Contrat CNRS-CPE-UCBL- Synthopétrol n° 053571 1
Production d’hydrocarbures
liquides à partir de la biomasse
forestière :
Etude bibliographique
Dr. Paul S. Campbell
Responsable : Dr. C. C. Santini
Université de Lyon, Institut de Chimie de Lyon,
UMR 5265 CNRS-Université de Lyon 1 -ESCPE Lyon,
C2P2, Equipe Chimie Organométallique de Surface
43 Boulevard du 11 Novembre 1918, 69616 Villeurbanne,
Fax: 33(0)472431795; Tel: 33(0)472431810;
E-mail : santini@cpe.fr
Contrat CNRS-CPE-UCBL- Synthopétrol n° 053571 2
Biomass to Liquid Project
Prior to the discovery of inexpensive fossil fuels, our society was dependent on plant
biomass to meet its energy demands. The discovery of crude oil, in the 19th century, created an
inexpensive liquid fuel source that helped industrialize the world and improved standards of
living. Now with declining petroleum resources, combined with increased demand for petroleum
1
by emerging economies, and political and environmental concerns about fossil fuels, it is
imperative to develop economical and energy-efficient processes for the sustainable production
of fuels and chemicals. In this respect, plant biomass is the only current sustainable source of
organic carbon, and biofuels, fuels derived from plant biomass, are the only current sustainable
source of liquid fuels. Biofuels generate significantly less greenhouse gas emissions than do
fossil fuels and can even be greenhouse gas neutral if efficient methods for biofuels production
are developed.1
Biomass is thus an important feedstock for the renewable production of fuels, chemicals,
and energy. As of 2005, over 3% of the total energy consumption in the United States was
supplied by biomass, and it recently surpassed hydroelectric energy as the largest domestic
source of renewable energy. Similarly, the European Union received 66.1% of its renewable
energy from biomass, which thus surpassed the total combined contribution from hydropower,
wind power, geothermal energy, and solar power. In addition to energy, the production of
chemicals from biomass is also essential; indeed, the only renewable source of liquid
transportation fuels is currently obtained from biomass.2
Wood is one of the most important sources of biomass and will be the focus of this
project. It is constituted principally of polymeric crystalline cellulose, amorphous hemicelluloses
and lignin as depicted in Figure 1.
Contrat CNRS-CPE-UCBL- Synthopétrol n° 053571 3
Figure 1. Structures of different biomass fractions (lignocellulose, cellulose, lignin and hemicellulose)
before and after reactions. (Lignocellulose structure adapted from Hsu et al.).3, 4
Contrat CNRS-CPE-UCBL- Synthopétrol n° 053571 4
Cellulose, as shown in Figure 1, consists of a linear polysaccharide with α-1,4 linkages of D-glucopyranose monomers.5
Unlike starch, cellulose is a
crystalline material with an extended, flat, 2-fold helical conformation.5 Hydrogen bonds help
maintain and reinforce the flat, linear conformation of the chain. The top and bottom of the
cellulose chains are essentially completely hydrophobic. The sides of the cellulose chains are
hydrophilic and capable of hydrogen bonding, because all the aliphatic hydrogen atoms are in
axial positions, and the polar hydroxyl groups are in equatorial positions. The degree of
polymerization of cellulose is approximately 10 000 to 15 000 glucopyranose monomer units in
wood and cotton, respectively.28 Upon partial acid hydrolysis, cellulose is broken into cellobiose
(glucose dimer), cellotriose (glucose trimer), and cellotetrose (glucose tetramer), whereas upon
complete acid hydrolysis it is broken down into glucose.4
2
Hemicellulose is a sugar polymer that typically constitutes 20-40 wt % of biomass.4 In
contrast to cellulose, which is a polymer of only glucose, hemicellulose is a polymer of five
different sugars. This complex polysaccharide occurs in association with cellulose in the cell
walls. It contains five-carbon sugars (usually xylose and arabinose) and sixcarbon sugars
(galactose, glucose, and mannose), all of which are highly substituted with acetic acid. The most
abundant building block of hemicellulose is xylan (a xylose polymer linked at the 1 and 4
positions). Hemicellulose is amorphous because of its branched nature and it is relatively easy to
hydrolyse to its monomer sugars compared to cellulose.
