synthesis of methanol and dimethyl ether from syngas

2116 Catal. Sci. Technol., 2012, 2, 2116–2127 This journal is c The Royal Society of Chemistry 2012

Cite this: Catal. Sci. Technol., 2012, 2, 2116–2127

Synthesis of methanol and dimethyl ether from syngas over

Pd/ZnO/Al2O3 catalysts

Vanessa M. Lebarbier,*aRobert A. Dagle,*

aLibor Kovarik,

a

Jair A. Lizarazo Adarme,bDavid L. King

aand Daniel R. Palo

b

Received 11th May 2012, Accepted 16th June 2012

DOI: 10.1039/c2cy20315d

A Pd/ZnO/Al2O3 catalyst was developed for the synthesis of methanol and dimethyl ether (DME)

from syngas with temperatures of operation ranging from 250 1C to 380 1C. High temperatures

(e.g. 380 1C) are of interest when combining methanol and DME synthesis with a methanol to

gasoline (MTG) process in a single reactor bed. A commercial Cu/ZnO/Al2O3 catalyst, utilized

industrially for the synthesis of methanol at 220–280 1C, suffers from a rapid deactivation when

the reaction is conducted at high temperature (4 320 1C). On the contrary, a Pd/ZnO/Al2O3

catalyst was found to be highly stable for methanol and DME synthesis at 375 1C. The Pd/ZnO/

Al2O3 catalyst was thus further investigated for methanol and DME synthesis at P = 34–69 bar,

T = 250–380 1C, GHSV = 5000–18 000 h�1, and molar feeds H2/CO = 1, 2, and 3. Selectivity

to DME increased with decreasing operating temperature, and increasing operating pressure.

Higher space velocity and H2/CO syngas feed ratios also enhanced DME selectivity. Undesirable

CH4 formation was observed, however, it could be lessen through choice of process conditions

and by catalyst design. By studying the effect of the Pd loading and the Pd : Zn molar ratio the

formulation of the Pd/ZnO/Al2O3 catalyst was optimized. A catalyst with 5% Pd and a Pd : Zn

molar ratio of 0.25 : 1 has been identified as the preferred catalyst. Results indicate that PdZn

particles are more active than Pdo particles for the synthesis of methanol and less active for CH4

formation. A correlation between DME selectivity and concentration of acid sites has been established.

Hence, two types of sites are required for the direct conversion of syngas to DME: (1) PdZn particles

are active for the synthesis of methanol from syngas, and (2) acid sites which are active for the

conversion of methanol to DME. Additionally, CO2 formation was problematic as PdZn was found to

be active for the water-gas-shift (WGS) reaction, under all the conditions evaluated.

Introduction

Strained fossil fuel reserves and the increasing demand of

emergent countries make the use of renewable energies attrac-

tive. Sun, wind and biomass are promising sources of renew-

able energies. Several technologies exist for converting

biomass to energy such as heat, electricity or fuel. One

approach involves biomass gasification to produce syngas.

Syngas can be burned in gas turbines like natural gas, or it

can be converted to high quality chemicals and fuels, such as

methanol and dimethyl ether (DME). Methanol and DME are

key intermediates in the methanol-to-gasoline (MTG) and

methanol-to-olefins (MTO) processes.1,2

DME is often produced in a two-step process where methanol

is first generated from syngas followed by methanol dehydration

over a solid acid catalyst to produce DME. Alternatively, in a

single-step conversion, DME is produced directly from syngas

over a bifunctional or hybrid catalyst system employing both a

methanol synthesis function and a methanol dehydration

function.3,4 Producing DME directly from syngas has

many economic and technical advantages, provided suitable

catalyst(s) exist.5 In addition, thermodynamically, DME

production from syngas is favored over methanol. Direct

DME synthesis involves several competing reaction pathways.

Methanol synthesis (eqn (1) and (2)) and the water-gas-shift

(WGS, eqn (3)) are equilibrium reactions occurring over metal

or mixed metal catalysts:

CO + 2H2 2 CH3OH DH0 = �92.0 kJ mol�1

(1)

CO2 + 3H2 2 CH3OH + H2O DH0 = �49.5 kJ mol�1

(2)

CO + H2O 2 CO2 + H2 (WGS) DH0 = �41.1 kJ mol�1

(3)

a Institute for Integrated Catalysis, Pacific Northwest NationalLaboratory, Richland, WA 99352, USA.E-mail: [email protected], [email protected]

bMicroproducts Breakthrough Institute, Pacific Northwest NationalLaboratory, Corvallis, Or 97330, USA

CatalysisScience & Technology

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This journal is c The Royal Society of Chemistry 2012 Catal. Sci. Technol., 2012, 2, 2116–2127 2117

Dehydration of methanol to DME (eqn (4)) is also equilibrium

controlled and occurs over solid acid catalysts. By producing

DME in the same step, methanol is continuously consumed in

the reactor, pushing the conversion of syngas beyond that

which would be achieved with methanol synthesis alone.

2CH3OH 2 CH3OCH3 + H2O (dehydration)

DH0 = �23.6 kJ mol�1 (4)

Methane forming reactions could occur from the hydrogeno-

lysis of DME (eqn (5)), hydrogenolysis of methanol (eqn (6))

or from methanation reactions (eqn (7) and (8)) over metal

active sites. When operating at temperatures lower than

approximately 280 1C, as is typically the case for direct

DME synthesis, these reactions are not usually observed to

any great extent.5

CH3OCH3 + 2H2 - 2CH4 + H2O (hydrogenolysis)

DH0 = �207.5 kJ mol�1 (5)

CH3OH + H2 - CH4 + H2O (hydrogenolysis)

DH0 = �115.4 kJ mol�1 (6)

CO + 3H2 2 CH4 + H2O (CO methanation)

DH0 = �206 kJ mol�1 (7)

CO2 + 4H2 2 CH4 + 2H2O (CO2 methanation)

DH0 = �165 kJ mol�1 (8)

