Pyridine immobilised on magnetic silica as an efficient solid base catalyst for Knoevenagel condensation of furfural with acetyl acetone

Article abstract of DOI:10.1016/j.catcom.2019.03.005

Novel heterogeneous pyridine immobilised magnetic silica (Fe₃O₄@SiO₂-Py) was found to be an efficient, greener and heterogeneous solid base catalyst for the Knoevenagel condensation of furfural with acetylacetone under optimized reaction conditions. The Knoevenagel condensation product 3-(2-furylmethylene)-2,4-pentanedione (FMP), a jet fuel precursor, was produced in high yield of 85% with 94% conversion of furfural at 100 °C within a period of 4 h. Fe₃O₄@SiO₂-Py catalyst showed excellent stability and recyclability without losing its initial activity.

Full text of DOI:10.1016/j.catcom.2019.03.005

Accepted Manuscript
Pyridine immobilised on magnetic silica as an efficient solid base
catalyst for Knoevenagel condensation of furfural with acetyl
acetone
Komal Tarade, Suhas Shinde, Sachin Sakate, Chandrashekhar
Rode
PII:
S1566-7367(19)30072-X
DOI:
Reference:
https://doi.org/10.1016/j.catcom.2019.03.005
CATCOM 5643
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
14 January 2019
20 February 2019
5 March 2019
Please cite this article as: K. Tarade, S. Shinde, S. Sakate, et al., Pyridine immobilised
on magnetic silica as an efficient solid base catalyst for Knoevenagel condensation of
furfural with acetyl acetone, Catalysis Communications, https://doi.org/10.1016/
j.catcom.2019.03.005
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ACCEPTED MANUSCRIPT
Pyridine Immobilised on Magnetic Silica as an Efficient Solid Base Catalyst
for Knoevenagel Condensation of Furfural with Acetyl acetone
Komal Tarade, Suhas Shinde, Sachin Sakate and Chandrashekhar Rode*
Chemical Engineering and Process Development Division, CSIR-National Chemical
Laboratory, Dr. Homi Bhabha Road, Pune, India-411008.
*Email: cv.rode@ncl.res.in, Fax: +9120 2590 2621, Phone: +9120 2590 2349.
Abstract
Novel heterogeneous pyridine immobilized magnetic silica (Fe3O4@SiO2-Py) was found to
be an efficient, greener and heterogeneous solid base catalyst for the Knoevenagel
condensation of furfural with acetylacetone under optimized reaction conditions. The
Knoevenagel condensation product 3-(2-furylmethylene)-2,4-pentanedione (FMP), a jet fuel
o
precursor, was produced in high yield of 85 % with 94 % conversion of furfural at 100 C
within a period of 4 h. Fe3O4@SiO2-Py catalyst showed excellent stability and recyclability
without losing its initial activity.
Key words: Knoevenagel condensation, furfural, silica immobilised pyridine, solid base, jet
fuel.
1. Introduction
Currently chemicals and liquid fuels are largely produced from crude oil however, the
sources of crude oil will be on the verge of diminishing in near future. In addition, during
crude oil refining process, huge amount of CO2 is expelled out causing irreparable air
pollution. Fortunately, these threats can be overcome by utilising lignocellulosic biomass
which is as a sustainable carbon source and green alternative to crude oil [1]. Furfural is the
most important platform molecule produced by the hydrolysis and dehydration of xylan
contained in lignocellulosic biomass. Diesel-range long chain alkanes can be produced from
furfural via acid or base catalyzed C−C bond formation followed by hydrodeoxygenation [2,
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3]. In this context, aldol, Knoevenagel condensations [4, 5], Baylis−Hillman, coupling
reactions [6, 7], and HAA reaction [8-10] can be performed for carbon upgradation of
furfural. These upgraded products are carbon-rich and can be converted into liquid
hydrocarbons via ring-opening−dehydration−hydrodeoxygenation processes [11, 12]. In
particular, the Knoevenagel condensation of furfural is not studied much. For instance, Ding
et al. demonstrated enzyme (lipoprotein lipase) catalyzed Knoevenagel condensation between
furfural and acetyl acetone at 35 °C. However, this catalytic system required long reaction
time of 48 h and gave moderate yield (76 %) in dimethyl sulfoxide solvent [13]. Very
recently, Appaturi et al. developed alanine functionalized MCM-41 catalyst for Knoevenagel
condensation between furfural with acetyl acetone. Under the solvent-free condition and in a
short reaction time (30 min) this catalyst system provided 100% selectivity to α,β-unsaturated
products with 92% conversion of furfural [5]. Several organic–inorganic hybrid materials
containing N-containing organic functionalities such as pyridine-functionalized porous
polymers [14], DABCO based ionic liquid [15], primary amine functionalized MCM-41 [16,
17] and organic polymer [18] have been found successful for the Knoevenagel condensation
of active methylene compounds with aldehydes.
