Water-promoted surface basicity in FeO(OH) for the synthesis of pseudoionones (PS) and their analogues

Article abstract of DOI:10.1016/j.jcat.2019.08.026

Use of Iron oxyhydroxide (γ-FeO(OH)) as a robust catalyst for the synthesis of important intermediates like pseudoionones and their analogues through the C-C bond formation reactions like knoevenagel and aldol condensation is explored. These motifs are the building blocks for the construction of the sesquiterpenes as well as the diterpenes such as retinoic acid, Vitamin A etc. Iron oxyhydroxide (γ-FeO(OH)) was synthesized and well characterized using XRD, FT-IR, TEM, XPS and adsorption studies to establish the catalytic activity. A thorough investigation on the nature of basic sites and the role of water as a promoter was explored based on dye adsorption, in situ methanol dissociation and CO₂ adsorption studies. The catalyst also showed a wide range of substrate scope with active methylene groups involving various functional groups such as cyanides, esters and acetophenones along with its stability and reproducibility.

Full text of DOI:10.1016/j.jcat.2019.08.026

Journal of Catalysis 378 (2019) 80–89
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Water-promoted surface basicity in FeO(OH) for the synthesis of
pseudoionones (PS) and their analogues
d,
Dnyanesh Vernekar a,b, Sachin S. Sakate a,c, Chandrashekhar V. Rode a,b, , Dinesh Jagadeesan
⇑
⇑
a Chemical Engineering and Process Development Division, CSIR-National Chemical Laboratory, Pashan, Pune 411008, India
b Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India
c P. E. Society’s Modern College of Arts, Science and Commerce (Autonomous), Shivajinagar, Pune 411005, India
d Department of Chemistry, Indian Institute of Technology Palakkad, Palakkad 678 557, Kerala, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Use of Iron oxyhydroxide (c-FeO(OH)) as a robust catalyst for the synthesis of important intermediates
like pseudoionones and their analogues through the C-C bond formation reactions like knoevenagel
and aldol condensation is explored. These motifs are the building blocks for the construction of the
sesquiterpenes as well as the diterpenes such as retinoic acid, Vitamin A etc. Iron oxyhydroxide (c-FeO
Received 8 March 2019
Revised 17 August 2019
Accepted 19 August 2019
(OH)) was synthesized and well characterized using XRD, FT-IR, TEM, XPS and adsorption studies to
establish the catalytic activity. A thorough investigation on the nature of basic sites and the role of water
as a promoter was explored based on dye adsorption, in situ methanol dissociation and CO2 adsorption
studies. The catalyst also showed a wide range of substrate scope with active methylene groups involving
various functional groups such as cyanides, esters and acetophenones along with its stability and
reproducibility.
Keywords:
Pseudoionones
Iron (III) oxyhydroxide
Knoevenagel condensation
Brønsted basicity
Ó 2019 Elsevier Inc. All rights reserved.
Surface hydroxyl groups
1. Introduction
from 60% to >90% as compared to the catalysts with just O2– sites.
Unfortunately, these intercalated OH– ions have a tendency to
Pseudoionones (PS) are special monoterpenoids used in the
synthesis of industrially important chemical intermediates such
leach away into the solvent due to their non-covalent bonding,
leading to a severe loss in the activity. Taking a cue from this work,
we wanted to explore the use of Iron oxyhydroxides FeO(OH) as a
catalyst for the synthesis of PS. FeO(OH) are metastable intermedi-
ates during the synthesis of iron oxides from its hydroxide gel pre-
cursors. It naturally contains abundant concentration of potential
Brønsted basic sites such as AOꢁ and Lewis basic sites such as
O2– in the crystal structure (Scheme 2a). [13] Most importantly,
the basic sites (Oꢁ & O2–) are chemically bonded to the metal
and occur in close proximity to O2– sites of FeO(OH) crystal. The
structure is therefore suitable for a high and stable activity in the
synthesis of PS [14]. For the first time, we report the use of FeO
(OH) as stable and highly active catalyst for the synthesis of PS.
