Applied Catalysis, 42 (1988) 15-27 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHG Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 15 zyxwvut Diffusion, Cracking and Coking on HZSM-5 of Various Morphologies JOACHIM VoLTER*, JURGEN CAR0 and MARTIN BELOW Zentralinstitut fiirphysikalische Chemie der Akademie der Wissenschaften der D.D.R., Rudower Chaussee 5, 1199 Berlin (G.D.R.) BA RBA RA FA HLKE Zentralinstitut fiir anorganische Chemie der Akademie der Wissenschaften der D.D.R., Rudower Chaussee 5, 1199 Berlin (G.D.R.) and JORG KA RGER and MICHAEL HUNGER Sektion Physik der Karl-Marx-Universitait Leipzig, Linnestr. 5, 7010 Leipzig (G.D.R.j (Received 10 April 1987, accepted 10 March 1988) A BSTRA CT Coked HZSM-5 samples of polycrystalline spherical and polyhedral morphology were studied using an NMR self-diffusion technique and n-hexane cracking. It was concluded that, in addition to the first order pores of the regular zeolite channel network, the polycrystalline grain contained second order pores. Second order pores exhibited two remarkable properties: self-diffusion w as enhanced, and coke was deposited preferentially within them. The presence of empty second order pores allowed coking to occur without significant poisoning. The cracking rate decreased with increasing particle size. INTRODUCTION zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Elucidation of the mechanism of coke formation on pentasil type zeolites represents a challenging task to both basic and industrial research workers. Recently, selectedproblems of coke deposition on HZSM-5 have been resolved, e.g. the interdependence of coking and deactivation rates [l-3], the role of zeolite pore structure in determining the chemical composition of the coke [l- 31 and the poisoning of acid sites by coke [ 41. Still contradictory, however, are the results from experiments examining the influence of crystal size and mor- phology [ 5 ] on the coking behaviour of HZSM-5 and those from experiments where the problem of where the coke is deposited are dealt with. Whereas in some previous papers HZSM-5 was assumed to produce coke preferentially on the external surface of the crystals [ 6, 71 recent studies give clear evidence of 0166-9834/ 88/ $03.50 0 1988 Elsevier Science Publishers B.V.