液态有机杂环化合物的储氢
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MINIREVIEW www.rsc.org/ees|Energy &Environmental Science
Hydrogen storage in liquid organic heterocycles
Robert H. Crabtree
Received 3rd April 2008, Accepted 13th May 2008
First published as an Advance Article on the web 11th June 2008DOI:10.1039/b805644g
Hydrogen storage in liquid organic heterocycles is feasible thermodynamically and is attractive in terms of simplicity, safety, scalability, heat management and economy, but extensive catalyst development is needed to bring it to fruition.
In his 1908book, Worlds in the Making , Svante Arrhenius 1expanded on his 1896prediction that industrial CO 2production would eventually raise average global temperature. Doubling atmospheric CO 2would, he believed, cause a 4 C rise, a value within the presently accepted range. Perhaps because he was writing from a cool northern city, however, he felt this rise would be a good thing.
We often hear lamentations that the coal stored up in the earth is wasted by the present generation without any thought of the future . We may finda kind of consolation that here, as in every other case, there is good mixed with the evil. By the influenceof the increasing percentage of carbonic acid in the atmosphere, we may hope to enjoy ages with more equable and better climates, espe-cially as regards the colder regions of the earth, ages when the earth will bring forth more abundant crops than at present, for the benefitof a rapidly propagating mankind. 1a
Few informed individuals still retain this optimistic view of climate change, now much more often considered deeply prob-lematic. 2Average energy consumption for the planet as a whole has been estimated at 2kW h per person, 3more than 80%based on fossil fuel, leading to an injection of ca. 1013kg of CO 2per year into the atmosphere.
Efforts to alleviate CO 2production confront several problems. Costs are obvious, prompt and local while benefitsare putative, delayed and global. It can be little wonder that the research
Yale University, Chemistry Department, 225Prospect Street, P.O. Box 208107, New Haven, CT 06520-8107, USA. E-mail:[email protected]
Robert H :Crabtree
Educated at Oxford and Sussex Universities and CNRS Natural Products Institute in Paris. Now, a Professor of Chemistry at Yale, he works extensively on catalysis, both organometallic and bioinorganic. Appointed Dow lecturer at Berkeley, Sabatier Lecturer at Toulouse, and will be Osborn Lecturer at Strasbourg and Mond Lecturer in the UK. He has been awarded ACS and RSC prizes for organometallic chemistry.
budgets in this fieldare dwarfed by those for health and defense. As the reality of the situation sinks in and climate disruption becomes more obvious, climate change and alternative energy may well become the dominant scientificproblems of the century. While a number of technical solutions appear plausible in principle, the challenge of applying them on the required global scale is daunting. For instance, to take just one likely scenario, in a world based on nuclear power, transport would require either light, highly efficientstorage batteries or else some transportable fuel that can be safely stored on board. In either case the principle is the same—electricalenergy is stored in chemical form.
Taking a global view, transport is an increasingly energy-intensive area, particularly with China and India rapidly indus-trializing. Hydrogen has been suggested as a possible energy carrier using an internal combustion engine (ICE)or fuel cell for the motive power. Efficiencyin an ICE is limited by the physics of the Carnot cycle to approximately 25%while fuel cells escape from this limitation and can have efficienciesabove 50%.4H 2is currently generated mainly from fossil fuel with release of CO 2, so the relatively widespread demonstration transport vehicles, usually advertised as ‘green’,do not yet achieve the stated goal. Both nuclear-to-H 2and solar-to-H 2schemes have been proposed to remedy this defect. Assuming H 2can be generated efficientlyby a CO 2-free route or with CO 2sequestration, very hard problems in themselves, 5–7we would next need a method for hydrogen storage in vehicles.
Numerous reports have treated this problem 3,4,7,8so it will only be necessary to summarize the major prior H 2storage approaches. These are physical:(i)high pressure (HP)tanks; (ii)cryogenic methods; or chemical:(iii)reversible absorption in metal or main group hydrides; (iv)reversible absorption on solids; and (v)storage in the form of metals that can liberate H 2from H 2O. Current plans for future vehicles employ one of three approaches, either (i),(ii)or (iii).
