Structure-induced Lewis-base Ga4B2O9 and its superior performance in Knoevenagel condensation reaction

Article abstract of DOI:10.1016/j.mcat.2020.110914

Solid Lewis-base catalysis is important in the production of fine chemicals. A Lewis-base Ga₄B₂O₉ was synthesized by high temperature solid state reactions. It exhibited a high yield (90 %) and a high stability in Knoevena

Full text of DOI:10.1016/j.mcat.2020.110914

MolecularCatalysis490(2020)110914
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Structure-induced Lewis-base Ga4B2O9 and its superior performance in
Knoevenagel condensation reaction
Yao Yanga, Duo Wanga, Pengfei Jianga, Wenliang Gaoa,*, Rihong Conga,b, Tao Yanga,b,
*
a College of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 401331, People’s Republic of China
b Chongqing Key Laboratory of Theoretical and Computational Chemistry, Chongqing University, Chongqing, 401331, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Solid Lewis-base catalysis
Knoevenagel
Condensation reactions
Gallium borates
Structure-activity relationship
Solid Lewis-base catalysis is important in the production of fine chemicals. A Lewis-base Ga4B2O9 was synthe-
sized by high temperature solid state reactions. It exhibited a high yield (90 %) and a high stability in
Knoevenagel condensation reactions, where several aldehydes were combined with malononitrile to form α, β-
unsaturated compounds through nucleophilic addition reactions. Reaction kinetics analyses indicated
Knoevenagel condensation reactions over Ga4B2O9 catalyst obeyed a second-order characteristics and the cal-
culated activation energy was ∼61.6 kJ/mol, suggesting that Langmuir-Hinshelwood absorption pathway was
probably employed. The structural evolution from Ga4B2O9→GaBO3→β-Ga2O3 evidenced that the structure-
induced basicity of Ga4B2O9 attributed to the special μ3-O atoms linked exclusively to 3 five-coordinated Ga3+
.
This investigation proves a convincible structure-property correlation in Ga-based solid base materials and helps
developing an alternative avenue towards the design of new intrinsic solid base catalysts.
Introduction
localized structure in the framework, which makes Ga4B2O9 very stable
and durable without some additional post-modifications. Ga4B2O9 is a
Solid-base catalysts play a decisive role in green chemistry by pro-
viding an environmentally benign and sustainable route for the synth-
eses of fine chemicals and chemical intermediates [1–3]. In the late
1950s, the first solid base catalyst was reported in the isomerization of
olefins with both an excellent conversion and selectivity, where alu-
mina was treated with molten sodium in an inert atmosphere to provide
Lewis basic sites [4]. Since then, solid-base-catalyzed organic reactions
have been an interesting topic [5–18]. Typically, the traditional five-
step synthetic route of 4-methylthiazole was improved into two steps
with an environmentally operation by applying a cesium-loaded zeolite
catalyst [11]; a robust recyclable solid-base catalyst CsF·Al2O3 catalyzes
a 1,4-addition reaction from glycine derivatives to glutamic acid deri-
vatives in mild conditions [12]. Although a great effort has been made,
this field is less developed compared to solid acid catalysis. The func-
tionality of solid bases and catalytic mechanism, especially in the pre-
cise preparation of solid materials with tunable electronic and physical
structure, are always a challenge [19–23], thus the development of new
types of solid bases is highly desired.
