Influence of the Distance between Two Catalysts for CO2 to Dimethyl Ether Tandem Reaction [original]

Influence of the Distance between Two

Catalysts for CO

to Dimethyl Ether Tandem

Reaction

Tandem reactions, with their great potential for reduction of capital and opera-

tional costs, reaction time, and byproducts are becoming more prominent. Among

possible tandem reaction systems for direct conversion of CO

to green fuel,

dimethyl ether synthesis via methanol route is of vital importance. However, there

are several parameters which must be considered for the combination of two cata-

lysts to achieve promising performance, of those, the distance between the two

catalysts is a crucial one. This distance can affect the overall performance of the

tandem system by causing external mass transfer limitations or the poisoning of

the catalysts. This work presents a systematic study on the influence of the

distance between the two catalysts for the CO

to dimethyl ether tandem system.

Keywords: Dimethyl ether tandem system, HZSM-5 deactivation, Mass transfer, Methanol,

Tandem catalysis

Received: November 02, 2022; revised: December 22, 2022; accepted: January 20, 2023

DOI: 10.1002/ceat.202200541

1 Introduction

The escalating effects of global warming and its drastic impact

on the climate in recent years are leaving us with no other

options but to look for production routes for environment-

friendly energy resources. In recent decades, exponential devel-

opment has been observed in the research focused on CO

cap-

ture and utilization, a major greenhouse gas [1–3]. Several

routes have been proposed for direct and indirect conversion

of CO

to fuels and other value-added chemicals, including

hydrogenation of CO

to syngas, methanol (MeOH), and for-

mic acid followed by their subsequent conversion to a variety

of secondary products [4–8].

Among several conversion routes of CO

, its conversion to

MeOH has attracted more attention in the last couple of

decades. MeOH, with its wide range of applications as a chemi-

cal feedstock, has earned significant importance in the chemi-

cal industry and energy sector. It can also be directly used as

fuel for internal-combustion engines [9, 10]. Direct synthesis of

MeOH from CO

follows a heterogeneous catalysis route. In

the last couple of decades, the synthesis process has been well-

explored, and various catalysts have been studied [11]. The

reaction system involves an exothermic reaction of hydrogena-

tion of CO

to MeOH and an endothermic reverse water-gas

shift reaction (RWGS) as a side reaction which produces CO.

The equilibrium CO

conversion and the yield of MeOH are

pressure- and temperature-dependent. Calculations with ther-

modynamic properties reveal that lower temperatures and

higher pressures shift the MeOH synthesis reaction equilibrium

towards the forward direction [10]. Cu-based catalysts have

been extensively studied for CO

to MeOH reaction, and

significant progress has been made in understanding the reac-

tion mechanism and structure-activity relation and locating the

active sites [7, 8, 10].

Several metals and metal-oxides have also been studied as

promoters for Cu-based catalysts. Baode et al. investigated the

effect of K and Ba on high-pressure CO

hydrogenation to

MeOH over Cu/Al

catalyst and observed enhanced adsorp-

tion of CO

for the catalysts with promoters. However, only

the Ba-promoted catalyst seemed to have a positive affect and

exhibited a 20 % increment in the selectivity of MeOH [12].

Researchers have also studied several other promoters for

Cu-based catalysts, like Zn, Ga, Mg [13], Cr, Mn, Ag, Zr [14]

and noticed different effects on the catalytic activity. Among

different types of promoters for Cu-based MeOH synthesis

catalysts, ZnO has been reported to exhibit promising results.

Recent research works about Cu/ZnO-based catalysts propose

that besides fully reduced metallic Cu sites, the activation of

molecules for MeOH synthesis is associated to an Cu-ZnO

synergistic effect at the interface of partially reduced CuO

and

ZnO, as well [15].

Chem. Eng. Technol. 2023,46, No. 6, 1163–1169 ª2023 The Authors. Chemical Engineering & Technology published by Wiley-VCH GmbH www.cet-journal.com

Mudassar Javed

Georg Bro

¨sigke

Reinhard Schoma

¨cker

Jens-Uwe Repke

This is an open access article under

the terms of the Creative Commons

Attribution-NonCommercial-

NoDerivs License, which permits

use and distribution in any

medium, provided the original

work is properly cited, the use is

non-commercial and no

modifications or adaptations are

made.

