Bifunctional Catalysts for One-Step Conversion of Syngas into Aromatics with Excellent Selectivity and Stability

Article

Bifunctional Catalysts for One-Step

Conversion of Syngas into Aromatics with

Excellent Selectivity and Stability

Kang Cheng, Wei Zhou, Jincan

Kang, ..., Yang Pan, Wu Wen, Ye

Wang

wangye@xmu.edu.cn

HIGHLIGHTS

One-step conversion of syngas to

aromatics with striking selectivity

was achieved

2

Zn-ZrO /H-ZSM-5 catalyst

showed 80% aromatic selectivity

and 1,000 hr stability

Zn/Zr ratio, strong acidity, and

proximity of catalyst components

are key factors

Self-promotion effect of CO

contributes to selective formation

of aromatics

Wang and colleagues successfully designed a powerful bifunctional catalyst

composed of Zn-doped ZrO nanoparticles and zeolite H-ZSM-5 for one-step

2

conversion of syngas into aromatics. Aromatics with 80% selectivity were obtained

at 20% CO conversion. No catalyst deactivation was observed in 1,000 hr.

Methanol and dimethyl ether were formed as reaction intermediates on Zn-doped

2

ZrO , which were subsequently transformed into aromatics on H-ZSM-5 via oleﬁns.

This work offers a highly selective and stable non-petroleum route for the synthesis

of aromatics.

Cheng et al., Chem 3, 1–14

August 10, 2017 ª 2017 Elsevier Inc.

http://dx.doi.org/10.1016/j.chempr.2017.05.007

Please cite this article in press as: Cheng et al., Bifunctional Catalysts for One-Step Conversion of Syngas into Aromatics with Excellent Selectivity

and Stability, Chem (2017), http://dx.doi.org/10.1016/j.chempr.2017.05.007

Article

Bifunctional Catalysts for One-Step

Conversion of Syngas into Aromatics

with Excellent Selectivity and Stability

Kang Cheng,¹^,³Wei Zhou,¹^,³Jincan Kang,¹^,³Shun He, Shulin Shi, Qinghong Zhang, Yang Pan,

1

2

Wu Wen, and Ye Wang¹^,⁴^,*

2

SUMMARY

The Bigger Picture

Syngas (CO/H

2

) is a key platform for chemical utilization of non-petroleum car-

Syngas chemistry has become a

hot research area because of the

increasing interest in the

bon resources. Among syngas transformation routes, the direct synthesis of ar-

omatics, which are among the most important bulk chemicals, is less successful

because of the limited selectivity and poor catalyst stability. We report a suc-

utilization of non-petroleum

carbon resources, such as natural

gas or shale gas, coal, biomass,

2

cessful design of bifunctional catalysts composed of Zn-doped ZrO nanopar-

ticles dispersed on zeolite H-ZSM-5 for one-step conversion of syngas to aro-

matics with high selectivity and stability. Aromatics with 80% selectivity at CO

conversion of 20% were achieved, and there was no catalyst deactivation in

2

and CO for sustainable

production of clean fuels and

chemicals. Signiﬁcant progress

has recently been achieved in the

selective synthesis of gasoline,

diesel, and lower oleﬁns from

syngas by modiﬁcation of the

classic Fischer-Tropsch process.

However, no success has been

achieved in the direct conversion

of syngas into aromatics, key

building blocks for the production

of various polymers, with

1

,000 hr. Methanol and dimethyl ether were formed as major intermediates

on Zn-doped ZrO , which were subsequently converted into aromatics on

2

H-ZSM-5 via oleﬁns. We discovered a self-promotion mechanism of CO in the se-

lective formation of aromatics. As well as being a reactant, CO facilitates the

removal of hydrogen species formed on H-ZSM-5 in the dehydrogenative

aromatization of oleﬁns.

INTRODUCTION

Syngas (a mixture of hydrogen and carbon monoxide) chemistry has become

increasingly important because of the increasing interest in the utilization of non-pe-

troleum carbon resources to replace diminishing resources of crude oil for sustain-

attractive selectivity. Here, we

report a highly selective syngas-

methanol-aromatics (SMA)

1

–5

able production of liquid fuels and chemicals.

Syngas can be produced from

various carbon resources such as natural gas (including conventional and non-con-

ventional natural gas such as shale gas and biogas), coal, biomass, and even waste.

Carbon dioxide can also be a feedstock of syngas via dry reforming of methane or via

process, that is, a one-step

conversion of syngas to aromatics

via methanol by a reaction

6

,7

renewable electricity. From syngas, a variety of products such as hydrocarbons,

which can be fuels (gasoline, diesel fuel, and jet fuel) or chemicals (oleﬁns and

aromatics), and oxygenates (methanol and CR2 oxygenates) can be obtained

coupling method. We

demonstrate that the successful

design of bifunctional catalysts

not only enables process

(

Scheme 1). Fischer-Tropsch (FT) synthesis is a well-established process for the trans-

formation of syngas, but the products of FT synthesis are typically straight-chain hy-

8

–14

drocarbons following the Anderson-Schulz-Flory (ASF) distribution.

progress has recently been achieved in the development of new catalysts or new

Signiﬁcant

intensiﬁcation but also solves the

problems of limited selectivity and

catalyst deactivation that besets

the synthesis of aromatics.

1

5–18

1,2,4,14,19,20

strategies to increase the selectivity of liquid fuels

that breaks the ASF distribution.

and lower oleﬁns

Aromatics are among the most important bulk chemicals, constituting about one-

2

1

third of the market for commodity petrochemicals. For example, the global con-

sumptions of benzene and p-xylene were estimated at more than 40 and 28

million tons in 2010. These aromatics are used mainly for the production of

Chem 3, 1–14, August 10, 2017 ª 2017 Elsevier Inc.

