REVERSIBLE LIQUID ORGANIC SYSTEM, METHOD AND PROCESS FOR LOADING AND DISCHARGING HYDROGEN BASED ON N-HETEROCYCLES

The present invention provides a system, a process and a method of storing hydrogen (H2) and releasing it on demand, comprising and making use of N-heterocycles as liquid organic hydrogen carriers (LOHCs).

FIELD OF THE INVENTION

The present invention provides a system, a process and a method of storing hydrogen (H2) and releasing it on demand, comprising and making use of N-heterocycles as liquid organic hydrogen carriers (LOHCs).

BACKGROUND OF THE INVENTION

The process of industrialization has brought prosperity and wealth to large parts of humanity during the last centuries. However, one fundamental obstacle associated with these processes is the ever-increasing exhaustion of fossil resources, along with the generation of waste and emissions. This directly has an adverse environmental impact that might drastically threaten global living conditions in the future. The search for alternative and sustainable energy systems to replace the current fossil fuel-based technologies has thus become one of the central scientific challenges of our society. In this context, hydrogen has long been regarded as an ideal alternative clean energy vector, which possesses an extremely high gravimetric energy density (lower heating value: 33.3 kWh/kg) and produces water as the sole byproduct upon combustion. These intrinsic properties of hydrogen make it a particularly attractive candidate for both stationary and mobile applications.

Recently, significant advances have been made in hydrogen-powered fuel cells. Nonetheless, hydrogen as energy vector has not yet been universally applied, which might be due to problems related to its storage and transport. Efficient storage of hydrogen is both crucial and challenging, due to its low volumetric energy density. Traditionally, hydrogen is stored physically in gas tanks under high pressure or as a liquid at cryogenic temperatures. However, the high energy input needed for storage, the low volumetric energy density, and potential safety issues largely limit applications using molecular hydrogen. Although extensive efforts have been made to store hydrogen in nanostructured materials, metal organic frameworks and metal hydrides, these systems suffer from low hydrogen storage capacities (HSC), harsh conditions, low energy efficiency, and high cost.

In recent years, storing hydrogen in chemical bonds has gained much attention, and is regarded as a promising pathway for a future “hydrogen economy”. Several kinds of metal hydrides, metal complexes and organic compounds have been investigated in this regard. Particularly interesting are liquid organic compounds as hydrogen carriers, which can have high hydrogen capacities, close to the U.S. Department of Energy (DOE of US, 5.5 wt % hydrogen storage capacity) and European Union (EU, 5 wt % hydrogen storage capacity) target for 2020. Moreover, liquid organic hydrogen carriers (LOHC) can have good stability, can be stored for a long time and readily transported, and may have potential applicability for on-board usage in vehicles.

A LOHC system is based on catalytic dehydrogenation of a hydrogen rich organic liquid, forming a H2-lean compound, which upon catalytic hydrogenation can regenerate the H2-rich compound. In early studies, aromatic hydrocarbons and their hydrogenated products were investigated as couples of H2-lean and H2-rich compounds. These LOHC systems can have a wide liquid range, excellent thermal stabilities, at low price. However, the dehydrogenation of H2-rich hydrocarbons for producing H2and corresponding H2-deficient aromatic hydrocarbons are thermodynamically unfavorable, which require harsh reaction conditions (temperatures above 250° C.).

The presence of nitrogen atom in LOHC systems based on N-heterocycles can reduce the enthalpy in hydrogenation and dehydrogenation, hence several N-heterocyclic LOHC systems have been investigated. One of the most attractive examples is the N-ethylcarbazole (NEC)/dodecahydro-N-ethylcarbazole (H12—NEC) system, which has 5.80 wt % theoretical hydrogen storage capacity (HSC), developed by Pez and his co-workers (FIG. 1A). However, NEC/H12-NEC system also has significant drawbacks: the reverse hydrogenation needs a different catalyst Ru/LiAlO2and high H2pressure (68 bar); the H2-lean compound N-ethylcarbazole is solid at room temperature (melting points 70° C.); and the thermal lability of the N-ethyl group leads to undesired side products.

In 2017, Fujita and his co-workers reported an interesting homogeneous Ir-complex catalyzed LOHC system based on 2,5-dimethylpyrazine/2,5-dimethylpiperazine in p-xylene/H2O or solvent-free, which has 5.3 wt % theoretical hydrogen storage capacity (FIG. 1B). However, under solvent-free conditions, the hydrogenation of 2,5-dimethylpyrazine to 2,5-dimethylpiperazine could not be completely achieved (78% conversion) under 30 bar of H2. Moreover, the high melting point of 2,5-dimethylpiperazine is also a disadvantage. Some recently reported N-based heteroaromatic/heteroalicyclic LOHC systems are 2,6-dimethyl-1,5-naphthyridine/2,6-dimethyldecahydro-1,5-naphthyridine (Fujita et. al.J. Am. Chem. Soc.2014, 136, 4829-4832), phenazine/tetradecahydrophenazine (Forberg et. al.Nat. Commun.2016, 7, 13201), 4-aminopiperidine/4-aminopyridine (Cui et. al.New J. Chem.,2008, 32, 1027-1037), 2,6-di-tert-butylpiperidine/2,6-di-tert-butylpyridine (Dean et. al.New J. Chem.,2011, 35, 417-422) and 2-(N-methylbenzyl)pyridine/2-[(n-methylcyclohexyl)methyl]piperidine (Oh et. al.Chem Sus Chem2018, 11, 661-665). However, almost all of them suffer from high melting points. Besides, the 2,6-dimethyl-1,5-naphthyridine/2,6-dimethyldecahydro-1,5-naphthyridine and phenazine/tetradecahydrophenazine systems use an expensive catalyst and solvent; the 4-aminopiperidine/4-aminopyridine system has problems in balancing the conversion and selectivity; the 6-di-tert-butylpiperidine/2,6-di-tert-butylpyridine system has low hydrogen storage capacity; 2-[(n-methylcyclohexyl)methyl]piperidine/2-(N-methylbenzyl)pyridine has low melting points, but requires high temperature (>230° C.) for dehydrogenation.

In 2015, a new LOHC, based on the dehydrogenative amide bond formation, which is thermodynamically favorable, was developed, but solvent was required (Hu et. al.Nat. Commun.2015, 6, 6859). Developing a LOHC system based on inexpensive organic liquid (which has a wide liquid range, low melting point) with high hydrogen storage capacity, solvent-free, under mild conditions, ideally using a single catalyst catalyzed for both hydrogenation and dehydrogenation is of great importance and challenge.

Piperidines were considered as potential ideal candidates of LOHCs, because they are abundant and inexpensive, have low melting points, wide liquid ranges, high theoretical hydrogen storage capacities (exceeding the targets of the US DOE and European Union), as shown inFIG. 1C. The acceptorless dehydrogenation of piperidines so far requires harsh conditions, as listed in Table 1.

Clearly, the development of inexpensive and abundant organic compounds with potentially high capacity to store and release hydrogen, ideally using the same catalyst for both loading and discharging hydrogen under relatively mild conditions, is a major challenge with no acceptable solutions known at this time. Therefore, developing a suitable catalytic system for mild, solvent-free, reversible dehydrogenation/hydrogenation of piperidines/pyridines is not only of theoretically significant, but also of practical interest, and a LOHC systems based on piperidines, using a single heterogeneous catalyst for both dehydrogenation and hydrogenation under mild conditions, with high hydrogen storage capacities, is highly desirable.

SUMMARY OF THE INVENTION

Thus, in the first aspect of the present invention, this invention provides a reversible hydrogen loading and discharging system comprising: at least one N-heterocycle; and one transition metal catalyst, or transition metal catalyst precursor, wherein the system does not comprise any solvent and is functional under mild temperatures and pressures. In some embodiments, the temperatures are between 50° C. and 180° C., the pressures are between 1 and 80 bar, or combination thereof. In some embodiments, the temperatures are between 130° C. and 180° C., the pressures are between 1.5 and 8 bar, or combination thereof. In some embodiments, the N-heterocylce is a H2-rich compound, an H2-lean compound or a combination thereof, and wherein the H2-rich compound is a substituted or unsubstituted piperidine, and the H2-lean compound is a substituted or unsubstituted pyridine. In some embodiment, the H2-rich compound is a substituted piperidine, and the H2-lean compound is a substituted pyridine. In some embodiments, the N-heterocycle is a liquid at least between a temperature of 15° C. and 100° C. In some embodiments, both the substituted or unsubstituted piperidine and the substituted or unsubstituted pyridine are liquids at room temperature. In some embodiments, the substituted piperidine is 2,6-dimethylpiperidine or 2-methylpiperidine. In some embodiments, the transition metal catalyst is palladium on activated carbon (Pd/C).

