Designed sol-gel precursors for atomically dispersed Nb and Pb within TiO2as catalysts for dihydroxyacetone transformation

Article abstract of DOI:10.1039/d0dt03726e

The ‘bottom-up’ synthesis of homogeneously doped metal oxide nanoparticles (NPs) with precision at the atomic/molecular level offers many advantages. We report here the synthesis, structural characterization and hydrolytic behavior of newN-methyldiethanol

Full text of DOI:10.1039/d0dt03726e

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Designed sol–gel precursors for atomically
dispersed Nb and Pb within TiO as catalysts
2
Cite this: Dalton Trans., 2021, 50,
1604
for dihydroxyacetone transformation†
a,b
Received 29th October 2020,
Accepted 14th January 2021
a,b
Antonio de Brito Santiago Neto, Márcia Gabriely Alves da Cruz,
c
b
a
DOI: 10.1039/d0dt03726e
Erwann Jeanneau, Alcineia Conceição Oliveira, Nadine Essayem and
a
rsc.li/dalton
Shashank Mishra
*
The ‘bottom-up’ synthesis of homogeneously doped metal oxide acid catalysts has been reported in the literature, with some
7
–9
nanoparticles (NPs) with precision at the atomic/molecular level applications to lactate synthesis in alcoholic medium.
offers many advantages. We report here the synthesis, structural Recent studies have reported the strong dependence of the
characterization and hydrolytic behavior of new N-methyl- acidic nature of the catalysts when the reaction is carried out
3
,8,10
diethanolamine-modified precursors of Ti(IV), Nb(V) and Pb(II) and in water for lactic acid production.
The ‘bottom-up’ syn-
their application as excellent sol–gel precursors for obtaining thesis of nanomaterials using well-defined molecular precur-
atomically dispersed Nb and Pb within TiO
areas (253–355 m g−1) and tunable acidic properties. Preliminary including access to these materials at much lower tempera-
2
having high surface sors has many advantages over traditional synthetic routes
2
1
1
results on the use of these doped TiO as water-tolerant catalysts tures. The advantages of the sol–gel (SG) process as a soft
2
in transforming dihydroxyacetone to lactic acid are presented.
solution route for the elaboration of homogeneously doped
metal oxide nanomaterials are well documented. Metal alk-
1
2
Dihydroxyacetone (DHA) is an attractive triose sugar currently
obtained by the valorization of crude glycerol from the pro-
oxides are the preferred sol–gel precursors for metal oxide cer-
13
14
amics and catalytic materials, but the use of classical metal
alkoxides in the SG process is unattractive due to their extre-
mely high susceptibility towards hydrolysis and poor stability
of the resulting colloidal solutions. The modification of classi-
cal metal alkoxides by functional alcohols bearing additional
ether or amino functionalities not only alter their physical and
chemical properties such as hydrolysis susceptibility, solubi-
lity, viscosity, etc. but also, due to their action as surfactants,
1
–3
duction of biodiesel. DHA is used as an active ingredient in
sunless tanning lotions and as a precursor for the preparation
of an increasingly sought-after commodity chemical, lactic
4
,5
acid (LA). The DHA transformation into LA proceeds via de-
hydration of the DHA triose yielding pyruvaldehyde (PA) as an
intermediate – a first step described to be catalyzed by both
Lewis and Brønsted acids (Scheme 1). The subsequent rehydra-
tion step of pyruvaldehyde to lactic acid is reported to be pro-
moted by Lewis acids where key hydride transfer shifts are
15
stabilize the colloidal solutions obtained during hydrolysis.
In this regard, we have shown previously that aminoalcohols
are not only easily modified to modulate their steric and elec-
3
,5,6
required.
The use of heterogeneous Brønsted and Lewis
1
6
tronic properties, but their metal derivatives are very attrac-
tive sol–gel precursors for metal oxide nanomaterials because
of the favorable hydrolysis and condensation characteristics
a
Université Claude Bernard Lyon 1, CNRS, UMR 5256, Institut de Recherches sur la
Catalyse et l’Environnement de Lyon (IRCELYON), 2 avenue Albert Einstein,
6
9626 Villeurbanne, France. E-mail: shashank.mishra@ircelyon.univ-lyon1.fr;
and their transformation into oxides at relatively low
Fax: +33-472445399; Tel: +33 472445322
17–20
temperatures.
