Low temperature conversion of linear C4 olefins using acid ZSM-5 zeolites with aluminum concentration gradient

Two ZSM-5 materials with high and low overall Si/Al ratio, each with an Al concentration gradient across the crystals (G-samples) have been synthesized, physico-chemically characterized with several techniques (XRD, SEM, N2 adsorption, 27Al MAS NMR, NH3-TPD, FT-IR and TOF-SIMS), and catalytically tested in the low-temperature dimerization of n-butenes, paying attention to coke formation. Their acidic properties were found consistent with their framework aluminum content, except for a lower than expected number of Brønsted acid sites as measured by FTIR of adsorbed pyridine. The dimerization – cracking – realkylation mechanism, previously observed for n-butenes dimerization over acid ZSM-5 with homogeneous aluminum distribution, was confirmed over G-type catalysts. The influence of hard coke content, observed in the previous work, on product selectivity and distributions was also found for G-type catalysts. The siliceous G-type catalyst, with low amount of hard coke and micropores partly accessible to reactants, shows shape-selectivity properties consistent with dominating intra-crystalline catalysis. The aluminous G-type catalyst, with high amount of hard coke, and micropores completely filled with aromatic carbonaceous residue, shows thermodynamically equilibrated products selectivity and distributions in agreement with extra-crystalline catalysis.

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► n-Butenes dimerization at 220 °C over ZSM-5 materials with a siliceous rim.
► Sufficiently high amount of aromatic coke to totally fill micropores in aluminous ZSM-5.
► Extra-crystalline catalysis for aluminous ZSM-5, thus low activity.
► Micropores partially blocked by coke for siliceous ZSM-5: intra-crystalline catalysis, thus low changes.