Silicon enrichment on iron contaminated fluid catalytic cracking catalyst particle surface
Graphical abstract
Introduction
As the workhorse of current oil refinery, fluid catalytic cracking (FCC) catalysts are responsible for a significant portion of the global gasoline production. Due to the well-known severe detrimental effects of added iron on the performance of FCC catalysts [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], many techniques have been utilized to characterize equilibrium catalysts (Ecats) from commercial units in order to better understand this iron poisoning issue, including scanning electron microscopy (SEM) [6], [10], [11], [12], [13], [14], [15], [16], energy dispersive X-ray spectroscopy (EDX or EDS) [10], [11], [12], [13], [14], [15], [16], Auger electron spectroscopy (AES) [12], electron paramagnetic resonance spectroscopy (EPR) [12], [17], transmission electron microscopy (TEM) [10], [11], [15], [16], magnetic susceptibility [11], [15], atomic force microscopy (AFM) [13], X-ray photoelectron spectroscopy (XPS) [13], [14], electron probe microanalyzer (EPMA) [14], optical microscopy [14], and Mössbauer spectroscopy [17]. In addition, several non-invasive methods based on X-ray probe were recently developed to study single catalyst particles, including X-ray fluorescence tomography [18], [19], transmission X-ray nanotomography [8], [9], [20], X-ray ptychography [19], [21], [22], and Tomographic X-ray Absorption Spectroscopy [23]. Although these efforts greatly enhanced our understanding, the exact mechanism of iron poisoning is still not very clear.
Most of the studies focused on iron and other contaminant metals only, and not much work has been dedicated to investigating the matrix surrounding these contaminants. Previously, based on the observation of small crystalline iron-rich particles on the catalyst surface with TEM [10], [11], Wieland and Chung [11] speculated that some iron-alkali (primarily calcium and sodium) eutectics formed on the surface, resulting in surface vitrification and poor catalyst performance. Yaluris et al. [14] made a similar proposal, but suggested that silicon should also be involved in the eutectics. More recently, van Bokhoven’s group reported that a dense amorphous silica–alumina (ASA) shell formed close to the particle exterior due to amorphization of zeolites, and suggested that iron contaminants contributed to the formation of this ASA envelope [16], [19], [22], [23]. In our previous paper [15], we showed that the iron-poisoned catalyst surface consisted of iron enriched crystalline nanoparticles that were relatively well dispersed in a silicon enriched amorphous matrix, and proposed that the gradual deposition and accumulation of both Si-bearing species and Fe contaminants in the surface layer were necessary to induce any detrimental effects on catalysts.
To further test the idea of concomitant deposition of Fe- and Si-bearing species on the catalyst surface, we examined many Ecat samples with different iron content from different refineries that suffered iron poisoning by SEM-EDX method. The expectation was that overall the Si/Al ratio on the catalyst surface would be higher for an Ecat sample with higher added iron content, since on average the added layer enriched with Si and Fe would be thicker. In addition, for a specific Ecat sample, the Si/Al ratio determined by EDX would decrease as the accelerating voltage increases, since it is well known that in SEM electron beam with a higher accelerating voltage can penetrate deeper into the sample. The experimental data were further complemented with Monte Carlo simulations [24]. The results confirm that surface Si enrichment indeed correlates well with the Fe content in the Ecat samples, strengthening the argument that both Fe contaminants and Si accumulation on the catalyst surface need to be considered to better understand iron poisoning mechanism.
Section snippets
Samples
The Ecat samples used in this study were obtained from different commercial FCC units that suffered typical iron poisoning symptoms due to iron contaminants in the feed. For many FCC units, their Ecats are usually mixtures of base catalyst particles with different age and other additives. To develop and validate the current method, it was preferred to use density separated relatively clean fractions of the base catalysts to minimize the possible interference from additives. Density separation
SEM-EDX mapping of Sink-float fractions of an Ecat sample
To address the concern about possible interference from additives when base catalyst is studied, density separation of Ecat samples [25] is usually performed. To test the feasibility of the suggested SEM-EDX method, it was preferred to use some sink float fractions from one Ecat sample. An Ecat from Refinery A that suffered iron poisoning was separated into four fractions for the base catalysts. As shown in Table S1, from the first to the last fraction, the iron content increases from 0.79 to
Discussion
In our previous study of iron-poisoned Ecat [15], it was found that iron and some other contaminant metals existed as well-dispersed crystalline nanoparticles, while the surrounding amorphous matrix was enriched in Si and depleted in Al. We proposed that both Si-bearing species and Fe contaminants were deposited and accumulated on the catalyst surface gradually and continuously over time. This is different from the idea from van Bokhoven’s group [16], [19], [22], [23], which claimed that the
Conclusions
Previously, we have reported that Fe, as well as some other contaminant metals such as Ni, Mg and Ca, and Si were enriched on iron-poisoned catalyst surface, and proposed that this enrichment was due to their gradual and continuous accumulation (deposition) on the surface [15]. To continue our investigation on the iron poisoning issues, surface Si enrichment was further studied with many Ecat samples from different refineries. The data was acquired with SEM-EDX measurements of catalyst
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper
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