These characteristics and the interesting
b
Universidade Federal do Ceará, Campus do Pici-Bloco 940, Departamento de
structural features that play a role in determining the pro-
Química Analítica e Fisioquímica, 60.000.000 Fortaleza, Ceará, Brazil
c
Université Claude Bernard Lyon 1, Centre de Diffractométrie Henri Longchambon,
5
rue de La Doua, 69100 Villeurbanne, France
†
Electronic supplementary information (ESI) available: Experimental section,
FT-IR and single crystal X-ray refinement data of the new precursors, TG-DTG
curves of as-prepared undoped and Nb- or Pb-doped TiO NPs obtained from the
hydrolysis of the precursors, Raman spectra, STEM images and N adsorption–
desorption isotherms of the Nb- and Pb-doped TiO , DHA conversion profiles
catalyzed by these undoped or doped TiO and 3 X-ray crystallographic files in
2
2
2
2
CIF format. CCDC 1558717, 2034796 and 2013426 for 1, 2a and 3, respectively.
For ESI and crystallographic data in CIF or other electronic format see DOI:
Scheme 1 Dihydroxyacetone conversion to lactic acid via pyrulvalde-
10.1039/d0dt03726e
hyde formation.
1604 | Dalton Trans., 2021, 50, 1604–1609
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perties of the final material produced have prompted us to (μ-OEt)(OEt)
3
] 1, which crystallizes with a solvated ethanol
investigate the designed metal aminoalkoxides for the solu- molecule in the space group P ˉ1 , two titanium centers are
1
5–20
tion- and vapor-phase synthesis of metal oxides.
By bridged by one alkoxide arm of a bridging–chelating mdea
extending the above studies, we report here the synthesis, ligand and one ethoxide group (Fig. 1a). The remaining alkoxide
structural characterization and hydrolytic behavior of new arm and the nitrogen center of the bridging–chelating mdea
N-methyldiethanolamine-modified precursors of titanium(IV), ligand are coordinated to the Ti2 atom, which is additionally
niobium(V) and lead(II). The choice of these metals was motiva- substituted by two terminal OEt ligands. The other titanium
ted by the aim to synthesize water-tolerant TiO nanoparticles atom (Ti1) in the dimer is coordinated to one chelating mdea
2
doped with Nb or Pb, dispersed homogeneously and at the ligand and a terminal OEt group. Thus, each titanium is six-co-
atomic scale within the TiO matrix to tune its catalytic acid– ordinated with a TiO N coordination sphere. The terminal
2 5
base properties. Indeed, the water tolerance of TiO2 is well ethoxy groups are most strongly bound [Ti–Oterminal 1.812(2)–
documented in the field of heterogeneous catalysis, where it 1.818(2) Å], followed by the aminoalkoxide [Ti–Omdea 1.860(2)–
has been employed as a stable support of noble metals or as 1.899(2) Å], while the bridging interactions are, unsurprisingly,
2
1
solid Lewis acids for carbohydrate conversion. The access to weaker [Ti–Obridging 2.018(2)–2.057(2) Å]. The Ti–N bond dis-
the designed metal aminoalkoxide precursors appears as a tances, 2.349(3)–2.374(3) Å, are typical of these species. The
2
unique tool to boost the Lewis acidity of nanometric TiO in a octahedral environment around Ti1 and Ti2 centers is con-
controlled way, via its doping with low amounts of metal siderably distorted as indicated by the cis O–M–O and O–M–N
centers capable of providing Lewis acid centers. The oxides of angles ranging from 69.8(8) to 109.2(1)°. These discrete
2
2
Nb and Pb were chosen due to their reported Lewis acidity.
dimeric entities do not have any considerable interaction
The use of these derivatives as sol–gel precursors not only among themselves but form H-bonding (2.977 Å) with the sol-
resulted in an excellent homogeneous dispersion of the vated EtOH molecules. Titanium(IV) derivatives with the
dopant Nb or Pb within TiO
ensured a high surface area in the range of 250–355 m g
and an enhanced Lewis acidity as demonstrated by their appli- of mdeaH
2
at the atomic level but also N-methyldiethanolamine ligand have previously been
2
−1
23–25
i
studied.
