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Hydrogen delivery through liquid organic hydrides:Considerations for a potential technology

Anshu Shukla, Shilpi Karmakar, Rajesh B. Biniwale*

National Environmental Engineering Research Institute, Council of Scientific and Industrial Research, Environmental Materials Unit,

Nehru Marg, Nagpur, Maharashtra 440020, India

a r t i c l e i n f o

Article history:

Received 27 February 2011

Received in revised form

12 April 2011

Accepted 13 April 2011

Available online 17 May 2011

Keywords:

Hydrogen transportation

Cycloalkanes

Dehydrogenation

Clean energy

Catalysts

* Corresponding author. Tel.: þ91 712 224988E-mail address: rb_biniwale@neeri.res.in

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.04.107

a b s t r a c t

Carrying hydrogen in chemically bounded form as cycloalkanes and recovery of hydrogen

via a subsequent dehydrogenation reaction is a potential option for hydrogen transport

and delivery. We have earlier reported a novel method for transportation and delivery of

hydrogen through liquid organic hydrides (LOH) such as cycloalkanes. The candidate

cycloalkanes including cyclohexane, methylcyclohexane, decalin etc. contains 6 to 8 wt%

hydrogen with volume basis capacity of hydrogen storage of 60e62 kg/m3. In view of

several advantages of the system such as transportation by present infrastructure of

lorries, no specific temperature pressure requirement and recyclable reactants/products,

the LOH definitely pose for a potential technology for hydrogen delivery. A considerable

development is reported in this field regarding various aspects of the catalytic dehydro-

genation of the cycloalkanes for activity, selectivity and stability. We have earlier reported

an account of development in chemical hydrides. This article reports a state-of-art in LOH

as hydrogen carrier related to dehydrogenation catalysts, supports, reactors, kinetics,

thermodynamic aspects, potential demand of technology in field, patent literature etc.

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction cyclohexane, methylcyclohexane and decalin are listed in

In order to meet the ever increasing energy demand without

causing further damage to the environment, zero carbon

emission fuel such as hydrogen is required [1,2]. There is

consensus on the hydrogen as a clean energy option. An

efficient method for hydrogen storage, transportation and

delivery to point of usage is a prerequisite for any hydrogen-

fueled energy system [2]. Among wide variety of hydrogen

storage technologies liquid organic hydrides provide several

advantages such as relatively higher hydrogen capacity on

both the weight and volume basis [1,2]. The candidate liquid

organic hydrides reported comprise cyclic alkanes such as

methylcyclohexane, cyclohexane, decalin, etc. The physico-

chemical properties and hydrogen storage capacities for

5, þ91 9822745768(mobile(R.B. Biniwale).2011, Hydrogen Energy P

Table 1.

Considering the boiling point and melting point, cyclo-

alkanesare inliquidphaseatambientconditionswithprevailing

temperature of 20e40 �C. This facilitates the transportation of

cycloalkanes using simple transport means such as lorries.

Further, properties of methylcyclohexane, as one of the candi-

date media for hydrogen storage, (Table 2) are comparable to

that of gasoline and diesel whichmakes it possible to transport

using present fuel transportation methods.

Due to simple reaction mechanism, the dehydrogenation

reaction is considered as favorable process for hydrogen

abstraction from cycloalkanes. The mechanism involves

adsorption of cycloalkane over metal catalyst (particularly Pt)

with either simultaneous or rapid subsequent dissociation of

).

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

Table 1 e Hydrogen storage capacities for cycloalkanes such as cyclohexane, methylcyclohexane and decalin and theirphysical properties.

Sr. No. Properties Cyclohexane Methylcyclohexane Decalin

1. Melting point (�C) 6.5 �126.6 �30.4

2. Boiling point (�C) 80.74 100.9 185.5

3. Density (g/ml) 0.779 0.77 0.896

4. Standard formation enthalpy in

dehydrogenation of cyclic hydrocarbons (101.3 kPa) DH (kJ mol�1)

þ205.9 þ204.8 þ319.5

5. Theoretical hydrogen storage- weight basis (%) 7.2 6.2 7.3

6. Theoretical hydrogen storage evolume basis (1028 mol/m3) 3.3 2.8 3.8

8. Product of dehydrogenation Benzene Toluene Naphthalene

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 3 7 1 9e3 7 2 63720

hydrogen atoms via pi-bond formation [3]. Dehydrogenation

results in formation of hydrogen and aromatic. The reaction is

highly selective over Pt catalysts and there have been reports for

formation of no by-products or partially dehydrogenated prod-

ucts. Dehydrogenation and its subsequent hydrogenation of

aromatics are easily reversible [1, 2, and 4]. A typical concept of

transportationofhydrogen fromhydrogenproduction facility to

fueling station using methylcyclohexane is depicted in Fig. 1.