In some embodiments, the catalyst is commercially available or is generated in-situ from a catalyst precursor. In some embodiments, the catalyst precursor is Pd(OAc)2. In some embodiments, the same catalyst is used both for hydrogen loading (hydrogenation) and hydrogen discharging (dehydrogenation) processes. In some embodiments, the system further comprises a catalytic amount of at least one acid. In some embodiments, the acid is selected from: acetic acid, benzoic acid, carboxypolystyrene and polyacrylic acid. In some embodiments, the catalyst or the catalyst precursor is present in an amount of between 0.05% to 5% w/w.

In the second aspect of the present invention, this invention provides a reversible process for the storage and release of hydrogen (H2) upon demand comprising the steps of:a. when hydrogen storage is desired, reacting a substituted or unsubstituted pyridine with molecular hydrogen (H2) in the presence of a first catalyst, under conditions sufficient to generate a substituted or unsubstituted piperidine derivative; andb. when hydrogen release is desired, reacting a substituted or unsubstituted piperidine with a second catalyst, under conditions sufficient to release hydrogen, thereby generating the corresponding substituted or unsubstituted pyridine derivative and molecular hydrogen (H2);
wherein the first and the second catalyst are the same, the process does not comprise any solvent, and the process is carried out under mild temperatures and hydrogen pressures. In some embodiments, both the substituted/unsubstituted piperidine and the substituted/unsubstituted pyridine are liquids at room temperature. In some embodiments, the substituted piperidine is selected from: piperidine, 2-methylpiperidine, 3-methylpiperidine, 4-methylpiperidine, 3,4-dimethylpiperidine 2,4-dimethylpiperidine, 2,5-dimethylpiperidine and 2,6-dimethylpiperidine. In some embodiments, the catalyst is palladium on activated carbon (Pd/C). In some embodiments, the catalyst is commercially available or is generated in-situ from a catalyst precursor. In some embodiments, the catalyst precursor is Pd(OAc)2. In some embodiments, the process further comprises at least one acid. In some embodiments, the acid is selected from: acetic acid, benzoic acid, carboxypolystyrene and polyacrylic acid. In some embodiments, the catalyst precursor is present in an amount of between 0.05% to 5% w/w.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

As contemplated herein, a fundamentally new, reversible system that can load and unload H2with a potentially high hydrogen storage capacity has been developed.

Accordingly, the present invention provides a method, a process and system for storing hydrogen (H2) and releasing it on demand, based on the reaction of hydrogen rich N-heterocycles with at least one transition metal catalyst to form the corresponding hydrogen lean N-heterocycle and at least one, at least two, or at least three hydrogen molecules (for each molecule of the hydrogen rich N-heterocycle); each represents a separate embodiment according to this invention. In some embodiments, the catalyst is a palladium on activated carbon (Pd/C). In other embodiment, the catalyst (for hydrogenation and dehydrogenation) is Pd/C, Pd(OAc)2/activated carbon. In some embodiment the catalyst for hydrogenation is Ru/Al2O3. In some embodiments, the system does not comprise any solvent. In some embodiments, the hydrogen storage/loading is carried out in mild temperature (e.g., between 50-200° C.). In some embodiments, the hydrogen storage/loading is carried out under mild hydrogen pressure (e.g., between 1.5 and 8 bar). In some embodiments, the hydrogen release/unloading is carried out under mild pressure (e.g., atmospheric pressure). In some embodiments, the hydrogen release/unloading is carried out in mild temperatures (e.g. between 100-200° C.). In other embodiments, the same catalyst is reused (as is) in a reversible process for the storage and release of hydrogen (H2). In other embodiments, the same catalyst is recycled and used in the storage and/or release process of hydrogen (H2).

Upon loading of hydrogen molecules, the hydrogen lean N-heterocycle is reacted with at least one hydrogen molecule to form hydrogen rich N-heterocycle. Preferably, the hydrogen lean N-heterocycle is a substituted or unsubstituted pyridine and the hydrogen rich N-heterocycle is a substituted or unsubstituted piperidine. In another embodiment, the hydrogen lean N-heterocycle is a substituted pyridine and the hydrogen rich N-heterocycle is a substituted piperidine.

Accordingly, the present invention further provides a method, a process and a system for storing hydrogen (H2) and releasing it on demand, based on the hydrogenation of substituted/unsubstituted pyridines and the dehydrogenation of the corresponding substituted/unsubstituted piperidines liquid-organic hydrogen carriers (LOHCs). More specifically, the invention relates to substituted/unsubstituted piperidine LOHC. The process for hydrogen storage of this invention has a potential high hydrogen storage capacity.

FIGS. 1A-1Fpresents examples of some of the processes and methods for releasing and loading hydrogen molecules in N-heterocylces.FIG. 1Fpresents examples of the processes and methods described herein.

H2-rich N-heterocycles (e.g., substituted/unsubstituted piperidines) undergo catalytic dehydrogenation to form the corresponding H2-lean N-heterocycles (e.g., substituted/unsubstituted pyridines) with release of hydrogen. The H2-lean N-heterocycles (e.g., substituted/unsubstituted pyridines) may be hydrogenated back to the corresponding H2-rich N-heterocycles (e.g., substituted/unsubstituted piperidines), which function as a hydrogen storage system. These reactions may be catalyzed by a variety of catalytic systems, including transition metals, transition metal-based compounds and complexes, heterogeneous and homogeneous catalysts, with or without polymer and/or insoluble matrices support. Examples of suitable catalysts are Palladium (Pd), Platinum (Pt), Ruthenium (Ru), Copper (Cu), Iron (Fe), and compounds and complexes containing these metals, among others, preferably Palladium (Pd), Platinum (Pt), and Ruthenium (Ru) and compounds and complexes containing these metals. Preferably, the catalyst is heterogeneous catalyst such as Pd/C, Pd(OAc)2/activated carbon, Ru/Al2O3. In some embodiments, the catalyst is formed in situ by using a catalyst precursor instead, which may be either soluble (homogeneous) or insoluble (heterogeneous) in the reaction mixture of said system, process or method. In some embodiments, the catalyst precursor becomes an active catalyst upon entering the reaction mixture, and/or interaction with the N-heterocycle substrate (either H2-rich or H2-lean substrate) or H2. In some embodiments, the catalyst precursor is Pd(OAc)2.

In one embodiment, “transition metal catalyst precursor” refers to a transition metal compound that can transform to an active catalyst useful for the purpose of this invention in situ. Examples of transition metal catalyst precursors include but not limited to: Pd(OAc)2, PdCl2, Pd(TFA)2, Pd(acac)2, and Pd2(dba)3Pt(OAc)2, Pt(TFA)2, Pt(acac)2, PtCl2, PtO2, Ru(OAc)2, RuCl3. In some embodiments, upon entering the reaction mixture, which comprises the N-heterocycle and a solid support (e.g., activated carbon), the Pd(OAc)2catalyst precursor transforms in situ to palladium supported on the solid support (e.g., Pd/C). In other embodiments, the Pd(OAc)2catalyst precursor transforms in situ to palladium supported on the solid support (e.g., Pd/C) when the reaction mixture is heated to a temperature of about 100-200 C.

In other embodiments, the same catalyst is reused (as is) in a reversible process for the storage and release of hydrogen (H2). In other embodiments, the same catalyst is recycled and used in the storage and/or release process of hydrogen (H2).

In one embodiment, a catalyst being “reused (as is) in a reversible process for the storage and release of hydrogen (H2)”, refers to a process or system, where the storage and release of hydrogen occurs upon demand, and the same catalyst is used in both storage and release step without any further treatment.

In one embodiment, a catalyst being “recycled and used in the storage and/or release process of hydrogen (H2)”, refers to a process or system, where the storage and release of hydrogen occurs upon demand, and the catalyst is recycled (i.e. the catalyst is being isolated by for example centrifugation or filtration under inert atmosphere) and used again upon demand.

Thus, in some embodiments, the present invention relates to the use of an H2-rich substituted N-heterocycle, e.g., 2-methylpiperidine and/or 2,6-dimethylpiperidine, as a liquid organic hydrogen carrier (LOHC) to store hydrogen (H2) and release it on demand.