With this background in this study, we report a magnetically separable silica immobilised
pyridine (Fe3O4@SiO2-Py) (Scheme 2) as an effective solid base catalyst that shows excellent
performance for the Knoevenagel condensation of furfural with acetyl acetone (Scheme 1).
Fe3O4@SiO2-Py catalyst showed the highest conversion of furfural (94 %) with 85 % yield of
o
condensation product at 100 C in the absence of solvent. This novel catalyst showed
reusability up to six times without significant decrease in activity and selectivity.
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Scheme 1. Synthesis of Jet fuel precursors from furfural and acetyl acetone
2. Experimental
2.1. Materials
Furfural, N,N-dimethylaminopyridine, furfurylamine, DBU, 4-hydroxypyridine, 3-
pyridinecarbaldehyde 3-aminopropyltriethoxysilane, tetraethyl orthosilicate, FeCl2 and FeCl3
were purchased from Sigma-Aldrich. Sodium hydroxide, triethylamine, zinc oxide, calcium
oxide and aq. ammonia (10%) were obtained from Thomas-Baker. Acetylacetone was
purchased from Merk, India. Methanol, toluene, pyridine, dichloromethane, ethyl acetate and
petroleum ether and silica gel (230-400 mesh) were purchased from Chem Labs, India.
2.2. Preparation of 3-aminopropylsilica
To a solution of 3-aminopropyltriethoxysilane (3.5 g) in toluene, pyridine (1:1, 20 mL), SiO2
(2 g, 230-400 mesh) were added. The resulting mixture was refluxed for 24 h under N2
atmosphere after which the reaction mixture was filtered and residue was washed with
toluene (20 mL x 1) and dichloromethane (20 mL x 1). The residue was dried in oven for 24
h to afford off a white powder (4.8 g).
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2.3. Preparation of silica-pyridine (SiO2-Py)
Step 1(1-(pyridin-3-yl)-N-(3-(triethoxysilyl)propyl)methanimine): A mixture of 1.8 g of 3-
aminopropyltriethoxysilane (AMPSi), 3-pyridinecarbaldehyde (0.877 g) and dichloromethane
o
(25 mL) was stirred at 50 C for 24 h and then the solvent was evaporated on rotary
evaporator to afford a colorless oil (2.5 g).
Step 2 (1-(pyridin-3-yl)-N-(3-(triethoxysilyl)propyl)methanimine-silica): To a solution of 1-
(pyridin-3-yl)-N-(3-(triethoxysilyl)propyl)methanimine (2.5 g) in toluene (20 mL), SiO2 (1.7
g, 230-400 mesh) was added. The resulting mixture was refluxed for 24 h under N2
atmosphere and then filtered to give a residue which was washed with toluene (20 mL x 1)
and dichloromethane (20 mL x 1). The residue was dried in oven for 24 h to afford off white
powder (4.0 g).
Step 3 (N-(pyridin-3-ylmethyl)propylamino-silica)[19]: A mixture of 1-(pyridin-3-yl)-N-(3-
o
(triethoxysilyl)propyl)methaniminesilica (3.5 g) and methanol (25 mL) was cooled at 0 C
and NaBH4 (0.619 g) was added. The resultant mixture was stirred at room temperature for
24 h then evaporated to afford off white powder (3.2 g).
