High-Purity Hydrogen Generation via Dehydrogenation of Organic

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High Purity Hydrogen Generation via Dehydrogenation of Organic Carriers: a Review on the Catalytic Process Elia Gianotti, Melanie Taillades-Jacquin, Jacques Rozière, and Deborah J. Jones ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04278 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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ACS Catalysis

High

purity

hydrogen

generation

via

dehydrogenation of organic carriers: a review on the catalytic process Elia Gianotti, Mélanie Taillades-Jacquin*, Jacques Rozière, Deborah J. Jones Institut Charles Gerhardt Montpellier, UMR CNRS 5253, Agrégats, Interfaces et Matériaux pour l’Energie, Université de Montpellier, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France

(*) corresponding author: [email protected]

ACS Paragon Plus Environment

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ABBREVIATIONS LOHC(s) – Liquid organic hydrogen carrier(s) PDH – Partial dehydrogenation CHE – Cyclohexane MCH – Methylcyclohexane DEC – Decalin DBT – Dibenzyltoluene 18H-DBT – Perhydrodibenzyltoluene NEID – N-ethylindole 8H-NEID – Octahydro-N-ethylindole NEC – N-ethylcarbazole 12H-NEC – Dodecahydro-N-ethylcarbazole 4MP-4-methylpiperidine PFR – Plug flow reactor CSTR – Continuous stirred tank reactor TOS – Time on stream GHSV – Gas hourly space velocity WHSV – Weight hourly space velocity MTBE – Methyl tertbutyl ether X - Conversion S - Selectivity Y – Yield AC – Activated carbon Deact – Deactivation PEM – Polymer electrolyte membrane EtOH – Ethanol MeOH - Methanol EtAC – Ethyl acetate FA – Formic acid


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ACH - Acetaldehyde

ABSTRACT High purity hydrogen delivery for stationary and mobile applications using fuel cells is a subject of rapidly growing interest. As a consequence, the development of efficient storage technologies and processes for hydrogen supply is of primary importance. Promising hydrogen storage techniques rely on the reversibility and high selectivity of liquid organic hydrogen carriers

(LOHCs),

for

example

methylcyclohexane,

decalin,

dibenzyltoluene

or

dodecahydrocabazole. LOCHs have high gravimetric and volumetric hydrogen density, and involve low risk and capital investment because they are largely compatible with the current transport infrastructure used for fossil fuels. A further advantage comes from the high purity (close to 100%) of the hydrogen generated by dehydrogenation, suitable to directly feed fuel cells without the need for bulky purification modules. Partial dehydrogenation (PDH) of liquid fuels has recently emerged as a transition technology for hydrogen delivery purposes. The principle is to extract from fossil fuels a small fraction of the available hydrogen, which can be used for fuel cell applications, while the dehydrogenated hydrocarbon mixture maintains suitable properties for its use as fuel. With this technology, the large energy demand of dehydrogenation processes can be satisfied by implementing a heat exchanger between the engine and the dehydrogenation reactor, overcoming one of the main constraints associated with the use of organic liquids as hydrogen carriers. This method qualifies itself as a transition technology towards more electrified transportations, in which the main propulsion is still obtained by fuel combustion, although the electrical utilities or auxiliary propulsion are powered by fuel cells.


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This paper provides a review of the effort that has been directed towards the utilization of organic liquids as hydrogen carriers, with particular focus on the design of the catalytic dehydrogenation process and on the recent approach of fuel partial dehydrogenation.

KEYWORDS Liquid Organic Hydrogen Carriers, Fuels, Hydrogen production, Dehydrogenation catalyst, Fuel cell.

1. INTRODUCTION The necessity of satisfying ever-increasing energy consumption, while limiting its environmental impact, requires both a growth in energy generation capacity and an increase of efficiency in the energy conversion processes. The search for alternative power sources with low pollutant emissions and affordable cost has driven the great research effort in the last decade. In this context, fuel cells are a very attractive technology due to their high efficiency and absence of polluting emissions, making them a promising candidate for integration into the energy system. Despite the advantages that fuel cells present, the question of hydrogen storage is often considered to be a bottleneck for the realization of a hydrogen based energy system. Currently, physical storage devices like cryogenic liquid H2 tanks or highly compressed gaseous H2 tanks are the most mature technologies and are close to the U.S. Department of Energy (storage capacity 5.5 wt% and 40 g/L of hydrogen by 2020)1,2 and European Union (storage capacity 5 wt% and 22 g/L of hydrogen by 2020)3 specifications, but they have some drawbacks: compressed H2 requires large volume tanks (depending on the pressure) and the higher the tank pressure, the thicker have to be the tank walls, with consequent rise of costs, weight and safety issues. Cryogenic liquid H2 is suitable for large scale storage, nevertheless for portable and on-


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ACS Catalysis

board applications, the evaporation losses and the cost of liquefaction process become important disadvantages1,4. Consequently, due to the significant technical concerns and volumetric constraints of both cryogenic liquid hydrogen and compressed hydrogen systems, the use of fuel cells in portable electronic devices and vehicles merits further investigation of means to store hydrogen. Cryo-compressed hydrogen tanks are one of the latest technologies in the domain of physical hydrogen storage. The principle is to combine cryogenic tanks and high-pressure tanks to increase the overall capacity and at the same time to avoid some of the inconveniences such as evaporation losses in cryogenic tanks or extended dormancy period with respect to high-pressure tanks5–7. Further possibilities for hydrogen storage are its absorption in a high surface material, the most common being carbon based, like nanotubes, nanorods, graphite and activated carbon, but more recently, studies on mesoporous silica (MCM-41) and metallic organic frameworks (MOFs) have emerged. The main drawback is that their gravimetric hydrogen capacity is lower than 1% at ambient temperature and high pressure (50-100 bar)8,9. Chemical storage of hydrogen is also gaining increasing attention; a variety of metal and complex hydrides are under investigation, but currently the thermodynamic and kinetics of H2 storage/release involves drastic conditions in order to achieve sufficient capacity8,9. For these reasons, the use of liquid organic hydrogen carriers (LOHC) appears to be the most mature technology in this sector at the moment. The first research activities towards hydrogen storage using LOHC systems start in the 1980’s with Paul Scherrer Institute’s researchers10, then with a work in the early 2000s, with a first patent by Pez, Scott, Copper and Cheng from Air Products11 and after continuous intensive research in the last decade, storage systems based on LOHCs have


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become a very promising technology for hydrogen storage and transport. These systems are formed by couples of organic compounds: the dehydrogenated state is an unsaturated compound (usually an aromatic or hetero-aromatic compound), while the hydrogen-rich state is typically a cyclic or heterocyclic compound. Since LOHCs are in most cases liquid at ambient conditions and have physical properties similar to gasoline or diesel, they are relatively safe and easy to handle. The existing infrastructures created for the transportation and storage of fossil fuels can be easily adapted to LOHCs with very low investment. The dehydrogenation and hydrogenation processes required for the H2 cycle using LOHCs are very well known and the catalysts are already at an advanced state of optimization. The existence of very large hydrogenation plants worldwide is a further advantage for the integration of LOHCs in the energy system12–23. Despite the technical, environmental and economic advantages, the concept of hydrogen storage in liquid organic carriers is still not widely commercially developed, mainly because of the limitations related to the amount of energy required to extract the hydrogen from organic liquids and for the reverse process of hydrogenation. Recent experimental and theoretical studies indicate that the incorporation of nitrogen atoms or a boron-nitrogen couple into the cyclic compound facilitates the dehydrogenation process by decreasing the endothermicity of the reaction, thus making these heterocycles potentially viable as a hydrogen storage substrate. The main drawback in the use of heterocycles is their degradation by C–N cleavage, disproportionation, alkyl transfer and other side reactions that occur during the dehydrogenation and hydrogenation12,13,24–26. In parallel to the reversible dehydrogenation/hydrogenation of LOHC, the partial dehydrogenation (PDH) of liquid fuels is emerging as a transition technology for hydrogen delivery purposes. The principle is to extract from fossil fuels a small fraction of hydrogen for use in a fuel cell, while the dehydrogenated hydrocarbon mixture maintains properties suitable for use as fuel. In this


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ACS Catalysis

approach the large energy demand of the dehydrogenation process can be satisfied through heat exchange between the engine and the dehydrogenation reactor, overcoming one of the main constraints in the use of LOHCs27–35. This method qualifies itself as a transition technology towards more electrified transportations, in which the main propulsion is still obtained by fuel combustion, but all the electrical utilities and possibly some auxiliary propulsion is powered by highly efficient fuel cells. Another attractive application for military and civilian purposes, is that of portable fuel cell based power units running on logistic hydrocarbon fuels, such as military aviation fuel (JP-8), kerosene (Jet A-1), diesel or gasoline. A fuel converter based on PDH technology can be used as a practical and portable energy supply for emergency response situations, sea expeditions and military operations in rural settings, where grid power is unreliable or unavailable36. The advantage, besides the possibility of using the dehydrogenated mixture as fuel, include higher gravimetric energy density, better efficiency, lower noise and emissions than a conventional internal combustion generator, while having longer operating time than batteries. Considering systems used only for hydrogen delivery, without implementation of a corresponding re-hydrogenation step, the dehydrogenation of ethanol, methanol and formic acid are also promising technologies. Similarly to fuel partial dehydrogenation, the generated hydrogen may be used in a fuel cell while the byproducts of the reaction are used for different purpose: for ethanol and methanol dehydrogenation the liquid effluents can be used as fuel, while in the case of formic acid the byproduct is CO2, which may be stored and used for different purposes. This paper provides a review of the effort that has been directed toward the utilization of LOHCs and focuses in particular on the catalytic dehydrogenation process and on the new


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approach of partial dehydrogenation of liquid fuels. Generally, the term liquid LOHC is coined only for condensable liquids that do not require the atmosphere to close the storage, however other fuels like ethanol, methanol and formic acid have been added to this review. Various review articles on the subject of hydrogen storage in LOHCs have been published; some of them focus on the most suitable LOHCs to use in the process12,37–39, some investigate the economic and energetic feasibility of LOHCs systems environmental

and

health

impact46

or

15,20,39–45

the

, while still others consider the possible

reversibility

and

stability

during

the

hydrogenation/dehydrogenation cycles17,42. The purpose of this work is to summarize the very latest advancements, with particular focus on the catalytic process involved in the dehydrogenation step. A variety of catalysts, reagents, reactor types and operating conditions are compared in detail in order to give a comprehensive overview of the limitations and advantages related to the hydrogen generation via dehydrogenation reaction.

