(623c) Fluorine-Modified Cu/Zn/Al/Zr Catalysts Via Hydrotalcite-Like Precursors for the CO2 Hydrogenation to Methanol

Fluorine-modified
Cu/Zn/Al/Zr catalysts via hydrotalcite-like precursors for the CO₂
hydrogenation to methanol

Peng Gao¹, Feng Li¹,Fukui Xiao¹, Ning Zhao¹, Wei Wei^1,* and
Yuhan Sun^1,2,*

¹State Key
Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of
Sciences, Taiyuan, PR China, weiwei@sxicc.ac.cn

²Low
Carbon Energy Conversion Technology Center, Shanghai Advanced Research
Institute, Chinese Academy of Sciences, Shanghai, PR China, yhsun@sxicc.ac.cn

It is well known
that carbon dioxide emissions have induced global warming. As a cheap, nontoxic
and abundant C1 feedstock, chemical utilization of CO₂ is a
challenge and important topic. Methanol is a starting material for several
important chemicals and can also be used as a fuel additive. It can also be
converted to high-octane gasoline, aromatics, ethylene, as well as other useful
petrochemicals.[1] The Cu/ZnO catalyst is well-known for high activity
and selectivity for CO₂ hydrogenation to methanol. Support such as
Al₂O₃ can further increase the catalytic performance on
methanol synthesis. Furthermore, the promoter such as Zr is known to affect the
copper dispersion and surface basicity, which in turn affects the activity and
the adsorption of CO₂.[2, 3]

In order to further improve their catalytic
performance, the Cu/Zn/Al/Zr mixed oxides modified by various ions had
been developed. However, to
the best of our knowledge, there
were few reports on the fluorine-modified Cu/Zn/Al/Zr mixed
oxides and their applications. The Cu/Zn/Al/Zr mixed oxides derived from
hydrotalcite-like compounds (HTlcs) possess intrinsic
basicity, good dispersion of metal cations at an atomic level, stability against sintering and synergetic effects between the elements. [4,
5] Thus, in the present work, the fluorine-modified Cu/Zn/Al/Zr mixed oxides were prepared from the precursors of fluorine-containing Cu/Zn/Al/Zr hydrotalcites and were used as catalysts for methanol
synthesis from CO₂ hydrogenation.

Experimental

The Cu₂Zn₂Al_0.7Zr_0.3
HTlcs intercalated with CO^3- (HTs-CO₃) were synthesized by
coprecipitation method, and corresponding calcined HTlcs(CHTs-CO₃)
were obtained. The fluorine-containing Cu₂Zn₂Al_0.7Zr_0.3
hydrotalcites (HTs-F) were prepared using the ''memory effect'' according to
the literature. Briefly, the as-prepared mixed metal oxide (CHTs-CO₃)
were treated separately with an aqueous solution of NaF under N₂
atmosphere for 48 h. To prevent CO₂ from contaminating the aqueous
solution, the deionized decarbonated (DD) water was used here. The obtained HTs-F
was further calcined in an oven at 500 ^oC for 4 h under a flowing
stream of pure N₂. Then the fluorine-modified mixed oxides (CHTs-F)
were obtained.

Results and Discussion

The XRD patterns
of the precursor and calcined materials shown in Fig. 1 are typical for
hydrotalcite-like compounds. Generally, the hydrotalcite characteristic could
be depicted by two important cell parameters: c (3d₀₀₃) and
a (2d₁₁₀). The c value of HTs-F
hydrotalcite increased but the a value kept invariable compared to the
two parameters of HTs-CO₃
hydrotalcite. The increase in c value could be ascribed to the less
electrostatic force of F^- with respect to CO₃^2-
anions.
Thermal
decomposition of these precursors results in the formation of poorly
crystallized CuO phase.

Fig.
2 shows the CO₂ desorption profiles of the reduced samples. All
profiles are able to be deconvoluted into three Gaussian peaks, which could be
assigned to the weak (¦Á peak), moderate (b peak) and strong (¦Ã peak) basic
sites, respectively. The intensity of ¦Ã peak for CHTs-F
sample significantly increased compared to that for pure CHTs-CO₃ sample. For CHTs-CO₃ sample, the strong basic sites
were ascribed to the unsaturated O^2-. The presence of fluorine anions
in HTs-F interlayer led to the formation of the coordinatively unsaturated F^-
ions during calcination, which increased the strong basic sites. Similar results
also have been reported by Wu et al.[6]

Table 1 BET surface area and the
catalytic performance for CO₂ hydrogenation to methanol over the
catalysts.

Reaction conditions: P = 5.0 Mpa, GHSV = 4000 h^-1, H₂:CO₂
(atomic) = 3:1.

The
catalytic performance of catalysts in CO₂ hydrogenation to methanol
is summarized in Table 2. It is noteworthy that the CH₃OH selectivity for CHTs-F is markedly higher than that for CHTs-CO₃ sample. According
to Guo et al.[7],
the amount of strong (¦Ã) basic sites is an important parameter for methanol
synthesis. The methanol selectivity increases linearly with the increase in the
fraction of ¦Ã basic site. This relationship can be interpreted in terms of the
bifunctional (dual-site) mechanism of CO₂ hydrogenation, which is currently accepted. As this mechanism
stated, both methanol and CO are produced dominantly via the common formate
intermediate. It is possible that comparing with the formate adsorbed on b
basic site, the formate adsorbed on ¦Ã basic site prefers to hydrogenate further
to form methanol rather than dissociate to form CO. Therefore, the introduction
of fluorine into Cu/Zn/Al/Zr mixed oxides significantly improved the strong
basic sites, and then sharply increased the CH₃OH selectivity in the methanol synthesis from CO₂
hydrogenation.

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