J.G.A.B. Silva et al.
Molecular Catalysis 507 (2021) 111567
and physical properties, aiming at improving the pore size and elec-
tronic structure [21–23]. Other types of supports are amorphous alloys,
zeolites and ordered mesoporous silicas (MCM), which allowed a better
diffusion of glucose molecules, better metallic dispersions and the use of
lower pressure reaction conditions, resulting in advances in the syn-
thesis of materials and higher selectivity and conversion [19,24–27].
The use of oxides as supports for more active metals than nickel al-
lows the reduction of costs with pre and post-reaction steps, longer
recycling times with the use of more stable materials, without the loss of
material to the reaction solution by leaching. Oxide supports appear as
an option, since they have exceptional durability during the process of
hydrogenation of glucose, favoring the reaction with the adsorption of
hydrogen on its surface, thereby favoring the reaction [28]. Other re-
actions also use oxide catalyst supports and involve the transformation
of glucose such as the oxidation of glucose to gluconic acid in gold
catalysts supported on Al2O3, CeO2 and ZrO2 [29]; and dehydration of
glucose to 5-HMF, using Al2O3-B2O3 [30].
amorphous and crystalline phases in the hydrogenation of glucose to
sorbitol.
2. Experimental section
2.1. Catalyst synthesis
The preparation of ruthenium catalysts supported on niobium oxide
was carried out by the wet impregnation method with 5 wt% Ru, using
ruthenium(III) chloride salt (RuCl3.xH2O, Sigma Aldrich, 45ꢀ 55 wt%
Ru) as a metal precursor and hydrated niobium oxide (Nb2O5.xH2O, HY-
˜
340, CBMM – Companhia Brasileira de Metalurgia e Mineraçao), as
support.
Firstly, the pretreatment of the support at 393 K for 6 h in a muffle
furnace was carried out to remove moisture and contaminants (Nb2O5 in
amorphous phase (Nb2O5_A)). Then, a part of the pre-treated oxide was
subjected to heat treatment for 4 h at 873 K, also in a muffle furnace to
obtain the crystalline phase of Nb2O5 (Nb2O5_C). Separately, the
resulting oxides were placed in evaporation flasks together with aliquots
for the required ruthenium content, under agitation for 24 h, with
subsequent drying at 343 K using a vacuum pump.
Maris et al. [31] evaluated the use of the Ru/SiO2 catalyst in the
hydrogenation of glucose and observed that due to the inert nature of
the support, there was a minimization of the formation of by-products,
obtaining 100 % selectivity to sorbitol. However, the increase in the
particle size of the active metal through sintering, had a negative impact
on the reaction rate, obtaining lower glucose conversions. Mishra et al.
[32] reported the use of mixed oxides of TiO2/NiO, obtaining an in-
crease in selectivity (92.6–96.6 %) to sorbitol when compared to TiO2
supports. The difficulties presented by oxide supports occurred due to
the formation of agglomerates with the growth of the size of the Ru
particle in long reaction times. Moreover, the poisoning by adsorption of
reaction by-products negatively affected the catalytic activity of the
material, the mechanical stability and the yield to sorbitol.
After drying, the prepared samples were calcined in a muffle for 4 h
at 673 K with a heating rate of 10 K minꢀ 1, in order to reduce the con-
tent of precursor salt in the synthesized samples and formation of
ruthenium oxide. The synthesized samples were named as indicated in
Table 1:
The catalysts were stored under ambient conditions for subsequent
catalytic reduction and hydrogenation.
The different structural and chemical properties presented by
niobium oxide, such as the support effect, in which there is a strong
metallic interaction with the active phase (SMSI - strong metal-support
interaction), polymorphism and promoter effect; may cause variations
in yields to reaction products and catalytic activity, ensuring its appli-
cations in numerous types of reactions [33–40].
2.2. Catalyst characterization
The catalysts, as well as the supports, were characterized by different
techniques.
The TG/DTA curves were obtained using a DTG-60 Shimadzu
equipment in the temperature range of 298ꢀ 1073 K at a rate of
10 K minꢀ 1 and conducted under inert air atmosphere using a mass of
approximately 3 mg of the sample.
The use of thermal processes in the generation of crystalline forms of
niobium oxide and application as support in Fischer-Tropsch reaction
resulted in different catalytic activities and selectivities. For crystalline
niobia, despite the decrease in specific surface area and pore volume
after heat treatment, a selectivity of 70 % for C5+ was achieved, whereas
when using amorphous niobia only 49 % was obtained [41].
X-ray powder diffraction (XRD) patterns of the synthesized catalysts
were performed in a Shimadzu XRD 6100 X-ray diffractometer with
CuKα tube operating at 40 kV and 30 mA. Firstly, the materials were
analyzed in the range of 2θ = 10–80 ◦ and a scanning rate of 2◦ minꢀ 1 to
obtain the crystallographic profile of the samples. Then, the crystalline
samples were scanned again in the range of 26◦ and 38◦ with scanning
rate 0.25◦ minꢀ 1 for better identification of the peaks indexed to RuO2
and calculation of the crystallite size from the Scherrer equation (Eq. 1):
Regarding the use of Nb2O5 in sugar processing, the use of niobium
oxide in the amorphous form to produce furfural from the dehydration
of xylose using a water/toluene medium, proved to be determinant as
the most active (93 % conversion) and selective (48 %) catalyst for the
reaction. The presence of Lewis acid sites in its structure, easy separation
of the catalyst and high reusability showed as essential for these results
when compared to homogeneous catalysts (HCl and Sc(OTf)3) [42].
Kreissl et al. [43] evaluated the correlation between the tunable acidity
quantity and strength of niobium oxides and their morphologies as well
as their application in sucrose transformation to 5-HMF. The authors
noticed that the presence of both acid types and variation of strength
lead to different 5-HMF yield. Furthermore, for the conversion of su-
crose, glucose and fructose components, the mesoporous niobium oxide
presented the highest HMF yield (36 % for each sugar) due to the
balanced quantity of Brønsted and Lewis acid sites and strengths. Luo
et al. [44] carried out a study of the application of Nb2O5 as a support
alternative for Ru in the hydrogenation of levulinic acid at 473 K and
40 bar of H2. The use of Nb2O5 promoted a 61.8 % yield in the formation
of γ-valerolactone (GVL) and low selectivity to other reaction products
such as methyltetrahydrofuran (MTHF). It was also noted that there was
excellent stability of the material with low leaching rates of Ru for the
reaction medium (maximum of 2%).
Kλ
β1/2cosθ
dhkl
=
(1)
where dhkl is the mean diameter of the crystallite size (nm), λ is the cell
wavelength used by the XRD equipment (0.1541 nm), K is the Scherrer
constant, related to the shape of the crystallite (for spherical shaped
particles, value equal to 0.89), β1/2 is the corrected peak width for the
spread of the equipment in radians (in radians), cosθ is the cosine of the
angle of the refractive peak.
The determination of the particle size related to Ru0, the following
correlation was used (Eq. 2):
Table 1
Nomenclature of prepared samples.
SAMPLE NAME
CATALYST
Nb2O5_A
Nb2O5 in amorphous phase
Nb2O5_C
Nb2O5 in crystalline phase
5RuNb2O5_A
5RuNb2O5_C
5% Ru/Nb2O5 in amorphous phase
5% Ru/Nb2O5 in crystalline phase
Therefore, the present work aimed at the synthesis, characterization
and evaluation of ruthenium catalysts supported on niobium oxide in
2