For a fuel-efficientautomobile, 4–8kg of H 2need to be stored to match current consumer needs and expectations. 3,4High pressure tanks are heavy and voluminous and may pose practical problems. Just this month (Feb2008) a major interstate highway in the writer’sstate was closed down for the whole day as a result of an otherwise minor mishap without gas release involving a commercial vehicle carrying high pressure H 2. Nevertheless, this is a cheap and readily reversible storage method and it is available today.
Given the low critical temperature, 33K, cryogenic storage requires very low temperatures, implying a large energy loss
from
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the liquefaction step. Apart from the weight penalty of the cooling module, this method is not ideal for vehicles used only intermittently because energy input is constantly required to maintain cooling.
Chemical methods are advantageous in binding H 2at ambient temperature and pressure but can suffer from lack of complete and easy reversibility. The H 2release enthalpies that appear to be most appropriate for automobile H 2storage lie in the range 15–25kJ mol À1and correspond to release temperatures in the range of 0–100 C. Carbon-based solids 9such as single walled nanotubes 10(SWNTs)or microporous metal–organicframe-works 11have shown attractive performance in room temperature hydrogen absorption but the performance still needs to be much improved before practical application can be envisaged. Metal hydrides 12,13such as LaNi 5, Mg 2Ni or MgH 2have also been very extensively studied. Although they have many attractive prop-erties, the fact that they tend to employ relatively high atomic weight elements means that few such metal hydrides are close to meeting the very ambitious criteria set by the US Department of Energy (DOE):9%hydrogen by weight by 2015. For compar-ison, one H stored per C atom leads to a gravimetric capacity of 7.1%but LaNi 5H 6stores only 1.37%.
This gravimetric criterion has focused attention on the light atoms of the Periodic Table, notably Li and B and on compounds that hold more than one H per non-hydrogen atom. One compound that fulfilsboth points is BH 3NH 3(ammonia–borane, or AB) 14and related materials. 15From its molecular weight and polarity, AB would be a gas except for the strong proton–hydrideinteractions present in the solid (mpt, 109 C). 16If it were to lose all its H 2(19.6%theoretical capacity) it would form boron nitride, an unpromising material for the regeneration step needed for any storage/releasecycle.
Thermal H 2release from BH 3NH 3is straightforward, and catalytic methods can also be successfully applied. Acid-cata-lyzed, as well as transition metal-catalyzed release have recently been reported. 14Catalytic AB dehydrogenation can give BH 3NH 2BH 2NH 3, (H2NBH 2) n (n ¼3,5), (HBNH)3or (HBNH)n polymer. The regeneration step is not at all straightforward, however. 17
A related strategy based on light element salts involves boro-hydrides, 18,19amidoborates 20or aluminohydrides. For example, Zuettel and coworkers have investigated LiBH 4, a hydride salt containing 18mass%of hydrogen. Hydrogen desorption was catalyzed with SiO 2and 13.5mass%of hydrogen was liberated over the range 200–350 C; LiBH 4regeneration proved chal-lenging, however. 21Bogdanovic and Schwickardi 19demonstrated reversible hydrogen storage with Ti-doped NaAlH 4, where the Ti acts as the catalyst. The LiBH 4–LiNH2system in which the principal phase present is Li 4BH 4(NH2) 3F, has also been proposed. 22
Perhaps not sufficientlyconsidered in current approaches is scalability. Any new technology would have to be applied on a vast scale to make any impact on global climate. Assuming a material having a 10%gravimetric capacity, 102kg of material would plausibly be needed for each of an assumed 109vehicles worldwide for a total of 1011kg of storage material. On this basis, elements that are not available on this scale may be unrealistic candidates. According to the US Geological Survey Mineral Resources Program, 23the 2007world production values for
selected elements are:La,
Liquid storage materials and heat management strategy
Whatever the storage material, liquids have significantengi-neering advantages over solids. They can be readily pumped not just for distribution and delivery but also within the vehicle during operation. This means that instead of heating the whole storage tank, only a small aliquot would be pumped into the catalytic dehydrogenation chamber for heating to reaction temperature at any given time. This is also a safety consideration in a collision, where it would be undesirable to have a large mass of hot, reactive—perhapseven pyrophoric—materialpresent in the vehicles involved. Once dehydrogenated, the spent storage material would pass back into the fuel tank and a partition would move across the tank to allow the spent material that is being pumped in to displace, but not mix with, the fresh material that is being pumped out. At the fuel station, the spent material would be off-loaded and the fresh material substituted in a similar manner. Trucks that deliver fresh material to the fuel stations would return the spent material for recharging with hydrogen. The liquid strategy also employs a simple, light fuel tank, as today, not a heavy duty tank capable of taking high pressure and temperature as would be needed if the whole storage bed had to be heated.