mullite-type borate, and its crystal structure and the related properties
have been reported elsewhere [24–27]. As shown in Fig. 1, Ga4B2O9 has
four types of building blocks, such as GaO6, GaO5, BO4, BO3, among
which, GaO6 octahedra share edges to form one-dimensional chains
along [010] direction. These edge-sharing chains along b axis are fur-
ther cross-linked by three inter-chain groups, i.e. GaO5, BO3 and BO4
units, into a three-dimensional structure [24]. In fact, its structure
within ac plane is disordered, but ordered along b axis. A preliminary
study with DFT calculation suggests the μ3-O atoms linked with three 5-
coorindated Ga3+ act as the donor of electrons (see Fig. 1c) [25]. In
general, the traditional solid bases include the defect-induced solids
(alkaline metal treated oxides), Brönsted bases and metal-organic fra-
mework (MOF)-based solids. The major drawback is their low dur-
ability and thermal stability. Here Ga4B2O9 was prepared with high-
temperature solid state reactions, and its basic sites are structure-in-
duced, intrinsically originating from the μ3-O atoms, thus is supposed to
be a new type of Lewis base with an outstanding stability. In the current
stage, the confirmation of the basicity from such μ3-O atoms is urgent.
Knoevenagel condensation reaction is a general route to the for-
mation of C]C bond using active methylene compounds and alde-
hydes/ketones as reactants. It is also commonly used as a probe reac-
tion or a determined proof to evaluate basic capacity in the base-
Recently, a potential Lewis base Ga4B2O9 was reported to have some
interesting structure-induced basic sites. Different with some pre-
viously-reported solid-base catalysts, Ga4B2O9 is an intrinsic solid base,
and its base-sites are reasonably proved to generate from a special
⁎ Corresponding authors at: College of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 401331, People’s Republic of China.
E-mail addresses: gaowl@cqu.edu.cn (W. Gao), taoyang@cqu.edu.cn (T. Yang).
https://doi.org/10.1016/j.mcat.2020.110914
Received 28 January 2020; Received in revised form 19 March 2020; Accepted 21 March 2020
2468-8231/©2020ElsevierB.V.Allrightsreserved.

Y. Yang, et al.
MolecularCatalysis490(2020)110914
Fig. 1. (a) Structure illustration of Ga4B2O9 with unsaturated coordination along b-axis; (b) octahedral chains along a-axis; (c) μ3-O atoms linked to five-coordinated
Ga3+
.
catalyzed reactions, in which basic sites attract proton atoms from
adsorbed methylene molecules to form carbanion intermediate [28].
Furthermore, Knoevenagel condensation reaction is a key step to pro-
duce several therapeutic drugs such as niphendipine and nitrendipine,
as well as pharmacological products of calcium channel blockers and
antihypertensives [29]. Traditionally, Knoevenagel condensation reac-
tion was performed in a homogeneous liquid alkali system [30–32].
However, their applications are limited due to the environmental pro-
blems, including the post-treatment of waste liquid, equipment corro-
sion and product separation [33]. Given these restrictions, many het-
erogeneous catalysts have been attempted in Knoevenagel condensation
reactions recently, including amine-functionalized molecular sieves
[34,35], metal oxides [36,37], MOFs [38–41], N-doped carbon [42,43],
and metal cation-exchanged zeolites [44].
As a successful alternative to homogeneous catalysis, heterogeneous
catalysis has also been widely used in various type of catalytic organic
reactions in recent years, such as Strecker reaction, acetylation reac-
tion, N-formylation of amines and nitroarenes, alcohols dehydrogena-
tion or dehydration, et al. [45–52]. Nevertheless, the complexity of
heterogeneous catalyst system poses a new challenge to the under-
standing of the related reaction mechanism, and it is especially difficult
for the insufficiently-investigated solid-bases system. Their feasibility
and the structure-property relationship between catalytic sites and the
involved active species still need a further explorations [53,54].
