1 catalytic carbon dioxide hydrogenation

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Contents lists available at ScienceDirect

Chemical Engineering Research and Design

j ourna l h omepage: www.elsev ier .com/ locate /cherd

eview

atalytic carbon dioxide hydrogenation toethanol: A review of recent studies

uhas G. Jadhava, Prakash D. Vaidyaa,∗, Bhalchandra M. Bhanageb,yeshtharaj B. Joshia,c

Department of Chemical Engineering, Institute of Chemical Technology, Nathalal Parekh Road, Matunga, Mumbai00019, IndiaDepartment of Chemistry, Institute of Chemical Technology, Nathalal Parekh Road, Matunga, Mumbai 400019,

ndiaHomi Bhabha National Institute, Anushaktinagar, Mumbai 400094, India

a b s t r a c t

Methanol demand is continuously increasing in the chemical and energy industries. It is commercially produced from

synthesis gas (CO + CO2 + H2) using CuO/ZnO/Al2O3 catalysts. Today, much effort is being put on the development of

technologies for its production from carbon dioxide (CO2). In this way, the Greenhouse effect may be mitigated. Over

the years, several useful works on CO2 hydrogenation to methanol have been reported in the literature. In this article,

we present a comprehensive overview of all the recent studies published during the past decade. Various aspects

on this reaction system (such as thermodynamic considerations, innovations in catalysts, influences of reaction

variables, overall catalyst performance, reaction mechanism and kinetics, and recent technological advances) are

described in detail. The major challenges confronting methanol production from CO2 are considered. By now, such

a discussion is still missing, and we intend to close this gap in this paper.

© 2014 Published by Elsevier B.V. on behalf of The Institution of Chemical Engineers.

Keywords: Methanol; Syngas; Carbon dioxide; Carbon Monoxide; Hydrogenation; Cu/ZnO Catalyst; Pd Catalyst

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ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2. Methanol from catalytic hydrogenation of CO2 . . . . . . . . . .3. Thermodynamic analysis of catalytic CO2 hydrogenatio4. Discussion on catalytic features . . . . . . . . . . . . . . . . . . . . . . . . . .

4.1. Recent advances in Cu-based catalysts . . . . . . . . . . .

4.2. Pd-based catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.3. Other catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5. Knowledge on reaction pathway . . . . . . . . . . . . . . . . . . . . . . . . .

6. Recent technological advances . . . . . . . . . . . . . . . . . . . . . . . . . . .

Please cite this article in press as: Jadhav, S.G., et al., Catalytic carbon dioEng. Res. Des. (2014), http://dx.doi.org/10.1016/j.cherd.2014.03.005

6.1. Current industrial status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author. Tel.: +91 22 33612014; fax: +91 22 33611020.E-mail addresses: [email protected], [email protected]

ttp://dx.doi.org/10.1016/j.cherd.2014.03.005263-8762/© 2014 Published by Elsevier B.V. on behalf of The Institution

xide hydrogenation to methanol: A review of recent studies. Chem.

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m (P.D. Vaidya).

of Chemical Engineers.

dx.doi.org/10.1016/j.cherd.2014.03.005

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6.2. Reactor innovations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 006.3. Alternate catalytic CO2 hydrogenation techniques for methanol synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

7. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

1. Introduction

Methanol is a primary liquid petrochemical which is of con-siderable importance in the chemical and energy industries.This large-volume product is in big demand, due to the easein its storage and transportation. For example, it was antici-pated that global methanol consumption will reach 58.6 MMTby 2012 (Centi and Perathoner, 2009). Methanol is commonlyused as solvent and feedstock for the production of chemi-cals (such as formaldehyde, acetic acid, methyl methacrylate,dimethyl terephthalate, methylamines and chloromethanes)and fuel additives (such as methyl tertiary butyl ether and fattyacid methyl esters) (Ortelli et al., 2001). Light olefins such asethylene and propylene, which can be used for manufactur-ing polymers and hydrocarbon fuels, are produced using themethanol-to-olefins process (Pop et al., 2004). Dimethyl car-bonate, which is a useful intermediate for derivatives usedin polycarbonates and polyurethanes, is synthesized frommethanol in supercritical CO2 (Ballivet-Tkatchenko et al.,2006). Methanol is a liquid energy-carrier suitable for trans-portation applications. It is an excellent alternative fuel, andit can also be blended with gasoline (Olah, 2005); moreover, itcan be used in fuel cells, too (Palo et al., 2007).

Methanol is commercially produced from natural gasthrough a syngas route. Steam methane reforming produces amixture of CO, CO2 and H2 according to Eqs. (1) and (2). Syngasis then converted to methanol in the ranges of temperature,250–300 ◦C, and pressure, 5–10 MPa, using CuO/ZnO/Al2O3 cat-alyst (see Eq. (3)).

CH4 + H2O ↔ CO + 3H2 �H25◦C = 206 kJ/mol (1)

CH4 + 2H2O ↔ CO2 + 4H2 �H25◦C = 165 kJ/mol (2)

CO2 + 3H2 ↔ CH3OH + H2O �H25◦C = −49.5 kJ/mol (3)

Today, CO2 is added up to 30% of the total carbon in syn-gas (Aresta and Dibenedetto, 2007). The addition of CO2 in theCO/H2 feed significantly improves the methanol yield and theenergy balance. CO2 is directly converted to methanol withouta preliminary reduction to CO (Saito et al., 1996). To facili-tate methanol synthesis, the CO in syngas is converted to CO2

through the water-gas shift (WGS) reaction:

CO + H2O ↔ CO2 + H2 �H25◦C = −41 kJ/mol (4)

Reactions represented by Eqs. (3) and (4) are exothermic. Theoverall reaction for methanol synthesis is given by the sum ofthese reactions:

CO + 2H2 ↔ CH3OH �H25◦C = −90.5 kJ/mol (5)

The theoretical single-pass CO conversion is limited to ∼20%under commercial operating conditions (Strelzoff, 1970; vonder Decken et al., 1987). Today, this process is well established


and several companies such as Lurgi, Topsoe and Mitsubishioffer commercial technology solutions.