Ten to twenty-five weight percent of biomass is typically composed of lignin which is a
highly branched, substituted, mononuclear aromatic polymer found in the cell walls of certain
biomass, particularly woody biomass. Lignin is often associated with the cellulose and
hemicellulose materials making up lignocellulose compounds. The manner in which it is
produced from lignocellulose affects its structure and reactivity. Figure 1 shows the structural
monomer units of lignin. Softwood lignins are formed from mostly coniferyl alcohol. Hardwood
lignins have both coniferyl and sinapyl alcohol as monomer units in roughly equal quantities.
Grass lignin contains coniferyl, sinapyl, and coumaryl alcohol.6 Lignin is an irregular polymer,
which is formed by an enzyme-initiated free-radical polymerization of the alcohol precursors.
The bonding in the polymer can occur at many different sites in the phenylpropane monomer
Contrat CNRS-CPE-UCBL- Synthopétrol n° 053571 5
due to electron delocalization in the aromatic ring, the double bond-containing side chain,
and the oxygen functionalities.7 Some lignin structural linkage units are shown in Figure 2. 8
Figure 2. Common lignin linkages found naturally in lignin. 8
The biorefinery of lignocellulosic material is summarised in Figure 3. Lignocellulosic
material can be converted into liquid fuels by three primary routes including syn-gas production
by gasification, bio-oil production by pyrolysis or liquefaction or hydrolysis of biomass to
produce sugar monomer units. Synthesis gas can be used to produce hydrocarbons through
Fischer Tropsch (diesel or gasoline), methanol, and other fuels. Bio-oils must be upgraded if they
are to be used as transportation fuels. Transportation fuels such as ethanol, gasoline, and diesel
fuel can be produced from sugar and associated lignin intermediates.1
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Figure 3. Strategies for production of fuels from lignocellulosic biomass adapted from Huber and Dumesic. 9
3
A more detailed description of biorefinery strategies is depicted in Figure 4, focusing
particularly on lignin, which despite accounting for only up to 25 wt %, contains up to 40 % of
the potential energy of lignocellulosic material.2 It can be seen that the initial step involves a
pretreatment of the material, with the aim of drying, separating and/or depolymerising the
lignocellulosic components to facilitate the subsequent treatments. Common pretreatments
include:10
the Kraft Lignin Process – treatment at high pH (NaOH, Na2S (aq)) at 423-453 K,
a base-catalysed depolymerisation
the Lignosulphonate Lignin Process – treatment with calcium or magnesium
sulphite, leads to sulphonated aromatics
the Organosolv process – dissolution in various organic solvents. No use of
sulphides or harsh conditions, results in a low sulphur content in the product
Pyrolysis – heating to high temperatures in oxygen-poor conditions
Steam explosion – steam impregnation at high pressure followed by rapid pressure
release – separated lignocellulosic components and ruptures the lignin structure
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Dissolution in ionic liquids – ionic liquids may be used to separate lignocellulosic
components and subsequent treatment may take place in these novel media.11
Figure 4. Lignocellulosic biorefinery scheme with particular emphasis on the lignin stream. Components from the
cellulose and hemicelluloses streams are integrated within the lignin framework, but the process arrows are not fully depicted for
clarity. 12
Three strategies exist for the refinement of the treated biomass. As mentioned above, this
may be gasified to produce syn-gas which can then undergo Fischer Tropsch to produce fuel, a
similar process to that undertaken with coal. The disadvantages of this process are that a
maximum of only 40% of the carbon is converted to fuel which is neither energetically nor
economically efficient. Furthermore the Fischer Tropsch catalysts are found to be sensitive to
deactivation through sulphur poisoning.
The second and third strategies depicted in Figure 4 involve passing directly from
pretreated biomass to liquid products, using new technology and new catalysts, either passing via
simple platform chemicals which in a second step are transformed to fuels and/or fine chemicals
(Strategy 2) or passing directly to the latter in one step (Strategy 3). The catalysts
4
Contrat CNRS-CPE-UCBL- Synthopétrol n° 053571 8
required in these cases must aid the depolymerisation and then either reduction or
oxidation of the resulting monomer units, depending on the products desired. For example, as
shown in Figure 5, oxidative degredation of lignin can lead to fine chemicals and such as vanillin
for the pharmaceutical industry, whereas reductive degredation leads us to simple aromatics
which can be used to produce both fuels and fine chemicals.2
Figure 5. Products from oxidative (left) and reductive (right) degredation of lignin.