In another embodiment, the present invention relates to a process for the release hydrogen (H2), the process comprises the step of reacting a substituted/unsubstituted piperidine derivative (e.g., 2-methylpiperidine or 2,6-dimethylpiperidine) with a catalyst (e.g., Pd/C, Pd(OAc)2/activated carbon), under conditions sufficient to release hydrogen, thereby generating the corresponding substituted/unsubstituted pyridine derivative (e.g., 2-methylpyridine (2-picoline) or 2,6-dimethylpyridine (2,6-lutidine)) and molecular hydrogen (H2). In some embodiments, the process is carried out with a catalytic amount of an acid. In some embodiments, the process is carried out under mild temperatures (e.g., between 100-200° C.). In some embodiments, the process is carried out under mild pressure (e.g., atmospheric pressure). In some embodiments, the process does not comprise any solvent.

In the context of this invention, the terms “release”, “discharging” and “unloading” are used interchangeably and correspond to the generation of hydrogen molecules from hydrogen rich organic compounds, by a catalytic dehydrogenation process of the compounds.

In another embodiment, the present invention relates to a process for the storage of hydrogen, the process comprises the step of reacting a substituted/unsubstituted pyridine derivative (e.g., 2-methylpyridine (2-picoline) or 2,6-dimethylpyridine (2,6-lutidine)) with molecular hydrogen (H2) in the presence of a catalyst (e.g., Pd/C, Pd(OAc)2/activated carbon, Ru/Al2O3), under conditions sufficient to generate a substituted/unsubstituted piperidine derivative (e.g., 2-methylpiperidine or 2,6-dimethylpiperidine) as a hydrogen storage system. In some embodiments, the process is carried out with a catalytic amount of an acid. In some embodiments, the process is carried out under mild temperatures (e.g., between 100-200° C.). In some embodiments, the process is carried out under mild hydrogen pressure (e.g., between 1.5 and 8 bar). In some embodiments, the process does not contain any solvent.

In the context of this invention, the terms “storage”, “charging” and “loading” are used interchangeably and correspond to the incorporation of hydrogen molecules into hydrogen lean organic compounds, by a catalytic hydrogenation process of the compounds.

In another embodiment, the present invention relates to a process for the storage and release of hydrogen (H2) upon demand, comprising the steps of: (a) when hydrogen storage is desired, reacting a substituted/unsubstituted pyridine derivative (e.g., 2-methylpyridine (2-picoline) or 2,6-dimethylpyridine (2,6-lutidine)) with molecular hydrogen (H2) in the presence of a first catalyst (e.g., Pd/C, Pd(OAc)2/activated carbon, Ru/Al2O3), under conditions sufficient to generate a substituted/unsubstituted piperidine derivative (e.g., 2-methylpiperidine or 2,6-dimethylpiperidine); and (b) when hydrogen release is desired, reacting a substituted/unsubstituted piperidine derivative (e.g., 2-methylpiperidine or 2,6-dimethylpiperidine) with a second catalyst (e.g., Pd/C, Pd(OAc)2/activated carbon), under conditions sufficient to release hydrogen, thereby generating the corresponding substituted/unsubstituted pyridine derivative (e.g., 2-methylpyridine (2-picoline) or 2,6-dimethylpyridine (2,6-lutidine)) and molecular hydrogen (H2). The first and second catalyst may be the same or different. In a preferred embodiment, the first and second catalysts are the same. In other embodiments, the same catalyst is reused (as is) in a reversible process for the storage and release of hydrogen (H2). In other embodiments, the same catalyst is recycled and used in the storage and/or release process of hydrogen (H2).

In some embodiments, the process is carried out with a catalytic amount of an acid. In some embodiments, the process is carried out under mild temperatures (e.g., between 100-200° C., 130-180° C., 150-170° C.). In some embodiments, the process for storing hydrogen is carried out under mild hydrogen pressure (e.g., between 1.5 and 8 bar). In some embodiments, the process for releasing hydrogen is carried out under mild pressure (e.g., atmospheric pressure). In some embodiments, the process does not comprise any solvent.

This invention provides a reversible hydrogen loading and discharging system comprising: at least one N-heterocycle; and at least one transition metal catalyst, or transition metal catalyst precursor.

This invention provides a reversible hydrogen loading and discharging system comprising: at least one N-heterocycle; and one transition metal catalyst, or transition metal catalyst precursor.

The invention further provides a reversible hydrogen loading and discharging method, comprising the steps of:(a) hydrogen releasing process: contacting at least one substituted N-heterocycle H2-rich compound with at least one transition metal catalyst under conditions that allow dehydrogenation of the substituted N-heterocycle H2-rich compound, thereby forming three hydrogen molecules and at least one substituted N-heterocycle H2-lean compound;(b) hydrogen loading process: contacting said at least one substituted N-heterocycle H2-lean compound with said at least one transition metal catalyst and three hydrogen molecules under conditions for hydrogenation of said substituted N-heterocycle H2-lean compound, thereby forming at least one substituted N-heterocycle H2-rich compound.

The invention further provides a reversible hydrogen loading and discharging method, comprising the steps of:(c) hydrogen releasing process: contacting at least one substituted/unsubstituted piperidine with at least one transition metal catalyst under conditions that allow dehydrogenation of the substituted/unsubstituted piperidine, thereby forming three hydrogen molecules and at least one substituted pyridine;(d) hydrogen loading process: contacting said at least one substituted/unsubstituted pyridine with said at least one transition metal catalyst and three hydrogen molecules under conditions for hydrogenation of said substituted/unsubstituted pyridine, thereby forming at least one substituted piperidine.

In some embodiments, the N-heterocycle in the system, process and/or method according to this invention, is an H2-rich compound. In other embodiments, the H2-rich compound is a substituted/unsubstituted piperidine. In other embodiments, the H2-rich compound is a substituted piperidine. In other embodiments, the N-heterocycle is an H2-lean compound. In other embodiments, the H2-lean compound is a substituted/unsubstituted pyridine. In other embodiments, the H2-lean compound is a substituted pyridine. In other embodiments, the N-heterocycle is a combination of an H2-rich compound and an H2-lean compound. In other embodiments, the N-heterocycle is a combination of a substituted/unsubstituted pyridine and substituted/unsubstituted piperidine.

In some embodiments, the N-heterocycle H2-rich compound is selected from: piperidine, 2-methylpiperidine, 3-methylpiperidine, 4-methylpiperidine, 2,6-dimethylpiperidine, 2,4-dimethylpiperidine, 2,3-dimethylpiperidine, 2,5-dimethylpiperidine, 3,4-dimethylpiperidine, 3,5-dimethylpiperidine, 2,5-dimethylpiperidine, 2,4,6-trimethylpiperidine or any combination thereof; each represents a separate embodiment according to this invention. In some embodiments, the N-heterocycle H2-rich compound is piperidine. In some embodiments, the N-heterocycle H2-rich compound is 2-methylpiperidine. In some embodiments, the N-heterocycle H2-rich compound is 2,6-dimethylpiperidine. In some embodiments, the N-heterocycle H2-rich compound is piperidine. In some embodiments, the N-heterocycle H2-rich compound is 3-methylpiperidine. In some embodiments, the N-heterocycle H2-rich compound is 4-methylpiperidine. In some embodiments, the N-heterocycle H2-rich compound is 2,4-dimethylpiperidine. In some embodiments, the N-heterocycle H2-rich compound is 2,3-dimethylpiperidine. In some embodiments, the N-heterocycle H2-rich compound is 2,5-dimethylpiperidine. In some embodiments, the N-heterocycle H2-rich compound is 3,4-dimethylpiperidine. In some embodiments, the N-heterocycle H2-rich compound is 3,5-dimethylpiperidine. In some embodiments, the N-heterocycle H2-rich compound is 2,5-dimethylpiperidine. In some embodiments, the N-heterocycle H2-rich compound is 2,4,6-trimethylpiperidine.

