Diatomite Stabilized KOH: An Efficient Heterogeneous Catalyst for Cyclopentanone Self-condensation

Article abstract of DOI:10.1002/cctc.202001538

Aldol condensation of cyclopentanone is an attractive reaction for bio-jet fuel production. KOH is an efficient homogeneous catalyst for aldol condensation but suffering the problem of separation. In this paper, a heterogeneous catalyst, diatomite stabilized KOH (KOH/diatomite) was prepared in a simple way (dry-mixed impregnation process), and exhibited good activity (overall yield: 85.0 %) and reusability. To clarify the support effect of diatomite, KOH/SiO₂ catalyst was also prepared. According to characterizations of these two catalysts, we suggest that the excellent catalytic activity of KOH/diatomite can be attributed to the formation of K?O?Si bond. In addition, the impurity elements existed in diatomite (e. g. CaO, Na₂O, Al₂O₃) and higher BET surface areas may also benefit to this reaction. This study provides a new possibility for converting homogeneous catalyst (KOH) to heterogeneous catalyst (KOH/diatomite). In this way, KOH/diatomite performed excellent catalytic activity and exhibited stable reusability, which can avoid the problem of separation and pollution.

Full text of DOI:10.1002/cctc.202001538

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Diatomite Stabilized KOH: An Efficient Heterogeneous Catalyst
for Cyclopentanone Self-condensation
Authors: Qianqian Xu, Xueru Sheng, Haiyuan Jia, Na Li, Jian Zhang,
Haiqiang Shi, Meihong Niu, and Qingwei Ping
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01/2020

10.1002/cctc.202001538
ChemCatChem
Diatomite Stabilized KOH: An Efficient Heterogeneous
Catalyst for Cyclopentanone Self-condensation
Qianqian Xu1, Xueru Sheng1,*, Haiyuan Jia 2, Na Li1, Jian Zhang1, Haiqiang Shi1,
Meihong Niu1, Qingwei Ping1,*
1College of Light Industry and Chemical Engineering, Dalian Polytechnic University,
Dalian 116034, Liaoning, China
2Shandong Provincial Key Laboratory of Molecular Engineering, School of Chemistry
and Pharmaceutical Engineering, Qilu University of Technology — Shandong Academy
of Sciences, No. 3501, Daxue Road, Jinan 250353, China
Corresponding author: Xueru Sheng, Qingwei Ping
E-mail: shengxr@dlpu.edu.cn, Tel/Fax: +86 411 86323327-603
pingqw@dlpu.edu.cn,
Tel/Fax: +86 411 86324620
Abstract
Aldol condensation of cyclopentanone is an attractive reaction for bio-jet fuel
production. KOH is an efficient homogeneous catalyst for aldol condensation but
suffering the problem of separation. In this paper, a heterogeneous catalyst, diatomite
stabilized KOH (KOH/diatomite) was prepared in a simple way (dry-mixed
impregnation process), and exhibited good activity (overall yield: 85.0%) and
reusability. To clarify the support effect of diatomite, KOH/SiO2 catalyst was also
prepared. According to characterizations of these two catalysts, we suggest that the
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excellent catalytic activity of KOH/diatomite can be attributed to the formation of K-O-Si
bond. In addition, the impurity elements existed in diatomite (e.g. CaO, Na2O, Al2O3) and
higher BET surface areas may also benefit to this reaction. This study provides a new
possibility for converting homogeneous catalyst (KOH) to heterogeneous catalyst
(KOH/diatomite). In this way, KOH/diatomite performed excellent catalytic activity and
exhibited stable reusability, which can avoid the problem of separation and pollution.
Key words:
Biomass; Diatomite; Heterogeneous catalysts; KOH; Self-condensation
1. Introduction
With the great concern of fossil fuel and environmental issues, renewable
biomass-derived fuel components conversion has gradually become an important
research topic [1]. Besides, the demand of high-density jet fuel is sharply increasing in
aerospace aircraft [2]. Compared with other synthesis pathways, the preparation of
high-density jet fuel from renewable biomass-based materials (such as lignocellulose,
starch, lipid, etc.) has been a relatively promising route [3]. Furthermore, it should be
pointed out that among biomass-based materials, lignocellulose can be obtained from an
economical way, such as the source of agricultural waste and forest residues. In the past
[5]
years, great advances have been made in this field pioneered by Dumesic [4], Huber
Corma [6] and other researchers [7].
