Permanently polarized hydroxyapatite for selective electrothermal catalytic conversion of carbon dioxide into ethanol

Article abstract of DOI:10.1039/d0cc07989h

Conversion of CO2 into valuable chemicals is not only a very challenging topic but also a socially demanding issue. In this work, permanently polarized hydroxyapatite obtained using a thermal stimulated polarization process is proposed as a highly selective catalyst for green production of ethanol starting from CO2 and CH4.

Full text of DOI:10.1039/d0cc07989h

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Permanently polarized hydroxyapatite
for selective electrothermal catalytic conversion
Cite this: DOI: 10.1039/d0cc07989h
of carbon dioxide into ethanol†
Received 8th December 2020,
Accepted 15th April 2021
Jordi Sans,a Guillem Revilla-Lopez,a Vanesa Sanz,b Jordi Puiggalı,
ac
´
´
acd
Pau Turon *b and Carlos Aleman
´
*
DOI: 10.1039/d0cc07989h
rsc.li/chemcomm
Conversion of CO2 into valuable chemicals is not only a very and electrochemical properties.22 Herein, we report the use of such a
challenging topic but also a socially demanding issue. In this work, bioceramic catalyst, p-HAp, for CO2 fixation under mild conditions,
permanently polarized hydroxyapatite obtained using a thermal affording a selective catalytic synthesis of ethanol that involves
stimulated polarization process is proposed as a highly selective hydrogenation of reduced CO2 and C–C bond construction as well.
catalyst for green production of ethanol starting from CO2 and CH4.
HAp was prepared by the hydrothermal method using
(NH4)2HPO4 and Ca(NO3)2 as reagents. The obtained samples
Carbon dioxide (CO2) is considered the primary greenhouse gas were sintered for 2 h at 1000 1C in air and, subsequently,
and the main cause of global climate warming. Therefore, its polarized by applying 3 kV cmꢀ1 at 1000 1C for 1 h. The success
efficient utilization as a C1 feedstock to synthesize high value of the polarization process is proved in Fig. S1 (ESI†), which
chemical and industrial products, such as carbon monoxide,1–3 compares the Raman n1 peak in the 930–990 cmꢀ1 interval
methanol,4–6 ethanol,7–9 formic acid,10–12 ureas,13 and carbo- before and after HAp polarization. Thermally stimulated polari-
nates,14,15 among others, is drawing increasing attention. Tran- zation produces an improvement in the definition of the
sition metals and, more recently, complexes that can act as vibrational modes and a reduction of the full width at half
transition metals dominate the catalysis associated with CO2 maximum (FWHM) from 9 cmꢀ1 to 5 cmꢀ1. This fact is
activation and fixation.16–19 However, environmentally friendly, consistent with a reduction of HAp crystal imperfections and
simpler and cheaper catalysts are highly desirable. Moreover, a decrease in the content of amorphous calcium phosphate and
there is an urgent demand for engineering and manufacturing b-tricalcium phosphate phases. Additionally, p-HAp was con-
catalysts for CO2 recycling under mild conditions (i.e. o10 bar firmed through other previously described electrical and struc-
pressure and o250 1C temperature) with lower levels of environ- tural assays, which are routinely conducted to distinguish the
mental contamination and reduced cost.
polarized catalyst from non-polarized HAp (np-HAp).20,21
p-HAp can be described as nanoparticles of approximately
Hydroxyapatite (HAp) is a naturally occurring mineral form of
calcium apatite, exhibiting similarity with human hard tissues in 100–300 nm that aggregate forming agglomerates of up to 1 mm
morphology and composition. Polarized HAp (p-HAp), which is in size (Fig. 1a). Additional characterization of p-HAp by wide
obtained by applying a thermally stimulated polarization process to angle X-ray diffraction (WAXD), high-resolution transmission
HAp,20,21 presents electrocatalytic behaviour due to its capacitive electron microscopy (HRTEM) and X-ray photoelectron spectro-
scopy (XPS) is provided in Fig. S2–S4 (ESI†). Recently, we reported
a
´
Departament d’Enginyeria Quımica and Barcelona Research Center for Multiscale
a multi-component electrophotocatalyst made of p-HAp coated
with zirconyl chloride, which hydrolyses into zirconia under
reaction conditions, and aminotris(methylenephosphonic acid)
(ATMP).22,23 The catalyst, hereafter named p-HAp/coat, yielded
glycine and alanine (D/L racemic mixture) by fixing nitrogen from
N2 and carbon from CO2 and CH4 at 95 1C and 1 bar. The molar
yield of amino acids was duplicated when the pressure increased
up to 6 bar, maintaining the temperature at 95 1C. Conversely,
amino acids were not formed when p-HAp was replaced by
np-HAp and/or when the coating was not deposited onto the
surface of p-HAp.
