Bioenergy systems for the future 7 catalysts for conversion of synthesis gas

Bioenergy systems for the future 7 catalysts for conversion of synthesis gas Bioenergy systems for the future 7 catalysts for conversion of synthesis gas Bioenergy systems for the future 7 catalysts for conversion of synthesis gas Bioenergy systems for the future 7 catalysts for conversion of synthesis gas Bioenergy systems for the future 7 catalysts for conversion of synthesis gas Bioenergy systems for the future 7 catalysts for conversion of synthesis gas

Catalysts for conversion of

synthesis gas

V Palma, C Ruocco, M Martino, E Meloni, A Ricca

University of Salerno, Salerno, Italy

Nomenclature

BTL biomass to liquid

CSTR continuous stirred tank reactor

HTFTS high-temperature Fischer-Tropsch synthesis

HTS high-temperature shift (or high-temperature water-gas shift)

LTFTS low-temperature Fischer-Tropsch synthesis

LTS low-temperature shift (or low-temperature water-gas shift)

MMT million metric tons

MTBE methyl tert-butyl ether

MTO methanol to olefin

OCNT oxygen-functionalized carbon nanotube

PEMFC proton exchange membrane fuel cell

PROX preferential oxidation

PSA pressure swing adsorption

RWGS reverse water-gas shift

Bioenergy Systems for the Future http://dx.doi.org/10.1016/B978-0-08-101031-0.00007-7

© 2017 Elsevier Ltd All rights reserved.

SSA specific surface area

STP standard temperature and pressure

The wide use of the syngas, in recent decades, has been favored by the low cost offossil sources; however, the increasingly severe restrictions on CO2emissions, thegrowing worry of public opinion on climate changes, and the spread of alarming data

on the reserves of crude oil have pushed the interest toward the use of alternativesources The best alternative to the fossil fuels are the biomass, renewable materialsthat contain considerable quantities of carbon, hydrogen, and oxygen, restorable byphotosynthetic reaction (Maschio et al., 1994) Certainly, there are many disadvan-tages in using biomass with respect the fossil fuel, the presence of contaminants, avariable hydrogen to carbon ratio due to the composition of the sources, a low energydensity and high costs make them uncompetitive; however, nowadays, there are noother real alternatives Theoretically, all biological materials (both animal and vege-table) represent a biomass; however, only cheap materials and wastes are convenientlyconverted into syngas, and wastes from wood processing, energy crops, agriculturalresidues, by-products from processing of biological materials, municipal and sludgewastes, and food industry wastes are normally used as raw materials for the syngas

Fisher Tropsch process (synthetic fuels)

Fig 7.1 Syngas—production and transformation

production There exist two processes for converting the biomass in biofuel and power, the fermentation that produces mainly ethanol (biogas) and the thermochem-ical conversion to syngas The variability of the sources requires a flexibility inprocessing steps; however, the most of biomass contain a significant amount of water,

bio-so a preliminary drying process is commonly performed before going to the sion processes; alternatively, a hydrothermal processing directly degrades the biomass(Tekin et al., 2014); similarly, the product gas stream, from thermochemical process(pyrolysis, combustion, and gasification), contains many unwanted by-products to beremoved before going to next process (Kumar et al., 2009):

conver-l Particulate, removed according to the size by cyclone separators, wet scrubbers, static precipitators, and barrier filters

electro-l Alkali compounds, removed by barrier filters (Turn et al., 2001)

l Nitrogen compounds, removed at high temperature with dolomites, Ni-based catalysts andFe-based catalysts (Lepp€alahti and Koljonen, 1995)

l Sulfur compounds, removed by limestone, dolomite or calcium oxide

l Tar compounds, removed directly in the gasifier with the use of specific catalysts (Han andKim, 2008) or alternatively in a separate reactor

The fate of the syngas depends on the process in which it is involved and the desiredfinal product (Fig 7.1); this chapter wants to provide a general overview on the pri-mary catalytic systems involved in the most widespread conversion processes of thesyngas, focusing on the results of the latest research The main process is the methanolsynthesis that produces one of the most flexible chemical commodities and energysources (Ajay et al., 2014) Methanol is used as feedstock in synthesis of formalde-hyde and acetic acid; additives in adhesive, foams, plywood subfloors, and windshieldwasher; methyltert-butyl ether (MTBE), a gasoline component; and dimethyl ether(DME), a clean-burning fuel Methanol is also used as additive into gasoline or asvehicle fuel itself

The Fischer-Tropsch process (FT) converts the syngas into a mixture of productsrefined to synthetic fuels, lubricants, and petrochemicals (de Klerk, 2000); dependingfrom the sources, the overall process, from the raw material to the final product, isnamed GTL (gas to liquid), CTL (coal to liquid), or BTL (biomass to liquid) (van

de Loosdrecht and Niemantsverdriet, 2013)

The Haber-Bosch process (Haber, 2002) allows to obtain ammonia by reactingnitrogen with pure hydrogen, usually obtained from syngas by removal of carbonmonoxide with the water-gas shift reaction (Palma et al., 2016) eventually coupledwith methanation (R€onsch et al., 2016), preferential oxidation or by a hydrogenpermselective membrane reactor (Piemonte et al., 2010), and CO2 sequestration.Ammonia is an important commodity for the fertilizers industry, is the precursor ofurea and ammonium salts (nitrates and phosphates), of the nitric acid and polyamides.The carbonylation processes allow to introduce the carbonyl group (CO) intoorganic or inorganic substrate, by reacting with pure carbon monoxide, obtained fromsyngas by reverse water-gas shift reaction (De Falco et al., 2013) Interesting exam-ples of carbonylation are the Monsanto process (Paulik and Roth, 1968), that allows toprepare the acetic acid by carbonylation of methanol and, the Mond process for theextraction and purification of Nickel (Mond et al., 1890)

7.2 Fischer-Tropsch synthesis

The Fischer-Tropsch synthesis (FTS) is an important catalytic process used for theconversion of syngas (derived from coal, natural gas, biomass, or other carbon-containing species) to hydrocarbons with different chain lengths The product selec-tivity is strongly dependent on temperature, pressure, and catalyst choice(Ghareghashi et al., 2013) Generally, low-temperature FTS is carried out overCo-based catalyst in the temperature interval of 190–250°C and at 20–40 bar yieldingproducts with high average molecular weight (middle distillates and waxes) Con-versely, iron-catalyzed process are driven at 340°C and 20 bar, with the aim ofobtaining short-chain hydrocarbons (fuels and petrochemicals) (Delparish andAvci, 2016; Shin et al., 2013) In order to face the reaction exothermicity and avoidhot spots or rapid catalyst deactivation, multitubular fixed bed, with external cooling(for LTFTS), and slurry bubble column reactors (for HTFTS) are commerciallyselected for the FTS process (Park et al., 2011) In the first case, considerable fraction

of the liquid reaction products has to be recycled to the reactor to remove the reactionenthalpy, thus increasing pressure drops and making the reactor trickier to be operatedand less flexible to be scaled On the other hand, in slurry reactors, the temperature isuniform and pressure drops are low, being related to the hydrostatic pressure of liquid,and the internal mass transfer limitations are ruled out by loading the catalyst as finepowder However, high aspect ratios (reactor height/diameter) and staging have to beused to limit back-mixing phenomena Also, particular attention has to be paid, espe-cially during the size-scaling processes, both to the prevention of the catalyst attritionand to the design of an efficient tool for the separation of the catalyst from the liquid(Visconti et al., 2011) In addition, during FT synthesis, even though the reactants are

in the gas phase, the pores of the catalyst are filled with liquid products, and the fusion rates in the liquid phase are typically three orders of magnitude slower than inthe gas phase; the increasing transport limitations may result in CO depletion andlower C5 + selectivity In a fixed-bed reactor, the selectivity problem can be solved

dif-by using catalyst pellets where the catalytic material is deposited in a thin outer layer(eggshell catalysts), while in a slurry reactor, the selectivity issue is faced by usingsmall catalyst particles (Liu et al., 2009) However, in the attempt of overcomingthe drawbacks of commercially available technologies for FT synthesis, differenttechnologies including the adoption of structured fixed-bed reactors, based on honey-comb monolith foams, knitted wires, or cross flow structures, and microchannel reac-tors have been recently proposed (Pangarkar et al., 2009; Twigg and Richardson,2002; Cao et al., 2009) For example, monolithic catalysts assure low pressure drop,high gas-liquid mass transfer rates in two-phase flow, the possibility of using high liq-uid and gas throughputs, and a good temperature control (Kapteijn et al., 2005; Hilmen

et al., 2001) For intensification of mass transfer between synthesis gas, liquid ucts and solid catalysts, alternative catalyst geometries like honeycombs, structuredpackings, and foams have also been developed (Guettel et al., 2008a)

prod-All group VII metals have noticeable activity for the hydrogenation of carbon oxide to hydrocarbons However, only ruthenium, iron, cobalt, and nickel have

mon-catalytic characteristics that allow considering them for commercial production ever, nickel catalysts, under practical conditions, produce too much methane; more-over, ruthenium is too expensive, and its worldwide reserves are insufficient(Khodakov et al., 2007) Cobalt and iron were proposed as the first catalyst by Fischerand Tropsch Fe-based catalysts, despite being less expensive than Co ones (Jin andDatye, 2000; van Berge et al., 2000), strongly suffer for deactivation by coke and pro-mote H2O formation by water-gas shift reaction (WGS) Sulfur content lower than 0.2and 0.1 ppm are mandatory for normal operation of Fe- and Co-based catalysts,respectively (Khodakov et al., 2007) Co catalysts are generally more resistant to attri-tion and are widely preferred for use in slurry-type reactors Despite the industrial-scale development of FTS process, the activity and stability of the catalyst need to