2. Methanol from catalytic hydrogenationof CO2

Much effort is now being put on CO2 conversion to methanol(see Eq. (3)). This method is a useful strategy of CO2 utilizationand a practical approach to sustainable development (Song,2006). It is technically competitive with the industrial pro-duction of methanol from syngas (Aresta and Dibenedetto,2007). The production of methanol and its derivatives by alter-native routes and their use as fuels and chemicals is thecore of the methanol economy, a concept earlier proposedby Olah and co-workers (see Olah, 2005; Olah et al., 2009a,b,2011). In this conception, CO2 is captured from any natural orindustrial source, human activities or air by absorption andchemically transformed into methanol, dimethyl ether andvaried products including synthetic hydrocarbons. Accordingto Olah (2005), methanol production from CO2 is advanta-geous owing to the usage of non-fossil fuel sources (unlikesyngas), avoidance of CO2 sequestration (which is expensive)and the opportunity for mitigation of the Greenhouse effect(by effective recycling of CO2). Olah et al. (2009a) emphasizedthat the chemical recycling of CO2 to methanol (and dimethylether) provides a renewable, carbon-neutral, unlimited sourcefor efficient transportation fuels, for storing and transportingenergy, as well as convenient feedstock for producing ethyleneand propylene and from them, synthetic hydrocarbons andtheir products. Thus, it essentially substitutes petroleum oiland natural gas. It allows the lasting use of carbon-containingfuels and materials and avoids excessive CO2 emissions caus-ing global warming (Olah et al., 2009b).

The methanol economy concept is based on the chemicalanthropogenic carbon cycle proposed by Olah et al. (2011). Itcombines carbon capture and storage with chemical recycling.While renewable feedstock such as water and CO2 are avail-able in plenty, the energy required for the synthetic carboncycle can come from any alternative energy source such assolar, wind, geothermal, and nuclear energy. According toOlah et al. (2011), this cycle supplements the natural carboncycle and offers a way of assuring a sustainable future forhumankind when fossil fuels become scarce.

Interestingly, CO2 is non-toxic, non-corrosive and non-flammable and it can be easily stored in liquid form undermild pressure. Therefore, the problem of process safety doesnot appear in the case of CO2 application. Besides, the pro-cess can be easily integrated in existing syngas conversionplants without any significant modification (Arakawa, 1998).Feedstock CO2 is inexpensive and abundant. Existing andproposed plants for carbon sequestration and storage (CSS)are candidate sources of CO2. Other resources are flue gasfrom coal-fired and natural gas-fired electric power plants,gaseous streams in several industrial processes (such asammonia and hydrogen manufacturing, coal gasification,WGS units, cement factories, aluminium production and fer-mentation plants) and CO2 accompanying natural gas andgeothermal energy producing wells. After effective separa-tion from air (e.g., by membrane separation or selective


absorption technique), excess atmospheric CO2 offers another

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Table 1 – Plausible thermo-chemical routes for H2production.

Reaction schemes Reactions

Sulfur–iodine process

H2SO4850 ◦C−→ SO2 + H2O + 1/2 O2

SO2 + I2 + 2 H2O → 2 HI + H2SO4

2 HI → H2 + I2

Overall : H2O → H2 + 1/2 O2

Copper–chlorine process

2 Cu + 2 HCl450 ◦C−→ 2 CuCl + H2

2 CuCl2 + H2O400 ◦C−→ Cu2OCl2 + 2 HCl

2 Cu2OCl2500 ◦C−→ 4 CuCl + O2

2 CuCl → CuCl2 + CuOverall : H2O → H2 + 1/2 O2

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easible alternative. When the appropriate conditions aresed, methanol synthesis by hydrogenation of atmosphericO2 is regarded as the most economic way after oil and gas

Olah, 2005).Even so, the sustainable and cost-effective production and

tilization of H2 is a major challenge (Raudaskoski et al.,009). Today, H2 is commercially produced by steam methaneeforming, coal gasification and partial oxidation of light oilesidues. As a result, fossil fuels are depleted and net atmo-pheric CO2 emissions are increased. Methanol productionrom H2 and CO2 will be deemed as environmentally benignnly if this process utilizes CO2 more than that produced in H2

anufacturing. Raudaskoski et al. (2009) discussed other can-idate methods for H2 production such as dry reforming andlectrolysis of water using renewable electrical energy. Clearly,hey have their limitations, e.g., high CO content of the syn-as in dry reforming process and high electricity cost. Biomassasification together with WGS and biomass fast pyrolysisoupled with steam reforming of the resulting bio-oil repre-ent further renewable routes for producing H2. Yet anotherotential route for H2 production is a thermo-chemical routehere the energy required for the splitting of water is supplied

y atomic energy or solar energy. Some representative reactionchemes, namely, sulfur–iodine process and copper–chlorinerocess are shown in Table 1. It is worthy of note that no car-on source, either from fossil- or biomass-origin, is used in thehermo-chemical route. Even so, more work is essential beforene can deduce the best possible route for H2 production.

. Thermodynamic analysis of catalytic CO2ydrogenation to methanol

O2 is a highly oxidized, thermodynamically stable com-ound having low reactivity. To activate CO2, it is necessary tovercome a thermodynamic barrier. Therefore, its chemicaltilization constitutes an important challenge. Today, therere very few organic syntheses using CO2, e.g., manufacturef urea (for nitrogen fertilizers and plastics), salicylic acid (aharmaceutical ingredient), and polycarbonates (for plastics).ong (2006) discussed thermodynamic considerations of CO2

onversion and highlighted the necessity of high energy input,ffective reaction conditions and active catalysts for CO2 con-ersion.

The catalytic hydrogenation of CO2 to methanol producesater as a by-product (see Eq. (3)). A third of the H2 is thus

onverted to water, which is much higher than that converteduring the commercial production of methanol via synthesis


as (Mikkelsen et al., 2010). Furthermore, the thermodynam-cs for methanol production from H2 and CO2 are not as

favourable as those for production of methanol from H2 andCO. For example, the equilibrium yield of methanol from CO2

at 200 ◦C is slightly less than 40% whereas the yield from COis greater than 80% (Arakawa, 1998).

