Catalysis Volume 22

A Review of Recent Literature

By J. J. Spivey, K. M. Dooley

The Royal Society of Chemistry

Copyright © 2010 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-1-84755-951-7

Contents

Preface James J. Spivey and Kerry M. Dooley, v,
Bioethanol reforming for H2 production. A comparison with hydrocarbon reforming Nicolas Bion, Florence Epron and Daniel Duprez, 1,
Catalytic reforming of liquid hydrocarbons for on-board solid oxide fuel cell auxiliary power units Johannes W. Schwank and Andrew R. Tadd, 56,
Coupling kinetic and spectroscopic methods for the investigation of environmentally important reactions F. C. Meunier, 94,
Oxidative conversion of lower alkanes to olefins K. Seshan, 119,
Asymmetric hydrogenation of activated ketones József L. Margitfalvi and Emília Tálas, 144,
Gold catalysis in organic synthesis and material science Cristina Della Pina, Ermelinda Falletta and Michele Rossi, 279,

CHAPTER 1

Bioethanol reforming for H2 production. A comparison with hydrocarbon reforming

Nicolas Bion, Florence Epron and Daniel Duprez

DOI: 10.1039/9781847559630-00001

Hydrogen is essentially produced by steam reforming (SR) of hydrocarbon fractions (natural gas, naphtha, …) on an industrial scale. Replacing fossil fuels by biofuels for H2 production has attracted much attention with an increased interest for bioethanol steam reforming. Kinetics and mechanisms of hydrocarbon-SR and alcohol-SR present some similarities but also some very important differences due to alcohol reactivity much more complex than that of hydrocarbons. The scope of this report is to compare the two processes in terms of reaction mechanisms. Attention will also be paid to the case of crude bioethanol.

1. Introduction

Whereas hydrogen is the most abundant element of the Universe, it is relatively rare on Earth (0.9 atom % in the outer shell of our planet). Virtually, it does not exist as dihydrogen: it is associated with oxygen in water, with carbon in fossil hydrocarbons, both with oxygen and carbon in bioresources (carbohydrates, cellulosic and lignocellulosic matter, lignin, …) and more rarely with other elements. Water is by far the main source of hydrogen on Earth (Table 1).

The stock of hydrogen available in fresh waters (lakes and rivers) is then of 1.3 × 1013 tons while the total ressources in hydrogen in oceans ans seas amount to 1.5 × 1017 tons. Comparatively, the ressources in hydrogen available in fossil fuels are modest (Table 2).

Assuming a mean H/C atomic ratio of 1.66 in crude oil, of 3.8 in natural gas and of 0.8 in coal, the stock of hydrogen in fossil fuels would not exceed 111 × 109 T (23 in crude oil, 30 in natural gas and 58 GT in coal reserves), i.e. two orders of magnitude less than the amount of hydrogen contained in fresh waters. Unfortunately, water is a stable molecule needing a high energy input to recover hydrogen as H2. This energy may be provided by (i) a chemical source by oxidizing the carbon of an hydrocarbon into CO and CO2 (steam reforming); (ii) by electricity (water electrolysis) or (iii) by photons (water splitting). Steam reforming is by far the main process for H2 production and only this way of hydrogen production will be examined in this Chapter. On an environmental point of view, steam reforming is not a green process since all (or almost all) the carbon of the hydrocarbons is transformed into carbon dioxide. To avoid this drawback, fossil fuels may be replaced by biofuels. Carbon dioxide is still produced but it may be recycled to new biomolecules by photosynthesis. The annual production of biomass in the World would be comprised between 150 and 420 × 109 metric tons. The mean hydrogen content in biomass being comprised between 5 and 7 wt-%, the stock of hydrogen in this renewable matter would be close to 11 × 109 T/year. In other words, ten years of biomass production would be sufficient to recover all the hydrogen content of fossil fuels. However, a great part of this biomass is composed of wood, difficult to transform into valuable products. For that reason, only products derived from cellulosic and hemicellulosic biomass have been considered for hydrogen production. Since ten years, intensive researches have been devoted to the steam reforming of bioethanol which is a fuel well-adapted to the production of hydrogen. This Chapter deals for a great part with this process, with a special attention paid to the use of crude bioethanol. In a first part, however, the steam reforming of hydrocarbons (aromatics and alkanes) will be reviewed as the model of many mechanistic investigations. This will allow one to compare the steam reforming of hydrocarbons with the steam reforming of ethanol, an alcohol leading to more complex kinetic schemes.

