catalysts

Article
Multi-Scale Analysis of Integrated C 1 (CH 4 and CO 2 )
Utilization Catalytic Processes: Impacts of Catalysts
Characteristics up to Industrial-Scale Process
Flowsheeting, Part I: Experimental Analysis of
Catalytic Low-Pressure CO 2 to Methanol Conversion
Hamid Reza Godini 1 , 2 , *, Mohammadali Khadivi 1 , Mohammadreza Azadi 1 , Oliver Görke 3 ,
Seyed Mahdi Jazayeri 1 , Lukas Thum 4 , Reinhard Schomäcker 4 , Günter W ozny 1 and
Jens-Uwe Repke 1
1 Process Dynamics and Operation, T echnische Universität Berlin, Straße des 17. Juni 135, Sekr . KWT -9,
D-10623 Berlin, Germany; [email protected] (M.K.); [email protected] (M.A.);
[email protected] (S.M.J.); guenter [email protected] (G.W .); j.r [email protected] (J.-U.R.)
2 Inorganic Membrane and Membrane Reactors, Department of Chemical Engineering and Chemistry ,
Eindhoven University of T echnology (TU / e), Den Dolech 2, 5612AD Eindhoven, The Netherlands
3 Advanced Ceramic Materials, Institute of Materials Science and T echnology , T echnische Universität Berlin,
Hardenber gstr . 40, 10623 Berlin, Germany; oliver [email protected]
4 Department of Chemistry , T echnische Universität Berlin, Straße des 17. Juni 124, D-10623 Berlin, Germany;
[email protected] (L.T .); [email protected] (R.S.)
* Correspondence: h.r [email protected] ; T el.: + 31 40 2476195; Fax: + 49-30-31426915
Received: 28 March 2020; Accepted: 1 May 2020; Published: 4 May 2020
      
  

Abstract:
A multi-aspect analysis of low-pr essur e catalytic hydr ogenation of CO
2
for methanol
pr oduction is r eported in the first part (part I) of this paper . This includes an extensive
r eview of distinguished low-pr essur e catalytic CO
2
-hydr ogenation systems. Specifically , the
r esults of the conducted systematic experimental investigation on the impacts of synthesis and
micr o-scale characteristics of the selected Cu / ZnO / Al
2
O
3
model-catalysts on their activity and
stability ar e discussed. The performance of the investigated Cu / ZnO / Al
2
O
3
catalysts, synthesized
via di ff er ent methods, were tested under a tar geted range of operating conditions in this resear ch.
Specifically , the performances of these tested Cu / ZnO / Al
2
O
3
catalysts with r egar d to the impacts of
the main operating parameters, namely H
2
/ CO
2
ratio (at stoichiometric -3-, average -6- and high
-9- ratios), temperature (in the range of 160–260
◦
C) and the lower and upper values of physically
achievable gas hourly space velocity (GHSV) (corr esponding to 200 h
− 1
and 684 h
− 1
, r espectively),
wer e analyzed. It was found that the catalyst pr epar ed by the hydrolysis co-pr ecipitation method,
with a homogenously distributed copper content over its entir e surface, pr ovides a pr omising
methanol yield of 21% at a r eaction temperatur e of 200
◦
C, lowest tested GHSV , highest tested H
2
/ CO
2
ratio (9) and operating pr essur e (10 bar). This is in line with other promising r esults so far reported
for this catalytic system even in pilot-plant scale, highlighting its potential for large-scale methanol
pr oduction. T o analyze the findings in more details, the thermal-r eaction performance of the system,
specifically with r egar d to the impact of GHSV on the CO
2
-conversion and methanol selectivity , and
yield wer e experimentally investigated. Mor eover , the stability of the selected catalysts, as another
crucial factor for potential industrial operation of this system, was tested under continual long-term
operation for 150 h, the r eaction-reductive shifting-atmospher es and also even after intr oducing
oxygen to the catalyst surface followed by hydrogen r eduction-reaction tests. Only the latter state
was found to a ff ect the stable performance of the screened catalysts in this r esearch. In addition, the
r eported experimental r eactor performances have been analyzed in the light of equilibrium-based
calculated achievable performance of this r eaction system. In the performed multi-scale analysis
Catalysts 2020 , 10 , 505; doi:10.3390 / catal10050505 www .mdpi.com / journal / catalysts

Catalysts 2020 , 10 , 505 2 of 22
in this r esear ch, the requir ements for establishing a selective-stable catalytic performance based
on the catalyst- and r eactor-scale analyses have been identified. This will be combined with the
techno–economic performance analysis of the industrial-scale novel integrated process, utilizing the
selected catalyst in this r esear ch, in the form of an add-on catalytic system under 10 bar pressur e
and H
2
/ CO
2
ratio (3), for e ffi ciently r educing the overall CO
2
-emission fr om oxidative coupling of
methane r eactors, as r eported in the second part (part II) of this paper .
Keywords:
low-pr essur e methanol synthesis; catalytic CO
2
-hydr ogenation; catalyst synthesis;
equilibrium-based calculation; systematic performance analysis; add-on CO 2 -utilization pr ocess
1. Introduction
E ffi cient conversion of generated carbon dioxide in industries to valuable fuels or chemicals is
evolving fr om being a pr omising alternative to becoming a necessity due to the ever gr owing CO
2
emission rate as well as the cost and limitations of CO
2
storage [
1
–
3
]. Less undesired bypr oduct CO
2
will be generated in the first place if less energy and fuels ar e utilized [
4
], or if the selective performance
of the catalytic and non-catalytic base-pr ocess is impr oved.
Having consider ed the potential of supplying hydrogen either using r enewable sources or via
the r eforming of hydrocarbons, CO
2
-hydr ogenation to methanol becomes one of the promising
CO
2
-utilization concepts as extensively investigated elsewher e [
5
,
6
]. On the other hand, methanol is
an important pr oduct and an equally important intermediate chemical for producing mor e valuable
chemicals such as olefins, dimethyl-ether (DME), fuels and solvents in general. Industrial-scale
methanol pr oduction is mainly based on medium- to high-pr essur e catalytic conversion technologies
of syngas (CO / H 2 with and without CO 2 ) even utilizing waste r esour ces [ 7 ].
Guiding the conceptual design of any r etr ofitted CO
2
hydr ogenation pr ocess fr om an
industrial-operating point of view , it should be taken into consideration that the carbon dioxide
containing gas str eams often have a low delivery pressur e, being either the product str eam of
CO
2
-r emoval-stripping section or the pur ged and flue gas str eams. Ther efore, CO
2
hydr ogenation
to methanol even under atmospheric pressur e has been investigated, but the observed levels of CO
2
conversion and the methanol yield in that case are not high enough for the perspective of possible
industrial-application [
8
,
9
]. Moreover , industrial-scale catalytic reactors usually do not operate below
5–10 bar pr essur e, which is needed to secur e feed flow along the reactor and downstr eam units.
Incr easing the operating r eaction pr essur e up to medium and high levels (mor e than 50 bar) and
pr ocessing the pr essurized carbon dioxide and hydr ogen with the conventional methanol pr oduction
technologies ar e usually very expensive and operationally challenging because their CO
2
content is low .
This has been demonstrated to be a crucial conceptual design aspect also for combining the syngas
pr oduction and methanol synthesis [
10
]. Compr essing the carbon oxides and unreacted hydr ogen
separated in the downstr eam units and r ecycling them back in to the medium and high level pr essur e
methanol r eactor is also very costly . Specifically , some condensable corrosive components pr esent in the
feed str eams make the r equir ed compr ession and the requir ed equipment very expensive. Therefor e,
the r elatively low pr essur e (up to 20 bar) CO
2
-catalytic conversion pr ocesses ar e especially attractive
fr om a techno–economic point of view .
In the curr ent study , catalytic hydrogenation of carbon dioxide to methanol specifically ar ound
10 bar pr essur e, which can be easily integrated with many industrial processes pr oducing CO
2
, was
investigated. Specifically , minimizing the CO
2
generation in the upstr eam methane activation
pr ocess [
11
,
12
] and the e ffi cient conversion of CO
2
to methanol in the downstr eam add-on
CO
2
–hydr ogenation pr ocess will be secured for e ffi cient utilization of methane and carbon dioxide
(C 1 : CH 4 and CO 2 ) via integrated oxidative coupling of methane (OCM) pr ocesses to be discussed in
detail in part II of this paper .

