Effect of desilication of H-ZSM-5 by alkali treatment on catalytic performance in hexane cracking
TL;DR: In this article, the effects of external surface and acid properties of desilicated H-type ZSM-5 zeolites (H-ZSM5) on their catalytic performance in hexane cracking were investigated.
Abstract: The effects of external surface and acid properties of desilicated H-type ZSM-5 zeolites (H-ZSM-5) on their catalytic performance in hexane cracking were investigated. H-ZSM-5 with two different crystallite sizes of 100 nm and 1 μm were treated with NaOH solution of different concentrations. The external surface area (SEXT) was increased with an increase in the NaOH concentration, because of the formation of mesopores inside the H-ZSM-5 crystallites as a result of desilication. The increase in the SEXT of the H-ZSM-5 catalysts contributed to mitigating the catalyst deactivation during the hexane cracking. Although the amount of coke deposited on the alkali-treated H-ZSM-5 was larger than that on the parent H-ZSM-5, the micropore volume of the alkali-treated H-ZSM-5 decreased less due to coke deposition than that of the parent. The deactivation rate and the decrease in the micropore volume of the small-sized H-ZSM-5 catalysts were smaller than those of the large-sized catalysts, because they had shorter average diffusion path lengths. Thus the activity of the alkali-treated H-ZSM-5, especially small-sized one is less sensitive to coke deposition. Lewis acid sites (LASs) were generated by treating with NaOH of high concentrations. The selectivities to benzene, toluene and xylene (BTX) in the hexane cracking were increased with an increase in the LASs amount at high reaction temperatures (≥873 K). The LASs on the alkali-treated H-ZSM-5 were selectively removed by acid treatment. The resultant H-ZSM-5 exhibited a slightly lower hexane conversion and a lower selectivity to BTX but a small amount of coke compared to the parent and alkali-treated H-ZSM-5 catalysts, suggesting that LASs on alkali-treated H-ZSM-5 accelerated the dehydrogenation including hydride transfer and aromatization, forming BTX, which would be precursors of coke.
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TL;DR: In this paper, the effects of desilication of ZSM-5 zeolite on its catalytic properties in catalytic fast pyrolysis (CFP) of lignocellulosic biomass were investigated.
Abstract: This study investigated the effects of desilication of ZSM-5 zeolite on its catalytic properties in catalytic fast pyrolysis (CFP) of lignocellulosic biomass. A series of mesoporous ZSM-5 zeolites were prepared by desilication of a conventional microporous ZSM-5 zeolite with NaOH solutions of varying concentrations (0.1–0.5 M). The creation of mesopores improved the diffusion property of the desilicated ZSM-5 zeolites and their catalytic activity for cracking bulky oxygenates (e.g., syringols derived from the lignin component of biomass). Consequently, the desilicated zeolites produced more aromatic hydrocarbons (carbon yields of 26.2–30.2%) and less coke (39.9–41.2%) in CFP of beech wood than the parent microporous ZSM-5 (23.2% aromatics and 44.4% coke). The highest aromatic yield (30.2%) and lowest coke yield (39.9%) were obtained in CFP of beech wood with mildly desilicated zeolite treated with 0.3 M NaOH solution. However, desilication with a greater concentration, 0.5 M NaOH, decreased the aromatic yield to 26.2% due to a considerable loss of microporosity in the severely desilicated zeolite. The results indicate that carefully controlled desilication of zeolite can improve the conversion of lignocellulose to valuable aromatic hydrocarbons and decrease the formation of undesired coke, thus improving the product distribution in CFP of lignocellulose.
265 citations
TL;DR: In this article, the effects of the organic structure-directing agents (OSDAs) and Na cations for the synthesis of ZSM-5 zeolites were investigated.
