Structural features of core–shell zeolite–zeolite composite and its performance for methanol conversion into gasoline and diesel

Abstract

Zeolite-zeolite composite composed of alumina-rich hierarchically porous ZSM-5 cores and high-silicon MFI shells was prepared by a hydrothermal synthesis procedure, in which a commercial ZSM-5 zeolite with a SiO 2 /Al 2 O 3 of 36 was treated by an alkaline solution and then used as a supporter for epitaxial growth of a polycrystalline Silicalite-1 zeolite shell (denoted as MMZsa). Acid sites associated with framework Al on exterior surfaces of ZSM-5 zeolite cores are therefore passivated in different degrees by the epitaxial MFI zeolite shell. The structural, crystalline, and textural properties of the as-synthesized samples were characterized by x-ray powder diffraction (XRD), energy-dispersive x-ray spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), N 2 adsorption-desorption, in situ IR spectra of pyridine and NH 3 -TPD. Aluminum species were observed to transfer from the alumina-rich cores to the high-silica shells. The adjustable thickness and SiO 2 /Al 2 O 3 ratio of the shell offer the as-synthesized composite a potential and high-efficiency catalyst for methanol conversion into gasoline and diesel. As compared with the commercial ZSM-5 zeolite, the composite catalyst exhibits excellent catalytic performances with a longer catalytic life as well as a higher conversion and a slightly higher yield of diesel oil.

Synthesis, Types, and Applications of Zeolite Capsule Catalysts

“…In the process of hydrothermal synthesis, factors affecting the growth of the zeolite shell are the type of aluminum source, [ 75 ] the concentration of template, water content, the ratio of silicon to aluminum, crystallization temperature and time. [ 76 ] Among them, crystallization temperature and Si/Al ratio are two important factors to control whether the core–shell structure is formed. As shown in Figure , [ 77 ] the area of overlapping area between crystallization temperature and Si/Al ratio is inversely proportional to the difficulty of forming core–shell structure.…”

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 …”

“…35 The advantage of tuning the surface acidity of the core−shell catalysts in methanol-to-hydrocarbon reactions and pyrolysis was reported as well. 36−39 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, 40 ketonization and cracking reactions, 41 and alkylation of toluene. [20][21][22]42,43 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 surfacepassivated catalysts with the highly acid zeolitic core.…”

Synthesis of mesoporous ZSM-5 zeolite encapsulated in an ultrathin protective shell of silicalite-1 for MTH conversion

“…Another strategy that has been investigated to resolve this problem is controlling the external surface acidity by covering the zeolite crystal with a non-active layer such as silicalite-1 [32][33][34][35][36]. Silicalite-1 shell has been reported to be formed around ZSM-5 zeolite by dealuminating the outer surface [37], treating ZSM-5 zeolite crystals with silicalite-1 seeds [38] and one-step synthesis in alkaline or fluoride medium [39].…”

“…The aforementioned surface passivation of acidic zeolites has been demonstrated by different groups to enhance the catalytic performance in MTH [8,35]. Kustova et al synthesized a mesoporous ZSM-5 core-shell via a sophisticated method.…”