Recycl. Catal. 2015; 2: 78–86
Mini-review
Open Access
Malte Behrens*
Chemical hydrogen storage by methanol:
Challenges for the catalytic methanol synthesis
from CO2
DOI 10.1515/recat-2015-0009
Received May 26, 2015; accepted July 29, 2015
Abstract: Methanol is a very promising chemical hydrogen
carrier molecule. The well-established industrial methanol
synthesis process is a reference case for the desired
sustainable synthesis from CO2 and “green” hydrogen.
The catalyst employed in this process has been studied
intensively and recent results demonstrate significant
progress in the understanding of methanol synthesis from
CO2, which are surveyed in this contribution. The next
step is the employment of this new knowledge basis for
the development of new and better catalytic materials.
The major challenges are related to synthetic inorganic
chemistry for an increased Cu dispersion, defect generation
in metallic nanoparticles for a higher concentration of
active sites, and surface/interface design between the two
major catalyst components Cu and ZnO, which seems to
be controlled to some extent by the presence of suitable
promoters in the ZnO lattice.
Keywords: Methanol synthesis, CO2 conversion, Cu/ZnO
catalyst, heterogeneous catalysis, water gas shift
1 Introduction
Hydrogen is an obvious primary chemical energy carrier
and the “hydrogen economy” has been discussed for
many years as a key for the dream of emission-free
energy technology [1]. Hydrogen has the potential to be
a non-fossil energy carrier for the desired reduction of
greenhouse gas emissions [2, 3]. It can be produced from
water by electrolysis using renewable energy or it must be
*Corresponding author: Malte Behrens: Universität Duisburg-Essen,
Fakultät für Chemie und CENIDE, Universitätsstr. 7, 45141 Essen,
Germany, E-mail: [email protected]
made available through efficient photocatalytic processes
that still need to be developed and optimized [4-6]. The
electrolysis technology makes hydrogen an attractive
energy storage molecule if surplus electric energy from
wind or solar power is used for its generation, and if the
electrolysis can be operated in a dynamic, but robust
manner to compensate for the volatility of these energy
sources. However, also this powerful energy storage
molecule must be stored itself to enable the hydrogen
economy and direct hydrogen storage remains connected
to a number of challenges [7].
Among the various hydrogen storage strategies,
chemical conversion into small hydrogenated molecules
is a very attractive option [8]. Hydrogenated carbon-based
molecules have the advantage over pure hydrogen that the
experience and the infrastructure for their distribution,
end-user handling and storage are readily available in
form of pipelines, road trucks and a grid of filling stations
[9, 10]. In addition, as these molecules often are not only
interesting synthetic fuels, but also base chemicals for
the chemical industry, an interface of the energy sector
and the chemical industry would be available that allows
various downstream options with great flexibility. For
on-board energy applications combustion engines can be
used as well as alternative technology such as fuel cells
that operate either directly with the small molecule or
after on-board reforming with the re-liberated hydrogen
[11].
The price to pay for these advantages is that such
indirect hydrogen technology is not CO2-free any longer
and requires the simultaneous development of carbon
capture [12] and utilization (CCU) strategies to convert
CO2. Small C1 molecules like methane or methanol can
be synthesized by hydrogenation of COx. For methanol
synthesis a large-scale industrial process is already in
operation [13, 14]. This so-called low-pressure methanol
synthesis process utilizes carbon, (oxygen) and hydrogen
from fossil sources in the form of synthesis gas (CO/CO2/
H2). Syngas chemistry is a well-established and important
© 2015 Malte Behrens licensee De Gruyter Open.
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License.
Chemical hydrogen storage by methanol: Challenges for the catalytic methanol synthesis from CO2
technology that includes its production, conditioning and
conversion into alcohols or alkanes. Herein, I will briefly
describe the industrial methanol synthesis process,
identify challenges that are related to the transfer of this
existing technology away from the utilization of coal- or
natural gas-derived syngas toward mixtures of “green”
hydrogen and captured anthropogenic CO2 and highlight
some recent results that contribute to the understanding
of the methanol synthesis catalyst. This paper contains a
brief review of the recent results from our lab and other
groups and will focus on the industrial catalyst Cu/ZnO
and the challenges for catalyst development.