,
Cyclopentanone is one of the biomass platforms compounds towards bio-high density
fuel synthesis, which can be produced by hydrolysis, dehydration and selective
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hydrogenation of hemicellulose [8]. Due to its cyclic structure and C=O groups, cyclic
dimer can be produced as bio-high density fuel precursor after cyclopentanone
self-condensation or with furfural aldol condensation [9]. In previous work, different
homogeneous or heterogeneous catalysts were explored and used for this reaction. As
for homogeneous catalysts, Wang et al. reported that KOH aqueous solution exhibited
well catalytic activity (83.3%) with the mixture of Raney Co. However, KOH aqueous
solution suffered the problem of subsequent separation [10]. As for heterogeneous
catalysts, Deng et al. found that 98.1% selectivity of product was obtained (conversion:
46.1%) over MOF-encapsulation phosphotungstic acid [11]. Meanwhile, metal oxides
catalysts also have been reported. Yang et al. reported that up to 86% yield was obtained
over MgAl-HT [12]. Liang et al. showed that C10 yield reached 84.6% with 88.2%
cyclopentanone conversion over MgZr41 catalyst [13]. However, the preparation process
of above heterogeneous catalysts is either expensive or complex. Thus, it will be
attractive to design a new heterogeneous catalyst combined KOH with supporting
materials, which possesses both catalytic activity and economic efficiency.
Among various catalytic materials, finding supporting materials are higher efficiency
for the conversion of cyclopentanone that is an appealing objective. Meanwhile,
materials are stable to disperse loading with lower cost should be considered. Diatomite,
which is derived from diatoms remains, is a natural and much cheaper material. It
exhibits unique microporous structure, large specific surface area, good adsorptivity,
strong acid resistance and good thermal stability [14]. Due to these characteristics,
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diatomite application has been described in the literature for a lot, such as composite
[14c]
super absorbent [14a], filter aids [14b], thermal insulation materials
etc. On the other
hand, diatomite exhibits amorphous structure and good dispersion, which has been
applied as an efficient heterogeneous catalyst support to investigate in some reactions
[15]. For instance, Zhang et al. investigated the Heck and Suzuki reactions could be
catalyzed by diatomite-supported Pd NPs [16]. Besides, Shan et al showed biodiesel
[17]
production achieved 92.4% conversion by using diatomite-supported CaO catalyst
.
Otherwise, Rabie et al. reported diatomite@CaO/MgO catalyst for the converting of
waste cooking oil [18]
.
Herein, the main goal of this contribution is to synthesis and characterize a
heterogeneous diatomite stabilized KOH catalyst (KOH/diatomite), which can be
further applied into cyclopentanone self-condensation. Great catalytic activity was
achieved compared with KOH/SiO2 catalyst.
2. Materials and Methods
2.1 Materials and catalyst preparation
2.1.1 Materials
Cyclopentanone, KOH were commercial available (Sinopharm Chemical Reagent
Co.,Ltd). Diatomite was collected from Changbai Mountain, Jilin, China. Fumed SiO2
was obtained from Aladdin Reagent Co.,Ltd.
2.1.2 Catalyst synthesis
Diatomite and KOH were mixed in a round bottom flask (500ml) with a rotor (400
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r/min) at 60 °C for 24 h, which mass ratios (KOH/diatomite) range from 1:6 to 1:1. In
the dry-mixed impregnation process, a magnetic heating stirrer was used to heat and
mix the catalyst, and round bottom flask was fitted with frosted bottle cap. After that,
the sample was calcined in muffle furnace at desired temperature (range from 300 °C to
700 °C) for 4 h. For the reusability test, after 8 h cyclopentanone self-condensation
reaction, catalyst was separated from the reaction mixture and washed with ethanol
solution, then calcined at 500 °C for 4 h before next reaction.
2.2 Characterization of catalysts
2.2.1 Scanning Electron Microscope (SEM) and Energy Spectrum Analysis (EDS)
The surface morphology of KOH/SiO2 and KOH/diatomite catalysts was observed by
SEM using JSM-7800 scanning electronic microscope. Elemental mapping scan was
obtained by Energy Spectrometer X-Max50.
2.2.2 X-ray diffraction (XRD)
Catalysts were characterized by Shimadzu X-ray diffractometer XRD-6100.
Continuous scans were collected at a step rate of 5 ° min-1 with a Cu target (over a 2θ
range of 10°- 80°).