Thus, in view of this researchers are trying to demonstrate

hydrogen storage and supply using liquid organic hydrides

mainly cycloalkanes such as cyclohexane, methylcyclohex-

ane and decalin. Hodoshima and co-workers reported that the

decalin should be considered as a potential candidate for fuel

cell vehicles [5]. The advantages related to decalin are inves-

tigated by Lazaro and group as negligible evaporation loss, low

toxicity, no parallel reactions, low cost and low dehydroge-

nation energy which allows reaction to perform at relatively

low temperatures [6]. Themethylcyclohexane exhibits similar

advantages as a candidate medium. The advantages expla-

ined by Oda and group include relatively high boiling point

and no carcinogenic products are formed [7]. Hodoshima and

co-workers also reported that the tetralin can be a potential

option over decalin due to 4e5 times higher reaction rate

under superheated liquid film condition [8].

In an earlier article we have given an account of chemical

hydrides in general and LOH in particular for hydrogen storage

and supply [2]. This article covers the state-of-art develop-

ment on several other aspects such as kinetics, thermody-

namics, patent-review regarding LOH technology.

2. Dehydrogenation catalysts

Several catalysts are reported for dehydrogenation of cyclo-

alkanes. Coughlan et al. in 1990 reported that the dehydrogena-

tionofcyclohexaneonnickelexchangedY-zeolitesproceededvia

Table 2 e Comparison of properties of methylcyclohexane wit

Sr. No. Properties D

1. Reid vapor pressure (psi) 0.2

2. Auto ignition temperature (�F) 446

3. Flash point (�F) 165

4. Peak flame temperature (�F) 372

5. Density (g/cc) 0.8

6. Flammable limit in air LELeUEL 1.3

series of consecutive dehydrogenation steps [9]. Kobayashi et al.

reported Pt/Al2O3 as an efficient catalyst for dehydrogenation of

iso-propanol [10]. In an earlywork related to this reaction, 3.82wt

% Pt/PCC (Pt supported on petroleum coke carbon) catalyst was

studied for dehydrogenation of methylcyclohexane and decalin

byKariyaet al. [4]. They concluded that themajorproperties such

as hydrogen spillover and hydrogen-recombination to be taken

into consideration while selecting the catalysts. The same group

in 2003 reportedmethylcyclohexanedehydrogenation over 10wt

% Pt/ACC (Pt supported on activated carbon cloth) wherein the

hydrogen evolution rate was reported as 0.52 mmol/gmet/min at

298 �C [1]. They proved that the particle size of catalyst have

profound effect on dehydrogenation reaction. Hodoshima and

group in one of the report concluded shorter Pt distance with its

neighbor can affect dehydrogenation of decalin. They even

proposed tomodify catalystandstudy theeffectofalkali reagents

on catalytic activity and fine structure of Pt species [11].

Bi-metallic catalysts have shown considerably higher

activity for dehydrogenation than monometallic catalysts as

has been reported by Ichikawa and group [4]. They concluded

that PteMo/PCC, PteW/PCC and PteRe/PCC exhibit compara-

tively better hydrogen evolution rate than monometallic Pt/

PCC for cyclohexane dehydrogenation. They also suggested

that physical mixing of Pd/PCC and Pt/PCC improves catalytic

activity, since Pd helps in suppressing reverse reaction and Pt

helps in hydrogen-recombination ability. These conclusions

are based on the hydrogen evolution activity and there is no

evidence of arrangement of metal presence on support.

Hodoshima and co-workers explained catalyst selection is

based on its ability to CeH dissociation for alkane and inves-

tigated PteRe/AC (PteRe supported on activated carbon) to

have excellent activity for dehydrogenation of decalin with

nearly 100% conversion [12]. In the same year 2005, Hodosh-

ima et al. have reported dehydrogenation of tetralin over

carbon supported on NieRu and suggested that this can be

alternate catalyst to costly Pt. Biniwale et al. have reported an

h gasoline and diesel.

iesel Gasoline Methylcyclohexane

8e15 1.61

572 482

�45 25

9 3591

3 0.75 0.81

e6.0% 1.4e7.4% 1.1e6.7%

Fig. 1 e Concept for hydrogen transportation using

methylcyclohexaneetoluene pair.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 3 7 1 9e3 7 2 6 3721

enhancement in the activity of catalyst by addition of a small

amount of Pt to the Ni based catalyst. Activated carbon was

used as the support with synergistic effect of NiePt catalysts

(with Ni:Pt ratio of 40:1 on the weight basis) [13].