1

Please cite this article in press as: Cheng et al., Bifunctional Catalysts for One-Step Conversion of Syngas into Aromatics with Excellent Selectivity

and Stability, Chem (2017), http://dx.doi.org/10.1016/j.chempr.2017.05.007

Scheme 1. Typical Reaction Processes Related to Syngas Chemistry

The red-line route demonstrates our SMA process for the direct synthesis of aromatics via reaction

coupling.

polymers such as polystyrene, phenolic resins, polycarbonate, nylon, polyure-

thane, and polyethylene terephthalate ﬁbers, resins, and ﬁlms. The world-wide

demand for benzene derivatives and polyester ﬁbers and resins is growing faster

than 6% each year. In the current chemical industry, the production of aromatics

heavily relies on petroleum. Approximately 70% of the global supply of benzene,

toluene, and xylenes (BTX) comes from the catalytic reforming of petroleum-

based naphtha, followed by pyrolysis of gasoline by-product from ethylene plants

(

ꢀ23%). Few studies have reported successful synthesis of aromatics from alterna-

22

tive carbon sources.

The direct conversion of syngas via conventional FT synthesis cannot result in

2

3

aromatics with a considerable selectivity (<15%). The modiﬁcation of conventional

Fe- or Co-based FT catalysts by addition of a zeolite could increase the fraction of

24–26

aromatics in products, but the aromatic selectivity was <40%.

Many studies

have been devoted to developing novel processes for the synthesis of aromatics

2

7

28

29,30

from alternative carbon feedstocks such as propane, methane, and methanol.

The methane and propane aromatization reactions suffer from low single-pass yields

and poor catalyst stability. The conversion of methanol to hydrocarbons, including

3

1–35

aromatics, has become a hot topic in recent years.

However, the selective forma-

tion of aromatics from methanol is still challenging. Although zeolite H-ZSM-5 cata-

lyzes the conversion of methanol to aromatics (MTA) because of its unique topol-

2

9–32

29,30

ogy,

it undergoes quick deactivation as a result of coke deposition.

Furthermore, compared with the two-step process, i.e., syngas to methanol (methanol

synthesis) and MTA, the direct conversion of syngas into aromatics would be more en-

ergy and cost efﬁcient. Here, we report a one-step conversion of syngas to aromatics

via methanol intermediate (syngas-methanol-aromatics [SMA] in Scheme 1) by a reac-

tion coupling method. The reaction coupling method has resulted in a signiﬁcant

¹State Key Laboratory of Physical Chemistry of

Solid Surfaces, Collaborative Innovation Center

of Chemistry for Energy Materials, National

Engineering Laboratory for Green Chemical

Productions of Alcohols, Ethers and Esters,

College of Chemistry and Chemical Engineering,

Xiamen University, Xiamen 361005, China

2

,19

breakthrough in the synthesis of lower oleﬁns.

We demonstrate the successful

design of bifunctional catalysts for the SMA process with excellent selectivity and ultra-

high stability. This work not only provides a promising route for the synthesis of aro-

matics but also opens a new avenue for syngas chemistry.

²National Synchrotron Radiation Laboratory,

University of Science and Technology of China,

Hefei 230029, China

RESULTS

³These authors contributed equally

⁴Lead Contact

Designing Active Components of the Bifunctional Catalyst

Using one catalyst to integrate methanol synthesis and the MTA reaction in tandem

is a challenging task. Typically, the MTA reaction proceeds at R673 K because of the

*Correspondence: wangye@xmu.edu.cn

http://dx.doi.org/10.1016/j.chempr.2017.05.007

3

1

kinetic requirement, but methanol synthesis is thermodynamically unfeasible at

2

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Please cite this article in press as: Cheng et al., Bifunctional Catalysts for One-Step Conversion of Syngas into Aromatics with Excellent Selectivity

and Stability, Chem (2017), http://dx.doi.org/10.1016/j.chempr.2017.05.007

Figure 1. Effect of Temperature on Catalytic Performance

(

A) Zn–ZrO

B) Zn–ZrO

2

for syngas conversion.

2

/ZSM-5 for syngas conversion.

Reaction conditions: catalyst weight, 0.10 g for Zn–ZrO

2

and 0.30 g for Zn–ZrO

/CO, 2:1; 3 MPa; 25 cm min . Selectivity was calculated on a molar carbon basis for CO

hydrogenation. Black arrows denote CO conversion. See Table S3 for detailed product

distributions. The formation of CO by the WGS reaction is also shown in Table S3.

2

/H-ZSM-5; 573–

3

À1

7

23 K; H

2

such high temperatures. Our analysis clariﬁes that the MTA reaction can thermody-

namically drive the syngas conversion at around 673 K according to the Le Chate-

lier’s principle (see Figure S1). Nevertheless, the challenge remains to ﬁnd a catalyst

capable of catalyzing the selective formation of methanol or dimethyl ether (DME) at

R673 K. We clariﬁed that although methanol/DME were formed with high selectivity

over a typical methanol synthesis catalyst such as Cu–Zn–Al or Cr–Zn–Al oxide at a

2 4

lower temperature (623 K), undesirable methane and lower (C –C ) parafﬁns

became the predominant products at higher temperatures (Table S1). The combina-

tion of Cu–Zn–Al or Cr–Zn–Al oxide with H-ZSM-5 caused the formation of a large

4 2 4

fraction of CH and C –C parafﬁns, although aromatics could also be formed with

low selectivity (Table S1). Some early studies on the conversion of syngas to gasoline

reported the formation of product mixtures containing a fraction of aromatics by

combining a supported metal catalyst or a mixed metal oxide such as Pd/SiO

2

or

Cr–Zn oxide with a zeolite, but undesirable CH and C –C parafﬁns were formed

4

2

4

3

6–38

with high selectivity (Table S2).

