Ind. Eng. Chem. Res. 2005, 44, 8033-8038
8033
Synthesis of Hydrated Aluminum Phosphate, AlPO4‚1.5H2O (AlPO4-H3), by Controlled Reactive Crystallization in Sulfate Media Felipe Lagno and George P. Demopoulos* Department of Mining, Metals and Materials Engineering, McGill University, 3610 University Street, Montreal, QC, Canada H3A 2B2
A new method for the synthesis of hydrated aluminum phosphate, AlPO4‚1.5H2O, in a semibatch reactor is reported. The synthesis involves direct reactive crystallization induced by pH adjustment to a predetermined supersaturation level. Precipitation was conducted in sulfatebased solutions with Al(III):P(V) molar ratio of 1 and at 95 °C. Homogeneous precipitation tests led to the definition of the metastable zone of aluminum phosphate. This zone was used to design the supersaturation-controlled seeded test. XRD, TGA, and chemical analysis confirmed the product to be AlPO4‚1.5H2O. SEM and particle size analysis revealed the product to consist of aggregates of submicron-size crystallites of 3.5 µm average size (homogeneously formed product) grown to 16.5 µm after three cycles of seeded crystallization. 1. Introduction Aluminum phosphates are of interest in both environmental and technological fields. With reference to the former, the formation of aluminum phosphates helps remove phosphate from wastewaters or, in other instances, their dissolution helps regulate the release of phosphate in acidic soils.1,2 In terms of technological relevance, aluminum phosphates are important in the area of catalysis, due to their surface acidity and thermal stability.3-5 Few well-crystallized aluminum phosphates, with an Al2O3 to P2O5 molar ratio of 1, have been observed in nature.6 These include anhydrous aluminum phosphate, berlinite (the quartz form of AlPO4), tridymite, and cristobalite. Two hydrated aluminum phosphates are also included: variscite and metavariscite, both compositionally AlPO4‚2H2O.7,8 Variscite is orthorhombic, 2/m2/m2/m, whereas metavariscite is monoclinic, 2/m. In addition to the naturally occurring aluminum phosphates, a number of synthetic hydrated aluminum phosphates have been reported: so-called AlPO4-Hi (i ) 1, 2, 3, 4). AlPO4-H1 and -H2 exhibit properties of molecular sieves; however, AlPO4-H3 and -H4 do not represent microporous materials, although AlPO4-H3 shows some prominent water adsorption characteristics.9 According to McCusker and co-workers,10 AlPO4H1 is hexagonal, having a molecular formula Al18P18O72‚ 42H2O (AlPO4‚2.33H2O); AlPO4-H2 structure is orthorhombic and its molecular formula is Al6P6O24‚4H2O (AlPO4‚0.67H2O).11 The crystal structure of AlPO4-H3, as determined by Pluth and Smith,12 is orthorhombic with a chemical formula Al16P16O64‚24H2O (AlPO4‚ 1.5H2O). The crystal and molecular structure of AlPO4H4 were recently determined by Poojary et al.13 to be monoclinic with composition AlPO4‚H2O. Variscite, metavariscite, and the hydrates of AlPO4-H1 to AlPO4-H4 show important structural differences. In variscite and metavariscite all the aluminum atoms are AlO4(H2O)2 octahedral;7,8 in AlPO4-H1 and AlPO4-H2, 33.3% of the * To whom correspondence should be addressed. Tel.: (514) 398-2046. Fax: (514) 398-4492. E-mail: george.demopoulos@ mcgill.ca.