Okada and co-workers reported 0.1 wt% k þ 0.6 wt% Pt/

Al2O3 catalyst with a view to perform the reaction at relatively

lower temperature [14]. The initial reactions were reported at

nearly 375 �C to avoid coke formation on the catalyst surface.

They have investigated use of g-alumina as a support for Pt

and optimized pH for impregnation of Pt for preparation of the

catalyst. The catalyst was reported with relatively higher

hydrogen evolution rate of about 744mmol/gmet/min. Further,

a hydrogen evolution rate of 958 mmol/gmet/min at 90 min

with 350 �C was reported during dehydrogenation of methyl-

cyclohexane over a proprietary catalyst [15].

Dehydrogenation of methylcyclohexane on partially

reduced metal oxide namely MoO3 has been reported earlier

[16]. This can be a cost effective catalyst option for dehydro-

genation reaction, as the reaction is performed in absence of

noble metal catalyst. They have reported that the catalytic

dehydrogenation of methylcyclohexane prevails at relatively

higher temperatures of 300 �C and 380 �C. They report metal

sites present in MoO2(Hc)ac are responsible for the catalytic

dehydrogenation of methylcyclohexane.

Accordingly, Pt based catalysts are reported for having

higher activity and selectivity. The literature seems to drive

the catalyst’s search toward bi-metallic catalysts exploring

hydrogen spillover and recombination properties of two co-

existing different metals [17]. The attempts to use Pt in

combination with Ni, Pd etc. have been reported as the

potential catalysts. The benchmark activity of 744 mmol/g/

min in terms of hydrogen evolution rate and selectivity of

about 99% for catalyst is reported with use of a fixed bed

reactor system and bi-metallic catalyst [14]. We have reported

a relatively higher activity in terms of hydrogen evolution rate

of 958 mmol/g/min [16].

Although, several efficient bi-metallic catalysts have been

reported with their detailed characterization and possible

synergistic effects none of the reports describes the mecha-

nism by which dehydrogenation is improved over a particular

catalyst. There is a scope for fundamental and theoretical

studies providing direct evidences from characterization to

explain and guide the further designing of the catalysts.

3. Selection of supports for catalysts

Dehydrogenation reaction is particularly favorable on the

well-dispersed catalysts. When a reforming catalyst such as

Ni is used the lower dispersion may lead to side reactions

such as hydrogenolysis [13]. The supports used for disper-

sion of metal catalysts have a major role to play in the

dehydrogenation reactions. Various materials that can be

employed as supports for metal catalysts include carbon

materials, metal oxides, perovskites, zeolites, silica etc

[1,2,4,5,6,15,16,17,18,19]. Due to high surface area and

inertness to the side reaction carbon based supports such

as activated carbon granules and activated carbon cloth are

widely studied by various groups [1,2,5].

Kariya et al. and Biniwale et al. reported that the conduc-

tive support like alumite (alumina layer formed through

surface oxidation of aluminum using anodization) favors high

and uniform catalyst surface temperature [1,20]. Lazaro and

co-workers have reported that 0.25 wt% Pt/CNF is effective for

dehydrogenation of cyclohexane [6]. They have claimed that

similar activity and selectivity was exhibited by 0.25 wt% Pt/

CNF catalyst as compared to 1 wt% Pt/Al2O3. This can be

attributed to better dispersion on Pt and the open structure of

CNF/CNTwhich allows the liquid to have easy accessibility for

catalyst sites and desorption of product is favored.

Okada et al. reported that thedeactivationof catalyst canbe

suppressed by high dispersion of Pt on pore controlled g-Al2O3

[14]. They also reported that the pH for impregnation has to be

optimized for chloroplatinic acid. A pH swing method was

applied for controlled pore distribution of g-Al2O3. One of the

reports wherein, dynamics of hydrogen spillover on carbon

based materials is discussed. Reports suggest metal oxide

favors small energy barrier of H-migration from catalyst to

substrates and favors subsequent proton diffusion [15]. Belatel

et al. investigated determination of fermi level for explaining

metallic character of the catalyst for reduced MoO2 [17]. They

explained that the dissociation of hydrogen can take place on

reduced MoO2 which can also lead to formation of Bronsted

MoeOH group. Thismechanism is not favored in carbon based

materials and thus, hydrogen spillover is suppressed. A study

reports metal oxides as support for Pt favoring hydrogen

spillover and in turns lowers the thermodynamic equilibrium

and thus performs better that activated carbon [15]. Biniwale

and group studied various metal oxide supports for Pt. The

different metal oxides supports studied include TiO2, La2O3,

CeO2, ZrO2, Fe2O3, Al2O3, MnO2 and LaNiO3. Further, perovskite

type oxides have been used as support for Pt catalysts. They

found1wt%Pt/La0.7Y0.3NiO3 exhibit hydrogenevolution rate of

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 3 7 1 9e3 7 2 63722

45 mmol/gmet/min during dehydrogenation of methyl-

cyclohexane at 350 �C [15].

