Relationship between Acid–Base Properties and the Activity of ZrO2-Based Catalysts for the Cannizzaro Reaction of Pyruvaldehyde to Lactic Acid

Article abstract of DOI:10.1002/cctc.201700305

The Cannizzaro reaction of pyruvaldehyde to lactic acid is investigated in a flow reactor with ZrO₂ catalysts with different structures and acid–base properties. The results show that a difference in the crystalline structures of two ZrO₂ polymorphs strongly affects the conversion of pyruvaldehyde. The monoclinic phase of zirconia is the most active for this reaction. A good correlation is observed between the reaction rate and the concentration of Lewis acid sites of sufficient strength, which shows that these sites play a major role in the reaction. A reaction mechanism is proposed involving coordinatively unsaturated Zr⁴⁺ cations as sites for activating pyruvaldehyde molecules, whereas Zr⁴⁺?O^2? pairs generate terminal OH groups through water dissociation.

Full text of DOI:10.1002/cctc.201700305

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Accepted Article
Title: Relationship between acid-base properties and the activity of
ZrO2 catalysts for the Cannizzaro reaction of pyruvaldehyde to
lactic acid
Authors: Elise M. Albuquerque, Luiz E.P. Borges, Marco A. Fraga, and
Carsten Sievers
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To be cited as: ChemCatChem 10.1002/cctc.201700305
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Relationship between acid-base properties and the activity of
ZrO2–based catalysts for the Cannizzaro reaction of
pyruvaldehyde to lactic acid
Elise M. Albuquerque,[a,b,c] Luiz E. P. Borges,[b] Marco A. Fraga*[a,b] and Carsten Sievers*[c]
Abstract: The Cannizzaro reaction of pyruvaldehyde to lactic
acid is investigated in a flow reactor using ZrO2 catalysts with
different structures and acid-base properties. The results show
that the difference in crystalline structures of two ZrO2
polymorphs strongly affects the conversion of pyruvaldehyde.
The monoclinic phase of zirconia is the most active for this
reaction. A good correlation is observed between the reaction
rate and the concentration of Lewis acid sites of sufficient
strength, showing that these sites play a major role in the
volume of biodiesel produced.[4] The amount of glycerol
produced in this process exceeds the demand for its
conventional uses by far.
Many efforts have thus been made to develop chemical process
technologies to convert glycerol into value-added products
through
acetalization[8] and condensation.[9]
As three-carbon backbone molecule, glycerol could be
oxidation,[5]
dehydrogenation,[6]
etherification,[7]
a
converted to lactic acid, an important product with a gradually
increasing demand due to its many end-use applications in food
and beverages, personal care, pharmaceuticals and plastic
manufacturing.[10] Lately, the production of biodegradable
poly(lactic acid), PLA, has become a major driving force for the
growth of the global lactic acid market.[11]
reaction.
A
reaction mechanism is proposed involving
coordinatively unsaturated Zr4+ cations as sites for activating
pyruvaldehyde molecules, while Zr4+-O2- pairs generate terminal
OH groups through water dissociation.
Hydrothermal conversion of glycerol into lactic acid through
homogeneous processes has been reported, but it only
proceeds in the presence of large amounts of inorganic
hydroxides,[12] like NaOH and KOH, which are not recycled or
recovered. This matter impacts the environmental footprint of the
process and increases production costs.
Lactic acid can also be obtained from glycerol over
heterogeneous catalysts. The direct reaction was reported over
mono-metallic and bimetallic catalysts based on Au, Pd and Pt
in the presence of NaOH.[13] High lactic acid selectivity (85%)
has been reached,[13] but the use of undesirable inorganic
hydroxides is still necessary.
Alternative approaches use cascade reactions to produce lactic
acid from glycerol. For example, glycerol may be initially
dehydrated to hydroxyacetone[14] on solid acid catalyst and then
this ketone can be converted into lactic acid by
oxidation/intramolecular rearrangement, which may occur in
alkaline medium[15] or on solid basic catalysts, such as
hydrotalcites and hydrotalcites derived mixed oxides.[16] Glycerol
can also be converted to 1,2-propanediol by hydrogenolysis,[17]
which can be oxidized to lactic acid.[18] Selective oxidation
(dehydrogenation) of glycerol to dihydroxyacetone[19] followed by
dehydration/intramolecular rearrangement[19b,20] is also a feasible
process.
