Oxidative dehydrogenation of n-octane using dealuminated faujasite zeolites

Abstract

Commercially obtained NaY was modified by a dealumination process using different solutions containing specific concentrations of citric acid, ethylenediaminetetraacetic acid disodium (EDTA-2Na), and a mixture of both EDTA-2Na and citric acid. The three prepared catalysts were then used to study the effect of dealumination on oxidative dehydrogenation (ODH) of n-octane. Dealumination was carried out to strengthen the zeolites acid sites, which will lead to an increased activity towards the oxidation of n-octane. Results from these showed that removing the framework and extra framework aluminium in the NaY zeolites increases the intrinsic acidity, which then led to increased conversion in the ODH of n-octane, with the catalyst prepared by the mixture of both EDTA-2Na and citric acid recording the highest conversion of just above 10 %. The conversion was ascribed to the removal of both the non- and framework aluminium, which leads to improved pore volumes and surface area. All the three prepared catalysts were mostly selective towards the carbon oxides (COx) products which was due to the absence of the ODH promoting metal. To induce the ODH promoting properties on the prepared catalysts (NaY[AL] for acid leaching using citric acid, NaY[CAT] for chelating agent treatment using EDTA-2Na and NaY[CT] for complex treatment using citric acid and EDTA-2Na), they were then further modified by introducing gallium using a modified ionic exchange procedure. Gallium has been shown by previous studies to be a promising ODH active metal. All the prepared catalysts were exchanged by 2 %wt of gallium which contributed to the increase of about 2 % for all the prepared catalysts when they tested were under similar ODH conditions with the first three batches of dealuminated NaY. The introduction of framework gallium also decreased side reactions such as the cyclization and cracking reactions, due to the reduction in medium to strong acid sites. The COx selectivity for the catalysts decreased from above 90 % to just below 85 %. This was not the case for the highest acidic NaY[AL], which showed no decrease in the COx selectivity. Though there was a slight improvement on the olefins selectivity after gallium introduction, COx production was still favoured by the catalysts. To mitigate this, barium, a basic metal was also introduced into the prepared Ga-NaY catalysts. Barium was also introduced using ionic exchange, and 1.5 %wt was introduced. Barium is known for inducing basic sites which facilitates the quick desorption of ODH products, leading to suppressed overoxidation. Due to subsequent ionic exchange procedures done on the catalysts, the morphology of the catalysts was altered, with the NaY zeolites losing their cubic shape and also clustering. The effect of Ba for all the catalysts was not much, as only BaGa-NaY[CT] showed a 4 % decrease in the COx activity. All the catalysts showed increased cracked products and oxygenates, but not much improvement in the olefins and aromatic products. The findings of this study showed that the behavior of the zeolite catalysts in the ODH of alkanes depends both on the physical and chemical properties of the catalyst. Acidity is a big factor when dealing with zeolites, as the activity and selectivity of the catalysts depends on the alteration of this feature. A high concentration of strong acid sites alone can activate the paraffin of interest, however Lewis acid sites induced by a reducible gallium metal are responsible for improved octene activity, selectivity and stability of the catalysts through the facilitation of coke burning on the surface of the catalyst.