From the literature, the conclusion can be drawn that the

metal oxides are potential support for metal catalysts for dehy-

drogenation of methylcyclohexane. A strong metalesupport

interaction helps in better hydrogen spillover and therefore

improving forward rate of reaction. Further, metal oxides under

reduced conditions form additional active sites on the surface

for dehydrogenation reaction.

4. Kinetic aspects of dehydrogenation ofcycloalkanes

Ichikawa and co-workers reported that the rate of reaction

dehydrogenation of cycloalkanes varies profoundly with

change in temperature, supports, initial feeding rates and use

of bi-metallic catalysts [4]. They studied the rate constant (k)

for dehydrogenation of cyclohexane and decalin and found

increase in rate constant with increase in temperature. The

retardation constants (K) observed to be declining with

increase in temperature. They suggested that this was due to

reduced adsorption of aromatics (product) on catalytic surface

at higher temperature. They also studied the various Pt based

catalysts supported on PCC, CC, Al2O3, FSM-16, HZSM-5 for

estimating rate constants for dehydrogenation. The variations

in support for Pt catalyst suggest Pt/PCC and Pt/CC resulted in

higher value for (k) with the feed of cyclohexane at 1.2 and

1.10 ml. The value for (k) with Pt/PCC at 1.20 ml was observed

as 4.5 mmol/min. The rate constant (k) calculated for Pt/CC

with the reactant of 1.10 ml was 4.3 mmol/min. The use of

different active carbons results in variation in reaction rate

depending on their particle size distribution, pore size distri-

bution and surface area, etc. They also reported that there is

no specific co-relation between rate of reaction and nature of

carbon. Very low rate of reaction for Pt/FSM, Pt/HZSM-5 and

Pt/Al2O3 catalysts was observed. In context with reactant feed,

the initial feeding rate of cyclohexane andmethylcyclohexane

has been reported to have profound effect on the reaction rate

constant. However, in the case of decalin initial feeding rate

was not found to have significant effect. Thus, retardation

constant data as explained by Ichikawa’s group; suggest dec-

alin have higher retardation constant (K) value than methyl-

cyclohexane and cyclohexane. This is because of naphthalene

adsorption property over catalytic surface. The adsorption of

naphthalene resulted in blockage of active catalytic sites.

Hodoshima and co-workers also found declination of the

reaction rate with use of decalin and gave same conclusion of

naphthalene adsorption over catalytic sites. Kariya et al.

reported Langmuir-type equation for reaction rate (�rA) and

rate constants (k) [1]. They discussed change in temperature of

the catalysts surface under alternate wet-dry reaction condi-

tions with conclusion that the major reason for decrease in

temperature is due to evaporation of liquid reactant droplets

on the catalysts surface. They reported negligible retardation

reaction rate (K) and suggested that the dehydrogenation

reaction to be first order reaction [1]. In an article Hodoshima

and group reported kinetics for dehydrogenation of tetralin in

superheated and liquid film state. They found higher value for

rate constant (k) and lower retardation constant (K) [12]. They

also suggested that higher reaction rates can be achieved by

sharp temperature gradient at catalystereactant interface.

The continuous removal of adsorbed hydrogen from catalysts

surface can also help in achieving higher forward rate of

reaction. Biniwale and co-workers, studied the dehydrogena-

tion of methylcyclohexane with Pt supported on different

metal oxides and reported reaction to be zero-order reaction

[15]. They found no difference in value of rate constant (k) for

Pt/LaNiO3 and Pt/La0.7Y0.3NiO3. The substitution of yittrium at

A-site of LaNiO3 has effect only on selectivity toward

hydrogen and toluene.

From the above discussions it can be concluded that higher

temperature of catalyst surface improves kinetics of the

reaction. The higher reaction temperature helps in rapid

removal of hydrogen from the surface of catalyst avoiding

blockage of active sites. The rate of reaction may be altered by

change in support used for the Pt catalyst.