2.4. Synthesis of nano-magnetite particles (Fe3O4)
A solution of FeCl2 (5.4 g) and FeCl3 (2 g) in aqueous hydrochloride acid (2 M, 25 mL) were
sonicated at room temperature until the salts dissolved completely. To this solution, an
aqueous ammonia solution (25%, 40 mL) was added slowly under argon atmosphere
followed by 30 minute stirring. The Fe3O4 nanoparticles were separated by external magnet
and washed with deionized water (3 x 50 mL) and ethanol (3 x 50 mL) and dried under
vacuum (Scheme 2).
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2.5. Synthesis of silica-coated magnetite nanoparticles (SMNP)
The synthesized Fe3O4 suspended in 35 mL ethanol and 6 mL deionized water and sonicated
for 15 min then 1.5 mL of tetraethyl orthosilicate (TEOS) was added slowly and again
sonicated for 10 min. Then aqueous ammonia (10%, 1.4 mL) was added slowly under stirrer
o
and mixture was heated at 40 C for 12 h. The iron oxide nanoparticles with a thin layer of
silica (Fe3O4@SiO2) were separated by an external magnet and washed three times with
ethanol and dried under vacuum.
2.6. Preparation of Fe3O4@SiO2-Py
To a solution of 1-(pyridin-3-yl)-N-(3-(triethoxysilyl)propyl)methanimine (2.5 g) in toluene
(20 mL), Fe3O4@SiO2 (10 g) was added lot-wise. The resulting mixture was refluxed for 24 h
under N2 atmosphere. Reaction mixture was filtered; residue was washed with toluene (20
mL x 1) and dichloromethane (20 mL x 1). Then the residue was dried in oven for 24 h to
afford off white powder (4.0 g) of 1-(pyridin-3-yl)-N-(3-(triethoxysilyl)propyl)methanimine
magnetic
silica.
Further
a
mixture
of
1-(pyridin-3-yl)-N-(3-
(triethoxysilyl)propyl)methanimine magnetic silica (4.0 g) and methanol (25 mL) was cooled
at 0 oC and NaBH4 (0.69 g) was added. The resultant mixture was stirred at room temperature
for 24 h then evaporated to afford off white powder (3.6 g) of Fe3O4@SiO2-Py (surface are of
138 m2/g).
2.7. Catalyst Test
A mixture of furfural (0.192 g, 2 mmol), acetylacetone (0.2 g, 2 mmol) and Fe3O4@SiO2-Py
o
(0.1 g) was stirred at 100 C for 4 h after which the catalyst was separated by an external
magnet and the resultant reaction mixture was diluted with ethyl acetate (20 mL x 1) and
subsequently washed with water (20 mL x 1) and brine solution (20 mL x 1). Then reaction
mixture was dried over sodium sulfate and evaporated under reduced pressure. The obtained
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crude oil was purified by column chromatography using ethyl acetate: petroleum ether (2: 98;
v/v) eluent to afford yellowish brown liquid. After reaction catalyst was hold with external
magnet and reaction mass was decanted and the catalyst was subsequently washed with ethyl
acetate and reused for next run.
Scheme 2. Synthesis routes for 3-Aminopropylsilica, Fe3O4@SiO2-Py and SiO2-Py and
Fe3O4@SiO2
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3. Results and discussion
3.1. Catalyst characterisation
3.1.1. FTIR
Structures of organosilica catalysts were confirmed by FTIR analysis (Figure 1). In the
Fe3O4@SiO2-Py catalyst, intense peaks between 580 cm-1 and 630 cm-1 were attributed to the
stretching vibration mode associated with metal-oxygen (Fe-O) bonds in the crystalline
lattice of Fe3O4. A band at 1647 cm-1 was attributed to –C=N- of pyridine moiety and band at
3375 cm-1 was for hydroxyl functionality. FTIR pattern of Fe3O4@SiO2-Py and SiO2-Py was
almost similar except that Fe3O4 pattern. Aminopropylsilica showed bands at 1020 cm-1 and
1472 cm-1 for Si-O-Si and –NH2 functionality, respectively.