The main objective of this work is to understand the structure
sensitivity of Lewis-base Ga4B2O9 in Knoevenagel condensation reac-
tion and thus reveals the structure-property correlation in the Ga-based
borates. A high yield (90 %) was achieved in a short time interval under
the mild reaction conditions when using benzaldehyde and mal-
ononitrile as substrates, and the structural comparison along with the
catalytic activity between Ga4B2O9, GaBO3 and β-Ga2O3 provided a
solid proof to ascertain the actual active sites in Ga4B2O9. Based on our
observations and some previous reported results, a catalytic mechanism
was proposed. More importantly, this work also hinted some gallium-
based compounds with similar crystal structure may behave as the
potential candidates in base-catalyzed reactions, which not only opens
an avenue towards the development of new intrinsic solid base cata-
lysts, but also a more in-depth understanding of reaction mechanism via
a detailed insight on structure-property correlations.
powder by gently heating in a hot plate. Afterward, 1.2360 g of boric
acid powder was charged and the mixture was ground in an agate
mortar. Finally, the resultant powder was placed in a corundum cru-
cible and calcined in muffle furnace at 873 K for 10 h. After heating, the
product was washed thoroughly with water to remove any soluble re-
siduals. The pure Ga4B2O9 sample was obtained after drying in an oven.
Catalyst characterization
Powder X-ray diffraction (XRD) data were collected with
a
PANalytical Empyrean powder diffractometer equipped with a PIXcel
1D detector (Cu Kα radiation 1.5406 Å). The cell parameters were re-
fined by Le Bail fitting using TOPAS software package [55]. Scanning
electron microscopy (SEM) was performed on a JSM-7800 F electron
microscopy at an accelerating voltage of 2 kV and a working distance of
4.0 mm. Transmission electron microscopy (TEM) was employed on
JEOL JEM-2100 F to observe the morphologies of as-prepared materials
at an accelerating voltage of 200 kV. The Brunauer-Emmett-Teller
(BET) method was employed to measure the surface area of Ga4B2O9
with nitrogen adsorption-desorption at 77 K using a Quantachrome
Quadrasorb SI analyzer. Temperature-programed desorption of carbon
dioxide (CO2-TPD) was carried out in a Quantachrome ChemBET Pulsar
instrument equipped with a thermal conductivity detector. Ga4B2O9
catalyst (0.1 g) was pre-heated in He gas flow (90 mL·min–1) at 673 K
for 1 h and then was exposed to CO2 flow at the rate of 90 ml/min at
323 K for 1 h, following by He flushing for 1 h, finally CO2 desorption
data was executed in the temperature range of 323−923 K. The amount
of CO2 desorbed were determined using 0.01 M NaOH absorbent, fol-
lowing a titration with 0.005 M HCl standard solution in the presence of
indicator.
Catalytic performance
Catalytic activity of the as-prepared Ga4B2O9 catalyst was evaluated
using Knoevenagel condensation reaction. Benzaldehyde and mal-
ononitrile were selected as model substrates to afford benzalmalono-
nitrite (1a), and the reaction equation was given in Scheme 1. Typi-
cally, 1 mmol of benzaldehyde and 1.2 mmol of malononitrile were
dissolved into 4 ml anhydrous DMSO solvent in a 10 ml flask. After
adding 30 mg of Ga4B2O9 catalyst into the reaction system, catalytic
reaction was kept at 313 K in an oil bath under stirring. Meanwhile,
inert protection gas N2 was forced to pass through the flask during the
reaction in order to prevent the oxidation of benzaldehyde. After a
certain time interval of 30 min, reaction mixture was drawn out with a
syringe equipped with an ultra-thin filter membrane to remove solid
catalyst, the obtained liquid were analyzed with chromatography-mass
spectrometry (GC–MS, Agilent 7890 N) to identify the molecular
structure of the obtained product, and the mass spectrum was shown in
Experimental section
Catalyst preparation
Ga4B2O9 was prepared by high temperature solid state reactions,
which was also described elsewhere [27]. In a typical run, 1 mmol of β-
Ga2O3 (0.1874 g) was dissolved in 2 ml of concentrated HNO3 in a
closed system, then the obtained aqueous solution was dried into white
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Y. Yang, et al.
MolecularCatalysis490(2020)110914
Scheme 1. Knoevenagel condensation reaction
between benzaldehyde and malononitrile to
produce benzalmalononitrile in the presence of
Ga4B2O9 catalyst.
