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Applied Catalysis A: General 475 (2014) 155– 160

Contents lists available at ScienceDirect

Applied Catalysis A: General

j ourna l h om epage: www.elsev ier .com/ locate /apcata

irect synthesis of LPG from carbon dioxide over hybrid catalystsomprising modified methanol synthesis catalyst and �-type zeolite

ongming Li, Xingdong Yuan, Kaoru Fujimoto ∗

apan Gas Synthesis Co. Ltd, 5-20, Kaigan 1-Chome, Minato-Ku, Tokyo, 105-8527, Japan

r t i c l e i n f o

rticle history:eceived 6 November 2013eceived in revised form 8 January 2014

a b s t r a c t

Hydrogenation of carbon dioxide to C3 and C4 paraffins (LPG) was studied by using hybrid catalystscomposed of Zr-modified Cu–Zn catalyst with Pd-modified � zeolite. Various factors that affect cata-lyst activity, including reaction temperature, space velocity, pressure, and the ratio of H2 to CO2 were

ccepted 10 January 2014vailable online 21 January 2014

eywords:arbon dioxide

investigated thoroughly. The hybrid catalysts exhibited stable activity over 100 h and extraordinary highselectivity of LPG synthesis (especially iso-butane) even under low temperature (260 ◦C) and low pressure(2 MPa). Conversion of CO2 and the yield of hydrocarbon reached 25.2% and 13.3%, respectively, whilekeeping the selectivity of LPG and CH4 75% and 1%, respectively. The iso-butane/n-butane ratio was about16/1, whose value was much higher than that of thermodynamic equilibrium (about 1/1).

so-butane

ybrid catalysts

. Introduction

Carbon dioxide is one of the most abundant and widely dis-ributed carbon resources, and has become the focus of attentionecause of its greenhouse effects [1–3]. Therefore, the utilization ofO2 is desirable for the effective method for reducing CO2 emission.any kinds of strategies with this regard have been proposed to

olve this intractable problem. Among various methods, recyclingechnology by the hydrogenation of CO2 has attracted additionalttention as a possible way to manufacture fuel material or usefulhemicals. The conversion of CO2 to methanol by its hydrogenationas been the subject of intense study over the past decades [4–12].owever, the reaction is regulated by the severe limitation of ther-odynamic equilibrium. To overcome such a limitation, the in situ

ransformation of methanol to stable materials is a reasonable wayo improve the reaction performance. One of the typical methodso make hydrocarbon is the methane formation, which is the onlyhermodynamically favorable reaction. Another is the modifiedischer–Tropsch (F–T) reaction [13]. However, the F–T type reactionf CO2 makes CH4-rich hydrocarbon mixture. Liquefied petroleumas (LPG), a general description of propane and butane, has envi-onmentally benign characteristics and widely used as clean fuels.esides, LPG is a good substitute for fuel in spark ignition engines,nd used as a replacement for aerosel propellants and refrigerants,

tc. [14–16]. So LPG synthesis from carbon dioxide and hydrogen isn important selection to fixation of carbon dioxide to fuels as wells useful chemicals. One of the present authors has shown that the

∗ Corresponding author. Tel.: +81 695 3202; fax: +81 695 3309.E-mail address: fujimoto070@gmail.com (K. Fujimoto).

926-860X/$ – see front matter © 2014 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apcata.2014.01.025

© 2014 Elsevier B.V. All rights reserved.

hydro-condensation of methanol or dimethylether (DME) on mod-ified �-zeolite can make LPG-rich paraffin mixture with little drygas (CH4 + C2H6) formation at temperature as low as 300 ◦C or lower[17]. The hybrid catalyst composed of methanol synthesis cata-lyst and modified zeolite give LPG-rich paraffins selectively fromsynthesis gas with much higher conversion for methanol synthesisreaction. The catalyst could work over 2000 h without regeneration[18]. However, the reaction of syngas to LPG gave only half of theconverted CO to target product because of the CO shift reaction ofthe product . Direct synthesis of LPG from CO2 hydrogenation is animportant choice for converting the most common and unlimitedcarbon resource to high-value-added products. We have reportedpreviously that Zr-modified Cu–Zn–Al catalyst (CZZA) show highactivity and good stability for methanol synthesis [19]. As catalystcomponent for hydro-conversion of methanol to LPG, a modified�-zeolite was regarded as the most suitable one in our previouswork [20,21].

