Tuning product selectivity in the catalytic oxidation of glycerol by employing metal-ZSM-11 materials

Article abstract of DOI:10.1039/c9nj04106k

Copper and chromium supported on ZSM-11 (MEL-type structure) microporous zeolites were investigated as catalysts for glycerol oxidation towards dihydroxyacetone and lactic acid. ZSM-11 was synthesized by hydrothermal crystallization. Subsequently, alkaline treatment of desilication was carried out in order to generate zeolites with micro-mesoporosity. Ion exchange with ammonium chloride was performed to recover acidic sites and then, Cr(iii) and Cu(ii) species were incorporated onto these materials. Finally, thermal treatment at 500 °C was carried out. These materials were characterized by different techniques and then evaluated in the glycerol oxidation reaction by employing H₂O₂ as an oxidizing agent. The maximum conversion of glycerol (～70%) was reached over the Cr-ZSM-11 catalyst with a selectivity of ～28% towards dihydroxyacetone. Meanwhile, Cu-ZSM-11 offered 50% glycerol conversion with a selectivity of 68% towards lactic acid. In both the cases, the optimal reaction conditions were studied to maximize the selectivity towards the products mentioned above.

Full text of DOI:10.1039/c9nj04106k

View Article Online
View Journal
NJC
New Journal of Chemistry
Accepted Manuscript
A journal for new directions in chemistry
This article can be cited before page numbers have been issued, to do this please use: E. Diguilio, E.
Galarza, M. E. Domine, L. Pierella and M. S. Renzini, New J. Chem., 2020, DOI: 10.1039/C9NJ04106K.
Volume 42
Number
March 2018
Pages 3963-4776
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
6
21
NJC
New Journal of Chemistry
rsc.li/njc
A journal for new directions in chemistry
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 1144-2546
PAPER
Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
prepared via a microwave-assisted hydrothermal route using
H2O2 and KMnO4 as oxidizing agents
rsc.li/njc

Page 1 of 35
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
View Article Online
Tuning products selectivity in the catalytic oxidat
glycerol employing Metal-ZSM-11 materials
D
i
O
o
I:
n
10.10
o
39/C9NJ04106K
f
1
1
2
1
Diguilio, Eliana ; Galarza, Emilce D. ; Domine, Marcelo E. ; Pierella, Liliana B.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
and Renzini, María S.1
1Centro de Investigación y Tecnología Química (CITeQ), UE CONICET- Universidad
Tecnológica Nacional, Facultad Regional Córdoba, Maestro Lopez esq Cruz Roja
Argentina, Ciudad Universitaria, (5016) Córdoba, Argentina. E-mail:
ediguilio@frc.utn.edu.ar
2
Instituto de Tecnología Química (UPV-CSIC), Universitat Politècnica de València,
Consejo Superior de Investigaciones Científicas, Avda. de los Naranjos s/n, 46022,
Valencia, España.
Abstract.
Copper and chromium supported on ZSM-11 (MEL type structure) microporous zeolites
were investigated as catalysts for glycerol oxidation towards dihydroxyacetone and lactic
acid. ZSM-11 was synthesized by hydrothermal crystallization. Subsequently an alkaline
treatment of desilication was carried out, in order to generate zeolites with micro-
mesoporosity. An ionic exchange with ammonium chloride was done to recover acids
sites, and then, Cr (III) and Cu (II) species were incorporated onto these materials. Finally,
thermal treatment at 500 ºC was carried out. These materials were characterized by
different techniques, and then, evaluated in glycerol oxidation reaction, employing H O
2
2
as an oxidizing agent. The maximum conversion of glycerol (≈70%) was reached over
Cr-ZSM-11 catalyst, with selectivity to dihydroxyacetone of ≈28%. Meanwhile Cu-ZSM-
1
1 offered 50% of glycerol conversion with selectivity to 68% towards lactic acid. In both
cases, the optimal reaction conditions were studied to maximize the selectivity towards
the products above-mentioned.
1

New Journal of Chemistry
Page 2 of 35
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
View Article Online
DOI: 10.1039/C9NJ04106K
Key Words. Glycerol Oxidation, Cu, Cr-Zeolites; Dihydroxyacetone, Lactic Acid.
1
. Introduction
In recent years, biodiesel industry has increased its production worldwide, mainly due to
the decline in fossil fuel reserves with the consequent increase in the price of oil. Biodiesel
is synthesized via the transesterification of triglycerides (vegetable oils) with methanol to
produce the corresponding fatty acid methyl esters (FAMEs). The raw materials
commonly employed as source of triglycerides used to be colza, sunflower, soybean and
palm oil, among other, and the biodiesel obtained can be applied directly as fuel or
blended with petroleum-derived diesel fractions in different ratios 1.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
USA, Brazil, Argentina, Indonesia, and Malaysia are between the top biodiesel producer
countries in the world, followed by others such as Thailand, Uruguay, and Colombia.
Exterior Agriculture Service (USDA, Bs. As.) of Argentina has reported national
production of biodiesel in 2018 in more than 2, 0 million of tons, close to the production
of 2, 3 million tons generated in 2017, and 2, 6 million tons in 2016, which was the record
year. Exports of biodiesel remained unchanged in 2018 compared to the previous year (1,
4
million of tons/year), while domestic consumption decreased in ≈ 22%.
The main by-product in biodiesel production is glycerol (10% in weight of total
production). The exportation of crude glycerol in December 2018, has diminish in 25,4
%
respect to December 2017, with a decrease in its price due to the high supply in the
market according was informed by the Argentina Biofuel Camera (Report 2018, 2019)2 .
Therefore, in recent years, the interest in find alternatives to increase the productivity of
the biodiesel chain has arisen, generating added value to the main by-product. Moreover,
glycerol accumulation in biodiesel plants can cause environmental problems, becoming
into an inconvenient waste.
2

Page 3 of 35
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
View Article Online
DOI: 10.1039/C9NJ04106K
Glycerol (GLY) is a useful chemical with many industrial applications such as cosmetics
(
soaps, creams), lubricants, paints and food industry, as well as intermediate in the
production of synthetic fibers. It is considered as one of the most important building block
molecules derived from biomass, from which can be obtained a large variety of higher
value-added compounds 3.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
In recent years, numerous glycerol valorization routes have been studied. One of the most
important and non-commercialized routes for glycerol valorization is the selective
oxidation of glycerol (GLY) in liquid phase. In this process, a large number of products
are generated mainly due to the high functionality of glycerol molecule. One of these
products is dihydroxyacetone (DHA), a simple carbohydrate with highest commercial
value due to its use in pharmaceutical and cosmetic industry. Other products from
glycerol oxidation are glyceraldehyde (GLYALD), as well as glyceric (GLYAC),
glycolic (GLYCAC), lactic acid (LA) and hydroxy-pyruvic (HYPAC) acids; most of
these compounds result interesting for applications in cosmetic, plastic and
pharmaceutical industries 4 56.
This work is focused on the production of DHA and LA from glycerol, since both are
relevant chemicals for several industries.
On one hand, DHA is a carbohydrate with three carbon atoms, widely used in cosmetic
industry to produce sunscreens. In addition, DHA combined with pyruvate results in an
effective nutritional supplement than can help to eliminate fat and increase muscle mass7.
Nowadays, DHA is produced from the microbial fermentation of glycerol employing
Gluconobacter oxidans with 87-94% yield after ∼32 h. The productivity to DHA is
relatively low due to the low glycerol concentration in aqueous solution and the long
89
reaction times applied .The direct catalytic oxidation of glycerol to DHA over Au, Pt
and Pd supported catalysts has been studied a few years ago. Nevertheless, the use of
3