How-be improved At that hand, the addition of suitable promoters and the selection ofproper supports provide a reasonable route for the enhancement of FTS catalyst per-formances In the following sections, the performances of Co- and Fe-based catalystswere reviewed and discussed

7.2.1 Co-based catalysts

Al2O3, despite having lower surface area than SiO2and TiO2, is commonly used assupport material for Co due to the strong metal-oxide interactions, its good mechanicalperformances, and resistance to attrition (de la Osa et al., 2011; Prieto et al., 2009) Onthe other hand, when Al2O3is selected as support, it is mandatory to avoid the forma-tion of hardly reducible cobalt aluminates that are responsible for activity reduction(Jongsomjit et al., 2001) Iglesia et al (Iglesia, 1997) found that for large cobalt par-ticles supported on Al2O3, SiO2, and TiO2, the FT reaction rate depends on the number

of available cobalt surface atoms and that hydrocarbon selectivity is only slightlyaffected by cobalt dispersion A different phenomenon is observed for small cobaltparticles, having a strong impact on product selectivity: Particle size in the range

60–80 A˚ improves olefinic products yields, while for smaller dimensions, C5+ tivity also decreased (Khodakov, 2009) The synthesis of Co-based catalystssupported on alumina nanofibers was shown to assure a homogenous metalparticle-size distribution After catalyst ultrasonication, more active cobalt particleswere generated, which marked improved C5 + selectivity lowering methane produc-tion In addition, even at high reaction temperature and under much higher CO con-version (79%), a quite stable activity was observed at 230°C, 20 bar, and H2/CO¼2over 300 h of reaction (Liu et al., 2016) Flame spray pyrolysis technique was alsosuccessfully employed for controlling catalytically active Co particles deposition

selec-on Al2O3: a good catalytic activity was recorded at the above operative conditions(Minnermann et al., 2013) Conversely, for Co/γ-Al2O3catalysts, it was shown thatpartial pores prefilling by incipient wetness impregnation of Al(NO3)3resulted in cat-alyst similar to theeggshell systems (Jacobs et al., 2016), which are able to decreaseC1–C4 light gas selectivity, improving, at the same time, C5+ selectivity

Due to the need of limiting pressure drop and the consequent necessity of adopting

“big” catalyst pellets, low-temperature Fischer-Tropsch synthesis in industrial bed reactors may suffer of strong intraparticle mass transport limitations, which are

fixed-known to result in decreased CO conversion rate and C5 + selectivity Upondecoupling the pellet diameter and the diffusive length,eggshell catalysts represent

an engineering solution for the intensification of the Fischer-Tropsch reactors In thisregard, Fratalocchi et al (Fratalocchi et al., 2015) showed that 600μm pellets, withcatalytically active layers, 75μm thick, grant a remarkable combination of high COconversion rate and high C5 + selectivity at 220–240°C, 25 bar, and H2/CO ratio of1.73, thus resulting extremely interesting for operations in reactors 3–6 m long.Concerning the impact of cobalt aluminate formation on catalyst activity, Moodley

et al (Moodley et al., 2011) found an enhancement of aluminate content with waterpartial pressure during FTS at 230°C, 10 bar, and H2/CO¼1.5 The presence of water

is also regarded as one of the main causes of catalyst sintering during Fischer-Tropschprocess (Bezemer et al., 2010) Sintering mechanism for Co/Al2O3catalysts in slurryreactors was accelerated by the formation of intermediate surface cobalt-oxide spe-cies, and the deactivation was favored by increasing H2/CO ratios in syngas or bythe presence of even small amounts of water (Sadeqzadeh et al., 2013) Besidesintering and aluminates formation, the activity drop commonly observed over FTScatalyst is related to the deposition of carbonaceous species on catalyst surface.The research of Pena et al (Pen˜a et al., 2014) was focused on the identification ofthe molecular structure of carbon species formed over a Co/Al2O3catalyst in a slurryreactor Carbon adsorbed on spent catalyst was mainly constituted by α-olefins,n-paraffins, branched alkanes/alkanes, aldehydes, and ketones, while carboxylic acidswere mostly detected at high water partial pressures In particular, the increase in COconversion enhanced the isomerization of α-olefins favoring the formation ofbranched alkanes/alkanes Carbon species are probably nucleated on the cobalt par-ticles and then migrate to alumina support and coke localized on the support showedhigh reluctance toward hydrogenation

The addition of promoters to Co/Al2O3catalysts was shown on one hand, to preventCo-aluminate formation due to establishment of an intimate contact with the metal(Nabaho et al., 2016a) and, on the other hand, to limit catalyst deactivation.Park et al (Park et al., 2012) investigated the impact of phosphorous addition toCo/Al2O3 catalyst on deactivation induced by lumps formation in the presence ofwater.γ-Al2O3, due to its hydrophilic properties, can undergo phase transformation

to pseudoboehmite (with low attrition resistance) in the presence of water vapor duced during FT reaction, forming fragmented fine catalyst powder The deposition ofheavy hydrocarbons on these fragments causes the formation of aggregate catalystlumps Conversely, phosphorous addition is able to suppress alumina hydrophilicproperties and reduce heavy hydrocarbon deposition on catalyst, thus preventing itsdeactivation (Fig 7.2) It was also reported (Tan et al., 2011) that the deposition ofsmall boron quantities on Co/γ-Al2O3catalyst can hinder the deposition, nucleation,and growth of resilient coke on catalyst surface, without affecting initial activity andselectivity at 240°C and 20 bar

pro-The addition of noble metal promoters (Pd, Pt, Re, and Ru) was shown to improveactivity and stability of Co/Al2O3catalysts (Ma et al., 2012) Improved CO conversionwas observed at 220°C, 22 bar, and H2/CO¼2 over the promoted catalysts, with Ptand Pd enhancing oxygenate formation and Re and Ru slightly decreasing it At fixed

CO conversion (50%), Re and Ru improved CH4and C5 + selectivity, whereas theopposite effect was observed for Pt and Pd promoters The latter metals also increased2-C4 olefins selectivity and WGS activity of the final catalyst On the other hand, Ptaddition had a negligible effect on C4 olefin isomerization Concerning Co catalystssupported on Al2O3or SiO2prepared via plasma technology (Chu et al., 2015), thepromotion by noble metals was shown to improve both cobalt dispersion and reduc-ibility, thus enhancing the FT reaction activity.

Other porous supports (including SiO2, TiO2, activated carbon, and zeolite) areusually selected for commercial Co-based catalysts (Lu et al., 2015; Shi et al.,2012; Eschemann et al., 2015) However, the combination of two types of oxideswas found to improve the pore structure, cobalt dispersion, and reducibility The effect

of alumina incorporation (0–3 wt%) into a Co/SiO2catalysts on product gas tion was investigated bySavost’yanov et al (2017) Trace of alcohols and olefins wereonly detected over the undoped catalyst and their content increase with alumina load-ing (Table 7.1) Over the 1 wt% sample, the molecular weight distribution becamenarrower, increasing the C8–C25 fraction A further reduction of SiO2/Al2O3ratiocaused the opposite effect Combustion synthesis method was employed for the prep-aration of Co/SiO2and Co/SiO2-Al2O3catalysts, and a strong impact of preparationmethod on product gas distribution was observed (Ail and Dasappa, 2016): the inno-vative catalyst increased the yield to C6 + products at 230°C, 30 bar, and H2/CO¼2.3,resulting in the formation of long-chain hydrocarbons waxes (C24 +) with respect tothe middle distillates (C10–C20), normally generated over impregnated catalysts.However, the overall C6 + yield was further increased by Al2O3 addition, due tothe marked improvement (48%) in cobalt dispersion promoted by alumina

distribu-Venezia et al (Venezia et al., 2012) modified SiO2support by TiO2grafting andobserved an improvement in C5 + selectivity especially at high space velocities

(GHSV¼7200 h1at 210°C, 20 bar, and H2/CO¼2) In addition, CoO oxide action with the doped support was enhanced, avoiding the particle mobility that canlead to catalyst deactivation by sintering A bimodal ZrO2-SiO2support was selectedfor jet fuel direct synthesis via FTS reaction with different 1-olefins as additives(Li et al., 2016a) Olefins cofeeding effectively shifted the product distribution towardjet fuel range, markedly suppressing CH4, CO2, and light hydrocarbons (C2–C4) for-mation The large pores of the bimodal support, in fact, provided efficient pathwaysfor reactants conversion and products diffusion, while the newly formed small poresassured high metal dispersion.

inter-Rare-earth oxides, able to remarkably enhance catalyst reducibility, were shown to

be beneficial for improving long-chain hydrocarbons selectivity (Spadaro et al.,

2005) CeO2addition (5 and 10 wt%) to Co/ZrO2catalysts promoted the formation

of larger cobalt particles, inhibiting water reoxidation and Co particles aggregationduring the reaction and resulting in a better resistance toward deactivation (Zhang

et al., 2016a) Fig 7.3displays CO conversion with time onstream for doped andundoped catalysts: CeO2-modified samples assured a good stability during 100 h ofreaction However, the deactivation rate was affected by the amount of ceria added.However, a rapid deactivation was observed in the initial stage of the reaction over the

10 wt% catalysts, caused by the initial smaller pores that were filled by the liquidwaxes produced during the FTS synthesis

Carbon materials, including activated carbon, carbon nanotubes and nanofibers,carbon spheres, and mesoporous carbon, are also reported as catalytic support for

Co, due to their several benefits with respect to the conventional oxides supports: thesematerials display high purity, high mechanical strength and thermal stability, andlarge surface area (Fu and Li, 2015) Moreover, having a hardly reducible surface,

