M.S. Nunes et al.
Inorganica Chimica Acta xxx (xxxx) xxx
dimensional (2D) supramolecular layers or 3D supramolecular networks
via intermolecular hydrogen bonding interactions in conjunction with
solution of Na2SiO3⋅5H2O (8.30 mL, 1.70 mmol) was added dropwise,
and the initially colorless solution turned yellow. The reaction mixture
other weak contacts such as
π
-
π
stacking interactions. Concerning the
was then left to stir at 80 ◦C for 30 min to enable isomerization (β →
α) of
possible applications of these class I hybrid POM salts, the electro-
chemical [9,14,16,17] and (photo)catalytic [10,14,16,17,20,21,23]
the Keggin-type silicododecamolybdate anion. Subsequently, ptz (0.91
g, 6.20 mmol) was added to the mixture, leading to the gradual pre-
cipitation of a yellow solid. The solid was filtered, washed with deion-
ized water (9 × 10 mL) and diethyl ether (5 × 10 mL), and finally
vacuum-dried. Green crystals of 1 with suitable quality for single crystal
X-ray determination were grown from the aqueous mother liquor.
Combined yield: 1.37 g, 33% (based on Na2MoO4⋅2H2O). Anal. Calcd for
C24H24N20Mo12O40Si⋅6H2O (2520.02): C, 11.44; H, 1.44; N, 11.12.
properties have received particular interest. Compounds containing the
4-
polyanion [SiMo12O40
]
were shown to be effective catalysts for the
oxidation of sulfides [20,23] and the epoxidation of alkenes [24,25]
using peroxide-type oxidants. The results with (Bu4N)4[SiMo12O40] and
[Zn(dipp)(L)]4[SiMo12O40] (L = 1-methyl-4,4′-bipyridinium, dipp =
2,6-di-isopropylphenylphosphate) for cyclohexene/H2O2 epoxidation
[24] and H4[SiMo12O40] for cis-cyclooctene/TBHP epoxidation [25]
(TBHP = tert-butylhydroperoxide) motivated us to study hybrid salts
containing the silicododecamolybdate anion for the valorization of
biomass-derived olefins to useful renewable products.
Found: C, 11.65; H, 1.43; N, 10.80%. FT-IR (KBr, cmꢀ 1):
ν = 3500 (br),
3085 (m), 1641 (sh), 1616 (s), 1534 (m), 1476 (m), 1461 (m), 1384 (vs),
1350 (m), 1292 (m), 1245 (m), 1169 (m), 1145 (w), 1037 (m), 1012
(sh), 948 (vs), 900 (vs, br), 866 (sh), 788 (vs, br), 744 (vs), 531 (m), 500
The use of biomass derived raw materials to produce marketable bio-
based products, which may complement or substitute fossil fuel-derived
petrochemicals, is in line with the circular bioeconomy concept, being
crucial forthe development ofmore sustainablechemical processes[26].
Methyl oleate (Ole) and methyl linoleate (LinOle) are unsaturated fatty
acid methyl esters (FAMEs)that are obtainedby the transesterification of
vegetable oils. Epoxidized FAMEs are of industrial interest to produce
specialty and commodity chemicals. For instance, epoxidized Ole
(OleOx) is a versatile intermediate in the synthesis of surfactants [27],
polyurethane polyols [28,29], components for lubricants [30,31], ad-
hesives [32] and phthalate-free, non-toxic plasticizers [28,33]. Another
important biomass derived unsaturated compound is limonene (Lim), an
abundant low-cost monoterpene which is extracted from the peel of cit-
rus fruits [34]. The main oxygenated forms of Lim are 1,2-epoxy-p-
menth-8-ene (LimOx), limonene diepoxide (LimDiOx) and 1-methyl-4-
(1-methylethenyl)-1,2-cyclohexanediol (LimDiol), which have assumed
importance as renewable building blocks. LimOx has been employed as a
monomer for the synthesis of biodegradable polymers such as poly
(limonene carbonates) as alternatives to conventional polycarbonates
made by the condensation of bisphenol-A and phosgene [35]. Another
example of the value of LimOx in organic synthesis is its use as an in-
termediateinthepreparationof(+)-perillylalcohol[36], anintranasally
delivered anticancer compound [37]. A recent study suggested that
LimOx may also treat inflammation and pain-related disorders [38].
The stoichiometric peracid processes used in industry to produce
epoxidized FAMEs and terpenes present safety and environmental issues
(e.g., corrosion effects, acid waste), and reduced selectivity [26]. These
factors have motivated the search for selective catalytic processes that
employ safer oxidants and produce less waste. The resistance of POMs to
oxidative degradation and hydrolysis, combined with their rich redox
properties and environmental compatibility, makes them preeminent
oxidation catalysts [39]. In the present work, the hybrid POM salt
(Hptz)4[SiMo12O40] (1) (ptz = 5-(2-pyridyl)tetrazole) has been pre-
pared, structurally characterized, and explored for the epoxidation of
biomassderivedolefins(Ole,LinOle,Lim)withTBHPasoxidant.TBHPis
a relativelyclean oxidantsince itsconsumptiongives the co-producttert-
butanol that may be repurposed as a solvent, ethanol denaturant, paint
remover ingredient, freeze-drying agent in the manufacture of phar-
maceuticals, and as an intermediate to produce alkyl tert-butyl ethers.