New Journal of Chemistry
Page 4 of 35
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
View Article Online
DOI: 10.1039/C9NJ04106K
these metals results expensive and has some drawbacks such as the catalyst deactivation,
mainly due to the sintering of metal nano-particles and the leaching of metals in the
reaction medium 1011. In this sense, it is of great interest the design of heterogeneous
catalysts possessing transition metals, with low cost and high resistance under reaction
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
conditions that also result efficient and selective towards the desired product .
Hu et al. have investigated glycerol selective oxidation to dihydroxyacetone in a semi-
batch reactor over Pt-Bi/C catalyst, with a maximum DHA yield reported of 48% at 80%
13
glycerol conversion . Recently, Crotti et al. have tested an “in situ” prepared iron
complex [Fe (BPA) (OTf) ] as alcohol oxidation catalyst, employing H O/CH CN as
2
2
2
3
reaction medium, and obtaining DHA selectivity of 53% after 90 min. of reaction at 25
14
°
C . Other authors have studied a series of sulfonato-salen metal complexes intercalated
into Mg-Al layer double hydroxide (LDH) with transition metals for the selective
oxidation of glycerol to produce dihydroxyacetone (DHA). The glycerol conversion was
4
0.3% and DHA selectivity reached 52.9% over Cu(II) sulfonato-salen-intercalated LDH
catalyst (at 60 °C, during 4 h and pH = 7), while glycerol conversion was 57.0%
employing Cr(III) sulfonate-salen-intercalated LDH catalyst and DHA selectivity was
1
4
42.3% under similar reaction conditions .
On the other hand, LA (2-hydroxypropanoic acid or lactic acid) is an organic acid widely
distributed in nature, which is actually being a molecule of great interest in the booming
bio-refinery industry. Due to its different properties 15, lactic acid is an important
industrial product with several applications as food additive, as well as in cosmetics and
pharmaceuticals. Currently, more attention has been paid on the potential use of LA as
monomer in the production of a biodegradable poly-lactic acid or polylactide,
thermoplastic aliphatic polyester with a wide range of applications in the polymer
industry. Li et al. have investigated Ni promoted noble metal catalysts for the glycerol
4

Page 5 of 35
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
View Article Online
DOI: 10.1039/C9NJ04106K
oxidation to LA in the presence of NaOH solution, achieving practically 100% glycerol
conversion and 62.6% LA selectivity in the first run of the reaction. However, the glycerol
1
6
conversion declined from ≈100% to 57.2% during the recycling experiments . In
addition, Zhang et al. 17 have studied the use of a series of copper-promoted platinum
supported on activated carbon (Cu-Pt/AC) catalysts for glycerol oxidation to lactic acid.
At 90°C, the average glycerol conversions were 56% with selectivity to LA lower than
72%.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Considering the scarcity and price of noble metals, exploring and developing novel,
efficient, and resistant catalysts with lower cost (by using non-noble metals) is highly
desirable. Following this idea, here is proposed the study of the catalytic oxidation of
GLY using modified ZSM-11 zeolites by alkaline treatment, generating a combined
micro-mesoporosity system. Each porosity in the hierarchical structure exhibits
differences, while microporous hold catalytically active sites, the access to the pores is
facilitated by mesoporous. Hierarchical zeolites exhibits improved accessibility to the
active sites, which facilitates mass transport of molecules, and they result more resistant
to deactivation by coke formation. Consequently, these materials show a higher catalytic
activity than conventional zeolites, in particular in those reactions that include bulky
molecules because prevent diffusional limitations. Furthermore, the secondary porosity
allows greater access to the active phases (cations, metal oxides, metal salts and metal
complexes) located in the external surface or micropores of the zeolites 418. Subsequently
the synthesized micro/mesoporous materials will be impregnated with transition metals
of Copper (II) and Chromium (III), to be evaluated as catalysts in the oxidation of glycerol
in liquid phase.
The optimal reaction conditions are properly studied and discussed in the following
sections. The adequate selection of metal and its presence in the tailor-made ZSM-11
5

New Journal of Chemistry
Page 6 of 35
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
View Article Online
DOI: 10.1039/C9NJ04106K
zeolite structure allow us tuning catalyst properties and driving reaction mechanism to
the attainment of dihydroxyacetone (DHA) or lactic acid (LA) desired products.
2. Experimental
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2.1 Catalysts preparation
ZSM-11 zeolite was synthetized by hydrothermal crystallization, at 140 ºC, using
1
9
tetrabutyl ammonium hydroxide (TBAOH) as a structure-directing agent . The obtained
gel was extracted by filtration, washed with deionized water, and then dried at 100 °C
during 24 h. To remove TBAOH, a thermal treatment was carried out in N atmosphere
2
(
20 ml/min) at programmed temperature (20° C/min) from ambient temperature up to 500
°C. Then, the solid was calcined in static air at 500 °C during 8 h to obtain the Na-ZSM-
1 material. NH -ZSM-11 form of the zeolite was prepared by ion exchange of the as-
1
4
prepared Na-zeolite with 1 M ammonium chloride aqueous solution at 80 °C during 40
h. Finally, thermal treatment (calcination) at 500 ºC under a N stream was applied to the
2
NH -zeolite to finally obtain the protonic form of the H-ZSM-11 zeolite.
4
2
.1.1 Alkaline post-synthesis treatment
The generation of mesoporous in microporous zeolites was carried out by post-synthesis
2
0
desilication treatment . The methodology consisted in the hydrolysis of the Si-O-Si
bonds by reacting the microporous zeolite with an alkaline solution of NaOH. This
procedure allows the generation of mesoporosity while the acidic properties of the zeolite
were maintained. For this purpose, 1 g of Na-ZSM-11 was placed in a flask and treated
with 30 ml of NaOH aqueous solution (0.3 M). The system was immersed in a water bath
with constant magnetic stirring to maintain a constant temperature of 65 °C during 30
min. After reaction, the solid material was recovered by filtration, and then dried at 100
°C overnight. Finally, three successive exchanges were performed employing NH Cl
4
aqueous solution (0.5 M) at 80 °C during 1 h. The solid was again filtered under vacuum,
6