We found that a Zn-doped ZrO

:200) was unique in keeping methanol/DME in a wider temperature range (Fig-

ure 1A), although CO conversion was low. The major by-products were C –C oleﬁns

oleﬁns could also work

is a better candidate as the

active component in methanol synthesis to match the subsequent MTA active

component working at around 673 K. The integration of Zn–ZrO with H-ZSM-5

weight ratio of Zn–ZrO to H-ZSM-5, 1:2) increased CO conversion, in particular

at higher temperatures, because of the thermodynamic driving force (Figure S1). Ar-

omatics were formed as major products along with C –C hydrocarbons instead of

methanol/DME (Figure 1B). Upon increasing the reaction temperature, the selec-

tivity of C –C oleﬁns decreased and that of aromatics increased. The aromatic

selectivity reached ꢀ75% at 673 K, but a further increase in reaction temperature

led to a decrease in the selectivity of aromatics and an increase in that of C –C par-

afﬁns. It is noteworthy that the water-gas-shift (WGS) reaction (CO + H O / CO

) can also occur over our catalysts. The selectivity of CO , which was evaluated

separately from the hydrogenation of CO, depended on the temperature and the

2 2

(denoted as Zn–ZrO ) catalyst (Zn/Zr molar ratio,

1

2

4

=

0

(

C

2–4 ) other than CH

4 2 4 2 4

and C –C parafﬁns (C2–4 ), and C –C

as intermediates for aromatic formation. Thus, Zn–ZrO

2

(

2

4

2

4

2

4

2

+

H

2

values were 34% and 40% for the Zn–ZrO

respectively, at 673 K (Table S3).

2 2

and the Zn–ZrO /H-ZSM-5 catalysts,

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Figure 2. Effect of Contact Time on Catalytic Performance

(

A) Zn–ZrO

B) Zn–ZrO

2

for syngas conversion.

2

/H-ZSM-5 for syngas conversion.

C) Reaction scheme over the bifunctional catalyst.

3

À1

Reaction conditions: catalyst weight, 0.010–3.0 g; 673 K; H

selectivity was calculated on a molar carbon basis for CO hydrogenation. Black arrows denote CO

2

/CO, 2:1; 3 MPa; 25 cm min . The

conversion. See Table S4 for detailed product distributions. The formation of CO

reaction is also shown in Table S4.

2

by the WGS

To gain insights into the reaction paths and in-depth understanding of the roles of the

two components, we have compared the catalytic behaviors of Zn–ZrO and Zn–ZrO

H-ZSM-5 catalysts at 673 K as a function of contact time, which is expressed by the

amount of Zn–ZrO contained in the catalyst divided by the feed rate of reactant gas.

2

/

2

Figures 2A and 2B show that CO conversions increase almost linearly with contact

time, indicating that the reaction proceeds steadily over both catalysts. The detailed

product distributions and the formation of CO

times are displayed in Table S4. Over Zn–ZrO

products at shorter contact times, and prolonging the contact time decreased the

selectivity of CH OH/DME and increased that of C –C oleﬁns, CH , and C –C parafﬁns

–C oleﬁns became major products at shorter contact times over the

/H-ZSM-5 catalyst (Figure 2B). The increase in contact time signiﬁcantly

2

by the WGS reaction at different contact

2

, methanol/DME were formed as major

3

2

4

2

4

(

Figure 2A). C

2

4

Zn–ZrO

2

increased the selectivity of aromatics. Aromatics with a selectivity of 81% were attained

at a CO conversion of 21% at 673 K at a longer contact time. Thus, the formation of

aromatics proceeds via methanol and DME and C

bifunctional catalyst. Our results also reveal that methanol and DME are mainly formed

on Zn–ZrO , whereas aromatics are formed on H-ZSM-5 via C –C oleﬁns from methanol

and DME coming from Zn–ZrO (Figure 2C).

2 4

–C oleﬁn intermediates over the

2

4

2

We have investigated the effect of the H

the Zn–ZrO /H-ZSM-5 catalyst. A higher H

sion of CO, but a lower H

S5). Upon increasing the H

2

/CO ratio on the catalytic performance of

2

/CO ratio was beneﬁcial to the conver-

2

/CO ratio favored the selectivity of aromatics (Table

/CO ratio from 0.5:1 to 3:1, CO conversion increased

2

from 11% to 24%, and the selectivity of aromatics decreased from 86% to 74%. At

a H /CO ratio of 2:1, the selectivity of aromatics was >80% with a reasonably high

CO conversion. These results indicate that our bifunctional catalyst is applicable

to the conversion of syngas with different H /CO ratios and thus could allow the uti-

2

lization of syngas derived from different carbon resources such as natural gas or

shale gas, coal, and biomass.

Information on active sites is the key to the rational design of an efﬁcient bifunctional

catalyst. We have clariﬁed that the Zn/Zr molar ratio is a key factor determining the

4

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Figure 3. Effect of Key Catalyst Factors on the Performance of Bifunctional Catalysts

(

A) Zn/Zr molar ratio.

B) ZrO /H-ZSM-5 with different densities of strong acid sites.

/CO, 2:1; 3 MPa; 25 cm min . C5+ denotes

aliphatic hydrocarbons with carbon numbers R5. Selectivity was calculated on a molar carbon basis

for CO hydrogenation. The formation of CO by the WGS reaction also occurred and the selectivity

of CO was in the range of 38%–42% over these catalysts. Black arrows denote CO conversion.