Co particle reducibility can be improved Carbon porous structure can also be properlycontrolled in order to promote the cobalt dispersion (Ha et al., 2013).Dı´az et al (2013)

carried out FT reaction over Co catalysts supported on carbon nanofibers prepared atthree different calcination temperatures (750°C, 600°C, and 450°C, denoted as Sam-ples 1, 2, and 3, respectively) At 250°C, 20 bar, and H2/CO ratio of 2, the Samples 1and 2, having a medium pore radius, displayed high catalytic activity without

Table 7.1 C5 + product distribution in the FTS over Co/SiO2-Al2O3catalysts at 210 °C, 20 bar, and H2/CO ratio of 2

deactivation; however, the promotion of WGS and methanation reaction led to icant CO2and CH4production The Sample 3, which was less active and suffered fromdeactivation, showed an improvement in C5 + selectivity Metal sintering wasobserved over all the catalysts and, especially, over the Sample 3, due to its lowerstructural order with respect to the other two catalysts The performances ofCo-based catalysts supported on carbon nanotubes (CNT) and graphene nanosheets(GNS) for FT reaction at 220°C, 18 atm, and H2/CO¼2 were compared in order toevaluate the effect of morphology and structure on catalyst stability (Karimi

signif-et al., 2015)

The difference, in terms of SSA (Table 7.2), between the bare GNS and CNT can beattributed to the nature and textural properties of graphene nanosheets Higher poros-ity was also observed over the GNS samples, which can be related to the interlayerspacing of them After 480 h of reaction, a specific area reduction of 20% and 3%,respectively, for the CNT and GNS catalysts, was observed Moreover, the extent

of pore blockage for the CNT catalyst is higher than that of the GNS sample, as a sequence of its higher rates of sintering and clusters growth During stability tests, dCo

(dotted line), Co/(5)CeO2-ZrO2

(dash-dotted line), and Co/(10)CeO2-ZrO2(dashed line);

220°C, 20 bar, and H2/CO¼2

Table 7.2 Specific surface areas (SSA), porous volume (Vp),

and cobalt average crystallite sizes (dCO) for Co/CNT

and Co/GNS catalysts ( Karimi et al., 2015 )

increased for both the catalysts; however, a more significant crystal growth wasrecorded over the Co/CNT catalyst The latter sample displayed a drop of activity

of 15.8% in the first 120 h of reaction, while in the remaining period, it only drops

of 4.9% Conversely, due to the lower extent of sintering phenomena and GNS philic properties that limit water deposition and cobalt reoxidation, only a conversionreduction of about 2.7% in the first period and 1.2% between 120 and 480 h wasobserved over the Co/CNT catalyst The relationship between C5 + selectivity andmetal particle sizes for Co catalysts supported on carbon nanotubes and sphereswas also studied (Xiong et al., 2011) In order to limit water effect, measurementswere carried out at a low CO conversion (4%) The change in C5 + selectivity withparticle sizes (Fig 7.4) can be explained considering that large Co particles promotethe formation of bridge-type adsorbed CO, more active and more easily dissociated,which enhanced the chain growth and C5 + selectivity Co supported over spheres,however, displayed slightly higher C5 + selectivity for a similar sized particle In fact,over nonporous carbon spheres, all cobalt particles are dispersed on the outer surface

hydro-of the support Therefore, the hydrogen concentration around the active cobalt cles is the same as that in the reactor, thus leading to less light hydrocarbon production

parti-Chernyak et al (2016)investigated the effect of oxidation time (1–15 h) selectedduring Co supported over carbon nanotubes preparation on their performances for FTsynthesis at 190°C, atmospheric pressure, and H2/CO¼2 The sample oxidized for 9 hdisplayed the highest CO conversion and yield to C5 + products Selectivity to heavyhydrocarbons is almost the same over the catalyst treated for 3, 9, and 15 h whileincreased over the sample oxidized for 1 h

Syngas mixture, especially when produced from coal or biomass, can be CO2rich, and carbon dioxide can significantly affect FT activity.Dı´az et al (2014)stud-ied the effect of CO2 cofeeding on the catalytic performances of a Co catalystsupported over carbon nanofibers at 220–250°C, 20 bar, and H2/CO¼2 Increasingtemperatures assure higher catalytic activity and the rate of undesired reactions(WGS and methanation) However, once reaction temperature was fixed at 250°C,

-Cobalt particle size (nm)

0 92 93 94 95 96 97 98 99 100

Fig 7.4 Influence on Co particle

size on C5 + selectivity over Co/

carbon nanotubes (square) and

spheres (triangle); 225°C, 8 bar,

and CO/H2ratio of 0.5

the presence of CO2in the feed gas was demonstrated to affect the rate of catalytichydrogenation of CO and product distribution H2/CO2, in fact, behaves as a mildoxidizing agent on Co/CNFs under selected conditions In the absence of CO, second-ary catalytic activity decayed and methanation process raised a maximum Therefore,the decrease of CO conversion and C5 + selectivity with CO2addition was attributed

to the lower reactivity of this component CO2also competed with Co for the tion on catalytic sites and C7–C20 hydrocarbon product distribution was shiftedtoward lower-molecular-weight hydrocarbons by feeding higher amounts of CO2,mainly caused by the easily desorption of the chains Besides CO2, biomass-derivedsyngas may contain different organic and inorganic impurities, including NH3 For Cocatalysts supported on Al2O3, TiO2, and SiO2, the addition of 10 ppmv of NH3during

adsorp-FT synthesis at 220°C, 19 bar, and H2/CO¼2 caused a significant deactivation forall supported cobalt catalysts, but the rate of deactivation was higher for the silica-supported catalysts relative to the alumina- and titania-supported catalysts(Pendyala et al., 2016) Ammonia addition had a positive effect on product selectivity(i.e., lower light gas products and higher C5 +) for alumina- and titania-supportedcatalysts compared with ammonia-free conditions, whereas, the addition of ammoniaincreased lighter hydrocarbon (C1–C4) products and decreased higher hydrocarbon(C5 +) selectivity compared with ammonia-free synthesis conditions for the silica-supported catalyst However, after H2treatment, both titania- and alumina-basedcatalysts were completely regenerated, while over Co/SiO2samples, the loss of activ-ity was irreversible, due to the formation of inactive cobalt-support compounds

Co catalysts supported over manganese oxides were also employed in FT due totheir high yields toward light hydrocarbons and low CO2 and methane selectivity(Zhou et al., 2015) However,Iqbal et al (2016)found that the use of activated carbonsupport for CoMnOx catalysts improved both the activity and selectivity to C2 +hydrocarbons, while further lowering methane (from 22.1% to 7.0%) and carbon diox-ide selectivity (from 37.0% to 20.4%) with respect to unsupported catalysts at 240°C,

6 bar, and CO/H2ratio of 1

In addition, carbon materials have been considered for the synthesis of hybridcomposites, such as carbon nanotubes-Al2O3and carbon nanofibers-SiO2 In partic-ular, carbon nanotubes enhanced the degree of reduction of Co, thus limiting C5 +production (Zaman et al., 2009).Chernyak et al (2015)also proved that the increase

in oxygen-containing groups on catalyst surface by H2O2 oxidation of Co/carbonnanotubes-Al2O3 assured a redistribution of pore sizes, with smaller Co particlesand a reduction on C5 + yield

Multifunctional catalysts, containing different types of active sites, have alsobeen proposed for FT synthesis For example, zeolite, due to the acidity ascribable

to Al in the structure, promoted secondary reactions that include the formation oflighter hydrocarbons, both aromatic and branched In addition, depending on thetype of zeolite, it is possible to restrict the chain growth, thus yielding to lighterhydrocarbons (Plana-Pallejà et al., 2016).Xing et al (2015) developed a properpreparation method to create hierarchical pores in a mesoporous zeolite for facilelytuning the product distribution during Fischer-Tropsch synthesis A series of cata-lysts was prepared by acid and basic leaching for different leaching times, showing

that, for a leaching time of 4 h, isoparaffin selectivity, at 260°C and 9.9 atm, reached

up to 52.4% and middle hydrocarbons become the main products due to theoptimized hydrocraking and isomerization function afforded by the hierarchicalzeolite structure It was also demonstrated (Xing et al., 2016) that the employment

of hierarchically spherical (0.5–1 μm) Co-based zeolite catalysts, having aggregatenanorods structure, improved isoparaffin and C5–11 hydrocarbons selectivity

On the other hand, CH4 and C12 + selectivities were lower than those observedfor a commercial HZSM-5 supported Co catalyst (at 240°C, 10 bar, and H2/COratio of 2) Lee et al (2010) evaluated the effect of the temperature and time ofhydrogen treatment on the performances of Co-supported zeolite catalysts Thesample treated at 500°C for 18 h (catalyst A) displayed finally dispersed metalclusters inside zeolite pores Increasing the temperature and the treating time, moreand more metal clusters inside zeolite cages migrated out of the pores and agglom-erate into large, immobile aggregates at the external surface of zeolite crystals

As a consequence, the sample A displayed low hydrogen chemisorption, showing amore difficulty in olefins hydrogenation to the corresponding paraffins at 270°Cand H2/CO ratio of 2 As a result, the slow hydrogenation ability enhanced chainpropagation, thus improving C5 and higher hydrocarbon formation A series of

Co catalysts supported on physically mixed ZSM-5/SBA-15 were tested for FTS

at 240°C and 20 bar (Wu et al., 2015) The composite-supported catalysts displayedimproved catalytic performances over the respective single material-supported

Co catalysts In particular, the sample containing 20 wt% of ZSM-5 reachedthe maximum CO conversion (90.6%,Table 7.3), the highest C5–C22 hydrocarbonselectivities (70.0%), and the minimum formation of light hydrocarbons (13.3%for CH4 and 7.0% for C2–C4 alkenes) Martı´nez et al (2007) investigated theperformances of a hybrid catalyst prepared by mixing Co/SiO2 and zeolite At

250°C, 20 bar, and H2/CO¼2, zeolite promoted the cracking of C13+ long-chainn-paraffin formed on the Co particles, mainly yielding to gasoline-range branc-hed products However, the accumulation of carbonaceous species caused a reduc-tion in the latter products yield with time onstream It was observed that the

Table 7.3 CO conversion (XCO) and selectivity results for the FTS over Co/ZSM-5/SBA-15 catalysts; 240 °C, 20 bar, and H2/CO 52 ( Wu et al., 2015 )

deactivation rate was little affected by zeolite acidity and increased with the zeolitepore dimension Coke molecules mainly comprised two- and three-ring aromatics inlarge pore zeolites, while it was predominantly of paraffinic nature in the most sta-ble HZSM-5 Aromatic coke is likely formed from light olefins produced in the FTsynthesis through consecutive oligomerization, cyclization, and dehydrogenationreactions.

