Synthesis, Characterization, and Catalytic Activity of Uniformly Crystalline LaPO4 Nanofiber Catalysts for Ethanol Dehydration

Nanostructured lanthanum phosphates (LaPO4) with different P/La ratios ranging from 0.5 to 2.0 were prepared using aqueous solutions of La(NO)3·6H2O and NH4H2PO4 as La and P sources, respectively, by a sol−gel precipitation method. These prepared catalysts were thoroughly characterized by various techniques, such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy, temperature-programmed desorption of ammonia (NH3-TPD), laser Raman spectroscopy, and N2 physisorption. XRD results of all the samples showed the crystalline features to be mainly attributed to the hexagonal Rhabdophane type of LaPO4 phase. SEM micrographs showed uniformly crystalline nanofiber structures for the samples with high P/La ratios of 1.0 and 2.0. For a sample with a P/La ratio of 0.5, the acidity measured by NH3-TPD showed two peaks with maxima at 210 and 750 °C. The high temperature peak at 750 °C completely disappeared for the higher P/La ratio samples. These catalysts were tested for selective ethanol dehydration to ethylene in the temperature range of 200−450 °C. LaPO4 catalysts showed higher ethylene selectivities as compared to other metal phosphates, such as AlPO4 and ZrPO4. The catalytic properties were found to be influenced by the P-to-La ratio. Higher P/La ratio catalysts exhibited higher ethanol conversion as well as ethylene selectivity.