Fig. 2. (a) Le Bail fitting of powder XRD pattern. Blue circles, red and black lines represent the observed, calculated patterns, and the difference between them,
respectively. The green bars below are the expected reflection positions; (b) SEM and (c) TEM images for the as-synthesized Ga4B2O9.
Fig. S1 in the Supporting Information (SI). Similarly, the qualitative
analyses of other 7 similar compounds were done and their corre-
sponding mass spectra were also provided in Figs. S2-S8. The quanti-
tative analysis of these involved reactants and products were performed
on a gas chromatography (SHIMADZU GC2010 plus) equipped with an
AOC20i auto-injector. A capillary column (HP-VOC, length 60 m; inner
diameter 0.32 mm; film thickness 1.80 μm) was chosen to separate the
mixed components at the following operation condition: 513 K for ca-
pillary column, 553 K for injector port, 563 K for FID detector. The total
yield of product 1a is calculated by multiplying benzaldehyde conver-
sion and product selectivity, while turnover number (TON) is calculated
with the division of the moles of consumed substrate by the moles of
used catalyst and reaction time. Nuclear magnetic resonance (NMR)
spectra were employed to identify the molecular structure of some
obtained products. 1H NMR spectra were recorded on Agilent VNMRS
(400 MHz) instrument using Deuterated dimethyl sulfoxide as the sol-
vent and using tetramethylsilane (TMS) as an internal standard, and the
obtained data were shown in Figs. S9-S16.
Results and discussion
Syntheses and structure of catalysts
The phase purity of as-synthesized Ga4B2O9 sample was confirmed
by powder XRD. As indicated in Fig. 2a, no impurity diffraction peak
was observed for the as-synthesized Ga4B2O9 sample, and all diffraction
peaks can be indexed to a monoclinic lattice system with the space
group C2/m. Le Bail fitting to the XRD pattern was performed to obtain
the unit cell parameters (a =15.406 Å, b =5.765 Å, c =11.016 Å,
β = 135.24°), which are consistent with the previously-reported data
[25,27]. The wide XRD diffraction peaks indicate that Ga4B2O9 may
crystallize in an extremely small particles. The SEM image in Fig. 2b
and TEM image in Fig. 2c confirmed the above assumption, where the
crystalline grains of Ga4B2O9 are clustered-like and are comprised of
some compact aggregates of nano-scaled needle, the length of the
needle-shaped crystal is at micron level and the diameter is about tens
of nanometers. Though this sample was prepared at a high temperature,
Ga4B2O9 has a large specific surface area. As shown in Fig. S17, the BET
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Y. Yang, et al.
MolecularCatalysis490(2020)110914
Fig. 3. The yield of product 1a and TON using benzaldehyde (BA) and malononitrile (MN) as reactants under different reaction conditions: (a) the influence of
different solvents: 1 mmol benzaldehyde, 1.2 mmol malononitrile, 4 ml solvent, 30 mg Ga4B2O9 catalyst, N2, 313 K, 30 min; (b) the influence of different temperature:
1 mmol benzaldehyde, 1.2 mmol malononitrile, 4 ml DMSO solvent, 30 mg Ga4B2O9 catalyst, N2, 30 min; (c) the influence of different molar ratio (MN/BA): 4 ml
DMSO solvent, 30 mg Ga4B2O9 catalyst, N2, 313 K, 30 min; (d) the influence of different solid base catalyst: 1 mmol benzaldehyde, 1.2 mmol malononitrile, 4 ml
DMSO solvent, 30 mg catalyst, N2, 313 K, 30 min. The value right above the histogram is the measured basic-amount of these solid base catalysts.
surface area of Ga4B2O9 is estimated to be 40.7 m2/g according to N2
adsorption-desorption experiments. This high value of specific surface
area is prominent in most bulk materials.
the relative proportion of malononitrile, but it maintained the same
level when their molar ratio exceeded 1.2/1. For the purpose of high-
lighting the predominant role of Ga4B2O9 catalyst, other five basic
oxides with different basic amounts, such as CeO2, γ-Al2O3, ZnO, SnO2
and TiO2, were also evaluated with Knoevenagel condensation reaction
under the similar reaction conditions. As shown in Fig. 3d, Ga4B2O9 has
the moderate amount of basic sites, but exhibited the optimal catalytic
activity. For example, the maximum TON over Ga4B2O9 is nearly 32
times higher than using TiO2.