In addition to that, we also provide an overwhelming evidence that
H2O promotes the catalytic activity of FeO(OH).
as
a-, b- and c-ionones [1]. PS are synthesized in large scale by
aldol condensation between citral and acetone in the presence of
a base (Scheme 1) [2]. The product yields vary between 50 and
80% depending upon the reaction conditions and the catalysts
used. Heterogeneous base catalysts such as alkaline metal oxides
[3], basic zeolites [4], and Mg-Al hydrotalcites [5] are preferred
over homogeneous bases due to non-toxicity, non-corrosion, min-
imal waste and recyclability. Recently, LiOHꢀH2O [6], aluminophos-
phates (ALPO) [7], anionic clays [8], choline hydroxide/MgO [9],
KF-Al2O3 [10] and Mg-Al mixed oxides containing rare earth ele-
ments [11] have been shown to catalyze the synthesis of PS. Among
various catalysts, hydrotalcites are quite interesting as the studies
on the reconstructed hydrotalcites (Mg6Al2(OH)18ꢀ4H2O) showed
that a combination of O2– (Lewis basic) and intercalated OH–
anions (Brønsted base) provide a high activity [12]. The presence
of charge-compensating Brønsted base (OH– ions) in the Mg-Al
hydrotalcite matrix was shown to increase the selectivity of PS
2. Experimental section
2.1. Materials & methods
Ferrous chloride (FeCl2ꢀ4H2O) and Sodium hydroxide (NaOH)
were purchased from Thomas-Baker. Citral and other active
methylene substrates were obtained from Sigma-Aldrich.
⇑
Corresponding authors.
E-mail addresses: cv.rode@ncl.res.in (C.V. Rode), d.jagadeesan@iitpkd.ac.in
(D. Jagadeesan).
https://doi.org/10.1016/j.jcat.2019.08.026
0021-9517/Ó 2019 Elsevier Inc. All rights reserved.

D. Vernekar et al. / Journal of Catalysis 378 (2019) 80–89
81
Scheme 1. Flowchart showing the synthesis of ionones from citral via pseudoionones [15]
AlPO, Al2O3) were prepared using standard preparation methods
from literature.
2.1.2. General experimental procedure for the synthesis of PS
In a 50 mL round bottom flask citral (0.152 g, 1 mmol) and ace-
tone (0.116 g, 2 mmol) were taken along with specially dried 1,4-
dioxane (5 mL) as a solventꢀH2O (1 mL) was used as an additive,
the catalyst amount was fixed at 0.100 g. The round bottom flask
was connected to a water cooled Liebig condenser which was
tightly sealed at the other end to prevent any evaporation. The
reaction mixture was heated up to 100 °C (reflux) in an oil bath.
The reaction was monitored by using thin layer chromatography.
After the completion of the reaction, reaction mixture was filtered,
water removed using anhydrous MgSO4 and the mixture was ana-
lyzed by Gas Chromatograph equipped with Mass Detector and/or
isolated using column chromatography and confirmed by 1H NMR.
2.1.3. In situ CO2-IR & methanol dissociation studies
In situ CO2/MeOH adsorption/dissociation Infrared Spectroscopy
measurements were done using PerkinElmer 2000 Frontier FT-IR
equipped with Praying MantisTM assembly using MCT (HgCdTe)
detector. CO2/MeOH were injected into the port from which the
injected molecule was driven under nitrogen flow (50 mL/min)
and adsorbed on the sample. The samples were then analyzed
using the spectrum software and compared with the blank
measurements.
Scheme 2. (a) Crystal structure of c-FeO(OH). (b) The three coordination modes of
2.1.4. Fluorescence studies using Rhodamine 6G
OH on FeO(OH) surface.
In a 10 mL volumetric flask, 0.002 M Rhodamine 6G solution
was prepared in distilled water. 4 mL of this solution was taken
Analytical grade acetone was obtained from Merck. Methanol,
ethyl acetate, petroleum ether and silica gel (100–230–400 mesh)
were obtained from Chem Labs, India.