While the reaction of Ti(OPr ) with 2 equivalents
4
2
ligand under reflux afforded a fully substituted
2
3
cation as excellent water-tolerant acid catalysts for the model product [Ti(mdea)
reaction of dihydroxyacetone conversion into lactic acid. mdeaH actually gave an unsymmetrical dimer [Ti (mdea)
New titanium(IV) and niobium(V) aminoalkoxides 1 and 2 were (OPr )
2
],
its reaction with 1 equivalent of
2
2
i
24
6
] (Scheme S1†). This reaction was thought to proceed
synthesized by ethoxo-aminoalkoxo exchange reactions in through a transitory coordinatively unsaturated and insuffi-
i
toluene using [Ti(OEt) ]/[Nb(OEt) ] and N-methyldiethanolamine ciently shielded species [Ti(mdea)(OPr ) ] (A), which further
4
5
2
i
as starting reagents, whereas lead(II) derivative 3 was prepared by reacted with Ti(OPr )
4
to finally afford a sterically controlled
(mdea)(OPr ) ]. The OPr groups, which are sterically
2 6
i
i
refluxing PbO with N-methyldiethanolamine in toluene product [Ti
(Scheme 2). While the derivatives 1 and 3 are crystalline solids, more crowded than the OEt groups, apparently prevented the
the niobium(V) derivative 2 is a viscous oil. The heteroleptic more straightforward dimerization of A, which would also have
i
derivatives 1 and 2, which have residual ethoxide groups, were given a coordinatively saturated product. On replacing OPr with
also studied for their microhydrolysis behavior, which produced less-hindered OEt, dimerization becomes feasible as confirmed
oxo clusters of the compositions [Ti(O)(mdea)] (1a) and [Nb(O) by the isolation and structural characterization of 1 here. This
(mdea)(OEt)] (2a), respectively (Scheme 2). While the heteroleptic structure, therefore, completes a missing link in the library of
derivatives 1 and 2 show good solubility in common organic sol- titanium derivatives with the N-methyldiethanolamine ligand,
1
5
vents such as toluene, THF, chloroform and alcohols, the homo- which is an important modifier in the sol–gel procedure. The
leptic derivative 3 and the oxo-clusters 1a and 2a are insoluble in structure of 1 also underlines the versatility in the bonding
hexane, only moderately soluble in chloroform but soluble in capabilities of the mdea ligand, which adopts here both chelat-
alcohols.
Crystals suitable for single crystal X-ray studies of 1, 1a, 2a matched well with the previously reported [Ti
and 3 were obtained as described in the Experimental section obtained through the hydrolysis of the homoleptic derivative [Ti
ing and bridging modes. The unit cell parameters of 1a
6
(O) (mdea)
6
6
]
2
5
in the ESI.† In the dimeric structure of [Ti
2
(μ-mdea)(mdea) (mdea)
compound. The molecular structure of [Nb
2a) is based on a cyclic Nb O core (Fig. 1b). It crystallizes in
2
]. Therefore, no further data were collected for this
4
(μ-O) (mdea) (OEt)
4
4
4
]
(
4
4
the monoclinic space group I2. Four niobium atoms and four
bridging oxo ligands lie in a plane forming a square with two
trans Nb⋯Nb distances being almost equal (5.383 and 5.386 Å).
The niobium-oxo bond lengths vary from 1.831(7) to 2.015(7) Å,
4 4
thus resulting in a slightly asymmetrical Nb O ring. The brid-
ging oxo ligands are slightly shifted out from the square sides,
as indicated by an angle of about 165° at them. Each niobium
centre has a chelating mdea ligand, bonded via two oxygen
atoms and a nitrogen atom, and a terminal ethoxo ligand to
Scheme 2 Synthesis and partial hydrolysis behavior of new N-methyl
diethanolaminates of Ti(IV), Nb(V) and Pb(II).
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as well as of 1 is further confirmed by the presence of absorp-
tion bands due to ν(OH) in the 3100–3500 cm− region. All the
1
new derivatives reported here also show new bands in the
75–415 cm− region for ν(M–O) and ν(M–N) stretching
1
6
1
vibrations. The room temperature H NMR spectra of these new
derivatives are either simple or poorly resolved, indicating a
fluxional behavior for them under these conditions. The signals
due to OEt and/or mdea ligands appear in the regions and inte-
grated intensity ratios expected for them, although their multi-
plicity is sometimes not so well resolved [e.g., δ 1.23 (CH
.29 (N–Me), 2.61 (NCH2), 3.67 (OCH2) and 4.32 ppm
CH CH O) in 1]. This fluxional process does not cease even at
3 2
CH O),
2
(
3
2
low temperature.