The properties of hydrogen make it most suitable for handling within commercial facilities by trained personnel. A liquid storage material thus has the further advantage that there would be no free hydrogen in the public sphere. H 2has an exceptionally high diffusivity, leading to an enhanced risk from leaks. It can cause embrittlement of metals, generating greater potential for leaks and complicating the engineering. A hydrogen flameis essentially invisible thus presenting greater dangers than a flamefrom other fuels. As an illustration, a standard method of detecting H 2flamesis to advance cautiously with a piece of paper in an outstretched hand, the flamebeing located when the piece of paper begins to burn.
An even more compelling argument for the liquid strategy is its heat management benefits.When any storage material is hydro-genated, large amounts of heat are necessarily generated. If this happens in a solid bed of storage material, the exotherm will require dissipation by cooling while the vehicle stands at the fuelling station. Not only is this an energy loss to the global energy balance of the strategy, but it means fillingthe vehicle will be prolonged, and incompatible with the level of patience usually encountered among the driving public, as well as detracting from the profitabilityof the fuel station. If cooling is applied via refrigeration to speed the fillingprocess, the energy input required would further degrade the energy balance of the system as a whole. This contrasts with the liquid strategy in which fillingthe vehicle takes no more time than today and involves no exotherm. If the hydrogenation step occurs in a commercial facility on a very
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Fig. 1The liquid strategy showing how the presence of free hydrogen is avoided in the public sphere. The exotherm of rehydrogenation of the spent storage material can also be more efficientlyrecovered if produced in a central facility on a large scale.
large scale, as in the liquid strategy, then the exotherm is produced in one place, where it can be at least partially recovered, rather than in fuel stations throughout the city where it will typically be lost. Fig. 1illustrates the liquid strategy in more detail.
Organic heterocycle strategy
The possibility of storage of hydrogen in organic compounds has been widely excluded from consideration because reversible, low temperature H 2release has not been thought feasible. For example, according to a standard review of the field.3
The second important criterion is reversibility of hydrogen uptake and release. This criterion excludes all covalent carbon hydrogen compounds as hydrogen is only released from carbon hydrogen compounds if they are heated to temperatures above 800 C or if the carbon is oxidized.
While this was a plausible argument based on much of the data available at the time (2004),this analysis does not consider the twin possibilities of catalytic H 2release and of chemical modifi-cation of the ‘‘carbonhydrogen compounds’’to favor low temperature H 2release.
Catalysis was firstconsidered for alkane–arenepairs, such as decalin–naphthalene,but the endothermicity of the release step is such that elevated temperatures are required for the thermody-namics to become favorable. Saito and coworkers 24proposed ‘liquidfilmstate’conditions, a nonequilibrium technique which allows much higher hydrogen production rates than in a batch reaction. Similarly, a pulse-spray mode reactor has been adopted by Ichikawa and coworkers. 25The highest rate, an impressive 3800mmol g À1Pt min À1, was obtained in the dehydrogenation of cyclohexane over Pt/alumiteheated at 375 C with a cyclohexane feed of 190mmol min À1with 3.5mmol pulses at 1.0s intervals. A bimetallic Pt–Rhcatalyst showed higher activity than a simple Pt catalyst on the same support.
The release temperatures are still rather high and it would be useful to lower the endothermicity of release and thus bring down the equilibrium release temperature.