In some embodiments, the N-heterocycle H2-lean compound is selected from: pyridine, 2-methylpyridine (2-picoline), 3-methylpyridine, 4-methylpyridine, 2,6-dimethylpyridine (2,6-lutidine), 2,4-dimethylpyridine, 2,3-dimethylpyridine, 2,5-dimethylpyridine, 3,4-dimethylpyridine, 3,5-dimethylpypridine, 2,5-dimethylpyridine, 2,4,6-trimethylpyridine or any combination thereof; each represents a separate embodiment according to this invention. In some embodiments, the N-heterocycle H2-lean compound is pyridine. In some embodiments, the N-heterocycle H2-lean compound is 2-methylpyridine (2-picoline). In some embodiments, the N-heterocycle H2-lean compound is 2,6-dimethylpyridine (2,6-lutidine). In some embodiments, the N-heterocycle H2-lean compound is pyridine. In some embodiments, the N-heterocycle H2-lean compound is 3-methylpyridine. In some embodiments, the N-heterocycle H2-lean compound is 4-methylpyridine. In some embodiments, the N-heterocycle H2-lean compound is 2,4-dimethylpyridine. In some embodiments, the N-heterocycle H2-lean compound is 2,3-dimethylpyridine. In some embodiments, the N-heterocycle H2-lean compound is 2,5-dimethylpyridine. In some embodiments, the N-heterocycle H2-lean compound is 3,4-dimethylpyridine. In some embodiments, the N-heterocycle H2-lean compound is 3,5-dimethylpypridine. In some embodiments, the N-heterocycle H2-lean compound is 2,5-dimethylpyridine, 2,4,6-trimethylpyridine.

In some embodiments, the N-heterocycle has a wide liquid range.

According to this invention, “liquid range” of a substance refers to a possible range of temperature where liquid is allowed to exist. The value of its liquid range is obtained by subtracting melting point from its boiling point. If the boiling point is higher than its melting point, the liquid range is said to be positive; if the boiling point is lower than its melting point, the liquid range is negative, meaning that liquid is not possible to form unless pressure applied on the substance is raised. In some embodiments according to this invention, “wide liquid range” refers to a temperature range of at least 80 degrees between the melting point and the boiling point of a substance. In other embodiments, at least 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 220, 250, 300 degrees between the melting point and the boiling point of a substance; each represents a separate embodiment according to this invention.

In some embodiments, both the H2-lean N-heterocycle and the H2-rich N-heterocycle are liquids at room temperature. In some embodiments, both the substituted/unsubstituted pyridine and the substituted/unsubstituted piperidine are liquids at room temperature.

In some embodiments, the system, process or method according to this invention is functioning under a temperature range of between about 130° C. and about 180° C., between about 50° C. and about 180° C., between about 100° C. and about 180° C., between about 100° C. and about 250° C., between about 140° C. and about 180° C., between about 100° C. and about 200° C., between about 130° C. and about 220° C., between about 150° C. and about 170° C., between about 130° C. and about 200° C.; each is a separate embodiment according to this invention. In some embodiments, the system, process or method according to this invention is functioning under a temperature of 170° C., 150° C., 119° C., 117° C., or 130° C.; each is a separate embodiment according to this invention.

In some embodiments, the hydrogenation process that is carried out in the system, process or method according to this invention, takes place under low pressure of hydrogen. In some embodiments, the hydrogenation process that is carried out in the system, process or method according to this invention, takes place under a hydrogen pressure of between about 1 bar and about 80 bar, between about 15 bar and about 60 bar, between about 1.5 bar and about 59 bar, between about 1 bar and about 59 bar, between about 4 bar and about 50 bar, between about 1.3 bar and about 50 bar, between about 1.5 bar and about 20 bar, between about 1.5 bar and about 10 bar, between about 1.5 bar and about 8 bar, between about 1.2 bar and about 6 bar, between about 1 bar and about 8 bar, between about 1.6 bar and about 5 bar, between about 2.6 bar and about 5 bar, between about 1.2 and about 8 bar, between about 30 bar and about 50 bar, between about 3.5 bar and about 5 bar, between about 2.5 bar and about 5 bar, between about 1.5 bar and about 5 bar, between about 1.5 and about 7 bar; each represent a separate embodiment according to this invention. In some embodiments, the hydrogenation process that is carried out in the system, process or method according to this invention, takes place under a hydrogen pressure of 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, or 8.0 bar; each represent a separate embodiment according to this invention.

In some embodiments the transition metal catalyst is the same in the hydrogen releasing and hydrogen loading reactions in a system, process or method of the invention. In some embodiments, the same catalyst is used for both hydrogen loading (hydrogenation) and hydrogen discharging (dehydrogenation) processes.

In other embodiments, the same catalyst is reused (as is) in a reversible process for the storage and release of hydrogen (H2). In other embodiments, the same catalyst is recycled and used in the storage and/or release process of hydrogen (H2).

In some embodiments the transition metal is selected from Mn, Fe, Co, Ni, Ru, Rh, Pd, Pt, Cu, Ag. In some embodiments the transition metal is selected from Ru, Pd, and Pt. In some embodiments the transition metal catalyst is heterogeneous. In some embodiments the transition metal catalyst is homogeneous. In some embodiments, the transition metal catalyst is palladium on activated carbon (Pd/C). In some embodiments, the transition metal catalyst is Pd(OAc)2. In some embodiments, the transition metal catalyst is Ru/Al2O3. In some embodiments, the catalyst is commercially available. Examples of commercially available catalysts include but not limited to: 5 wt % Pt/C, 10 wt % Pt/C, 1 wt % Pt/C, 3 wt % Pt/C, 30 wt % Pt/C, 0.5 wt % Pt/Al2O3, 1 wt % Pt/Al2O3, 4 wt % Pd/MCM-48, 5 wt % Pd/SiO2, 0.5 wt % Pd/Al2O3, 1 wt % Pd/Al2O3, 5 wt % Pd/Al2O3, 10 wt % Pd/Al2O3, 0.6 wt % Pd/C, 1 wt % Pd/C, 3 wt % Pd/C, 5 wt % Pd/C, 10 wt % Pd/C, 20 wt % Pd/C, 30 wt % Pd/C, 5 wt % Pd/BaSO4, 10 wt % Pd/BaSO4, 5 wt % Pd/BaCO3, 5 wt % Pd/CaCO3, 10 wt % Pd/CaCO3, 5 wt % Pd/SrCO3, 5 wt % Pd/TiO2, 5 wt % Ru/C, 5 wt % Ru/Al2O3etc. In some embodiments, the catalyst is generated in-situ from a catalyst precursor. Examples of catalyst precursors include but are not limited to: Pd(OAc)2, PdCl2, Pd(TFA)2, Pd(acac)2and Pd2(dba)3. In some embodiments, the catalyst is supported on insoluble matrices (or solid support, as described hereinbelow), such as inorganic oxides (for example alumina or silica optionally attached via tether) or organic insoluble polymers (such as for example cross-linked polystyrene). More examples of insoluble matrices include but not limited to: activated carbon, dried acidic activated carbon, SiO2, BaSO4, BN, γ-Al2O3or CeO2. In some embodiments, the active catalyst in the system, method and/or process of the invention is palladium on carbon, or Pd/C, wherein the palladium metal is supported on activated carbon in order to maximize its surface area and activity.

In some embodiments, the catalyst, or the catalyst precursor, each represents a separate embodiment according to this invention, is present in an amount of between 0.05% and 5% w/w based on the H2-rich compound, between 0.01% and 1% w/w, between 0.1% and 1% w/w, between 0.15% and 0.5% w/w, between 0.1% and 0.7% w/w, between 0.1% and 0.5% w/w, between 0.05% and 0.5% w/w between 0.15% and 0.3% w/w, 0.15% w/w, 0.2% w/w, 0.3% w/w; each represents a separate embodiment according to this invention.

In some embodiments, the catalyst, or the catalyst precursor, each represents a separate embodiment according to this invention, is present in an amount of between 0.05% and 5% w/w based on the H2-lean compound, between 0.01% and 1% w/w, between 0.1% and 1% w/w, between 0.15% and 0.5% w/w, between 0.1% and 0.7% w/w, between 0.1% and 0.5% w/w, between 0.05% and 0.5% w/w between 0.15% and 0.3% w/w, 0.15% w/w, 0.2% w/w, 0.3% w/w; each represents a separate embodiment according to this invention.

In some embodiments, the process/method of any of the embodiments of the present invention as described herein is conducted under neat conditions. In some embodiments, said system, process and/or method of the invention does not comprise any solvent. In some embodiments, said system and method of the invention further comprises at least one organic solvent. In some embodiments said at least one organic solvent is selected from benzene, toluene, o-, m- or p-xylene, mesitylene (1,3,5-trimethyl benzene), dioxane, THF, DME, DMSO, diglyme, DMF (dimethylformamide), valeronitrile, DMAC (dimethylacetamide), NMM (N-methylmorpholine), pyridine, n-BuCN, anisole, cyclohexane and combination thereof. In some embodiments, said system and method of the invention further comprises one organic solvent. In other embodiments said system and method of the invention further comprises a mixture of at least two organic solvents.

