Nitrogen-Containing Liquid Organic Hydrogen ... - ACS Publications

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Nitrogen-Containing Liquid Organic Hydrogen Carriers: Progress and Prospects Robert H. Crabtree* Chemistry Department, Yale University, 225 Prospect Street, New Haven, Connecticut 06520-8107, United States Energy Sciences Institute, Yale University, 810 West Campus Drive, West Haven, Connecticut 06516, United States ABSTRACT: Nitrogen heterocycles have advantages as hydrogen storage materials, including easier H2 release, better biodegradability, and low vapor pressure. The challenges that face practical applications, for example to vehicular transport, are discussed. Beyond heterocycles, amine/amide and amine/nitrile strategies are also covered.

KEYWORDS: Hydrogen storage, Ionic liquids, LOHCs, Dehydrogenation catalysis

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Solid metal hydrides such as Mg2Ni/Mg2NiH4 work well16 but have low gravimetric loading and tend to pulverize on cycling. Here, we look at liquid organic hydrogen storage compounds (LOHCs) that can undergo reversible hydrogenation−dehydrogenation.13,17,18 The capacity is high, and LOHC transport is relatively safe. However, catalysis is needed for the storage and release steps, and these reactions are often slow. Alicyclic pairs such as toluene/methylcyclohexane (MCH) attracted early attention, for example, and dibenzyltoluene is promising.19 Storage takes place by arene hydrogenation and release by alkane dehydrogenation, both steps requiring catalysis, along with high pressure for the hydrogenation. By analogy with the prior storage method involving metal hydrides such as MgH2,20 such compounds were originally given the name “liquid organic hydrides” (LOHs), a term that persists even today. Hydrocarbons do not normally qualify as hydrides, however, so the term liquid organic hydrogen carrier (LOHC) is more common. The LOHC term is best reserved for cases where the hydrogenation and dehydrogenation are both single-step but not where the dehydrogenation is alone possible. For example, ammonia-borane releases H2 thermally21 but is hard to regenerate. Where the regeneration becomes a multistep process, a better term might be “hydrogen source material” to identify the situation. Easily reversible LOHC pairs are of course preferred for many applications. Candidate vehicular LOHCs need to meet stringent conditions including (i) an acceptable toxicity profile to ensure

he 1973 oil crisis, taken together with the 1970 peak in U.S. domestic petroleum production,1 led to rising interest in alternative energy. Climate change only became important later because in the 1970s global cooling even seemed possible.2 It was early recognized that intermittent sources such as solar power would require improved energy storage options, and nuclear power, if ever dominant, as then seemed possible, would all require conversion to a liquid fuel for transportation.3 Now that the global fleet comprises ∼109 vehicles, this problem needs a solution if fossil fuels are ever to be retired for transportation. Batteries can be slow to recharge compared with conventional gasoline refueling and may be costly.3 Global scale battery production might run up against the availability limits of elements such as Li4 and Co5 for global-scale electric transportation. Elemental hydrogen became a prominent option as a future energy carrier in the 1970s, signaled by the founding of the International Journal of Hydrogen Energy in 1976. The idea of a “hydrogen economy”, and the need for “hydrogen storage”,6 both raised by Bockris,7 were implicit in Haldane’s 1923 lecture,8 and Jules Verne9 even mentioned H2 as a fuel from water splitting in his Mysterious Island of 1875: “Water is the coal of the future”. The topic regained attention from 2000 in connection with climate change.10 Carbon taxes, now becoming a live option, should greatly accelerate a move to renewable fuels.11 Hydrogen might be stored in many ways,12−15 and each may well prove best for specific applications. High pressure storage is rapid and cheap, but the equipment is heavy and metal embrittlement poses risks. Cryogenic storage has high energy density, but energy input is needed to keep the hydrogen liquid. High surface area materials such as metal−organic frameworks and carbon nanotubes are interesting but also require cooling. © 2017 American Chemical Society

