CO2 수화반응에서 혼성촉매의 중간산점

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한국과학기술원 화학공학과

Medium Acidity of Hybrid Catalysts in CO2 Hydrogenation

Young-Kwon Park and Son-Ki Ihm

Dept. of Chem. Eng., KAIST

Introduction

The catalytic transformation of CO2 is an interesting process for the production of methanol and hydrocarbons [1-3]. By using a mixture of catalysts consisting of metallic and acidic component, called hybrid or composite catalyst, and carrying out the conversion of CO or CO2 under suitable conditions, the methanol formed on the metallic catalyst can easily be transformed into gasoline by the acidic catalyst [4-12].

Even if the hydrocarbon synthesis by CO2 hydrogenation over hybrid catalyst have been extensively investigated [3, 7-12], there have been very few reports on the interactions which might happen between methanol synthesis catalyst and methanol conversion catalyst.

In our previous work [3] on hydrocarbon synthesis over hybrid catalyst, it was reported that the hydrocarbon yield depended upon the amount of zeolites acid sites. The properties of Cu/ZnO/ZrO2 were not modified due to the physical mixing for the preparation of hybrid catalyst. Even if modified, the catalytic activity of methanol synthesis catalyst remained almost the same. Further study about the catalyst modification is highly desirable to elucidate the catalytic active sites of hybrid catalyst. In this work, the interaction between methanol synthesis and zeolites will be evidenced in terms of the medium acidity identified through ammonia TPD, and correlated with catalytic activity for the synthesis of hydrocarbon (H.C.).

Experimental

2.1. Catalyst preparation

Cu/ZnO/ZrO2 (60:30:10 wt%) precursors were prepared by co-precipitation, and zeolites such as ZSM-5 and SAPO-34 were obtained through hydrothermal synthesis [3]. Hybrid catalysts were prepared by physically mixing equal weight of copper catalyst and zeolite. They were pelletized and crushed to obtain the desired size of 60-100mesh.

2.2. Catalyst characterization

The surface areas were measured by the nitrogen BET method using a Micromeritics ASAP2000. The exposed copper surface area was measured by nitrous oxide titration following the procedure described by Chinchen et al. [13]. The structures of HZSM-5 and SAPO-34 were confirmed by their powder X-ray diffraction (XRD) patterns using Cu K radiation. The coke deposit was measured through thermogravimetric analyses (TGA) using Cahn microbalance. Temperature programmed desorption (TPD) measurements were carried out to analyze the acidic properties of zeolites by using ammonia as an adsorbate [3].

2.3. Activity measurements

Carbon dioxide hydrogenation was performed in a continuous-flow microreactor with a fixed catalyst bed under a total gas pressure of 28 atm at a temperature 400oC [3]. Conversion and product distribution were obtained from analyses by Gas Chromatograph. The activity was measured with time on stream.

Results and discussion

3.1. Medium acidity in hybrid catalysts

NH3 TPD profiles of hybrid catalysts are shown in Fig. 1. Hybrid catalysts showed a new peak corresponding to the medium acidity which did not appear in each of Cu/ZnO/ZrO2 and zeolite. This new medium acidity might be ascribed to the interaction between methanol synthesis catalyst and zeolite. Especially, the amount of medium acidity of hybrid catalyst having SAPO-34 was much larger than that of hybrid catalyst having HZSM-5. The amount of medium acidity seemed to be depending upon the acidity of zeolite in the hybrid catalyst. The amount of original Br□nstead acid sites of SAPO-34 was larger than that of HZSM-5 [3].

3.2. Activity in CO2 hydrogenation

CO2 hydrogenation was carried out over hybrid catalysts with two different mixing ratios (8 : 2 and 5 : 5) of methanol synthesis catalyst and zeolite (Table 1). Since methanol formation is a rate-controlling step for the synthesis of dimethyl ether (DME), i.e. for the hydrocarbon formation, the 8 : 2 hybrid catalyst with higher mass fraction of methanol synthesis catalyst could be expected to show higher yield of hydrocarbons. On the contrary, the hybrid catalyst with equal weight (5 : 5) showed a higher hydrocarbon yield than the 8 : 2 catalyst. It was suggested that methanol conversion to hydrocarbon and methanol decomposition into CO are competing with each other [10]. Even for a hybrid catalyst with a higher yield of oxygenates, its hydrocarbon yield could not be higher if most of methanol decomposed into CO. For higher yield of hydrocarbon, it is desirable to make most of methanol molecules converted into hydrocarbons instead of CO. The 5 : 5 hybrid catalyst must have more potential to convert methanol into hydrocarbons. In our previous work [3], it was suggested that the hydrocarbon yield was highly dependent on the amount of zeolites acid sites. This suggestion can be elaborated in terms of the medium acidity. The more medium acidity appeared, the higher hydrocarbon yields were obtained. The hydrocarbon yield at 400oC could well be correlated with the amount of medium acidity characterized by the NH3 TPD. The peak intensity for the medium acidity of the 5 : 5 catalyst was larger than that of the 8 : 2 catalyst.

References

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Fig. 1. NH3 TPD of catalysts

Table 1. CO2 hydrogenation over hybrid catalysts composed of Cu/ZnO/ZrO2 and zeolite

Catalyst	A+HZSM-5	A+HZSM-5	A+SAPO-34	A+SAPO-34
*Mixing ratio	8 : 2	5 : 5	8 : 2	5 : 5
CO2 conv.(%)	40.0	38.4	35.3	33.9
Yield (wt%)
H.C.	0.9	2.7	8.3	12.2
CO	38.2	34.7	26.4	20.5
MeOH	0.9	1.0	0.6	1.2

A : Cu/ZnO/ZrO2, * :(Cu/ZnO/ZrO2 : zeolite), in weight

Reaction conditions : 28atm, 400oC, H2/CO2:3, W/F:20gcat□h/mol