In another embodiment, the catalyst is absorbed on a solid support and the storing/loading and releasing/discharging hydrogen is done without a solvent.

In some embodiments, said system and method of the invention further comprise at least one acid. Examples of acids that may be included in the system, process and/or method of the invention include but not limited to: acetic acid (HOAc), benzoic acid (BA), carboxypolystyrene (CPS), polyacrylic acid (PAA), 4-methylbenzenesulfonic acid (p-TsOH) and mixtures thereof. In some embodiments, the acid is present in catalytic amount. In some embodiments, the acid is present in an amount of 0.01%, 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 5%, 10%, 15%, 25%, 35% w/w based on the amount of the N-heterocycle (e.g., substituted/unsubstituted pyridine, or substituted/unsubstituted piperidine); each represent a separate embodiment according to this invention. In some embodiments, the acid is a weak acid.

In some embodiments, the discharging of hydrogen is achieved by reacting a substituted/unsubstituted piperidine with a transition metal catalyst; thereby forming three hydrogen molecules and a substituted pyridine.

In some embodiments, the loading of hydrogen is achieved by reacting a substituted/unsubstituted pyridine with three hydrogen molecules and a transition metal catalyst; thereby forming a substituted/unsubstituted piperidine.

In some embodiments, the reversible hydrogen loading and discharging system, process and/or method of the invention has hydrogen storage capacity of at least 4%, at least 5%, at least 5.2%, at least 5.3%, at least 5.5%, at least 6%, at least 6.1%; each represents a separate embodiment according to this invention. In other embodiments, the reversible hydrogen loading and discharging system, process and/or method of the invention has hydrogen storage capacity of between about 4% to about 6.5%, between about 4% to about 6.3%, between about 5% to about 6.5%, between about 5.3% to about 6.1%, between about 4% to about 6.1%; each represents a separate embodiment according to this invention.

The reaction pathway for utilizing substituted/unsubstituted piperidines of the invention as a LOHC is outlined inFIG. 1D, which describes a LOHC system based on Pd catalyzed dehydrogenation and hydrogenation of N-heterocycles (substituted/unsubstituted piperidines/pyridines). Two couples of H2-lean/H2-rich compounds were found to be promising LOHCs for future use. One is 2-picoline/2-methylpiperidine system, which has 6.1 wt % theoretical hydrogen storage capacity, which satisfies the US DOE onboard hydrogen storage target 2020 very well. The other one is 2,6-lutidine/2,6-dimethylpiperidine system, which has 5.3 wt % theoretical hydrogen storage capacity, which is close to the US DOE target. All the compounds have wide liquid ranges and lower than 0° C. melting points. Both dehydrogenation and hydrogenation were achieved in excellent yields using the same catalyst under mild conditions. Particularly, for 2,6-lutidine/2,6-dimethylpiperidine system, the complete dehydrogenation of 2,6-dimetylpiperidine could be performed at 170° C., with fast and reliable H2release rate. Mechanistic studies revealed the special role of acids and acidic groups on the surface of activated carbon. Noteworthy, the reverse hydrogenation only required low pressure of H2(2-7 bar for 2-picoline, and 1.6-5 bar for 2,6-lutidine), which is the lowest known. Catalyst recycling, and interconversion experiments demonstrated that catalyst, 2,6-dimethylpiperidine and 2,6-lutidine have good stability.

System

A reversible hydrogen loading and discharging system of this invention refers to any type of arrangement capable to holding the reactants of the reactions performed in said system, wherein the discharging and loading of hydrogen molecules is performed using N-heterocylces and at least one transition metal catalyst as described hereinabove, preferably the N-heterocycles are substituted/unsubstituted piperidines (for hydrogen discharging), and substituted/unsubstituted pyridines (for hydrogen loading).

Upon reaction of substituted/unsubstituted piperidine with said at least one transition metal catalyst hydrogen molecules are released, to form the corresponding substituted/unsubstituted pyridine and hydrogen molecules.

Upon loading of hydrogen molecule, the substituted/unsubstituted pyridine is reacted with hydrogen molecules to form substituted/unsubstituted piperidine.

In one embodiment, this invention is directed to a LOHC system for the storage/loading and release/discharging of hydrogen (H2) on demand, the system comprises N-heterocycle; and at least one transition metal catalyst.

In some embodiments, this invention relates to a liquid organic hydrogen carrier (LOHC) system for the storage of hydrogen (H2), the system comprises (i) a substituted/unsubstituted pyridine and (ii) a transition metal catalyst, wherein said substituted/unsubstituted pyridine is capable of reacting with hydrogen (H2) in the presence of said catalyst, under conditions sufficient to generate the corresponding substituted/unsubstituted piperidine, as a hydrogen storage system.

In some embodiments, this invention relates to a liquid organic hydrogen carrier (LOHC) system for releasing hydrogen (H2), the system comprises (i) substituted/unsubstituted piperidine; and (ii) a transition metal catalyst, wherein said substituted/unsubstituted piperidine, is capable of being dehydrogenated in the presence of said catalyst, under conditions sufficient to generate substituted/unsubstituted pyridine, and molecular hydrogen.

In some embodiments, this invention relates to a liquid organic hydrogen carrier (LOHC) system for the storage and release of hydrogen (H2) upon demand, the system comprises (i) a substituted/unsubstituted pyridine; (ii) a substituted/unsubstituted piperidine; and (iii) a first catalyst and a second catalyst as described hereinabove, wherein the first catalyst is capable of reacting with the substituted/unsubstituted pyridine under conditions sufficient to store hydrogen, and wherein the second catalyst is capable of reacting with a substituted/unsubstituted piperidine, under conditions sufficient to release hydrogen, upon demand as desired, and wherein the first and second catalyst may be the same or different.

In one embodiment, the discharging/release of hydrogen is achieved by reacting substituted/unsubstituted with said at least one transition metal catalyst; thereby forming three molecules of hydrogen and a substituted/unsubstituted pyridine.

In one embodiment, the loading/storage of hydrogen is achieved by reacting said substituted/unsubstituted pyridine with three molecules of hydrogen in the presence of said transition metal catalyst; thereby forming substituted/unsubstituted piperidine.

In one embodiment, this invention is directed to a LOHC system. In another embodiment, the LOHC system is used for a hydrogen fuel cell. In another embodiment, the LOHC system is used for fueling internal combustion engine. The LOHC of this invention release hydrogen on-board in vehicles powered by a hydrogen fuel cell, for internal combustion engine, or the LOHC systems store and release hydrogen at service stations, garages, central fleet refueling stations, and in residential individuals' homes, or other points of use. The release of the hydrogen is an on-site generation; and can be produced in individuals' homes or other points of use. Following the release of hydrogen, dehydrogenated compounds are taken to a specialized hydrogenation facility and the LOHC is recovered upon treatment with pressurized hydrogen and a catalyst.

In one embodiment, the LOHC system of this invention is used for dispensing and monitoring hydrogen-based fuel in a vehicle. The system is configured to store, release and dispense the hydrogen in the vehicle. The system also includes a fuel delivery system on the vehicle configured to deliver the hydrogen to the engine, and a control system configured to control the producing system and to monitor the use of the hydrogen by the vehicle.

This invention provides a method for releasing hydrogen gas from the LOHC of this invention and using the hydrogen gas for vehicles powered by a hydrogen fuel cell and/or for internal combustion engine.

In one embodiment, the LOHC can be pumped or poured for distribution to holding tanks and storage vessels. The liquid is easily transported using conventional methods for liquid transport and distribution (pipelines, railcars, tanker trucks). The hydrogen is generated on-site in the vehicle or by a dehydrogenation reactor system that delivers hydrogen and recovers the dehydrogenated substrate in a hydrogenation reactor site.

In one embodiment, the LOHC system of this invention for use in a vehicle comprises a reaction chamber configured to collect the LOHC and the catalyst of the invention; a heating element configured to heat the LOHC and the catalyst to release hydrogen; a buffer tank in flow communication with the reaction chamber configured to collect and temporarily store the hydrogen; a compressor system in flow communication with the buffer tank configured to pressurize the hydrogen to a selected pressure; a storage system in flow communication with the compressor system configured to store a selected quantity of the hydrogen the selected pressure; a dispensing system in flow communication with the storage system configured to dispense the hydrogen to the hydrogen fuel cell or to the internal combustion engine. A second dispensing system in flow communication with the reaction chamber configured to dispense spent of the reaction to a spent tank, wherein the dehydrogenated substrate is recovered in the presence of pressurized hydrogen. The recovery of the dehydrogenated substrate is done on-board or off-board.