and 10 mg of
c-FeO(OH) was added and the resultant mixture
was stirred for 6 h at room temperature. The catalyst was then
removed and the resultant filtrate was subjected to fluorescence
measurements at the excitation wavelength of 500 nm and emis-
sion wavelength centered around 562 nm. The fluorescence emis-
sion spectra of Rh-6G were analyzed using PTI Quanta MasterTM
steady state spectrofluorometer. The resultant FeO(OH)-Rh-6G
complex was obtained by thoroughly drying over Buchi rotavap
and its FT-IR was recorded and compared with bare FeO(OH) and
Rh-6G solids. To quantify the amount of Rh-6G adsorbed in
dioxane-water mixtures, four different solutions constituting
various ratios of dioxane: water were prepared (10 mL). 20 mg of
Rh-6G was dissolved in each of these and 40 mg of FeO(OH) was
2.1.1. General experimental procedure for the preparation of
(OH)
c-FeO
4 g FeCl2ꢀ4H2O was dissolved in distilled water. To this 1 M
NaOH was added until the pH of the solution reached 6.5–6.8. After
reaching this pH, greenish black deposits were formed. Oxygen
was bubbled through this solution for 20 min to obtain orange
deposits i.e.
c-FeO(OH) which were separated by filtration and
washed with ethyl alcohol and distilled water several times and
dried at 60 °C. All the other control catalysts (CaO, MgO, CeO2,

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D. Vernekar et al. / Journal of Catalysis 378 (2019) 80–89
dispersed in 5 mL of the above prepared Rh-6G solutions and stir-
red at room temperature for 1 h. The fluorescence measurements
were done after removing FeO(OH) and immediately quantified
based on the calibration plots for the various solution mixtures.
an X-Ray Source- Al K
and 12 kV.
a (Monchromatic) with 6 mA beam current
2.1.7. Analysis of catalysis reaction products
The catalysis reaction was monitored and quantified using Shi-
madzu GC–MS QP 2010 Ultra using Restek FFAP/HP-5 columns
with the ion source temperature maintained at 210 °C. The injec-
tion and interface temperature were set at 220 °C. The carrier gas
used was Helium. The column ramping was 20 °C/min starting
from 30 °C till 210 °C with 40 min hold at the final temperature.
The catalysis reaction involving different substrates were moni-
tored on TLC (Thin Layer Chromatography) and the products were
isolated using column chromatography and confirmed by 1H NMR.
2.1.5. FeO(OH) activation studies, hot filtration, & recyclability test
c
-FeO(OH) activation studies were performed by taking a two
neck round bottom flask containing 100 mg FeO(OH), 5 mL 1,4-
dioxane and 1 mL water. One neck of the flask was attached to
the liebeg condenser and the other was sealed using a rubber sep-
tum. The reaction mixture was refluxed for 2 h. After 2 h of reflux,
acetone and citral were introduced in the flask simultaneously
using a syringe though the rubber septum. Aliquots were removed
hourly and analyzed using GC–MS. For Hot filtration test, the reac-
tion was carried out normally but in a two neck round bottom
flask. The reaction was stopped after 4 h and the catalyst was fil-
tered using a syringe filter and the supernatant liquid was further
allowed to reflux without any catalyst. Recycle studies were per-
formed by washing the filtered catalyst with 1,4-dioxane and
MeOH after every cycle (12 h reaction) and then dried to be used
for next cycle.
3. Results and discussion
3.1. Material characterization
The crystal structure of FeO(OH) sample was studied using X-
ray Diffraction pattern and was found to be consistent with the
reported pattern of
c-FeO(OH) (JCPDS card no 73-2326) [16]
(Fig. 1a). It has an orthorhombic structure with arrays of close-
packed anions (O2–/OH–) stacked along the [1 5 0] direction and
Fe3+ ions occupying the octahedral interstices (Scheme 2a). FT-IR
spectra (Fig. 1b) showed a broad absorption band in the range of
3350–3400 cmꢁ1 due to O-H stretching characteristic of the FeO
(OH) polymorph. Bands in the wave number regions of 620–
680 cmꢁ1 are due to Fe-O symmetric stretching. Characteristic
bands in 750–1000 cmꢁ1 are due to AOH bending and stretching
vibrations [17].
2.1.6. Material characterization
X-ray diffraction measurements were made using Phillips PAN
analytical diffractometer with Cu-K
a radiation (k = 1.5406 Å).
Transmission electron microscopy (TEM) images were obtained
using FEI Tecnai-20ST electron microscope. Specimens of the sam-
ples were prepared by drop-casting well-dispersed isopropyl parti-
cle suspensions onto a carbon coated copper grid. The surface area
measurements were performed using Quantachrome Autosorb-iQ
gas sorption device. The binding energies of the elements on the
catalyst were analyzed by X-ray photoelectron spectroscopy
(Thermo Fisher Scientific Instruments UK, Model K ALPHA+) with
The surface composition of FeO(OH) was studied using X-ray
Photoelectron Spectroscopy (XPS). Deconvoluted O1s XPS spectra
(Fig. 1c) confirmed contributions due to three kinds of oxygen
Fig. 1. (a) XRD pattern of c-FeO(OH). (b) Infrared Spectra of c-FeO(OH). (c) Deconvoluted O1s XPS spectra. (d) Fe 2p XPS spectra of FeO(OH).