2 The industrial process and its scale
Industrial methanol is a technologically mature starting
point for energy applications. In 2013, worldwide
production of methanol was more than 60 million
tons [15]. It is used as solvent and mainly consumed
in downstream processes by conversion into various
chemical intermediates such as formaldehyde [16]. It can
be dehydrated into dimethylether and with the methanolto-gasoline technology, a process exists that allows
converting methanol into fuel over zeolite catalysts. Direct
fuel and additive usage accounts for approximately 15% of
the demand but is expected to rise.
The low-pressure methanol synthesis process was
introduced by ICI in the 1960s and operates at 240 °C–260
°C and 50–100 bar [17] employing a promoted Cu/ZnO
catalyst. “Low pressure” refers to the previous BASF
process that used ZnO/Cr2O3 catalysts at higher pressure
and temperature [13]. Interestingly, the replacement of
this old process was not triggered by progress in catalyst
development – copper catalysts were known to be active
in methanol synthesis since long – but rather due to
a breakthrough in feedstock conditioning technology
resulting in particular in a lower sulphur level, that allowed
to make use of these catalysts without poisoning issues
[18]. The process yields methanol with a >99% selectivity
and 75% energy efficiency [9]. It is one of mankind’s major
chemical processes. It has to be mentioned, however, that
the chemical industry is clearly smaller than the energy
industry and accounts for the consumption of only ca.
3% of energy resources [19]. The amount of produced
methanol today represents only ca. 0.01% of the gasoline
production. Thus, even a process like methanol synthesis
needs further strong up-scaling if it is to contribute to
fuel production in addition to base chemical synthesis.
However, the scale of methanol synthesis is nearly
equivalent to the total biodiesel and bioethanol production
79
and this starting point is clearly more advanced compared
many other emerging technologies that are discussed in
the context of solar fuel production.
The feedstock for methanol synthesis is synthesis gas
derived from the reforming of mainly natural gas, other
hydrocarbons or coal gasification [9]. A typical syngas
composition for methanol synthesis is characterized
by a modulus value M = ([H2]-[CO2])/([CO]+[CO2]) of
approximately 2 and contains hydrogen and both CO
and CO2. The production and purification of synthesis
gas accounts for 50%–80% of the total cost of methanol
production [9, 16] indicating that the market price of
methanol is not dominated by the actual COx hydrogenation
step. However, alternative synthesis gas generation
from mixing of captured anthropogenic CO2 and H2 from
electrolysis is clearly more costly at the moment.
The reaction of carbon oxides to methanol is described by
the following three equilibrium equations:
CO2 + 3H2  CH3OH + H2O∆H0=-50 kJ/mol (1)
CO + 2H2  CH3OH∆H0 = -91 kJ/mol (2)
CO2 + H2  CO + H2O∆H0 = 41 kJ/mol (3)
Methanol synthesis from CO2 (1) and CO (2) is mildly
exothermic and proceeds under volumetric contraction.
The slightly endothermic reverse water-gas shift (rWGS)
reaction (3) occurs as an important side reaction to
methanol synthesis.
The question of the carbon precursor for methanol
synthesis from a CO/CO2 mixture has been debated, but
has been solved already many years ago using isotope
labelling experiments [20, 21]. These have shown that at
sufficiently high space velocities, where the label is not
scrambled by the consecutive water gas shift equilibrium,
CO2 is the source for methanol over the industrial Cu/
ZnO catalyst. Even for a much lower CO2 concentration
compared to CO, reaction (1) was found to proceed much
faster than reaction (2) [20]. This result has been recently
confirmed also for Zinc-free catalysts [22] and renders
methanol synthesis a large scale CO2 conversion process
suitable for CCU applications. Thus, the question remains
if this technology can be readily applied in CO2/H2 feed
gas. What is the role of CO?