In order to examine the universality of Ga4B2O9 in Knoevenagel
condensation reaction, another 7 different aromatic aldehyde substrates
were employed under the same reaction conditions, and the obtained
results are summarized in Table1. In most cases, Ga4B2O9 catalyst ex-
hibited the superior yield and product selectivity (> 99 %). When these
aromatic aldehydes with an electron donor group were used, such as
methyl or methoxy, (entry 7–8 in Table 1), catalytic activity decreased
accordingly, which attributes to their stronger capacity of giving elec-
trons and thus suppresses the combination between the substituted
benzaldehydes and carbanions intermediate [20]. On the contrary, if
the aldehydes with electron-withdrawing groups were selected as sub-
strates, such as eNO2, eBr, eOH groups, the product yield will remain
at a high level.
Catalytic performance
Catalytic properties of Ga4B2O9 were assessed using benzaldehyde
and malononitrile as two substrates. The operation parameters, in-
cluding solvent medium, temperature, molar ratio of reactants and
catalyst type, were investigated in order to obtain an optimal reaction
condition. First, five solvents with different polarity, including dimethyl
sulfoxide (DMSO), N,N-dimethyl formamide (DMF), methyl alcohol
(MeOH), ethyl alcohol (EtOH), propyl alcohol (PA) were chosen as the
reaction medium because of their good compatibility with reactants. As
shown in Fig. 3a, a minimum yield (∼44 %) was obtained in DMF,
while the highest one (∼90 %) was achieved in DMSO. The reason of
supreme yield in DMSO solvent may originate from the so-called “ion
pair mechanism”, that is, the high polarity of DMSO solvent is favorable
to stabilize the formed carbon anion intermediates [56]. The influence
of reaction temperature on Knoevenagel condensation reaction was also
studied by ranging temperature from 293 to 333 K. As shown in Fig. 3b,
reaction temperature has a positive effect on the reaction rates. The
yield of product 1a and TON were significantly enhanced by increasing
temperature from 293 to 313 K, indicating Knoevenagel condensation
reaction was temperature-sensitive. Fig. 3c gives the influence of initial
molar ratio of malononitrile and benzaldehyde changing from 0.6/
1–1.4/1. The yield of 1a as well as TON rapidly increased by increasing
Catalytic recyclability and reaction kinetics
Catalyst recyclability is one of the most important aspects to de-
termine catalyst stability and life time. In this regard, the reusability
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Y. Yang, et al.
MolecularCatalysis490(2020)110914
yield of product 1a after 10 cycles. The powder XRD pattern of the
recovered Ga4B2O9 (Fig. 4b) remained almost the same with the fresh
catalyst, confirming an excellent structure stability of Ga4B2O9 in
Knoevenagel condensation reaction.
The kinetic parameters of Knoevenagel condensation reaction were
calculated from the linear fitting on the experimental data obtained
under different temperatures, and the fitted curves were plotted in
Fig. 5. The time-dependent curve of product yield in Fig. 5a gave a
typical characteristics of the second-order reaction. After a nonlinear
fitting, an empirical rate equation can be established as following:
Table 1
Knoevenagel condensation reaction between some aldehydes and malononitrile
in the presence of Ga4B2O9 catalyst.