2. CYCLOALKANE DEHYDROGENATION Initial efforts to develop liquid organic hydrogen carriers were primarily focused on cycloalkanes. The advantages of a high volumetric density, liquid state at ambient temperature, low toxicity, high boiling point and CO free hydrogen production, made the cycloalkanes suitable candidates for hydrogen storage. The major drawback is the high energy required for the dehydrogenation step: the reaction is strongly endothermic and, in order to achieve high hydrogen production, is carried out in gas phase reactors at a temperature range of 150-350 °C. However, it has been shown that for stationary applications, the hydrogenation heat for charging the LOHC can be used for dehydrogenation, and so high temperature hydrogenation of LOHC is


8

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ACS Catalysis

feasible47. Examples of liquid phase dehydrogenation and mixed phase dehydrogenation processes are reported, but the hydrogen evolution rate is considerably lower than for the gasphase catalytic reactors48,49. The compounds most studied so far are cyclohexane (CHE)49–53, methylcyclohexane (MCH)54–62 and decalin (DEC)63–70; they have a theoretical hydrogen capacity of 7.2 wt%, 6.2 wt% and 7.3 wt% respectively (corresponding to 56.0 gH2/L, 47.4 gH2/L and 65.3 gH2/L by volume) and a heat of desorption of 68.6 kJ/molH2, 68.3 kJ/molH2 and 63.9 kJ/molH2. The corresponding dehydrogenation products are benzene, toluene and naphthalene, although the carcinogenic toxicity of benzene, together with the lower heat of desorption of methylcyclohexane and decalin, makes cyclohexane a poor substrate for this reaction. A problem arises also for naphthalene, which is solid at ambient conditions, thus complicating the implementation of a storage system based on decalin. The most recent studies focus on perhydro-dibenzyltoluene (18H-DBT) as LOHC47, 19,40,46,71,72

. This compound, and the related aromatics, have the advantages of being non-toxic

and non-explosive, are liquid and stable at ambient pressure over a broad range of temperatures (-30 to 360 °C) and have a theoretical storage density of 6.2 wt% with a dehydrogenation enthalpy of 65.4 kJ/molH2, which is lower than that of methylcyclohexane. One of the issues identified with 18H-DTB, is the possibility of forming different intermediates during its dehydrogenation and hydrogenation steps, which may lead to an incomplete degree of dehydrogenation/hydrogenation, depending on the catalyst used and on the process conditions. This effect would translate to a storage capacity lower than the theoretical value. Nevertheless, multiple modules for hydrogen storage or hydrogen delivery, based on perhydrodibenzyltoluene (18H-DBT), are already commercialized by Hydrogenious Technologies73.


9

ACS Catalysis

To visualize the effect of the energy required for the dehydrogenation process, calculations using MCH as a model LOHC have been performed. As shown in Fig. 1, the energy required is calculated as the sum of the energy required to bring liquid MCH from 25 °C to 101°C (boiling point, BP), the energy required for MCH vaporization at 101°C, the energy for required for heating MCH vapor from 101°C to the reaction temperature, and the dehydrogenation enthalpy required for a certain degree of MCH conversion. In Fig. 2 the total efficiency of a LOHC/fuel cell system is plotted as function of MCH conversion, using the assumption that methylcyclohexane is dehydrogenated in the gas phase.

6 Energy Required (kJ/h)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4

2

0 100

150

200 250 300 Reaction Temperature (°C)

Total Heat Required MCH dehydrogenation enthalpy Heating Vapor MCH up to reaction T

350

400

MCH latent Heat Heating Liquid MCH up to 101°C

Figure 1 – Energy required to vaporize and heat MCH up to reaction temperature (MCH feed 0.033 mol/h).

The energy produced by the process is calculated assuming that the hydrogen generated has a gravimetric energy density of 120 MJ/kg that is converted into electricity by a fuel cell with a process efficiency of 50%. The total efficiency plotted in Fig. 1 is then defined as following:


10

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− ℎ ∗ 100 ℎ % = ℎ The efficiency was calculated for two different dehydrogenation temperatures for MCH and compared to the efficiency that would be achieved with systems where a heat recovery system is implemented. The assumption is that is the systems including heat recovery, the energy required for heating the liquid MCH from 25 °C to 101 °C is entirely provided by the heat generated from the fuel cell.

50 Total efficiency (%)

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ACS Catalysis

40 30 20 10 0 0 150°C

20

40 60 MCH Conversion (%)

150° with heat recovery

200°C

80

100

200°C with heat recovery

Figure 2 – Efficiency of system for MCH dehydrogenation coupled with a fuel cell as function of the conversion and heat recovery.

The plot of Fig. 1 shows that the dehydrogenation enthalpy has the highest impact on total energy requirement, but it also indicates the reaction temperature contribution, which should be as low as possible. In Fig. 2 the efficiency curves indicate that even a small heat contribution has a great impact on the system. In fact, just the subtraction of the energy required for heating liquid MCH (heat recovery from the fuel cell), that is the lowest contribution to the total energy,


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increases the efficiency considerably. For MCH conversion of 100% at 150 °C, the efficiency increases from 34.8% to 42.6% with the heat recovery. These calculations highlight the importance of developing a catalyst that allows high conversion at the lowest temperature and at the same time to use as substrate an LOHC that possesses low dehydrogenation energy. Table 1 gathers a comprehensive summary of the molecules studied as possible LOHCs, including their dehydrogenated form and main physio-chemical properties74–77. a

Table 1 – Most studied LOHCs and their physico-chemical properties - Conflicting data on the literature Reaction

H2 Melting T (°C) Boiling T (°C) Flash point (°C) Autoignit. (°C) ΔH capacity (kJ/mol) H2 rich H2 lean H2 rich H2 lean H2 rich H2 lean H2 rich H2 lean (wt%)

Cyclohexane → Benzene 7,2

68,6

7

6

81

80

-20

-12

245

498

6,2

68,3

-126

-95

101

111

-3

6

258

535

7,3

63,9

-37a

79

189

218

57

80

250

525

7,3

62,8

4

70

235

254

92

113

245

540

6,2

65,4

/

-34

/

398

/

190

/

/

6,0

55,7

/

158

65

273

45

164

/

640

5,8

/

/

60

178

272

58

141

/

510

5,8

51,9

/

95a

180

238

60

110

/

/

Me-cyclohexane → Toluene

Decalin → Napthalene

Bicyclohexyl → Byphenyl

Perhydro-dibenzyltoluene → dibenzyltoluene

4-amino-piperidine → 4-amino-piridine

2-methyperhydrolindole → 2-methylindole

N-methylperhydroindole → N-methylindole


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ACS Catalysis

Dodecahydro-N-ethylcarbazole → N-ethylcarbazole 5,8

50,6

-85

70

/

166

/

186

/

619

3. N-HETEROCYCLE DEHYDROGENATION The principle drawback for hydrogen storage in LOHC is the high energy required in the dehydrogenation process. Generally, a possible solution is the utilization of a related class of organic compounds. The introduction of hetero-atom(s) into cyclic organic rings to form heterocycloalkanes, reduces the enthalpy of the dehydrogenation reaction and allows it to be carried out at lower temperature than that required for the corresponding cycloalkane12. Even though hetero-LOHCs allow full catalytic hydrogenation/dehydrogenation there are some drawbacks correlated to their properties. The first is the slightly lower H2 gravimetric density than the cycloalkanes described in the previous section: amongst the most promising hetero-cycloalkanes are octahydro-N-ethylindole (8H-NEID) and perhydro-N-ethylcarbazole (12H-NEC) with a hydrogen capacity of 5.2 wt% and 5.8 wt% respectively. Another issue related to the application may be caused by the physical state of some of the dehydrogenated products: N-ethylcarbazole (NEC) and N-ethylindole (NEID) have melting points of 70°C and 51°C respectively; this means that some products involved in the dehydrogenation/hydrogenation cycle are solid at ambient temperature, a factor that must be taken into account in process development. A further consideration come from the possibility of secondary reaction such as N-C bond cleavage, alkyl transfer or disproportionation occurring during the hydrogenation or dehydrogenation reactions. It has been reported that these side reactions can be minimized by the steric effect of ligands on the heteroatom, such as the methyl and ethyl groups in NEID and NEC. For this reason, nonprotected heterocompounds such as carbazole and indoline have not been considered here.


