Assessment of coke deposits on lamellar metal-modified MFI zeolites in ethylene transformation to aromatic liquids
Graphical abstract
Introduction
Ethylene, one of the most important chemical intermediates in the chemical and petrochemical industry, is mainly produced by steam cracking of fossil-based feedstocks [[1], [2], [3]]. However, the depletion of the fossil energy resources and their harmful environmental effects have promoted the production of ethylene from renewable energy sources such as biomass [4]. Bio-ethanol obtained from fermentation of biomass can be dehydrated to produce ethylene. The resulting ethylene can be transformed to high energy density hydrocarbons through oligomerization, cyclization, and aromatization to be used in the transportation fuel market as a replacement for petroleum-based fuels [[5], [6], [7], [8]].
MFI-type zeolites modified with different metal precursors such as gallium (Ga) and zinc (Zn) are the most widely used catalysts for highly selective conversion of ethylene to aromatics [[9], [10], [11], [12], [13], [14]]. Shape selectivity of the MFI catalyst due to its crystalline microporous structure with 10-membered ring openings (∼ 0.5 nm) together with the bifunctional effect of metallic sites and acidic sites of the zeolite provide a high aromatics production yield. The metal loading increases the Lewis acidity of the zeolite catalyst and promotes the dehydrogenation of the acid-catalyzed oligomerization and cyclization products to facilitate the aromatics formation. The nature of the metal, the metal content, and the metal dispersion in the zeolite structure are important parameters affecting the performance of the catalyst [[11], [12], [13]].
Coke formation in zeolite-catalyzed hydrocarbon conversion reactions is one of the major reasons for catalyst deactivation [[15], [16], [17]]. The carbonaceous coke species block the zeolite pores and prevent the diffusion of reactants and products into and out of the zeolite channels or/and poison the active sites of the catalyst during the deactivation process upon coke deposition [[18], [19], [20]]. Catalyst characteristics, operating conditions, and the chemical reaction itself affect the coke structure significantly [[21], [22], [23], [24]]. The formation of coke species inside the micropores (internal coke) and on external surface of zeolites (external coke) involves different reaction steps such as condensation, rearrangement, hydrogen transfer, and dehydrogenation [19,25]. There are many techniques available for characterization of the coke deposits on zeolites [15,23,[26], [27], [28], [29]]. For example, thermogravimetric analysis (TGA) is used to determine the total amount of coke [30,31], while spectroscopic analysis methods such as Fourier transform infrared (FTIR) [21,24,[32], [33], [34]], ultraviolet-visible (UV–Vis) [21,[35], [36], [37]], UV-Raman [38,39], electron spin resonance (ESR) [40], and nuclear magnetic resonance (NMR) [24,29,32,33,41,42], are used to study the nature of the coke. Coke location (internal versus external) can be identified by combination of TGA and gas adsorption-desorption measurements [[43], [44], [45]], while the combination of coke extraction, gas chromatography-mass spectrometry (GC–MS), and transmission electron microscopy (TEM) techniques provides better understanding of the coke composition [42,[46], [47], [48], [49], [50]].
The spent catalyst after formation and deposition of coke species on it can be regenerated by oxidative coke combustion and reused for the catalytic reaction, but such regeneration cycles lead to a gradual loss of crystallinity of the zeolite due to steam treatment at high temperatures and reduce the lifetime of the catalyst for industrial applications [[51], [52], [53]]. Therefore, in addition to the optimization of the reaction conditions, it is necessary to tune the catalyst design parameters for having a better control on coke formation and catalyst regeneration.
The morphology and pore size of the microporous zeolite can affect both the coke amount and coke composition [54]. Zeolites with larger pores usually produce a higher amount of coke that mainly consists of aromatic species [21]. The metal-modification has also been reported to decrease the formation and deposition of carbonaceous coke species responsible for microporous zeolite catalyst deactivation [11,[55], [56], [57]]. More recently, the number of studies showing that zeolite or metal-modified zeolites containing mesoporosity or macropososity in their structure have higher coke resistance and a longer lifetime has been increased [44,48,[58], [59], [60]]. The slower deactivation rate for these hierarchical catalysts is usually correlated to the fast diffusion of coke precursors out of the zeolite structure and an increase of external coke fraction in these catalysts compared to their solely microporous zeolite counterparts. The agglomeration of microporous zeolite crystals with a meso- and macroporous matrix has also been shown to be effective in attenuating the deactivation of the catalyst by coke species [61,62].