Entry
1
Aldehyde
Product
Yield (%)
90.1
2
3
4
5
6
7
98.5
94.9
90.1
80.8
80.2
63.2
1
b( a− x)
k2t =
ln
a− b a( b− x)
(1)
where x is the concentration of product 1a, t is reaction time, k2 is
reaction rate constant, a and b represent initial concentration of mal-
ononitrile and benzaldehyde, respectively. It can be drawn from the
plots that there has a good linear relation between reaction time t and
concentration expression of ln[(a-x)/(b-x)], indicating Knoevenagel
condensation reaction over Ga4B2O9 obeys the second-order reaction
kinetics [57]. Fig. 5b shows some kind of function relation of tem-
perature T and the corresponding reaction rate constant k, and there
has a quasi-linear plot between ln(k) and T–1. According to Arrhenius
empirical formula [ln k = ln A - Ea/(RT)], the apparent activation
energy Ea can be deduced from the slope of the fitting curve after a
linear regression, the final calculated Ea is ∼61.6 kJ/mol. The lower
apparent activation energy Ea suggests Ga4B2O9 can efficiently activate
the involved substrate molecules and thus haste the reaction rates in
Knoevenagel condensation reaction.
Active sites and reaction mechanism
8
67.5
In order to identify the actual active sites in the framework of
Ga4B2O9, a successive thermal treatment was performed under the
ambient pressure to demonstrate the evolution process of the involved
crystal phase. Fig. 6a gives the powder XRD patterns of thermally-
treated Ga4B2O9 sample from 873 K to 1033 K. Ga4B2O9 has a high
thermal stability and remained its original structure until temperature
reached 993 K. Beyond this critical temperature, Ga4B2O9 will firstly
decompose into GaBO3, which occurs within a very narrow temperature
range from 1013 to 1033 K, then it further decomposed into β-Ga2O3
above 1033 K. Fig. 6b gives the corresponding catalytic yields over
Ga4B2O9, GaBO3 and β-Ga2O3 catalysts. Surprisingly, the results in-
dicated catalytic activity have a tremendous change for these three Ga-
containing catalysts. The yield was 90 %, 17 %, 17 % for Ga4B2O9,
Reaction conditions: 1 mmol aldehyde, 1.2 mmol malononitrile, 4 ml DMSO
solvent, 30 mg Ga4B2O9 catalyst, N2, 313 K, 30 min.
and stability of Ga4B2O9 catalyst was examined by the repetitive usage
under the same reaction conditions. After each cycle, the solid catalyst
was separated by centrifugation followed by water-washing. The re-
covered catalyst was dried and activated at 373 K for 60 min prior to
the next run. As shown in Fig. 4a, there is no obvious decrease in the
Fig. 4. (a) Catalytic behavior of Ga4B2O9 for 10 consecutive cycles; (b) powder XRD for fresh and recovered catalyst, respectively. Reaction conditions: 1 mmol
benzaldehyde, 1.2 mmol malononitrile, 4 ml DMSO solvent, 30 mg Ga4B2O9 catalyst, N2, 313 K, 30 min.
5

Y. Yang, et al.
MolecularCatalysis490(2020)110914
Fig. 5. (a) Time-dependent of product concentration in Knoevenagel condensation catalyzed over Ga4B2O9 at 313 K and the obtained empirical equation from the
linear fitting; (b) the apparent activation energy Ea calculated from Arrhenius formula though the linear regression of ln k and T–1. Reaction conditions: 1 mmol
benzaldehyde, 1.2 mmol malononitrile, 4 ml DMSO solvent, 30 mg Ga4B2O9 catalyst, N2, 313 K.
Fig. 6. (a) X-ray diffraction patterns of powdered Ga4B2O9 treated at different temperatures and (b) the corresponding yield of the target product in Knoevenagel
condensation reactions. Reaction conditions: 1 mmol benzaldehyde, 1.2 mmol malononitrile, 4 ml DMSO solvent, 30 mg Ga4B2O9 catalyst, N2, 313 K, 30 min.