Taking the cyclohexane–benzenepair as a model, the endo-thermicity of H 2release is such that a temperature of ca. 600K is required to bring the reaction to a D G value of zero. At this point the unfavorable enthalpy is exactly compensated by the favor-able entropy of H 2release. We refer to this temperature as T d , the point at which D G ¼0. Intermediate dehydrogenation products (e.g. cyclohexene) that do not benefitfrom the aromaticity of benzene are even more strongly disfavored than the finalarene. The overall reaction could still be accessible if these intermediates were sufficientlystabilized by binding to the catalyst. Even in very endothermic cases, product formation is experimentally possible, however. For example, alkane dehydrogenation to alkenes and free H 2has been observed with numerous homoge-neous catalysts by us and others even at temperatures of 90–150 C. These reactions are driven by refluxof the alkane, because the H 2is swept out of the solvent and the equilibrium continually displaced. 26
The organic heterocycle H 2storage strategy, firstproposed by Alan Cooper and Guido Pez at Air Products, appeared in a series of key patents. 27,28Our later, but independent computational work on this problem, largely in collaboration with Eric Clot and Odile Eisenstein, 29identifiedsome general trends for design of the heterocycles to favor low temperature H 2release. In summary, the storage step involves catalytic hydrogenation of an aromatic heterocycle to give the corresponding hydrogenated product. Release is effected by heating the hydrogenated form in the presence of a catalyst. Both directions must therefore be viable, implying that the endothermicity of the dehydrogenation has to be moderate. This endothermicity translates into a temperature T d at which D G ¼0and the results are therefore discussed in terms of T d .
The all-carbon systems (e.g. , cyclohexane–benzene)have a T d that is far too high but introduction of nitrogen atoms into the organic ring dramatically favors the thermodynamics of H 2release:in extreme cases, T d can now go below 50K. The most important design consideration for a low T d is a move to a 5-membered ring, when aromatic stabilization can be achieved after cleavage of only four C–Hbonds (eqn(1)),not six as for cyclohexane–benzene,always provided a NH or NR is present in the 1-position to permit aromaticity. Cyclopentane–cyclopentadiene shows no such
advantage.
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(1)
Incorporation of N into a six-membered ring also favors dehydrogenation because the N–Hbond that is now broken is weaker than the C–Hbond it replaced. In addition, C–Hbonds adjacent to a N atom are also weakened relative to a C–Hbond in a pure carbocycle. Nitrogen substituents are also
effective.
(2)
The thermodynamic data calculated {DFT(B3PW91)}by Eric Clot and Odile Eisenstein allow useful structure-activity trends
to
be made up with newly manufactured material with a consequent economic penalty. Catalysts that can meet the severe selectivity and activity requirements must be further developed. Since it is not yet clear which HSM to choose, the HSM and the catalyst will have to be optimized together.
Limitations
Gravimetric capacity for typical HSMs of the type proposed are in the range from 6to 8%,not as good as BH 3NH 3. To meet the more aggressive DOE goals, more than one H would need to be stored per heavy atom, for example C(CH2NH 2) 4–C(CN)4has an 11.7%capacity. The volumetric capacity for typical hetero-cycles is also satisfactory, although in energy terms not as good as gasoline, because only some of the hydrogen atoms of the HSM end up as H 2O and all the carbon remains uncombusted. The situation for gasoline, denoted (CH2) n , undergoing complete combustion versus the same (CH2) n , but now acting as an HSM with the hydrogen released ultimately undergoing oxidation with air, can be represented as follows.
(CH2) n +1.5n O 2¼n CO 2+n H 2O (CH2) n +0.25n O 2¼(CH)n +0.5n H 2O
(4)(5)
Scheme 1Thermodynamic hydrogen release temperatures (temperature{K}at which D G ¼0) for selected model compounds by DFT(B3PW91)calculations of Clot and Eisenstein. 22
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be identified.Scheme 1shows the T d values for a number of key cases. The most effective way of lowering T d is moving to a 5-membered ring, with incorporation of N-substituents and N ring atoms in a 1,3-arrangement being somewhat less effective strat-egies. As an aside, the data also suggest that the long known resistance of certain azoles (e.g. , imidazole) to hydrogenation is the result of thermodynamic rather than purely kinetic factors. In their patent, Pez and coworkers 28demonstrate reversible hydrogenation–dehydrogenationof heterocyclic liquids. For example, N-ethyl carbazole is hydrogenated with 72atm H 2and a Pd catalyst at 160 C to form a mixture of isomers of the fully hydrogenated species (eqn(3)).Dehydrogenation gave pure H 2with Ru at 50–197 C and at least 5cycles can be run without HSM
degradation.
