(435j) Mechanism, Reaction Kinetics and Multi-Scale Modelling of CO2 Hydrogenation to Methanol over Trimetallic Heterogeneous Catalysts

Carbon dioxide is a source of potential carbon-containing raw
material and also one of the major greenhouse gases [1]. In the past decades, the potential use of
CO₂ as an alternative feedstock replacing CO in methanol production
has received attention as an effective way of CO₂ utilization [1,2].

A highly efficient catalyst is the key for methanol synthesis via
CO₂ hydrogenation. Experiments with the
aid of the isotope-labelled CO and spectroscopy have demonstrated that methanol
was produced by the hydrogenation of CO₂ rather than CO, while the commonly-used
ternary Cu–Zn–Al oxide catalyst for CO hydrogenation was not particularly active
for the conversion of pure CO₂ at 5.0–10.0 MPa and 200–250 °C [3]. Mostly used catalyst type for CO₂
hydrogenation was the modified methanol synthesis analogue for CO hydrogenation
and the existing studies addressed its chemical composition, supports,
additives, different preparation methods/conditions and morphology. Notably, zinc oxide can improve the dispersion and stabilization of
copper. ZnO possesses lattice oxygen vacancies, consisting of an electron pair
in the lattice, which is active for methanol synthesis. The basicity of ZnO
influences the catalyst activity through affecting the dispersion of copper via
the precipitate phase of copper [4]. Studies on the precipitate of
Cu-containing hydroxocarbonates revealed that some of its phases played
important roles in detrmining catalytic performance. The mixed metal oxides,
obtained by the controlled thermal decomposition of hydrotalcite-like compounds
(HTlcs), with the general formula of [M²⁺_1–xM³⁺_x(OH)₂]^x+(A^y-)_x/y
× zH₂O, also exhibited promising results. M²⁺ and
M³⁺, divalent and trivalent cation, respectively, possess homogeneous
microstructure, a good dispersion of M²⁺ and M³⁺ at
atomic level, an enhanced metal–oxide interaction after reduction, the stability
against sintering and a high specific surface area, as well as strong basic
properties [5]. Therefore, HTlcs are among the most
investigated catalyst precursors considering the remarkable properties of the
final catalysts. In the present study, the influence of the basicity of
alkaline earth metal (i.e. Ba, Ca, Mg and Sr) catalysts, which were
prepared via the hydrotalcite route on the selectivity of the methanol
production in CO₂ hydrogenation, is reported. The catalysts with the
molar ratio of Cu:M:Al = 6:3:1 (M = Ba, Ca, Mg, Sr) were prepared and
characterized by various characterization techniques such as scanning (SEM) and
transmission (TEM) electron microscopy, X-ray diffraction (XRD), X-ray
photoelectron spectroscopy (XPS), CO₂ and NH₃
temperature-programmed desorption (TPD), and N₂O chemisorption.

Table 1: Properties
of Cu/M/Al catalysts

Powder XRD
patterns suggested that the isolated Cu²⁺ species interacted strongly
with alumina, to an extent that it may have even formed the surface copper
aluminate phase. However, the presence of a highly-dispersed surface CuO was
present as well, distributed as smaller crystallites. The SEM and TEM images of
the catalysts also supported the findings, obtained by XRD; specifically, no
extensive agglomeration of CuO was observed in the respective images. Weak,
strong and some moderate basic sites were attributed to the three peaks present
in the CO₂ TPD profile of the prepared catalysts, while basicity was
in the following order: Ba > Ca > Sr > Mg. The basic character of the
catalysts was strongly dominated by the presence of the alkali metal when compared
to copper and alumina.

The catalysts
were consequently tested in parallel reactor system, which comprised five fixed-bed
vessels (Figure 1). The catalytic performance of the alkaline earth metal-containing
catalysts was compared with the commercial methanol synthesis catalysts (by Alfa
Aesar and Lurgi). Methanol selectivity and CO₂ conversion were
examined under the continuous flow in the temperature (T) range between
200 °C and 400 °C. The catalysts were exposed to different space velocities
(GHSV) (from 2000 to 6000 h^–1). Prior to
catalytic measurements, fresh catalysts were reduced in the stream of H₂ at 300 °C for 3 h under the atmospheric
pressure (P; 1 bar). Reactors were then cooled to the ambient
temperature (25 °C) introducing reactant gas (N₂:CO₂:H₂
= 1:2:6 molar ratio) flow and raising the pressure to 2.0 MPa.