Catalysts 2020 , 10 , 505 3 of 22
Instead of investigating and comparing the impacts of all catalyst materials suggested and tested
for this application, the model-catalysts synthesized in di ff erent r ecipes r esulting in distinguished
r eported performances have been selected to be further investigated in this r esear ch. In the performed
literatur e r eview as well as the conducted analysis in the current study , the main focus has been on the
Cu / ZnO catalyst family with or without extra support such as alumina, because this catalyst has been
extensively investigated as a model catalyst for this r eaction system. Moreover , ther e are some r eports
on the pilot-plant scale testing-operation of this type of catalyst with promising r esults highlighting its
industrial implementation pr ospect, for instance as demonstrated by Saito [ 13 ].
Distinguished types of Cu / ZnO catalysts wer e selected to be compr ehensively investigated in this
r esear ch in order to study the impacts of the catalysts’ characteristics, established by implementing
di ff er ent synthesis methods, and the targeted range of operating conditions on the CO
2
conversion,
methanol selectivity and stability of the catalyst. These catalysts wer e all characterized using X-ray
powder di ff raction (XRD), BET surface ar ea and the por e size distribution measur ements and a scanning
electr on micr oscope (SEM) equipped with ener gy-dispersive X-ray spectr oscopy (EDX). These tests
allowed comparing of the general characteristics of the selected catalysts and their possible impacts on
the observed catalytic performances.
After scr eening the catalysts and analyzing the impacts of operating conditions on the r eactor
performance, the observed selected catalytic performances wer e subjected to the model-based
techno–economic analysis of this catalytic technology as an add-on process in an industrial-scale
integrated OCM pr ocess, the results of which ar e reported in details in part II of this paper .
In this integrated process, the CO
2
and the r equired hydr ogen for converting it to methanol are
the undesir ed pr oduct of the OCM r eactor and the enriched pr oduct of r eforming the r emaining
unr eacted methane, r espectively .
2. State-of-the-Art and Literature Review
An overview of the r eported r esear ch activities on the catalytic hydr ogenation of CO
2
to methanol,
especially the studies performed at low-pressur e using Cu / ZnO model-catalyst family , has been
summarized in T able S1 as Supplementary Materials. The catalyst’s type and synthesis appr oach,
r eduction conditions and the r eactor performance indicators including the selectivity and yield towards
the main pr oducts as well as the r eactors’ operating conditions including the temperatur e, pr essur e,
feed composition, flow and dilution, etc. have been all r eported there. It is not easy and straightforwar d
to compar e the r eported cases ther e one by one and conclude in terms of quantitative contributions of
these factors. However , the observed trends can be qualitatively compar ed.
The analysis starts by r eviewing the r eported catalytic materials tested for this system [
14
]
including Au, Ag, Pd, Pt, Ga, etc. with the focus on Cu and Zn as the main catalytic components.
Having r eviewed the r eported data in literatur e, Cu, Zn, Pd and Cr ar e among the most common
components r ecommended to be used for synthesizing and promoting the CO
2
hydr ogenation catalysts
capable of minimizing pr oduction of the by-pr oducts as well as maximizing the methanol selectivity
and yield [
13
–
18
]. Cu / ZnO catalyst with 47Cu / 47ZnO / 6Al
2
O
3
(wt%) composition has been shown to
be capable of r esulting in a very pr omising methanol yield of up to 28% [
17
] even at the r elatively
low pr essur e of 13 bar , using feed ratio H
2
/ CO
2
= 3 (mol / mol). Fr om an industrial point of analysis,
this is a very encouraging r esult because such a high methanol yield can be theor etically expected to be
achieved in much higher operating pr essur es as seen in the r eports highlighted in r ows 3–9 in T able S1.
Ther efor e, this catalyst was selected to be further investigated her e in the study .
Contributions of the support materials (e.g., alumina) and the implemented synthesis methods
(e.g., hydr olysis method) as well as the catalysts’ compositions on impr oving the methanol selectivity
and yield can be highlighted for instance by reviewing the r eported performances of the non-supported
Cu-ZnO catalysts (e.g., item 3) in comparison to the performances of the supported catalysts or the
ones synthesized via other methods, while tested under similar operating conditions.

Catalysts 2020 , 10 , 505 4 of 22
C u / Z n O / A l
2
O
3
, C u / Z n O / Z r O
2
, C u / Z r O
2
, C u / Z n O / G a
2
O
3
c a ta l y st s a n d ot h e r mu l t i -c o m po n e nt - o xi d e s
supports and pr omotors have been widely investigated for this application [
18
,
19
]. Among them,
Al
2
O
3
supported Cu-ZnO catalyst has shown a pr omising performance in terms of selectivity towar ds
methanol for the practically r elevant high range of carbon dioxide conversion so that the methanol
yield of more than 20% can be secur ed [
16
–
18
]. All the studies r eporting mor e than 20% methanol
yield have been highlighted in T able S1.
Continue following the r eported studies listed in T able S1, after r eviewing the r eported
performances of the Cu / ZnO catalysts under low–medium pr essur e (listed r eports in r ows10–19),
the performance of the alumina supported catalysts (listed r eports in r ows 20–42) show the highest
methanol yield as expected to be achieved at the highest operating pressur e of 110 bar . Here, the
impacts of operating pr essur e, H
2
/ CO
2
ratio, synthesis method as well as the catalyst composition
(even some commer cial catalysts) can be tracked. Similarly , the listed reports 43–65 (all having alumina
in their supports) and 66–109 in T able S1 can be compared, through which the impacts of mixed
supports and pr omotors can be highlighted. Performances of the catalysts containing Cu, ZnO or
their combinations over di ff er ent supports can be analyzed by r eviewing the r eported data listed
in r ows 110–177 in T able S1. Similarly the performances of other catalysts’ active components and
supports under wide range of conditions can be compar ed by analyzing the r est of r eported data in
this table. Even though that the main focus in the curr ent r esear ch study is on the low-pr essur e CO
2
hydr ogenation over Cu-based catalysts, some selected comparable catalytic performances on higher
pr essur es (cover ed range of up to 110 bar) have been also listed in T able S1 to highlight the impact of
operating pr essur e.
Having r eviewed these r eported performances, it can be concluded that tar geting 20–30%
methanol yield via low-pr essure CO
2
-hydr ogenation would pr esent this technology as an attractive
alternative, even as a competing technology for medium-to-high pressur e syngas to methanol processes.
Inexpensive hydr ogen supply , for instance from r enewable resour ces, is a key aspect her e.
In addition, reviewing the r epresentative mechanisms and the kinetic data for this catalytic system
hints how selective r eactions paths can be intensified [
16
]. Especially the quantitative and qualitative
contributions of the Cu on the catalytic activity have been extensively investigated.
One should however be awar e of the di ffi culties and limitations of comparing the reported r esults
with each other while r eviewing the observed tr ends and the impacts of operating conditions in
pr evious studies. Therefor e, one of the objectives of current study is to select a set of catalysts and test
their performances under comparable conditions. This also enables the determining of the tar geted
set of conditions under which, this catalytic system can be utilized in the form of industrial-scale
low-pr essur e CO 2 hydr ogenation as added / integrated part of the upstr eam CO 2 -generating pr ocess.
Concluding this r eview , it should be emphasized that the selected catalysts to be the subject of
further experimental studies in this resear ch are not necessarily the most e ff ective catalysts known
for this system. Nevertheless, these selected catalysts repr esent di ff erent synthesis r ecipes of Cu / ZnO
catalysts and analyzing their performances will pr ovide valuable information to better understand
and e ffi ciently utilize the catalytic CO
2
-hydr ogenation as add-on low-pr essur e (e.g., 10 bar) methanol
pr oduction pr ocess. The main challenges to be addressed in this context ar e the low CO 2 -conversion,
methanol selectivity and the stability of the catalyst. In or der to addr ess these challenges and scr een the
catalysts and their performances with r egar d to the e ff ects of the catalysts characteristics and operating
conditions, the selected catalysts were characterized and tested in a standar d fixed-bed reactor under
the tar geted range of operating conditions explained in the next sections.
As a r esult of the performed literatur e r eview summarized in T able S1, beside the main targeted
catalyst synthesized via hydr olysis co-pr ecipitation method [
17
], the conventional-, carbonate-, and
gel-copr ecipitation methods r eported in r efer ences [
20
,
21
] wer e selected to be applied in this r esear ch
for synthesizing the Cu / ZnO catalyst. In addition, the citrate impregnation method as r eported in
r efer ences [
22
,
23
] was also applied to synthesize the catalyst and test it along with the above mentioned

