Catalytic cracking and skeletal isomerization of n‐Hexene on ZSM‐5 Zeolite

Abstract

The catalytic cracking and skeletal isomerization of n‐hexenes on 60/80 mesh ZSM‐5 zeolite were studied in the temperature range 350–405°C. By applying the time‐on‐stream theory, the products of the reaction were identified as primary, secondary or both according to their optimum performance envelope (OPE) curves on product selectivity plots. The products of cracking were found to be almost exclusively mono‐olefins and those in the range C3‐C5 were found to be stable primary, or primary plus secondary products. No C1 was found, and only traces of C2 as ethylene. The observed product distributions can be explained by a dimerization‐cracking mechanism with no product species having more than twelve carbon atoms. The probability of a fragment undergoing further cracking before desorption increased with temperature and the observed initial selectivities must be corrected to account for this process. Methylpentenes, formed as unstable primary products, were the main isomers produced by skeletal rearrangement, with those derived from more stable carbenium ions predominating. Paraffins, coke and aromatics were found in small amounts only.

Kinetics and selectivity of reactions of 1‐alkenes on HY zeolite: The influence of chain length

“…We have previously reported the cracking behaviour of 1-hexene on HY at 400°C (Abbot and Wojciechowski, 1985a), as well as on amorphous silica-alumina (Abbot and Wojciechowski, 1985b) and HZSM-5 (Abbot and Wojciechowski, 1985~). Table 6 shows that C4 and C5 fragments are observed as initial products, and as there are no corresponding C2 and CI species formed initially a dimerization-cracking process has been postulated to explain the observed product distribution.…”

Selectivity control through fundamental mechanistic insight in the conversion of methanol to hydrocarbons over zeolites

“…This observation is in agreement with the high selectivity for C 3+ alkenes (up to >98%) mentioned previously. All C 3+ alkenes show identical 13 C contents, indicative of the equilibrating nature of cycle II, whereas ethene lies consistently marginally below, which could point to a very minor contribution from the aromatics based cycle I. Cracking of hexene over H-ZSM-22 catalyst at high temperatures has been shown to yield ethene as cracking product [38] and since ethene contains a lot more 13 C than the retained material during the MTH reaction over the H-ZSM-22 catalyst, the majority of the ethene probably comes from the cracking of higher alkenes. Previous studies have shown that the methylation rate of 12 C alkenes with 13 C methanol increases in the order of ethene, propene and butene [39,40].…”

Using kinetics to study intermediates in acid catalyzed reactions on solids

“…A recent selectivity study revealed that 1-hexene produces the products of skeletal rearrangement listed in Table I with initial selectivRies as shown [6]. This information can be used to develop the reaction network shown in Figure 4.…”