Received: March 31, 2017 Revised: April 28, 2017 Published: May 4, 2017 4491

DOI: 10.1021/acssuschemeng.7b00983 ACS Sustainable Chem. Eng. 2017, 5, 4491−4498

ACS Sustainable Chemistry & Engineering

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N-ETHYL CARBAZOLE AND RELATED MOLECULES Pez33 and colleagues identified N-ethyl carbazole (NEC) as the best LOHC in their extensive heterocycle series. This can store 5.8 wt % hydrogen with an energy density of 1.9 kWh/kg. Hydrogenation occurs at 130−160 °C at 70 bar on Ru on alumina, while dehydrogenation requires supported Pt or Pd at 200−230 °C at ambient pressure.34,35 They found a 99.9% yield of hydrogen having only very minor impuritiesCH4 (10 ppm) and C2H6 (80 ppm). The very high selectivity for formation of NEC on dehydrogenation permitted multiple recycling of the LOHC. The NEC nitrogen atom is endocyclic to the 5-membered ring and exocyclic to both 6-membered rings. Computational work by Pez30,35 and Clot and Eisenstein32 suggested that both structural factors destabilize the hydrogenated form and thermodynamically facilitate release. This work also identified the temperature at which the unfavorable ΔH of reaction was fully compensated by the −TΔS entropy term so that Kequil = 1. This temperature, defined as Td, was taken to be a measure of the relative thermodynamic suitability of any LOHC within a series. Only the starting and final structures were compared, although a number of partial dehydrogenation intermediates normally form during the process, complicating the analysis. The Td values given for a series of model compounds in Figure 1 show the very high

safety in accidental release such as in automobile collisions, (ii) thermal stability against undesired decomposition pathways, (iii) good biodegradability, (iv) ability to liberate H2 of sufficient purity for the intended application and return the dehydrogenated product in sufficient purity for multiple recycling, (v) good kinetics for reversible H2 release, catalyzed as necessary, (vi) cheap manufacture on a global scale, (vii) low melting point, and (viii) good gravimetric and volumetric capacity.15,22 The purity of the liberated H2 can be important, for example, to prevent poisoning of hydrogen fuel cells by such contaminants as CO. The elemental availability limits may apply to nickel and boron in the otherwise attractive hydrides such as Mg2NiH4 and H3NBH3. The global production figures22 range from 1−2 × 109 kg/yr for each element, posing problems for global transport where an estimated22 ∼1011 kg is needed for the worldwide vehicle fleet. As emphasized by Markiewicz,23 an acceptable health and safety profile is critical for any LOHCs intended for the public sphere. To be sure, gasoline and diesel fuel are not benign, but the public and the regulators tend to be much less tolerant of novel over familiar hazards. Transport applications will inevitably lead to road collisions that breach the fuel tank. In a road tunnel, the fuel vapors only slowly dispersean unsettling situation that the present author has personally experienced. Toxicity, flammability, and explosion hazard therefore become relevant. Formic acid/CO2 is an attractive LOHC candidate, but the human exposure limits for formic acid are very low (5 ppm), making vehicular adoption problematic.23 N2/NH3 is another attractive pair,24 but the toxicity of ammonia leads to a permitted human short-term exposure limit in the range of 25−50 ppm, and ammonia also forms explosive air−NH3 mixtures within a 15− 28% composition range.25 Ammonia derivatives such as urea and aliphatic amines could well prove very useful, however. Light hydrocarbons and hydrogen form explosive mixtures with air,26 but a high MW LOHC having low volatility would avoid this problem if the bulk of the LOHC is in a fuel tank well away from the catalyst in the dehydrogenation reactor. Indeed, both flammability and toxicity problems would be minimized by low volatility. The heavier hydrocarbons (e.g., Ph2CH2/ (C6H11)2CH2 (bp >250 °C)27) now under consideration fulfill this criterion, as do many of the N-heterocycles discussed here. Biodegradability also tends to be more efficient for heteroatoms containing organic compounds over alicyclics.28 The 2017 H2 capacity targets29 designated by the U.S. Department of Energy are 1.8 kWh/kg, 5.5 wt % H2, 1.3 kWh/L, and 40 gH2/L. The delivery temperature range of −40 to 85 °C will be hard to meet because the thermodynamics of such LOHCs as MCH/toluene require higher release temperatures even with a perfect catalyst. To be sure, a well-engineered system could achieve higher temperatures at the catalyst site without compromising efficiency and safety, and some systems described below lend themselves to this possibility. The first suggestion that N-heterocycles might be more suitable for hydrogen storage came from Pez30 and colleagues at Air Products. Their advantages are ready reversibility, easier H2 release than alicyclics, and they are kinetic and thermodynamic, as well as having low vapor pressure. In addition, they only contain C, H, and N elements that are widely available. We31,32 independently came to similar conclusions and explored how judicious placement of N atoms can greatly lower release temperatures in LOHCs. These advantages have led to their attracting much recent attention, and they are the main focus here.