As observed above, the porous characteristics of the support significantly affectactivity and hydrocarbon selectivity in FT synthesis (Xiong et al., 2008) In thisregard, it was found (Wei et al., 2016) that three-dimensional mesoporous silica foamsnot only provided cavities to suppress Co particles growth but also enhanced metalsdispersion and reducibility In particular, foams having large- and open-pore structurefavored the occurrence of secondary reactions, leading to higher C5 + selectivity At

210°C, 10 bar, and H2/CO¼2, a stable behavior was observed However, for highertemperatures (250°C), cobalt sintering and the formation of Co-silica compounds cau-sed rapid deactivation, and improvements in endurance performances were reached bycarbon coating or Al doping It was also shown (Labuschagne et al., 2016) that Cosupported over porous SiC catalysts, calcined at 550°C, assured higher stability duringlong-term tests than the Co/Al2O3counterparts Moreover, an aggressive acid washing

of the catalyst led to superior activities, even at high water partial pressures.Noble metals addition is a well-known route to enhance activity of Co-basedFischer-Tropsch catalysts, due to the improvement of cobalt-oxide reduction andthe increase in the active sites number (Das et al., 2003).den Otter et al (2016)eval-uated the effect of Pt addition to the activity of Co/γ-Al2O3and Co/Nb2O5catalysts

at 220°C At 1 bar, the cobalt-weight-normalized activity of Co/γ-Al2O3and Co/Nb2O5was found to increase by a factor of 1.7 and 2.8 upon Pt promotion (Fig 7.5, left) ForCo/γ-Al2O3, low activity and only slight influence of Pt promotion were observed at

1 bar At 20 bar, no large influence of Pt promotion on the cobalt-weight-normalizedactivity of Co/γ-Al2O3was observed, whereas for Co/Nb2O5, a factor of 2.4 increase inthe activity per unit weight of cobalt was measured, without affecting C5 + selectivity(Fig 7.5, right) Similar catalytic activity for FT reaction was observed over PtCo and

0 0 5 10 15 20

20 40 60 80 100 120 140 160

Time (h)Fig 7.5 Cobalt-weight-normalized activity in Fischer-Tropsch synthesis at 1 bar, COconversion 1–5% (left), and 20 bar, CO conversion 21–34% (right), 220°C and H/CO¼2

AuCo catalysts supported on Al2O3(Nabaho et al., 2016b) However, a key factor toconsider for the commercial viability of these catalysts is their activity and selectivityover extended time onstream In particular, Au-promoted samples reduce less readilythan Pt-promoted samples with each oxidation-reduction regeneration cycle, whichimplies that Au-Co/Al2O3may be a less attractive option in commercial applications.

Pendyala et al (2014)evaluated the effect of PtCo/Al2O3catalyst particle (sieve) onperformances for Fischer-Tropsch synthesis Four different size ranges were selected(20–63, 63–106, 106–180, and 180–355 μm) The increase of catalyst sieve size wasaccompanied by losses in CO conversion, except for the smallest sieve range Suchlosses can be related to the filling of the interior of the catalyst particles with heavywaxes, thereby blocking catalytically active sites Due to the presence of waxes, thesmall particles tended to flocculate to larger clusters or were small enough to move atthe speed of the liquid The effect of sulfur poisoning on PtCo/Al2O3FT catalysts wasalso studied (Barrientos et al., 2016) S clearly affected catalyst activity and stability at

210°C, 20 bar, and H2/CO¼2.1, decreasing long-chain hydrocarbons selectivity Inaddition, sulfur significantly enhanced secondary hydrogenation of olefins However,such effects were clearly related to S content, and the poisoning is negligible atmoderately low sulfur coverages (0–250 ppm)

Au addition to Co/Al2O3and Co/SiO2catalysts was shown to improve Co particlesreduction and catalyst activity (Jalama et al., 2011) This positive effect was found

to be similar to that observed with other noble metals such as Ru and Re However,

as a drawback, Au loadings below or equal to 1 wt% on alumina and in the range0.5–5 wt% on silica and titania increased methane selectivity Pirola et al (2014)

observed that the addition of low Ru or Pt loadings to Co/SiO2catalysts improved both

CO conversion and total yield toward the desired products (C2 + species that exclude

CH4and CO2) at 220°C, 20 bar, and H2/CO¼2 This improvement is related on onehand to the formation of a Ru1–yCoysolid solution and on the other hand to the gen-eration of a PtCo intrametallic compound Ru and Re were also added to Co/SiO2cat-alysts prepared by conventional drying and high-temperature supercritical drying Theinnovative preparation method led to the production of a less-reducible cobalt silicate,and the noble metal addition improved cobalt species reducibility and dispersion, par-tially suppressing the Co particles coverage by SiO2layers As a result, higher olefinproduction was measured, while the C11 + selectivity was lower than that observedover the conventional catalysts at 230°C, 20 bar, and H2/CO¼2 (Iida et al., 2013)

Osakoo et al (2013) prepared silica-supported catalysts by impregnation andcoprecipitation using a reverse micelle technique Over latter sample, tested at

230°C, 5 bar, and H2/CO¼2, lower Co3O4 particle sizes were measured, whichreduced methane and C2–C4 selectivities However, the addition of Pd in the range

0–1 wt% increased methane formation and negatively affected CO conversion: thebest results were observed for the 0.2 wt% catalyst prepared by coprecipitation, whichdisplayed 34.8% of CO conversion and high mole fraction (0.38) of paraffins in thegasoline range (C5–C9) The effect of the addition of different noble metals (Ag, Au,and Rh) to Co/SiO2catalysts was investigated in a work ofYan et al (2011) Au and

Rh showed a promoting effect on the FT activity, whereas the addition of Ag had adetrimental effect Moreover, the addition of small amounts of Rh (0.1–0.5 wt%)

improved CO conversion by 50% without affecting catalyst selectivity For Co/TiO2catalysts promoted by Ru, it was observed that the interaction in the bimetallic par-ticles can reduce site blockage by carbonaceous species (Eschemann et al., 2016).Similarly, for Re-promoted catalysts, the improved activity and stability are due tothe interaction of noble metal with cobalt Noble metals were also shown to be moreactive hydrogenation catalysts than Co, thus accelerating CO hydrogenation The per-formances of a Co/CeO2catalyst were compared with a Pt-promoted sample duringFTS at 220°C, 1 bar, and H2/CO¼2 (Lorito et al., 2016) Similar activities wereobserved in the two cases, as Pt was poorly active for CO hydrogenation under theseconditions The improvement of Co dispersion promoted by Pt was probably respon-sible for the increase in methane and the decrease in propene formation.

In order to lower the catalyst price, the substitution of noble metals with cheapercomponents was also proposed For example, bimetallic Co-Ni catalysts showedhigh activity for FT reaction, displaying high selectivity to gasoline-range hydrocar-bons (C5–C12) (Calderone et al., 2011).Shimura et al (2015)investigated the influ-ence of Co/Ni ratio and impregnation sequence on the activity of Al2O3supportedcatalysts Low Ni loading slightly increased CO conversion rate at nearly constantC5 + selectivity, but excess Ni loading largely decreased CO conversion rate andC5 + selectivity at 230°C, 10 bar, and H2/CO¼1.91 The catalytic activity did notdepend on impregnation sequence, when Ni loading amount was low However, when

Co was substituted with large amount of Ni, catalysts prepared by sequential nation method (Ni first and then Co) showed higher activity than those prepared bycoimpregnation method and reverse sequential impregnation method (Co first andthen Ni) These results indicate that catalysts with a Co-rich surface would be betterthan those with a Ni-rich surface The best catalyst was 19%Co-1%Ni/Al2O3thatexhibited 1.6 times as high activity as 20%Co/Al2O3catalyst prepared by a conven-tional impregnation method Co-Cu catalysts supported on Al2O3were also investi-gated for Fischer-Tropsch reaction at 220°C, 16 bar, and H2/CO ratio of 2 (Jacobs

impreg-et al., 2009) As shown inTable 7.4, the increase in Cu loading decreased CO sion, due to a poisoning of surface Co sites At similar conversion levels, the growth in

conver-Cu content slightly increased methane production reducing, at the same time, C5 +yield However, a further improvement in Cu loading led to a prohibitive increase

in methane selectivity (21.6% vs 9.2%) and precipitous drop in C5 + selectivity(47.4% vs 80.6%)

selectivity (Si) over mono- and bimetallic Co/Al2O3 catalysts;

220 °C, 16 bar, and H2/CO ratio of 2 ( Jacobs et al., 2009 )