Supporting Information

available online

–

Mudassar Javed https://orcid.org/0000-0002-4106-4663,

Dr. Georg Bro

¨sigke, Prof. Jens-Uwe Repke

([email protected])

Technical University Berlin, Process Dynamics and Operations Group,

Strasse des 17. Juni 135, 10623 Berlin, Germany.

Prof. Reinhard Schoma

¨cker

Technical University Berlin, Institute for Chemistry, Strasse des 17. Ju-

ni 135, 10623 Berlin, Germany.

Research Article 1163

Nowadays, the field of tandem catalysis, where multiple reac-

tions occur over a multifunctional catalyst or a mixture of dif-

ferent types of catalysts in one reactor, is becoming more and

more prominent. Tandem catalysis has a significant potential

of reducing the capital and operational cost of the process

while improving the yield and the selectivity of the targeted

product [16]. Depending on the thermodynamics of the

reactions and the type of the catalysts being used, few

secondary reactions can also be combined with the CO

MeOH reaction for direct synthesis of dimethyl ether (DME),

dimethyl carbonate, lower olefins, gasoline, and diesel etc.

[6, 7, 15, 16].

Among the products of CO

and MeOH-based tandem sys-

tem, DME is one of the most valuable and demanded product

[17, 18]. DME finds its applications in several chemical indus-

tries, including the production industries for lower olefins

[19, 20], dimethyl sulfate [21], ethanol, aromatics [22], and

several other valuable chemicals [23, 24]. MeOH synthesis reac-

tion can be combined with DME synthesis for direct conver-

sion of CO

to DME. Many researchers have discussed the

parameters affecting the performance of this tandem reaction

system [25, 26]. DME is produced by the dehydration of

MeOH on an acidic catalyst [27].

Researchers have proposed a variety of catalysts for DME

synthesis, which include several types of zeolites like ZSM-5,

ZSM-22 [28–31], SAPO-11, SAMP-34 [32–34], different types

of heteropoly acids (HPAs) [35, 36], g-Al

[37], aluminosili-

cates and a number of other ordered structures containing

acidic sites [38, 39]. Extensive discussions on the reaction

mechanism and the common challenges accompanied by DME

synthesis catalysts, mainly deactivation of the acidic sites dur-

ing the reaction, poisoning the catalyst with water and low

product selectivity, have been reported in recently published

review articles [25, 26, 39].

For direct conversion of CO

to DME, several parameters

need to be considered, including the type of both catalysts and

their ratio, particle size and distance between the two catalysts,

and so on. De Jong et al. [40] and Jian et al. [26] have studied

the effect of the nanometer- to millimeter-scale distance be-

tween the two types of active sites of bifunctional catalysts and

observed a great influence of it on the overall catalytic perfor-

mance. A recent theoretical study by Bro¨sigke et al. [41] also

discusses the design of a core-shell type bifunctional catalyst

with optimal distance between different active centers.

In this work, separate synthesis of MeOH and DME as well

as direct conversion of CO

to DME via tandem catalysis were

investigated. Moreover, the role of the distance between the

MeOH synthesis and DME synthesis catalysts on the produc-

tion of dimethyl is studied. The outcomes of this research work

can play a vital role in the design and synthesis of bifunctional

catalysts for tandem catalytic reaction systems.

2 Materials and Methods

2.1 Materials

KNO

from Carl Roth GmbH + Co. KG was used as the pre-

cursor for the K-based DME synthesis catalyst. Deionized

water was taken for all synthesis procedures. All chemicals

were employed without any further purification steps.

2.2 Catalyst Preparation

The commercially available MeOH synthesis catalyst (CuOZnO/

) with CuO:ZnO:Al

:MgO = 63.5:24.7:10.1:1.3 was used

for MeOH synthesis. For the DME synthesis reaction, commer-

cially available ZSM-5 (ammonium) type zeolite was employed

as a base material to synthesize all DME synthesis catalysts.