D. Vernekar et al. / Journal of Catalysis 378 (2019) 80–89
83
species a) oxygen involved in Fe-O-Fe (529.9 eV) b) the surface
3.2. Knoevenagel condensation
hydroxyl groups Fe-OH (531.3 eV) and c) chemisorbed and physi-
sorbed water (532.3 eV) [18,19]. The XPS signal of Fe2p (Fig. 1d)
showed two peaks due to Fe2p3/2 and Fe2p1/2 at 710.6 eV and
724.1 eV respectively. Weak satellite peaks attributed to Fe(III)
ions owing to the energy transfer processes of the excited core
electrons were observed around 718.0 eV due to Fe2p3/2 and at
PS was synthesized by condensation of citral with acetone (and
other active methylene compounds). To prevent citral from self-
condensation, the acetone and other active methylene compounds
were taken twice (2 mmol) as much as citral (1 mmol). The reac-
tion setup was sealed to prevent the formation of the oxidation
product nerolic acid. A blank control experiment that involved
732.8 eV due to Fe2p1/2. The structure of
probed using Thermogravimetric Analyzer (Fig. S1). The thermal
decomposition profile of -FeO(OH) indicated weight loss due to
dehydration in the low temperature region (150–250 °C) and
transformation of the phase to - Fe2O3 at higher temperatures
-FeO(OH) was
c-FeO(OH) was further
refluxing of all the reactants for 12 h in the absence of
c-FeO
c
(OH) catalyst (Table S2, entries 1, 2 and 3), did not show the forma-
tion of PS in solvents such as CH3OH, CH2Cl2 and 1,4-dioxane at
their reflux temperature respectively. Neither was any activity
a
(250 °C onwards) [20]. BET surface area of
c
observed even in the presence of
CH3OH and CH2Cl2 (Table S2, entries 4, 5 & 6). However, the pres-
ence of -FeO(OH) in 1,4- dioxane showed a marginal conversion of
25%. Interestingly, when 1 mL of H2O was added to 5 mL of 1,4-
dioxane solvent containing -FeO(OH), the conversion of the sub-
c-FeO(OH) in solvents such as
247 m2 gꢁ1 with a pore volume of 0.58 cc gꢁ1 and a pore radius
of 15.8 Å (Fig. S1b). The observed hysteresis showed typical type
IV characteristics which could be due to interparticle spaces.
c
Microstructure of
c-FeO(OH) was studied using TEM which
c
showed rod-shaped crystals of 250–500 nm in length and 20–
50 nm in diameter (Fig. 2a, b & c). Frequently, the rods occurred
in stacks as shown in the regions (I), (II) and (III) of Fig. 2a. In
Fig. 2b, a magnified image of stack of rods (I) can be seen. Fig. 2c
shows the high-resolution TEM image of a nanorod with clearly
visible lattice fringes. The spacing between the fringes 0.33, 0.26
and 0.16 Å correspond to the (1 2 0), (0 3 1) and (1 5 1) planes of
strate increased to 96% with 100% selectivity to PS. In the absence
of the catalyst, a mixture of dioxane-H2O (5:1) as solvent showed
only 12% conversion under identical reaction conditions
(Table S2, entry 9). Thus,
essential to achieve a high conversion of citral to PS.
We compared the reaction rates due to -FeO(OH) with other
c-FeO(OH) in the presence of H2O was
c
well-known solid bases both in the presence and the absence of
water (Fig. 3a). In the presence of conventional solid bases the
reaction rates remained below 0.012 mmol Lꢁ1hꢁ1 even when
water was used. In other words, H2O caused a marginal decrease
in the rates of the reactions when CaO, MgO, AlPO, Al2O3 or CeO2
c-FeO(OH) respectively. The electron diffraction pattern of an iso-
lated stack of FeO(OH) indicated the single crystalline nature of the
sample which could be indexed to the planes of
(Fig. 2d).
c-FeO(OH)
Fig. 2. (a) TEM image of c-FeO(OH) rods. (b) High magnification TEM image of (I), encircled in Fig. 2a c) High resolution TEM images of rods in region (I) & d) Electron
diffraction pattern.

