Imidazole and Nickel (II) Modified SAPO-34 and Its Catalytic Activity in CO₂ Hydrogenation to Ethylene

Imidazole and nickel (II) modified SAPO-34 was synthesized according to the mass ratio of aluminum isopropoxide: phosphoric acid: silica sol: tetraethylammonium hydroxide:water:modified agents = 1.0:0.8:0.6:2:52:0.01. Specified amount of aluminum isopropoxide was mixed with distilled water and phosphoric acid was added to present the white homogenous gel. Tetraethylammonium hydroxide was Also mixed with specified amount of silica sol to form homogenous solution. The homogenous solution was added to the white homogenous gel. After stirring at 30℃ for 4 h, specified amount of imidazole and nickel nitrate (mole ratio = 1 : 1) was added in and kept at 60℃ with a constant stirring speed for 8 h to result in a semi-transparent homogenous white gel. The white gel was transferred into a high-pressure stainless steel autoclave and maintained at 200℃ for 24 h without stirring. As precipitate was recovered from the autoclave, it was dried at 120℃ for 12 h and calcined at 550℃ for 6 h to acquire imidazole and nickel modified SAPO-34. It was characterized by means of N₂ adsorption/desorption, X-ray powder diffraction (XRD), scanning electronic microscopy (SEM), and energy dispersive spectroscopy (EDS). The modified SAPO-34 was mechanically mixed with CuO-ZnO-Al₂O₃ prepared by co-precipitation process to form composited catalyst (CZA/SAPO-34_x). The activity of CZA/SAPO-34_x for CO₂ hydrogenation to light olefins was investigated in a differential fixed reactor. Under reaction pressure of 3.0 Mpa, temperature of 400℃, space velocity (SV) of 1800 mL/(g_cat·h), volume ratio of CO₂ to H₂ of 1 : 3, and composited catalyst mass of 1.0 g, the one-way conversion of CO₂ and selectivity of ethylene are 73.8% and 67.7%, respectively. Compared with SAPO-34, imidazole and nickel (II) modified SAPO-34 keeps high catalytic activity when reaction temperature rises to 450℃, showing the high temperature resistance.