Combined methanol/DME synthesis typically necessitates the

use of a mixed catalyst system. Traditionally utilized are

CuZnAl-type methanol synthesis catalysts and acidic zeolite

or alumina to facilitate methanol dehydration.6,7 The system

operates at high pressure (5–10 MPa) and relatively low

temperature (200–280 1C). The well known Topsoe Integrated

Gasoline Synthesis (TIGAS) process has demonstrated

successful integration of methanol and DME synthesis.8 The

TIGAS process consists of the methanol/DME production in

one step and then in a second step methanol/DME is con-

verted to gasoline using the zeolite-catalyzed MTG process.8

Two separate synthesis reactors are used because the optimal

temperatures are different for the two operations. Methanol

synthesis favors temperatures around 250 1C, using con-

ventional Cu methanol catalysts, whereas MTG proceeds at

350–400 1C.2

Being a capital intensive process, combining methanol/

DME synthesis and gasoline production into a one synthesis

reactor operation would be economically attractive. Several

groups have attempted this integration using conventional

methanol synthesis and zeolite materials.9,10 However, several

technical hurdles exist. For one, there is a mismatch in

pressure and temperature when combining conventional

methanol synthesis and MTG. The high pressure (5–10 MPa)

operating conditions required to synthesize methanol/DME

will affect the final product distribution of the gasoline. The

conversion of DME to gasoline requires relatively high

temperature (350–400 1C),2 operating conditions at which

methanol/DME synthesis by itself is thermodynamically

unfavorable. Furthermore, the conventional Cu/ZnO/Al2O3

catalyst used for methanol synthesis suffers from instability at

temperatures greater than approximately 270 1C.11 Hence, a

catalyst both active and stable for the synthesis of methanol at

high temperature is lacking for the direct conversion of syngas

to gasoline.

In recent years a Pd/ZnO-type catalyst has been developed

for the methanol steam reforming reaction (MSR). Pd/ZnO

has been shown to be quite active and especially selective to

direct production of H2 and CO2.12,13 Beyond methanol

steam reforming, the PdZn-type catalyst was also found to

be very active for both water gas shift14 and also for methanol

synthesis,15 the latter having operating as high as 350 1C.

Pd/ZnO catalysts have been shown to be significantly more

stable for methanol steam reforming at higher temperatures

(e.g. 4280 1C) as compared to the conventional CuZnAl-type

methanol catalyst.16 Hence, the more stable PdZn-type

catalyst is a potential candidate for use in the direct synthesis

of gasoline from syngas.

The present study investigated a Pd/Zn/Al2O3 bifunctional

catalyst for the direct conversion of syngas to methanol and

DME over a wide temperature range, 250–380 1C. The higher

temperatures incorporate a regime that is not optimal for the

methanol synthesis reaction since this reaction is thermo-

dynamically limited. However, in a single step gasoline

synthesis process equilibrium constraint to hydrocarbon

product is alleviated, as described in more detail elsewhere

(e.g. integrating Pd/ZnO/Al2O3 with zeolite).17,18 The, objec-

tives here are to (1) demonstrate that the Pd/ZnO/Al2O3

catalyst is more stable than the Cu/ZnO/Al2O3 catalyst for

methanol synthesis at high temperature (375 1C), and (2) to

study the effect of the Pd loading and Pd : Zn molar ratio on

catalyst performance in order to optimize catalyst formulation

and understand what parameters affect the selectivity.

Operating temperature, pressure, gas hour space velocity

(GHSV), and syngas ratio (H2/CO) were process variables

explored. Catalytic compositions were altered by varying Pd

and Zn loadings on the alumina substrate, affecting catalytic

activity and selectivity. The relationship of the bifunctional

PdZn metal and acid sites and their impact on catalytic

performance were investigated. A combination of synthesis

experimentation and material characterization has led to

insights regarding the reaction pathways.

Experimental section

1. Catalyst preparation

Two series of Pd/ZnO/Al2O3 catalysts were prepared by

incipient wetness impregnation of an Al2O3 support (Engelhard,

AL-3945E) with a Pd nitrate solution (21.21 wt% Pd in nitric

acid) to which a Zn nitrate precursor (Sigma Aldrich) was

added, as reported in a previous study.19 The catalysts were

dried at 110 1C for 8 h and calcined at 350 1C for 3 h. For the

first series, the Pd loading was equal to 8.8 wt% and the

Pd : Zn molar ratio varied from 0.25 : 1 to 0.75 : 1. For the

second series, the Pd loading varied from 2.5 to 20 wt% and

the Pd : Zn molar ratio was kept constant and equal to 0.25 : 1.

A Pd/Al2O3 catalyst with 8.8 wt% Pd was prepared according

to the same method with no addition of Zn nitrate precursor

to the Pd solution. The Pd/ZnO/Al2O3 catalysts were labeled

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xPd/ZnO/Al2O3 � y were x stands for the Pd loading and y

stands for the Pd/Zn molar ratio. For example, 8.8Pd/ZnO/

Al2O3�0.25 indicates a Pd loading of 8.8 wt% and a

Pd : Zn content of 0.25 : 1 (molar). For comparison purpose

a commercial Cu/ZnO/Al2O3 catalyst (Synetix, F51-8 PPT)

was tested under the same conditions as the supported Pd

catalysts. Note that the term ‘‘spent’’ refers to the catalyst

after reaction.

2. BET surface area

Nitrogen adsorption was measured at 77 K with an automatic

adsorptiometer (Micromeritics ASAP 2000). The samples were

pretreated at 150 1C for 12 h under vacuum. The surface areas

were determined from adsorption values for five relative

pressures (P/P0) ranging from 0.05 to 0.2 using the BET

method. The pore volumes were determined from the total

amount of N2 adsorbed between P/P0 = 0.05 and P/P0 =

0.98. Prior to BET measurements the catalysts have been

reduced under 10% H2/N2 at 400 1C for 2 h.

3. X-ray Diffraction (XRD)

XRD analysis of the spent catalysts (i.e. after methanol

synthesis reaction conditions) was conducted using a Philips

X’pert MPD (Model PW3040/00) diffractometer with a copper

anode (Ka1 = 0.15405 nm) and a scanning rate of 0.011

per second between 2y = 101–701. The diffraction patterns

were analyzed using Jade 5 (Materials Data Inc., Livermore,

CA) and the Powder Diffraction File database (International

Center for Diffraction Data, Newtown Square, PA). Particle

sizes of the samples were determined from the XRD patterns

using the Debye-Sherrer relation (d = 0.89l/ Bcos y, where lis the wavelength of Cu-Ka radiation, B is the calibrated half-

width of the peak in radians, and y is the diffraction angle of a

crystal face). The metal dispersion was estimated from the

particle size by assuming hemispherical geometry using the

equationD=1/d (D=dispersion and d=metal particle size).20

4. Scanning transmission electron microscopy (S/TEM)

Scanning Transmission Electron Microscopy (S/TEM) was

performed with FEI Titan 80–300 operated at 300 kV. The

FEI Titan is equipped with CEOS GmbH double-hexapole

aberration corrector for the probe-forming lens, which allows

imaging with B0.1 nm resolution in scanning transmission

electron microscopy (STEM) mode. The STEM images were

acquired on High Angle Annular Dark Field (HAADF) with

inner collection angle of 52 mrad. In general, the TEM sample

preparation involved mounting of powder samples on copper

grids covered with lacey carbon support films and immediate

loading them into the TEM airlock to minimize an exposure to

atmospheric O2. Note that the samples were analyzed by

S/TEM after ex situ reduction under 10% H2/N2 at 400 1C

for 2 h.