5. Thermodynamics aspects ofdehydrogenation of cycloalkanes

Okada et al. reported that the dehydrogenation of decalin is

comparatively easier than methylcyclohexane and cyclo-

hexane [14]. Methylcyclohexane with a side chain is easily

dehydrogenated than cyclohexane as reported by Okada and

co-workers. They suggested that for the same equilibrium

conversion the temperature required for decalin is less than

as required for methylcyclohexane and cyclohexane. They

also reported that 99% conversion of methylcyclohexane can

be obtained with temperature maintained around 330 �C.Lazaro et al. reported a relation of the theoretical thermody-

namic equilibrium compositions vs. reaction temperature

using Gibbs energy minimization method [6]. They reported

data on decalin conversion with respect to various tempera-

tures suggesting that the increase in reaction temperature

increases thermodynamic conversion. They have estimated

that a 90% conversion of decalin is achievable at temperature

of ca 265 �C. As an optimum temperature they performed

dehydrogenation of decalin at ca 240 �C. Since the dehydro-

genation of cycloalkanes is endothermic reaction it is favor-

able at higher temperatures and the equilibrium conversion is

achievable with reaction temperatures in the range of

250e375 �C depending on the reactant, catalysts and reactor

system used.

6. Reactor systems

Dehydrogenation being endothermic reaction demands

supply of heat [1e2 and 4]. Thus, different reactor systems are

being developed and demonstrated by many groups. The

steady and unsteady state reactors have been studied for

dehydrogenation reaction. The advantages and disadvantages

for different reactor system employed for dehydrogenation of

cycloalkanes are shown in Table 3.

Initially a couple of studies reported steady state reactors

for dehydrogenation reactions [14,18]. Newson and co-

workers reported membrane reactor using Pd based catalyst

for hydrogen storage [5].

Table 3 e The strengths and weakness for different reactor systems employed for dehydrogenation of cycloalkanes.

Sr. No. Reactors Strengths Weakness

1. Batch reactor High conversion is achieved Liquid reaction is favored

2. Fixed bed reactor Continuous product formation Favors reverse reaction and

liquid phase reaction

3. Wet-dry multiphase system

or spray pulse system

1) Efficiency of reactant supply is high Commercial feasibility process,

intensification of process is crucial,

considering available volume in vehicle or

portable device is very limited

2) Catalyst is kept at high temperature

3) Suppression of reverse reaction

4. Monolithic reactor system 1) Structured catalyst reactor No direct experimental data available

2) Avoidance of granular catalyst

3) High catalytic geometric surface area,

maximizing working volume

4) Alternate wet-dry condition and thin liquid film

5) Regeneration step can be included

6) Exploitation of observed initial

5. Micro-reactor 1) Compact design Complicated design

2) Overcomes heat transfer limitation

6. Membrane reactor Reaction and separation carried out in one step Higher cost and maintenance

7. Liquid-film state condition Suspension of liquid reactant

with catalyst can be avoided

Thin film liquid always covers the

catalyst surface and does not favor drying

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 3 7 1 9e3 7 2 6 3723

The unsteady state reaction mechanism as thin liquid film

was reported by Kameyama and co-workers for dehydroge-

nation of iso-propanol on Pt/Al2O3 at 95 �C [10]. In order to

improve the conversion of cycloalkanes several attempts have

been reported employing unsteady state reactor systems.

In this context, Ichikawa and co-workers studied the

dehydrogenation of cycloalkanes using a spray-pulsed reactor

and reported optimization of parameters such as tempera-

ture, reactants, support, monometallic-bi-metallic catalyst,

and reactant/catalyst ratio [4]. Also, Bordeje and group

proposed a rotating monolithic reactor system so as to over-

come complexities of spray-pulse reactor system [6]. Ichikawa

and group reported use of wet-dry multiphase condition

wherein reactant and catalyst contact is effectively achieved

[4]. They also explained how this method is advantageous

over conventional method of reactant flow. The higher rate of

reaction for methylcyclohexane was reported as compared to

dehydrogen of decalin. It is explained that the dehydro-

genated product from decalin i.e. naphthalene requires higher

energy for removal from the catalyst surface due to its high

boiling point and affinity toward carbon. They proved efficacy

of reactant catalyst contact was improved, catalyst main-

tained a high temperature which helped in avoiding reverse

reaction. However, the rate of production of hydrogen was

dependent on reaction conditions like reactant feed rate,

temperature and catalyst support.

The same group studied the reaction with spray pulse

mode of reactant flow. The hydrogen production is dependant

on rate of reactant flow. Hodoshima and co-workers

concluded superheated liquid film conditions can be consid-

ered as better option than conventional batch process over

wide ranges of feed rate for decalin [17]. Roumanie et al.

demonstrated the development of silicon micro-structured

reactor for dehydrogenation of methylcyclohexane [19]. They

reported Pt/Al2O3 prepared by conventional method exhibited

better performance than that made by vapor deposition of

platinum film (PVD). They suggest the problem of elevated

endothermic heat of reaction can be resolved by proper design

of a micro-structured reactor with height in the range of

100 mm. The uniform temperature was successfully main-

tained at 400 �C with a preheating zone at 80 �C, using such

micro-reactor. Okada and co-workers reported the use of fixed

bed reactor with higher hydrogen evolution rates [14]. Ichi-

kawa and co-workers studied dehydrogenation of cyclo-

hexane with a different view of thermographic analysis [20].

