Simultaneous Enhancement of Aromatic Products and Catalytic Lifetime for Catalytic Re-forming of Lignite Pyrolysis Volatiles over a Self-Assembled Hierarchical Core–Shell ZSM-5@Silicalite-1 Zeolite

Abstract

Catalytic re-forming of lignite pyrolysis volatiles to aromatics over zeolites is a promising method for value-added utilization of coal tar. However, to achieve a significant enhancement in both the yields of light aromatics and the catalytic lifetime by adjusting the structure of ZSM-5 zeolite and to understand its catalytic mechanism comprehensively are still great challenges. Herein, a self-assembled hierarchical core−shell ZSM-5@silicalite-1 with controlled mesopores and acidity as well as an ultrathin nonacidic protective shell was synthesized through a mild hydrothermal method of alkaline treatment with tetrapropylammonium hydroxide or tetraethylammonium hydroxide (TPAOH or TEAOH). Both the number and pore size distribution of mesopores can be regulated by adjusting the hydrothermal treatment time. These generated mesopores not only facilitate the mass transfer of reactants and products but also improve the utilization efficiency of the acid site. In addition, the newly fabricated nonacidic shell, which coats on the surface of ZSM-5 as a chain mail, plays a critical role in suppressing the coke formation by passivation of the acid sites on the external surface. In this work, the pore properties and acid distribution of ZSM-5 zeolite can be regulated synchronously through a desilication−recrystallization process. In addition, enhanced yields of light aromatics and catalytic lifetime were obtained simultaneously. The construction of this hierarchical core−shell zeolite with redistributed acid sites contributes to the highly efficient catalytic upgrading of macromolecular reactants.

Rational Design of Ultrasmall PtZn Nanoparticles Supported on Zeolite Composites as Bifunctional Catalysts for n-Butane Dehydroisomerization

“…Obviously, the self-assembly of the stacked-nanosheet morphologies was observed in the case of SN1 (Figure 2a) with an average particle size of 117.2 ± 13.1 nm, and it was similar to what has been described in previous studies. 26 In strong contrast, the pristine FER has different morphologies than the aggregated ellipsoidal crystals with an average particle size of 1.1 ± 0.3 μm, as shown in Figure 2b. However, after the pretreatment of the pristine FER with TBAOH in the preliminary recrystallization step (step (i)), the morphology of the FER/TBAOH was comparable to that of the pristine FER (Figure 2c).…”

“…The reason for the improved catalytic activity relates to the following two reasons: (i) combining zinc species with Pt, diminishing the platinum sintering, which eventually suppresses the excessive number of n-butane molecules that are adsorbed on the surface for further side reactions; 12 (ii) the change in electron density of Pt in the presence of Zn species, eventually quashing side reactions due to the facile desorption of butene on the catalyst surface. 26 However, the n-butane conversion decreases to 40% when using higher zinc content (1Pt2Zn-SN1) with 2 wt.%. This relates to the fact that an excessive zinc concentration results in an intensely high electron density of the Pt sites, which prohibits n-butane adsorption on the Pt surfaces.…”

“…Typically, the development of butene selectivity via BDH is essential to further promote the isobutene yield in n -butane dehydroisomerization. Previously, the PtZn-based catalysts have been reported as one of the promising candidates providing high catalytic activity in propane dehydrogenation . Herein, to initially evaluate the influence of Pt and Zn on the BDH reaction, the catalytic activities among different metals and alloys, including monometallic Zn and Pt, as well as the PtZn alloys, were compared.…”

“…25 Recently, the strategy of core−shell zeolite composite synthesis containing silicalite-1 zeolite deposited on the outermost surface of a zeolite has been investigated to reduce the acidity. For instance, Feng et al 26 reported the synthesis of self-assembled hierarchical ZSM-5@silicalite-1 core−shell structures synthesized via a mild hydrothermal alkaline treatment. This innovation enhances the catalytic reforming of lignite pyrolysis volatiles by tuning mesopores and acidity in ultrathin silicalite-1 layers over hierarchical Zeolite Socony Mobil (ZSM)-5 crystals.…”

Passivated Surface of High Aluminum Containing ZSM-5 by Silicalite-1: Synthesis and Application in Dehydration Reaction

“…The favorable effect of such a passivation technique is presented in different reactions: improved stability of ZSM-5/silicalite-1 catalyst, containing Mo, in methane de-hydro-aromatization was reported; the catalyst exhibited an excellent shape selectivity without lowering the conversion significantly as a consequence of the reduced acidity . The advantage of tuning the surface acidity of the core–shell catalysts in methanol-to-hydrocarbon reactions and pyrolysis was reported as well. − Other examples for ZSM-5 zeolites coated with a layer of silicalite-1 were reported and employed in other applications such as methanol conversion into gasoline and diesel, ketonization and cracking reactions, and alkylation of toluene. − ,, However, the Si/Al ratio in the ZSM-5 core used was relatively high, ranging from 25 to more than 100, which hinders to broaden the application of the surface-passivated catalysts with the highly acid zeolitic core …”