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4. DEHYDROGENATION OF OTHER LOHCs Considering the hydrogen generation step only, there are other LOHC substrates besides cycloalkanes and heterocycloalkanes that can be efficiently dehydrogenated for hydrogen delivery purposes. The most promising are ethanol, methanol and formic acid, having a theoretical hydrogen capacity for their dehydrogenation of 4.3 wt% 6.25 wt% and 4.4 wt% respectively. The process involving the utilization of these LOHCs is different from that described for cycloalkanes and heterocycles, since the reverse hydrogenation step is not implemented: ethanol dehydrogenation byproducts are mainly used as fuel or fuel additives. The hydrogenation of CO2 generated from formic acid dehydrogenation on the other hand is currently a not viable process. The dehydrogenation of ethanol, ideally, should lead to the formation of diethylacetate and hydrogen only, while the dehydrogenation of formic acid should lead to the formation of CO2 and H2 only: 2

! "#

2

!#

→ →

##

→

! ##" % ! ## "

+2

+ 2

"

"

+ #"

The standard reaction enthalpy for ethyl acetate and hydrogen formation from ethanol is 72 kJ/mol, which is slightly higher than the dehydrogenation enthalpy of the cycloalkanes examined in the previous section. The reaction enthalpy for methyl formate and hydrogen formation from


14

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ACS Catalysis

methanol is 98,9 kJ/mol and the reaction enthalpy for formic acid decomposition to H2 and CO2, is considerably lower, 29.3 kJ/mol. For ethanol dehydrogenation, most studies reveal that the selectivity towards hydrogen and diethyl acetate is difficult to control78–82. When complete ethanol conversion is achieved, a contribution of side reactions is reported that lead to formation of byproducts including aldehydes, ketones, diethyl ether, butanol and other higher molecular weight compounds. These byproducts are caused by a certain degree of activity towards C-C bond cleavage, meaning that the purity of the hydrogen generated is affected as well, because of the formation of CO2 and CO in the gas phase, besides the liquid byproducts. With the most selective catalysts, ethanol conversion is reduced (50-80%), but an increase of the reaction pressure (to 20-30 bar, from atmospheric) allows selectivity to ethyl acetate of 95-99%, almost eliminating acetaldehyde and other byproducts formation82. Some examples of the direct use in fuel cells of the hydrogen produced by ethanol catalytic dehydrogenation are known, but the voltage generated is considerably lower than with a fuel cell fed with pure H283. The liquid products of ethanol dehydrogenation, on the other hand, can be used as fuel or as fuel additive because of its suitable combustion properties84. This solution is generally preferred to a hydrogenation step to regenerate ethanol, probably because of the increasing abundance of bio-ethanol produced by fermentation of sugar cane. A further reason is the difficulty of the hydrogenation process, since the total conversion of ethanol is hardly achieved and the products formed (mainly ethyl acetate and acetaldehyde) are highly flammable and possibly carcinogenic. Methanol/methyl formate couple has been suggested as a chemical hydrogen storage and carrier system as early as 199885 and some studies are reported in the literature86–88. For the direct dehydrogenation of methanol, methyl formate and hydrogen should be formed as the only


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product of the reaction. Formaldehyde formed in a first dehydrogenation step, then reacts with a second molecule of methanol to give methyl formate.

"#

!#

+

→

!#

→

"#

+

##

" !

+

"

The hydrogenolysis of methyl formate to give methanol is a relative easy reaction. But due to the high basicity of the catalyst used and the high reaction temperature (200 – 280°C), the selectivity can be limited by the degradation of methyl formate to CO and H2 as side products.

The dehydrogenation of formic acid is characterized by its complete decomposition to CO2 and H2, similarly to a reforming process. In this case, examples reported in the literature show that the selectivity is easier to control and only the presence of CO traces are reported in some of the studies89–96. Nevertheless, the direct utilization of the produced gas inside a fuel cell is not viable without purification, since CO2 and H2 are generated in equal amount and the performance of a fuel cell fed with diluted hydrogen at 50 mol%, is substantially lower than with pure hydrogen. Even though the implementation of hydrogen generators from ethanol and formic acid has some drawback, these technologies show great potential because of substrate availability. In ethanol dehydrogenation, the research on catalysts allowing complete conversion and at the same time high selectivity to ethyl acetate, would make a practical application more realistic. The main advantage for the formic acid based process is its low reaction enthalpy and low temperatures required (30-60 °C). On the other hand, the major drawback is the presence of a high concentration of CO2 in the hydrogen generated (50:50), requiring the implementation of an


16

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ACS Catalysis

efficient and compact CO2 elimination step, before entering the fuel cell. Despite the presence of a variety of studies on formic acid dehydrogenation for hydrogen production, no examples of catalytic processes are described here, since the lack of information (hydrogen purity or amount of catalyst used) did not allow the calculation of catalyst specific activity and comparison to the other processes presented in the following section. Recently a promising route was reported using the reversible dehydrogenative coupling of amines and alcohols: ' # + " (

"

→ " ( #' + 2

"

This reaction has been studied for peptide synthesis, but it can be applied also to hydrogen delivery and storage. Some examples of this reaction applied to hydrogen production are reported in the literature. The group of Hu et al.97 investigates the coupling of aminoethanol to form a cyclic dipeptide and hydrogen (Figure 3).

Figure 3. Reaction of Aminoethanol to cyclic dipeptide and hydrogen.

This route is interesting since the hydrogen capacity for the system based on ethanol amine is 6.56 wt% and both the dehydrogenation and hydrogenation reaction seem possible at mild conditions. They also developed and reported98 a homogeneous LOHC system using ethylenediamine and ethanol as hydrogen carriers with excellent conversion for both hydrogenation and dehydrogenation reactions. In the same way, Kothandaraman et al.99


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developed a reversible hydrogen carrier system by coupling 1,2 diamine and methanol with a good conversion (86 % of H2) and with no contamination of the gaz mixture.

5. CATALYSIS OF LOHC DEHYDROGENATION 5.1 FUNDAMENTAL ASPECTS OF CATALYTIC DEHYDROGENATION There are a large number of publications concerning the development of catalysts for hydrocarbon dehydrogenation. Since the 80s, noble metal catalysts, mainly Pt based, have been studied in this reaction. The active metal particles are usually dispersed on a porous support, such as silica, alumina or carbon, in order to maximize the metal surface area. Platinum is the most used component in dehydrogenation catalysts due to its high capacity to activate C-H bonds, combined with a lower capacity in the rupture of C-C bonds, resulting in intrinsically higher selectivity towards the dehydrogenation reaction than C-C cleavage. Platinum based catalysts have relative high abundance of sites that catalyze the C-H bond breakage, but depending on the size of the metal clusters and surface acidity, they are able to catalyze the C-C rupture as well. This is an even more important factor when using as reagent compounds that contain a heteroatom(s), such as nitrogen or boron, since C-N or C-B bonds are easier to break than C-C, thus requiring extremely selective catalyst and appropriate reaction conditions, in order to display high dehydrogenation selectivity. The side reactions that occur in addition to the dehydrogenation are deleterious, since they can lead to catalyst deactivation by carbon coke deposition (fouling) and to a decrease in the purity of the hydrogen generated, by formation of light gaseous alkanes such as methane, ethane, ethene and others. Hydrogenolysis and coke formation are more structure sensitive than


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dehydrogenation, and as a consequence, the addition of any inactive species on the catalyst surface, would act as a diluent for the Pt sites, increasing the selectivity towards dehydrogenation. This effect, often called “geometrical effect”, is well documented and has been observed in a number of bimetallic catalysts using a combination of Pt with Sn, In, K, Na and others. Pt alloys or binary compounds with one of these elements, results in average in higher Pt dispersion, with smaller atomic clusters that have lower or no activity towards C-C cleavage. The incorporation of other metals, for example Sn27,35,100–127, In32,100,113,114,128–130, and/or additives like K108,111,115, Na27,112,122,127 has become a common procedure, as it has been proven to confer superior stability to the catalyst. In addition to the dilution of Pt sites, there are other types of interactions that take place between the Pt and a second metal. It is proven that Sn, In and some other transition metals, besides the geometrical effect, have an electronic interaction with Pt100,116,119,127,129,131,132. These interactions can reduce the Pt-H binding energy, limiting deep dehydrogenation and as consequence coke formation. There are reports of Sn favoring hydrogen spill-over and coke migration from Pt sites to the support116, further contributing to catalyst stability. Significant differences in the magnitude of electronic interactions have been extrapolated for a series of Pt catalysts modified by the addition of different “promotors”, which are by themselves inactive for the dehydrogenation. This corroborates the coexistence of geometrical and electronic effects, which is of different extent for each promotor and related to the electronic configuration. Examples of cyclic hydrocarbon dehydrogenation catalysts, based on metals other than Pt, such as Ag, Ir and base metals, such as Ni, are found in the literature50–52,57,79,133. The activity of Ir is in some cases comparable to that of Pt, while Ag based catalysts often require the addition


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of a small fraction of noble metal to achieve sufficient activity. Base metal catalysts based on Ni are on average less active in the dehydrogenation reaction, but the availability and lower price of this metal make these systems interesting. Increasing the Ni loading on the catalyst and/or increasing the process temperature, allow complete conversion of cyclic hydrocarbons, but the selectivity towards the dehydrogenation product is low and the reactions of hydrogenolysis and cracking compete with the dehydrogenation. Similarly to Pt-Sn bimetallic catalysts, the combination of Ni with other metals, such as Cu, significantly increases selectivity towards hydrogen release and decreases the activity in C-C bond cleavage. The above active metals are not utilized as catalyst by themselves, but are usually dispersed on a high surface area support material, in order to maximize the metal dispersion. The choice of the support is another key parameter for the catalyst stability. Properties like surface area, pore shape and diameter, surface acidity and surface potential strongly influence the catalyst stability and selectivity. The interaction that is created during catalyst preparation (often involving thermal treatment) between the support and the active metals can reduce the loss of activity by particle sintering. It is a known phenomenon that small metal particles of a catalyst, when kept for a time at a sufficiently high temperature, tend to agglomerate to form bigger clusters. This sintering effect is disadvantageous, in the case of the dehydrogenation, since, as explained earlier, the selectivity decreases with the increasing particle size. The surface acidity is very important for the reaction selectivity, as the presence of a high density of strong acid sites shifts the selectivity towards cracking reactions, leading to formation of methane, ethane or other light hydrocarbons. These will reduce the purity of generated hydrogen and, in case of olefins formation, also cause the formation of coke precursors. The porosity of the support is important mainly for stability reasons. The deactivation of


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ACS Catalysis

dehydrogenation catalysts is commonly due to coke formation and deposition on active sites. As a consequence, the presence of large mesopores (6-15 nm), should avoid channel clogging and reduce the loss of accessibility to the active sites. This information allow a preliminary selection for an appropriate heterogeneous catalyst for LOHCs dehydrogenation. The desired characteristics are an active phase with intrinsically high selectivity for dehydrogenation and an activity sufficient to generate the required hydrogen. This active phase should be dispersed on a high surface area support with sufficiently large pores to withstand coking, which have the lowest amount of strong acid sites on the surface. The combination of suitable catalyst properties, and an appropriate reactor working at the optimal operating conditions, determines the results in terms of conversion, selectivity and H2 yield, as schematized in Fig. 4.

