Engineering of ZSM-5 zeolite crystals for enhanced lifetime in the production of light olefins via 2-methyl-2-butene cracking
The selective cracking of low-value C5 olefins such as 2-methyl-2-butene (2M2B), a growing by-product of deep catalytic cracking, over ZSM-5 zeolites is an attractive route for the on-purpose production of ethylene and propylene. Yet, several aspects, including the deactivation behaviour and the effect of the crystal properties (i.e., the size, morphology, amount of defects, and mesoporosity) on the performance, still lack understanding. Here, the relative impact of these key crystal variables on the catalyst stability and selectivity is examined by preparing a series of tailored materials with equivalent acidic properties within the optimal range for this process. Specifically, zeolites with micron- and nanosized crystals of rounded-boat and coffin-shaped morphology and different defect concentrations are synthesised in hydroxide and fluoride media, whereas intracrystalline mesopores are subsequently introduced by their controlled desilication. In agreement with the comparable active site distributions, evaluation in 2M2B cracking evidences similar initial conversion and product distributions over all catalysts, with high propylene:ethylene ratios and high butylene yields compatible with a dimerisation–cracking mechanism. In contrast, major differences in the deactivation behaviour are demonstrated, clearly illustrating the dominant impact of the mesopore (external) surface area compared to the defect concentration or crystal morphology. Comparative evaluation in the widely studied conversion of methanol to olefins (MTO) reveals qualitatively similar trends. Quantitatively, however, a striking six-fold extension of the lifetime (347 versus 54 h) and a 3-fold higher light olefin space-time yield (6.67 versus 2.25 gC2–4= gzeolite−1 h−1) are observed over the best catalyst in 2M2B cracking with respect to MTO.