3 CO2 Hydrogenation versus
Industrial Methanol Synthesis
from Syngas
According to thermodynamics, high pressure and low
temperature favor methanol synthesis, but in addition to
80 M. Behrens
pressure and temperature also the feed gas composition
will affect the thermodynamically possible methanol yield
[23]. The addition of CO to the feed mixture has a positive
effect on the equilibrium yield of methanol, as shown in
Figure 1.
In the light of the equilibrium limitations due to
the CO2 content in the feed and the exothermicity of the
reaction, the need for operation at lower temperature
becomes apparent. This requires a more active CO2
hydrogenation catalyst. Even at temperatures associated
with the low-pressure process, the equilibrium constant
of reaction (1) lies between 10–5 and 10–6, allowing for
a single-pass methanol yield of 15%–25% and thus
necessitating the implementation of costly recycling loops
to achieve similar yields as in the syngas conversion in the
presence of CO [23].
the true intrinsic forward rate of CO2 hydrogenation under
differential conditions, a yield as low as 0.3% should
not be exceeded. Under these conditions, the methanol
formation rate was found to scale linearly with the CO2
concentration in CO2/CO/H2 mixtures supporting the
results of the aforementioned isotope labelling studies.
Under integral conversions however, the two effects
of increasing rate and stronger product inhibition
compete with increasing CO2 concentration explaining
the maximum in rate at low CO2 content that is usually
observed [26-28].
Thus, while the WGS equilibrium helps in the syngas
conversion to keep the surface of the Cu/ZnO catalyst
clean of adsorbed water, it poses another problem in
the absence of CO. The WGS reaction now will change
direction and the rWGS comes into play. As this reaction
produces more water and consumes valuable hydrogen
that cannot be used for methanol formation, it is an
undesired competitive reaction and CO2 hydrogenation
suffers from CO selectivity due to rWGS.
While indeed CO2 hydrogenation is the dominant
reaction in industrial methanol synthesis, the role of CO
is highly important for the efficiency of the process as it
has a beneficial thermodynamic effect on the methanol
yield and drives the WGS in the forward direction which
suppresses product inhibition.
4 Goals for Catalyst Development
Figure 1: Equilibrium yield of methanol at 50 bar and 250 °C from 3:1
(CO,CO2)/H2 mixtures as a function of CO2:CO ratio [23]. With increasing CO2 content in the syngas, the methanol yield goes down.
A second problem is kinetic in nature and related to
the occurrence of product inhibition by water. In the
CO2 hydrogenation (1), water is a coupled by-product of
methanol formation. The water molecules tend to stick to
the catalyst’s surface and to block the methanol forming
surface sites [24]. Only, in the presence of CO does the
much faster forward shift reaction clean the surface by
scavenging surface water [25] and converting it into CO2,
thus generating new precursor molecules for methanol
synthesis. Hence, macroscopically the industrial methanol
synthesis is the sum of CO2 hydrogenation (1) and forward
shift (reverse 3) yielding formally reaction (2), although
microscopically this reaction does hardly proceed directly.
The problem of water inhibition in CO-free CO2/H2 has
been nicely shown in the space velocity variation study of
Sahibzada et al. [24]. The authors conclude that to measure
Low-pressure methanol synthesis relies almost exclusively
on catalysts based on copper, zinc oxide, and alumina
[13], which acts as a promoter in this catalyst rather
than as a classical support [29]. Industrial catalysts
typically contain 50–70 mol% CuO, 20–50% ZnO, and
5–20% Al2O3 and can be purchased in form of pellets
with approximately centimetre dimensions. Instead of
alumina, chromium oxide and rare earth oxides have
also been used as promoters. The catalyst is reduced
with dilute hydrogen in the reactor to its active form,
metallic copper crystallites interspersed by a ZnO/Al2O3
or ZnO:Al matrix with a typical lifetime of about 2 years
[23]. Although alternative catalyst materials like the Ni-Ga
system [30] are also researched, most of the studies on
methanol synthesis focus on copper based catalysts close
to the industrially used material.
