Optimization of ZSM-48 zeolite desilication parameters for catalytic conversion of methanol to hydrocarbon using experimental design and response surface methodology

The MTH is considered one of the widely used technologies for production liquid hydrocarbon from biomass, natural gas, and coal [1]. The most important limitation in microporous zeolites is the diffusion limitation which is the consequence of blocking the micropores [2]. The existence of mesopores in zeolites promotes the reactants access to active sites [3]. The medium pore zeolites have been widely used for conversion of methanol to aromatics due to suitability of medium size pores for the ring formation in aromatic production. The present work reports ZSM-48 catalyst design for the methanol to liquid hydrocarbon conversion. Mesoporous ZSM-48 zeolite was prepared through desilication at optimized conditions by employing central composite design (CCD) under response surface methodology (RSM). The temperature of desilication ranges between 40 and 80 °C and NaOH concentration was varied between 0.1 and 0.3 M. The software gave thirteen (13) runs experiment within the conditions. Optimal results showed that to achieve a high BET surface area and medium pore diameter, the temperature of treatment and concentration of NaOH should be at 60 °C and 0.15 M respectively. The reliability of the model used was successfully validated by additional tests. Analysis of variance (ANOVA) study suggested the equation to be significant for the desilication with major impact of sodium hydroxide concentration on the desilication than temperature. Statistical results showed a R2 of 0.95 and 0.98 for temperature and NaOH concentrations, respectively. The influence of the post-synthesis desilication on the pore characteristics, crystallinity, morphology and acidity of the catalyst obtained under the optimal conditions was examined using N2-adsorption, XRD, SEM and NH3-TPD. Optimised catalyst presented a BET surface area of 260 m g-1 and mean pore diameret of 5 nm. The prepared catalyst was used to methanol to hydrocarbon conversion in a fixed-bed stainless-steel reactor at 390 °C and weight hourly space velocity of 2.5 h-1. Over modified ZSM-48 zeolite, C4 and aromatic production was promoted. The yield of liquid hydrocarbon over this catalyst was increased about 35% and catalytic life time increased about 8% relative to microporous ZSM-48.