GaBO3 and β-Ga2O3, respectively. In fact, if the crystal structure and the
coordination microenvironment of these three catalysts have been
considered, these vastly different catalytic results can also be well
elucidated.
indicates that the framework of β-Ga2O3 is constructed by two type of
building units, i.e. 4-coordidated GaO4 units and 6-coordidated GaO6
units, these two type of building units are further interconnected into a
three-dimensional structure through the vertex-sharing oxygen atoms.
DFT calculations for Ga4B2O9 revealed the oxygen atoms adjacent to
three GaO5 groups have a strong electronegativity and incline to act as
the donors of electron pair, thus being active basic sites in the frame-
work of Ga4B2O9 [25]. After a high-temperature calcination, all GaO5
units in Ga4B2O9 were re-arranged into GaO6 and/or GaO4 units, such a
coordination style transformation has resulted into the complete loss of
basic centers in the GaBO3 and β-Ga2O3. The structural evolution of
these three compounds also provide a powerful evidence to ascertain
the actual catalytic sites in Ga4B2O9 for Knoevenagel condensation re-
action.
As described above, the structure of Ga4B2O9 features with 5-fold
coordinated Ga3+ (GaO5) and 6-fold coordinate Ga3+ (GaO6). The μ3-O
atoms linked exclusively to three 5-coordinated Ga3+ are intrinsic and
seem to function as the potential basic sites. For other two Ga-con-
taining compounds, GaBO3 and β-Ga2O3 have no similar GaO5 co-
ordination units. GaBO3 crystallizes in a trigonal crystal system with
lattice constants a =4.568 Å, c =14.182 Å and V = 256.3 Å3 in space
group R-3c [58]. As shown in Fig. 7a, GaBO3 has only one type of Ga-
based building unit (GaO6), and these GaO6 octahedra-chains are inter-
linked by sharing the vertex of triangular BO3 to form a three-dimen-
sion structure. β-Ga2O3 can be indexed with a monoclinic lattice (space
Temperature-programmed desorption (TPD) using CO2 as probe
molecules was employed to characterize surface basicity in the purpose
of further elucidating the origin of the structure-induced Lewis basicity
group C2/m) and the cell lattice parameters are
a =12.214 Å,
b = 3.037 Å, c =5.798 Å, β = 103.83° and V = 208.85 Å3 [59]. Fig. 7b
6

Y. Yang, et al.
MolecularCatalysis490(2020)110914
Fig. 7. Schematic illustration for the structure of (a) GaBO3 and (b) β-Ga2O3 along the b-axis, purple octahedron is GaO6, yellow tetrahedron is GaO4.
Fig. 8. (a) CO2-TPD profiles of as-synthesized Ga4B2O9, GaBO3 and β-Ga2O3; (b) the linear dependence on product yield and the added amount of hydrogen chloride.
Reaction conditions: 1 mmol benzaldehyde, 1.2 mmol malononitrile, 4 ml DMSO solvent, 30 mg Ga4B2O9 catalyst, N2, 313 K, 30 min.
in the above-mentioned three catalysts. As shown in Fig. 8a, compared
to GaBO3 and β-Ga2O3, Ga4B2O9 possesses an obvious characteristic
desorption peak at 721 K, indicating the presence of numerous basic
sites. After a phase-transition from Ga4B2O9 into GaBO3 and β-Ga2O3,
nearly all the surface basic sites disappeared, indicating the superior
base-catalyzed property in Ga4B2O9 largely comes from the special
connection mode of GaO5 units. On the contrary, if these basic active
sites are selectively occupied by a small amount of acidic poison, cat-
alytic activity will presumably have a deep drop [13,60]. According to
this assumption, hydrochloric acid, as an acidic impurity reagent, was
added to reaction system. As shown in Fig. 8b, a linear decreasing on
the product yield can be observed along with continuously adding hy-
drochloric acid. When exceeding a critical point, the product yield was
kept to nearly zero. In conclusion, catalytic activity in Knoevenagel
condensation reaction largely depends upon catalyst basicity. As the
actual catalytic centers, GaO5 coordination mode functions as a highly
effective activation sites in base-catalyzed reactions and therefore
provides an alternative and potential candidate.
