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. 1. XRD patterns of the prepared samples. (°ñ) CuO	Fig. 2. CO2-TPD curves of reduced samples.

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.

Sample	SBET		Temperature	CO2 Conversion.	Selectivity, (C-mol%)			CH3OH Yield
	(m2/g)		(oC)	(%)	CO	HC	MeOH	(g/ml.h)
CHTs-CO3	69		230	15.5	45.1	1.1	53.9	0.11
			250	22.5	52.2	0.7	47.1	0.14
			270	26.6	56.6	0.6	42.9	0.16
CHTs-F	38		230	14.3	35.0	1.1	63.9	0.12
			250	20.8	48.1	0.7	51.2	0.15
			270	25.8	54.7	0.5	44.8	0.17

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.

Reference

[1]       G.A. Olah, A. Geoppert, G.K.S. Prakash. Beyond Oil and Gas: The Methanol Economy (2006)

[2]       X. An, J.L. Li, Y.Z. Zuo, Q. Zhang, D.Z. Wang, J.F. Wang, Catal. Lett.118 (2007) 264.

[3]       D. Tichit, N. Das, B. Coq, R. Durand, Chem. Mater.14 (2002) 1530.

[4]       M. Behrens, I. Kasatkin, S. Kuhl, G. Weinberg, Chem. Mater.22 (2010) 386.

[5]       Y.X. Liu, K.P. Sun, H.W. Ma, X.L. Xu, X.L. Wang, Catal. Commun.11 (2010) 880.

[6]       G.D. Wu, X.L. Wang,  B. Chen, J.P. Li, N. Zhao, W. Wei, Y.H. Sun, Appl. Catal. A:Gen.329 (2007) 106.

[7]      X. Guo, D. Mao, G. Lu, S. Wang, G. Wu, J. Mol. Catal. A: Chem.345 (2011) 60.