5. Infrared spectroscopy

IR spectra were recorded with a Bruker spectrometer,

equipped with a MCT detector (resolution: 4 cm�1, 256 scans).

The samples pressed into a pellet were first pretreated under

H2 for 2 h at 400 1C. During this pretreatment, the sample was

alternatively exposed to H2 for 30 min and evacuated under

vacuum for 15 min to simulate flow conditions. After that, the

temperature was cooled down to room temperature and small

doses of CO were progressively added until saturation of the

catalyst surface occurred.

6. NH3 temperature programmed desorption (TPD)

NH3-TPD experiments were performed on an automated

catalyst characterization unit (Micromeritics Autochem

2910) equipped with a TCD detector. The catalyst (0.1 g)

was loaded in a U-type quartz tube. Then a 10% H2/Ar

mixture was passed through the sample starting from 20 1C

and heating up to 400 1C with a ramp of 5 1C min�1 and held

at this temperature for 2 h under 10% H2/Ar mixture and one

more hour under He. The temperature was cooled down to 100 1C

under He flow and the adsorption of NH3 (16% NH3/He) was

carried out at 100 1C for 2 h. After that, He flowed for 2 h at

the same temperature, to remove the physisorbed NH3 from

the surface of the catalyst. The catalyst was then heated to

650 1C (ramp 5 1C min�1) and held at this temperature for 1 h.

7. Catalytic activity

Catalytic activity tests were conducted in a 7.8 mm inner

diameter fixed-bed reactor. The catalyst (0.6 g), diluted with

SiC (3 g), was loaded between two layers of quartz wool inside

the reactor. A dual K-type thermocouple was placed in the

reactor for the measurement of inlet and catalyst bed

temperatures. The catalyst was reduced at 400 1C for 2 h,

using 10% H2/N2 gas mixture, prior to the test. A premixed

gas containing H2, CO, CO2 and N2 was fed into the system

using a Brooks Mass Flow Controller (5850E series). Four

different premixed gas compositions with syngas ratios H2/CO=

1, 2 or 3 (mol) were used and are listed in Table 1. The

catalysts were tested at temperatures between 250–380 1C over

a range of gas hour space velocity (5000–20 000 h�1) and

pressures (34.5–69 bar). The gas products were separated using

MS-5A and PPU columns and analyzed on-line by means of

an Agilent Micro GC equipped with a TCD detector. The

catalysts activities were compared using the following definitions:

CO conversion ð%Þ ¼ moles of CO reacted

moles of CO fed� 100

Selectivity ð%Þ ¼ moles Pi � uiP

i

moles of Pi � ui� 100

where Pi is a certain product and u is the number of carbon

atoms/molecule in Pi. e.g. if P = CO2, uCO2= 1, while for P=

CH3OCH3, uCH3OCH3= 2. Chemcad (version 5.6) was used to

estimate the equilibrium CO conversion.

Table 1 Premix gas compositions

H2/CO H2 (%) CO (%) CO2(%) N2 (%)

Premix 1 2 59.73 32.18 4.15 3.94Premix 2 1 41.50 41.44 12.97 4.09Premix 3 2 58.03 28.98 8.89 4.10Premix 4 3 66.11 21.94 7.75 4.20

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Results and discussion

1. Catalyst compositions and textural properties

Compositional information and the surface area and pore

volume of the catalysts are shown in Table 2. For a given

Pd loading, the surface area and pore volume increase with an

increase of Pd : Zn ratio (i.e. decrease in the ZnO content).

This could be due to a blocking of the pores of the Al2O3

support by ZnO and by the PdZn particles. The PdZn particle

size decreases with the Pd : Zn ratio (see Table 3). Similarly,

the decrease of the surface area and pore volume with the

increase of the Pd loading from 2.5 to 20% is likely due to the

increase of the PdZn particle size and ZnO content.

2. X-ray diffraction

For the 8.8Pd/ZnO/Al2O3�0.38 catalyst exposed to methanol

synthesis reaction conditions, the results have shown that the

PdZn particles size increases during the first 12 h on stream

(from 4.0 nm to 7.5 nm) but does not significantly increase for

TOS 4 12 h. We have thus examined the spent catalysts by

XRD to determine the PdZn particle size. Fig. 1(a) shows the

XRD patterns for the spent Pd/ZnO/Al2O3 catalysts with

different Pd loading and same Pd : Zn molar ratio, measured

at 2y= 401 to 481. For the samples with a Pd loading4 2.5%,

peaks characteristic of bimetallic PdZn at 41.21 and 44.11 are

observed. When the Pd loading increases these peaks become

more intense and their bandwidths decrease indicating an

increase of the PdZn particle size (see Table 3). For the

2.5Pd/ZnO/Al2O3�0.25 catalyst, one broad peak is detected

between 2y = 40.51 and 42.51. It is likely that this peak is

characteristic of Al2O3. However, the presence of small PdZn

particles that could contribute to this broad peak is not ruled out.

None of the XRD patterns shows peaks characteristic of Pdo

(expected at 40.21), suggesting that the samples present only

bimetallic PdZn particles. Fig. 1(b) displays the XRD patterns

for the Pd/ZnO/Al2O3 catalysts with 8.8% Pd and different

Pd :Zn molar ratios. The XRD pattern obtained for the 8.8Pd/

Al2O3 catalyst is presented in Fig. 1(b) as well. The XRD

patterns for the Pd/ZnO/Al2O3 catalysts show only peaks char-

acteristic of PdZn, again suggesting the absence of metallic Pdo

particles. Note that the bandwidth of the PdZn peak at 41.21

increases with the Pd :Zn ratio, indicating a decrease in the

bimetallic PdZn particle size. The PdZn particle size and

dispersion calculated from these XRD measurements are pre-

sented in Table 3. The dispersion increases with the increase in

Pd :Zn molar ratio and decreases with an increase in the Pd

loading.

