´
A.S. Granato et al.
Catalysis Today xxx (xxxx) xxx
2.6H2O (80 mmol, 2 equiv.) and Al(NO3)3.9H2O (40 mmol, 1 equiv.).
For LDH-02 system it was considered an aqueous solution of Mg(NO3)
2.6H2O (80 mmol, 2 equiv.) and Al(NO3)3.9H2O (40 mmol, 1 equiv.).
Finally, for the LDH-03 system it was considered an aqueous solution
containing Zn(NO3)2.6H2O (40 mmol, 1 equiv.), Mg(NO3)2.6H2O
(40 mmol, 1 equiv.) and Al(NO3)3.9H2O (40 mmol, 1 equiv.).
findings also support I.R. data indicating the breakage of LDH lamellar
structures resulting in a thin powder composed by a mixture of metal
oxides. After impregnation of ammonium niobium oxalate solution to
each MMO little to no difference were observed in XRD patterns, sug-
gesting a good dispersion of niobium species over the surface of MMO
systems. Additionally, the Solid State 27Al NMR reveals signals close to
50 and 70 ppm for MMO and NbCAT, absent in LDH series, suggesting
the formation of a distorted tetrahedral sites and AlO4 respectively, after
calcination of LDH to obtain the MMO. Further details and analyses
related to NbCAT catalysts development and characterization can be
found in our recent manuscript [6].
For each LDH system, both metallic and basic solutions were mixed
in a dropwise manner under continuous stirring, kept for 24 h. The
white precipitate observed was filtered in paper filter and washed with
distilled water until pH 7. The filtrates were dried in oven at 50 ◦C for
24 h. The dried materials were macerated and calcinated at 500 ◦C in a
ceramic muffle furnace for 5 h, leading to the mixed metal oxides MMO-
01 (Zn+2 0.67 M/Al+3 0.33 M) MMO-02 (Mg+2 0.67 M/Al+3 0.33 M)
and MMO-03 (Zn+2 0.33 M/ Mg+2 0.33 M/Al+3 0.33 M).
4.2. Reactions towards benzylamine
Finally, a solution of ammonium niobium oxalate (6.3 mmol), was
prepared in 5 ml of distilled water and added in a dropwise manner to
5 g of each MMO support under maceration. The resulting materials
were calcinated at 500 ◦C for another 3 h aiming a final supported
amount of 10 % m/m of Nb2O5, leading to the mixed oxide-supported
niobium-based catalysts NbCAT-01, NbCAT-02 and NbCAT-03.
Our research investigation initiated by taking the NbCAT-01 catalyst
in the presence of benzylamine, using 30 % aqueous H2O2 as oxidizing
agent and MeOH as solvent and at room temperature. Additionally,
based on previous results from our group, indicating higher catalytic
activities when in presence of low visible spectrum light, the reactions
were carried out under irradiation of violet light (380 nm) in a reaction
chamber equipped with a 50 W LED (Table 1).
3. Theoretical methodology
The first series of results indicated a good conversion of starting
material using 4 equivalents of H2O2 within 72 h of reaction under
continuous irradiation of violet light (Table 1, entries #1 and #2).
However, only modest selectivity for oxime production was achieved,
being identified the formation of benzylidene benzylamine as side
product. These results may relate to the dehydrogenation of amine
mediated by transition metal oxides. The hydrolysis of the correspond-
ing imine, followed by a condensation with the primary amine may lead
to the dimer, as suggested in the literature [20].
Ab Initio calculations were performed using the Quantum-Espresso
package [14]. The electronic structure calculations were based on
density functional theory (DFT) implemented with periodic boundary
conditions [15,16]. The effect of the exchange-correlation (XC) potential
was explored by generalized gradient approximation with PW91 func-
tional. Kohn-Sham orbitals were expanded in a plane wave basis set to a
kinetic energy cut-off of 50 Ry for all structures [17]. For MgO:Al,Zn
terrace calculations, the unit cell of the MgO(001) surface was replicated
four times on the x axis (a = 8.52) and three times on the y axis
(b = 6.39). A vacuum layer of 25 was used in the slab models to avoid
interactions between the periodic images perpendicular to the surface.
The minimum energy path (MEP) was constructed in order to obtain
the transition state, the reaction barrier and the main structural modi-
fications involved in the process of the tautomerism. The calculation of
the MEP connecting different minimum geometries is based on the
climbing image nudged elastic band (CI-NEB) method [18,19] which
accurately describes the MEP between the initial and the final states of a
reaction, and evaluates the transition state and the resulting activation
energy barriers. A total of 11 configurations were used to compute the
MEP.
Interestingly, when the reactions were performed in absence of light
(entry #3) or even under room light (entry #4), led to slightly inferior
conversion rates. These data appear to differ from previous findings in
the literature, indicating the oxidation of benzylamine into the corre-
sponding imine when Nb2O5 was used as photocatalyst. This conversion
was achieved even wavelengths up to 460 nm, but poorly 10 registered
in the absence of light. In addition, previous results in our research
group, indicated the light dependence of these reactions [6].
On the other hand, as mentioned before, a background reaction by
Table 1
Reactions under violet light.
4. Results and discussion
4.1. Catalysts characterization
Entry
Parameters
Amine conversion (%) *
Selectivity for Oxime (%) *
The materials from LDH, MMO and NbCAT series were analyzed
through I.R., Powder X-Ray diffraction and 27Al Solid State NMR (data
Influence of H2O2
#1
#2
4 eq. H2O2
6 eq. H2O2
89
82
51
45
available in supporting information). The Infrared analysis for each LDH
ꢀ 1
–
system indicated O H stretching signals at 3500 cm
from water
Influence of light and time
ꢀ 1
–
#3
#4
#5
Dark
70
73
71
44
53
47
molecules and hydroxyls and C O stretching at 1400 cm from car-
bonate present in LDH interlayers. Those signals were abrogated after
calcination step leading to the obtention of the mixed metal oxides as
result from the obliteration of LDH lamellar structures. Similar patterns
were maintained after impregnation of ammonium niobium oxalate
solution to each MMO with wide signals verified at 500ꢀ 600 cm-1 which
may relate to a symmetric stretching metal-oxygen bonding. The XRD
diffractograms showed broad patterns for MMO series when compared
to its LDH precursors indicating the obtention of a low crystallinity
powder. The presence of peaks 2θ (CuKα) around 33◦ (100), 34◦ (002),
36.5◦ (101), 48◦ (102), 57◦ (110), 63◦ (103) and 69◦ (112) for MMO-01
Room Light
48 h
Influence of catalyst
#6
#7
#8
#9
#10
No catalyst
42
46
76
71
97
37
30
37
45
55
Pure Nb2O5
MMO-01
MMO-02
ZnO
a) Standard reaction conditions: 10 % weight catalyst (NbCAT-01), violet light, 4
equivalents of H2O2 and MeOH as solvent, unless otherwise mentioned. Re-
actions kept for 72 h unless otherwise mentioned. *Calculated over GC–MS; b)
NbCAT-01 consisted in 10 % m/m of Nb2O5 impregnated over MMO-01. MMO-
01 consisted in Zn+2/Al+3 (0.67/0.33 M) based mixed metal oxide while MMO-
02 consisted in Mg+2/Al+3 (0.67/0.33 M) based mixed metal oxide.
and MMO-03, which consists in Zn+2/Al+3 (0.67/0.33 M) and Zn+2
/
Mg+2/Al+3 (0.33/0.33/0.33 M) mixed oxides, respectively, suggests
–
Zn O bonding formation after calcination of LDH systems. These
3