지르코니아-담지 니켈촉매상에서 1,3-부타디엔의 선택적 수소화

류재옥, 장종산, 김영호, 김성현,1 박상언

한국화학연구소 분자활성공학 연구실, 1고려대학교 화학공학과

Selective Hydrogenation of 1,3-Butadiene over Zirconia-Supported Nickel Catalysts

J. O. Ryu, J.-S. Chang, Y. H Kim, S. H. Kim,1 and S.-E. Park

Catalysis Center for Molecular Engineering, KRICT, Taejeon 305-600, Korea

1Dept. of Chem. Eng., Korea University, Seoul, Korea

Introduction

Zironium oxide and its solid solution are interesting materials of current technological and scientific interest [1]. They find application as support or promotors in environmental catalysis, selective hydrogenation, high temperature fuel cell and so on [2]. Zirconia is also considered to be a heterogeneous basic catalyst. Zirconia has recently been utilized as a commercial basic catalyst in the reduction of aromatic carboxylic acid to produce the corresponding aldehyde and the dehydration of 1-cyclohexylethanol to vinylcyclohexane [3].

Selective hydrogenations play an important role for the nearly complete removal of alkadienic or alkynic compounds from the C2, C3 and C4 cuts in the petrochemical industry. 1,3-Butadiene has now became a surplus product, and its selective hydrogenation, namely to 1-butene, is carried out on an industrial scale.    A number of studies have been devoted to the selective hydrogenation of 1,3-butadiene to butene since the 1960s [4]. Modern Industrial process use the selective hydrogenation over supported Pd catalysts for the purification of petrochemical products generated from cracking units by removing acetylene in the ethylene cut and 1,3-butadiene in the butene-rich cut. Even though Pd/Al2O3 is superior in the selective hydrogenation of 1,3-butadiene, the development of low-cost catalyst is still required since Pd is very expensive. Many supported nickel catalysts are widely utilized for the hydrogenation of olefins as well as functionalized hydrocabons, but they are generally non-selective. For this reason, the application of nickel catalysts to the selective hydrogenation has been so rare. Therefore, the selection of catalyst support, promoters and the preparation technique may be essential in order to develop selective nickel catalysts to the hydrogenation. Zirconia is a good candidate for catalyst support of nickel catalyst because it gives favorable interaction toward nickel.

In the present work, we have studied zirconia-supported nickel catalysts prepared by two different methods and their catalytic activity in the selective hydrogenation of 1,3-butadiene.

Experimental

Zirconia-supported Ni catalysts were prepared by different methods and loading. The catalysts by co-precipitation, Ni/ZrO2(cop), were prepared by adding NH4OH slowly into aqueous Ni(NO3)2□6H2O solution and Zr(NO3)2□xH2O solution mixture until pH of 8~9 was reached. The precipitate was filtered and washed with distilled water. After Drying in oven for overnight, the precursor was calcined in air at 600oC for 6h. The catalysts by impregnation, Ni/ZrO2(imp), were prepared by incipient wetness with aqueous solution of Ni(NO3)2□6H2O, followed by drying at 100oC for overnight and calcination in air at 600oC for 6h. Hydrous zirconia (MEL Co.) was used for a support of the impregnated catalysts.

The physical and chemical properties of prepared catalysts were analyzed by XRD, TPR, XPS, UV/VIS-DRS, EPR, and Raman.

The selective hydrogenation of 1,3-butadiene to butenes was carried out in a conventional fixed-bed microreactor made of quartz in the temperature range of 70 - 200oC at atmospheric pressure. Before catalytic measurement, the catalyst was reduced with 5% H2/N2 at 400oC for 1h. The reaction was carried out while introducing mixture of 1,3-butadiene (99.0%) and pure hydrogen in helium balance. The 1,3-butadiene was vaporized for the mixing with gases. The effluent stream from the reactor was analyzed by an on-line gas chromatograph (HP 5890 Series II) equipped with a FID detector and a DB-wax capillary column.