3. Scanning transmission electron microscopy (STEM)

The Pd/ZnO/Al2O3 catalysts were analyzed using STEM.

Fig. 2a shows bimetallic PdZn particles supported by ZnO/

g-Al2O3 for the 2.5Pd/ZnO/Al2O3�0.25 catalyst. Detailed

high-resolution imaging in Fig. 2b confirms that the particles

are PdZn bimetallic and have an ordered tetragonal structure

with L10 type ordering. Nevertheless, some of the PdZn

particles, such as the one shown in Fig. 2b, exhibit a contrast

variation at the nanoscale indicating some compositional or

structural inhomogeneities. The observations of bimetallic

PdZn particles with tetragonal L10 type ordering are common

for all of the analyzed Pd/ZnO/Al2O3 catalysts.

4. Infrared spectroscopy

Fig. 3(a) shows the infrared spectra recorded between

1800–2125 cm�1, after saturation of the catalyst surface with

CO at room temperature, for the Pd/ZnO/Al2O3�0.25 cata-

lysts with different Pd loadings. For all the catalysts, the IR

spectra present one main band between 2069–2077 cm�1

ascribed to the vibration of CO linearly adsorbed on the PdZn

alloy particles.21,22 The shift observed between the spectra for

the different catalysts for the band at 2069–2077 cm�1 is not

understood yet. In addition, a closer look at the spectrum

obtained for the 2.5Pd/ZnO/Al2O3�0.25 catalyst shows one

broad band between 1800–2000 cm�1 due to multi-bonded

CO species21 and characteristic of Pd1 particles.22 Note that

the band at 1800–2000 cm�1 is hardly detectable for the

2.5Pd/ZnO/Al2O3�0.25 catalyst, suggesting that the amount

of Pd1 is low compared to the amount of bimetallic PdZn

particles. Note that the presence of a band due to linearly CO

species adsorbed on Pd1 for the catalysts with Pd 4 2.5% can

be ruled out since for Pd1/ZnO/Al2O3 catalyst band due to

linearly adsorbed CO between 2100–2000 cm�1 is accompa-

nied by a more intense band between 2000–1800 cm�1.22

Fig. 3(b) shows the infrared spectra recorded between

1800–2125 cm�1, for the 8.8Pd/ZnO/Al2O3 catalysts with

different Pd : Zn molar ratios. All the spectra present one band

at 2069–2077 cm�1 characteristic of PdZn alloy. The spectrum

recorded for the 8.8Pd/ZnO/Al2O3�0.75 catalyst also shows

Table 2 Catalysts composition and textural properties

CatalystPd loading(wt%)

Pd : Zn(molar)

Surface area(m2 g�1)

Pore volume(cm3 g�1)

8.8Pd/ZnO/Al2O3�0.25

8.8 0.25 : 1 171.8 0.37


8.8 0.38 : 1 183.4 0.43


8.8 0.75 : 1 197.2 0.46

8.8Pd/Al2O3 8.8 1 : 0 229.2 0.552.5Pd/ZnO/Al2O3�0.25

2.5 0.25 : 1 213.6 0.59

5Pd/ZnO/Al2O3�0.25

5.0 0.25 : 1 192.8 0.5

20Pd/ZnO/Al2O3�0.25

20 0.25 : 1 81.05 0.12

Table 3 Particles size of the Pd/ZnO/Al2O3 catalysts determined byXRD and dispersion measurements

Catalyst PdZn Particle size (nm) Dispersiona (%)

8.8Pd/ZnO/Al2O3�0.25 13.8 7.28.8Pd/ZnO/Al2O3�0.38 8.7 11.58.8Pd/ZnO/Al2O3�0.75 7.3 13.72.5Pd/ZnO/Al2O3�0.25 o4b NA5Pd/ZnO/Al2O3�0.25 8.6 11.620Pd/ZnO/Al2O3�0.25 14.8 6.8

a dispersion calculated from PdZn particle size using the equation

D = 1/d with D = dispersion and d = PdZn particle size. b Peaks

characteristic of bimetallic PdZn particles were not detected by XRD

indicating that the particle size is below the XRD detection limit

(i.e. o 4 nm).

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one more band between 1800–2000 cm�1 which is attributed to

Pd1. These results suggest that the amount of Pd1 increases

with the Pd : Zn molar ratio. Note that contrary to the IR

measurements, the XRD patterns did not indicate the presence

of Pd1 for the 8.8Pd/ZnO/Al2O3�0.75. It can be due to the

fact that the Pd1 particles are too small to be detected by XRD

or to the fact that IR spectroscopy is sensitive to the surface

composition of the catalyst, whereas XRD technique informs

on the structure/bulk composition of the catalyst.

5. Catalytic activity

5.1 Thermodynamics of methanol synthesis and dehydration

reactions. Fig. 4(a) presents the equilibrium CO conversion

for the synthesis of methanol at T = 225–400 1C and P =

34.5–69 bar and H2/CO/CO2 of 2/1/0.13 (premix 1 in Table 1).

For these calculations two cases are considered: (1) methanol

was the only product, and (2) both methanol and DME as

products. For the first case, equilibrium CO conversion

decreases with increasing temperature from 75% at 225 1C,

to 0% at 400 1C. For the second case, with both methanol and

DME as products, the same trend of decreasing conversion

with increasing temperature is seen, but overall conversions

are higher. This demonstrates the benefit in thermodynamic

driving force when both methanol synthesis and methanol

dehydration are employed in tandem. Fig. 4(a) also shows how

equilibrium CO conversion increases with pressure. For

example, at 375 1C, CO conversion increases from 10% at

P = 34.5 bar, to 38% at P = 69 bar.

Fig. 4(b) shows the equilibrium CO conversion and selectiv-

ities to methanol, DME, and CO2 at T= 225–400 1C and P=

69 bar, when considering both methanol and DME as products.

Equilibrium selectivity to DME decreases from 64% at 225 1C,

Fig. 1 XRD patterns for the spent Pd/ZnO/Al2O3 catalysts with different Pd loading and same Pd : Zn molar ratio (a) and for the catalysts with

8.8% Pd and different Pd : Zn molar ratio (b).