(

2

3

À1

Reaction conditions: catalyst weight 1.0 g; 703 K; H

2

catalytic behavior of the bifunctional catalyst. ZrO

omatic selectivity, but its CO conversion was low. Doping of a small amount of Zn

or ZnO into ZrO signiﬁcantly increased CO conversion while maintaining high aro-

matic selectivity (Figure 3A). When the Zn/Zr ratio exceeded 1:200, however, the

selectivity of aromatics decreased and that of C –C parafﬁns increased. Undesir-

able C –C parafﬁns and CH became the major products at a Zn/Zr ratio of 1:5.

2

/H-ZSM-5 also displayed high ar-

2

+

2

4

2

4

Thus, the optimum Zn/Zr ratio for the formation of aromatics was 1:200. Our charac-

terizations using X-ray diffraction and transmission electron microscopy (TEM) tech-

2

niques revealed that ZrO particles were mainly in tetragonal structure with a mean

size of 6.4 nm (Figures S2 and S3). ZnO crystalline phases could not be observed

even for the sample with a Zn/Zr ratio of 1:5. The mean particle size evaluated by

TEM slightly increased after Zn doping (Figure S3). High-resolution TEM (HRTEM)

showed lattice fringes that could only be ascribed to ZrO

Zn/Zr ratio of 1:200 (Figure S4). The lattice fringes belonging to ZnO particles ap-

peared when the Zn/Zr ratio increased to 1:100. These results suggest that ZrO

2

for the sample with a

2

+

with highly dispersed Zn or small ZnO clusters accounts for the enhancement in ac-

tivity without sacriﬁcing the aromatic selectivity.

Early studies indicated that ZrO

2

could work for the activation of CO with its oxygen

vacancy sites to form formate species, which was then hydrogenated to methoxide

3

9–41

species in the presence of H

because of its low H activation ability, and the presence of a component that is

capable of activating H

is well known that H

2

.

2

ZrO itself shows low methanol synthesis activity

2

4

1,42

2

can accelerate methoxide and methanol formation.

It

2

can be activated on –Zn–O– pairs on ZnO surfaces via hetero-

and the hydrogen species formed can work for hydrogenation

4

3,44

lytic dissociation,

4

5

reactions. We performed H

studies to gain information on H

Zn/Zr molar ratios. The results showed that the addition of a small amount of Zn or

ZnO clusters enhanced the adsorption of H (Figure S5). ZrO itself could also adsorb

. Methoxide species could be formed from syngas on pure ZrO

2

temperature-programmed desorption (H

2

-TPD)

2

adsorption on Zn–ZrO nanoparticles with different

2

+

2

3

9

H

2

6

,

and ZrO

particles were reported to catalyze the conversion of syngas. We observed that

ZrO /H-ZSM-5 catalyzed the conversion of CO, and the activity was signiﬁcantly

2

nano-

4

2

+

enhanced by the presence of a small amount of Zn or ZnO clusters (Figure 3A).

On the basis of these insights, we propose that CO is mainly activated on oxygen

2 2 2

vacancies on ZrO (Figure S6). H can also be activated on ZrO to form H species

Chem 3, 1–14, August 10, 2017

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and Stability, Chem (2017), http://dx.doi.org/10.1016/j.chempr.2017.05.007

for CO hydrogenation, but the ability of ZrO

2 2

to activate H is weak. The –Zn–O– pair

on Zn–ZrO nanoparticles accelerates H activation to provide more H species for

2

the hydrogenation of the chemisorbed CO species (Figure S6), and thus enhances

CO conversion. However, the H species on –Zn–O– pairs could also work for the hy-

drogenation of lower oleﬁns, intermediates for the formation of aromatics. A higher

Zn/Zr ratio (>1:200) could lead to a too high surface density of hydrogen species,

accelerating the hydrogenation of lower oleﬁn intermediates to lower parafﬁns,

and thus is unbeneﬁcial to aromatic selectivity (Figure 3A). We believe that too

strong hydrogenation ability is the major reason why conventional methanol synthe-

sis catalysts such as Cu–Zn–Al oxide are not suitable as the active component for the

SMA process. Thus, control of the hydrogenation ability is important for obtaining

high selectivity of aromatics.

We have investigated the effect of the toplogy of zeolites on the catalytic behaviors

of bifunctional catalysts. The product distributions depended signiﬁcantly on the to-

pology of the zeolites used in the bifunctional catalysts (Table S6). High selectivity of

aromatics could only be obtained over the Zn–ZrO

2

/H-ZSM-5 catalyst. This is consis-

tent with the fact that only H-ZSM-5 has shown considerable selectivity in the MTA

3

2,47,48

reaction.

Zn–ZrO

pected. Relatively higher selectivity of C

Zn–ZrO /H-MOR catalyst, whereas C –C parafﬁns were the major products when

H-beta was used in the bifunctional catalyst, possibly because of the facile occur-

rence of hydrogenation of C –C oleﬁns.

2

/SAPO-34 provided high selectivity of C

2 4

–C oleﬁns as ex-

1

9

2

–C oleﬁns was also observed over the

4

2

4

2

4

The acidity of H-ZSM-5 is another crucial catalyst factor. It is accepted that the MTA

reaction is an acid-catalyzed reaction, and relatively strong acid sites are required

3

0,31,49

for the formation of aromatics.

NH -TPD) measurements for H-ZSM-5 samples with different Si/Al ratios show

two NH desorption peaks (Figure S7). This indicates acid sites with different

strengths on H-ZSM-5 exist, and the high-temperature peak corresponds to the

3

NH temperature-programmed desorption

(

3

5

0

strong acid sites. The quantiﬁcation clariﬁes that the density of strong acid sites

changes systematically with the Si/Al ratio. Figure 3B shows that the increase in

the density of strong acid sites signiﬁcantly increases CO conversion over the

ZrO

2

/H-ZSM-5 catalyst because of the thermodynamic driving force. The selectiv-

ities of methanol/DME and C

2

–C

4

oleﬁns decreased and that of aromatics

À1

increased with an increase in the density of strong acid sites to 43 mmol g . This

conﬁrms that the strong acid site is responsible for the aromatization reaction. How-

À1

ever, a larger density of strong acid sites (>43 mmol g ) reduced the selectivity of

aromatics and increased that of C

occurrence of hydrogenation of C

2

–C

4

parafﬁns. This is probably because of the

5

1,52

2

4

oleﬁns on strong Brønsted acid sites.