This project will focus on the reductive strategies as viable way of producing fuel from
biomass. The ideal catalyst sought will:
Depolymerise
Deoxygenate (for fuel applications less than 5% oxygen content is required)
Exhibit a certain hydrogenation activity (to produce aliphatics)
Be active at moderate temperatures and pressure (≤ 50 bars, ≤ 350 °C)
Be able to deal with untreated biomass (wood chips) to eliminate the cost
associated with pretreatment
Cope with large amounts of water (from undried biomass and water produced
through deoxygenation)
Cope with sulphur from biomass feeds
Contrat CNRS-CPE-UCBL- Synthopétrol n° 053571 9
Dissolution of biomass in Ionic Liquids
Dissolution is a useful process in biomass refinement, in order to disrupt the complicated
structure into its components. It has a complicated intertwined 3-D architecture, interlinking the
lignin and cellulosic material, which protects the plant from microbial attack and provides
resistance to the elements and thus renders it resistant to chemical attack. This lignocellulose
structure varies tremendously with the plant species, plant parts and growth conditions. For
instance hydrophobic π-stacking interactions, resistant to attack, are found in softwood. Ionic
liquids (low temperature molten salts) have become popular solvents for the dissolution of
biomass as the anions disrupt H-bonds between polymer chains.11 Some key results are listed
below:
BMICl was found to dissolve up to 10% weight cellulose. BMICl is able to
dissolve both cellulose and lignin. Precipitation agents lead to isolation of cellulose from other
components. 11, 13
5
EMIOAc able to selectively extract lignin from wood.14
allylMICl found to well dissolve wood chips – π-interactions with allyl species.
ILs with non-coordinating anions (PF6 etc.) do not dissolve lignin – MeSO4 is a
good choice
Recyclability of the IL is necessary due to their high price.15
Substrate and product extraction are challenging. π- π interactions enhance
solubility of lignin derived moieties in ILs.16
scCO2 may be a good choice for the extraction of substrates and products from
ILs. –Large quantities of CO2 may dissolve in the IL, but no measurable quantity of IL dissolve
in CO2. This is also an environmentally benign solvent.17
A range of transformation reactions have been carried out on biomass dissolved in
ILs.
Contrat CNRS-CPE-UCBL- Synthopétrol n° 053571 10
Catalytic Refinement of Biomass – State-of-the-art
1. Catalytic Pyrolysis, Cracking and Hydrolysis
Disruption of the complicated lignocellulosic polymers into smaller subunits is an
important step for lignin refinement. Amen-Chen and co-workers have published a review of the
production of monomeric phenols by thermochemical lignin conversion, describing several routes
to phenolic compounds, including the pyrolysis of monomeric, dimeric, and trimeric compounds,
in addition to the effects that different conditions have on forming methane, methanol, and
various compounds from biomass.18
Several transition metal catalytic processes have also been reviewed, including kraft
lignin pyrolysis in alkaline conditions and with ZnCl2.18 As previously mentioned, alkaline
conditions catalyse the depolymerisation of lignin through cleavage of ether bonds. On the other
hand, acidic conditions were found to lead to carbonyl containing products.18 Dorrestijn and coworkers published a review detailing the pyrolysis of lignin with a brief discussion of pyrolysis,
catalytic hydrogenation, and oxidation.19 Britt et al. studied flash vacuum pyrolysis of methoxysubstituted β-O-4 lignin model compounds in order to provide mechanistic insight into the
relevant reaction pathways.20 The reactions were dominated by free radical reactions, molecular
rearrangements, and concerted eliminations.20 Misson and co-workers investigated the
6
pretreatment of empty palm fruit bunches with NaOH, H2O2, and Ca(OH)2 before catalytic
pyrolysis using Al-MCM-41 and H-ZSM-5 to give phenolic yields of 90 and 80 wt % yield,
respectively.21 Li and co-workers studied the depolymerisation/repolymerisation of lignin during
steams treatment of aspen wood.22 They found that addition of a carbenium ion scavenger, such
as 2-naphthol, suppresses the repolymerisation reaction to give a more uniform and more easily
extractable lignin of low molecular weight.22 As indicated above, controlling the repolymerisation
of the monomer is important for selective biomass refinement.