Fig. 2 (a) General view of the supported PdZn particles for the spent

2.5% Pd/ZnO/Al2O3 0.25 : 1 catalyst. (b) High resolution HAADF image

revealing the crystallographic nature of the PdZn intermetallic particles.

Fig. 3 Infrared spectra recorded after CO adsorption at room temperature, after saturation of the surface by CO for the Pd/ZnO/Al2O3 catalysts

with different Pd loading and Pd : Zn molar ratio.

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to 40% at 400 1C. On the other hand, equilibrium selectivity to

CO2 increases from 31% at 225 1C, to 54% at 400 1C.

Equilibrium methanol selectivity remains quite level across

the entire temperature range at approximately 5%.

5.2 Comparison of catalytic activity for commercial Cu/

ZnO/Al2O3 and 8.8Pd/ZnO/Al2O3 catalysts for methanol and

DME synthesis from syngas. Industrially, the Cu/ZnO/Al2O3

catalyst is used for the synthesis of methanol from syngas.11

We have compared a commercial Cu/ZnO/Al2O3 catalyst to

the 8.8Pd/ZnO/Al2O3�0.38 catalyst for the synthesis of

methanol, focusing on catalyst stability at relatively high

pressure (i.e. 69 bar) and temperature (i.e. 375 1C). These

reaction conditions are suitable for the direct conversion of

syngas17 to gasoline and the temperature is significantly higher

than that employed in conventional methanol synthesis. Fig. 5

presents CO conversion versus time for a period of 125 h on

stream for the two catalysts. Note that the stability test was

conducted at higher GHSV (i.e. 8340 h�1) for the 8.8Pd/ZnO/

Al2O3�0.38 catalyst to ensure the CO conversion to be below

the equilibrium CO conversion. It is clear that the Cu/ZnO/

Al2O3 suffers from rapid deactivation under these conditions.

This deactivation is not surprising and is due to sintering of

the Cu particles.23 This is in stark contrast with the trend

observed for the 8.8Pd/ZnO/Al2O3�0.38 catalyst. CO conver-

sion is quite stable with time-on-stream for the supported

PdZn catalyst. Catalytic activity as a function of temperature

is shown in Fig. 6(a) for both catalysts. Note that for each

catalyst, the temperature was increased progressively from

250 1C to 380 1C. The activity was measured after a 12 h

plateau at 250 1C and a 3–4 h plateau at 310 1C, 330 1C, 355 1C

and 380 1C. For the 8.8Pd/ZnO/Al2O3�0.38 catalyst, CO

conversion increases with temperature until 360 1C, beyond

which it decreases due to equilibrium constraints. At tempera-

ture Z 330 1C the CO conversion is greater for the 8.8Pd/

ZnO/Al2O3�0.38 catalyst than for Cu/ZnO/Al2O3. For

both catalysts, CO2, C2H6, CH4, methanol and DME were

produced during reaction. For Cu/ZnO/Al2O3, the CO con-

version is relatively flat, as compared to the 8.8Pd/ZnO/

Al2O3�0.38 catalyst. This is due to a progressive deactivation

occurring during data collection. The activity was measured

over a 24 h period during which the catalyst deactivates as

evidenced from Fig. 5.

Fig. 6(b) shows the evolution of the methanol, DME, and

CH4 selectivities as a function of temperature. The remaining

selectivity (not shown) is for CO2. Note that for simplification

and because both are the desired products, methanol

and DME are lumped together and shown in the graph as

‘‘methanol + DME’’. The compositional breakdown between

methanol and DME is described below and is also shown in

Table 4. As shown in Fig. 6(b) selectivity to both methanol

and DME decreases dramatically with increasing temperature.

For the Cu/ZnO/Al2O3 catalyst, methanol and DME selecti-

vity decreases from 75% at 240 1C, to 4.3% at 380 1C. For the

8.8Pd/ZnO/Al2O3�0.38 catalyst, the selectivity to methanol

Fig. 4 (a) Evolution of the equilibrium CO conversion as a function of the temperature for methanol synthesis considering the formation of

CH3OH and DME as products for P = 69 bar, P = 51.8 bar, P = 34.5 bar, and considering only the formation of CH3OH for P = 69 bar,

H2/CO/CO2 = 2/1/0.13 (premix 1, Table 1). (b) Evolution of the CO conversion, CO2 selectivity, DME selectivity and CH3OH selectivity at

equilibrium as a function of the temperature for P = 69 bar and H2/CO = 2, considering the formation of CH3OH and DME as products.

Fig. 5 Evolution of the CO conversion with time on stream for the

Cu/ZnO/Al2O3 and the 8.8P/ZnO/Al2O3�0.38 catalysts. Reaction

T = 375 1C, P = 69 bar, H2/CO = 2 (premix 3), GHSV = 3500 h�1

for Cu/ZnO/Al2O3 and GHSV = 8340 h�1 for 8.8Pd/ZnO/Al2O3�0.38.

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and DME decreases from 36% at 240 1C, to 26% at 380 1C,

but is significantly higher than for the Cu/ZnO/Al2O3 catalyst

at 380 1C. At 380 1C, the selectivity to methanol is 1.8% and

4.5% for the Cu/ZnO/Al2O3 and 8.8Pd/ZnO/Al2O3�0.38catalysts, respectively, whereas the DME selectivity is 2.5

and 21.5%, respectively. This represents a seven-fold selectiv-

ity advantage for the 8.8Pd/ZnO/Al2O3�0.38 catalyst at

380 1C. For both catalysts, selectivity to undesirable CH4

increases with temperature and reaches 32% and 21.4% at

380 1C for the Cu/ZnO/Al2O3 and 8.8Pd/ZnO/Al2O3�0.38catalysts, respectively. CH4 production is an undesired bypro-

duct and could also lead to the formation of coke. At

temperatures above 350 1C, the Cu/ZnO/Al2O3 catalyst

produces a substantially greater amount of CH4 and lesser

amount of methanol and DME compared to the 8.8Pd/ZnO/

Al2O3�0.38 catalyst.

These results clearly show the 8.8Pd/ZnO/Al2O3�0.38catalyst to be preferred over the Cu/ZnO/Al2O3 catalyst for

methanol and DME synthesis from syngas at temperatures

above 350 1C. In contrast to the Cu/ZnO/Al2O3 catalyst, the

8.8Pd/ZnO/Al2O3�0.38 catalyst does not suffer from deacti-

vation at these relatively high temperatures. In addition,

higher selectivity to desirable DME and methanol, and lower

selectivity to undesired CH4 is observed for the 8.8Pd/ZnO/

Al2O3�0.38 catalyst. Optimizing the formulation of the

Pd/ZnO/Al2O3 catalyst in order to potentially suppress the

production of CH4 while facilitating methanol and DME

formation appears thus very relevant.