The precursor 1 was hydrolyzed using a modified procedure
of a previously published report on obtaining metal oxides
2
9
with high surface areas. In a typical procedure, a toluene
solution of 1 was added to a calculated amount of an aqueous
solution of acetic acid, followed by refluxing and aging for 3 h.
The powder obtained after the removal of the solvents using a
rotary evaporator was capped by N-methyldiethanolamine
ligand, as confirmed by the FT-IR and TG-DTA studies on it
Fig. 1 Perspective views of 1 (a), 2a (b) and 3 (c) with displacement
ellipsoids drawn at the 30% probability level. Carbon atoms are drawn as
ball and stick and the H-atoms on alkyl groups are omitted for clarity.
Selected bond lengths (Å) and angles (°): (1) Ti1–O1 1.899(2), Ti1–O3
(Fig. S2†). Since the as-obtained NPs were poorly crystallized,
1
.818(2), Ti1–O4 2.057(2), Ti1–O5 2.053(2), Ti1–N1 2.374(3), O4–Ti1–O5
i
6
2
9.78(8), N1–Ti1–O3 164.42(10), O1–Ti1–O2 104.55(11); (2a) Nb1–O8
.015(7), Nb2–O8 1.831(7), Nb2–O6 1.771(11), Nb2–O5 2.027(12), Nb1–
N1 2.442(11), Nb–N2 2.459(10), O1–Nb1–N1 168.0(5), O2–Nb1–O4
these were calcined at 500 °C to remove the remaining of the
organics and to make them well crystallized. The XRD of this cal-
cined sample revealed it to be well crystallized with all the peaks
indexing well with the PDF file 00-021-1272 of the pure anatase
i
i
1
1
60.1(4), O2–Nb1–O8 76.0(5), Nb1–O1–Nb2 164.9(4), Nb2–O8–Nb1
65.1(6); (3) Pb1–O1 2.676(7), Pb2–O4 2.174(6), Pb1–N1 2.569(9), Pb2–
2
phase (Fig. 2). To obtain Nb- or Pb-dispersed TiO , the precursors
N2 2.763(8), O2–Pb1–O1 134.6(2), O2–Pb1–N1 69.5(3), Pb2–O1–Pb1
i
1
1
02.4(2), Pb2–O4–Pb2 110.1(3). Symmetry elements: (2a) (i) 1 − x, y, 1 and 2 (or 1 and 3) were taken in an appropriate molar ratio in
− z; (3) (i) 1 − x, 1 − y, 1 − z.
toluene, stirred for 1 h to obtain a homogeneous solution, and
then hydrolyzed as described above. The as-obtained NPs, which
are capped with the N-methyldiethanolamine ligand (Fig. S3 and
complete the 6-coordination sphere around it. The Nb–O bond S4†), were calcined at 500 °C for 4 h for the complete removal of
lengths of these ligands vary from 1.771(11) to 2.027(12) Å, the organic ligands. The XRD peak positions of the doped Nb/
while the Nb–N bond lengths are in the range of 2.442(11)– TiO or Pb/TiO were the same as that of the undoped TiO , with
2
2
2
2
.459(10) Å. While the Nb–O distances are in agreement with all the peaks indexing well with the pure anatase phase (Fig. 2).
