As-made Zeolites Research Articles

Structure determination of as-made zeolite ITQ-52 by three-dimensional electron diffraction

The complete crystal structures of two as-made zeolites with the ITQ-52 structure (IFW framework type), where an alkylphosphonium and an aminophosphonium dications have been used as their respective organic structure-directing agents, have been solved by three-dimensional electron diffraction. Both organic cations have also been located within the 10-ring that connects the cavities of the IFW framework using the diffraction data.

F−@d4r, a New Type of Acidic Catalytic Site in Zeolite

As a solid acid, zeolite has been widely used in many fields such as tail gas treatment, petrochemical engineering, and the fine chemical industry. F− has been widely used in the synthesis of pure silica or high silica zeolites. To balance the charge of organic structure directing agents (OSDA), F− is often found located at the center of the double-4-rings (d4r) of the as-made zeolites. During calcination, fluorine ion is removed with the OSDA. We screened a series of composition building units and found that d4r is capable to retain F− in zeolite structure. We introduce the F− back after the calcination to create an unprecedented type of acid site, i.e., F−@d4r in pure silica zeolites ITQ-12. The F−@d4r species is thermal stable up to 300 °C. ITQ-12 with F−@d4r shows substantial catalytic activity in Biginelli reaction. Furthermore, the catalytic performance is proved to be positive correlated with the presence of F−@d4r, indicating the mild acid catalytic property of F−@d4r.

Enhanced pro-coagulant hemostatic agents based on nanometric zeolites

Micro-and nanometric faujasite zeolites were tested as hemostatic coagulant agents. The as-made zeolites and their calcium ion exchanged derivatives were characterized by XRD, SEM, AFM, TGA, and DSC. Zeta potential measurements of the micro and nanometric zeolites, as a function of pH (1–12.5), revealed the hemostatic potential of the materials. The isoelectric point for FAU, FAU/Ca, Nano-FAU, and Nano-FAU/Ca were measured at pH 2.0, 1.9, 3.2, and 2.5, respectively. The hemostatic activity was confirmed by the TEG technique for both micro and nanometric zeolites, however a superior activity were observed for the nanometric materials. FAU and FAU/Ca exhibited a reduction of the R parameter (defined as period of time of latency from the start of test to initial fibrin formation for the formation of a clot of an amplitude of 2 mm) from 8.6 ± 0.7 min (control) to 3.2 ± 0.7 min and 2.3 ± 0.1 min, respectively. On the other hand, for the Nano-FAU, and Nano-FAU/Ca the observed decrease were to 2.4 ± 0.6 min, and 1.1 ± 0.2 min, respectively. The amounts of heat released were 105.60 J/g (FAU), 65.8 J/g (FAU/Ca), 85.48 J/g (Nano-FAU), and 78.21 J/g (Nano-FAU/Ca). Statistical analysis using one-way analysis of variance showed a global value of p < 0.0001. Tukey-Kramer multiple comparisons revealed significant (p < 0.0001) differences for FAU, FAU/Ca, Nano-FAU, and Nano-FAU/Ca, relative to the control. Nano-FAU/Ca showed the most important reductions of the R with significant differences compared to the control (p < 0.0001 and p < 0.0001, respectively) and FAU (p = 0.0071 for the R parameter).

Steam-dealuminated, OSDA-free RHO and KFI-type zeolites as catalysts for the methanol-to-olefins reaction

RHO and KFI-type zeolites are synthesized in the absence of organic structure-directing agents (OSDAs), post-synthetically dealuminated via high temperature (600–800 °C) steam treatments, and evaluated as catalysts for converting methanol-to-light olefins (MTO). The proton forms of the as-made zeolites deactivate rapidly when tested for the MTO reaction (conducted at 400 °C and atmospheric pressure) due to their high aluminum content. Steam treatments lead to improvements in olefin selectivities and catalyst lifetimes with samples steamed at 600 °C giving the best combination of lifetime and olefin selectivity. Zeolite characterizations by 27Al NMR, 29Si NMR and argon physisorption indicate that the steam treatments extract framework aluminum, leading to reductions in the total number of Brønsted acid sites and the creation of mesopores that can facilitate transport of reactants. Poisoning of the acid sites external to the 8MR pores of the zeolites by treatments with trimethylphosphite results in longer catalyst lifetimes but do not affect the observed olefin selectivities, suggesting that olefins do not undergo secondary dimerization or methylation reactions at these sites.

ChemInform Abstract: Selectivities in Post‐Synthetic Modification of Borosilicate Zeolites

AbstractReview: 173 refs.