The present paper is the results of the trial of the selective syn-thesis of LPG over the hybrid catalysts consisting of a Zr-modifiedCu–Zn–Al methanol catalyst with modified � zeolite (Pd-�). Theresults show that the catalyst is active at low temperature, stableand selective for the synthesis of LPG-rich paraffins from CO2. Theprocess of hydrocarbon formation and the product distribution arediscussed in detail.

2. Experimental

2.1. Catalyst preparation

Modified Cu–Zn methanol synthesis catalyst was prepared bythe co-precipitation sedimentation method. For convenience, these

1 A: General 475 (2014) 155– 160

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LPG (C3 + C4) selectivity of about 75% accompanying lower methaneselectivity (1.2%). However, its level was much lower than that ofthe syngas conversion (45.5%) under the same reaction conditions.These subjects will be discussed in the later section. The low content

Table 1CO2 hydrogenation over hybrid catalysts.

Catalysts CZA/Pd-� CZZ/Pd-� CZZA/Pd-� CZZA/Pd-� CZZA

CO2 Conv. (%) 21.2 22.0 25.2 82.8a 19.0b

Yield(%)H.C. 7.5 11.2 13.3 45.5 trCO 13.7 10.8 11.9 37.2 17.2MeOH 0 0 0 0 1.8DME tr tr tr tr 0

H.C. compositionC1 3.7 3.8 1.2 0.1 –C2 5.0 4.9 2.8 0.5 –C3 21.1 17.8 12.0 5.8 –C4 60.4 56.9 62.8 85.2 –C5 10.3 12.3 14.8 6.4 –C6 tr 2.3 4.5 1.2 –C7+ tr tr 1.9 0.7 –i-C4/n-C4 9.8 9.1 16.0 46.7 –

56 C. Li et al. / Applied Catalysis

atalysts will be designed by an abbreviation, CZA (Cu/ZnO/Al2O3),ZZA (Cu/ZnO/ZrO2/Al2O3), CZZ (Cu/ZnO/ZrO2) throughout thisaper [19]. The resulting catalyst has the composition of CZACu/ZnO/Al2O3 = 4/3/3), CZZA (Cu/ZnO/ZrO2/Al2O3 = 4/3/1.5/1.5),ZZ (Cu/ZnO/ZrO2 = 4/3/3) by weight. A Pd-modified � zeoliteNH4-�, Si/Al = 38) (Pd/�) was prepared by ion exchange with nitriccid solution of PdNO3 for 24 h, then dried at 100 ◦C overnight andalcining at 500 ◦C for 4 h. The loading amount of Pd was 0.1 wt%.ethanol synthesis catalysts and the Pd-� were independently

elletized, crushed and sieved to the particles of 0.36–0.71 mm,espectively. The two kinds of particles were then mechanicallyixed well to form hybrid catalyst of CZA/Pd-�, CZZA/Pd-� and

ZZ/Pd-�.

.2. Catalyst characterization

The XRD (X-ray diffraction analysis) measurement was per-ormed with a Rigaku diffractometer equipped with a Cu–K�adiation. The specific surface area of the catalysts was determinedy BET method using a Micromeritics ASAP 2010. Temperature pro-rammed reduction of methanol synthesis catalyst were conductedy the procedure written in the preceding paper [19].