Process

This invention further relates to a process for the storage and release of hydrogen (H2) upon demand, comprising the steps of:(a) when hydrogen storage is desired, reacting a substituted/unsubstituted pyridine, with hydrogen (H2) in the presence of a first catalyst, under conditions sufficient to generate the corresponding substituted/unsubstituted piperidine; and(b) when hydrogen release is desired, reacting a substituted/unsubstituted piperidine, with a second catalyst, under conditions sufficient to generate the corresponding substituted/unsubstituted pyridine, and hydrogen (H2), wherein the first catalyst and the second catalyst may be the same or different.

In some embodiments, the first catalyst and the second catalyst are the same. In some embodiments, the catalyst is a Pd/C. In some embodiments, the catalyst is Pd(OAc)2. In some embodiments, the catalyst is Ru/Al2O3. In some embodiments, the process does not comprise any solvent. In some embodiments, the hydrogen storage is carried out in mild temperature (e.g., between 50-180° C.). In some embodiments, the hydrogen storage is carried out under mild hydrogen pressure (e.g., between 1.5 and 8 bar). In some embodiments, the hydrogen release is carried out mild pressure (e.g., atmospheric pressure). In some embodiments, the hydrogen release is carried out in mild temperatures (e.g. between 100-180° C.).

In some embodiments, the catalyst is attached to a solid support, or in other embodiments, the catalyst is embedded on a solid support, or in other embodiments located on the surface of a solid support.

Chemical Definitions

As used herein, the term alkyl, used alone or as part of another group, refers, in one embodiment, to a “C1to C8alkyl”, “C1to C3alkyl” or “C1to C10alkyl” denotes linear and branched, groups, Non-limiting examples are alkyl groups containing from 1 to 3 carbon atoms (C1to C3alkyls), or alkyl groups containing from 1 to 4 carbon atoms (C1to C4alkyls). Examples of saturated alkyl groups include, but are not limited to, methyl, ethyl, n-propyl, isopropyl, n-butyl, iso-butyl, sec-butyl, tert-butyl, amyl, tert-amyl, and hexyl.

The following examples are presented in order to more fully illustrate the preferred embodiments of the invention. They should in no way, however, be construed as limiting the broad scope of the invention.

EXAMPLES

General Information

All experiments with metal complexes were carried out under an atmosphere of purified nitrogen in a Vacuum Atmospheres glove box equipped with a MO 40-2 inert gas purifier or using standard Schlenk techniques. All solvents were reagent grade or better. All non-deuterated solvents were purified according to standard procedures under argon atmosphere. Deuterated solvents were used as received. All solvents were degassed with N2and kept in the glove box. Most of the chemicals used in the catalytic reactions were purified according to standard procedures.

1H NMR spectra were recorded at 300 MHz, using a Bruker AMX-300 NMR spectrometer. Measurements were done at ambient temperature, as noted for each experiment.1H NMR chemical shifts are referenced to the residual hydrogen signals of the deuterated solvent. GSMS analysis was performed on Agilent 7820A/5975C GCMS system with MS detector, and helium as carrier gas. EDS was recorded at EDS Bruker XFlash/60 mm, using Zeiss Ultra 55 Scanning Electron Microscope. Transmission electron microscopes (TEM) were recorded at JEM-2100 Electron Microscope.

Dehydrogenation of 2-methylpiperidine to 2-picoline

2-Methylpiperidine was chosen as the model H2-rich compound, since it has 6.1 wt % potential hydrogen storage capacity, and the electron-donating methyl group at 2-position could provide some benefits regarding both electronic and steric effects. Several kinds of heterogeneous catalysts were screened in p-xylene as a solvent (more details hereinbelow). Palladium on activated carbon (Pd/C, 4 wt %) was found to be the most efficient catalyst for the acceptorless dehydrogenation of 2-methylpiperidine to 2-picoline. Then, Pd/C was studied as a catalyst for solvent-free dehydrogenation of 2-methylpiperidine. Applying commercial Pd/C (5 wt %) resulted in 42-48% yield of 2-picoline and 9-10% side products, depending on the supplier (Table 2, entries 1 and 2). Importantly, addition of a catalytic amount of acetic acid (entry 3) improved the reaction in both yield (60%) and selectivity (only ˜3% of byproducts were formed).

Further, an in situ catalyst generation strategy was planned. Pd(OAc)2was chosen as the palladium precursor, dried acidic activated carbon (Darco@KB, surface area: 1500 n/g, pHPZC=4.25: pH value at the point of zero charge) as the support for a preliminary attempt, in which Pd/C and 2 equivalent of acetic acid (with respect to Pd) could potentially be generated. This in situ generated palladium catalyst is more active than the pre-prepared commercial ones, resulting in 72% yield of 2-picoline (entry 4). Other palladium sources including PdCl2, Pd(TFA)2, Pd2(dba)3, and Pd(acac)2had much lower catalytic activities than Pd(OAc)2(entries 5-8). Next, Pd(OAc)2was chosen as the best palladium precursor to study the effect of supports (Table 2, entries 9-12). Replacing activated carbon by SiO2, BN, γ-Al2O3or CeO2, very low yields of 2-picoline were obtained. In the absence of support, only 3% of 2-picoline was detected, and a palladium mirror was observed at the bottom of Schlenk tube, showing that a Pd0species was generated in the reaction system. Thus, the combination of Pd(OAc)2and activated carbon is the most efficient catalyst for 2-methylpiperidine dehydrogenation.

Next, the reaction was connected to a gas collection system (FIG. 3), for recording the time-dependent H2release curves. As shown inFIGS. 2A-2B, under the catalysis of Pd(OAc)2/activated carbon (Pd(OAc)2=0.03 mmol, AC=40 mg), at 170° C. (bath temperature, internal temperature is 119° C.), 27.1 mmol (90% yield) of H2was attained after 51 hours (entry 1, triangle ▴). The dehydrogenation was also achieved at a lower bath temperature; at 150° C. (internal temperature is also 119° C.), after 117 hours, H2in 84% yield was collected (entry 2, square ▪). Moreover, with the same amount of catalyst, doubling the scale of 2-methylpiperidine, the dehydrogenation (entry 3, dot ●) was even faster, and produced 54.5 mmol (91% yield) of H2after 94 hours. Gratifyingly, the reverse hydrogenation could be accomplished by directly pressurizing the same reaction mixture with H2. Under 2-7 bar of H2, 2-picoline in the mixture was fully converted to 2-methylpiperidine in 90% yield (FIG. 4). These results indicate that dehydrogenation and hydrogenation were catalyzed by the same single metal heterogeneous catalyst, and the total amount of byproducts was less than 10%.

In order to understand the process of side reactions, the mixture of side products was analyzed by GC-MS. These byproducts have similar retention times (some of them partially overlap, see hereinbelow) and similar molecular weights, likely py-py and py-pi, which were probably formed via the palladium catalyzed addition of enamine to imine (FIG. 5A). Therefore, restraining the addition of enamine to imine might be an effective method to improve the selectivity. The strategy was to increase the steric hindrance of both nucleophilic and electrophilic intermediates (FIG. 5B).

Dehydrogenation of 2-methylpiperidine to 2-picoline in a Solvent:

In a glovebox, catalyst, t-BuOK (0.2 mmol), 2-methylpiperidine (1 mmol) and solvent (1 mL) were added to a Schlenk tube. The Schlenk tube was equipped with a condenser and the solution was refluxed with stirring in an open system under argon flow on the top of the condenser for the specified time. After cooling to room temperature, the conversion and yield were determined by GC using n-heptane as an internal standard.

In a glovebox, 2-methylpiperidine (10 mmol), 43 mg of 5 wt % Pd/C or (4.5 mg of Pd(OAc)2and 50 mg of support) were added to a Schlenk tube. The Schlenk tube was equipped with a condenser and the solution was refluxed with stirring in an open system under argon flow for the specified time. After cooling to room temperature, the conversion and yield were determined by GC using n-heptane as an internal standard (results are given in Table 2 and Table 4).

Solvent-Free Dehydrogenation of 2-methylpiperidine (Connected to a Gas Collection System):

In a glovebox, 2-methylpiperidine and the catalyst were added to a Schlenk tube (see Table 5). The Schlenk tube was equipped with a condenser, which was connected to a gas collection system (FIG. 3). The volume of H2was recorded by the collection system, and time-dependent H2release curves are shown inFIG. 10. Yields and conversions were determined by GC based on the peak area. Yield of H2was calculated on basis of H2volume with respect to full conversion of 2-methylpiperidine to 2-picoline (10 mmol of 2-methylpiperidine can produce 30 mmol of H2, 740 mL at 28° C.).

