Dissertation•
Zeolite deactivation during hydrocarbon reactions: characterisation of coke precursors and acidity, product distribution
01 Feb 2008-
TL;DR: In this article, the deactivation of USHY zeolite catalysts as a result of coke deposition has been investigated in large scale petroleum-refining processes and three novel techniques, coke classification, thermogravimetric method for characterising coke precursors and indirect temperature programmed desorption (TPD) for catalyst acid sites characterisation were developed to further study catalyst deactivation mechanism.
Abstract: The catalytic conversion of hydrocarbons over zeolites has been applied in large scale petroleum-refining processes. However, there is always formation and retention of heavy by-products, called coke, which causes catalyst deactivation. This deactivation is due to the poisoning of the acid sites and/or pore blockage. The formation of coke on hydrocarbon processing catalysts is of considerable technological and economic importance and a great deal of work has been carried out to this study. The main aim of this work is to understand the deactivation of zeolite catalysts as a result of coke deposition. The deactivation by coke of USHY zeolite was investigated during catalytic conversion of hydrocarbons – 1-pentene, n-heptane and ethylbenzene – as representatives of olefins, paraffins and aromatics respectively, at different reaction temperatures, time-on-streams and composition. Three novel techniques, coke classification, thermogravimetric method for characterising coke precursors and indirect temperature programmed desorption (TPD) for catalyst acid sites characterisation were developed to further study catalyst deactivation mechanism. Product distribution, coke formation, characterisation of coke precursors, as well as the role of strong acid sites on hydrocarbon reactions are presented and discussed. During catalytic reactions of 1-pentene over USHY zeolite, cracking and hydride transfer were the predominant reactions in initial stage which deactivated rapidly allowing isomerisation to become the main reaction afterwards. Deactivation studies showed that coke formation was very strong initially which is in good correlation with the initial rapid deactivation. The hydrogen freed during this initial time from the formation of high C/H ratio coke components contributed to the formation of hydride transfer products. The amount of coke precursors decrease with increasing reaction temperature due to the higher desorption of coke precursors into gas phase while hard coke amount increased with temperature as expected from an activated process. The coke amount formed was not proportional to the reactant feed composition, because of a strong pseudo-zeroth- order initial coking on strong acidic sites. The thermogravimetric method provides insight into the chemical character of coke precursor components in terms of the mode of their removal and allows further classification of coke precursors into small and large coke precursors. The concentration and strength of acid sites of coked catalysts were studied by the TPD methodology. Besides, characterisation of coke precursors was also revealed. The initial deactivation preferentially on strong acid sites is very fast. The concentration of free acid sites is inversely correlated well with the total concentration of coke rather than individual coke groups. Coke precursors tend to be more stable at higher reaction temperatures. Furthermore, by selectively poisoning strong acid sites of USHY zeolite, it shows conclusively that strong acid sites are responsible for cracking and hydride transfer reactions as well as strong coke formation while weak acid sites can only catalyse double bond isomerisation.
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1,003 citations
04 Apr 2019
TL;DR: In this paper, a modified microporous to mesoporous zeolite catalysts were characterized using different analytical techniques, including X-ray diffraction (XRD), nitrogen porosimetry, Fourier transform infrared spectroscopy (FT-IR), temperature-programmed desorption (ammonia-TPD), and field emission scanning electron microscopy (FE-SEM) to analyze the crystallographic structure, surface acidity, surface area, porosity, morphology, and particle size.
Abstract: This work highlights how the treatment of ZSM-5 (parent Zeolite Socony Mobil–5, Si/Al = 23) with different surfactant templates and alkaline solution, improved the catalytic performance in the Friedel-Crafts acylation of anisole with a propionic anhydride to obtain p-methoxypropiophenone. The modified microporous to mesoporous zeolite catalysts were characterized using different analytical techniques, including X-ray diffraction (XRD), nitrogen porosimetry, Fourier-transform infrared spectroscopy (FT-IR), temperature-programmed desorption (ammonia-TPD) and field emission scanning electron microscopy (FE-SEM) to analyze the crystallographic structure, surface acidity, surface area, porosity, morphology, and particle size. The results showed that the formed mesoporous zeolite by NaOH solution had smaller mesopores (ca. 3.7 nm) as compared to the mesoporous zeolites obtained by surfactant templates, such as, CTAB (ca. 14.9 nm), TPAOH (ca. 11.1 nm) and mixture of CTAB/TPAOH (ca. 15.2 nm). The catalytic acylation reaction was conducted in a batch glass reactor at various temperatures and the products were analyzed using off-line gas chromatography–mass spectrometry (GC-MS). It was found that the activity of treated ZSM-5 with mixed surfactant templates (CTAB/TPAOH) exhibited enhanced selectivity towards the main product (p-methoxypropiophenone) by a factor 1.7 or higher than unmodified ZSM-5 due to its increased surface area by 1.5 times and enhanced acid sites.
32 citations
TL;DR: In this paper, the authors investigated the catalytic ethanol dehydration and dehydrogenation over γ-Al2O3-P catalysts with Pd modification and found that the 0.5Pd/Al 2O3P catalyst produced the highest yield of diethyl ether (ca. 38.4%) at 350°C attributing to increased portion of weak/moderate to strong acid site.