Herein, they recorded the temperature profile of reaction

temperature and estimated heat transfer flux under transient

conditions. They investigated in wet-dry reaction condition of

reactant on catalyst surface, the catalyst surface experiences

phase change for reactant due to evaporation. This resulted in

better heat transfer due to alternate wet-dry conditions

formed on the surface with better solid-liquid contact. Lazaro

et al. reported successful dehydrogenation of decalin over Pt/

CNF (Pt supported on carbon) using fixed bed reactor and

proposed use of rotatingmonolithic reactor system [6]. Shukla

et al. reported the dehydrogenation of methylcyclohexane

with Pt supported on different metal oxides using spray pulse

mode of reactant flow [15]. In succeeding report, they reported

nearly 100% conversion of methylcyclohexane with consid-

erable high hydrogen evolution rate of about 968 mmol/gmet/

min [16]. A high purity hydrogen production using amorphous

silica membranes with membrane reactor was reported by

Oda et al. [7]. They reported one step method to produce

hydrogen from methylcyclohexane. In 2010, Oda et al.

demonstrated hydrogen generationwith high purity of 99.95%

in absence of sweep gas or carrier gas using membrane

reactor. They reported the activity was almost similar to the

simulated data.

Accordingly, a considerable improvement in the cata-

lytic reactors for carrying out endothermic dehydrogena-

tion reactions has been reported in the literature. Further

scope of improvement relates to minimizing the heat

requirement by improvement in heat transfer and

improving the surface area of the catalysts. Up-scaling of

reactor and its study for dehydrogenation of cycloalkanes

also need to be pursued.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 3 7 1 9e3 7 2 63724

7. Theoretical studies

There are limited theoretical studies wherein hydrogen

adsorption on different materials is reported. The hydrogen

spillover on to carbon based materials is well studied by

Cheng and co-workers. They concluded that the spillover of

hydrogen is favored onto nanostructured carbon materials

only if H-atom generated via dissociation by metal catalyst

remains physisorbed. They reported molecular dynamics

(MD) calculations revealed efficacy of graphene materials can

be improved by using carbon materials with curved surfaces.

Chen et al. reported hydrogen spillover on the MoO3 in pres-

ence of Pt catalyst using periodic density functional theory

(DFT) [21]. They found relocation of adsorbed H-atom from Pt6to MoO3 (010) to go through a transition from repulsive elec-

trostatic to attractive protoneoxygen interactions. There is

a need for understanding of catalytic dehydrogenation

phenomena at atomistic level.

8. Patents on hydrogen storage by chemicalhydrides

Various methods for hydrogen storage and delivery have

been patented. A summary of the patents available is listed in

Table 4. Patents have been claimed on the basis of dehydro-

genation catalyst, reactants, reactor system, and reaction

conditions. Patents claimed on various chemical hydrides as

reactants include the use of solid hydrides, liquid hydrides

and liquid organic hydrides. Various solid hydrides like

lithium hydride, magnesium hydride has been reported for

hydrogen storage, magnesium based alloys have been repor-

ted for storage of hydrogen [22].

Table 4 e Summary of patents available for hydrogen storage,

Month/year Patent no.

Jan 2010 US 2010/0010280 A1 Catalyst for

May 2009 US 2009/0118557 A1 Reactant deh

April 2008 US 7351395 B1 Hydrogen st

Sep 2008 US 7429372 B2 Hydrogen st

June 2006 US 2006/0135831 A1 Dehydrogen

July 2006 US 2006/0143981 A1 Dehydrogen

Jan 2006 US 2006/0009668 A1 Process for t

Sep 2006 US 7,101,530 B2 Hydrogen st

Jan 2005 US patent application 2005002857 A1 Hydrogen st

June 2005 US 2005/0119515 A1 Partial dehy

Oct 2004 US patent application 20040199039 Dehydrogen

April 2004 US patent application 20040074759 Catalytic pro

Jan 2003 US patent application 20030014917 Chemical hy

incorporatin

May 2003 US patent application 20030091876 Chemical hy

a common h

May 2003 US patent application 20030091879 Chemical hy

incorporatin

June 2003 US patent application 20030113259 Chemical hy

May 2002 JP-2002134141 Hydrogen st

and supply b

April 2001 JP-2001110437 Hydrogen fu

A Japanese patent No. JP20001110437 describes catalytic

production of hydrogen from benzene, toluene, xylene,

mesitylene, naphthalene, anthracene, biphenyl, phenan-

threne and their alkyl derivatives by using Pt supported on

high surface area substrate as catalyst [23]. US Patent Appli-

cation 20050002857 explains the process where extended pi-

conjugated substrates are used to store and release

hydrogen by means of reversible catalytic hydrogenation [24].