Figure 4 – Scheme of the parameters influencing conversion, selectivity and yield in the catalytic dehydrogenation of LOHCs.

5.2 CATALYTIC PROCESSES FOR LOHC DEHYDROGENATION


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The catalytic dehydrogenation reaction, finalized to hydrogen generation from LOHCs, is also subject of a large number of publications in the last decades. In the present work we summarize and compare a variety of different catalytic systems and their performance. The data shown in Table 2 were extracted from the most relevant articles on this subject, selecting those providing sufficient data to calculate the activity in terms of mmol of hydrogen generated per hour per mass of catalyst. The values have been recalculated here in order to present a set of data with homogeneous units, therefore facilitating comparison between the different reactors, LOHCs and operating conditions. The GHSV is defined as volumetric feed flow (ml*h-1) divided by the catalyst volume (mL), while the WHSV is expressed as mass feed flow (g*h-1) divided by the catalyst mass (g).

Table 2 – Comparative table of most relevant catalytic processes for LOHCS dehydrogenation Catalyst

Reagent

CuCrO4/CuO/Cu /BaCrO4/Al2O3 (BASF Cu-12341/16-3F)

EtOH

10 wt% Cu/ZrO2

EtOH

5 wt% Pt/AC

DEC

5 wt% Pt/AC

DEC

5 wt% Pt-Ir/AC Pt/Ir = 4

DEC

5 wt% Pt-W/AC Pt/W = 1

DEC

5 wt% Pt-Re/AC Pt/Re = 2

DEC

Conditions -1 WHSV = 0.06 h feed: 50.7 wt% N2 + 0.3 wt% H2 + 49.0 wt% EtOH P = 20 bar T = 220°C -1 WHSV = 0.4 h feed: 58 wt% EtOH + 42 wt% Ar P = ambient T = 250°C Cat/DEC = 0.3g/mL Theater = 210°C P = ambient Cat/DEC = 0.3g/mL Theater = 280°C P = ambient Cat/DEC = 0.3g/mL Theater = 210°C P = ambient Cat/DEC = 0.3g/mL Theater = 210°C P = ambient Cat/DEC = 0.3g/mL Theater = 210°C

Reactor

Results

PFR like reactor

Χ = 47% S to EtAC = 97% -1 -1 H2 = 14 mmol*h *gcat

PFR like reactor

Χ = 80% S to EtAC = 72% S to ACH = 20% -1 -1 H2 = 24 mmol*h *gcat

CSTR like

CSTR like

CSTR like

CSTR like CSTR like

Χ@2.5h=40.0% S to Naphtalene not specified -1 -1 Avg H2 = 31.5 mmol*h *gcat Χ@2.5h=93.0% S to Naphtalene not specified -1 -1 Avg H2 =73.2 mmol*h *gcat Χ@2.5h=77.0% S to Naphtalene not specified -1 -1 Avg H2 = 60.7 mmol*h *gcat Χ@2.5h=96.0% S to Naphtalene not specified -1 -1 Avg H2=75.3 mmol*h *gcat Χ@2.5h=100.0% S to Naphtalene not specified

Ref 82

83

134

134

134

134

134


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Mo/SiO2 Mo/Si = 10

MCH

P = ambient -1 WHSV = 92.4h H2recycle/MCH vol. = 250 P = 2,2 MPa T = 400°C

-1

Avg H2 =78.8 mmol*h *gcat PFR like

-1

1.9 wt% Ir/USY

MCH

WHSV = 92.4h H2recycle/MCH vol. = 250 P = 3 MPa T = 250°C

MCH


MCH


MCH


PFR like

-1

1.2 wt% Pd /USY

PFR like

-1

1.6 wt% Pt/USY

PFR like

-1

2.7 wt% Ni/USY

PFR like

-1

X not specified Toluene S = 90.0% -1 -1 Initial H2 = 362 mmol*h *gcat X@1h TOS = 9.7% Toluene S@1h TOS = 89.0% H2@1h TOS = -1 -1 95 mmol*h *gcat Deact@4h TOS = -28% X@1h TOS = 2.9% X@3h TOS = 3.5% Toluene S@1h TOS = 1.0% H2@1h TOS = -1 -1 0.1 mmol*h *gcat X@1h TOS = 3.9% X@3h TOS = 2.4% Toluene S@1h TOS = 21.0% H2@1h TOS = -1 -1 3 mmol*h *gcat X@1h TOS = 5.5% X@3h TOS = 3.3% Toluene S@1h TOS = 10% H2@1h TOS = -1 -1 2 mmol*h *gcat

60

133

133

133

133

-1

NiCu/SiO2 Ni = 17.3 mol% Cu = 3.6 mol%

CHE

NiCu/SiO2 Ni = 17.3 mol% Cu = 3.6 mol%

CHE

4.9 wt% Ni 3.5 wt% Cu /SBA-15

CHE

5 wt% Pd/Al2O3

8H-NEID

5 wt% Pd/Al2O3

12H-NEC

5 wt% Pt/ACC

MCH

5 wt% Pt/ACC

4MP

2

3g.m Pt /Alumite

CHE

WHSV = 2.3h H2recycle/CHE mol = 25/1 P = ambient T = 350°C -1 WHSV = 2.3h H2recycle/CHE mol = 25/1 P = ambient T = 325°C -1 WHSV = 2.3h H2recycle/CHE mol = 25/1 P = ambient T = 350°C Cat/8H-NEID = 0,1 g/g Treflux = 190°C P = ambient Cat/12H-NEC = 0.1 g/g Treflux = 180°C P = ambient Spray = 0.3 Hz - 10 ms Eq. to ≈ 0.42 mL/min -1 WHSV ≈ 137 h T = 350°C P = ambient Spray = 0.3 Hz - 10 ms Eq. to ≈ 0.42 mL/min -1 WHSV ≈ 137 h T = 350°C P = ambient Pulse size=3,5 mmol Interval 1s -1 Feed : 190 mmol .min T = 375 °C P = ambient

PFR like

Initial X = 95.0% Benzene Initial S = 99.4% -1 -1 Initial H2 = 58 mmol*h *gcat

PFR like

Initial X = 87.8% Benzene Initial S = 99.9% -1 -1 Initial H2 = 54 mmol*h *gcat

PFR like

Initial X = 99.7% Benzene Initial S = 99.0% -1 -1 Initial H2= 61 mmol*h *gcat

CSTR like

CSTR like

Χ@6h=100.0% S to NEID = 100.0% -1 -1 Avg H2 = 45.9mmol*h *gcat Χ@3.25h=100.0% S to NEC = 100.0% -1 -1 Avg H2= 175 mmol*h *gcat

Spray Pulse

Χ@3h=26.6% S to Toluene not specified -1 -1 H2@3h = 158 mmol*h *gcat

Spray Pulse

Χ@3h=22.2% S to 4-me-pyridine not specified -1 -1 H2@3h = 412 mmol*h *gcat

Spray Pulse

X = 27 % -1 -1 H2= 3800 mmol*gPt *min

52

52

50

135

136

137

137

138


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5 wt% Pt/ACC

5 wt% Ru/Al2O3

DEC

Spray freq. = 0.3 Hz Spray duration = 10 ms Eq. to ≈ 0.42 mL/min T = 350°C P = ambient

12H-NEC

Cat/12H-NEC = 0.1 g/g Treflux = 180°C P = ambient

12H-NEC

Flow rate 0,2-0,8 ml*min T = 261 °C P = 1 bar

12H-NEC

Cat/12H-NEC = 0.1 g/g Treflux = 180°C P = ambient

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Spray Pulse

Χ@3h=8.4% S not specified -1 -1 H2@3h = 425 mmol*h *gcat

CSTR like

Χ@3h=100.0% S to NEC = 77.3% S to 4H-NEC = 22.7% -1 -1 Avg H2 = 103 mmol*h *gcat

PFR

Y = 20,9 % H2 = 5000 mmol*min-1*gPt-1

137

139

-1

-2

0,4 g m Pt /Al2O3

5 wt% Rh/Al2O3

CSTR like

-1

1 wt% Pt0.6 wt% Sn /AC

Pd/AC Metal loading unknown

DEC

MCH

WHSV = 3.2 h -1 GHSV = 1 h 100% decalin feed P = ambient T = 320°C -1 WHSV = 3.7 h -1 GHSV = 1.2 h 100% MCH feed P = ambient T = 200°C

PFR like Microwave heated

PFR like Microwave heated

Χ@3h=100.0% S to NEC = 11.3% S to 4H-NEC = 85.6% -1 -1 Avg H2 = 76 mmol*h *gcat X@5h TOS = 78% Naphtalene S@5h TOS = 97% H2@5h TOS = -1 -1 88 mmol*h *gcat Deact@4h TOS = -83% X@1h TOS = 97% Toluene S@1h TOS = 99% H2@1h TOS = -1 -1 110 mmol*h *gcat