Honeycomb monoliths have been developed as support for the cobalt phase by eral researchers CoRe/γ-Al2O3monolithic and powder catalysts were tested for FTreaction, demonstrating that, at similar methane selectivity, the structured catalystsassure higher reaction rates In fact, the reaction rate enhancement is most probablycaused by the advantageous mass transfer characteristics of the monolithic catalyst(Guettel et al., 2008b) Moreover, for tests carried out between 210°C and 232°C,

sev-at 25 bar, and H2/CO ratio of 2, higher C5–C18 liquid fractions and olefin/paraffinratios are obtained by conducting the FT reaction over the ceramic monolith catalystcoated with the bimetallic catalyst In addition, if significant wax formation wasobserved with the packed particle bed, wax was not detected in the liquid productsfor FT synthesis over monoliths (Liu et al., 2009) Ceramic foam, which could be

an alternative to ceramic monoliths, has also been extensively developed for the

FT reaction The advantage of foam versus a straight-channel monolith is the highdegree of radial mixing, which improves reactant distribution and convective heattransfer (Liu et al., 2014).Lacroix et al (2011)compared the FT activity of two foamcatalysts (Co/SiC and Co/Al2O3) at 220°C, 40 bar, and H2/CO¼2 At medium con-version (<50%), the two catalysts display similar C5+ selectivity, indicating that theintrinsic selectivity between the two catalysts is close from each other However, whenthe CO conversion was increased to 70%, a significant difference in terms of the C5 +selectivity was observed between the two catalysts, that is, 80% on the Co/SiC and54% on the Co/Al2O3, which indicate that under severe FTS reaction conditionsthe SiC seems to be more suitable support than alumina Additional catalytic test con-ducted on a hybrid support, that is, Al2O3-coated SiC foam, again confirmed the highC5 + selectivity under a similar severe reaction conditions in the presence of a SiCstructure underneath of the alumina layer that plays a role of heat disperser

7.2.2 Fe-based catalysts

Iron catalysts are efficiently employed for olefin synthesis via Fischer-Tropsch tion for several reasons, which includes high selectivity, low methanation activity,availability, low price, and lower sensitivity to poisons (Torres Galvis and de Jong,

reac-2013) Iron catalysts can be also efficient for utilization of syngas produced from coaland biomass that have lower hydrogen content (H2/CO<2) In fact, Fe catalysts pro-mote WGS reaction, providing additional H2 Moreover, according to the promotersand the operative conditions used, Fe also assures high flexibility, modulating theselectivity to alkenes or branched hydrocarbons (Cano et al., 2017).α-Al2O3, SiO2,and carbon materials are commonly employed as support substances for Fe-based cat-alysts Such weakly interactive supports were selected due to their capability in pro-moting iron species activation for the formation carbides, which improved FTperformances Generally, fresh Fe catalysts mainly containα-Fe2O3, which can beconverted to different reduced iron species (such as Fe3O4, FeO, or metallic iron)under H2 atmosphere Furthermore, the reduced Fe3O4 is transformed continually

to metastable FeO phase or reduced directly to metallic iron It is generally acceptedthat the strong interaction of metal support in the supported iron catalysts may stabi-lize the metastable FeO Under CO or syngas atmosphere, these reduced iron species

could be converted to different types of iron carbides, which are deemed as activephases for FTS (Ding et al., 2015).

In order to highlight the impact of metal-support interaction on activity and ity for FT synthesis, the performances of an Fe bulk catalyst were compared with

stabil-an Fe/Al2O3sample, prepared by a combination of coprecipitation and method (Xu et al., 2016) As shown inFig 7.6, Fe catalyst had higher FTS activityand deactivated slowly with time onstream, whereas the CO conversion of Fe/Al2O3

spray-dried-catalyst is stable or even increases slowly Apparently, incorporation of Al2O3intoiron catalyst decreases the catalyst activity but improves the catalyst stability Thelower activity of Al2O3 incorporated catalyst is probably due to the strongFe-Al2O3interaction, which inhibited the reduction and the carburization of ironspecies

Xu et al (2016)proposed an Fe-α-Al2O3catalyst modification in view of limitingtheir poisoning problem, which is an issue problem in FT synthesis The addition ofsulfur to the catalyst significantly decreased activity, due to the inhibition of COdissociation and the limited formation of iron carbide phases The presence of sulfuralso suppressed the formation of C5 + hydrocarbons and shifted the products toC2–C4 hydrocarbons At the same time, the olefin to paraffin (O/P) ratio of theC2–C4 hydrocarbons decreased with increasing S/Fe molar ratio (operative condi-tions, 200°C, 20 bar, and H2/CO¼1) However, the resistance to sulfur poisoning

of the Fe/α-Al2O3catalyst was found to improve with increasing reaction temperaturefrom 310°C to 350°C

Several authors have shown that FTS rates of Fe-based catalysts increase with

K and Cu addition and that the best performances can be achieved when both

Fig 7.6 Activity and stability of Fe and Fe/Al2O3 catalysts; 260°C, 15 bar, and H2/CO¼0.67

K and Cu are present Such species, in fact, promote the formation of Fe carbide withhigher dispersion (Li et al., 2002) By investigating the effect of calcination temper-ature, in the interval 250–650°C, on the performances of Fe/Cu/K/Al2O3catalysts forFTS, it was found that high calcination temperature can result in more Al2O3into ironoxide lattice, which further enhanced the metal-support interaction and led to the sep-aration of iron oxide and CuO Moreover, it weakened the promotional effect of CuOand K2O, thereby severely suppressing the reduction and carburization of the catalyst(Wan et al., 2007).

A beneficial effect of Na addition to Fe-based catalysts was also observed: thealkali metal enhanced iron carbidization, suppressing the secondary olefin hydroge-nation and increasing the chain growth probability (Torres Galvis et al., 2013) Cheng

et al evaluated the effect of sodium addition on the activity of iron catalysts supported

on alumina, silica, CMK-3, and carbon nanotubes for Fischer-Tropsch reaction.Sodium promotion, as displayed inTable 7.5, leads to higher olefin-to-paraffin ratiosfor both light and long-chain hydrocarbons, lower methane selectivity, and higherchain growth probability The selectivity effects were more pronounced for the cata-lysts supported by carbon nanotubes The impact of sodium promotion was less sig-nificant on silica and CMK-3 supported catalysts

Very little influence of sodium promotion on the catalytic performance wasobserved in the alumina-supported iron catalysts because of formation of sodium alu-minates, having a relative small effect for Fischer-Tropsch synthesis In more inert

Table 7.5 Activity of Na-promoted Fe-based catalysts for FTS

at 300 °C, 20 bar, and H2/CO 52 ( Cheng et al., 2015 )

Sample

XCO

(%)

Olefin/paraffinsratio C2–C4

Product distribution (%, CO2-free)

CH4

C2–C4olefins

C2–C4paraffins C5 +

supports such as silica and CMK-3, the optimum sodium contact could be in the range0.3–0.5 Na/Fe Fe/CNT demonstrates very strong effect of sodium on the selectivityalready at the small amounts of promoter (0.1 Na/Fe) with further decrease in theactivity at the higher sodium content (Cheng et al., 2015) The performances of a cat-alyst consisting of SiO2nanowires and highly dispersed Fe2O3(denoted NW-FS) werecompared with the FTS activity of an industrial spherical Fe2O3/SiO2catalyst (den-oted indus-FS) at 320°C and H2/CO¼1 NW-FS sample exhibited a high selectivityfor light olefins, especially for ethylene in the Fischer-Tropsch synthesis This wasbecause of the highly dispersed Fe2O3and low diffusion resistance of its open struc-ture The C2–C4 olefin/paraffin ratio was 3.3, which was higher than that of indus-FS,equal to 1.9 (Chen et al., 2014a).Sudsakorn et al (2001)investigated the effect of theaddition of small concentration of precipitated SiO2on the performances of a spray-dried Fe catalyst SiO2addition decreased catalyst particle density, thus resulting inlower attrition resistance The best results, in terms of high active surface area, gooddispersion in the slurry, and high attrition resistance, were measured for a SiO2loading

of 10 wt% The effect of the activation atmospheres on the FTS activity of aγ-Fe2O3catalyst supported on SBA-15 was investigated in a work ofPerez De Berti et al.(2016) The experimental results, carried out at 330°C, 20.3 bar, and H2/CO¼2,showed that activation with pure H2produces a catalyst more active and less selective

to methane In order to explain the different catalytic properties, the schematic sentation shown inFig 7.7was proposed Activation in H2/CO would occur following

repre-a “shrinking-core” model Insterepre-ad, pure H2would lead to expose suddenly the surface

of Fe3O4to a carburizing mix (H2/CO) This situation would produce a great number

of iron carbide nuclei The Fe oxide nanoparticles in the catalysts would have greatnumber of iron carbide “nodules” with smaller size if pure H2is used as activationatmosphere Therefore, a larger number of sites for CO adsorption and dissociationand shorter diffusion paths would be obtained, and the catalyst will be more active

Iron carbides Iron carbides

Li et al (2015)evaluated the effect of alkali metals on the FTS activity of an Fecatalyst supported over SiO2at 260°C, 15 bar, and H2/CO¼2 The amount of CH4

gradually declined with increasing the periodic orders of alkali metals, while theamount of ethane increased Alkali metals significantly decreased the surface concen-tration of hydrogen, inhibited the hydrogenation capability, and improved the COadsorption and C–C coupling reaction with increasing the periodic orders of alkalimetals These results were being ascribed to the increasing basic strength of alkalioxides in the order of Li2O<Na2O<K2O<Rb2O

Copper is typically selected as a promoter for Fe catalyst to facilitate the reduction

of iron oxides, while small amounts of potassium are commonly added to promote theformation of iron carbides