Both catalysts were purchased from Alfa Aesar (Thermo

Fischer (Kandel) GmbH). The ammonium-type ZSM-5 was

calcined at 600 C for 6 h to obtain HZSM-5. For KZSM-5,

0.0245 g of KNO

was dissolved in deionized water and im-

pregnated on 10 g of HZSM-5 via the wetness-impregnation

method following a similar approach as reported in the litera-

ture [42]. The sample was dried in an oven at 80 C overnight

and then calcined at 350 C for 5 h.

2.3 Catalyst Characterization

Physiochemical properties of the catalysts were investigated

with X-ray diffractometry (XRD), scanning electron micros-

copy (SEM), transmission electron microscopy (TEM), pyri-

dine-adsorption infrared spectroscopy (Py-IR), and thermo-

gravimetric analysis (TGA). Specifications of the equipment

used and analysis procedures for all the characterization tech-

niques are available in the Supporting Information (SI) section.

2.4 Catalytic Testing

All the catalysts were tested in a lab-scale Berty reactor. The

specification and schematic diagram of the testing rig as well as

the experimental procedure are presented in the Supporting

Information (SI) section.

In this work, along with the monofunctional catalysts for

separate MeOH and DME synthesis reactions, three conforma-

tions of bifunctional catalysts based on the distance between

CZA and KZSM-5 were used for tandem reactions. For pow-

der-mix conformation, powdered forms of both catalysts were

ground together with a mortar and pestle and then pelletized

into particles of 300–500 mm. In the case of granule-mix con-

formation, 300–500 mm particles of both catalysts were physi-

cally mixed, whereas for the two-bed conformation, the beds of

both catalysts were separated with a layer of quartz wool. The

three conformations are schematically presented in Scheme 1.

WHSV (weight hour space velocity), conversion, yield, and

selectivity were calculated by Eqs. (1), (2), (3), and (4), respec-

tively.

Chem. Eng. Technol. 2023,46, No. 6, 1163–1169 ª2023 The Authors. Chemical Engineering & Technology published by Wiley-VCH GmbH www.cet-journal.com

Scheme 1. Three conformations of the tandem catalyst (CZA:

yellow; KZSM-5: grey; CZA+KZSM-5: brown)

Research Article 1164

WHSV mLsg1

cat h1



¼Fin

mcat

(1)

Xr%ðÞ¼

nin

rnout

nin

·100 (2)

Yp%ðÞ¼ Yactual

Ytheoratical

·100 (3)

Sp%ðÞ¼ nc

nctotal

·100 (4)

3 Results and Discussion

3.1 Catalyst Characterization

The effect on structural changes and crystallinity on commer-

cial ZSM-5 after calcination and the post treatment was studied

with XRD analysis. Fig. 1 presents XRD patterns of the calcined

HZSM-5 and KZMS-5. For both catalysts, peaks between

2q= 21.5

–24

correspond to the typical peaks for MFI type

structure of zeolite (JCPDS No. 42–24), i.e., ZSM-5. It is worth

noting that neither any significance disappearance of peaks nor

appearance of new peaks is observed after post treatment of

HZMS-5 to synthesize KZSM-5. Therefore, the crystalline

structure of ZSM-5 is retained after the post-treatment. More-

over, no peaks ascribed to the K-containing phase are detected,

which suggests that K is well dispersed in the structure.

The dispersion of K in the structure of KZSM-5 was also

investigated with SEM elemental mapping, and results are dis-

played in Fig. 2. Figs. 2a, 2b, 2c, and

2d display the distribution of Si, Al,

O, and K in the sample, and it is

observed that, besides other ele-

ments, K is also homogeneously

distributed in the structure of the

ZSM-5, which is also in agreement

with the XRD analysis of the sam-

ple.

