L. Pierri, et al.
Molecular Catalysis 489 (2020) 110946
sample heating. The spectra were recorded for 3 s with 10 accumula-
tions to obtain sufficient signal-to-noise ratio.
cyclooctene was used as substrate as well and the reaction progression
II
was followed in each 30 min until 4 and 2.5 h for Mn -L-SS550ox and
II
EPR: A continuous-wave Brucker ER200D, equipped with an Agilent
Mn -L-SW550ox, respectively.
5310A frequency counter was used. The spectrometer was running
The reusability of heterogeneous catalysts is one of the main points
under a homemade software based on LabView. EPR spectra of the
pristine biochars were obtained with the following conditions:
Modulation Amplitude 4 Gptp; Modulation Frequency 100 kHz; Scans
Accumulation 3; Microwave Power 1.26 mW; EPR spectra of the as-
prepared catalysts were obtained with the following conditions: Solvent
to be evaluated concerning catalysts’ behavior. For this purpose, the
II
II
recyclability of Mn -L-SS550ox and Mn -L-SW550ox were evaluated
after obtaining the equilibrium time by the kinetic test mentioned be-
fore. The catalysts were used in a first catalytic run for 1.5 and 2 h,
II
II
respectively, for Mn -L-SS550ox and Mn -L-SW550ox. In sequence, the
ethanol/acetone mixture 8/9 v/v; Modulation Amplitude 10 Gptp
,
solid was separated by centrifugation and washed three times with
II
Modulation Frequency 100 kHz; Scans Accumulation 1; Microwave
Power 20 mW; EPR spectra of the recycled catalysts were obtained with
the following conditions: Modulation Amplitude 10 Gptp, Modulation
Frequency 100 kHz; Scans Accumulation 3; Microwave Power 20 mW.
TG/DTA: Shimadzu DTG-60 analyzer, using a heat rate of 10 °C
MeOH. After drying, the same amount of catalyst (7.3 mg for L-Mn -
II
SS550ox and 4.3 mg for L-Mn -SW550ox) was weighted and used in a
new catalytic run, using the same catalytic conditions described. This
procedure was repeated as many times as epoxide yields were pro-
duced.
−
1
3
−1
min and a flow rate of synthetic air as a carrier gas of 50 cm min
.
To better understand the data from recyclability experiments, EPR
spectra were recorded for both the recovered catalytic materials and the
FTIR: Thermo Scientific Nicolet iS5 spectrophotometer with trans-
mission module iD1. The materials were analyzed after pellets pre-
paration with KBr in a PerkinElmer manual hydraulic press at 6 tons for
II
filtrates, after the second and third use of the catalysts Mn -L-SS550ox
II
and Mn -L-SW550ox, respectively.
3
min.
BET: The N
2
adsorption-desorption isotherms were measured at 77
Results and discussion
2
K on a NOVAtouch LX Quantachrome porosimeter. Specific surfaces
areas were determined with the Brunauer-Emmett-Teller (BET) method
using adsorption data points.
Material characterization
XRD
Catalytic process
XRD diffractograms of pristine biochars as well as their functiona-
lized correspondents showed the presence of amorphous C (sp ), ac-
3
The experiments were conducted in vials and followed a molar ratio
of 1:2000:1000:1000 μmol (catalyst:oxidant:substrate:additive), using a
mixture of acetone/MeOH (450 μL/400 μL) as the solvent system and
with the addition of an internal standard (acetophenone/bromo-
benzene, 1000 μmol). The sequence of reactants were: i) acetone/
cording to the broad peak centered between 22° and 24° 2θ, (plane
(002)) (Fig. 2, black lines). The less intense peaks at 28.4° 2θ and 29.5°
2θ for the biochar SS550 and SW550 respectively, can be related to
2
graphitic C (sp ), since the plane (002) refers to the stacking of layers of
3
aromatic structures [21]. After oxidation with HNO , though, these and
the peaks of metal oxides disappeared (Fig. 2, red lines).