2. The steam reforming of hydrocarbons

2.1 Thermodynamics

For methane, four main reactions can occur:

1. The steam reforming reaction leading to CO and H2

CH4 + H2O -> CO + 3H2 ΔH0298 = +206 kJ mol-1 (1)

2. The steam reforming reaction leading to CO2 and H2

CH4 + 2H2O -> CO2 + 4H2 ΔH0298 = +165 kJ mol-1 (2)

3. The water gas shift reaction (WGS)

CO + H2O -> CO2 + H2 ΔH0298 = -41 kJ mol-1 (3)

4. The coking reaction

CH4 -> C + 2H2 ΔH0298 = +75 kJ mol-1 (4)

For higher hydrocarbons, a similar set of reactions may be written; for instance the reactions of n-heptane and of toluene leading to CO and H2 become:

C7H16 + 7H2O -> 7CO + 15H2 ΔH0298 = +1107 kJ mol-1 (5)

C7H8 + 7H2O -> 7CO + 11H2 ΔH0298 = +869 kJ mol-1 (6)

Except for the WGS reaction, all the reactions involved in the hydrocarbon steam reforming are strongly endothermic with an increase of the number of molecules. They are thus favored at high temperatures and low pressures. The temperature effect is illustrated in Fig. 1 which shows the change with T of the gas composition at equilibrium in the methane steam reforming (initial conditions H2O/CH4=1, P=1 bar). Calculations were carried out by minimizing the sum of the Gibbs free energies of formation of all the compounds (reactants and products) while keeping constant the number of moles of each element (here C, H and O). Details of the procedure are given in Perry’s Handbook. Thermodynamic data (molar Gibbs free energy of each compound) are taken from Stull et al. Maximal H2 production is observed around 700 °C. Above 700 °C, the H2 mol% does no longer increase because of the preferential formation of CO at high temperature. This is coherent with the WGS equilibrium: reaction 3 being exothermic, CO2 is favored at low temperature while the reverse reaction (RWGS) yielding CO is favored at high temperature. At 900 °C, total conversion of methane can be achieved yielding quasi-exclusively a syngas with the composition given in equation 1 (75% H2 + 25% CO).

The equilibrium gas compositions in methane, n-heptane and toluene steam reforming at 700 °C are compared in Table 3. The initial state is a steam/ hydrocarbon mixture with a molar ratio corresponding to the stoichiometry of equations 1, 5, 6 written with H2 and CO as products of steam reforming.

Methane and steam conversions are very high but not total even at 700 °C. The vol.% of hydrogen expected in dry gases amounts to 70%. The formation of methane and CO is favored in the steam reforming of C7 compounds at 700 °C, which tends to decrease the hydrogen content in dry gases. In every cases, the CO-to-CO2 molar ratio is remarkably constant (around 7.7) whathever the starting hydrocarbon. The conversion of C7 hydrocarbons (not reported in Table 3) is total whatever the temperature. Thermodynamics predicts that H2 production from C2+ hydrocarbons is controlled by methane formation. If, kinetically, methane formation can be avoided, the equilibrium gas composition is significantly changed. An example is shown in Fig. 2 for toluene steam reforming without CH4 formation. The complete conversion of toluene only occurs around 550–600 °C and the maximal H2 formation is observed at 500 °C, much below the corresponding value when methane can be formed. Interestingly, benzene formation can then be observed with a maximum around 400 °C. If methane formation may be avoided or suppressed, toluene dealkylation can occur in the 400–500 °C range of temperature: this is the so-called toluene steam dealkylation (TSDA) which has been largely studied in the past as a way to produce benzene from toluene (and more generally from alkylbenzenes) without H2 consumption.

2.2 Kinetics and mechanisms

Methane being a very stable molecule, the steam reforming of natural gas should be carried out at high temperatures (around 600–700 °C) and it can be expected that methane activation is a critical step of the reaction (see Section 2.2.3). Heavier hydrocarbons are more reactive and there are many indications in the literature that water activation may be the rate determining step in the steam reforming of these compounds, specially at lower temperatures (400–600 °C).

2.2.1 Aromatics

2.2.1.1 Reaction scheme and catalytic activity. Toluene will be chosen as model hydrocarbon illustrating this class of compounds. As mentioned in Section 2.1, the toluene steam reforming may lead to dealkylation (7) in the 400–500 °C temperature range where methane formation is kinetically unfavored (8, 9).

C7H8 + H2O -> C6H6 + CO + 2H2 ΔH0298 = 164 kJ mol-1 (7)

C7H8 + H2 -> C6H6 + CH4 ΔH0298 = -42 kJ mol-1 (8)

C7H8 + 10H2 -> 7CH4 ΔH0298 = -574 kJ mol-1 (9)

Equations 8, 9 clearly show that methane formation decreases the hydrogen yield. If the reactions leading to benzene, CO, CO2 and CH4 are considered (i.e. 3, 5, 7–9), the following relationship between the product yields could be established:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (10)

Intrinsic activity and selectivity to benzene of Group 8-9-10 metals (ex Group VIII) supported on γ-Al2O3 (210 m2 g-1) are reported in Table 4.