Page 7 of 35
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
View Article Online
DOI: 10.1039/C9NJ04106K
and dried at 100 °C during 12 h. Thermal treatment (calcination) at 500 ºC under a N2
stream was carried out to remove residual salts, thus obtaining the NH -ZSM-11 (T)
4
material. Later on, this material will be modified by incorporation of transition metals.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
.1.2 Incorporation of transition metals
Synthesized zeolites were modified by introduction of transition metals such as Cr (III)
and Cu (II). These metals were incorporated to the NH -zeolite form by wet impregnation
4
method. To this purpose, the catalyst was suspended in aqueous solution of the
corresponding metal precursor, Cl Cr.6H O (Fluka, 98%) for Cr-zeolite and Cu
3
2
(
C H O ) .H O (Mallinckrodt, ACS) for Cu-zeolite. Then, the samples were evaporated
2 3 2 2 2
at 80 °C under vacuum, until complete dryness. After that, the solids were dried at 100
C in an oven during 24 h. Finally, Cr-zeolite and Cu- zeolite were thermally treated under
N flow (20 ml/ min) from ambient temperature up to 500 °C, with a heating ramp of
°
2
2
0°C/min, during 8 h, followed by calcination in static air under the same conditions.
2
.2 Catalysts Characterization
Catalytic materials were characterized by different techniques:
X-ray diffraction (XRD) was employed to determine the structure and the crystallinity
-
level of the zeolitic materials, by using a Philips PW 3020 diffractometer using CuKα
radiation. Diffraction data were recorded from 2θ = 2º - 60º at an interval of 0.1º and
scanning speed of 2 º/min was used. Relative crystallinity of the zeolitic materials was
calculated by using the following formula (where Pattern refers to a standard sample of
ZSM-11 zeolite synthesized in our research group) 21.
퐴푟푒푎 푢푛푑푒푟 푡ℎ푒 푝푒푎푘푠 푠ℎ표푤푠 푋
%
퐶푟푦푠푡푎푙푙푖푛푖푡푦 =
∗ 100
퐴푟푒푎 푢푛푑푒푟 푡ℎ푒 푝푒푎푘푠 푠ℎ표푤푠 푃푎푡푡푒푟푛
Infrared spectroscopy analysis were performed in a JASCO 5300 FTIR spectrometer.
7

New Journal of Chemistry
Page 8 of 35
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
−1
View Article Online
DOI: 10.1039/C9NJ04106K
For the structural characterization in the lattice vibration region (400−1800 cm ), the
samples were mixed with KBr at 0.05% and pressed to form wafers. To determine the
type and concentration of acidic sites, pyridine (probe molecule) adsorption experiments
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
were carried out on self-supporting wafers (10−20 mg/cm ) of the zeolites, using a
thermostatted cell with CaF windows connected to a vacuum line.
2
−
4
Pyridine (3 Torr) was adsorbed at room temperature and desorbed (10 Torr) at 250, 350,
and 400 °C during 1 h. Finally, the number of Brønsted and Lewis acid sites was
calculated from the maximum intensity of the adsorption bands (1545 and 1450−1460
−
1
cm , respectively). These values were calculated by using literature data of the integrated
molar extinction coefficients, which are independent of the catalysts or strength of the
sites 22.
-
BET surface area determinations were carried out with ASAP 2000 equipment with N2
absorption at 77 K.
-
Nitrogen sorption isotherms were measured in Micromeritics apparatus Model ASAP
010 equipment. It was determined the total pore volume of zeolites at relative gas
2
pressure (p/p ) = 0.98, assuming complete filling of pores. Micropore volume (Vmicro) was
0
determined by t-plot method. Mesopore volume was calculated by difference between
total and micropore volume (VTotal – Vmicro = Vmeso).
-
The metal content and Si/Al relation in the zeolites were determined by an inductively
coupled plasma-atomic emission spectrometer ICP Varian 715ES.
UV–Vis-DR spectra of the materials were recorded using a Perkin Elmer precisely –
-
Lambda 35 spectrophotometer, to which a diffuse reflectance chamber Labsphere RSA-
PE-20 with an integrating sphere of 50 mm diameter and internal Spectralon coating is
attached.
8

Page 9 of 35
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
View Article Online
-
Transmission electron microscopy measurements were performed on JEM 2010 Plus
DOI: 10.1039/C9NJ04106K
electron microscope, equipped with EDS Oxford X-MAX 65 T operating at 200 kv. The
samples were pre-treated by ultrasonic dispersion in ethanol, and then dropped onto grids.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
.3 Catalytic experiments
The catalytic activity of the metal-zeolites here prepared was evaluated in the selective
oxidation of GLY (Merck, ACS reagent) in liquid phase. The reaction was carried out in
a glass reactor (25 ml) with two necks, and equipped with a condenser and a magnetic
stirrer. The system was submerged in a water bath that allowed maintaining a constant
temperature (60 °C) all along the experiment. An aqueous solution of GLY (0.5 M) was
placed in the reactor, with H O as oxidizing agent (30% v/v) and 200 mg of catalyst. The
2
2
reaction products were identified and quantified by liquid chromatography (HPLC), in a
Jasco UV-975/ PU-980. An Aminex HPX - 87H column set at 50 °C, a UV-VIS detector
(
210 nm) and a Refractive Index detector coupled in series were employed. The mobile
phase was sulfuric acid (5mM) at 0.6 ml/min. The identification and quantification of the
oxidation products was done by comparison with the pure standards using calibration
curves.
The optimal reaction conditions were studied in this work employing the Cr-ZSM-11 and
Cu-ZSM-11 zeolites as catalysts, with the aim to obtain the maximum selectivity to DHA
and LA, respectively. The reaction temperature was varied in a range of 60 to 80 °C,
reaction times from 60 a 240 min, and the amount of catalyst from 0.1 to 0.3 g, whereas
the substrate/H O molar ratio (R) was also studied (R = 0.3 - 0.5 - 075 - 1).
2
2
In some cases, the efficiency of H O (mols of oxidating agent that are consumed to
2
2
transform GLY in oxydated products) was calculated by means of iodometric titration
employing sodium thiosulfate as titrant solution. This H O efficiency (Eff H O ) was
2
2
2
2
obtained as a percentage by applying the following equation:
9

