Carbon Dioxide Utilisation -The Formate Route

UIDB/50006/2020 CEEC-Individual 2017 Program Contract.The relentless rise of atmospheric CO2 is causing large and unpredictable impacts on the Earth climate, due to the CO2 significant greenhouse effect, besides being responsible for the ocean acidification, with consequent huge impacts in our daily lives and in all forms of life. To stop spiral of destruction, we must actively reduce the CO2 emissions and develop new and more efficient “CO2 sinks”. We should be focused on the opportunities provided by exploiting this novel and huge carbon feedstock to produce de novo fuels and added-value compounds. The conversion of CO2 into formate offers key advantages for carbon recycling, and formate dehydrogenase (FDH) enzymes are at the centre of intense research, due to the “green” advantages the bioconversion can offer, namely substrate and product selectivity and specificity, in reactions run at ambient temperature and pressure and neutral pH. In this chapter, we describe the remarkable recent progress towards efficient and selective FDH-catalysed CO2 reduction to formate. We focus on the enzymes, discussing their structure and mechanism of action. Selected promising studies and successful proof of concepts of FDH-dependent CO2 reduction to formate and beyond are discussed, to highlight the power of FDHs and the challenges this CO2 bioconversion still faces.publishersversionpublishe

Liquid Organic Hydrogen Carriers LOHCs Towards a Hydrogen free Hydrogen Economy

The need to drastically reduce CO$_{2}$ emissions will lead to the transformation of our current, carbon-based energy system to a more sustainable, renewable-based one. In this process, hydrogen will gain increasing importance as secondary energy vector. Energy storage requirements on the TWh scale (to bridge extended times of low wind and sun harvest) and global logistics of renewable energy equivalents will create additional driving forces toward a future hydrogen economy. However, the nature of hydrogen requires dedicated infrastructures, and this has prevented so far the introduction of elemental hydrogen into the energy sector to a large extent. Recent scientific and technological progress in handling hydrogen in chemically bound form as liquid organic hydrogen carrier (LOHC) supports the technological vision that a future hydrogen economy may work without handling large amounts of elemental hydrogen. LOHC systems are composed of pairs of hydrogen-lean and hydrogen-rich organic compounds that store hydrogen by repeated catalytic hydrogenation and dehydrogenation cycles. While hydrogen handling in the form of LOHCs allows for using the existing infrastructure for fuels, it also builds on the existing public confidence in dealing with liquid energy carriers. In contrast to hydrogen storage by hydrogenation of gases, such as CO$_{2}$ or N$_{2}$, hydrogen release from LOHC systems produces pure hydrogen after condensation of the high-boiling carrier compounds.This Account highlights the current state-of-the-art in hydrogen storage using LOHC systems. It first introduces fundamental aspects of a future hydrogen economy and derives therefrom requirements for suitable LOHC compounds. Molecular structures that have been successfully applied in the literature are presented, and their property profiles are discussed. Fundamental and applied aspects of the involved hydrogenation and dehydrogenation catalysis are discussed, characteristic differences for the catalytic conversion of pure hydrocarbon and nitrogen-containing LOHC compounds are derived from the literature, and attractive future research directions are highlighted.Finally, applications of the LOHC technology are presented. This part covers stationary energy storage (on-grid and off-grid), hydrogen logistics, and on-board hydrogen production for mobile applications. Technology readiness of these fields is very different. For stationary energy storage systems, the feasibility of the LOHC technology has been recently proven in commercial demonstrators, and cost aspects will decide on their further commercial success. For other highly attractive options, such as, hydrogen delivery to hydrogen filling stations or direct-LOHC-fuel cell applications, significant efforts in fundamental and applied research are still needed and, hopefully, encouraged by this Account

LOHC-bound hydrogen for catalytic NOx reduction from O2-rich exhaust gas

The present study demonstrates a novel method for the NOx reduction by H2 in lean exhaust gases using H2 released from a Liquid Organic Hydrogen Carrier (LOHC). The concept implies the simultaneous H2 production and H2-deNOx reaction, which both take place on the same catalyst. In a first approach, the catalyst was suspended in the LOHC, while the exhaust flowed through the slurry. The experiments performed with a O2-rich model exhaust and perhydro dibenzyltoluene as LOHC as well as Pd/C, Pt/C and Pt/Al2O3 catalysts evidenced the feasibility of this transfer hydrogenation. As a result, the Pd/C catalyst revealed best H2-deNOx performance providing NOx conversions up to ??% and N2 selectivities of ??°C above 200°C. The characterization of the catalysts by temperature-programmed desorption of CO suggested that the superiority of the Pd/C sample is associated with its pronounced number of active Pd sites. Furthermore, the investigations also showed some LOHC degradation releasing CO, CO2 and hydrocarbons. However, additional experiments excluded significant participation of the formed CO in the H2-deNOx reaction at 210°C and above

Hydrogenation of aromatic and heteroaromatic compounds – a key process for future logistics of green hydrogen using liquid organic hydrogen carrier systems