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D. Vernekar et al. / Journal of Catalysis 378 (2019) 80–89
Fig. 4. Study of effect of H2O on the activation of c-FeO(OH) (i) in the absence of
water (ii) hot filtration test: flat line represents no further progress in reaction after
the removal of the catalyst (iii) in the presence of water (iv) reactants introduced
after 2 h of stirring FeO(OH) in dioxane-water mixture.
ingly, the conversion trends showed an improved spike with 60%
conversion after just 2 h of adding the reactants. On the other hand,
almost 7 h was needed to attain this conversion value, when the
reactants were present from the beginning i.e. 0 h. The observation
strongly suggests that H2O plays a crucial role in activating c-FeO
(OH) by deprotonating surface AOH to form AOꢁ. A surface con-
centration of Oꢁ groups builds up over a period of 2 h which makes
c-FeO(OH) a more efficient catalyst in the condensation reaction.
Water appeared to have no such promoting effect in the catalytic
activity of other solid bases indicating operation of a different nat-
ure of active site on c-FeO(OH).
3.3. Active sites
Fig 3. (a) Rate of the reaction for the various catalysts screened in the presence and
absence of water for the synthesis of the PS. Reaction Conditions: citral (0.152 g,
1 mmol), acetone (0.116 g, 2 mmol), specially dried 1,4-Dioxane (5 mL), H2O (1 mL),
time (12 h), reflux, sealed conditions. (b) Plot of TON Vs duration of reaction in the
3.3.1. Basicity in FeO(OH) and role of H2O
Our previous studies have shown that surface AOH groups on
metal oxyhdroxides have a strong role in the acid-base catalyzed
reactions [14,21]. To further probe the activity of surface hydroxyl
groups, we studied the changes in FT-IR absorption spectra of AOH
vibrations due to interactions with CO2 molecule. It has been
shown that interaction with CO2 can alter the O-H stretching fre-
quencies of FeO(OH) samples upto 150–175 cmꢁ1 [22]. The differ-
ence spectra (Trace iii in Fig. 5 a, b) was obtained by subtracting
presence of c-FeO(OH) with and without H2O.
were used as catalysts. Interestingly, in the presence of water, c-
FeO(OH) showed an impressive four-fold increase in the rate of
the reaction making it the most active catalyst. Turnover number
(TON) gives the number of reactant molecules converted per min-
the absorption bands of neat c-FeO(OH) sample from that of the
ute per catalytic active site present on 1 g of
c
-FeO(OH) under
CO2-adsorbed FeO(OH). The anti-symmetric and symmetric modes
of the CAO stretching region of carbonate & (bi) carbonate species
were formed on the surface of FeO(OH). Interaction of CO2 with
surface AOH and its deprotonated AOꢁ groups resulted in strong
bands at 1512 cmꢁ1 and 1314 cmꢁ1 corresponding to the above
modes respectively (Fig. 5b) [22]. This observation confirmed the
basicity of the Oꢁ sites on the surface. Further, FeO(OH) surface
may have three kinds of hydroxo groups 1) singly („FeOH, AOH)
given the reaction conditions. The number of sites was calculated
based on the concentration of surface AOH groups on FeO(OH)
using O1s XPS spectra. The change in the TON of the reaction in
the presence of
pared with the reactions carried out in the absence of water. At
the end of 1 h, TON of
-FeO(OH) gave a value of just 15 mmol gꢁ1
c-FeO(OH) at the end of 1, 4 and 6 h were com-
c
in the presence of H2O (Fig. 3b). The TON of the reaction increased
with time to 161 and 258 mmol gꢁ1 after 4 h and 6 h respectively.