17 citations
TL;DR: In this paper, the performance of two types of molecular sieves (HYD10B and LQ-MS-13X) in a batch system was investigated and it has been illustrated that HYD10b type has more adsorption capacity than LQMS- 13X type to separate ethyl mercaptan from simulated liquid mixture.
8 citations
TL;DR: In this article, the effect of alumina phases on the catalytic performance of platinum (Pt) on chlorinated alumina catalysts studied in the combined light naphtha isomerization and naphthenic ring opening reaction.
Abstract: The effect of alumina phases on the catalytic performance of platinum (Pt) on chlorinated alumina catalysts studied in the combined light naphtha isomerization and naphthenic ring opening reaction. The results showed that the type of alumina had a great impact on the catalytic performance of the Pt on chlorinated alumina catalysts. The support prepared by a mixture of eta and gamma alumina with the ratio of 70:30 shows better surface area, acidity and catalytic performance. Extra pre-chlorination of catalysts had a positive effect on the catalytic performance in the butane isomerization reaction. Activities of prepared catalysts tested in a pilot plant with two consecutive fixed bed reactors under different operating conditions. Under the experimental conditions of this study, temperatures greater than 135 °C had a reverse effect on the octane number of the products. It was found that the high LHSV has a negative effect on the strong acidic sites and with returning the LHSV to the initial value, 22DMB/ΣC6 ratio and ring opening reaction rate could not be restored completely.
6 citations
Cites background from "Zeolite deactivation during hydroca..."
...Therefore, by combining of an appropriate amount of ring contraction and ring opening, one can maximize RON and MON of the products [29]....
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References
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"Zeolite deactivation during hydroca..." refers background in this paper
...In contrast, for ZSM-5, hexane cracking ability increases linearly with increasing aluminum content, leading to the conclusion that the maxinum in acidity is a function not only the zeolite structure but also the surroundings of the aluminum atoms in the framework (Humphries et al., 1993)....
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TL;DR: In this paper, a variety of strategies involving modification of catalyst surface composition such as the use of polymetallic catalysts and manipulation of the reaction environment which often limits the yield due to thermodynamic constrains (i.e., high hydrogen pressures, etc.).
Abstract: Catalyst deactivation due to coke formation is an important technological and economic problem in petroleum refining and in the petrochemical industry. Remedies to catalyst deactivation are sought by a variety of strategies involving modification of catalyst surface composition such as the use of polymetallic catalysts and/or by manipulation of the reaction environment which often limits the yield due to thermodynamic constrains (i.e., high hydrogen pressures, etc.). In the limit, when the activity reaches unacceptable limits, regeneration by burning off carbon residues can usually be attained, Regeneration can take place in situ, as in fixed-bed reactors, or in an adjacent reactor to which the catalysts is transported to, such as in moving-bed reactors or in fluidized-bed reactors. In the first case intermittent operation is required, whereas in the second case the operation is continuous, but a second regeneration reactor is required. The choice of the proper process cycle is an economic optimi...
204 citations
TL;DR: In this article, a model that incorporates a change in the predominant cracking reaction mechanism as a function of alkane conversion and the very different rates of these mechanisms is proposed to explicitly account for coking and deactivation of the catalyst.
Abstract: Steamed HY or ultrastable Y (H-USY) zeolites are active hydrocarbon cracking catalysts. The high activity of H-USY compared to HY zeolite has been previously explained by the generation of unusually strong and active Bronsted acid sites, or an increase in the number of accessible sites in a micropore diffusion-controlled reaction. However, neither model explains the accumulated literature observations. A model is proposed that incorporates a change in the predominant cracking reaction mechanism as a function of alkane conversion and the very different rates of these mechanisms. Additionally, an oligomeric cracking mechanism is introduced to explicitly account for coking and deactivation of the catalyst. The model is capable of accounting for most literature results. It concludes that the large enhancement in cracking activity by steaming is due to a proportionally smaller increase in external surface area of the zeolite crystals and possibly a small increase in the specific initiation activity of each site. These small changes lead to a much larger overall effect because of the sensitive dependence of oligomeric cracking and, to a lesser extent, bimolecular cracking on the alkene partial pressure.
144 citations
78 citations
TL;DR: In this paper, the authors investigated catalytic cracking of butene to propene and ethene over HMCM-22 zeolite and proposed a simple reaction pathway for forming the propene, ethenes and by-products from butene cracking.
Abstract: Catalytic cracking of butene to propene and ethene was investigated over HMCM-22 zeolite. The performance of HMCM-22 zeolite was markedly influenced by time-on-stream (TOS) and reaction conditions. A rapid deactivation during the first 1 h reaction, followed by a quasi-plateau in activity, was observed in the process along with significant changes in product distributions, which can be attributed to the fast coking process occurring in the large supercages of MCM-22. Properly selected reaction conditions can suppress the secondary reactions and enhance the production of propene and ethene. According to the product distribution under different butene conversion, we propose a simple reaction pathway for forming the propene, ethene and by-products from butene cracking. HMCM-22 exhibited similar product distribution with the mostly used high silica ZSM-5 zeolite under the same conversion levels. High selectivities of propene and ethene were obtained, indicating that the 10-member ring of MCM-22 zeolite played the dominant role after 1 h of TOS. However, MCM-22 exhibited lower activity and stability than that on high silica ZSM-5 zeolite with longer time-on-stream.
69 citations