These extended pi-conjugated substrates includes large

polycyclic aromatic hydrocarbons, polycyclic aromatic

hydrocarbons with nitrogen hetero-atoms, polycyclic

aromatic hydrocarbons with oxygen hetero-atoms, polycyclic

aromatic hydrocarbons with alkyl, alkoxy, nitrile, ketone,

ether or polyether substituent. The hydrogen storage capacity

was relatively low in these chemicals for their economical use.

A process describing a hydrogen generation system including

an energy system have been stated in United States Patent

Application 20030014917, wherein they have used a group of

chemical hydride solute consisting of: NaBH4, LiBH4, KBH4,

RbBH4 to react with water in the presence of a catalyst (Ru, Co,

Pt or alloy thereof) to generate hydrogen [25]. The chemical

hydride has a shelf life under alkali conditions where alkaline

additive used is 0.1%NaOH and the chemical hydrides can be

stored as well [26]. Another patent on liquid organic hydrides

made to US Patent No. 6,074,447 describes a process of dehy-

drogenation ofmethylcyclohexane, decalin, dicyclohexyl, and

cyclohexane to toluene, naphthalene, biphynl and benzene,

respectively, in the presence of particular iridium based

molecular complex catalyst at preferably 190 �C or higher [27].

In an attempt to design reactors for endothermic dehydro-

genation reaction US patent application no. 20060143981

describes the use of micro channel catalytic reactor for dehy-

drogenation of liquid fuel for hydrogen generation. They have

used reactants including hydrogenated form of extended pi-

transportation and delivery using chemical hydrides.

Title

dehydrogenation of hydrocarbon

ydrogenation of alkyl aromatics

orage by reversible hydrogenation of pi-conjugated substrates.

orage by reversible hydrogenation of pi-conjugated substrates.

ation process

ation of liquid fuel in micro channel catalytic reactor

he dehydrogenation of an unsaturated hydrocarbon.

orage by reversible hydrogenation of pi-conjugated substrates.

orage by reversible hydrogenation of pi-conjugated substrates

drogenation method using continuous heterogeneous catalyst

ation reactions in narrow reaction chambers and integrated reactors

cess for the treatment of organic compounds.

dride hydrogen generation system and an energy system

g the same

dride hydrogen generation system and fuel cell stack incorporating

eat transfer circuit

dride hydrogen generation system and an energy system

g the same

dride hydrogen reactor and generation system

orage and supply system and liquid organic hydrogen storage

ody.

el supply system for fuel cell.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 3 7 1 9e3 7 2 6 3725

conjugatedsubstratewithhetro-atomsother thannitrogen,pi-

conjugated monocyclic substrates with multiple nitrogen

hetro-atoms, pi-conjugated organic polymers and oligomers

and ionic pi-conjugated substrate and polycyclic aromatic

hydrocarbon [28]. The reaction temperature reported for

dehydrogenation in this patent was in the range of 60e300 �C.The process described involves reversible catalytic hydroge-

nation and dehydrogenation of the liquid organic compound

followed by separation of the liquid phase dehydrogenated

organic compound and gaseous hydrogen and their recovery.

Many catalysts are reported to be used as hydrocarbon

conversion catalyst like Pt (0.1e5 wt%)-second metal alloy

(secondmetalmaybeCo,Ni, Fe, Cu, Sn, Pd, Cd, Ir, Rh, Ru, Ag, Bi,

Hg, Pb) supported on a refractory support by impregnation or

ion exchange technique. They have been employed for dehy-

drogenation of C3eC25 hydrocarbons and the patent relates to

platinum containing catalyst and their use for the catalytic

conversion of hydrocarbons [29]. A Japanese patent JP-

2002134141 describes catalytic dehydrogenation of aromatic

derivatives and hydrogenation of the respective aromatic by

catalyst containing at least onemetal fromNi, Pd, Pt, Rh, Ir, Ru,

Mo, Re,W, V, Os, Cr, Co and Fe [30]. A US patent applicationNo.

20100010280 demonstrates a stationaryor fluid bed catalyst for

dehydrogenation of hydrocarbons containing an alumina

carrier, with chromium and alkali metals consisting of only

sodium and potassium oxides, added as promoters [31]. US

Patent Application 20040199039, reports reactor configuration

for dehydrogenation of propane and isobutane to propylene

and isobutene, respectively in narrow reaction chamber and

integrated reactors [32]. Whereas another US Patent Applica-

tion 20050013767 describes a method of delivering a reversible

hydrogenstorage toamobileor stationaryapplicationusingpi-

conjugated substrate [33].