Eq. (3) is highly exothermic and results in a reduction in thenumber of molecules. Therefore, it is facilitated at high pres-sure and low temperature. Separation of the reaction products,viz., methanol and water results in high methanol yield.CuO/ZnO/Al2O3 catalysts, which facilitate methanol produc-tion from syngas, exhibit poor activity for CO2 hydrogenationat low temperature (T < 250 ◦C) (Inui et al., 1997; Saito et al.,1996). The increase in temperature facilitates CO2 activation;then again, undesirable CO and H2O are formed by reverseWGS. As a result, additional H2 is consumed and methanolproduction is reduced (Skrzypek et al., 1990). What is more,water accelerates the crystallization of Cu and ZnO in the cat-alyst, thus resulting in rapid sintering and deactivation (Wuet al., 2001). Other hydrogenated products such as higher alco-hols and hydrocarbons are often formed along with methanol.Thus, the usage of a highly selective catalyst is essential. Usinga H2/CO2 ratio in feed equal to 3, the values of equilibrium CO2

conversion and methanol selectivity at 250 ◦C and 5 MPa are27 and 68%, respectively (Gallucci et al., 2004). According toMahajan and Goland (2003), ∼30 MPa pressure is required toachieve high CO2 conversion (>80%) at T = 125 ◦C.

To evaluate the efficacy of CO2 capture from point-emissionsources via its transformation to methanol by catalytic hydro-genation, Fornero et al. (2011) performed process simulationin a reacting system with a provision for recycling of the non-condensable gases (i.e. CO, CO2 and H2). They found that highoverall CO2 capture (>50%) could be achieved under indus-trial operating conditions by using catalysts which facilitatethe occurrence of Eq. (3) and reverse of Eq. (4) at near ther-modynamic equilibrium conditions. Further, complete CO2

capture (i.e. CO2 conversion to methanol) could be achievedusing a molar recycle ratio equal to 5 at T = 250 ◦C (P ≥ 4 MPa)or T = 235 ◦C (P ≥ 3 MPa). Based on their work, they proposedincreased efforts for improving catalytic activity (i.e. specificproductivity) rather than selectivity.

4. Discussion on catalytic features

Liu et al. (2003) reviewed the advances in catalysts formethanol synthesis via hydrogenation of CO and CO2. How-ever, this review was published nine years ago and thus, anevaluation of recent developments is desirable. In a review onrecent advances in catalytic hydrogenation of CO2, Wang et al.(2011) briefly discussed methanol production from CO2. Here,we present a comprehensive overview of all the relevant inves-tigations on CO2 conversion to methanol which are publishedsince 2003.

Over the years, several catalysts for CO2 hydrogenation tomethanol have been reported in the literature. According toLim et al. (2009), Cu, Zn, Cr, and Pd are commonly used tominimize by-product formation (i.e. hydrocarbons) and max-imize methanol yield and selectivity. Among these, Cu/ZnOcatalyst is well-known for its high activity and selectivity forthe methanol synthesis reaction. A support such as Al2O3 canfurther increase the activity and selectivity. Furthermore, apromoter such as Zr is known to enhance the copper disper-sion and catalytic activity of methanol synthesis catalysts (Lim


et al., 2009). A detailed description of the efficacy of the activemetal (viz. Cu) and the support (such as ZnO and ZrO2) was

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earlier reported by Liu et al. (2003), and thus, is excluded fromthe scope of this work.

Since 2003, Cu- and Pd-based catalysts have been exten-sively studied; these investigations are described in detailhere. Besides, few works on other promising catalysts arehighlighted, too.

4.1. Recent advances in Cu-based catalysts

Cu-based catalysts (e.g., CuO/ZrO2, CuO/ZnO/ZrO2,CuO/ZnO/Ga2O3, modified CuO/ZnO/Al2O3 and multi-component catalysts) have been extensively studied.Furthermore, many Cu-based catalysts promoted by B, Vand Ga have been investigated, too. To illustrate the potentialof catalysts containing Cu and Zr in methanol synthe-sis, Raudaskoski et al. (2009) reviewed recent work doneby various researchers. Even so, we are describing thesestudies in further detail. Liu et al. (2001) found that thecatalyst composition and the catalyst preparation meth-ods and conditions have an enormous influence on thesurface structure of catalysts. For example, CuO/ZrO2 cat-alyst prepared by deposition-precipitation (DP) had finerparticles and higher catalytic activity than the catalystsprepared by impregnation or co-precipitation. When DPcatalyst (CuO/ZrO2 = 30/70 by wt.) calcined at 350 ◦C was used,methanol yield of 0.36 g/(gcat h) was obtained at T = 240 ◦C,P = 2 MPa, space velocity = 5400 1/h and H2/CO2 = 3 (mol/mol).Sloczynski et al. (2003) studied the effect of addition of Mgand Mn promoters on the catalytic activity and adsorptiveproperties of CuO/ZnO/ZrO2. A catalyst preparation methodwhich required decomposition of the citrate complexes of themetals was used. With the addition of these promoters, Cudispersion was enhanced. The surface layers were depletedof Cu and enriched in Zn and Zr. In effect, the promoters werepreferentially accumulated on the surface of the catalysts.A correlation between the adsorptive properties and thecatalytic activity was established. An overall factor combiningthe catalytic activity and the adsorptive properties favouringmethanol synthesis was considered. It was found that thisfactor increases in the order CuZnZr < CuZnZrMg < CuZnZrMn.Using a successive precipitation technique, Yang et al. (2006)prepared CuO/ZnO catalyst doped with ZrO2. They found thatthe presence of ZrO2 led to higher copper dispersion whichwas distinctive from that of CuO/ZnO. The CO2 conversion(26.4%) and methanol yield (0.22 g/(mL h)) using Zr–CuO/ZnOat T = 250 ◦C, P = 5 MPa, space velocity = 4000 1/h and H2/CO2 = 3(mol/mol) were considerably higher than those obtained usingCuO/ZnO (16% and 0.14 g/(mL h)). Jung and Bell (2002) studiedthe effect of zirconia phase and copper to zirconia surfaceon catalyst activity. The catalysts prepared using m-ZrO2

support were 4.5 times more active than those prepared usingt-ZrO2 support. Due to higher concentrations of the activeintermediates, the rate of methanol synthesis on Cu/m-ZrO2

was higher. Raudaskoski et al. (2007) studied the effect ofageing time during co-precipitation. It was found that theprolonged suspension ageing time during the preparation ofCuO/ZnO/ZrO2 is advantageous for catalytic activity. As theageing time increased, the sodium content of the catalystdecreased and finer crystallite structures were formed.