Selectivities in Post-Synthetic Modification of Borosilicate Zeolites

The chemistry of post-synthetic modification of borosilicate zeolites, and their understanding and use in catalytic applications are reviewed. First, a survey is given on the practical application of solid state NMR to follow the structural properties of boron. Then, the hydrolytic chemistry of boron in zeolites is summarized with a focus on selective reactivities of trigonal boron created by counterions, and stepwise extraction of boron from the framework. The reinsertion of other heteroatoms (mainly Al is discussed here) can be carried out by three different methods with different selectivities. First, the extent of Al reinsertion can be tailored quantitatively by controlling boron removal and vacancy formation with appropriate counterions. Secondly, the substitution of Al for B can be kinetically retarded by using uncalcined as-made zeolites with organic structure directing agents still occluded in the pores. Although these zeolites are not porous, quantitative replacement of framework atoms (Al for B) is possible, most likely by a solid state diffusion mechanism. Finally, Al reinsertion can be efficiently achieved in aqueous Al(NO3)3 solution under acidic conditions and is pore selective. This is due to the finding that the hydrated aluminum cations are too bulky for medium-sized pores, so that preferably 12-membered ring pores can be aluminated, and 10-membered ring pores do not substitute Al for B. This substitution can be quantitatively followed by the pH change in the zeolite/Al(NO3)3 slurry. A variety of catalytic test reactions are used to characterize the catalytic properties of the aluminosilicate zeolites prepared via post-synthetic treatment of their borosilicate counterparts.

Post-synthetic modifications of as-made zeolite frameworks near the structure-directing agents

As-made zeolite B-Beta was converted to the Al form with aqueous 1 M Al(NO3)3 solutions at 448, 423 and 393 K under hydrothermal conditions. Almost complete removal of framework boron was observed at 448 K in seven days while tetraethylammonium cations (TEA+) are in the pore system. At the same time aluminium was inserted in the zeolite. 1H–11B and 1H–27Al REAPDOR NMR data show that TEA+–BO4/2− ion pair interactions are quantitatively replaced by TEA+–AlO4/2− interactions. This exchange reaction is a thermally activated process which consists of three contributions. The initial reaction in the first step removes defect sites near the crystal surface by aluminium insertion within the first day at 448 K. Subsequently, boron is replaced by aluminium in the bulk of the crystals in the second step. The estimated activation energy of this second process is Ea = 80 ± 15 kJ mol−1. This reaction step is consistent with a solid state migration of framework atoms under hydrolysis and condensation of bonds and/or a coupled dissolution–reprecipitation mechanism. In the last step, a faster exchange reaction takes place due to the formation of mesopores in the crystals. The reverse reaction on Al-Beta does not exchange boron for aluminium, using boric acid solutions, but boron is inserted into the framework while defect sites are removed. As-made B-ZSM-12 shows similar B/Al exchange as B-Beta. No reaction was observed for zeolite B-ZSM-5 with tetrapropyl-ammonium cations in the pores.

Nanozeolites: New strategies for designing ultra small silicalite crystals with very few framework defects

Framework defect-free nanosized silicalite-1 crystals (ca. 150nm) have been obtained by post-synthesis modification of a zeolite prepared under alkaline pH conditions. Several methods have been proposed, all of them being based on dissolution/recrystallization processes that keep the size of the zeolite crystals unchanged. Treatment of as-made zeolites with NH4F led to a significant decrease in the density of siloxy framework defects but silanol groups could not be totally eliminated, even after calcination. On the other hand, a succession of treatments in TPAOH and NH4F led to materials with very few framework defects and more or less regular hollow structures. The best solids were obtained when treatments were performed on calcined zeolites with solid NH4F. Under such conditions, zeolites were first dissolved following a mechanism similar to that observed under alkaline pH conditions but hollow structures thus obtained could not be stabilized and they progressively disappeared with time. Despite that, the crystallinity remained unaltered and treated zeolites were characterized by highly resolved 29Si NMR spectra, characteristic of defect-free all-silica frameworks. Modifications of the crystal shape with time have been explained by a partial recrystallization of the zeolite in the presence of ammonium ions.