(3)
The carbazole fulfilssome of the thermodynamic requirements of Scheme 1in that the nitrogen is a substituent to two rings and a member of the central 5-membered ring.
In this vision, a nitrogen-containing organic liquid is preferred as a hydrogen storage material (HSM)on several counts. Both C and N are available in very large amounts, with 2007world production figuresfor nitrogen 23of 1.2Â1011kg and just counting carbon in the form of coal, 5Â1014kg (WorldCoal Institute). 30The material must not only be readily available but also be distributed economically to users. The Sallan Founda-tion 31estimates the infrastructure costs of the distribution system for petroleum based transport fuels at several hundred billion dollars. A liquid organic HSM can plausibly be distributed via the existing gasoline infrastructure with minimal modification,saving vast capital costs. Ideally the HSM would be minimally volatile, minimally toxic, and biodegradable.
One advantage of the heterocyclic liquid strategy is that the liquid can in principle be repurifiedwhen necessary, so that the inevitable non-regenerable fraction of the HSM does not build up on the vehicle—aweight and capacity penalty. For this step, vacuum distillation may be desirable and if so, the components will need to have appreciable volatility.
Efficiencyof HSM regeneration will be a key point because any shortfall would raise purificationcosts and require HSM to
In general terms, the exothermicity of a combustion reaction of H 2or of a hydrocarbon is simply proportional to the amount of O 2consumed. Comparison of the O 2used in the two equations above suggests that the energy content of an organic HSM is ca. 17%of the value that the same material would have if completely combusted as a normal fuel. Of course, complete combustion would form CO 2, contrary to the requirement for carbon dioxide abatement.
Design of the HSM requires attention to numerous points including (i)toxicity; (ii)thermal stability against undesired decomposition pathways; (iii)safety in accidental release such as in automobile collisions; (iv)biodegradability; (v)thermody-namic tendency to liberate H 2; (vi)kinetic facility for reversible H 2release; (vii)cheap manufacture on a 1011kg scale. Points (i)–(vii)severely restrict the types of materials that can be used, not only for the liquid organic strategy but for all others as well. Azaheterocycles can be of low toxicity, 1-decyl pyridinium chloride is used as a mouthwash in almost all common formu-lations and is thus in intimate contact with humans on a daily basis as well as being released into the environment on a large scale. Thermal decomposition is a serious issue and work will be needed to understand the thermolysis pathways of candidate HSMs and findways to guard against them by suitable design. Liquid organic HSMs can be of low volatility and have a high flashpoint, minimizing accidental release problems, and they may be biodegradable, particularly in their hydrogenated form, thanks to their heteroatom content. Quantitative structure-activity relationships (QSAR),now being developed both for toxicity and for biodegradation of N-heterocycles. 32
A HSM that readily releases H 2is of necessity hard to hydrogenate. ‘VirtualH 2storage’could avoid this step by using the electrical power source not to produce free H 2but to directly reduce the HSM electrocatalytically, but precedent is lacking. Likewise, a direct fuel cell could allow the conversion of the
fresh
HSM to motive power without H 2production. With air as the oxidant, the driving force of eqn (5)would be greatly enhanced and the high T d problem circumvented.
Conclusion
The liquid heterocycle strategy is worth greater emphasis because it has a number of advantages of simplicity, safety, scalability, heat management, and economy. Catalyst development is needed for further progress, however, since catalytic heterocycle dehy-drogenation is a neglected topic.
Acknowledgements
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I thank Eric Clot and Odile Eisenstein for their many insightful contributions to this and other problems of mutual interest, Peter Hall (Strathclyde)for the direct fuel cell idea, the referees for useful suggestions and DOE and NSF for funding our catalysis work.
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