Analysis of nitrogen fixation by a catalyst capable of transforming N2, CO2 and CH4 into amino acids under mild reactions conditions
Graphical abstract
Introduction
Nitrogen is an indispensable element of the life and ecosystems on Earth [1]. The main source of nitrogen, dinitrogen (N2), is the largest single component of Earth atmosphere (∼78 %). The strong NN triple bond (226 kcal/mol), the large HOMO-LUMO gap (10.8 eV), and the non-polarity render the N2 molecule inert towards most reagents [2]. However, the reduction of atmospheric N2 to ammonia (NH3), an important precursor for nitrogen-containing compounds, as fertilizers, is one of the most essential processes for nitrogen fixation. In microbiological organisms this process is catalyzed at room temperature and atmospheric pressure by the nitrogenase enzymes [3,4], which operate as follow: a N2 molecule bonded and activated at the nitrogenase cofactor is protonated and reduced by electrons that stem from a reducing agent.
From an industrial perspective, the production of NH3 is dominated by the Haber-Bosch process, in which high purity streams of N2 and H2 react at high temperatures (∼500 °C) and high pressures (200−300 atm) over iron- or ruthenium-based catalysts [[5], [6], [7]]. Approximately, 20 % of the NH3 synthesized through the Haber-Bosch process, which enables ∼50 % of the world’s food production, is upgraded to nitrogen-containing organic molecules [8]. In addition of its high energetic cost (i.e. the Haber-Bosch process accounts 1.4 % of world’s annual energy consumption) [9], fossil fuels used for heating and pumping the H2 produce large amount of greenhouse gases. Therefore, it is of great significance to copy natural systems and develop green and sustainable strategies for nitrogen fixation.
The catalytic reduction of N2 under mild conditions was achieved in early studies using Mo-based catalysts, a mixture of N2H4 and NH3 being obtained [10]. Also, N2 was converted into NH3 with excellent yield using protons [11]. Au-surfaces have been used for N2 reduction, forming N2Hy species [12]. In a very recent study, Misawa and co-workers [13] reported a two-electrode plasmon-induced NH3 synthesis by reducing N2. This was composed of a strontium titanate photocatalytic anode in which the plasmon effect is expressed by plasmonic gold nanoparticles and a zirconium cathode. However, the nitrogen fixation from N2 at mild reaction conditions is still considered a very hot topic. For example, Nishibayashi recently developed novel reaction systems for the catalytic transformation of molecular dinitrogen into NH3 and N2H3 using Mo–, Fe–, Co– and V–dinitrogen complexes under mild reaction conditions [14]. In a very recent work [15], we developed an electrophotocatalyst based on hydroxyapatite thermally and electrically stimulated, hereafter denoted permanently polarized hydroxyapatite (p-HAp), and coated with zirconyl chloride (ZC), which supposedly hydrolyzed into zirconia, and aminotris(methylenephosphonic acid) (ATMP) [16]. The new catalyst, p-HAp/ZC/ATMP, allows simultaneously fixation of nitrogen from N2 and carbon from CO2 and CH4 to obtain both glycine and alanine (D/L racemic mixture), the two simplest amino acids, using mild reaction conditions (i.e. atmospheric pressure and 95 °C). With such multi-component catalyst, glycine and alanine molar yields with respect to CH4 or CO2 are about 1.9 % and 1.6 %, respectively, growing up to 3.4 % and 2.4 % when the gas mixture pressure increases to 6 bars and the reaction temperature is maintained at 95 °C.
The synthesis of amino acids by direct fixation of nitrogen and carbon from gas mixtures opens new doors for the development of industrial processes based on nitrogen fixation. However, although the general mechanism was hypothesized in our previous study [16], further details about the N2 cleavage, which is a crucial step for the electrophotosynthesis of glycine and alanine, remained unveiled. Given the importance of understanding the breaking the NN triple bond on the p-HAp based catalyst, in this manuscript we report experimental and computational investigations aimed to determine different aspects of this unusual transformation.
Section snippets
Experiments
p-HAp discs were prepared by applying a constant DC of 500 V at 1000 °C for 1 h to previously sintered mineral discs. For this purpose, HAp was synthesized by chemical precipitation, the resulting powder being uniaxially pressed at 620 MPa for 10 min. The resulting discs were sintered by heating at 1000 °C for 2 h in air [15].
In order to prepare the p-HAp/ZC/ATMP catalyst, p-HAp discs were sequentially coated with three layers: two layers of ATMP separated by an intermediate layer of ZC [16].
Hydrolysis of zirconyl chloride into zirconia
In our previous work, the ZC (ZrOCl2·8H2O) coating was assumed to hydrolyze into zirconia since the environment employed for the catalytic transformation of N2, CO2 and CH4 into amino acids is suitable to force, at least partially, the hydrolysis [16]. This hypothesis, which is crucial for the modeling of the photo-active surface, has been corroborated in the present work by comparing a layer of ZC as prepared and after treatment for 1 h at 95 °C in a water vapor atmosphere. The latter
Conclusions
Investigations into the mechanism of p-HAp/ZC/ATMP induced N2 cleavage under mild conditions to from amino acids in presence of CO2 and CH4 have been carried out. The nitrogen fixation has been studied using isotopic labeled 15N2 in the feeding mixture and, subsequently, monitoring the formation of 15N labeled glycine and alanine by 2D 1H–15N HSQC NMR. Computational studies based on classical force-field MD simulations and DFT calculations have been used to determine the role of the different
CRediT authorship contribution statement
Guillermo Revilla-López: Methodology, Validation, Investigation. Jordi Sans: Methodology, Validation, Investigation. Jordi Casanovas: Methodology, Validation, Investigation. Oscar Bertran: Methodology, Validation, Investigation. Jordi Puiggalí: Conceptualization, Investigation. Pau Turon: Conceptualization, Writing - review & editing, Supervision, Project administration, Funding acquisition. Carlos Alemán: Conceptualization, Writing - original draft, Writing - review & editing, Supervision,
Declaration of Competing Interest
Authors declare that the preparation and application of permanently polarized hydroxyapatite was patented by the Universitat Politcnica de Catalunya and B Braun Surgical S.A. (EP16382381, EP16382524, P27990EP00, PCT/EP2017/069437).
Acknowledgements
This work was supported by MINECO-FEDER (RTI2018-098951-B-I00 and RTI2018-101827-B-I00), by B. Braun Surgical S.A. through a joint research agreement with UPC, and by the Agència de Gestió d'Ajuts Universitaris i de Recerca (2017SGR359 and 2017SGR373). This work is integrated within a wider research project supported by B. Braun Surgical S.A., UPC, ICS and ICFO. Support for the research of C.A. was received through the prize “ICREA Academia” for excellence in research funded by the Generalitat
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