Abstract: The effects of the organic structure-directing agents (OSDAs) and Na cations for the synthesis of ZSM-5 on the location of Al atom in the framework as well as the acidic and catalytic properties were investigated. To achieve these purposes, ZSM-5 zeolites were synthesized by using four kinds of OSDAs including tetrapropylammonium hydroxide cations, dipropylamine, cyclohexylamine, and hexamethylenimine with or without Na cations. In situ FT-IR spectroscopy using CO as probe molecule was applied to the evaluation of the acid property of the ZSM-5 zeolites. The location of Al atoms was examined by high resolution 27Al MAS and MQMAS NMR techniques. The constraint index (CI) has also been used to estimate the distribution of acid sites in the micropores. The location of acid sites was investigated based on the difference in the transition-state shape-selectivity through the cracking of n-hexane and 3-methylpentane. Furthermore, the cracking of various types of paraffins and the conversion of aromatic compounds...
206 citations
TL;DR: In this article, the authors discuss a selected group of emerging strategies in zeolite engineering that have great prospects for research and that they consider could impact the sector in the near future, including advances in crystal engineering and hierarchization achieved through bottom-up and top-down approaches, composite materials, tuning of the location of active sites among the different crystallographic positions available, and, importantly, how to characterize these modifications and th...
Abstract: Propene is a key building block for the petrochemical industry whose demand is increasing strongly in recent years, even faster than that of ethene. The availability of propene is limited, and therefore, efforts to optimize its production are being pursued. On the occasion of the 75th anniversary of the first FCC unit, we analyze some recent advances that have been achieved in the understanding and development of zeolites aiming to increase the production of light olefins as petrochemical building blocks by means of catalytic cracking. We discuss a selected group of emerging strategies in zeolite engineering that have great prospects for research and that we consider could impact the sector in the near future. These include advances in crystal engineering and hierarchization achieved through bottom-up and top-down approaches, composite materials, tuning of the location of active sites among the different crystallographic positions available, and, importantly, how to characterize these modifications and th...
154 citations
TL;DR: In this paper, a modification of Na-ZSM-5 zeolite catalyst applied converting methanol to gasoline (MTG) was studied in a fixed-bed tubular reactor at 400°C and WHSV of 28h−1 under ambient pressure.
116 citations
TL;DR: In this paper, a comparative assessment of the type, concentration, and strength of acid sites is attained through the infrared study of adsorbed probe molecules (pyridine, 2,4,6-trimethyl pyridine and d3-acetonitrile), the temperature-programmed surface reaction (TPSR) of n-propylamine and the cracking of nhexane.
99 citations
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TL;DR: In this article, the integrated molar extinction coefficients for infrared absorption bands of pyridine adsorbed on acid sites in Si/Al-based catalysts were determined for infrared spectra of five zeolites and two amorphous silica-aluminas.
2,429 citations
TL;DR: In this article, the infrared spectrum of pyridine coordinately bonded to the surface of acid solids has been determined and a rough estimate of the strength of surface Lewis sites can be inferred.
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852 citations
TL;DR: In this paper, the influence of various employed promoters on the chemical properties of the modified ZSM-5 and the performance of resulting catalyst in enhancing the selectivity to light olefins, have been addressed.
Abstract: Steam cracking of hydrocarbons has been the major source of light olefins for more than half a century. The recent studies have reported that ethylene and propylene can also be produced through the cracking of hydrocarbons over modified ZSM-5 zeolites in a considerable amount. This paper highlights the important current ideas about acid-catalyzed hydrocarbon cracking that has resulted in high yield of ethylene and propylene. Light olefin production via catalytic cracking of various industrial feedstocks, ranging from heavy hydrocarbons to ethane, over modified ZSM-5 zeolites, has been reviewed in the present paper. Furthermore, the influence of various employed promoters, i.e., alkali and alkaline earth, transition, rare earth elements, and phosphorus, on the chemical properties of the modified ZSM-5 and the performance of resulting catalyst in enhancing the selectivity to light olefins, have been addressed. Moreover, the influences of different factors, including the zeolite acidity, Si/Al ratio and the temperature, on the light olefin production and the reaction scheme have been specified. The role of incorporated element in the catalytic cracking mechanism is also summarized.
657 citations
TL;DR: In this paper, the modes of coking and deactivation of zeolites during n-heptane cracking at 723 K were established on the basis of the composition of the carbonaceous compounds responsible for deactivation (coke), the deactivating effect of the coke molecules and reduction by coke of the volume accessible to nitrogen and to n-hexane.
582 citations