Preparation And Performance Of ZnZrO_x/SAPO-34 Catalysts For One-Step Conversion Of Syngas To Light Olefins

With the continuous development and progress of science and technology and chemical industry,low-carbon olefins,as the most basic chemical raw materials,their preparation methods have received widespread attention.At present,the most commonly used method for preparing low-carbon olefins is pyrolysis naphtha.In recent years,due to the increase in fossil fuel consumption,the process technology of syngas production of low-carbon olefins with simple operation and low raw material price has received more and more attention.In this paper,the synthesis gas one-step method was used to prepare the low-carbon olefin（STO）bifunctional catalytic route,and the ZnZrOx/SAPO-34 bifunctional catalyst was used for STO reaction:（1）ZnZrO_x catalysts with different Zn/Zr ratios were prepared by sol-gel method and bifunctional catalysts were formed with SAPO-34 for the synthesis of light olefins in one-step conversion of syngas.The results showed that appropriately increasing the content of Zn could effectively increase the adsorption and dissociation of H₂ by ZnZrOx and form the hydrogen species which could participate in hydrogenation reactions,consequently promoting the conversion of CO and improving catalytic performance of ZnZrOx/SAPO-34.Among them,the ZnZrOx with a Zn/Zr molar ratio of 1:16 showed the best catalytic performance,with CO conversion and light olefins selectivity of 24.0%and 78.2%,respectively.（2）The effects of mass ratio of oxide and molecular sieve and reaction conditions such as reaction temperature,reaction pressure,space speed of the feed gas on catalytic performance ZnZrO_x/SAPO-34 were investigated.With the increase of the mass ratio of oxide to molecular sieve,the conversion of CO increased gradually,the selectivity of light olefins increased first and then decreased,and a mass ratio of 1:1,excellent CO conversion and low-carbon olefin selectivity were obtained.With the increase of reaction temperature,the conversion rate of CO increased gradually.The selectivity of light olefins increased first and then decreased,and the best catalytic performance was obtained at 400℃.With the increase of reaction pressure,the conversion rate of CO gradually increased,the selectivity of low-carbon olefins gradually decreased,and the excellent conversion of CO and light olefins selectivity were obtained at 3 MPa.With the increase of the space speed of the feed gas,the conversion rate of CO gradually decreased,and the selectivity of light olefins increased first and then basically unchanged.The optimum catalytic performance was obtained when the space speed of the feed gas was 3600 m L·g^-1·h^-1.（3）The effects of alkali post-treatment of SAPO-34 zeolite on catalytic performance of ZnZrO_x/SAPO-34 were explored.Firstly,SAPO-34 molecular sieve was treated by different concentrations of triethylamine and the results showed that with the increase of treatment concentration,the crystallinity of SAPO-34 gradually decreases,the acid strength of strong acid sites gradually weakens,the formation of multi-level pores gradually deepens,and when the treatment concentration is too high,some of the crystal structure of SAPO-34 will be damaged.The formation of multi-level pores on the surface can accelerate the diffusion rate of the product and improve the conversion of CO,and the decreased acid strength of the strong acid site can inhibit the secondary hydrogenation of olefins.The bifunctional catalyst with sample S-1-006 which was obtained by treating SAPO-34 with 0.06 mol·L^-1 triethylamine exhibits the CO conversion rate of26.4%and the light olefins selectivity of 84.7%.Secondly,SAPO-34 molecular sieve was treated with three different alkali solutions.All of them multi-level pores formation on its surface,and the acid strength of the strong acid site was weakened.However,some crystals of S-2-006 treated with tetramethylammonium hydroxide were damaged,which resulted in the decrease of the catalytic performance of ZnZrO_x/SAPO-34.The sample S-3-006 treated with tetraethyl ammonium hydroxide was also used in the STO reaction,and 30.6%CO conversion and 77.1%light olefins selectivity were obtained.（4）The effects of acid post-treatment of SAPO-34 zeolite on catalytic performance of ZnZrO_x/SAPO-34 were explored.Firstly,SAPO-34 molecular sieve was treated with different concentrations of nitric acid solution.With the increase of treatment concentration,the crystallinity of SAPO-34 gradually decreased,the formation of surface multi-level pores gradually deepened,and the acid density of weak acid sites gradually increased,which was conducive to improving the catalytic performance of ZnZrO_x/SAPO-34.In addition,some crystal structures of SAPO-34 were damaged when the concentration of nitric acid solution was too high.Using 0.06 mol·L^-1 nitric acid treated sample SP-1-006 in the reaction,the CO conversion rate of 29.7%and light olefins selectivity of 76.9%were obtained.Secondly,SAPO-34 molecular sieve was treated with three different acid solutions.The three samples were found to have pores on the surface,and the acid density of the weak acid site was increased.However,the acid strength of the samples SP-2-006and SP-3-006 modified by oxalic acid and citric acid was decreased.Therefore,samples SP-2-006and SP-3-006 not only improved CO conversion,but also improved the selectivity of light olefins to 80.9%and 81.8%,respectively.