Figure 1. Calculations by Clot and Eisenstein32 of Td in K (see text) for the ring systems indicated, in their dehydrogenated forms, showing the big effect of N atoms, both endocyclic and exocyclic.

sensitivity of Td to the structure. The high Td values of pure alicyclics make them hard to dehydrogenate, but the presence of N atoms greatly lowers the Td values In replacing a CH2 by NH, a strong CH is replaced by a weaker NH bond, and the CH bonds adjacent to N are also weakened.22 A mechanistic study of H12-NEC dehydrogenation under high vacuum conditions over Pt(111) by IR spectroscopy, XPS, and molecular beam methods36,37 identified H 8-NEC as an intermediate that is formed slowly even at 223−273 K. Heating to 380 K gave NEC itself, but undesired C−N bond scission in the N−Et group took place above 390 K. This scission proved to be highly structure-sensitive, favored by low coordinate Pt in particles but disfavored over Pt(111).38,39 More relevant to the conditions of practical hydrogen storage, Dong et al.40,41 find that at 140−170 °C with a pH2 of 1 atm over Pd/Al2O3, H12-NEC gives H8-NEC and H4-NEC as intermediates on the way to NEC (Figure 2). H4-NEC was the principal product, in line with the final step, H4-NEC to NEC, being rate limiting, having a barrier of 17.5 kcal/mol. These studies were complicated by H12-NEC being a mixture of isomers, so the rate of the first dehydrogenation step results from a conflation of different component rates. These differ substantially, the three isomers present in their material having 4492


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(85%), reversible hydrogenation−dehydrogenation was achieved. In a promising development, Matsuda et al.48 have converted H12-carbazole into an ionic liquid (IL) by linking it to an imidazolium cation (Figure 4). Dehydrogenation starts at ∼180

Figure 2. Dehydrogenation of H12-NEC.

Figure 4. H12-NEC form of an ionic liquid LOHC.

reaction barriers of 13.3, 14.6, and 17.0 kcal/mol for H12-NEC going to H8-NEC; after that point, only a single isomer need be considered. The same intermediates appeared in the hydrogenation of NEC over Ru black, except that here, H6-NEC was also detected.42 The fully hydrogenated form again occurred as a mixture of isomers. In a detailed study of the NEC hydrogenation over supported heterogeneous Ru and Rh catalysts,43 four isomers were identified shown as H12-NECa−c in Figure 3.

°C and is complete by 230 °C on Pd(111) or Pt(111) as revealed by UHV surface science techniques. Conversion of favored but often solid LOHCs to ILs may well prove more generally useful. The Smith group tested the effect of the introduction of a nitrogen atom into alicyclic ring systems by comparing the release rates for H12-NEC, H12-carbazole, and H12-fluorene over Pd/C catalyst. The release rates decreased in the order H12-NEC > H12-carbazole > H12-fluorene in line with the expectations discussed above.49,50 The 3-fold rate enhancement of H12-NEC over H12-carbazole was ascribed to the poisoning effect of the N lone pair in H12-carbazole. Other N-Heterocycles. The Jessop group51 explored a wide range of N-heterocyclessome examples are shown in Figure 5and confirmed that exocyclic electron donors favor

Figure 3. Isomers seen for H12-NEC.