From the previous section, three goals for the further
optimization of this complex catalyst for the application in
methanol synthesis from CO2/H2 can be concluded. Firstly,
the activity at low temperatures needs to be improved to
counteract unfavorable thermodynamics. Secondly, the
Chemical hydrogen storage by methanol: Challenges for the catalytic methanol synthesis from CO2
robustness of the catalyst against product inhibition needs
to be improved. And thirdly, the rWGS activity should be
suppressed. Due to the endothermicity of reaction (3),
progress in the first task will also automatically help the
last task.
Tackling these problems with rational approaches
requires knowledge of the mode of operation and the
mechanistic details of industrial methanol synthesis,
which is currently a frontier of research. One general
problem of the industrial catalyst however, becomes
obvious: Cu/ZnO catalyzes the WGS reaction, which
exhibits faster kinetics than methanol formation. In
surface science studies on clean Cu(110) for instance, the
CO formation rate was shown to be faster by a factor of
103 than methanol synthesis from CO2/H2 at approximately
5 bar [31]. In a recent study of the kinetic isotope effect
(KIE) upon H2/D2 exchange on Cu/ZnO catalysts, markedly
different KIEs were observed for methanol synthesis
compared to CO formation. This result suggests that both
reactions do not share a common intermediate in the
rate-determining step, but proceed in a parallel manner
[32]. The different extent of product inhibition indicates
furthermore that both reactions likely occur on different
surface sites. This conclusion is also in agreement with
the observation that the two reactions react differently on
Cs addition [33], which is a promoter for WGS and a poison
for methanol synthesis [34].
In the industrial process the shift reaction plays an
important role as described above, but has turned into an
enemy in CO2 hydrogenation in the absence of CO harming
the selectivity of our desired product. Thus, although
reaction (1) is common to both processes, the industrial
Cu/ZnO catalyst is per se not the optimal catalysts for
methanol formation in the absence of CO [32]. It is,
however, a suitable starting point for the development
of CO2 hydrogenation catalysts that still holds many
challenging and exciting open scientific questions.
5 The Mode of Operation of Cu/ZnO
Catalysts
The aforementioned methanol synthesis studies using
labelled CO2 in the syngas provide clear evidence that
CO2 is the preferred carbon source for methanol under the
investigated conditions close to the industrial process. A
proposal for the mechanism includes CO2 hydrogenation
to formate as the rate determining step [32, 35, 36], which is
usually found by spectroscopy to be an abundant surface
species [37, 38]. Recent experimental and theoretical data
provide support for this view [22, 32]. However, the exact
81
mechanism is still debated today based on experimental
results indicating existence of other intermediates on
Cu/SiO2 [39, 40] or supporting the consecutive RWGS
and CO hydrogenation path on Cu/CeO2 [41]. On Cu/ZnOtype catalysts a pressures of 360 bar, markedly higher
than employed in industry, might also favour this latter
mechanism [42].
It is very interesting to look at the role of ZnO in
this reaction. ZnO is a structural as well as an electronic
promoter for Cu-based methanol synthesis catalysts [43].