`
Science and Engineering, Universitat Politecnica de Catalunya, EEBE,
C/Eduard Maristany 10-14, 08019 Barcelona, Spain.
E-mail: carlos.aleman@upc.edu
b Research and Development. B. Braun Surgical, S.A.U., Carretera de Terrasa 121,
´
08191 Rubı (Barcelona), Spain. E-mail: pau.turon@bbraun.com
c Institute for Bioengineering of Catalonia (IBEC), The Barcelona Institute of Science
and Technology, Baldiri Reixac 10-12, 08028 Barcelona, Spain
d
`
Laboratory of the Environmental Center (LCMA), Universitat Politecnica Pau
Turon de Catalunya, EEBE, C/Eduard Maristany 10-14, 08019 Barcelona, Spain
† Electronic supplementary information (ESI) available: Experimental proce-
dures, Raman spectra and 1H NMR spectra are included in the supporting
information. See DOI: 10.1039/d0cc07989h
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properties, electrochemical activity and ability to store charge;
zirconia is a well-known photocatalyst with a relatively wide
band gap value and high negative value of the conduction band
potential,24–26 which was found to be essential for fixation of
nitrogen from N2; and ATMP weakly attracts the N2 molecules
towards the surface of the catalysts.22,23 In order to confirm the
electrocatalyzed fixation of the CO2, the zirconia and ATMP
coatings were eliminated from the catalyst. Analysis of the
reaction products obtained directly using p-HAp under
the same experimental conditions led to (Fig. 1c): ethanol
(19.4 ꢁ 3.8 mmol gꢀ1), acetone (0.9 ꢁ 0.1 mmol gꢀ1) and acetic
acid (0.6 ꢁ 0.1 mmol gꢀ1). This represents a 20% increment in
the yield of ethanol, while methanol and malonic acid were not
detected. The selectivity increased from 2.0 to 12.9, while the
conversion of feeding gases into products decreased to 0.07%.
Moreover, the influence of the polarization process on the
catalytic activity of conventional (non-polarized) HAp is proved
in Fig. 1d, which only produced a very low amount of ethanol
(1.9 ꢁ 0.5 mmol gꢀ1 catalyst) under the same reaction condi-
tions (i.e. yields of acetone and acetic acid were o0.1 mmol gꢀ1
catalyst). In the absence of any solid support acting as a catalyst
(negative control 1 in Fig. S6, ESI†), the yield of ethanol was
practically null (o0.1 ꢁ 0.05 mmol gꢀ1). Such a small amount,
which was attributed to eventual photo-induced CO2 reduction
and water splitting, totally disappeared in the absence of UV
radiation (negative control 2 in Fig. S6, ESI†).
Chemical shifts observed after the dissolution of the p-HAp/
coat were slightly de-shielded with respect to the peaks of
products derived from p-HAp (Fig. 1b–d). This effect was
attributed to the ATMP, which increased the acidity of the
medium, causing downfield shifts that are not detected for
p-HAp and HAp, independently of the conditions (Fig. 1). On
the other hand, the elimination of the coating from the catalyst
not only increases the conversion into ethanol by 20% but also
maximizes its selective synthesis of ethanol as the major
Fig. 1 (a) Scanning electron microscopy micrograph of p-HAp. (b–d)
1H NMR spectra of the solutions obtained by dissolving the catalysts after
the reaction: (b) p-HAp/coat, (c) p-HAp and (d) HAp. The reactions in (b–d)
were conducted for 72 h using CO2 (3 bar), CH4 (3 bar) and H2O (1 mL) at
95 1C under UV radiation.
