Study On The MgAl₂O₄-Based Materials With Both Acidic And Basic Sites

As the sulfur content in crude oil for fluid catalytic cracking (FCC) process is increased and the environmental legislation becomes more stringent, the removal of SOx from FCC units has been the subject of a considerable amount of attention over the past few years. To date, the least costly means is the addition of 2%?3% of sulfur-transfer agent which generally contains MgAl2O4. However, the stronger basic property for the spinel makes it to be less active for cracking hydrocarbons even compared with conventional inert matrices of FCC catalysts such as clays. Since the amount of sulfur-transfer agents required for effective SOx emission reduction is increased due to the increasing sulfur content in crude oil and the more stringent environmental regulations, there is no doubt that their dilution effect on FCC catalysts would be stronger.Therefore, an assumption of preparing the MgAl2O4-based materials which combine the basic property of MgAl2O4 with acidic property of zeolites was put forward. In this way, the addition amount of spinel would be increased without much affecting the catalytic performance of the FCC base catalyst. This paper mainly focused on two aspects. One way is physically mixing MgAl2O4 with ZSM-5 zeolite in order to obtain the bifunctional additives for enhancing propylene yield and removing SO2 in FCC units. For the bifunctional additives, ZSM-5 zeolite was employed as the active component and MgAl2O4 applied as matrix.The other method is to introduce the zeolitic building units into the mesopores of the spinel in light of the synthesis of meso-microporous zeolite. The obtained modified spinels with both basic and acidic sites would have much higher cracking ability compared to the parent spinel due to the improvement of Bronsted and Lewis acidity which can survive the severe hydrothermal treatment. Moreover, there would be still basic sites remained active for SO2 adsorption. Besides, the capacity of SO2 picking-up of MgAl2O4 is closely related to surface area. The increase in surface area can lead to the improvement in SO2 uptake capacity. So the synthesis of MgAl2O4 with high surface area was also explored in the paper.The common point of the bifunctional additive for enhancing propylene yield and removing SO2 and the modified spinel with both basic and acidic sites is that both combine the acidic property for the zeolite and the basic property for the spinel into a single particle. But there is big difference between the two kinds of materials. When the bifunctional additive is added into the FCC base catalyst, the propylene and LPG yield increase at the cost of gasoline yield, and the function of removing SO2 is also attached. While the introduction of a larger amount of modified spinel into the FCC system would guarantee the effective SO2 removal on the condition that it affects little the catalytic property of FCC catalyst (especially gasoline and diesiel oil yileds).As to the bifunctional additive for enhancing propylene yield and removing SO2, the bottleneck is that the existence of magnesium though in the form of stable spinel would interact with ZSM-5 during calcination and severe hydrothermal conditions, resulting in lower ZSM-5 activity. Moreover, the relatively higher HTC value and coke selectivity of MgAl2O4 also contribute to the lower ZSM-5 acitivity of the bifunctional additives for increasing propylene yield. However, the negative effect of Mg2+ could be counteracted by modification of ZSM-5 with both La and P, leading to the improvement of ZSM-5 hydrothermal stability and activity. On the other hand, modifying MgAl2O4 with P could not only inhibit the change in ZSM-5 acidity caused by the existence of MgAl2O4 but also largely decrease the hydrogen transfer activity of spinels, thereby boosting the ZSM-5 effectiveness in increasing propylene yield. And P doping also distinctly promoted the SO2 uptake capacity of bifunctional additives. What's more, pretreating MgAl2O4 with NH4Cl or HCl solutions or further exchanging bifunctional additives with amonium cations could also increase the ZSM-5 activity for increasing propylene yield and improve the DeSOx activity. The pretreatment of MgAl2O4 with NH4Cl or HCl solutions not only changed the ZSM-5 acidity and increased the weak acidic hydroxyl groups of additives, but also decreased the selective hydrogen transfer activity of spinels, which may both be responsible for the increase in ZSM-5 activity.Concerning the modified spinel with both acidic and basic sites, the structural building units of zeolite can be introduced into the mesopore of the spinel by employing the nanocluster solutions of zeolite Y or ZSM-5 mother solution. Compared to the parent spinel, the acidity and cracking ability of spinel were greatly increased, but the DeSOx activity was obviously decreased. When modifying the Ce/MgAl2O4 with ZSM-5 mother solution under neutral condition (pH=7), the building units of zeolite could be introduced into the spinel with the preservation of the CeO2 phase, obtaining the modified Ce/MgAl2O4 with both improved cracking ability and well remained DeSOx activity.The introduction of divalent transition metals Cu, Co or trivalent transition metals Fe, Cr into MgAl2O4 could improve the SO2 uptake capacity of the spinel and the reducibility of the sulfate formed during removal SO2 in different extents. Also it could increase the conversion of VGO cracking, but the non-selective hydrogen transfer activity (esp. coke selectivity) was evidently increased.Suitable treatment of Mg- or Al-rich magnesium-aluminate spinel prepared under acidic conditions with HNO3 solutions could obtain spinels with high surface areas of 301 and 311 m2/g, respectively. But the obtained spinels revealed poor hydrothermal stability.