| The coal-based methanol to olefins(MTO)process is one of the most important contents of novel coal chemical industry,which is also a powerful means and way to solve severe deficiency of petroleum resources for China.SAPO-34 molecular sieve is considered to be the best of MTO catalysts.In MTO reaction,people pay more attention to the life of the catalyst and light olefin selectivity.In order to improve the performance of catalyst,previous studies mainly focus on diffusion.However,the main factors which affecting the catalyst performance has two aspects:diffusion and acid.In this study,SAPO-34 molecular sieve with different acidity was synthesized by changing the source of aluminum,the order or the way of adding aluminum source.Moreover,the connection of product selectivity with acidity and the correlation between lifespan and acidity were also investigated so as to reveal the inherent law of acidity for catalyst life and low carbon olefin selectivity and the related reaction mechanism.It will provide a reference for the industrial synthesis of zeolites with a specific acidic.The corresponding results are as follows:(1)In this study,to hydrothermally synthesize the SAPO-34 molecular sieve with pseudo-boehmite as aluminum source,the factors influencing crystalline phase and the crystallinity were studied,including silica-alumina ratio and crystallization temperature,the following conditions(optimized)should be used:feeding silica-alumina ratio of 0.2(molar ratio)and then crystallized it at 170℃.(2)Based on the optimum synthetic process conditions,the effects of aluminum source on the physicochemical properties and catalytic performance of SAPO-34 molecular sieve were studied.It was found that properties of the aluminum source had great influence on the crystallization process and the chemical and physical properties of SAPO-34 molecular sieve.Among them,SAPO-34 molecular sieve with pseudo-boehmite as aluminum source was not only with high crystallinity,but also the particle size distribution was concentrated on about 500nm.When aluminum hydroxide was aluminum source,the gained SAPO-34 molecular sieve was with the highest crystallinity and the best growth orientation of(111)crystal plane,but its particle size was uneven distributed.However,SAPO-34 molecular sieve had the lowest crystallinity,imperfect crystal morphology with aluminum isopropanol as aluminum source.Due to smaller particle size,high specific surface area and moderate acid density,SAPO-34 molecular sieve with pseudo-boehmite as aluminum source showed better activity stability and high light olefin selectivity in MTO reaction.Its reaction condition as follows:the reaction temperature was 450℃,atmospheric pressure and methanol WHSV was 3.0h-1.(3)The influence of adjusting the accession method of the aluminum source upon SAPO-34 molecular sieve with pseudo-boehmite as aluminum source was studied after determining the best aluminum source.It seems that SAPO-34 molecular sieve with different acid density were synthesized successfully.MTO reaction results showed that the age of SAPO-34 molecular sieve were extended gradually with the increase of acid density under the system of low silicon(0.2SiO2:Al2O3).(4)After revealing the laws between acid density and catalyst life,this paper attempts to research the relationship between acid species and strength and light olefin selectivity by the separation of Bronsted acid and Lewis acid.Samples with different Bronsted acid and Lewis acid were obtained through ion exchange process and investigated based on the NH3-TPD and NH3-IR in order to measure the amount of Bronsted acid and Lewis acid accurately,combined with the MTO catalyst performance analysis,it would draw a conclusion that ethylene selectivity depended on the amount of Bronsted acid and weak acid controlled propylene selectivity.It was confirmed that Bronsted acid would converted Lewis acid by calcination treatment.Through the characterization of NH3-TPD and NH3-IR.Finally,MTO result confirmed again the above conclusion. |
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