Recent Trends in Tailoring External Acidity in Zeolites for Catalysis

Abstract

Zeolites are crystalline aluminosilicates with well-defined microporous structures that have found several applications in catalysis. In recent years, great effort has been devoted to defining strategies aimed at tuning structural and acidity properties to improve the catalytic performance of zeolites. Depending on the zeolitic structure, the acid sites located inside the crystals catalyze reactions by exploiting the internal channel shape-selectivity. In contrast, strong acid sites located on the external surface do not offer the possibility to control the size of molecules involved in the reactions. This aspect generally leads to a loss of selectivity toward desired products and to the uncontrolled production of coke. Passivating surface acidity is a promising way to overcome deactivation issues and to enhance the catalytic performance of zeolites. This Mini-Review aims to provide, for the first time, a complete overview of the techniques employed in recent years to neutralize strong external acid sites. Both chemical and liquid vapor deposition of silicates have been widely employed to passivate the external surface acidity of zeolites. In recent years, the epitaxial growth of layers of aluminum-free zeolite, e.g., silicalite-1, over the surface of the acidic zeolite has been proposed as a new approach to neutralize strong external acid sites controlling diffusional phenomena. NH₃-TPD, FT-IR, SEM-EDX, and other techniques have been used to provide information about the level of control of the external strong acidity of passivated zeolites. In this Mini-Review, both passivation treatments and characterization techniques are compared and advantages and disadvantages deeply discussed to elucidate the effect of passivation procedures on physical features and especially the catalytic behavior.

Figure 1

2,6-DTBPy FT-IR spectra of parent zeolite and core–shell samples. Reprinted with permission from ref (9). Copyright 2022 MDPI.

Figure 2

Toluene conversion and p-xylene selectivity as a function of time (TOS) in a toluene alkylation reaction over an untreated H-ZSM-5 sample and HZSM5@X@S-1 core–shells. Reprinted with permission from ref (9). Copyright 2022 MDPI.

Figure 3

TEM images of (a) ZSM-5 and (b) Z5@S(1.0). (c) TEM image of ZSM-5 and the corresponding EDS mapping images representing (d) Si and (e) Al. (f) TEM image of Z5@S(1.0) and the corresponding EDS mapping images representing (g) Si and (h) Al. Reprinted with the permission from ref (11). Copyright 2023 John Wiley and Sons.

Figure 4

n-Hexane conversion and product selectivity for parent zeolite and passivated samples. Reprinted with the permission from ref (11). Copyright 2023 John Wiley and Sons.

Figure 5

n-Hexane conversion and product selectivity for core–shell Z5@S(2.0) and a mechanical mixture (Z5/S). Reprinted with the permission from ref (11). Copyright 2023 John Wiley and Sons.

Figure 6

EM images of (a, b) HZSM-5 and (c, d) CS2 samples, where the sporadic silicalite-1 crystals (C), the incomplete silicalite-1 layer (B), and the layer incompletely covered by layer B (A) are indicated. Reprinted with the permission from ref (12). Copyright 2018 Elsevier.

Figure 7

1,3,5-TIPB and acetic acid conversion over untreated zeolite and passivated samples. Reprinted with the permission from ref (12). Copyright 2018 Elsevier.

Figure 8

Product distributions over untreated HZSM-5 and CS2 as a function of catalyst cycle number: carbon yields of BTX, olefins, and BTX + olefins. Reprinted with the permission from ref (12). Copyright 2018 Elsevier.

Figure 9

Methanol conversion over Conv-ZSM-5, Meso-ZSM-5, and Shell-Meso-ZSM-5 catalysts as a function of time. Reprinted with the permission from ref (13). Copyright 2020 Elsevier.

Figure 10

GC-MS spectra of spent catalysts. Reprinted with the permission from ref (13). Copyright 2020 Elsevier.

Figure 11

Benzyl alcohol conversion and yields of 1,3,5-trimethyl-2-benzylbenzene (TMBB) and dibenzyl ether (DBE) obtained testing parent zeolite (FER-Core) and core–shell sample (CS-FER): the BA conversion (gray) and yields of products DBE (blue) and TMBB (orange) from H-form FER-Core and CS-FER catalysts. Reprinted with the permission from ref (14). Copyright 2023 Elsevier.

Figure 12

(a) Methanol conversion and (b) ethylene/propene selectivity as a function of time on stream (TOD) in a MTO reaction over untreated SAPO-34 samples (SAPO-34-B) and samples treated with TEOS CLD (SAPO-34-L) and acetic acid etching (SAPO-34-H). Reaction condition: 450 °C, methanol WHSV = 5.0 h^–1. Reprinted with the permission from ref (17). Copyright 2020 Jhonson Wiley and Sons.

Figure 13

Influence of time on stream (TOS) on the catalytic yields of M-ET and P-ET over HM, HM–M, and their modified zeolites. The maximum standard deviation of each data point was <2.0%. (a) HM and modified HM samples. (b) HM–M and modified HM–M samples. Reprinted with the permission from ref (19). Copyright 2011 Royal Society of Chemistry.

Figure 14

(a) Propane conversion and (b) BTX selectivity as a function of time on stream for HZSM-5, Ga/HZSM-5 (Ga/Z5) and silylated samples with different TEOS amounts, i.e., 2wt % (Ga/Z5–2Si), 4 wt % (Ga/Z5–4Si), 6 wt % (Ga/Z5–6Si) and 8 wt % (Ga/Z5–8Si); The selectivity of (c) cracking products (methane and ethane) and (d) propene at 4.5 h. Reaction conditions: atmospheric pressure, 540 °C, WHSV = 6000 mL/(g·h), and N₂/C₃H₈ = 2. Reprinted with the permission from ref (20). Copyright 2021 Elsevier.

Figure 15

Effects of the phosphorus content on the catalytic behavior of HZSM-5 and phosphorus-modified samples during toluene methylation: (a) HZSM-5, (b) MZS-0.5P, (c) MZS-1.0P, (d) MZS-1.5P, (e) MZS-2.0P, and (f) HZSM-5–1.5P. (A) Toluene conversion. (B) Selectivity of p-xylene. (C) Selectivity of xylene. (D) Yield of xylene and p-xylene. Reprinted with permission from ref (21). Copyright 2019 American Chemical Society.

Figure 16

Time on stream tests of DME-to-olefin reactions over H-ZSM-5 (Z0), H-ZSM-5 modified with disilane (Z10), and calcined modified H-ZSM-5 (Z10-C). Reprinted with the permission from ref (23). Copyright 2019 Elsevier.