Our pyridine-adsorbed Fourier transform infrared (FT-IR) spectroscopic studies

demonstrated that the density of Brønsted acid sites, which are generally believed

5

0

to be strong acid sites in H-ZSM-5, increased with a decrease in the Si/Al ratio

Figure S8). This tendency is similar to that for the strong acid sites evaluated by

NH -TPD. Thus, the bifunctional catalyst should have an appropriate density of

(

3

strong acid sites.

2

We found that the weight ratio of Zn–ZrO nanoparticles to H-ZSM-5 crystallites

also affected the catalytic performance (Table S7). A decrease in the weight ratio

of Zn–ZrO to H-ZSM-5 from 2:1 to 1:4 decreased CO conversion. The selectivity

of aromatics was lower at a higher weight ratio of Zn–ZrO to H-ZSM-5. These obser-

vations conﬁrm that Zn–ZrO and H-ZSM-5 are responsible for the activation of CO

and the formation of aromatics, respectively. At the typical weight ratio of Zn–ZrO

2

6

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Figure 4. Effect of the Proximity of the Two Components

(

A) (i) Dual bed (0.33 g Zn–ZrO

sizes of 250–600 mm; (iii) Zn–ZrO

crystallites; (iv) nanocomposites of Zn–ZrO

2

+ 0.67 g H-ZSM-5); (ii) mixing of granules of two components with

2

nanoparticles dispersed on micrometer-sized H-ZSM-5

2

and H-ZSM-5. Reaction conditions: catalyst weight

3

À1

1

.0 g; 673 K; H

2

/CO, 2:1; 3 MPa; 25 cm min

.

(

B) Scanning electron microscopy images for the sample with Zn–ZrO

micrometer-sized H-ZSM-5 crystallites. Scale bar, 1 mm.

C) Transmission electron microscopy images for the nanocomposite of Zn–ZrO

Scale bar, 50 nm.

2

nanoparticles dispersed on

(

2

and H-ZSM-5.

to H-ZSM-5 used in this work (1:2), higher selectivity of aromatics and reasonable CO

conversion could be obtained.

Modulating the Proximity of the Two Active Components

The spatial arrangement of different active sites plays a pivotal role in bifunctional

5

3,54

catalysis.

nanoparticles (mean size, ꢀ7 nm) containing Zn or small ZnO clusters dispersed

in ZrO for CO hydrogenation and zeolite H-ZSM-5 for C-C coupling and formation

Our catalyst is composed of two major components, i.e., Zn–ZrO

2

+

2

of aromatics. We studied the effect of the proximity of these two active components

on catalytic behavior. We found that separation of the two components by quartz

wool not only lowered CO conversion because of the lack of the thermodynamic

driving force but also dramatically decreased the selectivity of aromatics (Fig-

ure 4A, i). CH

because of the longer contact time of syngas with Zn–ZrO

products formed on Zn–ZrO at longer contact times were difﬁcult to convert to ar-

4

and C

2

–C

4

hydrocarbons became the major products probably

2

(Figure 2A), and these

2

omatics over the downstream H-ZSM-5. The increase in proximity by changing the

manner of integration from dual-bed to the granule mixing (Figure 4A, ii) and further

2

to the dispersion of Zn–ZrO nanoparticles on H-ZSM-5 (Figure 4A, iii), which is typi-

cally used in this work, signiﬁcantly increased both the conversion of CO and the

selectivity of aromatics (Figure 4A). In the catalyst typically used in our work, Zn–

2

ZrO nanoparticles were dispersed on H-ZSM-5 crystallites with sizes of a few micro-

meters (Figure 4B). To further increase the intimacy of the two components, instead

of the micro-sized H-ZSM-5 crystallites, we synthesized nano-sized H-ZSM-5 (Fig-

5

ure S9) by using amphiphilic organosilane as a structure-directing agent. The

use of nanocomposites containing Zn–ZrO nanoparticles in contact with nano-sized

2

H-ZSM-5 (Figures 4A, iv, and 4C) slightly increased the selectivity of aromatics as well

as CO conversion. The closer proximity of the two components enables faster trans-

3 2

fer of CH OH/DME intermediates formed on Zn–ZrO nanoparticles to H-ZSM-5

Chem 3, 1–14, August 10, 2017

7

Please cite this article in press as: Cheng et al., Bifunctional Catalysts for One-Step Conversion of Syngas into Aromatics with Excellent Selectivity

and Stability, Chem (2017), http://dx.doi.org/10.1016/j.chempr.2017.05.007

Figure 5. Stability of the Zn–ZrO

2

/H-ZSM-5 Catalyst

/CO, 2:1; 3 MPa; 25 cm min

3

À1

.

Reaction conditions: catalyst weight 3.0 g; 673 K; H

2

before the occurrence of unfavorable consecutive reactions, thus favoring the selec-

tive formation of aromatics.

Stability of the Bifunctional Catalyst

Catalyst stability is crucial to the potential application of our SMA process for direct

conversion of syngas to aromatics. A short catalyst lifetime is a common issue for the

5

6

synthesis of aromatics via different routes, including methane aromatization, pro-

5

7

29,30

pane aromatization, and MTA

processes because of the coke deposition.

Although the choice of an appropriate reactor or engineering technique, e.g., the

5

8

use of a ﬂuidized-bed reactor with continuous generation technique is helpful,

the process becomes more complicated with remarkably higher operation costs.