Cracking is an industrial process commonly used in the petroleum industry to convert
heavy hydrocarbons into more valuable products. Hydrocracking of biomass combines a support
active in the cracking process (such as zeolites or amorphous SiO2-Al2O3 with various
compositions) where C-C bond cleavage is achieved in an acid catalysed reaction and a noble
Contrat CNRS-CPE-UCBL- Synthopétrol n° 053571 11
metal for hydrogenation (i.e. Co, W, Pd, Ru, Ni, etc.).23 As the cracking is acid-catalysed,
a higher abundance of acid sites on the support leads to more hydrocarbons. 24 and excess water
in the feedstock has an adverse effect on reactivity by blocking these acid sites. 25 Using zeolites
leads to more aromatics, however increases coking. An increased pore-size reduces this coking.24
Char and tar formation, a result of reploymerisation of heavy oil components can be
reduced by working at elevated temperatures however this also leads to decomposition to gaseous
components. 24, 25
Pt/Al2O3-SiO2 and sulphided Co-Mo/Al2O3, Ni-W/Al2O3 and Ni-Mo/Al2O3 have all been
used as cracking catalysts, Pt being the most effective for cracking and simultaneous oxygen
removal. The conditions used were 623 K and 10.34 MPa H2. 26 Also Pt superacid catalysts, e.g.
Pt/SO42-/ZrO2, Pt/WO42-/ZrO2, or Pt/SO42-/TiO2 both unsupported and supported, have been
patented as highly effective hydrocracking catalysts.27
Biomass may also be hydrolysed, using highly alkaline conditions to cleave the ether
linkages as previously mentioned, instance using KOH or Rb2CO3. The feedstock does not need
drying and the organic moieties may be extracted into a solvent such as toluene.28
Supercritical water has also been used to hydrolyse biomass, the advantages being that it
is completely miscible with light gases, hydrocarbons and aromatic compounds. Unfortunately
the severe conditions required to produce supercritical water (647.2 K, 22.1 MPa) lead to high
running costs.29
7
2. Reduction through hydrotreatment
Hydrodeoxygenation
Hydrocarbons with less than 5 wt % oxygen is needed for fuel applications. For biomass
feeds, oxygen content can be as high as 50%. Some of the O-containing compounds in the feed
readily polymerise and cause thus poor fuel stability and performance during combustion.
Hydrotreatment increases both the fuel energy content and the stability (eliminating
polymerisable entities) as well as the volatility. The viscosity of the resulting liquid is also
reduced due to the removal of oxygen and lowering of the molecular mass. The high-pressure
Contrat CNRS-CPE-UCBL- Synthopétrol n° 053571 12
needed for hydrotreating could be produced from the biomass itself in an integrated biorefinery.2
Conventional crudes only contain less than 2 wt. % O, however attention had been already
drawn to hydrodesulphurisation (HDS) and hydrodenitrogenation (HDN), driven by the need to
reduce NOx and SOx emissions on combustion. EU standards currently stipulate that less that
sulphur content in fuels must be below 10 ppm and these standards are only likely to become
stricter. Hydrodeoxygenation (HDO) occurs simultaneously with these reactions, producing
environmentally benign H2O, therefore HDO catalysts needed for biomass feedstocks are
generally derived from traditional HDS and HDN catalysts, i.e. CoMo/Al2O3 or NiMo/Al2O3.30
Full HDO of biomass feeds generally takes place in a multi-stage operation, Scheme 1.
Firstly at low temperatures (< 573 K), methoxyphenols, biphenols and ethers are converted to
phenols in a stabilising step, which must be removed in a second stage above 623 K. Ocompounds such as alcohols, esters ketones and carboxylic acids may also be involved.30
Scheme 1. The multi-stage HDO of biomass feeds.
The feedstock for HDO is generally bio-oils produced by flash pyrolysis or liquefaction of
biomass and therefore tests are generally performed on model compounds found in these sources,
i.e. mostly substituted phenols as seen in Scheme 2.