2.2.3 Thermogravimetric Analysis (TGA).
TGA was analyzed by TGA Q500 with the temperature process of 10 °C/min raised
to 700 °C.
2.2.4 Physical adsorption
ASAP 2020 Plus was used to measure the BET surface area at 77 K by nitrogen
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physisorption. Before test, the catalysts were evacuated at 393 K for 4 h.
2.2.5 FT-IR spectra
The FT-IR spectra of catalysts were performed for the range from 4000 cm−1 to 400
cm−1 using PerkinElmer Frontier Spectrometer.
2.3 Activity test/ Methods
The cyclopentanone, catalysts were mixed at a mass of 4 g and 1 g respectively in a
20 mL Teflon-lined batch reactor. Before test, N2 was passed through the reactor for 30 s.
Then the reactor was sealed, followed by stirring at 180 °C for 2 h. After that, the
product was analyzed by Varian 450-GC gas chromatography.
3. Results and Discussion
3.1 Catalyst Characterization
As the major constituent of diatomite is bio-SiO2 [19], so it is ineluctable to compare
the activity difference with commercial SiO2. KOH/diatomite and KOH/SiO2 catalysts
were prepared for this reaction. To investigate the structure-activity difference between
KOH/SiO2 and KOH/diatomite, several characterizations were conducted on catalysts.
Firstly, FT-IR was used to analyze the characteristic adsorption peaks of the spectra of
KOH/SiO2, SiO2, KOH/diatomite and pure diatomite (As shown in Fig. 1). Among the
multi-peaks, the spectral peaks at the position of ca. 1630 /810 /790 cm-1 and 3430 cm−1
can be related to the symmetric stretching and out of plane bending vibrations of
Si-O-Si, -OH, respectively [20]. Thus, all samples contain the Si-O-Si and -OH groups.
The major difference of KOH/diatomite and KOH/SiO2 are described as following: (i)
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In KOH/diatomite, a new obvious peak is observed visible at ca. 1440 cm-1, which is
owing to K-O-Si group stretching vibration [21]; (ii) In KOH/SiO2, a peak appeared at
the position of ca. 621 cm-1, which can be ascribed to the existence of Si-H stretching
vibration and this group differs from others. Above-mentioned indicates that both
diatomite and SiO2 react with KOH after KOH loading. However, different substances
are formed. In addition, to further investigate the effect of calcination on the catalyst,
KOH/diatomite catalyst without calcination was also prepared and named as
KOH/diatomite-UC. Based on the FT-IR analysis carried on KOH/diatomite-UC, a new
absorption peak is observed visible at ca. 899 /1383 cm-1, which is denoting the Si-OH
functional group. The formation of this group could be attributed to the absorbed
moisture during mix process. After calcination, this signal disappeared and the
absorption peak of 1440 cm-1 enhanced, which revealed that the Si-OH was transformed
to K-O-Si group during calcination.
1630
1440
1383
810
790
621
899
3430
KOH/SiO2
SiO2
KOH/diatomite
KOH/diatomite-UC
pure diatomite
4000 3500 3000 2500
2000
1500
1000
500
Wavenumber(cm-1)
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Figure 1. The FT-IR spectra of KOH/SiO2, SiO2, KOH/diatomite, KOH/diatomite-UC
and pure diatomite.
Subsequently, X-ray diffraction (XRD) was performed (As shown in Fig. 2 (a) and Fig.
S1). XRD patterns of both fumed SiO2 and diatomite exhibited amorphous SiO2
structure. After KOH loading, the diffraction peak of KOH/diatomite became wider and
lower than pure diatomite. This indicates that KOH is well-distributed over the
diatomite support with amorphous structure maintaining. However, the intensity of SiO2
diffraction peak of KOH/SiO2 was obviously enhanced. This situation is closely
associated with following facts that fumed, amorphous SiO2 was dehydrated and
transformed to crystalline SiO2 during this process. To further verify the above
hypothesis, a new catalyst (KOH/diatomite-AW catalyst) was prepared and tested (As
shown in Fig. S3-S5). During the mix process of this catalyst, 10 wt.% additional water
was added. As was expected, compared with KOH/diatomite, the activity of
KOH/diatomite-AW catalyst was declined. According to the FT-IR result, the spectra of
KOH/diatomite-AW catalyst becomes similar with KOH/SiO2: the absorption peak of
1440 cm-1, which is attributed to the K-O-Si active site, is reduced obviously. This above
phenomenon indicates that absorbed water may be the root cause of this catalytic
difference.