Isomers b and b′ were inseparable on the column employed so that only three isomers seem to be present. Finally, from IR and XPS data and DFT calculations, Subota et al. suggest that the initial CH cleavage in H12-NEC on Pd nanoparticles already occurs at 170 K at the bridgehead site α to the heteroatom, which is also expected to be the weakest CH bond in the molecule.44 Dong et al.45 prefer N-ethylindole (NEI) to NEC. The hard step in Figure 2 is thus avoided, and there are only two possible H8-NEI isomers. Hydrogenation of NEI was carried out over a 5 wt % Ru/Al2O3 catalyst from 160−190 °C at a pH2 of 89 atm. The dehydrogenation was achieved over a 5 wt % Pd/Al2O3 catalyst from 160−190 °C with the production of a very pure H2 stream, only trace water being detected. The low melting point of 17.8 °C and the acceptable gravimetric density of 5.23 wt % H makes NEI a promising new candidate for future development. 2Methylindole (capacity 5.7 wt %) behaves similarly with the H2 and H4 intermediates being detected in the dehydrogenation step.46 Manas et al.47 reported the dehydrogenation of H4-2methylquinoline to the parent heterocycle with a homogeneous iridium catalyst at 135 °C; although the H2 yields were low

Figure 5. Some Jessop heterocyclic LOHCs.51

dehydrogenation. They also found that exocyclic conjugated substituents can favor dehydrogenation to a degree beyond that expected from their Hammett parameter, for example, piperidine-4-carboxamide. They hoped to find a case where all the rings of their LOHC would have an equal facility for dehydrogenation, but attempts to realize this strategy ran up against stability problems principally resulting from C-heteroatom cleavage.

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AMINE−AMIDE STRATEGY Milstein52 has developed a novel strategy based on the reversible dehydrogenative coupling of alcohols and amines to give amides with a pincer ruthenium catalyst under reflux with base (Figure 6). In this relatively new reaction type,53 the alcohol is activated by dehydrogenation to form a bound aldehyde, and the amine 4493


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resulting in a 6.32 wt % value. Thermal dehydration or aldol reactions in the hydrogenated form are likely to pose problems for H14-NAC, however, so this capacity advantage may be only notional. They conclude that energy losses of Ru > Pt > Pd, with H4-NEC and H8-NEC being the only intermediates seen, with the H4-NEC to NEC step again being rate limiting.41 4494


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HOMOGENEOUS CATALYSIS A number of homogeneous catalysts successfully dehydrogenate NEC, for example, Brayton and Jensen65 find that the Ir(POCOP) pincer catalyst 1 is effective but they call attention to the need for a relatively high temperature, 180 °C, for the onset of H2 evolution and to the fact that the dehydrogenation product is a solid. They, therefore, prefer n-butyl pyrrolidene (H4-NBP, Figure 8) for its lower onset temperature (140 °C)

catalyst 2 (Figure 9) as an effective LOHC dehydrogenation catalyst at 2% loading at 110 °C over 24 h. They also followed the

Figure 9. Structure of catalyst 2.

