丁烯催化裂解反应中的C—H键型副反应和化学平衡问题

Catalytic Cracking of Butenes:on the C—H Bond related side-reactions and Chemical Equilibrium

用小型固定床反应器以及氨气程序升温脱附(NH3-TPD)和吡啶吸附红外光谱(Py-IR)研究了醚后碳四液化气中的丁烯在改性纳米ZSM-5沸石催化剂上的催化裂解反应.结果表明,丁烯催化裂解反应的丙烯选择性主要受氢转移和脱氢反应制约.这些C—H键型反应生成烷烃和芳烃副产物,并导致催化剂积炭.低温有利于氢转移(尤其异丁烯的氢转移反应),高温有利于脱氢.脱氢反应需要强酸中心,但氢转移反应在弱酸中心上也能进行.因此,通过改性降低催化剂的酸度虽然可以有效抑制C—H键型副反应,甚至消除脱氢副反应,却不能完全阻止氢转移副反应.在450℃以上的高温下,产物丙烯生成丁烯和乙烯的歧化反应,以及正丁烯向异丁烯的异构化反应导致了丁烯转化率的降低.

The cracking of butenes in post-MTBE C4LPG over modified nano-sized HZSM-5 zeolite catalyst was investigated with small fixed-bed reactor,ammonia temperature-programmed desorption（NH3-TPD） and pyridine-adsorption infrared spectroscopy（Py-IR）.Results show that,the propylene selectivity of the butenes catalytic cracking is mainly limited by hydrogen-transfer and dehydrogenation reactions.These C—H bond related side-reactions produce by-products such as alkanes and aromatics,and are responsible for the coking of the catalyst.The decrease of the reaction temperature favors hydrogen-transfer（especially the hydrogen-transfer of isobutene）,while the increase of the reaction temperature favors dehydrogenation.On the other hand,dehydrogenation needs strong acid sites,whereas hydrogen-transfer can occur with weak acid sites.As a result,the weakening of catalyst acidity can suppress the C—H bond related side-reactions,and even completely stop dehydrogenation when all the strong acid sites disappear,but it is impossible to avoid hydrogen-transfer under the conditions of butene catalytic cracking.When the catalytic cracking was carried out at 450 ℃ and above,the decrease of butene conversion occurs because of the disproportionation of propylene to butene and ethylene,and the isomerization of normal butenes to isobutene.