Abstract
Modification of ZSM-5 zeolite catalyst (A) reported by Mobil Oil Co. gave a zeolite catalyst (B) which showed higher selectivity to yield C2-C4 olefins from methanol. The X-ray diffraction patterns of catalyst A and Catalyst B are shown in Fig. 1. The lattice parameters of these catalysts were in agreement with those presented in the literature17). Scanning electron micrographs of catalyst A and B are shown in Fig. 2. From these pictures, the crystal aggregate of catalyst B was about 4 times larger than that of catalyst A. Table 1 summarizes the SiO2/Al2O3 ratio, B.E.T. area, and proton exchange quantity of the catalysts used for methanol conversion. Figure 3 shows the variation in methanol conversion with time on stream. The activities of catalyst A and B (SiO2/Al2O3 ratio=50) were almost unchanged, while the activity of HY zeolite to form hydrocarbons decreased gradually. The results of methanol conversion over catalyst A, B and HY are shown in Table 2. The activity of catalyst B was the highest among the three. While aromatics produced on catalysts A and B mainly consisted of C7-C10, mostly C11 and C12 on HY zeolite. Figure 4 shows the relation between the selectivity of catalysts A and B to yield C2-C4 olefins and the degree of methanol conversion to hydrocarbons. At the same conversion level, the selectivity of catalyst B was about 1.5 times higher than that of catalyst A. The time of crystallization in the preparation of catalyst B was 6 times shorter than that of catalyst A. Thus, the shorter time of crystallization of catalyst B may lead to the difference in the pore structure between catalyst A and B. The diffusion rate of the product olefins from the pores could probably be faster on catalyst B to account for the higher selectivity of C2-C4 olefins production.
The effect of SiO2/Al2O3 ratio in catalyst B on methanol conversion was studied. The X-ray diffraction patterns of catalyst B with different SiO2/Al2O3 ratios, are shown in Fig. 5. The peak height at 8.0° and at 8.9° (2θ) increased with increasing SiO2/Al2O3 ratio. The results of methanol conversion presented in Table 3 show that the activity decreased as SiO2/Al2O3 ratio increased. Variations in the selectivities for C2-C4 olefins are aromatics productions with varying degree of methanol conversion are shown in Figs. 6 and 7, respectively. With increasing SiO2/Al2O3 ratio, the selectivity to yield C2-C4 olefins production increased and vice versa for aromatics when compared at the same conversion level. The concurrent increase in the olefin/paraffin ratios of the C2-C4 hydrocarbons is probably due to the suppression of hydrocarbon transfer to olefins accompanied by the formation of aromatics. Decreasing yields of aromatic hydrocarbons with increasing SiO2/Al2O3 ratio are shown in Fig. 8, in which the formation of xylenes scarcely decreased, while that of triand tetramethylbenzenes were drastically suppressed. It was concluded that alkylation of aromatic rings with methanol was retarded by a catalyst of high SiO2/Al2O3 ratio, and that methanol was effective for production of C2-C4 olefins by dehydration.