Besides, several other useful works using CuO/ZnO/ZrO2

are reported in the literature, too. These works aresummarized in Table 2 (Arena et al., 2007; Guo et al.,


2009, 2011; Lachowska and Skrzypek, 2004). Lachowskaand Skrzypek (2004) found that CuO/ZnO/ZrO2 had

higher catalytic activity than CuO/ZnO/Al2O3. The addi-tion of Mn to CuO/ZnO/ZrO2, even at low concentration(2 wt%), resulted in increased methanol production rates. AtT = 220 ◦C, P = 8 MPa, GHSV = 3400 l/h and H2/CO2 = 3 (mol/mol),the methanol yield and selectivity were 138 g/(kgcat h) and91%, respectively. Guo et al. (2011) prepared CuO/ZnO/ZrO2

catalysts via a route of solid-state reaction. They investigatedthe effects of calcination temperature on the physicochemi-cal properties of the catalysts and found that Cu dispersiondecreases with an increase in the calcination temperature.A phase transformation of ZrO2 from tetragonal to mono-clinic was observed. The highest activity was achieved usingthe catalyst which was calcined at 400 ◦C. For example, atT = 240 ◦C, P = 3 MPa, space velocity = 3600 1/h and H2/CO2 = 3(mol/mol), the values of CO2 conversion, methanol selectivityand yield were 15.7, 58 and 9.1%, respectively. In anotherinvestigation, Guo et al. (2009) synthesized CuO/ZnO/ZrO2

catalysts using urea-nitrate combustion method. They stud-ied the effects of the urea/nitrate ratio on catalyst propertiesand performance. From their results, they concluded thatthe investigated catalyst exhibits optimum behaviour when50% of the stoichiometric amount of urea is used. Using thiscatalyst at T = 240 ◦C, P = 3 MPa, space velocity = 3600 1/h andH2/CO2 = 3 (mol/mol), the values of CO2 conversion, methanolselectivity and yield were 17, 56.2 and 9.6%, respectively.Arena et al. (2007) used a novel synthesis route based onreverse co-precipitation under ultrasound irradiation toprepare CuO/ZnO/ZrO2 catalysts. This method provided asignificant improvement in the total surface exposure and thedispersion and surface area of the active metal phase. ZnOhad a strong promoting effect on the texture of the catalyst.The investigated reaction was deemed structurally sensitive,due to the fact that TOF changed appreciably with metaldispersion. The activity of CuO/ZnO/ZrO2 was compared withthat of the conventional methanol synthesis catalyst (viz.CuO/ZnO/Al2O3) over the ranges in temperature, 160–260 ◦C,and pressure, 1–3 MPa. Due to a stronger affinity to water, theperformance of the Al2O3-based catalyst was poorer. Froma thermodynamic analysis, it was concluded that methanolformation proceeds via CO2 hydrogenation.

Recent investigations using catalysts containing Cu, Zn andAl are represented in Table 3 (An et al., 2007; Gallucci et al.,2004; Hong et al., 2002; Melian-Cabrera et al., 2002a; Saito andMurata, 2004). Gallucci et al. (2004) used a zeolite membranereactor that combined catalytic reaction over CuO/ZnO/Al2O3

with separation properties of zeolite membranes. They foundthat the values of CO2 conversion, methanol selectivity andmethanol yield were higher than those obtained in a tradi-tional reactor. Hong et al. (2002) reported high catalytic activityand selectivity of ultrafine CuO/ZnO/Al2O3 catalyst whichwas prepared by using a novel gel-network-coprecipitationmethod. Melian-Cabrera et al. (2002a) studied the effect of Pdon the performance of a CuO/ZnO/Al2O3 catalyst. When cata-lysts containing 4 and 10 wt% Pd were used at P = 4 MPa andW/F = 0.042 kg h/m3 over the temperature range 160–200 ◦C,values of the intrinsic methanol yield (mol methanol/h/molexposed Cu) were substantially higher than those obtainedusing the catalyst without Pd. It was found that Pd improvesthe reducibility of CuO. The intrinsic promoting effect of Pdwas attributed to a hydrogen spill over mechanism. Saitoand Murata (2004) found that multi-component catalysts suchas Cu/ZnO/ZrO2/Al2O3 and Cu/ZnO/ZrO2/Al2O3/Ga2O3 were


highly active for methanol synthesis from CO2 and H2. Thestability of these catalysts vastly improved even when a small

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Table 2 – Recent works on CO2 conversion into methanol using CuO/ZnO/ZrO2 catalyst.

Catalysts Preparation Salient features of work Reference

CuO/ZnO/ZrO2

Mn-promoted-CuO/ZnO/ZrO2

Thermal decomposition of citrates CuO/ZnO/ZrO2 has higher catalyticactivity than CuO/ZnO/Al2O3.Addition of Mn to CuO/ZnO/ZrO2

results in increased methanolproduction rates.

Lachowska and Skrzypek (2004)

CuO/ZnO/ZrO2 Solid-state reaction Cu dispersion decreases with anincrease in the calcinationtemperature.The highest activity is achievedusing the catalyst, which iscalcined at 400 ◦C.

Guo et al. (2011)

CuO/ZnO/ZrO2 Urea-nitrate combustion method The catalyst exhibits optimumbehaviour when 50% of thestoichiometric amount of urea isused.

Guo et al. (2009)

CuO/ZnO/ZrO2 Reverse co-precipitation underultrasound irradiation

This method provides a significantimprovement in the total surfaceexposure and the dispersion andsurface area of the active metalphase.CO2 conversion into methanol is

Arena et al. (2007)

adcm(dparr(f

rep

mount of colloidal silica was added. Crude methanol pro-uced in this way in a bench-scale unit (purity 99.9%) wasleaner than that produced in a commercial syngas-basedethanol plant. Using co-precipitation technique, An et al.