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Scheme 2. Substituted phenols typical of bio-oil feeds
During HDO several C-O bonds must be cleaved. This process is energetically costly,
therefore activation energies are high. It can be seen from the relative bond strengths tabulated in
Table 1 that cleaving a CAR-O bond is much more difficult than cleaving a CAL-O bond (CAR =
8
aromatic carbon, CAL = aliphatic carbon). If we were to hydrogenate the cycle to the
corresponding cycloalkane, this would facilitate HDO, and oxygen-free cycloalkanes. In fuel
production this is not necessarily desired as aromatics increase the octane number, and
furthermore full hydrogenation is costly in H2.
Table 1. Carbon-oxygen bond dissociation energies
B
ond Type
Bond dissociation
energies (kJ/mol)
R
339
R
422
R
385
A
468
O-R
O-Ar
-OH
r-OH
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Much attention has been devoted to the study of the mechanism of HDO of substituted
phenols. The substitution position was found to be an important factor. For example in the HDO
of cresols over sulphided CoMo/Al2O3 the following order of reactivity was estabilished by
Odebunmi’s group: meta>para>ortho. Toluene and cyclohexane were the main products, toluene
being predominant at lower temperatures and pressures. 31 Such a pattern for both HDO and
hydrogenation has been established for other catalysts. In certain cases, partially hydrogenated
cycles have been observed as minority products or intertmediates. This could be explained by a
mechanism whereby the ring is first hydrogenated before the elimination of H2O as shown in
Scheme 3. Dealkylation and alkylation are also provoked by the catalyst surface leading to small
amounts benzene, xylenes and xylenols in the products, depending on the conditions and
catalyst.30
Scheme 3. HDO pathways of cresols. (dashed arrow represents major pathway)
The HDO of ether linkages could be a potentially important reaction in the refinement of
biomass as this could be key in depolymerising and hydrotreating biomass feeds in one step.
However, studies so far carried out on model substrates such as diphenylether over sulphided
CoMo/Al2O3 that phenol is produced as an intermediate which must undergo subsequent HDO
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9
under harsher conditions, Scheme 4.32 Dinaphthylether and xanthene were also studied
leading to similar conclusions.
Scheme 4. HDO of biphenylether under mild conditions.32
The Hydrotreatment Catalysts
The catalyst most often studied used consists of MoS2 slabs often supported on alumina
(Al2O3). The crystal structure of MoS2 is shown in Figure 6. It can be seen that the molybdenum
(IV) sites adopt a trigonal prismatic geometry in the bulk, each one bound to 6 sulphide ions. At
the edge of the structure are coordinatively unsaturated sites or sulphide vacancies, and it is these
sites that are thought to be responsible for the activity. These sites can coordinate unpaired
electrons of molecules such as pyridine, NH3 etc., i.e. are Lewis acidic.
Figure 6. MoS2 structure. Blue = Mo, Yellow = S.
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A promoter metal does not affect the size of the particles nor does it increase the number of sulphide vacancies.
However the vacancies associated to the promoter metals are considerably more active than those associated with MoS 2 alone.
A study has been carried out by Shabtai et al. out to find the best promoter metal for the HDO
catalyst who screened a selection of transition metals. In Figure 7, the results show that using Co
as a promoter gave the most active catalyst, and in this case, the largest amount or aromatic
hydrogenation was also observed. In Figure 8 are depicted AFM images alongside computer
models of MoS2 slabs, pure and doped by Co.33 It can be seen that Co occupies edge sites,
opening them up for coordination. Ru gave a relatively active catalyst which was more selective
towards aromatics. Ni is interesting due to its relatively low cost.32
Figure 7. Activities of MoS2 catalyst for HDO with different promoter metals.32
Contrat CNRS-CPE-UCBL- Synthopétrol n° 053571 17
Figure 8. Computer simulations (left) and AFM images (right) of MoS2 slabs, doped with Co (a) and non-doped (b).33
A recurring problem with these catalysts is deactivation due to:
1. Formation of water during HDO, blocking the active sites
2. Loss of sulphur
3. Coke formation
Loss of sulphur and water coordination can be reduced by reacting in the presence of H2S.
