperature range. Evidently, the H2 and CH3CHO
desorption peaks at high temperatures were shifted toward lower
temperatures on the samples with ZnO, suggesting that the increase of
ZnO loadings facilitated the hydrogen production via ethanol dehy-
drogenation, as shown in Fig. 8a and b. It was also observed in Fig. 8c
that ethanol dehydration occurred competitively with its dehy-
drogenation on MgSi catalyst surface. The relative ratio of CH3CHO,
C2H4, 1,3-BD in the total desorbed carbon species are calculated by
deconvoluting the peaks in Fig. 8 and presented in Table S3. As shown
in Table S3, the relative ratio of CH3CHO, C2H4 and 1,3-BD were largely
dependent on the loading amounts of Zn in MgSi catalysts. The addition
of Zn effectively passivated the surface acidic sites and resulted in
dramatically decreased ethylene selectivity via ethanol dehydration.
Moreover, small amounts of ZnO loading favored a highly enhanced
1,3-BD yield while high loadings of Zn inhibited the aldol condensation
step and resulted in poor 1,3-BD yield (Fig. 8d). It is concluded that the
variations in the Zn loadings amounts on the catalyst surface may have
great impact on the surface adsorbed/desorbed species and the control
over their stabilized gaseous products [52]. The results obtained by
ethanol-TPD can be well correlated with the reactivity data in this
study.
Adsorption and desorption of isopropanol has long been considered
as
a chemical probe reaction for surface acid–base properties.
Therefore, the temperature-programmed desorption (TPD) was carried
out to probe the modification of surface acidity and basicity by ZnO
addition (Fig. 9). Isopropanol tends to dehydrate to propylene over
surface acidic site, and isopropanol tends to dehydrogenate to form
acetone on surface basic site [53]. Therefore, the alteration of surface
acid-basic strength on the catalyst surface can be determined by relative
ratio of desorbed propylene and acetone. As shown in Table 2, the re-
lative ratio of MgSi, ZnMgSi-0.2, and ZnMgSi-5 catalysts were de-
termined to be 2.8, 4.2 and 4.4, respectively, showing a up tendency
trend. With the small amount of ZnO addition, the catalytic dehydra-
tion activity of ZnMgSi-0.2 is much reduced with respect to MgSi
sample while the catalytic dehydrogenation activity is markedly fa-
cilitated with the increasing surface basicity, as evidenced by the che-
misorption data. A further increase of ZnO loading led to highly en-
hanced dehydrogenation reactivity, accompanying with the increased
surface basicity. The IPA-TPD study confirms that the surface acidity of
the MgSi sample is considerably passivated by the ZnO addition, re-
sulting the amount of produced propylene is significantly decreased.
The results of IPA-TPD can be well correlated with the results of NH3
and CO2-TPD.
In order to better understand the effect of ZnO addition on inter-
facial MgeOeSi chemical bonds and catalytic activity, the 29Si MAS
NMR was used to explore the chemical environment around Si. The 29Si
MAS NMR spectra of ZnMgSi catalysts with different proportions are
shown in Fig. 10. It is known in 29Si MAS NMR spectra that the strength
of 29Si MAS NMR signal depends on the density of 1H around the 29Si
core and the distance between 1H-29Si, and therefore, a higher 29Si MAS
NMR signal strength predicts shorter distances and greater density of 1H
around the 29Si. Obviously, some strong signals appear at −113 ppm,
−103 ppm, −92 ppm, −80 ppm, and −75 ppm. The three catalysts
with different components generally showed the subtle but the clear
differences within the chemical shift from −75 ppm to −100 ppm. It
was noted that pure MgSi catalysts showed only one peak of −75 ppm.
With the addition of ZnO, the intensity of this broad peak started to
decrease at −75 ppm, meanwhile new broads peaks emerged at
−80 ppm and at -92 ppm, representing a minor ringwoodite component
and siloxane groups [54,55]. According to previous literature reports
[25,56], three broad signals at −113 ppm, −103 ppm, −92 ppm are
Fig. 7. NH3-TPD (a) and CO2-TPD (b) profiles for the ZnMgSi-X catalysts.
Table 2
Amount of acidic and basic sites for the different ZnMgSi-X catalysts.
Samples
Number of acidic
sites (Ta)a (mmol/
cm2)
Number of basic
sites (Tb)b (mmol/
cm2)
Tb/Ta Acetone/
Propylenec
MgSi
40.5
68.8
78.6
97.7
103.7
112.3
1.7
2.0
4.8
5.3
9.8
2.8
–
4.2
–
ZnMgSi-0.1 39.9
ZnMgSi-0.2 20.5
ZnMgSi-0.8 19.7
ZnMgSi-5
11.5
4.4
a
Number of acid sites is quantified by NH3-TPD.
Number of basic sites is quantified by CO2-TPD.
Number of acetone/propylene sites is quantified by IPA-TPD.
b
c
basic sites provide a driving force for the dehydrogenation of ethanol to
acetaldehyde, and the eventual 1,3-BD production from ethanol can
only be achieved with an appropriate ratio of acid-base sites.
