title: Unraveling the crystallization mechanism of CoAPO-5 molecular sieves under hydrothermal conditions.
abstract: The hydrothermal crystallization of CoAPO-5 molecular sieves has been studied using time-resolved in-situ SAXS/WAXS, UV-vis, Raman, and XAS. Data collected during heating to 180 degrees C allowed the observation of different steps occurring during the transformation of the amorphous gel into a crystalline material from a macroscopic and atomic perspective. Raman spectroscopy detected the initial formation of Al-O-P bonds, whereas SAXS showed that these gel particles had a broad size distribution ranging from ca. 7 to 20 nm before crystallization began. WAXS showed that this crystallization was sharp and occurred at around 160 degrees C. Analysis of the crystallization kinetics suggested a one-dimensional growth process. XAS showed that Co(2+) transformed via a two-stage process during heating involving (i) a gradual transformation of octahedral coordination into tetrahedral coordination before the appearance of Bragg peaks corresponding to AFI, suggesting progressive incorporation of Co(2+) into the poorly ordered Al-O-P network up to ca. 150 degrees C, and (ii) a rapid transformation of remaining octahedral Co(2+) at the onset of crystallization. Co(2+) was observed to retard crystallization of AFI but provided valuable information regarding the synthesis process by acting as an internal probe. A three-stage, one-dimensional crystallization mechanism is proposed: (i) an initial reaction between aluminum and phosphate units forming a primary amorphous phase, (ii) progressive condensation of linear Al-O-P chains forming a poorly ordered structure separated by template molecules up to ca. 155 degrees C, and (iii) rapid internal reorganization of the aluminophosphate network leading to crystallization of the AFI crystal structure.
authors.affiliation: Department of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands.
authors.forename: Didier
authors.initials: D
authors.lastname: Grandjean
authors.forename: Andrew M
authors.initials: AM
authors.lastname: Beale
authors.forename: Andrei V
authors.initials: AV
authors.lastname: Petukhov
authors.forename: Bert M
authors.initials: BM
authors.lastname: Weckhuysen
mesh_terms: D017607:Aluminum Compounds / Q000737:chemistry*; D003035:Cobalt / Q000737:chemistry*; D003460:Crystallization; D008958:Models, Molecular; D010100:Oxygen / Q000737:chemistry*; D010710:Phosphates / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet / Q000379:methods; D013057:Spectrum Analysis / Q000379:methods; D013059:Spectrum Analysis, Raman / Q000379:methods; D013696:Temperature; D013997:Time Factors; D014965:X-Rays
chemical_list: D017607:Aluminum Compounds; D010710:Phosphates; D003035:Cobalt; C012714:aluminum phosphate; D010100:Oxygen