Figure 1. FTIR of 3-Aminopropylsilica, Py-SiO2 and Fe3O4@SiO2-Py catalysts
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3.1.2. Solid state NMR
The structure of SiO2-Py catalyst was confirmed by solid state 13C NMR (Figure S3) in
which, three well resolved lines at 50.38, 22.23, and 10.53 ppm were observed and assigned
to –CH2- groups as d, c, b and a, respectively. The bands at 125, 136.13, 138.24 and 148.56
ppm are assigned to –CH- groups of pyridine with carbon atoms numbered e, f, g and h,
respectively.
3.1.3. XRD
To confirm the presence of Fe3O4 phases, XRD pattern of Fe3O4@SiO2-Py was compared
with the standard Fe3O4 (Figure S2). A series of characteristic peaks such as (2 2 0), (3 1 1),
(4 0 0), (4 2 2), (5 1 1) and (4 4 0) were observed in the Fe3O4@SiO2-Py sample which are in
well accordance with the inverse cubic spinel phase of Fe3O4 (JCPDS 85-1436).
3.1.3. TEM
Morphology and particles size of Fe3O4@SiO2, Fe3O4@SiO2-Py and Fe3O4@SiO2-Py(reused)
samples was investigated by using TEM analysis (Figure S1). The results indicate that
Fe3O4@SiO2, Fe3O4@SiO2-Py and Fe3O4@SiO2-Py(reused) have almost a hexagonal shape
with an average particle size of 36, 40 and 41 nm, respectively. Figure S1 clearly showed that
nanoclusters of Fe3O4 were successfully coated with a thin layer of SiO2 leading to core/shell
structure of Fe3O4@SiO2 NPs. Morphology and particle size of Fe3O4@SiO2-Py sample has
been retained even after its reuse which proves stability of the catalyst (Figure S1).
3.1.2. Catalyst screening for Knoevenagel condensation of furfural with acetylacetone
The Knoevenagel condensation reaction is usually facilitated by using basic catalysts. In this
work, several basic catalysts were tested for the Knoevenagel condensation of furfural with
acetylacetone (Table 1). Initially, furfural was treated with acetylacetone in presence of
o
NaOH at 80 C for 4 h to give the Knoevenagel product (FMP) in 78 % yield with 95 %
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conversion of furfural (Table 1, entry 1). Indeed, use of homogeneous base catalyst is
unattractive due to downsides such as non-reusable, hazardous waste generation and it also
need to be neutralized with acids which ultimately increases the process operations.
Therefore, easily recoverable and environmentally compatible solid bases are usually most
preferred. In this direction, we performed Knoevenagel condensation over series of solid
bases. For instance, with hydrotalcite (Mg/Al = 3) at 100 oC, FMP was formed in 31 % yield
with only 56 % conversion of furfural (Table 1, entry 2). In presence of CaO, FMP was
produced in 41 % yield with 67 % conversion of furfural (Table 1, entry 3). With ZnO-400
catalyst, FMP obtained was in 29 % yield with lower conversion (41%) of furfural (Table 1,
entry 4). Although, the basic metal oxides gave 100 % selectivity for FMP but the yield was
low to moderate due to lower conversion of furfural. Generally, the Knoevenagel
condensation is more prone over amine containing organic bases. In presence of catalytic
amount of triethylamine, 79 % yield of FMP was achieved with 89 % conversion of furfural
(Table 1, entry 5). Biomass derived furfurylamine as a catalyst showed similar activity as of
triethylamine (Table 1, entry 6). Further, with N,N-dimethylaminopyridine (DMAP) the FMP
was obtained in 77 % yield (Table 1, entry 7). In presence of 1,8-Diazabicyclo[5.4.0]undec-7-
ene (DBU) catalyst, the FMP was formed in 78 % yield (Table 1, entry 8). With 4-
hydroxypyridine, furfural was consumed upto 95 % and 83 % yield of FMP was achieved
(Table 1, entry 9). Amine functionalized homogeneous bases gave good yield and 99 %
selectivity to the FMP with almost 90 % conversion of furfural. Homogeneous bases are not