2007) prepared a series of Cu/Zn/Al/Zr catalysts containingifferent ratios of Al/Zr. It was found that these catalysts com-rised Cu/Zn crystallites in a fibrous structure. The dispersionnd stability of the crystallites for these catalysts were supe-ior to those for a commercial CuO/ZnO/Al2O3 catalyst. As aesult, CO2 hydrogenation was enhanced. When Zr was added5%), the methanol space time yield was 80% higher than thator a commercial catalyst.

Few studies using other CuO/ZnO-based catalysts areeported in the recent literature. For example, Melian-Cabrera


t al. (2002b) compared the efficacy of CuO/ZnO with Pd-romoted CuO/ZnO catalyst prepared by using sequential

Table 3 – Recent studies using catalysts containing Cu, Zn and

Catalysts Preparation

CuO/ZnO/Al2O3 Commercial

CuO/ZnO/Al2O3 Gel-network-coprecipitationmethod

Pd-promoted CuO/ZnO/Al2O3 Impregnation

CuO/ZnO/Al2O3

Cu/ZnO/ZrO2/Al2O3/Ga2O3

Co-precipitation technique

Cu/Zn/Al/Zr Co-precipitation technique

structurally sensitive.

precipitation technique. As a result of Pd incorporation,methanol yield was considerably higher. Using this catalystat T = 240 ◦C, P = 6 MPa and W/F = 0.0675 kg h/m3, it was foundthat the values of CO2 conversion and methanol selectivitywere 9.2 and 66.2%, respectively (Melian-Cabrera et al., 2002c).Toyir et al. (2001a) prepared CuO/ZnO/Ga2O3 catalyst by co-impregnation of methoxide-acetylacetonate precursors frommethanolic solutions onto ZnO support. Due to the presence ofGa2O3 promoter and high Cu dispersion, this catalyst had highactivity, selectivity and stability. At T = 270 ◦C and P = 2 MPa, therate of methanol formation was 378 g/(kgcat h), whereas theselectivity to methanol was 88%.

Besides these works, there are some other investigationsusing �-Al2O3 supported Cu-based catalysts. Zhang et al.


(2006) investigated the effect of addition of Zr to a CuO/�-Al2O3

catalyst using impregnation technique. Due to the enhanced

Al.

Salient features of work Reference

A zeolite membrane reactorprovides higher values of CO2

conversion, methanol selectivityand methanol yield with respect toa traditional reactor.

Gallucci et al. (2004)

By this way, ultrafine catalystswith high catalytic activity andselectivity can be prepared.

Hong et al. (2002)

Pd improves the reducibility ofCuO.The promoting effect of Pd isbecause of a hydrogen spill overmechanism.

Melian-Cabrera et al. (2002a)

Multi-component catalysts havehigh catalytic activity.Catalyst stability is improved bythe addition of colloidal silica.

Saito and Murata (2004)

These catalysts comprise Cu/Zncrystallites in a fibrous structure.The dispersion and stability of thecrystallites are superior to thosefor a commercial CuO/ZnO/Al2O3

catalyst.

An et al. (2007)

dx.doi.org/10.1016/j.cherd.2014.03.005



dispersion of CuO, both the catalytic activity and selectiv-ity towards methanol improved after Zr addition. In anotherwork (Zhang et al., 2007), the effect of addition of V wasstudied, too; the efficacy of Cu–V/�-Al2O3 was higher thanthat of Cu/�-Al2O3. The influence of reaction variables on theperformance of 12%Cu–6%V/�-Al2O3 was studied, and it wasfound that the most favourable conditions for methanol for-mation are T = 240 ◦C, space velocity = 3600 1/h and H2/CO2

ratio = 3 mol/mol. Wang et al. (2002) found that the perfor-mance of CuO/CeO2/�-Al2O3 and CuO/YDC/�-Al2O3 (whereYDC denotes yttria-doped ceria) is superior than that ofCuO/�-Al2O3. They attributed this enhanced efficacy to thesynergistic effect between CuO and surface oxygen vacanciesof CeO2.

Interestingly, catalysts containing Cu and Ga are reportedin the literature, too. For instance, Toyir et al. (2001b) foundthat CuO/Ga2O3/SiO2 catalysts prepared by impregnationmethod were highly selective and stable in the temperaturerange, 250–270 ◦C. The use of hydrophobic SiO2 enhanced theactivity, selectivity and stability of the catalyst. The modi-fication of properties of Cu particles was attributed to thepresence of very small particles of Ga2O3 on the surface.Liu et al. (2005) used nano-crystalline ZrO2 as support forthe preparation of CuO/Ga2O3/ZrO2 and CuO/B2O3/ZrO2 cata-lysts. At T = 250 ◦C, P = 2 MPa and space velocity = 2500 1/h, thevalues of methanol yield for the Ga- and B-containing cata-lysts were 1.93 and 1.8 mmol/(g h), respectively. It was foundthat nano-crystalline ZrO2 changes the electronic structureand affects the metal and support interactions on the cata-lyst. Consequently, this results in facile reduction, intimateinteraction between Cu and ZrO2, more corner defects andoxygen vacancies on the surface of the catalyst. The valuesof CO2 conversion and methanol selectivity were higher thanthose obtained using catalysts prepared by conventional co-precipitation technique.

From all the above-mentioned studies, it may be noted thatCu-based catalysts are promising for methanol production viaCO2 hydrogenation.

4.2. Pd-based catalysts

Pd has high efficacy for CO2 hydrogenation to methanol (Maet al., 2009); however, the activity and selectivity of Pd-basedcatalysts depend on the type of support (Shen et al., 2001), andthe method of catalyst preparation (Kim et al., 2003). Severalstudies using Pd have recently been published.