The organic component Hptz+ is well suited to supramolecular chemis-
try owing to its rich store of free N and NH groups, which can form
hydrogen bonds. The catalytic performanceof 1hasbeencompared with
that of (Bu4N)4[SiMo12O40] (2) and literature data for different POMs
tested using the model cis-cyclooctene (Cy) epoxidation reaction.
(s), 459 (m), 378 (vs), 340 (s). Raman (cmꢀ 1):
ν = 3113 (sh), 3092 (w),
1644 (m), 1570 (w), 1553 (w), 1535 (w), 1486 (w), 1464 (w), 1432 (w),
1236 (w), 1168 (w), 1116 (w), 1006 (m), 975 (vs), 951 (s), 883 (m), 783
(w), 710 (m), 682 (w), 622 (s), 499 (w), 460 (m), 367 (m), 299 (w), 248
(s), 214 (m), 161 (m), 114 (s). 1H NMR (300.13 MHz, 25 ◦C, (CD3)2CO):
δ = 9.02 (br m, 1H, pyr-H6), 8.54–8.68 (overlapping br m, 2H, pyr-
H3,4), 8.06 (br m, 1H, pyr-H5) ppm.
2.2. Catalytic tests
The catalytic reactions with TBHP (in decane, referred to as TBHPD)
were carried out in closed borosilicate batch reactors (ca. 10 mL ca-
pacity), equipped with a PTFE-lined sampling valve and a PTFE-coated
magnetic bar for stirring at 1000 rpm. The reactor was loaded with
catalyst (18 µmol), solvent (1 mL) and olefin (1.80 mmol), and then
immersed in a thermostatically controlled oil bath heated at 70 ◦C. After
◦
10 min, preheated (70 C) oxidant (2.75 mmol for Cy and Lim; 4.57
mmol for Ole and LinOle) was added to the reactor, which was consid-
ered as the initial instant of the catalytic reaction.
The catalytic tests using aqueous H2O2 or TBHP (TBHPW) were car-
ried out in tubular borosilicate batch reactors with pear-shaped bottoms
(ca. 12 mL capacity), equipped with a valve and a PTFE-coated magnetic
bar for stirring at 1000 rpm, under reaction conditions similar to those
indicated above for TBHPD, namely 70 ◦C, initial Mo:substrate:oxidant
molar ratio = 1:100:153. Individual catalytic experiments were carried
out for each reaction time.
The evolutions of the reactions were monitored by gas chromatog-
raphy (GC) analysis through regular sampling (for the TBHPD reactions)
or after cooling of the reactors (for TBHPw and H2O2 reactions). An
Agilent 7820A GC equipped with a HP-5 capillary column (30 m ×
0.320 mm × 0.25 µm) and a flame ionization detector (H2 as carrier gas)
was used. Quantification was performed using extensive calibration
curves with undecane, nonane and methyl decanoate as the internal
standards for analyses of Cy, Lim and Ole/LinOle reactions, respectively.
The products were identified by GC–MS (Trace GC 2000 Series Thermo
Quest CE Instruments GC; Thermo Scientific DSQ II), with He as the
carrier gas. Product identifications were based on commercial mass
spectrometry databases (Wiley6, NIST2.0, NIST Chemistry WebBook,
MAINLIB) and mass spectral matching to available literature data.
For an assessment of the homogeneous catalytic contribution to the
overall reaction, a contact test (CT) was carried out for the system 1/
TBHPD as follows: A mixture of 1, TBHPD and α,α,α-trifluorotoluene
(TFT) was stirred for 24 h at 70 ◦C under conditions similar to those used
for a normal catalytic test, but without substrate. Subsequently, the
mixture was cooled to ambient temperature and centrifuged (3500 rpm,
10 min). The liquid phase (L) was separated from the solid phase by
centrifugation and passed through a filter equipped with a 0.2 µm PTFE
membrane, giving the solution denoted as 1-CT-L. The model substrate
Cy was added to this solution to give an initial substrate concentration of
1.1 M. The mixture Cy/1-CT-L was stirred for 24 h at 70 ◦C, and then
analyzed by GC.
2. Experimental
2.1. Synthesis of (Hptz)4[SiMo12O40]∙nH2O (1)
Concentrated HNO3 (12.62 M, 6.20 mL) was added to a 1 M solution
of Na2MoO4⋅2H2O (20 mL, 20 mmol) with stirring. Next, a 0.2 M
2