The performances of promoted and unpromoted Fe-containing catalysts prepared

by the sonochemical method and supported on SiO2were compared with the FT ity of traditional impregnated catalysts at 250°C, 20 bar, and H2/CO¼2 (Comazzi

activ-et al., 2017) Catalytic activity was higher in sonochemically prepared catalysts overimpregnated samples with the same amount of active metal and promoters; in partic-ular, an increase of about 5 times was observed for the 10 wt% Fe sample All thesonicated samples had lower selectivity to methane; in particular, methane selectivity

is 6 times lower for the 10 wt% Fe catalyst, while the 30 wt% Fe and the KCu moted catalysts displayed a decrease in methane selectivity of about 50% with respect

pro-to the impregnated samples The measured selectivity pro-to CO2is lower for the cated unpromoted catalysts, while the sample containing Cu and K presented thehighest selectivity value toward carbon dioxide All the catalysts synthesized bythe sonochemical method showed higher selectivity to>C7 species with respect tothe impregnated samples The improved reactant conversion and selectivity valuesobserved over the innovative catalysts can be attributed to the production of nano-structured materials with better surface and morphological properties Cano et al.(2017)evaluated the performances of Fe catalysts supported on SiO2and SBA-15,doped with Cu and K, at 270°C, 10 bar, and H2/CO¼2 The results of the catalytictests performed showed firstly the importance of porosity in silica supports forFTS catalysts, since Fe/SBA-15 showed a higher activity, major chain growth forma-tion of the products and more selectivity to olefins than the Fe/SiO2catalyst More-over, the effect of K and Cu as promoters showed that the addition of K can enhancethe catalytic activity and favor the selectivity to olefins In fact, it was observed thatthe presence of K and Cu in the vicinity of Fe over the support surface creates newactive sites, with the creation of conductor interfaces with different electronic densitydistribution On the other hand, the addition of Cu apparently increases the stability ofthe catalysts As a result, the best performance in the FT reaction was obtained withFeCu/SBA-15 and FeK/SBA-15 In view of investigating poisoning phenomena, theeffect of H2S on activity and stability of an Fe/SiO2catalyst promoted by K and Cuwas investigated in a CSTR between 230°C and 270°C, at 13 bar, and for a H2/COratio ranging between 0.67 and 0.77 (Ma et al., 2016) Adding 0.1 ppm H2S for

soni-72 h led to a small deactivation; for example, the deactivation rate based on averagepercentage loss in CO rate per day at 270°C was 0.28%; increasing the H2S level from0.2 to 1.0 ppm linearly increased the deactivation rate from 0.57% to 4.6% The H S

limit for the iron catalyst at which nearly zero deactivation rate can be achieved wasdetermined to be 50 ppb The cofeeding of different levels of H2S also altered theproduct selectivities Adding sulfur for about 400 h was found to gradually decrease

CH4selectivity and increase C5 + selectivity The added sulfur improved the ities of the secondary reactions of olefins and the WGS reaction even though the ratesfor these declined Moreover, the Fe/S ratio decreased dramatically from 13.5 to 6.0when the temperature was increased from 230°C to 270°C The results suggest thatsulfur poisoning of the Fe catalyst was exacerbated at lower temperatures Todic

selectiv-et al (2016) investigated the effect of process conditions on the FT activity of anindustrial FeKCu/SiO2catalyst in a stirred tank slurry reactor, finding a reduction

of methane production and increase of C5 + products by decreasing temperature (from

260°C to 220°C), inlet H2/CO ratio (2:0.67), and/or increasing pressure (8–25 bar).Moreover, overall selectivity toward methane and C5 + did not show significantchanges with variations in residence time For an Fe-Mn-K-SiO2catalyst tested at

262°C and 5 bar (Ding et al., 2013), it was found that activation in higher H2/CO ratiopromoted the reduction ofα-Fe2O3to Fe3O4, whereas decreasing H2/CO ratio facil-itated the formation of iron carbides on the surface of magnetite formed and surfacecarbonaceous species During the FT synthesis reaction, the catalyst reduced in lower

H2/CO ratio presented higher catalytic activity, which may be attributed to the tion of iron carbides (especiallyχ-Fe5C2) on the surface layers, providing the moreactive sites for FT synthesis The surface carbonaceous species formed had a negli-gible effect in keeping the FT synthesis activity and stability In addition, pretreatment

forma-in higher H2/CO ratio facilitated the product distributions shifting toward molecular-weight hydrocarbons

lower-The performances of a coprecipitated Fe-Cu-K catalysts were compared with theresults achieved after the employment of silica and alumina as structural promoters(Rafati et al., 2015) The doubly promoted Fe-Cu-K-Si-Al catalyst achieved higher

CO and CO2conversions than the K catalyst and singly promoted K-Al and Fe-Cu-K-Si catalysts at 350°C, 10 bar, and H2/CO¼2 The CO and CO2

Fe-Cu-conversions of the syngas with 54% H2/10% CO/29% CO2/7% N2over the doublypromoted catalyst were 88.3% and 25.2%, respectively, compared with 81.8% and18.5% for the Fe-Cu-K catalyst In this case, the C5 + selectivity of the doubly pro-moted catalyst was 71.9%, which was slightly lower than 75.5% for the Fe-Cu-K cat-alyst The CO2was converted to hydrocarbons using the doubly promoted catalystwhen the CO2/(CO + CO2) ratio was higher than 0.35 for H2-balanced syngas at

H2/(2CO + 3CO2)¼1.0 and 0.5 for H2-deficient syngas at H2/(2CO + 3CO2)¼0.5.The increase of hydrogen content in the syngas increased the methane selectivity

at the expense of decrease in the liquid hydrocarbon selectivity

Zn addition to Fe-based catalysts was shown to enhance light olefins selectivityand to decrease carbon selectivity (Gao et al., 2016).Ning et al (2013)studied theeffect of K and Cu addition to FeZn catalysts for FTS at 230°C, 16 bar and

H2/CO¼2.4 The CO conversion of the unpromoted sample decreases with timeonstream The addition of Cu the latter catalyst did not show any improvement How-ever, the simultaneous presence of Cu and K assured a more evident increasing COconversion during activity test In contrast with the FeZn catalyst, Cu increased the

CH4selectivity, while K decreased it The CH4selectivity of the four catalysts is ied in the order of Cu/FeZn>FeZn>KCu/FeZnK/Fe The sequence of CO2selec-tivity is K/FeZnKCu/FeZn>Cu/FeZnFeZn In fact, the water-gas shift activitywas increased upon Cu addition onto pure iron catalyst (Martinelli et al., 2014) Inorder to improve the reactivity of CO2, potassium was added to an Fe catalyst con-taining Zn and Cu K-promoted iron catalysts, at 220°C, 30 bar, and H2/CO¼1,favored CO2 adsorption, thus enhancing selectivity toward middle distillates CO2also had a key role in preventing the CO shift to CO2, thus improving the overall econ-omy of the conversion process and avoiding a net CO2production Interestingly, uponincreasing the K loading, the CO conversion rate is decreased, both in the presence and

var-in the absence of CO2, possibly as a result of the very strong CO adsorption on thecatalytic surface

Beside silica and alumina, other high surface area oxide, including ZrO2, TiO2,MnO, and MgO, have been selected as supports for Fe-FT catalysts By varyingthe pretreatment temperature in hydrogen atmosphere of Fe/ZrO2catalysts, differentactivity was observed during FTS at 320°C, 20 bar, and H2/CO¼2 (Al-Dossary andFierro, 2015) The pretreatment temperature affected the particle sizes of iron oxide,and the FT activity of the catalysts was strongly affected by this dimension In par-ticular, the CO conversion rate during FTS increases as a function of the increase

in particle size, reaching a maximum value for mean Fe particle size of approximately

7 nm, obtained after pretreatment at 900°C.Qing et al (2011)showed the beneficialeffects of ZrO2incorporation into Fe/SiO2catalysts for FT reaction.Fig 7.8displays

CO conversion and iron carbide content as a function of TOS at 280°C, 1 bar, and

0 0 15

Fig 7.8 CO conversion and iron

carbides content for Fe/ZrO2

(continuous line), Fe/SiO2

(dashed line), and Fe/SiO2ZrO2

catalysts (dotted line) during FT

reaction at 280°C, 1 bar, and

H2/CO¼1

H2/CO¼2 It is known that the interconversion between iron oxides and carbides isreversible upon the FT environment At high H2O and CO2partial pressures, iron car-bide will be oxidized to Fe3O4, which leads to the deactivation of iron-based FTScatalysts Accordingly, the Fe3O4could be recarburized to iron carbides when the

CO partial pressure was high For FeZr catalysts, high iron carbide content after vation ensured high initial activity; thus, a large amount of H2O would be produced,which in turn oxidized the iron carbides Consequently, the iron carbide contentdecreased gradually with TOS, which deactivated the catalyst Meanwhile, the deac-tivation caused by the deposition of inactive carbonaceous compounds on the cata-lysts’ surfaces cannot be ruled out However, theχ-Fe5C2content of FeSi catalystsincreased slightly in the initial stage and then became stable Therefore, the amount

acti-of iron carbide did not change significantly, and no obvious change in the CO sion level of FeSi catalysts was observed On the other hand, the reduction andcarburization ability of FeSi catalysts were enhanced by ZrO2 addition: Probably,more iron carbide would be formed in FeZrSi catalysts, and these species would bemore stable in terms of H2O oxidation than FeSi catalysts during FTS As a result,the χ-Fe5C2 content of Fe/SiO2ZrO2 catalysts increases gradually with TOS andthe highest CO conversion was recorded.Zhang et al (2016b)compared the perfor-mances of conventional Fe-Mn catalysts with the FT activity of Fe2O3@MnO2core-shell catalysts at 280°C, 20 bar, and H2/CO¼1 The latter catalysts enhanced catalyticperformances, especially in C5 + hydrocarbon selectivity In fact, Mn promoter canaccelerate the dissociation of CO and thus enhanced the concentration of active inter-mediates for chain growth Moreover, compared with the pure Fe2O3(Mn-free) cat-alyst, the selectivity toward C5 + hydrocarbons over Fe2O3@MnO2 catalyst wasincreased from 44.6 to 66.6 wt% Meanwhile, the undesired CH4was decreased from16.8 to 8.9 wt%

conver-Carbon materials have also been selected as suitable supports for Fe-based FT alysts.Abbaslou et al (2010)found that the iron oxide particles on a wide pore (WP)carbon nanotubes support were larger than those supported on narrow pore (NP)nanotubes As a consequence, during FT tests carried out at 275°C, 20 bar, and

cat-H2/CO¼2, the activity of the NP catalyst (%CO conversion of 30) was 2.5 times that

of WP (%CO conversion of 12) In addition, the Fe catalyst supported on WP nanotubewas more selective toward lighter hydrocarbons with a methane selectivity of 41%compared with that of NP sample with methane selectivity of 14.5% Deposition ofmetal particle on the carbon nanotubes with narrow pore size resulted in more activeand selective catalyst due to higher degree of reduction and higher metal dispersion