Optimization of an N-Heterocycle-Based Solvent-Free, Liquid to Liquid LOHC System

The Effect of Substitution on the Dehydrogenation Process

With the idea of increasing steric hindrance, the effect of the substituent group was studied. Piperidine, 3-methylpiperidine, 4-methylpiperidine and 2,6-dimethylpiperidine were investigated (Table 7). Using piperidine or 3-methylpiperidine as the H2-rich compounds resulted in good selectivity, but low yields of the H2-lean products (entries 1 and 2). Using 4-methylpiperidine resulted in 46% yield of 4-picoline and about 7% of byproducts (entry 3).

TABLE 7Dehydrogenation of piperidine and methylpiperidinesEntryH2-rich compoundH2-lean compound (%)Byproducts1apiperidinepyridine (11)trace2b3-methylpiperidine3-picoline (11)trace3c4-methylpiperidine4-picoline (46)74b,c2,6-dimethylpiperidine2,6-lutidine (>99)05c,d2,6-dimethylpiperidine2,6-lutidine (100)0aConditions: piperidines (10 mmol), catalyst (0.2 mol % of [Pd]), activated carbon (Darco @ KB, 50 mg), 170° C. (oil bath temperature), open system under argon flow on the top of condenser, with cold water circulation. Yields and conversions were determined by GC, using n-heptane as an internal standard.bDetermined by on1H NMR, using mesitylene as an internal standard.cDetermined by GC, based on the peak areas.dOpen system, argon atmosphere, the condenser was connected to a gas collection system.

Using 2,6-dimethylpiperidine resulted in more than 99% yield and 100% selectivity (entry 4). Importantly, the quantitative dehydrogenation of 2,6-dimethylpiperidine could also be achieved without argon flow and gave 100% yield of pure H2gas (H2purity>99.99%, confirmed by GC with thermal conductivity detector, no impurity was observed). Thus, it is also promising to establish a LOHC system on basis of 2,6-lutidine/2,6-dimethylpipeidine with the same catalyst system.

A LOHC system based on 2,6-lutidine/2,6-dimethylpipeidine has a maximum 5.3 wt % gravimetric capacity, which is higher than the European Union target, and very close to the US DOE target. In addition, physicochemical properties of 2,6-lutidine and 2,6-dimethylpiperidine meet all the requirements of an ideal LOHC. Thus, an LOHC system based a 2,6-lutidine/2,6-dimethylpieridine based LOHC system is attractive.

The time-dependent H2release curves for 2,6-dimethylpiperidine dehydrogenation was recorded by the gas collection system (FIGS. 6A-6B). Under the catalysis of Pd (Pd(OAc)2=6.7 mg, 0.3 mol %, AC=40 mg), the dehydrogenation of 2,6-dimethylpiperidine worked very well, yielding 97% of H2after 13 hours, and 100% of H2after 23 hours (FIG. 6A). Interestingly, the H2release rate was constant before reaching 95% yield of H2(FIG. 6A, inset) in 12 hours. These results suggest a zero-order reaction in 2,6-dimethylpiperidine, as a result of saturation of the surface of catalyst by 2,6-dimethylpiperidine. Besides, by measuring the H2release rates using different catalyst loadings, a first-order rate dependence in Pd is likely.

Moreover, the reverse hydrogenation of 2,6-lutidine to 2,6-dimethylpiperidine regeneration was achieved in 100% yield by pressurizing the mixture with 1.6-5 bar of H2, at 150° C. for 18 hours (FIG. 6B, right column 1). The resulting mixture could be reused for the second round of dehydrogenation (93% yield,FIG. 6B, left column 2) and hydrogenation (100% yield,FIG. 6B, right column 2), and no decomposition of 2,6-dimethylpiperidine and 2,6-lutidine took place. Additionally, with 0.3 mol % of catalyst, under H2pressure of 30-50 bar, at 150° C., the hydrogenation of 2,6-lutidine yielded 2,6-dimethylpiperidine in 87%, 95% and 98% yields, after 1.5 h, 2 h and 3 h, respectively, which enables fast H2loading (Details see hereinbelow).

Thus, an N-heterocycle-based solvent-free, liquid to liquid LOHC system was established, catalyzed by a single catalyst for both dehydrogenation and hydrogenation under relatively mild conditions.

Solvent-Free Dehydrogenation of 2,6-dimethylpiperidine to 2,6-lutidine (Connected with Gas Collection System):

In a glovebox, 2,6-dimethylpiperidine (10 mmol) and catalyst were added to a Schlenk tube equipped with a condenser, which was connected to a gas collection system (FIG. 3). The solution was refluxed with stirring in an open system for the specified time. Yield of H2was calculated on basis of the formed H2volume with respect to full conversion of 2,6-dimethylpiperidine to 2,6-lutidine (30 mmol of H2, 721 mL at 20° C., 740 mL at 28° C.). After cooling to room temperature, the conversions and yields were determined by1H NMR using mesitylene as an internal standard, or by GC based on the peak area.

For the Calculation of Average Turnover Frequency within 90% Yield (ATOF90):

In a glovebox, 2,6-dimethylpiperidine (10 mmol), Pd/CHS ([Pd]=0.03 mmol) and acid (0.06 mmol) were added to a Schlenk tube equipped with a condenser, which was connected to a gas collection system (FIG. 3). The solution was refluxed with stirring in an open system for the specified time. Yield of H2was calculated on basis of the formed H2volume with respect to full conversion of 2,6-dimethylpiperidine to 2,6-lutidine (30 mmol of H2, 721 mL at 20° C., 740 mL at 28° C.). After cooling to room temperature, the conversions and yields were determined by GC. The calculation of ATOF90s were based on the H2release rates (not shown).

General Procedure for Hydrogenation of Pyridines

In a glovebox, a 50 mL stainless steel autoclave lined with a Teflon tube (or a 90 mL Fisher Porter tube) containing a stir bar was charged with the catalyst (Pd or Ru) and 2,6-lutidine. After purging with H2(10 atm×2), the autoclave was pressurized with H2(pressure see Table 10). The mixture was stirred at 150° C., H2decreased to a specified pressure, then cooled to room temperature and pressurized with H2again (repeat a few times). Table 11, provides results of 5 repeats of hydrogenation. When the pressure didn't decrease, the autoclave was cooled to room temperature, and H2released carefully. Then centrifugation to separate the catalyst and product, 50 μL of the clear solution was withdrawn and measured by1H NMR in CDCl3and GC. After distillation, pure 2,6-dimethylpipridine was obtained for further use.

Mechanistic Investigation of the Catalytic Process—Control Experiments

A few control experiments were carried out to find out if the catalytic dehydrogenation involves homogeneous or heterogeneous catalysis. When 0.3 mol % of Pd(OAc)2, 40 mg of activated carbon, and 10 mmol of 2,6-dimethylpiperidine were heated at 170° C. (bath temperature, internal temperature is 128° C.) for 5 hours, 46% yield of H2was collected, which match the yield (46.2%) of the formed 2,6-lutidine very well (FIG. 7A). The mixture was filtered using a Teflon syringe filter (0.22 μm pore size PTFE) to give a colorless mixture of 2,6-dimethylpiperidine and 2,6-lutidine. Heating the obtained mixture at 170° C. for another 20 hours, no gas was formed, and the amount of 2,6-lutidine also didn't change (45.9%,FIG. 7B). Thus, a homogeneous catalysis process is unlikely. Analyzing the final mixture by ICP-MS showed only 0.172 ppm of Pd, indicating that virtually no Pd leaching to the reaction solution took place. Thirdly, a reaction without using a support gave only 5% yield of 2,6-lutidine and generated a Pd mirror (FIG. 7C). Taken together these results show that a homogeneous process can be ruled out.

In a glovebox, 2,6-dimethylpiperidine (1.13 g, 10 mmol), Pd(OAc)2(6.7 mg, 0.03 mmol) and activated carbon (40 mg) were added to a Schlenk tube. The Schlenk tube was equipped with a condenser, which was connected to a gas collection system. The solution was refluxed with stirring in an open system for 5 hours. H2was collected (331 mL, 46% yield). After cooling to room temperature, the reaction mixture was filtered using a Teflon syringe filter (0.22 μm pore size PTFE) to give a colorless mixture of 2,6-dimethylpiperidine (53.8%) and 2,6-lutidine (46.2%). Then the mixture of 2,6-dimethylpiperidine and 2,6-lutidine was transferred to a new Schlenk tube, and refluxed for another 20 hours, but no gas was collected. After cooling to room temperature, samples were taken for GC and ICP-MS analysis.