.3. Catalytic reaction test

A pressurized flow type reaction apparatus with a fixed-bedeactor was used for this study. The apparatus was equipped withn electronic temperature controller for a furnace, a tubular reactorith an inner diameter of 8 mm, thermal mass flow controllers for

as flows and a back-pressure regulator. A thermocouple was sett the axial center of the tubular reactor. 1 g of catalyst was placedn the reactor with inert quartz sands above and under the cata-yst. All catalysts were reduced in a flow of 5% H2 in nitrogen at50 ◦C for 4 h before the reaction. The steady-state activity mea-urements were taken after 15 h on the stream. All the productsrom the reactor were introduced in gaseous state and analyzed byas chromatograph (GC). CO, CH4 and CO2 were analyzed by a GCquipped with the thermal conductivity detector (TCD) and a col-mn of activated charcoal, and light hydrocarbons were analyzedy another GC equipped with the flame ionization detector (FID)nd a capillary column of Porapak-Q.

. Results and discussion

.1. Catalyst characterization

The calcined CZZA exhibited the highest surface area of06.3 m2/g, and CZZ (90.9 m2/g) and CZA (78.3 m2/g) followed. Theesult of N2O titration indicate that the aluminum-containing cat-lyst (CZA) had the highest Cu surface area, while TPR result showshe reduction peak of CZA obviously shifted to high temperatureompared with CZZA. All these phenomena suggest that as it isell-known the added Al2O3 promotes the CuO dispersion and sup-resses its reduction, whereas the added ZrO2 in CuO/ZnO-basedatalyst promote the reduction of CuO with H2 [19].

.2. Catalytic results

For the direct synthesis of hydrocarbons from CO2, there arehree important reactions involved: (i) methanol formation fromO2 hydrogenation and/or CO hydrogenation, (ii) dehydration ofethanol to DME, (iii) hydrocondensation of DME to hydrocarbons,

nd (iv) reversed water gas shift (RWGS) reaction . Since the targetf our study is the synthesis of LPG from CO2, the hydrocarbonshould be C3 and C4 parraffin with low C1 + C2 paraffins. As shownn Fig. 1, when butane is synthesized from CO2 with the ratio H2/CO2

Fig. 1. Equilibrium conversion of CO2 for different products as a function of tem-perature. Pressure: 2.0 MPa, H2/CO2 = 3/1 (mole ratio).

of 3 in the reaction (1), the equilibrium conversion of CO2 is higherthan 80%, while those of CH3OH or DME is much lower.

4CO2 + 13H2 → C4H10 + 8H2O (1)

It has been shown that the hydro-condensation of methanolcan make C3 and C4 hydrocarbons with high selectivity [22]. Theformation of hydrocarbons relieves the thermodynamic constraintinherent to methanol synthesis by in situ transforming methanolinto hydrocarbons. Thus, the high conversion of CO2 to hydrocar-bons can be expected.

3.3. CO2 hydrogenation to LPG on a variety of hybrid catalysts

Table 1 showed the catalytic performance at the steady state inCO2 hydrogenation over various hybrid catalysts. Neither alkenesnor aromatics were obtained over the hybrid catalysts. It is apparentthat CZZA/Pd-� exhibits the highest conversion of CO2 and espe-cially the yield of hydrocarbon was almost two times higher thanthat of Zr-free catalyst (CZA/Pd-�). This is probably due to higherproductivity of methanol synthesis over CZZA catalyst than thatover CZA [19].

The hydrocarbon yield obtained over CZZA/Pd-� at 260 ◦C wasmuch higher than that obtained at higher temperature (Fig. 4) for

LPG Sel. (%) 80.5 76.7 74.8 91.0 –

Reaction temp., 260 ◦C; pressure, 20 atm; W/F, 10 g cat h/mol; H2/CO2, 3/1.a H2/CO = 2.0.b Without Pd-� zeolite.