Liang et al. (2009) developed Pd/ZnO catalysts supportedon multi-walled carbon nanotubes (MWCNTs), which showedexcellent performance for CO2 hydrogenation to methanol. AtP = 3 MPa and T = 250 ◦C, the observed TOF for CO2 hydrogena-tion reached 1.15 × 10−2 1/s using 16% Pd0.1Zn1/CNTs (h-type).Using Pd/ZnO at 0.1 MPa pressure, Iwasa et al. (2004) reportedhigher TOF and methanol selectivity than Cu/ZnO. PdZn alloyswere formed by catalyst reduction at high temperature. Thesealloys served as active sites for methanol synthesis.

Collins et al. (2004) investigated the interaction of CO2

and H2/CO2 with pure �-Ga2O3 and Pd/�-Ga2O3. They pro-posed that the addition of Pd to the oxide support increasesthe hydrogenation rate of all the carbon-containing speciesbonded to the �-Ga2O3 surface by spill over of atomic hydrogenfrom metallic Pd to Ga2O3.

Bonivardi et al. (2000) found that the addition of


Ga to Pd/SiO2 resulted in enhanced catalytic perfor-mance. For example, the values of TOF using Pd/SiO2 and

Ga2O3-promoted Pd/SiO2 at T = 250 ◦C and P = 3 MPa were0.0017 and 0.45 1/s, respectively. The initial methanol selec-tivity at these conditions was 17 and 62%, respectively. Thisunusually high catalytic behaviour was attributed to the inter-action of Pd crystallites with reduced Ga species. According toCollins et al. (2002, 2005), the successful promotion of Pd/SiO2

by Ga is due to the closeness of the Ga2O3–Pd functions andhydrogen spill over onto the SiO2 support.

Thus, the performance of Pd as a catalyst for CO2 conver-sion to methanol appears promising. In our opinion, mucheffort should be focused on Pd-based catalysts in future.

4.3. Other catalysts

Transitional metal carbides, which are metal-derived com-pounds comprising carbon in the metal lattice, are potentiallyattractive catalysts for CO2 hydrogenation (Ma et al., 2009).They are characterized by high melting point and hardness,and have good thermal and mechanical stability. Their effi-cacy for hydrogenation reactions is comparable to those ofnoble metals such as Pt and Rh (Levy and Boudart, 1973;Oyama, 1992). Besides, their hydrogen adsorption, activationand transfer capabilities are superior to those of metal sulp-hides. Dubois et al. (1992) investigated CO2 hydrogenationusing transition metal carbides. Mo2C and Fe3C showed highCO2 conversion and methanol selectivity at T = 220 ◦C. Cuaddition to Mo2C lowered the hydrocarbon selectivity. A con-siderable amount of dimethyl ether was produced over WC.It was found that TaC and SiC were almost inactive. It is thusobvious that much effort should be focused for further improv-ing their performance.

The AB1−xBxO3 perovskite catalyst, which contains mixedvalence ions and catalytic active sites, has high activity for CO2

conversion to methanol. For example, Jia et al. (2009) reportedthat the catalytic activity (XCO2 = 10.4% and SMeOH = 90.8%) ofa pre-reduced lanthanum chromite perovskite catalyst dopedwith Cu (viz. LaCr0.5Cu0.5O3) was superior to that of 13%Cu/LaCrO3 catalyst (XCO2 = 4.8% and SMeOH = 46.6%) at 250 ◦C.The catalytic activity was higher, due to the fact that H2 wasadsorbed on the Cu˛+ sites and CO2 was activated on themedium basic sites.

Clearly, novel catalysts described in this section are effec-tive; even so, further investigations are essential before onecan deduce the practical feasibility of such catalysts.

5. Knowledge on reaction pathway

There are few previous works describing the reaction pathwayof methanol from syngas using Cu/ZnO catalyst (Chinchenet al., 1986, 1987; Edwards and Schrader, 1984; Klier et al., 1982;Lim et al., 2009; Sahibzada, 2000). Klier et al. (1982) reportedthat methanol production was promoted even at low CO2

concentration in CO/CO2/H2 mixtures, although the synthe-sis was inhibited in CO2-rich atmospheres. A CO2/CO redoxmechanism was employed to describe the kinetic promo-tion/inhibition with respect to the CO2/CO fraction (Chinchenet al., 1986; Klier et al., 1982). According to this mecha-nism, Cu is reduced or oxidized by the adsorption of CO andCO2, respectively (Klier et al., 1982). Interestingly, Chinchenet al. (1987) investigated methanol synthesis from CO/CO2/H2

using isotope-labelled 14CO2 and established that most of the


methanol was produced from CO2. Then, Sahibzada (2000)found that CO2 did not inhibit methanol synthesis. Rather,

dx.doi.org/10.1016/j.cherd.2014.03.005



tto(factpt(sm(Caarmiha

H

H

C

w

waWdCaf

gZchpunTft

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egsdtt

he progressive inhibition in CO2-rich atmosphere was dueo the increasing product water concentration as a resultf Eq. (3) (CO2 + 3H2 ↔ CH3OH + H2O) and the reverse of Eq.

4) (H2 + CO2 ↔ CO + H2O). The activity of Cu/ZnO catalystor methanol synthesis from CO2/H2 was improved by theddition of Pd, due to the fact that H2 spill-over from Pdounteracted the inhibition by water. At low water concentra-ion, it is possible that, in a CO-rich atmosphere, methanol isroduced by CO hydrogenation in addition to CO2 hydrogena-ion using Cu/ZnO (Sahibzada, 2000). Edwards and Schrader1984) showed that CO is adsorbed in the form of a carbonylpecies, and the formate species is a usual intermediate forethanol synthesis and WGS reaction. Recently, Lim et al.

2009) developed a kinetic model for methanol synthesis usingu/ZnO/Al2O3/ZrO2 catalyst. They considered that CO and CO2

re adsorbed on different sites on Cu (here denoted as s1

nd s3). From their results, they concluded that the surfaceeaction of a methoxy species, the hydrogenation of a for-

ate intermediate (HCOO), and the formation of a formatentermediate are the rate-determining steps for CO and CO2

ydrogenation and the WGS reaction, respectively; these stepsre represented as:

3CO s1 + H s2 ↔ CH3OH + s1 + s2 (6)

CO2 s3 + H s2 ↔ H2COO s3 + s2 (7)

O2 s3 + H s2 ↔ HCO2 s3 + s2 (8)

here H2 and H2O are adsorbed on the active site s2.Furthermore, they found that the CO2 hydrogenation rate

as much lower than the CO hydrogenation rate, and thisffected methanol production. However, CO2 decreased theGS reaction rate; this prevented methanol conversion into

imethyl ether, a by-product. By this way, a small fraction ofO2 accelerated the production of methanol indirectly within

limited range, showing a threshold value of the CO2 fractionor maximum methanol synthesis.