That is, reflections at 40.5° and 50.3° 2θ for the planes (113) and
2 3
(024) of Fe O detected in the pristine materials were absent after
MeOH; ii) H
2
O
2
(30 %, v/v), added by a digitally controlled syringe
−1
pump (SP101IZ WPI) in a rate of 13,3 μL min ; iii) substrate; iv)
II
CH
3
COONH
4
, used as an additive; and v) catalyst (7.3 mg for Mn -L-
II
SS550ox and 4.3 mg for Mn -L-SW550ox, based on the metal-complex
loadings previously calculated by TG/DTA). Ammonium acetate was
chosen as an additive because of its proton acceptor/donor behavior,
acting as co-catalyst, and promoting higher Mn oxidation states [7].
Blank experiments showed that in the absence of catalyst or co-catalyst
or the presence of just the biochars, the epoxidation reactions do not
take place (in any case, the observed yields were lower than 1%).
Cyclohexene, cyclopentene, cyclooctene, α-pinene, limonene,
styrene, cis-stilbene, trans-β-methylstyrene and 1-hexene were studied
as substrates. For each olefin, the catalytic reaction was prepared and
studied by quantifying the yield of products. Each vial’s content was
centrifuged and 0.2 μL of the supernatant was injected in a gas chro-
matographer coupled to a mass spectrometer (GC–MS, Shimadzu
QP2010 SE). The integrals of the retention peaks were compared with
the internal standard’s one to obtain the substrate conversion and the
yield of the oxidation products. When alcohol (from limonene and cy-
clohexene oxidation) and ketone (from cyclohexene oxidation) were
detected, as allylic oxidation products, Shul’pin’s procedure [20] was
run to be sure that these products were not resulting from former hy-
droperoxides formation. The turnover numbers (TON) were calculated
by the ratio between the moles of product formed and the moles of
catalyst added.
oxidation, which means that both biochars had their ash impurities
removed. This was also confirmed by EPR spectroscopy (not shown)
since the originally Fe and Cu present in both pristine biochars were
removed after the strong acid treatment. Thus, according to the XRD
data, ongoing from the pristine materials (SS550/SW550) towards the
II
II
final hybrids (Mn -L-SS550ox/ Mn -L-SW550ox) the synthetic protocol
leads to a metal-free carbon matrix bearing the grafted catalyst.
Raman Spectroscopy
The spectra of pyrolytic biochars typically exhibit a complex pattern
−
1
−1
of intense peaks covering the region 1600 cm
to 1250 cm
[15].
Biochars produced at low pyrolysis temperatures < 700 °C do not show
−
1
−1
only the “classical” 1580 cm
and 1350 cm
Raman peaks, which
2
3
are indicative of the “graphitized” sp - and sp -hybridized graphitic C-
atoms, respectively [22,23]. A careful Raman study of Li et al [15]
demonstrated that in biochars pyrolyzed at low temperature, < 600 °C,
the Raman spectrum contains distinct molecular signatures of [i] small
−1
3
aromatic structures, up to 6–8 rings at 1450-1550cm and [ii] sp -rich
−1
alkyl-type chains at 1240−1350 cm . These peaks can be observed on
the Raman spectra of both pristine materials and even more clearly on
the Raman spectra of their functionalized counterparts (see Fig. 3 panel
A for SS550 and panel B for SW550).
The catalytic kinetics was evaluated by the same reaction afore-
mentioned, using cyclohexene as a substrate. Samples were taken at
3
S-band sp -type carbons
times T
0
, T30, T60, T120, T180, T240, T300, T420 and T1440 (minutes),
) was followed by a
The Raman spectra for the two pristine materials SS550 and SW550
analyzed by GC–MS and the redox potential (E
h
(black lines in Fig. 3A & B), respectively show the presence of con-
3
Metrohm platinum electrode (type 6.0401.100) versus standard hy-
drogen electrode (SHE), previously calibrated with a Ferri/Ferro solu-
tion. The turnover frequencies (TOF) were calculated by the ratio be-
tween TONs and the equilibrium time needed to stabilize the olefin
conversion. In order to confirm the equilibrium time for each catalyst,
siderable amounts of alkyl-based or carboxyl-based sp units i.e.
−1
marked as S-band at 1140−1280 cm according to Li et al. [15]. After
oxidation of the materials by HNO (red lines in Fig. 3A & B), the band
is enhanced in SS550ox while in SW550ox the change is
minor. Oxidation of the materials is inducing the generation of
3
−
1
at 1140cm
3