The acid properties of the zeolite

framework play a key role in the

DME synthesis reaction. Therefore,

understanding the strength and

distribution of the acidic sites in

the structure of the catalyst is of

crucial importance. Furthermore,

the technique of modification of

acid sites via the introduction of

alkali metal ions is well-known

[43]. With the help of pyridine as a

probe molecule, the surface acidity

was characterized by Fourier trans-

form infrared (FTIR) spectroscopy.

The spectra after saturation of the

surface of HZSM-5 and KZSM5 are

depicted in Fig. 3.

The spectral region for the pyridine ring vibrations (Fig. 3a)

shows a redistribution of the vibrations associated with Lewis

acid sites (1650–1590 and 1455–1441 cm

–1

) after introduction

of the potassium ions. While reducing the total intensity of me-

dium to strong Lewis acid centers (1650–1600 and 1455 cm

–1

an increase is observed for the vibration of weak Lewis sites

(1592 and 1441 cm

–1

). However, the introduction of potassium

not only effects the Lewis acidity but also lowers the Brønsted

acidity of the zeolite, which can be seen by the lower intensity

of the vibration at 1545 cm

–1

[44, 45].

The lowered Brønsted acidity is additionally confirmed by

the lower intensity of the OH stretching vibrations assigned

to terminal SiOH (~3740–3720 cm

–1

) as well as bridged

Al(OH)Si units (3606 cm

–1

) upon pyridine coordination

(Fig. 3b) [46].

Chem. Eng. Technol. 2023,46, No. 6, 1163–1169 ª2023 The Authors. Chemical Engineering & Technology published by Wiley-VCH GmbH www.cet-journal.com

Figure 1. XRD patterns of calcined HZSM-5 and KZSM-5.

Figure 2. SEM-EDX elemental mapping of calcined KZSM-5: (a) Si, (b) Al, (c) O, and (d) K.

Research Article 1165

3.2 Catalytic Performance Testing

MeOH synthesis reaction was carried out on reduced CZA cat-

alyst, and catalytic performance results are presented in Fig. 4.

Fig. 4a exhibits CO

conversion rates at 20 bar pressure and a

range of temperature between 180–280 C. It can be observed

that the experimental conversion of CO

increases with rising

the temperature of the reactor and is always below the equilib-

rium conversion which confirms that all the experimental data

is collected in the kinetic regime. On the other hand, the yield

of MeOH first increases with higher temperature and achieves

a maximum value of 1.56 % at 240 C and then starts decreas-

ing with further raise in the reactor temperature (Fig. 4b). This

is because higher temperature is more favorable for the endo-

thermic RWGS reaction which produces CO. Hence, product

selectivity also shifts from 99 % MeOH at 180 C to 89.6 % CO

at 280 C.

The KZSM-5 catalyst was used for DME synthesis reaction

as it showed a higher stability than HZSM-5 for the long-run

activity test. Catalytic performance results are presented in

Fig. S2. It is noticeable from Fig. 5 that the MeOH conversion

rates show an increase with raising the reactor temperature and

reach a maximum conversion of 77 % at the highest tempera-

ture, i.e., 281 C. However, the yield of DME is observed to be

generally increasing with higher temperature and shows a

sharp decrease after 260 C due to the formation of a range of

unidentified hydrocarbons at high-

er temperatures.

A comparison of the catalytic

performance results of MeOH syn-

thesis and DME synthesis reaction

at 20 bar pressure (Figs. 4 and 5)

suggests that the operational win-

dow for combining both reactions

for the tandem catalysis at 20 bar is

roughly between 230–270 C, as

both MeOH and DME show the

maximum yields in this tempe-

rature range. Therefore, tandem

catalysis at 20 bar pressure and

230–270 C reactor temperature for

direct conversion of CO

to DME

via MeOH route by combining

Chem. Eng. Technol. 2023,46, No. 6, 1163–1169 ª2023 The Authors. Chemical Engineering & Technology published by Wiley-VCH GmbH www.cet-journal.com

Figure 3. FTIR spectra of fresh HZSM-5 and KZSM-5. (a) Pyridine adsorption, (b) OH stretching region.