140

139

64

141

The catalysts used in ethanol and methanol dehydrogenation are mainly Cu based materials supported on metal oxides, such as alumina or zirconia. The studies by Sato et al.83 and Carotenuto et al.82 investigate the heterogeneous catalytic dehydrogenation of ethanol in a PFR like reactor. These studies highlight how the selectivity towards ethyl acetate, acetaldehyde or other heavier products is strongly dependent on the operating conditions. The formation of ethyl acetate is one of the rate determining steps and the mechanism goes through an acetaldehyde intermediate. For this reason, low values of WHSV are preferable, since the selectivity towards ethyl acetate is increased and acetaldehyde formation, which is a potential carcinogen, is reduced. Both the pathways lead to the formation of 1 mol of hydrogen per mol of acetaldehyde/ethylacetate, while the formation of heavier products decreases the hydrogen purity by formation of CO/CO2. The examples considered also highlight that, even in diluted conditions (≈ 50 wt% EtOH in the feed), ethanol conversion is low (< 50%) and if the conversion is


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increased, a drop in selectivity to EtAC and ACH is observed. The specific activity towards hydrogen formation is lower than for the majority of cycloalkanes and heterocycloalkanes, being 14 mmol*h-1*gcat-1 and 24 mmol*h-1*gcat-1. The most recent studies on cycloalkanes and hetero-cycloalkanes, use as catalyst Pt, Pd, Ni, Rh and Mo incorporated on a variety of supports, such as Al2O3, SiO2, AC, zeolite and others. Heterogeneous catalysis seems to be the predominant choice for this process, mainly because using a homogeneous catalyst in the liquid phase, the H2 evolution ratio is insufficient for practical applications. However examples of LOHC dehydrogenation with a homogeneous catalyst have been reported, the first being by Gupta et al.142 in 1997, for cycloalkanes dehydrogenation. Wang et al.143 and Brayton et al.25 further developed the concept and applied it to the 12H-NEC dehydrogenation, using an Ir based pincer type homogeneous catalyst. The results in terms of H2 evolution have not been calculated here because of insufficient data, but the hourly production must be low since the time of reaction is of the order of 24-34 h, in order to achieve a high LOHC conversion. Furthermore, many of the reagents need to be manipulated using Schlenk techniques, complicating the perspective for practical applications. Homogeneous systems for dehydrogenations of alcohols and amines with Ru based catalysts have been reviewed by Gunanathan and Milstein. Their review shows that efficient and controlled release of hydrogen from alcohols and amines using Ru- pincer catalysts is a promising route of hydrogen production144 and some papers reported relatively high yield of H2 obtained with methanol, under low temperature (100 °C), with no decrease of catalytic activity145–147

Heterogeneous catalysts used in CSTR like reactors, at reflux conditions, generally display higher H2 evolution ratios in the liquid-film than the homogeneous pincer catalyst in the liquid-


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phase. Hodoshima et al.134 studied the effect of the temperature and the addition of a second metal to a 5 wt% Pt/AC catalyst for decalin dehydrogenation in a CSTR like reactor. The temperature had a great impact on decalin conversion and is a key parameter in this reaction, because it determines how much energy is required by the process, affecting the overall efficiency. On the monometallic Pt/AC the conversion increases from 40% to 93% when the temperature of external heating is changed from 210 °C to 280 °C, with a consequent increase of the H2 generated by 58% (from 37.3 to 73.2 mmol*h-1*gcat-1). The addition of a second metal is beneficial and it seems to decrease the energy required for the hydrogen desorption reaction. The activity increases in the following order: Pt < Pt-Ir < Pt-W < Pt-Re. The hydrogen evolution ratio of a Pt-Re/AC catalyst at 210 °C is comparable to that of a monometallic Pt/AC catalyst at 280 °C, suggesting that the interaction and/or alloy formation between Pt and a second metal decreases the energy required for hydrogen release. In the dehydrogenation of heterocyclic hydrocarbons, Pt is used to a lesser extent, while the preferred catalysts are often based on Pd, Rh and Ru. The main reason appears to be the lower energy required for the dehydrogenation of heterocycles compared to cycloalkanes. The activity of platinum, would in fact be too high for heterocyclic compounds, catalyzing side reactions of disproportionation, dealkylation and C-heteroatom bond cleavage, besides dehydrogenation (Sabatier principle). The use of other catalysts (Pd, Rh, Ru based), allow heterocycle dehydrogenation without substrate degradation and with high selectivity towards hydrogen, especially when an alkyl protecting group is present on the heteroatom (like in NEC). Yang et al. report in three different publications135,136,139 the possibility of dehydrogenating heterocyclic compounds using Pd, Rh and Ru heterogeneous catalysts in a CSTR like reactor. More precisely, they investigated a 5 wt% Pd/Al2O3 for the dehydrogenation of octahydro-N-


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ethylindole and dodecahydro-N-ethylcarbazole, of which we report here the most relevant results. At mild temperatures of 180 °C and 190 °C for 8H-NEID and 12H-NEC respectively, activities in terms of H2 evolution of 45.9 mmol*h-1*gcat-1 and 175 mmol*h-1*gcat-1 were reached with an high degree of purity (only traces of H2O detected beside H2). Better results in terms of H2 release rate (425 mmol*h-1*gcat-1) were reported by Patil et al.137 for decalin dehydrogenation with a 5 wt% Pt/AC catalyst in a pulse spray reactor. In that work the continuous conversion of decalin is lower (24.5%), probably because of the high WHSV imposed. It may be supposed that in this case the selectivity to naphthalene must be higher for the pulse spray reactor than the batch reactor, but because of the lack of comparable data a deeper analysis is not possible. The same catalyst 5 wt% Pt/AC has been studied in the dehydrogenation of MCH and 4-methylpyridine with high H2 productivity (373 mmol*h-1*gcat-1 and 412 mmol*h-1*gcat-1). Very high productivity of 3800 mmol*min-1*gPt-1 has been reported in a spray pulse reactor on a Pt/alumite catalyst138. In general, these high activity values are related to the use of a spray reactor that is operated at high gas space velocities and low catalyst loading. These conditions are optimal to perform kinetic studies that must be carried out at low reagent conversion, but for a practical application, the target for conversion would be as close as possible to 100%. For this reason the results obtained using PFR like gas-phase reactors, presented in the following part, are considered the best option for a hydrogen delivery prototype. Optimisation of Pt supported egg-shell catalysts has resulted in a productivity of 5000 mmol*min-1*gPt-1 which is the highest reported up to now and can be considered as the current state of the art140. The investigation by Cromwell et al. for MCH dehydrogenation in a PFR like reactor, similarly to the previous example with spray pulse reactor, is carried out at very high WHSV and low conversion, in order to evaluate the kinetics and to be able to discriminate between initial


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activities of the different active metals. With a USY zeolite as catalyst support; the activity towards hydrogen generation followed the trend: Ir > Pt > Pd > Ni. Surprisingly Ir/USY catalyst is the only one showing high H2 selectivity, while the Pt and Pd, that are usually the most efficient, were reported to produce high amounts of CHE and methane by a demethylation mechanism. The results reported by Xia et al.50,52 are the most promising amongst the examples of Table 2, because the type of reactor and operating conditions are appropriate for a scale-up and, considering the use of a non-noble metal active phase (Ni-Cu), the H2 evolution ratio observed is very high (54-61 mmol*h-1*gcat-1). At temperature of 350 °C the Ni-Cu/SiO2 catalyst displays high conversion (95%) and selectivity (99.4%). The results are improved when using an SBA-15 support, giving 99.7% conversion and 99.0% selectivity. The acidity of SiO2 and SBA-15 is similar and is generally very low, as consequence the activity and selectivity improvement have been attributed to the higher surface area of SBA-15, which should lead to a higher metal dispersion. Unfortunately the characterization data reported are insufficient for a deeper analysis. In terms of H2 evolution rate, as expected, highest activity is observed for Pt and Pd based catalysts. Suttisawat et al.64 performed the dehydrogenation of decalin with a Pt-Sn/AC catalyst obtaining a H2 evolution of 88 mmol*h-1*gcat-1 with 99% selectivity towards naphthalene. Horikoshi et al.141 report an activity of 110 mmol*h-1*gcat-1 for MCH dehydrogenation on a Pd/AC catalyst. The two latter examples involve the use of a PFR like fixed-bed reactor heated using microwave radiation, the effect of which appears to be only a more efficient heating of the catalytic bed. The polarity of LOHCs is very low, as consequence the microwaves mainly heat the catalyst. The LOHC is vaporized at the catalyst interface and the endothermic effect of the dehydrogenation is balanced by the direct heating of the active sites by microwaves irradiation.


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This leads to a lower temperature gradient at the interface of the catalyst during reaction, increasing the conversion in comparison to a conventional heating reactor (for equal T setpoints). It is reported in numerous studies that the use of a membrane reactor, or the coupling of a reactor with a membrane separator, can increase considerably the conversion of the LOCHs and the purity of H2 generated53,58,59,61,148,149. We mention this possibility as it shows great potential for the process optimization, but since it is not strictly a part of the dehydrogenation catalysis, the details of the processes using this technology are not included in this review.

6. FUEL PARTIAL DEHYDROGENATION 6.1 FUNDAMENTAL ASPECTS OF FUEL PARTIAL DEHYDROGENATION This approach is not considered as a hydrogen storage means, but more as a localized or on-board H2 generation process, involving the dehydrogenation reaction. The reaction is carried out on a complex mixture of hydrocarbons such as kerosene, diesel, naphtha or gasoline and a fraction of the fuel contained in the vehicle tanks is partially dehydrogenated to produce the hydrogen required. Indeed through a controlled dehydrogenation, the properties of the dehydrogenated fuel are not expected to change considerably and it will still be appropriate for use in an internal combustion engine. The electricity generated using fuel cells instead of turbines or alternators will increase the system efficiency and optimize the fuel utilization. A simplified scheme of the fuel PDH process is provided in Fig. 5.


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Electrical power

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Fuel Cell

High purity H2

PDh reactor

Fuel

Heat exchange

Propulsion

Dehydrogenated Fuel

Engine

Figure 5 – Fuel partial dehydrogenation scheme.