Figure 1: Parallel reactor system.

The results
showed that Cu/Mg/Al catalyst was at least as good as the commercial catalysts
(Figure 2), while the main products detected were
methanol, water and carbon monoxide. Selectivity for methanol was between 10 and 40%, depending on
reaction conditions and the catalyst used. The
experimental data, paralleled by model predictions showed that CO₂
conversion increased with elevated temperature, while on the contrary, methanol
selectivity dropped with increasing temperature.

Figure
2: Methanol selectivity and CO₂ conversions over different catalysts
at GHSV = 2000 h^–1, T = 200 and 250 °C, and P = 20
bar.

It is well-known
that copper metallic surface area is an important parameter for the methanol
synthesis by CO₂ hydrogenation. The relationship between copper
surface area and the catalytic activity for the reactions over the copper-based
catalysts has been studied extensively, though some controversies still remain.
In the present study, the effect of the surface area of the metallic copper on
the activity of the methanol synthesis through CO₂ hydrogenation was
established and the results are presented in Figure 3. It is observed that the
attained catalytic activity increases analogously with surface area versus
the amount of basic sites. This indicates that among the preparation techniques
applied, ultra-sonic method improves the dispersion of copper particles without
changing the intrinsic activity of CuO–ZnO catalyst. This observation was also
supported by the results, obtained by N₂O dissociation reaction and
XPS.

Figure
3: Catalyst activity versus metallic copper surface area and basic sites
for catalytic CO₂ hydrogenation at T = 250 °C and P =
20 bar.

In parallel to
extensive experimental trials (varying catalysts and process operating
conditions), density functional theory (DFT) calculations were used to
determine elementary reaction steps and their activation energies, as well as
pre-exponential factors (Figure 4), yielding the rate constants for different
operating conditions. These parameters were used in the kinetic Monte Carlo
(KMC) and micro-kinetic modelling to obtain the model for a packed bed reactor
(PBR). The two critical variables for the design of PBR, the packing void
fraction and the pressure drop across PBR, are usually predicted using
empirical correlations that were and still are a matter of an ongoing
discussion among researchers. The generalization of typical parameters brings
about a high error risk and leads to oversized reactors due to the capacity
safety factor. In contrast to the correlations and experimental investigations
of adaptive parameters, several numerical approaches are available today to
investigate the flow within particle packings, mostly based on computational
fluid dynamics (CFD), which were also applied in this study. The common
work-flow for the CFD simulations of a realistic packed bed thus consisted of
reactor bed packing, domain meshing, boundary conditions setting, simulation
running, and finally, collecting the results (Figure 5), coupling CFD with
reaction kinetics upon operating outside a fully-developed turbulent regime, in
which the solution could have been approximated by plug flow.

Figure
4: Reaction mechanisms and pathways employed.

Figure
5: CFD simulations of packed bed reactors.

In overall, the
study comprised the preparation of various non-noble metal heterogeneous catalysts
with differing in respective surface composition. While a typical Cu–Zn–Al process performance was modelled in an
inherently multi-scale manner; specifically, obtaining a full reaction kinetic
parameters set from DFT (in turn coupling the latter with a realistic reactor
operation using transport phenomena and fluid mechanics), for other
heterogeneous catalytic formulations, the micro-kinetic reaction model was
maintained in its original form, adopting a regression of the pinpointed rate
determining step(s) (RDS(s); also by DFT), whereas each reaction kinetic term
was de facto correlated with the corresponding sites available (meaning
Cu, ZnO, Al₂O₃, etc.). As a result of this it is
therefore possible to, to some extent, reverse engineer heterogeneous catalytic
material, or adopt the kinetic scheme proposed, should a new catalytic
formulation be applied, respectively. The developed modeling framework also
allows for a detailed pathway analysis also upon shifting the desired product
selectivity, e.g. to syngas via the reverse water–gas shift
reaction (RWGS), or varying process conditions applied.

Acknowledgement

The presented work was partially funded by the EU
Framework Programme for Research and Innovation Horizon 2020 under the grant
agreement No 637016 (MefCO2). The authors also gratefully
acknowledge the financial support of the Slovenian Research Agency (ARRS)
through the Programme P2–0152.

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