However, there were still some diffraction peaks (ca. 26°-35°) with low intensity
which cannot be attributed in KOH/SiO2 and KOH/diatomite-AW. To get deep insight of
this signal which may disclose the interaction between KOH and SiO2 during this
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process, high KOH loading KOH/SiO2 (2:1) was prepared and characterized. According
to the result, new peaks have occurred both KHSiO3 and K2Si4O9 were identified as
major products. Based on this result and FT-IR spectra of KOH/SiO2, diffraction peaks
(ca. 26°-35°) of KOH/SiO2 can be attributed to these two types of silicates, e.g. KHSiO3
and K2Si4O9. According to the literature, (1) K2Si4O9 is a stable, glass type of wadeite
mineral containing high (4-6) coordinated Si atoms [22]. However, medium or strong
solid basic/acid sites are necessary for cyclopentanone self-condensation [23], which
K2Si4O9 does not possess. (2) KHSiO3 is not stable and can be transformed to colloidal
type (OH)2SiO22− component in water [24]. It is inevitable that KHSiO3 component can
absorb water either from air or during reaction. The visible alterations of appearance of
used KOH/SiO2 catalyst is also in agreement with this (As shown in Fig. S2). Thus, the
poor activity of KOH/SiO2 can be reasonably linked to the inactive and unstable
K2Si4O9 and KHSiO3 components in KOH/SiO2 catalyst. In addition, the XRD data
illustrated that KOH/diatomite-AW maintained amorphous structure (As shown in Fig.
S5). However, as similar with KOH/SiO2 catalyst, some diffraction peaks with low
intensity appeared in ca. 26°-35°. It can be assumed from this result that inactive and
unstable K2Si4O9 and KHSiO3 components were synthesized in KOH/diatomite-AW
catalyst, which may decrease the catalytic activity directly.
TGA results were also consistent with XRD and FT-IR results as shown in Fig. 2 (b).
The weight loss of KOH/diatomite could be explained by evaporation of water (~100 °C),
formation of K-O-Si group (~200 °C) and reached stable above 300 °C. For KOH/SiO2,
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the obvious decrease (~100 °C) was related to the dehydration of fumed SiO2 and the
slight and long-range reduction (>100 °C) may be attributed to the transformation of
KOH and formation of K2Si4O9 and KHSiO3.
pure diatomite
100
90
80
70
60
50
.
(b)
(a)
 SiO
 KHSiO
. K Si
2
3
.
2
4
O
9
KOH/diatomite
KOH








KOH/SiO2-(2:1)
SiO
2

KOH/SiO2-(1:3)
SiO2
KOH/diatomite
pure diatomite
KOH/SiO
2
20
40
60
80
100
200
300
400
500
600
700
2 Theta ( o
)
Temperature(oC)
Figure 2. (a) XRD pattern and (b) TGA analysis of pure diatomite, KOH/diatomite, SiO2
and KOH/SiO2.
Furthermore, the BET surface areas, total pore volume and pore size areas distribution
of catalysts are summarized in Table. 1, and SEM analysis is showed in Fig. S6. Based
on the analysis results, there was obvious difference between these two supports. For
diatomite, the surface area increased markedly from 13.11 m2 g−1 to 22.27 m2 g−1.
Whereas, the surface area of SiO2 fell sharply from 205.02 m2 g−1 to only 10.04 m2 g−1
after KOH loading. KOH/diatomite possessed higher surface area, larger pore volume
and pore size. It can be assumed that KOH was well-distributed into diatomite
compared with KOH/SiO2. These results were corroborated with XRD results (Fig. 2(a))
and SEM images (Fig. S6). Namely, KOH was well-distributed over the diatomite
support with amorphous structure maintaining, but fumed amorphous SiO2 was
dehydrated and transformed to crystalline SiO2.
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Table 1. BET surface areas, total pore volume and pore size areas of the support or
catalyst.
Support or
BET surface
areas (m2 g−1)
13.11
Pore volume
(cm3 g−1)
0.012
Pore size
areas(nm)
5.49
catalyst
diatomite
KOH/diatomite
SiO2
22.27
0.028
17.75
205.02
0.572
11.43
KOH/SiO2
10.04
0.004
3.70
To confirm the dispersity of KOH/SiO2 and KOH/diatomite catalysts, SEM-EDS
analysis was conducted and presented in Fig. 3. According to the results, the EDS
mapping of (a) KOH/SiO2 and (b) KOH/diatomite both showed that K, Si, O elements
were detected on support with well dispersity. Moreover, Al2O3 (5.10%), Na2O (0.33%),
CaO (0.30%) were found as minor impurity components over KOH/diatomite.