As a structural support, it helps to keep Cu in a highly
dispersed state enabling exposure of a large specific Cu
surface area. This effect is most efficiently utilized in the
industrial synthesis recipe of Cu/ZnO/Al2O3 via a zincian
malachite solid solution precursor (Cu,Zn)2CO3(OH)2
[44]. As a result of this synthesis protocol, porous
aggregates of small Cu and ZnO particles are formed
by mild thermal treatment of the properly substituted
precursor phase (Figure 2a,b). The interwoven aggregates
of the zincian malachite precursor needles possess a
favourable mesostructure to form a porous catalyst. The
solid solution containing Cu2+ and Zn2+ in the precursor
will segregate on a nanoscopic level during the thermal
decomposition, because the corresponding oxides are not
well miscible (nanostructuring). The solid state chemistry
of the Zn-substituted malachite phase seems to constrain
the Zn-substitution to values around 30 mol% [45], which
leads to quite high Cu concentrations and a non-optimal
efficiency in the utilization of Cu due to relatively large
particles (10 - 15 nm). On the other hand, it has been
shown that surface defects play an important role for
methanol synthesis and are likely involved in the active
sites [46]. These defects are likely stabilized by under-lying
bulk defects, which are present in large quantity on the
industrial catalysts synthesized via the zincian malachite
precursor [47]. Figure 2c shows how the pattern of twin
boundaries in the bulk of a Cu nanoparticle is reflected in
changes in the termination of the surface.
The additional electronic effect of ZnO is related to
its partial reducibility and to the resulting strong metal
support interaction [50-52]. This electronic effect can also
be triggered on Cu model catalysts [53] or Cu nanoparticles
supported on different irreducible supports by postsynthesis addition of small amounts of ZnO in the sense
of a classic catalyst promotion [43]. For example, Cu
nanoparticles that were supported on irreducible MgO
were shown to act as CO2 hydrogenation catalyst only after
impregnation with small amount of ZnO (Figure 3). Recent
mechanistic insights reveal that a partially reduced
Zn-species is involved in the active surface ensemble for
CO2 hydrogenation to methanol [22, 32]. In a simplified
82 M. Behrens
a)
b)
c)
100
110
111
}
Disordered
ZnO xoverlayer
Twin boundaries
Figure 2: HRTEM images of a Cu/ZnO/Al2O3 methanol synthesis catalyst consisting of porous aggregates (a, [48]) of metallic Cu and ZnO
nanoparticles (b [49]) showing details of the surface faceting, decoration, and defect structure (c [46]).
bi-functional model of methanol synthesis, these highly
oxophilic sites seem to be responsible for formate
activation, which can react into finally methanol with
dissociatively chemisorbed hydrogen from the metallic
Cu surface sites. Interestingly, many of the experimental
observations made on Cu/ZnO and ZnO-free Cu catalysts
in different feed gases (Figure 3) can be satisfactorily
explained by DFT calculations using a simple model of the
active site that involves step sites on a Cu surface that are
decorated with metallic Zn atoms [22].
In real applied catalysts, such geometric situation
might be formed in a self-assembled manner by dynamic
strong metal support interaction, which has been
reported in this Cu/ZnO system [46, 54], and that can lead
to surface decoration of the Cu nanoparticle with ZnOx
layers as shown in Figure 2c. The analytic access to this
effect is complicated by the dynamic wetting/de-wetting
observed in different atmospheres [52]. Recent advanced
model catalysts combined with sophisticated TEM [55] and
surface analysis techniques [56] and, last but not least,
catalytic experiments were shown to be very promising
Figure 3: Comparison of the methanol formation rate of an industrial
like Cu/ZnO/Al2O3 catalyst, a Cu/MgO catalyst of comparable specific Cu surface area and the latter catalyst after impregnation with 5
wt% ZnO as a function of feed gas composition [22].
Chemical hydrogen storage by methanol: Challenges for the catalytic methanol synthesis from CO2
for a final elucidation of the surface composition and
structure of Cu/ZnO-based methanol synthesis catalysts
[57].
The structural complexity that is related to this effect
was recently demonstrated by the successful identification
of a new surface phase of ZnO, a distorted “graphitic”
polymorph [58]. It was found to wet the Cu metal surface
on partially oxidized brass model surfaces, but recently
also observed as a result of the reduction of real calcined
CuO/ZnO catalysts by TEM [55]. The exact role of this
and other possible “bi-functional” surface terminations
remains unclear and further work is certainly needed to
fully understand this important reaction in all details.
