The invention relates to liquid mixtures comprising two or more compounds selected from isomers of benzyltoluene and/or dibenzyltoluene in catalytic processes to bind hydrogen to and/or release hydrogen from the mixture and to a method of using same as hydrogen stores for supplying a consumer with motor fuel.
Currently considered scenarios for large-scale energy production from renewable sources, for example wind parks in the North Sea or Desertec, are technically predicated on suitable ways to ideally transport and store large amounts of energy without loss in order to rectify seasonal variations in generation and to achieve efficient transportation of useful energy over large distances.
A particularly attractive way to meet the challenges described above is to develop new “energy-carrying substances” and to provide technologies for their efficient energy loading and unloading. The use of “energy-carrying substances” presupposes that the energy provided at an “energy-rich” place, at an “energy-rich” time is used, for example, to convert an energy-poor liquid A into an energy-rich liquid B. B is then storable without loss for long periods and transportable at high energy density. When and where energy is required, the conversion of the energy-rich liquid B back into A then releases useful energy. A may be a liquid or a gaseous substance. When A is a liquid as well as B, the concept envisages the possibility of A being returned back to the place of energy generation and reloaded.
A preferred approach to technically realizing an energy transportation and energy storage system on the basis of “energy-carrying substances” involves loading the energy-poor substance A with hydrogen to form the energy-rich substance B, while the hydrogen needed for this is provided from an electrolysis of water by means of preferably renewably generated electrical energy. This operation of energy loading typically takes the form in the prior art of a catalytic hydrogenation reaction under superatmospheric pressure. Substance B is energy unloaded by catalytic dehydrogenation at low pressures and high temperatures. The hydrogen which is released again in the process is useful as energy source in a fuel cell or in a combustion engine for example. Hydrogen released on board a vehicle is directly usable to operate the vehicle. Prior art examples comprise energy storage in the form of CH4, NH3 or methanol. On hydrogen unloading, these compounds give rise to the gaseous substances CO2, in the case of methane and methanol, and nitrogen in the case of NH3.
DE 10 2008 034 221 A1 describes an alternative known concept where the energy-poor form A is a liquid and so the energy unloading reproduces a liquid. The energy-poor form A can in this case be stored and transported as liquid in order to be reloaded with hydrogen at an energy-rich time and at an energy-rich place. Systems of this type are known as liquid organic hydrogen carriers (LOHCs). Examples of such LOHCs are disclosed in patent application EP 1 475 349 A2.
Prior art LOHC systems are preferably substance pairs wherein the energy-poor substance A is a high-boiling functionalized aromatic compound which is hydrogenated in the energy-loading step. One particularly preferred example disclosed relates to using the substance pair N-ethylcarbazole/perhydro-N-ethylcarbazole, which allows energy loading typically at around 140° C. and elevated pressures and energy unloading at temperatures between 230 and 250° C. The energy-rich substance perhydro-N-ethylcarbazole has a hydrogen capacity of around 5.8% by mass of hydrogen in the system referred to. The energy stored in the releasable hydrogen of 100 kg of perhydro-N-ethylcarbazole is accordingly sufficient for a motor vehicle range of about 500 km, while the burning of the hydrogen on board for energy results in the almost exclusive formation of water vapor as combustion product. Hence this approach represents a technically interesting alternative to other energy storage concepts for mobile applications.
Reaction systems for catalytic release of hydrogen from liquid energy storage molecules in the prior art consist of fixed bed reactors or slurry phase reactors. Reactors are also being developed to release hydrogen from a hydrogen-carrying liquid compound which comprise a pressure- and heat-resistant reactor vessel in which at least one function to provide the hydrogen is performable in that the reactor vessel contains at least one body having a metallic supporting structure coated with a solid high-porosity layer containing catalytically active substances for the release of hydrogen from liquid hydrogen-carrying compounds. The hydrogen-carrying liquid compound here may advantageously also be a mixture of hydrogen-carrying liquid compound in an appreciable proportion and other compounds.
Some LOHC systems known from the prior art have heteroatom-carbon compounds. This structural feature activates the systems for the catalytic unloading of hydrogen. However, the distinctly higher loading of the nitrogen-carbon bond—as compared with the carbon-carbon or carbon-hydrogen bond—is also responsible for the fact that the thermal stability of any LOHC systems comprising nitrogen-carbon bonding is limited to temperatures up to 280° C. However, even minimal thermal decomposition of the LOHC system is relevant for technical application, since it adversely affects the fixed point of the LOHC system and the catalytic loadability/unloadability. A high thermal stability allows inter alia reaction temperatures above 280° C. in the catalytic release of hydrogen, resulting in a higher volumetric productivity of hydrogen release compared with lower temperatures.
The problem addressed by the present invention is that of devising a liquid compound which is useful as a hydrogen store, which is providable in large amounts and which is simple to use in technical facilities since it does not have the abovementioned disadvantages in that it is exclusively constructed of carbon and hydrogen and is thermally stable above and beyond 280° C. A method of supplying a consumer with hydrogen by using this liquid compound shall also be provided.