New Journal of Chemistry
Page 10 of 35
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
푚표푙푒푠 푐표푛푣푒푟푡푒푑
View Article Online
DOI: 10.1039/C9NJ04106K
퐸푓푓 퐻2푂2 =
푥 100
푚표푙푒푠 푐표푛푠푢푚푒푑
Finally, the reuse and recyclability of the catalysts (Cr-ZSM-11 and Cu-ZSM-11) was
evaluated. For that, the used solid catalyst was recovered from the reaction mixture by
filtration, and then washed with water. Afterwards, the catalyst was used in a new
experiment without any regeneration treatment.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
. Results and Discussion
3.1 Catalysts characterization
X-ray diffraction patterns of Cr-Zeolites and Cu-Zeolites obtained by incorporation of
transition metals in both conventional and post-treated (T) ZSM-11 zeolites are shown in
Figure 1. As can be seen, all of the as-obtained solids showed the presence of a highly
crystalline MEL-type zeolitic structure with well-defined diffraction peaks of a high
structural order (2θ = 7-8° and 2θ = 21-22°). XRD patterns clearly show that zeolitic
structures were not influenced by the incorporation of transition metals using the
impregnation method. For Cr-zeolites, the characteristic peaks of the chromium oxide
+3
Cr O (*) were observed, which is an evidence of the presence of Cr species in the
2
3
zeolite. In the case of the Cu-Zeolite samples, no characteristic CuO signals are observed,
which could be an indication that the Cu has been almost completely exchanged, the
percentage of metal deposited as oxide being very low (not detectable by XRD).
1
0

Page 11 of 35
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
View Article Online
DOI: 10.1039/C9NJ04106K
Cr-ZSM-11 (T)
H-ZSM-11 (T)
*
*
*
*
*
*
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Cr-ZSM-11
H-ZSM-11
*
*
*
*
a)
5
10
15
20
25
30
35
40
45
50
55
60
2

Cu-ZSM-11 (T)
H-ZSM-11 (T)
Cu-ZSM-11
H-ZSM-11
b)
5
10
15
20
25
30
35
40
45
50
55
60
2

Figure 1. XRD patterns of a) Cr-ZSM-11 and Cr-ZSM-11 (T), b) Cu-ZSM-11 and
Cu-ZSM-11 (T).
The main physicochemical and textural properties of the zeolites measured by different
techniques such as ICP, N sorption isotherms; XRD and FTIR are summarized in Table
2
1
. The content of metal effectively incorporated in the zeolites was very close to the
theoretical calculated values. Surface area determined by means of BET method of the
2
conventional ZSM-11 was around of 390 m /g, while it was slightly increased in the
1
1

New Journal of Chemistry
Page 12 of 35
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
View Article Online
DOI: 10.1039/C9NJ04106K
micro-mesoporous ZSM-11 (T). The relative crystallinity was higher than 97-99% in all
cases, which indicates that the crystalline structure of the zeolites is maintained with high
purity after both the chemical and thermal treatments.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 1. Main physicochemical and textural properties of the catalysts.
Si/Al
molar
ratio
Cu, Cr
Surface
area (m /g)
Relative
Crystallinity (%)
B/Lc
Catalysts
2
(wt. %)
ICP
18.02
15.59
18.04
ICP
-
BETa
390
430
392
430
390
433
XRD
100
>97
100
>97
100
>97
FTIRb
>99
FTIR
9
1
0
0
2
0
.20
.56
.64
.08
.96
.79
H-ZSM-11
H-ZSM-11 (T)
Cu-ZSM-11
-
>97
3.11
2.90
2.98
2.99
>99
Cu-ZSM-11 (T) 16.72
Cr-ZSM-11 18.01
Cr-ZSM-11 (T) 17.91
>97
>99
>97
a
b
c
Surface area values calculated from N sorption isotherms by means of BET method.
FTIR in the fingerprint zone of the materials (400−1200 cm ).
Lewis acid sites (µmol Py/g of catalyst) and Lewis/Brønsted sites ratio obtained from FTIR
2
−1
spectra of pyridine absorbed at room temperature and desorbed at 400°C.
The textural properties of these materials before and after desilication treatment were
obtained by N sorption isotherms at -196 °C. On one hand, the isotherms of the
2
synthesized zeolites were isotherms type I (see Supporting Information Figure S1),
characteristics of microporous structures according to the IUPAC classification. On the
other hand, the hierarchical zeolites, obtained after desilication treatment, exhibit
isotherms combining type I-IV, with greater adsorption at P/P between 0.4-1.0, this
0
indicating the existence of voids between particles with irregular shape with broad size
2
3
distribution .
3
The total Pore Volume (cm /g) is high in zeolites with alkaline treatment (Cr- and Cu-
ZSM-11 (T)), which is directly related to the increase of mesopore volume and the small
decrease in micropore volume; all these observations clearly indicate the generation of
mesoporosity in these materials. Determination of the pore size distribution by following
1
2

Page 13 of 35
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
View Article Online
DOI: 10.1039/C9NJ04106K
BJH method was also possible, the obtained values being between 2 and 10 nm
(Supporting Information Figure S2).
The FTIR spectra of pyridine adsorbed on conventional and hierarchical metal-zeolites
-1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
are depicted in Figure 2, only showing the wavelength zone from 1400 to 1700 cm .
In Table 1 it is shown the Brønsted to Lewis acid sites ratio of the zeolites calculated
from FTIR spectra of pyridine absorbed at room temperature and desorbed at 400°C.
2
3
4
50°
50°
00°
Cr-ZSM-11
250°
350°
Cr-ZSM-11 (T)
4
00°
a)
1400
b)
1400
1700
1650
1600
1550
1500
1450
1700
1650
1600
1550
1500
1450
Wave number (cm-1)
Wave number (cm-1)
2
50°
350°
00°
Cu-ZSM-11
250°
350°
400°
Cu-ZSM-11 (T)
4
c)
1400
d)
1400
1700
1650
1600
1550
1500
1450
1700
1650
1600
1550
1500
1450
Wave number (cm-1)
Wave number (cm-1)
Figure 2. FTIR spectra of pyridine absorption experiments at room temperature and
desorption at 250, 350 and 400ºC, a) Cr-ZSM-11, b) Cr-ZSM-11 (T), c) Cu-ZSM-11
and d) Cu-ZSM-11 (T).
−
1
−
The band at 1638 cm , which is assigned to structural OH ion vibration, is indicative of
+
the interaction of pyridine with Brønsted acid sites (PyH ). Bands at wavenumbers
1
620−1623 cm−1 would result from strong Lewis sites, whereas bands at lower
−1
wavenumbers (1614 cm ) would indicate medium to strong sites. Interestingly, the
1
3