FT reaction was also studied at 340°C, 25 bar, and H2/CO¼1 over iron oxidenanoparticles supported on untreated oxygen-functionalized carbon nanotubes(OCNTs) and nitrogen-functionalized CNTs (NCNTs), as well as thermally treatedOCNTs (Chew et al., 2016) An activity loss for iron nanoparticles supported onuntreated OCNTs was observed, originated from severe sintering and carbon encap-sulation of the iron carbide nanoparticles under reaction conditions Conversely, thesintering of the iron carbide nanoparticles on thermally treated OCNTs and untreatedNCNTs during reaction was far less pronounced The presence of more stable surfacefunctional groups in both thermally treated OCNTs and untreated NCNTs is assumed

to be responsible for the less severe sintering of the iron carbide nanoparticles duringreaction As a result, no activity loss for iron nanoparticles supported on thermallytreated OCNTs and untreated NCNTs was observed, which even became graduallymore active under reaction conditions The activity of a K-promoted iron/carbon nan-otubes composite, prepared by a redox method, was compared with the performances

of two Fe catalysts, prepared by impregnation, unpromoted or promoted by K (Duan

et al., 2016).Table 7.6presents a comparison of the FT selectivity of the three lysts under similar CO conversions The catalyst prepared by the redox methodexhibited much higher selectivity of hydrocarbons (i.e., 70.4%C), ascribable to thesmaller size iron nanoparticles and higher degree of carbidization Moreover, the lattersample exhibited much lower CH4selectivity and improved C5 + selectivity than theimpregnated catalysts, suggesting higher chain growth probability Such parameter, infact, increases with Fe particle sizes and K addition favors chain growth (α) More-over, the innovative catalyst displayed the highest olefin-to-paraffin ratio The effect

cata-of three alkali metal promoters (Li, Na, and K) on the catalytic performances cata-of Fecatalysts supported on carbon nanotubes for Fischer-Tropsch reaction at 275°C,

8 bar, and H2/CO ratio of 2 was studied in a work ofXiong et al (2015) The addition

of alkali promoters led to an increase in crystallite size of the iron oxide and decreasedsurface areas, as compared with the unpromoted Fe/CNT catalyst The presence of Naand K promoters slightly hindered catalyst reducibility by increasing the reductiontemperature of the iron oxide, while the potassium-promoted catalyst showed the mostpronounced effect, and no effect was observed for Li The sodium- and potassium-promoted catalysts were found to decrease the methane selectivity, increase the olefinproduction, and shift the product selectivity to higher-molecular-weight hydrocarbonsduring FTS Furthermore, Na and Li greatly increased the CO conversion, while theaddition of K suppressed the activity As a result, the catalyst promoted by Na resulted

in the largest increase in FTS reactivity compared with Li and K It was also reported(Li et al., 2016b) that K addition to Fe catalysts supported over graphite promoted thereduction behavior and enhanced the selectivity to liquid hydrocarbons significantly at

260°C, 20 bar, and H2/CO¼1 Products were mainly composed of C4–C10 α-olefinswith little methane, whose distributions changed with time onstream Theα-olefins in

catalyst (B) and the KFe catalysts prepared by redox method (C);

300 °C, 20 bar, and H2/CO 51 ( Duan et al., 2016 )

the liquid phase reaction media promoted the selectivity of C5 + distillate up to near90% while suppressing the formation of lighter hydrocarbons with higher COconversion.

Ma et al (2007) investigated the effect of K addition to an Fe-Cu-Mo catalystsupported on activated carbon Temperatures ranging between 260°C and 300°C,20.8 bar, H2/CO¼0.9, and the promotion by 0.9 wt% of potassium improved both

FT and WGS activity, while an opposite trend was observed for the 2 wt% of K.The potassium promoter significantly suppresses formation of methane and methanoland shifts selectivities to higher-molecular-weight hydrocarbons (C5 +) and alcohols(C2–C5) Meanwhile, the potassium promoter changes paraffin and olefin distribu-tions At least for carbon numbers of 25 or less, increasing the K level to 0.9 wt%greatly decreases the amount of n-paraffins and internal olefins (i.e., those with thedouble bond in other than the terminal positions) and dramatically increases branchedparaffins and 1-olefins, but a further increase in the K level shows little additionalimprovement The addition of potassium changes the effect of temperature on theselectivity to oxygenate In the absence of K, oxygenate selectivity decreases withtemperature However, when K is present, the selectivity is almost independent ofthe temperature

Many examples dealing with the use of Fe catalysts combined with zeolites areavailable in recent literature For FT synthesis at 280°C, 10 bar, and H2/CO¼1, itwas observed (Yoneyama et al., 2005) that, before adding zeolite, to Fe, FTS productsmainly contained normal paraffins with long chain from C1 to C16 After adding thezeolite, heavy hydrocarbons disappeared, and light hydrocarbons from C1 to C10 rich

in isoparaffins were produced Methane selectivity of Fe hybrid catalyst was very low,compared with Co hybrid catalyst at the same conditions, as Fe FTS catalyst had low

CH4 selectivity at higher temperature such as zeolite’s best reaction temperature.These results indicated that the hybrid catalysts containing Fe FTS catalysts andH-ZSM-5 for producing isoparaffins at one-step reaction were very effective

Baranak et al (2013)evaluated the influence of preparation method on the FT activity

of ZSM-5 supported iron catalysts Zeolite-supported catalysts were synthesized byusing incipient wetness impregnation method, and hybrid catalyst was prepared byphysical admixing of ZSM-5 and base iron At 280°C, 19 bar, and H2/CO¼2, all cat-alysts displayed a CO conversion higher than 40%; the selectivity toward C5–C11hydrocarbons of catalyst prepared by impregnation method was determined to be50%–74% The selectivity of the hybrid catalyst toward the same fraction was about45% No wax was detected in the products during the FT process using zeolite-supported iron catalysts However, the impregnated catalyst displayed a stable behav-ior for 260 h of time onstream without any activity loss In addition, the choice of lowacidity ZSM-5 support lead to lower selectivity for the light hydrocarbon and highselectivity for gasoline-range components The effect of Si/Al ratio, which influencedsupport acidity, in ZSM-5 for FT reaction over Fe-based catalysts was also studied(Plana-Pallejà et al., 2016) In fact, zeolite acidity is responsible for the cracking ofheavy hydrocarbons, and the formation of aromatics through oligomerization, cycli-zation, and dehydrogenation of primary short olefins In particular, the increment inacidic sites (low Si/Al ratios) induced the formation of more complex aromatic

structures, while a higher Si/Al ratio in the zeolite led to higher selectivity toward thegasoline-range products.Bae et al (2009)studied the effect of Cu promoter on thecatalytic performances of an Fe-Cu/ZSM-5 catalyst at 300°C, 10 bar, and H2/CO

of 2, finding an increase of initial activity and no significant effect at steady-state tion Moreover, the 2 wt% Cu was the optimum loading on the Fe-Cu-K/ZSM-5 cat-alyst wherein uniform mixing of copper and iron oxides can be achieved Thiscomposition also assured maximum density of moderate acid sites, thus showinghigher activity and selectivity to C2–C4 hydrocarbons Higher concentration of cop-per above 2 wt% led to iron oxide segregation eventually leading to decreased cata-lytic performances Fe-Cu-K/ZSM-5 catalysts at low Si/Al ratio were also found to besuperior to the other catalysts in terms of better C2–C4 selectivity in the FTS productsand higher olefin/(olefin + paraffin) ratio in C2–C4 because of the facile formation ofiron carbide during FTS reaction and also due to a larger number of weak acidic sitesthat are present in these catalysts (Kang et al., 2010) The study of Fe-Cu/ZSM-5 deac-tivation was also carried out in the work ofNakhaei Pour and Housaindokht (2013).Fischer-Tropsch synthesis was investigated at 300°C, 1 bar, and H2/CO¼1 for

reac-1400 h It was observed that before the regeneration, selectivity of the light productsincreased with time, in contrast to the heavy products However, after the catalystregeneration, selectivity to C5 + drastically increased in contrast to the CH4selectiv-ity Before the regeneration, the amount of aromatics declined with time onstream incontrast to olefins and paraffins due to the deactivation of the zeolite component Nev-ertheless, the regeneration process had positive effect on the paraffins production incontrast to aromatics

The choice of Fe-based bimetallic catalysts for FT synthesis is also of great interest.Noble metals (Pt, Pd, Ru, Ir, and Rh) addition to Fe/SiO2catalysts was shown to favorbimetallic clusters formation, which enhanced the low-temperature reducibility of theiron particles and improved catalyst activity for FT reaction (Niemantsverdriet et al.,

