Selective dealumination of HZSM-5 zeolite boosts propylene by modifying 1-butene cracking pathway

An HZSM-5 zeolite was dealuminated by steaming with its self-adsorbed water at mild temperatures (300, 400 or 500 °C), characterized and tested in the transformation of 1-butene into propylene. The structural, porous-surface and acidic properties of the catalysts (unmodified and steamed) were characterized using N₂ adsorption-desorption isotherms, tert-butylamine adsorption-TPD, XRD, ²⁹Si and ²⁷Al MAS-NMR, and FTIR spectroscopies, and the results pointed to the following evidences: (i) dealumination took place transforming specific strong acid, framework tetrahedral Al sitings (located within the micropores) into distorted and extra-framework species; and (ii) the porous-surface properties were less sensible to the treatment. The acid site density was shifted towards the intersections of the zeolite micropores (of weaker acidity), resulting in the selective inhibition of hydrogen transfer pathway, thus lowering the selectivity of paraffins, aromatics and ultimately, coke deposition. Overall, the performance of the 500 °C-steamed catalyst compared with the parent one was enhanced: propylene selectivity was boosted 69%, coke deposition dropped by 34%, whereas the initial conversion only fell 6%.