The development of new processes without catalyst deactivation or novel catalysts

5

9

with high intrinsic stability would be more promising. Figure 5 shows that our Zn–

ZrO /H-ZSM-5 catalyst is very stable in the direct conversion of syngas to aromatics.

The selectivity of aromatics and the conversion of CO were sustained at 80% and

0%, respectively, after 1,000 hr of reaction at 673 K. The selectivity of undesirable

CH remained at <3%. As far as we know, such a high catalyst stability has not been

2

4

achieved in the synthesis of aromatics from various feedstocks such as syngas, meth-

anol, propane, or methane.

The following factors could contribute to the high stability of our bifunctional cata-

2

lyst. As one major catalyst component, ZrO has high thermal stability under

6

0

oxidizing and reducing atmospheres. The electron microscope measurements

for the catalyst after reactions clariﬁed that the morphology of bifunctional catalysts

did not undergo signiﬁcant changes (Figure S10). As mentioned above, coke depo-

sition is a major reason for catalyst deactivation in the syntheses of aromatics from

methanol, propane, and methane over H-ZSM-5-based catalysts. H-ZSM-5 with a

low Si/Al ratio (%40) or a high density of strong acid sites is typically used in these

4

9,61,62

processes to increase the selectivity of aromatics.

However, a higher density

6

3

of strong acid sites can cause faster coke deposition. On the other hand, we could

obtain high selectivity of aromatics by using H-ZSM-5 with a relatively higher Si/Al

ratio (120) in our bifunctional catalyst. Our studies demonstrated that the ZrO

H-ZSM-5 catalyst with a lower Si/Al ratio (24) suffered from faster deactivation during

the conversion of syngas to aromatics, whereas the ZrO /H-ZSM-5 catalyst with a

Si/Al ratio of 120 was signiﬁcantly more stable (Figure S11). Furthermore, we

2

/

2

8

Chem 3, 1–14, August 10, 2017

Please cite this article in press as: Cheng et al., Bifunctional Catalysts for One-Step Conversion of Syngas into Aromatics with Excellent Selectivity

and Stability, Chem (2017), http://dx.doi.org/10.1016/j.chempr.2017.05.007

Figure 6. Tuning the BTX Fraction in Aromatics by Silylation of H-ZSM-5 with TEOS

3

À1

(

A) Fractions of products in aromatics. Reaction conditions: catalyst weight 1.0 g; 703 K; H

B) 2,6-DTBPy-adsorbed FT-IR.

2

/CO, 2:1; 3 MPa; 25 cm min . See also Table S8.

C) Scheme for suppressing the formation of heavier aromatics by selective silylation of external Brønsted acidity.

speculate that the presence of H

conﬁrmed that the conversion of methanol over H-ZSM-5 under an H

showed less coke deposition and higher catalyst stability than that under an N

2

in our case can suppress the coke formation. We

atmosphere

at-

2

mosphere (Figure S12). Thus, besides the robustness of the catalyst, the lower den-

sity of strong acid sites in H-ZSM-5 used in our bifunctional catalyst and the presence

2

of H are believed to play crucial roles for the excellent stability.

We further investigated the effect of the presence of a small amount of CO

S in syngas on the stability of the Zn–ZrO /H-ZSM-5 catalyst. It is surprising

that the presence of 1.0 vol % CO or 10 ppm H S in syngas did not exert a sig-

niﬁcant effect on the catalytic performance (Figure S13). The catalyst was stable

for 100 hr of reaction, even for the conversion of CO - or H S-containing syngas.

and H S tolerant at least

2

or

H

2

These observations indicate that our catalyst is also CO

for 100 hr.

2

Tuning the Fraction of BTX in Aromatic Products

Among the aromatic products, BTX are particularly large-scale feedstocks for

numerous chemical industrial processes. The fraction of BTX in the aromatics formed

on our Zn–ZrO

were formed as major products with a fraction of 57% together with 12% heavier

R10) aromatics. Considering the sizes of the product molecules (C >0.65 nm)

2

/H-ZSM-5 catalyst was ꢀ30% (Figure 6A). Trimethylbenzenes (C

9

)

(

C

9

and pores of H-ZSM-5 (0.55 nm), we speculate that larger aromatics could be formed

through the alkylation of BTX, which are mainly formed inside the micropores and

3

diffuse out, with CH OH/DME on the acid sites in the pore-mouth region or on

external surfaces of H-ZSM-5. To increase BTX selectivity, we preferentially passiv-

ated the external Brønsted acid sites by silylation of H-ZSM-5 through chemical

liquid deposition (CLD) of tetraethoxysilane (TEOS), a molecule larger than the

6

4

pore size of H-ZSM-5. The FT-IR spectra of adsorbed di-tert-butyl-pyridine

DTBPy), a larger basic probing molecule that cannot enter the pore of H-ZSM-5,

(

clariﬁed that the density of external Brønsted acid sites, which could be assessed

À1

by the intensity of the band at 1,616 cm , decreased signiﬁcantly after modiﬁcation

of TEOS by CLD in two cycles (CLD 3 2) (Figure 6B). The total amounts of strong acid

3

evaluated by NH -TPD (Figure S14) decreased only slightly at the same time. Thus,

the CLD of TEOS with two cycles could passivate external Brønsted acidity. Our cat-

alytic studies showed that CO conversion decreased slightly and aromatic selectivity

remained almost unchanged with the use of TEOS-modiﬁed H-ZSM-5 in the

Chem 3, 1–14, August 10, 2017

9

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and Stability, Chem (2017), http://dx.doi.org/10.1016/j.chempr.2017.05.007

Figure 7. Effect of CO on Methanol Conversion over the Zn–ZrO

2

/H-ZSM-5 Catalyst

(

A) Effect of CO on product selectivity in methanol conversion.