RCH2NH2 dehydrogenation to look for undesired formation of the imine RCHNCH2R. This arises from amine attack on the RCHNH partial dehydrogenation intermediate but is suppressed in the case of long chain aliphatic amine such as dodecylamine. For recycling, their catalyst was immobilized on reduced graphene oxide via the pyrene group attached to the Nheterocyclic ligand. Storage Applications of LOHCs. Krieger et al.74 have modeled the introduction of an LOHC energy storage system into a cement plant located in Düsseldorf, Germany, to see if a wind energy park could be successfully combined with heat and LOHC energy storage modules to lower the carbon footprint of the plant via increased efficiency. One special advantage of this situation is that the waste heat from the cement plant can be used to improve the efficiency of the H2 release step. This is calculated to result in an increase of the overall efficiency from 16% to 21.9%; the addition of a supplementary heat storage system further increased the calculated efficiency to 28.5%. For a capital cost of €3.5 M, an annual saving of €1 M in electricity costs was anticipated. This example suggests that innovative engineering strategies can provide significant efficiency enhancements. Direct Fuel Cell Applications of LOHCs. The standard procedure contemplated for energy release is catalytic dehydrogenation of the LOHC, followed by introduction of the resulting H2, purified if necessary, into a fuel cell to produce motive power. The two steps of this procedure could, in principle, be telescoped into one if a direct fuel cell could dehydrogenate the LOHC at the anode and reduce aerial O2 at the cathode, leading to a similar power output but with fewer components. This possibility was considered early,22 and experimental efforts in this direction were pioneered in a research effort led by GE. Indeed, highly selective electrocatalytic dehydrogenation of a model case4-methoxybenzyl alcohol to p-anisaldehydeproved possible at a 94% Faradaic efficiency with Pt electrodes at low applied potential, 0.2 V vs FcH/FcH+, catalyzed by the Ir(I) organometallic complex, 3 (Figure 10).75 A similar strategy was applied to H12-NEC dehydrogenation by Driscoll et al.,76 but instead of a dehydrogenation catalyst, a

Figure 8. H4-NBP and catalyst 1.

and because the pyrrole dehydrogenation product is a liquid at room temperature. The H wt % value was only 3.14%, however. The kinetic data suggested that steric inhibition was responsible for the poorer reactivity of NEC versus NBP. Catalyst stability was maintained up to 200 °C testifying to the unusual robustness of 1 (Figure 8). Photocatalysis has also been applied to the problem. The Beller group66 have applied the 1988 Saito-type67 alkane photodehydrogenation system to H12-NEC to liberate H2. The catalyst, RhCl(CO)(PPh3)2 at 0.1% loading, gave a 75% yield (TON = 640), primarily of H8-NEC, after irradiation at 320−500 nm for 7 h. In this case, the activation energy for release comes from the irradiation, so high temperatures are no longer needed. Kato et al.68 have reported a number of multicomponent hybrid catalyst systems that release H2 from a series of tetrahydronaphthalene and isoquinoline LOHCs with light. An acridinium dye is the photoredox initiator, a Pd(II) salt is the dehydrogenation catalyst, and a thiophosphoric imide organocatalyst regenerates the dye for the next light absorption event. A total of two H2 molecules are eliminated to give the parent naphthalene or isoquinoline as the final organic product. Engineering aspects have been considered by Peters et al., who have designed, built, and tested specialized reactors69 suitable for hydrogen release as well as eggshell catalysts having a Pt/γ-Al2O3 layer on an α-Al2O3 core as an improved design.70

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NONHETEROCYCLIC N-CONTAINING LOHCS Grellier and Sabo-Etienne71 argue that RCN/RCH2NH2 has, at least for the −CN/−CH2NH2 part of the molecule, a 2:1 ratio of released H atoms to LOHC heavy atoms resulting in a 13.3 wt % gravimetric loading for that substructure (eq 2). They argue that

butylamine, propionitrile, 1,5-diaminopentane, and pentanedinitrile have a wide liquid range and still have good gravimetric loadings in spite of the presence of the aliphatic chain. Szymczak’s72 demonstration of reversible nitrile hydrogenation/dehydrogenation, albeit using two different homogeneous Ru catalysts, brings the possibility of a nitrile/amine LOHC pair closer. The hydrogenation requires 3 atm H2 and proceeds at room temperature, while the dehydrogenation occurs at a relatively modest 110 °C. The Mata group73 introduced the Ru