It seems that on the surface of a working catalyst, an
intermediate situation between an alloyed brass surface
and segregated Cu-ZnO patches of the type Cu-ZnOx/
support might be important (0 < x < 1, structural support
can be ZnO, but might also be another oxide). Whether
such situation prevails only at certain interfacial sites, at
the boundaries of (porous and dynamic) surface patches
or within surface alloys remains an open question. Recent
progress has led to refined tools for the identification
and quantification of reduced surface sites on Cu/ZnO
catalysts. It has been shown in two independent studies
that the N2O chemisorption technique [59] does in fact
deliver information not only about metallic Cu surface
atoms, but in addition is also sensitive to reduced Zn sites
[56, 60]. Thus, the N2O chemisorption capacity should be
treated with care if used as a measure of the Cu surface
area. Results obtained by transient adsorption [56] or
chemisorption of hydrogen [60] can deviate from those
of the N2O technique for highly reducing conditions. The
dependence on the hydrogen partial pressure [61] suggest
that pre-treatment of the catalyst plays an important
role for the contribution of reduced Zn species. It should
be kept in mind, however, that the N2O chemisorption
capacity still turned out to be the best measure and
reliable indicator for high catalytic activity – likely just
because it is capable of probing the important synergetic
Cu-ZnOx surface sites.
Based on the well-established and new insights
regarding the importance of synergetic ZnO-promotion,
high Cu dispersion and surface defects for the lowtemperature activity of methanol synthesis catalysts,
one can think of three major strategies for further
optimization of Cu/ZnO-based for CO2 hydrogenation: 1.
Making smaller Cu particles; 2. improving the beneficial
interaction between ZnOx and Cu; 3. Increasing the defect
concentration and amount of surface steps and edges of
the Cu phase.
83
An increase in Cu dispersion calls for new synthesis
strategies for smaller Cu nanoparticles. New precursor
phases [62], advanced impregnation and thermal
treatment methods [63-65] or new synthesis approaches
like for instance flame spray pyrolysis [66] should be
in the focus of research here. For an improvement of
synergetic Cu-ZnOx interaction, doping of ZnO with
small amounts of trivalent ions like Al3+, Ga3+ or Cr3+ has
been shown to lead to promising results likely due to
the enhanced reducibility of doped ZnO:M [29, 67]. Also
zirconia has been successfully used as a promoter for CO2
hydrogenation over Cu/ZnO catalysts [68, 69]. Finally,
the key for a targeted defect design in Cu nanoparticles
might lie in the solid state chemistry of CuO reduction to
Cu metal [70].
In all three cases, a lot of work remains to be done
to develop a successful CO2 hydrogenation catalyst based
on Cu/ZnO. The experimental groups working on new
materials are encouraged to compare their results to
the existing benchmark systems like the FHI-Standard
catalysts Cu/ZnO:Al that can be obtained free of charge
for academic purposes from the Department of Inorganic
Chemistry at the Fritz-Haber-Institute in Berlin [71]. It is
important to monitor the progress made in the different
fields to find the overarching mechanisms and to converge
lessons learned from new materials with the insights
made in model catalysis and theory.
6 Conclusion
Methanol is a very promising chemical hydrogen carrier
molecule and the well-established industrial methanol
synthesis process is a reference case for its desired
sustainable synthesis from CO2 and “green” hydrogen.
The catalyst employed in this process has been studied
since decades and recent results demonstrate significant
progress in the understanding of methanol synthesis
from CO2. The next step is the employment of this new
knowledge basis for the development of new and better
catalytic materials. The major challenges are related
to synthetic inorganic chemistry, defect generation in
metallic nanoparticles and surface/interface design
between the two major catalyst components Cu and ZnO.
Acknowledgements: I am in particular grateful to my
former co-workers at the Fritz-Haber-Institute in Berlin
and to Robert Schlögl for many fruitful discussions. Our
work on this catalyst has enormously benefited from
the insightful collaboration with Martin Muhler, Olaf
Hinrichsen, Jens K. Nørskov and Felix Studt.
84 M. Behrens
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