5.3 Comparison of catalytic activity for the commercial Cu/

ZnO/Al2O3 catalysts, 8.8Pd/ZnO/Al2O3 catalyst and Al2O3

support for methanol dehydration to DME. As discussed above

for both the Cu/ZnO/Al2O3 and 8.8Pd/ZnO/Al2O3�0.38catalysts, formation of DME was observed. It is well known

that dehydration of methanol proceeds over acidic sites offered

by solid acid catalysts such as alumina and zeolites.24 Thus,

the acid sites of the Al2O3 support likely promote DME

formation via methanol dehydration once methanol is formed

from the syngas. To investigate this further, we compared

catalytic activity for the methanol-to-DME dehydration

reaction (eqn (4)) at 250–425 1C and at 1 bar. Activities of

Cu/ZnO/Al2O3, 8.8Pd/ZnO/Al2O3�0.38, and Al2O3 alone

(the support used for the PdZn catalysts) were compared.

Fig. 7 presents methanol conversion versus temperature for the

three catalysts. Conversion increases dramatically with

increasing temperature for both Cu/ZnO/Al2O3 and 8.8Pd/

ZnO/Al2O3�0.38 and complete methanol conversion was

achieved at approximately 350 1C for both. For the Al2O3

support, conversion increases just slightly with temperature,

from 29% to 35%. The significant increase of the conversion

with the temperature for the Cu/ZnO/Al2O3 and 8.8Pd/ZnO/

Al2O3�0.38 catalysts, compared to the Al2O3 alone, is due to a

considerable increase of their activity for methanol decom-

position. Fig. 8(a) and (b) present selectivities for the different

products (DME, CH4, CO and CO2) as a function of the

temperature. For the Al2O3 support, as expected, DME is the

only product observed up to 400 1C. At 425 1C, CH4 is produced

Fig. 6 Evolution of (a) the CO conversion and (b) the methanol, DME and CH4 selectivity as a function of the temperature for the Cu/ZnO/

Al2O3 (Cu) and the 8.8Pd/ZnO/Al2O3�0.38 catalysts. opened symbols: 8.8Pd/ZnO/Al2O3�0.38 (PdZn), filled symbols: Cu/ZnO/Al2O3. P= 69 bar,

GHSV = 10 000 h�1 and H2/CO = 2 (premix 1).

Table 4 Effect of the temperature, pressure and GHSV, on the conversion and selectivity for the 8.8Pd/ZnO/Al2O3�0.38 catalyst

Temperature (1) Pressure (bar) GHSV (h�1) CO conversion (%)

Selectivity (%)

CO2 CH4 C2H6 Methanol DME

307 69 10 000 22.9 54.2 7.6 2.3 8.8 27.1332 69 10 000 41.9 53.5 12.9 3.9 8.4 21.1352 69 10 000 49.7 51.6 15.8 4.4 6.0 22.0380 69 10 000 44.2 52 17.5 4.5 4.5 21.4380 34.5 10 000 20.6 66.1 16.4 2.2 3.3 12380 51.8 10 000 39 57.4 19.6 4.2 4.1 14.7380 69 5000 63.7 57.7 28.4 8.0 2.1 3.8380 69 18 000 40.3 52.8 15.1 3.2 5.2 23.7

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in addition to DME. Also at this temperature a small amount

of H2 and CO2 was detected. It is thus possible that CH4 was

produced, at least in part, from DME hydrogenolysis or CO2

methanation. Note that H2 and CO2 were likely produced

from methanol steam reforming rather than methanol decom-

position since no CO was detected. This is in contrast with the

results obtained for the Cu/ZnO/Al2O3 catalyst where CH4 is

produced along with CO and CO2, with little DME formation.

It appears that methanol is primarily decomposed to CO and

H2 (the reverse of eqn (1)). The small amount of CO2 formed is

likely due to the water-gas-shift reaction (eqn (3)). The CH4 is

produced from CO (and/or CO2) methanation, DME decom-

position or DME hydrogenolysis. Interestingly, for the

Pd/ZnO/Al2O3�0.38 catalyst, formation of CH4 is not

observed over the entire range investigated. However, DME

formation is observed, with an optimum temperature at

approximately 310 1C (20% selectivity) and decreases with

increasing temperature. Like the Cu/ZnO/Al2O3 catalyst, the

majority product formed over the entire temperature range is

CO, as a result of methanol decomposition.

These results indicate that the Pd/ZnO/Al2O3�0.38 catalyst

is more active for the dehydration of methanol to DME as

compared to commercial Cu/ZnO/Al2O3. This explains why

when syngas feed is used (see Fig. 6b), the selectivity to DME

is higher for the Pd/ZnO/Al2O3�0.38 catalyst than the

Cu/ZnO/Al2O3 catalyst. As seen in Fig. 8, the selectivity

toward DME is lower than the selectivity toward CO and

CO2 whatever the temperature (between 250–410 1C), for the

Pd/ZnO/Al2O3�0.38 catalyst. This is due to the fact that the

methanol dehydration reaction experiments were conducted

at atmospheric pressure. At high pressure (i.e. 69 bar), equili-

brium selectivity to methanol from syngas is favored via

methanol synthesis (eqn (2)). Thus, with increased methanol

synthesis, as opposed to methanol reforming, increased DME

production will result.

5.4 Effect of the temperature, pressure and gas hour space

velocity (GHSV) on the syngas conversion and selectivity for the

8.8Pd/ZnO/Al2O3�0.38 catalyst. The effects of the tempera-

ture, pressure, and GHSV on the reactivity for syngas

conversion to methanol and DME were examined for the

8.8Pd/ZnO/Al2O3�0.38 catalyst and the results are presented

in Table 4. The effect of temperature on conversion and

selectivity has already been discussed above. As expected,

CO conversion increases with pressure. Keeping the operating

temperature constant at 380 1C, CO conversion increases from

20.6% to 44.2% when increasing the pressure from 34.5 bar to

69.0 bar. Selectivity to methanol and DME does increase

somewhat with pressure. Methanol selectivity increases from

3.3% to 4.5% and DME increases from 12.0% to 21.4%. An

increase in methanol production is indeed predicted as the

forward methanol synthesis reaction rates (eqn (1) and (2)) are

favored with increasing pressure. CH4 and C2H6 production

did not increase substantially with pressure. When the pressure

increased from 34.5 bar to 69 bar the CH4 and C2H6

selectivities increased only slightly from 16.4% to 17.5% and

from 2.2% to 4.5%, respectively.

