Enhancing the activity of acidic catalysts with supercritical reaction media.

摘要

The problem of catalyst deactivation due to coking is prevalent in many industrial reactions. Undesirable side reactions produce olefinic oligomers, which consolidate into coke. Coke blocks catalytic sites and reduces activity, thereby placing an economic burden on the process. The objective of this work is to minimize catalyst coking.; Elimination of catalyst coking requires a balance between the oligomer formation and removal rates. Oligomer formation decreases when organic peroxide catalyzed, oxygen-induced, and metal catalyzed reactions are reduced. Oligomer removal increases when oligomers are solubilized and rapidly transported out of the catalyst pores. Supercritical fluids are well suited for in situ oligomer removal by virtue of their liquid-like densities and gas-like transport properties.; Investigations of 1-hexene isomerization on an industrial {dollar}rm Pt/gamma{dollar}-{dollar}rm Alsb2Osb3{dollar} catalyst are conducted at temperatures from 235 to {dollar}310spcirc{dollar}C (1.01-1.16T{dollar}rmsb{lcub}c{rcub}){dollar} and pressures from 35-70bar (1.1-2.2P{dollar}rmsb{lcub}c{rcub}){dollar} at a WHSV of {dollar}rm 135gsb{lcub}hexene{rcub}/gsb{lcub}catalyst{rcub}/h.{dollar} Supercritical operation with reduced oligomer formation results in virtually constant catalyst activity for 42 hours time-on-stream. In contrast, complete deactivation occurs in 8 hours without oligomer reduction at subcritical conditions.; The supercritical decoking concept and the oligomer formation mitigation methods are extended to an industrially significant alkylation reaction. Iso-butane/1-butene alkylation produces high-octane gasoline blending agents. A solid-acid catalyst replacement for the conventional acids {dollar}rm(Hsb2SOsb4,{dollar} HF) is desirable for environmental, safety, and economic reasons.; Conventional supercritical operation requires high operating temperatures {dollar}rm({lcub}sim{rcub}135spcirc C),{dollar} which promote undesirable reactions. By employing an inert, low {dollar}rm Tsb{lcub}c{rcub}{dollar} diluent {dollar}rm(COsb2),{dollar} the reaction is performed supercritically, at temperatures below the reactants critical temperature. Low temperature {dollar}rm({lcub}<{rcub}100spcirc C){dollar} supercritical operation results in virtually steady alkylate production on both microporous zeolitic (H-USY, beta) and mesoporous (sulfated zirconia, Nafion) solid acid catalysts. At a WHSV of {dollar}rm 0.25gsb{lcub}butene{rcub}/gsb{lcub}catalyst{rcub}/h, COsb2{dollar}:isobutane:olefin ratio of 86:8:1, {dollar}50spcircrm C{dollar} and 155bar, butene conversion on a USY catalyst is 20% and alkylates account for 5% of the products at steady state. Under similar conditions using Nafion as catalyst, alkylates account for 20% of the products at steady state.; The carbon dioxide-based, fixed-bed, solid-acid alkylation process shows promise as an environmentally superior alternative to conventional processes. The major remaining challenge is to increase alkylate selectivity to levels competitive with current technology.