New Journal of Chemistry
Page 14 of 35
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
View Article Online
DOI: 10.1039/C9NJ04106K
intensity of these bands was greatly affected by desilication treatment, especially in Cu-
ZSM-11 (T), due to a great dissolution of Si-OH groups. The band at 1490 cm−1
corresponds to the vibration of pyridine adsorbed over Brønsted and Lewis acid sites. The
−
1
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
band at 1455 cm corresponds to the interaction of pyridine with Lewis acid sites (PyL),
−
1
and the band at 1545 cm is indicative of the interaction of pyridine with Brønsted acid
+
sites (PyH ). It can be observed that the intensity of this band decreases in metal-zeolites
2
4
after alkaline treatment, until total disappearance in Cu-ZSM-11 (T) . Upon metal cation
introduction, the Lewis acid sites were significantly increased in both metal- zeolites (Cr,
Cu) with respect to H-ZSM-11 catalyst, resulting in a higher ratio of acid sites Lewis to
Brønsted mostly in Cu-ZSM-11 (T).
In literature it was proposed that in desilicated zeolites the majority of Lewis sites are
created from dehydroxylation of the Si–OH–Al groups, after the reinsertion of some Al
extracted during alkaline treatment. Consequently, the new Lewis sites formed are
primarily located on the mesopore surfaces, allowing accessibility of larger molecules.
Some Al may remain in extra- framework positions, modifying the acidity of these
materials. Some catalytic application may require an Al species extra-framework such as
Lewis acid centers or a reduced concentration of Brønsted acid sites. These reactions
generally are benefited by micro-mesoporosity for improved transport of molecules 1825
.
The morphology, distribution and the particle size of metal-based catalysts were
determined by Transmission electron microscopy (TEM) measurements. TEM images of
the samples are presented in Figures 3 and 4, where the dark spots of the images would
be the particles of Cu and Cr that have been incorporated in the zeolite ZSM-11 by wet
impregnation. In order to investigate the dispersion and average size of the particles, the
TEM micrograph was analysed by using Image J NIH program, thus counting more than
5
0 metal nanoparticles in each case. In this way the particle size distribution of the
1
4

Page 15 of 35
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
View Article Online
DOI: 10.1039/C9NJ04106K
samples was obtained too. The application of the procedure above mentioned resulted in
Cu nanoparticle sizes between 0.5 and 3.5 nm, with greater distribution between 1.0 and
2.5 nm, with an average size of 1.8 nm, showing a heterogeneous dispersion of the crystals
on the solid surface. Small particle size favours that metal nanoparticles have a greater
area exposed to the reactant molecules, thus increasing the catalytic activity of this
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
7
material in the conversion of GLY .
Cu-ZSM-11
0,5
1,0
1,5
2,0
2,5
3,0
3,5
Particle size (nm)
Figure 3. TEM images at different scales (top) and distribution of particle sizes (down)
of Cu-ZSM-11 catalyst.
1
5

New Journal of Chemistry
Page 16 of 35
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
View Article Online
DOI: 10.1039/C9NJ04106K
TEM images and the particle sizes distribution of the Cr-ZSM-11 catalyst are shown in
Figure 4. The images allow us to observe the dispersion of the metal particles on the
surface of the crystals, as well as their small size when Cr was incorporated into the
zeolite. Newly, the procedure described above was done to measure metal particles size
from the TEM micrograph, by counting more than 50 particles in each case. It can be
observed that Cr nanoparticles are less dispersed than Cu nanoparticles on the surface of
the crystals. In addition, the formation of agglomerates of Cr nanoparticles was observed,
this increasing their average size, which ranged from 1.6 nm to 5.0 nm, majority between
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
-3 nm, with an average size of ≈3.5 nm, higher than the case of Cu nanoparticles
incorporated on zeolite.
1
6

Page 17 of 35
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
View Article Online
DOI: 10.1039/C9NJ04106K
Cr-ZSM-11
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
10
11
Particle size (nm)
Figure 4. TEM images at different scales (top) and distribution of particle sizes (down)
of Cr-ZSM-11 catalyst.
3
.2. Catalytic Activity Results
The catalytic performances of the as-prepared zeolites were investigated in the selective
oxidation of GLY performed in liquid phase and using aqueous H O as oxidant. In these
2
2
experiments, the effect of the operational parameters of reaction and the type of catalysts
on the selective transformation of GLY to DHA and LA was analyzed. In all the cases,
any organic solvent or additive was used.
The evolution of the GLY conversion and the selectivity to different products with
reaction time for different catalysts are shown in Figure 5. Results were attained by
employing 0.2 g of each catalyst, with a substrate/H O molar ratio of 0.5, at 60 ºC and
2
2
in the absence of solvent (see also Supporting Information Figure S3). A negligible GLY
conversion of (~1 mol %) was detect in the absence of catalyst (data not shown). The
reaction catalyzed by the H-ZSM-11 conventional zeolite without metal loading only
gave a GLY conversion of 29.8 mol% at 4 h of reaction. The oxidation products detected
in this case were GLA (20.6 mol %), GA (20.3 mol %) and FA (54.5 mol %).
Interestingly, the GLY conversion increase notably when conventional ZSM-11 zeolites
modified with copper and chromium are used, which indicates that the presence of these
1
7

New Journal of Chemistry
Page 18 of 35
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
View Article Online
DOI: 10.1039/C9NJ04106K
metals, and more importantly, the increase of Lewis acid sites in the solid catalysts could
benefit such reaction.
In general, the zeolites with alkaline treatment resulted slightly more actives than the
traditional ones, in terms of GLY conversion, at short reaction times (≤60-90 min of
reaction). The lower Si/Al ratio in hierarchical zeolites (compared to starting zeolite)
would be responsible for this behavior, and this could lead to a decrease in the strength
of acid sites. For low Si/Al ratios in the framework of zeolites, the Brønsted /Lewis ratio
generally decreases, which makes them more prone to deactivation in the early reaction
stages, possibly due to the deposition of carbonaceous compounds within the pores of the
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
8
zeolites. .
In the case of Cu-containing zeolites, Cu-ZSM-11 (T) hierarchical material was more
active than Cu-ZSM-11 zeolite (until 120 min), when GLY conversion reaches a
stationary plateau. This could be due to the lower ratio Brønsted/Lewis acid sites,
resulting in a faster deactivation of them by blocking the pores with carbonaceous
compounds.
In the case of Cr-zeolites, both Cr-ZSM-11 and Cr-ZSM-11 (T) catalysts lead to similar
GLY conversion at shorter reaction times (< 60 - 90 min of reaction). Nevertheless, Cr-
ZSM-11 offered higher GLY conversion than its analogous Cr-ZSM-11 (T) sample after
this reaction time, which would also be related to the Brønsted/Lewis relationship of acid
sites.
According to these data and considering the FTIR results above mentioned (see Figure
3
+
2
), it is possible to adjudicate this behavior to the presence of Cr species, since they can
3
5
capture two electrons of atomic oxygen (valence shell structure from 3d to 3d ), thus
promoting the activation of H O and increasing the catalytic activity. Similar behavior
2
2
1
8