framework aluminum atoms are AlO4(H2O)2 octahedral and 66.7% of Al atoms are tetrahedrally (AlO4) coordinated.14,15 On the other hand, AlPO4-H3 and AlPO4H4 contain equal amounts of AlO4 tetrahedral and AlO4(H2O)2 octahedral.12,13 Several synthesis methods have been described in the literature in the production of one or more of these aluminum phosphate phases. These methods can be classified in two main routes: the first one involves crystallization of aluminophosphate gels and the second one involves homogeneous precipitation of amorphous aluminum phosphate solids followed by their conversion to crystalline phases via aging. In both routes chloride and nitrate solutions were used. d’Yvoire was the first to report the synthesis of AlPO4-Hi (i ) 1, 2, 3, 4) along with variscite and metavariscite16 using aluminophosphate gels (Al2O3:P2O5 ) 2.73) heated at temperatures around 100 °C and subsequent refluxing. According to this procedure different hydrated aluminum phosphates are obtained by varying the amounts of Al(III) and P(V) in solution. Thus, concentrated solutions ([Al(III)] )1.96 mol/L and [P(V)] ) 5.35 mol/L) were found to produce metavariscite, while dilute solutions ([Al(III)] ) 0.0032 mol/L and [P(V)] ) 0.0089 mol/L) were found to produce variscite. However, for solutions of intermediate concentration, d’Yvoire observed the formation of metastable AlPO4-Hi hydrates that upon prolonged equilibration converted to variscite or a mixture of variscite and metavariscite. d’Yvoire, though, was not able to isolate AlPO4-H1, -H2, -H3, or -H4 as single pure phases. Duncan et al.9 reinvestigated the synthesis of these metastable hydrated aluminum phosphates, H1 through H4, by working at hydrothermal conditions. Thus, they were able to produce highly crystalline AlPO4-H1 and AlPO4-H4 by reacting gels of different molar amounts of Al2O3, P2O5, and HCl (Al2O3:nP2O5:mHCl:50H2O n ) 0.6, 0.8, 1.0, 1.2 and m ) 0, 0.5, 1.0, 1.5, 2.0) at 140 °C. It was reported that AlPO4-H1 crystallized as a pure phase within 4 h followed by its transformation to AlPO4-H4 after an extended period of time (18 h). They were not able to isolate pure AlPO4-H2 material, but they managed to produce pure AlPO4-H3 phase using
10.1021/ie0505559 CCC: $30.25 © 2005 American Chemical Society Published on Web 09/10/2005
8034
Ind. Eng. Chem. Res., Vol. 44, No. 21, 2005
an aluminophosphate gel system with the additions of 1.0Al2O3:1.2P2O5:1.0HCl:50.0H2O at 140 °C after 4 h. Garcı´a-Carmona et al.15 reported the synthesis of aluminum phosphate hydrates following the aluminophosphate gel route again, but using a quaternary ammonium template. This template appears to control the structure of the synthesized material.17 Their synthesis method involved reacting gels at controlled pH of composition n-dipropylamine:P2O5:Al2O3:40H2O at the solution boiling point and atmospheric pressure. The reaction products varied with pH: (1) at pH 3.5-3.8 variscite was the predominant phase, (2) in the pH intervals 3.8-5.2 and 5.4-5.7 AlPO4-H3 was the major phase, and (3) at pH 5.2-5.4 AlPO4-H1 and -H2 were obtained. More recently Kunii et al.18 reported the synthesis of AlPO4-H1, -H2 and -H3 via microwave heating of aluminophosphate gels made up of different kinds of aluminum sources, namely amorphous aluminum hydroxide, gibbsite, and boehmite. More specifically, the authors were able to produce pure AlPO4-H1, -H2, and -H3, by crystallizing gels at 125, 118, and 90 °C containing aluminum hydroxide, gibbsite, and boehmite, respectively. The second synthesis route involving homogeneous precipitation of amorphous aluminum phosphates followed by their conversion to crystalline phases via aging has been limited to the preparation of variscite. Thus, Cole and Jackson19 reported that upon addition of 0.1 M sodium hydroxide to slightly acid solutions of aluminum chloride containing a 2:1 excess of phosphate (as KH2PO4 or NaH2PO4) an amorphous aluminum phosphate precipitated, which after aging at 90 °C from 8 h to 2 days yielded variscite, AlPO4‚2H2O, contaminated with small amounts of sterrettite, Al6(PO4)4(OH)6‚5H2O. Similarly, Bache20 reported the synthesis of variscite by mixing dilute solutions of AlCl3 and NaH2PO4 in the molar ratio Al(III):P(V) ) 0.4 with the precipitation pH adjusted to 4.0 and aging at 90 °C for 14 days. Finally, Matijevic´ and co-workers21,22 reported the production of colloidal crystalline variscite of several morphologies by aging at elevated temperatures of 80-98 °C acidified solutions containing Al(NO)3 and K2HPO4 at pH