Interestingly, at the end of 4 h the increase in TON was 10-fold
when the reaction was carried out in the presence of H2O as com-
pared to 2-fold in the absence of water. This indicated that active
2) doubly („Fe2OH,
l-OH) and 3) triply coordinated („Fe3OH,
l3-OH) (Scheme 2) [23]. From the plot in Fig. 5a, one can infer
the nature of the surface AOH groups depending on the site at
which the carbonate & (bi) carbonate species are coordinated. At
sites were generated on the surface of the
c-FeO(OH) by H2O dur-
a triply coordinated site („Fe3OH,
l3-OH) and at doubly coordi-
ing the course of the reaction. However, generation of the active
sites gradually flattened off after 4 h. A time-dependent plot of cat-
alyst performance in the presence and absence of water is pre-
sented in Fig. 4 (trace i & iii). The dotted line shows the
conversion trend in an experiment when the catalyst was filtered
off under hot conditions after 6 h of reaction (Fig. 4, trace ii).
Absence of further conversion indicated that there was no leaching
of ions from the catalyst making a homogeneous contribution to
catalysis under the reaction conditions.
nated site („Fe2OH, -OH), the absorption bands appear at
l
3597 cmꢁ1 and 3627 cmꢁ1 respectively [22,23]. The single coordi-
nated („FeOH, AOH) species which usually shows bands above
3650 cmꢁ1 was reasonably intense in our sample suggesting its
presence in large number as compared to species with other coor-
dination modes. Due to the difference in the coordination number
of the underlying Fe atoms, the surface hydroxyl groups that are
bonded to it exhibits difference in basicity and hence difference
in reactivity. Past literature reports based on adsorption and charg-
ing studies indicate double coordinated hydroxyl groups to be
inert, whereas single and triple coordinated species are reactive
with highest reactivity for single coordinated species in proton
The role of water as an activating agent was studied by allowing
FeO(OH) to stir in dioxane-water mixture for 2 h at reflux temper-
ature and then introducing the reactants (Fig. 4, trace iv). Interest-

D. Vernekar et al. / Journal of Catalysis 378 (2019) 80–89
85
Fig 5. Infrared Spectra of (i) neat FeO(OH) (ii) CO2 adsorbed on FeO(OH) and (iii) their difference spectra (a) changes in the hydroxyl group stretch (b) changes in the CAO
stretch.
adsorption measured by pressure jump relaxation methods
[24,25]. The presence of all three types of surface AOH groups
Since the pKa of water (pKa = 14) is higher than the protons on
FeO(OH) (pKa > 10), it can deprotonate FeO(OH) to give FeOOꢁ
according to Eq. (1) [17]. The Brønsted basicity was investigated
by means of methanol deprotonation followed by in situ Infrared
Spectroscopy (Fig. 6) Earlier studies on hydroxylated MgO show
that MeOH interacts with the surface basic sites forming monoden-
tate methoxy and bidentate methoxy species at 1115 cmꢁ1 and
1092 cmꢁ1, whereas molecularly adsorbed methanol involving H-
bonding gives an absorption band at 1064 cmꢁ1. [26] The various
interactions with FeO(OH) surface can be visualized in Fig. 6c. In
explains the catalytic activity of
c-FeOOH. For example, studies
on (0 0 1) plane of -FeO(OH) had shown equal numbers of single
c
and triply coordinated hydroxyls whereas, (0 1 0) plane contained
doubly coordinated surface hydroxyls exclusively [17]. Thus, the
deprotonated surface hydroxyl groups show exceedingly high con-
version as compared to other metal oxide catalysts.
FeOðOHÞ þ H2OꢀFeOðOꢁÞ þ H3Oþ
ð1Þ
Fig 6. In situ FT-IR spectra obtained by subtracting MeOH adsorbed FeO(OH) and neat FeO(OH) spectra (a) in the absence of water (b) in the presence of water. Trace (i) (ii) &
(iii) denote scans taken after 5 min, 15 min & 25 min respectively. (c) Vibration modes of MeOH dissociation on FeO(OH) surface: Molecularly adsorbed species (1064 cmꢁ1),
Bidentate methoxy species (1092 cmꢁ1) and Monodentate methoxy species (1115 cmꢁ1). (d) Relative MeOH dissociation on FeO(OH) in the presence and absence of water w.
r.t molecularly adsorbed MeOH.

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D. Vernekar et al. / Journal of Catalysis 378 (2019) 80–89
Fig. 6a & b, we observe the bands centered at 1064 cmꢁ1 and
1115 cmꢁ1 indicating molecularly adsorbed MeOH and the forma-
tion of monodentate methoxy species on FeO(OH) surface. The for-
mation of monodentate methoxy species on FeO(OH) is a clear
indication of the presence of basic sites Oꢁ [26,27]. Incidentally,
these bands were more intense in the absence of water (Fig. 6a).