However this must be carefully measured as an excess of sulphur can have an adverse effect on
the overall HDO, by filling all vacancies and transforming HDO sites into hydrogenation sites.34
Other HDO Catalysts
10
In Table 2 are summarised the advantages and disadvantages of three types of catalyst
used for HDS in industry. These include the previously discussed sulphide catalysts, as well as
oxides and noble metals. Sulphide catalysts are widely studied due to their resistance to sulphur
impurities however are not the most active. In industrial HDS processes, HDS generally takes
place in 2 steps – a first step involving sulphided catalysts, at which point sulphur levels as
Contrat CNRS-CPE-UCBL- Synthopétrol n° 053571 18
little as 50 ppm can be reached, before the removal of the H2S formed and the treatment
by a second catalyst, more active but less resistant to sulphur poisoning, such as noble metals or
their oxides. 33
Table 2. Hydrodesulphurisation catalysts
HDS
(hydrodesulphu
Sulphide
d Metals
Nobl
e Metals
Meta
l Oxides
risation)
Activity
+/-
++
+
Running
↑ (623K)
↓↓(52
↓
Temperature
3K)
Resistance to
++
--
-
Used in
1st step
2nd
2nd
sulphur
Step
step
Whiffen and Smith recently compared the activity of unsupported MoS2 catalysts to that
of MoO2 and MoP for the HDO of 4-cresol under the same conditions, finding that the oxide
catalysts exhibited similar activity to that of the sulphide. Oxides of course are resistant to water
content. The phosphides exhibited the highest activity, but require highly toxic PH3 for their
synthesis.35
Traditional hydrogenation catalysts such as Pd/C and Ru/C are found to be more active
than sulphided Mo systems – therefore can be used at lower temperatures. Therefore these have
been used in hydrodeoxygenation of lignin. Here aromatic hydrogenation is favoured and
therefore hydrogenated aliphatics and also alcohols and carbonyls result.36
11
Ni-W/Al2O3 has been used as a combined hydrocracking and HDO catalyst on lignin
streams in the presence of tetralin, a hydrogen donor solvent. Under severe conditions the
conversion did not exceed 50%.23
A patent from BASF tungsten carbide unsupported hydrogenates lignin in a single-stage
process under relatively mild conditions. The catalyst can cope with sulphur-rich and sulphurpoor lignin streams. Low molecular weight oligomers (dimers and trimers) of coniferyl and
coumaryl alcohols result, i.e. full deoxygenation does not occur.37
The Support
The support used has also an effect on the reactivity. For instance, alumina alone was
found to convert 37 % of guaiacol to pyrocatechol. When we switch to more neutral supports
Contrat CNRS-CPE-UCBL- Synthopétrol n° 053571 19
such as silica or carbon, activity was decreased, proving the importance of the acid sites
available only on alumina. The carbon supported catalyst however produced a catalyst more
selective towards the production of phenols. Coking is thought to be also due to the acid sites in
alumina. Alumina remains the most widely used support for this process however due to
mechanical and textural properties as well as low cost.2
3. Hydrogen from methane
Steamreforming:Water-gasshift:Desiredreaction:Catalysts: Fe, Co, NiNi-Fe-Cu/
Al2O3Temperature: ≥1123 K≥973 KRapiddeactivationdue to CokeNatural Gasusedas feed3.5%
Yieldprolongedcatalystlifetimein rotatingreactor70% H2concentration atoutletInt. J. Hydr.
Energy, 34 2009, 9730Int. J. Hydr. Energy, 34 2009, 2979
4. Methane Dehydroaromatisation
Gasification of biomass feedstocks to methane is an established process. In a two step
process it may be possible to reconvert this methane into benzene using a dehydroaromatisation
(DHA) catalyst.
Using a zeolite catalyst, typically ZSM-5, loaded with 5 wt. % of a metal such as
molybdenum, DHA of methane occurs at 973 K under non-oxidising conditions. Original CH4
activation occurs on Mo2C leading to C2H4 which then oligomerises on the acid sites of the
zeolite to produce benzene. Ethane and ethylene are typical by products of this reaction.38, 39
12
Addition of a few % of CO and CO2 to the CH4 feed promotes benzene production and
significantly improves the stability of the catalyst. Modifying the catalyst with metals such as Fe
or Co was also seen to have a positive effect on catalyst stability whilst inhibiting coke
formation.40, 41 Other metals investigated for this process include Mn42 and W,43 however none
Contrat CNRS-CPE-UCBL- Synthopétrol n° 053571 20
were found to be active at temperatures much inferior to 973 K, a temperature too high for
the scope of this project.
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13
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