B) Functioning mechanism of CO.

1

3

13

3

C) Incorporation of C into toluene during the conversion of CH OH under CO.

3

Catalytic reactions were performed at 673 K with 100% CH OH conversion in each case. Experimental details are shown in Supplemental Information.

bifunctional catalyst (Table S8). However, the use of modiﬁed H-ZSM-5 (CLD 3 2)

signiﬁcantly increased the fraction of BTX in aromatics from 30% to 63% (Figure 6A).

At the same time, the fractions of trimethylbenzenes and heavier benzenes

decreased to 31% and 5.9%, respectively. Thus, selective passivation of external

Brønsted acidity suppresses the alkylation of BTX (Figure 6C), contributing to the

enhancement in BTX selectivity.

Self-Promotion Mechanism of CO

To gain more insights into the reaction mechanism, we performed the conversion of

CH

3 2 2

OH over the Zn–ZrO /H-ZSM-5 and H-ZSM-5 catalysts at 673 K under N . Unex-

pectedly, aromatics were only formed with low selectivity under an N

2

atmosphere

2 4

C –C oleﬁns were

2

9,30

(

Table S9), which is typically used for the MTA reaction.

formed as major products together with C

2

–C

4

2

parafﬁns, aromatics, and C5+

9,30

65

aliphatic hydrocarbons. In the MTA reaction, ZnO

or Ga

2

O

3

is usually added

into the zeolite catalyst to increase aromatic selectivity by facilitating dehydrogen-

ative aromatization of oleﬁn intermediates through removal of H species. The co-ex-

istence of Zn–ZrO

enhance the selectivity of aromatics under N

H-ZSM-5 instead of micro-sized H-ZSM-5 to tune the proximity between Zn–ZrO

and zeolite did not show a signiﬁcant promoting effect on the selectivity of aromatics

Table S10). These observations indicate that the ability of Zn–ZrO to remove H spe-

cies was negligible under N . We further found that the change in the reaction atmo-

sphere from N to H did not affect the selectivity of aromatics but decreased the

selectivity of C –C oleﬁns and increased that of C –C parafﬁns (Table S9). Surpris-

ingly, the presence of CO instead of N enhanced the formation of aromatics, and

the co-existence of Zn–ZrO played a key role in this enhancing effect of CO (Table

S9). We found that an increase in CO pressure signiﬁcantly increased the selectivity

of aromatics and decreased that of C –C oleﬁns as well as C5+ aliphatic hydrocar-

bons (Figure 7A). When the gas ﬂow rate was increased from 6.0 (Table S9) to

2

nanoparticles with H-ZSM-5 in our case, however, did not

2

(Table S9). The use of nano-sized

2

(

2

4

2

4

2

4

3

À1

2

5 cm min , the promotion effect of CO on the selectivity of aromatics could still

be observed (Table S11). Thus, there exists a self-promotion effect of CO on the

selective formation of aromatics in the conversion of syngas, a mixture of CO and

6

H

2

. The concept of self-promotion has been proposed in a few catalytic reactions.

10

Chem 3, 1–14, August 10, 2017

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and Stability, Chem (2017), http://dx.doi.org/10.1016/j.chempr.2017.05.007

To the best of our knowledge, this is the ﬁrst report to demonstrate the self-promo-

tion effect in syngas chemistry.

We have already demonstrated that aromatics are formed from syngas via CH

DME and C –C oleﬁn intermediates over the bifunctional Zn–ZrO /H-ZSM-5

catalyst (Figure 2C). C –C oleﬁns could undergo oligomerization, cyclization, and

3

OH/

2

4

2

4

3

4

dehydrogenation to form aromatics. The removal of H species is believed to be

a rate-limiting step for the formation of aromatics from lower oleﬁns on

6

7

H-ZSM-5. The promoting effect of CO on aromatic selectivity (Figure 7A) suggests

that CO could work as a ‘‘hydrogen acceptor’’ to accelerate the removal of surface H

species during dehydrogenative aromatization. It is likely that the H species

generated on H-ZSM-5 migrate to the interface and react with the CO molecules

chemisorbed on Zn–ZrO

2 3

nanoparticles, forming CH OH, which participates in the

reaction cycle for the formation of aromatics (Figure 7B). To conﬁrm this speculation,

1

3

we performed the conversion of CH

ZrO /H-ZSM-5 catalyst. We observed the formation of aromatic products with

incorporated (Figures 7C, S15, and S16). This clearly suggests that CO not only func-

tions as a reactant for the formation of CH OH intermediate but also as a ‘‘hydrogen

3

OH under a CO atmosphere over the Zn–

1

3

2

C

3

acceptor’’ to accelerate dehydrogenative aromatization of oleﬁn intermediates,

which is a crucial step determining the selectivity of aromatics. The self-promotion

effect of CO is also consistent with the result obtained from the study on the effect

of the proximity of the two active components (Figure 4). The intimate contact of the

two components is beneﬁcial to the removal of H species on H-ZSM-5 by the CO

molecules chemisorbed on the nearby Zn–ZrO

selectivity of aromatics.

2

nanoparticles, thus favoring the

DISCUSSION

Our work has demonstrated that the SMA process can overcome the problems of

limited selectivity and catalyst deactivation, and thus is a promising route for the syn-

thesis of aromatics. The successful design of efﬁcient bifunctional catalysts is the key.