All the catalysts deactivate with time-on-stream. It was shown that the selectivities vary very little when the catalysts are deactivating and that the flow rate of dry gases (H2 + CO + CO2 + CH4) is proportional to the toluene conversion. This property allows one to extrapolate the running activity to zero-time. Turnover frequencies given in Table 4 are the intrinsic activities determined by this technique. The deactivation is due to a carbon deposit on the catalyst (metal and support). It was proved that toluene steam dealkylation is a relatively “structure” insensitive reaction: the carbon deposit does not change the turnover frequency per free metal atom which remains very close to the turnover frequency extrapolated at zero-time. Free metal surface area in coked catalysts were measured according to a procedure detailed in refs. Initial selectivity to benzene is the selectivity extrapolated at zero conversion. An interesting feature of the toluene steam dealkylation reaction is the increase of SB with conversion. For instance, rhodium selectivity reaches 86% at 10% conversion and 92% around 40% conversion. This is due to a complex behaviour of the catalyst including two phenomena: (i) though the dealkylation is structure insentive, the total steam reforming of toluene (6) is not and is strongly poisoned by coke deposits; (ii) CO is an inhibitor of both deal-kylation and total reforming but the former reaction is much less affected than the latter one. The relative activities reported in Table 4 show that the steam dealkylation reaction is not very sensitive to the nature of metal: there is less than one order of magnitude difference between the most active metal (Rh) and the less active one (Ir). The metal ranking found by Duprez et al. is close to that reported by Grenoble in similar conditions (440 °C, alumina support of 175 m2 g-1), except the water-totoluene ratio (3.25 in the Grenoble’s study). The only difference lies in the position of Ru found more active by Grenoble. Kim showed that promoting alumina by vanadium oxide did not change significantly the metal ranking.

2.2.1.2 Support effects and reaction mechanism on rhodium catalysts. Contrasting with the low sensitivity to nature of metal, the steam dealkylation reaction appeared as very sensitive to nature of support. This is illustrated in Table 5 with the rhodium as active metal. There are two to three orders of magnitude difference between the activity of rhodium catalysts supported on chromia or aluminochromia catalysts and those supported on silica or carbon. The data of Table 5 show that the differences are not due to metal dispersion effects. Catalyst characterization carried out after test also confirmed that there was no sintering during the kinetic measurements affecting more certain supports. A bifunctional mechanism was proposed by Grenoble and Duprez et al. to explain this dramatic support sensitivity. In this mechanism schematized on Fig. 3, the hydrocarbon molecule would be adsorbed and activated on metal sites while the water molecule would be activated on support sites.

The molecule of toluene undergoes a dissociative chemisorption on a metal site M leading to a molecule of benzene and an alkyl fragment (11). This reaction should require two metal sites. However, benzene being less strongly adsorbed than toluene on metals, it is immediately desorbed once formed.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (11)

Water is activated on support sites (S–O–S) according to equation 12:

H2O + S-O-S -> 2S-OH (12)

The final step is a transfer of OH groups to metal particles where they react to form carbon oxides and hydrogen:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (13)

The global reaction in this catalytic cycle is the dealkylation to benzene, CO and H2. It is implicitely assumed that CO2 is formed by water gas shift.

The reaction rate (per gram of catalyst) derived from equation 13 is:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (14)

where θCHx is the surface coverage of CHx fragments on metal sites while [xi]OH is the OH group coverage on support sites. As step (13) implies a transfer of OH groups through the metal/support interface, it is assumed that k=KI0, I0 being the length of the particle perimeter per gram of catalyst. Surface coverages are deduced from equations 11, 12 and Langmuir-type hyperbolic expressions for equilibium adsorption of molecules A can be approximated to power-law expressions using the following equation:

KAPA/1 + KAPA = α(KAPA)n (15)

Inserting power-law expressions for the surface coverages in equation 14 leads to the following rate equation:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (16)

where M0 and S0 are the numbers of metal sites and of support sites, respectively, per gram of catalyst; PT and PW are the partial pressures of toluene and water. Knowing the weight loading (xm %) and the dispersion D0 (%) of metal, the specific perimeter I0 can be calculated by:

I0 = βD20xm (17)

with β = 1.6 × 10-7 ρA2mol/M (cubic particles) (18)

ρ being the metal density (gm-3), Amol the molar surface of metal (m2 mol-1) and M, the atomic weight (gmol-1). For Rh, ρ=12.45 × 106gm-3, Amol=47633m2 mol-1 (equidistribution of low index faces) and M=102.9 g mol-1, which gives β = 4.28 × 105 mg-1. It is worth noting that, for a catalyst commonly used in steam reforming (0.6%Rh, mean dispersion of 50%), I0 amounts to 6.4 × 108 mg-1, a length greater than the Earth-Moon distance. This justifies the great impact the reactions at metal/support interfaces may have in Catalysis.

(Continues…)Excerpted from Catalysis Volume 22 by J. J. Spivey, K. M. Dooley. Copyright © 2010 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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