Catalysts 2020 , 10 , 505 5 of 22
synthesized catalysts under comparable testing conditions. Some of the benchmark commercial
catalysts have been synthesized similarly [ 20 ].
On the other hand, a comprehensively-analyzed catalyst with known performance trajectory
should be also chosen and tested as a refer ence catalyst along with these selected catalysts in a
comparative study . Therefor e, a benchmark r esearch catalyst fr om Fritz-Haber Institute, which is
the r esults of several years of r esear ch and optimization [
24
–
26
], was synthesized and supplied by
Fritz-Haber Institute and tested in this r esear ch. The current study ther efore aims to systematically
r eview and complete the previously r eported catalyst studies, and to consolidate the possible conclusions
made based on analyzing the impacts of the catalysts’ characteristics as well as the operating conditions
on their activity , selectivity and stability . This further improves our understanding of the parameters
shaping the performance of the CO
2
-hydr ogenation catalysts. The generated added-value information
ther eby supports e ffi cient utilization of these catalysts in the tar geted range of operating conditions in
this r esear ch in the context of multi-scale analysis of the integrated catalytic pr ocess.
3. Results and Discussion
In this section, first the r esults of catalyst characterizations for catalysts MET1-MET7 defined in
Section 4 ) ar e pr esented and discussed.
3.1. Catalyst Characterizations
The BET r esults show that except for MET6 and MET7, the nitrogen adsorption-desorption
isotherms for all other catalysts (as typically observed for MET2), are type IV with the hyster esis loop,
which usually is an indication for a mesoporous str uctur e with slit-shaped por es. Detailed BET results
of these samples have not been reported her e for the sake of shortening the paper , but they have been
observed to r epr esent r elatively flat isotherms, which is an indication for the porosity of the samples
caused by a dense agglomeration of metal oxides. On the other side, type II isotherms wer e identified
for the samples MET6 and MET7, which generally is an indication for their macro por ous characteristic.
The specific surface ar eas and the por e size distributions of all samples, which have been calculated
by (Barr ett–Joyner –Halenda) BJH method, ar e shown in T able 1 . These should be considered while
analyzing the performance of the catalytic samples.
T able 1. Surface area (BET) and pore diameter of all catalyst samples.
Catalyst S BET (m 2 g − 1 ) Mean Pore Size (nm)
MET1 79 7
MET2 45 6
MET3 19 5.5
MET4 22 5.5
MET5 52 5
MET6 11 6.5
MET7 28 6
All the peaks indicated in this figur e ar e r elated to CuO and ZnO. All the samples have very
clear CuO (111) r eflection based on which the size of copper oxide crystallites could be calculated.
The br oadened peaks of MET1 and MET5 indicate a rather small crystallite size in the nanometer range.
This is also indicated thr ough their r elatively lar ger specific surface ar ea as r eported in T able 1 .
The r elation between the catalyst selectivity and the size of the Cu particles is known and has been
fully explained elsewher e [
22
]. More details on the implementation of di ff er ent methods to calculate
the copper oxide particle sizes and their impacts on the performance of the catalysts have been also
alr eady discussed extensively and can be found elsewhere [
21
,
23
]. However , it should be mentioned
that by using the XRD data for analyzing the surface characteristics of the samples, usually only
the lar ge particle-sizes ar e observed, while the impacts of the small particle sizes needs to be also

Catalysts 2020 , 10 , 505 6 of 22
taken into analysis. The presence of such small particles has already been shown and pr oven via the
pr eviously r eported characterization r esults of these catalysts. Figure 1 shows the XRD pattern for the
catalyst-samples MET 1–6.
Figure 1. X—ray di ff raction (XRD) patterns of the calcined catalysts.
SEM-EDX mapping of the MET1, MET2 and MET6 are pr esented in Figure 2 , thr ough which also
the distribution of the active components over the surface of the catalysts can be observed.
Figure 2. Cont.

Catalysts 2020 , 10 , 505 7 of 22
Figure 2.
Scanning electr on microscope-ener gy dispersive x-ray spectroscopy (SEM-EDX) mapping of
(
a
) MET1, (
b
) MET2, (
c
) MET6 (top left: SEM pictur e; top right: EDX visualization of copper distribution;
bottom left: EDX visualization of zinc distribution; bottom right: EDX visualization of aluminum
distribution).

Catalysts 2020 , 10 , 505 8 of 22
The EDX pictur es for MET1 and MET2 show a homogenous distribution of copper over the entire
surface of these catalyst samples established by the implemented synthesis methods. On the contrary ,
the dispersion of copper over the surface of MET6 and MET7 is not homogenous. It is known that the
better distribution of the metal species over the catalyst surface results in r elatively lower local metal
loading and incr eases the portion of the strong basic sites, which positively contribute to selective
methanol formation [
27
,
28
]. Having generally r eviewed all these and without going into details,
the main focus in this step is to explain the observed di ff er ent selectivity of the investigated catalysts.
For instance as will be discussed in next section, the observed r elatively higher methanol selectivity of
MET1 and MET2 (synthesized via co-pr ecipitation method) can be mainly attributed to the pr oven
r elatively homogenous distribution of the Cu and Zn species over the surface of these catalysts.
3.2. Catalytic Performance
T esting the catalytic performance of the samples in this resear ch was designed to be conducted in
four steps in or der to step-by-step scr een the catalysts’ activity , selectivity , and stability with the view
on their possible industrial-scale operation:
Step one: comparative performance analysis of all selected catalysts under the operating range
r ecommended in the original r efer ences.
Step two: comparative sensitivity analysis of the short-listed catalysts under the preferr ed range
of operating conditions (GHSV and temperatur e) determined after the first step (step one).
Step thr ee: sear ching for the best catalytic performance through a well-designed full-factorial
experimentation, r epr esented in T ables 2 and 3 , to analyze the impacts of the operating temperature
and H
2
/ CO
2
ratios on the performance of the final selected catalysts. Results are evaluated also in
r efer ence to the equilibrium-based calculated achievable performance of the CO
2
-hydr ogenation under
these conditions.
Step four: stability tests under (a) long-term hydrogenation r eaction, (b) sequence of
r educing-r eaction atmospher es and (c) befor e and after exposing the catalysts to oxygen.
In the first step (step one) of the experimentation, all identified Cu / ZnO catalysts (MET1–MET7)
wer e tested close to the range of operating conditions r ecommended for each one in the original
r efer ences, under which their best observed performance have been observed. In this manner and in
or der to compar e the performance of these catalysts, the GHSV was set to 684 (h
− 1
) while they wer e
tested in operating temperatur e range of 200–260
◦
C at H
2
/ CO
2
ratios of 3, 6 and 9. All experiments
wer e r epeated at least thr ee times and the standar d deviations wer e calculated to be below 5%.
For all tested catalysts, methanol and CO wer e observed to be the main pr oducts and only a trace
of CH 4 was detected via the GC. The observed r esults of this scr eening step ar e r eported in T able 2 .
Most of the r eported r esults in T able 2 follow the expected trends as also pr eviously recor ded for
these catalysts [
16
,
17
], confirming that an increase in the operating temperatur e causes an increase
in the CO
2
conversion and CO selectivity and ther eby a decr ease on the methanol selectivity . This
could be explained based on the impact of temperature on the rates of competing CO
2
hydr ogenation
r eaction to methanol and the r everse water gas shift (R WGS) r eaction. These reactions ar e r epresented
as followings:
CO 2 + 3H 2  CH 3 OH + H 2 O ∆ H 298 = − 49.5  kJ.mol − 1  (R-1)
CO 2 + H 2  CO + H 2 O ∆ H 298 = 41.2  kJ.mol − 1  (R-2)
Incr easing the temperatur e is mor e favorable for the R WGS reaction due to its endothermic
thermal characteristic. Ther efor e, for the next step of the experimentation, narr ower lower range of
temperatur e (200–230 ◦ C) was applied.