We have gained the following insights that can guide the design of efﬁcient bifunc-

tional catalysts for the direct conversion of syngas to aromatics. First, control of the

hydrogenation ability of the methanol synthesis catalyst component is key to obtain-

ing high aromatic selectivity. Our studies suggest that the activation of CO takes

place mainly on the surface oxygen vacancy site related to ZrO

vated on ZrO , but the presence of surface –Zn–O– pairs accelerates the activation of

through heterolytic dissociation. The presence of Zn species enhances CO

2 2

. H can also be acti-

2

H

2

conversion by facilitating hydrogenation of chemisorbed CO species, but a higher

surface density of Zn species leads to a higher density of H species, increasing the

selectivity of undesirable C

lective aromatic formation. The unique hydrogenation ability of H species on Zn–

ZrO surfaces and their controlled surface density should limit the formation of

–C parafﬁns. Second, the strong acid sites in H-ZSM-5 account for the conversion

of methanol/DME to aromatics via C –C oleﬁns. However, the major side reaction,

i.e., hydrogenation of C –C oleﬁns, can also occur on the strong Brønsted acid

sites. Thus, control of the density of strong acid sites is another key to success. Third,

the longer contact time of syngas with Zn–ZrO nanoparticles could convert meth-

anol/DME intermediates to undesirable CH and C –C parafﬁns besides C –C ole-

ﬁns. The closer proximity between the two active components could facilitate the

transfer of reaction intermediates from Zn–ZrO nanoparticles to H-ZSM-5; this is

2 4

–C parafﬁns. There exists an optimum Zn/Zr ratio for se-

2

C

2

4

2

4

2

4

2

4

2

4

2

4

2

the third key to high aromatic selectivity. Furthermore, the use of H-ZSM-5 with

controlled density of strong acid sites in our bifunctional catalyst and the presence

Chem 3, 1–14, August 10, 2017

11

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and Stability, Chem (2017), http://dx.doi.org/10.1016/j.chempr.2017.05.007

2

of H are believed to contribute to suppressing coke deposition and thus the high

catalyst stability.

A fascinating feature of the present system is the self-promoting mechanism of CO.

Although H-ZSM-5 mainly accounts for the aromatization, it is unexpected that the

conversion of methanol under N

over our H-ZSM-5 or the Zn–ZrO

2

only results in a lower selectivity of aromatics

/H-ZSM-5 catalyst. The presence of CO plays a

2

pivotal role in obtaining high aromatic selectivity in the conversion of methanol,

and the co-existence of Zn–ZrO with H-ZSM-5 is necessary for such a self-promotion

2

effect. We have uncovered that CO works as a ‘‘hydrogen acceptor’’ to remove the

hydrogen species generated on H-ZSM-5 in the dehydrogenative aromatization of

oleﬁn intermediates, which is a rate-determining step for aromatic formation. This

contributes to the excellent aromatic selectivity in the conversion of syngas over

the simple but ingeniously designed bifunctional catalyst.

Many processes such as the MTA, methane aromatization, and propane aromatiza-

tion have been investigated as important alternatives for the synthesis of aromatics,

but they are not very successful because of poor catalyst stability or limited aromatic

selectivity. The optimized productivity of aromatics of our bifunctional catalyst is

cat^À¹

À1

0

.12 g g

hr (Table S12). This value is higher than that obtained for methane

cat^À¹

À1 68

cat^À¹

aromatization (ꢀ0.04 g g

hr ), but lower than those for MTA (ꢀ0.2 g g

À1 69

À1

À1 70

hr

)

and propane aromatization (ꢀ0.6 g gcat hr ). Although the productivity

of our catalyst for direct conversion of syngas to aromatics is not outstanding, the

selectivity and catalyst stability are very unique. Improving productivity should be

one major research target in the future.

In summary, our present work demonstrates a simple but powerful bifunctional cata-

2

lyst, Zn–ZrO /H-ZSM-5, for one-step conversion of syngas into aromatics with an

excellent selectivity of 80% at CO conversion of 20% at 673 K. The selectivity of un-

desirable methane is lower than 3%. The catalyst is very stable, and no catalyst deac-

tivation has been observed in 1,000 hr. The fraction of BTX in aromatics can be tuned

to >60% by selective passivation of external Brønsted acidity of H-ZSM-5. Our work

not only offers a simple intensiﬁed one-step SMA process for the synthesis of aro-

matics but also solves the problems of catalyst deactivation and limited product

selectivity that beset the synthesis of aromatics.

EXPERIMENTAL PROCEDURES

Full experimental procedures are provided in the Supplemental Information.

SUPPLEMENTAL INFORMATION

Supplemental Information includes Supplemental Experimental Procedures, 16 ﬁg-

ures, and 12 tables and can be found with this article online at http://dx.doi.org/10.

1

016/j.chempr.2017.05.007.

AUTHOR CONTRIBUTIONS

K.C. and W.Z. performed most of the experiments and analyzed the experimental

data. J.K. conducted the stability experiments and studied the effect of the manner

of integration. S.H. and S.S. performed catalyst characterizations. Q.Z. analyzed the

data and co-wrote the paper. Y.P. and W.W. performed the isotopic tracing study on

1

3

the conversion of CH OH under CO. Y.W. designed the study, analyzed the exper-

imental results, and co-wrote the paper. All authors discussed the results and

analyzed the manuscript.

12

Chem 3, 1–14, August 10, 2017

Please cite this article in press as: Cheng et al., Bifunctional Catalysts for One-Step Conversion of Syngas into Aromatics with Excellent Selectivity

and Stability, Chem (2017), http://dx.doi.org/10.1016/j.chempr.2017.05.007

ACKNOWLEDGMENTS

This work was supported by the National Basic Research Program of China (no.

2

9

013CB933102), the National Natural Science Foundation of China (nos.

1545203, 21433008, 21403177, 21503174, and 21673188), and the China Postdoc-

toral Science Foundation (no. 2016T90596).

Received: February 25, 2017

Revised: March 20, 2017

Accepted: May 15, 2017

Published: August 3, 2017

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