Cellulose-Nanofiber-Mediated Sorption-Benefitting Holed Silicalite-1 Crystals

Abstract

Silicalite-1-type zeolites with unique intracrystal holes or cracks were successfully prepared using a cellulose nanofiber (CNF) as an additional mediating material, and their vapor phase adsorption properties toward methyl tert -butyl ether (MTBE) and n -nitrosodimethylamine (NDMA) were examined. It was found that the mixing protocol of CNF and structure-directing agents (SDAs), the addition amount of CNF, and the CNF/SDAs amount ratio play important roles in forming the holed silicalite-1. The synthesis route that preliminarily mixes CNF with SDAs in a series of controlled conditions is particularly beneficial for the formation of the holed silicalite-1 with mesoporosity and larger pores because the CNF-SDAs composite structure benefits the zeolite growth closely encompassing CNF inside the crystal structure. It also promotes the preferential formation of the orthorhombic phase vicinal to the CNF surface, namely, the surface of the formed internal holes or cracks, with the twin-type crystal size reduced as compared to the non-CNF-templated sample. On the contrary, the synthesis route that mixes CNF with SDAs–silicate composite ions tends to modify the twin-type crystal shape at the same time to form small but uniform well-crystallized particles with less holes or cracks and a dominative monoclinic phase. It was considered that both the inter-subunit structural defect and silanol defect whose content is increased with CNF addition influence the adsorptivity of MTBE and NDMA. Owing to the small twin-type crystal size, the smaller crystal subunits, and the favored short path from the surface of internal holes or cracks, the holed silicalite-1 derived from the CNF and SDA premixture assures the easiest access of adsorbate molecules to the most energetically favored sites and is most appropriate for the adsorption of both MTBE and NDMA among the examined zeolites.

In Situ Hydrogen Peroxide Production for Selective Oxidation of Benzyl Alcohol over a Pd@Hierarchical Titanium Silicalite Catalyst

“…Notably, all curves show homologous characteristic peaks at 2θ = 7.9°, 8.9°, 23.1°, 23.9°, and 24.4°, which indicates that all four catalysts are a well-crystallized structure (topological structure MFI, defined by the Structure Commission of the International Zeolite Association). 24 − 28 Meanwhile, no diffraction peak of crystalline metal Pd was clearly observed for Pd-modified TS-1 catalysts. The existence of metallic Pd was confirmed by ICP-OES, indicating that the average sizes of Pd particles are very small and well dispersed within the framework of the support or low loading.…”

Dissolution–Recrystallization Formation of Huge Thin 2D Silicalite Lamella for Promoted Sorption

“…Particularly, it seems that MTBE is only able to fill the half of micropore volume in the parent silicalite‐2 (consequently adsorption amounts of MTBE in the region is comparable for the two materials irrespective of the smaller micropore volume for L‐silicalite_423‐360), implying that some serious blockade of its pores in MTBE adsorption is present at the lower adsorption temperature. Certain structural defects together with the difficulty for the large MTBE molecule to enter into the inner adsorption sites through the longer micropore channel path may be considered for the reason. Thus, the results strongly suggest that the beneficial adsorption sites such as channel intersections becomes more exposed to be easier access by MTBE for L‐silicalite_423‐360 because of the peculiar 2D structure of h‐nanoplatelets.…”

“…This process continues until the filling‐up of the microchannels where an adsorption energy summit takes place. The energy summit is usually attributed to rearrangement or phase change of adsorbates in zeolite microchannels to achieve stabilization in energy, which is often accompanied by certain change in zeolitic framework structure, thus serving as another unfavored factor for desorption. For L‐silicalite_423‐360, adsorption enthalpy of MTBE is almost the same as that of the parent silicalite‐2 at the initial stage (up to ≈7.6 mL‐STP g −1 ) irrespective of a greater adsorption amount, implying that indeed it is primarily the increased number of favored adsorption sites in h‐nanoplatelets rather than the enhanced adsorption energy which is responsible for the increased adsorption.…”

“…Besides, increased number of deficient sites such as internal deficient silanol groups (Figure S6, Supporting Information) can also be considered as such defect sites show greater affinity toward NDMA containing the more polar NO groups than MTBE containing the less polar O group. Therefore, the difference in adsorption behavior of MTBE and NDMA is determined by their different molecular size, geometric configuration, and polarity . The greater micropore size, shorter diffusion path, and insignificant zeolite structural distortion upon adsorption are possibly the contributing factors as a whole to benefit desorption recovery property of the 2D lamellar silicalite toward the two organic contaminants.…”