only non-recyclable but they are soluble in organic solvent hence, they could contaminate the
product. However, these catalysts can be heterogenized by anchoring amine functionalized
organic amines on silica. For instance, 3-Aminopropylsilica was used for this reaction
however a moderate yield (76 %) of FMP with 99 % selectivity was noticed with almost 90
% conversion of furfural (Table 1, entry 10). Pyridine anchored on silica (SiO2-Py) showed
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better catalytic performance than 3-Aminopropylsilica. With SiO2-Py catalyst, a 94%
conversion of furfural was achieved to obtain 85% yield of FMP (Table 1, entry 11). In case
of SiO2-Py catalyst, pyridine moiety is planar and the lone pair of electrons are present
perpendicular to the plane of pyridine. Hence, hydrogen from the active methylene group of
acetylacetone can be easily approached. On the contrary, in 3-Aminopropylsilica, catalyst
amine functionality is distorted tetrahedral structure which creates some hinderance for the
reactant as compared to pyridine. Due to these reasons SiO2-Py showed better activity than 3-
Aminopropylsilica. Fe3O4@SiO2-Py catalyst showed similar activity to that of SiO2-Py
(Table 1, entry 12). Use of magnetic catalyst (Fe3O4@SiO2-Py) is advantageous over SiO2-Py
because, the former can be easily separated with help of external magnate. While, in case of
SiO2-Py catalyst recovery is done by filtration which is associated with handling losses.
Table 1. Catalyst screening for Knoevenagel condensation of furfural with
acetylacetone [a]
Entry Catalyst
Loading
T
Conv.[b] Yield[b] Sel.[b]
[oC] [%]
80 95
[%]
78
31
41
29
79
80
77
78
83
76
85
85
[%]
92
100
100
100
99
99
98
99
99
1
NaOH
10 mol%
0.1 g
0.1 g
2
3
Hydrotalcite
CaO
100 56
100 67
100 41
100 89
100 90
100 85
100 87
100 95
100 81
100 94
100 94
4
ZnO-400
0.1 g
5
6
7
Triethylamine
Furfurylamine
DMAP
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
0.1 g
8
DBU
9
4-Hydroxypyridine
3-Aminopropylsilica
SiO2-Py
10
11
12
99
100
100
0.1 g
0.1 g
Fe3O4@SiO2-Py
[a] Reaction condition: Furfural (0.196 g, 2 mmol), catalyst, acetylacetone (0.2
g, 2 mmol), 4 h. [b] Yields are determined using HPLC.
3.1.3. Effect of temperature
o
The yield of FMP was increased gradually with rise in temperature from 80-100 C and
o
o
reached maximum (85%) at 100 C. Further increase in temperature to 110 C, the yield of
FMP not improved (Figure 2).
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Figure 2. Influence of temperature. Reaction conditions: Furfural (0.192 g, 2 mmol), acetyl
acetone (0.2 g, 2 mmol), Fe3O4@SiO2-Py (0.1 g), 4 h.
3.1.4. Effect of time
Up to 85% furfural conversion with 78% yield of FMP was observed within just 1.5 h. After
that conversion of furfural and yield of FMP was slowly increased. At the end of 4th hour
maximum conversion of furfural and yield of FMP was achieved. Extending reaction beyond
4 h did not show any improvement in reaction progress (Figure 3).
Figure 3. Influence of time. Reaction conditions: Furfural (0.192 g, 2 mmol), acetylacetone
(0.2 g, 2 mmol), Fe3O4@SiO2-Py (0.1 g), 0-5 h.
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3.1.5. Effect of catalyst amount
Influence of catalyst amount in the range of 0.05-0.150 g was investigated for this reaction
(Figure 4). With 0.05 g of Fe3O4@SiO2-Py the yield of FMP was only 61% and while,
increasing the amount to 0.075 g resulted in enhancement of the yield of FMP (79%). The
lower catalyst amount gave lower yield due to insufficient active site. However, with catalyst
amount 0.15 g the yield of FMP was comparable with that of 0.1 g.
Figure 4. Influence of catalyst amount. Reaction conditions: Furfural (0.192 g, 2 mmol),
acetylacetone (0.2 g, 2 mmol), Fe3O4@SiO2-Py (0.05 - 0.15 g), 4 h.