Page 19 of 35
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
2+
View Article Online
DOI: 10.1039/C9NJ04106K
was observed for Cu , since it can easily get one electron from H O to complete its
2
2
1
0 26
valence shell structure (3d ) .
Commonly, the three types of reactions are involved during the oxidation of GLY
dehydrogenation/hydrogenation reactions, C-O bond cleavage (dehydration) and C-C
bond cleavage. Brønsted acid sites are crucial for dehydration of DHA/GLA
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
intermediates) to produce pyruvic aldehyde (not detected here) and then by
rearrangement it generates LA, while Lewis acid sites catalyze hydrogenation and C-C
cleavage of molecules producing compounds of lower number of carbon (C and C ),
2
1
such as glycolic (GA), acetic (AA) and formic acid (FA) 4.
With respect to the products distribution, the main products detected during the GLY
oxidation reaction employing Cu and Cr-Zeolites under the reaction conditions used
herein were dihydroxyacetone (DHA), glyceraldehyde (GLA), glycolic acid (GA), lactic
acid (LA), acetic acid (AA), and formic acid (FA), as well as other by-products
(
quantified but not identified). The GLY conversion and the products distribution
determined over the time for Cu- and Cr-zeolites during the GLY oxidation are shown in
Figure 5.
GLY
DHA
GLA
GA
FA
AA
Cr - ZSM-11
8
7
6
5
4
3
0
0
0
0
0
0
80
70
60
50
40
30
20
10
0
80
70
60
50
40
30
20
10
0
GLY
DHA
GLA
GA
FA
AA
Cr-ZSM-11 (T)
80
70
60
50
40
30
20
10
0b)
20
10
0
a)
0
30
60
90
120
150
180
210
240
0
30
60
90
120
150
180
210
240
Reaction Time (min)
Reaction Time (min)
1
9

New Journal of Chemistry
Page 20 of 35
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
View Article Online
DOI: 10.1039/C9NJ04106K
100
90
100
90
100
90
80
70
60
50
40
30
20
10
0
100
GLY
DHA
GLA
GA
Cu-ZSM-11
GLY
DHA
GLA
GA
Cu-ZSM-11 (T)
90
80
70
60
50
40
30
20
10
0 d)
80
70
60
50
40
30
20
10
0
80
LA
LA
70
60
50
40
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
30
20
10
0 c)
0
30
60
90
120
150
180
210
240
0
30
60
90
120
150
180
210
240
Reaction Time (min)
Reaction Time (min)
Figure 5. GLY conversion and selectivity (mol%) towards different products
employing Metal-Zeolites. a) Cr-ZSM-11; b) Cr-ZSM-11(T); c) Cu-ZSM-11 and d) Cu-
ZSM-11 (T).
As can be seen, glyceraldehyde (GLA) is one of the primary products formed at the
beginning of the reaction, together with dihydroxyacetone (DHA), and also glycolic acid
(
GA, secondary product) when Cr-zeolites are employed (see reaction network S4 in the
Supporting Information). The production of DHA reaches a maximum for Cr-ZSM-11
and Cr-ZSM-11 (T) around 120 min of reaction, then it continues being oxidized, so that
over-oxidation products (mainly AA and FA) appear in the final reaction mixture.
Interestingly, the appereance of the over-oxidation products is rather delayed in Cr-ZSM-
1
1 (T) catalyst compared to Cr-ZSM-11 sample.This could be attributed to the higher
content of Lewis acid sites present in the Cr-ZSM-11 sample that would be responsible
to the formation of products generated by over-oxidation.
A different catalytic behavior is observed when employing Cu-ZSM-11 and Cu-ZSM-11
(T), where the main product of the reaction is the lactic acid (LA). In both cases, LA is
generated at the beginning of the reaction together with GLA and DHA, and also GA, the
two latter being formed in quite low amounts. The maximun production of LA is obtained
at 60 min of reaction for both Cu-modified zeolites. Then, lactic acid is transformed into
acetic and formic acids after 4 hours of reaction. In fact, the production of LA after 1 h
of reaction was ~70 mol% (36% yield ) when employing Cu-ZSM-11, that was higher
2
0

Page 21 of 35
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
View Article Online
DOI: 10.1039/C9NJ04106K
than LA selectivity (56.5 mol%, ~20% yield) obtained with Cu-ZSM-11 (T), always
working under the same reaction conditions above-mentioned. This performance was in
2
7 28
concordance with data published in the literature , .
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Moreover, another effect like “the confinement effect” in the microporous zeolites can be
considered in these cases. This effect was corroborated by using zeolites with larger pore
size, such as Cu-Beta, Cu-Y and Cu-MCM-41 that gave very low results in terms of
glycerol conversion, reaching no more than 30% mol of GLY conversion at same reaction
conditions, without detecting any product of interest (DHA or LA) 29.
Table 2 summarizes the selectivity to the more desired products DHA and LA at 60 and
120 min, since the majority generation of both products takes place at low reaction times.
Indeed, it can be observed that DHA and LA selectivity values are low at the end of the
reaction; this is mainly due to the continuous oxidation of these products into others such
as acetic and formic acids as above-mentioned (see Figure 3). In subsequent studies, we
decided to focus on DHA and LA since they have a higher commercial value than the rest
of the products.
Table 2. GLY conversion and selectivity (mol %) towards DHA and LA over different
Metal-Zeolites
DHA
Reaction Time GLY Conversion
Selectivity
(mol%)
11.4
LA Selectivity
Catalyst
(min)
(mol%)
13.7
40.6
23.8
42.5
27.1
42.5
24.1
31.5
(mol%)
Cr-ZSM-11
60
--
1
20
28.3
18.0
27.5
19.3
25.2
3,0
5,0
--
Cr-ZSM-11 (T)
Cu-ZSM-11
60
10.2
5.3
68.0
49.0
51.4
41.5
1
1
20
60
20
Cu-ZSM-11 (T)
60
120
Reaction conditions: Sol. GLY 0.5 M, GLY/H O 0.5 (mol/mol), T = 60°C, reaction time = 60,
2
2
1
20 min.
Considering the results exposed here, Cr-ZSM-11 showed the best catalytic behavior for
the conversion of GLY to DHA, while Cu-ZSM-11 showed the major yields to LA,
2
1

New Journal of Chemistry
Page 22 of 35
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
View Article Online
DOI: 10.1039/C9NJ04106K
respectively. Thus, these two zeolites were chosen to study the production of DHA or LA
by optimizing different reaction parameters.
.2.1 Selective production of DHA over Cr-zeolites
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 4 shows the change in the GLY conversion and DHA selectivity at 120 min of
reaction time over Cr-ZSM-11 zeolite by analyzing the influence of different reaction
parameters, such as tempertaure, amount of catalyst, and GLY/H O molar ratio, this
2
2
allowing elucidating the optimal reaction conditions to maximize DHA yields.
6
0
60
50
40
30
20
10
0
GLY
DHA
FA
GLY
DHA
5
4
3
2
0
0
0
0
50
40
30
20
10
50
40
30
20
10
0
10
0
0b)
a)
40
50
60
70
80
0,0
0,1
0,2
0,3
Temperature (°C)
Catalyst mass (g)
6
5
4
0
60
50
GLY
DHA
0
0
4
0
0
3
30
20
20
10
10
0
0
C)
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
1,1
Molar ratio substrate/ H O2
2
Figure 6. Influence of the reaction parameters on conversion and selectivity using Cr-
ZSM-11 zeolite, at 120 min of reaction time: a) temperature; b) amount of catalyst; c)
GLY/H O molar ratio.
2
2
The effect of reaction temperature on catalytic activity of Cr -ZSM-11 for the conversion
of GLY and selectivity to DHA was studied by varying this parameter from 40 to 80 °C
(Figure 6a). In these experiments, GLY (0.5 M), H O (30 %v/v), and mass of catalyst
2
2
(
0.2 g) were kept constant. With the increase of the reaction temperature, the GLY
2
2