Figure 4. (a) CO

conversion rates at 20 bar and different temperatures over CZA catalyst;

(b) MeOH yield and product selectivity of MeOH and CO, for reaction in a Berty reactor at

2000 rpm, P= 20 bar, H

:CO

= 1:3:1, m

cat

= 30 mg, WHSV = 100 000 mL

cat

–1

Figure 5. MeOH conversion rates and yield of DME at 20 bar

and different temperatures over KZSM-5 catalyst for reaction

in a Berty reactor at 2000 rpm, P= 20 bar, CH

OH/N

= 0.02,

cat

= 30 mg, WHSV = 201 818 mL

cat

–1

Research Article 1166

CZA and KZSM-5 catalysts in three different conformations

was studied. The three conformations were based on the dis-

tance between the two catalysts with the distance ranging from

nanometers to millimeters. The catalytic performance results

for the tandem catalysis are presented in the Fig. 6.

conversion for the distance-based series of tandem cata-

lysts increases with rising reactor temperature and shows simi-

lar rates for all three catalysts (Fig. 6a). However, a significant

difference in the yield of MeOH and DME is observed for all

three catalysts (Fig. 6b). For DME yield, granule-mix catalyst

conformation with mm-range distance between CZA and

KZMS-5 particles outperforms the other two conformations

where the distance is either too small (nm range in powder

mix) or too large (mm range in two beds).

The powder-mix catalyst, with nm-range distance between

the two catalysts, shows the lowest yield of DME at 230 C

which then increases sharply until 260 C. This could be associ-

ated with high local concentration of water within the catalyst

particle, since one particle contains active sites for both MeOH

and DME and both reactions produce water which partially

poisons KZSM-5 at lower temperatures. With an increase in

the temperature, the poisoning effect of water is reduced which

is also suggested by the fast drop in the yield of MeOH with

increasing temperature, since more MeOH is consumed for

DME synthesis. Fig. 6c shows that, in general, the selectivity of

both MeOH and DME drops with increasing temperature

which is because of the dominance of RWGS reaction at higher

temperature.

In order to further understand the influence of the distance on

the overall catalytic performance of the CO

-to-DME tandem

reaction, a systematic study by changing external mass transfer

conditions within the reactor was conducted and the outcome is

presented in Fig. 7. The external mass transfer within the reactor

was altered by gradually increasing the number of rpm (rota-

tions per minute) of the stirrer of the Berty reactor.

Fig. 7a shows that no significant influence by increasing the

rpm is observed on CO

conversion rates for all three confor-

mations of the catalysts, whereas a remarkable influence on the

yield of MeOH and DME is noticed, where the granule-mix

conformation of the catalysts shows the highest yield for DME

even at lower numbers of rpm. Generally, the DME yield

increases with higher rpm of the stirrer for both granule-mix

and two-bed conformation and reaches a maximum value at

2500 rpm for granule-mix and 3000 rpm for two-bed confor-

mation and then slightly drops.

A possible explanation for that is the external mass transfer

limitations caused by the distance between the two catalysts

which are avoided by increasing the number of rpm of the

stirrer. For the granule-mix conformation, these limitations are

avoided at comparably lower rpm of the stirrer than for the

Chem. Eng. Technol. 2023,46, No. 6, 1163–1169 ª2023 The Authors. Chemical Engineering & Technology published by Wiley-VCH GmbH www.cet-journal.com

Figure 6. Catalytic performance results for direct conversion of CO

to DME over bifunctional catalysts for reactions in a Berty reactor at

2000 rpm, P= 20 bar, H

:CO

= 1:3:1, m

cat

= 90 mg CZA + 30 mg KZSM-5, WHSV = 50 000 mL

cat

–1

. (a) CO

conversion rates;

(b) yield of MeOH and DME; (c) selectivity of MeOH and DME.

Figure 7. Catalytic performance results for direct conversion of CO

to DME over bifunctional catalysts with increasing rpm of the stirrer

of a Berty reactor at P= 20 bar, H

:CO

= 1:3:1, m

cat

= 90 mg CZA + 30 mg KZSM-5, WHSV = 50 000 mL

cat

–1

. (a) CO

conversion

rates; (b) yield of MeOH and DME; (c) selectivity of MeOH and DME.