Catalysts 2020 , 10 , 505 9 of 22
T able 2.
Screening and comparative performance analysis of the catalysts (P = 10 bar and gas hourly
space velocity (GHSV) = 684 h − 1 ).
X CO 2 S MeOH Y MeOH
H 2 / CO 2 369369369
T( ◦ C) Coprecipitation method Cu / ZnO / Al 2 O 3 (MET1)
200 7.9 14.5 18.6 66.1 77.6 77.6 5.2 11.3 14.4
230 16.9 25.4 31 33.7 38.8 43.7 5.7 9.9 13.5
260 17.1 24.7 29.3 0 0.2 0 0.0 0.0 0.0
T( ◦ C) Hydr olysis method-Cu / ZnO / Al 2 O 3 (MET2)
200 9.6 14.7 18.4 74.3 79.4 82.5 7.1 11.7 15.2
230 15.7 21.4 25.9 43.2 52.7 52.8 6.8 11.3 13.7
260 19 23.8 - 13.4 17.4 - 2.5 4.1 -
T( ◦ C) Copreci pitation method-Cu / ZnO / Al 2 O 3 (MET3)
200 8 11.9 16.8 80.3 76.8 79.2 6.4 9.1 13.3
230 15.2 22.8 29.6 39 46.5 44.7 5.9 10.6 13.2
260 20.6 26.9 34.3 18.8 15.1 14.9 3.9 4.1 5.1
T( ◦ C) Gel copr ecipitation method-Cu / ZnO / Al 2 O 3 (MET4)
200 8.8 13.4 18.6 74 75.6 78.6 6.5 10.1 14.6
230 16.1 24.8 31.2 37.8 41.9 45.1 6.1 10.4 14.1
260 19.8 27.5 34.2 10.6 12.5 16.1 2.1 3.4 5.5
T( ◦ C) Carbonate coprecipitation method-Cu / ZnO / Al 2 O 3 (MET5)
200 11 18 24.7 66.2 73.8 73.3 7.3 13.3 18.1
230 17.2 24.8 33.6 33.6 42.2 38.9 5.8 10.5 13.1
260 20.8 30.2 37.5 11.6 13.3 16.8 2.4 4.0 6.3
T( ◦ C) Citric and impregnation method-Cu / ZnO (MET6)
200 3.3 4.9 4.4 97.4 98 100 3.2 4.8 4.4
230 4.4 7.1 15.3 78.8 80 63.2 3.5 5.7 9.7
260 14.5 20.6 22.6 21.8 26.4 14.6 3.2 5.4 3.3
T( ◦ C) Impregnation method-Cu / Y AG (MET7)
200 1.4 2.7 3.7 96.9 98.3 87.2 1.4 2.7 3.2
230 3.7 6.3 8.7 64.9 68.2 69.7 2.4 4.3 6.1
260 8.8 14.2 18.3 33.6 37.4 40.3 3.0 5.3 7.4
T able 3.
Comparative analysis of the selected catalysts under improved operating conditions
(GHSV = 200 h − 1 ).
Cat T( ◦ C) X CO 2 S MeOH Y MeOH
H 2 / CO 2 3 6 9 3 6 9 3 6 9
MET1 200 11 17.1 21.1 51.2 65.4 69.2 5.6 11.2 14.5
230 18.5 25.5 31.9 32.3 38.7 39.1 6 9.8 12.5
MET2 200 15.4 25 31.1 60.3 66.1 67.9 9.3 16.5 21.1
230 17.5 25.8 32.7 33.7 41 47.3 5.9 10.6 15.5
MET3 200 14.6 21.5 28.2 54.5 63.4 65.6 7.9 13.6 18.5
230 18.2 26 31.5 33.4 39.2 45.1 6.1 10.2 14.2
MET4 200 14.7 22.5 29.1 53.4 60.7 63.4 7.8 13.6 18.4
230 16.5 24.6 30.8 26.9 32.7 39.7 4.4 8 12.2
MET5 200 5.6 8.7 11.5 75.9 79.7 80.5 4.2 6.9 9.3
230 11 16.4 20.7 36.8 41 43 4 6.7 8.9
Having consider ed the stoichiometry of the reactants in these r eactions implies that CO
2
hydr ogenation to methanol is a mor e pr eferr ed path in case of using excess H
2
. In fact, increasing

Catalysts 2020 , 10 , 505 10 of 22
the hydr ogen content in the feed significantly incr eases the CO
2
conversion and methanol yield and
decr eases the CO selectivity . This has been explained also elsewhere via quantitative analyzing of the
r elative impact of the partial pr essur e of CO
2
and hydr ogen on the formation rate of methanol [
16
].
Interaction of the adsorbed species with the Cu and the oxide components, and in general the
contribution of the Cu particle size and Cu surface ar ea on the activity and selectivity of these catalysts
have been also discussed ther e [
16
] as well as in many other refer ences [
22
], suggesting that for the
given Cu content and oxide supports, the smaller size Cu particles cause higher Cu surface ar ea and
thus a higher yield towar ds methanol. This was observed for the catalyst samples MET3, MET4
and MET5.
After analyzing the experimental r esults of the first step, in or der to highlight the main practical
theme of this r esear ch, the catalysts showing a methanol yield of higher than 4% using H
2
/ CO
2
= 3
and temperatur e of 200
◦
C wer e selected to be further investigated in the next steps. It was found
that MET6 and MET7 catalysts synthesized by the impr egnation method wer e not active enough,
as their conversion was relatively low . The relatively lower Cu content (only 15%), heter ogeneous
dispersion of the Cu components on the surface of these catalysts, confirmed via several SEM-EDX
images of di ff er ent parts of the catalysts samples, and their r elatively less mesopor ous structur e are
believed to be the main r easons for r elatively low activity–selectivity of these catalysts. Therefor e
these catalysts (MET6 and MET7) wer e not further investigated and the analysis continued with the
r emaining catalysts.
Having fixed the operating pr essur e and temperatur e as well as the size of catalytic bed, less feed
and ther efor e lower GHSV could be applied in order to impr ove the conversion of carbon dioxide
in the next step of the experimentation. Ther efor e, comparative performance tests of the r emaining
catalysts (MET1–MET5) under the lowest range of feed flow wer e conducted. The tar geted feed flow
was ther efor e determined, based on the targeted feed composition and by considering the lowest range
of flow in the mass flow contr ollers, which ultimately led to establishing the actual GHSV of 200 h
− 1
inside the catalytic bed. The results of the performed comparative tests under this low GHSV (200 h
− 1
),
enabled analyzing the impacts of varying temperatur e and H
2
/ CO
2
ratios on the CO
2
conversion and
methanol selectivity of the investigated catalysts under such intense r eaction envir onment as r eported
in T able 3 .
Mainly due to the longer contact time established in this set of experiments, the CO
2
conversion
was incr eased. It should be taken into consideration however , that decreasing the gas hourly space
velocity may incr ease the intensity of r eaction, resulting in higher r eaction temperatur e along the
catalytic bed, and also because of that it may a ff ect the CO
2
conversion and r eaction performance
in general. Having consider ed such interactive e ff ects, reducing the GHSV does not always favor
methanol formation and the R WGS reaction can be intensified in this way and ther eby more CO can be
pr oduced. This indicates that not only the activity , but also the selectivity of the reaction system will be
a ff ected by varying the GHSV or the contact time. Therefor e, beside considering the involved catalytic
kinetic and mechanism aspects, thermal e ff ects of the reactor design should be also taken into analysis
for describing the performance of this r eaction system.
Having analyzed the performance of all catalysts in these series of experiments, MET1 and MET2
catalysts showed the highest methanol yield under H
2
/ CO
2
ratio of 3, which repr esents the lowest
stoichiometric ratio of expensive-to-inexpensive educts and ther efor e the most practically r elevant
conditions for this catalytic r eaction system in its industrial-scale operating perspective. Among the
synthesized catalysts, these two catalysts also have shown relatively the most homogenous distribution
of the surface–active components, as earlier discussed. In fact, the best r ecor ded methanol yield for
low-pr essur e CO
2
-hydr ogenation has been pr eviously r eported for catalyst MET2 [
17
]. As expected,
the r esear ch-benchmark MET1 catalyst has also performed very well. Therefor e, the observed results
for these catalysts in curr ent r esear ch r econfirm their pr omising potential.