The partial dehydrogenation of fuels is a complex process and it presents more difficulties than the reaction on a single molecule. The main reason is the combined reactivity of all the classes of compound contained in the mixture: beside the simple dehydrogenation of paraffins to olefins and cyclic to aromatics, there are many different potential reaction pathways. The formation of intermediate products that can react with each other via condensations, polymerizations to carbon coke and also the presence of undesired reactions like cracking and hydrocracking, make difficult the prediction of the efficiency and the products of this type of reaction. Bashin et al. [46] performed a detailed study on the dehydrogenation reaction, explaining the possible alternative pathways observed on acidic sites (A) and Pt metal sites. In Fig. 6 the reactivity of hydrocarbons is schematized.


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Pt iso-alkenes

iso-alkanes A

A, Pt A, Pt

Cracking products A, Pt

n-alkanes

A A

Pt n-alkenes

Pt

A

A, Pt

A, Pt Cycloalkanes

Pt n-dienes

n-trienes

A

Cycloalkenes

A Oligomers

A, Pt

Pt aromatics

A, Pt

Coke A, Pt

Figure 6 – Reaction pathways for diffent classes of hydrocarbon.

A further limitation related to the composition of fuels is the possible presence of sulfurcontaining compounds and additives. Apart from the fuels synthesized by Fisher-Tropsch reaction, those obtained by oil fractionation contain a certain amount of sulfurous compounds which can cause catalyst poisoning. Commercial fuels also contain a range of additives like antioxidants, metal blockers, static dissipaters, corrosion inhibitors, icing inhibitors and biocides. The effect of these additives on the catalytic process have to be considered.

6.2 CATALYTIC PROCESSES FOR FUEL PARTIAL DEHYDROGENATION The catalysts and a summary of the partial dehydrogenation results discussed here are schematized in Table 3. The very first reported example of fuel partial dehydrogenation, in which a kerosene surrogate was used as reagent, is from Wang et al.150 in 2008. This group performed a study using Pt/γ-Al2O3, Pt-Sn/γ-Al2O3, Pt/USY and Pt-Sn/γ-Al2O3-ZrO2/SO4 as catalysts for the dehydrogenation of a surrogate of kerosene Jet-A1 (wt% = 30% decane, 35% dodecane, 14% methylcyclohexane, 6% decalin, 10% t-butylbenzene, 5% 1-methylnaphthalene). This reaction, and all the reactions described in this section, were carried in PFR like, tubular,


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fixed-bed reactors. Wang’s group carried out their reactions in diluted conditions (67 vol% of N2 in the feed) and imposed a simulated hydrogen recycle (H2 feed = 17 vol%), achieving hydrogen yields in the range of 4-12%, depending on the catalyst and the reaction conditions. Although the purity of H2 is not specified, this study shows the feasibility of generating hydrogen from a LOHC mixture, but it also highlighted a very quick deactivation already in the first two hours of reaction, due to the high quantity of coke produced.

Table 3 - Comparative table of most relevant catalytic processes for fuel partial dehydrogenation Catalyst

Properties

0.8 wt% Pt /Al2O3

No data available

0.8 wt% Pt /Al2O3

No data available

0.8 wt% Pt /Al2O3

No data available

0.8 wt% Pt /USY

No data available

0.8 wt % Pt0.3 wt% Sn /Al2O3-ZrO2-2 SO4

No data available

5 wt% Pt- 1 wt% Sn / Al2O3 5 wt% Pt- 1 wt% Sn- 1 wt% Na /Al2O3

Specific Area = 2 -1 184 m *g Width = 5.1 nm Surface acidity = -1 284 μmolNH3*g Specific Area = 2 -1 163 m *g Width = 5.0 nm Surface acidity = -1 284 μmolNH3*g

1 wt% Pt / Al2O3

Specific Area = 2 -1 138 m *g Width = 9.1 nm

1 wt% Pt- 1 wt% Sn

Specific Area = 2 -1 135 m *g

Conditions H2/JetA-1 = 10/1 mol T = 500°C P = ambient N2 in feed = 67 vol% H2/JetA-1 = 4/1 mol T = 500°C P = ambient N2 in feed = 67 vol% H2/JetA-1 = 10/1 mol T = 425°C P = ambient N2 in feed = 67 vol% H2/JetA-1 = 10/1 mol T = 425°C P = ambient N2 in feed = 67 vol% H2/JetA-1 = 10/1 mol T = 425°C P = ambient N2 in feed = 67 vol%

Reagent

Results

Jet A-1 surrogate (No Sulfur)

H2 Y@3h TOS = 9% Deact. = -20% in 3h Purity and activity not available

Jet A-1 surrogate (No Sulfur)

H2 Y@3h TOS = 5.7% Deact. = -54% in 3h Purity and activity not available

Jet A-1


Jet A-1


Jet A-1


100% JetA-1 feed T = 450°C P = 5 bar -1 GHSV = 1800 h

Low sulfur Jet A-1 (S < 3 ppmw)

H2@4h TOS= -1 -1 2 mmol*h *gcat Deact. = -68% in 4h H2 purity = 95.7 vol.%

100% JetA-1 feed T = 450°C P = 5 bar -1 GHSV = 1800 h



100% JetA-1 feed T = 450°C P = 5 bar -1 GHSV = 1800 h 100% JetA-1 feed T = 450°C

Jet A-1 surrogate (No Sulfur) Jet A-1 surrogate

H2@4h TOS= -1 -1 39 mmol*h *gcat Deact. = -57% in 4h Purity not available H2@4h TOS= -1 -1 134 mmol*h *gcat

Ref 150

150

150

150

150

27

27

28

28


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/ Al2O3

Width = 9.0 nm

1 wt% Pt- 3 wt% Sn / Al2O3


1 wt% Pt- 1 wt% Sn /Al2O3



1 wt% Pt- 1 wt% Sn /CeO2-Al2O3

1 wt% Pt- 1 wt% Sn /MgO-Al2O3

1 wt% Pt- 1 wt% Sn /BaO-Al2O3

1 wt% Pt- 1 wt% Sn /Al2O3 1 wt% Pt- 1 wt% Sn /Al2O3 Dechlorinat ed 1 wt% Pt- 1 wt% Sn /Al2O3


1 wt% Pt- 1 wt% Sn /Al2O3 1 wt% Pt- 1 wt% Sn-0.5 wt% K

Specific Area = 2 -1 130 m *g Width =12 nm Surface acidity = -1 120 μmolNH3*g Specific Area = 2 -1 180 m *g Width = 5-8 nm Surface acidity = -1 171 μmolNH3*g Specific Area = 2 -1 230 m *g Width = 7 nm Surface acidity = -1 195 μmolNH3*g Specific Area = 2 -1 100 m *g Width = 18 nm Surface acidity = -1 101 μmolNH3*g Specific Area = 2 -1 153 m *g Width = 7.4 nm Surface acidity = -1 327 μmolNH3*g Specific Area = 2 -1 153 m *g Width = 7.4 nm Surface acidity = -1 273 μmolNH3*g Specific Area = 2 -1 135 m *g Width = 9.0 nm Surface acidity = -1 120 μmolNH3*g Specific Area = 2 -1 135 m *g Width = 9.0 nm Surface acidity = -1 120 μmolNH3*g Specific Area = 2 -1 135 m *g Width = 9.0 nm Surface acidity = -1 120 μmolNH3*g Specific Area = 2 -1 138 m *g Width = 8.7 nm

P = 5 bar -1 GHSV = 1800 h 100% JetA-1 feed T = 450°C P = 5 bar -1 GHSV = 1800 h 100% JetA-1 feed T = 450°C P = 5 bar -1 GHSV = 1800 h

(No Sulfur) Jet A-1 surrogate (No Sulfur) Jet A-1 surrogate (S = 50 ppmw)

Deact. = -24% in 4h Purity not available H2@4h TOS= -1 -1 102 mmol*h *gcat Deact. = -11% in 4h Purity not available H2@4h TOS= -1 -1 50 mmol*h *gcat Deact. = -39% in 4h Purity not available

93% JetA-1-7% H2 feed T = 450°C P = 10 bar -1 GHSV = 1800 h


















93% JetA-1-7% H2 feed T = 450°C P = ambient -1 GHSV = 1800 h

Jet A-1 surrogate (No sulfur)

H2@4h TOS= -1 -1 22 mmol*h *gcat Deact. = -65% in 4h Purity not available


Jet A-1 surrogate (No sulfur)





93% JetA-1-7% H2 feed T = 450°C P = 10 bar


H2@4h TOS = -1 -1 63 mmol*h *gcat Deact. = -10% in 4h

28

28

30

30

30

30

35

35

34

34

34

34


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/Al2O3

Surface acidity = -1 23 μmolNH3*g

1 wt% Pt- 1 wt% Sn- 0.5 wt% In /Al2O3



Specific Area = 2 -1 100 m *g Width = 18 nm


Specific Area = 2 -1 100 m *g Width = 18 nm

5 %wt Ni2P0.5 %wt K /SiO2


1 wt% Pt- 1 wt% Sn /Al2O3 1 wt% Pt- 1 wt% Sn 0.5 wt% In /Al2O3 1 wt% Pt- 1 wt% Sn 0.5 wt% In /Al2O3

Specific Area = 2 -1 167 m *g Width = 19 nm Surface acidity = -1 209 μmolNH3*g Specific Area = 2 -1 226 m *g Width = 5 nm Surface acidity = -1 96 μmolNH3*g Specific Area = 2 -1 226 m *g Width = 5 nm Surface acidity = -1 96 μmolNH3*g Specific Area = 2 -1 187 m *g Width = 7.5 nm Surface acidity = -1 100 μmolNH3*g Specific Area = 2 -1 187 m *g Width = 7.5 nm Surface acidity = -1 100 μmolNH3*g

-1

Purity not available

GHSV = 1800 h

93% JetA-1-7% H2 feed T = 450°C P = 10 bar -1 GHSV = 1800 h 93% JetA-1-7% H2 feed T = 450°C P = 10 bar -1 GHSV = 1800 h 93% JetA-1-7% H2 feed T = 450°C P = 10 bar -1 GHSV = 1800 h