In SituTransformation of TON Silica Zeolite into the Less Dense ITW: Structure-Direction Overcoming Framework Instability in the Synthesis of SiO2Zeolites

Under specific synthesis conditions the crystallization of a dense silica zeolite (TON) is followed by its in situ transformation into a less dense and, in the absence of occluded species, less stable zeolite (ITW). Periodic ab initio calculations including energy corrections for van der Waals interactions as well as zero-point and thermal effects are used first to assess the relative stability of both SiO(2) (calcined) phases and then to investigate host-guest interactions in the as-made zeolites, as well as their relative stability. The less dense SiO(2)-ITW is less stable than SiO(2)-TON, with an energy difference that is significantly larger than expected from their difference in molar volume. This extra destabilization is ascribed to the strained double 4-ring units of silica tetrahedra (D4R). Regarding the as-made materials, the organic cation fills in more efficiently the zeolitic voids in ITW than in TON, bringing about a larger stabilization in the former owing to the extension of the long-range addition of dispersion force contributions. On the other hand, fluoride induces a polarization of the silica framework that is highly localized in TON (showing pentacoordinated [SiO(4/2)F](-) units) but has a large global character in ITW (where fluoride is encapsulated into D4R units). We argue that the structure-directing role toward D4R materials that has been proposed for fluoride consists fundamentally in the ability to induce a global polarization of the silica framework that allows relaxation of the strain associated with these units. In this sense, fluoride stabilizes the otherwise strained D4R-SiO(2) frameworks making them reachable for crystallization. This work documents a case in which the structure directing agents "choose" a structure not kinetically but through stabilization.

Negative Poisson’s ratios in siliceous zeolite MFI-silicalite

Brillouin scattering measurements of the single-crystal elastic properties of the as-made zeolite silicalite mid R:(C(3)H(7))(4)NFmid R:(4)[Si(96)O(192)]-MFI provides the first experimental evidence for on-axis negative Poisson's ratios (auxeticity) in a synthetic zeolite structure. MFI laterally contracts when compressed and laterally expands when stretched along x(1) and x(2) directions in the (001) plane (nu(12)=-0.061, nu(21)=-0.051). The aggregate Poisson's ratio of MFI, although positive, has an anomalously low value nu=0.175(3) compared to other silicate materials. These results suggest that the template-free MFI-silicalite [Si(96)O(192)] might have potential applications as tunable sieve where molecular discriminating characteristics are adjusted by application of stress along specific axes.

Solid-State NMR Studies of the Fluoride-Containing Zeolite SSZ-44

Magic-angle spinning NMR has been used to probe the local structure of as-made zeolite SFF containing occluded fluoride and N,N-diethyl-2,6-dimethylpiperidinium. The Si−F bond distance has been measured using two methods: variable contact time cross polarization studies (1.74 A) and spinning sideband intensity fitting at relatively slow spinning speeds (1.79 A). Both measurements yield results that are significantly shorter than the internuclear Si−F distance as measured by single-crystal X-ray diffraction (∼1.9 A), which is based on the average positions of the atoms due to disorder and hence does not reflect the true bond distance.

１‐ブタノール存在下でのゼオライト合成におけるシリカ源の影響

Synthesis of high-silica zeolites in the presence of 1-butanol(n-BuOH) was conducted under various conditions to explore the role of silica source, aging time, and n-BuOH.It was found that the solubility of the silica source in the aqueous phase was a critical factor for the formation of zeolite precursor when KOH was employed as an alkali source. MFI(ZSM-5) type zeolites were obtained when fumed silica with small BET specific surface area was employed, whereas TON(Theta-1) type zeolites were formed when fumed silica with large BET specific surface area was employed. It was also found that the addition of n-BuOH suppressed the formation of cristobalite. This is probably due to the reduction of the Si concentration in the aqueous phase of the synthesis gel. FT-IR and 13C CP MAS NMR measurements of as-made zeolites suggested that n-BuOH molecules were occluded in the zeolitic pores of TON and MFI type zeolites at the initial stage of crystallization.

ゼオライト合成における結晶化調整剤の影響

To get more information concerning zeolite synthesis in the presence of 1-butanol (n-BuOH), influences of the alkali metal cation and the synthesis conditions on the zeolite synthesis were investigated. It was found that highly crystalline TON (Theta-1), MER (Merlinoite) and LTL (L) type zeolites were synthesized when KOH was employed as an alkali source, whereas MFI (ZSM-5) and MOR (Mordenite)type zeolites for NaOH. Characterization of zeolites obtained was conducted using X-ray diffraction, scanning electron microscopy, FT-IR, thermal analysis, 13C and 27Al MAS NMR. From the results of FT-IR and 13C CP MAS NMR measurements of as-made zeolites, it was concluded that n-BuOH molecules were occluded in the zeolitic pores of TON and MFI type zeolites, while no n-BuOH molecules for MOR, MER and LTL type zeolites.