In this work a carbon fixation reaction was catalysed by reaction product (from 67% to 93%). This was attributed to
p-HAp/coat in an inert reaction chamber using a CO2 and CH4 the fact that the exposed surface is greater and the surface
gas mixture (3 bar each) and liquid H2O (1 mL) under irradia- charge distribution is more homogeneous for p-Hap than for
tion of an UV lamp illuminating directly the catalyst at 95 1C. p-HAp/coat.23
1
Fig. 1b shows the H-NMR spectrum obtained after 72 h reac-
In order to explore the role of the UV radiation and electro-
tion from the catalyst dissolved in deuterated water containing chemical activity imparted by the polarization process in the
100 mM HCl and 50 mM NaCl, which allowed us to identify the reaction catalyzed by p-HAp, the process was repeated without
following reaction products on the surface of the catalyst (yields UV illumination at 95 1C (Fig. 2a and b). The yield of ethanol
expressed as mmol of product per gram of catalyst): ethanol decreased drastically, from 19.4 ꢁ 3.8 to 0.2 ꢁ 0.03 mmol gꢀ1
,
(16.1 ꢁ 3.2 mmol gꢀ1), methanol (4.9 ꢁ 1.0 mmol gꢀ1), malonic while acetone and acetic acid were not detected. By increasing
acid (1.6 ꢁ 0.2 mmol gꢀ1), acetone (0.8 ꢁ 0.2 mmol gꢀ1) and the temperature to 140 1C, which is the maximum temperature
acetic acid (0.6 ꢁ 0.1 mmol gꢀ1). The predominant product, allowed by the reactor, the yield of ethanol, acetone and acetic
ethanol was identified by the quartet (CH2) and the triplet (CH3) acid increased up to 17.8 ꢁ 1.8, 0.9 ꢁ 0.2 and 0.7 ꢁ 0.1 mmol gꢀ1
at 3.53 and 1.06 ppm, respectively, since the OH peak at catalysts, respectively, these values being very similar to those
4.65 ppm overlapped with the intense water band (Fig. S5, achieved by using UV radiation and 95 1C (Fig. 2c). This feature
ESI†). The selectivity, expressed as the ratio of ethanol to the indicates that the thermally activated electrocatalytic CO2 con-
rest of the products, was 2.0, while the conversion of CO2 and version predominates over the photocatalytic reaction and that
CH4 into products was 0.08%.
UV radiation can be used to lower the free energy barrier to form
In the synthesis of amino acids, the role attributed to the intermediates once CO2 is adsorbed onto the catalyst surface.
different components of the p-HAp/coat catalyst can be broken Moreover, the selectivity towards ethanol at 140 1C without UV
down as follows: p-HAp is an electrocatalyst due to its electrical radiation was 11.1. Accordingly, the whole of the results
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Fig. 3 Representative protonated forms of CO2 adsorbed molecules on
the OHꢀ vacancy of the p-HAp. Numerical values (in eV) stand for the
calculated adsorption energies.
Fig. 2 1H NMR spectra of the solutions obtained by dissolving the
catalysts after the reaction. All the reactions were conducted for 72 h
using CO2 (3 bar), CH4 (3 bar) and H2O (1 mL), p-HAp as catalyst, and the
following specific conditions: (a) 95 1C under UV radiation; (b) 95 1C
without UV radiation; and (c) 140 1C without UV radiation.