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Low sulfur Jet A-1 (S < 3 ppmw) Jet A-1 (S = 288 ppmw) Jet A-1 fraction Lighter 5wt% (S = 4 ppmw)

H2@4h TOS = -1 -1 122 mmol*h *gcat Deact. = -10% in 4h H2 purity = 97.8 vol.% H2@4h TOS = -1 -1 13 mmol*h *gcat Deact. = -32% in 4h H2 purity = 98.6 vol.% H2@4h TOS = -1 -1 54 mmol*h *gcat Deact. = -13% in 4h H2 purity = 99.0 vol.% H2@4h TOS = -1 -1 45 mmol*h *gcat Deact. = -4% in 4h H2 purity@5h TOS =99.0 vol% H2 purity@24h TOS=75.0vol%


Jet A-1 (S = 250-500 ppmw)

93% Gasoline-7% H2 feed T = 400°C P = 10 bar -1 GHSV = 1800 h

Gasoline SP95 surrogate

H2@4h TOS = -1 -1 80 mmol*h *gcat H2 purity@4h TOS =99.3 vol%

93% Diesel-7% H2 feed T = 400°C P = 10 bar -1 GHSV = 1800 h

Diesel surrogate

H2@4h TOS = -1 -1 147 mmol*h *gcat H2 purity@4h TOS =99.6 vol%


Gasoline SP95

H2@150h TOS = -1 -1 105 mmol*h *gcat H2 purity 150h TOS =84.9 vol% Deact = -15% in 150h TOS


Gasoline SP95E10

H2@150h TOS = -1 -1 121 mmol*h *gcat H2 purity 150h TOS =92.3 vol% Deact = -9% in 150h TOS

32

31

31

151

29

29

The first patent describing this process was filed in 2008 by Airbus Deutschland Gmbh152. Resini et al.27 reported the use of Pt-Sn/γ-Al2O3 and Pt-Sn-Na/γ-Al2O3 catalysts for hydrogen production by dehydrogenation of standard and a low sulfur (S < 3 ppmw) kerosene Jet A-1. The best overall results were obtained with the Pt-Sn-Na catalyst at 350 °C, 5 bar, 100% fuel feed (no information on GHSV). Using the standard Jet A-1, the initial activity registered was already very low (0.3 mmol*h-1*gcat-1) because of instantaneous deactivation caused both by sulfur poisoning and coking. It is interesting to note the absence of H2S in the produced gas, suggesting


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that the sulfur may be completely retained on the solid catalyst. On feeding the low sulfur Jet A1, the initial hydrogen productivity was 10 mmol*h-1*gcat-1 and the hydrogen purity 92.1 vol%. The 7.9 vol% of impurities in the product gas were light hydrocarbons in the range C1-C6, generated by side reactions of dealkylation and cracking. As a consequence, and similar to what was observed by Wang et al.150, the catalyst stability was poor due to carbon coke deposition, with an activity loss of 62.5% in only 4.5 hours TOS. The deactivation by carbon coke deposition is one of the main cause of activity loss in this reaction using metal based catalysts supported on porous materials, but there are other effects that have to be considered. A scheme of the different deactivation pathways, which will be discussed in this section, is schematized in Fig. 7.

Figure 7 – Supported metal catalyst deactivation pathways.

Lucarelli et al.28 tested different compositions of Pt-Sn/γ-Al2O3 for the dehydrogenation of two kerosene surrogate mixtures : one containing a sulfurous compound (S = 50 ppmw) and the other sulfur free. The volumetric composition of the surrogates was the following: dodecane 65%; methyl-cyclohexane 14%; tert-butylbenzene 10%; decalin 6%; tetralin 5%. The most prospective results have been observed for the catalyst containing 1 wt% of Pt – 1 wt% of Sn at


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the optimal operating conditions of 450 °C, 5 bar, GHSV = 1800 h-1 , 100% fuel feed. With the sulfur containing surrogate the initial activity is 81 mmol*h-1*gcat-1 but the deactivation is still rapid (-39% in 4 h). The initial activity for the sulfur free surrogate is 176 mmol*h-1*gcat-1 and the deactivation is reduced to a loss of 24% activity in 4 hours. The activity is considerably higher for both feeds, comparing these to the previous results by Resini et al.27, even though for Lucarelli et al.28 no hydrogen purity data is available. The catalysts used are both Pt-Sn based and the difference are attributed principally to the higher reaction temperature and feed used by Lucarelli et al28. They worked with a 5 components surrogate, while in the previous case the fuel was a commercial Jet A-1, containing more than 300 compounds. This indicates how much the presence of such many different compounds makes the catalytic process difficult to optimize; on the other hand the studies on surrogates are very useful, since they allow an easier understanding of the reactivity of each class of compound during the dehydrogenation of a mixture. The experiments on surrogates by Wang et al.150 and Lucarelli et al.28,34 show that the cyclic hydrocarbons (decalin, tetralin, methylcyclohexane) are completely dehydrogenated to the corresponding aromatics (naphthalene, toluene), while the paraffins (dodecane, decane) display much lower conversion. The conversion of cyclics to aromatics is also confirmed by Resini et al.27 who report an increase of aromatics compounds from 15.6 vol% to 20.1 vol%. These first investigations highlight that the Pt-Sn couple is effective in dehydrogenating cyclic hydrocarbons with high selectivity, but also that there is a combined effect of Pt-Sn with the catalytic support chosen, its acidity and porosity. In 2013, Taillades-Jacquin et al.30 analyzed the effect of the support on the catalytic partial dehydrogenation of low sulfur Jet A-1. The active phase Pt-Sn was impregnated on a series of heteroatom modified γ-Al2O3 and the effect of the acidity and pore structure on the reaction were discussed. The supports were mixed oxides of


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Mg-Al, Ce-Al and Ba-Al with 230 m2*g-1, 180 m2*g-1, 100 m2*g-1 specific surface area and 7 nm, 5 nm, 18 nm average pore diameter respectively. The total acidity was 195 µmolNH3*g−1 171 µmolNH3*g−1 101 µmolNH3*g−1 respectively. These materials were compared to a reference PtSn/γ-Al2O3 catalyst (130 m2*g-1, 12 nm, 120 µmolNH3*g−1. The highest H2 evolution rates after 4 hour TOS, were given by the Pt-Sn/CeO-Al2O3 and Pt-Sn/BaO-Al2O3 catalysts (70 mmol*h1

*gcat-1 and 80 mmol*h-1*gcat-1 respectively), while the Pt-Sn/MgO-Al2O3 show similar initial

activity, but fast deactivation (-58% in 4 h). This effect is attributed to the higher acidity of the MgO-Al2O3 support and particularly to the presence of strong acidic sites that lead to a higher amount of deposited coke. The CeO-Al2O3 and BaO-Al2O3 supports have lower acidity and are less prone to coking deactivation, displaying a low deactivation ratio throughout the experiment (-8% and -12% in 4 h). This study highlights that the acidity of the material is another key parameter to optimize for fuel partial dehydrogenation. The intrinsic Pt activity combined with the presence of strong acid sites seems to lead to a high level of coke deposition and fast deactivation. Comparing these results to the previous study on low sulfur Jet A-1, a considerable increase in activity (80 mmol*h-1*gcat-1 instead of 6 mmol*h-1*gcat-1) and stability (-12% in 4 h instead of -43% in 4 h) were observed with the mixed oxide supports and the composition of 1wt% for both Pt and Sn. Comparing the low sulfur Jet A-1 PDH on 1 wt% Pt-1 wt% Sn/Al2O3 to the study of 5 wt% Pt-1 wt% Sn/Al2O3 by Resini et al.27, we can observe that the 1:1 ratio between Pt:Sn is more appropriate than 5:1, considering the price and availability of Pt and the H2 evolution ratio. The Al2O3 supports in the previous studies have different acidity, surface area and pore diameter and the results allow to draw conclusion on how these parameters to influence the stability: the higher density of strong acid sites, the more coke is formed during the reaction. High surface area seems to increase resistance to coke fouling but the most important


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characteristic is the average pore diameter; which appear to be optimal for this kind of reaction at a value range of 6-9 nm. More narrow pores than this range are easily clogged by carbon, hindering access to active sites and accelerating the deactivation phenomena. Wider pores do not have beneficial effects and are often accompanied by a decrease in surface area. These observations are in agreement with studies already conducted on similar catalysts used in industrial processes and find confirmation in the study by Reyes-Carmona et al.35 on the partial dehydrogenation of low sulfur Jet A-1 using Pt-Sn supported on a sucrose templated γAl2O3 with an optimal pore size of 7 nm. The reactions were carried out using catalysts obtained by impregnation and others using the same catalyst after steam treatment. This treatment was used to dechlorinate the catalyst (chlorinated precursors were used in the preparation) and reduce the surface acidity (from 327 µmolNH3*g−1 of original catalyst to 273 µmolNH3*g−1). The dechlorinated catalysts show a slightly lower H2 evolution ratio after 4 h TOS (33 mmol*h-1*gcat1

instead of 41 mmol*h-1*gcat-1), but a higher stability (after 4 h TOS 18% lost activity instead of

38%), giving further confirmation to what deducted from the previous studies. The same effect of the acidy was observed by Lucarelli et al.34 comparing a Pt-Sn/Al2O3 catalyst with a Pt-SnK/Al2O3 catalyst. The catalysts have very similar porosity to the previous study and they observed that doping with a potassium salt suppresses the majority of strong acid sites, leading to higher stability and less coke deposition. More specifically, the strong acid sites are eliminated, while the population of weak-mild acid sites remain unchanged. Isomerization and dehydrocyclization reactions still take place on weak acid sites, while the lack of strong acid sites suppresses the cracking reactions that can lead to coke formation. Another proof is the higher H2 purity obtained with the dechlorinated catalyst, explained by reduced formation of methane by cracking.