Research Article 1167

two-bed one since the catalyst’s particles are generally closer to

each other. In case of the powder-mix conformation, since both

active sites are present in one granule, the internal mass transfer

limitations are more dominant than the external ones.

A decrease in the yield of DME after 2000 rpm for the

powder-mix catalyst is probably ascribed to high local concen-

tration of water within the catalyst’s particle because of the syn-

thesis of both MeOH and DME within one particle. This high-

er local content of water poisons the acidic sites of KZSM-5

which is also suggested by increasing yield and selectivity of

MeOH with higher number of rpm for the powder-mix confor-

mation.

In summary, this work reveals the significant role of the dis-

tance between different types of active centers in multifunc-

tional catalysts and provides some fundamental guidance for

the design of such catalysts.

4 Conclusion

A systematic experimental approach was followed to find out the

operating window for a tandem system of direct conversion of

to DME which involved CO

hydrogenation reaction over a

CuZnO/Al

(CZA) catalyst and MeOH dehydration reaction

over a KZSM-5 catalyst. The commercially available HZSM-5

showed deactivation behavior for the long-term stability test

because of the carbon deposition, whereas the KZSM-5 catalyst

maintained its activity and exhibited significant lower amount of

carbon deposition than HZSM-5 after the reaction.

When combining MeOH synthesis with DME synthesis in a

tandem system, the effect of the distance between the two cata-

lysts on overall catalytic performance was thoroughly investi-

gated. Three types of confirmations of the combination of CZA

and KZMS-5 were studied and the one with the micrometer-

range distance between the two catalysts (granule mix) showed

the highest yield at all temperature points compared to the

other two conformations with nanometer-range (powder mix)

and millimeter-range distance (two beds). The larger distance

seems to cause external mass transfer limitation whereas the

shorter distance leads to poisoning of the DME synthesis cata-

lysts due to high local concentration of water around the active

sites. This study on the role of the distance in overall perfor-

mance of the bifunctional catalyst provides some fundamental

guidelines for the design on multifunctional catalysts for tan-

dem reaction systems.

Supporting Information

Supporting Information for this article can be found under

DOI: https://doi.org/10.1002/ceat.202200541.

Acknowledgment

This research work was funded by the Deutsche Forschungs-

gemeinschaft (DFG, German Research Foundation) under

Germany’s Excellence Strategy – EXC 2008 – 390540038 –

UniSysCat and Einstein Foundation Berlin. We acknowledge

DFG for funding the JEM-ARM300F2 – GZ: INST 131/789-1

FUGG, as well. Authors also express their gratitude to

Dr. Annette Trunschke and Dr. Nils Pfister from Fritz Haber

Institute of the Max Planck Society for conducting the Py-IR

measurements for us and Prof. Aleksander Gurlo (TU Berlin)

for the XRD measurements. Open access funding enabled and

organized by Projekt DEAL.

The authors have declared no conflict of interest.

Symbols used

[mL

–1

] total flow of reactants

cat

[g] mass of catalyst

nin

r[mol] number of moles of reactant in feed

nout

r[mol] number of moles of reactant in

product

p[mol] number of moles of any carbon-

containing product

nctotal

p[mol] total number of moles of all carbon-

containing product

[%] selectivity of any product

WHSV [mL

cat

–1

] weight hour space velocity

[%] conversion of any reactant

[%] yield of any product

Yactual

p[%] actual yield of the product

Ytheoretical

p[%] theoretical yield of the product

Abbreviations

DME dimethyl ether

FTIR Fourier transform infrared

MeOH methanol

Py-IR pyridine-adsorption infrared spectroscopy

RWGS reverse water-gas shift reaction

SEM scanning electron microscopy

TEM transmission electron microscopy

TGA thermogravimetric analysis

XRD X-ray diffractometry

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Chem. Eng. Technol. 2023,46, No. 6, 1163–1169 ª2023 The Authors. Chemical Engineering & Technology published by Wiley-VCH GmbH www.cet-journal.com

Research Article 1169