C. Li et al. / Applied Catalysis A: Ge

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3.5.2. W/FThe conversion of CO2 and yield of hydrocarbons increased

monotonously with increasing the time factor (W/F) as shown in

ig. 2. Stability of CO2 hydrogenation to LPG over CZZA/Pd-� catalyst at 260 ◦C,/F = 10 g cat h/mol, H2/CO2 = 5/1.

f methane in the products is quite favorable from the industrialtand point.

.4. Catalyst stability

As shown in Fig. 2, hybrid catalysts keep a constant activityCO2 conversion) up to 100 h from the start of the reaction. How-ver, with the time on stream, the DME becomes the main productith trace methanol while keeping CO2 conversion constant, which

hould be ascribed to the poisoning of acid site by water moleculen the zeolite. Successing the CO2 hydrogenation, the CO2–H2 gasas swiched to syngas (H2/CO/CO2 = 8/3/1) and operated the syn-

as reaction at 260 ◦C and W/F = 10 g cat h/mol for 30 h and againO2–H2 gas was fed to see what happened. It was interesting toote that the activity was recovered without any obvious activitynd selectivity attenuation after 100 h stability.

Fig. 3 shows the result of XRD measurement of the used hybridatalyst after 100 h run. Results obtained suggest the presence of

CuO phase (2� = 35.5◦, 38.5◦) for calcined sample. After reduc-ion, the peaks at 43◦ and 50◦ were attributed to metallic Cu, whicherformed as a role as active component for methanol synthesis23,24]. The peak attributed to CuO was observed at 36.4◦ after

ig. 3. XRD patters of catalysts: (1) calcined; (2) reduced; (3) after 100 h reaction,nd (4) Pd-� zeolite.

neral 475 (2014) 155– 160 157

reaction, suggesting a part of Cu metal was oxidized during thereaction. By using Debye–Scherrer formula, the crystallite sizegrowth of Cu occurred during stability test, which could be esti-mated to be 39.2 nm, which grew from 9.4 nm of fresh catalyst,larger than 100 h test of methanol synthesis (11.4 nm) [19], stillkeeping the CO2 conversion constant.

3.5. Effect of reaction conditions

3.5.1. TemperatureThe most suitable reaction temperature for methanol synthesis

over the current catalysts was approximately 230 ◦C, which differssubstantially from that of the methanol conversion reaction overzeolite. The catalytic activity was tested using the composite cata-lyst CZZA/Pd-� (1/1 weight ratio) at 260, 270 and 280 ◦C as shownin Fig. 4. With increasing reaction temperature from 260 to 280 ◦C,the conversion of CO2 decreased slightly, while the yield of hydro-carbons markedly decreased and that of CO increased. Typically,the methanol synthesis function is provided by a Cu-based cata-lyst while the dyhydrogenation of methanol to DME and then tohydrocarbons is afforded by modified zeolite. In general, zeoliteis preferred at higher temperature for a substantially high activ-ity, while methanol synthesis is favored for lower temperaturefrom the thermodynamically standpoint as shown in Fig. 1, and itwas realized in the experiment [19]. In order to attain a good syn-ergy between the two catalysts, a suitable reaction temperature isimportant. Below 260 ◦C, almost the entire product was DME andCO (not shown), while higher temperature do also tend to favor COformation. So we think 260 ◦C is beneficial to obtaining high activityand higher LPG selectivity.

It was found that with increasing the reaction temperature, theLPG selectivity decreased and the methane selectivity increased.Therefore, we consider that the relative lower temperature is suit-able for obtaining high yield of LPG.

Fig. 4. Effect of reaction temperature on CO2 hydrogenation to LPG over CZZA/Pd-�catalyst at 2 MPa, W/F = 10 g cat h/mol, H2/CO2 = 3/1.

158 C. Li et al. / Applied Catalysis A: General 475 (2014) 155– 160

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ig. 5. Effect of W/F on CO2 hydrogenation to LPG over CZZA/Pd-� catalyst at 260 ◦C, MPa, H2/CO2 = 3/1., * and ×: CO2 hydrogenation to methanol under the same reaction conditions.

ig. 5. At short contact time below 5 g cat h/mol, CO was the mainroduct. With an increase in the contact time beyond 5 g cat h/mol,O did not increase but slightly decreased to reach a constant level.his means that CO2 is converted to CO and hydrocarbon, then COs successively converted to hydrocarbon.