GHSV was varied to determine the time dependence of

conversion and selectivity, with the results shown in Table 4.

As expected, CO conversion decreases with increasing GHSV.

GHSV’s of 5000, 10 000, and 18 000 h�1 resulted in CO

conversions of 63.7%, 44.2%, and 40.3%, respectively.

Selectivities to CO2, CH4 and C2H6 increase with decreasing

GHSV, whereas selectivity toward DME and methanol

Fig. 7 Methanol-to-DME reaction. Evolution of the CH3OH con-

version with the reaction temperature for P=1 bar, GHSV= 5000 h�1

and. CH3OH = 36.1% in N2.

Fig. 8 Methanol-to-DME reaction. Evolution of the DME selectivity (a), CH4 selectivity (a), CO selectivity (b) and CO2 selectivity (b) as a

function of the temperature for the dehydration of methanol. P = 1 bar, GHSV = 5000 h�1, CH3OH = 36.1% in N2. Filled symbols represent

DME (a) and CO (b) selectivities. Opened symbols represent CH4 (a) and CO2 (b) selectivities.

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decrease. For example, at GHSVs of 5000 and 18 000 h�1

selectivity to DME was 3.8% and 23.7%, respectively. Hence,

shorter residence time favor the formation of DME relative to

side products. Methane formation reactions such as DME

hydrogenolysis (eqn (5)) become increasingly dominant at

longer residence time. The fact that increased throughputs

enhance selectivity to methanol and DME is an important

finding from a practical standpoint.

One can see from Table 4 that a considerable amount of

CO2 is produced during reaction. Indeed, whatever the

reaction temperature (300–380 1C), pressure (34–69 bar) and

GHSV (5000–18 000 h�1), the selectivity to CO2 is always

approximately 50%. Under the present reaction conditions,

the water-gas-shift activity is significant, limiting methanol

and DME formation.

5.5 Effect of the Pd :Zn molar ratio on the reactivity of the

Al2O3 supported catalysts with 8.8% Pd. Changes in Pd/Zn

composition and the resulting effect on catalytic activity were

also examined. Fig. 9(a) presents the evolution of the conver-

sion as a function of the Pd : Zn molar ratio at 380 1C and 69

bar. CO conversion increases from 36% to 44% when the

Pd : Zn molar ratio increases from 0.25 : 1 to 0.38 : 1. CO

conversion decreases for higher Pd : Zn molar ratios and is

equal to 26% for the 8.8Pd/Al2O3 catalyst with a Pd : Zn =

1 : 0. As evident from Fig. 9(a) there exists an optimum in

Pd : Zn ratio for CO conversion. The higher CO conversion

observed for the 8.8Pd/ZnO/Al2O3�0.38 catalyst, compared

to the 8.8Pd/ZnO/Al2O3�0.25 catalyst, is probably due to a

higher PdZn dispersion (see Table 3). From Fig. 9(a), it can be

seen that for Pd : Zn 4 0.25 : 1, the CO conversion decreases

with an increase in Pd : Zn ratio. The IR measurements have

shown an increase of the amount of Pdo with the increase of

the Pd : Zn ratio. Consequently, these results strongly suggest

Pdo particles to be less active than PdZn particles for the

synthesis of methanol.

The effect of Pd : Zn molar ratio on product selectivity is

displayed in Fig. 9(b). The CO2 selectivity is similar for all

Pd : Zn molar ratios investigated and is equal to B54%. One

can also see that the CH4 and DME selectivities follow

opposite trends. Indeed, the DME selectivity decreases from

24.5% to 10.5% with increasing Pd : Zn molar ratio. Consis-

tent with this, a significant increase of the CH4 selectivity,

from 13 to 33%, is observed with increasing Pd : Zn molar

ratio. As the Pd : Zn ratio increases from Pd : Zn = 0.38 to

1.0 more Pdo sites are present on the surface of the catalyst, as

shown by the IR results described above. Hence, these results

show Pdo to facilitate CH4 formation.

Methanol dehydration to DME is catalyzed by acid cata-

lysts. Al2O3 alone is active for the formation of DME from

methanol.25 However, Pd/ZnO is inactive for the dehydration

of methanol to DME.26,27 It is thus reasonable to assume that

for the Pd/ZnO/Al2O3 catalysts, the alumina support is the

source of acidity. NH3-TPD experiments were conducted to

determine the concentration of the acid sites for the

8.8Pd/ZnO/Al2O3 catalysts and the 8.8Pd/Al2O3 catalyst. The

NH3-TPD profiles (not shown) have indicated the presence of

one single peak located at 180 1C for all the Pd/ZnO/Al2O3

catalysts. Fig. 11(a) shows the evolution of the amount of NH3

desorbed as a function of the Pd : Zn ratio (for the catalyst

with 8.8% Pd loading). The amount of NH3 desorbed

decreases with increasing Pd : Zn ratio and is the lowest for

the 8.8Pd/Al2O3 sample (with Pd : Zn = 1 : 0). This signifies

that the concentration of acid sites decreases with increasing

Pd : Zn ratio. Since the dispersion increases with the Pd : Zn

ratio (see Table 3), the coverage of the Al2O3 support increases

and the number of accessible acid sites decreases. Note that

there is a correlation between the DME selectivity and the

amount of acid sites. It indicates that for the 8.8Pd/ZnO/Al2O3

catalysts, the acid sites of the Al2O3 support are active for the

dehydration of methanol to DME.

5.6 Effect of the Pd/Zn loading on the reactivity of the

Pd/ZnO/Al2O3 catalysts with Pd :Zn = 0.25 : 1. Catalytic

activity of several Pd/ZnO/Al2O3 catalysts with the same

Pd : Zn molar ratio (Pd : Zn = 0.25 : 1) and different Pd

loadings, varying from 2.5 to 20 wt%, were examined. The

results obtained at 380 1C and P = 69 bar are presented in

Fig. 10. CO conversion ranges from 41% to 47% for all the

Pd loadings tested. As shown in Fig. 10(b) the CO2 selec-

tivity is somewhat stable for all the Pd loadings tested and

methanol selectivity is less than 7% for all the catalysts.