Results and discussion

A monoclinic phase is generally a dominant one of zirconia at lower temperature than 800oC. However, a tetragonal phase can be also obtained by the selection of preparation method and condition. The presence of foreign species plays an important role to direct the zirconia structure. According to the preparation method of Ni/ZrO2 catalysts after calcination, the catalysts with the different structure were obtained. Fig. 1 illustrates the XRD patterns for the 3 wt.% Ni/ZrO2 catalysts with depending on the preparation method. The Ni/ZrO2 catalyst obtained from impregnation has a monoclinic zirconia phase (Fig. 1(A)). On the other hand, the co-precipitation method for the Ni/ZrO2 catalyst leads to the formation of metastable tetragonal zirconia (Fig. 1(B)). The tetragonal phase was obtained for Ni/ZrO2(cop) catalysts more than 1wt.% Ni loading. The presence of nickel is known to favor the formation of the tetragonal phase. In the tetragonal zirconia, nickel species can be incorporated in the zirconia structure and hence they form the solid solution with the zirconia framework. As a consequence of the solid solution, the tetragonal modification is favored at the expense of the thermodynamically stable monoclinic phase. The solid solution of NiO-ZrO2 may prevent the aggregation of nickel oxide on the zirconia surface because the nickel in the dispersed state of nickel oxide enters the solid solution of zirconia lattice. In this respect, the formation of the tetragonal phase may provide strong interaction of nickel and the zirconia support.

TPR experiment has been carried out to determine the reducibility of Ni and interaction between nickel and support. As shown in Fig. 2, the TPR profiles of these catalysts indicate that the zirconia structure of the Ni/ZrO2 catalysts according to the preparation method greatly affects the reduction behavior of the nickel. The Ni/ZrO2(imp) catalyst shows two intense peaks of the reduction at 405oC and 510oC (Fig. 2(A)). Considering reduction temperature (250 - 350oC) of bulk NiO, these peaks are ascribed to weak interaction and rather strong interaction of nickel oxide and monoclinic zirconia. The Ni/ZrO2(cop) catalyst produces three different reduction peaks shifted into high temperature as compared to the impregnated catalyst (Fig. 2(B)). It is obvious that the Ni/ZrO2(cop) catalyst has much stronger interaction of nickel and the zirconia support due to the formation of NiO-ZrO2 solid solution.

To distinguish the catalytic properties of Ni/ZrO2 catalysts according to the preparation method, the selective hydrogenation of 1,3-butadiene to butenes was investigated. 1,3-Butadiene is hydrogenated to a mixture of 1-butene, cis-2-butene and tras-2-butene. 2-Buenes among butenes isomers are obtained from bond migration of 1-butene as formed first and cis/trans isomerization. Table 1 compares catalytic activity and selectivities to hydrogenated products of the Ni supported catalysts for the selective hydrogenation of 1,3-butadiene. It is evident that the impregnated catalysts yield higher catalytic activity, while the co-precipitated catalysts give higher selectivities to a mixture of butenes. Ni/ZrO2(cop) with 3 wt.% Ni loading exhibits the highest butenes selectivity among the catalysts. Lower activity of the co-precipitated catalysts are ascribed to hard reduction of nickel species in the solid solution. Higher selectivities of these catalysts may be due to well dispersion of the reduced nickel species into the solid solution. High trans/cis ratios (4 - 12) of the 2-butenes in the products have been generally observed for supported Pd catalysts [5]. On the other hand, the Ni/ZrO2 catalysts give much lower trans/cis ratios (1.0 - 2.6), close to the equilibrium ratio. This suggests a different mode of adsorption of the partially hydrogenated intermediate, which is assumed to be a □-allyl species in the case of Pd catalyst. Butenes produced by hydrogenation may either undergo desorption or subsequent hydrogenation. In general, 1,3-Butadiene is more strongly adsorbed on group VIII metals than butenes formed by hydrogenation. The desorption rate of butenes on catalyst surface which is mainly determined by the relative difference in the adsorption strength of 1,3-butandiene and butenes depends markedly on the surface structure of the active nickel sites. The highest butenes selectivity on 3 wt.% Ni/ZrO2(cop) may be due to the high desorption rate of intermediate butenes. This work will be further discussed in detail on the nature of nickel sites in the solid solution or external surface of zirconia in terms of the influence of on the selectivity in the hydrogenation of 1,3-butadiene.

References

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