M.J. da Silva and A.A. Rodrigues
MolecularCatalysis493(2020)111104
supported Au nanoparticles [27,28]. Production at large scale of alkyl
furoates starting from xyloses present in biomass residues may make
these products attractive and economically viable for fine chemical
industries [29]. In this regard, the oxidative esterification in one pot is
an attractive alternative to the traditional processes that employ metal
salts as stoichiometric oxidant and sulfuric acid as the catalyst [30].
Keggin heteropolyacids (HPAs) are a versatile class of metal-oxygen
clusters active in acid-catalyzed or oxidations reactions [31,32]. Va-
nadium-Molybdenum heteropolyacids were successfully used as the
catalysts in aerobic oxidations of 5-hydroxymethyl furfural to maleic
acid and anhydride [33]. However, HPAs are soluble in a polar solvent,
hampering their reuse. The conversion of HPAs to insoluble salts has
been an approach that overcomes this drawback [34]. In addition,
when the saturated heteropolyanion (i.e., PW12O403−, SiW12O404-) are
converted to the lacunar anion (i.e., PW11O396−, SiW11O397−), these
compounds become catalysts highly active in oxidations with hydrogen
2.3. Characterization of the catalysts
The infrared spectra (FT-IR/ ATR) were recorded on Varian 660-IR
spectrometer 660-IR model Fourier transformed infrared coupled to the
attenuated total reflectance technique. The patterns of X-rays diffrac-
tion (XRD) of the salts were analyzed using a XRD-ray Diffraction
System model D8-Discover Bruker using Ni filtered Cu-kα radiation (λ
=1.5418 Å), working at 40 kV and 40 mA, with a counting time 1.0 s in
the diffraction angle (2θ) ranging from 5 to 80 degrees.
Crystallite sizes were calculated using the Scherrer equation:
K. λ
L =
β. cosθ
(1)
Where λ is the X-ray wavelength in nanometer (nm), β is the peak
width of the diffraction peak profile at half maximum height resulting
from small crystallite size in radians, and
crystallite shape, normally taken as 0.89.
K
is a constant related to
Another modification on the Keggin HPAs composition that has
been successfully performed is their conversion to salts, where the
acidic protons are partially or totally exchanged by metal cations [40].
This is an effective strategy mainly if the metal cation itself has de-
monstrated to be catalytically active; this is the case of Tin(II) cations,
which were very active catalysts in esterification and etherification
reactions [41–43]. Metal transition exchanged HPA salts were efficient
catalysts in acetalization reactions of the glycerol with HOAc, mainly
the Fe4/3SiW12O40 salt [44]. Moreover, Al4/3SiW12O40 salts were effi-
cient catalysts in oxidation reactions of camphene with hydrogen per-
In this work, we synthesized a series of metal silicotungstate salts
(i.e., M4/nSiW12O40; Mn+ = Co2+, Ni2+, Cu2+, Al3+, and Fe3+) and
evaluating their catalytic activity in furfural oxidation reactions with
hydrogen peroxide in alcohol solutions. No organic acid was used as co-
oxidant. Different than other oxidation processes of furfural, the main
reactions were obtained without opening of the furan ring [24–26,46].
The impacts of the main reaction variables were assessed. Furfuryl acid
and their alkyl esters were selectively obtained.
The porosity properties of the catalysts were evaluated by H2 ad-
sorption/ desorption isotherms in a NOVA 1200e High Speed, auto-
mated surface area and pore size analyzer Quantachrome instrument.
Before the analysis, the samples were degassed by 5 h. The surface area
was calculated by Brunauer-Emmett-Teller equation applied to the
isotherms.
Catalyst acidity was estimated by potentiometric titration, as de-
scribed in the literature [48]. The electrode potential variation was
measured with a potentiometer (i.e. Bel, model W3B). Typically, 50 mg
of the salt was dissolved in CH3CN, magnetically stirred by 3 h and then
titrated with n-butylamine solution in toluene (ca. 0.025 mol L−1).
The scanning electron microscopy (SEM) images acquired in a JEOL
JSM-6010/LA microscope. An energy dispersive spectrometry system
(EDS) was used to analyze the chemical composition of catalysts. SEM
images and EDS spectra were recorded using 10 mm working distance
and 20 KV acceleration voltage. Thermal analyses were carried out in a
Perkin Elmer Simultaneous Thermal Analyzer (STA) 6000. Typically, a
sample (ca. 10 mg) was heated at a rate of 10 K/ min under N2 flow.
Temperature range of TG curves was 303–973 K.
2.4. Catalytic runs
2. Experimental section
The catalytic reactions were carried out in a 25 mL three-necked
glass flask, fitted with a sampling system and a reflux condenser.
Typically, furfural (2.5 mmol) was solved in CH3OH solution (ca.
10 mL), magnetically stirred and heated to 323 K. After to add the
catalyst (ca. 1.0 mol %, 25.0 μmol) and hydrogen peroxide (5.0 mmol),
the reaction was started and monitored during 1 h through GC analyses
(Shimadzu 2010, FID, Carbowax column) of aliquots periodically col-
2.1. Chemicals
All the chemicals were acquired from commercial sources and used
without previous treatment. All the alcohols were Sigma 99 wt. %. All
the metal salts, FeCl3·6 H2O (97 wt. %), AlCl3·6H2O (99.5 wt. %),
CuCl2·2 H2O (99 wt. %), CoCl2·6 H2O (98 wt. %), and NiCl2·6 H2O
(97 wt. %) were Vetec. Furfural was purchased from Sigma-Aldrich
(99 wt. %). NaHCO3 and Na2WO4·2 H2O were Vetec (99 wt. %). HCl(aq)
(33 wt. %) was purchased from Dinâmica and Na2SiO3 (99 wt. %) was
from Sigma-Aldrich. CH3OH was acquired from Sigma (99 wt. %)
Aqueous hydrogen peroxide was purchased from Alphatec (35 wt. %).
lected. The profile of temperature was as follows: 80 °C min−1
,
10 °C min−1 up to 210 °C, hold time of 13 min. Temperatures of the
injector and detector were 250 °C.
Conversion and selectivity were calculated by matching of GC peaks
of compounds into respective calibration curves. Dodecane was the
internal standard. The main reaction products were identified by mass
spectrometry analysis in GC–MS Shimadzu ultra mass spectrometer
operating in an electronic impact mode at 70 eV.
2.2. Synthesis of transition metal exchanged Keggin heteropolyacid salts
The oligomers were quantified through mass balance of reactions,
comparing the sum of the GC peak areas of the products with the GC
peak area of the furfural consumed (Equation 1).
The syntheses of the transition metal exchanged heteropoly salts
were performed through metathesis of the silicotungstic acid with a
diluted solution of the metal chlorides as already described the litera-
ture [47]. Typically, a solution of H4SiW12O40 (ca. 1 g) was dissolved in
water (30 mL) and heated to 333 K with constant magnetic stirring. To
this solution was slowly added drop to drop another solution containing
metal chloride in stoichiometric amount. The resulting solution was
stirred and heating to 333 K/ 3 h. Afterward, the solution was evapo-
rated releasing HCl and giving the solid salt, which was dried at 383 K/
3 h.
% oligomers = ((A0 – Ai) − ∑Ap) × 100
(1)
Where:
A0 = GC peak area of furfural (time = t0);
Ai = GC peak area of furfural (time = t);
∑Ap = total sum of GC peak areas of the products after the cor-
rection by the response factor, which was determined comparing the GC
peak of the pure products with the GC peak of furfural at the same
2