The problem addressed by the invention is solved by a method of using a room temperature liquid mixture comprising two or more compounds selected from isomers of benzyltoluene and/or dibenzyltoluene in catalytic processes to bind hydrogen to and/or release hydrogen from the mixture. Other advantageous embodiments and advantageous methods of supplying a consumer with hydrogen are provided herein.
According to the invention, a room temperature liquid mixture of two or more compounds constructed exclusively of the elements carbon and hydrogen and forming in individual known compositions a synthetic mix of substances which is usable as a heat transfer liquid is characterized in that the mixture contains at least one compound having two or more non-fused non-pi-conjugated aromatic units and is used in catalytic processes to bind hydrogen to and/or release hydrogen from the mixture.
The mixture which in the hydrogen-poor form is already used as heat transfer oil, for example under the brandname Marlotherm LH or Marlotherm SH (from SASOL for example), in certain variants to be used as a liquid hydrogen storage and transportation system, is novel and inventive because the possibility of hydrogen loading the mixture has hitherto not been considered anywhere and the hydrogen loading makes possible a novel use through an inventive process. This also holds for the process of hydrogen release from the previously unknown use of the hydrogen-rich form as hydrogen carrier. This is because particularly the mixture has many important, previously unknown advantages over the prior art liquid hydrogen storage and transportation systems, viz., a high hydrogen capacity, low vapor pressure, high chemical and thermal stability coupled with inexpensive industrial availability, known and unconcerning toxicity and eco-toxicity, and compatibility with all sealant and tank materials. The use of heat transfer oils as LOHC system is an advantageous way to avoid the disadvantageous behavior of previously used LOHC systems due to their limited thermal stability.
An advantageous method of fully or partially supplying a consumer with hydrogen, by using a mixture of the present invention, is notable in that a first storage tank for the hydrogen-carrying mixture supplies said mixture to a reactor via a feed line and the mixture dehydrogenated in the reactor at high temperature and low pressure is discharged from the reactor into a second storage tank via a discharge line, wherein the reactor supplies a consumer with hydrogen via a connecting line. Such a method is particularly advantageous to use when the consumer is an internal combustion engine or at least a fuel cell and, more particularly, contributes to the energy supply of a motor vehicle. The first and second storage tanks may be in communication with one another, even to the point where their respective contents become commingled.
A preferred method according to the invention is characterized in that the mixture is contacted in the reactor with a metal-containing catalyst and binds or releases hydrogen in the process, wherein the metal-containing catalysts used for hydrogen loading and hydrogen unloading are identical or different solid catalysts comprising one or more of the metals palladium, nickel, platinum, iridium, ruthenium, cobalt, rhodium, copper, gold, rhenium and iron in finely divided form on a porous apolar carrier.
What is common to all the aforementioned methods is that hydrogen is released in the reactor from the hydrogen-laden mixture by catalytic dehydrogenation at high temperature and low pressure.
The concept of energy-carrying heat transfer oils which finds application here has the advantage that it is technically close to our existing energy supply with fossil fuels and therefore allows useful existing infrastructure, such as ships, refineries or gas stations, to be used. More particularly, energy-carrying heat transfer oils can store energy surpluses from renewable production and link them to the energy required for mobility, heating and transportation in the present-day infrastructure. These energy stores further have the following advantages: a nearly unlimited lossless storage capacity, a high energy density and low costs. They are further useful as a long-term store and transportation form of energy.
Tests using commercially available heat transfer oils such as Marlotherm LH or Marlotherm SH (from SASOL for example) as a hydrogen-unloaded form of an LOHC system have shown that the mixture which forms the hydrogen storage and transportation system should in the hydrogen-poor form A advantageously contain compounds having two or more non-fused aromatic units in a mass fraction between 5% and 100%, preferably between 60 and 100% and more preferably between 90 and 100%. It is further advantageous for the mixture to comprise above 50%, preferably above 90%, of different compounds which all contain at least two non-fused aromatic units.
Dibenzyltoluene may advantageously be a compound in the hydrogen-poor form of the mixture forming the hydrogen storage and transportation system. It is more advantageous for the mixture to comprise above 50%, preferably above 90% of different dibenzyltoluenes. This ensures that the hydrogen-poor form is convertible via a catalytic hydrogenation reaction, by hydrogen uptake, into a hydrogen-rich form in which the loaded hydrogen may be present in a chemically bonded form at a mass fraction of at least 6%, in which case the hydrogen pressure in the catalytic hydrogenation is between 5 and 200 bar, preferably between 10 and 100 bar and ideally between 30 and 80 bar and the reaction temperatures of the catalytic hydrogenation are between 20° C. and 230° C., preferably between 50° C. and 200° C., but ideally between 100 and 180° C.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.