2
6
the literature, the Nb–N distances are slightly longer in 2a However, these peaks were slightly broader and less intense,
than those found in related 6-coordinated Nb(V) complexes with
2
7
N-containing ligands. The structure of the solvated compound
Pb(mdea)]·mdeaH (3) reported here is quite similar at the
molecular level to the previously described non-solvated
[
2
28
[
Pb(mdea)], although both crystallize in different space groups
(I2/a and C2/c, respectively) and, therefore, have different mole-
cular packings. Similar to the non-solvated [Pb(mdea)], the
structure of the compound 3 also corresponds to a polymer
based on 5-coordinate lead centers having a trigonally distorted
octahedral stereochemistry (if the electron lone pairs are taken
into account) (Fig. 1c). The Pb–O [2.174(6)–2.676(7) Å], Pb–N
[
2.569(9)–2.763(8) Å] and Pb⋯Pb [3.86–3.91 Å] distances also
28
compare well with the reported structure. While the IR spectra
of the new mixed-ligand derivatives 1 and 2 exhibit character-
istic bands of the parent ligands, N-methyldiethanolamine and
ethoxide, the homoleptic 3 and oxo-derivative 2a show only the
bands for the parent aminoalkoxide ligand (Fig. S1†). All the
bands of the mdea ligand appears as twice in close vicinity for
Fig. 2 Powder XRD patterns of undoped TiO
NPs obtained from the hydrolysis of 1 alone or a mixture of 1 + 10% 2 or
mdeaH ligand in its lattice. The solvated nature of this species 1 + 10% 3, respectively, followed by calcination at 500 °C.
2 2 2
, Nb/TiO and Pb/TiO
3, which is consistent with its crystal structure having a free
2
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2
indicating a lower degree of crystallization in Nb/TiO and
Pb/TiO . No peaks for doped niobium or lead oxides could be
2
observed in the XRD patterns, thus suggesting the ultradisper-
sion of these species on the surface and/or within the TiO
matrix. The Rietveld refinement of powder XRD of these
samples reveals that the TiO lattice becomes more strained
2
2
upon doping with Nb or Pb, even though there is no signifi-
cant change in the lattice parameters (Table S2†). The Raman
spectra, which display typical vibrational modes of the anatase
1
−1
−1
−1
TiO
A1g + B ), and 634 cm (E ), also show a slight broadening
2 g g
at 144 cm− (E
), 195 cm (E ), 394 cm (B1g), 514 cm
−1
(
1g
g
2 2
and decreased intensity for the Nb/TiO and Pb/TiO catalysts,
thus indicating a decrease in the crystallinity as well as an
increase in the defects for these samples (Fig. S5†). Moreover,
the absence of any detectable vibrations due to niobium or
lead oxide confirms the extremely high dispersion of Pb and
Nb within the TiO matrix. These studies also suggest that the
2
5
+
2+
niobium and lead atoms are in the Nb and Pb states,
respectively. The atomic-level ultradispersion in these catalysts
is further confirmed by the high-resolution STEM images,
which show the absence of particles larger than 2 nm on the
two doped TiO
ultradispersed in TiO2 at the atomic level, the dispersion of
the Nb dopant within the TiO matrix is slightly less homo-
2
samples (Fig. 3, S6†). While the Pb dopant is
2
geneous, as marked by the coexistence of isolated atoms and
clusters of varying sizes (0.5 nm to 2 nm). The above studies
indicate a major inclusion of Nb and Pb inside the TiO lattice
2
although a part of these species, especially for Nb, also exists
as small clusters of less than 2 nm, well dispersed on the TiO2
nanoparticles, thanks largely to the milder sol–gel conditions
3
0
2
Fig. 3 STEM-ADF images of Nb/TiO (a) and Pb/TiO2 (b).
employed here for the synthesis of these catalysts. The N
adsorption–desorption isotherms of the two doped samples
Nb/TiO and Pb/TiO are typical of mesoporous materials, dis-
playing type IV isotherms with hysteresis in the range of 0.4 < to the homogeneous dispersion of the acidic metal centers in
P/P < 0.8 (Fig. S7†). A significantly high BET surface area for the TiO framework, the DHA consumption increases signifi-
and cantly up to 89% conversion for the Nb/TiO catalyst after
for the undoped 4 hours of reaction. Over Pb/TiO , a lower DHA conversion is
TiO ) can be attributed to the smaller crystallite size (9.9 and achieved, which is 75% after 4 h. These values are well above
2
2
2
0
2
2
−1
the two doped samples (253 and 355 m g for Nb/TiO
Pb/TiO
2
2
2
−1
2
, respectively, as against 40 m
g
2
2
1
2 2
6.6 nm in Nb–TiO and in Pb–TiO
2
, respectively, versus 20 nm the 49% conversion after 4 h, obtained for dopant-free TiO .