The competitive interaction between MeOH and H2O with surface
hydroxyl groups is probably responsible for the phenomenon. Band
at 1092 cmꢁ1 due to bidentate methoxy species was not predomi-
nant except at the initial 5 min scan in the absence of water Fig. 6a
(i). However, this band disappeared with time because of weak
bidentate methoxy interaction with the surface. To quantify the
effect of water on basicity, the relative ratio of peak intensities of
monodentate methoxy and molecularly adsorbed MeOH in the
absence and presence of water were calculated (Fig. 6d). The aver-
age value gave the measure of methanol dissociation on FeO(OH).
We invariably observed that the value was higher (0.85) for the
system in the presence of H2O than in the absence of H2O (0.45).
This shows that H2O helps to increase the number of monodentate
methoxy species by increasing the Oꢁ species on the FeO(OH)
surface.
To support the conclusion that H2O promotes the in situ forma-
tion of FeOOꢁ, we carried out the adsorption of a positively charged
dye Rhodamine 6G (Rh-6G) Fig. 7a. This dye is known to adsorb on
a negatively charged surfaces e.g. on the deprotonated OH on gra-
phene which can be monitored using florescence spectroscopy
[28]. The details are presented in the experimental section.
The „FeOOꢁ species is a negatively charged surface and hence
has an affinity to adsorb a positively charged Rh-6G (Fig. 7c). The
supernatant solution in FeO(OH)-Rh-6G after removing the FeO
(OH) from the solution showed a marked lower florescence as
compared to the standard solution. This indicates that the surface
of FeO(OH) has negative charge due to the presence of FeOOꢁ
(Fig. 7b) and hence the concentration of Rh-6G in the solution
decreased as seen by the lower fluorescence (red trace, Fig. 7c).
Fig. 7e show a comparative plot of FeO(OH) and FeO(OH)-Rh-6G
complex. One noticeable change is the shift in OH stretching fre-
quency of FeO(OH)-Rh-6G complex from 3134 cmꢁ1 in pure FeO
(OH) to 3172 cmꢁ1. This red shift can be attributed to the interac-
tion of the deprotonated surface hydroxyl groups with the posi-
tively charged nitrogen of Rh-6G similar to that, reported for
graphene oxide-Rh-6G complex [28]. A comparision of FT-IR bands
Fig 7. (a) Structure of Rhodamine ꢁ6G. (b) Interaction between FeO(OH) and Rh-6G. (c) Fluorescence spectrum for Rh-6G and FeO(OH)-Rh-6G (Excitation-500 nm) in water.
(d) Quantification of amount of dye adsorbed on FeO(OH) in various dioxane: water mixtures. (e) FTIR spectra for FeO(OH) and FeO(OH)-Rh-6G complex. (f) FT-IR spectra for
Rh-6G and FeO(OH)-Rh-6G complex.

D. Vernekar et al. / Journal of Catalysis 378 (2019) 80–89
87
Table 1
in the fingerprint region of Rh-6G and FeO(OH)-Rh-6G is shown in
Fig. 7f. The bands of Rh-6G showed a slight blue shift in the FeO
(OH)-Rh-6G complex further confirming strong interaction
Condensation reactions of Citral with active methylene compounds using
as catalyst.
c
-FeO(OH)
a
between the positively charged nitrogen of the dye and the nega-
tively charged FeO(Oꢁ) species as shown in Fig. 7b. The dye adsorp-
tion on FeO(OH) was also studied using a similar fluorescence
experiment in different dioxane-water compositions matching
the solvent conditions. The dye concentration in the supernatant
decreased from 0.03 mmol to 0.01 mmol as water content
increased (Fig. S3) indicating that Rh-6G adsorbs more on FeO
(OH) in excess water conditions due to formation of FeO(Oꢁ). The
amount of Rh-6G adsorbed on FeO(OH) in the dioxane water mix-
tures was quantified and the results are shown in Fig. 7d. The Rh-
6G adsorption values showed an increasing trend with increase in
water content. For example, 0.29 mmol of Rh-6G was adsorbed per
g of FeO(OH) at 4.5:0.5 dioxane-water ratio. This value reached
0.76 mmol/g of FeO(OH) at 1:1 dioxane-water ratio. Thus, it is con-
firmed that surface AOH group gets deprotonated in the presence
of H2O to form the Oꢁ sites responsible for adsorption of Rh-6G.