Catalysts 2020 , 10 , 505 11 of 22
3.3. Equilibrium-Limited Achievable Performance
The finally selected catalysts MET1 and MET2 were tested in wide range of operating temperatur es
(160–260
◦
C) to track the impact of temperatur e on the observed methanol selectivity and CO
2
conversion, while comparing these values with their ultimate achievable values calculated based
on the thermodynamic equilibrium limitations and the Gibbs fr ee ener gy minimization of products’
formation. These values were calculated using Aspen Plus simulator 8.8 and SR-Polar equation of state
for a given set of r eaction conditions (T , P , H
2
/ CO
2
ratio). In such calculations, all involved r eactions
including the equilibrium CO
2
and CO hydr ogenations and water gas shift r eaction have been taken
into consideration.
As shown in Figur e 3 , by tracking the tr ends showing the e ff ect of operating temperatur e on the
CO
2
-conversion and methanol selectivity , it can be observed that for the GHSV of 200 h
− 1
, a pr essur e
of 9 bar and H
2
/ CO
2
ratio 3, the highest methanol yield can be secur ed by operating at temperatur e of
200 ◦ C.
Figure 3.
The impacts of reaction temperatur e on the CO
2
conversion (top) and MeOH
selectivity (bottom) for the ultimate selected catalysts and comparing their performances with the
thermodynamic-equilibrium limited achievable performance; Reaction conditions: H
2
/ CO
2
= 3, T =
160–260 ◦ C, P = 9 bar g, GHSV = 200 h − 1 .
Although achieving higher methanol selectivity and yield is theor etically pr edicted at lower
temperatur es fr om the sole equilibrium point of view , the temperatures lower than 200
◦
C cannot
activate the catalyst enough to secur e a significant CO
2
-conversion and ther eby high methanol yield.
The r eaction temperatur es of higher than 200
◦
C, thermodynamically favor unselective conversion.
Ther efor e, at higher temperatur es, the r esults show a decr ease in methanol yield mainly because of the
poor selectivity . This is the case for both the investigated catalysts, MET1 and MET2.

Catalysts 2020 , 10 , 505 12 of 22
Thermal performance of the r eactor and the temperatur e distribution along the catalytic bed should
be also taken into consideration while interpr eting these observed data, especially the ones which are
very close to the equilibrium performance. In this manner , the actual equilibrium data correspond to
the actual local r eaction temperatur e which might be higher or lower than the ones demonstrated in
Figur e 3 . This again indicates that the observed performances ar e not solely r eflecting the involved
catalytic kinetic and mechanisms but also they reflect the impacts of the thermal characteristic of
the r eactor system as an important extrinsic parameter . Thermal characteristics of reactor operation
will a ff ect the r eactions selectivity specially the contribution of R WGS reaction and the achievable
CO
2
-conversion and methanol yield as they are a function of actual local r eaction temperature along
the r eactor .
3.4. Stability T est and Analysis
In the last step of the experimentation, the stability of the selected catalysts was tested thr ough
two di ff er ent appr oaches:
Durability: the final selected catalysts (MET1 and MET2) wer e tested each for 150 h and no
significant r eduction of activity was observed. This has been the case even when the reaction atmosphere
and temperatur e have been r epeatedly switched between the hydrogenation r eaction, the inert and the
r educing atmospher es. Based on these observations it can be concluded that no significant deactivation
along the time and / or due to the temperatur e changes has been observed. Similar findings have been
also r eported by Kar elovic and Ruiza [ 22 ].
Reducibility: in order to test the r educibility of the catalysts, the performance of the fr eshly
r educed catalysts and the performance of the r educed catalysts after being exposed to oxygen were
tested and compar ed. In this manner , first the fr esh catalysts were r educed and tested for the
designed r efer ence experiments. Then, 100 mL / min air was introduced to the catalytic bed for 1 h.
The catalysts wer e then reduced with the same r ecipe and tested under the same set of designed
r efer ence experiments.
As it can be seen in Figur e 4 , there is a significant di ff er ence between the performances of the
fr eshly r educed catalysts in normal operation and the r educed catalysts after being exposed to oxygen
even for a short time. In order to explain this, it should be highlighted that the copper content of the
calcined catalyst in its body and also on its surface are in CuO (oxide) form in a dynamic interaction with
the ZnO and Al
2
O
3
components [
28
]. Reducing the catalyst converts the copper oxide to copper . Then,
under CO
2
-hydr ogenation r eaction, copper is converted back to its original CuO status. Therefor e,
the available copper on the catalyst surface and its interaction with other oxide species does not
change because of the temperatur e shock or after several cycles of hydrogenation-r eduction reactions.
This is the r eason why no significant deactivation during the long-term (150 h) experimentation has
been observed.
After intr oducing the oxygen to the catalytic bed, however , the copper content over the catalyst
surface is transformed to a di ff erent copper -zinc-alumina state, but this time there is no interaction
between some of these CuO components and the other separate oxides, namely ZnO and Al
2
O
3
.
Ther efor e, such formed material state cannot be easily reduced to its original form and a significant
deactivation was observed in the corr esponding experiments.
Figur e 4 shows the observed stability and r educibility of the catalysts MET1 and MET2.
These experiments have been conducted for H
2
/ CO
2
= 3, P = 9 bar g and T = 200
◦
C. The catalyst
activity-selectivity mechanism discussed above and elsewher e, highlight the crucial impact of interaction
of hydr ogen and carbon dioxide with reducible CuO and other oxide species. Fine distribution of the
copper over the catalyst body is an important factor in this r egar d.

Catalysts 2020 , 10 , 505 13 of 22
Figure 4.
S t ab il it y an d r ed u c i bi li ty o f (
a
) M E T1 , (
b
) M E T2 ; r ea ct io n co n d i ti on : H
2
/ C O
2
= 3 ,
T = 2 0 0– 24 5 ◦ C
,
P = 9 bar g, GHSV = 200 h − 1 .