Page 23 of 35
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
View Article Online
DOI: 10.1039/C9NJ04106K
conversion increased from <15% until 60%, as expected. However, excessive
temperature (>60 ºC) exhibited an adverse effect on DHA selectivity, indicating the
presence of over-oxidation process. This behavior is evidenced by the increase in the
1
3 30
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
selectivity of FA , .
Catalyst mass introduced in the reaction was varied from 0.1 to 0.3 g by keeping the rest
of operational parameters unchanged, and the results are given in Figure 6b. GLY
conversion and DHA selectivity values increase together until catalyst mass of 0.2 g,
where a maximum is achieved in both cases (GLY conversion ≈40%, and DHA selectivity
≈
30%, respectively). Then, a slight decrease could be observed at higher amount of
catalyst. This tendendy may be is related to an auto decomposition of hydrogen peroxide
31
that gives water and oxygen as products (H O → H O + ½ O ) , thus reducing GLY
2
2
2
2
oxidation. This indicates that excessive amount of catalyst was unfavorable to the
selective oxidation reaction of GLY to DHA. So, since DHA selectivity were
considerably higher with 0.2g of catalyst, attempts to optimize reaction conditions were
done employing this amount of catalyst.
As can be observed in Figure 4.c, the use of different concentrations of H O (GLY/H O
2
2
2
2
molar ratios from 0.3 to 1.0) as oxidant generates a maximum DHA selectivity of ~28%
at 2 h of reaction. The increase in the molar concentration of hydrogen peroxide produces
not significantly changes in GLY conversion at 2 h, obtaining the best compromise
between GLY conversion and DHA selectivity with substrate/H O molar ratio of 0.5.
2
2
Interestingly, it could be observed that working with higher concentration of hydrogen
peroxide (substrate/H O molar ratio = 0.3) a selectivity to DHA of ≈24 mol% was
2
2
reached 15 min after the reaction has started, although this DHA production is strongly
diminished at higher reaction times due to the favored over-oxidation processes taking
place at higher oxidant concentrations.
2
3

New Journal of Chemistry
Page 24 of 35
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
View Article Online
To study the consumption of H O due to the Cr-based catalyst loading the GLY
2
2
,
DOI: 10.1039/C9NJ04106K
oxidation was tested by slowly addition of the oxidant agent (at the beginning, at 60 min
and at 120 min of reaction). It was observed that GLY conversion is mantained at ~70%
after 240 min, while DHA selectivity increases up to 43% after 30 min of reaction (see
supponting information S5). In addition, the fast appearance of formic acid (FA) could
also be observed in this case from the beginning of the reaction, reaching 50% of FA
selectivity at the end of the process (240 min of reaction).
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Summarizing, it is possible to achieve ≈30% of DHA selectivity at ≈40% of GLY
conversion by working at 60 ºC with 0.2 g of Cr-ZSM-11 catalyst and a GLY/H O molar
2
2
ratio of 0.5. The slowly addition of H O in the reaction system allows increasing the
2
2
DHA selectivity up to 43% at 30 min of reaction by keeping GLY conversion at adequate
level.
3
.2.2 Selective production of LA over Cu-zeolites
A study of main reaction parameters similar to that performed with Cr-ZSM-11 was
carried out to analyze and maximize the conversion of GLY and the selectivity to LA
when using Cu-ZSM-11 as catalyst (Figure 7). In this case, the conversion of GLY
increases by increasing the reaction temperature from 40 to 80 °C, while the selectivity
to LA remains close to 60%. Nevertheless, FA begins to be produced at 60-80 ºC, due to
the over-oxidation reactions occurring at higher temperatures than 60° C (Figure 7a).
On the other hand, a similar behavior in terms of desired product selectivity was observed
for Cu-ZSM-11 catalyst compared to Cr-ZSM-11 when analyzing the influence of the
catalyst mass (Figure 7b). The conversion of GLY remains constant independently of the
mass of catalyst used; while the selectivity to LA reaches a maximum when 0.2 g of
catalyst is used, and then decreases with higher catalyst`s content.
2
4

Page 25 of 35
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
View Article Online
DOI: 10.1039/C9NJ04106K
In the case of varying GLY/H O molar ratio, a maximum in LA selectivity (close to
2
2
7
0%) is achieved over Cu-ZSM-11 catalyst with a substrate/H O molar ratio of 0.5 at 60
2 2
min of reaction, when working at GLY conversion around 33% (Figure 7c).
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
As can be seen in the reported results, copper plays a fundamental role in the selective
transformation of GLY towards LA. The best values of LA selectivity (~70 mol%) here
attained with Cu-ZSM-11 catalyst at 60 min of reaction are superior to those reported in
1
7
literature for other copper-based materials .
80
70
60
50
40
30
20
10
0
80
70
60
50
40
30
20
10
0
50
40
80
70
60
50
40
30
GLY
GLY
LA
FA
LA
30
20
10
0
2
1
0
0
0
a)
b)
40
50
60
70
80
0,00
0,05
0,10
0,15
0,20
0,25
0,30
Temperature (°C)
Catalyst mass (g)
60
50
40
30
20
10
GLY
LA
80
70
6
0
0
5
40
30
20
10
c)
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
1,1
Molar Ratio Substrate/ H O2
2
Figure 7. Influence of the reaction parameters on conversion and selectivity using Cu-
ZSM-11 zeolite, at 60 min of reaction time: a) temperature; b) amount of catalyst; c)
GLY/H O molar ratio.
2
2
In order to evaluate the level of profitance of the oxidating agent in the catalytic process
by using different metal-based zeolites, the efficiency of H O was calculated by
2
2
iodometric titration (see Experimental section).
2
5