In order to verify if externally added H2O promoted the catalytic
activity in FeO(OH) by introducing additional surface AOH groups,
the FeO(OH) sample was stirred in water for 6 and 12 h. After the
stipulated time the solid was filtered and dried. Previous studies on
interaction of FeO(OH) with salt water had shown the formation of
Fe(OH)3 and [Fe(OH)]+2 on FeO(OH) surface [29]. IR transmittance
levels of the dried samples between 3670 cmꢁ1 and 2458 cmꢁ1
corresponding to the surface hydroxyl groups and physisorbed
water were studied. Interestingly, these bands remained consistent
with that of bare sample untreated with H2O (Fig. S4). Thus, H2O
does not promote activity in FeO(OH) by increasing the number
of surface hydroxyl groups on FeO(OH) within the duration of
the reaction.
Entry Reactant b
Product c
Isolated Yield of Product c (%)
84
1
2
3
4
72
68
74
5
6
82
74
7
8
70
75
Reaction conditions: citral (0.152 g, 1 mmol), Reactant b (2 mmol), 1,4-
Dioxane (5 mL), H2O (1 mL), time (12 h), reflux 1,4-Dioxane was dried over
molecular sieves and then used. Products isolated by column chromatography
and confirmed by 1H NMR.
3.4. Substrate scope
To understand the efficiency of the activity of
c-FeO(OH), we
tested it for the Knoevenagel condensation reactions of various
active methylene group compounds to synthesize PS, their ana-
logues and other products. All the reactions were carried out using
specially dried 1,4-dioxane as a solvent with 1 mL of externally
added water. The reaction of acetone with the citral (Table 1, entry
1) gave the aldol condensation product with 84% isolated yield in
12 h. Encouraged by this, we used substituted acetophenones with
various functional groups exerting -I and -R effects to condense
with citral. It was observed that the para substituted -Cl group in
4-chloroacetophenone on reaction with citral gave an isolated
yield of 72% due to higher -I as well as -R effect (Table 1, entry
2). In comparison, 3-bromoacetophenone gave a 68% yield (Table 1,
entry 3) of the condensation product. The activating ANO2 group
in 4-nitro acetophenone gave 74% yield of condensation product
(Table 1, entry 4). Further extension of the catalyst activity testing
was carried out to study Knoevenagel condensation reactions with
compounds having active methylene groups. Firstly, malononitrile
on reaction with citral yields 82% condensation product in (Table 1,
entry 5). Ethylcyanoacetate on reaction with citral gave the con-
densation product in 74% yield (Table 1, entry 6). Similarly, the
Knoevenagel condensation reaction of citral was further studied
for nitroethane and nitromethane having active methylene groups
which yields up to 70% and 75% condensation product respectively
(Table 1, entry 7& 8) under similar reaction conditions.
Fig. 8. Recyclability of c-FeO(OH) for PS synthesis.
dioxane and dried at 100 °C and then used for eight subsequent
runs. Fig. 8 shows that the citral conversion was stable (96–92%)
even after five recycles with minor losses. The selectivity of PS
dropped down from 100 to 83% after 8th recycle due to competi-
tive formation of Nerolic acid. Also, the X-ray diffraction pattern
of the catalyst appeared stable after 8th recycle under the reaction
conditions with minor changes (Fig. S5).
3.5. Catalyst recycle study
In a plausible mechanism shown in Scheme 3, the first step
involves the deprotonation of the surface hydroxyl groups of FeO
(OH) to give FeOOꢁ using water. These FeOOꢁ species act as base
centers to abstract the proton from active methylene compounds
The efficiency of c-FeO(OH) catalyst was established by its recy-
cle studies (Fig. 8). After completion of the first run, the catalyst
was separated by filtration, washed several times with 1,4-

88
D. Vernekar et al. / Journal of Catalysis 378 (2019) 80–89
Scheme 3. Plausible reaction mechanism for the synthesis of the Pseudoionones and their analogues.
(Step 2), thus paving way for the attack of the formed carbanion to
the carbonyl group of citral (Step 3). Loss of water molecule
assisted by FeOOꢁ finally yields the pseudoionone (Step 5).
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