Catalysts 2020 , 10 , 505 14 of 22
4. Material and Methods
4.1. Selected Catalysts and Synthesis Methods
The r ecipes and the chemicals used for synthesizing these selected types of Cu / ZnO catalysts ar e
r eported in this section and the list of utilized chemicals is pr ovided in T able 4 .
T able 4. List of chemicals used for the catalysts’ synthesis.
Used Basic Material CAS-Number Supplier
Copper II nitrate trihydrate 10031-43-3 Sigma-Aldrich (Darmstadt, Germany)
Zinc nitrate hexahydrate 10196-18-6 Sigma-Aldrich(T aufkirchen, Germany)
Citric acid anhydrous 77-92-9 Sigma-Aldrich (T aufkirchen, Germany)
Oxalic acid 144-62-7 Sigma-Aldrich (T aufkir chen, Germany)
Aluminium nitrate 7784-27-2 Sigma-Aldrich (Darmstadt, Germany)
Sodium carbonate 497-19-8 Sigma-Aldrich (T aufkir chen, Germany)
The details of the implemented synthesis appr oaches including all r equir ed information allowing
to r epr oduce these catalysts, ar e r eported in this section and can be also found in mor e details in the
pr ovided original r efer ences in each case.
4.1.1. Co-Precipitation Method for Pr eparation of Cu / ZnO / Al 2 O 3 Catalyst
This type of Cu / ZnO catalyst was pr epar ed by the copr ecipitation method described by
Behr ens et al. [ 26 ]
. Molar aqueous solutions of Cu(NO
3
)
2 ·
3H
2
O, Zn(NO
3
)
2 ·
6H
2
O and Al(NO
3
)
3
wer e pr epar ed. They wer e then mixed together to r esult in an overall metal composition of 68:29:3
(Cu:Zn:Al). An aqueous solution of one molar sodium carbonate was added to the mixture under
stirring at 65
◦
C to r each the pH of 6.5. In this manner , the resulted 2000 cc metal nitrate solution
was acidified with 15 cc concentrated HNO
3
and the carbonate solution as a basic pr ecipitating agent,
dosed in 1200 cc deionized water . The pr ecipitation was established at the same temperatur e for 30 min
and the color of the mixtur e was changed fr om gr een to bluish gr een. The pr ecipitate was centrifuged
and washed with deionized water and dried overnight at 110
◦
C and then calcined at 330
◦
C for 4 h
with the heating ramp of 2 ◦ C per minute. Here, the r esulted catalyst is referr ed to as MET1.
4.1.2. Hydrolysis Method for Pr eparation of Cu / ZnO / Al 2 O 3 Catalyst
The selected r ecipe for pr eparing Cu / ZnO / Al
2
O
3
catalyst using hydr olysis method has been
r eported by Xu et al. [
17
]. Cu(OH)
2
-Zn(OH)
2
pr ecipitate was pr epar ed by adding one molar Na
2
CO
3
to the pr emixed solution of Cu(NO
3
)
2
and Zn(NO
3
)
2
, pr epar ed by mixing a liter of one molar solution
of each of them, to reach the pH of 8 under stirring and ambient temperatur e. The precipitate was
then washed with deionized water . In parallel, a pr oportional amount of one molar solution of
Na
2
CO
3
was added to the pr epar ed 254 cc one molar solution of Al(NO
3
)
3
to r each the pH of 8
under stirring and ambient temperatur e. The pr ecipitate Al(OH)
3
(gel form) was also washed with
deionized water . Pr ecipitated Cu(OH)
2
-Zn(OH)
2
and Al(OH)
3
wer e then mixed in a mortar and stirr ed
to get a homogenous mixtur e to obtain the hydr oxide mixtur e containing Cu(OH)
2
, Zn(OH)
2
and
Al(OH)
3
in a molar ratio of 47, 47 and 6, respectively . The mixtur e was then dried at 120
◦
C for 12 h.
The dried pr oduct was calcined at 350
◦
C for 3 h followed by further calcination at 500
◦
C for 1 h with
the heating ramp of 3 ◦ C per minute. Here, the pr epared catalyst is r eferred to as MET2.
4.1.3. Coprecipitation Method for Pr eparation of Cu / ZnO / Al 2 O 3 Catalyst
The thir d type of Cu / ZnO / Al
2
O
3
catalyst was pr epar ed by the coprecipitation method described
by Jingfa et al. [
21
]. Aqueous solutions of Cu(NO
3
)
2 ·
3H
2
O, Zn(NO
3
)
2 ·
6H
2
O and Al(NO
3
)
3
each in 0.1
concentration wer e pr epar ed and mixed together to r each the metal composition of 45Cu:45Zn:10Al in
the pr ecipitate phase. An aqueous solution of 1 molar oxalic acid was added rapidly to the prepar ed

Catalysts 2020 , 10 , 505 15 of 22
mixed solution under stirring and ambient temperature. In this manner , a mixtur e solution of the
Cu(NO
3
)
2 ·
3H
2
O, Zn(NO
3
)
2 ·
6H
2
O, Al(NO
3
)
3
and oxalic acid was composed of 45 / 45 / 10 / 120 volume
units of each single solution, r espectively . The mixture was aged for 30 min and the pr ecipitate was
centrifuged and washed with deionized water . It was then dried overnight at 110
◦
C and calcined
step-wise at 150, 200, 250 and 300 ◦ C each for 1 h and finally at 360 ◦ C for 4 h with the heating rate of
2.5 ◦ C per minute. In this paper , this resulted catalyst is r eferred to as MET3.
4.1.4. Gel-Coprecipitation Method for Pr eparation of Cu / ZnO / Al 2 O 3 Catalyst
This catalyst was pr epar ed by the gel-copr ecipitation method, which is very similar to the typical
copr ecipitation method applied for synthesizing MET3. For gel-coprecipitation, however , ethanol was
used as the solvent in all corr esponding solutions [
21
]. Reduction of the volume of the resulted dried
particles indicates its gel characteristic. Here in this paper , the resulted catalyst is r eferr ed to as MET4.
4.1.5. Conventional Carbonate Coprecipitation Method for Pr eparation of Cu / ZnO / Al 2 O 3 Catalyst
This catalyst was pr epar ed by the conventional carbonate copr ecipitation method using the same
pr ecursors and solutions used for synthesizing MET3 and MET4 to reach the same pr ecipitate phase
composition [
21
]. For carbonate coprecipitation however , 100 CC of 0.1 mol sodium carbonate solution
was added and the pH was measur ed during this synthesis to be kept constant at 6.5–7. Her e in this
paper , the pr epar ed catalyst is r eferr ed to as MET5.
4.1.6. Citrate and Impregnation Method for Pr eparation of Cu / ZnO Catalyst
As the first step of synthesizing this catalyst, ZnO particles were pr epar ed by citrate method.
In or der to do so, the pr oper amount of 1 molar citric acid solution was added dr opwise to a 0.5 mol
solution of Zn(NO
3
)
2 ·
6H
2
O while it was moderately stirring [
23
]. The solution was left being stirred at
r oom temperatur e overnight, and after that it was transferr ed to a r otary evaporator . The evaporator
was working under 40
◦
C and a partially vacuum atmosphere. The solution was transformed to a mor e
viscous fluid which was further dried in a vacuum oven at 80
◦
C. These conditions caused a significant
incr ease in the volume of the solidified mixture befor e it was dried for more than 48 h. The sample was
then calcined at 350 ◦ C with the heating rate of 2.5 ◦ C per minute.
In or der to pr epar e the solution for impr egnation, a proportional amount of Cu(NO
3
)
2 ·
3H
2
O was
dissolved in deionized water and the alr eady synthesized ZnO was added to it while it was stirring at
r oom temperatur e to establish 15 wt % Cu in the final catalyst. The mixture was kept being stirr ed for
5 h and the water was r emoved fr om it using a rotary evaporator under vacuum and temperatur e of
40
◦
C. The dried r esulted particles wer e calcined at 350
◦
C for 4 h with the heating rate of 2.5
◦
C per
minute to achieve the final powder catalyst her e in this paper r eferr ed to as MET6.
The same pr ocedur e was used to impr egnate the pur chased powder of Yttrium aluminium garnet
(Y AG, Y 3 Al 5 O 12 ) and the prepar ed catalyst in this manner is referr ed to as MET7.
4.2. Catalyst Characterization
Reviewing the physical–chemical pr operties and the performance of the wide range of the
investigated catalysts (Cu-ZnO / Al
2
O
3
) in this r esear ch enables one to highlight the striking featur es of
the desir ed catalysts for CO
2
hydr ogenation to methanol. For instance, tracking the phase compositions
and the dispersion of Cu can pr ovide valuable information in this r egar d. Based on the performed
characterizations, Cu + and ZnO in the solid solution of the investigated catalyst MET2 have been
suggested to be the active centers under the investigated r eaction conditions [
17
]. However , it should
be emphasized that in this resear ch it was not intended to find any structure–activity r elationship or
draw any fundamental conclusion with r egar d to the understanding of the catalytic mechanism or
behavioural pattern of the studied systems, as it was out of the scope of the curr ent paper and would
have r equir ed extra characterization and tests and even studying other types of distinguished catalysts
synthesized by other types of support or active components [ 29 – 31 ].

