Synthesis of aluminophosphate molecular sieves with AFI topology substituted by alkaline earth metal and their application to solid acid catalysis

Abstract

Metalloaluminophosphate molecular sieves with AFI topology, in which a part of aluminum is substituted isomorphously to alkaline earth metals: Mg, Ca, Sr, and Ba (MAPO-5; M = Mg, Ca, Sr and Ba), were synthesized by hydrothermal synthesis (HTS) method and two types of dry–gel conversion (DGC) methods: vapor-phase transport (VPT) and steam-assisted conversion (SAC) methods, using triethylamine (Et 3N) as a structure-directing agent (SDA). Systematic studies were carried out by varying the synthesis parameters such as gel composition including content of alkaline earth metals and SDA, amount of external bulk water, and heating time. It has been observed that MO/Al 2O 3 ratio of starting gel plays an important role in the crystallization of the molecular sieves. The ratios from 0.025 to 0.10 gave pure MAPO-5 with AFI topology; however, MAPO-34 with CHA topology was observed as an impurity with further increase of the Mg content. The amount of Et 3N also played important role for the crystallization: Et 3N/Al 2O 3 ratios from 0.38 to 0.76 gave pure MgAPO-5 and CaAPO-5, whereas SrAPO-5 and BaAPO-5 were obtained as pure phase from 0.76 to 1.52. MAPO-5 molecular sieves were crystallized even in the absence of external bulk water. Period for crystallization was studied from 3 to 48 h; the highest intensities were observed between 12 and 18 h for all cases. The occluded SDA was removed by calcination at 550 °C. The molecular sieves were characterized by XRD, ICP, TG-DTA, SEM, FT-IR, N 2 and pyridine adsorption, and NH 3-TPD. Alkaline earth metals, at least a part of them, are isomorphously substituted in the framework of AFI topology judging from NH 3-TPD, unit cell parameters, and their FT-IR spectra of adsorbed pyridine. The substitution of alkaline earth metal with aluminum produced acidic sites in the molecular sieves: substitution of Al 3+ to Mg 2+ enhanced the appearance of strong acidity; however, only weak acidities appeared for CaAPO-5, SrAPO-5, and BaAPO-5 in small amount. The isopropylation of benzene and biphenyl was examined over MAPO-5 by VPT method to characterize their acidic properties. Their catalytic activity decreases in the order: MgAPO-5 ≫ CaAPO-5 ⩾ SrAPO-5 ⩾ BaAPO-5; ≫ APO-5; moderate to good selectivity for 4,4′-DIPB was observed in the level of 50–70% for all MAPO-5. These catalytic properties support strongly the incorporation of alkaline earth metals in the AFI framework.