According to the literature, these minor components can also catalyze the reaction[25]
.
Thus, it can be concluded that there is synergistic effect between KOH and these
impurity components of diatomite.
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Figure 3. SEM images (magnification: 2000) with EDS mapping of (a) KOH/SiO2 (b)
KOH/diatomite with 500 °C calcination.
3.2 Catalytic Activity
To investigate the effect of KOH loading amount on diatomite for cyclopentanone
self-condensation, various mass ratio of KOH/diatomite catalyst was designed in the
experiment. Obviously, both conversion, total yield of dimer and trimer improved, while
the KOH loadings changing from 1:6 to 1:1 (As shown in Fig. S7). However, the yield
of trimer increased rapidly when KOH/diatomite mass ratio a further increase from 1:3
to 1:1, which led to the low selectivity of dimer. Based on a balance between dimer
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yield and selectivity, KOH/diatomite-1:3 was selected as the optimized mass ratio of
KOH/diatomite.
100
80
60
40
20
0
400
Calcination temperature (°C)
300
500
600
700
Figure 4. Effect of different calcination temperature of KOH/diatomite catalyst.
Cyclopentanone conversion (blue bar), yield of dimer (orange bar) and trimer (green
bar), selectivity of dimer (yellow bar) and trimer (gray bar) over KOH/diatomite
catalyst. Reaction conditions: 1.0 g KOH/diatomite catalyst; 4.0 g cyclopentanone;
180 °C; 2 h.
Subsequently, the calcination temperature of KOH/diatomite was also investigated
(As shown in Fig. 4). It is clear that the conversion, yield and selectivity did not change
greatly over different calcination temperature. This result is consistent with TGA and
XRD (As shown in Fig. 2 (b) and Fig. S1) results. The weight loss of KOH/diatomite
reached stable above 300 °C. XRD results illustrated that no significantly difference
was detected on different calcination temperature of KOH/diatomite catalysts.
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Figure 5 shows the activity comparison of KOH/SiO2 and KOH/diatomite catalysts.
Unexpectedly, under KOH/SiO2 catalyst, there was no cyclopentanone conversion with
three repeated experiments conducted. While KOH/diatomite catalyst exhibited
relatively high dimer yield (55.9%) under the same reaction conditions. It can be
inferred from this result that diatomite support is more suitable for KOH than
commercial SiO2. Meanwhile, above characterizations of FT-IR, SEM-EDS and BET
have explained the reason of obviously different catalytic activity. we suggest that the
excellent activity of KOH/diatomite catalyst can be attributed to the formation of
K-O-Si bond. In addition, the impurity elements existed in diatomite (e.g. CaO, Na2O,
Al2O3) and higher BET surface areas may also benefit to this reaction.
100
80
60
40
20
0
KOH/diatomite
KOH/SiO
2
Figure 5. Comparison of KOH/SiO2 and KOH/diatomite catalysts.
Cyclopentanone conversion (blue bar), yield of dimer (orange bar) and trimer (green
bar), selectivity of dimer (yellow bar) and trimer (gray bar) over KOH/diatomite
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catalyst. Reaction conditions: 1.0 g KOH/support catalyst (KOH/support=1:3); 4.0 g
cyclopentanone; 180 °C; 2 h; 500 °C calcination.
To improve the yield of dimer, reaction time was extended from 2 h to 8 h (As shown
in Fig. S8). With the extension of reaction time, the total yield improved to 85.0%,
which is relatively high for catalytic reaction. Furthermore, the reusability of catalyst is
crucial parameter to assess catalytic activity for reaction. Thus, reusability test of
KOH/diatomite catalyst was carried out (As shown in Fig. S9). Dimer yield declined
slightly after three runs, but there is no significant microstructure difference among
KOH/diatomite catalyst after three times reaction cycles (As shown in Fig. S10). Based
on the XRD results of spent catalysts, the diffraction peaks (ca. 26°-35°) were appeared
(As shown in Fig. S11), which were similar to the inactive and unstable K2Si4O9 and
KHSiO3 components in KOH/SiO2 catalyst. This inactivation may be caused by the
H2O generated during reaction. Compared to KOH aqueous solution, KOH/diatomite
heterogeneous catalyst exhibited reusability and was more environmentally friendly.