Also hydrocarbon distribution vs. contact time (Fig. 5) showshat it is almost constant but C6+ hydrocarbon is graduallyecreased and C3 hydrocarbon increased.

.5.3. H2/CO2 ratioThe ratio of H2/CO2 had a great effect on the activity and selectiv-

ty of the catalyst. The stoichiometric ratio of H2 of H2/CO2 to formaraffin from CO2 is between 3 and 4 as described in the followingquation.

O2 + (3 + 1/n)H2 → 1/n(CnH2n+2) + 2H2O (n = 4forC4H10)

Effects of the molar ratio of H2 to CO2 are given in Fig. 6. As theolar ratio of H2 to CO2 increased, the apparent conversion of CO2

ncreased slightly while the relative selectivity of CO decreased,robably because that of methanol (which is the intermediatepecies for hydrocarbon synthesis) increased with increasing the2 concentration from the stand point both thermodynamics andinetics [19]. The hydrocarbon distribution was also affected ashown in Fig. 6. Increasing the H2/CO2 ratio increased the LPG selec-ivity from 74.8% H2/CO2 ratio of 3 to 78.6% H2/CO2 ratio of 4, andhen kept a relatively stable LPG selectivity when continuing the2/CO2 ratio.

.5.4. PressureFrom the dynamics stand point, a high pressure is beneficial

or the methanol synthesis from CO2 hydrogenation and for the

Fig. 6. Effect of H2 to CO2 ratio on CO2 hydrogenation to LPG over CZZA/Pd-� catalystat 260 ◦C, 2 MPa, W/F = 10 g cat h/mol.

next catalytic reaction of methanol to hydrocarbons. The catalyticbehavior as a function of total pressure of H2/CO2 feed mixture wasinvestigated between 1 and 2.5 MPa as shown in Fig. 7. The conver-sion of CO2 is promoted by increasing the reaction pressure. It isworth noting that this catalyst exhibits high activity even underrelative low pressure. At 1.0 MPa, the CO2 conversion was 23.3%and slightly increased to reach 32.5% at 2.5 MPa, while the hydro-carbon yield increased from 4.7% at 1.0 MPa to 21.4% at 2.5 MPa.Since the process was controlled by the kinetics, a high pressure,which meant that the high concentration of reactants was obtainedon the surface of catalyst. At the same time, increasing reactionpressure actually prolonged the residence time of reactant in thecatalyst bed, which is preferable for close contact between reactantand catalyst, resulting in the high conversion of CO2 to hydrocar-bons. The selectivity of LPG reaches as high as 80% in the pressure

Fig. 7. Effect of pressure on CO2 hydrogenation to LPG over CZZA/Pd-� catalyst at260 ◦C, W/F = 10 g cat h/mol, H2/CO2 = 5/1.

C. Li et al. / Applied Catalysis A: Ge

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ig. 8. Reaction path of CO2 hydrogenation to hydrocarbons over hybrid catalysts.

. Discussions on the CO2 hydrogenation on the hybridatalyst

The overall reaction path of the hydrocarbon synthesis can behown in Fig. 8. As previously reported [19], the methanol for-ation from CO2 on CZZA catalyst was assumed to pass through

wo routes: one is the direct hydrogenation of CO2 to methanol;nother is the route which passes through the CO formation,hile the conventional methanol synthesis catalyst (CZA) passes

hrough the CO formation. Methanol is formed by CO2 hydro-enation or via CO over the methanol catalyst, then methanol isonverted to DME and then converted directly to C1–C7 paraffinsn the acidic sites of the Pd-� zeolite according to mechanismf chain growth with the strong promotion effect of spillover ofydrogen [25]. The spillover of hydrogen makes it possible to con-ert methanol into paraffinic hydrocarbons at temperature as lows 260 ◦C.