1984) Fischer-Tropsch synthesis was also investigated over bimetallic catalysts(selected among Fe, Co, and Ni) supported on TiO2as is and in combination with

a HZSM-5 zeolite at 250°C, 10 bar, and H2/CO¼1 (Arai et al., 1984) Alloying ofmetals resulted in a significant enhancement in CO conversion without an increase

in methane selectivity A 50:50 weight ratio Co-Ni catalyst physically mixed withHZSM-5 gave the highest CO conversion (45.2%) at the conditions tested This com-pares to conversion of 8.9% and 10.5% with Co-only and Ni-only catalysts, respec-tively Mixing the Co-Ni catalyst with HZSM-5 also resulted in a significant reduction

in methane selectivity and a significant increase in C4 + selectivity The aromatic tion increased from 1.5 to 8.1 wt%, the C2 + olefins were nearly eliminated, andi-C4H10increased from 2.3 to 58.5 wt% in the C4 fraction

frac-In addition, several authors found that a mixture of the two most active catalysts, Feand Co, have generated product streams in the FT reaction richer in olefins and alco-hols than expected from either Fe or Co catalysts For mono- and bimetallic Co-Fecatalysts supported on carbon nanotubes, tested at 220°C, 20 bar, and H2/CO¼2, itwas found (Tavasoli et al., 2009) that the monometallic iron catalyst had the minimumFTS and maximum water-gas shift (WGS) rates On the other hand, the monometalliccobalt catalyst exhibited high selectivity (85.1%) toward C5 + liquid hydrocarbons,

while addition of small amounts of iron did not significantly change the product tivity Monometallic iron catalyst showed the lowest selectivity for 46.7% to C5 +hydrocarbons The olefin to paraffin ratio in the FTS products increased with the addi-tion of iron, and monometallic iron catalyst exhibited maximum olefin to paraffinratio of 1.95 The bimetallic Co-Fe catalysts proved to be attractive in terms of alcoholformation The introduction of 4 wt% iron in the cobalt catalyst increased the alcoholselectivity from 2.3% to 26.3%, proving that the Co-Fe alloys appear to be responsiblefor the high selectivity toward alcohol formation For catalysts supported onγ-Al2O3tested at 210°C, 20 bar, and H2/CO¼1, it was observed that alloying Co with small/moderate amounts of Fe improved the FT activity compared with the 100% Co cat-alyst at low conversion levels (L€ogdberg et al., 2009) Alloying Fe with small/mod-erate amounts of Co lowered the FT activity but increased the relative water-gas shift(WGS) activity compared with the 100% Fe catalyst.

selec-The bimetallic catalysts showed essentially no synergy effects with respect to HCselectivities and olefin/paraffin ratios, which partly can be explained by the use of

a substoichiometric H2/CO ratio as feed The higher the Fe content, the lower werethe C5 + selectivity and C3 olefin/paraffin ratio Water addition increased the C5 +selectivity and C3 olefin/paraffin ratio and reduced the CH4selectivity The effect

of Fe/Co ratio on catalyst activity was also investigated for bimetallic catalystssupported on SiO2and tested at 10 bar and H2/CO¼1.61 (Ma et al., 2009) As shown

inTable 7.7, the high iron content inhibited the activity, whereas high cobalt contentenhanced the activity of the Fe-Co/SiO2 catalyst When the CO conversion reachedapproximately the same value, the reaction temperature for the catalysts with high ironcontent was higher than that of the catalysts with high cobalt content At the samereaction temperature, activity of the latter was higher Concerning hydrocarbon dis-tribution, the total C2–C4 fraction of the 10Fe/6Co, 10Fe/10Co, 6Fe/10Co, and2Fe/10Co increased from 10.65% to 26.78%, but C5 + fraction decreased from75.75% to 57.63% at 250°C.Braganc¸a et al (2012)compared the performances ofbimetallic Fe-Co catalysts supported on SBA-15 and HMS mesoporous silicas at

220°C, 20 bar, and H2/CO¼2 HMS supported Co-Fe catalyst showed the highestactivity and C5 + hydrocarbon selectivity, while CoFe/SBA-15 catalyst revealedthe highest selectivity to alcohols Both bimetallic catalysts were more active towardthe C2–C4 hydrocarbon fraction, with an enhancement in the selectivity to C2, C3,and 1-C4 olefins In addition, greater chain growth probability values than the mono-metallic iron-based catalysts were observed, although their performance in catalytictests were more close to the iron catalyst The integration of Co and Fe in La-basedperovskites (La1–yCo0.4Fe0.603–δ) was also shown to be efficient for Fischer-Tropschsynthesis At the same CO conversion (5%) molar selectivity to CO2was stronglyaffected by lanthanum deficiency, varying between 26% fory¼0.2 and y¼0.3 and19% for y¼0.4 Hydrocarbon fraction was always the major part of the products(around 67%–69% of molar selectivity) The molar selectivity into oxygenated prod-ucts (mainly methanol and ethanol) ranged from 6.4% fory¼0 and 12.1% for y¼0.4

In the hydrocarbon fraction, the ratio [C2–C4]/C5+ was independent of y and equals2.4 (weight distribution,Fig 7.9) For lanthanum deficiency up toy¼0.3, methanewas the main hydrocarbon fraction (accounting for 47%–52% of the total weight

Table 7.7 Catalytic results of Fe-Co catalysts—CO conversion (XCO), CO2selectivity (SCO2), hydrocarbon distribution, and olefin/paraffin ratio (O/P) in C2 –C4 fraction; 10 bar and H2/CO 51.61 ( Ma et al., 2009 )

Hydrocarbon distribution (% by mass)

of the hydrocarbon fraction) For the most deficient catalyst (y¼0.4), decreasing thereaction temperature (to 230°C) induced a significant decrease in methane formation,

to only 32% of the total weight For this catalyst, at CO conversion of 5%, the mainfraction was the C2–C4 fraction (48% of the total weight) The decrease in reactiontemperature changed the olefin proportion in the C2–C4 fraction Olefins were clearlyfavored fory¼0.4 (230°C), because the olefin/paraffin ratio (O/P) reached the value

of 3 (75% of olefins in this fraction) La0.6Co0.4Fe0.6O3–δexhibited high CO sion at moderate reaction temperatures (21% conversion at 255°C), with such temper-atures allowing high selectivity toward light olefins

conver-7.3 Methanol synthesis

Methanol is one of the main build blocks of chemical industry Nowadays, over

90 methanol plants worldwide have a combined production capacity of about

110 MMT, able to satisfy the current world demand of about 80 MMT However,the methanol demand is constantly rising, mainly due to the massive growing of Asianmarket, for which methanol demand increased from 10 to 50 MMT in the last

15 years, and forecasted to overcome 70 MMT in the next 5 years Methanol is widelyemployed in the synthesis of not only formaldehyde but also methyl methacrylate(MMA), dimethyl terephthalate (DMT), and other chemicals; moreover, it can be used

as solvent or gasoline extender In addition, the methanol employment in the methanol

to olefin and methanol to paraffin (MTO/MTP process) is growing with an averagegrowth rate of almost 7%, expecting to become the second largest methanol derivate.Methanol is industrially obtained by catalytic conversion of syngas, mainly fromfossil fuel reforming, in which carbon monoxide and carbon dioxide are partiallyreduced by hydrogen to methanol:

2

0 1 60

Fig 7.9 FT product distribution over Co- and Fe-based perovskites reduced at 750°C, 10 bar,and H2/CO¼1

Of course, the reaction(7.1)may be accompanied by the water-gas shift reaction(7.3),

in which anyway carbon monoxide is oxidized to carbon dioxide, to be converted inmethanol in the reaction(7.2):

CO + H2O>CO2+ H2 Δo

Both reactions have an exothermic nature and generated a reduction of mole number;for these reasons, they are promoted at low temperature and high pressure Suchconditions counter several issues in terms of reaction kinetics and plants fixed andoperative costs It is worth to consider that the methanol synthesis from CO2 ischaracterized by a less pronounced adiabaticity and by a minor volume decreasing,

so resulting in a less relevancy of temperature and pressure conditions On theother hand, the desired reduction of by-product requires a defined operating condi-tions window and an appropriate catalytic system Industrial processes are carriedout on copper and zinc oxides based catalysts, with an operating temperature of around

200–300°C and quite high pressures (from 5 to 30 MPa, depending on processparameters)

The first industrial process for heterogeneously catalyzed methanol synthesiswas commercialized by BASF in the 1923 (Mittasch et al., 1925), according tothe process described by Patart (Patart, 1922) few years earlier The process, verysimilar to the ammonia synthesis technology introduced by BASF in the sameperiod, was carried out at 300–360°C and 15–25 MPa on zinc and chromium oxidescatalysts Copper-based catalysts were introduced by Fr€ohlich and coworkers(Frolich et al., 1928; Lewis and Frolich, 1928), that evidenced the higher activity

of Cu with respect to other catalysts and that it was industrially used in 1960s

by Imperial Chemical Industries (Davies et al., 1965) only after syngas purificationtechnologies were effectively developed By this date, Cu/ZnO/Al2O3catalysts arealmost exclusively used in industrial methanol synthesis, with a Cu/Zn molar ratioaround 4

Despite methanol technology is nowadays widely consolidated, the necessity inprocess intensification aimed to reduce production costs by improving processes effi-ciency resulted in the need of a wide knowledge of catalytic process, in terms of reac-tion mechanism and catalyst components role in the methanol synthesis process

7.3.1 Thermodynamic evaluations

It is widely accepted that methanol synthesis process could be thermodynamicallydescribed by two of (7.1), (7.2), and (7.3)reactions The equilibrium constant ofthe reactions(7.1)and(7.2)could be described by the equations(7.4)and(7.5), inwhichpAandφAare, respectively, the partial pressure and the fugacity coefficient

of the species A (Lee, 1990; Kjær, 1972)

Kð Þ 1 ¼ φCH 3 OH

φCO φ2  pCH3OH