Based on this result, the cost accounting of our KOH/diatomite catalyst and traditional
KOH aqueous solution was conducted. Because of the reusability, the cost of
KOH/diatomite catalyst decreases 47.0% at least (The calculative process was listed in
Supporting information). According to the above cost benefit analysis, the use of
KOH/diatomite catalyst is more economical and environmentally friendly than KOH
aqueous solution.
On the basic of the catalytic evaluations and the overall structural characterizations, a
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proposed catalyst synthesis pathway and reaction mechanism of KOH/diatomite and
KOH/SiO2 is showed in Scheme 1. For KOH/diatomite catalyst, K-O-Si bond was
formed during dry-mixed impregnation process and identified as the active group for
cyclopentanone self-condensation. Moreover, the minor impurity components (e.g. CaO,
Na2O, Al2O3) in diatomite may also benefit to this reaction. However, for KOH/SiO2
catalyst, inactive and unstable K2Si4O9 and KHSiO3 were formed during preparation
process, which led to the inactivation of the reaction.
Scheme 1. Proposed catalyst synthesis and reaction pathway of KOH/diatomite and
KOH/SiO2.
Furthermore, a proposed reaction mechanism for cyclopentanone aldol condensation
on K-O-Si bond is showed in Scheme 2. It is well-known that aldol condensation of
cyclopentanone is an effective reaction for C–C coupling with simultaneous elimination
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of O-atom. Meanwhile, the elementary steps include the following sequence of steps:
adsorption of reactants, enolization/α-C–H abstraction, C–C coupling, dehydration and
desorption to the final α, β–unsaturated oxygenates. As shown in Scheme 2, due to the
acidic of carbonyl compounds, the basic O anion are able to abstract the α-C–H from
the ketone and forms an enolate, which is stabilized by delocalizing the negative charge
onto an electronegative O. Then, the enolate was formed in this base-route which could
readily attack another carbonyl to form a C-C bond, because of the negatively charged.
In this step, the second carbonyl can be either adsorbed on a vicinal K-cation or from
the liquid phase as shown below.
Scheme 2. Proposed reaction mechanism for cyclopentanone aldol condensation on
K-O-Si bond.
4. Conclusions
This work presents the synthesis and characterization of a heterogeneous diatomite
stabilized KOH catalyst and exhibited relatively high yield (85.0% total yield) and good
reusability. Through multiple characterizations, it can be noticed that KOH is
well-distributed over the diatomite support with amorphous structure maintaining. The
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excellent catalytic activity of KOH/diatomite can be attributed to the formation of K-O-Si
bond as active site. In addition, the impurity elements existed in diatomite (e.g. CaO,
Na2O, Al2O3) and higher BET surface areas may also benefit to this reaction. Compared
to KOH aqueous solution, KOH/diatomite heterogeneous catalyst exhibited good
reusability and was more environmentally friendly. Because of the reusability, the cost of
KOH/diatomite catalyst decreases 47.0% at least. Meanwhile, this work may
demonstrate a new approach to stabilization KOH on heterogeneous support by using
natural diatomite and has a significant fall in catalyst cost compared with traditional
homogeneous KOH catalyst.
Author Contributions:
Qianqian Xu performed the experiments; Na Li, Jian Zhang, Meihong Niu and
Haiqiang Shi analyzed the data; Qingwei Ping contributed reagents and analysis tools;
and Xueru Sheng conceived, designed the experiments and reviewed the paper.
Acknowledgments:
This work was supported by National Natural Science Foundation of China [No.
21808023]; Natural Science Foundation of Liaoning [No. 20180550559]; Liaoning
Revitalization Talents Program [XLYC1802025].
Conflicts of Interest:
The authors declare no conflict of interest.
Indicate：
The color should be used for figures in print.
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Scheme 1. Proposed catalyst synthesis and reaction pathway of KOH/diatomite and
KOH/SiO2.
Key words:
Biomass; Diatomite; Heterogeneous catalysts; KOH; Self-condensation
A heterogeneous catalyst (KOH/diatomite) was prepared in a simple way, and
exhibited good activity (overall yield: 85.0%). We suggest that the excellent catalytic
activity of KOH/diatomite can be attributed to the formation of K-O-Si bond, the
impurity elements existed in diatomite and higher BET surface areas may also benefit.
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