Synthesis and characterisation of zeolites for potential use as supports for low-temperature fischer-tropsch wax hydrocracking catalysts

Abstract

In this study, the use of 10-member ring zeolites with different silica-to-alumina ratios (SARs) as supports for palladium (Pd) to produce hydrocracking catalysts was investigated. The syntheses of zeolites ZSM-22, ZSM-48, ZBM-30 and ZSM-23 were carried out under hydrothermal conditions, and the resulting materials were characterised with XRD, SEM and BET surface area measurements, prior to activity tests. Successful synthesis of ZSM-48 and ZBM-30 remained elusive for the major part of this study, and these zeolites could therefore not be catalytically tested. The XRD patterns showed pure ZSM-22 materials with different SARs (60, 80 and 120) were successfully synthesised using hexamethylenediamine (HMDA) as a structure-directing agent (SDA) and a synthesis gel with pH 12. However, at synthesis gel pH 13, cristobalite and ZSM-5 impurity phases tend to form in addition to ZSM-22. Relative % XRD crystallinity of the materials prepared at synthesis gel pH 12 decreased with a decrease in SAR, and there was no specific trend for the response of a particular SAR to changes in pH from 12 to 13. SEM micrographs showed needle-shaped crystals with lengths in the range 0.6 - 1.2 μm. The BET surface area of the ZSM-22 with SAR of 60 was found to be 189 m2/g, which is around the theoretical BET surface area of ZSM-22 materials and the presence of impurities lowered the surface area of the ZSM-22 materials. The synthesis of ZBM-30 using triethylenetetramine (TETA) and a (1 TETA : 1 pyrrolidine) mixture as SDAs was also attempted. The XRD patterns showed that a completely amorphous material was obtained when using TETA as SDA and ZSM-39 was produced when using the mixture as SDA. The XRD patterns revealed that impurity-free ZSM-23 materials were successfully synthesised with SAR > 60, and that with SAR of 60, ZSM-5 was produced instead. Relative % XRD crystallinity of the impurity-free ZSM-23 materials increased with an increase in SAR from 80 to 120. SEM micrographs of the impurity-free ZSM-23 materials showed needle-shaped crystals of around 0.9 μm in length. The predominantly ZSM-5 material had the highest BET surface area compared to the impurity-free ZSM-23 materials. ZSM-48 synthesis was attempted using HMDA (produced ZSM-22), pyrrolidine (produced ZSM-23) and hexamethonium bromide (HMBr2) as SDAs. The XRD and SEM vi analysis showed only HMBr2 successfully directed the synthesis of impurity-free crystalline ZSM-48 at prolonged synthesis time of 168 h. ZSM-48 crystals were also needle-shaped and 4.2 - 11.3 μm in length. The incipient wetness impregnation method was used to achieve 0.5 wt. % Pd loadings on the catalysts. The hydrocracking of n-hexadecane (n-C16) over the catalysts was studied at conditions typical of catalytic cracking of LTFT products. At 225 oC, the Pd/ZSM-22 (80) and Pd/ZSM-23 (80) were highly selective to cracking products, with excessive secondary cracking occurring over these catalysts, as indicated by the C4/C12 ratios of 11.3 and 5.2, respectively. Excessive secondary cracking (C4/C12 = 11.7) was also observed over Pd/ZSM-23 (60). However, the Pd/ZSM-22 (60) and Pd/ZSM-23 (120) catalysts achieved a C4/C12 ratio close to 1.0, suggesting closeness to ideal hydrocracking behaviour. The Pd/ZSM-22 (60) (C4/C12 of 1.9) catalyst, was physically mixed with Pd/ZSM-5 (90) (C4/C12 = 6.4) and catalytically tested for the hydrocracking of n-C16. This Pd/ZSM-5/ZSM-22 catalyst achieved a remarkable C4/C12 = 1.1, which is less than what was achieved over the individual catalysts. On the basis of the C4/C12, this catalyst’s behaviour is close to that of an ideal hydrocracking behaviour. In summary, Pd/ZSM-22 (80), Pd/ZSM-22 (120) and Pd/ZSM-23 (80) catalysts are promising for diesel-selective catalysis and need further exploration.