Catalysts 2020 , 10 , 505 16 of 22
It should be mentioned that most of the investigated catalysts in this resear ch have been also
characterized in pr evious studies. For instance, general characterization techniques such as XRD,
TEM and thermal analysis (TG / DTG) have been previously [
21
] applied for the most of the selected
catalysts in this r esear ch. In or der to conduct a comparative study , the characteristics of the selected
catalyst samples, using XRD, BET and SEM characterizations, have been analysed and r eported and
conceptually compar ed with each other in the current r esearch. These r esults can be also dir ectly
compar ed with the r eported characteristics of similar catalytic systems in literature. This can be
generalized for the other measur ed / r eported characteristics, except for the few parameters such as
por e dimeters of the MET3 and MET4 which have been measur ed to be 50% smaller than their original
r eported values of 10–14 nm [
21
]. The results of these basic characterization techniques (i.e., XRD,
BET , SEM) can be also used for studying the impacts of the structur e and the morphology of the
catalysts as well as their involved crystalline phases on their catalytic performances. Moreover , the
r esults of the physical–chemical characterization methods which ar e sensitive enough to identify the
characteristics of the external surface, for instance for detecting the copper species befor e and after the
r eaction using XPS analysis, and the ones determining the reactivity of the solids (chemisorption, IR)
for some of these catalysts ar e available elsewher e [ 17 , 23 ].
Calcined catalysts samples wer e characterized by applying X-ray powder di ff raction (XRD)
analysis technique in the 10
≤
2
θ ≤
90 range on a Brucker D8 advance di ff ractometer via Co-Ko
radiation (using 1.541 Å, 40 kV , 35 mA, Berlin, Germany) at a scanning rate of 2K / min to identify the
crystalline phases. The BET surface area and the por e size distribution of all catalysts were calculated
based on the N
2
adsorption-desorption measur ements at
−
196
◦
C. The pr oper amount of catalyst
samples (corr esponding to 10–100 m
2
surface) wer e degassed under vacuum atmospher e. During the
pr e-tr eatment period, each sample was heated under a step-wise rising temperature pr ofile starting
with 80
◦
C, followed by being heated under 120 and 180
◦
C each for one hour and finally at 220
◦
C for
4 mor e hours with the heating rate of 3
◦
C per minute in order to clean the surface and por es of the
sample. After degassing the BELpr ep-V ACII preparation equipment (Berlin, Germany), the sample
was weighed again to quantify the dried mass of the materials. The cell was then placed in the
BELSORP-mini II measuring equipment (Berlin, Germany) and then into a Dewar vessel of liquid
nitr ogen. N
2
was stepwise intr oduced to the samples until r eaching the ambient pr essur e and then was
step-wise vacuumed in or der to measur e the N
2
adsorption and desorption behaviour of the samples.
In or der to study the surface morphology and the homogeneity of the samples, a Zeiss Gemini
Leo 1530 Field Emission Scanning Electr on Micr oscope (FESEM, Berlin, Germany) was used, which is
equipped with ener gy-dispersive X-ray spectr oscopy (EDX).
5. Experimentation for Catalyst T esting
The specification of the experimental setup and the design of experiments as well as the pr ocedur e
of testing the catalysts ar e r eported in this section.
Experimental Setup
The schematic of the experimental setup utilized for testing the catalyst is depicted in Figur e 5 .
A pictur e of the setup and the visualization interface of the contr ol system can also be seen ther e.
In this setup, the operating temperature, pressur e and feed composition are r espectively controlled
via an electrical tube furnace, back pressur e regulator and several mass flow contr ollers. The set-points
and the measur ed values of the feed flow rates, operating pr essur es and the applied temperatur e of the
thr ee electrical elements / zones in the furnace are set and monitor ed online through the user -interface
PCS-ILS contr ol system as shown in Figur e 5 .

Catalysts 2020 , 10 , 505 17 of 22
Figure 5.
Experimental setup: (
a: left
) picture of the r eactor , (
b: middle
) schematic flow diagram,
( c: right-top ) monitor-contr ol system.
The temperatur e could be set either as the temperature of the furnace (local-mode) or the
temperatur e inside the catalytic bed (cascade-mode), which was the case for most of the experiments
in this r esear ch. The thermocouple located inside the tube was in touch with the top of the catalytic
bed inside the vertical reactor , where feed gas enters fr om the top. The volumetric feed flow rates of
nitr ogen, carbon dioxide and hydrogen wer e controlled using Br onkhorst F-series mass flow controllers
(MFC) with a 0.8% pr ecision range ar ound the measur ed values. The operating pressur e inside the
r eactors was contr olled using an electr onic back-pr essur e contr oller .
For each experiment, a 20 cm long catalytic bed, made of 200–400 micr on sieved powder of
each catalyst, was located inside a stainless steel fixed-bed reactor with an inner diameter of 8 mm.
The bed was positioned in the height of the middle electrical heating element / zone. In order to fix the
location of the catalytic bed, the bottom part of the r eactor was filled with inert packing and quartz
wool. For calculating the requir ed amount of inert packing and the catalyst for each run and in or der
to ensur e pr oper filling the r eactor , the locations of the heating zones as well as the values of inner
diameter of the r eactor and the density of catalysts wer e taken into calculation.
After placing the r eactor inside the furnace and fixing the connections, the feed-mixture composed
of the desir ed portions of nitr ogen (or air solely used for specific type of stability tests), hydr ogen and
CO
2
wer e tuned for establishing the targeted r educing, purging and r eaction atmospheres. The requir ed
overall gas feed flow rate to be intr oduced to the r eactor for r epr esenting the tar geted actual gas hourly
space velocity (GHSV) inside the catalytic bed is calculated using Equation (1):
GHSV = 60 × Q × ( 273.15 + T cb )
V cb × P × ( 273.15 + T a ) (1)

