Probing the Influence of SSZ‐13 Zeolite Pore Hierarchy in Methanol‐to‐Olefins Catalysis by Using Nanometer Accuracy by Stochastic Chemical Reactions Fluorescence Microscopy and Positron Emission Profiling

Abstract

[a] Increasingly,m ethanol is considered as an important chemical intermediate between coal and natural gas feedstocks and olefinic building blocks used extensively in the chemical industry.Currently,t he methanol-to-olefins (MTO) reaction is the most important in terms of the installed capacity of all methanol-tohydrocarbon reactions. [1][2][3][4] Am icroporous silicoaluminophosphate materialw ith the chabazite (CHA) framework topology (H-SAPO-34)i st he preferred catalystc ommercially.[5] One concern is the predicted scarcity of phosphorus,w hich is contained in typical silicoaluminophosphates up to 20 wt %. [6,7] The higher acidity of SSZ-13, the isostructural phosphorous-free aluminosilicate analogue of H-SAPO-34, presentsa no pportunity to operate the MTO reactiona talower temperature and, in this way,t oi mprove process economics.[8] However,t he stronger acidityr esults in more rapid catalystd eactivation caused by multiring aromaticst hat block the pore system of the zeolite.[9] These multiring aromatics derive from the fusion of aromatic hydrocarbon pool species, which are key reactioni ntermediates of the MTOr eactioni ns mall-pore zeolites.[10] Catalyst stability is impeded by the fact that these deposits form in the outer regions of the zeolite crystalsa nd, accordingly,p reventt he accesso fm ethanolt oi nternal regions.T he introduction of mesopores within zeolite crystals is an elegant way to reduce these negative effects of diffusionl imitations and improve the catalytic performance. [11][12][13] Common and scalable approaches such as steaming and alkaline leaching have an egative effect on MTO performance if applied to SSZ-13 zeolite. [14,15] An understanding of the role of the hierarchical pore architecture of SSZ-13z eoliteso nt he catalytic performance in the methanol-to-olefins (MTO) reactioni sc rucial to guide the design of better catalysts. We investigated the influence of the space velocity on the performance of am icroporous SSZ-13 zeolite and severalh ierarchically structured SSZ-13 zeolites. Single catalytic turnovers, as recorded by nanometera ccuracy by using stochastic chemical reactions (NASCA) fluorescence microscopy verified that the hierarchical zeolitesc ontain pores larger than the 0.38 nm apertures native to SSZ-13z eolite.T he amount of fluorescent events correlated well with the additional pore volume availableb ecause of the hierarchical structuring of the zeolite. Positron emission tomography (PET) using 11 C-labeled methanolw as used to map the 2D spatial distribution of the deposits formed during the MTO reaction in the catalystb ed. We used PET imaging to demonstrate that hierarchical structuring not only improves the utilization of the availablem icroporous cages of SSZ-13 but also that the aromatic hydrocarbon pool species are involved in more turnovers beforet hey condense into larger multirings tructures that deactivate the catalyst.

Rapid synthesis of hierarchical SSZ-13 zeolite microspheres via a fluoride-assisted in situ growth route using aluminum isopropoxide as aluminum source

“…4 To date, various zeolite catalysts have been used for the conversion of methanol, among which SSZ-13 zeolite with CHA framework topology exhibits high MTO catalytic performance, mostly contributed to its large surface area, useful pore structure in terms of shape selectivity, moderate acidity, and high thermal/hydrothermal stability. [5][6][7] However, the mass transfer limitation and coke species formation in the cages result in a low catalytic efficiency and the rapid deactivation of SSZ-13. 8 In order to prolong the catalytic lifetime and suppress the deactivation of SSZ-13 catalyst in the MTO reaction, various methods have been used to decrease the crystal size, which can signicantly reduce the diffusion path length and enhance the accessibility of reactants to the external active sites.…”

“…12,13 Typical strategies for the preparation of hierarchical zeolite materials, such as alkaline treatment, does not enhance MTO catalytic performance when applied to SSZ-13, because of the decrease of surface area and acid sites. 7,12 Hensen et al 13,14 synthesized mesoporous SSZ-13 crystals by a dually structure-directing synthetic route using N,N,N-trimethyl-1adamantanammoniumhydroxide (TMAdOH) and diquaternary ammonium type surfactant as templates. They further synthesized hierarchical SSZ-13 zeolites by adding surfactant C 22À4À4 Br 2 and sodium uoride to the synthesis gel.…”

Mild dealumination of template-stabilized zeolites by NH4F

“…[10][11][12] More recently, it has also been reported that introduction of NaF during the synthesis of high-silica CHA zeolites results in additional (intracrystalline) microporosity, which can greatly enhance mass transport of reactants to active sites during catalysis. 13,14 Framework etching with HF or NH 4 HF 2 is another method to introduce additional porosity in zeolites. [15][16][17][18] Furthermore, substitution of surface hydroxyls with fluoride groups involving for instance organosilane grafting can be employed to enhance the surface hydrophobicity.…”

Performance Enhanced SAPO-34 Catalyst for Methanol to Olefins: Template Synthesis Using a CO2-Based Polyurea

“…Methanol to lower olefins reaction is a critical process for conversion of nonoil resources to chemicals through methanol, and light olefins especially ethylene and propylene have been widely applied in polymerization reaction and petrochemical industry [1][2][3][4][5][6]. Several types of molecular sieves for the MTO reaction have been intensively studied, and the most typically are SAPO-34 [7,8], ZSM-5 [9][10][11], and SSZ-13 [12][13][14]. Among these catalysts, SAPO-34 has drawn the most attention due to its special framework with a CHA cage (9.4 Å) and 8-ring pore openings (0.38 Å 0.38 Å) [15], as well as mild acidity [16][17][18] which result in excellent catalytic performance [3,[19][20][21].…”