Abstract
Abstract Hierarchical SAPO-5 molecular sieves were synthesized with three different mesopore structure-directing agents (meso-SDAs) (cetyltrimethylammonium bromide (CTAB), myristyltrimethylammonium bromide (MTMAB) and [3-(trimethoxysilyl)propyl] dimethyloctadecylammonium chloride (TPOAC)) based on a soft-template hydrothermal synthesis procedure. To investigate the modified porosity of the hierarchical SAPO-5s, they were characterized thoroughly with the results being compared to the conventional microporous SAPO-5. Nitrogen sorption measurements revealed considerable hysteresis loops for the hierarchical SAPO-5s as well as larger mesopore volumes (≥0.15 cm3 g-1) compared to the conventional SAPO-5 (0.05 cm3 g-1). The relative number of acid sites for each sample was calculated from FTIR adsorption data and was in the order of C-SAPO-5>H-CTAB>H-MTMAB>H-TPOAC. The hierarchical SAPO-5s had a significantly increased lifetime (>150 h) in the methanol to hydrocarbons (MTH) model reaction compared to the conventional SAPO-5 (
Highlights
The silicoaluminophosphate-5 (SAPO-5) with the AFI framework type is an acidic, microporous zeotype which is composed of 12membered rings aligned in parallel, being a large-pore, onedimensional structure where the micropores have a diameter of 7.3 Å [1, 2]
The X-ray powder diffraction (XRD) results confirmed that the AFI structure was obtained in all the SAPO-5 samples and the BET results confirmed the presence of both micropores and mesopores for the hierarchical SAPO-5s
While Carbon monoxide (CO) adsorption confirmed that comparably strong acid sites were present in all samples, the relative number of acid sites in the samples varied ac cording to the type and length of meso-structure-directing agents (SDAs) that was used, with the quaternary ammonium surfactant CTAB giving the largest and organo silane surfactant TPOAC giving the smallest number of Brønsted acid sites
Summary
The silicoaluminophosphate-5 (SAPO-5) with the AFI framework type is an acidic, microporous zeotype which is composed of 12membered rings aligned in parallel, being a large-pore, onedimensional structure where the micropores have a diameter of 7.3 Å [1, 2]. In an effort to ameliorate this, the introduction of larger pores (typically mesopores) into the microporous structures to make so-called hierarchical materials has recently gained traction [7]. There are several pathways to make hierarchical SAPOs, often divided into either top-down or bottom-up approaches Examples of the former include post-synthesis modification by e.g. dealumination or desilication, while examples of the latter include hard- or softtemplating techniques [8,9]. In the bottom-up approaches, the larger pore system is typically introduced into the structure during synthesis with large molecular templates or so-called structure-directing agents (SDAs). Danilina et al [3] have previously attempted to do comparison studies between different surfactants (organosilane and quaternary ammonium), how ever syntheses with the quaternary ammonium surfactant did not yield phase pure SAPO-5 and the sample was not further analyzed
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