This review deals with the chemical storage of green hydrogen in the form of Liquid Organic HydrogenCarrier (LOHC) systems. LOHC systems store hydrogen by an exothermal catalytic hydrogenationreaction that converts the hydrogen-lean compounds of the LOHC system to their hydrogen-richcounterparts. All compounds of a technically suitable LOHC system are liquids and this offers theadvantage of simple logistics of chemically bound hydrogen in the existing infrastructure for fuels. Ondemand, hydrogen can be released from the hydrogen-rich LOHC molecule in an endothermal catalyticdehydrogenation at low hydrogen pressure (typically below 5 bar). Our contribution deals first withavailable sources of green hydrogen for a future hydrogen economy and then describes establishedtechnical processes to produce clean hydrogen from technically hydrogen-rich gas mixtures.Subsequently, the review focuses on the hydrogenation of aromatic and heteroaromatic compounds asthe key step of the LOHC-based hydrogen storage cycle. Special emphasis is given to the hydrogenchargingof hydrogen-lean LOHC compounds with various gas mixtures demonstrating that sucha Mixed Gas Hydrogenation (MGH) process offers the technical potential to selectively extract hydrogenin a chemically bound form that enables very efficient hydrogen logistics. In this way, low cost hydrogensources can be connected to high value hydrogen application, e.g. hydrogen filling stations for cleanmobility applications, to enable a future hydrogen economy

Operational Stability of a LOHC-Based Hot Pressure Swing Reactor for Hydrogen Storage

Apart from hydrogen logistics, stationary hydrogen storage applications using Liquid Organic Hydrogen Carrier (LOHC) systems are also of significant interest. In contrast to the traditional use of separate hydrogenation and dehydrogenation reactors, our so‐called oneReactor technology offers the advantages of a simpler storage unit layout and high dynamics in switching from hydrogen charging to hydrogen release. Here we report repeated hydrogenation and dehydrogenation cycles with one batch of liquid carrier for LOHC stability tests under defined hydrogenation and dehydrogenation conditions. We demonstrate up to 13 hydrogenation/dehydrogenation cycles over a total of 405 h of operation including two long dehydrogenation sequences over weekends. In general, longer dehydrogenation runs, i. e. exposure of the LOHC to catalyst at low hydrogen pressure and elevated temperatures (>280 °C), showed negative effects on both activity of the subsequent cycles and by‐product formation. Concerning catalyst activity and hydrogen productivity, stable productivity was achieved (within 3 to 9 cycles) under all conditions tested. Longer hydrogenation runs led to significantly higher stability of the reaction system

Catalytically activated stainless steel plates for the dehydrogenation of perhydro dibenzyltoluene

Hydrogen storage and transport via Liquid Organic Hydrogen Carriers (LOHC) is gaining increasing attention. In this study, we present catalytically activated stainless steel plates as a promising alternative to the commonly used pellet catalysts for the dehydrogenation of perhydro dibenzyltoluene (H18-DBT). These plate catalysts promise better heat transport to the active sites. For improved performance, we modified our Pt/alumina plate catalysts by using i) platinum sulfite impregnation and ii) post-treatment with (NH4)2SO4. Post-treatment with (NH4)2SO4 resulted in a less active catalyst with lower formation of high-boiling side products compared to the S-free plate catalyst. Catalysts prepared with platinum sulfite showed both >35% higher activities and 90% reduction in high-boiler formation compared to the S-free plate catalysts. Our findings pave the way for the development of catalytically activated heat transfer plates that would allow the incorporation of LOHC dehydrogenation units into the geometry of future high temperature fuel cell stacks

Hydrogenation of the liquid organic hydrogen carrier compound dibenzyltoluene – reaction pathway determination by 1 H NMR spectroscopy

The catalytic hydrogenation of the LOHC compound dibenzyltoluene (H0-DBT) was investigated by $^{1}$H NMR spectroscopy in order to elucidate the reaction pathway of its charging process with hydrogen in the context of future hydrogen storage applications. Five different reaction pathways during H0-DBT hydrogenation were considered including middle-ring preference (middle-side-side, MSS), side-middle-side order of hydrogenation (SMS), side-ring preference (SSM), simultaneous hydrogenation of all three rings without intermediate formation and statistical hydrogenation without any ring preference. Detailed analysis of the $^{1}$H NMR spectra of the H0-DBT hydrogenation over time revealed that the reaction proceeds with a very high preference for the SSM order at temperatures between 120 °C and 200 °C and 50 bar in the presence of a Ru/Al$_{2}$O$_{3}$-catalyst. HPLC analysis supported this interpretation by confirming an accumulation of H12-DBT species prior to full hydrogenation to H18-DBT with middle ring hydrogenation as the final step

Experimental determination of the hydrogenation/dehydrogenation - Equilibrium of the LOHC system H0/H18-dibenzyltoluene

Liquid organic hydrogen carrier (LOHC) systems store hydrogen through a catalystpromotedexothermal hydrogenation reaction and release hydrogen through an endothermalcatalytic dehydrogenation reaction. At a given pressure and temperature theamount of releasable hydrogen depends on the reaction equilibrium of the hydrogenation/dehydrogenation reaction. Thus, the equilibrium composition of a given LOHC system isone of the key parameters for the reactor and process design of hydrogen storage andrelease units. Currently, LOHC equilibrium data are calculated on the basis of calorimetricdata of selected, pure hydrogen-lean and hydrogen-rich LOHC compounds. Yet, real reactionsystems comprise a variety of isomers, their respective partially hydrogenatedspecies as well as by-products formed during multiple hydrogenation/dehydrogenationcycles. Therefore, our study focuses on an empirical approach to describe the temperatureand pressure dependency of the hydrogenation equilibrium of the LOHC system H0/H18-DBT under real life experimental conditions. Because reliable measurements of the degree of hydrogenation (DoH) play a vital role in this context, we describe in thiscontribution two novel methods of DoH determination for LOHC systems based on 13C NMRand GC-FID measurements