Preparation Of Pd / SAPO - 41 Bifunctional Catalyst And Study On Hydrogenation Isomerization Performance

Microporous SAPO-41 molecular sieve samples were synthesized by using silica sol, pseudoboehmite with different properties and di-n-butylamine (DBA) or di-n-propylamine (DPA) as the sources of aluminum, silica and template, respectively. Hierarchical SAPO-41 molecular sieve samples were synthesized by adding different amount of glucose and PEO-PPO-PEO (P123) as mesopore templates into the intial gels, respectively. Morever, SAPO-11 and SAPO-31 were synthesized with the template of DBA. The bifunctional catalysts loaded with noble metal were prepared by the incipient wetness impregnation technique with acid supports of as-synthesized molecular sieve samples. The structure, acidity and metal dispersion degree of the samples were characterized by XRD, N2 physical adsorption, SEM, TEM, XRF,29Si MAS NMR, Py-IR and H2 chemisorption. The catalytic performance of the catalysts was evaluated in hydroisomerization of n-decane used as probe-reaction on the fixed bed micro-reaction apparatus.The influence of pore structure and acidity of acid supports of Pd/SAPO-n bifunctional catalysts on catalytic performance were studied by using one-dimensional molecular sieve with different topology structures and compositions, SAPO-11, SAPO-31 and SAPO-41. Metal sites of the bifunctional catalysts were adjusted by using different noble metal, metal contents, metal salt precursors and adding the second metal as metal additive. Hence, the relationship between the catalytic performance and cooperation function of acid sites and metal sites were investigated.The results indicated that the morphology, structure and acidity of the SAPO-41 molecular sieve samples and catalytic performance of the corresponding catalysts were affected by the different silicon contents, properties of aluminum source and templates. Excessive SiO2/Al2O3 in the intial gels tended to the generation of Si domains in the samples. Compared to the SAPO-41 molecular sieve samples synthesized by using DPA as template, samples synthesized by using DBA favored the distribution of silicon in the crystallization products. SAPO-41 molecular sieve samples synthesized by using pseudoboehmite with lower reactivity as aluminum sources had more Bronsted acid sites. Hierarchical SAPO-41 molecular sieve sample synthesized by adding appropriate P123 possessed larger mesopore volume, which contributed to the formation and diffusion of multibranched isomers. Therefore, the selectivity for dibranched isomers was significantly increased on this catalyst.Compared to SAPO-11 and SAPO-41 with elliptical pore, bifunctional catalyst Pd/SAPO-31 prepared by SAPO-31 as acid supports with circular pore and appropriate acidity, exhibited better catalytic performance and isomers selectivity, because it promoted the formation and diffusion of monobranched isomers.It preserved the highest catalytic activity and isomers selectivity on the bifunctional catalyst 0.3Pd/S41(0.6) prepared by Pd(NO3)2 as metal salt precursor among the samples with the acid supports of SAPO-41 and equal noble metal loadings. On this catalyst, the conversion of n-decane was 87.7%, while the selectivity of iso-decane reached 89.6% under the reaction conditions that H2 flow rate was 40 mL/min, reaction temperature was 340℃, reaction pressure was 2.0 MPa, and WHSV(weight hourly space velocity) was 2.5 h-1. The addition of moderate rare metal additive in the preparation of bifunctional catalyst promoted the dispersion of Pd, meanwhile it accelerated the desorption of the carbenium ion from the acid sites, and restrained effectively the cracking reactions. It enhanced notably the selectivity for isomers by adding 0.2 wt.% Ce into the samples with 0.1 wt.% Pd.