Advanced Powder Technology, Vol.31, No.1, 402-408, 2020
Magnetically recoverable catalyst based on porous nanocrystalline HoFeO3 for processes of n-hexane conversion
Nowadays, a wide class of rare earth orthoferrites is of great interest among acid-base catalysts due to their high activity and the possibility of magnetic recovery. However, the synthesis of chemically and phase-pure rare-earth orthoferrites in the form of nanocrystals with developed surface and microstructure is still a difficult task. In this work, we report for the first time a successful synthesis of phase-pure holmium orthoferrite (HoFeO3) nanocrystals with a porous microstructure and a branched surface via simple glycine-nitrate combustion approach. Holmium orthoferrite nanocrystals were characterized by using energy-dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), powder X-ray diffraction (PXRD), Fe-57 Mossbauer spectroscopy and low-temperature sorption-desorption of nitrogen. Acid-base properties of the surface and catalytic activity of porous HoFeO3 in process of n-hexane conversion were analyzed as well. Finally, a magnetic recovery procedure was performed for the spent catalyst to analyze its efficiency for synthesized HoFeO3. The elemental composition of the sample corresponds to holmium orthoferrite (48.8 and 51.2 at. %, for Ho and Fe, respectively) with the absence of any impurities as it was shown by EDX. The PXRD results show that the synthesized HoFeO3 nanocrystals have an orthorhombic structure (space group Pbnm) and an average crystallite size of 62 +/- 5 nm. The Rietveld method was used to refine the unit cell parameters as follows: a = 5.254(1) angstrom, b = 5.575(3) angstrom, c = 7.598 (4) angstrom; R-wp = 2.45%. The Mossbauer spectrum of HoFeO3 evidenced the magnetically ordered state of nanocrystals and it is represented by a sextet with quadrupole splitting (QS), of 0 mm/s, an isomeric shift (IS) of 0.36 mm/s, and an effective magnetic field (H-eff) of 497 kOe. Sorption-desorption of nitrogen shows that the holmium orthoferrite has a predominantly macro- and mesoporous structure, also confirmed by SEM, with specific surface area and total porosity values of 31 m(2)/g and 0.071 cm(3)/g, respectively. The analysis of the acid-base properties of the surface of the porous HoFeO3 evidences a high concentration of the aprotic (Lewis) acidic (pKa = 14.2) and basic (pKa = -4.4) sites, as well as of the Bronsted weak acid (pKa = 6.4) sites. The catalytic activity of holmium nanocrystalline in process of n-hexane conversion was analyzed and compared with commercial dehydrogenation (CD-1) and cracking (ZSM-5) catalysts. The n-hexane conversion and cracking products selectivity displayed by HoFeO3 are comparable to those of the reference catalysts, and the selectivity to dehydrogenation and especially isomerization products significantly exceeds the values of CD-1 and ZSM-5; low n-hexane oligomerization product selectivity of HoFeO3 catalyst was observed as well. A high recovery rate (similar to 97%) achieved as a result of the magnetic separation of the spent o-HoFeO3-based catalyst from the reaction mixture, followed by heat treatment at 600 degrees C for 1 h. These results allow us to conclude that synthesized porous HoFeO3 can be considered as a promising basis for new catalytic materials for cracking, dehydrogenation and isomerization processes with a possibility of highly efficient magnetic recovery. (C) 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.