Ethanol-TPD experiments were carried out over a chemisorption
instrument coupled with mass spectrometry [17,51] to explored the
interaction of ethanol molecule with the surface of MgSi, ZnMgSi-0.2,
and ZnMgSi-5. As shown in the Fig. 8, the main desorption species were
identified to be H2, CH3CHO, C2H4 and C4H6 (1,3-BD). Analysis of
desorbed products in gas phase showed differences between the
6

X. Wang, et al.
AppliedCatalysisA,General598(2020)117565
Fig. 8. Ethanol-TPD profiles over ZnMgSi-X catalysts. (m/e: a, hydrogen = 2 b, acetaldehyde = 44 c, ethylene = 27 d, 1,3-BD = 54).
attributed to siloxane groups ((SiO)4eSi), simple silanol
((SiO)3eSieOH), and geminal silanol ((SiO)2eSie(OH)2), indicating
the presence of unreacted SiO2. The broad minor ringwoodite compo-
nent showed up at −80 ppm. −75 ppm is attributed to Si*(OM-
g)2(OSi)2, which are the characteristic signs of MgeOeSi interfacial
bonds. Combining previous FT-IR spectra, we may have a reasonable
conclusion. With the small amount of Zn addition, Zn is atomically
dispersed in replacement of Mg in MgeOeSi interfacial structures
(Si*(OMg)2(OSi)2), and the chemical environment near Si in particular
the surface basicity has been altered by the intervention of zinc.
Nevertheless, small amounts of zinc do not lead to the great destruction
of the Si*(OMg)2(OSi)2 interfacial bonds, and the catalyst still retains
the key interfacial structures required for CeC bond growth. After
loading 5 wt.% ZnO onto MgSi catalyst, the excess ZnO loadings destroy
the Si*(OMg)2(OSi)2 interfacial bonds over the MgSi composite cata-
lysts, which are the key structures required for CeC bond growth.
MgeO bonds break off from their original structure and some of MgeO
bonds recombined with Si atoms to form ringwoodite component.
Furthermore, (SiO)2eSie (OH)2 is a combination of hydroxyl radical
and Sie(OSi)2 which is Si*(OMg)2(OSi)2 bonds after MgeO bond have
separated. The results of 29Si MAS NMR can be well correlated with the
results of FT-IR spectra.
of Zn offered more basic sites, making the surface reaction on the cat-
alyst more favorable for acetaldehyde via ethanol dehydrogenation,
which is a key intermediate for 1,3-BD formation. Our results are in
good agreement with the recent theoretical DFT prediction made by An
[57] and experimental work by Resasco [58] who showed that strong
basic sites are beneficial to CeC bond growth via aldol-condensation.
The product selectivity toward either acetaldehyde or 1,3-BD formation
can be related to the MgeOeSi interfacial structure. It was noted that
MgeOeSi interfacial structures remained almost intact with a small
amount of Zn addition, while the destruction of MgeOeSi interfacial
bonds by 5 wt.% Zn addition is very obvious from the FT-IR and 29Si
MAS NMR spectra. The excess ZnO loadings destroy the MgeOeSi in-
terfacial bonds over the MgSi composite catalysts, which are the key
structures required for CeC bond growth, leading to stabilized forma-
tion of acetaldehyde with selectivity of 78 % and the dramatically
lowered 1,3-BD yield in the product effluents.
4. Conclusions
In this work, a series of catalysts with different zinc loadings were
prepared by a simple impregnation method and evaluated for ethanol
transformation. It was found that MgSi with a low loading of Zn
(< 0.2 wt.%) exhibited an enhanced 1,3-BD selectivity. A further in-
crease of Zn loading led to the considerably enhanced acetaldehyde
selectivity. Multiple characterizations have shown that a highly dis-
persed zinc oxide species were predominant in the MgSi samples of low
Correlating with the results of TPD, the variation of surface acid-
base ratios and acetone/propylene ratios reveal that the addition of Zn
passivated the acidic sites and suppressed significantly the surface
acidity on the catalyst surface. Moreover, the addition of small amounts
7

X. Wang, et al.
AppliedCatalysisA,General598(2020)117565
Fig. 10. 29Si MAS NMR patterns of the ZnMgSi-X catalysts.
Zn loading amounts, favoring
a highly enhanced ethanol dehy-
drogenation capability and 1,3-BD yield due to the stronger surface
basicity in MgeOeSi interfacial structures, while the bulk ZnO species
and the destruction of MgeOeSi interfacial structures in the MgSi
samples of higher Zn loading effectively suppressed the CeC bond
growth and facilitated acetaldehyde production via ethanol dehy-
drogenation. The MgeOeSi interfacial structures were shown to be the
key structure-selectivity controller over product distribution.
CRediT authorship contribution statement
Xuefei Wang: Validation, Formal analysis, Investigation, Writing -
original draft. Yong Men: Conceptualization, Methodology,
Investigation, Writing - original draft. Jinguo Wang: Writing - review &
editing. Shuang Liu: Writing - review & editing. Qiaoling Song:
Validation. Mei Yang: Writing - original draft.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgments
This research was supported by the Talent Program of Shanghai
University of Engineering Science, Science and Technology Commission
of Shanghai Municipality (18030501100), National Natural Science
Foundation of China (21503133, 21776274), Shanghai Talent
Development Foundation (2017076) and the Program for Professor of
Special Appointment (Eastern Scholar) at Shanghai Institutions of
Higher Learning.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcata.2020.117565.
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