As compared to articles reported in journals, patent liter-

ature covers a more comprehensive class of potential

aromatic compounds which can be used as a hydrogen carrier

in the hydrogenated form. It is obvious to cover as many as

potential candidates in the patent literature in order to protect

the commercial interest. However, a systematic data on

reactions kinetics, thermodynamics and catalyst activity for

these extended potential reactants is not available.

Based on the patent literature available following are the

claims related to various aspects of hydrogen storage through

chemical hydrides or liquid organic hydrides.

< Reactant: methylcyclohexane, cyclohexane, decalin, pi-

conjugated substrates.

< Catalyst: Pt and Pt based catalyst with second metal

(secondmetalmay be Co, Ni, Fe, Cu, Sn, Pd, Cd, Ir, Rh, Ru,

Ag, Bi, Hg, Pb)

9. LOH state-of-art

Considerably high evolution rate of about 3800 mmol/gmet/

min was reported Ichikawa and co-workers by dehydrogena-

tion of cyclohexane at 375 �Cwith Pt/alumite as catalyst [1]. In

the same year the hydrogen evolution rate ranging from 1500

to 500 mmol/gmet/min was reported by several researchers.

However, these rates are for initial several minutes of the

reactions, typically for 5e20min.Whereas, reports for time on

series data for a considerably longer period in several hours

are rare.

Considering hydrogen demand for PEMFC, Hodoshima and

co-workers demonstrated 50 kW of dehydrogenation system

using superheated liquid film condition as a feasible option for

practical purpose. They have achieved 70% conversion of

cycloalkane inonepass levelhaving reactionareaofabout 1m2.

There are several reports on high performance of dehy-

drogenation catalyst establishing the potential of the method

of hydrogen delivery using LOH for practical application,

Okada et al. reported methylcyclohexane conversion of 95%

with 99% selectivity toward toluene with stability upto 6000 h.

The hydrogen production cost of 64.7 U/Nm3 of H2 was

reported by Okada et al. A feasibility study by Biniwale and

group estimated the hydrogen production cost at 7.57 USD/kg

of hydrogen. They have reported the catalyst stability up to

200 h with conversion of ca 60% in a single step [16].

Many research groups in collaboration with industries

anticipated in demonstrating liquid organic hydrides system.

The first organic hydride vehicle was successfully driven by

Japanese based industries and Prof. Masaru Ichikawa from

Hokkaido University in 2008 [34]. They demonstrated fuel

efficiency was improved by 30% and CO2 emission was

reduced by 30% as compared to the base line emissions of

vehicle using only gasoline as fuel. In this experiment they

have used gasoline for initial period of drive and then

switched over to hydrogen. The use of gasoline in initial cold-

engine conditions attributed to emissions from the vehicle. If

a fuel cell vehicle is used by using hydrogen obtained from on-

board dehydrogenation of LOH then it is expected to result

into zero tail-pipe emissions.

Although emphasis is on transportation sector application

for hydrogen economy however, it is apparent that there are

several other potential markets of liquid organic hydride for

near future. Several potential applications for LOH technology

to meet hydrogen supply include;

� Use of hydrogen in industrial applications such as float

glass, sorbital manufacture etc.

� Auxiliary power generation units for remote application

such as telephone towers.

� Hydrogen for electrolysers.

� Large reforming units.

� Hydrogen in transportation.

� Use of hydrogen in IC engines.

10. Conclusions

Based on the literature and our groups studies it is evident

that LOH is a potential technology for transportation and

delivery of hydrogen. The advantages related to the LOH

systems include relatively higher hydrogen storage capacity,

possibility of hydrogen transportation at near ambient

conditions and using simple lorries. In order to realize the

hydrogen transportation by LOH system considerable devel-

opment have been reported in dehydrogenation catalysts,

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 3 7 1 9e3 7 2 63726

heat transfer in reactor systems, use of unsteady states etc.

Nearly equilibrium conversion with relatively lower temper-

ature requirement has been reported. However, challenges

regardingminimizing heat losses, use of renewable energy for

providing heat for dehydrogenation, minimizing evaporative

losses, effective separation of products particularly liquid

product/unreacted reactant are need to be resolved as

a prerequisite for a step toward up-scaling of the technology.

Acknowledgment

The part of the work was carried out under the project spon-

sored by Ministry of New and Renewable Energy, New Delhi.

One of the author Ms. Anshu Shukla acknowledged a Senior

Research fellowship supported by CSIR, New Delhi.

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