Studies on reaction mechanism and kinetics of CO2 hydro-enation are fewer (Chiavassa et al., 2009; Tang et al., 2009;hao et al., 2011). As was reported by Wang et al. (2011), twolasses of reaction routes for CO2 conversion to methanolave been proposed in the literature. One route is the formateathway, where the formation of the intermediate HCOO issually considered as the rate-determining step. This mecha-ism suggests that CO is formed by methanol decomposition.he other route is the reverse WGS mechanism, where CO is

ormed by reverse WGS and converted to methanol accordingo Eq. (5).

Tang et al. (2009) applied first principles kinetic Montearlo simulation for studying reaction kinetics using Cu/ZrO2

atalyst. They established a Cu/ZrO2 interface model andnvestigated the reaction network of CO2 hydrogenation. As

entioned above, two reaction channels to methanol weredentified: first, a reverse WGS via CO2 decomposition to CO,nd second, the well-regarded mechanism via a formate inter-ediate.Using periodic density functional theory calculations, Zhao

t al. (2011) investigated the reaction network for CO2 hydro-enation to methanol on Cu(1 1 1). They listed the elementaryteps in the formate hydrogenation route (see Table 4), namely:issociative adsorption of H2, direct interaction of CO2 with


he surface H to form HCOO via an ER mechanism, hydrogena-ion of HCOO to H2COO, deoxygenation of H2COO to H2CO via

the H2COOH intermediate, and further hydrogenation of H2COto H3CO and H3COH. They concluded that methanol synthe-sis from direct hydrogenation of the formate on Cu(1 1 1) isnot feasible due to the high activation barriers for some of theelementary steps. Contrarily, it was suggested that the CO2

hydrogenation route to hydrocarboxyl (trans-COOH) is kineti-cally more favourable than formate in the presence of H2O viaa unique hydrogen transfer mechanism. First, the trans-COOHis converted into hydroxymethylidyne (COH) via dihydrox-ycarbene (COHOH) intermediates. Next, three consecutivehydrogenation steps occur, thereby resulting in the formationof hydroxymethylene (HCOH), hydroxymethyl (H2COH), andmethanol (see Table 4).

Chiavassa et al. (2009) modelled methanol synthesis fromCO2/H2 over Ga2O3–Pd/SiO2 catalyst, along with the reverseWGS reaction. They found that hydrogenation of the formateintermediate and its decomposition on the Ga2O3 surface werethe rate determining steps, respectively. From their results,they concluded that a competitive adsorption mechanism,where adsorbed atomic hydrogen occupies the same activesites as other oxygenated surface intermediates on Ga2O3, ismost appropriate.

Clearly, there is scarce information in the literature andadditional work on the reaction pathway and kinetics of CO2

hydrogenation to methanol is essential for a comprehensiveinsight into methanol synthesis chemistry.

6. Recent technological advances

6.1. Current industrial status

Olah et al. (2011) discussed the current industrial status ofmethanol production from CO2; this state-of-art is repre-sented here. A major milestone was achieved when LurgiAG and Sud–Chemie together developed a highly active andselective catalyst for producing methanol from CO2 and H2

at T = 260 ◦C. The activity of this catalyst decreased at aboutthe same rate as the activity of the commercial methanolsynthesis catalyst (Goehna and Koenig, 1994). In anothermajor development, the first pilot plant for the production ofmethanol (50 kg/h) from CO2 and H2 was built in Japan using aSiO2-modified Cu/ZnO catalyst. Recycling the feed produced aspace-time yield of methanol around 600 g/(L h), with 99.9%selectivity over 8000 h operation at 250 ◦C and 5 MPa (Saito,1998). Yet another pilot plant producing methanol from CO2

and H2 with an annual capacity of 100 tonnes is being builtby Mitsui Chemicals in Japan. To accomplish this target, H2

will be generated by photochemical splitting of water usingsolar energy (Tremblay, 2008). For the first time, a liquid-phasemethanol synthesis process was also developed, which allowsa CO2 and H2 conversion to methanol of about 95% with veryhigh selectivity in a single pass (Air Products, 2003). Then, thefirst commercial CO2 to methanol recycling plant using locallyavailable cheap geothermal energy is presently being builtafter successful pilot plant scale operation in Iceland by thecompany Carbon Recycling International. This plant is basedon the conversion of CO2, a significant by-product accompa-nying local geothermal energy sources or industrial sources(aluminium production). H2 is produced by water electrolysis(vide infra) (Shulenberger et al., 2007).

The technical feasibility of methanol production from CO2


has also been demonstrated in pilot plants using a two-stepapproach, viz. KIST (CAMERE) process (reverse WGS separate

dx.doi.org/10.1016/j.cherd.2014.03.005



Table 4 – Elementary reaction steps in various methanol synthesis routes from CO2 according to Zhao et al. (2011).

Reaction routes Reactions

Formate reaction route

H2 → H + HCO2 + H → mono-HCOO → bi-HCOOmono-HCOO → CO + OH and bi-HCOO → HCO + Obi-HCOO + H → H2COO and bi-HCOO + H → HCOOHH2COO + H → H2COOH and HCOOH + H → H2COOHHCO + H → H2CO and H2COO → H2CO + O andH2COOH → H2CO + OHH2CO + H → H3COH3CO + H → H3COH

Hydrocarboxyl reaction route

H2 → H + HCO2 + H → trans-COOHtrans-COOH → cis-COOHtrans-COOH + H → t,t-COHOH → t,c-COHOH → c,c-COHOHc,c-COHOH → COH + OH and t,c-COHOH → COH + OHCOH → CO + H and HCO → CO + H and cis-COOH → CO + OHCOH + H → HCOH and HCO + H → HCOHHCOH + H → H2COH and H2CO + H → H2COHH2COH + H → H3COH

from methanol synthesis) or a single step approach proposedby NIRE/RITE (the two stages integrated in a single reactor)(Centi et al., 2008). The first approach seems preferable interms of higher catalyst productivity, lower gas recycle andreactor size (Centi and Perathoner, 2009).