Figure 10. Structure of catalyst 3. 4495


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combustion of the original LOHC. For example, the calculated free energy release (300 K) for (C6H12 + 1.5O2)/(C6H6 + 3H2O) is only 18.8% of the release from full combustion (C6H12 + 9O2)/ (6CO2 + 6H2O). This makes the energy density of the LOHC much inferior to that of the hydrocarbon. Much the same will hold for any N- or O-heteroatom containing LOHC that releases one H atom per heavy atom (C, N, or O). The next difficulty is the slow release rates and high temperatures so far needed for H2-regenerable LOHCs. Here, there is hope because improved catalysts and more kinetically labile LOHCs would make a big difference. Hydrogen storage is a hard problem, so a big challenge is development of a direct fuel cell that converts the LOHC into electrical power. This would allow a much lower temperature operation because O2 reduction at the fuel cell cathode would make the dehydrogenation thermodynamically allowed at room temperature; once again, good catalysts will be needed. If these points can be resolved, it will be time to move on to optimizing the more detailed points (i−viii) mentioned above. It seems inevitable that, whether sooner or later, some form of solar, wind, and perhaps nuclear energy economy must arise. Since practical development of such a system is a major challenge that will take many decades to bring about, we must continue right now to establish the required science and engineering basis.

simple redox transfer agent, ferrocene or decamethylferrocene, proved effective at 0.53 V vs Ag/Ag+ in the presence of a base. Indoline and benzylaniline were also dehydrogenated, reactions that are easier to analyze because they avoid the multiple intermediates and geometrical isomers found in the NEC case. A drawback in the case of indoline was an undesired dimerization at the 5 position. The high potential quinone, 2,3-dichloro-5,6dicyano-1,4-benzoquinone (DDQ), also proved effective as a catalyst in the case of benzylaniline, where a hydride transfer mechanism was suggested. Flow Battery Applications. Beyond transport applications, the intermittency problem of solar and wind power raises the question of energy storage at night for domestic power production. Flow batteries77 have attracted attention in this context. A flow battery typically stores excess electrical power by electrochemical reduction/oxidation of a solution phase redox carrier pair, Qox/Qred as the anolyte and Q′ox/Q′red as the catholyte, the Q and Q′ solutions being kept apart by a membrane in the cell and stored separately in external tanks. Its low energy density means the device best suited for stationary over mobile applications. When energy release is required, the flow is reversed and the anode and cathode swap identities, reversing the chemical changes that occurred during charging; efficiencies of 70% are typical. An early rechargeable flow battery of this kind used aqueous CrII/CrIII and FeII/FeIII as redox couples.78 One suggested embodiment, however, involves another potential application of N-heterocycle LOHC ideas because quinoxalines, dipyridylketones, and viologens have all been suggested as suitable redox carriers.79 Economic Analysis of LOHCs. Given the highly applied character of LOHC work, we need to be guided by economic analysis to avoid lines of research that cannot possibly become economically viable. Eypasch80 and colleagues have made a techno-economic evaluation of an electricity storage system based on LOHCs. Dibenzoyltoluene was the LOHC chosen for the study, but the results are likely to be broadly applicable to related molecules such as NEC. The complete system had separate hydrogenation and dehydrogenation modules together with an electrolyzer for the energy input and a combined heat and power module for the output. The full device was simulated with plausible assumptions of relevant variables for making cost comparisons with likely competitor strategies. The best compromise between complete reliance on the grid and complete self-sufficiency occurred at 60% self-sufficiency. The estimate was made for Germany with its specific energy price and subsidy structure, so this aspect is likely to differ for different locations. However, the work indicated that the LOHC system was potentially viable in real world situations. An economic analysis by Teichmann et al.81 points out the advantages of a combined heat and power (CHP) system for distributed domestic or small-scale commercial storage. Heat losses from the LOHC storage module now become available for heating or cooling, thus enhancing overall efficiency in a way that would not be available to a centralized storage system. The authors identify numerous sources of efficiency. For example, the hydrogenation of NEC is highly exothermic and so typically occurs at about 150 °C; waste heat from the electrolyzer and fuel cell would be available at ∼80 °C. The same group also looked at LOHCs for importing solar power from North Africa to Europe.82 Limitations and Prospects. The biggest and seemingly inevitable problem with LOHCs is the low energy density, estimated here by comparing combustion of the released H2 with

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Robert H. Crabtree: 0000-0002-6639-8707 Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS I thank Paul Anastas for discussions and the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy (Grant DEFG02-07ER15909) and the TomKat Charitable Trust for support of some of our work in this area.

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REFERENCES

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