The meso- or macro-porosity is usually introduced to the structure of hierarchical zeolites by post-modification methods such as desilication, but controlling the synthesis process for production of uniform meso- or macro-pores is very difficult [[63], [64], [65]]. Another efficient method for production of hierarchical meso-/microporous zeolites with uniform mesopore sizes is synthesis of two-dimensional (2D) lamellar zeolites such as MFI by using organic surfactant molecules as templates [44,[66], [67], [68]]. Metal-modification of 2D lamellar MFI might be an effectual strategy to control the amount, nature, composition, and location of the coke that is formed on the catalyst during hydrocarbon conversion reactions.
In our previous work [69], the effect of metal-modification and lamellar zeolite structure on the catalytic conversion reaction of ethylene-to-aromatic liquids (ETA) was studied. The hierarchical meso-/microporous lamellar MFI was synthesized by the dual template synthesis method developed before [70] and then the zeolite was loaded with Ga or Zn (2 wt.%) using the wet impregnation technique. Catalytic performances of the synthesized Lamellar MFI, 2%Ga-Lamellar MFI, and 2%Zn-Lamellar MFI were analyzed and compared to those obtained for their microporous zeolite counterparts. 2%Zn-Lamellar MFI produced the highest amount of aromatic liquids with the higher selectivity toward mono-benzene alkylated aromatics while a lower fraction of coke precursor species (such as naphthalene) was obtained. The lowest amount of total coke was also obtained for this catalyst based on TGA measurements. The improved performance of this bifunctional catalyst was explained by the combined effect of tuning the textural properties of MFI zeolite through introduction of mesoporosity to its structure and modification of its Brønsted/Lewis acidity ratio by metal-modification which facilitates the aromatics production and also controls the coke formation in the catalyst [69]. The aim of the present work is to analyze the coke deposits and determine the amount, nature, composition, and location of the coke in the above mentioned lamellar zeolite catalysts after ethylene conversion reaction to aromatic liquids over them in comparison to their microporous zeolite analogues. Different techniques, including temperature programmed oxidation (TPO), FTIR, MS, UV–Vis, coke extraction followed by GC–MS, and Ar adsorption-desorption, have been used to better understand the effect of lamellar meso-/microporous zeolite structure and its metal-modification on characteristics of the coke deposits.
Section snippets
Catalyst synthesis
Lamellar MFI zeolite was synthesized using the following recipe as reported in our previous publication [70]: 30Na2O/1Al2O3/100SiO2/10C22-6-6/5TPAOH/4000H2O/18H2SO4. Synthesis steps for C22-6-6 as structure directing agent, the chemicals that were used, and details of the dual template synthesis technique for lamellar zeolite, template removal by calcination, and ion-exchange of the resulted zeolite have been discussed in the supporting information. Commercial MFI zeolite from Alfa Aesar was
Effect of the zeolite structure and its metal-modification on the coke amount
MS-TPO was employed to determine the coke content of the spent MFI zeolite catalysts in the ETA reaction. The change of CO signal intensity recorded by MS during the TPO process was not significant for any of the studied zeolite samples. MS collected data for CO2 were used to determine the CO2 production rate as discussed in the supporting information. The MS-TPO profiles corresponding to the combustion of the coke deposited on the MFI zeolite catalysts have been shown in Fig. 1. Two types of
Conclusions
In summary, the effects of meso-/microporous structure and metal addition (Zn or Ga) in lamellar MFI zeolite catalysts on properties of the coke formed in ethylene conversion to aromatic liquids were systematically studied. Meso-/microporous lamellar MFI zeolite was synthesized using the dual template method and then impregnated with gallium or zinc (2 wt.%). Commercial MFI zeolite was also modified by gallium or zinc (2 wt.%) using the same impregnation method for comparison purpose. The spent
CRediT authorship contribution statement
Laleh Emdadi: Conceptualization, Methodology, Investigation, Writing - original draft, Visualization. Luther Mahoney: Investigation. Ivan C. Lee: Conceptualization. Asher C. Leff: Investigation. Wei Wu: . Dongxia Liu: Investigation. Chi K. Nguyen: Investigation. Dat T. Tran: Supervision.
Acknowledgements
Research was sponsored by the U.S. Army Research Laboratory and was accomplished under Cooperative Agreement Numbers W911NF-16-2-0008 and W911NF-16-2-0085. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the U.S. Army Research Laboratory or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes
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