(0.7 ꢁ 0.1 mmol gꢀ1) and acetic acid (2.0 ꢁ 0.5 mmol gꢀ1), which
probably converts into acetone on the catalyst surface, formic
obtained using p-HAp/coat and p-HAp catalysts, which were acid (1.9 ꢁ 0.6 mmol gꢀ1) and methanol (1.5 ꢁ 0.3 mmol gꢀ1) can
achieved using different experimental conditions (i.e. 95 and be identified as well. This feature suggests that both the CO2
140 1C and with/without UV radiation) indicate that the selective activation and the formation of the reduced *COOꢀ intermedi-
conversion of CO2 into ethanol is due to an electrothermal ate do not occur on the surface of the solid catalyst but close to
catalytic process. It should be noted that thermal activation is it. Thus, when the intermediate incorporates water, which is in
necessary due to the fact that the capacitive behaviour of p-HAp, contact with the p-HAp catalyst, it does not evolve towards
which is a ceramic, is much lower than that of conventional ethanol but protonates to yield formic acid. Finally, methanol
metallic electrocatalysts.20 Furthermore, comparison of the can be originated as a result of the *CH3 radicals trapped in
Raman spectra of p-HAp before and after the reaction demon- water or from formic acid. In any case, the impact of the
strates the structural stability of the catalyst, which remains reaction products dissolved in the water phase on the ethanol
unaltered (Fig. S7, ESI†).
selectivity is negligible.
In order to support the p-HAp fixation mechanism based on
The reaction with p-HAp at 95 1C and UV radiation was
the formation of carboxylates, DFT calculations were performed repeated considering the two following scenarios: (i) complete
at the PBE-D3 level. The (0001) facet, which is the most stable absence of water; and (ii) an excess of initial water (20 mL) in
HAp surface,27 and an isodesmic model in which H2 is used as the reactor. In the latter case, the catalyst was partially
a source of protons were considered for the calculations. immersed in water and, during the reaction, a water vapor rich
Adsorption energies for different protonation products of CO2 atmosphere was expected. The results show how the regulation
were calculated by inserting the molecules in the hydroxyl of the initial amount of water is very important for the efficient
vacancy of the mineral. The results proved that the protonation reduction of CO2 (Fig. S9, ESI†). Although both ethanol and
of CO2 to yield formic acid is exothermic in the gas phase by acetone were obtained in the absence of water, their yield was
ꢀ3.1 kcal molꢀ1 but it is more exothermic when adsorbed on very low (4.7 ꢁ 0.9 and 0.1 ꢁ 0.06 mmol gꢀ1, respectively) in
p-HAp substrate, by ꢀ32.7 kcal molꢀ1. Yet, all protonated comparison to that achieved when 1 mL of water was intro-
species displayed endothermic adsorption energies; the one duced in the reaction chamber. This difference was attributed
for the protonated formic acid was very small (0.2 kcal molꢀ1
)
to the difficulties in generating and stabilizing active radical
while the one for CO2 was 5.1 kcal molꢀ1. Other adsorption species in the catalyst when it is dry (i.e. only with water
sites were checked on the p-HAp displaying higher energies absorbed from atmospheric moisture). On the other hand,
(Fig. 3), thus making this pathway unfeasible to be followed the excess of water also caused a notable reduction in the yield
completely and shifting the catalysis location elsewhere close to of ethanol and acetone (7.0 ꢁ 0.1 and 0.3 ꢁ 0.07 mmol gꢀ1
,
the surface but not directly adsorbed.
respectively), which again were the only products that are
Analysis of the liquid water incorporated into the reaction formed. However, in this case the low yield was associated with
chamber (Fig. S8, ESI†) provided additional information on the the fact that the water saturated atmosphere makes gas
total yield and confirmed that the catalytic reaction takes place exchange more difficult at the hydrophilic catalyst interface.
over the p-HAp surface, supporting the theoretically proposed
As a proof of concept the reaction was repeated at atmo-
pathway. Interestingly, an additional small amount of ethanol spheric pressure using contaminated air taken from the
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Conflicts of interest
The authors declare that the preparation and application of
permanently polarized hydroxyapatite as catalyst was
a
patented by the Universitat Politcnica de Catalunya and B
Braun Surgical S.A. (EP16382381, EP16382524, P27990EP00,
PCT/EP2017/069437, and P58656 EP).
Notes and references
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The authors acknowledge MINECO/FEDER (RTI2018-098951-
0
`
´
B-I00 and RTI2018-101827-B-I00), the Agencia de Gestio d Ajuts
Universitaris i de Recerca (2017SGR359 and 2017SGR373) and B.
Braun for financial support. Support for the research of C.A. by
ICREA Academia program for excellence in research is gratefully
acknowledged.
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