3.1.6. Recycle study
The stability of Fe3O4@SiO2-Py was evaluated by performing recycle runs (Figure S6). It was
found that the catalytic activity of Fe3O4@SiO2-Py was very much consistent after being used
for six times.
4. Conclusions
We presented here a simple and robust Fe3O4@SiO2-Py catalyst system for the Knoevenagel
condensation of furfural with acetyl acetone to achieve high yield of FMP. The condensation
product FMP is a unit of C10 and could be converted into branched alkanes (Jet fuel) through
hydrodeoxygenation process. The stability of the Fe3O4@SiO2-Py is confirmed by its several
reuses.
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Acknowledgements
S.H.S is grateful to CSIR-National Chemical Laboratory, AcSIR and CSIR.
References
[1] J. C. Serrano-Ruiz, J. A. Dumesic, Energy Environ. Sci. 4 (2011) 83−99
[2] J.-P. Lange, E. van der Heide, J. van Buijtenen, R. Price, ChemSusChem, 5 (2012)
150–166.
[3] W. Dabelstein, A. Reglitzky, A. Schutze, K. Reders, Automotive Fuels. Ullmann’s
Encyclopedia of Industrial Chemistry; Wiley-VCH: Weinheim, 2000.
[4] G. Liang, A. Wang, X. Zhao, N. Lei and T. Zhang, Green Chem., 18 (2016) 3430–
3438
[5] J. N. Appaturi, M. Selvaraj, S. B. A. Hamid, Microporous and Mesoporous Materials,
260 (2018) 260-269
[6] A. V. Subrahmanyam, S. Thayumanavan, G. W. Huber, ChemSusChem 3 (2010)
1158−1161.
[7] C. M. Nicklaus, A. J. Minnaard, B. L. Feringa, J. G. de Vries, ChemSusChem 6
(2013) 1631−1635.
[8] A. Corma, O. de la Torre, M. Renz, N. Villandier, Angew. Chem., Int. Ed. 50 (2011)
2375−2378.
[9] M. Balakrishnan, E. R. Sacia, A. T. Bell, ChemSusChem 7 (2014) 1078−1085.
[10]S. H. Shinde, C. V. Rode, Green Chem 19 (2017) 4804−4810
[11]G. W. Huber, J. N. Chheda, C. J. Barrett, J. A. Dumesic, Science 308 (2005)
1446−1450.
[12]K. S. Arias, M. J. Climent, A. Corma, S. Iborra, Energy Environ. Sci. 8 (2015)
317−331.
13

ACCEPTED MANUSCRIPT
[13]P. Oliveira, A. Machado, A. M. Ramos, I. Fonseca, F. M. B. Fernandes, A. M. B. d.
Rego, J. Vital, Microporous Mesoporous Mater. 120 (2009) 432-440.
[14]Y. L. Zhang, S. Liu, S. Y. Liu, F. J. Liu, H. Y. Zhang, Y. Y. He, F. S. Xiao, Catal.
Commun. 12 (2011) 1212–1217.
[15]D. Z. Xu, Y. Liu, S. Shi, Y. M. Wang, Green Chem. 12 (2010) 514–517.
[16]J. Mondal, A. Modak, A. Bhaumik, J. Mol. Catal. A: Chem. 335 (2011) 236–241.
[17]K. M. Parida, D. Rath, J. Mol. Catal. A: Chem. 310 (2009) 93–100.
[18]A. Modak, J. Mondal, A. Bhaumik, Appl. Catal. A: General 459 (2013) 41–51.
[19]O. C. Navarro, A. Corma, and S. I. Chornet, Top Catal. 52 (2009) 304–314.
14

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Highlights
 Solid base catalyst prepared by anchoring pyridine on magnetic silica
 Fe3O4@SiO2-Py catalyzed Knoevenagel condensation of furfural with acetyl acetone
 Structural characterization of Fe3O4@SiO2-Py confirmed by NMR and FTIR
 The catalyst successfully recycled for six consecutive runs
15