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ACS Catalysis

The pressure is another important parameter to control in this reaction; the dehydrogenation reaction is thermodynamically favored at low pressure (Le Chatelier principle), but heterogeneous catalytic processes are complex and the kinetics of some steps or mass transfer limitation depend on the pressure. This means that the ideal pressure may not be atmospheric and that an evaluation of these effects is required. Lucarelli et al.34 investigated the effect of the pressure on the partial dehydrogenation of a 5 component Jet A-1 surrogate with Pt-Sn/Al2O3 as catalyst. The results shows a great increase in the catalytic activity and stability, when increasing the pressure from atmospheric to 5 bar. No considerable changes are observed instead between 5, 7 and 10 bar. The results for the 1 and 10 bar investigations are reported in Table 3. This effect of the pressure has been attributed to the reagents concentration that reach the active sites. In the gas phase, the concentration of reactant increase with the pressure and the diffusion of molecules intra-pores may be favored at higher pressure. This would explain the wide gap in H2 evolution ratio when passing from 1 to 5 bar. Similarly, when introducing a hydrogen recycle, like in this study, H2 partial pressure on the catalyst increases with the absolute pressure. The presence of hydrogen can increase the stability, since the dehydrogenation equilibrium get shifted towards the reagent, limiting the deep dehydrogenation that lead to coke formation. The hydrogen can also contribute to partial elimination of the coke deposits, via methanization or hydrogenation of coke precursors adsorbed on the active sites. In a previous studies carried out by the authors, the use of bimetallic Pt-Sn/Al2O3 and trimetallic Pt-Sn-In/Al2O3 catalysts, for the dehydrogenation of low sulfur Jet A-1 has been investigated. Comparing the reaction on the bimetallic catalyst, with the study by TailladesJacquin et al.30, carried out at the same conditions with a very similar catalyst, a considerable


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increase in the H2 evolution (99 mmol*h-1*gcat-1 instead of 15 mmol*h-1*gcat-1) and stability is observed (after 4 h TOS, 17% lost activity instead of 57%). Catalysts with such similar characteristics at the same reaction conditions should display similar activity. This substantial difference is explained by the preparation method: the catalyst giving better results was prepared by co-impregnation of Pt and Sn, while the other by successive impregnations of Sn and Pt. The co-impregnation solution containing Pt, Sn and HCl leads to the formation of the complex PtCl2(SnCl3)2-, which induces a more homogeneous dispersion of Pt and Sn atoms on the surface during the impregnation phase. As consequence, during the final catalyst thermal treatment, the formation of bimetallic Pt-Sn cluster and alloys is increased. The use of the trimetallic catalysts allow a further increase in stability and activity, showing, at the optimal metal weight ratio of 1:1:0.5 (Pt:Sn:In), a hydrogen evolution of 122 mmol*h-1*gcat-1 and a deactivation of 10% in the first 4 hours. The authors prove that In have an electronic interaction with Pt and Sn and suggest an increase in the formation of Pt-Sn alloys, as well as the possibility of forming Pt-In alloys. The suppression of reactions leading to coke formation and an increased coke migration from the active phase to the support, is detailed with characterization of spent catalysts and coke deposit. Most studies use a desulfurized fuel as feed to limit catalyst poisoning. To investigate the combined effect of coking and sulfur poisoning, a study31 was conducted using commercial Jet A-1 (S = 228 ppmw) and fractions obtained by its distillation. The investigation show how with a rectification step before the partial dehydrogenation reaction it is possible to reduce the sulfur content and increase the catalyst activity and stability. Feeding the commercial Jet A-1 the activity after 4 h TOS is 13 mmol*h-1*gcat-1 with a 32% loss of activity in that time span. When using the lighter fraction of kerosene (lighter 5 wt% of Jet A-1) the sulfur content is reduced to 5


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ppmw, therefore increasing considerably the activity and the stability (after 4h TOS, 54 mmol*h1

*gcat-1, 13% loss of activity). The deactivation by sulfur poisoning has been attributed to the

formation of PtS and SnS2 species, which is a partially reversible process and the equilibrium depends on the temperature, on the feed sulfur content and on the H2 partial pressure. The deactivation by carbon coke deposition is also decreased, as the lighter fractions exhibit a lower tendency to form coke at this reaction conditions. The analysis carried out on the partially dehydrogenated fuel fractions show small changes in composition (mainly cyclic converted to aromatic), to an extent that allow the spent fuel to be used in an internal combustion engine. Further investigation on thio-tolerant catalysts for fuels dehydrogenation have been carried out by Albonetti et al.151 that studied Ni2P and CoP based catalysts for the dehydrogenation of commercial Jet A-1 (sulfur content 250-500 ppmw). These phosphatized metal are known for their good activity in hydrotreatment process in presence of sulfur rich mixtures. In this study the support used is a low acidity SiO2 (Cab-O-Sil). All the catalysts display a good initial activity towards dehydrogenation, but in the course of reaction the selectivity to H2 drops significantly. For the catalyst 5 wt% Ni2P-0.5 wt% K/SiO2 , showing the higher hydrogen evolution ratio that is stable at 44 mmol*h-1*gcat-1 for 24 h, but the purity drops from 92.7% to 75.3% throughout the 24 h TOS. The drop in selectivity towards hydrogen, is attributed to catalyst sulfur poisoning with the formation of NiS. Nickel sulfide particles have low activity in the dehydrogenation, but it can in some extent catalyze hydrocracking and cracking reactions153, leading to a decrease in hydrogen purity. Nevertheless, these results show the possibility of producing CO free hydrogen from fuels containing considerable amounts of sulfurous compounds. Most studies concern the partial dehydrogenation of kerosene, but some investigations that utilize other fuels are reported29,154. A study was made of the feasibility of partial


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dehydrogenation of gasoline and diesel, using a Pt-Sn/Al2O3 catalyst. The catalytic process conditions were optimized for each fuel, since, as also observed with kerosene, the hydrogen selectivity is strongly affected by the fuel composition, temperature and contact time. The best results of hydrogen rate are registered for the gasoline SP95 surrogate at 400 °C, 10 bar and 7 vol% of hydrogen recycling, with 80 mmol*h-1*gcat-1 and 99.3 vol% purity after 4 h TOS. Although this results is encouraging, the surrogate used does not contain oxygenated compounds that are present in commercial SP95 in considerable amounts (minimum 2.3 wt%)155. The presence of oxygen in the fuel may lead to formation of CO and CO2, which would provide a constraint in the use of the product to feed a fuel cell without intermediate purification steps. The H2 rates obtained with the diesel surrogates are considerably higher than with gasoline, displaying a maximum of 147 mmol*h-1*gcat-1 at 400 °C, 10 bar and 7 vol% of hydrogen recycling, with 99.6 vol% purity after 4 h TOS. On the other side the deactivation by coking observed with diesel surrogate is more rapid than for gasoline. The presence of larger hydrocarbons, in particular the products of complete dehydrogenation of bi-cyclic molecules (naphthenes), lead to the formation of high amounts of organized coke on the active phase, via a polycondensation mechanism. This effect was not observed for the gasoline surrogate, containing smaller hydrocarbon and mono-cyclic compounds (C6-C8). A study carried out by the authors, prove the possibility of producing hydrogen by PDH of commercial gasoline with a similar catalyst. A Pt-Sn-In/Al2O3 was used in the dehydrogenation of SP95 and SP95E10 gasoline. The optimal pressure for gasoline dehydrogenation appears to be in the range 5-10 bar and the temperature in the range 350 °C – 400 °C. Increasing temperature tends to increase the amount of gas produced but the hydrogen purity decreases as consequence of CO and light hydrocarbons formation. The high content of oxygenated compounds in the


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gasoline, which are mostly ethers like MTBE for SP95 and bioethanol for SP95E10, lead to the formation of CO via ether decomposition or ethanol dehydration mechanism. The choice compromise between production rate and purity was obtained for both SP95 and SP95E10 at 370°C, 8 bar, GHSV = 3600 h-1 and a feed composition of 93 vol% gasoline/ 7 vol% H2. The H2 evolution ratio for SP95 and SP95E10 are respectively 105 mmol*h-1*gcat-1 and 121 mmol*h1

*gcat-1, with purity of 84.9 vol% (3.8 vol% CO) and 92.3 vol% (2.5 vol% CO). In this study,

long term reactions of 150 hours were carried out, with start and stop procedure each 6 hours, in order to evaluate the feasibility of an intermittent hydrogen demand. This work highlights the possibility of recovering the initial activity during the stop procedure; leaving the catalyst under H2 pressure overnight favor the migration of coke deposit from the active phase towards the support and a part of it is eliminated by methanization. This allowed to run the reactions for 150 hours TOS with very low activity losses of -15% and -9% for SP95 and SP95E10 respectively. Analyses on the partially dehydrogenated SP95E10 show that the physio-chemical properties were not altered considerably and that the combustion enthalpy is increased with respect to the original fuel. This result is very encouraging for the possibility of using the dehydrogenated gasoline as fuel.

7. CONCLUSIONS AND OUTLOOK Various classes of liquid organic hydrogen carriers were compared in this review, including cycloalkanes, heterocyclic hydrocarbons, hydrocarbon mixtures and transportation fuels. In addition to a comparison between the properties of the various substrates, such as hydrogen content, boiling point and dehydrogenation enthalpy, an analysis of the catalyst


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development and reactor design was carried out for hydrogen generation by catalytic dehydrogenation. For successful implementation of a hydrogen delivery/storage cycle the LOHCs must be a non-toxic stable liquid both in their hydrogen rich and hydrogen lean forms. Besides, the hydrogen content must be high enough (> 5.5 wt%) while maintaining a low dehydrogenation enthalpy (

High-Purity Hydrogen Generation via Dehydrogenation of Organic

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