The essential factors for the selective synthesis of LPG:

1) excellent activity of methanol formation and resistance of CZZAto steam [19];

2) easy transfer of methanol from CZZA to Pd-� zeolite;3) large pores in beta zeolite (∼0.7 nm) enable the quick diffu-

sion of methanol and product hydrocarbon which suppress thesecondary reactions within the interior of the pore system;

4) introduction of H species (H+ and or H−) to Pd-� zeolite, by thehydrogen spillover [26].

The high selectivity of C3 and C4 paraffins are interpreteds follows: the active species of the methanol conversion toydrocarbons are accepted as carbene CH2 and carbenium ions.he thermodynamical stability of carbenium ion is the ascend-ng order: CH3

+ « C2H5+ < sec- C3H7

+ < sec-C4H9+ « tert-C4H9

+. Thesearbenium ions come out of the zeolite through two possible routesas shown in Fig. 9). One is the de-protonation for making olefin:

4H9+ → C4H8 + H+ (2)

Another is the hydrogenation to make parraffin:

4H9+ + H− → C4H10 (3)

It is clear that reaction (2) is much more endothermic than reac-

ion (3). The olefin formation from methanol (MTG reaction, whichnclude reaction (2) required high temperature (>350 ◦C). On thether hand, reaction (3) is large exothermic reaction, and thus pro-eeds at lower temperature. The difference in the thermodynamic

ig. 9. Reaction scheme of CO2 hydrogenation to hydrocarbons over Pd-� zeolite.

[[[

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neral 475 (2014) 155– 160 159

makes the large temperature difference for the hydrocarbon for-mation with Pd (for hydrogen activation) containing zeolite andPd-free zeolite catalyst [27,28]. Thus the reactivity of three activespecies with carbene is in the reverse order. Only small amount ofactive species of C1 (CH3

+) may come out of the zeolite as methanebecause of its high reactivity [29].

Active species of C3 or higher, may have chance to come outof zeolite surface as hydrocarbons. In the case of C4 species, twotypes of active species could exist, that is secondary butyl carbe-nium ion and tertiary butyl carbenium ion. The latter species is byabout 58.6 kJ/mol more stable than the former one and thus may bepredominant in its concentration over the former one. Its predomi-nance is demonstrated by the fact that the amount of iso-butane inthe product is by about 16 times higher than that of n-butane. Andthe tertiary butyl carbenium ion would be very low in its reactivityfor chain growth and thus most of them come out of catalyst surfaceas branched C4 hydrocarbons without growing to C5+ species.

5. Conclusion

Hybrid catalysts consisting of Zr modified Cu–Zn catalyst withPd-modified � zeolite were found to show high and stable conver-sion for CO2 hydrogenation. The stable activity of CO2 conversioncan be attributed to the high water resistant of CZZA. The hybrid cat-alyst consisting of CZZA with Pd-� was effective for LPG synthesiswith selectivity as high as 75% and with little methane formation.Catalytic activity was stable over 100 h. Due to the poisoning of acidsite by water molecule on the zeolite, DME became the main prod-uct with trace methanol with the time on stream. But the bad effectscould be overcome if the Pd-� was exposed to the “dry” conditions(CO–H2 reaction). Iso-butane could be obtained in extra-ordinaryhigh selectivity and in good yields over these novel composite cat-alysts by the hydrogenation of carbon dioxide. The essentials ofthe high activity at low temperature and the high selectivity ofaimed products may be caused by the promotion effect of hydro-gen spillover and its effective reaction with the intermediates onzeolite. These results indicated that the proper use of methanolcatalyst with zeolite enabled us to produce specified hydrocarbonsfrom CO2.

Acknowledgement

This work was supported by New Energy and Industrial Tech-nology Development Organization (NEDO).

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