in undoped TiO
2
) and the empty space among the loosely coa- The initial rates of DHA conversion (Table 1) increased by
gulated primary particles. The properties of the final materials factors of 1.6 and 2.7 upon doping with Pb and Nb, respect-
depend on the nature of the precursors and the preparative ively, without correlation with the surface enhancement. This
methods employed. The high surface area and atomic dis- enhanced activity can be explained by the superficial acidic
persion observed here are due to the surface passivating properties achieved through the doping of TiO with niobium
2
effects of the N-methyldiethanol aminate ligand and the sol– and lead. According to the generally accepted mechanism
gel process employed resulting in small nanoparticles and the (cf. Scheme 1), pyruvaldehyde (PA) is the intermediate product
imperfect packing of these nanoparticles. Achieving finely dis- that is produced by DHA dehydration catalyzed by the
persed doped metal oxide nanoparticles with a remarkably Brønsted or Lewis acid sites, whereas the final product lactic
high surface area here thus validates the use of the designed acid (LA) is formed by hydration of PA and further hydride
precursors and the modified sol–gel procedure.
transfers promoted by the Lewis acid sites solely
5
,8,31
The above-obtained highly dispersed Nb/TiO
2
and Pb/TiO
2
(Fig. S8†).
The production of lactic acid is due to the pres-
catalysts having high surface areas were evaluated for the ence of accessible Lewis acid sites in the solid structure, and
model reaction of dihydroxyacetone transformation into lactic the increased LA yield with the progress of the reaction is
acid and compared to the undoped TiO
utions of DHA consumption as a function of reaction time for observed for Pb/TiO
different solids are shown in Fig. 4a. It is noteworthy that, due were reported to be effective for DHA dehydration into PA but
2
catalyst. The evol- related to the increase of Lewis acidity of TiO
2
after doping as
2
(Fig. 4b). Since the Brønsted acid sites
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Conclusions
New homo- and heteroleptic N-methyldiethanolamine-modi-
fied precursors of Ti(IV), Nb(V) and Pb(II) showing interesting
structural diversity and sol–gel characteristics are reported.
Their partial and complete hydrolysis leads to the isolation of
well-defined molecular oxo-clusters or metal oxide nano-
particles, respectively, the latter forming a promising approach
to obtain highly dispersed catalysts containing Nb and Pb,
mainly as isolated cations within the TiO
2
matrix, but also as
small oxo clusters of less than 2 nm on the TiO NPs. These
2
Nb/TiO
2 2
and Pb/TiO catalysts with high surface areas
2
−1
(
253–355 m g ) and tunable acidic properties show improved
reactivity for aqueous phase dihydroxyacetone conversion.
Author contributions
S. M. and N. E. conceived the work and supervised the
materials and catalysis parts, respectively. S. M. also syn-
thesized and characterized the precursors and wrote the
manuscript. A. B. S. N. and M. G. A. C. synthesized the cata-
lysts by the sol–gel method, characterized them and measured
their catalytic activity. A. C. O. supervised the catalysis
part. E. J. solved the single crystal X-ray structures of the pre-
cursors. All authors discussed the results and commented on
the final manuscript.
Conflicts of interest
Fig. 4 (a) DHA conversion profiles as a function of time, and (b) LA
yield profiles as a function of DHA conversion in the presence of
There are no conflicts to declare.
2
undoped and doped TiO .
Table 1 Initial rate of DHA conversion catalyzed by TiO
Pb–TiO
2
, Nb–TiO
2
and Acknowledgements
2
Financial support from the Capes/Brazil to A. B. S. N. (Grant
Catalysts
TiO
2
Nb–TiO
2
Pb–TiO
2
No. 88881.190581/2018-01) and M. G. A. C. (Grant No.
9999.006712/2015-00) is gratefully acknowledged. The
authors thank M. Aouine (HR-TEM) and Y. Aizac (XRD) of
IRCELYON.
9
r° DHA consumption
5.2
14.4
8.2
mmol DHA h gcat−1)
−
1
(
2
−1
S BET (m g
)
40
253
355
ineffective for pyruvaldehyde rehydration into LA, one can con-
Notes and references
clude that the enhanced activity of Nb/TiO is due to an increase
2
in its Brønsted acidity as compared to the dopant-free TiO
2
. The
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2
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1608 | Dalton Trans., 2021, 50, 1604–1609
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