Mechanistic Investigation—the Effect of the Acid on the Dehydrogenation Process

Pd/CHS(Pd/CHS=palladium on activated carbon for Hydrogen Storage) was prepared from Pd(OAc)2and acidic activated carbon (Darco@KB) to study the effect of acid. The prepared Pd/CHSwas characterized by Fourier-transform infrared spectroscopy (FTIR), scanning electron microscope (SEM) and transmission electron microscope (TEM). It was found that the palladium nanoparticles bound to the carboxyl groups of the activated carbon surface were distributed uniformly, the average diameter of Pd nanoparticles was about 1.93±0.44 nm.

Then, the catalytic activity of Pd/CHS for 2,6-dimethylpiperidine dehydrogenation was tested; the time-dependent H2release curves and average turnover frequency within 90% yield (ATOF90=85 mol H2per mol Pd per hour) are shown inFIG. 8. The ATOF90of using Pd/CHS as a catalyst is close to that of the Pd(OAc)2/activated carbon system (ATOF90=91).

The effect of acid was investigated by adding 2 equivalents (with respect to Pd) of acetic acid (pKa=4.76), benzoic acid (pKa=4.20), 4-methylbenzenesulfonic acid (pKa=1.99), carboxypolystyrene (monomer pKa=4.35) or polyacrylic acid (pKa=4.75) into the reaction mixture (FIG. 8, entries 2-6). Acetic acid, benzoic acid, and the polymer carboxypolystyrene showed positive effects on the dehydrogenation process, resulting in ATOF90s of 103, 128 and 107, respectively. Using polyacrylic acid as an additive, the ATOF90slightly decreased to 80, and use of the strong acid 4-methylbenzenesulfonic acid had a negative effect, resulting in the ATOF90decrease to 45. However, treatment of Pd/CHS with t-BuOK before use, upon which the interaction of the carboxyl groups and Pd nanoparticles might be broken by the base, the catalyst became completely inactive (FIG. 8, entry 8). These results indicate that carboxylic acids and the carboxyl group on the activated carbon surface accelerate the palladium catalyzed dehydrogenation of 2,6-dimethylpiperidine. Furthermore, Pd/CHS and carboxypolystyrene could be easily recovered by centrifugation without loss of catalytic activity (ATOF90=101,FIG. 8, entry 9).

Based on the control experiments, preliminary mechanistic studies, and the literature, a plausible dehydrogenation mechanism (FIG. 9A) and pathway (FIG. 9B) of 2-picoline and 2,6-lutidine formation is proposed. Coordination of the N-heterocycle to Pd(II) nanoparticles results in an increase in the acidity of N—H group, which could be deprotonated by the counter carboxylate anions of Pdn2+to generate an amido-palladium species and carboxylic acid (FIG. 9A). Then β-hydride elimination occurs to generate an imine and a palladium hydride species. Finally, with the assistance of carboxylic acid, H2is released and the active catalyst Pdn2+is regenerated. Another role of the carboxylic acid in the reaction system is acceleration of the tautomerization of imines to key intermediate enamines (FIG. 9B). The cascade steps of dehydrogenation and tautomerization produce the final H2-lean product 2-picoline or 2,6-lutidine and H2.

Procedure for treating Pd/CHS with t-BuOK: In a glovebox, Pd/CHS (43 mg, [Pd]=0.03 mmol), t-BuOK (16.8 mg, 0.15 mmol) and THF (1 mL) were added to a Schlenk tube. After stirring at room temperature for 12 hours, the catalyst was separated by centrifugation, washed with THF (1 mL×2), and dried under vacuum.

2-Methylpiperidine as a LOHC System

Reversible Interconversion of 2-methylpiperidine and 2-picoline

In a glovebox, 2-methylpiperidine (20 mmol), Pd(OAc)2(6.7 mg, 0.03 mmol) and activated carbon (40 mg) were added to a Schlenk tube. The Schlenk tube was equipped with a condenser, which was connected to a gas collection system (FIG. 3). The solution was refluxed with stirring in an open system. When no more gas was generated, the reaction was stopped. The yield of H2(91%) was calculated based on H2volume (1344 mL) with respect to full conversion of 2-methylpiperidine to 2-picoline (Table 11). After cooling to room temperature, the Schlenk tube was brought into a glovebox, and the mixture was transferred into a 90 mL Fisher-Porter tube for hydrogenation.

The Fisher-Porter tube was taken out of the glovebox, pressurized with 7 bar of H2, and heated at 150° C. After 6 hours, the H2pressure decreased to 2 bar and the reaction mixture was cooled to room temperature, then pressurized with H2(7 bar) for the second time, and heated at 150° C. for another 6 hours. The Fisher-Porter tube was pressurized with H2(7 bar) for the third time and heated at 150° C. for 12 hours. After the fourth pressurizing (7 bar), the Fisher-Porter tube was heated at 150° C. for the last 12 hours. Then, the Fisher-Porter tube was cooled to room temperature, and hydrogen was released carefully, 950 mg of n-heptane was added as an internal standard, a sample of 100 μL was taken (diluted with Et2O) for GC analysis (Table 12).

TABLE 12Interconversion of 2-methylpiperidine and 2-picolineConversion (%)2-methylpipridine (%)2-picoline (%)H2mL (%)Dehydrogenation98b—81b1344 (91)cHydrogenation100b90a(94b)——aGC yield, using n-heptane as an internal standard.bDetermined by GC, based on the peak area.cYield of H2was calculated on basis of H2volume with respect to full conversion of 2-methylpiperidine to 2-picoline (60 mmol of H2, 1480 mL at 28° C.).

2,6-dimethylpiperidine as a LOHC System

Reversible Interconversion of 2,6-dimethylpiperidine and 2,6-lutidine and Catalyst Reuse

2,6-Dimethylpiperidine (50 mmol), Pd(OAc)2(33.6 mg, 0.15 mmol) and activated carbon (200 mg) were added to a Schlenk tube. The Schlenk tube was equipped with a condenser, which was connected to a gas collection system (FIG. 3). The solution was refluxed with stirring in an open system for 23 hours. After cooling to room temperature, the Schlenk tube was brought into a glovebox, and the mixture was transferred into a 90 mL Fisher-Porter tube for hydrogenation.

The Fisher-Porter tube was taken out of the glovebox, pressurized with 7 bar of H-2 (the first time) and heated at 150° C. After 6 hours, the H2pressure decreased to 1.6 bar and the Fisher-Porter tube was cooled to room temperature and pressurized with H2(7 bar) for the second time. After 8 times pressurization and heating, the Fisher-Porter tube was cooled to room temperature, hydrogen was released carefully, then the tube with the reaction mixture was taken into the glovebox, and the mixture was transferred to a Schlenk tube for dehydrogenation.

After three times dehydrogenation and two times hydrogenation, the solid catalyst was separated via centrifugation (4000 rpm for 40 mins), and was washed with n-pentane (4 mL×4), then dried under vacuum for 8 hours. The recovered catalyst was used for the fourth and fifth dehydrogenation.

FIG. 12demonstrates 100% conversion of the catalyst in the following reversible process steps dehydrogenation→hydrogenation→dehydrogenation-→hydrogenation→dehydrogenation, wherein the catalyst was not recycled and was used as is. Following the last step of the dehydrogenation, the catalyst was recycled and was used again for additional two dehydrogenation steps (the catalyst was recycled between each of the last dehydrogenation steps). In all processes the catalyst demonstrated 100% conversion.

Preparation of Catalyst Pd/CHS

In a glovebox, Pd(OAc)2(100.8 mg), activated carbon (600 mg, Darco@KB, surface area: 1500 m2/g, pHPZC=4.25), 2,6-dimethylpiperidine (170 mg), and i-PrOH (15 mL) were added to a 90 mL Fisher-Porter tube. The Fisher-Porter tube was pressurized with 5 bar of H2, and the mixture was stirred at 120° C. for 5 hours, then stirred at room temperature for another 5 days. The solvent was removed under vacuum and the residue was treated with H2flow (40 mL/min) at 200° C. for 2 hours. The obtained catalyst Pd/CHS must be kept under inert atmosphere.