New Journal of Chemistry
Page 26 of 35
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
View Article Online
For GLY oxidation employing Cu-ZSM-11 as catalyst, the calculated H O efficiency
2
2
DOI: 10.1039/C9NJ04106K
was 22.5%, while by employing Cr-ZSM-11 as catalyst the oxidant eficiency was higher
reaching 80.4%. These results are in agreement with catalytic activity (mol% GLY
Conversion) previously mentioned with theses materials.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
.5 Reusability of the Cr-ZSM-11 and Cu-ZSM-11 catalyst
The possibility of reusing the heterogeneous catalysts is another key aspect for evaluating
their catalytic performance. In this sense, the recyclability of Cr-ZSM-11 and Cu-ZSM-
1
1 in the GLY oxidation with aqueous solution of H O as oxidant was evaluated by
2 2
performing several re-uses of the catalysts; the latter were recovered from the reaction
mixture, and then used in a new experiment without any regeneration (see Experimental
section).
As can be seen in Figure 8a, a partial decrease in the conversion of GLY can be observed
after using five consecutive times the Cr-ZSM-11 zeolite under the same reaction
conditions afore-mentioned. Thus, GLY conversion was ~70% by employing fresh
catalyst, while a second use of the catalyst diminished GLY conversion to close to 53%.
Then, in the third, fourth and fifth consecutive uses of the catalyst glycerol conversion
remained practically constant at 48%. It is worth noting that these results were obtained
without any regeneration treatment (for example, heat-treatment as calcination) all along
the consecutive re-uses. Simultaneously, the selectivity to DHA (measured at 120 min)
diminished from ~28% to ~13% after five catalytic cycles.
All these data clearly indicate that zeolite Cr-ZSM-11 is still active after five re-uses in
the GLY oxidation reaction under mild conditions used herein. The slightly drop on GLY
oxidation and DHA selectivity might be related to the increase of carbonaceous
compounds adsorption onto the external surface of the solid catalyst (and also onto the
internal channels), this making difficult the accessing of reactants to the active sites. In
2
6

Page 27 of 35
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
View Article Online
DOI: 10.1039/C9NJ04106K
addition, the active sites can be deactivated due to the presence of these carbonaceous
molecules that do not diffuse towards the surface, and then to the liquid reaction mixture.
To confirm these facts, structural (by XRD measurements) and acid (by FTIR
spectroscopy with absorbed pyridine) properties of the used catalyst were determined.
By FTIR analysis, the B/L acid sites ratio for the Cr-ZSM-11 catalyst diminished from
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
.96 to 2.73 µmol of pyridine/g of catalyst after the first cycle, this implicating a relative
reduction of the acidity of only ~8%. After that, the B/L acid sites ratio remained
unalterable after five uses, this meaning that the acid properties of the Cr-based catalyst
are practically unchanged during reuses (see Supporting Information S6). In addition, no
loss of crystallinity was observed in the used Cr-ZSM-11 zeolite with respect to the fresh
catalyst (XRD patterns shown in Supporting Information S7) after five cycles of reaction.
These evidences allow inferring that the Cr-ZSM-11 catalyst preserved its zeolitic
structure and maintained a relative stable catalytic behavior during five reaction cycles.
In addition, the chromium content in the catalyst after the consecutive re-uses determined
by ICP measurements was 2.71wt%, only slightly minor to the content of the fresh
material (2.90wt%, see Table 1). From this result, it can be inferred that no leaching of
the metal has taken place during reaction.
In summary, the reported results indicate that the Cr-ZSM-11 zeolite had long durability
and was relatively resistent to catalytic deactivation. These results were similar to some
reported results in literature 32 33, where the incorporation of chromium into the zeolite
catalysts increased not only its catalytic activity but also its thermal stability.
2
7

New Journal of Chemistry
Page 28 of 35
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
View Article Online
DOI: 10.1039/C9NJ04106K
80
100
100
GLY
DHA
GLY
LA
70
60
50
40
80
80
60
40
20
0
60
40
20
0
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
30
2
0
0
0
1
a)
b)
1
2
3
4
5
1
2
3
Reuses
Reuses
Figure 8. Reusability of Cr-ZSM-11(a) and Cu-ZSM-11 (b). Reaction conditions: GLY
0.5 M), H O (30% v/v), catalyst 0.2 g, at 60 °C, 4 h duration of each cycle.
(
2
2
In the case of Cu-ZSM-11 (Figure 8b), the GLY conversion attained at the end of the first
cycle was ~50% whereas a slight decrease (~38%) is observed for the second cycle; after
that the GLY conversion remained constant in the third catalytic use. On the contrary,the
LA selectivity was drastically decreased from 68 to 3% from the first to the third catalytic
cycle, which means a decrease of more than 95% in LA selectivity. This behavior could
be related to the strong decrease of both Lewis and Brønsted acid sites observed when
studying the Cu-ZSM-11 used catalyst (after three reaction cycles) by means of FTIR
spectroscopy with adsorbed pyridine. Indeed, the L/B acid sites ratio decreased from 1.55
µmol of pyridine/g of catalyst for the fresh sample to 0.78 in the case of the used catalyst
(
see Supporting Information S8).
On the other hand, the UV–vis–DRS spectroscopy is known to be a very sensitive probe
for the identification and characterization of metal ion coordination and its existence in
framework and/or in extra-framework position of metal containing zeolites. A major
aspect that should be taken into account is the modifications induced in the structure of
the catalytic sites due to the interaction with the reaction medium (see Supporting
Information S9)
2
8

Page 29 of 35
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
View Article Online
DOI: 10.1039/C9NJ04106K
To evaluate that, the Cu-ZSM-11 catalyst used for three cycles has been calcined and
used in a new oxidation reaction, under the same conditions. The GLY conversion
achieved (47.8%) were very close to those obtained with the fresh catalyst (50%). This
would confirm our hypothesis about the deposition of carbonaceous compounds in the
catalyst during the catalytic cycle.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
In addition, ICP measurements of the used catalyst sample allow determining a small
decrease (≈ 20wt %) of the copper metal content copper with respect to the fresh material.
4
. Conclusions
Microporous zeolite ZSM-11 was synthetized and modified by alkali treatment to
generate mesoporosity (ZSM-11 (T)), achieving higher percentages of crystallinity and
purity. Then, these catalytic materials were modified by the incorporation of transition
metals (Cu and Cr) employing wet impregnation at 80° C, in rotatory evaporator. The
textural and physicochemical properties was studied and analyzed in this work, using
different characterization techniques as XRD, FTIR, TEM, N sorption isotherms, ICP,
2
TPR, among others. These catalysts showed a great potential for the conversion of GLY
to produce DHA and LA employing H O as an oxidant and mild reaction conditions.
2
2
The reaction catalyzed by H-ZSM-11 without metal loading only gives a 29.8% of GLY
conversion at 4 h of reaction without production of products of interest, such as DHA or
LA. Instead, the GLY conversion increased notably by employing copper and chromium
modified zeolites, due to the modification of its acid properties (Brønsted/Lewis ratio).
Although all the Metal-ZSM-11 modified samples studied had a similar metal content (~3
wt %), the Cr-ZSM-11 was more active than the other catalysts giving ~70% of GLY
conversion with a maximum DHA selectivity of ~28% at 120 min of reaction. On the
other hand, Cu-ZSM-11 catalyst resulted more selective towards the LA formation,
reaching 68% of LA selectivity with a GLY conversion of 55% at 60 min of reaction.
2
9