Effect of phosphorus on novel bifunctional additives for enhancing the production of propylene and removal of SO2 in FCC process

Magnesium-aluminate spinels were introduced into the matrix of typical propylene additives (ZSM-5 as the active component) to obtain a series of novel bifunctional additives for enhancing the production of propylene and removal of SO2. Their performance as additives to a commercial equilibrium USY FCC catalyst for VGO (vacuum gas oil) cracking and activity in oxidative adsorption of SO2 under similar FCC regenerator conditions were investigated. The bottleneck of the novel bifunctional additives is that the substitution of MgAl2O4 for Kaolin clay in propylene additives causes the migration of Mg2+ into ZSM-5 zeolite, which evidently lowers the activity of ZSM-5 as an FCC additive. However, pretreating MgAl2O4 with phosphoric acid either by extraction or by impregnation method which weakened the interaction between MgAl2O4 and ZSM-5 during the steaming and calcination processes, respectively, was proved to be effective in enhancing the efficiency of ZSM-5 for increasing propylene yield. Concurrently, the unusally high hydrogen transfer activity of MgAl2O4 in VGO cracking, which also contributed to the decrease in ZSM-5 activity, was sharply reduced by P doping. Moreover, the phosphorus modification also promoted the SO2 uptake capacity of bifunctional additives.

Graphical abstractThe adsorption of SO2 over different novel bifunctional additives. M-the bifunctional additive containing MgAl2O4; M-P1-the bifunctional additive containing MgAl2O4 modified by P via extraction method; M-P2-the bifunctional additive containing MgAl2O4 modified by P via impregnation method; M-P1-i.e.-the bifunctional additive obtaining by ion exchange of M-P1 with NH4+.Figure optionsDownload full-size imageDownload high-quality image (129 K)Download as PowerPoint slideHighlights► During the substitution of MgAl2O4 for Kaolin clay, matrix of a typical propylene additive, ZSM-5 interacted with MgAl2O4. ► The interaction between MgAl2O4 and ZSM-5 caused the decrease of ZSM-5 activity in improving propylene yield. ► The particularly higher hydrogen transfer activity of MgAl2O4 than Kaolin clay in VGO cracking also contributed to the lower ZSM-5 activity for improving propylene yield. ► The modification of MgAl2O4 with P retarded the interaction between MgAl2O4 and ZSM-5. ► The P doping into the spinel also reduced the hydrogen transfer activity of MgAl2O4.