The above results evidenced the instinct basic-site structure loca-
lized in the framework of Ga4B2O9 catalyst, these basic sites can be
accessible by the reactant molecules and are responsible to activate
malononitrile. Based on our results and some previous literatures
[19,57,60], a base-catalyzed mechanism has been presented in Fig. 9.
Because the O2− atom in the distorted GaO5 polyhedron exhibits a
relatively lower Bader charge, which conclude Lewis base sites are lo-
cated at the oxygen atoms linked with 5-coordinated Ga3+, mal-
ononitrile, as an electrophilic molecule, is favorably adsorbed and ac-
tivated in the sites of μ3-O atom, then a proton is abstracted from this
activated methylene molecule and promote the formation of the cor-
responding carbanions. Because the 5-fold Ga3+ of Ga4B2O9 can par-
tially polarize the C]O bond in benzaldehyde molecule, therefore, this
formed carbanion subsequently will approach an activated
7

Y. Yang, et al.
MolecularCatalysis490(2020)110914
Fig. 9. Proposed mechanism for Knoevenagel condensation reaction.
benzaldehyde molecule, which is absorbed in the neighboring Ga site in
the GaO5 group, to undergo a nucleophilic attack and form a new CeC
bond. After a subsequent molecular rearrangement, the cleavage of
water from the formed benzylidene malononitrile intermediate will
lead to the formation of α,β–unsaturated compound with C]C bond
[26,40,61].
- review & editing, Supervision. Rihong Cong: Supervision. Tao Yang:
Writing - review & editing, Supervision, Project administration.
Declaration of Competing Interest
There are no conflicts to declare.
Acknowledgements
Conclusion
This work was supported by the National Natural Science
Foundation of China (21671028, 21771027, 21805020), Natural
Science Foundation of Chongqing (cstc2019jcyj-msxmX0312,
cstc2019jcyj-msxmX0330), Fundamental Research Funds for the
Central Universities (2019CDXYHG0012). We also acknowledge the
support from the Chongqing Postdoctoral Science Special Foundation
(XmT2018004), and Postdoctoral Research Foundation of China
(2018M643402).
In summary, a mullite-type Ga4B2O9 catalyst, as an intrinsic Lewis
solid base, has been prepared through high temperature calcination
method. The special connection mode in Ga4B2O9, i.e. μ3-O atoms
bonding with 3 neighboring Ga atoms in GaO5 groups, has endowed
this material with a large amount of basic sites in the framework, thus
an excellent catalytic performance (90 % yield) was achieved in
Knoevenagel condensation reaction under the mild reaction conditions.
Compared to other conventionally-used solid-base catalysts, Ga4B2O9
exhibited a superior catalytic efficiency. The kinetic analysis indicated
that Knoevenagel condensation reaction is characteristic of second-
order reaction, and also has a lower apparent activation energy in the
presence of Ga4B2O9. From the structural evolution from
Ga4B2O9→GaBO3→β-Ga2O3, as well as the experiment of active basic
sites poisoning by acid additive species, the actual active sites were
further ascertained and evidenced. Based on above observations, the
relationship between the local structure of Ga4B2O9 and its catalytic
activity has been well established, and a plausible mechanism was
proposed. Despite of still some limitations in the observation of active
intermediates through in-situ method, this investigation will bring a
deep understanding on some special structure-induced property and
thus helps to design and fabricate some similar-type solid base catalysts,
a future effort is still needed to enhance the basicity of O2− in some
bulk-type solid Lewis bases, either through morphology controllable
synthesis or transition metal-doping, thus extends the wide application
in the industrial field of acid-base catalysis chemistry.
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.mcat.2020.110914.
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CRediT authorship contribution statement
Yao Yang: Writing - original draft, Investigation. Duo Wang:
Investigation. Pengfei Jiang: Formal analysis. Wenliang Gao: Writing
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