Very recently, researchers at the Institute of Bioengineeringand Nanotechnology (IBN) in Singapore used organocatalyststo activate CO2 in a mild and non-toxic process for producingmethanol. Certainly, all these developments are very encour-aging.

6.2. Reactor innovations

There are few recent works which consider innovations inreactors; these are briefly discussed in this subsection. Tocombine catalytic reaction with the separation properties ofzeolite membranes, Gallucci et al. (2004) used a zeolite mem-brane reactor. They found that CO2 conversion (XCO2 = 11.6%),methanol selectivity (SMeOH = 75%) and yield (YMeOH = 8.7%)at T = 206 ◦C were higher than those obtained in a con-ventional reactor (XCO2 = 5%, SMeOH = 48% and YMeOH = 2.4%)at T = 210 ◦C, P = 2 MPa, H2/CO2 ratio = 3 mol/mol and spacevelocity = 6000 1/h. The methanol yield was higher, due tothe fact that the products (methanol and water) wereselectively removed from the reaction system. From reac-tor simulations at T = 210 ◦C and P = 1 MPa, Barbieri et al.(2002) found that reactors with an organophilic (X = 22.7%,SMeOH = 60.2%, YMeOH = 13.7%) and hydrophilic membrane(X = 23.9%, SMeOH = 54.2%, YMeOH = 13%) have better perfor-mance than that of a conventional tubular reactor (X = 14.2%,SMeOH = 40.5%, YMeOH = 5.8%). They highlighted the fact thatincreased methanol yield in a membrane reactor reduces theconsumption of reactant and facilitates operation at lowerpressures and higher temperatures. This, in turn, favours reac-tion kinetics by reduction in the residence time and the reactorvolume.

Rahimpour (2008) investigated a two-stage catalyst bedconcept for CO2 conversion to methanol. A system with twocatalyst beds instead of one single catalyst bed was consid-ered. In the first catalyst bed, the synthesis gas was partly


converted to methanol in a conventional water-cooled reac-tor. This bed was operated at higher than normal operating

temperature and at high yield. In the second bed, the reac-tion heat was used to pre-heat the feed gas to the first bed.The continuously reduced temperature in this bed providedincreasing thermodynamic equilibrium potential. In this bed,the reaction rate was much lower, and hence, was the reac-tion heat. This feature resulted in milder temperature profilesin the second bed because less heat was released as com-pared to the first bed. In this way, the catalysts were exposedto less extreme temperatures and, catalyst deactivation viasintering was avoided. The two-stage catalyst bed systemcould be operated with higher conversion and longer cata-lyst life time than a conventional single-bed reactor. Thus,it is an interesting candidate for application in methanolproduction.

6.3. Alternate catalytic CO2 hydrogenation techniquesfor methanol synthesis

Joo et al. (1999) reported CAMERE (CO2 hydrogenation to formmethanol via a reverse WGS reaction) process to convert CO2

into methanol. This process comprised a reverse WGS reactionand a methanol synthesis reaction. CO2 and H2 were convertedto CO and H2O by the reverse WGS, and then the gaseous mix-ture of CO/CO2/H2 was fed into the methanol reactor afterremoving water. Several useful papers on the CAMERE pro-cess were published during the past decade (Park et al., 2000,2001; Joo and Jung, 2003).

Low temperature methanol synthesis in a liquid mediumis a candidate technique reported in the literature. Accord-ing to Xu et al. (2009), this technology is expected to producemethanol more efficiently than the conventional methanolproduction processes using Cu/ZnO-based catalysts, due tothe fact that it has several advantages over the conventionalprocess, i.e., thermodynamically favourable low temperatureoperation and efficient removal of the heat of reaction due tothe large heat capacity of a liquid medium. Recently, Liu et al.(2007) employed a novel low-temperature route in an auto-clave for the efficient conversion of CO2 into methanol. Usingthis process, 25.9% CO2 conversion and 72.9% methanol selec-


tivity was achieved at a low temperature of 443 K and pressureof 5 MPa using alcohol as solvent.

dx.doi.org/10.1016/j.cherd.2014.03.005



7

ItmaoacCciabcgu

mmvmtttastt

A

SN

R

A

A

A

A

A

B

B

B

. Conclusions

n this work, recent advances in catalytic CO2 hydrogena-ion to methanol are comprehensively discussed. Since 2003,

uch effort is focused on investigating the efficacy of Cu-nd Pd-based catalysts. It is found that the performancef CuO/ZnO/ZrO2 catalysts prepared by novel routes suchs solid-state reaction, urea-nitrate combustion and reverseo-precipitation under ultrasound irradiation is promising.uO/ZnO-based catalysts promoted with Pd and Ga are espe-ially useful. As a result of Pd incorporation, methanol yields enhanced. The presence of Ga2O3 promoter enhances thectivity, selectivity and stability of CuO/ZnO. Among the Pd-ased catalysts, Pd/ZnO catalysts supported on multi-walledarbon nanotubes show excellent performance for CO2 hydro-enation to methanol. The addition of Ga to Pd/SiO2 results innusually high catalytic behaviour.

The reaction pathway of catalytic CO2 hydrogenation toethanol is discussed. Commonly, two reaction routes toethanol are described in literature: first, a reverse WGS

ia CO2 decomposition to CO, and second, the well-regardedechanism via a formate intermediate. There are a few reac-

or innovations recently described in literature; among these,he zeolite membrane reactor which combines catalytic reac-ion with the separation properties of zeolite membranes isttractive. Finally, alternate catalytic techniques to methanolynthesis are discussed, and the CAMERE (CO2 hydrogenationo form methanol via a reverse WGS reaction) process seemso be a promising option.

cknowledgement

uhas G. Jadhav is grateful to University Grants Commission,ew Delhi, for the financial support.

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