Catalysts 2020 , 10 , 505 18 of 22
Her e, V
cb
is the volume of the catalytic bed, P is the pressur e in bar , T
cb
is the temperatur e inside
the catalytic bed and Q is the set feed flow rate by the mass flow contr ollers, calibrated for standard
r efer ence pr essur e and temperatur e (STP: P = 1 bar and T a = 0 ◦ C).
In the r eduction period, hydr ogen flow of 80 mL / min was used to r educe the catalyst under
ambient pr essur e and temperature of 300
◦
C for six hours. The r eactor was then cooled down to the
tar geted r eaction temperatur e for each experiment using pr oper flow rate of nitr ogen while pr essur e
was set to 10 bar . The wall-temperature of the r eactors’ outlet pipe-lines to the GC were kept at
170
◦
C to pr event condensation, which a ff ect the mass balance and can cause a fluctuation of the
operating pr essur e.
After r eaching ar ound the desir ed temperatur e range in the catalytic bed, the gas feed was
intr oduced to the r eactor and a period of approximately 30 min was consider ed as the stabilization
time. Then, the gas stream was intr oduced to the gas chromatograph (GC) analyzer . The gases wer e
analyzed by a Schimadzu 2014 A TF GC (Berlin, Germany) equipped with a methanizer , a thermal
conductivity detector (TCD), a flame ionization detector (FID) and two packed columns (HayeSep Q
and Molecular sieve 13X) used for analyzing H
2
, N
2
, CO
2
, CO, CH
4
and methanol in all gas str eams.
Ar gon was used as the carrier gas for both columns.
The setup is equipped with a multi-position valve to send the desired str eam to the gas
chr omatograph. This was especially instrumental in conducting the experiments while two OCM
and methanol r eactors ran consecutively as coupled-r eactors configurations, reported in part II of
this paper .
In or der to calculate the CO
2
conversion, pr oducts’ selectivity and yield, a small amount of
nitr ogen was fed as an inert r efer ence species, which does not participate in any r eaction and therefor e
the same amount of nitr ogen will appear in the pr oduct str eam. Knowing its molar inlet flow
(x in × Q in )
and by measuring the outlet nitr ogen mole fraction (x
out
) with GC, the molar flow rate of the total
pr oduct str eam can be calculated using the following equation.
Q out =
x N in
2
× Q in
x N out
2
(2)
The value of the calculated outlet flow has been r echecked time to time using a standar d calibrated
flowmeter under the outlet temperatur e and pr essur e. The molar flow rate of each component in the
pr oduct steam is then calculated by multiplying the above-calculated total outlet flow rate with the
mole fraction of each component measur ed by the GC. Based on these data, the CO
2
conversion (X),
methanol selectivity and yield (S and Y) could be calculated as following:
Conversion of carbon dioxide (X) is calculated using Equation (3).
X ( % ) =
F CO in
2
− F CO out
2
F CO in
2
(3)
Selectivity towar ds methanol (S) is calculated using Equation (4).
S ( % ) = F CH 3 OH out
F CO in
2
− F CO out
2
(4)
Methanol yield (Y) is calculated using Equation (5).
Y ( % ) = F CH 3 OH out
F CO in
2
(5)
Having consider ed the pr ecision of the measur ements and the contr ol devices, the observed
selectivity , conversion and yield in average have
±
10% mar gin of err or in r efer ence to their r eported

Catalysts 2020 , 10 , 505 19 of 22
values in this manuscript. This means that for instance, the actual value of CH
3
OH-Y ield lies in the
range of 0.9 × CH 3 OH-Y ield r eported < CH 3 OH-Y ield actual < 1.1 × CH 3 OH-Y ield r eported .
6. Conclusions
A compr ehensive r eview analysis on the low-pr essur e performances of the selected
CO
2
-hydr ogenation catalysts has been provided in this paper . The performed experimentation
and testing of the selected CO
2
-hydr ogenation catalysts showed that some r eceipts of Cu / ZnO / Al
2
O
3
catalysts exhibit a stable active catalytic performance pr omising for industrial-scale operation. It was
observed that the coprecipitati on catalyst synthesis approach, which is known for its potential of
establishing a homogenous distribution of active components over the whole catalytic body , provided
the highest methanol selectivity and yield. All tested catalysts have shown their best performance, in
term of methanol yield, for the highest applied H
2
/ CO
2
ratio and lowest GHSV . A methanol yield of
21% using the H
2
/ CO
2
ratio of 9 and the temperatur e of 200
◦
C under 10 bar pr essur e was the best
catalytic performance achieved.
After testing the stability of the selected catalysts under long-term operation as well as the
r eaction-r educing shifting-atmospheres and even after intr oducing oxygen to the catalyst surface
followed by reducing-r eaction tests, it was found that exposing the catalyst with oxygen can significantly
a ff ect the stability of some of the catalysts. Moreover , it was found that the performance of some of the
selected catalytic systems, especially in the average range of temperatur es, can come very close to the
ultimate achievable performance pr edicted by considering the thermodynamic-equilibrium limitations.
Thermal characteristics of the r eactor operation wer e shown to also be important in determining the
ultimate achievable yield of methanol in this system.
The experimental analysis conducted in this r esear ch (part I) will support the novel e ffi cient
implementation strategy for industrial-scale utilization of the CO
2
-hydr ogenation catalytic system
(r epr esented by the selected catalyst MET2) as an add-on pr ocess to be integrated with the case-study
pr ocess oxidative coupling of methane (OCM). This will be analyzed in part II of this manuscript.
Supplementary Materials:
The following are available online at http: // www .mdpi.com / 2073- 4344 / 10 / 5 / 505 / s1 ,
T able S1: A selective overview of the reported performances for catalytic hydr ogenation of CO
2
to methanol
(Specially the studies performed at low-medium pressur e).
Author Contributions:
Conceptualization, H.R.G. and S.M.J.; Formal analysis, L.T . and O.G.; Investigation,
H.R.G., M.K., M.A. and O.G.; Methodology , H.R.G., M.K. and S.M.J.; Resources, R.S., G.W . and J.-U.R.; Software,
M.A.; Supervision, H.R.G., O.G., S.M.J., L.T ., R.S., G.W . and J.-U.R.; V isualization, O.G.; W riting—original draft,
H.R.G. and M.A.; W riting—review & editing, H.R.G., O.G., G.W . and J.-U.R. All authors have read and agr eed to
the published version of the manuscript.
Funding: This resear ch received no external funding.
Acknowledgments:
Catalyst sample preparation by Elias Fr eie (Department of Inorganic Chemistry , Fritz
Haber Institute of the Max Planck Society , Berlin, Germany) is sincerely acknowledged. The authors
acknowledge the financial support from the Cluster of Excellence UniCat “Unifying Concepts in
Catalysis” coordinated by the T echnische Universität Berlin and funded by the German Research
Foundation—Deutsche Forschungsgemeinschaft.
Conflicts of Interest: The authors declare no conflict of inter est.

Catalysts 2020 , 10 , 505 20 of 22
Abbreviations
BET Measuring the specific surface based on
Brunauer –Emmett–T eller theory
BJH Barrett–Joyner –Halenda
Cat Catalyst
CCSU Carbon capture separation utilization
Dilu. Dilution
DME Dimethyl ether (Methoxymethane)
EDX Energy-dispersive X-ray spectr oscopy
FESEM Field emission scanning electr on micr oscopy
Gas Gas phase
GC Gas chromatography
In Inlet stream
MeOH (CH3OH) Methanol
MET Methanol catalysts prepar ed with di ff erent methods
Out Outlet stream
PC-ILS Pr ocess contr ol system—integrated lab solution
R WGS Reverse water gas shift
UniCat “Unifying Concepts in Catalysis” (a resear ch group in Berlin)
XRD X-ray di ff raction
Nomenclature
A Ambient -
Cb catalytic bed -
D Diameter or equivalent diameter nm
F Molar flow rate mol / min
GHSV Gas hourly space velocity L / h
P Pressur e bar
Q T otal flow rate Nml / min
S
(Selectivity)
Portion of the whole consumed carbon
dioxide which appears in the (desired)
products
-
T T emperature ◦ C
V V olume ml
X
(CO2 Conversion)
Portion of the inlet carbon dioxide
converted to the desired and undesir ed
products
-
X Mole fraction -
Y (Y ield)
Amount of the converted carbon dioxide
appears in each product per whole total
amount of the inlet carbon dioxide
-
∆ HR Reaction enthalpy kJ / mol
P Density kg / m 3
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©
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