Fig. 9 Evolution of (a) the conversion and (b) the selectivity with the Pd : Zn molar ratio for the supported 8.8% Pd catalysts. Reaction T =

380 1C, P = 69 bar, H2/CO = 2 (premix 1) and GHSV = 10 000 h�1.

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Interestingly, DME selectivity goes through a maximum of

28% at 5% Pd loading. This trend in DME selectivity is

opposite to the trend in CH4 selectivity. The CH4 selectivity

is at its lowest (11.2%) for 5Pd/ZnO/Al2O3�0.25. Contrary to

5Pd/ZnO/Al2O3�0.25, the IR spectra recorded for 2.5Pd/

ZnO/Al2O3�0.25 suggest the presence of Pd1. The higher

CH4 selectivity observed for 2.5Pd/ZnO/Al2O3�0.25, com-

pared to 5Pd/ZnO/Al2O3�0.25 is thus attributed to the

presence of Pd1. Since the CH4 selectivity increases with the

Pd loading for Pd Z 5% and no Pd1 was detected by IR

Fig. 10 Evolution of (a) the conversion and (b) the products selectivity as a function of the Pd loading for the Pd/ZnO/Al2O3 catalysts with a

Pd : Zn molar ratio equal to 0.25 : 1. Reaction T = 380 1C, P = 69 bar, H2/CO = 2 (premix 1) and GHSV = 10000 h�1.

Fig. 11 Evolution of the DME selectivity and the amount of NH3 desorbed from the catalysts surface as a function of (a) the Pd : Zn molar ratio

and (b) the Pd loading. Reaction T = 380 1C, P = 69 bar, GHSV = 10 000 h�1 and H2/CO = 2 (premix 1).

Fig. 12 Effect of the feed gas composition on (a) the conversion and (b) on the selectivity for the 5Pd/ZnO/Al2O3�0.25 catalyst. For H2/CO= 1

(premix 2), for H2/CO = 2 (premix 3) and for H2/CO = 3 (premix 4). Reaction T = 380 1C, P = 69 bar and GHSV = 10 000 h�1.

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spectroscopy for the higher loadings one can speculate that the

CH4 formation is facilitated on bigger PdZn particles.

Fig. 11(b) shows the NH3 TPD results as a function of Pd

loading. The amount of NH3 desorbed increases with the Pd

loading from 2.5 to 8.8% Pd and decreases for the highest

loading. One can see that the DME selectivity and the amount

of NH3 desorbed (i.e. amount of acid sites) from the catalyst

surface, follow the same trend with the increase of the Pd

loading. This further confirms the acid sites of the catalysts are

active for the production of DME.

From the analysis of the influence of the Pd loading and

Pd : Zn molar ratio on the reactivity of the Pd/ZnO/Al2O3

catalysts, we can conclude that under these reaction conditions

the sample with 5% Pd and a Pd : Zn molar ratio equal to

0.25 : 1 is preferred for the syngas conversion to methanol

and DME.

5.7 Effect of the feed syngas ratio on the catalytic activity of

the 5Pd/ZnO/Al2O3�0.25 catalyst. The effect of feed syngas

ratio on the catalytic performance of this composition is

examined on 5Pd/ZnO/Al2O3�0.25, identified above as the

most selective catalyst under these conditions. H2/CO molar

ratios of 1.0, 2.0, and 3.0 were evaluated and the results are

shown in Fig. 12. CO conversion increases whereas the CO2

selectivity decreases with the increase of the H2/CO ratio

[Fig. 12(a)]. DME selectivity increases from approximately

11% to 32% when increasing the H2/CO ratio from 1.0 to 3.0.

The methanol selectivity is approximately 4% at H2/CO = 1

then approaches zero at higher H2/CO ratios. Interestingly,

CH4 production also decreases with feed syngas ratio, from

approximately 6% to 2% for H2/CO= 1.0 and H2/CO= 3.0,

respectively. Methanol synthesis and subsequent methanol

dehydration is thus favored over CO methanation or DME

hydrogenolysis under these higher hydrogen-containing feeds.

Conclusion

In this work we have demonstrated the use of a Pd/ZnO/Al2O3

catalyst for the high-temperature production of methanol and

DME from syngas and contrasted its activity with that of a

commercial Cu/ZnO/Al2O3 catalyst. The PdZn-formulation

outperforms the Cu-based formulation at these conditions

which are directly relevant to the potential single-step syngas-

to-gasoline concept, which combines methanol/DME synth-

esis with MTG in a single reactor. By studying the influence of

the Pd loading and Pd : Zn molar ratio, a catalyst with 5% Pd

and a Pd : Zn molar ratio of 0.25 : 1 has been identified as the

best performing catalyst. Since Pdo promotes the formation of

methane over methanol it is critical that catalyst synthesis

avoid generation of Pdo. A direct relationship between DME

selectivity and concentration of acid sites was shown. Hence,

two types of sites are required for the direct conversion of

syngas to DME: (1) PdZn particles which are active sites for

the synthesis of methanol from syngas, and (2) acid sites which

are active for the conversion of methanol to DME. The results

of this study have shown that a non negligible amount of

undesired CH4 is produced during reaction at high tempera-

ture of operation. Also, under the conditions tested, the PdZn

particles were quite active for the water-gas-shift reaction,

leading to the formation of a large amount of CO2 at the

expense of DME formation. To consider Pd/ZnO/Al2O3 as a

catalyst for direct conversion of syngas to gasoline it will be

necessary to further investigate the parameters that could

favor the methanol synthesis reaction and further lower

methane formation.

Acknowledgements

The authors gratefully acknowledge financial support for this

work provided by the Energy Conversion Initiative, funded

internally by the Pacific Northwest National Laboratory

(PNNL). This work was also supported by the National

Advanced Biofuels Consortium (NABC) which is funded by

the Department of Energy’s Office of Biomass Program with

recovery act funds. PNNL funding was provided under

contract DE-AC05-76RL01830. Finally, the authors would

like to acknowledge that a portion of this work was done in

the Environmental Molecular Sciences Laboratory (EMSL),

a DOE sponsored user facility located at PNNL in

Richland, WA.

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synthesis of methanol and dimethyl ether from syngas

Documents

synthesis of methanol

dme synthesis

dme selectivity

methanol dehydrationover

theconversion of methanol

selectivityto dme

catalyst design

preferred catalyst