Controllable Orientation Growth of ZSM-5 for Methanol to Hydrocarbon Conversion: Cooperative Effects of Seed Induction and Medium pH Control

Abstract

The growth orientation of ZSM-5 zeolites strongly affects product selectivity in methanol conversion reaction. Here, we proposed a versatile synthetic strategy by introducing seeds and controlling medium pH to achieve controllable orientation growth of ZSM-5. The systematic analysis of the crystallization process indicated that the introduction of seeds ensured successful crystallization in a quasi-neutral solution and the dissolution rate of seeds and aluminosilicate determined the growth orientation of ZSM-5. In the quasi-neutral solution, the slow dissolution of seeds and aluminosilicate enhanced growth advantages along the c axis. The ratio between the length of the c axis and b axis (L c /L b ) of the obtained ZSM-5 at pH of 7 could reach 8.1, much higher than 1.8 obtained at pH of 11. No obvious impact of seed added amount on growth orientation was found, while with increasing seed crystal size, the obtained ZSM-5 showed preferred growth along the c axis. The L c /L b of the sample adding seeds with a size of 355 nm reached 7.9, much higher than 2.1 of the sample adding seeds with a size of 70 nm. The obtained ZSM-5 with specific growth orientation exhibited potential shape selectivity in methanol to aromatics and olefin reaction. This work opens new possibilities to tailor the orientation growth of ZSM-5 based on the seed-induced strategy under mild conditions.

Enhancing sorption kinetics by oriented and single crystalline array-structured ZSM-5 film on monoliths

“…This is especially true considering the anisotropic pore distribution of ZSM-5. The anisotropic channel of ZSM-5 plays an essential role in determining the catalytic properties besides the acid properties and micropore size and geometry 53 . ZSM-5 contains two types of interconnected 10-membered ring channels: the straight channel along the b -axis (5.6 × 5.3 Å) and the sinusoid one alone the a -axis (5.5 × 5.1 Å).…”

“…Typically, decreasing the b- axis channel length could shorten the path length of molecular diffusion and enhance accessibility to internal acid sites, which could enhance the activity of the catalyst in the acid-catalyzed reaction and reduce deactivation. Therefore, most of the literature reported work focuses on controlling ZSM-5 b -axis oriented growth to achieve a very thin thickness along b -axis, ranging from serval nanometers to hundreds of nanometers 53 – 56 . Guest molecules such as methanol, benzene, n-heptane, and phenol etc.…”

Urea-assisted morphological engineering of MFI nanosheets with tunable b-thickness

“…The final crystal morphology is governed by both the intrinsic nature of the zeolite topology and many synthetic parameters, such as chemical composition, solvent, additives, crystallization temperature, etc. To restrict the growth rate along the b-axis of MFI zeolite, many strategies, as illustrated in Scheme 1, have been developed including 1) the design of novel organic structuredirecting agents (OSDA) to prevent the stacking of extra pentasil layers, as a result, 1-2 pentasillayered MFI zeolite is synthesized [15,16]; 2) the use of fluoride anion promoting the plate-like crystal formation [17,18], and the b-thickness of MFI crystals reaches down to around 10 nm [19,20]; 3) the introduction of additive molecules to inhibit the growth along b-axis or promote the growth in other directions [21,22]; and 4) the variation of the synthetic conditions, for instance, gel composition, seeds, pH, and recrystallization, can also produce MFI crystal with short b-thickness [23][24][25]. A summary of developed methods regarding safety, cost, availability, and efficiency issues is presented.…”