Independently of the process and the reaction pathway,
pyruvaldehyde is a key intermediate towards lactic acid in most
reports.[13,15-16,18,19b] Some researchers even showed that more
selective lactic acid production is accomplished when glycerol is
initially and directly converted to pyruvaldehyde instead of
dihydroxyacetone and then rearranged to lactic acid.[19b]
However, despite such advantages of a two-step process with
pyruvaldehyde as the only isolated chemical intermediate, there
are just a few studies focused on its rearrangement into lactic
acid.[21]
Introduction
The world energy supply is still dominated by fossil resources
(such as coal, oil and natural gas). The continued use of these
resources leads to severe environmental problems like global
warming, escalating the necessity to develop new technologies
for generating energy and chemicals from renewable
resources.[1] Biofuels have attracted strong interest as means of
sustainable fuel production and providing value from agricultural
residues.[1] Since their combustion characteristics are similar,
they can replace the fossil fuels, like gasoline and diesel, with
limited modification to the engines.[2]
In particular, biodiesel gained worldwide attention, since it offers
advantages like a sustainable production chain, low toxicity,
biodegradability, so that it can be considered less polluting than
conventional petrodiesel.[3] However, the production of glycerol
as a by-product is inevitable and accounts for about 10% of the
[a]
[b]
[c]
Dr. E. M. Albuquerque, Dr. M. A. Fraga
Divisão de Catálise e Processos Químicos
Instituto Nacional de Tecnologia/MCTIC
Av. Venezuela, 82/518, Saúde, Rio de Janeiro/RJ 20081-312, Brazil
E-mail: marco.fraga@int.gov.br
Dr. E. M. Albuquerque, Prof. L. E. P. Borges, Dr. M. A. Fraga
Seção de Química
Instituto Militar de Engenharia
Praça Gen Tibúrcio, 80, Praia Vermelha, Urca, Rio de Janeiro/RJ
22290-270, Brazil
Dr. E. M. Albuquerque, Prof. C. Sievers
School of Chemical & Biomolecular Engineering
Georgia Institute of Technology
311 Ferst Dr. NW., Atlanta, Georgia 30332-0100, United States
E-mail: carsten.sievers@chbe.gatech.edu
Supporting information for this article is given via a link at the end of
the document.
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Pyruvaldehyde conversion to lactic acid occurs via its
intramolecular disproportionation, also known as Cannizzaro
reaction (Scheme 1). It is well reckoned that the rearrangement
of α-keto aldehydes leads to α-hydroxy carboxylic acids in strong
basic medium.[22] Such transformation is also possible in the
presence of acids. However, Brönsted acids (HCl, H2SO4) are
not as effective as Lewis acids.[21b] Koito et al. reported that only
water-tolerant homogeneous Lewis acids, such as scandium,
yttrium and ytterbium triflates, achieved high catalytic activity in
the intramolecular Cannizzaro disproportionation with lactic acid
yields higher than 94%.[21b] As mentioned above, the
corrosiveness and toxicity of alkaline or acidic solutions is cause
for environmental concerns. Therefore, the design of a greener
heterogeneously-catalyzed process is imperative to allow for the
valorization of glycerol via lactic acid production in a truly
sustainable way. However, various challenges have to be
overcome to understand and control the surface chemistry and
stability of heterogeneous catalysts for this process.[23]
1525705) crystalline structures (Figure S1a). The doped
zirconias, Y-ZrO2 and La-ZrO2, showed diffraction patterns
similar to that of a tetragonal phase. However, a small shift to
lower Bragg angles can be noticed, which is made clear by
enlarging the angular region corresponding to the hkl line (111)
(Figure S1b). Despite these structural distortions all diffractions
matched those expected for a tetragonal structure, revealing
that either no isolated crystalline Y2O3 or La2O3 phases were
present or that they were below the XRD detection limit.
Table 1. Surface area (SBET), pore volume (Vp), average diameter (dp) and
the amount of dopant oxide (DO) of all zirconia catalysts.
Catalyst
SBET
Vp
dp
(Å)
DO
(%)
(m2.g-1)
(cm3.g-1)
m-ZrO2
t-ZrO2
103
129
102
94
0.304
0.165
0.163
0.209
0.290
90
46
49
72
111
-
-
A few recent studies used batch reactors to provide some insight
into the conversion of pyruvaldehyde to lactic acid over solid
Y-ZrO2
La-ZrO2
TiO2-ZrO2
5
8
35
catalysts such as MgO,[24] ZrO2 and Nb2O5.[21b] Acidic Nb2O5
[24]
was found to be quite efficient even with a low density of water-
tolerant Lewis acid sites.[21b] Its strong acidity, however,
promoted the formation of numerous polymerized species.
Amphoteric ZrO2 provided higher yield of lactic acid than highly
basic MgO in neutral or weakly acidic/basic aqueous medium,
whose pH was cautiously adjusted for the reaction.[24]
In this contribution, the Cannizzaro reaction of pyruvaldehyde
over zirconium oxide-based catalysts is investigated using
continuous flow reactor setup. These catalysts were chosen due
to their amphoteric properties and hydrothermal stability.
Furthermore, polymorphic structures of ZrO2 and their ability to
embed foreign cations allow for tailoring surface properties to
unveil the role of acid and basic surface sites on the reaction.
The reactions in this study were performed without any addition
of alkaline or acidic solutions.
49
On the other hand, TiO2-ZrO2 gave
a
more complex
diffractogram (Figure S1a and S1c), containing peaks
associated with both monoclinic and tetragonal phases of
zirconia, and also the anatase structure of TiO2 (COD 9009086).
The Raman spectrum of commercial m-ZrO2 (space group P21/c
)
exhibited bands at 101, 177, 189, 221, 307, 332, 346, 380, 476,
501, 535, 557, 616 and 632 cm-1 corresponding to its active
vibrational modes (9Ag+9Bg symmetry) (Figure S2).[25] For t-ZrO2
(space group P42/nmc), five bands were recorded at 150, 274,
317, 461 and 646 cm-1, which are related to its Raman active
vibrational modes (A1g+2B1g+3Eg).[25-26]
The spectra of Y-ZrO2 and La-ZrO2 doped zirconias were
comparable to that of t-ZrO2, exhibiting the same typical Raman
bands of a tetragonal lattice. No Raman shifts could be
associated with any isolated crystalline or amorphous Y2O3 or
La2O3 phases. However, in case of TiO2-ZrO2, the Raman
spectrum was totally different from the others; five bands were
observed at 150, 201, 398, 519 and 640 cm-1, which are well
related to the six vibrational modes A1g+2B1g+3Eg of titania
anatase phase (space group I4I/amd).[27] The spectrum also
contained a small band at 484 cm-1 that can be associated with
the monoclinic phase of zirconia.
Scheme 1. Reaction scheme for the Cannizzaro reaction of pyruvaldehyde to
lactic acid.
The porosity and surface area of the catalysts were assessed by
N2 physisorption (Table 1, Figure S3a). All oxides presented the
same type IVa isotherm, according to IUPAC classification,[28]
indicating the presence of mesopores. All catalysts, except for
TiO2-ZrO2, presented high surface areas above 90 m2.g-1 and
average pore diameters above 45 Å (Table 1). The pore size
distributions obtained from Barret-Joyner-Halenda (BJH) method
had a reasonably sharp peak centered around 40 Å for the
catalysts with a tetragonal structure and around 70-80 Å for the
Results
Catalyst characterization
The composition of the doped catalyst is summarized in Table 1.
All catalysts were characterized by XRD and Raman
spectroscopy to determine their structures.
Pure zirconia samples exhibited diffraction peaks characteristic
of either monoclinic (COD 9007485) or tetragonal (COD
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other two samples that are dominated by the monoclinic zirconia
phase (Figure S3b).
all samples contained base sites with a range of different
strengths, the relative abundance of weak and strong sites
varied between the samples. The m-ZrO2 catalyst possessed a
higher concentration of base sites than its polymorph t-ZrO2.
Doping zirconia with yttrium and lanthanum increased the
basicity of the t-ZrO2. Lastly, TiO2-ZrO2 showed the lowest
concentration of base site.
The concentration and strength of base sites was probed by
TPD of CO2 (Figure 1, Table 2). The catalysts CO2 desorption
profiles were similar with an intense desorption peak below
200 °C and a long tail extending to temperatures as high as
500 °C. Such traces indicate that the samples possess a wide
distribution of surface base sites in terms of their strength. While
Table 2. Concentration of weak, medium and strong base and acid sites of all ZrO2 catalysts as determined by CO2 and NH3 temperature-programmed
desorption analysis.
Catalyst
Base sites (µmolCO2.g-1)
Acid sites (µmolNH3.g-1)
Weak[a]
113
53
Medium[b]
Strong[c]
Total
246
89
Weak[d]
56
Medium[e]
Strong[f]
63
Total
187
142
126
143
181
m-ZrO2
t-ZrO2
79
26
86
55
26
54
10
65
67
9
68
54
57
54
66
54
34
Y-ZrO2
La-ZrO2
TiO2-ZrO2
101
75
252
197
71
38
31
76
13
36
55
61
[a] desorption temperatures up to 170 °C. [b] desorption temperatures between 150-300 °C. [c] desorption temperatures above 300 °C. [d] desorption
temperatures up to 200 °C. [e] desorption temperatures between 200-400 °C. [f] desorption temperatures above 400°C.
The characterization of acid sites by temperature-programmed
desorption of NH3 provided traces with three distinct desorption
peaks for all catalysts (Figure 2). The most intense peaks were
recorded below 250 °C, but the strength distribution of the acid
sites was much wider with a shoulder extending to much higher
temperatures. The m-ZrO2 catalyst had the highest density of
acid sites, whereas all catalysts with tetragonal structure
presented lower acid site concentrations (Table 2). The strength
distribution of the acid sites was analyzed based on
mathematical deconvolution of the TPD traces. The m-ZrO2 and
TiO2-ZrO2 catalysts had similar concentrations of each type of
acid sites (weak, medium and strong sites). t-ZrO2 contains
mostly weak and medium acid sites, while Y-ZrO2 has a higher
concentration of medium site, and La-ZrO2 showed a higher
density of weak acid sites and a lower concentration of strong
sites.
Figure 1. CO2 temperature-programmed desorption profiles of ZrO2-based
catalysts.
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